X‐ray crystal structures and MMA polymerization of cadmium(II) complexes with bidentate pyrazole ligands: the formation of monomers or dimers as a function of a methyl substituent on the pyrazole and aniline rings

Abstract

The reaction of CdBr2·4H2O with ancillary ligands, N,N‐bis(1H‐pyrazolyl‐1‐methyl)aniline (L1), N,N‐bis(1H‐pyrazolyl‐1‐methyl)‐p‐methylaniline (L2), N,N‐bis(1H‐pyrazolyl‐1‐methyl)‐3,5‐dimethylaniline (L3), N,N‐bis(3,5‐dimethyl‐1H‐pyrazolyl‐1‐methyl)aniline (L4) and N,N‐bis(1H‐pyrazolyl‐1‐methyl)‐2,6‐dimethylaniline (L5) in ethanol yields novel Cd(II) bromide complexes, [L1CdBr2]2, [L2CdBr2]2, [L3CdBr2]2, [L4CdBr2] and [L5CdBr2]. The X‐ray crystal structures of [L1CdBr2]2, [L2CdBr2]2 and [L3CdBr2]2 reveal a bromo‐bridged dimeric species with crystallographic inversion symmetry. Conversely, [L4CdBr2] and [L5CdBr2] exist as monomeric complexes, presumably due to the steric hindrance between the methyl substituents of the two pyrazole groups in the ligand and cadmium centre for [L4CdBr2], and crowding around the cadmium metal by methyl substituents on the aniline residue in the ligand for [L5CdBr2]. The geometry at each Cd(II) centre for [L1CdBr2]2, [L2CdBr2]2 and [L3CdBr2]2 is best described as a distorted trigonal bipyramid. A distorted trigonal bipyramid is achieved in [L4CdBr2] by coordinative interaction of the nitrogen atom of the aniline unit and the cadmium atom with a σ plane of symmetry, based on the bond length of Cd―Naniline (2.759(7) Å). [L5CdBr2] exists with a distorted tetrahedral geometry involving non‐coordination of the nitrogen atom of aniline and the Cd centre, resulting in the formation of an eight‐membered chelate ring. The catalytic activity of monomeric, five‐coordinated [L4CdBr2] in the polymerization of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) at 60°C resulted in a higher molecular weight and a narrower polydispersity index (PDI) than those obtained with dimeric [LnCdBr2]2 (Ln = L1, L2, L3) or monomeric tetrahedral [L5CdBr2]. Copyright © 2014 John Wiley & Sons, Ltd.