Cr(II) and Cr(I) PCP Pincer Complexes: Synthesis, Structure, and Catalytic Reactivity

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.