Theses and Dissertations from UMD
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Item NOVEL PLATINUM COMPLEXES SUPPORTED BY SULFONATED CNN PINCER LIGANDS RELEVANT TO AEROBIC METHANE FUNCTIONALIZATION CHEMISTRY(2021) Ruan, Jiaheng; Vedernikov, Andrei N; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mild and selective aerobic methane functionalization has always been a challenge. Shilov developed a PtII based system that has a potential to solve the problem, but the non-practical PtIV oxidant needs to be substituted by more accessible oxidants, such as O2. On pursuing this goal, several series of PtII and PtIV methyl complexes derived from two pre-ligands, Ph-dpms [(6-phenylpyridin-2-yl)(pyridin-2-yl)methanesulfonate, HL1-] and Ph-pcpps [7-(6-phenylpyridin-2-yl)-6,7-dihydro-5H-cyclopenta[b]pyridine-7-sulfonate, HL2-] were synthesized. Their reactivity in O2 and X-H (X = C, Si) bond activation (PtIIMe), and in CH3-X (X = O, N, C) reductive elimination (PtIVMe), was studied. The three steps of the proposed catalytic cycle suitable for aerobic methane functionalization were investigated.The reverse of the first step, methane C-H activation, was probed by reacting K[(L1)PtIIMe] with acid H(Et2O)2BArF4 {BArF4 = tetrakis[3,5- bis(trifluoromethyl)phenyl]borate} in CH2Cl2 at -78 °C. Although methane and no stable PtIV(Me)H species were detected at both -78 °C and room temperature, the resulting solutions were shown to activate Si-H bonds of Et3SiH and Me3SiH to form Pt hydrido complexes. The second step, aerobic oxidation of PtIIMe complexes, was investigated using K[(L1)PtIIMe] and K[(L2)PtIIMe]. K[(L1)PtIIMe] reacts with O2 in MeOH solutions to form diastereomeric (L1)PtIVMe2 complexes, which are barely reactive in CH3-X reductive elimination. Notably, K[(L2)PtIIMe] reacted with O2 in MeOH or acetone / TFE to selectively form three out of four possible diastereomeric (L2)PtIV(Me)OH complexes with (L2)PtIVMe2 as a minor by-product. The remaining fourth diastereomer of (L2)PtIV(Me)OH was prepared using H2O2 as oxidant. The third step, CH3-X reductive elimination, was studied using a series of PtIVMe species supported by L1 and L2. The PtIVMe(Y) species (Y = Cl, I, OCH2CF3, OH) having methyl trans- to sulfonate and one (L2)PtIV(Me)OH complex having pyridyl trans- to methyl demonstrated facile CH3-X reductive elimination (X = Me2SO+, OH, O2CCF3, and Me2NPh+) using the corresponding nucleophiles with yields of the CH3-X products up to 99%. Two other (L2)PtIV(Me)OH complexes having methyl ligand trans- to pyridyl formed predominantly C-C coupling products in aqueous DMSO solutions of CF3CO2H. Overall, this work demonstrates the potential of our novel sulfonated pincer ligands to support aerobic functionalization of methane at a Pt center.Item NEW LIGAND MOTIFS FOR PLATINUM-BASED `SHILOV CHEMISTRY' AND DETOURS INTO BASIC ORGANOMETALLIC RESEARCH(2009) Khaskin, Eugene; Vedernikov, Andrei N; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.