BISTABILITY AND LOW-ENERGY-ELECTRON TRANSFER IN COBALT COMPLEXES CONTAINING CATECHOLATE AND SEMIQUINONE LIGANDS

Abstract

Studies have been carried out on intramolecular cobalt-quinone electron transfer for a series of complexes of general form Co(III)(N-N)(SQ)(Cat), where N-N is 2,2'-bipyridine (bpy), N,N,N',N'-tetramethylethylenediamine (tmeda), 1,10-phenanthroline (phen), bis(2-pyridyl) ketone (PY)2CO), 5-nitro-1,10-phenanthroline (NO2phen), dipyrido-[3,2-a:2',3'-c]phenazine (dppz), and 4,5-diazafluoren-9-one (dafl) and SQ and Cat are the 3,5- or 3,6-di-tert-butylcatechol and semiquinone ligands. Optical absorptions that appear in the 2500-nm (4000-cm-1) region of the infrared for the Co(III) complexes are assigned as Cat --> Co(III) charge transfer transitions. Spectral changes observed for the complexes in toluene solution result from an equilibrium between Co(III)(N-N)(SQ)(Cat) and Co(II)(N-N)(SQ)2 redox isomers. Magnetic measurements on solid samples of Co(bpy)(3,5-DBSQ)(3,5-DBCat) and Co(phen)(3,6-DBSQ)(3,6-DBCat) show that the equilibrium exists in the solid state. The temperature range of the equilibrium is dependent upon the donation effect of the nitrogen coligand; values for the Co(III)/Co(II) transition temperature have been determined that follow coligand donor strength. Crystallographic characterization of Co(tmeda)(3,6-DBSQ)(3,6-DBCat) (Co(tmeda)(3,6-DBSQ)(3,6-DBCat): monoclinic, P2(1)/n, a = 11.379(2) angstrom, b = 34.510(5) angstrom, c = 17.583(3) angstrom, beta = 91.73(1)-degrees, V = 6902(2) angstrom3, Z = 8) has provided bond lengths showing that the metal is low-spin Co(III) at room temperature. Structural characterization of Co(NO2phen)(3,6-DBSQ)2 (Co(NO2phen)(3,6-DBSQ)2.2toluene: monoclinic, Pc, a = 12.626(2) angstrom, b = 11.297(2) angstrom, c = 17.974(3) angstrom, beta = 105.55(1)-degrees, V = 2470(l) angstrom3, Z = 2) and Co(dafl)(3,6-DBSQ)2 (Co(dafl)(3,6-DBSQ)2.2toluene: monoclinic, Pc, a = 12.385(5) angstrom, b = 11.230(5) angstrom, c = 17.892(6) angstrom, beta = 105.41(2)-degrees, V = 2400(2) angstrom3, Z = 2) has shown that the complex molecules are trigonal prismatic in structure at room temperature, with features that are consistent with the high-spin Co(II) charge distribution. Magnetic measurements on Co(NO2phen)(3,6-DBSQ)2 indicate that it remains in the Co(II) form at low temperature.