Записки Российского минералогического общества, 2021, T. 150, № 5, стр. 103-114

1. INTRODUCTION

In the recent years, there have been an explosive interest to structural chemistry and pro-perties of copper oxysalt minerals and their synthetic analogues, mostly due to the whole range of new mineralogical discoveries and continuing exploration of natural copper-based – compounds in solid state physics and material sciences (Botana et al., 2018; Inosov, 2018; Volkova, Marinin, 2018; Winiarski et al., 2019; Dey, Botana, 2020, etc.). The Tolbachik eruptions in the Kamchatka Peninsula, Far East, Russia, provided an unprecedented paragenetic suite of unique and variable copper mineralization with many new copper mineral species that have no analogues in synthetic material chemistry (Pekov et al., 2018). The K–Cu–Cl–H₂O system is of particular interest. Kahlenberg (2004) reported the synthesis of a new compound, K₂Cu₅Cl₈(OH)₄ · 2H₂O, prepared as a by-product in a hydrothermal experiment. The compound showed a remarkable similarity to avdoninite, a technogenic phase found in oxidation zones of sulfide deposits in Ural (Bushmakin, Bazhenova, 1998). In 2006, N.V. Chukanov and co-authors established the status of avdoninite as a separate mineral species based on its findings in Tolbachik fumaroles (Chukanov et al., 2006), whereas Pekov et al. (2015) demonstrated that the mineral is indeed a natural analogue of synthetic phase K₂Cu₅Cl₈(OH)₄ · 2H₂O first reported by Kahlenberg (2004). Owing to its unique structural architecture, avdoninite attracted attention as an interesting magnetic material (Volkova, Marinin, 2018). There are two other minerals in the K–Cu–Cl–H₂O system: mitscherlichite, K₂CuCl₄ · 2H₂O (Zambonini, Carobbi, 1925; Chidambaram et al., 1970), and romanorlovite, K₁₁Cu₉Cl₂₅(OH)₄ ⋅ 2H₂O (Pekov et al., 2016), both having a fumarolic origin. Herein we report the synthesis and structural characterization of ${{{\text{K}}}_{4}}{\text{C}}{{{\text{u}}}^{{2 + }}}{\text{Cu}}_{2}^{ + }{\text{C}}{{{\text{l}}}_{8}}\,\,\cdot\,\,2{{{\text{H}}}_{2}}{\text{O}}$, a new compound in the K–Cu–Cl–H₂O system, which remarkably contains Cu ions in two oxidation states, Cu⁺ and Cu²⁺. It is worthy to note that the only known mineral with well-defined Cu⁺ and Cu²⁺ ions is allochalcoselite, ${\text{C}}{{{\text{u}}}^{ + }}{\text{Cu}}_{5}^{{2 + }}$PbO₂(SeO₃)₂Cl₅, which was also disco-vered in Tolbachik fumaroles (Vergasova et al., 2005; Krivovichev et al., 2006).

The mixed-valence Cu⁺/Cu²⁺ systems are of particular attention in metal-organic chemistry, due to their interesting electrical, magnetic, luminescent, and catalytic properties (Tanaka et al., 2013; Nakatani et al., 2015; Vinogradova et al., 2016). 1D and 2D polymeric complexes of Cu⁺ and Cu²⁺ ions are of special interest due to the possibility of delocalized electron density and cuprophillic Cu–Cu interactions. The title compound is indeed based upon 1D copper chains consisting of alternating Cu⁺ and Cu²⁺ ions that may well communicate with each other along the extension of the chain.

2. EXPERIMENTAL

Single crystals of title compound were obtained as a by-product in our experiments using chemical vapor transport reactions (Binnewies et al., 2013) in the course of our study of the KCl–CuO–V₂O₅ system. Copper oxide (0.0397 g), vanadium oxide (0.0910 g) and potassium chloride (0.0370 g) in a 1 : 1 : 1 molar ration were ground in an agate mortar with further annealing of the mixture at 250 °C for 16 h in air. The mixture was loaded into a fused silica ampule (ca. 16 cm), which was evacuated to 10^–2 mbar before sealing. The ampule was placed horizontally into two-zone furnace and heated to 675 °C within 4 h. The temperature gradient between the source and deposition zones of the ampule was ~50 °C. After 2 days the ampule was cooled to 200 °C over a period of 64 h and then the furnace was switched off. Reddish brown single crystals of the title compound were found in the deposition zone covered by bluish transparent crystals of synthetic analogue of eriochalcite, CuCl₂ ∙ 2H₂O, in association with crystals of α- and β-Cu₂V₂O₇ phases. The probable cause for the presence of H₂O molecules in the title compound is a hygroscopic nature of potassium chloride used as a reagent in the synthesis.

Diffraction data for K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈(H₂O)₂ were collected using a using a Rigaku XtaLAB Synergy S X-ray diffractometer operated with monochromated microfocus MoKα tube (λ = = 0.71073 Å) at 50 kV and 1.0 mA, and equipped with a CCD HyPix 6000 detector. The frame width was 1.0° in ω, and a 190 s count time for each frame. A CrysAlisPro software (Agilent, 2014) was used for integration and correction of diffraction data for polarization, background and Lorentz effects. A Gaussian absorption correction was performed using a CrysAlisPro software on the basis of multifaceted crystal model. The unit-cell parameters were refined using the least-squares teсhniques. The structure was solved by a dual-space algorithm and refined as an inversion twin using the SHELX programs (Sheldrick, 2015a, b) incorporated in the OLEX2 program package (Dolomanov et al., 2009). The H atoms of H₂O molecules were located from the inspection of difference Fourier maps and were included in the refinement with U_iso(H) set to 1.5 U_eq(O) and an O–H bond length restrain of 0.98 Å. The final model includes coordinates and anisotropic displacement parameters.

The crystal data and experimental parameters of the X-ray diffraction experiment are given in Table 1, atom coordinates, and isotropic displacement parameters are provided in Table 2. Table 3 contains selected interatomic distances, whereas Table 4 provides information on the geometry of the hydrogen bonding system. Table 5 contains results of the bond-valence analysis (the bond-valence parameters for the Cu⁺–Cl, Cu²⁺–Cl, and K-ϕ bonds (ϕ = Cl, H₂O) were taken from Shields et al. (2000), Brese and O’Keeffe (1991), and Brown and Altermatt (1985), respectively.

In order to investigate electron-density distribution, the CRYSTAL14 software package was used to perform the solid-state DFT calculations (Dovesi et al., 2014). The Peintinger–Oliveira–Bredow split-valence triple-ζ (pob-TZVP) basis sets (Peintinger et al., 2013) were used for all atoms, along with the hybrid Becke-3–Lee–Yang–Parr (B3LYP) functional. The electron-density distribution function was calculated using experimentally observed geometries for each structure and analysed using the TOPOND14 software (Gatti, Casassa, 2013) with respect to the properties of the bond critical points in electron density distributions and scalar fields of the electron-density Laplacian (Gatti et al., 1994).

3. RESULTS

The crystal structure of K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O is shown in Fig. 1. It consists of the [Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈]^4– anionic chains extended along the a axis and linked through the K⁺ ions and H₂O molecules. The atomic structure of the chain is depicted in Fig. 2. There are three Cu sites in the title compound. The Cu1 and Cu3 sites are coordinated tetrahedrally by four Cl atoms each with the Cu–Cl bond lengths in the range 2.309–2.441 Å. The bond-valence calculations (Table 5) provide the bond-valence sums of 1.00 and 0.99 valence units (v. u.) for the Cu1 and Cu3 sites, respectively. The tetrahedral (Cu⁺Cl₄) is quite typical for Cu⁺ chloride compounds and was observed, for instance, in nantokite, CuCl (Pfitzner, Lutz, 1993), and synthetic CsCu₂Cl₃ (Meyer, 1984). The (Cu1Cl₄) and (Cu3Cl₄) tetrahedra share Cl^…Cl edge to form a (Cu₂Cl₆) dimer with relatively short Cu⋅⋅⋅Cu intermetallic distance of 2.585 Å. The dimers are linked into 1D chains through the Cu2 atoms in an octahedral coordination (Fig. 2). The coordination is [4 + 2] distorted in accord with the Jahn-Teller effect (Jahn, Teller, 1937): there are four short (2.281–2.318 Å) and two long (3.118–3.125 Å) Cu2–Cl bonds. The bond-valence calculations indicate that the Cu2 site is occupied by Cu²⁺ ions.

Fig. 1.

The crystal structure of K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O in projection along the a axis. Рис. 1. Кристаллическая структура K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O в проекции вдоль оси a.

Fig. 2.

The copper chloride [Cu⁺₂Cu²⁺Cl₈] chain in the crystal structure of K₄Cu²⁺Cu⁺₂Cl₈ · 2H₂O. Рис. 2. Медь-хлоридная цепочка [Cu⁺₂Cu²⁺Cl₈] в кристаллической структуре K₄Cu²⁺Cu⁺₂Cl₈ · 2H₂O.

There are four K sites in the crystal structure of K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O (Fig. 3). The K1 and K2 sites are coordinated by seven Cl atoms each to form (KCl₇) capped trigonal prisms. This type of coordination is one of the most common sevenfold coordination geometries in inorganic chemistry (Hoffmann et al., 1977). In contrast, the K3 and K4 sites have a bicapped trigonal prismatic coordination by six Cl atoms and two H₂O groups. The latter are arranged at the corners of a vertical edge of the (KCl₄(H₂O)₂) trigonal prism with two tetragonal faces capped by Cl atoms (Fig. 3). This type of an eightfold coordination is again quite common for inorganic and metal-organic compounds (Burdett et al., 1978).

Fig. 3.

The coordination of K⁺ ions in the crystal structure of K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O. Рис. 3. Координация ионов K⁺ в кристаллической структуре K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O.

The [Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈]^4– chains are linked via K–Cl and K–H₂O interactions as well as by H₂O^…Cl hydrogen bonds. Each H₂O group is coordinated by two K⁺ ions and donates two hydrogen bonds to two adjacent Cl^– ions. As a result, the H₂O groups, K⁺ and Cl^– ions form chains running parallel to the a axis (Fig. 4 ).

Fig. 4.

The chain of H₂O molecules, Cl^– anions and K⁺ cations in the crystal structure of K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O. Рис. 4 . Цепочка из молекул H₂O и ионов Cl^– и K⁺ в кристаллической структуре K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O.

4. DISCUSSION

The crystal structure of K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O was solved and refined in the non-centrosymmetric space group P2₁. It is remarkable that each [Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈]^4– chain is itself centrosymmetric with inversion centers located in each Cu3 site and in the midpoints of the Cl^…Cl edges shared between the (Cu1Cl₄) and (Cu3Cl₄) tetrahedra. The non-centrosymmetricity of the overall structure is the result of the shift of the adjacent chains relative to each other in the direction parallel to the a axis.

Another interesting aspect of the crystal structure is the arrangement of Cu ions along the extension of the chains. As it was noted above, the Cu1^…Cu3 distance across the shared Cl^…Cl edge is remarkably short (2.585 Å). The Cl–Cu1–Cl and Cl–Cu3–Cl3 valence angles are 114.2 and 116.2°, respectively. This is in drastic contrast with usual values of angles opposite to the shared edges in tetrahedral dimers: as a rule, they are shortened with respect to the “ideal” tetrahedral value of 109.5° (see, e.g., Krivovichev et al., 1998). The possible explanation for the existence of such a short Cu⁺⋅⋅⋅Cu⁺ contact in the observed coordination geometry would the presence of attractive interaction between the adjacent Cu⁺ centers, i.e. a cuprophillic interaction (Jansen, 1987; Carvajal et al., 2004). However, the analysis of the electron-density distribution shows that the midpoint Cu1^…Cu3 distance (the point with the coordinates x = –0.1248, y = –0.4793, z = –0.2419) is a (3, +1) critical point, which is a ring critical point that indicates the absence of bonding interactions between the adjacent Cu centers.

In terms of chemistry, the title compound is close to avdoninite, K₂Cu₅Cl₈(OH)₄ · 2H₂O, mitscherlichite, K₂CuCl₄ · 2H₂O, and romanorlovite, K₁₁Cu₉Cl₂₅(OH)₄ ⋅ 2H₂O. However, it differs from them in its mixed-valence character. However, since mixed-valent copper compounds can exist in volcanic fumaroles, and K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O was prepared by means of chemical transport reactions, its formation under natural conditions is likely. The analysis of the structural and chemical complexity parameters (Krivovichev, 2013; Krivovichev S.V., Krivovichev V.G., 2020) listed in Table 6 shows that K₄Cu²⁺${\text{Cu}}_{2}^{ + }$Cl₈ · 2H₂O possesses neither unusually high nor unusually low complexity and thus its possible formation as a secondary phase in fumaroles does correspond to the typical level of complexity observed in this geochemical environment.

We are grateful to D.Yu. Pushcharovsky for the useful comments on the manuscript. The reported study was funded by the Russian Science Foundation, project number 19-17-00038.