Synthesis, structure, redox and spectra of green iridium complexes of tridentate azo-aromatic ligands

Reactions of IrCl₃·xH₂O with the ligands, 2-(phenylamino)phenylazo]pyridine (HL^1a) and 2-(p-tolylamino)phenylazo]pyridine (HL^1b) produce Ir(L¹)₂]Cl (L¹ = L^1a, 1]Cl and L¹ = L^1b, 2]Cl) along with many unidentified products. The iridium complexes have been characterized by various techniques such as X-ray crystallography, mass spectrometry, ¹H and ¹³C NMR, cyclic voltammetry and absorption studies. The complex 1]ClO₄ crystallises in triclinic space group. The crystallographic data have been determined. Notably, the Ir-N (azo) lengths are short (av. 1·9875(4) ?) as compared to the remaining four Ir-N lengths (av. 2·052(5) ?). There is significant degree of ligand backbone conjugation in the coordinated ligands, which result in shortening of the C-N lengths on the other side of the middle phenyl ring and also in lengthening of the diazo (N=N) lengths. The complexes display multiple low energy transitions ranging between 1010 and 450 nm. These are electro active and show three reversible redox responses in the potential range, +1·5 V to −1·5 V. The cathodic potential responses are ascribed as ligand reductions, while the redox process at the anodic potential occurs at a mixed metal-ligand (HOMO) orbital.