Synthesis of a mononuclear, non-square-planar chromium(II) bis(alkoxide) complex and its reactivity toward organic carbonyls and CO2

In this paper, we report the synthesis and reactivity of a rare mononuclear chromium(II) bis(alkoxide) complex, Cr(OR')(2)(THF)(2), that is supported by a new bulky alkoxide ligand (OR' = di-t-butyl-(3,5-diphenyl-phenyl)methoxide). The complex is prepared by protonolysis of square-planar Cr(N(SiMe3)(2))(2)(THF)(2) with HOR'. X-ray structure determination disclosed that Cr(OR')(2)(THF)(2) features a distorted seesaw geometry, in contrast to nearly all other tetra-coordinate Cr(II) complexes, which are square-planar. The reactivity of Cr(OR')(2)(THF)(2) with aldehydes, ketones, and carbon dioxide was investigated. Treatment of Cr(OR')(2)(THF)(2) with two equivalents of aromatic aldehydes ArCHO (ArCHO = benzaldehyde, 4-anisaldehyde, 4-trifluorbenzaldehyde, and 2,4,6-trimethylbenzaldehyde) leads cleanly to the formation of Cr(IV) diolate complexes Cr(OR')(2)(O2C2H2Ar2) that were characterized by UV-vis and IR spectroscopies and elemental analysis; the representative complex Cr(OR')(2)(O2C2H2Ph2) was characterized by X-ray crystallography. In contrast, no reductive coupling was observed for ketones: treatment of Cr(OR')(2)(THF)(2) with one or two equivalents of benzophenone forms invariably a single ketone adduct Cr(OR')(2)(OCPh2) which does not react further. QM/MM calculations suggest the steric demands prevent ketone coupling, and demonstrate that a mononuclear Cr(III) bis-aldehyde complex with partially reduced aldehydes is sufficient for C-C bond formation. The reaction of Cr(OR')(2)(THF)(2) with CO2 leads to the insertion of CO2 into a Cr-OR' bond, followed by complex rearrangement to form a diamagnetic dinuclear paddlewheel complex Cr-2(O2COR')(4)(THF)(2), that was characterized by NMR, UV-vis, and IR spectroscopy, and X-ray crystallography.