Cuprophilic and π-stacking interactions in the formation of supramolecular stacks from dicoordinate organocopper complexes

Article abstract of DOI:10.1039/b417532h

The unsupported organocopper pyridine complexes C₆F ₅Cu(Py) (2) and [C₆F₅Cu]₂-bipy) (3) form supramolecular structures that are unprecedented in organocopper chemistry; one-dimensional chains of coppe

Full text of DOI:10.1039/b417532h

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Cuprophilic and p-stacking interactions in the formation of
supramolecular stacks from dicoordinate organocopper complexes{
Anand Sundararaman,a Lev N. Zakharov,b Arnold L. Rheingoldb and Frieder Ja¨kle*a
Received (in Columbia, MO, USA) 18th November 2004, Accepted 19th January 2005
First published as an Advance Article on the web 8th February 2005
DOI: 10.1039/b417532h
shift difference Dd(19Fmeta/para) from 16.6 ppm in 1 to 3.1 ppm for 2
and 2.7 ppm for 3 in the 19F NMR spectra is characteristic of
The unsupported organocopper pyridine complexes
C6F5Cu(py) (2) and [C6F5Cu]2(4,49-bipy) (3) form supramole-
coordination of nucleophiles and break-down of the tetrameric
aggregate.4 Low temperature NMR spectroscopy of 2 showed no
evidence of a dynamic process down to 280 uC.
cular structures that are unprecedented in organocopper
chemistry; one-dimensional chains of copper atoms with
…
˚
Cu Cu distances of 2.8924(3) A in the blue-luminescent
Complex 2 is thermally stable to ca. 150 uC and the polymorphs
of 3 start to decompose above 130 uC. Coordination of the
pyridine ligand thus leads to destabilization of the pentafluoro-
phenylcopper complex in comparison to the dioxane complex of 1
with Tdec 5 200–220 uC.3 Indeed, the isolation and crystal-
lographic characterization of organocopper pyridine complexes
have been hampered in the past by their relatively low thermal
stability.2a
complex 2 are likely associated with cuprophilic interactions,
whereas multiple perfluoroarene–arene interactions dominate
in the supramolecular assembly of 3.
Organocopper compounds form an intriguing variety of interesting
aggregates in the solid state as well as in solution.1 To date, a
number of well-defined homoleptic arylcopper species [ArCu]n
with varying degrees of association (n 5 2–8, ‘) have been isolated
and structurally characterized.1 Aggregation typically occurs
through bridging of two copper centers with an organic moiety.1
It is also well-known that treatment of organocopper species with
strongly coordinating ligands (L) can lead to break-down of the
aggregated structure.2 We show here a new case where, even
though aggregate break-down leads to formally dicoordinate
organocopper complexes RCuL, cuprophilic and p-stacking
interactions result in formation of supramolecular structures in
the solid state.
Pentafluorophenylcopper tetramer ([C6F5Cu]4; 1)3–5 was treated
with an equimolar amount of pyridine at ambient temperature.
Pale yellow crystals of 2 were obtained from CH2Cl2 solution at
238 uC in 81% isolated yield. Slow diffusion of a solution of
4,49-bipyridine in CH2Cl2 into a solution of 1 in CH2Cl2 led to
precipitation of 3 as a light-yellow solid (yield: 87%), which is only
sparingly soluble in non-coordinating solvents.
Crystal structures of complex 2 and of two polymorphs of 3
were obtained (Fig. 1–3).{{ The structure of 2 is unusual in that all
atoms, including the hydrogen atoms, reside on a crystallographic
mirror plane. The copper centers adopt in all three structures a
linear or nearly linear coordination geometry (2: 178.54(6)u; 3-a:
180u; 3-b: 178.17(8)u) and thus represent rare examples of
structurally characterized dicoordinate organocopper complexes
RCuL. The pentafluorophenyl groups and the pyridine rings are
either perfectly coplanar or nearly so (2: 0.0u; 3-a: 3.8u; 3-b: 9.5u).
The two structures of 3 differ mainly in the twist at the central C–C
bond connecting the two pyridine rings; for 3-a an interplanar
angle of 44.3u is observed, whereas the pyridine rings in 3-b adopt a
˚
coplanar conformation. The copper–carbon bonds of 1.8913(17) A
˚
for 2, 1.880(4) A for 3-a and 1.890(2) A for 3-b are shorter than
˚
˚
those in the tetrameric precursor 1 (1.957 A to 2.145 A)
˚
The coordination of pyridine and 4,49-bipyridine to copper was
confirmed by multinuclear NMR spectroscopy and the composi-
tion of 3 as a 2 : 1 complex of C6F5Cu and 4,49-bipyridine was
verified by elemental analysis. The strong decrease in the chemical
˚
Fig. 1 Packing diagram of 2. Selected interatomic distances (A) and
…
angles (u) for 2: Cu1–C1 1.8913(17), Cu1–N1 1.9022(15), Cu1 Cu1A
2.8924(3), C1–Cu1–N1 178.54(6), C7–N1–Cu1 121.11(12), C11–N1–Cu1
{ Electronic supplementary information (ESI) available: experimental
details, Ortep plots of 2, 3-a, 3-b, a packing diagram of 2. See http://
www.rsc.org/suppdata/cc/b4/b417532h/
121.30(11), C6–C1–Cu1 121.80(13), C2–C1–Cu1 124.61(14), C1–
…
…
…
…
Cu1 Cu1A 89.900(5), N1–Cu1 Cu1A 90.097(5), Cu1 Cu1A Cu1B
179.716(15).
*fjaekle@rutgers.edu
1708 | Chem. Commun., 2005, 1708–1710
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˚
Fig. 2 Packing diagram of 3-a. Selected interatomic distances (A) and angles (u) for 3-a: Cu1–C1 1.880(4), Cu1–N1 1.902(3), N1–C5 1.347(3), C1–Cu1–
N1 180.0, C5–N1–Cu1 121.3(2), C2–C1–Cu1 123.3(2).
coplanar to the pyridine rings of adjacent molecules and vice versa
providing a total of eight arene–perfluoroarene interactions per
molecule. The interplanar distance between the aromatic rings
˚
amounts to ca. 3.5 A, typical of strong arene–arene p-stacking
interactions.17 While the copper atoms are also aligned in 3-a, the
…
˚
Cu Cu distances of 3.639 A are much longer than those in 2. A
zigzag arrangement of the copper atoms results from a slight
lateral slippage of the aryl groups typical of perfluoroarene–arene
stacks with an angle of 70.8u between the molecular and the
Fig. 3 Overlap diagram of three molecules of 3-b. Selected interatomic
˚
distances (A) and angles (u) for 3-b: Cu1–C1 1.890(2), Cu1–N1 1.903(2),
N1–C7 1.324(3), N1–C11 1.327(3), C1–Cu1–N1 178.17(8), C7–N1–Cu1
121.0(2), C11–N1–Cu1 122.4(2), C2–C1–Cu1 124.1(2), C6–C1–Cu1
122.4(2).
…
stacking axis (angle Cu Cu1–C1). The polymorph 3-b shows very
…
…
˚
long Cu Cu distances of 5.033 A (Fig. 3). The longer Cu Cu
distances in 3-b compared to 3-a can be traced back to a much
larger offset between the aromatic rings that results in alignment of
the copper atoms at an angle of 43.4u relative to the plane of the
molecules.
and Power’s monomeric arylcopper solvate [C6H2-2,4,6-
t-Bu3Cu(Me2S)].6 However, they are similar to those found by
van Koten et al. for a dimeric complex containing a chelating
7
oxazolinyl group (Cu–C 5 1.899(5) A). The copper–nitrogen
…
Based on the short Cu Cu contacts in 2, which are close to the
˚
18
I
˚
distances are in a very narrow range from 1.902–1.903 A and are
˚
sum of the van der Waals radii of Cu centers of 2.80 A, the
presence of so-called ‘‘cuprophilic’’ interactions has to be
considered. Aurophilic and argentophilic interactions, the attrac-
tive forces between closed-shell d10 metal ions of gold and silver,
have long been recognized and are now commonly accepted.19
7
comparable to those in the oxazolinyl complex (Cu–N 1.902(4) A)
˚
8
and a related pyridylalkyl species [2-(SiMe3)2C(Cu)C5H4N]2
˚
(Cu–N 5 1.910(3) A).
Inspection of the extended structures of these dicoordinate
copper species shows that aggregation leads to supramolecular
stacks that are unprecedented in organocopper chemistry.
Intriguingly, the copper atoms in 2 are arranged in one-
I
I
…
Attractive interactions between closed-shell Cu Cu pairs on the
other hand have only recently been proposed based on experi-
mental and theoretical studies, and the existence of such
interactions is still highly controversial.9 Based on MP2 calcula-
tions Schwerdtfeger et al. determined that cuprophilic interactions
between neutral pairs [RCuL]2 should be attractive by up to
…
dimensional chains with Cu Cu distances of 2.8924(3) A, which
˚
I
I
…
are among the shortest reported for unsupported Cu Cu
contacts (Fig. 1).9 Moreover, 2 represents the first structure of
an organocopper species that is aggregated into linear metal chains
extending throughout the entire crystal lattice.10 Only two
examples of linear polymeric copper chains, [Cu(NH3)2]+ Br2
and [Cu2terpy2]2+ 2X2 (terpy 5 terpyridine),11,12 have been
24 kcal mol21, and that the interaction potential for cuprophilic
˚
interactions is very shallow in the range from ca. 2.5 to 3.5 A.
20
Cuprophilic interactions are therefore likely to be present in 2 and
may also weakly contribute to the overall intermolecular attractive
forces in 3-a. The latter are, however, certainly to a large extent
governed by the multiple perfluoroarene–arene interactions, and
perfluoroarene–arene interactions are overruling any attractive
forces between the copper centers in 3-b.
I
I
…
reported previously. The Cu Cu distances in 2 are similar to
those reported by Wagner and co-workers for {[Cu(NH3)2]Br}n
11
(2.931(1) A). In contrast to these ionic CuI complexes, in which
˚
the counterions X2 may play a significant role in the bonding,9 the
individual units in 2 are neutral RCuL fragments.
Preliminary photophysical studies show that 2 displays strong
blue luminescence (lmax 5 460 nm) in the solid state at RT upon
excitation at l 5 330 nm (Fig. 4). A weaker long-lived emission
band extends to ca. 700 nm. In contrast, the polymorphs 3-a and
3-b are non-emissive at RT and no luminescence was found for 2
at RT in solution. The photophysical properties of CuI aggregates
have previously been discussed in context with cuprophilic
interactions.21 For instance, concentration dependent luminescence
has recently been reported for a trimeric copper pyrazole complex,
{[3,5-(CF3)2Pz]Cu}3.22 Moreover, the rich photoluminescent
The ligands in adjacent RCuL units of 2 adopt a staggered
conformation thus avoiding any p-stacking interactions.13,14 In
contrast, both crystallographically characterized polymorphs of 3,
3-a and 3-b, show supramolecular two-dimensional stacks as a
result of multiple perfluoroarene–arene p-interactions15 (Fig. 2 and
Fig. 3). The observed supramolecular isomerism16 may be traced
back to the flexibility of the central carbon–carbon bond within
the bipyridyl spacer and the linear and near-planar geometry of the
C6F5–Cu–Py fragments. The pentafluorophenyl groups in 3-a are
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retrieving.html (or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
deposit@ccdc.cam.ac.uk). See http://www.rsc.org/suppdata/cc/b4/
b417532h/ for crystallographic data in .cif or other electronic format.
1 (a) J. T. B. H. Jastrzebski and G. van Koten, in Modern Organocopper
Chemistry, ed. N. Krause, Wiley-VCH, Weinheim, 2002, pp. 1–44; (b)
E. Nakamura and S. Mori, Angew. Chem., Int. Ed., 2000, 39, 3750.
2 See for example: (a) A. Camus and N. Marsich, J. Organomet. Chem.,
1970, 21, 249; (b) T. Ikariya and A. Yamamoto, J. Organomet. Chem.,
1974, 72, 145; (c) G. van Koten, J. G. Noltes and A. L. Spek,
J. Organomet. Chem., 1978, 159, 441; (d) E. M. Meyer, S. Gambarotta,
C. Floriani, A. Chiesi-Villa and C. Guastini, Organometallics, 1989, 8,
1067.
3 A. Cairncross, W. A. Sheppard, E. Wonchoba, W. J. Guildford,
C. B. House and R. M. Coates, Org. Synth., 1980, 59, 122.
4 A. Sundararaman, R. A. Lalancette, L. N. Zakharov, A. L. Rheingold
and F. Ja¨kle, Organometallics, 2003, 22, 3526.
5 Structure of CuC6F5 supported by a bulky dialkyne ligand: H. Lang,
K. Ko¨hler, G. Rheinwald, L. Zsolnai, M. Bu¨chner, A. Driess,
G. Huttner and J. Stra¨hle, Organometallics, 1999, 18, 598.
6 X. He, M. M. Olmstead and P. P. Power, J. Am. Chem. Soc., 1992, 114,
9668.
7 E. Wehman, G. van Koten, J. T. B. H. Jastrzebski, M. A. Rotteveel and
C. H. Stam, Organometallics, 1988, 7, 1477.
8 R. I. Papasergio, C. L. Raston and A. H. White, J. Chem. Soc., Chem.
Commun., 1983, 1419.
Fig. 4 Solution absorption and solid-state excitation (lem 5 460 nm) and
emission spectra (lexc 5 330 nm) of 2 at RT.
behavior of stacks and extended chains of related AgI and AuI
species is commonly attributed to the presence of short inter-
molecular contacts.23 The solid-state luminescence of [Au(2-py)]3,
for example, has been related to the supramolecular aggregation of
individual trimers into offset stacks through short intermolecular
…
˚
9 For a recent review see: M. A. Carvajal, S. Alvarez and J. J. Novoa,
Chem. Eur. J., 2004, 10, 2117.
Au Au distances of 3.146(3) and 3.105(2) A based on a
comparison of solid state and solution properties.24 Further
studies on the origin of the photoluminescent behavior of 2
including low temperature measurements are currently in progress
and will likely provide insight into whether the luminescent
properties of 2 may be related to the presence of short copper–
copper contacts.
10 Offset stacks associated by short contacts between three- and four-
coordinate Cu(I) centers have been observed for the organocopper
trimer [Cu(2-py)]3: F. Garc´ıa, A. D. Hopkins, R. A. Kowenicki,
M. McPartlin, M. C. Rogers and D. S. Wright, Organometallics, 2004,
23, 3884.
11 G. Margraf, J. W. Bats, M. Bolte, H.-W. Lerner and M. Wagner, Chem.
Commun., 2003, 956.
12 M. Al-Anber, B. Walfort, S. Vatsadze and H. Lang, Inorg. Chem.
Commun., 2004, 7, 799.
13 The complex C6F5Au(3-picoline) shows p-stacking but no aurophilic
interactions: P. G. Jones and B. Ahrens, Z. Naturforsch., Teil B, 1998,
53, 653.
14 For related discussions on the competition of p-stacking and cuprophilic
interactions, see: (a) U. Siemeling, U. Vorfeld, B. Neumann and
H.-G. Stammler, Chem. Commun., 1997, 1723; (b) X.-M. Zhang,
M.-L. Tong, M.-L. Gong, H.-K. Lee, L. Luo, K.-F. Li, Y.-X. Tong and
X.-M. Chen, Chem. Eur. J., 2002, 8, 3187; (c) J.-P. Zhang, Y.-B. Wang,
X.-C. Huang, Y.-Y. Lin and X.-M. Chen, Chem. Eur. J., 2005, 11, 552.
15 J. H. Williams, Acc. Chem. Res., 1993, 26, 593.
16 B. Moulton and M. J. Zaworotko, Chem. Rev., 2001, 101, 1629.
17 C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885.
18 A. Bondi, J. Phys. Chem, 1964, 68, 441.
19 (a) H. Schmidbaur, Gold Bull., 1990, 23, 11; (b) P. Pyykko¨, Chem. Rev.,
1997, 97, 597.
20 H. L. Hermann, G. Boche and P. Schwerdtfeger, Chem. Eur. J., 2001, 7,
5333.
21 (a) P. C. Ford, E. Cariati and J. Bourassa, Chem. Rev., 1999, 99, 3625;
(b) V. W.-W. Yam and K. K.-W. Lo, Chem. Soc. Rev., 1999, 28, 323; (c)
V. W.-W. Yam, W.-K. Lee, K. K. Cheung, H.-K. Lee and W.-P. Leung,
J. Chem. Soc., Dalton Trans., 1996, 2889.
22 H. V. R. Dias, H. V. K. Diyabalanage, M. A. Rawashdeh-Omary,
M. A. Franzman and M. A. Omary, J. Am. Chem. Soc., 2003, 125,
12072.
In conclusion, coordination of pyridine or 4,49-bipyridine results
in complete breakdown of the tetrameric structure of 1, but
cuprophilic and multiple perfluoroarene–arene interactions in turn
lead to the formation of novel supramolecular structures and, in
the case of 2, the alignment of the copper atoms in one-
dimensional chains. Intriguingly, a comparison of the structures of
2 and 3 indicates that significant cuprophilic interactions in these
compounds are observed only if p-stacking between the aromatic
substituents is avoided as in 2. In the solid state 2 shows intense
blue luminescence, the origin of which we are currently further
investigating.
Acknowledgement is made to the National Science Foundation
(CAREER award to F.J.) and the donors of The Petroleum
Research Fund, administered by the ACS, for support of this
research. We are grateful to Prof. Piotr Piotrowiak and Cynthia
Pagba for help with luminescence measurements.
Anand Sundararaman,a Lev N. Zakharov,b Arnold L. Rheingoldb and
Frieder Ja¨kle*a
aDepartment of Chemistry, Rutgers University Newark, 73 Warren
Street, Newark, NJ 07102, USA. E-mail: fjaekle@rutgers.edu;
Fax: +1 973 353 1264
23 (a) R. L. White-Morris, M. M. Olmstead and A. L. Balch, J. Am. Chem.
Soc., 2003, 125, 1033; (b) E. J. Ferna´ndez, M. C. Gimeno, A. Laguna,
J. M. Lo´pez-de-Luzuriaga, M. Monge, P. Pyykko¨ and D. Sundholm,
J. Am. Chem. Soc., 2000, 122, 7287; (c) Y.-A. Lee and R. Eisenberg,
J. Am. Chem. Soc., 2003, 125, 7778; (d) see also related {Au–Tl}n chains:
E. J. Ferna´ndez, J. M. Lo´pez-de-Luzuriaga, M. Monge, M. E. Olmos,
J. Pe´rez, A. Laguna, A. A. Mohamed and J. P. Fackler, J. Am. Chem.
Soc., 2003, 125, 2022.
bDepartment of Chemistry and Biochemistry, University of California,
San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0358, USA
Notes and references
{ Crystallographic details are given in the Electronic Supplementary
Information (ESI), and CCDC-250638, CCDC-219364, and CCDC-
219363 contain the crystallographic data for 2, 3-a, and 3-b, respectively.
These data can be obtained online free of charge via www.ccdc.ac.uk/conts/
24 A. Hayashi, M. M. Olmstead, S. Attar and A. L. Balch, J. Am. Chem.
Soc., 2002, 124, 5791.
1710 | Chem. Commun., 2005, 1708–1710
This journal is ß The Royal Society of Chemistry 2005