Synthesis and structural characterization of 5-coordinate cobalt(II), copper(II) and 4-coordinate zinc(II) complexes containing N′-cyclopentyl substituted N,N-bispyrazolylmethylamine

Novel [L_nMCl₂] complexes (L_n = L_A, L_B; M = Co, Cu, Zn) containing the N,N-chelating ligands N,N-bis((1H-pyrazol-1-yl)methyl)cyclopentanamine (L_A) and N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)cyclopentanamine (L_B) have been synthesized and characterized. The X-ray crystal structures of the Co(II) and Cu(II) complexes revealed that the ligands are coordinated in an N,N,N-tridentate manner, resulting in distorted trigonal bipyramidal geometries through the coordinative interaction of the nitrogen atom of the cyclopentylamine moiety and the metal centers. [L_AZnCl₂] showed a distorted tetrahedral geometry involving a non-coordinative interaction of the N_amine atom and the Zn(II) center, resulting in the formation of an eight membered chelate ring. [L_ACoCl₂] exhibited the highest catalytic activity (7.67 × 10⁴ g PMMA/mol Co h) for the polymerization of MMA in the presence of MMAO at 60 °C and yielded PMMA with a high molecular weight (M_w) (11.2 × 10⁵ g/mol) and narrower polydispersity index (PDI) 1.01 compared to its Cu(II) and Zn(II) analogs. Syndiotactic enriched PMMA was obtained with T_g in the range 120-132 °C. The introduction of bulky methyl substituents played an influential role on controlling the catalytic activities while the syndiotacticities remained unaffected.