Template-directed assembly of polyoxothiometalate scaffolds into nanomolecular architectures.

Polyoxometalate clusters (POMs) represent some of the largest non-biologically derived molecules so far synthesized as they occupy the 1–6 nm size range and exhibit a diverse variety of structures, assembled from a large “virtual combinatorial library” of building blocks. Whilst many tungsten POM clusters can be built in a controlled way, the designed assembly of giant molybdates is more difficult due to the inherent lability of the resulting compounds. However, the thiometalate aqua cation [Mo2O2S2(H2O)6] 2+ has been explored systematically by Cadot et al. who showed that a large variety of planar ring clusters could be templated by organic ligands, and recently reported by us in the assembly of a Mçbius strip. The key problem with developing new approaches to nanosized clusters, especially in polyoxometalate chemistry, is due to the lack of structural stability of reactive building blocks. To address this we reasoned that the [Mo2O2S2] 2+ cation, containing two Mo atoms could act as a highly effective linking unit, especially when templated by a highly symmetrical structure-directing anion such as squarate. Herein we show that in the presence of excess of molybdate with [Mo2O2S2] 2+ and squarate, it is possible to cut the rings into fragments yielding much larger, protein-sized polyoxothiometalates, resulting in an unprecedented new family of compounds. Consequently, we deduced that this combination of building units would lead to a new family of architectures because, under reducing conditions, the molybdate-based building blocks can form a vast array of combinatorial units. This indeed proved correct since altering the pH value, the ratio of the starting material, and the counter cations resulted in a family of related structures, representing a new assembly type whereby “open” rings of [(Mo2O2S2)3(OH)4(C4O4)] (type A building blocks) can be combined with [Mo5O18] 6 (type B building blocks) and in some cases [(Mo2O2S2)2(OH)2(C4O4)] (type C building blocks) and [Mo2O2S2] 2+ units (type D building blocks) to construct a library of new compounds (1– 5, Figure 1); 1: {Mo45} = {A5B3} = {[(Mo2O2S2)3(OH)4(C4O4)]5(Mo5O18)3} 18 , 2 : {Mo47} = {A5B3D1} = {[(Mo2O2S2)3(OH)4(C4O4)]5(Mo5O18)3(Mo2O2S2)(H2O)4]} 16 , 3 : {Mo55} = {A5B3C2D1} = {[(Mo2O2S2)3(OH)4(C4O4)]5 (Mo5O18)3[(Mo2O2S2)5(OH)8(C4O4)2]} 20 , 4 : {Mo96} = {A9B6C3} = {[(Mo2O2S2)3(OH)4(C4O4)]9[(Mo2O2S2)2(OH)2 (C4O4)]3(Mo5O18)6} 36 (previously reported in Ref. [11]). The