Silver coordination networks and cages based on a semi-rigid bis(isoxazolyl) ligand

Article abstract of DOI:10.1039/c1dt10029g

The semi-rigid ligand 1,4-bis((3,5-dimethylisoxazol-4-yl)methyl)benzene (bisox) reacts with a range of silver(i) salts to give products in which the anions dictate the structure. The reactions with AgNO₃ and AgO ₂CCF₃ both lead to compounds in which the anions are coordinated to the silver centres. Thus, the structure of [Ag ₂(NO₃)₂(bisox)] 1 contains helical silver-nitrate chains that are linked into sheets by bridging bisox ligands, whereas the structures of [Ag(O₂CCF₃)(bisox)]·0.5X (2a, X = MeOH; 2b, X = MeCN) consist of sheets in which Ag₂(μ- O₂CCF₃)₂ dimers act as 4-connecting nodes. In these structures bisox adopts the S-conformation, with the nitrogen donor atoms anti to each other. The reactions of bisox with AgClO₄ and AgBF ₄ in methanol give the compounds [Ag₂(bisox) ₃]X₂ (3, X = ClO₄; 5, X = BF₄), the structures of which contain triply-interpenetrated sheets with Borromean links and ligands in the S-conformation. Recrystallisation of these compounds from acetonitrile-diethyl ether gives [Ag₂(bisox)₃]X ₂·xEt₂O (4, X = ClO₄, x = 1; 6, X = BF₄, x = 1.2). The structures of 4 and 6 contain similar triply-interpenetrated sheets to those in 3 and 5, though these are sandwiched between sheets of discrete Ag₂(bisox)₃ cages, in which the bisox ligands are in the C-conformation, with the nitrogen donor atoms syn to each other. Diethyl ether molecules project through the faces of the cages and template cage formation. Both 4 and 6 lose diethyl ether on heating in vacuum, and convert into 3 and 5, respectively. This solid state transformation requires a change in conformation of half the bisox ligands, with conversion of 6 into 5 occurring more readily than conversion of 4 into 3. The reactions of bisox with AgPF₆ and AgSbF₆ in methanol give mixtures of products from which [Ag(bisox)₂]X·0.5bisox (7, X = PF₆; 8, X = SbF₆) can be isolated. Both 7 and 8 have structures containing one-dimensional chains, in which the bisox ligands adopt C-conformations and interconnect distorted tetrahedral silver centres in a pairwise manner generating macrocycles. Additional uncoordinated bisox molecules lie within half of these macrocyclic rings. Recrystallisation of the crude AgSbF ₆/bisox reaction mixture from acetonitrile-diethyl ether gives [Ag(bisox)₂]SbF₆9, the structure of which consists of a triply-interpenetrated flattened diamondoid network. A similar structure was observed for [Ag(bisox)₂]CF₃SO₃10, which is formed from the reaction of AgO₃SCF₃ and bisox in methanol. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt10029g

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PAPER
Silver coordination networks and cages based on a semi-rigid bis(isoxazolyl)
ligand†
Andrew D. Burrows,*a David J. Kelly,a Mary F. Mahon,a Paul R. Raithby,a Christopher Richardsona,b and
Anna J. Stevensona
Received 7th January 2011, Accepted 7th March 2011
DOI: 10.1039/c1dt10029g
The semi-rigid ligand 1,4-bis((3,5-dimethylisoxazol-4-yl)methyl)benzene (bisox) reacts with a range of
silver(I) salts to give products in which the anions dictate the structure. The reactions with AgNO3 and
AgO2CCF3 both lead to compounds in which the anions are coordinated to the silver centres. Thus, the
structure of [Ag2(NO3)2(bisox)] 1 contains helical silver-nitrate chains that are linked into sheets by
bridging bisox ligands, whereas the structures of [Ag(O2CCF3)(bisox)]·0.5X (2a, X = MeOH; 2b, X =
MeCN) consist of sheets in which Ag2(m-O2CCF3)2 dimers act as 4-connecting nodes. In these structures
bisox adopts the S-conformation, with the nitrogen donor atoms anti to each other. The reactions of
bisox with AgClO4 and AgBF4 in methanol give the compounds [Ag2(bisox)3]X2 (3, X = ClO4; 5, X =
BF4), the structures of which contain triply-interpenetrated sheets with Borromean links and ligands in
the S-conformation. Recrystallisation of these compounds from acetonitrile–diethyl ether gives
[Ag2(bisox)3]X2·xEt2O (4, X = ClO4, x = 1; 6, X = BF4, x = 1.2). The structures of 4 and 6 contain similar
triply-interpenetrated sheets to those in 3 and 5, though these are sandwiched between sheets of discrete
Ag2(bisox)3 cages, in which the bisox ligands are in the C-conformation, with the nitrogen donor atoms
syn to each other. Diethyl ether molecules project through the faces of the cages and template cage
formation. Both 4 and 6 lose diethyl ether on heating in vacuum, and convert into 3 and 5, respectively.
This solid state transformation requires a change in conformation of half the bisox ligands, with
conversion of 6 into 5 occurring more readily than conversion of 4 into 3. The reactions of bisox with
AgPF6 and AgSbF6 in methanol give mixtures of products from which [Ag(bisox)2]X·0.5bisox (7, X =
PF6; 8, X = SbF6) can be isolated. Both 7 and 8 have structures containing one-dimensional chains, in
which the bisox ligands adopt C-conformations and interconnect distorted tetrahedral silver centres in
a pairwise manner generating macrocycles. Additional uncoordinated bisox molecules lie within half of
these macrocyclic rings. Recrystallisation of the crude AgSbF6/bisox reaction mixture from
acetonitrile–diethyl ether gives [Ag(bisox)2]SbF6 9, the structure of which consists of a
triply-interpenetrated flattened diamondoid network. A similar structure was observed for
[Ag(bisox)2]CF3SO3 10, which is formed from the reaction of AgO3SCF3 and bisox in methanol.
porosity properties. Potential applications in gas storage are
Introduction
especially topical,2 but porous coordination networks are also
attractive for a range of other applications including separations,3
catalysis4 and drug delivery.5
Coordination network structures in which bridging ligands link
metal centres or aggregates into extended structures are currently
attracting considerable attention.1 Much of this interest has
arisen because these materials, often referred to as metal–organic
frameworks, frequently retain their structures and crystallinity on
removal of solvents, which enables them to display interesting
While rigid ligands are often used to generate inflexible frame-
works, there is an increasing recognition that including a degree
of flexibility into a network can allow it to reversibly respond to
the presence or absence of a guest.6 One way of achieving this
is to introduce flexibility into a bridging ligand. If the ligand is
completely flexible, then it is likely that it would be able to adjust
its conformation in a network to produce a non-porous structure.
In contrast, use of semi-rigid ligands,7 which contain only a limited
degree of flexibility, form an attractive strategy towards porous but
non-rigid frameworks.
aDepartment of Chemistry, University of Bath, Claverton Down, Bath, BA2
7AY, UK
bSchool of Chemistry, University of Wollongong, Northfields Avenue, Wol-
longong NSW, 2522, Australia
† CCDC reference numbers 804679–804686. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt10029g
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Networks formed by silver(I) and ditopic nitrogen donor ligands
have attracted interest for a number of reasons.8 The 4d10 silver(I)
centre can adopt coordination numbers from 2 to 6, with no strong
energetic preference for any particular geometry. A wide range
of silver(I) salts are readily available, which makes it relatively
straightforward to assess the effects of the counter-ion on the
structure of the network formed between the metal and a given
ligand.9,10 In addition, weak d10 ◊ ◊ ◊ d10 interactions can also have a
structurally defining effect.
In this paper, we report our studies into the silver(I) net-
works formed by the semi-rigid bis(isoxazolyl) ligand 1,4-bis((3,5-
dimethylisoxazol-4-yl)methyl)benzene, bisox. We have previously
communicated the results with AgClO4.11
Imidazolyl and triazolyl12 ligands with a similar geometry
to bisox have been studied previously. Most notably, the re-
actions between silver(I) and the bis(pyrazolylmethyl)benzene
ligands 1,4-bis(imidazol-1-ylmethyl)benzene (bisim) and 1,4-
bis(2-methylimidazol-1-ylmethyl)benzene (bismim) have attracted
attention, with examples of polyrotaxanes,13 coordination
networks14,15 and macrocycles16 illustrating the potential afforded
by semi-rigid ligands.
The work reported in this paper represents, to the best of our
knowledge, the first use of a bis(isoxazolyl) ligand in forming
coordination networks. However, there have been a few examples
of networks formed from substituted isoxazole ligands. Silver(I)
coordination polymers have been prepared using isoxazolyl lig-
ands containing either amido or sulfide groups,17 and a copper(II)
network has been formed from isoxazole-4-carboxylate.18
The bisox ligand was prepared by the reaction of 3,3¢-(1,4-
phenylenebis(methylene))bis(4-hydroxypent-3-en-2-one)19 with
hydroxylamine, and characterised by a combination of NMR
spectroscopy, mass spectrometry and microanalysis. In order
to keep the number of variables to a minimum, the reactions
between bisox and silver(I) salts were carried out in methanol,
and slow evaporation of the solvent was used to facilitate
crystal growth. The resultant solids were also recrystallised from
acetonitrile–diethyl ether.
The bisox ligand contains methylene spacers between the isoxa-
zole rings and the central benzene ring, allowing it to adopt several
conformations. Of these, molecular mechanics suggests the lowest
energy conformation is of Ci symmetry, with the nitrogen donor
atoms oriented away from each other. For ease of visualization,
we denote this the S- conformation. A rotation of 180◦ about one
of the benzene-methylene C–C bonds generates a conformation of
Ch symmetry, which we denote the C- conformation. The S- and
C- conformations are shown in Fig. 1. While other conformations
are also possible by rotation about the C(sp3)–C(sp2) bonds, the
presence of the central benzene ring and two isoxazole rings limit
the ligand flexibility.
Reaction of silver(I) nitrate with bisox
The reaction between bisox and silver(I) nitrate in methanol
gave crystals that were too small for single crystal analysis.
When this compound was recrystallised from acetonitrile–diethyl
ether, crystals of [Ag2(NO3)2(bisox)] 1 that were suitable for
a single crystal X-ray analysis resulted, though the powder
X-ray diffraction pattern suggests an unidentified impurity is also
present.
The asymmetric unit of 1 contains a silver(I) centre, a coordi-
nated nitrate anion and half of a bisox ligand, with the remainder
of this ligand generated by inversion symmetry. Thus, the bisox
ligand adopts an S- conformation in this structure. Each silver(I)
centre has a distorted trigonal planar geometry, coordinated to
two oxygen atoms from different nitrate ions and one nitrogen
atom from a bisox ligand. The bond angles around the silver
centre range from 101.16(10)◦ to 137.42(10)◦ and the sum of the
angles is 355.5◦. The nitrate ions bridge between silver(I) centres in
1
1
the m-k O,k O¢- coordination mode to form helical chains. These
chains are cross-linked by the bridging bisox ligands to form two-
dimensional sheets, as shown in Fig. 2a. There are two sets of these
parallel sheets in the crystal structure, with an angle of 57◦ between
the least squares planes of each set. Similar silver-nitrate chains
have been previously observed with 3,6-bis(2-pyrazinyl)-1,2,4,5-
tetrazine and 1,4-bis(2-pyridylaminomethyl)benzene co-ligands.20
Each sheet contains 44-membered metallomacrocyclic rings
which, because of the ligand conformation, adopt ‘hour-glass’
shapes. Two bisox ligands from the other set of sheets are
directed through each of the rings in an example of 2D inclined
interpenetration, as shown in Fig. 2b. The isoxazole rings of the
interpenetrating ligands lie equidistant between pairs of isoxazole
rings in the same metallomacrocyclic ring, and distances between
˚
the isoxazole planes of 3.3–3.8 A suggest the presence of p ◊ ◊ ◊ p
interactions.
Fig. 1 (a) The S-conformation and (b) the C-conformation for bisox.
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Fig. 2 The structure of [Ag2(NO3)2(bisox)] 1 showing (a) the sheet structure, and (b) the 2D inclined interpenetration of the sheets. Hydrogen atoms
˚
have been removed for clarity. Selected bond lengths and angles: Ag(1)–N(1) 2.195(3), Ag(1)–O(2) 2.307(3), Ag(1)–O(3)¢ 2.468(3) A, N(1)–Ag(1)–O(2)
137.42(10), N(1)–Ag(1)–O(3)¢ 101.16(10), O(2)–Ag(1)–O(3)¢ 116.93(9)◦. Primed atoms generated by the symmetry operation x + 1/2, -y + 1, -z + 1/2.
Reaction of silver(I) trifluoroacetate and bisox
The bridging bisox ligands link the dimers into a (4,4)-
connected two-dimensional grid as shown in Fig. 3b, containing
64-membered metallomacrocyclic rings. These rings are sup-
The reaction between bisox and silver(I) trifluoroacetate in
methanol gave [Ag(O2CCF3)(bisox)]·0.5MeOH 2a. The asymmet-
ric unit of 2a contains a silver(I) centre, a trifluoroacetate anion,
coordinated by one of its oxygen atoms, two independent halves of
bisox ligand and a fraction of an included methanol molecule. The
remaining halves of the bisox ligands are generated by symmetry,
and as with 1, the ligands adopt S- conformations. A symmetry
centre proximate to the silver centre leads to the formation of
˚
ported by C–H ◊ ◊ ◊ O hydrogen bonds [C(14) ◊ ◊ ◊ O(4) 3.355 A,
◦
˚
H(14A) ◊ ◊ ◊ O(4) 2.40 A, C(14)–H(14A) ◊ ◊ ◊ O(4) 165 ]. A combi-
nation of the partial occupancy methanol molecules and interdig-
itation by the trifluoroacetate groups of neighbouring sheets fill the
rings and reduces the potential for porosity within the structure.
The included methanol molecules also form hydrogen bonds with
the fluorine atoms of the trifluoroacetate groups. Ag2(O2CCF3)2N4
units such as those observed in 2a have been observed previously
in discrete structures,21 but this is the first example in an extended
network.
˚
Ag2(m-O2CCF3)2 dimers, with a Ag ◊ ◊ ◊ Ag distance of 3.0640(5) A.
Each silver centre is 4-coordinate, and bonded to two bisox ligands
in addition to the oxygen atoms of two carboxylates. This means
that each Ag2(m-O2CCF3)2 dimeric unit acts as a 4-connecting
node, with a distorted square-planar geometry, as shown in Fig.
3a.
Recrystallisation of 2a from a acetonitrile–diethyl ether mixture
gave crystals of [Ag(O2CCF3)(bisox)]·0.5MeCN 2b, which has a
similar gross structure to that of 2a. Analysis of the unit cell
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Fig. 3 The structure of [Ag(O2CCF3)(bisox)]·0.5MeOH 2, showing
(a) the dimeric Ag2(O2CCF3)2 unit and (b) the sheet structure.
Included methanol molecules and minor disordered anion compo-
nents have been removed for clarity. Selected bond lengths and an-
gles: Ag(1)–N(1) 2.335(3), Ag(1)–N(2) 2.337(3), Ag(1)–O(4) 2.377(3),
˚
Ag(1)–O(3)¢ 2.303(3) A, N(1)–Ag(1)–N(2) 108.25(10), O(3)¢–Ag(1)–N(1)
99.88(10), O(3)¢–Ag(1)–N(2) 116.19(12), N(1)–Ag(1)–O(4) 91.07(9),
N(2)–Ag(1)–O(4) 95.35(11), O(3)¢–Ag(1)–O(4) 140.68(11)◦. Primed atoms
generated by the symmetry operation -x + 1, -y + 2, -z + 1.
parameters reveals one of the cell lengths of 2a is approximately
twice that of the corresponding parameter in 2b.
Reaction between silver(I) perchlorate and bisox
The reaction between bisox and silver(I) perchlorate in methanol
gave crystals of [Ag2(bisox)3](ClO4)2 3, whereas recrystallisation of
the crude product from acetonitrile–diethyl ether gave the diethyl
ether solvate [Ag2(bisox)3](ClO4)2·Et2O 4.
Fig. 4 The structure of [Ag2(bisox)3](ClO4)2 3 showing (a) one of the (6,3)
Ag2(bisox)3 layers present in the structure, (b) the triply interpenetrated
network with the independent sheets shown in different colours, and (c) the
position of the perchlorate anions with respect to the triangular pores, with
C–H ◊ ◊ ◊ O hydrogen bonds shown in red and white, and the hydrogen atom
groups not involved in these interactions omitted for clarity. Selected bond
The asymmetric unit of 3 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. The bisox ligands
adopt S- conformations, and link the silver centres into (6,3) sheets,
as shown in Fig. 4a. The hexagonal ‘pores’ within these sheets have
◦
˚
lengths and angles: Ag(1)–N(1) 2.225(2) A, N(1)–Ag(1)–N(1)¢ 119.888(7) .
Primed atoms generated by the symmetry operation -y, x - y, z.
˚
internal widths of 16.8 A, and unsurprisingly interpenetration
The structure of 3 is similar to that observed previously for the
1,4-bis(2-methylimidazol-1-ylmethyl)benzene (bismim) networks
[Ag2(bismim)3](BF4)2 and [Ag2(bismim)3](CF3SO3)2.14,22 Subtle
differences were seen in the two silver-bismim networks, with a
change in orientation of the methylimidazolyl rings facilitating
the presence of the larger triflate anion at the expense of Ag ◊ ◊ ◊ Ag
interactions. The bisox ligand has methyl groups on both sides
occurs to reduce the free space. The structure is triply parallel
interpenetrated as shown in Fig. 4b. The layers contain Borromean
links, which involve three interlocked rings. The triangular pores
generated by this interpenetration are sandwiched between per-
chlorate anions, which are involved in C–H ◊ ◊ ◊ O hydrogen bonds
with both methyl and aromatic CH groups, as shown in Fig. 4c.
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of the five-membered ring, and lies approximately parallel to the
positions to either side of the interpenetrated sheets, and are
involved with C–H ◊ ◊ ◊ O hydrogen bonds with CH donors on both
the sheets and the cages.
silver coordination plane. The Ag ◊ ◊ ◊ Ag distance in 3 is 2.9990(7)
10
A, consistent with a d ◊ ◊ ◊ d10 interaction.
˚
The asymmetric unit of 4 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. This generates the overall formula for 4 as
[Ag2(bisox)3](ClO4)2·Et2O. Both independent silver centres have
trigonal planar geometry, 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, whereas that coordinated to Ag(1) is in the
C- conformation. As a consequence, Ag(2) forms triply parallel
interpenetrated (6,3) sheets similar to those in 3, whereas Ag(1)
forms discrete Ag2(bisox)3 cages, as depicted in Fig. 5a. The diethyl
ether molecules project through the faces of these cages.
Reaction of silver(I) tetrafluoroborate with bisox
The reactions of bisox with AgBF4 proceed in a similar manner
to the reactions with AgClO4. Thus the product from the reaction
in methanol was [Ag2(bisox)3](BF4)2 5, which exhibits a largely
identical X-ray powder diffraction pattern to that of 3. Recrystalli-
sation of the crude methanol reaction product from acetonitrile–
diethyl ether yielded the solvate [Ag2(bisox)3](BF4)2·1.2Et2O 6,
which is isostructural with 4. In 6, the portion of diethyl ether
in the asymmetric unit refines to 0.40 occupancy, giving a slightly
greater proportion of included solvent in the structure than in
4, but otherwise there are no significant differences between the
structures.
Ag2L3 cages have not been reported for the related
bisim ligands, though Ag2L2 macrocycles have been ob-
served in the structures of [Ag2(bismim)2](BF4)2·2MeCN16 and
[Ag2(Fbisim)2](BF4)2·H2O·MeOH (Fbisim = 1,1¢-(perfluoro-1,4-
phenylene)bis(methylene)-bis(1H-imidazole).23 Interestingly, the
acetonitrile molecules in [Ag2(bismim)2](BF4)2·2MeCN occupy
the centre of the macrocycles, and can be removed by heating
to 80 ◦C. The resulting desolvated material retains the crystal
structure, and has been shown to adsorb N2, H2, CO2 and CH4 at
◦
30 C. [Ag2(Fbisim)2](PF6)2·2MeCN has recently been shown to
lose solvent in a single-crystal-to-single-crystal transformation.24
The reaction between silver(I) tetrafluoroborate and bisox was
carried out in acetonitrile in the absence of diethyl ether to see
whether analogous Ag2(bisox)2 cages could be formed. However,
the powder X-ray diffraction pattern of the product revealed it
to be [Ag2(bisox)3](BF4)2 5, which suggests that diethyl ether is
important in templating formation of the Ag2(bisox)3 cages.
Reaction of silver(I) hexafluorophosphate with bisox
The reaction of bisox with silver(I) hexafluorophosphate in
methanol gave a mixture of products from which crystals of
[Ag(bisox)2]PF6·0.5bisox 7 were isolated. The asymmetric unit
of 7 contains one silver centre, two and a half bisox molecules
and one hexafluorophosphate anion. The two complete bisox
molecules adopt C-shaped conformations and are coordinated,
bridging between silver centres. Each silver centre coordinates to
four ligands, and adopts a distorted tetrahedral geometry, with
bond angles ranging from 100.14(7) to 117.09(7)◦. The bisox
ligands connect the silver centres into one-dimensional chains of
macrocycles, as shown in Fig. 6. The remaining bisox molecules,
generated from the half molecule in the asymmetric unit, are
uncoordinated, and lie within half of the macrocyclic rings, so
that alternate rings are filled and empty. The uncoordinated bisox
molecules are in the S- conformation. The hexafluorophosphate
anions lie between the chains.
7 can be regarded as a polypseudorotaxane, and it has some
features in common with the two-dimensional polyrotaxane net-
work formed by [Ag2(bisim)3](NO3)2 (bisim = 1,4-bis(imidazol-1-
ylmethyl)benzene).13 In this compound, similar macrocyclic rings
to those seen in 7 also encompass additional ligands, although
in the bisim case, in contrast to 7, the additional ligands are
coordinated.
Fig. 5 The structure of [Ag2(bisox)3](ClO4)2·Et2O 4, 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. Perchlorate anions and hydrogen atoms have been omitted
for clarity. Selected bond lengths and angles: Ag(1)–N(1) 2.234(4),
˚
Ag(2)–N(2) 2.216(4) A, N(1)–Ag(1)–N(1)¢ 119.841(14), N(2)–Ag(2)–N(2)¢
119.745(17)◦. Primed atoms generated by the symmetry operation -y + 1,
x - y - 1, z.
In the gross structure, the Ag2(bisox)3 cages lie directly above
the silver centres of the interpenetrated sheets forming layers,
as shown in Fig. 5b. As a consequence, there are two types of
short Ag ◊ ◊ ◊ Ag contact—those between the silver atoms within
˚
the sheets [Ag(2) ◊ ◊ ◊ Ag(2)¢ 3.0444(11) A (Ag(2)¢ generated by
symmetry operation x + 2, -y, -z + 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 8.004(1) A. The perchlorate anions occupy
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Fig. 6 The structure of [Ag(bisox)2]PF6·0.5bisox 7 showing the one-dimensional chains and the included bisox molecules. Hydrogen atoms are omitted for
clarity, and included bisox molecules are shown in orange. Selected bond lengths and angles: Ag(1)–N(1) 2.2813(19), Ag(1)–N(3) 2.2532(19), Ag(1)–N(4)¢
˚
2.2983(19), Ag(1)–N(2)” 2.348(2) A, N(3)–Ag(1)–N(1) 114.72(7), N(3)–Ag(1)–N(4)¢ 117.09(7), N(1)–Ag(1)–N(4)¢ 111.42(7) N(3)–Ag(1)–N(2)” 105.55(7),
N(1)–Ag(1)–N(2)” 105.97(7), N(4)¢–Ag(1)–N(2)” 100.14(7)◦. Primed atoms generated by the symmetry operation -x + 1, - y, -z. Double primed atoms
generated by the symmetry operation -x + 3, -y + 1, -z + 1.
Reaction of silver(I) hexafluoroantimonate with bisox
formed using the related bis(pyrazolyl) ligand bisim,14,22 and in all
cases the interweaving is facilitated by the relative positions of the
nitrogen donors in the S- conformation of both bisox and bisim.
The size and shape of the ClO4 and BF4 ions are important in
forming this structure, with the anions located proximate to the
triangular pores and involved in CH ◊ ◊ ◊ X interactions.
The reaction of bisox with silver(I) hexafluoroantimonate in
methanol gave a mixture of products from which crystals of
[Ag(bisox)2]SbF6·0.5bisox 8 were isolated. These were shown by an
X-ray single crystal analysis to be isostructural with 7, so will not
be discussed further. The crude reaction mixture was recrystallised
from acetonitrile–diethyl ether to give [Ag(bisox)2]SbF6 9 as the
sole product. The asymmetric unit of 9 consists of one silver
centre located on a crystallographic two-fold axis, two independent
ligand halves, the remainder of each generated via an inversion
centre in the middle of the six-membered ring, and half of
a hexafluoroantimonate anion fragment with the central atom
located at an inversion centre. Each silver centre coordinates to
four ligands, and adopts a distorted tetrahedral geometry, as
shown in Fig. 7a, with bond angles ranging from 94.46(8) to
126.85(6)◦. The bisox ligands adopt S- conformations, and link
the silver centres into a three-dimensional flattened diamondoid
network, as shown in Fig. 7b. This is triply-interpenetrated, as
shown in Fig. 7c. The SbF6- anions are accommodated in cavities,
and interact with the network through C–H ◊ ◊ ◊ F hydrogen bonds
involving the methyl and methylene groups.
-
-
Recrystallisation of 3 and 5 from acetonitrile–diethyl ether gives
compounds 4 and 6, the structures of which contain discrete
Ag2L3 cages in addition to sheets. These cages are not observed
in the absence of diethyl ether, and the proximity of the Et2O
molecules to the cages in the structures of 4 and 6 suggest the
diethyl ether molecules play a templating role. This is further
evidenced by the fact that carrying out the reaction between
silver(I) tetrafluoroborate and bisox in acetonitrile generates 5, in
which the Ag2L3 cages are absent. Ag ◊ ◊ ◊ Ag interactions may be
important in stabilising the structures of 3–6. Ag ◊ ◊ ◊ Ag distances
˚
of 2.9990(7), 3.0444(11) and 3.0597(12) A were observed between
the interwoven sheets in 3, 4 and 6 respectively, whereas Ag ◊ ◊ ◊ Ag
˚
distances of 3.3253(8) and 3.3236(9) A were observed between the
sheets and cages in 4 and 6.
The triply interpenetrated sheets observed in 3–6 are not
observed when using silver salts containing larger anions. The
lack of tetrahedral symmetry may also be a factor, as formation of
the C–H ◊ ◊ ◊ X interactions between the ligands and the anions
would orientate an octahedral anion or triflate in a sterically
unfavourable position, disfavouring formation of this structural
type with respect to one-dimensional chains in the case of 7 and 8,
or three dimensional diamondoid structures in the cases of 9 and
10.
Reaction of silver(I) triflate with bisox
The reaction of bisox with silver(I) triflate in methanol gave
[Ag(bisox)2]CF3SO3 10, which is isostructural to 9. The anion lies
on an inversion centre, which ensures disorder between the two
halves. It is involved in C–H ◊ ◊ ◊ O/F hydrogen bonds.
In order to see whether silver-bisox fragments would pref-
erentially crystallise with a particular anion, reactions between
bisox and a mixture of silver(I) salts were studied. In all cases,
the powder X-ray diffraction patterns suggested these reactions
yielded mixtures of products, suggesting selective crystallisation
does not occur with this series of compounds.
Discussion
Analysis of the crystal structures of 1–4 and 6–10 reveals a number
of interesting features. The nitrate and trifluoroacetate anions are
well established as being able to coordinate to silver(I) centres,9
and the products from the reactions between bisox and AgNO3 or
AgO2CCF3 both contain Ag-anion bonds. Indeed, these bonds
play an important role in determining the structures of these
compounds, and the ability of a carboxylate to bridge metal centres
into dimers is evident in the structures of 2a and 2b.
Solid state interconversions
Rearrangement of coordination networks via dissolution-
precipitation pathways is well established,25 though solid state
transformations are less common, and typically involve a change in
-
-
The tetrahedral anions ClO4 and BF4 give the isostructural
triply-interpenetrated sheet structures 3 and 5. Similar sheets are
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solvate molecules in 4 could be removed to leave empty cages, or
whether loss of solvent would facilitate a structural rearrangement.
The initial X-ray powder diffraction for 4 suggested the
compound was phase pure, with no other crystalline compounds
observed. The TGA is also consistent with the presence of the
Et2O molecules. A mass loss of 2.7% was observed between 100
and 120 ◦C which corresponds to the removal of half of the diethyl
ether. The remainder of the solvent is lost more gradually, with
the network decomposition beginning at around 280 ◦C.
A powdered sample of 4 was placed under vacuum for 4 h,
and then heated at 50 ◦C under vacuum for 4 h. During this
time, a powder X-ray diffraction pattern was recorded every hour.
These were all identical to that observed initially for 4, signifying
no structural rearrangement had occurred. TGA on the heated
samples confirmed that diethyl ether was still present, confirming
that no change had occurred. The sample was then heated at
75 ◦C under vacuum for 4 h, with a powder X-ray diffraction
pattern again recorded every hour. These patterns revealed that a
gradual conversion of 4 into 3 was occurring (Fig. 8) which was
completed after 4 h. The conversion appears to be clean, with
no other products observed in the powder diffraction traces. The
TGA confirmed the complete loss of the solvent, and furthermore
showed no mass loss until the onset of decomposition at 280 ◦C.
The conversion of 4 into 3 involves a significant rearrangement
of the network, so is remarkable given the material remains
crystalline throughout, although single crystals do not survive this
transformation. The bisox ligand is required to rearrange from the
C-conformation in 4 to the S-conformation in 5, which requires a
rotation of 180◦ of one of the isoxazolyl rings about a methylene
carbon atom. This, in turn, involves the ring nitrogen atom moving
˚
through an arc of approximately 10.7 A. This change also requires
breaking and re-forming of half of the Ag–N bonds (Fig. 9). The
conversion of the cages into the 2D sheets also necessitates a
substantial movement of the silver atoms, which is consistent with
a 17% shrinkage in the c direction of the cell.
A similar investigation was carried out with the isostructural
tetrafluoroborate analogue [Ag2(bisox)3](BF4)2·1.2Et2O 6. In this
case, conversion of 6 into [Ag2(bisox)3](BF4)2] 5 occurs more
readily than with the perchlorate compounds 4 and 3, and does not
require any heating. Indeed, a crystalline sample of 6 was shown by
powder X-ray diffraction to convert completely into 5 on standing
in the atmosphere for 1 week at room temperature. This conversion
was speeded up by placing the sample under vacuum—after 1 h a
sample of 6 had been almost completely converted to 5, and after
3 h under vacuum the transformation was complete.
Fig. 7 The structure of [Ag(bisox)2]SbF6 9 showing (a) the geom-
etry about the silver(I) centre, (b) one of the flattened diamondoid
networks, and (c) the three triply-interpenetrated networks shown in
different colours, with the hexafluoroantimonate ions in orange and
hydrogen atoms omitted for clarity. Selected bond lengths and angles:
Ag(1)–N(1) 2.3089(15), Ag(1)–N(1)¢ 2.3089(15), Ag(1)–N(2) 2.3075(15),
˚
Ag(1)–N(2)¢ 2.3075(15) A, N(2)–Ag(1)–N(1) 103.71(5), N(1)–Ag(1)–N(1)¢
103.92(7), N(2)–Ag(1)–N(2)¢ 94.46(8), N(2)¢–Ag(1)–N(1) 126.85(6),
N(2)–Ag(1)–N(1)¢ 126.85(6), N(2)¢–Ag(1)–N(1)¢ 103.71(5)◦. Primed atoms
generated by the symmetry operation -x + 3, y, -z + 1/2.
The more facile loss of diethyl ether from 6 when compared with
4 may be related to differences in the intermolecular interactions.
Although the diethyl ether molecules reside in very similar
positions in the cages of 4 and 6, the intermolecular contacts
with the anions are slightly shorter in the perchlorate case. Thus
for 4, there are two C–H ◊ ◊ ◊ O interactions [C(102) ◊◦◊ ◊ O(5) 3.935,
the metal coordination environments.26 Interconversions between
1D, 2D and 3D silver-polynitrile coordination polymers have been
shown to be promoted by anion exchange,27 whereas conversion
between 1D ladders and 2D bilayers in the [Zn2(4,4¢-bipy)3(NO3)4]
system has been facilitated by inclusion of guest molecules.28 De-
hydration of networks based on tris(carboxyethyl)isocyanurate29
and 5-sulfoisophthalate30 have both been demonstrated to initiate
reversible transformations involving additional coordination of
donor atoms from the ligands.
˚
H(10E) ◊ ◊ ◊ O(5) 3.04 A, C(102)–H(10E) ◊ ◊ ◊ O(5) 153 ] whereas for
6 there are two longer C–H ◊ ◊ ◊ F interactions [C(23) ◊ ◊ ◊ F(2) 4.032,
◦
˚
H(23B) ◊ ◊ ◊ F(2) 3.16 A, C(23)–H(23B) ◊ ◊ ◊ F(2) 149 ].
Conclusions
Compounds [Ag2(bisox)3](ClO4)2 3 and [Ag2(bisox)3](ClO4)2·
Et2O 4 have similar compositions and possess structural features in
common. Hence, it was of interest to see whether the diethyl ether
By using the semi-rigid ligand bisox, silver coordination polymers
with a wide range of structures were formed including examples
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Fig. 8 Powder X-ray diffraction patterns for the conversion of 4 into 3: (a) Simulated pattern for 4, (b) 4, (c) 4 heated at 75 ◦C under vacuum for 1 h,
(d) simulated pattern for 3.
Powder diffraction measurements were recorded using a Bruker
D8 powder diffractometer, fitted with Goebel mirrors, and using
˚
Cu-Ka radiation of wavelength 1.5414 A. Samples were placed in
0.3 mm to 0.7 mm diameter Lindemann capillaries, and measured
with a 2q range of 4–60◦. The step size was 0.02◦ with time per
step of 1.00 s.
Elemental analyses were conducted by Alan Carver (University
of Bath) and Gillian Maxwell (University College London). NMR
spectra were recorded at 298 K on a Bruker Avance 300 MHz
Fig. 9 The transformations required to convert the Ag2(bisox)3 cages in
NMR spectrometer, and referenced to residual protio solvent
4 and 6 into the triply-interpenetrated sheets in 3 and 5.
signals for 1H NMR spectra (CDCl3, d 7.24) and to solvent
1
resonances for 13C{ H} NMR (CDCl3, d 77.2). Mass spectra
were recorded on a Bruker MicrOTOF electrospray time-of-
with zero-, one-, two- and three-dimensional networks. The
occurrence of two distinct ligand conformations is a factor in the
broad structural chemistry observed, though the anion was also
shown to play a major role in determining the structure adopted.
The most unusual feature in this series of compounds is the ability
of diethyl ether to template the formation of Ag2(bisox)3 cages in
the solid state, leading to materials containing both interwoven
sheets and discrete cages.
flight (ESI-TOF) mass spectrometer (Bruker Daltonik GmbH)
coupled to an Agilent 1200 LC system (Agilent Technologies).
Thermogravimetric analyses were recorded on a Perkin Elmer
TGA 4000 analyser heating at 5 ◦C min-1 under an atmosphere of
nitrogen.
Safety note: Although no issues were observed during the course
of this study, care must be taken with using perchlorates as they
are potentially explosive.
Removal of the included diethyl ether molecules was obser-
ved for both [Ag2(bisox)3](ClO4)2·Et2O
4 and [Ag2(bisox)-
Synthesis of 1,4-bis((3,5-dimethylisoxazol-4-yl)methyl)benzene
(bisox)
3](BF4)2·1.2Et2O 6, with the former requiring more forcing con-
ditions. In both cases, the powder X-ray diffraction data shows
that solvent loss occurs with a solid state transformation to
give [Ag2(bisox)3](ClO4)2 3 and [Ag2(bisox)3](BF4)2 5 respectively.
These conversions involve rearrangement of cages into sheets, and
are accompanied by a change in conformation of half of the bisox
ligands from the C- to the S- conformation.
Hydroxylamine hydrochloride (2.11 g, 30 mmol), dissolved in 1
N NaOH solution (20 cm3, 20 mmol), was added to 3,3¢-(1,4-
phenylenebis(methylene))bis(4-hydroxypent-3-en-2-one) (4.08 g,
13.5 mmol) suspended in EtOH (50 cm3) at room temperature.
The mixture was heated at reflux for 3 h and the solvents removed
under reduced pressure. The residue was suspended in H2O (100
cm3) and the pH adjusted to 7. The solid material was collected
by filtration, washed with H2O (3 ¥ 20 cm3) and crystallised from
hot EtOH–H2O. Yield 3.59 g (89%). Found: C, 72.9; H, 6.78; N,
9.31. C18H20N2O2 requires C, 72.9; H, 6.81; N, 9.46. dH (300 MHz;
CDCl3) 2.06 (6 H, s), 2.28 (6 H, s), 3.63 (4 H, s), 7.06 (4 H, s).
dC (75.5 MHz, CDCl3) 10.27, 10.96, 27.67, 112.26, 128.25, 136.95,
159.86, 165.32. m/z (ESI) 297.1586 ([M + H]+. [C18H20N2O2 + H]+
requires 297.1603).
Experimental
General
3,3¢-(1,4-Phenylenebis(methylene))bis(4-hydroxypent-3-en-2-one)
was prepared following the previously reported method.19 All
other compounds were obtained commercially and used without
further purification.
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Synthesis of [Ag2(NO3)2(bisox)] 1
methanol. Yield 0.031 g (48%). Found: C, 50.02; H, 4.57; N,
6.57. C54H60Ag2B2F8N6O6 (5) requires C, 50.73; H, 4.73; N, 6.57%.
Compound 5 was recrystallised by dissolving it in the minimum
volume of MeCN and allowing Et2O to slowly diffuse into the
solution. Crystals of the diethyl ether solvate 6 of suitable size
were produced for single crystal X-ray diffraction.
bisox (0.029 g, 0.10 mmol) was dissolved in methanol (2 cm3) with
gentle heating and stirring. To this solution, AgNO3 (0.017 g,
0.10 mmol) dissolved in methanol (1 cm3) was added, rinsing
through with a further 0.5 cm3 methanol. The solution was stood at
room temperature overnight. A small amount of a brown precipi-
tate had formed, which was removed by filtration through a cotton
wool plug. The solution was left at room temperature for 3 days,
with the vial lid slightly open to allow approximately half of the
solvent to evaporate slowly. The crystalline product was then sep-
arated by filtration and washed with small amounts of methanol.
Yield 0.033 g. The crude product was recrystallised by dissolving
in the minimum amount of MeCN and allowing Et2O to diffuse
slowly into the solution. Crystals of suitable size for single crystal
X-ray analysis began to form after several days. The X-ray powder
diffraction pattern showed 1 to be the major product, with an
unidentified by-product also present.
Synthesis of [Ag(bisox)2]PF6·0.5bisox 7
bisox (0.073 g, 0.25 mmol) was dissolved in methanol (4 cm3)
with gentle heating and stirring. To this solution, AgPF6 (0.031 g,
0.12 mmol) dissolved in methanol (2 cm3) was added. The product
was allowed to crystallise overnight. The solid product was filtered,
and washed with the minimum quantity of methanol. Crystals of
suitable size were produced from the methanol filtrate for single
crystal X-ray diffraction. The X-ray powder diffraction pattern
revealed that 7 was not the only product of the reaction.
Synthesis of [Ag(bisox)2]SbF6·0.5bisox 8, and [Ag(bisox)2]SbF6 9
Synthesis of [Ag(O2CCF3)(bisox)]·0.5MeOH 2a and
[Ag(O2CCF3)(bisox)]·0.5MeCN 2b
bisox (0.060 g, 0.2 mmol) was dissolved in methanol (6 cm3)
with gentle heating and stirring. To this solution, AgSbF6
(0.036 g, 0.1 mmol) dissolved in methanol (1 cm3) was added. The
product was allowed to crystallise overnight at room temperature.
The crystalline product was then filtered and washed with small
amounts of methanol. Crystals of [Ag(bisox)2]SbF6·0.5bisox 8
were isolated from the filtrate, though X-ray powder diffraction
revealed the presence of another compound too. The crude
product was recrystallised by dissolving it in the minimum
amount of MeCN and allowing Et2O to slowly diffuse into
the solution. Powder X-ray diffraction confirmed that following
recrystallisation, [Ag(bisox)2]SbF6 9 was the sole product. Found:
C, 46.10; H, 4.31; N, 5.64. C36H40AgF6N4O4Sb requires C, 46.18;
H, 4.31; N, 5.98%.
bisox (0.029 g, 0.10 mmol) was dissolved in methanol (2 cm3)
with gentle heating and stirring. To this solution, AgCF3CO2
(0.022 g, 0.10 mmol) dissolved in methanol (1 cm3) was added,
rinsing through with a further 0.5 cm3 methanol. The solution
was allowed to stand at room temperature overnight. A small
amount of a brown precipitate had formed, which was removed
by filtration through a cotton wool plug. The solution was left
at room temperature for approximately 3 days, with the vial lid
slightly open to allow slow evaporation of the solvent. Crystals
of 2a were formed which were of suitable size for single crystal
X-ray analysis. Yield 0.024 g (45%). Found: C, 46.20; H, 4.15,
N, 5.21. C20.5H22AgF3N2O4.5 requires C, 46.17; H, 4.16, N, 5.25%.
Recrystallisation of the crude product from MeCN–Et2O yielded
crystals of 2b which were suitable for single crystal X-ray analysis.
Synthesis of [Ag(bisox)2]CF3SO3 10
Synthesis of [Ag2(bisox)3](ClO4)2 3, and
[Ag2(bisox)3](ClO4)2·Et2O 4
bisox (0.030 g, 0.10 mmol) was dissolved in methanol (2 cm3)
with gentle heating and stirring. To this solution, AgCF3SO3
(0.023 g, 0.10 mmol) dissolved in methanol (1 cm3) was added,
rinsing through with a further 0.5 cm3 methanol. The solution
was allowed to stand at room temperature overnight. A small
amount of a brown precipitate had formed, which was removed
by filtration through a cotton wool plug. The solution was left
at room temperature for approximately 3 days, with the vial lid
slightly open to allow half of the solvent to evaporate slowly.
Crystals formed which were suitable for single crystal X-ray
analysis. Yield 0.026 g (34%). Found: C, 52.25; H, 4.73; N, 6.30.
C37H40AgF3N4O7S requires C, 52.30; H, 4.75; N, 6.59%.
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 was filtered
and washed with methanol. Yield 0.146 g (93%). Found: C, 49.54;
H, 4.53; N, 5.91. C54H60Ag2Cl2N6O14 requires C, 49.75; H, 4.64;
N, 6.45%. Compound 3 was recrystallised by dissolving it in the
minimum volume of MeCN and allowing Et2O to slowly diffuse
into the solution. Crystals 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%.
Crystallography
Synthesis of [Ag2(bisox)3](BF4)2 5, and
[Ag2(bisox)3](BF4)2·1.2Et2O 6
bisox (0.061 g, 0.2 mmol) was dissolved in methanol (6 cm3) with
gentle heating and stirring. To this solution, AgBF4 (0.021 g,
0.1 mmol) dissolved in methanol (1 cm3) was added. The product
was allowed to crystallise overnight. The crystalline product
was separated by filtration and washed with small amounts of
X-ray data for compounds 1–4 and 6–10 were collected on a No-
nius Kappa CCD diffractometer using Mo-Ka radiation. Details
of the data collections, solutions and refinements are given in
Table 1. Unless noted below, all non-hydrogen atoms were refined
anisotropically in the final least squares run, and hydrogen atoms
were included at calculated positions. The structures were solved
using SHELXS-97 and refined using full-matrix least squares in
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SHELXL-97.31 Refinements were generally straightforward with
the following exceptions and points of note.
For 2a, the carbon atom of the fragment of methanol in
the asymmetric unit is situated at an inversion centre, which
necessitates disorder of the hydroxyl group over two sites. The
hydrogen atoms in this solvent moiety could not be reliably located
and hence were omitted from the final least squares. The electron
density map in the region of the fluorine atoms is very smeared,
also due to disorder. Optimal modelling was achieved by treating
each fluorine as being located over three sites. C–C and C–F
bond distance restraints served to converge the model. The largest
residual electron density maximum is also in this region. For 4,
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.
For 7, the fluorine disorder in the hexafluorophosphate anion was
successfully modelled in a 60 : 40 ratio, whereas for 10, the triflate
anion lies proximate to an inversion centre so is disordered in a
50 : 50 ratio.
Acknowledgements
The EPSRC, the Leverhulme Trust and the Cambridge Crystallo-
graphic Data Centre are thanked for financial support.
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