Examples of Reductive Azo Cleavage and Oxidative Azo Bond Formation on Re2(CO)10 Template: Isolation and Characterization of Re(III) Complexes of New Azo-Aromatic Ligands

A new example of simultaneous reductive azo bond cleavage and oxidative azo bond formation in an azo-aromatic ligand is introduced. The chemical transformation is achieved by the reaction of Re-2(CO)(10) with the ligand 2-[(2-N-Arylamino)phenylazo]pyridine (HL1). A new and unexpected mononuclear rhenium complex [Re (L-1)(L-3)] (1) was isolated from the above reaction. The new azo-aromatic ligand, H2L3 (H2L3 = 2, 2' -dianilinoazobenzene) is formed in situ from HL1. A similar reaction of Re-2(CO)(10) and a closely related azo-ligand, 2,4-ditert-butyl-6-(pyridin-2-ylazo)-phenol (HL2), resulted in a seven coordinated compound [Re(L-2){(L-4)(center dot-)}(2)] (2; HL4 = 2-amino-4,6-ditert-butyl-phenol) via reductive cleavage of the azo bond. The complexes have been characterized by using a host of physical methods: X-ray crystallography, nuclear magnetic resonance (NMR), cyclic voltammetry, ultraviolet-visible (UV-vis), electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT). The experimental structures are well reproduced by density functional theory calculations and support the overall electronic structures of the above compounds. Complex 1 is a closed shell singlet, while complex 2 exemplifies a singlet diradical complex where the two partially oxidized aminophenoleto ligands are coupled to each other, yielding the observed diamagnetic ground state. Complexes 1 and 2 showed two successive one-electron redox responses. EPR spectral studies in corroboration with DFT results indicated that all of the redox processes occur at the ligand center without affecting the trivalent state of the metal ion.