To What Extent Can Cyclometalation Promote Associative or Dissociative Ligand Substitution at Platinum(II) Complexes? A Combined Kinetic and Theoretical Approach
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- 作者：Maria Rosaria Plutino ; Luigi Monsù ; Scolaro ; Raffaello Romeo ; and Antonio Grassi
- 刊名：Inorganic Chemistry
- 出版年：2000
- 出版时间：June 26, 2000
- 年：2000
- 卷：39
- 期：13
- 页码：2712 - 2720
- 全文大小：248K
- 年卷期：v.39,no.13(June 26, 2000)
- ISSN：1520-510X
文摘
The ligand exchange rate constants for the reactions [Pt(bph)(SR2)2] + 2*SR2 MG SRC="/images/entities/rarr.gif"> [Pt(bph)(*SR2)2] + 2SR2 (bph =2,2'-biphenyl dianion; R = Me and Et) and cis-[PtPh2(SMe2)2] + 2*SMe2 
mages/entities/rarr.gif"> cis-[PtPh2(*SMe2)2] + 2SMe2 havebeen determined in CDCl3 as a function of ligand concentration and temperature, by 1H NMR isotopic labelingand magnetization transfer experiments. The rates of exchange show no dependence on ligand concentration andthe kinetics are characterized by largely positive entropies of activation. The kinetics of displacement of thethioethers from [Pt(bph)(SR2)2] with the dinitrogen ligands 2,2'-bipyridine and 1,10-phenanthroline (N-N) toyield [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 ofbehavior indicates that the rate-determining step for substitution is the dissociation of a thioether ligand and theformation of a three-coordinated [Pt(bph)(SR2)] intermediate. The value of the parameter a, which measures therate of ligand dissociation, is constant and independent of the nature of N-N, and it is in reasonable agreementwith the value of the rate of ligand exchange at the same temperature. Theoretical ab initio calculations wereperformed for both [Pt(bph)(SMe2)2] and cis-[PtPh2(SMe2)2], and for their three-coordinated derivatives upon theloss of one SMe2 ligand. The latter optimize in a T-shaped structure. Calculations were performed in the HFapproximation (LANL2DZ basis set) and refined by introducing the correlation terms (Becke3LYP model). Theactivation enthalpies from the optimized vacuum-phase geometries are 52.3 and 72.2 kJ mol-1 compared to theexperimental 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 ofnegative charge over the platinum atom which exerts a repulsion force on an axially incoming nucleophile. Onthe other hand, the strength of the organic carbanions trans to the leaving group and the stabilization of theT-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 dissociationfrom the complex containing the planar 2,2'-biphenyl dianion is easier than from its analogue with single arylligands. Electron back-donation from filled d orbitals of the metal to empty mages/gifchars/pi.gif" BORDER=0 >* of the in-plane cyclometalatedrings is weak or absent and is not operative in promoting an associative mode of activation.