Self-Sorting of Heteroanions in the Assembly of Cross-Shaped Polyoxometalate Clusters

Abstract

Heteroanion (HA) moieties have a key role in templating of heteropolyoxometalate (HPA) architectures, but clusters templated by two different templates are rarely reported. Herein, we show how a cross-shaped HPA-based architecture can self-sort the HA templates by pairing two different guests into a divacant {XYW₁₅O₅₄} building block, with four of these building block units being linked together to complete the cross-shaped architecture. We exploited this observation to incorporate HA templates into well-defined positions within the clusters, leading to the isolation of a collection of mixed-HA templated cross-shaped polyanions [(XYW₁₅O₅₄)₄(WO₂)₄]^32-/36- (X = H-P, Y = Se, Te, As). The template positions have been unambiguously determined by single crystal X-ray diffraction, NMR spectroscopy, and high-resolution electrospray ionization mass spectrometry; these studies demonstrated that the mixed template containing HPA clusters are the preferred products which crystallize from the solution. Theoretical studies using DFT calculations suggest that the selective self-sorting originates from the coordination of the template in solution. The cross-shaped polyoxometalate clusters are redox-active, and the ability of molecules to accept electrons is slightly modulated by the HA incorporated as shown by differential pulse voltammetry experiments. These results indicate that the cross-shaped HPAs can be used to select templates from solution, and themselves have interesting geometries, which will be useful in developing functional molecular architectures based upon HPAs with well-defined structures and electronic properties.

Figure 1

Structural representations of the {W₁₅} building block of the four cross-shaped POMs Te₄P₄, Se₄P₄, As₄P₄, and P₈. Color scheme: Purple: tellurium; green: selenium; pink: arsenic; orange: phosphorus; gray: tungsten; red: oxygen; black: hydrogen.

Figure 2

³¹P NMR spectra of [W₆₄O₂₀₀(HPO₃)₈]^32– (P₈) in 1 M NaCl/D₂O solution showing two doublets with ¹J_PH = 727 Hz and ¹J_PH = 779 Hz; [W₆₄O₂₀₀(TeO₃)₄(HPO₃)₄]^32– (Te₄P₄) in D₂O solution showing a doublet with ¹J_PH = 738 Hz; [W₆₄O₂₀₀(SeO₃)₄(HPO₃)₄]^32– (Se₄P₄) in 1 M NaCl/D₂O solution showing a doublet with ¹J_PH = 726 Hz; [W₆₄O₂₀₀(AsO₃)₄(HPO₃)₄]^36– (As₄P₄) in 1 M LiCl/D₂O solution showing a doublet with ¹J_PH = 718 Hz.

Figure 3

ESI-MS spectra of Te₄P₄ in water/acetonitrile (5%:95%) mixture solvent.

Figure 4

(a) DPVs of cross-shaped POMs in lithium acetate buffer (pH 4.3). (b) UV–vis spectra of one-electron reduction of Te₄P₄ and P₈ ranging from 400 to 1200 nm. (c) EPR spectrum of one-electron reduced Te₄P₄ and P₈.

Scheme 1

Self-Sorting and Self-Assembly of Heteroanion Templated Nanoscale Cross-Shaped Polyoxoanions [H₄P₄X₄W₆₄O₂₂₄]^32–/36– (X = Te, Se, As, P)^{a a}Color scheme: purple and orange: heteroanions XO₃; grey: tungsten; red: oxygen; black: hydrogen.