Oxidopolyborate anions templated by transition-metal complex cations: Self-assembled syntheses and structural studies (XRD) of [Co(NH3)6]2[B4O5(OH)4]3·11H2O, [Ni(phen)3][B7O9(OH)5].9.5H2O and [Zn(dac)2(H2O)2][B7O9(OH)5]·H2O

Abstract

The new oxidopolyborate salts [Co(NH3)6]2[B4O5(OH)4]3·11H2O (1), [Ni(phen)3][B7O9(OH)5].9.5H2O (2), and [Zn(dac)2(H2O)2][B7O9(OH)5]·H2O (3), were crystallized, in moderate yields, from aqueous solutions containing B(OH)3 and the hydroxide salt of the corresponding cationic metal complex (prepared in situ) in moderate yields. Complexes 1–3, characterized by spectroscopic (NMR, IR) and thermal (TGA) methods and by single-crystal XRD studies, were obtained through cation-templated self-assembly reactions. Compound 1 contains insular tetraborate(2-) anions (4-1:2Δ + 2T) and 2 and 3 both contain insular heptaborate(2-) anions (‘O+’ isomer, 6:(3Δ + 3T) + Δ). The X-ray structures show that in all three compounds the polyborate anions are H-bonded together into extended networks, and there are multiple cation-anion H-bond interactions present in 1 and 3. Compound 2 has cation-cation π-stacking interactions. These interactions are likely to be responsible for templating the observed structures. Thermal decomposition of 1–3 in air yielded amorphous anhydrous borates of composition CoB6O10.5, NiB7O11.5 and ZnB7O11.5, respectively.