Theses and Dissertations from UMD
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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 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 ALD-ENABLED CATHODE-CATALYST ARCHITECTURES FOR LI-O2 BATTERIES(2015) Schroeder, Marshall Adam; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Li-O2 electrochemical redox couple is one of the prime candidates for next generation energy storage. Known for its impressive theoretical metric for specific energy, even current practically obtainable values are competitive with state of the art Li-ion intercalation chemistries and the achievable performance of batteries featuring this nascent technology will continue to improve as fundamental scientific challenges in each component of the device are addressed. The positive electrode is particularly complicated by its role as a scaffold for oxygen reduction and evolution, exhibiting sluggish kinetics, poor chemical stability, and limited cyclability due to parasitic side reactions. Fortunately, recent Li-O2 research has shown some success in improving the performance and cyclability of these O2 cathodes by shifting toward nanostructured architectures with catalytic functionalizations. Atomic layer deposition (ALD) is one of the most promising enabling technologies for fabricating these complex heterostructures. Offering precise control of film thickness, morphology, and mass loading with excellent conformality, this vapor-phase deposition technique is applied in this work to deposit thin film and particle morphologies of different catalyst chemistries on mesostructured carbon scaffolds. This thesis dissertation discusses: (1) development of a lab-scale infrastructure for assembly, electrochemical testing, and characterization of Li-O2 battery cathodes including a custom test cell and a state of the art integrated system for fabrication and characterization, (2) design, fabrication, testing, and post-mortem characterization of a unique 3D cathode architecture consisting of vertically aligned carbon nanotubes on an integrated nickel foam current collector, (3) atomic layer deposition of heterogeneous ruthenium-based catalysts on a multi-walled carbon nanotube sponge to produce a freestanding, binder-free, mesoporous Li-O2 cathode with high capacity and long-term cyclability, (4) evaluation of dimethyl sulfoxide as an electrolyte solvent for non-aqueous Li-O2 batteries, and (5) investigation of the relative importance of passivating intrinsic defects in carbon redox scaffolds vs. introduction of heterogeneous OER/ORR catalysts for improving the long-term stability and cyclability of these Li-O2 electrodes.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 Synthesis and Reactivity of Monohydrocarbyl Palladium(IV) Complexes Using Hydrogen Peroxide as Oxidant in Protic Solvents(2011) Oloo, Williamson Njoroge; Vedernikov, Andrei N; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mild, and selective transition metal catalyzed processes for the functionalization of C-H bonds utilizing environmentally benign and inexpensive dioxygen and/ or HOOH oxidants are extremely attractive, as they render these transformations more atom economical and practical for large-scale syntheses. Our approach towards this end involves optimizing the oxidation and C-X reductive elimination steps of the proposed catalytic cycle using tridentate facially chelating ligands, which include 1-hydroxy-1,1-di(2-pyridyl)methoxide, a derivative of di(2-pyridyl)ketone (dpk) and 6-(2-pyridinoyl)pyridine-2-carboxylic acid (ppc). Oxidation of the dpk- and the ppc-ligated palladacycles with HOOH in water and acetic acid solvents produces the corresponding monohydrocarbyl Pd(IV) complexes quantitatively. The mechanism of oxidation of these complexes was investigated, and was proposed to involve addition of HOOH across the C=O bond of the ligand, followed by heterolytic cleavage of the O-O bond via nucleophilic attack of Pd(II) onto the hydroperoxo adduct. The dpk- and ppc-ligated monohydrocarbyl Pd(IV) complexes undergo C-O reductive elimination at room temperature in acetic acid and/ or water to produce the corresponding phenols and/ or aryl acetates quantitatively. Mechanistic studies led us to propose a C-O reductive elimination reaction that proceeds either from a 5-coordinate intermediate, produced upon dissociation of the pyridine group of the dpk chelate or from a 6-coordinate Pd(IV) species. Addition of HX (X=Cl, Br, and I) to aqueous solutions of the dpk-supported hydroxo-ligated monohydrocarbyl Pd(IV) complexes leads to quantitative formation of C-X bond-coupling products. Some of the corresponding X-ligated monohydrocarbyl Pd(IV) complexes were isolated from these solutions (X=Cl and Br), and could be independently prepared by oxidation of the hydrocarbyl Pd(II) precursors with the corresponding N-halogenosuccinimides (NXS). Palladium catalyzed C-H functionalization reactions were performed in the presence of tridentate, facially chelating bis(6-methyl-2-pyridyl)methanesulfonate ligand. Substituted 2-phenylpyridine substrates underwent predominantly C-C coupling reactions with minor C-O coupling products produced, while 2-benzyl- and 2-phenoxypyridine substrates that form 6-membered palladacycles produced the corresponding C-O coupling products selectively in high yields. These reactions were significantly slower in the absence of the ligand, and no reactions took place in the absence of palladium acetate.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.