Cycloplatination of aryl and ferrocenyl oximes by cis-[PtCl2(OSMe2)2] affording expected platinum(II) and unexpected platinum(IV) products

Abstract

Acetophenone, benzamide, ferrocenyl methyl ketone and ferrocenecarboxamide oximes reacted with cis-[PtCl₂(dmso)₂] (dmso = dimethyl sulfoxide) in refluxing methanol to afford the expected platina(II)cycles N,S-trans-[Pt(C–N)Cl(dmso)], where C–N stands for the corresponding cycloplatinated organic ligand. The highest yield of 51% was observed in the case of the acetophenone oxime complex. The structures of the complexes were established on the basis of analytical and ¹H NMR data, and confirmed by an X-ray diffraction analysis of the platinated ferrocenyl methyl ketone oxime. A cyclic voltammetry study of the latter indicated that cyclometallation decreases the redox potential of the ferrocene unit by ca. 200 mV. The reaction of a bulkier analog of acetophenone oxime, viz. 2-methyl-1-phenylbutan-1-one oxime, resulted in the formation of the unexpected platinum(IV) complex fac-[Pt(C–N)Cl₃(SMe₂)] revealing a unique combination of oxidative cycloplatination and dmso-deoxygenation processes which occur under rather mild conditions. The composition of the platinum(IV) compound was established by X-ray crystallography. The Pt–Cl bond distance trans to the phenyl carbon [2.464(4) Å] is rather large. As a result, the trans chloro ligand undergoes rapid solvolysis in aqueous solution, causing a pH drop. The deoxygenation of dimethyl sulfoxide that occurs within the metal co-ordination sphere can be viewed as a model of the action of the metal-dependent redox enzyme dimethyl sulfoxide reductase.