The first pentahaptofullerene metal complexes

Article abstract of DOI:10.1021/ja962681x

Cyclopentadienyl metal complexes have played important roles in chemistry owing to their unique structures and functional activities. Here we report the synthesis and characterization of fan entirely new class of cyclopentadienyl (Cp) metal complexes (η⁵-C₆₀Ph₅)MLn (MLn = Li, K, Tl, and Cu·PEt₃). In these molecules, the five Cp carbons represent one pentagon of C₆₀ isolated form the remaining 50 sp² carbon atoms by five surrounding sp³ carbon atoms each bearing a phenyl group. The X-ray crystal structure analysis of the thallium complex. Tl(η⁵-C₆₀Ph₅)·2.5THF revealed it unique and esthetically pleasing C₅ symmetrical molecular structure with the phenyl groups forming a chiral propeller array. The thallium atom is deeply buried in the cavity created by the phenyl groups, bonding to the five Cp carbons (η⁵-coordination) which an averaged Tl-C distance of 2.87 A. The key finding that we made in this research was a remarkable 5-fold addition of an organocopper reagent to C₆₀, which stands in contrast to the monoaddition reaction of Grignard or organolithium reagents.

Full text of DOI:10.1021/ja962681x

12850
J. Am. Chem. Soc. 1996, 118, 12850-12851
The First Pentahaptofullerene Metal Complexes
Masaya Sawamura,* Hitoshi Iikura, and Eiichi Nakamura*
Department of Chemistry, The UniVersity of Tokyo
Hongo, Bunkyo-ku, Tokyo 113, Japan
ReceiVed August 1, 1996
Cyclopentadienyl metal complexes have played important
roles in chemistry owing to their unique structures and functional
activities. Here we report the synthesis and characterization of
an entirely new class of cyclopentadienyl (Cp) metal complexes
(η5-C60Ph5)MLn (MLn ) Li, K, Tl, and Cu‚PEt3, 5-8). In these
molecules, the five Cp carbons represent one pentagon of C60,
isolated from the remaining 50 sp2 carbon atoms by five
surrounding sp3 carbon atoms each bearing a phenyl group. The
X-ray crystal structure analysis of the thallium complex Tl(η5-
C60Ph5)‚2.5THF revealed its unique and esthetically pleasing
C5 symmetrical molecular structure with the phenyl groups
forming a chiral propeller array. The thallium atom is deeply
buried in the cavity created by the phenyl groups, bonding to
the five Cp carbons (η5-coordination) with an averaged Tl-C
distance of 2.87 Å.
The key finding that we made in this research was a
remarkable 5-fold addition of an organocopper reagent to C60,
which stands in contrast to the monoaddition reaction of
Grignard or organolithium reagents.1 The reaction of C60 with
an excess amount of organocopper reagent prepared from
PhMgBr (16 equiv) and CuBr‚SMe2 (16 equiv) followed by
quenching with aqueous NH4Cl gave, in 94% isolated yield,
the pentaphenyl adduct C60Ph5H (1, reddish amorphous solid),2,3
which contains a cyclopentadiene moiety within the spherical
surface of C60 core (Scheme 1). Monitoring (HPLC) of the
reaction throughout the reaction period indicated direct conver-
sion of C60 to 1 without formation of detectable amounts of
mono- to tetra-adducts, and quenching of the reaction with D2O
afforded C60Ph5D with 96% deuterium incorporation. While
the detailed mechanism remains unclear, we speculate that the
five phenyl groups were introduced through sequential additions
of the cuprate Ph2Cu-, as shown in Scheme 1. Thus, the first
addition of one molecule of Ph2Cu- gives the 1,4-bisadduct 2,4
which is more reactive than C60 and accepts readily a second
equivalent of the copper reagent, and finally a third equivalent
introduces the fifth phenyl group to generate the [Cu(η5-C60-
Ph5)(Ph)]- (4) by 1,2-addition to the fulvene moiety in 3.
Figure 1. Molecular structure of Tl(η5-C60Ph5)‚2.5THF (7‚2.5THF)
determined by X-ray crystal structure analysis. Selected bond lengths
(Å): Tl-Cp(centroid), 2.60; Tl-C(1), 2.87(1); Tl-C(2), 2.88(1); Tl-
C(3), 2.90(1); Tl-C(4), 2.83(1); Tl-C(5), 2.85(1); Tl-O 3.16(2);
C(1)-C(2), 1.41(2); C(2)-C(3), 1.41(2); C(3)-C(4), 1.40(2); C(4)-
C(5), 1.45(2); C(5)-C(1), 1.34(2). (a) Front view with weakly
coordinated THF. Lattice-bound THF molecules and all hydrogen atoms
are omitted for clarity. (b) Top view. All THF molecules and hydrogen
atoms are omitted for clarity.
Treatment of a THF-d8 solution of the cyclopentadiene
derivative 1 with an alkali metal alkoxide such as LiOtBu and
KOtBu at room temperature caused color change of the solution
from red to dark red, which accompanied conversion of the Cs
symmetric C60Ph5H (1) to a new species possessing C5V
1
symmetry, as observed by the H and 13C NMR spectra. The
distinctive molecular symmetry and disappearance of the singlet
peak due to the CpH proton (δ 5.31 for 1, CDCl3) led us to
assign the new species as Li(η5-C60Ph5) (5) and K(η5-C60Ph5)
(6), respectively. Similarly a thallium(I) complex Tl(η5-C60-
Ph5) (7) was also prepared by the reaction with TlOEt in THF-
d8. The reaction of 1 with Cu(OtBu)(PEt3)5 in THF-d8 at -70
°C gave a phosphine-coordinated transition metal complex Cu-
(η5-C60Ph5)(PEt3) (8). These complexes (5-8) were found to
be air and moisture sensitive but thermally stable in solution at
room temperature.
(1) Hirsch, A.; Gro¨sser, T.; Skiebe, A.; Soi, A. Chem. Ber. 1993, 126,
1061.
(2) This sample has 99% purity by HPLC analysis. Satisfactory analytical
data was obtained as 1‚0.5toluene. See Supporting Information for
experimental details.
(3) Multistep synthesis of C60Ph5H Via electrophilic aromatic substitution
of benzene with C60Cl6 has been reported, see: Avent, A. G.; Birkett, P.
R.; Crane, J. D.; Darwish, A. D.; Langley, G. J.; Kroto, H. W.; Taylor, R.;
Walton, D. R. M. J. Chem. Soc., Chem. Commun. 1994, 1463.
(4) For examples of 1,4-addition to C60, see: (a) Nagashima, H.; Terasaki,
H.; Kimura, E.; Nakajima, K.; Itoh, K. J. Org. Chem. 1994, 59, 1246. (b)
Schick, G.; Kampe, K.-D.; Hirsch, A. J. Chem. Soc., Chem. Commun. 1995,
2023. (c) Miki, S.; Kitao, M.; Fukunishi, K. Tetrahedron Lett. 1996, 37,
2049. (d) Wang, G.-W.; Murata, Y.; Komatsu, K.; Wan, T. S. M. J. Chem.
Soc., Chem. Commun. 1996, 2059.
(5) Tsuda, T.; Hashimoto, T.; Saegusa, T. J. Am. Chem. Soc. 1972, 94,
658.
S0002-7863(96)02681-9 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 50, 1996 12851
Scheme 1
the phenyl groups at room temperature in solution, the solid
phase structure implies that the enantiomer separation may
become possible by a suitable modification of the aromatic
substituents for inhibition of the ring rotation. The structural
constraints of the fullerene core caused significant deformation
of local structure around the Cp ring, as observed for the five
sp3 carbon atoms connected to the Cp moiety, which are tilted
away from the plane of the Cp ring against the thallium atom
with the average angle of as much as 22°. In spite of the large
structural change around the Cp ring, the C60 core largely retains
the measure of its original spherical form (e.g., as to the distance
between the Cp pentagon and the bottom pentagon and the
diameter along the “equator”). The average lengths of the C-C
double bonds (1.38 Å) and the single bonds (1.44 Å) in the C50
moiety are similar to those of monofunctionalized C60 deriva-
tives.
The UV-vis spectra (CH2Cl2) of both the cyclopentadiene
derivative 1 and the thallium complex 7 show that the 50
π-electron C50 moiety beneath the Cp group is a chromophore
of intensity (ꢀ ) 1.1 × 105 M-1 cm-1 at 240 nm) comparable
to that of the parent C60. The absorption extends to fairly long
wavelengths (670 nm for 1, 720 nm for 7). These properties
suggest that these fullerene derivatives have electronic properties
similar to those of the parent C60.
We presented the first example of η5-fullerene metal com-
plexes. In other reported exohedral fullerene-transition metal
complexes, the fullerenes act as an olefinic ligand carrying a
low-valent, electron-rich metal center in an η2-fashion.8-10 Now
[60]fullerene has become a cyclopentadienyl ligand, which can
potentially accept a wide range of metal atoms and tolerate
various metal oxidation states. Efforts will be continued to
explore the applicability of the new class of cyclopentadienyl
metal complexes.
Crystals suitable for X-ray structure analysis were obtained
for the thallium(I) complex 7 as Tl(η5-C60Ph5)‚2.5THF (dark
red plates) by slow solvent-diffusion techniques with THF/
hexane. The molecular structure and selected bond lengths are
shown in Figure 1.6 The thallium atom is deeply buried in a
cavity created by the five phenyl groups and is equally bonded
to the five Cp carbons (C1-C5) (η5-coordination) with an
average bond length of 2.87 Å (2.83-2.90 Å). This length is
longer by 3% than the value reported for the Tl[η5-C5(CH2-
Ph)5].7 The distance between the thallium atom and the centroid
of the η5-coordinating cyclopentadienyl moiety is 2.60 Å. One
of the THF molecules may have a weak interaction with the
thallium atom (Tl-O, 3.16 Å) while other THF molecules have
no short contacts to the complex. The thallium complex adopts
approximately C5 symmetric conformation with the phenyl
Acknowledgment. We thank the Ministry of Education, Science,
Sports, and Culture for financial support, a Grant-in-Aid for Exploratory
Research.
Supporting Information Available: Experimental details for the
preparation of 1, the 1H and 13C NMR spectra for 5-8, analytical data
for 1 and 7, UV-vis spectra for 1 and 7, and details of the structure
determination of 7‚2.5THF (18 pages). See any current masthead page
for ordering and Internet access instructions.
1
groups arranged in a chiral propeller array. Although the H
and 13C NMR spectra of 7 in THF-d8 indicated free rotation of
(6) Crystal data for Tl(η5-C60Ph5)‚2.5THF (7‚2.5THF): Monoclinic, P21/
n, a ) 27.509(4) Å, b ) 17.498(8) Å, c ) 14.275(3) Å, â ) 104.57(1)°,
V ) 6650(5) Å3, Z ) 4. Data were collected at 25 °C on Rigaku AFC5R
diffractiometer using graphite-monochromated Mo KR radiation to a
maximum 2θ ) 55.2°, giving 11 663 unique reflections; the structure was
solved by direct method (SAPI 91), yielding R ) 0.079, Rw ) 0.110 for
4443 independent reflections with I > 3σ(I).
JA962681X
(8) Fagan, P. J.; Calabrese, J. C.; Malone, B. Acc. Chem. Res. 1992, 25,
134.
(9) Lerke, S. A.; Parkinson, B. A.; Evans, D. H.; Fagan, P. J. J. Am.
Chem. Soc. 1992, 114, 7807.
(7) Schumann, H.; Janiak, C.; Khan, M. A.; Zuckerman, J. J. J.
Organomet. Chem. 1988, 354, 7.
(10) Yamago, S.; Yanagawa, M.; Mukai, H.; Nakamura, E. Tetrahedron
1996, 52, 5091.