Redox induced electron transfer in doublet azo-anion diradical rhenium(II) complexes. Characterization of complete electron transfer series

Article abstract of DOI:10.1021/ic9016195

Reactions of dirhenium decacarbonyl with the two azoaromatic ligands, L^a = (2-phenylazo)pyridine and L^b = (4-chloro2-phenylazo) pyridine (general abbreviation of the ligands is L) afford paramagnetic rhenium(II) complexes, [Re^II(L^.-)₂(CO) ₂] (1) (S= 1/2 ground state) with two one-electron reduced azo-anion radical ligands in an octahedral geometrical arrangement. At room temperature (300 K) the complexes 1a-b, showed magnetic moments (μff) close to 1.94 μ_B, which is suggestive of the existence of strong antiferromagnetic interactions in the complexes. The results of magnetic measurements on one of the complexes, 1 b, in the temperature range 2-300 K are reported. The above complexes showed two cathodic and two anodic responses in cyclic voltammetry where one-electron oxidation leads to an unusual redox event involving simultaneous reduction of the rhenium(II) and oxidation of the second ligand via intramolecular electron transfer. The oxidized complexes Ia+ and Ib+ are air stable and were isolated as crystalline solids as their tri-iodide (I₃-) salts. The structures of the two representative complexes, 1 b and [1 b]I₃, as determined by X-ray crystallography, are compared. The anionic complexes, [1]^- and [1]^2- were characterized in solution by their spectral properties.

Full text of DOI:10.1021/ic9016195

Communication
pubs.acs.org/JACS
Azo Anion Radical Complex of Rhodium as a Molecular Memory
Switching Device: Isolation, Characterization, and Evaluation of
Current−Voltage Characteristics
Nanda D. Paul,† Utpal Rana,‡ Sreetosh Goswami,§ Tapan K. Mondal,∥ and Sreebrata Goswami*,†
†Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
‡Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
§Department of Electrical Engineering, Bengal Engineering and Science University, Howrah 711 103, India
∥Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India
S
* Supporting Information
phenyl ring of the ligand L) were synthesized in excellent yields
ABSTRACT: Two rare examples of azo anion diradical
complexes of Rh(III) are reported. These complexes
showed excellent memory switching properties with a large
ON/OFF ratio and are suitable for RAM/ROM
applications. Their electronic structures have been
elucidated using a host of physical methods, including X-
ray crystallography, variable-temperature magnetic suscept-
ibility measurement, cyclic voltammetry, electron para-
magnetic resonance spectroscopy, and density functional
theory. The results indicate a predominant triplet state
description of the systems with two ferromagnetically
coupled radicals.
(>90%) from the reaction of the substitutionally labile,
electron-rich precursor [RhI(COD)2]CF3SO3 with 2-(arylazo)-
pyridine [L; R = H, La = 2-(phenylazo)pyridine; R = Cl, Lb = 2-
(4 chlorophenylazo)pyridine] in a dry nitrogen atmosphere
(Scheme 1).
Scheme 1. Synthetic Reaction
Microanalytical positive-ion electrospray ionization mass
spectra of the two compounds convincingly support their
formulation [see the Supporting Information (SI)]. The X-ray
structure of the representative complex [1b]CF3SO3 was
solved, and the molecular view is displayed in Figure 1 (for
uring recent years, the invention of thin-film devices with
D
memristive properties1 has led to a revolutionary turn in
the research area of electrical switching technology.2 This is
because such a technology may enable functional scaling of
logic and memory circuits well beyond the limits of
complementary metal−oxide−semiconductors and have the
potential to simulate biological and neuromorphical processes
as well.1a,2,3 Herein we disclose the first-ever example of a
metal-stabilized organic radical system that exhibits memristive
properties.
Because of their diverse chemical and physical properties,
organic radicals are at the focus of contemporary research.4
They are highly reactive, and their isolation in the crystalline
state has always been a challenge. In this respect, a strategy of
using transition-metal ions with multivalent possibilities4a−c,5 as
a carrier of radical ligands has been successfully used.
Consequently, a large variety of transition-metal-stabilized
radical complexes have been synthesized. However, in most
cases the radical character of the coordinated ligand is obscured
through the coupling of the radical spin with unpaired electrons
on the metal or on another radical ligand.6 Thus, stable,
uncoupled radical complexes are rare in the literature.
Figure 1. Molecular view of the complex [1b]CF3SO3. H atoms and
CF3SO3 have been omitted for clarity. Selected bond lengths (Å):
Rh1−N1, 2.015(2); Rh1−N3, 2.034(2); Rh1−N4, 2.037(2); Rh1−
N6, 2.061(2); Rh1−N7, 2.015(2); Rh1−N9, 2.072(2); N2−N3,
1.341(3); N5−N6, 1.277(3); N8−N9, 1.332(3).
During our recent works on non-redox-innocent aromatic
azo ligands, we have developed a synthetic protocol6a−c,7 for
the synthesis of metal-stabilized azo anion radical complexes
using low-valent transition-metal precursors. Accordingly, the
diradical complexes [RhIII(L•−)2(L)]CF3SO3 ([1]CF3SO3; R =
H (1a), Cl (1b), where R is a substituent on the pendant
the ORTEP, see Figure S2 in the SI). The structural analysis
revealed that the complex [1b]CF3SO3 is hexacoordinate, with
Received: December 31, 2011
Published: March 30, 2012
© 2012 American Chemical Society
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the central metal ion residing in a distorted octahedral
environment. The most notable part of the structure is the
unusual elongation of the N−N (azo) bonds in two of the three
coordinated ligands; the three N−N bond lengths (dN−N) are
1.341(3), 1.332(3), and 1.277(3) Å (Table S3 in the SI).
Notably, the N−N distance is an excellent indicator of the
charge of an azo function. From the dN−N of ca. 1.24 Å for free
ligands,8 coordination by back-donating metals may shift this
value to ca. 1.25−1.30 Å. Real one-electron-reduced (i.e., anion
radical) ligands have dN−N ≈ 1.35 Å, while the two-electron-
radicals, and the third wave is attributed to the oxidation of
Rh(III) to Rh(IV); the corresponding cathodic responses are
ascribed to the stepwise reductions of the three coordinated
ligands.7 The uncoordinated free azo ligand, under identical
experimental conditions, showed two cathodic responses near
−1.06 and −1.86 V (Table S2 and Figure S4). These are
due6a−d,7 to its stepwise reduction to the azo anion radical and
dianionic hydrazido forms, respectively.
To elucidate further the electronic structure of the
complexes, broken-symmetry (BS) hybrid density functional
theory (DFT) calculations were conducted on [1b]CF3SO3 at
the B3LYP level.11 The calculated bond characteristics for the
triplet ground state (Table S3) are in reasonable agreement
with the experimental values. The singlet state was found to
reside much higher in energy (12.87 kcal mol−1) relative to the
triplet state. In [1b]CF3SO3, two ligand radicals were observed
in the spin density maps, which featured typical patterns for azo
anion radicals,6a indicating that two ligands carry one unpaired
electron each, leaving the third one in the unreduced neutral
form (Figure 3). The LUMOs are also localized on the ligands
reduced hydrazido(2−) ligands have single bonds with dN−N
>
1.40 Å. The observation of the two elongated azo
chromophores clearly is in accordance with the anion radical
form of the two coordinated ligands and, by implication, the
trivalent state for the metal in [RhIII(L•−)2(L)]CF3SO3.
The magnetic properties of the complexes were probed by
variable-temperature magnetic moment measurements (Figure
2a). At 300 K, the value of μeff (2.42 μB) is close to that
●
Figure 2. (a) Plot of μeff (black for experimental data, red line for
theoretical value) vs T for [1b]CF3SO3. (b) Cyclic voltammogram of
[1a]CF3SO3 in CH2Cl2/0.1 M Bu4NClO4 with a Pt working electrode
and a saturated Ag/AgCl reference electrode.
Figure 3. Spin density plot for [1b]CF3SO3 in the native S = 1 state
(S2 = 2.0222; spin populations: Rh, 0.0600; L, 0.1580; L−•, 0.8036;
L
−•, 0.9784).
9
1
calculated for two uncoupled S = /2 spins with g = 2.0. The
effective magnetic moment for [1b]CF3SO3 increases very
slightly with decreasing temperature and reaches a maximum
value of 2.44 μB near 100 K, revealing the presence of a weak
ferromagnetic interaction between the two radical magnetic
centers. At much lower temperatures (<50 K), the magnetic
moment decreases rapidly because of the intermolecular
antiferromagnetic interaction. Theoretical modeling of the
experimental data using the Bleaney−Bowers equation9 with
(Tables S5 and S6 and Figures S7 and S8). These results are in
line with the structural features of the complexes (Table S3)
and confirm the direct participation of the ligands in the redox
processes, as stressed by the electrochemical measurements.
The spin density map for the theoretical model of the native
[1b]CF3SO3 is shown in Figure 3. The calculated structural
parameters (Table S3), the composition of the molecular
orbitals (Tables S5 and S6 and Figures S7 and S8), and the spin
density plots of the redox partners [1b]2+, [1b]3+, [1b]4+, [1b],
[1b]−, and [1b]2− (Figures S9−S12) are consistent with ligand-
based redox processes, with the exception that the third
oxidation process occurs at the metal center [Rh(III) →
Rh(IV)]. Furthermore, the EPR spectra of the above
electrogenerated complexes also support these descriptions. A
detailed analysis of the spectroelectrochemical studies will be
published separately in a future publication.
Thus, the complexes [1]CF3SO3 with multiple reversible
electron transfer processes were anticipated to be suitable
candidates for possible application as a molecular memory
device.12 Accordingly, a device was prepared as shown in Figure
4a and tested for the above possible application. It consisted of
chloroform solution spin-coated [1b]CF3SO3 film (100 nm)
sandwiched between two gold electrodes (see the Experimental
Section in the SI). The selection of two identical electrodes
allowed us a clear demonstration of whether or not the origin
of the memory effect comes from an intrinsic property of the
̂
̂
̂
the isotropic exchange Hamiltonian (H = −JS1·S2) for the two
1
interacting S = /2 centers yielded J = 2.15 cm−1 (J is the
exchange constant between the two radical ligands) and θ =
−5.0 K (θ is the intermolecular exchange coupling constant)
(see the SI). The triplet spin state was further characterized by
electron paramagnetic resonance (EPR) spectroscopy in a
dichloromethane/toluene glass at 77 K (Figure S3). The EPR
spectra of [1a/b]CF3SO3 show a broad, nearly isotropic signal
with an additional weak spin-forbidden ΔmS = 2 transition at
half field.10 In variable-temperature measurements, the intensity
of the spectrum increased as the temperature was lowered to 77
K. This overall increase in intensity and especially the increase
in the low-field transition is consistent with the ferromagneti-
cally coupled diradical nature of [1a/b]CF3SO3.10c,d
Cyclic voltammetry of the complexes [1a/b]CF3SO3 in
CH2Cl2 exhibited six reversible one-electron redox responses
(three oxidation and three reduction) in the potential range 1.0
to −1.5 V (Figure 2b and Table S2). The first two anodic
responses are due to the oxidation of the two coordinated
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Figure 4. (a) Cross-sectional view of the device layout for the memory
device. (b) Current−voltage (I−V) characteristics of the Au/
[1b]CF3SO3/Au molecular device (semilogarithmic scale).
diradical complex [1b]CF3SO3.12a The thicknesses of the top
and bottom electrodes were 90 and 85 nm, respectively.
The current−voltage (I−V) characteristics of the device
(Figure 4b) were recorded in the range V = +5 to −5 V at a
scan rate of 10 mV/s using a potential sweep in the positive to
negative direction with a bias being applied on a film of
thickness 100 nm (Figure S6). In the voltage sweep from 0 to
+5 V, an abrupt increase in current was observed at a switching
threshold voltage of 2.75 V, indicating the device transition
from a low-conductivity (OFF) state to a high-conductivity
(ON) state (“writing” process), and this state was retained until
a negative bias threshold voltage of −2.05 V was applied, at
which there was an abrupt decrease in current indicating the
device transition from the ON state to the OFF state (“erasing”
process). The erased state (OFF) could be rewritten (ON) by
applying the switching threshold voltage. It is indeed interesting
to note that the distinct bielectrical states in the voltage range
−2.05 to +2.75 V allow any voltage in this range to read as an
OFF or ON signal depending upon the history of the voltage
sweep, which actually qualifies the device to be used as
nonvolatile memory device.
Figure 5. (a) Input applied voltage sequence and (b) output current
responses during the write/read/erase/ead (W/R/E/R) cycle of the
gold/[1b]CF3SO3/gold device. Voltages: W, +4.0; R, 1.0; E, −4.0; R,
1.0 V.
For comparison, the I−V characteristics of the free azo ligand
(Lb) (Figure S5) do not show any switching in the voltage
range V = +5 to −5 V. It may be concluded that the switching
phenomenon reported herein is a molecular property of the
diradical complex. The mechanism of the switching process
remains unresolved as yet. However, it may be proposed that
the switching-ON process is due to the shifting of electrode
Fermi levels with the applied potential and their alignment with
the donor and acceptor orbitals of the materials,12b,13 resulting
in a high-conducting state. In this state (ON), the device
behaves as an ohmic material during the voltage sweep from
+2.75 V to −2.05 V, at which (i.e., −2.05 V) the material
switches to the low-conducting (OFF) state.
The device can be used as random-access memory (RAM)
and read-only memory (ROM) as well. The endurance of the
device as RAM was examined using write/read/erase/read (W/
R/E/R) cycles (+4 V/1 V/−4 V/1 V) in pulse mode (Figure
5). More than 100 cycles with ON/OFF ratios of up to 103
were observed without any degradation of the device (Figure
6). In addition, after the device was turned ON by applying a
positive voltage greater than the threshold value, even with the
application of a very small voltage it was tested to maintain the
ON state for hours, and a similar kind of behavior was also
observed for the OFF state. This indicates the possibility that
the device can be used as ROM as well. It is worthwhile to
emphasize that the low threshold voltages (<3 V in magnitude)
for both the ON and OFF states are desirable for a memory
device because they result in low power consumption. They
Figure 6. Retention times of the ON- and OFF-state data of the gold/
[1b]CF3SO3/gold device, probed with currents under +1.0 V. The ON
and OFF states were induced by +4.0 (writing) and −4.0 V (erasing),
respectively.
may be attributed to the facile multiple redox responses of
[1]CF3SO3 in the narrow range of applied potential.
In summary, we have reported here a designed synthesis and
complete characterization of two unusual but stable triplet azo
anion diradical complexes of rhodium(III) differing only with
respect to substitution on the coordinated aromatic azo ligand.
The diradical complex [1b]CF3SO3 showed a pinched
hysteresis loop in the I−V plane, which is a typical feature of
a memristive device. On the basis of the simple gold−
molecule−gold device structure, we conclude that the switch-
ability and memory phenomena of the azo anion radical
memory device originate from the coordinated radical ligands.
The device can potentially be used as RAM and ROM, and it
exhibits very low area, reduced power consumption, and much
higher speed in comparison with devices in practice. While its
ON/OFF ratio is not comparable to those of available devices,
it is of course reasonably high and comparable to those of
several other memristive devices reported3c,12 in recent times.
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(9) (a) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
Though several useful applications of metal complexes of
redox-active radical ligands are currently being investigated,4a−c
our present results have indeed opened up a heretofore
unexplored avenue of research in similar systems.
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(11) Frisch, M. J.; et al. Gaussian 03, revision D.01; Gaussian, Inc.:
Wallingford, CT, 2004.
(12) (a) Lee, J.; Lee, E.; Kim, S.; Bang, G. S.; Shultz, D. A.; Schmidt,
R. D.; Forbes, M. D. E.; Lee, H. Angew. Chem., Int. Ed. 2011, 50, 4414.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures, X-ray crystallographic data
(CIF), ORTEP of [1b]CF3SO3, complete ref 11 (as SI ref 8),
and absolute energies and the coordinates of the DFT-
optimized structures. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
icsg@iacs.res.in
(13) Hsu, J.-C.; Chen, Y.; Kakuch, T.; Chen, W.-C. Macromolecules
2011, 44, 5168.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Department of Science and
Technology (DST), New Delhi (Project SR/S1/IC/0031/
2010). Sreebrata Goswami sincerely thanks DST for a J. C.
Bose National Fellowship. Crystallography was performed at
the DST-funded National Single Crystal Facility at the
Department of Inorganic Chemistry, IACS. We are thankful
to Dr. S. Malik, Polymer Science Unit, IACS. N.D.P. thanks the
Council of Scientific and Industrial Research for fellowship
support.
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