Transition metal complexes of a salen-fullerene diad: Redox and catalytically active nanostructures for delivery of metals in nanotubes

Article abstract of DOI:10.1002/chem.201300872

A covalently-linked salen-C₆₀ (H₂L) assembly binds a range of transition metal cations in close proximity to the fullerene cage to give complexes [M(L)] (M=Mn, Co, Ni, Cu, Zn, Pd), [MCl(L)] (M=Cr, Fe) and [V(O)L]. Attaching salen covalently to the C₆₀ cage only marginally slows down metal binding at the salen functionality compared to metal binding to free salen. Coordination of metal cations to salen-C₆₀ introduces to these fullerene derivatives strong absorption bands across the visible spectrum from 400 to 630 nm, the optical features of which are controlled by the nature of the transition metal. The redox properties of the metal-salen-C₆₀ complexes are determined both by the fullerene and by the nature of the transition metal, enabling the generation of a wide range of fullerene-containing charged species, some of which possess two or more unpaired electrons. The presence of the fullerene cage enhances the affinity of these complexes for carbon nanostructures, such as single-, double- and multiwalled carbon nanotubes and graphitised carbon nanofibres, without detrimental effects on the catalytic activity of the metal centre, as demonstrated in styrene oxidation catalysed by [Cu(L)]. This approach shows promise for applications of salen-C₆₀ complexes in heterogeneous catalysis. Copyright

Full text of DOI:10.1002/chem.201300872

FULL PAPER
DOI: 10.1002/chem.201300872
Transition Metal Complexes of a Salen–Fullerene Diad: Redox and
Catalytically Active Nanostructures for Delivery of Metals in Nanotubes
Maria A. Lebedeva, Thomas W. Chamberlain, E. Stephen Davies, Dorothꢀe Mancel,
Bradley E. Thomas, Mikhail Suyetin, Elena Bichoutskaia, Martin Schrçder,* and
Andrei N. Khlobystov*[a]
Dedicated to Professor Maurizio Prato on the occasion of his C60th birthday
Abstract: A covalently-linked salen–
C60 (H2L) assembly binds a range of
transition metal cations in close prox-
imity to the fullerene cage to give com-
plexes [M(L)] (M=Mn, Co, Ni, Cu,
Zn, Pd), [MCl(L)] (M=Cr, Fe) and
[V(O)L]. Attaching salen covalently to
the C60 cage only marginally slows
down metal binding at the salen func-
tionality compared to metal binding to
free salen. Coordination of metal cati-
ons to salen–C60 introduces to these
fullerene derivatives strong absorption
bands across the visible spectrum from
400 to 630 nm, the optical features of
which are controlled by the nature of
the transition metal. The redox proper-
ties of the metal–salen–C60 complexes
are determined both by the fullerene
and by the nature of the transition
metal, enabling the generation of a
wide range of fullerene-containing
charged species, some of which possess
two or more unpaired electrons. The
presence of the fullerene cage enhan-
ces the affinity of these complexes for
carbon nanostructures, such as single-,
double- and multiwalled carbon nano-
tubes and graphitised carbon nanofi-
bres, without detrimental effects on the
catalytic activity of the metal centre, as
demonstrated in styrene oxidation cat-
alysed by [Cu(L)]. This approach
shows promise for applications of
salen–C60 complexes in heterogeneous
catalysis.
Keywords: carbon nanotubes · cat-
alysis · fullerenes · salen · transition
metals
Introduction
metal atoms are trapped inside the fullerene cage, and vari-
ous solid-state co-crystallites[7] and charge-transfer systems,[8]
in which the fullerene cage and the metal are held together
by electrostatic or other intermolecular interactions. Transi-
tion-metal complexes, where the transition metal atoms are
bonded to the outside of the fullerene,[9] and so-called meta-
lated bucky-ligands, in which a metalated ligand is covalent-
ly attached to the fullerene cage, are also important targets.
The synthesis of fullerene derivatives containing metal
binding groups has received significant attention,[10] due in
part to the molecular nature of the species formed in which
the metal atoms are bound in discrete coordination environ-
ments. A large variety of ligand moieties have been attached
to C60, including macrocycles, such as porphyrins,[11] phthalo-
cyanines,[12] and crown ethers[13] and tri-, bi- and monoden-
tate metal binding groups, such as terpyridines, bipyri-
dines,[14] and dipyrrins,[15] and pyridines.[16] However, the
rapid advancement of this area has been hindered by diffi-
culties in the synthesis of functionalised fullerene ligands or
the poor solubility of the resultant metal complexes. There-
fore, no “universal” {fullerene}–{metal binding group} com-
bination has yet been identified that can coordinate a wide
variety of metal centres.
Fullerenes and their derivatives possess redox and optical
properties that make them ideal components in systems for
photoactivation, such as photovoltaics and photocatalysis.[1–3]
Transition-metal complexes also provide a wealth of redox,
optical and magnetic properties, and the combination of full-
erene with bound transition metal ions can be utilised to
form molecular or supramolecular systems with tunable
physicochemical properties. This represents an important
step towards the development of molecular electronic devi-
ces for data storage, molecular machines,[4] molecular
switches, sensors, photoconductors and photoactive dyads as
well as the next generation of light activated catalysts.[5] The
desire to fabricate such systems has led to the development
of several classes of fullerene and metal containing com-
pounds. These include endohedral fullerenes,[6] in which
[a] M. A. Lebedeva, Dr. T. W. Chamberlain, Dr. E. S. Davies,
Dr. D. Mancel, B. E. Thomas, Dr. M. Suyetin, Dr. E. Bichoutskaia,
Prof. Dr. M. Schrçder, Prof. Dr. A. N. Khlobystov
School of Chemistry, University of Nottingham
Nottingham NG7 2RD (UK)
We report herein the synthesis of a new {metal receptor}–
C60 dyad assembly incorporating the tetradentate N2O2-coor-
dinating Schiff-base ligand salen (N,N’-ethylenebis(salicyli-
mine)). We demonstrate that the salen moiety in this system
E-mail: M.Schroder@nottingham.ac.uk
Andrei.Khlobystov@nottingham.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300872.
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can bind to a wide range of transition metal cations
({IVV(O)}, CrIII, MnII, FeIII, CoII, NiII, CuII, ZnII and PdII) to
form stable and soluble metal complexes. The introduction
of redox-active metal centres enables fine-tuning of the elec-
tron-acceptor properties of the salen–C60 compounds and
significantly broadens the range of available reduced states,
which are both important for the application of fullerenes in
photovoltaics and electrocatalysis. Furthermore, many of the
reduced species possess two or more unpaired electrons pre-
senting an opportunity for the exploitation of the electro-
chemically controlled magnetic states of salen–C60 com-
plexes in quantum information processing and molecular
The synthesis of 5 (H2L) began with protection of 4,5-o-
phenylene diamine with
a tert-butyloxycarbonyl (Boc)
group[18] to give 1 in 80% yield. Subsequent bromination
with N-bromosuccinimide (NBS) in the presence of benzoyl
[19]
peroxide in CCl4 gave the bis(bromomethyl) derivative 2
in 20% yield. Compound 2 was treated with C60 fullerene in
the presence of KI and 18-crown-6 in dry toluene[20] to give
the Diels–Alder cycloaddition product 3 in 20% yield. De-
protection of the amine groups in 3 was achieved using a
large excess of TFA (10 equiv) in CHCl3 to give 4 (65%
yield). The diamine 4, which decomposes gradually over a
few days, was purified by column chromatography and char-
acterised spectroscopically. Condensation of freshly pre-
pared 4 and 3,5-di-tert-butyl salicylic aldehyde was carried
out in the presence of a catalytic amount of TFA in tetrahy-
drofuran (THF) instead of the more commonly used alco-
holic solvents[21] in order to improve the solubility of the
fullerene precursor. Mixtures of compounds 4 and 5 cannot
be readily separated and so the conversion of 4 to 5 was per-
formed with a significant excess of aldehyde to ensure full
conversion of the diamine (monitored by TLC and MALDI
MS). Any aldehyde present after the reaction was conven-
iently removed from the product material by washing with
methanol (MeOH).
spintronic devices. MACTHUNTRGNEUNG(salen)–C60 complexes exhibit catalytic
activity in reactions of (ep)oxidation, characteristic of
metal–salen complexes.[17] The presence of a fullerene cage
bound to a metal–salen moiety does not hinder catalysis but
facilitates the deposition of these catalytic centres onto a va-
riety of carbon-based support materials offering a new
methodology for engineering catalytically active surfaces at
nanoscale.
Results and Discussion
Ligand synthesis: We have developed a five-step procedure
for the synthesis of a salen–C60 metal receptor, 5, which in-
corporates a salen metal binding moiety and a C60 cage co-
valently linked via a cyclohexenyl bridge (Scheme 1). Four
tert-butyl groups were introduced into the aromatic back-
bone to provide solubility to 5 and its corresponding metal
complexes. Complexes of the corresponding salen–C60 sys-
tems not incorporating tert-butyl groups were found to be
only very sparingly soluble.
Metal complex formation: The efficacy of 5 to bind a range
of metal centres was investigated. The general procedure in-
volved the heating of 5 and a metal salt as a solution in a
10:1 v/v mixture of THF/MeOH under reflux. The majority
of the metal precursors utilised were MII salts (M=Mn, Co,
Ni, Cu, Zn, Pd) or {IVV(O)}, which formed neutral metal–
salen complexes upon complexation. For MCl3 salts (M=Cr
and Fe), treatment with 5 resulted in the formation of
metal–salen complexes having
an additional chloride ligand
bound
to
the
metal
(Scheme 1). For V(O), Cu and
Ni, the rate of metal complex
formation was significantly in-
creased in the presence of
triethyl amine as
a
base.[22]
Complexes 6–12 and 14 were
found to be highly soluble in
solvents typically used for solu-
bilising fullerenes (CS2, o-di-
chlorobenzene (ODCB)) and
displayed moderate solubility
in common organic solvents,
such as CHCl3, CH2Cl2, and
THF. In contrast the Zn com-
plex, 13, was found to be in-
soluble in all solvents including
DMF and pyridine, consistent
with other ZnII–salen systems;
this is attributed to the forma-
ꢀ ꢀ
tion of Zn O Zn bonds be-
tween two or more molecules,
Scheme 1. Synthesis of the fullerene–salen ligand, 5, and corresponding metal complexes 6–14.
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Transition Metal Complexes of a Salen–Fullerene Diad
FULL PAPER
which drastically reduces the solubility of the complex.[23,24]
MALDI-TOF MS for each of the complexes 6–14 shows a
molecular ion and peak distribution entirely consistent with
theoretical predictions and confirms the presence of 1:1
metal/ligand stoichiometries in all cases (see the Supporting
Information). The resultant complexes were found to be
stable under ambient conditions in air, with the exception of
the Mn complex, 8, which was observed to oxidise partially
to MnIII in air forming an oxo-bridged binuclear complex
ꢀ ꢀ
The reaction between the metal salts and H2L can be rep-
resented by:
H2L þ MX2 ¼ ML þ 2 HX
ð1Þ
in which X is anion. The salts MAHCTUNGTERG(NNUN OAc)2 (M=Co, Ni, Cu)
are six-coordinate, octahedral complexes in solution with
the metal centres bound by either four oxygen atoms of two
bidentate carboxylates and two solvent molecules in axial
positions as in the case of Cu
atoms of two monodentate carboxylates and four solvent
(OAc)2,[25] or by two oxygen
ACHTUNGTRENNUNG
[(Salen)Mn O MnACHTUNGTRENNUNG(Salen)] as confirmed by MALDI-TOF
MS and CV measurements (see the Supporting Informa-
tion).
[26]
molecules as found for Ni
(OAc)2.[21,27] In
ACHUTGTNNERNUG(OAc)2 and CoACHUTNGTRENNGUN
contrast, Pd
AHCTUNGTRENNUNG
Metal complex formation with salen–C60 (H2L) is accom-
panied by a change in the colour of the reaction mixture.
Therefore, the kinetics of the metal coordination process
can be conveniently followed by UV/Vis absorption spectro-
scopy. Additionally, for diamagnetic compounds, such as the
with the metal centre strongly bound to four equatorially
coordinated acetate oxygen atoms and two weakly coordi-
nated molecules of solvent in the axial positions. Therefore,
Equation (1) can be treated as ligand substitution of an oc-
tahedral complex according to the Eigen–Wilkins mecha-
nism (Scheme 3).
1
Pd (14) and Ni (11) complexes, H NMR spectroscopy can
be used to monitor complex formation by observing the loss
of the OH signal of the free ligand, H2L, at 13.42 ppm, the
shifts of the imine proton signal at 8.84 ppm in H2L to lower
values (8.36 ppm for [Ni(L)], 8.70 ppm for [Pd(L)] and the
shifts of the aromatic ring signals from 7.57 and 7.39 ppm in
H2L to higher values (8.19 and 7.53 ppm in [Pd(L)], and
8.00 and 7.38 ppm in [Ni(L)], respectively). Infrared spectro-
scopy can also confirm the deprotonation of the salen
Scheme 3. The Eigen–Wilkins mechanism for the substitution of an ace-
tate ligand by H2L at MII (M=Co, Ni, Cu, Pd).
ꢀ1
ꢀ
moiety through the loss of n
G
complexes. However, the C=N bond stretch observed be-
tween 1615–1620 cmꢀ1 experiences only a minor shift in
complexes 6–14 as compared to the free ligand H2L (see the
Experimental Section).
Within the framework of the Eigen–Wilkins formalism
(for details see the Supporting Information) the observed
rate constants (kobs) have been measured for each complex
formation at three different temperatures and subsequently
the activation energies (Ea) for complex formation were de-
termined from the Arrhenius equation for Co, Ni, Cu and
Pd complexes (Table 1, Table 2).
Metal binding studies: To quantify the kinetics of complexa-
tion, the coordination of Co, Ni, Cu and Pd cations to H2L
was investigated in detail. These data were compared to
those obtained for metal complexation to a reference salen
ligand with identical structure but not attached to C60 (com-
pound 15 (H2L’), Scheme 2) in order to evaluate the influ-
ence of the fullerene moiety on the ability of the salen to
Figure 1 and Figure 2 summarise the analyses for complex
formation as measured by UV/Vis and H NMR spectrosco-
1
py (see the Supporting Information for the control experi-
ments). Comparisons of the formation of complexes [M(L)]
and [M(L’)] (M=Co, Ni, Cu, Pd; Tables 1 and 2) confirms
that the presence of the C60 fullerene moiety slows down
metal binding by 5–15%, corresponding to a 10–20 kJmolꢀ1
increase in the activation barrier. The higher activation bar-
riers observed for complexation reactions for H2L can be re-
lated to an increase in steric bulk of the ligand, which inhib-
its the approach to the metal acetate molecule during the in-
itial stage of complex formation, or the electron withdraw-
ing (inductive) effect of the fullerene cage on the salen
moiety.
bind metals. Reactions between Co
Cu(OAc)2 and H2L and H2L’ were monitored by UV/Vis
spectroscopy in THF, and between Pd(OAc)2 and the same
ACHUTNGTNERNUG(OAc)2, NiACHTUNGTRNEN(UGN OAc)2, and
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
1
ligands by H NMR spectroscopy in deuterated THF.
DFT geometry optimisation of the complexes [M(L)] con-
firms that the salen group binds to the metal centres in a
square plane as observed in conventional metal–salen com-
plexes that do not incorporate the fullerene (Figure 3a).[29]
The donor atoms are arranged in a slightly distorted square
planar geometry around the Ni centre with bond angles de-
viating only marginally from 908 (Table 3). The cyclohexenyl
ring, which links salen and C60 moieties, adopts a boat-like
Scheme 2. Synthesis of complexes of a reference salen ligand without full-
erene 15 (H2L’).
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M. Schrçder, A. N. Khlobystov et al.
Table 1. Observed rate constants kobs [sꢀ1 mꢀ1] activation energies Ea [kJmolꢀ1] and ln
ACTHNUGTRENNG(U A·KE) parame-
ters for formation of [M(L)] and [M(L’)] (M=Cu, Co, Ni) determined by UV/Vis spectroscopy at vari-
ous temperatures [8C].
of complexes 6–12 and 14, no bands
that could be assigned to metal-to-
fullerene transitions were observed.
All measurements indicate that the
Parameter
T
H2L+CoII H2L’+CoII
T
H2L+NiII H2L’+NiII
T
H2L+CuII H2L’+CuII
[8C]
[8C]
[8C]
metal centre maintains
a square
kobs
25
35
40
0.369ꢁ
0.003
0.625ꢁ
0.003
30
40
50
0.024ꢁ
0.002
0.0249ꢁ
0.0003
25
30
35
1.485ꢁ
0.007
1.960ꢁ
0.009
planar or square-base pyramidal ge-
ometry in solution despite the close
proximity of the fullerene group to
the metal (Figure 3).
The transition metal centres also
provide redox features, which in com-
bination with the electron–acceptor
properties of the fullerene cage form
a basis for redox tuneable electron-
1.292ꢁ
1.327ꢁ
0.0549ꢁ
0.0719ꢁ
2.526ꢁ
2.875ꢁ
0.009
0.051
0.0002
0.0004
0.008
0.013
2.042ꢁ
0.019
2.501ꢁ
0.010
0.1389ꢁ
0.0001
0.1114ꢁ
0.0040
3.632ꢁ
0.013
3.905ꢁ
0.006
Ea
lnACHTUNGTRENNUNG(A·KE)
89.5ꢁ2.7
69.1ꢁ0.3
70.7ꢁ0.3 61.1ꢁ1.0
67.7ꢁ0.2
53.2ꢁ0.3
35.1
27.4
24.3 20.6
27.7
22.1
Table 2. Observed rate constants kobs [sꢀ1 mꢀ1
]
activation energies Ea
[kJmolꢀ1] and ln
G
spin active molecular systems. The electrochemical behav-
iours of compounds 5–12 and 14 have been investigated by
cyclic and square-wave voltammetry at a glassy carbon elec-
1
by H NMR spectroscopy at various temperatures [8C].
Parameter
kobs ꢂ103
T [8C]
H2L+PdII
H2L’+PdII
+
ꢀ
trode as ODCB solutions containing [NBu4 ]AHCUTNGERNUN[G BF4 ] (0.2m)
45
55
65
1.651ꢁ0.031
3.572ꢁ0.071
6.273ꢁ0.151
59.7ꢁ0.2
1.994ꢁ0.041
3.989ꢁ0.100
6.300ꢁ0.204
51.5ꢁ0.5
as the supporting electrolyte (Table 5). The nature of the
electron transfer for each redox process was assessed by
monitoring the peak-to-peak separation and the ratio of
peak currents at a range of scan rates between 0.02 and
0.3 Vsꢀ1. As a typical example the cyclic voltammogram of
[Cu(L)] 12 is presented in Figure 4, and the results for all
complexes are summarised in Table 5. These measurements
confirm that the potentials of the three observed quasi-re-
versible reductions of the fullerene moiety are essentially
unperturbed by variations of the metal centre and differ
only slightly from those in the metal-free ligand H2L
(Table 5). These values are similar to others reported previ-
ously for fullerene derivatives.[39,40] Clearly, the large dis-
tance (6.7 ꢁ) separating the metal centre of the salen
moiety from the surface of the fullerene cage (Figure 3) and
the lack of electronic conjugation between these centres is
sufficient to preclude any electronic interactions between
the metal and the fullerene.
In addition to the observed quasi-reversible fullerene-
based reductions, metal centres contribute extra reduction
processes. For example, 6 exhibits two quasi-reversible
metal-related reductions for the {V(O)}IV/III and {V(O)}III/II
processes at ꢀ1.43 and ꢀ1.66 V, respectively, but the CoII/I
process in 10 at ꢀ1.30 V is essentially irreversible. Com-
plexes 9 and 12 exhibit quasi-reversible reductions at
ꢀ0.53 V FeIII/II and ꢀ1.36 V CuII/I, respectively. The reduc-
tion of the metal centres in 11 and 14 observed at ꢀ1.59 V
NiII/I and ꢀ1.56 V PdII/I, respectively, are also quasi-reversi-
ble. Since 8 is prone to oxidation in air during purification
and storage, the process observed at ꢀ0.25 V was assigned
to the MnIII/II reduction in an oxo-bridged binuclear complex
[(L)Mn-O-Mn(L)], which forms over time as a result of par-
tial oxidation in air, consistent with the results of MALDI
MS for this complex (see the Experimental Section). The
CrIII complex 7 shows no metal-based reductions, which is
consistent with the control complex [Cr(L’)Cl] and the fact
that CrII is significantly less stable than CrIII, so that reduc-
tion to CrII occurs at a much more negative potential, out-
side of the range of potentials studied. Thus, the choice of
Ea
ln
N
25.6
13.3
conformation providing an angle of 1248 between the plane
defined by [Ni(salen)] and the fullerene cage (Figure 3b).
ACHTUNGTRENNUNG
Despite the angular conformation of the cyclohexenyl con-
nection, the metal centre is separated by about 6.7 ꢁ from
the fullerene, thus precluding any interactions between the
d-orbitals of the metal centre and the p-system of the
carbon cage. However, the proximity of the fullerene cage
to the donor atoms of the salen group is likely to have some
hindering effects on metal coordination, which is consistent
with the kinetic measurements discussed above.
Optical and redox properties of metal complexes: H2L ex-
hibits two bands at 710 and 436 nm in the UV/Vis spectrum
characteristic of a [6,6]-functionalised fullerene cage,[30] and
strong bands at 300–430 nm associated with p–p* and n–p*
transitions in the salen moiety.[31] Thus, the optical proper-
ties of H2L can be viewed as a superposition of the individu-
al fullerene and salen components. The coordination of a
metal centre to L2ꢀ results in the retention of absorption
bands associated with the fullerene moiety, albeit with a
shift to lower wavelengths compared to those of the free
ligand. The optical transitions associated with the metal–
salen centres appear between 366–630 nm, are metal de-
pendent and in some complexes overlap with the bands of
the fullerene moieties (Table 4).
The absorption bands for complexes 6–12 and 14 are as-
signed as either d–d transitions based on the metal centre,
or d–p* charge transfer transitions from the metal to the
ligand, typically with higher extinction coefficients for the
latter. These data correlate well with other reported salen
complexes[32–38] indicating that the electronic transitions
based on the metal centres remain largely unperturbed by
the presence of the fullerene cage. Although metal-to-ligand
optical transitions dominate the UV/Vis absorption spectra
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Transition Metal Complexes of a Salen–Fullerene Diad
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Figure 1. UV/Vis spectroscopic data for the titration of H2L into MX2 (left) and plots of lnACHTNUGTRENN[UG H2L]/AHCTUNGTERNN[UNG H2L]0 versus t used for the determination of the rate
constant (right) for the formation of: a) [Co(L)], 10, b) [Ni(L)], 11, and c) [Cu(L)], 12. Arrows in a)–c) (left) indicate the increase in concentration of
[M(L)]; squares, triangles and diamonds and the corresponding temperature values describe the rate constants determination at different temperatures
(right).
transition metal modifies the energy of the orbitals of the
metal–salen moiety with respect to the fullerene thus tuning
the electronic- and spin-states of the resultant fullerene/
{metal–salen} adduct. Incorporation of transition metals Fe,
Mn, Co, Ni, Cu, Pd or {V(O)} into H2L increase the capacity
of these fullerene derivatives for uptake of one or two elec-
trons, in addition to the three electrons that can be reversi-
bly deposited onto the fullerene cages in all studied com-
plexes (Scheme 4).
All of the metal complexes 6–12 and 14 exhibit oxidation
processes related to the formation of phenoxyl radicals on
the salen moiety or oxidation of the metal centre (Table 6).
For example, 6 and 10 show metal-based oxidations at
+0.57 V {V(O)}IV/V and +0.74 V CoII/III, respectively. Fur-
thermore, the two tert-butyl groups on the phenol part of
the ligand in the ortho and para positions are known to sta-
bilise phenoxyl radicals,[41] which form in H2L at +1.17 V.
Metal coordination to H2L stabilises the oxidised state of
the salen moiety, thus lowering the potential for phenoxyl
radicals formation to between +0.99 and +1.06 V.[42] Addi-
tionally, the CrIII, MnII, NiII and CuII centres in complexes 7,
8, 11 and 12 promote the formation of a second phenoxyl
radical in the salen as indicated by the oxidation potentials
observed at +1.12, +1.41, +1.42 and +1.29 V, respectively.
However, the presence of highly charged {V(O)}V, CoIII and
FeIII centres inhibits oxidation of salen and shifts the poten-
tial of phenoxyl radicals formation to more anodic values
(Scheme 4b).
Oxidation of the fullerene cage was not observed in our
experiments.[43] Typically, oxidised fullerene species are diffi-
cult to access and have only limited stabilities, but the pres-
ence of the metal–salen group attached to the fullerene cage
provides one or two reversible oxidations, some of which
lead to the formation of electron-spin active species, without
having to oxidise the fullerene cage (Scheme 4b). The abili-
ty to generate molecules with high total electron spin is de-
sirable for molecular electronics and magnetic applica-
tions.[44]
Catalytic properties: Metal–salen compounds are known as
catalysts for oxidation,[45] ring-opening of epoxides,[46] aldol
condensation[47] and epoxidation[48] reactions. We have
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M. Schrçder, A. N. Khlobystov et al.
Table 4. UV/Vis absorption bands for compounds 5–14 in CH2Cl2 solu-
tion.
Compound
UV/Vis, lmax [nm] (e [ꢂ10ꢀ3 dm3 molꢀ1 cmꢀ1])
fullerene moiety
metal–salen moiety
5
710 (0.413), 436
(6.587)
–
6
702 (0.31), 436
(14.87)
418 (14.68)
7
703 (0.31), 433
(4.20)
483 (4.36), 406 (4.45)
8
703 (0.68), 432
(7.32)
630 (1.06), 581 (1.68), 486 (6.97), 366
(20.18)
9
702 (0.65), 432
(11.54)
406 (13.66)
10
11
12
703 (0.729), 432
(15.555)
702 (0.20), 433
(4.98)
708 (0.62), 439
(22.88)
596 (3.784), 544 (6.655), 398 (25.184)
525 (4.35), 497 (5.43), 386 (15.28)
455 (23.53), 415 (21.92)
13
14
insoluble
706 (1.478), 436
(12.153)
insoluble
528 (9.885), 494 (10.882), 408
(16.819)
Figure 2. 1H NMR spectroscopic data for the titration of H2L into MX2
showing an increase in the size of the aromatic H peak at 8.56 ppm of the
complex [Pd(L)] and the decrease of the aromatic H peak at 7.81 ppm of
the free ligand with complex formation.
Table 5. Electrochemical data[a] for the reduction of compounds 5–12
and 14 (cyclic and square wave voltammograms are presented in the Sup-
porting Information).
Compound Fullerene reduction, E1/2 [V] Metal salen re- DE Fc+/Fc
(DE [V])
duction, E1/2 [V] [V][b]
(DE [V])
E1/2red1 E1/2red2 E1/2red3 E1/2red4
E1/
2red5
5
ꢀ0.61
(0.15)
ꢀ0.61
(0.15)
ꢀ0.60
(0.11)
ꢀ0.61
(0.10)
ꢀ0.61
(0.15)
ꢀ0.61
(0.16)
ꢀ0.61
(0.15)
ꢀ0.62
(0.13)
ꢀ0.61
(0.09)
ꢀ0.99
(0.17)
ꢀ1.00
(0.15)
ꢀ0.98
(0.10)
ꢀ0.98
(0.10)
ꢀ1.00
(0.15)
ꢀ0.99
(0.16)
ꢀ0.99
(0.15)
ꢀ1.00
(0.15)
ꢀ1.01
(0.10)
ꢀ1.52
(0.15)
ꢀ1.55
(0.13)
ꢀ1.52
(0.12)
ꢀ1.52
(0.10)
ꢀ1.52
–
–
0.14
6
ꢀ1.43
(0.15)
–
ꢀ1.66 0.12
7
–
–
–
–
–
–
–
0.11
0.07
0.10
0.11
0.12
0.12
0.07
8
ꢀ0.25
(0.10)
9
ꢀ0.53[b]
10
11
12
14
ꢀ1.55
(0.14)
ꢀ1.53
(0.08)
ꢀ1.55
(0.14)
ꢀ1.52
(0.10)
ꢀ1.29
(0.17)
ꢀ1.588
(0.09)
ꢀ1.36
(0.16)
ꢀ1.56
(0.09)
[a] Potentials (E1/2 =(Epa +Ep )/2) [V] quoted to the nearest 0.01 V. All
potentials are reported against the (h5-Me5C5)2Fe+/(h5-Me5C5)2Fe couple
[BF4] as the sup-
(h5-Me5C5)2Fe =
c
AHCTUNGTRENNUNG
for 0.5 mm solutions in ODCB containing 0.2m [nBu4N]ACHTUGNTRENNNUG
porting electrolyte (E1/2 ferrocene vs. (h5-Me5C5)2Fe+/
G
Figure 3. Calculated geometry of [Ni(L)], 11, illustrating: a) the square
a
c
planar geometry of the [Ni
to fullerene cage.
ACHTUNGTREN(NUNG salen)] moiety, and b) its position with respect
0.52 V). The anodic/cathodic peak separation (DE=Ep ꢀEp ) is given in
ACHTUNGTRENNUNG
brackets where applicable. [b] The DE for the (h5-Me5C5)2Fe+/(h5-
Me5C5)2Fe couple used as the internal standard.
Table 3. Selected bond lengths [ꢁ] and angles [8] of the [Ni(L)], 11.
chosen oxidation of styrene with tert-butyl peroxide leading
to benzaldehyde and styrene oxide as a model reaction to
ascertain the influence of the fullerene cage on the catalytic
properties of the metal centres. Cu-containing complex 12
demonstrates a catalytic activity comparable to fullerene-
free Cu–salen complex, 18, confirming that the addition of
Length [ꢁ]
Angle [8]
ꢀ
Ni N1
ꢀ
Ni N2
1.85
1.84
1.84
1.84
O3-Ni-O4
O3-Ni-N2
O4-Ni-N1
N1-Ni-N2
84.2
95.0
95.3
86.3
ꢀ
Ni O3
ꢀ
Ni O4
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Transition Metal Complexes of a Salen–Fullerene Diad
FULL PAPER
Table 6. Electrochemical data[a] for the oxidation of compounds 5–12 and
14. All potentials are quoted versus the (Me5Cp)2Fe+/
ACTHUNGTRNEUNG(Me5Cp)2Fe couple
(detailed electrochemical data are presented in the Supporting Informa-
tion).
Compound Metal oxidation E1/2 [V]
Salen oxidation DE Fc+/Fc
E1/2 [V] (DE [V]) [V][b]
E1/2ox2 E1/2ox3
(DE [V])
E1/2ox1
5
–
1.17
–
0.14
(0.15)
–
1.06
6
7
0.57 (0.14)
–
–
1.12
0.12
0.11
(0.12)
1.18
1.31
8
9
–
–
1.41
–
0.07
0.10
+
ꢀ
(0.15)
1.30
0.99
Figure 4. The cyclic voltammogram of 12 in ODCB with [NBu4 ]ACHTUNETGRG[NUNN BF4 ]
(0.2m) as supporting electrolyte at a scan rate of 0.1 Vsꢀ1
.
10
11
0.74 (0.18)
–
–
1.42
0.11
0.12
(0.15)
1.03
(0.15)
1.05
12
14
–
–
1.29
(0.16)
–
0.12
0.07
the fullerene cage does not have a detrimental effect on the
activity of the metal centre (Figure 5). The electron with-
drawing ability of fullerene depletes the electronic density
at the catalytic centre in complex 12 and may be responsible
for the slight increase in the observed catalytic activity of 12
as compared to the control compound 18. However, the ob-
served increase is small and only marginally greater than the
error of these measurements (Figure 5).
The ability to anchor catalytic centres to solid supports is
essential for the development of effective heterogeneous
catalysts. Thus, salen has been supported on polymer
beads,[49] zeolites,[50] and covalently linked to the termini of
carbon nanotubes.[51] The presence of the fullerene cage in
complex 12 offers a new mechanism for anchoring catalytic
(0.15)
[a] Potentials (E1/2 =(Epa +Ep )/2) [V] quoted to the nearest 0.01 V. All
potentials are reported against the (h5-Me5C5)2Fe+/(h5-Me5C5)2Fe couple
[BF4] as the sup-
(h5-Me5C5)2Fe =
c
AHCTUNGTRENNUNG
for 0.5 mm solutions in ODCB containing 0.2m [nBu4N]ACHTUGNTRENNNUG
porting electrolyte (E1/2 ferrocene vs. (h5-Me5C5)2Fe+/
G
a
c
0.52 V). The anodic/cathodic peak separation (DE=Ep ꢀEp ) is given in
ACHTUNGTRENNUNG
brackets where applicable. [b] The DE for the (h5-Me5C5)2Fe+/(h5-
Me5C5)2Fe couple used as the internal standard.
centres utilising noncovalent interactions. Fullerenes have
significant affinity for sp2-carbon nanostructures, such as
nanotubes and nanofibres, due to strong specific van der
Waals forces. Indeed, titration
experiments reveal that 2–3-
times more of complex 12 can
be loaded onto single-, double-
and multiwalled carbon nano-
tubes compared to the non-
fullerene
analogue
18
(Table 7). The impact of the
fullerene moiety on adhesion
efficiency is even more re-
markable
for
graphitised
carbon nanofibres (GNF)—cy-
lindrical structures with a set
of step-edges formed by rolled
up
sheets
of
graphene
(Figure 6)—as Cu–salen com-
plex 18 showed no measurable
adsorption on GNFs although
complex 12 affinity for GNFs
is comparable to that for nano-
tubes (Table 7).
High resolution transmission
electron
microscopy
(HRTEM) confirms that com-
plex 12 enters into hollow
nanofibres and adsorbs on the
graphitic step-edges forming
Scheme 4. Schematic representation of: a) the sequential reduction, and b) oxidation processes occurring for
complexes 6–12 and 14. The fullerene cage is represented as a circle and the metal–salen moiety as a rectangle,
where n is the number of d-electrons on the metal centre in the neutral complex.
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M. Schrçder, A. N. Khlobystov et al.
Table 7. Adsorption data for 18 and 12 on a variety of carbon nanostruc-
tures (single- (SWNTs), double- (DWNTs), and multiwalled carbon
nanotubes (MWNTs) and graphitised carbon nanofibres (GNFs)), quoted
in mmol of complex per g of carbon support material and measured by
UV/Vis spectroscopy in dichloromethane. Enhancement factor (last
column) is the increase of affinity due to the fullerene moiety in complex
12.
Carbon
structure
Complex 18
[mmolgꢀ1
Complex 12
[mmolgꢀ1
Enhancement
factor
G
]
N
]
SWNTs
DWNTs
MWNTs
GNFs
7.644ꢂ10ꢀ3
1.496ꢂ10ꢀ2
3.280ꢂ10ꢀ3
0
2.323ꢂ10ꢀ2
2.708ꢂ10ꢀ2
6.483ꢂ10ꢀ3
1.657ꢂ10ꢀ3
3.04
1.81
1.98
N/A[a]
[a] Not applicable.
Figure 5. Reaction profiles for the oxidation of styrene with tert-butyl hy-
drogen peroxide in the presence of 12 (circles), 18 (triangles) and with
no metal complex (squares).
carbon support materials leading to a new family of hybrid
nanostructures that show promise for applications in hetero-
geneous catalysts.
van der Waals contacts with GNF (Figure 6c). The fullerene
part of the complex can be clearly observed in HRTEM
images as a circle with diameter of approximately 0.7 nm,
but imaging of the Cu–salen part is more challenging be-
cause of the limited stability of organic groups in the elec-
tron beam. Energy dispersive X-ray (EDX) spectroscopy
performed for an isolated 12@GNF structure confirms the
presence of copper inside GNF (Figure 6d).
Conclusion
We have developed a five-step synthetic procedure to link
fullerene C60 and salen units within a ligand molecule. The
salen part binds a wide range of transition metals [{V(O)}IV,
CrIII, MnII, FeIII, CoII, NiII, CuII, ZnII and PdII] bringing them
in close proximity to the fullerene cage (c.a. 6–7 ꢁ). Kinetics
studies confirm that complexation reactions are only slightly
hindered by the presence of the bulky fullerene moiety
when compared to fullerene-free analogues of salen. Com-
Titration and electron microscopy measurements both
demonstrate that fullerene–salen complexes provide a facile
route for the secure anchoring of catalytic metal centres on
Figure 6. a) Schematic representation of complex 12 adsorbed on the step-edges in the internal channel of a graphitised carbon nanofibre. HRTEM
images of the 12@GNF composite show: b) the overall structure of the nanofibre, and c) a high-magnification image of a step-edge with adsorbed fuller-
ene molecule (~0.7 nm circle in the centre of the white circle). d) EDX spectrum of an individual 12@GNF structure confirming the presence of copper
complex (the Ni peaks are due to the TEM specimen grid).
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Transition Metal Complexes of a Salen–Fullerene Diad
FULL PAPER
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Experimental Section
The Supporting Information contains experimental data, 1H and
13C NMR spectra, MALDI mass spectra, electrochemistry results, DFT
geometry optimisation results, metal binding studies procedures and data
processing, nanotube absorption experiments results.
Acknowledgements
We thank EPSRC, Royal Society, Nottingham Nanoscience and Nano-
technology Centre and the University of Nottingham for support. A.N.K.
and E.B. acknowledge the award of ERC for Starter Grants, and M.S. ac-
knowledges support through an ERC Advanced Grant. D.M. acknowl-
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Received: March 6, 2013
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Published online: && &&, 0000
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
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These are not the final page numbers!

Transition Metal Complexes of a Salen–Fullerene Diad
FULL PAPER
Ball and chain: Salen-group linked to
Fullerenes
C
60 (see scheme) provides an effective
binding site for transition metals
tuning optical, redox and catalytic
properties of the fullerene complex.
This approach shows promise for
applications of salen–C60 complexes in
heterogeneous catalysis.
M. A. Lebedeva, T. W. Chamberlain,
E. S. Davies, D. Mancel, B. E. Thomas,
M. Suyetin, E. Bichoutskaia,
M. Schrçder,*
A. N. Khlobystov* ........... &&&& — &&&&
Transition Metal Complexes of a
Salen–Fullerene Diad: Redox and
Catalytically Active Nanostructures for
Delivery of Metals in Nanotubes
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
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&
ÞÞ
These are not the final page numbers!