Solid state interconversion of cages and coordination networks via conformational change of a semi-rigid ligandw

The reaction of silver(i) perchlorate with the semi-rigid ligand 1,4-bis((3,5-dimethylisoxazol-4-yl)methyl)benzene (bisox) in methanol gives a triply-interpenetrated sheet structure, but a structure containing both interpenetrated sheets and Ag(2)(bisox)(3) cages is formed on recrystallisation from acetonitrile-diethyl ether. Rearrangement of the cages into sheets occurs on heating under vacuum, a process which involves a solid state change in conformation of the bisox ligands.

The reaction of silver(I) perchlorate with the semi-rigid ligand 1,4-bis((3,5-dimethylisoxazol-4-yl)methyl)benzene (bisox) in methanol gives a triply-interpenetrated sheet structure, but a structure containing both interpenetrated sheets and Ag 2 (bisox) 3 cages is formed on recrystallisation from acetonitrile-diethyl ether. Rearrangement of the cages into sheets occurs on heating under vacuum, a process which involves a solid state change in conformation of the bisox ligands.
While bridging ligands are often used to generate rigid metalorganic frameworks, 1 introducing flexibility into a coordination network can allow it to reversibly respond to the presence or absence of a guest. 2 Metal-organic frameworks capable of 'breathing' are normally formed by employing metal centres that are amenable to distortions in the coordination sphere, but an alternative strategy is to incorporate the potential for flexibility into a bridging ligand. If the ligand conformations are totally unrestricted, it might be expected to adjust its conformation in a network to produce a non-porous structure. In contrast, the use of semi-rigid ligands, 3 with inherent but limited flexibility, represents an attractive strategy to form flexible frameworks.
The methylene spacers between the isoxazole rings and the central benzene ring in the bisox ligand allow it to adopt several conformations. Of these, molecular mechanics suggests that the lowest energy conformation is of C i symmetry, with the nitrogen donor atoms oriented away from each other. This is also the conformation adopted in the crystal structure of bisox (see ESIw), and for ease of visualisation, we denote this the S-conformation (Fig. 1a). A rotation of 1801 about one of the benzene-methylene C-C bonds generates a conformation of C h symmetry, which we denote the C-conformation (Fig. 1b).
The reaction between bisox and silver(I) perchlorate in methanol gave crystals of [Ag 2 (bisox) 3 ](ClO 4 ) 2 1 suitable for a single crystal X-ray analysis.z The asymmetric unit of 1 contains a third of a silver(I) centre, located on a three-fold rotation axis, half of a bisox ligand, proximate to an inversion centre, and a perchlorate anion fragment, in which the chlorine and one oxygen atom are located on the three-fold rotation axis. Each silver(I) centre is trigonal planar and coordinated to three bisox ligands, which all adopt S-conformations, and link the silver centres into (6, 3) sheets.
The hexagonal 'pores' within these sheets have internal widths of 16.8 Å , and unsurprisingly interpenetration occurs to reduce the void space. Overall, the structure is triply parallel interpenetrated as shown in Fig. 2a. The layers contain Borromean links, 4 which involve three interlocked rings. The interpenetration generates triangular pores, and the perchlorate anions reside towards the top and bottom of these, between the layers, as shown in Fig. 2b. These anions form C-HÁ Á ÁO hydrogen bonds with both methyl and aromatic CH groups.
The structure of 1 is related to those reported previously for the 1,4-bis(2-methylimidazol-1-ylmethyl)benzene (bismim) networks [Ag 2 (bismim) 3 ](BF 4 ) 2 and [Ag 2 (bismim) 3 ](CF 3 SO 3 ) 2 . 5 Subtle differences were seen in the two silver-bismim networks, with a change of orientation of the methylimidazolyl rings allowing accommodation of the larger triflate anion at the expense of AgÁ Á ÁAg interactions. The bisox ligand in 1 has methyl groups on both sides of the five-membered ring, and lies approximately parallel to the silver coordination plane. The intra-sheet AgÁ Á ÁAg distance is 2.9990(7) Å , consistent with a d 10 Á Á Ád 10 interaction, whereas the inter-sheet AgÁ Á ÁAg distance is 4.306 Å .
Recrystallisation of the crude product from the reaction in methanol from acetonitrile-diethyl ether gave the diethyl ether solvate [Ag 2 (bisox) 3 ](ClO 4 ) 2 ÁEt 2 O 2. The asymmetric unit of 2 contains two partial silver atoms, with one-third occupancy, two independent ligand halves proximate to inversion centres, two perchlorate fragments (central chlorines one-third occupancy) and a diethyl ether molecule with one-third occupancy. Both independent silver centres have trigonal planar geometry, each coordinated to three symmetry-related bisox ligands. However, the two independent bisox ligands have different conformations, with that coordinated to Ag(2) in the S-conformation, and that coordinated to Ag(1) in the C-conformation. As a consequence, Ag(2) forms triply parallel interpenetrated (6,3) sheets similar to those observed in 1, whereas Ag(1) forms discrete Ag 2 (bisox) 3 cages, as depicted in Fig. 3a. The diethyl ether molecules project through the faces of these cages.
In the gross structure, the Ag 2 (bisox) 3 cages lie directly above the silver centres of the interpenetrated sheets, as shown in Fig. 3b. As a consequence, there are two types of short AgÁ Á ÁAg contact-those between the silver atoms within the sheets [Ag(2)Á Á ÁAg(2) 0 3.0444(11) Å (Ag(2) 0 generated by symmetry operation x + 2, Ày, Àz + 1)] and those between a silver atom in a sheet and a silver atom in a cage [Ag(1)Á Á ÁAg(2) 3.3253(8) Å ]. The distance between the two silver atoms in a Ag 2 (bisox) 3 cage is larger, at 8.004(1) Å . The perchlorate anions occupy positions to either side of the interpenetrated sheets, and are involved in C-HÁ Á ÁO hydrogen bonds with CH donors on both the sheets and the cages.
The Ag 2 (bisox) 3 cages are not observed in the absence of diethyl ether, and when the reaction is carried out in acetonitrile, only 1 is observed. The proximity of the Et 2 O molecules to the cages in the structure of 2 implies that these molecules play a templating role in cage formation, and the data suggest the presence of C-HÁ Á ÁO hydrogen bonds.
Given the similarity in formulae of 1 and 2, and the correspondence in the relative positions of the silver centres in the two structures, we sought to investigate whether 2 could be converted to 1 by removal of diethyl ether, or whether desolvated cages could be obtained. No change was observed in the X-ray powder diffraction pattern of 2 on standing under vacuum for 4 h, or on heating at 50 1C under vacuum, but changes were observed when the sample was heated at 75 1C under vacuum (Fig. 4). On heating for 4 h, the sample slowly converted into 1, as witnessed by changes in the X-ray powder   diffraction pattern. These changes are consistent with complete and clean conversion of 2 to 1. The TGA for 2 showed a mass loss of 2.7% between 100 and 120 1C which corresponds to removal of half of the diethyl ether (calc. for complete removal 5.4%). The remainder of the solvent is lost more gradually, with the network beginning to decompose at 280 1C.
The conversion of 2 to 1 is remarkable, given the extent of conformational reorganisation of the bisox ligands required. Conversion from the C-conformation in 2 to the S-conformation in 1 requires a rotation of 1801 of one of the isoxazolyl rings about a methylene carbon atom which, in turn, involves the ring nitrogen atom moving through an arc of approximately 10.7 Å . This conformational change is accompanied by breaking and re-forming half of the Ag-N bonds, as shown in Fig. 5. Conversion of the cages into sheets also requires substantial movement of the silver centres, which is consistent with a shrinking along the c direction of 17%. Given this dramatic change in cell parameters between 2 and 1, it is perhaps unsurprising that single crystals do not survive the transformation.
Rearrangement of coordination networks via dissolutionprecipitation pathways is well established, 6 though solid state transformations are less common, and typically involve changes in the metal coordination environments. 7 Notably, interconversion between 1D, 2D and 3D silver-polynitrile coordination polymers have been shown to be promoted by anion exchange, 8 whereas conversion between 1D ladders and 2D bilayers in the [Zn 2 (4,4 0 -bipy) 3 (NO 3 ) 4 ] system has been facilitated by inclusion of guest molecules. 9 Dehydration of networks based on tris(carboxyethyl)isocyanurate 10 and 5-sulfoisophthalate 11 have both been demonstrated to initiate reversible transformations involving additional coordination of donor atoms from the ligands.
Further study of the transformation of 2 to 1 is currently underway, as is investigation of the reaction between bisox and other silver(I) salts.
The EPSRC, the Leverhulme Trust and the Cambridge Crystallographic Data Centre are thanked for financial support.

Notes and references
z Synthesis of 1 and 2: bisox (0.144 g, 0.49 mmol) was dissolved in methanol (10 cm 3 ) with gentle heating and stirring. To this solution, AgClO 4 (0.050 g, 0.24 mmol) dissolved in methanol (3 cm 3 ) was added. The product was allowed to crystallise overnight. This solid product was filtered and washed with methanol. Yield 0.146 g (93%).