Lu2@C2n (2n = 82, 84, 86): Crystallographic Evidence of Direct Lu-Lu Bonding between Two Divalent Lutetium Ions Inside Fullerene Cages

Article abstract of DOI:10.1021/jacs.7b04421

Although most of the M₂C_2n-type metallofullerenes (EMFs) tend to form carbide cluster EMFs, we report herein that Lu-containing EMFs Lu₂C_2n (2n = 82, 84, 86) are actually dimetallofullerenes (di-EMFs), namely, Lu₂@C_s(6)-C₈₂, Lu₂@C_3v(8)-C₈₂, Lu₂@D_2d(23)-C₈₄, and Lu₂@C_2v(9)-C₈₆. Unambiguous X-ray results demonstrate the formation of a Lu-Lu single bond between two lutetium ions which transfers four electrons in total to the fullerene cages, thus resulting in a formal divalent state for each Lu ion. Population analysis indicates that each Lu atom formally donates a 5d electron and a 6s electron to the cage with the remaining 6s electron shared with the other Lu atom to form a Lu-Lu single bond so that only four electrons are transferred to the fullerene cages with the formal divalent valence for each lutetium ion. Accordingly, we confirmed both experimentally and theoretically that the dominating formation of di-EMFs is thermodynamically very favorable for Lu₂C_2n isomers.

Full text of DOI:10.1021/jacs.7b04421

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Article
Lu2@C2n (2n = 82, 84, 86): Crystallographic Evidence of Direct Lu-Lu
Bonding between Two Divalent Lutetium Ions inside Fullerene Cages
Wangqiang Shen, Lipiao Bao, Yongbo Wu, Changwang Pan, Shasha Zhao,
Hongyun Fang, Yunpeng Xie, Peng Jin, Ping Peng, Fang-Fang Li, and Xing Lu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b04421 • Publication Date (Web): 05 Jul 2017
Downloaded from http://pubs.acs.org on July 6, 2017
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Lu2@C2n (2n = 82, 84, 86): Crystallographic Evidence of Direct
Lu-Lu Bonding between Two Divalent Lutetium Ions inside
Fullerene Cages
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Wangqiang Shen†, Lipiao Bao†, Yongbo Wu†, Changwang Pan†, Shasha Zhao†,
Hongyun Fang†, Yunpeng Xie†, Peng Jin‡,*, Ping Peng†,* FangꢀFang Li†,* and Xing
Lu†,*
†State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials
Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan
430074, P.R. China
‡School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R.
China
ABSTRACT:
Although most of the M2C2nꢀtype metallofullerenes (EMFs) tend to form carbide cluster
EMFs, we report herein that Luꢀcontaining EMFs Lu2C2n (2n = 82, 84, 86) are actually
dimetallofullerenes (diꢀEMFs), namely, Lu2@Cs(6)ꢀC82, Lu2@C3v(8)ꢀC82, Lu2@D2d(23)ꢀC84 and
Lu2@C2v(9)ꢀC86, respectively. Unambiguous Xꢀray results demonstrate the formation of a LuꢀLu
single bond between two lutetium ions which transfer four electrons in total to the fullerene cages,
thus resulting in a formal divalent state for each Lu ion. Population analysis indicate that each Lu
atom formally donates a 5d electron and a 6s electron to the cage with the remaining 6s electron
shared with the other Lu atom to form a LuꢀLu single bond, so that only four electrons are
transferred to the fullerene cages with the formal divalent valence for each lutetium ion.
Accordingly, we have confirmed both experimentally and theoretically that the dominating
formation of diꢀEMFs are thermodynamically very favorable for Lu2C2n isomers.
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INTRODUCTION
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Endohedral metallofullerenes (EMFs) are a class of novel hybrid molecules resulting from
the endohedral doping of fullerenes with metallic species.1–3 EMFs have attracted great attention
because of their fascinating properties and broad applications in materials science, photovoltaics,
electronics, and biomedicine.2,4 In addition to such conventional EMFs containing only metal
atoms, novel EMFs encapsulating a variety of metallic compounds, including metal carbide
(M2C2/M3C2/M4C2),5–7 metal nitride (M3N),8,9 metal oxide (M2O/M4O2/M4O3),10–12 metal sulfide
(M2S),13 and metal cyanide (M3CN/MCN)14,15 clusters have been obtained and structurally
confirmed in recent years.
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M2C2nꢀtype EMFs may exist as dimetallofullerenes (diꢀEMFs), M2@C2n, or as carbide cluster
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metallofullerenes (CCMFs), M2C2@C2n−2
.
Surprisingly, most of the M2C2nꢀtype EMFs that
have been crystallographically characterized prefer to take the carbide form. For instance, almost
all the Sc2C2nꢀtype EMFs are CCMFs, most probably because of the strong coordinating ability
and the small ionic radius of scandium that facilitate the formation of carbide clusters suitable for
encapsulation inside common fullerene cages.5,18–22 Recently, more and more M2C2nꢀtype
compounds are confirmed as CCMFs, instead of diꢀEMFs, such as Tm2C2@Cs(6)ꢀC82,23
M2C2@C1(51383)ꢀC84 (M = Y, Gd),24 Gd2C2@D3(85)ꢀC9225 and Tb2C2@Cs(6)ꢀC82.26 Furthermore,
it is found that the even larger La3+ ions also prefer the carbide structure to form some giant
cages,27–29 which are rationalized by considering a synergistic effect of inserting a C2ꢀunit on the
stabilization of CCMFs both electronically and geometrically.
Since lutetium possesses a similar ionic radius to scandium, it has been proposed that
Luꢀcontaining EMFs should also take the cluster structures as Scꢀcontaining EMFs do. Indeed,
Lu3N@Ih(7)ꢀC80 was synthesized with a comparable yield to that of Sc3N@Ih(7)ꢀC80 and was
revealed to possess nearly the same chemical properties.30 However, the unambiguous structural
characterization of other Luꢀcontaining EMFs has rarely been reported. For instance, Lu2@C76
was reported to have the Tdꢀsymmetric cage according to NMR results,31 and recently Lu2C2@C72,
Lu2C2@C74(I), Lu2C2@C80, and Lu2C2@C82(III) are reported to exist but no further experimental
evidence was available.32 In addition, Zhao et al. proposed theoretically that Lu2C2@C2(157)ꢀC96
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and Lu2C2@D2d(163)ꢀC96 are perfectly stable.33 However, we herein report that Luꢀcontaining
EMFs prefer to form diꢀEMFs, instead of the common carbide structures. Singleꢀcrystal Xꢀray
crystallographic results of these four new compounds show that they are Lu2@Cs(6)ꢀC82,
Lu2@C3v(8)ꢀC82, Lu2@D2d(23)ꢀC84 and Lu2@C2v(9)ꢀC86, respectively. More meaningfully,
theoretical results show that the encapsulated Lu2 cluster transfers four electrons to the cage with a
formal divalent valence for each lutetium ion. Thus, the dominating formation of diꢀEMFs instead
of CCMFs for Lu2C2n (2n = 82, 84, 86) are thermodynamically very favorable, by forming a
LuꢀLu single bond in these Lu2@C82ꢀ86 isomers.
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RESULTS AND DISCUSSION
Carbon soot containing lutetiumꢀEMFs was synthesized using a direct current arc discharge
method. Briefly, a coreꢀdrilled graphite rod filled with graphite/Lu2O3 (molar ratio: Lu/C=1:17)
was burned under 250 Torr helium and 50 Torr nitrogen atmosphere with a power of 100 A × 20 V.
Multistage HPLC separations gave pure isomers of Lu2@C82ꢀ86 (Figures S1ꢀS6, Supporting
Information). Figure 1 shows their HPLC chromatograms and the laserꢀdesorption ionization
timeꢀofꢀflight (LDIꢀTOF) mass spectra to confirm their purity as 99%.
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Figure 1. (a) HPLC chromatograms, (b) LDIꢀTOF mass spectra and (c) VisꢀNIR absorption
spectra of purified Lu2C82ꢀ86 isomers. (HPLC conditions: eluent = toluene, flow rate = 1.0 mL
minꢀ1, detection wavelength = 330 nm, room temperature).
Electronic structures of Lu2@C82ꢀ86 were characterized with absorption spectroscopy. (Figure
1c and Table S1). Lu2C82(I) exhibits distinct absorptions at 481, 646, 740, 813 and 1213 nm with
an onset at 1440 nm, corresponding to a small optical bandgap (0.86 eV). The spectrum of
Lu2C82(II) has two distinct peaks at 705 and 895 nm with an onset at 1024 nm, corresponding to a
relatively large optical bandgap (1.21 eV). Lu2C84 and Lu2C86 exhibit absorption onsets at 1513
and 1261 nm, respectively, thus suggesting their small bandgaps (0.82 eV for Lu2C84 and 0.98 eV
for Lu2C86). Furthermore, Lu2C84 shows distinct absorption bands at 422, 643, 686, 1115 and 1318
nm, and Lu2C86 shows characteristic bands at 429, 590, 679, 782, 950 and 1080 nm. Interestingly,
the spectra of the four compounds under study are similar to the respective curves of the
corresponding Scꢀcontaining CCMFs (Sc2C2@Cs(6)ꢀC82, Sc2C2@C3v(8)ꢀC82, Sc2C2@D2d(23)ꢀC84
4ꢀ
and Sc2C2@C2v(9)ꢀC86 ),5,19–21 which have been widely accepted to have the (Sc2C2)4+@(C82ꢀ86
)
configuration. In addition, the absorption spectra of the two Lu2@C82 isomers are also similar to
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the respective curves of the corresponding Er2@C82 isomers.34 Accordingly, it infers that Lu2C82ꢀ86
isomers may be diꢀEMFs instead of CCMFs, and the Lu2 cluster transfers four electrons to the
cages with a formal divalent valence for each lutetium ion.
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Figure 2. Positions of the disordered lutetium sites in (a) Lu2@Cs(6)ꢀC82, (b) Lu2@C3v(8)ꢀC82, (c)
Lu2@D2d(23)ꢀC84 and (d) Lu2@C2v(9)ꢀC86 relative to a cage orientation. Those Lu atoms labeled
with “A” are generated by crystallographic operation.
The four compounds under study were cocrystallized with NiII(OEP) (OEP
=
2,3,7,8,12,13,17,18ꢀoctaethylporphin dianion) to obtain good crystals suitable for Xꢀray
measurements. Their molecular structures are unambiguously determined with singleꢀcrystal
Xꢀray diffraction (XRD) crystallography. The results confirm that all these compounds are
actually diꢀEMFs instead of CCMFs, namely Lu2@Cs(6)ꢀC82, Lu2@C3v(8)ꢀC82, Lu2@D2d(23)ꢀC84
and Lu2@C2v(9)ꢀC86, respectively. Inside the fullerene cages, the Lu ions show some degree of
disorder. Details of the disorder are given in Figure 2 and Table S2. The crystal system of
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Lu2@Cs(6)ꢀC82•NiII(OEP) falls into the monoclinic space group P21/c. Up to thirteen Lu sites are
distinguished (Figure 2a). In comparison, the crystal systems of the other three EMFs belong to
the monoclinic space group C2/m where the asymmetric unit contains two halves of the carbon
cage. Accordingly, taking Lu2@C3v(8)ꢀC82 as an example, six disordered Lu sites are found in the
cage. Because some disordered Lu sites do not reside at the symmetric plane, three additional Lu
sites are generated by symmetric operation (Figure 2b and Table S2). Similarly, twentyꢀone Lu
sites and fifteen Lu sites are distinguished in the cages of Lu2@D2d(23)ꢀC84 and Lu2@C2v(9)ꢀC86
(Figure 2c, 2d and Table S2), respectively. In other midsized dimetallic EMFs that have been
crystallographically characterized, such as La2@Ih(7)ꢀC80, Er2@Cs(6)ꢀC82 and Er2@C3v(8)ꢀC82, the
disordered metal sites are found to be along a band of 10 contiguous hexagons,34–36 but this
phenomenon does not happen here in these Lu2@C82ꢀ86 isomers.
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Figures 3aꢀ3d portray the molecular structures of these EMFs showing the major components
together with the cocrystallized NiII(OEP) molecule. The shortest distances between Ni and a cage
carbon range from 2.851 Å to 3.045 Å, suggesting substantial π−π interactions between the
fullerene cage and NiII(OEP). Furthermore, the geometries of Lu2@Cs(6)ꢀC82, Lu2@C3v(8)ꢀC82
and Lu2@D2d(23)ꢀC84 allow only one of the two Lu ions to be positioned near the planar porphyrin,
whereas both Lu ions in Lu2@C2v(9)ꢀC86 reside near the planar porphyrin because of the unique
pyramidal shape of the cage. The Lu···Lu distances between any two opposite Lu sites with
similar occupancy values are in the range of 3.35ꢀ3.67 Å, 3.21ꢀ3.57 Å, 3.24ꢀ3.75 Å and 3.43ꢀ3.72
Å, for Lu2@Cs(6)ꢀC82, Lu2@C3v(8)ꢀC82, Lu2@D2d(23)ꢀC84 and Lu2@C2v(9)ꢀC86, respectively
(Table S4). These values are all comparable to a LuꢀLu single bond length (3.28ꢀ3.81 Å),37,38
confirming crystallographically the direct LuꢀLu bonding to result in a divalent state for each
lutetium ion in these cages.
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Figure 3. ORTEP drawings of (a) Lu2@Cs(6)ꢀC82•NiII(OEP), (b) Lu2@C3v(8)ꢀC82•NiII(OEP), (c)
Lu2@D2d(23)ꢀC84•NiII(OEP) and (d) Lu2@C2v(9)ꢀC86•NiII(OEP). Thermal contours are drawn at
the 10% probability level. Only one fullerene cage and the predominant metal sites are shown,
whereas minor sites and solvent molecules are omitted for clarity.
Theoretical calculations were conducted to rationalize the formation of the Lu－Lu bond in
these compounds. The geometries of the four molecules optimized at the M06ꢀ2X/6ꢀ31G*~SDD
level agree perfectly with their Xꢀray structures (Figure S8). They all bear a singlet ground state
(1A) with other spin states much higher in energy (Table S6).
As for Lu element, which has a [Xe]4f146s25d1 electronic configuration and may keep its 6s
electrons due to the relativistic contraction and excellent stabilization of the 6s atomic orbital.39
2
2
2
2
The Lu2 dimer has a triplet (6s)σg (6s)σu (5d)πu electronic structure with the σg orbital much
2
2
lower in energy than the σu and πu molecular orbitals.40 Natural bond order (NBO) analyses on
the Lu2@C82ꢀ86 isomers demonstrate that the Lu atoms form a LuꢀLu single bond with electron
occupancy of 1.97ꢀ1.98 e, which is supported by the calculated Wiberg bond orders (WBOs)
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ranging from 0.92 to 0.97 (Table 1). Consistent with the lowꢀlying Lu2 σg molecular orbital, the
hybrid compositions of LuꢀLu bonds reveal that the largest contribution to the metal bonding ꢀ
molecular orbitals stems from the Luꢀ6s orbitals, and each Lu atom formally donates one 5d
electron and one 6s electron to the cage with the 4f electrons remaining intact. It is also
noteworthy that Cs(6)ꢀC82, C3v(8)ꢀC82 and D2d(23)ꢀC84 are all the lowestꢀlying isomers of the
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4ꢀ
corresponding charged C2n cage, and they are thus suitable to encapsulate tetravalent metal
species to form EMFs.5,21 The C2v(9)ꢀC86 cage has large (LUMO+2)ꢀ(LUMO+1) gap energies and
is very favorable to formally accept four electrons.41 Accordingly, the above results strongly
suggest that each Lu2@C82ꢀ86 isomer bears a (Lu2+)2@(C82ꢀ86
)
electronic state, resulting in a
4ꢀ
formal divalent state for each Lu ion. On the other hand, the unoccupied Lu atomic orbitals (6p, 6d
and 7p) have considerable electron populations, which are ascribed to their participation in the
metalꢀmetal bonding and/or the substantial electron backꢀdonation from the occupied cage π
orbitals (Table 1). Actually, our results are perfectly consistent with those from Zhao et al. and
Popov et al. who independently proposed that a metalꢀmetal bond may exist in dimetallofullerenes
40,42,43
between two metal atoms with low valence states,
and our crystallographic results confirm
unambiguously the presence of direct LuꢀLu bonding in Lu2@C82ꢀ86 isomers.
Table 1 Bond lengths (RLuꢀLu, Å), Wiberg bond orders (WBOs), electron occupancies (Occ., e),
natural population analysis (NPA) charges and natural electron configuration populations of the
LuꢀLu single bonds in Lu2@C82ꢀ86 isomers. The Lu atoms are numbered from left to right in
Figure S8.
Compound
RLu-Lu WBO Occ. Atom Charge
Population
Hybrid Composition
3.60
0.96
1.97
Lu1
Lu2
Lu1
1.06
1.10
1.09
6s0.545d0.306p0.62 s(48%)p(35%)d(17%)
6d0.517p0.01
Lu2@Cs(6)-C82
6s0.625d0.196p0.55 s(55%)p(31%)d(14%)
6d0.567p0.01
3.47
0.97
1.98
6s0.555d0.206p0.59 s(49%)p(35%)d(16%)
6d0.597p0.01
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Lu2@C3v(8)-C82
Lu2
Lu1
Lu2
Lu1
Lu2
1.10
1.09
1.09
1.11
1.07
6s0.605d0.276p0.57 s(53%)p(33%)d(14%)
6d0.487p0.01
4.00
0.92
1.97
6s0.635d0.316p0.16 s(57%)p(30%)d(13%)
7s0.016d0.437p0.40
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6s0.635d0.316p0.16 s(57%)p(30%)d(13%)
7s0.016d0.437p0.40
Lu2@D2d(23)-C84
3.70
0.94
1.97
6s0.595d0.196p0.54 s(53%)p(31%)d(16%)
6d0.587p0.01
Lu2@C2v(9)-C86
6s0.575d0.196p0.56 s(52%)p(31%)d(17%)
6d0.627p0.01
Moreover, as for Luꢀcontaining EMFs, Shinohara et al. proposed recently the structural
formulas of Lu2C2@C72, Lu2C2@C74(I), Lu2C2@C80, and Lu2C2@C82(III) according to a linear
relationship between the effective volume of the cage and the atom number.32 Meanwhile, Zhao
and coworkers reported the excellent stability of Lu2C2@C2(157)ꢀC96 and Lu2C2@D2d(163)ꢀC96
by DFT calculations.33 However, we did not obtain any Luꢀbased CCMFs in our experiment.
Accordingly, we investigated a series of possible Lu2C2@C2nꢀ2 isomers based on the reported
4ꢀ
lowꢀenergy C2nꢀ2 cages, including C2v(5)ꢀC80, D5h(6)ꢀC80, Ih(7)ꢀC80, Cs(6)ꢀC82, C3v(8)ꢀC82,
C2v(9)ꢀC82, D2d(23)ꢀC84 and C1(51383)ꢀC84. Figure 4 depicts their optimized structures and relative
energies. It is evident that the carbide cluster isomers are more than 6 kcal/mol higher in energy
than the corresponding Lu2@C82ꢀ86 isomers, suggesting that the dominating formation of diꢀEMFs
is thermodynamically very favorable.
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Figure 4. Optimized structures of Lu2@C82ꢀ86 isomers and their corresponding lowꢀenergy
Lu2C2@C80ꢀ84 isomers with relative energies (kcal/mol) and HOMOꢀLUMO gap energies (eV, in
parenthesis).
CONCLUSIONS
In summary, four unprecedented EMFs containing lutetium atoms have been isolated and
structurally determined to be diꢀEMFs, namely Lu2@Cs(6)ꢀC82, Lu2@C3v(8)ꢀC82,
Lu2@D2d(23)ꢀC84 and Lu2@C2v(9)ꢀC86 by singleꢀcrystal XRD crystallography. Our experimental
and theoretical results reveal that a LuꢀLu single bond is formed between the two lutetium ions
which transfer four electrons in total to the fullerene cages, thus resulting in a formal divalent state
for each Lu ion. Moreover, our theoretical calculations unambiguously reveal that the dominating
formation of diꢀEMFs are thermodynamically very favorable for Luꢀcontaining EMFs because of
the formation of a metalꢀmetal bond. Our study has not only added four new members to the
lessꢀexplored diꢀEMFs family, but also confirms that the metal-metal bonding can be achieved
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when the metals adopt low valence states inside fullerene cages, which may be a result from the
strong reductive atmosphere of carbon plasma during the arcꢀdischarging process.
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EXPERIMENTAL SECTION
Synthesis and Isolation of Lu2C82-86. Soot containing LuꢀEMFs was synthesized by a direct
current arc discharge method and was extracted using carbon disulfide. After removal of CS2, the
residue was dissolved in toluene and the solution was subjected to a threeꢀstage highꢀperformance
liquid chromatography (HPLC) separation. Further experimental details are described in
Supporting Information.
Single-Crystal XRD Measurements of Lu2C82-86. Crystalline blocks of Lu2C82ꢀ86 were obtained
by layering a benzene or a chloroform solution of NiII(OEP) over a nearly saturated solution of the
respective endohedral in CS2 in a glass tube. Over a 20ꢀday period, the two solutions diffused
together, and black crystals formed. XRD measurements were performed at 173 K on a Bruker D8
QUEST machine equipped with a CMOS camera (Bruker AXS Inc., Germany). The multiscan
method was used for absorption corrections. The structures were solved by direct method and
were refined with SHELXLꢀ2014/744. CCDCꢀ1539295 (Lu2@Cs(6)ꢀC82), CCDCꢀ1539296
(Lu2@C3v(8)ꢀC82), CCDCꢀ1539297 (Lu2@D2d(23)ꢀC84) and CCDCꢀ1539298 (Lu2@C2v(9)ꢀC86)
contain the supplementary crystallographic data for this paper. Details of the structural refinement
can be found in the Supporting Information.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the Internet at http://pubs.acg.org.
Additional crystal data for Lu2@Cs(6)ꢀC82•NiII(OEP)•2(C6H6)
Additional crystal data for Lu2@C3v(8)ꢀC82•NiII(OEP)•0.84(CHCl3) •1.16(CS2)
Additional crystal data for Lu2@D2d(23)ꢀC84•NiII(OEP)•2(CHCl3)
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Additional crystal data for Lu2@C2v(9)ꢀC86•NiII(OEP)•2(CHCl3)
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Experimental details, HPLC retention time and details of the visꢀNIR spectra of Lu2@C82−86
,
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crystallographic solution of Lu2@C82−86, the Xꢀray results of Lu2@C82−86, redox potentials (V vs
Fc/Fc+) of Lu2@Cs(6)ꢀC82 and Lu2@C2v(9)ꢀC86 and optimized geometries of Lu2C82ꢀ86 isomers.
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AUTHOR INFORMATION
Corresponding Author
*lux@hust.edu.cn
*china.peng.jin@gmail.com
*ffli@hust.edu.cn
*pputep@gmail.com
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
Financial support from NSFC (Nos. 51472095, 51672093, 51602112, 51602097 and 21103224)
and Program for Changjiang Scholars and Innovative Research Team in University (IRT1014) is
gratefully acknowledged. We thank the Analytical and Testing Center in Huazhong University of
Science and Technology for all related measurements.
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