NEW LIGAND MOTIFS FOR PLATINUM-BASED `SHILOV CHEMISTRY' AND DETOURS INTO BASIC ORGANOMETALLIC RESEARCH

Abstract

The C-H activation reaction at cationic platinum centers utilizing chelating aromatic N-type ligands has been widely studied in TFE (trifluoroethanol): a weakly coordinating solvent. In our laboratory, recent studies involving a modified dipyridine methane ligand revealed that benzene C-H activation in water, methanol and the activation of alkane substrates in TFE is possible. Anionic Pt(II) centers created via an anionic dipyridyl borate ligand present a new and promising direction towards realizing selective oxidation of alkanes. Rapid CH activation of alkanes and arenes is possible in biphasic water/hydrocarbon solvent mixtutes. In the course of CH activation studies with [dpbPtII(Me)2]- (dpb = di-2pyridyl-dimethyl-borate), the complex was found to yield olefin hydrides upon alkane activation. The yield of olefin hydride complexes with the dpb ligand proved low (30-40%). A lipophilic ligand (dtBupb = di-t-butylpyridyl-dimethyl-borate) activated various cyclic and linear olefins with near quantitative yields. The resultant olefin hydride complexes proved to be catalysts for transfer dehydrogenation of cyclic alkanes (TONs up to 13).

We found that in the presence of a hydroxylic solvent, a very rapid oxidation of [dpbPtII(Me)2]- complex towards a PtIV species was observed. The proposed reaction mechanism includes rapid coordination of O2 by the highly electron-rich metal complex with subsequent nucleophiilic substitution reaction at boron and a methyl group transfer from the boron atom to the PtIV center.

Oxidation with methyl iodide to give penta-coordinate dpbPtIVMe3 and its subsequent reaction with a hydroxylic solvent furnished the same product as under aerobic oxidation conditions. This proved that oxidation had to occur prior to methyl group transfer. Since in this case, our system can be considered as a mechanistic probe for Suzuki coupling, the insight into the nature of alkyl transfer provides a clear model of one the key steps of this widely-utilized transformation. Eventually, we were able to observe a reversible alkyl group transfer between PtIV and B in DMSO solutions.

To probe the transfer of an aryl group between PtIV and B, a dpbPtIVMePh2 complex and a PtIVMe3 complex supported by (dpydphb = dipyridyl-diphenyl-borate) were synthesized. While phenyl transfer from PtIV to B was facile already in THF, the reverse, B-to-PtIV phenyl transfer was not observed due to the greater stabilization conferred to the complex by a B-Ph---PtIV moiety. The feasibility of a B-to-PtIV phenyl transfer was demonstrated when [dpydphbPtIIMe2] was oxidized by O2 in isopropanol.