Crystal structure of poly[bis(ammonium) [bis(μ4-benzene-1,3,5-tricarboxylato)dizincate] 1-methylpyrrolidin-2-one disolvate]

The title three-dimensional metal–organic framework (MOF) material features an anionic framework constructed from Zn2+ cations and benzene-1,3,5-tricarboxylate (BTC) organic anions. Charge balance is achieved by outer sphere ammonium cations formed by degradation of di-n-butylamine in the solvothermal synthesis of the material.

1. Chemical context 1,3,5-Benzenetricarboxylic acid (H 3 BTC) has proved its efficacy as a versatile and powerful ligand for the construction of metal-organic frameworks (MOFs). Its three carboxylate groups and benzene ring can act as short and long bridges between metal ions, leading to three-dimensional assemblies with a large structural diversity (Eddaoudi et al., 2001;Almeida Paz & Klinowski, 2004;Liu et al., 2007). Since 1997 (Yaghi et al., 1997), the coordination chemistry of zinc ions and BTC ligands has represented one of the most extensively explored systems in efforts to synthesize new porous materials. The various aspects of the Zn-BTC system continue to being investigated, and diverse MOF structures have been reported. The published results reveal that the variation of starting compositions, solvents and templates as well as reaction conditions are significant and can result in the formation of completely different metal-organic framework compounds. A base is needed for deprotonation of H 3 BTC so that it can make use of its full coordination capacity. This base should have a low affinity for binding to metal ions to avoid competition with BTC, especially if the aim is the synthesis of porous materials. A wide range of different solvent systems and reaction conditions have been used in the construction of new coordination networks, including the use of ionothermal techniques (Xu et al., 2007), and conducting reactions in the presence of different surfactants as reaction media (Gao et al., 2014). ISSN 2056-9890 In our recent work (Ordonez et al., 2014), we reported 13 different Zn-BTC coordination networks that were formed as a result of the use of different cations as framework templates. Generally, only one type of secondary building unit (SBU) is observed in one compound; however, data from our and other groups (Ordonez et al., 2014;Xie, 2013;Hao et al., 2012) have shown the possibility of different SBUs in a single selfassembled system which can, in turn, result in distinct frameworks and topologies. In some cases, hydrothermal reaction conditions lead to decomposition of solvents or bases (Burrows et al., 2005), and fixation of the decomposition products in the systems can result in unexpected guests such as ammonium cations (Ordonez et al., 2014). Herein we report the structure of a new three-dimensional Zn-BTC MOF obtained serendipitously by reaction of the H 3 BTC ligand with zinc nitrate hexahydrate using 1-methylpyrrolidin-2-one (NMP) as a solvent and di-(n-butyl)amine as a base and a framework template. The main product of the reaction was the {Zn-BTC}{n-Bu 2 NH 2 } MOF, but a few single crystals of title compound were found as a byproduct.

Structural commentary
The asymmetric unit of the title compound, {(NH 4 ) 2 [Zn 2 (C 9 H 3 O 6 ) 2 ]Á2C 5 H 9 NO} n , contains two Zn II cations, two ammonium cations, two NMP molecules and two BTC residues (Fig. 1). The compound has a three-dimensional structure constructed from dimeric zinc carboxylate entities and BTC linkers (Fig. 2). The two zinc ions form a unit with six carboxylate units from the two symmetry-independent BTC ligands, and four additional BTC units created by the glide operations and translations. Each of the Zn II cations exhibits an O 4 coordination set defined by four oxygen atoms of four coordinating BTC residues. The Zn-O distances range within 1.927 (5)-1.982 (5) Å for Zn1 and 1.926 (5)-1.969 (5) Å for Zn2. Of the six BTC residues around the Zn 2 units, two act in bidentate bridging modes, and combine the two crystallographically unique Zn II ions in the binuclear cluster {Zn 2 (COO) 2 } that acts as the SBU in this compound. All of the other carboxylic oxygen atoms coordinate in a monodentate fashion (Fig. 1). The Zn1Á Á ÁZn2 separation within the SBU is 3.542 (5) Å . The connection of alternating zinc carboxylate units and BTC linkers results in an infinite three-dimensional (3,6)-connected net, which leads to the framework having the same topology as rutile, TiO 2 .

Database survey
A literature overview (Xu et al., 2007)  , Et 3 NH + , (PhCH 2 )Me 3 N + , and BMIM = 1-butyl-3-methylimidazole (Ordonez et al., 2014). As a result of the size of the templates, the reticular networks differ by the packing modes of the cations in the channels, and correspondingly by channel size within the framework. {Zn/Cd-BTC} networks with the same rtl topology have also been reported (

Synthesis and crystallization
A mixture of Zn(NO 3 ) 2 Á6H 2 O (0.343 g, 1.15 mmol), H 3 BTC (0.244g, 1.16 mmol), di-(n-butyl)amine (0.142 g, 1.10 mmol), and 1-methylpyrrolidin-2-one (NMP, 10 mL) was prepared in a capped vial. The solution was transferred to a 23 mL Teflonlined acid digestion vessel and placed in an oven at 423 K for four days. The crystals produced were collected in a vial, washed with fresh NMP, and sonicated to remove impurities from the crystals. The main product of the reaction was the MOF {Zn-BTC}{n-Bu 2 NH 2 }; only few single crystals of the title compound were found as a byproduct. Those crystals were plate shaped and colorless. Synthetic details are given in Ordonez et al. (2014).

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were calculated in geometrically idealized positions and refined riding on their parent atoms, with U iso (H) = 1.2U eq (C) (aromatic) and 1.5U eq (C) (methyl), and with C-H = 0.95 Å (aromatic) and 0.98 Å (methyl). The methyl H atoms were allowed to rotate around the corresponding C-C bond. N-bound H atoms in ammonium cations were found in a difference map and refined Three-dimensional structure in the unit cell viewed along the a axis. Hydrogen-bonding interactions are shown as dashed lines. C-bound H atoms in coordination network are omitted for clarity.
using geometrical restraints to fix the N-H distances, and with an isotropic displacement parameter of U iso (H) = 1.5U eq (N). One of the NMP molecules is disordered over two positions with partial occupancies 0.903 (8) and 0.097 (8). The geometries of the major and minor NMP moieties were restrained to be similar using a SAME command. The displacement parameters for the disordered NMP molecule were restrained to be similar to each other using a SIMU command with a standard deviation of 0.01 Å 2 .  Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT-Plus (Bruker, 2009); data reduction: SAINT-Plus (Bruker, 2009); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: publCIF (Westrip, 2010).

Poly[bis(ammonium) [bis(µ 4 -benzene-1,3,5-tricarboxylato)dizincate] 1-methylpyrrolidin-2-one disolvate]
Crystal data (NH 4  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. Refinement. Refined as a 2-component inversion twin.