Mapping the synthesis of low nuclearity polyoxometalates from octamolybdates to Mn-Anderson clusters

Abstract

A comprehensive study of the isomer-independent synthesis of TRIS ((HOCH₂)₃CNH₂) Mn-Anderson compounds from Na₂MoO₄·2H₂O, via the corresponding octamolybdate species, is presented. Three octamolybdate salts of [Mo₈O₂₆]⁴⁻ in the β-isomer form, with tetramethylammonium (TMA), tetraethylammonium (TEA) and tetrapropylammonium (TPA) as the counter cation, were synthesised from the sodium molybdate starting material. Fine white powdery products for the three compounds were obtained, which were fully characterised by elemental analysis, TGA, solution and solid state Raman, IR and ESI-MS, revealing a set ratio of Na and organic cations for each of the three compounds; (TMA)₂Na₂[Mo₈O₂₆] (1), (TEA)₃Na₁[Mo₈O₂₆] (2) and (TPA)₂Na₂[Mo₈O₂₆] (3), and the analyses also confirmed that the three compounds all consisted of the octamolybdate in the β-isomeric form. ESI-MS analyses of 1, 2 and 3 show similar fragmentation for these β-isomers compared to the previously reported study for the α-isomer ((TBA)₄[α-Mo₈O₂₆]) (A) in the synthesis of ((TBA)₃[MnMo₆O₁₈((OCH₂)₃CNH₂)₂]) (B), and compounds 1, 2 and 3 were successfully used to synthesise equivalent TRIS Mn-Anderson compounds: (TMA)₃[MnMo₆O₁₈((OCH₂)₃CNH₂)₂] (4), (TEA)₃[MnMo₆O₁₈((OCH₂)₃CNH₂)₂] (5) and (TPA)₂Na₁[MnMo₆O₁₈((OCH₂)₃CNH₂)₂] (6), as well as Na₃[MnMo₆O₁₈((OCH₂)₃CNH₂)₂] (7). This is the first example where symmetric organically-grafted Mn-Anderson compounds have been synthesised in DMF from anything but the {Mo₈O₂₆} α-isomer.