Cobalt(II) complexes containing N′-substituted N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine ligands: The formation of four-coordinate or five-coordinate complexes as a function of the N′-substitution group in N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine

•Co(II) complexes with N’-substituted N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine ligands are synthesized.•The Co(II) complexes showed 4- or 5-coordinated geometries around cobalt centre.•The Co(II) complexes proved to be active in MMA polymerization.

N,N-bis((1H-pyrazol-1-yl)methyl)(cyclohexyl)methanamine (LA), N,N-bis((1H-pyrazol-1-yl)methyl)-2,6-diethylbenzenamine (LB), N,N-bis((1H-pyrazol-1-yl)methyl)-4-bromobenzenamine (LC), N,N-bis((1H-pyrazol-1-yl)methyl)cyclohexanamine (LD), N,N-bis((1H-pyrazol-1-yl)methyl)(furan-2-yl)methanamine (LE), N,N-bis((1H-pyrazol-1-yl)methyl)-2-methoxyethanamine (LF), and N,N-bis((1H-pyrazol-1-yl)methyl)-3-(methylthio)propan-1-amine (LG) with [CoCl2·6H2O] in ethanol yield a novel series of Co(II) chloride complexes; i.e. [LnCoCl2] (Ln = LA–LG), respectively. The molecular structures of [LnCoCl2] (Ln = LA–LC) and [LnCoCl2] (Ln = LD–LG) exist as monomeric four-coordinated and five-coordinated complexes, respectively. [LnCoCl2] (Ln = LA–LC) showed a distorted tetrahedral geometry involving non-coordination of the nitrogen atom of the N′-substitution amine moiety and the Co(II) centre, resulting in the formation of an eight-membered chelate ring. The geometry at each Co(II) centre in [LnCoCl2] (Ln = LD–LG) were best described as a distorted trigonal bipyramid involving coordination of the nitrogen atom of the N′-substitution amine group and the Co(II) centre. Specifically, a distorted trigonal bipyramid was achieved in [LDCoCl2] through a coordinative interaction of the nitrogen atom of the cyclohexylamine unit and the Co(II) atom with an σ plane of symmetry based on the bond length of Co–Ncyclcohexylamine [2.466(4) Å]. Moreover, [LDCoCl2] showed the highest activity for methyl methacrylate (MMA) polymerization in the presence of modified methylaluminoxane (MMAO) at 60 °C and yielded poly(methylmethacrylate) (PMMA) with a high molecular weight (Mw) and narrower polydispersity index (PDI) compared to the other Co(II) complexes. However, all Co(II) complexes produced syndiotactic (PMMA), characterised using 1H NMR spectroscopy, with ca. 0.70.

Graphical abstractA new series of N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine supported Co(II) complexes have been synthesized and structurally characterized by X-ray diffraction studies. These Co(II) complexes exhibited diverse coordination geometries around the metal centre i.e. monomeric 4- or 5-coordinated structures determined by the steric encumbrance of the N′-substitution amine moiety. The catalytic activities of Co(II) complexes for methyl methacrylate polymerisation are investigated.Figure optionsDownload full-size imageDownload as PowerPoint slide