Mixed crystal of bis(ammonium/oxonium) tetraaqua-μ3-fluorido-dodecakis(μ2-trifluoroacetato)octahedro-hexaytterbiate(III) tetrahydrate, [(NH4)1–x (H3O) x ]2[Yb6F8(O2CCF3)12(H2O)4]·4H2O (x = 1/4), containing a hexanuclear ytterbium(III) carboxylate complex with face-capping fluoride ligands and comprising an unusual kind of substitutional disorder

By an oxidative synthesis in aqueous trifluoroacetic acid, the mixed ammonium/oxonium crystalline solid [(NH4)1–x (H3O) x ]2[Yb6F8(O2CCF3)12(H2O)4]·4H2O is obtained from ytterbium(II) trifluoroacetate. It is the first example of a substance containing an octahedro-hexanuclear ytterbium(III) complex with μ3-face-capping fluorido ligands and comprises an unusual kind of substitutional disorder. The effects of the disordered cation position on the coordination of additional O,O′-bridging carboxylato and aqua ligands are discussed in detail.

The reaction of ytterbium metal with ammonium trifluoroacetate in liquid ammonia resulted in a green substance comprising a substantial amount of ytterbium(II) trifluoroacetate that is a useful precursor for the oxidative synthesis of the new ytterbium(III) compound, [(NH 4 ) 1-x (H 3 O) x ] 2 [Yb 6 F 8 -(O 2 CCF 3 ) 12 (H 2 O) 4 ]Á4H 2 O (x = 1/4), in aqueous trifluoroacetic acid. This mixed ammonium/oxonium crystalline solid is the first example of a substance containing an octahedro-hexanuclear ytterbium(III) complex with 3 -facecapping fluorido ligands. The main structural features of its [Yb 6 F 8 ] core are non-bonding YbÁ Á ÁYb distances and Yb-F bond lengths of 3.7576 (3)-3.9413 (5) and 2.2375 (17)-2.3509 (17) Å , respectively. Yb-O bond lengths involving the O atoms of O,O 0 -bridging carboxylato ligands and vertexsubstituting aqua ligands are in the ranges 2.23 (4)-2.329 (2) and 2.448 (2)-2.544 (3) Å , respectively. These bond lengths are in accordance with expectations, taking into account lanthanoid contraction. Interestingly, there is a significant ammonium versus oxonium ion site dependence, not only of the hydrate water molecule positions within the solid's hydrogen-bonding framework, but also of the coordination sites of one carboxylato and one aqua ligand in the hexanuclear complex.

Chemical context
The stabilizing influence of liquid ammonia as a reaction medium on Ln II of certain lanthanoids (Ln) is well known (Warf & Korst, 1956;Warf, 1970). Selected ytterbium(II) compounds such as bis(cyclopentadienyl)ytterbium(II) (Fischer & Fischer, 1965;Hayes & Thomas, 1969), ytterbium(II) phosphide (Pytlewsky & Howell, 1967), ytterbium(II) amide (Hadenfeldt & Juza, 1969;Hadenfeldt et al., 1970;Gö rne et al., 2016) and ytterbium(II) halides (Howell & Pytlewski, 1969) can be obtained by precipitation reactions in liquid ammonia. Adapting this procedure in explorative attempts to synthesize ytterbium(II) trifluoroacetate, we obtained a green mixture of substances, the color of which indicating the presence of Yb II ions. By dissolution experiments in trifluoroacetic acid and subsequent crystallization under non-inert conditions, we obtained colorless crystals of the title compound. The formation of this substance requires not only redox reactions with the change of the oxidation state from 0 to +II and from +II to +III, but also an activation of the C-F bonds of the trifluoroacetate anion (Rillings & Roberts, 1974). This is evident not only from the presence of fluorido ligands as part of the octahedro-hexanuclear complex anion of the title compound, [(NH 4 ) 1-x (H 3 O) x ] 2 [Yb 6 F 8 (O 2 CCF 3 ) 12 -(H 2 O) 4 ]Á4H 2 O (x = 0.25), but also from the presence of ammonium fluoride in the greenish precipitate from the reaction of ytterbium metal with ammonium trifluoroacetate in liquid ammonia.

Structural commentary
In the course of the crystal-structure refinement, the crystal under investigation turned out to be a mixed crystal characterized by NH 4 + /H 3 O + substitution. However, the structure model with disorder of the cation sites is much more complicated because the disorder not only affects the latter, but also other parts of the crystal structure. Fig. 1 shows the asymmetric unit of the title compound, separated in terms of the NH 4 + -containing partial occupation site (part a) and in terms of the H 3 O + -containing partial occupation site (part b). Both partial occupation site units comprise three Yb II ions, four fluoride anions, six trifluoroacetate anions and two water molecules, all in general position and establishing one half of a centrosymmetric octahedro-hexanuclear [Yb 6 F 8 (O 2 CCF 3 ) 12 (H 2 O) 4 ] 2complex. Also in general positions, one NH 4 + or H 3 O + cation and two water molecules complete the asymmetric unit. The charge balance of the double-negatively charged complex ion is ensured by two symmetry-related cations. The most prominent moiety in both cases is the octahedro-hexanuclear anionic complex, formed by six Yb III ions with non-bonding YbÁ Á ÁYb distances of 3.7576 (3)-3.9413 (5) Å (mean 3.83 Å , see Table 1), the eight octahedral faces of which are capped by 3 -fluorido ligands. In the NH 4 + case, all twelve octahedral edges of the central [Yb 6 F 8 ] core are bridged by 2 -trifluoroacetato ligands. Yb1 is eightfold coordinated with a typical square-antiprismatic coordination polyhedron (Karraker, 1970). Water molecules additionally coordinate the octahedral vertices of the Yb2 and Yb3 sites and complete the coordination sphere of these Yb III ions, giving a ninefold coordination that results in monocapped square-antiprismatic coordination polyhedra (Fig. 2a).   -containing partial occupation site (a) and as related to the H 3 O + -containing partial occupation site (b), shown separately with the same view direction and the same scaling. Displacement ellipsoids are drawn at the 50% probability level, hydrogen atoms are drawn with an arbitrary radius. The CF 3 groups at C5 and C11 suffer from rotational disorder that is not related to the cation substitution; only F atoms of the major occupied sites are shown. The directions of further Yb-O and Yb-F bonds are given by truncated sticks, the directions of hydrogen-bonding by segmented blue sticks. Note the coincidence of most parts of the partial occupation site models and the significant differences in the cation/water region and the coordination spheres of Yb2 and Yb3.
In the H 3 O + case, one trifluoroacetato ligand binds to Yb2 monodentately only, while two water molecules coordinate to Yb3 in return ( Fig. 1), giving an eightfold coordination of Yb1 and Yb2 and a ninefold coordination for Yb3 (Fig. 2b). At first view, the nature of the cation seems to influence the remaining parts of the structure and even to some extent the ligand substitution pattern of the hexanuclear complex. However, we cannot exclude the possibility that the presence of the two isomeric anions (related to hydration) is the origin of the cation substitution. The Yb-O bond lengths of 2.23 (4)-2.329 (2) Å (mean 2.30 Å ), and the O-C-O 0 bond angles of 129.6 (3)-132.2 (3) (mean 129.9 ) of the trifluoroacetato ligands are in typical ranges for the bidentately bridging coordination mode of carboxylate ligands (Rohde & Urland, 2006). Relevant Yb-F and Yb-O bond lengths are given in Table 1, along with the corresponding empirical bond valences for each bond, s i . The Yb-F bond lengths and the bondvalence sums S of 3.01-3.13 valence units give striking struc-tural evidence for the presence of fluorido ligands. Comparisons of the complex anion with the one in the very recent crystal-structure determination of an octahedro-hexanuclear terbium(III) complex containing a [Tb 6 F 8 ] core (Ling et al., 2020) and with some europium(III) complexes containing [Eu 6 F 8 ] cores (Morsbach et al., 2022) Table 1 Selected structural parameters (Å ) and empirical bond valences s i (valence units) for Yb1-Yb3.

Figure 2
Central [Yb 6 F 8 ] core of the title structure with additional O atoms coordinating to Yb III ions, with the cation partial occupation site occupied by NH 4 + (a) and H 3 O + (b). For Yb III ions with square-antiprismatic coordination, polyhedra are drawn in red, for Yb III ions with monocapped square-antiprismatic coordination, polyhedra are drawn in blue; color code: O (red), F (green), Yb (gray).

Supramolecular features
Approximating the hexanuclear anionic complex as a bulky sphere, a distorted fcc packing of these voluminous anions can be recognized. As shown in Fig. 3 in more detail, in a strongly off-center mode the small cations occupy all tetrahedral interstices of this packing. The hexanuclear ytterbiate(III) anions as well as all other moieties are engaged in an extended hydrogen-bonded supramolecular network (Table 2). All hydrogen bonds have medium to weak strengths. A remarkable segment of this network is established by two symmetryrelated pairs of water molecules around a center of inversion.  Table 2 Hydrogen-bond geometry (Å , ).  (3) 134 (5) Symmetry codes:

Figure 3
Schematic packing diagram. The bulky fluoridocarboxylate anions are represented by octahedra, the cation positions are given by dot-centered circles, and the closest anionÁ Á Ácation contacts are indicated by dashed lines. The distorted fcc-packing of the bulky anions can easily be recognized. Note the offset of the cations from the centers of the tetrahedral intersticial regions that are indicated by eight translucent circular areas. With respect to the primitive unit cell, this offset is along [101] or in the reverse direction and to a lesser extent along [010] or in the reverse direction. Thus, each cation is significantly closer to one of the four anions establishing a tetrahedral hole than to the other three.
Depending on the nature of the cation, the positions and orientations of these water molecules are significantly different, as shown in Fig. 4. Note, that the partial occupation sites occupied by O2 and O3 are related to NH 4 + and those occupied by O2A and O3A are related to H 3 O + . In both cases, the graph set descriptor R 4 4 (8) can be assigned to the hydrogen-bond motif (Etter et al., 1990). However, a different orientation of the hydrogen-bond-donor direction is given within the ring-shaped system. In the NH 4 + case, with the exception of four H atoms at the vertices (H5, H7, H5 ii and H7 ii ), the (H 2 O) 4 unit is almost planar (Fig. 4a) (Taft et al., 1978;Saielli, 2010), the primary hydrogen-bonding interaction of H 3 O + is significantly stronger than that of NH 4

Synthesis and crystallization
All chemicals were obtained from commercial sources and used as purchased. In a representative experiment, 0.584 g (0.337 mmol) of ytterbium were dissolved in approximately 50 ml of liquid ammonia (dried over sodium) to which 0.903 g (0.675 mmol) of ammonium trifluoroacetate were added. The ammonia was evaporated, and the residue was dried in vacuo until a pressure of 10 À3 hPa was reached. 0.816 g of a greenish powder were obtained. 100 mg of this powder were stirred in 2 ml of anhydrous trifluoroacetic acid, and the insoluble portions were allowed to settle overnight. The supernatant solution was transferred into an ampoule and stored open in air. Colorless crystals of the title compound grew within one week. A suitable single crystal for X-ray crystal structure determination was selected directly from the mother liquor. An IR spectrum was recorded with a Spectrum Two FT-IR spectrometer (Perkin Elmer Inc., 2008), equipped with a LiTaO 3 detector (4000-350 cm À1 ) and an ATR unit. Band assignments were made according to metal trifluoroacetate salts (Baillie et al., 1968;Faniran & Patel, 1976

Refinement
Crystal data along with data collection and structure refinement details are summarized in Table 3. After having completed the primary structural model, (a) physically nonmeaningful anisotropic displacement parameters, (b) features appearing in the difference-electron density map in the course of further refinement cycles and (c) analysis of potential hydrogen-bonding orientations clearly indicated disorder that refers to: (i) position and nature of the cation (NH 4 + vs. H 3 O + ), (ii) position and coordination mode of the complete carboxylato ligand with atoms O8 and O9, (iii) position (coordination site) of the aqua ligand with O17, (iv) orientation of the aqua ligand with O16, (v) rotational orientation of four of the six CF 3 groups and (vi) position and orientation of the two hydrate water molecules. The refinement of siteoccupation factors finally proved the disorder according to (i), (ii), (iii), (iv), (vi) and the rotational orientations of three of the four CF 3 groups addressed in (v) to be directly dependent. In the final stages of a converging refinement, for these dependent sites a common occupation factor was introduced and refined to 0.749 (4) for NH 4 + and its related partial occupation site moieties, giving 0.251 (4) for H 3 O + and its related moieties. When involved in disorder, NH 4 + and H 3 O + ions can hardly be distinguished in a structure refinement based on X-ray diffraction data alone. All substances related to the class of the title compound showed somewhat too high proportions for N in the combustion analysis, and due to the complex vibration spectra, an identification of O-H or N-H stretching modes in the IR spectrum is not possible. In consequence, the nature of the cations could not be determined by chemical analysis or spectroscopic studies. Even though the crystal structure model is therefore based only on the results of structure refinement and comparative structural considerations, the final choice of occupation with NH 4 + and H 3 O + is unambiguous for the following reasons: the partial occupation site related to N1 with 75% occupation shows four tetrahedrally arranged residual electron-density maxima, which are identified as H atoms on the basis of their heights and spacings; at the site related to O1 with 25% occupation, clear electron-density maxima could not be identified, as ]. Finally, if the lower-occupied site were assumed to be a NH 4 + ion, no suitable hydrogen-bond acceptor could be identified for an additional, fourth hydrogen bond. All disordered parts of the structure were subjected to appropriate bond lengths and angles and anisotropic displacement restraints or constraints. The C-F bond lengths of the disordered CF 3 groups related to C4, C10, C12, (C6) were restrained to 1.32 Å within a s. u. of 0.02 Å (0.002 Å ), combined with default FÁ Á ÁF same distance and with strongly restrictive isotropic displacement restraints for all F atoms. No restraints were needed for the two CF 3 groups not suffering from disorder. For the CF 3 group related to C6, which suffers from both dependent positional and independent rotational disorder, more restrictive C-F bond lengths restraints (see above) had to be used and the C-C bond length was restrained to 1.52 Å within a s. u. of 0.02 Å . For atoms at partial occupation sites in close proximity, in an approximative manner equivalent anisotropic displacement constraints have been applied, namely for the pairs N1/O1, O2/O2A, O3/O3A, O8/O8A, O9/O17A, C12/C12A. The NH 4 + ion was treated in the refinement as a rigid group with idealized tetrahedral shape and N-H bond lengths constrained to 0.91 Å . The H 3 O + cation was included as a rigid flat pyramid with O-H bond lengths constrained to 0.84 Å and the pyramidalization defined by HÁ Á ÁH distances constrained to 1.39 Å . The hydrate water molecules related to O2 and O2A were treated as rigid groups with O-H bond lengths of 0.83 Å and H-O-H angles adjusted to 105.4 . The O-H bond lengths of the aqua ligands including O16, O16A, O17, O17A and of the hydrate water molecules including O3 and O3A were restrained to 0.83 Å within an s.u. of 0.02 Å , the corresponding HÁ Á ÁH distances to 1.32 Å within an s.u. of 0.04 Å defining H-O-H angles of 105 (4)-109 (4) . U iso (H) values of all H atoms were set to 1.5U eq of the parent atoms. Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2020), SHELXTL (Sheldrick, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).