Coordination chemistry in the solid state: synthesis and interconversion of pyrazolium salts, pyrazole complexes, and pyrazolate MOFs

Abstract

Solid pyrazole reacts with HCl gas to form pyrazolium chloride [H₂pz]Cl, which reacts in the solid state, under grinding, with metal chlorides MCl₂ (M = Co, Zn, Cu) to form the pyrazolium tetrachlorometallate salts [H₂pz]₂[MCl₄] (M = Co 1, Zn 3, Cu 5). Salt 5 cannot be made in solution, and upon standing at room temperature spontaneously emits HCl to give the coordination compound [CuCl₂(Hpz)₂] (6). Compounds 1 and 3 do not exhibit this behaviour, but can be ground together with bases such as KOH or K₂CO₃ to effect the elimination of HCl and afford their respective [MCl₂(Hpz)₂] compounds (M = Co 2, Zn 4). 2, 4 and 6 can also be synthesised in the solid-state by direct reaction of the appropriate metal chloride with pyrazole, or by reaction of a basic metal salt such as the carbonate or hydroxide with pyrazolium chloride. 4 and 6 {and their nickel analogue [NiCl₂(Hpz)₂]} can be ground with a further two equivalents of base to make the known polymeric metal pyrazolates [M(pz)₂]_n (M = Ni 7, Cu 8, Zn 9); the same reaction appears to work for the cobalt analogue 2, but the presumed product [Co(pz)₂]_n10 then decomposes by oxidation. The imidazolate complexes [M(im)₂] (M = Ni, 11; Cu, 12; Zn, 13; Co, 14) were similarly prepared by grinding the appropriate [M(Him)₂Cl₂] precursor with KOH.