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

Matthias G Käfer; Wolfgang Eder; Jan Pecak; Berthold Stöger; Marc Pignitter; Luis F Veiros; Karl Kirchner

doi:10.1007/s00706-023-03128-6

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

Monatsh Chem. 2023;154(11):1263-1273. doi: 10.1007/s00706-023-03128-6. Epub 2023 Oct 10.

Authors

Matthias G Käfer¹, Wolfgang Eder¹, Jan Pecak¹, Berthold Stöger², Marc Pignitter³, Luis F Veiros⁴, Karl Kirchner¹

Affiliations

¹ Institute of Applied Synthetic Chemistry, TU Wien, Getreidemarkt 9/163-AC, 1060 Vienna, Austria.
² X-Ray Center, TU Wien, Getreidemarkt 9/163-AC, 1060 Vienna, Austria.
³ Department of Physiological Chemistry, Faculty of Chemistry, University of Vienna, Althanstraße 14, 1090 Vienna, Austria.
⁴ Centro de Química Estrutural, Institute of Molecular Sciences, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049 001 Lisbon, Portugal.

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 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.

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

Keywords: Chromium; Hydrosilylation; Ketones; Pincer complexes; Silanes.