Platinum(IV) DMSO Complexes: Synthesis, Isomerization, and Agostic Intermediates

Oxidation of cyclometalated Pt(II) complexes with S-bound DMSO ligands initially results in Pt(IV) complexes that retain the S-bound DMSO ligands in the same relative position. Isomerization reactions result in a rearrangement of the ligands to give O-bound DMSO complexes, with the DMSO trans to a cyclometalated carbon. X-ray structures representing the only two known examples of Pt(IV) complexes with O-bound DMSO ligands have been solved. The rate of isomerization of complexes without a pendant alkyl chain is strongly solvent dependent, consistent with the need to stabilize a coordinatively unsaturated intermediate. Pt(IV) complexes with a pendant alkyl chain show little dependence on isomerization rate with solvent, with solution NMR data strongly suggesting the presence of agostic complexes. DFT calculations provide support for the presence of agostic complexes, with the same interactions being used to account for the loss of DMSO from the O-bound DMSO complexes.