Copper pyromellitates: A complex story

Article abstract of DOI:10.1039/c5ce00470e

Reaction of copper(ii) salts with pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid) and bases using slow vapour and gel diffusion techniques produces a variety of complexes containing copper and pyromellitate ions and solvent molecules, both coordinated (usually as terminal ligands) and uncoordinated. In some cases molecules of base are also coordinated, and organic or alkali metal cations may be present. Most of the complexes are 2- or 3-dimensional polymeric networks as expected, though there are also some discrete molecular complex ions, which are unusual for pyromellitate complexes of metals. Coordination of copper(ii) is most often square pyramidal with a long apical bond, but with some examples also of square-planar coordination supplemented by additional secondary interactions with carboxylates or solvent molecules above and below the plane to give distorted octahedral geometry. Pyromellitate is fully deprotonated in all but one product, and usually coordinates to 2, 4 or 6 copper ions, and additionally to 8 alkali metal ions where these are present. Hydrogen bonding plays a significant role in all the structures, its role ranging from formation of discrete clusters of coordinated and uncoordinated water molecules to extensive 3-dimensional networks. The impressive structural variety arises from the combination of rigidity of the benzene ring and individual carboxylate groups with torsional flexibility in the connections of these groups, together with the availability of different coordination geometries for copper(ii). The results illustrate the considerable structural versatility of pyromellitate ligands and the unreliability of structural design in benzenepolycarboxylate coordination chemistry.

Full text of DOI:10.1039/c5ce00470e

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Copper pyromellitates: a complex story†‡
*
William Clegg, James M. Holcroft§ and Nicola C. Martin¶
Cite this: DOI: 10.1039/c5ce00470e
Reaction of copperIJII) salts with pyromellitic acid IJbenzene-1,2,4,5-tetracarboxylic acid) and bases using
slow vapour and gel diffusion techniques produces a variety of complexes containing copper and
pyromellitate ions and solvent molecules, both coordinated (usually as terminal ligands) and uncoordinated.
In some cases molecules of base are also coordinated, and organic or alkali metal cations may be present.
Most of the complexes are 2- or 3-dimensional polymeric networks as expected, though there are also
some discrete molecular complex ions, which are unusual for pyromellitate complexes of metals. Coordina-
tion of copperIJII) is most often square pyramidal with a long apical bond, but with some examples also of
square-planar coordination supplemented by additional secondary interactions with carboxylates or solvent
molecules above and below the plane to give distorted octahedral geometry. Pyromellitate is fully
deprotonated in all but one product, and usually coordinates to 2, 4 or 6 copper ions, and additionally to
8 alkali metal ions where these are present. Hydrogen bonding plays a significant role in all the structures, its
role ranging from formation of discrete clusters of coordinated and uncoordinated water molecules to
extensive 3-dimensional networks. The impressive structural variety arises from the combination of rigidity
of the benzene ring and individual carboxylate groups with torsional flexibility in the connections of these
groups, together with the availability of different coordination geometries for copperIJII). The results illustrate
the considerable structural versatility of pyromellitate ligands and the unreliability of structural design in
benzenepolycarboxylate coordination chemistry.
Received 6th March 2015,
Accepted 9th March 2015
DOI: 10.1039/c5ce00470e
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and in carborane research, including a particularly attractive
cyclic tris-carborane compound that featured on the cover of
Introduction
In the early 1980s I (Bill Clegg) began collaborative research
with Ken Wade, whom I had first met while I was working on
a fixed-term contract in Newcastle and he was in nearby Dur-
ham. At first the collaboration was at a distance, but it devel-
oped further when I returned to Newcastle from Göttingen in
1984 and continued then for almost a decade, during which
there were no crystallographic facilities in the Chemistry
Department at Durham. First results were in lithium chemis-
try1 and led to a long-term fruitful interaction with research
groups in Strathclyde (principally Ron Snaith, then Rab
Mulvey and Eva Hevia), initiated and encouraged by Ken.
Many other applications followed in main-group chemistry
my undergraduate crystallography textbook2 and was dubbed
by Ken a ‘Wagnerian Ring Cycle’, although ‘cluster cycle’ was
the nearest to this that could appear in the published report.3
A Durham project investigating some mixed copper-zinc
carboxylates as potential precursors to catalysts for methanol
synthesis, as well as for the structural and magnetic proper-
ties of metal benzenepolycarboxylate complexes, provided a
few samples for crystallographic study, the results of which
have not yet been published except in the PhD thesis of
Gavin Walker;4 this work was led by Ken's group, including
contributions of Hugh MacBride and Stan Hauptman, who
prepared and supplied the samples of copper and mixed
copper-zinc pyromellitates for structure determination. At
this stage the targeted study of benzenepolycarboxylates as
linkers in the formation of metal–organic frameworks and
reticular families of porous structures had not yet begun.5
The fascination and beauty of the results of our early
Durham-Newcastle collaboration in this area led to a more
systematic investigation of transition metal complexes of
benzenepolycarboxylic acids in my research group over the
following years. The area proved to be very complex, both in
terms of the variety of products formed from reactions under
different or even very similar or supposedly identical condi-
tions and with respect to the structural properties and
School of Chemistry, Newcastle University, Newcastle upon Tyne NE1 7RU, UK.
E-mail: bill.clegg@ncl.ac.uk
† Dedicated to the memory of Ken Wade, who first introduced me to the beauty
of benzenepolycarboxylate chemistry, and who valued precise structural results
and used them with such enormous impact.
‡ Electronic supplementary information (ESI) available: Full formatted tables of
crystallographic results. CCDC 1044116–1044125 contain the supplementary
crystallographic data for this paper. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c5ce00470e
§ Current address: School of Chemical Engineering and Advanced Materials,
Newcastle University, Newcastle upon Tyne NE1 7RU, UK.
¶ Current address: Natrium Capital Limited, The Euston Office, One Euston
Square, 40 Melton Street, London NW1 2FD, UK.
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relationships of these products, some of them made more
challenging by the incidence of structural disorder, twinning
and pseudo-symmetry.
network complex with empirical formula corresponding to a
decahydrate Cu2IJpm)IJOH2)6Ĵ4H2O9 and the discrete molecu-
lar complex Cu2IJpm)IJ9-ane-N3)2Ĵ4H2O, where 9-ane-N3 is
the three-fold-symmetric terdentate aza-crown macrocycle
1,4,7-trimethyl-1,4,7-trizacyclononane.10 Many others have
appeared since then, and the most relevant are referenced
below in the discussion. Some of the more recent reports are
of structures that we have ourselves also obtained in this
work; in most cases our results represent a significant
improvement in precision and interpretation, particularly of
hydrogen bonding, and we are able to correct some errors in
the previous accounts. One of the complexities we have found
in the developing story is that the same product can be
obtained by different routes (sometimes during the attempt
to prepare a different compound, but with one or more
intended reactants finding no place in the resulting struc-
ture), while parallel or repeated syntheses using similar or
supposedly identical procedures and conditions sometimes
lead to two or more different products, these occasionally
being obtained as a mixture from a single experiment. Simi-
lar observations have been made by others.4,11,12
In order to reduce this complexity, and that of the struc-
tural aspects, we present our results here in a logical rather
than a chronological order, grouping the reported com-
pounds and related literature results under four headings: (a)
complexes containing only Cu, pyromellitate and solvent; (b)
structures incorporating alkali metal cations; (c) anionic com-
plexes with protonated bases as counterions; (d) complexes
in which unprotonated bases serve as ligands. The generation
of copper pyromellitate complexes involves the deprotonation
of H4pm by an appropriate base so that it can coordinate to
the metal. Commonly used bases are ammonia, organic
amines (both aliphatic and aromatic), and alkali metal
hydroxides or carbonates, and the bases themselves or their
cations are incorporated in the structures of groups IJb)–IJd),
whereas they are absent in group (a). In presenting the struc-
tures below, the primary graphical representation of each
gives the crystallographic asymmetric unit, to which are
added sufficient symmetry-equivalent atoms to show the
immediate environment of each of the unique metal centres
and pyromellitate ligands. Further figures are provided as
appropriate, to illustrate key features of the structures.
Descriptions include the coordination geometry of copper,
the dihedral angles between carboxylate groups and benzene
rings of pyromellitate, the coordinating function (usually
multiple) of the pyromellitate ligands, the resulting dimen-
sionality of the structure, and the hydrogen bonding. Bond
lengths and angles for the copper coordination geometry in
all the structures are collected in Table 1. Full details includ-
ing complete bond lengths and angles, torsion angles, and
tables of hydrogen bonding are provided in the ESI.†
We have previously reported some of the results, concerning
selected complexes of isophthalic IJbenzene-1,3-dicarboxylic)6
and mellitic IJbenzene-1,2,3,4,5,6-hexacarboxylic)7 acids. Here
we collect together most of the results we have obtained on cop-
per complexes of pyromellitic IJbenzene-1,2,4,5-tetracarboxylic)
acid, including the original structures obtained in the collabora-
tion with Ken Wade, as an appropriate tribute to his memory
and our shared research interests (ESI†).
Pyromellitic acid (H4pm) has been widely used in syn-
thetic and structural research. It is the most symmetrical of
the three benzenetetracarboxylic acids. In terms of known
crystal structures, it vastly surpasses the other two in entries
in the Cambridge Structural Database (CSD).8 In our survey
of the CSD using the version with updates to February 2014,7
we found 840 entries for complexes, salts and derivatives of
H4pm, this figure being surpassed among the benzenepoly-
carboxylic acids only by the most symmetrical tricarboxylic
acid (trimesic, 1,3,5-substituted), by the three dicarboxylic
acids, and by benzoic acid. A major point of structural inter-
est in considering the most symmetrically substituted iso-
mers of increasingly substituted acids in this series is that, in
contrast to the lower members terephthalic (1,4) and trimesic
(1,3,5) acids, the carboxylate groups in H4pm are prevented
from achieving coplanarity with the benzene ring by mutual
steric interaction, a factor taken to its extreme in the fully
substituted and much less studied mellitic acid. The only excep-
tion is the possibility of hydrogen bonding between adjacent
carboxylate/carboxylic acid groups and this, leading to coplanar-
ity, is found in some, but by no means all, of the 100 or so
structures containing doubly-deprotonated H2pm2− anions.
The CSD (version 5.35 with updates to May 2014) has 136
entries containing copper and partially or fully deprotonated
pyromellitate anions (ranging from H3pm− to pm4−; two
structures also contain H4pm, but none have the parent acid
without anions also present). The structures contain also a
wide range of other components, including further ligands
coordinated to copper, and many incorporate additional
metal ions. For comparison with our own results here, we
restrict coverage to those compounds in which no other
metal ions are present apart from alkali metals, and in which
the other components are solvent molecules and mono-
dentate ligands, leaving aside the structures in which auxil-
iary ligands serve as bridges between copper centres; there
are 28 of these, including a number of reports of the same
structure in different publications. The additional ligands are
nitrogen heterocycles, ammonia, methylamine, hydroxide,
and solvents (water and DMF).
Results and discussion
(a) Complexes containing only Cu, pyromellitate and solvent
At the time when the work reported here began, there were
only two known crystallographically characterized copper
pyromellitate complexes: the two-dimensional polymeric
Three of our complexes contain only Cu2+, either pm4− or
H2pm2−, and H2O (both coordinated and uncoordinated).
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Table 1 Selected bond lengths (Å) and angles (°) for compounds 1–10
1
Cu–O1
Cu–O4b
Cu–O6
2.2319IJ17)
1.9589IJ18)
1.984IJ2)
Cu–O2a
Cu–O5
1.9592IJ18)
1.9636IJ18)
O1–Cu–O2a
O1–Cu–O5
O2a–Cu–O4b
O2a–Cu–O6
O4b–Cu–O6
102.39IJ7)
O1–Cu–O4b
O1–Cu–O6
O2a–Cu–O5
O4b–Cu–O5
O5–Cu–O6
88.40IJ7)
101.58IJ9)
166.19IJ8)
89.02IJ8)
81.70IJ8)
91.22IJ7)
93.62IJ8)
93.15IJ8)
166.45IJ9)
Symmetry operations: a = −x, −y + 1, −z; b = x − 1, y − 1, z
2
Cu–O2
Cu–O5
Cu–O7
1.9553IJ12)
1.9548IJ13)
2.2432IJ13)
Cu–O4a
Cu–O6
2.0090IJ12)
1.9517IJ13)
O2–Cu–O4a
O2–Cu–O6
O4a–Cu–O5
O4a–Cu–O7
O5–Cu–O7
175.22IJ5)
O2–Cu–O5
O2–Cu–O7
O4a–Cu–O6
O5–Cu–O6
O6–Cu–O7
86.44IJ6)
92.90IJ5)
93.60IJ6)
173.57IJ6)
90.71IJ5)
91.15IJ6)
88.90IJ5)
86.48IJ5)
95.36IJ5)
Symmetry operation: a = −x + 1/2, y − 1/2, −z + 1/2
3
Cu–O2
Cu–O9
Cu–O11
1.9555IJ16)
2.220IJ3)
1.985IJ2)
Cu–O6a
Cu–O10
1.9430IJ15)
1.986IJ2)
O2–Cu–O6a
O2–Cu–O10
O6a–Cu–O9
O6a–Cu–O11
O9–Cu–O11
176.91IJ11)
90.82IJ8)
92.55IJ9)
88.55IJ10)
95.19IJ12)
O2–Cu–O9
O2–Cu–O11
O6a–Cu–O10
O9–Cu–O10
O10–Cu–O11
84.38IJ8)
91.46IJ9)
89.98IJ8)
100.16IJ12)
164.64IJ10)
Symmetry operation: a = x, y − 1, z
4
Cu–O2
1.9354IJ18)
1.9771IJ18)
91.27IJ8)
91.03IJ8)
177.28IJ8)
Cu–O4a
Cu–O7c
O2–Cu–O5b
O4a–Cu–O5b
O5b–Cu–O7c
1.9907IJ18)
1.9476IJ19)
177.13IJ8)
86.56IJ8)
Cu–O5b
O2–Cu–O4a
O2–Cu–O7c
O4a–Cu–O7c
91.19IJ8)
Symmetry operations: a = −x + 1, −y, −z + 1; b = x, y, z + 1; c = −x + 1, −y + 1, −z + 1
5
Cu–O2
Cu–O4b
O2–Cu–O2a
O2a–Cu–O4b
O2a–Cu–O4c
1.9397IJ10)
1.9551IJ9)
84.98IJ6)
167.72IJ4)
92.64IJ4)
Cu–O2a
Cu–O4c
O2–Cu–O4b
O2–Cu–O4c
O4b–Cu–O4c
1.9397IJ10)
1.9551IJ9)
92.64IJ4)
167.72IJ4)
92.14IJ6)
Symmetry operations: a = −x + 1, y, −z + 3/2; b = −x + 1, −y + 1, −z + 1; c = x, −y + 1, z + 1/2
6
Cu–O2
Cu–O6a
1.940IJ4)
1.926IJ4)
Cu–O4
Cu–O8a
1.927IJ4)
1.945IJ4)
Cu–O9
2.278IJ4)
O2–Cu–O4
O2–Cu–O8a
O4–Cu–O6a
O4–Cu–O9
O6a–Cu–O9
89.42IJ16)
177.11IJ15)
171.63IJ15)
93.89IJ15)
94.46IJ15)
O2–Cu–O6a
O2–Cu–O9
O4–Cu–O8a
O6a–Cu–O8a
O8a–Cu–O9
89.67IJ16)
91.64IJ15)
90.12IJ17)
90.37IJ16)
91.23IJ15)
Symmetry operation: a = −x + 1, −y + 1, −z + 1
7
Cu–O2
Cu–O6a
Cu–O9
O2–Cu–O4
O2–Cu–O8a
O4–Cu–O6a
1.9426IJ19)
1.9320IJ19)
2.2658IJ19)
88.69IJ9)
177.12IJ8)
171.61IJ8)
Cu–O4
Cu–O8a
1.9352IJ19)
1.9497IJ19)
O2–Cu–O6a
O2–Cu–O9
O4–Cu–O8a
90.31IJ9)
94.74IJ8)
90.42IJ9)
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Table 1 (continued)
O4–Cu–O9
O6a–Cu–O9
96.67IJ8)
91.71IJ8)
O6a–Cu–O8a
O8a–Cu–O9
90.18IJ9)
88.08IJ8)
Symmetry operation: a = −x + 1, −y + 1, −z + 1
8
Cu1–O2
Cu1–N2
Cu2–N4
Cu2–N5
O2–Cu1–N1
O2–Cu1–N3
N1–Cu1–N3
N4–Cu2–N4a
N4a–Cu2–N5
N4a–Cu2–N5a
2.024IJ2)
2.009IJ3)
2.031IJ3)
2.040IJ3)
87.88IJ12)
88.84IJ11)
175.21IJ13)
180.00
92.10IJ14)
87.90IJ14)
Cu1–N1
Cu1–N3
Cu2–N4a
Cu2–N5a
O2–Cu1–N2
N1–Cu1–N2
N2–Cu1–N3
N4–Cu2–N5
N4–Cu2–N5a
N5–Cu2–N5a
2.018IJ3)
1.982IJ3)
2.031IJ3)
2.040IJ3)
175.24IJ11)
91.71IJ14)
91.85IJ14)
87.90IJ14)
92.10IJ14)
180.00
Symmetry operation: a = −x + 1, −y + 1, −z + 1
9
Cu1–O2
Cu1–N1
Cu1–O9
Cu2–O8a
Cu2–N4
O2–Cu1–O6
O2–Cu1–N2
O6–Cu1–N1
O6–Cu1–O9
N1–Cu1–O9
O4–Cu2–O8a
O4–Cu2–N4
O8a–Cu2–N3
O8a–Cu2–O10
N3–Cu2–O10
1.968IJ5)
2.002IJ6)
2.293IJ6)
1.968IJ5)
2.012IJ6)
Cu1–O6
Cu1–N2
Cu2–O4
Cu2–N3
1.960IJ5)
1.986IJ6)
1.966IJ5)
2.019IJ6)
2.290IJ5)
87.4IJ2)
92.95IJ19)
92.1IJ2)
168.2IJ3)
88.9IJ2)
88.1IJ2)
91.44IJ19)
87.5IJ2)
170.7IJ3)
97.2IJ2)
Cu2–O10
175.4IJ2)
O2–Cu1–N1
O2–Cu1–O9
O6–Cu1–N2
N1–Cu1–N2
N2–Cu1–O9
O4–Cu2–N3
O4–Cu2–O10
O8a–Cu2–N4
N3–Cu2–N4
N4–Cu2–O10
90.6IJ2)
89.2IJ2)
90.9IJ2)
102.8IJ2)
178.2IJ2)
91.2IJ2)
92.9IJ2)
90.09IJ19)
92.0IJ2)
Symmetry operation: a = x + 1, y − 1, z
10
Cu1–O2
Cu1–N1
Cu1–O9
Cu2–O4a
Cu2–N3a
Cu3–O8b
Cu3–N4b
O2–Cu1–O6
O2–Cu1–N2
O6–Cu1–N1
O6–Cu1–O9
N1–Cu1–O9
O4–Cu2–O4a
O4–Cu2–N3a
O4a–Cu2–N3
O8–Cu3–O8b
O8b–Cu3–N4b
O8b–Cu3–N4
1.971IJ2)
2.028IJ3)
2.357IJ3)
1.961IJ2)
2.047IJ3)
1.973IJ2)
2.023IJ3)
169.89IJ10)
95.07IJ10)
85.60IJ10)
84.32IJ9)
98.16IJ11)
180.00
Cu1–O6
Cu1–N2
Cu2–O4
Cu2–N3
Cu3–O8
Cu3–N4
1.946IJ2)
2.026IJ3)
1.961IJ2)
2.047IJ3)
1.973IJ2)
2.023IJ3)
O2–Cu1–N1
O2–Cu1–O9
O6–Cu1–N2
N1–Cu1–N2
N2–Cu1–O9
O4–Cu2–N3
O4a–Cu2–N3a
N3–Cu2–N3a
O8–Cu3–N4
O8–Cu3–N4b
N4–Cu3–N4b
88.60IJ10)
88.33IJ9)
92.53IJ10)
166.01IJ12)
95.44IJ11)
92.69IJ10)
92.69IJ10)
180.00
88.44IJ10)
91.56IJ10)
180.00
87.31IJ10)
87.31IJ10)
180.00
88.44IJ10)
91.56IJ10)
Symmetry operations: a = −x + 1, −y + 2, −z; b = −x + 2, −y + 2, −z + 1
The first (1) is one of the compounds originally provided by
Ken Wade's project. Empirically it is a hexahydrate of
Cu2IJpm), but the formula is more informatively written
Cu2IJpm)IJOH2)4Ĵ2H2O. The same structure has previously
been reported,13 but the data for that publication were mea-
sured at room temperature, H atoms were not included for
any of the water molecules, the final R factor was 0.064, and
standard uncertainties are higher, e.g. by a factor of about 3
for Cu–O bond lengths.
The copper ion occupies a general position, while the
pm4− anion lies on an inversion centre. Copper has a square
pyramidal coordination geometry, with two terminal aqua
ligands arranged cis in basal positions, carboxylate O atoms
of two different pm4− ligands in the other two mutually cis
basal positions, and a considerably longer apical bond to a
third anion, as shown in Fig. 1; all other Cu⋯O distances are
>3 Å. Square pyramidal coordination with a long apical bond
is found in most of the structures reported here. Each pm4−
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sheets consisting of direct IJaqua)O–H⋯OIJpm) interactions as
well as interactions via uncoordinated water molecules.
The second structure is that of the higher hydrate
Cu2IJpm)IJOH2)6Ĵ4H2O (2), which we have obtained by vapour
diffusion of pyridine into a solution of copperIJII) nitrate and
H4pm in aqueous methanol; for related structures incorporat-
ing pyridine as a ligand, see later. It can also be produced by
use of triethylamine as base in a gel diffusion experiment. Its
synthesis has been reported twice previously, once using
potassium hydroxide as base together with H4pm and a
copperIJII) salt under careful pH control,9,11 and once much
more recently by refluxing a mixture of H4pm, CuO and
Gd2O3 in water, the role of the gadolinium oxide in the reac-
tion being unclear.14
The 1982 report of this structure was based on room-
temperature diffraction data, the R factor is 0.052, and not
all H atoms were included; O–H distances range from 0.77 to
1.30 Å. The 2013 report has many disconcerting features: it
makes no reference to the earlier structure determination,9
nor to the 2002 publication of compound 1,12 and claims that
this is a new compound; it incorrectly states that the pm4−
anion lies on an inversion centre instead of a C2 axis; and it
claims that the uncoordinated water molecules could not be
reliably located and refined, which is surprising given their
location in the earlier study, our own straightforward refine-
ment, and the extensive network of hydrogen bonding hold-
ing them in place.
Fig. 1 The asymmetric unit of compound 1, with additional atoms to
complete the environment of Cu and pm4−
.
ligand is coordinated to six copper centres, with two mutually
trans carboxylate groups bonded to one metal each and the
other two to two metal ions each, making use of both O
atoms. The dihedral angles between carboxylate groups and
the benzene ring are two of 30.4 and two of 65.9°, the singly
coordinating groups being twisted more out of the ring
plane. The copper and pm4− ions generate a polymeric
sheet structure, as shown in Fig. 2; this is a space-filling rep-
resentation with van der Waals radii for the atoms and
includes the coordinated aqua ligands but not the un-
coordinated water molecules, which lie between the sheets,
and it demonstrates that the previous description of this as a
2-dimensional porous structure13 is misleading, there being
no significant voids within the layer.
The environments of the copper and pm4− ions are shown
in Fig. 3. Once again the copper coordination geometry is
square pyramidal, but here the apical ligand is aqua and the
pairs of aqua and pm4− ligands in the basal plane are each
arranged trans. Each carboxylate group of pm4− coordinates
one copper ion, though for two of them the second O atom
(O3 and its symmetry-equivalent on the adjacent carboxylate)
Hydrogen bonding involves all six water O–H bonds as
donors; the acceptors are all the carboxylate O atoms and the
uncoordinated water molecule. A 3-dimensional network is gen-
erated, the connections between covalently bonded polymer
Fig. 2 The polymeric sheet structure of compound 1 in a space-filling
representation.
Fig. 3 The asymmetric unit of compound 2, with additional atoms to
complete the environment of Cu and pm4−
.
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forms a secondary Cu⋯O interaction with length 2.676 Å in
the space below the basal plane opposite the apical aqua
ligand; all other Cu⋯O distances are >3.2 Å. The dihedral
angles between the carboxylate groups and the benzene ring
are two of 52.8 and two of 42.1°. Covalently bonded poly-
meric sheets are formed, which in this case contain substan-
tial spaces surrounded by O and H atoms, as shown in Fig. 4.
These are occupied by the uncoordinated water molecules.
Contrary to the 2013 description of this structure,14 the
water-filled spaces are not continuously connected, in either
the a-axis or c-axis direction to form channels, because
the successive sheets separated by the c-axis spacing of
9.5816IJ6)° are interleaved with symmetry-equivalent sheets
at half the spacing, which are displaced laterally so that
benzene rings of one layer lie above the spaces of the next.
Nevertheless it is true that hydrogen bonding, considered
alone without the covalent bonding within the polymeric
sheets and involving coordinated and uncoordinated
water, generates networks that extend along the c axis but
do not connect with each other in the other two dimen-
sions. All O–H bonds of the coordinated and uncoordinated
water molecules serve as hydrogen-bond donors; the accep-
tors are all the carboxylate O atoms, the two uncoordinated
water molecules, and the apical (but not the basal) aqua
ligand.
The empirical formula of 3 corresponds to exchange of
one Cu2+ in 1 for two protons retained by H2pm2−, main-
taining 6 water molecules per anion: CuIJH2pm)IJOH2)3Ĵ3H2O.
All atoms lie in general positions in this, the only structure
in the series to have a non-centrosymmetric (but achiral)
space group. The two carboxylic acid groups, which are
mutually trans, do not coordinate copper, and the two
trans-related carboxylate groups act as monodentate donors
to copper ions, linking these together into a polymeric chain
parallel to the b axis, a segment of which is shown in Fig. 5.
The dihedral angles between the substituents and the ben-
zene ring are 84.8 and 86.9° for the coordinating carboxylate
groups, and 6.2 and 3.9° for the carboxylic acid groups,
which thus achieve near-coplanarity with the ring at the
expense of near-orthogonality to the ring for the carboxylate
groups.
The coordination of copper in 3 is very similar to that in 2
and is best described as square pyramidal, though there is a
larger difference between the two basal trans angles in this
case. The τ parameter, defined as the difference between
these two angles divided by 60°,15 is 0.03 for 2 and 0.20 for 3;
it is 0 for ideal square pyramidal, and 1 for ideal trigonal
bipyramidal geometry. The structure has been reported twice
before,12,16 both times from room-temperature data with R =
0.033 or 0.037 and with H atoms constrained in a riding
model without orientational freedom, leading to some poor
geometry (both covalent and hydrogen bonding); one of these
reports describes the copper coordination as slightly
distorted trigonal bipyramidal.12 The next-nearest carboxylate
O atom is at a distance of 2.881 Å.
Comparison of these two Cu/pm4−/H2O structures shows
that the larger proportion of water in 2 means that the five
coordination sites of Cu are occupied in the ratio 3 : 2 by
water and carboxylate ligands instead of 2 : 3, and there are
twice as many uncoordinated water molecules in the struc-
ture; concomitantly the connectivity of the pm4− ligand is
reduced from 6 to 4 metal centres.
We find, with H atoms refined subject to soft distance
restraints, that all 3 coordinated and all 3 uncoordinated
water molecules donate their H atoms in hydrogen bonds, as
do the two carboxylic acid functions; the acceptors are all 4
carboxylate O atoms, the two carboxylic CO groups (but not
the carboxylic OH groups), and the 3 uncoordinated water
molecules (one of them as a double acceptor), the 3 aqua
ligands accepting no hydrogen bonds in this structure. A
3-dimensional hydrogen-bonding network is generated. The
Fig. 4 The polymeric sheet structure of compound 2 in a space-filling
representation.
Fig. 5 The asymmetric unit of compound 3, with additional atoms to
complete the environment of Cu and H2pm2−
.
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shortest hydrogen bonds are between the carboxylic acid OH
groups (in which the H atom positions were freely refined
without restraints) and one of the water molecules (the dou-
ble acceptor O14), with O⋯O distances of 2.618(2) and
2.625(3) Å.
Comparison of the hydrogen bonding in these three struc-
tures shows that, when potential donors outnumber accep-
tors, all donors are used together with all the carboxylate O
atoms as acceptors, and uncoordinated water molecules
accept hydrogen bonds preferentially over aqua ligands.
There is one other known Cu/pyromellitate/H2O structure
containing no other components, the lower hydrate Cu2IJpm)-
IJOH2)4, this formula corresponding to that of 1 with all
uncoordinated water removed.17 The structure is in marked
contrast to those of 1–3, consisting of a 3-dimensional network
of copper and pm4− ions in which copper has an almost regular
trigonal bipyramidal coordination with terminal aqua ligands
in one axial and one equatorial site, the others being occupied
by carboxylate O atoms. Hydrogen bonding uses all four O–H
bonds of the two aqua ligands, and the acceptors are 3 carboxyl-
ate O atoms and one of the aqua ligands. There is also one
Cu/pyromellitate/solvent structure in the CSD (two separate
reports18) for which the solvent is not water but DMF: Cu2IJpm)-
IJDMF)2. This is a 2-dimensional sheet polymer containing
‘paddle-wheel’ units in which a pair of copper ions is bridged
by four pm4− ligands in the well-known Cu2IJcarboxylate)4
arrangement, each Cu has also an apical DMF ligand to give
square pyramidal coordination, and each pm4− anion links
four such paddle-wheel units to generate the polymer sheet.
Fig. 6 The asymmetric unit of compound 4, with additional atoms to
complete the environment of Cu, K1, K2 and pm4−
.
coordination of two of the carboxylate groups, as shown in
Fig. 6.
The two crystallographically independent potassium cations
have different coordination environments. K1 is 7-coordinate
(K1–O4 is obscured behind K1–O2 in Fig. 6) with an approxi-
mate pentagonal bipyramidal geometry: the axial bonds are to
a bridging water molecule O9 and a carboxylate O atom (sym-
metry-equivalent to O3), and the equatorial plane contains
four carboxylate O atoms and the terminal aqua ligand O10.
K2 is 8-coordinate with an irregular geometry; it may be
regarded as based on 7-coordinate pentagonal bipyramidal
with water O9 and a symmetry-equivalent of carboxylate O6 posi-
tioned axially, with 3 carboxylate O atoms and a bridging water
molecule (O11) in 4 equatorial sites, and with a fifth single
equatorial atom replaced by a symmetry-equivalent water O11
and a further carboxylate O atom on either side of the equato-
rial plane (these are the two bonds to K2 pointing closest to
the bottom edge of Fig. 6, and the K–carboxylate bond is sig-
nificantly longer than the other 7 in this coordination sphere,
at 3.216 Å; the next-nearest O atom is at 3.698 Å).
(b) Complexes with structures incorporating
alkali metal cations
Complexes 1 and 2 have been produced by reactions of
copperIJII) salts with H4pm and potassium hydroxide in aque-
ous solution.4,11 The product obtained is dependent on the pH
control during the experiment. Complex 4, CuK2IJpm)Ĵ4H2O, is
another product obtainable from this system as the conditions
are varied; larger crystals suitable for single-crystal X-ray diffrac-
tion were generated by slow diffusion rather than from a
single reaction mixture.
All components of this structure lie in general positions.
This is the only structure in the series for which we have not
been able to locate and refine all the water H atoms reliably;
one uncoordinated water molecule (O12) has markedly
higher displacement parameters than the others and may
be subject to incomplete site occupancy, and directions for
its O–H bonds to give consistent appropriate hydrogen bond-
ing cannot be found, suggesting that at least the orientation
may be disordered. The rest of the structure, however, is
completely clear.
The pm4− ligand coordinates to no fewer than 12 different
metal centres, of which 4 are copper, 4 are K1 and symmetry-
equivalents, and 4 are K2 and symmetry-equivalents. Each
O atom forms two bonds to metal ions, and the carboxylate
groups serve both bridging and chelating functions. The
dihedral angles between carboxylate groups and the benzene
ring are 39.8, 63.3, 58.6 and 42.4°. The copper and pm4− ions
alone form a polymeric sheet, and the sheets are connected
together through the potassium ions.
In contrast to the first three structures, the primary coor-
dination of copper here is square planar, with carboxylate O
atoms as donors. There are two further O atoms 2.601
and 2.611 Å above and below the coordination plane, giving a
very distorted octahedral environment with a semi-chelating
The four water molecules all have different functions and
environments. O9 is a bridge between K1 and K2 with a wide
K–O–K angle of 164.59IJ12)°. O10 is a terminal ligand for K1.
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O11 is an asymmetrical bridge between two symmetry-
equivalent K2 ions, generating a centrosymmetric parallelo-
gram with two K2 and two O11 as its vertices and K–O–K
angles of 102.94IJ14)°. O12 is not coordinated. O9, O10 and
O11 form hydrogen bonds, the acceptors being carboxylate O
atoms and O12. Hydrogen bonding in this structure gener-
ates discrete clusters of four water molecules surrounded by
carboxylate groups.
there is hydrogen bonding also within the completely planar
H2pm2− anion.
(c) Anionic complexes with protonated bases as counterions
We have three of these, without literature precedents among
copper pyromellitate complexes as far as we are aware, and
the structures show some unusual and surprising features.
Vapour diffusion of triethylamine into a solution of a
copperIJII) salt and H4pm in ethanol produces compound 6.
The triethylamine deprotonates H4pm and the resulting pro-
tonated base serves as counter-cations to the dimeric anion
containing Cu, pm4− and ethanol (Fig. 8). Discrete (non-poly-
meric) metal complexes of pm4− are much less common than
polymeric structures, and in most of them coordination sites
of the metal centres are blocked by bi- or multidentate
ligands that serve as terminators, preventing further associa-
tion into chains, sheets or networks. The few entries in the
CSD for discrete structural units that contain only metal ions,
pm4− and coordinated solvent are all neutral dinuclear spe-
cies in which a single pm4− coordinates through two opposite
Complex 5 has the formula CuNa2IJpm)Ĵ4H2O. Although
this is empirically the same as 4 with sodium replacing potas-
sium, the structure is completely different. Cu lies on a C2
axis, Na in a general position, and pm4− on an inversion
centre; the sodium and pm4− ions give a covalently connected
3-dimensional framework even without consideration of the
sodium ions and water molecules. The pm4− anion is bonded
to 4 copper ions (one through each carboxylate group) and to
8 sodium ions. The dihedral angles between the carboxylate
groups and the benzene ring are two of 53.9° and two of 56.3°.
As in 4, Cu has a primary square-planar coordination,
bonded to four different pm4− ligands, and has one longer
Cu⋯OIJpm) contact on each side of the plane, both at 2.767
Å, as shown in Fig. 7. Sodium is 6-coordinate in a distorted
octahedral geometry, with two cis terminal aqua ligands and
four carboxylate O atoms. The two independent water mole-
cules form four hydrogen bonds, the acceptors being three
carboxylates and one of the water molecules, generating dis-
crete hydrogen-bonded units of two water molecules and
three surrounding carboxylate groups.
The important common features of the structures of 4
and 5 are that copper is coordinated only by carboxylate
groups, water is coordinated only to the alkali metal ions,
and pyromellitate coordinates 12 different cations (4 Cu and
8 alkali metal). By contrast, the other known structure with
only Cu, alkali metal, pyromellitate and water as ingredients
is CuNa2IJH2pm)IJOH2)8Ĵ2H2O, in which copper is exclusively
coordinated by water (4 bridging and 2 terminal in a Jahn–
Teller-elongated octahedral geometry with long bonds to
two trans bridges), H2pm2− forms only two coordinative
bonds and thus bridges two sodium ions, and the structure
is a chain polymer with a linear ⋯Na⋯Na⋯Cu⋯ repeat
unit;19 the chains are connected by hydrogen bonding, and
2+
carboxylate groups to two MIJOH2)5 centres (M = Mg, Mn,
Co, Ni).20
The centrosymmetric anion of 6 has two parallel pm4−
ligands with their benzene rings separated by 4.155 Å
(too far for any significant π-stacking interactions). The car-
boxylate groups are twisted out of the benzene ring plane by
42.8, 48.2, 56.0 and 39.2°, and the four O atoms lying on one
side of the plane coordinate the two copper ions, leaving the
other four O atoms protruding on the opposite side and not
Fig. 7 The asymmetric unit of compound 5, with additional atoms to
Fig.
8 The unit cell contents of compound 6. Minor disorder
complete the environment of Cu, Na and pm4−
.
components are not shown.
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engaging in coordination. Each pm4− anion acts as a bis-
bidentate ligand, adjacent carboxylate groups being attached
to the same copper ion. Each of the two copper ions is coor-
dinated in this way by two pm ligands, forming the basal
plane in a square pyramidal coordination geometry, the api-
cal position of which is occupied by a molecule of ethanol
(with its methyl group disordered over two orientations); the
apical bond is considerably longer than the basal bonds, as
in the other square pyramidal structures reported here.
Such a ‘lantern’ structure is unprecedentated among metal
pyromellitate complexes. There are five previously reported
structures in which pm4− coordinates in a bis-bidentate
fashion with each pair of adjacent carboxylate groups coordi-
nated to a single metal ion.10,21 In each of these there is only
one pm4− ligand and the dinuclear molecule is precisely or
approximately centrosymmetric, with the two metal centres
lying on opposite sides of the benzene ring.
The ethyl groups of one of the two crystallographically
independent Et3NH+ cations are disordered; the N–H groups,
however, are ordered and are oriented such as to form
N–H⋯O hydrogen bonds to 4 of the 8 uncoordinated carbox-
ylate O atoms of the anion. Further O–H⋯O hydrogen bonds
are formed between the ethanol apical ligands and carboxyl-
ate O atoms of two adjacent anions, linking the anions
together in a hydrogen-bonded chain parallel to the b axis.
Two of the uncoordinated O atoms do not serve as hydrogen
bond acceptors.
shows no disorder, and uncoordinated methanol is incorpo-
rated in the crystal structure. The unit cells of the two struc-
tures are very similar, with the same space group, and the
arrangements of the cations and anions are almost the same,
the smaller methanol molecule providing space for extra sol-
vent inclusion that causes a small increase rather than the
expected decrease in the unit cell volume. The separation
between the two parallel benzene rings is 4.188 Å, and the
dihedral angles between the carboxylate groups and the ben-
zene ring within each ligand are 46.4, 49.3, 57.6 and 36.6°.
The hydrogen bonding pattern is the same as in compound
6, with the addition that the uncoordinated methanol mole-
cules are hydrogen-bonded to the remaining carboxylate O
atoms that do not serve as acceptors in 6.
Compound 8 results from the use of ammonia as base
to deprotonate H4pm; ammonia is incorporated in the struc-
ture both as a ligand and as an ammonium cation (Fig. 10).
If only primary coordination is considered (bonds to Cu
under 2.5 Å in length), the structure contains relatively
+
simple discrete units: NH4 and centrosymmetric square-
planar ijCuIJNH3)4]2+ cations, ijCuIJpm)IJNH3)3]2− anions in which
a terminal pm4− ligand replaces one NH3 of the tetra-
amminecopperIJII) cation, and uncoordinated water molecules.
As far as we are aware, this is the first occurrence of pm4− as a
terminal ligand coordinating through only one O atom. The
dihedral angles between the carboxylate groups and benzene
ring are 67.2 (coordinated), 18.3, 58.3 and 68.9°.
Compound 7, obtained in a similar reaction carried out
in methanol rather than ethanol solution, has essentially
the same structure with only minor differences (Fig. 9): there
is more extensive disorder of one of the cations, methanol
Each of the two copper ions has two O atoms above and
below its coordination square plane at distances representing
secondary bonding in an elongated octahedral geometry. For
Cu2, lying on an inversion centre in the ijCuIJNH3)4]2+ cation,
the additional ligands are water molecules, with Cu–O =
2.505 Å. A water molecule is also one of the extra ligands of
Cu1, at a distance of 2.644 Å; opposite it (O–Cu–O 165.3°) lies
a symmetry-equivalent of the coordinating carboxylate atom
O2, at 2.581 Å. Two Cu1-containing anions thus form a cen-
trosymmetric pair linked by a Cu2O2 parallelogram with sides
of 2.024 and 2.581 Å and angles at O of 106.9°. These
Cu⋯OH2 and Cu⋯OIJpm) secondary interactions link the
Fig.
9
The unit cell contents of compound 7. Minor disorder
Fig. 10 The asymmetric unit of compound 8, with additional atoms to
components are not shown.
complete the environment of Cu2.
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various ions (other than ammonium) and water molecules
into a polymeric chain in the [101] direction, as shown in
Fig. 11.
These chains are connected by an extensive 3-dimensional
network of hydrogen bonds, in which all but one of the 19
N–H bonds and both water O–H bonds serve as donors and
all O atoms are acceptors.
The only other reported copper pyromellitate complex
containing ammonia is the 2-dimensional polymeric
Cu2IJpm)IJNH3)4Ĵ2H2O, in which copper has a square pyrami-
dal coordination and each pm4− anion coordinates 6 Cu cen-
tres.22 Use of methylamine instead of ammonia generates
instead a 3-dimensional polymer Cu2IJpm)IJNH2Me)4IJOH2)2
(note the very similar formula overall) with elongated octahe-
dral Cu coordination, bonding of each pm4− ligand to 4 Cu
centres, and water now coordinated as a bridging ligand
between pairs of copper ions.
Fig. 12 The asymmetric unit of compound 9, with additional atoms to
complete the environment of Cu2 and pm4−
.
coordinated pyridine molecules, two coordinated and four
uncoordinated water molecules. The coordination of the two
copper ions is the same: square pyramidal with two trans pyr-
idines and two trans carboxylates in the basal plane and a
long apical bond to an aqua ligand. Each pm4− ligand coordi-
nates 4 copper ions (two Cu1 and two Cu2), one attached to
each carboxylate group in a monodentate fashion. The dihe-
dral angles between the carboxylate groups and the benzene
rings are two of 83.2° and two of 6.0° in one ligand, and two
of 86.3° and two of 2.8° in the other, a combination of near-
coplanarity and near-orthogonality not found in the other
complexes in this series except in the H2pm2− ligand of com-
pound 3. Hydrogen bonding is confined to the clusters of 6
water molecules (2 of them being aqua ligands) and 4 sur-
rounding carboxylate groups, described previously, and does
not generate a polymeric hydrogen-bonding network.
(d) Complexes in which unprotonated bases serve as ligands
The two uncharged polymeric species just mentioned are
examples of this class of complex, which is otherwise
unrepresented in the literature except for the pyridine-
containing Cu2IJpm)IJpy)4IJOH2)2Ĵ4H2O.23 We have obtained
the same compound 9 by a gel diffusion experiment, and a
related compound Cu2IJpm)IJpy)4IJMeOH)Ĵ2MeOH (10) using
vapour diffusion of pyridine into a solution in methanol of a
copperIJII) salt and H4pm. The two structures have many simi-
larities, but there are some significant differences.
The structure of 9 has been described, with the main
focus on the presence of hydrogen-bonded clusters of 6 water
molecules.23 Although the publication states that water H
atoms were refined as riding, examination of the deposited
CIF suggests that they were actually refined freely. We
include our own essentially identical result (of similar preci-
sion) here for comparison with the other structures in the
series, using consistent refinement treatment and atom label-
ling schemes (Fig. 12). The asymmetric unit contains two
copper ions in general positions, half each of two centrosym-
metric pm4− ions (the previous report implies there is a com-
plete pm4− ion without crystallographic symmetry), four
The asymmetric unit of compound 10, in a unit cell of
similar volume to that of 9 and the same space group, also
contains half each of two centrosymmetric pm4− ligands and
four coordinated pyridine molecules, but has three copper
ions of which two lie on inversion centres (instead of two in
Fig. 11 A section of the polymeric chain of compound 8 including
secondary coordination bonds.
Fig. 13 The asymmetric unit of compound 10, with additional atoms
to complete the environment of Cu2, Cu3 and pm4−
.
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general positions) and three methanol molecules, one of
which is clearly a ligand and the other two may be described
as semi-coordinating (i.e. half the number of solvent mole-
cules) (Fig. 13). Cu1 has a square pyramidal coordination
geometry, similar to that found in compound 9 but with
methanol replacing the apical aqua ligand: the basal ligands
are two trans pyridines and two trans carboxylates. The next
shortest Cu1⋯O distance is 2.781 Å, which may be consid-
ered as rendering one carboxylate group semi-chelating. Cu2
and Cu3 both have primary square planar coordination with
two trans pyridines and two trans monodentate carboxylates;
methanol molecules lie above and below the square plane in
each case, at 2.536 and 2.521 Å, thus generating elongated
octahedral coordination. As for 9, the Cu and pm4− ions gen-
erate 2-dimensional polymer sheets in which pyridine and
water or methanol serve only as terminal ligands decorating
the sheet. Hydrogen bonding links methanol to carboxylate
groups within the sheets and also connects each sheet to
those above and below by methanol⋯methanol interactions.
Ignoring the Cu1⋯O1 contact, each pm4− ligand has the
same connectivity as in 9, with one copper ion attached to
each carboxylate group. The dihedral angles between carbox-
ylate groups and benzene rings are two of 55.7° and two of
57.1° for one ligand, and two of 61.7° and two of 31.8° for
the other.
Principal comparisons and patterns
In most of the structures described here, the fully
deprotonated pyromellitate anion pm4− coordinates 4 or 6
Cu2+ ions, at least one for each carboxylate group, and addi-
tionally 8 alkali metal cations in two of the structures. Rarely
do carboxylate groups lie anywhere near coplanar with the
benzene ring of pyromellitate ligands, most being twisted out
of the ring plane by 30–70°. The most common coordination
of copper is square pyramidal with a long apical bond,
though there are some examples of primary square planar
coordination, which is supplemented by two weaker Cu⋯O
Fig. 14 Sections of the 2-dimensional Cu/pm4− nets of compounds 1 (a), 2 (b), 4 (c), 9 (d), 10 (e) and Cu2IJpm)IJOH2)2IJMeNH2)4 (f).
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interactions to give a distorted octahedral geometry. Between
1 and 4 coordination sites are taken by carboxylate groups,
the others being occupied by solvent or nitrogen base mole-
cules as ligands. Coordinated solvent molecules are usually
terminal ligands, though in some cases water bridges two
metal ions. Structures range from discrete molecular ions
through chains and sheets to 3-dimensional polymeric net-
works, sheet polymers being the most common. Hydrogen
bonding makes a significant contribution to most of the
structures, usually with all potential donors satisfied and a
surfeit of acceptors.
One particular feature of interest in the 2-dimensional
polymeric structures described here concerns the network
formed by only the Cu2+ and pm4− ions, comparison of
which is facilitated by ignoring all the other components of
the structures (other ions, and both coordinated and un-
coordinated uncharged molecules). A section of each of the
Cu/pyromellitate networks is shown in Fig. 14 (a–e) for com-
pounds 1, 2, 4, 9, and 10, together with Cu2IJpm)IJOH2)2-
IJMeNH2)4 (f),22 which is similar in this respect. It can be seen
that the number of pm4− anions coordinating each copper
ion is 2 (linear, b, d, e, f), 3 (pyramidal, a), or 4 (square pla-
nar, c), and the number of copper ions coordinated by each
pm4− anion is either 4 (b–f) or 6 (a); the ratio of these is 1 : 2
except for structure 4 (c), where it is 1 : 1, in accordance with
the stoichiometric pyromellitate : Cu ratio of each compound.
In each structure, the polymeric network displays a grid of
either one unique (b, d, e, f) or equal numbers of two differ-
ent (a, c) rings made up of alternating copper centres and
segments of pm4− ligands. The unique rings contain 4 Cu
and 4 pm4− ligands, while the others contain 2 of each, which
are smaller. These large rings do not constitute channels or
voids in the structures, as they are occupied and surrounded
by the counterions, coordinated and uncoordinated mole-
cules that have been omitted from the network diagrams.
manually on the basis of optical examination (crystal shape
and colour).
X-ray crystallography
Crystals were examined and diffraction data collected on
three different instruments over a period of several years: a
Stoe-Siemens AED2 four-circle diffractometer (1 and 4), an
Agilent Technologies Gemini A Ultra diffractometer (10), and
a Bruker SMART 1K diffractometer (all others); the radiation
was Mo-Kα in each case (λ = 0.71073 Å). For some com-
pounds more than one data set was measured, using crystals
from different syntheses and sometimes with different in-
struments, and the best result is presented here. Selected
crystal data are shown in Table 2. Standard diffractometer
control software was used, complemented by locally written
programs.24 Absorption corrections were based on symmetry-
equivalent and repeated reflections measured in different
positions and thus subject to different X-ray path
lengths.24c,25 The structures were solved and initially refined
with SHELXTL;25,26 final refinements used SHELXL-2014.27
Hydrogen atoms were included in idealised positions bonded
to C atoms, and refined as riding; most of those bonded to O
and N atoms were located in difference density maps and/or
assigned to give appropriate hydrogen bonds, and were sub-
sequently refined with restraints on N/O–H and H⋯H dis-
tances (providing orientational degrees of freedom) and with
displacement parameters tied to those of their parent atoms.
Synthetic procedures
Compound 1.4 The copper hydroxynitrate Gerhardtite,
Cu2IJNO3)IJOH)3 prepared from potassium carbonate and
copperIJII) nitrate (0.649 g, 2.6 mmol), was added to a hot
aqueous solution of H4pm (0.792 g, 2.7 mmol in 45 ml); car-
bon dioxide was evolved and the solution changed from light
green to turquoise. The resulting suspension was refluxed for
1 h, to give a blue crystalline solid. Calc. for C10H14Cu2O14: C
24.8, H 2.9; found: C 25.1, H 2.8%.
Compound 2. Solutions were prepared of CuIJNO3)2Ĵ4H2O
(0.079 g, 0.33 mmol) in methanol (10 ml) and H4pm (0.043 g,
0.17 mmol) in water (10 ml). A small sample vial containing
0.5 ml of each solution was almost sealed with plastic film
and placed inside a larger closed vial containing pyridine
(0.02 ml, 0.11 mmol). Blue block crystals formed after a
few days. Calc. for C10H22Cu2O18: C 21.6, H 4.0; found: C
21.7, H 4.3%.
Experimental
Materials and methods
All starting materials were commercial products and were
used as supplied; water was distilled, and other solvents were
not dried. Chemical analyses were performed with a Carlo
Erba 1108 Elemental Analyser in the Newcastle University
School of Chemistry Analytical Services Unit. Most syntheses
were carried out several times in parallel or on different occa-
sions, using slow diffusion methods in order to obtain single
crystals suitable for X-ray diffraction analysis; the methods
employed involved either slow diffusion of reactant solutions
from opposite sides through a freshly prepared silica gel, dif-
fusion of one solution into a gel containing a non-volatile
base, or vapour diffusion of a volatile base into a solution of
other reactants, with details given below. In cases where gel
synthesis gave more than one product, these were usually
generated in different parts of the gel and were collected
separately, or crystals of mixed products were separated
Compound 3. Silica gel was prepared from tetra-
methoxysilane (5%) and water and allowed to set for 24
hours. On one side of the gel was placed a solution of H4pm
(0.042 g, 0.17 mmol) and CuCl2Ĵ6H2O (0.045 g, 0.33 mmol) in
methanol (1 ml), and on the other triethylamine (0.67 ml).
Blue crystals formed over a period of two weeks. Calc. for
C10H16CuO14: C 28.3, H 3.8; found: C 28.6, H 3.6%.
Compound 4.11 Aqueous solutions of CuSO4Ĵ5H2O and
K4pm (prepared from KOH and H4pm in 4 : 1 molar ratio)
were allowed to diffuse together slowly through a fine pyrex
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sinter, giving blue crystals. Calc. for C10H10CuK2O12: C 25.9,
H 2.2; found: C 26.0, H 2.5%.
susceptible to a balance of influences of other uncharged
ligands, including solvent, of organic or alkali metal cations
providing overall charge balance, and of hydrogen bonding.
Since markedly different structures can be generated from
the same or similar ingredients under broadly similar condi-
tions, and the same structure is sometimes found for prod-
ucts from quite different synthetic routes, rational design is
unreliable as a guide in benzenepolycarboxylate metal com-
plexes; we have come to the same conclusion based on a
series of results for metal mellitate complexes.7 Nevertheless,
the research area provides much interest and the opportunity
for recognition of patterns and some rationalization of the
fascinating structures it generates.
Compound 5. A 10% silica gel was prepared, containing
sodium carbonate (0.212 g, 2 mmol) and H4pm (0.254 g,
1 mmol) in 5 ml water, and allowed to set for 24 hours.
CuCl2Ĵ6H2O (0.134 g, 1 mmol) in 1 ml ethanol was added on
top of the gel. Blue crystals formed in three weeks. Calc. for
C10H10CuNa2O12: C 27.8, H 2.3; found: C 27.5, H 2.1%.
Compound 6. A solution of CuCl2Ĵ6H2O (0.045 g, 0.33
mmol) and H4pm (0.042 g, 0.17 mmol) in ethanol (3 ml) was
placed in a small sample vial, which was almost sealed with
plastic film and placed inside a larger closed vial containing
triethylamine (0.67 ml). Blue crystals formed in several days.
Calc. for C48H80Cu2N4O18: C 51.1, H 7.2, N 5.0; found: C 50.7,
H 7.7, N 4.9%.
Compound 7. The same method and quantities were used
as for compound 6, but with methanol instead of ethanol as
the solvent. A few blue crystals formed in several days,
embedded in a pale blue pasty precipitate of heterogeneous
appearance; there was insufficient product for reliable chemi-
cal analysis.
Acknowledgements
We thank EPSRC and its predecessor SERC for funding (stu-
dentship for NCM and three equipment grants). Compounds
6 and 7 were first prepared and the structures solved as part
of an undergraduate research project of Colette Harvey at
Newcastle, jointly supervised by Dr Mark Elsegood (now at
Loughborough University). Compounds 1 and 4 were pro-
vided by Drs Hugh MacBride and Stan Hauptman working
with Professor Ken Wade at Durham University, and their
experimental procedures are included here. We also thank
Drs Mark Fox (Durham) and Gavin Walker (now at Notting-
ham University) for additional information, including the
PhD thesis of Gavin Walker.
Compound 8. A solution of CuCl2Ĵ6H2O (0.045 g, 0.33
mmol) and H4pm (0.042 g, 0.17 mmol) in methanol (2.5 ml)
was placed in a small sample vial, which was almost sealed
with plastic film and placed inside a larger closed vial
containing 1M aqueous ammonia (1 ml). Blue-purple crystals
formed in several days. Calc. for C20H46Cu3N12O18: C 25.7, H
5.0, N 18.0; found: C 26.0, H 4.7, N 17.7%.
Compound 9. Silica gel was prepared from tetra-
methoxysilane (5%) and water and allowed to set for 24 hours.
On one side of the gel was placed a solution of H4pm (0.042 g,
0.17 mmol) and CuCl2Ĵ6H2O (0.045 g, 0.33 mmol) in ethanol
(2.5 ml), and on the other pyridine (1 ml). Blue crystals formed
over a period of two weeks. Calc. for C30H34Cu2N4O14: C 44.9,
H 4.3, N 7.0; found: C 44.9, H 4.0, N 6.8%.
Compound 10. Solutions were prepared of CuIJNO3)2Ĵ4H2O
(0.079 g, 0.33 mmol) in methanol (10 ml) and H4pm (0.043 g,
0.17 mmol) in methanol (10 ml). A small sample vial
containing 0.5 ml of each solution was almost sealed with
plastic film and placed inside a larger closed vial containing
pyridine (0.05 ml, 0.22 mmol). Blue block crystals formed
after a few days. Calc. for C33H34Cu2N4O11: C 50.2, H 4.3, N
7.1; found: C 49.9, H 4.1, N 6.8%.
Notes and references
1 (a) W. Clegg, R. Snaith, H. M. M. Shearer, K. Wade and G.
Whitehead, J. Chem. Soc., Dalton Trans., 1983, 1309; (b) D.
Barr, W. Clegg, R. E. Mulvey, R. Snaith and K. Wade,
J. Chem. Soc., Chem. Commun., 1986, 295; (c) D. R.
Armstrong, D. Barr, W. Clegg, R. E. Mulvey, D. Reed, R.
Snaith and K. Wade, J. Chem. Soc., Chem. Commun.,
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R. E. Mulvey, K. Wade and D. Reed, J. Chem. Soc., Dalton
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2 W. Clegg, Crystal Structure Determination, Oxford University
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3 W. Clegg, W. R. Gill, J. A. H. MacBride and K. Wade, Angew.
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4 G. S. Walker, PhD thesis, University of Durham, 1995, http://
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5 (a) J. Kim, B. Chen, T. M. Reineke, H. Li, M. Eddaoudi, D. B.
Moler, M. O'Keefe and O. M. Yaghi, J. Am. Chem. Soc.,
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Conclusions
The pyromellitate anion displays considerable versatility in
its formation of complexes with copperIJII), as a result of the
availability of four carboxylate groups, each capable of coordi-
nating one or more metal ions in a variety of modes, and the
flexibility of orientation of the carboxylate groups relative to
the benzene ring and to each other, subject to steric restric-
tions. While copperIJII) shows a preference for square pyrami-
dal coordination in these complexes, it can also adopt a
square planar geometry, usually with additional secondary
coordination on each side of this plane. Structural details are
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