To what extent can cyclometalation promote associative or dissociative ligand substitution at platinum(II) complexes? A combined kinetic and theoretical approach

The ligand exchange rate constants for the reactions [Pt(bph)(SR2)(2)] + 2* SR2 --> [Pt(bph)(*SR2)(2)] + 2SR(2) (bph = 2,2'-biphenyl dianion; R = Me an d Et) and cis-[PtPh2(SMe2)(2)] + 2*SMe2 --> cis-[PtPh2(*SMe2)(2)] + 2SMe(2) have been determined in CDCl3 as a function of ligand concentration and te mperature, by H-1 NMR isotopic labeling and magnetization transfer experime nts. The rates of exchange show no dependence on ligand concentration and t he kinetics are characterized by largely positive entropies of activation. The kinetics of displacement of the thioethers from [Pt(bph)(SR2)(2)] with the dinitrogen ligands 2,2'-bipyridine and 1,10-phenanthroline (N-N) to yie ld [Pt(bph)(N-N)]. carried out in the presence of sufficient excess of thio ether and N-N to ensure pseudo-first-order conditions, follow a nonlinear r ate law k(obsd) = a[N-N]/(b[SR2] + [N-N]). The general pattern of behavior indicates that the rate-determining step for substitution is the dissociati on of a thioether ligand and the formation of a three-coordinated [Pt(bph)( SR2)] intermediate. The value of the parameter a, which measures the rate o f ligand dissociation, is constant and independent of the nature of N-N, an d it is in reasonable agreement with the value of the rate of ligand exchan ge at the same temperature. Theoretical ab initio calculations were perform ed for both [Pt(bph)(SMe2)(2)] and cis-[PtPh2(SMe2)(2)], and for their thre e-coordinated derivatives upon the loss of one SMe2 ligand. The latter opti mize in a T-shaped structure. Calculations were performed in the I-IF appro ximation (LANL2DZ basis set) and refined by introducing the correlation ter ms (Becke3LYP model). The activation enthalpies from the optimized vacuum-p hase geometries are 52.3 and 72.2 kJ mol(-1) compared to the experimental v alues in CDCl3 solution, 80 +/- 1 and 93 +/- 1 kJ mol(-1) for [Pt(bph)(SMe2 )(2)] and cis-[PtPh2(SMe2)(2)], respectively. The electrostatic potential m aps of both parent compounds show a remarkable concentration of negative ch arge over the platinum atom which exerts a repulsion force on an axially in coming nucleophile. On the other hand, the strength of the organic carbanio ns trans to the leaving group and the stabilization of the T-shaped interme diate in the singlet ground state may also rationalize the preference for t he dissociative mechanism. All of the kinetic and theoretical data support the latter hypothesis and indicate, in particular, that dissociation from t he complex containing the planar 2,2'-biphenyl dianion is easier than from its analogue with single aryl ligands. Electron back-donation from filled d orbitals of the metal to empty pi* of the in-plane cyclometalated rings is weak or absent and is not operative in promoting an associative mode of ac tivation.