Electrochromic platinum(II) complexes derived from azobenzene and zwitterionic quinonoid ligands: Electronic and geometric structures

Article abstract of DOI:10.1021/om4009179

The ligands azobenzene (az) and the zwitterionic 4-(isopropylamino)-6- (isopropyliminio)-3-oxocyclohexa-1,4-dien-1-olate (Q) were used to synthesize the mononuclear complex [(Q_-H)Pt(az_-H)] (1), and the dinuclear complex [(Q_-H)Pt(μ-az_-2H)Pt(Q_-H)] (2). Structural characterization of the complexes shows a distorted-square- planar environment around the Pt(II) centers and localization of the double bonds within the Q_-H ligand on metal coordination. Furthermore, the N=N azo bond is elongated in the metal complexes in comparison to free az, owing to π back-bonding from Pt(II) to az. Complexes 1 and 2 display multiple reversible reduction steps in their cyclic voltammograms. The complexes also exhibit strong absorptions in the visible region, the position and intensity of which can be influenced by the chromophore [(Q_-H)Pt]. UV-vis-near-IR spectroelectrochemical studies show that the absorption of these complexes in the visible as well as the near-IR region can be controlled by electron transfer steps. Depending on the charge state of the complexes, they are found to be either transparent in the near-IR region but strongly absorbing in the visible or vice versa, thus displaying strong electrochromic behavior. EPR spectroelectrochemical studies together with DFT calculations and comparison with the complex [(Q_-H)Pd(az_-H)] (3) are used to locate the site of electron transfer in these complexes and to elucidate their electronic properties in the various redox states. Complex 2 is a rare example where doubly deprotonated azobenzene acts as a bridging ligand.

Full text of DOI:10.1021/om4009179

Article
pubs.acs.org/Organometallics
Electrochromic Platinum(II) Complexes Derived from Azobenzene
and Zwitterionic Quinonoid Ligands: Electronic and Geometric
Structures
Naina Deibel,† Stephan Hohloch,‡ Michael G. Sommer,‡ David Schweinfurth,‡ Fabian Ehret,†
Pierre Braunstein,§ and Biprajit Sarkar*,‡
†Institut fur Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
̈
̈
̈
‡Institut fur Chemie und Biochemie, Freie Universitat Berlin, Fabeckstraße 34-36, D-14195 Berlin, Germany
̈
§Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universite
67081 Strasbourg Cedex, France
́
de Strasbourg, 4 rue Blaise Pascal,
S
* Supporting Information
ABSTRACT: The ligands azobenzene (az) and the zwitterionic
4-(isopropylamino)-6-(isopropyliminio)-3-oxocyclohexa-1,4-dien-
1-olate (Q) were used to synthesize the mononuclear complex
[(Q‑H)Pt(az‑H)] (1), and the dinuclear complex [(Q‑H)Pt(μ-
az‑2H)Pt(Q‑H)] (2). Structural characterization of the complexes
shows a distorted-square-planar environment around the Pt(II)
centers and localization of the double bonds within the Q‑H ligand
on metal coordination. Furthermore, the NN azo bond is
elongated in the metal complexes in comparison to free az, owing
to π back-bonding from Pt(II) to az. Complexes 1 and 2 display
multiple reversible reduction steps in their cyclic voltammograms.
The complexes also exhibit strong absorptions in the visible
region, the position and intensity of which can be influenced by
the chromophore [(Q‑H)Pt]. UV−vis−near-IR spectroelectrochemical studies show that the absorption of these complexes in the
visible as well as the near-IR region can be controlled by electron transfer steps. Depending on the charge state of the complexes,
they are found to be either transparent in the near-IR region but strongly absorbing in the visible or vice versa, thus displaying
strong electrochromic behavior. EPR spectroelectrochemical studies together with DFT calculations and comparison with the
complex [(Q‑H)Pd(az‑H)] (3) are used to locate the site of electron transfer in these complexes and to elucidate their electronic
properties in the various redox states. Complex 2 is a rare example where doubly deprotonated azobenzene acts as a bridging
ligand.
properties of these and related ligands on various surfaces have
been explored.8 Metal complexes of these ligands in their
deprotonated forms have displayed unusual bonding and
electronic properties.7,9 Such complexes have found use in
homogeneous catalysis,10 supramolecular chemistry,7c and
electron transfer research.11 The “para” counterparts of these
ligands have also been explored by us12 and others13 in recent
years in various areas of coordination and organometallic
chemistry. Our continuing interest in the redox and electronic
properties of metal complexes containing quinonoid and azo
type ligands, and the knowledge of the excellent light
absorption properties of az, Q, and their deprotonated forms
led us to the present project. Here we have combined these two
classes of ligands on the platform of a d8 Pt(II) center which is
predestined to form square-planar complexes. We present
below the mononuclear complex [(Q‑H)Pt(az‑H)] (1) and the
INTRODUCTION
■
The compound azobenzene (az) has been extensively used for
“molecular switching” purposes because of its ability to undergo
reversible light-induced isomerism.1 Intriguingly, az was also
one of the first ligands where C−H activation was observed in
its Pd(II) and Pt(II) complexes.2 Azo ligands in general have
been widely used in coordination and organometallic chemistry
because of their redox non-innocent character and their ability
to strongly absorb light in the visible or the near-IR region in
their metal-coordinated forms.3 Metal complexes of azo ligands
such as phenylazopyridine and 2,2′-azobispyridine4 have been
extensively studied, and those with az‑H as a chelating ligand
have also been investigated.1c−e,5 However, the use of az‑2H as a
bis-chelating bridging ligand toward metal centers has been
only sporadically reported.6
The zwitterionic molecule 4-(isopropylamino)-6-(isopropy-
liminio)-3-oxocyclohexa-1,4-dien-1-olate (Q) and other mem-
bers of the series with different N substituents have been shown
previously to possess many fascinating properties.7 Recently,
Received: September 13, 2013
© XXXX American Chemical Society
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Scheme 1. Synthesis of Complexes 1 and 2
dinuclear complex [(Q‑H)Pt(μ-az‑2H)Pt(Q‑H)] (2), with the
monodeprotonated form of Q and the mono- and dideproto-
nated forms of az. A combination of structural, electrochemical,
and UV−vis−near-IR and EPR spectroelectrochemical studies
and support from DFT calculations are used to shed light on
the electronic structures of these complexes in their various
accessible redox forms. We explore below the possibility of
switching the absorption properties of these complexes in the
visible and the near-IR region through redox processes. Insights
from the combined studies are used to understand this
electrochromic behavior and to locate the sites of electron
transfers in these molecules. The known complex [(Q‑H)Pd-
(az‑H)] (3)11f will be used for comparison purposes and for a
better understanding of the complexes presented in this study.
n-hexane followed by slow diffusion at ambient temperature.
The same was achieved for 2·0.5C3H6O by slow diffusion of
diethyl ether into an acetone solution (see the Experimental
Section). 1·0.5CH2Cl2 crystallizes in the monoclinic C2/c space
group and 2·0.5C3H6O in the orthorhombic Pbcn space group
(Table S1, Supporting Information). The Pt(II) center in 1·
0.5CH2Cl2 is in a distorted-square-planar environment, being
coordinated by the C and N centers of the az‑H ligand and the
O and N centers of the Q‑H ligand (Figure 1). The distortion is
imposed by the chelating nature of both ligands (Table 1).
RESULTS AND DISCUSSION
■
Synthesis and Crystal Structures. The mono- and
dinuclear complexes 1 and 2 were both synthesized by the
reaction of Q11d with [(az‑H)Pt(μ-Cl)2Pt(az‑H)]2 in the
presence of a base (Scheme 1). Extraction with n-heptane led
to the separation of red 1 from blue 2, and subsequent
recrystallization led to the isolation of pure products for both
complexes (see the Experimental Section).
Figure 1. ORTEP view of 1·0.5CH2Cl2. Ellipsoids are drawn at 50%
probability. The hydrogen atoms and solvent molecules have been
omitted for clarity.
The identity and purity of the products were established by
1H and 13C NMR spectroscopy, elemental analysis, and mass
spectrometry. The N−H proton of Q‑H, which appears at 6.58
and 6.64 ppm for 1 and 2, respectively, appears as a doublet
(Figure S1, Supporting Information). The appearance of the
doublet is a result of the coupling of the N−H proton with the
C−H proton of the isopropyl substituent on the nitrogen atom
of Q‑H. While identical reaction conditions were used to
synthesize the Pd(II) analogues, only the mononuclear complex
3 could be isolated, as reported previously.11f Increasing the
amount of base, the reaction time, or the temperature did not
lead to the formation of a dinuclear complex in the case of
Pd(II). Thus, Pt(II) is seen to favor the double deprotonation
of az, which then acts as a bridging ligand in 2. It should be
noted here that although az in its monodeprotonated, ortho-
metalated form has been extensively used as a chelating ligand
in many metal complexes since the pioneering work by Cope
and Siekman,2 its use as a bridging ligand in its doubly
deprotonated form has been rare.6 Attempts at deprotonating
the remaining N−H protons of 2, with the aim of generating
multinuclear complexes, were unfortunately not successful.
Such reactions led to the formation of 1 and other unidentified
decomposition products.
Table 1. Selected Bond Angles (in deg) in the Complexes
1·
2·
a
0.5CH2Cl2
0.5C3H6O
3
O1−Pt1−N1/O1−Pd1−N1
N1−Pt1−C13/N1−Pd1−C13
C13−Pt1−N3/C13−Pd1−N3
N3−Pt1−O1/N3−Pd1−O1
N3−Pt1−N1/N3−Pd1−N1
C13−Pt1−O1/C13−Pd1−O1
78.81(8)
105.14(9)
78.28(9)
98.07(7)
175.15(8)
173.98(8)
78.4(2)
103.6(3)
78.7(3)
78.8(1)
107.7(1)
78.5(1)
101.7(2)
168.3(3)
168.4(3)
95.2(1)
172.1(1)
173.5(1)
a
From ref 11f.
In 2·0.5C3H6O, both Pt(II) centers are in a distorted-square-
planar environment, each of them being coordinated by the C
and N atoms of the az‑2H ligand and the O and N donors of the
Q‑H ligands (Figure 2). The az‑2H ligand thus acts as a bridge
between the two Pt(II) centers in 2·0.5C3H6O. The molecule
possesses an inversion center at the NN azo bond.
The Pt−O, Pt−N, and Pt−C bond lengths in both
complexes are in the expected range (Table 2). The Pt1−N3
distance to the azo-N is slightly shorter than the Pt1−N1
distance to the Q‑H ligand. As reported previously, in the free
Single crystals of 1·0.5CH2Cl2 suitable for X-ray diffraction
studies were grown by layering a dichloromethane solution with
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C4−C5 bond distance of 1.373(3) Å is shorter than the C5−
C6 distance of 1.404(3) Å; the C−N bond distances, however,
are more similar. Thus, monodeprotonation and coordination
to a single metal center leads to bond localization within Q‑H.
The C3−C4 and C6−C1 distances of about 1.5 Å are in the
range of a single bond, just as in the case of free Q (Table 2).
The bonding situation within the Q‑H ligand in 2·0.5C3H6O is
exactly the same as in 1·0.5CH2Cl2. Similar to the case for 1·
0.5CH2Cl2, in 2·0.5C3H6O, Q‑H is bound to a single metal
center, and hence bond localization is observed within that
ligand in 2·0.5C3H6O (Table 2). Similar trends have been
observed for Q‑H in 311f and in other related complexes where
Figure 2. ORTEP plot of 2·0.5C3H6O. Ellipsoids are drawn at 50%
probability. The hydrogen atoms and solvent molecules have been
omitted for clarity.
Q
‑H is coordinated to a single metal center.11 The NN bond
length in az‑H in 1·0.5CH2Cl2 is 1.278(3) Å. The corresponding
value within az‑2H in 2·0.5C3H6O is 1.30(1) Å. Coordination of
metal centers to az‑H or az‑2H leads to back-bonding from the
filled dπ orbitals of Pt(II) to the empty π* orbitals of the azo
ligand, leading to an elongation of the NN bond in
comparison to free azobenzene.3,4,5b The intramolecular Pt−
Pt distance in 2·0.5C3H6O is 4.78 Å.
The presence of uncoordinated “O” and “N−H” sites within
Q‑H in both 1·0.5CH2Cl2 and 2·0.5C3H6O leads to
intermolecular hydrogen bonding in these molecules in the
solid state. In the mononuclear complex 1·0.5CH2Cl2, hydro-
gen bonding between complementary N−H and O groups of
two adjacent molecules leads to the formation of dimers in the
solid state (Figure 3). The N−H···O distance is 2.30(2) Å. For
the dinuclear complex 2·0.5C3H6O, the presence of two Q‑H
ligands within the complex allows hydrogen bonding in both
directions. Such weak intermolecular interactions lead to the
formation of a three-dimensional “steplike” structure in the
solid state (Figure 3). The N−H···O distance between two
molecules of 2·0.5C3H6O is 2.5(1) Å.
Cyclic Voltammetry. The redox properties of complexes 1
and 2 were investigated by cyclic voltammetry. 1 displays two
reversible reduction steps at −1.58 and −1.93 V in CH2Cl2/0.1
M Bu4NPF6 (Figure 4 and Table 3). The difference between
these reduction potentials is 350 mV. For comparison, the
corresponding mononuclear Pd(II) complex 3 shows two
reduction steps at −1.56 and −1.85 V, respectively, with a
potential difference of 290 mV.11f Hence, the replacement of
Pd(II) by Pt(II) on going from 3 to 1 appears to increase the
thermodynamic stability of the one-electron-reduced form; the
comproportionation constant Kc values are on the order of 105
Table 2. Selected Bond Lengths in the Complexes (in Å)
a
1·0.5CH2Cl2
2·0.5C3H6O
3
Pt−O1/Pd1−O1
Pt1−C13/Pd1−C13
Pt1−N3/Pd1−N3
Pt1−N1/Pd1−N1
C1−O1
C3−O2
C1−C2
C2−C3
C3−C4
C4−C5
C5−C6
C6−C1
C6−N1
2.082(2)
1.995(2)
1.993(2)
2.024(2)
1.317(3)
1.241(3)
1.364(3)
1.420(4)
1.524(4)
1.373(3)
1.404(3)
1.480(4)
1.331(3)
1.328(3)
1.278(3)
1.446(3)
1.394(3)
2.092(5)
1.973(7)
2.015(6)
2.035(6)
1.298(9)
1.23(1)
1.37(1)
1.41(1)
1.52(1)
1.37(1)
1.40(1)
1.50(1)
1.330(9)
1.33(1)
1.30(1)
1.42 (1)
1.42(1)
2.058(2)
1.995(3)
2.018(3)
2.078(3)
1.286(4)
1.236(4)
1.363(5)
1.404(5)
1.519(5)
1.371(5)
1.411(5)
1.509(5)
1.317(5)
1.331(5)
1.269(4)
1.437(4)
1.398(5)
C4−N2
N3−N4/N3 − N3
N3−C19/N3−C18
N4−C18/N3−C18
a
From ref 11f.
ligand Q, electronic delocalization is observed in the “upper”
and “lower” parts of the molecule, which are connected to each
other with two C−C single bonds.7 Such a situation results in a
6π + 6π system for ligands such as Q. For 1·0.5CH2Cl2, the
C1−O1 bond distance within Q‑H is 1.317(3) Å and the C3−
O2 distance is 1.241(3) Å. Accordingly, the C1−C2 distance is
1.364(3) Å and the C2−C3 distance is 1.420(4) Å. A similar
trend is observed in the “lower” part of Q‑H in 1·0.5CH2Cl2: the
Figure 3. Intermolecular hydrogen bonding in 1·0.5CH2Cl2 (top) and 2·0.5C3H6O (bottom).
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Figure 4. Cyclic voltammograms of 1 and 2 in CH2Cl2/0.1 M
Bu4NPF6 at 295 K. Scan rate: 100 mV/s. The ferrocene/ferrocenium
couple was used as an internal standard.
Table 3. Redox Potentials of Complexes from Cyclic
a
Voltammetry
d
d
d
compd
Eox1 (V)
Ered1 (V)
Ered2 (V)
Ered3 (V)
1
2
−1.58 (90)
−1.08 (65)
−1.56
−1.93 (95)
−1.62 (65)
−1.85
−1.94 (70)
b
c
3
0.80
a
Half-wave potentials from cyclic voltammetric measurements in
CH2Cl2/0.1 M Bu4NPF6 for reversible processes at 298 K and a scan
Figure 5. X-band EPR spectra of chemically generated 1•− (top) in
CH2Cl2 and of electrochemically generated 12− (bottom) in CH2Cl2/
0.1 M Bu4NPF6 at 295 K.
rate of 100 mV s−1, with ferrocene/ferrocenium used as an internal
b
c
d
standard. From ref 11f. Epa for the irreversible process. Peak to peak
separations in mV are given in parentheses.
EPR signal with g = 2.002 and detectable hyperfine coupling to
and 106 for 3•− and 1•−, respectively. The uncoordinated ligand
Q is known to display only irreversible reduction processes.
Complex 2, which contains two “Pt(Q‑H)” units and a doubly
deprotonated az‑2H bridging ligand, displays three reversible
reduction steps, the first of which appears at −1.08 V (Figure
4). Thus, going from the mononuclear Pt(II) complex 1 to the
dinuclear Pt(II) complex 2 results in an anodic shift of 500 mV
for the first reduction step (Table 3). Furthermore, the removal
of a proton from az‑H and the addition of the second “Pt(Q-H)”
unit on going from 1 to 2 also results in the appearance of an
additional, third reduction step. Metal coordination is known to
result in a positive shift of the reduction potentials of azo
ligands.3,4 The difference between the first and second
reduction processes of 2 is 540 mV, and this translates to a
Kc value on the order of 109 for the one-electron-reduced form
of 2, a value that is substantially higher than that of the one-
electron-reduced form of the corresponding mononuclear
complex 1. For the mononuclear Pd(II) complex 3, an
irreversible oxidation process was observed at 0.80 V.11f For
1 and 2, no oxidation step was detected within the
dichloromethane solvent window. In order to further shed
light on the electronic structures of the various redox forms,
and to possibly locate the site of electron transfer in 1 and 2,
spectroscopic signatures were collected for the various redox
forms of the complexes, and DFT calculations were carried out.
EPR Spectroscopy and Spin-Density Calculations. The
one-electron- and two-electron-reduced forms of the complexes
were generated either chemically or electrochemically (see the
Experimental Section), and the X-band EPR spectra of those
species were recorded. The one-electron-reduced form of the
mononuclear Pt(II) complex 1•− in CH2Cl2 displays an
isotropic signal at 295 K centered at g = 1.986 (Figure 5).
No hyperfine structure was detected for this species. The g-
value and the appearance of the EPR signal in fluid solution at
295 K are the first signs of a ligand-centered spin.14 The
corresponding mononuclear Pd(II) complex 3•− displays an
the 14N and 105Pd nuclei (Table 4).11f We were also able to
Table 4. EPR Parameters of the Complexes from
a
Measurements in X-band
compd
giso
1•−
12−
2•−
3•−
1.986
2.003
1.988
2.002
bc
,
2a(195Pt) = 50 G
d f
−
a(14N) = 4.5, 6 G; a(105Pd) = 5 G
a
X-band EPR data obtained from in situ generated species in CH2Cl2/
b
0.1 M Bu4NPF6 at 298 K. Isotropic hyperfine coupling constants in
c
gauss obtained from simulation. Hyperfine coupling to 195Pt, I = /2,
1
d
natural abundance 33.3%. Hyperfine coupling to 105Pd, I = 5/2,
e
f
natural abundance 22.2%. Hyperfine coupling to 14N, I = 1. From ref
11f.
record the EPR spectrum of the two-electron-reduced species
12−. The latter displays a line-rich EPR spectrum at 295 K in
CH2Cl2/0.1 M Bu4NPF6 centered at g = 2.003 (Figure 5). The
poor signal-to-noise ratio, coupled with the complex nature of
the signal, precluded a reasonable simulation of this spectrum.
For 12−, which contains two spins, either the singlet or the
triplet state could be the ground state of this molecule. The
detection of an EPR signal at 295 K proves that the triplet state
is populated at 295 K. Molecule 1 contains two redox-active
ligands, az‑H and Q‑H. Since the EPR spectrum of 1•− does not
contain any hyperfine structure, we turned to DFT calculations
to further investigate the electronic structures of that redox
form.
Structure-based DFT calculations were carried out for the
various redox forms of 1. Structural optimization of 1 with the
BP-86 functional reproduced the bond lengths and bond angles
with reasonable accuracy (Tables S2 and S3, Supporting
Information). A look at the spin densities calculated according
to the Lowdin population analysis for 1•− shows that the spin
̈
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density is equally distributed between the Q‑H and az‑H ligands
(Figure 6). Thus, taken together, the EPR and DFT
(Tables S2 and S3, Supporting Information). A look at the spin
density calculated according to the Lowdin population analysis
̈
shows about 70% spin density on the az‑2H part in 2•−. Hence
2•− can be best formulated as [(Q‑H)PtII(μ-az‑2H)•−PtII(Q‑H)]•−
(Figure 7).
For the two-electron-reduced form 22−, DFT calculations
delivered a broken-symmetry solution with an energy difference
between the triplet and the singlet state of 1168 cm−1, thus
clearly establishing the singlet state as the ground state for that
molecule. Taking together the DFT-supported stabilization of
the singlet ground state, the EPR silence of 22−, and the known
tendency of azo-based ligands to undergo facile reduction upon
double metalation, 22− is best formulated as [(Q‑H)PtII(μ-
az‑2H)2−PtII(Q‑H)]2−. The two-electron reduction thus results in
a hydrazido type of bridge. The EPR results and spin-density
calculations thus show that the locus of reduction in such
complexes can be shifted from a delocalized situation to a
localized one by changing the metal center (1•− vs 3•−) or on
going from a mononuclear to a dinuclear complex (1•− vs 2•−).
UV−Vis-Near-IR Spectroelectrochemistry and TD-DFT
Calculations. Complex 1 exhibits several absorption bands in
the visible and ultraviolet region. The main bands in the visible
and UV regions are observed at 527, 486, 446, 387, and 354
nm, all of which are intense (Figure 8 and Table 5). TD-DFT
Figure 6. DFT calculated spin density plot of 1•−
.
calculations suggest that the 1•− form is best described as a
c o m b i n a t i o n o f [ ( a z ‑ H ) P t I I ( Q ‑ H
)
]
a n d
• − • −
[(az‑H)•−PtII(Q‑H)]•−. Interestingly, the corresponding Pd(II)
complex 3•− was formulated by us as [(az‑H)•−PdII(Q‑H)]•− on
the basis of hyperfine information from EPR spectroscopy.11f
Calculation of spin density for 3•− shows about 80% spin
density on the az‑H part and supports the [(az‑H)•−PdII(Q‑H)]•−
formulation (Figure S2, Supporting Information). Hence, on
changing the metal center from Pd(II) in 3•− to Pt(II) in 1•−,
the locus of the first electron transfer changes from a
predominantly az‑H-localized to a ligand-delocalized situation.
For 12−, we were not able to generate reasonable calculated
data. This problem is likely associated with the high negative
charge of 12−, which makes calculations on such systems
inaccurate.
The one-electron-reduced form 2•− of the dinuclear complex
displays an EPR signal in CH2Cl2 at 295 K centered at g =
1.988 (Figure 7). This signal could be simulated by considering
Figure 8. Changes in the UV−vis−near-IR spectrum of 1 during the
reduction steps from OTTLE spectroelectrochemistry in CH2Cl2/0.1
M Bu4NPF6.
Figure 7. X-band EPR spectrum of 2•− in CH2Cl2 at 295 K together
with a simulation (top). DFT calculated spin density plot of 2•−
(bottom).
calculations on 1 help in the assignment of these bands. The
relevant frontier orbitals are shown in Figure 9, and the data are
summarized in Table 6. All of these bands possess a mixed
ligand to ligand charge-transfer (LLCT) and/or metal to ligand
charge-transfer (MLCT) character. This description fits with
the intuitive idea of the Q‑H- and Pt-based orbitals
predominantly contributing to the filled orbitals and the Q‑H-
and az‑H-based orbitals contributing to the empty receptor
orbitals in 1. Additional bands in the UV region are assigned to
a π−π* ligand-centered transition. The absorption bands for
the corresponding mononuclear Pd(II) complex are much
1
a hyperfine coupling to two 195Pt (I = /2, natural abundance
33.3%) nuclei of 50 G. The two-electron-reduced form 22−
turned out to be EPR silent. Structure-based DFT calculations
were also carried out on the various redox forms of 2.
Optimization of the structure of 2 was carried out with the
BP86 functional, and the calculated bond lengths and bond
angles show a reasonable match with the experimental values
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transparent in the near-IR region, its reduction to 1•− leads to
the appearance of a strong absorption band at 1720 nm. This
Table 5. UV−vis−Near-IR Absorption Data of the
a
Complexes in the Various Redox States
near-IR band is assigned to a π to π* transition from within the
λ (nm) (ε (103 M−1 cm−1))
•−
resonance-stabilized forms [(az‑H)PtII(Q‑H)•−
]
and
10
234 (23.4); 261 (19.8); 354 (19.2); 387 (19.2); 446 sh; 486 (18.9);
527 (10.1); 617 sh
[(az‑H)•−PtII(Q‑H)]•− (see EPR Spectroscopy and Spin-Density
Calculations). Further reduction to 12− leads to a drastic loss in
intensity in all the vis−near-IR bands of 1•−, except for that at
339 nm (Figure 8). Furthermore, for 12− a new intense band
appears at 854 nm. This band is tentatively assigned to a metal-
to-ligand-charge-transfer (MLCT) transition within the com-
plex [(az‑H)•−PtII(Q‑H)•−]2−.
1•−
12−
20
227 (34.6); 270 sh; 352 (20.4); 466 sh; 562 sh; 604 (15.4);
768 (2.1); 854 (2.6); 1068 sh; 1427 sh; 1720 (11.0)
227 (35.5); 280 sh; 339 (18.4); 366 sh; 469 sh; 559 sh; 604 (5.1);
854 (10.8); 1074 sh; 1416 (2.0); 1729 (2.0)
234 (44.4); 273 sh; 340 (23.8); 404 (33.6); 550 (29.0); 612 (41.7);
913 (3.2)
2•−
22−
228 (74.9); 367 (44.6); 412 sh; 567 sh; 612 (32.9); 1350 (9.0)
The reduction of the dinuclear complex 2 to 2•− leads to a
loss in intensity of all the bands in the visible region (Figure 9).
Additionally, a broad band centered at 1350 nm appears in the
229 (75.5); 305 (33.1); 361 (41.6); 615 (15.5); 953 (7.3); 1302 sh;
1775 (26.0)
23−
229 (77.2); 311 (38.1); 339 (37.9); 444 sh; 582 (11.1); 954 (16.8);
1072 (13.8); 1321 sh; 1768 (16.9)
•−
near-IR region. This band is assigned to a (az‑2H
)
to (Q‑H)
LLCT transition within the formulation [(Q‑H)PtII(μ-
az‑2H)•−PtII(Q‑H)]•−. Reduction of 2•− to 22− leads to an
increase in intensity of the near-IR band, and its bathochromic
shift to 1775 nm. This band is also LLCT in character. The
bathochromic shift and the increase in intensity of this band can
be explained by considering the formulation [(Q‑H)PtII(μ-
b
30
3•−
32−
359 (20.50); 407 (17.50); 485 sh; 560 (0.82)
b
b
360 sh; 397 (19.70); 557 (3.50); 650 sh; 1415 (1.650)
329 (14.70); 386 (12.50); 490 (4.80); 605 sh; 908 (3.40)
a
From OTTLE spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6 at
b
295 K. From ref 11f.
az‑2H)2‑PtII(Q‑H)]2−, with the transition taking place from
weaker in intensity (Table 5). Hence, replacing Pd(II) in 3 by
Pt(II) in 1 results in an improvement of the oscillator strengths
of the transitions, possibly due to better orbital overlap in 3.11f
Introduction of a second “Pt(Q‑H)” unit on going to 2 leads to
a doubling of intensity of all the absorption bands (Figure S3
(Supporting Information), Figure 10, and Table 5). Addition-
ally, all absorption bands (except those appearing in the UV
region) exhibit a bathochromic shift on going from the
mononuclear complex 1 to the dinuclear complex 2; the
main bands in the UV−vis region for 2 appear at 612, 550, 404,
and 340 nm. Additionally, there is a weak band at 913 nm in
the near-IR region. The band patterns are similar for both
complexes. Relevant calculated frontier orbitals for the
transitions observed in 2 are displayed in Figure 11. The
LUMO and LUMO+1 orbitals of 2 are localized on the az‑2H
and Q‑H orbitals, respectively. TD-DFT supports the assign-
ment of the band at 612 nm as a predominantly dπ(Pt) to
π*(az‑2H) MLCT transition (Figure 11 and Table 6). The band
at 550 nm has a mixed MLCT/LLCT character, and that at 404
nm has a predominantly dπ(Pt) to π*(Q‑2H) MLCT character.
The one-electron reduction of 1 leads to a slight decrease in
the intensity of the band at 354 nm. The bands at 486 and 527
nm disappear on one-electron reduction, and a new band
appears at 604 nm (Figure 8 and Table 5). This band is
assigned to a mixed MLCT/LLCT transition. Whereas 1 is
2−
(az‑2H
)
to (Q‑H). The presence of an additional electron on
az‑2H on going from 2•− to 22− (see EPR Spectroscopy and
Spin-Density Calculations) would account for the bath-
ochromic shift and the increase in intensity of the LLCT
band. All bands in the visible region lose intensity on reducing
2•− to 22−. Reduction of 22− to 23− leads to a decrease in the
intensity of the LLCT band, as would be expected for a
formulation [(Q‑H)•−PtII(μ-az‑2H)2‑PtII(Q‑H)]3−, which contains
one hole less on Q‑H than in 22−. Two new bands appear for
23− at 954 and 1072 nm, and these are tentatively assigned to
MLCT transitions.
CONCLUSIONS
■
Summarizing, we have presented here the mononuclear Pt(II)
complex 1 and the dinuclear Pt(II) complex 2, which contain
different redox-active ligands. 2 is a rare example of a metal
complex where the ligand az acts as a bridging ligand in its
doubly deprotonated form. Structural characterization of the
complexes shows bond localization within the Q‑H ligand in
comparison to free Q. A combination of electrochemical, EPR
spectroelectrochemical, and DFT studies was used to unravel
the electronic structures of these complexes in the various
redox states (together with the Pd(II) analogue 3), and the
results can be summarized as shown in Scheme 2.
Figure 9. Relevant calculated frontier orbitals (canonical orbitals) for the optical transitions observed in 1 (see Table 6 for details).
F
dx.doi.org/10.1021/om4009179 | Organometallics XXXX, XXX, XXX−XXX

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Article
Table 6. Main TD-DFT Calculated Transitions of 1 and 2 in Comparison with Experimental Data
a
transition maximum
(nm)
oscillator
strength
exptl transition maximum
molar abs coefficient (M−1 cm−1), band
a
compd main contributing excitation (%)
(nm)
assignment
1
HOMO-1 → LUMO (57)
HOMO-2 → LUMO (22)
HOMO-2 → LUMO (46)
HOMO-1 → LUMO (25)
HOMO-2 → LUMO (34)
HOMO-3 → LUMO (31)
HOMO-4 → LUMO (35)
HOMO-1 → LUMO+1 (25)
HOMO-4 → LUMO+1 (40)
HOMO-2 → LUMO (80)
HOMO-4 → LUMO (72)
HOMO-3 → LUMO+1 (53)
mixed
502
473
438
398
0.172
0.163
0.218
0.106
527
486
446
387
10100, M/L → L
18900, M/L →L
sh, M → L
19200, M/L → L
325
599
520
420
331
0.135
0.678
0.349
0.187
0.228
354
612
550
404
340
19200, M/L → L
41700, M → L
29000, M/L → L
33600, M → L
23800
2
a
Data from OTTLE spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6.
simple electron-transfer steps. Absorption in the visible as well
as the near-IR regions can be switched on and off by
performing redox processes in the complexes, making them
highly electrochromic.15 It is the combination of different
redox-active chromophores that makes this electrochromic
behavior possible. We note that all electron transfer processes
that lead to the electrochromic behavior in these complexes are
ligand based.
EXPERIMENTAL SECTION
■
General Considerations. The ligand Q11d and the complex
[(az‑H)Pt(μ-Cl)2Pt(az‑H)]2 were prepared according to reported
procedures. All other reagents are commercially available and were
used as received. All solvents were dried and distilled using common
techniques unless otherwise mentioned.
Instrumentation. 1H and 13C NMR spectra were recorded at
JEOL LAMBDA 400 (400 MHz) instrument. Cyclic voltammetry was
carried out in 0.1 M Bu4NPF6 solution using a three-electrode
configuration (glassy-carbon working electrode, Pt counter electrode,
Ag wire as pseudoreference) and PAR 273 potentiostat and function
generator. The ferrocene/ferrocenium (Fc/Fc+) couple served as
internal reference. The EPR spectroelectrochemistry for 1•− and 12−
was carried out in a cell using a laminated Pt-mesh working electrode,
a Pt-wire counter electrode, and a silver-wire quasi-reference electrode.
The cell was filled with a solution in a glovebox and then tightly closed
and transferred into the cavity of an X-band EPR spectrometer (EMX
Bruker, Germany), where the spectroelectrochemical measurements
were done. Potentials were controlled by a HEKA potentiostat/
galvanostat system (HEKA Elektronik, Lambrecht/Pfalz, Germany).
Chemical generation of paramagnetic 2•− was carried out by reaction
of the substance with cobaltocene under an argon atmosphere. EPR
spectra were recorded with a Bruker System EMX instrument.
Simulations of EPR spectra were done using the EasySpin program.16
UV−vis−near-IR absorption spectra were recorded on an Avantes
spectrometer system: Ava Light-DH-BAL (light source), AvaSpec-
ULS2048 (UV−vis detector), and AvaSpec-NIR256-2.5TEC (near-IR-
detector). Spectroelectrochemical measurements were carried out
using an optically transparent thin-layer electrochemical (OTTLE)
cell.17 Elemental analysis was performed on an ELEMENTAR Vario
EL III instrument. Mass spectrometry experiments were carried out on
an Agilent 6210 ESI-TOF mass spectrometer (Agilent Technologies,
Santa Clara, CA).
Figure 10. Changes in the UV−vis−near-IR spectrum of 2 during the
reduction steps from OTTLE spectroelectrochemistry in CH2Cl2/0.1
M Bu4NPF6.
The locus of reduction can thus be shifted by changing either
the metal center (1 vs 3) or the nuclearity of the metal
complexes (1 vs 2). Furthermore, the complexes have been
shown to absorb light in either the visible or the near-IR region
depending on their redox states. Thus, the position and
intensity of light absorption in these complexes can be tuned by
Synthesis of Complexes 1 and 2. A mixture of ligand Q (46 mg,
0.2 mmol) and KO-t-Bu (24 mg, 0.2 mmol) in tetrahydrofuran (15
mL) was stirred for 8 h at room temperature. The suspension changed
from deep red to yellow-orange. The solvent was removed in vacuo.
The complex [Pt(az‑H)(μ-Cl)]2 (82 mg, 0.1 mmol) and dichloro-
methane (15 mL) were added to the yellow precipitate. The reaction
G
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Figure 11. Relevant calculated frontier orbitals (canonical orbitals) for the optical transitions observed in 2 (see Table 6 for details).
Scheme 2. Redox Scheme
mixture was stirred for 24 h at room temperature. The solution turned
black. The solution was filtered, and the solvent was evaporated. The
black residue was extracted with 10 mL of n-heptane, the suspension
was filtered, and the filtrate was concentrated. The deep red complex 1
was recrystallized from dichloromethane/n-hexane (2 mL/10 mL).
The precipitate was filtered, washed with n-hexane, and dried in vacuo.
Yield: 44.4 mg (0.069 mmol, 34.5%). 1H NMR (400 MHz, CDCl3): δ
1.29 (d, 6H, J = 6.5 Hz, i-Pr); 1.66 (d, 6H, J = 7.1 Hz, i-Pr); 3.65
(multiplet, 1H, J = 6.6 Hz, i-Pr); 4.90 (sept, 1H,3J = 7.1 Hz, i-Pr); 5.42
(s, 1H, quinone); 5.47 (s, 1H, quinone); 6.58 (d, 1H, J = 7.8 Hz, NH);
7.24 (m, 1H, azobenzene); 7.35 (m, 1H, azobenzene); 7.49 (m, 4H,
azobenzene); 7.91 (m, 2H, azobenzene); 8.07 (dd, 1H, J = 7.7 Hz, J =
1.4 Hz, azobenzene). 13C{1H} NMR (100 MHz, CDCl3): δ 21.67;
21.97; 31.01; 44.39; 57.33; 87.60; 103.41; 124.34; 124.94; 128.35;
129.56; 130.97; 132.31; 132.75; 140.47; 145.83; 151.67; 165.53;
170.92; 178.94; 182.21. Anal. Calcd for C24H26N4O2Pt·0.5CH2Cl2: C,
45.98; H, 4.25; N, 8.75. Found: C, 45.86; H, 4.24; N, 8.68. HRMS
(ESI): calcd for C24H27N4O2Pt ([M + H]+), m/z 598.1778; found, m/
z 598.1798.
of 2. Intensity data were collected at 100(2) K on a BRUKER Smart
AXS diffractometer (graphite-monochromated Mo Kα radiation, λ =
0.71073 Å). Crystallographic and experimental details for the
structures are summarized in Table S1 (Supporting Information).
Structures were solved by direct methods (SHELXS-97) and refined
by full-matrix least-squares procedures (based on F2, SHELXL-97).18
CCDC 933111 and 933110 contain CIF files for this paper. All data
can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_requests/cif.
DFT Calculations. DFT calculations were performed with the
ORCA 2.9.1 program package19 using the BP86 and B3LYP
functionals for the geometry optimization and single-point calcu-
lations, respectively.20 Empirical van der Waals corrections were
utilized for the geometry optimization.21 Default convergence criteria
were set for the geometry optimization (OPT) and tight convergence
criteria for SCF calculations (TIGHTSCF). Relativistic effects were
included with the zeroth-order relativistic approximation (ZORA).22
Triple-ζ valence basis sets with polarization functions (TZVPP-
ZORA)23 were employed for all atoms. The resolution of the identity
approximation24 was used with matching auxiliary basis sets. Time-
dependent DFT (TD-DFT) was employed to calculate the low-lying
excitation energies. Solvent effects were taken into account with the
conductor-like screening model (COSMO)25 for all calculations. Spin
After the extraction with n-heptane, the blue residue was
recrystallized from chloroform/n-hexane (5 mL/15 mL). The blue
compound 2 was filtered, washed with n-hexane, and dried in vacuo.
1
Yield: 24.3 mg (0.024 mmol, 24%). H NMR (400 MHz, CDCl3): δ
densities were calculated according to the Lowdin population
̈
1.32 (d, 6H, J = 6.4 Hz, i-Pr); 1.66 (d, 6H, J = 7.1 Hz, i-Pr); 3.67
(multiplet, 1H, J = 6.6 Hz, i-Pr); 5.02 (sept, 1H, J = 7.1 Hz, i-Pr); 5.50
(s, 1H, quinone); 5.65 (s, 1H, quinone); 6.64 (d, 1H, J = 8.1 Hz, NH);
7.27 (m, 2H, azobenzene); 7.49 (m, 1H, J = 7.7 Hz, azobenzene); 9.14
(m, 1H, azobenzene). 13C{1H} NMR (100 MHz, CDCl3): δ 14.19;
21.90; 21.98; 22.73; 31.66; 44.49; 53.49; 57.95; 88.20; 103.43; 125.37;
126.75; 131.75; 145.85; 178.99; 182.22. Anal. Calcd for
C36H42N6O4Pt2: C, 42.69; H, 4.18; N, 8.30. Found: C, 42.76; H,
4.41; N, 8.30. HRMS (ESI): calcd for C36H43N6O4Pt2 ([M + H]+), m/
z 1013.2630; found, m/z 1013.2625.
analysis.26 Molecular orbitals and spin densities were visualized with
the Molekel program.27
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files and tables giving the crystallographic details for 1 and
2, figures giving 1H NMR spectra of 1 and 2, a spin density plot
of 3•−, and UV−vis−near-IR spectra of 1 and 2, and tables
showing a comparison of calculated and experimental bond
lengths and bond angles for 1 and 2 and coordinates of DFT
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
X-ray Crystallography. Single crystals of 1 were obtained by
layering a dichloromethane solution of 1 with n-hexane (1/5) and
allowing for slow diffusion at ambient temperature. Single crystals of 2
were grown by slow diffusion of diethyl ether into an acetone solution
H
dx.doi.org/10.1021/om4009179 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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(b) Kauf, T.; Braunstein, P. Inorg. Chem. 2011, 50, 11472.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for B.S.: Biprajit.sarkar@fu-berlin.de.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are indebted to the Carl-Zeiss Stiftung (doctoral stipend for
N.D.) and the Fonds der Chemischen Industrie (Chemiefonds-
stipendium for M.G.S.) for financial support of this project. The
ZEDAT, FU Berlin, is acknowledged for access to the
computing center. We are grateful to Prof. Dr. L. Dunsch for
access to the instrumental facilities of IFW Dresden for the
EPR measurements of 1•− and 12−.
́
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