Iso- and hetero-polyoxovanadium glycolates [V IV6O6(glyc)6MIII(OH)6]3− (M = V, Cr, Fe, and Al) encapsulating metal hydroxides

Abstract

Polyoxovanadium glycolates (NH₄)₃[V₆O₆(glyc)₆V(μ-OH)₆]·10H₂O (1-V₇) (H₂glyc = glycolic acid) and its derivatives added with ammonium sulfates (NH₄)₃[V₆O₆(glyc)₆M(μ-OH)₆][(NH₄)₂SO₄]₂·xH₂O (M = Cr, x = 6, 2-CrV₆; M = Fe, x = 7, 3-FeV₆; M = Al, x = 6, 4-AlV₆) were obtained through self-assembly and fully characterized. Compounds 1–4 are composed of the same fully reduced cyclic {V^IV₆O₆} unit bridged by six glycolate ligands. The framework encapsulated an octahedral metal(III) hydroxide {M(OH)₆} (M = V³⁺, Cr³⁺, Fe³⁺, and Al³⁺) in the center, forming an iso- or hetero-heptanuclear Anderson-type structure. Moreover, two sulfate anions capped the two sides of cyclic {V₆O₆} units in compounds 2–4. Each sulfate formed a strong triple hydrogen bond (2.657–2.829 Å) with the μ₃-OH group of the hydroxide. The sulfate anions played important roles in the formation of layered structures in 2–4, while the clusters were stacked compactly through strong hydrogen bonds in 1-V₇. Because of the different central metals, these heptanuclear clusters exhibited distinct electronic structures as well as redox and magnetic properties. Magnetic studies showed that compounds 1–3 exhibited weak antiferromagnetic interactions in decreasing order with Fe³⁺, V³⁺ and Cr³⁺, while 4-AlV₆ displayed weak ferromagnetic interactions. Their relationships with the local environment of the FeV-cofactor in V-nitrogenase are also discussed.