Manganous heteropolytungstates. Synthesis and heteroatom effects in Wells–Dawson-derived sandwich complexes

Abstract

A tetranuclear manganous Wells–Dawson sandwich-type polyoxometalate has been synthesized by the reaction of α-Na₁₂(As₂W₁₅O₅₆) with an aqueous solution of MnCl₂·4H₂O. The structure of this complex, αββα-Na₁₆(Mn^IIOH₂)₂Mn^II₂(As₂W₁₅O₅₆)₂·55H₂O (Na1), was determined by single-crystal X-ray crystallography (a = 14.5230(12) Å, b = 14.7104(13) Å, c = 19.8927(17) Å, α = 84.326(2)°, β = 81.709(2)°, γ = 65.584(2)°, triclinic, R₁ = 6.2%, based on 26721 independent reflections) and is similar to the phosphorus analogue, αββα-Na₁₆(Mn^IIOH₂)₂Mn^II₂(P₂W₁₅O₅₆)₂ (Na2). Magnetization studies confirm that both Na1 and Na2 show antiferromagnetic coupling of the four Mn(II) centers. Despite the structural similarities, electrochemical studies reveal that the presence of arsenic shifts the Mn waves to more positive potentials. Catalytic studies confirm that 1 is a significantly better catalyst than 2 for the H₂O₂-based epoxidation of cis-cyclooctene, cyclohexene, and 1-hexene.