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.