James P. Collman

Oxidative-Addition Reactions of d8 Complexes

Publisher Summary The rapid development of organo-transition-metal chemistry over the past decade has been dominated by considerations of bonding and structure. The reactions of such organo-metallic compounds have been discussed to a lesser extent and usually in the context of a particular metal or ligand. The apparent parallel between oxidative addition of covalent molecules to unsaturated d complexes and chemisorption of these molecules to the latent valences on transition metal surfaces may be significant. Several organic reactions that are homogeneously catalyzed by unsaturated d8 complexes involve oxidative addition FS a key step in the mechanism. An alternative pathway for five-coordinate complexes to undergo oxidative additions is through prior dissociation of a ligand to form a more reactive four-coordinate complex. Labilization of a ligand may be brought about by heating or irradiating a five-coordinate complex. The tendency for d complexes to undergo oxidative additions depends markedly on the nature of the central metal ion and the ligands attached to it.

Application of a polymer solid electrolyte for the vapor-phase electrocatalysis of dioxygen reduction by some cobalt porphyrins

Abstract An electrochemical cell suitable for the study of dioxygen reduction electrocatalysis by films of cobalt metalloporphyrins on electrodes in the absence of liquid electrolytes is described. The glassy carbon disk of a (stationary) ring-disk electrode is coated with either an electropolymerized film of cobalt tetra(o-aminophenyl)porphyrin or an adsorbed multimolecular layer of a dicobalt cofacial porphyrin, Co2 FTF4. Then the disk, the ring (serving as an auxiliary electrode), and a peripheral silver wire (serving as a pseudo-reference electrode) are coated with a thin film of an ionically conducting polymer: poly(ethyleneoxide) containing a 16:1 ether oxygen/Li+ concentration of lithium triflate. Bathing this solid state cell in moist, acidic or neutral bathing gases containing dioxygen produces electrocatalytic currents for dioxygen reduction. The fraction of the reduction leading to H2O2 was determined with a twin electrode sandwich modification of the above cell in which a Au film electrode atop a Nucleopore membrane is contacted to the top surface of the PEO16·Li(CF3SO3) film. No H2O2 was produced in the case of the Co2FTF4 catalyst in a neutral bathing gas; this electrocatalysis was also quite stable.

Epoxy‐Encapsulated Ceramic Superconductor Microelectrodes

A procedure is outlined for fabricating well-behaved microelectrodes from ceramic pellets of YBa{sub 2}CU{sub 3}O{sub 7} and Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} which involves systematic polishing of an epoxy-encapsulated superconductror chip, under Et{sub 4}NCIO{sub 4}/acetonitrile solution, to a potentiometric end point. Voltammetry of the resulting microelectrodes in acetronitrile is illustrated and compared to that arising from alternative superconductor electrode geometries. The microelectrodes have active electrode surface areas ranging from 2 {times} 10 {sup {minus} sup 6} to 3 {times} 10 {sup {minus} sup 4}cm{sup 2}, as characterized electrochemically and microscopically. The results discussed herein are steps toward developing the methodology necessary to study the electrochemical response of high temperature superconductor phases at temperatures below theirtheir superconductor critical temperature.