Supramolecular assembly of oxalatomolybdates controlled by the hydrogen bonding potential of Co(iii)-ammine cations

Abstract

A systematic study of the possibility of controlling the formation of oxalatomolybdate anions by the choice of a counterion and adjustment of pH was presented. Using four Co^III(ammine) macrocations ([Co(NH₃)₆]³⁺, [Co(NO₂)(NH₃)₅]²⁺, [Co(en)₃]³⁺ and [Co(ox)(en)₂]⁺) in acidic aqueous solutions, ten crystalline solids were obtained. They were found to contain a total of six different oxalatomolybdate species, three of which, a cyclic tetramer, [Mo₂O₄(μ-O)₂(ox)₂]₂⁸⁻, a linear tetramer [Mo₄O₈(μ-O)₃(ox)₄(H₂O)₂]⁶⁻ and a polymer, [Mo₄O₁₁(ox)₄(H₂O)]_n^6n– have not been reported to date. The oxalatomolybdate species found in solids mostly do correspond with the predominant species in solution at the pH of the solution from which they were obtained. Condensation of those species can be correlated with the hydrogen bonding ability of the used macrocation, a complex cation of Co(III).