Making Heterometallic Metal–Metal Bonds in Keggin-Type Polyoxometalates by a Six-Electron Reduction Process

Abstract

Polyoxometalates (POMs) represent a promising class of molecular electron reservoirs. However, their multielectron reduction gives rise to intricate physical–chemical phenomena that must be fully understood for their future use in energy-storage devices. Herein, we show that bulk electrolysis of the archetypal Keggin-type POM [Si(WVI 2MoVIO10)(WVI 3O10)3]4– in aqueous solution leads to the six-electron-reduced derivative [Si(WIV 2MoIVO7(H2O)3)(WVI 3O10)3]4– (notated SiW 11 Mo-VI′) in which the mixed-metal triad acts as a storage unit for six electrons and six protons. X-ray diffraction analysis and multinuclear NMR (183W and 95Mo) studies reveal that this electron-rich species represents the first example of POMs containing heterometallic metal–metal bonds between addenda centers. This electron-rich POM can be further reduced through multielectronic events, while its full oxidation restores the structure of the oxidized parent ion. Remarkably, the formation of SiW 11 Mo-VI′ results from a fast clustering process compared to that observed for the entirely W-based analogue, revealing that the formation of metal–metal bonds in the mixed-metal Mo/W POM is facilitated because the reaction rate is not limited by a slow disproportionation step. Last, we evaluate the supramolecular properties of SiW 11 Mo-VI′ using a method based on the cloud-point measurement of a nonionic surfactant. This investigation demonstrates that the clustering process has dramatic consequences on the solution behavior of the POM, canceling its superchaotropic character due to a local structuring effect of the hydration shell. These fundamental results pave the way for applications using the massive electron-storage properties of mixed-metal POMs.

Clustering Six Electrons within “Dawson‐Like” Polyoxometalate: An Open Route toward Its Post‐functionalization

“…Another curious fact is that multiple proton‐coupled reductions of Keggin‐type anions [XW 12 O 40 ] n− (X=Si, B, or H 2 ) offer the possibility to store up to two electrons per metal center. Actually, this class of super‐reduced POMs commonly referred to as “ browns ” or “ heteropoly browns ” owing to their brownish color, exbibit one of their four edge‐sharing trimetallic units which store both six electrons and six protons [14–16] . Within this electron‐rich unit, the metal centers M IV are connected through metal‐metal bonds.…”

“…Actually, this class of super-reduced POMs commonly referred to as "browns" or "heteropoly browns" owing to their brownish color, exbibit one of their four edge-sharing trimetallic units which store both six electrons and six protons. [14][15][16] Within this electronrich unit, the metal centers M IV are connected through metal-metal bonds. Although the first descriptions of this clustering process were reported in the 70s, [17,18] no other POM architecture has been identified to store electrons through a similar mechanism.…”

Chemical diversity and versatility of polyoxometalate ligands: homologous series of twenty-five new structures with polyoxometalates binding to alkali, alkaline earth, lanthanide, and actinide ions

“…Several examples of heteroelements have been incorporated in the Keggin structure, with the most readily isolable being the ones containing B 3+ , Si 4+ , P 5+ , Ga 3+ , or Ge 4+ . [47][48][49][50] More complex synthesis procedures [46,51] and use of stabilizing elements have also led to Keggin ions XW11 with X = Be 2+ , Al 3+ , S 4+ , Cr 3+ , Co 2+/3+ , Ni 2+ , Cu 2+ , Zn 2+ , As 5+ , Sb 5+ , and Bi 5+ (Figure 2). This likely represents the most versatile ligand system in which various heteroelements can be encapsulated and eventually be used as an integral part of the XW11 metal chelator.…”

Modulation of the Band Gap and Redox Properties by Mixed Addenda in Sandwich Polyoxometalates

“…Cronin et al effectively applied this strategy in tetrasubstituted sandwich polyoxometalates to reduce over potential in water oxidation . Various studies on mixed addenda Lindqvist, Lacunary, Keggin, and Wells–Dawson POMs with combinations like (W/V, W/Mo, Mo/V, Nb/W) have shown improved activity in applications like photocatalysis, energy storage, and conversions. For example, lacunary POMs were engineered for visible-light-driven photocatalytic aerobic oxidation of sulfides to sulfoxides .…”