Cr(II) and Cr(I) PCP Pincer Complexes: Synthesis, Structure, and Catalytic Reactivity
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In the current investigation, the reaction of [Cr(CO)6] with the ligand precursor PO(C-Br)OP-tBu (1a) was investigated. When a suspension of [Cr(CO)6] and 1a in toluene was transferred into a sealed microwave glass vial and stirred for 3 h at 180 °C, the square-planar Cr(II) complex [Cr(POCOP-tBu)(Br)] (2a) was obtained. With PO(C-Br)OP-iPr (1b) the dibromo Cr(III) complex trans-[Cr(POCOP-iPr)(Br)2(THF)] (2b) was formed, after recrystallization from THF. Treatment of 2a with 1 equiv of LiBH4 in THF led to the formation of the borohydride complex [Cr(POCOP-tBu)(κ2-BH4)] (3). Exposure of a toluene solution of 3 to NO gas (1 bar) at room temperature afforded the Cr(I) complex [Cr(POCOP-tBu)(NO)(κ2-BH4)] (4). Alternatively, 4 was also obtained by reacting [Cr(POCOP-tBu)(NO)(Br)] (5) with LiBH4. On the basis of magnetic and EPR measurements as well as DFT calculations, compounds 4 and 5 adopt a low-spin d5 configuration and feature a nearly linearly bound NO ligand, suggesting CrINO+ rather than CrIINO• character. The reaction of 2a with 1 equiv of Li[CH2SiMe3] in toluene afforded the square-planar alkyl complex [Cr(POCOP-tBu)(CH2SiMe3)] (6) in 57% yield. This compound is catalytically active for the hydrosilylation of ketones at room temperature with a catalyst loading of 0.5 mol %. X-ray structures of all complexes are presented.
