One-dimensional ladder gallium coordination polymer

The one-dimensional [Ga(HPDC)(OH)(H2O)]n ladder-type coordination polymer can be prepared using three distinct synthetic approaches (hydrothermal, microwave-assisted and a one-pot process) with crystallite size varying according to the method employed.


Chemical Context
Research on coordination polymers (CPs) and metal-organic frameworks (MOFs) remains a topical area in chemistry, particularly the study of their crystal structures (Cui et al., 2016). These crystalline materials are typically obtained from a combination of metal ions and organic molecules, giving rise to one-, two-or three-dimensional structures (Chaplais et al., 2009). A wide variety of synthetic methods have been reported for the preparation of CPs/MOFs (Stock & Biswas, 2012;Yuan et al., 2018) ranging from ambient-temperature synthesis to conventional [hydrothermal (HT) and one-pot processes (OP)] and microwave (MWAS) synthesis. In addition, other less common techniques such as electrochemistry (EC), mechanochemistry (MC) and ultrasonic (US) synthesis can be used (Rubio-Martinez et al., 2017;Stock & Biswas, 2012).
A large number of MOF-containing divalent transitionmetal ions have been described (Stock & Biswas, 2012;Devic & Serre, 2014). Examples of CPs/MOFs containing maingroup elements, such as Al 3+ , Ga 3+ , or In 3+ , remain scarce (Stock, 2014). A search in the Cambridge Structural Database unveils around 100 Ga 3+ -bearing CP/MOF structures, for example. Remarkably, most of these structures were solved using powder X-ray diffraction (PXRD) techniques (Reinsch & De Vos, 2014;Volkringer et al., 2009). Such materials exhibit high thermal and chemical stability and are ideal candidates for a wide variety of applications (Silva et al., 2015;Yuan et al., 2018;Ajoyan et al., 2018;Howarth et al., 2016). As a result of the close similarity of the coordination chemistry of gallium and aluminium, most of the Ga 3+ -CP/MOFs published are isotypic with well-known Al 3+ -CP/MOFs, and also with the much rarer In 3+ -CP/MOFs (Schilling et al., 2016). This may, in part, explain why most of the studies found in the literature of Ga 3+ -CP/MOFs report only their structures. Furthermore, most of the applications that have been studied are related to those that are also found for Al 3+ -CP/MOFs (Banerjee et al., 2011;Zhang et al., 2018;Reinsch & De Vos, 2014;Canivet et al., 2014;Zhou et al., 2012). For certain applications, Ga 3+ -CP/ MOFs excel, even surpassing the performance of the Al 3+ -CP/ MOFs (Coudert et al., 2014;Ramaswamy et al., 2017;Weber et al., 2016;Gao et al., 2014). Furthermore, gallium materials possess low toxicity and are found in applications such as gas and water adsorption, shock-absorber technology and semiconductors (Ramaswamy et al., 2017;Coudert et al., 2014;Schilling et al., 2016) Following our interest in CP/MOFs, we have attempted the preparation of MOF-303 (Fathieh et al., 2018) with Ga 3+ . In this crystallographic report we describe these studies, which culminated in the isolation of a compact one-dimensional ladder coordination polymer, [Ga(HPDC)(OH)(H 2 O)] (I), prepared by the self-assembly of Ga 3+ and the organic linker 3,5-pyrazoledicarboxylic acid monohydrate (H 3 PDCÁH 2 O). Compound I was obtained using a variety of methods (hydrothermal, microwave and a one-pot process) and a survey of the literature revealed that it is isotypic with a compound published in 2008 (Chen et al., 2008), which is not unprecedented (Volkringer et al., 2009;Finsy et al., 2009;Volkringer et al., 2008).

Crystal Morphology and Characterization
Compound I was prepared by hydrothermal (HT), microwave (MWAS) and one-pot (OP) synthesis. The general experimental conditions were similar (solvent, molar quantities and temperature). The compound is isotypic with [V(HPDC)(OH)(H 2 O)] (Chen et al., 2008), which was prepared using harsher conditions. Our attempts to obtain the analogous V 3+ -bearing material using the conditions described here were unsuccessful.
MWAS produces phase pure coordination polymers much faster than the HT and OP approaches. PXRD studies have confirmed the same structural features (see Figure S1 in the supporting information). The crystal morphology, however, varied depending on the method employed ( Fig. 1). Crystals typically exhibit irregular shapes. MWAS allowed a faster preparation of I when compared to the other methods (a reduction from 24 h to just 1 h) with a significantly smaller average crystal size (ca 3-5 mm) and a more uniform plate-like morphology. The HT method, on the other hand, afforded larger crystals (ca 15-65 mm) with a more block-type morphology, while the OP method resulted mainly in agglomerated particles with a plate-like morphology.
FT-IR spectroscopy supports the structural features revealed by the X-ray diffraction studies ( Figure S2 in the supporting information). Compound I exhibits two broad bands centred at 3280 and 3159 cm À1 attributed to the O-H stretching vibrational modes from the coordinated water molecules and to the N-H stretching vibrations of the pyrazole ring. In the central region of the spectrum, between ca 1700 and 1300 cm À1 , it is possible to discern the typical C-O, C-C and C-N stretching vibrational modes arising from the pyrazole rings and the bending vibration of water molecules.  The materials showed similar thermal decomposition profiles between ambient temperature and ca 1000 K ( Figure  S3 in the supporting information). Between ambient temperature and ca 548 K, there is almost no weight loss, which is indicative of good thermal stability. The weight loss registered between ca 548 and 618 K is 14.9, 14.6 and 14.8% for the OP-, HT-and MWAS-derived materials, respectively, and is attributed to the release of the water of coordination and the decomposition of the hydroxyl group (theoretical weight loss 14.0%). The subsequent weight loss (ca 44.9%) is attributed to the decomposition of the ligand, resulting in the formation of Ga 2 O 3 .

Structural Commentary
Compound I was formulated by single-crystal X-ray diffraction as [Ga(HPDC)(OH)(H 2 O)] from a crystal obtained using hydrothermal synthetic conditions (see Experimental section for further details). This compound crystallizes in the centrosymmetric P1 space group with the asymmetric unit being composed of one Ga 3+ metal centre, one HPDC 2À anionic organic linker, one hydroxyl group and one coordinated water molecule, as depicted in Fig. 2a.
The anionic organic linker HPDC 2À has two distinct coordination modes: forming a N,O-chelate with the crystallographically independent Ga 3+ metal centre [bite angle of 78.02 (11) ], and bridging with an adjacent metal centre through a syn interaction involving the carboxylate group, imposing a GaÁ Á ÁGa intermetallic distance of ca 8.52 Å (i.e. the length of the c axis of the unit cell). The octahedral {GaNO 5 } coordination sphere is completed by one water molecule and two-symmetry related 2 -bridging hydroxyl groups, which are the responsible for the formation of a centrosymmetric dimer, as depicted in Fig. 2b (intermetallic distance of ca 2.97 Å ).
range, 1.832-2.475 Å ) and 2.023 Å for the Ga-N bond (CSD range 1.798-3.275 Å ). The aforementioned connectivity promotes the formation of a one-dimensional ladder-type coordination polymer along the [001] direction (Fig. 3a), which close pack in a parallel fashion in the ab plane of the unit cell mediated by various supramolecular contacts (see the following section). Although the organic linkers are stacked within these ladders, the intercentroid distance is 4.442 (3) Å , indicating the absence of significantsupramolecular interactions.

Supramolecular Features
Compound I contains several functional groups that can promote the formation of various hydrogen-bonding interactions (Fig. 4, Table 2). The coordinated water molecule is engaged in two strong and directional O-HÁ Á ÁO hydrogenbonding interactions with neighbouring carboxylate groups from adjacent one-dimensional chains: d DÁ Á ÁA distances of 2.643 (4) and 2.803 (4) Å and <(DHA) angles in the 164-165 range (Fig. 4, Table 2). These interactions may be described by the graph set motifs, R 2 2 (12) and R 2 2 (20). The independent 2bridging hydroxyl group also donates a hydrogen atom to a neighbouring carbonyl group (from the N,O-chelated moiety) in a strong interaction: d DÁ Á ÁA distance of 2.751 (4) Å and <(DHA) angle of 175 . This contact leads to the formation of a supramolecular chain C 1 1 (10) across neighbouring polymers. Like the 2 -bridging hydroxyl group, the pyrazole ring is involved in a N-HÁ Á ÁO interaction with a N,O-chelated ligand, leading to the formation of a distinct type of supramolecular chain, C 1 1 (6) [d DÁ Á ÁA distance of 2.773 (4) Å and <(DHA) angle of 152 ].
These supramolecular interactions lead to a close packing of individual polymers and to a compact crystal structure of I, as shown in Fig. 5.

Synthesis and Crystallization Procedures
Chemicals were purchased from commercial sources (Merck and TCI) and used without any further purification. The methods and molar quantities described here were based on a methodology described by Yaghi and coworkers for the preparation of MOF-303 (Fathieh et al., 2018).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms bound to carbon and nitrogen were placed at idealized positions using the HFIX 43 instruction in SHELXL2018/3 and included in subsequent refinement with C-H = 0.95 Å and N-H = 0.88 Å with the isotropic thermal displacement parameters fixed at 1.2U eq of the atom to which they are attached. Hydrogen atoms from the coordinated water molecule and the hydroxyl group were directly located from difference-Fourier maps and included in the final structural models with the O-H and HÁ Á ÁH distances restrained to 0.95 (1) and 1.55 (1) Å , respectively, in order to ensure a chemically reasonable environment. These hydrogen atoms were modelled with the isotropic thermal displacement parameters fixed at 1.5U eq (O).  Data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2015); data reduction: SAINT (Bruker, 2015); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL2018 (Sheldrick, 2015b) and PLATON (Spek, 2009).

Poly[[(µ 2 -hydroxido)(µ 2 -1H-pyrazole-3,5-dicarboxylato)gallium(III)] monohydrate]
Crystal data [Ga(C 5  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.