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(L^1 )2]Cl (L^ 1 = L^1a , [1]Cl and L 1 = 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, 1 H and 13 C 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) Å) as compared to the remaining four Ir–N lengths (av. 2⋅052(5) Å). 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.