New zirconium complexes supported by N-heterocyclic carbene (NHC) ligands: Synthesis and assessment of hydroamination catalytic properties

•[H3L2]Br crystalizes in the S shape conformation.•Zirconium complexes 7 and 8 exhibit a fluxional process in solution at room temperature.•THF dissociation promotes interconversion between the two S-shape twisted forms of the ligand.•7 and 8 react with 2,2-diphenylpent-4-en-1-amine in a 1:1 ratio but not under catalytic conditions.

Reactions of 2-(bromomethyl)-4,6-di-tert-butylphenol or 2-(bromomethyl)-4,6-bis(2-phenylpropan-2-yl)phenol with 1H-imidazole led to syntheses of N,N-disubstituted imidazolium bromides presenting two methylene-bis(2,4-di-tert-butylphenol) ([H2L1]Br) or methylene-bis(2,4-di-tert-butylphenol) ([H2L2]Br) appended groups. Treatment of Zr(NMe2)4 with [H2L1]Br or [H2L2]Br gave tethered NHC zirconium complexes of general formula [ZrL(NMe2)(THF)Br] (L = L1, 7; L2, 8). The bonding of the tridentate ligands to the metal adopts S-shape conformation. In solution a fluxional process between the left- and right-handed forms of the ligand is observed for both complexes. In agreement with the NMR spectra, the optimised structure obtained by DFT revealed octahedral geometry around zirconium with mutually trans Br and Ccarbene donors. Complexes 7 and 8 react with 2,2-diphenylpent-4-en-1-amine in a 1:1 ratio to give the hydroamination product 2-methyl-4,4-diphenylpyrrolidine. In catalytic conditions, the systems deactivate and catalytic conversions are not observed.

Graphical abstractN,N-Disubstituted imidazolium bromides and tethered NHC zirconium derivatives of general formula [ZrL(NMe2)(THF)Br] (7, 8) are described. In solution a fluxional process involving ligand twist is observed for both complexes. 7 and 8 react with 2,2-diphenylpent-4-en-1-amine in a 1:1 ratio to give 2-methyl-4,4-diphenylpyrrolidine but the systems deactivate in catalytic conditions. Figure optionsDownload full-size imageDownload as PowerPoint slide