SYNTHETIC UTILITY OF CHROMIUM(II)/(III) AND COBALT(II)/(III) REDOX CHEMISTRY MEDIATED BY COORDINATED LIGANDS (AGOSTIC, GLUCOSE TOLERANCE, CARBON-HYDROGEN ACTIVATION)

The reactivity of transition metal ions can be greatly influenced by coordinated ligands either through electronic or steric factors. The redox chemistry of M(II)/M(III) where M = Co and Cr has provided extensive information on the behavior of transition metals due to the kinetic inertness of the trivalent state. The slow substitution rates of Co(III) and Cr(III) have allowed detailed investigations of reaction products and intermediates which effectively are "snapshots" of reaction pathways. In this investigation it was found that by appropriate balance of steric and electronic factors associated with coordinated ligands and the M(II)/M(III) redox couples, new insights into cobalt and chromium chemistry were obtained. The results could make a significant impact on the role of metal ions in biological systems and in industrial catalysis.;A synthetic technique for producing stable nitrogen coordinated Cr(III)-nicotinic acid complexes was developed by taking advantage of the poised intramolecular redox couple which exists between Cr(II) and (nitrogen coordinated) nicotinic acid. This method, which we have termed "ligand reduction redox trapping," is not a generally new synthetic technique, but the application to Cr(III)-nicotinic acid complexes may prove very useful since Cr(III) in conjunction with nicotinic acid has been implicated in the maintenance of normal glucose metabolism in mammalian systems.;In the second study, the unique stereochemical requirements of the (coordinated) ligand dacoda (1,5-diazacyclooctane N,N'-diacetic acid) were combined with Co(II)/(III) redox chemistry resulting in intramolecular carbon-hydrogen activation via electrophilic attack of the Co(III) center on the C(2) carbon of the coordinated dacoda ligand. The reaction pathway proceeds via an intermediate containing a three-center, two-electron Co(III)-H-C (agostic) interaction. This system is the first example of a well-characterized, classical Werner-type complex capable of C-H activation and will certainly shed light on the general problem of alkane activation and the role agostic interactions in C-H activation systems.