Palladium azoimidazoles. Solvatochromic studies of catecholato complexes

Abstract

N(1)-alkyl-2-(arylazo)imidazoles, p-RC6H4N=NC3H2 NN(1)X [RaaiX; X=Me(3), Et(4); R=H(a), Me(b), Cl(c)] have been synthesized and, upon reaction with Pd(MeCN)2Cl2, yield orange–red complexes Pd(RaaiX)Cl2 (5),(6) which in turn were reacted with catechols in the presence of Et3N to yield the ternary complexes Pd(RaaiX)(O,O) where O,O=pyrocatechol [cat, (7),(11)], 4-t-butylcatechol [tbcat,(8),(12)], 3,5-di-t-butylcatechol (dtbcat) (9),(13) and tetrachlorocatechol [tccat,a (10),(14). The complexes were characterized by elemental analysis, i.r., u.v.–vis. and 1H-n.m.r. data. Electrochemical studies on (7)–(14) suggest four successive redox responses which are highly sensitive to the nature of the substituents. These complexes were found to exhibit a ligand-to-ligand charge transfer (l.l.c.t.) transition and a negative solvatochromic effect. The band position is largely dependent upon the substituent on the catechol frame and is qualitatively assigned as the 3b1(cat)→π*(RaaiX) transition.