Cadmium(II) complexes containing N′-substituted N,N-di(2-picolyl)amine: The formation of monomeric versus dimeric complexes is affected by the N′-substitution group on the amine moiety

Abstract

The reaction of [CdBr2.4H2O] with N′-substituted N,N-di(2-picolyl)amine ligands such as 1-cyclohexyl-N,N-bis(pyridin-2-ylmethyl)methanamine (LA), N,N-bis(pyridin-2-ylmethyl)aniline (LB), 2-methoxy-N,N-bis(pyridin-2-ylmethyl)ethan-1-amine (LC) and 3-methoxy-N,N-bis(pyridin-2-ylmethyl)propan-1-amine (LD) in ethanol yielded cadmium(II) [Cd(II)] complexes, i.e. [LnCdBr2]m (Ln = LA, m = 2; LB, m = 1; LC, m = 1; LD, m = 2, respectively). The molecular structures of Cd(II) complexes revealed that 6-coordinate dimeric [LACd(μ−Br)Br]2 and [LDCd(μ−Br)Br]2 showed a distorted octahedral geometry through coordination of nitrogen in the N′-substituted amine moiety (cyclohexylmethylamine and 3-methoxypropylamine, respectively) to the Cd metal centre and a bromide bridge ligand. However, the 5-coordinate monomeric [LBCdBr2] was square pyramidal through the coordination of nitrogen in the N′-phenyl-substituted amine residue to the Cd metal centre. The molecular structure of 6-coordinate monomeric [LCCdBr2] had a distorted octahedral geometry achieved through bidentate coordination of nitrogen and oxygen in the N′-methoxyethylamine group to the Cd metal centre. However, for dimeric [LDCd(μ−Br)Br]2, although LD has a bidentate coordination ability in the N′-methoxypropylamine group, it generated a distorted octahedral geometry through coordination of nitrogen and non-coordination of oxygen in the N′-methoxyethylamine moiety to the Cd metal centre. Monomeric complex [LBCdBr2] [6.70 × 104 gPMMA·(molCd·h)−1] showed higher catalytic activity for the polymerisation of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) than the rest of Cd(II) complexes in this study. The catalytic activity of dimeric complex [LACd(μ−Br)Br]2 [7.00 × 104 gPMMA·(molCd·h)−1] and [LDCd(μ−Br)Br]2 [7.07 × 104 gPMMA·(molCd·h)−1] increased by twofold due to the presence of two Cd metal centres compared to the corresponding reference complex [CdBr2.4H2O] [2.85 × 104 gPMMA·(molCd·h)−1] or anhydrous [CdCl2] [3.53 × 104 gPMMA·(molCd·h)−1]. All Cd(II) complexes resulted in syndiotactic poly(methylmethacrylate) (PMMA) with ca. 0.70.