Theses and Dissertations from UMD
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Item Mechanistic investigations of stoichiometric and catalytic Pt-mediated oxidative functionalization at a proximal boron center(2013) Pal, Shrinwantu; Vedernikov, Andrei N; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.Item LIGAND-ENABLED PLATINUM--CARBON BOND FUNCTIONALIZATION UTILIZING DIOXYGEN AS THE TERMINAL OXIDANT(2009) Khusnutdinova, Julia; Vedernikov, Andrei N.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of organotransition metal complexes for selective functionalization of hydrocarbons is of great importance. Dioxygen is the most practical oxidant for large-scale applications in the petroleum industry. The focus of this work is the development of ligand-modulated platinum-based systems that can utilize O2 or air for selective transformation of organoplatinum(II) derivatives into alcohols, diols, aminoalcohols and epoxides in aqueous media. We found that the hemilabile tripod ligand dipyridylmethanesulfonate (dpms) enables facile aerobic functionalization of various PtIIMe complexes and some olefin hydroxo PtII complexes in hydroxylic solvents such as water and alcohols. Complexes LPtII(R)(HX) (L = dpms; R = Me, Ph; HX = H2O, MeOH, PhNH2) are oxidized by O2 to yield virtually quantitatively LPtIV(R)(X)(OH). Some of the derived PtIV alkyls LPtIV(Alk)(X)(OH) (X = OH, OMe) can reductively eliminate methanol in high yield. The mechanism of C-O elimination from LPtIV(Me)(X)(OH) (X = OH, OMe) in acidic aqueous media involves two concurrent pathways: an SN2 attack by water and an SN2 attack by a hydroxo or methoxo ligand of another PtIV species. In the latter case dimethyl ether is produced. The complex (dpms)Pt(ethylene)(OH) is oxidized by O2 in water to give a PtIV hydroxyethyl derivative that reductively eliminates ethylene oxide and ethylene glycol in aqueous solutions. The complexes derived from cyclic alkenes, cis-cyclooctene, norbornene, benzonorbornadiene, (dpms)PtII(cy-alkene)(OH), undergo olefin oxoplatination to give 1,2-oxaplatinacyclobutanes (PtII oxetanes). The derived PtII oxetanes are easily oxidized by O2 to produce PtIV oxetanes. The latter eliminate cleanly the corresponding epoxides by the mechanism of direct C(sp3)-O reductive eliminations, unprecedented in organoplatinum chemistry. The 1,2-azaplatinacyclobutanes (PtII azetidines) LPtII(CH2CH2NHR-&kappaC,&kappaN) (R = t-Bu, Me) are oxidized by O2 in the presence of acids to give PtIV azetidine complexes, [LPt(CH2CH2NHR-&kappaC,&kappaN)(OH)]+. The latter undergo reductive elimination of N-alkyl ethanolammonium salts, HOCH2CH2NH2R+, in acidic aqueous solutions at elevated temperatures. Efficient catalytic systems based on palladium acetate, di(6-pyridyl)ketone and 6-methyldi(2-pyridyl)methanesulfonate ligands, suitable for selective oxidation of ethylene with H2O2 to glycol acetates were developed. Glycol acetates were obtained in high selectivity and high yield on H2O2 under mild reaction conditions.