Effects of Phosphine–Carbene Substitutions on the Electrochemical and Thermodynamic Properties of Nickel Complexes

A phosphine-free molecularly-defined Ni(II) complex in catalytic hydrogenation of CO2

The development of base metal catalysts capable of CO2 hydrogenation is a challenge and a necessity to progress from the scarce noble metal catalysts. In this regard, we report herein the first non-phosphine-based Ni complex, supported by a "carbazolato-bis-NHC" pincer ligand framework, for efficient catalytic hydrogenation of CO2 to formate. A tailored combination of the Ni complex as a catalyst, DBU as a base, and Zn(OAc)2 as an additive offered enhanced activity leading to a TON up to 5476 and an excellent yield up to 92% for the formate product from a reaction on ∼27 mmol scale.

Structure and Thermodynamic Hydricity in Cobalt(triphosphine)(monophosphine) Hydrides

The mononuclear cobalt hydride complex [HCo(triphos)(PMe3)], in which triphos = PhP(CH2CH2PPh2)2, was synthesized and characterized by X-ray crystallography and by 1H and 31P NMR spectroscopy. The geometry of the compound is a distorted trigonal bipyramid in which the axial positions are occupied by the hydride and the central phosphorus atom of the triphos ligand, while the PMe3 and terminal triphos donor atoms occupy the equatorial positions. Protonation of [HCo(triphos)(PMe3)] generates H2 and the Co(I) cation, [Co(triphos)(PMe3)]+, and this reaction is reversible under an atmosphere of H2 when the proton source is weakly acidic. The thermodynamic hydricity of HCo(triphos)(PMe3) was determined to be 40.3 kcal/mol in MeCN from measurements of these equilibria. The reactivity of the hydride is, therefore, well suited to CO2 hydrogenation catalysis. Density functional theory (DFT) calculations were performed to evaluate the structures and hydricities of a series of analogous cobalt(triphosphine)(monophosphine) hydrides where the phosphine substituents are systematically changed from Ph to Me. The calculated hydricities range from 38.5 to 47.7 kcal/mol. Surprisingly, the hydricities of the complexes are generally insensitive to substitution at the triphosphine ligand, as a result of competing structural and electronic trends. The DFT-calculated geometries of the [Co(triphos)(PMe3)]+ cations are more square planar when the triphosphine ligand possesses bulkier phenyl groups and more tetrahedrally distorted when the triphosphine ligand has smaller methyl substituents, reversing the trend observed for [M(diphosphine)2]+ cations. More distorted structures are associated with an increase in ΔGH-°, and this structural trend counteracts the electronic effect in which methyl substitution at the triphosphine is expected to yield smaller ΔGH-° values. However, the steric influence of the monophosphine follows the normal trend that phenyl substituents give more distorted structures and increased ΔGH-° values.

Thermodynamic targeting of electrocatalytic CO2 reduction: advantages, limitations, and insights for catalyst design

Herein is reported the electrocatalytic reduction of CO₂ with the complex [Ni(bis-NHC)(dmpe)]²⁺ (1) (bis-NHC = 1,l':3,3'-bis(1,3-propanediyl)dibenzimidazolin-2,2'-diylidene; dmpe = 1,2-bis(dimethylphosphino)ethane). The hydricity of 1 was previously benchmarked to be , equating to a driving force of a minimum of ∼3.4 kcal mol^-1 for hydride transfer to CO₂. While hydride transfer to CO₂ is thermodynamically favorable, electrocatalytic and infrared spectroelectrochemical (IR-SEC) experiments reveal that hydride transfer is blocked by direct reactivity with CO₂ in the reduced, Ni(0) state of the catalyst, yielding CO via reductive disproportionation (2CO₂ + 2e^- = CO + CO₃^2-) and concomitant catalyst degradation. Although thermodynamic scaling relationships provide guidance in catalyst targeting, the findings herein illustrate the fundamental kinetic challenges in balancing substrate reactivity and selectivity in the design of CO₂ reduction electrocatalysts. Advantages and limitations of this scaling relationship as well as approaches by which divergence from it may be achieved are discussed, which provides insight on important parameters for future catalyst design.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Mono- and dimeric complexes of an asymmetric heterotopic P,CNHC,pyr ligand

An asymmetric heterotopic ligand (S-N(Me)CP) containing a central bicyclic, expanded-ring NHC with one pyridyl and one phosphine exo-substituent has been synthesised and its coordination chemistry with selected late transition metals investigated. The amidinium precursor [S-N(Me)CHP]PF6 shows variable coordination modes with Ag(i), Cu(i) and Au(i) depending on the L : M ratio. The reaction of two mols of [S-N(Me)CHP]PF6 with [Cu(MeCN)4]BF4, AgBF4 or Au(THT)Cl gives the bis-ligand complexes [Cu(κ-P-N(Me)CHP)2(CH3CN)2]BF4·(PF6)2, 1, and [M(κ-P-N(Me)CHP)2]X·(PF6)2 (3: M = Ag, X = BF4; 6: M = Au, X = Cl) respectively. The 1 : 1 reaction of [S-N(Me)CHP]PF6 with AgOTf gave the head-to-tail dimer H,T-[Ag2(μ-N,P-N(Me)CHP)2(μ-OTf)2](PF6)2, 2, whereas the analogous reaction with Au(THT)Cl gave monomeric [Au(κ-P-N(Me)CHP)Cl]PF6, 5. Complex 2 was converted to H,T-[Ag2(μ-C,P-N(Me)CP)2](PF6)2, 4, upon addition of base, while 6 gave [Au(κ-C-N(Me)CP)2]Cl, 8, when treated likewise. Reaction of [S-N(Me)CHP]PF6 with Ni(1,5-COD)2 gave the oxidative addition/insertion product [Ni(κ(3)-N,C,P-N(Me)CP)(η(3)-C8H13)]PF6, 9, which converted to [Ni(κ(3)-N,C,P-N(Me)CP)Cl]PF6, 10, upon exposure of a CHCl3 solution to air. Complex 10 showed conformational isomerism that was also present in [Rh(κ(3)-N,C,P-N(Me)CP)(CO)]PF6, 14, prepared from the precursor complex [Rh(κ-P-N(Me)CHP)(acac)(CO)]PF6, 13, upon heating in C6H5Cl. [Pt(κ(3)-N,C,P-N(Me)CP)(Cl)]PF6, 12, derived from trans-[Pt(κ-P-N(Me)CHP)2(Cl)2](PF6)2, 11, was isolated as a single conformer.

Thermodynamic Hydricity of Transition Metal Hydrides

Transition metal hydrides play a critical role in stoichiometric and catalytic transformations. Knowledge of free energies for cleaving metal hydride bonds enables the prediction of chemical reactivity, such as for the bond-forming and bond-breaking events that occur in a catalytic reaction. Thermodynamic hydricity is the free energy required to cleave an M-H bond to generate a hydride ion (H(-)). Three primary methods have been developed for hydricity determination: the hydride transfer method establishes hydride transfer equilibrium with a hydride donor/acceptor pair of known hydricity, the H2 heterolysis method involves measuring the equilibrium of heterolytic cleavage of H2 in the presence of a base, and the potential-pKa method considers stepwise transfer of a proton and two electrons to give a net hydride transfer. Using these methods, over 100 thermodynamic hydricity values for transition metal hydrides have been determined in acetonitrile or water. In acetonitrile, the hydricity of metal hydrides spans a range of more than 50 kcal/mol. Methods for using hydricity values to predict chemical reactivity are also discussed, including organic transformations, the reduction of CO2, and the production and oxidation of hydrogen.

Synthesis and structures of ruthenium-NHC complexes and their catalysis in hydrogen transfer reaction

Ruthenium complexes [Ru(L1)2(CH3CN)2](PF6)2 (1), [RuL1(CH3CN)4](PF6)2 (2) and [RuL2(CH3CN)3](PF6)2 (3) (L1= 3-methyl-1-(pyrimidine-2-yl)imidazolylidene, L2 = 1,3-bis(pyridin-2-ylmethyl)benzimidazolylidene) were obtained through a transmetallation reaction of the corresponding nickel-NHC complexes with [Ru(p-cymene)2Cl2]2 in refluxing acetonitrile solution. The crystal structures of three complexes determined by X-ray analyses show that the central Ru(II) atoms are coordinated by pyrimidine- or pyridine-functionalized N-heterocyclic carbene and acetonitrile ligands displaying the typical octahedral geometry. The reaction of [RuL1(CH3CN)4](PF6)2 with triphenylphosphine and 1,10-phenanthroline resulted in the substitution of one and two coordinated acetonitrile ligands and afforded [RuL1(PPh3)(CH3CN)3](PF6)2 (4) and [RuL1(phen)(CH3CN)2](PF6)2 (5), respectively. The molecular structures of the complexes 4 and 5 were also studied by X-ray diffraction analysis. These ruthenium complexes have proven to be efficient catalysts for transfer hydrogenation of various ketones.

Synthesis and structural studies of nickel(0) Tetracarbene complexes with the introduction of a new four-coordinate geometric index, τδ

The synthesis and characterization of two homoleptic chelating nickel(0) tetracarbene complexes are reported. These are the first group 10 M(0) (M = Ni, Pd, Pt) tetracarbene complexes. These species have geometries intermediate between C2v sawhorse and tetrahedral and show high UV-vis absorption in the 350-600 nm range, with extinction coefficients (ϵ) between 5600 and 9400 M(-1) cm(-1). Density functional theory analysis indicates that this high absorptivity is due to metal-to-ligand charge transfer. In order to better describe the unusual geometries encountered in these complexes, an adjustment to the popular τ4 index for four-coordinate geometries is introduced in order to better delineate between sawhorse and distorted tetrahedral geometries.