Synthesis,Characterization,and Electrochemical Properties of Dinuclear Complexes Assembled from Asymmetric Co~(III) Bis(dioximates) and Boronic Acids

摘要

Bis(methylphenylglyoximate)cobalt(III) complexes exist both as cis and trans isomers due to the asymmetry of the equatorial ligand,and,when the axial ligands are different,the trans isomer is chiral.The reaction of racemic trans-[CH_3Co(mpgH)_2py] (1) with either 3- or 4-pyridylboronic acid affords dimeric units arranged on a crystallographic symmetry center such that the pyridyl nitrogen of one moiety coordinates to the Co atom of the symmetry-reiated unit.In principle,three structurally different dimeric species (two homodimers and one heterodimer) can be obtained.Time-resolved ~1H NMR spectra of a 1:1 mixture of racemic 1 and either 3- or 4-pyridylboronic acid in CDC1_3/CD_3OD show that the reaction does not converge toward a unique species in solution.Nevertheless,X-ray structures show that the hetero-chiral dimers are the only products that crystallize from the reaction mixture.The nature of the dioximate side groups does not affect the geometry of the dimeric arrangements assembled by 4-pyridylboronic acid ("molecular box").On the contrary,the geometry of the species assembled by 3-pyridyl-boronic acid varies from the "molecular parallelogram" obtained from the bis(dimethylglyoximates) to the highly squeezed "molecular box" obtained from bis(methylphenyl-glyoximates).Cyclic voltammetry studies show that the metal centers in the dimeric species do not interact with each other and undergo a simultaneous redox process.However,depending on the geometry of the systems,the redox process involves a single four-electron reduction for 3 and 5 or two consecutive two-electron reduction steps for 4 and 6.