Electronic and molecular characterization of an air-stable Cr(II) complex containing azo-anion-radicals

摘要

A new mononuclear homoleptic chromium complex has been synthesized by reaction of one equiv. of Cr(CO)(6) with two equiv. of the azoaromatic pincer ligand namely 2,6-bis[(4-methylphenyl)azo]pyridine [L-Me] in n-octane under reflux condition. The complex has been fully characterized by ESI-MS, H-1-NMR, FT-IR and UV/Vis spectroscopy. The X-ray structure of the complex reveals that the Cr-metal is coordinated to the central pyridine N-atom and two azo N-atoms of each tridentate pincer ligand in an octahedral environment. The H-1-NMR spectrum of the complex is indicative of diamagnetic ground state. The elongation of N-N bond lengths [d(N-N)(av) = 1.3275 angstrom] in the complex is consistent with the presence of azo-anion-radical character in the ligand. Since 2,6-bis(phenylazo)pyridine and its 4-Me-derivative are redox non-innocence in nature and therefore can coordinate to the chromium metal center as neutral (L-0) or as mono-anionic mono-radical (L-center dot)(1-) or as di-anionic di-radical (L-center dot center dot)(2-) or even as tri-anionic mono-radical (L-center dot)(3-) resulting an ambiguity on the true oxidation state of the metal center. Thus the present work nicely elaborates the importance of suitably designed bis-azopyridine containing pincer ligand in accessing an air-stable Cr(II) complex starting with low-valent metal carbonyl precursor via electron transfer from the metal center to the highly pi-acidic ligand leading to stable azo-anion-radicals. The stability of azo-anion-radical bound Cr(II) complex has been investigated by DFT calculations. (C) 2020 Elsevier B.V. All rights reserved.