Reactivity of tertiary phosphines toward a cyclometallated platinum(II) complex: evaluation of steric and electronic contributions

Abstract

An extended series of phosphines (L), having widely different steric and electronic properties, was used to study the kinetics of chloride substitution on the cyclometallated complex [Pt(N-N-C(Cl] (N-N-CH=6-(1-methylbenzyl)-2,2′-bipyridine). The final cationic products [Pt(N-N-C)(L)]+ were fully characterised by 1H and 31P{1H} NMR spectroscopy in CDCl3 solution. The values of the 1J(PtP) coupling constants for these complexes showed interesting dependencies upon the size and the σ-donor ability of the ligands. The reaction, in dichloromethane as solvent at 298.2 K, takes place by way of a direct bimolecular attack of the ligand on the substrate, with no evidence of a significant solvent contribution. The order of reactivity is that expected on the basis of the nucleophilic reactivity constants (nptp), derived from previous studies. The values of the second-order rate constants (k2 M−1 s−1) can be resolved quantitatively into steric, electronic and aryl contributions of the phosphine ligands, by use of a four-parameter equation log k2 = α+β(pKn′)+γ(θ)+δ(Ear) where pKa′ measures the σ-donor ability of the phosphines, θ (Tolman's cone angle) is a measure of the size of the P-donor ligand, and Ear is related to the number of aryl groups attached to the ligand phosphorus atom. This is the first reported example of an aryl effect contributing to the nucleophilicity of phosphines as entering ligands on square-planar complexes. No steric threshold is observed. Steric and electronic profiles for the substitution process can be plotted showing the dependence of rates on the size and the σ-donor ability of the phosphines. The sensitivity of the rates to pÅa′ is significant (β = 0.20 ± 0.04) and this is related to the structural properties of the substrate.