Water-Soluble Mono- and Dimethyl N-Heterocyclic Carbene Platinum(II) Complexes: Synthesis and Reactivity

Abstract

A family of water-soluble dimethyl complexes of formula cis-[PtMe2(dmso)(NHC·Na)] (2), in which NHC is an anionic N-heterocyclic carbene bearing a sulfonatopropyl chain on one of the nitrogen atoms and a sulfonatopropyl (a), methyl (b), mesityl (c), or 2,6-diisopropylphenyl group (d) on the other, have been prepared. The hydrolytic stability of the Pt–C bonds in these complexes under different neutral, alkaline, and acidic aqueous conditions has also been studied. Complexes 2 were found to be quite stable at room temperature in water under neutral or alkaline conditions. Degradation occurred at higher temperatures but involved C sp3–H activation and C–C reductive elimination processes in addition to Pt–Me bond hydrolysis. Hydrolytic cleavage of the platinum–methyl bonds was favored by good nucleophiles. Thus, the addition of KCN to an aqueous solution of 2 resulted in formation of the monomethyl complexes K[PtMe(CN)2(NHC·Na)] (9), whereas the dimethyl complexes K[PtMe2(CNR)(NHC·Na)] (10) were formed with the isocyanide CNCH2COOK. The addition of stoichiometric amounts of protic acids to aqueous solutions of 2 resulted in the clean cleavage of one or both platinum(II)–methyl bonds. Thus, the reaction of 2 with HCl afforded the complexes [PtClMe(dmso)(NHC·Na)] (3) and [PtCl2(dmso)(NHC·Na)] (4), whereas [PtMe(OH2)(dmso)(NHC)] (5) and [Pt(OH2)2(dmso)(NHC)][BF4] (7) were obtained upon treatment with HBF4. The crystal structure of 9a is remarkable in light of the longitudinal channels around 6 Å in diameter internally decorated with Pt–Me bonds.