A Chromium-Substituted Polyoxoniobate with High Ionic Conductivity

Abstract

A rare and novel Cr-substituted polyoxoniobate (PONb), [Cr2.5Nb27.5O66(OH)20(H2O)2]7–, has been synthesized by a combination of hydrothermal and conventional solution methods. The PONb shows an unknown tetrameric structure, which is the largest Cr-containing PONb to date. Interestingly, every two PONb tetramers can be joined together by alkali-metal cations to form a discrete cubelike ionic cluster. The obtained PONb not only enriches the very limited members of the Cr-substituted PONb family but also exhibits high ionic conductivity.

Polyoxometalates: Tailoring metal oxides in molecular dimension toward energy applications

“…Polyoxoniobate‐based composites such as those coupled with polyoxovanadate are used as efficient catalysts . Cobalt‐containing and chromium‐containing polyoxoniobates such as Na 6 K 12 [H 2 Co 8 O 4 (Nb 6 O 19 ) 4 ]·39H 2 O are produced to demonstrate their novel electronic and magnetic properties . Novel polyoxoniobate structures have been constructed; for example, polyoxoniobates with a large number of niobium atoms based on Nb 114 , Nb 81 , Nb 52 , Cu 3 Nb 78 , and Cu 4 Nb 78 have been synthesized, while an even larger‐size polyoxoniobate Nb 288 O 768 (OH) 48 (CO 3 ) 12 has been successfully produced .…”

Strategic Capture of the {Nb7} Polyoxometalate

“…The latter always assembles into larger species, due to the high reactivity of the Nb(O,OH,OH 2 ) 3 appendix‐unit on the hexaniobate core. Larger assemblies of the {Nb 7 } unit include heterometal clusters with Cu, or Cr, and most commonly {Nb 24 } stabilized and crystallized with copper amine adducts,– and recently with europium . Not surprising, the {Nb 7 } unit has never been isolated in the solid state without two of the three above‐mentioned terminal O/OH/OH 2 ‐ligands stabilized by polymerization.…”

“…Broadly speaking, despite 17 years of expansion of heteropolyniobate chemistry that includes incorporation of first‐row transition metals, and even acid‐stable Nb/Ta‐POMs, Nb‐POM chemistry lacks the rational synthetic approaches that are employed for group VI POM chemistry. Advances have ensued through hydrothermal processing to obtain higher Nb oxide solubility at lower pH, since many studies report 10.5 as close to ideal .…”

“…Broadly speaking, despite 17 years of expansion of heteropolyniobate chemistry that includes incorporation of first-row transition metals, [16,18] and even acid-stable Nb/Ta-POMs, [19] Nb-POM chemistry lacks the rational synthetic approaches that are employed for group VI POM chemistry.A dvances have ensued through hydrothermal processing to obtain higher Nb oxide solubility at lower pH, [15] since many studies report 10.5 as close to ideal. [12] Additionally,i ncorporating open-shell transition metals in Nb-POMs wasa chieved by rigorous exclusion of alkali counter-cationst hat otherwise promote precipitation of these base-insoluble metal oxides.…”

Capturing the Heptaniobate {Nb7O22}9– Anion by Covalent Bond Formation: Synthesis, Crystal Structure, and Selected Properties of {[Fe(cyclam)]3Nb7O22}· ≈ 19 H2O

“…Inside the tetrameric moiety no guests are located due to the limited void space . The compound [Na 2 (H 2 O) 9 ] 2 {[Na 4 K 6 (H 2 O) 10 ] [Cr 2.5 Nb 27.5 O 66 (OH) 20 (H 2 O) 2 ] 2 } · ≈ 100 H 2 O features the tetrameric unit with some Nb 5+ cations being substituted by Cr 3+ cations, and these units are interconnected by only two instead of four [Nb/CrO 6 ] octahedra.…”