Polymorphism and Pseudopolymorphism of the [Ni ... - ACS Publications

Polymorphism and Pseudopolymorphism of the [Ni(4-Methylpyridine)4(NCS)2] Werner Complex, the Compound that Led to the Concept of “Organic Zeolites”†

CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 6 1185-1194

D. V. Soldatov,*,#,‡ G. D. Enright,# and J. A. Ripmeester# Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa K1A 0R6, Canada, and Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090 Russia Received June 11, 2004

ABSTRACT: The inclusion chemistry of the title complex was revisited in the context of three topical problems of present-day crystal engineering: conventional and pseudo-polymorphism, the design of organic zeolite mimics, and the creation of “third generation” porous metal-organic frameworks. The crystal structures of dense (R) and microporous (β) polymorphs of the complex were redetermined and two new crystal structures were studied: β- and γ-inclusion phases of the complex (1:1 and 1:2 host-to-guest ratio, respectively) with benzene as the simplest aromatic guest. Although both the β- and γ-crystal architectures display a remarkable adaptability toward inclusion of various guests, the mechanisms of this adaptability are essentially different. The microporous β-phase is capable of expanding by 14.5% in response to the size of the included guest or variations in temperature. This flexibility is the highest ever observed for a metal-organic microporous framework and is responsible for the zeolitic behavior of the material. In contrast, the γ-phase shows an ability to transform to a number of architectures that are topologically similar but crystallographically different. Introduction Polymorphism was first described by Mitscherlich in 1822 as the existence of different crystalline forms of the same compound.1 Extensive research in the area of inclusion compounds, conducted over half a century,2,3 has contributed to the further development of polymorphism as a fundamental concept of the chemical science. Polymorphism turns out to be closely related to the inclusion phenomenon. Many versatile hosts demonstrate conventional polymorphism along with a strong affinity for forming inclusion compounds. Striking examples of such hosts are water (at least 10 guest-free polymorphs),4,5 gossypol (seven guest-free polymorphs),6 hydroquinone (three guest-free polymorphs),7 4,5-bis(4-methoxyphenyl)-2-(4-nitrophenyl)-1H-imidazole (three guest-free polymorphs),8 and bis(dibenzoylmethanato)bis(4-vinylpyridine)cobalt(II) (three guest-free polymorphs).9 Pseudopolymorphism is most commonly defined as the existence of structurally different crystalline modifications of a host compound, embracing both unsolvated forms and various solvates (inclusion compounds) of the host.10-13 Indeed, in a broad sense, inclusion compounds may be considered as polymorphic modifications of the host component, which are stabilized by the presence of a templating guest. While some hosts persistently form the same architecture with a wide range of * Corresponding author: Dr. D. V. Soldatov, A. V. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Ac. Lavrent’ev Av. 3, Novosibirsk, Russian Federation 630090. Tel.: +7 3832 391 346. Fax: +7 3832 344 489. E-mail: [email protected]; [email protected]. † Preliminary results of this study were reported on IXth International Seminar on Inclusion Compounds, Novosibirsk (Russia), June 2003. # National Research Council of Canada. ‡ Siberian Branch of the Russian Academy of Sciences.

guests,14-18 others may exhibit a number of inclusion architectures to accommodate different guests.19-23 In addition, there are examples where the same host forms several crystalline clathrates with the same guest,24-36 this phenomenon also being referred to as solvatomorphism.37 Finally, some inclusion compounds have exactly the same composition but different crystal structures, thus being true polymorphs.38,39 The concept of “organic zeolites” was inspired by a remarkable physicochemical behavior of the title complex and its analogues.40,41 The term itself arose as jargon in the Barrer’s research group to define solids able to reversibly and selectively absorb large amounts of hydrophobic (organic) species while showing poor tendency toward sorption of inorganic compounds.42 In other words, the “zeolites” label was used to mark permanent porosity of the new materials (a characterictics of true zeolites43), while the “organic” label was used to stress the hydrophobic nature of the interior pore surface (rather than the organic nature of the host itself). At present, there is a great deal of interest in the design of organic and metal-organic materials mimicking zeolite behavior.44-67 The intrinsic characteristic feature of the permanently porous materials to remain intact upon guest removal helps to justify the concept of pseudopolymorphism showing that many host frameworks may exist independently of the guest solvent, even though in most cases these host frameworks are not stable thermodynamically and exist due to kinetic reasons.41 The preparation and remarkable clathration ability of [Ni(4-MePy)4(NCS)2] (4-MePy ) 4-methylpyridine) and other Werner complexes was first reported in 1957.68 Later studies revealed that the complex exists in either of two polymorphic modifications: a dense

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Crystal Growth & Design, Vol. 4, No. 6, 2004

R-form69 or a microporous β-form.70 Numerous inclusion compounds of the complex fall into three basic types corresponding to three basic architectures. First are the cage-type clathrates which are trigonal (typical space group R3 h ) with a 1:2/3 host-to-guest ratio. The cage-type clathrates form with nitrotoluene and chloronitrobenzene isomers.71 Details of this architecture were obtained from studies of isostructural clathrates of the Mg(II),24,72 Mn(II),73 Cu(II),73 Zn(II),25 and Cd(II)73 host analogues with a 4-MePy guest. As a rule, guest molecules can be extracted from this type of host structure only with chemical decomposition of the host. The second type, which gives rise to “organic zeolites”, the so-called β-phases, are channel-type clathrates which are tetragonal (space group I41/a) with a typical 1:1 host-to-guest limiting stoichiometry. This type of structure forms with a wide range of guests, including p- and m-xylenes,74 methanol,74 p-cymene,75 4-MePy,76 and others.77 This structure is also formed by other similar complexes, including [M(4-MePy)4(NCS)2] with Mg(II),24,72 Fe(II),78,79 Co(II),79,80 and Zn(II)25 as M, [M(4vinylpyridine)4(NCS)2] with Co(II)81 and Ni(II),82 and [Ni(4-ethylpyridine)4(NCS)2].83-85 A remarkable property of β-phases of the title complex and some of its analogues is that they survive total guest removal, producing empty microporous β-polymorphs. The third type, the so-called γ-phases, are layer-type clathrates which are triclinic or monoclinic with a typical 1:2 host-to-guest ratio. The γ-phases form with o-xylene,86,87 p-terphenyl,88 azulene,89 naphthalene,90 and its derivatives.89,91,92 A similar structure was also reported for [Cu(4-MePy)4(NCS)2]*2(4-MePy).93 In these compounds, guest removal is followed by the collapse of the structure. In this study, we determined crystal structures of two inclusion compounds of the title complex with benzene, a β-type clathrate [Ni(4-MePy)4(NCS)2]*(C6H6) and a γ-type clathrate [Ni(4-MePy)4(NCS)2]*2(C6H6). The compounds are interesting as β- and γ-type clathrates containing the simplest aromatic guest. Although the compounds were known,94 their structures have not yet been reported. The first compound exhibits very fast guest loss, while the second presents special problems for the single-crystal X-ray analysis associated with a large unit cell and twinning. We also report redetermined crystal structures of both the dense (R) and the microporous (β) polymorphs of the title complex and discuss the relationship among the various architectures of the complex in the context of the general problem of designing microporous solids. Experimental Preparations. [Ni(4-MePy)4(NCS)2] was synthesized by dissolving Ni(SCN)2 (4.37 g, 25 mmol) in hot ethanol/water (40/60 v/v; 300 mL) and adding to this a solution of 4-MePy (9.13 g; 100 mmol) in ethanol/water (100 mL). The reaction mixture was left to cool while stirring and a blue finecrystalline product was separated. Yield: 10.2 g (18.7 mmol, 75%). This product was the R-polymorph. Large blue prisms of the R-polymorph were obtained by crystallization of the product from nitromethane (cooling of a saturated solution filtered at 80-90 °C). A pure β-polymorph was obtained by decomposition of β-[Ni(4-MePy)4(NCS)2]*(C6H6) in air for one week. Anal.: Calcd. for C26H28N6NiS2 (%): C, 57.05; H, 5.16; N, 15.35. Found for the R-polymorph: C, 57.05; H, 5.23; N, 15.61. Found for the β-polymorph: C, 56.86; H, 5.22; N, 15.24.

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Figure 1. Crystalline samples of phases formed by the [Ni(4-MePy)4(NCS)2] complex: (a) R-polymorph, guest-free; (b) β-polymorph, guest-free phase left after guest benzene release from 1:1 inclusion compound; (c) γ-phase, 1:2 host-to-benzene ratio; (d) sample (c) after 30 min in air.

Figure 2. Scheme showing transformations between different phases formed by the [Ni(4-MePy)4(NCS)2] complex. Phase purity of the polymorphs was confirmed with powder XRD analysis and helium pycnometry measurements. The inclusion compounds were obtained by cooling of a hot saturated solution of the R-polymorph (solubility of the complex in benzene at room temperature is 0.0125 mol/L, or 6.82 mg/mL). Crystals of [Ni(4-MePy)4(NCS)2]*2(C6H6) (γ-phase) formed reproducibly first as violet prisms. After several weeks, they totally regrew into truncated blue octahedra of [Ni(4MePy)4(NCS)2]*(C6H6) (β-phase). Crystalline samples of the isolated forms of the complex are shown in Figure 1. A scheme of the observed phase transformations is given in Figure 2. Single-Crystal XRD. Single crystals of isolated phases were studied on a Bruker SMART CCD X-ray diffractometer (MoKR radiation, λ ) 0.71073 Å, graphite monochromator) equipped with a LT-2A low-temperature device. Crystals of inclusion compounds were studied at low temperature to prevent guest loss. All three experiments on β-phases were run with the same single crystal. First it was removed from the mother solution, instantly frozen, and studied as such at 173 K. After this experiment, the crystal was allowed to lose guest benzene at room temperature for two weeks. To make the process slower and thus to prevent cracking of the crystal,94 it was initially placed together with bulk inclusion material in a loosely closed vial and was exposed to air only after one week. The guest-free β-structure was then determined successively at 298 and 173 K. The integration of the diffraction profiles and an empirical absorption correction utilized the SAINT95 and SADABS96

Polymorphism of a Werner Complex

Crystal Growth & Design, Vol. 4, No. 6, 2004 1187

Table 1. Single-Crystal XRD Experimental Parameters and Crystallographic Data for Polymorphs and Inclusion Compounds of [Ni(4-MePy)4(NCS)2] (“H”) compound temp of study (K) refined guest: host ratio empirical formula formula weight crystal system space group unit cell dimensions: a (Å) b (Å) c (Å) β (°) V (Å3) Z Dcalc (g/cm3) µ (MoKR) (cm-1) crystal color and shape crystal sizes (mm) goodness of fit on F2 final R1, wR2 (I > 2σ(I)) res. extrema (e/Å3)

H, R-form

H, β-form

H, β-form

H*(C6H6), β-form

H*2(C6H6), γ-form

H*2(C6H6), γ-form

298

298

173

173 0.941(3)

173 2.02(7)

125 2.01(4)

C26H28N6NiS2

C26H28N6NiS2

C26H28N6NiS2

547.4 monoclinic P21/c

547.4 tetragonal I41/a

547.4 tetragonal I41/a

C26H28N6NiS2, 0.94(C6H6) 620.9 tetragonal I41/a

C26H28N6NiS2, 2(C6H6) 703.6 monoclinic P21/c

C26H28N6NiS2, 2(C6H6) 703.6 monoclinic P21/c

19.226(3) 9.749(2) 16.791(3) 113.62(1) 2884(1) 4 1.261 8.42 blue prism

16.657(2)

16.597(2)

16.857(2)

22.704(2)

22.610(3)

23.099(3)

6228(1) 8 1.168 7.79 blue truncated octahedron 0.3 0.25 0.15


15.128(1) 15.737(1) 50.041(3) 107.56(1) 11358(1) 12 1.234 6.56 violet prism

0.4 0.25 0.25


15.156(2) 15.747(2) 50.557(6) 107.48(1) 11509(3) 12 1.218 6.48 violet prism 0.5 0.4 0.4

0.4 0.3 0.3

1.020

1.023

1.049

1.058

1.069

0.941

0.040, 0.097

0.033, 0.078

0.028, 0.078

0.028, 0.077

0.071, 0.192

0.051, 0.107

-0.42, +0.52

-0.21, +0.21

-0.17, +0.38

-0.16, +0.56

-0.89, +0.93

-0.39, +1.21

routines, respectively. The unit cell parameters were calculated from the entire data sets. The structures were solved (direct methods) and refined (difference Fourier synthesis) using the SHELXTL package.97 The structural refinement was performed on F2 and applied to all data with positive intensities. Non-hydrogen atoms were refined anisotropically. Selection of crystal data and experimental parameters is summarized in Table 1, and further details as well as full lists of derived results are listed in Supporting Information. Solution difficulties were encountered with [Ni(4-MePy)4(NCS)2]*2(C6H6). The first set of data collected at 173 K led to a preliminary solution with a unit cell a ) 15.156(2), b ) 15.748(2), c ) 48.220(6) Å and all angles close to 90°. A solution in the orthorhombic system with many restrictions on molecular geometry was unstable and left unreasonably high residual extrema near the Ni(II) atoms. To obtain data with higher resolution, we collected another data set (with a different crystal) at 125 K and with the XRD detector moved from the crystal from 4.5 to 9 cm. Analysis of the collected data led to a different solution, with a monoclinic cell derived from the original unit cell setting by a transformation matrix (1 0 0 0 1 h 0 1 h 0 1 h ). The structure was solved in the P21/c monoclinic space group as a twin with the orientation of the minor twin component defined by the orientation matrix (1h 0 0 0 1h 0 2 0 1). The final solutions led to the following results: At 125 K: R ) 0.051 for 57589 unique reflections with I > 2σ(I) and 1283 refined parameters, with the minor fraction of the twin component of 0.377(1). At 173 K: R ) 0.071 for 78660 reflections with I > 2σ(I) and 1283 refined parameters, with the minor fraction of the twin component of 0.441(1). Helium Pycnometry. Measurements were performed on a AccuPyc 1330 gas pycnometer (Micromeritics) at 300 K. The accuracy of the density measurement was tested with NaCl (d ) 2.165 g/cm3), with average of three independent determinations of 2.163(2) g/cm3. Sample mass was 150-250 mg. Crystals of both polymorphs were ground to facilitate sufficient equilibration rate (measurements performed on 2-mm crystals of β-polymorph gave irreproducible results with up to 1.5% deviation in the measured values). The space fraction (%) in the materials accessible to the helium gas was calculated from (1 - dcalc/d(He))*100%, where dcalc is the density calculated from the single-crystal XRD experiment and d(He) is the density of

the metal-organic framework body taken from the pycnometry measurement. Other Measurements. DSC experiments were run on a TA 2920 instrument with 10 mg samples sealed in aluminum pans. The heating rate was 5°/min. The instrument was calibrated using indium. TGA experiments were run on a TA 2050 thermogravimetric analyzer in a linear heating mode (5°/min) under a nitrogen purge. Experiments were started with crystals of the inclusion compounds wetted with mother liquid to prevent guest escape before the experiments. The quantities of the samples were calculated from the mass of the final product, which was assumed to be Ni(SCN)2 (M ) 174.88).

Results and Discussion Isolated Forms and Their Properties. Zeolitic vs Normal Desolvation Behavior. The three forms of the [Ni(4-MePy)4(NCS)2] complex isolated in this work are shown in Figure 1: R (a), β (b), and γ (c) phases. Crystallographic parameters of the phases are listed in Table 1. The R-phase (monoclinic) crystallizes in the form of blue needlelike prisms. The β-phase (tetragonal) may exist either as a guest-free polymorph and as an inclusion compound; from benzene it crystallizes as blue truncated octahedra with a 1:1 host-to-guest ratio. The γ-phase exists only when filled with guest; from benzene it crystallizes as violet prisms with a 1:2 host-to-guest ratio. A scheme showing transformations of the title complex between different forms is given in Figure 2. The pure R-polymorph of the complex may be obtained by crystallization of the complex from nitromethane or ethanol. Crystallization from benzene produces the 1:2 γ-inclusion compound, which slowly regrows into the 1:1 β-inclusion compound. The crystals of the β-phase exhibit zeolitic behavior retaining their microporous

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Figure 3. TGA thermograms of β-[Ni(4-MePy)4(NCS)2]*(C6H6) (1) and γ-[Ni(4-MePy)4(NCS)2]*2(C6H6) (2) plotted as mass/n vs temperature (n is the number of moles of each compound calculated from the mass of the final Ni(SCN)2 product). Each experiment starts with crystals of an inclusion compound wetted with benzene. Sample quantities: (1) 0.037 mmol; (2) 0.060 mmol. Purge gas: N2, 40 mL/min. Heating rate: 5°/min.

structure upon guest release. Fast release of the guest is followed by severe cracking of the crystals due to a significant contraction of the structure (∼5%). Slow guest release produces single crystals of high quality as shown in Figure 1b. In contrast, guest release from the γ-phase causes it to collapse. Fast release yields a dense R-polymorph, while slow release produces significant quantities of the β-inclusion compound, due to secondary interaction of the dense polymorph with benzene vapors, with the final product being the empty β-phase. Visually, collapse of the γ-structure looks like bleaching of the crystals due to their regrowing into a new fine-crystalline phase starting at the surface (Figure 1d). The difference in behavior of the β- and γ-phases upon guest release is also apparent in TGA experiments (Figure 3). Heating of wet crystals falls into several distinctive steps: (1) evaporation of excess solvent; (2) release of guest benzene; (3) release of first 4-MePy ligand; (4) release of second 4-MePy ligand; (5) release of the remaining two 4-MePy ligands. The final product, [Ni(SCN)2], was used to calculate expected intermediate phase compositions in the process as shown in Figure 3. Step 2, corresponding to the release of the guest from the structure, essentially differs for these two inclusion compounds. Guest release from the γ-phase occurs in a rather narrow temperature range surrounded by two plateaus corresponding to the initial and final phases. Such desolvation, characteristic of a vast majority of inclusion compounds,98 is followed by collapse of the inclusion γ-phase into the dense R-polymorph, and the dissociation equilibrium involves two solid phases:

γ-[Ni(4-MePy)4(NCS)2]*2(C6H6)solid ) R-[Ni(4-MePy)4(NCS)2]solid + 2(C6H6)gas In contrast, guest release from the β-phase is continuous over the whole range until the decomposition of the host complex itself. Such desolvation behavior is characteristic of zeolites;40-42 the inclusion β-phase does not

Soldatov et al.

Figure 4. The molecule of the [Ni(4-MePy)4(NCS)2] complex as found in the R-polymorph. Thermal ellipsoids are at 50% probability level.

collapse, and the dissociation equilibrium involves only one solid phase:

β-[Ni(4-MePy)4(NCS)2]*x(C6H6)solid ) β-[Ni(4-MePy)4(NCS)2]*(x-δ)(C6H6)solid + δ(C6H6)gas (x ) 0÷1; δ ) 0÷x) Comparison of r- and β-Guest-Free Polymorphs. The R-polymorph is a dense polymorph of the complex. Its density calculated from XRD data (1.261(1) g/cm3) and the density obtained from helium pycnometry (1.266(2) g/cm3) are in good agreement, indicating negligible space inside the structure (0.4(1)%) accessible to helium. The β-polymorph is microporous with crystallographic density of 1.154(1) g/cm3 and density of the “host framework body” obtained from helium pycnometry of 1.268(3) g/cm3, that is, 9.0(1)% of the space in the structure is freely accessible to the helium gas. DSC experiments indicate that the R-polymorph is stable up to 482 K (in a sealed pan), in good accordance with previous studies.99 The β-polymorph is metastable; it shows an exotherm at ∼385 K collapsing into the more stable R-polymorph. The enthalpy of the β f R transition of 5.0(2) kJ/mol (385 K) is slightly higher than that determined by other methods at room temperature, 3.5(1) kJ/mol.100 These values are comparable to the differences in stability between β- and R-polymorphs of other known hosts, 0.5(1) kJ/mol for hydroquinone101 and 1.31(5) kJ/mol for a copper diketonate.102 The increased stability of the R-polymorph may be attributed to its closer packing. The crystal structures of both polymorphs are of the van der Waals type. The molecules of the complex adopt a propeller-type conformation in both structures (Figure 4). The nickel(II) atom is octahedrally coordinated by six N atoms, four from 4-MePy ligands in the equatorial plane and two from the isothiocyanate groups situated axially. The 4-MePy planes form dihedral angles of 5060° with the equatorial plane of the complex. The geometry of the molecule in the two polymorphs is similar and will not be discussed in more detail. A tabulated comparison is given in Supporting Information. The crystal packing in the R- and β-polymorphs is illustrated in Figures 5 and 6. In the R-polymorph, chains of interdigitating molecules (separately shown



Figure 5. Crystal packing in R-[Ni(4-MePy)4(NCS)2]: (a) projection along c; (b) projection along b; (c) chain of interdigitating molecules along c.

in Figure 5c) stretch along the c-axis with the distance between nickel atoms of 8.4 Å. These chains are packed in a roughly hexagonal fashion (Figure 5a). In the β-polymorph, a similar packing fragment may be seen, but the chains are truncated into dimers with a distance between nickel atoms of 8.5 Å in the dimer and 14.2 Å between molecules of adjacent dimers. These “chains” are organized in a square-grid packing arrangement (Figure 6a). It is easy to comprehend a smooth transition between the undulated layer in the R-polymorph (Figure 5b) into the flat layer in the β-polymorph (Figure 6b) and vice versa. Such a transition would involve slight translational movement of every second chain to form a flat layer and the extension of the structure along the chain direction at the expense of cutting the chains into dimers. However, in the R-polymorph the adjacent layers are related by b translation, while in the β-polymorph they are related by centrosymmetry. In other words, the phase transition would require extreme translational and rotational movement and so must be difficult kinetically. Therefore, the existence of two guest-free polymorphs for the title complex is possible due to the relatively small difference in thermodynamic stability and a significant kinetic barrier arising from essentially different packing modes. The kinetic stability of the microporous β-polymorph is surprising as the structure is built entirely from van der Waals forces linking simple structural units. Current approaches to the design of porous architectures typically utilize strong coordination and covalent bonds to prevent their collapse into dense structures. Examples of such architectures include 1D,103-120 2D,64,121-128 and 3D51-54,129-135 host species and host frameworks. Robust microporous architectures built

upon discrete units are less common.44-48,136-148 Most of these discrete structures either comprise cumbersome macrocycles,136-143 or make use of interactions stronger than van der Waals, such as intermolecular hydrogen bonds44,45,145,146 or secondary 3D coordination assembly.48 Structure of β-[Ni(4-MePy)4(NCS)2]*(C6H6) and Flexibility of the β Matrix. Compared to the empty β-polymorph, the structure of the benzene inclusion compound is expanded in volume by 5.4%. This expansion occurs because of elongation of both a (0.26 Å) and c (0.49 Å) parameters. The guest benzene molecules are located in zigzag channels running in all three dimensions (Figure 7). The system of channels has a distorted diamondoid topology. The point where four channels meet, having -4 point symmetry, was previously referred to as the “large” cavity, and a segment between two such intersections, having -1 point symmetry, was referred to as the “small” cavity (in Figure 7b the small cavities are represented by rods). Guest molecules exclusively occupy small cavities in the structure, with an occupation of 0.941(3), close to the limiting 1:1 guestto-host molar ratio. The results of this and previous studies make it possible to rationalize data on the flexibility of the β-matrix. The comparison of structural parameters of the β-matrix found in various inclusion compounds is given in Table 2. In this comparison, we take the structure of the empty β-polymorph at 298 K as a reference standard. It should be noted that earlier determination of the structure70 reported a unit cell volume 0.8% larger (6350 Å) than that obtained from our results. Such a larger volume might be observed because of the presence of residual guest quantities in the structure undetectable by the structural analysis at that time. As can be seen from Table 2, the thermal

1190


Figure 6. Crystal packing in β-[Ni(4-MePy)4(NCS)2]: (a) projection along c; (b) half of unit cell content projected along b.

contraction of the matrix upon cooling on 125 K is 1.13(2)%. Inclusion of benzene and larger aromatic molecules causes expansion of 5.4 to 13.2%, and the entire range is 14.5%. Flexibility and microporosity are somewhat contradictory properties. Flexibility is facilitated by weaker interactions in the materials, while microporosity requires robustness and overall stability of the architecture. Various strategies were utilized to provide an appropriate balance of strong and weak interactions within a designed architecture to make it robust and flexible at the same time. Van der Waals packing of organic 1D polymeric molecules was extensively used,103-118 although overall sorption in most such materials is, in fact, a sum of sorption and dissolution. Organic and metal-organic 1D polymers forming socalled “wheel-and-axle” or “ladder-and-platform” architectures also have been reported.119,120 There are few reports on flexible microporous architectures of higher dimensionality.48,64,124-134 For example, a 2.5% unit cell volume contraction was reported, due to a scissor-like shift in interlocked layers of a 2D coordination polymer.124 A 3% contraction was reported for a 3D coordi-

Soldatov et al.

Figure 7. Crystal packing in β-[Ni(4-MePy)4(NCS)2]*(C6H6): (a) guest benzene species filling zigzag channels. Host molecules are blue. Guest molecules are from yellow (nearer) to red (farther). (b) Pseudo-diamondoid 3D system of channels in the structure outlined by the rods. Table 2. Selected Parameters of β-Phases Formed by [Ni(4-MePy)4(NCS)2] included guest

T (K)

a (Å)

a (Å)

V (Å3)

volume change (%)a -1.13(2)b

no guest

173 16.597(2) 22.610(3)

6228(1)

no guest

298 16.657(2) 22.704(2)

6299(1) 0

benzene

173 16.857(2) 23.099(3)

6564(1) +5.39(1)

p-xylene 4-MePy p-cymene m-xylene total change

298 298 298 298

b

16.98(1) 17.09(1) 17.105(9) 17.28(1) 0.68

23.62(4) 23.44(1) 23.84(1) 23.87(2) 1.20

6810(20) 6846(17) 6974(11) 7128(14) 900

+8.1(3) +8.7(3) +10.7(2) +13.2(2) 14.5

ref this work this work this work 74 76 75 74

a With respect to the empty β-phase at the same temperature. With respect to the empty β-phase at room temperature.

nation polymer, due to distortion in a coordination polyhedron of Ag(I).149 More flexible 3D architectures utilized angular flexibility of secondary coordination bonds, to give 8.1%150 and 8.6%48 changes in the unit cell volume. In an overall comparison of such structures,



crystallographically distinct host molecules (A, B, and C). The second and fifth layers are built by C molecules. The rest of the layers are formed by A and B molecules in the same layer. The isothiocyanate groups pillar the interlayer space dividing it into a square-grid system of channels (Figure 8b). Unlike the β-phase, the γ-phases of the title complex vary in terms of crystallographic parameters such as crystal system, space group, and unit cell dimensions. Nevertheless, topologically they all are isomorphous. The distance between host metal centers in the layer varies within 10.8-11.9 Å and the interlayer distance varies within 7.8-8.4 Å. Ten known structures, including the one reported here, fall into four groups on the number of translationally inequivalent layers: (1) First order (adjacent layers are related by translation). Inclusion compounds with 2-methylnaphthalene,91 2-bromonaphthalene,91 and p-terphenyl.88 All three crystallize in the P1 h space group, with the unit cell volume roughly at 1000 Å3. The latter compound has a 1:1 host-to-guest ratio due to size of the guest. (2) Second order (every second layer is translationally equivalent). No examples of the title complex were reported; this type has been reported for [Cu(4-MePy)4(NCS)2]*2(4-MePy).93 P21 space group, unit cell volume about 2000 Å3. (3) Fourth order. Compounds with 1-methylnaphthalene,92 1-bromonaphthalene,89 azulene,89 o-xylene,86,87 and naphthalene.90 The first four crystallize in the P21/c (P21/n) space group with unit cell volume of more than 4000 Å, the last one in C2/c with unit cell volume of more than 8000 Å3. (4) Sixth order. [Ni(4-MePy)4(NCS)2]*2(C6H6) studied in this work is the only reported example. Space group P21/c and a unit cell volume close to 12000 Å3. The above comparison demonstrates the adaptability of the γ-architecture which easily transforms into several distinct modifications to accommodate various guests. Conclusion

Figure 8. Crystal packing in γ-[Ni(4-MePy)4(NCS)2]*2(C6H6): (a) projection along b. Guest benzene molecules are outlined as stick-and-balls. (b) A layer of guest species (orange). Host molecules are blue.

the β-matrix of the title complex remains the most flexible of all microporous crystal architectures reported so far. Structure of γ-[Ni(4-MePy)4(NCS)2]*2(C6H6) and Comparison of γ-Phases. This inclusion compound belongs to the layer type (Figure 8). Although all intermolecular interactions are van der Waals, there are distinct layers of closely packed host molecules and guest molecules filling the space between these layers (Figure 8a). The host layer is formed by 4-MePy moieties. The distance between closest host metal centers in the layer varies within 10.8-11.2 Å. The unit cell comprises six layers stacked on different levels on the c axis at a distance of 8.0 Å. There are three

In this work, we revisit inclusion chemistry of a Werner type host complex [Ni(4-MePy)4(NCS)2]. The variety of architectures and the specificity of physicochemical behavior of the materials it forms are relevant to three topical problems of present-day crystal engineering: conventional and pseudopolymorphism,10-13,31-39,151-155 design of organic zeolite mimics,40,42,44-67,119,156,157 and creation of “third generation” porous metal-organic frameworks.3,66,98,158,159 The title complex forms a great number of inclusion compounds, but even compounds with the same guest demonstrate essentially different physicochemical properties. The β-phase of the complex shows zeolitic behavior, surviving upon total guest removal, and a remarkable flexibility, expanding by 5-14% upon guest inclusion. In contrast, the γ-phase of the complex collapses upon desolvation and undergoes a series of minor phase transformations to include various guests. In conclusion, it should be noted that some properties of the title complex and its analogues may well be of practical interest. The utilization of inclusion materials for gas storage has become a reality as the first industrial pilot plant started the production of natural

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gas hydrates in 2003.160 According to Allison and Barrer,40 the β-forms of [Ni(4-MePy)4(NCS)2] and [Co(4-MePy)4(NCS)2] absorb significant quantities of gases (O2, N2, CO2, CS2, CH4, C2H6, CH3Cl, Ar, Kr, Xe). For example, the Ni-complex absorbs up to ∼5.8 wt % (∼82 mL/g) of methane (approximately 1:2 complex-tomethane ratio). This value is comparable to methane quantities absorbed by zeolite NaX (88 mL/g),161 zeolite 13X (40 mL/g),98 and a series of recently developed coordination polymers (15-85 mL/g).98,122,129 Acknowledgment. D.V.S. acknowledges support of this research received during his work at the Steacie Institute. Supporting Information Available: Table comparing geometry of the [Ni(4-MePy)4(NCS)2] molecule in studied compounds. Crystal data in the CIF format for six structures studied (as listed in Table 1). This material is available free of charge via the Internet at http://pubs.acs.org.

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Polymorphism and Pseudopolymorphism of the [Ni ... - ACS Publications

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