Strong interfullerene electronic communication in a bisfullerene- hexarhodium sandwich complex

Article abstract of DOI:10.1021/ja047290u

Reaction of Rh₆(CO)₁₂(dppm)₂ (dppm = 1,2-bis(diphenylphosphino)methane) with 1.4 equiv. of C₆₀ in chlorobenzene at 120 °C affords a face-capping C₆₀ derivative Rh₆(CO)₉(dppm)₂(μ₃- η²,η²,η²-C₆₀) (1) in 73% yield. Treatment of 1 with excess CNR (10 equiv., R = CH₂C ₆H₅) at 80 °C provides a bisbenzylisocyanide- substituted compound Rh₆(CO)₇(dppm)₂(CNR) ₂(μ₃-η²,η², η²-C₆₀) (2) in 59% yield. Reaction of 1 with excess C₆₀ (4 equiv.) in refluxing chlorobenzene followed by treatment with 1 equiv. of CNR at room temperature gives a bisfullerene sandwich complex Rh₆(CO)₅(dppm)₂(CNR)(μ₃- η²,η²,η²-C₆₀) ₂ (3) in 31% yield. Compounds 1, 2, and 3 have been characterized by spectroscopic and microanalytical methods as well as by X-ray crystallographic studies. Electrochemical properties of 1, 2, and 3 have been examined by cyclic voltammetry. The cyclic voltammograms (CVs) of 1 and 2 show two reversible one-electron redox waves, a reversible one-step two-electron redox wave, and a reversible one-electron redox wave, respectively, within the solvent cutoff window. This observation suggests that compounds 1 and 2 undergo similar C ₆₀-localized electrochemical pathways up to 1⁵ and 2 ^5-. Each redox wave of 2 appears at more negative potentials compared to that of 1 because of the donor effect of the benzylisocyanide ligand. The CV of compound 3 reveals six reversible well-separated redox waves due to strong interfullerene electronic communication via the Rh₆ metal cluster bridge. The electrochemical properties of 1, 2, and 3 have been rationalized by molecular orbital calculations using the density functional theory (DFT) method. In particular, the molecular orbital (MO) calculation reveals significant contribution of the metal cluster center to the unoccupied molecular orbitals in 3, which is consistent with the experimental result of strong interfullerene electronic communication via the Rh₆ metal cluster spacer.

Full text of DOI:10.1021/ja047290u

Published on Web 07/17/2004
Strong Interfullerene Electronic Communication in a
Bisfullerene-Hexarhodium Sandwich Complex
Kwangyeol Lee,*,† Yoon Jeong Choi,‡ Youn-Jaung Cho,‡ Chang Yeon Lee,‡
Hyunjoon Song,‡ Chang Hoon Lee,† Yoon Sup Lee,*,‡ and Joon T. Park*,‡
Contribution from the Department of Chemistry, Korea UniVersity, Seoul, 136-701, Korea and
National Research Laboratory, Department of Chemistry and School of Molecular Science (BK
21), Korea AdVanced Institute of Science and Technology, Daejeon, 305-701, Korea
Received May 10, 2004; E-mail: kylee1@korea.ac.kr, YoonSupLee@kaist.ac.kr, joontpark@kaist.ac.kr
Abstract: Reaction of Rh6(CO)12(dppm)2 (dppm ) 1,2-bis(diphenylphosphino)methane) with 1.4 equiv. of
C
60 in chlorobenzene at 120 °C affords a face-capping C60 derivative Rh6(CO)9(dppm)2(µ3-η2,η2,η2-C60) (1)
in 73% yield. Treatment of 1 with excess CNR (10 equiv., R ) CH2C6H5) at 80 °C provides a
bisbenzylisocyanide-substituted compound Rh6(CO)7(dppm)2(CNR)2(µ3-η2,η2,η2-C60) (2) in 59% yield.
Reaction of 1 with excess C60 (4 equiv.) in refluxing chlorobenzene followed by treatment with 1 equiv. of
CNR at room temperature gives a bisfullerene sandwich complex Rh6(CO)5(dppm)2(CNR)(µ3-η2,η2,η2-C60)2
(3) in 31% yield. Compounds 1, 2, and 3 have been characterized by spectroscopic and microanalytical
methods as well as by X-ray crystallographic studies. Electrochemical properties of 1, 2, and 3 have been
examined by cyclic voltammetry. The cyclic voltammograms (CVs) of 1 and 2 show two reversible one-
electron redox waves, a reversible one-step two-electron redox wave, and a reversible one-electron redox
wave, respectively, within the solvent cutoff window. This observation suggests that compounds 1 and 2
undergo similar C60-localized electrochemical pathways up to 15- and 25-. Each redox wave of 2 appears
at more negative potentials compared to that of 1 because of the donor effect of the benzylisocyanide
ligand. The CV of compound 3 reveals six reversible well-separated redox waves due to strong interfullerene
electronic communication via the Rh6 metal cluster bridge. The electrochemical properties of 1, 2, and 3
have been rationalized by molecular orbital calculations using the density functional theory (DFT) method.
In particular, the molecular orbital (MO) calculation reveals significant contribution of the metal cluster center
to the unoccupied molecular orbitals in 3, which is consistent with the experimental result of strong
interfullerene electronic communication via the Rh6 metal cluster spacer.
Introduction
or are separated by a single atom spacer. For organic-based
bisfullerenes with longer spacers, no electronic communication
Considerable research efforts have been devoted to the
electrochemical studies of various [60]fullerene derivatives due
to the ability of C60 to accept up to six electrons in a completely
reversible manner.1,2 In particular, bisfullerene compounds with
two electroactive fullerene centers are of special interest, since
the electronic communication between the two C60 moieties has
practical implications for future optical and electronic applica-
tions.3 A number of bisfullerene compounds with various spacers
have been prepared and characterized in order to effect the
electronic communication between the two C60 cages.3 Weak
through-space electronic communication, however, has been
observed only for C120O,4 C120(CH2)2,5 C120C,6 and C120Si-
(C6H5)2,7 where the fullerenes are directly bonded to each other
has been observed. Insertion of organic spacers between the
two C60 cages results in the transformation of the hybridization
of C60 carbon atoms involved in the spacer binding from sp2 to
sp3, and consequently the electronic communication can occur
only through space via overlapped π-orbitals between the two
separate C60 cages.
Metal cluster complexes of C60 exhibit remarkable thermal
and electrochemical stabilities and strong electrochemical
interaction between the two electroactive C60 and metal cluster
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† Department of Chemistry, Korea University.
‡ National Research Laboratory, Department of Chemistry and School
of Molecular Science (BK 21), Korea Advanced Institute of Science and
Technology.
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9
10.1021/ja047290u CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 9837-9844
9837

A R T I C L E S
Lee et al.
Result and Discussion
centers. Furthermore, the electronic communication between C60
and metal cluster centers can be readily fine-tuned with ligands
attached to the metal clusters.8 The metal-C60 π-interaction in
the C60-metal moieties little perturbs the C60 hybridization as
evidenced by earlier studies on self-assembled monolayers9 and
X-ray structural characterization of C60-metal π-complexes,10-12
which implies that the electronic properties of bisfullerene
complexes with a metal cluster spacer are drastically different
from those of organic-based bisfullerenes. In addition, C60-metal
cluster sandwich compounds should serve as direct models for
two carbon nanotubes connected by a heterogeneous inorganic
junction.13 Multiple coordination of C60 to a single metal center,
however, has not been accomplished prior to our study, even
though C60 with a cone angle of 120° is not an exceptionally
bulky ligand.14 Coordination of two electron-withdrawing C60
ligands on a single metal center has been considered to be
energetically unfavorable.
Synthesis and Characterization of 1-3. When a mixture
of Rh6(CO)12(dppm)217 and 1.4 equiv. of C60 in chlorobenzene
was heated at 120 °C, the color of the reaction mixture changed
over 2 h from dark red to green. Removal of the solvent in
vacuo and subsequent purification by column chromatography
(silica gel, CS2/CH2Cl2 ) 4/1) gave a major green solid (1, 73%)
following a purple band of unreacted C60. Reaction of 1 with
10 equiv. of CNR (R ) CH2C6H5) in chlorobenzene at 80 °C
slowly changed the solution from green to brown color over
1h. Removal of the solvent and purification by TLC (silica gel,
CS2/CH2Cl2 ) 4/1) provided a greenish brown solid (2, Rf )
0.1, 59%) as the major product along with several uncharac-
terized minor bands at lower Rf values. The two new compounds
were formulated as Rh6(CO)9(dppm)2(C60) (1) and Rh6(CO)7-
(dppm)2(CNR)2(C60) (2) on the basis of microanalytical and
positive FAB mass spectroscopic data.
Reaction of 1 with 4 equiv. of C60 in refluxing chlorobenzene
for 3 h formed a new green compound identified by analytical
TLC (silica gel), which could not be further characterized
because of its marginal solubility after solvent removal. The
reaction mixture was treated in situ with 1 equiv. of CNR at
room temperature for 90 min. Removal of the solvent and
purification by preparative TLC (silica gel, CS2/CH2Cl2 ) 7/1)
provided a green solid 3 as the major product (Rf ) 0.3, 31%).
Compound 3 exhibited increased solubility and was formulated
as Rh6(CO)5(dppm)2(CNR)(C60)2 on the basis of microanalytical
data and X-ray crystallographic study (vide infra).
The 31P NMR spectrum of 1 shows four multiplet signals of
equal intensities at δ 18.68, 13.94, 10.99, and 8.56 for four
phosphorus atoms of the two dppm ligands, indicating lack of
symmetry in the molecule. The phosphorus resonances of 2 and
3 in the 31P NMR spectra are extensively overlapped, which
renders the peak assignment impossible. The 1H NMR spectrum
of 2, however, shows four multiplets at δ 5.05, 4.89, 4.43, and
4.17 for the two sets of two diastereotopic methylene hydrogens
We have demonstrated that electron-withdrawing C60 cages
can be connected by a cluster bridge (Rh6 or Ir4), when the
cluster bridge is coordinated with electron-donating phosphine
ligands.15,16 The bisfullerene-metal cluster sandwich complexes,
15
Rh6(CO)5(dppm)2(CNR)(µ3-η2,η2,η2-C60)2 and Ir4(CO)3(µ4-
CH)(PMe3)2(µ-PMe2)(CNR)(µ-η2,η2-C60)(µ4-η1,η1,η2,η2-
C60)16 (R ) CH2C6H5), show the presence of unusually strong
electronic communication between the two C60 centers, which
is far stronger than that observed for organic-based bisfullerenes.
To understand the nature of this strong inter-fullerene com-
munication, we have carried out molecular orbital calculations
on a set of face-capping C60-Rh6 cluster compounds, Rh6(CO)9-
(dppm)2(µ3-η2,η2,η2-C60) (1), Rh6(CO)7(dppm)2(CNR)2(µ3-
η2,η2,η2-C60) (2), and Rh6(CO)5(dppm)2(CNR)(µ3-η2,η2,η2-C60)2
(3), in conjunction with their electrochemical property measure-
ments by cyclic voltammetry. Herein, we report full details of
synthesis, characterization, and electrochemical behaviors of
1-3 as well as their theoretical considerations.
in two dppm ligands and two AB patterns at δ 5.05 (JAB
)
16.7 Hz) and 4.70 (JAB ) 16.6 Hz) for the two sets of two
(8) (a) Song, H.; Lee, K.; Park, J. T.; Choi, M.-G. Organometallics 1998, 17,
4477-4483. (b) Song, H.; Lee, Y.; Choi, Z.-H.; Lee, K.; Park, J. T.; Kwak,
J.; Choi, M.-G. Organometallics 2001, 20, 3139-3144. (c) Babcock, A.
J.; Li, J.; Lee, K.; Shapley, J. R. Organometallics 2002, 21, 3940-3946.
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Chem. Commun. 2002, 2966-2967.
(10) (a) Fagan, P. J.; Calabrese, J. C.; Malone, B. Science 1991, 252, 1160-
1161. (b) Balch, A. L.; Catalano, V. J.; Lee, J. W. Inorg. Chem. 1991, 30,
3980-3981. (c) Fagan, P. J.; Calabrese, J. C.; Malone, B. J. Am. Chem.
Soc. 1991, 113, 9408-9409. (d) Bashilov, V. V.; Petrovskii, P. V.; Sokolov,
V. I.; Lindeman, S. V.; Guzey, I. A.; Struchkov, Y. T. Organometallics
1993, 12, 991-992. (e) Balch, A. L.; Lee, J. W.; Noll, N. C.; Olmstead,
M. M. Inorg. Chem. 1993, 32, 3577-3578. (f) Hsu, H.-F.; Du, Y.; Albrecht-
Schmitt, T. E.; Wilson, S. R.; Shapley, J. R. Organometallics 1998, 17,
1756-1761.
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Rouvinen, J. J. Am. Chem. Soc. 1993, 115, 4901. (b) Mavunkal, I. J.; Chi,
Y.; Peng, S.-M.; Lee, G.-H. Organometallics 1995, 14, 4454-4456.
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(b) Lee, K.; Hsu, H.-F.; Shapley, J. R. Organometallics 1997, 16, 3876-
3877. (c) Lee, K.; Shapley, J. R. Organometallics 1998, 17, 3020-3026.
(d) Lee, K.; Lee, C. H.; Song, H.; Park, J. T.; Chang, H. Y.; Choi, M.-G.
Angew. Chem., Int. Ed. 2000, 39, 1801-1804. (e) Lee, K.; Choi, Z.-H.;
Cho Y.-J.; Song, H.; Park, J. T. Organometallics 2001, 20, 5564-5570.
(f) Lee, K.; Song, H.; Park, J. T. Acc. Chem. Res. 2003, 36, 78-86.
(13) (a) Zhang, Y.; Ichihashi, T.; Landree, E.; Nihey, F.; Iijima, S. Science 1999,
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F. G.; Green, M. L. H. Chem. Commun. 2002, 276-277.
diastereotopic benzylic hydrogens in two inequivalent benzyl
1
isocyanide ligands. The H NMR spectrum of 3 exhibits four
multiplets at δ 5.33, 4.90, 4.57, and 3.96 for the four diaste-
reotopic methylene hydrogens in two dppm ligands and an AB
pattern at δ 5.06 (JAB ) 16.7 Hz) for the two diastereotopic
benzylic hydrogens in the benzyl isocyanide ligand.
X-ray Crystallographic Studies of 1-3. Selected crystal-
lographic details for 1, 2, and 3 are shown in Table 1. The
metal-metal bond lengths and the selected distances for the
C60 ligands are listed in Tables 2 and 3, respectively.
Compound 1 has two symmetrically unrelated, but nearly
identical, molecular units in the crystal lattice, and one of them
is shown in Figure 1. The Rh6 octahedral metal framework of
the starting material Rh6(CO)12(dppm)2 remains intact, and the
C60 ligand is face-capping Rh1-Rh2-Rh3 triangle as a µ3-
η2,η2,η2-ligand. The geometries for the remaining ligands are
similar to those in Rh6(CO)12(dppm)2.17
One dppm ligand bridges the Rh3-Rh4 edge and the other
dppm ligand the Rh5-Rh6 edge. Four face-capping µ3-CO
ligands are observed and remaining carbonyls are all terminally
(14) Balch, A. L. In Applications of Organometallic Chemistry in the Preparation
and Processing of AdVanced Materials; Harrod, J. F., Laine, R. M., Eds.;
Kluwer Academic Publishers: Boston, MA, 1995; p283.
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Soc. 2002, 124, 2872-2873.
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2003, 125, 13 920-13 921.
(17) Foster, D. F.; Nicholls, B. S.; Smith, A. K. J. Organomet. Chem. 1982,
236, 395-402.
9
9838 J. AM. CHEM. SOC. VOL. 126, NO. 31, 2004

Bisfullerene−Hexarhodium Sandwich Complex
A R T I C L E S
Table 1. Crystallographic Data for 1, 2, and 3
Table 3. Selected Interatomic Distances (Å) for C60 Ligand in 1,
2, and 3 (1′ Denotes the Symmetrically Unrelated Molecule in the
Crystal Lattice of 1)
t
1
2
3
formula
fw
cryst system
space group
a, Å
b, Å
c, Å
R, deg.
â, deg.
γ, deg.
V, Å3
C
119H44O9P4Rh6
C
133H58N2O7P4Rh6
C
183H47NO5P4Rh6‚
1
1′
2
C6H4Cl2‚2H2O
3415.84
monoclinic
P2(1)/n
17.59(3)
23.98(5)
31.90(6)
90
2358.88
triclinic
P1h
2537.15
triclinic
P1h
Rh1-C1
Rh1-C2
Rh2-C3
Rh2-C4
Rh3-C5
Rh3-C6
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C1
2.19(3)
2.19(3)
2.20(3)
2.13(3)
2.17(3)
2.18(3)
1.35(4)
1.53(4)
1.42(4)
1.49(4)
1.47(4)
1.43(4)
2.24(3)
2.13(3)
2.17(3)
2.16(3)
2.10(3)
2.19(3)
1.46(4)
1.53(4)
1.41(4)
1.58(4)
1.45(4)
1.61(4)
2.23(2)
2.18(2)
2.14(2)
2.20(2)
2.24(2)
2.22(2)
1.48(2)
1.51(2)
1.44(2)
1.48(2)
1.42(2)
1.53(2)
15.499(3)
19.415(4)
33.223(7)
85.07(3)
83.43(3)
73.24(3)
9495(3)
4
14.678(1)
20.035(1)
20.336(1)
95.625(1)
96.939(1)
111.310(1)
5465(1)
2
96.75(4)
90
13360(45)
4
Z
D
calcd, g cm-3
1.650
1.542
1.698
T, K
293(2)
0.71073
1.149
293(2)
0.71073
1.003
20146
12924
0.0860
1.040
0.1029
0.2873
293(2)
0.71073
0.944
109452
18592
0.3367
1.091
0.0848
0.2290
λ (Mo KR), Å
µ, mm-1
3
no. of rflns measd 32181
no. of unique rflns 19943
Rint
Rh1-C1
Rh1-C2
Rh2-C3
Rh2-C4
Rh3-C5
Rh3-C6
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C1
2.19(2)
2.20(1)
2.17(1)
2.14(2)
2.19(2)
2.19(1)
1.42(2)
1.49(2)
1.41(2)
1.49(2)
1.36(2)
1.48(2)
Rh1-C1′
Rh1-C2′
Rh4-C3′
Rh4-C4′
Rh5-C5′
Rh5-C6′
C1′-C2′
C2′-C3′
C3′-C4′
C4-C5′
2.14(2)
2.15(2)
2.21(2)
2.19(2)
2.21(2)
2.19(2)
1.45(2)
1.42(2)
1.40(2)
1.48(2)
1.42(2)
1.50(2)
0.1894
0.941
0.1036
0.2154
goodness of fit
a
R1
wR2b
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑ω(Fo - Fc )2/∑ω(Fo )2]1/2
.
Table 2. Metal-Metal Distances (Å) for 1, 2, and 3 (1′ Denotes the
Symmetrically Unrelated Molecule in the Crystal Lattice of 1)
C5′-C6′
C6′-C1′
1
1′
2
3
Rh1-Rh2
Rh1-Rh3
Rh1-Rh4
Rh1-Rh5
Rh2-Rh3
Rh2-Rh5
Rh2-Rh6
Rh3-Rh4
Rh3-Rh6
Rh4-Rh5
Rh4-Rh6
Rh5-Rh6
2.734(4)
2.725(4)
2.770(4)
2.792(4)
2.768(4)
2.785(4)
2.790(4)
2.732(4)
2.860(4)
2.757(4)
2.789(4)
2.765(4)
2.751(4)
2.730(4)
2.795(4)
2.786(4)
2.772(4)
2.771(4)
2.794(4)
2.722(4)
2.868(4)
2.759(4)
2.799(4)
2.768(4)
2.774(2)
2.769(2)
2.808(2)
2.813(2)
2.802(2)
2.830(2)
2.861(2)
2.748(2)
2.881(2)
2.810(2)
2.837(2)
2.813(2)
2.690(4)
2.649(5)
2.676(5)
2.698(5)
2.743(4)
2.760(4)
2.772(5)
2.746(4)
2.793(6)
2.757(4)
2.751(4)
2.764(5)
bonded. One µ3-CO ligand is capping the Rh(4, 5, 6) triangle
which is trans to the C60 coordinated on the Rh(1, 2, 3) triangle,
and the other three µ3-CO ligands are disposed in a fashion to
form a tetrahedron composed of four µ3-CO ligands as observed
in Rh6(CO)12(dppm)2.17 The molecular structure of 2 is shown
in Figure 2, and the general structural feature is similar to that
of compound 1. The only difference is that two terminal
carbonyl ligands on Rh2 and Rh4 are displaced by two terminal
CNR ligands. The Rh-C (C60) distances in 2 do not alternate
in length, while the C-C distances of the ligated C6 ring of
C60 ligand reveal bond alternation (av. 1.45 and 1.50 Å,
respectively). Overall, the bonding parameters for the µ3-
η2,η2,η2-C60 ligands in 1 and 2 are similar to those in other
related µ3-η2,η2,η2-C60 cluster systems.12
The molecular structure of 3 is shown in Figure 3. The two
µ3-η2,η2,η2-C60 ligands are face-capping Rh(1, 2, 3) and Rh(1,
4, 5) triangles, respectively, of the Rh6 octahedral metal
framework. The coordination environments of the two C60
ligands are different from each other; the Rh(1, 2, 3) triangle is
coordinated by an isocyanide ligand and a phosphorus atom
(P(1)) of a dppm ligand, while the Rh(1, 4, 5) triangle is
coordinated by two phosphorus atoms (P(2,3)), each from the
two dppm ligands. Interestingly, the Rh1 atom is bonded to both
of the face-capping C60 ligands in an η2- mode. Although the
Rh1 atom is coordinated by two C60 ligands, the Rh1-C (C60)
Figure 1. Molecular geometry and atomic-labeling scheme for 1. Phenyl
groups except ipso carbons are removed for clarity.
bond distances (Rh1-C1 ) 2.18(2) Å; Rh1-C2 ) 2.18(1) Å;
Rh1-C1′ ) 2.15(2) Å; Rh1-C2′ ) 2.14(2) Å) are comparable
to the other Rh-C (C60) distances (av. 2.18 Å) in 3, implying
that the electron-deficient nature of C60 is much compensated
for by the remote CNR and dppm donor ligands. The ligated
C6 ring of the C60 ligand on Rh(1, 2, 3) triangle shows
alternation in C-C bond distances (av. 1.45 and 1.48 Å,
respectively), but no systematic bond alternation is observed
either in the C6 ring of the other C60 ligand on the Rh(1, 4, 5)
triangle or in the Rh-C (C60) distances. A terminal isocyanide
ligand is coordinated on the Rh2 atom, and the geometries for
the remaining ligands are similar to those in 1 and 2.
Electrochemical Studies. Electrochemical properties of Rh6-
(CO)12(dppm)2, 1, 2, 3 as well as Os3(CO)8(PPh3)(µ3-η2,η2,η2-
C60) 8a and Os3(CO)8(CNCH2C6H5)(µ3-η2,η2,η2-C60)18 in chlo-
robenzene have been examined by cyclic voltammetry with
9
J. AM. CHEM. SOC. VOL. 126, NO. 31, 2004 9839

A R T I C L E S
Lee et al.
Figure 4. CVs of Rh6(CO)12(dppm)2, 1, and 2 in chlorobenzene (scan rate
) 10 mV/s).
Figure 2. Molecular geometry and atomic-labeling scheme for 2. Phenyl
groups except ipso carbons are removed for clarity.
Figure 5. CV of compound 3 in chlorobenzene (scan rate ) 10 mV/s).
Table 4. Half-Wave Potentials (E1/2 vs E°Fc/Fc+) of C60
,
Rh6(CO)12(dppm)2, 1, 2, and 3 in Chlorobenzene
1
2
3
4
5
6
compound
E
E
E
E
E
E
1/2
1/2
1/2
1/2
1/2
1/2
C60
-1.06
-1.43 -1.91
-2.38
Rh6(CO)12(dppm)2 -1.96a,b
1
2
3
-1.23
-1.35
-1.19
-1.58 -1.94a -2.57
-1.70 -2.25 a -2.71
-1.38 -1.62
-1.86 -2.12 -2.41
a Two-electron process. b Peak potential of irreversible process.
processes, but the third redox wave corresponds to a two-
electron process. The first two reductions for 1 and 2 occur at
much more positive potentials compared to those of the Rh6
metal center (Ered ) -1.96 V for the parent molecule Rh6(CO)12-
(dppm)2), and thus can be confidently assigned to the C60-
localized successive reductions. The first two waves of 1 are
shifted to more negative potentials by 0.17 and 0.15 V,
respectively, relative to those of free C60. Such cathodic shifts
are consistent with the presence of strongly electron-donating
dppm ligands. The first two waves of 2 are further shifted to
more negative potentials by 0.12 V compared to 1, respectively,
due to the additional donor effect of the two isocyanide ligands.
The third (two-electron reduction) and the fourth (one-electron
reduction) waves in 1 and 2 are assigned to the redox processes
also localized at the C60 ligand, based on the molecular orbital
calculation results of these molecules (vide infra). It is notable
that complexes 1 and 2 demonstrate high electrochemical
stability without showing a metal cluster reduction process
within the solvent cutoff window. The CV of the parent
molecule Rh6(CO)12(dppm)2, however, exhibits a one-step two-
electron reduction at -1.96 V and two consecutive oxidation
steps (-1.42 and -1.29 V) at the Rh6 metal center. Similar
irreversible behaviors have been observed for several metal
carbonyl clusters, where loss of a carbonyl and structural change
are generally accompanied by the two-electron reduction step.19
The face-capping µ3-η2,η2,η2-C60 bonding interaction, appar-
ently, greatly alters the electronic nature of the Rh6 cluster unit,
Figure 3. Top: Molecular geometry and atomic-labeling scheme for 3.
Phenyl groups except ipso carbons are removed for clarity. Bottom:
Expanded view of the cluster core.
tetrabutylammonium perchlorate as the supporting electrolyte.
Cyclic voltammograms (CVs) of Rh6(CO)12(dppm)2, 1, and 2
are shown in Figure 4 and that of 3 is shown Figure 5. Half-
wave potentials (E1/2) of free C60, Rh6(CO)12(dppm)2, 1, 2, and
3 are summarized in Table 4.
The CVs of both 1 and 2 exhibits similar four reversible redox
couples up to 15- and 25- within the solvent potential window.
The first two and the fourth redox waves are due to one-electron
(18) Song, H.; Lee, C. H.; Lee, K.; Park, J. T. Organometallics 2002, 21, 2514-
2520.
9
9840 J. AM. CHEM. SOC. VOL. 126, NO. 31, 2004

Bisfullerene−Hexarhodium Sandwich Complex
A R T I C L E S
Table 5. Half-Wave potentials (E1/2 vs E°Fc/Fc+) of C60
,
Table 6. DFT Calculation Results for 1 and 2
Os3(CO)9(µ3-η2,η2,η2-C60) (4), Os3(CO)8(PMe3)(µ3-η2,η2,η2-C60) (5),
Os3(CO)8(PPh3)(µ3-η2,η2,η2-C60) (6), and
orbital
C
60
cluster
character (%)
Os3(CO)8(CNR)(µ3-η2,η2,η2-C60) (7) in Chlorobenzene
orbital no.
energy (eV)
character (%)
1
2
1
2
compound
E
1/2
E
ref
1/2
499(LUMO + 2)
498(LUMO + 1)
497(LUMO)
-2.5062
-2.7995
-2.9767
-4.9285
91
85
89
19
9
15
11
81
C60
4
5
6
7
-1.06
-0.98
-1.06
-1.07
-1.06
-1.43
-1.33
-1.42
-1.43
-1.43
this work
8a
8a
496(HOMO)
this work
this work
547(LUMO + 2)
546(LUMO + 1)
545(LUMO)
-2.3862
-2.5157
-2.8305
-4.4834
89
92
83
17
11
8
17
83
resulting in the observed C60-localized reductions and high
electrochemical stabilities of 1 and 2. Similar increased elec-
trochemical stability has been previously demonstrated for
related C60-metal cluster complexes with a face-capping µ3-
η2,η2,η2-C60 ligand.8a,b
544(HOMO)
The CV of 3 exhibits six well-separated reversible, one-
electron redox waves as shown in Figure 5. Each redox wave
of 3 is ascribed to sequential, pairwise addition of six electrons
into the two C60 centers to form C60--Rh6-C60, C60--Rh6-
3-
C60-, C602--Rh6-C60-, ..., and ultimately C603--Rh6-C60
.
All three pairs of redox waves are shifted to more negative
potentials (0.13 and 0.32 V; 0.19 and 0.43 V; 0.21 and 0.50 V)
relative to free C60. The absence of Rh6 cluster reduction wave
for 3 in the solvent window could be explained by significant
decrease in electron affinity of the cluster framework due to
coordination of two, electron-rich polyanionic C60 ligands, which
are generated during electrochemical studies. This observation
suggests a strong electronic communication between the metal
cluster and C60 centers. There is little difference between donor
effects of a phosphine ligand and a benzyl isocyanide ligand in
C60-metal cluster complexes, which lead to the negative shifts
(0.08∼0.09 V) of C60 reduction potentials compared to the
parent carbonyl complex; Os3(CO)9(µ3-η2,η2,η2-C60), Os3(CO)8-
(PMe3)(µ3-η2,η2,η2-C60), Os3(CO)8(PPh3)(µ3-η2,η2,η2-C60), and
Os3(CO)8(CNCH2C6H5)(µ3-η2,η2,η2-C60) exhibit C60-localized
first redox wave (E1/20/-1) at -0.98, -1.06, -1.07, and -1.06
V, respectively, as shown in Table 5. The donor effect of a
phosphorus end of a dppm ligand should be very similar to that
of a benzyl isocyanide ligand, and thus the electronic environ-
ment for the two C60 centers is essentially the same despite the
Figure 6. Diagrams for frontier MOs of 1(a) and 2(b).
compounds, 1, 2, and 3, in an effort to understand electrochemi-
cal properties of these complexes.
Table 6 shows energies and contribution of component parts
for selected molecular orbitals (MOs) of monofullerene com-
plexes, 1 and 2. The diagrams for MOs of 1 and 2 are displayed
in Figure 6 and the similarity between both sets of MOs is
clearly seen. Highest occupied molecular orbitals (HOMOs) for
both compounds are mainly metal cluster-based, but with
nonnegligible atomic orbital contributions (19% (1) and 17%
(2)) from metal bonded carbon atoms in the C60 unit, which
implies a strong ground-state interaction between C60 and Rh6
cluster centers. The unoccupied MOs are C60-based, but the
metal cluster contributions to these MOs are not negligible (11%,
15%, and 9% for 1; 17%, 8%, and 11% for 2; see Table 6).
Metal cluster contribution to C60-based unoccupied orbitals to
a similar extent has been previously reported for a related C60-
cluster complex Ru3(CO)9(µ3-η2,η2,η2-C60).21
difference in coordination spheres around the two C60 centers.
3
The large peak separations (∆(E1/21, E1/22) ) 0.19 V, ∆(E1/2
,
E1/24) ) 0.24 V, ∆(E1/25, E1/26) ) 0.29 V) in the three redox
pairs of the two C60 ligands, therefore, reflect a strong electronic
The four reversible redox waves of 2 (Figure 4) are shifted
to negative potentials by 0.12, 0.12, 0.31, and 0.14 V as
compared to those of 1 due to the presence of electron-donating
benzylisocyanide ligands. While the values of negative potential
shift for the three one-electron redox waves are similar to one
another, the shift value of 0.31 V for the third, one-step two-
electron reduction wave in 2 is abnormally high. Assuming that
the C60-based unoccupied orbitals are sequentially filled by
added electrons, this behavior could be rationalized by compar-
ing the energies of unoccupied orbitals of the two compounds
as following. The LUMO and LUMO + 1 are responsible for
the first two one-electron and the third two-electron reduction
steps, respectively. Much larger energy difference was observed
between LUMO (orbital No. 545) and LUMO + 1 (orbital No.
546) of 2 compared to those (orbitals No. 497 and 498) of 1:
communication between the two C60 centers via the Rh6 spacer.
3
Much smaller peak separations (∆(E1/21, E1/22) ) 0.09 V, ∆(E1/2
,
E1/24) ) 0.08 V, ∆(E1/25, E1/26) ) 0.14 V) have been observed
7
for C120Si(C6H5)2 , which represents the strongest electronic
interaction reported for noncluster-bridged bisfullerene com-
pounds.
Theoretical Calculations of 1-3. Density functional theory
(DFT) using B3LYP20 hybrid functional was employed to
investigate the molecular orbitals of fullerene-metal complex
(19) (a) Geiger, W. E.; Connelly, N. G. AdV. Organomet. Chem. 1985, 24, 87-
130. (b) Johnson, B. F. G.; Lewis, J.; Nelson, W. J. H.; Nicholls, J. N.;
Puga, J.; Raithby, P. R.; Rosales, M. J.; Schroder, M.; Vargas, M. D. J.
Chem. Soc., Dalton Trans. 1983, 2447-2457. (c) Clark, R. J. H.; Dyson,
P. J.; Humphrey, D. G.; Johnson, B. F. G. Polyhedron 1998, 17, 2985-
2991. (d) Shephard, D. S.; Johnson, B. F. G.; Harrison, A.; Parsons, S.;
Smidt, S. P.; Yellowlees, L. J.; Reed, D. J. Organomet. Chem. 1998, 563,
113-136.
(20) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(21) Lynn, M. A.; Lichtenberger, D. L. J. Cluster Sci. 2000, 11, 169-188.
9
J. AM. CHEM. SOC. VOL. 126, NO. 31, 2004 9841

A R T I C L E S
Lee et al.
Table 7. DFT Calculation Results for 3
680 and No. 681, respectively, suggest that the electronic
information of one C60 ligand is efficiently communicated
through the Rh6 cluster to the other C60 ligand. More interest-
ingly, the two C60 ligands contribute more or less evenly to the
higher lying MOs, No. 682 (61% and 28%) and No. 683 (32%
and 51%), in conjunction with nonnegligible metal cluster
contributions of 11% and 17%. The electrons in these MOs are
delocalized over the entire molecule to result in stronger
repulsion between electrons in these MOs, which in turn would
lead to the observed large peak separation in the third redox
pair in the CV of 3. This delocalization was not anticipated in
our earlier interpretation based on CV data alone.
C60 character (%)
orbital
cluster
character (%)
orbital no.
energy (eV)
C60 (left)
C
60 (right)
685(LUMO + 5)
684(LUMO + 4)
683(LUMO + 3)
682(LUMO + 2)
681(LUMO + 1)
680(LUMO)
-2.1848
-2.2509
-2.3037
-2.3511
-2.4232
-2.5693
-4.3745
75
4
32
61
1
9
80
51
28
69
1
16
16
17
11
30
38
76
61
12
679(HOMO)
12
Conclusion
Compounds 1 and 2 have been proposed to undergo C60-
localized reversible redox processes up to 15- and 25- within
the solvent cutoff window, based on cyclic voltammetric studies
and MO calculations. The bisfullerene metal sandwich complex
3 reveals an interesting structural feature of a Rh metal atom
interconnecting two C60 cages and unusually strong electronic
communication between the two C60 centers. The MO calcula-
tion of 3 reveals that the strong interfullerene electronic
interaction has been possible by the strong contribution of the
cluster unit, which acts as a passage for electronic communica-
tion, to the unoccupied MOs of this compound. The unique
electronic behavior of the C60-metal cluster sandwich compound
and its theoretical consideration by MO calculation described
herein should have a direct relevance to two carbon nanotubes
connected by a heterogeneous inorganic junction, which might
find useful applications in future electronic materials.
Figure 7. Diagrams for frontier MOs of 3.
∆E (orbitals No. 497, 498) ) 0.1771 eV for 1 and ∆E (orbitals
No. 545, 546) ) 0.3148 eV for 2.
Calculated results for selected MOs for 3 are shown in Table
7, and the diagrams for these MOs are depicted in Figure 7.
The HOMO for 3 is mainly metal cluster-based (76%) with
some atomic orbital contributions (12% and 12%) from metal
bonded carbon atoms in the two C60 units as similarly observed
for 1 and 2. The unoccupied MOs are C60-based, but again
significant metal cluster contributions to the MOs are observed
(38%, 30%, 11%, 17%, 16%, and 16%). In addition, contribu-
tions from the two C60 units show a pairwise behavior in the
three sets of adjacent unoccupied MOs (No. 680 and 681; No.
682 and 683; No. 684 and 685), which explains the alternating
electron additions to the two C60 units in 3. The spin pairing
energies, apparently, exceeds the energy gaps among unoccupied
MOs of 3.
Experimental Section
General Comments. All reactions were carried out under a
nitrogen atmosphere with use of standard Schlenk techniques.22
Solvents were dried appropriately before use. C60 (99.5%,
Southern Chemical Group, LLC.), Rh6(CO)16 (Strem Chemi-
cals), and CNR (R ) CH2C6H5, Aldrich) were used without
further purification. Rh6(CO)12(dppm)2,17 Os3(CO)8(PPh3)(µ3-
η2,η2,η2-C60),8a and Os3(CO)8(CNR)(µ3-η2,η2,η2-C60)18 were
prepared by the literature methods. Preparative thin layer plates
were prepared with silica gel GF254 (Type 60, E. Merck).
Infrared spectra were obtained on a Bruker EQUINOX-55
FT-IR spectrophotometer. 1H (400 MHz) spectra were recorded
on a Bruker Avance-400 spectrometer. 31P (122 MHz) NMR
spectra were recorded on a Bruker AM-300 spectrometer. All
m/z values are referenced to 103Rh.
Preparation of Rh6(CO)9(dppm)2(µ3-η2,η2,η2-C60) (1). A
chlorobenzene (150 mL) solution of Rh6(CO)12(dppm)2 (600
mg, 0.339 mmol) and C60 (350 mg, 0.486 mmol) prepared in a
250 mL three-neck flask equipped with a reflux condenser was
heated at 120 °C for 2 h. The solution color gradually changed
from red to green. The solvent was removed under vacuum,
and the dark residue dissolved in minimal amount of carbon
disulfide was placed on top of a silica gel column (5 × 20 cm).
Elution by CS2/CH2Cl2 (4/1) afforded a brownish green solid
of 1 (582 mg, 0.247 mmol, 73%) following a purple band of
C60. IR (CS2) νCO 2029(vs), 2006(s), 1984(m), 1743(vs), 1719-
(vs) cm-1; 1H NMR (1,2-C6D4Cl2, 298 K) δ 8.2-7.2 (m, 40H),
The second redox wave in each pair in the CV of 3 (Figure
5) becomes increasingly separated from the first wave as the
reduction proceeds (∆(E1/23, E1/24) - ∆(E1/21, E1/22) ) 0.05 V;
∆(E1/25, E1/26) - ∆(E1/23, E1/24) ) 0.05 V). Similar trend was
4
7
3
also observed in C120O and C120(SiPh2) (C120O: ∆(E1/2
,
E1/24) - ∆(E1/21, E1/22) ) 0.02 V; ∆(E1/25, E1/26) - ∆(E1/23, E1/2
)
4
) 0.08 V; C120(SiPh2): ∆(E1/23, E1/24) - ∆(E1/21, E1/22) ) -0.01
V; ∆(E1/25, E1/26) - ∆(E1/23, E1/24) ) 0.06 V), and was proposed
to result from the effects of increasing Coulombic repulsion.
The increase in the separation within the redox pairs of 3,
however, cannot be explained solely by the stronger Coulombic
repulsion, because the increase in redox pair separation of C120O4
and C120(SiPh2)7 with much shorter interfullerene distances of
∼1.5 Å are only comparable, if not smaller. Careful examination
of the MOs of 3 gives further insight on the observed
electrochemical behavior of this compound. The significant
metal cluster contributions of 38% and 30% to the MOs of No.
(22) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-SensitiVe
Compounds, 2nd ed.; Wiley: New York, 1986.
9
9842 J. AM. CHEM. SOC. VOL. 126, NO. 31, 2004

Bisfullerene−Hexarhodium Sandwich Complex
A R T I C L E S
5.03 (m, 1H), 4.75 (m, 1H), 4.32 (m, 1H), 4.21 (m, 1H); 31P-
{1H} NMR (1,2-C6D4Cl2, 298 K) δ 18.68 (m, 1P), 13.94 (m,
1P), 10.99 (m, 1P), 8.56(m, 1P); MS (FAB+) m/z 2358 (M+).
Anal. Calcd. for C119H44O9P4Rh6: C, 60.59; H, 1.88. Found:
C, 60.34; H, 2.01.
Preparation of Rh6(CO)7(dppm)2(CNR)2(µ3-η2,η2,η2-C60)
(R ) CH2C6H5) (2). A chlorobenzene (20 mL) solution of 1
(40.0 mg, 0.017 mmol) and CNR (20.0 mg, 0.171 mmol) was
prepared in a 100 mL three-neck flask equipped with a reflux
condenser. The reaction mixture was heated at 80 °C for 1 h,
and the solution color slowly changed from green to brown.
Solvent removal in vacuo and separation by TLC (silica gel,
CS2/CH2Cl2 (4/1)) gave a greenish brown solid of 2 (Rf ) 0.1,
24.9 mg, 0.010 mmol, 59%) as the major product. IR (CH2Cl2)
crystallographic data are given in Table 1. The structures were
determined by direct methods;24 positions for the rhodium atoms
were deduced from an E map. One cycle of isotropic least-
25
squares refinement (SHLEX97) followed by an unweighted
difference Fourier synthesis revealed positions for the rhodium
and remaining non-hydrogen atoms. Hydrogen atoms were not
included in the final structure factor calculations. Successful
convergence of full-matrix least-squares refinement on F2 was
indicated by the maximum shift/error for the final cycle. The
final difference Fourier map had no significant features. The
metal-metal distances and the selected distances for C60 ligands
in 1, 2, and 3 are shown in Table 2 and Table 3, respectively.
Electrochemical Measurements. Cyclic voltammetry and
chronoamperometry were carried out a BAS 100B (Bioanalytical
Systems, Inc.) electrochemical analyzer using the conventional
three electrode system of a platinum working electrode (1.6 mm
diameter disk, Bioanalytical Systems, Inc.), a platinum counter
wire electrode (5 cm length of 0.5 mm diameter wire), and a
Ag/Ag+ reference electrode (0.1 M AgNO3/Ag in acetonitrile
with a Vycor salt bridge). All measurements were performed
at ambient temperature under nitrogen atmosphere in a dry
deoxygenated 0.1 M chlorobenzene solution of [(n-Bu)4N]ClO4.
The concentrations of compounds were ca. 3 × 10-4 M. All
potentials were referenced to the standard ferrocene/ferrocenium
(Fc/Fc+) scale. Relative number of electrons involved in each
reduction process was obtained from the graph of current vs
(time)-1/2 according to the Cottrell equation.26
ν
CO 1996(s, sh), 1990(s), 1970(m, sh), 1734(vs), 1714(vs) cm-1
νCN 2160(m), 2138(w, sh) cm -1; H NMR (1,2-C6D4Cl2, 298
K) δ 8.2-7.2 (m, 50H), 5.05 (2H, JAB ) 16.7 Hz, CNCH2Ph),
5.05 (overlapped, m, 1H) 4.89 (m, 1H), 4.70 (2H, JAB ) 16.6
Hz, CNCH2Ph), 4.43 (m, 1H), 4.17 (m, 1H); 31P{1H} NMR
(1,2-C6D4Cl2, 298 K) δ 19.37-8.37 (m, 4P); MS (FAB+) m/z
2536 (M+). Anal. Calcd. for C133H58O7N2P4Rh6: C, 62.96; H,
2.30; N, 1.10. Found: C, 62.64; H, 2.56; N, 0.98.
1
Preparation of Rh6(CO)5(dppm)2(CNR)(µ3-η2,η2,η2-C60)2
(3). A chlorobenzene (50 mL) solution of 1 (50.0 mg, 0.021
mmol) and C60 (61.2 mg, 0.085 mmol 4equiv) prepared in a
100 mL three-neck flask equipped with a reflux condenser was
heated at 132 °C for 3 h. The reaction mixture was cooled to
room temperature. To the reaction mixture was added dropwise
1 equiv of CNR in 1 mL chlorobenzene, and the solution was
stirred for 90 min. Solvent removal in vacuo and separation by
TLC (silica gel, CS2/CH2Cl2 (7/1)) gave a greenish brown solid
of 3 (Rf ) 0.3, 19.9 mg, 0.006 45 mmol, 31%) as the major
Computational Details. Density functional theory (DFT)
using B3LYP20 hybrid functional was employed to investigate
the molecular orbitals of fullerene-metal complex compounds.
B3LYP functional has been widely used to calculate molecular
orbitals of fullerene and fullerene-derivatives.27,28 The coordi-
nates for 1, 2, and 3 were taken directly from the respective
crystal structures without further structural optimization.
1
product. IR (CS2) νCO 1987 (s, br) cm-1; H NMR (1,2-C6D4-
Cl2, 298 K) δ 8.4-6.9 (m, 45H), 5.33 (m, 1H), 5.06 (2H, J )
16.7 Hz, CNCH2Ph), 4.90 (m, 1H), 4.57 (m, 1H), 3.96 (m, 1H);
31P{1H} NMR (1,2-C6D4Cl2, 298 K) δ 18.3-12.9 (m, 3P), 5.96
(m, 1P); Anal. Calcd. for C183H51O5NP4Rh6: C, 71.25; H, 1.67;
N, 0.45. Found: C, 71.08; H, 2.01; N, 0.46.
To reduce the number of electrons treated, relativistic effective
core potentials with corresponding basis sets were used for Rh
atoms.29 The 3-21+G* basis set was used for C, N, O, and P
atoms. The carbon atoms of the C60 ligand were calculated using
6-311G* basis set for compounds 1 and 2 and 6-31G* basis set
for 3.
X-ray Crystallographic Studies. Crystals of 1, 2, and 3
suitable for an X-ray analysis were grown by slow solvent
diffusion; for 1 of heptane into carbon disulfide/chloroform (1/
1) at room temperature, for 2 of methanol into carbon disulfide/
1,2-dichlorobenzene (1/1) at room temperature, and for 3 of
methanol into carbon disulfide/1,2-dichlorobenzene (1/1) at 10
°C. The data crystals were mounted on glass fibers, transferred
to a Siemens SMART diffractometer/CCD area detector em-
ploying 3 kW sealed tube X-ray source operating at 2 kW, and
centered in the beam. Data were collected at room temperature
for 12 h. Preliminary orientation matrix and cell constants were
determined with a set of 20 data frames with 30 s collection
per frame, followed by spot integration and least-squares
refinement. A hemisphere of data was collected using 0.3° ω
scans at 30 s per frame. The raw data were integrated (XY spot
spread ) 1.60, Z spot spread ) 0.6) and the unit cell parameters
were refined using SAINT.23 Data analysis and absorption
correction were performed by using Siemens XPREP. The data
were corrected for Lorentz and polarization effects, but no
correction for crystal decay was applied. Details of the relevant
All DFT calculations were carried out with the GAUSSIAN98
program package.30
(24) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-473.
(25) Sheldrick, G. M. SHELX97, Program for Crystal Structure Refinement;
University of Go¨ttingen: Germany, 1997.
(26) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley &
Sons: New York, 1980; pp 142-146.
(27) Xie, R.-H.; Bryant, G. W.; Jensen, L.; Zhao, J.; Smith, V. H., Jr. J. Chem.
Phys. 2003, 118, 8621-8635.
(28) Patchkovskii, S.; Thiel, W. J. Am. Chem. Soc. 1998, 120, 556-563.
(29) LaJohn, L. A.; Christiansen, P. A.; Ross, R. B.; Ermler, W. C. J. Chem.
Phys. 1987, 87, 2812-2824.
(30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
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A R T I C L E S
Lee et al.
Acknowledgment. K.L. thanks a Korea University Grant and
Center for Electro- and Photo-Responsive Molecules for finan-
cial support of this research. Y.S.L. thanks the support by Center
for Nanotubes and Nanostructured Composite and Center for
Nanomechatronics and Manufacturing. J.T.P. thanks the Na-
tional Research Laboratory (NRL) Program of Korean Ministry
of Science & Technology (MOST) and the Korea Science
Engineering Foundation (Project No. 1999-1-122-001-5).
Supporting Information Available: X-ray crystallographic
files for 1, 2, and 3 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
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