Synthesis, structures and reactivity of cobalt(II) complexes supported by N,N,N′,N″-tetradentate N′-substituted bis((1H-pyrazol-1-yl)methyl)amine

A series of cobalt(II) complexes, namely [L_nCo(NO₃)]⁺Y⁻ (L_n = L_A–L_D; Y = NO₃⁻, BPh₄⁻) was prepared by the reaction of the corresponding metal starting materials with N,N-bis((1H-pyrazol-1-yl)methyl)-2-(piperidin-1-yl)ethanamine (L_A), N,N-bis((1H-pyrazol-1-yl)methyl)-2-morpholinoethanamine (L_B), N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-2-(piperidin-1-yl)ethanamine(L_C) and N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-2-morpholinoethanamine (L_D), respectively. X-ray diffraction studies revealed that the coordination geometry around the Co(II) centre in [L_nCo(NO₃)]BPh₄ (L_n = L_A, L_B and L_D) can be best described as distorted trigonal bipyramidal, whereas [L_CCo(NO₃)]NO₃ showed a distorted octahedral geometry around the Co(II) centre due to the bidentate binding mode of the ligand NO₃⁻. Specifically, the presence of methyl substituents at the pyrazole moieties plays an influential role in determining the anionic exchange capabilities of these complexes. The anion exchange reaction was found to be highly selective; the substitution of the smaller anion NO₃⁻ to the bigger anion BPh₄⁻ was achieved easily, whereas the attempted anion exchange of BPh₄⁻ with NO₃⁻ proved unsuccessful.