Anion mediated switching from mono- to polymer structure in copper(II) complexes with 4,6-dimethylpyrimidinylhydrazone 1-phenyl-3-methyl-4-formylpyrazol-5-one

[Display omitted] •Cu(II) Complexes were synthesized based on hetarylhydrazone pyrazol-5-one derivative.•The structure of the compounds were studied by experimental and theoretical methods.•Mono- or polymer structure of complexes Cu(II) is dependent on salt anion. The novel hetarylhydrazone (H2L) has been synthesized by condensation of 2-hydrazino-4,6-dimethylpyrimidine with 1-phenyl-3-methyl-4-formylpyrazol-5-one and its complexes with CuCl2 and Cu(ClO4)2 have been obtained. The structures of the compounds were studied by means of NMR, IR, ESR spectroscopy, magnetic susceptibility measurements and X-Ray diffraction methods. The quantum-chemical modelling of the geometry and electronic structure of the hetarylhydrazone’s tautomers has been carried out. The influence of the nature of acido-ligand on the structure of complex compounds has been established. Copper(II) chloride gives mono-nuclear complex [Cu(H2L)Cl2].H2O, while copper(II) perchlorate yields the polymeric structure ([Cu(HL)(CH3OH)]ClO4)n. For polymer compound no magnetic exchange interactions were detected.