Crystal structure of a compact three-dimensional metal–organic framework based on Cs+ and (4,5-dicyano-1,2-phenylene)bis(phosphonic acid)

The three-dimensional metal–organic framework compound prepared from Cs+ and the organic linker 4,5-dicyano-1,2-phenylene)bis(phosphonic acid is based on an irregular CsO8N2 coordination center comprising a single monodentate hydronium O-atom donor, together with multiple bridging links to the two phosphonate O-atom donors and to the two nitrile N-atom donors.


Chemical context
The area of metal-organic frameworks (MOFs) and coordination polymers (CPs) has proven to be of great importance, not only in academic research but also for industrial applications . The simple and easy preparation of these materials, allied with the enormous variety of building blocks (either metal atoms or organic linkers) make these materials ideal to be employed in different applications: gas sorption/separation (Sumida et al., 2012), as heterogeneous catalysts (Mendes et al., 2015), luminescence (Heine & Mü ller-Buschbaum, 2013), batteries and as corrosion inhibitors (Morozan & Jaouen, 2012), among many others. Most of these compounds are obtained by mixing transition metal cations with carboxylic acids. The use of other oxygen-based donor groups such as phosphonic acids has seen a great resurgence in recent years. The use of mixed oxygen-nitrogen donor organic linkers is relatively less common, as confirmed by a search of the Cambridge Structural Database (CSD) (Groom et al., 2016).
Although alkali-metal cations are of great interest due to their abundance in biological systems, there is a surprisingly small number of MOFs/CPs based on these elements. Cs +based materials are not as common as other alkali metals, ISSN 2056-9890 especially when coordinated by either phosphonic or sulfonic acid residues. Reports on these structures are directed to solely structural descriptions rather than to applications. Nevertheless, these compounds can be used as functional materials in batteries, either as proton conductors (Bazaga-Garcia et al., 2015) or as insulators (Tominaka et al., 2013).
Following our interest in this field of research, we report the preparation of a new compact and dense MOF network, [Cs(H 2 cpp)(H 3 O)] n , prepared by the self-assembly of Cs + and the organic linker (4,5-dicyano-1,2-phenylene)bis(phosphonic acid), (H 4 cpp), previously reported by our group (Venkatramaiah et al., 2015). The title compound, [Cs(H 2 cpp)(H 3 O)] n (I), was assembled under atmospheric conditions and represents, to the best of our knowledge, the first reported MOF or CP based on an amino/cyano phosphonate with caesium as the metal cation, and the crystal structure is reported herein.

Structural commentary
The asymmetric unit of (I) comprises one Cs + atom coordinated by a dianionic H 2 cpp 2À ligand, together with a monodentate hydronium cation (Fig. 1). The irregular CsO 8 N 2 coordination polyhedron is defined by the O atom of one monodentate hydronium molecule, six hydrogen phosphonate O-atom donors and two cyano N-atom donors. The Cs-O bond-length range is 3.159 (4)-3.410 (3) Å and for Cs-N, 3.234 (7) and 3.334 (6) Å (Table 1). These values are in good agreement with those reported for other phosphonate-based materials as found in a search in the Cambridge Structural Database (CSD; Groom et al., 2016): mean value of 3.24 Å for the Cs-O bond (CSD range, 3.01-3.41 Å ), and 3.28 Å for the Cs-N bond (CSD range, 2.35-3.79 Å ).
The crystallographic independent H 2 cpp 2À residue in (I) acts as a linker connecting seven symmetry-related Cs + metal atoms. The coordination modes between cyano and phosphonate groups are, as expected, different. While the cyano groups connect to two different metal atoms, each in a simple 1 coordination mode, the two phosphonate groups coordinate to the remaining metals by 1 -O, 2 -O and 2 -O,O coordination modes. This high coordination of the phosphonate groups is responsible for the formation of a metallic undulating inorganic layer lying in the ac plane of the unit cell. Within this layer, the intermetallic CsÁ Á ÁCs distances range from 5.7792 (4) to 7.8819 (5) Å (Fig. 2). The cyano groups are, on the other hand, responsible for the inter-layer connections along the [010] direction. In this case, the intermetallic CsÁ Á ÁCs distances between layers range from 9.7347 (6) to 9.9044 (6) Å . Although the organic linkers are stacked, the The asymmetric unit of [Cs(H 2 cpp)(H 3 O)] n (I) showing all non-hydrogen atoms represented as displacement ellipsoids drawn at the 50% probability level and hydrogen atoms as small spheres with arbitrary radius. The coordination sphere of Cs + is completed by generating (through symmetry) the remaining oxygen and nitrogen atoms. For symmetry codes, see Table 1. Symmetry codes: (i) Àx þ 1; y þ 1 2 ; Àz þ 3 2 ; (ii) Àx þ 2; y þ 1 2 ; Àz þ 3 2 ; (iii) Àx þ 1; Ày þ 1; Àz þ 1; (iv) Àx þ 1; Ày þ 1; Àz þ 2; (v) Àx þ 2; Ày þ 1; Àz þ 1; (vi) Àx þ 2; Ày þ 1; Àz þ 2.  The unusual presence of a coordinating H 3 O + ion in this Cs + structure is confirmed by the location of the three hydrogen atoms associated with this cation, which were clearly visible from difference-Fourier maps and by the presence of the double charge with respect to the delocalized P1-O1, P1-O2 and P2-O4, P1-O6 bonds [1.499 (4), 1.509 (3) Å and 1.497 (4), 1.495 (3) Å , respectively]. The P1-O3 and P2-O5 bond lengths for the protonated groups are 1.558 (4) and 1.572 (4) Å , respectively. In addition, although the distance between O1W and O4 is very short, suggesting a possible O4-HÁ Á ÁO1W interaction, a calculated site for such a hydrogen was found to be sterically impossible in the crowded environment about Cs. Not only that, but any attempts to refine this molecule as a coordination water molecule proved to be not as successful as the hydronium cation. When the proton is connected to the adjacent phosphonic residue, the bond is only possible by restraining the O-H distance between O4 and the proton. Also there was still a residual charge near O1W, which corroborated the initial refinement.

Topology
The various coordination modes of the ligand and the presence of a compact undulating inorganic layer formed by the metal atoms to form the MOF architecture can be better understood from a pure topological perspective. Based on the recommendations of Alexandrov et al. (2011), any moiety (ligand, atom or clusters of atoms) connecting more than two metallic centers ( n ) should be considered as a network node. For (I), all crystallographically independent moieties comprising the asymmetric unit, both the Cs + cation and the anionic H 2 cpp 2À ligand, should therefore be considered as nodes. Using the software package TOPOS (Blatov & Shevchenko, 2006), (I) could be classified as a seven-connected uninodal network with an overall Schä fli symbol of {4 17 .6 4 }. Fig. 2 illustrates the breakdown of the network of (I) into nodes and connecting rods, with the individual connectivity of each node being superimposed into the crystal structure itself ( Fig. 2a and 2b). The metal atom and the organic linker are connected to each other in every direction of the unit cell (Fig. 2c), forming a compact and robust three-dimensional network (Fig. 3). The absence of water molecules of crystal-lization leads to this very compact structure having no solventaccessible pores: only 0.2% of the unit cell volume [calculated using Mercury (Macrae et al., 2006)] corresponds to voids.
hydrogen bonds are confined within the inorganic undulating layer (Fig. 4).

Database survey
Although unusual in the case of Cs, in the Cambridge Structural Database (CSD) a total of 45 structures in which coordination between the metal cation and the hydronium cation is present, e.g. among the metal complexes (Reyes-Martínez et al., 2009;Jennifer et al., 2014;Teng et al., 2016;Hu & Mak, 2013) and coordination polymer/metal-organic frameworks (Yotnoi et al., 2015;Wang et al., 2013;Humphrey et al., 2005). Wang et al. (2013) in fact reported the structures of an isotypic series of crystal materials involving lanthanides (Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Y), in which the presence of the coordinating hydronium cation was confirmed.

Synthesis and crystallization
Chemicals were purchased from commercial sources and used without any further purification steps. (4,5-Dicyano-1,2phenylene)bis(phosphonic acid) (H 4 cpp) was prepared according to published procedures (Venkatramaiah et al., 2015). Synthesis of [Cs(H 2 cpp)(H 3 O)] n , (I): H 4 cpp (29 mg, 0.1 mM) was dissolved in 4 ml of methanol. A 1 ml aliquot of a methanolic caesium hydroxide solution (45 mg, 0.3 mM; Sigma Aldrich, puriss p.a. ! 96%) was added slowly. The resulting mixture was stirred at ambient temperature for 10 min for uniform mixing. The final solution was allowed to slowly evaporate at ambient temperature. White transparent crystals of the title compound were obtained after one week. Crystals were filtered and dried under vacuum.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms bound to carbon were placed at idealized positions with C-H = 0.95 Å and included in the final structural model in a riding-motion approximation with the isotropic displacement parameters fixed at 1.2U eq (C). Hydrogen atoms associated with the H 3 O + moiety and the phosphonate groups were clearly located from difference-Fourier maps and were included in the refinement with the O-H and HÁ Á ÁH (only for the cation) distances restrained to 0.95 (1) and 1.55 (1) Å , respectively, in order to ensure a chemically reasonable environment for these moieties. These hydrogen atoms were modelled with the isotropic displacement parameters fixed at 1.5U eq (O). In order to avoid a close proximity between the H atoms associated with the POH group and the H 3 O + cation and the central Cs + ion in the crystal structure, an antibump restraint [3.5 (1) Å )] was included in the overall refinement. Computer programs: APEX2 and SAINT (Bruker, 2012), SHELXS (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and DIAMOND (Brandenburg, 1999).

Figure 4
Schematic representation of a portion of the undulating inorganic layer comprising the crystal structure of (I), emphasizing the various strong and directional supramolecular O-HÁ Á ÁO hydrogen-bonding interactions (orange dashed lines) present within this layer. For geometrical details and symmetry codes, see Table 2. program(s) used to solve structure: SHELXS (Sheldrick, 2015); program(s) used to refine structure: SHELXL2014/6 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL2014/6 (Sheldrick, 2015).

Poly[[µ 7 -dihydrogen (4,5-dicyano-1,2-phenylene)diphosphonato](oxonium)caesium]
Crystal data 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.