1. Chemical context

Transition-metal clusters have high potentials and synergetic effects in the fields of biotechnology, catalysis, or sensor applications (Nguyen et al., 2022). The use of these clusters as supra­molecular building units is advantageous because of their unique structural, chemical and physical properties (Zhou & Lachgar, 2007). For example, charge-transfer (CT) solids with an anti-perovskite crystal structure have been derived from molybdenum cluster units by electro-crystallization (Hiramatsu et al., 2015), or octa­hedral cluster units of niobium have been widely used as raw materials for the preparation of novel compounds with inter­esting structures and magnetic properties (Naumov et al., 2003; Zhang et al., 2011).

A large number of binary, ternary and quaternary niobium compounds with octa­hedral clusters based on the [Nb₆Lⁱ₁₂L^a₆] unit (L = halogen or oxygen ligands) have been reported previously (Perrin et al., 2001). In this cluster unit, the edge of the Nb₆ octa­hedron is bridged by twelve inner ligands (Lⁱ) while the other six outer ligands (L^a) are located at apical positions (Schäfer & von Schnering, 1964). The number of electrons involved in the formation of metal–metal bonds in the cluster is called the valence electron count (VEC). The ideal VEC value per cluster is 16 for chloride compounds and 14 for oxide compounds. In chlorides, the cluster units are inter­linked by involving outer ligands (Perrin, 1997), whereas in oxides, the connectivity between the units is achieved through the inner ligands (Köhler et al., 1991).

Mixing of halogen and oxygen as ligands for Nb₆ cluster compounds is a very inter­esting topic for in-depth studies to enrich our knowledge about new materials and their physical and chemical properties. It has been reported that the structural, magnetic and electronic properties of octa­hedral clusters of niobium oxychlorides are influenced by oxygen ligands (Fontaine et al., 2011). In this respect, preparation and characterization of new oxychloride compounds with octa­hedral Nb₆ clusters were reported by Perrin et al. with one Oⁱ (Cordier et al., 1996), two Oⁱ (Gulo & Perrin, 2000), three Oⁱ (Cordier et al., 1994, 1997; Gulo & Perrin, 2002), and six Oⁱ (Gulo et al., 2001) ligands per cluster unit. Other oxychloride compounds containing four Oⁱ (Anokhina et al., 1998, 2000) and six Oⁱ (Anokhina et al., 2001) ligands per cluster unit are also known.

The niobium oxychloride compound Cs₂UNb₆Cl₁₅O₃ was synthesized and structurally characterized many years ago (Cordier et al., 1997). We have now prepared a related compound with composition Cs₂GdNb₆Cl₁₅O₃ and have determined its crystal structure where gadolinium occupies the same position as uranium in the previous compound. In the current communication, the crystal structure, inter­atomic distances, and the role of monovalent and trivalent cations in Cs₂GdNb₆Cl₁₅O₃ are compared with other niobium oxychlorides containing octa­hedral Nb₆ clusters.

2. Structural commentary

The structure of Cs₂GdNb₆Cl₁₅O₃ is isotypic with Cs₂UNb₆Cl₁₅O₃ and displays the Nb₆ octa­hedron as the basic cluster motif. The asymmetric unit comprises seven sites: one Cs (site symmetry 3.., multiplicity 4, Wyckoff letter e), one Gd (3.2, 2 c), one Nb (1, 12 i), one O (..2, 6 h) and three Cl (Cl1: 6 h; Cl2 12 i; Cl3: 12 i). Six symmetry-equivalent niobium atoms build up the octa­hedral cluster (centered at a position with site symmetry 3.2, 2 a). Each niobium atom is surrounded by one oxygen (O) inner-ligand, three chlorine (Cl1 and Cl2) inner-ligands, and one chlorine (Cl3) outer-ligand. Every edge of the Nb₆ octa­hedron is bridged by a chlorine or oxygen ligand as inner-ligands, and six other chlorine ligands are attached in apical positions as outer ligands, as shown in Fig. 1. This cluster motif can be written as a developed unit, [(Nb₆Clⁱ₉Oⁱ₃)Cl^a₆]^5–.

Figure 1 The [(Nb6Cli9Oi3)Cla6]5– unit in the crystal structure of Cs2GdNb6Cl15O3. Displacement ellipsoids are shown at the 60% probability level.

The length of the intra­cluster Nb—Nb bonds range from 2.7686 (5) to 3.0317 (5) Å corresponding to the edge bridged by the Oⁱ and Clⁱ ligands, respectively; the average bond length is 2.954 Å. Thus, the Nb₆ octa­hedron undergoes distortions as observed in other niobium oxychloride compounds. The Nb—Nb distances in this compound are significantly shorter than those observed in other compounds containing two or fewer Oⁱ ligands but are significantly longer than those observed in compounds containing four or more Oⁱ ligands (Gulo & Perrin, 2012). In the various oxychloride compounds that have been isolated so far, it seems that an increase in the number of Oⁱ ligands per formula leads to a decrease in the length of intra­cluster Nb—Nb bonds. This difference is due to a stronger steric effect as observed, for example, in PbLu₃Nb₆Cl₁₅O₆ (Gulo et al., 2001) with six Oⁱ ligands. Cs₂GdNb₆Cl₁₅O₃ has three Oⁱ ligands per cluster. They are localized at trans-inner positions relative to the Nb₆ cluster, similar to the arrangement of three Oⁱ ligands in Na_0.21Nb₆Cl_10.5O₃ where the cluster exhibits point group symmetry 3 (Gulo & Perrin, 2002). In contrast, the three Oⁱ ligands in ScNb₆Cl₁₃O₃ occupy a cis-inner position relative to the Nb₆ octa­hedron to produce a cluster motif with 2 symmetry (Cordier et al., 1994). In the title compound, the Nb₆ clusters are arranged in (001) layers with an …AA′A… stacking along [001] (Fig. 2).

Figure 2 The …AA′A… stacking of the Nb6 clusters in the crystal structure of Cs2GdNb6Cl15O3. Cl and O atoms are omitted for clarity

The Nb—Clⁱ distances vary from 2.4543 (7) Å to 2.4802 (7) Å (average 2.468 Å) while the Nb—Cl^a bond is longer, 2.5728 (7) Å. In general, the Nb—L (L = O, Cl) bond lengths in Cs₂GdNb₆Cl₁₅O₃ are not significantly different from that of other niobium oxychloride compounds (Naumov et al., 2003).

In the crystal structure of Cs₂GdNb₆Cl₁₅O₃, the [(Nb₆Clⁱ₉Oⁱ₃)Cl^a₆]^5– units are inter­connected through the Cs^I and Gd^III atoms that are located in between the layers of Nb₆ clusters (Fig. 2). The existence of such discrete cluster units or the absence of inter­cluster connectivity has also been observed in PbLu₃Nb₆Cl₁₅O₆ (Gulo et al., 2001) and Cs₂Ti₄Nb₆Cl₁₈O₆ (Anokhina et al., 2001) where Nb₆-clusters likewise are formed by six symmetry-equivalent Nb atoms in contrast to CsNb₆Cl₁₂O₂ (Gulo & Perrin, 2000) where the Nb₆-octa­hedron is formed by three different Nb atoms. In the crystal structure of the latter, the the cluster units are linked together via bridging O and Cl ligands.

The Gd^III atom in Cs₂GdNb₆Cl₁₅O₃ has a coordination number of 9, defined by three Oⁱ and six Cl^a ligands provided by three nearby cluster units (Fig. 3), with bond lengths of Gd—O = 2.322 (3) Å and Gd—Cl = 3.0994 (8) Å. In comparison, in the crystal structure of PbLu₃Nb₆Cl₁₅O₆, the Lu^III atom is surrounded by only six ligands, viz. two O and four Cl atoms, defining Lu₂Cl₂ entities (Gulo et al., 2001). The Nb₆-clusters in PbLu₃Nb₆Cl₁₅O₆ connect to each other via these Lu₂Cl₂ entities whereby each cluster is surrounded by six Lu₂Cl₂ entities, and each of them bridging four adjacent clusters via O and Cl ligands. A related motif is found in Ti₂Nb₆Cl₁₄O₄ where Ti^III atoms form zigzag chains of edge-sharing [TiCl₄O₂] octa­hedra (Anokhina et al., 2000). In other cases, the trivalent ions, such as Sc^III in ScNb₆Cl₁₃O₃ (Cordier et al., 1994) or Ti^III in Cs₂Ti₃Nb₁₂Cl₂₇O₈ (Anokhina et al., 2000), have a coordination number of five, defined by three O and two Cl ligands. In another case, the Gd^III atom in RbGdNb₆Cl₁₈ is octa­hedrally surrounded by six Cl ligands from six neighboring cluster units (Gulo et al., 2023). In general, in the series of niobium oxychloride compounds containing octa­hedral Nb₆ clusters, the crystallographic sites associated with trivalent cations are always fully occupied and are surrounded by Cl and O ligands (Gulo & Perrin, 2012). Only in Cs₂GdNb₆Cl₁₅O₃ and the isotypic uranium analogue, the trivalent cation occupy the center of a triangle formed by three adjacent cluster units and are bonded to nine ligands.

Figure 3 The environment of the GdIII atom in Cs2GdNb6Cl15O3.

In Cs₂GdNb₆Cl₁₅O₃, the monovalent Cs^I atom is surrounded by six Nb₆ clusters and coordinated by twelve Cl ligands (Fig. 4). The lengths of Cs—Cl bonds range from 3.5074 (8) to 3.9770 (6) Å. A similar environment around Cs is found in CsNb₆Cl₁₂O₂ with Cs—Cl distances between 3.330 (5) and 3.862 (4) Å (Gulo & Perrin, 2000). In contrast, the Cs^I atom in Cs₂LuNb₆Cl₁₇O is surrounded by four Nb₆-clusters and is bonded to twelve chlorine ligands with Cs—Cl distances in the range 3.567 (1) to 3.619 (1) Å (Cordier et al., 1996). In RbGdNb₆Cl₁₈ with its smaller monovalent cation Rb^I, the coordination number is likewise 12. Here, the cation is surrounded also by four Nb₆ clusters, and the Rb—Cl bond lengths range from 3.471 (1) Å to 3.557 (2) Å with an average of 3.512 Å (Gulo et al., 2023). The sites of monovalent cations encountered in the crystal structures of oxychlorides with Nb₆ clusters are always surrounded by Cl ligands with the exception of Cs₂LuNb₆Cl₁₇O where an O atom statistically occupies a site among the twelve inner ligands defining the coordination environment of Cs. On the other hand, the sites associated with the (large) monovalent cation often show partial occupancy. For example, in Na_0.21Nb₆Cl_10.5O₃, the corresponding Na site has an occupancy of only 42.6% (Gulo & Perrin, 2002) and the three Cs sites in Cs₂Ti₄Nb₆Cl₁₈O₆ have occupancies of 38.1%, 57.0% and 6.9% (Anokhina et al., 2001).

Figure 4 The environment of the CsI atom in Cs2GdNb6Cl15O3.

The VEC in Cs₂GdNb₆Cl₁₅O₃ is 14 per cluster unit, as observed in most oxide (Köhler et al., 1991) and oxychloride compounds (Gulo & Perrin, 2012). The number of Oⁱ ligands per cluster can affect the VEC value. Compounds containing one Oⁱ ligand (Cordier et al., 1996) or two Oⁱ ligands (Gulo & Perrin, 2000) exhibit VEC values of 16 and 15. However, niobium oxychloride compounds containing three or more Oⁱ ligands per cluster unit have always a VEC of 14 per cluster unit.

3. Synthesis and crystallization

Cs₂GdNb₆Cl₁₅O₃ was prepared by solid-state reactions, starting from a stoichiometric mixture of CsCl (Prolabo, purity 99.5%), Gd₂O₃ (Rhône Poulenc), Nb₂O₅ (Merck, Optipur), NbCl₅ (Ventron, purity 99.998%) and niobium powder (Ventron, purity 99.8%). A total of 300 mg of the mixture was mashed and then loaded in a silica tube under argon atmos­phere in a glove box. The silica tube sample was then sealed under vacuum condition. The sample was heated in a vertical heating furnace at 973 K for two days, followed by slow cooling to room temperature. Brown single crystals with a block-like form suitable for structural determination were obtained this way.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The remaining maximum and minimum electron density peaks are located 0.19 Å from Nb and 0.54 Å from Cs, respectively.

Table 1 Experimental details Crystal data Chemical formula Cs2GdNb6Cl15O3 Mr 1560.28 Crystal system, space group Trigonal, P1c Temperature (K) 293 a, c (Å) 9.1318 (1), 17.1558 (2) V (Å3) 1238.95 (3) Z 2 Radiation type Mo Kα μ (mm−1) 9.83 Crystal size (mm) 0.08 × 0.07 × 0.05   Data collection Diffractometer Nonius KappaCCD Absorption correction Multi-scan (DENZO and SCALEPACK; Otwinowski & Minor, 1997) Tmin, Tmax 0.004, 0.017 No. of measured, independent and observed [I > 2σ(I)] reflections 7607, 2020, 1790 Rint 0.026 (sin θ/λ)max (Å−1) 0.833   Refinement R[F2 > 2σ(F2)], wR(F2), S 0.029, 0.068, 1.15 No. of reflections 2020 No. of parameters 44 Δρmax, Δρmin (e Å−3) 1.38, −1.35 Computer programs: COLLECT (Nonius, 1999), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b), DIAMOND (Brandenburg, 2014), and publCIF (Westrip, 2010).