Hexahapto metal coordination to curved polyaromatic hydrocarbon surfaces: The first transition metal corannulene complex

Full text of DOI:10.1021/ja964380t

J. Am. Chem. Soc. 1997, 119, 4781-4782
4781
Chart 1
Hexahapto Metal Coordination to Curved
Polyaromatic Hydrocarbon Surfaces: The First
Transition Metal Corannulene Complex
T. Jon Seiders, Kim K. Baldridge,
Joseph M. O’Connor,* and Jay S. Siegel*
Department of Chemistry and Biochemistry - 0358
UniVersity of California, San Diego, 9500 Gilman DriVe
La Jolla, California 92093-0358
San Diego Supercomputer Center
0100 Johns Hopkins AVenue, La Jolla, California 92137
Although 1 is a structural subunit of 2, and the smallest bowl-
shaped PAH, its surface curvature is substantially less than that
of 2. The carbon substituents on a six-membered ring of 2 are
displaced from the mean plane of the ring by 80 pm, whereas
for 1 that displacement is ca. 50 pm.9 This distinct difference
in surface curvature suggested 1 to be an ideal starting point
for the study of hexahapto metal coordination to curved
networks of trigonal carbon atoms. In addition, 1 also affords
the possibility of metal coordination to either the concave (endo)
or the convex (exo) surface of the carbon network.
ReceiVed December 20, 1996
ReVised Manuscript ReceiVed April 4, 1997
Curved networks of trigonal carbon atoms,1 such as that found
in corannulene (1, Chart 1),1a-e have attracted increased interest
as fragments of buckminsterfullerene (C60, 2) and related carbon
clusters.2 Substantial efforts have been directed toward the syn-
thesis of metal-complexed fullerenes, and a number of dihapto
transition metal complexes of 2 have been prepared.3 Specif-
ically, Fagan et al. reported the reaction of (C5Me5)Ru-
(NCMe)3+O3SCF3- (3) and 2 to give {[(C5Me5)Ru(NCMe)2]3-
C60}3+(O3SCF3-)3 (4), in which each ruthenium is η2-bound to
C60.3a This result is striking in that 3 readily forms η6-arene
complexes with flat polyaromatic hydrocarbons such as coro-
nene (5),4 as well as with highly electron deficient arenes.5,6
Indeed, there have been no examples of hexahapto mononuclear
metal coordination to 27 or any other curved network of trigonal
carbon atoms.8 To this point, we report the reaction of 1 with
3 to give the first transition metal complex of corannulene and
the first example of η6-coordination to a curved polynuclear
aromatic hydrocarbon (PAH) surface.3h-j
When a room temperature dichloromethane-d2 solution of
ruthenium cation 3 (12 mg, 0.03 mmol) and 1 (7.7 mg, 0.03
mmol) is monitored by 1H NMR spectroscopy, the corannulene
resonance at δ 7.84 decreases in intensity and a new set of
-
resonances attributed to (η6-corannulene)Ru(C5Me5)+O3SCF3
(6) grow in at δ 1.65 (s), 6.59 (s), 7.60 (d, J ) 9.0 Hz), 7.95
(d, J ) 9.0 Hz), 8.11 (d, J ) 9.0 Hz), and 8.18 (d, J ) 9.0 Hz).
After approximately 20 h, the ratio of 1/6 stabilized at ca. 1:1
1
and no further change is observed by H NMR spectroscopy
(Keq ) [6][CH3CN]3/[1][3] ) ca. 102 M2). Evaporation of the
volatiles and addition of fresh dichloromethane-d2 solvent leads
to complete conversion of 1 to 6, which was spectrally
characterized by HRMS and various NMR techniques (Figure
1).10,11 Although both electron-rich8a,b and electron-deficient5
(η6-arene)Ru(C5Me5)+O3SCF3- complexes are air-stable, com-
plex 6 decomposes upon exposure to air in both the solid state
and in solution.
(1) (a) Barth, W. E.; Lawton, R. G. J. Am. Chem. Soc. 1966, 88, 380.
(b) Scott, L. T.; Hashemi, M. M.; Meyer, D. T.; Warren, H. B. J. Am.
Chem. Soc. 1991, 113, 7082. (c) Borchardt, A.; Fuchicello, A.; Kilway, K.
V.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1992, 114, 1921. (d)
Zimmermann, G.; Nuechter, U.; Hagen, S.; Nuechter, M. Tetrahedron Lett.
1994, 35, 4747. (e) Liu, C. Z.; Rabideau, P. W. Tetrahedron Lett. 1996,
37, 3437. (f) Scott, L. T.; Bratcher, M. S.; Hagen, S. J. Am. Chem. Soc.
1996, 118, 8743. (g) Sygula, A.; Abdourazak, A. H.; Rabideau, P. W. J.
Am. Chem. Soc. 1996, 118, 339. (h) Seiders, T. J.; Baldridge, K. K.; Siegel,
J. S. J. Am. Chem. Soc. 1996, 118, 2754.
(2) For leading references, see: (a) Seiders, T. J.; Siegel, J. S. Chem.
Br. 1995, 31, 307. (b) Faust, R. Angew. Chem., Int. Ed. Engl. 1995, 34,
1429. (c) Rabideau, P. W.; Sygula, A. Acc. Chem. Res. 1996, 29, 235. (d)
Scott, L. T. Pure Appl. Chem. 1996, 68, 291. (e) Haddon, R. C. Acc. Chem.
Res. 1992, 25, 127.
Complex 4 has been suggested to undergo exchange of free
and bound acetonitrile, concurrent with migration of ruthenium
on the C60 surface.3a Addition of acetonitrile to dichloro-
methane-d2 solutions of isolated 6 leads to arene-nitrile ex-
change and regeneration of 1 and 3. Under ambient conditions,
exchange occurs faster than the time required to obtain the NMR
spectrum. Previously, Mann et. al. established a second-order
rate expression for exchange of arene ligands in (η6-arene)Ru-
(C5Me5)+ complexes with acetonitrile, but the reactions were
much slower.12 Intermolecular arene exchange between the
(3) (a) Fagan, P. J.; Calabrese, J. C.; Malone, B. Science 1991, 30, 3980.
(b) Fagan, P. J.; Calabrese, J. C.; Malone, B. Acc. Chem. Res. 1992, 25,
134. (c) Fagan, P. J.; Calabrese, J. C.; Malone, B. J. Am. Chem. Soc. 1991,
113, 9408. (d) Balch, A. L.; Catalano, V. J.; Lee, J. W. Inorg. Chem. 1991,
30, 3980. (e) Koefed, R. S.; Hudgens, M. F.; Shapley, J. R. J. Am. Chem.
Soc. 1991, 113, 8957. (f) Douthwaite, R. E.; Green, M. L. H.; Stephens, A.
H. H.; Turner, J. F. C. J. Chem. Soc., Chem. Commun. 1993, 1522. (g)
Balch, A. L.; Costa, D. A.; Olmstead, M. M. J. Chem. Soc., Chem. Commun.
1996, 2449. (h) For µ2-η2,η2-C60 diruthenium complexes, see: Mavunkal,
I. J.; Chi, Y.; Peng, S.-M.; Lee, G.-H. Organometallics 1995, 14, 4454. (i)
For a µ2-η2,η2-C60 di(iridium) complex, see: Rasinkangas, M.; Pakkanen,
T. T.; Pakkanen, T. A.; Ahlgre´n M.; Rouvinen, J. J. Am. Chem. Soc. 1993,
115, 4901. (j) For a µ3-η2,η2,η2-C60 triruthenium complex, see: Hsu, H.-
F.; Shapley, J. R. J. Am. Chem. Soc. 1996, 118, 9192.
(8) An interesting class of hexahapto arene metal complexes with the
ring substituents bent out of the plane of the arene is found with cyclophane
ligands: (a) Fagan, P. J.; Ward, M. D.; Calabrese, J. C. J. Am. Chem. Soc.
1989, 111, 1698. (b) Ward, M. D.; Fagan, P. J.; Calabrese, J. C.; Johnson,
D. C. J. Am. Chem. Soc. 1989, 111, 1719. (c) Laganis, E. D.; Finke, R. G.;
Boekelheide, V. Proc. Natl. Acad. Sci. U.S.A. 1981, 78, 2657. (d) de Meijere,
A.; Reiser, O.; Sto¨bbe, M.; Kopf, J.; Adiwidjaja, G.; Sinnwell, V.; Khan,
S. I. Acta Chem. Scand. A 1988, 42, 611. (e) Plitzko, K.-D.; Rapko, B.;
Gollas, B.; Wehrle, G.; Weakey, T.; Pierce, D. T.; Geiger, W. E., Jr.;
Haddon, R. C.; Boekelheide, V. J. Am. Chem. Soc. 1990, 112, 6545.
(9) The six-membered rings in buckminsterfullerene are symmetry
constrained to a plane, whereas for corannulene the six-membered rings
adopt a shallow boat structure with the bow and stern of the boat ca. 10
pm above the boat’s base, as defined by the hub and rim carbons of 1; the
fold angle is approximately 10°.
(4) Cha´vez, I.; Cisternas, A.; Otero, M.; Roma´n, E. Z. Naturforsch. 1990,
45b, 658.
(5) (a) Dembek, A. A.; Fagan, P. J. Organometallics 1995, 14, 3741.
(b) Dembek, A. A.; Fagan, P. J. Organometallics 1996, 15, 1319.
(6) An explanation of this preference for η2 binding can be seen by
considering that the carbon-carbon double bonds of 2 are ideally disposed
toward binding in an η2-fashion with the four carbons attached to the double
bonds bent back by 31°, approximately the same degree of bending found
in many η2-alkene complexes.3a In contrast, the ring substituents of
hexahapto arene complexes show only minor deviations from the plane of
the ring atoms compared to the deviations seen in 2.
(10) For 6: 1H NMR (500 MHz, CD2Cl2) δ 1.63 (s, 15H), 6.59 (s, 2H),
7.60 (d, J ) 9.0 Hz, 2H), 7.95 (d, J ) 9.0 Hz, 2H), 8.11 (d, J ) 9.0 Hz,
2H), 8.18 (d, J ) 9.0 Hz, 2H); 13C{1H} NMR (125 MHz, CDCl3) δ 10.17,
83.94, 93.54, 96.40, 101.63, 124.16, 128.83, 129.79, 132.52, 133.97, 136.89,
140.13; UV-vis (CH2Cl2) λmax 227 nm (ꢀ ) 4.86 × 103), 245 nm (ꢀ )
4.27 × 103), 292 (ꢀ ) 2.59 × 103); HRFABMS M+ actual 487.0986, calcd
487.100
(11) Addition of extra equivalents of 3 resulted in the appearance of a
new set of corannulene 1H NMR resonances presumably due to the
formation of one isomer of (Cp*Ru)2-corannulene2+, but the complex was
not isolated.
(7) Shapley has pointed out3j that the gas phase ion CoC60+ may have a
η6-structure: Kan, S. Z.; Byun, Y. G.; Freiser, B. S. J. Am. Chem. Soc.
1994, 116, 8815.
(12) McNair, A. M.; Mann, K. R. Inorg. Chem. 1986, 25, 2519.
S0002-7863(96)04380-6 CCC: $14.00 © 1997 American Chemical Society

4782 J. Am. Chem. Soc., Vol. 119, No. 20, 1997
Communications to the Editor
Figure 2. Ab initio (RHF 6-31G(d) plus Ru core potentials) structures
of exo- and endo-7 (H’s unlabled).
of ruthenium. We therefore carried out ab initio computations
on the cyclopentadienyl analogue of 6 at the RHF/6-31G(d) level
of theory with split valence effective core potentials for Ru.15
The computational results on (η6-corannulene)Ru(C5H5)+ (7)
predict the exo-7 isomer to be ca. 6 kcal/mol more stable than
the endo-7 isomer (Figure 2); this energy difference at a Tc
comparable to that measured for free 1 would lead to only one
form being observed. The closest nonbonding interaction
between the C5H5 ligand and the corannulene ring in endo-7 is
3.5 Å. Assuming normal bond lengths, the methyls of endo-6
would come into van der Waals contact with the umbrella of
the corannulene fragment. The Ru-to-arene centroid distance
is predicted to be slightly extended in the complexes of 7 as
compared to simple benzene analogues. The cyclopentadienyl
and arene rings remain essentially parallel, and the plane-to-
plane angles for endo-7 and exo-7 is 2.9° and 0.5°, respectively.
The bond lengths of the ring of corannulene bound to Ru show
the normal Dewar-Chatt slight elongation, but nothing signifi-
cant can be said about changes in bond alternation throughout
the corannulene network.
Figure 1. NMR spectral assignments for 6 deduced from COSY, NOE,
and HMQC data. Chemical values in ppm relative to TMS. Relative
NOE intensities are consistent with computed geometry.
corannulene ligand in 6 and benzene also occurs readily. Thus,
addition of a excess of benzene (8.7 × 10-1M) to CD2Cl2
solutions of 6 (8.53 × 10-3 M) at room temperature results
in clean conversion to free corannulene and (η6-C6H6)Ru-
-
(C5Me5)+O3SCF3 over the course of 24 h.
In principle, two types of intramolecular fluxional processes
are possible for 6: migration of the metal on the corannulene
surface and bowl-to-bowl inversion (exo/endo interconversion).
The barrier for bowl-to-bowl inversion in alkyl substituted
corannulenes has been measured at ∼10 kcal/mol.2d,13,1h Al-
though arene migration in RuCp* systems is notoriously slow,
one might assume that the air sensitive character of 6 and its
anomalous propensity toward exchanging the corannulene
fragment for nitriles betray faster intramolecular chemical
dynamics. To address the dynamic behavior of 6, we carried
out variable-temperature 1H NMR spectroscopic studies between
-90 °C (CD2Cl2) and 140 °C (C2D2Cl4). The 1H NMR
resonances for 6 remained unchanged across the entire temper-
ature range, with onset of a slow irreversible decomposition
occuring at 150 °C. Contrary to the scenario described above
based on reactivity, the high-temperature results rule out
intramolecular arene exchange in 6 on the NMR time scale (∆Gq
> 20 kcal/mol). The low-temperature studies are consistent
with either a rapid exo/endo interconversion on the NMR time
scale or a slow exo/endo interconversion for which the popula-
tion of the minor isomer is too low for detection by 500 MHz
NMR spectroscopy.
The results found here impact upon the general question of
how metals bind to curved surfaces. The outlook is promising
that this phenomenon will be further elucidated experimentally
and computationally through organometallic studies of more
highly curved polynuclear aromatic hydrocarbon surfaces.
Acknowledgment. Support by the National Science Foundation
(CHE9628565, GER9550398 [VPW to K.K.B.], CHE9520213) and
ACS-PRF is acknowledged. A gift of 3 was provided by Dr. Paul
Fagan (DuPont). See any current masthead page for ordering and
Internet access instructions.
Supporting Information Available: Spectra (1H and 13C{1H}
NMR, COSY, and HMQC 2-D NMR, HRMS) (6 pages). See any
current masthead page for ordering and Internet access instructions.
Fagan et al. have pointed out that the p-orbitals of 2 are tilted
away from the normal of the six-membered rings which may
weaken overlap of the HOMO and LUMO with ruthenium-
centered unfilled and filled d-orbitals.3a,12 Assuming no unusual
rehybridization effects, the p-orbitals in corannulene tilt out on
the convex face but in on the concave face. On the basis of
Haddon’s POAV analysis, the degrees of p-orbital tilting in 2
and 1 are 11.6° and 8.7°, respectively.14 The spectroscopic data
for 6 do not distinguish between endo- and exo-coordination
JA964380T
(15) The molecular structure of 1, exo-7, and endo-7 have been
determined at the restricted Hartree-Fock (RHF) SCF level of theory with
the analytically determined gradients and the search algorithms contained
within GAMESS,16 using the 6-31Gl(d)17 split valence basis set for all
second row atoms and the Stevens, Basch, Krauss split valence effective
core potentials18 for Ru. The nature of each stationary point was uniquely
characterized by calculating and diagonalizing the matrix of energy second
derivatives (Hessian) to determine the number of imaginary frequencies.
(16) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.;
Su, S.; Windus, T. L.; Elbert, S. T. J. Comput. Chem. 1993, 14, 1347.
(17) Dunning, T. H., Jr.; Hay, P. J. Modern Theoretical Chemistry;
Plenum: New York, 1976; pp 1-28.
(13) Scott, L. T.; Hashemi, M. M.; Bratcher, M. B. J. Am. Chem. Soc.
1992, 114, 1920.
(14) In this context, we define tilt angle as POAV-90, see: (a) Haddon,
R. C. Acc. Chem. Res. 1988, 21, 243. (b) Haddon, R. C. Science 1993,
261, 1545. (c) Haddon, R. C. J. Am. Chem. Soc. 1990, 112, 3385.
(18) Kahn, L. R.; Baybutt, P.; Truhlar; D. G. J. Chem. Phys. 1979, 65,
3826-3853.