Metal-Promoted Aromatic Ring Amination and Deamination Reactions at a Diazo Ligand Coordinated to Rhodium and Ruthenium

Reactions of MCl3·3H2O (M = Rh and Ru) with the ligand 2-[(2-N-arylamino)phenylazo]pyridine [HL1; NH4C5NNC6H4N(H)C6H4(H) (HL1a), NH4C5NNC6H4N(H)C6H4(CH3) (HL1b), and NH4C5NNC6H4N(H)C5H4N (HL1c)] in the
presence of dilute NEt3 afforded multiple products. In the case of rhodium, two green compounds, viz. [Rh(L1)2]+
([2]+) and [RhCl(pap)(L1)]+ ([3]+), where L1 and pap stand for the conjugate base of [HL1] and 2-(phenylazo)pyridine, respectively, were separated on a preparative thin layer chromatographic plate. The reaction of RuCl3·3H2O, on the other hand, produced two brown compounds, viz. [RuCl(HL1)(L1)] (4) and [RuCl(pap)(L1)] (5), respectively,
as the major products. The X-ray structures of the representative complexes are reported. Except for complex 2,
and 4, the products are formed due to the cleavage of an otherwise unreactive C(phenyl)−N(amino) bond. In
complex 4, one of the tridentate ligands (HL1) does not use its maximum denticity and coordinates as a neutral
bidentate donor. Plausible reasons for the differences in their modes of coordination of the ligands as in 2 and 4
have been discussed. The ligand pap in the cationic mixed ligand complex [3]+ reacts instantaneously with ArNH2
to produce an ink-blue compound, [RhCl(HL2)(L1)]+ ([6]+) in a high yield. The ligand HL2 is formed due to regioselective
fusion of ArNH2 residue at the para carbon of the phenyl ring (with respect to the azo fragment) of pap in [3]+. The
above complexes are generally intensely colored and show strong absorptions in the visible region, which are
assigned to intraligand charge transfer transitions. These complexes undergo multiple and successive one-electron-transfer processes at the cathodic potentials. Electrogenerated cationic complexes of ruthenium(III), [4]+ and [5]+,
showed rhombic EPR spectra at 77 K.