A collection of robust methodologies for the preparation of asymmetric hybrid Mn–Anderson polyoxometalates for multifunctional materials

Abstract

Here we report a suite of approaches for the isolation of asymmetrically grafted organic–inorganic hybrid Mn–Anderson polyoxometalate compounds (TBA)₃[MnMo₆O₁₈((OCH₂)₃CNHR₁)((OCH₂)₃CNHR₂)] (where TBA = tetrabutylammonium). Both a “pre-functionalization” route (for compound 1 – R₁ = –COC₁₄H₉, R₂ = –H) using two different TRIS-based ligands ((HOCH₂)₃CNHR), and a “post-functionalization” of the preformed TRIS Mn–Anderson compound (R₁ = R₂ = –H) were demonstrated. Compounds 2 (R₁ = –COC₁₅H₃₁, R₂ = –CO(CH₂)₂COOH) and 3 (R₁ = –COC₁₅H₃₁, R₂ = –H) are some of the first reported examples of asymmetric Mn–Anderson compounds to have been synthesized by the latter route. The reliable and broadly applicable chromatographic method used to isolate these compounds relies on the difference in affinity of compounds' organic moieties for reverse phase (RP) media; the target asymmetric cluster will have an intermediate affinity, between that of the two symmetric by-products. For instances where this is not the case, we have prepared and isolated a “universal” asymmetric Mn–Anderson precursor 4 (R₁ = –C(O)OC₁₄H₁₁, R₂ = –H), which can be used as a precursor to synthesize practically any asymmetric Mn–Anderson system. The use of 4 as an “universal” precursor was successfully demonstrated in the synthesis and isolation of compound 5 (R₁ = –COC₂H₅, R₂ = –H), which would not be accessible by a simple ‘one pot’ approach. In addition to removing a significant barrier to the exploitation of asymmetric Mn–Anderson clusters as new functional materials, the methods presented here should be applicable to a range of other hybrid organic–inorganic clusters.