81572-41-0Relevant academic research and scientific papers
Electron transfer. 54. Remote attack in the reductions of carboxylato-bridged dicobalt(III) complexes
Srinivasan, Vangalur S.,Singh,Wieghardt, Karl,Rajasekar,Gould
, p. 2531 - 2537 (2008/10/08)
Twelve carboxylato-bridged dicoalt(III) complexes of type I have been prepared and their reductions with the metal centers V2+, Eu2+, Cr2+, Ru(NH3)62+, and U3+, as well as with the radical (Rb?) and the dihydro derivative (RbH2) of riboflavin, have been examined. In each of the dimeric oxidants, the carbonyl of the carboxyl group has been tied off by coordination, blocking off inner-sphere reduction of the type often encountered with monomeric carboxylato oxidants, but seven of the dimers feature a pendent carbonyl group in conjugation with the bridging carboxyl. Specific rates in each case appear to be determined by the reduction of the first of the two Co(III) centers. For those dimeric oxidants having no free carbonyl function, rate patterns for reactions with the five metal-center reductants correspond closely to those observed for other systems where only outer-sphere paths are possible. Incorporation of the conjugated carbonyl group enhances the rates of reduction with Ru(NH3)62+, Rb?, and RbH2 (all outer-sphere reductants) only modestly (2+, Eu2+, and U3+ by 102-107, indicating strongly that a second (remote inner-sphere) path has come into play with these metal-center reductants. These experiments provide the first reported evidence for remote attack by U3+. Reductions, using Eu2+, of the carbonyl-substituted dimers yield considerably less than 1:1 Co2+:Eu2+ even when the oxidant is taken in excess, indicating that reduction of the carbonyl group (and subsequent dimerization of the derived radical) competes with reduction of bound Co(III); this side reaction appears to occur both with the original dimer and with the monomeric Co(III) intermediate. In contrast, 1:1 stoichiometry is observed in reductions with Ru(NH3)62+ (which is too weak a reductant to reduce the carbonyl group) and with Cr2+ and U3+, the oxidized forms of which (CrIII and UIV) presumably remain bound to the carbonyl function after the initial act of electron transfer. Rate comparisons between reductions of the dimers and carbonyl-substituted monomeric oxidants of the type RCO2Co(NH3)52+ have been used to estimate the distribution between alternate reaction paths when carbonyl substitution in the monomers allows reduction, in part, by remote attack. It has thus been calculated that the remote path for Cr2+ reduction of the monomeric pyruvato complex comprises only 0.2% of the total reaction whereas that for the 2-formylbenzoato complex constitutes 15%.
