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

Abstract

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 NiPr2via CH2-linkers is described. Reacting the in situ lithiated ligand precursor N(C-Br)NCH2-Et (1) with [CrCl3(THF)3] resulted in the formation of the Cr(III) complex trans-[Cr(κ3NCN-NCNCH2-Et)(Cl)2(THF)] (3). Upon reaction of lithiated N(C-Br)NCH2-iPr (2) with a suspension of anhydrous CrCl2, the Cr(II) complex [Cr(κ2NC-NCNCH2-iPr)2] (4) is formed featuring two NCN ligands bound in κ2NC-fashion. In contrast, when lithiated 2 is reacted with a homogeneous solution of anhydrous CrX2 (X = Cl, Br), complexes [Cr(κ3NCN-NCNCH2-iPr)X] (5a,b) are obtained. Treatment of 5a with 1 equiv of PhCH2MgCl and LiCH2SiMe3 afforded the alkyl complexes [Cr(κ3NCN-NCNCH2-iPr)(CH2Ph)] (6) and [Cr(κ3NCN-NCNCH2-iPr)(CH2SiMe3)] (7). All Cr(II) complexes exhibit effective magnetic moments in the range of 4.7–4.9 µB which is indicative for d4 high spin systems. If a solution of lithiated 2 is treated with CrCl2, followed by addition of an excess of Na[HB(Et)3], the dimeric complex [Cr(κ2NC-NCNCH2-iPr)(µ2-H)]2 (8) is obtained bearing two bridging hydride ligands. [Cr(κ3NCN-NCNCH2-iPr)(CH2SiMe3)] (7) 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.