Main Group Lewis Acid-Mediated Transformations of Transition-Metal

Review pubs.acs.org/CR

Main Group Lewis Acid-Mediated Transformations of TransitionMetal Hydride Complexes Ayan Maity and Thomas S. Teets* Department of Chemistry, University of Houston, Lamar Fleming Jr. Building, 3585 Cullen Boulevard, Room 112, Houston, Texas 77204-5003, United States ABSTRACT: This Review highlights stoichiometric reactions and elementary steps of catalytic reactions involving cooperative participation of transition-metal hydrides and main group Lewis acids. Included are reactions where the transition-metal hydride acts as a reactant as well as transformations that form the metal hydride as a product. This Review is divided by reaction type, illustrating the diverse roles that Lewis acids can play in mediating transformations involving transition-metal hydrides as either reactants or products. We begin with a discussion of reactions where metal hydrides form direct adducts with Lewis acids, elaborating the structure and dynamics of the products of these reactions. The bulk of this Review focuses on reactions where the transition metal and Lewis acid act in cooperation, and includes sections on carbonyl reduction, H2 activation, and hydride elimination reactions, all of which can be promoted by Lewis acids. Also included is a section on Lewis acid−base secondary coordination sphere interactions, which can influence the reactivity of hydrides. Work from the past 50 years is included, but the majority of this Review focuses on research from the past decade, with the intent of showcasing the rapid emergence of this field and the potential for further development into the future.

CONTENTS 1. Introduction and Scope of This Review 2. Adducts of Transition-Metal Hydrides and Lewis Acids 2.1. Background 2.1.1. Titanium 2.1.2. Niobium 2.1.3. Tantalum 2.1.4. Molybdenum 2.1.5. Tungsten 2.1.6. Rhenium 2.1.7. Iron 2.1.8. Ruthenium 2.1.9. Osmium 2.1.10. Rhodium 2.1.11. Iridium 2.1.12. Nickel 2.1.13. Platinum 2.2. Summary and Outlook 3. Lewis Acid-Assisted Carbonyl Reduction 3.1. Background 3.2. Reduction of Organic Carbonyl Compounds 3.2.1. Ketone Hydrogenation 3.2.2. Hydroboration of Aldehydes and Ketones 3.3. Carbon Monoxide Reduction 3.3.1. Intermolecular Interactions of Metal Formyls and Lewis Acids 3.3.2. Carbon Monoxide Reduction Facilitated by Pendant Lewis Acids 3.3.3. Carbonyl Reduction with Appended Hydrogen-Bonding Groups © 2016 American Chemical Society

3.4. Carbon Dioxide Reduction 3.4.1. CO2 Hydroboration 3.4.2. Lewis Acid-Assisted CO2 Hydrogenation 3.5. Summary and Outlook 4. Lewis Acid-Assisted H2 Activation 4.1. Background 4.2. Intramolecular H2 Activation by Transition Metal/Lewis Acid Pairs 4.2.1. H2 Cleavage by Metalloborane Complexes 4.2.2. H2 Cleavage by Metalloboryl Complexes 4.2.3. H2 Cleavage Facilitated by Non-Boron Lewis Acids 4.3. Intermolecular H2 Activation by Transition Metal/Lewis Acid Pairs 4.3.1. H2 Cleavage by a Platinum(0)/Borane Lewis Pair 4.3.2. H2 Cleavage by a Ruthenium/Imidazolium Lewis Pair 4.4. Summary and Outlook 5. Transformations of Metal Hydrides Promoted by Lewis Acid Interactions with Supporting Ligands 5.1. Background 5.2. Homogeneous Catalysts 5.2.1. Nickel-Based Olefin Polymerization Catalysts 5.2.2. Rhenium Hydrogenation Catalysts Activated by Nitrosyl−Borane Interactions

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Special Issue: Metal Hydrides

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Received: January 18, 2016 Published: May 10, 2016 8873

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DOI: 10.1021/acs.chemrev.6b00034 Chem. Rev. 2016, 116, 8873−8911

Chemical Reviews 5.3. Stoichiometric Reactions 5.3.1. Borane-Protected Hydrogenase Models 5.3.2. Crown Ether-Functionalized Iridium Hydride Pincer Complexes 5.4. Summary and Outlook 6. Lewis Acid-Assisted Hydrogen Elimination Reactions 6.1. Background 6.2. Lewis Acid-Promoted α-Hydrogen Elimination 6.2.1. AlMe3-Promoted α-Hydrogen Elimination from Alkylidene Complexes 6.2.2. Borane-Induced α-Elimination from a Cyclopentadienyl Ligand 6.3. β-Hydrogen Elimination 6.3.1. β-Elimination from Pentamethylcyclopentadienyl Ligands 6.3.2. β-Elimination Reactions of Nickelalactones 6.4. Summary and Outlook 7. Other Reactions of Metal Hydrides Mediated by Lewis Acids 7.1. Background 7.2. Hydride Migration 7.2.1. Borane-Mediated Hydride Migration in Disilazido Zirconium Compounds 7.3. C−H Activation and Elimination 7.3.1. Deprotonation of Iridium Hydrides by Lewis Acidic Alkyls 7.3.2. Intramolecular C−H Activation Initiated by Triphenylborane 7.3.3. Intermolecular C−H Activation by a Nickel Hydride/Lewis Acid Adduct 8. Conclusions Author Information Corresponding Author Notes Biographies Acknowledgments References

Review

involving a transition-metal hydride complex as either a reactant or a product. We include discussions of both stoichiometric and catalytic transformations involving transition-metal hydrides, where one or more key steps involve both the hydride and a Lewis acid acting in concert. Hydride/proton transfer reactions involving exclusively main group reagents, which are especially numerous as the concept of frustrated Lewis pairs gains prominence in small-molecule-activation and catalysis, are beyond the scope of this Review and will not be considered here.26−33 This Review is organized by the reaction type, where each section highlights a distinct role that Lewis acids can play in mediating reactions of metal hydrides. We start in section 2 with the simplest reaction between a metal hydride and a Lewis acid, a Lewis acid−base interaction where the hydride coordinates the Lewis acid in some fashion. Although much of the work in this area focuses on the isolation and characterization of a variety of structure types that result from such interactions, we also highlight some of the effects these interactions can have on the structural dynamics and reactivity of the adducts. In section 3, Lewis acid-assisted carbonyl reduction is reviewed, where most often the Lewis acid engages in an interaction with the carbonyl oxygen, facilitating hydride transfer to the carbonyl. This section includes discussions of organic carbonyl, carbon monoxide, and carbon dioxide reduction reactions. In section 4 we describe reactions where Lewis acids and transition metals react cooperatively to heterolytically split H2. In these types of reactions, metal hydrides are formed as products, with a formal proton transfer from H2 to the metal center occurring; the hydride equivalent migrates to the Lewis acid. Section 5 describes several examples where Lewis acid−base interactions on the periphery of a metal complex influence the reactivity of a metal hydride. In this case, there is no direct interaction between the hydride and the Lewis acid or between the substrate and the Lewis acid, but instead a remote interaction that either results in an open coordination site or alters the electronic structure of the hydride to promote reactivity. In section 6, we describe hydrogen elimination reactions influenced by Lewis acids; these include α- and β-eliminations from hydrocarbyl ligands and β-eliminations from nickelalactones, and again involve formation of metal hydrides as a product. Finally, in section 7, we highlight a few other relevant research efforts that fit the topic of this Review but are not satisfactorily categorized in any of the above sections. Each section of this Review provides relevant introductory information and references, so we refrain from a more detailed background discussion here.

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1. INTRODUCTION AND SCOPE OF THIS REVIEW Transition-metal hydride complexes have been known for almost 100 years, with many review articles available highlighting different aspects of their preparation, structure, and reactivity.1−23 Many catalytic transformations include transition-metal hydrides as possible intermediates; these include but are not limited to (de)hydrogenation catalysis, electrocatalytic proton reduction, carbon dioxide/monoxide reduction, aerobic oxidation catalysis, polymerization, and hydroboration. New applications of transition-metal hydride complexes continue to be uncovered, and they are especially important players in a number of stoichiometric and catalytic transformations relevant to renewable fuels production, solar energy storage, and efficient utilization of carbon-based resources.24,25 To define a reasonable scope for this Review, we are primarily restricting our coverage to reactions involving transition-metal hydrides in concert with main group Lewis acids. Although we do not intentionally limit the scope of the Lewis acids surveyed, the nature of the field dictates that a majority of the examples involve boron Lewis acids. Most of the reactions can be classified as either hydride or proton migration reactions,

2. ADDUCTS OF TRANSITION-METAL HYDRIDES AND LEWIS ACIDS 2.1. Background

In this first section, we describe what could be thought of as the simplest transformation involving metal hydrides and Lewis acids, one where a direct Lewis acid−base interaction occurs between the hydride and the Lewis acid. Related adducts between transition-metal hydrides and Brønsted acids have also been characterized, and while they are not considered here there are some notable reviews on the topic.22,34,35 Often the products that form from interactions between hydrides and Lewis acids are stable, isolable species with no notable reactivity, although by virtue of their stability they permit 8874


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characterization of the variety of structures that can be formed across many transition metals. Nevertheless, as will be emphasized at various points throughout this Review, some of these adducts appear as catalytically active intermediates (section 4), and others are observed as off-cycle species in catalytic reactions (section 3.4.1). These metal hydride-Lewis acid adducts most often involve borane Lewis acids, and out of convenience are frequently prepared by metathesis with borohydride reagents. In an effort to appropriately limit the scope and keep with the theme of this Review, this section only focuses on examples where the adducts were prepared by reaction of a transition-metal hydride with a main group Lewis acid; reactions involving borohydride reagents and nonhydride precursors are not considered, and have been reviewed previously.36 In addition, noteworthy examples of reactions between transition-metal hydrides and coinage metal Lewis acids are not reviewed here.37−44 Borane Lewis acids coordinate to early and late transition-metal polyhydrides to form several structural types (Figure 1), all of which will be considered in

Scheme 1

1973 by Tebbe.47 Niobocene hydrides with various spectator ligands (L) were allowed to react with a variety of main group (AlEt3, GaEt3) and transition-metal-based (ZnEt2, CdEt2, and Hf(CH2Ph)4) Lewis acids. Complexes generated as shown in Scheme 2 all showed a 1H hydride chemical shift that is 5−7

Figure 1. Various structures resulting from coordination of HBR2 on LnMH fragment.

Scheme 2 this section. Many of the structures described here were validated by X-ray crystallography, and although the large systematic errors associated with determining hydrogen positions often obviate precise comparisons of bond metrics, with high-quality structures it is possible to locate the hydrides in the difference map and assign one of the structural types shown in Figure 1. Structures of this type can also be observed when transition metals and boranes cooperatively activate H2; these examples will be covered in section 4, which deals with hydrogen cleavage. The present section is organized by transition metal, working from left-to-right through the dblock, and within each section a number of different binding modes of the Lewis acid can be seen. 2.1.1. Titanium. An early report of a Lewis acid bound hydride was published back in 1966 by James and Wallbridge.45 Alkyl titanate (Ti(OR)4) was treated with diborane to generate [Ti(OR)(BH4)2]. The IR stretching frequency indicated the presence of a double hydride bridge, although it was not clearly established if a titanium hydride intermediate was involved in this transformation. More recent work has investigated such adducts of titanium supported by tripodal ligand platforms.46 Reduction of acetophenone by borohydride has been shown to be catalyzed by titanium alkoxide complexes. To elucidate the reaction mechanism, tris(phenolate) titanium isopropoxide complex 1 was allowed to react with BH3·THF (Scheme 1). Spectroscopic evidence and crystallographic data suggest the formation of titanium(IV) tetrahydroborate complex 2 through initial borane binding to the alkoxide group (1a) and isopropoxide hydride exchange (1b). The fluxional behavior of the BH4 unit at room temperature has been confirmed by the presence of a single proton resonance in the 1H NMR spectrum and the appearance of a quintet at δ 16.3 ppm with 1JB−H = 86 Hz in the 11B NMR spectrum. 2.1.2. Niobium. The first report of Lewis acid binding by dicyclopentadienylniobium (niobocene) hydrides appeared in

ppm upfield relative to the respective starting material. This, along with a negligible perturbation of νCO in the carbonyl derivative, suggest binding of the Lewis acid by the hydride and not by the carbonyl oxygen. However, when niobocene trihydride ([CpNbH3], Cp = η5-cyclopentadienyl) was treated with AlEt3, ZnEt2, or [Hf(CH2Ph)4], ethane (in the case of alkyl complexes) or toluene (in the case of the benzyl complex) was eliminated, but adducts between the reduced niobium dihydride anion and the Lewis acid cation were proposed (Scheme 3). Villasenor and co-workers have described the reactivity of silyl-substituted niobocene hydrides, [Nb(η5-C5H4SiMe3)2H(L)] (3) (L = 2,6-dimethylphenylisocyanide, cyclohexylisocyanide, and CO) with the Lewis acids BPh3 and BF3.37−39 The niobocene hydrides of type 3, ligated with isocyanides or carbon monoxide, were treated with 1 equiv of either B(C6F5)3 Scheme 3

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borabicyclononane (9-BBN) (Scheme 6). X-ray crystallographic parameters and spectroscopic studies indicate a significant Nb−B interaction in the case of 7. The Nb−B bond length, 2.292(5) Å, is shorter than that of a previously reported related d0 metallocene hydridoborate (>2.5 Å).49 The extremes in the continuum are shown in Figure 2. Detailed

or BF3 resulting in the formation of air-sensitive zwitterionic species 4 (Scheme 4). Although no crystal structure was Scheme 4

Figure 2. Extremes in the bonding continuum for 7 and 8.

studies of complexes 7 and 8 using 1H NMR, 11B NMR, and isotopic perturbation measurements indicate complex 7 exists as a rapid equilibrium between structures 7a and 7b, whereas complex 8, with enhanced B−H interactions, remains in the other end of the continuum as a hydridoborate (structure B in Figure 2). These conclusions demonstrate that the identity of the Lewis acid can influence structural dynamics when transition-metal hydride complexes form adducts with Lewis acids. 2.1.3. Tantalum. Tebbe47 has reported Lewis acid adducts of bis(cyclopentadienyl)tantalum trihydrides. Reaction of Cp2TaH3 with various Lewis acids such as AlEt3, GaEt3, ZnEt2, CdEt2, or [Hf(CH2Ph)4] formed the simple Cp2TaH3· LA adducts, except for [Hf(CH2Ph)4], where toluene was liberated at room temperature. Reaction of [Cp2TaH3] with AlEt3 and GaEt3 was found to be irreversible, and the adducts were isolable, but in the case of divalent Lewis acids ZnEt2 and CdEt2 the interactions are weaker, as summarized in Scheme 7. The author proposes a single hydride bridge between Ta and the Lewis acid, as depicted in Scheme 7.

reported, IR spectra and NMR studies indicate the formation of a hydride bridged species. An infrared band at 2260 cm−1 in all of the complexes has been ascribed to νNb−H−B. No proton signal for the bridging hydride was observed due to the high quadrupolar moments of 11B and 93Nb. However, their 11B NMR resonances appear as doublets (at ca. −20 ppm) with 1 JB−H = 85 Hz, between bridging B−H (1JB−H = 60 Hz) and terminal B−H (1JB−H = 100 Hz). On a different note, when silyl-substituted niobocene trihydride (5) was combined with B(C6F5)3 in acetone, no adduct between the hydride and borane was observed. One equivalent of dihydrogen was evolved, and concurrently the acetone solvent was reduced to form the isopropoxideborate anion, which coordinated to the electron-deficient niobocene (Scheme 5). Two coordination Scheme 5

Scheme 7

modes of the counteranion have been proposed and are shown as 6a and 6b (Scheme 5). The presence of only three sets of 19F signals, even at −90 °C, favors the O−Nb binding mode as shown in 6b. Hartwig and co-workers48 have shown a continuum between hydridoborate and boryl complexes when niobocene trihydride was allowed to react with catecholborane (HBcat) and 9Scheme 6

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2.1.4. Molybdenum. A series of 1:1 complexes between [Cp2MoH2] and AlR3 (R = Me, Et, or Ph) were reported by Storr and Thomas.50 Some of these complexes are reported to be unstable and liberate hydrogen and alkane over time. No detailed structures were provided, but on the basis of IR and NMR data the authors proposed a direct metal−metal interaction. Corresponding complexes of AlH3 are unstable due to the reduced Lewis acidity of AlH3 as compared to that of AlR3. 2.1.5. Tungsten. One of the earliest demonstrations of the Lewis basicity of transition-metal hydrides was reported in 1963,51 when Shriver demonstrated reactivity between [CpWH2] and BF3. On the basis of infrared spectroscopy, which showed very little perturbation of the W−H stretching frequencies upon interaction with boranes, the authors proposed a direct tungsten−boron bond, as shown for the

Scheme 10

observed at −3.71, −2.58, and +4.61 ppm in a 1:1:2 ratio in the H NMR spectrum. The structure of one such complex, [transW(η2-BH4)(CO)(NO)(PMe3)2], has been authenticated by Xray crystallography. It is interesting to note that no reaction was observed between 16 and weaker Lewis acids such as BEt3, B(OMe)3, and BPh3, whereas the stronger Lewis acid B(C6F5)3 abstracts the hydride from tungsten and generates the solvated tungsten cation. Berke and co-workers54 have also synthesized tungsten hydride complexes bearing an ambiphilic phosphanylborane ligand. Compound 18 was synthesized in two steps, with ligand substitution on the precursor [W(CO)4(NO)(ClAlCl3)] followed by treatment with excess NaHBEt3 (Scheme 11) to 1

Scheme 8

Scheme 11

structure of 13 in Scheme 8. However, subsequent results from 1 H and 11B NMR and X-ray structures of related complexes dispelled this notion and instead demonstrated formation of ionic complex 14 in this reaction. Reaction between [Cp2WH2] and AlR3 (R = Me, Et, or Ph) likewise yielded a series of 1:1 complexes of type [Cp2WH2·AlR3] (15), which were originally reported to feature metal−metal bonds.50 The X-ray crystal structure of 15 (R = Me) instead indicates the presence of two bridging hydrides connecting W and Al, as shown in Scheme 9.52

install the hydride. The isomer of 18 where the two phosphines are arranged cis to each other and the two carbonyls are likewise cis arranged isomerizes to produce the more stable trans isomer shown in Scheme 11. Interestingly, the X-ray crystal structure of 18 indicates that the hydride ligand on tungsten is engaged in secondary coordination sphere σcoordination to the Lewis acidic B(C8H14) group, resulting in a three-center two-electron (3c−2e)55 W−H−B bond; the W−H (1.66(6) Å) and B−H (1.60(6) Å) bond distances are indistinguishable, and suggest a relatively weak B···H interaction. The other B(C8H14) unit remains as a free borane. Following a similar reaction protocol, compound 19 was synthesized, which contains a diphosphine ligand with a Lewis acidic borane center connected to it via a flexible arm (Scheme 12). The X-ray crystal structure of 19 displays a stronger (C8H14)B···H interaction in the secondary coordination sphere when compared to 18, with the B−H internuclear distance observed to be 1.38(3) Å in this case. An analogous compound with a more strained borane arm, 20 (Scheme 12), was synthesized, and 1H NMR and 31P NMR spectroscopy indicates formation of a hydride complex. The accurate position of hydride, however, could not be located due to the twinned nature of the crystal. 2.1.6. Rhenium. A Lewis basicity trend for the hydride center of rhenium complexes was also tested by Berke and coworkers.56 Two isomers each of [Re(CO)(PMe3)4H] (21 and 22) and [Re(CO)2(PMe3)3H] (23 and 24), along with [Re(CO)3(PMe3)2H] (25) (phosphines are trans oriented), were allowed to react with BH3 and 9-BBN. Complexes 21 and 22 afforded η2-BH4 and η2-BBN complexes as shown in

Scheme 9

Berke and co-workers53 have systematically investigated the reactivity of phosphine-substituted tungsten-hydride-nitrosyl complexes with boranes. The σ-basicity and π-acidity of the phosphine ligands were varied by using various alkyl and arylphosphines. Tungsten hydrides of type [trans,trans,transW(CO)2(PR3)2H(NO)] (16, R = Me, Et, Ph, OMe, OiPr, and OPh) were allowed to react with borane (BH3·L; L = THF, SMe2) as shown in Scheme 10. The complex [trans-W(η2BH4)(CO)(NO)(PR3)2] (17) was isolated as the major product from the reaction when R is Me, Et, Ph, or OiPr, while in the case of OMe and OPh only borane-phosphite adducts were observed. The η2-binding mode of BH4 unit has been indicated by presence of three sets of upfield singlets 8877


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2.1.7. Iron. Complexes of iron with borohydride ligands, which can be thought of as adducts between an iron hydride and a borane, are well-known in homogeneous catalysis. One example, which features an η1-BH4 linkage and is supported by a pyridine-based PNP pincer ligand, was prepared by Milstein and co-workers,58 and a related example, initially developed by Beller and co-workers, utilizes an alkyl-spaced PNP supporting ligand.59 Both of these complexes are typically prepared by metathesis reactions involving halide precursors and NaBH4, but a recent account by Jones et al. showed that borohydride complex 32 can be accessed by trapping trans-dihydride complex 31 with BH3, as outlined in Scheme 16. 2.1.8. Ruthenium. Binding modes of common boranes such as HBpin (pinacolborane), HBcat, and 9-BBN with ruthenium hydride complexes have been investigated by SaboEtienne and co-workers.60 The stoichiometric reaction between [RuH 2 (η 2 -H 2 ) 2 (PCy 3 ) 2 ] (33) and HBpin leads to an equilibrium between σ-borane complex [RuH2(η2-HBpin)(η2H2)(PCy3)2] (34) and 33 (Scheme 17). The identity of 34 has been confirmed by NMR spectroscopy and also by low temperature X-ray crystallography. From the X-ray structure, the dihydrogen ligand (σ-H2) was found to be perpendicular to the equatorial plane and slightly activated (d(H−H) 0.79(3) Å). The most conclusive proof of σ-borane ligation in 34 comes from the angle at which the Bpin group orients itself toward the metal center. The angle between the center of [O,O], B, and Ru in 34 is found to be 170.0°, similar to the σ-borane angle (171.5°) in [RuH[(μ-H)2Bpin](η2-HBpin)(PCy3)2] (35), which has both dihydroborate and σ-borane linkages.61 Complex 35 is accessed by reaction of bis(dihydrogen) complex 33 with excess HBpin (Scheme 17).61 Formulation of the structure has been supported by variable-temperature NMR spectroscopy and X-ray crystallography. Reaction of 33 with Bcat leads to the formation of similar σ-borane complex 36 with evolution of 1 equiv of dihydrogen, with complex 36a isolated as a minor product (Scheme 18). Reaction of 33 with 0.5 equiv of 9-BBN dimer leads to slow formation of [RuH[(μH)2BBN](η2-H2)(PCy3)2] (37) featuring the hydridoborate coordination mode of the BBN ligand. Finally, reaction with excess 9-BBN affords the 16e− bis(dihydridoborate) complex [Ru[(μ-H)2BBN]2(PCy3)] (38).62 These results indicate that Bpin and Bcat are not acidic enough to abstract a hydride and stabilize the symmetrical dihydrido borate mode of coordination, instead preferentially forming σ-borane complexes with weak H/BH cis interaction. In contrast, the more acidic 9-BBN forms the dihydridoborate complex. Klankermayer and co-workers63 have disclosed syntheses, structures, and a concise reactivity study of bifunctional ruthenium hydride/borane complexes. A series of ruthenium hydride complexes of the type [CpRuH(CO)(PR2R′)], where R′ is an ethyl group terminated with B(C6F5)2 or 9-BBN pendant boranes, were synthesized (Scheme 19). Cp was also substituted with Cp* (C5Me5−). Structural identities of all three complexes 39, 40, and 41 were established by NMR spectroscopy and X-ray crystallography. The B−H and Ru···B distances are approximately 0.1 Å shorter for 40 and 41, indicating a stronger binding between the stronger Lewis acidic B(C6F5)2 and the hydride. Weaker binding with the hydride in the case of 9-BBN also has been observed by comparing the hydride signals in the compounds. Broad signals in the high field region of proton NMR, indicative of H−B interaction, are only observed at −50 °C in the case of 39, while this broad signal is already evident for 40 and 41 at room temperature.

Scheme 12

Scheme 13. The η2-binding mode has been confirmed by NMR spectroscopy and X-ray crystallography. In a similar way, 23 Scheme 13

and 24 were converted to corresponding η2 complexes by reaction with BH3·THF and 9-BBN (Scheme 14). The reaction of 24 with BH3·THF led to an equilibrium with the adduct formed at the OCO terminus, as revealed by VT IR studies, while 25 does not react with 9-BBN. Scheme 14

Reaction of the ambiphilic phosphinoborane Ph2PCH2CH2BR2 (BR2 = 9-borabicyclo[3,3,1]nonanyl) with the unsaturated dimer [H2Re2(CO)8] led to the formation of [HRe(CO)4(κ2B,P-Ph2PCH2CH2BR2)] (30) as the sole product (Scheme 15).57 From X-ray crystallography and DFT calculations, it has been established that the borane in the ambiphilic ligand coordinates to the Re−H bond to form a 3c− 2e Re−H−B bond. 8878


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Scheme 15

Scheme 16

Scheme 19

Scheme 17

Scheme 20

Scheme 18

2.1.9. Osmium. Bell and co-workers64 reported that HgCl2 forms adducts with [OsH4(PEtPh2)3], and elemental analysis predicted almost 9 mol of HgCl2 per mole of osmium complex. After that, the stoichiomet ric reaction between [OsH6(PPhiPr2)2] and HgCl2 was reported by Connelly et al.41 (eq 1). The reaction progressed without evolution of hydrogen. The formula of the resulting complex was predicted to be [HgCl2{OsH6(PPhiPr2)2}] based on elemental analysis and proton NMR integration. The proton signal appears as a triplet at δ −8.57 ppm with 199Hg satellites. HgCl2

OsH6(PPhiPr 2) ⎯⎯⎯⎯⎯→ HgCl2[OsH6(PPhiPr 2)]

(1)

2.1.10. Rhodium. Some common diphosphine-ligated rhodium(I) hydrides are capable of completely transferring a hydride to a borane, and while it is likely that an interaction between the hydride and borane forms prior to hydride transfer, no stable adducts between these hydride complexes and boranes have been reported.65,66 Kameo and Nakazawa have demonstrated that adding a diphosphine-borane ligand to a rhodium(I) hydride complex forms a stable Rh−(μ-H)−B interaction in the isolated complex. As summarized in Scheme

Complexes 39 and 40 react with HBF4 to afford the cationic dihydrogen complexes 42 and 43, while the same reaction yields dihydride complex 44 for 41 (Scheme 20). When treated with deuterated methanol, fast H/D exchange was observed for 40, while this process was not completed even after weeks for 39. This has been explained by the cooperative activation of Lewis basic methanol by the strong pendant Lewis acidic B(C6F5)2 moiety in 40. 8879


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21, when the 5-coordinate complex [Rh(CO)H(PPh3)3] is treated with a diphosphine-borane ligand, two PPh3 ligands are

Scheme 23

Scheme 21

substituted to form complex 45, where the hydride occupies a bridging position between the rhodium and the borane. The complex was crystallographically characterized, with the hydride position located in the difference map. In addition, pyramidalization of the borane is clearly observed in the X-ray structure, and solution 1H NMR studies show considerable broadening of the Rh−H resonance, consistent with an interaction between the hydride and the borane. Complex 45 was found to be a competent precatalyst for the hydrogenation of ethyl phenyl ketone, using isopropanol as the hydrogen source. 2.1.11. Iridium. Heinekey et al.67 have investigated the interaction of some of the most common boranes, BH3, HBpin, and 9-BBN, with phosphine supported iridium pincer compounds. As depicted in Scheme 22, complex 46 can be

Scheme 24

Similar to complex 46, complex 49 shows two broad resonances for the protons that neighbor boron and a sharp signal for the terminal iridium hydride. The bulky structure of 9-BBN has been predicted to prevent the complete formation of 49. Braunschweig and co-workers68 have disclosed syntheses and structural characterization of a series of iridium pincer compounds with dihydroborate ligands featuring bulky aryl groups (Scheme 25). Addition of H2B-Dur (Dur = 2,3,5,6-

Scheme 22

Scheme 25

synthesized in two different ways, either reaction of NaBH4 with [(tBuPOCOP)IrHCl] (tBuPOCOP = κ3P,C,P′-C6H3-1,3[OP(tBu)2]2) or reaction of BH3 with [(tBuPOCOP)IrH2]. Complex 46 is stable at room temperature. Out of the various possible binding modes, the structure of 46 has been assigned as shown in Scheme 22. Variable-temperature NMR studies and neutron diffraction experiments predict the σ-borane binding mode of 46. With an activated B−H bond distance of 1.45(5) Å, 46 is considered to be the first example of transition-metal σcomplex of borane. To explore the binding modes with substituted borane analogues, [(tBuPOCOP)IrH2] was treated with a THF solution of HBpin. At low temperature (−20 °C), clean formation of 47 was observed (Scheme 23), and when the temperature was raised slow evolution of hydrogen was observed along with the formation of 48. Because of its instability, the structure of 47 could not be accurately determined, whereas the structure of 48 was ascertained by NMR and X-ray diffraction experiments. The proximity of hydride ligand to both boron and iridium in the structure of 48 indicates a 3c−2e bond as shown. In a similar reaction, when an equimolar amount of [(tBuPOCOP)IrH2] and 9-BBN was mixed together in THF, an equilibrium between the starting material and the σ-complex 49 was established (Scheme 24).

Me4C6H) to a benzene solution of [(tBuPCP)IrH2] (PCP = κ3P,C,P′-C6H3-1,3-[P(tBu)2]2) afforded the bidentate dihydroborate complex [(tBuPCP)IrH(κ2H,H′-BH3Dur)] 50 in good yield (Scheme 25). In the 1H NMR spectrum of 50, the bridging hydrides appear as broad signals at δ −4.57 and −6.48 ppm, whereas the terminal B−H appears at δ 9.00 ppm. The terminal Ir−H appears at δ −21.1 with H−P coupling of 12.9 Hz. However, the upfield chemical shift of the 11B NMR resonance indicates a discernible Ir···B interaction in the complex, although in the X-ray crystal structure of 50 the Ir−B internuclear distance of 2.283(2) Å is significantly longer than the sum of their covalent radii (2.14 Å for Ir/B).69 Compounds [(tBuPOCOP)IrH(κ2H,H′-BH3Ar)] (Ar = Dur (51), Ar = Mes (52)) were synthesized by stoichiometric reaction between [(tBuPOCOP)IrH2] and H2BAr. The NMR data and X-ray crystal structure of compound 51 indicate the presence of a hydroborate binding mode. In a similar reaction between [Cp*IrCl2]2 and H2B-Dur, a complex mixture of compounds resulted, limiting full spectroscopic identification (Scheme 26). X-ray crystallography of the isolated product, however, established the identity of the complex 53, where a chloride 8880


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been determined by X-ray diffraction. The Ni···B internuclear distances in 55−57 (2.1280(3)−2.214(3) Å) are significantly longer than that in a previously described PNP-pincer boryl complex (1.9091(18) Å).72 Other boranes such as 9-BBN and HBcat were observed to form dihydridoborate adducts with pincer complexes of sterically smaller phosphine ligands. However, the reactions are reversible, reforming the parent nickel hydride complex. 2.1.13. Platinum. To our knowledge, there are no examples of well-defined products resulting from reaction of platinum hydride complexes with main group Lewis acids, although such an adduct has been proposed as a transient intermediate in a linkage isomerism reaction. The N-bound complex trans[HPt(PEt3)2NC·BR3] (58, R = Ph, p-tolyl, benzyl, o-tolyl, 1naphthyl) isomerizes to the more stable C-bound complex trans-[HPt(PEt3)2CN·BR3] (59), as summarized in Scheme 29.73 The rate of isomerization depends on the identity of the bound borane, and was found to be catalyzed by added triaryl borane. To account for these observations, the authors proposed a mechanism, also shown in Scheme 29, where the added borane coordinates to the hydride ligand, pulling electron density toward the hydride and weakening the C− BR3 bond and allowing isomerization to occur.

Scheme 26

instead of a hydride has migrated from the iridium to the borane. 2.1.12. Nickel. To understand the affinity of BH3 for nickel hydrides, the reaction between pincer complex [(tBuPCP)NiH] and the Lewis acid adduct H3B·THF was reported by Peruzzini et al.70 NMR spectroscopy suggested coordinated tetrahydridoborate in complex 54. Structural characterization by X-ray crystallography was not possible due to the instability of 54. However, DFT studies propose a η1-BH4 binding mode present Scheme 27

2.2. Summary and Outlook

in the structure (Scheme 27). The reactivity of the related R POCOP type pincer complexes of nickel hydride with various boranes including BH3·THF, 9-BBN, and HBcat has been investigated by the Guan group (Scheme 28).71 The size of the

The reactions and structures described in the preceding section highlight the expansive chemistry of transition-metal hydrides with main group Lewis acids. Interestingly, no periodic trend is apparent in the chemistry, and we conclude that the chemistry described in this section is general over a range of transition metals. Metal hydrides, which are formally d0 (TiIV, NbV) all of the way to d8 (NiII, PtII), can bind boranes and form one or more bridging M−μ-H−LA interactions. Many of the fundamental aspects of the preparation and structures of these adducts were established 30 or more years ago, but the observation and characterization of transition-metal hydrides bridged to main group Lewis acids continues today, particularly as such complexes are recognized as key intermediates in catalytic reactions like amine-borane dehydrogenation and olefin hydrogenation (see sections 4.2.1 and 4.2.3), which remain of active interest. As we show below, particularly in section 4, structures of the type described in this section persist in other contexts and can be prepared by alternative means. Having outlined numerous examples of simple adducts between transition-metal hydrides and Lewis acids, we now move on to describe and contextualize other reaction types where these two partners participate.

Scheme 28

R group on the phosphorus was also varied to evaluate the effect of the phosphine’s steric parameters on the reactivity of the nickel hydride. Reaction of [(RPOCOP)NiH] with BH3· THF produced the hydrido complexes [(RPOCOP)Ni(η2H2BH2)] 55, 56, and 57 in good yields. The presence of a dihydro binding mode has been indicated by the two sets of IR absorptions, terminal νB−H at 2370−2420 cm−1 and bridging νB−H at 1790−2070 cm−1. Solid-state structures of 55−57 have Scheme 29

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3. LEWIS ACID-ASSISTED CARBONYL REDUCTION

Scheme 30

3.1. Background

Transition-metal hydrides are involved in a number of hydrogenation and reduction reactions of carbonyl compounds, and Lewis acids can mediate stoichiometric and catalytic reduction reactions of organic carbonyls, carbon dioxide, and carbon monoxide. In most cases, the Lewis acid, either added exogenously or incorporated into a supporting ligand, interacts with the carbonyl oxygen either in the substrate or in a key intermediate. This interaction serves to activate the substrate and/or stabilize a transition state or intermediate, influencing the thermodynamics and/or kinetics of a reaction or elementary step. In the following section, we describe developments in the area of Lewis acid-assisted, transitionmetal hydride-mediated carbonyl reductions. Some organic transformations are considered, although the majority of hydride-assisted organic reductions utilize main-group hydride reagents like aluminum hydrides or borohydrides, a research area that has been reviewed extensively74−76 and will not be further elaborated here. 3.2. Reduction of Organic Carbonyl Compounds

Scheme 31

3.2.1. Ketone Hydrogenation. A prominent example of Lewis acid participation in carbonyl reduction comes from studies of Noyori’s hydrogenation catalyst, which can hydrogenate ketones under very mild conditions with low catalyst loadings, excellent product yields, high chemoselectivity for ketones over olefins, and enantiomeric excess (ee) up to 99%.77,78 A systematic kinetic study on the hydrogenation of acetophenone by this catalyst revealed that Lewis acidic cations, which accompany the strong alkoxide base that is used to assist H2 activation, are required not only for catalyst initiation but also for turnover.79 The authors screened a variety of alkali metal tert-butoxide bases, and found that catalyst activity depended on the identity of the alkali metal cation, in the order K > Na ≈ Rb > Li. More significantly, they found complete loss of activity when the cations were sequestered with crown ethers, or when the organic base 1,8-diazabicycloundec-7-ene (DBU) was used instead. However, activity was restored when DBU was used in concert with an alkali metal tetraarylborate additive. The authors proposed two roles for the Lewis acid, as highlighted in Scheme 30 for catalyst 60. The strong base deprotonates the amine supporting ligand, with the remaining cationic Lewis acid coordinated to the ensuing amide (i). The first role of the coordinated Lewis acid is to direct substrate binding (ii), positioning, and activating the carbonyl group for hydride transfer (iii). The coordinated Lewis acid’s second function is to make the catalytic ruthenium center more electrophilic, which facilitates binding of H2 (iv) prior to product release (v). With some modification of the proposed structures, these findings were recently corroborated computationally, and in particular the dual role of the Lewis acid in ketone activation and H2 cleavage was confirmed.80 In a similar fashion, more recent work has demonstrated that sodium cations serve as a cocatalyst for outer-sphere ketone hydrogenation catalyzed by iridium.81 The experimental observation that NaOMe is required for catalytic activity motivated a computational investigation of the catalytic mechanism, where again the Na+ counterion was found to play a role. The key reaction steps where the sodium participates are given in Scheme 31. In this case, the Na+ cation interacts with the anionic tetrahydride catalyst 61 via bridging hydride interactions, but serves a similar role as was

proposed for Noyori’s catalyst. The ketone binds to the coordinated sodium cation, positioning the substrate for hydride transfer. The generated alkoxide remains bound to the sodium cation, and is positioned to assist in the heterolytic cleavage of H2, which generates the product alcohol and reforms the tetrahydride catalyst. 3.2.2. Hydroboration of Aldehydes and Ketones. In addition to hydrogenation, the hydroboration of such substrates, which involves a hydride transfer to carbonyl, can be facilitated by interaction with Lewis acids. The well-known Shvo catalyst82 mediates hydrogenation or transfer hydrogenation reactions of carbonyl compounds and imines in an outer-sphere fashion,83−85 via concerted hydride transfer from the metal and proton transfer from a ligand hydroxyl group.86−93 Clark and collaborators recently developed a boron-substituted version of Shvo’s catalyst, 62, which catalyzes the hydroboration of aldehydes, ketones, and imines, furnishing alcohols or amines after workup.94 Aldehydes were the most reactive substrate, and with 2% catalyst loading and 1.5 equiv of HBpin at 50 °C high yields of the primary alcohol products could be obtained. Imines required higher temperatures (70 °C) and longer reaction times, but good yields of secondary amines could be obtained. In contrast, ketones reacted very sluggishly, and only one substrate, 4-nitroacetophenone, was studied in detail. On the basis of analogy with the Shvo catalyst, 8882


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theme that has persisted in subsequent studies where formyl complexes are generated by hydride delivery to a bound carbonyl. Another demonstration of Lewis acid-stabilized formyl complexes, also with iron, came from Nakazawa, Miyoshi, and co-workers in 2000.103 They found that Cp-ligated iron phosphonate complex 64 reacts with 9-BBN to furnish stabilized formyl complex 65 (Scheme 33). This reaction is

the authors proposed the mechanism shown in Scheme 32, where the key hydride transfer step involves simultaneous Scheme 32

Scheme 33

interaction of the carbonyl oxygen with the tethered borane, in the same manner as depicted in Schemes 30 and 31 where coordination of the carbonyl oxygen to a Lewis acid functionality promotes the reduction of the carbonyl group.

noteworthy in that the thermodynamic driving force brought on by chelation of the BBN fragment allows for a relatively weak hydride donor to be used to generate the reduced carbonyl. The authors propose that the phosphonate oxygen interacts with the borane and activates the B−H bond, allowing hydride transfer to occur. Complex 65 is quite stable, and the authors report no decomposition on heating to 60 °C in benzene. The complex was also structurally characterized by single-crystal X-ray diffraction. The two B−O bonds differ by ca. 0.05 Å, with the bond to phosphonate the shorter and stronger of the two. The formyl Fe−C distance is shorter and C−O distance longer than those in a structurally related acyl complex,104 leading the authors to conclude that there is significant contribution of a boroxycarbene resonance structure in complex 65. Subsequent examples of formyl complexes, particularly those in the group 7 metals, were frequently prepared by hydride transfer using strong hydride donors.105 Borohydride reagents, such as LiHBEt3 (Super Hydride), were especially popular, and these reagents both possess a lithium cation and leave behind a three-coordinate borane, which could serve as Lewis acids to interact with the newly generated formyl moiety. Early examples noted the difficulty of removing the borane byproduct, although chromatographic purification furnished the formyl free of borane contamination.106 Much later, Bercaw et al. spearheaded renewed efforts to characterize the role of Lewis acids in the reduction of carbon monoxide, and to utilize such interactions advantageously to promote unique or enhanced reactivity. In one study, the effect of exogenous borane Lewis acids on the reactivity of manganese and rhenium formyl complexes was investigated.107 The formyl complexes in this work were prepared by hydride delivery from Super Hydride, and the Li+ and BEt3 byproducts were not reported to interact with the formyl oxygen. However, the stronger Lewis acids BF3 and B(C6F5)3 interacted with the formyl, generating structures that are described as resonance hybrids somewhere between a borane-capped formyl and a boroxycarbene, as outlined in Scheme 34. A later study by the same group108 characterized the interaction of [(PPh3)2Re(CO)3(CHO)] (66-Re) with trialkylboranes. The reaction of [(PPh3)2Re(CO) 4 ] + with [HPt (dmpe) 2 ] + (dmpe = 1,2-bis(dimethylphosphino)ethane) is sluggish and likely thermodynamically unfavorable, but is promoted by the addition of

3.3. Carbon Monoxide Reduction

3.3.1. Intermolecular Interactions of Metal Formyls and Lewis Acids. With the long-standing goal of enhancing mechanistic understanding of existing heterogeneous catalysts95,96 for syngas (CO + H2) conversion while concomitantly developing homogeneous alternatives, metal formyl complexes have been studied as key intermediates in CO-reduction schemes.97 These intermediates could potentially be generated by migratory insertion of a metal−hydride into a metal-bound carbonyl (eq 2), but with rare exception98−101 such a direct

transformation is thermodynamically unfavorable, and in some of these previous examples98,99 the metal center that the formyl is coordinated to is bound to both the formyl carbon and the oxygen, stabilizing the formyl in much the same was as external Lewis acids do (see below). Collman and Winter prepared the first isolable transition-metal formyl [Fe(CO)4(CHO)]− (63), which was accessed by addition of acetic formic anhydride to the nucleophilic carbonyl anion [Fe(CO)4]2−.102 They observed that the formyl’s 13C NMR shift depended on the solvent environment and the identity of the countercation, as described in Figure 3, which points to significant ion pairing and is proposed to involve interaction of the formyl oxygen with the counterion. Although this first formyl complex was not prepared by hydride transfer, the role that Lewis acids could play in stabilizing the formyl by was recognized early on, a

Figure 3. Structures of an iron formyl complex and the effect of Lewis acid binding on 13C chemical shift. 8883


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Scheme 34

Scheme 35

external trialkylborane Lewis acids. In solution, the interaction of 66-Re with tBu(CH2)2B(C8H14) is favorable, inducing an upfield shift of the ReCHO 1H resonance from δ 15.21 to δ 14.60, with the equilibrium constant for Scheme 34 determined to be 100 M−1 for this borane. Increasing the Lewis acidity of the borane does have a significant effect on the bond metrics for the formyl/boroxycarbene structures. As summarized in Figure 4, for 66-Re·BEt3, the Re−C, C−O, and B−O bond

through intermediacy of alkoxyborane complex 70, which was not observed. Complex 69 is cleanly and rapidly generated by reaction of 67 with 2 equiv of either NaHBEt3 or [HPt(dmpe) 2]+, and again this reactivity contrasts that of [(PPh3)2Re(CO)4]+, which only reacts with a single equivalent of borohydride and as mentioned above is unreactive toward the platinum hydride. These results show that tethered Lewis acids can dramatically influence the hydride-acceptor properties of metal carbonyls. Along the same lines as the organic carbonyl reactivity described in section 3.2.1, the Lewis acid coordinates to a carbonyl oxygen, this time from a formyl group, and activates the carbonyl toward further reactivity. Building on these results, the possibility of using H2 to generate the hydride equivalents necessary to initiate the reduction of CO was investigated. Base-assisted, heterolytic cleavage of H2 to generate metal formyl complexes was first proposed in 2002 by DuBois and collaborators, in an account of detailed thermodynamic parameters for manganese, ruthenium, and rhenium formyl complexes.110 They showed, as summarized in Scheme 36, that the combination of [CpRe(NO) (CO)2]+, H2, Proton Sponge (a strong base), and [Pt(dmpp)2]2+ (dmpp = 1,3-bis(dimethylphosphino)propane)

Figure 4. Selected structural metrics for 66-Re·BEt3 and 66-Re·BF3, determined from single-crystal X-ray structures. Distances are reported in angstroms (Å) with s.u. values in parentheses.

distances are 2.126(3), 1.252(3), and 1.638(3) Å, respectively.108 With the stronger Lewis acid in 66-Re·BF3, the Re−C and B−O distances shorten to 2.096(3) and 1.544(4) Å, respectively, whereas the C−O distance lengthens to 1.270(4) Å. All of these observations are consistent with more boroxycarbene character with the stronger Lewis acid, and highlight the effect of Lewis acid strength on the structure and bonding in stabilized formyl complexes. 3.3.2. Carbon Monoxide Reduction Facilitated by Pendant Lewis Acids. Further transformations of formyl complexes and an even greater impact on the initial hydridetransfer reactivity were achieved when boranes were appended to supporting phosphine ligands in rhenium carbonyl complexes. Significantly, the carbonyl reduction event is triggered by hydride transfer from a platinum hydride [HPt(dmpe)2]+, which, unlike borohydride reagents, can be generated by heterolytic cleavage of H2. The complex [(PPh3)2Re(CO)4]+ only reacts with [HPt(dmpe)2]+ in the presence of external BEt3 when the counteranion and solvent are chosen to favor precipitation of the [Pt(dmpe) 2]2+ byproduct. In contrast, an analogous complex with pendant boranes, [(PPh2(CH2)2B(C8H14))2Re(CO)4]+ (67), reacts cleanly with the platinum hydride.109 As outlined in Scheme 35, hydride transfer first generates a borane-stabilized formyl, formulated as a boroxycarbene (68). This boroxycarbene is prone to dimerization through intermolecular interaction of the formyl oxygen with the pendant borane, generating a dimeric form of 68, which was crystallographically characterized. Although the reversibility of the dimerization was not established, the authors found that intermediate 68 disproportionates via hydride transfer, generating a stable boroxy(boroxymethyl)carbene complex 69, which includes a new C−C bond. The C−C coupling event was proposed to occur

Scheme 36

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product, instead giving product 73, which the authors describe as a “confused” alkyl (Scheme 38). Thus, in this case, the Lewis

generated the formyl complex [CpRe(NO)(CO)(CHO)] in low yields (9%). The Bercaw group explored a similar strategy utilizing borane-appended rhenium complex 67 as the hydride acceptor. The combination of 67, [Pt(dmpe)2]2+, H2 (1−4 atm), and a strong bulky phosphazene base (P1) generated the C−C coupled product 69 in high yield over the course of a few days at room temperature.111 However, by careful evaluation of the reaction kinetics, and by systematically varying the hydride mediator and finally leaving it out altogether, the authors concluded that [Pt(dmpe)2]2+ was not required to mediate C− C coupling, and actually inhibited the transformation. Instead, a frustrated Lewis pair (FLP) mechanism112 was proposed, which is highlighted in Scheme 37. In this sequence, H2 cleavage is

Scheme 38

acid promotes reaction with a second hydride equivalent (the PPh3 analogue reacts only with a single equivalent), but does not promote C−C bond formation, and instead the substituted alkyl migrates to one of the boranes, with the other borane supporting a bridging alkoxide interaction with the rhenium center. Interestingly, if complex 74, which only consists of one methylene-spaced borane-appended phosphine, is treated with two hydride equivalents, C−C coupling is observed, giving alkoxy-substituted acyl complex 75 (Scheme 39). An analogous

Scheme 37

Scheme 39

product 77 is obtained if complex 76, which contains one ethylene-spaced ligand paired with PPh3, is treated with 2 equiv of hydride. Finally, as part of this study, the mechanism for the formation of C−C coupled product 69 was re-evaluated, and on the basis of low-temperature NMR data an alternative for the mechanism shown in Scheme 35 was proposed. Instead of the second hydride attacking the boroxycarbene, it adds to a second carbonyl, generating two boroxycarbene units (complex 78), which couple via 79 to form the C−C bond, followed by hydrogen migration to furnish 69. This alternative mechanism is summarized in Scheme 40. More recently, Berke et al. described stoichiometric CO reductions using borane-appended diphosphine supporting ligands.113 The authors claimed that the cis-enforcing diphosphine ligands lead to more labile metal-bound CO ligands. When a small bite-angle diphosphine was used, supporting complex 80, reduction gave a mixture of formyl (81) and hydride (82) products; the formyls were unstable, and no evidence for stabilization by the pendant borane was observed (Scheme 41). Presumably, the inflexible nature of the small bite-angle diphosphine precluded the formation of a stabilized formyl product. In contrast, both the manganese and the rhenium versions of complex 83, with the larger diphosphine, were reduced to generate robust borane-stabilized formyl complexes 84, as shown in Scheme 42. The manganese analogue was characterized by single-crystal X-ray diffraction, confirming the interaction of the formyl oxygen with the tethered borane. The formyl C−O and B−O bond lengths in 84-Mn are very similar to the borane capped rhenium formyls (66-Re) described above (Figure 4). The rhenium analogue of

mediated by the strong base and the pendant borane, which interact only weakly on account of the sizable steric bulk of P1 and can effect the heterolytic cleavage of H2. The borohydride thus generated (71) shuttles the hydride to the carbonyl and initiates the reduction/migration sequence that leads to the formation of 69, analogous to that described in Scheme 35. Thus, in this case the Lewis acid plays two roles in the hydride transfer chemistry, mediating the formal H− transfer from H2, and stabilizing the reduced boroxycarbene products that result from hydride attack on the bound carbonyl. In a very in-depth follow-up study, Miller, Labinger, and Bercaw evaluated the effect of the ligand structure and other variables on the reductive CO coupling chemistry.108 The spacer length between the phosphine and the tethered borane was varied, and complexes featuring only one of the phosphinoborane ligands, either in a pentacarbonyl complex or partnered with PPh3, were also investigated. The findings in this study are extensive, and are only highlighted briefly here. It was found that complex 72, a methylene-spaced analogue of 67, reacts with 2 equiv of hydride but does not produce a C−C coupled 8885


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Scheme 40

Scheme 42

appended to supporting ligands in rhenium phosphine complexes. The base was intended to facilitate the heterolytic cleavage of H2, accepting a proton with the hydride equivalent attacking an electrophilic carbonyl. The protonated base could then interact with the formyl oxygen via hydrogen bonding, replacing the prohibitively strong B−O bond in boraneappended complexes with a much milder interaction. The first class of complexes studied for this purpose has the general formula [Re(κP-PPh2∼B:)(CO)5]n (85, n = 0, +1), where Ph2P∼B: is a monodentate phosphine with a tethered strong base.116 The equilibrium depicted in Scheme 43, where H2 is Scheme 43

84 was not crystallographically characterized, but the authors mentioned that this complex was stable in solution for “a few weeks”, highlighting the enhanced stability brought on by interaction of the formyl oxygen with the pendant borane Lewis acid. 3.3.3. Carbonyl Reduction with Appended HydrogenBonding Groups. In the previous section, we highlighted many advances in Lewis acid-assisted reduction of carbon monoxide with hydride reagents or H2-derived hydride equivalents. This work revealed many important features of such a strategy, highlighting the multifarious roles that Lewis acids, specifically boranes, could play in both the initial hydride transfer to the metal-bound carbonyl to form a formyl, and the subsequent reactivity and fate of that reduced carbonyl intermediate. The major drawback of the borane-assisted strategy, which to date has precluded any realization of catalytic transformations, is that the primary driving force for these transformations is provided by the strong B−O bond that forms upon reduction of the carbonyl. In the context of C−C bond formation via methyl-to-carbonyl migration, the Bercaw group examined other ligand platforms, which could support weaker Lewis acids, such as zinc114 or alkaline earth cations,115 in the secondary coordination sphere of rhenium carbonyl complexes. These constructs have not been utilized to promote hydride transfer reactions, and as such are not highlighted in detail here. Related to the theme of this Review, an alternative strategy was investigated by Bercaw to assist the cleavage of H2 and delivery of a hydride to a carbonyl. Strong bases were

cleaved to form hydrogen-bond-stabilized formyl complex 86, was evaluated as a function of the ligand structure. The stabilized formyl complex 86 could be prepared by sequential hydride and proton transfer, and was found to be unstable with respect to H2 loss. The free energy change was determined using thermodynamic cycles, and in all cases the forward reaction was thermodynamically unfavorable, by at least +8(2) kcal/mol. The origin of the unfavorable thermodynamics lies primarily in the attenuation of the base strength when the phosphine is coordinated to [Re(CO)5]+; in all cases, the carbonyl complexes had pK(HB+) values in acetonitrile that were 5−11 orders of magnitude smaller than for the free ligands. In an effort to improve the thermodynamics of H2 cleavage, a strong guanidine base was appended to a cyclopentadienyl rhenium carbonyl/nitrosyl complex, which is a substantially better hydride acceptor than complex 85. As summarized in Scheme 44, the heterolytic cleavage of H2 and CH3OH was studied for guanidine-appended complex 87.117 In this case,

Scheme 41

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enabling hydride transfer to CO2, which would otherwise be thermodynamically unfavorable128−130 or kinetically sluggish. That said, unlike the case with carbon monoxide, the insertion of carbon dioxide into a metal hydrogen bond is often thermodynamically favorable, driven by the formation of a formate complex with a metal−oxygen bond. Often, Lewis acids like boranes are used to cleave the O-bound formate from the catalytic active site. In some cases, hydridoborates are used as reagents in CO2 reduction catalysis, which can serve the dual role of providing a hydride equivalent and serving as a Lewis acid site. 3.4.1. CO2 Hydroboration. The insertion of CO2 into metal−hydride bonds is a well-known elementary step in CO2 hydrogenation, and there are numerous examples where this step has been observed stoichiometrically or proposed as a key step in catalytic CO2 hydrogenation.131−133 In more recent developments, Lewis acids have been utilized to either promote hydride transfer to or release the bound formate from the transition-metal center following insertion. Often the primary role of the hydridoborane is as a hydride donor, but in some cases the Lewis acid character of the borane plays a part and interactions of the borane substrate with the transition-metal hydride or other intermediates are possible. Recent work on nickel-catalyzed CO2 hydroboration highlights the multifarious roles played by hydridoboranes in CO2 reduction catalysis. Work by Guan and co-workers demonstrated that [(tBuPOCOP)NiH] rapidly inserts CO2 to form a formate complex (89) in the absence of Lewis acid (step (i) in Scheme 45).134 When the Lewis acid/hydride donor HBcat was

Scheme 44

tethering the base to the Cp ligand attenuates the electrophilicity of the carbonyl ligand, and complex 87 is a ca. 3 kcal/ mol worse hydride acceptor than the parent complex [Re(Cp)(CO)2(NO)]+.110 As such, the thermodynamics of H2 cleavage are improved relative to phosphine complex 85, but still unfavorable at +3.3(6) kcal/mol. The related heterolytic O−H cleavage of methanol, forming methoxycarbonyl complex 88b, proceeds rapidly at high concentrations of MeOH and was found to be nearly thermoneutral. By comparing these results to hypothetical bimolecular H 2 activation where complex 87 serves as its own base, it was determined that tethering the base in the secondary coordination sphere makes the H2 cleavage reaction ca. 6 kcal/mol more favorable. This thermodynamic boost is likely a combination of factors, the Coulombic stabilization brought on by placing the hydride and proton equivalents in close proximity, and any hydrogen-bonding stabilization, which occurs via interaction of the formyl oxygen with the conjugate acid of the tethered base (this interaction was not directly observed). Although the tethered-base strategy was ultimately not successful at bringing about productive CO hydrogenation, it remains an appealing alternative to Lewis acid activation by offering much milder and more tunable interactions with the reduced carbonyl, and this work also clearly demonstrated the quantitative thermodynamic benefit of intramolecular over intermolecular stabilization of formyl complexes, an insight that was only broached qualitatively in previous studies on Lewis acid-functionalized complexes.109,111,113

Scheme 45

3.4. Carbon Dioxide Reduction

In much the same vein, the reduction of CO2 mediated by transition-metal hydrides can be assisted by Lewis acids. Boranes are again popular choices as Lewis acids for these transformations, and a recent review highlights advances in borane-mediated activation of CO2.118 Many recent examples of Lewis acid-assisted CO2 functionalization utilize main-group frustrated Lewis pairs (FLPs), a topic that is certainly noteworthy but is beyond the scope of this Review.119−122 In addition, there are several recent examples of Lewis acidassisted CO2 electroreduction, although none of these accounts specifically invoke a hydride intermediate that is impacted by the Lewis acid additive.123−126 Finally, Byers and co-workers have demonstrated enhanced activity for CO2 hydrogenation when a variety of potassium salts are added, although the Lewis acidic cation was not proposed to play a role in facilitating catalysis.127 Nevertheless, there are several recent examples of Lewis acid-assisted CO2 reduction mediated by transition-metal hydrides. In many cases, the Lewis acid serves a role similar to that seen above for reduction of carbon monoxide, facilitating hydride transfer by stabilizing the reduced CO2 (usually a formate if direct hydride attack is involved). Again, it is the oxophilicity of the Lewis acid that drives this interaction,

introduced, formate-for-hydride exchange regenerated [(tBuPOCOP)NiH] and formed HCOOBcat (ii), which was reduced further with another equivalent of HBcat to form formaldehyde (iii). The formaldehyde also reacts with [(tBuPOCOP)NiH] to form methoxide intermediate 90 (step (iv)), which then reacts with HBcat to release the CH3OBcat product and regenerate [(tBuPOCOP)NiH] (step (v)). Thus, in this catalytic system it was demonstrated that the transitionmetal hydride could mediate two reduction steps, and although the Lewis acid was not proposed to be required for either insertion reaction to occur, it did participate in both steps by liberating the reduced CO2 and regenerating the hydride. The authors also claim an interaction between hydride complex [(tBuPOCOP)NiH] and HBcat, forming an off-cycle species (91), which must release the borane prior to entering the catalytic cycle. The authors propose that formate intermediate 8887


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capable of hydride donation to CO2, with transfer of the formate to the pendant borane to form 93 (Scheme 46). The

89 interacts with the catBOBcat byproduct, suggesting another role for the Lewis acid additive. No structure was given for this adduct, which is also off-cycle and inhibits catalysis when HBcat is depleted. Hydridoborane adducts of the nickel hydride [(tBuPOCOP)NiH] were subsequently prepared and characterized (see section 2.1.12), and confirmed to be dormant species in borane-assisted CO2 reduction catalysis.71 Along the same lines as this previous example, there are several other accounts of transition-metal-mediated hydroboration of CO2, where the insertion of CO2 is thermodynamically favorable and the added hydridoborane abstracts the bound formate and closes the catalytic cycle by delivery of a hydride. Since the theme of this Review is Lewis acid-assisted transformations of metal hydrides, these examples will only be covered briefly, because in most of them the hydride transfer is not proposed to be facilitated by the Lewis acid. In a reaction very similar to that described above, Hazari et al. found that a Pd(II) pincer complex with a tridentate phosphinosilyl ligand catalyzed the hydroboration of CO2. In this case, HBpin was used, and the major product was the singly reduced HCOOBpin, with only traces of further reduction noted.135 A study on a very similar system showed that the same palladium hydride and its platinum congener form an adduct with the Lewis acids BPh3 and B(C6F5)3, and that these adducts are active catalysts for the reduction of CO2 to methane, when silane reductants are employed.136 In this case, the borane stabilizes the formate that forms when CO2 inserts into the metal−hydrogen bond, through interaction with the formate oxygen. Interestingly, the unstabilized formate does not react with silane, and reduction to methane is only observed when the borane-stabilized precatalysts are used, but again the borane does not play an active role in the reactivity of the transitionmetal hydrides, which are reactive with CO2 in the absence of any additive. Earlier, Sabo-Etienne’s group had found that a ruthenium hydride/H2 complex, previously observed to reversibly insert CO2,137 catalyzes the hydroboration of CO2 to a mixture of C1 and C2 products, with the HBpin serving to extrude the bound formate and promote further reduction.138 Sgro and Stephen also reported a ruthenium-catalyzed CO2 hydroboration scheme, using a tripodal tris(aminophosphine) ligand.139 A noteworthy feature of this work was the cooperativity between the ruthenium hydride and the ligand in the activation of CO2, with the hydride first migrating to an iminium functionality concomitant with CO2 insertion. The HBpin then transfers two hydrides to the bound CO2 and extrudes one oxygen atom to form CH3OBpin as the final product. Shintani and Nozaki have disclosed an example of copper-catalyzed CO2 hydroboration, where a NHC-ligated copper hydride inserts CO2 to form a two-coordinate copper formate, with HBPin extruding the product in the form of HCOOBpin.140 3.4.2. Lewis Acid-Assisted CO2 Hydrogenation. In the above examples of CO2 hydroboration, the hydridoborane delivered the hydride equivalent and extruded the oxygenated product(s) from the transition metal, but the key step involving the metal hydride, the insertion of CO2 to form a formate, proceeded without Lewis acid assistance. There are also examples of Lewis acid-assisted CO2 hydrogenation, where in some cases the initial hydride transfer step is not favorable in the absence of Lewis acid. Miller, Labinger, and Bercaw reported a study of stoichiometric CO2 reduction facilitated by trialkylboranes.141 As part of this study, complex 92, the dimeric form of boroxycarbene complex 68, was shown to be

Scheme 46

authors also showed that complex 93 was formed when cationic carbonyl complex 67 was treated with CO2 and the hydride donor [HNi(dmpe)]+, but found that intermediate 92 was not involved in this transformation. Instead, they discovered that trialkylboranes could assist in the direct transfer of a hydride to CO2. The reaction between [HNi(dmpe)2]+ and CO2 is predicted to be unfavorable by ∼7 kcal/mol, and accordingly reaction of [HNi(dmpe)2]+ with CO2 in MeCN only gave ∼5% yield of formate. Conversely, as depicted in Scheme 47, when Scheme 47

the nickel hydride and CO2 were reacted in the presence of the triakylborane tBu(CH2)2B(C8H14), much higher yields were observed, 60% with 1 equiv of borane and quantitative with 10 equivalents. NMR confirms the interaction of the borane with the formate product. H2 gas could be used as a reactant when the transformation was mediated by [Rh(dmpe)2]+, which oxidatively adds H2 to furnish [H2Rh(dmpe)2]+, a hydride donor similar in strength to [HNi(dmpe)2]+. The mixture of [Rh(dmpe)2]+ and 1:1 CO2/H2 (1 atm total pressure) resulted in no formate formation, but formate was observed when t Bu(CH2)2B(C8H14) was included, Scheme 48. Although no catalytic transformations were realized using this strategy, these results did show that Lewis acids can be used to overcome thermodynamic barriers in the hydride transfer step of CO2 reduction. In a more recent account, Berke et al. provided an example of borane-assisted CO2 reduction that could be made catalytic.142 The five-coordinate rhenium hydride complex [ReH(Br)(NO)(PR3)2] (94, R = iPr, Cy) is unreactive toward CO2, despite the presence of an open coordination site. However, as depicted in Scheme 49, in the presence of B(C6F5)3 the complex binds 8888


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Scheme 48

Scheme 50

Scheme 49

106 were achieved when LiBF4 was added at 10 mol %, a 105 molar excess relative to catalyst 99. In the case of CO2 hydrogenation, the authors examined both secondary-amine pincer ligands (e.g., 99-H) and tertiary amine pincer ligands (99-Me). Both versions of the catalyst are promoted by Li+ additives, with the tertiary amine versions performing better overall. Mechanistically, in both formic acid dehydrogenation and CO2 hydrogenation, the Lewis acid interacts with the formate intermediate, as shown in Scheme 51. In the case of Scheme 51

CO2 immediately, forming complex 95 where the CO2 is bound cooperatively between the rhenium and the borane. Over the course of several hours, the hydride migrates to the bound CO2, forming dimeric formate complex 96. Complex 96 can be cleaved by H2, forming the “compressed” dihydride 97. This system can be rendered catalytic in two different ways. Addition of Et3SiH to a mixture of 94, B(C6F5)3, and CO2 produces (Et3SiO)2CH2 as the exclusive product with a turnover number (TON) of 35; using 96 or 97 as the precatalyst and higher CO2 pressures (5 atm) resulted in higher TON (89−95) and still good selectivity for (Et3SiO)2CH2. By adding a base, catalytic formate production was observed, and a screen of different bases revealed secondary amines like HNiPr2 to be the best choices. In the proposed catalytic cycle, the borane is involved in the initial CO2 binding and hydride transfer events, as depicted in Scheme 49, and dissociates along with formate, with the base serving to deprotonate dihydride intermediate 97. Lewis acids besides boranes can promote CO2 hydrogenation, as well as the reverse reaction, the dehydrogenation of formic acid. Hazari and Bernskoetter have developed a series of pincer-ligated iron hydride complexes (98), which insert CO2 to form a formate complex (99), as shown in Scheme 50. With the related secondary-amine pincer complex 100, the formate complex can be prepared by direct addition of formic acid. Complexes of this type can dehydrogenate formic acid to H2 and CO2143 and in the presence of a strong base like DBU can catalyze the conversion of CO2 and H2 to formate.144 Both of these transformations are promoted by Lewis acid additives, with Li+ being the optimum additive in both cases; the catalytic transformations are summarized in eqs 3 and 4. For the dehydrogenation of formic acid, turnover numbers approaching

formic acid dehydrogenation, the cation is proposed to interact with the formate in a κ2-fashion, stabilizing an H-bound formate, which is positioned for decarboxylation via hydride transfer to iron.143 This same mode of cation-assisted formateto-metal hydride transfer is proposed to be involved in the dehydrogenation of methanol catalyzed by complex 99, which forms CO2 and H2 as the final products.145 In this scheme, formate is formed as an intermediate, and is dehydrogenated in the same fashion described in Scheme 51. For CO 2 hydrogenation, the alkali cation assists in the release of formate, opening a coordination site for base-assisted heterolytic H2 activation.144 0.001 mol % 99 ‐ H,10 mol % LiBF4

HCO2 H ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H 2 + CO2 8889

(3)


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interactions with boranes are especially prominent.154 This “Z-type” metal−ligand interaction155 involves the formal donation of two electrons to the acceptor, and as such is most frequently seen with late transition metals in low oxidation states.156 A common method for preparing metal− borane complexes is to add low-valent transition-metal precursors to borohydride-containing proligands, and as such the ability for hydrogen to migrate between boranes and transition metals in such complexes has long been recognized.157 There are earlier examples of H2 cleavage by a terminal ruthenium borylene (Ru=BR) complex,158 but the first reported example of H2 activation by a transition-metal borane complex came later.159 Rhodium-borane complex 101 is prepared by treatment of the borohydride ligand precursor with [RhCl(NBD)] 2 (NBD = bicyclo[2.2.1]hepta-2,5diene),160 followed by hydride migration to the metal and hydrogenation of NDB. As shown in Scheme 52, 2 equiv of H2

0.30 − 0.78 mmol of 99 ‐ Me

CO2 + H 2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [DBUH+][HCO2−] DBU, LiOTf

(4)


In this section, most of the reactions we describe involve transfer or migration of a hydride from a transition metal to a carbonyl-containing substrate. All of the work in this section revolved around mid to late transition metals, which we presume is because earlier metals would tend to form prohibitively strong M−O bonds with reduced carbonyl intermediates, which would obviate catalysis. That said, the primary role of the Lewis acid in this chemistry is to stabilize reduced carbonyl intermediates, which brings about a similar issue, and a persistent challenge in this chemistry is avoiding the stable “thermodynamic sinks” that can form when the reduced carbonyl interacts with the Lewis acid. As such, attempts to render CO reduction catalytic have been ineffective (see section 3.3.2), and many examples of CO2 reduction involve forming products with strong B−O or Si−O bonds (see sections 3.4.1and 3.4.2). It has been shown recently, as just described at the end of section 3.4.2, that CO2 hydrogenation can be promoted with weaker Lewis acids like alkali metal cations, and this strategy appears to be a promising way forward.

Scheme 52

4. LEWIS ACID-ASSISTED H2 ACTIVATION 4.1. Background

In this section, we consider one class of reactions where Lewis acids are involved in the formation of metal hydrides. There are numerous routes available to synthesize transition-metal hydride complexes, many of which have been known for several decades.1 Chief among these are oxidative addition of H2, reduction with alcohols or borohydrides, and oxidative addition of HX. Heterolytic cleavage of H2 has emerged as a popular strategy for accessing transition-metal hydrides. This reaction has been known for some time,146 and conventionally involves base-assisted H2 cleavage where the base picks up a proton equivalent from H2 and the hydride is delivered to a transition metal without change in oxidation state. In a sense, the transition metal is acting as a Lewis acid in these transformations. DuBois, Bullock, et al. have evaluated thermodynamic aspects of heterolytic H2 cleavage147 and demonstrated its importance in the design of molecular electrocatalysts for oxidation of H2 and related transformations.148−150 The use of non-transition-metal Lewis acids to assist in the cleavage of H2 has gained traction in the past decade or so. There are many examples of FLP-based H2 cleavage, using a Lewis base in concert with a main group Lewis acid, usually a borane, to split H2 into proton and hydride equivalents.112,151 Again, the chemistry of FLPs is beyond the scope of this Review, but in this section we review an emerging area of hydride chemistry where Lewis acids in combination with transition metals heterolytically cleave H2, a topic that has been reviewed briefly in the recent literature.152,153 Here, the transition metal serves as a base and is protonated, with the hydride migrating to the Lewis acid. This inverted strategy for H2 activation has begun to show promise in catalytic applications, as highlighted below.

adds to this complex, in the presence of a trialkylphosphine donor ligand. One hydride is added to rhodium via hydrogenolysis of the alkyl ligand, and a second equivalent adds via heterolytic cleavage, facilitated by the borane ligand. This results in the addition of two hydride ligands to the rhodium, and a third that is bound to the boron and bridges the boron and the rhodium, furnishing product 102. The authors of this work have not yet reported any catalytic applications for this system. An analogous strategy for H2 activation, using primarily firstrow transition metals, has proven to be fruitful in catalytic applications. Harman and Peters have described a lowcoordinate, low-valent nickel-borane complex 103, stabilized by an arene−nickel interaction.161 Complex 103 reacts reversibly with H2, as shown in Scheme 53, and at room temperature with 1 atm of H2 an ca. 80% yield of H2-addition product 104 is achieved. This corresponds to a Keq of ∼5, and complex 103 is regenerated quantitatively when H2 is removed, confirming reversibility. NMR spectra for complex 104 are consistent with the presence of two distinct hydrides, one Scheme 53

4.2. Intramolecular H2 Activation by Transition Metal/Lewis Acid Pairs

4.2.1. H2 Cleavage by Metalloborane Complexes. Interactions of transition metals with Lewis acids have long captured the attention of coordination chemists, and 8890


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108, where L = N2, CO, or CNtBu, reversibly adds H2 to form iron-hydride/borohydride complex 109. When L = N2, it is possible to displace N2 with H2 to form 110, but in all cases only 1 equiv of H2 is cleaved. In this work, three of the H2 adducts (109, L = N2, CNtBu, and 110) were characterized by single-crystal X-ray diffraction, providing further confirmation of the structures. Complex 108 was found to be an effective precatalyst for the hydrogenation of alkenes or alkynes, using 1 atm of H2 and 30 equiv of the substrate at room temperature, with near quantitative conversions and substrate-dependent turnover frequencies ranging between 0.16 and 15 h−1. In conceptually related work, Cowie and Emslie have demonstrated cooperative H2 activation by a diposphine-linked platinum-borane complex.166 The formally platinum(0) borane complex 111, which is stabilized by η3-BCC-arylborane coordination, was studied for the stoichiometric activation of H2, Scheme 56. Both complex 111 and its CO adduct 112 react

borohydride that bridges the nickel center, and a terminal hydride localized on the nickel, similar to previously characterized intermolecular nickel(II) hydride/borohydride adducts.162,163 The Lewis acidic borane serves to facilitate the oxidative addition of H2 to Ni(0), an otherwise unobserved reaction. Adduct 104 is able to hydrogenate the alkenes styrene and tert-butylethylene to their respective alkanes, and with only 1 atm of H2, turnover numbers in excess of 100 could be achieved with styrene. A subsequent study, using complex 105, a more electron-rich analogue of complex 103 with isopropyl substituents on the phosphines, established the mechanism of H2 cleavage, which is summarized in Scheme 54.164 This work Scheme 54

Scheme 56

with H2 to form the same product, 113, which similar to nickel complexes 104 and 107 consists of both a terminal metalhydride and a borohydride that bridges to the transition metal. The hydrogenation reaction of 111 is likewise reversible, precluding crystallographic characterization, but multinuclear NMR (1H, 31P, and 11B), in combination with IR spectroscopy and DFT computations, established the structure of 113 that is shown in Scheme 56. This system was not found to be active in any catalytic applications. Many of the above examples of cooperative Lewis acid/ transition metal H2 activation utilize phosphine-based supporting ligands, although recently Figueroa et al. have demonstrated cooperative H2 activation by low-coordinate Pt complexes featuring bulky terphenyl isocyanide ligands.167 Starting with the two-coordinate complex [Pt(CNArDipp2)2] (ArDipp2 = 2,6(2,6-(iPr)2C6H3)2-C6H3), hydroboration of one of the isocyanide ligands converts it into an imine donor with a tethered borane, generating complex 114. A short Pt−B interaction of 2.224(2) Å is observed crystallographically, and 11B NMR confirms the existence of this interaction in solution. Complex 114 reacts readily with H2 at room temperature, forming platinum hydride/borohydride complex 115 (Scheme 57), a

10

demonstrated that a d Ni−H2 adduct (106) was formed prior to borane-assisted heterolytic cleavage, forming 107. This adduct was characterized spectroscopically (NMR and UV−vis absorption), with variable-temperature studies and labeling studies with D2 being especially informative. DFT computations showed that the borane played a role in stabilizing the H2 adduct; the nickel d-orbital, which is σ-antibonding with respect to H2, is simultaneously stabilized by interaction with the empty borane p-orbital, which lies approximately trans to the H2 ligand. Further mechanistic calculations show that the borane is involved in the H2-cleavage transition state. The barrier for oxidative addition of H2 was 9 kcal/mol higher than a transition state involving simultaneous B−H bond formation and H−H cleavage, favoring a cooperative activation of H2 involving both Ni(0) and the borane. Thus, the borane plays two parts in cooperative H2 activation, stabilizing the initially formed H2 adduct via a σ-backbonding interaction, and then lowering the transition state for the cleavage of the H−H bond. The Peters group has also demonstrated analogous reactivity using iron boratrane complexes supported by tripodal phosphine ligands.165 As summarized in Scheme 55, complex

Scheme 57

Scheme 55

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reaction that the authors claim is irreversible. The complex was characterized by single-crystal X-ray diffraction, and although the terminal platinum hydride was not located in the difference map, 1H NMR spectra show two distinct, coupled hydride resonances, with the most upfield resonance (δ = −15.00 ppm) attributed to the terminal platinum hydride. Notably, the reactivity of complex 114 with H2 is distinct from that of the free boryl(iminomethane) ligand, which undergoes hydrogenation of the CN bond when exposed to H2. In addition to these previous examples with late transition metals, early metal/borane pairs have also been demonstrated to be capable of intramolecular H2 addition. Erker et al. have prepared a Cp-ligated, formally zirconium(II) complex with a pendant borane via a circuitous route.168 Starting with Schwartz’s reagent, [Cp2ZrHCl], sequential insertion of tertbutylacetylene and metathesis with trimethylsilylacetylide gave the product [Cp2Zr(CHCHtBu)(CCSiMe3)]. Transmetalation of the acetylide to HB(C6F5)2, followed by reductive elimination of tert-butylethylene, furnished complex 116 featuring a π-coordinated alkynylborane ligand. Complex 116 reacted rapidly with H2, as shown in Scheme 58, forming the

[Mn(CO)5 (B(NMe2)2 )] + H 2 → [Mn(CO)5 H] + HB(NMe2)2

(5)

[CpFe(CO)2 (BPh 2)] + H 2 → [CpFe(CO)2 H] + HBPh 2 (6)

To develop a system where H2 activation is reversible, and to prevent dissociation of the boron-containing group after activation, Lin and Peters described a cobalt complex supported by a boryl-containing pincer ligand, and studied its reactivity with H2.171 Using a bisphosphino-boryl pincer ligand first described by Nozaki,172 the low-valent cobalt complex 118 was prepared, and exposure to H2 immediately generated a new species. The room-temperature 1H NMR integration suggested the activation of 2 equiv of H2, a notable feature of this system, and on the basis of variable-temperature NMR, X-ray crystallography, and DFT calculations the product was concluded to be the η2:η2-dihydridoboratocobalt dihydride complex 119, as shown in Scheme 59. Both equivalents of H2 Scheme 59

Scheme 58

isolable and structurally characterized H2-addition product 117. The authors propose two possible mechanisms for H2 cleavage, and on the basis of DFT calculations conclude that the reaction proceeds via cooperative activation of H2 by the zirconium center and the borane, as opposed to oxidative addition of H2 followed by hydride migration. This mechanism is very similar to that which was established for H2 activation by nickel− borane complexes, summarized in Scheme 54, and again demonstrates how the formation of transition-metal hydrides can be aided by boranes, with the Lewis acid accepting a hydride equivalent and avoiding the formation of high-energy oxidative addition intermediates. 4.2.2. H2 Cleavage by Metalloboryl Complexes. In addition to the many examples of cooperative H2 activation involving transition metals with tethered borane groups, there have been a few accounts of H2 activation and metal−hydride formation involving boryl (BR2−) ligands. In contrast to cooperative metal−borane H2 activation, where the metal center is protonated and the formal oxidation state increases by 2, cooperative activation of H2 by a metal−boryl pair results in no change in oxidation state at the metal, and in some cases (vide infra) the product metalloborane is able to activate a second equivalent of H2, in accord with the examples given in section 4.2.1. Early examples focused on hydrogenolysis reactions of metal boryl complexes, and in these cases the hydridoborane that forms dissociated from the metal. For example, the complex [Mn(CO)5(B(NMe2)2)], described in a 1966 paper,169 reacts with H2 to form pentacarbonylmangenese hydride and the corresponding hydridoborane (eq 5). Similarly, Hartwig and Huber described the reactivity of the Cp-ligated iron boryl complex [CpFe(CO)2(BPh2)], and showed that the iron−boryl bond could be cleaved by H2 to form CpFe(CO)2H (Fp−H) and HBPh2 (eq 6).170

could be displaced under an N2 atmosphere, regenerating 118 and establishing the reversibility of the H2 cleavage reaction. Complex 118 was applied as a catalyst for the hydrogenation of olefins. Complex 119 was observed to persist under catalytic conditions, and with 2 mol % loading of the cobalt catalyst both styrene and 1-octene were quantitatively hydrogenated in a period of 3 min at room temperature. Complex 118 was also found to be an active catalyst for the dehydrogenation of the amine borane HMe2N−BH3, with modest turnover frequencies but good selectivity for the formation of the cyclic product (Me2N−BH2)2, releasing 1 equiv of H2. This system was recently investigated computationally, largely confirming the findings of the above-mentioned experimental work.173 One interesting finding to come out of the computed mechanism is that complex 119, the product of cleaving 2 equiv of H2, is not a catalytically relevant species, and that the product of a single H2 cleavage, with one terminal hydride and a hydridoborane, is an on-cycle intermediate that binds the olefin substrate. In other words, coordinatively saturated complex 119 must dissociate 1 equiv of H2 prior to olefin binding. Subsequent work by Lin and Peters established the olefin hydrogenation reactivity of bimetallic cobalt and nickel borylpincer complexes.174 Using the cyclohexyl-substituted bisphosphino-boryl ligand,175 reactions similar to those that afforded complex 118 instead produced bimetallic complex 120 in good yield, which already contains one borohydride and one bridging cobalt hydride derived from the hydridoborane ligand precursor. Complex 120 is also reactive toward H2, and addition of 1 atm of H2 again resulted in an immediate color change and the formation of product 121. This paramagnetic complex eluded characterization by X-ray diffraction, but the authors proposed a bis(hydridoborane) Co−(μ2-H)2−Co complex, as shown in Scheme 60. A dinickel analogue, 122, 8892


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Scheme 60

Scheme 61

Lu et al. have introduced a class of dinucleating supporting ligands capable of binding late transition metals and trivalent cations (i.e., AlIII, GaIII, InIII) simultaneously, facilitating interactions between and cooperative transformations involving transition metals and a variety of Lewis acids.177 Figure 5 shows

was also prepared. Treatment of the phosphine-substituted hydridoborane precursor with [Ni(COD)2] (COD = 1,5cyclooctadiene) afforded a dinickel complex lacking any hydride or borohydride moieties. Complex 122 is proposed to be a metal−metal bonded NiI−NiI species, and consistent with this formulation it is diamagnetic and displays a crystallographic Ni−Ni internuclear distance of 2.2421(9) Å. Complex 122 likewise cooperatively activates H2, with Scheme 60 also summarizing this reaction. H2 is added across each nickel−boryl bond, forming a bridging hydride structure (123), which is analogous to the proposed structure for cobalt complex 121. Diamagnetic 123 was amenable to characterization by NMR, and with the aid of selective decoupling experiments, deuterium labeling, and T1 measurements a nonclassical H2 adduct was ruled out and the structure shown for 123 was favored. Dicobalt complex 120 was found to be active for ethylene hydrogenation, and the rate law was consistent with a bimetallic active catalyst. Whereas dinickel analogue 122 was not active for olefin hydrogenation, a monometallic nickel−boryl complex was found to be active, although the involvement of cooperative nickel−boryl H2 activation in the catalytic cycle was not conclusively established. 4.2.3. H2 Cleavage Facilitated by Non-Boron Lewis Acids. Having shown above that many systems with appended borane or boryl functional groups can cooperatively activate H2, we now highlight a couple of recent examples where Lewis acids not containing boron were involved in H2 cleavage. Tobita et al. have prepared ruthenium complex 124 by treating [RuHCl(PPh3)3] with the free phosphino(silyl)xanthene ligand.176 Complex 124 can be dehydrogenated with styrene to access silyl complex 125 in high yield, as shown in Scheme 61. Addition of 1 atm of H2 to complex 125 regenerated complex 124 rapidly and quantitatively, via cooperative activation of H2 analogous to the activation of H2 by metal− boryl complexes described in section 4.2.2. Complex 124 was found to be an active catalyst for styrene hydrogenation, and after 2 days at 60 °C quantitative conversion was noted. The authors proposed a hydrogenation mechanism that involved the cooperative cleavage of H2 by the ruthenium center and the silyl moiety, analogous to the stoichiometric reaction outlined in Scheme 61.

Figure 5. General structure for nickel complex 126 featuring Lewis acidic group 13 metalloligands.

the general structure for a class of complexes 126 recently shown to be active for the hydrogenation of alkenes.178 Whereas the nickel complex without the bound Lewis acid and the version of 126 where M = AlIII were inactive catalysts for styrene hydrogenation, the version of 126 where M = Ga was modestly active, giving quantitative yields of ethylbenzene after 24 h at room temperature and a TOF of 2.4(1) h−1. Much lower but non-negligible activity (12 ± 5% yield after 24 h, TOF = 0.10(4) h−1) was observed when M = In. The authors propose two roles for the Lewis acids. The presence of the nearby electron-deficient Lewis acid reduces the electron density at the nickel center, facilitating the binding of H2 and stabilizing the nonclassical H2 adducts, which were characterized by NMR spectroscopy for M = Al and In. Once the H2 adduct enters the catalytic cycle, the authors propose that H2 cleavage generates a reactive HNi(μ-H)M intermediate, analogous to those described in section 4.2.1, although in this case no direct evidence for this type of structure is provided. 4.3. Intermolecular H2 Activation by Transition Metal/Lewis Acid Pairs

4.3.1. H2 Cleavage by a Platinum(0)/Borane Lewis Pair. Despite the numerous above-referenced examples of cooperative H2 activation and metal−hydride formation involving a tethered Lewis acid that is part of the supporting ligand, until recently there were no examples of cooperative bimolecular hydrogen activation involving a Lewis acid and a transition metal. Pringle, Wass, et al. showed that Pt0 complex 127, when partnered with B(C6F5)3, could activate a number of small molecules cooperatively.179 When H2 is the substrate, the platinum hydride complex 128 is formed, paired with the HB(C6F5)3− anion as shown in Scheme 62. Two possible pathways for H2 cleavage were proposed: (i) oxidative addition of H2 to form a platinum(II) dihydride followed by hydride transfer, and (ii) cooperative, FLP-like activation of H2 where 8893


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Scheme 62

The reactivity described in this section, whereby main group Lewis acids and transition metals cooperatively split H2, represents a relatively recent development that seems to be gaining popularity. Because the metal is formally oxidized by 2 electrons during such a reaction, this chemistry is mostly limited to late transition metals with high d-electron counts, although one example shows that a formally d2 ZrII complex168 can undergo analogous reactivity. In addition to being a fundamentally interesting mode of reactivity, it has been shown that heterolytic H2 cleavage with transition metals and Lewis acids can be incorporated into catalytic cycles, in particular olefin hydrogenation, and could be a way to ultimately replace precious metal catalysts with more earth-abundant versions (although of course the cost of the borane or borane-containing ligand must also be considered). Two potentially interesting future developments, which have yet to be implemented, would be to apply this reactivity to the discovery of hydrogenation catalysts for substrates other than alkenes, as well as to prepare chiral versions of these catalysts, which are capable of asymmetric hydrogenation.

H2 is simultaneously bound by 127 and B(C6F5)3 prior to H− H bond cleavage. The reaction of 127 with H2 by itself is slow and leads to numerous side products,180 and this observation coupled with preliminary DFT calculations lead the authors to favor possibility (ii), the cooperative FLP-like activation of H2, which is shown in Scheme 62. 4.3.2. H2 Cleavage by a Ruthenium/Imidazolium Lewis Pair. Inspired by the recently discovered monoiron hydrogenase, which cooperatively activates H2 using an organometallic iron cofactor and a methenyltetrahydromethanopterin (methenyl-H4MPT+) hydride acceptor,181 Meyer and collaborators developed a system employing a transition-metal complex and organic hydride acceptor to split H2 heterolytically.182 The relatively simple anionic ruthenium carbonyl complex K[CpRu(CO)2] (129), in combination with the imidazolium salt 1,3-bis(2,6-difluorophenyl)-2-(4-tolyl)imidazolium bromide (130), results in undesirable aromatic substitution by the anionic ruthenium complex on one of the aryl fluorides. However, an insoluble, amorphous polymeric form of 129, abbreviated here as 129′, is unreactive toward 130, but when H2 is introduced the hydride complex [HCpRu(CO)2] (131) and the corresponding imidazoline (132) were formed slowly, as depicted in Scheme 63. Under optimized

5. TRANSFORMATIONS OF METAL HYDRIDES PROMOTED BY LEWIS ACID INTERACTIONS WITH SUPPORTING LIGANDS 5.1. Background

In the preceding sections, we have seen numerous ways in which Lewis acids can mediate the formation of or transformations involving metal hydrides. In most of these preceding examples, the Lewis acid interacted with the metal hydride itself (section 2), or the substrate of interest (sections 3 and 4). In this section, we consider another potential role of the Lewis acid, which is to interact with an ancillary ligand, and indirectly influence the reactivity of a transition-metal hydride. There are other examples of complexes with supporting ligands, which can bind Lewis acids and modulate reactivity,183,184 including some that have been referenced previously,114,115,178 but in these previous examples the bound Lewis acid directly interacted with the transition metal and/or participated in substrate activation. Here, we focus on Lewis acid−base interactions in the secondary coordination sphere where the role of the Lewis acid is strictly to modulate the electronic structure of the metal center, and not to participate in the chemistry of interest. These types of secondary coordination sphere interactions have been exploited as a means of systematically tuning the reactivity of a complex, by modulating the electronic properties of the supporting ligand and by extension the metal center. As a representative example outside the field of transition-metal hydride chemistry, Bergman and Tilley recently showed that interaction of B(C6F5)3 with the external nitrogen atoms of a bipyrazine(bpz)-ligated platinum diaryl complex Pt(bpz)(4-CF3-Ph)2 induced a 64 000-fold increase in the rate of biaryl reductive elimination, as summarized in Scheme 64.185 Analogous interactions of supporting ligands with Lewis acids can also promote the formation and modulate the reactivity of transition-metal hydrides, as will be discussed in this section.

Scheme 63

conditions, 96% conversion of 130 was observed in 1 d, as monitored by 19F NMR. The authors noted some decomposition of [HCpRu(CO) 2], which precluded accurate determination of its yield, and the heterogeneous nature of the reaction impeded detailed kinetic analysis. Nevertheless, the results demonstrated successful cleavage of H2, with the organic imidazolium as the hydride accepting Lewis acid, and the Lewis basic anionic carbonyl complex 129′ as the proton acceptor.

5.2. Homogeneous Catalysts

5.2.1. Nickel-Based Olefin Polymerization Catalysts. Perhaps the most widely studied catalysts that can be activated by secondary coordination sphere Lewis acid−base interactions 8894


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Scheme 64

are nickel-based olefin polymerization catalysts. Equation 7 shows the general mechanism for olefin polymerization, which

involves sequential alkene insertion steps (i) followed by βhydride elimination (ii) to release the growing polymer chain.186 Thus, metal−hydride intermediates can be formed during olefin polymerization, and by controlling the rate of βhydride elimination relative to the insertion steps, the length of the polymer chain can be influenced. (It should also be noted that chain transfer to monomer can terminate polymerization, avoiding the formation of a metal−hydride altogether.) Many late-metal catalysts are selective for the formation of lowmolecular-weight oligomers instead of large polymers, due to the propensity of late metal alkyls to undergo facile β-hydrogen elimination (step (ii)) relative to insertion (step (i)).187,188 One of the best-known examples of a nickel oligomerization catalyst is the “SHOP” catalyst, which converts ethylene into a distribution of oligomeric products.189 In more recent work, a few groups have demonstrated that the reactivities of a number of SHOP-type catalysts and related derivatives are enhanced by Lewis acid additives. Bazan et al. spearheaded this effort with their discovery that nickel complex 133, which is selective for the dimerization of ethylene to 1butene, is much more active than the borane-free version.190 Coordination of Lewis acids reduces electron density at the metal center, which can have two effects: (i) the affinity for olefin substrate increases, leading to enhanced olefin insertion, or (ii) the rate of β-hydride elimination increases. As such, Lewis acid additives often lead to an increase in the rate of olefin consumption, and can either increase or decrease the selectivity for low-molecular weight oligomers. Figure 6 shows the structures of a number of such catalysts where carbonyl or cyano groups not directly coordinated to the metal center can interact with Lewis acids and influence reactivity with olefins. In addition to their original account of 133, Bazan et al. have also studied 134,191 a faster-initiating benzyl derivative of 133, and have pioneered nickel catalysts with α-iminocarboxamidato (135),192 pyridinecarboxamidato (136),193 and α-keto-βdiimine ligands (137),194,195 which can bind Lewis acids with their external carbonyl groups and enhance reactivity with olefins. In related work from other groups, Lee et al. developed

Figure 6. Structures of nickel olefin oligomerization/polymerization catalysts with activities that are enhanced by interactions with Lewis acids.

2-(alkylideneamino)benzoate complexes 138,196 Piers reported β-diketiminate nickel complexes that bind boranes through aryl-cyano substituents (139 and 140),197 and Chen described a phosphine-sulfonate ligated nickel catalyst (141) that binds B(C6F5)3 at the sulfonate.198 In all of these examples, the activities for olefin oligomerization/polymerization were affected, but the role that the Lewis acid played in the formation of a metal−hydride intermediate or the subsequent reactivity of the hydride was not determined. In contrast, recent related work by Rojas and collaborators directly implicated secondary coordination-sphere interactions as being responsible for increased rates of β-hydride elimination and the increased selectivity for ethylene dimerization under certain conditions.199 Complex 142, supported by a pyridyldimethoxybenzimidazole ligand, is active for ethylene oligomerization, with good selectivity for 1-butene at room temperature but very low activity (maximum TON = 83, TOF = 252 h−1 at 50 °C). In the presence of B(C6F5)3 and BF3·Et2O, a large enhancement in activity is observed. With 5 equiv of B(C6F5)3, a maximum TOF of 2684 h−1 was obtained at 50 °C, and with 20 equiv of BF3·Et2O, the TOF = 427 h−1 at 50 °C. More significantly, with BF3·Et2O as the additive, higher selectivity for 1-butene was observed than that for B(C6F5)3, and with either Lewis acid present (and no ethylene present) instantaneous isomerization of 1-octene occurred. The authors concluded that the Lewis acids can interact with the ether functional groups on the supporting ligand, and that these interactions facilitate the β-elimination step depicted in Scheme 65. 5.2.2. Rhenium Hydrogenation Catalysts Activated by Nitrosyl−Borane Interactions. Recent work by Berke et al. has demonstrated that Lewis acids can coordinate to nitrosyl 8895


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formally 14-electron, five-coordinate complex readily binds and inserts olefin substrate, initiating catalytic hydrogenation via an Osborne-type scheme.201 In this case, binding of the Lewis acid to the ancillary nitrosyl ligand both promotes the formation of the key hydride intermediate and enhances its reactivity, leading to very active and robust catalysis. In a follow-up study, the same group generalized the “catalytic nitrosyl effect” in hydrogenation chemistry by studying a five-coordinate, rhenium dinitrosyl complex 146, which can bind a variety of Lewis acids through the nitrosyl oxygen.202 As before, the function of the Lewis acid is to induce bending of the nitrosyl, which creates electron-deficient, coordinatively unsaturated hydride intermediates that can bind and insert alkenes and/or activate H2. This feature is especially important for rhenium catalysts, because rhenium tends to form stable, substitutionally inert 18-electron complexes that are not active catalysts, particularly for hydrogenation, which requires the formation of olefin πcomplexes and/or H2 σ-complexes. In this expanded work, the authors showed that complex 146 can be functionalized with neutral B(C6F5)3 or cationic Et+ and Et3Si+ Lewis acids, forming complexes 147a−c, all of which are active catalysts for the hydrogenation of olefins. Figure 7 gives the structures of

Scheme 65

oxygen atoms, turning on olefin hydrogenation activity by inducing a structural change and enhancing the electrophilicity of the catalytic center.200 Rhenium nitrosyl complex 143, a nonclassical hydrogen adduct, shows negligible hydrogenation activity at room temperature in the presence of 10 bar of H2 and ca. 104 equiv of substrate. In contrast, catalysis is activated by the presence of silylium cocatalysts, with turnover frequencies in excess of 104 h−1 under the same conditions. Through a combination of multinuclear NMR (1H, 31P, 29Si, 15 N) and DFT calculations, the authors postulated an activation pathway and catalytic cycle for this system, with the key steps summarized in Scheme 66. Stoichiometric reactions demonScheme 66

Figure 7. Structures of complexes 146 and 147a−c.

146 and 147a−c. In the absence of additional Lewis acid additive, the activity tracks in the same order as the ONO−LA binding strength, with 147b > 147c > 147a. By adding 5 equiv of Lewis acid cocatalyst, the performance of the borane- and silane-capped catalysts improved significantly, and in the case of the cyclohexyl-substituted versions of 147a and 147c a greater than one order-of-magnitude increase in TOF was observed. The authors propose two different catalyst activation pathways induced by the nitrosyl-bound Lewis acid. These reaction sequences are summarized in Scheme 67. The structurally characterized versions of complexes 147a−c all have linear nitrosyls in their structures, but the authors propose that bending of the nitrosyl initiates the activation. For cationic Lewis acid adducts 147b and 147c, [HPR3]+ is detected as a reaction product after catalysts, suggesting an activation sequence that is analogous to that in Scheme 66, where the Lewis acid induces bending of the nitrosyl, and the 16-electron intermediate 148 binds H2 to form 149. The coordinated H2 is acidified by virtue of binding to the Lewis acidic rhenium, and deprotonation by a coordinated phosphine forms hydride complex 150, which binds olefin and enters the catalytic cycle. In contrast, when neutral borane-capped complex 147a is used as a catalyst, no phosphonium byproduct is observed, implicating an alternative activation pathway. The same bentnitrosyl intermediate 148 is formed, but instead of H2 binding

strate that 143 reacts with in situ generated silylium Lewis acids to generate silylium-capped complex 144. The interaction of the nitrosyl oxygen with the silylium induces a bending of the nitrosyl, formally decreasing the electron count from 18 to 16.155 The enhanced electrophilicity of the rhenium center renders the coordinated H2 more acidic, promoting the formation of reactive hydride intermediate 145, which as a 8896


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Scheme 67

for the two compounds are similar. The bent nitrosyl shows a Re−NO angle of ∼130° in each case, with the bent NO bond length observed to be 1.30 Å in both complexes, a full 0.13 (152) or 0.27 (153) Å longer than the linear NO distance in the same molecule. Complexes 152 and 153 were active catalysts for the hydrogenation of 1-hexene, with activity increased in the presence of 5 equiv of the Lewis acid cocatalyst, although the most noteworthy aspect of this publication was the first conclusive demonstration that Lewis acids can induce nitrosyl bending, which the authors posit is responsible for the enhanced hydrogenation activity of the Lewis acid-capped complexes.

phosphine dissociation forms the highly reactive 14-electron species 151, which can bind olefin and enter the catalytic cycle. Notwithstanding the differences in the two pathways, the role of the Lewis acid in each case is to increase the electrophilicity of the hydride complex and permit the binding of H2 and olefin, which is critical for catalysis. Stable bent nitrosyls of rhenium were observed when the hydride in complex 146 is replaced with a chloride prior to Lewis acid binding, furnishing complexes 152 and 153, which contain both a linear and a bent nitrosyl.203 Figure 8 depicts the chemical structures of 152 and 153, as well as key structural metrics of the linear and bent nitrosyls. In comparing the two crystal structures of 152 and 153, the longer Re−N and shorter NO distances in the latter are consistent with weaker back bonding in cationic 153, but in general the structural metrics

5.3. Stoichiometric Reactions

5.3.1. Borane-Protected Hydrogenase Models. In addition to the catalytic reactions described in section 5.2, there are recent examples of stoichiometric reactions involving transition-metal hydrides, which are promoted by remote binding of Lewis acids on the periphery of the metal complex. Hydrogenases are natural enzymes that interconvert protons and H2, and several synthetic chemistry groups have targeted structural and functional models of the hydrogenase active site, which can contain iron, diiron, or iron−nickel centers depending on the class of the enzyme.204 Manor and Rauchfuss recently described a set of model complexes for [NiFe] hydrogenases, and were able to turn on biomimetic reactivity by coordinating the nitrogen atoms of bound cyanide ligands to Lewis acids.205 In the absence of Lewis acids, complexes of the type 154 (Scheme 68) are unreactive toward H2, on account of their coordinative saturation, and are also unable to be chemically or photochemically decarbonylated to produce more reactive structures; UV irradiation of 154 produces a complex mixture of products, and no reaction was observed with the decarbonylating agent Me3NO. Both complexes 154 react with a variety of Lewis acids, and the reactivity of B(C6F5)3-capped version 155 was explored in detail. In contrast

Figure 8. Chemical structure of bent nitrosyl complexes 152 and 153, and key structural metrics determined from single-crystal X-ray structures. 8897


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reaction, with side products and instability of the hydride isomers caused by protonation of an iron-bound cyanide.209 In addition to increasing the thermal stability of the hydride product 158, the bound boranes influence key physical properties. The pKa of the borane-free analogue of 158 is estimated to be 16, whereas the pKa of 158 was significantly diminished; with B(C6F5)3 a pKa of 10.8(2) was observed in MeCN, and with the weaker Lewis acid B(2,4,6-C6H2F3)3 the pKa was measured to be 13.5(1). The Lewis acids make the metal center more electron deficient, increasing the acidity of the bridging hydride. The bound Lewis acids also have an influence on the redox potentials of 158. Borane-free 158 could not be evaluated electrochemically because of its instability, but the identity of the borane did influence the reduction potentials of the borane-capped complexes. With B(C6F5)3 a reduction potential of −1.65 V vs Fc+/0 was noted, whereas the potentials were recorded at −1.74 V for B(2,4,6-C6H2F3)3 and BPh3. 5.3.2. Crown Ether-Functionalized Iridium Hydride Pincer Complexes. Polyether functionalized ligands have been known for some time,210−213 and in recent developments115,214 their impact on stoichiometric and catalytic transformations upon binding cationic Lewis acid additives has been highlighted. The Miller group has reported a crownether-substituted PCN pincer ligand, which modulates the reactivity of an iridium hydride complex.215 The hydridochloride complex 159 reacts with Na(BArF4) via halide abstraction, with the open coordination site being occupied by a hemilabile ether group in complex 160 (Scheme 70). Both

Scheme 68

to complex 154, complex 155 reacts readily with Me3NO, and in the presence of H2 heterolytic cleavage of H2 is observed, and bridging hydride complex 156 forms rapidly. The primary function of the borane Lewis acid is to promote labilization of the carbonyl, allowing for the binding and activation of H2, much in the same way as Lewis acids promote the formation of open coordination sites in the rhenium hydrogenation catalysts described in section 5.2.2. Furthermore, the boranes permit access to anionic FeNi hydrides, which are oxidized at milder potentials than neutral or cationic derivatives,206,207 and make them promising candidates for synthetic H2 oxidation catalysts. The same group also showed that binding boranes to diiron hydrogenase models can influence the formation and properties of biologically relevant hydride complexes.208 Borane-capped complex 157 protonates readily with HCl to form hydridebridged complex 158, which exists as a mixture of isomers, Scheme 69. The hydride products are stable at room temperature, and over the course of several days the isomeric ratio changes but no decomposition is noted. In contrast, the protonation of the borane-free analogue of 157 is not a clean

Scheme 70

coordinated ethers in 160 can be displaced by donor ligand like MeCN, and upon their displacement sodium cations can be incorporated into the crown ether, inducing a distinct change in the 1H chemical shift for the hydride. The reactivity of the hydride is also influenced by added cations. In CD2Cl2/Et2O solvent mixture at room temperature, introduction of complex 160 to 1 atm of D2 leads to gradual conversion to the deuterated analogue 160-D. In the absence of any cationic additives, the half-life for this transformation is 160 h. Introduction of M(BArF4) (M = Li, Na) into the reaction mixture leads to dramatic increases in the rate of H/D exchange. With Na+, the half-life decreases to 8 h when 0.3 equiv is added, and drops further to 2 h when 1.2 equiv is present. The rate enhancement with added Li+ is even more dramatic, and after addition of only 0.4 equiv of Li+ the t1/2 drops to 40 min. Notably, complex 160 was the only observed species by NMR in these experiments, with no intermediates detectable. This suggests that cation-induced dissociation is kinetically accessible but not thermodynamically favorable, indicating a mechanism summarized in Scheme 71 involving dynamic hemilability where the cation binding and ether decoordination are rapid and reversible. Similar to the examples in sections 5.2.2 and 5.3.1, the role of the Lewis acid in this work is to induce lability of a supporting ligand, opening a

Scheme 69

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aryl, affording 162. In contrast to this complex reactivity, treatment of 161 with Zn(OTf)2, La(OTf)3, or Sc(OTf)3 results in clean protonation of the iridium(I) complex, yielding cationic iridium(III) hydride complex 163. The pendant crown ether moiety is not a prerequisite for this type of reactivity, as the analogue of 161 where the aza-crown is replaced with NEt2 is protonated by water in the presence of Ca(OTf)2 or La(OTf)3 to yield the corresponding iridium(III) hydride, and in the case of the latter the yield is quantitative, just as it is in Scheme 72. Here, it appears the crown ether is not required to promote protonation of the iridium(I) species, but instead the Lewis acid is simply coordinating and acidifying water, allowing the hydride to form.

Scheme 71


The use of external Lewis acid “triggers” to modulate reactivity has been around for some time and explored in several different contexts. Here, we highlighted a few selected examples where the remote Lewis acid additives influence the reaction chemistry of hydride complexes. One potential advantage of this approach is that it can in principle be applied to any number of stoichiometric or catalytic transformations, and by changing the identity of the Lewis acid it could be possible to optimize the same catalyst for different reactions or different substrates. Although at times the precise role of the Lewis acid is not known (see section 5.2.1), and unintended side reactions involving the Lewis acid can sometimes plague the chemistry (see section 5.3.2), further work in this area will likely reveal new transformations where Lewis acid-responsive supporting ligands play a key role. The work described in sections 5.2.2 and 5.3.1 is particularly promising, in that the promoting effects of Lewis acid additives were realized without the need to synthesize elaborate supporting ligands to accommodate the Lewis acid additive. Thus, it may be possible to develop other catalytic reactions, including those that do not invovle hydrides at all, where the enhanced electrophilicity or decreased formal electron count brought on by coordination of Lewis acids to cyanide, nitrosyl, or related sypporting ligands can promote the desired reactivity.

coordination site for reaction, the key difference here being that the association of the Lewis acid with the ancillary ligand is dynamic and not static. In subsequent work, Miller et al. examined the reactivity of iridium carbonyl complexes supported by the same crownether-appended pincer ligand described above.216 In this work, the Lewis acidic cations were found to influence the formation of iridium(III) hydride complexes, but further reactivity of these hydrides and the effects of Lewis acids on such reactions were not elaborated. When treated with metal halide salts NaI or LaI3, iridium(III) hydride complexes of the crown-ethersubstituted PCN pincer ligand either underwent halide substitution or reversibly bound the cation in the crown ether, but no transformations of the hydride were promoted by the Lewis acid additives. In contrast, iridium(I) carbonyl complex 161, which is stable toward water, is protonated by water in the presence of certain Lewis acidic cations. With LiOTf, the only reaction observed is association of the crown ether with the Li+ cation, but otherwise the complex is unchanged. Transformations at the metal center are observed when stronger Lewis acids are added, as summarized in Scheme 72. When Ca(OTf)2 is added, the aza-crown dissociates from the iridium center and is oxidized to an iminium, and the iridium concomitantly migrates to the other side of the pincer

6. LEWIS ACID-ASSISTED HYDROGEN ELIMINATION REACTIONS 6.1. Background

In this section, we review another class of reactions that results in the formation of a transition-metal hydride. Hydrogen elimination (often referred to as hydrogen abstraction or hydride elimination) reactions are involved in a number of important stoichiometric and catalytic reactions, and can commonly occur from the α or β position of metal alkyl217−219 or amide complexes220−222 as well as from the β position of alkoxide complexes.223,224 Most of these reactions are driven thermally, and are thermodynamically favorable. However, there are select examples of Lewis acids initiating or participating in hydrogen elimination reactions, and some of these transformations are key parts of catalytic cycles. There are examples of both α- and β-hydrogen elimination reactions in which Lewis acids play a key role.

Scheme 72

6.2. Lewis Acid-Promoted α-Hydrogen Elimination

6.2.1. AlMe3-Promoted α-Hydrogen Elimination from Alkylidene Complexes. Schrock et al. have extensively studied hydrogen elimination reactions of metal alkyls and alkylidenes, and some of these transformations can be initiated 8899


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cationic alkylidene is generated when pyridine is added to complex 167. 6.2.2. Borane-Induced α-Elimination from a Cyclopentadienyl Ligand. The examples above demonstrated Lewis acid-induced α-elimination from alkylidene complexes, and it is also possible, by a different mechanism, to promote αhydrogen elimination from a cyclopentadienyl ligand using boranes. (Note: Although the term α-hydrogen elimination is not typically used to describe this transformation, we term it as such here because the hydride originates from a carbon that is bonded directly to the metal through an η5-Cp interaction). Work by Braunschweig and Wagner showed that treatment of the tungsten hydride complex [Cp2WH2] with BRCl2 (R = tBu, i Pr) results in the formation of zwitterionic trihydride complex 168, where borane attaches to the Cp-ring and a hydrogen (formally a proton) migrates from the Cp to the tungsten center (Scheme 75), converting the dihydride starting material into a trihydride product.228

by aluminum alkyl reagents. In an earlier example of aluminumpromoted alkylidyne formation, the AlMe3 served as an alkylating agent, installing a methyl ligand from which two rounds of α-hydrogen elimination furnished the methylidyne.225 In contrast, when a preformed tantalum alkylidene complex [Ta(CHtBu) (dmpe)2Cl] (164) is treated with AlMe3, all aluminum−carbon bonds remain intact, and instead of alkylating the complex the AlMe3 serves as a Lewis acid, abstracting the chloride ligand and inducing α-hydrogen elimination.226 This transformation is summarized in Scheme 73. The resulting product is a 7-coordinate alkylidyne hydride Scheme 73

Scheme 75 complex [Ta(CtBu)H(dmpe)2(ClAlMe3)] (165), where the Lewis acid remains associated with the chloride ligand. Formation of alkylidyne hydrides from alkylidenes is favored when more loosely bound, poorer π-donor ligands are trans to the alkylidene, which leads to the conclusion that the role of the aluminum is to abstract the strongly donating chloride ligand and polarize the metal toward elimination. On the basis of Xray crystallography and spectroscopic comparisons with structurally related complexes, the authors conclude a pentagonal bipyramidal geometry. Sequestration of the aluminum by adding THF reforms complex 164, establishing the reversibility of the Lewis acid-induced α-hydrogen elimination. In related and more recent work on ruthenium, Stephan et al. demonstrated gallium-induced conversion of a pincer-supported ruthenium alkylidene complex to the corresponding alkylidyne hydride complex.227 This reaction is summarized in Scheme 74, and the role of the Lewis acid is once again to

More recently, Malcolm Green et al. generalized the reaction shown in Scheme 75 to other boranes and metals, and also proposed a mechanism for these transformations.229 In their work, [Cp2WH2] reacted with boranes of the type B(C6F5)2R (R = C6F5, Ph, H, or Cl) to form complex 169, which is analogous to complex 168. A mechanism was proposed, summarized in Scheme 76, whereby the borane undergoes exoScheme 76

Scheme 74

abstract a halide. Ruthenium complex 166 is prepared by substituting the POP pincer ligand onto a preformed ruthenium alkylidene precursor. The alkylidene is characterized by a downfield 1H NMR signal at 20.03 ppm, and upon treatment of 166 with GaX3 (X = Cl, Br) a new species forms in which the alkylidene 1H resonance is lost, and a new Ru−H resonance grows in at −7.91 ppm, characteristic of alkylidyne hydride complex 167. Complex 167 was characterized crystallographically, and unlike the structure of tantalum alkylidyne 165, no association was observed between the cationic alkylidyne complex and the anion, which forms from halide abstraction. Addition of Lewis bases reverses the reaction, with the hydride migrating back to the alkylidyne to reform the alkylidene. Complex 166 can be regenerated by adding a halide to complex 167 (in the form of NR4X), and a

addition to the Cp, forming a diene intermediate, which oxidatively adds to form 169. As above, this transformation results in proton transfer to the metal and formal oxidation by 2 electrons, and as such adding B(C6F5)3 to [Cp2MoH2], which is more difficult to oxidize than [Cp2WH2], only results in adduct formation between the hydride and the borane, as described in detail in section 2.1.5. Similarly, treating [Cp2TaH3] with B(C6F5)3 results in adduct formation, because the d0 complex cannot be oxidized further. The borane-induced α-elimination reaction also occurs for the d4 hydride complex [Cp2ReH], forming the dihydride 170 (Scheme 77). In contrast, addition of B(C6F5)3 to [Cp2ReCH3] results in a more complex reaction sequence to form product 171. A mechanism is proposed for this transformation, as outlined in Scheme 78, which involves 8900


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the hydride is first transferred to the Sm or migrates directly to the borane is not clear. 6.3.2. β-Elimination Reactions of Nickelalactones. In addition to the reduction reactions of CO2 described in section 3.4, reactions where CO2 is coupled with small organic molecules to form valuable commodity chemicals or precursors are potentially impactful on the future of sustainable chemical enterprise.25,232,233 One promising reaction that has emerged is the coupling of CO2 and ethylene to form acrylate, an important monomer for the polymerization industry, although developing homogeneous catalysts for this transformation remains a challenge.234,235 With early group VI transition metals, coupling of CO2 and ethylene is facile to form acrylates, but the strong oxophilicity of the metals precludes removal of the products from the metal’s coordination sphere.236−238 In contrast, late transition metals like nickel are less reactive in the initial coupling step, and β-hydride elimination from the nickelalactone intermediates is sluggish and only promoted under harsh conditions.239−241 Recently, Bernskoetter et al. have discovered that addition of Lewis acids to nickelalactone complexes can induce β-hydrogen elimination, as summarized in Scheme 80.242 Complex 173 was prepared, and treatment

Scheme 77

Scheme 78

the same α-hydrogen elimination step described for the hydride complex in Scheme 77. Oxidative α-elimination forms a methyl hydride intermediate, which reductively eliminates methane to form an unobserved zwitterionic intermediate, from which boron-to-rhenium transmetalation of C6F5 generates the final product.

Scheme 80

6.3. β-Hydrogen Elimination

6.3.1. β-Elimination from Pentamethylcyclopentadienyl Ligands. Along the same lines as described in the previous section, Lewis acids have been shown to interact with the Cp* ligand, albeit in a lanthanide complex. Evans et al. have studied the reactions of [Cp*3Sm], a catalyst for ethylene polymerization,230 with commonly used activators in the polymerization reactions.231 They found that treatment of [Cp*3Sm] with Al2Me6 resulted in methylation with transfer of one Cp* to the aluminum. In contrast, the reaction of [Cp*3Sm] with B(C6F5)3 resulted in the formation of [Cp*2Sm][κ3H,F,F′-B(C6F5)3] (172), along with tetramethylfulvene, which is described in Scheme 79. The reaction sequence is proposed to involve Lewis acid-assisted β-hydrogen elimination from a putative [Cp*2Sm(η1-Cp*)] intermediate, although the precise role of the Lewis acid and how it interacts to induce the elimination reaction is not discerned, and whether

with B(C6F5)3 initially gave adduct 174, where the borane interacts with one of the carboxylate oxygen atoms. Over the course of a few hours, isomeric complex 175 evolves as the major product, which is proposed to form by Lewis acidinduced β-hydrogen elimination followed by reinsertion in a 2,1-fashion. Thus, the role of the borane is to weaken the stable five-membered lactone chelate, facilitating elimination. Acrylate could be slowly formed by deprotonating complex 175 with strong organic bases, but the acrylate remained bound to the formally nickel(0) complex 176. Attempts to regenerate 174 from 176, using CO2 and ethylene, were unsuccessful, and catalysis was not observed. The same research group has also shown that the weaker Lewis acid Na+ can promote the rearrangement of nickelalactones that proceed via β-hydrogen elimination.243 In this report, complexes 177 and 178, supported by 1,2-bis(dicyclohexylphosphino)ethane and bis(dicyclohexylphosphino)methane, respectively, undergo rearrangement to complexes 179 and 180 in the presence of Na(BArF4), although the reaction does not go to completion and Keq is observed to be 0.28(2) for the rearrangement of 177

Scheme 79

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in this section are of interest from a fundamental standpoint and in select cases are relevant to catalysis. Many of the reactions described in this section were discovered >15 years ago and have not seemingly received attention since then. The Cp-activation chemistry described in section 6.2.2 could potentially be generalized even further, and may lead to unusual reactivity of the zwitterionic hydride complexes, although it does result in an increase in the coordination number at the metal and it is unclear if the high valent metal hydrides that result will have any notable chemical properties. In the work described in section 6.3.2, productive catalysis was not observed, and as highlighted in section 3 the recurring problem of stable adducts between reduced carbonyls and Lewis acids obviated catalysis. Even by switching to weaker alkali metal Lewis acids, the stability of the Lewis acid-stabilized nickelalactones was too great to overcome, and so it remains to be seen if Lewis acid-promoted β-hydrogen elimination can be incorporated into a catalytic cycle.

and 0.13(3) for the rearrangement of 178 (Scheme 81). Complex 177, with the ethylene-spaced diphosphine, was Scheme 81

observed to undergo partial conversion to the rearranged nickelalactone even in the absence of sodium cations. Nevertheless, a computational investigation established a plausible mechanism for the rearrangement and found that in the presence of Na+ all of the key intermediates were stabilized, including the high-energy olefin-hydride, which was stabilized by almost 9 kcal/mol. The mechanism, which involves the same four elementary steps with and without sodium present, is depicted in Scheme 82. Much the same as in Scheme 81, the

7. OTHER REACTIONS OF METAL HYDRIDES MEDIATED BY LEWIS ACIDS

Scheme 82

7.1. Background

In this section, we highlight a few other works relevant to the topic of this Review, which do not fall into any of the above reaction categories. This section includes select examples of hydride migration, C−H activation, and C−H reductive elimination reactions, which are all initiated by some interaction between a main group Lewis acid and a transition-metal hydride complex. 7.2. Hydride Migration

7.2.1. Borane-Mediated Hydride Migration in Disilazido Zirconium Compounds. In recent work from Sadow’s group, the reactivity of Cp-ligated zirconium silazido complexes with Lewis acids and Lewis bases was studied.245 A large suite of reactions were studied using complexes of the type [Cp2Zr(X)(N(SiHMe2)2)], where X is a hydride, alkoxide, triflate, or chloride ligand. Relevant to the focus of this Review, the reaction of hydride complex 181 with B(C6F3)3 was studied. When complex 181 is treated with B(C6F5)3, the cationic complex [Cp2Zr(N(SiHMe2)2)]+ (182) is formed quantitatively. Although the net reaction is simply extraction of the hydride ligand from 181 by the Lewis acid, which has been demonstrated in numerous systems,246−248 analogous reactions with alkyl analogues of 181 evince the plausibility of an alternative sequence, where β-hydride abstraction from silicon produces a silylium, with the hydride then migrating from zirconium to the silicon, as described in Scheme 83.

rearrangement involves β-hydrogen elimination followed by 2,1-insertion; the low-coordinate intermediates that precede and follow the olefin-hydride are stabilized by agostic interactions. DFT-computed energies, determined using a truncated model where the cyclohexyl groups are replaced with methyls, are also shown in Scheme 82. The numbers in parentheses refer to the energy values with sodium present, and those in brackets refer to the energy levels absent any Lewis acid. The reactions in this section were independently interrogated by computational methods, largely confirming the findings described above where Lewis acids, both boranes and Na+, decrease the barrier for lactone contraction via βhydrogen elimination but do not promote formation of acrylic acid.244

Scheme 83


Although seemingly less common than other classes of reactions described here, the hydrogen eliminations covered 8902


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7.3. C−H Activation and Elimination

Scheme 86

7.3.1. Deprotonation of Iridium Hydrides by Lewis Acidic Alkyls. Work by Bergman and Andersen has demonstrated that late transition-metal hydrides display some acidity and can be deprotonated by main group alkyl or aryl complexes.249 In further studies on this reaction, they proposed the possibility of a Lewis acid/base adduct between the iridium and the organometallic Lewis acid, which precedes the deprotonation reaction.250 One piece of evidence for this is the isolation of a stable adduct (183) of the iridium(III) hydride complex [Cp*Ir(PMe3)H2] with AlPh3, as shown in Scheme 84. Treatment of [Cp*Ir(PMe 3 )H 2 ] with can drive the equilibrium in Scheme 86 but at the expense of eventually forming zwitterionic product 189, which results when a second equivalent of BPh3 extracts another PMe3, and the ruthenium-hydride migrates to the third equivalent of BPh3. 7.3.3. Intermolecular C−H Activation by a Nickel Hydride/Lewis Acid Adduct. C−H activation is a very important elementary step in a number of valuable chemical transformations, and most frequently is mediated by secondand third-row transition metals,252−255 notwithstanding some notable recent developments.256 Liang et al. have found that pincer-ligated nickel(II) hydride complex [(PCP)NiH], which is unreactive toward benzene by itself, activates the C−H bond of benzene in the presence of B(C6F5)3, forming nickel phenyl complex 190 (Scheme 87) in low yields, along with C6F5

Scheme 84

MgPh2(THF)2 results in elimination of 1 equiv of benzene and the formation of a dimeric iridium−magnesium complex 184. With the more basic aluminum reagent AlEt3, double deprotonation of [Cp*Ir(PMe3)H2] occurs, and dimeric iridium−aluminum complex 185 forms. These latter two reactions are summarized in Scheme 85. This work shows Scheme 85

Scheme 87

complex 191.257 The authors observed an adduct between [(PCP)NiH] and B(C6F5)3, which could polarize the Ni−H bond and favor C−H activation, or it is possible that complete hydride abstraction could precede benzene activation. In any case, the fate of the hydrogen atoms (Ni−H and C−H) was not determined, and many mechanistic questions remain about this transformation. Cleaner formation of 190 was observed when an analogous nickel methyl complex was activated with AlMe3, but again the precise function of the Lewis acid was not unveiled.

that electron-rich late transition metals can coordinate to Lewis acids through the transition metal (and not through bridging hydride interactions as described in section 2.1.11), and that such adducts can be precursors to the elimination of C−H bonds via deprotonation of the hydride species. 7.3.2. Intramolecular C−H Activation Initiated by Triphenylborane. One common role that Lewis acids play in initiating reactivity is opening a coordination site either by direct ligand extraction or by rendering a ligand more substitutionally labile; this has been demonstrated in some of the previous work covered in this Review.200,202,203,206,215,226,227 In work by Berry et al., the ability of boranes to initiate intramolecular C−H activation by ligand extraction was demonstrated.251 The 18-electron silyl complex [Ru(PMe3)4H(SiMe3)] (186) reacts rapidly at room temperature with BPh3 to form a complex equilibrium involving 16-electron complex 187 and dihydride complex 188, where the SiMe3 group has undergone intramolecular C−H activation (Scheme 86). The equilibrium is solvent dependent and is more favorable in cyclohexane than in benzene, presumably because of the lower solubility of the Me3P·BPh3 byproduct in the former. Attempts to drive the reaction forward by using stronger Lewis acids like B(C6F5)3 resulted in nonspecific decomposition. Excess BPh3

8. CONCLUSIONS In this Review, we have outlined a number of different reaction types, which involve cooperative participation of transitionmetal hydrides and Lewis acids. We started in section 2 with a discussion of the variety of ways that metal hydrides and Lewis acids can form adducts with one another, showing that hydride complexes of many transition metals have characterized adducts with Lewis acids. The bulk of this Review focused on reaction chemistry of metal hydrides involving Lewis acids; some of these involve a direct interaction of the transition-metal hydride and the Lewis acid, but many involve interaction of the Lewis acid with a substrate molecule or supporting ligand. Reactions 8903


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discovery of new Lewis acid-promoted electrocatalysts for CO2 reduction and H2 oxidation, respectively. Whereas interactions between metal hydrides and Lewis acids have been recognized for some time (the oldest work cited herein appeared in 196351), much of the work in this Review is very recent, having been disclosed within the last 10 years, which speaks to the possibility of this field being ripe for future development. In particular, the work highlighted in sections 4 and 5 represents particularly recent findings that have a lot of potential for growth. The mode of H2 activation described in section 4, which thus far has been utilized catalytically only for olefin hydrogenation, could potentially be applied to other reactions involving H2. In addition to hydrogenation of other organic substrates like carbonyl compounds or nitriles, it is possible that Lewis acid-assisted heterolytic H2 cleavage could be a key elementary step in CO2 hydrogenation reactions, or in electrocatalytic H2 oxidation, a reaction in which “conventional” heterolytic H2 cleavage using transition metals and bases is an important step.148 The concept outlined in section 5, whereby external Lewis acids interact with supporting ligands to bring about changes in reactivity, could certainly be applied to a large number of transformations. Homogeneous catalyst design often involves careful design and extensive screening of supporting ligands, and using Lewis acid additives to alter ligand structure and electronic properties is rapid and simple to implement and could lead to more subtle and systematic changes to the properties of the catalyst. Moving forward, a combination of ligand design, catalyst screening, and experimental and theoretical mechanistic studies will be needed to continue to better understand and exploit the many ways that main group Lewis acids can influence the formation and reactivity of transition-metal hydride complexes. As the principles of these types of reaction become more mainstream, the research directions should become more targeted and the advances more systematic. We hope this Review systematizes the variety of elementary transformations that occur with metal hydrides and Lewis acids, and will contribute to an enhanced appreciation and understanding of the cooperativity between hydrides and Lewis acids.

where the hydride is either a reactant or a product have been described, but in all cases the reactivity is uniquely dependent on the presence of a Lewis acid, which promotes transformations that are otherwise thermodynamically and/or kinetically unfavorable. In section 3, we reviewed numerous examples where the Lewis acid could promote an unfavorable hydride transfer event, facilitate further reactions of the reduced carbonyl, or assist in the cleavage of the reduced carbonyl product from the catalytic center. In section 4, we summarized many examples of H2 addition enabled by Lewis acids, and several of the examples included first-row transition metals where the Lewis acid engendered precious-metal-like reactivity in metals where H2 binding and cleavage is otherwise not observed. Sections 5−7 described a variety of organometallic reactions where Lewis acids give rise to unique reactivity, some of which only occurs with Lewis acid participation. The sheer variety of the chemistry described here is significant, and even with the tightly defined scope of this Review (molecular transition-metal hydrides and main group Lewis acids), a number of reaction types spanning most of the transition metals were observed. The breadth of chemistry involving hydrides and Lewis acids expands well beyond what we have covered here. We have not included reactions involving exclusively main group hydride/Lewis acid pairs, as well as bimetallic systems involving two transition metals where one metal could be thought of as a Lewis acid, both of which are related to the general theme presented here. The reactivity described here presents several challenges and future research directions. In all of the work we covered, boranes, especially B(C6F5)3, are the most popular choice for a Lewis acid. While these fluorinated boranes are very reactive and can be valuable for certain transformations,248 they are rather expensive additives for large-scale processes and suffer from poor compatibility with polar reagents and solvents. Another limitation with boranes, which surfaced at various points in this Review, is the strong bonds between boron and oxygen that form with oxygen-containing substrates, which often hinders catalysis due to the formation of thermodynamic sinks or only allows isolation of boron-containing products. So one major challenge in this area is to design more platforms where desired reactions can be promoted by simpler Lewis acids, such as main group cations, which are inexpensive and can operate in a variety of solvent environments. Some promising work from the recent literature has met this challenge and was highlighted herein,79,81,115,143,144,178,215,216,243 Transfer for the Interaction ofbut there are still opportunities for continued development of ligand platforms and reaction conditions that can accommodate Lewis acids besides boranes. Another major challenge, which was highlighted in some of the sections, is that often the precise role of the Lewis acid is difficult to discern, especially in catalytic reactions. Many catalytic reactions involving hydride intermediates benefit from Lewis acid additives, and while in catalysis the most important objective is to develop efficient catalysts, a more clear understanding of the role that Lewis acids play could lead to a more rational development of catalytic schemes. Finally, electrocatalytic reactions were notably absent from the discussion in this Review, notwithstanding some notable examples of electrocatalytic CO2 reduction promoted by Lewis acids, which do not involve hydride intermediates.123−126 Many electrocatalytic reactions involve hydride intermediates, and one could envision the fundamentals outlined in sections 3 and 4 leading to the

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Ayan Maity was born in Lalat, a small town near Kolkata, India, on February 16, 1985. In 2006, he received his Bachelor of Science degree from Narendrapur Ramakrishna Mission Residential College, Kolkata, and obtained his Master of Science degree from Indian Institute of Technology, Madras (IITM), in 2008. After graduating from IITM, he moved to Cleveland, OH, and obtained his Ph.D. degree in 2014 from Case Western Reserve University, under the guidance of Thomas G. Gray. His Ph.D. work focused on organometallics synthetic methodology development relevant to OLED phosphors and nucleoside transport biomarkers. Currently, Ayan is conducting his postdoctoral research at the University of Houston under the supervision of Thomas S. Teets, where his research focuses on the development of transition-metal-based electrocatalysts enhanced by Lewis acid−base interactions. 8904


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Thomas S. Teets (born 1984) is currently an Assistant Professor of Chemistry at the University of Houston. He spent his childhood in Amherst, OH, and completed undergraduate studies at Case Western Reserve University in 2007, where he earned a B.S. degree in chemistry and did undergraduate research with Prof. Thomas G. Gray. He obtained his Ph.D. degree in inorganic chemistry from Massachusetts Institute of Technology in 2012. There he worked with Prof. Daniel G. Nocera on photochemical halogen elimination and the aerobic oxidation chemistry of late-transition-metal hydride compounds, and was supported by a graduate research fellowship from the Fannie and John Hertz Foundation. He then spent two years at California Institute of Technology as a postdoctoral scholar, working under the guidance of Prof. John E. Bercaw and Dr. Jay A. Labinger on the thermodynamics of cooperative heterolytic H2 cleavage mediated by rhenium carbonyl compounds with pendant organic bases. He began his independent career at UH in 2014, with research interests that include the design of new cyclometalated organometallic phosphors, the coordination chemistry of redox-active formazanate ligands, and homogeneous catalysis.

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