Platinum(ii) and platinum(iv) complexes stabilized by abnormal/mesoionic C4-bound dicarbenes

Abstract

Platinum(II) complexes comprising abnormal diimidazolylidene ligands were synthesized from cis-PtMe₂(DMSO)₂ using microwave-assisted double C–H bond activation. NMR analysis revealed an unusual solvolysis process, induced by coordinating solvents such as DMSO and MeCN, which has not been observed in related normal dicarbene complexes. NMR and IR spectroscopy and crystallographic analysis of the mono-substituted DMSO complex indicate a sulfur-bonding of the DMSO ligand to the platinum(II) center. Analysis of the DMSO exchange kinetics provided for the first time a quantitative measure of the trans effect of abnormal carbene ligands. The kinetic exchange rate in these bidentate abnormal dicarbene complexes is 0.050(±2) s⁻¹ and thus similar to analogous platinum(II) complexes containing phenylpyridine, yet significantly slower than that induced by pyridylidene pyridine. Reaction of the dicarbene platinum(II) complexes with PhICl₂, Br₂ and I₂ afforded the corresponding platinum(IV) complexes. Linkage isomerism of the Pt^IV-bound DMSO was observed when the bromination reaction was performed in DMSO solution. Moreover, solvolysis was less pronounced in the platinum(IV) complexes than in the corresponding platinum(II) analogues.