INTRODUCTION

Polyoxometalates are a large and very diverse group of compounds, and a large number of them have unique physicochemical properties [1-4]. Their major applications are analytical chemistry, medicine, biology, homogeneous and heterogeneous catalysis, and others. The structural diversity is based on the ability of metal oxo complexes (for the most part, of the fifth and sixth groups) to combine in different ways depending on the synthesis conditions. Polyanions formed mainly consist of MO6 octahedra, but sometimes they also involve MO5 (square pyramid) and MO4 (tetrahedron). As a result, the number of possible configurations is truly enormous. In each such case, the polyanion has a certain charge, hence, the second aspect of the crystal structure diversity are the cation charge and structure. These factors open up great opportunities for the synthesis of more and more new crystal structures. This work considers two structures with octamolybdate anions (hereafter, OMA).

At present, the data on only few crystal structures of complex salts containing OMA and inorganic cations have been published. In their vast majority, hexaaqua cations of rare earth metals act as cations: [Eu(H2O)6]2Mo8O27·6H2O [5; No. 71216], [Sm(H2O)6]2Mo8O27·6H2O [5; No. 79725], [Dy2(H2O)12]Mo8O27·8H2O [5; Nos. 247333-247336]. There are much more complex salts containing OMA and organic cations [6].

Octamolybdate anions form when eight distorted MoO6 octahedra (in rare cases, together with MoO4 tetrahedra) link through vertices and edges. The characteristics of the known OMA isomers with the composition Mo8O26 are given in [7], but the list does not exhaust all possible variants of polyanions containing eight Mo atoms. For example, in the structure of (NH4)4Mo8O26·4H2O [5; No. 427451] isolated OMA has a βconfiguration; eight MoO6 octahedra are involved in the organization. However, when the crystallization conditions change, the additional O atoms linking the neighboring OMA into infinite 1D chains can appear in the OMA composition; as a consequence, (NH4)6Mo8O27·4H2O [5; No. 2017] salt forms where OMAs have other compositions and charges.

The terminal O atoms can be involved in the coordination environment of metal atoms belonging to the cations and, hence, form a variety of moieties: infinite 2D ribbons {[Co(H2O)4]2Mo8O27}∞ [8]; {3D-[Cu(DIE)2][1D-Mo8O26]0.5} (DIE is diimidazoloethane) [9]; [Mo8O26(BiCl3)2)]4– [10]. Some complexes based on OMA, the Ag+ cation, and organic ligands are described in [11]. The list of different structural variants with OMA can be continued.

A special role in combining OMAs is played by monatomic cations, in particular Na+. When infinite chains form, its coordination environment can include different number of O atoms from two neighboring anions: 4 + 4 [2; NEQPUV], 4 + 2 [2; NOKGEZ], etc.

This work is devoted to the synthesis and study of crystal structures containing OMA and Na+ and [Pd(NH3)4]2+ cations.

EXPERIMENTAL

In the synthesis, an aqueous Na6Mo7O24 solution prepared according to the technique [12] by neutralization of a mixture of Na2MoO4 and Na2CO3 solutions (3:1 volume ratio) was used. The freshly prepared Na6Mo7O24 solution (0.5 mmol in 35 mL of Н2O, pH ~ 5.5) was mixed with aqueous [Pd(NH3)4](NO3)2 and NaNO3 solutions in the following ratio:

Na6Mo7O24 + 2[Pd(NH3)4](NO3)2 + 2NaNO3.

A glass beaker with the reaction mixture was tightly covered with a film and left for crystallization. In four months, a fine crystalline white product formed on the bottom. It was filtered off, washed with acetone, and dried in the air (hereafter, product A). The product synthesized was a fine crystalline white powder. The mother liquor was left for further crystallization, and in two months a newly formed precipitate (hereafter, product B) was isolated in a similar way.

The single crystal XRD analysis was performed on a Bruker DUO diffractometer (MoKα radiation, graphite monochromator, CCD detector). From product A, several crystals suitable for the XRD study were selected. All were [Pd(NH3)4]2Mo8O26 (hereafter, I). The same crystals were found in product B, but along with them, single crystals with another faceting were extracted. These crystals were target complex salt Na2[Pd(NH3)4]2Mo8O27·5H2O (hereafter, II). The additional syntheses aimed at obtaining II in its pure form (variations in the ratio of initial solutions and pH) were not successful.

The structures of I and II were determined using the SHELXT-2014/5 program [13] and refined in the anisotropic (isotropic for hydrogen atoms) approximation. The hydrogen atoms were calculated geometrically. The structure was refined using the SHELXL-2018/3 program [14]. The atomic coordinates and thermal parameters have been deposited with the Cambridge Crystallographic Data Centre [2] and are available by request at the address: www.ccdc.cam.ac.uk/structures/. The crystallographic data, experimental conditions, and characteristics of the crystal structure refinement are listed in Table 1.

Table 1 Crystallographic Characteristics and the Results of the Structure Refinement for [Pd(NH3)4]2Mo8O26 and Na2[Pd(NH3)4]2Mo8O27·5H2O
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The XRD analysis of the obtained products was performed on a Bruker D8 Venture diffractometer (Incoatec IμS 3.0 microfocus tube, CuKα radiation, three-circle goniometer, PHOTON III CPAD detector) using the Debye–Scherrer scheme at Т = 298 K. The XRD pattern of product A (Fig. 1а) was indexed from the single crystal XRD data for [Pd(NH3)4]2Mo8O26; the absence of “excess” reflections evidences that it is single phase. On the XRD pattern of B weak reflections from II were observed. The IR spectrum of product A (Fig. 1b) in a KBr pellet was recorded in a wavenumber range from 400 cm–1 to 4000 cm–1 on a Scimitar FTS 2000 IR Fourier spectrometer.

Fig. 1
figure 1

Results for product A: the XRD pattern (the dashed line corresponds to the experimental curve, the solid line corresponds to the theoretical curve for [Pd(NH3)4]2Mo8O26) (а); the IR spectrum (b).

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RESULTS AND DISCUSSION. CONCLUSIONS

Crystal structure of [Pd(NH3)4]2Mo8O26. The structure of [Mo8O26]4– OMA is shown in Fig. 2а. According to the nomenclature [7], OMA is a β-isomer. It has such a configuration in many structures, for example, (NH4)4Mo8O26·4H2O [1; No. 427451]. Mo2 atoms related by the symmetry element have one terminal O atom at a distance of 1.696 Å, the other six Mo atoms have two terminal O atoms; the Mo–Oterm distances are in a range of 1.693-1.733 Å (Table 2). Six O atoms are the inner bridges for the pairs of Mo atoms (Mo2–Mo4, Mo3–Mo4, Mo1–Mo4) and symmetrical to them pairs; the Mo–Oμ2 range is 1.755-2.228 Å. Four O atoms link three Mo atoms each (Mo–Oμ3 of 1.927-2.390 Å), the other two link five Mo atoms each (Mo–Oμ5 of 2.143-2.501 Å). Note that the presence of Oμ5 atoms is the distinguishing feature of the Mo8O26 β-configuration among ten others currently known.

Fig. 2
figure 2

Fragments of the crystal structure of [Pd(NH3)4]2Mo8O26: the OMA structure at 150 K, the interatomic distances are given in Å (а); the anion packing motif (b).

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Similar to the previously studied IR spectra of polymolybdates [15-18], the bands in a range of 935-970 cm–1 refer to ν(Mo=O) stretching vibrations (Fig. 1b). A band at 905 cm–1 corresponds to ν(O–Mo–O) vibrations; and two bands at 725 am–1 and 695 cm–1 correspond to ν(Mo–O–Mo) vibrations where the O atom is connected with three Mo atoms. The δ(O–Mo–O) bending vibrations occur at 552 cm–1, whereas δ(Mo–Oμ–Mo), in which the Oμ atom is connected with two of three Mo atoms, is at 522 cm–1. In the IR spectra, the vibrations corresponding to the [Pd(NH3)]2+ cation are also observed: ν(N–H) in a range of 3300-3180 cm–1, δ(NH3) at 1626 cm–1, 1340 cm–1, 1312 cm–1, ρ(NH3) at 842 cm–1 and 807 cm–1, and Pd–N at 479 cm–1 [15]. The geometry of the complex cation is fairly standard: the Pd–N distances are 2.04 Å, the ∠N–Pd–N angle is 90°.

The general motif of the crystal structure was identified by the method described in [19, 20]. Given that [Pd(NH3)4]2+ cations are much lighter than OMAs, we focused on the search for their mutual arrangement. To this end, the XRD pattern was calculated with regard to the positions of only Mo atoms. This approach was previously employed in the analysis of packings of some structures, for example, in [21] for [CoEn3]2W7O24·6H2O. The most symmetrical sublattice was chosen using the software [22], which takes into account symmetry-related planes. Eventually, the anion sublattice (the centers of gravity of OMAs should correspond to its nodes) is formed by the intersection of the families of {1 0 –1}, {0 1 1}, {0 –1 1} planes and is spanned by the vectors aт = а, bт = а/2 + b/2 + c/2, cт = а/2 – b/2 + c/2. The subcell metrics (at = 10.40, bt = ct = 9.03 Å, αt = 67.97, βt = γt = 70.67°) allow us to consider the mutual arrangement of anions in terms of the three-layer close packing based on a distorted rhombohedron: aT ≈ 9.5 Å, αT ≈ 69.8°. Fig. 2b depicts one of the planes forming the anion sublattice.

Crystal structure of Na2[Pd(NH3)4]2Mo8O27·5H2O. The Na2[Pd(NH3)4]2Mo8O27·5H2O crystals formed in a very small amount during the crystallization of the mother liquor remained after the extraction of product A. We conducted a number of additional syntheses which led only to a slight increase in the fraction of II rather than the formation of a single-phase product.

The structure of the polyanion is shown in Fig. 3а; previously, such a configuration was observed in the structures of (NH4)6Mo8O27·4H2O [5; No. 2017], [Eu(H2O)6]2Mo8O27·6H2O [5; No. 71216], [Sm(H2O)6]2Mo8O27·6H2O [5; No. 79725], (NH4)4(Mo8O24(O2)2(H2O)2)·4H2O [5; No. 403077], [Dy2(H2O)12]Mo8O27·8H2O [5; Nos. 247333-247336]. In all cases, OMAs form ([Mo8O27]6–)∞ chains. In the structure of II, Mo atoms are in total bonded to 16 (4×2 + 2×1 + 2×2.5) terminal O atoms, the distances to which are in a range of 1.705-1.901 Å. The distance ranges of Mo–O with bridging O atoms connecting two, three, and four Mo atoms are given in Table 2. No O atoms bonding five Mo atoms, as in the case of I, have been identified.

Table 2 Characteristics of OMAs in the Structures of [Pd(NH3)4]2Mo8O26 and Na2[Pd(NH3)4]2Mo8O27·5H2O
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Fig. 3
figure 3

Fragments of the crystal structure of Na2[Pd(NH3)4]2Mo8O27·5H2O: the OMA structure, interatomic distances are given in Å (a); the packing motif (b).

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Since the structure of II is triclinic and Z = 1, the anion subcell coincides with the unit cell. Pseudohexagonal anion layers (a ≈ c ≈ 9.2 Å, ∠β ≈ 120°) are perpendicular to the Y axis. The Na+ cations and the crystallization water molecules occupy the cavities between them (Fig. 3b). Some terminal O atoms are involved in the coordination of Na+ cations, as a result of which, a complex 3D structure forms. The Na+ coordination polyhedron is a distorted octahedron; the Na…O distances are in a range of 2.315-2.693 Å.

Fig. 4
figure 4

Fragments of the crystal structures of (NH4)6Mo8O27·4H2O [5; No. 2017] (a), [Eu(H2O)6]2Mo8O27· ·6H2O) [5; No. 71216] (b); (NH4)2[Co(H2O)4]2Mo8O27·6H2O [8] (c).

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Thus, in this work, it is shown that fractional crystallization from a mixture of aqueous Na6Mo7O24, [Pd(NH3)4](NO3)2 and NaNO3 solutions results in the formation of two new complex salts containing OMAs with fundamentally different structures: [Mo8O26]4– and ([Mo8O27]6–)∞.