Extension to Oligomeric and Polymeric Host Receptors with

2 Jan 2018 - DBMs), are extended to polynuclear host receptors by connecting metal DBM units with bridging ligand 4,4′-dipyridyl (bipy). Series of n...
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Modified Metal Dibenzoylmethanates for Soft Supramolecular Materials: Extension to Oligomeric and Polymeric Host Receptors with Nanosized Void Spaces† D. V. Soldatov,‡ P. Tinnemans,§ G. D. Enright, C. I. Ratcliffe, P. R. Diamente,| and J. A. Ripmeester* Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6 Received October 2, 2002. Revised Manuscript Received April 9, 2003

Recently reported molecular host complexes, modified metal dibenzoylmethanates (metal DBMs), are extended to polynuclear host receptors by connecting metal DBM units with bridging ligand 4,4′-dipyridyl (bipy). Series of new solvent-free as well as inclusion materials were prepared and studied with XRD, NMR, and other methods. Template-directed synthesis gave inclusion compounds with three distinct types of host receptors: dimeric [Zn2(bipy)(DBM)4], trimeric [Zn3(bipy)2(DBM)6], and polymeric [Zn(bipy)(DBM)2]n complexes. The dimeric complex was isolated as a solvent-free compound and as inclusion compounds with tert-butylbenzene and fluorobenzene (host per guest molar ratios of 1:1 and 1:2/3, respectively). The trimeric complex was prepared as an inclusion compound with DMSO (1:5 molar ratio). The polymeric complex was observed in inclusion compounds with fluorobenzene (host unit per guest molar ratio 1:2), tetrahydrofuran (1:2), 2-pentanone (1:2), bipy/DMSO (1:1/2:1/2), and bipy/tert-butylbenzene (1:1:2); a solvent-free material was obtained upon desolvation of tetrahydrofuran and 2-pentanone inclusions. The isolated host species have five-coordinated terminal zinc centers (two chelating DBMs and a singly coordinating bipy) and octahedrally coordinated intrinsic zinc centers (two chelating equatorial DBMs and two singly coordinating bipys). The bipy ligands bridge the zinc centers to give the di-, tri-, and polynuclear complexes. Nanosized voids between the DBM units coordinated to adjacent zinc centers form the basis for cages and channels in the inclusion materials. These voids have, in many cases, a typical size of between 1 and 2 nm and accommodate up to five molecules per single cavity or two rows of guest molecules in a single channel. When compared to previously reported metal DBMs, the multinuclear metal DBM receptors reported here exhibit stronger inclusion affinity and enhanced inclusion capacity.

Introduction Modified metal dibenzoylmethanates (metal DBMs)1-5 constitute a new class of host complexes that exhibit a remarkable versatility for producing soft supramolecular materials. The most attractive features of such soft materials are the wide variability in structure and selectivity, their easy conversion from one structure to * Corresponding author. E-mail: [email protected]. Fax: (613) 998-7833. † This is part VI of the series Modified Metal Dibenzoylmethanates and Their Clathrates; for the previous contributions see refs 1-5. ‡ Permanent address: Institute of Inorganic Chemistry, Lavrentyeva 3, Novosibirsk, 630090 Russia. § On leave of absence from the Fontys University of Professional Education, Eindhoven, The Netherlands. | Research conducted as a part of study program at Carleton University, Ottawa, Canada. (1) Soldatov, D. V.; Enright, G. D.; Ripmeester, J. A. Supramol. Chem. 1999, 11, 35-47. (2) Soldatov, D. V.; Ripmeester, J. A. Supramol. Chem. 2001, 12, 357-368. (3) Soldatov, D. V.; Ripmeester, J. A. Chem. Eur. J. 2001, 7, 29792994. (4) Soldatov, D. V.; Enright, G. D.; Ratcliffe, C. I.; Henegouwen, A. T.; Ripmeester, J. A. Chem. Mater. 2001, 13, 4322-4334. (5) Soldatov, D. V.; Enright, G. D.; Ripmeester, J. A. Chem. Mater. 2002, 14, 348-356.

another, and their transient response to external stimuli. These attributes provide good prospects for the use of these materials as “third-generation microporous frameworks”,6 “functional organic zeolite analogues”,7 “metrically engineered host frameworks”,8 supramolecular ion exchange materials,9,10 artificial ion conductors,11 “flexible sorbents”12 and “smart sorbents”,13 and materials switchable on the molecular level through coordination,14,15 isomerization,16 and other reversible in situ changes.17 (6) Kitagawa, S.; Kondo, M. Bull. Chem. Soc. Jpn. 1998, 71, 17391753. (7) Endo, K.; Sawaki, T.; Koyanagi, M.; Kobayashi, K.; Masuda, H.; Aoyama, Y. J. Am. Chem. Soc. 1995, 117, 8341-8352. (8) Holman, K. T.; Pivovar, A. M.; Swift, J. A.; Ward, M. D. Acc. Chem. Res. 2001, 34, 107-118. (9) Min, K. S.; Suh, M. P. J. Am. Chem. Soc. 2000, 122, 6834-6840. (10) Dalrymple, S. A.; Shimizu, G. K. H. Chem. Eur. J. 2002, 8, 3010-3015. (11) Tanaka, Y.; Kobuke, Y.; Sokabe, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 693-694. (12) Cussen, E. J.; Claridge, J. B.; Rosseinsky, M. J.; Kepert, C. J. J. Am. Chem. Soc. 2002, 124, 9574-9581. (13) Nossov, A. V.; Soldatov, D. V.; Ripmeester, J. A. J. Am. Chem. Soc. 2001, 123, 3563-3568. (14) Albrecht, M.; Lutz, M.; Spek, A. L.; Koten, G. Nature 2000, 406, 970-974.

10.1021/cm020987z CCC: $25.00 Published 2003 by the American Chemical Society Published on Web 09/06/2003

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Figure 1. Schematic representation of metal dibenzoylmethanate complexes: designations for (a) metal(II), (b) DBM anionic ligand, (c) pyridine-type ligand and (d) 4,4′-dipyridyl ligand; (e) monomeric complex extended with pyridine; (f) oligoor polymeric complex with metal centers connected by dipyridyl (dimer at k ) 0, trimer at k ) 1, and polymer at k .1).

The metal dibenzoylmethanate hosts studied so far are mononuclear complexes, as shown schematically in Figure 1e, where an M(II) center (a) is chelated by two DBM anions (b) in the equatorial plane and coordinated by two axial pyridine ligands (c). Modification of the archetype by changing the M(II) center and the pyridine ligand produced a variety of molecular host complexes with the ability to enclathrate a wide range of organic guests in their van der Waals lattices.1-5 Now this new family of materials has been explored further in a series of compounds in which the positioning of the metal DBM planar units is restricted by connecting them with a bidentate bridging ligand (Figure 1f); 4,4′-dipyridyl (Figure 1d) has been selected in the present work. Most studies on coordination polymers employing di- and polypyridyl ligands reported to date were targeted toward constructing rigid frameworks and cavities, as this strategy allowed for a high degree of predictability.18-22 In contrast, the purpose of this work was to create materials with flexible architecture and at the same time to enhance the ability to entrap guest species. Some noncrystalline organic poly(15) Ma¨kinen, S. K.; Melcer, N. J.; Parvez, M.; Shimizu, G. K. H. Chem. Eur. J. 2001, 7, 5176-5182. (16) Soldatov, D. V.; Ripmeester, J. A.; Shergina, S. I.; Sokolov, I. E.; Zanina, A. S.; Gromilov, S. A.; Dyadin, Yu. A. J. Am. Chem. Soc. 1999, 121, 4179-4188. (17) Soldatov, D. V.; Ripmeester, J. A. Stud. Surf. Sci. Catal. 2002, 141, 353-362.

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mers possess this type of bulk property and have been studied already;23-27 however, they do not display the propensity for easily controlled assembly-disassembly inherent to most coordination polymers. Enhancing the inclusion propensity of known types of molecular hosts by extending them to oligomeric or polymeric species seems attractive. Surprisingly, so far only a few such attempts have been made. Most work in this field was performed on the macrocyclic metalloporphyrin hosts.28-33 Chelated complexes seem to have greater potential as they are more numerous and easier to synthesize, but only two reports on metal xanthates are available.34,35 This work shows the extension of the metal chelate hosts as a tool for the systematic generation of new oligomeric and polymeric materials with a very strong affinity for guest inclusion. Our group initiated the work on oligomerization with nickel and cobalt DBMs. The materials prepared initially displayed an obvious ability to reversibly sorb a variety of organic components, sometimes with a color change evident. The characterization of these materials proved difficult as only powderlike impure mixtures were isolated, and neither composition nor structure could be determined properly. Substantial progress in this work was achieved after zinc DBM, which forms weaker coordination compounds, was utilized. In many cases, zinc materials were of acceptable quality, with a number of crystalline inclusion products being isolated. This paper reports selected results clearly demonstrating the templating ability of a guest solvent in the formation of certain oligomeric or polymeric host complexes. Experimental Section Preparations. Chemicals. Zinc dibenzoylmethanate (Zn(DBM)2) was prepared as reported elsewhere.36 The main chemicals and solvents were from Aldrich, with purity of 98% for 4,4′-dipyridyl (bipy) and 99% or higher for tert-butyl(18) Robson, R. J. Chem. Soc., Dalton Trans. 2000, 3735-3744. (19) Zaworotko, M. J. Chem. Commun. 2001, 1-9. (20) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 16291658. (21) Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.; Kusukawa, T.; Biradha, K. Chem. Commun. 2001, 509-518. (22) Dinolfo, P. H.; Hupp, J. T. Chem. Mater. 2001, 13, 3113-3125. (23) Ichiraku, Y.; Stern, S. A.; Nakagawa, T. J. Membr. Sci. 1987, 34, 5-18. (24) Witchey-Lakshmanan, L. C.; Hopfenberg, H. B.; Chern, R. T. J. Membr. Sci. 1990, 48, 321-331. (25) Volkov, V. V. Polym. J. 1991, 23, 457-466. (26) Srinivasan, R.; Auvil, S. R.; Burban, P. M. J. Membr. Sci. 1994, 86, 67-86. (27) Ilinitch, O. M.; Fenelonov, V. B.; Lapkin, A. A.; Okkel, L. G.; Terskikh, V. V.; Zamaraev, K. I. Microporous Mesoporous Mater. 1999, 31, 97-110. (28) Kumar, R. K.; Balasubramanian, S.; Goldberg, I. Inorg. Chem. 1998, 37, 541-552. (29) Kumar, R. K.; Diskin-Posner, Y.; Goldberg, I. J. Inclusion Phenom. 2000, 37, 219-230. (30) Diskin-Posner, Y.; Dahal, S.; Goldberg, I. Angew. Chem., Int. Ed. 2000, 39, 1288-1292. (31) Goldberg, I. Chem. Eur. J. 2000, 6, 3863-3870. (32) Imamura, T.; Fukushima, K. Coord. Chem. Rev. 2000, 198, 133-156. (33) Diskin-Posner, Y.; Patra, G. K.; Goldberg, I. Chem. Commun. 2002, 1420-1421. (34) Krueger, A. G.; Winter, G. Aust. J. Chem. 1971, 24, 13531359. (35) Gable, R. W.; Hoskins, B. F.; Winter, G. Inorg. Chim. Acta 1985, 96, 151-159. (36) Soldatov, D. V.; Henegouwen, A. T.; Enright, G. D.; Ratcliffe, C. I.; Ripmeester, J. A. Inorg. Chem. 2001, 40, 1626-1636.

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Table 1. Low-Temperature Single-Crystal XRD Analysis: Experimental Parameters and Crystallographic Data compound

host/guest ratio (ideal) host/guest ratio (refined) gross formula formula unit mass crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dcalc, g cm-3 µ(Mo KR), cm-1 T, °C normal crystal habit crystal size, mm reflns collected unique obs reflns (I > 2σ(I)) refined parameters R1/wR2 (obsd data) GOF on F2 res density, e Å-3

[Zn2(bipy)(DBM)4]‚ (tert-butylbenzene)

[Zn2(bipy)(DBM)4]‚ 2/ (fluorobenzene) 3

[Zn3(bipy)2(DBM)6]‚ 5(DMSO)

[Zn(bipy)(DBM)2]n‚ 2n(fluorobenzene)

1:1 1:0.994(4) (C70H52N2O8Zn2), (C10H14) 1314.1 triclinic P-1 (No. 2) 10.737(2) 12.277(2) 13.456(2) 88.45(1) 82.59(1) 68.14(1) 1632.0(5) 1 1.337 7.95 -100 plate 0.2 0.3 0.3 19535 6695 485 0.037/0.093 1.035 +0.37/-0.50

1:2/3 1:0.67(1) (C70H52N2O8Zn2), 2/ (C H F) 3 6 5 1243.9 triclinic P-1 (No. 2) 12.242(2) 13.629(2) 27.183(3) 80.45(1) 87.45(1) 88.37(1) 4467(1) 3 1.387 8.69 -100 block 0.1 0.3 0.3 53254 14312 1157 0.068/0.138 1.030 +0.69/-0.45

1:5 1:4.91(2) (C110H82N4O12Zn3), 5(C2H6OS) 2238.6 triclinic P-1 (No. 2) 12.820(1) 13.074(1) 18.821(3) 72.92(1) 81.78(1) 62.54(1) 2675.6(5) 1 1.389 8.34 -100 prism 0.2 0.2 0.2 33965 11312 790 0.041/0.095 1.050 +0.48/-0.38

1:2 1:1.983(5) (C40H30N2O4Zn), 2(C6H5F) 860.2 orthorhombic Pccn (No. 56) 19.299(3) 19.866(3) 22.892(3) 90 90 90 8777(2) 8 1.302 6.16 -100 prism 0.2 0.2 0.5 100344 7038 567 0.046/0.120 1.024 +0.60/-0.53

compound [Zn(bipy)(DBM)2]n‚ 2n(THF) host/guest ratio (ideal) host/guest ratio (refined) gross formula formula unit mass crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dcalc, g cm-3 µ(MoKR), cm-1 T, °C normal crystal habit crystal size, mm reflns collected unique obs reflns (I > 2σ(I)) refined parameters R1/wR2 (obsd data) GOF on F2 res density, e Å-3 a

1:9/4 1:1.97(1) (C40H30N2O4Zn), 1.97(C4H8O) 809.8 triclinic P-1 (No. 2) 12.785(2) 17.880(2) 20.050(3) 74.33(1) 79.43(1) 69.20(1) 4106(1) 4 1.310 6.50 -100 needle 0.1 0.2 0.5 48866 14864 1400 0.046/0.133 1.129 +0.60 / -0.46

[Zn(bipy)(DBM)2]n‚ xn(2-pentanone) 1:9/4a (C40H30N2O4Zn), 2(C5H10O)a triclinic P-1 (No. 2)a 12.70(1) 17.58(2) 20.26(3) 74.45(7) 79.70(8) 70.42(8) 4086(8) 4a 1.37a 6.6a -100 needle 0.05 0.3 0.5 67

[Zn(bipy)(DBM)2]n‚ 1 2n(bipy)‚ /2n(DMSO)

1/

1:1/2:1/2 1:0.505(1):0.498(1) (C40H30N2O4Zn), 1/ (C H N ), 1/ (C H OS) 2 10 8 2 2 2 6 785.2 triclinic P-1 (No. 2) 13.006(2) 13.322(2) 22.970(3) 99.79(1) 95.59(1) 102.04(1) 3799(1) 4 1.373 7.25 -100 prism 0.2 0.3 0.5 45243 16383 993 0.034/0.078 1.034 +0.34 / -0.32

[Zn(bipy)(DBM)2]n‚ n(bipy)‚2n(tert-butylbenzene) 1:1:2 1:1.010(3):2.001(7) (C40H30N2O4Zn) (C10H8N2), 2(C10H14) 1092.6 monoclinic C2/c (No. 15) 23.109(3) 25.834(4) 10.185(2) 90 104.61(1) 90 5884(2) 4 1.233 4.70 -100 prism 0.3 0.3 0.4 34629 6014 455 0.051/0.113 1.068 +0.47 / -0.30

Assumed values based on apparent isostructurality with THF inclusion.

benzene, fluorobenzene, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and 2-pentanone. The remaining solvents were of reagent grade quality or higher. General Crystallization Procedure. The synthesis of most products was performed in one step. Zn(DBM)2 (256 mg, 0.5 mmol) was dissolved in a minimal amount of hot solvent (∼10 mL or more). To this solution was added a hot solution of bipy (86 mg, 0.55 mmol) dissolved in 5 mL of the same solvent. The solution was allowed to cool slowly. Depending on the solvent used, crystalline products formed either instantly or after several hours and, in some cases, after several days. Each crystallization was performed at least twice for consistency. With some solvents, a variable Zn(DBM)2-to-bipy ratio was used.

Crystallizations from tert-Butylbenzene. More than 20 crystallizations were performed from this solvent under various conditions, but satisfactory reproducibility was not achieved. Crystals of two inclusion compounds for single-crystal XRD analysis were found by random selection. For elucidating the composition of the solid phase in this system, a series of six samples was prepared and studied by the powder XRD method. Each sample contained 0.5 mmol (256 mg) of Zn(DBM)2, 70 mmol (9.4 g) of tert-butylbenzene, and a variable amount of bipy: 0.5, 0.75, 1.0, 1.5, 2.5, and 5.0 mmol for samples 1-6, respectively. A drop of pyridine was added to each sample to accelerate ligand exchange. The samples were equilibrated with stirring for 2 days at 50 °C and for the next 2 days at room temperature. It was assumed that the system

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Table 2. Room-Temperature Unit Cell Parameters from Single-Crystal XRD Experiments compound

a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Za Dcalc, g cm-3 a no. of used reflns normal cryst habit crystal size, mm a

[Zn2(bipy)(DBM)4]‚ (tert-butylbenzene)

[Zn2(bipy)(DBM)4]‚ 2/ (fluoro3 benzene)

10.770(3) 12.372(3) 13.553(5) 88.25(2) 82.74(3) 68.65(2) 1668(1) 1 1.308 341 plateb 0.2 0.3 0.3b

12.371(3) 13.676(3) 27.386(7) 80.90(3) 87.01(2) 88.29(2) 4568(2) 3 1.357 247 block 0.1 0.3 0.4

[Zn3(bipy)2-(DBM)6]‚ 5(DMSO)

[Zn(bipy)(DBM)2]n‚ 2n(fluorobenzene)

[Zn(bipy)(DBM)2]n‚ 1/ n(bipy)‚ 2 1/ n(DMSO) 2

[Zn(bipy)(DBM)2]n‚ n(bipy)‚ 2n(tert-butylbenzene)

12.952(3) 13.194(4) 18.868(5) 73.04(2) 81.71(2) 62.75(2) 2742(1) 1 1.356 270 prism 0.2 0.3 0.4

19.559(4) 20.121(4) 22.932(4) 90 90 90 9025(3) 8 1.266 488 prism 0.3 0.5 0.5

13.120(2) 13.402(3) 23.052(5) 99.77(2) 95.62(2) 102.01(2) 3870(1) 4 1.348 470 prismb 0.2 0.3 0.5b

23.203(5) 26.241(7) 10.264(2) 90 104.65(2) 90 6046(2) 4 1.200 344 prism 0.2 0.3 0.4

Assumed values. b The same crystal as for the low-temperature XRD experiment was used.

attained equilibrium as no significant change was detected upon further stirring. The solid phase was separated and studied with powder XRD. By comparing experimental and theoretical powder diffractograms, solid phases from samples 1 and 6 corresponded to pure [Zn2(bipy)(DBM)4]‚(tert-butylbenzene) and [Zn(bipy)(DBM)2]n‚n(bipy)‚2n(tert-butylbenzene), respectively (see Supporting Information). Composition of the solid phase from other samples was estimated from intensities of five reference reflections belonging to each of the two phases. [Zn(bipy)(DBM)2]. Zn(DBM)2 (1.02 g, 2 mmol) and bipy (0.328 g, 2.1 mmol) were dissolved in hot THF (∼20 mL); cooling of the clear solution resulted in needlelike crystals of the inclusion compound with THF. The crystals were separated and desolvated in air for several hours to give 1.1 g (80%) of final product. See Results and Discussion for characterization. Methods. Thermogravimetric Analysis. A 2050 thermogravimetric analyzer (TA Instruments) was utilized. Samples of 10-20 mg were studied in a linear heating mode (5 deg/ min) under a nitrogen purge. Crystals of the inclusion compounds were taken from the corresponding mother solutions, crushed, and pressed to dryness between two pieces of blotting paper. Powder XRD Analysis. Phase analyses were performed with a Rigaku Geigerflex diffractometer (Co KR radiation, λ ) 1.7902 Å) in a 5-30° 2θ range, with a 0.02° step scan with 1 s/step. Inclusion compounds with volatile guests were recorded in an atmosphere of the correspondent guest solvent. For theoretical powder diffractograms, the low-temperature singlecrystal analysis results (Table 1) were used with unit cell dimensions determined at room temperature (Table 2). Single-Crystal XRD Analysis. Single-crystal diffraction experiments were performed with crystals, or chips cut therefrom, taken directly from under their respective mother solutions and cooled immediately to -100 °C. A Siemens SMART CCD X-ray diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.7107 Å) was used to collect diffraction data. An empirical absorption correction SADABS37 was applied. The final unit cell parameters were obtained using the entire data sets. The structures were solved by direct methods followed by differential Fourier synthesis, using the SIR9238 and SHELXTL 37 packages. The structural refinement was performed on F2 using all data with positive intensities. Crystal data and experimental details of the low-temperature experiments are listed in Table 1. (37) Sheldrick, G. M. SHELXTL PC, Ver. 4.1. An Integrated System for Solving, Refining and Displaying Crystal Structure from Diffraction Data; Siemens Analytical X-ray Instruments, Inc.: Madison, WI, 1990. (38) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Gualardi, A. J. Appl. Crystallogr. 1993, 26, 343-350.

The results of guest occupancy refinement are shown in Table 1.39 Significant deviations from ideal stoichiometry were observed only for the THF inclusion compound; therefore, for all the remaining structures, total guest occupancy was fixed at ideal values in final cycles of refinement.40 The largest residual extrema on the final difference map were about zinc atoms or were associated with disordered guest molecules. Specific solution problems encountered in most difficult cases are summarized below. [Zn2(bipy)(DBM)4]‚(tert-butylbenzene): Residual centers on the electronic density difference map indicated disordering of the host dimer; a satisfactory solution was obtained with the Zn-bipy-Zn fragment disordered over two positions. [Zn3(bipy)2(DBM)6]‚5(DMSO): Testing a preliminary solution of the structure with PLATON41 indicated missed translation symmetry corresponding to a halved c axis; the smaller unit cell was finally selected. Disordered guest DMSO molecules were treated as rigid molecules, but in some cases, only sulfur atoms with partial occupancy could be resolved. [Zn(bipy)(DBM)2]n‚2n(fluorobenzene): To find starting coordinates, an exhaustive direct method search procedure was necessary. The final solution was tested with PLATON,41 but despite the two similar host chains, no additional symmetry was indicated. [Zn(bipy)(DBM)2]n‚2n(THF): The presence of translational pseudosymmetry made the application of direct methods difficult. The structure was initially solved in space group P1 and subsequently transformed to P-1 using PLATON.41 No additional symmetry, which might be suspected from the presence of two chemically equivalent fragments,42,43 was found. Minor orientations of disordered guest molecules were represented as isoelectronic cyclopentane molecules. For six compounds studied, the room-temperature unit cell dimensions were also measured (Table 2). To accomplish this, several dozens of reflections were found randomly using 120 or more frame ω scans, 0.3° wide, starting at three different φ positions. NMR Spectroscopy. 13C Cross-polarization/magic angle spinning (CP/MAS) NMR spectra were obtained at 75.43 MHz at room temperature on a Bruker AMX300 spectrometer equipped with a Doty Scientific 5-mm CP/MAS probe. Samples of compounds with included fluorobenzene were recorded with (39) As all inclusion compounds were prepared with the maximum possible concentration of guest component, full guest occupancy might be suspected a priori. (40) Because the guest occupancy depended on the rigid model employed, the real errors in the occupancy would thus be expected to be somewhat greater than those determined in this refinement. (41) Spek, A. L. PLATON, A Multiple Crystallographic Tool; Utrecht University: Utrecht, The Netherlands, 2001. (42) Marsh, R. E. Acta Crystallogr. 1995, B51, 897-907. (43) Marsh, R. E.; Kapon, M.; Hu, S.; Herbstein, F. H. Acta Crystallogr. 2002, B58, 62-77.

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Figure 2. Powder XRD patterns of guest-free solids: (a) [Zn(DBM)2]; (b) [Zn2(bipy)(DBM)4] (crystals from 2-butanol); (c) [Zn(bipy)(DBM)2] (after desolvation of the THF inclusion). Radiation: Co KR, λ ) 1.7902 Å.

Figure 3. TGA thermogram of the [Zn(bipy)(DBM)2] complex prepared by decomposition of its inclusion compound with THF. Heating rate, 5 deg/min. Sample mass, 16.9 mg. To is onset temperature of the mass loss curve (formed by the intersection of the tangents). an excess of guest (a drop was added to the sample in the NMR rotor before capping) to prevent them from desolvation. A standard CP pulse program was used with fixed-amplitude 1H decoupling during signal acquisition. 1H 90° pulse lengths were 2.7-3.05 µs, CP times were 0.7-3 ms, and recycle times were 2-16 s, depending on the sample. Dipolar dephased spectra44 were obtained by interrupting the 1H decoupling for 40 µs immediately after the CP sequence and before starting the data acquisition. Chemical shifts were measured relative to external solid hexamethylbenzene and then corrected to the TMS scale. Spinning speeds were set in the range 5.7-6.54 kHz to avoid overlap of spinning sidebands with isotropically shifted lines as far as possible.

Results and Discussion Isolated Materials. Solvent-Free Products. Two solvent-free products were isolated in this work: [Zn2(bipy)(DBM)4] and [Zn(bipy)(DBM)2]. Powder diffractograms of the initial [Zn(DBM)2] (monomeric form of zinc dibenzoylmethanate)36 and the two isolated complexes are shown in Figure 2. Both complexes were white and stable in air at room temperature. At elevated temperatures (>140 °C), [Zn(bipy)(DBM)2] decomposed to [Zn2(bipy)(DBM)4] (Figure 3), while the latter decomposed irreversibly at >245 °C. [Zn2(bipy)(DBM)4] precipitated as a fine-crystalline product from n-heptane, cyclopentane, methylcyclohexane, pinene, 1,2-dichloropropane, hexachloropropene, 2-bromobutane, 1-bromohexane, 1-bromooctane, 2,2(44) Opella, S. J.; Frey, M. H. J. Am. Chem. Soc. 1979, 101, 58545856.

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dichloro-1,1,1-trifluoroethane, 1-butanol, 2-butanol, 1-heptanol, pinacolone, methylformate, n-butyl acetate, and nitromethane. The products from all these solvents were identical, displaying very similar powder XRD (Figure 2b) and TGA (Figure 3) patterns. Attempts to obtain the complex as larger crystals failed. Microscope images suggested formation of small needlelike crystals in all cases, with typical dimensions of 4 × 90 µm (see Figure S1 in Supporting Information). The morphology of the crystals suggested the formation of a single phase rather than a mixture. As attested by powder XRD (Figure 2b), the same phase formed in the course of a TGA experiment and displayed a plateau of thermal stability in the 168-247 °C temperature range (cf. Figure 3). From its stoichiometry and by comparison with the structures of some inclusion compounds reported later in this work, it is proposed that the complex forms dinuclear molecules with two zinc centers connected with bridging bipy and two DBMs chelated to each zinc center completing its coordination number to five. The [Zn(bipy)(DBM)2] complex was prepared by desolvation of inclusion compounds with THF or 2-pentanone, which are described later in this work. The product was a powder, with a particle size of CdO peaks. B, peaks in range 165-136 ppm. C, peaks in range 136-110 ppm. D, DMSO methyls or tert-butyl group methyls and quaternary peaks. c sum of C + D.

Multiplicities. NMR can serve as a check on the structural analysis. Chemically equivalent carbons can be crystallographically, and therefore potentially mag-

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netically, distinct. This gives rise to a multiplicity of resonances for a particular type of carbon that cannot be greater than the number of crystallographically distinct atoms in the asymmetric unit. It is possible to say from the multiplicities of the DBM CdO and CsH resonances for the empty dimer and polymer that these must have at least two and four DBM ligands in their asymmetric units, respectively. For the inclusion compounds, those multiplicities that could be distinguished (without complications due to overlap with other lines) were all consistent with the crystal structures determined. Examples are given in the details below. The trimeric structure with DMSO guest has a very rich spectrum due to the large number of atoms in the asymmetric unit, which includes 3 DBM, 1 bipy, and 2.5 DMSO. Though there are five crystallographically distinct DMSO methyl groups, they are not distinguished in the single line at 40.6 ppm. The DBM methanate C-H carbons show as a 1:2 doublet and the CdO carbons as a 1:2:2:1 quartet (with one of the doublets showing a very slight splitting). These results are quite consistent with there being three DBMs in the asymmetric unit. The observed intensity ratio of DMSO carbons to the total host carbons is 1:11.1 (cf. expected 1:11). In the polymeric structure with fluorobenzene, the C-F carbon resonance is a doublet (164.9, 161.5 ppm), which might have been taken as distinguishing the two distinct guest molecules; however, the splitting (∼256 Hz) is of the same order as the reported J(13C-19F) coupling (245.3 Hz) for the liquid, which should still be present in the solid. The DBM methanate C-H shows as a doublet, consistent with the two separate polymer chains A and B in the structure. The spectrum of the less stable dimer with fluorobenzene is markedly different and less well resolved, which may reflect some inhomogeneity arising from partial desolvation. Nevertheless the methanate C-H shows as a quartet of lines, consistent with the structural data. Dynamics. Residual intensity for proton-bearing carbons in the dipolar dephased spectra indicates the presence of dynamics. Methyl reorientation is quite common, and thus the methyl carbons of DMSO and tert-butylbenzene are readily seen. The fluorobenzene molecule also shows dynamics in both of the complexes studied. There are also indications of bipy reorientation in the polymer with fluorobenzene and the polymer with tert-butylbenzene/bipy. As was found previously for the Zn/DBM complexes, there is evidence of reorientation of the DBM phenyl rings about their 2-fold axes in that the ortho and meta resonances show residual intensity but the para resonance does not. General Discussion. As the most important result, this work demonstrates that new metal DBM host complexes may be generated not only by exchanging metal and ligands in the basic unit but also by connecting these units together. The resulting oligo- and polymeric host receptors show enhanced inclusion ability and enlarged size of the cavity space, as compared with the mononuclear metal DBM complexes reported previously.1-5 The ability of modified metal DBM complexes to entrap solvent molecules is the result of the poor ability of these complexes to pack efficiently in three dimen-

Soldatov et al.

sions. The entrapped molecules improve the packing efficiency. Extension of the complexes to multinuclear species appears to be a new way to reduce their ability to pack efficiently. This strategy appears to be more effective than increasing size of the neutral ligand used, as it decreases the translational and rotational freedom of the species. Consequently, the nuclearity of the host complex may be expected to correlate with its inclusion ability. Results of the present work are in good accordance with this expectation. The dimeric complex produced inclusion compounds with tert-butylbenzene and fluorobenzene but precipitated as a solvent-free form from 17 other solvents tested. In contrast, the polymeric complex produced inclusion compounds with a wide range of solvents but never crystallized in a solvent-free form.53 The size of the host complexes increases dramatically upon oligomerization. The maximum van der Waals dimension of the dimeric [Zn2(bipy)(DBM)4] complex exceeds 27 Å and that of the trimeric [Zn3(bipy)2(DBM)6] complex exceeds 38 Å. The void space between adjacent zinc DBM units is significantly greater than the shallow pockets found in mononuclear metal DBMs. Interdigitation of the host oligomeric molecules or host polymeric chains does not result in close packing, and the residual cavity space is large enough to accommodate up to three average-sized guest molecules for each zinc DBM unit. When the space between adjacent zinc DBM units is fully available for inclusion, up to five guest molecules may be accommodated in one cavity, or two rows of guest molecules may fit into one channel. The dimension of the cavity space can vary up to 1-2 nm in many cases. It is clear therefore that the polynuclear metal DBMs have the potential for generating supramolecular materials with nanosized pore space. An interesting observation of this work is that guest molecules can be associated with characteristic pocket space already available on a host molecule. Also, in many cases, the guest species are concentrated in groups inside large cavities rather than evenly distributed within the structure. There are two types of host molecules:54 Molecules of the first type do not possess cavity space themselves but create it upon assembling in solid state; gossypol, urea, Werner complexes, and water are a few examples of this type.55 Molecules of the second type already possess cavity space and often are referred to as “receptors”; calixarenes, carcerands,56 and cyclodextrins57 are among the most common representatives. The host molecules examined in this study are intermediate between these two types, but their resemblance to the second type is greater than for other metal complexes, including mononuclear metal DBM (53) Further ongoing research in our laboratory has revealed that crystallization from more than 20 aromatic solvents produced inclusion compounds of the polymeric host of at least 7 types, but no guest-free product was isolated. The results of that study will be published later. (54) More generally, these types correspond to two types of selfassembly that result either in infinite lattices or in discrete species. See, for example: Gibb, C. L. D.; Gibb, B. C. J. Supramol. Chem. 2001, 1, 39-52. (55) Comprehensive Supramolecular Chemistry, Vol. 6, Solid-state Supramolecular Chemistry: Crystal Engineering; MacNicol, D. D., Toda, F., Bishop, R., Eds.; Pergamon: Oxford, 1996. (56) Comprehensive Supramolecular Chemistry, Vol. 2, Molecular Recognition: Receptors for Molecular Guests; Vogtle, F., Ed.; Pergamon: Oxford, 1996. (57) Comprehensive Supramolecular Chemistry, Vol. 3, Cyclodextrins; Szejtli, J., Osa, T., Ed.; Pergamon: Oxford, 1996.

New Host Receptors

hosts. This observation allows us to use term “receptor” even though no data on inclusion in single host molecule are available. Despite the structural variety of compounds reported in this work, common features in the host molecular structure and crystal packing may be readily recognized. The growth of the host units occurs due to insertion of host monomeric units with six-coordinated zinc between terminal host units with five-coordinated zinc. The former unit is always trans-configured, which necessarily produces a linear arrangement of the host molecules. In the crystal structure, the host molecules are always aligned in parallel, as has been previously noted for other elongated5,29,30,58,59 and polymeric35,60-62 complexes. The needle- or prismlike habit of most of the isolated crystals, which seems to reflect this form of crystal packing, is most noticeable upon fast growth of the crystals. A parallel alignment of the host species is understandable, as it allows interdigitation. Still, the host species are not able to pack efficiently despite the intrinsic high flexibility of that metal coordination environment that can be dramatically distorted in some cases. The observed loose packing leads to residual cavity spaces that often combine in one or both other directions of the crystal structure. Another important result of this work is that the formation of the host species of specific nuclearity is determined by the guest template. tert-Butylbenzene, DMSO, and THF exemplify specific templates inducing formation of dimeric, trimeric, and polymeric host species, respectively. Guest-induced change of the molecular structure of host complexes is an attractive but still not well explored way to switch bulk properties of supramolecular materials. 17 Guest-supported stabilization in supramolecular solids of molecular conformations,2,3,63-66 spatial isomers,16,67-69 and the molecules themselves70-72 was traced in the literature, but there seemed to be very few examples of guest-supported stabilization of an oligomeric host complex.36,73 (58) Bond, D. R.; Jackson, G. E.; Nassimbeni, L. R. S. Afr. J. Chem. 1983, 36, 19-26. (59) Schultheiss, N.; Barnes, C. L.; Bosch, E. Cryst. Growth Des. 2003, 3, 573-580. (60) Kumar, R. K.; Balasubramanian, S.; Goldberg, I. Chem. Commun. 1998, 1435-1436. (61) Huang, W.; Gou, S.; Hu, D.; Chantrapromma, S.; Fun, H. K.; Meng, Q. Inorg. Chem. 2001, 40, 1712-1715. (62) Liao, J.-H.; Chen, P.-L.; Hsu, C.-C. J. Phys. Chem. Solids 2001, 62, 1629-1642. (63) Lipkowski, J. J. Mol. Struct. 1981, 75, 13-28. (64) Nassimbeni, L. R.; Niven, M. L.; Taylor, M. W. J. Chem. Soc., Dalton Trans. 1989, 119-125. (65) Soldatov, D. V.; Grachev, E. V.; Lipkowski, J. J. Struct. Chem. 1996, 37, 658-665. (66) Soldatov, D. V.; Suwinska, K.; Lipkowski, J.; Ogienko, A. G. J. Struct. Chem. 1999, 40, 781-789. (67) Nassimbeni, L. R.; Niven, M. L.; Zemke, K. J. Acta Crystallogr. 1986, B42, 453-461. (68) Adams, R. P.; Allen, H. C.; Rychlewska, U.; Hodgson, D. J. Inorg. Chim. Acta 1986, 119, 67-74. (69) Guilard, R.; Siri, O.; Tabard, A.; Broeker, G.; Richard, P.; Nurco, D. J.; Smith, K. M. J. Chem. Soc., Dalton Trans. 1997, 34593463. (70) Dyadin, Yu. A.; Kislykh, N. V. Mendeleev Commun. 1991, 134136. (71) Dyadin, Yu. A.; Soldatov, D. V.; Logvinenko, V. A.; Lipkowski, J. J. Coord. Chem. 1996, 37, 63-75. (72) Lipkowski, J.; Kislykh, N. V.; Dyadin, Yu. A.; Sheludyakova, L. A. J. Struct. Chem. 1999, 40, 772-780. (73) Cariati, E.; Bu, X.; Ford, P. C. Chem. Mater. 2000, 12, 33853391.

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tert-Butylbenzene acts as a specific template for the dimeric [Zn2(bipy)(DBM)4] (Zn:bipy ) 1:1/2) host complex. An excess of bipy was applied in an attempt to shift the equilibrium into formation of the polymeric [Zn(bipy)(DBM)2]n (Zn:bipy ) 1:1) complex with included tert-butylbenzene. However, tert-butylbenzene alone is not able to support the formation of the polymer, and instead, the compound [Zn(bipy)(DBM)2]n‚n(bipy)‚2n(tertbutylbenzene) (Zn:bipy ) 1:2) forms. This compound contains bipy included as guest, thus playing the role of a polymer template, together with tert-butylbenzene. Inclusion of the same molecule both as a part of the host complex and as a guest in the same compound has been observed and discussed before,1,64,71,72,74-78 and the inclusion of bipy in such instance was recently demonstrated.79-81 It is quite surprising though that inclusion of bipy occurs even upon application of 2 orders molar concentration excess of tert-butylbenzene. Both guest bipy and tert-butylbenzene interact with the host lattice by van Waals forces only, and the resulting structure must reflect the spatial complementarity. These studies have shown that fluorobenzene directs oligomerization of zinc DBM units into either dimers or polymers but never induces the formation of trimers. DMSO is the only template that we have found for the trimeric host species, and it is very specific. The inclusion compound [Zn3(bipy)2(DBM)6]‚5(DMSO) (Zn: bipy ) 1:2/3) forms as the only product in a range of concentrations of up to 3-fold excess of bipy over zinc DBM. Increased excess bipy again leads to inclusion of the ligand as guest molecule to produce the solvated host polymer, [Zn(bipy)(DBM)2]n‚1/2n(bipy)‚1/2n(DMSO). The polymeric host thus forms in DMSO only upon templating with these two molecules, bipy and DMSO, together. THF and 2-pentanone are examples of guest types that induce formation of a solvated polymeric host, and the polymeric structure of the host complex seems to remain upon desolvation. Two solvent-free and eight inclusion compounds reported in this work elucidate a rich supramolecular chemistry in the metal DBM-bipy-guest systems. These compounds exhibit significant chemical, stoichiometric, and structural diversity. The zinc DBM-to-bipy ratio takes five different values ranging from 1:1/2 to 1:2; the host-to-guest ratio takes six different values ranging from 1:1 to 1:5. Dimeric, trimeric, or polymeric host complexes form in the system depending on the initial concentrations and specific geometry of the guest template. The crystal structures reveal complexity and dramatic differences in topology. At the same time, (74) Graddon, D. P.; Heng, K. B.; Watton, E. C. Aust. J. Chem. 1968, 21, 1, 121-135. (75) Lipkowski, J.; Soldatov, D. V. J. Inclusion Phenom. 1994, 18, 317-329. (76) Soldatov, D. V.; Lipkowski, J. J. Struct. Chem. 1995, 36, 979982. (77) Soldatov, D. V.; Ripmeester, J. A. Supramol. Chem. 1998, 9, 175-181. (78) Soldatov, D. V.; Logvinenko, V. A.; Dyadin, Yu. A.; Lipkowski, J.; Suwinska, K. J. Struct. Chem. 1999, 40, 757-771. (79) Carlucci, L.; Ciani, G.; Proserpio, D. M.; Sironi, A. J. Chem. Soc., Dalton Trans. 1997, 1801-1803. (80) Tong, M. L.; Cai, J. W.; Yu, X. L.; Chen, X. M.; Ng, S. W.; Mak, T. C. W. Aust. J. Chem. 1998, 51, 637-641. (81) Noro, S.; Kondo, M.; Ishii, T.; Kitagawa, S.; Matsuzaka, H. J. Chem. Soc., Dalton Trans. 1999, 1569-1574.

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along with their complexity and diversity, the systems demonstrate the remarkably high inclusion potential of oligomeric and polymeric metal DBMs. The complexes may serve as building elements for coordination oligoand polymeric materials possessing interesting properties and important functions as those reported in the very recent literature.82-87 Supporting Information Available: Figure showing needlelike microcrystals of [Zn2(bipy)(DBM)4] complex obtained (82) Kristiansson, O.; Tergenius, L. E. J. Chem. Soc., Dalton Trans. 2001, 1415-1420. (83) Kim, I.; Kwak, B.; Lah, M. S. Inorg. Chim. Acta 2001, 317, 12-20. (84) Tabares, L. C.; Navarro, J. A. R.; Salas, J. M. J. Am. Chem. Soc. 2001, 123, 383-387.

Soldatov et al. from pinacolone; figures comparing experimental and calculated powder diffractograms for [Zn2(bipy)(DBM)4]‚(tert-butylbenzene), [Zn2(bipy)(DBM)4]‚2/3(fluorobenzene), [Zn3(bipy)2(DBM)6]‚5(DMSO), [Zn(bipy)(DBM)2]n‚2n(fluorobenzene), [Zn(bipy)(DBM)2]n‚1/2n(bipy)‚1/2n(DMSO), and [Zn(bipy)(DBM)2]n‚ n(bipy)‚2n(tert-butylbenzene); table listing occupancy of disordered guest orientations (PDF). CIF files of eight structures studied in this work (14 files for low- and room-temperature measurements). This material is available free of charge via the Internet at http://pubs.acs.org. CM020987Z (85) Lu, J. Y.; Babb, A. M. Chem. Commun. 2002, 1340-1341. (86) Seki, K. Phys. Chem. Chem. Phys. 2002, 4, 1968-1971. (87) Noro, S.; Kitaura, R.; Kondo, M.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Yamashita, M. J. Am. Chem. Soc. 2002, 124, 25682583.