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

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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 N i Pr 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 - i Pr with a suspension of anhydrous CrCl 2 , the Cr(II) complex [Cr(κ 2 NC -NCN CH2 - i Pr) 2 ] is formed featuring two NCN ligands bound in κ 2 NC -fashion. In contrast, when lithiated N(C–Br)N CH2 - i Pr is reacted with a homogeneous solution of anhydrous Cr X 2 ( X = Cl, Br), complexes [Cr(κ 3 NCN- NCN CH2 - i Pr)X] are obtained. Treatment of [Cr(κ 3 NCN- NCN CH2 - i Pr)Cl] with 1 equiv of PhCH 2 MgCl and LiCH 2 SiMe 3 afforded the alkyl complexes [Cr(κ 3 NCN -NCN CH2 - i Pr)(CH 2 Ph)] and [Cr(κ 3 NCN -NCN CH2 - i Pr)(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 - i Pr 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 - i Pr)(μ 2 -H)] 2 is obtained bearing two bridging hydride ligands. [Cr(κ 3 NCN -NCN CH2 - i Pr)(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.