Behaviour of α-functional organotransition metal compounds: nuclear magnetic resonance investigation of a novel Friedel–Crafts metallacyclisation of halogenomethyl(arylphosphine)platinum(ii) complexes

Abstract

Reaction of complexes trans-[Pt(CH₂X′)XL₂] with 2 equivalents of AgBF₄ in the presence of triarylphosphine (L′) affords cationic metallacyclic species [Pt{PPh₂(2-C₆H₄CH₂)}L(L′)]BF₄ [X = Cl, Br, or I; X′ = Cl or I; L = PPh₃; L′ = PPh₃ or P(C₆H₄Me-4)₃]. When L′ = P(C₆H₄Me-4)₃ the isomeric metallacycle [Pt(P(C₆H₄Me-4)₂(4-MeC₆H₃CH₂-2)}L₂]BF₄ is also formed in significant amount. In the absence of additional phosphine the cyclic product [Pt{PPh₂(2-C₆H₄CH₂)]L(S)]BF₄ is observed instead (S = solvent molecule). This novel metallacyclisation route features a remarkably facile intramolecular electrophilic substitution by the metal-bound methylene on an aryl substituent of a co-ordinated phosphine. The extent to which this involves the incoming phosphine is controlled by the rate of isomerisation of intermediates. Multinuclear n.m.r. characteristics of the products and intermediates are reported and mechanistic implications discussed.