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To What Extent Can Cyclometalation Promote Associative or Dissociative Ligand Substitution at Platinum(II) Complexes? A Combined Kinetic and Theoretical Approach
journal contribution
posted on 2000-06-09, 00:00 authored by Maria Rosaria Plutino, Luigi Monsù Scolaro, Raffaello Romeo, Antonio GrassiThe ligand exchange rate constants for the reactions [Pt(bph)(SR2)2] + 2*SR2 → [Pt(bph)(*SR2)2] + 2SR2 (bph =
2,2‘-biphenyl dianion; R = Me and Et) and cis-[PtPh2(SMe2)2] + 2*SMe2 → cis-[PtPh2(*SMe2)2] + 2SMe2 have
been determined in CDCl3 as a function of ligand concentration and temperature, by 1H NMR isotopic labeling
and magnetization transfer experiments. The rates of exchange show no dependence on ligand concentration and
the 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
yield [Pt(bph)(N−N)], carried out in the presence of sufficient excess of thioether and N−N to ensure pseudo-first-order conditions, follow a nonlinear rate law kobsd = a[N−N]/(b[SR2] + [N−N]). The general pattern of
behavior indicates that the rate-determining step for substitution is the dissociation 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 of ligand dissociation, is constant and independent of the nature of N−N, and it is in reasonable agreement
with the value of the rate of ligand exchange at the same temperature. Theoretical ab initio calculations were
performed for both [Pt(bph)(SMe2)2] and cis-[PtPh2(SMe2)2], and for their three-coordinated derivatives upon the
loss of one SMe2 ligand. The latter optimize in a T-shaped structure. Calculations were performed in the HF
approximation (LANL2DZ basis set) and refined by introducing the correlation terms (Becke3LYP model). The
activation enthalpies from the optimized vacuum-phase geometries are 52.3 and 72.2 kJ mol-1 compared to the
experimental values 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 maps of both parent compounds show a remarkable concentration of
negative charge over the platinum atom which exerts a repulsion force on an axially incoming nucleophile. On
the other hand, the strength of the organic carbanions trans to the leaving group and the stabilization of the
T-shaped intermediate in the singlet ground state may also rationalize the preference for the dissociative mechanism.
All of the kinetic and theoretical data support the latter hypothesis and indicate, in particular, that dissociation
from the 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 π* of the in-plane cyclometalated
rings is weak or absent and is not operative in promoting an associative mode of activation.
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Dissociative Ligand SubstitutionSMe 2CDCl 3 solutionparent compounds showBecke 3LYP modelsinglet ground statemagnetization transfer experimentsSMe 2 ligandPtHF1 H NMRligand concentrationnonlinear rate law k obsdLANL 2DZ basisCyclometalation Promote AssociativeTheoretical ab initio calculations72.2 kJ mol2 SR 22 SMe 2ligand exchange rate constantsPtPh
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