DOI: 10.1021/cg900824n
Concomitant Conformational Polymorphism: Mechanochemical Reactivity and Phase Relationships in the (Methanol)cis-dioxo(N-salicylidene-2-amino-3-hydroxypyridine)molybdenum(VI) Trimorph
2009, Vol. 9 5327–5333
K. Uzarevic,*,† M. Rubcic,† I. ^ilovic,† Z. Kokan,‡ D. Matkovic-Calogovi c,† and † M. Cindric †
Laboratory of General and Inorganic Chemistry, Chemistry Department, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia, and ‡Laboratory for Solid-State and Complex skovi c”, Bijeni cka 54, 10000 Zagreb, Croatia Compounds Chemistry, Institute “Ru{er Bo
Received July 17, 2009; Revised Manuscript Received September 1, 2009
ABSTRACT: The title compound, [MoO2(L)(CH3OH)], crystallizes as a concomitant conformational trimorph. The occurrence domains, under which these forms can be prepared, separated, and/or interconverted, were explored and determined in terms of the crystallization conditions and solid-state reactions. Solvent-free reactions resulted in dissociation of coordinated methanol, producing two new complexes, monomeric [MoO2(L)] and polymeric [MoO2(L)]n. They both bind methanol molecule from its vapor, thus providing a pure R-form of the title complex. All forms have been characterized and compared to each other by a variety of methods including elemental analysis, Fourier transform IR, thermal methods, X-ray powder diffraction, and single-crystal X-ray structure determination.
Introduction Polymorphism, a phenomenon where a single substance displays different crystal structures, has been one of the main topics in the field of solid-state chemistry for over a century.1 More than ever, the fact that different polymorphs show unique physicochemical properties (solubility, bioavailability, melting point, etc.) makes them interesting from an industrial point of view, especially pharmaceutical,2 as well as academic.3 Concomitant polymorphs are denoted as a special case of polymorphism in which more solid-state arrangements of the same chemical entity occur simultaneously.4 Among the first examples of denoted polymorphism was the case of two concomitant species of benzamide.5a However, the occurrence of more than two forms is not often perceived.4,5b,6 Although it can be interpreted as a problematic event, a batch sample of incoherent physical properties offers valuable information on the thermodynamic and kinetic aspects of the explored system that are not available if only one phase crystallizes. Comprehending these factors broadens knowledge of the crystallization phenomenon (nucleation, crystal growth, solid-state phase transformations). Various principles have been applied in an attempt to achieve control over the crystallization process.7 Recently, the importance of solvent-free reactions8 has been recognized in the field of solid-state chemistry not only as a simple path to materials practically impossible to obtain by classical solution synthesis,9 but also as a powerful tool for directing the reaction toward the desired form (enantiomorph, polymorph, or solvate).10 Herein we report synthesis and thorough solid-state characterization of the (methanol)cis-dioxo(N-salicylidene-2amino-3-hydroxypyridine)molybdenum(VI), [MoO2(L)(MeOH)], a coordination compound proven to crystallize as a concomitant conformational11 trimorph. Part
of this work was focused on the correlation between crystal packing interactions and molecular geometry. The occurrence of different crystalline phases was investigated in terms of different synthetic approaches and crystallization conditions under which these crystals can be prepared, separated, and/or interconverted. Additionally, influence of assorted precursors and additives on the reactions’ outcome was explored. Upon grinding or heating, all forms (or their mixture) lose the coordinated methanol molecule thus producing two kinds of coordinatively unsaturated products, GP and HP (the abbreviations GP and HP refer to desolvated molybdenum complexes), of the same empirical formulas, [MoO2(L)]. Stability and conversion conditions between these two products were explored. Their exposure to methanol vapor as well as conventional solvent crystallization methods led always to the one phase of the title complex. Experimental Section
*To whom correspondence should be addressed. Telephone (þ)385 1 4606350. Fax: (þ)385 1 4606341. E-mail:
[email protected].
Materials. Salicylaldehyde, acetylacetone, 2-amino-3-hydroxypyridine, and (NH4)6Mo7O24 4H2O were obtained from SigmaAldrich and used without further purification. [MoO2(acac)2] ([MoO2(acac)2] = cis-dioxobis(2,4-pentanedionato)molybdenum(VI), [MoO2(C5H7O2)2]) and [MoO2(sal)2] ([MoO2(sal)2] = cisdioxobis(salicylaldehydato)molybdenum(VI), [MoO2(C7H5O2)2]) were prepared according to the literature methods.12,13 Solvents and concentrated acids (p.a. grade) were purchased from Kemika, Zagreb. Synthesis of N-Salicylidene-2-amino-3-hydroxypyridine, H2L. Equimolar amounts (0.82 mmol) of 2-amino-3-hydroxypyridine and 2-hydroxybenzaldehyde were added in 5 mL of ethanol and refluxed for 1 h. Orange crystalline product was isolated after 3 days standing at room temperature. Yield: 61%. Elemental analysis (Calc. (Found) for C12H10N2O2): C 67.28 (66.97); H 4.71 (4.52); N 13.08 (13.14). IR (KBr, cm-1): 3000-2600 br (νO-H, νC-H); 1611, 1584, 1556 (νCdN and νCN). Method A. Synthesis of [MoO2(L)(MeOH)] by the Reaction of [MoO2(sal)2] and 2-Amino-3-hydroxy-pyridine. A methanolic solution of 2-amino-3-hydroxy-pyridine (88 mg, 0.80 mmol; 5 mL) was
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Table 1. Crystallographic Data for the R-, β-, and γ-Form R-form
β-form
γ-form
)
)
C13H12N2O5Mo C13H12N2O5Mo empirical formula C13H12N2O5Mo 372.19 372.19 372.19 formula weight (g mol-1) crystal system triclinic orthorhombic monoclinic 0.30 0.10 0.10 0.26 0.24 0.22 0.30 0.25 0.20 crystal size (mm3) crystal habitus needle prism prism crystal color yellow orange red Pbca P21/c space group P1 unit cell dimensions (A˚, ) a 7.6245(9) 12.0547(7) 7.3420(2) b 9.3366(8) 14.6678(5) 9.9789(3) c 9.5643(12) 15.3009(9) 18.1447(5) R 78.426(9) 90 90 β 78.886(11) 90 97.438(3) γ 89.541(8) 90 90 3 654.17(13) 2705.4(2) 1318.19(7) volume (A˚ ) Z 2 8 4 1.890 1.828 1.875 Dcalc (g cm-3) -1 1.027 0.993 1.019 μ (mm ) F (000) 372 1488 744 reflections collected/unique 6092/2293 13100/2513 11481/2441 data/restraints/parameters 2293/0/195 2513/1/195 2441/1/195 2 0.949 0.949 0.965 goodness-of-fit on F , S a 0.0302/0.0648 0.0315/0.0788 0.0323/0.0717 R/wR [I > 2σ (I)] 1.100, -0.553 1.172, -0.453 0.936, -0.469 largest diff peak and hole (e A˚-3) P P P P P a R= Fo| - |Fc / Fo, w = 1/[σ2(Fo2) þ (g1P)2 þ g2P] where P = (Fo2 þ 2Fc2)/3, wR2 = [ w(Fo2 - Fc2)2/ (Fo2)2]1/2, S = [w(Fo2 - Fc2)2/ 1/2 (Nobs - Nparam)] .
added to a methanolic solution of [MoO2(sal)2] (150 mg, 0.41 mmol; 10 mL). The solution immediately changed color from yellow to dark orange. The reaction mixture was refluxed for 1 h, cooled to room temperature, and the product was filtered off under vacuo. It was observed that the product was a mixture of three morphologically different crystalline species: yellow needles (R), orange prisms ( β), and red prisms (γ). Overall yield (mixture of all three forms): 98%. It was possible to mechanically separate the crystals on the basis of their shape and color. Elemental analyses were performed after mechanical separation. R-form. Elemental analysis (Calc. (Found) for C13H12N2O5Mo): C 41.95 (41.77); H 3.25 (3.39); N 7.53 (7.41); Mo 25.78 (24.92). IR (KBr, cm-1): 3441-2532 br (νO-H, νC-H); 1606, 1568, 1542 (νCdN); 1015 (νC-O, methanol); 937, 914 (νModO, terminal). β-form. Elemental analysis (Calc. (Found) for C13H12N2O5Mo): C 41.95 (41.63); H 3.25 (3.22); N 7.53 (7.67); Mo 25.78 (25.07). IR (KBr, cm-1): 3444-2536 br (νO-H, νC-H); 1604, 1565, 1541 (νCdN and νCN); 1019 (νC-O, methanol); 937, 907 (νModO, terminal). γ-form. Elemental analysis (Calc. (Found) for C13H12N2O5Mo): C 41.95 (42.05); H 3.25 (3.11); N 7.53 (7.31); Mo 25.78 (25.13). IR (KBr, cm-1): 3440-2555 (broad) (νO-H, νC-H); 1601, 1565, 1541 (νCdN and νCN); 1013 (νC-O, methanol); 935, 909 (νModO, terminal). Method B. Synthesis of [MoO2(L)(MeOH)] by the Reaction of [MoO2(acac)2] and H2L. [MoO2(acac)2] (75 mg; 0.23 mmol) was added to a methanolic solution (5 mL) of H2L (50 mg; 0.23 mmol) and refluxed. After 5 min, the yellow precipitate was filtered off and dried. Only R-form was observed (confirmed by X-ray powder diffraction, XRPD). Yield: 78%. Elemental analysis ((Calc. (Found) for C13H12N2O5Mo): C 41.95 (42.07); H 3.25 (3.50); N 7.53 (7.50); Mo 25.78 (24.40). Synthesis of Pure γ-Form of [MoO2(L)(MeOH)]. A methanolic solution of 2-amino-3-hydroxy-pyridine (44 mg, 0.40 mmol; 15 mL) was added to a methanolic solution of [MoO2(sal)2] (75 mg, 0.21 mmol; 10 mL). To the prepared reaction mixture an excess of 4,4bipyridine (330 mg, 2 mmol) was added. The solution immediately changed color from yellow to brown. The vessel was left at 4 C for a week. The product was filtered off and washed with cold methanol. Yield: 10%. Synthesis of [MoO2(L)], GP. Form R was ground using a mortar and a pestle for 5 min. After 30 s, the yellow powder changed color to dark orange. The process was repeated under inert atmosphere (argon) with the same result. Grinding of β- and γ-form yielded the same product. Amorphous (XRPD) substance thus prepared,
[MoO2(L)], was collected and analyzed. Elemental analysis ((Calc. (Found) for C12H8N2O4Mo): C 42.37 (41.13); H 2.37 (2.49); N 8.24 (8.10). IR (KBr, cm-1): 2923, 2851 (νC-H) 1602, 1565, 1545 (νCdN and νCN); 934, 912 (νModO, terminal). Synthesis of [MoO2(L)]n, HP. Method I. A mixture of polymorphs was heated from 25 to 230 C (5 C min-1) and slowly cooled to room temperature. In all cases this resulted in a brown product, [MoO2(L)]n. The same product was obtained by heating of pure phases of R-, β-, or γ- polymorphs or their mixture (XRPD). Elemental analysis ((Calc. (Found) for C12H8N2O4Mo): C 42.37 (42.13); H 2.37 (2.02); N 8.24 (8.40).IR (KBr, cm-1): 2923, 2851 (νC-H) 1602, 1565, 1545 (νCdN and νCN); 947 (νModO, terminal); 798 (νModO Mo). Method II. [MoO2(L)] was heated from 25 to 130 C (5 C min-1) and cooled to room temperature. Analytical and spectral data for the isolated product obtained by this method are in agreement with those of the compound prepared according to Method I. Methods. Elemental analyses were performed by Central Analytical Service, “Ru{er Boskovic” Institute, Zagreb. IR spectra were recorded on PerkinElmer Spectrum RXI FT-IR spectrometer (KBr pellet technique, 4000-400 cm-1 range, 2 cm-1 step). Thermogravimetric analyses were performed on a Mettler-Toledo TGA/SDTA851e thermobalance using aluminum crucibles under nitrogen or oxygen stream with the heating rate of 5 C min-1. In all experiments, the temperature ranged from 25 to 600 C. The results were processed with the Mettler STARe 9.01 software. DSC measurements were performed on the Mettler-Toledo DSC823e calorimeter with STARe SW 9.01 in the range from 25 to 600 C (5 C min-1) under the nitrogen stream. X-ray Diffraction Experiments. The single-crystal X-ray data of the three polymorphs14 were collected on an Oxford Diffraction Xcalibur CCD diffractometer with graphite-monochromated MoKR radiation in a nitrogen vapor stream at 100 K using ω-scans. Details of data collection and crystal structure refinement are presented in Table 1. Programs CrysAlis CCD and CrysAlis RED15 were employed for data collection, cell refinement, and data reduction. The structures were solved by direct methods. The refinement procedure by full-matrix least-squares methods based on F2 values against all reflections included anisotropic displacement parameters for all nonH atoms. The positions of hydrogen atoms, except H5O, were positioned geometrically and refined applying the riding model [C-H = 0.95-0.98 A˚ and with Uiso(H) = 1.2 or 1.5Ueq(C,N)]. The position of the H5O atom in each polymorph was determined from the difference Fourier map and was included in the refinement
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Scheme 1. Preparation and Transformation Conditions of the Three Polymorphic Forms of [MoO2(L)(MeOH)] and Their Desolvated Products, [MoO2(L)] and [MoO2(L)]n
process with an isotropic thermal parameter. Calculations were performed with SHELXS9716 and SHELXL9717 (both operating within the WinGX18 program package). Geometry calculations were done using PLATON19 and PARST,20 and the molecular graphics were done with ORTEP21 and Mercury.22 Selected bond lengths and valence angles are listed in Table S1, Supporting Information. X-ray powder diffraction experiments were performed on a Philips PW 3710 diffractometer using CuKR radiation, with a zero background sample holder in the Bragg-Brentano geometry; tension 40 kV, current 40 mA. The patterns were collected in the angle region between 4 and 40 (2θ) with a step size of 0.02 and 1.0 s counting per step.
Results and Discussion Two synthetic approaches were employed to obtain the title complex (Scheme 1). Method A yielded concomitantly three morphologically different crystal species: yellow needles (R), orange prisms (β), and dark red prisms (γ) (Figure 1, Scheme 1). Method B momentarily afforded the pure R-form of the complex [MoO2(L)(MeOH)]. Since the method A provided three solid forms of the molybdenum complex, it was of interest to explore the influence of the reaction conditions, as well as introduction of an additive, on the occurrence of each of the polymorphs. The following results23 were obtained: (i) By crystallization from the hot and concentrated reaction mixture, method A afforded predominantly crystals of the R-form. On the other hand, the cooled and diluted reaction mixture favored growth of β-form’s big crystals (diameter 1-3 mm), together with the R- and γ-form. Method B yielded almost momentarily the R-form. These observations suggest that quick crystallization favors the R-form. Slow recrystallization of pure polymorphic phases yielded a mixture of all forms. (ii) Although the seeding technique has been recognized as an efficient tool in obtaining desired forms of the investigated material,24 in our system it failed to yield pure forms. (iii) When an excess of 4,40 -bipyridine, as an additive, was introduced in the reaction mixture prepared by method A, pure γ-form was obtained.
(iv) To establish the ability of polymorphs to interconvert between the R-, β-, and γ-form, grinding and thermal experiments (DSC and TGA) were employed. These methods failed to detect direct interconversion due to the decomposition of starting compounds, but resulted in interesting new products as will be discussed later. Crystal Structure Description. Crystallographic data for the polymorphs are given in Table 1. All forms crystallize in centrosymmetric space groups. The molecular structure, together with atom labeling scheme, is depicted in Figure 2a. The comparison of molecular structures of the complex in the R-, β-, and γ-form is presented in Figure 2b. Molybdenum(VI) ion displays a distorted octahedral arrangement, with two oxo groups in the cis-orientation and an equatorially coordinated dianionic O,N,O ligand. The sixth coordination site is occupied by the methanol molecule. Bond lengths and angles in the ligand are in agreement with the fact that, upon chelation, the system of conjugated bonds spreads over the five- and six-membered chelate heterocycles (Table S1, Supporting Information). All three forms are conformational polymorphs, easily distinguished by the relative positions of the phenyl and the pyridyl rings. In the γ-form, the chelating ligand is considerably more planar than in the other two forms, which can be easily seen from the dihedral angles between the two rings (Figure 2b). The same building block, a dimer, is found in the crystal structures of all three forms (Figure 3). The dimer consists of molecule pairs mutually connected by two hydrogen bonds of the O-H N type [denoted with the graph-set26 notation R22(12)] and, in R- and γ-form, face-to-face π-π interactions (Table 2).27 The relatively small changes in the chelating ligand geometry are a result of the molecule’s flexibility to accommodate several different crystal packing motifs that are similar in lattice energies. Considering the positions of the functionalities in dimer, it is obvious that all strong hydrogen bond donors are involved in hydrogen bonding and, consequently, only weaker interactions are disposable for the three-dimensional expansion.
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Figure 1. Concomitant polymorphs of [MoO2(L)(MeOH)]. Form R - yellow needles; β - orange prisms; γ - red prisms.
Figure 3. Discrete dimers formed through intermolecular O-H N hydrogen bonds for the R-polymorph. The same motif is repeated in the β- and γ-form. C gray, N blue, O red, H white, Mo magenta; dashed yellow lines indicate hydrogen bonds.
Figure 2. (a) ORTEP-POV-Ray25 rendered view of the molecule of the R-form. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms are presented as spheres of arbitrary small radii; (b) overlapped molecular structures of R- (light gray), β- (black), and γ-form (red). Dihedral angles between the planes of the phenyl (1) and pyridyl (2) rings are 11.32(16) (R), 19.79(15) (β), and 5.44(18) (γ). Planes are defined by atoms: (1) C2-C3C4-C5-C6-C7 and (2) N2-C8-C9-C10-C11-C12.
In the R-form, the dimers are arrayed on top of each other through C-H O interactions, thus forming infinite pillars, parallel to the crystallographic a axis (Figure 4, Table S2, Supporting Information). Similar interactions govern emerging of a two-dimensional brick-wall network (parallel to the bc plane) characteristic for the β-form architecture. Dominant contacts between the dimers that build γ-form architecture are π-π interactions. In the γ-form, the phenyl and pyridine moieties are nearly coplanar (5.44), the corresponding interplanar and centroid-to-centroid distances between the two rings being 3.224 and 3.820(2) A˚, while the slip angle (ψ) amounts to 27.02. Such a parallel displaced arrangement of the dimers leads to a slanting column architecture. Solid-State Experiments and Spectroscopic Characterization. In the course of the phase-transition investigation of the
polymorphs, interesting thermal and mechanochemical behavior was revealed. Forms R- and β- were stable up to ∼90 C after which they started to decompose in three steps (Figure S1a, Supporting Information).23 In the case of the γ-form decomposition started at 140 C, presumably due to the nature of the forces stabilizing the crystal structure. Heating of any of the forms or their mixture to 230 C resulted in the loss of the coordinated methanol molecule and formation of a brown crystalline product (HP). No thermal events that could be associated to the phase transition between the polymorphs were observed before that temperature. Upon manual grinding, all forms gave a dark orange and amorphous product (GP) (Figures 5 and S2b, Supporting Information). HP and GP products have the same composition (Scheme 1). IR spectra of both GP and HP lack the strong band associated with C-O stretching vibrations of the methanol molecule (Figure 6). Absence of methanol molecule at the sixth coordination place of GP was additionally confirmed by thermogravimetric measurements (Figure S1b, Supporting Information). Furthermore, spectra revealed differences in the coordination sphere of the central molybdenum cation (Figure 6). In the case of cis-dioxomolybdenum complexes, it is commonly observed that in the absence of a suitable donor for the sixth coordination place they tend to form dinuclear or polymeric chain-like structures, rather than leaving the structure pentacoordinated.28,29 The polymeric structure is realized through ModO Mo interaction of the neighboring mononuclear units. In the IR spectra, this kind of solid-state arrangement is easily distinguishable from the mononuclear entities by the strong and broad band in the range of 750800 cm-1, attributed to the vibration of the ModO Mo moiety. Additionally, spectra of compounds with such polymeric chain-like structure exhibit only one band characteristic for the terminal ModO vibration, indicating involvement of the other ModO unit in the interaction. IR spectra of GP revealed two bands in the region characteristic for the vibration of the cis-MoO2 group (939 cm-1 and
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Table 2. Intermolecular O5-H5O N2 Hydrogen Bonds and π-π Interactions Involved in the Formation of Dimersa (O-H N)
d(O-H)/A˚
R-polymorph β-polymorph γ-polymorph π-π R-polymorph(i) γ-polymorph(ii)
0.80(4) 0.94(3) 0.96(3) Cg(1) Cg(2)/A˚b 3.911(2) 3.861(2)
d(H N)/A˚ 1.93(4) 1.82(3) 1.75(3) j/c 11.32 5.44
D(O N)/A˚ 2.721(4) 2.755(4) 2.693(4) Cg(1)-perp/A˚d 3.369 3.372
— (O-H N)/
symmetry operator on N
175(4) 170(3) 166(4) ψ /e 24.10 27.20
2 - x, 1 - y, 2 - z 1 - x, -y, 1 - z 1 - x, 1 - y, 1 - z
Equivalent position code: (i) 2 - x, 1 - y, 2 - z; (ii) 1 - x, 1 - y, 1 - z. Cg(1) = N2-C8-C9-C10-C11-C12; Cg(2) = C2-C3-C4-C5-C6-C7. Cg Cg, distance between ring centroids. c j, dihedral angle between planes of the rings. d Cg(1)-perp, perpendicular distance from the first centroid onto the second ring plane. e ψ, angle between the Cg Cg vector and the normal to the plane through ring (1). a
b
Figure 5. Mechanochemical and thermal transformations of the polymorphs: (a) R-form; (b) product after 2 min of grinding of R-form; (c) product of grinding after 2 min at 130 C.
Figure 4. Structural motifs in the R-, β-, and γ-forms. C gray, N blue, O red, H white, Mo magenta; dashed lines indicate π-π interaction.
912 cm-1), but their intensities were reduced in comparison with the spectra of [MoO2(L)(MeOH)]. Moreover, strong or wide peaks indicative of the polymeric structure were not observed in the spectra. On the basis of the previous considerations, it can be concluded that grinding of all forms of the starting complex [MoO2(L)(MeOH)] produces the coordinatively unsaturated complex [MoO2(L)]. In the case of HP, disappearance of one of the ModO (912 cm-1) stretching bands and appearance of a strong broad band (798 cm-1) indicate that this product is a polymer, [MoO2(L)]n. To explore the possibility of conversion between these two species and their relative stability, GP, [MoO2(L)] was carefully heated up to 150 C, and the process was monitored by means of DSC. During heating, an exothermic peak at ∼125 C was observed, indicating formation of a new phase (Figure S1c, Supporting Information). The product thus formed was analyzed by XRPD and IR-spectroscopy and determined to be HP, [MoO2(L)]n. The temperature of the conversion of GP to HP is approximately 100 C lower than the temperature of the formation of the HP from
Figure 6. IR spectra for R-form (black, top), GP (red, middle) and HP (blue, bottom) in the range of 1100-450 cm-1. Bands important for discussion are denoted with an asterisk (*). Polymorphs have identical spectra in this range.23
[MoO2(L)(MeOH)]. However, the process was not reversible, meaning that the polymer [MoO2(L)]n once formed could not be again converted to the starting metastable [MoO2(L)] complex. It is worth mentioning that we tried to prepare the pentacoordinated [MoO2(L)] complex by holding the [MoO2(L)(MeOH)] at the 105 C, temperature lower than the temperature of conversion of the pentacoordinated to the polymeric structure. We were not able to prepare monomeric [MoO2(L)], on each occasion a mixture containing starting [MoO2(L)(MeOH)] and polymeric
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[MoO2(L)]n products was isolated, as confirmed by the XRPD analysis. It is well-known that the polynuclear ModO ModO chains are easily cleaved by a variety of donor molecules (D) which can complete the molybdenum coordination sphere, thus forming hexacoordinated complexes of the general formula [MoO2(L)(D)]. This behavior can be expected to be even more pronounced in the case of the coordinatively unsaturated complex [MoO2(L)]. Therefore, we conducted reactions involving the addition of the methanol molecule on the polymeric or pentacoordinated complexes to learn how the process influences the polymorph occurrence. The experiments were performed in two ways, one by exposure of the starting complexes to the methanol vapors and the other by dissolving the complexes in methanol. By vapor uptake, [MoO2(L)] and [MoO2(L)]n yielded exclusively the R-form of the complex [MoO2(L)(MeOH)]. The same polymorph was momentarily obtained by addition of a drop of methanol. Compounds capable of absorbing or reversibly reacting with the molecules in the gas phase are tempting due to their potential use, for example, as extractants of the environmentally hazardous molecules. Furthermore, when bonding involves measurable response, such compounds can be utilized in detection and signal transduction. Conclusion Three concomitant conformational polymorphs of the [MoO2(L)(MeOH)] were synthesized and characterized. The basic building block in all forms is a supramolecular dimer bound through O-H N hydrogen bonds. Close inspection of the polymorphs’ molecular structures revealed that the conformational differences of the bound ligand are in a remarkable interlude with each form’s unique crystal packing and color. Arrays of weak noncovalent interactions between the dimers lead to the following architectures: pillar (R-form), 2D brick-wall (β-form), and slanting column (γ-form). Fine tuning of the solution reaction conditions afforded crystals of pure R- and γ-form. Grinding and heating of the polymorphs yielded two new products: [MoO2(L)] and [MoO2(L)]n, respectively. At higher temperatures, the metastable [MoO2(L)] irreversibly transforms to [MoO2(L)]n. These solvent-free approaches demonstrate an attractive and efficient green route toward synthesis of new compounds. Furthermore, both compounds easily coordinate the methanol molecule from its vapors, producing exclusively the Rform. Future studies will address the possible selectivity and discriminating properties of the coordinatively unsaturated complexes toward various neutral donors, emphasizing donors’ influence on the crystal architecture and properties of obtained materials. Acknowledgment. We gratefully acknowledge invaluable intellectual contributions of Dr. Ivan Halasz to this work. We are grateful to Ms. Sanja Mestrovic for her expertise in graphical solutions. We thank Dr. Gordana Pavlovic for helpful discussions. This work was supported by the Ministry of Science, Education and Sports of the Republic of Croatia (Grant Nos. 119-1191342-1082 and 119-1193079-1084). Supporting Information Available: Thermogravimetric and infrared spectra analyses of three mononuclear cis-dioxomolybdenum(VI) polymorphs, R, β, and γ. Detailed description of solvent-free
Uzarevic et al. reactions. Thermogravimetric analyses of solvent-free reactions products, GP and HP. Comparison of powder patterns for all above-mentioned products. Selected geometrical parameters and intermolecular C-H O interactions for R-, β-, and γ-forms. This material is available free of charge via the Internet at http://pubs. acs.org.
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