Preparation and Crystal Structures of Formato Complexes of the [M^IV₃O₄]⁴⁺ and [M^IV₃S₄]⁴⁺ (M = Mo, W) Clusters. Convenient Precursors to the Corresponding Aqua Complexes

In the aqueous chemistry of molybdenum(IV) and tungsten(IV), trinuclear, incomplete cubane-like, oxo and sulfido clusters of the type [M3E4]4+ (M = Mo, W; E = O, S) play a central role. We here describe how formato complexes of all these cluster cores can be prepared in high yields by crystallization from methanol−water or ethanol−water mixtures. Since potassium and ammonium formate are very soluble in these alcohol−water mixtures, high formate concentrations could be accomplished in the solutions from which the corresponding salts of cluster formato complexes crystallized. The [Mo3O4]4+ compounds could be synthesized without requiring the use of noncomplexing acids in the process. Some [M3E4]4+ compounds were characterized by single-crystal structure determinations. [NH4]3.20[K]0.80[H3O][Mo3O4(HCO2)8][HCO2]·H2O was triclinic, space group P1̄ (No. 2) with a = 11.011(2) Å, b = 13.310(2) Å, c = 9.993(1) Å, α = 106.817(7)°, β = 91.651(9)°, γ = 88.340(9)°, and two formula units per cell. [K]6[W3S4(HCO2)9][HCO2]·2.27H2O·0.73CH3OH was monoclinic, space group C2/m (No. 12) with a = 19.605(6) Å, b = 14.458(7) Å, c = 13.627(5) Å, β = 118.94(2)°, and four formula units per cell. Generally, the nine coordination sites of [M3E4]4+ were occupied either by a mixture of monodentate and μ2-bridging formato ligands or by monodentate formato ligands only. By dissolution in noncomplexing strong acid, all the formato complexes immediately hydrolyzed to form [M3E4(H2O)9]4+ aqua complexes. This allows, for example, high concentrations of [Mo3S4(H2O)9]4+ in CF3SO3H to be obtained and these solutions to be used for the synthesis of bimetallic clusters containing the cubane-like motif Mo3M‘S4.