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.