Auxiliary proton-influenced metal-mediated reactions: dioxygen activation, anion coordination, and catalysis
Studies on the incorporation of hydrogen-bonding groups into the second coordination sphere of Cu complexes have revealed that O2 reactivity is enhanced in the presence of these functional groups. The effects of intramolecular hydrogen-bonding on the oxygen affinity of CuN3+ derivatives, where N3 = tris(2-picolin-6-yl)methane (tripic) and analogues are presented in Chapter 2. The key synthetic methodology relies on Pd-catalyzed coupling reactions of lithiated 6-methyl-2-pyridone with bromopyridyl derivatives. These building blocks allow the preparation of tridentate N3 ligands decorated with OH and OMe substituents flanking the fourth coordination site of a tetrahedral complex. Crystallographic characterization of the ligand complemented with two methyl and one OH group revealed that it exists as the pyridone tautomer with a bifurcated hydrogen-bond. Coupling of these tridendate ligands gives the corresponding hydroxy- and methoxy-functionalized bis(tripodal) ligands. Cu[bis(2-methylpyrid-6-yl)(2-hydroxypyrid-6-yl)methane](NCMe)+ oxidizes readily in air to afford a mixed valence type III Cu1.5 dimer, and can be reversed through the addition of decamethylferrocene and acid. The reactivity with dioxygen requires the hydroxyl substituent: neither [Cu(tripic)(NCMe)]+ nor the methoxy complexes display O2 reactivity. A similar mixed valence dimer was found to form upon exposing the dicopper(I) complex of a tetrahydroxy bis(tridentate) ligand to air. The dicopper(I) complex of the analogous tetramethoxy N6-ligand instead reversibly binds O2. Deprotonation of Cu[bis(2-methylpyrid-6-yl)(2-hydroxypyrid-6-yl)methane](CO)+ afforded the neutral Cu(I) carbonyl complex. Deprotonation of Cu[bis(2-methylpyrid-6-yl)(2-hydroxypyrid-6-yl)methane](NCMe)+ gave a Cu(I) dimer that does not contain MeCN, which can be reoxidized, reprotonated, and carbonylated. The structures of bis(2-methylpyrid-6-yl)(2-pyridon-6-yl)methane, a bimetallic Cu(I)(NCMe) complex of the N6-tetramethoxylated ligand, and two mixed valence Cu dimers were confirmed by single crystal X-ray diffraction. Electrochemical O2 reduction studies of these Cu complexes were undertaken, in addition to various Cu complexes reported in literature. The oxygen reduction reaction (ORR) onset potential of Cu[bis(2-methylpyrid-6-yl)(2-hydroxypyrid-6-yl)methane](NCMe)+ (0.46 V) was found to be more positive than its methoxylated derivative (0.29 V), in addition to hemocyanin model complexes [Cu(tripic)(NCMe)]+ (0.34 V) and its tethered bimetallic derivative, [Cu2(bistripic)(NCMe)2]2+ (0.40 V) (all potentials vs RHE at pH = 2). [Cu(tpa)](ClO4)2 (tpa = tris(2-pyridylmethyl)amine) was found to have the highest reported onset potential for the ORR (0.53 V) for a Cu complex at low pH. The ORR activity of Cu(II)(ClO4)2 salts ligated to tpa derivatives were studied, including a new tpa ligand functionalized with a ferrocenyl group (tpa-Fc). All derivatives were shown to display similar or lower catalytic activity relative to the parent complex [Cu(tpa)](ClO4)2. A thiol-tethered tpa derivative forms a monolayer on a Au surface. Cu(II)(ClO4)2 coordinates the surface-immobilized tpa ligand, however ORR activity of the complex was found to be unresolved from competitive reduction by the Au electrode. Solution cyclic voltammetry studies of [Cu(tpa)](ClO4)2 using a glassy carbon electrode identify that the active catalyst species adsorbs to the electrode’s surface. A survey of the binding of anions to the unsaturated 16e– Lewis acid, [Cp*Ir(TsDPEN)]+ ([1H]+) was performed, where TsDPEN is racemic H2NCHPhCHPhNTs–, is presented in Chapter 4. The derivatives Cp*IrX(TsDPEN) were characterized crystallographically for X– = CN– and Me(C=NH)S–. [(1H)2(μ-CN)]+ forms from [1H]+ and 1H(CN). Aside from 2-pyridone, amides generally add reversibly and bind to Ir through N. Thioacetamide binds irreversibly through sulfur. Compounds of the type Cp*IrX(TsDPEN) generally form diastereoselectively, with absolute configuration at Ir being opposite to that of the TsDPEN-– ligand. For the strong ligands (X = CN–, H–, or Me(C=NH)S–), two diastereomers were observed by NMR spectroscopy. Diastereomerization to the stable isomers was first order in iridium with modest solvent effects. The diphenyl groups are generally diequatorial for the stable diastereomers. Cp*Ir(SC(NH)Me)TsDPEN is the first example of a κ1-S-thioamidato complex. The addition of H3PO4 to Cp*Ir(TsDPEN-H) was found to be a simple method to obtain a water-soluble hydrogenation catalyst capable of reducing aromatic ketones to their corresponding alcohols in aqueous solutions. Key to the reactivity is the low affinity of the coordinatively unsaturated [Cp*Ir(TsDPEN)]+ for oxoanions (H2PO4–), unlike its Ru congener, [(p-cymene)Ru(OPO3H2)(TsDPEN)]+ that was found to coordinate the dihydrogenphosphate anion. Catalyst degradation is proposed to proceed via protonation of the tosylamido ligand, as was established by the crystallographic characterization of the tosylamine complex [Cp*Ir(NCMe)(HTsDPEN)]2+. Protonation studies of Cp*IrH(TsDPEN) (the proposed active species in hydrogenation catalysis) support the hemi-lability of the resulting HNTs amine center.
Dioxygen Reduction, Second Coordination Sphere, Hydrogen Bonding, Transfer Hydrogenation
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign