Reversible Phase Transitions in a Coordination 1D-Polymer

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Reversible phase transitions in a coordination 1D-polymer containing an unusual hexatungstate building block. Reinaldo Atencio, Gustavo Liendo, Alexander Briceño, Pedro Silva, Isabelle Beurroies, Philippe Dieudonne, and Haci Baykara Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00157 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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Crystal Growth & Design

Reversible phase transitions in a coordination 1D-polymer containing an unusual hexatungstate building block. Reinaldo Atencio*,,,† Gustavo Liendo,‡,ξ Alexander Briceño,ξ Pedro Silva,Ϟ Isabelle Beurroies,¥ Philippe Dieudonné,§ and Haci Baykara. ,† 

Centro de Investigación y Tecnología de Materiales, Instituto Venezolano de Investigaciones Científicas (IVIC-Zulia) Maracaibo, Edo. Zulia, Venezuela. de Oriente, Núcleo de Sucre, Cumaná 6101, Venezuela. ξ Centro de Química and Ϟ Centro de Física, Instituto Venezolano de Investigaciones Científicas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela. ¥ Université de Provence, Centre de Saint Jérôme 13397, Marseille Cedex 20, France. § Laboratoire Charles Coulomb (L2C), UMR 5221 CNRS, Université de Montpellier II, Montpellier Cedex 05, France.  Departamento de Ciencias Químicas y Ambientales, and † Centro de Investigación y Desarrollo de Nanotecnología (CIDNA), Escuela Superior Politécnica del Litoral, ESPOL, Campus Gustavo Galindo km 30.5 Vía Perimetral, Guayaquil, Ecuador. ‡ Universidad

KEYWORDS. Hexatungstate, Bimetallic Cu/W-polymer, Phase transitions, Polyoxometalate. Supporting Information ABSTRACT:

Two novel compounds {[Cu(bpy)2]3H2W12O40}·3H2O (1) and {{[Cu(OH2)(bpy)][Cu(bpy)]2W6O21}·4H2O}n (2) were isolated under hydrothermal conditions. 1 is an α-metatungstate Keggin unit decorated by three "{Cu(bpy)2}2+" fragments. 2 is a thermodynamically stable 1D-polymer built-up from an unusual hexatungstate moiety decorated by three Cucomplexes. Compound 2 can be synthesized either from the starting reagents or by the thermal transformation from crystals of 1 (180 °C, 7 d). The hexatungstate in 2 represents the first example containing a W-O bonding pattern with thirteen terminal oxygen ligands, four 2-Obridging, two 3-Otryplebridging and two 4-Ocorner, symbolizing the fourth W-hexamer geometry characterized so far. 2 provides a rare example that withstands a water desorption-readsorption process (80-130 °C) and a subsequent reversible phase transition (190-230 C). Both events appear accompanied by a unit-cell contraction, which occurs in the b- and c-direction involving the shortening of the separation of 1D-polymers. EPR studies support the shortening in the Cu(II)···Cu(II) distances.

INTRODUCTION The importance of the organic-inorganic hybrid materials based on polyoxometalates (POMs) has been drawn the attention thanks to the unusual chemical properties associated with its structural diversity.1–6 Their potential applications are spread over in many fields including medicine, catalysis, solidstate technology, and chemistry.7–9 Polyoxomolybdates have significant industrial applications as catalytic precursors in the hydroprocessing of oil feedstock, such as hydrodesulfurization.10,11 An impressive structural property of POMs is their capacity to form functional materials with intrinsic porous feature. Two different structures, zeolitic tetrahedral polyoxometalate metal-organic frameworks (denoted Z-POMOFs),12 or open frameworks (POM-OFs),13,14 if they are combined with an organic linker and inorganic cation, can be formed. POMs can also generate highly stable allinorganic framework solids involving transition metal linkers

that reflect the zeolitic nature (called POMzites)2 of each family member and may be designed using topological and reactivity principle similar to those used for metal-organic frameworks.15 A striking feature of the POMs is their anionic nature, which means that they can be combined with cationic metal-organic complexes that act as charge compensating agents.14,16 In principle, the presence of a second metal center in these materials affords the opportunity to enhance and/or tuning some specific property in comparison to the individual constituents. From the structural viewpoint, metal-organic systems may force to an intrinsic synergism between this entity and the POM leading to unique hybrid supramolecular architectures with valuable properties.16 This strategy may drive an avenue towards managing well-known polyoxometalate structures and generating new ones. Hydrothermal synthesis appears to be exceptional and represents an invaluable opportunity for accessing route to new POM-containing materials with significant properties for a variety of applications.17 Under these conditions, the impact of structure-directing by organic pyridinium cations have been well-studied to obtain a great variety of metastable phases with different dimensionality.16,18 Despite the vast number of examples reported in the literature, precise control over resulting product composition and structure often remains elusive so far. Very few studies are known to deal with the mechanisms and the intermediate phases responsible for driving the final product.19 These studies have revealed that within a given system, parameters as time and the reaction temperature play a critical role in the degree of polycondensation of the POM subnetwork. These phases can be sensible to single or multiple steps transformations with remarkable changes on the structural dimensionality from 0D to complex 3D networks, solely with the variation in a few tens of degrees or increasing time of the reaction.4,18 Thus, such transformations represent a crucial insight to reach a better understanding and potential control of the factors that govern the assembly of POM solids.16

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Remarkably, as the structural diversity of hybrid polyoxomolybdates15 has widely been exploited, the isopolyoxotungstate based materials remain mostly less explored, except for structures containing well-known stable clusters, mainly meta- and para-dodecatungstate-type anions and other containing Lindqvist or Dawson oxo-clusters.20–27 In this context, as part of our ongoing studies on the self-assembly of novel hybrid POMs, we have extended this approach for the synthesis of two unique hybrid structures based on polyoxotungstates. Herein, we report a new example of a metastable phase in which "{Cu(bpy)2}2+" fragments serve as a decorating agent of a Keggin anion to lead a novel bimetallic compound {[Cu(bpy)2]3H2W12O40}·3H2O (1, bpy= 2,2'bipyridine). Compound 1 became unstable with the rise of the temperature, leading to a 1D-polymer {[Cu(OH2)(bpy)][Cu(bpy)]2W6O21}·4H2O]n, (2) via hydrothermal treatment at 180 oC. Besides, the compound 2 is a channeled crystal structure formed from a new hexatungstate based building block. The packing of 2 withstands a dehydration-rehydration process and a subsequent reversible structural phase transition maintaining a very high degree of its crystallinity.

EXPERIMENTAL SECTION Physical measurement. C, H and N elemental analyses were performed on an EA1108 CHNS-O Fison Instrument. Elemental analyses for W and Cu were determined by using an inductively coupled plasma (ICP) Thermo Scientific spectrometer model iCAP 6000. FTIR spectra were obtained as KBr pellets on a Bruker spectrometer model IFS113V provided with a device for variable temperature, standard Globar source and an MCT detector. The TGA measurements were performed on a DuPont 990 analyzer. The sample was contained within alumina crucibles and heated at a rate of 10 °C min-1 from room temperature to 800 °C under N2 atmosphere. Surface area and porosity were measured with a Micromeritics ASAP 2010 apparatus. An adsorption isotherm was obtained using N2 as adsorbate (77 K). The EPR spectra were measured at room temperature in a Varian E-line spectrometer working in the Xband (9.3 GHz) with a homemade cylindrical cavity and coaxial microwave coupler for the X-band. Experimental conditions (microwave power and modulation field) were adjusted to avoid saturation of signal intensity. Synthesis of {[Cu(bpy)2]3H2W12O40}·3H2O (1). A suspension prepared by mixing CuCl2 (0.0185 g, 0.138 mmol), 2,2’bipyridine (0.0215 g, 0.138 mmol), Na2WO4·2H2O (0.1537 mg, 0.466 mmol) and H2O (8.00 ml, 0.444 mol) acidified with two drops of HCl conc. was heated for five days at 140 °C under hydrothermal condition. Only blue crystals (see Figure S1) of 1 were obtained in 44.2% yield based on tungsten. Anal. Calcd for C60H56N12O43W12Cu3: C, 17.88; H, 1.50; N, 4.17; Cu, 4.73; W, 54.69 Found: C, 18.08; H, 1.15; N, 4.12; Cu, 4.79; W, 54.75. IR (KBr, cm-1): 3600-3200 (HO–H), 1666.4 (H–O–H), 1602.2; 1444.1; 1164.5 and 1105.2 (bpy C=C and C=N), 943.8; 763.4; 877.8 (W=Ot, W-Otb-W, W-Ob-W). Full FTIR spectrum is displayed in Figure S12. Synthesis of {{[Cu(OH2)(bpy)][Cu(bpy)]2W6O21}·4H2O}n, (2). This compound was prepared using the same starting materials and amount as mentioned before for 1. In this case, the initial suspension was heated for seven days at 180 °C. Blue crystals of 2 were collected in 7% yield based on tungsten and separated from a water-insoluble light blue unidentified powder (see Figure S2). However, highly pure crystals of compound 2 could be hydrothermally obtained directly from

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crystals of compound 1 at 180 °C for seven days with a yield of 56.4% (based on W). This structural purity of the final solid of 2 was demonstrated by comparison the bulk pattern vs. simulated diffractogram from single-crystal data of 2 (see Figure S3). Anal. Calcd for C30H34N6O26W6Cu3: C, 16.47; H, 1.57; N, 3.84; Cu, 8.71; W, 50.41 Found: C, 16.80; H, 1.26; N, 3.84; Cu, 8.76; W, 50.45. IR (KBr, cm-1): 3600-3200 (HO–H), 1666.8 (H– O–H), the intricate set of bands in the range 1620-1000 (bpy C=C and C=N); 800-1000 s (W=Ot), 720-830 m (W-O-W). X-Ray Crystallography. X-Ray powder diffraction (XRPD) patterns were carried out in a transmission configuration on solid powders. A copper rotating anode X-ray source coupled with a multilayer focusing ‘Osmic’ monochromator giving high flux (108 photons.s-1) and punctual collimation was used as the source (= 1.5418 Å). An ‘Image plate' was used as a 2D detector. Diffracted intensities were corrected by exposition time, transmission and intensity background. Intensity data for single crystals were recorded at room temperature on a Rigaku AFC-7S diffractometer equipped with a CCD detector using monochromated Mo(Kα) radiation (λ= 0.71073 Å). An empirical absorption correction (multi-scan) was applied using the CrystalClear28 package. The structures were solved by Direct Methods and refined by full-matrix leastsquares on F2 using the SHELXL29 package. Hydrogen atoms on the carbon atoms were placed at fixed positions using the HFIX instruction. They were refined using the riding model with isotropic displacement parameters set to 1.2×Ueq of the attached atom. The non-hydrogen atoms of the organic molecules were refined by restraining the ADP components in the direction of bond to be equal within a standard deviation of 0.01 and 0.001 for 1 and 2, respectively. Crystal data for 1: Monoclinic, space group P21/n, a= 14.328(3), b= 25.528(5), c= 22.596(5) Å, = 96.49(3)°, V= 8212(3) Å3, Z= 4, T= 298(2) K, = 17.589 mm-1, Dx= 3.253 g·cm-3. 86627 data collected (max=28.13°), 16311 unique (Rint= 7.40%) and 11650 observed [I> 2(I)], R1= 0.0576, wR2= 0.1302. Crystal data and refinement parameters are register in Table S1. Crystal data for 2: Triclinic, space group P(1), a= 8.4859(17), b= 12.304(3), c= 21.667(4) Å, α= 90.60(3)°, = 99.36(3)°, γ= 94.79(3)°, V= 2223.7(8) Å3, Z= 2, T= 298(2) K, = 16.951 mm-1, Dx= 3.253 g·cm-3. 25847 data collected (max=28.04°), 8409 unique (Rint= 5.37%) and 6374 observed [I> 2(I)], R1= 0.0555, wR2= 0.1362. Crystal data and refinement parameters are register in Table S1.

RESULTS AND DISCUSSION Both compounds 1 and 2 were separated as air-stable and water-insoluble blue crystals from the hydrothermal reactions of CuCl2, bpy, Na2WO4·2H2O, and H2O as starting materials in a ratio (1:1:4.7:3217). With plenty of parallel experiments, it was found that heating temperature, molar ratio, and initial pH play a crucial role in the yielding optimization of these solids. As the molecular compound 1 was obtained setting the initial pH to a close range of 5-6 and heating the starting materials at 140 oC for five days, the polymer 2 was observed in the final solid at low yield (7%) using the same condition but increasing the temperature to 180 oC for seven days. It should be quoted that, by using other combination of pH and heating temperature, the final compounds were either not observed or obtained in a very low yield. A more striking feature, however, is that crystals of compound 1 could be forced to transform into crystals of 2 with high structural purity (see Figure S3) through the use of hydrothermal conditions at 180 oC during seven days. From a


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mechanistic point of view, this 12 crystal transformation is not so obvious, but the difference in the molar ratio found in compound 1 (W/Cu=4) versus that in polymer 2 (W/Cu=2; vide infra) suggests that the metastable compound 1 must be defragmented before the formation of the thermodynamic product 2. Crystal structure of 1. The asymmetric unit contains a discrete molecular cluster [Figure 1 and Figure S11(a)] and three lattice water molecules. This cluster can be described as consisting of a core of one α-metatungstate Keggin unit [H2W12O40]6- which is decorated by three "{Cu(bpy)2}2+" moieties. There are twelve {WO6}-octahedra arranged in four groups of three {W3O13} units with edge-sharing octahedra. These trimeric entities, in turn, are linked together by corner-sharing octahedra. It is noted the absence of heteroelement in the central cavity of the Keggin anion. However, two of the triply-bridging oxygen atoms (3-Otryple-bridging) located in the central cavity of the cluster, show low valence sums30 (O11: 1.04 and O25: 1.02 uv) as compared with the expected value of 2, and they are, therefore, assumed to be protonated. The W-O distances in the {WO6}-octahedra (see Tables S2 and S3) lie in the range 1.683(11)-1.789(11) Å for terminal oxo-ligand (W-Ot), 1.817(11)-2.103(11) Å for 2-Obridging atoms (W-Ob), and 2.021(11)-2.372(12) Å for 3-Otryple-bridging that form the central cavity of the cluster (W-Otb). The W-Ob-W and W-Otb-W bond angles are in the range 109.5(6)-155.8(7)o and 92.9(4)100.1(4)o, respectively. The surface of the anionic cluster is decorated by secondary metal-ligand "{Cu(bpy)2}2+" subunits (see Figure 1). There exist three crystallographically independent Cu(II) ions on the Keggin core. Two Cu-centers (Cu1, Cu3) exhibit a highly distorted octahedral geometry with four nitrogen donors of two bidentate bpy ligands, and the remaining positions occupied by one oxo-terminal and one 2-Obridging atom of the cluster. On these Cu-sites, an axial distortion [Cu1-N4: 2.241(17) Å, Cu1-O31: 2.684(11) Å and Cu3-N6: 2.113(16) Å, Cu3-O22: 2.673(11) Å, respectively] of the M-L bond lengths vs that in the equatorial plane [Cu1-N(mean): 2.006 Å, Cu1-O30: 1.989(12) Å and Cu3-N(mean): 2.011 Å, Cu3-O3: 2.051(11) Å, respectively].is observed which may be attributed to a JeanTeller effect. In this coordination geometry, the distances between the sites Cu(II) and 2-Obridging of the cluster [CuOb(mean): 2.679 Å] are significantly longer than the distances between the copper ion and terminal oxygen atom (CuOt(mean): 2.020 Å]. However, it is considered that the orientation of sites Cu(II) concerning 2-Obridging atom on the cluster is not accidental, so this interaction although quite weak should be significant. Other researchers have described Cu-O bond interactions with comparable31 or even longer32 lengths. The third Cu-center (Cu2) shows a distorted "[CuN4O]" trigonal bipyramid coordination environment, defined by one oxoterminal group [Cu2-O19: 2.115(11) Å] from the polyoxoanion core and four nitrogen atoms from two bpy ligands [CuN(range): 1.954-2.059 Å]. Bipyridine rings on "{Cu(bpy)2}2+" fragments appear responsible for linking together the discrete molecular clusters via a complex number of edge-to-face CH···π (range: 2.9-3.3 Å) and two different π···π interactions (shortest Cg···Cg distances: 3.38 and 4.12 Å; see Figure S4 in the supporting information). Additionally, C-H···O [C···O(range): 2.98-3.48 Å] hydrogen bond interactions, involving crystallization water molecules and oxo groups, help to sustain the final assembly. Crystal structure of 2. The packing of 2 is constructed from 1Dpolymers in which the building block that can be formulated as

"{[Cu(OH2)(bpy)][Cu(bpy)]2W6O21}" [Figure 2a and Figure S11(b)] contains an unusually decorated hexatungstate [W6O21]6-. This hexatungstate core (see scheme 1a) of 2 can be described as a pair of [W3O11] trimers constructed from edgesharing {WO6}-octahedra (W1, W5, W3 and W2, W4, W6) in such a way that the central octahedron of each trimer (W5 and W4, respectively) is connected via edge-sharing to the two remaining {WO6}-octahedra, while these last two octahedra share a corner between them. Both trimers assembly each other involving only edge-sharing (see scheme 1b) such that one octahedron (W3) is linked to the three octahedra of the other trimer (W2, W4, W6), the W5-octahedron connect to W2and W4-octahedron, while W1-octahedron is joined only to the W2-octahedron. In this geometry (see Table S4) the crystallographically independent hexamer entity shows thirteen terminal oxygen ligands [range: W-Ot: 1.723(10)1.869(9) Å], four 2-Obridging [range: W-Ob: 1.814(9)-2.053(10) Å], two 3-Otryple-bridging [range: W-Otb: 2.020(9)-2.244(10) Å] and two 4-Ocorner [range: W-Oc: 1.977(8)-2.483(9) Å]. Although, it should be emphasized that one of the terminal Oatom [W1-O18i-W6i: W1-O18i: 1.920(9) Å; W6-O18: 1.869(9) Å (i)= 1+x,y,z] is co-responsible for the extension of the polymer structure. This hexatungstate sits on a local site involving a very close C2-symmetry, in which the two-fold axis goes across the middle point of the line between W4...W5 and that one of the W2....W3 line (see Scheme 1b). However, the presence of the decorating "[Cu(OH2)(bpy)]" fragment (vide infra) break down that C2-symmetry to the final C1-symmetry obtained in the crystal structure. The geometry for the hexatungstate [W6O21]6entity observed in 2 represents the first example containing the previously described W-O bonding pattern in a very close C2symmetry. It is noteworthy that common synthetic conditions have so far led hexatungstates with Lindqvist type structure33, other Whexamer cores have been scarcely reported. Even though the existence of hexatungstate has been invoked from almost nine decades ago34 to the best of our knowledge only two crystal structures involving W-hexamers (different to the well-known Lindqvist type) have been known so far. From these two crystal structures, the first one was published 25 years ago by Hartl et al.35 and was obtained through the reaction of Na2WO4 and WO3 from basic solutions. Such an entity corresponds to a Cssymmetric hexamer [H3W6O22]5-. Unlike to the [W6O21]6moiety found in 2, the Hartl's hexatungstate contains thirteen terminal oxygen ligand, five 2-Obridging, two 3-Otryple-bridging and two 4-Ocorner. The second W-hexamer isolated in a crystal structure was reported by Kortz et al.36 and was described as a Ci-symmetric [W6O22]8-, which was stabilized by using dimethyltin as an electrophile and crystallized in acidified tungstate solutions. More recently, a third crystal structure was reported as a rare [W6O22]8- fragment captured in an aqueous solution of [W6O19]2- by using a transition metal complex and isolated as the first example of a transition-metal coordination 2D-polymer based on the [W6O22]8- cluster in the building block [Cu4(W6O22)(L)2(OH2)2]·2H2O (L= 2-amino-4,6-bis(2pyridyl)pyrimidine).37 As claimed by authors, in such a complex, the Ci-symmetric hexatungstate corresponds to the Kortz’s [W6O22]8- fragment, which can be described as two fused W3O13 trimers linked via edge-sharing of {WO6}octahedra and containing eight 2-O, two 4-O and twelve terminal O-atoms, but the linking fashions of the hexatungstate are different in the two structures.37 It must also be quoted that at least eighteen possible geometric forms, derived from {WO6}-octahedra, for hexatungstate ions and the probability criteria for their existences have been



reported.38 Additionally, the possible formation mechanisms for hexatungstates have also been considered.39 Interestingly, only the well-known Lindqvist's type (Fuch's [W6O19]2anions40) and the two structures mentioned before (Hartl and Kortz's W-hexamer) from that pool of eighteen geometric forms of hexatungstate cores have been isolated in a crystal structure. Therefore, the W-hexamer found in 2 symbolizes the fourth geometry form of an isolated and fully structurally characterized so far, and it corresponds to the geometric configuration labeled with the number 19 theoretically proposed by Tytko and Glemser.38 In the 1D-polymer structure observed in 2, the complete building unit "{[Cu(OH2)(bpy)][Cu(bpy)]2W6O21}" is selfassembled through two crystallographically independent "[Cu(bpy)]2+" entities and corner-sharing {WO6}-octahedra to generate the polymeric chain running parallel to a-axis (Figure 2b). A third "[Cu(OH2)(bpy)]2+" fragment acts as a decorating moiety on the surface of the hexatungstate via two monodentate Cu-Ot coordination. All three Cu(II) centers display a slightly distorted square-pyramidal coordination geometry. In Cu1, one chelating bpy ligand [Cu1-N1: 2.018(14) and Cu1-N2: 1.989(13) Å], one oxo-terminal group [Cu1-O13: 1.938(9) Å] and one oxygen atom from a coordinated water and Cu1-N2: 1.989(13) Å], one oxo-terminal group [Cu1-O13: 1.938(9) Å] and one oxygen atom from a coordinated water molecule [Cu1-O1w: 1.964(11) Å] form the basal plane, whereas the apical position is occupied by another oxo-terminal from the same polyoxometalate cluster [Cu1-O20: 2.275(9) Å]. In Cu2 and Cu3, however, the basal plane is defined by two nitrogen donors of a bpy ligand [Cu2-N3: 2.002(12)/Cu2-N4: 1.996(12) Å and Cu3-N5: 1.981(13)/Cu3-N6: 2.001(12) Å], and two oxo-terminal but from two different neighboring clusters [Cu2-Ot(mean): 1.919 Å and Cu3-Ot(mean): 1.918 Å]. The apical position on each of these Cu-centers is occupied by one oxoterminal [Cu2-O10ii: 2.309(10) Å and Cu3-O8i: 2.348(10) Å, respectively; (i) 1+x,y,z; (ii) -1+x,y,z]. Each polymeric unit in 2 is surrounded by other three polymers running parallel in such a way that the planar bpy entities from all Cu-complexes are intercalated, via face-to-face interactions, in a zipper-like manner and accommodated in the b- (Figure 3b) and roughly in the bc-direction (Figure 3c). This array leads channels along the a-axis where water molecules are hosted (Figure 3a), in which zipper-like intercalation is supposed to be strong as suggested by the face-to-face separations [mean: 3.921 Å; range: 3.620-4.382 Å; see Figure S5]. The polymers that enclose the channels are accommodated in the bc-plane forming a kind of layers, which in turn are stacked in the cdirection by hydrogen bonding interactions involving the remaining lattice water molecules and that one coordinated to the Cu1-center (see Figure 3 and Figure S5). Thermal Analysis of 1 and 2. Thermal decompositions of 1 and 2 are supported by its crystal structures. TGA of 1 (Figure S6) shows the first loss below 190 oC of 1.3% corresponding to three crystallization water molecules (Calc. 1.34%). After, a plateau is reached in a wide range (190-340 oC) which is followed by the decomposition and/or sublimation (found. 23.8%) of the organic component (Calc. 23.10%, 340-870 oC). TGA of 2 (Figure S6) gives a first loss of 3.6% in the range 20170 oC, which is attributed to four water molecules of crystallization and one coordinated water molecule (Calc. 4.1%). The water loss is followed by similar thermal stability (plateau: 170-340 oC) to that observed for 1. The next weight loss of 22.2% in the range 340-850 oC arises from the decomposition and/or sublimation of the bpy ligands (Calc. 22.9%). DSC curve of 2 (see Figure S6) shows an endothermic

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process in the range 80-130 oC corresponding to the water loss. A striking finding is the existence of an exothermic phase transition in the DSC range 190-230 oC, which is associated to a second structural change without any weight loss. Phase transformation in 2. As mentioned before, the DSC curve indicates only two processes in the range of 20-230 °C for the channeled crystal packing of 2. As the first process (80-130 °C) is associated with the desorption of crystallization water molecules, the second one occurs in the range 190-230 °C, which is related to a structural phase transition without weight loss. It was found that a sample of 2 after heated at 230 oC readily re-adsorbed water molecules in few (3-4) hours under exposure to room atmosphere. A new TG analysis on the same rehydrated sample demonstrated that the amount of water readsorbed in 4 h corresponds to the amount lost during the heating. These observations suggest a reversible water desorption/re-adsorption process (vide infra). The evolution of the phase transformations observed in 2 was also monitored by FTIR spectroscopy. IR-spectra of 2 at different temperatures are collected in Figure 4. It is noted the decreasing of the intensity in the absorption bands centered at 3404 and 1666 cm-1, which are attributed to the vibrational modes corresponding to the stretching (O-H) and bending (HO-H), respectively, of crystallization water molecules. Also, it can also be observed that the window in the range of 980 and 650 cm-1, characteristic of the polyoxoanion, remains unchanged in the interval of temperature (25-220 °C), which is indicative that the molecular structure of the hexatungstate moiety remains substantially unaltered. The structural features of these transformations in 2 were also followed by a set of X-ray powder diffraction (XRPD) experiments measuring diffractograms at 22, 80, 140, 180, 230 °C and again at 22 °C after the rehydration on the same sample (Figure 5). It is noted that the pattern for a fresh sample is quite comparable to that for a rehydrated specimen, which suggests that both diffractograms correspond to the same crystal packing confirming the reversibility of the events; the desorption-readsorption water process and the structural phase transition. The first endothermic process (80-130 oC) occurs with moderate structural changes as observed in the patterns at 80 and 140 oC, respectively. However, the exothermic transition (190-230 °C) appear to lead to more significant structural changes than that observed during the desorption-readsorption water process, although the corresponding patterns at 180 and 230 °C still show pronounced peaks what suggests that the packing of 2 withstands the dehydration process and the subsequent phase transition without significant loss of crystallinity. On the other hand, the reflections (001) and (010) observed at room temperature disappear in the patterns at 180 and 230 °C. Unlike, in the fresh pattern at 22 °C the reflections (010), (002) and (011) are found overlapped under the band at 8.24°. But, after increasing the temperature to 180 °C, the (002) and (011) peaks are shifted from 8.27° and 8.40° to higher values up to 8.80° and 8.90°, respectively. This behavior appears because of a contraction of the volume of the unit-cell associated with the liberation of water molecules during the heating process. As anticipated, all these reflection planes cut the water molecules positions in the crystal structure at room temperature. Such results also suggest that the shrinking occurs in the b- and cdirection involving the shortening of the separation of the 1Dpolymers in the crystal structure. It should be mentioned that although our attempts have been unsuccessful for the characterization of the anhydrous structural phase found


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above 180 °C, the results obtained so far suggest a kind of structure that preserves the integrity of the 1D-polymer maintaining closed its channels after the water liberation. Nitrogen adsorption measurement of 2. The reversible water sorption behavior of 2 prompted us to evaluate whether the channeled crystal structure is still accessible to uptake nitrogen molecules after the water desorption. Attempts were made to obtain adsorption isotherms with nitrogen (see supporting information, Figure S7). Prior to the adsorption experiment, the solid was maintained to 180 °C overnight and rapidly carried into the apparatus where was outgassed at 180 °C until a static vacuum of 3 × 10−3 Torr. Under this condition, the solid did not adsorb nitrogen in substantial amounts (~4.0 cm3mg-1 at P/P0= 0.9) probably due to the significant diminishing of the pore volume after the water evacuation and/or the quite poor interaction of the non-polar nitrogen molecule inside the ionic channels.41 This result was supported by the type III isothermal adsorption (Figure S7), characteristic for poor adsorbate-adsorbent interactions.41 In contrast, the rapid water reincorporation previously described can be favored by the formation of the intricate hydrogen bonding observed between the water molecules and the hydrophilic structural topology of the channels in the packing of 2. Such a reversible water sorption behavior was also observed for nanosized discrete POM compounds containing Keggin-42 , Dawson-26 or {Mo36}-anions43, in which the crystalline solid phase withstands several water sorption-desorption cycles. EPR Study of 2. The room temperature EPR spectrum (Figure S8) for 2 is a very noisy typical spectrum for a Cu(II) in a square-pyramidal geometry interacting with a non-equivalent Cu(II) in its neighborhood. This interaction is responsible for the lack of resolution in the hyperfine coupling in the EPR spectrum. From the spectrum two 𝑔 values 𝑔 ⊥ = 2.1248 and 𝑔 ∥ = 2.2413 can be obtained. In order to study the temperature dependence of the EPR parameters, spectra were recorded in the temperature range 300 ≤ T ≤ 520 K (27 ≤ T ≤ 247 °C). The peak to peak linewidth of the EPR spectra shows a temperature dependence (Figure S9), in which three nearly linear regions can be distinguished. The first one from room temperature up to 90 °C, after that an abrupt change in slope can be observed, which is in accordance with the water desorption-readsorption process observed in the crystal structure. In this region, a linear increase in the peak to peak linewidth is indicative of the shortening in the Cu(II)···Cu(II) distances. Close to T = 170 °C a decrease in the peak to peak linewidth can be associated with the subsequent reversible structural exothermic phase transition observed in the crystal structure. Both transition processes appear accompanied by a contraction of the unit-cell volume, whose shrinking is responsible for the continuous increase in the peak to peak linewidth after each solid transition. As anticipated, when the temperature is lowered it is observed, within the experimental error, that the system recovers its original state according to the reversibility of the transitions previously studied. A slightly and continuous decrease of the resonance field as the temperature is increased also observed (Figure S10), with small changes in slope near the temperatures where the phase transitions are observed. This behavior is, in general, consistent with the observed in the linewidth. Such a decrease in the resonance field agrees with the increase in the exchange interaction. CONCLUSION

In summary, two novel hybrid compounds have been isolated and structurally characterized. Structural studies demonstrated that compound 1 is a Cu-decorated Keggin moiety and it is a metastable phase in function of temperature, which can be transformed under hydrothermal conditions at 180 °C into stable thermodynamic 1D-polymeric structure observed in 2. The building block of this 1D-polymer is based on a new W-hexamer moiety with a close C2-symmetry. Also, compound 2 provides a rare example of a solid that suffers two distinctive reversible transitions. Both processes appear accompanied by a contraction of the volume of the unit-cell associated with the liberation of water molecules during the heating. X-ray diffraction results suggest that the shrinking occurs in the b- and c-direction involving the shortening of the separation of the 1D-polymers in the crystal structure. The approach to use hydrothermal methods to force phase transformations is currently being extended.18,44 Preliminary results indicate that the variation of the supramolecular domain of the organic bidentate ligand (diethylenediamine instead of bpy) avoids obtaining hexamer moieties like observed in 2 but also lead to polymerization of a decorated Keggin entity.44 The temperature dependence on the peak to peak linewidth of the EPR spectra with three nearly linear regions is consistent with the shortening in the Cu(II)···Cu(II) distances expected by the cell volume systematic contraction and the consequent environment changes around the paramagnetic Cu(II) ions. ASSOCIATED CONTENT

Supporting Information Supporting Information available: Powder x-ray experimental and simulated patterns, other representations of the crystal structures, TG analysis of 1 and 2, DSC of 2, nitrogen adsorption/desorption isotherm of 2, EPR spectrum of 2, FTIR spectrum of 1, Tables for crystal data and refinements and selected bond distances and angles for both compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

Accession Codes

CCDC 640862−640863 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION Corresponding Author

* E-mail: [email protected] Author Contributions The manuscript was written through the contributions of all authors. All authors have approved the final version of the paper.

Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS



This work was funded in part by grants from FONACITMppEUCT (Project No. LAB-97000821 and G-2005000431) and PCP Exchange Program (Project No. PCP-2010000306). REFERENCES (1)

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Figure 1 Representation of the molecular structure of 1.

Figure 2 Graphical representation of the (a) building block of the (b) 1D-polymer observed in the crystal structure of 2; Cu2, Cu3, O1, O7, O8, O9, O10, O17, and O18 are connecting centers to extend the polymer along the a-axis. (i)= 1+x,y,z and (ii)= -1+x,y,z.


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Crystal Growth & Design (b)

(a)

W4

W6

W5

W3

W1 W2

Scheme 1. Graphical representation of the hexatungstate moiety found in 2. (a) 3D-Polyhedra model obtained from crystallographic data and (b) 2D-representation in which W-labels indicate the corresponding {WO6}-octahedra.

(b)

(c)

Figure 3 (a) View along a-axis of the channels and layers observed in the crystal structure of 2 and where water molecules are hosted (discrete red balls). (b) and (c) Zipper-like intercalation accommodated parallel to the b- and roughly in the bc-direction, respectively.

Figure 4 FTIR-spectra of 2 in function of the temperature.


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Figure 5 XRPD patterns of 2 in function of the temperature. Vertical lines correspond to the theoretical pattern as modeled from single-crystal data. Top pattern (22 C, recovered) was registered after the dehydrated sample at 220 C cooled down and exposed to room ambient.


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For Table of Contents Use Only

Reversible phase transition in a coordination 1D-polymer containing an unusual hexatungstate building block. Reinaldo Atencio,* Gustavo Liendo, Alexander Briceño, Pedro Silva, Isabelle Beurroies, Philippe Dieudonné, and Haci Baykara

180 °C 7 days 1

2

Crystal structure 2 can be obtained from the direct hydrothermal transformation of 1. Compound 2 contains a 1Dpolymers built up from a new W-hexamer building block and provides a rare solid that withstands two reversible transformations; an adsorption-readsorption process and a distinctive structural phase transition.


11

Reversible Phase Transitions in a Coordination 1D-Polymer

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