Composition Space Analysis in the Development of Copper Molybdate

Article pubs.acs.org/IC

Composition Space Analysis in the Development of Copper Molybdate Hybrids Decorated by a Bifunctional Pyrazolyl/1,2,4Triazole Ligand Andrey B. Lysenko,*,† Ganna A. Senchyk,† Liliana V. Lukashuk,† Konstantin V. Domasevitch,† Marcel Handke,‡ Jörg Lincke,‡ Harald Krautscheid,‡ Eduard B. Rusanov,§ Karl W. Kram ̈ er,∥ Silvio Decurtins,∥ and Shi-Xia Liu*,∥ †

Inorganic Chemistry Department, Taras Shevchenko National University of Kyiv, Volodimirska Street 64, Kyiv 01033, Ukraine Institut für Anorganische Chemie, Universität Leipzig, Johannisallee 29, D-04103 Leipzig, Germany § Institute of Organic Chemistry, Murmanska Street 5, Kyiv, 02660, Ukraine ∥ Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland ‡

S Supporting Information *

ABSTRACT: A bitopic ligand, 4-(3,5-dimethylpyrazol-4-yl)1,2,4-triazole (Hpz-tr) (1), containing two different heterocyclic moieties was employed for the design of copper(II)− molybdate solids under hydrothermal conditions. In the multicomponent CuII/Hpz-tr/MoVI system, a diverse set of coordination hybrids, [Cu(Hpz-tr)2SO4]·3H2O (2), [Cu(Hpztr)Mo3O10] (3), [Cu4(OH)4(Hpz-tr)4Mo8O26]·6H2O (4), [Cu(Hpz-tr)2Mo4O13] (5), and [Mo2O6(Hpz-tr)]·H2O (6), was prepared and characterized. A systematic investigation of these systems in the form of a ternary crystallization diagram approach was utilized to show the influence of the molar ratios of starting reagents, the metal (CuII and MoVI) sources, the temperature, etc., on the reaction products outcome. Complexes 2−4 dominate throughout a wide crystallization range of the composition triangle, while the other two compounds 5 and 6 crystallize as minor phases in a narrow concentration range. In the crystal structures of 2−6, the organic ligand behaves as a short [N−N]-triazole linker between metal centers Cu···Cu in 2−4, Cu···Mo in 5, and Mo···Mo in 6, while the pyrazolyl function remains uncoordinated. This is the reason for the exceptional formation of low-dimensional coordination motifs: 1D for 2, 4, and 6 and 2D for 3 and 5. In all cases, the pyrazolyl group is involved in H bonding (H-donor/H-acceptor) and is responsible for π−π stacking, thus connecting the chain and layer structures in more complicated H-bonding architectures. These compounds possess moderate thermal stability up to 250−300 °C. The magnetic measurements were performed for 2−4, revealing in all three cases antiferromagnetic exchange interactions between neighboring CuII centers and long-range order with a net moment below Tc of 13 K for compound 4.

■

INTRODUCTION During the last decades the development of new polyoxometallate hybrid solids decorated by N-donor ligands has received particular interest in the field of inorganic chemistry and functional materials.1 Among them, organic−inorganic hybrid polyoxomolybdates represent an especially rich structural subclass cross-linking with practical applications in catalysis, magnetism, etc.2 Appropriate choice of accessible heterocyclic ligands, their cationic complexes, and polyoxomolybdate anions as fundamental building units on the one hand and the exploration of a convenient hydrothermal synthetic approach, on the other hand, opened up a straightforward way to tremendous diversity of coordination solids. A few hundreds of their crystal structures deposited at the Cambridge Crystallographic Data Center demonstrate representative © XXXX American Chemical Society

archetypes of multilevel structural interactions between inorganic and organic entities. Despite major progress in the field, far less attention has been devoted to a systematic synthetic methodology applied to polyoxomolybdate hybrids. Many crucial practical details, like the influence of different reactant sources and their proportions, temperature conditions, and pH on the final reaction outcome (such as the obtained products isolated, the coordination modes of organic ligands, etc.), still remain outside the frame. A principal criticism can be often addressed to poorly performed hydrothermal reactions, usually proceeding in a few milligram scale only, that, in general, cannot be accepted as an Received: September 23, 2015

A

DOI: 10.1021/acs.inorgchem.5b02188 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

magnetic exchange interactions between paramagnetic cations in coordination compounds, which is a subject of topical interest in molecular magnetism.10

appropriate synthetic protocol. In the case of multicomponent systems, it typically results in a mixture of products having different degrees of crystallinity, unknown compositions, and crystal structures. These factors prompted us to initiate the comprehensive study of polyoxomolybdate hybrids decorated by N-heterocyclic ligands employing composition space analysis.3 As shown by Poeppelmeier et al., the proposed concept, widely known in physical chemistry and chemistry of metals and alloys, can be successfully applied for the investigation of AgI/VIV,V oxide/oxyfluorides4 and pyridinebased mixed-metal oxyfluorides.5 More recent publications were focused on employing other chelating and bridging heterocyclic ligands like pyrazole, 2,2′-bipy, 4,4′-bipy, 1,2-di(4pyridyl)ethane, 1,2,4-triazole, etc.6 In a similar context, Stock et al.7 introduced a systematic investigation of a series of metal− organic frameworks based on acidic linkers using a highthroughput methodology. In fact, the composition space approach seems to be indispensable for many organic−inorganic systems studied, especially in the case when multifunctional Nn-donors are used. The organic building blocks with several N-binding sites allow a wide variety of metal−ligand interactions that could lead to rich structural hybrid motifs with potential applications. Herein, we introduce 4-(3,5-dimethylpyrazol-4-yl)-1,2,4triazole (Hpz-tr) as a very illustrative example of a multidentate ligand for the investigation of CuII/MoVI oxide systems. The organic ligand unites two basic heterocyclic termini of 1,2,4triazole and pyrazole which generally enable formation of coordination clusters supported by either short [N−N]-tr/pz bridges or terminal coordination.8,9 The expected twisted configuration of Hpz-tr (Scheme 1) can be also regarded as an

■

EXPERIMENTAL SECTION

All chemicals were of reagent grade and used as received without further purification. The 4-(3,5-dimethylpyrazol-4-yl)-1,2,4-triazole (Hpz-tr) ligand was synthesized as described earlier.11 The crystals of free ligand monohydrate Hpz-tr·H2O (1) were crystallized from its aqueous solution at rt. Anal. Calcd for C7H11N5O: C, 46.40; H, 6.12; N, 38.65. Found: C, 46.30; H, 6.19; N, 38.55. Synthesis of the Coordination Compounds. Four reaction systems (CuSO 4 ·5H 2 O/Hpz-tr/MoO 3 , CuSO 4 ·5H 2 O/Hpz-tr/ (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, Cu(OAc) 2 ·H 2 O/Hpz-tr/(NH 4 ) 6 Mo 7 O 24 · 4H2O, and Cu(OAc)2·H2O/Hpz-tr/MoO3) differing in the choice of metal-component sources were thoroughly examined under hydrothermal conditions. The following synthetic procedures for complexes 2−6 were selected as the most appropriate ones through analyzing many reaction parameters. [Cu(Hpz-tr)2SO4]·3H2O (2). A mixture of CuSO4·5H2O (100 mg, 0.400 mmol), Hpz-tr (81.5 mg, 0.500 mmol), and MoO3 (14.4 mg, 0.100 mmol) in 5 mL of H2O (molar ratio CuII/Hpz-tr/MoVI = (4:5:1) was placed in a Teflon reactor and intensively stirred for 1 min. The reactant mixture was heated in a steel autoclave up to 165 °C, kept for 12 h, and cooled to rt for 54 h. The resulting solid product consisted of two crystalline phases: large light-blue needles of complex 2 as a major phase and blue precipitate of 4. The mixture was filtered off, washed with water, and dried at rt. Complex 2 was separated from compound 4 in CH2Cl2/CHBr3 on the basis of their different densities. The yield of 2 was 56% (76.0 mg). Anal. Calcd for C14H24CuN10O7S: C, 31.14; H, 4.48; N, 25.94. Found: C, 31.28; H, 4.09; N, 25.72. IR (KBr discs, cm−1): 455 m, 505 m, 614 m, 660 m, 691 w, 768 m, 796 w, 896 m, 973 s, 1046 vs, 1073 vs, 1113 vs, 1168 vs, 1241 s, 1313 m, 1386 w, 1449 m, 1517 w, 1550 m, 1613 m, 1640 m, 1772 w, 2924 m, 3119 s, 3188 s, 3224 s, 3404 s, 3468 s. Complex 2 was also prepared without adding the molybdenum component to the synthesis. Starting from CuSO4·5H2O (0.30 mmol, taken in excess) and Hpz-tr (0.20 mmol) in 3 mL of H2O at 190 °C, compound 2 crystallized in ∼50% yield. [Cu(Hpz-tr)Mo3O10] (3). A mixture of CuSO4·5H2O (100 mg, 0.400 mmol), Hpz-tr (32.6 mg, 0.200 mmol), and MoO3 (57.6 mg, 0.400 mmol) in 5 mL of H2O (molar ratio CuII/Hpz-tr/MoVI = 4:2:4) was placed in a Teflon reactor and intensively stirred for 1 min before being sealed. The autoclave was heated using the same temperature regime as described for 2. Green prisms were filtered off, washed with water, and dried. Yield: 88% (79.0 mg). Anal. Calcd for C7H9CuMo3N5O10: C, 12.46; H, 1.34; N, 10.38. Found: C, 12.36; H, 1.38; N, 10.19. IR (KBr discs, cm−1): 419 m, 528 vs, 655 vs, 837 vs, 878 s, 909 m, 950 s, 1023 w, 1050 w, 1086 m, 1123w, 1246 m, 1290 w, 1318 m, 1404 w, 1440 w, 1550 m, 1608 w, 1640 w, 1777 w, 3019 w, 3071 m, 3441 s, 3473 m. [Cu4(μ-OH)4(Hpz-tr)4Mo8O26]·6H2O (4) was prepared in two ways, as follows. Method A. A mixture of Cu(OAc)2·H2O (40.0 mg, 0.200 mmol), Hpz-tr (97.8 mg, 0.600 mmol), and MoO3 (28.8 mg, 0.200 mmol) in 5 mL of H2O (molar ratio CuII/Hpz-tr/MoVI = 2:6:2) was placed in a Teflon reactor and intensively stirred for 1 min before being sealed. The autoclave was heated using the same temperature regime as described for 2. Yield: 72% (41.0 mg). Method B. A mixture of Cu(OAc)2·H2O (40.0 mg, 0.200 mmol), Hpz-tr (32.6 mg, 0.200 mmol), and (NH4)6Mo7O24·4H2O (106 mg, 0.0858 mmol) in 5 mL of H2O (molar ratio CuII/Hpz-tr/MoVI = 2:2:6) was placed in a Teflon reactor and intensively stirred for 1 min before being sealed. The autoclave was heated using the same temperature regime as described for 2. Blue crystals of 4 were formed as a major phase together with a small quantity of green compound 3, which was separated in CH2Cl2/CHBr3 on the basis of their different densities. Yield: 70% (79.1 mg).

Scheme 1. (a) 4-(3,5-Dimethylpyrazol-4-yl)-1,2,4-triazole, the Heterobifunctional Ligand Utilized in This Study, Adopts a Twisted Conformation; (b and c) Four Possible Coordination Modes of 4-Substituted 1,2,4-Triazole- and Pyrazole-Type Ligands Toward CuII−MoVI Centers

important design tool for preprogramming the further coordination through the direction of M−N(tr/pz) bond vectors. Unlike bridging bipyridine derivatives, which in most cases facilitate the formation of CuII/MoVI hybrid solids with discrete octamolybdates, the presence of the tr/pz donor sets can be beneficial to the generation of homo- and heteroleptic coordination subunits (Cu−[N−N]−Cu, Mo−[N−N]−Mo, and Cu−[N−N]−Mo). This can also afford the mixed-metal oxide matrix to be bound more tightly. Moreover, the tr/pz bridges provide a remarkable ability to efficiently transmit B


Article

Inorganic Chemistry Anal. Calcd for C28H52Cu4Mo8N20O36: C, 14.84; H, 2.31; N, 12.36. Found: C, 14.80; H, 2.51; N, 12.28. IR (KBr discs, cm−1): 451 w, 523 m, 555 m, 650 s, 669 m, 718 s, 737 s, 846 s, 904 vs, 955 vs, 1050 m, 1077 m, 1132 w, 1245 m, 1313w, 1440 w, 1558 m, 1618 m, 2929 w, 3119 m, 3325 s, 3467 s. [Cu(Hpz-tr)2Mo4O13] (5). A mixture of CuSO4·5H2O (25.0 mg, 0.100 mmol), Hpz-tr (81.5 mg, 0.500 mmol), and MoO3 (57.6 mg, 0.400 mmol) in 5 mL of H2O (molar ratio CuII/Hpz-tr/MoVI = 1:5:4) was placed in a Teflon reactor and intensively stirred for 1 min before being sealed. The autoclave was heated using the same temperature regime as described for 2. The resulting product contained green crystals of complex 3 as a major phase and blue-green blocks of compound 5 as a minor component. Since the crystal densities of complexes 3 and 5 are too similar, we were unable to separate them using a CH2Cl2/CHBr3 solution. Blue-green crystals (∼5 mg) of 5 were partially separated from this mixture by hand. The product was washed with water, dried at rt, and analyzed. Anal. Calcd for C14H18CuMo4N10O13: C, 17.13; H, 1.85; N, 14.27. Found: C, 17.16; H, 1.97; N, 14.33. [Mo2O6(Hpz-tr)]·H2O (6). A mixture of CuSO4·5H2O (25.0 mg, 0.100 mmol), Hpz-tr (48.9 mg, 0.300 mmol), and MoO3 (86.4 mg, 0.600 mmol) in 5 mL of H2O (molar ratio CuII/Hpz-tr/MoVI = 1:3:6) was placed in a Teflon reactor and intensively stirred for 1 min before being sealed. The autoclave was heated using the same temperature regime as described for 2. The resulting product contained green crystals of complex 3 as a major phase, blue-green blocks of 5 as a minor phase, and small colorless needles of 6. Under a microscope the colorless needles of complex 6 (∼7 mg) were partially isolated by hand, washed with water, dried, and then analyzed. Anal. Calcd for C7H11Mo2N5O7: C, 17.92; H, 2.36; N, 14.93. Found: C, 17.86; H, 2.44; N, 14.84. The composition-space diagrams were introduced for the four related systems: CuSO4·5H2O/Hpz-tr/MoO3, CuSO4·5H2O/Hpz-tr/ (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, Cu(OAc) 2 ·H 2 O/Hpz-tr/(NH 4 ) 6 Mo 7 O 24 · 4H2O, and Cu(OAc)2·H2O/Hpz-tr/MoO3. The CuSO4·5H2O/Hpztr/MoO3 ternary crystallization diagram was performed using three dozen hydrothermal reactions between the corresponding components in the same quantity of H2O (5 mL). The initial individual concentrations of the reactants were taken in the range 20−160 mM, while the total concentration was equal to 200 mM (Table S15). The reaction mixture was placed in a 20 mL Teflon reactor and intensively stirred for 1 min. The resulting slurry was heated in a steel autoclave up to 165 °C, kept for 12 h, and cooled to rt for 54 h. The product precipitates formed were collected by filtration, washed with water, dried in air, and analyzed by PXRD (solutions were neglected) and crystal habit. Similarly, the other three CuII/Hpz-tr/MoVI reaction systems were investigated. Measurements. Elemental analysis was carried out with a Vario EL-Heraeus microanalyzer. IR-spectra (400−4000 cm−1) were measured with a PerkinElmer FTIR spectrometer (KBr pellets) and with a Mattson 7000 spectrometer using a globar source, a DTGS detector, and KBr cells, with 2 cm−1 resolution and triangular apodization. The room-temperature (rt) powder X-ray diffraction patterns (PXRD) were measured using a Stoe STADIP (Cu Kα1 using a linear PSD detector). The temperature-dependent X-ray measurements were recorded on a Stoe STADIP with a high-temperature attachment and an image plate detector system. Simultaneous thermogravimetric/differential thermal analysis/mass spectrometry (TG/DTA-MS) studies were carried out on a Netzsch F1 Jupiter device connected to an Aeolos mass spectrometer. Samples were heated at a rate of 10 K min−1. Magnetic susceptibility data were recorded using a Quantum design MPMS-5XL SQUID magnetometer in the temperature range 300−1.9 K and at a field of 2 kG. Experimental data were corrected for sample holder, diamagnetic contributions, and TIP contribution for CuII. X-ray Crystallography. The diffraction data were collected with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) using a Stoe Image Plate Diffraction System (for 1 and 4−6) and a Bruker APEXII CCD area-detector diffractometer (ω scans) (for 2 and 3). The data were corrected for Lorentz-polarization effects and for the

effects of absorption (multiscans method for 2 and 3, numerical absorption correction using X-RED and X-SHAPE12 for 1 and 4−6). The structures were solved by direct methods and refined by fullmatrix least-squares on F2 using the SHELX-97 package.13 All nonhydrogen atoms were refined anisotropically. The OH and NH hydrogen atoms were located and then fixed with Uiso = 1.5Ueq(O, N). The CH hydrogen atoms were added geometrically [Uiso = 1.2Ueq(C)] and refined as riding. In the structure of 2, the outer 3,5dimethylpyrazolyl groups of the triazole-N-coordinated ligands are equally disordered over two orientations related by a mirror plane. The disorder was refined with restrained coplanar situation of the ring atoms and a set of similarity restraints for anisotropic thermal parameters. One of the solvate water molecules in the structure is also equally disordered. Two corresponding contributions were refined anisotropically, but the hydrogen atoms were not added to this fragment. In the structure of 4, the solvate water molecules are badly disordered over multiple positions. This electron density was modeled using a Squeeze routine implemented in PLATON.14 In the structure of 6, the solvate water molecule is unequally disordered over two closely separated positions. The partial occupancy factors (0.75 and 0.25) were derived from refinement of isotropic thermal parameters, and both positions of the disordered oxygen atom were refined anisotropically. Graphical visualization of the structures was made using the program Diamond 2.1e.15 The crystallographic material can be obtained from the CCDC, the deposition numbers being CCDC 1423076−1423081.

■

RESULTS AND DISCUSSION Synthesis and Composition Space Diagram. Polycomponent systems, especially those including molybdenum oxide species, are distinguished by a broad diversity of metal− molybdate−organic hybrids. One of the most powerful approaches to a systematic study is sequentially varying molar ratios of reagents and, as a result, constructing ternary crystallization diagrams. Employing such a strategy to the complicated CuII/Hpz-tr/MoVI system gives many benefits in understanding inorganic/organic interactions under hydrothermal reaction conditions through the influence of numerous parameters on the product formation and phase purity. In order to find the optimal synthetic conditions, which lead to the formation of well-crystalline products and minimize undesirable admixtures, a few fast screens were made using different CuII and MoVI sources (CuSO4/Cu(OAc)2 and MoO3/(NH4)6Mo7O24) and different temperatures. It was observed that a temperature over 170 °C yielded a higher content of inorganic metal−oxides and amorphous admixtures, while low-temperature conditions (below 150 °C) were not sufficient due to the limited dissolution of inorganic components and intermediates (MoO3, Cu3(OH)2(MoO4)2).3 Thus, a 165 °C temperature regime (keeping for 12 h with cooling to rt within 54 h) was found to be optimal. For the CuSO4/Hpz-tr/MoO3 system, which was taken as a basic one, the ternary crystallization diagram (Figure 1) was constructed by placing different molar ratios of the reactants from 1 to 8 (0.1−0.8 mmol, 20−160 mM) through 36 experimental points investigated (for more details see, Table S15, Figure S15). The solid products were collected by filtration, dried on air, and analyzed by PXRD to confirm the presence of crystalline phases of the compounds reported below; the solution contents were neglected (Figures S16−S23). The ternary diagram of the CuSO4·5H2O/Hpz-tr/MoO3 system (Figure 1) shows five distinct but strongly overlapping regions of crystallization of complexes 2−6. Compound 3 is a dominant phase, and its crystallization field covers almost the whole triangle, except for a small area at the upper corner with C


Article

Inorganic Chemistry

high Hpz-tr and low MoO3 molar ratios. Formation of complex 4 is observed at low MoO3 content through changing the molar ratio of CuII/Hpz-tr from 1 to 8, and its phase occupies onethird of the diagram (ratio diapasons CuSO4:Hpz-tr:MoO3 = 1−8:1−8:1−3). Compound 2 crystallizes simultaneously with 4, but it is mostly located in the upper part of the triangle within the molar ratio of CuSO4:Hpz-tr:MoO3 in a range of 1− 6:3−8:1−4. The crystallization field of 5 occurs in the upper left side in a small range at low CuII concentration (CuSO4:Hpz-tr:MoO3 = 1−2:3−7:2−6) and includes the region of compound 6. The latter was only formed at two points with ratios of CuSO4:Hpz-tr:MoO3 = 1:3:6 and 1:4:5. In the lower left corner related to high MoO3 concentration (140−160 mM) the presence of unreacted molybdenum(VI) oxide accompanied by an unknown white phase was detected through analyzing the PXRD patterns. The set of diffraction peaks of complex 6 and the unknown phase exhibit similar patterns that may indicate some structural similarities between the compounds (Figure S22).

Figure 1. Triangular space diagram constructed for the threecomponent CuSO4/Hpz-tr/MoO3 system indicates crystallization fields of several new compounds.

Figure 2. Evolution of crystalline phases along one axis of the composition triangle confirmed by PXRD patterns: (a) molar concentration of CuSO4 was kept constant; (b) molar concentration of Hpz-tr was kept constant. D


Article

Inorganic Chemistry Trying to understand some peculiar trends in the crystallization area distribution, we followed the evolution on PXRD patterns along one triangle axis (Figure 2). Along the direction running parallel to the MoO3 axis the molar ratio of the CuII component is constant (Figure 2a), while two others (Hpz-tr, MoO3) are changeable. The PXRD studies demonstrate a gradual appearance or disappearance of diffraction peaks belonging to certain compounds. At the lowest CuSO4 concentration (point Cu/Hpz-tr/MoO3 = 1:8:1), the interplay between MoO3 and Hpz-tr results in switching the structural types from [Cu(Hpz-tr) 2 SO 4 ]·3H 2 O (2) and [Cu 4 (μOH)4(Hpz-tr)4Mo8O26]·6H2O (4) with a high ligand−CuII content to compounds [Cu(Hpz-tr)Mo 3 O 10 ] (3) and [Mo2O6(Hpz-tr)]·H2O (6) with a high molybdenum oxide content. In the case of 60 mM Hpz-tr concentration, being kept constant (Figure 2b), similar crystallization features from “copper-rich” species 2 and 4 through complexes 3 and 5 to “copper-free” complex 6 are observed. Nevertheless, the composition of solid products does not necessarily match the molar ratios of starting materials used; for instance, compound 4 has additional μ2-OH bridges generated in situ upon hydrothermal hydrolysis of CuII salts. Considering the influence of metal sources, three additional systems CuSO4·5H2O/Hpz-tr/(NH4)6Mo7O24·4H2O, Cu(OAc)2·H2O/Hpz-tr/(NH4)6Mo7O24·4H2O, and Cu(OAc)2· H2O/Hpz-tr/MoO3 were examined. Quick screen experiments showed dramatic differences in the crystallization field distribution of main phases (compounds 2−4) inside the ternary diagrams (Figure 3). A comparison of two related systems based on copper(II) sulfate, CuSO4/Hpz-tr/MoO3 and CuSO4/Hpz-tr/(NH4)6Mo7O24, allows one to assess the impact of slight changes in pH values caused by different molybdenum sources. In the CuSO4/Hpz-tr/(NH4)6Mo7O24 system, crystallization areas of all three compounds 2−4 have almost equivalent sizes of one-third of the triangle diagram, but the position of 4 is totally inverted. Thus, compound 2 occupies the right side and compound 3 the left one, whereas in contrast to the CuSO4/Hpz-tr/MoO3 concentration triangle, compound 4 covers the bottom part. This difference can be accounted for by the more acidic properties of MoO3 than (NH4)6Mo7O24, an important factor influencing the presence of hydroxide species. The transition to copper(II) acetate in Cu(OAc)2·H2O/Hpztr/MoO3 and Cu(OAc)2·H2O/Hpz-tr/(NH4)6Mo7O24·4H2O aimed to avoid the formation of compound 2 and to increase the pH to less acidic values. Even so, the crystallization area positions of compounds 3 and 4 in Cu(OAc)2·H2O/Hpz-tr/ MoO3 and Cu(OAc)2·H2O/Hpz-tr/(NH4)6Mo7O24·4H2O diagrams are adjusted by molybdenum salts. Unfortunately, a major disadvantage of these systems is associated with the low thermal stability of Cu(OAc)2, which decomposes into copper(II, I) oxides under hydrothermal conditions (Figure 3 b and 3c). At the same time, these two systems possess the fields of pure compound 4 that was impossible to achieve for copper(II) sulfate systems. Crystal Structures. The organic ligand Hpz-tr unites two heterocyclic functions of 1,2,4-triazole and pyrazole that define the appropriate acido−basic properties, whereas the “inner” C− N single bond between triazole and pyrazole provides a rigid angular disposition of N-donor centers. As evidenced by the crystal structure of the hydrate Hpz-tr·H2O (1), the two heterocyclic termini have a torsion angle of ∼52° (Figure 4a) that might originate from the repulsive interactions between CH3 and triazolyl groups. As demonstrated for the related

Figure 3. Product distribution in three systems: CuSO4·5H2O/Hpztr/(NH4)6Mo7O24·4H2O (a), Cu(OAc)2·H2O/Hpz-tr/ (NH4 )6 Mo 7O 24 ·4H2 O (b), Cu(OAc)2 ·H2 O/Hpz-tr/MoO 3 (c); where light-blue squares belong to [Cu(Hpz-tr)2SO4]·3H2O (2), green circles to [Cu(Hpz-tr)Mo3O10] (3), and deep-blue triangles to [Cu4(μ-OH)4(Hpz-tr)4Mo8O26]·6H2O (4). Orange lines in b and c define areas of copper(II, I) oxides; gray line in c indicates the presence of unreacted MoO3. (The total concentration of the initial CuII/Hpz-tr/MoVI components was equal to 200 mM.)

tetramethyl-4,4′-bipyrazole ligand, this interplanar angle can be responsible for further topological motifs of coordination polymers.16 In 1, the Hpz-tr molecule can be regarded as a tecton with one hydrogen-bonding donor and three hydrogenbonding acceptor sites (NH of pz, three N atoms of tr). To balance excess of H-bond acceptors over H-bond donors, formation of a structure requires additional polar guest molecules like H2O serving as a universal H-donor/acceptor E


Article

Inorganic Chemistry

bonding and π−π stacking interactions (Table S2). This can be explained by noticeable differences between the pKa values of 1,2,4-triazolyl and dimethylpyrazolyl fragments of the organic ligand. Unlike the 1,2,4-triazolyl group (pKb ≈ 2.39), the dimethypyrazolyl group (pKb for dimethylpyrazole ≈ 4.38) is partially protonated in aqueous solution at pH ≈ 4−6 under the given reaction conditions. The crystal structure of compound [Cu(Hpz-tr)2SO4]·3H2O (2) represents the key coordination role of the organic ligand Hpz-tr in the formation of polymeric motifs. The coordination environment of CuII consists of four N atoms from [−N−N−]bridging triazolyl groups in the equatorial plane and two O atoms from μ2-O,O′-sulfate in axial positions. Being connected through oxygen atoms of the opposite vertices, the square bipyramidal polyhedra {CuN4O2} are arranged into a straight line along the b axis (Figure 5). As mentioned above, the pyrazolyl fragment appears to be uncoordinated, whereas the ligand configuration has a stronger tendency, compared with Hpz-tr·H2O, to be orthogonal (the corresponding tr-pzH twistangle is of ca. 72°). These factors facilitate the intercolumn pzNH···OSO32− hydrogen-bonding and π−π stacking interactions. The latter are expressed in four types (Table S2) defined by the corresponding pzH---pzH ring shift and centroid−centroid distances and play a decisive role in the crystal packing. The packing mode, reminiscent of CuX2/1,4phenylene-4,4′-bi(1,2,4-triazole) complexes (X = Cl−, Br−),17 facilitates the formation of rhombic channels having a 3 × 6 Å2 van der Waals cross section and being filled with water molecules. The chemistry of copper(II)−molybdenum(VI) oxides itself represents a unique subclass of inorganic compounds in which the combination of different oxidation states, polyhedral types, and their connectivity defines the structural variety of geometric forms.18 The diversity of structural types becomes far richer when organic ligands are introduced. For the CuII/N-donor ligand/MoVI hybrid systems, the metal−oxide component can interact with the organic part in a different manner, resulting in various polynuclear moieties, from typical discrete β−Mo8O264− to polymeric ones, as partially shown in Scheme 2. Though the triazole heterocycle exhibits a strong affinity to the first-row

Figure 4. Formation of 3D H-bonding arrays between pyrazolyl/ triazolyl groups of the organic ligand and water molecules in the crystal structure of monohydrate Hpz-tr·H2O (1).

amphoteric module. Thus, the desirable self-assembly of Hpz-tr and H2O molecules resulted in a 3D network (Figure 4b) which is built by a diverse set of O−H···N(tr/pz), N−H··· N(pz), and C(tr)−H···O hydrogen-bonding interactions. In the series of coordination compounds 2−6, the Hpz-tr ligand displays some common features. Only the 1,2,4-triazole group acting as a [N−N]-bidentate bridge is involved in coordination, while the pyrazole moiety stays in a noncoordinated neutral form that is responsible for hydrogen-

Figure 5. (a) Chain structural motifs observed in the crystal structure of [Cu(Hpz-tr)2SO4]·3H2O (2); (b) intercalation of the neighboring columns is supported by numerous intercolumn H-bonding interactions (pzN-H...O−SO32−) and π−π stacking of the uncoordinated pyrazolyl termini (uncoordinated water molecules are not shown). F


Article

Inorganic Chemistry

interlayer bifurcated hydrogen bonds with two terminal molybdenyl groups (NH···OMo, 3.173(7) Å). The pyrazolyl and triazolyl moieties of each ligand exhibit an interplanar angle of 85°. Thus, the organic pillar modules completely fill the interlayer volume, leaving no space to incorporate even a solvent molecule. These structural features facilitate the interlayer packing at the distance of 11.47 Å (between CuII ions) (Figure 6b). The overall alternation of mixed-metal− oxide sheets and stacked organic walls of Hpz-tr can be regarded as −ABA···ABA− sequence, where B is the inorganic layer and A is the interlayer space occupied by organic pillars. Unlike compound 3, the structure of [Cu4(OH)4(Hpztr)4Mo8O26]·6H2O (4) contains four crystallographically independent copper(II) ions that are assembled into {CuII(μ2-OH)[N−N]-tr} chains with intrachain Cu···Cu distances of 3.362(14)−3.419(15) Å (Figure 7). Such

Scheme 2. Three Types of [N−N]-Triazole Bridging Modes Toward Molybdenum(VI) Oxide Subtopological Types That Are Formed in the CuII/Hpz-tr/MoVI System

transition-metal cations (CuII), it is interesting to note the existence of rare triazolyl-based molybdenum(VI) oxide complexes.19,20 In the present work, we demonstrate three possible coordination modes of the tr ligand, including even a more exclusive heteronuclear connection (Scheme 2). Compound [Cu(Hpz-tr)Mo3O10] (3) can be considered to be built up from two polymeric parts: CuII-1,2,4-triazole coordination chains and inorganic {Mo3O10}n2n− ribbons which are interconnected at a distance of 8.83 Å (Figure 6a). The first one consists of corner-sharing square bipyramids Cu{N2O4} supported by the [−N−N−]-tr bridging functions and μ2-{OMoO5} polyhedra. The inorganic moiety is constructed from two kinds of edge-sharing {MoO6} octahedra. The structure of the polymeric {Mo3O10}n2n− anion was described earlier for simple salts A2Mo3O10 (A+ = NH4,21a Na,21b K21c or with organic diamines as counter cations22), but only in a few cases its partial coordination to transition metals was observed.23 In contrast, the close interaction between CuII and MoVI centers predetermines the layered motifs in compound 3, running along the ab plane. The uncoordinated H-pyrazolyl groups are orientated perpendicularly to the metal−oxide planes to form

Figure 7. {CuII(μ-OH)(Hpz-tr)} chain in compound 4 supported by double μ2-tr and μ2-OH− bridges (octamolybdate anions are not shown).

polymeric motifs supported by means of a double 1,2,4triazole/hydroxide bridge were reported earlier for a few 1,2,4tr-based complexes.24 In general, the 1,2,4-tr/OH− combination may also lead to discrete hydroxo clusters,25 where water

Figure 6. (a) View of the 2D crystal structure of 3: interconnection of CuII/tr and {Mo3O10} chains; (b) close packing of the hybrid layers 3 through the weak N−H interactions of pyrazolyl groups with molybdenum oxide matrix. G


Article

Inorganic Chemistry

the corner-sharing octamolybdate parts, was reported.27,22b,23c In certain cases, organic N-donor ligands partially occupy oxygen positions in the molybdenum octahedral environment, stabilizing neutral or anionic motifs.28 In the latter case, the charge compensation can be reached by introducing first-row transition-metal cations, while short [N−N]−tr links can provide heterometallic interconnections between the cationic and the anionic species. This principle is realized in compound [Cu(Hpz-tr)2Mo4O13] (5) (Figure 9). The polymeric molybdenum oxide anion is built of strongly distorted {MoO5N} octahedral and square-pyramidal {MoO5} polyhedra. First, they are organized in centrosymmetric {Mo4O13N2} fragments in an edge-sharing manner. Then they are further interconnected through common O corners of {MoO5N}, forming a ribbon along the a axis. The {Mo4O13}n2n− anions together with CuII linking “islands” generate the neutral inorganic layer {CuMo4O13} in the ab plane. The CuII atoms adopt a square bipyramidal coordination environment {N2O2 + O2} with two elongated apical positions from terminal molybdenyl oxygen atoms of {MoO5} (Cu1− O6, 2.425(12) Å). Each independent {Mo4O13} unit is coordinated to four adjacent copper ions maintaining the closest intralayer Cu···Cu distances at 7.75 and 8.18 Å (Figure 10). The formation of such a mixed-metal−oxide motif is

molecules or anions complement axial sites in the square pyramidal environment. In complex 4, both tr/OH− linkers form the equatorial {N2O2} plane around the CuII centers while the elongated axial positions are occupied by oxygen atoms of β-Mo8O264− anions (Cu−O, 2.564(5)−2.873(6) Å). The octamolybdate serves as a multitopic charge-compensating and space-filling agent that interconnects the copper hydroxide chains into the 2D layer running in the ab plane (Figure 8).

Figure 8. Fragment of the crystal structure of 4: cationic [Cu(μ2-OH) tr]nn+ chains are interlinked into 2D motifs by multidentate binding of β-Mo8O264−.

Moreover, the highly hydrophilic anion β-Mo8O264− is also a large H-bond acceptor platform for intermolecular interactions (O-H···OMo = 2.8407−2.9646 Å; (pz)N−H···OMo = 2.8959−3.0251 Å, for details see Figures S9 and S10). Among octamolybdates, the β-Mo8O264− type is the most favorable unit. However, under hydrothermal conditions the discrete species demonstrate well-known intermolecular reorganizations due to the lability of the Mo−O bond. Therefore, polymerization of molybdenum oxides, e.g., monolithic {Mo15O47}n4n− with all edge-sharing {MoO6} octahedra26 or {Mo8O26}n4n−/{Mo4O13}n2n− formed through

Figure 10. 2D {CuIIMoVI4O13} inorganic layer in 5 supported by 1,2,4triazole functional groups of the Hpz-tr ligand.

Figure 9. (a) View of the independent fragment in compound 5; the triazole group acts as a N,N-bridge between two different metal sites (MoVI and CuII) in the heteronuclear chain motif. The uncoordinated pyrazolyl functions form the H-bonding interactions with molybdenum oxide fragments that are responsible for packing in the crystal. (b) {MoO5N} and {MoO5} polyhedra; molybdenyl bonds are shown in white. H


Article

Inorganic Chemistry

mass loss corresponding to Hpz-tr decomposition is observed as well as loss of sample crystallinity. Compound 3, which has no water molecules in the structure, shows higher thermal stability until 300 °C, as confirmed by the TG curve and TDPXRD data. Complex 4 realizes a stepwise mass loss in the range from rt to 180 °C that corresponds to the elimination of six water molecules and also exhibits a slight shift of diffraction patterns in TD-PXRD. The decomposition process starts at 230 °C, followed by further water release taken from the OH− bridges. Thus, the thermal stability of compound 4 is apparently lower than those of complexes 2 and 3 (Table 1, Figures S27−S29).

supported by [N−N] coordination of tr ligand, which links two different metal centers (CuII−MoVI) at a distance of 3.485(15) Å. To the best of our knowledge, this is a fourth structural precedent of a CuII/MoVI heterometallic pair bridged by the tr heterocycle.29 Despite the low value of the twist angle (47°) between linked triazolyl and pyrazolyl fragments, the overall crystal packing of complex 5 is closely related to that of complex 3, and the presence of interlayer (pz)NH···OMo hydrogen-bonding contacts stabilizes this behavior. The large excess of MoVI over CuII in the CuII/Hpz-tr/MoVI system led to the isolation of a copper-free complex having the composition [Mo2O6(Hpz-tr)]·H2O (6) (Figures 11, 12). In 6,

Table 1. TD-PXRD and TGA Results for Coordination Compounds 2−4 complex 2

complex 3

Figure 11. Formation of the inorganic MoO3 ribbon supported by up and down [N−N]-tr bridges in the crystal structure of 6.

complex 4

TD-PXRD

TGA

130 °C → obvious pattern shifts 270 °C → decomposition of the structure 300 °C → decomposition of the structure 130 °C → slight pattern shifts

25−130 °C → release of 3H2O (10.00% theor./9.99% exp.) 270 °C → decomposition of Hpz-tr

230 °C → decomposition of the structure

300 °C → decomposition of Hpz-tr 25−140 °C; 140−180 °C → two steps of the release of 6H2O (4.76% theor/ 5.10% exp) 250 °C → decomposition of Hpz-tr

Magnetic Properties. Compounds 2−4 were investigated by magnetic susceptibility measurements. The χMT(T) plot (Figure 13a) of a polycrystalline sample of 2 exhibits a rt value

Figure 12. Crystal packing of the [Mo2O6(Hpz-tr)] chains in 6 along the a axis (water molecules are omitted).

Figure 13. Thermal variation of χmT for 2 (a) and 3 (b), respectively; solid line is a fit.

the polymeric MoO3 itself is organized into inorganic double chains by μ3-O atoms. The motif is supported by [−N−N−] triazole bridges sitting up and down on the molybdenum trioxide ribbon. Thus, the MoVI atom adopts a distorted octahedral {O5N} coordination environment. Recently, we explored a series of bitriazolyl ligands for engineering molybdenum−oxide organic frameworks (MOOFs).20 It is interesting to note that the ribbon-type structure of polymeric MoO3 can be supported by carboxylate bridges in nicotinic and isonicotinic acids.30 Thermal Stability. The thermal stability of compounds 2− 4 was analyzed by temperature-dependent PXRD and TGA. Both methods are in good agreement and complement each other. For compound 2, TGA data indicate immediate release of crystallization water until 130 °C and then until 270 °C no mass loss occurs. The TD-PXRD patterns in the range from rt to 270 °C show slight shifts of diffraction peaks due to release of water from the rhombic channels. Above 270 °C, a sharp

of 0.32 cm3 K mol−1 (theoretical values χT = 0.375 cm3 K mol−1 for S = 1/2 and g = 2) which is, however, continuously decreasing toward zero with lowering temperature. This result indicates strong antiferromagnetic coupling between neighboring CuII centers along the chain structure. Compound 3, while also exhibiting the characteristics of antiferromagnetic exchange interactions, though with weaker coupling between adjacent CuII centers, shows a rt χMT value of 0.44 cm3 K mol−1 which decreases more pronounced only below 100 K (Figure 13b). The data of compounds 2 and 3 was modeled following the theoretical approach of Rojo et al.31 based on a spin Hamiltonian H = −JΣSi·Si+1, which accounts for nearestneighbor exchange interactions within a chain structure. A fit of the data over the whole temperature range results in the following parameters: g = 2.3 (fixed), J = −254(2) K (−177(1) I


Article

Inorganic Chemistry cm−1) for 2, and g = 2.217(4), J = −32.1(2) K (−22.3(1) cm−1) for 3. In contrast, compound 4 shows a distinctly different χMT(T) plot (Figure 14) with strongly decreasing χMT values (rt χMT

Figure 16. Plot of the field dependence of the magnetization for 4 at 1.9 and 20 K.

with a net magnetization which may result from a spin canting or distinct magnitudes of the ordered moments on the four CuII sites. A visualization of the planes of the magnetic orbitals of the CuII centers for complexes 2−4 is given in Figure 17. A

Figure 14. Thermal variation of χmT for 4 per CuII4; solid line is a fit for the temperature range 200−300 K.

value of 0.95 cm3 K mol−1 per CuII4 unit) but with a sharp increase below 20 K and a sharp turn with decreasing values below 10 K. Again, the higher temperature regime indicates strong antiferromagnetic interactions between the CuII centers, and at low temperature (

Composition Space Analysis in the Development of Copper Molybdate

Recommend Documents