Self-ordering of metallogrid complexes via directed hydrogen-bonding

Article abstract of DOI:10.1039/c2dt31384g

Reaction of imidazole aldehydes with dihydrazino derivatives of 2-phenylpyrimidine provides a family of bis(acylhydrazone) ligands which form [2 × 2] metallogrid complexes with transition metal ions including Fe(ii), Co(ii), Cu(ii) and Zn(ii). The free ligands show H-bonding interactions, both donor and acceptor, largely involving the imidazole units, while binding of the metal ions occupies all the acceptor sites and leaves only the pyrrolic-NH site as an H-bond donor, although its deprotonation by a strong base can regenerate an acceptor. These H-bonding interactions have been studied by ¹H NMR spectroscopy in solution and in the solid state by means of several crystal structure determinations. The Fe(ii) grids appear to be exclusively high-spin species over a wide temperature range in solution. In the solid state various forms of spin-crossover behaviour can be observed between 1.8 and 300 K, which has been rationalised in terms of the varied forms of hydrogen-bonding possible in the crystalline state.

Full text of DOI:10.1039/c2dt31384g

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Selfꢀordering of metallogrid complexes via directed hydrogenꢀbonding
Artur R. Stefankiewicz,a# Guillaume Rogez,b Jack Harrowfield,a Alexandre N. Sobolev,c Augustin
Madalan,a† Juhani Huuskonen,d Kari Rissanend and Jean-Marie Lehn a*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Reaction of imidazole aldehydes with dihydrazino derivatives of 2ꢀphenylpyrimidine provides a family of
bis(acylhydrazone) ligands which form [2x2] metallogrid complexes with transition metal ions including
Fe(II), Co(II), Cu(II) and Zn(II). The free ligands show Hꢀbonding interactions, both donor and acceptor,
largely involving the imidazole units, while binding of the metal ions occupies all the acceptor sites and
10 leaves only the pyrrolicꢀNH site as an Hꢀbond donor, although its deprotonation by strong base can
1
regenerate an acceptor. These Hꢀbonding interactions have been studied by H NMR spectroscopy in
solution and in the solid state by means of several crystal structure determinations. The Fe(II) grids
appear to be exclusively highꢀspin species over a wide temperature range in solution. In the solid state
various forms of spinꢀcrossover behaviour can be observed between 1.8 and 300 K, which has been
15 rationalised in terms of the varied forms of hydrogenꢀbonding possible in the crystalline state.
Introduction
in the solid state particularly. While this is indeed the case,
Hydrogen bonding and metal ion coordination are two of the
more important nonꢀcovalent interactions implemented for the
20 designed construction of functional supramolecular systems.1,2
Among metallosupramolecular architectures, metallogrid
complexes3 provide an egregious example of the rational
exploitation of coordination preferences to provide a wide range
of oligonuclear species with various possible applications.4
25 Although generally observed to be prominent within the solid
state lattices of metallogrids, hydrogenꢀbonding has more often
been an incidental5 rather than a programmed6 aspect of
metallogrid chemistry. The synthetic pathways to gridꢀforming
ligands based on dihydrazone and di(acylhydrazone)
30 formation,3(b),5 however, are particularly readily adapted to the
introduction of both Hꢀbond acceptor and donor units (amongst
other things) as external substituents to the grids ultimately
formed. In the present work, we have explored this approach
through the use of azole (imidazole, thiazole) entities to provide
35 Hꢀbonding sites as well as being part of the coordination network.
further variations arise in that the NH acidity of the imidazole
50 derivatives is sufficiently enhanced by coordination for the bound
ligands to be readily converted to anionic forms and thus for an
Hꢀbond donor site to be converted to an Hꢀbond acceptor.
The ligands involved are shown in Figure
1 in the
conformation presenting two bis(tridentate) subunits suited to the
formation of grids with octahedral metal ions, although this is not
the conformation actually adopted by the free ligands (vide infra).
40 In all but one case, the potential of the hydrazone unit to act as an
Hꢀbond donor (or, after deprotonation, as an acceptor) has been
blocked by methylation, a modification which means that
formally these ligands can behave only as neutral species, giving
cationic grids. Their necessary accompaniment by counter anions
45 thus adds an extra dimension to consideration of grid interactions
Fig. 1 Bis(hydrazone) ligands L1ꢀL6 used in the present work for
55 metallogrid formation. (For ligand L1, some limited data are also given
on the properties of a complex where the pyrrolic NH has been replaced
by an nꢀhexadecyl group.)
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bridging of the NH sites by dmsoꢀO (solventꢀbridging, in a
45 different manner, being seen in the solid state structure, Figure 4).
Results and discussion
Ligand synthesis and characterisation
5
The ligands were all obtained in good yields from reaction
between commercially available azoleꢀaldehydes and either 4,6ꢀ
dihydrazinoꢀ2ꢀphenylpyrimidine7 (giving L1) or 4,6ꢀdi(1’ꢀ
methylhydrazino)ꢀ2ꢀphenylpyrimidine8 (giving L2 – L6) in
ethanol with or without acid catalysis. For ligands L1 and L5, the
10 solvent was heated at reflux to dissolve the rather insoluble
aldehyde. All the ligands readily precipitated from the final
(cooled) reaction mixtures as analytically pure materials. Indeed,
the solubility of the ligands, particularly L3 and L4, was low in
most solvents and severely limited opportunities to compare their
15 NMR spectra in a common solvent. The broadest comparison that
in fact proved to be possible was that of ligands L1, L2, L5 and
L6 in dimethylsulfoxide (Figure 2) and while these spectra
indicated that each species was at least preponderantly present as
a single isomer in this solvent, the marked differences in
20 chemical shifts for equivalent proton resonances also indicated
that the electronic structure of the ligands was sensitive to the
different substitution patterns. It is conceivable that isomers
differing with respect to the configuration about the hydrazone
double bonds9 were present in the various cases but crystal
25 structure determinations (vide infra) provided no support for this
hypothesis.
Fig. 3 The NOESY specturm (d6ꢀDMSO, 298 K, 400 MHz) of ligand L5;
the correlations observed and the conformation deduced are shown in the
inset.
50
Whatever the details of the dynamics in this (and the other)
system(s), it is clear that the preferred configuration of the free
ligand is not that present in its metallogrid derivative, as indeed
appears to be the case generally for bis(tridentate) ligands of the
55 hydrazone and acylhydrazone types.3,10
Solid state structures of the ligands L2, L4 and L5
Slow
evaporation
of
chloroform/methanol
(L2)
or
60 chloroform/ethanol (L4, L5) solutions provided crystals suitable
for Xꢀray diffraction measurements. Crystals could be obtained of
the other ligands but none proved suitable for such
measurements. In the case of L5, the crystal diffracted weakly
and a rather unsatisfactory data set was obtained, giving a high R
65 factor, although the structure solution was considered adequate
for the purpose of comparison with the structures of L2 and L4.
Note that L4 crystallised as monohydrate, presumably as a
consequence of the presence of water in the ethanol, whereas L2
crystallised as a methanolate and L5 as an ethanolate. The free
70 ligands possess both Hꢀbond donor and acceptor sites, and one
expression of this feature is that in the pincerꢀshaped
configuration of the molecules resulting from the expected
transoid arrangement of the N(pyrimidine)ꢀCꢀNꢀN= units,5(a),10
the solvent molecules of crystallisation are chelated by the
75 terminal imidazole groups, which encircle the solvent molecules
in such a way that one is an Hꢀbond donor and the other an
acceptor (Figure 4).
1
Fig. 2 Part of the 400 MHz H NMR spectra of ligands L1, L2, L5 and
L6 in d6ꢀDMSO, presenting differences in the proton signals distribution
30 in the aromatic region (note that the depicted ligands are all shown in the
conformation best suited for tridentate binding, though this is not the
conformation adopted by the free species).
In the case of ligand L5, a NOESY spectrum (Figure 3) showed
35 correlations between the NCH3 protons (HMe) and both the
hydrazone CH proton H3 and the phenyl group proton H5,
consistent with the configuration shown in the figure. The
transoid arrangement of the N(pyrimidine)ꢀCꢀNꢀN= unit is as
expected from studies of related ligands,5(a),10 and a trans
40 configuration about the exocyclic C=N bond is again as expected
from the general chemistry of imines.11 The imidazoleꢀCH
resonances are broad at room temperature, indicating possibly a
slow rotation about the CꢀC(azole) bond, perhaps retarded by
2
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Fig. 4 Xꢀray solid state molecular structures of ligands L2, L4 and L5,
imidazole units project in alternation to both sides of the sheets of
zigꢀzag chains. This feature leads to an exception to the otherwise
exclusive involvement of only the imidazole units in Hꢀbonding
in the present structures, as Hꢀbonded dimers are formed by
50 oppositely oriented molecules in adjacent sheets via OH
interactions with one of the pyrimidineꢀN atoms in each. These
interactions are associated with numerous short atomic contacts
between the two nearꢀparallel ligand molecules, indicating that πꢀ
stacking, here, may be an important interaction. In L5, it is again
55 possible to dissect the lattice into sheets of molecules, here lying
parallel to the (102) plane. Within these sheets, the molecules
form linear chains which lie in pairs in alternating headꢀtoꢀhead
and tailꢀtoꢀtail arrangements. There is a partial disorder of the
ethanol solvate molecules which is such that the ethyl tails which
60 project into the “headꢀhead” region are disordered over two sites
while those which project into the “tailꢀtail” region are not
disordered. L5 molecules in one sheet project upon oppositely
oriented and laterally displaced molecules in an adjacent sheet in
such a way that there are short contacts between atoms of the
65 heterocyclic rings and hydrazone units but only relatively long
contacts to atoms of the phenyl groups. Inꢀplane contacts in both
the “headꢀhead” and “tailꢀtail” regions are all remote, indicating
that πꢀstacking controls interactions both transverse and
perpendicular to the direction of the Hꢀbond chains.
with bound solvate molecules, methanol, water and ethanol, respectively.
This is so even in the case of L4, where water is the bridge and
thus could act as a double Hꢀbond donor towards two imidazole
acceptors, although it may be noted that the splaying of the
imidazole groups is considerably greater here than in either of the
other two systems, possibly as a result of repulsions between the
methyl substituents on the imidazole groups in L4. Although all
10 three ligand molecules are close to planar in their lattices, all are
in fact slightly domed as a result of the Hꢀbonding interactions
with the entrapped solvent molecules which involve a degree of
tilting of the imidazole units towards the oxygen atoms.
The common arrangement of the imidazole groups about a
15 solvent molecule produces a residual, divergent array of one
Hꢀbond donor and one Hꢀbond acceptor on these groups, and an
expression of this feature is the formation of different
supramolecular polymers12 through selfꢀrecognition in the solid
state. Quite dramatic consequences of the seemingly fairly minor
20 structural differences between the ligands are seen in that the
polymers are either helical (L2), zigꢀzag (L4) or linear (L5)
(Figure 5).
5
70
Complex formation by the ligands
The ligands L1 – L6 are, in the appropriate conformation,
bis(tridentate) species expected to form [2x2] grids with
75 octahedral metal ions. Although it was subsequently established
that the complexes could be formed by template reactions15
(component selfꢀassembly5(a),16), all the species described were
first formed via the conventional procedure of reacting the preꢀ
formed ligand with a simple metal salt (see Table 1 in the ESI).
80 The optimum conditions for the syntheses, although generally
mild, were determined by the solubility of both the ligand and the
final complex, and one of the reasons for the use of nitromethane,
for example, as a reaction medium was to avoid precipitation of
the product before dissolution of the ligand was complete. In all
85 but one instance, use of a 1:1 M:L reaction stoichiometry resulted
in crystallised materials of the same composition, consistent with
the desired grid formation. In the case of Cu(II) and ligand L5, a
crystal structure determination established that the cation present
was binuclear [Cu2(L5)3]4+ (described elsewhere17), its formation
90 possibly reflecting the fact that true octahedral coordination is
essentially unknown for Cu(II). In all cases, the formation of a
complex was signalled by the development of an intense colour
characteristic of the particular metal ion involved.
Fig. 5 The spacefilling representations of ligands L2, L4 and L5 showing
25 three types of Hꢀbondingꢀinduced supramolecular assemblies of the
molecules found in the crystal lattice: a) helical for L2; b) zigꢀzag for L4;
c) linear for L5.
In L2, the (flattened) helices are of ellipsoidal crossꢀsection, with
30 the long axis of the ellipse defining a direction in which the
helices partially interpenetrate to give sheets of homochiral
species lying parallel to the (101) plane, the chirality alternating
from one sheet to the text. An alternative view of the lattice is
that inclined helices of alternating chirality form sheets parallel to
35 the ab plane. In both situations, there are various fairly remote
interꢀhelix contacts which appear to be of CH3ꢀπ13 and/or faceꢀtoꢀ
face (stacking) and edgeꢀtoꢀface aromatic interaction types.14 In
L4, the zigꢀzag chains involving imidazole unit Hꢀbonding are
essentially what would be obtained by complete flattening of the
40 helices found in L2 and indeed sheets (again parallel to the (101)
plane) of interlocking zigꢀzag chains are formed corresponding to
flattening of the homochiral sheets in L2. The chains show a
slight undulation due to the fact that the doming resulting from
the intramolecular Hꢀbonding to water alternates in direction
45 from one unit to the next. The OH bonds not involved in bridging
95 Characterisation of the complexes as grids
(a) In solution: As no detailed thermodynamic analysis of any
grid formation system has yet been conducted, it is not possible to
use stability constants to predict the speciation resulting when a
100 solid established by Xꢀray crystallography to contain grid units is
dissolved in a particular solvent. In general,3(b),3(c),5ꢀ8 however,
mass spectrometry has enabled establishment of the fact that
tetranuclear species, presumably grids, are predominant down to
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concentrations at least as low as 10ꢀ3 M (again with certain
exceptions involving Cu(II)17) in weakly coordinating solvents.
must be quite far from the spin crossover point.
The hydrogenꢀbonding ability of the present grids was evident
For the present complexes as 5x10ꢀ3 M solutions in CH3CN, 40 when a good Hꢀbond acceptor, triphenylphosphine oxide,19 was
electrospray mass spectra showed in all cases peaks for the
species [M4L4]8+ associated with various numbers (6, 5 or 4) of
anions. In the case of [Cu4(L6)4](CF3SO3)8, however, a strong
peak corresponding to the “grid corner” species
{[Cu(L6)2] ](CF3SO3)}+ was also evident (see ESI).
added to their solutions. In the case of the grid [Zn4(L1)4]8+ , both
hydrazoneꢀ and imidazoleꢀNH donor centres are present and
significant shifts in the 1H NMR resonances of both were
observed on addition of (C6H5)3PO to a solution of the grid in
45 acetonitrile (Figure 7). While shifts were also observed in the
spectra of the paramagnetic Fe(II) grids, the interactions involved
were not sufficiently strong to produce any detectable effect on
the spin state of the metal ions.
5
At the higher concentrations used to obtain NMR spectra,
10 tetranuclear species must therefore be present and at least in the
case of diamagnetic complexes, specific features of the NMR
spectra are conclusive indices of their grid structure. The 1H
spectrum of [Zn4(L2)4](CF3SO3)8 (Figure 6a) is typical of all the
present Zn(II) species in showing five separate resonances for the
15 protons of the pendent phenyl group,5,18 consistent with its slow
rotation within the confines of the grid. The NOESY spectrum
(Figure 6b) of [Zn4(L6)4](CF3SO3)8 also shows the correlations
expected due to the rearrangement of the bound ligand to a
bis(tridentate) form.
1
50 Fig. 7 a) HꢀNMR titration data (MeCN, 298 K, 400 MHz) of the grid
complex [Zn4(L1)4](OTf)8 with triphenylphosphine oxide showing shifts
of NH from imidazole (red square) and NH from hydrazone (blue square);
b) schematic representation of Hꢀbonding motif between grid and
triphenylphosphine oxide.
55
Binding of an imidazole group through the pyridineꢀlike N to a
metal is expected to enhance the acidity of the pyrrolic NH to a
degree where its deprotonation can be readily achieved,20 as was
observed for all the Fe(II) grids presently studied. Thus, in the
60 case of the grid [Fe4(L2)4]8+, which forms a deep yellowꢀorange
solution in CH3CN, addition of NaOMe resulted initially in a
colour change towards a deep red, followed by the formation of a
redꢀorange precipitate, most abundant when only four equivalents
of the base had been added. Further base addition up to the point
65 of eight equivalents in fact led to the dissolution of the precipitate
and the formation of a deep red solution. These changes could be
20
Fig.
6
a) 1HꢀNMR spectrum (MeCN, 298 K, 400 MHz) of
[Zn4(L2)4](CF3SO3)8 indicating five distinct resonance from phenyl group
on the pyrimidine (green stars); b) NOESY specturm (MeCN, 298 K, 400
MHz) of [Zn4(L6)4](CF3SO3)8 indicating NOE interactions characteristic
25 for hydrazone based gridꢀtype architectures.
In the cases of paramagnetic complexes giving well resolved but
strongly shifted H spectra (complexes of Fe(II) and Co(II) but
1
1
not of Cu(II)), the loss of spinꢀspin coupling made peak
30 assignment difficult but the number of peaks was consistent again
with the grid structure. The spectra of the Fe(II) complexes of L1
– L6 (see ESI) show that the actual peak displacements are highly
sensitive to the ligand structure and the very large shifts show, of
course, that all these complexes are highꢀspin species in solution.
35 Recording of the spectra down to ꢀ70 °C in CH3CN resulted in
negligible changes in their form, indicating that the coordination
environment of the Fe(II) centres in the dissolved complexes
readily monitored by H NMR spectroscopy (Figure 8), the final
spectrum showing that the highꢀspin state of the Fe(II) was
retained in the complex of the deprotonated ligand, so that the
70 lack of any change as a result of simple Hꢀbonding as described
above was unsurprising. The remarkable insolubility of the
fourfoldꢀdeprotonated grid, however, is presumably at least in
part due to direct Hꢀbonding between undeprotonated and
deprotonated imidazoleꢀNH units of different grid cations.
4
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(a)
(b)
5
Fig. 8
1HꢀNMR spectra (MeCN, 298 K, 400 MHz) of complex
[Fe4(L2)4](OTf)2 showing reversible acidꢀbase switching between the a)
fully protonated grid complex, 0 equiv. of NaOMe; b) halfꢀdeprotonated
grid complex, 4 equiv. of NaOMe; c) fully deprotonated grid complex, 8
equiv. of NaOMe.
10
(b) In the crystalline solid state: Crystals suitable for Xꢀray
diffraction studies were obtained of the complexes
[Fe4(L2)4](BF4)8•12CH3CN•3.5H2O,
45
(c)
Fig. 9 a) A perspective view of the grid unit present in the lattice of
[Fe4(L2)4](BF4)8, showing the N…F contacts, ranging from 2.791(6) to
3.183(6) Å, indicative of Hꢀbonding between the cation and adjacent
tetrafluoroborate anions; b) A perspective view of the core FeN6 units of
[Zn4(L2)4](ClO4)8•3.5CH3CN•4H2O,
15 [Co4(L2)4]Cl(PF6)7•4CH3CN
[Co4(L5)4](CF3SO3)8•6CH3CN•6H2O. Solution of their structures
revealed variety of features determined by the ligand
and
50 the grid, with distortions from regular octahedral symmetry about the
metal ions obvious. Fe1…Fe4 = Fe2…Fe3 = 6.381 Å, Fe1…Fe3 = 6.269
Å, Fe2…Fe4 = 6.371 Å, Fe1ꢀFe3ꢀFe2 = 85.3 °, Fe1ꢀFe4ꢀFe2 = 84.4 °,
Fe3ꢀFe2ꢀFe4 = 94.7 °, Fe3ꢀFe1ꢀFe4 = 95.6 °; (c) A view the links
between adjacent grids involving tetrafluoroborate anions forming
55 NH…FBF…HC bridges (B…C 3.157(6), 3.395(6) Å; B…N 2.815(6),
2.805(6) Å).
a
functionality, the metal ions and the counter anions, though
establishing that all were true grid species. Note that the
20 formulations given for these materials are those deduced from the
crystal structures, since drying of the crystals prior to elemental
analysis invariably resulted in the loss of some solvent. Problems
of data quality and disorder led to an inability to fully resolve
structures of [Mn4(L2)4](CF3SO3)8, [Co4(L5)4](PF6)8 and
25 [Cu4(L2)4](ClO4)8, though in all cases their grid nature was
confirmed.
In [Fe4(L2)4](BF4)8•12CH3CN•3.5H2O, the Fe4 unit is not
precisely square and shows a just perceptible distortion towards a
“butterfly” form, as is usually observed to be the case for [2x2]
30 grid complexes of ligands of the present type.5 Given that the
structure (Figure 9) was determined at 173 K, it is possible that
part of this distortion may arise from a spin state change at the
metal ion (see ahead), although the FeꢀN bond lengths for all four
centres in the grid unit (all being inequivalent in the crystal) are
35 consistent with a highꢀspin state. As is commonly the case for
grid complex structures, full definition of solvent molecules of
crystallisation proved difficult. Optimum displacement
parameters were obtained in the present case by treating some
solvent molecule atoms as isotropic and including some disorder,
40 geometrical constraints also being applied to all the acetonitrile
molecules.
Since all Hꢀbond acceptor sites of the free ligand are occupied by
the metal ion, the grid complex can act only as an Hꢀbond donor
60 through the NH centres of the imidazole units. (Note that the
NCH3 centres of the hydrazone units are trigonal, indicating that
the lone pair formally present is delocalised into the π system.) In
the lattice, there are three species, H2O, CH3CN and [BF4]ꢀ,
which could, in principle, act as Hꢀbond acceptors but in fact it is
65 the tetrafluoroborate anions which are the exclusive partners of
the laterally directed imidazoleꢀNH groups of the complex
cations (Figure 9a). Perhaps because, even considering just the
anions, the number of acceptor sites considerably exceeds the
number of donor sites, the Hꢀbonding array is complicated and
70 does not, for example, simply involve intermolecular bridging of
opposed NH centres (to give a linear array of grids) by a single
anion, though one anion does form an angular bridge between
two grids. The tetrafluoroborate anions appear to bind to NH
units in both chelate and unidentate fashions, although this may
75 just be a reflection of the dynamic nature of the interaction.
While, as just stated, they do not bridge directly opposed NH
units, they do form bridges between grids as a result of F…HC
interactions in addition to the F…HN (Figure 9c), generating a
very complicated network, bolstered by BF…CH3CN and
80 BF…OH2 interactions, which can be regarded as a threeꢀ
dimensional polymer. A dissection of the lattice which is useful
in terms of comparison with the other complexes presently
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characterised is to consider that the grid units lie in columns
parallel to the b axis (with these columns sideꢀbyꢀside in a sheet
prallel to the ab plane); The mean Fe4 planes of the grids alternate
conformation than the Fe4 and two of the ligand units deviate
considerably from planarity (Figure 10). Both these differences
may be consequences of a greater mismatch between the ligand
in their inclination towards the b axis, so that they can be 25 structure and the ideal coordination requirements of the metal ion
5
considered as forming a zigꢀzag array down the column, an array
which is presumably largely determined by the particular form of
the interactions with the counteranions.
but it should be noted that in closely related grid species derived
from bis(acylhydrazone) ligands,5(a) variations in such structural
features are observed even for a particular metal ion as a function
of changes in the ligand substituents, so that the differences in
30 counterꢀanion and solvent content in the present cases may also
be significant factors. Unlike the Fe complex, the Zn cation forms
NH hydrogenꢀbonds to both the perchlorate counterꢀanions and
lattice water and here some of the anions do serve as simple
bridges between NH units of different grid cations (Figure 10c).
35 The overall Hꢀbonding network is complicated to describe
because of disorder involving both waterꢀmolecule and
perchlorate oxygen atoms but it is such as to result in sheets of
grids parallel to the ac plane. Considering only the major
components of the disordered array, Hꢀbonding to the NH units
40 results in simple bridges within the sheets involving perchlorate
anions and more complicated cyclic bridges involving water and
perchlorate anions encircling an acetonitrile molecule (Figure
10d). Between sheets, O3ClO…HC interactions of the Nꢀmethyl
groups generate a third dimension to the lattice.
45
Although precipitation of the Co(II) complex of L2 as its
hexafluorophosphate salt provided crystals which diffracted well,
modelling of the anion component of the lattice proved difficult,
involving not only disorder and partial occupancy but also
50 requiring the composition [Co4(L2)4]Cl(PF6)7•4CH3CN for the
optimum solution. Thus, while the cation units here appear to
contain perfectly square Co4 entities (Figure 11), the nature of the
cationꢀanion interactions is somewhat obscure. Intriguingly,
interactions of the [PF6]ꢀ anions appear to involve exclusively
55 contacts to CH atoms and not to NH, the closest contacts to the
NH protons being the Nꢀatoms of acetonitrile molecules,
although a full investigation of these aspects would require
superior crystalline material.
60
10
Fig. 11 Views, perpendicular to the Co4 plane, of the cationic grid unit
Fig. 10 a) and b) Orthogonal views of the cationic grid unit present in the
lattice of [Zn4(L2)4](ClO4)8 and of the core ZnN6 units; c) A view of the
chain of grid units bridged by NH bonding to perchlorate anions only; d)
A view of the cyclic Hꢀbonding array involving water molecules and
15 perchlorate anions in interaction with grid NH sites and encircling an
acetonitrile molecule.
containing true Co4 square and its CoN6 components, present in the lattice
of [Co4(L2)4]Cl(PF6)7. Co…Co (edge) = 6.3882(7) Å.
65
The consequences of changing both the ligand isomer and the
counteranion are nicely illustrated by the structure of
[Co4(L5)4](CF3SO3)8•6CH3CN•6H2O. Although the Co4 unit
retains a form very close to a true square, there are in fact two
grid units, each containing four inequivalent Co atoms, in the
The
[2x2]
grid
form
of
the
complex
[Zn4(L2)4](ClO4)8•3.5CH3CN•4H2O was again established by its
70 asymmetric unit, and the ligands show marked distortions from
planarity.
20 solid state structure determination, although the Zn4 unit shows a
somewhat more marked distortion towards
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Fig. 12 a) A view, perpendicular to the mean Co4 plane, of the cationic
grid unit present in[Co4(L5)4](CF3SO3)8 in which Hꢀbonding interactions
of the grid NH entities appear to involve only triflateꢀO; b) A similar view
of the other cationic grid unit in which NH interactions involve both
waterꢀO and triflateꢀO acceptors; (c) A view of the triflate anion which
bridges the inequivalent grid units through O…π and O…HN
interactions.
5
10
The two grid units differ in that (amongst other things) in one, the
NH units form Hꢀbonds to triflateꢀO exclusively whereas in the
other, two of the NH centres form bonds to water (Figure 12a, b).
As with [BF4]ꢀ in the Fe grid discussed above, there are no direct
Fig. 13 Magnetic susceptibility data for the Fe(II) grids formed by the
45 various ligands shown inset on the plots. Horizintal axis: temperature/K;
vertical axis χMT/emu K molꢀ1.
15 bridges involving one anion connecting NH units in different
grids though the different grids are linked through a triflate which
Hꢀbonds to one and form an O…π link to the other (Figure 12c).
The effects of various weak interactions in not only distorting the
grid units but also in controlling their juxtapositioning in the
20 lattice, may be important in determining properties such as the
magnetism of the crystalline materials. In the present work we
therefore decided to examine the Fe(II) complexes of the various
Hꢀbonding ligands to see if spinꢀcrossover behaviour might be
usefully subject to such effects.
At room temperature, the susceptibility values are close to that
50 expected for the presence of four highꢀspin Fe(II) centres (some
variability in the values probably reflecting the fact that all the
complexes crystallise as solvated materials which tend to lose
solvent at different rates). This is consistent with the solution
properties as measured by NMR spectroscopy. Interestingly, it is
55 the complex of ligand L5, where the NH units are axially
oriented, that shows the most dramatic change in susceptibility as
a function of temperature, the value at 50 K corresponding to two
residual highꢀspin centres per grid. Two HS Fe(II) out of four in
this complex undergo a gradual spin transition centered around
60 215 K. For the complex of ligand L6, only one HS Fe(II)
undergoes a very gradual spin transition centered around 140 K.
25
Magnetism of the Fe(II) complexes of ligands L1, L2, L3, L5
and L6
Grid complexes provide interesting systems for study of spinꢀ
30 crossover4,21,22 in that there is the possibility not only of
cooperative behaviour within a given grid unit but also between
grid units in a lattice. The present work was, however, intended to
be no more than a characterisation of the magnetic properties of
the various grids as a preliminary step in the choice of a system
35 for detailed study. The results of susceptibility measurements
between 1.8 and 300 K for the complexes [Fe4(L1)4](OTf)8,
[Fe4(L2)4](BF4)8, [Fe4(L3)4](OTf)8, [Fe4(L5)4](OTf)8 and
[Fe4(L6)4](OTf)8 and [Fe4(NꢀC16H33L1)4](OTf)8 are shown in
Figure 13, it being clear that the magnetism is indeed sensitive to
40 changes in the Hꢀbonding characteristics of the ligand and
probably also to the nature of the counteranion.
In
contrast,
the
complexes
[Fe4(L1)4](OTf)8
and
[Fe4(L3)4](OTf)8, with laterally oriented Hꢀbond donor sites,
appear to remain completely highꢀspin over the whole
65 temperature range studied. The crystal structures of the various
Fe(II) grids are unknown, except in the case of L2 and where the
anion is tetrafluoroborate. If a parallel with the structures
described above is legitimate in the sense that the ligand L5 may
give rise to a hydrogenꢀbonding network of especially high
70 symmetry, it may be that this case is particularly susceptible to
distortion induced by a spin state change, explaining the
generation of a 2HS:2LS species at low temperatures.
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3662; (c) L. N. Dawe, K. V. Shuvaev, L. K. Thompson, Chem. Soc.
Rev. 2009, 38, 2334ꢀ2359.
Conclusions
4
5
6
(a) M. Ruben, JꢀM. Lehn, P. Müller, Chem. Soc. Rev. 2006, 35, 1056ꢀ
1067; (b) V. A. Milway, S. M. T. Abedin, V. Niel, T. L. Kelly, L. N.
Dawe, S. K. Dey, D. W. Thompson, D. O. Miller, M. S. Alam, P.
Müller, L. K. Thompson, Dalton Trans. 2006, 2835ꢀ2851; (c) K. S.
Murray, Aust. J. Chem. 2009, 62, 1081ꢀ1101.
(a) XꢀY. Cao, N. KyritsakasꢀGruber, J. M. Harrowfield, A. Madalan,
J. Nitschke, J. Ramirez, K. Rissanen, L. Russo, AꢀM. Stadler, G.
Vaughan, JꢀM. Lehn, Eur. J. Inorg. Chem. 2007, 2944ꢀ2965; (b) A.
R. Stefankiewicz, G. Rogez, J. M. Harrowfield, M. Drillon, JꢀM.
Lehn, Dalton Trans. 2009, 5787ꢀ5802.
(a) U. Ziener, E. Breuning, JꢀM. Lehn, E. Wegelius, K. Rissanen, G.
Baum, D. Fenske, G. B. M. Vaughan, Chem. Eur. J. 2000, 6, 4132ꢀ
4139; (b) E. Breuning, U. Ziener, JꢀM. Lehn, E. Wegelius, K.
Rissanen, Eur. J. Inorg. Chem. 2001, 1515ꢀ1522; (c) A. R.
Stefankiewicz, JꢀM. Lehn, Chem. Eur. J. 2009, 15, 2500ꢀ2503.
M. Ruben, JꢀM. Lehn, G. B. M. Vaughan, Chem. Commun. 2003,
1338ꢀ1339.
L. H. Uppadine, JꢀP. Gisselbrecht, N. Kyritsakas, K. Nättinen, K.
Rissanen, JꢀM. Lehn, Chem. Eur. J. 2005, 11, 2549ꢀ2565.
M. N. Chaur, D. Collado, JꢀM. Lehn, Chem. Eur. J. 2011, 17, 248ꢀ
258.
Incorporation of pyrrolic (imidazole) NH units into grids formed
from bis(acylhydrazone) ligands is clearly an effective means of
ensuring directional hydrogenꢀbonding by such complexes and
although there is no situation where a simple, direct comparison
of the Hꢀbond donor ability of the free ligand and its complexes
can be made, this ability is presumably enhanced by the
proximity of a metal cation, as indicated by the greater Brønsted
acidity. In solution, it appears that the interaction of the grids
10 with an Hꢀbond acceptor like (C6H5)3PO is largely independent of
the isomeric form and substitution pattern of the imidazole entity,
whereas in the solid state, where anion interactions are
predominant, a variety of forms arises, leading to threeꢀ
dimensional arrays of the grid units which differ considerably.
15 These differences may be at the origin of the different
temperature dependence of the magnetism of the Fe(II) grids
presently studied, though further structural investigations would
be necessary to fully explore the subtleties of this behaviour.
65
70
75
80
5
7
8
9
85 10 (a) J. Ramirez, AꢀM. Stadler, J. M. Harrowfield, L. Brelot, J.
Huuskonen, K. Rissanen, L. Allouche, JꢀM. Lehn, Z. Anorg. Allg.
Chem. 2007, 633, 2435ꢀ2444; (b) AꢀM. Stadler, J. M. Harrowfield,
Inorg. Chim. Acta 2009, 362, 4298ꢀ4314.
20
11 C. GodoyꢀAlcantar, A. K. Yatsimirsky, JꢀM. Lehn, J. Phys. Org.
90
Chem. 2005, 18, 979ꢀ985.
12 (a) J.ꢀM. Lehn, Makromol. Chem., Macromol. Symp., 1993, 69, 1ꢀ17;
(b) J. D. Fox, S. J. Rowan, Macromolecules 2009, 42, 6823ꢀ6835; (c)
T. F. A. De Greef , M. M. J. Smulders, M. Wolffs, A. P. H. J.
Schenning, R. P. Sijbesma, E. W. Meijer, Chem. Rev. 2009, 109,
5687ꢀ5754; (d) A. Ciferri Supramolecular Polymers, Second Edition,
Taylor and Francis, London, 2005; (e) JꢀM. Lehn, Aust. J. Chem.
2010, 63, 611ꢀ623.
Notes and references
aInstitut de Science et d’Ingénierie Supramoléculaires, Université de
Strasbourg, 8, allée Gaspard Monge, 67083 Strasbourg, France
25 #,Present address: University Chemical Laboratory, University of
Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom.
bInstitut de Physique et Chimie des Matériaux de Strasbourg, 23 rue du
Loess, 67034 Strasbourg, France
95
13 M. Nishio, M. Hirota, Y. Umezawa The CH/π Interaction
–
Evidence, Nature and Consequences, WileyꢀVCH, New York, 1998
(Methods in Stereochemical Analysis, A. P. Marchand, Series
Editor).
c Chemistry, M313, University of Western Australia, Crawley, WA 6009,
30 Australia
100
105
d Department of Chemistry, Nanoscience Center, University of Jyväskylä,
P.O. Box 35, 40014 JYU, Finland
14 W. B. Jennings, B. M. Farrell, J. F. Malone, Acc. Chem. Res. 2001,
34, 885ꢀ894.
† Present address: Inorganic Chemistry Laboratory, Faculty of
Chemistry, University of Bucharest, Str. Dumbrava Rosie 23, 020464
35 Bucharest Romania.
15 C. A. Schalley, F. Vögtle, K. H. Dötz (Eds) Templates in Chemistry I
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2005. See in particular D. H. Busch, Ch.1, Vol. 249, pp. 1ꢀ65.
16 (a) J. R. Nitschke and J.ꢀM. Lehn, Proc. Natl. Acad. Sci. USA, 2003,
100, 11970ꢀ11974; (b) J. R. Nitschke, Acc. Chem. Res. 2007, 40, 103ꢀ
112.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
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110 17 A. Même, A. R. Stefankiewicz, J. M. Harrowfield, XꢀY. Cao, J.
Huuskonen, K. Rissanen, JꢀM. Lehn, N. Potier, E. Leize, Eur. J.
Inorg. Chem. 2012, 647ꢀ654.
18 J. Rojo, F. J. RomeroꢀSalguero, JꢀM. Lehn, G. Baum, D. Fenske, Eur.
J. Inorg. Chem. 1999, 1421ꢀ1428.
115 19 M. H. Abraham, P. L. Grellier, D. V. Prior, J. J. Morris, P. J. Taylor,
J. Chem. Soc., Perkin Trans 2 1990, 521ꢀ529.
40
1
For a few general presentations, see: (a) J. L. Atwood, J. E. D. Davies,
D. D. MacNicol, F. Vögtle and J.ꢀM. Lehn, eds., Comprehensive
Supramolecular Chemistry, Pergamon, Oxford, 1996; (b) Perspectives
in Supramolecular Chemistry, edited book series, since Vol. 1, 1994,
Wiley, Chichester; (c) J.W. Steed, P. A. Gale, eds., Supramolecular
Chemistry: From Molecules to Nanomaterials, Wiley, Chichester,
2012. (d) J.ꢀM. Lehn, Supramolecular Chemistry : Concepts and
Perspectives, VCH, Weinheim, 1995; (e) K. Ariga and T. Kunitake,
45
50
55
20 R. B. Martin, Proc. Nat. Acad. Sci. USA 1974, 71, 4346ꢀ4347.
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120 22 P. Gütlich, H. A. Goodwin, Top. Curr. Chem. 2004, 233, 1ꢀ47.
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Nudelman, B. M. Stoltz, J. E. Bercaw, K. I. Goldberg,
Organometallics 2010, 29, 2176ꢀ2179; (b) H. E. Gottlieb, V. Kotylar,
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Supramolecular Chemistry
– Fundamentals and Applications,
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Wiley, Chichester, 2nd edition 2009.
2
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(a) C. B. Aakeröy, K. R. Seddon, Chem. Soc. Rev. 1993, 22, 397ꢀ407;
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25 Z. Otwinowski and W. Minor, Methods in Enzymology, vol. 276:
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26 G. M. Sheldrick, “SHELXSꢀ97 Program for Crystal Structure
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27 G. M. Sheldrick, (1999) “SHELXLꢀ97”, Universität Göttingen,
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60
130
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Table of contents entry
Introduction of azole substituents on metallogrids provides H-bonding sites suitable for
controlling the solid state arrays and their magnetic properties.