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

Article abstract of DOI:10.1039/c0cc00826e

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 Ag₂(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 Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0cc00826e

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Chemical Communications
www.rsc.org/chemcomm
Volume 46 | Number 28 | 28 July 2010 | Pages 5029–5176
ISSN 1359-7345
COMMUNICATION
Andrew D. Burrows et al.
Solid state interconversion of
cages and coordination networks
via conformational change of a
semi-rigid ligand

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Solid state interconversion of cages and coordination networks via
conformational change of a semi-rigid ligandw
Andrew D. Burrows,*a Christopher G. Frost,a Mary F. Mahon,a Paul R. Raithby,a
Christopher Richardsonab and Anna J. Stevensona
Received 8th April 2010, Accepted 25th May 2010
First published as an Advance Article on the web 8th June 2010
DOI: 10.1039/c0cc00826e
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 Ag2(bisox)3
cages is formed on recrystallisation from acetonitrile–diethyl
ether. Rearrangement of the cages into sheets occurs on heating
Fig. 1 (a) The S- and (b) C-conformations for bisox.
under vacuum, a process which involves a solid state change in
conformation of the bisox ligands.
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 Ch
symmetry, which we denote the C-conformation (Fig. 1b).
The reaction between bisox and silver(I) perchlorate in
methanol gave crystals of [Ag2(bisox)3](ClO4)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.
While bridging ligands are often used to generate rigid metal–
organic 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 hexagonal ‘pores’ within these sheets have internal
widths of 16.8 A, 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–Hꢀ ꢀ ꢀO
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 [Ag2(bismim)3](BF4)2 and [Ag2(bismim)3](CF3SO3)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 Agꢀ ꢀ ꢀAg 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 Agꢀ ꢀ ꢀAg distance is 2.9990(7) A, consistent
with a d10ꢀ ꢀ ꢀd10 interaction, whereas the inter-sheet Agꢀ ꢀ ꢀAg
distance is 4.306 A.
In this communication, we report our initial studies into the
silver(I) networks formed by the semi-rigid bis(isoxazolyl)
ligand 1,4-bis((3,5-dimethylisoxazol-4-yl)methyl)benzene (bisox)
on reaction with silver(I) perchlorate.
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 Ci symmetry, with
the nitrogen donor atoms oriented away from each other. This
is also the conformation adopted in the crystal structure of
a Department of Chemistry, University of Bath, Claverton Down,
Bath BA2 7AY, UK. E-mail: a.d.burrows@bath.ac.uk;
Fax: +44 (0)1225 386231; Tel: +44 (0)1225 386529
b School of Chemistry, University of Wollongong, Wollongong,
NSW 2522, Australia
w Electronic supplementary information (ESI) available: Synthesis of
bisox, TGA for 2, crystallographic details for bisox, 1 and 2. CCDC
772528–772530. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc00826e
Recrystallisation of the crude product from the reaction in
methanol from acetonitrile–diethyl ether gave the diethyl ether
solvate [Ag2(bisox)3](ClO4)2ꢀEt2O 2. The asymmetric unit of 2
contains two partial silver atoms, with one-third occupancy,
two independent ligand halves proximate to inversion centres,
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Fig. 2 The structure of [Ag2(bisox)3](ClO4)2 1 showing (a) the triply
interpenetrated network with the independent sheets shown in different
colours, and (b) the position of the perchlorate ions, shown in red,
between the interpenetrated layers.
Fig. 3 The structure of [Ag2(bisox)3](ClO4)2ꢀEt2O 2, showing (a) the
Ag2(bisox)3 cages, with the included diethyl ether molecules shown in
orange, and (b) the alternation of triply interpenetrated sheets with
rows of cages, shown in dark blue, with perchlorate anions shown in
red. Hydrogen atoms have been omitted for clarity.
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 Ag2(bisox)3 cages, as depicted in Fig. 3a. The diethyl
ether molecules project through the faces of these cages.
templating role in cage formation, and the data suggest the
presence of C–Hꢀ ꢀ ꢀO 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
In the gross structure, the Ag2(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
Agꢀ ꢀ ꢀAg contact—those between the silver atoms within the
sheets [Ag(2)ꢀ ꢀ ꢀAg(2)0 3.0444(11) A (Ag(2)0 generated by
symmetry operation x + 2, ꢂy, ꢂz + 1)] and those between
a silver atom in a sheet and a silver atom in a cage
[Ag(1)ꢀ ꢀ ꢀAg(2) 3.3253(8) A]. The distance between the two
silver atoms in a Ag2(bisox)3 cage is larger, at 8.004(1) A. The
perchlorate anions occupy positions to either side of the
interpenetrated sheets, and are involved in C–Hꢀ ꢀ ꢀO hydrogen
bonds with CH donors on both the sheets and the cages.
The Ag2(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 Et2O molecules to the
cages in the structure of 2 implies that these molecules play a
Fig. 4 Powder X-ray diffraction patterns for the conversion of 2 to
1: (a) simulated pattern for 2, (b) 2, (c) 2 heated at 75 1C under vacuum
for 1 h, and (d) simulated pattern for 1.
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was filtered and washed with methanol. Yield 0.146 g (93%). A crystal
of 1 suitable for X-ray diffraction was obtained from the filtrate.
Found: C, 49.54; H, 4.53; N, 5.91%. C54H60Ag2Cl2N6O14 requires C,
49.75; H, 4.64; N, 6.45%. The crude material from the methanol
reaction was recrystallised by dissolving it in the minimum volume of
CH3CN and allowing Et2O to slowly diffuse into the solution. Crystals
of 2 of suitable size were produced for single crystal X-ray diffraction.
Found: C, 50.60; H, 4.74; N, 6.09%. C58H70Ag2Cl2N6O15 requires C,
50.56; H, 5.12; N, 6.10%. Crystal data for 1: C54H60Ag2Cl2N6O14
,
ꢀ
M = 1303.72, trigonal, P3, a = 14.8380(3) A, c = 7.3050(1) A, U =
1392.84(4) A3, T = 150 K, Z = 1, 20 782 reflections collected of which
2127 are independent [Rint = 0.0712]. R1 = 0.0362, wR2 = 0.0808 for
1611 data with I > 2s(I). GOF = 1.052 based on F2. Crystal data for
2: C58H70Ag2Cl2N6O15, M = 1377.84, hexagonal, P63/m, a =
14.4400(2) A, c = 35.3990(6) A, U = 6392.28(17) A3, T = 150 K,
Z = 4, 67 463 reflections collected of which 3828 are independent
[Rint = 0.1774]. R1 = 0.0455, wR2 = 0.1075 for 2001 data with
I > 2s(I). GOF = 0.937 based on F2. The C–C and C–O distances
within the diethyl ether molecule were restrained to being individually
similar, and restraints were also applied to the associated atomic
displacement parameters.
Fig. 5 The transformations required to convert the Ag2(bisox)3 cages
in 2 into the triply-interpenetrated sheets in 1.
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.
1 O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae,
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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 A. 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 un-
surprising that single crystals do not survive the transformation.
Rearrangement of coordination networks via dissolution–
precipitation 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 [Zn2(4,40-bipy)3(NO3)4] system has been
facilitated by inclusion of guest molecules.9 Dehydration
of networks based on tris(carboxyethyl)isocyanurate10 and
5-sulfoisophthalate11 have both been demonstrated to initiate
reversible transformations involving additional coordination
of donor atoms from the ligands.
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Notes and references
z Synthesis of 1 and 2: bisox (0.144 g, 0.49 mmol) was dissolved in
methanol (10 cm3) with gentle heating and stirring. To this solution,
AgClO4 (0.050 g, 0.24 mmol) dissolved in methanol (3 cm3) was added.
The product was allowed to crystallise overnight. This solid product
11 B. Xiao, P. J. Byrne, P. S. Wheatley, D. S. Wragg, X. Zhao,
A. J. Fletcher, K. M. Thomas, L. Peters, J. S. O. Evans,
J. E. Warren, W. Zhou and R. E. Morris, Nat. Chem., 2009, 1,
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5066 | Chem. Commun., 2010, 46, 5064–5066