Coexistence of Two Thermally Induced Intramolecular Electron Transfer Processes in a Series of Metal Complexes [M(Cat-N-BQ)(Cat-N-SQ)]/[M(Cat-N-BQ)(2)} (M = Co, Fe, and Ni) bearing Non-Innocent Catechol-Based Ligands: A Combined Experimental and Theoretical Study

The different thermally induced intermolecular electron transfer (IET) processes that can take place in the series of complexes [M(Cat-NBQ)(Cat-N-SQ)/[M(Cat-N-BQ)2], for which M=Co (2), Fe (3) and Ni(4), and Cat-N-BQ and Cat-N-SQ denote the mononegative (Cat-N-BQ(-)) or di-negative (Cat-N-SQ(2-)) radical forms of the tridentate Schiff-base ligand 3,5-ditert-butyl-1,2-quinone-1-(2-hydroxy-3,5- di-tert-butylphenyl)imine, have been studied by variable-temperature UV/Vis and NMR spectroscopies. Depending on the metal ion, rather different behaviors are observed. Complex 2 has been found to be one of the few examples so far reported to exhibit the coexistence of two thermally induced electron transfer processes, ligand-to-metal (IETLM) and ligand-to-ligand (IETLL). IETLL was only found to take place in complex 3, and no IET was observed for complex 4. Such experimental studies have been combined with ab initio wavefunction-based CASSCF/CASPT2 calculations. Such a strategy allows one to solicit selectively the speculated orbitals and to access the ground(-) states and excited-spin states, as well as charge-transfer states giving additional information on the different IET processes.