Solvated-ion-pairing-sensitive molecular bistability based on copper(I)-coordinated pyrimidine ring rotation

Article abstract of DOI:10.1021/ic302141z

We describe herein the effect of solvated ion pairing on the molecular motion of a pyrimidine ring coordinated on a copper center. We synthesized a series of heteroleptic copper(I) complex salts bearing an unsymmetrically substituted pyridylpyrimidine and a bulky diphosphine. Two rotational isomers of the complexes were found to coexist and interconvert in solution via intramolecular ligating atom exchange of the pyrimidine ring, where the notation of the inner (i-) and outer (o-) isomers describes the orientation of the pyrimidine ring relative to the copper center. The stability of the pyrimidine orientation was solvent- and counterion-sensitive in both 2·BF ₄ {2⁺ = [Cu(Mepypm)(dppp)]⁺, where Mepypm = 4-methyl-2-(2′-pyridyl)pyrimidine and dppp = 1,3-bis(diphenylphosphino) propane} and previously reported 1·BF₄, which possesses a bulky diphosphine ligand (1⁺ = [Cu(Mepypm)(DPEphos)]⁺, where DPEphos = bis[2-(diphenylphosphino)phenyl] ether). Two rotational isomers of 2⁺ were separately obtained as single crystals, and the structure of each isomer was examined in detail. Both the enthalpy and entropy values for the rotation of 2·BF₄ in CDCl₃ (ΔH = 6 kJ mol^-1; ΔS = 25 J K^-1 mol^-1) were more positive than that tested under other conditions, such as in more polar solvents CD₂Cl₂, acetone-d₆, and CD₃CN. The reduced contact of the anion to the cation in a polar solvent seems to contribute to the enthalpy, entropy, and Gibbs free energy for rotational isomerization. This speculation based on solvated ion pairing was further confirmed by considering the rotational behavior of 2⁺ with a bulky counterion, such as B(C₆F₅)₄^-. The findings are valuable for the design of molecular mechanical units that can be readily tuned via weak electrostatic interactions.

Full text of DOI:10.1021/ic302141z

Article
pubs.acs.org/IC
Solvated-Ion-Pairing-Sensitive Molecular Bistability Based on
Copper(I)-Coordinated Pyrimidine Ring Rotation
Michihiro Nishikawa, Kuniharu Nomoto, Shoko Kume,*,† and Hiroshi Nishihara*
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
ABSTRACT: We describe herein the effect of solvated ion pairing on
the molecular motion of a pyrimidine ring coordinated on a copper
center. We synthesized a series of heteroleptic copper(I) complex salts
bearing an unsymmetrically substituted pyridylpyrimidine and a bulky
diphosphine. Two rotational isomers of the complexes were found to
coexist and interconvert in solution via intramolecular ligating atom
exchange of the pyrimidine ring, where the notation of the inner (i-) and
outer (o-) isomers describes the orientation of the pyrimidine ring
relative to the copper center. The stability of the pyrimidine orientation
was solvent- and counterion-sensitive in both 2·BF4 {2+ = [Cu(Mepypm)(dppp)]+, where Mepypm = 4-methyl-2-(2′-
pyridyl)pyrimidine and dppp = 1,3-bis(diphenylphosphino)propane} and previously reported 1·BF4, which possesses a bulky
diphosphine ligand (1+ = [Cu(Mepypm)(DPEphos)]+, where DPEphos = bis[2-(diphenylphosphino)phenyl] ether). Two
rotational isomers of 2+ were separately obtained as single crystals, and the structure of each isomer was examined in detail. Both
the enthalpy and entropy values for the rotation of 2·BF4 in CDCl3 (ΔH = 6 kJ mol−1; ΔS = 25 J K−1 mol−1) were more positive
than that tested under other conditions, such as in more polar solvents CD2Cl2, acetone-d6, and CD3CN. The reduced contact of
the anion to the cation in a polar solvent seems to contribute to the enthalpy, entropy, and Gibbs free energy for rotational
isomerization. This speculation based on solvated ion pairing was further confirmed by considering the rotational behavior of 2+
−
with a bulky counterion, such as B(C6F5)4 . The findings are valuable for the design of molecular mechanical units that can be
readily tuned via weak electrostatic interactions.
luminescence36−48 but also for use in applications.39−42
A
INTRODUCTION
■
family of [Cu(diimine)(DPEphos)]+ (DPEphos = bis[2-
(diphenylphosphino)phenyl] ether) complexes has been
particularly well studied owing to their intense lumines-
cence.43−46 The luminescence of [Cu(diimine)(dppp)]+
[dppp = 1,3-bis(diphenylphosphino)propane] derivatives has
also been examined in detail.47
Solvated ion pairing is valid for both traditional and recent
topics of chemistry, such as electron transfer,1−3 general
reactions,4,5 charge separation,6 molecular machines,7−12 ion
sensing,13−15 and ionic liquids.16 An ion pair consists of an
equilibrium between several different states that include the
anion and cation present as a solvated contact ion pair (CIP), a
solvent-shared ion pair, a solvent-separated ion pair (SSIP), and
unpaired solvated ions.17 Because ion pairing between the
metal complex cation and counteranion has often been found
to play a key role in the functionalization of molecular systems,
detailed studies on the solvation are valid for the development
of promising materials.18−30 In the present study, we
investigated ion-pair effects on a metal complex bistability
caused by intramolecular ligating atom exchange using simple
heteroleptic copper(I) complexes bearing an unsymmetrically
substituted pyridylpyrimidine and a bulky diphosphine ligand.
The copper(I) state prefers to adopt a tetrahedral geometry
because of its d10 electronic configuration.26−52 The bidentate
diimines on copper(I) undergo ligand-substitution reactions at
slow rates under ambient temperatures.28−32 In addition, the
labile Cu−N bond rearrangement is valid for applications in
nanoscience.20−22,33−35 Furthermore, the photophysics of
simple heteroleptic copper(I) complexes bearing a diimine
and a diphosphine ligand has attracted significant attention not
only for fundamental studies pertaining to their intense
We introduced a bidentate ligand consisting of a coordinated
pyrimidine moiety that could effectively alternate between two
possible coordination geometries at the copper center via
rotational isomerization.53−59 The ring rotation has been
exploited for modulation of the electrode potential of
[Cu(Mepypm)(Lanth)]BF4 [Mepypm = 4-methyl-2-(2′-
pyridyl)pyrimidine; Lanth = 2,9-bis(9-anthryl)-1,10-phenanthro-
line],53 for the manipulation of intramolecular electron
transfer,54 for the development of a redox potential switch
based on photodriven rotation,58 and in the demonstration of
the dual-luminescence behavior of [Cu(Mepypm)(DPEphos)]-
BF4 (1·BF4),57 considering well-established copper(II/I) redox
properties.20−22,33−35,60−63 We have also reported that the
equilibrium of 1·BF4 between the two rotational isomers in the
copper(I) state depends significantly on both the solvent and
temperature, as confirmed by analysis in chloroform-
Received: October 2, 2012
Published: December 13, 2012
© 2012 American Chemical Society
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d1(CDCl3), acetone-d6, and acetonitrile-d3 (CD3CN) at
variable temperature.57
The rotational bistability of 2·BF4 in CDCl3, dichloro-
methane-d2 (CD2Cl2), acetone-d6, acetonitrile-d3 (CD3CN),
2·B(C6F5)4 in CDCl3, 1·BF4 in CDCl3, CD2Cl2, acetone-d6, and
In the present paper, we investigate in detail the effect of ion
pairing on the rotational dynamics of [Cu(Mepypm)(dppp)]+
(2+). The coordination geometry of the diphosphine moiety
differs from that previously described. Two kinds of non-
1
CD3CN, and 1·B(C6F5)4 in CDCl3 was characterized using H
NMR analysis to monitor chemical exchange (Figure 2 and the
coordinative counterions are considered,64−67 BF4 and
−
−
B(C6F5)4 , where the latter is much larger than the former.
The chemical equilibrium of the isomers is illustrated in Figure
1, where the notation of the inner (i-) and outer (o-) isomers
1
Figure 2. Aromatic H NMR signal of a Mepypm moiety in 2·BF4 in
CDCl3 at several temperatures. The signals derived from i- and o-
isomers are represented as red and blue, respectively. Illustration of the
ratios and the rate of the rotational equilibrium are also described on
the right side.
Figure 1. Conceptual diagram showing the effect of ion pairing on the
chemical equilibrium of pyrimidine ring rotational isomerization.
1
Supporting Information, Figures S1−S10). Integration of H
NMR spectra along with line-shape analysis are well-established
methods68,69 for investigating the thermodynamics and kinetics
of a chemical equilibrium.70−78 Two clearly resolved sets of
describes the orientation of the pyrimidine ring. To elucidate
the ion-pairing sensitivity, we describe the rotational equili-
brium for both 1·BF4 and 1·B(C6F5)4. We found that the
dynamics requires a consideration of the relationship between
the solvated ion pairing and the pyrimidine ring rotation.
1
signals were observed in the H NMR spectrum of 2·BF4 in
CDCl3 at 253 K. Signal splitting was also observed in the
aromatic proton signals of the Mepypm ligand (Figure 2). The
ratio of o-isomers, estimated from integration of the minor and
major signals, is represented as xo %. The ratio of i-isomer is
thus (100 − xo) %. Consider, for instance, that at 253 K the xo
value was determined to be 59%; the reason for higher xo values
at higher temperature is described in the later sections. Upon
heating, these signals broadened, indicating that i- and o-
isomers interconverted in solution on a time scale that is
commensurate with that of the 1H NMR measurement (Figure
2). The rate constant for the i → o isomerization, k, was
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Rotational
Equilibrium in Solution. 1·B(C6F5)4, 2·BF4, and 2·B(C6F5)4
were synthesized using a modified literature method (Scheme
1).44,47,57 In this approach, tetrakis(acetonitrile)copper(I) salt
was reacted with a diphosphine ligand and a Mepypm ligand in
dichloromethane at room temperature. The obtained com-
pounds were characterized by 1H NMR and elemental analysis.
1
estimated from simulated fittings of the broadened H NMR
Scheme 1. Synthesis of 2·BF4 and Reference
[Cu(diimine)(diphosphine)]X Complexes
spectra (Supporting Information, Figure S1b). For example, the
value of k was 100 s−1 at 303 K; a faster rate of rotation at
higher temperature is normal for chemical equilibrium. These
parameters, at several temperatures and under various
conditions, were determined by the same analysis as that
described above.
The i/o ratios and their heat sensitivities, along with their
isomerization rates, are dependent on the solvent and
counterion (Supporting Information, Figures S1−S10). How-
ever, a complete description of the observed effects requires a
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close consideration of the geometry of the complexes, as
highlighted in later sections.
This interpretation is supported by the fact that the shape of
the peaks in the H NMR spectrum of 1·BF4 in acetone-d6 at
1
The interconversion between the two rotational isomers is
room temperature displayed no dependence on the concen-
tration (Supporting Information, Figure S12). The evidence of
intramolecular rotational isomerization is valid to drive our
previous rotational system53−58 in the single molecular level.
Crystallography. We performed single-crystal X-ray
structural analysis of 1·B(C6F5)4, 2·BF4, and 2·B(C6F5)4
(Figure 4 and the Supporting Information, Figures S13−S17
1
generally an intramolecular process, as confirmed by H NMR
analysis of a mixed solution of 1·BF4 and [Cu(bpy)-
(DPEphos)]BF4 (3·BF4; bpy = 2,2′-bipyridine), which does
not undergo rotation,57 This strategy is based on a modified
method described in the literature.70b The rate of interconver-
sion between i- and o-isomers (k) via an intramolecular process
is denoted as kintra, which via an intermolecular process is kinter
(k = kintra + kinter). The value of kinter is evaluated from the rate
of interconversion between 3·BF4 and 1·BF4 because both
processes require intermolecular diimine ligand exchange. A 1H
NMR spectrum of 3·BF4 in acetone-d6 at room temperature
displayed one set of signals derived from the diimine moiety
(Figure 3, middle). Two sets of broadened signals derived from
Figure 3. (left) Partial 1H NMR spectra of 1·BF4 (top), 3·BF4
(middle), and a mixture of 1·BF4 and 3·BF4 (bottom) in acetone-d6
in the dark at room temperature (right). Illustration of the
interconversion between species in a mixed solution of 1·BF4 and
3·BF4.
1
i- and o-isomers were observed in a H NMR spectrum of
1
1·BF4 under the same conditions (Figure 3, top). A H NMR
spectrum of a mixed solution of 3·BF4 and 1·BF4 under the
same conditions was simply a superposition of the two spectra
(Figure 3, bottom), suggesting that the intermolecular
interconversion between 3·BF4 and 1·BF4 via a diimine ligand
exchange reaction was extremely slow compared with the time
scale of the 1H NMR measurement (Figure 3, right). In
1
addition, the H NMR spectrum of the solution showed no
signal broadening derived from 3·BF4 at 313 K (Supporting
Information, Figure S11), where the estimated rate constant for
interconversion between i- and o-isomers (k) was 300 s−1. The
rate constant for the intermolecular process under the given
conditions (kinter) was smaller than ca. 1 s−1 (k ≫ kinter).
Therefore, interconversion between i- and o-isomers in typical
organic solvents at ambient temperature is dominated by the
intramolecular process (kintra ≫ kinter). Possibilities of a faster
intermolecular process (kintra ∼ kinter or kintra ≪ kinter) can be
denied from small differences in the signal broadness between
three species derived from i- and o-isomers of 1·BF4 and 3·BF4.
Figure 4. Side (left) and front (right) views of the crystal structures of
o-2·BF4·0.5MeOH (a), i-2·B(C6F5)4 (b), o-1·BF4·CHCl3 (c), and o-
1·B(C6F5)4·1.5hexane (d). The carbon atoms in the Mepypm moiety
are blue or red, which correspond to o- and i-isomers, respectively.
The Mepypm moiety, copper atom, and counterion are drawn as a
space-filling model, whereas the diphosphine moiety is included as a
capped stick model. For clarity, some molecules are omitted: (a) a
−
complex cation, a BF4 ion, and a methanol molecule; (c) a
chloroform molecule; (d) hexane molecules.
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and Table S1). The previously reported crystal structure of
1·BF4 was also employed for comparison. The ratio of the
(ΔH), entropy (ΔS), and Gibbs free energy (ΔG) for the i → o
rotation were obtained using van’t Hoff plots that were
estimated from integration of the H NMR spectra at various
57
1
complex cation to the counterion in the asymmetric unit was
1:1, suggesting that the oxidation state of the metal atom was
copper(I) in the single crystal. The geometric parameters of the
complex cations in 1·BF4, 1·B(C6F5)4, 2·BF4, and 2·B(C6F5)4
such as the bond lengths were in agreement with the ones of
the family of [Cu(diimine)(diphosphine)]+ complexes (Sup-
porting Information, Table S2).43−47 The methyl group of the
Mepypm moiety in 2·BF4 is oriented outward of the metal
center, and no disorder was found in the coordination mode.
This result suggests that all 2+ species in the single crystal exist
solely as the o-isomer, which corresponds to a pyrimidine
moiety orientation that is identical with that of the previously
reported crystal structure of 1·BF4 (Figures 4a,c).57 In contrast,
the methyl group in 2·B(C6F5)4 is directed toward the metal
center, indicating that all 2+ species exist as the i-isomer in the
single crystal (Figure 4b). Furthermore, all 1+ species in
1·B(C6F5)4 exist as the o-isomer (Figure 4d), which is identical
with 1·BF4 from the viewpoint of pyrimidine orientation. On
the basis of the orientation of pyrimidine in the crystal as well
as considering the solvents to have negligible interaction with
the complex cation, the four single crystals are denoted as o-
2·BF4·0.5MeOH, i-2·B(C6F5)4, o-1·BF4·CHCl3, and o-1·B-
(C6F5)4·1.5hexane.
temperatures in different solvents (Figure 5). A negative ΔG
Figure 5. van’t Hoff plots for the rotational equilibrium with the
equilibrium constant, K, set equal to [o-isomer]/[i-isomer].
corresponds to a predominance of the o-isomer compared with
the i-isomer. A positive ΔH value corresponds to an enthalpic
stabilization of the i-isomer and a positive ΔS value to an
entropic stabilization of the o-isomer. The significant heat
sensitivity of the isomer ratios (Figure 2) indicated the large
value of ΔH. We denote ΔG and xo at 298 K as ΔG298 and
In the case of o-1·BF4·CHCl3, the copper center is
surrounded by the coordinating nitrogen atoms, a bulky
−
diphosphine moiety, and a BF4 ion. The interaction of the
−
BF4 ion with the copper center in o-2·BF4·0.5MeOH is
significantly stronger than that in o-1·BF4·CHCl3, considering
the Cu−B distance. These results reflect the reduced bulkiness
of dppp, owing to the presence of fewer phenyl groups,
compared with DPEphos. In contrast, the methyl group in i-
2·B(C6F5)4 is located nearer to the copper atom than the
x
o,298, respectively. Selected parameters are tabulated in Table 1
and the Supporting Information, Table S3.
Table 1. Selected Thermodynamic Parameters for the
Rotational Equilibrium of [Cu(Mepypm)(diphosphine)]+
Complexes under Various Conditions
−
counteranion. B(C6F5)4 is distant from the copper center
−
because of a large steric repulsion between B(C6F5)4 and the
complex cation. Because of a coordination geometry similar to
that of o-2·BF4·0.5MeOH, the proximity of the counterion is
seemingly the primary cause for destabilization in the i-isomer.
On the other hand, the methyl group did not cover the copper
center in 1·B(C6F5)4, which is similar to o-1·BF4·CHCl3. The
results suggest that the position of the counterion has a small
effect on the orientation of pyrimidine in 1+ most likely because
of the bulkiness of the diphosphine moiety. As a result, the
effects of the counterion on the rotational dynamics of 2+ are
expected to be more significant than those operative in 1+. The
aforementioned trend is characteristic of the [Cu(Mepypm)-
(diphosphine)]+ family because of the geometry of the
diphosphine ligand. Such behavior was not observed when
bulky diimines were employed as the auxiliary ligand, such as in
[Cu(Mepypm)(Lanth)]+.53 Examination of the crystal packing in
the presented complexes reveals that the proximity of other
species, such as solvent molecules or another complex salt, was
less than that of the nearest counterion described above
(Supporting Information, Figures S14−S17). The obtained
crystal structures enable us to effectively construct a reasonable
model for the sensitivity of rotational equilibrium on a weak
interaction in a solution state, which is described in the
following section.
a
b
c
d
solvent
ΔH
ΔS
ΔG298
xo,298
1·BF4
1·BF4
1·BF4
1·BF4
1·B(C6F5)4
2·BF4
2·BF4
2·BF4
2·BF4
CDCl3
0.0
1.3
8
−2.6
−2.1
−2.1
−1.6
−2.2
−1.9
−0.6
−0.8
0.0
74
70
70
65
71
68
56
58
50
57
CD2Cl2
acetone-d6
CD3CN
CDCl3
11
7
0.0
0.2
6
−0.2
5.6
6
CDCl3
25
15
11
10
13
CD2Cl2
acetone-d6
CD3CN
CDCl3
4.0
2.5
3.0
2·B(C6F5)4
3.3
−0.7
a
b
Enthalpy for i → o rotation,/kJ mol−1. Entropy for i → o rotation,/J
c
K−1 mol−1. Gibbs free energy for i → o rotation at 298 K/kJ mol−1.
d
Molar ratio of the o-isomer at 298 K/%.
The results reveal two key points: (a) the values of both ΔH
and ΔS of 2·BF4 in CDCl3 were large and positive and (b) the
values of xo,298 were reduced in more polar solvents or with use
of a larger counterion.
Both the enthalpy and entropy values for the rotation of
2·BF4 in CDCl3 (ΔH = 6 kJ mol−1 and ΔS = 25 J K−1 mol−1)
were more positive than that tested under other conditions
(Table 1), such as in more polar solvents CD2Cl2 (ΔH = 4 kJ
mol−1 and ΔS = 15 J K−1 mol−1), acetone-d6 (ΔH = 3 kJ mol−1
and ΔS = 10 J K−1 mol−1), and CD3CN (ΔH = 3 kJ mol−1 and
ΔS = 10 J K−1 mol−1). The parameters of 2·B(C6F5)4 in CDCl3
Results for the Thermodynamics of Rotation in
Solution. The thermodynamics of the rotational equilibrium
of 2·BF4 and 2·B(C6F5)4, along with the reference compounds
1·BF4 and 1·B(C6F5)4, was examined. The values of enthalpy
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(ΔH = 3 kJ mol−1 and ΔS = 13 J K−1 mol−1) were more
negative than those of 2·BF4. On the other hand, the values of
ΔH and ΔS of 1+ under the same aforementioned conditions
fell within ranges of 0−1 kJ mol−1 and 6−11 J K−1 mol−1,
respectively, indicating that the solvent and counterion effects
on the values of both ΔH and ΔS of 1+ were less than those of
2+. In all tests, the values of ΔH and ΔS of 2+, particularly ΔH,
were more positive than those of 1+. These results suggest that
the diphosphine moieties considerably affect the enthalpy and
entropy for the rotational isomerization.
35.94 for CD3CN). Roughly linear relationships between the
parameters and (εr − 1)/(2εr + 1) indicate that the solvent
polarity contributes considerably to the thermodynamics of the
systems considered. The slopes of the four 2·BF4 plots, based
on a linear least-squares regression fitting, were much larger
than those of 1·BF4, suggesting that 2+ is more sensitive to
solvent selection than 1+. The slopes of ΔG298 for both 1·BF4
and 2·BF4 are positive, indicating that the mechanism of the
solvent dependency is similar throughout the [Cu(Mepypm)-
(diphosphine)]+ family.
A lower population of the o-isomer, particularly in 2+, was
observed when a larger counterion was employed. The xo,298
values of 2·BF4 and 2·B(C6F5)4 in CDCl3 were 68% and 57%,
respectively. Significant counterion effects were not observed in
1+; the ratio of 1·B(C6F5)4 in CDCl3 (xo,298 = 71%) is slightly
less than that of 1·BF4 in CDCl3 (xo,298 = 74%). A lower
population of the o-isomer in the polar solvent, similar to that
observed in a previous report,57 was found not only for 1·BF4
but also for 2·BF4. The values of xo,298 for 2·BF4 in CD2Cl2,
acetone-d6, and CD3CN were 56%, 58%, and 50%, respectively,
which are less than the values obtained in CDCl3. These trends
are approximately observed in the full temperature range (see
the van’t Hoff plots in Figure 5). The population of the
entropically favored o-isomer is reduced at lower temperature
(xo at 233 K = 54%; ΔG = −0.3 kJ mol−1). The two isomers
therefore have comparable stabilities. Because the values of ΔH
under the other conditions were relatively small, the temper-
ature dependences of xo and ΔG were less significant than that
for 2·BF4 in CDCl3.
The aforementioned weak interaction dependence of the
parameters ΔH, ΔS, ΔG298, and xo,298 cannot be explained by
the simple consideration of chemical structures alone. For
completeness, with the aid-generated crystal structures, we
constructed a model that considers the geometries of both the
ion pair and the six-membered chelating dppp moiety, as
highlighted in the following section.
Discussion for the Thermodynamics of Rotation. The
geometric features of 2+ and reference compound 1+ are
illustrated at the top of Figure 7. The major bonding surface of
the copper center in both 1+ and 2+ is occupied by the
coordinating nitrogen atoms and diphosphine moiety. The
bonding area taken up by coordination in 2+ is reduced
compared with that in 1+, which provides additional space that
a counterion or a methyl group on the pyrimidine moiety can
occupy. Contributions of the CIP state in CDCl3 are larger than
those in more polar solvents such as CD3CN.17 In other words,
the population of the SSIP state in a polar solvent is larger than
that in a less polar solvent. For clarity, we focus on the
representative two ion-pairing states and omit the others. It
should be noted that ion pairing between a monovalent metal
complex cation and a monovalent counteranion is strengthened
in CDCl3, moderate in CD2Cl2, and weakened in more polar
solvents such as acetone-d6.23−26 On the other hand, with
The parameters ΔH, ΔS, ΔG298, and xo,298 were plotted
against the Kirkwood function,17 (εr − 1)/(2εr + 1), where εr
corresponds to the relative permittivity of the solvent (Figure 6;
εr = 4.89 for CDCl3, 8.93 for CD2Cl2, 20.56 for acetone-d6, and
−
respect to the size of the anion, BF4 is significantly smaller
−
than B(C6F5)4 , which is comparable with the size of the
copper complex cation.
The value of ΔH for solvated i- and o-isomers is linked to
intrinsic preference caused by the induction effect of the methyl
group, electrostatic interactions, solvation, electrostatic attrac-
tion between the electron cloud of the methyl group and the
positively charged metal core of the complex, and other
enthalpic factors present. The ΔS values are a reflection of their
respective freedom of rotation of the functional groups, along
with the vibrational freedom of the ligand moieties, solvent, and
counterion. For example, the large positive values of 2·BF4 in
CDCl3 indicate that the behavior of the solvated i-isomer differs
significantly from the solvated o-isomer. In addition, the
relatively small absolute values of both ΔH and ΔS of other
conditions, 2·BF4 in a polar solvent, 1·BF4 in all solvents as we
tested, 2·B(C6F5)4 in CDCl3, and 1·B(C6F5)4 in CDCl3,
indicate that the differences in the behaviors between the
solvated i- and o-isomers are trivial.
The rotational equilibrium in solution is represented as a
model, displayed in Figure 7. Six conditions are represented:
(a) 2·BF4 in CDCl3, (b) 2·BF4 in CD3CN, (c) 2·B(C6F5)4 in
CDCl3, (d) 1·BF4 in CDCl3, (e) 1·BF4 CD3CN, and (f)
1·B(C6F5)4 in CDCl3. The equilibrium behaviors in CD2Cl2
and acetone-d6 can be interpreted as a range from those in less
polar CDCl3 to those in more polar CD3CN, considering their
moderate values of ΔH, ΔS, ΔG, and relative permittivity. Each
Figure 6. Correlation between the Kirkwood function, (εr − 1)/(2εr +
1), and the thermodynamic parameters ΔH (a), ΔS (b), ΔG298 (c),
and xo298 (d) of 1·BF4 and 2·BF4 in CDCl3, CD2Cl2, acetone-d6, and
CD3CN.
result has two points: (i) ΔH and ΔS; (ii) ΔG298 and xo,298
.
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Figure 7. Illustrations of the effect of ion pairing on the rotational bistability of a [Cu(Mepypm)(diphosphine)]+ family. (top, left) Geometric
features of the diphosphine moieties of 1+ and 2+. (top, right) The copper(I) complexes in CIP and SSIP states. (a−f) Selected chemical equilibrium
between the solvated i- and o-isomers: (a) 2·BF4 in CDCl3; (b) 2·BF4 in CD3CN; (c) 2·B(C6F5)4 in CDCl3; (d) 1·BF4 in CDCl3; (e) 1·BF4 in
CD3CN; (f) 1·B(C6F5)4 in CDCl3.
Six Conditions a−f Explained as Follows. (a-i) The
values of ΔH and ΔS of 2·BF4 in CDCl3 are larger in
magnitude and more positive than those under other
conditions. In the CIP state of the o-isomer, there is no steric
repulsion between the counterion and methyl group (Figure
7a). In contrast, the CIP state of the solvated i-isomer competes
for the BF4− ion and methyl group on the pyrimidine moiety as
a result of steric repulsion. This repulsion can be correlated to
destabilization of the solvated i-isomer owing to the loss of
freedom of motion in both the complex cation and counterion.
The steric repulsion can cause more positive ΔH. The
enhanced solvated i-isomer preference based on a combination
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of several enthalpic factors described above can contribute to
ΔH because its movement is strongly limited by the
competition. (a-ii) The negative value of ΔG298 indicates that
the population of the entropically favorable solvated o-isomer,
mentioned in part a-i, is larger than that of the i-isomer at 298
K. At a low temperature such as 233 K, where the entropic
factor is less dominant, both isomers have comparable stability.
The comparable stabilization effects in the viewpoint of
enthalpy and entropy cause the bistability based on the
rotational equilibrium.
(b-i) The effects of solvent polarity are described in this
paragraph. The ΔH and ΔS of 2·BF4 are positively small in
polar solvents such as CD3CN (Figure 7b). The steric repulsion
between the counterion and methyl group on the pyrimidine
moiety, mentioned in part a-i, is relatively small in the SSIP
states of both the solvated i- and o-isomers. Therefore, the
absolute values of ΔH and ΔS are also small.
interaction between the methyl group and the phenyl groups
on the diphosphine can also contribute to positive ΔH values.
The entropy loss of the i-isomer originates from a crowded
coordination geometry, which reduces the freedom of motion
of the ligand moiety, thus contributing to positive values of ΔS.
The intrinsic factors also contribute to the values of xo.
Consequently, all results related to thermodynamics can be
reasonably understood based on the proposed model,
suggesting that the present rotational bistability, particularly
in 2+, arises from solvated ion pairing.
1
Notes about the Model. It should be noted that the H
NMR signals can be the average of many kinds of
conformations with negligible differences in chemical shifts,
such as the resonances of the solvent, counterion, and ligands
including the diphosphine moiety. The conceptual diagrams,
displayed in Figure 7, are representative of the real equilibrium.
Because the crystal structures of the [Cu(Mepypm)(diimine)]+
family53 were very different from those of the present
complexes, particularly from the viewpoint of the coordination
structure and location of the counterion, the ion-pairing
sensitivity is characteristic of [Cu(Mepypm)(diphosphine)]+
structures.
(b-ii) The xo,298 values of 2·BF4 in CD3CN are smaller than
those in CDCl3 because destabilization of the solvated i-isomer,
mentioned in paragraph a, is considerably minimized in the
SSIP state.
(c-i) The effects of the counterion size are described in this
paragraph. The ΔH and ΔS of 2·B(C6F5)4 in CDCl3 are more
negative than those of 2·BF4 in CDCl3, which is characteristic
Changes in the dipole moment, hydrogen-bonding inter-
action, and hydrophobic interactions between states are often
attributed to solvent sensitivity not only in traditional axial/
equatorial conformational equilibrium17 but also in chemistry
for promising nanomaterials such as molecular machines based
on supramolecules.7,8,71−74 Because the i- and o-isomers are
similar with respect to the coordination structure, the polarity
differences between the complex cations of the two isomers are
expected to be small. On the other hand, the absolute value of
the dipole moment of the solvated copper(I) compounds is
expected to be proportional to the distance between the copper
atom and counterion center (Cu−X). Therefore, the polarity of
the i-isomer in the CIP state is expected to be larger than that
of the o-isomer, considering that the Cu−X distance is
determined by steric repulsion. This is an additional effect
caused by ion pairing, as described in Figure 7. The difference
in polarity between the two isomers in the CIP state can
contribute to the preference of the i-isomer for a more polar
solvent. This rationale is incomplete for three reasons: (1) the
population of the CIP state in the polar solvent is small, (2) the
difference of the Cu−X distance between i- and o-isomers in
the SSIP state is small, and (3) the large ΔH and ΔS of 2·BF4
in CDCl3 cannot be appropriately explained by this reasoning.
If a solvent molecule, such as CD3CN, coordinates to the
copper(I) center, steric repulsion between the methyl group
and solvent molecule would cause a decrease of the i-isomer
ratio in CD3CN compared with CDCl3. This assumption can
be totally denied because of both the experimental results and
copper(I) electronic configuration. The copper(I) center
already has a coordination number of 4, so it cannot
accommodate any additional ligand.
−
of the other conditions (Figure 7c). Therefore, the B(C6F5)4
ion causes a small destabilization of the i-isomer in the CIP
state, as described in paragraph a, because of its size.
(c-ii) The inhibition of destabilization of the i-isomer also
causes a decrease in xo,298 for the B(C6F5)4− ion compared with
−
the BF4 ion.
(d-i) The effect of the geometry of the diphosphine ligand is
described in paragraphs d−f. Despite the absolute values of ΔH
and ΔS of 2·BF4 in CDCl3 being relatively large, those of 1·BF4
in CDCl3 are small because the bulkiness of the diphosphine of
1+ reduces the steric repulsion between the counterion and
methyl group of the solvated i-isomer, as mentioned in part a-i
(Figure 7d). The reason for nearly zero ΔH value of 1+ can be
interpreted as a small difference in solvation between the i- and
o-isomers (Figure 7d).
(d-ii) The value of ΔG298 is negative because the bulkiness of
the diphosphine contributes to destabilization of the i-isomer
via steric repulsion between the methyl group and diphosphine
moiety. The bulkiness is also the reason why xo,298 of 1·BF4 in
several solvents is larger than that of 2·BF4.
(e-i) The absolute values of both ΔH and ΔS of 1·BF4 in
CD3CN are small because of a combination of phenomena
described in parts b-i and d-i (Figure 7e).
(e-ii) The negative ΔG stems from the steric bulk of the
diphosphine as described in part d-ii. The lower xo,298 of 1·BF4
in polar solvents compared with that of 1·BF4 in CDCl3 was
attributed to ion-pairing effects, as highlighted in the above
comparison of parts a and b.
(f-i) The absolute values of both ΔH and ΔS of 1·B(C6F5)4
in CDCl3 are small due to a combination of the c-i and d-i
phenomena (Figure 7f).
(f-ii) Because the ion-pairing effects are slightly retained,
xo,298 of 1·B(C6F5)4 was lower than that of 1·BF4.
The values of both ΔH and ΔS contain a contribution of the
effects mentioned above as well as the following intrinsic
factors. For example, stabilization of the i-isomer stemming
from the electronic structure of Mepypm, which can be derived
from induction effects, thus contribute to positive ΔH values.
Stabilization of the i-isomer caused by an easing of the C−H
We assume that the volume occupied by 2+ (Figure 7) can be
modified in CDCl3, which can destabilize the i-isomer via steric
repulsion between the methyl group and solvent molecule.
Because this assumption does not explain the decrease in the o-
isomer molar ratio when a larger counterion is used, the effects
of the size of the solvent molecule are small.
Rate for Isomerization in a Solution State. The rate
constants for the i → o isomerization, k, at variable temperature
were estimated from Arrhenius plots based on simulation of the
1
broadened H NMR spectra (Figure 8 and the Supporting
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CDCl3. The resemblance also seems to relate to the model
provided in Figure 7. The CIP effect can contribute to the
kinetics of rotation, but this effect seems to be smaller than that
of the solvent coordination. The solvent sensitivity trends,
mentioned above, are basically observed over the full
temperature range tested (see the Arrhenius plots in Figure
8). The k value varied from 10−6 s−1 (frozen motion) to 103 s−1
over the temperature and solvent ranges considered (Support-
ing Information, Figure S18). These slow rates, compared to
many of chemical equilibrium, are one of the requirements for
switching between equilibrium and metastable states, which
play a key role for our systems with electric signal response.
Physical Properties of 2+. In our previous systems,
[Cu(Mepypm)(Lanth)]+ and their derivatives were found to
exhibit reversible copper(II/I) redox activities that were
synchronized with rotational isomerization.53−55,58 Cyclic
voltammograms of 2·BF4 in 0.1 M Bu4NBF4−CH2Cl2 show
irreversible oxidation waves at approximately 0.7 V vs Ag+/Ag
(Figure 9a). This irreversibility seems to be caused by an
Figure 8. Arrhenius plots and rate constant at 298 K for the i → o
interconversion of 1+ (a) and 2+ (b) under a variety of conditions.
Information, Figures S1−S10). The values of k at 298 K, k298
,
which are the parameters most representative of the solvent-
and counterion-sensitive kinetics, are also summarized in Figure
8. Other parameters are tabulated in the Supporting
Information, Table S3.
Kinetic analysis supports the validity of thermodynamic
analysis. The magnitude of the k values at the given
temperature, where 1H NMR spectra for thermodynamic
analysis were conducted, falls within a range of 10−2−100 s−1
(Supporting Information, Figure S18). Qualitatively, the values
indicate that the system reaches equilibrium within 102 s. For
1
the H NMR spectra conducted with ca. 103 s intervals, the
signal integrations reflect the ratio of the i- and o-isomers in
equilibrium. Signal broadening is observed when k exceeds ca.
1
101 s−1. The H NMR spectra at high temperature are not
employed for thermodynamic analysis.
The much larger k298 values obtained in coordinating solvent
such as CD3CN, compared with noncoordinating solvents such
as CDCl3, were observed not only for 1·BF4 but also for
Figure 9. (a) Cyclic voltammogram of 2·BF4 (0.4 mM) in 0.1 M
Bu4NBF4−CH2Cl2 (top) and 2·B(C6F5)4 (0.4 mM) in 0.1 M
Bu4NB(C6F5)4−CH2Cl2 (bottom) at room temperature at a scan
rate of 100 mV s−1. (b) Absorption spectrum of 2·BF4 in acetone at
room temperature. (c) Fluorescence microscopy images of solids of
2·BF4 (left) and 2·B(C6F5)4 (right) in a bright field (top) and under
blue-light excitation (bottom) at room temperature. (d) Emission
spectra of 2·BF4 (blue) and 2·B(C6F5)4 (red, dotted line) in the solid
state at room temperature.
57
2·BF4 (50 s−1 in CDCl3, 20 s−1 in CD2Cl2, 200 s−1 in acetone-
d6, and 500 s−1 in CD3CN). The promotion of isomerization by
a coordinating solvent molecule, which assists the dissociation
of the pyrimidine nitrogen atoms from the copper center,57 was
generally found throughout the [Cu(Mepypm)(diphosphine)]+
family. The rate constant associated with 1·BF4 in CD2Cl2 (k298
= 30 s−1) was comparable with that of 1·BF4 in CDCl3 because
the coordination abilities of these solvents are similarly weak. In
contrast, a small decrease of k298 in CD2Cl2 compared with
CDCl3 was observed for 2·BF4. The solvent sensitivity seems to
be consistent with the model illustrating that the ion-pairing
sensitivity of 2+ is larger than that of 1+, shown in Figure 7. The
value of k298 in a CDCl3 solution of 2·B(C6F5)4 is 10 s−1, which
is smaller than that of 2·BF4. The decrease of k298 resulting
from the use of a bulky counteranion suggests that the weak
instability of the oxidized [Cu(Mepypm)(diphosphine)]+,
which can undergo rapid adverse chemical reactions. Redox
irreversibility is also observed in some [Cu(diimine)-
(diphosphine)]+ complexes.42 The redox potential in a solution
−
−
with B(C6F5)4 is more positive than that with BF4 , and the
reversibility of the redox wave in the solution with B(C6F5)4− is
better than that with BF4 (Figure 9a). Stabilization of the
oxidized form by noncoordinative bulky B(C6F5)4 could
attribute to these results. The electrolyte effects of B(C6F5)4
−
−
−
coordination ability of BF4 can slightly assist in Cu−N bond
−
dissociation. This behavior seems to be consistent with the
−
model describing the BF4 ion approach in the CIP state.
on the reversibility of some simple redox-active molecules have
1·B(C6F5)4 in CDCl3 (k298 = 40 s−1) is very similar to 1·BF4 in
been reported in the literature.79
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The environment-sensitive molar ratio of rotational isomers
can cause negligible change in absorption in solution. The UV−
vis absorption spectrum of 2·BF4 in acetone displayed a charge-
transfer (CT) absorption band characteristic of [Cu(diimine)-
(diphosphine)]+ complexes (Figure 9b). The maximum
wavelength (λmax) and the molar extinction coefficient (ε) at
λmax of the CT absorption were 401 nm and 2.4 × 103 M−1
cm−1, respectively. The red shift of the absorption maximum of
for rationally accounting for the effect of the weak interaction
on the rotational equilibrium. Because the major part of the
copper center bonding surface was occupied by ligands, the
position of the counterion affects the orientation of the
pyrimidine moiety. The findings described herein are valuable
for the design of photoactive and/or redox-active molecular
mechanical units that can be readily functionalized via weak
electrostatic interactions.
57
2·BF4 compared with 1·BF4 is caused by previously reported
diphosphine effects.44,47,49 The absorption maximum wave-
lengths of 2·BF4 and 2·B(C6F5)4 in acetone were identical
(Supporting Information, Figure S19). Because the effects of
methyl substitution on the absorption maxima of copper(I)
diimine complexes are known to be within a few nanome-
ters,44,50 the difference in absorption between the i- and o-
isomers is also expected to be small. The small difference in
color is consistent with our previous findings pertaining to the
methylpyrimidine-based rotational isomers.53,58 The lumines-
cence of 2·BF4 and 2·B(C6F5)4 in acetone at room temperature,
which is the same experimental condition as that reported for
the luminescence of 1+,58 was not detected; luminescence
quantum yields of 1·BF4 and 2·BF4 were 0.02% and less than
0.002%, respectively. The negligible luminescence of 2+
compared with 1+ is attributed to the different steric demands
of the two diphosphines; DPEphos offers an enhanced
protection toward structural deformations in both the ground
and excited states and additionally blue-shifts the absorption
through electronic effects, leading to a better emission
performance for 1+.43−47 The values of the luminescence
quantum yields are consistent with the well-established
photophysics of a family of [Cu(diimine)(diphosphine)]43−47
because of the large structural rearrangement that occurs in the
photoexcited state of complexes containing less bulky Mepypm
ligands compared with bulky diimines such as 2,9-dimethyl-
1,10-phenanthroline44,47 and the high possibility of solvent
coordination quenching due to the presence of the less bulky
Mepypm.44,47 Solids of 2·BF4 and 2·B(C6F5)4 were yellow on
appearance and exhibited orange luminescence under UV and
blue-light excitation (Figure 9c). The emission spectra of 2·BF4
and 2·B(C6F5)4 solids displayed CT luminescence characteristic
of [Cu(diimine)(diphosphine)]+ systems (Figure 9d).36−41,47
The maximum emission wavelength of 2·BF4 (λmax = 597 nm)
was blue-shifted compared with that of 2·B(C6F5)4 (λmax = 620
nm; Figure 9d). An enhancement in the solid-state
luminescence over the solution luminescence, which is typical
for emissive copper(I) diimine complexes, was observed. The
enhancement is attributed to inhibited structural relaxation in
the solid state and/or additional solvent coordination to
photoexcited species that occur in solution.39−41 The differ-
ences in the orientation of the pyrimidine ring, position of the
counterion, and other types of packing effects can contribute to
the blue shift of 2·BF4 relative to 2·B(C6F5)4.39,40,51,52
EXPERIMENTAL SECTION
■
Materials. 1·BF4 was obtained by methods described previously.57
4-Methyl-2-(2′-pyridyl)pyrimidine (Mepypm),80 tetrakis(acetonitrile)-
copper(I) tetrafluoroborate ([Cu(MeCN)4]BF4),64 and tetrakis-
(acetonitrile)copper(I) tetrakis(pentafluorophenyl)borate ([Cu-
(MeCN)4]B(C6F5)4)65 were prepared according to literature proto-
cols. Bis[2-(diphenylphosphino)phenyl] ether (DPEphos) was
purchased from Wako Pure Chemical Industries, Ltd. 1,3-Bis-
(diphenylphosphino)propane (dppp) was purchased from Kanto
Chemicals. Other chemicals were used as purchased.
Synthesis of [Cu(Mepypm)(DPEphos)]B(C6F5)4 [1·B(C6F5)4].
The heteroleptic copper complex 1·B(C6F5)4 was synthesized
according to a modified literature procedure.44,47,57 In this synthesis,
[Cu(MeCN)4]B(C6F5)4 (80 mg, 0.088 mmol) was added to DPEphos
(59 mg, 0.11 mmol) in 5 mL of dichloromethane. Mepypm (14 mg,
0.082 mmol) in 5 mL of dichloromethane was then added, upon which
the reaction solution immediately turned yellow. The reaction mixture
was subsequently stirred for an additional 30 min. Hexane was then
added to the solution in order to precipitate the product as a yellow
solid, which was filtered and washed with hexane. Reprecipitation from
a dichloromethane and hexane mixture afforded 1·B(C6F5)4 as a
yellow solid with a yield of 70% (83 mg). 1H NMR (500 MHz, CDCl3,
253 K): δ 8.76 (m, o-1H + i-1H), 8.68 (d, J = 7.7 Hz, i-1H), 8.37 (d, J
= 5.5 Hz, o-1H), 8.29 (d, J = 5.5 Hz, o-1H), 8.26 (d, J = 4.8 Hz, i-1H),
7.99 (t, J = 7.8 Hz, o-1H), 7.95 (t, J = 7.6 Hz, i-1H), 7.4−6.7 (m), 2.63
(s, o-3H), 2.29 (s, i-3H). Elem anal. Calcd for C70H37N3P2OCuBF20:
C, 57.89; H, 2.57; N, 2.89. Found: C, 58.16; H, 2.87; N, 2.77.
Synthesis of [Cu(Mepypm)(dppp)]BF4 (2·BF4). 2·BF4 was
synthesized using a procedure similar to that described for 1·B(C6F5)4
with the exception that diethyl ether was used in place of hexane.
[Cu(MeCN)4]BF4 (67 mg, 0.21 mmol), dppp (104 mg, 0.25 mmol),
and Mepypm (34 mg, 0.20 mmol) were used. Yield: yellow solid (56%,
82 mg). 1H NMR (500 MHz, CDCl3, 253 K): δ 8.97 (d, J = 5.0 Hz i-
1H), 8.93 (d, J = 7.9 Hz, i-1H), 8.85 (d, J = 7.9 Hz, o-1H), 8.79 (d, J =
5.5 Hz, o-1H), 8.47 (d, J = 5.1 Hz, o-1H), 8.35 (d, J = 5.2 Hz, i-1H),
8.20 (t, J = 7.8 Hz, i-1H), 8.11 (t, J = 7.7 Hz, o-1H), 7.69 (dd, J = 7.5
and 5.1 Hz, i-1H), 7.62 (dd, J = 7.5 and 5.2 Hz, o-1H), 7.5−7.1 (m),
2.88 (m, br), 2.7 (br), 2.69 (s, o-3H), 2.45 (t, br), 2.32 (s, i-3H), 2.10
(m, br). Elem anal. Calcd for C37H35N3P2CuBF4: C, 60.54; H, 4.81; N,
5.73. Found: C, 60.52; H, 4.92; N, 5.49.
Synthesis of [Cu(Mepypm)(dppp)]B(C6F5)4 [2·B(C6F5)4]. 2·B-
(C6F5)4 was synthesized using a procedure similar to that described for
1·B(C6F5)4 by employing [Cu(MeCN)4]B(C6F5)4 (185 mg, 0.20
mmol), dppp (92 mg, 0.22 mmol), and Mepypm (35 mg, 0.20 mmol).
1
Yield: yellow solid (40%, 108 mg). H NMR (500 MHz, CDCl3, 253
K): δ 8.91 (d, J = 8.0 Hz, i-1H), 8.88 (m, i-1H + o-1H), 8.29 (d, J =
5.1 Hz, o-1H), 8.26 (d, J = 4.7 Hz, i-1H), 8.18 (d, J = 5.5 Hz, o-1H),
8.14 (t, J = 7.8 Hz, i-1H), 8.09 (t, J = 7.9 Hz, o-1H), 7.59 (dd, J = 7.4
and 5.3 Hz, i-1H), 7.50 (dd, J = 7.3 and 5.4 Hz, o-1H), 7.4−7.1 (m),
2.87 (m, br), 2.7 (br), 2.69 (s, o-3H), 2.57 (m, br), 2.35 (m, br), 2.23
(s, i-3H), 2.14 (m, br). Elem anal. Calcd for C61H35N3P2CuBF20: C,
55.24; H, 2.66; N, 3.17. Found: C, 55.10; H, 2.86; N, 3.13.
X-ray Structural Analysis. Yellow single crystals of o-1·B-
(C6F5)4·1.5hexane, o-2·BF4·0.5MeOH, and i-2·B(C6F5)4 were ob-
tained by the slow diffusion of hexane into a dichloromethane solution
of 1·B(C6F5)4, diethyl ether into a methanol solution of 2·BF4, and
hexane into a chloroform solution of 2·B(C6F5)4, respectively.
Diffraction data were collected on an AFC10 diffractometer with
graphite-monochromated Mo Kα radiation (λ = 0.7107 Å). Lorentz
polarization and numerical absorption corrections were performed
CONCLUSION
■
A series of simple copper(I) complexes bearing Mepypm and
diphosphine ligands, namely, 1·B(C6F5)4, 2·BF4, and 2·B-
(C6F5)4, were synthesized. Two rotational isomers of the
complexes coexist and interconvert in solution via intra-
molecular ligating atom exchange of the pyrimidine ring, as
previously reported for 1·BF4. The values of ΔH, ΔS, and ΔG
for rotation are strongly dependent on the steric demands of
the diphosphine, polarity of the solvent, and size of the
counterion. Consideration of solvated counterion pairing is key
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with the CrystalClear 1.3.6 program. The structure was solved by direct
methods using SIR 92 software81 and refined against F2 using
SHELXL-97.82 WinGX software was used to prepare the material for
publication.83 Crystallographic data are listed in the Supporting
Information, Table S1. Disordered counterions in o-2·BF4·0.5MeOH
were analyzed by the PART, SIMU, and SADI options of SHELXL-
97.82 The disordered methanol molecule in o-2·BF4·0.5MeOH was
analyzed by the PART, EADP, and SADI options of SHELXL-97.82
Instruments. NMR spectra at several temperatures in the dark
were recorded on a Bruker DRX 500 spectrometer, using a ca. 20 min
data-recording interval. The experimental 1H NMR spectra were
simulated using iNMR 2.6.5 software. The reported chemical shifts of
the solvent peaks were used for calibration of the NMR spectra in
CDCl3 (tetramethylsilane, δ 0), CD2Cl2 (δ 5.32), acetone-d6 (δ 2.05),
and acetonitritrile-d3 (CD3CN, δ 1.94).84 UV−vis absorption spectra
were recorded with a Hewlett-Packard 8453 spectrometer. Steady-
state-corrected emission spectra were obtained with a Hitachi F-4500
spectrometer. The luminescence quantum yield was estimated from
tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate in acetonitrile
under air (1.8%) as a standard.85 Solid-state luminescence images were
measured under blue-light excitation using an Olympus BX51
fluorescence microscope. Electrochemical measurements were ac-
quired with an ALS 750A electrochemical analyzer (BAS Co. Ltd.).
The working electrode was a 0.3-mm-o.d. glassy carbon electrode; a
platinum wire served as the auxiliary electrode, and the reference
electrode was an Ag+/Ag electrode (a silver wire immersed in 0.1 M
Bu4NClO4/0.01 M AgClO4/CH3CN). The solutions were deoxy-
genated with argon prior to measurement.
Thermodynamic and Kinetic Analyses. Analysis was performed
using the aromatic 1H NMR signals of the Mepypm moiety. The
results of 1·BF4 in CDCl3, acetone-d6, and CD3CN were comparable
with previously reported values, which were based on the methyl
group of the Mepypm moiety.57 The solution-state molar ratios of the
isomers at several temperatures were determined from 1H NMR signal
integration. The broad spectra acquired at room temperature were
excluded from thermodynamic analysis. The generated van’t Hoff
plots86 were based on an equilibrium constant corresponding to the
value of [o-isomer]/[i-isomer]. The molar ratios of the o-isomers, xo,
at variable temperatures were calculated by extrapolation of the van’t
Hoff plots. The thermodynamic parameters for the i → o rotation
(ΔH, ΔS, and ΔG), K, and xo can be represented by the following
van’t Hoff equations:
AUTHOR INFORMATION
Corresponding Author
*E-mail: kume@chem.s.u-tokyo.ac.jp (S.K.), nisihara@chem.s.
u-tokyo.ac.jp (H.N.).
■
Present Address
†Department of Chemistry, Graduate School of Science,
Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima
739-8526, Japan.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid from MEXT of
Japan [Grants 20750044, 20245013, and 21108002; area 2107
(Coordination Programming)], JST (Research Seeds Quest
Program), a Research Fellowship of the Japan Society for the
Promotion of Science for Young Scientists, and the Global
COE Program for Chemistry Innovation.
REFERENCES
■
(1) Marcus, R. A. J. Phys. Chem. B 1998, 102, 10071−10077.
(2) Kavarnos, G. J.; Turro, N. J. Chem. Rev. 1986, 86, 401−449.
(3) Wasielewski, M. R. Chem. Rev. 1992, 92, 435−461.
(4) (a) Rueping, M.; Uria, U.; Lin, M.-Y.; Atodiresei, I. J. Am. Chem.
Soc. 2011, 133, 3732−3735. (b) Rauniyar, V.; Lackner, A. D.;
Hamilton, G. L.; Toste, F. D. Science 2011, 334, 1681−1684.
(5) Macchioni, A. Chem. Rev. 2005, 105, 2039−2073.
(6) (a) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.;
Ito, O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607−
2617. (b) D’Souza, F.; Chitta, R.; Ohkubo, K.; Tasior, M.; Subbaiyan,
N. K.; Zandler, M. E.; Rogacki, M. K.; Gryko, D. T.; Fukuzumi, S. J.
Am. Chem. Soc. 2008, 130, 14263−14272.
(7) Balzani, V.; Credi, A.; Venturi, M. Molecular Devices and Machines,
2nd ed.; Wiley-VCH: Weinheim, Germany, 2008.
(8) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007,
46, 72−191.
(9) Laursen, B. W.; Nygaard, S.; Jeppesen, J. O.; Stoddart, J. F. Org.
Lett. 2004, 6, 4167−4170.
(10) Credi, A.; Dumas, S.; Silvi, S.; Venturi, M.; Arduini, A.; Pochini,
A.; Secchi, A. J. Org. Chem. 2004, 69, 5881−5887.
(11) Gibson, H. W.; Jones, J. W.; Zakharov, L. N.; Rheingold, A. L.;
Slebodnick, C. Chem.Eur. J. 2011, 17, 3192−3206.
(12) Evans, N. H.; Rahman, H.; Leontiev, A. V.; Greenham, N. D.;
Orlowski, G. A.; Zeng, Q.; Jacobs, R. M. J.; Serpell, C. J.; Kilah, N. L.;
Davis, J. J.; Beer, P. D. Chem. Sci. 2012, 3, 1080−1089.
(13) Kim, S. K.; Sessler, J. L.; Gross, D. E.; Lee, C.-H.; Kim, J. S.;
Lynch, V. M.; Delmau, L. H.; Hay, B. P. J. Am. Chem. Soc. 2010, 132,
5827−5836.
ln[xo/(100 − xo)] = ln K = −ΔG/RT = −ΔH/RT + ΔS/R
(1)
Here, T corresponds to the absolute temperature and R is the gas
constant (8.314 J K−1 mol−1). The value of xo,298 was estimated from
the predicted value of ln K at 1/T = 1/298 K−1 using single linear
regression of the van’t Hoff plots. The error of xo,298 was less than 1%,
based on the root-mean-square error and t test in 95% significance
level. The rate constants for the i → o isomerization, k, at several
temperatures were determined from simulation analysis of the NMR
spectra using the equilibrium constants determined from the van’t
Hoff plots. k values at variable temperatures were calculated by
extrapolation of the Arrhenius plots:86
(14) Jones, J. W.; Gibson, H. W. J. Am. Chem. Soc. 2003, 125, 7001−
7004.
(15) Tobey, S. L.; Anslyn, E. V. J. Am. Chem. Soc. 2003, 125, 10963−
10970.
ln k = −Ea/RT + ln(A)
(16) Hou, J.; Zhang, Z.; Madsen, L. A. J. Phys. Chem. B 2011, 115,
4576−4582.
(17) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry,
3rd ed.; Wiley-VCH: Weinheim, Germany, 2003.
(2)
Here, Ea and A correspond to the activation energy and frequency
factor of the i → o isomerization, respectively.
(18) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163−170.
ASSOCIATED CONTENT
* Supporting Information
Crystal structure data in CIF format (CCDC 902739, 902740,
and 902741) and spectral data. This material is available free of
charge via the Internet at http://pubs.acs.org. Crystallographic
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
(19) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.;
■
Petta, J. R.; Rinkoski, M.; Sthena, J. P.; Abruna, H. D.; McEuen, P. L.;
̃
S
Ralph, D. C. Nature 2002, 417, 722−725.
(20) Collin, J.-P.; Dietrich-Buchecker, C.; Gavina, P.; Jimen
Molero, M. C.; Sauvage, J.-P. Acc. Chem. Res. 2001, 34, 477−487.
́
ez-
̃
(21) Armaroli, N.; Balzani, V.; Collin, J.-P.; Gavina, P.; Sauvage, J.-P.;
Ventura, B. J. Am. Chem. Soc. 1999, 121, 4397−4408.
(22) Poleschak, I.; Kern, J. M.; Sauvage, J.-P. Chem. Commun. 2004,
474−476.
378
dx.doi.org/10.1021/ic302141z | Inorg. Chem. 2013, 52, 369−380

Inorganic Chemistry
Article
(23) Pregosin, P. S.; Kumar, P. G. A.; Fernan
105, 2977−2998.
́
dez, I. Chem. Rev. 2005,
(51) Kovalevsky, A. Y.; Gembicky, M.; Novozhilova, I. V.; Coppens,
P. Inorg. Chem. 2003, 42, 8794−8802.
(24) Moreno, A.; Pregosin, P. S.; Veiros, L. F.; Albinati, A.; Rizzato,
S. Chem.Eur. J. 2009, 15, 6848−6862.
(52) Cunningham, C. T.; Moore, J. J.; Cunningham, K. L. H.;
Fanwick, P. E.; McMillin, D. R. Inorg. Chem. 2000, 39, 3638−3644.
(53) Nomoto, K.; Kume, S.; Nishihara, H. J. Am. Chem. Soc. 2009,
131, 3830−3831.
(25) Kumar, P. G. A.; Pregosin, P. S.; Goicoechea, J. M.; Whittlesey,
M. K. Organometallics 2003, 22, 2956−2960.
(26) (a) Rezende, M. C.; Guerrero, J. Magn. Reson. Chem. 2009, 47,
505−510. (b) Martinez-Viviente, E.; Pregosin, P. S. Inorg. Chem. 2003,
42, 2209−2214.
(54) Kume, S.; Nomoto, K.; Kusamoto, T.; Nishihara, H. J. Am.
Chem. Soc. 2009, 131, 14198−14199.
(55) Kume, S.; Nishihara, H. Chem. Commun. 2011, 47, 415−417.
(56) Kume, S.; Nishihara, H. Dalton Trans. 2011, 40, 2299−2305.
(57) Nishikawa, M.; Nomoto, K.; Kume, S.; Inoue, K.; Sakai, M.;
Fujii, M.; Nishihara, H. J. Am. Chem. Soc. 2010, 132, 9579−9581.
(58) Nishikawa, M.; Nomoto, K.; Kume, S.; Nishihara, H. J. Am.
Chem. Soc. 2012, 134, 10543−10553.
(27) Lacour, J.; Moraleda, D. Chem. Commun. 2009, 7073−7089.
(28) Hebbe-Viton, V.; Desvergnes, V.; Jodry, J. J.; Dietrich-
Buchecker, C.; Sauvage, J.-P.; Lacour, J. Dalton Trans. 2006, 2058−
2065.
(29) Desvergnes-Breuil, V.; Hebbe, V.; Dietrich-Buchecker, C.;
Sauvage, J.-P.; Lacour, J. Inorg. Chem. 2003, 42, 255−257.
(30) Hutin, M.; Nitschke, J. R. Chem. Commun. 2006, 1724−1726.
(31) Riesgo, E.; Hu, Y.-Z.; Bouvier, F.; Thummel, R. P. Inorg. Chem.
2001, 40, 2541−2546.
(32) Frei, U. M.; Geier, G. Inorg. Chem. 1992, 31, 187−190.
(33) Kume, S.; Murata, M.; Ozeki, T.; Nishihara, H. J. Am. Chem. Soc.
2005, 127, 490−491.
(34) Umeki, S.; Kume, S.; Nishihara, H. Chem. Lett. 2010, 39, 204−
205.
(59) Example for other types of coordinated ligand rotational
isomerization such as those based on the Re−C double bond.
McCormick, F. B.; Kiel, W. A.; Gladysz, J. A. Organometallics 1982, 1,
405−408.
(60) Ruthkosky, M.; Kelly, C. A.; Castellano, F. N.; Meyer, G. J.
Coord. Chem. Rev. 1998, 171, 309−322.
(61) Miller, M. T.; Gantzel, P. K.; Karpishin, T. B. Inorg. Chem. 1998,
37, 2285−2290.
(62) Rorabacher, D. B. Chem. Rev. 2004, 104, 651−697.
(63) Le Poul, N.; Campion, M.; Douziech, B.; Rondelez, Y.; Le
Clainche, L.; Reinaud, O.; Le Mest, Y. J. Am. Chem. Soc. 2007, 129,
8801−8810.
(64) Merrill, C. L.; Wilson, L. J.; Thamann, T. J.; Loehr, T. M.; Ferris,
N. S.; Woodruff, W. H. J. Chem. Soc., Dalton Trans. 1984, 2207−2221.
(65) Liang, H.-C.; Kim, E.; Incarvito, C. D.; Rheingold, A. L.; Karlin,
K. D. Inorg. Chem. 2002, 41, 2209−2212.
(66) Letko, C. S.; Rauchfuss, T. B.; Zhou, X.; Gray, D. L. Inorg. Chem.
2012, 51, 4511−4520.
(35) Kume, S.; Kurihara, M.; Nishihara, H. Inorg. Chem. 2003, 42,
2194−2196.
(36) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Top. Curr.
Chem. 2007, 280, 69−115.
(37) Lavie-Cambot, A.; Cantuel, M.; Leydet, Y.; Jonusauskas, G.;
Bassani, D. M.; McClenaghan, N. D. Coord. Chem. Rev. 2008, 252,
2572−2584.
(38) McMillin, D. R.; McNett, K. M. Chem. Rev. 1998, 98, 1201−
1219.
(39) (a) Zhang, Q.; Zhou, Q.; Cheng, Y.; Wang, L.; Ma, D.; Jing, X.;
Wang, F. Adv. Mater. 2004, 16, 432−436. (b) Armaroli, N.; Accorsi,
G.; Holler, M.; Moudam, O.; Nierengarten, J.-F.; Zhou, Z.; Wegh, R.
T.; Welter, R. Adv. Mater. 2006, 18, 1313−1316.
(40) Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.;
Chou, P.-T.; Chang, C.-H.; Chen, P.-Y. J. Am. Chem. Soc. 2011, 133,
12085−12099.
(67) Lee, Y.; Lee, D.-H.; Park, G. Y.; Lucas, H. R.; Narducci Sarjeant,
A. A.; Kieber-Emmons, M. T.; Vance, M. A.; Milligan, A. E.; Solomon,
E. I.; Karlin, K. D. Inorg. Chem. 2010, 49, 8873−8885.
(68) Bain, A. D. Prog. Nucl. Magn. Reson. Spectrosc. 2003, 43, 63−103.
(69) Sandstrom, J. Dynamic NMR Spectroscopy; Academic Press:
̈
London, 1982.
(70) (a) Hiraoka, S.; Okuno, E.; Tanaka, T.; Shiro, M.; Shionoya, M.
J. Am. Chem. Soc. 2008, 130, 9089−9098. (b) Hiraoka, S.; Hisanaga,
Y.; Shiro, M.; Shionoya, M. Angew. Chem., Int. Ed. 2010, 49, 1669−
1673.
(41) Smith, C. S.; Branham, C. W.; Marquardt, B. J.; Mann, K. R. J.
Am. Chem. Soc. 2010, 132, 14079−14085.
(42) Linfoot, C. L.; Richardson, P.; Hewat, T. E.; Moudam, O.;
Forde, M. M.; Collins, A.; White, F.; Robertson, N. Dalton Trans.
2010, 39, 8945−8956.
(71) Kilbas, B.; Mirtschin, S.; Scopelliti, R.; Severin, K. Chem. Sci.
2012, 3, 701−704.
(43) Cuttell, D. G.; Kuang, S. M.; Fanwick, P. E.; McMillin, D. R.;
Walton, R. A. J. Am. Chem. Soc. 2002, 124, 6−7.
(44) Kuang, S. M.; Cuttell, D. G.; McMillin, D. R.; Fanwick, P. E.;
Walton, R. A. Inorg. Chem. 2002, 41, 3313−3322.
(45) Yang, L.; Feng, J. K.; Ren, A. M.; Zhang, M.; Ma, Y. G.; Liu, X.
D. Eur. J. Inorg. Chem. 2005, 1867−1879.
́
(72) Leigh, D. A.; Morales, M. A. F.; Perez, E. M.; Wong, J. K. Y.;
Saiz, C. G.; Slawin, A. M. Z.; Carmichael, A. J.; Haddleton, D. M.;
Brouwer, A. M.; Buma, W. J.; Wurpel, G. W. H.; Leon, S.; Zerbetto, F.
́
Angew. Chem., Int. Ed. 2005, 44, 3062−3067.
(73) Gong, C.; Gibson, H. W. Angew. Chem., Int. Ed. Engl. 1997, 36,
2331−2333.
(46) Costa, R. D.; Tordera, D.; Ortí, E.; Bolink, H. J.; Schonle, J.;
̈
(74) Wilmes, G. M.; France, M. B.; Lynch, S. R.; Waymouth, R. M.
Organometallics 2004, 23, 2405−2411.
Graber, S.; Housecroft, C. E.; Constable, E. C.; Zampese, J. A. J. Mater.
Chem. 2011, 21, 16108−16118.
(75) Tafazzoli, M.; Ziyaei-Halimjani, A.; Ghiasi, M.; Fattahi, M.;
Saidi, M. R. J. Mol. Struct. 2008, 886, 24−31.
(47) Saito, K.; Arai, T.; Takahashi, N.; Tsukuda, T.; Tsubomura, T.
Dalton Trans. 2006, 4444−4448.
(76) Gennari, M.; Lanfranchi, M.; Cammi, R.; Pellinghelli, M. A.;
(48) (a) Vorontsov, I. I.; Graber, T.; Kovalevsky, A. Y.; Novozhilova,
I. V.; Gembicky, M.; Chen, Y.-S.; Coppens, P. J. Am. Chem. Soc. 2009,
131, 6566−6573. (b) Iwamura, M.; Watanabe, H.; Ishii, K.; Takeuchi,
S.; Tahara, T. J. Am. Chem. Soc. 2011, 133, 7728−7736. (c) Siddique,
Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K. Inorg. Chem. 2003, 42,
6366−6378.
̀
Marchio, L. Inorg. Chem. 2007, 46, 10143−10152.
(77) Nakafuji, S.; Kobayashi, J.; Kawashima, T.; Schmidt, M. W.
Inorg. Chem. 2005, 44, 6500−6502.
(78) Blom, R.; Swang, O. Eur. J. Inorg. Chem. 2002, 411−415.
(79) LeSuer, R. J.; Geiger, W. E. Angew. Chem., Int. Ed. 2000, 39,
248−250.
(49) (a) McCormick, T.; Jia, W.-L.; Wang, S. Inorg. Chem. 2006, 45,
147−155. (b) Sakaki, S.; Mizutani, H.; Kase, Y.-I.; Inokuchi, K.-J.; Arai,
T.; Hamada, T. J. Chem. Soc., Dalton Trans. 1996, 1909−1914.
(50) (a) Everly, R. M.; Ziessel, R.; Suffert, J.; McMillin, D. R. Inorg.
Chem. 1991, 30, 559−561. (b) Cunningham, C. T.; Cunningham, K.
L. H.; Michalec, J. F.; McMillin, D. R. Inorg. Chem. 1999, 38, 4388−
4392. (c) Del Paggio, A. A.; McMillin, D. R. Inorg. Chem. 1983, 22,
691−692.
(80) (a) Medwid, J. B.; Paul, R.; Baker, J. S.; Brockman, J. A.; Du, M.
T.; Hallett, W. A.; Hanifin, J. W.; Hardy, R. A.; Tarrant, M. E.; Torley,
L. W.; Wrenn, S. J. Med. Chem. 1990, 33, 1230−1241. (b) Lafferty, J.
J.; Case, F. H. J. Org. Chem. 1967, 32, 1591−1596.
(81) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435.
(82) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122.
379
dx.doi.org/10.1021/ic302141z | Inorg. Chem. 2013, 52, 369−380

Inorganic Chemistry
Article
(83) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838.
(84) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.;
Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I.
Organometallics 2010, 29, 2176−2179.
(85) Suzuki, K.; Kobayashi, A.; Kaneko, S.; Takehira, K.; Yoshihara,
T.; Ishida, H.; Shiina, Y.; Oishi, S.; Tobita, S. Phys. Chem. Chem. Phys.
2009, 11, 9850−9860.
(86) Atkins, P.; De Paula, J. Physical Chemistry, 8th ed.; W. H.
Freeman: New York, 2006.
380
dx.doi.org/10.1021/ic302141z | Inorg. Chem. 2013, 52, 369−380