Three Polymorphic Forms of a Monomeric Mo(VI) Complex: Building

Article pubs.acs.org/crystal

Three Polymorphic Forms of a Monomeric Mo(VI) Complex: Building Blocks for Two Metal−Organic Supramolecular Isomers. Intermolecular Interactions and Ligand Substituent Effects Višnja Vrdoljak,*,† Biserka Prugovečki,† Dubravka Matković-Č alogović,† Tomica Hrenar,† Renata Dreos,‡ and Patrizia Siega‡ †

Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102A, 10000 Zagreb, Croatia Department of Chemical and Pharmaceutical Sciences, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy

‡

S Supporting Information *

ABSTRACT: Three polymorphic forms of the molybdenum(VI) complex [MoO2(L)(EtOH)] (1α, 1β, and 1γ) (L2− = 4methoxy-2-oxybenzaldehyde isonicotinylhydrazonate) were synthesized by the reaction of H2L with the dioxobis(acetylacetonato)molybdenum(VI) complex, [MoO2(acac)2], in ethanol. Removal of the coordinated ethanol molecule upon grinding or heating led to the solid-state transformation of the polymorphs 1α, 1β, or 1γ into the coordination polymer [MoO 2 (L)] n (2a). The square inclusion complex [MoO2(L)]4⊃CH2Cl2·4CH2Cl2 (2b⊃CH2Cl2·4CH2Cl2) was obtained by a self-assembly reaction in dichloromethane. Standard Gibbs energies of binding for molybdenum(VI) compounds [MoO2(L)(D)] with the sixth coordination site occupied by a nitrogen or an oxygen donor D were estimated using quantum chemical calculations. Crystal and molecular structures of the molybdenum(VI) compounds [MoO2(L)(EtOH)] (1α, 1β, and 1γ), [MoO2(L)]4⊃CH2Cl2·4CH2Cl2 (2b⊃CH2Cl2·4CH2Cl2), [MoO2(L)(γ-pic)]·γ-pic (3·γ-pic), [MoO2(L)(py)] (4), and [MoO2(L)(DMSO)] (5) were determined by the single crystal X-ray diffraction method. The compounds were further characterized by chemical analysis, thermogravimetric, and differential scanning calorimetry measurements, IR, UV−vis, one- and two-dimensional NMR spectroscopies, and the powder X-ray diffraction method.

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INTRODUCTION Polymorphism has been receiving an ongoing interest over decades. Different packing arrangements or conformations can lead to significant changes of the physical and chemical properties of polymorphs, which makes them attractive in various fields.1 Although polymorphism has been extensively investigated in the organic field, particularly focused on drug design, growing attention has been devoted to the polymorphism of transition metal complexes,2 coordination polymers,3 and organometallic compounds. 4 Nevertheless, polymorphs based on dioxomolybdenum(VI) complexes are still scarce. To the best of our knowledge, only the structures of the conformational trimorph [MoO2(C12H8N2O2)(CH3OH)]5 and of the dimorph [MoO2(C9H9NO2)(C9H11NO2)],6 containing the {MoO2}2+ unit and the N-salicylidene-2-amino-3-hydroxypyridine or Nsalicylidene-2-aminoethanol ligand, respectively, were reported so far. At the same time, metal−organic frameworks (MOFs) are attracting much attention not only in connection with great structural diversity7 but also because of their potential applications in catalysis,8 optics,9 magnetism,10 gas adsorption,11 and so on. Recently, we have published the first © XXXX American Chemical Society

examples of supramolecular architectures [MoO2(L)]x (where x = 4, 6, or n) containing the cis-{MoO2}2+ core with the linker group being the izonicotinoyl part of the aroylhydrazone ligand L2−. They consist of discrete cyclic assemblies, molecular square (x = 4),12 or interwoven hexagon (x = 6)13 or of infinite onedimensional zigzag chains (x = n).12,13 Our previous studies were focused on possible supramolecular isomerism. The great tendency for polymerization through the MoOt···Mo interaction14 is the reason why this kind of dioxomolybdenum(VI) supramolecular compound is very rare. The appearance of polymorphs as well as of MOFs greatly depends on the interplay between kinetic and thermodynamic factors.1,15 Suitable conditions, for example, solution concentration, temperature of crystallization, rate of evaporation, choice of solvents, or pressure are of crucial importance for their synthesis.16 It is considerably more difficult to assemble MOFs in the solid state. In general, reactions in solids are accompanied by limited movements of building blocks17 and Received: May 20, 2013 Revised: June 29, 2013

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dx.doi.org/10.1021/cg400782c | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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loop and put on the goniometer head into nitrogen vapor at 150 K. The data reduction was performed using the CrysAlis software package.20 Solution, refinement, and analysis of the structures were done using the programs integrated in the WinGX system.21 The structures were solved using SHELXS by the Patterson method, and the refinement procedure was performed by the full-matrix leastsquares method based on F2 against all reflections using SHELXL.22 The non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in calculated positions and refined using the riding model. Exceptions were hydrogen atoms from the hydroxyl groups of the coordinated ethanol molecule in 1α, 1β, and 1γ, which were placed as found in the difference Fourier map and refined, and the hydrogen atoms on the CH2Cl2 molecule near the 2-fold axis in the center of the tetramer 2b⊃CH2Cl2·4CH2Cl2 that could not be placed and are not included in the model. All three crystallographically independent molecules of CH2Cl2 in 2b⊃CH2Cl2·4CH2Cl2 were found to be disordered, and two positions were modeled for two of them, while the third CH2Cl2 molecule is disordered at two positions generated by the 2-fold axis. The geometrical calculations were done using PLATON.23 The structure drawings were prepared using PLATON and MERCURY24 programs. The crystallographic data are summarized in Table 1, whereas the selected bond distances and angles are listed in Table S1 (in the Supporting Information). X-ray Crystallography. Powder Diffraction. The powder X-ray diffraction data were collected by the Panalytical X′Change powder diffractometer in the Bragg−Brentano geometry using Cu Kα radiation. The sample was contained on a Si sample holder. Patterns were collected in the range of 2θ = 5 − 50° with a step size of 0.03° and at 1.5 s per step. The data were collected and visualized using the X’Pert programs Suite.25 Calculations. Quantum chemical calculations were performed using the Gaussian 09 program package.26 Geometry optimizations for ground states were performed at the B3LYP/Def2-TZVP levels of the theory.27 For all optimized structures, harmonic frequencies were calculated to ensure that obtained geometries correspond to a local minimum on the potential energy surface. Solvation effects were incorporated in the calculations using the reformulation of polarizable continuum model (PCM)28 known as integral equation formalism within the SMD solvation model of Truhlar and co-workers.29 The NMR shieldings in vacuo and in solvent were calculated using GIAO and PCM GIAO30 methods on the previously optimized geometries. Differences in standard Gibbs energies of binding ΔΔrG°binding were calculated at T = 298.15 K and p = 1 atm by subtraction of Gibbs energies for different complexes from the Gibbs energy of cis{MoO2}2+ complex with ethanol. Principal component analysis on the covariance matrix of time dependent NMR spectra was carried out using our own FORTRAN code31 based on the NIPALS algorithm.32 Preparative Part. The starting complex [MoO2(acac)2]33 and the ligand H2L (anhydrous form)34 were prepared as described in the literature. Isonicotinyl hydrazine, 2-hydroxy-4-methoxybenzaldehyde, glacial acetic acid, acetic anhydride, dichloromethane, dimethyl sulfoxide, pyridine, and γ-picoline were of reagent grade and used as purchased. Ethanol was dried using magnesium turnings and iodine and then distilled. All compounds were characterized by elemental analyses, thermal, IR, UV−vis, and NMR spectroscopies (data are given in the Supporting Information). Synthesis of 1α. The complex [MoO2(acac)2] (0.1 g, 0.30 mmol) was added under vigorous stirring at 35 °C to an ethanolic solution of H2L (0.09 g, 0.30 mmol in 20 mL) containing glacial acetic acid (200 μL). After 2 h of continuous stirring, the obtained red crystalline product 1α was isolated by filtration and washed with cold ethanol. The filtrate was slowly evaporated at room temperature. After a few days, dark red crystals suitable for X-ray structure analysis were obtained. Yield: 0.1 g; 75%. Synthesis of 1β and 1γ. Anhydrous form of H2L (0.09 g, 0.30 mmol) was dissolved in dry ethanol (20 mL) containing acetic anhydride (200 μL), and the solution was left at room temperature for 3 h. The solution was cooled down to 10 °C. The complex [MoO2(acac)2] (0.1 g, 0.30 mmol) was added to the solution without stirring or shaking. The reaction led to a mixture of forms 1β (red

therefore require suitably arranged reactants with reacting centers close to each other.18 Herein, we present synthetic, crystallographic, computational, and spectroscopic studies of three dioxomolybdenum(VI) polymorphs [MoO2(L)(EtOH)] (1α, 1β, and 1γ), two metal−organic supramolecular isomers: coordination polymer [MoO2(L)]n (2a) and molecular square inclusion complex [MoO2(L)]4⊃CH2Cl2·4CH2Cl2 (2b⊃CH2Cl2·4CH2Cl2) and three mononuclear complexes [MoO2(L)(γ-pic)]·γ-pic (3·γpic), [MoO2(L)(py)] (4), and [MoO2(L)(DMSO)] (5) (L2− = 4-methoxy-2-oxybenzaldehyde isonicotinylhydrazoate, Scheme S1, Supporting Information). In order to investigate whether the mononuclear complex [MoO2(L)(D)], with a nitrogen (γ-pic, py, or H2L) or an oxygen donor ligand D (EtOH, DMSO) could be formed preferentially, standard Gibbs energies of binding were calculated. The influence of the methoxy substituent position was also studied. Intermolecular interactions have also been investigated not only in the context of different crystal packing19 but also considering their role in the supramolecular architecture formation. Structural transformations in the solid state of different complexes triggered by mechanochemical or thermal reactions are reported. Structural unit {MoO2(L)} present in dioxomolibdenum(VI) compounds is shown in Figure 1. The sixth coordination place is available for coordination of ligand D.

Figure 1. The structural unit {MoO 2 (L)} present in dioxomolibdenum(VI) compounds.

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EXPERIMENTAL SECTION

Elemental analyses were provided by the Analytical Services Laboratory of the Ruđer Bošković Institute, Zagreb. Thermogravimetric (TG) analysis was carried out with a Mettler TG 50 thermobalance using aluminum crucibles. All experiments were recorded in a dynamic atmosphere with a flow rate of 200 cm3 min−1. Heating rates of 5 K min−1 were used for all investigations. Differential scanning calorimetry (DSC) measurements were carried out with a Mettler-Toledo DSC823e calorimeter and analyzed by the Mettler STARe 9.01. software. Fourier transform infrared spectra (FTIR) were recorded in KBr pellets with a Perkin-Elmer 502 spectrophotometer. Spectra were recorded in the spectral range between 4500−450 cm−1. UV−vis spectra were recorded at 25 °C on a Cary 100 UV/vis spectrophotometer. The spectra were recorded in ethanol. Solid-state UV−vis spectra were recorded with a Shimadzu UV−vis-NIR spectrophotometer (model UV-3600) using the integrated sphere. For the solid-state optical spectra the samples were spread on BaSO4 powder. 1H NMR, COSY, and ROESY spectra were recorded with Jeol EX-400 (1H at 400 MHz) and Varian INOVA 500 (1H NMR at 499 MHz) instruments with the solvent as internal reference. X-ray Crystallography. Single Crystal Diffraction. The singlecrystal X-ray diffraction data of 1α, 1β, 1γ, 2b⊃CH2Cl2·4CH2Cl2, 3·γpic, 4, and 5 were collected by ω-scans on an Oxford Diffraction Xcalibur 3 CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Crystals of 2b⊃CH2Cl2·4CH2Cl2 were transferred from the mother liquor to paratone-N, a highly viscous, inert cryoprotectant. After a few seconds, it was scooped with a cryoB



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Table 1. Crystallographic Data for Compounds [MoO2(L)(EtOH)] (1α), [MoO2(L)(EtOH)] (1β), [MoO2(L)(EtOH)] (1γ), [MoO2(L)]4⊃CH2Cl2·4CH2Cl2 (2b⊃CH2Cl2·4CH2Cl2), [MoO2(L)(γ-pic)]·γ-pic (3·γ-pic), [MoO2(L)(py)] (4), [MoO2(L)(DMSO)] (5) chemical formula Mr crystal color, habit crystal size (mm3) crystal system space group unit cell parameters a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Dcalc (g cm−3) temperature (K) wavelength (Å) μ (mm−1) F(000) number of unique data number of data [Fo ≥ 4σ(Fo)] number of parameters R1a, [Fo ≥ 4σ(Fo)] wR2b goodness of fit on F2, Sc min and max electron density (e Å−3) chemical formula Mr crystal color, habit crystal size (mm3) crystal system space group unit cell parameters a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Dcalc (g cm−3) temperature (K) wavelength (Å) μ (mm−1) F(000) number of unique data number of data [Fo ≥ 4σ(Fo)] number of parameters R1a, [Fo ≥ 4σ(Fo)] wR2b goodness of fit on F2, Sc min and max electron density (e Å−3)

a

1α

1β

1γ

2b⊃CH2Cl2·4CH2Cl2

C16H17MoN3O6 443.27 dark-red, plate 0.47 × 0.33 × 0.15 monoclinic C2/c

C16H17MoN3O6 443.27 red, plate 0.48 × 0.22 × 0.14 triclinic P1̅

C16H17MoN3O6 443.27 orange, plate 0.34 × 0.21 × 0.06 triclinic P1̅

C56H44Mo4N12O20, 5(CH2Cl2) 1006.71 orange, plate 0.30 × 0.26 × 0.15 monoclinic C2/c

8.2387(13) 14.450(2) 14.9022(18) 87.511(11) 83.424(12) 87.411(13) 1759.3(4) 4 1.674 150(2) 0.71073 0.784 896 7615 5257 477 0.054 0.159 0.99 −1.07, 0.68 4

28.8951(7) 9.6243(2) 27.6282(6) 90 90.607(2) 90 7682.8(3) 4 1.741 150(2) 0.71073 1.062 4008 9249 8134 545 0.034 0.075 1.09 −0.58, 0.84

28.0444(10) 9.7439(3) 13.8917(6) 90 115.716(5) 90 3420.1(2) 8 1.722 150(2) 0.71073 0.807 1792 3694 3281 239 0.023 0.063 1.06 −0.41, 0.88

6.6894(2) 7.9510(3) 16.5450(4) 89.648(2) 88.077(2) 86.684(3) 878.01(5) 2 1.677 150(2) 0.71073 0.786 448 3797 2957 239 0.030 0.051 0.92 −0.31, 0.38 3·γ-pic

5

C20H18MoN4O5, C6H7N 583.45 orange, plate 0.48 × 0.26 × 0.16 monoclinic P21/n

C19H16MoN4O5 476.30 orange, plate 0.47 × 0.28 × 0.10 monoclinic P21/c

C16H17MoN3O6S 475.34 orange-yellow, plate 0.22 × 0.20 × 0.16 triclinic P1̅

12.8761(7) 11.2127(5) 17.9660(8) 90 100.514(5) 90 2550.3(2) 4 1.520 150(2) 0.71073 0.561 1192 5499 4619 334 0.034 0.081 1.08 −0.36, 0.48

12.1954(2) 10.7126(2) 14.7474(2) 90 97.317(2) 90 1910.98(5) 4 1.656 150(2) 0.71073 0.726 960 4156 3710 262 0.027 0.075 1.07 −0.48, 0.34

7.4961(4) 11.6335(6) 11.9380(6) 110.363(5) 102.375(4) 98.315(4) 925.61(8) 2 1.705 150(2) 0.71073 0.860 480 4033 3677 244 0.024 0.057 1.06 −0.41, 0.31

R = Σ||Fo| − |Fc||/Σ|Fo|. bwR = [Σ(Fo2 − Fc2)2/Σw(Fo2)2]1/2. cS = Σ[w(Fo2 − Fc2)2/(Nobs − Nparam)]1/2. C



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crystals) and 1γ (orange crystals). Individual crystals of each polymorph were carefully separated manually and used as seeds. The procedure described above was repeated and seeds of 1β together with [MoO2(acac)2] were added to the reaction mixture. The suspension was left to stand at 10 °C for 24 h. The red crystals of 1β were collected and washed with ethanol. They lose their EtOH molecules on prolonged standing at room temperature. Therefore, after filtration, the crystals of 1β were transferred into a desiccator and then placed in a freezer (at −15 °C). Yield: 0.08 g; 60%. The polymorph 1γ was synthesized in the same way as described for 1β, using seeds of 1γ instead of 1β during the reaction. The orange crystals of 1γ were collected and washed with ethanol. They lose their EtOH molecules on prolonged standing at room temperature. Therefore, after filtration, the crystals of 1γ were transferred into a desiccator and then placed in a refrigerator (at 10 °C). Yield: 0.06 g; 60%. Synthesis of 2a. A mixture of [MoO2(acac)2] (0.1 g, 0.30 mmol) and H2L (0.09 g, 0.30 mmol) in ethanol (20 mL) was heated under reflux for 2 h. Yellow-orange product was obtained. Yield: 0.09 g; 75%. Synthesis of 2b⊃CH2Cl2·4CH2Cl2. A mixture of [MoO2(acac)2] (0.05 g, 0.15 mmol) and H2L (0.04 g, 0.15 mmol) was stirred in dichloromethane (50 mL) for 6 h. The solution was slowly evaporated at room temperature. After a few days, the orange crystals suitable for X-ray structure analysis were obtained. Crystals of 2b⊃CH2Cl2·4CH2Cl2 easily lose dichloromethane molecules at room temperature. They were left up to constant weight and analyzed as 2b·2CH2Cl2. Yield: 0.04 g; 61%. Conversion of 2a or 2b·2CH2Cl2 to 3·γ-pic. Complex 2a (0.1 g, 0.25 mmol) was dissolved in γ-picoline (5 mL). The solution was allowed to evaporate at ambient temperature for 4−5 days. An orange product was filtered and dried. Yield: 0.08 g; 67%. The same product was also obtained by conversion of 2b·2CH2Cl2. Conversion of 2a or 2b·2CH2Cl2 to 4. Complex 2a (0.1 g, 0.25 mmol) was dissolved in pyridine (5 mL). The solution was allowed to evaporate at room temperature for 4−5 days. An orange product was filtered and dried. The orange crystals lose their solvent molecules on prolonged standing at room temperature. Therefore, after filtration, the crystals of 4 were transferred into a desiccator and then placed in a freezer (at −15 °C). Yield: 0.07 g; 48%. The same product was also obtained by conversion of 2b·2CH2Cl2. Conversion of 3·γ-pic or 4 to 5. The compound 3·γ-pic (0.15 g, 0.25 mmol) was dissolved in dimethyl sulfoxide (5 mL). The solution was very slowly evaporated at room temperature. An orange-yellow product was filtered and dried. Yield: 0.03 g; 25%. The same product was also obtained by conversion of 4.

Scheme 1. Synthesis of 1α, 1β, and 1γ (Reactions on the Left Half of the Scheme) and Synthesis of 2a and 2b⊃CH2Cl2·4CH2Cl2 (Reactions on the Right Half of the Scheme)

ethanol was stirred at various temperatures, and the resulting solids were filtered and dried. They were analyzed by PXRD, which revealed that all three forms remained unchanged at 10 °C. Form 1α was unaffected also at room temperature, whereas 1β and 1γ almost completely converted into 2a after two days. It was expected that a single polymorphic form of 1β or 1γ could be obtained by adding seed crystals of the desired form at the beginning of the reaction. Indeed, seeding assisted synthesis at 10 °C resulted mainly in the formation of forms 1β or 1γ. A very small amount of the alternative phase could be visually detected and removed manually. A comparison of powder X-ray diffraction patterns was used to identify the polymorphic form in the bulk sample (Figure 2).

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RESULTS AND DISCUSSION Synthesis of the Mononuclear Molybdenum(VI) Complex, [MoO2(L)(EtOH)] (Forms 1α, 1β, and 1γ). Three polymorphs of the mononuclear molybdenum(VI) complex [MoO2(L)(EtOH)] (1α, 1β, and 1γ) have been prepared by the reaction of [MoO2(acac)2] with a stoichiometric amount of H2L in ethanol (Scheme 1). Addition of acetic acid into the reaction mixture or its generation in situ was necessary for preparation of the polymorphs. The synthesis performed in ethanol under strong stirring at 35 °C for 2 h produced 1α. No evidence for any supramolecular assembly or other polymorphs was found. The reaction carried out in very dry conditions without stirring or shaking at 10 °C led to a mixture of forms 1β (red crystals) and 1γ (orange crystals). Anhydrous H2L and acetic anhydride was used in order to eliminate traces of moisture. Individual crystals of each polymorph were carefully separated manually and used in further experiments. Before any attempt to control the formation of the polymorphic forms via seeding, their stability in solution was examined. A suspension of each of the pure polymorphs in dry

Figure 2. PXRD patterns of 1α (a and b); 1β (c and d); 1γ (e and f). The colored lines indicate patterns obtained by powder diffraction, while the black lines indicate patterns calculated from the X-ray singlecrystal structures of the corresponding polymorphs. D



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the nonsolvated forms up to the temperature related to the ligand decomposition, it could be assumed that molecules of the three polymorphs immediately interlinked after losing ethanol molecules. To the best of our knowledge, self-assembly of different cisdioxomolybdenum(VI) polymorphs into the same supramolecular isomer has not been reported so far. The formation of the same compound by the solid-state reaction can be explained by the relatively short distance between the isonicotinyl nitrogen atom and the molybdenum atom of the neighboring molecule (Figure S7, Supporting Information).18 The usual manner of polymerization through the MoOt···Mo interaction14 was not observed since the MoOt group is further away than the isonicotinyl nitrogen from the Mo coordination site of the neighboring molecule. Synthesis of the Supramolecular Isomers: Coordination Polymer [MoO2(L)]n (2a) and Molecular Square [MoO2(L)]4⊃CH2Cl2·4CH2Cl2 (2b⊃CH2Cl2·4CH2Cl2). Compound 2a can be synthesized from the reaction of [MoO2(acac)2] with H2L in ethanol at an elevated temperature (Scheme 1) or by a self-assembly reaction after dissolution of 1α, 1β, or 1γ in hot ethanol. Despite many attempts, we were not able to prepare crystals of 2a suitable for the X-ray single crystal diffraction experiment. Each time, the obtained crystals were extremely thin plates. TG study on a sample of 2a carried out in the temperature range of 25−320 °C showed neither desolvation nor decomposition. Upon heating of 2a, only weight loss in the range 320−500 °C corresponding to complex decomposition occurred (Figure S5, Supporting Information). Conversion of [MoO2(L)(EtOH)] in dichloromethane at ambient temperature yielded a small amount of orange crystals (Scheme S2, Supporting Information). The product was identified by X-ray crystallography and NMR spectroscopy, and it was found that a square molecular inclusion complex [MoO2(L)]4⊃CH2Cl2·4CH2Cl2 (2b⊃CH2Cl2·4CH2Cl2) was obtained. It can be assumed that the appropriate concentration of building units35 in CH2Cl2 under a given temperature and choice of solvent are related to its formation. The same inclusion complex can also be synthesized as the only product through direct reaction of [MoO2(acac)2] with H2L in CH2Cl2 at room temperature (Scheme 1). Single-crystal X-ray analysis reveals that one dichloromethane molecule is accommodated inside the square of the tetranuclear host [MoO2(L)]4. Encapsulation of guest molecules by a supramolecular dioxomolybdenum(VI) host has not been reported so far. Crystals of 2b⊃CH2Cl2·4CH2Cl2 crack even at low temperatures. It is estimated that upon exposure to air at room temperature ≈60% of the solvent molecules leave the crystal corresponding to three CH2Cl2 molecules (as found by TG measurements). The powder pattern of 2b·2CH2Cl2 becomes only to some extent diverse from that of the calculated for 2b⊃CH2Cl2·4CH2Cl2 (Figure 3), which indicates that the square tetranuclear structure is maintained after partial loss of the solvent molecules. However, after a longer grinding time (ca. 30 min) crystals lose solvent molecules completely and turn into an amorphous solid. The porous framework can be desolvated completely upon heating of a sample of 2b·2CH2Cl2. TG analysis showed a weight loss of 9.9% within the range of 165−185 °C consistent with loss of the remaining two dichloromethane molecules (Figure S5 in the Supporting Information). Removal of solvent molecules from 2b·2CH2Cl2 resulted in structural transformation and formation of 2a (Figure 3). This thermally

While initial grinding of 1β and 1γ resulted in a partial release of the coordinated solvent molecule (according to TG measurements the weight loss was about 1−2%), this was not the case with 1α. However, this change was not significant, and PXRD patterns could still be used for unambiguous phase identification. Structural Transformations upon Grinding. All three polymorphs were subjected to solid-state grinding by means of a mortar and pestle at room temperature. Samples of 1α, 1β, or 1γ lose coordinated EtOH molecules depending on the grinding time as confirmed by TG measurements. A sample obtained upon 30 min of grinding of 1α showed significant loss of crystallinity, whereas under the same conditions 1β or 1γ turned into amorphous solids (Figures S1−S3, Supporting Information). An exposure of the samples (obtained after 30 min grinding) to ethanol vapors caused an increase in the degree of crystallinity. Thus, the product obtained from 1α having a vacant coordination site can be reversed to the starting polymorph, whereas solvent-vapor exposure of samples obtained from 1β or 1γ resulted in crystalline products distinct from the starting polymorphs (Scheme S2, Figures S1−S4, Supporting Information). In both cases, they were identified to be 2a. Thermally Induced Structural Transformations in the Solid State. The conversion between 1α, 1β, and 1γ was not observed at room temperature. Therefore, we were interested in studying possible structural changes caused by temperature variation. TG measurements were carried out to determine the thermal stability of compounds. Crystals of all samples were used for TG analysis without grinding in order to avoid the loss of the coordinated solvent molecule. All three polymorphs showed similar thermal behavior (Figure S5, Supporting Information). The first weight loss in the TG curves of 1α, 1β, or 1γ was related to the solvent molecule release. In the atmosphere of pure oxygen, weight loss (10.08−10.21%) occurred in the range 141−170 °C (1α); 118−163 °C (1β); and 126−139 °C (1γ). Afterward, a nonsolvated product was stable up to 320 °C, when it started to decompose and afforded MoO3 as the final residue (at ca. 500 °C). Sample of each polymorph was subjected to DSC analysis. All samples were heated from the ambient temperature at various heating rates (2, 5, 10, 20, 50 °C min−1). No peaks were observed up to the temperature range corresponding to the ethanol molecule release. This result indicates that polymorphic transition does not take place upon heating. The desolvation process was observed as an endothermic peak in the DSC curve (55.6 kJ mol−1 (1α); 58.9 kJ mol−1 (1β); and 56 kJ mol−1 (1γ)). Afterward, no additional peaks were observed in DSC thermographs up to the temperature range corresponding to the ligand decomposition. We were interested to determine structural changes caused by the desolvation of these polymorphs. The crystalline samples of 1α, 1β, and 1γ were heated for 1 h at 220 °C, and the obtained products were examined by PXRD. Interestingly, all three polymorphs exhibit the same structural transformation into 2a (Scheme S2, Figure S4, Supporting Information). On the basis of mass spectrometry measurements, the polymeric structure of 2a can be assumed (Figure S6, Supporting Information). This is consistent with the preference for the formation of the polymer at high concentrations of building units.35 Since no peaks were observed in DSC thermographs of E



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complex (Table 2). These Gibbs energies are in accordance with the experimentally observed trends of the distances from Table 2. Calculated Standard Gibbs Energies of Binding at 298.15 K and 1 atm for [MoO2(L)(D)] and [MoO2(VIH)(D)] Relative to the Complex [MoO2(L)(EtOH)]a ΔΔrG°binding/kcal mol−1 complex

in DMSO (εr = 46.826)

[MoO2(L)(EtOH)] [MoO2(L)(DMSO)] [MoO2(L)(γ-pic)] [MoO2(L)(py)] [MoO2(L)(H2L)] [MoO2(VIH)(DMSO)] [MoO2(VIH)(γ-pic)] [MoO2(VIH)(py)]

0.00 −2.66 −1.68 −1.72 −1.98 −1.89 −2.42 −1.35

a B3LYP/def2-TZVP level of the theory; solvent effects were incorporated using the SMD method.

the molybdenum atom to the ligand’s O atom (Table S1, Supporting Information). When the nitrogen donor ligand completes the sixth coordination place of the molybdenum atom ΔΔrG°binding decreases. The value found for [MoO2(L)(H2L)] where H2L is coordinated through the izonicotinyl moiety is lower than in [MoO2(L)(EtOH)] (ΔΔrG°binding is −1.98 kcal mol−1). This is in accordance with the fact that polymerization of the mononuclear building blocks occurs in EtOH. These results were compared to those obtained for [MoO2(VIH)(D)] with the methoxy group of the ligand in ortho position. The mononuclear complex [MoO2(VIH)(DMSO)] has the standard Gibbs energy of binding lower for −1.89 kcal mol−1. This is again in accordance with experimental findings that polymerization of [MoO2(VIH)(DMSO)] does not occur in DMSO. Crystal and Molecular Structures. In all compounds, the molybdenum atoms exhibit distorted octahedral coordination spheres. The ligand is coordinated in the dianionic form to the cis-{MoO2}2+ core via the O,N,O donor atoms (phenolicoxygen, azomethine-nitrogen, and enolic-oxygen). The remaining sixth coordination site is occupied either by the nitrogen or the oxygen atom from the solvent molecule (ethanol in three polymorphic forms 1α, 1β, and 1γ; γ-picoline in 3·γ-pic, pyridine in 4 and dimethyl sulfoxyde in 5) or by the nitrogen atom of the izonicotinyl moiety of the neighboring molecule forming discrete cyclic assemblies, molecular squares (in 2b⊃CH2Cl2·4CH2Cl2). The X-ray single-crystal structure data obtained for all compounds are tabulated in Table 1. ORTEP plots of the crystal structures of 1α, 1β, 1γ, 3·γ-pic, 4 and 5 are given in the Supporting Information (Figure S8). The distance from the molybdenum atom to the O/N atom from the solvent molecule represent the largest bond length within the distorted octahedron (Table S1, Supporting Information).36,12,13 The C1−N1 bond of the ligand in molybdenum(VI) complexes varies from 1.279(3) Å to 1.294(3) Å and is not significantly different then in the free ligand (1.2791(18) Å and 1.276(2) Å).34,37 The bond length N2−C2 is shortened in the complexes (1.281(6) to 1.308(5) Å) in comparison to the free ligands (1.3552(18) Å and 1.339(2) Å),34,37 while the N1−N2 bond is lengthened (1.394(3) Å to 1.404(3) Å) in comparison to the free ligands

Figure 3. PXRD patterns of (a) 2b⊃CH2Cl2·4CH2Cl2 calculated from the X-ray single crystal structure; (b) 2b·2CH2Cl2 obtained after conversion of 1α in CH2Cl2; (c) 2b·2CH2Cl2 obtained by reaction of [MoO2(acac)2] with H2L in CH2Cl2; (d) 2a obtained by reaction of [MoO2(acac)2] with H2L in EtOH; (e) 2a obtained upon heating of 2b·2CH2Cl2 up to 200 °C.

induced transformation can be explained by the cleavage of the Mo−Nisonicotinyl bonds in the square and formation of new Mo− Nisonicotinyl bonds. Synthesis of the Mononuclear Complexes [MoO2(L)(D)], D = γ-pic (3·γ-pic), py (4), and DMSO (5). The construction of 2a or 2b⊃CH 2Cl2·4CH2Cl2 from the mononuclear units via solvent-mediated transformations or by the solid-state structural conversions indicates that the mononuclear building blocks have a great tendency to aggregate into supramolecular architectures. The reverse transformation can be achieved by dissolving 2a or 2b·2CH2Cl2 in stronger coordinating solvents D. Thus, complexes [MoO2(L)(γ-pic)]·γ-pic (3·γ-pic) and [MoO2(L)(py)] (4) were obtained by conversion of 2a or 2b·2CH2Cl2 using γ-picoline or pyridine, respectively. Monomerization is related to the coordination of solvent D to the cis-{MoO2}2+ core yielding [MoO2(L)(D)]. When oxygen donor solvents such as DMF or DMSO were used, compound 2a precipitated again. Interestingly, an analogous compound [MoO2(VIH)]4 (where VIH = 3-methoxy-2-oxybenzaldehyde izonicotinoyl hydrazonate)12 upon dissolution in DMSO gave [MoO2(VIH)(DMSO)]·DMSO.12 Efforts have been made to obtain crystals of [MoO2(L)(DMSO)] by using different mononuclear molybdenum(VI) complexes. After prolonged standing of 3·γ-pic or 4 in a dimethyl sulfoxide solution, substitution of γ-pycoline or pyridine, respectively, occurred yielding yelloworange crystals of [MoO2(L)(DMSO)] (5). Quantum Chemical Calculations. Standard Gibbs energies of binding were calculated for [MoO2(L)(D)], where D = EtOH, DMSO, γ-pic, py, or H2L. Calculated values revealed that the stability of the [MoO2(L)(EtOH)] complex is the lowest one, and therefore we present all differences in standard Gibbs energies of binding ΔΔrG°binding relative to this F



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(1.3791(19) Å and 1.3724(15) Å),34,37 due to electron delocalization. Polymorphs 1α, 1β, and 1γ. The asymmetric unit in 1α and 1β contains one molecule, while in 1γ it contains two symmetrically independent molecules. Analysis of the bond lengths and distances revealed only small or insignificant differences in the crystal structure. Differences in the molecular structure of the three polymorphs 1α, 1β, and 1γ are revealed by their overlay (Figure 4) and by their dihedral angles (Table 3).

A common feature of all three polymorphic forms is the association of the molecules by hydrogen bonds O6−H2···N3 into centrosymmetric dimers, graph set R22 (18), Figure 5. The shortest hydrogen bond distance is in form 1α (Table S2, Supporting Information). In all three polymorphic forms, the centrosymmetric hydrogen-bonded dimers are also held together by π···π interactions between the two pyridine rings, while the neighboring dimer molecules are additionally connected by π···π interactions between the two phenyl rings into endless chains (Figure 6, Table S2, Supporting

Figure 4. Superposition of the molecules of 1α (red), 1β (blue), and 1γ (molecule 1 - green and molecule 2 - yellow).

Table 3. Angle between the Phenyl and the Pyridyl Moieties, φ (°) and Angle between the Five- and Six-Membered Chelate Rings, ψ (°) for Compounds 1α, 1β, 1γ (Two Independent Molecules), 2b⊃CH2Cl2·4CH2Cl2, 3·γ-pic, 4 and 5 1α 1β 1γ 2b⊃CH2Cl2·4CH2Cl2 3·γ-pic 4 5

φ/°

ψ/°

22.09(9) 8.84(12) 11.9(2), 31.6(2) 28.03(14), 19.92(14) 18.89(12) 11.58(15) 8.52(12)

11.75(7) 9.04(8) 7.99(15), 9.66(15) 10.98(10), 10.07(10) 7.98(9) 8.91(8) 8.52(12)

Figure 6. Partial structural motif of endless chains in 1α, 1β, and 1γ. Hydrogen-bonds are shown by blue dotted lines. Green dashed lines indicate π···π interactions between two pyridyl rings Cg3···Cg3[centrosymmetric pair] (Cg3 is the centroid of the pyridyl ring; red sphere) and phenyl rings Cg4···Cg4[centrosymmetric pair] (Cg4 is the centroid of the phenyl ring; blue sphere).

The ligand is not planar with the largest deviation from planarity being that of the phenyl and the pyridyl moieties (φ angle). The largest dihedral angle between the planes of the five- and six-membered chelate rings (ψ angle) is in 1α (Table 3, Figure S9, Supporting Information). The difference is also in the orientation of the methoxy group. The dihedral angle C13− C12−O5−C14 is 16.9(3)° in 1α and 7.9(3)° in 1β, and −174.1(4)° and −177.0(5)° in 1γ.

Information). The chains are connected by weak C−H···O interactions that are different in the three polymorphs (packing of the molecules of 1α, 1β, and 1γ are given in Figure S10, Table S3, Supporting Information). One unique feature of 1α is an additional π···π interaction between pyridyl rings of the adjacent chains, thus forming an interwoven three-dimensional network (Figure 7). This polymorph has the highest density.

Figure 5. Discrete dimers formed through intermolecular O6−H2···N3 [−x, 1 − y, −z] hydrogen bonds in 1α. Hydrogen bonds are shown as blue dashed lines. G



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diagonal of the fingerprint plot as a green area with (di, de) ≈ 1.7−1.9 Å (labeled 3). Mononuclear Complexes [MoO2(L)(D)] (3·γ-pic, 4 and 5). Overlaying the molecules of the three mononuclear complexes 3·γ-pic, 4, and 5 reveals the differences in their molecular structures (Figure 9), the greatest being at the coordinated solvent molecule or the orientation of the methoxy group.

Figure 9. Superposition of the molecules of 3·γ-pic (blue), 4 (red), and 5 (green).

The ligand is not planar, the largest deviation from planarity is that of the phenyl and the pyridyl moieties, while the fiveand six-membered chelate rings form the largest angle in 4 (Table 3). There are no classical hydrogen bonds in the crystal structures of 3·γ-pic, 4, and 5, and only weak C−H···O and C− H···N hydrogen bonds as well as C−H···π interactions are present (Figures S11−S13, Table S4, Supporting Information) Metal−Organic Supramolecular Isomer 2b⊃CH2Cl2·4CH2Cl2. Complex 2b⊃CH2Cl2·4CH2Cl2 consists of discrete cyclic assemblies, molecular squares, in which the linker ligand connects the metal ions at the corners of the square (Figure 10). The largest deviation from planarity can again be seen between the phenyl and the pyridyl moieties, while the five- and six-membered chelate rings form angles of 10.98(10)° and 10.07(10)° (Table 3). The packing of molecules (Figure 10) is characterized by weak C−H···O and C−H···N interactions (Table S5, Supporting Information). Four dichloromethane molecules lie between the squares that are stacked along the baxis, while one is within the square. The solvent molecules are weakly bound and can easily leave the crystal accounting for its instability in air. All of them were found to be disordered. Analysis of the accessible solvent area in the model without the dichloromethane molecule situated within the center of the molecular square shows discontinuous spheres of accessible solvent area amounting to 348.7 Å3. However, when all solvent molecules are removed, channels extending along the b-axis are

Figure 7. Packing in the unit cell of 1α, 1β, and 1γ. Yellow chains in 1α are in the lower layer, while the blue chains are in the upper layer. In 1γ the chains 1γ-1 are given in blue, and 1γ-2 are in yellow.

The atom−atom contacts in the three polymorphic forms were also investigated via visualization of their respective Hirshfeld surfaces.38 Fingerprint plots were obtained for each of the polymorphs using the program Crystal Explorer.39 The resulting two-dimensional fingerprint plots40 (di vs de), are shown in Figure 8. Blue color points correspond to the low frequency of occurrence of a (di, de) pair.41 For consistency and clarity, all two-dimensional fingerprint plots are generated showing only the region between 0.4 Å and 2.8 Å. The plots for all three polymorphs exhibit a pair of long sharp spikes at the bottom left of the plot with short di and de values, the upper one associated with the donor atom and the lower one with the acceptor representing the strong hydrogen bond O6−H2···N3 (labeled 1a for the acceptor and 1b for the donor), and the pattern of diffuse points in between these spikes occurs only for cyclic hydrogen bonds.42 The C−H···O interactions in all three polymorphs (labeled 2a and 2b) can be seen in the green coloring toward the top of the spikes and as small spikes between them. The π···π stacking appears on the

Figure 8. 2-D fingerprint plots derived from Hirshfeld surfaces for 1α, 1β, and 1γ; di is the distance from the surface to the nearest atom in the molecule itself, and de measures the distance from the surface to the nearest nucleus in another molecule. H



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stretching bands having considerably lower intensities appear in the same region, but they either overlap with the asymmetric one or appear as shoulders. Two vibration bands belonging to CNimine and C−Ophenolic groups (at 1627 cm−1 and 1387 cm−1, respectively) seen in the IR spectrum of H2L, are shifted to ca. 1600 cm−1 and ca. 1350 cm−1 in the spectra of all of the complexes.44 This indicates coordination of the ligand to the cis-{MoO2}2+ core through the nitrogen and oxygen atoms of these two groups. The band at ≈1335 cm−1 belongs to the C−O group of the hydrazone moiety. In the mononuclear complex, the sixth coordination site is available for a monodentate solvent molecule. This is confirmed by the presence of the C−O band around 1040 cm−1 assigned to the coordinated ethanol molecule. On the other hand, in 2a and 2b·2CH2Cl2 this band is missing (Figure S16, Supporting Information.). The CNisonicotinyl absorption band (around 1610 cm−1) seen in the IR spectra of 1α, 1β, and 1γ is shifted and overlapped with the band (at 1600 cm−1) belonging to CNimine in the IR spectrum of 2a and 2b·2CH2Cl2, thus proving that the octahedral coordination around Mo atom is completed by the nitrogen atom from the isonicotinyl moiety. This is additionally supported by the absence of a strong broad band at ≈850 cm −1 , which is characteristic for an intermolecular MoOt···Mo interaction. UV−vis Absorption Spectra. Crystals of each polymorph when grinded into a fine powder have an orange red (1α and 1β) or dark yellow color (1γ) (Figure S17, Supporting Information). The color difference is reflected in the different absorption spectra of the solid samples (Figure S18, Supporting Information). Absorption bands at about 330 and 420 nm in the solid-state UV−vis spectra of each of the forms are accompanied by a 490 nm shoulder found for 1α and 1β. This low energy band is not present in the absorption spectra of 1α, 1β, and 1γ in the solution, which indicates that different molecular packing of the three crystal forms and different intermolecular interactions that occur in the solid state are responsible for the color difference. Unlike the solid-state absorption spectra, dissolution of any of the three forms in ethanol gave a pale yellow solution with similar absorption spectra (Figure S18, Supporting Information). Electronic spectra of 1α, 1β, 1γ, and 2a and 2b exhibited N(pπ)− Mo(dπ) and O(pπ)−Mo(dπ) LMCT bands at about 410 and 320 nm,45 respectively, in the similar region in comparison with those observed in the solid-state spectra. NMR Spectroscopy. The 1H NMR spectra of 1α, 1β, 1γ, 3·γ-pic, and 4 in DMSO-d6 show the same set of signals arising from the chelating ligand L (Figure S19, Supporting Information). 1α presents also a minor set of resonances, which will be discussed below. All 1H NMR spectra present a singlet at 3.83 ppm due to the OCH3 group, five resonances in the aromatic region due to the protons of the phenyl and pyridyl groups, and a singlet at 8.91 ppm arising from the azomethine proton. The absence of the signals due to N−NH and OH is attributed to the double deprotonation of H2L (Scheme S4, Supporting Information). The downfield shift of CHN resonance (+0.33 ppm) is a result of the electron redistribution in the ligand caused by the complexation.12,13 Signals arising from free EtOH are observed in the spectra of 1α, 1β, and 1γ (about 1 equiv), while resonances due to free γpicoline (about 2 equiv) and py (about 1 equiv) are present in the spectra of 3·γ-pic and 4, respectively. These data suggest substitution of these ligands by the solvent, leading to the

Figure 10. (a) ORTEP plot of 2b⊃CH2Cl2·4CH2Cl2 (CH2Cl2 molecules are omitted for clarity), displacement ellipsoids of nonhydrogen atoms are drawn at the 50% probability level; (b) packing arrangement of 2b⊃CH2Cl2·4CH2Cl2 in the unit cell. Only one position of the disordered CH2Cl2 molecules is shown.

formed (Figure S14, Supporting Information). The total potential volume for solvent molecules per unit cell volume is 2338.6 Å3, which amounts to 30% of the unit cell volume. Therefore, removal of the first surface solvent molecules forms a pathway for easy removal of the neighboring ones. However, not all escape as described previously. Spectroscopic Characterization. FT-IR Spectra. The polymorphic forms show small differences in the spectra, which nevertheless allow the crystal forms to be distinguished (Figure S15, Supporting Information). All forms show a slightly different pattern of peak positions and intensities of the bands in the 1000−600 cm−1 region. Compounds were identified by the appearance of the stretching frequencies characteristic for the νasym(OMo−OEtOH) (found at 931 cm−1 for 1α, 930 cm−1 for 1β, 941 cm−1 for 1γ), νasym(MoO2) (found at 905 cm−1 for 1α, 907 cm−1 for 1β, 908 cm−1 for 1γ, 927 cm−1 for 2a and 935 cm−1 for 2b·2CH2Cl2), and νasym(OMo−Nisonicotinyl) (found at 900 cm−1 for 2a and 911 cm−1 for 2b·2CH2Cl2).43 The corresponding symmetric I



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show chemical exchange cross-peaks with two small broad resonances at 8.10 and 7.60 ppm. They suggest that the pyridyl group is involved in chemical exchange processes probably involving the coordination to the Mo atom of another molecule. A variable temperature 1H NMR study of 1α in CD2Cl2 was undertaken in the range of temperature from 25 to −40 °C (see Figure S22, Supporting Information). By lowering the temperature, the broad signals of ortho and meta protons of the pyridyl group become sharper. The signals of the phenyl moiety become initially broader and then sharp again. New sets of small signals appear in the aromatic region. Cross-peaks in the COSY experiment at −40 °C (Figure S23, Supporting Information) allowed establishing the presence of at least three minor sets of pyridyl and one minor set of phenyl signals. Furthermore, the ROESY experiment at −40 °C (Figure S24, Supporting Information) showed chemical exchange crosspeaks between the signals of the major and the minor sets. Therefore, the presence in solution of exchanging assemblies with different nuclearity could be inferred.

formation of the mononuclear DMSO-d6 derivative. The latter product is obtained also by dissolving supramolecular isomers in DMSO-d6. As mentioned above, immediately after dissolution in DMSO-d6, 1α gives rise to a second set of resonances characterized by small downfield shifts with respect to those of the major set, varying from +0.13 ppm (for H−C3) to +0.21 for CHN, except for the H−C5 signal (Figure S20, Supporting Information). The two sets of signals display an integrated intensity ratio of about 1:0.17, but the smaller one decreases with time and is not present after few days. This minor set is attributed to residual 1α with EtOH that remained coordinated. In order to confirm this fact, principal component analysis on a set of time-dependent NMR spectra was performed. The first principal component (PC1) was covering 78.30% of total variations among the set (Figure 11, Figure S21, Supporting Information).

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CONCLUSIONS Substitution of the acetylacetonate ligands in [MoO2(acac)2] by 4-methoxy-2-oxybenzaldehyde isonicotinylhydrazonate (L2−) gives rise to either three polymorphs of [MoO2(L)(EtOH)] (1α, 1β, and 1γ), a supramolecular polymer, [MoO 2 (L)] n (2a), or a square inclusion complex [MoO2(L)]4⊃CH2Cl2·4CH2Cl2 (2b⊃CH2Cl2·4CH2Cl2), depending on the reaction conditions. According to quantum chemical calculations among the mononuclear complexes [MoO2(L)(D)] having nitrogen (γ-pic, py, or H2L) or oxygen donor ligands (EtOH, DMSO), the complex [MoO2(L)(EtOH)] is the least stable, whereas the complex [MoO2(L)(H2L)] is more stable. This is in accordance with the fact that mononuclear units readily aggregate into the supramolecular architectures containing the cis-{MoO2}2+ cores with the linker group being the izonicotinoyl part of the ligand. Despite this propensity, we have prepared three polymorphs of the complex [MoO2(L)(EtOH)]. In these polymorphs, a common feature is the association of the molecules by hydrogen bonds into centrosymmetric dimers. Solid-state reactions lead to the transformation of the mononuclear polymorphs or the supramolecular square into the coordination polymer. The formation of 2a regardless of the starting polymorph can be explained by the relatively short distance between the isonicotinyl nitrogen atom and molybdenum atom of the neighboring molecule present in all three forms. This is also consistent with the preference for the formation of the polymer at high concentration of building units. The square-molecular complex 2b⊃CH2Cl2·4CH2Cl2 can be obtained from a dilute dichloromethane solution due to a lower concentration of building units in CH2Cl2 and suitable reaction conditions.

Figure 11. PC1 loadings in the range of 5.0 to 0.0 ppm for a set of four time-dependent NMR spectra of 1α dissolved in DMSO-d6.

The highlighted changes in EtOH signals show variations in intensities and slight shifts in the position. These variations arise as a result of EtOH/DMSO exchange, during which the signals of coordinated EtOH in the complex are changed to signals of free EtOH in solution. The same trend in variations was observed in the calculated values of isotropic shieldings. After coordination of DMSO, EtOH remains in the solution and cannot replace DMSO because of the difference in standard reaction Gibbs energies of bindings (calculated value is −2.66 kcal mol−1 in the favor of DMSO, Table 2). 1α is slightly soluble in CD2Cl2, and the attempt to dissolve it in this solvent leads to a diluted yellow solution and a microcrystalline red solid on the bottom of the NMR tube. Nevertheless, it was possible to record the spectrum of this sample (Figure S22, Supporting Information, 25.0 °C). After some days, the small crystals at the bottom of the NMR tube changed color from red to orange (see X-ray section for characterization of the square inclusion complex). 1H NMR spectrum of the supernatant solution did not show significant changes, only resonance due to free EtOH increasing with time. 1 H NMR spectrum shows, besides the presence of about one equivalent of free EtOH, sharp signals due to the phenyl ring and CHN. Interestingly, the signals of ortho and meta protons of the pyridyl group (8.52 and 7.76 ppm) are quite broad. In a room temperature ROESY experiment, these signals

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S Supporting Information *

(1) Schemes for ligand and Mo compounds, (2) analytical and spectral data, (3) XRPD patterns, (4) TG curves, (5) figures for compounds. This information is available free of charge via the Internet at http://pubs.acs.org/. Crystallographic data sets for the structures 1α, 1β, 1γ, 2b⊃CH2Cl2·4CH2Cl2, 3·γ-pic, 4 and 5 are available through the Cambridge Structural Database with deposition numbers J



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934502−934508, respectively. Copies of this information may be obtained free of charge from the director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44 1223 336 033; email: [email protected] or http://www.ccdc.cam.ac.uk).

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Corresponding Author

*Tel: ++385-1-4606353. Fax: ++ 385-1-4606341. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this research was provided by Ministry of Science and Technology of the Republic of Croatia (Grant Nos. 119-1191342-1082, 119-1193079-1084, 119-11913422959). We thank to Dr. M. Ristić for recording the solidstate UV−vis spectra. We thank Dr. S. Kazazić for mass spectrometry experiments.

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