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 NEt(2) and NiPr(2) via CH(2)-linkers is described. Reacting the in situ lithiated ligand precursor N(C–Br)N(CH2)-Et with [CrCl(3)(THF)(3)] resulted in the formation of the Cr(III) complex trans-[Cr(κ(3)NCN-NCN(CH2)-Et)(Cl)(2)(THF)]. Upon reaction of lithiated N(C–Br)N(CH2)-iPr with a suspension of anhydrous CrCl(2), the Cr(II) complex [Cr(κ(2)NC-NCN(CH2)-iPr)(2)] is formed featuring two NCN ligands bound in κ(2)NC-fashion. In contrast, when lithiated N(C–Br)N(CH2)-iPr is reacted with a homogeneous solution of anhydrous CrX(2) (X = Cl, Br), complexes [Cr(κ(3)NCN-NCN(CH2)-iPr)X] are obtained. Treatment of [Cr(κ(3)NCN-NCN(CH2)-iPr)Cl] with 1 equiv of PhCH(2)MgCl and LiCH(2)SiMe(3) afforded the alkyl complexes [Cr(κ(3)NCN-NCN(CH2)-iPr)(CH(2)Ph)] and [Cr(κ(3)NCN-NCN(CH2)-iPr)(CH(2)SiMe(3))]. All Cr(II) complexes exhibit effective magnetic moments in the range of 4.7–4.9 µ(B) which is indicative for d(4) high spin systems. If a solution of lithiated N(C–Br)N(CH2)-iPr is treated with CrCl(2), followed by addition of an excess of Na[HB(Et)(3)], the dimeric complex [Cr(κ(2)NC-NCN(CH2)-iPr)(μ(2)-H)](2) is obtained bearing two bridging hydride ligands. [Cr(κ(3)NCN-NCN(CH2)-iPr)(CH(2)SiMe(3))] 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. GRAPHICAL ABSTRACT: [Image: see text] SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00706-023-03128-6.