Steric interference on the redox-conjugated pyrimidine ring rotation of monoand dinuclear copper complexes with (4-Methyl-2-pyrimidinyl)imine ligands

Article abstract of DOI:10.1246/cl.140238

A mononuclear copper(I) complex with N-[(4-methyl-2- pyrimidinyl)methylene] -p-toluidine (1?BF₄) and a dinuclear copper(I) complex with N,N'-bis[(4-methyl-2-pyrimidinyl)- methylene]-p-phenylenediamine (2?(BF₄)₂) were synth

Full text of DOI:10.1246/cl.140238

Received: March 18, 2014 | Accepted: April 4, 2014 | Web Released: April 10, 2014
CL-140238
Steric Interference on the Redox-conjugated Pyrimidine Ring Rotation of Mono-
and Dinuclear Copper Complexes with (4-Methyl-2-pyrimidinyl)imine Ligands
³
³³
Yohei Hattori, Michihiro Nishikawa, Tetsuro Kusamoto, Shoko Kume, and Hiroshi Nishihara*
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(E-mail: nisihara@chem.s.u-tokyo.ac.jp)
A mononuclear copper(I) complex with N-[(4-methyl-2-
pyrimidinyl)methylene]-p-toluidine (1¢BF4) and a dinuclear
copper(I) complex with N,N¤-bis[(4-methyl-2-pyrimidinyl)-
methylene]-p-phenylenediamine (2¢(BF4)2) were synthesized
¹
as BF4 salts to evaluate the influence of the imine moiety on
the pyrimidine ring rotation isomerism. 1¢BF4 existed in
solution as a mixture of two isomers; 2¢(BF4)2 was present as
a mixture of three isomers. The redox potentials of the copper
centers were changed by pyrimidine ring rotation. Comparison
of 1+ and 22+ indicated that increasing the steric congestion
around the copper center increased the o/i isomer ratio; the
redox potentials of both the o- and i-isomers shifted in the
positive direction, and the CuII/I redox reaction became slower.
Molecular electronic devices and machines such as mem-
ories,1 switches,2 and motors3 have been developed as the
components of ultimately accumulated computing system in the
molecular scale. Molecular systems controlled by electrical
stimuli are particularly expected to be important in future
nanoelectronics.
Copper complexes are promising candidates for the mo-
lecular machinery system that is controlled by redox reactions
(i.e., electrical stimuli) because of their reversible CuII/I redox
activities, labile coordination bonds, and strong correlations
between the coordination structure and CuII/I redox potential.
Tetrahedral coordination is preferred by a CuI center, and square
planar or five- and six-coordinated structures are preferred by a
CuII center. Artificial molecular motors and muscles driven by
the redox reactions of copper centers were reported by Sauvage
Figure 1. (a) i- and o-Isomers of 1¢BF4. (b) N-[(4-Methyl-2-
pyrimidinyl)methylene]-p-toluidine (L1). (c) N,N¤-Bis[(4-methyl-2-
et al.4
Our group previously reported the isomerizing motion of a
copper complex [(Lanth)Cu(L3)]¢BF4 (3¢BF4, Lanth: 2,9-bis(9-
anthryl)-1,10-phenanthroline, L3: 4-methyl-2-(2¤-pyridyl)pyrim-
idine).5 This complex exists in solution as a mixture of two
isomers, an inner i-isomer and an outer o-isomer via 4-
methylpyrimidine ring rotation, where i- and o- indicate the
position of the methyl group on the pyrimidine ring. Although
the structural difference between the isomers is solely the
position of the methyl group, their Cu II/I redox potentials differ
by as much as 0.15 V. Therefore, on the basis of “coordination
programming,”6 this molecule can behave as a molecular device
that converts the rotational motion of the molecule into a redox
potential. The copper complexes containing the 4-methyl-2-(2¤-
pyridyl)pyrimidine derivative ligands have been coupled with
various auxiliary ligands, functional groups, or redox-active
units to exhibit interesting properties; for example, electronic,
magnetic, luminescent, mechanical, and light-driven properties.7
In this study, a mononuclear copper(I) complex [(Lanth)-
Cu(L1)]¢BF4 (1¢BF4 (Figure 1a), L1: N-[(4-methyl-2-pyrimidi-
nyl)methylene]-p-toluidine (Figure 1b)) was synthesized to
pyrimidinyl)methylene]-p-phenylenediamine (L2). (d) oo-, io-, and ii-
isomers of 2¢(BF4)2. (e) Molecular structure of i-1+ in [i-1]¢BF4¢Et2O
crystal. (f) Molecular structure of one of o-1+ in 2[o-1]¢2BF4¢
3CH2Cl2¢1.5p-xylene crystal. (g) Molecular structure of oo-22+ in
[oo-2]¢(BF4)2¢x(solvents) crystal. Hydrogen atoms, BF4 anions and
solvents are omitted for clarity. Methyl carbons on pyrimidine ring are
shown as ball models.
evaluate the influence of the imine moiety on the pyrimidine
ring rotation isomerism. We also prepared a dinuclear copper
complex [(Lanth)2Cu2(¯-L2)](BF4)2 (2¢(BF4)2 (Figure 1d), L2:
N,N¤-bis[(4-methyl-2-pyrimidinyl)methylene]-p-phenylenedi-
amine (Figure 1c)), in which two rotating 4-methylpyrimidine
units are connected with a conjugated diimine skeleton. The aim
of this study is to investigate the way in which the connection of
the two units influences their rotational behavior and the redox
characteristics of the copper centers. We report herein the
synthesis, single-crystal X-ray crystallographic investigations,
isomerization behavior, and redox properties of 1¢BF4 and
2¢(BF4)2.
Chem. Lett. 2014, 43, 1037–1039 | doi:10.1246/cl.140238
© 2014 The Chemical Society of Japan | 1037

Figure 2. (a) 1H NMR signals of methyl groups of 1¢BF4 in CD2Cl2
(pm: methyl group on pyrimidyl group, tol: methyl group of tolyl
1
group). (b) H NMR signals of methyl groups of 2¢(BF4)2 in CD2Cl2.
Figure 3. Cyclic voltammograms of 1¢BF4 at 293 K (a), 1¢BF4 at
243 K (b), 2¢(BF4)2 at 293 K (c), and 2¢(BF4)2 at 243 K (d) in
dichloromethane with 0.1 M Bu4NBF4 as supporting electrolyte. Scan
rates are 0.5 (red line), 0.25 (orange line), 0.1 (green line), 0.05 (blue
L1 and L2 (Figures 1b and 1c) were each prepared from
4-methylpyrimidine-2-carbaldehyde with either p-toluidine or
1,4-phenylenediamine, respectively. 1¢BF4 was synthesized by
mixing a 1:1:1 molar ratio of [Cu(MeCN)4]BF4,9 Lanth, and L1 in
CH2Cl2 at room temperature. 2¢(BF4)2 was yielded from a 2:2:1
mixture of [Cu(MeCN)4]BF4, Lanth, and L2. The bulky 9-anthryl
groups of Lanth impeded homoleptic complexation, resulting in
the selective formation of heteroleptic complexes.10
¹1
line), and 0.025 V s (purple line).
Decreasing the scan rate reduced the anodic peak current for
the i-isomer more rapidly than for the o-isomer, indicating that
the isomerization from the i-isomer to the o-isomer occurred
over a time scale similar to the scan rate. The CVs of 1¢BF4
recorded at 243 K show that the relative degree of peak current,
indicating o/i was independent of the scan rate (Figure 3b).
These results demonstrate that the ring inversion was blocked
Single-crystal X-ray diffraction studies revealed the struc-
tures of two linkage isomers of 1¢BF4 (Figures 1e and 1f,
Table S1).11 The crystals obtained from a CH2Cl2-Et2O solution
contained two crystallographically independent i-1+ ions, two
¹
1
BF4 ions, and two Et2O molecules in the unit cell. The crystals
at this temperature, as evidenced by H NMR.
¹
from CH2Cl2-p-xylene contained four o-1+ ions, four BF4 ions,
Overall, 1¢BF4 is shown here to work as a molecular device
that converts rotational motion into a shift of redox potential in a
way similar to that as previously reported for 3¢BF4.5 Although
they have similar functions, some differences in their properties
emerge owing to structural factors.
three p-xylene molecules, and six Et2O molecules. The labels
i- and o- indicate the isomers of 1+ with the methyl group in the
inner and outer positions, respectively. These results indicate
that the two isomers were similarly thermodynamically stable.
This represents the first selective crystallization of both the
isomers of the pyrimidine rotation system.
XRD revealed distortion on the tetrahedral coordination
environment of the copper center in 1¢BF4. The angle ¡ between
the phenanthroline plane and the pyrimidine plane was inves-
tigated for the isomers as a measure of the degree of distortion:
o-1+ and i-1+ each formed a distorted tetrahedral coordination
environment with ¡ = 77 and 76°, respectively, while the
pyrimidine plane was almost perpendicular to the phenanthro-
line plane (¡ = 87°) in a previously reported i-3¢BF4 crystal.5
The distortions are presumably due to the intramolecular π-π
stacking interaction between the pyrimidinyl and anthryl groups.
In the 2-(2¤-pyridyl)-4-methylpyrimidine complex, such stacking
would occur between the pyrimidinyl and anthryl groups and
between the pyridyl and anthryl groups.
The 1H NMR signals of 1¢BF4 in CD2Cl2 were grouped into
two sets, each attributable to one of the two isomers (i and o).
The separation of the signals was supported by the 1H-1H COSY
spectrum (Figure S1). The signals were assigned on the basis
of the shielding effect of the anthracene moieties,5 with a
greater high-field shift of the methyl protons expected for the
i-isomer (¤ = 1.44 ppm) than for the o-isomer (¤ = 2.36 ppm)
(Figure 2a). Variable-temperature 1H NMR spectra of 1¢BF4
were measured from 203 to 303 K. The o/i ratio was calculated
to obtain van’t Hoff plots (Figure S2), which showed that ring
inversion appeared to be frozen at below 263 K.
Cyclic voltammograms (CVs) of 1¢BF4 recorded at 293 K
show two redox waves at E°¤ = 0.56 and 0.71 V vs. Ag+/Ag
(Figure 3a), indicating that 1¢BF4 exists as a mixture of two
Structural effects also appear to influence the ratio of the
isomers, with 1¢BF4 showing a lower ratio (i/o = 1.1) than did
3¢BF4 (i/o = 2.0) at 293 K owing to greater structural distortion
of 1¢BF4 causing greater steric congestion in the i-isomer.
Electrochemical measurements revealed the E°¤ values of 1¢BF4
(0.56 and 0.71 V vs. Ag+/Ag) to be more positive than those
of 3¢BF4 (0.38 and 0.55 V vs. Ag+/Ag).5 The difference was
also attributed to steric effects; the tolyl group adjacent to the
coordinating N atom prevents the formation of a favorable
structure of CuII.
1
isomers in the solution, corroborating the H NMR results. The
redox couple at more positive potential can be ascribed to the
i-isomer and the other to the o-isomer. Because the methyl group
of the i-isomer adjacent to the coordinating N atom prevents
the formation of a favorable square-planar structure in the CuII
state,12 the i-isomer shows a more positive E°¤ value than did the
o-isomer.
1038 | Chem. Lett. 2014, 43, 1037–1039 | doi:10.1246/cl.140238
© 2014 The Chemical Society of Japan

Single crystals of 2¢(BF4)2 obtained from a CH2Cl2-Et2O
solution contained only the oo-isomer oo-22+ (Figure 1g,
Table S2), which formed a symmetric molecular structure with
an inversion center. The unit cell contains disordered solvent
three, as expected from the single or double inversion of the
pyrimidine rings. CVs show that the negative redox potential
shifts from i- to o- through pyrimidine inversion also works in
these structures. The inversion in 1¢BF4 was supported by
crystal structure analysis, which allowed the selective isolation
of both i- and o-isomers in the crystalline state. Independent
rotation at each unit explained the behavior of 2¢(BF4)2. The
different inversion and redox behaviors of 3¢BF4, 1¢BF4, and
2¢(BF4)2 were explained by intramolecular steric effects. The
behavior of 2¢(BF4)2 was particularly affected by the steric
interference between its two monomer units. These results
provide new insights into the assembly of redox-active molecu-
lar machinery units within short distances.
¹
molecules in the void surrounded by oo-22+ and two BF4 ions.13
The coordination tetrahedron of 22+ was more distorted
(¡ = 78°) than that of i-3+ (¡ = 87°). Although the pyrimidine
plane and the adjacent C-N double bond were almost coplanar,
the phenylene linker between the two imine moieties is distorted
with a Cu-N(imine)-C(1-phenylene)-C(2-phenylene) dihedral
angle of 27° (Figure S3). This distortion probably originates
from the avoidance of atomic contacts between the bulky anthryl
groups.
1H NMR spectra of 2¢(BF4)2 in CD2Cl2 displayed four
methyl signals. Consideration of shielding effects and integrated
values suggests that one (¤ = 2.38 ppm) is attributed to an
oo-isomer, another one (¤ = 1.82 ppm) to an ii-isomer, and
the remaining two (¤ = 2.39 ppm, 1.75 ppm) to an io-isomer
(Figure 2b). The peak shifts and integrated values of the other
The authors acknowledge Grants-in-Aid from MEXT of
Japan (Nos. 20750044, 20245013, and 21108002; area 2107
(Coordination Programming)), the Global COE Program for
Chemistry Innovation.
1
signals and the H-1H COSY spectrum were well assigned and
References and Notes
³
Present address: Department of Materials and Life Science, Seikei
University, 3-3-1 Kichijoji-kitamachi, Musashino, Tokyo 180-8633
agreed with the interpretation of the methyl signals (Figure S4).
The three isomers coexisted with an oo-:io-:ii- ratio of 49%:
42%:9% at room temperature. This ratio corresponds to the
statistic distribution of o:i = 7:3, considering that io- is doubled
by io- and oi- configuration (49%:42%:9% = (0.7 © 0.7):(2 ©
0.7 © 0.3):(0.3 © 0.3)). The i/o ratio of 2¢(BF4)2 (i/o = 0.4) is
smaller than that of 1¢BF4 (i/o = 1.1), suggesting that the cop-
per center is sterically more crowded in 2¢(BF4)2 than in 1¢BF4.
The CV of 2¢(BF4)2 recorded at 293 K shows two redox
waves at E°¤ = 0.61 and 0.74 V vs. Ag+/Ag (Figure 3c). The
redox couple at a more positive potential is assignable to the
copper centers with an i-oriented methylpyrimidine ring of the
ii- and io-isomers. Another redox wave is assigned to the copper
centers with an o-oriented methylpyrimidine ring of the oo- and
io-isomers. No additional redox waves were detected in the CV,
confirming the absence of a mixed-valence state in 23+ (i.e., the
one-electron-oxidized form of 22+). The absence is possibly
explained by the twisting of the phenylene linker in the L2
ligand, as mentioned above in the section concerning crystal-
lography. The twist causes weak or almost no conjugation on the
two imine bonds, yielding negligible electronic communication
between the two copper ions. The slight positive shift of the
redox potential from 1¢BF4 (E°¤ = 0.56 and 0.71 V) to 2¢(BF4)2
(E°¤ = 0.61 and 0.74 V) can also be attributed to the congested
structure, which prevents the formation of a favorable structure
of CuII.
The CV waves of 2¢(BF4)2 appear broader than those of
1¢BF4 (Figure 3c). The broader waves and larger separation
between the oxidation and reduction peaks were likely due to the
slower electron-transfer rate in 2¢(BF4)2 than in 1¢BF4. The
slower electron transfer might be attributable to the congested
structure around each copper atom, which would interfere
structurally with each other, thereby disturbing the structural
rearrangement for the redox process. The CVs of 2¢(BF4)2
recorded at 243 K show further broadening of the redox peaks
owing to the slower electron-transfer rate at low temperatures
(Figure 3d).
³³ Present address: Department of Chemistry, Graduate School of
Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-
Hiroshima, Hiroshima 739-8526
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In conclusion, we synthesized mono- and dinuclear 4-
methyl-2-pyrimidinyl)imine complexes 1¢BF4 and 2¢(BF4)2,
respectively. 1¢BF4 formed two isomers, and 2¢(BF4)2 formed
Chem. Lett. 2014, 43, 1037–1039 | doi:10.1246/cl.140238
© 2014 The Chemical Society of Japan | 1039