Synchronized motion and electron transfer of a redox-active rotor

Article abstract of DOI:10.1039/c0dt01084g

We have constructed a series of copper complexes with asymmetric 4,6-substituted 2-pyrimidyl coordination units, which form two coordination isomers via rotation of the pyrimidine ring. Redox-active ferrocenyl moieties were introduced at the 4-positions of the pyrimidine as rotating units in these complexes. The stability and dynamics of the rotative interconversion could be tuned using the structure of the rotor to accommodate a large ferrocene unit within the inner space of the complex. Among these compounds, a complex with a ferrocenylvinyl substituent showed synchronized intramolecular electron transfer between the copper and the ferrocenyl moiety with rotational motion. That is, the oxidation state of the ferrocenyl unit depended on its position within a cyclic trajectory.

Full text of DOI:10.1039/c0dt01084g

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Dalton
Transactions
An international journal of inorganic chemistry
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Volume 40 | Number 10 | 14 March 2011 | Pages 2077–2392
ISSN 1477-9226
COVER ARTICLE
Kume and Nishihara
Synchronized motion and electron transfer of a redox-active rotor

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PAPER
Synchronized motion and electron transfer of a redox-active rotor†
Shoko Kume* and Hiroshi Nishihara
Received 23rd August 2010, Accepted 18th November 2010
DOI: 10.1039/c0dt01084g
We have constructed a series of copper complexes with asymmetric 4,6-substituted 2-pyrimidyl
coordination units, which form two coordination isomers via rotation of the pyrimidine ring.
Redox-active ferrocenyl moieties were introduced at the 4-positions of the pyrimidine as rotating units
in these complexes. The stability and dynamics of the rotative interconversion could be tuned using the
structure of the rotor to accommodate a large ferrocene unit within the inner space of the complex.
Among these compounds, a complex with a ferrocenylvinyl substituent showed synchronized
intramolecular electron transfer between the copper and the ferrocenyl moiety with rotational motion.
That is, the oxidation state of the ferrocenyl unit depended on its position within a cyclic trajectory.
preferred by the Cu(I) centre, and square planar structure (or 5-
and 6- coordination) is preferred by Cu(II) centre.10 Coordination
structures have been studied in association with their function as
redox centres in electron transfer proteins.11
Introduction
Molecular systems that permit large-scale relative motions be-
tween molecular units were seminally developed and form a
basis of molecular machines.1 Precise motions, such as rotation,2
translation,3 or hinge-like4 motions have been demonstrated to
date. The functionalities of these molecules are not limited to
dispersed solution chemistry, and applications for molecular
memory devices with electromagnetic response have also been
described.5
In natural systems, correlations between electronic states and
the spatial positioning of molecular components are especially
important, as in, for example, motor proteins,6 that converts
directional signal into energy and vice versa. Proton gradients
across membranes produce rotational motions of the F0 unit in the
ATPase, which synthesizes ATP molecules. In the F0 unit, several
proton receptors toggle their affinity according to position, which
convert the proton gradient into a torque on the protein unit.7
Inspired by these natural motors, an artificial molecular motor
that converts electronic voltages into unidirectional rotational
motion was proposed by Rapenne et al.8 In their design, the redox
activity of a ferrocenyl moiety was converted to unidirectional
rotational motion through electrostatic interactions with an
external electrode.
The specific correlation between coordination structures and
Cu(II)/Cu(I) redox states, together with coordination lability have
been used to toggle a molecular structure via redox stimulation.12
From the reversed point of view, the structure-redox correlation
in the Cu(II)/Cu(I) system can be used to construct an electron
transfer system controlled by a structural conversions. If electron
transfer occurs in a manner controlled by molecular mechanics,
it can be coupled to a wide variety of output elements, such
as optical, magnetic, or electrostatic response structures. The
transferred electron may be directed in the context of a specific
chemical reaction or involved in a conduction through a molecular
wire.
To enable such structurally regulated systems, we selected a
pyridylpyrimidine (pmpy) ligand with asymmetrically substituted
pyrimidines.13–15 Because both nitrogen atoms in the pyrimidine
ring can bind to a copper centre, differential steric effects, induced
by introducing substitution adjacent to the nitrogen atom, are
expected to affect the coordination structure. These coordination
isomers can interconvert by rotation of the pyrimidine ring,
accompanied by a relatively large redox potential shift (ca. 0.2 V).
The notable feature of this rotational conversion is that the
rotational motion can be turned ON or OFF in the solution
state, because interconversion requires bond dissociation and
reconstruction between the copper and nitrogen atoms. Placing
a wall in the rotational trajectory enables the precise tuning
of the dynamics.13 Up to now, we successfully demonstrated an
intramolecular electron transfer induced by the mechanical rota-
tional motions of the a copper complex structure.14 Conversion of
rotational signal can be applied not only to ground state electron
transfer phenomena, but also to its excited state and emission
behavior.15
Electron transfer phenomena in copper complexes are unique
due to the strong dependence of the redox chemistry on the
coordination structure.9 For example, tetrahedral coordination is
Department of Chemistry, School of Science, The University of Tokyo, 7-
3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan. E-mail: kume@chem.s.u-
tokyo.ac.jp; Fax: +81 3 5841 4489
† Electronic supplementary information (ESI) available: View of structural
comparison for [Cu(RFcpmpy)(LAnth)]+ compounds (Fig. S1), crystallo-
graphic data (Table S1), cyclic voltammograms (Fig. S2 and S3), and
EPR spectra (Fig. S4). CCDC reference numbers 788142 and 798395. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt01084g
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2299–2305 | 2299
©

Table 1 Chemical shifts of selected signals of the isomers
(i/o isomer, in acetone-d6, 293 K)
In this work, we introduced a redox-active ferrocenyl unit
as the rotating unit. Synchronization between the rotational
motion and intramolecular electron transfer, enabled tuning of
the oxidation state of the ferrocenyl moiety by specific positioning
of the coordination structure. In such a molecule, highly selective
molecular electron transfer is expected through redox mediation
by the ferrocenyl unit. We used two approaches to construct
appropriate rotor structures that showed dynamic rotational
behavior.
Ha
Hb
Hc
HFcpmpy
4.09
-/4.08
3.89/3.93
4.02/3.94
4.05/3.96
3.88/-
4.57
-/4.66
4.41/4.63
4.44/4.65
4.37/4.63
4.27/-
5.22
-/5.03
3.77/5.06
3.81/5.06
3.84/5.09
3.75/-
[Cu(HFcpmpy)(LAnth)]+
[Cu(4-tolFcpmpy)(LAnth)]+
[Cu(1-NaphFcpmpy)(LAnth)]+
[Cu(9-AnthFcpmpy)(LAnth)]+
[Cu(tBuFcpmpy)(LAnth)]+
Fcvipmpy
4.20
3.99/4.31
4.42
4.31/4.56
4.59
3.53/4.76
[Cu(Fcvipmpy)(LAnth)]+
a bulky substituent opposite the ferrocene (at the 6-position
of the pyrimidine) in place of a hydrogen atom. A family of
[Cu(RFcpmpy)(LAnth)]+ compounds was prepared with various
sizes of hydrocarbons (R = 4-tolyl, 1-naphtyl, 9-anthryl and tert-
butyl) at the 6-position. Another strategy involved extending
the tethering moiety between pmpy and the ferrocenyl moiety.
[Cu(Fcvipmpy)(LAnth)]+ was synthesized based on the second
strategy, with vinyl moiety as a tethering moiety with its stiffness
and p-conjugation. [Cu(tolvipmpy)(LAnth)]+ was also prepared as
a reference compound.
Crystallography
The isotropic d10 electronic structure of a Cu(I) complex tends
to assume a tetrahedral coordination geometry to avoid mutual
repulsion of the ligands.10 [Cu(HFcpmpy)(LAnth)]+ took an essen-
tially tetrahedral geometry, with almost orthogonal coordination
of two ligands (Fig. 1). As the substituent at the inner position
became larger, two molecular deformations were observed. First,
the pmpy ligand moved to the upper side of the molecule by
rocking, and bulky substituents were removed from the space
enclosed by the two anthryl units. Second, the bond between the
Results and discussion
Strategy for designing the rotation convertible system
We first investigated the basic [Cu(HFcpmpy)(LAnth)]+ complex in
which the ferrocenyl moiety was directly attached at the pyrimidine
2-position. Assignment of each isomer of the Cu(I) complex was
1
made using H NMR spectroscopy. Table 1 shows the selected
chemical shifts of the various pmpy ligands and Cu(I) complexes at
room temperature. The signals of the substituents at the pyrimidine
4-position of the o-isomers were found in positions that were
similar to those observed for the uncoordinated ligand. (We denote
the isomer in which the ferrocenyl moiety is directed to the copper
centre as the i-isomer, and otherwise the o-isomer). In contrast, the
signals of the i-isomer were shifted upfield, due to the significant
shielding effects at the copper center and the aromatic moieties in
LAnth
.
[Cu(HFcpmpy)(LAnth)]+ showed only one set of peaks, which
were assigned to the o-isomer. The bulkiness of the ferrocenyl
moiety, prohibited conversion of the [Cu(HFcpmpy)(LAnth)]+
because formation of the unstable i-isomer was inhibited. To
construct an interconvertible structure, the two isomers were
tuned structurally to have comparable energies via two approaches.
The first approach attempted to ‘press in’ the ferrocenyl moiety
into the inner space. This was accomplished by substituting
Fig.
1
Crystal structures of [Cu(HFcpmpy)(LAnth)]+ and
[Cu(1-Naphpmpy)(LAnth)]+ from different point of views. Anions
and solvent molecules are omitted for clarity.
2300 | Dalton Trans., 2011, 40, 2299–2305
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©

˚
Table 2 Cu–N bond lengths (A) of the Cu complexes
Table 3 Molar ratio of the isomers at 293 K in acetone-d6
Cu–N(pm)
Cu–N(py)
i-: o -isomer
o-[Cu(HFcpmpy)(LAnth)]+
2.032(4)
2.128(6)
2.103(6)
2.107(2)
2.076(3)
2.127(3)
2.096(2)
2.029(5)
2.035(4)
2.028(4)
2.005(6)
2.016(6)
1.999(2)
2.015(3)
1.992(2)
1.978(3)
2.028(5)
2.005(4)
[Cu(HFcpmpy)(LAnth)]+
0 : 100
18 : 82
13 : 87
73 : 27
100 : 0
49 : 51
49 : 51
o-[Cu(4-tolFcpmpy)(LAnth)]+a
[Cu(4-tolFcpmpy)(LAnth)]+
[Cu(1-NaphFcpmpy)(LAnth)]+
[Cu(9-AnthFcpmpy)(LAnth)]+
[Cu(tBuFcpmpy)(LAnth)]+
[Cu(Fcvipmpy)(LAnth)]+
o-[Cu(1-NaphFcpmpy)(LAnth)]+
i-[Cu(9-AnthFcpmpy)(LAnth)]+
i-[Cu(tBuFcpmpy)(LAnth)]+a
[Cu(ptolvipmpy)(LAnth)]+
o-[Cu(Fcvipmpy)(LAnth)]+
i-[Cu(ptolvipmpy)(LAnth)]+
the compounds in this series of work crystallized in the isomer
which dominants in solution as described below.
pm = pyrimidine, py = pyridine.a Two independent complexes were found
in the crystal.
Rotational thermodynamics and kinetics
phenanthroline-anthryl groups bent and hinged to increase the
inner space. These deformations can be clearly observed by com-
paring [Cu(HFcpmpy)(LAnth)]+ and [Cu(1-NaphFcpmpy)(LAnth)]+
in Fig. 2, and other [Cu(RFcpmpy)(LAnth)]+ families (Figure
S1). Despite these deformations, the complexes still assumed 4-
coordinate structures with pmpy ligands and LAnth, but with an
elongated Cu–N(pm) bond (Table 2). In contrast, the distance
Cu–N(py) tend to contract through these deformations. In [Cu(1-
NaphFcpmpy)(LAnth)]+, obvious p-stacking between the naphthyl
and phenanthroline groups formed, pmpy displacement in rocking
motion is enhanced by the attractive forces.
The 1H NMR spectral signals for the [Cu(RFcpmpy)(LAnth)]+
family could be divided into two set of peaks in different ratio that
varied depending on the size of the R substituent. The i-isomer
was preferred in the order H > 1-Naph > 4-tolyl > ferrocenyl >
9-Anth > tBu (Table 3). The size of the substituent was a primary
factor for the rotor behaviors, and a variety of ferrocene rotors
t
ranging from all-o (R = H) to all-i (R = Bu), were observed.
[Cu(1-NaphFcpmpy)(LAnth)]+ deviated from the size trend, to form
an excessively small amount of the i-isomer. This was attributed to
the attractive forces between the naphtyl and phenanthroline rings
through p–p* stacking, as described above. The isomeric ratio was
sensitive to the weak interactions in the outer-coordination sphere,
suggesting that rotational motion may also be controlled by these
types of interactions.
In contrast, [Cu(Fcvipmpy)(LAnth)]2+ contained nearly equal
amounts of the i- and o- isomers (Table 3). The ratio was
almost identical to that observed for the reference compound,
[Cu(tolvipmpy)(LAnth)]+. The similarity between the isomeric ratios
suggested that the ferrocenyl and 4-tolyl groups did not introduce
additional steric repulsion into the inner space, and the isomeric
ratio was determined by competition between -H and -C C- bond
replacement during rotation. In the case of the previously reported
Mpmpy complex,13 the methyl group had as larger preference
for the inner space than hydrogen atom. Induction effects of
methyl group overcame the small steric repulsion. The vinyl group
is considered to have a smaller induction effect due to a less-
extended sp2 configuration, and the large substituent requires some
entropically disfavourable structural element.
Fig.
2
Crystal
structures
of
[Cu(Fcvipmpy)(LAnth)]+
and
[Cu(tolvipmpy)(LAnth)]+. Anions and solvent molecules are omitted
for clarity.
These structural variation with large substituents are suggestive
for the intermediate structure of the rotational motion. As the
interconversion requires a dissociation of Cu–N(pm) bond and
a larger space for the twisted pyrimidine ring, three-coordinated
intermediate complex may be formed through the deformations
noted above, even in the complexes with tetrahedral structure at
the equilibrium position.
Substituents that featured intervening vinyl groups (Fig. 2),
showed almost no such deformations. In [Cu(tolvipmpy)(LAnth)]+,
the vinyl group penetrated the narrow gap between the anthryl
groups, and the p-tolyl group was placed in the open space.
[Cu(tolvipmpy)(LAnth)]+ was found only in the i-isomer, in contrast
with [Cu(Fcvipmpy)(LAnth)]+ which was present only as the o-
isomer. However, it seems to an intermolecular packing effect
determining isomer forms in these vinyl complexes. In solution,
these isomers were characterized by comparable energy (1 : 1 ratio)
and the energetic difference between [Cu(tolvipmpy)(LAnth)]+ and
[Cu(Fcvipmpy)(LAnth)]+ is almost negligible (Table 4). Generally,
Among all compounds tested, [Cu(9-AnthFcpmpy)(LAnth)]+,
[Cu(tolvipmpy)(LAnth)]+ and [Cu(Fcvipmpy)(LAnth)]+ were further
investigated with respect to their dynamics because they showed
in appropriate equilibrium properties with moderate isomeric
ratios in solution. The equilibrium ratios of the isomers were
evaluated by VT-1H NMR measurement. The isomeric ratio
near room temperature showed a correlation that followed the
equilibrium behavior, but the data deviated at low temperatures
(Fig. 3). The deviation was attributed to freezing of the structural
interconversion during rotational motions.
[Cu(9-AnthFcpmpy)(LAnth)]+ showed the lowest freezing point
(ca. 233 K). As discussed in previous section, this compound
˚
showed longer Cu–N(pm) bonds (2.08 A) than the other two
compounds in the crystal structure. Because both isomers ac-
commodated large substituents in the inner space, close contact
between the copper and nitrogen atoms was prevented. With other
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Table 4 Electrochemical potentials of Cu complexes
CuII/I
Fc+/Fc
[Cu(HFcpmpy)(LAnth)]+a
[Cu(4-tolFcpmpy)(LAnth)]+a
[Cu(1-NaphFcpmpy)(LAnth)]+a
[Cu(9-AnthFcpmpy)(LAnth)]+a
[Cu(tBuFcpmpy)(LAnth)]+a
[Cu(Fcvipmpy)(LAnth)]+b
0.07
0.41
0.45
0.44
0.41
0.02(o-)
0.03(o-)
0.26
0.17
0.18
0.23
0.21
0.13
0.27(i-)
0.27(i-)
[Cu(ptolvipmpy)(LAnth)]+c
Potentials are calibrated with ferrocene/ferrocenium as a standard. Data
were collected ata 298 K, b 251 K, and c 254 K.
compounds produced a much smaller redox potential shift
compared with the previously described Mpmpy structure.13,14
The potential shift was caused by the mutual repulsion of the
substituents in the Cu(II) state, which preferred square planar
geometry, and the repulsive effect was almost saturated during
the transition from the hydrogen to the methyl substitution.17
Note that the size effects discussed here related to the Cu(II)
state, not the Cu(I) state, which was discussed in the previous
section.
A cyclic voltammogram of [Cu(tolvipmpy)(LAnth)]+ at low
temperatures clearly showed splitting in the two waves of the
Cu(II)/Cu(I) cycle, which were assigned to the i- (0.27 V vs.
Fc+/Fc) and o- (0.03 V) isomers (Fig. 4). The voltammogram
of [Cu(Fcvipmpy)(LAnth)]+ indicated the presence of a ferroce-
nium/ferrocene redox couple at the midpoint of the Cu(II)/Cu(I)
couples for both isomers. In these vinyl complexes, the size
difference between the hydrogen and vinyl moiety was appro-
priate for producing a large Cu(II)/Cu(I) potential shift. The
order of these redox waves indicated a rotation and electron
transfer synchronization, because the Cu(II)/Cu(I) redox potential
crossed over the ferrocenium/ferrocene potential during the
course of rotation. The similar Cu(II)/Cu(I) redox potentials
Fig. 3 van’t Hoff plot of the interconversion between the i- and o- isomers
in acetone-d6, recorded during the heating process after rapid cooling.
Asterisks denote the point at which interconversions were frozen.
displacement of the ligands discussed, this complex seems to take
an equilibrium form similar to that of the inversion intermediate.
For [Cu(tolvipmpy)(LAnth)]+ and [Cu(Fcvipmpy)(LAnth)]+, freezing
occurred at higher temperatures (ca. 253 K). The elongated vinyl
structure hardly destabilized the coordinated structure in the stable
state, but seemed to prevent structural deviations toward the
intermediate in which the Cu–N(pm) bond was dissociated.
Electrochemical properties
The electrochemical potentials of the complexes are shown in
Table 4. In the family of [Cu(RFcpmpy)(LAnth)]+ compounds, only
two redox waves were observed and no splitting with respect
to the rotational isomers was observed in the range of 220 K–
298 K (Fig. S2†). The assignment of these redox waves was
judged from deformation of waves at low temperature, due to
the sluggishness of the Cu(II)/Cu(I) redox process. In these
complexes, only [Cu(HFcpmpy)(LAnth)]+ showed Cu(II)/Cu(I)-
ferrocenium/ferrocene oxidation order, in contrast, the other four
showed successive ferrocenium/ferrocene-Cu(II)/Cu(I) oxidation
processes. Although precise analysis showed a small degree of
splitting for the ferrocenium/ferrocene redox couple of [Cu(9-
AnthFcpmpy)(LAnth)]+ at low temperatures,16 rotation of these
Fig.
4
Cyclic voltammograms of [Cu(Fcvipmpy)(LAnth)]+ and
[Cu(tolvipmpy)(LAnth)]+ recorded in 0.1 M nBu4NBF4-acetone at ca.
250 K. Scan rate: 50 mV s-1.
2302 | Dalton Trans., 2011, 40, 2299–2305
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©

of [Cu(tolvipmpy)(LAnth)]+ and [Cu(Fcvipmpy)(LAnth)]+ supported
our speculation that the vinyl structure mainly determined the
degree of steric repulsion, and the 4-tolyl and ferrocenyl sub-
stituents had no significant effects on the coordination structure.
The ferrocenium/ferrocene couple was hardly split, unlike the
previously discussed complex which had ferrocenyl group directly
bound to the 5-position of the pyridine moiety.14 A longer distance
between the copper and ferrocenyl moieties seemed to cause a
less positive charge repulsion during the course of oxidation.
We obtained the relative stabilities of the isomers at dication
state using the equilibrium constant in the monocation state
and the Nernst equation. The equilibrium constant Kio = [o-
isomer]/[i-isomer] at 251 K was 1.1 for [Cu(Fcvipmpy)(LAnth)]+,
and 120 for the monooxidized [Cu(Fcvipmpy)(LAnth)]2+. The strong
preference for the o-isomer in the oxidized state causes dynamic
shift toward the o-isomer upon oxidation at room temperature,
observed as an asymmetric waves of isomers (Fig. S3†). However,
the processes in the monooxidized and dioxidized species were
not separable due to the proximity of the ferrocene redox
waves.
Fig. 5 UV-Vis absorption spectral changes of copper complexes upon
oxidation with (NH4)2[Ce(NO3)6] in acetone at 213 K. Spectra a–c
were recorded in successive procedures, during stepwise oxidation of
[Cu(Fcvipmpy)(LAnth)]+ (0–0.57 equiv., a; 0.57–1equiv; b) and after warm-
ing to room temperature (c). Spectra (d) show stepwise oxidation of
[Cu(tolvipmpy)(LAnth)]+.
Intramolecular electron transfer synchronized with rotational
motion
From the electrochemical properties, [Cu(Fcvipmpy)(LAnth)]+
is a sole candidate for intramolecular electron transfer from
copper to ferrocene, synchronized with the rotational motion
in the monooxidized state, i.e. i-[CuI(Fc+vipmpy)(LAnth)]2+
to o-[CuII(Fc0vipmpy)(LAnth)]2+. The 1e- oxidation of
[Cu(Fcvipmpy)(LAnth)]+ at 213 K, at a temperature at which ring
inversion was thermally prohibited, afforded the two successive
processes (Fig. 5a,b). The first spectral change (0–0.57 eq) showed
a significant decrease in the copper(I) MLCT band at 490 nm, with
emergence of a shoulder band around 580 nm. During the second
change (0.57–1.0 eq), a broad weak ferrocenium LMCT band
appeared around 700 nm. These results highlighted the differences
between the first oxidation centres in each isomers; Cu(II)/(I) for
the o-isomer, and ferrocenium/ferrocene for the i-isomer. The
assignment was in accordance with the electrochemical behaviour
and EPR spectrum which shows a partial Cu(II) spin density
(0.58), upon 1e- oxidation at 193 K.
When the sample with 1 : 1 i-[CuI(Fc+vipmpy)(LAnth)]2+ and o-
[CuII(Fc0vipmpy)(LAnth)]2+ warmed from 213 K to room tempera-
ture, the absorption spectrum converged to a distinct absorption
at 550 nm (Fig. 5c) to indicate the thermal activation of the
i- to o- conversion. A similar procedure was performed with
EPR measurements. The Cu(II) spin density measured at 80 K
increased from 0.58 to 0.91 upon warming to room temperature
(Fig. S4a†). The increase of spin density was not observed in
[CuII(tolvipmpy)(LAnth)]2+(Fig. S4b†) through the same procedure,
which has an almost quantitative Cu(II) spin density (0.88)
immediately after the oxidation.
accompanied by the movement of the ferrocene to the outer space
of the complex.
Absorption spectra of the isomers
As shown in Fig. 5c, the absorption spectra of the
[Cu(Fcvipmpy)(LAnth)]2+ isomers are very different, which is natu-
ral because the valence electronic configurations differ consider-
ably. The spectra were characterized by the ferrocene-Cu(II) charge
transfer band (o-isomer, 550 nm), and ferrocenium LMCT band (i-
isomer, 700 nm). The charge transfer band was assigned because no
strong band appeared during oxidation of [Cu(tolvipmpy)(LAnth)]+
(Fig. 5d). The very weak band of [Cu(tolvipmpy)(LAnth)]2+ at
630 nm was considered to be a d-d transition in Cu(II), and no sub-
stantial differences between the i- and o-isomer was observed upon
heating. Similar to the chemical oxidation at low temperatures, we
prepared o-[Cu(tolvipmpy)(LAnth)]+ and o-[Cu(Fcvipmpy)(LAnth)]+
in quantitative isomer ratio by reduction of the equilibrated o-
Cu(II) complexes at low temperature. Trapping of the unequi-
librated o-[Cu(tolvipmpy)(LAnth)]+ and o-[Cu(Fcvipmpy)(LAnth)]+
5
isomers by reduction with Fe(h -Me5Cp)2 was confirmed by the
significant increase in the MLCT absorption compared with the
initial state, which reversed to the initial state upon heating to room
temperature (Fig. 6). The absorption differences and molar ratio
of the isomers at equilibrium enabled us to calculate the absorption
spectra of the pure i-isomer state of these complexes (Fig. 6, dotted
line). The MLCT absorption band was significantly enhanced in
the o-isomer, which was not observed in the complex without
vinyl substituents.13 Absorption enhancement was attributed to
extension of the p* orbital of the pmpy ligand to the vinyl moiety.
In the o-isomer, this extension increased the transition dipole
moment of MLCT absorption. On the other hand, the effect was
reversed in the i-isomer by its position on the vinyl moiety, to
decrease the transition dipole moment.18
These spectral changes were similar to those observed for the
previous complex, which consisted of a fixed ferrocene-copper
tethered structure and a rotating 4-metyl-2-pyrimidyl ring.14 The
notable feature of [Cu(Fcvipmpy)(LAnth)]+ in this study is that
the ferrocene was in the oxidized form in the inner space, but
it was reduced by electron transfer from the copper(I) centre,
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ferrocenecarbaldehyde (214 mg, 1 mmol), tetrabutylammonium
hydrogen sulfate (34 mg, 0.1 mmol) and sodium hydroxide (1 g,
25 mmol) were suspended in 5 mL water and refluxed for 1 h.
After cooling to room temperature, 10 mL water were added and
extracted with ethyl acetate. The crude mixture was purified by
almina column chromatography eluted with ethyl acetate–hexane
(1 : 1) to remove impurities, successively eluted with ethyl acetate
to collect the product, and recrystallized from ethanol–water to
give orange-red microcrystals (yield: 211 mg, 57%). 1H-NMR
(500 MHz, CDCl3) d 8.87(d, J = 4.2 Hz, 1H), 8.79(d, J = 5.2 Hz,
1H), 8.58(d, J = 7.9 Hz, 1H), 7.89(t, J = 7.7 Hz, 1H), 7.76(d,
J = 16 Hz, 1H), 7.28(d, J = 5.1 Hz, 1H), 6.84(d, J = 16 Hz, 1H),
4.59(d, 2H), 4.43(s, 2H), 4.18 (s, 5H). Elemental Analysis Calcd.
for C21H17FeN3·0.2EtOH, C: 68.28, H: 4.87, N: 11.16. Found, C:
68.41, H: 4.98, N: 10.94.
Fig. 6 UV-Vis absorption spectral changes of o- [Cu(Fcvipmpy)(LAnth)]+
(a) and o-[Cu(tolvipmpy)(LAnth)]+ (b) upon warming. Complexes of
o-isomer were prepared by 1 equiv. oxidation with (NH4)2[Ce(NO3)6] at
213 K, warming to room temperature, cooled to 213 K and reduced with
5
Fe(h -Me5Cp)2. Dotted line shows the spectra of i-isomer calculated from
Synthesis of [Cu(tolvipmpy)(LAnth)]BF4. Under a nitrogen
the equilibrated molar ratio of i- and o- isomers. Asterisk denotes the
atmosphere, 2,9-bis(9-anthracenyl)-1,10-phenanthroline (LAnth
,
5
absorption of [Fe(h -Me5Cp)2]+.
53 mg, 0.1 mmol) and [Cu(CH3CN)4]BF4 (31 mg. 0.1 mmol)
was stirred for 1 h in 5 mL dichloromethane. To the resulting
orange clear solution, tolvipmpy (27 mg, 0.1 mmol) was added
and the solution immediately changed to brown. The solution was
filtered, and diethyl ether was added to the filtrate to obtain the
product as a brown solid. (yield: 82 mg, 86%). A single crystal
suitable for crystallography was obtained by slow diffusion of
benzene into a dichloromethane solution. Elemental Analysis
Calcd. for C58H39BCuF4N5·2H2O, C: 70.20, H: 4.37, N: 7.06.
Found, C: 70.50, H: 4.42, N: 6.97. ESI-TOF-MS m/z = 868.25
([Cu(tolvipmpy)(LAnth)]+).
Conclusion
A series of pyridylpyrimidine copper complexes with redox-
active rotating ferrocenyl units were constructed. Two effective
approaches to tuning the stability and conversion dynamics of the
isomers were demonstrated. Extension of the linker moiety by a
vinyl group proved to be an effective approach to induce a large
Cu(II)/Cu(I) redox potential shift, and [Cu(Fcvipmpy)(LAnth)]+
showed an oxidation state change in the ferrocene moiety through
electron transfer from the copper, accompanied by rotational
motion. The presented system demonstrated a new molecular
mechanics mode, in which the oxidation states of molecular
components depend on its positioning within a fixed range of
motion.
Synthesis of [Cu(Fcvipmpy)(LAnth)]BF4. Under
a nitrogen
atmosphere, 2,9-bis(9-anthracenyl)-1,10-phenanthroline (LAnth
,
53 mg, 0.1 mmol) and [Cu(CH3CN)4]BF4 (31 mg. 0.1 mmol)
were stirred until an orange clear solution was formed. Fcvipmpy
(37 mg, 0.1 mmol) was added and the solution was filtered.
Diethyl ether was added to the filtrate to precipitate the
product as a red solid (yield: 93 mg, 89%). A single crystal
suitable for crystallography was obtained by slow diffusion of
p-xylene into a dichloromethane solution. Elemental Analysis
Calcd. for C61H41BCuF4FeN5·2H2O, C: 67.45, H: 4.18, N: 6.45.
Found, C: 67.28, H: 4.22, N: 6.34. ESI-TOF-MS m/z = 962.20
([Cu(Fcvipmpy)(LAnth)]+).
Experimental
Materials
Synthetic procedures for the RFcpmpy ligands and copper com-
plexes were described elsewhere.16 2,9-Bis(9-anthracenyl)-1,10-
phenanthroline(LAnth)
19 and [Cu(CH3CN)4]BF420 were prepared by
the procedures described in the literature. Other chemicals were
used as purchased.
Synthesis of 2-(2-pyridyl)-4-(4-methylstyryl)pyrimidine (tol-
vipmpy). 2-(2-pyridyl)-4-methylpyrimidine (171 mg, 1 mmol), 4-
tolaldehyde (120 mg, 1 mmol), tetrabutylammonium hydrogen
sulfate (34 mg, 0.1 mmol) and sodium hydroxide (200 mg, 5 mmol)
were suspended in 5 mL water and refluxed for 2 h. The product
was filtered off, washed with water, and recrystallized from
ethanol–water to give off-white microcrystals (yield 87 mg, 32%).
1H-NMR (500 MHz, CDCl3) d 8.86(m, 2H), 8.60(d, J = 7.6 Hz,
1H), 7.94(d, J = 16 Hz, 1H), 7.89(t, J = 7.7 Hz, 1H), 7.55(d, J =
8.1 Hz, 1H), 7.42(d, J = 5.1 Hz, 1H), 7.35(d, J = 5.1 Hz, 1H),
7.23(d, J = 7.8 Hz, 2H), 7.19(d, J = 16 Hz, 1H), 2.40(s, 3H).
Elemental Analysis Calcd. for C18H15N3·0.2EtOH, C: 78.22, H:
5.78, N: 14.87. Found, C: 78.48, H: 5.72, N: 14.82.
X-ray structural analysis
Single crystal X-ray diffraction data were collected with an
AFC10 diffractometer coupled with a Rigaku Saturn CCD
system equipped with a rotating-anode X-ray generator that pro-
˚
duced graphite-monochromatic Mo-Ka radiation (l = 0.7107 A).
Lorentz-polarization and numerical absorption corrections were
performed using the program Crystal Clear 1.3.6. The structures
were solved by the direct methods using the SIR 92 program21
and refined against F2 using SHELXL-97.22 WinGX software
was used to prepare the material for publication.23 Disorders
-
in the positions of the BF4 anion and solvent molecules in
the crystals, which were treated using the PART option of the
SHELX-97 program. The crystallographic data are listed in
Table 3.
Synthesis
of
2-(2-pyridyl)-4-(2-ferrocenylvinyl)pyrimidine
(Fcvipmpy). 2-(2-pyridyl)-4-methylpyrimidine (171 mg, 1 mmol),
2304 | Dalton Trans., 2011, 40, 2299–2305
This journal is
The Royal Society of Chemistry 2011
©

Instrumentation
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NMR spectra were recorded using a Bruker DRX 500. UV-
vis absorption spectra were recorded using a Hewlett-Packard
8453 equipped with a temperature controller (UNISOKU USP-
203A), using a quartz cell with 1 cm optical path length.
ESI-TOF-MS spectra were recorded using an LCT Micromass
spectrometer. Cyclic voltammograms were recorded using an
ALS 650B electrochemical analyzer. The working electrode was
a 3 mm f glassy carbon electrode, a platinum wire served as the
auxiliary electrode, and the reference electrode was an Ag+/Ag
electrode (silver wire immersed in 0.1 M Bu4NClO4/0.01 M
AgClO4/CH3CN), calibrated with a ferrocene/ferrocenium redox
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©