title: 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.
authors.affiliation: Department of Chemistry, Wayne State University, 5101 Cass Ave, Detroit, MI 48202, USA. lordri@gvsu.edu.
authors.forename: Maryam
authors.initials: M
authors.lastname: Yousif
authors.affiliation: Department of Chemistry, Wayne State University, 5101 Cass Ave, Detroit, MI 48202, USA. lordri@gvsu.edu and Department of Chemistry, Grand Valley State University, Allendale, MI 49401, USA. groysman@chem.wayne.edu.
authors.forename: Alyssa C
authors.initials: AC
authors.lastname: Cabelof
authors.affiliation: Department of Chemistry, Wayne State University, 5101 Cass Ave, Detroit, MI 48202, USA. lordri@gvsu.edu.
authors.forename: Philip D
authors.initials: PD
authors.lastname: Martin
authors.affiliation: Department of Chemistry, Grand Valley State University, Allendale, MI 49401, USA. groysman@chem.wayne.edu.
authors.forename: Richard L
authors.initials: RL
authors.lastname: Lord
authors.affiliation: Department of Chemistry, Wayne State University, 5101 Cass Ave, Detroit, MI 48202, USA. lordri@gvsu.edu.
authors.forename: Stanislav
authors.initials: S
authors.lastname: Groysman