Département de chimie moléculaire

Square-planar nickel(II) complexes of salen ligands, N,N'-bis(3-tert-butyl-(5R)-salicylidene)-1,2-cyclohexanediamine, in which R = tert-Bu (1), OMe (2), and NMe2 (3), were prepd. and the electronic structure of the 1-electron-oxidized species [1-3]+.bul. was investigated in soln. Cyclic voltammograms of 1-3 showed two quasi-reversible redox waves that were assigned to the oxidn. of the phenolate moieties to phenoxyl radicals. From the difference between the first and second redox potentials, the trend of electronic delocalization 1+.bul. {\textgreater} 2+.bul. {\textgreater} 3+.bul. was obtained. [1-3]+.bul. Exhibited isotropic g tensors of 2.045, 2.023, and 2.005, resp., reflecting a lower metal character of the singly occupied MO (SOMO) for systems that involve strongly electron-donating substituents. Pulsed-EPR spectroscopy showed a single population of equiv. imino nitrogen atoms for 1+.bul., whereas two distinct populations were obsd. for 2+.bul.. The resonance Raman spectra of 2+.bul. and 3+.bul. displayed the $ν$8a band of the phenoxyl radicals at 1612 cm-1, as well as the $ν$8a bands of the phenolates. In contrast, the Raman spectrum of 1+.bul. exhibited the $ν$8a band at 1602 cm-1, without any evidence of the phenolate peak. Previous work showed an intense near-IR (NIR) electronic transition for 1+.bul. ($Δ$$ν$1/2=660 cm-1, $ε$=21,700 M-1 cm-1), indicating that the electron hole is fully delocalized over the ligand. The broader and moderately intense NIR transition of 2+.bul. $Δ$$ν$1/2=1250 cm-1, $ε$=12,800 M-1 cm-1 suggests a certain degree of ligand-radical localization, whereas the very broad NIR transition of 3+.bul. $Δ$$ν$1/2=8630 cm-1, $ε$=2550 M-1 cm-1 indicates significant localization of the ligand radical on a single ring. Therefore, 1+.bul. is a Class III mixed-valence complex, 2+.bul. is Class II/III borderline complex, and 3+.bul. is a Class II complex according to the Robin-Day classification method. By employing the Coulomb-attenuated method (CAM-B3LYP) the authors were able to predict the electron-hole localization and NIR transitions in the series, and show that the energy match between the redox-active ligand and the metal d orbitals is crucial for delocalization of the radical SOMO. [on SciFinder(R)]