The competition between acetate and pyrazolate in the formation of polynuclear Zn(ii) coordination complexes

Abstract

Hydrated zinc(II) acetate reacts with pyrazole (Hpz) and, depending on the reaction conditions, forms different pyrazole-containing species, i.e. [{Zn(CH₃COO)(µ-pz)(Hpz)}₂] (1), [{Zn(CH₃COO)₂(Hpz)₂·CH₃COOH}] (2), [{Zn(µ-pz)₂}_n] (3), and [{Zn(µ-CH₃COO)(µ-pz)}_n] (4). Their structural models have been derived from single-crystal X-ray diffractometry as well as from less conventional ab-initio X-ray powder methods. All species contain tetrahedrally coordinated Zn(II) ions, with Zn–N and Zn–O bond distances close to 2.0 Å. The existence of the {Zn(µ-pz)}₂ core in the species 1, 3 and 4 indicates the propensity for the formation, in the presence of pyrazolate ligands, of well-defined dinuclear entities (with Zn⋯Zn contacts in the range 3.6–3.8 Å). The latter can mutually interact, in the crystals, through either hydrogen-bonding of ancillary ligands (as in 1) or coordinative bonds (via acetates, as in 4, or by self-complementarity, as in 3). The interconversion paths among these species have been studied, employing chemical and thermal methods. In particular, the topotactic and quantitative transformation of 1 into 3 by moderate heating is likely based on a solid-state cooperative condensation mechanism of the dangling pyrazolates toward neighbouring zinc(II) ions, with concomitant acetic acid extrusion.