Coordination chemistry of 4-aminopyrazole: Structure and physicochemical properties of cobalt(II) chlorido complexes, and amino group reactivity towards a ketone to yield an imine bond

The in situ reaction of the terminal amine groups in the mononuclear cobalt(II) complex with acetone leads to imine bond formation and the construction of a one-dimensional coordination polymer. [Display omitted] •A methodology utilised to form COFs was successfully employed to construct a MOF.•A cobalt(II) chlorido complex ligated by pyrazole with an amine substituent was characterised.•The in situ construction of a coordination polymer after reaction with acetone was demonstrated.•The coordinated pyrazole amine was easily reacted with a carbonyl group to form an imine bond. An outline of the in situ construction of a coordination polymer by reacting a mononuclear cobalt(II) complex, functionalised with an amine substituent, with acetone is given. The methodology involved 4-amino-3,5-diisopropyl-1-pyrazole (denoted as L1pzHNH2) as the precursor, putative linker molecule. Reaction of the ligand with anhydrous cobalt(II) chloride, yielded a four-coordinated cobalt(II) chlorido complex, [CoCl2(L1pzHNH2)2]. When the complex was reacted with acetone, a four-coordinated cobalt(II) chlorido coordination polymer, [CoCl2{L1pzHN=C(CH3)2}]n, with an imine bond, was obtained. The complexes were characterised by various spectroscopic techniques. X-ray crystallography shows [CoCl2(L1pzHNH2)2]·2(thf) to comprise a tetrahedrally coordinated cobalt(II) centre with the donors being two chlorido and two pyrazolyl-N atoms. Similarly, [CoCl2{L1pzHN=C(CH3)2}]n features a Cl2N2 donor set defining an approximately tetrahedral geometry with the imine-N donors being derived from a pyrazolyl ring and a symmetry related exocyclic imine-N atom with the result that a one-dimensional coordination polymer with a helical topology is formed. When the uncoordinated precursor molecule, L1pzHNH2, was reacted with acetone, the formation of the imine was confirmed by 1H NMR measured in (CD3)2CO solution. However, from the 1H NMR measured for the imine in CDCl3 solution, a 70:30 equilibrium between the imine and amine was established. It is concluded that the nucleophilicity of the amine group is enhanced by the binding of cobalt(II) to the pyrazole nitrogen in [CoCl2(L1pzHNH2)2]·2(thf), thereby favouring irreversible imine formation. The facile formation of [CoCl2{L1pzHN=C(CH3)2}]n in the solid-state also correlates with the reduced solubility of the coordination polymer structure in organic solvents.