Pulsed Laser Photolysis and Thermodynamics Studies of Intramolecular Electron Transfer in Valence Tautomeric Cobalto-Quinone Complexes

Abstract

Four valence tautomeric complexes with the general composition [Co(3,5-DTBSQ)2(N⌒N)] have been investigated, where 3,5-DTBSQ- is the semiquinonate form of 3,5-di-tert-butyl-o-quinone. The diiminium ligand N⌒N is one of the following ligands: phen (1,10-phenanthroline, complex 1); bpy (2,2‘-bipyridine, complex 2); dmbpy (4,4‘-dimethyl-2,2‘-bipyridine, complex 3); dpbpy (4,4‘-diphenyl-2,2‘-bipyridine, complex 4). All four of the complexes undergo a valence tautomerism converting from low-spin (ls) [CoIII(3,5-DTBSQ)(3,5-DTBCAT)(N⌒N)] at low temperatures to high-spin (hs) [CoII(3,5-DTBSQ)2(N⌒N)] at high temperatures, where the 3,5-DTBCAT2- ligand is the catecholate form of the o-quinone ligand. Variable temperature (150−340 K) magnetic susceptibility data are presented for toluene solutions of all four complexes. These data were least-squares fit to give ΔH and ΔS values that characterize the valence tautomerism equilibrium. The ΔH values range from 21.33 to 38.36 kJ mol-1, whereas the ΔS values range from 60...