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

摘要

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-N-pyridine bond lying trans to Re NC6H4Y-p motif is lengthened by similar to 0.2 angstrom compared to Re-N-imine bond and is attributed to trans influence of imide nitrogen. The complexes, I 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 (g(iso) similar to 1.945, A(av) similar to 493 G) and magnetic susceptibility measurement indicates strong orbital coupling consistent with one electron paramagnetic nature (similar to 1.45 mu(B)). Re-VI/Re-V responses for 1 appear at higher potential (similar to 0.95 V) that those observed for 2 (similar to 0.12 V). Type 2 complexes are reduced (low Re-VI/Re-V 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 (Delta H-not equal = 11.61 kcal mol(-1), Delta S-not equal = -31.22 eu using N2H5+ and Delta H-not equal = 10.74 kcal mol(-1), Delta S-not equal = -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.