Synthesis and characterization of mononuclear and binuclear chromium(III) complexes of α-benzoin oxime

Irradiation of a THF solution containing Cr(CO)(6) and alpha-benzoin oxime (H2BNO) under various atmospheric conditions, of either argon or vacuum, gave two dichromium(III) complexes of the formulae [Cr(mu-OH)(BNIM)(HBNO)](2) (1) and [Cr(BNIM)(HBNO)(THF)](2)O (2) where BNIM is the alpha-benzoin imine radical anion formed as a result of homolytic cleavage of the oxime N-OH bond. When a related reaction of alpha-benzoin oxime was carried out with Cr(NO)(3) in open atmosphere, Cr(HBNO)(3) (3) was obtained. Variable-temperature magnetic susceptibility measurements show a magnetic moment of 8.12 B.M. for the mu-dihydroxo complex (1) at room temperature. This high magnetic moment value strongly confirms the formulation of a dimer consisting of two benzoin imine radicals (S=1/2 coordinated to bridged dichromium(III) (S=3/2) ions. In contrast, the mu-oxo complex (2) shows a magnetic moment of 3.10 B.M. which is even smaller than that observed for the monomeric complex Cr(HBNO)(3) (3) at room temperature. Moreover, IR spectral analysis was very informative in assigning the vibrations due to Cr-2(OH)(2)(4+) and Cr2O4+ moieties for the complexes (1) and (2), respectively. Thermal gravimetric analysis (TGA) of complex (2) has shown decompositions due to coordinated THF molecules, followed by gradual decomposition of bezoin imine ligands. Interestingly, a low-energy transition in the Visnear-IR region assigned as BNIM-->Cr(III) LMCT transition was observed for the [Cr(BNIM)(HBNO)(ThF)](2)O complex.