A reassignment of the EPR spectra previously attributed to Cu@C-60

Abstract

EPR spectra attributed to the endohedral metallofullerene Cu@C60 are better explained by the previously characterized Cu(II) dithiocarbamate family of compounds.

Full text

A reassignment of the EPR spectra previously attributed to Cu@C
60
Bevan Elliott,
a
Keqin Yang,
b
Apparao M. Rao,
b
Hadi D. Arman,
a
William T. Pennington
a
and
Luis Echegoyen*
a
Received (in Cambridge, UK) 9th January 2007, Accepted 26th January 2007
First published as an Advance Article on the web 22nd February 2007
DOI: 10.1039/b700320j
EPR spectra attributed to the endohedral metallofullerene
Cu@C
60
are better explained by the previously characterized
Cu(II) dithiocarbamate family of compounds.
Though many endohedral mono-metallofullerenes having rela-
tively large carbon cages have been isolated, characterized, and
chemically functionalized, M@C
60
and M@C
70
species have so far
remained largely elusive due to their low solubility in common
organic solvents and their general instability in air. Recently, a
report of the synthesis, isolation, and characterization of a new
endohedral fullerene, Cu@C
60
, was published by Huang et al in
this journal.
1
In their synthesis, copper ions sputtered into a
nitrogen rf-plasma collided with sublimated C
60
, and the resulting
deposit was solubilized in CS
2
and subsequently examined by EPR
spectroscopy. The two observed overlapping patterns of four
completely resolved hyperfine lines having slightly different
hyperfine splittings and the correct isotopic intensities gave
unequivocal proof of paramagnetism arising from unpaired
electron density on
63,65
Cu nuclei. On the basis of a TOF-MS
spectrum with a main peak at m/z783, the g-value of the EPR
hyperfine pattern, and its m
I
-dependent line widths, the assignment
to a carbon-60 cage containing an off-center Cu
2+
ion was made.
In 2005, Dinse and co-workers published both room-temperature
and liquid nitrogen-frozen CS
2
solution EPR spectra of a radical
which was produced in a standard Kra¨tschmer–Huffman (K–H)
arcing apparatus by unintentional addition of copper from the
graphite electrode mounts.
2
Since their room-temperature solution
spectrum was identical to that in the previous study, Dinse and co-
workers also assigned it to Cu@C
60
, though they did not report a
mass spectrum of the compound. The m
I
-dependent spectral line
widths and the temperature behavior, as well as DFT structure
optimizations of the radical, were believed to be indications that
the off-center Cu
2+
endohedral ion was ‘internally docking’ to the
inner carbon cage on the EPR timescale. Also in 2005, a
theoretical paper motivated by the first report of Cu@C
60
was
published.
3
In it, energy optimization calculations using the
Hartree–Fock method yielded a model of Cu@C
60
with an off-
center copper ion inside a deformed carbon cage and a small but
positive binding energy. In this paper, we report attempts to
synthesize Cu@C
60
by K–H arc in large enough quantities for
complete characterization, and the surprising finding that the
isolated radical corresponds to a dithiocarbamate copper complex,
not Cu@C
60
. However, the radical’s EPR spectrum is identical to
that assigned to Cu@C
60
.
In an effort to duplicate the published results, a modified K–H
apparatus was utilized.
4
In a 300 Torr helium atmosphere, an 80 A
dc arc was struck between a graphite anode and a core-drilled
cathode into which a piece of copper metal had been inserted. The
soluble portion of the soot produced from this synthesis was
extracted in CS
2
, and EPR spectra showed the presence of the
copper radical previously reported, but in very small amounts. To
increase the yield (monitored by the intensity of the EPR signal),
variations in the elemental composition and configuration of the
rods, helium pressure, arc current, and extraction solvents were
explored. Cathode composition included pure graphite, core-
drilled graphite with an inserted copper slug, and pure copper.
Anodes were either pure graphite or composite carbon/copper
powder annealed molded rods. Extraction solvents consisted of
CS
2
, pyridine, or mixtures of the two, since M@C
60
species have
been shown to be preferentially extractable in pyridine and
aniline.
5
Yields of the copper-centered radical remained low until
both yttrium and nickel were added in catalytic amounts to the
anode. Milligram quantities of the radical compound were then
collected when a 1 : 1 mixture of CS
2
–pyridine was used to extract
soot produced by arcing a carbon/Y/Ni composite anode against a
pure copper cathode.
The compound was purified by column chromatography in a
1 : 1 eluent ratio of toluene : CS
2
after first evaporating the CS
2
–
pyridine mixture and redissolving and filtering in CS
2
. Two yellow
bands having similar R
f
values by TLC were eluted separately, and
the band containing the paramagnetic species was collected, dried,
and redissolved in CS
2
for EPR studies. Fig. 1 shows the X-band
first-derivative EPR absorption spectrum of the radical in solution
at room temperature. With g-values of 2.0483 and 2.0477 and
hyperfine coupling constants of 77.4 and 82.7 G for
63
Cu and
65
Cu
nuclei, respectively, in an isotopic ratio of 2.25, this spectrum
is identical to those reported by Huang et al,
1
and Dinse and
co-workers.
2
MALDI-TOF-MS of this compound did not yield the expected
peak corresponding to Cu@C
60
despite repeated attempts. Instead,
shown in Fig. 2, the most intense feature of the spectrum was a
double peak at m/z383 and 385 with an isotopic ratio indicative of
a copper-containing species. Cyclic voltammetry in o-dichloroben-
zene (Fig. 3) showed only two reversible waves at 0.11 and
21.04 V vs. Fc/Fc
+
within the entire solvent window, a behaviour
that is not typical of fullerenes. At this point, it was discovered that
the compound was highly soluble in many organic solvents
ranging in polarity from CS
2
to methanol. Again, no known
pristine fullerenes behave this way. The compound was then
dissolved in THF, and UV-Vis spectra collected. The spectrum in
Fig. 4 shows an intense absorption at 272 nm with a shoulder at
287nmwhichdominatesthespectrum,andabroadbandwith
a
Department of Chemistry, Clemson University, Clemson, SC, USA.
E-mail: luis@clemson.edu; Fax: +1 (864) 656 6613;
Tel: +1 (864) 656 0778
b
Department of Physics, Clemson University, Clemson, SC, USA
COMMUNICATION www.rsc.org/chemcomm | ChemComm
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Chem. Commun.
, 2007, 2083–2085 |2083

l
max
at 435 nm followed by a very broad, weaker, tailing band that
extends past 700 nm.
As no clear evidence indicating the presence of Cu@C
60
was yet
present except for the EPR spectrum in our characterization of this
compound, crystallization and X-ray diffraction were attempted.
Slow evaporation of a concentrated solution in CS
2
yielded flat
reddish-black parallelpipeds which were examined by single-crystal
X-ray diffraction at room temperature. The resulting structure, a
copper ion coordinated to two thiocarbamate ligands with
terminal piperidine groups, shown in Fig. 5, was quite
unexpected.{A search of single-crystal structures yielded a result
identical to ours,
6
namely, bis(piperidine-1-dithiocarbamato)
copper(II), the divalency of the copper causing the paramagnetism
of the compound. The complexation of CS
2
ligands by Cu
2+
is
easily explained, but the origin of our piperidine N-terminal
ligands is not as clear. One possible source is the pyridine used in
the extraction.
More information about the Cu(II) dithiocarbamate family of
compounds was obtained from an extensive literature search.
Those reports mentioned herein are merely representative, not
exhaustive, since the literature abounds with such examples. The
first report of electron spin resonance of a divalent copper
dithiocarbamate compound was published in 1959 in Nature.
7
The
reported X-band benzene solution spectrum of the Cu(II)
diisopropyl dithiocarbamate complex is in all respects identical
Fig. 1 X-Band EPR spectra in CS
2
of the purified Cu
2+
radical
compound.
Fig. 2 MALDI-TOF-MS spectrum of the purified Cu
2+
radical
compound.
Fig. 3 Cyclic voltammogram in oxygen-free THF with TBAPF
6
as the
supporting electrolyte. Pt working and auxiliary electrodes with a Ag/Ag
+
pseudo-reference electrode were utilized in a one-compartment cell.
Fig. 4 UV-Vis spectrum in THF of the purified copper compound.
Fig. 5 Single-crystal X-ray structure of bis(piperidine-1-dithiocar-
bamato)copper(II). Thermal ellipsoids are shown at 50% probability and
solvent molecules are omitted for clarity. The central metal atom lies on an
inversion center and the atoms generated by the inversion are labelled with
suffix (A).
2084 |
Chem. Commun.
, 2007, 2083–2085 This journal is ßThe Royal Society of Chemistry 2007

to that published by Huang et al., and Dinse and co-workers,
and as shown in this report. According to this early article,
exchanging the terminal groups on the dithiocarbamate
ligand (ethyl, isopropyl and methyl phenyl) had little or no effect
on the solution EPR spectrum. Also interesting in this article
was the authors’ explanation of the m
I
-dependent line
widths, which were attributed to insufficient averaging of
anisotropy in the resonance structure. Another solution EPR
study of a series of Cu(II) dithiocarbamate complexes offered a
different interpretation: that the variation in linewidths is due to
the orientation of the copper nuclear spin with respect to the
applied magnetic field.
8
EPR spectra of solid-phase cupric dithiocarbamate complexes
have also been reported in the literature. Room-temperature
EPR spectra of thin films of PVC doped with 0.5% Cu(II)
diethyldithiocarbamate
9
are identical to the low-temperature
spectra reported by Dinse and co-workers, as are liquid-
nitrogen solid-solution spectra of the same molecule in another
article.
10
Previously reported voltammetry and UV-Vis spectra of
cupric dithiocarbamate complexes with a series of different
N-terminal ligands showed qualitative similarity with our results,
with the N,N-dipiperidine species quantitatively identical to our
data.
11
Though theoretically the endohedral mono-metallofullerene
Cu@C
60
is a stable molecule,
3
theonlyevidencetodateofthe
existence of Cu@C
60
is the mass spectrum reported by Huang
et al.
1
However, the EPR spectra assigned to Cu@C
60
can be
accounted for by the compounds formed by the complexation of
CS
2
in solution by Cu
2+
ions. Despite repeated attempts under a
wide variety of conditions, we failed to detect the presence of
Cu@C
60
. More research, both experimental and theoretical, is
needed to ascertain the stability and existence of divalent d-block
metals encapsulated by small fullerenes.
We thank the U.S. National Science Foundation (Grants
CHE0509989 and DMI0304019) for support of this work. Thanks
to Amit Palkar for assistance in the early stages of this
investigation. This material was based on work supported by the
National Science Foundation, while working at the Foundation.
Any opinions, findings, conclusions, or recommendations
expressed in this material are those of the authors, and do not
necessarily reflect the views of the National Science Foundation.
Notes and references
{The unit cell parameters for [Cu(C
6
H
10
NS
2
)
2
] synthesized in this study
are as follows: space group = P2
1
/c,a= 6.1726(16), b= 5767(2), c=
15.266(4) A
˚,b= 95.442(10)u,V= 804.5589 A
˚
3
.
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This journal is ßThe Royal Society of Chemistry 2007
Chem. Commun.
, 2007, 2083–2085 |2085