μ6-η2:η2:η2:η2:η2:η2 As a new bonding mode for benzene [19]

Full text of DOI:10.1021/ja0016598

J. Am. Chem. Soc. 2000, 122, 8335-8336
8335
metal centers.15,16 However, higher coordination of benzene has
never been achieved. It occurred to us that complexes in which
benzene is coordinated to more than three metals might be
prepared if two adequately chosen polydentate Lewis acid
molecules interact concomitantly with a unique benzene molecule.
The affinity of mercury for aromatic compounds is well docu-
mented. While electrophilic mercuration reactions17 and π-com-
plex formations18 substantiate the high affinity of Hg2+ cations
for arenes, weaker but measurable interactions also occur between
arenes and organomercurials.19 In this contribution, we report that
the reaction of benzene with trimeric o-tetrafluorophenylene
mercury (1)20 leads to the formation of a supramolecule that
contains sandwiched µ6-η2:η2:η2:η2:η2:η2-benzene.
µ6-η2:η2:η2:η2:η2:η2 As a New Bonding Mode for
Benzene
Mitsukimi Tsunoda and Franc¸ois P. Gabba¨ı*
Chemistry Department, Texas A&M UniVersity
3255 TAMU, College Station, Texas 77843-3255
ReceiVed May 15, 2000
With the advent of polyfunctional Lewis acids, the multiple
coordination of electron-rich species has flourished into an area
of relevance to both molecular recognition1-5 and catalysis.6-8
Examination of the factors that govern these chemistries has led
to the discovery of electrophilic complexes that feature electron-
rich species in unusual coordination environments and
geometries.3-5 In particular, both molecular and supramolecular
anionic complexes that contain tetra-, penta-, and hexa-coordinated
halide and pseudohalide anions bound to mercury polydentate
Lewis acids have been reported.3,4,9-11 Such phenomena are not
limited to the case of anions and also include neutral electron-
rich molecules that undergo multiple coordination to the binding
sites of polydentate Lewis acids. While organic substrates such
as nitriles,3 ketones,12 formamides,5,12 and sulfoxide13 have been
involved in such complexes, the interaction of arenes with main
group polydentate Lewis acids has never been studied.
Compound 1 dissolves in boiling benzene. Upon cooling,
followed by slow evaporation of the solvent, crystallization occurs
to afford a quantitative yield of 1‚C6H6 (2). Initial information
on the composition of 2 was gained from elemental analysis21
and thermogravimetric analysis which revealed that, between 70
and 110 °C, benzene loss occurred (6.5% of original weight). As
indicated by MAS 13C NMR spectroscopy, the benzene resonance
of 2 (133.3 ppm) is slightly deshielded when compared to free
benzene (128.0 ppm). Also, this resonance is sharp and suggests
that benzene in 2 exists in a very symmetrical environment. The
infrared spectrum of 2 was dominated by absorption bands of 1.
While the CdC stretching bands of benzene in 2 could not be
observed due to interference with those of 1, the spectrum
permitted the detection of an intense out-of-plane (oop) C-H
deformation band (mode ν4) at 716 cm-1 (525 cm-1 for 1‚C6D6;
νΗ/νD ) 1.36). When compared to that of free benzene, this oop
deformation is shifted to higher energy by δνoop 42 cm-1. It is
interesting to note that this shift is stronger than that observed
when benzene is adsorbed on metal surfaces such as Pd(111)
(δνoop 21 cm-1).22
In an effort to model the sorption of benzene on metal
surfaces,14 the synthesis of molecular complexes that feature
multiply bridging benzene molecules has been investigated. These
studies led to the isolation of a series of compounds in which
benzene interacts in a µ3-η2:η2:η2 fashion with three transition
(1) (a) Shriver, D. F.; Biallas, M. J. J. Am. Chem. Soc. 1967, 89, 1078-
1081. (b) Katz, H. E. J. Org. Chem. 1985, 50, 5027-5032. (c) Horner, J. H.;
Squatritto, P. J.; McGuire, N.; Riebenspies, J. P.; Newcomb, M. Organome-
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F. P. Inorg. Chem. 1997, 36, 5706-5711. (e) Altmann, R.; Jurkschat, K.;
Schuermann, M.; Dakternieks, D.; Duthie, A. Organometallics 1998, 17,
5858-5866. (f) Uhl, W.; Hannemann, F. J. Organomet. Chem. 1999, 579,
18-23.
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Commun. 1998, 897-898.
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(b) Hawthorne, M. F. Pure Appl. Chem. 1994, 66, 245-254.
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T.; Yanovsky, A. I.; Tikhonova, I. A.; Shur, V. B. J. Organomet. Chem. 1997,
536, 413-424.
The result of a single-crystal X-ray analysis23 of 2 revealed
extended stacks that run parallel to one another. Each stack
consists of nearly parallel, yet staggered molecules of 1 that
sandwich benzene molecules (Figure 1). These stacks are rather
(5) (a) Wuest, J. D. Acc. Chem. Res. 1999, 32, 81-89. (b) Vaugeois, J.;
Simard, M.; Wuest, J. D. Coord. Chem. ReV. 1995, 95, 55-73.
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11307-11308.
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M.; Braga, D.; Grepioni, F. J. Mol. Catal. 1992, 74, 61-72. Dyson, P. J.;
Johnson, B. F. G.; Lewis, J.; Martinelli, M.; Braga, D.; Grepioni, F. J. Am.
Chem. Soc. 1993, 115, 9062-9068.
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J. J. Organomet. Chem. 1982, 226, 105-114.
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724 and references therein.
(20) Sartori, P.; Golloch, A. Chem. Ber. 1968, 101, 2004-2009.
(21) Anal. Calcd (found) for C24 H6F12Hg3: 25.64 (25.63); H, 0. 53 (0.51).
(22) (a) Grassian, V. H.; Muetterties, E. L. J. Phys. Chem. 1987, 91, 389-
396. (b) Eng, J., Jr.; Bent, B. E.; Fruhberger, B.; Chen, J. G. J. Phys. Chem.
B 1997, 101, 4044-4054.
(23) Crystal data for 2: C24H6F12Hg3, M 1124.06, rhombohedral, space
group R3hc, a ) 10.9331(13) Å, R ) 105.383(17)°, V ) 1133.0(2) Å3, Z ) 2,
Fcalc ) 3.295 g cm-3. Siemens SMART-CCD area detector diffractometer,
Mo KR radiation (λ ) 0.71069 Å), T ) -163 °C. Crystal size 0.14 × 0.12
× 0.03 mm3, ω-scan mode, measurement range 2.34 e θ e 24.97°, 651 unique
reflections, 587 reflections with I > 2σ(I), µ ) 20.389 mm-1. The structure
was solved by direct methods and refined by full-matrix least squares against
F2 using the SHELXTL/PC (version 5.10) package, 60 parameters, R1 )
0.0365, wR2 ) 0.0865 (all data).
(7) Lee, H.; Diaz, M.; Hawthorne, M. F. Tetrahedron Lett. 1999, 40, 7651-
7655.
(8) (a) Jia, L.; Yang, X.; Stern, C.; Marks, T. J. Organometallics 1994,
13, 3755-3757. (b) Williams, V. C.; Irvine, G. J.; Piers, W. E.; Li, Z.; Collins,
S.; Clegg, W.; Elsegood, M. R. J.; Marder, T. B. Organometallics 2000, 19,
1619-1621.
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Petrovskii, P. V.; Panov, S. Y.; Furin, G. G.; Vol’pin, M. E. J. Organomet.
Chem. 1991, 418, C29-C32. (b) Shur, V. B.; Tikhonova, I. A.; Yanovskii,
A. I.; Struchkov, Y. T.; Petrovskii, P. V.; Panov, S. Y.; Furin, G. G.; Vol’pin,
M. E. Dokl. Akad. Nauk SSSR 1991, 321, 1002-1004. (c) Shur, V. B.;
Tikhonova, I. A.; Dolgushin, F. M.; Yanovsky, A. I.; Struchkov, Y. T.;
Volkonsky, A. Y.; Solodova, E. V.; Panov, S. Y.; Petrovskii, P. V.; Vol’pin,
M. E. J. Organomet. Chem. 1993, 443, C19-C21.
(11) Lee, H.; Diaz, M.; Knobler, C. B.; Hawthorne, M. F. Angew. Chem.,
Int. Ed. Engl. 2000, 39, 776-778.
(12) Tschinkl, M.; Schier, A.; Riede, J.; Gabbai F. P. Organometallics 1999,
18, 1747-1753.
(13) (a) Schmidbaur, H.; O¨ ller, H.-J.; Wilkinson, D. L.; Huber, B. Chem.
Ber. 1989, 122, 31-36. (b) Gabba¨ı, F. P.; Schier, A.; Riede, J.; Tschinkl M.
T. Angew. Chem., Int. Ed. Engl. 1999, 38, 3547-3549.
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10.1021/ja0016598 CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/11/2000

8336 J. Am. Chem. Soc., Vol. 122, No. 34, 2000
Communications to the Editor
Figure 1. Left: Side view of a portion of a stack in the structure of 2.
Color code: fluorine, green; carbon, gray; and mercury, purple. H atoms
omitted for clarity. Right: ORTEP view (50% ellipsoids) showing the
sandwiched benzene molecule. F and H atoms are omitted for clarity.
Figure 3. Schematic interaction diagram depicting the bonding molecular
orbitals between benzene and two molecules of 1. A D3d symmetry is
assumed.
of the two juxtaposed molecules of 1. Assuming a D3d symmetry,
we propose that these interactions result in the formation of a
molecular orbital of A2u symmetry and two degenerate molecular
orbitals of Eg symmetry as shown in Figure 3. These molecular
orbitals result from π-electron donation of the benzene into sets
of available 6p orbitals of mercury. As suggested by the location
of the fluorine secondary ligands, these mercury 6p orbitals do
not point directly toward the center of each C-C bond but are
orientated inward at approximately 30° from the vector defined
by the mercury atom and the C-C bond centroid. While Kekule´-
type distortions have been observed in transition-metal complexes
that feature µ3-η2:η2:η2 benzene ligands,15,16 all benzene C-C
bond lengths in 2 are equivalent and no lengthening could be
established within the error of the X-ray measurement. This
situation reflects the symmetry of the interactions that affect each
C-C bond equally. It also substantiates the modest character of
the interactions that occur between organomercurials and
arenes.19,27
Figure 2. View of the mercury primary and secondary coordination
sphere. Selected bond length (Å) and angles (deg): Hg(1)-C(1) 2.058-
(8) and C(1)-Hg(1)-C(1A), 175.9(5).
compact and exhibit a short centroid distance of 3.24 Å.24 With
a dihedral angle of 8.1° formed between the phenylene ring and
the plane containing the three mercury atoms, the molecules of 1
deviate slightly from planarity and adopt a propeller conformation.
This distortion is only minute and the stacks approach a D3d
symmetry reminiscent of that recently described by Hawthorne
et al. in an octahedral iodide electrophilic sandwich complex.11
The mercury atom is linearly coordinated to the two carbon atoms
of the bound tetrafluorophenylene rings. The secondary coordina-
tion sphere of the mercury center contains two benzene molecules
and two fluorine atoms, the latter belonging to molecules of 1 in
neighboring stacks (Figure 2). The Hg‚‚‚F distance (3.273 Å) falls
within the sum of the van der Waals radii (rvdw(F) ) 1.30-1.38
Å,25 rvdw(Hg) ) 1.73-2.00 Å).26 The distance between the C-C
bond centroid (X) and the mercury center (3.36 Å) is comparable
to those distances found in organomercurial-aromatic complexes
that feature intramolecular mercury-π secondary interactions.19
Further inspection of the secondary coordination sphere of the
mercury center reveals a nearly right F′‚‚‚Hg‚‚‚F′′ angle (92.8°)
and an obtuse X′-Hg-X′′ angle (149.7°).
In conclusion, we report the formation of a supramolecule that
features sandwiched µ6-η2:η2:η2:η2:η2:η2-benzene in a double
face-capping mode. We are presently investigating the interaction
of 1 with acetylene.
Acknowledgment. We acknowledge the support of the Department
of Chemistry at Texas A&M University. We thank Prof. Clearfield for
TGA measurements. M.T. is grateful to the Universidade Federal de Sa˜o
Carlos for allowing his research stay at Texas A&M. The purchase of
the CCD X-ray diffractometer was made possible by a grant from the
NSF (CHE-98 07975).
Note added in proof: We have been informed by Prof. John P.
Fackler, Jr., that his group has observed the formation of stacked structure
involving 1 and trinuclear cyclic gold complexes such as [Au(µ-C2,N3-
bzim)]3 (C2,N3-bzim ) 1-benzylimidazolate). An account of this work
should soon appear in the literature.
It is interesting to note that each of the six C-C bonds of the
benzene molecule interacts with one of the six mercury centers
(24) Distance between centroid of benzene and the centroid of the three
mercury atoms of 1.
(25) Nyburg, S. C.; Faerman, C. H. Acta Crystallogr. Sect. B 1985, 41,
274-279.
(26) Canty, A. J.; Deacon, G. B. Inorg. Chim. Acta 1980, 45, L225-L227.
(27) (a) Kiefer, E. F.; Waters, W. L.; Carlson, D. A. J. Am. Chem. Soc.
1968, 90, 5127-5131. (b) Kitching, W.; Drew, G. M.; Alberts, V. Organo-
metallics 1982, 1, 331-335.
Supporting Information Available: X-ray crystal analysis data for
2 (PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
JA0016598