Transition Metal Complexes of 1, 3-bis (2'-pyridylimino) Isoindolines and Their Use as Alcohol Oxidation Catalysts

Citation

Marks, David Neil (1982) Transition Metal Complexes of 1, 3-bis (2'-pyridylimino) Isoindolines and Their Use as Alcohol Oxidation Catalysts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/swwe-jf28. https://resolver.caltech.edu/CaltechTHESIS:07122018-160904951

Abstract

Condensation of 1, 2-dicyanobenzene and 2-amino-4-methylpyridine resulted in the formation of a ligand, 4'-MeLH, which in its anionic or neutral form functions as a tridentate chelate. Metal complexes were prepared with metal to ligand ratios of 1:1 and 1:2. The 1:1 complexes also contain acetate or chlorides as ligands while in the 1:2 complex the two tridentate ligands provide a pseudooctahedral environment about the metal ion. An analogous binucleating ligand was prepared by reaction of 1, 2, 4, 5-tetracyanobenzene and 2-amino-4-s-butylpyridine. This ligand, as a dianion, is capable of binding two metal ions, providing three coordination sites for each. Complexes were prepared in which the remaining coordination sites are either occupied by 4'-MeLH, resulting in two six-coordinate metal ions, or by an acetate ligand. Mononuclear and binuclear complexes were prepared with Mn(II), Fe(II), Co(II), Co(III), Ni(II), Cu(II), Zn(II), Ru(II), and Ru(III). Mixed-metal complexes containing Ru(II) were also prepared. Magnetic, spectral and electrochemical properties of these molecules were investigated.

The ruthenium complex, (4'-MeLH)RuCl₃, catalyzes the autoxidation of alcohols in basic alcoholic solution. The reaction is general, resulting in the oxidation of primary and secondary alcohols, with the principal products being aldehydes and ketones. The catalytic reaction is affected by the strength of the base used and its coordinating ability. The best results were obtained using sodium ethoxide as a base. Turnover numbers of 10-30 per day were observed in one atmosphere of oxygen at ambient temperature; with larger turnover numbers at higher temperatures. More than 200 turnovers were observed in the oxidation of ethanol at the ambient temperature with little or no loss of catalytic activity. Dimeric complexes of Ru(II) and Ru(III) were isolated from ethanolic solutions. These complexes were characterized and their role in the catalytic reaction is discussed.

The Ru(III) complex, (4'-MeLH)RuCl₃, also catalyzes the electrochemical oxidation of alcohols in basic alcoholic solution. The best results were obtained with 2, 6-lutidine as the base. The electrochemical oxidation was carried out at a carbon electrode immersed in an alcoholic solution at 0.8-1.0 V vs. nhe. In the absence of catalyst, negligible current was observed. The catalyzed electrochemical oxidation was general, oxidizing primary and secondary alcohols, with the principal products being aldehydes and ketones. More than 20 catalytic cycles were completed with the current remaining at 75% of its initial value. The Ru(III) complex exhibits reversible one-electron oxidation waves in nonalcoholic solvents in the presence or absence of 2, 6-lutidine. The electrochemistry in alcohols and nonalcohols is discussed, and a possible pathway for the catalytic electrochemical oxidation is presented.

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