Stepwise Guest Exchange in a Cluster-Supported Three-Dimensional Host Wen-Hua Zhang,† Jian-Ping Lang,*,†,‡ Yong Zhang,† and Brendan F. Abrahams§ School of Chemistry and Chemical Engineering, Suzhou UniVersity, Suzhou 215123, Jiangsu, P. R. China, State Key Laboratory of Coordination Chemistry, Nanjing UniVersity, Nanjing 210093, Jiangsu, P. R. China, and School of Chemistry, UniVersity of Melbourne, ParkVille, Victoria 3010, Australia
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 399–401
ReceiVed NoVember 16, 2007; Accepted January 2, 2008
ABSTRACT: Reactions of [Et4N]2[MoS4(CuCN)2] · H2O with bis-(4-pyridyl)propane (bpp) and [Cu(MeCN)4][ClO4] in aniline produced
[MoS4Cu4(bpp)2(CN)2] · 1.5(aniline) (1a). 1a consists of a new three-dimensional [MoS4Cu4]-supported open coordination polymer which has a relatively large channel with a volume of ca. 618 Å3 per unit cell. 1a underwent a two-step host–guest process in which the aniline molecules residing in its channels could be partly exchanged with MeCN molecules in 3d and then completely with dimethylformamide molecules in a month, forming {[MoS4Cu4(bpp)2(CN)2] · (aniline) · (MeCN)}n (1b) and {[MoS4Cu4(bpp)2(CN)2] · (DMF) · (MeCN)}n (1c), respectively.
Interest in open coordination polymers has arisen because of their potential applications in areas such as storage and separation processes,1 catalysis,2 drug delivery,3 and sensor technology.4 The wide variety of chemical building blocks that may be employed in the generation of open coordination networks offers great scope for tailoring materials to perform specific tasks in regard to host–guest chemistry. The successful generation of stable and robust crystalline networks has allowed investigations of kinetic and thermodynamic factors affecting guest exchange processes. In such processes, it has been shown that secondary bonding interactions such as hydrogen bonding or π-π stacking5 between host and guest can play a key role. Such interactions often have little effect on the structure of the host but sometimes can cause distortion or rupture of the host framework.6 Regardless of the extent of host deformation, it is the interaction between host and guest that generally dictates the sorption and desorption processes. Herein we report a stepwise exchange process in which the identity of the resident guest affects the ability of the host to incorporate other guest species. Addition of an aniline solution containing [Et4N]2[MoS4(CuCN)2] · H2O7 and bis-(4-pyridyl)propane (bpp) to an aniline solution of [Cu(MeCN)4][ClO4] produced a large amount of a powdery red precipitate within a dark-red homogeneous solution. The red solid was filtered off, and the filtrate was left to stand in darkness. After a few days, the solution yielded black prisms of [MoS4Cu4(bpp)2(CN)2] · 1.5(aniline) (1a).8a Elemental analysis revealed that the red powder had the same composition as 1a; however, X-ray powder diffraction (XRPD) (see Supporting Information) indicated that the initial precipitate was amorphous. An X-ray analysis9 of 1a revealed the formation of a pentanuclear saddle-like [MoS4Cu4] core. The asymmetric unit of 1a contains two independent Cu atoms with similar coordination environments. Each Cu center is tetrahedrally coordinated by two µ3-S atoms from the tetrahedral [MoS4]2- moiety, one N atom from a bpp ligand and one C or N atom from a disordered bridging cyanide unit (Figure 1a). Each [MoS4Cu4] cluster core is coordinated by four bridging cyanide ions and four bridging bpp ligands. These eight ligands extend to eight crystallographic equivalent clusters, six of which lie at the corners of a trigonal antiprism, while the other two are situated along the midpoints of two opposing triangular edges as indicated in Figure 1b. * To whom correspondence should be addressed. Fax & Tel: Int. code +86 512 65880089. E-mail:
[email protected]. † Suzhou University. ‡ Nanjing University. § University of Melbourne.
Figure 1. (a) Part of the polymeric structure of 1a with only one set of atoms for the disordered cyanides shown. (b) The connection of the 8-connecting [MoS4Cu4] node to equivalent nodes in 1a. Hydrogen atoms have been omitted for clarity.
Figure 2. Perspective view of the repeating units of 1a, showing (a) the [MoS4Cu4] clusters are linked by CN ions in the ac plane to form a (4, 4) net; (b) the [MoS4Cu4] clusters are linked by bpp ligand in the ab plane to form two interwoven (4, 4) net.
From a topological perspective, each cluster may be considered as an 8-connecting node in a three-dimensional (3D) network. The connectivity of the network may be understood in terms of two types of intersecting sheets each involving only one type of bridging ligand. If only the cyanide bridges between the clusters are considered then a 2D (4,4) square grid extending in the ac plane is apparent (Figure 2a). Adjacent cyanide sheets are linked by bridging bpp ligands. If only the bpp bridges between clusters are considered then two interwoven two-dimensional (2D) (4,4) networks that extend in the ab plane are formed (Figure 2b). If both types of bridging ligand are taken into account, the result is a single 3D (42266) network (Figure 3a), which to the best of our knowledge has not been identified before.10
10.1021/cg701131t CCC: $40.75 2008 American Chemical Society Published on Web 01/11/2008
400 Crystal Growth & Design, Vol. 8, No. 2, 2008
Figure 3. Crystal structure of 1a. (a) A schematic representation of the 3D network; the [MoS4Cu4] cluster cores are represented by red spheres; the bridging cyanide and bpp ligands are indicated by green and light blue connections, respectively. (b) A space filling model viewed down the a axis. Hydrogen atoms have been omitted for clarity.
Figure 4. Space-filling models showing location of solvent molecules within the two solvent sites (site A and site B) of the channels in (a) 1a, (b) 1b, and (c) 1c. Only the cyanide bridged clusters of the channel walls are shown. Only one orientation of the solvent molecules in each of the sites is shown. Hydrogen atoms have been omitted for clarity.
Of particular importance in this structure are one-dimensional (1D) channels that run parallel to the a axis (Figure 3b). The internal surface of these almost oval-shaped channels is provided by cyanide-bridged clusters at the top and bottom and by bpp ligands at the sides. Each channel has a volume of ca. 618 Å3 per unit cell that corresponds to an average cross-sectional area for the channels of ∼64 Å2. The channels represent ca. 29% of the total crystal volume.11 Although the aniline molecules as guests in the channel exhibit disorder around crystallographic 2-fold axes, it is clear that each of them is located in one of two clearly distinct sites within the channel (Figure 4a). One of these sites (site A) is situated between pairs of clusters that are face-on to each other, while the other site (site B) is between a pair of bridging cyanide ions. In the crystal structure refinement, site B was found to be only 50% occupied. When single crystals of 1a were immersed in a DMF/MeCN (v/v ) 1:1) solution for 3 days,8b no change in size or color of these crystals was observed; however, the Fourier transform IR (FTIR) spectrum showed changes in the 2100–2200 cm-1 region (see Supporting Information). A band at 2129 cm-1 was assigned to MeCN guest molecules based upon the observation that this band diminished in intensity upon heating the crystals (see Supporting Information). An X-ray analysis9b revealed that the aniline underwent partial exchange with MeCN to produce crystals of composition, [MoS4Cu4(bpp)2(CN)2] · (aniline) · (MeCN) (1b). Elemental analysis, thermogravimetric analysis (TGA), and XRPD supported this composition (see Supporting Information). The crystal structure reveals that the MeCN displaced the aniline from site A in the structure, with the linear molecule appearing to fit snugly between the opposed clusters (Figure 4b). B-type sites within the channels are now fully occupied by aniline molecules. The fact that site B is only partially occupied by aniline in 1a but is fully occupied in 1b suggests that the affinity of MeCN for site A forces the aniline molecules to occupy a less preferred site (site B). The alternating arrangement of aniline and MeCN molecules in the channels
Communications suggests that the formation of 1b upon immersion in DMF/MeCN, involves partial loss of aniline accompanied by a process in which the relatively small MeCN molecules “squeeze” past the larger aniline molecules that are retained within the channels. Immersion of 1b in the MeCN/DMF solution for a period of three days did not lead to further exchange as indicated by infrared spectroscopy and single-crystal X-ray diffraction. After a month immersed in the solution the crystals became fragile but still retained their single-crystal character.8c The FT-IR spectrum of these crystals, clearly showed a new band at 1673 cm-1 (see Supporting Information), consistent with the incorporation of DMF molecules into the host framework. An X-ray analysis9c indicated that the channels are now occupied by DMF and MeCN molecules in a 1:1 ratio. In this new compound, [MoS4Cu4(bpp)2(CN)2] · (DMF) · (MeCN) (1c), the original host framework is retained, but the bpp ligand exhibits 2-fold positional disorder which is of little consequence to the gross framework structure. The composition of 1c was confirmed by elemental analysis, TGA and XRPD. In the structure of 1c the DMF molecules now occupy site A positions, while the MeCN molecules are relegated to site B positions (Figure 4c). The fact that DMF displaces MeCN from site A suggests that DMF has a strong affinity for site A. As far as the solvent exchange kinetics is concerned, the incorporation of DMF into the channels is much slower than the partial replacement of aniline by MeCN. The difference in the rate of uptake of MeCN compared to DMF may reflect the relative sizes of the MeCN and DMF molecules. We propose that the MeCN molecules are able to move past aniline molecules in the channels with relative ease; however, in the case of the relatively large DMF molecule, the aniline is an obstacle that provides a significant barrier to the incorporation of DMF. Continued exposure of 1c to the MeCN/DMF did not result any further changes. TGA of 1a showed the release of aniline molecules with increasing temperature up to 200 °C. The decomposition of the host framework immediately followed the loss of the aniline from the channels. In contrast, TGA indicated that guest free phases of 1b and 1c could be produced by heating the samples to temperatures of 175 °C (see Supporting Information). In conclusion, we have demonstrated the formation of a novel cluster-based 3D host-framework that exhibits stepwise guestexchange without the loss of single-crystal character. In all three structures considered the structural features of the host are retained; however, the position of guest molecules within the channels shows considerable variation. The physical basis behind such exchange behaviors might be the various weak interactions between host and guest species or the guests themselves (see Supporting Information). The properties exhibited by the framework in this preliminary study suggest that broad and rich host–guest chemistry with the potential to be rationally controlled may thus be anticipated. Investigations of the host behavior of the desolvated crystals are currently underway.
Acknowledgment. This work was supported by the National Natural Science Foundation of China (20525101), the NSF of Jiangsu Province (BK2004205), and the State Key Laboratory of Coordination Chemistry of Nanjing University. Supporting Information Available: Crystallographic data for 1a, 1b, and 1c in CIF format; FT-IR spectra for 1a, 1b, 1c in the 1565–2250 cm-1 region, and for 1b at 175 °C with different time intervals; TGA for 1a, 1b, and 1c; XRPD patterns for the amorphous powder, 1a, 1b, and 1c. These materials are available free of charge via the Internet at http://pubs.acs.org.
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Crystal Growth & Design, Vol. 8, No. 2, 2008 401 Found: C, 41.07; H, 3.64; N, 9.74. IR (KBr pellet): 2927(w), 2858(w), 2138(s), 1933 (w), 1611(s), 1557(w), 1499(m), 1424(s), 1382(w), 1222(m), 1114(w), 1070(m), 1015(m), 800(m), 613(w), 573(w), 490(m), 452(s) cm-1. (b) The black crystals 1b were obtained by immersing 1a in a mixture of DMF/MeCN solution for 3 days. Anal. Calcd for C36H38Cu4MoN8S4: C, 40.75; H, 3.61; N, 10.5;56. Found: C, 40.48; H, 3.72; N, 10.46. IR (KBr pellet): 2923(w), 2859(w), 2129(s), 2104(s),1939. (w), 1611(vs), 1556(w), 1498(m), 1458(w), 1424(s), 1385(w), 1274(w), 1223(m), 1118(w), 1085(m), 1067(m), 1013(s), 915(w), 798(s), 753(m), 693(w), 614(w), 579(w), 508(m), 453(s) cm-1. (c) The black crystals 1c were obtained by immersing 1a in a mixture of DMF/MeCN solution for 30 days. Anal. Calcd for C33H38Cu4MoN8OS4: C, 38.07; H, 3.68; N, 10.76. Found: C, 38.27; H, 3.54; N, 10.76. IR (KBr pellet): 2923(w), 2856(w), 2127(s), 2107(s), 1942(w), 1673(vs), 1610(s), 1556(w), 14500(m), 1458(w), 1423(s), 1384(m), 1254(w), 1223(s), 1086(m), 1067(m), 1012(s), 917(w), 861(w), 823(w), 805(m), 756(w), 659(w), 612(w), 612(w), 578(w), 516(m), 490(w), 453(s) cm-1. (9) (a) Crystal data for 1a: monoclinic, space group P2/c, a ) 9.6701(19) Å, b ) 11.742(2) Å, c ) 18.852(4) Å, β ) 97.91(3)°, V ) 2120.2(7) Å3, Z ) 2, Fcalcd ) 1.654 g/cm3, µ(Mo KR) ) 2.494 mm-1, F000 ) 1049, R ) 0.0921, wR ) 0.2014, GOF ) 1.207. (b) Crystal data for 1b: monoclinic, space group P2/c, a ) 9.6250(19) Å, b ) 11.780(2) Å, c ) 18.714(4) Å, β ) 98.18(3)°, V ) 2100.2(7) Å3, Z ) 2, Fcalcd ) 1.662 g/cm3, µ(Mo KR) ) 2.517 mm-1, F000 ) 1044, R ) 0.0745, wR ) 0.2105, GOF ) 1.115. (c) Crystal data for 1c: monoclinic, space group P2/c, a ) 9.6362(19) Å, b ) 11.462(2) Å, c ) 19.147(4) Å, β ) 97.98(3)°, V ) 2094.3(7) Å3, Z ) 2, Fcalcd ) 1.635 g/cm3, µ(Mo KR) ) 2.524 mm-1, F000 ) 1024, R ) 0.0885, wR ) 0.2155, GOF ) 1.126. (10) (a) Wells, A. F. Three-Dimensional Nets and Polyhedra; WileyInterscience, New York, 1977. (b) Blatov V. A. Multipurpose crystallochemical analysis with the program package TOPOS. Newsletter Commission on Crystallographic Computing of International Union of Crystallography, 2006, Issue 7, pp 4–38. (11) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht University, Utrecht, the Netherlands, 2001.
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