Nickel Hydride Complex Research Articles

Synthesis and Reactivity of Nickel Hydride Complexes of an α-Diimine Ligand

Reaction of L(0)NiBr(2) with 2 equiv of NaH yielded the Ni(II) hydride complex [(L(•-))Ni(μ-H)(2)Ni(L(•-))] (1) (L = [(2,6-iPr(2)C(6)H(3))NC(Me)](2); L(0) represents the neutral ligand, L(•-) is its radical-anionic form, and L(2-) denotes the dianion) in good yield. Stepwise reduction of complex 1 led to a series of nickel hydrides. Reduction of 1 with 1 equiv of sodium metal afforded a singly reduced species [Na(DME)(3)][(L(•-))Ni(μ-H)(2)Ni(L(•-))] (2a) (DME = 1,2-dimethoxyethane), which contains a mixed-valent core [Ni(μ-H)(2)Ni](+). With 2 equiv of Na a doubly reduced species [Na(DME)](2)[L(2-)Ni(μ-H)(2)NiL(2-)] (3a) was obtained, in which each monoanion (L(•-)) in the precursor 1 has been reduced to L(2-). By using potassium as the reducing agent, two analogous species [K(DME)(4)][(L(•-))Ni(μ-H)(2)Ni(L(•-))] (2b) and [K(DME)](2)[L(2-)Ni(μ-H)(2)NiL(2-)] (3b) were obtained. Further treatment of 3b with 2 equiv of K led to a trinuclear complex [K(DME)(THF)](2)K(2)[L(2-)Ni(μ-H)(2)Ni(μ-H)(2)NiL(2-)] (4), which contains one Ni(II) and two Ni(I) centers with a triplet ground state. When 1 and 3a were warmed in toluene or benzene, respectively, three reverse-sandwich dinickel complexes, [(L(•-))Ni(μ-η(3):η(3)-C(7)H(8))Ni(L(•-))] (5) and [Na(DME)](2)[L(2-)Ni(μ-η(3):η(3)-C(6)H(5)R)NiL(2-)] (6: R = CH(3); 7: R = H), were isolated. Reaction of 1 with Me(3)SiN(3) gave the N(3)-bridged complex [(L(•-))Ni(μ-η(1)-N(3))(2)Ni(L(•-))] (8). The crystal structures of complexes 1-8 have been determined by X-ray diffraction, and their electronic structures have been fully studied by EPR/NMR spectroscopy.

Hydrosilylation of Aldehydes and Ketones Catalyzed by an N‐Heterocyclic Carbene‐Nickel Hydride Complex under Mild Conditions

AbstractHalf‐sandwich N‐heterocyclic carbene (NHC)‐nickel complexes of the general formula [Ni(NHC)ClCp†] (Cp†=Cp, Cp*) efficiently catalyze the hydrosilylation of aldehydes and ketones at room temperature in the presence of a catalytic amount of sodium triethylborohydride and thus join the fairly exclusive club of well‐defined nickel(II) catalyst precursors for the hydrosilylation of carbonyl functionalities. Of notable interest is the isolation of an intermediate nickel hydride complex that proved to be the real catalyst precursor.

Alkene Hydrogenation Catalyzed by Nickel Hydride Complexes of an Aliphatic PNP Pincer Ligand

AbstractTo investigate metal–ligand cooperativity as a strategy for promoting nickel‐catalyzed alkene hydrogenation, cationic and neutral nickel(II) hydride complexes of the aliphatic pincer ligand PNHPCy {PNHPCy = HN[CH2CH2P(Cy)2]2} have been synthesized and characterized. Cationic hydride complex [(PNHPCy)Ni(H)]BPh4 (2) catalyzed the hydrogenation of styrene and 1‐octene under mild conditions. Only low conversion was observed in the hydrogenation of 3,5‐dimethoxybenzaldehyde using 2. The neutral hydride complex (PNPCy)Ni(H) (3) was also found to be an alkene hydrogenation catalyst. Mechanistic experiments suggest that for catalyst 2, the hydrogenation reaction proceeds through a pathway involving initial insertion of the alkene into the Ni–H bond. Contrary to the initial hypothesis, reactivity comparisons with the methyl‐substituted hydride complex [(PNMePCy)Ni(H)]BPh4 {PNMePCy = (CH3)N[CH2CH2P(Cy)2]2} suggest that metal–ligand cooperativity is not involved in these rare examples of mild and homogeneous nickel hydrogenation catalysis.

Synthesis, Reactivity, and Catalytic Application of a Nickel Pincer Hydride Complex

The nickel(II) hydride complex [(MeN2N)Ni-H] (2) was synthesized by the reaction of [(MeN2N)Ni-OMe] (6) with Ph2SiH2 and was characterized by NMR and IR spectroscopy as well as X-ray crystallography. 2 was unstable in solution, and it decomposed via two reaction pathways. The first pathway was intramolecular N–H reductive elimination to give MeN2NH and nickel particles. The second pathway was intermolecular, with H2, nickel particles, and a five-coordinate Ni(II) complex [(MeN2N)2Ni] (8) as the products. 2 reacted with acetone and ethylene, forming [(MeN2N)Ni-OiPr] (9) and [(MeN2N)Ni-Et] (10), respectively. 2 also reacted with alkyl halides, yielding nickel halide complexes and alkanes. The reduction of alkyl halides was rendered catalytically, using [(MeN2N)Ni-Cl] (1) as catalyst, NaOiPr or NaOMe as base, and Ph2SiH2 or Me(EtO)2SiH as the hydride source. The catalysis appears to operate via a radical mechanism.

Divergent Carbonylation Reactivity Preferences of Nickel Complexes Containing Amido Pincer Ligands: Migratory Insertion versus Reductive Elimination

The reactivity of a series of nickel(II) hydride, alkyl, and anilide complexes supported by amido diphosphine ligands, including symmetrical [N(o-C6H4PR2)2]− (R = Ph (1a), iPr (1c), Cy (1d)) and unsymmetrical [N(o-C6H4PPh2)(o-C6H4PiPr2)]− (1b), with carbon monoxide is described. Exposure of a benzene solution of [1a–d]NiH to carbon monoxide under ambient conditions leads to reductive elimination of diarylamine to give quantitatively zerovalent nickel dicarbonyl complexes [H·1a–d]Ni(CO)2 (2). Migratory insertion of CO into the Ni-R bonds of [1]NiR in benzene solutions affords Ni(II)-acyl derivatives [1]NiC(O)R (R = Me (3), Et (4), n-hexyl (5), 2-norbornyl (6)). Interestingly, further carbonylation of acyl 3a and 4a generates [RC(O)N(o-C6H4PPh2)2]Ni(CO)2 (R = Me (7a), Et (8a)) as a result of C–N bond-forming reductive elimination whereas no reaction occurs for 3b–c or 4b–c under similar conditions. Carbonylation of the anilide complexes [1]NiNHPh produces carbamoyl [1]Ni[C(O)NHPh] (9) as the final product i...

Synthesis and Hydride Transfer Reactions of Cobalt and Nickel Hydride Complexes to BX3Compounds

Hydrides of numerous transition metal complexes can be generated by the heterolytic cleavage of H(2) gas such that they offer alternatives to using main group hydrides in the regeneration of ammonia borane, a compound that has been intensely studied for hydrogen storage applications. Previously, we reported that HRh(dmpe)(2) (dmpe = 1,2-bis(dimethylphosphinoethane)) was capable of reducing a variety of BX(3) compounds having a hydride affinity (HA) greater than or equal to the HA of BEt(3). This study examines the reactivity of less expensive cobalt and nickel hydride complexes, HCo(dmpe)(2) and [HNi(dmpe)(2)](+), to form B-H bonds. The hydride donor abilities (ΔG(H(-))°) of HCo(dmpe)(2) and [HNi(dmpe)(2)](+) were positioned on a previously established scale in acetonitrile that is cross-referenced with calculated HAs of BX(3) compounds. The collective data guided our selection of BX(3) compounds to investigate and aided our analysis of factors that determine favorability of hydride transfer. HCo(dmpe)(2) was observed to transfer H(-) to BX(3) compounds with X = H, OC(6)F(5), and SPh. The reaction with B(SPh)(3) is accompanied by the formation of dmpe-(BH(3))(2) and dmpe-(BH(2)(SPh))(2) products that follow from a reduction of multiple B-SPh bonds and a loss of dmpe ligands from cobalt. Reactions between HCo(dmpe)(2) and B(SPh)(3) in the presence of triethylamine result in the formation of Et(3)N-BH(2)SPh and Et(3)N-BH(3) with no loss of a dmpe ligand. Reactions of the cationic complex [HNi(dmpe)(2)](+) with B(SPh)(3) under analogous conditions give Et(3)N-BH(2)SPh as the final product along with the nickel-thiolate complex [Ni(dmpe)(2)(SPh)](+). The synthesis and characterization of HCo(dedpe)(2) (dedpe = Et(2)PCH(2)CH(2)PPh(2)) from H(2) and a base is also discussed, including the formation of an uncommon trans dihydride species, trans-[(H)(2)Co(dedpe)(2)][BF(4)].

Catalytic properties of nickel bis(phosphinite) pincer complexes in the reduction of CO 2 to methanol derivatives

A new nickel bis(phosphinite) pincer complex [2,6-(R 2PO) 2C 6H 3]NiCl (L RNiCl, R = cyclopentyl) has been prepared in one pot from resorcinol, ClP(C 5H 9) 2, NiCl 2, and 4-dimethylaminopyridine. The reaction of this pincer compound with LiAlH 4 produces a nickel hydride complex, which is capable of reducing CO 2 rapidly at room temperature to give a nickel formate complex. X-ray structures of two related nickel formate complexes L RNiOCHO (R = cyclopentyl and isopropyl) have shown an “in plane” conformation of the formato group with respect to the coordination plane. The stoichiometric reaction of nickel formate complexes L RNiOCHO (R = cyclopentyl, isopropyl, and tert-butyl) with catecholborane has suggested that the reaction is favored by a bulky R group. L RNiOCHO (R = tert-butyl) does not react with PhSiH 3 at room temperature; however, it reacts with 9-borabicyclo[3.3.1]nonane and pinacolborane to generate a methanol derivative and a boryl formate species, respectively. The catalytic reduction of CO 2 with catecholborane is more effectively catalyzed by a more sterically hindered nickel pincer hydride complex with bulky R groups on the phosphorus donor atoms. The nickel pincer hydride complexes are inactive catalysts for the hydrosilylation of CO 2 with PhSiH 3.

Nickel(ii) hydride and fluoride pincer complexes and their reactivity with Lewis acids BX3·L (X = H, L = thf; X = F, L = Et2O)

Reaction of the pincer hydride complex ((tBu)PCP)Ni(H) [(tBu)PCP = 2,6-C(6)H(3)(CH(2)P(t)Bu(2))(2)] with BH(3)·thf in THF at 190 K generates the corresponding borohydride complex ((tBu)PCP)Ni(BH(4)). The kinetically stable (but thermodynamically unstable) species undergoes reversible borane loss. The related fluoride complex ((tBu)PCP)Ni(F) shows the same reactivity towards BF(3)·Et(2)O, producing ((tBu)PCP)Ni(BF(4)) as the main final product. The processes were followed through multinuclear NMR spectroscopy and DFT calculations, at the M06//6-31+G(d,p) level of theory.

Amino olefin nickel(i) and nickel(0) complexes as dehydrogenation catalysts for amine boranes

A rare paramagnetic organometallic nickel(I) olefin complex can be isolated using the ligand bis(5H-dibenzo[a,d]cyclohepten-5-yl)amine. This complex and related nickel(0) hydride complexes show very high catalytic activity in the dehydrogenation of dimethylamino borane with release of one equivalent of dihydrogen.

Propylene dimerization in the presence of nickel hydride complexes formed in situ

We study the influence of nickel hydride complexes formed in situ by reaction nickel(0) complexes having phosphorus-containing ligands with Bronsted acids in the presence of various modifiers on a catalyst turnover and selectivity in propylene dimerization. The activating action of boron trifluoride etherate is considered.

Noninnocent Behavior of Ancillary Ligands: Apparent Trans Coupling of a Saturated N-Heterocyclic Carbene Unit with an Ethyl Ligand Mediated by Nickel

Oxidative addition of the tridentate N-heterocyclic carbene (NHC) diphosphine ligand precursor ([PCP]H)PF(6) (1) {[PCP] = o-(i)Pr(2)PC(6)H(4)(NC(3)H(4)N)o-C(6)H(4)P(i)Pr(2)} to Ni(COD)(2) results in the formation of the nickel(II) hydride complex ([PCP]NiH)PF(6) (2). This hydride undergoes a rapid reaction with ethylene to generate a nickel(0) complex in which an ethyl group has been transferred to the carbene carbon of the original NHC-diphosphine ligand. If the first intermediate is the anticipated square-planar nickel(II) ethyl species, then the formation of the product would require a process that involves a trans C-C coupling of the NHC carbon and a presumed Ni-ethyl intermediate. Deuterium-labeling studies provide evidence for migratory insertion of the added ethylene into the Ni-H bond rather than into the Ni-carbene linkage; this is based on the observed deuterium scrambling, which requires reversible beta-elimination, alkene rotation, and hydride readdition. However, density functional theory studies suggest that a key intermediate is an agostic ethyl species that has the Ni-C bond cis to the NHC unit. A possible transition state containing two cis-disposed carbon moieties was also identified. Such a process represents a new pathway for catalyst deactivation involving NHC-based metal complexes.

Catalytic Hydrosilylation of the Carbonyl Functionality via a Transient Nickel Hydride Complex

We report the syntheses and characterization of two new sterically demanding chelate ligands, denoted PNMe3 (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) and PNiPr3 (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-triisopropylanilide), as well as the preparation of their nickel(II) halide complexes [(PNiPr3)Ni(μ2-Br)]2 (1) and [(PNMe3)Ni(μ2-Cl)]2 (2). The combination of complex 1 with KOtBu and Et3SiH as the hydride source can promote the hydrosilylation of a variety of carbonyl compounds in excellent yield and in a catalytic manner. The reactivity for a scope of substrates and a plausible mechanism along the hydrosilylation reaction are discussed in this work.

Syntheses and properties of molecular nickel(II) hydride, methyl, and nickel(I) complexes supported by trimethylphosphane and (2-diphenylphosphanyl)thiophenolato and -naphtholato ligands

Abstract (2-Diphenylphosphanyl)thiophenol (P^pSH) or (3-diphenylphosphanyl)-2-thionaphthol (P^nSH) react with Ni(PMe3)4 to form NiH(P^pS)(PMe3)2 (1) or NiH(P^nS)(PMe3)2 (2). 1,3-Bis(diphenylphosphanyl)propane (P^P) replaces the monodentate phosphane ligands to give NiH(P^pS)(P^P) (3). NiMe(OMe)(PMe3) or NiMe2(PMe3)3 react with P^pSH to form NiMe(P^pS)(PMe3) (4) and NiMe(P^pS)(PMe3)2 (5), respectively, and P^nSH affords NiMe(P^nS)(PMe3)2 (6), NiMe(P^nS)(PMe3) (7). Dissociation of PMe3 ligands induces transformation of 1 to Ni(P^pS)(PMe3)2 (8) and Ni(P^pS)2. Crystal and molecular structures are given for 1, 5–8, and dynamic solution spectra (NMR, EPR) are discussed.

A Nickel Hydride Complex in the Active Site of Methyl-Coenzyme M Reductase: Implications for the Catalytic Cycle

Methanogenic archaea utilize a specific pathway in their metabolism, converting C1 substrates (i.e., CO2) or acetate to methane and thereby providing energy for the cell. Methyl-coenzyme M reductase (MCR) catalyzes the key step in the process, namely methyl-coenzyme M (CH3-S-CoM) plus coenzyme B (HS-CoB) to methane and CoM-S-S-CoB. The active site of MCR contains the nickel porphinoid F430. We report here on the coordinated ligands of the two paramagnetic MCR red2 states, induced when HS-CoM (a reversible competitive inhibitor) and the second substrate HS-CoB or its analogue CH3-S-CoB are added to the enzyme in the active MCR red1 state (Ni(I)F430). Continuous wave and pulse EPR spectroscopy are used to show that the MCR red2a state exhibits a very large proton hyperfine interaction with principal values A((1)H) = [-43,-42,-5] MHz and thus represents formally a Ni(III)F430 hydride complex formed by oxidative addition to Ni(I). In view of the known ability of nickel hydrides to activate methane, and the growing body of evidence for the involvement of MCR in "reverse" methanogenesis (anaerobic oxidation of methane), we believe that the nickel hydride complex reported here could play a key role in helping to understand both the mechanism of "reverse" and "forward" methanogenesis.

Phosphorus and Olefin Substituent Effects on the Insertion Chemistry of Nickel(II) Hydride Complexes Containing Amido Diphosphine Ligands

A series of nickel(II) hydride complexes supported by amido diphosphine ligands, including symmetrical [N(o-C6H4PR2)2]− ([R-PNP]−; R = Ph, iPr, Cy) and unsymmetrical [N(o-C6H4PPh2)(o-C6H4PiPr2)]− ([Ph-PNP-iPr]−), have been prepared for the investigation of olefin insertion chemistry. The unsymmetrical ligand precursor H[Ph-PNP-iPr] (1d) that features different substituents (phenyl and isopropyl) at the two phosphorus donors was prepared in 53% yield as colorless crystals. Treatment of Ni(COD)2 (COD = cycloocta-1,5-diene) with H[R-PNP] (R = Ph (1a), iPr (1b), Cy (1c)) or 1d produced the corresponding four-coordinate nickel hydride complexes 2a−d. Attempts to isolate 2a led instead to the cyclooct-4-en-1-yl complex [Ph-PNP]Ni(η1-C8H13) (3a) as a consequence of COD insertion into the Ni−H bond of 2a. The reactions of 2a,d with ethylene, 1-hexene, and norbornene, respectively, generated cleanly the corresponding ethyl (4a,d), n-hexyl (5a,d), and 2-norbornyl (6a,d) complexes. The quantitative formation of 5a,d is indicative of exclusive 1,2-insertion of 1-hexene. In contrast, styrene inserts into the Ni−H bond of 2d in an exclusively 2,1-manner to afford [Ph-PNP-iPr]NiCH(Me)Ph (7d) quantitatively. The selective 2,1-insertion products [Ph-PNP]NiCH(Me)CO2Me (8a), [iPr-PNP]NiCH(Me)CO2Me (8b), [Cy-PNP]NiCH(Me)CO2Me (8c), and [Ph-PNP-iPr]NiCH(Me)CO2Me (8d) were also isolated from the reactions of methyl acrylate with the corresponding nickel hydride complexes 2a−d. The effects of the phosphorus and olefin substituents on the reactivity and regioselectivity of the olefin insertion reactions are discussed. In addition to solution NMR spectroscopic data for all new compounds, X-ray structures of 1d, 2b−d, 3a, and 5d−7d are reported.

A low-coordinate nickel(ii) hydride complex and its reactivity

The preparation of a novel dinuclear nickel(ii) hydride complex and its reactivity that often leads to nickel(i) compounds is described.

Nature of hydrogen interactions with Ni(II) complexes containing cyclic phosphine ligands with pendant nitrogen bases

Studies of the role of proton relays in molecular catalysts for the electrocatalytic production and oxidation of H(2) have been carried out. The electrochemical production of hydrogen from protonated DMF solutions catalyzed by [Ni(P(2)(Ph)N(2)(Ph))(2)(CH(3)CN)](BF(4))(2), 3a (where P(2)(Ph)N(2)(Ph) is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane), permits a limiting value of the H(2) production rate to be determined. The turnover frequency of 350 s(-1) establishes that the rate of H(2) production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. In the electrochemical oxidation of hydrogen catalyzed by [Ni(P(2)(Cy)N(2)(Bz))(2)](BF(4))(2), 3b (where Cy is cyclohexyl and Bz is benzyl), the initial step is the reversible addition of hydrogen to 3b (K(eq) = 190 atm(-1) at 25 degrees C). The hydrogen addition product exists as three nearly isoenergetic isomers 4A-4C, which have been identified by a combination of one- and two-dimensional (1)H, (31)P, and (15)N NMR spectroscopies as Ni(0) complexes with a protonated amine in each cyclic ligand. The nature of the isomers, together with calculations, suggests a mode of hydrogen activation that involves a symmetrical interaction of a nickel dihydrogen ligand with two amine bases in the diphosphine ligands. Single deprotonation of 4 by an external base results in a rearrangement to [HNi(P(2)(Cy)N(2)(Bz))(2)](BF(4)), 5, and this reaction is reversed by the addition of a proton to the nickel hydride complex. The small energy differences associated with significantly different distributions in electron density and protons within these molecules may contribute to their high catalytic activity.

Insertion of Alkynes into Ni−H Bonds: Synthesis of Novel Vinyl Nickel(II) and Dinuclear Vinyl Nickel(II) Complexes Containing a [P, S]-Ligand

Reactions of alkynes with nickel hydride complexes bearing a [P, S]-ligand and supported by trimethylphosphine were investigated. Tetracoordinate vinyl nickel(II) complexes 3, 5, and 6 with square-planar geometry were obtained by reacting phenylacetylene, trimethylsilylacetylene, and 1-hexyne with the hydrido nickel complex 1. Reaction of 1,4-bis(trimethylsilylethynyl)benzene with complex 1 proceeds as a monoinsertion and afforded complex 7, while reaction of 1,4-bis(ethynyl)benzene with 1 leads to the dinuclear vinyl nickel(II) complex 8.

Isolated [Ni2H7]7− and [Ni4H12]12− Ions in La2MgNi2H8

Isolated and complex: La2MgNi2H8 contains isolated dinuclear [Ni2H7]7- and tetranuclear [Ni4H12]12- anions (see picture). These are the first polynuclear solid-state nickel hydride complexes. Their formation upon hydrogenation of the intermetallic compound La2MgNi2 is associated with a metal-insulator transition. (Figure Presented). © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.

Structural Factors Influencing Linear M−H−M Bonding in Bis(dialkylphosphino)methane Complexes of Nickel

Structural data for four closely related dinuclear nickel hydride complexes have been compared in order to gain insight into the factors governing the Ni-H-Ni geometries. The derivatives [(dippm)2Ni2X2](mu-H) [dippm = 1,2-bis(diisopropylphosphino)methane] were found to contain a linear Ni-H-Ni bridge, whereas the derivatives [(dcpm)2Ni2X2](mu-H) [dcpm = 1,2-bis(dicyclohexylphosphino)methane] were found to contain a bent Ni-H-Ni bridge. The number of internal and interatomic CH-to-halide contacts of the former were much shorter and more numerous than the latter, suggesting an important role of external forces in bridging hydride geometries.