40326-49-6Relevant academic research and scientific papers
Synthesis of [Ni(η2-CH2C6H 4R-4){PPh(CH2CH2PPh2) 2}]+ (R = H, Me or MeO) and protonation reactions with HCl
Autissier, Valerie,Brockman, Ellie,Clegg, William,Harrington, Ross W.,Henderson, Richard A.
, p. 1763 - 1771 (2007/10/03)
The complexes [Ni(η2-CH2C6H 4R-4)(triphos)]BPh4 {R = H, Me or MeO; triphos = PhP(CH2CH2PPh2)2} have been prepared and characterised by spectroscopy and X-ray crystallography. In all cases the coordination geometry of the nickel is best described as square-planar with an η2-benzyl ligand occupying one of the positions. The orientation of the η2-benzyl ligand is dictated by the steric restrictions imposed by the phenyl groups on the triphos ligand, so that the phenyl group on the unique secondary phosphorus and the aromatic group of the benzyl ligand (which are trans to one another) are oriented in the same direction. [Ni(η2-CH2C6H4R-4)(triphos)] + react with an excess of anhydrous HCl in MeCN to form [NiCl(triphos)]+ (characterised as the [BPh4]- salt by X-ray crystallography) and the corresponding substituted toluene. The kinetics of the reaction of all [Ni(η2-CH2C 6H4R-4)(triphos)]+ and HCl in the presence of Cl- have been determined using stopped-flow spectrophotometry. All reactions exhibit a first-order dependence on the concentration of complex and a first-order dependence on the ratio [HCl]/[Cl-]. Varying the 4-R-substituent on the benzyl ligand shows that electron-withdrawing substituents facilitate the rate of the reaction. It is proposed that the mechanism involves initial rapid protonation at the nickel to form [NiH(η2-CH2C6H4R-4)(triphos)] 2+, followed by intramolecular proton migration from nickel to carbon to yield the products.
Kinetic evidence for intramolecular proton transfer between nickel and coordinated thiolate
Clegg, William,Henderson, Richard A.
, p. 1128 - 1135 (2008/10/08)
The complexes [Ni(YR)(triphos)]BPh4 {Y = S, R = Ph or Et or Y = Se, R = Ph; triphos = (Ph2PCH2CH2)2PPh} have been prepared and characterized, and the X-ray crystal structure of [Ni(SPh)(triphos)]BPh4 has been solved. In MeCN, [Ni(YR)(triphos)]+ are protonated by [lutH]+ (lut = 2,6-dimethylpyridine) to give [Ni(YHR)(triphos)]2+. Studies on the kinetics of these equilibrium reactions reveal an unexpected difference in the reactivities of [Ni(SPh)(triphos)]+ and [Ni(SEt)(triphos)]+. In both cases, the reactions exhibit a first-order dependence on the concentration of complex. When R = Ph, the dependence on the concentrations of [lutH+] and lut is given by kobs = k1Ph[lutH+] + k-1Ph[lut], which is typical of an equilibrium reaction where k1Ph and k-1Ph correspond to the forward and back reactions, respectively. Analogous behavior is observed for [Ni(SePh)(triphos)]+. However, for [Ni(SEt)(triphos)]+, the kinetics are more complicated, and kobs = {k1k2[lutH+] + (k-2 + k2)}/(k1[lutH+] + k--1[lut]), which is indicative of a mechanism involving two coupled equilibria in which the initial protonation of the thiolate is followed by a unimolecular equilibrium reaction that is assumed to involve the formation of an η2-EtS-H ligand. The difference in reactivity between the complexes with alkyl and aryl thiolate ligands is a consequence of the {Ni(triphos)}2+ site "leveling" the basicities of these ligands. The pKa's of the PhSH and EtSH constituents coordinated to the {Ni(triphos)}2+ are 16.0 and 14.6, respectively, whereas the difference in pKa's of free PhSH and EtSH differ by ca. 4 units. The pKa of [Ni-(SeHPh)(triphos)]+ is 14.4. The more strongly σ-donating EtS ligand makes the {Ni(triphos)}2+ core sufficiently electron-rich that the basicities of the sulfur and nickel in [Ni(SEt)(triphos)]+ are very similar; therefore, the proton serves as a bridge between the two sites. The relevance of these observations to the proposed mechanisms of nickel-based hydrogenases is discussed.
