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

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.