Identifying Protonated Decavanadate Polyanions

Substitutional/positional disorder of biguanide and guanylurea in the structure of a decavanadate complex [(Bg)(HV10O285-)]0.4[(HGU+)(V10O286-)]0.6(H2Met2+)2(H3O+)·8H2O

A hydrated salt of decavanadate containing diprotonated metforminium(2+) (H2Met2+), hydronium (H3O+) and either neutral biguanide (Bg) or monoprotonated guanylurea (HGU+) exhibits a previously seen complex charge-stabilized hydrogen-bonded network [Chatkon et al. (2022). Acta Cryst. B78, 798-808]. Charge balance is achieved in two ways through substitutional disorder: a 0.6 occupied HGU+ cation is paired with a V10O286- anion, and a 0.4 occupied neutral Bg molecule is paired with a HV10O285- anion, with the remaining charge in both cases balanced by two H2Met2+ dications and one H3O+ monocation. Bg/HGU+ moieties exhibit bifurcated N-H...O hydrogen bonding to the H3O+ cation and are substitutionally/positionally disordered along with the H3O+ cation about an inversion center. The HGU+ V10O286- synthon seen in the previous study occurs again. Bg exhibits bifurcated hydrogen bonding from two amino groups to two rows of cluster O atoms running diagonally across the equatorial plane of the HV10O285- anion with a return hydrogen bond from the cluster H atom to the imino N atom of the Bg. Thus, a Bg...cluster synthon similar to the HGU+...cluster synthon previously reported is found. The disordered moieties occupy spaces with excess volume in the 3-D network structure. Interestingly, when the crystallographic unit cell of the current compound, whose X-ray data was collected at 100 K, is compared with that of a previous compound exhibiting the same supramolecular framework, unit-cell parameter c does not shorten as a and b expectantly do because of the lower data collection temperature. The lack of contraction on unit-cell parameter c is possibly due to the supramolecular structure.

Polyoxometalate clusters in minerals: review and complexity analysis

Most research on polyoxometalates (POMs) has been devoted to synthetic compounds. However, recent mineralogical discoveries of POMs in mineral structures demonstrate their importance in geochemical systems. In total, 15 different types of POM nanoscale-size clusters in minerals are described herein, which occur in 42 different mineral species. The topological diversity of POM clusters in minerals is rather restricted compared to the multitude of moieties reported for synthetic compounds, but the lists of synthetic and natural POMs do not overlap completely. The metal-oxo clusters in the crystal structures of the vanarsite-group minerals ([As³⁺V⁴⁺₂V⁵⁺₁₀As⁵⁺₆O₅₁]^7-), bouazzerite and whitecapsite ([M³⁺₃Fe₇(AsO₄)₉O_8-;n(OH)_n]), putnisite ([Cr³⁺₈(OH)₁₆(CO₃)₈]^8-), and ewingite ([(UO₂)₂₄(CO₃)₃₀O₄(OH)₁₂(H₂O)₈]^32-) contain metal-oxo clusters that have no close chemical or topological analogues in synthetic chemistry. The interesting feature of the POM cluster topologies in minerals is the presence of unusual coordination of metal atoms enforced by the topological restraints imposed upon the cluster geometry (the cubic coordination of Fe³⁺ and Ti⁴⁺ ions in arsmirandite and lehmannite, respectively, and the trigonal prismatic coordination of Fe³⁺ in bouazzerite and whitecapsite). Complexity analysis indicates that ewingite and morrisonite are the first and the second most structurally complex minerals known so far. The formation of nanoscale clusters can be viewed as one of the leading mechanisms of generating structural complexity in both minerals and synthetic inorganic crystalline compounds. The discovery of POM minerals is one of the specific landmarks of descriptive mineralogy and mineralogical crystallography of our time.

Polyoxometalate chemistry at volcanoes: discovery of a novel class of polyoxocuprate nanoclusters in fumarolic minerals

Polyoxometalate (POM) chemistry is an important avenue of comprehensive chemical research, due to the broad chemical, topological and structural variations of multinuclear polyoxoanions that result in advanced functionality of their derivatives. The majority of compounds in the polyoxometalate kingdom are synthesized under laboratory conditions. However, Nature has its own labs with the conditions often unconceivable to the mankind. The striking example of such a unique environment is volcanic fumaroles – the natural factories of gas-transport synthesis. We herein report on the discovery of a novel class of complex polyoxocuprates grown in the hot active fumaroles of the Tolbachik volcano at the Kamchatka Peninsula, Russia. The cuboctahedral nanoclusters {[MCu₁₂O₈](AsO₄)₈} are stabilized by the core Fe(III) or Ti(IV) cations residing in the unique cubic coordination. The nanoclusters are uniformly dispersed over the anion- and cation-deficient NaCl matrix. Our discovery might have promising implications for synthetic chemistry, indicating the possibility of preparation of complex polyoxocuprates by chemical vapor transport (CVT) techniques that emulate formation of minerals in high-temperature volcanic fumaroles.