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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

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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 [L n CdBr2]2 (L n  = L1 , L2 , L3 ) or monomeric tetrahedral [L5CdBr2]. Copyright © 2014 John Wiley & Sons, Ltd. The crystal structure and methyl methacrylate (MMA) polymerization of dimeric or monomeric Cd(II) complexes with N,N‐bis(1‐pyrazolyl)methyl‐based ligands were investigated.
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Keywords: bispyrazolyl; dimeric Cd(II) complex; methyl methacrylate polymerization; syndiotacticity

Document Type: Research Article

Publication date: June 1, 2014

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