Chemistry & Biochemistry Theses and Dissertations
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Item DESIGN AND SYNTHESIS OF NEW SULFONATED CNN LIGAND SCAFFOLDS FOR PLATINUM CATALYZED H/D EXCHANGE APPLICATIONS(2023) Kramer, Morgan; Vedernikov, Andrei N; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of platinum group metals for the activation and functionalization of C-H bonds has been a topic of substantial interest over the past 60 years. Specifically, platinum-based complexes represent a particularly promising avenue due to their ability to form air- and water-stable species that are capable of reacting with some of the most inert C-H bonds within organic substrates. Over the decades of research contributing to this field, platinum complexes have frequently been angled towards fundamental mechanistic analysis of homogeneous C-H bond activation. In turn, the development of homogenous PtII-based catalytic systems has remained underdeveloped for the practical applications in C-H bond functionalization and, in particular, deuteration of complex organic molecules, including pharmaceuticals. The latter direction is now attracting a significant interest by the pharmaceutical industry. In this work the kinetic and thermodynamic selectivity of our new catalyst, a Pt(II) sulfonated CNN-pincer complex 1.5, in the H/D exchange reaction between aromaticsubstrates and wet TFE-d1 was screened across thirty-four aromatic substrates with the catalysts TON up to 300 (Chapter 2). A kinetic preference of 1.5 for electron-rich C-H bonds and substrates was firmly established and a novel scale of Hammett-like σXM constants was introduced to characterize the reactivity of the substrates’ C(sp2)–H bonds in transition-metal-mediated C-H activation. To greatly enhance our PtII catalysts’ useful life, we used their rigid covalent immobilization to mesoporous silica nanoparticles (immobilized complex 3.5). The resulting robust material served as an efficient H/D exchange catalyst utilizing cheaper sources of exchangeable deuterium, AcOD-d4, and D2O, with the catalyst’s TON up to 1600 (Chapter 3). To understand our novel catalyst’s structure – activity relationship, a series of benzene fragment – R-substituted analogs of 1.5 (R = MeO, tBu, iPr, F, Cl, CF3) were synthesized and explored in the H/D exchange of a series of aromatic compounds (Chapter 4). Surprisingly, the complex 4.1-tBu (R = tBu) stood out as a most robust homogeneous catalyst compatible with AcOD-d4 and D2O at 120 oC as deuterium sources that can work under air. Thanks to this finding, the substrates scope for the H/D exchange with AcOD-d4 catalyzed by 4.1-tBu was expanded to include eight pharmaceuticals, some alkenes, with signs of engagement of some C(sp3)-H bond donors. A novel photo-induced (violet light) room temperature H/D exchange catalyzed by 4.1-OMe was discovered with a substantially different substrate selectivity, as compared to the thermal reaction at 80 oC. These observations may provide some important insight into the mechanism of PtII-mediated C-H activation. Finally, Chapter 5 summarizes the results of this work and suggests some future directions for this area of research.Item NEW LIGAND SCAFFOLDS FOR COMBINING ARENE C-H ACTIVATION AND AEROBIC OXIDATION AT PLATINUM(2018) Watts, David; Vedernikov, Andrei N; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Extensive research over the past half-century has proven the utility of late transition metal complexes in the activation and functionalization of alkanes and arenes. Homogeneous platinum compounds have been particularly promising as they readily form air and water stable complexes that can react with some of the strongest C-H bonds (e.g., CH3-H, Ph-H) under relatively benign conditions. Yet, the development of methods for the application of O2 or air as the terminal oxidant in the oxidative functionalization of inert C-H bonds remains an elusive but important goal. The focus of this work is to enable the direct involvement of O2 with PtII-mediated C-H activation processes through computation directed intelligent ligand design, with the end goal of selective aerobic C-H functionalization. Prior experience with the hemi-labile tripodal ligand di-(2-pyridyl)methanesulfonate (dpms) lead us to develop a new class of sulfonated k3-CNN pincer pre-ligand, 6-phenyl-di-(2-pyridyl)methanesulfonate (ph-dpms). The ph-dpms derived PtII-aqua complex, (C6H4-dpms)PtII(H2O), was shown to be an especially active hydrogen/deuterium exchange catalyst with arene substrates. While facile, the arene C-H activation chemistry was also selective for aryl C(sp2)-H bonds over benzyl C(sp3)-H, despite severe steric protection of the former in some cases. Ph-dpms also supports the aerobic oxidation chemistry for which the earlier generation ligand, dpms, was engineered. An anionic [PtII(Ph)]- complex derived from ph-dpms undergoes relatively fast oxidation in trifluoroethanol (TFE) solvent resulting in oxidative C-C coupling between the phenyl substituent and ligand. Changing the solvent to MeOH allows for isolation of the PtIV-Ph intermediate. Furthermore, the ability to support both C-H and O2 activation was combined in the one-pot aerobic C-H oxidation of both electron rich and electron poor arenes to give (C6H4-dpms)PtIV(Aryl)(OH) complexes from (C6H4-dpms)PtII(H2O); a feat never accomplished before by a PtII complex without the use of co-catalysts or reagents to mediate the O2 chemistry. The limits of the new ligand scaffold were then explored through the reactivity of PtII chloro and aqua complexes derived from more rigid analogs of ph-dpms. The rigidity of the ligand was found to be intimately tied to both C-H and O2 activation chemistry as well as some detrimental bimolecular decomposition pathways.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 REDUCTIVE ELIMINATION OF (DPMS)PTIV COMPLEXES DERIVED FROM ISOMERIC 2-BUTENEs AND 2-BUTYNE (DPMS=DI(2-PYRIDYL)METHANE SULFONATE)(2012) Liu, Xiaohang; Vedernikov, Andrei; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The (dpms)PtII complexes (dpms = di(2-pyridinyl)methanesulfonate)derived from some cyclic olefins can be readily oxidized to PtIV oxetanes, followed by reductive elimination to produce corresponding epoxides. A catalytic version of this reaction can potentially be achieved if decomposition of active species responsible for olefin substitution is avoided. Several attempts were made to solve this problem, and a more hydrophilic analog of the dipyridinemethanesulfonate ligand was obtained. Furthermore, the reductive elimination step of PtIV oxetanes was studied by using diastereomeric cis- and trans-2-butene derivatives. We believe that two mechanisms of C O reductive elimination may be involved in these reactions and that steric repulsion between substituents at the oxetane carbon atoms may play a major role in determining the predominant of the two competing mechanisms. Platinum(IV) η1-butanone complex was synthesized and characterized, which was found to undergo different types of elimination reaction to give a series of butane derivatives as products.