Concurrent anion...π interactions between a perchlorate ion and two π-acidic aromatic rings, namely pentafluorophenol and 1,3,5-triazine

Article abstract of DOI:10.1039/b803817a

The rational design of a ligand containing two electron-poor π-rings, i.e. a triazine and a pentafluorophenoxy groups, has allowed the preparation of a copper complex where both the anticipated anion...π interactions are present. The Royal Society of Chem

Full text of DOI:10.1039/b803817a

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Concurrent anionꢀ ꢀ ꢀp interactions between a perchlorate ion and two
p-acidic aromatic rings, namely pentafluorophenol and 1,3,5-triazinew
a
b
c
c
a
¨
Rens J. Gotz, Arturo Robertazzi, Ilpo Mutikainen, Urho Turpeinen, Patrick Gamez*
and Jan Reedijk*a
Received (in Cambridge, UK) 5th March 2008, Accepted 18th April 2008
First published as an Advance Article on the web 21st May 2008
DOI: 10.1039/b803817a
The rational design of a ligand containing two electron-poor
p-rings, i.e. a triazine and a pentafluorophenoxy groups, has
allowed the preparation of a copper complex where both the
anticipated anionꢀ ꢀ ꢀp interactions are present.
aromatic ring, respectively a phenoxy and a pentafluorophe-
noxy unit. The presence of these electronically opposite
p-substituents is aimed at verifying the sole involvement of
the p-acidic one in the expected anionꢀ ꢀ ꢀp interactions. The
third substituent, i.e. the dipyridylamine group, would co-
ordinate to a metal cation to ensure charge neutrality of the
host–guest system.
Noncovalent contacts such as hydrogen bonds,1 C–Hꢀ ꢀ ꢀp,2
pꢀ ꢀ ꢀp,3 and cationꢀ ꢀ ꢀp4 interactions are widely regarded as
stabilizing interactions for a number of bio(macro)molecules,
molecular recognition, and supramolecular assemblies. Since
the past 5 years, a new type of supramolecular bonding force is
being considered by the scientific community, namely the
anionꢀ ꢀ ꢀp interaction with electron-deficient aromatic rings.5,6
After having being clearly evidenced by theoretical studies,7–9
the first crystallographic examples of anionꢀ ꢀ ꢀp interactions
recognized as such were reported in 2004.10,11 This field of
supramolecular chemistry has subsequently developed and
exciting new examples of anionꢀ ꢀ ꢀp binding associations are
now increasingly described in the literature.12–14
Single crystals of the ligand dppftz were obtained which
were analysed by X-ray diffraction (see Fig. S1 and ESIw for
X-ray data). Remarkably, the solid-state structure of the free
ligand dppftz already reveals the exceptional p-acidic charac-
ters of the 1,3,5-triazine and the pentafluorophenol rings. The
crystal packing of dppftz is shown in Fig. S2 (ESIw). The
crystal structure of the compound is composed of two slightly
distinct dppftz molecules (indicated by the triazine rings A and
A0 in Fig. S2, ESIw). This differentiation most likely arises
from the packing of the dppftz molecules which involves lone
pairꢀ ꢀ ꢀp interactions.19 As is evidenced in Fig. S2, the fluoride
atom F138 is strongly interacting with the triazine ring of a
neighbouring molecule of dppftz (see Table S2, ESI,wfor all
distances between F138 and each atom of the triazine ring A).
The F138ꢀ ꢀ ꢀcentroid A distance is 2.941(8) A and the angle
F138–ring centroid–aromatic plane amounts to 78.7(3)1. Both
the distance (which is below the sum of the F and C, and of the
F and N van der Waals radii,21 respectively 3.17 and 3.02 A)
and the angle close to 901 are indicative of a significant
bonding association. This value is just above the calculated
distance (2.6 A) for the anionꢀ ꢀ ꢀp interaction between a
fluoride ion and a simple triazine ring.7 Lone pair (l.p.)ꢀ ꢀ ꢀp
interactions between acetonitrile and a triazine ring have been
previously observed with experimental and calculated lone
pair(acetonitrile)ꢀ ꢀ ꢀp(triazine) contact distances of 3.087 and
3.116 A, respectively.22 Interestingly, this electron-poor penta-
fluorophenol p-ring (ring B in Fig. S2) is acting as a lone pair
host as well, through its interaction with the nitrogen atom
N217 of a second neighbouring dppftz molecule (Fig. S2). The
N217ꢀ ꢀ ꢀcentroid B separation distance is 3.122(9) A and the
Both 1,3,5-triazine and perfluoro aromatic rings are elec-
tron-poor rings whose anion binding properties have been
comprehensively, independently investigated, both theoreti-
cally and experimentally.7,15,16 These two categories of aro-
matic groups appear to represent prime electron-deficient
hosts for anion guests,17,18 and more generally for electron-
rich molecules.19
In the present study, these two types of p-acidic moieties
have been combined within a single molecule to examine
its potential joint anion binding properties. The ligand 2-(N,N-
di(pyridin-2-yl))-4-(perfluorophenoxy)-6-phenoxy{1,3,5}tria-
zine (dppftz; Scheme 1) was prepared from 2,4,6-trichloro-
{1,3,5}triazine, applying an experimental procedure described
earlier (see ESIw).20 Ligand dppftz possesses a 1,3,5-triazine
core bearing three different functional groups as substituents.
Thus, dppftz holds both an electron-rich and an electron-poor
a Leiden Institute of Chemistry, P. O. Box 9502, 2300 RA Leiden,
The Netherlands. E-mail: p.gamez@chem.leidenuniv.nl;
Fax: +31 71 5274671
b International School for Advanced Studies and CNR-INFM-
Democritos National Simulation Center, Trieste, Italy
c University of Helsinki, Department of Chemistry, Laboratory of
Inorganic Chemistry, 00014 Helsinki, Finland
w Electronic supplementary information (ESI) available: Experimental
procedures for the preparation of dppftz and complex 1, illustrative
schematic representations of dppftz and complex 1, selected bond
lengths and angles for 1, contact distances reflecting the lone pairꢀ ꢀ ꢀp
(dppftz) and anionꢀ ꢀ ꢀp (complex 1) interactions, computational details,
X-ray crystal data. CCDC 679994 (dppftz) and 679693 (1). For ESI
and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b803817a
Scheme 1 Representation of the ligand dppftz.
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angle N217–ring centroid–aromatic plane is 79.5(4)1 (see
Table S2 for all distances between N217 and each atom of
the pentafluorophenol ring B). In comparison, the theoretical
distance for an anionꢀ ꢀ ꢀp interaction between a cyanide ion
and hexafluorobenzene is 2.80 A (for an ideal anion–cen-
troid–ring plane angle of 901).16 In the same way, the triazine
ring A undergoes lone pairꢀ ꢀ ꢀp interactions through its nitro-
gen atom N117 with the pentafluorophenol unit B0 (distance
N117ꢀ ꢀ ꢀB0 = 3.202(3) A and angle N117–centroid B0–ring
plane = 75.9(3)1), and the pentafluorophenol group B0 is
associated, through its fluoride atom F234, to the triazine ring
A0 (distance F234ꢀ ꢀ ꢀA0 = 3.093(9) A and angle F234–centroid
A0–ring plane = 83.1(4)1). These four lone pairꢀ ꢀ ꢀp orbitals
give rise to the formation of an infinite one-dimensional
supramolecular chain.
orientations in the crystal lattice with occupancy factors of
ꢂ
0.5. The two crystallographically independent ClO4 ions are
labelled {Cl1O11–O14} and {Cl2O21–O24}. As is evidenced in
Fig. 1, the oxygen atom O11 is strongly interacting with an
intramolecular triazine ring (see Table S5, ESI,w for all
separation distances between O11 and each atom of the
triazine ring A). The O11ꢀ ꢀ ꢀcentroid A distance is 3.203(8) A
and the angle O11–ring centroid–aromatic plane amounts to
61.4(3)1. The perchlorate oxygen atom O12 is also in contact
with the triazine ring A (O12ꢀ ꢀ ꢀcentroid A = 3.356(11) A;
O12–ring centroid–aromatic plane = 73.8(4)1). Finally, the
oxygen atom O14 is weakly interacting with an intermolecular
pentafluorophenol ring (ring B in Fig. 1 and Table S5, ESIw).
The O14ꢀ ꢀ ꢀcentroid B distance is 3.651(6) A and the angle
O14–ring centroid–aromatic plane amounts to 53.6(4)1. The
second perchlorate orientation experiences anionꢀ ꢀ ꢀp contacts
as well. The oxygen atom O21 shows significant interactions
with the intramolecular triazine unit A (Fig. 1; O21ꢀ ꢀ ꢀcentroid
A = 3.188(7) A and O21–ring centroid–aromatic plane =
60.1(4)1). O22 exhibits a very strong bonding association with
the triazine ring A (see Table S5) with a short distance
O22ꢀ ꢀ ꢀcentroid A of 2.939(10) A and an angle O22–ring
centroid–aromatic plane of 75.1(3)1. In addition, O22 is in
contact with the intermolecular ring B (Fig. 1; O22ꢀ ꢀ ꢀcentroid
B = 3.601(8) A and O22–ring centroid–aromatic plane =
63.4(3)1). The two spatial orientations of the ClO4ꢂ ions in the
lattice are most likely induced by competitive interactions
between the two different electron-poor rings. This experi-
mental observation has been investigated by theoretical calcu-
lations to confirm the energetical equivalence of these two
anion positions.
The reaction of one equivalent of Cu(ClO4)2ꢀ6H2O with two
equivalents of dppftz in methanol produces the coordination
compound [Cu(dppftz)2(ClO4)2](H2O)5 (1), whose X-ray crys-
tal structure is depicted in Fig. S3 (see Table S3, ESI,w for
X-ray data).1,2 Our selection for the perchlorate ion is based
on a search at the Cambridge Structural Database (CSD)
which reveals 1933 examples of anion–p close contacts
between this anion and nitrogen-containing six-membered
aromatic rings.23 Thus, this ion is the most commonly
observed for anion–p interactions in the CSD, and is therefore
a candidate of choice for anion binding studies.
Complex 1 is constituted of two dppftz ligands coordinated
at the equatorial plane of an octahedral copper atom which
lies on an inversion center (Fig. S3, ESIw). The axial positions
of the octahedron around Cu are occupied by two perchlorate
ions, at normal distances for an elongated octahedral geome-
try. The Cu–N bond lengths (Table S4, ESIw) are in the usual
range for such CuN4O2 chromophore. The in-plane angles are
very close to the ideal value of 901 for a perfect octahedron
(Table S4).
While most popular DFT functionals are not suitable for
systems governed by dispersive forces,24–28 high-level ab initio
methods such as MP2 and CCSD,28–30 which correctly de-
scribe these forces, require a computational cost that is only
affordable for relatively small molecules. For complexes such
as those studied here, a valuable alternative is represented by
the use of the hybrid BHandH functional, which has recently
been shown to describe surprisingly well geometries and
energies of a variety of systems for which dispersive forces
are crucial.24,31–35
The fascinating crystal packing of the solid-state structure
of 1 is shown in Fig. 1. As anticipated, the two p-acidic rings
are involved in anion–p interactions, while the phenoxy group
is not. Indeed, each perchlorate ion exhibits intramolecular
close contacts with a triazine ring (ring A in Fig. 1) and
intermolecular anion–p interactions with a pentafluorophenol
group from an adjacent dppftz molecule (ring B in Fig. 1).
Interestingly, the perchlorates adopt two crystallographic
The BHandH/6-31+G(d) level of theory has therefore been
employed in order to estimate the interaction energies of
‘‘model A’’ and ‘‘model B’’ corresponding to the two crystal-
ꢂ
lographic positions of the ClO4 ions in the crystal lattice of
complex 1 (see Fig. 2 and ESIw). In addition, by means of the
Bader’s Atom-in-Molecule theory (AIM),36 intra- and inter-
molecular interactions were characterized involving the per-
chlorate group. It is worth mentioning that the electron
density at the so-called bond critical points (blue and red balls
for inter- and intramolecular interactions, respectively; Fig. 2;
for further details see the ESIw) roughly correlates with the
strength of hydrogen bonds,37,38 covalent bonds39 and pꢀ ꢀ ꢀp
interactions.24 As a reference, BHandH calculated values for
bond critical points in the water, argon and benzene dimers are
0.0343, 0.0059 and 0.0141 au, respectively.
First, single-point calculations with BHanH/6-31+G(d)
confirmed that both configurations have a similar interaction
energy, differing by o3 kJ molꢂ1 (Table S6, ESIw). Second, the
Fig. 1 Crystal packing of complex 1 illustrating the anionꢀ ꢀ ꢀp con-
tacts shown by the two crystallographically independent perchlorate
ions.
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Fig. 2 Schematic views of Atoms in Molecules topology of (A) ‘‘model A’’ and (B) ‘‘model B’’. The blue lines represent the bond paths. The red
and blue balls are the bond critical points of the intra- and intermolecular interactions, respectively. For the sake of clarity, only the electron
density relative to the interactions involving the perchlorate group is displayed. Further details on AIM theory and topology are reported in the
ESI.w
AIM analysis revealed that different interactions contribute to
these values (Fig. 2). In particular two Oꢀ ꢀ ꢀC lone pairꢀ ꢀ ꢀp-
interactions were found between the perchlorate group and the
pentafluorophenol ring in model A, with an overall electron
density at the bond critical point of 0.00660 au. In addition, an
Oꢀ ꢀ ꢀH–C hydrogen bond was also found between the per-
chlorate and the phenoxy ring as displayed in Fig. 2. By
contrast, model B shows one Oꢀ ꢀ ꢀC and one strong Oꢀ ꢀ ꢀN
lone pairꢀ ꢀ ꢀp interactions with the pentafluorophenol and
triazine rings, respectively, the overall electron density being
equal to 0.0165 au (Table S6). Notably, the electron density of
the inter-molecular interactions between the perchlorate and
the pentafluorophenol rings in models A and B are virtually
equivalent (0.00660 vs. 0.00571 au), giving a rationale for the
similar binding energies and therefore the equal probability of
the two perchlorate orientations in the crystal structure.
Supramolecular assemblies are generally stabilized by a
range of weak dispersive interactions. The comprehension of
these various interactions is crucial for the rational design of
supramolecular structures. The present experimental and
theoretical study has clearly shown that l.p.ꢀ ꢀ ꢀp and
anionꢀ ꢀ ꢀp interactions should be considered by the supra-
molecular chemist to build intricate molecular architectures.
We are grateful to the Graduate Research School Combina-
tion ‘‘NRSC Catalysis’’ for financial support.
11. P. de Hoog, P. Gamez, H. Mutikainen, U. Turpeinen and
J. Reedijk, Angew. Chem., Int. Ed., 2004, 43, 5815–5817.
12. M. Mascal, I. Yakovlev, E. B. Nikitin and J. C. Fettinger, Angew.
Chem., Int. Ed., 2007, 46, 8782–8784.
13. A. Garcia-Raso, F. M. Alberti, J. J. Fiol, A. Tasada, M. Barcelo-
Oliver, E. Molins, D. Escudero, A. Frontera, D. Quinonero and
P. M. Deya, Inorg. Chem., 2007, 46, 10724–10735.
14. K. V. Domasevitch, P. V. Solntsev, I. A. Gural’skiy,
H. Krautscheid, E. B. Rusanov, A. N. Chernega and J. A.
K. Howard, Dalton Trans., 2007, 3893–3905.
15. T. J. Mooibroek and P. Gamez, Inorg. Chim. Acta, 2007, 360,
381–404.
16. I. Alkorta, I. Rozas and J. Elguero, J. Am. Chem. Soc., 2002, 124,
8593–8598.
17. P. U. Maheswari, B. Modec, A. Pevec, B. Kozlevcar, C. Massera,
P. Gamez and J. Reedijk, Inorg. Chem., 2006, 45, 6637–6645.
18. O. B. Berryman, F. Hof, M. J. Hynes and D. W. Johnson, Chem.
Commun., 2006, 506–508.
19. M. Egli and S. Sarkhel, Acc. Chem. Res., 2007, 40, 197–205.
20. P. de Hoog, P. Gamez, W. L. Driessen and J. Reedijk, Tetrahedron
Lett., 2002, 43, 6783–6786.
21. A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
22. T. J. Mooibroek, S. J. Teat, C. Massera, P. Gamez and J. Reedijk,
Cryst. Growth Des., 2006, 6, 1569–1574.
23. T. J. Mooibroek, C. A. Black, P. Gamez and J. Reedijk, Cryst.
Growth Des., 2008, 8, 1082–1093.
24. M. P. Waller, A. Robertazzi, J. A. Platts, D. E. Hibbs and
P. A. Williams, J. Comput. Chem., 2006, 27, 491–504.
25. J. van deVondele, R. Lynden-Bell, E. J. Meijer and M. Sprik,
J. Phys. Chem. B, 2006, 110, 3614–3623.
26. J. Cerny and P. Hobza, Phys. Chem. Chem. Phys., 2005, 7,
1624–1626.
27. M. Swart, T. van der Wijst, C. F. Guerra and F. M. Bickelhaupt,
J. Mol. Model., 2007, 13, 1245–1257.
28. P. Hobza and J. Sponer, Chem. Rev., 1999, 99, 3247–3276.
29. P. Hobza and J. Sponer, J. Am. Chem. Soc., 2002, 124,
11802–11808.
30. P. Jurecka and P. Hobza, J. Am. Chem. Soc., 2003, 125,
15608–15613.
31. W. Z. Wang, M. Pitonak and P. Hobza, ChemPhysChem, 2007, 8,
2107–2111.
32. A. Magistrato, A. Robertazzi and P. Carloni, J. Chem. Theory
Comput., 2007, 3, 1708–1720.
33. J. Overgaard, M. P. Waller, R. Piltz, J. A. Platts, P. Emseis,
P. Leverett, P. A. Williams and D. E. Hibbs, J. Phys. Chem. A,
2007, 111, 10123–10133.
Notes and references
1. G. R. Desiraju and T. Steiner, The Weak Hydrogen Bond In
Structural Chemistry and Biology, Oxford University Press, New
York, 1999.
2. M. Nishio, M. Hirota and Y. Umezawa, The C–H/p Interaction:
Evidence, Nature and Consequences, Wiley-VCH, New York, 1998.
3. S. K. Burley and G. A. Petsko, Science, 1985, 229, 23–28.
4. J. C. Ma and D. A. Dougherty, Chem. Rev., 1997, 97, 1303–1324.
5. B. L. Schottel, H. T. Chifotides and K. R. Dunbar, Chem. Soc.
Rev., 2008, 37, 68–83.
6. P. Gamez, T. J. Mooibroek, S. J. Teat and J. Reedijk, Acc. Chem.
Res., 2007, 40, 435–444.
7. M. Mascal, A. Armstrong and M. D. Bartberger, J. Am. Chem.
Soc., 2002, 124, 6274–6276.
34. X. L. Dong, Z. Y. Zhou, L. J. Tian and G. Zhao, Int. J. Quantum
Chem., 2005, 102, 461–469.
8. D. Quinonero, C. Garau, C. Rotger, A. Frontera, P. Ballester,
A. Costa and P. M. Deya, Angew. Chem., Int. Ed., 2002, 41,
3389–3392.
9. D. Kim, P. Tarakeshwar and K. S. Kim, J. Phys. Chem. A, 2004,
108, 1250–1258.
35. M. Meyer, T. Steinke and J. Suhnel, J. Mol. Model., 2007, 13,
335–345.
36. R. F. W. Bader, Chem. Rev., 1991, 91, 893–928.
37. R. J. Boyd and S. C. Choi, Chem. Phys. Lett., 1986, 129, 62–65.
38. S. J. Grabowski, Chem. Phys. Lett., 2001, 338, 361–366.
39. S. T. Howard and O. Lamarche, J. Phys. Org. Chem., 2003, 16,
133–141.
10. S. Demeshko, S. Dechert and F. Meyer, J. Am. Chem. Soc., 2004,
126, 4508–4509.
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3386 | Chem. Commun., 2008, 3384–3386