Cr(II) and Cr(III) NCN pincer complexes: synthesis, structure, and catalytic reactivity

The synthesis, characterization, and reactivity of several new Cr(II) and Cr(III) complexes featuring an NCN pincer ligand with an arene backbone connected to amine donors NEt₂ and NiPr₂ via CH₂-linkers is described. Reacting the in situ lithiated ligand precursor N(C-Br)N^CH2-Et with [CrCl₃(THF)₃] resulted in the formation of the Cr(III) complex trans-[Cr(κ³NCN-NCN^CH2-Et)(Cl)₂(THF)]. Upon reaction of lithiated N(C-Br)N^CH2-iPr with a suspension of anhydrous CrCl₂, the Cr(II) complex [Cr(κ²NC-NCN^CH2-iPr)₂] is formed featuring two NCN ligands bound in κ²NC-fashion. In contrast, when lithiated N(C-Br)N^CH2-iPr is reacted with a homogeneous solution of anhydrous CrX₂ (X = Cl, Br), complexes [Cr(κ³NCN-NCN^CH2-iPr)X] are obtained. Treatment of [Cr(κ³NCN-NCN^CH2-iPr)Cl] with 1 equiv of PhCH₂MgCl and LiCH₂SiMe₃ afforded the alkyl complexes [Cr(κ³NCN-NCN^CH2-iPr)(CH₂Ph)] and [Cr(κ³NCN-NCN^CH2-iPr)(CH₂SiMe₃)]. All Cr(II) complexes exhibit effective magnetic moments in the range of 4.7-4.9 µ_B which is indicative for d⁴ high spin systems. If a solution of lithiated N(C-Br)N^CH2-iPr is treated with CrCl₂, followed by addition of an excess of Na[HB(Et)₃], the dimeric complex [Cr(κ²NC-NCN^CH2-iPr)(μ₂-H)]₂ is obtained bearing two bridging hydride ligands. [Cr(κ³NCN-NCN^CH2-iPr)(CH₂SiMe₃)] turned out to be catalytically active for the hydrosilylation of ketones at room temperature with a catalyst loading of 1 mol%. X-ray structures of all complexes are presented.

The online version contains supplementary material available at 10.1007/s00706-023-03128-6.