Metal-promoted aromatic ring amination and deamination reactions at a diazo ligand coordinated to rhodium and ruthenium

Abstract

Reactions of MCl₃·3H₂O (M=Rh and Ru) with the ligand 2-[(2-N-arylamino)phenylazo]pyridine [HL¹; NH₄C₅N=NC₆H₄N(H)C₆H₄(H) (HL^1a), NH₄C₅N=NC₆H₄N(H)C₆H₄(CH₃) (HL^1b), and NH₄C₅N=NC₆H₄N(H)C₅H₄N (HL^1c)] 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.