Auxiliary proton-influenced metal-mediated reactions: dioxygen activation, anion coordination, and catalysis

Dioxygen Reduction;Second Coordination Sphere;Hydrogen Bonding;Transfer Hydrogenation

Letko, Christopher

关键词: Dioxygen Reduction;    Second Coordination Sphere;    Hydrogen Bonding;    Transfer Hydrogenation;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/29511/Letko_Christopher.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

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.