Coinage Metal Hydrides: Synthesis, Characterization, and Reactivity

Review pubs.acs.org/CR

Coinage Metal Hydrides: Synthesis, Characterization, and Reactivity Abraham J. Jordan,‡ Gojko Lalic,*,† and Joseph P. Sadighi*,‡ †

Department of Chemistry, University of Washington, Seattle, Washington 98195, United States School of Chemistry & Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive NW, Atlanta, Georgia 30332, United States

‡

ABSTRACT: Hydride complexes of copper, silver, and gold encompass a broad array of structures, and their distinctive reactivity has enabled dramatic recent advances in synthesis and catalysis. This Review summarizes the synthesis, characterization, and key stoichiometric reactions of isolable or observable coinage metal hydrides. It discusses catalytic processes in which coinage metal hydrides are known or probable intermediates, and presents mechanistic studies of selected catalytic reactions. The purpose of this Review is to convey how developments in coinage metal hydride chemistry have led to new organic transformations, and how developments in catalysis have in turn inspired the synthesis of reactive new complexes.

CONTENTS 1. Introduction 2. Synthesis, Characterization, and Key Reactions of Coinage Metal Hydrides 2.1. Copper Hydrides 2.1.1. Binary Copper Hydride 2.1.2. Neutral (Phosphine)copper Hydride Oligomers 2.1.3. N-Heterocyclic Carbene and Cyclic Alkylamino Carbene Ligands 2.1.4. Hydride-Bridged Oligocopper Cations 2.1.5. Larger Copper Hydride Clusters 2.2. Silver Hydrides 2.2.1. Clusters Supported by Dichalcogen Donor Ligands 2.2.2. (N-Heterocyclic Carbene)silver Hydrides 2.2.3. (Phosphine)silver Hydrides 2.3. Gold Hydrides 2.3.1. (N-Heterocyclic Carbene)gold(I) Hydrides 2.3.2. (Phosphine)gold(I) Hydrides 2.3.3. A Stable Gold(III) Hydride 3. Organic Transformations Mediated by Coinage Metal Hydride Complexes 3.1. Reduction Reactions Mediated by Copper Hydride Complexes 3.1.1. Hydrogenation and Hydrosilylation of Carbonyl Compounds 3.1.2. Reduction of Alkenes and Alkynes 3.1.3. Reductions of CO2 3.1.4. Reductions of Other Functional Groups 3.2. Hydrofunctionalization Reactions Mediated by Copper Hydride Complexes 3.2.1. Hydrofunctionalization of Conjugated Alkenes and Alkynes 3.2.2. Hydrofunctionalization of Aryl- and Heteroaryl-Substituted Alkenes © 2016 American Chemical Society

3.2.3. Hydrofunctionalization of Alkynes 3.2.4. Hydrofunctionalization of Alkenes 3.3. Silver Hydrides in Catalysis 3.4. Gold-Hydride-Catalyzed Transformations 4. Mechanistic Studies of Coinage Metal Hydrides in Catalysis 4.1. Mechanisms of Copper-Hydride-Catalyzed Reactions 4.1.1. Conjugate Reduction of α,β-Unsaturated Ketones 4.1.2. Hydrosilylation of Ketones 4.1.3. (NHC)CuH-Catalyzed Semireduction of Alkynes 4.1.4. Hydrocarboxylation of Alkynes 4.1.5. Hydroalkylation of Alkynes 4.1.6. 1,3-Halogen Migration/Borylation 4.1.7. Asymmetric Hydroamination of Styrene 4.2. Silver-Catalyzed Dihydrogen Activation 4.3. Gold Hydrides in Catalytic Alcohol Silylation 5. Conclusions Author Information Corresponding Authors Notes Biographies Acknowledgments References Note Added in Proof

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1. INTRODUCTION The coinage metals, copper, silver, and gold, form hydride complexes with a remarkable range of architectures, presenting

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DOI: 10.1021/acs.chemrev.6b00366 Chem. Rev. 2016, 116, 8318−8372

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represent partial covalent bonding between a metal and hydrogen, or two metal centers. Exceptions occur in early cases where researchers tentatively proposed a structure, and the use of this notation would be anachronistic. The notation M···M represents a close approach between metal centers, in which the intermetallic distance suggests a possible bonding interaction but does not require it.

thought-provoking interactions between metal and hydrogen, and often between metals as well. The metals in these hydrides are almost always monovalent. Unlike the earlier transition metals, their d-subshells are thus fully occupied; unlike the group 12 or later metals, their d-electrons remain energetically accessible, with consequences for their bonding and reactivity. The coinage metal hydrides thus represent a middle ground between transition metal and main group metal hydrides. The relatively low electropositivity of these metals results in rather covalent metal−hydrogen bonds. A theoretical study comparing group 1 and group 11 metal hydrides,1 for example, found a cubic tetrameric structure [MH]4 for the more ionic alkali metal hydrides. For the coinage metal hydrides, the reduced dipole moment and small charge separation in the metal− hydrogen bonds contribute to a strong preference for planar cyclic structures. Combined spectroscopic and theoretical studies have found that hydrogen can mimic the coinage metals in small clusters.2,3 The distinctive properties of copper−, silver−, and gold−hydrogen bonds confer unique reactivity, which underpins a broad and growing array of catalytic transformations. The purpose of this Review is to explore copper, silver, and gold hydrides from three basic perspectives: (1) a synthetic and structural perspective, focused mainly on the nature of these complexes in themselves; (2) a catalytic perspective, exploring the range of organic chemistry they enable; and (3) a mechanistic perspective, examining how they do so. A key goal of this Review is to present the great expansion of coinage metal hydride chemistry from 2008 to the present, while placing it in the context of longstanding results. A recent account by Liu and co-workers reviews the synthesis and structures of copper hydrides.4 In this Review, more emphasis will be placed on solution-phase characterization and reactivity where available, and the scope expands to include silver and gold hydrides. Earlier reviews published in 20075 and 20086 focused on copper hydrides in the catalysis of organic reactions. New reaction pathways7 have since been achieved in copper hydride chemistry, while the study of silver and gold hydrides, then in its infancy, has accelerated dramatically. Another goal is to convey the increasingly rapid iteration among the synthesis, characterization, catalysis development, and mechanistic study of these hydrides. This focus produces a heavy emphasis on complexes that form readily under ordinary laboratory conditions, and persist long enough to undergo intermolecular reactions. Theoretical studies, and experimental studies under matrix isolation or in the gas phase, will be discussed where they elucidate the synthetic studies, but will not be covered systematically. Although the chemistry of heterometallic hydride clusters containing coinage metals is a rich field, with examples including other transition metals as well as main group elements, these complexes will be discussed only in cases where they arise from, or give rise to, purely coinage-metal-bound hydrides. This constraint serves to focus the Review on the properties and reactivity specific to these metals. A recurring motif in the coinage metal hydride series is the presence of at least one hydride bridge. All structurally characterized copper and silver hydrides prepared under ordinary conditions, as opposed to matrix isolation or the gas phase, feature such bridges, and gold hydrides studied so far include several hydride-bridged examples. Different philosophies exist on how best to depict these three-center, twoelectron arrangements.8 This Review will use dashed lines to

2. SYNTHESIS, CHARACTERIZATION, AND KEY REACTIONS OF COINAGE METAL HYDRIDES 2.1. Copper Hydrides

2.1.1. Binary Copper Hydride. Uniquely among the coinage metals, copper forms an isolable binary hydride under ordinary conditions. In 1844, Wurtz reported the reduction of cupric sulfate with hypophosphorous acid in aqueous solution to form a red-brown precipitate. Investigating its chemical composition and its reactivity, he concluded that this product consisted of copper combined with 1200 times its volume of hydrogen gas, and noted that decomposition with hydrochloric acid produced twice the volume of hydrogen as simple thermal decomposition.9 Some 35 years later, in a note titled, “On the Substance Designated Under the Name of Copper Hydride”, Berthelot questioned the nature of this compound, characterizing it instead as a mixture of elemental copper, copper hydroxide, and copper phosphate.10 A rapid and pointed exchange followed,11 with Berthelot stating, “It is incumbent on Mr. Wurtz, and not on me, to obtain pure copper hydride and to establish that oxygen and phosphorus are not inherent in its composition...if he wants this hydride to continue to figure in Science.”12 Wurtz began his retort, “I am pleased to note that the substance designated under the name of copper hydride has again become copper hydride, at least in the title of the new note by Mr. Berthelot.” Assembling the data in support of his original assignment, he noted the special and remarkable properties of copper hydride, expressed pride in his finding, and thanked Berthelot for calling attention to it.13 Revisiting Wurtz’s synthesis, Müller and Bradley obtained a product with the composition of Cu1H0.82 using Wurtz’s conditions (Wurtz’s report of 1200 volumes of hydrogen per copper corresponds to Cu1H0.76), and Cu1H0.97 under modified conditions. Using X-ray powder diffraction, they found a hexagonal close-packed structure, with calculated cell parameters a = 2.89 Å and c/a = 1.59−1.60.14 Subsequent investigations by both X-ray and neutron diffraction confirmed the Wurtzite structure of copper hydride (Figure 1).15 Analysis of copper deuteride, with the composition Cu1D0.73H0.07, by solid-state 2H and 63Cu NMR spectrometry revealed a temperature-independent chemical shift for 63Cu of 780 ppm relative to CuCl, as compared to 2350 ppm for pure metallic copper. The latter signal was observed as well, but at the exact intensity obtained from the empty probe, and was attributed to copper wiring present in the instrument. Despite the substoichiometric composition of copper hydride/deuteride, the authors thus concluded that no copper metal was present in the sample.16 Copper hydride is unstable, however, with a free energy of formation under standard conditions calculated at +51 kJ/mol.17 Its decomposition to the elements is complete within a day at ambient temperature.18 Recently, a structural reinvestigation, using both X-ray and neutron diffraction, was carried out on both copper hydride and copper deuteride prepared by various routes. The refinement data were consistent with statistical occupation by deuteride of fewer 8319


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What could be the nature of copper hydride itself in solution? The impurities found by Dilts and Shriver were substantial, but not enough to require a fundamentally different stoichiometry. Later computational studies of molecular copper hydrides at the B3LYP level of theory found that oligomers [CuH]n (n = 3−6) should adopt stable, planar ring structures (Figure 2) with H−Cu−H moieties approaching linearity, and multicenter bonding resulting in short Cu···Cu contacts, from 2.404 Å in the trimer to 2.556 Å in the hexamer.22,23 Calculated 1 H NMR chemical shifts for the bridging hydrides reflect significant effects from subtle structural changes, with a pronounced upfield shift as the Cu···Cu distances lengthen and the Cu−H distances contract.22 Ab initio calculations on the dimeric hydride [Cu2(μ-H)2] find no direct copper−copper bond, but delocalized bonding among the copper and hydrogen centers results in the shortest predicted copper−copper distance: 2.15 Å, as compared to 2.22 Å for σ-bound Cu2.24 Cryoscopic evidence for monomeric [CuH] in solution might suggest sufficiently strong ligation by pyridine to overcome the formation of strong hydride bridges, yet most of the pyridine is readily lost on drying. 2.1.2. Neutral (Phosphine)copper Hydride Oligomers. Cryoscopic studies on solutions of copper hydride with added phosphine, phosphite, and bipyridine donors gave results consistent with the formation of ligated complexes, including dinuclear copper hydrides in the case of the most basic donors.26 In the presence of n-Bu3P, for example, the molality of the copper hydride reached a minimum at a phosphine:copper hydride ratio of 1:2. The authors explained the implied empirical formula of [(n-Bu3P)(CuH)2] in terms of multicenter bonding (Figure 3). All of the ligated copper hydrides proved thermally sensitive, and decomposed substantially during attempted isolation.

Figure 1. Crystal structure of copper hydride (deuteride). Gray spheres represent copper positions; black spheres represent hydrogen (deuterium) positions for an ordered model; white spheres represent alternate positions in a disordered model, in which nearest black/white pairs cannot both be occupied. Reproduced with permission from ref 19. Copyright 2014 Wiley-VCH.

than one-half [0.368(6)] of the tetrahedral voids in a hexagonal close-packed structure (Figure 1). If the voids within facesharing tetrahedra were occupied simultaneously by hydrides, the H−H distance would be too short at 0.939(5) Å, but with less than 50% occupancy this condition can be avoided.19 A variation on the preparation of binary copper hydride appeared in 1952, when Wiberg and Henle treated a solution of cuprous iodide in pyridine with a solution of LiAlH4 in mixed diethyl ether and pyridine. The reaction product did not precipitate until an excess of ether was added. Redissolution in pyridine, followed by reprecipitation with ether and drying under vacuum, gave a product described as identical to that of Wurtz’s preparation, except that no water was present. Thus, this copper hydride reduced benzoyl chloride to benzaldehyde, whereas the precipitate from aqueous solution, even after drying, hydrolyzed it to benzoic acid.20 Revisiting this synthesis and analyzing the product in more detail, Dilts and Shriver confirmed the formation of copper hydride with compositions ranging from Cu1H0.58 to Cu1H0.96, averaging Cu1H0.81.21 They also found the presence of pyridine, 4−20%, lithium iodide, 1− 2%, and cuprous iodide, 10−20%, in reprecipitated, dried samples. Cryoscopic measurements on solutions in pyridine, ranging in concentration from 0.03−0.18 M, suggested the presence of monomeric copper hydride, with apparent molecular weights of 60 ± 6 g/mol. These researchers were unable to observe copper-bound hydrides in these solutions by infrared or 1H NMR spectroscopy. Base-mediated decomposition of [CuD] in methanol gave rise to 73% D2 and 24% HD, indicating the operation of competing mechanisms for hydrogen release.

Figure 3. Structures proposed for [(n-Bu)3P(CuH)2]; composition inferred from cryoscopic measurements. Adapted from ref 26. Copyright 1969 American Chemical Society.

The preparation of pyridine-soluble copper hydride was modified further by Whitesides and co-workers, who used diisobutylaluminum hydride as the hydride source and copper(I) bromide as the starting copper complex. The resulting product after isolation contained ∼25% pyridine, but less than 0.5% aluminum or bromide, and corresponded to

Figure 2. Calculated structures for cyclic [CuH]n oligomers. Distances (in pm) and angles (deg) are given nearest the feature to which they refer; nucleus-independent chemical shift25 values are circled; δH are the predicted 1H NMR chemical shifts, in ppm, for the bridging hydrides. Image kindly provided by Prof. C. A. Tsipis. Copyright 2003 American Chemical Society. 8320


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a composition of Cu1H0.92−Cu1H1.0. These researchers then prepared solutions of this hydride in the presence of 1 equiv of n-Bu3P, to study its reactivity. They described the hydrodehalogenation of iodobenzene to form benzene in 80% yield, and demonstrated the stereospecific formation of C−D bonds by reaction of [(n-Bu3P)CuD]) with copper alkyls (Scheme 1).27 No reaction was observed between [(n-Bu3P)CuH] and alkenes such as 1- or 2-butene or norbornene. Scheme 1. Stereospecific Formation of a C−D Bond from a Copper(I) Deuteride and a Copper(I) Alkyl27

Figure 4. Two views of the [PCuH]6 core of {[(p-tolyl)3P]CuH}6, as determined by neutron diffraction: In (a), two hydrides (center) appear symmetric with respect to Cu3 faces; (b), from a different angle, suggests edge-bridging as the closer description. Reproduced with permission from ref 33. Copyright 2014 American Chemical Society.

The first molecular copper hydride to be isolated in pure form was [(Ph3P)CuH]6, prepared by the reduction of (triphenylphosphine)copper(I) chloride with sodium trimethoxyborohydride in N,N-dimethylformamide (DMF) solution, from which it forms bright red crystals of a DMF adduct.28,29 As with solid [CuH] and the soluble variants with and without added ligands, the copper-bound hydrides were not detected by either infrared or NMR spectroscopy; however, decomposition of the product using PhCO2D gave rise to mixtures of H2 and HD. The solid-state structure obtained by X-ray diffraction showed a distorted octahedron of copper centers, each bearing an apical phosphine ligand. Six relatively long Cu···Cu contacts, 2.632(6)−2.674(5) Å, and six relatively short ones, 2.494(5)− 2.595(5) Å, were identified. Although even the longer distances fall within two van der Waals radii for copper (2.8 Å),30 the authors hedged their case for copper−copper bonding, citing complexes with similar Cu···Cu contacts in which such bonding is unlikely. Although the data did not permit refinement of the hydride positions, the authors discussed three plausible binding modes, doubly bridging along each long edge, doubly bridging each short edge, or triply bridging over the six short faces of the octahedron, and argued for the first of these modes as most likely. As the reactivity of [(Ph3P)CuH]6 became more widely explored (see below), interest in its exact structure grew. In a structural study of cyclohexyllithium, Stucky and co-workers found a distorted octahedron of lithium centers with notably short Li···Li contacts, and triply bridging carbanions over the six shorter faces; they suggested that the bridging hydrides in [(Ph3P)CuH]6 might likewise lie over faces rather than edges.31 Finding {[(p-tolyl)3P]CuH}6 more amenable to the growth of suitable crystals, Bau, Koetzle, and co-workers carried out a neutron diffraction study and found all hydrides capping the six small triangular faces of the octahedron, with an average Cu−H distance of 1.76(3) Å.32 In 2014, however, a study combining neutron diffraction analysis of {[(p-tolyl)3P]CuH}6, inelastic neutron scattering (INS) spectroscopy on [(Ph3P)CuH]6, and ab initio calculations on a model complex found the hydrides to be edge-bridging.33 The Cu−H distances in the neutron diffraction study of {[(p-tolyl)3P]CuH}6 were 1.72, 1.73, and 1.99 Å; those derived from INS spectroscopy on [(Ph3P)CuH]6 were 1.67 and 1.92 (±0.06) Å. Figure 4 shows two views of the [PCuH]6 core of {[(p-tolyl)3P]CuH}6, with view (a) showing how the hydrides might appear face-capping, but (b) suggesting more nearly edge-bridging geometry. The

authors acknowledge the possibility of semiface-bridging, however, and the long Cu−H distance of 1.99 Å does not preclude a significant bonding interaction. In any case, these data strongly suggest a more complicated picture than simple, symmetrically bound μ3-hydrides. In 1981, exploring the potential of the (phosphine)copper(I) hydride hexamer for synthesis gas conversion chemistry,34 Goeden and Caulton reported findings that underpinned many later developments in copper hydride chemistry.35 Their analysis of {[(p-tolyl)3P]CuH}6 by 1H NMR spectroscopy revealed for the first time, in addition to the p-tolyl-derived resonances, a broad, structured multiplet at δ 3.50 ppm. This resonance could be resolved to five lines of the septet predicted if each hydride were to couple to six phosphorus centers, and indeed collapsed to a singlet under 31P-decoupling. The apparent equivalence of the phosphines in coupling to the hydride indicated a fluxional structure in solution, interpreted in terms of intramolecular hydride migration. The positive chemical shift for this hydride was compared to that of several d0 hydrides, and contrasted to the highly negative shifts of metal hydrides with partially filled d-subshells. In the presence of 1 equiv of free phosphine per copper, phosphine exchange was not observed on the NMR time scale, and the hydride resonance retained its line shape, thus ruling out the generation of [(Ar3P)2CuH]n oligomers or monomer in significant amounts under these conditions. Interested in understanding the conversion of CO to methanol in heterogeneous systems, these authors examined the interaction of the copper hydride hexamer with formaldehyde, a plausible intermediate in this reaction, and observed catalytic disproportionation to methyl formate. When the corresponding copper deuteride was employed, the deuterium appeared only in the methyl group, resulting in the stoichiometric formation of HCO2CH2D, and catalytic formation of HCO2CH3. They proposed the catalytic cycle shown in Scheme 2 to account for these results. Notably, this cycle includes a hydride shift from the methoxymethyl group to copper to regenerate the hydride and form the carbonyl. Having observed the stability of the copper hydride hexamer in the presence of excess methanol in benzene solution, these authors demonstrated the hydrogenolysis of copper(I) tertbutoxide, in the presence of excess tri-p-tolylphosphine, to form 8321


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Scheme 2. Catalytic Disproportionation of Formaldehyde to Methyl Formate35

the (phosphine)copper(I) hydride hexamer, and confirmed the liberation of free tert-butanol. This finding represents the first hydrogenolysis of a metal alkoxide to form a metal hydride, and reinforced the inference by Halpern and Peters, from kinetic results, that copper(II)36 and copper(I) salts37 could heterolyze dihydrogen to form transient copper hydride intermediates. Hydrogenolysis of copper(I) tert-butoxide in the presence of phosphine proved a versatile method for the synthesis of new copper hydride clusters (Scheme 3).38 The ligand P(NMe2)3,

Figure 5. Structural representation of [(dppp)4Cu8H8]. Note that bridging hydrides were not refined. Image prepared from published crystallographic information file for ref 38. Copyright 1985 American Chemical Society.

Scheme 3. Copper Hydride Clusters from Copper Alkoxide Hydrogenolysis35,38

A remarkable change in outcome for the hydrogenolysis of copper(I) tert-butoxide was observed when dppp, CH 2 (CH 2 PPh 2 ) 2 , was replaced with triphos, CH 3 C(CH2PPh2)3: the formation of a Cu2H2 dimer, in which each triphos serves as a bidentate chelating ligand, with one pendant −CH2PPh2 arm.39 The 31P NMR spectrum showed inequivalent phosphine resonances for free and bound phosphines, in the expected ratio, and these resonances did not coalesce on warming to the sample decomposition temperature. The 2D NMR spectrum of the corresponding copper deuteride dimer displayed a singlet resonance at δ 1.83 ppm. The infrared spectrum revealed an absorbance at 950 cm−1, assigned to ν(CuH); this absorbance was replaced by one at 680 cm−1 in the spectrum of the copper deuteride dimer. In the solid state, the Cu···Cu distance of [(κ2-triphos)Cu(μ-H)]2 was remarkably short at 2.371(2) Å; the hydride positions were refined from the X-ray crystal data, with Cu−H distances of 1.66(8) and 1.81(8) Å. Like [(Ph3P)CuH]6,40 the hydride dimer reacted with CO2 to afford a copper formate. In this case, the formation of [(κ3-triphos)Cu(κ1-O2CH)] (Scheme 4) proved that this ligand can readily bind in terdentate fashion to give

for example, gave rise to the hexameric cluster {[(Me2N)3P]CuH}6. A hydride resonance at δ 2.81 ppm was resolved using 31 P-decoupled 1H NMR spectroscopy at −70 °C, and was compared to the deuteride resonance (δ 2.57 ppm) in the 2H NMR spectrum of {[(Me2N)3P]CuD}6, recorded at the same temperature. The solid-state structure, obtained through X-ray diffraction, was broadly similar to that of {[(p-tolyl)3P]CuH}6: The copper centers formed a distorted Cu6 octahedron, with six relatively long and six relatively short Cu···Cu contacts. For the first time, however, copper-bound hydrides could be located and refined. These hydrides were found to be triply bridging over the six short faces of the Cu6 core, consistent with the early structural suggestion for [(Ph3P)CuH]6 by Stucky,31 and contrasting with the much later findings by Parker and coworkers.33 Use of the bidentate and potentially chelating ligand 1,3-bis(diphenylphosphino)propane (dppp) likewise resulted in hydrogenolysis, but formed a very different cluster, the octanuclear [(dppp)4Cu8H8]. Figure 5 shows the [Cu8] core of this complex, with each bisphosphine bridging two copper centers. Although the hydride ligands could not be refined from the X-ray crystallographic data, the presence of face-capping hydrides was proposed to give a dodecahedral form. Like the hexanuclear clusters, the Cu8 core featured equal numbers of relatively long [2.676(3)−2.738(3) Å] and relatively short [2.453(3)−2.517(3) Å] copper···copper contacts. The 1:1 ratio of phosphine resonances in the 31P NMR spectrum, unchanged over the range of +20 to −70 °C, was consistent with stereochemical rigidity in the (phosphine)copper core.

Scheme 4. A Hydride Bridge Preferred to a Viable Third Cu−P Bond39

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four-coordinate copper(I), and that the Cu−H bridges formed by dimerization of hypothetical (κ3-triphos)CuH are stronger than the Cu−P bonds lost. Further investigation of the reaction between (phosphine)copper(I) chloride precursors and hydride reagents revealed new variations on (phosphine)copper(I) hydride clusters. Healy, White, and co-workers found that the addition of KSelectride (potassium tri-sec-butylborohydride) to a 1:1 mixture of triphenylphosphine and copper(I) chloride in THF afforded [(Ph 3 P)CuH] 6 , and also a probable oxo complex [(Ph3P)6Cu6H4O], described tentatively as the product of partial hydrolysis. Hydride transfer to (Ph3P)3CuCl under similar conditions led to crystallization of the new pentameric hydride [(Ph3P)CuH]5, and subsequently of the familiar hexamer. The Cu5 core of the pentamer formed a distorted trigonal bipyramid in the solid state (Figure 6), with distances between adjacent Cu centers ranging from 2.431(7) to 2.608(7) Å.41

Figure 7. Storable (phosphine)copper hydrides, generated as solutions.45,46 Note: Solution nuclearity is unknown.

complex is typically depicted as a monomer, although a dimeric hydride-bridged structure appears plausible. Building on this work, Lipshutz and co-workers examined the effect of the rigid bidentate ligand 1,2-bis(diphenylphosphino)benzene (o-BDPPB, variously shortened to BDP or denoted as dppbz) in hydrosilylation catalysis, and found that it, like DTBM-Segphos, gave rise to a copper hydride complex (Figure 7b) that retained high catalytic activity after long periods at ambient temperature in solution.46 Preparation of this species by addition of dppbz to a solution of [(Ph3P)CuH]6 in C6D6 solution resulted in the disappearance of the hydride resonance at δ 3.55 ppm, and the appearance of a new resonance at δ 1.49 ppm. In marked contrast, Norton and co-workers observed this resonance initially during the preparation of a (dppbz)copper(I) hydride by hydrogenolysis of a tert-butoxide intermediate, generated in situ, but isolated a product complex with a copper hydride resonance at δ 0.60 ppm. Structural analysis of this product by X-ray crystallography revealed a trimeric structure (Figure 8), featuring a triangular Cu3 core with edge-bridging hydrides.47 The Cu···Cu distances of 2.555(1), 2.564(2), and 2.619(1) Å fall within the range bracketed by [(Ph3P)CuH]6.28 The new clusters {[(p-anisyl)3P]CuH}6 and [(i-PrPh2P)CuH]6 were also prepared by hydrogenolysis, and crystallographically characterized.

Figure 6. Structural representation of [(Ph3P)CuH]5. Note that bridging hydrides were not refined. Image prepared from published crystallographic information file for ref 41. Copyright 1989 American Chemical Society.

While exploring the copper-catalyzed 1,2-hydrosilylation of ketones and imines, and the conjugate hydrosilylation of enones,42 Lipshutz and co-workers found that nonracemic biphenyl-derived bisphosphines enabled highly efficient and enantioselective catalysis, which will be discussed in section 3.1.1. These findings built on earlier work by Buchwald and coworkers on asymmetric hydrosilylation using (BINAP)copper systems.43,44 On reaction with a suitable silane, precatalysts such as copper(II) acetate monohydrate, in combination with the ligand DTBM-Segphos (DTBM = 3,5-di-tert-butyl-4methoxyphenyl), formed a copper(I) hydride that was stable in solution at ambient temperature, a storable “CuH in a bottle” (Figure 7a).45 The 1H NMR spectrum of this species in C6D6 solution displayed a rather broad singlet resonance at δ 2.55 ppm, assigned to the copper-bound hydride. The nuclearity of this complex remains unknown, and for practical purposes the

Figure 8. A trimeric copper hydride: thermal ellipsoid representation at 50% probability. Reproduced with permission from ref 47. Copyright 2013 American Chemical Society. 8323


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This study also presented the one-electron oxidation of (triarylphosphine)copper hydride hexamers. Cyclic voltammetry of [(Ar3P)CuH]6 in CH2Cl2 solution gave reversible waves with E1/2 = −1.01 V (Ar = Ph), −1.20 V (Ar = p-tolyl), and −1.15 V (Ar = p-anisyl) relative to Fc/Fc+ [Fc = [(η5C5H5)2Fe]. The chemical reversibility of these oxidations suggested that the radical cations [(Ar3P)CuH]6+• were stable on the time scales of these measurements, and indeed the cation [(Ph3P)CuH]6+•, although unstable, was found to persist for at least 30 min. The spectroelectrochemisty of [(Ph3P)CuH]6 at an applied potential of −0.800 V versus Fc/Fc+ showed a new absorbance for the green oxidized product at λmax 655 nm, as compared to a λmax of 524 nm for the neutral hexamer. During the reduction of 1-(phenoxycarbonyl)pyridinium cation by [(Ph3P)CuH]6, stopped-flow UV−vis measurements demonstrated the appearance of the 655 nm absorbance, indicating that the net hydride transfer involves a single-electron transfer prior to C−H bond formation (Scheme 5).

solution was consistent with the formation of (EtO)3SiOt-Bu and a product corresponding to [(IDipp)CuH], with a singlet resonance at δ 2.67 ppm integrating to one hydrogen per IDipp ligand. The intense yellow color suggested an oligomeric form; for comparison, the monomeric (IDipp)CuCH3 is colorless. The infrared spectrum of the hydride product displayed an absorbance at 881 cm−1, which shifted to 638 cm−1 in the spectrum of the corresponding deuteride. Assigned to the Cu− H and Cu−D stretching modes, these peaks occurred at somewhat lower frequencies than those of Caulton’s dimeric [(κ2-triphos)Cu(μ-H)]2.39 This copper hydride displayed limited stability in solution, with some deposition of copper metal visible after an hour at ambient temperature. Single crystals suitable for X-ray diffraction were obtained by vapor diffusion of hexamethyldisiloxane into a solution of the copper hydride in n-pentane at −40 °C. The solid-state structure (Figure 9) revealed the

Scheme 5. One-Electron Oxidation of [(Ar3P)CuH]6 Complexes and Hydride Transfer47

Figure 9. Solid-state structure of [(IDipp)Cu(μ-H)]2, shown as 50% ellipsoids. For clarity, only one molecule is shown, of four in the asymmetric unit. Note: The bridging hydrides were not refined, and the Cu(1)−Cu(2) vector does not denote a full bond. Selected bond lengths and angles: Cu(1)−C(1), 1.878(6) Å; Cu(2)−C(28), 1.882(6) Å; Cu···Cu, 2.3059(11); C(1)−Cu(1)−Cu(2), 169.4(2)°, Cu(1)−Cu(2)−C(28), 170.33(19)°. Reproduced with permission from ref 52. Copyright 2004 American Chemical Society.

2.1.3. N-Heterocyclic Carbene and Cyclic Alkylamino Carbene Ligands. The N-heterocyclic carbene (NHC) ligand tends to confer low coordination numbers on copper(I) centers. For example, IDipp [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene,48 originally and more commonly denoted IPr], supports the two-coordinate, linear monomers (IDipp)CuCl49,50 and (IDipp)CuO2CCH3.51 The reaction of monomeric (IDipp)CuOt-Bu with triethoxysilane (Scheme 6) or 1,1,3,3-tetramethyldisiloxane afforded an intense yellow solution. The 1H NMR spectrum of the product mixture in C6D6

dimeric arrangement [(IDipp)Cu(μ-H)]2, with four distinct molecules crystallized in the asymmetric unit, and apparent πstacking between ligand N-aryl groups in neighboring dimers. Although the hydrides could not be refined, their presence was known from the 1H NMR spectrum and reactivity of the product complex. The most striking feature of the structure was the short Cu···Cu contacts, ranging from 2.2946(11) to 2.3093(11) Å. The Cu−CNHC distances ranged from 1.878(6) to 1.898(7) Å. Insertion of 3-hexyne into [(IDipp)Cu(μ-H)]2, generated in situ, resulted in clean formation of the stable, isolable copper vinyl complex [(IDipp)Cu(3-hexenyl)].52 A vinyl intermediate was earlier inferred from isotopic labeling results in the reduction of diphenylacetylene to cis-stilbene by [(Ph3P)CuH]6,53 which will be discussed further in section 3.1.2. It is not yet known why a dimeric [(NHC)Cu(μ-H)]2 undergoes clean 1,2-insertion of alkyne, whereas the betterknown [(Ph3P)CuH]6 effects a net hydrogenation. One possibility is that the dimeric structure is preferred in the solid state, whereas in solution a significant or even

Scheme 6. Synthesis and Alkyne Reactivity of an (NHC) Copper Hydride52

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monomer−dimer equilibrium lay far on the side of the dimers.57 The inferred (NHC)copper hydrides in many catalytic processes are fleeting intermediates. While studying the copper-catalyzed hydroalkylation of terminal alkynes, however, Lalic and co-workers demonstrated that the reaction of (5Dipp)CuF [5Dipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, also commonly abbreviated SIPr] with 1,1,3,3-tetramethyldisiloxane (Scheme 7) resulted in the rapid

predominant fraction of the hydride is present as a monomer. In their study of a tandem reductive aldol reaction, Nolan, Riant, and co-workers found that the reaction of (IDipp)Cu(DBM) (DMB = dibenzoylmethanide) with any of several silanes gave rise to a sharp singlet resonance at δ 4.46 ppm in the 1H NMR spectrum. Contrasting this chemical shift to that observed for [(IDipp)Cu(μ-H)]2, they ascribed the difference to an interaction of the copper hydride with the silylated dibenzoylmethane byproduct, although the nature of this interaction was not pursued.54 A theoretical study at the SV(P)/PBE-D3 level found the dimerization of (IPh)CuH (IPh = 1,3-diphenylimidazol-2-ylidene) to be nearly thermoneutral with reversion to monomer favored principally by entropy, and ΔE for dimerization of +5 kJ/mol.55 Likewise, in an experimental and theoretical investigation of ketone hydrosilylation catalyzed by (NHC)copper(I) complexes, where the NHCs were IDipp, IMes, or IAd (I = 1,3-disubstituted imidazol-2-ylidene; Dipp = 2,6-diisopropylphenyl, Mes = 2,4,6-trimethylphenyl, Ad = 1-adamantyl), Leyssens and coworkers inferred from kinetic data that any [(NHC)CuH]2 dimers were fully dissociated to monomers in solution. Their theoretical findings suggested that the dimers are only modestly preferred in the gas phase, with Keq for dimerization of ∼1− 100, and they argued that these equilibria should be shifted strongly in favor of monomers in solution.56 On the other hand, Schomaker and co-workers used NMR experiments to establish the nature of [(IDipp)CuH] and closely related species (Figure 10) in THF solution. These related species bore

Scheme 7. A Copper(I) Hydride Bearing a SaturatedBackbone NHC58

and quantitative formation of [(5Dipp)CuH] as a bright yellow solution in C6D6, with a singlet resonance integrating to one hydride per 5Dipp ligand at δ 1.93 ppm.58 Although the nuclearity of this new hydride was not established, its intense color is consistent with that observed for the dimeric [(IDipp)Cu(μ-H)]2. The [(5Dipp)CuH] solution was observed to be stable at ambient temperature for at least 30 min, suggesting that an NHC containing a saturated backbone conferred at least as much stability on the ligated copper hydride as its unsaturated imidazol-2-ylidene analogue. A marked increase in copper hydride stability, relative to that observed using 5-membered NHC ligands, was achieved through the use of a cyclic alkylamino carbene (CAAC) ligand.59 After treatment of a (CAAC)copper(I) tert-butoxide precursor with Li+[HBEt3]− solution at low temperature, Bertrand and co-workers were able to isolate the corresponding [(CAAC)Cu(μ-H)]2 dimer as an orange powder in 84% yield (Scheme 8). This complex proved stable at ambient temperScheme 8. Induced Carbene Insertion of a Stable [(CAAC)Cu(μ-H)]259

Figure 10. Molecular weights of (N,N′-diaryl-NHC)copper hydrides studied by DOSY NMR in THF-d8 solution: (a) calculated molecular weight for monomer; (b) calculated molecular weight for dimer; (c) molecular weight(s) determined experimentally. The presence of two species in solution for the larger ligands, and the disparities between measured and calculated weights for the dimers, are ascribed to THFsolvation.57

3-pentyl or 4-heptyl groups in lieu of isopropyl groups on the ligand N-aryl substituents, and exhibited greater stability: Whereas the thermal decomposition of [(IDipp)Cu(μ-H)]2 in THF-d8 solution proceeded with a half-life of 72 min at 30 °C, the 3-pentyl-substituted analogue exhibited a half-life of 98 min at 58 °C. The 4-heptyl-substituted analogue displayed intermediate stability, with a half-life of 80 min at 45 °C, consistent with some slight destabilization of the dimer by the bulkiest alkyl group in the series. Diffusion (DOSY) experiments, using pulse gradient spin echo (PGSE) NMR to measure translational motion of molecules through solution, gave no evidence for significant monomer formation. Although other findings pointed to [(NHC)CuH] monomers as key reactive intermediates, these authors concluded that the

ature in the solid state and in solution. The hydride resonance could not be identified in the 1H NMR spectrum, due to overlapping resonances from the CAAC ligand, but the infrared spectrum displayed an absorption band at 981 cm−1 assigned to νCuH. This frequency is significantly higher than that measured for [(IDipp)Cu(μ-H)]2. The solid-state structure obtained through X-ray crystallography (Figure 11) revealed very short distances for the Cu···Cu contact [2.3058(5) Å] and the Cu−C bonds [1.861(3) and 1.862(3) Å]. The hydride positions could be refined isotropically; curiously, this refinement revealed 8325


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Scheme 9. Formation and Reactions of (Expanded-Ring NHC)copper Hydrides62

Figure 11. Solid-state structure of a stable [(CAAC)Cu(μ-H)]2. Hydrogen atoms except for hydrides are omitted for clarity. Selected bond lengths and angles: C(1)−Cu(1), 1.861(3) Å; Cu(1)−H(1), 1.71(6) Å; Cu(1)−H(2), 1.73(5) Å; Cu(1)···Cu(2), 2.3058(5) Å; H(1)−Cu(1), 1.55(5) Å; H(2)−Cu(2), 1.52(6) Å; Cu(2)−C(28), 1.862(3) Å; C(1)−Cu(1)−H(1), 133.9(17)°; C(1)−Cu(1)−H(2), 142.7(18)°Å; H(1)−Cu(2)−C(28), 130(2)°; H(2)−Cu(2)−H(2), 133(2)°. Reproduced with permission from ref 59. Copyright 2011 Wiley-VCH.

rather asymmetric hydride bridges, with Cu−H bond lengths of 1.71(6) and 1.73(5) Å, versus 1.55(5) and 1.52(6) Å. Treatment of this dimer in solution with phosphines or an isonitrile resulted in the net 1,1-insertion of the CAAC ligand into the hydride to form an (aminoalkyl)copper complex. Expanded-ring N-heterocyclic carbenes, derived from sixmembered or larger cyclic amidinium salts, have recently been explored as supporting ligands for copper hydride complexes. The wider N−C−N bond angles of these NHCs relative to their five-membered analogues result in greater σ-basicity,60 and also in greater steric encumbrance about the metal for a given N-substituent.61 Whittlesey and co-workers examined two such ligands with similar steric demand but very different electronic properties: the electron-poor 6MesDAC, a diamidocarbene, and the far more electron-rich 6Mes [1,3-bis(2,4,6trimethylphenyl)tetrahydropyrimid-2-ylidene].62 Copper(I) tert-butoxide precursors bearing either ligand were found to react with triethylsilane to form the silyl tert-butoxide, but the expected copper hydrides could not be observed. In the case of the 6MesDAC ligand, insertion of the NHC into the copper hydride formed (1,1-diamidoalkyl)copper products (Scheme 9a), in analogy to the ligand-induced insertion of a CAAC into a Cu−H bond.59 A similar reaction also occurred during the attempted isolation of [(6Mes)CuH]. The more electron-rich 6Mes, however, inserted slowly enough into the Cu−H bond to permit intermolecular reactions to compete effectively (Scheme 9b). Thus, the reaction of (6Mes)CuOt-Bu with Et3SiH in the presence of 1-phenylpropyne afforded the alkenyl complex {(6Mes)Cu[C(Ph)C(Me)H]}; (6Mes)CuOt-Bu also proved an effective precatalyst for the 1,2-hydrosilylation of cyclohexanone and a competent, albeit slow, precatalyst for alkyne semireduction using a silane plus tert-butanol. Both of these reaction types are discussed in sections 3.1.1, 3.1.2, 4.1.2, and 4.1.3. The greater steric demand of 6Dipp [1,3-bis(2,6diisopropylphenyl)tetrahydropyrimid-2-ylidene] as compared to 6Mes results in dramatically increased copper hydride

stability.63 Treatment of (6Dipp)CuOt-Bu with pinacolborane in THF solution results in the appearance of an intense yellow color. The product crystallizes from THF−pentane solution in near-quantitative yield; its 1H NMR spectrum in C6D6 is consistent with its assignment as [(6Dipp)CuH], whether monomeric or oligomeric, with the hydride resonance observed as a singlet at δ 0.77 ppm (cf. δ 1.93 ppm for [(5Dipp)CuH]258). This hydride proved stable at ambient temperature both in the solid state and in solution, with no decomposition discernible by 1H NMR spectrometry after days in C6D6 solution under inert atmosphere. The analogous [(7Dipp)CuH] complex was similarly stable, and its hydride resonance appeared at still higher field in the 1H NMR spectrum, at δ 0.47 ppm in C6D6 solution. The infrared spectra of both hydrides displayed absorptions similar in frequency to the copper− hydride stretching mode of [(IDipp)Cu(μ-H)]2: 909 cm−1 for [(6Dipp)CuH], and 912 cm−1 for [(7Dipp)CuH]. The stability of these complexes also permitted solution infrared measurements, suggesting the possibility that dissociation of the dimer in solution would result in a discernible absorption for a terminal Cu−H bond stretch. The IR spectrum obtained from the presumed [(6Dipp)Cu(μ-H)]2 in toluene solution, however, showed no such absorption in the expected range, whereas the absorption assigned to the bridging hydride stretching modes remained prominently visible. For comparison, (η2-H2)CuH, prepared by laser ablation and studied in a solid neon matrix, gives rise to a terminal Cu−H stretching band at 1862.5 cm−1.64 The UV−vis spectra of these complexes in THF solution displayed intense absorptions (ε ≈ 11 000 M−1 cm−1), ascribed to the close interactions of d10 copper centers, with λmax of 453 nm for [(6Dipp)Cu(μ-H)]2 and 476 nm for orange [(7Dipp)Cu(μ-H)]2. 8326


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triazacyclohexane and chloride ligands, and bearing a μ6hydride ligand in the center of the Cu6 core (Scheme 11).65 The solid-state structure (Figure 12) revealed Cu···Cu contacts ranging from 2.4566(8) to 2.7049(8) Å. The distances

These hydrides undergo facile intermolecular insertion reactions (Scheme 10). For example, [(6Dipp)Cu(μ-H)]2 Scheme 10. Formation and Some Reactions of Stable (Expanded-Ring NHC)copper Hydrides63

Figure 12. Solid-state structure of a TACH-supported [Cu6(μ6-H)] complex, ellipsoids at the 50% probability level. For clarity, the anion has been omitted. Selected distances: Cu(1)−Cu(2), 2.7049(8) Å; Cu(1)−Cu(3), 2.4566(8) Å; Cu(2)−Cu(4), 2.6734(9) Å; Cu(1)− Cu(1A), 2.5103(11) Å; Cu(2)−Cu(2A), 2.5713(11) Å; Cu(1)−N(1), 2.048(4) Å; Cu(2)−N(2), 2.120(4) Å; Cu(2)−Cl(2), 2.5087(14) Å; Cu(1)−H, 1.84(7) Å. Reproduced with permission from ref 65. Copyright 2003 Royal Society of Chemistry.

reacts with 1-hexene in THF solution over a period of 24 h to form [(6Dipp)Cu(n-hexyl)]. This reaction represents a key difference between the expanded-ring NHC ligand and its fivemembered analogue: [(5Dipp)Cu(μ-H)]2 was also found to react with 1-hexene but more slowly, proceeding to the extent of 20% after 48 h, and its competing thermal decomposition became apparent early in this period. An intermolecular 1,1insertion was also feasible, as benzyl isonitrile reacted with [(6Dipp)Cu(μ-H)]2 to produce a stable copper formimidoyl, characterized spectroscopically and crystallographically. 2.1.4. Hydride-Bridged Oligocopper Cations. Early work with copper hydride in solution (see above) had suggested that nitrogen donors could serve as supporting ligands for copper hydrides, but the structures of the resulting complexes were unclear. In 2003, Köhn and co-workers, while synthesizing (triazacyclohexane)copper(I) complexes, observed the abstraction of hydride from ligand gem-diaminomethane fragments, and of chloride from the dichloromethane solvent, to form a complex hexanuclear cation linked by bridging

from each copper center to the assigned hydride position were all similar, averaging 1.85(3) Å. This complex was described as sensitive, with low solubility, precluding solution studies or bulk isolation. Nonetheless, the structural results, and definitive characterization of the amidinium salt resulting from hydride abstraction, corroborated the description of this complex as a hydride. In 2005, Che and co-workers described a unique phosphinesupported [Cu3(μ3-H)]2+ complex, formed by reaction between a bisphosphine-bridged dicopper(I) dication, with various noncoordinated anions, and methanol in basic solution (Scheme 12).66 The solid-state structure (Figure 13) revealed a triangular Cu 3 core bearing three bridging bis-

Scheme 11. Formation of a [Cu6(μ6-H)] Cluster through Hydride Abstractiona

a

Reproduced with permission from ref 65. Copyright 2003 Royal Society of Chemistry. 8327


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Scheme 12. Synthesis of a Trinuclear Copper Hydride Dication66

Scheme 13. Synthesis and Alkyne Insertions of {[(NHC)Cu]2(μ-H)}+ Cations67,69

Figure 13. Solid-state structure of [(κ2-(Cy2PCH2PCy2)3Cu3(μH)]2+[ClO4−]2. Hydrogen atoms, except copper-bound hydride, and anions are omitted for clarity. Phosphorus atoms are shown in purple. Reproduced with permission from ref 66. Copyright 2005 American Chemical Society.

and 122(3)°; the Cu···Cu contacts were short, at 2.5331(15) and 2.5354(15) Å. The theoretical investigation of a model complex, bearing the simple IMe (1,3-dimethylimidazol-2ylidene) ligand, found two fully occupied molecular orbitals to be bonding across the Cu−H−Cu core; the highest occupied molecular orbital was [Cu2]−H nonbonding, but Cu−Cu antibonding in character (Figure 14b). This picture suggests some partial Cu−Cu bonding, principally through the bridging hydride ligand, producing a Wiberg bond order68 in the Löwdin basis of 0.386. The hydride-bridged dicopper cation underwent facile methanolysis with CD3OD to produce H−D, identified by its distinctive 1:1:1 triplet resonance in the 1H NMR spectrum. This observation stands in contrast to the well-established hydrogenolysis of (phosphine)copper alkoxides discussed earlier. Reaction of this hydride with carbon dioxide produced a formate-bridged dicopper cation quantitatively. The hydride also underwent clean 1,2-insertion with a terminal alkyne, reacting with phenylacetylene to afford a trans-phenylvinylbridged dicopper cation (Scheme 13d). Seeking to understand the mechanism of a copper-catalyzed hydroalkylation of alkynes, using alkyl triflates, a fluoride source, and a silane, Lalic and co-workers carried out the reaction of [(IDipp)Cu(μ-H)]2 with alkyl triflate, 1 equiv per hydride. The reaction proceeded rapidly, but only one-half of the alkyl triflate was converted to alkane, and the neutral hydride was completely converted to the hydride-bridged cation {[(IDipp)Cu]2(μ-H)}+ (Scheme 13b). Having achieved this previously elusive partial hydride abstraction, these authors showed in a competition experiment that hydrocupration of a terminal alkyne by this hydride-bridged cation was much faster than hydride delivery to the alkyl triflate, in contrast to the behavior of the neutral hydride. This contrast suggested that the hydrocupration of alkyne by the hydride-bridged cation did not involve neutral hydride, which would react more rapidly with alkyl triflate, as an intermediate. They went on to show that the hydride-bridged dicopper cation was probably formed

(dicyclohexylphosphino)methane ligands. In the case of the ClO4− salt, the triply bridging hydride was located in the difference Fourier map, although the data did not permit determination of the precise hydride position. The average Cu···Cu distance in the perchlorate salt was 2.879(1) Å, significantly larger than in the hydride-bridged oligocopper species described above, and indeed larger than two van der Waals radii of copper.30 The 1H NMR signal for this hydride was partially obscured by those of the ligand-based cyclohexyl groups, but the authors identified six lines of the expected septet resonance, with 2JP−H = 16 Hz, centered at δ 2.2 ppm. These [(L2)3Cu3H]2+ complexes were also formed in basic ethanol or benzyl alcohol solutions, but not in isopropanol or tert-butanol solutions, indicating that methanol was indeed the hydride source. The synthesis of a singly hydride-bridged dicopper cation was later pursued for comparison both to the neutral dimer [(IDipp)Cu(μ-H)]252 as well as to complexes featuring [Ag2(μH)]+ and [Au2(μ-H)]+ cores (see sections 2.2.2 and 2.3.1). Initial attempts at hydride abstraction from [(IDipp)Cu(μ-H)]2 gave rise to complex mixtures and extensive decomposition, but treatment of the siloxy-bridged cation {[(IDipp)Cu]2(μOSiMe3)}+BF4− with pinacolborane afforded the corresponding hydride in good yield (Scheme 13a).67 In contrast to the neutral hydride-bridged dimers and many larger clusters, the hydride-bridged cation was colorless in solution and in the solid state. The 1H NMR spectrum of this complex displayed a singlet resonance for the bridging hydride at notably high field, with δ = −4.13 ppm in THF-d8 solution. The solid-state structure (Figure 14a), determined by X-ray crystallography, revealed the presence of two similar but crystallographically distinct forms of the cation. Both exhibited a bent arrangement about the hydride position, with Cu−H−Cu angles of 121(3)° 8328


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Figure 14. (a) Solid-state structure of {[(IDipp)Cu]2(μ-H)}+, with ellipsoids at 50% probability; (b) calculated orbital interactions in {[(IMe)Cu]2(μ-H)}+. Adapted with permission from ref 67. Copyright 2013 Wiley-VCH.

under the catalytic conditions by reaction of an in situ generated {[(IDipp)Cu]2(μ-F)}+ cation with silane (Scheme 13c), rather than by electrophilic hydride abstraction from the neutral hydride. Because the (5Dipp) ligand gave more efficient results in this catalysis, this study presented the synthesis, and the spectroscopic and crystallographic characterization of interrelated (5Dipp)-supported complexes, including {[(5Dipp)Cu]2(μ-H)}+ and an alkenyl complex produced from it by insertion of a terminal alkyne (Scheme 13d). The 1 H NMR spectrum of {[(5Dipp)Cu]2(μ-H)}+ in dichloroethane-d4 solution displayed a singlet resonance at δ −4.52 ppm; the X-ray crystal structure revealed a Cu···Cu contact of 2.541(2) Å. The catalytic competence of this intermediate, as well as the slow reaction of neutral [(5Dipp)CuH] with alkyne as compared to alkyl triflate, reinforced the interpretation that dicopper cations could be key intermediates in hydroalkylation catalysis (see also section 4.1.5).69 The combination of a tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm), with [(CH3CN)4Cu]+ salts (with BF4− or PF6− anions) and sodium borohydride gave rise to two new cationic copper hydride complexes, both characterized crystallographically: a dinuclear μ-hydrido complex, and a tetranuclear complex containing both μ2- and μ4-hydride bridges. In the dinuclear μ-hydrido cation, each tetraphosphine chain chelates one copper center via P1 and P3, and bridges to the other copper center via P3 and P4, with P2 remaining unbound. The 1 H NMR spectrum displays a broad signal for the hydride at δ 0.16 ppm, consistent with the signal observed for the corresponding deuteride complex by 2H NMR spectrometry. The solid-state structure (Figure 15a) obtained through X-ray crystallography showed a rather long Cu···Cu contact at 2.7948(7) Å. The hydride position was determined using difference Fourier syntheses, revealing Cu−H distances of 1.62 Å, and a Cu−H−Cu angle of 120°. In the case of the [Cu4H3]+ core (Figure 15b), each pair of copper centers is bridged by a μ2-hydride and the μ4-hydride, at a Cu···Cu distance of 2.465(2) Å; the two pairs are connected via bridging dpmppm ligands and the μ4-hydride, at a Cu···Cu distance of 2.789(1) Å.

Figure 15. Solid-state structures of (a) phosphine-supported [Cu2(μH)]+ and (b) [Cu4(μ2-H)2(μ4-H)]+ cations, shown as capped-stick models for clarity. Reproduced with permission from ref 70. Copyright 2014 Wiley-VCH.

The hydride positions, again refined from the difference Fourier syntheses, corresponded to Cu−H distances of 1.52 and 1.57 Å for the μ2-hydride, and 1.82 and 1.90 Å for the μ4-hydride. Both cations were found to react readily with CO2 under mild conditions to afford formate-bridged cations, and calculations on a simplified model of the dinuclear cation, using density functional theory (DFT), found a low barrier for essentially 8329


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Figure 16. Solid-state structures of dichalcogen-supported copper hydride clusters. (a) X-ray crystal structure of {Cu4(μ4-H)(μ3-Cu)4[Se2P(OiPr)2]6}[PF6]. Reproduced with permission from ref 74. Copyright 2009 American Chemical Society. (b) Core structure of {Cu4(μ4-H)(μ3Cu)[S2CNR2]6} (NR2 = aza-15-crown-5) obtained through neutron diffraction. Reproduced with permission from ref 76. Copyright 2012 American Chemical Society. (c) The (aza-15-crown-5)-substituted dithiocarbamate ligand.

resonances for these hydrides occur at significantly higher field than those of the dithiophosphate complexes: δ −0.58 ppm for {Cu4(μ4-H)(μ3-Cu)4[Se2P(Oi-Pr)2]6}[PF6], in acetone-d6 solution, as compared to δ 3.69 ppm for its dithiophosphate analogue. In both the dithiophosphate and the diselenophosphate complexes, attempts to exchange the encapsulated hydride for deuteride, or vice versa, proved unsuccessful. In a study of dithiocarbamate-supported complexes, however, Saillard, Liu, and co-workers found that the oxidation of {Cu8(H)[S2CN(iPr)2]6}+ by [NH4]2[CeIV(NO3)6] released the interstitial hydride as 0.5 equiv of H2, forming 3 equiv of {CuII[S2CN(iPr)2]2} with loss of the remaining copper. Indeed, addition of borohydride and [(CH3CN)4Cu]+ to the CuII dithiocarbamate regenerated the octanuclear hydride cation. Treatment of this cation with more borohydride resulted in the net removal of Cu+ to form the neutral {Cu7(H)[S2CN(i-Pr)2]6}; this reaction in turn could be reversed by titration with [(CH3CN)4Cu]+ (Scheme 14). Similar processes were observed using an aza-15crown-5-substituted dithiocarbamate. The neutral [Cu7(H)] complexes exhibited the form {Cu4(μ4-H)(μ3-Cu)[S2CNR2]6} as determined by both X-ray and neutron diffraction analysis of single crystals. The neutron diffraction structure (Figure 16b)

nucleophilic addition of the bridging hydride to CO2 to form the new C−H bond.70 2.1.5. Larger Copper Hydride Clusters. The scope and potential applications of copper hydride chemistry expanded considerably with the discovery that 1,1-dichalcogen-based chelating ligands, such as diselenophosphates, dithiophosphates, and vinylenedithiolates, support stable clusters of d10 metals containing encapsulated hydrides and other main-groupbased anions.71 In 2009, Liu and co-workers found that the reaction of [(CH 3 CN) 4 Cu] + PF 6 − with an ammonium dithiophosphate in a 4:3 ratio afforded the cubic octanuclear cations {Cu8[S2P(OR)2]6}2+ (R = i-Pr or Et) as their PF6− salts. Incorporation of hydride, by preparing the cluster in the presence of sodium borohydride, led to a significant structural rearrangement: The resulting cluster exhibited a tetracapped tetrahedral geometry, with the interstitial hydride bound to the inner four copper centers. This distortion from cubic geometry has been elucidated through combined theoretical and experimental studies on the encapsulation of hydride versus a range of other anions.72,73 These hydrides were thoroughly characterized using X-ray crystallography, NMR spectroscopy, mass spectrometry, and DFT studies using model complexes. In the solid-state structure (Figure 16a) of the diisopropoxydithiophosphate-ligated example, the Cu···Cu distances through the edge of the tetrahedron ranged from 2.908(2) to 3.116(3) Å; however, the distances from the copper vertices to the capping copper atoms were shorter, ranging from 2.6759(17) to 2.7512(19) Å. The 1 H NMR spectrum of {Cu4(μ4-H)(μ3-Cu)4[S2P(Oi-Pr)2]6}[PF6] in acetone-d6 solution showed a broad singlet resonance at δ 3.69 ppm for the encapsulated hydride, consistent with the 2 H NMR resonance at δ 3.71 ppm for the corresponding deuteride. The 31P NMR spectrum showed only one singlet resonance for the dithiophosphate ligands, reflecting the chemical equivalence of the dithiophosphate ligands in solution.74 Liu, Saillard, and co-workers described similar findings for diselenophosphate-stabilized clusters, contrasting the tetracapped tetrahedral structures of the μ4-hydrides with the cubic structures and μ8-anion bridges for the fluoride- and sulfide-encapsulated clusters.75 Interestingly, the 1H NMR

Scheme 14. Interconversion of [Cu8(H)] and [Cu7(H)] Coresa

a

Reproduced with permission from ref 76. Copyright 2012 American Chemical Society.

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Figure 17. Solid-state structures from single-crystal neutron diffraction. (a) The structure of {Cu20H11[S2P(Oi-Pr)2]9}: thermal ellipsoids shown at the 50% probability level, (Oi-Pr) groups omitted for clarity. (b) The [Cu20H11] core, showing μ3, μ4-tetrahedral, and μ4-square planar hydrides. Inner Cu···Cu distance: 2.302(7) Å. Reproduced with permission from ref 80. Copyright 2014 American Chemical Society.

clearly resolved the position of the μ4-bridging hydride, with Cu−H distances of 1.81(2), 1.84(2), 1.78(2), and 2.02(2) Å.76 Beyond their synthetic and structural novelty, complexes of this type show promise in several possible applications. Both [Cu8(μ4-H)]+ and [Cu7(μ4-H)] clusters supported by diselenocarbamate ligands have been investigated as single-source precursors to copper selenide nanocomposites.77 A dithiophosphate-supported [Cu8(μ4-H)]+ complex was found to act as an effective catalyst for azide−alkyne cycloaddition.78 Generally these clusters are air- and moisture-stable, increasing their utility as applications are found. This general synthetic approach proved remarkably versatile in preparing clusters of higher nuclearity, containing multiple hydrides. The reaction of [(CH3CN)4Cu]+PF6− with NH4+ [S2P(Oi-Pr)2]− and LiBH4 in tetrahydrofuran solution, in a 7:6:1 ratio, afforded the neutral cluster {Cu20H11[S2P(OiPr)2]9} in 73% yield. Described arrestingly as an elongated triangular orthobicupola framework of 18 Cu atoms encapsulating a [Cu2H5]3− ion, this complex contains three distinct types of hydride, as shown by the 2H NMR spectrum of the deuteride analogue. (The 1H NMR resonance for one of the hydride types overlapped with a ligand-based resonance.) The 11 deuterides gave rise to three broad singlet resonances at δ −0.89, 1.54, and 2.88 ppm, in the ratio of 6:2:3, in CHCl3 solution. Each of these resonances shifted downfield by at least 0.40 ppm for a sample prepared in toluene solution. A key finding for this new cluster was its ability to release H2 on treatment with weak acid, on mild thermolysis at 65 °C, or upon direct irradiation with sunlight.79 A single-crystal neutron diffraction study of the [Cu20H11] cluster confirmed the three types of hydride assigned using NMR spectroscopy, and revealed a surprising new binding mode. Six of the hydrides were present in a μ3-bridging mode, two μ4-hydrides were present in tetrahedral cavities, and three hydrides exhibited nearly square planar geometry (Figure 17). The distance between the two inner copper centers, bridged by these three μ4-hydrides, was only 2.302(7) Å.80 Still further synthetic advances permitted the isolation and characterization of the “Chinese Puzzle Ball” cluster [Cu 28 H 15 (S 2 CNR 2 ) 12 ] + PF 6 − , in which an irregular Cu 4 tetrahedron is encapsulated within a rhombicuboctahedral Cu24 cage, with six face-truncating hydrides bridging the inner

and outer subclusters (Figure 18). Like the [Cu20H11] cluster, this cluster released H2 on treatment with acid, on thermolysis,

Figure 18. Cubic {Cu28H15[S2CN(n-Pr)2]12}+ cation. Anion and hydrogens, except for hydrides, omitted for clarity. Carbon shown in gray, nitrogen shown in purple, sulfur in yellow, copper in blue, hydrides in red. Reproduced with permission from ref 81. Copyright 2014 Wiley-VCH.

or on solar irradiation.81 A still larger cluster, {Cu32H20[S2P(OiPr)2]12}, was isolated and fully characterized. Single-crystal neutron diffraction analysis assigned the positions and bridging types of all of the hydrides: 12 μ3-hydrides, six tetrahedral μ4hydrides, and two μ5-hydrides, variously present in capping and interstitial modes.82 Mixed phosphine and chalcogen donors can also stabilize polyhydride clusters of copper, as shown by van Leeuwen and co-workers in 2014.83 Beginning with 2-(diphenylphosphino)benzenethiol, they prepared first a copper(I) μ-iodide dimer, with the supporting ligand chelating through the phosphine and the neutral thiol. Addition of ethanolic sodium borohydride to a solution of this precursor in ethanol resulted in the immediate 8331


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unoccupied molecular orbital (ligand π*-based) in potential energy. In 2014, Hayton and co-workers observed a dramatic transformation resulting from the addition of a large excess of 1,10-phenanthroline to [(Ph3P)CuH]6 in CH2Cl2 solution.84 The initially red solution underwent a change in color to green, then to blue. After 18 h at ambient temperature, the dark blue product [Cu14H12(phen)6(PPh3)4]2+[Cl−]2 (phen = 1,10phenanthroline) was isolated as a crystalline solid in 80% yield (Scheme 15). When the reaction was run in CD2Cl2

appearance of a bright yellow color. After 16 h, the product nanoparticles were isolated as a yellow powder; analysis by transmission electron microscopy revealed a broad size distribution, but most of the particles ranged in size from 1.2−1.6 nm. Transmission infrared spectroscopy showed the disappearance of the thiol S−H absorption, consistent with the deprotonation of the copper-bound thiol to form a phosphinobenzenethiolate ligand. The 1H NMR spectrum of these nanoparticles in solution showed two resonances attributable to copper-bound hydrides, at δ 2.20 ppm and δ 4.50 ppm. The authors assigned the higher-field resonance to μ3-hydrides, and proposed a thiolatebound, μ2-hydride-bridged dicopper fragment as the source of the δ 4.50 ppm signal. These nanoparticles were found to evolve H2 under photolysis or thermolysis at 70 °C. A small fraction of the mixture crystallized from CH2Cl2/hexane solution as the neutral [Cu18H7L10I] (L = 2-diphenylphosphinobenzenethiolate), which was characterized by single-crystal X-ray diffraction (Figure 19). The structure displayed a central

Scheme 15. Formation of a Mixed (Phenanthroline) (Phosphine)copper Hydride Cluster84,a

a

Inferred stoichiometry; 26 equiv was added per [Cu6].

solution and monitored by 1H NMR spectrometry, intermediates exhibiting hydride resonances at δ 2.13 and 2.75 ppm were gradually replaced by the final product, in which the hydrides give rise to a singlet resonance at δ 3.47 ppm. A 2H NMR spectrum showed a resonance assignable to CHD2Cl, confirming the solvent as the source of the chloride counterions. The same dicationic cluster could be prepared as its trifluoromethanesulfonate salt by treatment of [(Ph3P)CuH]6 with 1,10-phenanthroline and [(CH3CN)4Cu]+OTf− (OTf− = trifluoromethanesulfonate) in CH3CN solution over 18 h. The solid-state structure of the dichloride complex (Figure 20a), as determined by single-crystal X-ray diffraction, revealed a tetrahedral Cu4 core inside an adamantane-like arrangement of 10 more copper centers (Figure 20b), bound to the

Figure 19. Solid-state structure of [Cu18H7L10I] (L = 2-diphenylphosphinobenzenethiolate) as determined by X-ray diffraction. Ellipsoids shown at 50% probability level. Minor disordered components and all hydrogen atoms omitted for clarity. Reproduced with permission from ref 83. Copyright 2014 Wiley-VCH.

core of eight Cu centers, with 10 surrounding copper centers connected to these by P and S donors. The hydrides could not be refined in these studies, and the complex did not afford crystals suitable for neutron diffraction. The presence of seven hydrides was inferred from charge balance, and from the agreement between the structure determined from X-ray diffraction and that calculated for a 2-(dimethylphosphino)benzenethiolate model complex, with seven hydrides and all copper centers in the formal +1 oxidation state. These calculations show a highest occupied molecular orbital derived principally from filled copper 3d orbitals, with copper 4s- and 4p-based molecular orbitals occurring well above the lowest

Figure 20. Solid-state structure of a mixed (phenanthroline)(phosphine)copper hydride cluster, [Cu14H12(phen)6(PPh3)4]2+ (phen = 1,10-phenanthroline). Ellipsoids shown at the 50% probability level. Chloride anions, cocrystallized solvent, and all hydrogen atoms omitted for clarity. Copper shown in yellow-orange, phosphorus in purple, nitrogen in blue. (a) Overall structure of cation. (b) Truncated view of outer [Cu10] core, with inner [Cu4] tetrahedron and ligands, except for donor atoms, omitted. Reproduced with permission from ref 84. Copyright 2015 Wiley-VCH. 8332


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of this complex gave results consistent with partial copper(0) character, leading the authors to propose the description of an n* = 2 superatom, with the closed-shell 1S2 electronic configuration.87 The [Cu25] complex decomposes slowly in CD 2 Cl 2 solution at ambient temperature, forming [Cu18H17(PPh3)10Cl], copper(0), [(Ph3P)3CuCl], HCD2Cl, and H2. The solid-state structure of the [Cu25] complex, obtained through X-ray diffraction of a hexane- and dichloromethanesolvated crystal, revealed a core of 13 copper atoms, arranged in a distorted centered-icosahedron (Figure 21). The central copper atom was thus coordinated to 12 copper atoms, as in bulk copper metal. The Cu···Cu distances within this cluster range from unusually short, at 2.389(3) Å, to well over two van der Waals radii, at 3.037(3) Å. The chloride was found to be present as an outer-sphere anion, disordered over two positions. The 31P{1H} NMR spectrum of this complex in CD2Cl2 solution displays a single resonance at δ −2.96 ppm, indicating high symmetry in solution as well as in the solid state. The hydride ligands gave rise to three broad resonances in the 1H NMR spectrum, observed at δ −0.92 ppm (4H), 1.55 ppm (12H), and 2.07 ppm (6H). The 31P NMR spectrum of a sample in C6D6 solution gave a complex spectrum consistent with the formation of a contact ion pair, of lower symmetry, between [Cu25H22(PPh3)12]+ and chloride ion. Another polyhydride cluster with partial copper(0) character is [(Cp*AlCu)6H4] [Cp* = C5(CH3)5, bound η5 unless otherwise specified].88 Reasoning that [Cp*AlI], isolobal with a carbenoid, should be able to stabilize transition metal clusters, Fischer and co-workers heated [(Ph3P)CuH]6 with (Cp*Al)4 in a 2:3 ratio and obtained the new complex as a brown-black crystalline solid in 78% yield. The corresponding deuteride was obtained likewise from [(Ph3P)CuD]6. The absence of phosphine in the product was confirmed by 31P NMR spectroscopy; the 1H NMR spectrum for a sample in C6D6 solution displayed a broad singlet resonance at δ −2.86 ppm, integrating to 4H with respect to the Cp* ligands. Interestingly, the deuteride complex gave rise to a 2H NMR resonance at a rather different chemical shift, δ −1.63 ppm, than the hydride. The solvent for this sample was n-hexane, however, and the change from aromatic to aliphatic solvent, plus any isotopic shift, might account for this difference. The 1H and 13C NMR data, consisting of single sets of resonances for the Cp* ligands, otherwise confirmed the two complexes as isotopologues. Thus,

phenanthroline and phosphine ligands. Within the tetrahedron, the Cu···Cu distances are the longest, averaging 2.893 Å; the distances from the tetrahedral vertices to the nearest phosphorus-bound copper averaged 2.655 Å, and those to the nearest nitrogen-bound copper averaged 2.514 Å. Distances from nitrogen-bound to phosphine-bound copper centers averaged 2.663 Å. This complex underwent a slow reaction with CH2Cl2 to form [(phen)(Ph3P)CuCl], CH3Cl, H2, and Cu°. Exposure of a CH2Cl2 solution of this complex to an atmosphere of CO2, in the presence of excess PPh3, resulted in the formation of the copper formate [(Ph3P)2Cu(O2CH)] in 37% isolated yield. This net insertion of CO2 proved that the hydrides could be accessible and reactive toward incoming small molecule substrates. In 2005, Lee and Yun showed that [(Ph3P)CuH]6 could be conveniently generated through the reaction of diphenylsilane with copper(II) acetate [Cu(OAc)2], which results in both reduction to copper(I) and hydride transfer, in the presence of triphenylphosphine.85 Recently, Scott, Hayton, and co-workers described a very different outcome from a modified approach.86 Reasoning that a deficiency of ligand should produce [CuH] oligomers, which could then be incorporated into nanoclusters, they treated 24 equiv of Cu(OAc)2, 1 equiv of CuCl, and 12 equiv of PPh3 with 13 equiv of Ph2SiH2, and separated two major principal products by crystallization: [Cu 2 5 H 2 2 (PPh 3 ) 1 2 Cl], isolated in 23% yield, and [Cu18H17(PPh3)10Cl], isolated in 14% yield (Scheme 16). Scheme 16. Synthesis of a Mixed-Valent Copper Polyhydride Cluster86,a

(a) Other products: [Cu18H17(PPh3)10]+Cl−, Cu°. (b) Reaction stoichiometry; 13 equiv was added.

a

Some elemental copper (17%) was also isolated. The [Cu25] complex is especially noteworthy for its unusual oxidation state: At a glance, the presence of neutral phosphines, 22 hydrides, and a single chloride would formally require the presence of two copper(0) centers for charge balance. Indeed, X-ray absorption spectroscopy and X-ray photoelectron spectroscopy

Figure 21. Solid-state structure of [Cu25H22(PPh3)12]+ as determined by single-crystal X-ray diffraction. Chloride anion, cocrystallized solvent molecules, and all hydrogens omitted for clarity. (a) Side view, with [Cu13] centered-icosahedron highlighted in blue. (b) Side view showing only Cu and P atoms. (c) Top view, showing only Cu and P atoms, looking down 3-fold rotation axis. Reproduced with permission from ref 86. Copyright 2015 American Chemical Society. 8333


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defined models for Hume−Rothery-type intermetallic clusters, and the clean insertion of benzonitrile pointed toward other possibilities for cooperative interaction between very different metals. Loss of Cp*H on thermolysis also suggested the potential for these complexes as precursors for the growth of larger Al/Cu clusters. Polyhydride clusters of copper are reviewed in detail, and discussed in the context of other copper hydrides, in the recent account by Liu and co-workers.4

the preparative reaction led not only to the net substitution of [Cp*Al] for PPh3, but the loss of 1 equiv of H2. Structural analysis by single-crystal X-ray diffraction was complicated by severe disorder, but the data permitted the overall connectivity of the complex to be established. This heterometallic hydride cluster was found to react with benzonitrile to deliver a hydride to carbon in near-quantitative yield, forming the aldiminate complex [(Cp*AlCu)6H3(N CHPh)] (Scheme 17). In the 1H NMR spectrum of this

2.2. Silver Hydrides

Scheme 17. Synthesis and Nitrile Insertion of an Aluminum Copper Hydride Cluster88

In 1956, kinetics studies provided indirect evidence for the formation of silver hydrides from silver salts and H2 in aqueous solution.89 (This process will be discussed further in section 4.2.) Much more recently, mass spectrometry experiments using collision-induced dissociation (CID) provided direct evidence not only for the existence and stability of oligonuclear silver hydride cations in the gas phase,90,91 but for their mediation of processes such as C−C bond formation from allylic halides.92,93 Such experiments have further demonstrated the generation of the [Ag2H]+ cation from silver ions complexed with fatty acids or glycerolipids,94 or with glycine.95 Investigations of silver-containing heterogeneous catalysts have implicated the participation of silver hydrides in alcohol dehydrogenation by γ-alumina-supported silver clusters,96 and in the activation of methane by the silver-modified zeolite HZSM-5.97 The potential for silver hydrides to mediate important transformations, combined with advances in copper and gold hydride chemistry, helped to inspire the design and synthesis of ligand-stabilized silver hydride complexes. 2.2.1. Clusters Supported by Dichalcogen Donor Ligands. The first isolable silver-only hydrides were reported by Liu and co-workers in 2010. 9 8 Treatment of [(CH3CN)4Ag]+[PF6]− in THF solution with an anionic diselenophosphate ligand, followed by sodium borohydride, afforded the octanuclear [Ag8(H){Se2P(OR)2}6]+[PF6]− (R = Et, i-Pr). The use of sodium borodeuteride afforded the corresponding silver deuteride complex. Like their copper analogues, these complexes exhibit a remarkable structure in which the silver atoms form a tetracapped tetrahedron, within an icosahedral skeleton formed by the 12 selenium donors. As observed in the copper series, this arrangement results in a μ4-coordination environment for the hydride in the solid state. For the [Se2P(O-i-Pr)2]-supported complex (Figure 23), the distances between the hydride and the four inner silver atoms range from 1.8723(13) to 2.0306(17) Å, somewhat longer than the sum of the covalent radii.30 The Ag−Ag distances in the inner tetrahedron range from 3.0841(18) to 3.235(4) Å, and from these to the capping silver centers, 2.9888(17) to 3.1411(19) Å. The intermetallic distances are somewhat shorter than two van der Waals radii of silver (3.40 Å), consistent with argentophilic interactions99 among formally nonbonded d10 silver centers. In solutions of this complex in acetone-d6, fluxionality results in the equivalence of all diselenophosphate ligands in the 31P and 77Se NMR spectra, and in coupling of the hydride ligand to all eight silver centers as observed by 1H NMR spectroscopy. Naturally occurring silver consists of a mixture of two isotopes, 107 Ag (52%) and 109Ag (48%), each with a nuclear spin of 1/2; the gyromagnetic ratio, to which dipolar couplings are proportional, is roughly 15% larger for 109Ag.100 In this case, the separate 1H−107Ag and 1H−109Ag couplings could not be resolved, and so an apparent nonet peak, consistent with

product the hydride resonance, now at δ −2.50 ppm, integrated to three hydrogens; the Cp* ligands gave rise to four signals, reflecting distinct chemical environments, and the aldiminate CH fragment gave rise to a distinctive singlet resonance at δ 7.70 ppm. The nitrile insertion product crystallized with three benzene molecules in the asymmetric unit, and these crystals proved well-ordered. The resulting high-quality structure (Figure 22) afforded insight not only into this product, but

Figure 22. Solid-state structure of [(Cp*AlCu)6H3(NCHPh)]. [Cu4] tetrahedron shown in orange; capping Cu atoms in green. Al shown in light blue, N in dark blue, C in gray. Hydrogen atoms omitted for clarity. Where used, ellipsoids are shown at 30% probability. Reproduced with permission from ref 88. Copyright 2014 Wiley-VCH.

into the parent hydride. Inside an octahedral shell of Cp*bound Al centers, the Cu6 core adopts a bicapped tetrahedral arrangement. Four Cp* ligands adopted the η5-binding mode, with the remaining two η1-bound. The Cu···Cu distances ranged from 2.3289(9) to 2.7027(9) Å, whereas the Al···Cu distances fell between 2.4027(14) and 2.7189(14) Å. The disposition of the hydrides with respect to copper and aluminum could not be determined from the structural data or from the NMR data. These complexes provided small, well8334


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Figure 23. Geometry of (a) [Ag8(H){Se2P(Oi-Pr)2}6]+ (i-Pr groups omitted for clarity), and (b) its [Ag8(μ4-H)] core. Reproduced with permission from ref 98. Copyright 2010 American Chemical Society.

Figure 24. Geometries of (a) [Ag7(μ4-H){Se2P(Oi-Pr)2}6]+ (i-Pr groups omitted for clarity). Reproduced with permission from ref 102. Copyright 2013 American Chemical Society. (b) [Ag11(μ5-H)(S2CNPr2)9]+ (Pr groups omitted for clarity; hydride not located). Reproduced with permission from ref 103. Copyright 2011 Royal Society of Chemistry. (c) The [Ag11(μ5-H)] core from (b), ibid, showing calculated hydride position.

by bridging dichalcogen-based donor ligands. These clusters include related dithiophosphate-ligated [Ag8(μ4-H)] complexes;101 [Ag7(μ4-H)] cores (Figure 24a), which serve as ready precursors to silver nanoparticles, supported by diselenoand dithiophosphate ligands;102 an [Ag11H] core supported by dithiocarbamate ligands, with a μ5-bridging mode calculated for the interstitial hydride (Figures 24b,c);103 and an [Ag8(μ4-H)] complex supported by the 1,1-dicyanoethylene-2,2-dithiolate (MNT) ligand.104 2.2.2. (N-Heterocyclic Carbene)silver Hydrides. In light of their ability to support well-defined hydride complexes of copper and gold, N-heterocyclic carbene (NHC) ligands were

coupling between the hydride nucleus and eight equivalent spin-1/2 nuclei, was recorded. This resonance is centered at δ 3.14 ppm at ambient temperature but shifts upfield, without significant change in line shape, by 0.5 ppm at −80 °C, consistent with contraction of the metal core upon cooling. The 109 Ag NMR spectrum, for a solution in acetone-d6 solution at ambient temperature, displays a doublet resonance at δ 1155 ppm. This observation is consistent with an averaged environment for the silver nuclei, each coupling only to the single interstitial hydride. The octanuclear silver hydrides became the first in a series of silver clusters containing an interstitial hydride, and supported 8335


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examined as supporting ligands for silver hydrides. One complicating factor was the tendency of (NHC)silver complexes to act as carbene transfer agents, whether to other metals or between silver centers.105 The ligand 5Dipp was found to have a lower tendency toward the formation of homoleptic [(NHC)2Ag]+ complexes106 than the unsaturated IDipp.107 Although a neutral (NHC)silver hydride remains elusive, the [Ag2(μ-H)]+ core proved amenable to stabilization. Treatment of an alkoxy-bridged disilver cation with PhSiH3 afforded the desired {[(NHC)Ag]2(μ-H)}+ cation; the reaction with PhSiD3 afforded the corresponding deuteride-bridged disilver cation (Scheme 18).108 Scheme 18. Synthesis of {[(NHC)Ag]2(μ-H)}+ Salts108

The 1H NMR spectrum of the μ-hydrido cation displays a single set of resonances for the 5Dipp ligands, as well as an apparent triplet of triplets centered at δ −1.18 ppm. The latter multiplet results from a superimposition of the resonances for each isotopologue: [(107Ag)2(μ-H)]+, 1:2:1 triplet, 1J107Ag−H = 116 Hz, 27% of total intensity; [(109Ag)2(μ-H)]+, 1:2:1 triplet, 1 109 J Ag−H = 134 Hz, 23% of total intensity; [(107Ag)(μH)(109Ag)]+, doublet of doublets, 1J107Ag−H = 116 Hz, 1 109 J Ag−H = 134 Hz, 50% of total intensity (Figure 25a). The 109 Ag spectrum displayed an apparent doublet of triplets centered at δ 519.3 ppm. Like the hydride signal in the 1H NMR spectrum, this multiplet actually comprises superimposed resonances: a 1:1 doublet (1JAg−H = 134 Hz) for the [(109Ag)2(μ-H)]+ isotopologue, and a 1:1 doublet of 1:1 doublets (J109Ag−H = 134 Hz; J109Ag−107Ag = 113 Hz) for the heteronuclear [(107Ag)(μ-H)(109Ag)]+ isotopologue (Figure 25b). As the starting alkoxy-bridged disilver cation displays just a singlet resonance (δ 541.4 ppm) in its 109Ag spectrum, the silver−silver coupling in the hydride-bridged complex has been attributed to metal−metal bonding resulting from the three-center, two-electron interaction.8 The intermetallic distance in the solid-state structure, 2.8087(4) Å, is consistent with significant bonding between the d10 silver centers. The {[(5Dipp)Ag]2(μ-H)}+ cation displays moderate stability, undergoing significant decomposition (roughly 25%) after 24 h in CD2Cl2 solution at ambient temperature. Exposure to ambient air does not accelerate this decomposition appreciably, and the addition of D2O does not result in protonolysis to form H−D. The dominant decomposition pathway appears to be the formation of H2, silver metal, and the homoleptic cation [(5Dipp)2Ag]+. Decomposition is accelerated in coordinating solvents such as CD3CN, or in the presence of added ligands such as PPh3. This observation is consistent with Lewis-base-mediated displacement of unstable, neutral (5Dipp)AgH from the [Ag2H]+ core, but could also reflect a direct transfer of NHC between the two silver centers, facilitated by the presence of coordinating ligand or solvent (Scheme 19). Although the {[(5Dipp)Ag]2(μ-H)}+ cation was not observed to react with aldehydes or ketones, it did react

Figure 25. 1H NMR (a) and 109Ag NMR (b) spectra of [(LAg)2(μH)]+ OTf− [L = 5Dipp, 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2ylidene]. Adapted with permission from ref 108. Copyright 2013 Royal Society of Chemistry.

Scheme 19. Proposed Paths for {[(NHC)Ag]2(μ-H)}+ Decompositiona

a

L = 5Dipp; L′ = e.g., CD3CN, PPh3.

sluggishly with CO2 to form a formate-bridged disilver cation. The reaction proceeded to ca. 5% conversion after 4 days under 1 atm CO2 in THF solution; that it proceeded at all was confirmed through the use of 13CO2, giving rise to a doublet resonance centered at δ 8.0 ppm in the 1H NMR spectrum, 8336


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with 1JC−H = 195 Hz. On the hypothesis that this cationic hydride was simply not hydridic enough to react readily with CO2,109−111 the reaction of {[(5Dipp)Ag]2(μ-H)}+ with a Lewis-basic CO2 derivative, the imizadolium 2-carboxylate zwitterion formed on reaction of a free NHC with CO2,112−115 was examined. In this case, the hydride transfer was complete within 5 min, as judged by 1H NMR spectroscopy, forming 2 equiv of heteroleptic bis(NHC)silver cation and formate. Because a complete and clean reaction required the delivery of one new NHC to each silver center, 1 equiv of CO2 was released, but the other was reduced to formate (Scheme 20).

informative 1H NMR signals) in CD2Cl2 solution with H2 (4.4 atm) proceeded slowly but smoothly, releasing tertpentanol and affording the hydride-bridged disilver cation in near-quantitative yield as judged by 1H NMR spectroscopy. The neutral silver(I) fluorides LAgF (L = 6Dipp or 7Dipp) also activate dihydrogen, cleanly forming the hydride-bridged disilver cations and the [HF2]− anion after several days (Scheme 21). This overall heterolysis of dihydrogen by two Scheme 21. Preparative Hydrogenolysis of Silver Alkoxide and Fluoride Complexesa

Scheme 20. Hydride Transfer from [(LAg)2H]+ to Unactivated (a) and NHC-Activated (b) 13CO2 (L = 5Dipp)a

a

Adapted with permission from ref 121. Copyright 2015 Wiley-VCH.

silver fluorides, reminiscent of the hydrogenolysis of ruthenium(II) fluorides,117−120 displays a first-order rate dependence on both silver and dihydrogen concentration. Although other sequences cannot be ruled out, this profile is consistent with initial rate-limiting heterolysis by a single LAgF molecule, with rapid subsequent formation of a hydride bridge and bifluoride.121 The hydride resonances in the 1H NMR spectra of [(LAg)2(μ-H)]+ salts shift substantially upfield with an increase in ring size, being centered at δ −1.18 ppm for L = 5Dipp, −2.15 ppm for L = 6Dipp, and −2.46 ppm for L = 7Dipp; the 109 Ag resonances shift from δ 519.3 to 524.6 to 532.0 ppm. These changes may result from a combination of the enhanced σ-basicity of the expanded-ring ligands, and the increased crowding of the [Ag2H]+ core by the aryl rings25 as the N−C− N angles expand. The solid-state structure of {[(6Dipp)Ag]2(μH)}+[HF2]− displays a slightly longer Ag···Ag distance than that in {[(5Dipp)Ag]2(μ-H)}+OTf−: 2.8948(9) Å, as compared to 2.8087(4) Å; both anions remain outside the metal coordination spheres in the solid state. 2.2.3. (Phosphine)silver Hydrides. Isolable (phosphine) silver hydride complexes were first reported in 2013, after Khairallah, Donnelly, O’Hair, and co-workers studied the reduction of silver trifluoroacetate with sodium borohydride in the presence of 1,1-bis(diphenylphosphino)methane (henceforth L2; L in the original).122 Solutions of the reaction products in methanol/chloroform, analyzed by electrospray ionization mass spectrometry (ESI−MS), showed peaks consistent with the presence of [Ag3H(L2)3]2+, [Ag3H(Cl)(L2)3]+, [Ag3(Cl)2(L2)3]+, and [Ag10(H)8(L2)6]2+. The use of sodium borodeuteride gave rise to the corresponding silver deuteride clusters, confirming that the borohydride was the hydride source. Building on their mass spectrometry results, these researchers refined their condensed-phase synthesis to allow the synthesis of [Ag3H(Cl)(L2)3]+BF4− from AgBF4. Analysis of this complex by both X-ray (Figure 26) and neutron diffraction established the μ3-bridging mode of both the hydride

a

Adapted with permission from ref 108. Copyright 2013 Royal Society of Chemistry.

In a catalytic reaction, the released CO2 could in principle be captured during a subsequent cycle. More problematic was the use of an energy-intensive silane or borane to deliver hydride to the silver centers. The use of dihydrogen, potentially derived from renewable sources, would be more relevant to broader challenges in CO2 functionalization.116 In light of Caulton’s results on the preparative hydrogenolysis of copper alkoxides (see section 2.1.2),38 and evidence from Halpern and others (see section 4.2) for silver hydride generation from silver salts and dihydrogen, the formation of an (NHC)silver hydride from dihydrogen and a suitable precursor appeared plausible. Moreover, salts of {[(5Dipp)Ag]2(μ-H)}+ were not observed to revert to their tert-butoxide-bridged disilver precursor upon addition of tert-butanol, raising the possibility that the attempted alcoholysis reaction might be unfavorable under standard conditions, and that the desired alkoxide hydrogenolysis should be favored. The reaction of {[(5Dipp)Ag]2(μ-Ot-Bu)}+BF4− in CD2Cl2 solution with H2 (3.0 atm) gave rise to a pseudotriplet-oftriplets resonance at δ −1.18 ppm, characteristic of a hydride bound to two silver centers, in the 1H NMR spectrum; however, the reaction proceeded only to a moderate extent (∼55%) before decomposition of the product to homoleptic [(NHC)2Ag]+ and Ag°(s), as described above, predominated. The use of the expanded-ring NHCs 6Dipp and 7Dipp, in contrast, suppressed this decomposition entirely. The greater σbasicity of these ligands,60 described in section 2.1.3, makes the Ag−C bond more inert, and their greater steric encumbrance61 hinders the formation of homoleptic byproduct. The reaction of [(LAg)2(μ-Ot-Pent)]+ (L = 6Dipp; Ot-Pent = OC(Me)2Et, chosen over Ot-Bu for its greater solubility and more 8337


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Figure 26. Solid-state structure of (a) the cation [Ag3(μ3-H)(μ3-Cl)(L2)3]+ [L2 = 1,1-bis(diphenylphosphino)methane]; and (b) the distorted trigonal bipyramidal core, with P-phenyl groups omitted for clarity. The hydride position was established by a neutron Laue diffraction analysis. Selected distances: Ag(1)−H, 1.91(2) Å; Ag(1)−Cl, 2.859(1) Å; Ag(1)−P(2), 2.4421(9) Å ; Ag(1)−Ag(1), 2.8988(2) Å. Figure reproduced with permission from ref 122. Copyright 2013 Wiley-VCH.

Figure 27. (a) Expansion of the hydride resonance taken from the 31P-decoupled 1H NMR spectrum of [Ag3(μ3-H)(L2)3]2+ (BF4−)2 [L2 = 1,1bis(diphenylphosphino)methane], taken in CD3CN solution; the singlet resonance is assigned to dissolved H2. (b) Solid-state structure of the cation [Ag3(μ3-D)(L2)3]2+; the hydride position was established by a neutron Laue diffraction study. Selected distances: Ag(1)−D(1), 1.85(2) Å; Ag(2)− D(1), 1.80(2) Å; Ag(3)−D(1), 1.85(3) Å; Ag(1)−Ag(2), 3.1511(2) Å; Ag(2)−Ag(3), 3.0655(2) Å; Ag(1)−Ag(3), 3.1415(2) Å. Reproduced with permission from ref 123. Copyright 2014 American Chemical Society.

resonance, centered at δ 4.75 ppm, for the hydride (Figure 27a); a corresponding multiplet at the same chemical shift was observed in the 2H NMR spectrum of the μ3-deuteride complex. The hydride resonance is consistent with an overlay of signals for the isotopologues arising statistically from naturally occurring 107Ag and 109Ag, with coupling constants J109Ag−H = 86 Hz and J107Ag−H = 75 Hz. Structural characterization of the μ3-deuteride complex by X-ray and neutron diffraction analysis reveals interesting contrasts with the [Ag3(μ3-H)(μ3-Cl)(L2)3]+ cation. The absence of the bridging chloride results in a more nearly planar [Ag3(μ3-H)] core (Figure 27b): The deuteride in [Ag3(μ3-D)(L2)3]2+ lies 0.31 Å above the Ag3 plane, as compared to 0.91 Å for the hydride in [Ag3(μ3-H)(μ3-Cl)(L2)3]+. The Ag−D distances of 1.85(1), 1.85(1), and 1.80(1) Å are significantly shorter than the Ag−H distances in [Ag3(μ3-H)(μ3-Cl)(L2)3]+, with the shortest distance found between the deuteride and the silver center furthest removed from the BF4− anions. The Ag−Ag distances range from 3.0655(2) to 3.1511(2) Å. Although still well

and the chloride ligands, with the bisphosphine ligands bridging adjacent silver centers. The silver−hydrogen distances, at 1.91(2) Å, are notably shorter than the silver−chlorine distances of 2.859(1) Å; the silver−silver distances within the Ag3 triangle, 2.8988(2) Å, are short enough to suggest attractive interactions among the silver(I) centers.99 Modifications of this synthetic approach enabled the isolation and characterization of other key clusters identified by ESI− MS. By carrying out the hydride transfer reaction in methanol and using freshly purified chloroform in the preparation of samples for mass spectrometry, the authors greatly reduced the fraction of μ3-chloro complexes formed, allowing the complexes [Ag3(μ3-H)(L2)3]2+(BF4−)2 or [Ag3(μ3-D)(L2)3]2+(BF4−)2 to be observed as major products and subsequently isolated in pure form.123 These dications are analogous to the [Cu3(μ3H)(L2)3]2+ [L2 = 1,1-bis(dicyclohexylphosphino)methane] reported by Che and co-workers (see above). The 31P-decoupled 1H NMR spectrum of [Ag3(μ3-H)(L2)3]2+(BF4−)2 displays an apparent quartet of quartets 8338


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Scheme 22. Synthesis of Terminal Gold Hydrides127−129

shorter than two van der Waals radii of silver, these distances are considerably longer than those in [Ag3(μ3-H)(μ3-Cl)(L2)3]+. The treatment of AgBF4 with bisphosphine ligand and excess borohydride in acetonitrile solution, at or below −10 °C, led to the isolation of a hydride- and borohydride-bridged trisilver cation, [Ag3(μ3-H)(μ3-BH4)(L2)3]+BF4− (Figure 28).124 The

resonances for IDipp, the 1H NMR spectra for the new complex displayed a rather broad singlet resonance integrating to one proton. Strikingly, this singlet was observed at δ 3.38 ppm in CD2Cl2 solution, but at 5.11 ppm in C6D6 solution. The infrared spectrum of the hydride product displayed a sharp, intense band at 1976 cm−1, consistent with the presence of a terminal hydride. The corresponding deuteride was prepared by reaction of (IDipp)AuCl with Li[DBEt3], forming (IDipp)AuD as the principal product and (IDipp)AuEt as a minor (∼10%) byproduct. The 1H NMR resonances for (IDipp)AuD were identical to those of (IDipp)AuH except for the absence of the singlet assigned to hydride; the 2H NMR spectrum displayed a singlet at δ 3.34 ppm. The infrared spectrum of (IDipp)AuD likewise showed the expected bands arising from the IDipp ligand, but the band at 1976 cm−1 was absent. On the basis of a reduced-mass calculation, a band for the Au−D stretch was predicted at 1407 cm−1 but not observed, as this frequency falls within a series of stretching modes arising from IDipp. The X-ray crystal structure of (IDipp)AuH (Figure 29) confirmed its assignment as a terminal hydride complex, with electron density for the hydride found in the difference Fourier synthesis of the structure. The Au−C distance of 2.045(3) Å is rather long for an (IDipp)AuI complex, reflecting the strong trans-influence of the hydride. The large distance between Au(I) centers, nearly 9 Å at the closest, precludes stabilization of the complex through aurophilic interactions. Nolan and co-workers subsequently prepared (IDipp)AuH in near-quantitative yield by reaction of the versatile precursor (IDipp)AuOH with (MeO)3SiH.128 More recently, (NHC)gold acetonyl complexes were shown to react with pinacolborane to afford the corresponding gold hydrides. This interesting reaction involves the loss of a C-bound enolate through attack of the oxophilic hydride source at the unbound enolate oxygen. Both the known (IDipp)AuH and the new (IMes)AuH are formed in high yield by this route.129 The (IDipp)AuH complex proved surprisingly robust, decomposing over a period of many hours in solution after exposure to air at ambient temperature (see below), but stable under inert atmosphere or in the solid state. Because suitable gold(I) complexes catalyze cyclopropanation using diazo complexes,130 the reaction of ethyl diazoacetate with (IDipp)AuH was explored.127 Instead of the expected insertion of diazo-derived carbene into the Au−H bond, to afford (IDipp)AuCH2CO2Et, this reaction afforded an α-aurated diazo compound as its major product, with a stretching band for the intact CNN moiety prominently visible at 2040 cm−1 in the infrared spectrum. Although the auration of ethyl diazoacetate by (IDipp)AuOt-Bu proved cleaner and more

Figure 28. Solid-state structure of the cation [Ag3(μ3-H)(μ3BH4)(L2)3]+ [L2 = 1,1-bis(diphenylphosphino)methane], with Pphenyl groups omitted for clarity. Selected distances: Ag(1)−H, 1.93(3) Å; Ag(1)−H(1a), 2.17(3) Å; B(1)−H(1a), 1.10(3) Å; B(1)− H(1b), 1.07(6) Å; Ag(1)−Ag(1), 2.9100(3) Å. Figure reproduced with permission from ref 124, under the terms of the Creative Commons License Deed, Attribution 3.0 Unported, https:// creativecommons.org/licenses/by/3.0/. Copyright 2015 Royal Society of Chemistry.

silver−silver and silver−hydride distances in this complex are closer to those in [Ag3(μ3-H)(μ3-Cl)(L2)3]+ than in [Ag3(μ3D)(L2)3]2+; the nature of the bonding and the metal−metal interactions were probed with density functional theory calculations. In light of the importance of borohydride to silver nanoparticle synthesis, the isolation of this mixed borohydride/ hydride cluster provides a link to previously reported silver hydride nanoclusters, and thus to the elemental nanoclusters. Collision-induced dissociation (CID) studies of these and related complexes in the gas phase have established their fragmentation patterns,125 and DFT studies have helped to elucidate these pathways. Interestingly, the cluster [Ag3(μ3H)2(L2)3]+ loses [L2AgH] from its ground state, but reductively eliminates H2 from a photoexcited state,126 raising the possibility of silver clusters as potential catalysts for the release of H2 from hydrogen storage materials. Similar studies, in turn, demonstrated that loss of ligand from the mixed borohydride/ hydride cluster [Ag3(μ3-H)(μ3-BH4)(L2)3]+ facilitates loss of BH3 to form the [Ag3(μ3-H)2]+ core.124 2.3. Gold Hydrides

2.3.1. (N-Heterocyclic Carbene)gold(I) Hydrides. The first gold-only hydride to be isolated under preparative conditions was (IDipp)AuH.127 In analogy with the synthesis of [(IDipp)CuH] 2, Tsui treated (IDipp)AuOt-Bu with trimethoxysilane in benzene or dichloromethane solution (Scheme 22), and obtained a single new NHC-containing species as judged by 1H NMR spectroscopy. In addition to the 8339


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(IDipp)AuH and ethylene in C6D6 solution, whether under 8 bar of ethylene at ambient temperature or 1 bar of ethylene at 80 °C. Phosphine- and NHC-supported alkyls and aryls required high temperatures for thermolysis, up to 180 °C in dimethyl sulfoxide solution, to achieve partial decomposition, and gave no evidence for gold hydride formation (Scheme 24). Scheme 24. Exploring 1,2-Insertion into a Gold(I) Hydride131,a

Figure 29. Solid-state structure of (IDipp)AuH. (a) Ellipsoid representation (at 50% probability); H atoms, except for hydride, and cocrystallized C5H12 omitted for clarity. N(1A) is a crystallographic symmetry equivalent of N(1). Au(1)−C(1), 2.045(3) Å. (b) Fo − Fc difference-electron density plot of the crystal structure of (IDipp)AuH; projection on the H(1), Au(1), C(1) plane. The map was generated assuming the absence of the hydride atom H(1). The highest residual electron density maximum, labeled Q(1), occurs along the Au(1)−C(1) bond. The second residual maximum was chosen as that of the hydride, and corresponds to the position of H(1) in the final model. Adapted with permission from ref 127. Copyright 2008 Wiley-VCH.

a (a) ln (CD3)2SO at 180 °C: partial decomp., no evidence for LAuH. (b) L = IDipp. For L = PPh3, Au° and free ligand observed.

In contrast, hydride abstraction reactions are feasible: (CAAC)gold(I) alkyls react with the triphenylcarbenium ion [Ph3C]+ to afford either cationic alkene complexes via β-abstraction, or cationic carbenoid complexes via α-abstraction, depending on the alkyl group.134 The nature of the decomposition of (IDipp)AuH under air was not identified during the initial study, but in 2015 Bochmann and co-workers demonstrated the reaction of (IDipp)AuH with O2 in C6D6 solution to form the hydroperoxide complex (IDipp)AuOOH (Scheme 25),135 and

convenient preparatively, this deprotonation of a diazo C−H bond by a gold hydride, liberating dihydrogen and forming a gold−carbon bond, is remarkable in light of the low basicity of the hydride toward more conventional Brønsted acids. It does not react with unactivated alkynes such as diphenylacetylene or 3-hexyne, but reacts readily with the highly activated dimethyl acetylenedicarboxylate to afford the corresponding vinylgold species (Scheme 23). Curiously, the product is that of trans-

Scheme 25. Reactions of a Gold(I) Hydride with O2 and with Free Radicals135

Scheme 23. Some Reactions of a Gold Hydride127

observed its competitive partial decomposition to (IDipp)AuOH128 with release of 0.5 equiv of O2. The kinetics of this oxidation were studied under O2 pressures of 4−9 bar. The reaction was found to be first-order in both [(IDipp)AuH] and [O2], and an Eyring plot over the temperatures 36−52 °C gave ΔH⧧ as −21.1 kJ/mol and ΔS⧧ as −251.5(2) J/mol·K. Comparison of the oxidation rate of (IDipp)AuH versus (IDipp)AuD revealed a kinetic isotope effect of kH/kD = 2.4, consistent with rate-limiting Au−H bond cleavage. These researchers further showed that addition of the stable radical TEMPO did not affect the oxidation rate, but the more reactive galvinoxyl radical abstracted hydrogen from the gold hydride, leading to decomposition. This observation allows the AuI−H

addition of gold and hydrogen across the double bond, whereas the concerted 1,2-insertion of an alkyne must afford a cis-vinyl complex, at least initially. A combined theoretical and experimental study explored the difficulty of β-hydride elimination from gold(I) alkyls, and of the microscopic reverse reaction, 1,2-insertion into a gold(I) hydride.131 Density functional theory using D3 dispersion parameters,132 and including scalar relativistic effects,133 predicted free-energy barriers of 152 kJ/mol for the insertion of ethylene into (IDipp)AuH, and of 208 kJ/mol for the elimination of ethylene from (IDipp)AuEt to form (IDipp)AuH. Experimentally, no reaction was observed between 8340


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equimolar LAuCl and LAuH in the presence of Na+[BArf4]−. By treating the neutral hydrides instead with tris(pentafluorophenyl)borane at −20 °C, and monitoring the reactions by 11B and 19F NMR spectroscopy, these researchers obtained convincing evidence for intermediates of the form [LAu···H−B(C6F5)3], in which the hydride is largely transferred to boron and the complex is essentially a tight ion pair. On attempted isolation, these species undergo aryl transfer to form isolable LAuC6F5 complexes and liberate HB(C6F5)2 (Scheme 26).

bond dissociation energy to be bracketed between 291 and 329 kJ/mol. Heterolytic cleavage of the Au−H bond in (IDipp)AuH was explored using Ph3C+ BF4− as the hydride abstraction reagent in CD2Cl2 solution.127 The expected formation of Ph3CH proceeded only halfway to completion, however, and a new gold complex was cleanly formed according to 1H NMR spectroscopy. The resulting spectrum displayed a single set of IDipp resonances and a new singlet resonance, integrating to one hydrogen per two IDipp ligands, at δ 0.42 ppm. The product complex was recognized as a salt of the hydridebridged cation {[(IDipp)Au]2(μ-H)}+, which is also readily prepared by the combination of equimolar (IDipp)AuH and (IDipp)AuOTf. Analysis of {[(IDipp)Au]2(μ-H)}+OTf− by Xray crystallography revealed distinct bending about the C−Au− Au vectors [158.25(12)° and 154.22(11)°]. The remarkably short Au···Au interaction of 2.7099(4) Å is consistent with three-center, two-electron bonding8 in an [Au2H]+ core. Hydride abstraction from (NHC)gold(I) hydrides was explored more thoroughly, both experimentally and theoretically, in a study on the generation of electrophilic gold cations from either hydride or chloride precursors.136 By treatment of expanded-ring (NHC)gold(I) chlorides with K+[HBEt3]−, Aldridge and co-workers prepared and fully characterized (6Mes)AuH, (6Dipp)AuH, and (7Dipp)AuH. The 1H NMR spectra for these complexes in C6D6 solution reveal marked upfield shifts of the hydride resonance as compared to that of 5.11 ppm for (IDipp)AuH: δ 3.36 ppm for (6Mes)AuH, δ 3.57 ppm for (6Dipp)AuH, and δ 3.15 ppm for (7Dipp)AuH. In the X-ray crystallographic analysis of each complex, the gold-bound hydride was located in the difference Fourier map, and could be refined isotropically. Protonolysis of LAuH (L = 6Dipp or 7Dipp) using Brookhart’s acid {[(Et 2 O) 2 H] + [BAr f 4 ] − , Ar f = 3,5-bis(trifluoromethylphenyl},137 at −80 °C in CD2Cl2 solution resulted in the appearance of new hydride resonances at high field, δ −1.47 ppm for the (6Dipp) complex and δ −1.95 ppm for the (7Dipp) complex, assigned to the hydride-bridged dinuclear cations [(LAu)2(μ-H)]+ (Figure 30). As in the abstraction of hydride from (IDipp)AuH,127 the presumed intermediate [LAu]+ cation is rapidly intercepted by unreacted neutral hydride to form the hydride bridge. The new cations bearing expanded-ring NHC ligands were isolated and fully characterized, and could also be synthesized by mixing

Scheme 26. Terminal and Bridging Hydrides of Gold Supported by Expanded-Ring Carbenes136

Density functional theory calculations, using [(Me2O)2H]+ as the acid and the simpler ligand 6Me (1,3-dimethyltetrahydropyrimid-2-ylidene) as a model, offer insight into likely key steps for the conversion of terminal gold hydride to hydridebridged digold cation by protonolysis (Scheme 27). Proton Scheme 27. Calculated Thermodynamics for Protonolysis of an (NHC)AuH Model Complex136

transfer from [(Me2O)2H]+ to (6Me)AuH, forming 2 equiv of Me2O and the dihydrogen-bound cation [(6Me)Au(η2-H2)]+, is exergonic, with ΔG° = −80.8 kJ/mol. Loss of dihydrogen from this intermediate, forming [(6Me)Au]+ and H2, presents a modest barrier of +40 kJ/mol. Subsequent coordination of the Au−H bond of (6Me)AuH to the [(6Me)Au]+ cation, forming {[(6Me)Au]2(μ-H)}+, is strikingly favorable, with ΔG° = −213 kJ/mol.

Figure 30. Solid-state structure of {[(6Dipp)Au]2(μ-H)}+[BArf4]− ellipsoids set at 50% probability. For clarity, anion and H atoms except hydride are omitted, and i-Pr groups are shown in wireframe view. The hydride was located in the difference Fourier map and refined isotropically. Selected bond lengths and angles: Au(1)−Au(2), 2.7571(3) Å; Au(1)−C(33), 2.040(5) Å; Au(2)−C(3), 2.049(5) Å; Au(1)−Au(2)−C(3), 165.7(1)°; Au(2)−Au(1)−C(33), 164.6°. Reproduced with permission from ref 136. Copyright 2014 Wiley-VCH. 8341


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displayed a doublet resonance for the phosphines at δ 33.8 ppm, with 2JP−H = 49.6 Hz. These observations, consistent with coupling between a hydride and four equivalent phosphines, were corroborated by the spectra for the corresponding deuteride complex, prepared from (xy-Xantphos)AuCl and PhSiD3. In the 1H NMR spectrum for this complex, the quintet assigned to the hydride was absent; the 2H NMR spectrum displayed a quintet resonance at δ 8.29 ppm (2JP−D = 7.4 Hz), and the 31P spectrum displayed a triplet resonance at δ 33.5 ppm (2JP−D = 7.4 Hz). The ratio of these coupling constants, 6.7, is close to the ratio of the gyromagnetic constants for 1H and 2H, 6.5. Although this species is clearly established as a (μhydrido)digold complex, and one relevant to the silylation of alcohols, its structure remains uncertain. The authors judged a neutral (μ-hydrido)(μ-chloro)digold complex, bearing chelating xy-Xantphos at each metal center, to be the most probable structure, but did not rule out (μ-hydrido) cations bearing either chelating or bridging xy-Xantphos ligands. 2.3.3. A Stable Gold(III) Hydride. The coinage metal hydride complexes discussed so far have featured d10 metal centers. Hydride complexes of d8 platinum(II), however, have been well explored since the first report by Chatt in 1957.140 Using a rigid and strongly σ-donating terdentate pincer framework derived from a 2,6-diarylpyridine, Bochmann and co-workers stabilized the Au(III) oxidation state, normally highly oxidizing, to obtain an isolable d8 gold hydride.141 Treatment of (C∧N∧C)AuOH (where C∧N∧C is derived from 4′,4″-di-tert-butyl-2,6-diphenylpyridine by metalation at the 2′ and 2″ positions) with Li+[HBEt3]− in toluene solution affords the (C∧N∧C)gold(III) hydride, which can be isolated as yellow crystals. The 1H NMR spectrum of this complex in CD2Cl2 solution displays a broad singlet resonance for the hydride at δ −6.51 ppm. Like that of (IDipp)AuH, the hydride resonance shifts considerably downfield in C6D6 solution, to δ −5.73 ppm. Interestingly, although splitting of the hydride resonance was not resolved, the resonance for the β-aryl protons is split weakly (4JH−H = 1 Hz) by the hydride, and this splitting is absent from the 1H NMR spectrum of the corresponding (C∧N∧C)AuD complex. This complex in CD2Cl2 solution displays a singlet resonance at δ −6.58 ppm in its 2H NMR spectrum, confirming the assignment of the hydride. Likewise, electron density for the gold-bound hydride was located on the density map from the X-ray crystallographic data; the solid-state structure is shown in Figure 31. The infrared spectrum of (C∧N∧C)AuH displays a sharp, intense absorbance at 2188 cm−1, significantly higher than that of (IDipp)AuH and approaching that of AuH itself (2226.6 cm−1 in a frozen argon matrix).142 Despite the presence of two aryl groups cis to the hydride, this Au(III) center does not reductively eliminate Ar−H readily, reflecting the rigidity and strong binding of the pincer framework. In solution, thermolysis or exposure to light led to significant decomposition. The Au(III) hydride was not observed to react with ethylene, with electron-rich alkynes such as 3-hexyne or phenylacetylene, or with the electron-poor dimethyl acetylenedicarboxylate. The allenes cyclohexylallene and dimethylallene, however, undergo 1,2-insertion to form stable gold(III) vinyl complexes. Remarkably, (C∧N∧C)AuH undergoes a reductive condensation reaction with (C∧N∧C)AuOH or (C∧N∧C)Au(O2CCF3), losing water or trifluoroacetic acid to form the covalently bound dimer (C∧N∧C)AuII−AuII(C∧N∧C) (Scheme 29).

2.3.2. (Phosphine)gold(I) Hydrides. Neutral phosphinesupported gold hydrides remain elusive, but in 2009 Jean, Le Floch, and co-workers reported a (μ-hydrido)digold cation supported by a Xantphos framework bearing 2,5-diphenylphosphole donors.138 This complex was synthesized through hydride abstraction from PhSi(Me)2H by the stable, isolable [(Xantphos-Phosphole)Au]+OTf− (Scheme 28a). Although the Scheme 28. Diphospholyl- (a)138 and Diarylphosphino(b)139 Xanthene Gold Hydride Complexes

hydride resonance was not observed in the 1H NMR spectrum of the resulting complex, the 31P NMR spectrum displayed a doublet resonance at δ 23.8 ppm with 2JP−H of 54.0 Hz. The corresponding deuteride complex, in CD2 Cl 2 solution, displayed a quintet resonance (2JP−D = 8.4 Hz) in its 2H NMR spectrum at δ 7.0 ppm, consistent with coupling of the deuteride to four phosphorus centers. The 31P NMR spectrum of this complex likewise displayed a triplet resonance (2JP‑D = 8.4 Hz) at δ 23.8 ppm. These findings indicated that a hydride or deuteride bridged two (Xantphos-Phosphole)gold(I) fragments, and that the resonance for the hydride proton in the 1H NMR spectrum was obscured by aromatic C−H resonances. The chemical shift of 7.0 ppm for the bridging deuteride is found at markedly lower field than that of the bridging hydride in {[(IDipp)Au]2(μ-deuteride)}+. Crystallographic analysis of the new hydride complex by Xray diffraction confirmed its solid-state structure as that of a hydride-bridged digold cation, with a short Au···Au distance of 2.7542(3) Å. The data permitted refinement of the hydride location, giving Au−H bond distances of 1.70(3) Å. Whereas in solution the phosphines are equivalent on the NMR time scale, in the solid state the two Au−P distances at each gold fragment differed substantially, at 2.308(1) and 2.446(1) Å; this difference was ascribed to the strong trans-influence of the hydride, which lies nearly opposite the more distant phosphine on each fragment. Concurrent with this work, Ito, Sawamura, and co-workers studied the dehydrogenative silylation of alcohols to silyl ethers using (Xantphos)gold(I) chloride precatalysts (see also sections 3.4 and 4.3). Using the ligand xy-Xantphos, in which the xanthene-bound phosphines bear 3,5-dimethylphenyl substituents, they observed by 1H and 31P NMR spectroscopy the formation of a hydride-bridged digold(I) intermediate formed by reaction of (xy-Xantphos)AuCl with PhSi(Me)2H (Scheme 28b). The hydride resonance was observed as a quintet centered at δ 8.29 ppm, even further downfield than in the phosphole-supported analogue (see above), with 2JP−H = 49.5 Hz; a 31P NMR spectrum obtained without proton decoupling 8342


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Scheme 30. Oxygen Atom Abstraction from Gold(III) Hydroperoxide and Hydroxide143

Figure 31. Solid-state structure of (C∧N∧C)AuIIIH, shown as 50% ellipsoids. Hydrogen atoms except hydride are omitted for clarity; the hydride position was located on the density map. Selected distances and angles: Au(1)−C(1), 2.074(4) Å; Au(1)−C(17), 2.073(4) Å; Au(1)−N(1), 2.035(3) Å; C(17)−Au(1)−C(1), 161.63(16)°. Reproduced with permission from ref 141. Copyright 2012 Wiley-VCH.

Scheme 29. Synthesis and Reactions of a Gold(III) Hydride

a

the Pt−H bond. Although long elusive under ordinary conditions, gold−hydrogen bonds are by no means intrinsically weak,144 and kinetic stabilization through ligand choice proved crucial to isolating gold hydride complexes.

a

3. ORGANIC TRANSFORMATIONS MEDIATED BY COINAGE METAL HYDRIDE COMPLEXES This section presents a detailed review of organic reactions mediated by coinage metal hydride complexes. By far the largest portion of this section is devoted to processes mediated by copper hydride complexes, which have been studied for decades. In contrast, the first homometallic, isolable hydride of gold was reported only in 2008,127 and that of silver in 2010.98 As a result, while silver and gold hydrides are invoked in many catalytic processes, direct evidence for their participation is often lacking. The few examples of reactions known to feature gold or silver hydride intermediates suggest the potential for important future developments, as these complexes often display very different reactivity from their copper analogues. The first subsection, 3.1, focuses on copper-catalyzed reduction reactions, and is followed by a discussion of the hydrofunctionalization reactions (3.2), with an emphasis on developments since 2008. Section 3.3 provides a review of organic transformations in which the activation of dihydrogen is proposed to lead to the participation of a silver−hydrogen bond or, conversely, in which a silver−hydrogen bond reacts with a proton source to form dihydrogen. Catalytic processes in which gold hydrides are probable intermediates, as supported by computational and/or experimental findings, are presented in section 3.4.

Adapted with permission from ref 141. Copyright 2012 Wiley-VCH.

The net insertion of O2 into the (C∧N∧C)Au−H bond to form a hydroperoxide (C∧N∧C)Au−OOH has not been observed; however, the reduction of (C∧N∧C)Au−OOH, synthesized independently, by excess phosphine gives rise to the characteristic hydride resonance at high field in the 1H NMR spectrum. The reduction is proposed to proceed stepwise, with oxidation of the first phosphine forming phosphine oxide plus (C∧N∧C)AuOH, then subsequent reduction of the hydroxide by phosphine forming hydride. Indeed, reduction of (C∧N∧C)AuOH by phosphines gave (C∧N∧C)AuH in at least 95% yield (Scheme 30), with the balance forming the (C∧N∧C)AuII dimer.143 The abstraction of an oxygen atom from a metal hydroxide to form a hydride was apparently unprecedented, and is relevant to potential water-splitting pathways. Like (IDipp)AuH, (C∧N∧C)AuH reacts with galvinoxyl radical but not with 2,2,6,6-tetramethylpiperidoxyl (TEMPO), indicating an Au−H bond enthalpy between 291 and 329 kJ/mol. Calculations using density functional theory find a dissociation energy of 317 kJ/mol for the Au−H bond in this complex, stronger than the Au−O bond in (C∧N∧C)AuOH (BDE = 279 kJ/mol). In contrast, for an analogous pair of d8 platinum complexes, the calculated dissociation energy was higher for the Pt−OH bond than for

3.1. Reduction Reactions Mediated by Copper Hydride Complexes

Most of the early applications of copper hydride complexes in organic chemistry resulted in a net reduction of the organic substrate. This focus on reductive transformations is evident in the last major review of the field from 2008,6 which was dominated by the discussion of such processes. Over the last eight years, significant advances have been made in expanding the range of functional groups that can be reduced using copper hydride complexes and in improving the selectivity (enantioand regioselectivity) of the known transformations. 3.1.1. Hydrogenation and Hydrosilylation of Carbonyl Compounds. Copper-catalyzed hydrogenation of carbonyl compounds was originally reported by Stryker in 2000145 and was followed in 2001 by the first copper-catalyzed hydro8343


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silylation reported by Lipshutz et al.146 (For a detailed discussion of the mechanism of copper-catalyzed hydrosilylation of carbonyls, see section 4.1.2.) These reports inspired further efforts devoted to the development of enantioselective methods for ketone reduction. Enantioselective reductions of carbonyls reported prior to 2008 were limited to aryl and diaryl ketones, which could be reduced with excellent enantioselectivities.6 A similar scope was later achieved in the enantioselective hydrosilylation of ketones reported by Beller et al. in 2010.147 In a series of papers,148−151 Chan and Wu also reported hydrosilylation of aryl and diaryl ketones using dipyridylphosphane ligands. It is worth noting that their system was not sensitive to air and moisture and that excellent results were obtained with α, β, and γ halo aryl ketones (Scheme 31).151

Scheme 33. Reduction of Alkenyl Ketones

Scheme 31. Reduction of Aldehydes and Ketones

Enantioselective reduction of aryl ketones could also be achieved by the copper-catalyzed hydrogenation reaction first reported by Shimizu et al. in 2007,152 and later by Beller et al. in 2011,153 and Johnson et al. in 2013154 (Scheme 32). The

selective 1,2-reduction observed by Lipshutz et al. was surprising considering the general preference for 1,4-reduction in copper-catalyzed reactions of enones.6 As a subsequent study by the same group revealed,156 the presence of an α substituent was essential for consistently high regioselectivity (1,2- vs 1,4reduction). Furthermore, results of a computational study pointed to the steric demand of the ligands used in the reduction as the key factor responsible for the observed 1,2selectivity.156 The regioselectivity in the reductions of enones was also explored by Gordon et al.157 In 2013, they reported a systematic study of ligands in the copper-catalyzed hydrosilylation shown in Scheme 34. The best selectivity was

Scheme 32. Enantioselective Reduction of Ketones

Scheme 34. 1,2-Reduction of Enones

obtained using a 1,2-bis(diethylphosphino)ethane ligand, which gave exclusively the product of 1,2-reduction. Sole formation of the 1,4-reduction product was observed with 1,2-bis(diphenylphosphino)benzene, in agreement with the previous reports by Lipshutz.46 It remains unclear how general the ligand effects described by Gordon et al. are considering that only a single substrate was used in the study. While most of the methods for the reduction of carbonyl compounds rely on copper catalysts supported by phosphine ligands, significant contributions have also been made using NHC copper catalysts. Most notably, in 2011, Gawley et al. reported a highly enantioselective hydrosilylation of a wide range of ketones, including dialkyl ketones (Scheme 35).158 With the (R)-CuPhEt catalyst previously developed by the same group,159 excellent enantioselectivity was obtained even with substrates featuring two alkyl substituents with similar

reaction conditions described in the three reports are quite similar, with hydrogen as a terminal reductant and isopropanol as a solvent. In all cases, a strong alkoxide base was necessary for the activation of the precatalyst. The scope of the enantioselective reductions of ketones was significantly extended in 2010 by Lipshutz et al. (Scheme 33).155 They reported a highly enantio- and regioselective 1,2reduction of α-substituted enones using catalysts derived from Cu(OAc)2 and DTMB-Segphos or BIPHEP ligands. In agreement with previously made observations, the selectivity was influenced by the hydride source, with best results obtained using (EtO)2MeSiH. Good enantioselectivities were obtained with a range of β-aryl-substituted and cyclic enones. The 8344


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Scheme 35. Enantioselective Reduction of Alkyl Ketones

Scheme 37. Semireduction of Alkynes

steric and electronic properties. The reaction was performed at room temperature, under mild reaction conditions, using diethyl or diphenyl silane as the hydride source. The products of the reduction were isolated as silyl ethers. In terms of scope and enantioselectivity, the hydrosilylation reported by Gawley et al. is one of the best available methods for enantioselective reduction of simple dialkyl ketones. A highly sterically hindered alkylfluorenyl-substituted NHC ligand L3 developed by Matt et al. was used to prepare achiral copper catalysts for hydrosilylation of ketones and aldehydes (Scheme 36).160 While slower than the catalysts previously

cases exclusively Z) and high yields. The same reaction conditions were also effective in the reduction of dialkylsubstituted alkynes, dienes, and enynes, but not terminal alkynes. Several examples of terminal alkyne reductions using a variety of reaction conditions and several NHC supported copper catalysts were also reported in the paper. In early 2013, Lalic et al. reported the semireduction of both terminal and internal alkynes using (IDipp)CuOt-Bu as the catalyst and PMHS as a terminal reductant (Scheme 38).166

Scheme 36. Reduction of Aldehydes and Ketones

Scheme 38. Semireduction of Alkynes

reported by Nolan,161,162 Matt’s catalyst showed improved TONs (up to 1000), while still affording excellent yields of the desired products. Improved durability of the catalyst was attributed to the increased steric shielding of the copper center by the ligand, which has a percent buried volume significantly higher than IDipp ligand. NHC supported copper catalysts have also been used in hydroboration of ketones recently reported by Mankad et al.163 3.1.2. Reduction of Alkenes and Alkynes. The first example of copper hydride-mediated reduction of alkynes to alkenes was described by Stryker in 1990.53 In the presence of a stoichiometric amount of [(Ph3P)CuH]6 (0.5 equiv), alkynes were selectively reduced to alkenes, using water as a proton source. The reaction was proposed to involve hydrocupration followed by the protonation of the alkenyl copper intermediate (see section 4.1.3 for a detailed discussion of the mechanism of copper-catalyzed reduction of alkynes). Considering this early report by Stryker, it is surprising that the first catalytic variant of this semireduction was reported only in 2012, by Tsuji et al.164 The reduction was accomplished using a catalyst prepared in situ from Cu(OAc)2 and CF3Ar-Xan ligand, with polymethyhydrosiloxane (PMHS) as a hydride donor and t-BuOH as a proton source, although the exact reaction conditions vary from substrate to substrate (Scheme 37). The focus of the study was the reduction of aryl- and diaryl-substituted alkynes, which are particularly prone to over-reduction using traditional methods, such as Lindlar’s catalyst.165 With both classes of compounds, exclusive formation of the alkene product was observed, together with excellent Z/E selectivity (>95/5, and in most

The reduction of a wide range of terminal alkynes was accomplished within an hour at 25 °C, using 0.5 mol % of a catalyst and i-BuOH as a proton source. The reaction could be accomplished in the presence of a wide range of functional groups, including ketones, internal alkynes, propargylic benzoates, nitroarenes, and strained alkenes. The desired products were obtained in excellent yields (generally >90%), without contamination from alkene over-reduction or isomerization. The reduction of internal alkynes required t-BuOH as a proton source, a 2 mol % catalyst loading, and a higher temperature (45 °C). Z-Alkenes were the exclusive products of the reduction and were obtained in high yields (>81%). Similar 8345


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tions provide a detailed and useful definition of the reaction scope. Copper catalysts together with the recently developed gold nanoparticles172−174 provide the most general and selective approaches to the semireduction of alkynes.165 Copper catalysts perform well with substrates that have traditionally suffered from over-reduction and isomerization of the product alkenes, such as aryl- and diary-substituted alkynes, terminal alkynes, and alkynes containing polar functional groups. The semireduction can be accomplished in the presence of a wide range of functional groups, and unlike most other catalytic systems, monitoring of the reaction progress is not necessary to avoid over-reduction. A rare example of copper-catalyzed reduction of alkenes was reported by Lam et al. in 2009. They developed the enantioselective copper-catalyzed reduction of azaarene-substituted alkenes shown in Scheme 40.175 Excellent enantiose-

results in the reduction of both terminal and internal alkynes were obtained on a large scale (>1 g) with commercially available and air-stable (IDipp)CuCl complex as a catalyst precursor. A detailed discussion of the mechanism of the semireduction reactions developed by the Tsuji and Lalic groups is presented in section 4.1.3. Since 2013, several other examples of the semireduction of alkynes have been reported in the presence of copper catalysts (Scheme 39). In 2014, Yin et al. reported the semireduction of Scheme 39. Semireduction of Alkynes

Scheme 40. Enantioselective Reduction of AzaareneSubstituted Alkenes

lectivities were observed with a wide range of azaarenes. The reduction was presumed to involve hydrocupration of the alkene, followed by protonation of the organocopper intermediate. The presence of the azaarene is postulated to allow facile hydrocupration of the otherwise unreactive alkenes. The essential role of the nitrogen atom of the azaarenes in the reaction was clearly demonstrated by the fact that only 2alkenyl azaarenes were reactive. 3.1.3. Reductions of CO2. In 2012, Baba et al. published the first in a series of papers describing copper-catalyzed reductive transformations of CO2 in the presence of silane. The first two publications176,177 demonstrated facile hydrosilylation of CO2 at low pressure (1 atm) and mild reaction conditions (25 °C) (Scheme 41). The initially formed silyl formate was

terminal alkynes at 130 °C in DMF using a catalyst prepared in situ from copper citrate and hexamethylenetetramine, with H3PO2 as both a reductant and a proton source.167 The same year, Zhong et al. accomplished the reduction of a wide range of aryl-substituted internal and terminal alkynes using a catalyst prepared in situ from air-stable precursors, Cu(OAc)2 and IDipp·HCl.168 In 2015, Nakao, Semba et al. reported the semireduction of internal alkynes using H2 gas (5 bar) as a terminal reductant, with i-PrOH as a proton source.169 Later, Teichert et al. demonstrated that an efficient semireduction of a range of internal aryl- and diaryl-substituted alkynes can be accomplished using H2 gas (100 bar) in the absence of protic additives.170 In this case, both hydrogen atoms from H2 gas are delivered to the alkyne by the same catalyst. The key for the success of the reaction is the presence of a pendant hydroxyl group in NHC ligand L4, shown in Scheme 39. Finally, early in 2016 Sawamura reported a related semireduction of both aryland alkyl-substituted internal alkynes.171 A detailed investigation of chemoselectivity identified a range of substrates not compatible with the reaction conditions, including nitro and bromo arenes and nitriles. Terminal alkynes could not be reduced under the reaction conditions, while certain substrates provided lower diastereoselectivities. Overall, these observa-

Scheme 41. Hydrosilylation of CO2

transformed into formic acid in the presence of water. Excellent TON (8600) and TOF (1400) were achieved using a catalyst developed by Lipshutz46 and formed in situ from Cu(OAc)2 and 1,2-bis(diphenylphosphino)benzene (BDP, R = Ph). The results could be further improved using 1,2-bis(diisopropylphosphino)benzene as a ligand (R = iPr) (TON = 62 000, TOF = 10 300). Hou et al. also accomplished a hydrosilylation of CO2, using (IDipp)CuOt-Bu as a catalyst.178 Shintani and Nozaki used the same catalyst in a closely related hydroboration of CO2, 8346


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reported in 2013.179 Transformations of CO2 catalyzed by NHC-copper complexes have been reviewed in 2013.180 Finally, in 2015, two related reports described the coppercatalyzed reduction of CO2 with H2 as a reductant.181,182 In 2013, Baba et al. described copper-catalyzed synthesis of formamides from CO2 and free amines, in the presence of PMHS (Scheme 42).183 Excellent results were obtained using

Scheme 44. Reduction of Propargylic Carbonates

Scheme 42. Catalytic Formylation of Amines

catalyst derived from BDP and Cu(OAc)2, with a wide range of primary and secondary amines. However, sterically hindered and electron-deficient amines were not suitable as substrates. A mechanistic study supports the direct formation of formamides without the intermediacy of silyl formates.184 In 2015, Cazin et al. described a related method for the methylation of amines.185 Unfortunately, a mixture of methylated and formylated amines was obtained in most cases. Interesting selectivity was observed by Mankad et al. in a CO2 reduction using heterobimetallic catalysts and HBpin as a reductant (Scheme 43).186 While the reduction mediated by

Scheme 45. Reduction of Phosphine Oxides

Scheme 43. Catalytic Reduction of CO2 to CO copper-catalyzed reduction of secondary phosphine oxides and a copper-catalyzed phosphination of aryl and alkyl halides. This method was used to prepare a number of tertiary aryl and alkyl phosphines. The results of stoichiometric experiments with Stryker’s reagent suggest that copper hydride complexes may not be intermediates in the reduction of phosphine oxides, although the exact mechanism of the reaction remains unclear. 3.1.4.3. Fluoroarenes. In 2013, Zhang et al. reported a copper-catalyzed monohydrodefluorination of electron-deficient fluoroarenes (Scheme 46).191 In some instances, the

(IDipp)CuH produced exclusively formate, using (NHC)Cu− [M] catalysts ([M] = metal carbonyl anion) led to the formation of CO as the major product. The exclusive formation of CO is observed with (IMes)CuMp catalyst. In the original publication, a detailed study of the reaction mechanism was also presented by Mankad et al. 3.1.4. Reductions of Other Functional Groups. 3.1.4.1. Propargylic Electrophiles. In 2007, Krause et al. reported the synthesis of α-hydroxyallenes from propargylic epoxides.187 Following up on their original work, the Krause group developed a more general method for the synthesis of allenes from propargylic carbonates (Scheme 44).188 Similar results were reported by Sawamura et al. using a catalyst prepared in situ from Cu(OAc)2 and Xantphos ligand in the presence of PHMS (Scheme 44).189 In both reactions, di- and trisubstituted allenes could be prepared with excellent chirality transfer (ct) and high anti SN2′ selectivity. 3.1.4.2. Phosphine Oxides. A copper-catalyzed reduction of phosphine oxides was reported by Beller et al. using (Me2HSi)2O (TMDS) as a terminal reductant (Scheme 45).190 The most effective catalyst precursor was Cu(OTf)2, which proved effective in the reduction of a wide range of tertiary phosphine oxides. Considering that none of the substrates reported by Beller were chiral at phosphorus, the stereochemistry of the reduction remains unclear. In the same report, Beller describes a one-pot procedure that combines a

Scheme 46. Monodefluorination of Polyfluorinated Arenes

para position relative to the electron-withdrawing group is substituted with good selectivity (up to >99:1, p:o selectivity), although concomitant hydrodefluorination in the ortho position is often observed. The best results were obtained with a catalyst prepared in situ from CuCl, KOt-Bu, and BDP ligand, using PhMe2SiH as a hydride source. The experimental examination of the reaction mechanism supports a copper hydride as the key reactive intermediate. On the basis of a theoretical study, the authors suggest that the key step of the reaction is a reaction involving copper hydride intermediate and aryl fluoride that proceeds through a four-membered transition state with no intermediates. 8347


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3.1.4.4. Alkyl Iodides and Sulfonate Esters. In 2014, Lalic et al. reported a method for the deoxygenation of primary alcohols that involves copper-catalyzed reduction of alkyl triflates shown in Scheme 47.192,193 The reduction proceeds under mild

process designed to accomplish similar hydrofunctionalization reaction was reported by Chiu et al.195 Their report of a reductive aldol reaction was then quickly followed by the development of a large number of hydrofunctionalization reactions involving enones and enoates. The significant developments reported prior to 2008 have been covered in the most recent reviews of copper hydride chemistry5,6 and will not be further discussed here. In late 2008, Lipshutz et al. reported a copper-catalyzed hydroboration of alkenyl esters, using Styker’s reagent as a catalyst (Scheme 48).196 The Z-boronic esters formed in the

Scheme 47. Reduction of Alkyl Triflates and Iodides

Scheme 48. Hydroboration of Alkynes

reaction conditions and can be accomplished in the presence of a wide range of functional groups, including nitroarenes, aryl halides, tosylates, esters, and nitriles. The best results were obtained using (IDipp)CuOt-Bu as a catalyst, and TMDSO as a hydride source. The key to the success of this reaction was the use of CsF as a turnover reagent. The reduction of primary and secondary alkyl iodides was also accomplished using similar reaction conditions. Intramolecular trapping experiments and experiments performed in the presence of TEMPO suggest that the reduction does not involve the formation of free-radical intermediates.192

reaction are generally not accessible using standard methods for the synthesis of organoboron compounds. Interestingly, a relatively high concentration of the substrate (1 M) was necessary for the reaction. At lower concentrations (0.1 M), only products of the reduction were obtained. It should be noted that a closely related hydrostannation of activated alkynes was previously reported by Chiu et al.197 Also in 2008, Lipshutz et al. reported the first example of a hydrofunctionalization in which both the hydrocupration and the subsequent functionalization step generated a new stereocenter (Scheme 49).198 The enantioselective reductive

3.2. Hydrofunctionalization Reactions Mediated by Copper Hydride Complexes

Scheme 49. Enantioselective Reductive Aldol Reaction

Arguably the most significant development in copper hydride chemistry over the last 8 years has been the emerging focus on hydrofunctionalization reactions of various classes of unsaturated compounds. The potential of copper hydride complexes to mediate hydrofunctionalization reactions was recognized already by Stryker. In 1990, he noted intramolecular hydroalkylation as a side reaction in the conjugated reduction of enones containing pendant alkyl halides.194 It was proposed that the hydroalkylation product was formed by the intramolecular alkylation of the copper enolate intermediate formed by hydrocupration of the substrate. The same general approach that involves hydrocupration and electrophilic functionalization of the organocopper intermediate (see section 4.1 for a more detailed discussion) has subsequently been used in the development of hydrofunctionalization reaction of a wide range of unsaturated compounds. Initial efforts in this area were focused on hydrofunctionalization of activated alkenes, which readily participate in a hydrocupration reaction. More recently, the discovery of copper hydride complexes that effect the hydrocupration of less reactive classes of unsaturated compounds enabled the development of hydrofunctionalization reactions of styrenes, allenes, alkynes, and alkenes. 3.2.1. Hydrofunctionalization of Conjugated Alkenes and Alkynes. A full eight years after the Stryker’s report of the accidental hydroalkylation of an enone, the first catalytic

aldol was accomplished using a catalyst prepared in situ from Cu(OAc)2 and Josiphos ligand (PPF-P(t-Bu)2) shown in Scheme 49. Cyclohexane products containing three contiguous stereocenters were formed in high yield and with excellent enantio- and diastereoselectivity using just 1 mol % of the catalyst. Five-membered products were formed with lower diastereoselectivity (59:41). More recently, several transformations relying on reductive aldol and Mannich reactions have been reported by the Chiu and Li groups. The desymmetrization of cyclic 1,3-diketones 8348


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accomplished by a reductive aldol reaction with α,β-unsaturated thioesters was reported by Chiu et al. in 2012 (Scheme 50).199

Scheme 52. Enantioselective Hydroboration of Styrenes

Scheme 50. Enantioselective Reductive Aldol Reaction

Scheme 53. Hydroborylative 1,3-Rearrangement of 2Bromostyrenes

Careful optimization of the hydride source, ligand, and thiolderived portion of the thioester allowed efficient synthesis of cyclohexanes containing three contiguous stereocenters with high enantio- and diastereoselectivity (>90% ee and >98:2 dr). Five-membered rings are formed in lower yield (90% ee) were obtained with a wide range of styrenes (Scheme 52). The presence of a meta substituent decreased the enantioselectivity, while the ortho and para substituents were well-tolerated. Hydroboration of βsubstituted styrenes proceeded with high enantioselectivity at a significantly reduced rate. An interesting 1,3-bromo rearrangement shown in Scheme 53 was observed by Schomaker et al.202 in an attempted 8349


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Scheme 54. Reductive Coupling of Alkenylazaarenes with Ketones

Scheme 56. Enantioselective Hydroamination of Styrenes

first report described the enantioselective synthesis of 2,3disubstituted indolines, as shown in Scheme 57.209 The best results were achieved using a catalyst prepared in situ from Cu(OAc) 2 and (S,S)-PhBPE ligand, together with (EtO)2MeSiH as a hydride source and an alcohol as a proton source. The reaction is believed to involve hydrocupration of the styrene, followed by the addition of the alkyl copper Scheme 57. Enantioselective Synthesis of 2,3-Disubstituted Indolines

Scheme 55. Reductive Coupling of Vinylazaarenes with NBoc Aldimines

section 3.2.4. for a related report by Buchwald et al.) Enantioselective hydroamination of styrenes was achieved using (PMHS) as a hydride source, and N,N-dialkyl-Obenzoylhydroxylamines as an electrophilic amination reagent. The best results were obtained using catalyst made in situ from CuCl and Duphos or BPE ligands. Good enantioselectivities and yields were obtained with a wide range of electrophiles and with a range of styrenes, including β-substituted styrenes. In the same paper, Miura et al. also described an effective method for the synthesis of racemic hydroamination products using an achiral diphosphine ligand. In 2015, Buchwald et al. reported two transformations in which the initial hydrocupration of a styrene was followed by the alkylation of the benzylic organocopper intermediate. The 8350


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intermediate to the aldimine and the protonation of the resulting adduct (Scheme 57b). To suppress the unproductive protonation of the alkyl copper intermediate, the authors relied on the KIE and used t-BuOD as a proton source. A catalytic amount of PPh3 had a positive effect on the catalyst TON and the yield of the reaction.210 The starting aldimines were prepared from 2-alkenyl anilines and an aryl aldehyde. Excellent syn diastereoselectivity (>99:1) was observed with all substrates, while the enantioselectivity was generally above 90% ee, except for the substrates derived from electrondeficient aryl aldehydes. β-Substituted styrenes were also successfully used in the reaction with minimal changes of the reaction conditions. In the second report, Buchwald et al. reported the enantioselective synthesis of aryl-substituted carbo- and heterocycles relying on hydrocupration of styrenes, followed by intramolecular alkylation of the organocopper intermediate (Scheme 58).211 This method is effective for the synthesis of

Scheme 59. Enantioselective Allylation of Vinyl Arenes

functionalization of styrenes reported by the Buchwald group, excellent enantioselectivities and yields are observed. Labeling experiments suggest that the reaction occurs with SN2′ regioselectivity. 3.2.3. Hydrofunctionalization of Alkynes. In 2011, Tsuji et al. reported the copper-catalyzed hydrocarboxylation of internal alkynes shown in Scheme 60.213 A number of

Scheme 58. Intramolecular Alkylation of Functionalized Styrenes

Scheme 60. Hydrocarboxylation of Alkynes

symmetrical diaryl alkynes were successfully transformed at 100 °C, with (IMes)CuF as a catalyst precursor and (EtO)3SiH as a hydride source (Scheme 60a). The same catalyst provided only a trace amount of product when nonsymmetrical alkynes featuring one alkyl and one aryl substituent were used. However, using (Cl2IDipp)CuF as a catalyst precursor, even with this class of substrates, useful regioselectivity and yields were obtained (Scheme 60b). Furthermore, nonsymmetrical dialkyl-substituted alkynes also provided good selectivity as long as the two alkyl substituents were sufficiently differentiated either sterically or electronically. The mechanism proposed by Tsuji et al. involves hydrocupration of the alkyne and carboxylation of the alkenyl copper intermediate, and is supported by a series of stoichiometric experiments that demonstrate the feasibility of the proposed elementary steps. The details of the reaction mechanism are presented in section 4.1.4. The following year, the same group reported a regioselective hydroboration of internal alkynes (Scheme 61).214 Tsuji et al. showed that in copper-catalyzed hydroboration, good selectivity could be obtained with internal alkynes containing electronically differentiated substituents. With substrates containing an alkyl substituent and an electron-withdrawing substituent, such as aryl or carbonyl group on the other side, selectivity greater

cyclobutanes, cyclopentanes, tetrahydropyrans, and piperidines, as well as indanes. The best results were obtained using a catalyst prepared from Cu(OAc)2 and DTMB-Segphos, with (MeO)2MeSiH as a hydride source. Uncharacteristically, LiOMe was uniquely effective as a turnover reagent. Uniformly high enantioselectivity (>93% ee) and diastereoselectivity (>20:1) were observed. The only exception were indenes, which were obtained with slightly lower enantioselectivity (85− 88% ee). In 2016, Buchwald et al. reported further development in hydrofunctionalization of styrenes. Using the same catalytic system used in synthesis of indolines, they achieved enantioselective hydrallylation of vinyl arenes (Scheme 59).212 Allyl phosphate and 2-substituted allyl phosphates can be used as electrophiles. Using electrophiles containing vinyl silanes, rapid enantioselective synthesis of allylic silanes can be accomplished. Styrenes and heteroaromatic vinyl arenes can be used as substrates. As in the previous example of hydro8351


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63).216 The best results were achieved using (IDipp)CuCl as a catalyst precursor and diphenyl silane as a hydride source. The

Scheme 61. Hydroboration of Alkynes

Scheme 63. Anti-Markovnikov Hydrobromination of Alkynes

use of 2-t-BuC6H4OK as a turnover reagent was essential for the success of the reaction, as was the use of 1,2-dibromo1,1,2,2-tetrachloroethane as the electrophilic source of bromine. While rarely used, 1,2-dibromo-1,1,2,2-tetrachloroethane is a convenient brominating reagent, as it is commercially available, cheap, and stable. Finally, the slow addition of the brominating reagent (over 0.5−1 h) was necessary to avoid an unproductive reaction of the copper hydride intermediate with the brominating reagent.7 The exploration of the substrate scope showed that anti-Markovnikov E-selective hydrobromination can be accomplished with aryl- and alkyl-substituted alkynes, in the presence of a wide range of functional groups, including esters, nitriles, epoxides, alkyl halides, aryl halides, alkenes, and aryl boronic esters. A detailed study of the mechanism of the hydrobromination reaction has also been reported by Lalic et al.7 In 2015, the Lalic group also reported a rare example of a direct reductive cross-coupling of alkynes with sigma (sp3) electrophiles shown in Scheme 64.58 (5Dipp)CuOTf was used

than 98:2 was observed. The selectivity was lower if the alkyl substituent was small (Me), or if the activating group was only mildly electron-withdrawing. Best results were obtained using a catalyst prepared in situ from MeAr-Xantphos ligand and CuCl in the presence of NaOt-Bu, although the exact reaction conditions varied slightly with the structure of the alkyne precursor. The proposed mechanism of the reaction is supported by a series of stoichiometric experiments. The synthesis of the other regioisomer of these products was reported in the same paper using a borocupration/protonation approach. An interesting diastereoselectivity in copper-catalyzed hydroboration of terminal aryl-substituted alkynes was recently reported by Yun et al. (Scheme 62).215 They observed that with

Scheme 64. Anti-Markovnikov Hydroalkylation of Alkynes

Scheme 62. Stereodivergent Hydroboration of Terminal Alkynes

as a catalyst precursor, with (Me2SiH)2O as a hydride source, and CsF as a turnover reagent. The reaction proceeded only with alkyl triflates as coupling partners, while alkyl tosylates, iodides, and bromides were inert under the reaction conditions. Good yields of the desired alkene products were obtained with primary alkyl and benzylic triflates in the presence of a range of common nonprotic functional groups. In all cases, excellent anti-Markovnikov regioselectivity and E diastereoselectivity were observed. Finally, it is interesting to note that slow addition of highly electrophilic alkyl triflates is not necessary in this case. A detailed mechanistic study reported by Lalic et al.69 identified a dinuclear copper hydride complex as the key intermediate responsible for the selective hydrocupration of an alkyne in the presence of alkyl triflates (see section 4.1.5 for details).

a proper choice of the catalyst, either Z or E alkenylboron products could be obtained selectively. A catalyst generated from (5Dipp)CuCl afforded exclusively E-alkenylboron products. The catalyst prepared in situ from CuTC and DPEphos, on the other hand, provided exclusively the Z diastereoisomer, albeit together with ∼15% of the alkylborane. Both electronrich and electron-poor aryl-substituted alkynes were used in both reactions with equal success. In 2014, Lalic et al. reported a copper-catalyzed antiMarkovnikov hydrobromination of terminal alkynes (Scheme 8352


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catalyst was the only one found to be competent in this reaction, while ethanol performed best as the additive. Arylsubstituted alkynes were converted into benzylic amines with excellent regio- and enantioselectivity (most examples over 95% ee). Terminal alkyl-substituted alkynes were converted into alkylamines with excellent anti-Markovnikov selectivity and good yields (71−88%), even in the presence of protic functional groups such as alcohols and amines. The reductive amination was proposed to occur through the sequential semireduction of alkyne according to the mechanism described by Tsuji,164 followed by the hydroamination of the generated alkene. The alcohol additive is responsible for the protonation of the initially formed alkenyl copper intermediate and the formation of the alkene intermediate. This proposal is supported by the finding that enamines are unreactive under the reaction conditions, as well as the fact that the same reaction conditions with the alcohol additive can be used to accomplish hydroamination of alkenes. 3.2.4. Hydrofunctionalization of Alkenes. Soon after the initial report by Miura in 2013 describing copper-catalyzed hydroamination of styrenes, Buchwald et al. reported the first example of an anti-Markovnikov hydroamination of terminal alkenes (Scheme 68a).219 The reaction is postulated to involve electrophilic amination of the alkyl copper intermediate obtained by the hydrocupration of the alkene. The best results

A formally related hydroalkylation of terminal dienes was reported by Kambe et al., also in 2015 (Scheme 65).217 Alkyl Scheme 65. Hydroalkylation of Dienes

fluorides were uniquely effective as coupling partners, and EtMgCl was used as a hydride source. In all cases, selective 1,2addition was observed favoring the terminal alkene product. The exploration of the substrate scope demonstrated that structurally varied terminal dienes could be used successfully, while the functional group compatibility seems limited by the use of a Grignard reagent at elevated temperature (50 °C). The authors proposed that the reaction involves the formation of a copper hydride intermediate through β-hydride elimination from the alkyl copper complex, followed by the hydrocupration of the diene, and the cross-coupling of the allyl copper intermediate with alkyl fluoride. The authors proposed a mechanism that features a series of highly reactive cuprate complexes as intermediates. However, the overall mechanism and the identity of the reactive intermediates remain to be firmly established. Two hydroamination reactions of alkynes were reported in 2015 by Buchwald et al. (Schemes 66 and 67).218 They found

Scheme 68. Copper-Catalyzed Hydroamination

Scheme 66. Hydroamination of Alkynes

Scheme 67. Reductive Hydroamination of Alkynes

that aryl-substituted internal alkynes could be transformed into E enamines in the presence of N,N-dialkyl-O-benzoylhydroxylamines, a copper catalyst, and (EtO)2MeSiH as a hydride source (Scheme 66). The best results were obtained using a catalyst prepared from Cu(OAc)2 and DTMB-Segphos ligand, although good results were also obtained with several other diphosphine ligands. In all cases, excellent regioselectivity was observed, favoring the formation of benzylic enamines. In the presence of an alcohol additive with minimal changes of the reaction conditions, alkyl amines were obtained through a reductive amination of alkynes.218 A DTMB-Segphos-derived 8353


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diverse alkenes. Furthermore, a variety of N,N-dialkyl-Obenzoylhydroxylamines was successfully used. With chiral 1,1disubstituted alkenes excellent catalyst control of the newly formed stereocenter was observed. Even in a mismatched case, useful diastereoselectivities were observed (6:1 or better). The authors also demonstrated that the reaction can be performed on a 10 mmol scale with 0.4 mol % catalyst loading with excellent results. Essential for the low catalyst loading was the presence of a secondary ligand (Ph3P), an effect originally identified by Lipshutz.210 The same reaction conditions were also highly effective in the enantioselective hydroamination of alkenylsilane reported by Buchwald et al. in 2015 (Scheme 71).222 Universally high

were obtained using a catalyst prepared in situ from Cu(OAc)2 and DTMB-Segphos ligand, together with (EtO)2MeSiH as a hydride source in THF. Excellent yields (generally above 85%) were obtained with alkenes containing amides, alkyl bromides, epoxides, protected alcohols, and protected amines. In all cases, excellent regioselectivity was observed. In the same paper,219 the authors also described a highly enantioselective hydroamination of styrenes using only slightly modified reaction conditions (0.5 M vs 1.0 M concentration) (Scheme 68b). Benzylic amines could be formed with high regio- and enantioselectivity (mostly above 90%) using either terminal or β-substituted styrenes as substrates. Hydroamination of both terminal alkenes and styrenes was limited to the formation of trialkylamines. In 2014, Miura et al. reported a more focused study on the enantioselective hydroamination of bicyclic strained alkenes (Scheme 69)220 using the same catalytic system they originally developed for the hydroamination of styrenes (see Scheme 56).

Scheme 71. Enantioselective Hydroamination of Silanes and Boranes

Scheme 69. Enantioselective Hydroamination of Strained Alkenes

enantioselectivities (92%−99% ee) were obtained with various combinations of different alkenyl silanes and N,N-dialkyl-Obenzoylhydroxylamines. The regioselectivity observed in these reactions was rationalized by the stabilizing α-silicon effect that governs the formation of the alkyl copper intermediate. Somewhat surprisingly, E and Z isomers of the alkenylsilanes provided the same enantiomer of the desired product, although E alkenyl silanes react significantly faster. Control experiments indicate that Z-alkenylsilanes are first isomerized to E isomers, which then undergo hydroamination. Similar reaction conditions were also used in the closely related enantioselective hydroamination of alkenyl dan boronates (dan = 1,8diaminonaphthyl) reported by Miura et al. in 2015 (Scheme 71).223 The aminoborane products have interesting application as substrates in palladium cross-coupling reactions and as direct precursors to bioactive α-aminoboronic acids. Initially, one of the major limitations of hydroamination reactions was that only trialkylamines could be prepared using this approach. Recently, Buchwald et al. reported the synthesis of N,N-dialkyl amines by hydroamination of styrenes using the modified electrophile shown in Scheme 72.224 The utility of the method was demonstrated by efficient syntheses of highly complex α-chiral benzyl amines shown in Scheme 72. Competition experiments demonstrated that the new electrophile is reduced by copper hydride at a significantly lower rate than the previously used N-alkyl-O-benzoylhydroxylamine. The slower reduction of the new electrophile is credited for its improved performance in the hydroamination of alkenes. A detailed discussion of the mechanism of the hydroamination reaction is presented in section 4.1.7. The same electrophile proved essential for the effective hydroamination of simple internal alkenes, which undergo hydrocupration significantly less readily than terminal or arylsubstituted alkenes. This extension of the substrate scope

The reaction conditions reported by Buchwald et al. in the 2013 paper219 proved to be quite general and, with minor modifications, were applied by the Buchwald group and others to the hydroamination of a wide range of substrates. Buchwald et al. reported excellent enantioselectivity in hydroamination of 1,1-disubstituted alkenes to produce β-chiral amines, which are often found in medically relevant compounds (Scheme 70).221 Detailed exploration of the substrate scope suggests excellent functional group compatibility and tolerance for structurally Scheme 70. Enantioselective Hydroamination of 1,1Disubstituted Alkenes

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Scheme 74. Hydroamination in the Synthesis of γ- and δChiral Amines

Scheme 72. Enantioselective Hydroamination of Styrenes

reaction conditions featuring a DTMB-Segphos-derived catalyst and (EtO)2MeSiH. An important extension of the hydroamination reaction that addresses some of the limitations associated with its application to simple 1,2-disubstituted alkenes was recently reported by Hartwig et al. (Scheme 75).227 They described a regioselective

allowed the development of an enantioselective hydroamination of simple symmetric 1,2-disubstituted alkenes (Scheme 73)225 using the same standard conditions used in other Scheme 73. Enantioselective Hydroamination of Internal Alkenes

Scheme 75. Regioselective Hydroamination of Internal Alkenes

and enantioselective hydroamination of nonsymmetrical 1,2disubstituted alkenes using the standard hydroamination conditions developed by Buchwald.219 Control experiments suggested that the observed regioselectivity was a result of subtle electronic effects exerted by remote electron-withdrawing substituents. Sulfamido, alkoxy, or acyloxy groups in the homoallylic position polarize alkenes sufficiently to afford useful levels of regioselectivity (>4:1), without direct interaction between the catalyst and the directing group. Good yields were obtained using alkenes as a limiting reagent. Excellent levels of enantioselectivity were observed with most substrates, further demonstrating the robustness of the DTMBSegphos catalyst system. However, only N,N-dibenzyl-Ohydroxylamines could successfully be used in this reaction, and a relatively high catalyst loading (10 mol %) was necessary. Considering the utility of this application of the hydroamination reaction, it would be interesting if these limitations could be overcome using the electrophiles shown in Scheme 72 developed by Buchwald et al. Extending their work on hydroamination of internal nonsymmetrical alkenes, Hartwig et al. have recently reported

hydroamination reactions reported by Buchwald et al. Excellent enantioselectivity (>97% ee) was observed with a wide range of alkenes and electrophiles. Remarkably, excellent selectivity was observed even with 2-butene. Furthermore, the diastereoselectivity in reactions with chiral electrophiles was effectively controlled by the catalyst. Moderate levels of regioselectivity were achieved in the hydroamination of nonsymmetrical 1,2disubstituted alkenes, based on the steric properties of the substituents, as shown in Scheme 73. The only major drawback of this transformation is the use of 3 equiv of the alkene substrate, which limits the applications of the reaction to transformations of simple feedstock alkenes. The most recent report from the Buchwald group describes the synthesis of γ- and δ-chiral amines by reductive amination of allylic alcohols and their derivatives (Scheme 74).226 The authors proposed that the initial hydrocupration of the allylic substrate is the enantiodetermining step. Subsequent elimination of the alkoxide leads to the formation of the alkene, which undergoes a standard hydroamination reaction. Excellent scope and selectivity are again achieved using the standard 8355


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transfer hydrogenation of aldehydes using formate as a hydride source.231 Similarly, the silver-catalyzed hydrosilylation of aldehydes has also been reported.232,233 Although this process may well involve a silver hydride intermediate, nonhydridebased mechanisms are also plausible.234 In addition, Mankad et al. propose a silver hydride intermediate in the hydrogenation of alkynes by a cooperative Ag/Ru heterobimetallic catalyst (Scheme 78).235 A transient

a highly regio- and enantioselective hydroboration reaction shown in Scheme 76.228 The authors found that the use of a Scheme 76. Enantioselective Hydroboration of 1,2Disubstituted Alkenes

Scheme 78. Semi-Hydrogenation of Alkynes by Ag/Ru Heterobimetallic Catalyst

strong base (KOt-Bu vs LiOt-Bu) resulted in a higher yield of the desired product, while the use of a nonpolar solvent (cyclohexane vs toluene) significantly increased the rate of the reaction. Excellent enantioselectivities and regioselectivities are observed with a range of substrates. Furthermore, with enantioenriched substrates, the absolute stereochemistry of the newly formed stereocenter was found to be controlled by the catalyst. The presence of a directing group in a homoallylic position was essential for the observed regioselectivity. Several directing groups were effective, including trifluorobenzoyl ester, triflamide, toluenesulfonamide, and pentafluorophenyl ether. The results of the NBO analysis of the ground and the possible transition states suggests that the regioselectivity is a result of a buildup of a negative charge in the transition state at the carbon forming the Cu−C bond. The inductive effect of the directing group stabilizes the transition state, leading to the formation of the observed regioisomer.

silver hydride is proposed to form upon the activation of H2 across the Ag−Ru bond. This system provides unusual Eselectivity to furnish the trans-alkene and remains selective for alkyne reduction in the presence of aldehydes, ketones, alkenes, and nitriles. For each of the Ag-catalyzed processes above, there is no experimental evidence for participation of silver hydrides. Recently, Bonačić-Koutecký, Dugourd, O’Hair and coworkers reported the selective catalytic extrusion of CO2 from formic acid by hydride-bridged disilver cations in the gas phase.236 Tethered phosphine ligands such as bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)benzene (dppbz), and bis(diphenylphosphino)ethane (dppe) are crucial for the selective extrusion of CO2 (Figure 32). Unligated [Ag2(μ-H)]+ and monoligated [(Ph3PAg)Ag(μ-H)]+ react with formic acid to form Lewis adducts, whereas [(Ph3PAg)2(μ-H)]+ is unreactive. In the catalytic pathway, the hydride-bridged disilver cation is initially protonated by formic acid to yield a formate-bridged disilver cation, with concomitant loss of H2 (Figure 32, step i). Collision-induced dissociation (CID) then regenerates the hydride-bridged disilver cation, releasing CO2 (Figure 32, step ii). DFT calculations are consistent with the weakening of Ag− H bonds when the bidentate bridging ligands enforce a nonlinear P−Ag−H angle, facilitating the protonation of the hydride. The CID and theoretical studies inspired solutionphase experiments in which AgBF4 and dppm, in a 2:1 ratio, were heated with 13C-formic acid and sodium formate in CD3CN solution, and the reaction monitored by 1H and 13C NMR spectrometry. These experiments revealed the selective decarboxylation of formic acid to CO2 and H2 at 70 °C.

3.3. Silver Hydrides in Catalysis

Of the coinage metal hydrides, silver hydrides are perhaps the least commonly invoked as reactive intermediates. In 2013, Li et al. reported the catalytic hydrogenation of aldehydes in water229 using a silver salt in combination with the sterically demanding dialkylphosphinobiphenyl ligand XPhos 230 (Scheme 77). The authors propose that H2 is heterolytically activated to form a silver hydride, followed by insertion of aldehyde to form a silver alkoxide. Subsequent heterolysis of H2 would regenerate a silver hydride and release the terminal alcohol. The same group later developed a silver-catalyzed Scheme 77. Silver-Catalyzed Hydrogenation of Aldehydes in Water

3.4. Gold-Hydride-Catalyzed Transformations

Gold(I) and gold(III) complexes can serve as hydrogenation precatalysts, which are thought to give rise to gold hydride intermediates. In 2005, Corma et al. described the gold(I)catalyzed enantioselective hydrogenation of alkenes and imines (Scheme 79).237 8356


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Figure 32. Role of ligand in the selective decarboxylation of formic acid. Adapted with permission from ref 236, under the terms of the Creative Commons Attribution 4.0 International License, https://creativecommons.org/licenses/by/4.0/. Copyright 2016 Nature Publishing Group.

Scheme 79. Gold-Catalyzed Enantioselective Hydrogenation

Scheme 80. Ionic Mechanism Proposed for Gold-Catalyzed Hydrogenation

Scheme 81. (Schiff Base)Au(III)-Catalyzed Alkene Hydrogenation

Computational studies suggest the reaction proceeds through an ionic mechanism in which the gold hydride intermediate is formed upon heterolytic cleavage of H2 by the gold center and a solvent molecule (Scheme 80).238 Corma later reported the hydrogenation of diethyl itaconate by Schiff-base Au(III) complexes, both in solution and on solid support (Scheme 81).239 Although no gold hydride intermediate is observed experimentally, computational findings are consistent with the formation of an Au(III)−H intermediate through an ionic mechanism, analogous to that proposed for Au(I) hydrogenation, in which the gold oxidation state remains unchanged throughout the catalytic cycle. Both the Au(I) and the Au(III) catalysts are highly active, and show rates are comparable to those of similar Pd and Pt complexes. Phosphine-supported, hydride-bridged digold cations (see section 2.3.2 for synthesis and characterization of these species) 8357


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It is apparent that there are still many opportunities to develop of the reactivity of both silver and gold hydrides in catalysis. The development and characterization of lownuclearity hydrides of silver and gold has accelerated in the past decade. As this area expands, the development of further organic transformations mediated by these complexes should follow suit.

are important intermediates in the dehydrogenative silylation of alcohols.139,240 Sawarmura et al. first reported the dehydrogenative silylation of alcohols in good yield using (Xantphos)AuCl as a precatalyst (Scheme 82).241 This system is highly selective Scheme 82. Dehydrogenative Silylation of Alcohols

4. MECHANISTIC STUDIES OF COINAGE METAL HYDRIDES IN CATALYSIS Advances in catalysis by coinage metal hydrides, and in the mechanistic understanding of such catalysis, have been iterative and symbiotic. As described above, the great preponderance of this research has focused on copper hydrides, but silver and gold hydrides have been the subject of renewed and increasing interest in recent years. Section 4.1 describes seven mechanisms of copper-catalyzed reactions that have been studied in detail. The first two are presented in a general sense, as these are classic copper hydride reduction processes that have been thoroughly developed over the past few decades under a multitude of conditions. The others are more recent examples in which the reaction-specific mechanisms were studied in depth. One general trait of copper hydrides is their propensity to form aggregates in solution (as described in sections 2.1.1−2.1.3), and the effect of copper hydride aggregation on reaction mechanisms is little understood. In these catalytic cycles, the active copper hydride species are depicted as monomers, but the monomeric form may well exist as the minor component of an equilibrium mixture with its oligomers. Investigations into the role of silver and gold hydrides in the transformation of organic substrates have provided little direct evidence for their intermediacy, even though hydrides of silver and gold are invoked in many catalytic processes (see sections 3.3 and 3.4). Nonetheless, kinetic evidence suggests the participation of transient silver hydrides in the activation of dihydrogen, and section 4.2 presents a key early study of silvercatalyzed hydrogenation. Experimental evidence supports the intermediacy of gold hydrides in the dehydrogenative silylation of alcohols, as discussed in section 4.3.

for dehydrogenative silylation, tolerating functional groups such as alkenes, alkynes, ketones, aldehydes, alkyl halides, conjugated enones, esters, and carbamates. Further mechanistic studies revealed the intermediacy of a hydride-bridged digold cation.139 Le Floch et al. likewise showed a phosphole-Xantphossupported, hydride-bridged digold cation to be a competent catalyst for this process.240 One application of NHC-supported gold hydrides is in the catalytic hydrodefluorination of perfluoroarenes. Initially, Zhang et al. found that gold(I) complexes supported by Xantphos-type ligands were highly effective precatalysts for hydrodefluorination, whereas (NHC)Au(I) precatalysts displayed no reactivity, but the proposed reactive gold hydride intermediate could not be isolated.242 The addition of 4dimethylaminopyridine (DMAP) to mixtures containing (NHC)gold(I) hydride catalysts, however, resulted in efficient hydrodefluorination reactivity.243 The NMR, UV−vis, and DFT studies suggest a π−π interaction between the perfluoroarene and DMAP, which lowers the activation barrier. Using (IMes)AuH as the catalyst, yields up to 90% could be achieved. This procedure tolerates a wide range of reducible functional groups (Scheme 83).

4.1. Mechanisms of Copper-Hydride-Catalyzed Reactions

The mechanisms of copper-hydride-catalyzed reactions vary extensively, but often share the same general framework and some of the same elementary steps (Scheme 84). For example, the active copper hydride often forms through a σ-bond metathesis mechanism,244 and subsequently adds to unsatu-

Scheme 83. Hydrodefluorination of Perfluoroarenes

Scheme 84. General Sequence for Many [CuH]-Based Catalytic Processes

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rated substrates. This hydrocupration step leads to an organocopper species that undergoes electrophilic functionalization to yield the product. The copper complex formed in the electrophilic functionalization step can sometimes directly engage in σ-bond metathesis to regenerate the copper hydride and turn over the catalytic cycle. More often, this complex has to undergo a ligand exchange in a reaction with an additive (usually metal alkoxide, phenoxide, or fluoride) before the hydride formation can be achieved. As discussed below, the actual reaction mechanisms are often much more subtle and elaborate, but this general reaction scheme applies to most organic transformations catalyzed by copper hydride complexes. 4.1.1. Conjugate Reduction of α,β-Unsaturated Ketones. The proposed mechanism of copper-hydride-catalyzed 1,4-reduction of Michael acceptor substrates is depicted in Scheme 85.6,231,245 First, delivery of a hydride to the β-carbon

Scheme 86. 1,4-Reduction in the Presence of Alcohols

In a study of the conjugate reduction of lactams and lactones, Buchwald and co-workers suggest that the slow σ-bond metathesis of the copper enolate could result from the inability of a C-bound copper enolate to undergo σ-bond metathesis, requiring this step to travel through an O-bound copper enolate (Scheme 87).246

Scheme 85. Proposed Mechanism of the 1,4-Reduction of α,β-Unsaturated Ketones

Scheme 87. Proposed Sequence for Formation of Silyl Ketene Acetals from Acrylate Esters

Interestingly, calculations do not suggest an O-bound copper enolate intermediate, but rather one that features two Cu−C bonds of 2.031 and 2.025 Å, to the carbonyl carbon and αcarbon, respectively.231 O-bound copper enolates have been isolated as the 1,4-addition products following the insertion into Cu−B and Cu−Si bonds, but O- and C-bound enolates are suggested to have only slight energetic differences.248−250 Computation investigations also suggest a six-centered transition state for the σ-bond metathesis step, not the commonly invoked four-centered transition state.231 Furthermore, the calculated rate-limiting step is delivery of the hydride to the β-carbon, rather than regeneration of the copper hydride. 4.1.2. Hydrosilylation of Ketones. Similarly, the proposed mechanism for the hydrosilylation of ketones, for both NHC- and phosphine-supported copper hydrides, proceeds through two key steps, 1,2-addition of copper hydride to the ketone, followed by σ-bond metathesis of the newly formed copper alkoxide (Scheme 88). Enantioselective ketone hydrosilylation processes using bidentate chiral phosphines are well developed and well studied (see section 3.1.1). Mechanistic studies are consistent with the first step being the enantio-determining step (EDS), proceeding through a four-centered transition state with a relatively low computed barrier.251 To assess whether the copper alkoxide exists in equilibrium with the starting copper hydride plus ketone, an enantiomerically pure copper alkoxide was prepared by reaction of racemic BINAP with CuCl and sodium (R)-1-phenylethanolate (99% ee).251 After 3 h, diphenylsilane was added. Subsequent hydrolysis of the resulting silyl ether produced phenylethanol quantitatively, with complete retention of stereochemistry, suggesting that βhydride elimination from the secondary alkoxide, the reverse of 1,2-insertion, does not occur. Theoretical investigations suggest that the σ-bond metathesis, which also proceeds through a four-

forms a copper enolate. Next, σ-bond metathesis with a stoichiometric hydride source, such as a silane or H2, regenerates the active copper hydride catalyst and releases an enol. Subsequent tautomerization of the enol, independent of the metal, affords the ketone product. The first step is suggested to involve an intermediate in which the copper and the CC bond form a π-complex.6 In contrast, coordination of the carbonyl oxygen to the copper center should result in the corresponding 1,2-reduction.231 Calculations reveal that a copper hydride π-complex is preferred over the O-bound isomer, a key factor in determining the regioselectivity.231 The σ-bond metathesis step should then pass through a four-centered transition state, regenerating the copper hydride and furnishing the product tautomer. The addition of bulky alcohols showed increased reaction rates: Alcoholysis of the copper enolate is thought to form the product tautomer and a copper alkoxide, which in turn reacts with the hydride source to regenerate the active copper hydride. The facile alcoholysis of the copper enolate and subsequent σbond metathesis of the copper alkoxide are believed to be much faster than the corresponding σ-bond metathesis of the copper enolate (Scheme 86).6,145,246 In support of this mechanism, labeling studies using t-BuOD reveal deuterium incorporation mainly at the α-position.247 8359


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alkynes.166,193 The proposed three-step mechanism begins with the generation of an NHC-supported copper hydride from a (NHC)copper(I) alkoxide and a silane. Hydrocupration results in a (NHC)copper(I) vinyl that is then protonolyzed by an alcohol, resulting in the regeneration of (NHC)copper(I) alkoxide and concomitant release of the Z-alkene (Scheme 90).

Scheme 88. Proposed Mechanism for the Hydrosilylation of Ketones

Scheme 90. Proposed Mechanism for the Semireduction of Alkynes

centered transition state, involves the highest energetic barrier along the reaction coordinate and is thus the rate-determining step (RDS). KIE experiments were also consistent with ratedetermining σ-bond metathesis. Surprisingly, Lipshutz et al.,210 corroborated by others,190,234 observed that propiophenone does not react with an excess of Stryker’s reagent and 20 mol % BIPHEP after 4 days in the absence of silane (Scheme 89), inconsistent with the Initial investigations into the semireduction of terminal alkynes afforded the desired product in only 19% yield. Three competing reactions were identified and systematically studied, leading to the development of optimal reaction conditions for the semireduction of terminal alkynes (Scheme 91). First,

Scheme 89. Influence of Silane on the Hydrosilylation of Propiophenone

Scheme 91. Competing Reactions during the Semireduction of Terminal Alkynes

mechanism proposed in Scheme 84. Upon addition of PMHS, clean reduction occurs; moreover, if enone is added to the mixture of Stryker’s reagent, BIPHEP, and propiophenone, complete conversion to the 1,4-adduct is observed. These results suggested the silane is essential to the insertion step, and resulted in the proposal of silyl-containing active species (Figure 33). DFT calculations by Bellemin-Laponnaz et al., (ICy)Cu(O-t-Bu) (ICy = 1,3-dicyclohexylimidazol-2-ylidene) efficiently catalyzes the formation of dihydrogen and silyl ether, presumably via protonolysis of copper hydride by isopropanol. Replacement of ICy with a more sterically encumbering NHC, IDipp, led to the suppression of this undesired reaction. Deprotonation of terminal alkyne by (NHC)copper(I) alkoxide, forming an inactive (NHC)copper acetylide, was overcome through the use of a more reactive silane source, PMHS, increasing the rate of hydride formation. In addition, deprotonation of terminal alkyne by the (NHC)copper(I) vinyl can occur, but use of the more acidic i-BuOH in lieu of t-BuOH increases the rate of alcoholysis, thus suppressing acetylide formation. Stoichiometric reactions previously described by Sadighi52 and Tsuji164 support the first two steps of the proposed

Figure 33. Proposed silyl-containing intermediates for the hydrosilylation of ketones.

however, suggest that these species are not viable intermediates, and along with kinetic, kinetic isotope effect, and isotopic labeling studies argue for the originally proposed mechanism.251 They go on to suggest that the lack of reactivity in the absence of silane may arise from the unfavorable equilibrium between monomeric and dimeric copper hydride species. 4.1.3. (NHC)CuH-Catalyzed Semireduction of Alkynes. A detailed mechanistic understanding was crucial to the development and generalization of the semireduction of 8360


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catalytic cycle. The hydrocupration step must occur in syn fashion, making this the stereoselective step; labeling studies using t-BuOD resulted in a single deuterium-labeled isomer. Furthermore, both (IDipp)CuH and (IDipp)copper(I) vinyl proved competent precatalysts for semireduction. Overreduction of the alkene to alkane was not observed, consistent with the lack of reactivity when terminal alkene was subjected to the reaction conditions for a prolonged period. Monitoring by 1H NMR revealed the catalytic resting state to be the copper vinyl species, suggesting rate-limiting protonolysis, consistent with observed rate increases when more acidic alcohols were used. These refined conditions for the semireduction of internal alkynes still resulted in poor yields of Z-alkene, with complete consumption of silane. The inference that the protonolysis of copper hydride is faster than the hydrocupration step was confirmed by a competition experiment. The combination of stoichiometric amounts of internal alkyne, isopropanol, and copper hydride resulted in the facile formation of dihydrogen. Use of the less acidic t-BuOH resulted in the increased formation of desired alkene. As monitored by 1H NMR spectroscopy, the catalyst resting state during the semireduction of internal alkynes is the copper hydride, rather than the (NHC)copper(I) vinyl as observed during the semireduction of terminal alkynes. 4.1.4. Hydrocarboxylation of Alkynes. Each step of the proposed mechanism for the hydrocarboxylation of alkynes (Scheme 92) can be achieved stoichiometrically, allowing the

the silacarboxylation and the hydrosilylation of alkynes, were conducted to help elucidate the selectivity.252,253 Scheme 93. Possible Side-Reactions in the Hydrocarboxylation of Alkynes

The silacarboxylation process, involving the insertion of alkyne into a copper formate, is both thermodynamically and kinetically unfavorable. In contrast, the hydrosilylation of alkynes is exoergic, but kinetically incompetent due to the barrier required for the σ-bond metathesis of (NHC)copper(I) vinyl and silane. Interestingly, the thermodynamically favorable hydrosilylation of CO2 possesses a lower barrier than the hydrocarboxylation of alkynes, despite the observed selectivity of the Tsuji system for hydrocarboxylation. Lin et al. suggest the homogeneity of the alkyne, versus the heterogeneity of CO2 under the reaction conditions, as a key factor in the selectivity.252 The system is not only operated at low CO2 pressure, using a balloon, but at elevated temperature. The resulting low concentration of CO2 favors the hydrocarboxylation pathway by allowing the alkyne to compete effectively with CO2 for insertion into the copper hydride. 4.1.5. Hydroalkylation of Alkynes. The originally proposed mechanism of the hydroalkyation of alkynes consisted of four key steps (Scheme 94). Starting with an (NHC)copper(I) triflate complex, formation of an (NHC)copper(I) fluoride allows the subsequent generation of copper hydride.58,69 Hydrocupration of a terminal alkyne yields a copper vinyl, which is alkylated with an electrophilic alkyl

Scheme 92. Proposed Mechanism for the Hydrocarboxylation of Alkynes

Scheme 94. Originally Proposed Mechanism for the Hydroalkylation of Alkynes

isolation of each expected intermediate.213 As in the semireduction of alkynes, the first step involves the 1,2-insertion of alkyne into an NHC-supported copper hydride. Insertion of CO2 into the resulting copper vinyl forms an (NHC)copper(I) carboxylate, which undergoes σ-bond metathesis with a silane to regenerate the copper hydride and form a silyl acrylate product. The selectivity for hydrocarboxylation of alkynes over the hydrosilylation of CO2 poses some interesting mechanistic questions. Subsequent to this work, the hydrosilylation of CO2 by (IDipp)CuH and HSi(OEt)3 yielded a TON of 7489 and TOF of 1248 (h−1) at low CO2 pressure (1 atm) under solventfree conditions.178 Theoretical investigations into this competing pathway, and into other possibilities (Scheme 93) such as 8361


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although the reaction proceeds to form the cross-coupling product in good yield upon addition of CsF, silane, and an alkyne. Importantly, if this additional alkyne differs from the alkyne inserted to form the bridging vinyl, the alkylated product results from functionalization of the bridging vinyl, not the additional alkyne. The slow phase transfer of fluoride, the ratelimiting step, is crucial to suppressing the pathway of alkyl triflate reduction. Indeed, the use of fluoride plus a phase transfer catalyst, in place of CsF, affords both alkane and alkyl fluoride, arising respectively from the reduction and fluorination of alkyl triflate. 4.1.6. 1,3-Halogen Migration/Borylation. In 2012, Schomaker and co-workers reported a remarkable halogen migration/borylation reaction, in which a 2-bromostyrene reacts with pinacolborane to form an arylboronate bearing a secondary benzylic bromide. The proposed mechanistic sequence of this reaction is the most complex of those presented here (Scheme 97).202,203 An initial pre-equilibrium

triflate, resulting in the formation of a new C−C bond and the regeneration of (NHC)CuOTf. This mechanism is consistent with stoichiometric reactivity, but the selectivity for hydroalkylation over the reduction of the alkyl triflate prompted the authors to investigate the mechanism in depth. Reaction of terminal alkyne, alkyl triflate, and (IDipp)CuH revealed no insertion of the alkyne but rather 0.5 equiv of alkane, derived from alkyl triflate reduction, and complete consumption of the copper hydride (Scheme 95). Scheme 95. Competition Experiments of Neutral and Cationic Copper Hydrides

Scheme 97. Proposed Mechanism of 1,3-Halogen Migration

The authors established the formation of hydride-bridged dicopper cation, accounting for the complete consumption of neutral copper hydride. The hydride-bridged dicopper cation selectively inserts alkyne to form a vinyl-bridged dicopper cation, rather than reducing the alkyl triflate. A new mechanism based on the intermediacy of bridged dicopper cations was proposed (Scheme 96).69 Scheme 96. New Reaction Mechanism for the Hydroalkylation of Alkynes

step of π-olefin complex formation between 2-bromostyrene and the copper hydride is followed by 1,2-Markovnikov insertion, producing a benzylcopper intermediate. The insertion step is rate-limiting, although the rate is independent of styrene concentration. This observation is rationalized by the pre-equilibrium: The concentration of the π-olefin intermediate depends solely on the concentration of copper hydride. After a formal 1,3-rearrangement, the benzylcopper species yields an arylcopper intermediate bearing a benzylic bromide. Stereochemistry is not maintained through the 1,3-rearrangement, indicating the existence of a stereoablative step. Alternatively, the benzylcopper can undergo σ-bond metathesis with pinacolborane to give the benzylic borylated product, consistent with work by Yun et al.201 The use of electronrich styrenes in combination with sterically encumbering ligands favors 1,3-halogen migration, whereas electron-deficient styrenes and smaller ligands give rise to the Markovnikov hydroboration product.203 Calculations suggest that as the steric bulk of the ligand is increased, the barrier of the σ-bond metathesis step that leads to the hydroboration product also increases, consistent with the observed selectivity. Following

The first step, the formation of (NHC)CuF, is also ratelimiting due to the sluggish phase transfer of fluoride from solid CsF. As a result,(NHC)CuF is generated in the presence of excess (NHC)CuOTf, resulting in the facile formation of a fluoride-bridged dicopper cation.67 The bridging fluoride reacts readily with silane to form the bridging hydride, which then inserts alkyne. The bridging fluoride can deliver fluoride to alkyl triflate, but its reaction with silane is much faster. The specific mechanism of product formation remains ambiguous, as direct reaction of the bridging vinyl with alkyl triflate is not observed, 8362


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the formal 1,3-migration, σ-bond metathesis between the arylcopper and pinacolborane results in the regeneration of copper hydride and formation of the 1,3-halogen migration/ borylation product. The authors hoped to elucidate the nature of the formal 1,3rearrangement step using computational methods.203 The originally proposed oxidative addition/reductive elimination pathway involved a high barrier (50 kcal/mol) to form a Cu(III) intermediate, and did not produce a stereoablative step. Likewise, radical pathways involving a Cu(II) aryl or benzylic radicals appeared unlikely due to their high energetic barriers. Rather, calculations suggest a dearomatization step, in which the ligated copper migrates to the bromine-bearing aryl carbon (Scheme 98). This is followed by a 1,2-bromide shift, restoring

copper hydride into styrene. Interception of the resulting benzylcopper intermediate by an electrophilic hydroxylamine derivative produces a new C−N bond and a copper benzoate. Transmetalation then regenerates the active copper hydride, completing the catalytic cycle. Hammett studies of para-substituted styrenes show a linear correlation with observed enantioselectivities, consistent with hydrocupration being the enantio-determining step, and labeling studies confirm this step to be irreversible. Kinetic studies reveal a zero-order rate dependence on styrene and amine concentration, a first-order dependence on silane concentration, and a fractional-order dependence on copper concentration, suggesting that the rate-limiting step is regeneration of copper hydride. The uncommon fractional order in catalyst was also observed by Bellemin-Laponnaz et al., who explain it in terms of the small fraction of active monomeric copper hydride in equilibrium with the favored dimeric form.251 Hammett studies using styrenes with electronic variations in the para position reveal similar rates, whereas there is a linear correlation for electronically varied para-substituted benzoates. The data suggest that more electron-donating benzoates react more rapidly, and are consistent with catalyst regeneration being rate-limiting. With this mechanistic insight, the authors screened silane sources and electrophilic amines. Using HSi(OMe)2Me and a hydroxylamine-O-pivalate electrophile, at a precatalyst loading of 2 mol % Cu(OAc)2, they achieved the hydroamination of styrene in 10 min, in a vessel open to air, to obtain the product in 91% yield and 95% ee. Taken together, these seven reaction mechanisms offer a survey, not comprehensive but representative, of the chemistry of copper hydrides. Although common features are evident across the series, generalizations are often difficult. For example, the addition of copper hydrides across an unsaturated bond is most often the enantioselective step, but in the case of 1,3halogen migrations the presence of a stereoablative step requires the enantioselectivity to be determined after the hydrocupration. Furthermore, the rate-limiting step changes from mechanism to mechanism, but can also change in the same system depending on the substrates used, as in the semireduction of alkynes. Consequently, the mechanistic understanding gained through these studies has furthered the development of copper hydride catalysis. The stoichiometric reactivity and isolation of proposed (NHC)copper(I) intermediates has been central to understanding the reaction mechanisms. The analogous steps and intermediates are frequently proposed in phosphine supported copper hydride catalysis, but the direct evidence of these elementary steps is currently unavailable, thus underlining the importance of further mechanistic studies.

Scheme 98. Computed Mechanism for the Formal 1,3Bromide Shift

aromaticity and poising the bromide to be delivered to the benzylic position. Furthermore, optimization of the transition states that lead to each enantiomer indicates a slightly higher barrier for formation of the S-product, consistent with the observed selectivity for the R-product. 4.1.7. Asymmetric Hydroamination of Styrene. The asymmetric hydroamination of olefins is proposed to consist of a three-step cycle (Scheme 99).219,254 A detailed mechanistic study by Buchwald and co-workers led to a greatly improved protocol, using (S)-DTBM-SEGPHOS as the supporting ligand.254 The reaction proceeds by Markovnikov insertion of Scheme 99. Proposed Mechanism for the Hydroamination of Styrene

4.2. Silver-Catalyzed Dihydrogen Activation

Early mechanistic studies conducted by Halpern et al.,255−259 and by others,260,261 on the homogeneous catalytic reduction of dichromate by Ag(I) salts and H2 revealed the intermediacy of reactive silver hydrides. The first step in this process is the activation of H2 by Ag(I) ions to form reactive silver hydrides. Aqueous Ag(I) ions activate H2 homolytically at low temperatures and high Ag concentrations, with a second-order rate dependence on silver concentration.89 At high temperature and low Ag concentrations, kinetics exhibit first-order dependence on silver concentration, consistent with heterolytic activation of dihydrogen.256 (Scheme 100). Basic ligands, such as acetate, 8363


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resting state, regardless of the starting catalyst.240 Interestingly, this observation seems inconsistent with the results of DFT calculations, which suggest the initial transmetalation step to be rate-limiting, but experimentally the counterion (X) exerts a large influence on the reaction rate. Of the weakly coordinating counterions studied, OTf− gave the best results, while PF6− and NTf2− proved somewhat more efficient than BF4−. The hydride-bridged digold complex supported by diaryl-Xantphos ligands (Ar = phenyl or 3,5-xylyl), with chloride as the counterion, was characterized by 31P and 1H NMR as well as ESI−MS but could not be structurally characterized. Therefore, the chloride counterion may be outer-sphere, or it could bridge the two gold centers. An alternative mechanism was envisioned where the hydride-bridged digold complex is protonolyzed by an alcohol, forming a gold alkoxide and releasing H2, before transmetalation with a silane regenerates the gold hydride and furnishes the silylether product (Scheme 102).

Scheme 100. Hydrogenation of Dichromate Catalyzed by Ag(I)

ethylenediamine, and fluoride, which can serve as proton acceptors, increase the rate of dihydrogen activation, enabling the heterolytic mechanism to predominate.258,260,261 Throughout the course of these reactions, the silver concentrations do not change, and there is no evidence of metallic silver until the complete reduction of Cr2O72− at which time the Ag(I) is reduced to Ag metal in the presence of H2.89

Scheme 102. Alternative Mechanism for Gold-Catalyzed Dehydrogenative Silylation

4.3. Gold Hydrides in Catalytic Alcohol Silylation

Gold catalysts achieve the dehydrogenative silylation of alcohols.241 Mechanistic studies by Ito et al.139 and later Le Floch et al.,240 both using Xantphos derived ligands, are consistent with gold(I) hydride intermediates. The proposed mechanism begins with a transmetalation between a silane and a gold(I) precursor [(Xantphos)Au]+[X]− to form a transient terminal (Xantphos)gold(I) hydride, which in the presence of more (Xantphos)gold(I) forms a hydride-bridged digold complex (Scheme 101; see also section 2.3.2 for the Scheme 101. Proposed Mechanism of Gold-Catalyzed Dehydrogenative Silylation This mechanism proved inconsistent with experiment: The hydride-bridged digold complex does not react with alcohols, which proves important to tolerating a wide range of functional groups such as alkenes, alkynes, ketones, aldehydes, alkyl halides, conjugated enones, esters, and carbamates.139 Furthermore, addition of bases such as 2,6-lutidine does not impact the formation of gold hydride but dramatically decreases the yield of hydrogenative silylation, presumably by the trapping of acid, inhibiting the protonolysis of gold hydride. Further mechanistic investigations into the function of silver and gold hydrides in catalysis are necessary, to better understand and develop their reactivity. As with copper hydride catalysts supported by phosphine ligands, the isolation, characterization, and stoichiometric reactivity of proposed intermediates provide an opportunity for future studies.

5. CONCLUSIONS This Review began with the earliest findings in the chemistry of copper hydride itself, and went on to present the first copper hydride complexes. These include solutions of Lewis base donors with copper hydride, generated in situ, as well as the isolable and well-defined [(Ph3P)CuH]6. Such long-known systems are discussed here to put in perspective the rapid and accelerating developments in coinage metal hydride chemistry since roughly 2008. Advances in organic synthesis using the classic copper hydride systems, and variations on them, continue to appear rapidly. Questions considered settled, like the hydride binding mode in [(Ph3P)CuH]6, have been

characterization of such species). The transmetalation step also yields a new silyl(X) species that reacts with the alcohol, furnishing the silyl ether product and an acid (HX). Protonolysis of the hydride-bridged digold complex by the acid regenerates [(Xantphos)Au]+[X]− with concomitant loss of H2. Monitoring the reaction by 31P NMR, Le Floch et al. revealed the hydride-bridged digold cation to be the catalyst 8364


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implications of mechanistic studies have spurred efforts to identify and synthesize key intermediates. This interplay has led to exciting and unexpected advances in the field, and will likely continue to do so.

reopened, and the electrochemistry of such clusters has only recently been explored. New copper hydride architectures have raised fascinating questions in structure and bonding, and led to surprising new directions in reactivity. Likewise, more recently isolated silver and gold hydrides present interesting parallels and contrasts to the structures of copper hydrides, and display distinctive reactivity. Among all three metals, terminal hydrides have only been isolated, or observed in solution at ordinary temperatures, in the case of gold. Indeed, the first molecular hydride of gold simply happened to be monomeric. In contrast, although many copper hydride reactions are believed to proceed through mononuclear intermediates, the copper hydrides characterized under ordinary conditions are either neutral oligomers or hydride-bridged cations, and in recent years these have come to include nanoclusters of increasing size. The silver hydrides isolated to date are hydride-bridged cations: Neutral hydrides are probably viable intermediates in silver hydride chemistry, but appear to be unstable with respect to elemental silver, hydrogen, and free ligand. Another unique aspect of gold in this series is its ability to form a stable, open-shell hydride,262 a terminal, neutral gold(III) hydride supported by a strongly donating pincer ligand. The gold hydrides characterized to date are rather stable, often requiring activated organic substrates such as electronpoor alkynes or diazo compounds for reaction. Nonetheless, several examples were identified in the mechanistic study of the catalytic dehydrogenative silylation of alcohols, and gold complexes can serve as precatalysts for the hydrogenation of electron-poor alkenes. Despite being among the first metals to catalyze hydrogen activation in solution, silver has been little explored synthetically for this purpose. Recent advances show that suitable silver complexes can be effective catalysts under conditions incompatible with existing copper systems, with substrates toward which gold hydrides are unreactive. Future developments will no doubt expand the subfield of silver and gold hydride catalysis, and establish classes of reactions to which they are particularly well suited. Still, copper continues to dominate the catalytic chemistry of the coinage metal hydrides. This dominance results in part from its relative abundance, and accordingly much lower cost, but also from the great synthetic versatility of copper hydrides. The systems studied to date are compatible with a much wider range of supporting ligands, enabling a wider-ranging screen for effective metal/ligand combinations. For a long time, copper hydrides were best known as reagents or as catalytic intermediates in selective reduction chemistry. This area, which continues as a very active area of copper catalysis, includes hydrosilylation, hydrogenation, and more recently hydroborylation. Key new directions include hydrofunctionalization reactions, and tandem hydroborylation/ halogen migration reactions. Moreover, although the reactions remain stoichiometric and separate for now, the demonstration of sunlight-induced hydrogen elimination from a copper hydride nanocluster, and of carbon dioxide hydrogenation by suitable copper precatalysts, suggests potential future developments in energy storage. A striking feature of coinage metal hydride chemistry, particularly within the past decade, has been the interplay among synthetic advances, catalyst development, and mechanistic study. In several cases, new structural motifs have been identified as intermediates in previously known reactions, enabling the rational design of further advances. The

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Gojko Lalic grew up in Belgrade, Serbia, where he completed his B.Sc. in chemistry. After he obtained a Ph.D. from Harvard University under the mentorship of Prof. Matthew Shair, he completed a postdoctoral fellowship with Prof. Robert Bergman at UC Berkeley. After another postdoctoral fellowship with Prof. E. J. Corey, he joined the faculty at the University of Washington as an assistant professor in fall 2008. Abraham J. Jordan grew up in central New York, obtaining his Bachelors of Science from Virginia Military Institute (VMI) in 2014. At VMI he conducted undergraduate research with Prof. Daren J. Timmons, working on the design and synthesis of flavone-based liquid crystals. He also spent a summer working in the lab of Prof. Daniel A. Clark at Syracuse University studying new methodologies to access substituted imidazo[4,5-b]pyridines. Currently, he is a Ph.D. student in the group of Joseph P. Sadighi, studying the synthesis and reactivity of new (NHC)copper(I) hydrides. His research interests include areas of organometallic chemistry related to catalysis, small molecule activation, fuels, etc. Joseph P. Sadighi was born in Pittsfield, MA, and received his B.A. in Chemistry from Williams College in 1994. He carried out his doctoral studies under the guidance of Prof. Stephen L. Buchwald at the Massachusetts Institute of Technology. After receiving his Ph.D. degree in 1999, he moved to the California Institute of Technology as an NIH postdoctoral scholar under the guidance of Prof. John E. Bercaw and Dr. Jay A. Labinger. In 2001 he joined the Chemistry faculty at MIT as an assistant professor. From 2007−2011 he served on active duty in the U.S. Army, and in 2011 he joined the School of Chemistry & Biochemistry at the Georgia Institute of Technology, where he is currently an associate professor. His research interests include the synthesis of late transition metal complexes with unusual bonding motifs to enable new catalysis.

ACKNOWLEDGMENTS We thank the U.S. National Science Foundation for financial support (NSF CAREER award no. 1254636 to G.L.; award CHE-1300659 to J.P.S.). Professor C. A. Tsipis (Aristotle University of Thessaloniki) graciously provided the image of cyclic hydrocoppers used in Figure 2. Dr. John Bacsa (Emory University, X-ray Crystallography Center) kindly provided the images in Figures 5 and 6, prepared from published crystallographic information files. REFERENCES (1) El-Hamdi, M.; Solà, M.; Frenking, G.; Poater, J. Comparison between Alkalimetal and Group 11 Transition Metal Halide and Hydride Tetramers: Molecular Structure and Bonding. J. Phys. Chem. A 2013, 117, 8026−8034. 8365


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NOTE ADDED IN PROOF After this Review was accepted, a report appeared describing a mixed-valence dicopper(I,II) hydride and its unusual small molecule reactivity (see reference 262).

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Coinage Metal Hydrides: Synthesis, Characterization, and Reactivity

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