Transition-Metal-Promoted C−N Bond-Formation Processes − Low-Spin FeIII, FeII, and NiII Complexes of 2-[(Arylamido)phenylazo]pyridine − X-ray Structure, Redox- and Spectroelectrochemistry

Abstract

The metal-promoted amination of an aromatic ring of coordinated L1 [L1, pap = 2-(phenylazo)pyridine] is described. Whereas the labile cobalt complex [Co(L1)3]2+ prefers ortho-amination, yielding L2H {2-[2-(arylamino)phenylazo]pyridine}, the corresponding [Fe(L1)3]2+ complex produces the para-aminated product L3H {2-[4-(arylamino)phenylazo]pyridine}. Both L2H and L3H have been isolated in the pure state and were fully characterised. They have low pKa values, for example pKa(L2aH) = 8.5 ± 1 and pKa(L3aH) = 9.1 ± 1. Upon deprotonation, the ligand L2H behaves as a potential tridentate N,N,N donor. It reacts with anhydrous FeCl3 and NiCl2·6H2O to produce cationic [Fe(L2)2]+ (1+) and neutral [Ni(L2)2] (2), respectively. The cationic ferric complex has been isolated as its perchlorate salt, which is paramagnetic with one unpaired electron (1.66 μB). It shows a rhombic EPR spectrum with g1 = 2.11, g2 = 2.08, g3 = 1.93. The room-temperature magnetic moment of the nickel complex is 2.89 μB, which confirms the presence of two unpaired electrons. The representative X-ray structures of [1]ClO4 and 2 are reported. In both cases the azo nitrogen atoms of the coordinated ligand, [L2]− approach the metal centres more closely, and there is a significant degree of ligand backbone conjugation. The complexes display multiple redox responses. Chemical reduction of 1+ with dilute hydrazine affords the corresponding ferrous complex [Fe(L2)2] (1) in almost quantitative yield. The spectral changes upon electrolysis of the above couples have been recorded in an OTTLE cell.