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

Abstract

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-diphenylphenyl)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 Cr₂(O₂COR′)₄(THF)₂, that was characterized by NMR, UV-vis, and IR spectroscopy, and X-ray crystallography.