New bis-o-semiquinonato cobalt complexes with 1,10-phenanthroline ligands

Abstract

Two novel cobalt compounds [Co(1,10-phen)(4-OMe-3,6-DBSQ)2] (Complex 1) and [Co(2,9-Me2-1,10-phen)(3,6-DBSQ)2] (Complex 2) (1,10-phen – 1,10-phenanthroline, 2,9-Me2-1,10-phen – 2,9-dimethyl-1,10-phenanthroline; 3,6-DBSQ – 3,6-di-tert-butyl-benzoquinone-1,2, 4-OMe-3,6-DBSQ – 4-methoxy-3,6-di-tert-butyl-benzoquinone-1,2) were obtained and investigated by means of X-ray structural analysis temperature variable magnetic susceptibility measurements and precise calorimetry methods. Inner coordination core of both complexes are close to octahedral. Stacking of molecules through phenanthroline ligand is present in both cases but it character is different. Complex [Co(1,10-phen)(4-OMe-3,6-DBSQ)2] (1) demonstrates thermally driven redox-isomeric (valent tautomeric) interconversion. So, at the temperatures down from 254K it is complex of low spin Co(III) connected with catecholate dianion and semiquinonato anion radical. Heating leads to it conversion into complex of high spin Co(II) connected with two semiquinonato anion radicals. This conversion can be clear monitored on the magnetic moment temperature dependence which curve has typical S-like shape. The interconversion of redox isomers is accompanied by phase transition. The heat capacity temperature dependence has λ-like peak. It temperature interval corresponds to maximal change of magnetic moment on the S-like curve. Compound [Co(2,9-Me2-1,10-phen)(3,6-DBSQ)2] (2) possesses no redox isomerism. It has no essential changes in magnetic properties. So, it seems that two methyl groups in 2,9-positions of 1,10-phenanthroline prevent bonds lengths shortening necessary for redox isomeric transformation.