Redox tuning the Weakley-type polyoxometalate archetype for the oxygen evolution reaction.

Water oxidation is a key reaction for the conversion of solar energy into chemical fuels, but effective water-oxidation catalysts are often based on rare and costly precious metals such as Pt, Ir or Ru. Developing strategies based on earth-abundant metals is important to explore critical aspects of this reaction, and to see whether different and more efficient applications are possible for energy systems. Herein, we present an approach to tuning a redox-active electrocatalyst based on the doping of molybdenum into the tungsten framework of [Co4(H2O)2(PW9O34)2]10-, known as the Weakley sandwich. The Mo-doped framework was confirmed by X-ray crystallography, electrospray ionization mass spectrometry and inductively coupled plasma optical emission spectrometry studies. The doping of molybdenum into the robust Weakley sandwich framework leads to the oxidation of water at a low onset potential, and with no catalyst degradation, whereby the overpotential of the oxygen evolution reaction is lowered by 188 mV compared with the pure tungsten framework.

Encapsulation of a {Cu16} cluster containing four [Cu4O4] cubanes within an isopolyoxometalate {W44} cluster.

We report a {Cu16} embedded within a {W44} cluster containing four cubane-like [Cu4O4] units within an isopolyoxotungstate (isoPOT) in a {Na4Cu4[(H2W11O38) (CH3COO)(OH)3]}4·88H2O (1) and a polyanion Cu-linked {W11} chain Na6Cu2[(H2W11O38)(CH3COO)(OH)]·26H2O (2). Electronically, the redox properties show that both compounds 1 and 2 undergo irreversible reductions resulting in the demetalation of the compounds, whilst the magnetic behavior of 1 and 2 shows a weak antiferromagnetic and a stronger ferromagnetic coupling, respectively.

A metamorphic inorganic framework that can be switched between eight single-crystalline states.

The design of highly flexible framework materials requires organic linkers, whereas inorganic materials are more robust but inflexible. Here, by using linkable inorganic rings made up of tungsten oxide (P8W48O184) building blocks, we synthesized an inorganic single crystal material that can undergo at least eight different crystal-to-crystal transformations, with gigantic crystal volume contraction and expansion changes ranging from -2,170 to +1,720 Å3 with no reduction in crystallinity. Not only does this material undergo the largest single crystal-to-single crystal volume transformation thus far reported (to the best of our knowledge), the system also shows conformational flexibility while maintaining robustness over several cycles in the reversible uptake and release of guest molecules switching the crystal between different metamorphic states. This material combines the robustness of inorganic materials with the flexibility of organic frameworks, thereby challenging the notion that flexible materials with robustness are mutually exclusive.

Investigating the Transformations of Polyoxoanions Using Mass Spectrometry and Molecular Dynamics.

The reactions of [γ-SiW10O36](8-) represent one of the most important synthetic gateways into a vast array of polyoxotungstate chemistry. Herein, we set about exploring the transformation of the lacunary polyoxoanion [ß2-SiW11O39](8-) into [γ-SiW10O36](8-) using high-resolution electrospray mass spectrometry, density functional theory, and molecular dynamics. We show that the reaction proceeds through an unexpected {SiW9} precursor capable of undertaking a direct ß â†’ γ isomerization via a rotational transformation. The remarkably low-energy transition state of this transformation could be identified through theoretical calculations. Moreover, we explore the significant role of the countercations for the first time in such studies. This combination of experimental and the theoretical studies can now be used to understand the complex chemical transformations of oxoanions, leading to the design of reactivity by structural control.

Assembly of Tungsten-Oxide-Based Pentagonal Motifs in Solution Leads to Nanoscale {W48}, {W56}, and {W92} Polyoxometalate Clusters.

We report an approach to synthesize molecular tungsten-oxide-based pentagonal building blocks, in a new {W21 O72} unit, and show how this leads to a family of gigantic molecular architectures including [H12W48O164](28-) {W48}, [H20W56O190](24-) {W56}, and [H12W92O311](58-) {W92}. The {W48} and {W56} clusters are both dimeric species incorporating two {W21} units and the {W56} species is the first example of a molecular metal oxide cluster containing a chiral "double-stranded" motif which is stable in solution as confirmed by mass spectrometry. The {W92} anion having four {W21} units is one of the largest transition metal substituted isopolyoxotungstates known.

Synthesis and characterization of a series of [M2(ß-SiW8O31)2](n-) clusters and mechanistic insight into the reorganization of {ß-SiW8O31} into {α-SiW9O34}.

Lacunary polyoxometalates of low nuclearity are difficult to synthesize in isolation. We report the facile synthesis of six {M2(B-ß-SiW8O31)2} clusters (M = Co/Mn/Ni/Zn/Cu(2+), Fe(3+)) that can be employed as building blocks for the formation of larger architectures. We show for the first time that such {B-ß-SiW8O31} lacunae are capable of reorganizing into larger Keggin lacunary species even in the absence of an external source of tungstate. We hypothesize, based on electrospray ionization mass spectrometry evidence obtained, not only that such a transformation is only possible via an initial decomposition of the {SiW8} precursor into a {SiW6}-based intermediate but also that it is this {SiW6} species that acts as the template for the growth of the larger fragments.

Controlling the minimal self assembly of "complex" polyoxometalate clusters.

Despite the vast number of polyoxometalate clusters now known, an ongoing and important challenge is to understand causality in the assembly of "complex" clusters at a mechanistic level, since this is the only way the rational, targeted synthesis of new compounds will ever be achieved. Often, the complexity of the reactions themselves makes such investigations near impossible, as very small changes can often make dramatic differences. Herein, we explore a very simple [A + B] binary synthetic system that gives rise to the facile assembly of two isomeric anions, [Fe(III)(H2O)2{γ-Fe(III)SiW9O34(H2O)}2](11-) (1) and [Fe(III)(H2O)2{γ-Fe(III)2SiW8O33(H2O)2}{γ-SiW10O35}](11-) (2), which can be formed as individual and dimeric species (3) and (4). Furthermore, the simple binary nature of this synthetic system allowed its investigation by a comprehensive time-resolved ESI-MS analysis, yielding unprecedented mechanistic information regarding the initial interactions and reorganizations of the {γ-SiW10} precursor in the presence of Fe(2+).

Nanoscale control of polyoxometalate assembly: a {Mn8W4} cluster within a {W36Si4Mn10} cluster showing a new type of isomerism.

Two near isomeric clusters containing a novel {Mn(8)W(4)} Keggin cluster within a [W(36)Mn(10)Si(4)O(136)(OH)(4)(H(2)O)8](24-) cluster are reported: K(10)Li(14)[W(36)Si(4)O(136)Mn(II)(10)(OH)(4)(H(2)O)(8)] (1) and K(10)Li1(3.5)Mn(0.25)[W(36)Si(4)O(136)Mn(II)(10)(OH)(4)(H(2)O)(8) ] (1'). Bulk characterization of the clusters has been carried out by single crystal X-ray structure analysis, ICP-MS, TGA, ESI-MS, CV and SQUID-magnetometer analysis. X-ray analysis revealed that 1' has eight positions within the central Keggin core that were disordered W/Mn whereas 1 contained no such disorder. This subtle difference is due to a differences is how the two clusters assemble and recrystallize from the same mother liquor and represents a new type of isomerism. The rapid recrystallization process was captured via digital microscopy and this uncovered two "intermediate" types of crystal which formed temporarily and provided nucleation sites for the final clusters to assemble. The intermediates were investigated by single crystal X-ray analysis and revealed to be novel clusters K(4)Li(22)[W(36)Si(4)Mn(7)O(136)(H(2)O)(8)]·56H(2)O (2) and Mn(2)K(8)Li(14)[W(36)Si(4)Mn(7)O(136)(H(2)O)(8)]·45H(2)O (3). The intermediate clusters contained different yet related building blocks to the final clusters which allowed for the postulation of a mechanism of assembly. This demonstrates a rare example where the use X-ray crystallography directly facilitated understanding the means by which a POM assembled.