Nickel Hydride Complexes - Chemical Reviews (ACS Publications)

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Nickel Hydride Complexes Nathan A. Eberhardt and Hairong Guan* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States ABSTRACT: Nickel hydride complexes, defined herein as any molecules bearing a nickel hydrogen bond, are crucial intermediates in numerous nickel-catalyzed reactions. Some of them are also synthetic models of nickel-containing enzymes such as [NiFe]-hydrogenase. The overall objective of this review is to provide a comprehensive overview of this specific type of hydride complexes, which has been studied extensively in recent years. This review begins with the significance and a very brief history of nickel hydride complexes, followed by various methods and spectroscopic or crystallographic tools used to synthesize and characterize these complexes. Also discussed are stoichiometric reactions involving nickel hydride complexes and how some of these reactions are developed into catalytic processes.

CONTENTS 1. Introduction 2. Synthetic Methods 2.1. From Main Group Hydrides 2.1.1. MBH4 as the Hydride Source 2.1.2. MR3BH as the Hydride Source 2.1.3. LiAlH4 as the Hydride Source 2.1.4. Other Main Group Hydrides 2.2. Via an Oxidative Addition Process 2.2.1. Protonation with Acids 2.2.2. Oxidative Addition of H2 2.2.3. Oxidative Addition of C−H Bonds 2.2.4. Oxidative Addition of Si−H Bonds 2.2.5. Oxidative Addition of Other Bonds 2.3. Through Heterolytic Cleavage of H2A Nonoxidative Pathway 2.4. From Alkyl Metal Reagents 2.5. σ-Bond Metathesis with a Silane or Borane 2.6. From Sodium Formate 2.7. η1-Borane Complexes 2.8. η2-Silane Complexes 2.9. Dihydrogen Complexes 2.10. Agostic Nickel Alkyl Complexes 3. Characterization 3.1. Nuclear Magnetic Resonance (NMR) Spectroscopy 3.2. Infrared (IR) Spectroscopy 3.3. X-ray Crystallography 3.4. Neutron Diffraction Study 4. Stoichiometric Reactions 4.1. Proton Transfer (Acidity) 4.2. Protonation by Acids 4.3. Loss of Hydrogen via Reductive Processes 4.4. Halogen Exchange Reactions 4.5. Insertion of Alkenes and Alkynes 4.6. Hydricity of Nickel Hydrides 4.7. Insertion of CO and CS Bonds 4.8. Reactions with Boranes and Silanes © 2016 American Chemical Society

4.9. Other Miscellaneous Reactions 5. Catalytic Reactions 5.1. Isomerization of Alkenes 5.2. Oligomerization or Polymerization of Alkenes/Alkynes 5.3. Hydrogenation or Hydroarylation of Alkenes/Alkynes 5.4. Hydrosilylation and Related Hydrofunctionalization Reactions 5.5. Reduction of CO2 and Dehydrogenation of Formic Acid 5.6. Electrocatalytic Oxidation or Reduction Reactions 5.7. Other Miscellaneous Catalytic Reactions 6. Conclusions and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

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8406 8406 8406 8407 8408 8410 8412 8414 8416 8418 8418 8418 8418 8418 8418 8418

1. INTRODUCTION Transition metal hydride complexes play important roles in organometallic chemistry especially related to homogeneous catalysis. They often function as key intermediates for transferring proton (H+), hydrogen atom (H•), or hydride (H−) between molecules. A number of reviews and books on this topic have been published, including two early comprehensive surveys (for hydrides known before 1983)1,2 and more recent ones focusing on structures and bonding3,4 or catalytic applications of hydride complexes of a specific metal

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Special Issue: Metal Hydrides Received: April 25, 2016 Published: July 20, 2016 8373

DOI: 10.1021/acs.chemrev.6b00259 Chem. Rev. 2016, 116, 8373−8426

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Table 1. Nickel Hydride or Borohydride Complexes Synthesized Using MBH4a NiH and NiBH4 complex trans-(Cy3P)2Ni(H)Cl trans-(Cy3P)2Ni(H)X (X = Br, SCN) trans-(iPr3P)2Ni(H)Cl trans-(iPr3P)2Ni(H)(CN) trans-(Cy3P)2Ni(H)(NO3) [{(dippe)NiH}2(μ-H)](BPh4) [{(dcpe)NiH}2(μ-H)](BPh4) (tBuPCPtBu)NiH (iPrPNPiPr-Y)NiH (Y = Me, F) (iPrPNPiPr-Me)NiH [(CyPNHPCy)NiH](BPh4) [(CyPNMePCy)NiH](BPh4) [(CyPNHPCy)NiH](PF6) [(NS3tBu)NiH](BPh4) [{(Me2-TPA)Ni}2(μ-H)2](PF6)2 trans-(Cy3P)2Ni(H)(BH4) trans-(iPr3P)2Ni(H)(BH4) (κ3-triphos)Ni(BH4) (κ3-Tp*)Ni(BH4) [(MeTACN)Ni(BH4)(CH3CN)](BPh4) (RPOCOPR)Ni(BH4) (R = tBu, iPr, cPe) (iPrPNCNPiPr-Me)Ni(BH4) (tBuPCPtBu)Ni(BH4) trans-(cyclam)Ni(BH4)2 [(N6S2)Ni2(μ-BH4)](BPh4)

precursor

solvent

ref

trans-(Cy3P)2NiCl2 trans-(Cy3P)2NiX2 trans-(iPr3P)2NiCl2 trans-(iPr3P)2Ni(CN)2 Cy3P, Ni(NO3)2·6H2O (dippe)NiBr2, NaBPh4 (dcpe)NiBr2, NaBPh4 (tBuPCPtBu)NiCl (tBuPCPtBu)NiBr (iPrPNPiPr-Y)NiCl (iPrPNPiPr-Me)NiBr [(CyPNHPCy)NiBr]Br, NaBPh4 Cy PNMePCy, Ni(diglyme)Br2, NaBPh4 [(CyPNHPCy)NiBr]Br, NaPF6 [(NS3tBu)NiCl](BPh4) [(Me2-TPA)Ni(NO3)](NO3), NH4PF6 trans-(Cy3P)2Ni(H)Cl trans-(Cy3P)2Ni(H)(NO3) trans-(iPr3P)2Ni(H)Cl (κ2-triphos)NiCl2 (κ3-Tp*)NiX (X = Cl, Br, NO3) [(MeTACN)Ni(CH3CN)2](ClO4)2, NaBPh4 (RPOCOPR)NiCl (iPrPNCNPiPr-Me)NiCl (tBuPCPtBu)NiBrb (cyclam)NiLx (Lx = SO4, (ClO4)2, (BF4)2)b [(N6S2)Ni2(μ-ClO4)](ClO4), NaBPh4c

THF−EtOH benzene−EtOH Et2O−EtOH hexane−EtOH benzene−EtOH EtOH EtOH benzene−EtOH benzene−EtOH THF−EtOH THF CH3OH THF−CH3OH THF−CH3OH THF H2O−CH3OH acetone−EtOH acetone−EtOH acetone−EtOH THF−EtOH CH3CN CH3CN toluene THF−CH3OH THF THF CH3CN

24, 25 25 25 26 26 31 31 46, 47 45 60 39 40 40 40 53 54 27 28 27 33 34 35 43 42 45 55 57

a

Unless otherwise mentioned, NaBH4 was used as the hydride source. Abbreviations for the ligands: dippe = iPr2PCH2CH2PiPr2, dcpe = Cy2PCH2CH2PCy2, NS3tBu = N(CH2CH2StBu)3, Me2-TPA = bis((6-methyl-2-pyridyl)methyl)(2-pyridylmethyl)amine, triphos = (Ph2PCH2)3CCH3, Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, MeTACN = 1,4,7-trimethyl-1,4,7-triazacyclononane, cPe = cyclopentyl, cyclam = 1,4,8,11tetraazacyclotetradecane; for structures of pincer and N6S2 ligands, see Schemes 5 and 8, respectively. bLiBH4 was used. cnBu4NBH4 was used.

(e.g., Fe5 and Zn6). General discussions of hydride as a ligand are available in organometallics textbooks.7,8 The chemistry of nickel hydride complexes has received increasing attention in recent years but, to our knowledge, has not been previously reviewed. The first time that nickel hydride complexes appeared in the literature was in a 1959 paper by Green, Street, and Wilkinson, who described “...a borohydride reduced solution of tripropylphosphine nickel chloride in tetrahydrofuran gives a high-field low intensity peak in the NMR spectrum attributable to the formation of a hydride species...”9 During the several decades following that report, studies of new nickel hydride complexes were sporadic, as suggested by limited discussion of these compounds in review articles.1−3 However, the past two decades have witnessed a tremendous growth of new structures and reactivity of nickel hydride complexes. The interests in this class of compounds have been primarily driven by the need to develop catalytic processes using earth-abundant metals including nickel,10−13 utilize dihydrogen as an energy carrier,14 and understand the active sites of nickel-containing enzymes such as [NiFe]hydrogenase15−17 and methyl-coenzyme M reductase.18 The main objective of this review is to provide readers with an up-to-date (January 2016) summary of the chemistry of nickel hydride complexes, which are defined here as molecules bearing a nickel hydrogen bond including those with a threecenter two-electron bond. The term nickel hydride has also been used in the literature to describe alloys made of nickel and hydrogen or a type of rechargeable batteries.19,20 These

materials as well as nickel nanoparticles with Ni−H bonds on the surface are not subjects for discussion here. This review is divided into four main sections: (1) methods to synthesize nickel hydride complexes, (2) spectroscopic and crystallographic tools to study these compounds, (3) stoichiometric transformations involving nickel hydrides, and (4) their catalytic applications.

2. SYNTHETIC METHODS In the 1970s and 1980s, the chemistry of nickel hydride complexes lagged way behind that of many other metal hydride complexes, largely due to the limited availability of well-defined systems. Early attempts to isolate nickel hydride complexes using the protocols well established for palladium and platinum analogs often failed, which was attributed to the low thermal stability for the nickel hydrides.9,21 The exploration of new ligand scaffolds and polydentate ones in particular has later on expanded the inventory of nickel hydrides and improved the landscape of this research field. Different synthetic strategies, including the failed ones, are detailed in the following subsections. The sequence of discussion in each subsection is as follows: monodentate ligand systems, bidentate, tridentate, etc. For those who wish for a quicker search of synthetic procedures, nickel hydride complexes that have been isolated (on a preparation scale) and characterized are tabulated in Tables 1−8. 8374


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2.1. From Main Group Hydrides

Scheme 2. Synthesis of [{(Diphosphine)NiH}2(μ-H)](BPh4)

A now frequently used method to synthesize nickel hydride complexes is through the reaction of a nickel halide, pseudohalide, or related complex with a main group hydride. The majority of these reactions proceed via a simple ligand substitution process, although in some cases Ni−H bond formation could be accompanied by reduction of the nickel center. This synthetic method could be advantageous, especially when nickel halide complexes or their derivatives are readily accessible. 2.1.1. MBH4 as the Hydride Source. Sodium borohydride is a relatively mild source of H− for transition metals.22 The success in making well-defined nickel hydride complexes from NaBH4 depends greatly on the supporting ligands. Simple Ni(II) salts (e.g., NiCl2, Ni(OAc)2) react with NaBH4 in aprotic solvents to form black precipitates of nickel boride and/ or some soluble but ill-characterized species that could be nickel hydrides.23 This material is often utilized to catalyze hydrogenation or reduction reactions, where a hydride intermediate might also be involved. One of the first stable, isolable nickel hydride complexes, reported by Green and Saito in 1969, bears two tricyclohexylphosphine (Cy3P) ligands.24 In that study, trans-(Cy3P)2NiCl2 was treated with 1 equiv of NaBH4, which resulted in trans-(Cy3P)2Ni(H)Cl in 46% isolated yield. This particular hydride is remarkably stable; when exposed to air, solid samples are stable for several hours, but in solution it decomposes readily. The synthetic method can be extended to nickel bromide, thiocyanate, and cyanide complexes as long as a bulky phosphine Cy3P or iPr3P is present (Scheme 1).25,26 The remaining chloride of trans-

The r eaction o f (κ 2 -t riph os)NiCl 2 (t riph os = (Ph2PCH2)3CCH3) with NaBH4 highlights the importance of the geometry (Scheme 3). The first equivalent of NaBH4 Scheme 3. Reaction of (κ2-Triphos)NiCl2 with NaBH4

reduces Ni(II) to Ni(I), resulting in a tetrahedral complex, (κ3-triphos)NiCl.32 The second equivalent of NaBH4 replaces the chloride of (κ3-triphos)NiCl with BH4−.33 A similar observation has been made to the reaction of (κ3-Tp*)NiX (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate; X = Cl, Br, or NO3) with NaBH4.34 Another example involving a C3symmetric tridentate ligand is [( Me TACN)Ni(κ 2 -BH 4 )(CH3CN)](BPh4) (MeTACN = 1,4,7-trimethyl-1,4,7-triazacyclononane), which has been recently made from an acetonitrile solution of [(MeTACN)Ni(CH3CN)2](ClO4)2 mixed with NaBH4 followed by salt metathesis with NaBPh4 (Scheme 4).35 Scheme 4. Synthesis of [(MeTACN)Ni(κ2BH4)(CH3CN)](BPh4)

Scheme 1. Synthesis of trans-(R3P)2Ni(H)X

(R3P)2Ni(H)Cl (R = Cy, iPr) can be displaced by other X-type ligands including Br−, I−, CN−, NO2−, SCN−, and OCN−.25−27 The nitrate ligand of trans-(Cy3P)2Ni(H)(NO3) (prepared in one pot from Cy3P, Ni(NO3)2·6H2O, and NaBH4) is more labile, providing a convenient entry to cationic nickel hydride complexes of the type trans-[(Cy3P)2Ni(H)(L)]BPh4 (L = pyridines, pyrazole, or imidazole).26 Both trans-(R3P)2Ni(H)Cl and trans-(Cy3P)2Ni(H)(NO3) react with NaBH4 further to yield trans-(R3P)2Ni(H)(BH4).27,28 The steric properties of Cy3P and iPr3P are crucial in stabilizing the hydride complexes. It was suggested that these ligands could effectively protect the Ni−H moiety and prevent geometric rearrangement of nickel from square planar to tetrahedral.24 Consistent with this rationale, synthesis of a less sterically hindered (R3P)2Ni(H)Cl (R = nPr, nBu, Ph, or PhCH2) from (R3P)2NiCl2 and NaBH4 has been unsuccessful. 9,29,30 A similar reaction of (dppe)NiCl2 (dppe = Ph2PCH2CH2PPh2) with NaBH4 also failed to produce a stable nickel hydride complex.29 However, with a chelating a lk y l p h o s p h i n e li g a n d s u c h a s d i p p e ( d i p p e = i Pr2PCH2CH2PiPr2) and dcpe (dcpe = Cy2PCH2CH2PCy2) and in the presence of an excess of NaBH4 (2.6 equiv), nickel bromide complexes are converted cleanly to Ni(II) trihydride complexes (Scheme 2).31

When a tridentate ligand binds to nickel in a meridional configuration, especially with a rigid backbone, it can provide a protective scaffold for the Ni−H bond, thus increasing the likelihood of isolating the nickel hydride species. A number of these so-called pincer-ligated nickel hydride complexes have been successfully made using NaBH4 as the hydride donor. As illustrated in Scheme 5, NaBH4 transfers either H− or BH4− to nickel depending on the type of pincer ligand.36 Of particular note is the reactivity difference between (tBuPCPtBu)NiCl and ( tBu P O C O P tBu )NiCl (phosphine vs phosphinite). Bis(phosphinite)-based pincer complexes in general have more trans void space as a result of the P−M−P angle being contracted more toward the pincer backbone37,38 and, therefore, are more likely to accommodate the BH4 ligand. The PNP-pincer nickel complexes (both neutral and cationic) have comparatively large P−Ni−P angles (174.2−174.4°).39−41 In contrast, the P−Ni−P angle of (iPrPNCNPiPr-Me)Ni(BH4) is 165.19°,42 only slightly larger than those of (RPOCOPR)Ni(BH4) (161.49−163.14°).43 At 60−80 °C in the presence of Et3N, both (tBuPOCOPtBu)Ni(BH4) and (iPrPNCNPiPr-Me)Ni(BH4) lose BH3 to form (pincer)NiH, which can be converted back to the borohydride complexes with the addition of BH3· THF. Removing BH3 from (RPOCOPR)Ni(BH4) (R = iPr, cPe) is much more challenging, further demonstrating that a more 8375


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Scheme 5. Reactions of Nickel Pincer Halide Complexes with NaBH4a

a

Color code: donor atoms in red, substituents on the donor atoms in blue, key heteroatoms in magenta, and signature substituents from the backbone in green.

accessible nickel center favors the borohydride complex. Two recent studies have shown that (tBuPCPtBu)Ni(BH4) is the intermediate en route to (tBuPCPtBu)NiH.44,45 The earlier synthesis of (tBuPCPtBu)NiH by Shaw and co-workers did not exhibit the anticipated hydride resonance in the 1H NMR spectrum.46 Nevertheless, pure (tBuPCPtBu)Ni(BH4) has been isolated, albeit in low yield, from the reaction of (tBuPCPtBu)NiBr with LiBH4 in THF at ambient temperature.45 Synthesis of less bulky (RPCPR)NiH (R = Cy, iPr) from (RPCPR)NiCl requires a different hydride source (section 2.1.2).47 NaBH4 converts (RPCPR)NiCl to a species that has been suspected to be ( R PCP R )Ni(BH 4 ). 47 Interestingly, NaBH 4 reduces [(tBuPONOPtBu)NiCl]+Cl− to (tBuPONOPtBu)NiICl,48 which to some extent resembles the process forming (κ3-triphos)NiCl (Scheme 3). The MeNNNMe-pincer nickel complex (Scheme 5) developed by the Hu group is apparently unreactive toward NaBH4.49 Nickel hydride complexes bearing a tetradentate ligand are also known in the literature. The 1:1:1 mixture of NiY2·6H2O (Y = BF4, ClO4, NO3), N(CH2CH2PPh2)3(NP3), and NaBH4 generates a material with the formula [(NP3)NiHx]Y (x = 0.18−0.80), as determined from the analysis of the hydride content.50 Magnetic moment data further support that this material is a mixture of Ni(I) and Ni(II) species. Treatment of Ni(NO3)2·6H2O and NP3 with 2 equiv of NaBH4 reduces nickel further to (NP3)Ni(0).51 A related tripodal ligand system involving N(CH2CH2SR)3 (NS3R, R = iPr, tBu) has been studied to mimic the reactivity of nickel in acetyl-CoA synthase. Nickel hydride [(NS3tBu)NiH](BPh4) can be prepared from [(NS3tBu)NiCl](BPh4) and NaBH4 in THF.52,53 The same reaction conditions applied to the isopropyl derivative gives intractable products, once again demonstrating that bulky substituents are crucial in stabilizing the nickel hydride. Interestingly, nickel complexes bearing a dimethyl-substituted TPA (TPA = tris(2-pyridylmethyl)amine) ligand prefer an octahedral geometry (Scheme 6).54 The nitrate complex reacts with NaBH4 to give a hydride-bridged dinickel species rather than a trigonal bipyramidal, mononuclear nickel hydride complex as observed for [(NS3tBu)NiH](BPh4). An attempt to develop catalysts for hydrogen storage/release has led to the preparation of (cyclam)Ni(BH4)2 (cyclam = 1,4,8,11-tetraazacyclotetradecane).55 Grinding the solid−liquid

Scheme 6. Synthesis of [{(Me2-TPA)Ni}2(μ-H)2](PF6)2

mixture of (cyclam)NiLx and LiBH4 with a mortar and pestle yields a complex with two trans-BH4 ligands, which gradually isomerizes to the ionic product when dissolved in CH3CN (Scheme 7). As expected for the latter isomer, nickel is lifted Scheme 7. Synthesis of Nickel Borohydride Complexes Bearing a Tetradentate Ligand

from the coordination plane formed by the four nitrogen atoms, which is made possible by the high flexibility of the cyclam backbone. One might have anticipated that bis-chelating phosphine Ni(II) complex [(dppe)2Ni]2+ would exhibit similar reactivity. A study by Chatt, Hart, and Watson suggests that NaBH 4 reduces [(dppe) 2 Ni] 2+ and other [NiL 2 ] 2+ to (dppe)2Ni and NiL2.56 Dinuclear nickel(II) complexes stabilized by a macrocyclic hexaaza-dithiophenolate ligand (N6S2) have been studied for the formation of nickel borohydride complexes (Scheme 8).57 It is an unusual case in the sense that the chloride complex [(N6S2)Ni2(μ-Cl)]ClO4 shows no reactivity toward nBu4NBH4, presumably due to chloride binding too tightly to the nickel centers. Using a more labile bridging ligand such as ClO4− (introduced by treating the chloride complex with Pd(ClO4)2) circumvents the problem. Examples of nickel hydride or borohydride complexes made from MBH4 are listed in Table 1. The majority of these studies 8376


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formation of bridging hydride in this case would not be favorable.

Scheme 8. Synthesis of a Dinuclear Nickel Borohydride Complex

Scheme 10. Synthesis of (iPrDPDBFphos)Ni(H)X

use NaBH4 as the hydride source. There are few cases employing LiBH4 or nBu4NBH4 instead, but the advantages or reasons are not obvious.45,55,57 Solvents used for the synthesis are also included in Table 1, as they could influence the reaction outcomes significantly. Although solvent effects on a nickel system have not been explicitly studied, it has been shown in an iron system that the reaction of trans-(dmpe)FeCl2 (dmpe = 1,2-bis(dimethylphosphino)ethane) with NaBH4 gives trans-(dmpe)Fe(H)(BH4) in THF58 but trans-[(dmpe)Fe(H)(H2)]+ in alcohols.59 2.1.2. MR3BH as the Hydride Source. Replacing three hydride ligands of MBH4 with alkyl or alkoxide groups renders the remaining hydride exceptionally hydridic.22 Accordingly, MR3BH are attractive reagents for the synthesis of nickel hydrides, especially when borohydride complexes are not desired. On the other hand, MR3BH are more reducing than MBH4 and, therefore, more likely to reduce the nickel center. For example, the reaction of trans-(iPr3P)2NiCl2 with LiEt3BH (Super-Hydride) forms a mixture of trans-(iPr3P)2Ni(H)Cl and (iPr3P)2NiCl.61 Separating the hydride from the mixture is possible, although the isolated yield is not as high as that obtained from the NaBH4 method.25 In contrast, the related trans-(Bn3P)2Ni(H)Cl (Bn = benzyl) cannot be synthesized from trans-(Bn3P)2NiCl2 and NaBH4, but it can be prepared in good yield if LiEt3BH is used as the hydride donor.30 The chloride ligand of trans-(R3P)2Ni(H)Cl (R = Bn, iPr, Cy) is exchangeable with NaOPh, thus offering the opportunity to synthesize hydrides of the type trans-(R3P)2Ni(H)(OPh). Reactions of diphosphine-ligated nickel complexes with MR3BH have been known for many years. As early as in 1970, {Cy2P(CH2)nPCy2}NiCl2 (n = 2−4) were reported to react with NaMe3BH to generate dinuclear Ni(I)−Ni(I) complexes with two bridging hydride ligands (Scheme 9).62 A

Nitrogen-based bidentate ligands can also stabilize nickel hydride species, provided that the nitrogen substituents are sufficiently bulky. The diimine-ligated nickel complex is a rare case where borane derived from LiEt3BH still binds to the Ni− H moiety (Scheme 11).69 The anionic β-diketiminate (also Scheme 11. Synthesis of a Diimine-Ligated Nickel Dihydridoborate Complex

known as NacNac70) system maintains the Ni(II) oxidation state after initial hydride transfer from KEt3BH, which gives rise to a bridging nickel hydride (Scheme 12).71 Without heating or a suitable donor, elimination of H2 from {(MeNacNac)Ni}2(μH)2 is not favorable, thus allowing isolation of this hydride. This complex can be reduced by 1 or 2 equiv of KC8, resulting in a dinuclear or trinuclear nickel hydride complex with mixed valence of Ni(I) and Ni(II).72 Introducing tert-butyl groups to the ligand backbone forces the aryl groups to exert more steric pressure on the hydride. As a result, {(tBuNacNac)Ni}2(μ-H)2 becomes an elusive species; elimination of H2 is facile even in the presence of a weak donor like N2.73 Anilinotropones are another type of anionic bidentate ligand that has been studied for nickel chemistry. The hydride derivatives are key intermediates responsible for chain transfer in nickel-catalyzed polymerization of ethylene.74 The triphenylphosphine adduct (N,O)Ni(PPh3)H (N,O = 2,6-diisopropylanilinotropone) can be isolated from the nickel chloride complex and Na(MeO)3BH (Scheme 13) but decomposes at room temperature.75 A related nickel hydride supported by a bidentate diarylamido phosphine ligand has been synthesized from the chloride complex and LiEt3BH (Scheme 14).76 Bis-chelating phosphine nickel hydride complexes are often synthesized via protonation of (diphosphine)2Ni(0) species (section 2.2.1). A less frequently used method involves the addition of MR3BH to cationic [(diphosphine)2Ni]2+, which has a nickel center already in the +2 oxidation state. As exemplified in Scheme 15, [(iPrP2PhN2)2Ni](BF4)2 reacts with Na(MeO) 3 BH to form a 5-coordinate nickel hydride complex.77 A variety of pincer-ligated nickel hydride complexes have been synthesized from MR3BH and the corresponding nickel chloride or bromide complexes (Figure 1). Reaction conditions

Scheme 9. Synthesis of {(Diphosphine)Ni}2(μ-H)2

more recent synthesis of {(dcpe)Ni}2(μ-H)2 used LiEt3BH as the hydride donor.63 The isopropyl analogs {(dippp)Ni}2(μH)264 and {(dippe)Ni}2(μ-H)265 were synthesized similarly by treating the corresponding nickel chloride complexes with KEt3BH and LiEt3BH, respectively. Mononuclear nickel hydrides with dppe (or other commonly used diphosphine ligands) have been proposed for the reaction between (dppe)NiCl2 and LiEt3BH; however, no spectroscopic evidence is available to support the presence of such species.66,67 A much more definitive example involves a conformationally flexible ligand that can span two trans positions (Scheme 10).68 Reductive elimination of HX and 8377


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Scheme 12. Reactions of β-Diketiminato Nickel Complexes with KEt3BH

hydride donor. The reactivity of cationic complex [(tBuPONOPtBu)NiCl]+Cl− (Scheme 5) is distinctively different from other pincer complexes. Instead of reducing Ni(II) to Ni(I) as observed with NaBH4, the first equivalent of LiEt3BH attacks the carbon para to the pyridine nitrogen to give (tBuPONOPtBu-H)NiCl, where the oxidation state of nickel is unchanged while the pyridine ring is dearomatized. The second equivalent of LiEt3BH attacks nickel to give (tBuPONOPtBuH)NiH.48 The phosphinimine-based cationic complexes [( P NN R N P )NiCl]PF 6 undergo ligand substitution with LiEt3BH more straightforwardly, forming cationic nickel hydride complexes (Figure 1).78 The pyrrole-based nickel pincer complexes have been studied by a number of research groups. With bulky or medium-sized alkyl groups on the phosphorus, nickel hydride complexes can be made using either LiEt3BH79 or NaEt3BH.80 Synthesizing the phenyl derivative using the same method is, however, problematic. The hydride complex is observable at −40 °C by 1H NMR but loses H2 readily at ambient temperature (Scheme 16).81 The reaction of phenyl-substituted PSiMeP-pincer complex (PhPSiMePPh)NiCl with LiEt3BH also yields a dimer, although the bridging atom is silicon instead of phosphorus.82 However, the cyclohexyl analog (Figure 1) is readily synthesized from the chloride complex and LiEt3BH.83 Several other nickel hydride complexes bearing a polydentate or polyhapto ligand have been made using the MR3BH route. Coordinatively saturated complexes CpNi(IMes)H and CpNi(IPr)H (IMes and IPr are N-heterocyclic carbenes or NHCs) are accessible from the reaction of nickel chloride complex with KEt3BH84 and LiEt3BH,85 respectively (Scheme 17). Bis(βdiketiminato)pyridine-ligated dinickel hydride complex (PYP)Ni(μ-H)Ni has a unique Ni(II)−H → Ni(I) core, which can be generated from (PYP)Ni(μ-Br)NiBr and KEt3BH (Scheme 18).86 It has been proposed that the initial product is a Ni(II)−μ-H−Ni(II)−H complex, which loses the terminal hydride as H2. 2.1.3. LiAlH4 as the Hydride Source. In addition to BH4− and R3BH−, hydrides of other main group elements are suitable for the synthesis of nickel hydride complexes (Table 3). In some cases, they could be more effective than the boron-based hydrides. Bis(phosphinite)- or bis(phosphinous amide)-based nickel pincer complexes have the tendency to form borohydride complexes when using NaBH4 as the hydride source (section 2.1.1). If the phosphorus substituents are bulky enough (e.g., t Bu), BH3 could be removed by heating in the presence of Et3N.42,43 Using MR3BH instead will probably generate terminal hydride complexes as observed with the palladium analogs,88 although such experiments have not been reported. In addition to these two types of main group hydrides, LiAlH4

Scheme 13. Synthesis of an Anilinotropone Ligated Nickel Hydride Complex

Scheme 14. Synthesis of a Diarylamido Phosphine-Ligated Nickel Hydride Complex

Scheme 15. Synthesis of Cationic Hydride [(iPrP2PhN2)2NiH](BF4)

Figure 1. Nickel pincer hydride complexes synthesized from MR3BH.

are listed in Table 2, and the structures of these hydrides are viewable in Figure 1. As mentioned earlier, it is not possible to convert (RPCPR)NiCl (R = Cy, iPr) to (RPCPR)NiH using NaBH 4 , likely due to the formation of borohydride complexes.47 This is usually not an issue with LiEt3BH as the 8378


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Table 2. Nickel Hydride Complexes Synthesized Using MR3BHa precursor

H− source

solvent

ref

trans-( Pr3P)2NiCl2 trans-(Bn3P)2NiCl2 {Cy2P(CH2)nPCy2}NiCl2 (dcpe)NiCl2 (dippp)NiCl2 (dippe)NiCl2 (iPrDPDBFphos)NiX2 (N−N)NiBr2 (MeNacNac)Ni(μ-Br)2Li(THF)2 (N,O)Ni(PPh3)Cl (N,P)Ni(PMe3)Cl [(iPrP2PhN2)2Ni](BF4)2 (RPNPR)NiCl (RPCPR)NiCl (tBuPONOPtBu-H)NiCl [(PNNRNP)NiCl](PF6) (CyPNPyPCy)NiCl (tBuPNPyPtBu)NiBr (CyPSiMePCy)NiCl CpNi(IMes)Cl CpNi(IPr)Cl (PYP)Ni(μ-Br)NiBr

LiEt3BH LiEt3BH NaMe3BH LiEt3BH KEt3BH LiEt3BH NaEt3BH LiEt3BH KEt3BH Na(MeO)3BH LiEt3BH Na(MeO)3BH LiEt3BH LiEt3BH LiEt3BH LiEt3BH LiEt3BH NaEt3BH LiEt3BH KEt3BH LiEt3BH KEt3BH

THF THF toluene benzeneb toluene benzeneb THF THF Et2O Et2O THF CH3CN THF tolueneb pentaneb CH2Cl2b toluene tolueneb THF THF THF Et2O

61 30 62 63 64 65 68 69 71 74 76 77 87 47 48 78 79 80 83 84 85 86

hydride complex i

trans-( Pr3P)2Ni(H)Cl trans-(Bn3P)2Ni(H)Cl {(Cy2P(CH2)nPCy2)Ni}2(μ-H)2 (n = 2−4) {(dcpe)Ni}2(μ-H)2 {(dippp)Ni}2(μ-H)2 {(dippe)Ni}2(μ-H)2 (iPrDPDBFphos)Ni(H)X (X = Cl, Br) (N−N)Ni(μ-H)2BEt2 {(MeNacNac)Ni}2(μ-H)2 (N,O)Ni(PPh3)H (N,P)Ni(PMe3)H [(iPrP2PhN2)2NiH](BF4) (RPNPR)NiH (R = iPr, Cy) (RPCPR)NiH (R = iPr, Cy) (tBuPONOPtBu-H)NiH [(PNNRNP)NiH](PF6) (R = H, Me) (CyPNPyPCy)NiH (tBuPNPyPtBu)NiHc (CyPSiMePCy)NiH CpNi(IMes)H CpNi(IPr)H (PYP)Ni(μ-H)Ni

i

a

Abbreviations for the ligands: Bn3P = tribenzylphosphine, dcpe = Cy2PCH2CH2PCy2, dippe = iPr2PCH2CH2PiPr2, iPrDPDBFphos = 4,6-bis(3diisopropylphosphinophenyl)dibenzofuran, N-N = (iPr2C6H3N)(MeCNC6H3iPr2)2, N,O = 2,6-diisopropylanilinotropone, N,P = o-(2,6Me2C6H3)NC6H4PiPr2, iPrP2PhN2 = 1,5-diphenyl-3,7-diisopropyl-1,5-diaza-3,7-diphosphacyclooctane, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; for structures of MeNacNac, pincer, and PYP ligands, see Scheme 12, Figure 1, and Scheme 18, respectively. bA small amount of THF is present because LiEt3BH is in a THF solution. cSome boron-containing byproduct is present.

Scheme 16. Reaction of Phenyl-Substituted PNPyP-Pincer Nickel Complexes with LiEt3BH

proves to be very effective in converting nickel chloride complexes to the terminal hydride complexes (Figure 2). For the resorcinol-derived pincer system, the compatible phosphorus substituents are usually bulky or medium-sized alkyl groups but not phenyl groups.89,90 The pincer backbone can be modified by introducing different substituents to the aromatic ring, 91 incorporating a cyclohexyl ring, 92 or using a phosphinous amide.42 The synthesis is typically carried out in toluene or benzene, and filtration through a short pad of Celite can separate the hydride from the unreacted LiAlH4. The cationic complex [(tBuPONOPtBu)NiCl]+Cl− shown in Scheme 5, though also a bis(phosphinite)-based pincer complex, reacts with LiAlH4 to form (tBuPONOPtBu)NiICl instead of a hydride species.48 For the MeNNNMe-pincer nickel complex described earlier (Scheme 5), the LiAlH4 route only leads to intractable products.49 Beyond pincer complexes, LiAlH4 has been used for the synthesis of nickel hydride clusters. The reaction of CpNi(NO) with AlCl3 and LiAlH4 leads to the isolation of a paramagnetic nickel cluster Cp4Ni4H3.93 2.1.4. Other Main Group Hydrides. Hydrides of other Group 13 elements are not as popular as borohydrides and aluminum-based hydrides for the synthesis of transition metal hydrides. Nevertheless, the reaction of Cp2Ni with (IMes)InH3 provides CpNi(IMes)H in good yield (Scheme 19).94 This study suggests that (IMes)InH3 could be a useful reagent for efficient transfer of both hydride and carbene ligands to a metal. For Group 14 elements, trimethylgermane (Me3GeH) has been used to prepare palladium hydrides from palladium chloride complexes. 21 The same method applied to (Et3P)2NiBr2 produces a hydride species based on the infrared spectrum (a band at 1937 cm−1) and its ability to reduce CCl4

Scheme 17. Synthesis of CpNi(NHC)H from MR3BH

Scheme 18. Synthesis of a Bis(β-diketiminato)pyridineLigated Dinickel Hydride Complex

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Table 3. Nickel Hydride Complexes Synthesized Using LiAlH4 or Other Main Group Hydridea hydride complex R O O R

t

i

precursor c

( P C P )NiH (R = Bu, Pr, Pe) (tBuPOCOPtBu-Y)NiH (Y = Me, MeO, tBu) (tBuPOCsp3OPtBu)NiH (iPrPNCNPiPr-Me)NiH Cp4Ni4H3 CpNi(IMes)H (dcpm)2Ni2Cl2(μ-H) (dippm)2Ni2Cl2(μ-H) trans-(iPr3P)2Ni(H)Cl (dcpm)2Ni2Cl2(μ-H) (α-diimine•−)2Ni2(μ-H)2 {Cp#Ni(μ-H)}2

R O O R

( P C P )NiCl (tBuPOCOPtBu-Y)NiCl (tBuPOCsp3OPtBu)NiCl (iPrPNCNPiPr-Me)NiCl CpNi(NO), AlCl3 Cp2Ni (dcpm)NiCl2 (dippm)NiCl2 trans-(iPr3P)2NiCl2 (dcpm)NiCl2 (α-diimine)NiBr2 Cp#Na, NiBr2·DME

H− source

solvent

ref

LiAlH4 LiAlH4 LiAlH4 LiAlH4 LiAlH4 (IMes)InH3 n Bu3SnH n Bu3SnH NaH NaH or LiH NaH NaH

toluene benzene toluene toluene THF toluene toluene benzene THF toluene Et2O THF

89, 90 91 92 42 93 94 95 96 61 95 97 98

a Abbreviations for the ligands: IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, dcpm = Cy2PCH2PCy2, dippm = iPr2PCH2PiPr2, α-diimine = {(2,6-iPr2C6H3)NC(Me)}2, Cp# = tetraisopropyl cyclopentadienyl, DME = 1,2-dimethoxyethane; for structures of pincer ligands, see Figure 2.

Figure 2. Nickel pincer hydride complexes synthesized from LiAlH4.

trans-(iPr3P)2NiCl2 and NaH gives a mixture of trans(iPr3P)2Ni(H)Cl and (iPr3P)2NiCl, which is analogous to the reaction of trans-(iPr3P)2NiCl2 with LiEt3BH (section 2.1.2).61 The bridging hydride (dcpm)2Ni2Cl2(μ-H) shown in Scheme 20 can also be made by replacing nBu3SnH with NaH or LiH and under heating; however, long reaction time leads to the cleavage of dcmp to form (Cy 2 MeP)Ni(μ-PCy 2 ) 2 Ni(PMeCy2).95 (α-Diimine)NiBr2 is reactive toward NaH, producing a dinickel bridging hydride complex with the αdiimine ligand being reduced (Scheme 21).97 Further reaction with Na or K results in three different nickel hydride species, all depending on the equivalent of alkali metal used. Reduction can occur to nickel converting Ni(II) to Ni(I) and/or to the ligand converting α-diimine radical anion to enediamido dianion. These reduction steps are similar to the chemistry observed with {(MeNacNac)Ni}2(μ-H)2.72 Cp#NiBr (Cp# = tetraisopropyl cyclopentadienyl), generated in situ from Cp#Na and NiBr2· DME (DME = 1,2-dimethoxyethane), reacts with NaH to form an unusual dinickel bridging hydride complex with a triplet ground state (Scheme 22).98 The intermediate spin is a result of delocalized Ni2(μ-H)2 bonding, because Ni(II) hydride dimers are typically diamagnetic or with a molar magnetic susceptibility close to the sum of two uncoupled high-spin Ni(II) ions.

Scheme 19. Synthesis of CpNi(IMes)H from (IMes)InH3

to CHCl3. However, the hydride, presumably (Et3P)2Ni(H)Br, cannot be isolated because of rapid decomposition. Tributylstannane (nBu3SnH) has been routinely used in organic synthesis involving halogen abstraction. With this hydride donor, both (dcpm)NiCl295 and (dippm)NiCl296 are converted to a formally NiINiII complex with a bridging hydride ligand (Scheme 20). The Ni−H−Ni bond is bent in the dcpm case Scheme 20. Synthesis of Dinuclear Nickel Hydride Complexes Using nBu3SnH

2.2. Via an Oxidative Addition Process

Another widely used strategy to synthesize transition metal hydrides is to oxidatively add a Y−H bond to a low-valent metal center. For nickel systems, this strategy is often implemented by using a Ni(0) species supported by the ligands of interest or using the commercially available Ni(COD)2 (COD = 1,5cyclooctadiene) where the COD ligand could be readily displaced by the Y group. In principle, Ni(I)80,99,100 and Ni(II) species101 can also undergo oxidative addition to form nickel hydrides, although examples of such processes are rare. 2.2.1. Protonation with Acids. Adding a substrate with an acidic Y−H moiety to a Ni(0) species could lead to the

but linear for the dippm derivative. Unlike other chelating phosphine systems described earlier (Schemes 2 and 9), dcpm and dippm here change their coordination mode from chelating to bridging upon mixing with nBu3SnH. Alkali metal hydrides such as NaH and LiH are also a suitable source of hydride for transition metals. The 1:1 reaction of 8380


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Scheme 21. Reaction of (α-Diimine)NiBr2 with NaH and Further Reduction by Na/K

However, their reaction with Et3Al leads to (Cy3P)2Ni(CH2CH2) instead of a hydrido nickel ethyl complex, suggesting that β-hydride elimination from the “Ni−Et” intermediate is facile. It should be mentioned that the ethylene complex could also serve as a precursor to nickel hydrides. (Bn3P)2Ni(CH2CH2) has been shown to react with HCl· dioxane and PhOH to give trans-(Bn3P)2Ni(H)Cl and trans(Bn3P)2Ni(H)(OPh)·HOPh, respectively.30 In the presence of a strong acid such as HCl, H2SO4, HClO4, or CF3CO2H, {(EtO)3P}4Ni is in an equilibrium with the 5coordinate complex [{(EtO)3P}4NiH]+.107 The analogous trimethyl or triphenyl phosphite complex behaves similarly. Isolating [{(EtO)3P}4NiH](HSO4) in an analytically pure form has been plagued by rapid decomposition of the compound at room temperature.108 Dissociation of (EtO)3P from nickel forms [{(EtO)3P}3NiH]+, which can be protonated to eliminate H2. In addition, free (EtO)3P reacts with acids to generate HPO(OEt)2 irreversibly, which facilitates the decomposition of the nickel hydride. A better behaved homoleptic system devoid of ligand degradation is (IMes)2Ni (Scheme 24).109 2,6-Lutidine·HX (X = Cl, Br) proves to be the

Scheme 22. Dinickel Bridging Hydride Complex with a Triplet Ground State

protonation of nickel and hence the formation of a Ni−H bond. At the end of the reaction, the Y group may or may not be attached to the nickel center. Potential risks for failure in such a process are as follows: (1) ligands on nickel could dissociate and compete with nickel for protonation, and (2) the resulting nickel hydride could be basic enough to be further protonated to form dihydrogen. One of the earliest successes in isolating nickel hydride complexes involved the protonation of {(Cy3P)2Ni}2(N2)102 with a variety of acids including HCl, CH3CO2H, PhOH, pyrrole, and cyclopentadiene (Scheme 23).103,104 This work by Scheme 23. Synthesis of Nickel Hydride Complexes from {(Cy3P)2Ni}2(N2)

Scheme 24. Synthesis of (IMes)2Ni(H)X

preferred acid with 2 equiv of 2,6-lutidine·HX needed for optimal selectivity. Reducing the amount of acid to 1 equivalent also provides (IMes)2Ni(H)X, but they are contaminated with (IMes)2NiX2 and (IMes)2NiX. The reaction of (IMes)2Ni with HCl·dioxane is less selective for (IMes)2Ni(H)Cl, preventing the hydride from being isolated in a pure form. Secondary phosphine oxides (R2P(O)H) can tautomerize to phosphinous acids (R2P−OH), especially upon coordination to a transition metal. A hydride species would form if the OH group could protonate the metal center. This is demonstrated in the reaction between Ph2P(O)H and Ni(PEt3)4, which gives a nickel hydride complex supported by a hydrogen-bonding network including Ph2PO− and Ph2P(OH) (Scheme 25).110 This particular hydride is not stable at ambient temperature but at −30 °C can be isolated in a pure form.

Jonas and Wilke appeared in 1969, shortly after Green and Saito published their NaBH4 method.24 The synthetic method was later extended to PhSH and p-CH3C6H4SH for the synthesis of hydrido nickel thiolate complexes.105 With these thiols as well as PhOH, pyrrole, and cyclopenadiene, the reaction pathway may not involve protonation of the metal but rather proceed via a concerted oxidative addition mechanism. A more recent investigation pertaining to {(Cy3P)2Ni}2(N2) shows that an iminium cation is also capable of protonating nickel, resulting in an air-stable cationic nickel hydride complex with imine bound to the nickel.106 The hydrido nickel acetate and phenoxide complexes can further react with (CH3)3Al or PhLi to give the hydrido methyl or phenyl complex.103 8381


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Ni(0) and C−N cleavage products (Scheme 29).122 The addition of Et3N·HCl to this mixture provides cationic hydride [(iPrPNN(H)NPiPr)NiH]Cl in high yield. Nickel complex (NP3)Ni adopts a trigonal pyramidal geometry with nitrogen occupying the apical position, which should allow H+ to conveniently approach from the opposite side of nitrogen.51 Protonation of (NP3)Ni with HCl (in ClCH2CH2Cl) surprisingly leads to H2 and some Ni(II) species. Using HBF4 or HClO4 as the acid affords [(NP3)NiHx] Y (Y = BF4, ClO4) with the value of x within the range 0.73− 0.90.50 The substoichiometric amount of hydride may suggest that [(NP3)NiH]Y is contaminated with some paramagnetic species [(NP3)Ni]Y. Synthesizing pure [(NP3)NiH]Y is possible, accomplished by protonation of (NP3)Ni with CH3CO2H followed by the addition of NaBPh4 (Y = BPh4).123 Square planar Ni(II) complex bearing a tetradentate P2S2 ligand (Scheme 30) can be reduced by 2 equiv of sodium amalgam, presumably to (P2S2)Ni, where the coordination mode of the P2S2 ligand has not been established.124 Subsequent protonation with HBF4·Et2O generates a nickel hydride consistent with the formula of [(P2S2)NiH](BF4). The tether between the two sulfur donors proves to be important for the stability of the nickel hydride. As a comparison, solutions of the bis-chelating complex [(Ph2PCH2CH2SEt)2NiH]+ (prepared following the same procedures in Scheme 30) lose H2 in hours,125 whereas solutions of [(P2S2)NiH]BF4 are stable for more than 6 days. Dithiolate-linked nickel−iron bridging hydride complexes have been studied as mimics of [NiFe]-hydrogenase active site. The synthesis involves the protonation of Fe(I)−Ni(I) species126 with HBF4·OEt2, after which the oxidation state of both metals becomes +2 (Scheme 31).127−129 Complex [(CO)3Fe(pdt)(μ-H)Ni(dppe)](BF4) (pdt = 1,3-propanedithiolate) has been further derivatized by displacing two CO ligands with dppe under photochemical conditions127 or displacing one CO ligand by (PhO)3P, Ph3P, and Ph2PPyr (Pyr = 2-pyridyl) under thermal conditions.128 The triphenylphosphine complex can be independently synthesized from protonation of (CO)3Fe(pdt)Ni(dppe) with [Ph3PH]BF4 (Scheme 31). Nickel clusters may be subjected to protonation to yield nickel hydrides without breaking the cluster core. For instance, protonation of a solution of [Ph3P(CH2Ph)]2[Fe3Ni(CO)12] in THF with 48% H3PO4 gives [Ph3P(CH2Ph)][Fe3Ni(CO)12(μ3H)].130 2.2.2. Oxidative Addition of H2. When H2 is added oxidatively to a low-valent Ni species, a nickel dihydride complex is expected as the initial product (Table 5). This has been demonstrated in the synthesis of {(dtbpe)Ni}2(μ-H)2 (dtbpe =1,2-bis(di-tert-butylphosphino)ethane), in which case the “(dtbpe)Ni(0)” precursor is either a complex with a labile benzene ligand or generated in situ from the reduction of (dtbpe)NiCl2 with Mg (Scheme 32).131 The dinickel bridging hydride complex can be viewed as the initial oxidative addition product (dtbpe)Ni(H)2 being trapped by (dtbpe)Ni(0). An analogous nickel bridging hydride complex bearing a βdiketiminato ligand has been synthesized from three different routes, all involving the oxidative addition of H2 to a Ni(I) species (Scheme 33).100 Activation of H2 by Ni(I) complex [(depe)2Ni](BF4) (generated from comproportionation of (depe)2Ni and [(depe)2Ni](BF4)2) leads to clean formation of the monohydride [(depe)2NiH](BF4).115 Similarly, mercurybridged nickel(I) pincer complex {(tBuPNPyPtBu)Ni}2(μ-Hg)

Scheme 25. Synthesis of a Nickel Hydride Complex from a Phosphine Oxide

The reactions of (dppe)2Ni and (Ph3P)4Ni with aqueous acids generate H2 and hydrated Ni(II) salts with no evidence for a nickel hydride species.111 The results under anhydrous conditions are quite different. Protonation of (dppe)2Ni with HCl-AlCl3, HBF4, or HCl gives nickel hydride [(dppe)2NiH]Y (Y = AlCl4, BF4, HCl2), which is evidently stable enough to be fully characterized.99,112 In benzene, protonation of (dppe)2Ni with CF3CO2H leads to the isolation of pure [(dppe)2NiH](OCOCF3).113 A wide range of other [(diphosphine)2NiH]+ complexes have been synthesized via the protonation of (diphosphine)2Ni.114−117 The structures of these diphosphine ligands are depicted in Figure 3, and the reaction conditions including

Figure 3. Diphosphine ligands studied for the protonation of (diphosphine)2Ni.

the specific acids used are summarized in Table 4. The Et PNMePEt 118 and iPrP2PhN277 systems contain dangling amine groups that could potentially compete with nickel for protonation. On the other hand, nitrogen could be the kinetic site for protonation and function as a proton relay. Ni(0) species can be stabilized by a single diphosphine ligand, like the one bearing a terphenyl backbone in which the central arene also coordinates to nickel (Scheme 26).119 Upon mixing with HCl in diethyl ether, (iPrPArPiPr)Ni is converted to a square planar (iPrPArPiPr)Ni(H)Cl. Abstraction of the chloride by TlOTf yields a cationic nickel hydride, which in acetonitrile is in equilibrium with the acetonitrile complex. The zwitterionic diphosphine ligand shown in Scheme 27 has an acidic hydrogen nestled between the two phosphorus centers. Its reaction with (Ph3P)4Ni (generated in situ from Ni(COD)2 and Ph3P) produces a nickel hydride complex that maintains the zwitterionic nature.120 Mixing the ligand with Ni(COD)2 alone results in no hydride species because the released COD molecule can insert into the Ni−H bond. The reaction of Ni(COD)2 with 1,2,3,6-tetrahydrophthalic anhydride (THPA) in THF−TMEDA produces an unusual dinickel species bridged by a hydride and a CO (Scheme 28).121 The presence of a dicarboxylate counterion is an indication that the anhydride is hydrolyzed by residual water in TMEDA and THF. Thus, the reaction can be viewed as protonation of “(TMEDA)Ni” by cis-4-cyclohexene-1,2-dicarboxylic acid. The bridging CO is probably originated from decarbonylation of the anhydride by a Ni(0) species. The protonation strategy has been seldom used for the synthesis of pincer-ligated nickel hydride complexes. Nevertheless, bis-aminophosphine ligand iPrPNN(H)NPiPr reacts with Ni(COD)2 to give an equilibrium mixture of (iPrPNN(H)NPiPr)8382


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Table 4. Nickel Hydride Complexes Synthesized via Protonation Reactionsa hydride complex

precursor

trans-(Cy3P)2Ni(H)Cl (Cy3P)2Ni(H)(OCOCH3) (Cy3P)2Ni(H)(OPh) (Cy3P)2Ni(H)(Py) CpNi(PCy3)(H) trans-(Cy3P)2Ni(H)(SAr) (Ar = Ph, p-CH3C6H4) trans-[(Cy3P)2Ni(H)(BnNCMe2](BPh4) trans-(IMes)2Ni(H)X (X = Cl, Br) (Et3P)2(Ph2PO···H···OPPh2)NiH [(dppe)2NiH]Y (Y = AlCl4, HCl2) [(dppe)2NiH](BF4)

{(Cy3P)2Ni}2(N2) {(Cy3P)2Ni}2(N2) {(Cy3P)2Ni}2(N2) {(Cy3P)2Ni}2(N2) {(Cy3P)2Ni}2(N2) {(Cy3P)2Ni}2(N2) {(Cy3P)2Ni}2(N2) (IMes)2Ni (Et3P)4Ni (dppe)2Ni (dppe)2Ni

[(dppe)2NiH](OCOCF3) [(dmpe)2NiH](PF6) [(depe)2NiH](PF6) [(dmpp)2NiH](PF6) [(dedpe)2NiH](BF4) [(dppv)2NiH](BF4) [(depp)2NiH](PF6) [(EtPNMePEt)2NiH](PF6) [(iPrP2PhN2)2NiH]+ (iPrPArPiPr)Ni(H)Cl [P,P-BPh3]Ni(PPh3)H [{(TMEDA)Ni}2(μ-H)(μ-CO)]+ [(iPrPNN(H)NPiPr)NiH]Cl [(NP3)NiH](BPh4) [(P2S2)NiH](BF4) [(Ph2PCH2CH2SEt)2NiH](BPh4) [(CO)3Fe(pdt)(μ-H)Ni(dppe)](BF4) [(Ph3P)(CO)2Fe(pdt)(μ-H)Ni(dppe)](BF4) [(CO)3Fe(pdt)(μ-H)Ni(dcpe)](BF4) [(CO)3Fe(edt)(μ-H)Ni(dppe)](BF4) [(CO)3Fe(edt)(μ-H)Ni(dcpe)](BF4) [Ph3P(CH2Ph)][Fe3Ni(CO)12(μ3-H)]

(dppe)2Ni (dmpe)Ni (depe)2Ni (dmpp)2Ni (dedpe)2Ni (dppv)2Ni (depp)2Ni (EtPNMePEt)2Ni (iPrP2PhN2)2Ni (iPrPArPiPr)Ni (Ph3P)4Ni Ni(COD)2, TMEDA Ni(COD)2, iPrPNN(H)NPiPr (NP3)Ni, NaBPh4 (P2S2)Ni (Ph2PCH2CH2SEt)2Ni,NaBPh4 (CO)3Fe(pdt)Ni(dppe) (CO)3Fe(pdt)Ni(dppe) (CO)3Fe(pdt)Ni(dcpe) (CO)3Fe(edt)Ni(dppe) (CO)3Fe(edt)Ni(dcpe) [Ph3P(CH2Ph)]2[Fe3Ni(CO)12]

acid

solvent

ref

HCl CH3CO2H PhOH PyH CpH ArSH [Me2CN(H)Bn](BPh4) 2,6-lutidine·HX Ph2P(O)H HCl-AlCl3 or HCl HBF4 HBF4·Et2O CF3CO2H NH4PF6 NH4PF6 NH4PF6 [p-(CH3O)C6H4(NH3)](BF4) [p-BrC6H4(NH3)](BF4) NH4PF6 NH4PF6 HOTf•DMF HCl (P,P-BPh3)-H THPA Et3N•HCl CH3CO2H HBF4·Et2O HBF4·Et2O HBF4·Et2O [Ph3PH]BF4 HBF4·Et2O HBF4·Et2O HBF4·Et2O H3PO4

toluene/Et2O toluene/Et2O toluene/Et2O toluene/Et2O toluene/Et2O unclear THF THF toluene toluene DME THF benzene THF THF THF THF THF CH3CN THF−EtOH CH3CN Et2O toluene THF THF THF−EtOH benzene−Et2O benzene−Et2O CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 THF

103 103 103 103 103 105 106 109 110 112 112 99 113 114 115 115 116 116 117 118 77 119 120 121 122 123 124 125 127 128 129 129 129 130

a

Abbreviations for the ligands: PyH = pyrrole, Bn = benzyl, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 2,6-lutidine = 2,6dimethylpyridine, DME = 1,2-dimethoxyethane, dppe = Ph2PCH2CH2PPh2, dmpe = Me2PCH2CH2PMe2, depe = Et2PCH2CH2PEt2, dmpp = Me2PCH2CH2CH2PMe2, dedpe = Et2PCH2CH2PPh2, dppv = cis-Ph2PCH = CHPPh2, depp = Et2PCH2CH2CH2PEt2, EtPNMePEt = Et 2 PCH 2 N(Me)CH 2 PEt 2 , iPr P 2 Ph N 2 = 1,5-diphenyl-3,7-diisopropyl-1,5-diaza-3,7-diphosphacyclooctane, iPr PArP iPr = p-bis(2diisopropylphosphinophenyl)benzene, TMEDA = tetramethylethylenediamine, THPA = 1,2,3,6-tetrahydrophthalic anhydride, iPrPNN(H)NPiPr = HN(CH2CH2NHPiPr2)2, NP3 = N(CH2CH2PPh2)3, P2S2 = Ph2P(CH2)2S(CH2)3S(CH2)2PPh2, pdt = 1,3-propanedithiolate, edt = 1,2ethanedithiolate, dcpe = Cy2PCH2CH2PCy2; for the structure of P,P-BPh3, see Scheme 27.

Scheme 26. Synthesis of Nickel Hydrides with a TerphenylBased Diphosphine Ligand

Scheme 27. Synthesis of a Zwitterionic Nickel Hydride Complex

Scheme 28. Bridging Hydride Complex Obtained from an Acid Anhydride

reacts with H2 at 60 °C to give (tBuPNPyPtBu)NiH (see Figure 1 for structure) and metallic Hg.80 The reaction of [(dppp)Ni(PhP2BnN2)](BF4)2 with H2 gives a nickel hydride complex with a proton bridging two pendant

amines (Scheme 34).101 Isolating this hydride species is challenging due to the reversibility of H2 addition. The overall 8383


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Ni(0) complex (Keq ≈ 5).132 Replacing the mesityl group with a phenyl group results in (PhDPBPh)Ni(THF) featuring (η2B,Cipso) interactions, which is evidently inactive toward H2. A very recent computational study based on the phenyl derivative (without THF) suggests that H2 activation proceeds via either a concerted pathway involving both boron and nickel or cis homolytic cleavage to form a dihydride intermediate followed by H− migration from nickel to boron.133 The concerted mechanism has a slightly lower kinetic barrier (29.7 vs 33.7 kcal/mol). Dihydrogen activation can also be assisted by an external borane. As a prototypical example, the reaction of (CO)3Fe(pdt)Ni(dppe) with H2 and B(C6F5)3 in CD2Cl2, though carried out in an NMR tube only, gives [(CO)3Fe(pdt)(μ-H)Ni(dppe)][HB(C6F5)3].128 The reaction of {(iPr3P)2Ni}2(μ-N2) with dihydrogen does not generate a dihydride complex or a dinuclear bridging hydride as observed for the dtbpe and β-diketiminato ligand systems. The isolated product is a nickel cluster with Ni5 forming a distorted square pyramid (Scheme 36).134 In solution, all hydride ligands are equivalent according to NMR spectroscopy; however, in the solid state four of them bridge the basal Ni−Ni edges while the other two cap two opposite faces of the pyramid. 2.2.3. Oxidative Addition of C−H Bonds. Certain C−H bonds are susceptible to activation by low-valent nickel species, resulting in nickel hydride complexes that contain a carbonbased ligand. The first synthesis of a nickel hydride using the C−H bond activation route is arguably the reaction of {(Cy3P)2Ni}2(N2) with cyclopentadiene for the synthesis of CpNi(PCy3)(H) (Scheme 23).103 A more recent example employs (IMes)2Ni (generated in situ from Ni(COD)2 and IMes) and an imidazolium salt (Scheme 37).135 The isolated cationic nickel hydrides are stable in refluxing THF and can be handled in air. In another study, premixing Ni(COD)2 with an amino-NHC ligand followed by the addition of a benzimidazolium salt also provides a cationic nickel hydride complex.136 The C−H bonds of polyfluorinated benzene derivatives undergo oxidative addition with (iPr3P)2Ni(η2-anthracene) to yield nickel hydride complexes with a trans-fluorinated aryl group (Scheme 38).137 The advantage of using the anthracene complex over Ni(COD)2 is that the byproduct anthracene does not insert into the newly formed Ni−H bond. The fluorine substituents are essential for hydride formation; arenes without fluorine groups (e.g., benzene, toluene, and mesitylene) simply displace anthracene to form η6-arene complexes. As observed for the reaction of trans-(R3P)2Ni(H)Cl with NaBH4 (section 2.1.1), bulky phosphines are needed here to stabilize the hydride complexes. Although the reaction of (Et3P)2Ni(η2phenanthrene) with 1,2,4,5-tetrafluorobenzene (HArF) leads to a species consistent with trans-(Et3P)2Ni(H)(ArF), it cannot be isolated due to an equilibrium with the starting materials.138 Oxidative addition of a C−H bond of a pincer ligand to Ni(COD)2 is an efficient way to synthesize square planar nickel pincer hydride complexes. As shown in Scheme 39, a diphosphine linked by an imidazolinium group leads to cationic complex [(PCNHCP)NiH](PF6) after oxidative addition.139 mPhenylene-bridged bis-NHC ligand displays a similar reaction pattern, except that the resulting nickel hydride is a neutral species.140 The reaction of nickelocene with sodium and a terminal alkene (in a 1:1.4:2.5 ratio) is a very complicated process, producing a mixture of at least four different products (Scheme 40).141 The synthetic utility of this reaction is probably limited.

Scheme 29. Synthesis of a Nickel Pincer Hydride Complex via Protonation Reaction

Scheme 30. Synthesis of a Nickel Hydride Complex Bearing a Tetradentate P2S2 Ligand

Scheme 31. Synthesis of Dithiolate-Linked Nickel−Iron Bridging Hydride Complexes via Protonation Reactions

transformation does not appear to be an oxidative addition process because the oxidation state of nickel is +2 before and after the reaction. However, low-temperature NMR experiments suggest that the kinetic product is a Ni(IV) dihydride complex, which transfers both hydrogens as protons to the pendant amines (a formally 4-electron reduction at the nickel center). Subsequent protonation of nickel by one of the two protons gives [(dppp)NiH(PhP2BnN2H)](BF4)2 as the thermodynamic product. The closely related compound [(iPrP2PhN2)2Ni(H)](BF4) (see Scheme 15 for structure) can be prepared from [(iPrP2PhN2)2Ni](BF4)2 and H2 in the presence of p-anisidine as an external base (in addition to using the synthetic methods already described in sections 2.1.2 and 2.2.1).77 A dihydride intermediate has not been observed in this case; thus, H2 activation could proceed via a concerted heterolytic cleavage by nickel and nitrogen centers or through a dihydrogen complex. More definitive examples following these nonoxidative pathways are discussed in section 2.3. An alternative way of cleaving H2 heterolytically while making a hydride complex is using a boron-based Lewis acid as hydride acceptor. Under such a scenario, the metal accepts H+ rather than H− and therefore undergoes oxidation in the process of activating H2. To date, two nickel systems are known to react with H2 following this pathway: one involving intramolecular H− abstraction by boron and the other one involving intermolecular H− abstraction. As shown in Scheme 35, (MesDPBPh)Ni activates H2 to afford a nickel hydride with a B−H−Ni bridge, which is in equilibrium with the starting 8384


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Table 5. Nickel Hydride Complexes Synthesized via Oxidative Addition of Y−H (Y = H, C, Si) Bonda hydride complex

reagents

Y−H bond

ref

{(dtbpe)Ni}2(μ-H)2 {(dtbpe)Ni}2(μ-H)2 {(MeNacNac)Ni}2(μ-H)2 {(MeNacNac)Ni}2(μ-H)2 {(MeNacNac)Ni}2(μ-H)2 [(iPrP2PhN2)2Ni(H)](BF4) {(iPr3P)Ni}5H6 CpNi(PCy3)H [(IMes)2Ni(H)(NHC)]X (X = BF4, PF6) [(amino-NHC)2Ni(H)(DMBI)](PF6) (iPr3P)2Ni(H)(2,3,5,6-C6F4H) (iPr3P)2Ni(H)(2,3,4,6-C6F4H) (iPr3P)2Ni(H)(C6F5) [(PCNHCP)NiH](PF6) (NHCCCCNHC)NiH (CpNi)3(μ3-CCH2R)(μ3-H) (R = nPr, nBu) {(MeNacNac)GaNi(C2H4)}2Ni(H)(CHCH2) (Cp*Al)3Ni(μ-H)Al(Ph)Cp* cis-(iPrIm)2Ni(H)(SiRPh2) (R = Ph, Me) {(iPrIm)Ni}2(μ-Ph2SiH)2 cis-(iPrIm)2Ni(H)(SiHMes2) cis-(iPrIm)2Ni(H)(SitBu2Cl) (Cp*Al)3Ni(H)(SiEt3) [(dppe)2NiH](SiCl3) (dtbpe)Ni(H)(SiAr2H) (Ar = Mes, Ph) (dtbpe)Ni(H)(SiPh2E) (E = CH3, Cl) (MesDPBPh-H)NiE (E = SiH2Ph, SiHPh2) (SiSiSi)(μ-H)Ni(depe) (PhP3Si)NiH (RP3Si)NiH (R = Ph, iPr)

{(dtbpe)Ni}2(μ-η2:η2-C6H6), H2 (dtbpe)NiCl2, Mg, H2 {(MeNacNac)Ni}2, H2 {(MeNacNac)Ni}2(N2), H2 {(MeNacNac)Ni(μ-Br)Li(THF)2}2, H2 [(iPrP2PhN2)2Ni](BF4)2, p-anisidine, H2 {(iPr3P)2Ni}2(μ-N2), H2 {(Cy3P)2Ni}2(N2) (IMes)2Ni, [NHC-H]X amino-NHC, Ni(COD)2, [DMBI-H](PF6) (iPr3P)2Ni(η2-anthracene), 1,2,4,5-C6F4H2 (iPr3P)2Ni(η2-anthracene), 1,2,3,5-C6F4H2 (iPr3P)2Ni(η2-anthracene), C6F5H [PCNHCP-H](PF6), Ni(COD)2 NHC CCCNHC-H, Ni(COD)2 Cp2Ni, Na, RCHCH2 (MeNacNac)Ga, Ni(C2H4)3 Ni(COD)2, (Cp*Al)4, benzene {(iPrIm)2Ni}2(COD), Ph2RSiH {(iPrIm)2Ni}2(COD), Ph2SiH2 {(iPrIm)2Ni}2(COD), Mes2SiH2 {(iPrIm)2Ni}2(COD), tBu2SiHCl Ni(COD)2, (Cp*Al)4, Et3SiH (dppe)2Ni,HSiCl3 {(dtbpe)Ni}2(μ-η2:η2-C6H6), H2SiAr2 {(dtbpe)Ni}2(μ-η2:η2-C6H6), HSiEPh2 (MesDPBPh)Ni, HE (depe)Ni(PEt3)2, SiHSiHSiH (Me3P)4Ni, PhP3Si−H Ni(COD)2, RP3Si−H

H−H H−H H−H H−H H−H H−H H−H C−H C−H C−H C−H C−H C−H C−H C−H C−H C−H C−H Si−H Si−H Si−H Si−H Si−H Si−H Si−H Si−H Si−H Si−H Si−H Si−H

131 131 100 100 100 77 134 103 135 136 137 137 137 139 140 141 142 150 147 148 148 148 150 146 151 151 152 153 154 155

a Abbreviations for the ligands: dtbpe = tBu2PCH2CH2PtBu2, iPrP2PhN2 = 1,5-diphenyl-3,7-diisopropyl-1,5-diaza-3,7-diphosphacyclooctane, IMes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, DMBI-H = 1,3-dimethyl-benzimidazolium, NHCCCCNHC = bis(diisopropylphenylimidazol-2ylidene)phenyl, iPrIm = 1,3-diisopropylimidazolin-2-ylidene, Mes =2,4,6-trimethylphenyl, dppe = Ph2PCH2CH2PPh2, MesDPBPh = MesB(oPh2PC6H4)2, depe = Et2PCH2CH2PEt2, SiHSiHSiH = bis(2-silylphenyl)silane, PhP3Si−H = (o-Ph2PC6H4)3SiH; see main text for structures of Me NacNac (Scheme 33 or 41), [NHC-H]X (Scheme 37), amino-NHC ligands (Scheme 37), and [PCNHCP−H]PF6 (Scheme 39).

synthesized on a preparation scale from a 2:3 mixture of (MeNacNac)Ga and Ni(ethylene)3. 2.2.4. Oxidative Addition of Si−H Bonds. The interaction between a silane Si−H bond and nickel could lead to three different scenarios: an oxidative addition product, an η2-silane complex, or an η1-silane complex (Figure 4).143 Compounds that have been described as η2-silane complexes in the literature will be discussed in section 2.8. Readers should be cautioned that distinguishing silyl hydride complexes from η2silane complexes is not always an easy task. Under many circumstances, the ambiguity is resolved neither by computational methods nor by X-ray crystallography, which can provide hydrogen location but with limited accuracy. The 29Si−1H spin−spin coupling constant could be informative; however, the border is not so clear cut as there is a structural continuum between the silyl hydride and η2-silane complexes. Moreover, the coupling constant can be greatly influenced by silicon substituents.144,145 Synthesis of nickel hydride complexes from (Ph3P)4Ni and silanes such as Ph3SiH, Ph2SiH2, PhSiH3, and HSiCl3 has not been successful. 1 4 6 In contrast, the reaction of {(iPrIm)2Ni}2(COD) with bulky tertiary or secondary silanes gives nickel silyl hydride complexes (Scheme 42) based on relatively small 2JSiH coupling constants (11−20 Hz),147,148

Scheme 32. Synthesis of a Diphosphine-Ligated Nickel Hydride Complex from H2

Nevertheless, cluster (CpNi)3(μ3-CCH2R)(μ3-H) can be isolated in 8−9% yield from the reaction mixture using column chromatography. The 2:1 reaction of a gallium−nickel ethylene complex and Ni(cdt) (cdt = 1,5,9-cyclododecatriene) is a high-yielding process for a hydrido gallium−nickel cluster, as confirmed by NMR spectroscopy (Scheme 41).142 The hydride and vinyl ligands stem from oxidative addition of an ethylene C−H bond to a Ni(0) species, while the rest of the molecule is essentially two (MeNacNac)GaNi(ethylene) “ligands” with the Ga−Ni moiety σ-bonded to the nickel. This particular cluster can be 8385


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Scheme 33. Synthesis of a β-Diketiminato-Ligated Nickel Hydride Complex from H2

Scheme 34. [(dppp)NiH(PhP2BnN2H)](BF4)2 Generated from H2 Activation

Scheme 38. C−H Bond Activation of Polyfluorinated Benzene Derivatives with Nickel

Scheme 35. Activation of H2 by (MesDPBPh)Ni

Scheme 39. Square Planar Nickel Pincer Hydride Complexes Synthesized via C−H Bond Activation

Scheme 36. Synthesis of a Nickel Cluster via H2 activation

Scheme 40. Synthesis of Nickel Clusters from Cp2Ni, Sodium, and Alkenes Scheme 37. Synthesis of Nickel Hydride Complexes from Imidazolium Salts

Scheme 41. Synthesis of a Hydrido Gallium−Nickel Cluster

although calculations suggest that there are still some residual Si···H interactions.149 At 60 °C, cis-(iPrIm)2Ni(H)(SiPh2Me) isomerizes to the thermodynamically more stable trans isomer. Interestingly, the reaction of {(iPrIm)2Ni}2(COD) with 2 equiv

of Ph2SiH2 at 110 °C generates a dinuclear complex with two bridging Ph2SiH groups.148 The same Ni(0) species mixed with 6 equiv of Ph2SiH2 at room temperature, however, gives rise to 8386


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Scheme 43. Si−H and C−H Bond Activation by Ni(COD)2− (Cp*Al)4

Figure 4. Complexes resulted from interaction between a Si−H bond and nickel.

(iPrIm)2Ni(SiPh2H)2 as the final product. Nickel silyl hydride (iPrIm)2Ni(H)(SiPh2H) has been detected as an intermediate during this reaction. Other less sterically hindered silanes such as (EtO)3SiH, Et2SiH2, Me2SiH2, PhSiH3, and CySiH3 also react with {(iPrIm)2Ni}2(COD) readily to form the corresponding nickel disilyl complexes. Cp*Al is a carbenoid compound showing properties similar to phosphines. Its tetramer reacts with Ni(COD)2 in n-hexane to yield Ni(AlCp*)4, which is isolobal to Ni(PPh3)4. However, in Et3SiH (as the solvent and reactant), the isolated product is a nickel silyl hydride resulting from Si−H oxidative addition to Ni(AlCp*)3 (Scheme 43).150 The silyl hydride complex loses Et3SiH in benzene upon heating, producing a new species with a hydride ligand bridging nickel and aluminum. The Ni−H−Al complex is more cleanly synthesized from Ni(COD)2 and Ni(AlCp*)4 using benzene as the solvent. The net reaction can be described as C−H activation of benzene by aluminum, although mechanistically it might involve breaking the C−H bond by nickel first. Unlike the monodentate phosphine case, (dppe)2Ni reacts with HSiCl3 to yield a nickel hydride that is consistent with the formula of [(dppe)2NiH](SiCl3).146 This process may be interpreted as HSiCl3 protonating the nickel center. In the absence of detailed structural information, a silyl hydride or an η2-HSiCl3 complex (accompanied by phosphine dissociation) cannot be ruled out. It should be mentioned that at 35 °C under neat conditions [(dppe)2NiH](SiCl3) reacts with HSiCl3 further to give [(dppe)2Ni(SiCl3)](SiCl3). In ethanol with a slight excess of HClO4 added, both [(dppe)2NiH](SiCl3) and [(dppe)2Ni(SiCl3)](SiCl3) are converted to [(dppe)2NiH](ClO4). Treatment of {(dtbpe)Ni}2(μ-η2:η2-C6H6) with various silanes provides nickel hydride complexes with structural features closer to the oxidative addition products (Scheme 44).151 NMR data support a rapid dynamic process (in solution) that renders the four tert-butyl groups equivalent. It has been proposed that the silyl and hydride groups exchange positions rapidly via reductive elimination to Ni(0)−η2-silane complexes, Si−H bond rotation, and then oxidative addition again. Oxidative addition of the Si−H bond of PhSiH3 or Ph2SiH2 with diphosphineborane complex (MesDPBPh)Ni gives a borohydrido nickel silyl complex (Scheme 45), which can be isolated in a pure form by crystallization.152 When dissolved in C6D6, (MesDPBPh-H)Ni(SiPh2H) establishes an equilibrium

Scheme 44. Si−H Bond Activation by {(dtbpe)Ni}2(μ-η2:η2C6H6)

Scheme 45. Si−H Bond Activation by (MesDPBPh)Ni

with free Ph2SiH2 and (MesDPBPh)Ni (Keq ≈ 960 M−1), similar to the oxidative addition product derived from H2 (Scheme 35). The reaction of bis(2-silylphenyl)silane with (depe)Ni(PEt3)2 represents a rare case for a Ni(IV)−H complex (Scheme 46).153 Key evidence supporting such a structure is Scheme 46. Activation of Si−H Bonds Involving a Ni(IV)− H Intermediate

the absence of coupling between the hydride and the central silicon, which is also confirmed by density functional theory (DFT) calculations. Although the oxidation addition product is

Scheme 42. Activation of Silanes with Nickel(0) Carbene Complexes

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bond, which was first developed by Ozerov and co-workers in an NMR-scale reaction using (iPrPNPiPr-Me)H (Figure 5).60

observable at −80 °C, at ambient temperature, the Si−H activation product is an η2-silane complex. A tripodal phosphine bearing a central SiH group shows the opposite temperature dependence during its reaction with (Me3P)4Ni. The kinetic product is a nickel η2-silane complex with two of the three phosphorus donors of the ligand being coordinated to nickel (Scheme 47).154 Heating the silane Scheme 47. Synthesis of Nickel Hydride Complexes from a Tripodal Phosphine Ligand

Figure 5. PNP-type pincer nickel hydrides synthesized from Ni(COD)2.

Many structural variations have been made since, including the modification of pincer backbone79,80,87,156 and the use of unsymmetrical pincer ligands.157,158 As mentioned in section 2.1.2, it is not possible to isolate (PhPNPyPPh)NiH due to facile formation of a dinuclear species (Scheme 16). The reaction of this particular pincer ligand with Ni(COD)2 gives the same dinickel complex in addition to cyclooctene and isomerized COD.79,81 Introducing phenyl groups to the phosphorus donors appears to create some challenge in the synthesis of nickel pincer hydride complexes. The amido diphosphine system produces (PhPNPPh)NiH as judged by in situ NMR, but after workup, the isolated compound is a COD insertion

complex to 80 °C results in a C3-symmetric nickel hydride complex (PhP3Si)NiH by expelling the final Me3P ligand. The hydride complex can also be synthesized from Si−H oxidative addition of the ligand to Ni(COD)2.155 2.2.5. Oxidative Addition of Other Bonds. The method of N−H oxidative addition (Table 6) was initially used to make (Cy3P)2Ni(H)(Py) (Scheme 23)103 and is now routinely utilized to synthesize nickel pincer hydride complexes with a central anionic nitrogen donor. The standard protocol involves the mixing of Ni(COD)2 with a pincer ligand bearing a N−H

Table 6. Nickel Hydride Complexes Synthesized via Oxidative Addition of a More Polarized Y−H Bonda hydride complex

reagents

H−Y bond

ref

(Cy3P)2Ni(H)(Py) (RPNPR)NiH (R = Cy, iPr) (PhPNPiPr)NiH (PhPNPR-Me)NiH (R = Cy, iPr) (RPNPyPR)NiH (R = Cy, iPr) (tBuPNPyPtBu)NiHb (Cbzdiphos)NiH (Cbzdiphol)NiH (bimca)NiH (MICCNCMIC)NiH (Cy3P)2Ni(H)(OPh) {κP,κO-Ph2PCH2C(CF3)2O}NiH(PCy3) {κO-Ph2PCH2C(CF3)2O}NiH(PCy3)2 [(dippe)Ni(H)(dippeO)]OH trans-(Cy3P)2Ni(H)(SPh) trans-(Cy3P)2Ni(H)(SC6H4CH3) trans-(iPrIm)2Ni(H)(StBu) (κP,κS-o-Ph2PC6H4S)NiH(PMe3)2 (κP,κS-o-Ph2PC10H6S)NiH(PMe3)2 (κP,κSe-o-Ph2PC6H4Se)NiH(PMe3)2 (C5R5)Ni(μ-H)(μ-CO)WCp2 (R = H, Me) (C5R5)Ni(μ-H)(μ-CO)MoCp2 (R = H, Me)

{(Cy3P)2Ni}2(N2), PyH Ni(COD)2, RPN(H)PR Ni(COD)2, PhPN(H)PiPr Ni(COD)2, PhPN(H)PR-Me Ni(COD)2, RPN(H)PyPR Ni(COD)2, tBuPN(H)PyPtBu Ni(COD)2, CbzdiphosH Ni(COD)2, CbzdipholH Ni(COD)2, [bimcaH3](BF4)2, MeLi NiCl2·DME, [MICC(H)N(H)C(H)MIC]+, (Me3Si)2NK {(Cy3P)2Ni}2(N2), PhOH Ni(COD)2, Ph2PCH2C(CF3)2OH, Cy3P Ni(COD)2, Ph2PCH2C(CF3)2OH, Cy3P (dippe)NiCl2, KOH {(Cy3P)2Ni}2(N2), PhSH {(Cy3P)2Ni}2(N2), p-CH3C6H4SH {(iPrIm)2Ni}2(COD), tBuSH Ni(PMe3)4, o-Ph2PC6H4SH Ni(PMe3)4, o-Ph2PC10H6SH Ni(PMe3)4, o-Ph2PC6H4SeH (C5R5)Ni(CO)2, Cp2WH2 (C5R5)Ni(CO)2, Cp2MoH2

N−H N−H N−H N−H N−H N−H N−H N−H N−H/C−H N−H/C−H O−H O−H O−H O−H S−H S−H S−H S−H S−H Se−H W−H Mo−H

103 87 157 158 79, 80 80 156 156 159 160 103 163 163 164 105 105 165 166 166 167 168 168

a

Abbreviations for the ligands: PyH = pyrrole, CbzdiphosH = 3,6-di-tert-butyl-1,8-bis(diphenylphosphinomethyl)-9H-carbazole, CbzdipholH = 3,6di-tert-butyl-1,8-bis(((2R,5R)-2,5-diphenylphospholan-1-yl)methyl)-9H-carbazole, bimca = 3,6-di-tert-butyl-1,8-bis(3-methylimidazolin-2-ylidene)carbazolide, DME = 1,2-dimethoxyethane, dippe = iPr2PCH2CH2PiPr2, iPrIm = 1,3-diisopropylimidazolin-2-ylidene, C10H6 = disubstituted naphthalene; for the structures of pincer ligands, see Figure 5 and Schemes 48 and 49. bRequires 5 bar of H2 and heating at 80 °C. 8388


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product.157 Replacing only one pincer arm with a Ph2P group leads to stable and isolable nickel hydrides.157,158 In contrast, having phenyl groups as the phosphorus substituents is not an issue with the carbazole-derived nickel pincer system.156 Bis(imidazolium)-based carbazole [bimcaH3](BF4)2 bear three acidic hydrogens: two from the imidazolium groups and one from the carbazole (highlighted in red in Scheme 48).

Oxidative addition of O−H bonds has also been explored as a viable synthetic method for nickel hydride complexes. The first study was probably the synthesis of (Cy3P)2Ni(H)(OPh) from {(Cy3P)2Ni}2(N2) and PhOH.103 Another early work intended to probe the intermediacy of nickel hydride in olefin oligomerization led to the isolation of two stable nickel hydrides that differ by the number of phosphine ligands (Scheme 51).163 A recent study of (dippe)NiCl2 in aqueous

Scheme 48. Synthesis of a Nickel Hydride from a Bis(imidazolium)-Based Carbazole

Scheme 51. Synthesis of Nickel Hydride Complexes via O− H Oxidative Addition

Following lithiation with 2.1 equiv of MeLi, this ligand coordinates to the nickel of Ni(COD)2 to yield a pincer nickel hydride complex.159 The overall transformation can be rationalized as N−H oxidative addition of a bis-carbene carbazole to a Ni(0) species. However, carbazole is more acidic than an imidazolium salt, suggesting that the NH group is deprotonated first followed by a C−H oxidative addition process. A related mesoionic carbene (MIC) system also forms a nickel pincer hydride complex except that a Ni(II) species is employed (Scheme 49).160

media shows that a nickel hydride can be isolated (as a PF6− salt) by treating (dippe)NiCl2 with 2 equiv of KOH for 1 week.164 According to the proposed mechanism (Scheme 52), Scheme 52. Nickel Hydride Complex Produced from (dippe)NiCl2 and KOH

Scheme 49. Synthesis of a Nickel Pincer Hydride Complex Bearing Mesoionic Carbenes

one of the dippe phosphorus centers of intermediate [{(dippe)Ni}2(μ-OH)2](OH)2 is attacked by free OH−, which is formally a reductive elimination process at nickel. Subsequent O−H oxidative addition or protonation of a Ni(0) center generates [(dippe)Ni(H)(dippeO)]+. Other chalcogen−hydrogen bonds have been shown to react with Ni(0) species to form nickel hydrides. Oxidative addition of ArS−H (Ar = Ph, p-CH3C6H4) to {(Cy3P)2Ni}2(N2) gives trans-(Cy3P)2Ni(H)(SAr).105 An aliphatic thiol tBuSH reacts with carbene-ligated nickel(0) complex {(iPrIM)2Ni}2(COD) in a similar fashion to yield trans-(iPrIM)2Ni(H)(StBu) (Scheme 53).165 Chelating [P,S]- and [P,Se]-ligands can be introduced to nickel through oxidative addition of S−H166 and Se−H bonds,167 respectively (Scheme 54). Adding dppp to the resulting 5-coordinate nickel hydrides generates new hydrides with the two Me3P ligands being displaced by dppp.166

Nickel hydrides with an anionic phosphorus donor can be prepared via oxidative addition of P−H bonds. For example, the reaction of (dtbpe)Ni(CH3)2 with 2,6-dimesitylphenylphosphine (DmpPH2) at 50 °C gives (dtbpe)Ni(H){PH(Dmp)} in 70% isolated yield (Scheme 50).161 Other Ni(0) precursors Scheme 50. Synthesis of a Nickel Hydride Complex via P−H Oxidative Addition

Scheme 53. Nickel Hydride Complex Synthesized via Oxidative Addition of an S−H Bond

including (dtbpe)Ni(CH2CH2) and (dtbpe)Ni(COD) are inert to DmpPH2 at 70 °C, but at 110 °C, the ethylene molecule of (dtbpe)Ni(CH2CH2) is inserted into the P−H bond, resulting in a free secondary phosphine DmpPH(Et). Not all primary phosphines undergo P−H bond activation with Ni(0). The reaction of MesPH2 with Ni(COD)2 gives (MesPH2)4Ni as a result of ligand substitution reaction.162 8389


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nonoxidative pathway has been shown in several studies pertaining to [(diphosphine)2Ni]2+ complexes115,117,118 or nickel complexes bearing a pincer169 or tetradentate ligand.155 Occasionally it is used as a method to make nickel hydrides on a preparation scale (Table 7). One notable example is the activation of H2 by a cationic PNP-type pincer complex, in which case nickel and nitrogen accept H− and H+, respectively (Scheme 56).170 The conversion of the cationic nickel hydride to a neutral one is made possible with the addition of a strong base LiNiPr2.

Scheme 54. Synthesis of Nickel Hydrides Bearing a Chelating [P,S]- or [P,Se]-Ligand

In principle, a metal−hydrogen bond could be oxidatively added to nickel to generate a heterobimetallic hydride complex. A process that might fall into this category is the reaction of Cp2MH2 with {(C5R5)Ni(CO)}2 under photochemical conditions (Scheme 55).168 The isolated Ni−W and Ni−Mo

Scheme 56. Heterolytic Cleavage of H2 by a Cationic, Bifunctional Nickel Pincer Complex

Scheme 55. Photochemical Synthesis of Heterobimetallic Hydride Complexes

hydride complexes can be regarded as a Cp2MH radical being added to (C5R5)NiI(CO). To complete the dinuclear oxidative addition reaction, (C5R5)Ni(CO)H is likely produced as the byproduct, although it was deemed to be too unstable to be detected. 2.3. Through Heterolytic Cleavage of H2A Nonoxidative Pathway

Hydrogenolysis of a tBuPBPtBu-pincer nickel methyl complex leads to a quantitative formation of a nickel hydride and methane (Scheme 57).171 The methyl group may be perceived as a base promoting the heterolytic cleavage of H2. DFT calculations however suggest that the boryl group plays a critical role in the hydrogenolysis process. It forms a B−H bond as methane elimination takes place, resulting in an Ni(0) η2-

Dihydrogen molecule may be heterolytically cleaved by nickel so that H− is delivered to the metal while H+ is transferred to an external or internal base. Under this mechanistic scenario, the oxidation state of nickel is unchanged, which is distinctively different from the mechanism discussed in section 2.2.2. The

Table 7. Nickel Hydride Complexes Synthesized by Other Methodsa hydride complex tBu Si

Si tBu

( P N P )NiH (tBuPBPtBu)NiH [(H2O)Ni(μ-SR)2(μ-H)Ru(C6Me6)](NO3) [(TsTACN-S2)Ni(μ-H)RhCp*](NO3) [(iPrTACN-S2)Ni(μ-H)RhCp*](NO3) (R3P)2Ni(H)(N2) (R = Et, nBu) (Ph3P)3Ni(H)Br (Ph3P)3Ni(H)Br (dippm)2Ni2Br2(μ-H) (dcpm)2Ni2Br2(μ-H) (N,P)Ni(PMe3)H [(NS3tBu)NiH](BPh4) (Cp*Ni)3(μ-H)(μ3-CH) trans-(iPrIm)2Ni(H)C6F5 trans-(iPrIm)2Ni(H)C6F4(p-CF3) (MeNNNMe)NiH (iPrPCPiPr)NiH Cp*Ni(PEt3)H [(TsTACN-S2)Ni(μ-H)RhCp*](NO3) [(TsTACN-S2)Ni(μ-H)IrCp*](NO3) [Ni(μ-meppp)(μ-H)MCp*](NO3) (M = Rh, Ir)

reagents tBu Si

Si tBu

NaBArF4,

i

( P N P )NiCl, H2, LiN Pr2 (tBuPBPtBu)NiCH3, H2 [Ni(μ-SR)2Ru(C6Me6)(OH2)](NO3)2, H2 [Ni(TsTACN-S2)RhCp*(NO3)](NO3), H2 [Ni(iPrTACN-S2)RhCp*Cl](NO3), H2 Ni(acac)2, iBu3Al, R3P Ni(acac)2, Et2AlBr, Ph3P (acac)(Ph3P)NiEt, Et2AlBr, Ph3P (dippm)NiBr2, AdZnBr (dcpm)NiBr2, AdZnBr (N,P)Ni(PMe3)Cl, RMgCl (R = Et, nBu) [(NS3tBu)NiCl](BPh4), EtMgBr Cp*Ni(acac), CH3Li trans-(iPrIm)2Ni(F)C6F5, PhSiH3 trans-(iPrIm)2Ni(F)C6F4(p-CF3), PhSiH3 (MeNNNMe)NiOMe, Ph2SiH2 (iPrPCPiPr)NiOMe, (EtO)3SiH Cp*Ni(PEt3)NHTol, Me3SiH [Ni(TsTACN-S2)RhCp*(NO3)](NO3), HCO2Na [Ni(TsTACN-S2)IrCp*(NO3)](NO3), HCO2Na [Ni(μ-meppp)MCp*(NO3)](NO3), HCO2Na

source of H−

ref

H2 H2 H2 H2 H2 i Bu3Al Et2AlBr Ni−Et AdZnBr AdZnBr RMgCl EtMgBr CH3Li PhSiH3 PhSiH3 Ph2SiH2 (EtO)3SiH Me3SiH HCO2Na HCO2Na HCO2Na

170 171 172 174 174 175 176 176 177 96 76 53 181 182 183 49 184 186 174 189 190

a

Abbreviations for the ligands: ArF = 3,5-(CF3)2C6H3, (μ-SR)2 = N,N′-dimethyl-N,N′-bis(2-mercaptoethyl)-1,3-propanediamine, RTACN-S2 = 1,4bis(2-mercaptoethyl)-7-R-1,4,7-triazacyclononane (R = Ts, iPr), Ts = p-toluenesulfonyl, acac = acetylacetonate, dippm = iPr2PCH2PiPr2, Ad = 1adamantyl, dcpm = Cy2PCH2PCy2, N,P = o-(2,6-Me2C6H3)NC6H4PiPr2, NS3tBu = N(CH2CH2StBu)3, iPrIm = 1,3- diisopropylimidazolin-2-ylidene, meppp =1,3-bis((mercaptoethyl)phenylphosphino)propane; for structures of pincer complexes, see Schemes 56 and 57 and Figure 6. 8390


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As mentioned in section 2.1.4 (Scheme 20), synthesis of (diphosphine)2Ni2Cl2(μ-H) (diphosphine = dippm, dcpm) from (diphosphine)NiCl2 has been accomplished by using n Bu3SnH as the hydride resource. Preparation of the bromide analogs requires the use of 1-adamantylzinc bromide as the hydride donor (Scheme 60).96,177 Diarylamido phosphine-

Scheme 57. Hydrogenolysis of a Nickel Pincer Methyl Complex

Scheme 60. Synthesis of Dinuclear Nickel Hydride Complexes Using 1-Adamantylzinc Bromide

borane intermediate. Oxidation addition the B−H bond gives the final pincer hydride complex. Bimetallic complexes of nickel and ruthenium bearing two thiolate bridges have been investigated as model compounds for [NiFe]-hydrogenase. Remarkably, a diamagnetic Ni(II)− Ru(II) aqua complex activates H2 readily in water to yield a new complex with a Ni−H−Ru core (Scheme 58).172 As the Scheme 58. Heterobimetallic Complexes Synthesized via H2 Activation

Scheme 61. Synthesis of a Diarylamido Phosphine-Ligated Nickel Hydride Complex

reaction proceeds, the pH of the solution decreases, indicative of the dissociation of H+. At pH 9, the aqua ligand of the bridging hydride complex is deprotonated, producing a neutral hydride complex (HO)Ni(μ-SR)2(μ-H)Ru(C6Me6).173 The analogous nickel−rhodium complexes supported by N3S2 dithiolate and Cp* ligands react with H2 in a similar fashion, producing Ni−Rh bimetallic complexes with a bridging hydride ligand (Scheme 58).174

ligated nickel hydride complex (Scheme 61) is available from (N,P)Ni(PMe3)Cl and a Grignard reagent such as EtMgCl and n BuMgCl (or LiEt3BH as shown in Scheme 14).76 Using MeMgCl instead provides an isomeric mixture of nickel methyl complexes rather than a hydride. Nickel pincer hydride complexes are rarely prepared from alkyl metal reagents because the nickel alkyl intermediates often resist β-hydride elimination.79,178,179 Less rigid and/or sterically demanding pincer ligands may allow nickel hydrides to be synthesized using this method. One promising study involves a PCP-pincer ligand with an sp3-hybridized carbon donor as well as bulky tBu groups on the phosphorus (Scheme 62).180 Mixing

2.4. From Alkyl Metal Reagents

Organolithium, Grignard, and other alkyl metal reagents sometimes can be used to prepare nickel hydride complexes if β-hydride elimination from the nickel alkyl intermediates is facile. Under a nitrogen atmosphere, the reduction of Ni(acac)2 (acac = acetylacetonate) with iBu3Al in the presence of a large excess of Et3P or nBu3P gives (R3P)2Ni(H)(N2) (Scheme 59).175 The isolated nickel(I) hydrides lose N2 gradually but

Scheme 62. Nickel Pincer Hydride Complex Generated from Chloride Complex and nBuLi

Scheme 59. Synthesis of Nickel Hydride Complexes Using i Bu3Al

show higher stability under nitrogen. The synthesis failed when the reaction was carried out at room temperature or used Ph3P or Ph2PEt as the ligand. In a closely related study, mixing Ni(acac)2 with Ph3P and Et2AlBr in a 1:1:0.16 ratio at −10 °C produces a nickel hydride species, which was proposed to be (Ph3P)3Ni(H)Br.176 The same hydride can be isolated from the reaction of (acac)(Ph3P)NiEt and Et2AlBr in the presence of 2 equiv of Ph3P.

the nickel chloride complex with nBuLi forms a hydride as suggested by NMR spectroscopy; however, isolating the hydride is challenging due to degradation of the species via Csp3−H reductive elimination. Tetradentate ligand systems typically face the same challenges as the pincer systems, unless one of the donor groups dissociates from nickel to provide a vacant coordination 8391


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site for β-hydride elimination. For a tripodal ligand like N(CH2CH2StBu)3, synthesizing the nickel hydride complex from the chloride complex is possible with EtMgBr (Scheme 63).53 The initially formed nickel ethyl complex undergoes β-

material!182 In solution, they gradually isomerize to trans(iPrIM)2Ni(F)(C6F3RH) as a result of C−F bond activation following C−H reductive elimination.183 Several nickel pincer hydrides have been made from nickel alkoxide or hydroxide complexes with silanes (Figure 6). Of

Scheme 63. Synthesis of [(NS3tBu)NiH](BPh4) Using EtMgBr

hydride elimination reversibly, and therefore, isolating the hydride requires a stream of nitrogen to purge the ethylene out of the reaction mixture. The 1:1 reaction of Cp*Ni(acac) and CH3Li results in a trinuclear nickel cluster bearing a bridging hydride and a bridging CH group (Scheme 64).181 This cluster is para-

Figure 6. Nickel pincer hydrides made from alkoxide/hydroxide complexes and silanes.

particular note is the synthesis of (MeNNNMe)NiH, which is unsuccessful with the LiAlH4, LiEt3BH, and oxidative addition methods described earlier. It proves to be possible to isolate this hydride from the reaction of (MeNNNMe)NiOMe and Ph2SiH2 in toluene kept at −70 °C.49 Synthesis of (iPrPCPiPr)NiH involves a series of transformations from (iPrPCPiPr)NiBr to (iPrPCPiPr)NiNH2 then to (iPrPCPiPr)NiOMe and finally to the hydride using (EtO)3SiH.184 Using LiEt3BH to convert (iPrPCPiPr)NiBr to the hydride, though effective, was reported to be a less desirable synthetic method due to the problems of removing boron-based byproducts. The pyridine dicarboxamide-based nickel hydride was not isolated but observable by NMR from the reaction of the corresponding nickel hydroxide complex with Ph2SiH2 in THF-d8.185 Compared to the silane route, the reaction of [(ArNCONCONAr)NiCl](NEt4) with LiBH4 has a much lower conversion (33% vs >99% in 1 h). Synthesis of nickel hydrides from nickel amido complexes directly is realistic, as demonstrated in the conversion of Cp*Ni(PEt3)NHTol to Cp*Ni(PEt3)H using Me3SiH.186 It has been noted that removing the residual Me3SiCl from commercial Me3SiH is needed in order to avoid the conversion to Cp*Ni(PEt3)Cl. Related NHC-tethered cyclopentadienyl nickel hydride is detectable by NMR from the reaction of the tert-butoxide complex and PhSiH3 (Scheme 66).187 However, isolating this hydride in an analytically pure form has been unsuccessful.

Scheme 64. Synthesis of a Trinuclear Nickel Cluster from Cp*Ni(acac) and CH3Li

magnetic with an oxidation state of +7/3 averaged by the three nickel centers. The mechanism for the hydride formation is not fully understood, although based on a deuterium-labeling experiment, it is certain that the hydride is originated from CH3Li. 2.5. σ-Bond Metathesis with a Silane or Borane

Cleavage of a Ni−Y (Y = F, O, N) bond by a silane or borane offers another opportunity to synthesize nickel hydride complexes. These reactions are often high yielding, driven by the formation of a strong Si−Y or B−Y bond. Removing the silicon- or boron-containing byproducts could complicate the purification of the hydrides, and in some cases silanes and boranes may also react with the newly formed hydrides. Perfluorinated aromatic compounds such as C6F6 and C6F5CF3 undergo C−F bond activation with {(iPrIM)2Ni}2(COD). The resulting nickel fluoride complexes react with an equimolar amount of PhSiH3 to generate nickel hydrides, which can be isolated in an analytically pure form (Scheme 65).182,183 Both compounds are highly sensitive to air, moisture, Et2O, THF, and halogenated solvents, and trans(iPrIm)2Ni(H)(C6F5) has been described as a pyrophoric

Scheme 66. Formation of (Cp*-NHC)NiH from (Cp*NHC)NiOtBu and PhSiH3

A tetradentate ligand system featuring one phosphorus and three sulfur donors has been explored to mimic [NiFe]hydrogenase. The reactions with [(PS3)NiX](PPN) (X = OPh, Cl; PPN+ = [Ph3PNPPh3]+) with commonly used hydride sources such as NaBH 4 , LiEt 3 BH, and LiAlH 4 give [(PS3)2Ni2]2− in which Ni(III) has been reduced to Ni(II).188 Treatment of [(PS3 )NiOPh](PPN) with pinacolborane (pinBH) at −80 °C generates a nickel(III) hydride (Scheme 67) based on reactivity (elimination of H2 and CS2 insertion). The low thermal stability of the hydride prevents it from being isolated from the reaction mixture.

Scheme 65. Synthesis of Nickel Hydride Complexes from the Fluoride Complexes and PhSiH3

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polydentate ligand supporting the nickel center is either a dithiolate derived from 1,4,7-triazacyclononane (TsTACNS2)174,189 or 1,3-bis((mercaptoethyl)phenylphosphino)propane (meppp).190 Given the lability of NO3−, it is highly likely that both HCO2− coordination and decarboxylation events take place at the Rh or Ir center.

Scheme 67. Hydride [(PS3)NiH](PPN) Generated in Situ Using a Borane

2.7. η1-Borane Complexes

A wide variety of η1-borane complexes of nickel have been synthesized for the purpose of modeling [NiFe]-hydrogenase or as a consequence of activating small molecules (Table 8). Well-defined nickel η1-borane complexes are compiled in Figure 7. The triaminotriethylamine-based complex [(tren)Ni(NCBH3)]2(BPh4)2 can be synthesized from the amine ligand, NiSO4·6H2O, and Na[NCBH3] followed by salt metathesis with NaBPh4.191 The bis(mercaptoimidazoyl)borate-ligated nickel complex (BmMe)2Ni is air stable in the solid state and readily available from the reaction of NiCl2·6H2O with 2 equiv of Na[BmMe] in H2O or CH3OH.192 The analogous (BtMe,Et)Ni193 and (BttMe)Ni194 can be prepared similarly from NiCl2· 6H2O and the sodium salt of the corresponding dihydridoborate in methanol. Synthetic procedures for the bis(pyrazolyl)borate complex (BptBu)2Ni are slightly different, requiring NiCl2(pyridine)4 and Tl[BptBu] as the starting materials (mixed in THF).195 In contrast to the other homoleptic complexes described above, the two Ni−H−B bonds of (BptBu)2Ni are trans to each other. The structural diversity of these η1-borane complexes can be further increased through the use of a different nickel precursor. For instance, the reaction of Ni(PPh3)2(NO)Br with Na[BmMe] or Na[BmtBu] (1 equiv)

2.6. From Sodium Formate

Sodium formate is potentially a good source of H− for the synthesis of nickel hydride complexes. Heterobimetallic complexes of Ni−Ir and Ni−Rh with a bridging hydride ligand have been synthesized in this way (Scheme 68). The Scheme 68. Synthesis of Heterobimetallic Complexes Using Sodium Formate as the Hydride Donor

Table 8. Nickel Borane, Silane, Dihydrogen, and Agostic Alkyl Complexesa hydride complex

type

reagents

ref

[(tren)Ni(NCBH3)]2(BPh4)2 (BmMe)2Ni (BtMe,Et)2Ni (BttMe)2Ni (BptBu)2Ni (BmR)Ni(NO)(PPh3) (R = Me, tBu) [(BmR)Ni(dppe)]X (R = Me, tBu; X = Cl, Br) [(TmR)Ni(dppe)]X (R = Me, tBu, p-Tol; X = Cl, Br) (MesDPBPh-H)NiE (E = SiH2Ph, SiHPh2) {(Cy3P)Ni}2(μ-Ph2SiH)2 {(dmpe)Ni}2(μ-Ph2SiH)2 {(iPrIm)Ni}2(μ-Ph2SiH)2 [(dtbpe)Ni(μ-H)(SiMes2)](BArF4) [(dtbpe)Ni(μ-H)(SiMes2)](BArF4) (η2-NHSiH−NHC)Ni(PMe3)2 (PhPSiPPh−H)Ni(PPh3) (PhPSiPPh−H)Ni(PMe3) (PhPSiPPh−H)Ni(CO) (PhP3Si−H)Ni(PMe3) [(tBuPCPtBu)Ni(H2)][B(C6F5)4] [(P−P)NiEt](BF4) (P−P = dtbpe, dtbpp, dtbpmb) [(dtbpe)Ni(CH2E(Me)2CH3)]X (E = C, Si; X = BArF4 or PF6) [(AnCNAr)2NiEt](BArF4) [(AnCNAr)2NiiPr](BArF4) (MeNacNacxy)NiR′ (R′ = Et, nPr)

η1-borane η1-borane η1-borane η1-borane η1-borane η1-borane η1-borane η1-borane η1-borane η2-silane η2-silane η2-silane η2-silane η2-silane η2-silane η2-silane η2-silane η2-silane η2-silane η2-H2 β-agostic γ-agostic β-agostic β-agostic β-agostic

NiSO4·6H2O, tren, Na[NCBH3] NiCl2·6H2O, Na[BmMe] NiCl2·6H2O, Na[BtMe,Et] NiCl2·6H2O, Na[BttMe] NiCl2(pyridine), Tl[BptBu] Ni(PPh3)2(NO)Br, Na[BmR] Ni(dppe)X2, Tl[BmR] Ni(dppe)X2, Tl[TmR] (MesDPBPh)Ni, HE Ni(COD)2, Ph2SiH2, Cy3P Ni(COD)2, Ph2SiH2, dmpe {(iPrIm)2Ni}2(COD), Ph2SiH2 (dtbpe)Ni(SiHMes2), [Cp2Fe](BArF4) [(dtbpe)Ni(neopentyl)](BArF4), Mes2SiH2 (NHSi-NHC)NiBr2, PMe3, KC8 Ni(PPh3)4, PhPSiPPh-H Ni(PMe3)4, PhPSiPPh-H (PhPSiPPh-H)Ni(PMe3), CO Ni(PMe3)4, PhP3Si−H (tBuPCPtBu)NiCl, [(Et3Si)2H][B(C6F5)4] (P−P)Ni(ethylene), HBF4·OEt2 (dtbpe)Ni(CH2E(Me)2CH3), [Cp2Fe]X (AnCNAr)2NiEt2, [H(OEt2)2](BArF4) (AnCNAr)2NinPr2, [H(OEt2)2](BArF4) (MeNacNacxy)NiR′(2,4-lutidine), BF3•OEt2

191 192 193 194 195 196 197 197 152 198 198 148 199 199 200 201 202 202 154 169 210 211 212 212 213

a

Abbreviations for the ligands: tren = N(CH2CH2NH2)3, MesDPBPh = MesB(o-Ph2PC6H4)2, dmpe = Me2PCH2CH2PMe2, iPrIm = 1,3diisopropylimidazolin-2-ylidene, dtbpe = tBu2PCH2CH2PtBu2, ArF = 3,5-(CF3)2C6H3, PhPSiPPh−H = (o-Ph2PC6H4)2Si(Me)H, PhP3Si−H = (oPh2PC6H4)3SiH, dtbpp = tBu2PCH2CH2CH2PtBu2, dtbpmb = o-(tBu2PCH2)2C6H4, 2,4-lutidine =2,4-dimethylpyridine; see main text for structures of η1-borane complexes (Figure 7), NHSi-NHC (Scheme 71), tBuPCPtBu-pincer (Figure 8), (AnCNAr)2 (Figure 9) and MeNacNacxy (Figure 9). 8393


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Figure 7. Well-defined nickel η1-borane complexes.

in CH3OH provides a 5-coordinate nickel complex (BmR)Ni(NO)(PPh3).196 Cationic complexes [(BmR)Ni(dppe)]X and [(TmR)Ni(dppe)]X can be synthesized by treating Ni(dppe)X2 with Tl[BmR] and Tl[TmR], respectively.197 Using sodium salts Na[BmR] and Na[TmR] results in lower yields or intractable products. It is interesting to note that in [(TmR)Ni(dppe)]X one of the mercaptoimidazolyl arms is not coordinated to nickel. Complexes (MesDPBPh-H)NiE (E = H, SiH2Ph, SiHPh2) are available from activation of H2 and silanes, which have already been described in sections 2.2.2 and 2.2.4 (Schemes 35 and 45).152

The nickel(I) silyl complex (dtbpe)Ni(SiHMes2) (available from the reaction of {(dtbpe)Ni(μ-Cl)}2 with Mes2Si(H)K) is essentially the bulky analog of the monomeric form of {(dmpe)Ni}2(μ-Ph2SiH)2. Its oxidation by [Cp2Fe]+ provides a diamagnetic bridging hydride complex, which based on the NBO analysis is best described as a protonated silylene complex (Scheme 70).199 Alternatively, this compound can be synthesized from metathesis reaction of the neopentyl complex with Mes2SiH2. Scheme 70. Synthesis of a “Protonated” Nickel Silylene Complex

2.8. η2-Silane Complexes

As mentioned in section 2.2.4, activation of silanes with nickel could lead to nickel η2-silane complexes. Most of these studies rely on a Ni(0) species for the synthesis. A representative example is the 1:1:1 reaction of Ni(COD)2 with Cy3P and Ph2SiH2 (Scheme 69), which forms a Ph2SiH-bridged dinickel complex in very high yield (97%).198 The Cy3P ligands can be substituted by a chelating phosphine such as dmpe to give {(dmpe)Ni}2(μ-Ph2SiH)2, which is also available by mixing Ni(COD)2 with dmpe and Ph2SiH2 directly. Both nickel complexes contain the same core structure as {(iPrIm)Ni}2(μPh2SiH)2, whose synthesis is already described in Scheme 42.

Nickel complexes bearing a mixed silylene−carbene ligand exhibit unprecedented reactivity with reducing agents. As demonstrated in Scheme 71, (NHSi−NHC)NiBr2 is reduced by KC8 (2 equiv) to a Ni(0) η2-silane complex that is stabilized by PMe3.200 The reaction stopped after a shorter time (3 h) leads to the isolation of another nickel complex, which remains at the Ni(II) state but already loses a hydrogen from the βdiketiminate part of the ligand. Bis(o-phosphinophenyl)silane complexes of nickel have been independently investigated by three research groups (Scheme 72). The reaction of the tridentate ligand with Ni(PPh3)4201 or Ni(PMe3)4202 does not lead to a pincer complex (via an oxidative addition pathway) but rather creates an η2-silane complex with one PPh3 or PMe3 ligand still coordinated. In fact, pincer hydride complex (PhPSiMePPh)NiH remains elusive;

Scheme 69. Synthesis of Ph2SiH-Bridged Dinickel Complexes

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yields a neutral nickel hydride complex. In contrast, the isopropyl analog does not undergo deprotonation by Et3N. Pincer complex [(tBuPCPtBu)Ni(H2)][B(C6F5)4] is synthesized in C6H5F by treating (tBuPCPtBu)NiCl with [(Et3Si)2H][B(C6F5)4] and then exposing the reaction mixture to H2.169

Scheme 71. Synthesis of a Nickel Silane Complex from a Mixed Silylene−Carbene Complex

2.10. Agostic Nickel Alkyl Complexes

Agostic interactions have been observed with numerous transition metal complexes.205,206 In nickel complexes specifically, such interactions play important roles in catalytic processes such as olefin polymerization207 and cross-coupling reactions.208 Most of the agostic hydrogens of nickel alkyl complexes show negative chemical shifts, resembling what is typically seen for a hydride resonance. Agostic nickel alkyl complexes often have the tendency to eliminate the agostic hydrogens to form nickel hydrides. For these reasons, they are included here for discussion. Agostic nickel alkyl complexes have been spectroscopically observed74,209 and even isolated in a number of studies (Figure 9). The diphosphine-ligated nickel ethyl complexes [(P− P)NiEt](BF4) can be synthesized from protonation of (P− P)Ni(ethylene) with HBF4·OEt2.210 Related [(dtbpe)Ni(CH2E(Me)2CH3)]X (E = C, Si; X = BArF4 or PF6) are available from oxidation of nickel(I) complexes (dtbpe)Ni(CH2E(Me)2CH3) by ferrocenium salts.211 The 1,2-acenaphthylenedione-derived α-diimine complexes [(AnCNAr)2Ni(CHR′CH 3)](BArF4) can be prepared by treating the corresponding nickel diethyl or di-n-propyl complexes with [H(OEt2)2](BArF4).212 It should be mentioned that in the latter case, the initially formed cationic nickel n-propyl complexes isomerize to the thermodynamically more stable nickel isopropyl complexes, presumably via β-hydride elimination followed by propylene insertion. The neutral nickel ethyl and n-propyl complexes (MeNacNacxy)Ni(CH2CH2R) with βagostic interactions are isolable through the removal of 2,4lutidine from (MeNacNacxy)Ni(CH2CH2R)(2,4-lutidine) using BF3·OEt2.213 In contrast to the cationic α-diimine system, the linear isomer in this case is thermodynamically more stable than the branched isomer. In addition to nickel alkyl complexes, nickel halide complexes may also have agostic interactions but often involve remote hydrogens. For example, the crystal structure of NiCl(η3allyl)(P(menthyl)(Me)(tBu)) shows that the methine hydrogen of the menthyl group has close contact with nickel.214 A similar phenomenon has been observed for nickel(II) complexes of benziporphyrins, where the rigid ligand structure brings a C−H bond to the close proximity of nickel for interaction.215

Scheme 72. Synthesis of Nickel Bis(ophosphinophenyl)silane Complexes

an attempt to prepare this hydride from (PhPSiMePPh)NiCl and LiEt3BH results in the isolation of (PhPSiMePPh)2Ni2, where the silicon donors bridge two nickels.82 However, the cyclooctenyl complex is regarded as a masked hydride, showing reactivity similar to a nickel pincer hydride.203 Its reaction with PPh3 gives the phosphine-trapped nickel η2-silane complex. The trimethylphosphine complex exchanges with CO, resulting in a new η2-silane complex.202 To some extent, the tripodal ligand (PhP3Si−H) illustrated in Scheme 47 behaves similarly to the tridentate bis(ophosphinophenyl)silane ligand mentioned above. The reaction of PhP 3Si−H with Ni(PMe3 ) 4 also yields an η 2-silane complex.154 The difference lies in the fact that in the tripodal system PMe3 can be removed upon heating, resulting in a terminal hydride bound to nickel. 2.9. Dihydrogen Complexes

Dihydrogen complexes of nickel are rare and often observed at low temperatures170 or in hydrogen-containing matrices.204 The two known nickel dihydrogen complexes stable at ambient temperature are cationic with a weakly coordinating counteranion (Figure 8). Complexes [(RP3Si)Ni(H2)](BArF4) are thermally stable and generated from an NMR reaction of [(RP3Si)Ni(N2)](BArF4) with H2 in CD2Cl2.155 The phenyl derivative is presumably more acidic; its deprotonation by Et3N

3. CHARACTERIZATION 3.1. Nuclear Magnetic Resonance (NMR) Spectroscopy

Diamagnetic nickel hydride complexes have been routinely studied by 1H NMR spectroscopy. The presence of a 1H resonance shifted upfield from tetramethylsilane (TMS) is often indicative of a hydride species. The chemical shift of a specific metal hydride complex may be calculated by considering both paramagnetic and diamagnetic contributions to the 1H screening constants, although the discrepancy between experimental values and calculated ones could be high.216,217 According to the reported data for nickel hydride complexes, the resonance for a terminal hydride appears within the range from −5 to −38 ppm; those with a hydride resonance close to either end of the range are depicted in Figure 10. Both

Figure 8. Nickel dihydrogen complexes stable at room temperature. 8395


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Figure 9. Isolated agostic nickel alkyl complexes.

Figure 10. Hydride resonances of selected nickel complexes.

trans-(Cy3P)2Ni(H)X and trans-(iPr3P)2Ni(H)X follow an increasing order of δNiH value (less negative): Cl < Br < I < CN, which is consistent with the trans influence series for X (Figure 11). These results also indicate that the ancillary ligands could drastically impact the hydride resonance.

these resonances could be difficult to locate or are absent, as reported for the RPSiPR-pincer system.203 A dynamic process involving the exchange between the bridging hydride and other hydrogens can potentially cause a significant shift of the hydride resonance. For example, (dtbpe)Ni(H)SiMes2H and (dtbpe)Ni(H)SiPh 2 H display peaks at −0.17 and 0.20 ppm, respectively, which are the averaged NiH and SiH resonances.151 By comparison, hydride resonances of (dtbpe)Ni(H)SiPh2(CH3) (−6.49 ppm) and (dtbpe)Ni(H)SiPh2Cl (−8.29 ppm) are typical of those for terminal hydrides due to the lack of exchangeable hydrogens on the silicon. Similar observations have been made with cis-(iPrIm)2Ni(H)(SiMes2H) (δH −1.86) and its analogs containing a tertiary silyl group (δH from −8.95 to −11.18).147,148 Variable-temperature NMR experiments can be useful in studying the aforementioned dynamic process. Like (dtbpe)Ni(H)SiMes2H, at room temperature, nickel phosphanido hydride complex (dtbpe)Ni(H){P(H)(Dmp)} (Scheme 50) gives a hydride peak at −3 ppm.161 However, at −70 °C, it is replaced with two sets of new peaks at 4.5 and −10.8 ppm, which are assigned to the PH and NiH resonances, respectively. Similarly, at room temperature, the NiH and SiH hydrogens of (SiSiSi)(μ-H)Ni(depe) (Scheme 46) appear as one broad resonance 2.61 ppm, but at −80 °C it is split into two sets of resonances at 4.79 (δSiH) and −6.7 ppm (δNiH).153 The 1H NMR of K2{(MeNacNac)Ni}2Ni(μ-H)4 (Figure 12) is temperature dependent not because of a dynamic process but due to a variable population of the triplet state.72 At 20 °C, the hydride resonance appears around −37 ppm as a broad peak. Raising the temperature increases the triplet state population, resulting in further broadening and peak shifting more upfield (ca. −52 ppm at 80 °C). Conversely, lowering the temperature

Figure 11. Hydride resonances of trans-(R3P)2Ni(H)X.

Chemical shifts of bridging hydrides (δNi−H−M) depend greatly on the nature of the interacting element M. Hydride resonances of diphosphine-supported Ni−H−Ni complexes typically appear between −9.3 and −13.5 ppm.31,62−65,131 Reduced α-diimine-ligated complexes [(enediamido)2Ni2(μH)2]2− have a hydride resonance at −14.37 (Na salt) or −14.78 ppm (K salt).97 Hydrides in cluster {(iPr3P)Ni}5H6 are highly fluxional, featuring a singlet at −26.5 ppm. Bridging hydrides in (C5R5)Ni(μ-H)(μ-CO)MCp2 (R = H, Me; M = Mo, W) can be found in a narrow range from −5.5 to −6.5 ppm. The [NiFe]-hydrogenase models illustrated in Scheme 31 display hydride resonances around −3.3 (pdt complexes) or −5.5 ppm (edt complexes), depending on the length of the bridging dithiolate.127−129 The RTACN-S2-ligated Ni−H−M (M = Ru, Rh, Ir) complexes and related compounds (Schemes 58 and 68) are paramagnetic due to the octahedral geometry at the nickel center.172,174,189 In contrast, the meppp-ligated Ni−H− M complexes bear a 5-coordinate nickel center (Scheme 68) and, hence, are diamagnetic, showing bridging hydride signals at −7.86 (Ni−H−Rh complex) and −11.04 ppm (Ni−H−Ir complex).190 Aluminum seems to have little effect on the hydride resonance of (Cp*Al)3Ni(μ-H)Al(Ph)Cp*, whose chemical shift (−11.1 ppm) is comparable to that of terminal hydride complex (Cp*Al)3Ni(H)SiEt3 (−12.8 ppm).150 The Ni−H−B resonance in (MesDPBPh-H)NiH (Scheme 35) appears at −6.16 ppm,132 which is shifted to ca. −2.5 ppm for the silyl complexes (MesDPBPh-H)Ni(SiPhRH) (Scheme 45).152 Diamagnetic nickel borohydride complexes usually show broad resonances from 0 to −2 ppm attributed to a rapid exchange of bridging and terminal hydrides.42−44 Sometimes

Figure 12. Trinuclear nickel hydride complexes. 8396


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to −60 °C gives rise to a sharp resonance around −30 ppm. Interestingly, the related complex [K2{(enediamido)2Ni3(μH)4}]2− shows 1H signals that are too broad to provide any structural information, possibly due to a higher population of the triplet state.97 Dihydrogen complexes of transition metals in general show hydride resonances that are broad and downfield shifted from the corresponding monohydride complexes. This is also observed with nickel complexes. Hydride resonances of [(tBuPCPtBu)Ni(H2)][B(C6F5)4] and (tBuPCPtBu)NiH appear at −3.21 and −10.1 ppm as a broad peak and a well-resolved triplet, respectively.169 For a tetradentate ligand system, hydride resonances of [(RP3Si)Ni(H2)][BArF4] can be found from −2.5 to −3.6 ppm, which are shifted downfield from (RP3Si)NiH (by 3−5 ppm).155 Hydride resonances of nickel η2-silane complexes usually fall into the range from −0.8 to −8.7 ppm.

The terminal Ni−H bond of [{(dippe)NiH}2(μ-H)](BPh4) is considerably short with a value of 1.1(1) Å, although the bridging Ni−H bond of 1.6(1) Å is more or less expected.31 Another study involving essentially the same cation (BEt4− as the counteranion) shows two bridging hydrides and one terminal hydride with Ni−H bond distances of 1.37−1.73 and 1.40(3) Å, respectively.218 The discrepancy observed in these two studies underscores that the Ni−H bonds should be interpreted with caution. Moreover, the Ni−H bonds in (iPrPNCNPiPr-Me)NiH [1.99(2) Å]42 and trans-(iPrIM)2Ni(H)(C6F5) [1.975(8) Å]182 are unusually long for a terminal hydride bound to nickel. DFT calculations of the latter hydride give 1.4919 Å for the Ni−H bond, implying that the crystallographic study could have overestimated the bond length.

3.2. Infrared (IR) Spectroscopy

A handful of homobimetallic/heterobimetallic complexes and clusters of nickel with a bridging hydride ligand (Figure 13)

3.4. Neutron Diffraction Study

Infrared spectroscopy could be a very useful characterization tool for identifying nickel hydrides. Nickel complexes with a terminal hydride ligand typically show a Ni−H stretching band at 1690−2000 cm−1, although sometimes the intensity of the band could be too low for a definitive identification. Of the reported values for terminal hydrides bound to nickel, the lowest ν(Ni−H) is 1695 cm−1 for CpNi(IMes)H94 while the highest one is 1994 cm − 1 for [(Cy 3 P) 2 Ni(H)(4methylpyridine)](BPh4)·CH2Cl2.26 Nickel−hydrogen bands of bridging hydride complexes are much weaker and fall into the fingerprint region that is often obscured by many bending vibrations. A commonly used strategy to identify the bands is to synthesize the corresponding nickel deuteride and observe the isotope shift [ν(Ni−H)/ ν(Ni−D) ≈ 1.4]. The Ni−H absorption band of {(dtbpe)Ni}2(μ-H)2 is broad and weak and shifted from 1280 to 920 cm−1 after deuteration.131 [{(dippe)NiH}2(μ-H)](BPh4) and [{(dcpe)NiH}2(μ-H)](BPh4) each has a broad band (dippe, 1673 cm−1; dcpe, 1654 cm−1), which could be attributed to a ν(Ni−H−Ni) or ν(Ni−H) vibration.31 The Ph2SiH-bridged dinickel complex {(iPrIM)Ni}2(μ-Ph2SiH)2 (Scheme 42) apparently has a very strong ν(Ni−H−Si) band at 1526 cm−1.148 In contrast, (SiSiSi)(μ-H)Ni(depe) (Scheme 46) shows an extremely broad band near that region (∼1600 cm−1), which could be assigned to the ν(Ni−H−Si) vibration.153 Several hydrido nickel clusters have been studied by infrared spectroscopy. In the case of (Cp*Ni)3(μ3-H)(μ3-CH), ν(Ni− H) of the triply bridging hydride ligand is observed at 981 cm−1, which is shifted to 682 cm−1 upon deuteration.181 The analogous nickel clusters (CpNi)3(μ3-CCH2R)(μ3-H) (R = nPr, n Bu) show a similar ν(Ni−H) band at 1004 cm−1.141 Nickel hydride cluster {(iPr3P)Ni}5H6 has both μ3-H and μ2-H ligands; its ν(Ni−H) band at 1235 cm−1 is broad and likely the average of all ν(Ni−H) vibrations.134

Figure 13. Nickel hydride complexes studied by neutron diffraction.

have been studied by neutron diffraction, which, compared to X-ray diffraction, provides more precise information about the Ni−H bond distance. The linearity of the Ni−H−Ni linkage in (dippm)2Ni2Br2(μ-H)177 and (dippm)2Ni2Cl2(μ-H)96 can also be more unambiguously established. The Ni−H bond distance of 1.859(7) Å in the Ni−Ru heterobimetallic complex (Figure 13) is much longer than other nickel hydrides.172 Despite a larger covalent radius with ruthenium, the Ru−H bond is shorter [1.676(8) Å] than the Ni−H bond, suggesting that the hydride is more engaged in the bonding with ruthenium. The structure of Cp4Ni4H3 features a tetrahedral Ni4 core with hydrides capping three of the four faces.219 Thus, the Ni−H bonds involving the apical nickel atom (in blue) are slightly longer than those involving the basal nickel atoms (in red). The mean for all Ni−H bonds is 1.691 (8) Å.

4. STOICHIOMETRIC REACTIONS 4.1. Proton Transfer (Acidity)

3.3. X-ray Crystallography

Some nickel hydride complexes behave as acids, especially those synthesized by the protonation method (section 2.2.1), transferring H+ to bases such as amines, anilines, pyridines, or even metal complexes. [(Ph 2PCH 2CH 2 SEt)2 NiH]+ and [(P2S2)NiH]+ deliver proton to nickel methyl complexes, resulting in CH4 elimination.124 With an appropriate base, the pKa value of a specific nickel hydride complex could be determined by measuring the equilibrium constant for the proton transfer reaction. Many of these data have been validated by DFT calculations220 or the method of adding ligand acidity constants.221 The reported pKa values for the

Despite the inherent uncertainty of the hydrogen atom positions derived from X-ray data, the hydride ligand has been located by X-ray crystallography in numerous systems. A recent search of the Cambridge Structural Database (CSD version 5.37 updates, January 2016) for molecules bearing a Ni−H bond reveals close to 300 structures, with only a few of them obtained from neutron diffraction studies (section 3.4). Most terminal hydride complexes have the Ni−H bond length in the range of 1.32−1.65 Å. Compounds with Ni−H−B moieties typically have a longer Ni−H bond (1.60−2.28 Å). 8397


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Table 9. Experimental and Calculated pKa Values of Nickel Hydride Complexesa complex [(dmpe)2NiH]

+

[(dmpp)2NiH]+ [(depe)2NiH]+ [(depp)2NiH]+ [(EtPNMePEt)2NiH]+ [(EtPNMePEt)2NiH]2+ [(dedpe)2NiH]+ [(dppe)2NiH]+ [(dppv)2NiH]+ [(CyP2BnN2)2NiH]+ [(PhP2PhN2)2NiH]+ [(PhP2BnN2)2NiH]+ [(MeP2PhN2)2NiH]+ [(dppp)Ni(H)(PhP2BnN2H)]+ (iPrPArPiPr)Ni(H)Cl [(CO)3Fe(pdt)(μ-H)Ni(dppe)]+ [(Ph3P)(CO)2Fe(pdt)(μ-H)Ni(dppe)]+ [(CO)3Fe(pdt)(μ-H)Ni(dcpe)]+ [(Ph3P)(CO)2Fe(edt)(μ-H)Ni(dppe)]+ [(CO)3Fe(edt)(μ-H)Ni(dppe)]+ [(CO)3Fe(edt)(μ-H)Ni(dcpe)]+

solvent

pKa (exp)

PhCN CH3CN PhCN CH3CN PhCN CH3CN PhCN CH3CN PhCN PhCN PhCN PhCN CH3CN PhCN CH3CN PhCN CH3CN CH3CN CH3CN CH3CN CH3CN PhCN PhCN PhCN PhCN PhCN PhCN

ref(exp)

24.4

115

24.0

115

23.8

115

23.3

117

22.2 7.7 ± 2.5 20.3 14.7 14.2 13.1 13.2 21.2 16.3 19.4 22.5 8.5 ∼18 10.7 14.9 ∼13.6 14.0 11.3 13.6

118 118 116 115 115 116 116 222 222 222 223 101 119 128 128 129 129 129 129

pKa (calcd)b 23.8 24.0 25.6 22.8 22.0

15.7 12.8

a

Abbreviations for the ligands: dmpe = Me2PCH2CH2PMe2, dmpp = Me2PCH2CH2CH2PMe2, depe = Et2PCH2CH2PEt2, depp = Et2PCH2CH2CH2PEt2, EtPNMePEt = Et2PCH2N(Me)CH2PEt2, dedpe = Et2PCH2CH2PPh2, dppe = Ph2PCH2CH2PPh2, dppv = cis-Ph2PCH CHPPh2, RP2R′N2 = substituted 1,5-diaza-3,7-diphosphacyclooctanes with R group on phosphorus donors and R′ group on nitrogen donors, dppp = Ph2PCH2CH2CH2PPh2, iPrPArPiPr = p-bis(2-diisopropylphosphinophenyl)benzene, pdt =1,3-propanedithiolate, dcpe = Cy2PCH2CH2PCy2, edt =1,2-ethanedithiolate; for structure of PhP2BnN2H, see Scheme 34. bFrom ref 220.

(iPrDPDBFPhs)Ni(H)Cl68 and [(dppe)2NiH](BF4),99 and tripodal complexes [(NP3)NiHx]Y.50 Protonation of nickel pincer hydride complexes is a useful strategy for the synthesis of other nickel pincer complexes. The reaction of (NHCCCCNHC)NiH with HCl·Et2O leads to the isolation of ( N H C CCC N H C )NiCl. 1 4 0 Protonation of (tBuPOCOPtBu)NiH224 and (tBuPCPtBu)NiH45 with HCO2H yields nickel formate complexes cleanly and efficiently. In addition to Bronsted acids, MeOTf also reacts with nickel hydrides to eliminate CH4, as demonstrated in the synthesis of (iPrPNPiPr-Me)NiOTf from (iPrPNPiPr-Me)NiH.225 Reactivity of nickel borohydride complexes toward acids can be quite similar to those of terminal hydride complexes. For example, protonation of the hexaaza-dithiolate complex [(N6S2)Ni2(μ-BH4)]+ by HCl and HCO2H generates chlorideand formate-bridged complexes, respectively (Scheme 73).57 Interestingly, H2O also reacts with the borohydride complex, resulting in a bridging hydroxide species. Tungsten complex CpW(CO)3H bears an acidic hydride and thus is capable of protonating nickel hydrides like (tBuPCPtBu)NiH (Scheme 74).226 The final product is a heterobimetallic complex featuring CpW(CO)3 bound to nickel via the oxygen end of a CO ligand. The reaction monitored at −83 °C reveals an interesting adduct that contains a dihydrogen bond. Elimination of H2 takes place readily when the temperature is raised above −43 °C.

synthetically accessible nickel hydride complexes are summarized in Table 9. Acetonitrile is often the preferred solvent for the pKa measurement of transition metal hydrides.115 However, in some cases for solubility reasons benzonitrile is chosen as the solvent. As demonstrated with [(dppe) 2 NiH] + and [(dppv)2NiH]+, values determined from these two solvents are quite similar. Some general trends can be drawn from the data listed in Table 9. Increasing the chelating ring size of [(diphosphine)2NiH]+ appears to slightly increase the acidity of the nickel hydride (dmpe vs dmpp or depe vs depp). Changing the phosphorus substituents from alkyl to aryl groups (depe or dcpe vs dppe) or replacing Ph3P with CO lowers the pKa value substantially. 4.2. Protonation by Acids

Protonation of nickel hydride complexes by acids are well documented in the literature. Part of the reason that [{(EtO)3P}4NiH](HSO4) cannot be isolated from the reaction of {(EtO)3P}4Ni with H2SO4 is because protonation of [{(EtO)3P}3NiH]+ can also occur, resulting in H2 elimination.108 The reaction of (IMes)2Ni with HCl·dioxane is not selective because the initially formed (IMes)2Ni(H)Cl undergoes further protonation to generate (IMes)2NiCl2 and H2.109 Upon mixing with strong acids, [(Ph2PCH2CH2SEt)2NiH]+ loses its hydride in the form of H2 while generating [(Ph2PCH2CH2SEt)2Ni]2+ quantitatively.125 Similar reactivity has been observed with diphosphine-ligated complexes 8398


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75).166 Such a decomposition process is accelerated by ethylene and CO.

Scheme 73. Reactions of [(N6S2)Ni2(μ-BH4)]+ with Acids

Scheme 75. Disproportionation of a Nickel Hydride Complex Bearing a Chelating [P,S]-Ligand

Scheme 74. Dihydrogen Bonding Observed with Nickel and Tungsten Hydride Complexes

Bridging hydride complex {(dcpe)Ni}2(μ-H)2 is thermally robust; in mesitylene, it loses H2 only at the boiling temperature (∼165 °C).62 However, when mixed with Ph3P, PhCN, PhCCPh, or an olefin (e.g., ethylene, propylene, and cis-2-butene), {(dcpe)Ni}2(μ-H)2 loses H2 readily at room temperature, resulting in the formation of a Ni(0) complex. The same hydride reacts with (dcpe)NiCl2 or I2 to yield (dcpe)NiX (X= Cl, I) along with H2. {(dtbpe)Ni}2(μ-H)2 shows similar reactivity toward ethylene and acetylene, giving (dtbpe)Ni(CH2CH2) and (dtbpe)Ni(CHCH), respectively.131 In the latter case, none of the reduced species such as C2H4, C2H6, (dtbpe)Ni(CH2CH2), (dtbpe)Ni(CH CH2), or (dtbpe)NiEt was observed, suggesting that the acetylene molecule simply displaces the hydrogen. The analogous {(dippe)Ni}2(μ-H)2 is equivalent to “(dippe)Ni(0)” and used to activate the C−S bonds of thiophenes,228 benzothiophenes,228 dibenzothiophenes,65,228,229 and biphenyl-2-thiol230 and the C−H or C−CN bonds of aryl231−233 and alkyl nitriles.234−237 Bridging hydride complex (dippm)2Ni2Br2(μ-H) shows unique reactivity toward exogenous ligands and bases (Scheme 76).177 Under 1 atm of CO, the hydride complex undergoes

4.3. Loss of Hydrogen via Reductive Processes

Nickel hydride complexes can lose their hydrogens under a variety of circumstances. Most of these processes are reductive as far as the nickel center is concerned, although oxidation of nickel hydrides could also lead to loss of hydrogen from nickel.91 This type of reactivity is the culprit for the low stability of many nickel hydride species but at the same time could be taken advantage of as an entry to interesting reactions. Without sufficient steric protection, monophosphine-based nickel hydride complexes decompose readily, presumably via a reductive elimination process.9,29,30 Complexes bearing bulky phosphines like trans-(R3P)2Ni(H)X (R = Cy, iPr) are stable unless heated to high temperatures. Thermolysis of trans(Cy3P)2Ni(H)SAr (Ar = Ph, p-CH3C6H4) at 140 °C leads to the breakdown of the complexes to form ArH, ArSAr, H2, and Cy3PS.105 The isomerization of trans-(iPrIM)2Ni(H)(C6F4R) to trans-(iPrIM)2Ni(F)(C6F3RH) at room temperature (completed in a few days) is an indication that the C−H reductive elimination for this system is not difficult.183 Carbon monoxide often accelerates the decay of nickel hydride complexes. For example, (Ph3P)3Ni(H)Br reacts with CO at as low as −20 °C to generate (Ph3P)2Ni(CO)2, (Ph3P)2NiBr2, free Ph3P, and likely H2.227 Under photochemical conditions, nickel hydride complexes lose H2 readily, a process implicated for photoproduction of H2 from protons. Photolysis of trans-(IMes)2Ni(H)X (X = Cl, Br) with λ > 295 nm produces H2 as well as (IMes)2NiX, although H2 yield is low ( 313 nm).68 The reaction carried out in THF-d8 reveals that the hydride exchanges with deuterium from the solvent. The formation of propylene, though in a small amount, implies hydrogen abstraction from the iPr groups of the ligand. Nickel hydrides supported by less sterically hindered bidentate ligands have the tendency to lose H2 and decompose to bis-chelating complexes. For ethylene polymerization catalyzed by anilinotropone-based nickel complexes, the deactivation pathway for the catalysts is the formation of (N,O) 2Ni from the hydride intermediate (N,O)NiH.75 Similarly, nickel hydride bearing a chelating [P,S]-ligand undergoes disproportionation while releasing H2 (Scheme

Scheme 76. Reactions of (dippm)2Ni2Br2(μ-H) with CO, NO, and Bases

disproportionation reaction to give (dippm)NiBr2 and (η1dippm)2Ni(CO)2. The fate of the hydride is not clear. Exposure of the complex to 1 atm of NO results in dissociation of dippm ligand, and it was proposed that the hydride was transferred to NO forming HNO. A strong base like KOtBu removes HBr and deprotonates the dippm ligand. In contrast, LiN(TMS)2 cleaves the P−C bond of dippm, giving a iPr2P-bridged complex. The related complex (dcpm)2Ni2Cl2(μ-H) reacts with LiH or NaH at 90 °C to form (Cy2MeP)Ni(μ-PCy2)2Ni(PMeCy2).95 The β-diketiminato nickel complex {(MeNacNac)Ni}2(μ-H)2 eliminates H2 in a number of ways (Scheme 77).71 For a toluene solution of the hydride, heating or adding KBEt3H results in a toluene-bridged Ni(I)−Ni(I) complex. Adding diphenylacetylene to the solution forms the same toluene complex except that the alkyne is reduced to stilbene and 8399


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Scheme 77. Elimination of H2 from {(MeNacNac)Ni}2(μ-H)2

Nickel pincer hydride with a reduced pyridine backbone is remarkably stable, even inert to CO or isocyanides at room temperature. In the presence of tBuNC (20 equiv) and under heating, the nickel hydride starts to decompose. One of the isolated products is an isocyanide complex (Scheme 80).48 The Scheme 80. Reaction of (tBuPONOPtBu-H)NiH with tBuNC

restoration of the pyridine structure suggests that in addition to the hydride ligand, hydrogen on the ring is also lost. Olefinic molecules such as COD can potentially insert into Ni−H bonds,140,157 thus complicating the synthesis of nickel pincer hydrides when Ni(COD)2 is used as the nickel precursor. The diphosphinoboryl-based nickel pincer hydride shows completely different reactivity with COD (Scheme 81).171 Instead

bibenzyl. Nitrogen-based ligands such as DMAP (DMAP = 4dimethylaminopyridine) and propionitrile are also capable of displacing the hydrogen. The α-diimine-based bridging hydrides resemble {(MeNacNac)Ni}2(μ-H)2 to a great extent. At 80 °C in benzene or toluene, both (α-diimine•−)2Ni2(μ-H)2 and [(enediamido)2Ni2(μ-H)2]2− lose H2 and form an arenebridged dinickel complex.97 Loss of hydrogen from nickel pincer hydrides is noted in a handful of studies. Phenyl-substituted PNPyP-pincer nickel hydride is detectable at −40 °C but upon warming to room temperature loses hydrogen as H2 and forms a dinickel complex (Scheme 16).81 The thermal stability of (MeNNNMe)NiH in solution is also low. Its degradation occurs primarily via N−H reductive elimination to yield the free pincer ligand and nickel particles (Scheme 78).49 A second decomposition pathway

Scheme 81. Reactivity of (tBuPBPtBu)NiH toward COD

Scheme 78. Degradation of (MeNNNMe)NiH of inserting into the Ni−H bond, COD coordinates to nickel through both CC bonds. In doing so, B−H reductive elimination occurs to yield a mixture of η2-borane complexes, which are converted to free pincer ligand, Ni(COD)2, and several unidentified products. Several nickel hydrides bearing a tetradentate ligand have been shown to lose hydrogen in the presence of an exogenous ligand. [(NS3tBu)NiH]+ reacts with CO rapidly to give [(NS3tBu)Ni(CO)]+, Ni(CO)4, [HNS3tBu]+, and a trace amount of H2.53 A similar reaction of [(NP3)NiH](BPh4) (NP3 = N(CH2CH2PPh2)3) with CO leads to nickel(0) complex [(HNP3)Ni(CO)](BPh4) as a result of N−H reductive elimination.123 In contrast, the analogous complex [(PP3)NiH](BPh4) (PP3 = P(CH2CH2PPh2)3) does not react with CO under the same conditions. Elimination of H2 from {(Me2-TPA)Ni}2(μ-H)2 is affected by the addition of MeCN.54 Kinetics study shows an overall first-order reaction, which is consistent with intramolecular reductive elimination of H2.

involves the conversion of (MeNNNMe)NiH to H2 and (MeNNNMe)Ni, which is further decayed to a 5-coordinate nickel complex and nickel particles. The phosphinimine-based cationic nickel hydride complexes are stable at ambient temperature. However, at 80 °C, the hydride ligand is lost due to orthometalation of the ligand (Scheme 79).78 Under this Scheme 79. Loss of Hydrogen from [(PNNRNP)NiH](PF6)

4.4. Halogen Exchange Reactions

Like other transition metal hydride complexes, nickel hydrides can react with halogenated compounds, especially those with weak C−X bonds such as CCl4 and CHCl3. These reactions have been used as indirect evidence supporting the presence of a Ni−H bond. The elusive nickel hydride trans-(Et3P)2Ni(H)Cl reacts with CCl4 to give CHCl3,21 as do the more stable trans(R3P)2Ni(H)Cl (P = Cy, iPr).25 Nickel borohydride complexes may have better resistance against CCl4, but there are also reports showing that they behave like terminal hydride complexes. As an example, Tp*Ni(BH4) reacts with CCl4,

condition, [(PNNHNP)NiH](PF6) (R = H) also loses the NH hydrogen, resulting in decomposition to nickel black and unidentified paramagnetic species. The reaction of the nickel hydrides with LiEt3BH and PhBr provides nickel phenyl complexes. It was suggested that LiEt3BH delivers another hydride to the nickel center to form a dihydride intermediate. Subsequent reductive elimination of H2 followed by oxidative addition of PhBr gives the nickel phenyl complexes. 8400


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4.5. Insertion of Alkenes and Alkynes

CBr4, and CHBr3 at room temperature and with CHCl3 at 50 °C; however, it does not react with CH2Cl2.34 Because of the halogen exchange reactions, CDCl 3 is usually not a recommended NMR solvent for studying nickel hydrides. Nickel pincer hydride complexes (iPrPNPiPr-Y)NiH60 and (bimca)NiH159 are known to react with CHCl3 to give the corresponding nickel chloride complexes. It is interesting to note that (bimca)PdH and (bimca)PtH react with CHCl3 at higher temperature (50 °C), perhaps suggesting that the Pd−H and Pt−H bonds are stronger than the Ni−H bond. In the case of nickel borohydride complexes, [(MeTACN)Ni(BH4)(CH3CN)]+ undergoes halogen exchange with CHCl3 or pXC6H4COCl to form [(MeTACN)NiCl]+.35 The C−Cl bonds in CH2Cl2 are stronger (74 vs 70 kcal/mol for CCl4), and therefore, some nickel hydrides may be dissolved in CH2Cl2 without the concern of a halogen exchange process.34,127−129 On the other hand, some nickel hydride or borohydride complexes do react with CH2Cl2, as demonstrated with [(PNNRNP)NiH](PF6)78 and (κ3-triphos)Ni(κ2-BH4).33 (MeNNNMe)NiH is particularly reactive toward primary alkyl halides, providing (MeNNNMe)NiX and alkane in less than 1 min.49 The rate for the halogen exchange reaction follows the decreasing order of alkyl iodides > alkyl bromides > alkyl chlorides. The halogen exchange reactions mentioned above are known to be radical processes,238 which should be influenced by the strength of the Ni−H bond. The free energies239 for the homolytic Ni−H bond cleavage (ΔGoH•) of many nickel hydrides have been determined experimentally and computationally. These data are summarized in Table 10.

Insertion of alkenes and alkynes into a metal−hydrogen bond is an important elementary reaction in organometallic chemistry. Reaction of alkenes with nickel hydrides, specifically, has major implications for ethylene oligomerization. DFT calculations on the acac ligand system suggest that a 3-coordinate nickel hydride (acac)NiH is the key intermediate, which adopts a planar geometry and allows ethane to coordinate cis to the hydride.240,241 Consistent with this mechanistic proposal, the related nickel hydride [κP, κO-Ph2PCH2C(CF3)2O]Ni(H)(PCy3) is inactive for catalytic ethylene oligomerization, but the mixture of Ni(COD)2 and Ph2PCH2C(CF3)2OH is catalytically active. The inertness of the isolated nickel hydride is likely due to PCy3 blocking the vacant coordination for ethylene. Nickel-catalyzed ethylene polymerization reactions involve a similar insertion step, which happens after the βhydride elimination from the propagating alkyl chain.74,75,207 Insertion of alkenes is common with many other nickel hydride complexes. Mixing Ni(COD)2 and iPr3P (2 equiv) with 1,2,4,5-tetrafluorobenzene generates a nickel η3-cyclooctenyl complex as shown in Scheme 82.137 The initial product from the insertion of COD into trans-(iPr3P)2Ni(H)(C6F4H) undergoes iterative β-hydride elimination and reinsertion until the π-allyl-type structure is formed. A similar process using 1,2,3,5- or 1,2,3,4-tetrafluorobenzene produces a mixture of two isomers. The 5-coordinate nickel hydrides [{(EtO)3P}4NiH]+ and [{(MeO)3P}4NiH]+ react with dienes at 0 °C to form nickel π-allyl complexes.242 The fast insertion rate is consistent with fact that phosphite dissociation from nickel is facile. Compared to the rapid reactions with the phosphite complexes, insertion of 1,3-butadiene into [(dppe)2NiH]+ is considerably slower (t1/2 ≈ 8 h at 50 °C), which has been attributed to slow dissociation of the phosphine arm.242 A related complex [(Ph2PCH2CH2SEt)2NiH]+ in which a more labile chelating ligand is present reacts with 1,3-butadiene rapidly even at −78 °C to yield a π-allyl complex.125 This result further demonstrates that the success of alkene insertion into these coordinately saturated nickel hydrides relies on the lability of the ligand. Square planar nickel hydride (dtbpe)Ni(H){P(H)(Dmp)} reacts with 1-hexene at 110 °C (Scheme 83).161 The isolated product is a secondary phosphine, suggesting that the insertion product reductively eliminates the hexyl and phosphanido groups. Some of the nickel pincer hydrides react with alkenes via an insertion mechanism. Under an ethylene atmosphere, both (MeNNNMe)NiH49 and [(PNNRNP)NiH](PF6)78 are converted to the nickel ethyl complexes. The insertion in the latter case is reversible, thus preventing the ethyl complex from being isolated. The reaction of NHCCCCNHC-H with Ni(COD)2 produces the desired (NHCCCCNHC)NiH along with an impurity that was proposed to be the COD insertion product.140 The PhPNPPh-pincer hydride reacts with a variety of alkenes (Scheme 84).157 Unlike the iPr3P-ligated nickel hydride, the COD-inserted product in the pincer system does not undergo isomerization. Other molecules that can insert a CC bond into this hydride include norbornene, ethylene, 1hexene, styrene, and methyl acrylate. The regioselectivity for the insertion reaction depends on how the CC bond is polarized. Electron-withdrawing substituents such as phenyl and ester groups favor the 2,1-insertion, whereas the electrondonating nBu group promotes the 1,2-insertion. The steric (and perhaps the electronic) environment around nickel is very important for these insertion reactions. At room temperature or

Table 10. Experimental and Calculated ΔGoH• (in kcal/mol) for Nickel Hydride Complexesa complex +

[(dmpe)2NiH] [(dmpp)2NiH]+ [(depe)2NiH]+ [(depp)2NiH]+ [(EtPNMePEt)2NiH]+ [(dedpe)2NiH]+ [(dppe)2NiH]+ [(dppv)2NiH]+ [(CyP2BnN2)2NiH]+ [(PhP2PhN2)2NiH]+ [(PhP2BnN2)2NiH]+ [(CO)3Fe(pdt)(μ-H)Ni(dppe)]+ [(Ph3P)(CO)2Fe(pdt)(μ-H)Ni(dppe)]+

ΔG°H• (exp)

ref

ΔG°H• (calcd)b

55.7 55.6 56.3 54.5 55.3 56.4 52.8 52.5 53.2 52.4 52.8 57 58

115 115 115 117 118 116 115 116 222 222 222 128 128

57.3 55.6 57.1 53.0 55.3 55.9 53.1

a

Abbreviations for the ligands: dmpe = Me2PCH2CH2PMe2, dmpp = Me 2 PCH 2 CH 2 CH 2 PMe 2 , depe = Et 2 PCH 2 CH 2 PEt 2 , depp = Et2PCH2CH2CH2PEt2, EtPNMePEt = Et2PCH2N(Me)CH2PEt2, dedpe = Et2PCH2CH2PPh2, dppe = Ph2PCH2CH2PPh2, dppv = cisPh2PCHCHPPh2, RP2R′N2 = substituted 1,5-diaza-3,7-diphosphacyclooctanes with R group on phosphorus donors and R′ group on nitrogen donors, pdt = 1,3-propanedithiolate; data are obtained from CH3CN. bFrom ref 220.

As seen from Table 10, the variation for the ΔG°H• values is small. It appears that phenyl-substituted phosphines such as dppe and dppv make the Ni−H bond slightly weaker in comparison with the all alkyl phosphines. As expected, the ΔG°H• values of the heterobimetallic complexes are higher than those of terminal hydrides. 8401


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Scheme 82. Reactions of Ni(COD)2 and iPr3P with Tetrafluorobenzene

Scheme 85. Reaction of trans-(iPr3P)2Ni(H)(C6F4H) with 3Hexyne

Scheme 83. Insertion of 1-Hexene into (dtbpe)Ni(H){P(H)(Dmp)}

Scheme 86. Reactions of (κP,κSe-o-Ph2PC6H4Se)NiH(PMe3)2 with Alkynes

Scheme 84. Various Insertion Reactions Involving (PhPNPPh)NiH

80 °C, neither (iPrPNPiPr)NiH nor (CyPNPCy)NiH is reactive toward COD, norbornene, ethylene, 1-hexene, and styrene. However, they interact with methyl acrylate at room temperature in the same way as (PhPNPPh)NiH. The unsymmetric pincer complex (PhPNPiPr)NiH reacts with all of the alkenes shown in Scheme 84 except COD. The closely related (PhPNPR-Me)NiH (R = Cy, iPr) do not react with COD either, based on the fact that they can be successfully synthesized from the pincer ligands and Ni(COD)2.158 Dioxgen might insert into the Ni−H bond to form (PhPNPR-Me)NiOOH, although identification of the products is challenging. Nickel hydrides supported by a tetradentate ligand have also been studied for alkene insertion. The reaction of ethylene with the 5-coordinate nickel hydride [(NS3tBu)NiH]+ is reversible.52,53 In fact, this allows [(NS3tBu)NiH]+ to be synthesized from [(NS3tBu)NiCl]+ and EtMgBr by purging ethylene out of the reaction. The β-hydride elimination and ethylene insertion steps probably require the dissociation of one of the thioether arms. The insertion of alkynes into nickel hydrides is relatively unexplored. The 1:1 reaction of trans-(iPr3P)2Ni(H)(C6F4H) with 3-hexyne results in primarily reductive elimination of 1,2,4,5-tetrafluorobenzene with less than 5% of the nickel species being the insertion product (Scheme 85).137 The reaction of the [P,Se]-ligated nickel hydride with PhCCH gives a 2,1-insertion product in 72% isolated yield (Scheme 86).167 The insertion product originated from PhCCMe, however, places nickel and the phenyl group trans to each

other. With PhCCPh, PhCCSiMe3, and Me3SiCH as the substrates, the isolated products are nickelacyclopropanes rather than the anticipated vinylic complexes. It is possible that the steric pressure exerted by the more bulky vinyl groups forces PMe3 to migrate from nickel to carbon. The insertion of PhCCH into (RPOCOPR)NiH (R = iPr, c Pe) is less selective, producing an isomeric mixture favoring the 2,1-insertion product (Scheme 87).243 The more bulky Scheme 87. Insertion of Phenylacetylene into (RPOCOPR)NiH

nickel hydride (tBuPOCOPtBu)NiH does not react with PhC CH at all. In contrast, the palladium analogs (RPOCOPR)PdH (R = tBu, iPr, cPe) react with PhCCH to give alkynyl complexes and H2 with only a negligible amount of insertion products.244 The insertion of PhCCH into [(NS3tBu)NiH]+ affords an (E)-alkenyl complex exclusively, although the 8402


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energies for hydride transfer (from synthetically accessible nickel hydrides) are summarized in Table 11. Some important trends can be drawn from the data listed in Table 11. The ΔGoH− value increases in the order of [(depe)2NiH]+ < [(dedpe)2NiH]+ < [(dppe)2NiH]+, suggesting that electron-donating substituents on the phosphorus donors increase the hydricity of the nickel hydride (smaller numbers mean better hydride donors). A good correlation exists between the hydricity and the twist angle between the two diphosphine ligands; the smaller the twist angle is, the better the hydride donor is. Thus, the fact that [(dmpe)2NiH]+ is a better hydride donor than [(depe)2NiH]+ is due to reduced steric interactions between the phosphine ligands. This sterics argument could also be used to explain the observation that a smaller chelate ring size favors the hydride transfer (i.e., [(dmpe) 2 NiH] + < [(dmpp) 2 NiH] + ; [(depe) 2 NiH] + < [(depp)2NiH]+). The ΔGoH− values for [(EtPNMePEt)2PdH]+ and [(EtPNMePEt)2PtH]+ have also been measured experimentally.247 Comparing these compounds with the nickel complex reveals a periodic trend of Pd > Pt > Ni; the nickel hydride is a poorer donor than the platinum hydride by 11 kcal/mol. The ΔGoH− values for the Ni−Fe heterobimetallic compounds are substantially higher, implying that the hydride ligand is not very hydridic.

conversion is low (30%) due to decomposition of the nickel species.53 The reaction of {(dmpe)Ni}2(μ-Ph2SiH)2 with 4 equiv of diphenylacetylene generates a disilane and a nickel alkyne complex (Scheme 88).198 The first step of the reaction may involve alkyne insertion into the Ni−H bonds or alternatively the insertion into the Ni−Si bonds. Scheme 88. Reaction of {(dmpe)Ni}2(μ-Ph2SiH)2 with Diphenylacetylene

4.6. Hydricity of Nickel Hydrides

One of the most common reactivities associated with transition metal hydride complexes is their ability to transfer H− to different electrophiles. In terms of nickel hydrides, they have been demonstrated to deliver the hydride to organic and organometallic compounds. For instance, [(depe)2NiH]+ and [(dmpp)2NiH]+ can transfer hydride to N-benzylnicotinamide hexafluorophosphate.115 The reaction between [(dmpe)2NiH]+ and [CpRe(CO)2(NO)]+ involves the attack of the coordinated CO by hydride to yield [(dmpe)2Ni]2+ and CpRe(CO)(NO)(CHO).114 For a better understanding of the reactivity of nickel hydrides, it is important to know their kinetic and thermodynamic hydricity. Kinetic information about hydride transfer from nickel to a noncoordinating hydride acceptor (e.g., Ph3C+)245 is virtually unknown in the literature. In contrast, thermodynamic hydricity of nickel hydrides has been extensively studied. The experimental and calculated free

4.7. Insertion of CO and CS Bonds

There has been some growing interest in developing nickelbased catalysts for the reduction of carbonyl compounds.10,13 Insertion of the carbonyl group into a Ni−H bond can be a crucial step for these reactions. Several nickel pincer hydride complexes are known to display this type of reactivity with aldehydes or ketones. The bis(phosphinite)-based pincer complex (iPrPOCOPiPr)NiH reacts with PhCHO readily to yield a nickel benzyloxide complex.89,248 The insertion reaction is sluggish with the more bulky hydride (tBuPOCOPtBu)NiH or with a ketone substrate. Studies of β-hydride elimination from

Table 11. Experimental and Calculated ΔGoH− (in kcal/mol) for Nickel Hydride Complexesa complex [(dmpe)2NiH]

+

[(dmpp)2NiH]+ [(depe)2NiH]+ [(depp)2NiH]+ [(EtPNMePEt)2NiH]+ [(dedpe)2NiH]+ [(dppe)2NiH]+ [(dppv)2NiH]+ [(iPrP2PhN2)2NiH]+ [(CyP2BnN2)2NiH]+ [(PhP2PhN2)2NiH]+ [(PhP2BnN2)2NiH]+ [(MeP2PhN2)2NiH]+ [(CO)3Fe(pdt)(μ-H)Ni(dppe)]+ [(Ph3P)(CO)2Fe(pdt)(μ-H)Ni(dppe)]+

solvent

ΔG°H−(expt)

ref(expt)

PhCN CH3CN PhCN CH3CN PhCN CH3CN CH3CN CH3CN CH3CN PhCN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN PhCN PhCN

50.7 48.9 62.1 61.2 56.0 56.0 67.2 66.0 59.8 62.8 62.7 66.4 63.8 60.7 59.0 57.1 54.0 79 79

115 115 115 115 115 115 117 118 116 115 115 116 77 222 222 222 223 128 128

ΔG°H−(calcd)b

ΔG°H−(calcd)c

50.5

52.0

63.9

61.2

56.0 60.9

58.3 63.0 64.7

63.2 65.9

a

Abbreviations for the ligands: dmpe = Me2PCH2CH2PMe2, dmpp = Me2PCH2CH2CH2PMe2, depe = Et2PCH2CH2PEt2, depp = Et2PCH2CH2CH2PEt2, EtPNMePEt = Et2PCH2N(Me)CH2PEt2, dedpe = Et2PCH2CH2PPh2, dppe = Ph2PCH2CH2PPh2, dppv = cis-Ph2PCH CHPPh2, RP2R′N2 = substituted 1,5-diaza-3,7-diphosphacyclooctanes with R group on phosphorus donors and R′ group on nitrogen donors, pdt = 1,3-propanedithiolate. bFrom ref 246. cFrom ref 220. 8403


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Figure 14. Nickel hydride complexes studied for CO2 insertion.

(iPrPCPiPr)Ni−OMe show that the reverse reaction, namely, the insertion of HCHO into (iPrPCPiPr)NiH, is thermodynamically downhill by 5.9 kcal/mol.249 This number is comparable to the 6.5 kcal/mol calculated for the reaction of HCHO with ( tBu P O C O P tBu )NiH. 250 The reaction of acetone with (MeNNNMe)NiH gives an insertion product (MeNNNMe)NiOiPr, albeit slowly and low yielding (∼50%).49 Treating (tBuPONOPtBu-H)NiH with PhCHO surprisingly generates benzoin (PhCH(OH)COPh) instead of an insertion product or PhCH2OH.48 A broad range of nickel hydride complexes has been explored for CO2 insertion (Figure 14). The reaction of trans(Cy3P)2Ni(H)CH3 with CO2 (1 atm) is fast at −50 °C, giving trans-(Cy3P)2Ni(H)(OCHO), CH4, C2H6, and some unidentified products.251 The isolated nickel formate complex exchanges with 13CO2 at 22 °C (t1/2 ≈ 150 min), confirming that CO2 insertion is reversible. The imine-ligated cationic nickel hydride is completely inactive under a CO2 pressure as high as ∼50 atm.106 The Me2Si-linked pincer complex (tBuPSiNSiPtBu)NiH (Figure 14) reacts with CO2 (1 atm) at a moderate rate (complete in hours to days at 25 °C).252 However, the isolated product is a hydrido nickel cyanate complex formed by an unusual N/O transposition process. In general, nickel pincer hydrides bearing a carbon-based central donor are highly reactive toward CO 2 (∼1 am), as demonstrated by the bis(phosphinite)-, 90,92,224 bis(phosphinous amide)-,42 and bis(phosphine)-based pincer systems.83,253 These reactions are typically complete in minutes or less than 1 h, forming nickel formate complexes. The fast insertion has been attributed to the strong trans influence of the carbon-based donor.83 Given that anionic silicon and boron donors are also strongly trans-influencing groups, it is not surprising that CO2 insertion reactions with (CyPSiMePCy)NiH83 and (tBuPBPtBu)NiH254 are fast as well. In contrast, nickel hydrides containing a nitrogen-centered pincer ligand react with CO2 at much slower rates. The reaction of (RPNPyPR)NiH (R = iPr, Cy) with CO2 (1−2.5 atm) requires up to 24 h to reach completion.79,80 The more bulky hydride (tBuPNPyPtBu)NiH gives a low conversion even when heated to 80 °C under ∼5 atm of CO2.80 The room-temperature reaction of (iPrPNPiPr-Me)NiH with CO2 (1 atm) takes 10 days to

complete, although changing the solvent from C6H6 to CH3CN shortens the reaction time to ∼7 h.255 According to the DFT calculations, using a polar solvent helps stabilize the transition state structure for the insertion. Calculations on (MePCPMe)MH and (MePNPMe)MH (truncated models; M = Ni, Pd) show that CO 2 insertion is kinetically and thermodynamically more favorable with the nickel systems.83 This result demonstrates that the contribution of the M−O bond strength to the energies of the transition state and the product should not be ignored for the pincer systems. As mentioned in the previous section, nickel hydrides are expected to be less hydridic than their palladium analogs; however, the ΔGoH− values listed in Table 11 do not factor in the coordination of the hydride acceptor. There is no need to do so for the coordinately saturated [(diphosphine)2NiH]+. Furthermore, after hydride transfer, HCO 2− does not coordinate to the nickel center, as demonstrated by [(dmpe)2NiH]+.256 Nickel borohydride complexes (Figure 15) may also react with CO2 to generate nickel formate complexes. The

Figure 15. Nickel borohydride complexes studied for CO2 insertion.

bis(phosphinous amide)-based complex (iPrPNCNPiPr-Me)Ni(BH4) shows similar reactivity as (iPrPNCNPiPr-Me)NiH under 1 atm of CO 2 . 42 The bis(phosphinite)-based complexes (RPOCOPR)Ni(BH4) are less reactive, requiring heating and longer reaction time.43 The dinickel complex [(N6S2)Ni2(μBH4)](BPh4) reacts with CO2 readily to afford a formatebridged complex.57 Carbon disulfide has more reactive π-bonds,257 and therefore is expected to insert into nickel hydride more rapidly. Examples for such reactions are surprisingly rare. The in-situ-generated [(PS3)NiH](PPN) undergoes CS2 insertion rapidly even at −80 °C to yield a dithioformate complex (Scheme 89).188 8404


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More Lewis acidic boron compounds such as B(C6F5)3 potentially abstract hydride from nickel, resulting in electrophilic nickel centers capable of activating inert chemical bonds. Treatment of (iPrPNPiPr)NiH with 1 equiv of B(C6F5)3 in benzene produces a mixture of three new nickel species along with the unreacted nickel hydride (Scheme 92).87 The presence

Scheme 89. Insertion of CS2 into [(PS3)NiH](PPN)

Scheme 92. Reaction of (iPrPNPiPr)NiH with B(C6F5)3 4.8. Reactions with Boranes and Silanes

The interaction between a nickel hydride complex and a borane can lead to several different outcomes, depending on the supporting ligands as well as the boranes used. Both (iPrPNCNPiPr-Me)NiH42 and (RPOCOPR)NiH43 abstract BH3 from BH3·THF to yield nickel κ2-BH4 complexes. The reaction of the bis(phosphine)-based pincer complex (tBuPCPtBu)NiH gives a κ1-BH4 complex,44 perhaps due to less trans void space created by this particular pincer ligand. The PhPSiMePPh-based η3-cyclooctenyl complex is a masked nickel hydride, as evidenced by its reaction with BH3·THF to form (PhPSiMePPh)Ni(κ2-BH4).203 Boranes other than BH3·THF have been investigated for their reactivity toward nickel hydride complexes. With 9-BBN (9-BBN = 9-borabicyclo[3.3.1]nonane) or HBcat (HBcat = catecholborane), complexes (RPOCOPR)NiH are in equilibria with κ2-dihydridoborate complexes.43 Sometimes these reactions could lead to the elimination of H2 and the formation of a nickel boryl complex, as demonstrated by (iPrPNPiPr-Me)NiH (Scheme 90).39

of (iPrPNPiPr)NiPh indicates activation of a benzene C−H bond. Heating the solution at 110 °C for 1 day leads to the isolation of (iPrPNPiPr)Ni(C6F5). The cyclohexyl derivative (CyPNPCy)NiH reacts with B(C6F5)3 similarly but at a slower rate. The C−H bond activation process is more favorable and also faster when AlMe3 is used as the Lewis acid. Under these conditions, both nickel hydrides are converted to the nickel phenyl complexes cleanly. Activation of other aromatic compounds such as toluene and m-xylene is also possible, and the selectivity favors the sterically more accessible Csp2−H bonds. This type of hydride abstraction reaction has been observed with nonpincer systems. With the assistance of Et3N, B(SPh)3 abstracts hydride from [(dmpe)2NiH]+ to form Et3N−H2B(SPh) and [(dmpe)2Ni(SPh)]+.258 The reactions between nickel hydride complexes and silanes are less explored, and the typical outcome involves the loss of hydrogen and the formation of nickel silyl complexes (Scheme 93). Converting (iPrPNPiPr-Me)NiH to (iPrPNPiPr-Me)Ni-

Scheme 90. Reaction of (iPrPNPiPr-Me)NiH with HBcat

Scheme 93. Reactions Between Nickel Hydride Complexes and Silanes

The reaction of {(diphosphine)Ni}2(μ-H)2 with BEt3− LiEt3BH appears to be a very complicated process, resulting in a nickel(0) species (diphosphine)Ni(η2-HBEt2) and a nickel(II) species [{(diphosphine)Ni}2(H)3](BEt4) (Scheme 91).218 Because mixing BEt3 with LiEt3BH produces 1/ 2(HBEt2)2 and LiBEt4, this reaction can be rationalized by considering the dissociation of {(diphosphine)Ni}2(μ-H)2 to (diphosphine)Ni(0) and (diphosphine)NiH2 as the first step. (Diphosphine)Ni(0) then binds HBEt2, while (diphosphine)NiH2 dimerizes to give [{(diphosphine)Ni}2(H)3]+ and H−. The structure of [{(diphosphine)Ni}2(H)3]+, though shown here to have two bridging and one terminal hydride ligands, may be identical to that of [{(diphosphine)NiH}2(μ-H)]+ in Scheme 2.

(SiPhH2) with PhSiH3 alone is extremely slow even at 120 °C (50−60% conversion after >14 days).259 Fortunately, the addition of a Lewis acid such as ZnI2 accelerates this process significantly, providing the silyl complex in a quantitative yield after 14 h (at 60 °C). When Ph2SiH2 is used as the silane, the silyl product is contaminated with (iPrPNPiPr-Me)NiI, which can be avoided by replacing ZnI2 with [PhNHMe2][B(C6F5)4]. The bis(phosphinite)-based hydride (iPrPOCOPiPr)NiH, which

Scheme 91. Reaction of {(Diphosphine)Ni}2(μ-H)2 with BEt3−LiEt3BH

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can be generated in situ from (iPrPOCOPiPr)NiOSiMe3 and PhSiH3, reacts with PhSiH3 slowly at room temperature. Nevertheless, after 8 days of stirring and occasional purging with N2 (to remove gaseous products), it is possible to isolate (iPrPOCOPiPr)Ni(SiPhH2) in a pure form.260 The conversion of (iPrPOCOPiPr)NiOSiMe3 to (iPrPOCOPiPr)Ni(SiPh2H) using Ph2SiH2 (via the nickel hydride) is substantially slower, requiring heating at 90 °C for a week. Synthesis of (Cp*NHC)NiH from (Cp*-NHC)NiOtBu and PhSiH3 is complicated by its further reaction with the silane.187 With an excess of PhSiH3 and heating at 60 °C for 12 h, the in-situ-generated nickel hydride is converted to (Cp*-NHC)Ni(SiPhH2).

Scheme 96. Reaction of (α-Diimine•−)2Ni2(μ-H)2 with Me3SiN3

4.9. Other Miscellaneous Reactions

entering into catalytic cycles that may or may not involve nickel hydrides.261,262 There is also a large body of literature in which nickel hydride complexes are proposed as intermediates during the catalytic reactions, although they have yet to be observed spectroscopically.263−265 For the scope of this review, only those with spectroscopic evidence supporting the catalytic role of nickel hydride species are discussed.

Nickel hydride complexes may exhibit some unusual reactivity that does not fall into any of the categories mentioned so far. The terphenyl-based hydride (iPrPArPiPr)Ni(H)Cl (Scheme 26) exchanges its hydride ligand with hydrogens of the central benzene ring via H− transfer between nickel and carbon.119 An intermolecular version of the reaction involves H/D exchange between [(PNNHNP)NiD](PF6) (see Figure 1 for structure) and LiEt3BH, in which case H− is transferred from boron to nickel first, followed by D− being transferred back to boron.78 One-electron oxidation of (tBuPOCOPtBu)NiH by Ce4+ results in H2 evolution (95% yield) and the formation of a cationic nickel complex trapped by the solvent molecule (Scheme 94).91

5.1. Isomerization of Alkenes

One of the important catalytic applications of nickel hydride complexes is nickel-catalyzed isomerization of alkenes. While many different mechanistic pathways exist, the most common one involves repeated alkene insertion and β-hydride elimination until an equilibrium mixture of the isomers is established (Scheme 97). An early study focusing on

Scheme 94. One-Electron Oxidation of (tBuPOCOPtBu)NiH

Scheme 97. General Scheme for Alkene Isomerization Catalyzed by a Nickel Hydride

The reaction of (dtbpe)Ni(H)(SiHAr2) with trityl cation surprisingly does not lead to hydride transfer products. Instead, elimination of the silane takes place, and the trityl group coordinates to nickel in an η3-fashion (Scheme 95), implying that Ph3C+ is reduced to Ph3C− by a Ni(0) species.151

{(EtO)3P}4Ni−H2SO4-catalyzed alkene isomerization shows that within just 5 min 95% of 1-butene is already isomerized to cis- and trans-2-butene.266 The cis/trans ratio is about 2.5 at the beginning of the isomerization process but decreases with time and eventually approaches the thermodynamic ratio of 1:3. Mechanistic investigation reveals that the isomerization reaction is inhibited by (EtO)3P, consistent with the hypothesis that [{(EtO)3P}4NiH]+ must dissociate a phosphinite ligand first to create a vacant coordination site for alkene coordination and insertion.267 Because both [{(EtO)3P}3NiH]+ and free (EtO)3P are unstable under the acidic medium, decomposition of the catalyst is a major concern. A successful strategy to improve catalyst stability (and catalytic performance) is through the immobilization of the nickel hydride on an ion-exchange resin, Amberlyst-15.268 Nickel hydride complexes bearing a labile chelating ligand may provide the needed vacant coordination site through ligand dissociation, thus serving as efficient catalysts for the isomerization of alkenes. Complex [(Ph2PCH2CH2SEt)2NiH](BPh4) is a good example as the thioether moiety is expected to dissociate readily from nickel. Such a complex has been shown to catalyze the isomerization of 1-pentene to cis- and trans-2pentene. 1 2 5 Similar to the reaction catalyzed by [{(EtO)3P}4NiH]+, at the beginning of the isomerization process, cis-2-pentene is the major product; however, with a sufficiently long reaction time, trans-2-pentene becomes the major product. In addition to 1-pentene, allylbenzene and allyl

Scheme 95. Reaction of (dtbpe)Ni(H)(SiHAr2) with Trityl Cation

The reactivity of dinickel bridging hydride complexes often features the dissociation of hydrogen as H2; however, it is also possible to transfer the hydride ligand to an electrophile. For example, the reaction of (α-diimine•−)2Ni2(μ-H)2 with Me 3 SiN 3 generates a nickel azide complex, which is accompanied by the formation of Me3SiH as a result of hydride transfer from nickel to silicon (Scheme 96).97

5. CATALYTIC REACTIONS Many nickel hydride complexes have been demonstrated as effective catalysts for various chemical transformations. They are either directly employed as catalysts or readily produced under catalytic conditions. Some nickel hydride complexes are merely precursors to low-valent nickel species (section 4.3) 8406


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on a nickel phenyl complex derived from Ni(COD)2 and a keto−phosphorus ylide (Scheme 100). A toluene solution of

cyanide are viable substrates for the nickel-catalyzed isomerization reactions, resulting in trans-β-methylstyrene and crotononitrile, respectively. The isomerization of allylic alcohol gives propionaldehyde due to rapid keto−enol tautomerization following CC bond isomerization. Pincer-ligated nickel hydrides, especially those with a rigid pincer backbone, are usually not particularly effective in catalyzing olefin isomerization. Olefin insertion into Ni−H bonds and β-hydride elimination from nickel alkyl species are often slow with this type of ligand system. The MeNNNMepincer system behaves differently, demonstrating catalytic activity for the isomerization of allylbenzene to trans-βmethylstyrene (Scheme 98).269 Although in this case a nickel

Scheme 100. Generation of the Catalytically Active Nickel Hydride Species

Scheme 98. Isomerization of Allylbenzene Catalyzed by Me NNNMe-Pincer Nickel Species the Ph3P-trapped complex catalyzes the oligomerization of ethylene (50 bar) at 50 °C, giving linear α-olefins almost exclusively.271 The catalytically active nickel hydride species can be generated via ethylene insertion into the nickel phenyl complex, followed by the elimination of styrene.270 The [P,O]chelating ligand system was later extended to functionalized phosphines such as Ph2PCH2CO2H and Ph2PCH2C(CF3)2OH. The reaction of Ph2PCH2CO2H with Ni(COD)2 forms a mixture of two cyclooctenyl complexes, which lose cyclooctadiene under the catalytic conditions to generate the active nickel hydride.272 The mixture of Ph2PCH2C(CF3)2OH with Ni(COD)2 is also an effective catalyst for the oligomerization of ethylene (50 bar, at 50 °C) to linear α-olefins.163 Monitoring the reaction at −10 °C confirms the presence of a nickel hydride species. In addition to the [P,O]-chelating ligand systems, nickel complexes bearing other bidentate ligands with X, Y (X, Y = O, S, N, P) donors have been studied as ethylene oligomerization catalysts.273−275 For the cationic nickel system bearing a 2-(2-pyridyl)benzimidazole ligand, the hydride intermediate has been identified by ESI-MS.275 It is generally believed that a 3-coordinate nickel hydride is needed to initiate the oligomerization process (Scheme 101).240,241 The vacant

propyl complex is used as the catalyst, mechanistic study suggests that β-hydride elimination is kinetically favorable. Thermodynamics however favor the reverse reaction, which is the insertion of olefins into (MeNNNMe)NiH. Complex (PhPSiMePPh)Ni(η3-C8H13) has been described as a masked nickel hydride, implying that the hydride species is also kinetically accessible.203 In fact, the 1,3-COD-inserted complex is an excellent catalyst for the isomerization of 1,5-COD and 1hexene (Scheme 99). Scheme 99. Olefin Isomerization Catalyzed by a Masked Nickel Hydride

Scheme 101. Catalytic Cycle for Nickel-Catalyzed Oligomerization of Ethylene

Five-coordinate nickel hydride complexes bearing a tetradentate ligand are coordinatively saturated. Perhaps the dissociation of one of the ligand arms would allow alkenes to be inserted into the Ni−H bonds and ultimately isomerized. Since [(NS3tBu)NiH](BPh4) is known to react with ethylene reversibly, it is expected that this hydride also catalyzes the isomerization of 1-hexene to cis- and trans-2-hexene.53 It is interesting to note that (iPrP3Si)NiH promotes the isomerization of 1-octene to cis- and trans-2-octene.155 The success of synthesizing this specific nickel hydride from iPrP3SiH and Ni(COD)2 (without seeing the COD-inserted products) may imply that alkene insertion is thermodynamically disfavored but kinetically accessible.

coordination site is first occupied by ethylene and then left open following ethylene insertion. β-Hydride elimination from the propagating alkyl chain gives α-olefins and regenerates the nickel hydride. The 4-coordinate nickel hydride {κO,κPOC(CF3)2CH2PPh2}Ni(H)(PCy3) is not an active catalyst for the oligomerization of ethylene, presumably due to the vacant coordination site being blocked by the phosphine.

5.2. Oligomerization or Polymerization of Alkenes/Alkynes

The Shell higher olefin process (SHOP) is arguably the most well-known catalytic process that involves a nickel hydride intermediate.270 The catalyst development was initially focused 8407


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BH3 from the reaction mixture has led to the hypothesis that the active species is not a nickel hydride but (Cy3P)Ni(butadiene). Nickel hydride [{(EtO)3P}4NiH]+ catalyzes the coupling of butadiene and ethylene at 100 °C to yield 1,4-hexadienes, a process closely related to the oligomerization of alkenes.280 Mechanistic studies including kinetics suggest that the hydride first interacts with butadiene to form [{(EtO)3P}3Ni(η3C4H7)]+ or a π-crotyl nickel complex. Subsequent dissociation of (EtO)3P creates the vacant coordination site for ethylene to bind and then insert into the Ni−C bond. Selectivity of the reaction favors trans-1,4-hexadiene (60−70%), which is accompanied by cis-1,4-hexadiene (∼10%), 3-methyl-1,4pentadiene (∼10%), and other isomers. Oligomerization of alkynes is usually catalyzed by low-valent metal species, and the isolated products are benzene derivatives.281 Using a nickel hydride as the catalyst provides pathways to completely different products because of the possibility of inserting the alkyne into the Ni−H bond. As demonstrated with [(dppe)2NiH](OCOCF3), the oligomerization of 3-hexyne gives a substituted cyclobutene as the major product (Scheme 104).113 A mechanism explaining the oligomerization products is outlined in Scheme 105. Two consecutive alkyne insertions give a butadienyl complex, which undergoes electrocyclic ring closure leading to the cyclobutene derivative. The cyclopentadiene-based byproducts stem from insertion of the third alkyne molecule followed by cyclization, 1,2-H shift, and β-hydride elimination.

Catalytic polymerization of ethylene proceeds via a similar mechanism as shown in Scheme 101, except that the propagating alkyl chain is significantly longer. This requires the insertion of ethylene to be substantially faster than the βhydride elimination. While the electronic and steric properties of the catalysts are important, solvent can play a critical role in modulating the reactivity of the nickel complexes. For the reaction of ethylene catalyzed by the keto−phosphorus ylidederived nickel phenyl complex, changing the solvent from toluene to n-hexane results in high molecular linear polyethylene rather than the linear α-olefins.271 α-Diimine-276 and anilinotropone-ligated74 nickel complexes developed by the Brookhart group are much more active catalysts for the polymerization of ethylene. All these catalytic systems involve a nickel hydride during the chain-transfer step. Discrete nickel hydrides have been employed as catalysts for alternating copolymerization of ethylene and CO. The zwitterionic hydride (P,P-BPh3)Ni(PPh3)H (or a catalyst generated in situ from (P,P-BPh3)-H and Ni(COD)2) in particular shows initial catalytic activity comparable to palladium catalysts.120 However, rapid decomposition of the catalyst under CO (Scheme 102) results in loss of catalytic activity. Scheme 102. Reaction of the Zwitterionic Hydride (P,PBPh3)Ni(PPh3)H with CO

5.3. Hydrogenation or Hydroarylation of Alkenes/Alkynes

Heterogeneous nickel catalysts are widely used for alkene hydrogenation. In contrast, very few homogeneous nickel catalysts have been developed for this process. As illustrated in Scheme 35, (MesDPBPh)Ni activates H2 reversibly to form a nickel hydride. Although the insertion of alkenes into this hydride has yet to be established, (MesDPBPh)Ni catalyzes the hydrogenation of styrene and tert-butyl ethylene at room temperature under 1 atm of H2.132 Mechanistic studies using styrene as the substrate show that the resting state of the catalytic cycle is (MesDPBPh)Ni(η2-styrene). The catalyst loading for the hydrogenation of styrene can be as low as 1 mol %. Cationic nickel hydride [(CyPNHPCy)NiH](BPh4) is also an active catalyst for the homogeneous hydrogenation of alkenes, although the temperature and H2 pressure are higher (Scheme 106).40 In addition to styrene and tert-butyl ethylene, α-methylstyrene and 1-octene are viable substrates for the hydrogenation reaction. In the latter case, after 24 h, 70% of 1octene is hydrogenated to n-octane whereas the rest of the substrate is isomerized to internal octenes. The insertion of 1octene into [(CyPNHPCy)NiH](BPh4) affords primary octyl complex, as confirmed by NMR spectroscopy. Hydrogenolysis of the octyl complex at 80 °C regenerates the cationic hydride while giving n-octane along with 1-octene due to the insertion being reversible. Hydrogenation of aldehydes such as 3,5dimethoxybenzaldehyde and cinnamylaldehyde with [(CyPNHPCy)NiH](BPh4) is feasible though not catalytic in nickel. The neutral nickel hydride derived from [(CyPNHPCy)NiH](BPh4) is also an active catalyst for the hydrogenation of styrene and 1-octene under similar conditions. The structure of the pincer ligand greatly influences the catalytic activity of the nickel hydride in alkene hydrogenation. At room temperature under 1 atm of H2, the bis(phosphino)boryl-based nickel hydride (tBuPBPtBu)NiH catalyzes the

Nickel complexes have been studied for catalytic oligomerization or polymerization of alkenes other than ethylene. Mixing Ni(COD)2 with hfacac-H (hfacac-H = CF3COCH2COCF3) generates (hfacac)Ni(cyclooctenyl) (analogous to those shown in Scheme 100), which catalyzes the oligomerization of 1-butene at 70−80 °C to linear octenes in 75−83% yield.277 A recent study demonstrates that methallyl complex {(η3-C4H7)Ni(μ-Cl)}2 mixed with hfacac-H is also an active catalyst for the oligomerization of 1-butene.278 The reaction temperature in this case is lower (30 °C), while the selectivity still favors the linear dimers (67% C8 vs 21% C12). In both catalytic systems, the catalytically active species is the 3coordinate (hfacac)NiH. An early work focusing on the oligomerization of butadiene showed that trans-(iPr3P)2Ni(H)Cl failed to catalyze this reaction.279 In contrast, trans-(iPr3P)2Ni(H)(BH4) is an active catalyst for this process, providing a mixture of cis-1,2divinylcyclobutane, 4-vinylcyclohex-1-ene, 1,5-cyclooctadiene, and 1,5,9-cyclododecatriene (Scheme 103). Interestingly, in the presence of MeOH, the conversion of butadiene is higher and the major product is 1,3,7-octatriene. The isolation of Cy3P· Scheme 103. Oligomerization of Butadiene Catalyzed by trans-(iPr3P)2Ni(H)(BH4)

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Scheme 104. Oligomerization of 3-Hexyne Catalyzed by [(dppe)2NiH](OCOCF3)

are capable of catalyzing the hydrogenation of phenylacetylene to styrene and ethylbenzene at 70 (in THF) or 100 °C (in toluene).243 Although the insertion of phenylacetylene is facile, hydrogenolysis of the alkenyl species is too slow to account for the overall catalytic reaction. Added elemental mercury retards the reaction significantly, implying that the hydrogenation is heterogeneous in nature. Biphenylene could be viewed as a dimer of benzyne. Various Group 10 metal complexes such as Pt(PEt 3 ) 3 , trans(Et3P)2PtH2, Pd(PEt3)3, and {(dippe)Ni}2(μ-H)2 show catalytic activity for the hydrogenolysis of biphenylene to biphenyl (1 atm of H2, 56−120 °C).283 The nickel precatalyst is most efficient with a turnover number (TON) of 16 over 24 h at 56 °C. The resting state species of the catalytic reaction is (dippe)Ni(2,2′-biphenyl), which is generated from C−C bond activation of biphenylene by (dippe)Ni(0) (produced via loss of H2 from {(dippe)Ni}2(μ-H)2). The intermediacy of a nickel hydride has not been established experimentally; however, the resting state species for the Pt system is a hydride complex trans-(Et3P)2Pt(α-biphenyl)H, implying that hydrides are also involved in the nickel-catalyzed hydrogenolysis reaction. Nickel hydrides have been proposed as key intermediates for nickel-catalyzed hydroheteroarylation of styrene derivatives with benzimidazole (Scheme 107).136 The mixture of Ni-

Scheme 105. Mechanism for Nickel Hydride-Catalyzed Oligomerization of 3-Hexyne

Scheme 106. Hydrogenation of Alkenes Catalyzed by [(CyPNHPCy)NiH](BPh4)

Scheme 107. Nickel-Catalyzed Hydroheteroarylation of Styrene Derivatives

hydrogenation of styrene to ethylbenzene with a turnover frequency (TOF) of 25 h−1 (Figure 16).282 High catalytic

(COD)2 and amino-NHC alone is catalytically active at 150 °C, resulting in the branched products exclusively. The selectivity is completely reversed with the addition of 10 mol % Me3Al as a cocatalyst, and under this condition, the reaction temperature can be lowered to 100 °C. Monitoring the catalytic reaction by 1 H NMR reveals a hydride resonance at δ −12.05 ppm. The involvement of a nickel hydride intermediate is indirectly supported by the isolation of [(amino-NHC)2Ni(H)(DMBI)](PF6) from the reaction of Ni(COD)2 and amino-NHC with [DMBI-H](PF6) (Scheme 37). Thus, the catalytic cycle begins with C−H bond activation of benzimidazole by (aminoNHC)Ni(0) (Scheme 108). In the absence of Me3Al, the insertion of styrene derivatives is governed by electronic effects, resulting in nickel being added to the internal carbon. Reductive elimination of the product reforms the Ni(0) species. Me3Al is expected to bind to the nitrogen, and consequently, the selectivity for the insertion is determined by steric effects. Related reactions are nickel-catalyzed addition of acidic C−H bonds of fluorinated aromatics284 or heterocycles285 across alkynes (Scheme 109). The proposed mechanism involves

Figure 16. Comparison of nickel pincer hydrides for catalytic hydrogenation of styrene.

activity of (tBuPBPtBu)NiH is also demonstrated in roomtemperature hydrogenation of 1-octene and tert-butyl ethylene, in which cases (tBuPCPtBu)NiH is a completely inactive catalyst. The fact that ( tBu PBP tBu )NiH can be prepared from (tBuPBPtBu)NiCl and (iPr)2Mg supports the reversibility of alkene insertion into the nickel hydride. Kinetics of (tBuPBPtBu)NiH-catalyzed hydrogenation of cis-cyclooctene are consistent with a mechanism involving reversible alkene insertion followed by the hydrogenolysis of the nickel alkyl intermediate. In principle, hydrogenation of alkynes could also be catalyzed by nickel hydride complexes following the insertion of alkynes and the hydrogenolysis of the resulting alkenyl complexes. Nickel hydrides (RPOCOPR)NiH (R = iPr, cPe, or cyclopentyl) 8409


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of aldehydes with Et3SiH.287 At 100 °C, aldehydes bearing different functional groups including F, Cl, and Me2N are reduced to their silyl ethers with TONs up to 50 and TOFs up to 287 h −1 . The TONs are lower (25−35) for the hydrosilylation of bulky aldehydes such as 2,6-dimethylbenzaldehyde or ketones such as cyclohexanone and benzophenone. According to the NMR studies, {(PNiPr3)Ni(μ-Br)}2 reacts with KOtBu and Et3SiH to form {(PNiPr3)Ni(μ-H)}2, which is likely to dissociate into monomeric (PNiPr3)NiH. Insertion of aldehydes into the 3-coordinate nickel hydride leads to nickel alkoxide complexes, which undergo metathesis reaction with Et3SiH to release the silyl ether products and regenerate (PNiPr3)NiH. Aldehydes such as 3-thiophenecarboxaldehyde and 2-pyridinecarboxaldehyde are not viable substrates for this catalytic system. Presumably, the heterocyclic rings could coordinate to the nickel center and prevent carbonyl insertion. The bis(phosphinite)-based nickel hydride (iPrPOCOPiPr)NiH is a more active hydrosilylation catalyst. With a catalyst loading as low as 0.2 mol %, a variety of aldehydes including those with pyridine and thiophene rings can be reduced by PhSiH3 or Ph2SiH2 at room temperature (TONs up to 500).89 As with the {(PNiPr3)Ni(μ-Br)}2 system, ketones such as cyclohexanone, acetophenone, and benzophenone are less reactive, giving TONs of 6−60 at 70 °C. Replacing the isopropyl groups of the catalyst with tert-butyl groups results in a less active catalyst, which is attributed to more sluggish insertion of aldehydes. The reaction mechanism consisted of two steps: carbonyl insertion and metathesis with silanes. Each step has been verified by NMR experiments. Other nickel pincer hydrides have been explored for catalytic hydrosilylation of aldehydes. The reduction of PhCHO with PhSiH3 is catalyzed by 8.3 mol % of (tBuPONOPtBu-H)NiH at room temperature.48 The chloride complex (tBuPONOPtBu-H)NiCl shows some catalytic activity; however, the time needed to fully convert PhCHO is much longer (2 d vs 10 h). A stoichiometric reaction between (tBuPONOPtBu-H)NiH and PhCHO surprisingly gives no insertion product but PhCH(OH)COPh as a result of benzoin condensation. The PNP-pincer hydrides (iPrPNPiPr-Y)NiH are inert toward PhCHO.287 They are also inactive for the catalytic hydrosilylation of PhCHO with Et3SiH at 100 °C. Several half-sandwich nickel complexes have been studied as hydrosilylation catalysts. For the reduction of PhCHO with Ph2SiH2, CpNi(IMes)Cl (5 mol %) is effective at 70 °C, resulting in >97% conversion of the substrates in 22 h.84 The catalytic performance is significantly enhanced with the addition of NaEt3BH, which is shown to convert CpNi(IMes)Cl to CpNi(IMes)H. Using the in-situ-generated nickel

Scheme 108. Mechanism for Nickel-Catalyzed Hydroheteroarylation of Styrene Derivatives

Scheme 109. Nickel-Catalyzed Hydroarylation and Hydroheteroarylation of Alkynes

oxidative addition of the C−H bond to a Ni(0) species to yield a nickel hydride intermediate. The catalytic cycle is completed by alkyne insertion and reductive elimination of the product. In the case of pentafluorobenzene, the C−H bond can also be added to 1-phenyl-1,3-butadiene and 2-vinylnaphthalene. The involvement of a nickel hydride intermediate is more unambiguously established in C−H addition of fluorinated aromatics to 3-hexyne catalyzed by Ni(COD)2/Et3P or Ni(COD)2/iPr3P.137 In fact, nickel hydride complexes can be isolated from the stoichiometric reactions between fluorinated aromatics and a phosphine-ligated Ni(0) species.138,286 5.4. Hydrosilylation and Related Hydrofunctionalization Reactions

Nickel-catalyzed hydrosilylation of aldehydes and ketones has received a great deal of attention in recent years. Many of these catalytic systems employ nickel hydrides directly as the catalysts, while others use catalyst precursors that are converted to nickel hydrides under the catalytic conditions (Figure 17). The dinickel complex {(PNiPr3)Ni(μ-Br)}2, when mixed with 2 equiv of KOtBu, is an excellent catalyst for the hydrosilylation

Figure 17. Nickel-based catalysts for the hydrosilylation of aldehydes and ketones. 8410


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hydride, the catalyst loading can be reduced to 0.5 mol %, the reaction time can be shortened to 1 h, and the temperature can be lowered to 25 °C. Compared to CpNi(IMes)H, catalytic activities of Cp*Ni(IMes)H and CpNi(IPr)H are lower. With 1 mol % of CpNi(IMes)H, a wide range of aldehydes (except those with OH and Br groups) are reduced to the corresponding alcohols in high yields following basic hydrolysis of the silyl ether products. Furthermore, in the presence of 5 mol % of CpNi(IMes)H, various ketones are readily reduced by Ph2SiH2 at 25 °C. The mechanism for this catalytic system is not well understood. The nickel hydride does not react with PhCHO or Ph2SiH2. In contrast, a closely related system using an NHC ligand tethered to the cyclopentadienyl ring shows insertion of aldehydes into (Cp*-NHC)NiH.187 However, mixing PhCHO, Ph2SiD2, and (Cp*-NHC)NiH in a 1:1:1 ratio gives PhCHD−OSiDPh2 with no deuterium incorporated into the nickel hydride, suggesting that the hydride is a spectator ligand. For convenience, the tert-butoxide complex (Cp*NHC)NiOtBu is used as the catalyst for the hydrosilylation reactions as it is readily converted to (Cp*-NHC)NiH by the silane. The hydrosilylation of aldehydes with PhSiH3 is typically completed within 5 min with 1 mol % of (Cp*-NHC)NiOtBu. The hydrosilylation of ketones, however, requires a higher temperature (100 °C) and longer reaction time (2−24 h). Hydrosilylation of aldehydes catalyzed by {(Me3P)(Me)Ni(μ-imine)}2 and (MesDPBPh)Ni appears to follow a mechanism without the involvement of a terminal hydride. The reaction of {(Me3P)(Me)Ni(μ-imine)}2 with Ph2SiH2 gives a mononuclear species with the silane σ-bonded to nickel (Scheme 110).288

Scheme 111. Mechanism for the Hydrosilylation of Aldehydes Catalyzed by (MesDPBPh)Ni

those bearing an electron-donating group such as Me2N, MeO, and Me. Of the nickel catalysts shown in Figure 17, (iPrPOCOPiPr)NiH, CpNi(IMes)Cl (mixed with NaEt3BH), and {(Me3P)(Me)Ni(μ-imine)}2 show selectivity favoring the reduction of CO over CC bonds. The opposite chemoselectivity has been observed with (MeNNNMe)NiOMe as the catalyst (Scheme 112).289 In the presence of Ph2SiH2, (MeNNNMe)NiOMe is Scheme 112. Chemoselective Hydrosilylation of CC Bonds Catalyzed by (MeNNNMe)NiOMe

Scheme 110. Mechanism for the Hydrosilylation of Aldehydes Catalyzed by {(Me3P)(Me)Ni(μ-imine)}2

converted to (MeNNNMe)NiH, which reacts with ethylene at a rate much faster than that of CO insertion. In addition to the substituted indole shown in Scheme 112, a large library of alkenes with different functional groups (e.g., Br, NH2, epoxide, ester, amide, carbamate) undergoes selective hydrosilylation at the CC bond. Because (MeNNNMe)NiH is also an alkene isomerization catalyst,269 hydrosilylation of internal alkenes with Ph2SiH2 provides terminal alkylsilanes as well. The efficiency of the catalytic system is remarkable; hydrosilylation of 1-octene can be catalyzed by 0.01 mol % of (MeNNNMe)NiOMe (TON = 10 000) and is complete in 2.5−3 min! A dinickel complex supported by a doubly reduced naphthyridine−dimine ligand (iPrNDI)Ni2(C6H6) is an effective hydrosilylation catalyst not only for aldehydes and ketones but also for alkenes, alkynes, and even amides.290 The reaction using Ph2SiH2 is typically conducted at room temperature (or 60 °C for acetophenone and N,N-dimethylbenzamide) and complete in hours with a 5 mol % catalyst loading. The dinickel reacts with Ph2SiH2 to yield a silane complex (Scheme 113), which might be involved in the catalytic cycle. Nickel hydride complexes play important roles in the hydrofunctionalization of alkenes and alkynes. A generalized mechanism begins with oxidative addition of the H−X (X = heteroatom) bond to a low-valent nickel species, followed by insertion of alkene/alkyne into the nickel hydride intermediate and then reductive elimination of the product. A well-known example is the addition of H−P(O) bonds to terminal alkynes catalyzed by phosphine-ligated Ni(0) complexes (Scheme

Insertion of aldehydes into the Si−H bond and reductive elimination of the silyl ethers complete the catalytic cycle. Among the dinickel complexes screened, the one with an omethyl-substituted phenyl group (Ar = o-MeC6H4) shows the best activity. With a 0.6 mol % catalyst loading at 45 °C, hydrosilylation of aldehydes with Ph2SiH2 is complete within 7 h. Functional groups such as F, Cl, MeO, furyl, and CC bond are tolerated under the catalytic conditions. (MesDPBPh)Ni reacts with Ph2SiH2 and PhCHO individually to give (MesDPBPh-H)Ni(SiPh2H) and (MesDPBPh)Ni(η2-PhCHO), respectively.152 Monitoring the catalytic reaction, however, reveals a new species that is more consistent with a siloxyalkyl complex. Thus, hydrosilylation of aldehydes catalyzed by (MesDPBPh)Ni is more likely to proceed via a mechanism featuring insertion of aldehydes into the Ni−Si bond (Scheme 111). With 5 mol % of (MesDPBPh)Ni at room temperature, hydrosilylation of benzaldehyde derivatives with Ph2SiH2 is complete within 1 h or a few days, depending on the substituents on the benzene ring. The reaction is faster for 8411


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Scheme 113. Reaction of a Naphthyridine−Dimine-Ligated Dinickel Complex with Ph2SiH2

Scheme 116. Catalytic Cycles for Nickel-Catalyzed Reduction of CO2 with HBcat

114).110 The regioselectivity can be altered by using different phosphine ligands. In particular, the catalytic mixture of

a methylene diolate bridging nickel and boron, [Ni]OCH2OBcat. β-Elimination of the OBcat group produces HCHO that can be further inserted into nickel hydride to form [Ni]OCH3. The reactions of [Ni]OBcat and [Ni]OCH3 with HBcat regenerate the nickel hydride, which is driven by the formation of strong B−O bonds. Less sterically hindered nickel hydrides (RPOCOPR)NiH (R = iPr, cPe) prove to be inferior catalysts, contradicting the notion that insertion of CO bonds (of CO2, HCO2Bcat, and HCHO) should be favored with sterically more accessible Ni−H bonds.90 Studies of (RPOCOPR)NiH and boranes suggest that with a less bulky pincer ligand, the nickel hydride is more likely to be trapped as a catalytically dormant dihydridoborate complex.43 Furthermore, decomposition of the nickel hydrides by HBcat is faster when the phosphorus substituents are smaller. Selection of the reducing agent is critical to the success of the catalytic reduction of CO2. For the aforementioned catalytic system, 9-borabicyclo[3.3.1]nonane (9-BBN) shows similar activity as HBcat, reducing CO2 to the methoxide level at room temperature.90 In contrast, BH3·THF fails to reduce CO2 in the presence of a catalytic amount of (RPOCOPR)NiH.43 At room temperature, the nickel hydrides are trapped by BH3·THF to form (RPOCOPR)Ni(BH4), which, unlike the dihydridoborate complexes generated from HBcat and 9-BBN, do not dissociate the borane readily to re-enter into the catalytic cycles. Using (tBuPOCOPtBu)NiH as the catalyst, pinacolborane (HBpin) reduces CO2 only to the formate level (i.e., HCO2Bpin). However, when the masked hydride (PhPSiMePPh)Ni(η3-C8H13) is employed as the catalyst, HBpin reduces CO2 beyond the formate level, resulting in pinBOCH2OBpin and pinBOCH3 in as short as 15 min (Scheme 117).203 Extending reaction time to 12 h leads to the disappearance of pinBOCH2OBpin, the formation of HCHO, more pinBOCH3 (13%) and pinBOBpin (85%), and some unidentified products.

Scheme 114. Nickel-Catalyzed Addition of H−P(O) Bonds to Terminal Alkynes

Ni(COD)2/PPhMe2/Ph2P(O)OH favors the Markovnikov addition of the H−P(O) bond. A similar catalytic mixture of Ni(PPh2Me)4 (5 mol %) and Ph2P(O)OH (10 mol %) catalyzes the addition of PhSH to 1-octyne to form the branched product (>91% selectivity). Nickel hydride made from Ni(PEt3)4 and Ph2P(O)H (Scheme 25) reacts with 1octyne rapidly to give the H−P addition products, although the selectivity is low (63% favoring the linear product). Nevertheless, this result supports the relevance of nickel hydrides during the catalytic reactions. Hydrophosphination of styrenes with Ph2PH has been reported to be catalyzed by 5 mol % of {(EtO)3P}4Ni at 130 °C in the presence of 1 equiv of Et3N.291 Although a nickel hydride intermediate has not been established, the related study161 with (dtbpe)Ni(0) (Scheme 50) suggests that the catalytic reaction may involve the hydride species. 5.5. Reduction of CO2 and Dehydrogenation of Formic Acid

As demonstrated in Figure 14, many well-defined nickel hydride complexes have been developed to reduce CO2 to nickel formate complexes. With an appropriate reducing agent, some of these hydrides are capable of reducing CO2 in a catalytic fashion. When catecholborane (HBcat) is used as the reducing agent, (tBuPOCOPtBu)NiH catalyzes the reduction of CO2 to a methoxyboryl species with a TON of 495 in 1 h (Scheme 115).224 The stoichiometric reaction between (tBuPOCOPtBu)NiOCHO and HBcat gives HCO2Bcat and (tBuPOCOPtBu)NiH, thus completing the first catalytic cycle of the overall reduction process (Scheme 116). According to the DFT calculations,250 subsequent CO insertion of HCO2Bcat into the Ni−H bond crosses the highest kinetic barrier to yield

Scheme 117. Reduction of CO2 with HBpin Catalyzed by (PhPSiMePPh)Ni(η3-C8H13)

Scheme 115. Reduction of CO2 with HBcat Catalyzed by (tBuPOCOPtBu)NiH

The calculated kinetic barrier for the reaction of (tBuPOCOPtBu)NiOCHO with PhSiH3 is high (42.6 kcal/ mol).250 Consistent with the calculations, (tBuPOCOPtBu)NiOCHO does not react with PhSiH3 at room temperature.90 Similarly, treatment of (tBuPBPtBu)NiOCHO with 1 equiv of Et3SiH shows no reaction.254 Interestingly, adding B(C6F5)3 and CO2 (4 bar) to the mixture and heating at 70 °C result in selective reduction of CO2 to (Et3SiO)2CH2. The major nickel 8412


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species observed during the reaction is an adduct of (tBuPBPtBu)NiOCHO and B(C6F5)3 (Scheme 118). This adduct

Scheme 121. Catalytic Cycle for Ni−Rh-Catalyzed Hydrogenation of CO2

Scheme 118. Interaction between (tBuPBPtBu)NiOCHO and B(C6F5)3

can be directly employed as a catalyst (0.05−0.6 mol % loading) for the reduction of CO2 with tertiary silanes (e.g., Et3SiH, Ph2MeSiH, and PhMe2SiH) at 70 °C with TONs up to 1200. The selectivity favoring (R3SiO)2CH2 over R3SiOCH3 and CH4 is remarkable, as B(C6F5)3 is known to catalyze the cleavage of C−O bonds with silanes.292 It thus has been suggested that B(C6F5)3 is engaged in the interactions with (tBuPBPtBu)NiOCHO and (tBuPBPtBu)NiH (highlighted in Scheme 119), preventing (R3SiO)2CH2 from being further reduced.

bar of H2 and CO2 (1:1) at 20 °C, aqueous solutions of [Ni(TsTACN-S2)RhCp*X]+ catalyze the hydrogenation of CO2 (or bicarbonate at pH 11) to formic acid (or formate) with TONs up to 38. When [Ni(TsTACN-S2)RhCp*(NO3)]+ is used as the catalyst, the TON increases from 10 to 34 as the pH value increases from 4 to 11. As expected, hydride complex [(TsTACN-S2)Ni(μ-H)RhCp*]+ is also an active catalyst, although the obtained TON is lower than the chloride or nitrate complex. Following a similar mechanism shown in Scheme 121, [Ni(TsTACN-S2)RhCp*(NO3)]+ catalyzes the hydrogenation of aromatic and aliphatic aldehydes in water (60 °C, pH2 = 1 bar, pH 7) with TONs of 13−46. It is interesting to note that the substituent on the axial nitrogen makes a big difference in modulating the catalytic activity. The iPrsubstituted complex [Ni(iPrTACN-S2)RhCp*Cl]+ shows minimal activity in catalyzing the hydrogenation of CO2 and PhCHO, which has been attributed to lower stability for the hydride or its inability to activate H2 under the catalytic conditions. The reduction of CO2 by the bridging hydride complexes should be reversible, as inferred by the fact that they can be prepared from HCO2Na (Scheme 68). It is therefore not surprising that [Ni(TsTACN-S2)RhCp*(NO3)]+ catalyzes transfer hydrogenation of PhCHO in water using HCO2H as the hydrogen donor (at 60 °C, pH 0.5−7).189 The catalytic activity for the iridium analog [Ni(TsTACN-S2)IrCp*(NO3)]+ is lower (maximum TON 18 vs 48), likely due to lower hydricity for the iridium hydride. The meppp-ligated system [Ni(μ-meppp)MCp*(NO3)] (M = Rh, Ir) shows low activity for transfer hydrogenation of PhCHO with HCO2H/Et3N (at 50 °C) in methanol, but the rhodium complex once again outperforms the iridium analog (TON 5 vs 3).190 Many nickel hydride complexes are known to be protonated by HCO2H to generate nickel formate complexes. If these formate complexes can undergo decarboxylation reaction or CO2 insertion into the nickel hydrides is reversible, either the formate or the hydride complexes should be able to catalyze the dehydrogenation of HCO 2 H. Nickel pincer hydride (tBuPCPtBu)NiH is anticipated as an active catalyst because both protonation and reversibility for CO2 insertion are well established for this compound. However, a base such as dimethyl-n-octylamine is needed to facilitate the dehydrogenation process. For this particular catalytic system a TON as high as 626 has been achieved when the dehydrogenation of HCO2H/nOctNMe2 (11:10) is catalyzed by (tBuPCPtBu)NiH and carried out in propylene carbonate (80 °C, 3 h).45 For some reason, the catalytic activity of (tBuPCPtBu)NiOCHO is

Scheme 119. Proposed Mechanism for the Reduction of CO2 with R3SiH Catalyzed by (tBuPBPtBu)NiOCHO−B(C6F5)3

Dihydrogen would be a more attractive reducing agent because it is cheaper than boranes and silanes and could potentially be generated from renewable sources of energy. However, hydrogenation of CO2 to methanol is a very challenging process. In terms of using nickel-based catalysts, it is difficult to convert CO2 even to formic acid or its derivatives. Despite facile CO2 insertion into (tBuPCPtBu)NiH, catalytic hydrogenation of CO2 (pCO2 = pH2 = 30 bar, in DMF, 150 °C) in the presence of dimethyl-n-octylamine as a base is unsuccessful.45 On the other hand, (tBuPCPtBu)NiH catalyzes the hydrogenation of NaHCO3 to HCO2Na with a TON of 3038 in 20 h (Scheme 120). Scheme 120. Hydrogenation of NaHCO3 Catalyzed by (tBuPCPtBu)NiH

Hydrogenation of CO2 to formic acid under ambient pressure is thermodynamically uphill in organic solvents. It is, however, favorable if a base is added to neutralize the acid or if the reaction is carried out in water. Heterobimetallic complexes [Ni(TsTACN-S2)RhCp*X]+ are able to activate H2 in water to yield bridging hydride complexes, which are reactive enough to reduce CO2 to formic acid (Scheme 121).174 Thus, under 20 8413


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0.65 V. [Ni(EtPNMePEt)2]2+ which contains a pendant base shows improvement with a TOF of 0.01−0.5 s−1 (under 1 atm of H2) at lower overpotentials ( 0.06 M. The maximum TOFs range from 107 s−1: the Medium Provides an Increase in Rate but not Overpotential. Energy Environ. Sci. 2014, 7, 4013−4017. (305) Rountree, E. S.; Dempsey, J. L. Potential-Dependent Electrocatalytic Pathways: Controlling Reactivity with pKa for Mechanistic Investigation of a Nickel-Based Hydrogen Evolution Catalyst. J. Am. Chem. Soc. 2015, 137, 13371−13380. (306) Wiedner, E. S.; Brown, H. J. S.; Helm, M. L. Kinetic Analysis of Competitive Electrocatalytic Pathways: New Insights into Hydrogen Production with Nickel Electrocatalysts. J. Am. Chem. Soc. 2016, 138, 604−616. (307) Ichikawa, K.; Nonaka, K.; Matsumoto, T.; Kure, B.; Yoon, K.S.; Higuchi, Y.; Yagi, T.; Ogo, S. Concerto Catalysis-Harmonising [NiFe]hydrogenase and NiRu Model Catalysts. Dalton Trans. 2010, 39, 2993−2994. (308) Rettenmeier, C.; Wadepohl, H.; Gade, L. H. Stereoselective Hydrodehalogenation via a Radical-Based Mechanism Involving TShaped Chiral Nickel(I) Pincer Complexes. Chem. - Eur. J. 2014, 20, 9657−9665. (309) Fontaine, F.-G.; Zargarian, D. Me2AlCH2PMe2: A New, Bifunctional Cocatalyst for the Ni(II)-Catalyzed Oligomerization of PhSiH3. J. Am. Chem. Soc. 2004, 126, 8786−8794. (310) Smith, E. E.; Du, G.; Fanwick, P. E.; Abu-Omar, M. M. Dehydrocoupling of Organosilanes with a Dinuclear Nickel Hydride Catalyst and Isolation of a Nickel Silyl Complex. Organometallics 2010, 29, 6527−6533.

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