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

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(SiMe 3 ) 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 (O 2 C 2 H 2 Ar 2 ) that were characterized by UV-vis and IR spectroscopies and elemental analysis; the representative complex Cr(OR′) 2 (O 2 C 2 H 2 Ph 2 ) 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 (OCPh 2 ) 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 CO 2 leads to the insertion of CO 2 into a Cr-OR′ bond, followed by complex rearrangement to form a diamagnetic dinuclear paddlewheel complex Cr 2 (O 2 COR′) 4 (THF) 2 , that was characterized by NMR, UV-vis, and IR spectroscopy, and X-ray crystallography. A rare non-square-planar mononuclear Cr( ii ) bis(alkoxide) complex Cr(OR′) 2 (THF) 2 is reported and its reactivity with organic carbonyls and CO 2 is investigated.