Discovery of gigantic molecular nanostructures using a flow reaction array as a search engine

Abstract

The discovery of gigantic molecular nanostructures like coordination and polyoxometalate clusters is extremely time-consuming since a vast combinatorial space needs to be searched, and even a systematic and exhaustive exploration of the available synthetic parameters relies on a great deal of serendipity. Here we present a synthetic methodology that combines a flow reaction array and algorithmic control to give a chemical 'real-space' search engine leading to the discovery and isolation of a range of new molecular nanoclusters based on [Mo(2)O(2)S(2)](2+)-based building blocks with either fourfold (C4) or fivefold (C5) symmetry templates and linkers. This engine leads us to isolate six new nanoscale cluster compounds: 1, {Mo(10)(C5)}; 2, {Mo(14)(C4)4(C5)2}; 3, {Mo(60)(C4)10}; 4, {Mo(48)(C4)6}; 5, {Mo(34)(C4)4}; 6, {Mo(18)(C4)9}; in only 200 automated experiments from a parameter space spanning ~5 million possible combinations.

Figure 1. Exploring the reaction space with search engine.

Individual experiments fixed the concentrations of three components while altering the concentration of two other components. {Mo₂} is an essential component, regardless of its selection from either squarate (C₄) or croconate (C₅), which acts as organic template components, and selects from either Mo or W that acts as addenda anion component. Colour code: Mo, blue spheres and cyan polyhedra; S, yellow spheres; C, black spheres; O, red spheres; Mo₆ template, sky blue polyhedron (M: Mo or W).

Figure 2. Diagram of the search engine system.

The exploration of the combinatorial space was based on running five different sets of experiments (a–e), using a common batch of stock solutions of the building blocks. The search approach in the experiments was based on changing ratio between C₄ and C₅ (a); between Mo and W (b); between Mo and C₅ (c); finally, a three-component variation between Mo, {Mo₂}²⁺ and potassium source/medium (d and e) was also used.

Figure 3. Co-crystallizing POTM-POMs discovered during screening.

(a) X-ray crystal structure of 2 {Mo₁₄(C₄)₄(C₅)₂} and (b) 3 {Mo₆₀(C₄)₁₀}. Colour code: Mo, blue spheres; S, yellow spheres; C, black spheres; O, red spheres; K, green sphere; {Mo(Mo₅)} pentagonal unit, cyan spheres.

Figure 4. The C₄-POTMs discovered during screening.

X-ray crystal structures of (a) 4 {Mo₄₈} 24.5 × 23.0 Å, (b) 5 {Mo₃₄} 22.5 × 23.7 Å and (c) 6 {Mo₁₈} 17.1 × 17.2 Å. Colour code: Mo, blue spheres; S, yellow spheres; C, black spheres; O, red spheres; K, green sphere; {Mo(Mo₅)} pentagonal/{Mo(Mo₄)} lacunary pentagonal unit, cyan spheres.

Figure 5. Plausible representation of formation of 4 {Mo₄₈}.

Two building blocks {Mo₆(C₄)} (purple) coordinate, at 90° relative to each other, to the pentagonal unit via Mo-O bonds to form the ‘wings of {Mo₄₈}. The two wings are bridged by two further {Mo₆(C₄)} units to construct {Mo₄₈} 4. (Pentagonal unit {Mo(Mo₅)}: cyan polyhedra; ball-and-stick representation for C: black; Mo^V: blue; S: yellow; O: red.) Supplementary Movie 1 shows how the cluster compound is constructed.