o-Iminobenzosemiquinonate and o-Imino-p-methylbenzosemiquinonate Anion Radicals Coupled VO2+ Stabilization

The diamagnetic VO2+-iminobenzosemiquinonate anion radical (LRIS•−, R = H, Me) complexes, (L−)(VO2+)(LRIS•−): (L1−)(VO2+)(LHIS•−)•3/2MeOH (1•3/2MeOH), (L2−)(VO2+)(LHIS•−) (2), and (L2−)(VO2+)(LMeIS•−)•1/2 LMeAP (3•1/2 LMeAP), incorporating tridentate monoanionic NNO-donor ligands {L = L1− or L2−, L1H = (2-[(phenylpyridin-2-yl-methylene)amino]phenol; L2H = 1-(2-pyridylazo)-2-naphthol; LHIS•− = o-iminobenzosemiquinonate anion radical; LMeIS•− = o-imino-p-methylbenzosemiquinonate anion radical; and LMeAP = o-amino-p-methylphenol} have been isolated and characterized by elemental analyses, IR, mass, NMR, and UV−vis spectra, including the single-crystal X-ray structure determinations of 1•3/2MeOH and 3•1/2 LMeAP. Complexes 1•3/2MeOH, 2, and 3•1/2 LMeAP absorb strongly in the visible region because of intraligand (IL) and ligand-to-metal charge transfers (LMCT). 1•3/2MeOH is luminescent (λext, 333 nm; λem, 522, 553 nm) in frozen dichloromethane−toluene glass at 77 K due to πdiimine→πdiimine* transition. The V−Ophenolato (cis to the VO) lengths, 1.940(2) and 1.984(2) Å, respectively, in 1•3/2MeOH and 3•1/2 LMeAP are consistent with the VO2+ description. The V−Oiminosemiquinonate (trans to the VO) lengths, 2.1324(19) in 1•3/2MeOH and 2.083(2) Å in 3•1/2 LMeAP, are expectedly ∼0.20 Å longer due to the trans influence of the VO bond. Because of the stronger affinity of the paramagnetic VO2+ ion to the LHIS•− or LMeIS•−, the V−Niminosemiquinonate lengths, 1.908(2) and 1.921(2) Å, respectively, in 1•3/2MeOH and 3•1/2 LMeAP, are unexpectedly shorter. Density functional theory (DFT) calculations using B3LYP, B3PW91, and PBE1PBE functionals on 1 and 2 have established that the closed shell singlet (CSS) solutions (VO3+-amidophenolato (LRAP2−) coordination) of these complexes are unstable with respect to triplet perturbations. But BS (1,1) Ms = 0 (VO2+-iminobenzosemiquinonate anion radical (LRIS•−) coordination) solutions of these species are stable and reproduce the experimental bond parameters well. Spin density distributions of one electron oxidized cations are consistent with the [(L−)(VO2+)(LRIQ)]+ descriptions [VO2+-o-iminobenzoquinone (LRIQ) coordination], and one electron reduced anions are consistent with the [(L•2-)(VO3+)(LRAP2-)]− descriptions [VO3+-amidophenolato (LRAP2−) coordination], incorporating the diimine anion radical (L1•2−) or azo anion radical (L23−). Although, cations and anions are not isolable, but electro-and spectro-electrochemical experiments have shown that 3+ and 3− ions are more stable than 1+, 2+ and 1-, 2- ions. In all cases, the reductions occur with simultaneous two electron transfer, may be due to formation of coupled diimine/azo anion radical-VO2+ species as in [(L•2-)(VO2+)(LRAP2−)]2−.