A family of double-bowl pseudo metallocalix[6]arene discs

Article abstract of DOI:10.1039/b926704b

We report the synthesis and magnetic characterisation of a series of planar [M₇] (M = Ni^II, Zn^II) disc complexes [Ni ₇(OH)₆(L₁)₆](NO₃) ₂ (1), [Ni₇

Full text of DOI:10.1039/b926704b

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www.rsc.org/dalton | Dalton Transactions
A family of double-bowl pseudo metallocalix[6]arene discs†
Sea´n T. Meally,a Cecelia McDonald,a Georgios Karotsis,b Giannis S. Papaefstathiou,c Euan K. Brechin,b Peter
W. Dunne,a Patrick McArdle,a Nicholas P. Powera and Leigh F. Jones*a
Received 17th December 2009, Accepted 30th January 2010
First published as an Advance Article on the web 10th March 2010
DOI: 10.1039/b926704b
We report the synthesis and magnetic characterisation of a series of planar
[M7] (M = NiII, ZnII) disc complexes [Ni7(OH)6(L1)6](NO3)2 (1), [Ni7(OH)6(L1)6](NO3)2·2MeOH (2),
[Ni7(OH)6(L1)6](NO3)2·3MeNO2 (3), [Ni7(OH)6(L2)6](NO3)2·2MeCN (4), [Zn7(OH)6(L1)6](NO3)2·
2MeOH·H2O (5) and [Zn7(OH)6(L1)6](NO3)2·3MeNO2 (6) (where HL1 = 2-iminomethyl-6-
methoxy-phenol, HL2 = 2-iminomethyl-4-bromo-6-methoxy-phenol). Each member exhibits a
double-bowl pseudo metallocalix[6]arene topology whereby the individual [M7] units form molecular
host cavities which are able to accommodate various guest molecules (MeCN, MeNO2 and MeOH).
Magnetic susceptibility measurements carried out on complexes 1 and 4 indicate weak exchange
between the NiII centres.
formed at the lower rim of the Calix[4]arene ligands) and spatial
separation of the individual [Mn4] units to promote magnetic
Introduction
Supramolecular chemistry has rapidly become a vast and multidis-
dilution.
ciplinary field with applications including catalysis,1 anion sensing
With this in mind we herein report the synthesis and character-
recognition,2 gas sequestration/storage3 and species transporta-
ization of a family of ferromagnetic planar disc [Ni7] complexes
tion towards drug delivery,4 and consequently is of interest to
which possess double-bowl metallocalix[6]arene topologies and
scientists of wide ranging disciplines. One particularly interesting
themselves exhibit host–guest behaviour allowing direct com-
parison to supramolecular calix[n]arene behaviour. The work
high degrees of local and extended topological control onto a
described in detail below serves as an extension to our initial
facet of supramolecular chemistry concerns the expression of
molecule (e.g. a host unit) in terms of it partaking in intermolecular
findings, namely the report on the founder members of this
extended family.11
interactions with other species (e.g. a guest molecule). This is
achieved via careful structural manipulation of these molecules
(and molecular assemblies) giving rise to complex molecular
architectures possessing targeted synergic chemical and physical
properties. Although supramolecular host–guest chemistry has
been readily exhibited and widely reported with a vast array
of organic receptor moieties,5 the engagement of magnetically
interesting inorganic host units is still relatively rare.6 The comple-
mentarity which has long existed between the fields of supramolec-
ular chemistry and molecular magnetism has fascinated scientists
for many years.7 This was once again highlighted recently in
the production of a tetranuclear [Mn4] Single-Molecule magnet8
and a [Mn4Gd4] magnetic cooler,9 each built entirely using bowl-
like calix[4]arene ligands - a cyclophane synonymous with host–
guest supramolecular chemistry.10 By employing this particular
ligand the authors have purposefully exercised site specific cluster
growth (i.e. the {Mn4} and {Mn4Gd4} motifs can only be
Results and discussion
The first complex of this series to be discovered was the heptanu-
clear complex [Ni7(OH)6(L1)6](NO3)2 (1) which was obtained via
the ethanolic reaction of Ni(NO3)2·6H2O and HL1 (Scheme 1)
in the presence of NaOH. The green hexagonal crystals of 1
crystallize in the trigonal space group P-3c1 in approximately
30% yield (Fig. 1).‡ The core in 1 is best described as a body
aSchool of Chemistry, University Road, National University of Ireland,
Galway, Ireland. E-mail: leigh.jones@nuigalway.ie; Tel: +353-091-49-3462
bSchool of Chemistry, Joseph Black Building, University of Edinburgh, West
Mains Road, Edinburgh, Scotland, UK
cLaboratory of Inorganic Chemistry, Department of Chemistry, National
and Kapodistrian University of Athens, Panepistimiopolis, Zografou 157 71,
Greece
† Electronic supplementary information (ESI) available: Full experimental
details on the synthesis of the ligands HL1 and HL2 available. CCDC
reference numbers 758960–758962. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b926704b
Scheme 1 Structure of the Schiff base ligands HL1 and HL2 utilised in
this work (R = H (HL1), Br (HL2)).
‡ For crystal structure data for complexes 1, 3 and 4 see CCDC 722288–
722290 respectively.†
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 4809–4816 | 4809
©

centred hexagon whereby six NiII ions surround a central NiII
centre to form a planar disc. Although topologically analogous
[Mn7],12 [Fe7]13 and [Co7]14 complexes are known, the synthesis
of 1 represents the first example for NiII. All NiII centres exhibit
distorted octahedral geometries. The six m3-bridging OH- ions
(O1 and symmetry equivalent, s.e) link the central nickel (Ni1)
located above and below the individual [Ni7] units with C–H ◊ ◊ ◊ O
bonding interactions between the NO3- oxygen atoms (one unique,
-
O4) and protons (H1A and H5) of the L1 ligands (H1A ◊ ◊ ◊ O4 =
2.59 A and H5 ◊ ◊ ◊ O4 = 2.44 A). The NO3- belts act as ‘molecular
˚
˚
zips’ by pairing up individual [Ni7] complexes to form molecular
3
15
˚
cavities each of approximate volume ~265.9 A , formed by two
juxtaposed pseudo metallocalix[6]arene [Ni7] bowl units (Fig. 3).
For complete molecular cavity dimensions in the crystal of 1 (and
all other analogues) see Table 1.
to the six peripheral nickel ions (Ni2 and s.e.). The central NiII
-
¯
ion is located at a site with imposed 3 symmetry with the NO3
nitrogen atom (N2) sitting on a threefold axis. The remainder of
the asymmetric unit comprises a second NiII centre (Ni2) along
with one L- unit and one hydroxy group (O1–H1) occupying
general positions. Each of the six trigonal pyramidal OH- ions are
situated alternately above and below the [Ni7] plane (Fig. 1). The
-
six singly deprotonated (at the phenolate site) L1 ligands bridge
1
2
1
the peripheral NiII centres via a m2-h :h :h coordination mode.
These ligands are situated alternately above and below the [Ni7]
plane which gives rise to a double-bowl conformation in which the
[Ni7] core is the basal plane, reminiscent of a metallocalix[6]arene
concave unit (Fig. 1). Full crystallographic parameters obtained
for 1 (and all other members) are documented in Table 2.
Fig. 2 (top) Crystal structure of 2 as viewed parallel to the [Ni7] cores;
(bottom) Crystal packing observed in 2. Dashed lines represent H-bonding
interactions in between the individual [Ni7] units in 2 (via the NO3- anions).
The guest disordered MeOH molecules are shown as space filled spheres
(O = red, C = grey).
Fig. 1 Molecular structures of complexes 1 (top) and 4 (bottom) viewed
perpendicular and parallel to the [Ni7] plane respectively. Colour code:
Ni = green, O = red, N = blue, C = silver, Br = yellow.
Close scrutiny of the double-bowl conformation in 1 shows
approximate bowl dimensions of 6.20 ¥ 4.21 ¥ 11.70 A (base ¥
Fig. 3 Molecular structures of 1, 3 and 4 highlighting (left): the empty
cavity and belt of NO3- anions in 1, (middle) the disordered guest MeNO2
molecules in 3 and (right) the two MeCN guests located inside the cavities
of 4.
˚
depth ¥ rim diameter). The [Ni7] units in 1 stack on top of one
another resulting in the formation of psuedo-superimposable 1D
columns whereby each moiety is spaced at a [Ni7]plane-[Ni7]plane
distance of 11.64 A.13 Furthermore the unit cell in 1 possesses four
Although void of guest moieties, the fascinating double bowl
metallocalix[6]arene cavities observed in 1 led to their investigation
as rare examples of paramagnetic host receptors. Initial work in-
volved the replacement of EtOH (used in production of the empty
˚
such 1D columns, each unit linked by a 120◦ rotation (Fig. SI3†).
The [Ni7] moieties are connected into 1D columnar arrays via zig-
zag shaped belts of NO3- anions (each comprising six NO3- ions),
4810 | Dalton Trans., 2010, 39, 4809–4816
This journal is
The Royal Society of Chemistry 2010
©

Table 1 Molecular cavity dimensions observed in the crystals of 1–4
1
2
3
4
3
˚
Cavity volume/A
265.9
6.20 ¥ 4.21 ¥ 11.70
11.64
293.7
6.20 ¥ 4.16 ¥ 11.81
11.57
283.8
6.20 ¥ 4.08 ¥ 12.04
11.37
155.9
6.22 ¥ 6.18 ¥ 11.90
11.14
˚
Cavity dimensions/A (base ¥ depth ¥ rim)
[M7]plane–[M7]plane dist./A
˚
Table 2 Crystallographic data for complexes 1–4
1
2
3
4
Formulaa
MW
C54H66N8O24Ni7
C56H66N8O26Ni7
C57H75N11O30Ni7
1709.25
Trigonal
P-3c1
13.933(2)
13.933(2)
22.742(5)
90
90
120
3823.4(11)
2
150
0.71070
1.485
1.762
2333/1835(0.0502)
0.1725
0.0604
C58H66Br6N10O24Ni7
2177.64
Monoclinic
C2/c
28.8575(14)
11.1352(3)
27.4079(13)
90
109.603(3)
90
8296.6(7)
4
150
0.71070
1.743
1.516
7348/3779 (0.0511)
0.1325
1622.12
Trigonal
P-3c1
13.806(2)
13.806(2)
23.270(5)
90
90
120
3841.2(11)
2
150
0.71070
1.402
1.749
2306/1376 (0.0802)
0.2446
0.1209
1.091
1678.14
Trigonal
P-3c1
13.913(2)
13.913(2)
23.141(5)
90
90
120
3879.6(11)
2
150
0.71070
1.437
1.736
2335/2034(0.0679)
0.2155
0.0751
1.047
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
T/K
b
˚
l /A
Dc/g cm-3
m(Mo-Ka)/mm-1
Meas./indep.(Rint) refl.
wR2 (all data)
d,e
R1
0.1090
0.852
Goodness of fit on F2
1.191
ꢀ
ꢀ
ꢀ
a Includes guest molecules. b Mo-Ka radiation, graphite monochromator. c wR2 = [ w(IFo2I - IFc2I)2/ wIFo2I2]1/2
.
d For observed data. e R1 = IIFoI -
ꢀ
IFcII/ IFoI.
host (1)) with the smaller MeOH solvent in order to encourage
guest occupancy. This proved successful with the isolation of
the host–guest complex [Ni7(OH)6(L1)6](NO3)2·2MeOH (2), which
crystallises in the same trigonal P-3c1 space group as 1 in ~40%
yield. This improved yield, when compared to the preparation
of 1, is presumably due to the increased solubility of NaOH in
significant signs of supramolecular interactions within or outside
its host cavity. This is not particularly surprising as this central
-
˚
MeOH guest moiety lies over 4 A away from the nearest m3-OH
˚
protons (O1(H1) ◊ ◊ ◊ O9 = 4.10 A).
It was decided to attempt the encapsualtion of a larger guest
molecule which would invoke H-bonding interactions within the
molecular cavity and promote affinity towards its host receptor.
It was decided to utilise MeNO2 unit as a potential guest on the
assumption that its O-atoms would interact with the H atoms
of the m3-OH- bridges within the [Ni7] host cavity units. This was
achieved, producing the complex [Ni7(OH)6(L1)6](NO3)2·3MeNO2
(3), formed by dissolution and recrystallisation of 1 from MeNO2
in ~15% yield.† In the crystals of 3 (Fig. 4) the host cavity
MeOH. Complex 2 also exhibits a central NiII (Ni1) of imposed
-
¯
3 symmetry and a N atom of the NO3 counter anion (N2)
located on a threefold rotation axis. The molecular cavities in 2
˚
are similar to those in 1 (6.20 ¥ 4.16 ¥ 11.81 A (base ¥ depth ¥ rim
˚
diameter)), while the [Ni7]plane-[Ni7]plane distance of 11.57 A is also
comparable to that of 1 (Table 1). Complex 2 differs with respect
to 1 only in that the H-bonded cavities in 2 (of calculated volume
3
3
˚
˚
of ~ 293.7 A ) are of the required dimensions to accommodate
has a calculated volume of 283.8 A and is occupied by three
two guest MeOH molecules. When small molecules are located
within such highly symmetrical molecular cavities, it is common to
observe crystallographic disorder and the MeOH guest molecules
in 2 are no exception.
MeNO2 guests which are related crystallographically via a three
fold rotation. As observed in 2, the trigonal planar MeNO2 guests
also experience disorder whereby the methyl carbon atoms (C10
and s.e) lie on a two fold axis. These orientations are most likely to
exist in the up-down-up antiparallel configuration with respect to
the three fold rotation symmetry they share, as any other spatial
arrangement would result in significant steric effects (Fig. 3). As
predicted the three interact within the cavity via hydrogen bonding
interactions between their O-atoms (O5 and O6) and the nearby
m3-OH- protons of the two [Ni7] units which line the cavity floors
The first disordered MeOH guest (atoms C20–O10) has only
1/6th occupancy and thus periodically occupies positions in
between the NO3- counter anions (which form the aforementioned
zig-zag belt) and partakes in H-bonding interactions (O4 ◊ ◊ ◊ O10 =
˚
2.86 A). The second MeOH guest (C21–O9) lies within the
molecular cavity in 2 at the midpoint between the two [Ni7] planes
(possessing a two-fold axis along the C–O vertex) where it is also
disordered over three sites with respect to the three fold rotation
axis inherent to the cell (Fig. 2). This central MeOH guest shows no
˚
˚
(O1 ◊ ◊ ◊ O5 = 3.08 A; O1 ◊ ◊ ◊ O6 = 3.25 A).
In order to alter the interior size and shape of the molecular
cavities highlighted in 1–3 towards subsequent modification
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 4809–4816 | 4811
©

a staggered arrangement (Fig. 3). The [Ni7]plane-[Ni7]plane distance
˚
inside the cavity is 11.14 A and represents a cavity height reduction
˚
of ~0.4 A cf. 1–3. We postulate that this is attributed to the
significant H-bonding affinity of the pendant Br-atoms (Br1) in 4,
producing a more tightly embraced cavity of approximate volume
3
3
˚
˚
155.9 A , which is significantly smaller than those of 1 (265.9 A ),
3
3
˚
˚
2 (293.7 A ) and 3 (283.8 A ). For full details on the individual
H-bonding parameters in complexes 1–6 see table SI2.†
Efforts to encapsulate MeCN and MeNO2 solvent guests inside
the cavities of 2 and 4, respectively, were unsuccessful. These
findings suggest that guest molecules can only be placed within
such cavities if and when they are able to orientate themselves into
certain topologies comprising symmetry elements compatible with
their host lattices. This hypothesis is supported by the formation
of complex 3 in which three MeOH guests, linked via a three-
fold rotation axis, are accommodated inside the trigonal P-3c1
cell whereas attempts at producing a MeOH guest analogue
to 4 (in a monoclinic C2/c cell) remain fruitless. It must be
noted that the size and shape of the molecular cavities in such
complexes will also reflect their resultant guests and must also be
considered here. Attempts at encapsulating larger organic moieties
(inc. fluorophores, amino acids and anions) have so far been
unsuccessful. Indeed all attempts at such encapsulation simply
resulted in the recrystallisation of complexes 1–4 (depending on
the synthon employed). We may postulate that this is due to the
ubiquitous nature of the solvent molecules present in our reaction
mixtures which will inevitably become guests within our host [Ni7]
discs.
Fig. 4 Crystal packing observed in the crystals of 3 (top) and 4 (bot-
tom) showing the molecular cavities accommodating guest MeNO2 (red
spheres), and MeCN (grey/blue spheres) solvent molecules respectively.
NO3 counter anions have been omitted for clarity in figure depicting
complex 4.
-
and/or control of guest preference, it was decided to attempt to
increase the bowl depth (in relation to complexes 1–3) by utilising
the Br-analogue of HL1, namely the pro-ligand 2-iminomethyl-4-
bromo-6-methoxy-phenol (HL2). To this end an ethanolic solution
of Ni(NO3)2·6H2O, the ligand HL2 and NaOH was left to stir for
4 h. The mother liquor was then left to evaporate slowly, but no
crystalline product was obtained. The subsequent green powder
produced was redissolved in numerous potential guest solvents
(MeOH, MeNO2, MeCN). Interestingly only the MeCN guest
species was successfully incorporated in the form of the complex
[Ni7(OH)6(L2)6](NO3)2·2MeCN (4), which was formed in ~23%
yield and crystallises in the monoclinic C2/c space group (cf. 1–
3).† In this case each cavity accommodates two MeCN molecules
which exhibit a head-to-tail conformation (Fig. 3 (right)). The
MeCN guests are held in place via H-bonding between their
N-atoms (N5) and a proton (H3A) of an m3-OH- bridging ion
belonging to the neighbouring [Ni7(OH)6] core (N5 ◊ ◊ ◊ H3A(O3) =
3D connectivity of 1–4
Complexes 1–3 crystallise in the trigonal P-3c1 space group and
only differ in terms of their guest occupancy and therefore are
analogous in terms of their 3D connectivity. The [Ni7] columns
in their unit cells are connected via H-bonds to adjacent 1D
[Ni7] columns. These connections are manifested by a myriad
-
of H-bonding interactions between the NO3 counter anions
and the individual [Ni7] moieties. More specifically each [Ni7] is
-
H-bonded to twelve NO3 counter anions which in turn connect
to six other [Ni7] units thus creating a (6,12)-connected net with
a (415)2(448.618)-alb topology (Fig. 5).16,17 As previously stated,
[Ni7(OH)6(L2)6](NO3)2·2MeCN (4) crystallises in the monoclinic
C2/c space group and as such possesses a 3-D connectivity
different to that of 1–3. The 1D columnar stacks of [Ni7] units in
4 are linked by C–H ◊ ◊ ◊ Br interactions through the Br atoms (Br2
and Br3) of the bridging L2- ligands and the –CH3 protons (H18B
˚
2.36 A).
The central NiII ion (Ni4) located at the centre of 4 lies on an
inversion centre with the remaining three metal centres (Ni1-3)
and all other atoms in the asymmetric unit occupying general
˚
and H27B) of adjacent [Ni7] moieties (H18B ◊ ◊ ◊ Br3 = 2.93 A,
-
˚
positions. The employment of L2 in the construction of 4 does
H27B ◊ ◊ ◊ Br2 = 2.70 A and s.e). The result is a 3-D connectivity
indeed significantly alter the cavity size and shape, as the crystal
comprising a 10-connected net with a (312.428.55)-bct topology
structure shows the formation of a deeper bowl of dimesions 6.22 ¥
(Fig. 5).16,17
˚
6.18 ¥ 11.90 A (base ¥ depth ¥ rim diameter). As observed in 1–3
IR spectroscopic studies on the host complexes 2, 3 and 4 were
performed to ascertain whether their guest molecules remained
within their respective H-bonded cavities on drying. CHN analysis
on samples of complexes 2–4 were consistent with guest residency.
The IR spectrum of [Ni7(OH)6(L1)6](NO3)2·2MeOH (2) showed
broad OH stretching bands (centred at 3416 cm-1) attributable
to both the m3-OH- bridges and MeOH guest solvent molecules.
However there was also a possibility that such bands were due
to adsorbed MeOH solvent molecules and therefore the same
the individual [Ni7] units in 4 again arrange into superimposable
1D columns, which propagate along the b direction of the unit
cell. The stacking of the [Ni7] units along b is supported by two
complementary O–H ◊ ◊ ◊ Br interactions which involve one m3-OH-
(H1) of a [Ni7] unit and the bromine atom (Br1) of a neighbouring
˚
[Ni7] moiety (H1 ◊ ◊ ◊ Br1 = 2.82 A). These hydrogen bonds direct
molecular cavities which differ from those in 1–3 as here they
are tilted with respect to the [Ni7] planes and are interlocked in
4812 | Dalton Trans., 2010, 39, 4809–4816
This journal is
The Royal Society of Chemistry 2010
©

In order to further probe potential affinities of our guest solvent
molecules for their [Ni7] hosts using NMR techniques the dia-
magnetic [Zn7] analogues [Zn7(OH)6(L1)6](NO3)2·2MeOH·H2O
(5) (Fig. SI6†) and [Zn7(OH)6(L1)6](NO3)2.3MeNO2 (6) (Fig. 6)
were synthesized (for crystallographic information see ESI).
The structure of [Zn7(OH)6(L1)6](NO3)2.3MeNO2 (6) is analo-
gous to that of its NiII analogue (complex 3) with a slightly
3
˚
larger cavity volume of 295.4 A ; however the connectivity
in [Zn7(OH)6(L1)6](NO3)2·2MeOH·H2O (5) differs slightly to
that of its counter part [Ni7(OH)6(L1)6](NO3)2·2MeOH (2). The
[Zn7] units in (5) are held in 2D layers running parallel to
-
the ab plane via the NO3 anions, which sit above and below
the individual heptanuclear complexes with C–H ◊ ◊ ◊ O bonding
-
interactions between the NO3 oxygen atoms (one unique, O4)
-
and protons (one unique, H5) of the L1 ligands (H5 ◊ ◊ ◊ O4 =
-
˚
2.46 A). In this arrangement, each [Zn7] is H-bonded to six NO3
anions with the latter being connected to three [Zn7] units thus
creating a (3,6) layer. Although the NO3- ions do not hold the [Zn7]
moieties of neighbouring layers, the (3,6) layers stack with the [Zn7]
forming columnar arrays like those found in complexes 1, 2, 3 and
5, giving rise to molecular cavites (each of approximate volume
3
˚
˚
~ 280.4 A with a [Zn7]plane-[Zn7]plane distance of 11.68 A), formed
by two juxtaposed pseudo metallocalix[6]arene [Zn7] bowl units.
Those cavities are of the required size and shape to accommodate
two guest MeOH and one H2O solvent molecules. These are related
crystallographically via a three fold rotation.
Fig. 5 (top) The (6,12)-connected net with a (415)2(448.618)-alb topology
in 1–3. Large blue spheres represent the 12-connected [Ni7] units. Smaller
red spheres represent the 6-connected NO3-. (bottom) The 10-connected
(312.428.55)-bct in 4 where the blue spheres represent the 10-connected
[Ni7] units.
sample was subsequently dried under vacuum for 2 h prior to
re-analysis of its IR spectrum. It was found that the initial broad
OH stretching band had lost intensity on drying and was now
centred at 3406 cm-1 (Fig. SI4†). Subsequent elemental analysis
on this same sample analysed as the empty [Ni7(OH)6(L1)6](NO3)2
(2) complex. Similarly the IR spectrum of 3 gave peaks at
1337 and 1555 cm-1 which are characteristic for the asymmetric
and symmetric NO stretching of the guest MeNO2 molecules
respectively. Subsequent vacuum drying of 3 (for 2 h) resulted
in the loss of these vibrational resonances indicating egress of
the MeNO2 guests which was also subsequently confirmed by
elemental analysis. The facile removal of the MeOH and MeNO2
guests in 2 and 3 respectively is consistent with their crystal
structures and more specifically in the way that their host cavities
are held only via weak hydrogen bonding interactions and by
no means form tightly bound enclosures. The IR spectrum of
[Ni7(OH)6(L2)6)(NO3)2·2MeCN (4) exhibits a weak resonance at
2256 cm-1 corresponding to a CN stretch indicative of the enclosed
MeCN guest molecules. Interestingly and unlike complexes 2 and
3 this peak at 2256 cm-1 is not lost upon substantial vacuum drying
(Fig. SI5†), while no significant change is seen in its subsequent
CHN microanalysis. This observation suggests the MeCN guests
cannot readily exit the host cavities in 4 which may be attributed
to its distinct (cf. 1–3) and more tightly locked double-bowl
enclosures formed via the H-bonding pendant Br-groups (vide
infra).
Fig. 6 Crystal structure of 6 as viewed along a 1D column of [Zn7] units.
Large space-fill spheres represent the three guest MeNO2 moieties.
By direct crystallographic comparison it becomes clear that the
3
3
3
3
˚
˚
˚
˚
cavities of 1 (265.9 A ), 2 (293.7 A ), 3 (283.8 A ), 5 (280.4 A )
3
˚
and 6 (295.4 A ) are very similar. The rather small differences are
due to the slightly different orientations of the [M7] molecules in
the crystals. From this we may draw the conclusion that the guests
-
and/or the H-bonding with the NO3 do not play an important
role in the resultant cavity size.
Solution studies
Despite the rather poor solubility of this class of com-
pound UV-vis studies on MeOH and MeCN solutions of
[Ni7(OH)6(L1)6](NO3)2·2MeOH (2) were successfully performed
(Fig. SI7†). Both solutions showed similar absorptions at approx-
imately 230, 270 and 356 nm. A fourth sharp transition in the
methanolic solution of 2 is observed at 205 nm which has been cut
off in the analogous MeCN solution spectrum. The transitions
at ~205, 230 and 270 nm are as a result of p → p* excitations
(e values ranging from 42.8–110.2 ¥ 103 dm3 mol-1 cm-1), while
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 4809–4816 | 4813
©

the absorptions at 356 nm correspond to the n → p* excitations
occurring in both solutions of 2. No absorptions indicative of d-
d transitions are observed and if present are presumably masked
-
by the presence of L1 in 2. This assumption is supported by the
UV-vis spectra of MeOH and MeCN solutions of the ZnII sibling
[Zn7(OH)6(L1)6](NO3)2·2MeOH·H2O (5), whose absorptions are
analogous to those observed in 2 (Fig. SI8†) with no possibility
of any significant d-d transitions. Furthermore the UV-vis spectra
obtained from MeOH and MeCN solution of HL1 and HL2 (Fig.
SI1 and SI2 respectively†) showed similar absorptions to those of
[Ni7] (2) and [Zn7] (5), although on comparison appearing less
defined presumably due to their more symmetric nature when
incorporated into the [M7] complex.
Magnetic susceptibility studies
Fig. 7 Plot of cmT vs. T for complexes 1(ꢀ) and 4 (᭺) in the 300–5 K
temperature range (measured in an applied field of 0.1 T). (Inset) Plot of
M/Nmb vs. H (G) carried out on complexes 1 in the 2–7 K temperature
range and 0.5–7 T magnetic field range.
Magnetic susceptibility measurements were obtained on
crystalline samples of [Ni7(OH)6(L1)6](NO3)2 (1) and
[Ni7(OH)6(L2)6](NO3)2·2MeCN (4) in the 300–5 K temperature
range (Fig. 7). The room temperature cmT values of 7.76 and
7.90 cm3 mol-1 K respectively are slightly larger than the value
Conclusions
expected for seven non interacting NiII ions (~ 7.7 cm3 mol-1
K
assuming g = ~2.1). As the temperature is decreased the values
of cmT increase slowly, reaching maximum values of ~8.5 cm3 K
mol-1 at 40 K for 1 and ~10 cm3 K mol-1 at 25 K for 4, before
decreasing below these temperatures to minimum values at 5 K
of ~5.5 cm3 K mol-1 (1) and ~7.9 cm3 K mol-1 (4). This behaviour
is suggestive of very weak ferromagnetic intra-molecular
exchange between the NiII ions in both complexes, with their low
temperature (T < 40 K) decreases in cmT ascribed to relatively
strong inter-molecular antiferromagnetic exchange, consistent
with the extensive H-bonding observed in the crystals of 1 and
4. The maxima in cmT for 1 and 4 are well below that expected
for an isolated S = 7 spin ground state which would give a cmT
value of approximately 31 cm3 K mol-1 (assuming g = 2.10).
Fitting of the 1/cm versus T data to the Curie–Weiss law using
only the 300–50 K data affords Weiss constants (H) of +18.7 K
(1) and +29.0 K (4) (Fig. SI9†), suggestive of weak ferromagnetic
exchange. Several factors proclude the fitting or simulation of the
susceptibility data: a) the presence of numerous different exchange
interactions; b) the presence of relatively strong intermolecular
interactions; c) the weak exchange between the metal centres and
the likelihood that J will be comparable to the single ion zfs (weak
exchange limit) and thus the presence of multiple low lying states
that cannot properly be described as total S states. This scenario
is supported by the magnetisation versus field data carried out in
the 2–7 K temperature and 0.5–7.0 T magnetic field ranges (Fig. 7
(inset) (1) and Fig. SI10 (4)†). For an isolated spin ground state
one would expect a rapid initial increase in M, with saturation
of the magnetisation achieved for relatively low fields. However
for both 1 and 4 M increases only slowly with H, indicative of
the population of low lying levels with smaller magnetic moment,
which only become depopulated with the application of larger
fields. Our ability to make the analogous ZnII complexes will
therefore prove important for future studies. Doping experiments
in which we make highly diluted [Ni7] clusters in [Zn7] matrices
and study their behaviour by both SQUID magnetometry and
EPR spectroscopy are currently in progress and will be reported
at a later date.
We have reported the syntheses, structures and solid and solution
state characterisation of a family of [Ni7] and [Zn7] planar
hexagonal disc complexes. This family may also be described as
pseudo metallocalix[6]arene complexes due to their double-bowl
topologies which have shown to act as host cavities accommodat-
ing numerous guest solvent molecules. The magnetic behaviour of
the NiII discs is complicated by the presence of relatively strong
intermolecular interactions and weak intramolecular exchange
and thus the isolation of the diamagnetic ZnII analogues and the
examination of the diluted [Ni7] discs promises to afford much
useful information. Further progress of the work will include the
functionalisation of the ligands HL1 and HL2 at their upper rim
positions (R group in Scheme 1) in order to a) preferentially attract
specific guest species and b) increase their solubility towards future
1H NMR host–guest titration studies. Solid state NMR studies will
also be investigated.
Experimental
Infra-red spectra were recorded on a Perkin Elmer FT-IR Spec-
trum One spectrometer equipped with a Universal ATR Sampling
accessory (NUI Galway). UV-visible studies were carried out on
a Cary 100 Scan (Varian) spectrophotometer. Elemental analysis
was carried at the School of Chemistry microanalysis service at
NUI Galway. Variable-temperature, solid-state direct current (dc)
magnetic susceptibility data down to 1.8 K were collected on a
Quantum Design MPMS-XL SQUID magnetometer equipped
with a 7 T dc magnet. Diamagnetic corrections were applied to the
observed paramagnetic susceptibilities using Pascal’s constants.
Syntheses
All reactions were performed under aerobic conditions and all
reagents and solvents were used as purchased.
Synthesis of [Ni7(OH)6(L1)6](NO3)2 (1). Ni(NO3)2·6H2O
(0.25 g, 0.85 mmol), HL1 (0.14 g, 0.85 mmol) and NaOH (0.034 g,
0.85 mmol) were dissolved in 30 cm3 EtOH and stirred for 4 h.
4814 | Dalton Trans., 2010, 39, 4809–4816
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The Royal Society of Chemistry 2010
©

The resultant green solution was then filtered and allowed to
stand. X-Ray quality crystals of 1 were obtained in 30% yield
upon slow evaporation. Elemental analysis calculated (%) for
C54H66N8O24Ni7 (1): C, 39.98; H, 4.10; N, 6.91; Found: C, 39.76; H,
4.18; N, 6.85. FT-IR (cm-1): 3415 (w), 2968 (w), 2932 (w), 1627 (s),
1602 (w), 1559 (w), 1459 (m), 1406 (w), 1315 (s), 1221 (s), 1171 (w),
1148 (w), 1072 (m), 1044 (w), 1018 (w), 963 (m), 864 (m), 828 (w),
793 (m), 743 (s).
1173 (w), 1148 (w), 1093 (m), 1077 (w), 1013 (w), 967 (m),
860 (m), 828 (w), 793 (m), 743 (s). UV/vis (MeOH): lmax [nm] (emax
103 dm3 mol-1 cm-1): 202 (108.3), 226.9 (146.4), 267 (71.1), 350
(27.9). (MeCN): lmax [nm] (emax 103 dm3 mol-1 cm-1): 227 (242.6),
267 (103.6), 350 (44.0).
[Zn7(OH)6(L1)6](NO3)2·3MeNO2 (6). Zn(NO3)2·6H2O (0.25 g,
0.85 mmol), HL1 (0.14 g, 0.85 mmol) and NaOH (0.034 g,
0.85 mmol) were dissolved in 30 cm3 EtOH and stirred for 3 h.
The resultant yellow solution was then filtered and evaporated
to dryness. The resultant solid was then redissolved in MeNO2
from which X-ray quality crystals of 6 were obtained in ~ 10%
yield. Elemental analysis calculated (%) for C57H75N11O30Zn7 (6):
C, 36.97; H, 4.08; N, 8.32; Found: C, 37.02; H, 4.17; N, 8.44. FT-IR
(cm-1): 3418 (w), 2967 (w), 2930 (w), 1625 (s), 1601 (w), 1561 (w),
1459 (m), 1405 (w), 1319 (s), 1220 (s), 1168 (w), 1149 (w), 1072 (m),
1041 (w), 1016 (w), 960 (m), 860 (m), 829 (w), 789 (m), 743 (s).
[Ni7(OH)6(L1)6](NO3)2·2MeOH (2). Ni(NO3)2·6H2O (0.25 g,
0.86 mmol), HL1 (0.145 g, 0.86 mmol) and NaOH (0.04 g,
1.00 mmol) were dissolved in 30 cm3 MeOH and stirred for 3 h.
The resultant green solution was then filtered and allowed to stand.
X-Ray quality hexagonal crystals of 2 were obtained in 40% yield
upon slow evaporation of the mother liquor. Elemental analysis on
‘wet’ sample of 2 calculated (%) for C56H74N8O26Ni7 (2·2MeOH):
C, 39.95; H, 4.10; N, 6.90; Found: C, 40.21; H, 4.43; N, 7.15.
Elemental analysis on vacuum dried sample of 2 calculated (%) as
C54H66N8O24Ni7 (2): C, 39.99; H, 4.10; N, 6.91; Found: C, 39.69; H,
4.37; N, 7.03. FT-IR (cm-1): 3402 (b), 2932 (w), 2817 (w), 1627 (s),
1603 (w), 1561 (w), 1459 (m), 1436(m), 1407 (w), 1338(m), 1314 (s),
1240(w), 1222 (s), 1169 (w), 1147 (w), 1073 (m), 1042 (w), 1018 (w),
965 (m), 864 (m), 828 (w), 789 (m), 741 (s). UV/vis (MeOH):
lmax [nm] (emax 103 dm3 mol-1 cm-1): 205 (84.6), 231 (108.5), 268
(46.3), 356 (21.4). (MeCN): lmax [nm] (emax 103 dm3 mol-1 cm-1):
230 (110.2), 268 (42.8), 356 (15.8).
Acknowledgements
We wish to thank the NUI Galway Millennium Fund (LFJ),
the Irish Research Council for Science and Technology (IRCSET
Embark Program (SM)) and the Special Account for Research
Grants of the National and Kapodistrian University of Athens
(GSP) for funding.
[Ni7(OH)6(L2)6](NO3)2·3MeNO2 (3). The reaction mixture
obtained from 1 was filtered and the filtrate left to evaporate to
dryness. The resultant green solid was then redissolved in 10 cm3
MeNO2 whereby green hexagonal crystals of 3 were obtained in
10% yield upon slow Et2O diffusion. Elemental analysis calculated
(%) for C57H75N11O30Ni7 (3.3MeNO2): C, 37.93; H, 4.19; N, 8.54;
Found: C, 38.31; H, 4.59; N, 8.29. FT-IR (cm-1): 3625(w), 2931(w),
1628(s), 1601(m), 1555(s), 1476(s), 1460(s), 1433(m), 1407(m),
1337(m), 1316(m), 1221(m), 1171(w), 1147(w), 1086(w), 1072(w),
1018(w), 865(w), 795(w), 743(w).
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4816 | Dalton Trans., 2010, 39, 4809–4816
This journal is
The Royal Society of Chemistry 2010
©