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Cite this: Chem. Commun., 2014, 50, 6585 Received 20th March 2014, Accepted 1st May 2014
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Solid-state polymerization in a polyrotaxane coordination polymer via a [2+2] cycloaddition reaction† In-Hyeok Park,a Raghavender Medishetty,b Shim Sung Lee*a and Jagadese J. Vittal*ab
DOI: 10.1039/c4cc02080d www.rsc.org/chemcomm
Alternate bpeb ligands in the polyrotaxane 2D coordination polymer [Cd(bpeb)(sdb)]DMA (1) selectively undergo polymerization via a [2+2] cycloaddition reaction to form a polyrotaxane based 3D structure [Cd(bpeb)0.5(poly-bppcb)0.5(sdb)]DMA (2) in a single-crystal-to-singlecrystal manner.
Crystalline organic polymers are an interesting class of materials with potential applications1 and these have been synthesized in the solid state using the monomers of diacetylenes,2 diolefins,3 triacetylenes,4 trienes,4a,5 quinodimethanes6 and 5-distyrylpyrazine.7 In contrast, crystalline metal complexes of organic polymers are virtually unknown although organic polymers containing pendent metal complexes have been explored extensively.8 As of today, traditional crystallization methods may not be a suitable route to obtain crystalline metal complexes of organic polymers, but may be obtained indirectly by the [2+2] cycloaddition reaction of conjugated diolefins in the solid state. One such organic polymer containing cyclobutane rings has been recently integrated into a metal–organic framework (MOF) using the precursor 1,4-bis[2-(40 -pyridyl)ethenyl] benzene (bpeb, Scheme 1a).9 The conjugated CQC bonds of the bpeb ligands were infinitely aligned closely spaced in a slip-stacked (‘‘out-of-phase’’) fashion in the MOF and the photo-dimerization of these pairs of CQC bonds in the solid state polymerized these diolefins to furnish the desired metal complex of the 1,3-(4,4 0 -bipyridyl)-2-phenylcyclobutane polymer (poly-bppcb, vide infra). The resultant crystalline compound is a metal–organo polymeric framework (MOPF) structure formed by the fusion of a 1D coordination polymer (CP) and the organic polymer ligand.9 a
Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, South Korea. E-mail: [email protected]; Fax: +82 55-753-7614; Tel: +82 55-772-1483 b Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543. E-mail: [email protected]; Fax: +65 6779 1691; Tel: +65 6516 2975 † Electronic supplementary information (ESI) available: Synthesis, XRPD patterns, TGA curves and crystal structures. CCDC 952014 (1) and 952015 (2). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc02080d
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Scheme 1
Structural diagrams of (a) bpeb and (b) H2sdb.
Molecular entanglement is one of the exciting fields of science. Of these, rotaxane based organic systems as well as metal containing rotaxanes have attracted enormous interest due their unusual structural topologies as well as potential applications, for example, as molecular motors and for switching and sensing abilities.10 In contrast, interpenetrating polyrotaxane structures in CPs were first reported by Robson in 1997.11 Interestingly, stilbene based pseudorotaxanes have been found to undergo [2+2] cycloaddition reactions both in the solid state and solution.12 However, we are not aware of any photoreactive interpenetrated polyrotaxane based CPs reported in the literature. In this work, we have shown that it is possible to align the alternate bpeb ligands in a [Cd(bpeb)] chain selectively to undergo photo-polymerization to produce a new MOPF structure by a [2+2] cycloaddition reaction. The polymerization in a photoreactive polyrotaxane containing a two-fold entangled MOF has been assisted by the CdII atoms which align the alternate bpeb ligands in the [Cd(bpeb)] zigzag chains to pack in a slip-stacked manner in a plane. Due to the rotaxane structure, the other bpeb ligand becomes photo-inactive. The conversion occurs in a single-crystalto-single-crystal (SCSC) manner, as shown in Scheme 2. The solidstate structure resulted in a rare tfc topology. The details are given below. Orange platy crystals of [Cd(bpeb)(sdb)]DMA (1) suitable for single crystal X-ray data collection were obtained under solvothermal conditions from Cd(NO3)24H2O, 4,4 0 -sulfonyldibenzoic acid (H2sdb, Scheme 1) and bpeb in a mixture of dimethylacetamide (DMA), dimethylsulfoxide (DMSO) and water at 100 1C, followed by slow cooling. X-ray crystallographic experiments carried out at 100 1C revealed that 1 is a 2D CP and crystallized in the triclinic space group P1% with Z = 2. The asymmetric unit
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Scheme 2 A Schematic representation of the SCSC formation of a 3D polyrotaxane MOPF by the polymerization of slip-stacked bpeb ligands via a [2+2] photo-cycloaddition reaction.
Fig. 1 (a) A perspective view showing the coordination sphere of Cd1 in 1. Symmetry code: (A) 2 x, 2 y, 2 z; (B) 1 x, 1 y, 1 z; (C) 2 x, 2 y, z. (b) A schematic representation of the 2-fold entanglement. Hydrogens are not shown.
contains one CdII atom, one sdb ligand and two halves of bpeb ligands (Fig. 1a). The Cd1 with a highly distorted octahedral CdN2O4 core [52.8(2)–142.4(3)1] is coordinated to the N atoms of two different bpeb ligands and chelated by two sdb anions in a bidentate fashion. A crystallographic inversion centre is located at the centre of each bpeb ligand. The two angular sdb ligands are bonded to two CdII atoms to form a [Cd2(sdb)2] ring13 with a crystallographic centre of inversion (Fig. 1a). Each of these rings is further linked by four bpeb ligands via Cd–N bonds (2.231(7) and 2.279(6) Å). Considering the [Cd2(sdb)2] unit as a node, 1 has the (4,4) net topology (see Fig. S5 in the ESI†). The large void produced in this connectivity is filled by 2-fold interpenetration (Fig. 1b). Despite this interpenetration, 1 has porous channels viewed along the a-axis with a calculated solvent accessible void14 of 22.2% occupied by the DMA solvents (Fig. S7, ESI†). Notably, of the two bpeb ligands bonded to each CdII centre (green node in Fig. 2a), the one with the N1 atom acts as an axle and entangles the [Cd2(sdb)2] ring (wheel) from the neighbouring (4 4) net to form a rotaxane structure and the other bpeb (with N2 atom) is photoreactive, as described below. Accordingly, the 2-fold entanglement15 due to the rotaxane arrangement generates the polyrotaxane array in 1. Such a polyrotaxane entanglement in 2D layers is uncommon.16 Fig. 2 shows the packing of the adjacent layers in 1, it can be seen that the bpeb spacer ligands which are not involved in the rotaxane formation (i.e., the bpeb ligand containing N2 atoms) are aligned in a parallel fashion. Interestingly, the [Cd(bpeb)] zigzag chains are packed one below the other forming a plane. As a result, the bpeb spacer ligands are slip-stacked (or "out-ofphase") relative to each other in such a way that each phenylene
6586 | Chem. Commun., 2014, 50, 6585--6588
Fig. 2 (a) The topological representation of the adjacent layers showing alignment of one of the bpeb ligands. CdII in pink, the reactive bpeb spacer in green and rotaxane axle bpeb in blue. (b) and (c) perspective views of the packing showing the alignment of CQC bonds. (d) ‘‘Slip-stacked’’ alignments of alternate bpeb spacers in the [Cd(bpeb)] chains of 1. Hydrogens are not shown for clarity.
ring in the bpeb ligand is closer to a neighbouring pyridyl group with a centroid-to-centroid distance of 3.84 Å (Fig. 2d). This indicates the presence of a strong face-to-face p–p interaction (Fig. 2b and c), and the distance between the centres of the adjacent CQC bonds was found to be 3.76 Å. The slip-stacked arrangement of the adjacent bpeb spacer ligands is congenial for photochemical polymerization via a [2+2] cycloaddition reaction, since only one CQC bond pair is aligned between any two adjacent bpeb ligands in the plane of [Cd(bpeb)] chains. It is apparent from Fig. 2a that such a [2+2] cycloaddition reaction will yield a cyclobutane-based organic polymer with a 3D polyrotaxane structure. Overall, it is expected that only 50% of the total bpeb ligands in 1 will be photoreactive (vide infra). Irradiating the orange single crystals of 1 under a Xe-lamp (l = 365 nm) for 48 h resulted in pale orange crystals of [Cd(bpeb)0.5(poly-bppcb)0.5(sdb)]DMA (2) (where poly-bppcb is a 1,3-(4,4 0 -bipyridyl)-2-phenylcyclobutane polymer, Fig. S11, ESI†) suitable for single crystal X-ray crystallographic analysis. Furthermore, unlike 1 (see Fig. S12, ESI†), characterization by routine solution 1H-NMR spectroscopy was not possible due to its insolubility even in strong acids, indirectly inferring that the expected organic polymer has formed. This behaviour is very similar to that reported recently.9a The X-ray crystallographic analysis of 2 revealed that a [2+2] cycloaddition reaction had indeed taken place in one of the
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Fig. 3 (a) A perspective view of 2 showing the coordination sphere of Cd1. Symmetry code: (A) 1 x, 1 y, 1 z; (B) 1 x, 1 y, 3 z; (C) 1 x, 2 y, 2 z. (b) A perspective view of the packing showing the poly-bppcb. (c) A side view showing poly-bppcb. (d) The 2D sheets formed by the polybppcb with bpeb and CdII atoms in 2. The hydrogen atoms and disorders are not shown for clarity.
bpeb spacer ligands in 1, resulting in the formation of the cyclobutane ring accompanied by the SCSC transformation (Fig. 3a). The cell parameters during this process changed very little and the same space group P1% was retained (Table S1, ESI†). The well-aligned [Cd(bpeb)] zigzag chains in 1 (Fig. 2d) were fused by a [2+2] cycloaddition reaction incorporating the poly-bppcb chain in 2 (Fig. 3d). As discussed above, the 2-fold entangled 2D polyrotaxane structure in 1 is now converted to a 2-fold interpenetrated MOPF with the polyrotaxane structure, due to the poly-bppcb chain associated with the formation of cyclobutane rings (Fig. 3c and d). The channel along the a-axis remains with a small reduction in the calculated solvent accessible void to 20.2% as compared to 1.14 It may be noted that the unreacted bpeb is disordered in 2 over two sites with an occupancy ratio of approximately 55 : 45. A crystallographic inversion centre is also located at the centre of the bppcb and the disordered bpeb ligands. The resultant unusual binodal net in 2 is built from the tetrahedral nodes of CdII atoms and four connected nodes of the cyclobutane rings of the poly-bppcb ligand. It is this infinite extension of the polymer ligand which results in a very rare (3,4) connected net with a tfc topology and one double edge that entangles as a rotaxane with the Point Symbol {83}2{8510} and the Vertex Symbol (88383)(8282828282*) (Fig. 4).17 Recently, solid-state structural transformation of a 2D interdigitated layer CP into a 3D MOF was reported by us.18 The transformation of a 2D bilayer structure to a 3D MOF via photo-cycloaddition was reported by Wu’s group.19 To the best of our knowledge, this is first example of the 2D polyrotaxane MOF transformed into a 3D polyrotaxane MOPF by a photochemical route. Once formed 2 cannot be thermally depolymerized back to 1 unlike the reported example.9a Although the solid-state polymerization in MOFs by g-irradiation20 and a thermal method21 has been reported, the resultant structures of these products were unfortunately unknown. While this work was in progress, Garai and Biradha reported another conjugated
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Fig. 4 A perspective view of the 2-fold entangled net, tfc viewed along the a-axis. The topological representation of the 3D framework created from herringbone style 2D sheets formed from the distorted tetrahedral Cd node (pink), bpeb and the poly-bppcb node (green) via the sdc spacer (yellow). The unreacted bpeb spacer (axle) is shown in pink.
diene ligand in four AgI CPs with different anions to undergo solidstate a [2+2] photo-polymerization reaction in an SCSC fashion.22 Polymerization of bpeb in a MOF appeared recently.23 These organic polymerization reactions in metal complexes, CPs or MOFs should not be confused with metal complex polymerization reactions.24 Previously we have shown that [Zn(bpeb)] zigzag polymer strands could be aligned one below the other for the polymerization of bpeb ligands through a [2+2] cycloaddition reaction forming an unprecedented MOPF structure.9 In this work, we have further demonstrated that the bpeb ligands can be juxtaposed in an infinitely slip-stacked manner in a 2D polyrotaxane. It appears that it is possible to align the bpeb ligands congenial for [2+2] cycloaddition polymerization in more than one ways in MOF compounds. Furthermore, 2 is an exceptional metal complex of an organic polymer for which the crystal structure has been determined unequivocally from the single crystal data.9 In summary, the well-known [2+2] cycloaddition reaction has been used for the photo-polymerization of a conjugated diene ligand, bpeb. The polymerization observed in a MOF is assisted by the CdII atoms which align the alternate bpeb ligands in the [Cd(bpeb)] zigzag chains to pack in a slipstacked manner in a plane. This is a rare photoreactive 2D polyrotaxane MOF formed by 2-fold entanglement. The structural transformation readily occurs between the well-aligned alternate bpeb ligands under UV light yielding another 3D polyrotaxane MOPF, 2 with a tfc topology in an SCSC manner. Compound 2 is also a unique MOF containing an unusual organic polymer as a linker along with the conventional bent dicarboxylate ligand. This photo-dimerized structure 2 has both organic polymer ligands fused together with two-fold entangled rotaxane formed by [Cd(bpeb)0.5(sdb)]. There were not many examples for structurally characterized metal complexes containing organic polymer ligands.9,22 The results described here appear to exemplify that the C–C bond formation by a [2+2] cycloaddition reaction is a general route to access crystalline metal complexes of organic polymer ligands. This work was supported by NRF (2012R1A4A1027750), South Korea and the Ministry of Education, Singapore through a NUS FRC grant R-143-000-562-112.
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Notes and references 1 (a) A. Sun, J. W. Lauher and N. S. Goroff, Science, 2006, 312, 1030–1034; (b) K. Sada, M. Takeuchi, N. Fujita and S. Shinkai, Chem. Soc. Rev., 2007, 36, 415–435; (c) D. J. Ahn and J. M. Kim, Acc. Chem. ´ndez, M. Stole, V. Stepanenko Res., 2008, 41, 805–816; (d) G. Ferna ¨rthner, Chem. – Eur. J., 2013, 19, 206–217; (e) T.-J. Hsu, and F. Wu F. W. Fowler and J. W. Lauher, J. Am. Chem. Soc., 2012, 134, 142–145; ( f ) S. Dawn, M. B. Dewal, J. A. Krause, P. J. Pellechia and L. S. Shimizu, J. Am. Chem. Soc., 2011, 133, 7025–7032; ( g) A. Matsumoto, T. Odani, K. Sada, M. Miyata and K. Tashiro, Nature, 2000, 405, 328–330; (h) M. Garai, R. Santra and K. Biradha, Angew. Chem., Int. Ed., 2013, 52, 5548–5551; (i) D. Beaudoin, T. Maris and J. D. Wuest, Nat. Chem., 2013, 5, 830–834. 2 (a) R. Xu, V. Gramlich and H. Frauenrath, J. Am. Chem. Soc., 2006, 128, 5541–5547; (b) T. Itoh, T. Shichi, T. Yui and K. Takagi, Langmuir, 2005, 21, 3217–3220; (c) S. M. Curtis, N. Le, F. W. Fowler and J. W. Lauher, Cryst. Growth Des., 2005, 5, 2313–2321; (d) S. B. Lee, R. Koepsel, D. B. Stolz, H. E. Warriner and A. J. Russell, J. Am. Chem. Soc., 2004, 126, 13400–13405; (e) X. Ouyang, F. W. Fowler and J. W. Lauher, J. Am. Chem. Soc., 2003, 125, 12400–12401. 3 (a) A. Matsumoto and T. Odani, Macromol. Rapid Commun., 2001, 22, 1195–1215; (b) G. W. Coate, A. R. Dunn, L. M. Henling, J. W. Ziller, E. B. Lobkovsky and R. H. Grubbs, J. Am. Chem. Soc., 1998, 120, 3641–3649; (c) L. Addadi and M. Lahav, J. Am. Chem. Soc., 1978, 100, 2838–2844. 4 (a) J. W. Lauher, F. W. Fowler and N. S. Goroff, Acc. Chem. Res., 2008, 41, 1215–1229; (b) J. Xiao, M. Yang, J. W. Lauher and F. W. Fowler, Angew. Chem., Int. Ed., 2000, 39, 2132–2135. 5 T. Hoang, J. W. Lauher and F. W. Fowler, J. Am. Chem. Soc., 2002, 124, 10656–10657. 6 S. Nomura, T. Itoh, H. Nakasho, T. Uno, M. Kubo, K. Sada, K. Inoue and M. Miyata, J. Am. Chem. Soc., 2004, 126, 2035–2041. 7 (a) M. Hasegawa, Pure Appl. Chem., 1986, 58, 1179–1188; (b) M. Hasegawa, Chem. Rev., 1983, 83, 507–518. 8 (a) J. M. Pollino and M. Weck, Chem. Soc. Rev., 2005, 34, 193–207; (b) M. O. Wolf, J. Inorg. Organomet. Polym. Mater., 2006, 16, 189–199; (c) T. Kaliyappan and P. Kannan, Prog. Polym. Sci., 2000, 25, 343–370. 9 (a) I.-H. Park, A. Chanthapally, Z. Zhang, S. S. Lee, M. J. Zaworotko and J. J. Vittal, Angew. Chem., Int. Ed., 2014, 53, 414–419; (b) I.-H. Park, A. Chanthapally, H.-H. Lee, H. S. Quah, S. S. Lee and J. J. Vittal, Chem. Commun., 2014, 50, 3665–3667. 10 (a) A. R. Pease, J. O. Jeppesen, J. F. Stoddard, Y. Luo, C. P. Collier and J. R. Heath, Acc. Chem. Res., 2001, 34, 433–444; (b) C. A. Schalley,
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Cite this: Chem. Commun., 2014, 50, 6585 Received 20th March 2014, Accepted 1st May 2014
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Solid-state polymerization in a polyrotaxane coordination polymer via a [2+2] cycloaddition reaction† In-Hyeok Park,a Raghavender Medishetty,b Shim Sung Lee*a and Jagadese J. Vittal*ab
DOI: 10.1039/c4cc02080d www.rsc.org/chemcomm
Alternate bpeb ligands in the polyrotaxane 2D coordination polymer [Cd(bpeb)(sdb)]DMA (1) selectively undergo polymerization via a [2+2] cycloaddition reaction to form a polyrotaxane based 3D structure [Cd(bpeb)0.5(poly-bppcb)0.5(sdb)]DMA (2) in a single-crystal-to-singlecrystal manner.
Crystalline organic polymers are an interesting class of materials with potential applications1 and these have been synthesized in the solid state using the monomers of diacetylenes,2 diolefins,3 triacetylenes,4 trienes,4a,5 quinodimethanes6 and 5-distyrylpyrazine.7 In contrast, crystalline metal complexes of organic polymers are virtually unknown although organic polymers containing pendent metal complexes have been explored extensively.8 As of today, traditional crystallization methods may not be a suitable route to obtain crystalline metal complexes of organic polymers, but may be obtained indirectly by the [2+2] cycloaddition reaction of conjugated diolefins in the solid state. One such organic polymer containing cyclobutane rings has been recently integrated into a metal–organic framework (MOF) using the precursor 1,4-bis[2-(40 -pyridyl)ethenyl] benzene (bpeb, Scheme 1a).9 The conjugated CQC bonds of the bpeb ligands were infinitely aligned closely spaced in a slip-stacked (‘‘out-of-phase’’) fashion in the MOF and the photo-dimerization of these pairs of CQC bonds in the solid state polymerized these diolefins to furnish the desired metal complex of the 1,3-(4,4 0 -bipyridyl)-2-phenylcyclobutane polymer (poly-bppcb, vide infra). The resultant crystalline compound is a metal–organo polymeric framework (MOPF) structure formed by the fusion of a 1D coordination polymer (CP) and the organic polymer ligand.9 a
Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, South Korea. E-mail: [email protected]; Fax: +82 55-753-7614; Tel: +82 55-772-1483 b Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543. E-mail: [email protected]; Fax: +65 6779 1691; Tel: +65 6516 2975 † Electronic supplementary information (ESI) available: Synthesis, XRPD patterns, TGA curves and crystal structures. CCDC 952014 (1) and 952015 (2). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc02080d
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Scheme 1
Structural diagrams of (a) bpeb and (b) H2sdb.
Molecular entanglement is one of the exciting fields of science. Of these, rotaxane based organic systems as well as metal containing rotaxanes have attracted enormous interest due their unusual structural topologies as well as potential applications, for example, as molecular motors and for switching and sensing abilities.10 In contrast, interpenetrating polyrotaxane structures in CPs were first reported by Robson in 1997.11 Interestingly, stilbene based pseudorotaxanes have been found to undergo [2+2] cycloaddition reactions both in the solid state and solution.12 However, we are not aware of any photoreactive interpenetrated polyrotaxane based CPs reported in the literature. In this work, we have shown that it is possible to align the alternate bpeb ligands in a [Cd(bpeb)] chain selectively to undergo photo-polymerization to produce a new MOPF structure by a [2+2] cycloaddition reaction. The polymerization in a photoreactive polyrotaxane containing a two-fold entangled MOF has been assisted by the CdII atoms which align the alternate bpeb ligands in the [Cd(bpeb)] zigzag chains to pack in a slip-stacked manner in a plane. Due to the rotaxane structure, the other bpeb ligand becomes photo-inactive. The conversion occurs in a single-crystalto-single-crystal (SCSC) manner, as shown in Scheme 2. The solidstate structure resulted in a rare tfc topology. The details are given below. Orange platy crystals of [Cd(bpeb)(sdb)]DMA (1) suitable for single crystal X-ray data collection were obtained under solvothermal conditions from Cd(NO3)24H2O, 4,4 0 -sulfonyldibenzoic acid (H2sdb, Scheme 1) and bpeb in a mixture of dimethylacetamide (DMA), dimethylsulfoxide (DMSO) and water at 100 1C, followed by slow cooling. X-ray crystallographic experiments carried out at 100 1C revealed that 1 is a 2D CP and crystallized in the triclinic space group P1% with Z = 2. The asymmetric unit
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Scheme 2 A Schematic representation of the SCSC formation of a 3D polyrotaxane MOPF by the polymerization of slip-stacked bpeb ligands via a [2+2] photo-cycloaddition reaction.
Fig. 1 (a) A perspective view showing the coordination sphere of Cd1 in 1. Symmetry code: (A) 2 x, 2 y, 2 z; (B) 1 x, 1 y, 1 z; (C) 2 x, 2 y, z. (b) A schematic representation of the 2-fold entanglement. Hydrogens are not shown.
contains one CdII atom, one sdb ligand and two halves of bpeb ligands (Fig. 1a). The Cd1 with a highly distorted octahedral CdN2O4 core [52.8(2)–142.4(3)1] is coordinated to the N atoms of two different bpeb ligands and chelated by two sdb anions in a bidentate fashion. A crystallographic inversion centre is located at the centre of each bpeb ligand. The two angular sdb ligands are bonded to two CdII atoms to form a [Cd2(sdb)2] ring13 with a crystallographic centre of inversion (Fig. 1a). Each of these rings is further linked by four bpeb ligands via Cd–N bonds (2.231(7) and 2.279(6) Å). Considering the [Cd2(sdb)2] unit as a node, 1 has the (4,4) net topology (see Fig. S5 in the ESI†). The large void produced in this connectivity is filled by 2-fold interpenetration (Fig. 1b). Despite this interpenetration, 1 has porous channels viewed along the a-axis with a calculated solvent accessible void14 of 22.2% occupied by the DMA solvents (Fig. S7, ESI†). Notably, of the two bpeb ligands bonded to each CdII centre (green node in Fig. 2a), the one with the N1 atom acts as an axle and entangles the [Cd2(sdb)2] ring (wheel) from the neighbouring (4 4) net to form a rotaxane structure and the other bpeb (with N2 atom) is photoreactive, as described below. Accordingly, the 2-fold entanglement15 due to the rotaxane arrangement generates the polyrotaxane array in 1. Such a polyrotaxane entanglement in 2D layers is uncommon.16 Fig. 2 shows the packing of the adjacent layers in 1, it can be seen that the bpeb spacer ligands which are not involved in the rotaxane formation (i.e., the bpeb ligand containing N2 atoms) are aligned in a parallel fashion. Interestingly, the [Cd(bpeb)] zigzag chains are packed one below the other forming a plane. As a result, the bpeb spacer ligands are slip-stacked (or "out-ofphase") relative to each other in such a way that each phenylene
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Fig. 2 (a) The topological representation of the adjacent layers showing alignment of one of the bpeb ligands. CdII in pink, the reactive bpeb spacer in green and rotaxane axle bpeb in blue. (b) and (c) perspective views of the packing showing the alignment of CQC bonds. (d) ‘‘Slip-stacked’’ alignments of alternate bpeb spacers in the [Cd(bpeb)] chains of 1. Hydrogens are not shown for clarity.
ring in the bpeb ligand is closer to a neighbouring pyridyl group with a centroid-to-centroid distance of 3.84 Å (Fig. 2d). This indicates the presence of a strong face-to-face p–p interaction (Fig. 2b and c), and the distance between the centres of the adjacent CQC bonds was found to be 3.76 Å. The slip-stacked arrangement of the adjacent bpeb spacer ligands is congenial for photochemical polymerization via a [2+2] cycloaddition reaction, since only one CQC bond pair is aligned between any two adjacent bpeb ligands in the plane of [Cd(bpeb)] chains. It is apparent from Fig. 2a that such a [2+2] cycloaddition reaction will yield a cyclobutane-based organic polymer with a 3D polyrotaxane structure. Overall, it is expected that only 50% of the total bpeb ligands in 1 will be photoreactive (vide infra). Irradiating the orange single crystals of 1 under a Xe-lamp (l = 365 nm) for 48 h resulted in pale orange crystals of [Cd(bpeb)0.5(poly-bppcb)0.5(sdb)]DMA (2) (where poly-bppcb is a 1,3-(4,4 0 -bipyridyl)-2-phenylcyclobutane polymer, Fig. S11, ESI†) suitable for single crystal X-ray crystallographic analysis. Furthermore, unlike 1 (see Fig. S12, ESI†), characterization by routine solution 1H-NMR spectroscopy was not possible due to its insolubility even in strong acids, indirectly inferring that the expected organic polymer has formed. This behaviour is very similar to that reported recently.9a The X-ray crystallographic analysis of 2 revealed that a [2+2] cycloaddition reaction had indeed taken place in one of the
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Fig. 3 (a) A perspective view of 2 showing the coordination sphere of Cd1. Symmetry code: (A) 1 x, 1 y, 1 z; (B) 1 x, 1 y, 3 z; (C) 1 x, 2 y, 2 z. (b) A perspective view of the packing showing the poly-bppcb. (c) A side view showing poly-bppcb. (d) The 2D sheets formed by the polybppcb with bpeb and CdII atoms in 2. The hydrogen atoms and disorders are not shown for clarity.
bpeb spacer ligands in 1, resulting in the formation of the cyclobutane ring accompanied by the SCSC transformation (Fig. 3a). The cell parameters during this process changed very little and the same space group P1% was retained (Table S1, ESI†). The well-aligned [Cd(bpeb)] zigzag chains in 1 (Fig. 2d) were fused by a [2+2] cycloaddition reaction incorporating the poly-bppcb chain in 2 (Fig. 3d). As discussed above, the 2-fold entangled 2D polyrotaxane structure in 1 is now converted to a 2-fold interpenetrated MOPF with the polyrotaxane structure, due to the poly-bppcb chain associated with the formation of cyclobutane rings (Fig. 3c and d). The channel along the a-axis remains with a small reduction in the calculated solvent accessible void to 20.2% as compared to 1.14 It may be noted that the unreacted bpeb is disordered in 2 over two sites with an occupancy ratio of approximately 55 : 45. A crystallographic inversion centre is also located at the centre of the bppcb and the disordered bpeb ligands. The resultant unusual binodal net in 2 is built from the tetrahedral nodes of CdII atoms and four connected nodes of the cyclobutane rings of the poly-bppcb ligand. It is this infinite extension of the polymer ligand which results in a very rare (3,4) connected net with a tfc topology and one double edge that entangles as a rotaxane with the Point Symbol {83}2{8510} and the Vertex Symbol (88383)(8282828282*) (Fig. 4).17 Recently, solid-state structural transformation of a 2D interdigitated layer CP into a 3D MOF was reported by us.18 The transformation of a 2D bilayer structure to a 3D MOF via photo-cycloaddition was reported by Wu’s group.19 To the best of our knowledge, this is first example of the 2D polyrotaxane MOF transformed into a 3D polyrotaxane MOPF by a photochemical route. Once formed 2 cannot be thermally depolymerized back to 1 unlike the reported example.9a Although the solid-state polymerization in MOFs by g-irradiation20 and a thermal method21 has been reported, the resultant structures of these products were unfortunately unknown. While this work was in progress, Garai and Biradha reported another conjugated
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Fig. 4 A perspective view of the 2-fold entangled net, tfc viewed along the a-axis. The topological representation of the 3D framework created from herringbone style 2D sheets formed from the distorted tetrahedral Cd node (pink), bpeb and the poly-bppcb node (green) via the sdc spacer (yellow). The unreacted bpeb spacer (axle) is shown in pink.
diene ligand in four AgI CPs with different anions to undergo solidstate a [2+2] photo-polymerization reaction in an SCSC fashion.22 Polymerization of bpeb in a MOF appeared recently.23 These organic polymerization reactions in metal complexes, CPs or MOFs should not be confused with metal complex polymerization reactions.24 Previously we have shown that [Zn(bpeb)] zigzag polymer strands could be aligned one below the other for the polymerization of bpeb ligands through a [2+2] cycloaddition reaction forming an unprecedented MOPF structure.9 In this work, we have further demonstrated that the bpeb ligands can be juxtaposed in an infinitely slip-stacked manner in a 2D polyrotaxane. It appears that it is possible to align the bpeb ligands congenial for [2+2] cycloaddition polymerization in more than one ways in MOF compounds. Furthermore, 2 is an exceptional metal complex of an organic polymer for which the crystal structure has been determined unequivocally from the single crystal data.9 In summary, the well-known [2+2] cycloaddition reaction has been used for the photo-polymerization of a conjugated diene ligand, bpeb. The polymerization observed in a MOF is assisted by the CdII atoms which align the alternate bpeb ligands in the [Cd(bpeb)] zigzag chains to pack in a slipstacked manner in a plane. This is a rare photoreactive 2D polyrotaxane MOF formed by 2-fold entanglement. The structural transformation readily occurs between the well-aligned alternate bpeb ligands under UV light yielding another 3D polyrotaxane MOPF, 2 with a tfc topology in an SCSC manner. Compound 2 is also a unique MOF containing an unusual organic polymer as a linker along with the conventional bent dicarboxylate ligand. This photo-dimerized structure 2 has both organic polymer ligands fused together with two-fold entangled rotaxane formed by [Cd(bpeb)0.5(sdb)]. There were not many examples for structurally characterized metal complexes containing organic polymer ligands.9,22 The results described here appear to exemplify that the C–C bond formation by a [2+2] cycloaddition reaction is a general route to access crystalline metal complexes of organic polymer ligands. This work was supported by NRF (2012R1A4A1027750), South Korea and the Ministry of Education, Singapore through a NUS FRC grant R-143-000-562-112.
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