Mechanistic investigations of stoichiometric and catalytic Pt-mediated oxidative functionalization at a proximal boron center
Vedernikov, Andrei N
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The focus of the work detailed in this dissertation is the investigation of mechanism and catalytic applications of Pt complexes supported by novel anionic di(2-pyridyl)borate ligands. It was found that oxidation of Me,MeBPy2-supported PtII complexes bearing no hydrocarbyl complexes directly generated dimethyl ether in quantitative yields, with one methyl originating from the MeB fragment. We also found that increasing formal charge on the metal center renders related complexes reluctant to undergo oxidation. Based on a proposed mechanism involving a transient PtIV-Me complex, we set out to develop a series of modified R,RBPy2 ligands to prevent such oxidatively induced hydrocarbyl transfer. We found that the strategy of replacing one hydrocarbyl (Me) group in the dmdpb ligand by methoxo (OMe) was not sufficient in completely preventing degradation of the borate center. However, derived mono- and di-hydrocarbyl PtII complexes could still be easily oxidized under aerobic conditions. Interestingly, oxidation products corresponding to both B-to-PtIV methyl migration and ligand retention were observed. We focused our attention to a unique 1,5-cyclooctanediylBPy2 ligand, which, we presumed, would prevent hydrocarbyl migration due to the rigid structure imposed by the bicyclic framework. The derived PtIVMe3 complex was found to exhibit `enhanced' BC-H agostic stabilization of the penta-coordinate PtIV center. Oxidation of derived PtII complexes results in hydride migration from the B-CH fragment onto the PtIV center, led to the formation of a series of (MeO),(MeO)BPy2 supported Pt complexes, and unanticipated C-C and C=C coupling at the borate center. The (MeO),(MeO)BPy2 ligand proved to be the first example of anionic facially chelating borate ligand capable of resisting oxidative degradation. The derived PtIV(Ph)2(OH) can be used for catalytic aerobic oxidation of NaBH(OMe)3 and NaBH4, with TOFs of 178/h and 216/h respectively. This may be of particular interest from the perspective of a direct-borohydride-fuel-cell (DBFC). We also found that the PtIV(Ph)2(OH) complex could be used as a catalyst to oxidize isopropanol to acetone under aerobic conditions with a TON of 3.8 after 56h at 80 °C. A mechanism involving selective hydride migration from a B-bound isopropoxy fragment to the PtIV center was proposed.