Synthesis, structure and properties of cobalt(II/III) seleno-bisphenolate complexes containing NN bidentate co-ligands: kinetic studies on the reduction of a cobalt(III) complex by ascorbic acid in 80% MeCN–20% water

摘要

Two cobalt complexes, [Co(LSe)(phen)] · CH2Cl2 (1) and [Co(LSe)(N,N-Me2en)(CH3COO?)] (2) have been synthesized and characterized by elemental analyses, magnetic measurements, i.r. studies etc. Single crystal X- ray studies reveal that in complex (1) cobalt atom is in +2 oxidation state with trigonal bipyramidal geometry, while in complex (2) it is in +3 oxidation state and surrounded octahedrally. The asymmetric unit of complex (2) contains two crystallographically independent discrete molecules. Complex (1) was found to be paramagnetic with μeff = 2.19 BM indicating a low spin cobalt(II) d7 system, whereas complex (2) is found to be diamagnetic with cobalt(III) in low spin d6 state. The kinetic studies on the reduction of (2) by ascorbic acid in 80% MeCN–20% H2O (v/v) at 25 °C reveal that the reaction proceeds through the rapid formation of inner-sphere adduct, probably by replacing the loosely coordinated AcO? group, followed by electron transfer in a slow step and is supported by a large Q (formation constant) value.