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