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

Abstract

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 [LMe] in n-octane under reflux condition. The complex has been fully characterized by ESI-MS, 1H-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 1H-NMR spectrum of the complex is indicative of diamagnetic ground state. The elongation of N-N bond lengths [d(N-N)av = 1.3275 Å] 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 (L0) or as mono-anionic mono-radical (L·)1− or as di-anionic di-radical (L··)2- or even as tri-anionic mono-radical (L·)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 π-acidic ligand leading to stable azo-anion-radicals. The stability of azo-anion-radical bound Cr(II) complex has been investigated by DFT calculations.