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.


Results
Two new mineral species, arsmirandite and lehmannite, have been found in the active Arsenatnaya fumarole 29,30 located at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption 1975-1976, Tolbachik volcano, Kamchatka Peninsula, Far-Eastern Region, Russia (55°41′N 160°14′E, 1200 m asl). The mineral assemblages encasing these halide-arsenates were formed in the temperature range between 500 and 700 °C 22,30 and were located inside the fumarole. The detailed mineralogical description of the minerals will be given elsewhere. Polyoxocuprates occur as aggregates of greyish-green well-shaped crystals resembling rhombic dodecahedra ( Fig. 1) that could easily be separated from the surrounding matrix. The chemical composition of arsmirandite and lehmannite were determined using electron-microprobe analysis (Table S0). The empirical formula of arsmirandite, calculated on the basis of 45 anions (O + Cl) pfu, is: Na 17  The crystal structures of arsmirandite and lehmannite (Tables S1-S7) are very similar, yet not identical. From the structural point of view, they are quite unusual and unique. The basic structural unit in both structures is a novel nanoscale (~1.5 nm across) polyoxocuprate cluster with the composition {[MCu 12 O 8 ](AsO 4 ) 8 } (M = Fe 3+ and Ti 4+ , for arsmirandite and lehmannite, respectively) shown in Fig. 2. The most peculiar feature of the nanocluster is the presence of Fe 3+ (arsmirandite) or Ti 4+ (lehmannite; the tetravalent state of Ti confirmed by XANES spectroscopy (Fig. S3)) in a cubic coordination (Fig. 2a): the central unit of the nanocluster represents the slightly distorted (MO 8 ) cube (Fig. 2a,d). The cubic coordination of Fe(III) and Ti(IV) has not been encountered so far in natural minerals, though several examples are known for synthetic compounds [31][32][33][34] . Each O atom of the (MO 8 ) configuration is further coordinated by three Cu 2+ cations (Fig. 2b) that have square-planar geometry by O atoms of the (AsO 4 ) groups (Fig. 2c,e,f). The metal-oxide core of the nanocluster can also be represented in terms of oxocentered (OCu 3 M) tetrahedra 27,28 that form an eightfold unit (Fig. 2g), which can be considered as a fragment of the crystal structure of fluorite, if the latter is described as a framework of (FCa 4 ) tetrahedra. The [O 8 MCu 12 ] core formed by eight oxocentered tetrahedra is surrounded by eight AsO 4 tetrahedra that are in the face-to-face orientation relative to the (OCu 3 M) tetrahedra 35 (Fig. 2h). The cubic nanoclusters are negatively charged and are surrounded by an array of Na + cations and X − anions (X = F, Cl) ( Fig. 3), which requires further remark. The analysis of the Na array in arsmirandite and lehmannite (Figs. S1 and S2) shows that it is in fact a highly deficient face-centered cubic (fcc) lattice as observed, e.g. in the crystal structure of halite, NaCl 36  , i.e. the structures of arsmirandite and lehmannite can be considered as the 2 × 4 × 4 supercell relative to the halite cell. The parameters of the halite-like cubic subcell, e.g., in lehmannite, are: a hal = 5.418, b hal = 5.273, c hal = 5.279 Å, which shows that, realtive to halite, the deficient Na fcc array is compressed and tetragonally distorted. www.nature.com/scientificreports www.nature.com/scientificreports/

Discussion
The difference between arsmirandite and lehmannite is in the nature of the M cation, which triggers the re-arrangement of the halogen substructure. Since Fe 3+ and Ti 4+ possess different charges, the Fe 3+ -for-Ti 4+ substitution requires the mechanism of charge compensation, which is achieved via the incorporation of the additional anionic site, F, in lehmannite (Ti 4+ species) relative to arsmirandite (mineral with Fe 3+ ). The charge compensation, therefore, is governed by the following substitution scheme: where the '□' sign stays for vacancy. In lehmannite, the substitution is realized through the following crystal chemical mechanism. The Cl1 site with the 2/m site symmetry with the coordinates (1/2,1/2,0) in arsmirandite is shifted along the a axis to the Cl1 site with the m site symmetry (approximate coordinates: 0.53,1/2,0) in lehmannite, resulting in the splitting of the Cl1 site into two mutually exclusive sites. The F site with the same m site symmetry is incorporated (approximate coordinates: 0.65,1/2,0) into the structure so that the Cl1 … F distance is equal to ca. 1.3 Å. The short distance precludes the simultaneous occupancy of the Cl1 and F sites with the maximum total occupancy F 0.50 Cl 0.50 . Note that the Cl1 … Cl1 distance in disordered configuration is very short (~0.6 Å in lehmannite), prohibiting more-than-50% occupancy of the Cl1 site, whereas the F … F distance (~3.1 Å) allows for the full occupancy of the two F sites assuming that the Cl1 site is empty. www.nature.com/scientificreports www.nature.com/scientificreports/ In lehmannite, the disordered configuration has the total occupancy F 0.52 Cl 0.34□0.16 or F 1.04 Cl 0.68□0.32 per formula unit (pfu) with the −1.72 total negative charge. The corresponding occupancy of the M site is calculated as Ti 4+ 0.69 Fe 3+ 0.28 Sn 4+ 0.03 with the total positive charge +3.72, which provides the formula electroneutrality. The full substitution of Fe 3+ by Ti 4+ in lehmannite would correspond to the total occupancy of the anionic sites F 0.50 Cl 0.50 or F 1.00 Cl 1.00 pfu with the −2 total negative charge.
The crystal chemical formula of arsmirandite determined on the basis of chemical analysis and crystal-structure refinement can be written as (Na 17 Taking into account the discussion given above, the general formula of the hypothetical arsmirandite-lehmannite series can be written as Na 18 (Ti 4+ 1-x Fe 3+ x )Cu 12 (AsO 4 ) 8 O 8 Cl 6-x-y F y . The ideal formula of arsmirandite corresponds to x = 1 and y = 0, whereas that of lehmannite requires x = 0 and y = 1. Note that 0 ≤ x ≤ 1, whereas 0 ≤ y ≤ 2. The experimental case of lehmannite corresponds to x = 0.28 and y = 1.04. The case y = 2 corresponds to the full occupancy of the F site and the complete emptiness of the Cl1 site and results in the 'theoretical' formula Na 18  ) tetrahedra sharing central Me atom are well-known in synthetic inorganic chemistry. It seems that the first detailed structural report was done for Me = Pb 2+ found in the crystal structure of Pb 13 O 8 (OH) 6 (NO 3 ) 4 37 . In this compound, the central 8-coordinated cation in the nanocluster is Pb 2+ , which has a stereochemically inactive lone-electron pair, in contrast to twelve peripheral Pb 2+ that possess strongly asymmetrical coordination environments. Later Kolitsch and Tillmanns 38 reported Pb 13 O 8 (OH) 6 (NO 3 ) 4 as an anthropogenic compound formed in old mine dumps due to the use of nitrate explosives. The phase Pb 13 O 8 (OH) 6 (NO 3 ) 4 forms in the Pb(NO 3 ) 2 -NaOH system at pH = 9-10 39 .
In 2008, Chubarova et al. 40 reported on the synthesis and structure of Na 8 {[Pd 13 O 8 ] (AsO 3 (OH)) 6 (AsO 4 ) 2 }·42H 2 O, the compound, which opened up the whole new field of polyoxopalladate chemistry 5 . The crystal structure of this compound is based upon {[Pd 13 O 8 ](AsO 3 (OH)) 6 (AsO 4 ) 2 } 8− nanoclusters structurally identical to those found in arsmirandite and lehmannite (which are also arsenates). Later it was found that arsenate groups can be replaced by phosphate 41 or selenite groups [42][43][44] . The central Pd 2+ cation in the Pd-based 13-nuclear nanoclusters (Fe 3+ and Ti 4+ in arsmirandite and lehmannite, respectively, play the same role as Pd 2+ in this compound) can be replaced by Na + 45 , REE 3+ (REE = rare-earth element 42,46 ), divalent or trivalent metal cations M 2+ (M = Sc 3+ , Mn 2+ , Fe 3+ , Co 2+ , Ni 2+ , Cu 2+ , or Zn 2+ ), and tetravalent Sn 4+ and Pb 4+ cations 47 . Very recently, Bhattacharya et al. 48 reported on the use of the 13-nuclear Pd-based nanoclusters, [Pd 13 O 8 (AsO 4 ) 8 H 6 ] 8− , incrustated by Ba 2+ cations for the construction of novel types of porous metal-organic frameworks. It is remarkable that, in arsmirandite and lehmannite, the role of Pd 2+ cations in peripheral metal positions is played by Cu 2+ cations, whereas the central cation in the metal-oxide core is either Fe 3+ or Ti 4+ , respectively. The observed stereochemical similarity is due to the similar square planar coordination by O 2− anions specific for both Pd 2+ and Cu 2+ . Our findings indicate the chemical possibility of the family of novel polyoxocuprate clusters with interesting functional properties. Yang and Kortz 5 indicate that the Pd nanoclusters are stable in the solid state, solutions (both aqueous and organic) and gases, which allows for their applications in catalysis, nanotechnology, molecular spin qubits and in biology as aqueous-phase macromolecular models. Kondinski 8 ] q− clusters (M = Fe 3+ , Ti 4+ ) in arsmirandite and lehmannite found in Tolbachik fumaroles provides a useful clue for the targeted synthesis of their analogues under laboratory conditions. As it has been demonstrated previously [49][50][51] , fumarolic minerals with oxocentered cores can be conveniently synthesized using chemical vapor transport techniques. The occurrence of the [M⊂Cu 12 O 8 (AsO 4 ) 8 ] q− nanoclusters in Tolbachik fumaroles testifies that they are stable under high temperatures (500-700 °C) at least in the gaseous and crystalline phases and may as well possess interesting physical and chemical properties. They could also play the role of metal transport forms in fumarolic gases, in agreement with the previous proposal 52 about the similar geochemical role of tetranuclear (OCu 4 ) clusters.
The geometrical similarity of the Na array in arsmirandite and lehmannite to that observed in halite, NaCl, allows to describe both minerals as consisting of cubic-shape nanoclusters periodically integrated into deficient NaCl matrix, a feature that is quite uncommon for inorganic materials. There has been a recent interest in salt-inclusion compounds (SICs), which possess hierarchical structures consisting of porous metal-oxide frameworks with voids filled with simple ionic salts 53 . The examples of natural SICs are averievite, Cu 5 O 2 (VO 4 )·nMCl x (M = Cu, Cs, Rb, K) [54][55][56][57] , and aleutite, Cu 5 O 2 (AsO 4 )(VO 4 )·(Cu 0.5□0.5 )Cl 25 . In the case of arsmirandite and lehmannite, we have the opposite situation, i.e., the incorporation of metal-oxide clusters into the salt matrix, which, as to our knowledge, had never been observed at least in minerals and mineral phases.