Ni(ii) and Pd(ii) complexes of a new redox-active pentadentate azo-appended 2-aminophenol ligand: Pd(ii)-assisted intraligand cyclization forms a phenoxazinyl ring

Square planar complexes of Ni(II) and Pd(II) of a new redox-active pentadentate azo-appended 2-aminophenol ligand (H4L = N,N '-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2 '-diamino-ortho-azobenzene) in three accessible redox levels [amidophenolate(2-), semiquinonate(1-) pi radical, and quinone(0)] were synthesized. The coordinated HL(3-) ligand provides four donor sites [two N(iminophenolates), an N '(azo), and an O(phenolate)], while the phenolic OH group remains free in the three complexes. Cyclic voltammetry on complex [Ni(L)] 1 and its corresponding Pd(II) analogue [Pd(L)] 2 in CH2Cl2 displayed three redox responses (two oxidative at E-1/2 = 0.06 V and E-pa (anodic peak potential) = 0.80 V and one reductive at -0.77 V for 1 and at E-1/2 = 0.08 V and E-pa = 0.85 V and at -0.74 V for 2 vs. Fc(+)/Fc). The chemical oxidation of 1 with AgSbF6 afforded [Ni(L)]SbF6<middle dot>2CH(2)Cl(2) (3<middle dot>2CH(2)Cl(2)). Complex [Pd(L*)] 4, which is coordinated by a phenoxazinyl derivative of L(4-), was obtained via intraligand cyclization in the parent complex 2 under basic oxidizing conditions. The molecular structures of 1, 2, 3<middle dot>2CH(2)Cl(2) and 4 were elucidated through X-ray crystallography at 100 K. Characterization using H-1 NMR, X-band EPR, and UV-VIS-NIR spectroscopy established that the complexes have [Ni-II{(L-ISQ)(center dot)(2-)}] 1, [Pd-II{(L-ISQ)(center dot)(2-)}] 2, [Ni-II{(L-IBQ)(-)}]SbF6/1(+)SbF(6)(-)(3), and [Pd-II{(L*(AP))(center dot)(2-)}] 4 electronic states. Complexes 1, 2, and 4 possess paramagnetic S-t (total spin) = 1/2 ground-state, whereas 3 is diamagnetic (S-t = 0). Density functional theory (DFT) electronic structural calculations at the B3LYP level rationalized the observed experimental results. Time-dependent (TD)-DFT calculations allowed us to identify the nature of the observed absorption spectra.