Synthesis, Structure, and Electrochemistry of Di- and Zerovalent Nickel, Palladium, and Platinum Monomers and Dimers Derived from an Enantiopure (S,S)-Tetra(tertiary Phosphine)

Abstract

The ligand (S,S)-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane, (S,S)-tetraphos, reacts with hexa(aqua)nickel(II) chloride in the presence of trimethylsilyl triflate (TMSOTf) in dichloromethane to give the yellow square-planar complex [Ni{(R,R)-tetraphos}](OTf)2, which has been crystallographically characterized as the square-pyramidal, acetonitrile adduct [Ni(NCMe){(R,R)-tetraphos}]OTf. Cyclic voltammograms of the nickel(II) complex in dichloromethane and acetonitrile at 20 degrees C showed two reduction processes at negative potentials with oxidative (E(p)(ox)) and reductive (E(p)(red)) peak separations similar to those observed for ferrocene/ferrocenium under identical conditions, suggesting two one-electron steps. The cyclic voltammetric data for the divalent nickel complex in acetonitrile at temperatures below -20 degrees C were interpreted according to reversible coordination of acetonitrile to the nickel(I) and nickel(0) complexes. The divalent palladium and platinum complexes [M{(R,R)-tetraphos}](PF6)2 and [M2{(R,R)-tetraphos}2](OTf)4 have been prepared. The reduction potentials for the complexes [M{(R,R)-tetraphos}](PF6)2 increase in the order nickel(II) < palladium(II) < platinum(II). The reaction of (S,S)-tetraphos with bis(cycloocta-1,5-diene)nickel(0) in benzene affords orange [Ni{(R,R)-tetraphos}], which slowly rearranges into the thermodynamically more stable, yellow, double-stranded helicate [Ni2{(R,R)-tetraphos}2]; the crystal structures of both complexes have been determined. The reactions of (S,S)-tetraphos with [M(PPh3)4] in toluene (M = Pd) or benzene (M = Pt) furnish the double-stranded helicates [M2{(R,R)-tetraphos}2]; the palladium complex crystallizes from hot benzene as the 2-benzene solvate and was structurally characterized by X-ray crystallography. In each of the three zerovalent complexes, the coordinated (R,R)-tetraphos stereospecifically generates tetrahedral M(PP)2 stereocenters of M configuration.

Electrocatalytic Hydrogen Production by a Nickel Complex Containing a Tetradentate Phosphine Ligand

“…A plausible assignment for the decomposition pathway is dissociation of one or more phosphines from Ni 0 (P 4 N 2 ), followed by formation of multimetallic species in which the P 4 N 2 ligand bridges two or more nickel centers. In support of this assignment, Ni 0 (tetraphos) has been shown to isomerize to [Ni 0 2 (tetraphos) 2 ], in which the tetraphos ligands bridge both Ni centers …”

“…Notably, 1 displays a square-pyramidal geometry in the solid state with an axial CH 3 CN ligand, with a τ 5 value of 0.02 . In contrast to 1 , the related complex [Ni­(tetraphos)­(CH 3 CN)] 2+ (tetraphos = ( R,R )-Ph 2 CH 2 CH 2 P­(Ph)­CH 2 CH 2 P­(Ph)­CH 2 CH 2 PPh 2 ) displays τ 5 values of 0.24 and 0.40 for two crystallographically independent molecules, indicating a moderately to strongly distorted square pyramidal geometry. This structural difference is a consequence of the cyclic P 2 N 2 fragment in the P 4 N 2 backbone, which possesses a small P–Ni–P angle of 80.23(2)°.…”

Force-Modulated C–C Reductive Elimination from Nickel Bis(polyfluorophenyl) Complexes

“…A number of parameters point to the potential for the differential force/rate response of reductive elimination for nickel compared to platinum. For example, nickel is smaller ( r = 1.24 Å) than platinum ( r = 1.36 Å), with concomitantly shorter M–P bonds (Δ d = 0.07 Å). − Nickel­(II) complexes are also more prone to adopt a tetrahedral geometry and are more easily reduced than are Pt­(II) complexes. − Importantly, energy decomposition analysis of ethylene dissociation from the group 10 bisphosphine complexes (PMe 3 ) 2 M­(π-H 2 CCH 2 ) (M = Ni, Pt) revealed that the preparation energy (Δ E prep ), which corresponds to the energy required to deform the two-coordinate (PMe 3 ) 2 M fragments from their equilibrium geometry to the geometry in the corresponding three-coordinate ethylene complexes, was significantly larger for platinum (Δ E prep = 30.2 kcal/mol) than for nickel (Δ E prep = 14.7 kcal/mol) . This suggests that the P–M–P bond angle of square-planar nickel complexes might be more pliable than the corresponding platinum complexes, leading to enhanced sensitivity to applied force.…”

Theoretical study of cobalt and nickel complexes involved in methyl transfer reactions: structures, redox potentials and methyl binding energies

“…This reaction proceeds according to the equation: The occurrence of Ni(0) [8,9] or Ni(I) [10] in reaction (1) of ACS was postulated. Since the mechanism of catalytic action of the ACS enzyme is not fully understood [8,[11][12][13], models of methylation reactions involving nickel complexes and various methylation factors are being examined experimentally [9,[14][15][16][17][18][19]. Likewise, many complexes relevant to ACS enzyme are investigated experimentally [20][21][22][23][24] and computationally [25][26][27][28][29].…”