Croconic acid and alkali metal croconate salts: Some new insights into an old story

Article abstract of DOI:10.1002/1521-3765(20020415)8:8<1804::AID-CHEM1804>3.0.CO;2-C

The solid-state structures of a series of alkali metal salts of the croconate dianion (C₅O₅^2-) and of croconic acid (H₂C₅O₅) have been determined. The alkali metal croconates were obtained

Full text of DOI:10.1002/1521-3765(20020415)8:8<1804::AID-CHEM1804>3.0.CO;2-C

FULL PAPER
Croconic Acid and Alkali Metal Croconate Salts:
Some New Insights into an Old Story
Dario Braga,*[a] Lucia Maini,[a] and Fabrizia Grepioni*[b]
Abstract: The solid-state structures of a
series of alkali metal salts of the cro-
conate dianion (C5O52À) and of croconic
acid (H2C5O5) have been determined.
The alkali metal croconates were ob-
tained by ring contraction of rhodizonic
acid (H2C6O6), upon treatment with
alkali metal hydroxides and recrystalli-
sation from water. The novel species
Na2C5O5 ¥ 2H2O, Rb2C5O5 and Cs2C5O5,
as well as the mixed hydrogencroconate/
croconate salt K3(HC5O5)(C5O5) ¥ 2H2O
are described and compared with the
interplanar separations lie in the narrow
range 3.12 3.42 ä and do not necessa-
rily reflect the presence of p-stacking
interactions. It is argued that the small
interplanar separation is the result of a
compromise between packing of flat
croconate units and the spherical cations
together with the water molecules that
fill the coordination spheres of the alkali
metal atoms.
‡
‡
‡
Li , K and NH4 salts. Single crystals of
croconic acid were obtained by crystal-
lisation of croconic acid in the presence
of HCl. Crystal structure determinations
2À
showed that the C5O5 ions tend to
organize themselves in columns. The
Keywords:
alkali
metals
¥
croconates ¥ crystal engineering ¥
oxocarbons ¥ stacking interactions
supramolecular aggregation of the building blocks rather than
from their nature.
Introduction
Crystal engineering, the bottom-up construction of crystalline
solids with desired arrangements of the component molecules
and ions,[1] has fuelled new interest in some old issues of
structural chemistry. Like supramolecular chemistry, defined
as chemistry beyond the molecule,[2] crystal engineering is
concerned with the assembly of molecules[3] and ions in
aggregates of higher complexity, with collective properties
that depend upon the plethora of intermolecular interactions
that are responsible for crystal cohesion and stability.[4] The
complex relationship between molecular size and shape (and
ionic charge in the case of ions) and the type and number of
intermolecular interactions often favours the utilisation of
(relatively) simple ions and molecules as building blocks.
Simplicity will (hopefully) provide better insight into the
factors that are truly important for crystal construction.
However provocative it may sound, chemical novelty is not a
necessary prerequisite for a crystal-engineering building block
to be useful. Chemical and physical novelty arises from the
Indeed most of the successful crystal-engineering experi-
ments have been conducted on simple systems, like guanidi-
nium and sulfonate ions,[5] organometallic acids,[6] halometal-
late systems,[7] copper halides,[8] nanoporous systems[9, 10] and
coordination networks.[11, 12] We have followed a similar type
of approach in one of our lines of crystal-engineering
research.[13] We used polyprotic organic and inorganic acids
to exploit the robustness and reproducibility of hydrogen-
bonding interactions. A family of such acids that has proved to
be particularly well suited are the oxocarbon acids, which
include rhodizonic (H2C6O6), croconic (H2C5O5), squaric
(H2C4O4) and deltic (H2C3O3) acids.[14] We have used the
monodeprotonation products of squaric acid to construct
interdigitated coordination complexes and hybrid organic
organometallic materials for magnetic studies.[15] We were
attracted by the structural characteristics of these simple and
elegant chemical systems.
Here we report the synthesis and structural characteriza-
tion of a series of salts of the croconate dianion (C5O52À) and
the structure of croconic acid (H2C5O5). While the solid-state
structures of deltic[16] and squaric acids[17] were established
long ago, that of croconic acid was only recently communi-
cated.[18a] The croconate dianion belongs, together with the
[a] Prof. D. Braga, Dr. L. Maini
Dipartimento di Chimica G. Ciamician
¡
Universita di Bologna
Via Selmi 2, 40126 Bologna (Italy)
Fax: (39)051-209-9456
E-mail: dbraga@ciam.unibo.it
2À
rhodizonate C6O62À, the squarate C4O4 and the deltate
[b] Prof. F. Grepioni
2À
C3O3 ions, to the family of oxocarbon dianions. The
Dipartimento di Chimica, Via Vienna 2
prototype of these dianions is the rhodizonate dianion
C6O6 ,
2À [18b] which has attracted the interest of many research-
¡
Universita di Sassari, 07100 Sassari, Italy
E-mail: grepioni@ssmain.uniss.it
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Chem. Eur. J. 2002, 8, No. 8

1804 1812
ers in view of the structural analogy with benzene. Rhodiz-
onate salts have found many applications, for example as
markers for lead, in the analysis of radium in fresh waters, and
also for their luminescence properties.[19] Yellow croconic acid
was discovered, together with orange potassium croconate
dihydrate (see below), by L. Gmelin in 1825.[20] Gmelin
pressed by the strong surrounding coulombic field. This study
provides further examples of charge-compressed stacking of
anions.
Results and Discussion
Â
himself derived the name ™croconic∫ from the Greek to
Â
c1ocon for saffron or egg yolk, because of the yellow and
Table 1 summarises the compounds that are described in this
paper and provides references to those reported by others.[23]
Relevant intra- and interionic structural parameters are
compared. While the structure of K2C5O5 ¥ 2H2O was reported
recently,[23b] the structures of the sodium, rubidium and cesium
salts are novel.
orange colours of croconic acid and of many of its compounds.
Gmelin wrote: ™Were it confirmed that it is a hydroxyacid,
then it should be given the name hydrocroconic acid and its
radical should be called crocon∫.[20]
We have utilised oxocarbon anions for evaluating some
fundamental aspects of hydrogen-bonding interactions be-
tween ions.[21] Structural investigation on rubidium and
cesium hydrogencroconates afforded some insight into the
relationship between anion anion hydrogen-bonding inter-
actions and the presence of a coulombic field generated by the
ions. It has been argued that interactions between ions are
often the result of a compromise between the need to achieve
maximum packing density and that of preserving weaker
intermolecular (or interionic) interactions, which, however
feeble with respect to the strength of the coulombic field
generated by small ions, are highly directional and contribute
to packing cohesion. This reasoning applies not only to
hydrogen bonding between ions but also to p stacking, as
shown previously for short interplanar separations observed
in crystals of squarate and hydrogensquarate salts.[22] In these
cases the weak noncovalent interactions are charge-com-
Croconic acid: Yellow transparent single crystals of croconic
acid were obtained by crystallisation of croconic acid from an
aqueous solution of HCl (1m; see Experimental Section). In
fact the first characterisation of the croconic acid molecule
was not performed on the pure molecular crystal, but on a
serendipitous product obtained in the course of the reaction
of the organometallic hydroxide [(h5-C5H5)2Co]OH with
rhodizonic acid, followed by acidification of the solution with
HCl,[18] which yielded the co-crystal [(h5-C5H5)2Co]Cl ¥
H2C5O5. Rhodizonic acid is known to undergo ring contrac-
tion upon treatment with bases, and this route yielded all the
croconate compounds described here.
The structure of croconic acid is shown in Figure 1 (top).
Importantly, all hydrogen atom positions could be obtained
from the diffraction data, and this allowed unambiguous
Table 1. Relevant intra- and intermolecular parameters [ä] in the hydrogencroconate and croconate salts.
‡
À
À
Salt
Interplanar Distance between Shift between C O
C C
Shortest O ¥¥¥ M
interactions [<rvdW(O) ‡ rvdW(M)]
Ocroconate ¥¥¥ Owater
distance
ring centroids
the centroids
Li2C5O5 ¥ 2H2O[23a]
3.30
3.46
1.04
1.268(3) 1.451(2) 1.917(3)
1.245(2) 1.469(2) 1.984(3)
1.235(2) 1.477(2)
1.990(4)[a]
1.994(3)[a]
2.747(2)
2.691(2)
Na2C5O5 ¥ 2H2O
3.12
3.30
4.36
3.43
3.0
1.248(2) 1.461(2) 2.318(1)
1.249(2) 1.470(2) 2.468(1)
1.242(2) 1.471(3) 2.507(1)
2.586(1)
2.827(2)
2.774(2)
2.320(1)[a]
2.400(1)[a]
K2C5O5 ¥ 2H2O[23b]
1.10
1.248(3) 1.474(2) 2.784(1)
1.241(2) 1.474(2) 2.870(1)
1.252(2) 1.465(3) 2.789(1)
2.864(1)
2.870(1)
2.783(2)
2.877(2)
3.006(1)
2.649(1)[a]
3.064(2)[a]
K3(HC5O5)(C5O5) ¥ 2H2O 3.19
4.23
2.78
1.234(4) 1.477(5) 2.661(3)
1.233(4) 1.450(5) 2.719(3)
1.289(4) 1.435(5) 2.738(3)
1.243(4) 1.476(5) 2.767(4)
1.231(4) 1.473(5) 2.820(3)
2.847(4)
2.910(3)
3.118(4)
3.352(5)
2.726(4)[a]
2.809(4)[a]
2.444(5)[b]
2.972(6)
2.786(6)
Rb2C5O5
Cs2C5O5
3.30
3.42
3.82
4.02
1.92
2.11
1.24(1)
1.458(7) 2.838(5)
2.892(5)
3.135(5)
2.958(6)
1.244(8) 1.463(9) 3.021(3)
1.240(8) 1.47(1)
1.231(9) 1.46(1)
1.226(8) 1.448(7) 3.126(4)
1.261(9) 1.46(2)
2.851(5)
3.106(3)
3.029(5)
3.077(7)
3.337(2)
3.117(7)
3.048(6)
3.235(6)
2.82(1)[c]
2.83(1)[c]
2.83(1)[c]
2.91(1)[c]
[23c]
(NH4)2C5O5
3.30
3.47
1.07
1.24(1)
1.27(1)
1.29(1)
1.46(1)
1.45(1)
1.45(2)
‡
[a] Owater ¥¥¥ M . [b] Ocroconate ¥¥¥ Ohydrogencroconate. [c] O ¥¥¥ N distance.
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D. Braga, F. Grepioni, and L. Maini
FULL PAPER
Figure 2. The flat tetramolecular rings formed by squaric acid can be
compared with the rings formed by croconic acid (see Figure 1, middle).
in the structure of croconic acid with respect to the more
relaxed molecular distribution in crystalline squaric acid or
some other electronic effect[24] is difficult to say on the basis of
the structural comparison alone.
We can now move to the alkali and ammonium salts
compared in Table 1. Some common structural features are
noteworthy:
1) The lithium, sodium and potassium croconates are all
known as dihydrate salts, whereas the ammonium, rubid-
ium and caesium salts crystallise in the anhydrous form.
The water molecules act as ™coordination fillers∫ in
crystallisation with the lighter alkali cations.
2) All the crystals show some degree of dianion stacking. The
distance between planes varies in a rather narrow range
(3.12 3.42 ä), but these data should not be taken as
indicative of significant overlap between the rings. It has
been pointed out by us[22] and others[25, 26] that examination
of the interplanar separation alone can be extremely
misleading when discussing small ions or molecules in
crystals. In the case of flat, discoidal fragments such as the
croconate ions, a complete topological analysis of the
relationship between the planes would require knowledge
of not less than four parameters, namely, the distance
between ring centroids, the shift between ring centroids,
the distance between the ring centroid and the underlying
plane, and the angle between the normals to the ring
planes. Because in all the examined cases this last quantity
is invariably zero or close to zero, only the first three
parameters have been calculated and reported in Ta-
ble 1.[26] On the basis of this information it is clear that the
Figure 1. Top: The structure of croconic acid. Middle: Aview of the crystal
down the a axis, showing the large tetrameric rings formed by the croconic
acid molecules. Bottom: The same molecular ensemble viewed down the c
axis, showing the accordion-type superstructure formed by the tetramo-
lecular rings.
asignnment of the hydrogen-bonding motifs. In the crystal
each molecule of croconic acid is linked to four other
molecules by two (crystallographically) independent types
of hydrogen bonds (O ¥¥¥ O 2.628(5), 2.617(5) ä) to give
sheets of large tetrameric rings (Figure 1, middle). The sheets,
however, are not flat but pleated in an ™accordion∫ pattern, as
can be seen in the side-on view in Figure 1 (bottom). The
angle between the croconic acid planes is 688, and the hinges
‡
À
are formed by one row of O H ¥¥¥ O hydrogen bonds. Squaric
acid H2C4O4 also forms large tetramolecular rings in the
short interplanar separation in the Na salt (3.12 ä) does
[17]
not correspond to ™super-tight∫ p stacking, because the
shift distance between the rings is large (3.0 ä) and
indicates that even if the planes containing the molecules
are very close, the two molecular units are not. In contrast,
solid state (Figure 2), but in this case the sheets are flat. The
hydrogen bonds in squaric acid (O(H) ¥¥¥ O 2.532(4),
2.544(4) ä) are slightly shorter than in croconic acid
(O(H) ¥¥¥ O 2.628(5), 2.617(5) ä). Whether this reflects strain
‡
‡
‡
the Li , K and NH4 salts show the best combination of
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Croconates
1804 1812
and shifts (1.92, 2.11 ä) correlate with the increase in the
Shift
O
2-
‡
‡
cation size on from Rb to Cs . This is also reflected by the
‡
O
increase in the O ¥¥¥ M distances (see Table 1).
x
O
O
O
Lithium croconate: The structure of lithium croconate was
determined by GonÁalves et al. in 1996[23a] Since we are
interested in comparing the arrangements of the croconate
dianions in this family of salts, we summarise here the main
structural features. Coordinates were retrieved from the CSD.
Distance between
ring centroids
Interplanar distance
2-
O
O
O
x
O
‡
O
Figure 3 (top) shows how the Li ion is linked to two
croconate dianions and two water molecules, and thus
achieves a tetrahedral coordination geometry. The water
molecules form hydrogen bonds to the oxygen atoms of the
dianions. The Ocroconate ¥¥¥ Owater separations are 2.691(2) and
2.747(2) ä. The water molecules knit together the dianions to
give the sheet shown in Figure 3 (middle). The dianion layer
interplanar separation (3.30, 3.24 and 3.30 ä) and ™shift∫
parameters (1.04, 1.10 and 1.07 ä; see also below).
3) The Rb and Cs salts, which are anhydrous, show ring
stacking. Both interplanar separations (3.30 and 3.42 ä)
‡
‡
‡
in the Li salt is related to that in croconic acid (see Figure 1,
Figure 3. Top: The tetrahedral disposition of two water molecules and two
croconate dianions around the Li ion in Li2C5O5 ¥ 2H2O. Middle: The
Figure 4. Top: Octahedral arrangement of four croconate dianions and
two water molecules around the Na ions in crystalline Na2C5O5 ¥ 2H2O.
‡
‡
water molecules in Li2C5O5 ¥ 2H2O bridge the oxygen atoms of the dianions
by hydrogen bonding. Bottom: The relative arrangement of the croconate
anions along the stacking.
Middle: Two water molecules join the croconate anions to form a sheet
similar to that observed in crystalline Li2C5O5 ¥ 2H2O (cf. Figure 3 middle).
Bottom: The superimposition of croconate dianions.
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D. Braga, F. Grepioni, and L. Maini
FULL PAPER
middle), whereby the water molecules expand the network,
and the Li ions link together the dianions, which can thus
adopt a flat arrangement. The croconate layers in the Li salt
are superimposed (see Figure 3, bottom) such that the
croconate dianions stack on each other at an interplanar
distance of 3.30 ä.
3.232(2) ä, considerably shorter than sum of the van der
Waals radii (3.40 ä). The close, and supposedly repulsive,
contact is a consequence of the charge compression effect
(see below).
‡
[23b]
Potassium croconate: As recalled by Dunitzet al.,
potas-
sium croconate was first isolated by Gmelin more than 175
years ago and reported together with croconic acid. In the
course of this study we also prepared K2C5O5 ¥ 2H2O. We do
not report our data here, but rather use those obtained by
Dunitzet al. The coordination around the potassium cation is
Sodium croconate: The coordination number of the sodium
cations in the salt Na2C5O5 ¥ 2H2O is six, and the oxygen atoms
of four croconate dianions and two water molecules occupy
‡
the vertices of an octahedron (Na ¥¥¥ O 2.318(1) 2.586(1) ä;
‡
Figure 4, top). Despite the different geometry around the
cation, the solid-state structure is closely related to that of
Li2C5O5 ¥ 2H2O: two water molecules join the croconate
anions in large tetrameric units (Figure 4, middle). The
relative orientation of the dianions is different from that in
Li2C5O5 ¥ 2H2O, but the supramolecular organisation still has
quite irregular (see Figure 5, top left), with six K ¥¥¥ Ocroconate
‡
and two K ¥¥¥ Owater contacts in the range 2.649(1)
‡
‡
3.064(1) ä (see Table 1). Contrary to the Li and Na salts,
the water molecules do not link the dianions in layers. The
larger cations can presumably no longer be accommodated
between contiguous anions within the same plane, and the
structure ™evolves∫ into the arrangement shown in Figure 5
(top, right). The croconate dianions form columns that extend
parallel to the c axis and are completely surrounded by cations
and water molecules, with these latter acting as ™pincers∫
towards the dianions (see Figure 5, bottom left). The short
interplanar separation (3.30 ä) is thus the result of external
‡
niches in which the Na ions are encapsulated. Figure 4
(bottom) shows how the croconate dianions are superimposed
‡
in crystalline Na2C5O5 ¥ 2H2O. In contrast to the Li salt, an
oxygen atom of one dianion is located almost over the centre
of the underlying croconate dianion. The shortest C ¥¥¥ C
separation between the two almost overlapping C atoms is
‡
Figure 5. Top left: The coordination of two water molecules and four croconate dianions around the K ion in crystalline K2C5O5 ¥ 2H2O. Top right: Aview of
the crystalline salt in the bc plane, showing how the dianions are stacked in columns surrounded by the cations and water molecules. Ocroconate ¥¥¥ Owater
interactions omitted for clarity. Bottom right: A projection in the ring plane along the column, showing how the dianions are staggered, and the centroids only
slightly shifted. Bottom left: A front view of the stacking of the anion, showing the interactions with the water molecules.
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Croconates
1804 1812
interactions that ™compress∫ the anions together. Figure 5
(bottom right) shows how two consecutive croconate dianions
along a column are staggered and slightly shifted (1.1 ä).
Ammonium croconate: In spite of the difference in chemical
composition and in the type of noncovalent interactions that
can be exploited, the ionic arrangement in ammonium
croconate[23c] is reminiscent of that discussed above for
potassium croconate. The structure of ammonium croconate
was determined by Baenziger et al. in 1964, and we summarise
here the main structural features. Figure 6 (top) shows how
the ammonium cations take the place of the potassium cations
between croconate layers and act as bridges between the
‡
À
À
anions by forming charge-assisted N H ¥¥¥ O hydrogen
bonds[27] (four interactions with N ¥¥¥ O separations in the
range 2.82(1) 2.91(1) ä). The anion stacking (Figure 6,
‡
bottom) is comparable to that in the K salt, both in terms
of interplanar distance and shift (3.30 and 1.07 versus 3.30 and
‡
‡
1.10 ä for the NH4 and K salts, respectively).
Rubidium and caesium croconates: In spite of the difference
‡
in cation size, reflected in the different O ¥¥¥ M separations
‡
‡
(see Table 1), the Rb and Cs croconates are isomorphous
and crystallise as anhydrous salts. The coordination around
the cations is complex (see Figure 7, top) with at least seven
‡
close O ¥¥¥ M interactions in the ranges 2.838(5) 3.135(5)
‡
‡
and 3.029(5) 3.337(2) ä for the Rb and Cs salts, respec-
tively. The middle stucture in Figure 7 depicts a projection in
the crystallographic ab plane; the stacks of flat croconate
dianions are surrounded by piles of large cations. The
interplanar separations in Rb2C5O5 (3.30 ä) and in Cs2C5O5
(3.42 ä) are comparable to that in K2C5O5 ¥ 2H2O (3.30 ä),
but the shifts between the ring centroids are larger (1.92 and
‡
‡
2.11 ä in the Rb and Cs salts, respectively, versus 1.10 ä in
‡
Figure 7. Top: The coordination around the Cs ion in crystalline Cs2C5O5
‡
(the Rb salt is isomorphous). Middle: A projection along the crystallo-
graphic c axis of the unit cell, showing how the anionic columns are
‡
surrounded by the Cs ions along the columns. Bottom: The relationship
‡
between two consecutive rings along the anionic stacking in the Cs salt.
‡
the K salt), so that the extent of overlap is smaller, as can be
seen in Figure 7 (bottom).
Potassium hydrogencroconate croconate dihydrate: In an
attempt to prepare potassium croconate we also obtained the
™mixed∫ salt K3(HC5O5)(C5O5) ¥ 2H2O, which formally con-
À
tains both the hydrogencroconate monoanion (HC5O5 ) and
the croconate dianion (C5O52À). To the best of our knowledge,
this the first example of such a derivative of croconic acid;
thus far only hydrogencroconate or croconate salts have been
isolated and structurally characterised. The interest in this
compound stems from the presence of a hydrogen-bonding
interaction that joins the two anions. Figure 8 (top) shows that
2À
the crystal contains discrete HC5O5À ¥¥¥ C5O5 units, which
would be more appropriately described as [C5O5 ¥¥¥ H ¥¥¥
C5O5]3À dimeric units. These could be regarded as super-
anions, and indeed the distinction between hydrogencrocon-
ate and croconate ions is only semantic: the hydrogen atom of
Figure 6. Top: The ammonium cations bridge the croconate anions
‡
À
À
through charge-assisted N H ¥¥¥ O hydrogen bonds. Bottom: The rela-
tionship between two consecutive rings along the anionic stacking.
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D. Braga, F. Grepioni, and L. Maini
FULL PAPER
(1.45 1.48 ä). These values are clearly intermediate between
that of aromatic systems and the average[28] value expected for
[28]
À
ˆ
À
ˆ
À
a cyclic C( O) C( O) system (1.537(2) ä from a CSD
search of diketonic cyclic systems). In croconic acid (Figure 1,
À
ˆ
À
ˆ
À
À
top), the two C( O) C( O) distances are C3 C4 1.508(6)
À
and C4 C5 1.518(6) ä, which are considerably longer than in
the croconate dianions and only slightly shorter than the
À
average value of 1.537(2) ä. However, the C1 C2 distance of
1.382(5) ä is appreciably shorter and in agreement with the
presence of a double bond. This comparison suggests that
there is some degree of aromaticity in the C5 ring. The average
À
C C bond lengths (1.460(1), 1.467(5), 1.472(4), 1.462(5),
1.455(7), 1.454(5) ä, for M ˆ Li, Na, K, Rb and Cs, respec-
tively) are, in fact, in reasonable agreement with that
calculated for the isolated oxocarbon dianion (1.488 ä).[29]
[23b]
In agreement with the observation by Dunitzet al.
the
geometry of all croconate dianions discussed herein deviate
only slightly from fivefold symmetry.
Conclusion
We have investigated the ion organisation in the family of
crystalline salts M2C5O5 (M ˆ Li, Na, K, Rb, Cs), (NH4)2C5O5
and
the
mixed
hydrogencroconate/croconate
salt
K3(HC5O5)(C5O5) ¥ 2H2O. As pointed out in the introduction,
2À
the croconate dianion C5O5 and the corresponding acid
H2C5O5, were first reported by Gmelin in 1825. His findings
were contemporary with the isolation by Faraday of benzene,
the prototype of aromatic molecules. Beside discussing the
À
geometry of the croconate dianions in terms of average C C
Figure 8. Top: The chains of water-bridged [C5O5 ¥¥¥ H ¥¥¥ C5O5]3À units.
distances, we sought the reason for the presence of short
‡
The K ions are located between the chains. Bottom: The [C5O5 ¥¥¥ H ¥¥¥
C5O52À ¥¥¥ C5O5 inter-ring separations in the crystals. The
2À
C5O5]3À units are superimposed along the stacking, parallel to the b axis.
Bottom: A side view showing the alternation of stacked anion/water chains
and cationic layers.
inter-ring distances are, in fact, shorter than in graphite and in
many other systems in which p p stacking is invoked.[30] It
has been pointed out previously, and supported by theoretical
calculations,[22] that, in many cases, short interplanar separa-
tions between ring systems (often called p-stacking interac-
tions) are the result of a complex compromise between the
need to occupy space efficiently and the need to optimise the
ensemble of intermolecular interactions between building
blocks.[31] When the packing problem is that of accommodat-
ing spherical objects (the alkali metal cations) and flat
discoidal units (the croconate dianions, but also the squarates,
the hydrogencroconates, etc.), the best compromise is often
attained by placing the disks ™flat-on-flat∫ in a stack and by
placing the spherical cations around the stack. If this is not
sufficient to complete the coordination sphere of the smaller
alkali metal cations, water molecules are incorporated in the
crystal to take the place of the large dianions as coordination
(and space) fillers. The fact that the discoidal dianions do not
show a preferential relative orientation and can slide over
each other indicates that the electronic gain from this type of
weak noncovalent interaction is either zero or too small to be
of any relevance in the presence of the much stronger
electrostatic interactions.
the bridge resides on a centre of inversion and is therefore
exactly midway between the two oxygen atoms, the O ¥¥¥ O
separation being as short as 2.444(5) ä. The [C5O5 ¥¥¥ H ¥¥¥
C5O5]3À units are bridged by two hydrogen-bonded water
molecules (Oanion ¥¥¥ Owater 2.786(6) and 2.972(6) ä; see Fig-
ure 8, top). The resulting chains are organized in stacks along
the b axis: The middle structure in Figure 8 depicts how the
dimeric units are superimposed along the stacking. The
packing in cristalline K3(HC5O5)(C5O5) ¥ 2H2O can thus be
described as consisting of layers of anion/water chains that are
intercalated with layers of cations, as shown in Figure 8
(bottom). The interanion O ¥¥¥ O separation within the dimer
is comparable to the values observed in other classes of
hydrogen-bridged anions, such as hydrogenoxalates and
22 ]
hydrogensquarates[21
(range 2.417 2.503 ä), as well as to
the O ¥¥¥ O separations in chains of hydrogencroconates in
RbHC5O5 (2.443(8) ä). The interplanar separation of 3.19 ä
is one of the shortest in this study, but, as evidenced by the
shift value of 2.78 ä, it does not correspond to a p-stacking
interaction.
The relatively large sample of croconate compounds now
Hence, it appears that the short interatomic/interplanar
separations in the croconate salts are the result of the
™electrostatic compression∫ arising from the attractive
available allows an evaluation of the bonding parameters. The
C C distances within the C5O52À ion lie within a narrow range
À
1810
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Chem. Eur. J. 2002, 8, No. 8

Croconates
1804 1812
‡
2À
‡
À
À
reported in Table 2. Both diffractometers were equipped with a graphite
monochromator (MoKa radiation, l ˆ 0.71073 ä). SHELXS97[35a] and
SHELXL97[35a] were used for structure solution and refinement based on
F 2. SCHAKAL99[35b] was used for the graphical depiction of the results.
CCDC 167861 167864 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge via www.ccdc.
cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (‡44)1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk).
M ¥¥¥ C5O5 interactions (or N H ¥¥¥ O interactions when
‡
the cation is NH4 ) that largely overcompensate for the
repulsive M ¥¥¥ M and C5O52À ¥¥¥ C5O52À interactions. Similar
reasoning has been applied to arene arene interactions in
crystals of arene complexes containing large transition metal
clusters, in which the compromise is between packing of the
arene fragments and packing of the tubular CO ligands,[32] and
‡
‡
‡
in crystals of salts containing the large PPh4 ions, which tend
to establish so-called phenyl embraces.[33] These classes of
different chemical systems all show preferential stacking or
herringbone arrangements of ring systems, even though the
dominant interactions are different. The relative role of
attraction and repulsion in molecular crystals of fused
aromatic hydrocarbons has also been discussed.[34] All these
investigations point to the fact that a packing analysis or a
study of molecular/ionic recognition focused only on pairwise
interactions can be misleading, as one may forget that it is the
overall balance of intermolecular interactions, some acting at
short range only, and others very long range, that accounts for
molecular organisation and ultimately for crystal cohesion.
Acknowledgements
We thank M.U.R.S.T. (projects Supramolecular Devices 1999 2000 and
Solid Supermolecules 2000 2001) and the Universities of Bologna (project
Innovative Materials) and Sassari for financial support.
[1] a) G. R. Desiraju, Angew. Chem. 1995, 107, 2541; Angew. Chem. Int.
Ed. Engl. 1995, 34, 2311; b) D. Braga, F. Grepioni, Acc. Chem. Res.
2000, 33, 601; c) Crystal Engineering: from Molecules and Crystals to
Materials (Eds.: D. Braga, F. Grepioni, A. G. Orpen), Kluwer
Academic, Dordrecht, 1999.
[2] J. M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,
VCH, Weinheim, 1995.
Experimental Section
[3] a) D. Braga, F. Grepioni, G. R. Desiraju, Chem. Rev. 1998, 98, 1375;
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[4] In 1 mm3 of ice there are approximately 1019 1020 hydrogen bonds
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[6] See, for example a) D. Braga, F. Grepioni, J. Chem. Soc. Dalton Trans.
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Pigge, Dalton 2000, 3855.
Syntheses: Croconic acid (Aldrich) was dissolved in HCl (1m). Yellow
crystals suitable for X-ray diffraction were obtained by slow evaporation of
the solution. All croconate salts described here prepared by ring
contraction of rhodizonic acid (Aldrich). In a typical procedure, rhodizonic
acid (0.4 mg) was dissolved in an aqueous solution of the metal hydroxide
(5 mL, 0.1m); crystals were obtained by slow evaporation of the solution.
K3(HC5O5)(C5O5) ¥ 2H2O was obtained by dropwise addition of HCl (3m)
to a yellow solution of potassium croconate; the solution was then allowed
to slowly evaporate.
Crystal structure determination: X-ray diffraction data of croconic acid and
Na2C5O5 ¥ 2H2O were collected on a Bruker AXS SMART diffractometer.
X-ray diffraction data for the potassium, rubidium and caesium salts were
collected on a Nonius CAD-4 diffractometer equipped with an Oxford
Cryostream liquid-N2 device. Crystal data and details of measurements are
[7] a) A. L. Gillon, A. G. Orpen, J. Starbuck, X.-M. Wang, Y Rodriguez-
Martin, C. Ruiz-Perez, Chem. Commun. 1999, 2287; b) L. Brammer,
E. A. Bruton, P. Sherwood, New J. Chem. 1999, 23, 965.
[8] C. B. Aakerˆy, A. M. Beatty, D. S. Leinen, K. R. Lorimer, Chem.
Commun. 2000, 935.
Table 2. Crystal data and details of measurements for H2C5O5, Na2C5O5 ¥ 2H2O, Rb2C5O5, Cs2C5O5 and K3(HC5O5)(C5O5) ¥ 2H2O.
H2C5O5
Na2C5O5 ¥ 2H2O
Rb2C5O5
Cs2C5O5
K3(HC5O5)(C5O5) ¥ 2H2O
formula
Mr
T [K]
system
space group
a [ä]
b [ä]
c [ä]
a [8]
b [8]
C2H2O5
142.07
273(2)
orthorhombic
Pca21
8.7108(8)
5.1683(5)
10.9562(9)
90
C5H4Na2O7
222.06
293(2)
monoclinic
C2/c
7.7442(8)
17.363(2)
6.4024(7)
90
111.799(3)
90
799.32(15)
4
C5O5Rb2
310.99
293(2)
monoclinic
C2/c
13.284(5)
7.826(2)
7.633(5)
90
114.22(5)
90
723.7(6)
4
C5O5Cs2
405.87
293(2)
monoclinic
C2/c
13.679(6)
8.0350(10)
8.023(2)
90
113.51(2)
90
808.6(4)
4
C10H5K3O12
434.44
223(2)
triclinic
≈
P1
4.234(6)
8.005(4)
11.260(7)
74.10(4)
83.94(8)
87.69(7)
365.0(6)
1
90
90
g [8]
V [ä3]
Z
493.25(8)
4
F(000)
q range [8]
m(MoKa) [mmÀ1
measured reflns
unique reflns
refined parameters
GOF on F 2
R1 [on F, I > 2s(I)]
wR2 (F 2, all data)
288
448
576
720
218
3 25
1.002
1342
1270
4
34
2
34
3
27
3
30
]
0.179
6525
1838
91
0.864
0.0549
0.1240
0.260
5721
1513
74
0.812
0.0406
0.0948
2.854
843
782
8.993
1339
1180
56
0.930
0.0314
0.0783
57
115
1.012
0.0407
0.1096
1.054
0.0464
0.1368
Chem. Eur. J. 2002, 8, No. 8
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0808-1811 $ 20.00+.50/0
1811

D. Braga, F. Grepioni, and L. Maini
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Â
Â
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nennen und ihr Radical: Krokon.∫
Received: July 30, 2001
Revised: December 27, 2001 [F3455]
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