Proton Transfer to Nickel−Thiolate Complexes. 2. Rate-Limiting Intramolecular Proton Transfer in the Reactions of [Ni(SC₆H₄R-4)(PhP{CH₂CH₂PPh₂}₂)]⁺ (R = NO₂, Cl, H, Me, or MeO)

Abstract

The protonation of [Ni(SC(6)H(4)R-4)(triphos)](+) (triphos = PhP[CH(2)CH(2)PPh(2)](2); R = NO(2), Cl, H, Me, or MeO) by [lutH](+) (lut = 2,6-dimethylpyridine) to form [Ni(S(H)C(6)H(4)R-4)(triphos)](2+) is an equilibrium reaction in MeCN. Kinetic studies, using stopped-flow spectrophotometry, reveal that the reactions occur by a two-step mechanism. Initially, [lutH](+) rapidly binds to the complex (K(2)(R)) in an interaction which probably involves hydrogen-bonding of the acid to the sulfur. Subsequent intramolecular proton transfer from [lutH](+) to sulfur (k(3)(R)) is slow because of both electronic and steric factors. The X-ray crystal structures of [Ni(SC(6)H(4)R-4)(triphos)](+) (R = NO(2), H, Me, or MeO) show that all are best described as square-planar complexes, with the phenyl substituents of the triphos ligand presenting an appreciable barrier to the approach of the sterically demanding [lutH](+) to the sulfur. The kinetic characteristics of the intramolecular proton transfer from [lutH](+) to sulfur have been investigated. The rate of intramolecular proton transfer exhibits a nonlinear dependence on Hammett sigma(+), with both electron-releasing and electron-withdrawing 4-R-substituents on the coordinated thiolate facilitating the rate of proton transfer (NO(2) > Cl > H > Me < MeO). The rate constants for intramolecular proton transfer correlate well with the calculated electron density of the sulfur. The temperature dependence of the rate of the intramolecular proton transfer reactions shows that deltaH() is small but increases as the 4-R-substituent becomes more electron-withdrawing [deltaH = 4.1 (MeO), 6.9 (Me), 11.4 kcal mol(-)(1) (NO(2))], while DeltaS() becomes progressively less negative [deltaS = -50.1 (MeO), -41.2 (Me), -16.4 (NO(2)) cal K(-)(1) mol(-)(1)]. Studies with [lutD](+) show that the rate of intramolecular proton transfer varies with the 4-R-substituent [(k(3)(NO)2)(H)/(k(3)(NO)2)(D) = 0.39; (k(3)(Cl))(H)/(k(3)(Cl))(D) = 0.88; (k(3)(Me))(H)/(k(3)(Me))(D) = 1.3; (k(3)(MeO))(H)/(k(3)(MeO))(D) = 1.2].

Four- and five-coordinate nickel(ii) complexes bearing new diphosphine–phosphonite and triphosphine–phosphite ligands: catalysts for N-alkylation of amines

“…These two values are slightly different from the values (0.18 and 0.064(1) Å, respectively) found in the closely related structure [NiCl(PhP{CH 2 CH 2 PPh 2 } 2 )] + (D) 34 and are attributed to the relatively smaller size of the oxygen atom in ligand 1 than the carbon atom of the ligand in D. The P1–Ni–P3 angle of 161.7(1)° and the P2–Ni1–Cl1 angle of 169.9(1)° are not differing much from the analogous angles (162.4(1)° and 171.6(1)°) in the reported complex D. The Ni–Cl bond distance of 2.1685(7) Å is close to 2.169(3) Å in complex D. The central phosphorus P2–Ni distance of 2.1067(7) Å is slightly shorter than the other two P–Ni distances (2.1978(6) Å and 2.1949(6) Å) owing to the stronger trans influence of phosphorus than the chlorine atom and to the nature of atoms (two oxygens and one aryl carbon to the central P2; two aryl and one alkyl carbons to P1 and P3) attached to the phosphorus atom. Similar trend has been observed in the reported four coordinate Ni( ii ) square planar complexes such as D, [Ni(SR)(PhP{CH 2 CH 2 PPh 2 } 2 )] + (R = Et, t Bu, Cy, 35 Bn 36 and Ph 37 ) and in the five coordinate square pyramidal Ni( ii ) complex C.…”

Cyclopentadienyl Ruthenium–Nickel Catalysts for Biomimetic Hydrogen Evolution: Electrocatalytic Properties and Mechanistic DFT Studies

“…www.chemeurj.org complexes that involve, for example, initial protonation at the nickel center or at the thiolate ligands. [30][31][32][33][34] Transient breakage of one RuÀS bond might also occur in such coordinatively saturated complexes, even if the DFT calculations carried out in this study never found such hemi-lability of…”

pH Dependent Reversible Formation of a Binuclear Ni2 Metal-Center within a Peptide Scaffold

“…An interesting feature of the nickel-thiolate moiety is its ability to support ligand protonation without subsequent protonolysis [12][13][14]. This is most often observed in (near) square planar Ni 2+ centers where the nucleophilic HOMO possesses significant S(3pπ) character, which effectively act as S-based lone-pairs.…”