Crystal structure of Cs2GdNb6Cl15O3 in the structural evolution of niobium oxychlorides with octahedral Nb6-cluster units

Cs2GdNb6Cl15O3 is an octahedral Nb-cluster compound containing three inner ligands of oxygen atoms. Individual Nb6 clusters are linked to each other via GdIII and CsI atoms, which exhibit a coordination number of 9 (three O and six Cl ligands) and 12 (twelve Cl ligands).


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 [Nb 6 L i 12 L a 6 ] unit (L = halogen or oxygen ligands) have been reported previously (Perrin et al., 2001).In this cluster unit, the edge of the Nb 6 octahedron is bridged by twelve inner ligands (L i ) while the other six outer ligands (L a ) are located at apical positions (Scha ¨fer & 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 (Ko ¨hler et al., 1991).
Mixing of halogen and oxygen as ligands for Nb 6 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 Nb 6 clusters were reported by Perrin et al. with one O i  (Cordier et al., 1996), two O i (Gulo & Perrin, 2000), three O i (Cordier et al., 1994(Cordier et al., , 1997;;Gulo & Perrin, 2002), and six O i (Gulo et al., 2001) ligands per cluster unit.Other oxychloride compounds containing four O i (Anokhina et al., 1998(Anokhina et al., , 2000) ) and six O i (Anokhina et al., 2001) ligands per cluster unit are also known.
The niobium oxychloride compound Cs 2 UNb 6 Cl 15 O 3 was synthesized and structurally characterized many years ago (Cordier et al., 1997).We have now prepared a related compound with composition Cs 2 GdNb 6 Cl 15 O 3 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 Cs 2 GdNb 6 Cl 15 O 3 are compared with other niobium oxychlorides containing octahedral Nb 6 clusters.

Structural commentary
The structure of Cs 2 GdNb 6 Cl 15 O 3 is isotypic with Cs 2 UNb 6 Cl 15 O 3 and displays the Nb 6 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) innerligands, and one chlorine (Cl3) outer-ligand.Every edge of the Nb 6 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, [(Nb 6 Cl i 9 O i 3 )Cl a 6 ] 5-.The length of the intracluster Nb-Nb bonds range from 2.7686 (5) to 3.0317 (5) A ˚corresponding to the edge bridged by the O i and Cl i ligands, respectively; the average bond length is 2.954 A ˚. Thus, the Nb 6 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 O i ligands but are significantly longer than those observed in compounds containing four or more O i ligands (Gulo & Perrin, 2012).In the various oxychloride compounds that have been isolated so far, it seems that an increase in the number of O i 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 PbLu 3 Nb 6 Cl 15 O 6 (Gulo et al., 2001) with six O i ligands.Cs 2 GdNb 6 Cl 15 O 3 has three O i ligands per cluster.They are localized at trans-inner positions relative to the Nb 6 cluster, similar to the arrangement of three O i ligands in Na 0.21 Nb 6 Cl 10.5 O 3 where the cluster exhibits point group symmetry 3 (Gulo & Perrin, 2002).In contrast, the three O i ligands in ScNb 6 Cl 13 O 3 occupy a cis-inner position relative to the Nb 6 octahedron to produce a cluster motif with 2 symmetry (Cordier et al., 1994).In the title compound, the Nb 6 clusters are arranged in ( 001  The Nb-Cl i distances vary from 2.4543 (7) A ˚to 2.4802 (7) A ˚(average 2.468 A ˚) while the Nb-Cl a bond is longer, 2.5728 (7) A ˚.In general, the Nb-L (L = O, Cl) bond lengths in Cs 2 GdNb 6 Cl 15 O 3 are not significantly different from that of other niobium oxychloride compounds (Naumov et al., 2003).
In the crystal structure of Cs 2 GdNb 6 Cl 15 O 3 , the [(Nb 6 Cl i 9 O i 3 )Cl a 6 ] 5-units are interconnected through the Cs I and Gd III atoms that are located in between the layers of Nb 6 clusters (Fig. 2).The existence of such discrete cluster units or the absence of intercluster connectivity has also been observed in PbLu 3 Nb 6 Cl 15 O 6 (Gulo et al., 2001) and Cs 2 Ti 4 Nb 6 Cl 18 O 6 (Anokhina et al., 2001) where Nb 6 -clusters likewise are formed by six symmetry-equivalent Nb atoms in contrast to CsNb 6 Cl 12 O 2 (Gulo & Perrin, 2000) where the Nb 6 -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 Cs 2 GdNb 6 Cl 15 O 3 has a coordination number of 9, defined by three O i and six Cl a ligands provided by three nearby cluster units (Fig. 3), with bond lengths of Gd-O = 2.322 (3) A ˚and Gd-Cl = 3.0994 (8) A ˚.In comparison, in the crystal structure of PbLu 3 Nb 6 Cl 15 O 6 , the Lu III atom is surrounded by only six ligands, viz.two O and four Cl atoms, defining Lu 2 Cl 2 entities (Gulo et al., 2001).The Nb 6 -clusters in PbLu 3 Nb 6 Cl 15 O 6 connect to each other via these Lu 2 Cl 2 entities whereby each cluster is surrounded by six Lu 2 Cl 2 entities, and each of them bridging four adjacent clusters via O and Cl ligands.A related motif is found in Ti 2 Nb 6 Cl 14 O 4 where Ti III atoms form zigzag chains of edgesharing [TiCl 4 O 2 ] octahedra (Anokhina et al., 2000).In other cases, the trivalent ions, such as Sc III in ScNb 6 Cl 13 O 3 (Cordier et al., 1994) or Ti III in Cs 2 Ti 3 Nb 12 Cl 27 O 8 (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 RbGdNb 6 Cl 18 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 Nb 6 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 Cs 2 GdNb 6 Cl 15 O 3 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.
In Cs 2 GdNb 6 Cl 15 O 3 , the monovalent Cs I atom is surrounded by six Nb 6 clusters and coordinated by twelve Cl ligands (Fig. 4).The lengths of Cs-Cl bonds range from 3.5074 (8) to 3.9770 (6) A ˚.A similar environment around Cs is found in CsNb 6 Cl 12 O 2 with Cs-Cl distances between 3.330 (5) and 3.862 (4) A ˚ (Gulo & Perrin, 2000).In contrast, the Cs I atom in Cs 2 LuNb 6 Cl 17 O is surrounded by four Nb 6clusters and is bonded to twelve chlorine ligands with Cs-Cl distances in the range 3.567 (1) to 3.619 (1) A ˚ (Cordier et al., 1996).In RbGdNb 6 Cl 18 with its smaller monovalent cation Rb I , the coordination number is likewise 12. Here, the cation is surrounded also by four Nb 6 clusters, and the Rb-Cl bond lengths range from 3.471 (1) A ˚to 3.557 (2) A ˚with an average of 3.512 A ˚ (Gulo et al., 2023).The sites of monovalent cations encountered in the crystal structures of oxychlorides with Nb 6 clusters are always surrounded by Cl ligands with the exception of Cs 2 LuNb 6 Cl 17 O 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 0.21 Nb 6 Cl 10.5 O 3 , the corresponding Na site has an occupancy of only 42.6% (Gulo & Perrin, 2002) and the three Cs sites in Cs 2 Ti 4 Nb 6 Cl 18 O 6 have occupancies of 38.1%, 57.0% and 6.9% (Anokhina et al., 2001).

Figure 3
The environment of the Gd III atom in Cs 2 GdNb 6 Cl 15 O 3 .

Figure 4
The environment of the Cs I atom in Cs 2 GdNb 6 Cl 15 O 3 .
The VEC in Cs 2 GdNb 6 Cl 15 O 3 is 14 per cluster unit, as observed in most oxide (Ko ¨hler et al., 1991) and oxychloride compounds (Gulo & Perrin, 2012).The number of O i ligands per cluster can affect the VEC value.Compounds containing one O i ligand (Cordier et al., 1996) or two O i ligands (Gulo & Perrin, 2000) exhibit VEC values of 16 and 15.However, niobium oxychloride compounds containing three or more O i ligands per cluster unit have always a VEC of 14 per cluster unit.

Synthesis and crystallization
Cs 2 GdNb 6 Cl 15 O 3 was prepared by solid-state reactions, starting from a stoichiometric mixture of CsCl (Prolabo, purity 99.5%), Gd 2 O 3 (Rho ˆne Poulenc), Nb 2 O 5 (Merck, Optipur), NbCl 5 (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.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes. Fractional Figure 1 The [(Nb 6 Cl i 9 O i 3 )Cl a 6 ] 5-unit in the crystal structure of Cs 2 GdNb 6 Cl 15 O 3 .Displacement ellipsoids are shown at the 60% probability level.

Figure 2
Figure 2 The . . .AA 0 A . . .stacking of the Nb 6 clusters in the crystal structure of Cs 2 GdNb 6 Cl 15 O 3 .Cl and O atoms are omitted for clarity

Table 1
Experimental details.
atomic coordinates and isotropic or equivalent isotropic displacement parameters(Å 2 )