Rhodium complexes of 1,3-diaryltriazenes: Usual coordination, N–H bond activation and, N–N and C–N bond cleavage

Reaction of 1,3-diaryltriazenes (R–C6H4–Ndouble bond; length as m-dashN–(NH)–C6H4–R, R = OCH3, CH3, H, Cl, NO2 at the para position) with [Rh(PPh3)3Cl] in ethanol in the presence of a base (NEt3) affords a family of yellow complexes (1-R) containing a PPh3, two de-protonated triazenes coordinated as bidentate N,N-donors, and an aryl (C6H4–R) fragment coordinated in the η1-fashion. A similar reaction in toluene yields a group of reddish-orange complexes (2-R) containing a PPh3, two N,N-coordinated triazenes, and a chloride. Structures of the 1-CH3 and 2-CH3 complexes have been determined by X-ray crystallography. All the 1-R and 2-R complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. The 1-R and 2-R complexes also fluoresce in the visible region under ambient condition while excited at around 400 nm. Cyclic voltammetry on these complexes shows a Rh(III)–Rh(IV) oxidation (within 0.76–1.68 vs. SCE), followed by an oxidation of the coordinated triazene ligand (except the R = NO2 complexes). An irreversible reduction of the coordinated triazene is also observed for all the complexes below −0.96 V vs. SCE. In the 1-R and 2-R complexes potential of the Rh(III)–Rh(IV) oxidation correlates linearly with the electron-withdrawing nature of the para-substituent (R).