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.

A 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 as PowerPoint slide