Reversible solid-state interconversion of rhodizonic acid H2C6O6 into H6C6O8 and the solid-state structure of the rhodizonate dianion C6O62- (aromatic or non-aromatic?)

Article abstract of DOI:10.1039/b107317f

Anhydrous rhodizonic acid H₂C₆O₆ is obtained by (fully reversible) thermal dehydration of solid 2,3,5,5,6,6-hexahydroxycyclohex-2-ene-1,4-dione, H₆C₆O₈, commonly known as rhodizonic acid dihydrate. Treatment of H₆C₆O₈ with RbOH yields crystals of Rb₂C₆O₆; the oxocarbon dianion C₆O₆^2- is shown to possess a flat, benzene-type structure, with C-C bonds shorter than expected for a (non-aromatic) ketonic-type structure.

Full text of DOI:10.1039/b107317f

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Reversible solid-state interconversion of rhodizonic acid H C O into
2 6 6
H C O and the solid-state structure of the rhodizonate dianion
6 6 8
6 6
C O 2— (aromatic or non-aromatic?)
Dario Braga,*a Gianna Cojazzi,b Lucia Mainia and Fabrizia Grepioni*c
L e t t e r
a Dipartimento di Chimica G. Ciamician, Universita di Bologna, V ia F. Selmi 2, 40126 Bologna,
Italy. E-mail: dbraga=ciam.unibo.it
b Centro CNR per la Fisica delle Macromolecole c/o Dipartimento di Chimica G. Ciamician,
Universita di Bologna, V ia F. Selmi 2, 40126 Bologna, Italy
c Dipartimento di Chimica, Universita di Sassari, V ia V ienna 2, 07100 Sassari, Italy.
E-mail: grepioni=ssmain.uniss.it
Received (in Cambridge, UK) 3rd July 2001, Accepted 10th August 2001
First published as an Advance Article on the web 4th September 2001
Anhydrous rhodizonic acid H C O is obtained by (fully
The single crystal X-ray structure of 1 has been determined
(see Fig. 1).9 The crystal is constituted of layers of molecules
linked by both intra-planar and inter-planar hydrogen bonds
between the six ÈOH groups [six OÉ É ÉO distances in the range
2.72È3.01(1) A]. It is interesting to note that the shortest
OÉ É ÉO interactions formed by the two ““axialÏÏ ÈOH groups in
each molecule are intra-layer interactions. Although not yet
proved, a reasonable ““OccamÏs razorÏÏ hypothesis is that the
dehydration process involves precisely these two ÈOH groups
occupying the intra-layer space. After proton abstraction from
adjacent ÈOH groups, the water molecules, once formed,
would be able to leave the crystal structure easily by di†using
between the layers.
2 6
6
reversible) thermal dehydration of solid 2,3,5,5,6,6-
hexahydroxycyclohex-2-ene-1,4-dione, H C O , commonly
6 6
8
known as rhodizonic acid dihydrate. Treatment of H C O
6 6
with RbOH yields crystals of Rb C O ; the oxocarbon dianion
2 6
8
6
C O 2— is shown to possess a Ñat, benzene-type structure, with
6
6
C–C bonds shorter than expected for a (non-aromatic) ketonic-
type structure.
One of the reasons for the continuing interest in the structure
of oxocarbon acids and of their deprotonation products is the
view that cyclic oxocarbon anions of the general formula
C O 2~ might show aromaticity, stabilised by electron delo-
n
n
calisation of the p electrons around the ring.1 The oxocarbon
anion family comprises the rhodizonate, C O 2~, croconate,
All attempts to grow single crystals of 2 have, thus far, been
unsuccessful. Treatment of 1 with RbOH in ethanol yields a
deep-red powder of Rb C O , which, upon recrystallisation
6
6
C O 2~, squarate, C O 2~ and deltate, C O 2~, dianions.2
5
5
4
4
3 3
2 6
6
The prototype for all these is the rhodizonate dianion,
from a small amount of water, a†ords dark-green crystals of
C O 2~, because of the structural analogy with benzene,
anhydrous Rb C O , 3.9 The product can assume di†erent
6
6
2 6
6
C H . Rhodizonate salts have found many applications, for
colours depending on subtle changes in the preparation pro-
cedure, thus appearing deep-red, purple or dark-green in
reÑected light, but it is always red in transmitted light.10 The
6
6
example as markers for lead, in the analysis of radium in fresh
waters, and also for their luminescence properties.3 We have
utilised oxocarbon acids, and the corresponding mono- and
dianions, in the course of our crystal engineering studies4 and
in the evaluation of some fundamental aspects of hydrogen
bonding interactions between ions.5 While the solid state
structure of croconic acid has been recently determined by
us,6a that of rhodizonic acid has been assigned on the basis of
solution studies.6b,c Analogously, there is no previous report
on the solid state structure of the rhodizonate dianion,
C O 2~.
Scheme 1 The reversible dehydration process.
6
6
In this preliminary communication, we report that when
crystalline H C O (2,3,5,5,6,6-hexahydroxycyclohex-2-ene-
8 6
8
1,4-dione) 1 is subjected to a thermogravimetric experiment
(TGA), the loss of two water molecules per formula unit is
observed at 180 ¡C, and an orange powder is obtained.7 The
product was identiÐed as rhodizonic acid H C O 2 on the
basis of mass spectrometric measurements.7 The same powder
material was subsequently obtained by thermal treatment of
solid 1 at 140 ¡C under vacuum (10~3 Torr).8 Although hydra-
tion of carbonyl groups is a well-known process, usually
studied in solution, the controlled and reversible dehydration
of a solid hexol has never been investigated before. The
process is fully reversible and uptake of water vapour by 2
leads quantitatively back to 1 in 3 min, as demonstrated by
powder X-ray di†raction. The process is summarised in
Scheme 1.
2 6
6
Fig. 1 A view of two layers of molecules in crystalline 1; two ÈOH
groups from each molecule protrude above and below the molecular
layers and form intermolecular hydrogen bonds that link the layers
together (only intermolecular hydrogen bonds are shown).
DOI: 10.1039/b107317f
New J. Chem., 2001, 25, 1221È1223
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This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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Crystallography
Single crystal X-ray di†raction data for 1 were collected on a
Bruker AXS SMART di†ractometer: C H O , T \ 223(2) K,
6
6 8
M \ 206.11, monoclinic, C2/c, a \ 11.216(1), b \ 9.612(1),
c \ 12.786(2) A, b \ 91.796(6)¡, V \ 1377.8(3) AŽ 3, Z \ 8,
F(000) \ 848, k \ 0.193 mm~1, h range 2È34¡, 9624 reÑec-
tions, 2589 independent, reÐnement on F2 for 152 parameters,
wR (F2, all reÑ.) \ 0.1462, R [I [ 2p(I)] \ 0.0592. The
1
powder X-ray di†raction pattern measured on the commercial
powder corresponds precisely to that calculated on the basis
of the single-crystal structure. Crystal data for 3 were collected
on a Nonius CAD4 di†ractometer equipped with an Oxford
Cryostream liquid-N device: Rb C O , T \ 223(2) K,
2
2 6 6
M \ 339.00, Monoclinic, C2/m, a \ 12.522(4), b \ 8.354(4),
c \ 3.761(3) A, b \ 96.93(5)¡, V \ 390.6(4)
AŽ 3, Z \ 2,
F(000) \ 316, k \ 12.528 mm~1, h range 3È28¡, 1001 reÑec-
tions, 501 independent, reÐnement on F2 for 36 parameters,
wR (F2, all reÑ.) \ 0.1061, R [I [ 2p(I)] \ 0.0374. Both dif-
1
fractometers equipped with a graphite monochromator (Mo-
Ka radiation, j \ 0.710 73 A). SHELXS-979a and SHELXL-
979a were used for structure solution and reÐnement based on
F2. SCHAKAL999b was used for the graphical representation
of the results.
CCDC reference numbers 163618 and 163619. See http://
www.rsc.org/suppdata/nj/b1/b107317f/ for crystallographic
data in CIF or other electronic format.
Fig. 2 (a) The distribution of C O 2~ dianions and Rb` cations in
6
6
the layers of crystalline Rb C O . (b) The layer stacking, note how
2
6 6
the C O 2~ dianions (interplanar distance 3.30 A) are shifted while
6
6
the Rb` cations are in close contact, forming a cationic pile. RbÉ É ÉO
distances in the range 2.904(6)È3.369(7) A.
Acknowledgements
We thank M.U.R.S.T. (projects ““Supramolecular DevicesÏÏ
1999È2000 and ““Solid SupermoleculesÏÏ 2000È2001) and the
Universities of Bologna (project ““Innovative MaterialsÏÏ) and
Sassari for Ðnancial support.
structure of 3 is shown in Fig. 2. Both the dianion and its
crystal possess some remarkable features: (i) the crystal is
formed of layers of rhodizonate dianions, C O 2~, organised
6
6
with pseudo-sixfold symmetry within the layer [see Fig. 2(a)];
the C O 2~ units lie Ñat on each other at an interplanar dis-
6
6
tance of 3.30 A; (ii) the Rb cations form cationic rows in
between the rhodizonate units [see Fig. 2(b)]. Similar stacking
has been recently observed in crystalline potassium croconate
dihydrate.11
Notes and references
1
2
3
(a) R. West, Oxocarbons, Academic Press, New York, 1980; (b) F.
Serratosa, Acc. Chem. Res., 1983, 16, 170.
(a) R. West and D. L. Powell, J. Am. Chem. Soc., 1963, 82, 6204;
(b) J. Aihara, J. Am. Chem. Soc., 1981, 103, 1633.
Within the dianion there are two independent CÈO dis-
tances [1.252(9) and 1.248(6) A] and two independent CÈC
distances [1.468(6) and 1.469(6) A]. These latter values are
intermediate between those of aromatic systems12a and those
expected for a cyclic ÈC(2O)ÈC(2O)È system [1.508(6), 1.518(6)
(a) M. R. Bartsch, H. J. Kobus and K. P. Wainwright, J. Forensic
Sci., 1999, 41, 1046; (b) E. J. Scharman and E. P. Krenzelok, J.
T oxicol., Clin. T oxicol., 1996, 34, 699; (c) E. J. Esswein, M. Boeni-
ger and K. Ashley, US Pat. 97-49352, 1997; (d) M. T. Ganzerli
Valentini, L. Maggi and V. Caramella Crespi, J. Radioanal. Nucl.
Chem., 1997, 221, 105; (e) M. T. Ganzerli Valentini, L. Maggi and
V. Caramella Crespi, Anal. Chem., 1999, 71, 162; ( f ) M. T. Gan-
zerli Valentini, L. Maggi and V. Caramella Crespi, J. Radioanal.
Nucl. Chem., 1997, 221, 109; (g) E. A. Terpetschning, Ger. Pat.
19815659, 1998; US Pat. 83820, 1998.
A in croconic acid,6a 1.537 A from a CSD12b search of dike-
tonic six-membered ring systems]. The value of 1.468(5) A is
2
also slightly shorter than predicted theoretically in a recent
computational study of isolated oxocarbon dianions.13
Whether this di†erence is due to a signiÐcant aromatic contri-
bution to the CÈC bonds, or to the fact that, in the solid state,
the rhodizonate anion interacts with the cations, needs further
investigation.
In this context, it is worth noting that the interplanar dis-
tance between the dianions along the stack is even shorter
than in graphite and in many other systems where pÈp stack-
ing is observed.14 We believe that this short separation is due
to the Rb cations, which pinch together dianions along the
stacking sequence, this being a manifestation of the ““charge
compressionÏÏ e†ect previously discussed in the cases of the
stacking of Ñat squarate and hydrogen squarate ions.15
Since hydration of rhodizonic acid can be achieved in a
reversible solid-state process, we are currently exploring the
possibility of using the uptake of nucleophilic molecules via a
heterogeneous process, to prepare other adducts.
4
5
D. Braga, L. Maini, L. Prodi, A. Caneschi, R. Sessoli and F. Gre-
pioni, Chem. Eur. J., 2000, 6, 1310.
(a) D. Braga and F. Grepioni, Acc. Chem. Res., 2000, 33, 601; (b)
D. Braga, L. Maini, F. Grepioni, F. Mota, C. Rovina and J. J.
Novoa, Chem. Eur. J., 2000, 6, 4536; (c) D. Braga, J. J. Novoa
and F. Grepioni, New J. Chem., 2001, 25, 226.
6
7
(a) D. Braga, L. Maini and F. Grepioni, CrystEngComm, 2001, 6;
http://www.rsc.org/ej/ce/2001/b100020l/index.htm; (b) R. T.
Bailey, J. Chem. Soc. B, 1971, 627; (c) R. I. Gelb, L. M. Schartz
and D. A. Laufer, J. Phys. Chem., 1978, 18, 1985.
2,3,5,5,6,6-Hexahydroxycyclohex-2-ene-1,4-dione was purchased
from Aldrich. The product of the dehydration process was identi-
Ðed as rhodizonic acid 2 on the basis of mass spectrometric mea-
surements (highest peak at m/z 205 for 1 and m/z 169 for 2).
W. Stadeli, R. Hollenstein and W. von Philipsborn, Helv. Chim.
Acta, 1977, 60, 948.
8
9
(a) G. M. Sheldrick, SHELX-97, Program for Crystal Structure
Determination, University of Gottingen, Germany, 1997; (b) E.
Keller, SCHAKAL99, Program for Graphical Representation of
Molecular Models, University of Freiburg, Germany, 1999.
Note added in proof. While this paper was being processed,
we have become aware of the report by Lam and Mak
of the structure of the rhodizonate dianion in the
salt [Bun N`] C O 2~ É 4PhNHCONH : C.-K. Lam and
10 P. W. Preisler and L. Berger, J. Am. Chem. Soc., 1942, 64, 67.
11 J. D. Dunitz, P. Seiler and W. Czechtizky, Angew. Chem., Int. Ed.,
2001, 40, 1779.
4
2 6
6
2
T. C. W. Mak, Chem. Commun., 2001, 1568.
1222
New J. Chem., 2001, 25, 1221È1223

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12 (a) F. H. Allen, O. Kennard, D. Watson, L. Brammer, A. G.
Orpen and R. Taylor, J. Chem. Soc. Perkin T rans. 2, 1987, S1; (b)
14 (a) C. A. Hunter, Chem. Soc. Rev., 1994, 101; (b) C. A. Hunter and
J. K. M. Sanders, J. Am. Chem. Soc., 1990, 112, 5525; (c) J. Har-
rowÐeld, J. Chem. Soc., Dalton T rans., 1996, 3165; (d) C. Janiak,
J. Chem. Soc., Dalton T rans., 2000, 3881 and references therein.
15 D. Braga, C. Bazzi, F. Grepioni and J. J. Novoa, New J. Chem.,
1999, 23, 577.
The Cambridge Structural Database was searched for cyclic C
6
rings containing the fragment ÈC(2O)ÈC(2O)È with uncoordi-
nated oxygen atoms (79 entries, 103 observations).
13 P. von Rague Schleyer, K. NajaÐan, B. Kiran and H. Jiao, J. Org.
Chem., 2000, 65, 426.
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