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COVER ARTICLEFalvello et al.Using the crystal to engineer the molecule: cis-trans-isomer selection in anionic bis(orotate) complexes
COMMUNICATIONBraga et al.Solvent effect in a “solvent free” reaction
Volume 9 | Number 10 | October 2007 | Pages 835–960
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View Article Online / Journal Homepage / Table of Contents for this issue
Using the crystal to engineer the molecule: cis-trans-isomer selection inanionic bis(orotate) complexes
Larry R. Falvello,*a Daniel Ferrer,a Marıa Piedrafita,a Tatiana Solerb and Milagros Tomas*a
Received 18th June 2007, Accepted 12th July 2007
First published as an Advance Article on the web 2nd August 2007
DOI: 10.1039/b709168k
Two chelating orotate22 ligands have been coordinated to the
same metal ion, producing the first dianionic bis-orotate
complex, [Ni(orotate)2(H2O)2]22, whose geometric isomer can
be selected by enabling or vitiating the formation of non-
covalent interactions in the solid in which the product is
isolated. This provides an important example of the influence
and potential use of non-covalent interactions for isomer
selection in the synthesis of coordination compounds, as well as
the manner in which such stereochemical selection can be
achieved by the appropriate use of counterions.
In contrast to the large body of work that has been dedicated to
the use of non-covalent interactions in preparing molecular crystals
with particular packing patterns,1 a much lesser amount of effort
has been directed at the challenging problem of using non-covalent
interaction-forming capability to direct the obtention of molecules
with particular shapes or coordination modes. While the field of
crystal engineering2 has experienced rapid growth—its goal being
the design of solid materials with specific properties based on
the supramolecular structures formed by molecules with given
topologies—the separate goal of isolating molecules with specific
or variable shapes by enabling non-covalent interactions in the
crystals in which those molecules are isolated, has received
relatively little attention.3,4
We report herein the isolation of two stereoisomers of the
first dianionic bis-orotate transition metal complex to have been
structurally characterized, cis- and trans-[Ni(HOr)2(H2O)2]22
(HOr = doubly deprotonated orotic acid). The crystal and
molecular structures demonstrate that the stereochemistry of the
isolated entity depends on the number of external non-covalent
interactions enabled during the synthesis and crystallization.
Coordination complexes that are trapped within a crystalline
environment dominated by non-covalent but directional forces—
hydrogen bonds, other electrostatic interactions, p–p interac-
tions—must sometimes adapt to those surroundings or to changes
produced in them by variation of temperature or pressure.5 At the
same time, the molecule is an active participant in the formation of
its crystalline environment; so unlike the better-known efforts at
crystal engineering, in which a molecule is designed in such a way
as to give a particular packing pattern, in our efforts to influence
molecular shape neither the target molecule nor the extended
crystal environment is known at the outset. Nevertheless, results
obtained to date demonstrate that much useful information is to
be had through systematic experimentation, and that unprece-
dented products are attainable as often as not. As an example,
we have isolated crystals of [Zn(k-N1-uracilate)(k-N3-uracilate)-
(NH3)2],6 a simple tetrahedral complex in which two different
tautomers of uracilate are simultaneously bound to the metal. This
result is demonstrably based on the hydrogen-bonding patterns
formed by the two tautomers. In crystals of trans-[Ni(k-N-cyan)2-
(NH3)4] (cyan = cyanurate, C3H2N3O32), a simple six-coordinate
complex, the molecule undergoes significant, reversible shape
changes as its single-component crystal suffers a second-order
phase transformation.7 In [Pt(CN)3(m-CN)Cu(NH3)4], immersion
of the molecular core in a hydrogen-bonded web forces a severe
bend in the CMN–Cu bond angle [120.1(6)u].8
Orotic acid, C5H4N2O4 (6-uracilcarboxylic acid, H3Or, vitamin
B13), Fig. 1, can be deprotonated, producing H2Or2 (pK1 = 2.09)
and HOr22 (pK2 = 9.28). Orotic acid and some of its derivatives
are involved in biological processes and have applications in
medicinal chemistry.9 The ligands H2Or2 and HOr22 are also
useful as ligands in coordination chemistry and molecular
materials science. Their utility as versatile polyfunctional ligands
derives from the relatively large fraction of their topologies
available for use in structure direction. Of the eleven non-hydrogen
atoms present, more than half (2 N and 4 O) can be employed in
coordination or in the formation of hydrogen bonds or other
electrostatic contacts. Orotates can coordinate to one, two, or three
transition metals at the same time,10 producing mononuclear,
dinuclear or polymeric compounds with a variety of coordination
types.11 The formation of a five-membered chelate is the most
common coordination behavior for HOr22, but no reports have
appeared to date of the isolation and structural characterization of
mononuclear bis-chelate complexes. The only two compounds
reported so far with two five-membered chelate HOr22 groups
bonded to the same transition metal are neutral polymers obtained
by hydrothermal synthesis.12
When NiCl2 and orotic acid are reacted in a 1 : 2 ratio in
the presence of CsOH,{ the dianionic bis-orotate chelate with
aUniversity of Zaragoza, C.S.I.C., Department of Inorganic Chemistryand Aragon Materials Science Institute, Zaragoza, Spain.E-mail: falvello@unizar.es; Fax: 34 976761187; Tel: 34 976 761179bServicios Tecnicos de Investigacion, Facultad de Ciencias Fase II, 03690S. Vicente de Raspeig, Alicante, Spain. E-mail: tatiana.soler@ua.es Fig. 1 Orotic Acid.
COMMUNICATION www.rsc.org/crystengcomm | CrystEngComm
852 | CrystEngComm, 2007, 9, 852–855 This journal is � The Royal Society of Chemistry 2007
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trans-dispositions of all ligands is isolated as Cs2[trans-Ni(HOr)2-
(H2O)2]?4H2O (1), Fig. 2. X-Ray diffraction analysis{ reveals a
centrosymmetric complex with the two chelating orotates in the
basal plane and two aqua ligands in the axial positions. One of the
useful particulars of this complex is its anionic character. Anionic
compounds are rare among structurally characterized orotate
complexes and are mainly complexes of metals in oxidation states
other than +2 – oxidation state 0 (Cr, W),13 +1 (Rh, Ir),14
+3 (Fe).11 [Cu(HOr)(NH2CH2COO)(H2O)]2 is the only struc-
turally characterized anionic M+2 orotate compound.15 The
anionic character, among other things, enables a systematic study
of the product- and structure-directing influence of non-covalent
forces, which can be varied by changing the counterion.
If Cs+ is substituted in the synthesis by the bulky nBu4N+
cation,§ capable of forming only weak non-covalent interactions,
the product is an aqua-cis-isomer," isolated as (NBu4)2[cis-
Ni(HOr)2(H2O)2]?2H2O (2), Fig. 3.IApart from their overall shapes, there are no major geometrical
differences in the covalent structures of (1) and (2). There are slight
variations in the Ni–L distances, which can be described through a
comparison with the previously reported neutral mono-orotato
complex [Ni(HOr)(H2O)4]?H2O.16 The anionic trans-isomer (1) has
the Ni–N distance [2.0922(15) A] slightly elongated with respect to
that found for the neutral complex [2.059(1) A], while the Ni–O
(orotate) distances do not vary [2.0264(12) and 2.023(1) A,
respectively]. For the cis-isomer (2), the Ni–N and Ni–O distances
for one of the orotate ligands are slightly elongated [both are
2.070(3) A] with respect to those in the neutral complex, while the
analogous distances for the second chelate [2.053(3) and 2.029(3) A]
are identical within experimental error between the anionic and
neutral complexes.
However, the non-covalent interactions are very different for (1)
and (2). The relative dispositions of the ligands in the cis-isomer (2)
permit the formation of two intramolecular hydrogen bonds, in
each of which an aqua ligand serves as a donor and the proximal
carbonyl oxygen atom of the uracilate fragment is the acceptor
(Fig. 3).** In addition, in (2) one H atom of an aqua ligand
(H2WA, attached to O2W), makes a contact of 2.71(5) A with the
center of gravity (Cg) of the chelate ring [Ni1/O5/C7/C6/N1] of the
same molecule. The trans-isomer (1) does not present any
intramolecular hydrogen bonds or H…Cg contacts.
Moreover, the remaining functional groups of one orotate
ligand in (2), namely the imine at the 3-position [N(3)] and the
carbonyl at the 4-position, O(4), are involved in intermolecular
self-recognition with a neighboring molecule at (12x, 2y, 2z),
forming a ring of R22(8) topology17 (Fig. 4). The second orotate
forms a second link, via a symmetry-related pair of hydrogen
bonds from N(13) to the ligated carboxyl oxygen of its congener at
(22x, 2y, 12z) [R22(12), Fig. 4]. In this way, and together with the
nickel atoms, an unbounded chain of anions is formed. Parallel
chains are cross-linked by hydrogen-bonded, unligated water
molecules, giving a 2-D aggregate which is stacked along the [101]
direction to leave open channels in which the nBu4N+ cations
reside (Fig. 5).
The all-trans isomer (1) displays an entirely different behavior
(Fig. 6), forming electrostatic interactions with the cesium atom
and interstitial water molecules. These interactions dominate and
obviate the formation of orotate self-recognition interactions,
although there is a single unique p–p interaction between
neighboring pyrimidine rings.
It is clear from a comparison of (1) and (2) that the presence of
large groups with little capacity for forming directed non-covalent
interactions favors aggregation by self-recognition. These results
demonstrate, further, that it is possible to select among geometrical
isomers when the number of atoms that can participate in directed
non-covalent interactions is a large fraction of the total number
of non-hydrogen ligand atoms. This conclusion has important
impli-cations for the design of procedures aimed at selecting
isomers in the preparation of molecular solids.
Further investigation is needed to determine whether the present
results are obtained by selection during the crystallization process,
or whether significant aggregation already occurs in solution for
one or both cases.
The anionic bis-orotate chelate complexes are expected to be
useful in several interesting applications, from the study of non-
covalent interactions—as here—to the obtention of solids with
varying properties of interest for physical or even medical
applications. Their ionic nature is particularly useful in this
context, because it enables the use of the counterion topology as a
further variable parameter for structure direction, as well as the use
of the anionic complex as a building block for the formation of
heterometallic structures.
Fig. 2 Representation of the anion in Cs2[trans-Ni(HOr)2(H2O)2]?4H2O,
(1).
Fig. 3 Drawing of the anion from the crystal structure of (nBu4N)2[cis-
Ni(HOr)2(H2O)2]?2H2O, (2).
Fig. 4 Chain formed by three [Ni(HOr)2]2- fragments linked by
hydrogen bonds between orotate groups in complex 2.
This journal is � The Royal Society of Chemistry 2007 CrystEngComm, 2007, 9, 852–855 | 853
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Funding from the Ministry of Education and Science of Spain
(Grant CTQ2005-03141) is gratefully acknowledged.
Notes and references
{ Preparation of (1): The addition of 0.168 g (1.27 mmol) of NiCl2 to anaqueous solution of 0.45 g (2.53 mmol) of H3Or?H2O and 4.65 mmol ofCsOH produced the precipitation of complex (1) after 6 h of stirring. Theblue solid was filtered and washed with isopropanol, 71% yield. Elementalanalysis calcd (%): C 16.21, H 2.16, N 7.56; found: C 16.39, H 2.35, N 7.56.IR: 3509, 3458, 3252, 3143, 1605, 1552, 1489, 1373, 1017, 953, 802, 775,528 cm21. Crystals suitable for X-ray diffraction studies were obtainedfrom an aqueous solution of complex 1 layered with isopropanol.{ Crystallographic data for (1): Cs2[Ni(HOr)2(H2O)2]?4H2O,C10H16Cs2N4NiO14, M = 740.80, triclinic, P-1, a = 7.1982(5), b =8.6340(6), c = 9.2042(7) A, a = 105.813(1), b = 99.892(1), c = 112.181(1)u,V = 484.83(10) A3, Z = 1, F(000) = 354, r = 2.537 g cm23, m = 4.783 mm21,
T = 100(1) K, total of 6020 reflections, 2299 unique reflections (Rint =0.0156), 2243 observed reflections [I . 2s(I)], R1(obs) = 0.0140, R1(all) =0.0147, wR2(all) = 0.0363. Structure solution by direct methods. Allhydrogen atoms were located in a difference Fourier map and refined freelywith isotropic displacement parameters. CCDC reference number 649149.For crystallographic data in CIF or other electronic format see DOI:10.1039/b709168k§ Preparation of (2): An aqueous solution of 0.200 g (1.15 mmol) ofH3Or?H2O, 4.2 ml of NBu4OH (2.3 mmol) in MeOH and 0.074 g(0.58 mmol) of NiCl2 was allowed to stand for 24 h at 6 uC.; then it wasfiltered, taken to dryness and dissolved in isopropanol. The slowevaporation of the isopropanol solution produced crystals suitable for anX-ray diffraction study. IR: 3500, 2960, 1643, 1623, 1584, 1564, 1465, 1363,1314, 1019, 878, 785 cm21." With two aqua ligands and two hybrid (N, O) chelates, six geometricisomers are possible. We can describe them as cis- or trans- with respect tothe three types of ligating atoms, in the following order: (1) O(aq); (2)N(Or); (3) O(Or). Compound (1) is the all-trans or ttt isomer, and
Fig. 5 Aggregate formed by the [cis-Ni(HOr)2(H2O)2]22 anion in 2, with the atoms of the (nBu4N
+) cations, shown as spheres of radius 0.9 A, residing in
the channels formed.
Fig. 6 Two-dimensional net formed by the supramolecular aggregation of [trans-Ni(HOr)2(H2O)2]22 anions. The Cs+ cations are shown in space-filling
form with atoms represented by spheres of radius 1.0 A.
854 | CrystEngComm, 2007, 9, 852–855 This journal is � The Royal Society of Chemistry 2007
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compound (2) is cis- for the oxygen ligands and trans-for nitrogen (ctc). Tokeep the present discussion simple, we use the dispositions of themonodentate aqua ligands to distinguish between trans-(1) and cis-(2).I Crystallographic data for (2): (nBu4N)2[Ni(HOr)2(H2O)2]?2H2O,C42H84N6NiO12, M = 923.86, triclinic, P-1, a = 12.1915(4), b =12.3098(6), c = 16.4178(7) A, a = 90.374(4), b = 96.009(3), c =95.849(3)u, V = 2437.24(18) A3, Z = 2, F(000) = 1004, r = 1.259 g cm23,m = 0.461 mm21, T = 100(2) K, total of 19690 reflections, 8544 uniquereflections (Rint = 0.0547), 6734 observed reflections [I . 2s(I)], R1(obs) =0.0758, R1(all) = 0.1034, wR2(all) = 0.1422. Structure solution by directmethods. Orotate H atoms and methylene H atoms of the cations wereplaced in calculated positions and refined as riders with Uiso set to 1.2 timesUeq of their parent atoms. Methyl H atoms of the cation were treatedsimilarly except that they were located in slant Fourier calculations andrefined as riders with a variable torsion angle for the methyl group as awhole. Bound water H atoms were refined freely. For the unligated waterat O3W, the hydrogen atoms were not located. For the unligated water atO4W, H atoms were located and refined with free positional parametersbut with Uiso constrained to 1.2 times Ueq of O4W. CCDC referencenumber 649150. For crystallographic data in CIF or other electronicformat see DOI: 10.1039/b709168k** One such intramolecular H-bond could exist for the other possible cis-aqua arrangement, the (ccc) isomer; but the ligand arrangement would notfavor two such interactions.
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