1. 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.
- Author
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Bhowmik, Saumitra, Sengupta, Arunava, and Mukherjee, Rabindranath
- Subjects
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MOLECULAR structure , *X-ray crystallography , *DENSITY functional theory , *QUINONE derivatives , *CYCLIC voltammetry , *ABSORPTION spectra , *RING formation (Chemistry) , *SCHIFF bases , *QUINONE - Abstract
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−) 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 CH2Cl2 displayed three redox responses (two oxidative at E1/2 = 0.06 V and Epa (anodic peak potential) = 0.80 V and one reductive at −0.77 V for 1 and at E1/2 = 0.08 V and Epa = 0.85 V and at −0.74 V for 2vs. Fc+/Fc). The chemical oxidation of 1 with AgSbF6 afforded [Ni(L)]SbF6·2CH2Cl2 (3·2CH2Cl2). 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, 3·2CH2Cl2 and 4 were elucidated through X-ray crystallography at 100 K. Characterization using 1H NMR, X-band EPR, and UV–VIS–NIR spectroscopy established that the complexes have [NiII{(LISQ)˙2−}] 1, [PdII{(LISQ)˙2−}] 2, [NiII{(LIBQ)−}]SbF6/1+SbF6−(3), and [PdII{(L*AP)˙2−}] 4 electronic states. Complexes 1, 2, and 4 possess paramagnetic St (total spin) = 1/2 ground-state, whereas 3 is diamagnetic (St = 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. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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