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 (H 4 L = N , N '-bis(2-hydroxy-3,5-di- tert -butylphenyl)-2,2'-diamino- ortho -azobenzene) in three accessible redox levels [amidophenolate(2-), semiquinonate(1-) π 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 CH 2 Cl 2 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 AgSbF 6 afforded [Ni(L)]SbF 6 ·2CH 2 Cl 2 (3·2CH 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, 3·2CH 2 Cl 2 and 4 were elucidated through X-ray crystallography at 100 K. Characterization using 1 H NMR, X-band EPR, and UV-VIS-NIR spectroscopy established that the complexes have [Ni II {(L ISQ )˙ 2- }] 1, [Pd II {(L ISQ )˙ 2- }] 2, [Ni II {(L IBQ ) - }]SbF 6 /1 + SbF 6 - (3), and [Pd II {(L* AP )˙ 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.