title: Electrochemical oxidation of CoCp(CO)2: radical-substrate reaction of a 17 e-/18 e- pair and production of a unique dimer radical.
abstract: Anodic oxidation of the important half-sandwich compound CoCp(CO)2, 1, has been studied under gentle electrolyte conditions, e.g., chlorinated hydrocarbons with weakly coordinating anion (WCA) supporting electrolyte anions. The 17-electron cation 1+ produced at E(1/2)(1) = 0.37 V vs FeCp2(0/+) undergoes a surprising reaction with neutral 1 to form the dimer radical cation [Co2Cp2(CO)4] +, 2+, which has a metal-metal bond unsupported by bridging ligands. The dimer radical is oxidized at a slightly more positive potential (E(1/2) = 0.47 V) to the corresponding dication 2(2+). Observation of the oxidation of 2+ is without precedent in confirming a radical-substrate (R-S) dimerization process by direct voltammetric detection of the R-S intermediate, K(eq) = 3 x 10(4) M(-1) for [2+]/[1][1+]. The R-S mechanism and the reaction products have been characterized by voltammetry, electrolysis, fiber-optic IR spectroscopy, and ESR measurements. DFT calculations indicate that removal of an electron from 1 results in rehybridization in 1+, thereby opening the metal center for interaction with the neutral compound 1, which has a relatively basic metal center. The LUMO of the dimer dication 2(2+) is metal-metal antibonding, and its half-occupancy in 2+ results in lengthening of the Co-Co bond from 2.64 A to 3.14 A. Inclusion of solvent in the (COSMO) calculations shows that solvation effects are necessary to account for the fact that E(1/2)(2) > E(1/2)(1). These results show the importance of medium effects in probing the fundamental redox chemistry of half-sandwich metal complexes.
authors.affiliation: Department of Chemistry, University of Vermont, Burlington, VT 05405, USA. Ayman.Nafady@sci.monash.edu.au
authors.forename: Ayman
authors.initials: A
authors.lastname: Nafady
authors.forename: Paulo Jorge
authors.initials: PJ
authors.lastname: Costa
authors.forename: Maria José
authors.initials: MJ
authors.lastname: Calhorda
authors.forename: William E
authors.initials: WE
authors.lastname: Geiger
mesh_terms: D000838:Anions / Q000737:chemistry; D003035:Cobalt / Q000737:chemistry*; D019281:Dimerization; D004563:Electrochemistry; D004573:Electrolytes / Q000737:chemistry; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry; D008956:Models, Chemical; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction
chemical_list: D000838:Anions; D004573:Electrolytes; D005609:Free Radicals; D009942:Organometallic Compounds; D003035:Cobalt