On the Border between Low-Nuclearity and One ... - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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On the Border between Low-Nuclearity and One-Dimensional Solids: A Unique Interplay of 1,2,4-Triazolyl-Based {CuII5(OH)2} Clusters and MoVI-Oxide Matrix Andrey B. Lysenko,*,† Oksana A. Bondar,† Ganna A. Senchyk,† Eduard B. Rusanov,‡ Monika Srebro-Hooper,*,§ James Hooper,§ Krunoslav Prsa,∥ Karl W. Kram ̈ er,⊥ Silvio Decurtins,⊥ Oliver Waldmann,∥ and Shi-Xia Liu*,⊥ †

Inorganic Chemistry Department, Taras Shevchenko National University of Kyiv, Volodimirska Street 64, Kyiv 01033, Ukraine Institute of Organic Chemistry, Murmanska Street 5, Kyiv 02660, Ukraine § Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland ∥ Physikalisches Institut, Universität Freiburg, Hermann-Herder-Strasse 3, D-79104 Freiburg, Germany ⊥ Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland ‡

S Supporting Information *

ABSTRACT: A pentanuclear CuII5-hydroxo cluster possessing an unusual linear-shaped configuration was formed and crystallized under hydrothermal conditions as a result of the unique cooperation of bridging 1,2,4-triazole ligand (trans-1,4cyclohexanediyl-4,4′-bi(1,2,4-triazole) (tr2cy)), MoVI-oxide, and CuSO4. This structural motif can be rationalized by assuming in situ generation of {Cu2Mo6O22}4− anions, which represent heteroleptic derivatives of γ-type [Mo8O26]4− further interlinked by [Cu3(OH)2]4+ cations through [N−N] bridges. The f r a m e w o r k st r u c t u r e o f t h e re s u l t i n g c o m p o u n d [Cu5(OH)2(tr2cy)2Mo6O22]·6H2O (1) is thus built up from neutral heterometallic {Cu5(OH)2Mo6O22}n layers pillared with tetradentate tr2cy. Quantum-chemical calculations demonstrate that the exclusive site of the parent γ-[Mo8O26]4− cluster into which CuII inserts corresponds with the site that has the lowest defect (“MoO2 vacancy”) formation energy, demonstrating how the local metal-polyoxomolybdate chemistry can express itself in the final crystal structure. Magnetic susceptibility measurements of 1 show strong antiferromagnetic coupling within the Cu5 chain with exchange parameters J1 = −500(40) K (−348(28) cm−1), J2 = −350(10) K (−243(7) cm−1) and g = 2.32(2), χ2 = 6.5 × 10−4. Periodic quantum-chemical calculations reproduce the antiferromagnetic character of 1 and connect it with an effective ligand-mediated spin coupling mechanism that comes about from the favorable structural arrangement between the Cu centers and the OH−, O2−, and tr2cy bridging ligands.

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INTRODUCTION The engineering of metal−organic frameworks (MOFs) employing secondary building units (SBUs) represents one of the most fundamental methods in developing and understanding the structures of coordination solids, an issue which is tightly linked with their applications in materials chemistry.1 MOFs comprising 1,2,4-triazolyl ligands (tr) and divalent transition-metal cations belong to a remarkable class of coordination polymers in which the formation of diverse solid-state architectures can be rationalized through the SBU approach.2 The 1,2,4-triazole derivatives in their neutral form provide short [N−N]-bridging and/or N-terminal binding modes suitable for sustaining various coordination environments in a well-defined cluster geometry, as discrete linear diand trinuclear motifs or polymeric chains in particular. In the hierarchy, the polymeric 1D motif is a result of self-assembly of © XXXX American Chemical Society

repeating low-nuclearity fragments (“bottom-up” construction, Scheme 1). Among the 1,2,4-triazolyl-based transition-metal clusters, the formation of nuclearities higher than three, arranged in the most popular linear configuration, is unusual and still remains terra incognita. Its development can be highly appealing and is yet a challenging subject for applications in molecular magnetism.3 In view of the outlined context, intriguing perspectives can be envisaged when dealing with CuII tr-based coordination polymers incorporating anionic polyoxomolybdates.4 First, the MoVI-oxide matrix usually exhibits a multifunctional chargecompensating, space-filling, and rich coordination behavior.4,5 While polyoxomolybdates form numerous transition-metalReceived: March 8, 2018

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DOI: 10.1021/acs.inorgchem.8b00616 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. Top: Hypothetical Structural Hierarchy of Triazolyl-Based Coordination Clusters Demonstrating the Assembly of Simple Discrete di-{M2}, tri-{M3} Clusters (lowest-nuclearity structures) and Polymeric 1D Chain {Mn} Motifs (highestnuclearity structure), while the Related Medium-Nuclearity Clusters such as {M4}, {M5}, {M6}, etc. Have Not Been Reported so Far. Bottom: Generation of Linear Clusters Can Be Rationally Achieved through the Polyoxometalate Approacha

a The transition-metal-incorporated molybdenum(VI)-oxide anion interacts with the cationic {M3(tr)4} unit through [N−N]-tr links leading to the {M5} cluster core with a unique linear architecture.

with bromoform, and then dichloromethane was slowly added on the top, leading to a middle mixed-layer solution having a density gradient from highest, 2.89 g/cm3, in CHBr3, to lowest, 1.33 g/cm3, in CH2Cl2. The test tube was left undisturbed for a few minutes until three phases stopped moving through the solvent layers (see Figure S2 in the Supporting Information, SI). Then the bottom of the test tube was frozen under liquid nitrogen to solidify the part containing just compound 1. The top liquid layer with [Mo2O6(tr2cy)] and [Cu(tr2cy)SO4]·7H2O was carefully decanted. The test tube with compound 1 was allowed to warm up to room temperature. The separation procedure was repeated several times to receive a pure fraction of 1 in CHBr3/CH2Cl2. The green crystals were filtered, washed with methanol, and dried. Yield: 0.9 mg (10%). For further analytical data and magnetic measurements, a larger amount of 1 was obtained after repeating the hydrothermal reaction about a hundred times. The cumulative amount of the products was collected together and then purified as mentioned above. Anal. Calcd for C20H42Cu5Mo6N12O30: C, 13.17; H, 2.32; N, 9.22. Found: C, 13.22; H, 2.25; N, 9.28. IR (KBr discs, selected bands, cm−1): 483s, 548s, 642s, 671s, 761s, 808s, 891vs, 932s, 995m, 1035w, 1071m, 1092m, 1204m, 1241w, 1306w, 1354w, 1426w, 1452w, 1503w, 1544m, 1558m, 1639w, 2869m, 2904m, 2955m, 3026m, 3117m, 3406s. The utilized hydrothermal conditions, reactant sources, and reactant concentrations were crucial for the crystallization of the title compound as for high-quality single crystals. Namely, under refluxing or mild hydrothermal conditions (below 140 °C) and 50−200 mM total concentration of CuII, tr2cy, and MoVI, the reaction yielded a mixture of only [Cu(tr2cy)SO4]·7H2O and [Mo2O6(tr2cy)]. Measurements. IR-spectra (400−4000 cm−1) were measured with a PerkinElmer FTIR spectrometer (KBr pellets). The temperaturedependent X-ray measurements were recorded on a Stoe STADIP with a high temperature attachment and an image plate detector system. Elemental analysis was carried out with a Vario EL-Heraeus microanalyzer. Magnetic susceptibility data were recorded using a Quantum design MPMS-5XL SQUID magnetometer in the temperature range of 300−1.9 K and at a field of 2 kG. The magnetic data were corrected for sample holder, diamagnetic contributions, and TIP contribution for CuII.

substituted motifs demonstrating polymeric structures ({CuMo2O7},6a {CuMo3O10},6b {CuMo4O13}6a,b), this tendency is not observed for discrete heteroleptic polyanions ([Cu2Mo6O22]4−, β-[CuMo7O24(H2O)2]4−).7,8 Second, the 1,2,4-triazolyl derivatives represent an exceptional class of ligands which can readily support such MoVI-oxide motifs in a predictable fashion while using a set of coordinating bridges: CuII−[N−N]−CuII,2 CuII−[N−N]−MoVI,6b,9 and MoVI−[N−N]−MoVI.10 This observation suggests that the interaction between low-nuclear tr-coordinated intermediates and metal-substituted polyoxomolybdates might proceed in a rational way, affording a novel type of clusters or SBUs of higher nuclearity. In this report, we present a novel approach toward mixedmetal CuII−MoVI coordination solids supported through bitopic trans-1,4-cyclohexanediyl-4,4′-bi(1,2,4-triazole) (tr2cy). The resulting new polyoxometalate-organic framework has been fully characterized via structural and magnetic studies. The structural and electronic features of the compound have been rationalized based on f irst-principles calculations.

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EXPERIMENTAL SECTION

All chemicals were of reagent grade and were used as received without further purification. The tr2cy ligand was prepared as described earlier.10a [Cu5(OH)2(tr2cy)2Mo6O22]·6H2O (1). A mixture of CuSO4·5H2O (7.5 mg, 0.030 mmol), tr2cy (6.5 mg, 0.030 mmol), and MoO3 (4.3 mg, 0.030 mmol) (1:1:1 molar ratio) in 5 mL of H2O was placed in a 10 mL heavy-walled Pyrex tube. The tube was sealed, placed inside an oven, and heated at 160 °C for 24 h with further cooling to room temperature in 48 h. After reaction a polycrystalline mixture of [Cu5(OH)2(tr2cy)2Mo6O22]·6H2O (green), [Mo2O6(tr2cy)]10a (colorless), and [Cu(tr2cy)SO4]·7H2O (light-blue) was obtained. The desired product was then purified as follows. The precipitate was sonicated in water using an ultrasonic bath, then filtered, washed with water and methanol, and dried. The sample was placed in a test tube B


Article

Inorganic Chemistry

Figure 1. Formation of the pentanuclear CuII5(OH)2 cluster core can be considered as a unique combination of heteroleptic {Cu2Mo6O22}4− anions and [Cu3(OH)2]4+ cations which are linked through [N−N] bridges.

solution.13 As evidenced from the thermal PXRD analysis and IR spectra of its calcined samples, compound 1 demonstrates sufficient thermal stability up to ∼230−250 °C while proceeding without formation of crystalline intermediate phases (Figures S4−S5). These data are in good agreement with those for related CuII−MoVI coordination solids.8,9 Compound 1 crystallizes in the triclinic centrosymmetric s p a c e g r o u p P 1̅ . T h e f r a m e w o r k s t r u c t u r e o f [Cu5(OH)2(tr2cy)2Mo6O22]·6H2O is built up from neutral heterometallic {Cu5(OH)2Mo6O22}n layered motifs pillared in the third direction with tr2cy linkers (Figure 1). The inorganic mixed-metal layer consists of centrosymmetric and linear-shaped cations {Cu5(μ2-OH)2} which are squeezed through two terminal Cu centers into anionic {Mo6O22}8− clusters. Each of the fragments provides double binding pockets as well as bridging modes toward terminal and penultimate Cu atoms of the {Cu5(μ2-OH)2} SBU. More likely, these interactions can be interpreted as coordination of heteroleptic {Cu2Mo6O22}4− anions toward linear [Cu3(OH)2]4+ cations. The occurrence of the former can be regarded as a result of the hydrothermal-induced substitution of two opposite octahedral MoO6 centers in the γ-type octamolybdate14 by two CuII cations in the square-pyramidal arrangement {O3N + O} (Figure 1, vide inf ra).6,7 Four [N−N]-triazole groups fit

X-ray Crystallography. The diffraction data were measured on a Bruker APEXII diffractometer using a CCD area-detector (λ = 0.71073 Å, ω scans). The data were corrected for Lorentz-polarization effects and for the effects of absorption (multiscans method). The structure was solved by direct methods and refined by full-matrix least-squares on F2 using the SHELX-97 and SHELXL-2014/7.11 Graphical visualization of the structures was made using the program Diamond 2.1e.12 A full description of the crystallographic studies is summarized in Table S1. Selected bond lengths and angles are given in Table S2. The crystallographic material can also be obtained from the Cambridge Crystallographic Data Centre through deposition number CCDC 1827952.

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RESULTS AND DISCUSSION Synthesis and Crystal Structure. The coordination polymer [Cu5(OH)2(tr2cy)2Mo6O22]·6H2O (1) was prepared in 10% yield by the hydrothermal reaction of CuSO4·5H2O, MoO3, and tr2cy, taken in a ratio of 1:1:1, in a sealed Pyrex tube at 160 °C. Prismatic deep-green crystals of the product were produced in a mixture with two other solids, blue [Cu(tr2cy)SO4]·7H2O (2) and colorless [Mo2O6(tr2cy)] (3)10a (see SI). Due to significantly large differences in densities of these phases (1−3), compound 1 was easily separated from the reaction mixture employing the flotation method in a CHBr3/CH2Cl2 C


Article

Inorganic Chemistry perfectly between CuII−CuII pairs cementing the heterometallic {Cu5(OH)2Mo6O22} ensemble. Moreover, the expected tetradentate coordination behavior and chair conformation of the organic ligand (tr-groups are aligned parallel to the mean equatorial plane,10 as the most stable form for trans-1,4cyclohexane derivatives) allows metal−metal separation across the tr2cy module to reach ca. 13.0 Å. Magnetic Properties. Compound 1 was investigated by magnetic susceptibility and magnetization measurements. Overall, the χT(T) data of a polycrystalline sample of 1 are consistent with an antiferromagnetic system (Figure 2). The

Figure 3. Visualization of the individual planes of the magnetic orbitals for 1.

Figure 2. Thermal variation of χT for 1. The solid line is the result of the grid calculations with optimal parameters compared to the experimental data (square markers).

values of χT(T) decrease from 1.3 cm3 K mol−1 at room temperature to a plateau of 0.5 cm3 K mol−1 at low temperatures, consistent with an S = 1/2 ground state (g = 2.3). Due to a strong coupling within the Cu5 chain (Figure 3), the Curie regime is not accessible with temperatures up to room temperature, restricting the measurements only the lowtemperature part of the χT(T) curve. The magnetic data were modeled based on the standard Heisenberg Hamiltonian for the centrosymmetric {Cu5} chain: H = −J1(S2S3 + S3S4) − J2 (S1S2 + S4S5)

Figure 4. Results of the grid calculations: Calculated χ2 for a given parameter pair (J1, J2) using the g-value obtained from least-squares fits. The magenta value denotes the best fit and the dark blue area the estimated uncertainty of parameters J1 and J2.

Here, J1 describes the coupling between Cu1 and Cu2, and J2 the coupling between Cu2 and Cu3. In order to determine the couplings J1 and J2 from the magnetic susceptibility data, we performed grid calculations. The theoretical magnetic susceptibility was computed using inhouse software on a three-dimensional grid of 10 K steps in the parameters J1 and J2 (both varied from −1000 to 0 K) and a step of 0.01 in the parameter g (varied from 2.2 to 2.7). The calculated values were compared to a reduced number of χT(T) data points (N = 33, from Tmin = 10 K to Tmax = 300 K) for determining the χ2 values. For each pair (J1, J2) the optimal g was found by least-squares minimization. The result of the grid calculation is shown in Figure 4. We observe a clear, unique minimum of χ2. The optimum g values for the grid calculation are shown in Figure 5 to be smooth functions of J1 and J2. The uncertainty of the parameters J1, J2, and g was then estimated by a boundary of constant χ2, with χ2 = 4 (χ2)min.15 Hence the

uncertainty of the parameters corresponds to an average doubling of the difference between experimental and simulated values on each temperature data point. The criterion chosen seems to be thus reasonable. The best agreement to the experimental data is obtained for the following set of parameters J1 = −500(40) K (−348(28) cm−1), J2 = −350(10) K (−243(7) cm−1), g = 2.32(2), χ2 = 6.5 × 10−4. Comparison of the best-fit theoretical susceptibility curve χT(T) to the experimental data is shown in Figure 2. In addition, another grid calculation was performed where the χT(T) data set was reduced to data points above 50 K (N = 26, from Tmin = 50 K to Tmax = 300 K) to check the robustness of D


Article

Inorganic Chemistry

1, density functional theory (DFT) calculations were performed for both periodic and molecular cluster models. See SI for computational details and additional computed data. The [Cu5(OH)2(tr2cy)2Mo6O22]·6H2O crystal structure’s atomic coordinates were first optimized within periodic DFT. Due to an uncertainty about which DFT functional could be expected to outperform others, several types of DFT methods were used, specifically the PBE+D3, PW91, SCAN+rVV10, and TPSS functionals. Different sets of calculations were set up such that they sampled different types of Cu−Cu magnetic coupling along the Cu5 chains. The corresponding energy results are shown in Figure 7, wherein each structure is sorted according

Figure 5. Results of the grid calculations: Best-fit g-values for each parameter pair (J1, J2).

the previous solution. The optimal values for J1 and J2 and g for this calculation were well within one standard deviation of the previous one. Since the fit results do not significantly deviate from the choice of the data points, the fit is indeed robust. Accordingly, we conclude that there is a unique solution for the parameters for the Cu5 molybdate chain system of compound 1 and that the exchange parameters could be reliably determined. The least-squares fit to the low-temperature magnetization M(B) at T = 1.9 K of 1 was performed using in-house software, assuming a ground state of S = 1/2. The best fit is shown in Figure 6. The extracted value g = 2.41(1) is reasonable for a CuII-based system.

Figure 7. Computed relative electronic energies of 1 as the type of Cu−Cu magnetic coupling is varied from ferromagnetic (FM) to antiferromagnetic (AFM). The FM state has zero AFM Cu−Cu contacts (marked as “0” on the x-axis of the plot) and the AFM state has four AFM Cu−Cu contacts. Results from the PBE+D3 (black), PW91 (red), TPSS (green), and SCAN+rVV10 (blue) calculations are shown, and the energies are given relative to the AFM state. A calculated estimate of an “average” Cu−Cu coupling constant, Jcalc, is also shown for the PW91 and SCAN+rVV10 functionals.

to the number of antiferromagnetic (AFM) nearest-neighbor Cu−Cu interactions that it has. For example, the state that has only ferromagnetic (FM) Cu−Cu interactions is shown to the far left of Figure 7 (since such a structure has zero AFM Cu− Cu contacts), and the state that has only AFM Cu−Cu interactions appears on the far right (since it has four AFM Cu−Cu contacts). All of the DFT functionals were shown to favor the AFM state over the FM state by −0.1 to −0.3 eV per simulation cell. Furthermore, the FM state was found to be unstable with the PBE+D3 and PW91 functionals when the number of unpaired electrons was allowed to freely optimize. This is all consistent with the experimental characterization of a more stable antiferromagnetic magnetic coupling along the Cu chain. Furthermore, the difference in energy between the AFM and FM states can be used to compute an estimate of an “average” Cu−Cu coupling constant, assuming for simplicity the same J for all of the Cu−Cu contacts and neglecting the magnetic interaction between the Cu5 chains, Jcalc = (E(AFM) − E(FM))*(1/4)*(1/2). The corresponding values fall in the range between −100 cm−1 and −300 cm−1. The estimated Jcalc is reasonably close to the couplings J1 (−348 cm−1) and J2 (−243 cm−1) determined based on the experimental data, further demonstrating an overall good agreement between experiment and the computational models. The relationship between the electronic energy and the nature of the differences between the J1 and J2 coupling was found to be quite sensitive to the level of theory used (note, for example, that the AFM state was not found to be the lowest in

Figure 6. Best-fit to the low-temperature magnetization data for 1.

The obtained magnetic coupling parameters can be compared with literature values, for instance with those of a Cu3-oximato metallacrown cluster with a J value of −430 cm−1.16 In the case of another pentameric CuII cluster, a J value of −250 cm−1 was determined.17 Quantum-Chemical Calculations. To provide a deeper insight into the structural and electronic features of compound E


Article

Inorganic Chemistry energy with the PBE+D3 functional in Figure 7). This suggests that the pure AFM state may not be “locked” in the sense that it may be isoenergetic with electronic states that have majority AFM Cu−Cu coupling. But the distribution of the unpaired electron spin density, Δρ = ρα − ρβ, is consistently described by all four DFT functionals, and it is shown, for the AFM state, in Figure 8. The sign of the

Figure 9. Crystal structure14 of γ-[Mo8O26]4− containing four symmetrically equivalent MoVI sites (top left) along with the corresponding {Cu2Mo6O22}4− units generated via substitution of Mo3- and Mo4-type sites (right). Numbers listed are relative energy values, ΔE in kcal/mol, calculated with the B3LYP and PW91 functionals for a set of {Cu2Mo6O22}4−-(N,Ni) clusters in their spintriplet electronic configuration. Figure 8. Visualization of the unpaired electron spin density, Δρ = ρα − ρβ (±0.003 au, yellow/blue denotes excess of α-spin/β-spin) in a computed broken-symmetry doublet configuration of a [Cu5(OH)2(tr2cy)2Mo6O22]·6H2O model with antiferromagnetic Cu−Cu coupling along the Cu5 chain. The regions around the Cu1−Cu2 (J1) and Cu2−Cu3 (J2) contacts are enlarged at the bottom.

block.7 This implies that there are some energetic effects that direct the CuII ions to Mo4-type sites in γ-[Mo8O26]4−. Accordingly, DFT calculations were performed for a set of molecular clusters generated from the γ-[Mo8O26]4− crystal structure14 in which four different MoVIN/MoVINi sites were substituted by CuII in a symmetric fashion (see Figure 9). As unconstrained geometry optimizations of the resulting {Cu2Mo6O22}4−-(N,Ni) units resulted in structures that strongly deviated from the initial γ-arrangement, energies of non-optimized clusters were analyzed. The corresponding results are presented in Figure 9 and Table S4. They demonstrate that CuII indeed prefers to be placed at the Mo4-type site; the computed electronic energy of such a cluster is ca. 7.5−9.5 kcal/mol (depending on the functional) lower than that obtained for the second most favorable substitution site, i.e. Mo3-type. The substitution at the remaining octahedral Mo2/Mo2i and square-pyramidal Mo1/Mo1i sites leads to much higher energies, >25 kcal/mol. See also SI for data obtained for selected asymmetric clusters (Figure S6 and Table S4). Such a large energetic preference is consistent with the 100% exclusivity of Mo4 substitution in the crystal structure. To shed some light on the demonstrated preference of the Mo4-type site substitution, additional energy calculations were performed for singly substituted (MoO2 → X = CuII, MgII, Ar, vacancy) γ-[Mo8O26]4− based clusters. It was found, as shown in Table S5 in the SI, that the general energetic preferences for substituting CuII at different sites in γ-[Mo8O26]4− are readily reproduced with MgII and Ar or by simply removing the MoO2 subunit to create the vacancy. This implies that the exclusive site of the parent γ-[Mo8O26]4− cluster into which CuII inserts corresponds with the site that has the lowest defect (“MoO2 vacancy”) formation energy.

spin density around, for example, the OH− is consistent with its two occupied nonbonding molecular orbitals interacting with the unoccupied 3d orbitals of the two Cu atoms on either side (i.e., an α-spin/β-spin density ligand → metal donation resulting in excess of β-spin/α-spin density on the ligand). Similar features, and therefore similar coupling interactions, are seen at the Cu−[N−N]−Cu and Cu−O2−−Cu bridges. The computed net spin density populations on the Cu centers are found to be smaller in the AFM configuration than in the FM configuration (by ∼0.09 to 0.16 e per Cu center), reflecting a stronger ligand → metal charge transfer in the AFM configuration. It can further be seen that the spin density at each contact (Cu1−Cu2 and Cu2−Cu3) is similar, but weaker and less symmetric at the Cu2−O2−−Cu3 contact than at the Cu1−OH−−Cu2 contact. This is consistent with the coupling J2 being less than that of J1. Summarizing, all these results indicate that the strong antiferromagnetic coupling observed in 1 appears due to the effective ligand-mediated spin coupling mechanism that may be associated with a favorable structural arrangement between the metallic centers and the bridging ligands (compare also with Figure 3).18 As mentioned above, the {Cu2Mo6O22}4− polyoxometalate units in 1 can be described as derivatives of γ-[Mo8O26]4− in which two symmetrically equivalent octahedral MoO6 fragments, labeled as Mo4 and Mo4i in Figure 9, are replaced by the distorted square pyramids of CuO4N. Interestingly, the same type of Mo → Cu substitution was also observed in a related compound that has the same {Cu2Mo6O22}4− building F


Inorganic Chemistry

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CONCLUSIONS We prepared a novel CuII−MoVI-oxide hybrid with an unusual Cu5-hydroxo cluster core stabilized by the bridging bitopic 1,2,4-triazole ligand tr2cy. The design principle is based on the ability of two copper(II) ions to be introduced into the molybdenum(VI)-oxide matrix under hydrothermal conditions to form heteroleptic {Cu2Mo6O22}4− anions. This fragment interacts with linear [Cu3(OH)2]4+ cations affording a rigid pentanuclear cluster which is integrated into the 3D framework through the tetradentate tr2cy linkers. Magnetic susceptibility measurements of the resulting compound [Cu5(OH)2(tr2cy)2Mo6O22]·6H2O 1 show strong antiferromagnetic coupling within the Cu5 chain with exchange parameters J1 = −500(40) K (−348(28) cm−1), J2 = −350(10) K (−243(7) cm−1) and g = 2.32(2), χ2 = 6.5 × 10−4. DFT calculations reveal that the strong antiferromagnetic character of 1 stems from a favorable structural arrangement between the metallic centers and the bridging ligands that promotes effective ligand-mediated spin coupling. Calculations further demonstrate that the site which CuII occupies in the synthesized {Cu2Mo6O22}4− cluster is driven by where the creation of an “MoO2 vacancy” is computed to be the most energetically stable. Collectively, the crystal’s rational structural and electronic properties demonstrate that the polyoxomolybdate synthetic approach utilized here can provide a new possibility for designing unprecedented cluster motifs and metal−organic frameworks with unique properties.

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REFERENCES

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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.8b00616. X-ray crystallography data, FT-IR and PXRD studies, computational details, and additional computed data (PDF) Accession Codes

CCDC 1827952 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 data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail address: [email protected] (Lysenko A. B.). *E-mail address: [email protected] (Srebro-Hooper M.). *E-mail address: [email protected] (Liu S.-X.). ORCID

Andrey B. Lysenko: 0000-0002-0342-5122 Monika Srebro-Hooper: 0000-0003-4211-325X Karl W. Krämer: 0000-0001-5524-7703 Shi-Xia Liu: 0000-0001-6104-4320 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.H. acknowledges the “Outstanding Young Scientist” scholarship from the Ministry of Science and Higher Education in Poland. G


Article

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