Structural diversity of ethylzinc derivatives of 3,5-substituted pyrazoles

Abstract

Equimolar reactions of Et₂Zn with 3,5-dimethylpyrazole (H-pz^Me2), 3,5-di-iso-propylpyrazole (H-pz^iPr2), 3,5-di-tert-butylpyrazole (H-pz^tBu2) and indazole (H-ind) were investigated in toluene or tetrahydrofuran (as a coordinating solvent). A series of diverse ethylzinc pyrazolates and indazolates were identified using advanced NMR spectroscopy and the single crystal X-ray diffraction techniques. The NMR experiments indicate that dimeric moieties of the general formula [EtZn(pz)]₂ or [Et₂Zn₂(pz)₂(THF)] are favoured in solution. Nevertheless, these types of complexes are kinetically labile and tend to undergo ligand scrambling reactions according to the Schlenk equilibrium. For example, the alkyl substituents in the pz^Me2 and pz^iPr2 ligands do not appear to be a strong determinant of the dimeric moieties and the composition of the isolated complexes by crystallisation from the parent reaction mixture varies between spiro-type tri- and tetranuclear aggregates, [Et₂Zn₃(pz)₄(THF)_x] (x = 0 or 2) and [Et₂Zn₄(pz)₆(THF)₂], respectively. The nonstoichiometric formula of these organozinc derivatives is likely related to both the Schlenk-type equilibria and solubility of the respective moieties. In turn, the high steric demands of the 3,5-di-tert-butylpyrazolate ligand promote the dimeric form in solution and the solid state. Interestingly, the ethylzinc indazolate complex also does not undergo a redistribution reaction and yields a dimer.