Chemistry at the nanoscale: Synthesis of an N@C60-N@C 60 endohedral fullerene dimer

Article abstract of DOI:10.1002/anie.201107490

Rattling the cage: The rapid one-pot double 1,3-dipolar cycloaddition reaction of the rare endohedral fullerene N@C₆₀ to an oligo(p-phenylene polyethylene) bis(aldehyde) using a novel amino acid derivative as an anchoring group is reported. The

Full text of DOI:10.1002/anie.201107490

Angewandte
Chemie
DOI: 10.1002/anie.201107490
Cage Compounds
Chemistry at the Nanoscale: Synthesis of an N@C60–N@C60 Endohedral
Fullerene Dimer**
B. J. Farrington, M. Jevric, G. A. Rance, A. Ardavan, A. N. Khlobystov, G. A. D. Briggs, and
K. Porfyrakis*
Endohedral fullerenes—carbon cages with one or more
heteroatoms incarcerated within them—are one of the most
exotic classes of molecules. They possess a wealth of
fascinating functional properties, including magnetism and
photoactivity. While the properties of individual endohedral
fullerene molecules are remarkable,[1] the full functional
potential of many of these species will likely be realized in
systems where two or more of these molecules are connected.
One prominent example is the endohedral fullerene radical
species N@C60. The highly symmetric central location of the
nitrogen atom within the C60 cage,[2] with minimal mixing
between the nitrogen atom and fullerene electron wave-
functions, imparts a degree of isolation to the nitrogen radical
that is usually only attainable using an ion trap or in an atomic
including the creation of controlled entanglement between
electron spins, an array of at least two N@C60 centers with
fixed separation is required to understand the interaction of
neighboring spin centers with one another and investigate
methods of selective manipulation of one of the spin centers.
Herein we describe a one-pot method, using a double 1,3-
dipolar cycloaddition, for the synthesis of a two-center N@C60
molecule with fixed spatial separation (Scheme 1), thus
providing a platform for future experiments to probe the
nature of the electron interaction between two N@C60
molecules to assess the potential capabilities of a 2 qubit
N@C60 quantum computer. To the best of our knowledge, this
is the first endohedral fullerene dimer comprising two
chemically linked N@C60 spin centers.
4
gas. This isolation allows the nitrogen center to retain a S3/2
A Prato 1,3-dipolar cycloaddition reaction was chosen to
link the two N@C60 molecules owing to its proven compat-
ibility with the stability[10] of N@C60 and the wide range of
amino acid and aldehyde derivatives that it has been reported
to be compatible with. A dibenzaldehyde-terminated oligo(p-
phenylene polyethylene) (OPE) molecule (Ald-3Bz) was
synthesized and used as the spacing unit because of its rigidity
and modular construction. The incorporation of long alkyl
chains on the central phenyl group enhances the inherent
poor solubility of the rigid, linear molecule, thus allowing the
reaction to be performed at high concentration and making
the product fullerene dimer (dimer-3Bz) soluble in common
solvents such as chloroform. Previous work has shown that
the structure of the amino acid derivative used makes
a significant difference to the rate and yield of the 1,3-dipolar
cycloaddition reaction.[11] Work within our group has found
that the N-(4-(hexyloxy)benzyl)glycine amino acid derivative
greatly enhances the dimer formation reaction rate compared
to more commonly used amino acid derivatives such as N-
(ethyl)glycine (see the Supporting Information). In addition,
the amino group produces a fullerene dimer that can be
purified using standard single-pass HPLC methods.
electron spin ground state[3] and to have an extraordinarily
long electron spin coherence time (T2e of 250 ms).[4] This
remarkable spin coherence time has lead many research
groups to study the potential and feasibility of an N@C60-
based quantum computer over the past decade.[5] Owing to
a number of synthetic challenges associated with N@C60,
experimental electron spin resonance studies have so far been
restricted to probing the interaction of ensembles of mole-
cules each containing a single N@C60 spin center. These
challenges center on the low-yielding production methods
available to synthesize N@C60, thus producing at best
a 500 ppm N@C60/C60 mixture,[2,6] and the time consuming
processing techniques currently required to enrich the ratio of
N@C60 to C60.[7] In addition, compared to C60, chemically
functionalized N@C60 derivatives have significantly lower
thermal and photo stability.[8,9] To further explore the
potential of N@C60 as a quantum computing element,
[*] B. J. Farrington, Prof. G. A. D. Briggs, Dr. K. Porfyrakis
Department of Materials, University of Oxford
Oxford OX1 3PH (UK)
E-mail: kyriakos.porfyrakis@materials.ox.ac.uk
We synthesized three grams of a N@C60/C60 mixture using
an ion implantation method,[12] thus yielding an average
purity of 50 ppm of N@C60 in C60. Extensive purification of
this sample followed. After 19 recycling HPLC runs,[13] a high
purity N@C60/C60 peak was isolated. The integrated area of
the N@C60/C60 peak indicated the total mass of N@C60/C60 to
be 10 mg. This sample then underwent quantitative ESR
analysis of the total number of N@C60 spin centers present,
thereby giving a lower bound for the mass of N@C60 to be
4.6 mg.
Dr. M. Jevric, Dr. G. A. Rance, Prof. A. N. Khlobystov
School of Chemistry, University of Nottingham
Nottingham NG7 2RD (UK)
Dr. A. Ardavan
CAESR, The Clarendon Laboratory, Department of Physics
Oxford University, Oxford OX1 3PU (UK)
[**] We acknowledge the EPSRC and NSF for funding (EP/F028806/01).
B.J.F. is supported by the EPSRC through a DTA studentship. A.A.
and A.N.K. are supported by the Royal Society. We thank Dr. Simon
Plant for valuable suggestions and discussions during the course of
this work.
Optimized reaction conditions and an efficient purifica-
tion method for the one-pot dimer synthesis were identified
and scaled down to work using only 20 mg of the C60 starting
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107490.
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Scheme 1. Reaction scheme for the synthesis of the dimer-3Bz using 1% N@C60 in C60 and >50% N@C60 in C60 (bold typeface). The scheme
indicates the predicted statistical distribution of the N@C60 in the dimers collected together as the product fraction. For the reaction using 1%
N@C60 in C60, the distribution was calculated using the number of N@C60 centers per fullerene cage in the starting material (0.01); C60–
C
60 =0.992, N@C60–C60 =0.01ꢀ0.99, N@C60–N@C60 =0.012. For the reaction using >50% N@C60 in C60, the distribution was calculated from the
number of N@C60 centers per fullerene cage in the product dimer fraction, measured using HPLC peak and ESR signal integration to be 0.43(5);
C60–C60 =0.5652, N@C60–C60 =0.435ꢀ0.565, N@C60–N@C60 =0.4352.
material. To minimize thermal degradation of the spin-active
endohedral fullerene dimer the reaction was additionally
optimized to produce a 45% yield of the dimer product using
only a five-minute reaction time (Figure 1). Using the final
reaction conditions with a partially enriched N@C60/C60
sample produced a 79% retention of spin centers in the
dimer product compared to the N@C60 starting material.
After demonstrating that the reaction conditions were
compatible with N@C60, a higher purity N@C60–C60 dimer
sample was synthesized using 150 mg of approximately 1%
N@C60 in C60 (Scheme 1). A frozen solution CW ESR
spectrum of the dimer product is shown in Figure 2. This
spectrum was simulated using the Easyspin software pack-
age[14] to characterize the environment of the three unpaired
electrons present in the N@C60–C60 dimer. The fitted simu-
lated model used the following parameters: hyperfine split-
ting term (Aiso = 15.736 MHz), axial zero-field term (D =
Figure 1. Reaction progress for the formation of C60–C60 dimer (dimer-
14.19, E = 0.74), line-width 0.121 G, and giso = 2.0027 with
3Bz) after 0, 2, 5, 10, and 60 min reaction times, followed by HLPC.
a) Unreacted bis(aldehyde), b) mono-3Bz, c) dimer-3Bz, d) unreacted
C60. Insert: quantification of dimer-3Bz yield and unreacted C60, based
a RMS residual fit of 0.014. The principal fitting error is
assigned to a minor component of the 15N@C60–C60 dimer
present in the sample. These fitted parameters were consis-
on integrated area of HPLC peaks.
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Figure 3. a) Frozen-solution CW ESR spectra taken at 90 K of, the
N@C60–C60 dimer (dimer-3Bz) from the 1% N@C60/C60 dimer-3Bz
reaction (dotted black line); a mixture of N@C60–N@C60 and N@C60–
Figure 2. Frozen-solution CW ESR spectra taken at 90 K of N@C60–C60
dimer (dimer-3Bz) from 1% N@C60/C60 dimer reaction (green dots)
fitted to a model spectrum (blue line) using the following parameters:
hyperfine splitting term (Aiso =15.736 MHz), axial zero-field term
(D=14.19, E=0.74), line-width 0.121 G, and giso =2.0027 with a RMS
residual fit of 0.014.
C60 dimers (dimer-3Bz) from the >50% N@C60/C60 dimer-3Bz reaction
(solid red line); and a difference plot of the two spectra (green) below.
b) Difference spectrum (solid green line) fitted to simulated spectra of
an N@C60–N@C60 model with a maximum dipolar coupling strength
between the two spin centers of 2.67 MHz (dotted blue line). The
model is corrected for a 5.(6)% 15N@C60 impurity present in the
N@C60–C60 spectra.
tent with those found for previously reported pyrrolidine-
functionalized N@C60 adducts.[10,15]
Finally the reaction was repeated using the high purity
N@C60 sample shown in Scheme 1 and the dimer component
isolated by HPLC (see the Supporting Information). The
presence of a doubly filled dimer was probed using room
temperature (RT) and frozen-solution CW ESR spectroscopy.
By quantifying the number of moles of dimer produced using
the reaction HPLC trace (500 ng) and the total number of
N@C60 spin centers present, calculated from the integral of
the RT CW ESR spectra, the average number of spin centers
per cage was calculated to be 0.43(5). By assuming similar
chemical reactivity of N@C60 and C60, the fraction of N@C60–
N@C60 dimer and N@C60–C60 dimer can be calculated to be 19
and 49%, respectively (Scheme 1). A low temperature
frozen-solution ESR spectrum of the dimer fraction was
recorded and compared to a sample that only contained
N@C60–C60 dimer molecules (Figure 3).
The principal features of the two spectra are similar, with
inhomogeneous line broadening and the addition of zero-field
splitting peaks resulting from the reduction in the symmetry
of the fullerene cage after exohedral functionalization. By
subtracting the contribution of the N@C60–C60 dimer in the
mixed dimer sample the resulting ESR difference spectra can
be fitted to the simulated spectrum of a N@C60–N@C60 dimer
using the parameters fitted to the N@C60–C60 spectra shown in
Figure 2, with the addition of a dipolar coupling parameter
resulting from the interaction of the two spin centers. The best
simulation was obtained using an isotropic distribution of
molecular orientations with a maximum dipolar coupling
magnitude of 2.67 MHz, which corresponds to a separation of
the two N@C60 centers of 2.7 nm. This value is in excellent
agreement with the expected average distance between the
two N@C60 moieties in the fullerene dimer, which is estimated
by molecular modeling to be between 2.63 nm to 2.75 nm (see
the Supporting Information for details). This is the first
evidence of dipolar coupling between two covalently bonded
N@C60 molecules.
Herein we demonstrate the enhanced 1,3-dipolar cyclo-
addition reaction rate that can be achieved using the amino
acid derivative N-(4-(hexyloxy)benzyl)glycine to form a full-
erene dimer in one pot. A 45% yield of fullerene dimer
product could be recovered after a mere five-minute reaction
time. We also show that this reaction can be scaled down to
work using a 0.014 mmol scale, thus providing a route to
chemically functionalized, high purity, rare endohedral ful-
lerenes, such as N@C60.
The combination of low thermal stability of chemically
functionalized N@C60 and the requirement (assuming the
absence of a catalyst) for elevated temperature to activate the
1,3 dipolar cycloaddition reaction made optimizing the rate of
the dimer formation reaction key to the successful synthesis of
an N@C60–N@C60 dimer. A comparison of the product
mixture at a number of reaction time intervals using the
chosen amino acid N-(4-(hexyloxy)benzyl)glycine or the
more commonly used N-(ethyl)glycine in the dimer-forming
reaction (see the Supporting Information) shows that our
chosen amino acid provides a substantial enhancement of the
reaction rate. The addition of an aromatic substituent to the
amino group can greatly enhance the rate of 1,3-dipolar
cycloaddition to nanotubes;[16] rationalized by a preorganiza-
tion of the ylide with the sp2 nanotube surface through p–p
stacking. The same preorganization is likely to be present
between the fullerene cage and N-(4-(hexyloxy)benzyl)gly-
cine. This could then produce a double rate enhancement:
firstly, the probability of the three reagents meeting to react is
enhanced, and secondly, the electrophilic fullerene could
stabilize the formation of the ylide intermediate. It is also
possible that the presence of the o-hexyl substituent enhances
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quenched in an ice bath. The product mixture was passed through
a short silica plug to remove amino acid residues and then purified
using HPLC (4.6 mm ꢀ 250 mm BuckyprepM, eluent: 35% n-hexane,
65% toluene). A 3% yield (500 ng) of the product was recovered.
the reaction rate, firstly by increasing the solubility of the
amino acid in the reaction solvent, thus permitting a greater
stoichiometric ratio to be used, and secondly by enhancing the
p–p interaction between the fullerene and the amino acid
benzene ring through mesomeric donation of the oxygen
atom into the benzene ring.
Received: October 24, 2011
Published online: March 1, 2012
The ESR spectrum arising from a two-spin-center N@C60–
N@C60 molecule suggests that the presence of two N@C60
functionalized molecules in close proximity to one another
does not significantly affect the stability of the endohedral
nitrogen species. This stability is a fundamental requirement
for the successful implementation of N@C60 as a quantum
logic element.
Since our reaction can be made to work with microgram
quantities, the way is open for further chemical functionali-
zation of high purity N@C60. This method can be used to
create two-spin-center N@C60 molecules with the required
narrow coupling strength distribution that will allow N@C60 to
be exploited for quantum information processing applica-
tions. The next step is the ability to orient the molecules with
respect to a magnetic field. We are currently pursuing
experimental methods to accomplish this.
Keywords: cage compounds · cycloaddition · dimerization ·
fullerenes · synthesis design
.
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Experimental Section
All ESR spectra where recorded in toluene, using a Bruker EMX
spectrometer fitted with a liquid helium cooled cryoprobe.
Synthesis of C60–C60 dimer (dimer-3Bz). Ald-3Bz (10 mg; 1.5 ꢀ
10–2 mmol), C60 (32 mg; 4.4 ꢀ 10–2 mmol), and N-(4-(hexyloxy)ben-
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round bottom flask. The mixture was then sealed and degassed with
a flow of argon. Dry toluene (20 mL) was added to a 50 mL schlenk
tube, degassed using three freeze/pump/thaw cycles, and then
transferred by cannula to the flask containing the solid reagents.
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condenser and heated to 1108C for 1 hour in a preheated oil bath. The
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product mixture was passed through
a short silica plug then
concentrated and purified using HPLC (buckyprepM column (20 ꢀ
250 mm); eluent: toluene; flow rate: 18 mLminÀ1; retention time:
5.7 min). The product yield was 40%.
Synthesis of double-spin-labeled N@C60–N@C60 dimer (dimer-
3Bz). N-(4-(hexyloxy)benzyl)glycine (20 mg; 7.5 ꢀ 10À2 mmol), ca.
50% pure N@C60 (10 mg; 1.37 ꢀ 10À2 mmol) in C60 dissolved in toluene
(100 mL), and of Ald-3Bz (50 mL; 0.137 mmolLÀ1) in toluene were
added to a new, 7 inch 5 mm NMR tube. The reaction mixture was
concentrated to a final volume of 25 mL using a flow of argon gas and
a 2 ꢀ 5 mm magnetic stirrer bar was added. The reaction mixture was
heated to 1108C for 5 min in a preheated oil bath then removed and
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