Synthesis and characterization of a neutral tricoordinate organoboron isoelectronic with amines

Article abstract of DOI:10.1126/science.1207573

Amines and boranes are the archetypical Lewis bases and acids, respectively. The former can readily undergo one-electron oxidation to give radical cations, whereas the latter are easily reduced to afford radical anions. Here, we report the synthesis of a

Full text of DOI:10.1126/science.1207573

Synthesis and Characterization of a Neutral Tricoordinate
Organoboron Isoelectronic with Amines
Rei Kinjo et al.
Science 333, 610 (2011);
DOI: 10.1126/science.1207573
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REPORTS
8. J. Y. Park, P. A. Thiel, J. Phys. Condens. Matter 20,
314012 (2008).
9. C. Lee et al., Phys. Status Solidi B 246, 2562 (2009).
10. C. Lee et al., Science 328, 76 (2010).
between the tip and graphene. Puckering deforma- created by the radial stress around the particle
tion due to the tip pushing the ripple crest forward are manifested in the formation of domains ex-
along the scanning direction explains the 180° hibiting friction anisotropy with similar 180° pe-
anisotropy. Therefore, if Q is the relative angle riodicity (Fig. 2). The 60° shift between ripple
between the ripple line and the scan direction, we lines of adjacent domains supports our assump-
can then expect minima and maxima separated tion that the ripple lines are related to the crys-
by 90° with an angular dependence following a tallographic direction of the graphene. The friction
|sin Q| relation, because the in-plane force com- contrast in the domains also became weaker af-
ponent along the direction perpendicular to the ter thermal annealing at 200°C and disappeared
11. H. Lee, N. Lee, Y. Seo, J. Eom, S. W. Lee, Nanotechnology
20, 325701 (2009).
12. T. Filleter et al., Phys. Rev. Lett. 102, 086102 (2009).
13. A. C. Ferrari et al., Phys. Rev. Lett. 97, 187401 (2006).
14. J. Y. Park, D. F. Ogletree, P. A. Thiel, M. Salmeron,
Science 313, 186 (2006).
15. D. S. Grierson, E. E. Flater, R. W. Carpick, J. Adhes. Sci.
Technol. 19, 291 (2005).
ripple line of the AFM tip varies in this way.
completely after annealing at 400°C (fig. S4).
16. M. Dienwiebel et al., Phys. Rev. Lett. 92, 126101 (2004).
17. J. Y. Park et al., Science 309, 1354 (2005).
18. M. Liley et al., Science 280, 273 (1998).
19. R. W. Carpick, D. Y. Sasaki, A. R. Burns, Tribol. Lett. 7, 79
(1999).
20. K. Min, N. R. Aluru, Appl. Phys. Lett. 98, 013113 (2011).
21. A. Sakhaee-Pour, Solid State Commun. 149, 91 (2009).
22. S. V. Morozov et al., Phys. Rev. Lett. 97, 016801
(2006).
A high loading force induces a large contact The initial anisotropy was a characteristic of the
area (100 to 1000 nm2), which can decrease the graphene flake previous to equilibration. Because
relative contribution of ripple deformations to two-dimensional structured graphene has a neg-
the friction and thus the dependence on Q. This ative thermal expansion coefficient, heating and
is because puckering takes place at the exposed cooling processes can restore the graphene to
graphene in contact proximity to the tip (Fig. 4C). equilibrium, changing a stress-induced ripple (4).
Although the contact area increases at the high
Our results indicate that friction mapping on
Acknowledgments: Supported by the National Research
Foundation of Korea funded by the Ministry of Education,
Science, and Technology (National Research Laboratory
program grant 2008-0060004; World Class University
program grants R31-2008-000-10057-0 and R31-2008-
000-10055-0; Basic Science Research program grants
KRF-2008-314-C00111, KRF-2010-0005390, 2010-0015035,
2011-0014209, and 2011-0017605; and Quantum
Metamaterials Research Center grant R11-2008-053-03002-0).
M.B.S. was supported by the Office of Basic Energy Sciences,
Division of Materials Sciences and Engineering, U.S.
Department of Energy, under contract DE-AC02-05CH11231.
J.S.C. was supported by a Hi Seoul Science/Humanities
Fellowship from the Seoul Scholarship Foundation.
load, the contribution of the ripple deformation the graphene layer constitutes a powerful tool to
around the tip in relation to the contact area de- study the ripple structure formed on graphene
creases, and thus the bending of graphene becomes during mechanical exfoliation processes. Anoth-
less dependent on the scan direction. We can er interesting outcome of our research is that the
expect, therefore, more isotropic puckering and control of friction and anisotropic mechanical
friction at the high load. High-resolution friction properties, such as the one presented here with a
images showed the existence of lattice distortions large anisotropy ratio of 215%, could be ex-
that could be associated with the puckering (fig. ploited in solid lubrication of micro- or nano-
S1). These distortions were not observed on bulk electromechanical systems.
graphite images obtained under identical condi-
tions. Because bulk graphite can be considered
an extremely thick graphene, such distortion as-
sociated with puckering seems to decrease as the
graphene thickness increases.
As a further test of this model, we performed
FFM measurements on a graphene sheet acci-
dentally deposited over a particle in the sub-
strate. The images revealed mechanical wrinkles
and friction domains induced by stress in the
proximity of the particle (figs. S2 and S3). Ripples
References and Notes
1. K. S. Novoselov et al., Science 306, 666 (2004).
2. A. Hashimoto, K. Suenaga, A. Gloter, K. Urita, S. Iijima,
Nature 430, 870 (2004).
Supporting Online Material
www.sciencemag.org/cgi/content/full/science.1207110/DC1
Materials and Methods
Figs. S1 to S4
References
3. A. Fasolino, J. H. Los, M. I. Katsnelson, Nat. Mater. 6, 858
(2007).
4. W. Bao et al., Nat. Nanotechnol. 4, 562 (2009).
5. K. Xu, P. Cao, J. R. Heath, Nano Lett. 9, 4446
(2009).
6. C. H. Lui, L. Liu, K. F. Mak, G. W. Flynn, T. F. Heinz,
Nature 462, 339 (2009).
7. P. Y. Huang et al., Nature 469, 389 (2011).
18 April 2011; accepted 16 June 2011
Published online 30 June 2011;
10.1126/science.1207110
(CR2) and nitrenes (NR), have been spectro-
scopically characterized in solid inert gas matri-
ces at temperatures of a few K (10, 11) but to
date have eluded preparative isolation. Nonethe-
less, Braunschweig et al. (12) have shown that
borylenes can be incorporated into the ligand
sphere of stable, isolable transition metal com-
plexes (13).
In recent years, stable singlet carbenes such
as N-heterocyclic carbenes (NHCs) (14, 15) and
cyclic (alkyl)(amino)carbenes (CAACs) (16) have
proven as powerful as transition metal centers
for stabilizing highly reactive main group element
species (17, 18). In the boron series, Robinson
and co-workers (19, 20) have reported that re-
duction of the (NHC)BBr3 adduct A produced
the isolable stable neutral diborene B, which can
be regarded as a dimer of the parent borylene-
Synthesis and Characterization of a
Neutral Tricoordinate Organoboron
Isoelectronic with Amines
Rei Kinjo,1 Bruno Donnadieu,1 Mehmet Ali Celik,2 Gernot Frenking,2 Guy Bertrand1*
Amines and boranes are the archetypical Lewis bases and acids, respectively. The former can
readily undergo one-electron oxidation to give radical cations, whereas the latter are easily
reduced to afford radical anions. Here, we report the synthesis of a neutral tricoordinate
boron derivative, which acts as a Lewis base and undergoes one-electron oxidation into the
corresponding radical cation. These compounds can be regarded as the parent borylene (H-B:)
and borinylium (H-B+.), respectively, stabilized by two cyclic (alkyl)(amino)carbenes. Ab initio
calculations show that the highest occupied molecular orbital of the borane as well as the singly
occupied molecular orbital of the radical cation are essentially a pair and a single electron,
respectively, in the p(p) orbital of boron.
1UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957),
Department of Chemistry, University of California Riverside
(UCR), Riverside, CA 92521–0403, USA. 2Fachbereich Chemie,
Philipps-Universitat Marburg, Hans-Meerwein-Strasse, 35032
Marburg, Germany.
he chemistry of boron is dominated by numerous clusters involving hypervalent boron
compounds in which the element adopts centers are known (1). At the opposite extreme,
the +3 oxidation state and acts as a potent it is only recently that low-valent boron deriv-
T
electron pair acceptor, or Lewis acid. To com- atives have been thoroughly explored (2–9).
pensate its intrinsic electron deficiency, boron Among these species, borylenes (BR), the sub-
*To whom correspondence should be addressed. E-mail:
also often participates in multicenter bonds, and valent boron(I) derivatives analogous to carbenes guy.bertrand@ucr.edu
610
29 JULY 2011 VOL 333 SCIENCE www.sciencemag.org

REPORTS
carbene complex (Fig. 1). By using a similar syn- and a flexible cyclohexyl moiety as the second diffraction study were obtained by recrystalliza-
thetic approach, Braunschweig and co-workers carbene substituent. The first CAAC was installed tion from a dry tetrahydrofuran (THF) solution
(21) generated the parent borylene-carbene classically (19–21) by reaction of 1 with BBr3 in at room temperature. In the solid state (Fig. 3,
complex D, and, although they were not able hexane, which afforded the (CAAC)BBr3 ad- left), the carbene carbons C1 and C2, boron, and
to characterize it spectroscopically, trapping ex- duct 2 in 94% yield (26). Then, in the hope of the hydrogen H1 are in a perfectly planar ar-
periments left no doubt of its transient existence. trapping the putative (CAAC)BH adduct by a rangement (sum of the bond angles at B is
The extreme reactivity of borylenes is due to second CAAC, a fivefold excess of potassium 359.94°). The B1-C1 [1.5175 0.0015 Å] and
their two vacant orbitals as well as the presence graphite was added to 1/1 mixture of boron ad- B1-C2 [1.5165 0.0015 Å] bond distances are
of a lone pair of electrons. We reasoned that two duct 2 and CAAC 1 in dry toluene. The reac- equal and are halfway between typical B-C single
carbene ligands might simultaneously decrease tion mixture was stirred at room temperature (1.59 Å) and double (1.44 Å) bonds (28), sug-
the Lewis acidity while also accepting sufficient for 14 hours, and, from a complex mixture of gesting the delocalization of the lone pair of elec-
electron density to diminish the Lewis basicity unidentified products, compound 3 was iso- trons at boron to the empty p orbitals of the carbene
and thereby stabilize a borylene. Having already lated as a red powder in only 8% yield (Fig. 2). centers. Ab initio calculations performed on 3 at
shown that CAACs are slightly more nucleo- Unexpectedly, when the same experiment was the BP86/def2-SVP level of theory support this
philic but considerably more electrophilic than carried out in the absence of CAAC 1, the bonding analysis. The carbene→BH donation
NHCs (22–24), we considered them good can- (CAAC)2BH adduct 3 was also formed and occurs from the s lone pairs of carbene ligands
didates. Here, we report the isolation of the par- isolated in 33% yield. The 1H-decoupled 11B nu- into the empty in-plane molecular orbital at bo-
ent borylene supported by two CAACs. We show clear magnetic resonance (NMR) spectrum of 3 ron, affording two low-lying orbitals. The high-
that this neutral tricoordinate organoboron, fea- shows a broad signal at 12.5 parts per million est occupied molecular orbital (HOMO) of 3
turing boron in the +1 oxidation state, can be (ppm) with a half-width of 216 Hz. In the proton- (–3.34 eV) is essentially an electron lone pair in
oxidized to afford the corresponding stable rad- coupled 11B NMR spectrum, no clear splitting the p(p) orbital of boron, which mixes in a bond-
ical cation and even protonated to give the con- was observed, but a broadening of the signal ing fashion with the p(p) atomic orbital of the two
jugate acid.
with a half-width of 261 Hz was. The presence carbene carbons (Fig. 4, left). The charge exchange
In order to be able to protect the boron cen- of the hydrogen atom at boron was confirmed via s donation and p backdonation leaves the
ter while still having space for coordination of by an infrared absorption at 2455 cm−1 (fig. S1), BH moiety in 3 with a partial charge of +0.05 e.
two carbenes, we chose CAAC 1 (25), featuring which can be assigned to the B-H stretching For comparison, the BH fragment in (CH3)2BH,
a bulky 2,6-diisopropylphenyl group at nitrogen mode (27). Single crystals suitable for an x-ray which has two B-C electron-sharing bonds, car-
ries a positive charge of +0.61 e. Therefore, the
zwitterionic form 3b, featuring a dianionic boron
center (29), is a far weaker resonance contrib-
Fig. 1. (Top) Synthesis of
the neutral bis(carbene)-
utor than 3a, which shows the parent borylene
stabilized diborene B,
coordinated by two carbene ligands.
the dimer of the parent
borylene-NHC adduct, by
Robinson and co-workers
(19, 20) (Dipp indicates
The boron in compound 3 is in the formal
oxidation state +1 and is electron-rich. This was
confirmed by the cyclic voltammogram (fig. S2)
A
C
B
D
of a THF solution of 3 (0.1 M nBu4NPF6 elec-
trolyte), which shows a reversible one-electron
oxidation at E1/2 = –0.940 V versus Fc+/Fc (Fc is
ferrocene). Indeed, addition at room temperature
of two equivalents of gallium trichloride to a tol-
uene solution of 3 quantitatively afforded the
2,6-diisopropylphenyl;
Me, methyl group). (Bot-
tom) Generation of a tran-
sient parent borylene-NHC
adduct D by Braunschweig
and co-workers (21) (Np,
naphthalene).
radical cation [3+.]GaCl4 . A single crystal x-ray
–
diffraction study showed that the boron center of
[3+.]GaCl4– is in a perfectly planar arrangement,
as observed for its precursor 3 (Fig. 3, center).
Fig. 2. Synthesis of the parent borylene-bis(CAAC) adduct
3 represented under two canonical forms, its oxidation
with gallium trichloride to the stable radical cation 3+.
–
and protonation with trifluoromethanesulfonic acid to afford the boronium salt [3H+]CF3SO3 .
www.sciencemag.org SCIENCE VOL 333 29 JULY 2011
611

REPORTS
However, the boron-carbon and carbon-nitrogen Calculations using the natural bond orbital (NBO) toluene solution of compound 3 at room tem-
bond distances are longer and shorter, respec- method confirmed that the spin density is main- perature, and after work up the conjugate acid
–
tively, than those of 3, in line with the weaker ly located at boron (0.50 e) with some contribu- [3H+]CF3SO3 was isolated in 89% yield. The
electron-donation from boron to the carbene lig- tions of the nitrogen atoms (0.16 e and 0.17 e). proton-coupled 11B NMR spectrum of this salt
and. Compound [3+.]GaCl4 is one of very few The singly occupied molecular orbital (SOMO) shows a triplet (JBH = 83.5 Hz) at –21.8 ppm,
–
crystallographically characterized boron radicals (–7.30 eV) is essentially the boron p orbital, confirming the presence of two hydrogen atoms
(30) and molecules featuring boron in the for- weakly mixing with the p(p) atomic orbital of directly bonded to boron and thus the boronium
–
mal +2 oxidation state (31). The room-temperature the two carbene carbons (Fig. 4, right).
electron paramagnetic resonance spectrum in THF
nature (35) of [3H+]CF3SO3 . The solid-state
Because of the presence of a lone pair of structure confirmed the tetracoordination of bo-
solution displays a complex system (g = 2.0026) electrons at boron, bis(carbene)BH adduct 3 has ron. The boron-carbon and carbon-nitrogen bond
because of the couplings with the boron [a(11B) = the potential to react with electrophiles, which is distances are in the range of single and double
6.432 G], hydrogen [a(1H) = 11.447 G], and two quite unusual for tricoordinate boron compounds bonds, respectively, in line with the absence of
nitrogen nuclei [a(14N) = 4.470 G] (fig. S3). The (5–9, 34). No reactions of 3 were observed with back-donation from boron to the carbene lig-
values of spin couplings with the 11B and 1H nu- trimethylsilyl- or methyl-trifluoromethanesulfonate and. To quantify the basicity of 3, we calculated
clei are similar to those observed in the persistent even after heating at 80°C for 14 hours, prob- (BP86/def2-SVP+ZPE) its gas phase proton af-
(NHC)BH2 radical, whereas the coupling con- ably because of the presence of the two bulky finity: The 1108 kJ mol−1 value is much higher
stant with the 14N nuclei is greater than those in CAAC ligands, which shield the boron center. than that calculated for the free BH (856 kJ mol−1)
the NHC adducts (32, 33), in line with the higher To probe basicity further, we added an equimolar and comparable to the unsaturated free N-phenyl–
electron-acceptor ability of CAACs versus NHCs. amount of trifluoromethane sulfonic acid to a substituted NHC (1107 kJ mol−1) (36). In toluene
solution, we found that 3 is readily protonated by
BrCH2CO2H, whereas the reaction with PhCO2H
proceeded very slowly, and only trace amounts
of [3H+]PhCO2 were detected after 14 hours.
–
Boronium [3H+]CF3SO3– is rapidly deprotonated
by sodium ethoxide in a THF solution, giving
back 3 in 68% yield, although no reaction was
observed with strong but bulky bases such as po-
tassium hexamethyldisilazide, lithium diisopropyl-
amide, and t-butyllithium, confirming the steric
shielding of the boron center (37, 38).
Although the parent borylene adduct 3 and
the radical cation [3+.]GaCl4– are sensitive to air,
they are stable at room temperature under ar-
gon both in solution and in the solid state for
2 months at least (melting point of 3 is 328°C;
[3+.]GaCl4 , 278°C), which demonstrates the sta-
–
bilizing efficiency of CAACs. In marked con-
trast to the well-known tricoordinate boron(+3)
derivatives, compound 3, featuring a boron in
the +1 oxidation state, behaves as a Lewis base
and can readily be oxidized. Its reactivity with
electrophiles is hampered by the bulkiness of the
CAAC ligands, but the steric and electronic prop-
erties of carbenes can be substantially modulated.
Compounds of type 3 are isoelectronic with amines
Fig. 3. Molecular views (50% thermal ellipsoids are shown) of the parent borylene-bis(CAAC) 3 (left),
radical cation 3+. (center), and boronium 3H+ (right) in the solid state (for clarity, H atoms of the carbene
ligand and the counterions GaCl4– for 3+. and CF3SO3– for 3H+ are omitted). Selected bond lengths (Å)
and angles (°) are given in the table; for comparison, the calculated values at the (U)BP86/def2-SVP level
of theory are given in brackets.
and phosphines, and, because of the lower electro-
negativity of boron compared to those of nitrogen
and phosphorus, they are potential strong electron-
donor ligands for transition metals.
References and Notes
1. R. N. Grimes, J. Chem. Educ. 81, 657 (2004).
2. D. Vidovic, S. Aldridge, Chem. Sci. 2, 601 (2011).
3. M. Yamashita, K. Nozaki, J. Synth. Org. Chem. Jpn. 68,
359 (2010).
4. R. C. Fischer, P. P. Power, Chem. Rev. 110, 3877 (2010).
5. Y. Segawa, M. Yamashita, K. Nozaki, Science 314, 113
(2006).
6. T. B. Marder, Science 314, 69 (2006).
7. H. Braunschweig, Angew. Chem. Int. Ed. 46, 1946
(2007).
8. K. Nozaki, Nature 464, 1136 (2010).
9. M. S. Cheung, T. B. Marder, Z. Lin, Organometallics 30,
3018 (2011).
10. P. Hassanzadeh, L. Andrews, J. Phys. Chem. 97, 4910 (1993).
11. H. F. Bettinger, J. Am. Chem. Soc. 128, 2534 (2006).
12. H. Braunschweig, C. Kollann, U. Englert, Angew. Chem.
Fig. 4. Plot of the calculated HOMO (–3.34 eV) of the parent borylene-bis(CAAC) 3 (left) and SOMO
(–7.30 eV) of the radical cation 3+. (right).
Int. Ed. 37, 3179 (1998).
612
29 JULY 2011 VOL 333 SCIENCE www.sciencemag.org

REPORTS
13. H. Braunschweig, R. D. Dewhurst, A. Schneider,
Chem. Rev. 110, 3924 (2010).
14. F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 47, 3122
(2008).
26. Preparation methods and spectroscopic data for
38. M. Sigl, A. Schier, H. Schmidbaur, Chem. Ber. 130, 1411
compounds 2, 3, [3+.]GaCl4–, and [3H+]CF3SO3 and
computational details are available as supporting
material on Science Online.
(1997).
–
Acknowledgments: We are grateful to the NSF
(CHE-0924410), U.S. Department of Energy
(DE-FG02-09ER16069), and DFG for financial support
of this work and to the Japan Society for the Promotion
of Science for a postdoctoral fellowship (R.K.). We thank
E. Stoyanov for helpful advice in infrared spectroscopy.
Metrical data for the solid state structures of 3, [3+.]
GaCl4–, and [3H+]TfO– are available free of charge
from the Cambridge Crystallographic Data Centre under
reference numbers CCDC-822246, CCDC-822247,
and CCDC-822248, respectively. The authors and UC
Riverside have filed a patent.
15. D. Bourissou, O. Guerret, F. P. Gabbaï, G. Bertrand,
27. Attempts to prepare the B-D analog using deuterated
toluene as solvent failed, indicating that the hydrogen
atom is abstracted from an aryl group of a carbene.
28. M. M. Olmstead, P. P. Power, K. J. Weese, R. J. Doedens,
J. Am. Chem. Soc. 109, 2541 (1987).
29. J. Monot et al., Angew. Chem. Int. Ed. 49, 9166 (2010).
30. M. M. Olmstead, P. P. Power, J. Am. Chem. Soc. 108,
4235 (1986).
31. R. Dinda et al., Angew. Chem. Int. Ed. 46, 9110 (2007).
32. T. Matsumoto, F. P. Gabbaï, Organometallics 28, 4252
(2009).
33. J. C. Walton et al., J. Am. Chem. Soc. 132, 2350 (2010).
34. H. Braunschweig, C. W. Chiu, K. Radacki, T. Kupfer,
Angew. Chem. Int. Ed. 49, 2041 (2010).
Chem. Rev. 100, 39 (2000).
16. M. Melaimi, M. Soleilhavoup, G. Bertrand, Angew. Chem.
Int. Ed. 49, 8810 (2010).
17. Y. Z. Wang, G. H. Robinson, Chem. Commun. (Camb.)
2009, 5201 (2009).
18. D. Martin, M. Soleilhavoup, G. Bertrand, Chem. Sci. 2,
389 (2011).
19. Y. Wang et al., J. Am. Chem. Soc. 129, 12412 (2007).
20. Y. Wang et al., J. Am. Chem. Soc. 130, 3298 (2008).
21. P. Bissinger, H. Braunschweig, K. Kraft, T. Kupfer,
Angew. Chem. Int. Ed. 50, 4704 (2011).
22. V. Lavallo, Y. Canac, B. Donnadieu, W. W. Schoeller,
G. Bertrand, Angew. Chem. Int. Ed. 45, 3488 (2006).
23. O. Back, B. Donnadieu, P. Parameswaran, G. Frenking,
G. Bertrand, Nat. Chem. 2, 369 (2010).
Supporting Online Material
www.sciencemag.org/cgi/content/full/333/6042/610/DC1
Materials and Methods
Figs. S1 to S6
Tables S1 and S2
35. W. E. Piers, S. C. Bourke, K. D. Conroy, Angew. Chem.
Int. Ed. 44, 5016 (2005).
36. R. Tonner, G. Heydenrych, G. Frenking, ChemPhysChem
9, 1474 (2008).
24. G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller,
References (39–45)
G. Bertrand, Science 316, 439 (2007).
25. V. Lavallo, Y. Canac, C. Prasang, B. Donnadieu,
G. Bertrand, Angew. Chem. Int. Ed. 44, 5705 (2005).
37. Unsuccessful attempts to deprotonate bis(phosphine)BHX
27 April 2011; accepted 3 June 2011
10.1126/science.1207573
adducts (X: H, Br) were reported (38).
insertion of a molecule through it—wide var-
ieties of endohedral fullerenes could be synthe-
sized by organic synthesis because restoration of
the small opening should be easier than that of
the larger one. We report the macroscopic syn-
thesis of H2O@C60 using such a dynamic control
of the opening size with a series of organic re-
actions, as well as the structure and the properties
A Single Molecule of Water
Encapsulated in Fullerene C60
Kei Kurotobi and Yasujiro Murata*
Water normally exists in hydrogen-bonded environments, but a single molecule of H2O without
any hydrogen bonds can be completely isolated within the confined subnano space inside fullerene of the molecule in which a molecule of H2O is
C60. We isolated bulk quantities of such a molecule by first synthesizing an open-cage C60
derivative whose opening can be enlarged in situ at 120°C that quantitatively encapsulated one
water molecule under the high-pressure conditions. The relatively simple method was developed
to close the cage and encapsulate water. The structure of H2O@C60 was determined by
single-crystal x-ray analysis, along with its physical and spectroscopic properties.
completely isolated in a confined space.
The synthetic route for open-cage C60 deriva-
tives is outlined in Fig. 1A. Applying our previ-
ous synthetic route (13, 14), diketones 3a and
3b were synthesized in 29 and 43% isolated
yields, respectively, from the reaction of C60 with
he unusual properties of bulk water, such (ii) ion implantation to empty C60 and C70, and pyridazine derivatives 1a and 1b followed by
as its high boiling and melting points, high (iii) high-pressure and high-temperature treatments photochemical cleavage of one of the C=C double
dielectric constant, and ability to act as of empty fullerenes with rare gases (5). However, bonds on the rim of the opening on intermediates
T
both acid and base, arise from hydrogen bonding these methods are not suitable to obtain endohe- 2a and 2b, respectively (15) (figs. S1 to S12). The
in bulk. Clusters of H2O molecules, both in the dral fullerenes encapsulating small molecules. lowest unoccupied molecular orbital (LUMO) of
gas phase (1) and confined in nanoscale spaces The fourth method for synthesizing endohe- open-cage C60 with a similar opening motif is
defined by the hydrophobic interiors of carbon dral fullerenes is the molecular surgical approach mainly located on the conjugated butadiene
nanotubes (2) or a self-assembled coordination (6), which includes construction of an opening moiety (13), and sulfur (13, 14) and hydrazine
cage (3), also undergo hydrogen bonding. How- on an empty C60 or C70 cage, insertion of small derivatives (11) were reported to react with that
ever, a single molecule of H2O without any hy- molecules through it, and then restoration of the moiety. N-Methylmorpholine N-oxide (NMMO) is
drogen bonding to other organic molecule or original framework, retaining the encapsulated known as a nucleophilic oxidant (16), and we
coordination to metal is rare so far (4).
species (7). By using such an approach, hydro- found that the reaction of 3 with 2.3 equivalents
The inner space of the fullerene C60, which gen molecules have been entrapped in C60 and of NMMO in wet tetrahydrofuran (THF) at room
is spherical with a diameter of 3.7 Å, is suitable C70 in macroscopic quantities (8, 9). To apply temperature led to the synthesis of 5a and 5b,
to entrap a water molecule. When atoms or mol- this method to a larger molecule than H2, devel- respectively, in good yields after purification with
ecules are encapsulated in fullerenes, it is often opments of synthetic methods are needed to create silica gel chromatography (figs. S13 to S18).
possible to control the properties of the outer open-cage fullerenes with an opening large enough
The structure of 5a determined by single-
carbon cage as well as to study the isolated spec- for the small molecule to pass through (10). crystal x-ray analysis (Fig. 1B) shows that the
ies. Endohedral fullerenes encapsulating a wide Fullerenes with large openings to encapsulate an opening is constructed by the 13-membered ring
variety of species, including metal ions, rare gases, H2O molecule have been reported (11, 12), but containing two hemiacetal carbons, in addition to
and nitrogen atoms, have been synthesized with the encapsulated H2O is in an equilibrium, with two carbonyl carbons (table S1). This molecule
physical methods under harsh conditions such as outer H2O molecules existing in the system in can be considered as the hydrate of tetraketone
(i) arc discharge of carbon rods containing metal, large amounts. Furthermore, attempts to close 4a having the 16-membered-ring opening with
such large openings have not been reported.
four carbonyl carbons. Because wet THF was used
If the size of an opening on fullerenes can be in this reaction, a water molecule attacked one of
controlled dynamically—that is, a small opening the carbonyl carbons in 4a to give 5a. When the
changes into a larger one in situ under specific hydrate 5a was heated at reflux temperature in
conditions and regenerates itself again after toluene for 30 min, complete transformation of
Institute for Chemical Research, Kyoto University, Uji, Kyoto
611-0011, Japan.
*To whom correspondence should be addressed. E-mail:
yasujiro@scl.kyoto-u.ac.jp
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