Coordination Chemistry and Structural Dynamics of a Long and

Jun 21, 2016 - Chris S. Hawes , Gregory P. Knowles , Alan L. Chaffee , Keith F. White ... Adrian J. Emerson , Ali Chahine , Stuart R. Batten , David R...
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Coordination Chemistry and Structural Dynamics of a Long and Flexible Piperazine-Derived Ligand Chris S. Hawes,*,†,‡ Sophie E. Hamilton,† Jamie Hicks,† Gregory P. Knowles,† Alan L. Chaffee,† David R. Turner,*,† and Stuart R. Batten*,†,§ †

School of Chemistry, Monash University, Clayton, Victoria 3800, Australia Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

§

S Supporting Information *

ABSTRACT: A long and highly flexible internally functionalized dipyridyl ligand α,α′-p-xylylenebis(1-(4-pyridylmethylene)-piper-4-azine), L, has been employed in the synthesis of a series of coordination polymer materials with CoII, CdII, and AgI ions. In poly-[Cd(L)(TPA)] 1 and poly-[Co(L)(IPA)], 2, (TPA = terephthalate, IPA = isophthalate) the ligand adopts a similar linear conformation to that seen in the structure of the unbound molecule and provides a long (2.6 nm) metal−metal bridging distance. Due to the mismatch of edge lengths with that provided by the carboxylate coligands, geometric distortions from the regular dia and (4,4) network geometries for 1 and 2, respectively, are observed. In poly[Ag2(CF3SO3)2(L)], 3, the ligand coordinates through both pyridine groups and two of the four piperazine nitrogen donors, forming a high-connectivity 2-dimensional network. The compound poly-[Ag2(L)](BF4)2·2MeCN, 4, a porous 3-dimensional cds network, undergoes a fascinating and rapid single-crystal-to-single-crystal rearrangement on exchange of the acetonitrile guests for water in ambient air, forming a nonporous hydrated network poly-[Ag2(L)](BF4)2·2H2O, 5, in which the well-ordered guest water molecules mediate the rearrangement of the tetrafluoroborate anions and the framework itself through hydrogen bonding. The dynamics of the system are examined in greater detail through the preparation of a kinetic product, the dioxane-solvated species poly-[Ag2(L)](BF4)2·2C4H8O2, 6, which undergoes a slow conversion to 5 over the course of approximately 16 h, a transition which can be monitored in real time. The reverse transformation can also be observed on immersing the hydrate 5 in dioxane. The structural features and physical properties of each of the materials can be rationalized based on the flexible and multifunctional nature of the ligand molecule, as well as the coordination behavior of the chosen metal ions.

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INTRODUCTION Since the formal inception of the field by Hoskins and Robson in 1990,1 the study of coordination polymer materials has seen a dramatic growth into a multidisciplinary field comprising thousands of researchers worldwide, encompassing both academic and industrial viewpoints.2 The myriad applications of coordination polymer materials have led to great interest in the design and synthesis of functional coordination polymers from a materials science perspective, with physical properties tailored to a particular application.3 Although a considerable portion of these applications involve the adsorption and separation of gases or vapors within porous materials,4 other applications such as catalysis,5 proton conduction,6 chromatographic separations,7 and nonlinear optics are becoming ever more popular.8 While a number of robust structural motifs have been identified and successfully exploited using rigid linker molecules of well-defined dimensions,9 the behavior of highly flexible ligands in coordination polymer synthesis is considerably more complex. In particular, in the preparation of porous coordination polymer materials, there is a tendency to avoid ligands containing a high degree of flexibility for fear of creating highly interpenetrated or densely packed materials, materials © XXXX American Chemical Society

with unpredictable geometries and network topologies, or lowdensity materials which collapse on evacuation of the pores.10 As such, while ligands containing either one or two flexible sp3hybridized groups in their backbone are now becoming commonplace, very few porous coordination polymer materials are known containing large (>20 Å) and very highly flexible linker molecules.11 The choice of metal ion is also a crucial consideration for the formation of coordination polymer materials. The first-row dblock elements are a common choice due to their ready availability, kinetic lability, and relatively well-understood and predictable geometric parameters.12 More recently, interest has broadened to second-row transition metal ions such as zirconium, especially for applications requiring high stability to water,13 and the f-block elements for photophysical applications.14 A widely used tecton in crystal engineering, although not commonly found in porous architectures, the AgI ion is well known for its extreme lability toward soft electron donors and for adopting a wide range of coordination numbers Received: April 14, 2016

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DOI: 10.1021/acs.inorgchem.6b00933 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and geometries.15 As such, many silver-based assemblies are renowned for their intriguing and visually striking behaviors, topologies, and functionalities.16 The lability and coordination flexibility of AgI can also lend itself to dynamic behavior within coordination polymer materials, as Sumby and Bloch demonstrated in a series of materials based on AgI nodes and a flexible nitrogen donor ligand.17 Here we report the synthesis of an unusually long and highly flexible divergent mixed-donor ligand L, its structural and coordination chemistry, and the interesting behavior of a flexible Ag−L framework toward solvent exchange and structural transformation.

3.493(3) Å and C−H···N angles of 141° in both cases, these interactions are most likely a consequence of crystal packing effects, rather than any significant attractive force. Molecules of L undergo further interaction by way of C−H···π interactions between piperazine methylene groups and phenyl core groups from adjacent molecules and offset face-to-face π−π interactions between the pyridyl groups. Structure of poly-[Cd(L)(TPA)], 1. A large number of structurally interesting coordination polymer materials have been constructed from 1,4-bis(4-pyridylmethylene)piperazine and 1,4-bis(4-pyridylformyl)piperazine in combination with poly(carboxylic acid)s by LaDuca and others in recent years.20 With these reports in mind, L was first examined in the synthesis of coordination polymers with divalent d-block metal ions in the presence of dicarboxylate coligands. The reaction of L with cadmium nitrate and terephthalic acid (H2TPA) in DMF gave colorless crystals of poly-[Cd(L)(TPA)], 1, in good yield. The crystals were analyzed by single-crystal X-ray diffraction, and the data were solved and refined in the monoclinic space group C2/c, which revealed a threedimensional polymeric structure of CdII ions linked by L and TPA ligands into an 8-fold-interpenetrated (6,4) dia network. The asymmetric unit of 1 contains one cadmium ion occupying a crystallographic special position and halves of an L ligand and a TPA ligand. The cadmium ion is coordinated by two equivalent pyridine groups from L molecules and two equivalent bidentate carboxylate groups. Each ligand molecule bridges two equivalent cadmium ions, with metal−metal separations of 11.120(4) and 26.980(6) Å for TPA and L, respectively. Interestingly, the conformation adopted by L is very similar to the linear zigzag arrangement observed in the structure of the unbound molecule, with the distance between pyridine nitrogen atoms of 22.709(7) Å only 0.13 Å larger than that observed in the absence of metal coordination. The structure of 1 is shown in Figure 2. Bridging through L and TPA linkages, the extended structure of 1 adopts a 3-dimensional distorted diamondoid topology. In addition to the discrepancy in linker lengths, the irregular coordination geometry of the cadmium ion lends further distortion to the network dimensions, with Cd−Cd−Cd angles within the network of 127.61(2)°, 137.37(2)°, 85.57(3)°, and 83.54(3)°. The result of these distortions is substantial flattening of the individual diamondoid cages in one dimension (Figure 2). As would be expected given the very large size of the windows in each network, 8-fold interpenetration is observed within the structure of 1, removing any potential for porosity within the structure and leaving a densely packed material with no encapsulated solvent molecules or void volume. As neither the L nor the TPA ligands contain especially large or contiguous π-surfaces, this interpenetration is primarily supported by various van der Waals contacts and C−H···π interactions and is most likely favored on density considerations rather than due to any particular interaction between the frameworks. Structure of poly-[Co(L)(IPA)], 2. The reaction of L with cobalt nitrate and isophthalic acid (H2IPA) under hydrothermal conditions gave a small crop of red crystals of the twodimensional coordination polymer poly-[Co(L)(IPA)]. X-ray diffraction analysis provided a structure model in the triclinic space group P-1, where the asymmetric unit contains one cobalt(II) ion and complete L and IPA ligands. Each molecule of L coordinates to two cobalt ions through the pyridine nitrogen atoms. Each molecule of IPA coordinates to three

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RESULTS AND DISCUSSION The ligand L was prepared in good yield in three steps from 4formylpiperazine, following the procedure reported by Grant et al. for the formation of the intermediate p-xylylenebis(piper-1azine).18 In order to gauge the physical dimensions of the ligand, single crystals were prepared by slow evaporation of a 5:1 CH2Cl2:n-heptane mixture. The asymmetric unit of L contains one-half of one molecule, with the remainder generated by a crystallographic inversion center located within the central phenyl ring. The piperazine ring adopts the expected chair conformation with the substituents on the nitrogen atoms occupying equatorial positions. The molecule adopts an approximately linear orientation, with a distance of 22.571(8) Å between the two pyridyl nitrogen atoms. The structure of L is shown in Figure 1.

Figure 1. (Top) Structure of L with labeling scheme for unique heteroatoms. (Bottom) Crystal packing of L showing alignment of terminal pyridine groups. Hydrogen atoms are omitted for clarity.

With no conventional hydrogen-bond donor groups, the intermolecular interactions in the structure of L consist primarily of weak C−H···N and C−H···π motifs.19 Each pyridyl group interacts with two others via C−H···N interactions originating from the 2- and 6-positions of each pyridyl ring, with each pyridyl nitrogen atom participating in two C−H···N contacts. With C···N distances of 3.426(3) and B

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Figure 3. (Top) Coordination environment of the metal ions within the structure of 2 showing naming scheme for unique coordinating heteroatoms. Hydrogen atoms and the L ligand backbone are omitted for clarity. (Center) Parallel 4-fold aryl embrace (P4AE) interactions present within the structure of 2, with the involved moieties from two adjacent sheets colored separately. (Bottom) Extended structure of a single sheet of 2. Hydrogen atoms are omitted for clarity.

Figure 2. (Top) Coordination environment of compound 1, with numbering scheme for coordinating atoms. Hydrogen atoms are omitted for clarity. Symmetry codes used to generate equivalent atoms: (i) 1 − x, y, 1/2 − z. (Center) Single adamantoid cage within the extended structure of 1. Hydrogen atoms are omitted for clarity. (Bottom) Representation of a single diamondoid network constituting the structure of 1. The seven other interpenetrating networks are omitted for clarity.

interlayer packing, giving the overall 3-dimensional arrangement of the 2-dimensional sheets an appearance of alternating organic (L) and metal−organic ([CoIPA]) zones. Structure of poly-[Ag2(CF3SO3)2(L)], 3. Analyzing the structures of 1 and 2 above suggested little interaction of the amine-functionalized central segment of L toward cadmium or cobalt ions in the solid state under these conditions. Previous research into AgI complexes of substituted piperazines has shown the piperazine nitrogen atoms are capable of forming strong interactions with metal ions.22 On the basis of this work, L was reacted with AgI salts under a variety of conditions, in order to explore the impact of coordination to the piperazine groups and the effect on the conformation of the ligand and the resulting network topologies. Reaction of L with silver trifluoromethanesulfonate in acetonitrile gave colorless crystals on standing for 2 days, and analysis by single-crystal X-ray diffraction provided a structure model in the triclinic space group P-1. The structure model reveals a 2-dimensional polymeric network of the formula poly-[Ag2(CF3SO3)2(L)], in which silver ions are bridged by both triflate anions and L molecules. The asymmetric unit of 3 contains one silver ion

cobalt ions; one carboxylic acid group coordinates in a bidentate chelating mode, while the other bridges two cobalt ions in a μ2-κO:O′ coordination mode. As a result, the cobalt ion is coordinated in a distorted octahedral N2O4 geometry. The structure of poly-[Co(L)(IPA)], 2, is shown in Figure 3. The extended structure of 2 is a 2-dimensional sheet in which looped chains of [Co(IPA)] lying parallel to the crystallographic b axis are bridged in the perpendicular direction by L units (Figure 3). The linear zigzag conformation of L (with a pyridyl N···N distance of 22.422(7) Å) and metal−metal distance of 26.523(6) Å are again surprisingly similar to those noted in the structures of L and compound 1. Adjacent sheets associate via parallel offset π−π interactions between isophthalate rings with an interplanar distance of 3.25 Å, further supported by two edge-to-face C−H···π interactions between the isophthalate groups and the coordinating pyridyl groups. The interaction mode between adjacent metal sites is reminiscent of the well-known parallel 4-fold aryl embrace (P4AE) motif often observed in metal tris-phenanthroline complexes.21 These interactions give only a slight offset to the C

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where the trifluoromethyl groups from triflate anions on neighboring sheets protrude into the rectangular windows and prevent the formation of any solvent-accessible void space. As a result, no guest solvent molecules were observed within the structure of 3. Structure of poly-[Ag2(L)](BF4)2·2MeCN, 4. The reaction of L with silver tetrafluoroborate in acetonitrile gave light orange crystals after standing for 48 h. Analysis of these crystals by single-crystal X-ray diffraction provided a structure model in the monoclinic space group C2/c and revealed the overall structure as a 3-dimensional cds network of the formula poly[Ag2(L)](BF4)2·2MeCN, in which the ligand L acts as a fourconnecting node. The asymmetric unit of 4 consists of one-half of a molecule of L coordinating to two nonequivalent silver ions, both of which reside at crystallographic special positions. One complete tetrafluoroborate counterion is also present within the asymmetric unit and participates in weak F,F′ chelate-type interactions with silver ion Ag1, at Ag···F distances of 3.088(3) and 3.209(6) Å. Similar to the behavior observed in 3, each molecule of L coordinates to four silver ions; however, in this case the coordination takes place through the pyridine nitrogen atoms and the xylyl-substituted nitrogen atoms of the piperazine rings. The ligand adopts a crankshaft-like conformation with a pivot at each methylene link. This geometry contracts the distance between pyridine-bound metal ions to 18.906(3) Å and the distance between pyridine nitrogen atoms to 17.075(9) Å. Ignoring the weak Ag···F contacts at Ag1, each silver ion adopts a linear 2-coordinate geometry, coordinating to either two equivalent pyridine nitrogen atoms or two equivalent piperazine nitrogen atoms. While the pyridinebound silver ion Ag1 experiences weak interactions with the tetrafluoroborate anions, the piperazine-bound silver ion Ag2 is encapsulated on all sides by two intersecting ligand strands, preventing any further coordination. The structure of 4 is shown in Figure 5.

with a coordinating triflate anion and one-half of a molecule of L. Expansion through crystallographic symmetry operations reveals that each molecule of L coordinates to four silver ions, through the two pyridine nitrogen atoms and the two pyridylmethylene-substituted piperazine nitrogen atoms. Unlike in the previously discussed structures, L adopts an S-shaped conformation, pivoting about the methylene bridges nearest to the silver ions, most likely as a response to the steric influence of coordination. As a result, a shorter distance between pyridine-bound metal ions of 21.112(3) Å is observed, as well as a distance between pyridine nitrogen atoms of 18.414(7) Å. Each silver ion is coordinated by one pyridine nitrogen atom, one piperazine nitrogen atom, and two nonequivalent triflate oxygen atoms, in a sawhorse geometry. Each triflate anion weakly bridges two silver ions in a μ2-κO:O′ bridging mode, with Ag−O distances of 2.591(3) and 2.828(3) Å. The structure of 3 is shown in Figure 4.

Figure 4. (Top) Environment of the metal ions and ligands within the structure of 3. Hydrogen atoms are omitted for clarity. Symmetry codes used to generate equivalent atoms: (i) x − 1, +y, +z; (ii) 1 − x, 1 − y, 1 − z; (iii) 1 + x, +y, +z; (iv) −x, 3 − y, −z; (v) 1 − x, 3 − y, −z. (Bottom) Connectivity of a single two-dimensional sheet within the structure of 3. Hydrogen atoms are omitted for clarity.

The extended structure of 3 is a two-dimensional sheet which can best be described as adopting a (4,4) topology, taking each L molecule and each Ag2(CF3SO3)2 unit as four-connecting nodes (Figure 4). Held in close proximity by the triflate bridging mode, the coordinating pyridine groups within each sheet also experience offset parallel π−π interactions at an interplanar distance of 3.47 Å. Each sheet contains rectangular windows with corners defined by silver ions, with minimum (non-hydrogen) interatomic dimensions of 13 × 6 Å. These windows are filled by the offset stacking of adjacent sheets,

Figure 5. Environment of the two unique metal sites (top) and the ligand geometry and connectivity (bottom) within the structure of 4, with partial heteroatom-labeling scheme. Hydrogen atoms and symmetry-generated atom labels are omitted for clarity. D

DOI: 10.1021/acs.inorgchem.6b00933 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry When the local structure of 4 is expanded through the silver ion linkages, a three-dimensional network is evident, which can be visualized as undulating one-dimensional chains of L molecules linked through the pyridine groups by silver ions, which are repeatedly intersected on opposite sides by a series of perpendicular chains, joined at each piperazine group (Figure 6). Topologically, the network is equivalent to the (65·8) cds

Figure 7. Comparison of the structures of 4 and 5: (Top) Overlaid structures of 4 (green) and 5 (red) showing the similarity in ligand geometry; (bottom) comparison of the primary solvent channels within 4, showing disordered acetonitrile in red (left), and 5, showing lattice water molecules as red spheres (right).

Desolvation Studies of 4 and Preparation of poly[Ag2(L)](BF4)2·3H2O, 5, and poly-[Ag2(L)](BF4)2·2C4H8O2, 6. During our attempts to characterize the bulk material 4 and ascertain the exact content of the solvent channels, we observed a conversion to a new phase 5 upon exposure of dry, ground samples of 4 to air within a matter of minutes. X-ray powder diffraction experiments carried out within 10 min of isolating 4 from acetonitrile solution showed only peaks due to the second phase 5. In order to confirm the phase purity of 4, a slurried sample was coated in Paratone-N immersion oil in order to prevent solvent loss and subjected to X-ray powder diffraction analysis, which confirmed that the synthesis of 4 produced a homogeneous product (Supporting Information). Removing a batch of single crystals of 4 from acetonitrile solution, allowing them to stand in air, and periodically measuring single-crystal X-ray diffraction images on representative crystals, we found that after approximately 10 min of air drying, the reflections due to 4 became increasingly diffuse with evidence of a mixture of poorly crystalline phases being formed. After 2 h of air drying, a crystal was found to display diffraction characteristics of sufficient quality to allow indexing of a new unit cell and a structure solution. A structure model of 5 was generated in the monoclinic space group Cc with a unit cell volume approximately 10% smaller than that of 4 (Table 1). The single-crystal diffraction data for 5 was found to be entirely consistent with the X-ray powder diffraction pattern obtained from the measurement of the dried sample of 4. The asymmetric unit contains one molecule of L, two silver ions and their accompanying tetrafluoroborate counterions, and lattice water molecules disordered across three well-defined sites, each of 2/3 occupancy. The coordination mode of L is identical to that observed in 4, and overlaying the two structures reveals the conformations of L to be almost identical between 4 and 5, with only very slight variations in the angles around the silver ions and rotations around the unconstrained bonds within the L molecule (Figure 7). The distance between pyridine-bound metal ions is increased by roughly 5% in 5 compared to 4, at 20.073(7) Å.

Figure 6. Extended structure and connectivity of compound 4. (Top) Chemical structure of compound 4 viewed perpendicular to the primary solvent channels, with a single L unit highlighted in red for exmphasis. Hydrogen atoms are omitted for clarity. (Bottom) Simplified connectivity diagram for 4 viewed near-parallel to the primary solvent channels. Ag1 sites are colored red, and Ag2 sites are colored green, while the ligand molecules are represented as blue rods.

net, taking L molecules as four-connected nodes with coordination linkages through silver ions. As was the case with 1, the network itself is somewhat distorted due to the discrepancy of edge lengths caused by the large variations in bridging distances. Due to the irregular conformation of the flexible ligand placing stronger steric restraints in the vicinity of the aromatic groups, no substantial π−π interactions exist within the structure of 4. The structure of 4 contains onedimensional rectangular channels parallel to the c axis with minimum (non-hydrogen) interatomic dimensions of 6 × 11 Å (Figure 7). By virtue of the weak silver−anion interactions, these channels remain unobstructed by the tetrafluoroborate groups and are instead occupied by disordered acetonitrile molecules to which an occupancy of one acetonitrile molecule per silver ion, disordered over two positions, is assigned. E

DOI: 10.1021/acs.inorgchem.6b00933 Inorg. Chem. XXXX, XXX, XXX−XXX

F

largest diff. peak/hole (e Å−3) CCDC deposition number

final R indexes [all data]

empirical formula fw temp. (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) vol. (Å3) Z ρcalc (g/cm3) μ (mm−1) F(000) 2Θ range no. reflns collected no. of independent reflns/Rint/Rsigma no. of reflns obsd [I ≥ 2σ(I)] data/restraints/parameters goodness-of-fit on F2 final R indices [I ≥ 2σ(I)]

identification code

2321 3017/0/154 1.085 R1 = 0.0626, wR2 = 0.1549 R1 = 0.0815, wR2 = 0.1656 0.21/−0.21 1473707

C28H36N6 456.63 100 monoclinic P21/c 17.146(3) 6.1190(12) 12.350(3) 90 98.79(3) 90 1280.5(5) 2 1.184 0.072 492 2.404−55.778 14 359 3017/0.0564/0.0357

L

3552 3937/0/214 1.056 R1 = 0.0362, wR2 = 0.0900 R1 = 0.0407, wR2 = 0.0929 0.57/−0.99 1473708

C36H40CdN6O4 733.14 100 monoclinic C2/c 22.074(4) 12.515(3) 13.367(3) 90 116.98(3) 90 3290.8(15) 4 1.48 0.713 1512 3.858−55.946 53 825 3937/0.0449/0.0150

1

2

8203 9474/0/424 1.048 R1 = 0.0532, wR2 = 0.1353 R1 = 0.0613, wR2 = 0.1409 0.90/−0.80 1473709

C36H40CoN6O4 679.67 100 triclinic P-1 9.305(4) 10.066(2) 18.050(3) 96.719(10) 92.675(14) 104.812(7) 1618.1(8) 2 1.395 0.581 714 4.224−60.45 52 118 9474/0.1006/0.0603

Table 1. Crystallographic and Refinement Data for L and Compounds 1−6

3679 4282/0/236 1.04 R1 = 0.0398, wR2 = 0.1005 R1 = 0.0487, wR2 = 0.1063 0.83/−0.98 1473710

C30H36Ag2F6N6O6S2 970.51 100 triclinic P-1 9.386(2) 9.596(2) 11.148(2) 74.860(6) 85.802(10) 65.050(11) 878.0(3) 1 1.836 1.318 486 3.79−56.99 18 923 4282/0.0604/0.0417

3

4040 4699/23/260 1.089 R1 = 0.0494, wR2 = 0.1293 R1 = 0.0585, wR2 = 0.1349 0.95/−1.78 1473711

C32H42Ag2B2F8N8 928.09 100 monoclinic C2/c 12.282(3) 22.790(5) 13.580(3) 90 91.87(3) 90 3799.2(15) 4 1.623 1.105 1864 3.77−56.646 24 004 4699/0.0432/0.0287

4

4657 6483/24/442 1.045 R1 = 0.0790, wR2 = 0.2078 R1 = 0.1079, wR2 = 0.2310 1.65/−0.78 1473712

C28H40Ag2B2F8N6O2 882.02 100 monoclinic Cc 9.733(10) 22.323(7) 15.631(7) 90 93.89(6) 90 3388(4) 4 1.729 1.237 1768 3.65−51.908 14 275 6483/0.0791/0.0966

5

3410 4693/14/243 1.049 R1 = 0.0567, wR2 = 0.1520 R1 = 0.0808, wR2 = 0.1678 0.85/−1.16 1473713

C30H40Ag2B2F8N6O 890.04 100 monoclinic C2/c 12.975(8) 23.050(2) 13.155(3) 90 91.819(9) 90 3932(3) 4 1.503 1.065 1784 3.604−55.938 18 646 4693/0.0588/0.0430

6

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in a freshly prepared sample of 4 were exchanged for 1,4dioxane, a low-volatility solvent incapable of participating in the hydrogen-bonding interactions that are thought to have caused the pore collapse in 5. Exchange of the lattice acetonitrile molecules was achieved by soaking the freshly prepared compound 4, without allowing the crystals to dry, in 1,4dioxane (20 mL/50 mg). The exchange solvent was decanted and replaced with fresh solvent every 12 h for a 48 h period. After this exchange process, the crystals were again subjected to single-crystal X-ray diffraction, which showed that the original open framework structure was retained, maintaining the same space group and symmetry as in compound 4, with a ca. 4% increase in unit cell volume (Table 1). Whereas the lattice acetonitrile molecules in 4 and the lattice water molecules in 5 were relatively well ordered, the incorporated dioxane molecules in 6 displayed significant disorder and a complete description of their orientations within the asymmetric unit could not be unambiguously obtained. However, examination of the residual Fourier difference map (Fo2 − Fc2, threshold ±2 e·Å−3) after assignment of all framework atoms revealed two equally distributed, well-defined areas of residual electron density, approximately hexagonal prismatic in shape, within the primary solvent channels (Figure 9). This observation implies

The net effect of the geometric distortion in 5 is most notable when examining the primary solvent channels. The rectangular, one-dimensional solvent channels present in 4 are compressed within 5 by an expansion of the metal−organic pillars. Furthermore, whereas the tetrafluoroborate anions do not significantly obstruct the channels in 4, in 5 one of the two unique anions resides directly within the former channel. This loss of symmetry between the two formerly equivalent tetrafluoroborate anions is the most visible evidence of the reduction in symmetry of the overall structure, from centrosymmetric space group C2/c to the acentric Cc setting. The incorporated water molecules within 5 likely play a significant role in this transformation, as each of the three localized water positions is within hydrogen-bonding distance of a fluorine atom from a tetrafluoroborate anion, although the hydrogen atoms involved could not be unambiguously assigned from the single-crystal X-ray diffraction data. The disturbance of the tetrafluoroborate anions also affects the local environment of pyridine-bound silver ion Ag1, where each anion only displays one Ag···F interaction rather than two (with Ag···F distances of 2.886(13) and 3.13(3) Å for the two unique anions). In their place, an additional weak interaction with a partially occupied lattice water molecule at an Ag···O distance of 2.87(2) Å is introduced (Figure 8). The incorporation of

Figure 9. Three-dimensional map of F02 − Fc2 for the unmodified crystallographic data set of compound 5 prior to solvent modeling with a rendering threshold of 2 e·A−3, showing the presumed localization of the lattice dioxane molecules. Positive values of F02 − Fc2 are shown in red and negative values in green.

that the guest dioxane molecules reside in discrete positions, albeit over a wide range of orientations. While the residual electron density from one of these regions could be approximated by two unrestrained isotropic dioxane molecules, overlapping and with a total occupancy of 1/2 per silver ion, the other proved intractable and was accounted for using the SQUEEZE routine within PLATON in the final refinement24 to give more meaningful refinement statistics for the framework atoms. Elemental analysis of the dioxane-exchanged material suggested a total occupancy of one dioxane molecule per silver ion, consistent with the crystallographic findings. Although most of the infrared absorbances due to dioxane in 6 are obscured by those originating from the framework, a new absorbance at 889 cm−1 is observed, corresponding to a characteristic rocking mode of 1,4-dioxane,25 while the absorbances due to acetonitrile (2249 cm−1) and water (3600 cm−1) are both absent (Supporting Information). X-ray powder diffraction analysis of the dioxane-exchanged material 6 showed a bulk phase which matched that predicted from the single-crystal analysis and was closely related to that

Figure 8. Interactions between the lattice water molecules and the metal and anions adjacent to the Ag1 site within the structure of 5. Contacts of hydrogen-bonding or weak interactions are shown as green dashed lines. Hydrogen atoms are omitted for clarity. Symmetry codes used to generate equivalent atoms: (i) +x, 1 − y, z − 1/2; (ii) 1/ 2 − x, 1/2 − y, 1/2 − z; (iii) 1/2 + x, y − 1/2, +z; (iv) −1/2 + x, y − 1/2, +z; (v) x − 1/2, 1/2 − y, z − 1/2.

hydrogen-bonding water molecules was also observed using infrared spectroscopy (Supporting Information); on standing in air, the carbon−nitrogen triple bond stretching frequency from lattice acetonitrile molecules at 2249 cm−1 in 4 is lost, and a new broad peak emerges at ∼3600 cm−1. The prominent absorbance at 1047 cm−1 originating from the tetrafluoroborate anions also displays slight broadening upon exposure of 4 to air, consistent with the desymmetrization and addition of hydrogen-bond acceptor character to these groups.23 In order to probe the solvent exchange and structural rearrangement process further, the lattice acetonitrile molecules G

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room temperature or at 100 °C under dynamic vacuum for 72 h. The best uptake was observed for compound 6 when evacuated at room temperature, which showed steep uptake in the pressure region 0−0.2 bar, before rapidly flattening to reach a low total gas loading at 1 bar of 19 cc(STP)/g (Figure 11).

predicted from the original material 4. Furthermore, compound 6 proved considerably more stable under ambient conditions than 4 and could be easily handled in air for several minutes without substantial conversion to the hydrated form 5. A timecourse X-ray powder diffraction study was carried out on 6 to monitor the conversion into 5 on exposure to air. As shown in Figure 10, the dioxane phase begins disappearing almost

Figure 11. CO2 adsorption isotherms for compounds 4 and 6 in the pressure range 0−1 atm, measured at 273 K following 72 h of evacuation at the temperatures specified.

This process was extremely slow, with each of the first six data points in the low-pressure region requiring 3−4 h of equilibration time with an equilibration interval of 45 s for all measurements. The steep uptake and slow equilibration for the CO2 adsorption isotherm are suggestive of a very narrow and highly polar micropore environment within the sample.26 This observation indicates that while a channel structure may be retained within the evacuated material, the pores are most likely substantially contracted, not necessarily resembling those determined crystallographically. Such a contraction is likely through either mobility of the anions or flexing of the framework itself. By comparison, the CO2 adsorption isotherm for a sample of freshly isolated compound 4 following degassing at room temperature for 72 h showed a slow linear trend without a steep onset and substantially lower loading values across the entire pressure range. These observations are more consistent with surface or interparticle condensation rather than micropore filling for compound 4, indicating a loss of microporosity, presumably due to partial conversion to the hydrate 5 in the brief period following isolation from solution. Both compounds 4 and 6 displayed very low CO2 uptake when degassed at 100 °C in vacuo, suggestive of material decomposition from the necessarily long dwell period at the activation temperature. Unsurprisingly, the N2 adsorption isotherm measured at 77 K on compound 6 after evacuation at room temperature failed to show any substantial gas uptake in the micropore pressure range, with only a small uptake in the pressure range P/P0 = 0.9−0.99, consistent with condensation on the external surfaces of the crystallites (Supporting Information). An X-ray powder diffraction pattern measured in air after the gas adsorption experiment showed only peaks due to the hydrated phase 5, consistent with immediate conversion of the evacuated 6 to the hydrate 5 on air exposure after the measurement.

Figure 10. Sequential X-ray powder diffraction patterns of compound 6 in air immediately following removal from the exchange solvent (bottom trace). Data collections were carried out in 15 min intervals for the first 8 collections and then hourly with a total experiment time of 16 h.

immediately and is lost almost entirely after ca. 7 h, evidenced by the diminishing of the peaks at 2θ = 8.2°, 10.7°, 15.6°, and 17.0°. The other peaks corresponding to the dioxane phase are overlapped by those from the hydrated phase, the growth of which can be followed with the new peaks emerging at 2θ = 10.2°, 12.6°, 14.4°, and 22.2° and which reach a maximum within 16 h. A trend toward higher 2θ angles is observed where reflections are maintained between both phases and is consistent with the single-crystal observations of a decrease in unit cell volume from compound 6 to compound 5. As would be expected from such a transition from previously solventexchanged crystals, the diffraction peaks due to the new phase are also considerably broadened and lacking in much of the fine structure present from the original hydrate phase 5 and the dioxane-exchanged material 6. The behavior of each of the three solvates of 4 was also probed by thermogravimetric analysis. Each compound showed an immediate onset of mass loss under the experimental conditions, consistent with rapid solvent exchange under ambient conditions. As a result, the measured mass losses in the range 30−200 °C for 4, 5, and 6 of 5.9%, 3.1%, and 13.1%, respectively, were slightly lower than anticipated (8.8%, 4.1%, and 17%, respectively). In each case, the gradual solvent loss step leads directly into thermal decomposition with an onset of ca. 210 °C for all compounds, implying that complete thermal desolvation may not occur before framework decomposition. Although the framework was not expected to retain accessible solvent channels on complete evacuation due to the flexible nature of the framework and the mobility of the anions, CO2 and N2 adsorption isotherms were measured on both compounds 4 and 6. For comparison, CO2 adsorption isotherms were measured at 273 K with evacuation either at

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DISCUSSION Comparing the structures of L and compounds 1−6, it can be reasoned that the dominant conformation of the flexible ligand H

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Furthermore, the transition to the hydrate 5 can be observed on a time scale of several hours by exchanging the lattice acetonitrile guests in 4 for 1,4-dioxane, giving a new solvate 6, which is slowly converted to the hydrate 5 by standing in air in a similarly reversible transition. Although the structural changes to an individual molecule of L are relatively minor in this transition, the cumulative effects of the pore contraction and blockage by the mobile anions are sufficient to hinder the uptake of CO2 guests at low partial pressures. These results represent new progress in the use of very large and highly flexible ligands in the synthesis of porous and guest-responsive frameworks, an underdeveloped area which shows promise in the design of new adaptive materials.

L is linked to the degree of coordination taking place via the piperazine nitrogen atoms. The unbound species L and the two compounds 1 and 2 display effectively identical geometric parameters and are presumably indicative of a preferred geometry for cases where the piperazine groups are uninvolved in coordination. However, introduction of metal coordination on the piperazine nitrogen atoms in 3 and 4 (and the subsequent materials 5 and 6) induces a conformational change. Compound 3 contains a relatively crowded coordination sphere in which the pyridine groups are forced to adopt a different orientation by the steric bulk of the coordinating atoms from both silver ions within the triflate-bridged unit, while the central segment of the ligand retains a similar geometry to that previously observed. In contrast, no such obvious steric constraints are observed in the coordination geometry of 4 to dictate the ligand geometry. In this case, the ligand conformation is likely a result of crystal packing influences within the highly interconnected network structure, in which each ligand is tethered at four points. The combined flexibility of the ligand molecule and the labile coordination sphere of the silver ions presumably act in concert to allow the subtle structural changes necessary for the conversion of 4 to 5. The driving force behind the phase transition to 5 from either 4 or 6 is likely a combination of the volatility of the guests, which are lost irreversibly on standing in air at varying rates and the thermodynamic gains in the formation of multiple hydrogen-bonding interactions in the denser hydrated phase. There have been a range of examples of structural transformations taking place in coordination polymers upon the exchange or loss of encapsulated solvent molecules, and in some cases, such processes can be reversed by exposure to the original guest.27 To test the reversibility of the formation of 5, samples were immersed in acetonitrile or 1,4-dioxane for 2 days, and X-ray powder diffraction analysis (Supporting Information) showed only the presence of solvated phases 4 or 6, respectively. Even after only 1 h of solvent exchange with 1,4-dioxane, the reflections due to the hydrate 5 were almost completely absent, confirming the facile reversibility of the system.

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EXPERIMENTAL SECTION

Materials and Methods. All reagents, solvents, and starting materials were obtained from Sigma-Aldrich, Alfa-Aesar, or Merck and used as received. NMR spectra were recorded on a Bruker AVANCE spectrometer operating at 400 MHz for 1H and 100 MHz for 13C nuclei, with all samples dissolved in deuterated solvents as specified and signals referenced to the residual nondeuterated solvent peak. Melting points were recorded in air on an Electrothermal melting point apparatus and are uncorrected. Mass spectrometry was carried out using a Micromass Platform II ESI-MS instrument operated in positive and negative ionization mode. Thermogravimetric analyses were carried out with a Mettler-Toledo STARe TGA/DSC instrument with samples in aluminum crucibles of 40 μL capacity heated from 30 to 400 °C at a rate of 5 °C/min under a nitrogen purge flow of 20 mL/min. Microanalysis was performed by Campbell Microanalytical Laboratory, University of Otago, New Zealand, and the Science Centre, London Metropolitan University, London, England. Infrared spectra were obtained using an Agilent Cary 630 spectrometer equipped with an attenuated total reflectance (ATR) sampler. Gas adsorption analyses were carried out using a Micromeritics TriStar 3020 volumetric analyzer using ultra-high-purity gases, with samples prepared by evacuating under dynamic vacuum overnight at the temperatures specified, using a Micromeritics Vacprep 061 station. Bulk phase purity of all crystalline materials was confirmed with Xray powder diffraction patterns recorded with a Bruker X8 Focus powder diffractometer operating at Cu Kα wavelength (1.5418 Å), with samples mounted on a zero-background silicon single-crystal stage. Scans were performed at room temperature in the 2θ range 5− 55° and compared with predicted patterns based on low-temperature single-crystal data (Supporting Information). No baseline corrections were required. Time-course studies and patterns for oil-immersed samples of compound 5 were measured with a Bruker D8 Advance ECO instrument using Cu Kα radiation. Oil-immersed samples were subjected to a background subtraction routine to remove the diffuse scatter originating from the oil coating, while all other spectra are uncorrected. X-ray Crystallography. Structural and refinement information is presented in Table 1. All data sets were collected on the MX1 beamline at the Australian Synchrotron, Victoria, Australia,28 operating at 17.4 keV (λ = 0.7109 Å). Data collections were conducted using BluIce control software,29 and the data were reduced, processed, and multiscan absorption corrections applied using the XDS software suite.30 Dispersion corrections for the nonstandard wavelengths were applied in the final refinements using Brennan and Cowan data.31 All data sets were solved using direct methods with SHELXS32 and refined on F2 using all data by full-matrix least-squares procedures with SHELXL-201433 within the OLEX-2 package.34 Non-hydrogen atoms were refined with anisotropic displacement parameters, while hydrogen atoms were included in calculated positions with isotropic displacement parameters of either 1.2 or 1.5 times the isotropic equivalent of their carrier atoms. The structures of 4, 5, and 6 contained guest solvent molecules disordered over several positions and orientations. In each case, the occupancies of the disordered contributors were determined by free variable refinement and then

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CONCLUSIONS We have shown that a new internally functionalized dipyridyl ligand L, capable of bridging metal ions at distances of up to 2.7 nm, can be employed in a variety of binding modes to generate coordination polymer materials whose structures are representative of the geometric and chemical properties of the ligand. Compounds 1 and 2, containing rigid dicarboxylate coligands, display geometric distortions from idealized dia and (4,4) networks based on the roughly 3-fold discrepancy in bridging distances between the two linker molecules, while the flexible ligand L itself adopts similar conformations to that observed in the unbound structure. In compound 3, the piperazine functionality within the ligand backbone acts to provide additional donor sites for coordination to AgI ions, which in concert with bridging triflate groups form a two-dimensional structure of high connectivity. Compound 4, a three-dimensional cds-type coordination polymer, displays a fascinating single-crystal-to-single-crystal transformation on exchange of the lattice acetonitrile molecules for water molecules on standing in air. This transformation, involving a framework flexing and mobility of the tetrafluoroborate anions to accommodate new hydrogen-bonding interactions, is fully reversible by reimmersing the compound in acetonitrile. I

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Inorganic Chemistry rounded and fixed. Minimal geometric restraints were employed where necessary to ensure sensible chemical geometries were maintained for the low-occupancy guest species. In the case of compound 5, no sensible orientation of hydrogen atoms for the disordered lattice water molecules could be modeled; these groups were modeled as isolated, anisotropic oxygen atoms, and the hydrogen atoms were added to the chemical formula to allow correct determination of density, F000, and absorption coefficient. As discussed in the text, one position of the 1,4dioxane guests in compound 6 could not be satisfactorily modeled; as such, the SQUEEZE routine within PLATON was employed to give more meaningful refinement statistics for the framework atoms,24 and the solvation level of the bulk material was determined from elemental analysis. All other data sets were refined to completion using unmodified reflection data. A full explanation of the treatment of the disordered groups and restraints used is given in the _refine_special_details section for each structure in the combined crystallographic information file: CCDC 1473707−1473713. Synthesis of α,α′-p-Xylylenebis(1-(4-pyridylmethylene)piper-4-azine) L. The title compound was prepared by a similar method to that reported for the related compound N,N′-bis(4pyridylmethyl)piperazine.35 To a stirred mixture of α,α′-p-xylylenebis(1-piperazine) (830 mg, 3.0 mmol) and sodium hydroxide (490 mg, 12 mmol) in 100 mL of water was added 4-chloromethylpyridine hydrochloride (1.0 g, 6.1 mmol). The mixture was stirred at room temperature in an open flask for 3 days and then heated at 50 °C until the volume was reduced to 30 mL. The mixture was cooled to room temperature and filtered, and the pale pink solids were washed with a small quantity of ice cold water and dried in air. Yield 910 mg (66%); mp 112−115 °C. δH(400 MHz, CDCl3) 2.55 (br s, 16H), 3.53 (s, 4H), 3.58 (s, 4H), 7.28 (d, 4H, overlapping residual solvent signal), 7.32 (s, 4H), 8.55 (d, 4H, 3J = 6.3 Hz). δC(100 MHz, CDCl3) 52.93, 53.10, 61.65, 62.69. 123.82, 129.05, 136.74, 147.59, 149.63. m/z (ESMS) 229.2 ([M + 2H+] 100%, calcd for C28H38N62+ 229.2), 457.2 ([M + H+] 20%, calcd for C28H37N6+ 457.3). νmax(ATR)/cm−1 3444m br, 2946m, 2805m, 1601s, 1559m, 1511w, 1456m, 1416s, 1347m, 1302s, 1157s, 1130s, 1008s sh, 929m, 840s, 821m, 796s. Single crystals of L1 were prepared by slowly evaporating to dryness a solution of L1 in 5:1 DCM:heptane (10 mg/mL). Phase purity was confirmed by X-ray powder diffraction (Supporting Information). Synthesis of poly-[Cd(L)(TPA)], 1. A mixture of L (10 mg, 22 μmol), terephthalic acid (8 mg, 48 μmol), and cadmium nitrate tetrahydrate (14 mg, 45 μmol) in 2 mL DMF was sealed and heated to 100 °C. After dwelling at this temperature for 48 h, the mixture was cooled to room temperature and the colorless crystals filtered and dried in vacuo. Yield 9 mg (55%); mp >300 °C. Anal. Calcd for C36H40N6O4Cd: C, 58.98; H, 5.50; N, 11.46. Found: C, 58.82; H, 5.72; N, 11.53. νmax(ATR)/cm−1 2940m, 2808m sh, 1656m sh, 1612s, 1544s, 1502m, 1457w, 1427m, 1383s, 1344w, 1302w, 1225w, 1152m, 1130m, 1065m, 1010s, 930w, 839s sh, 745s. Phase purity was confirmed by X-ray powder diffraction. Synthesis of poly-[Co(L)(IPA)], 2. A suspension of L (10 mg, 22 μmol), isophthalic acid (4 mg, 24 μmol), and cobalt(II) chloride hexahydrate (5 mg, 21 μmol) in 2 mL of water was sealed within a 45 mL capacity Teflon-lined stainless steel autoclave and heated to 130 °C. The vessel was held at this temperature for 36 h, followed by cooling to room temperature over a period of 6 h. A small quantity of red crystals was recovered by filtration. Yield 2 mg (15%); mp > 300 °C. Anal. Calcd for C36H40N6O4Co: C, 63.62; H, 5.93; N, 12.37. Found: C, 63.62; H, 5.98; N, 12.24. νmax(ATR)/cm−1 2928w, 2800m, 2761m, 1607s, 1559m sh, 1541s, 1480w, 1444s, 1387s sh, 1333m, 1299m sh, 1156s, 1129s, 1064m, 1009s sh, 928m, 839s, 799s, 746s, 716s. Phase purity was confirmed by X-ray powder diffraction. Synthesis of poly-[Ag2(CF3SO3)2(L)], 3. A solution of silver trifluoromethanesulfonate (20 mg, 78 μmol) in 1 mL of acetonitrile was added to a suspension of L (10 mg, 22 μmol) in 1 mL of acetonitrile. The mixture was stirred briefly and left to stand in a sealed vial with the exclusion of ambient light. After 48 h, the colorless crystals were isolated by filtration. Yield 12 mg (56%); mp 206−208 °C (decomp). Anal. Calcd for C30H36N6O6F6S2Ag2: C, 37.13; H, 3.74; N, 8.66. Found: C, 36.91; H, 3.85; N, 8.96. νmax(ATR, cm−1) 2957w,

2854w. 2754m sh, 1611w, 1560w, 1511w, 1455m sh, 1456s, 1335m, 1279s, 1241s, 1221s, 1154s, 1114m, 1097m, 1024s, 994m, 834s, 801m, 638m. Phase purity was confirmed by X-ray powder diffraction. Synthesis of poly-[Ag2(L)](BF4)2·2MeCN, 4. A solution of silver tetrafluoroborate (65 mg, 340 μmol) in 2 mL of acetonitrile was added to a suspension of L1 (40 mg, 88 μmol) in 2 mL of acetonitrile. The mixture was stirred briefly and allowed to stand in a sealed vial with the exclusion of ambient light for 6 h, and then the mixture was agitated briefly. The mixture was left to stand undisturbed for a further 48 h, and the resulting colorless crystals were isolated by filtration. Yield 47 mg (57%). Due to the rapid conversion to compound 5 upon drying in air, melting point or microanalysis on the as-synthesized material could not be obtained. νmax(ATR, cm−1) 2934w, 2865w, 2797w, 2750w, 2249m, 1612s, 1453s, 1432s, 1320m, 1286m, 1226m, 1110s, 1047s sh, 984s, 921m, 878s sh, 807s sh, 748m, 671s. Phase purity was confirmed by X-ray powder diffraction. Synthesis of poly-[Ag2(L)](BF4)2·2H2O, 5. The title compound was prepared in quantitative yield by allowing a freshly isolated sample of 4 to stand in air at ambient temperature for 2 h; mp 192−196 °C (decomp). Anal. Calcd for C28H40B2N6O2F8Ag2: C, 38.13; H, 4.57; N, 9.53. Found: C, 38.12; H, 4.53; N, 9.40. νmax(ATR, cm−1) 3618m br, 2921w, 2861w, 2799w, 2748w, 1637m br, 1611s, 1453m, 1429s, 1315m, 1285m, 1224m, 1027s sh, 980s, 913m, 866m, 803s sh, 750m, 669s. Phase purity was confirmed by X-ray powder diffraction. Synthesis of poly-[Ag2(L)](BF4)2·2C4H8O2, 6. To a sample of 4 (ca. 50 mg) in a minimal quantity of the acetonitrile mother liquor was added 20 mL of dioxane. The solution was decanted to a minimum quantity of solvent to prevent drying of the crystals, and a further 20 mL of dioxane was added. The mixture was allowed to stand for 48 h, with the exchange solvent decanted and refilled with fresh solvent every 12 h. The crystals were isolated by filtration. Anal. Calcd for C36H52B2N6O4F8Ag2: C, 42.30; H, 5.13; N, 8.22. Found: C, 42.18; H, 4.93; N, 8.36. νmax(ATR, cm−1) 2916w, 2854m, 2794w, 2746w, 1611s, 1451m, 1431m, 1317w, 1282m, 1257m, 1225m, 1111s, 1023s sh, 983s, 910m, 888m, 867s, 806s sh, 749m, 670s. Phase purity was confirmed by X-ray powder diffraction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00933. X-ray powder diffraction data for all compounds, infrared spectra for the solvated compounds 4, 5, and 6, thermogravimetric analysis plots, and additional gas adsorption isotherm plots (PDF) (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Present Address ‡

C.S.H.: School of Chemistry and Trinity Biomedical Sciences Institute, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. J

DOI: 10.1021/acs.inorgchem.6b00933 Inorg. Chem. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported by the Science and Industry Endowment Fund. Portions of this work were carried out on the MX1 Macromolecular Crystallography beamline at the Australian Synchrotron, Victoria, Australia. D.R.T. acknowledges the Australian Research Council for a Future Fellowship.

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DOI: 10.1021/acs.inorgchem.6b00933 Inorg. Chem. XXXX, XXX, XXX−XXX