Irreversible addition of carbon nucleophiles to ethylene in cationic platinum(II) complexes

Abstract

The cationic complex [Pt(η2-C2H4)Cl(tmen)]+1(tmen =N,N,N′,N′-tetramethylethylenediamine) reacts with carbon nucleophiles [an acidic carbon substrate (HL) in the presence of a base (Na2CO3); HL = acetylacetone, HL1; ethyl acetoacetate, HL2; or diethyl malonate, HL3] to give the corresponding addition products [Pt(CH2CH2L)Cl(tmen)](L = L1, 3; L2, 4; or L3, 5) and [{(tmen)ClPtCH2CH2}2(L1– H)]6. Compound 3 exhibits a keto–enol tautomerism. The keto tautomer is kinetically and thermodynamically favoured and in solution it represents about 90% of the equilibrium mixture; on the contrary, the enol tautomer is the favoured form in the solid. In compound 4 the carbon atom of the nucleophile which has bound to ethylene is chiral and, as a consequence, the methylene protons exhibit a diastereotopic splitting which is greater for the methylene group next to platinum than for that next to the asymmetric carbon. Treatment of compounds 3–6 with acids (HCl or HClO4) does not reverse the carbon to carbon coupling but cleaves the platinum–carbon bond, forming the ethylated nucleophile and [PtCl2(tmen)] or [{PtCl(tmen)}2][ClO4]2 depending on the acid used. The cationic complex [Pt(η2-C2H4)(NO2)(tmen)]+2 reacts with carbon nucleophiles in a similar way to 1. Treatment of the addition products with HClO4 under an ethylene atmosphere gives the ethylated nucleophile and the starting complex 2.