RhCl(PPh3)3-mediated C–H oxyfunctionalization of pyrrolido-functionalized bisazoaromatic pincers: a combined experimental and theoretical scrutiny of redox-active and spectroscopic properties

Abstract

A potentially symmetrical NNN pyrrolido-functionalized pincer ligand, HL = 2,5-bis(phenylazo)-1H-pyrrole, reacts with [Rh^ICl(PPh₃)₃] in toluene in the presence of air, affording an emerald crystalline solid of the composition [Rh^III(L^O)Cl(PPh₃)₂]. A spontaneous C–H oxyfunctionalization of the aromatic ring with atmospheric oxygen leads to phenoxido functionalized organic transformation at room temperature. X-ray diffraction and MASS spectral analyses authenticate the unsymmetrical NNO coordination of the title ligand with a dangling phenylazo moiety. Cyclic voltammetry of redox innocent Rh(III) complexes exhibits a reversible oxidative response at E_1/2 ≈ 0.9 V vs. Ag/AgCl along with a quasi-reversible reductive response near −1.0 V. The electronic structures of the electro-active species are scrutinized by DFT calculations at the B3LYP-level of theory and both the responses are found to be ligand-centered (LC) in nature. Furthermore, an EPR study of the one-electron oxidized radical cation [Rh^III(L^O)Cl(PPh₃)₂]˙⁺ validates that the oxidation process is confined exclusively on the ligand framework (spin density: ρ_Phenoxido ≈ −0.50 and ρ_Pyrrolido ≈ −0.40). Moreover, an appreciable involvement of the pyrrolido function apart from the phenoxido group of the redox-active ligand (L^O) is apparent in the oxidation process from the nature of HOMO and thus, this type of ligand system provides two oxidizable domains within the single ligand backbone. A comparison of the relative oxidizability power between the two potential oxidizable centers viz. pyrrolido and phenoxido rings reveals that the former is somewhat less efficient for oxidation. In contrast, reductive response is mainly associated with both the coordinated and free azo chromophores. Time-dependent DFT and natural transition orbital (NTO) analyses on the complexes elucidate the origin of UV-vis absorptions.