Cu-PDC-bpa solid coordination frameworks (PDC=2,5-pyrindinedicarboxylate; bpa=1,2-DI(4-pyridil)ethane)): 2D and 3D structural flexibility producing a 3-c herringbone array next to ideal
Graphical abstract
Cu-PDC-bpa 3-c herringbone arrays.
Introduction
Solid coordination frameworks (SCF) [1], [2], [3], [4] represent one of the most studied material-family during the last decade thanks to the variety of structures that can be formed by using metal complexes as synthons.
Porous SCFs, in particular, can exhibit large cavities and high surface areas, so they have opened a wide range of applications in fields like gas storage [5], [6], [7], gas separation [8], [9], [10], drug delivery [11], [12], [13], chemical sensing [14], [15], heterogeneous catalysis [16], [17], [18], biomedical imaging [19] and many others [20], [21]. Therefore, they are receiving increasing attention as they can display a wide range of interesting functional properties like dielectric [22], [23], [24] behavior and water sorption for heat transformation [25], [26].
Furthermore, these materials can present flexible or soft porous networks, producing the third generation of porous coordination polymers [27], which are especially interesting as they can exhibit structural dynamism compared to the rigid MOFs (metal organic frameworks). This dynamic nature seems to be advantageous for the development of new materials, and can be provoked by the presence of external stimuli, as light, heat, guest removal, condensation, or reactions between the ligands. This property can be enhanced by means of flexible ligands, being able to accommodate structural changes upon the exchange of different solvents located as host molecules in the cavities of the guest framework.
On the other hand, the mobility of guest molecules in SCFs with interconnected channels can be expected to produce interesting dielectric response [28], [29], [30]. In this way, focus has also been directed to the possible appearance of electrical order or dielectric anomalies induced by order-disorder processes of polar and/or H-bonded guest molecules located in the channels of the host structures. Besides, phase transformations are of interest in this type of systems. Particularly, some crystalline-to-amorphous-to-crystalline (CAC) transformations in literature are outstanding [31], [32].
Polycarboxylate spacers are some of the most used ligands to built this kind of networks, being the 2,5-pyrindinedicarboxylate (PDC) our choice. This ligand is non-centrosymmetric, exhibits five potential donor atoms, and has been observed to produce up to twenty three coordination modes (we first reported four of them) [33], [34], [35], [36]. The dimensionality of the crystalline structures is influenced, in most of the cases, by the use of a secondary organic ligand that usually acts as a structural spacer. In these sense, we have focused our work on combinations of PDC with dypiridyl ligands. More specifically, the work herein presented is devoted to the unexplored PDC-bpa combination (bpa= 1,2-di(4- pyridil)ethane). As observed in Scheme 1, where the Lewis structure for both ligands has been drawn, bpa can be found as two geometric isomers, anti and gauche [37], [38].
Taking into account the above mentioned aspects, this work reports on two novel CuII-PDC-bpa compounds exhibiting 2D herringbone-arrays. The new solids are related to Cu-PDC-bpe compounds published by us elsewhere [39] where bpe is 1,2-di(4-pyridyl)ethylene). In that article we first established the different nature of the 4-c and 3-c herringbone arrays, and did identify the structural features for the 3-c ones. The interest of the present work lies on the fact that, as previously said, it first explores the PDC-bpa combination. The change from bpe [39] to bpa (current work) results in flexible herringbone-type SCFs related by a phase transformation. On the other hand, the lack of reversibility for this transformation is coherent with the fact that one of the compounds is the closest one to ideal 3-c herringbone-type arrangement reported so far. This work also discusses on the 2D and 3D flexibility making this phase transformation possible. In this sense, dielectric measurements have been performed in order to discuss the role of guest molecule mobility in the transformation.
Section snippets
Materials and general methods
All solvents and chemicals were used as received from reliable commercial sources. The reactants, 2,5-pyridinedicarboxylic acid (H2PDC), 1,2-di(4-pyridyl)ethane, copper (II) nitrate hydrated 99%, triethylamine (Et3N) and the solvent N,N-dymethylformamide (DMF) 99.8%, were purchased from Sigma-Aldrich Co. The nitric acid 65% (HNO3) and ethanol (EtOH) 96% were purchased from Panreac.
The thermogravimetric analysis (TGA) was performed under air atmosphere on a SDT 2960Simultaneous DSC-TGA TA
Crystal structures
Crystal structures for compounds [Cu2(PDC)2(bpa)(H2O)2]·3H2O·DMF(1), and [Cu2(PDC)2(bpa) (H2O)2]·7H2O (2) are quite similar, so they will be described together. In fact, both compounds consist of 2D arrays of the 3- c herringbone-type (Fig. 1). We have recently reported on similar arrays for Cu–PDC-bpe systems (bpe=(1,2-di(4-pyridyl)ethene) [39] For both compounds, Cu atoms have square pyramidal coordination environment, being coordinated to two oxygen atoms and a nitrogen atom (from two
Conclusions
The same synthon (Cu–PDC–0.5bpa–H2O) produces two different 3-c herringbone arrays thanks to the torsion flexibility of bpa ligand. Additionally, these planes get 3D packed according to distinct modes that are solvent dependent since the interlayer connections are based on hydrogen bonds. Therefore, 2D and 3D flexibility has been observed for the compounds herein studied. The role of the guest molecules has been determined, and a CAC transformation has been detected. The lack of reversibility
Acknowledgments
This work has been financially supported by the “Ministerio de Economía y Competitividad” (MAT2013-42092-R, FEDER MAT2010-21342-C02-01), the “Gobierno Vasco” (Basque University System Research Group, IT-630-13), “Xunta de Galicia” (Grupos de Referencia Competitiva GRC 2014/042) and UPV/EHU (UFI 11/15) which we gratefully acknowledge. SGIker (UPV/EHU) technical support is gratefully acknowledged. F. Llano-Tomé thanks the “Ministerio de Ciencia e Innovación” for a fellowship (BES-2011-045781).
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