Synthesis of silylene-bridged endohedral metallofullerene Lu3N? Ih-C80

Article abstract of DOI:10.1021/ja3073929

Functionalization of endohedral metallofullerenes has been shown to differ depending on photochemical or thermal pathways. We report that Lu ₃N?I_h-C₈₀ reacts with thermally generated bis(2,6-diethylphenyl)silylene with hig

Full text of DOI:10.1021/ja3073929

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Article
Synthesis of a Silylene-Bridged Endohedral Metallofullerene Lu3N@Ih-C80
Kumiko Sato, Masahiro Kako, Mitsuaki Suzuki, Naomi Mizorogi, Takahiro Tsuchiya,
Marilyn Morgan Olmstead, Alan L. Balch, Takeshi Akasaka, and Shigeru Nagase
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja3073929 • Publication Date (Web): 04 Sep 2012
Downloaded from http://pubs.acs.org on September 7, 2012
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Synthesis of a Silylene-Bridged Endohedral Metallofullerene Lu3N@Ih-C80
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Kumiko Sato,† Masahiro Kako,‡ Mitsuaki Suzuki,† Naomi Mizorogi,† Takahiro Tsuchiya,†
Marilyn M. Olmstead,|| Alan L. Balch,|| Takeshi Akasaka,*,† and Shigeru Nagase#
† Life Science Center of Tsukuba Advanced Research Alliance, University of Tsukuba, lbaraki
305-8577, Japan
‡ Department of Engineering Science, The University of Electro-Communications, Chofu
182-8585, Japan
|| Department of Chemistry, University of California, Davis, California 95616, United States
# Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan
Abstract:
Functionalization of endohedral metallofullerenes has been shown to differ depending on
photochemical or thermal pathways. We report that Lu3N@Ih-C80 reacts with thermally
generated bis(2,6-diethylphenyl)silylene with high selectivity and forms a mono-silylated
derivative 1b. Unexpectedly, 1b undergoes photochemical conversion to afford an isomer 1a
under ambient light. These adducts were characterized using NMR, visible-near-IR
spectroscopy, and MALDI-TOF mass spectrometry. Single crystal X-ray structure
determination of 1a reveals a rare example of an open 1,2-adduct at the [5,6]-ring junction of
the Ih-C80 cage. The electrochemical study reveals that the redox potentials of 1a and 1b are
shifted cathodically compared to those of pristine Lu3N@Ih-C80, and that mono-silylation is
effective to fine-tune the electronic properties of endohedral metallofullerenes as well as empty
fullerenes. Density functional theory calculations were also performed, which provide a
theoretical basis for the structures and the behavior of the encapsulated Lu3N cluster.
Introduction
Many research groups have investigated the chemical reactivity of endohedral
metallofullerenes (EMFs), the properties of which have been proven to differ greatly from those
of empty fullerenes.1 The electronic structures of EMFs are of great interest because the encaged
metal atoms and the carbon cages are regarded to have cationic and anionic characters,
respectively, as a result of electron transfer. After many years of study, exohedral chemical
functionalization1b-e is widely recognized as an effective method for modifying the physical and
chemical properties of EMFs, which can serve as functional materials in molecular electronics,
nanomaterials sciences,2 and biochemistry.3
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To design new valuable organofullerenes for these applications, it is necessary to understand
how exohedral chemical functionalization affects the electronic properties of fullerenes.
Introduction of heteroatoms such as electropositive silicon directly onto fullerene surfaces has
induced remarkable changes in the electronic characteristics of fullerenes.4 We have described
the derivatization of EMFs by the addition of reactive silicon compounds, such as disiliranes5
and siliranes.6 In fact, electrochemical analysis and theoretical calculation revealed that silylated
fullerene derivatives have more negative charge on the cage than the parent fullerenes have.
Moreover, we have demonstrated that the number of silicon atoms on the fullerene surface plays
an important role in the electronic properties and the movement of the encapsulated metal atoms.
For example, the three-dimensional movement of metal atoms in M2@Ih-C80 is restricted to the
two-dimensional movement because silylation alters the electrostatic potentials and thereby the
dynamic behavior of the cationic metals.5a–c Such regulation of the dynamic behavior of the
encaged species will be useful for the application of EMFs as functional materials in molecular
electronics. Nevertheless, silylation of EMFs has been hitherto limited to bis-silylation5 and
carbosilylation6 using disiliranes and siliranes. Therefore, it is valuable to develop convenient
and versatile synthetic methods for silylation to provide various silylated EMFs. As a candidate
for mono-silylating reagents for EMFs, silylenes, divalent silicon species, are promising
compounds because the addition of silylenes to unsaturated C–C bonds occurs readily.7
Carbenes, the carbon analogues of silylenes, are well known as common reagents for exohedral
functionalization of EMFs1,8 and hollow fullerenes.9 In fact, photochemically generated silylenes
react with C60 and C70 to afford the corresponding silylene adducts.10
This report describes the mono-silylation of Lu3N@Ih-C80,11 which is widely known as a
trimetallic nitride templated (TNT) endohedral metallofullerene. To achieve effective silylation,
we used a new silylene precursor, 9,9-bis(2,6-diethylphenyl)-9-silabicyclo[6.1.0]nonane 2,
which can generate silylene efficiently under both thermal and photochemical conditions
(Scheme 1). Additionally described is the detailed characterization of two obtained products
corresponding to Lu3N@Ih-C80(SiDep2). Of particular interest is the isolation of a
silylene-bridged silafulleroid structure, which represents a rare example of an open type of
1,2-adduct at the [5,6]-ring junction of the Ih-C80 cage. To the best of our knowledge, the only
other example is a recent report of azide addition to Sc3N@Ih-C80 to form a [5,6] azafulleroid.8g
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Results and Discussion
Synthesis of Lu3N@Ih-C80(SiDep2) 1a and 1b. The reaction of Lu3N@Ih-C80 with the
silylene precursor 2 (see Supporting Information) was conducted as follows (Scheme 1). A
solution of Lu3N@Ih-C80 (2.0 mg, 1.3 × 10-6 mol) and 2 (13.7 mg, 3.4 × 10-5 mol) in
1,2-dichlorobenzene (ODCB, 20 mL) was placed in a Pyrex tube and degassed using
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freeze–pump–thaw cycles under reduced pressure. It was then heated for 23 min at 180 °C
under an argon atmosphere in the dark. After cooling the solution, a small portion of the
solution was analyzed by HPLC using a Buckyprep column. The HPLC chromatogram revealed
a new fraction (peak F1) corresponding to a product with high intensity (Figure 1). In addition,
another fraction (F2) appeared at a slightly earlier retention time which gradually increased
when the reaction mixture was exposed to ambient light at room temperature, as shown in
Figure 2.
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Subsequently, the reaction mixture was separated by preparative HPLC using a Buckyprep
and a 5NPE column. Two compounds, 1a and 1b, were obtained from the HPLC fractions F2
and F1 (Figure S1, see Supporting Information). Additional analysis of 1a and 1b using
MALDI-TOF mass spectrometry showed weak peaks at m/z 1793, which correspond to the
molecular ion peak for the isomeric silylene adducts, Lu3N@Ih-C80(SiDep2) (see Supporting
Information, Figure S2). In addition, strong fragment peaks were observed at m/z 1499 for 1a
and 1b, which are assignable to the parent cage, Lu3N@Ih-C80. Therefore, it is reasonable to
assume that the silylene Dep2Si, generated by the thermal extrusion from 2, reacted with
Lu3N@Ih-C80 to afford 1b, which isomerized slowly to 1a under room light. It is noteworthy that
multi-adducts such as Lu3N@Ih-C80(SiDep2)2 were not observed. Consequently, silylation using
2 produces selective derivatization of Lu3N@Ih-C80. It is further confirmed that exposure of a
degassed ODCB solution of 1b to ambient light afforded 1a and pristine Lu3N@Ih-C80 within 24
h (Figure 3a). Under the same conditions, 1a in ODCB decomposed to give Lu3N@Ih-C80
(Figure 4a). Although 1a and 1b are photolabile, both isomers are stable in the dark at room
temperature (Figures 3b, 4b).
Although 2 efficiently generates the silylene Dep2Si by irradiation using a medium pressure
mercury lamp, UV irradiation of an ODCB solution of 2 and Lu3N@Ih-C80 gave neither 1a nor
1b, while starting materials were consumed. The cophotolysis of Lu3N@Ih-C80 and 2 is expected
to be disadvantageous for the synthesis of 1a and 1b because these adducts degrade to eliminate
the silylene addend by photoirradiation, as described above.
For comparison, we examined the reaction of Sc3N@Ih-C80,1e which is the most abundant and
well-explored TNT EMF, with Dep2Si. However, the addition reactions of Sc3N@Ih-C80 and
Dep2Si proceeded much less efficiently to afford poor yields of adducts under the thermal or the
photochemical conditions that are identical with those for Lu3N@Ih-C80. The isolation and
structural analyses were frustrated because of low quantities of the adducts, which were
confirmed only by MALDI-TOF mass spectrometry. Recently, Echegoyen et al. reported the
reactions of electrochemically generated M3N@Ih-C80 dianions (M = Sc, Lu) with electrophiles
and ascribed the superior reactivity of Lu3N@Ih-C80 dianion according to the localization of its
HOMO on the cage.13c Although the origin of the different reactivities of M3N@Ih-C80 toward
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Dep2Si has not been clarified at present, the HOMOs of M3N@Ih-C80 may play an important role
on the reactions with silylenes, weak electrophiles. We are currently investigating the
reactivities of M3N@Ih-C80 with several organosilicon compounds.
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Structural Determination of 1a and 1b. For the structural characterization of 1a and 1b,
NMR analysis and X-ray crystallography were performed. There are four possible isomeric
structures for the derivatized Ih-C80 cage resulting from 1,2–addition of a silylene bridge as
1
shown in Figure 5. The H NMR spectra of 1a and 1b are shown in Figure S3 (see Supporting
Information). The addition of Dep2Si at the [5,6]-ring junction will make the two Dep
substituents nonequivalent, whereas addition at the [6,6]-ring junction will produce equivalent
Dep moieties. Therefore, we specifically examined the 1H NMR signals of the para aryl protons
of the Dep rings. The spectrum of 1a displays a pair of triplets at 7.47 and 7.42 ppm, which are
assigned as nonequivalent para aryl protons, and suggests that the Dep2Si group is bridged over
the [5,6]-ring junction (Figure S3a). In contrast, the spectrum of 1b exhibits one triplet signal at
7.50 ppm, which reflects the equivalency of the two Dep groups (Figure S3b). These
observations suggest that 1a and 1b are Dep2Si adducts at the [5,6]-ring and [6,6]-ring junctions,
respectively. 13C NMR measurements can help to establish whether these compounds are
13
silacyclopropane derivatives or open cage silafulleroids. The C NMR spectrum of 1a showed
44 signals from the sp2 carbon atoms of the Ih-C80 cage, as well as four quaternary sp2 carbons of
the Dep groups between 70 ppm and 155 ppm (Figure S4, see Supporting Information). In
addition, four tertiary sp2 carbons and two sets of ethyl carbons of the nonequivalent Dep groups
1
were observed, which is consistent with H NMR results described above. These spectral data
suggest that 1a has Cs symmetry, resulting from 1,2–addition of Dep2Si at the [5,6]-ring
junction of the Ih-C80 cage. Consequently, the structure of 1a is determined to be the
silylene-bridged silafulleroid type, which represents the rare example of an [5,6]-open Ih-C80
EMF derivative. However, the 13C NMR spectrum of 1b showed two nonequivalent singlet
signals at 85.14 and 77.17 ppm, which are ascribed to the sp3 carbons of the silacyclopropane
ring (Figure S5 in Supporting Information). Previously, we reported the synthesis of the
10a
10b
fullerene–silylene adducts C60SiDip2 and C70SiDip2 (Dip = 2,6-diisopropylphenyl) with
silacyclopropane structures. The sp3 carbon signals of the silacyclopropane rings were observed
at 71.12 ppm for C60SiDip2 and 78.51 and 67.77 ppm for C70SiDip2, respectively. In the 13C
NMR spectrum of 1b, 45 sp2 carbon signals are assigned to 42 carbons of the Ih-C80 cage and
three quaternary aromatic carbons of the Dep groups. As for the tertiary sp2 ring carbons of the
Dep groups, one signal for the para-position and two signals for the meta-positions are observed,
respectively. An additional four sp3 carbon signals are also found for the two sets of ethyl
groups in the upfield region. The lowered spectral symmetry of the Dep groups is rationalized
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based on restriction of the free rotation of the two equivalent Dep rings in 1b as a result of steric
constraints. These observations are consistent with the structure of 1b with Cs symmetry.
Therefore, it is concluded that 1b should be a “closed” silamethano fullerene rather than a
silafulleroid (Figure S3b).
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Visible-near-infrared (Vis-NIR) spectroscopy is a useful method for characterization of the
electronic structures of hollow fullerenes and EMFs. Moreover, exohedrally functionalized
fullerenes show characteristic Vis-NIR spectra that are more dependent on the regiochemistry of
functionalization than on the type of functional group.6,8,11,15,16 For example, the spectra of
5b
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1,4-Sc3N@Ih-C80(Mes2Si)2CH2 and 1,4-Sc3N@Ih-C80(CH2C6H5)2 are quite similar, with
absorption maxima around 900 nm, although they have different substituents. The Vis-NIR
absorption spectra of 1a and 1b are shown with that of Lu3N@Ih-C80 in Figure 6. The spectrum
of 1b shows a distinctive absorption maxima around 700 nm, which resembles those of the
reported [6,6]-methano bridged Lu3N@Ih-C80 derivatives.13 This spectral similarity supports the
1
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assignment of 1b as the [6,6]-adduct, as determined by H and C NMR analyses. Meanwhile,
1a shows a broad and gently sloping absorption around 700 nm with no characteristic
absorption maximum. This finding also confirmed that Vis-NIR spectra of derivatized isomers
of EMFs are specific to the regiochemistry of derivatization. It is worthwhile noting that the
spectrum of 1a shows no resemblance to that of 1,4-Lu3N@Ih-C80(CH2C6H5)2, which was
reported to have a noticeable absorption maximum around 800 nm.12
Fortunately, a thin red crystal suitable for X-ray diffraction was obtained from a solution of
1a in a mixture of CS2/ODCB at 0 °C with the addition of hexane to lower the solubility of the
adduct. The structure of 1a was determined by X-ray structural analysis. A drawing of the
adduct is shown in Figure S6 (see Supporting Information), with selected bond lengths in Table
S1. The crystal structure of 1a shows some disorder both in the positions of the Lu3N cluster
and the cage carbon atoms. There are two cage orientations of the C80 cage that have a
pseudo-D5d symmetry of the derivatized Ih-C80. These cages are related by a crystallographic
mirror plane and occupy a common site. Such disorder is frequently observed when addition
occurs at [5,6] ring junctions.15b
Only one of these orientations and the primary Lu3N unit, which consists of N1, Lu1, Lu3,
and Lu3’ (symmetry code' = 1/2-x, 1/2-y, z), are shown in Figure S7. Lu1 is situated on the
crystallographic mirror plane with a fractional occupancy of 0.96. Lu3 and Lu3’ have fractional
occupancies of 0.85. Lu3’ is generated by reflection of Lu3 through a crystallographic mirror
plane.
The X-ray structure showed unambiguously that the addition of silylene to the Ih-C80 cage
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had occurred at the [5,6]-ring junction. In addition, the C1–C2 distance at the site of silylene
functionalization elongated to 2.25(5) Å, which confirms that 1a possesses an open structure.
The open structure with aza-bridged 4-isopropoxyphenynitrene at the [5,6] position of
Sc3N@Ih-C80 yielded a C1–C2 distance of 2.161(2) Å, in general agreement with the shorter
covalent radius of N relative to Si.8g Open structures at [6,6]-ring junctions on the Ih-C80 cage
have been reported for La2@Ih-C80(Ad) and Y3N@Ih-C80C(CO2CH2Ph)2, for which the
functionalized C–C distances are 2.164(3) Å and 2.30(3) Å, respectively.8e,14a The distance of
the C1–C2 separation at the [5,6]-ring junction in 1a is nearly as long as those of the
functionalized [5,6] and [6,6]-ring junctions. The Si–C bond lengths (Si1–C1, 1.87(3) Å;
Si1–C2, 1.88(3) Å) between the addend and the Ih-C80 cage are close to the normal Si–C single
bond lengths in organosilanes.
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The encapsulated Lu3N cluster shows disorder of the Lu atoms, whereas the N atom has one
site, which lies on a crystallographic mirror plane. The primary Lu3N unit is planar with the sum
of Lu–N–Lu bond angles being 360.0° (Lu1–N1–Lu3, 120.3(7)°; Lu1–N1–Lu3’, 120.3(7)°;
Lu3–N1–Lu3’, 119.4(7)°). The Lu–N bond lengths (Lu1–N1, 2.03(1) Å; Lu3–N1, 2.096(7) Å;
Lu3’–N1, 2.096(7) Å) are comparable to those reported for pristine Lu3N@Ih-C80, which range
from 1.980(6)–2.0819(8) Å (Table S1).11 The Lu1 atom is pointing to the silylated site, and is
almost collinear with the N1 and Si1 atoms, as indicated by the N1–Lu1–Si1 bond angle
(180.0(4)°).
The positional relation between the cage and Lu atoms was also investigated. As shown in
Figure 7, Lu3 and Lu3’ reside close to a hexagon and a pentagon highlighted respectively in
yellow-green and purple in the interior of the cage. The distances from Lu3 to the nearest
carbon atoms of the cage range from 2.37(3) Å to 2.47(3) Å, whereas those from Lu3’ fall
within a range from 2.23(3) Å to 2.56(3) Å. In addition, the distances between Lu1 and C1 and
C2, the [5,6]-ring carbons functionalized by silylene, are 2.54(3) and 2.53(3) Å, respectively.
These nonbonded Lu–C distances are slightly elongated compared to those of pristine
Lu3N@Ih-C80 (2.112(7)–2.220(7) Å),11 as a result of the addition of the silylene. The orientation
of the Lu3N cluster, in which one Lu atom is pointing to the functionalized site, is similar to
those of the methanofulleroid derivatives of M3N@Ih-C80 (M = Sc, Y).14
Electrochemical Properties of 1a and 1b. The redox potentials of 1a and 1b were
measured using cyclic voltammetry (CV) and differential pulse voltammetry (DPV), and are
listed in Table 1 along with those of related compounds. The voltammograms of 1a and 1b are
presented in Figure S8 (see Supporting Information). These silylated EMFs afforded irreversible
oxidation and reduction processes. The first oxidation of the silylated derivatives led to removal
of the addend from the carbon cage to give Lu3N@Ih-C80, as observed for other silylated EMFs
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such as Sc3N@Ih-C80(Mes2Si)2CH2.5b The first oxidation peak potentials of 1a and 1b were
shifted cathodically by 340 and 180 mV, respectively, compared to those of Lu3N@Ih-C80. The
first reduction peak potentials were also cathodically shifted by 40 mV for 1a and 130 mV for
1b, respectively. In addition, these cathodic shifts of the redox potentials of 1a and 1b,
compared to those of pristine Lu3N@Ih-C80, are smaller than those observed for the bis-silylated
EMFs, Sc3N@Ih-C80(Mes2Si)2CH2,5b and La2@Ih-C80(Dep2Si)2CH2.5c Similar weak effects on the
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redox
potentials
have
been
reported
for
the
carbosilylated
EMFs,
La2@Ih-C80Dep2Si(CH2)CH(t-BuPh).6 In contrast, the redox potentials of the carbene and the
Bingel–Hirsch derivatives of EMFs are shifted less cathodically than 1a and 1b, as expected for
electronically neutral carbon functional groups.2d,13 For example, the cathodic shifts of the first
oxidation potentials of carbon-bridged methano Lu3N@Ih-C80 derivatives are observed between
20–70 mV as reported by Echegoyen et al.13b,c These results demonstrate that introduction of
silicon atoms onto the Lu3N@Ih-C80 cages represents an effective way to raise the LUMO levels
of EMF derivatives. By comparing the redox data for 1a and 1b, it is noteworthy that the redox
potentials of the silylated EMFs depend on both the electronic properties of the exohedral
functionality and the π-conjugation of the cage structures.
Theoretical Calculations. To obtain insight into the structures of the silylated products,
theoretical calculations were conducted for 1a and 1b. The geometries of the compounds were
optimized at the M06-2X18/6-31G(d)19[C, H, N, Si], LanL2DZ20[Lu] levels as shown in Figure 8.
The optimized structure of 1a shows good agreement with the result of X-ray crystal analysis.
The important calculated bond lengths are shown in Table S1 for comparison with those
obtained from X-ray analysis. The silylene functionalized C–C distance is calculated as 2.261 Å,
and the Si–C bond length between the addend and the Ih-C80 cage is almost within regular values.
These results also confirm that the structure of 1a is an open silafulleroid, which strongly
supports the spectroscopic data. As shown in the X-ray structure of 1a, the Lu1 atom is pointing
at the silylated site with an almost collinear orientation of Si1, Lu1, and N1 atoms. Therefore,
we have also calculated four optimized structures I–IV, for isomers of 1a with varied
orientations of the encapsulated Lu3N cluster (Figure S9 in Supporting Information). The
relative energies of I–IV compared to that of 1a are in the narrow range of 4 kcal/mol (Table
S2). Thus, the Lu3N cluster can be assumed to rotate inside the cage around an axis passing
along the Lu-N bond that is pointing to the open C–C bond. In contrast, the isomer V, in which
the Lu3N cluster is positioned on the equatorial plane viewed from the silylated site as the pole,
was calculated as 18.4 kcal/mol less stable than 1a. The calculated structure of 1b is shown in
Figure 8b. Based on the relative energies, these results show that 1b is 19.2 kcal/mol less stable
than 1a. In the optimized structure of 1b, the Lu3N cluster is located almost coplanar with the
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silacyclopropane ring, and one of the Lu atoms is directed to the opposite side of the silylated
C–C bond. However, no energy minimum is obtained for 1b if the orientation of the Lu3N
cluster is reversed so that one of the Lu atoms is oriented to point at the silylated site. In this
case, the silylated C–C bond is broken during the structural optimization process.
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Conclusion
We have described the first example of a reaction of Lu3N@Ih-C80 with a thermally
generated silylene. A silylene adduct of Lu3N@Ih-C80, 1b, is obtained as the sole product during
the thermal reaction in the dark. It is particularly interesting that 1b is transformed slowly into
an isomeric product 1a under ambient light. Characterization using NMR, Vis-NIR, and MS
elucidated the structures of 1a and 1b, which were assigned respectively to [5,6]-silafulleroid
and [6,6]-silacyclopropane. The structure of 1a has been established unequivocally using
single-crystal X-ray analysis. In comparison with the Prato and the Bingel–Hirsch adducts of
M3N@Ih-C80 (M = Sc, Y), for which [5,6]-closed and [6,6]-open derivatives have been reported,
respectively, the silylene addition is remarkable because it leads to [5,6]-open and [6,6]-closed
structures. The theoretically optimized structure of 1a verified the structural features inferred
from X-ray analysis. From the calculated relative energies, 1a is postulated to be more stable
than 1b, which might offer some clue about the photochemical transformation of 1b into 1a.
The results of electrochemical examination indicate that, because of the electron-donating silyl
groups, 1a and 1b are more electronegative than pristine Lu3N@Ih-C80. In addition to the
electronic effect, exohedral functionalization affects the movement of the encapsulated cluster.
Thus, the development of silylation procedures broadens the scope of functionalization of EMFs
for future applications.
Experimental Section
General. All chemicals were of reagent grade, purchased from Wako Pure Chemical Inds.
Ltd. Lu3N@Ih-C80 was purchased from Luna Innovations Inc. 1,2-Dichlorobenzene (ODCB)
was distilled from P2O5 under vacuum before use. Reagents were used as purchased unless
otherwise specified. High-performance liquid chromatography (HPLC) was performed on a
LC-908 apparatus (Japan Analytical Industry Co. Ltd.) monitored using a UV detector at 330
nm. Buckyprep (i. d. 20 mm × 250 mm) and 5NPE (i. d. 20 mm × 250 mm) columns (Nacalai
Tesque Inc.) were used for preparative HPLC separation. Toluene was used as the eluent for
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HPLC. The H, C, and Si NMR measurements were conducted on spectrometers (AVANCE
500, 600; Bruker Analytik GmbH) with a CryoProbe system (Bruker Biospin K.K.) in
1,1,2,2-tetrachloroethane-d2. The outer standard reference was TMS for 29Si NMR.
Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass experiments
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were recorded (Biflex Ш; Bruker) with 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) as the matrix in
both positive and negative ion modes. Absorption spectra were measured using a UV
spectrophotometer (UV-3150; Shimadzu Corp.). Cyclic voltammograms (CVs) and differential
pulse voltammograms (DPVs) were recorded on an electrochemical analyzer (BAS CV50W;
BAS Inc.). The reference electrode was a saturated calomel reference electrode (SCE). The
glassy carbon electrode was used as the working electrode, and a platinum wire was used as the
counter-electrode. All potentials are referenced to the ferrocene/ferrocenium couple (Fc/Fc+) as
the standard. 0.1 M (n-Bu)4NPF6 in ODCB was used as the supporting electrolyte solution. The
CVs were recorded using a scan rate of 100 mV/s. The DPVs were obtained using a pulse
amplitude of 50 mV, a pulse width of 50 ms, a pulse period of 200 ms, and a scan rate of 50
mV/s.
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Thermal Reaction of Lu3N@Ih-C80 with 2. An ODCB solution (20 mL) of Lu3N@Ih-C80
(2.0 mg, 1.3 × 10-6 mol) and 2 (13.7 mg, 3.4 × 10-5 mol) was degassed using freeze–pump–thaw
cycles under reduced pressure in a Pyrex tube. The solution was heated at 180 °C for 23 min
under an argon atmosphere in the dark. When the reaction was finished, 1b was detected as the
sole product. Subsequently, slow photochemical transformation of 1b into 1a was observed
under ambient light. The reaction mixture was separated by preparative HPLC with Buckyprep
and 5NPE columns to isolate 1a and 1b, respectively, in 20% and 33% yields.
Data for 1a: red solid; 1H NMR (500 MHz, C2D2Cl4, 293 K) δ 7.47 (t, J = 8 Hz, 1H), 7.42 (t,
J = 8 Hz, 1H), 7.30 (d, J = 7.5 Hz, 2H), 7.21 (d, J = 8 Hz, 2H), 3.88−3.81 (m, 2H), 3.67−3.55
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(m, 4H), 3.29−3.21 (m, 2H), 1.46 (t, J = 7.5 Hz, 6H), 1.43 (t, J = 7.5 Hz, 6H); C NMR (125
MHz, C2D2Cl4, 293 K) δ 151.24 (2C; o-Ar), 151.10 (2C; o-Ar), 150.86 (2C), 148.28 (2C),
148.04 (1C), 147.76 (2C), 146.70 (2C), 145.82 (2C), 145.51 (1C), 145.23 (2C), 145.12 (2C),
144.92 (2C), 143.58 (1C), 143.43 (4C), 143.30 (2C), 143.00 (2C), 142.82 (2C), 142.63 (2C),
142.36 (2C), 142.24 (2C), 142.16 (2C), 141.69 (2C), 141.64 (2C), 141.55 (2C), 140.92 (2C),
140.63 (2C), 140.51 (2C), 140.26 (2C), 139.93 (2C), 139.63 (1C), 139.50 (1C), 139.25 (2C),
139.06 (2C), 138.31 (1C), 138.02 (2C), 136.85 (2C), 136.32 (2C), 136.06 (2C), 135.74 (2C),
133.11 (1C), 132.15 (1C), 131.79 (1C; p-Ar), 131.57 (1C; p-Ar), 126.80 (2C; m-Ar), 126.75
(2C; m-Ar), 125.01 (1C), 124.85 (1C), 123.80 (2C), 122.72 (2C), 110.82 (2C), 72.80 (2C; C80Si),
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31.42 (2C; CH2), 30.73 (2C; CH2), 16.16 (2C; CH3), 15.76 (2C; CH3); Si NMR (119.4 MHz,
C2D2Cl4, 298 K) δ −57.24; MALDI-TOF MS (TPB) m/z 1793 ([M]+), 1499 ([M − C20H26Si]+).
Data for 1b: red solid; 1H NMR (500 MHz, C2D2Cl4, 293 K) δ 7.50 (t, 2H, J = 7.5 Hz), 7.31
(br, 2H), 7.25 (br, 2H), 3.48 (br, 4H), 3.40 (br, 2H), 3.31 (br, 2H), 1.43 (br, 6H), 1.30 (br, 6H);
13C NMR (125 MHz, C2D2Cl4, 293 K) δ 152.22 (2C), 151.18 (2C; o-Ar), 151.00 (2C; o-Ar),
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150.05 (2C), 148.34 (2C), 146.76 (2C), 146.40 (2C), 146.05 (1C), 145.52 (2C), 145.40 (2C),
145.10 (2C), 144.74 (2C), 144.47 (2C), 143.97 (2C), 143.92 (2C), 143.66 (2C), 143.02 (2C),
142.99 (4C), 142.83 (2C), 142.72 (1C), 142.10 (2C), 141.90 (2C), 141.78 (2C), 141.31 (1C),
141.02 (2C), 140.86 (2C), 140.51 (2C), 140.44 (2C), 140.12 (2C), 139.60 (2C), 138.86 (2C),
138.29 (1C), 137.64 (2C), 135.43 (1C), 135.35 (2C), 135.23 (2C), 134.77 (2C), 134.05 (3C),
132.01 (2C; p-Ar), 127.87 (2C), 126.84 (2C; m-Ar), 126.70 (2C; m-Ar), 126.54 (2C), 125.81
(2C), 124.91 (2C), 122.33 (2C), 85.14 (1C; C80Si), 77.17 (1C; C80Si), 31.34 (2C; CH2), 31.13
(2C; CH2), 15.86 (2C; CH3), 15.44 (2C; CH3); 29Si NMR (119.4 MHz, C2D2Cl4, 298 K) δ
−53.12; vis-NIR (CS2) λmax 698 nm; MALDI-TOF MS (TPB) m/z 1793 ([M]+), 1499 ([M −
C20H26Si]+).
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X-ray Crystallography of 1a. Crystals of 1a were obtained using liquid–liquid bilayer
diffusion method of an CS2/ODCB = 1/1 solution of 1a using hexane as a poor solvent at 0 °C.
Single-crystal X-ray diffraction data of 1a were collected on a Bruker AXS machine equipped
with an Apex II CCD detector and a liquid-nitrogen low-temperature apparatus providing a
constant temperature at 90 K.
Crystal data for 1a, Lu3N@Ih-C80-SiDep2•CS2: C101H26Lu3NS2Si, Mr = 1870.35, red
platelet, 0.18 × 0.16 × 0.04 mm, orthorhombic, Pccn (No.56), a = 10.9576(10) Å, b =
17.2860(15) Å, c = 28.760(2) Å, V = 5447.5(8) Å3, Z = 4, λ = 0.71073 Å, µ(Mo Kα) = 5.564
mm-1, θmax = 28.69°, Dcalc = 2.281 g/cm3, T = 90 K, 34063 measured reflections, 6916
independent reflections, 473 refined parameters, R1 = 0.1221, wR2 = 0.2339 for all data, GOF =
1.278 for all data; R1 = 0.1043 for 6916 independent reflections [I > 2.0σ(I)] with 473 parameter
and 776 restraints.
Computational Method. Theoretical calculations were conducted for 1a and 1b. The
geometries of the compounds were optimized at the M06-2X18/6-31G(d)19[C, H, N, Si],
LanL2DZ20[Lu] level using the Gaussian 0921 program.
ASSOCIATED CONTENT
Supporting Information
X-ray crystallographic data for 1a in CIF format, spectroscopic data for 1a and 1b, synthesis of
2, complete list of authors for refs 2c, 2d, 2e, 3b, 4b, 5a, 5b, 5c, 8a, 8b, 8c, 8f, 12, 14b, 15b, 21.
This material is available free of charge via the Internet at http://pubs.acs.org .
AUTHOR INFORMATION
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Corresponding Author
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akasaka@tara.tsukuba.ac.jp
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ACKNOWLEDGMENTS
This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (No.
20108001, “pi-Space”), a Grant-in-Aid for Scientific Research (A) (No. 20245006) and (B) (No.
24350019), The Next Generation Super Computing Project (Nanoscience Project),
Nanotechnology Support Project, Grants-in-Aid for Scientific Research on Priority Area (Nos.
20036008 and 20038007), Specially Promoted Research (No. 22000009) from the Ministry of
Education, Culture, Sports, Science, and Technology of Japan, and The Strategic
Japanese−Spanish Cooperative Program funded by JST and MICINN.
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Scheme 1.
Figure 1. HPLC profiles of the crude reaction mixture. Conditions: Buckyprep column (φ 4.6 × 250
mm); eluent, toluene; flow rate, 1.0 mL/min; wavelength, 330 nm; temperature, room temperature.
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Figure 2. (a) HPLC profiles of the reaction mixture exposed to ambient light. Conditions:
Buckyprep column (φ 4.6 × 250 mm); eluent, toluene; flow rate, 1.0 mL/min; wavelength, 330 nm;
temperature, room temperature. (b) HPLC profile of the reaction mixture exposed to ambient light
for 12 h. Conditions: 5NPE column (φ 4.6 × 250 mm); eluent, toluene; flow rate, 1.0 mL/min;
wavelength, 330 nm; temperature, room temperature.
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Figure 3. HPLC monitoring of a solution of 1b in ODCB under degassed conditions at room
temperature (a) in the presence of ambient light and (b) in the absence of ambient light. Conditions:
5NPE column (φ 4.6 × 250 mm); eluent, toluene; flow rate, 1.0 mL/min; wavelength, 330 nm;
temperature, room temperature.
Figure 4. HPLC monitoring of a solution of 1a in ODCB under degassed conditions at room
temperature (a) in the presence of ambient light and (b) in the absence of ambient light. Conditions:
5NPE column (φ 4.6 × 250 mm); eluent, toluene; flow rate, 1.0 mL/min; wavelength, 330 nm;
temperature, room temperature.
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Figure 5. Addition patterns of Lu3N@Ih-C80(SiDep2).
Figure 6. Vis-NIR absorption spectra of 1a, 1b, and Lu3N@Ih-C80.
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Figure 7. Relation between cage and Lu atoms in 1a. Lu, red; N, blue, Si, green; C closest to
Lu3, yellow-green; C closest to Lu3’, purple.
Table 1. Redox Potentials (V)a of 1a, 1b, Lu3N@Ih-C80, and Silylated EMFs
compound
Eox
Ered
Ered
Ered
3
1
1
2
Lu3N@Ih-C80
+0.61
-1.39b -1.83b -2.16b
1a
1b
+0.27b -1.43b -1.72b -1.94b
+0.43b -1.52b -1.73b -1.99b
c
Sc3N@Ih-C80
+0.62
-1.22
c
Sc3N@Ih-C80(Mes2Si)2CH2
+0.08b -1.45
d
La2@Ih-C80
+0.56
-0.04b
-0.31
-0.70
-1.71
-2.13
e
La2@Ih-C80(Dep2Si)2CH2
La2@Ih-C80(Dep2Si)CH2CH(t-BuPh)-2Af +0.11b -0.50
La2@Ih-C80(Dep2Si)CH2CH(t-BuPh)-2Bf +0.13b -0.53
a Values obtained by DPV are in volts relative to ferrocene/ferrocenium couple. b Irreversible. c Data
from ref 5b. d Data from ref 17. e Data from ref 5c. f Data from ref 6.
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Figure 8. Optimized structures of (a) 1a and (b) 1b.
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SYNOPSIS TOC
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