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

Hydrated zinc(II) acetate reacts with pyrazole (Hpz) and, depending on the reaction conditions, forms different pyrazole-containing species, i.e. [[Zn(CH3COO)(mu-pz)(Hpz)]2] (1), [[Zn(CH3COO)2(Hpz)2.CH3COOH]] (2), [[Zn(mu-pz)2]n] (3), and [[Zn(mu-CH3COO)(mu-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 angstroms. The existence of the [Zn(mu-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 angstroms). 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.