Intramolecular amide → imine reduction in higher valent rhenium complexes stabilized by arylimido coligand: Rate studies and effect of reductants

The present work deals with the imidorhenium(V) complexes of type [ReCl3(NC6H4Y-p)(L)], with Y = OCH3(1d), CH3(1c), H(1b), Cl(1a) where L is the pyridylimine Schiff base ligand obtained from facile condensation of pyridine-2-carboxaldehyde and p-phenylenediamine. Structural authentication of one representative (1d) reveals meridional disposition of three Cl atoms around the metal center in a distorted octahedral ReCl3N3 coordination environment. Re–Npyridine bond lying trans to ReNC6H4Y-p motif is lengthened by ∼0.2 Å compared to Re–Nimine bond and is attributed to trans influence of imide nitrogen. The complexes, 1 are reactive towards dilute aqueous nitric acid furnishing amide bound hexavalent rhenium complexes, 2. Six lines EPR spectra have been recorded for 2 in solution phase at ambient condition (giso ∼ 1.945, Aav ∼ 493 G) and magnetic susceptibility measurement indicates strong orbital coupling consistent with one electron paramagnetic nature (∼1.45 μB). ReVI/ReV responses for 1 appear at higher potential (∼0.95 V) that those observed for 2 (∼0.12 V). Type 2 complexes are reduced (low ReVI/ReV reduction potential, +0.15 V) by N2H5+ and NH3OH+ species under mild condition to regenerate 1. The reduction with N2H5+ is nearly five times faster than NH3OH+. Rate study suggests an associative pathway (ΔH≠ = 11.61 kcal mol−1, ΔS≠ = −31.22 eu using N2H5+ and ΔH≠ = 10.74 kcal mol−1, ΔS≠ = −37.30 eu using NH3OH+) for amide → imine transformation. No such analogous amide → imine conversion has yet been achieved in metal free environment, accentuating the exclusive electronic role of variable metal valence. Further, the oxo complex, 6 does not exhibit intramolecular ligand oxidation suggesting decisive electronic role of the coligands (oxo/arylimido) in stabilizing higher metal valence.

Graphical abstractReduction of amide bound imidorhenium(VI) (2) into imine coordinated imidorhenium(V) (1) occurs in presence of proton cum electron donor nitrogenous reducing agents like N2H5+ and NH3OH+. The rate is first order on both the substrate (2) and the reductant. The reaction is associative and the rate with N2H5+ is nearly five times faster than NH3OH+.Figure optionsDownload full-size imageDownload as PowerPoint slide