Catecholato complexes of palladium(II)–naphthylazoimidazoles: synthesis, spectral characterization, electrochemistry and solvatochromic studies

[Pd(α-NaiR)(O,O)] [α-NaiR = 1-alkyl-2-(naphthyl-α-azo)imidazoles; R = Me (a), Et (b), CH2Ph (c) and O,O = pyrocatecholato (cat) (3), 4-t-butylcatecholato (tbcat) (4), 3,5-di-t-butylcatecholato (dtbcat) (5), and tetracholorocatecholato (tccat) (6)] complexes were characterized by elemental analyses, i.r. and 1H-n.m.r. spectral data. Studies by cyclic voltammetry suggest the existence of two redox couples, positive saturated calomel electrode (s.c.e.) that are due to catecholes to semiquinone, and semiquinone to quinone oxidations, respectively; two couples at negative to s.c.e. are associated with azo reductions. The complexes exhibit ligand-to-ligand charge-transfer transition in the near-i.r. region. The band position is largely dependent upon the substituent on the catechole frame and exhibits a negative solvatochromic effect. The transition is qualitatively assigned to the HOMO (cat) → LUMO (α-NaiR) transition.