Charge Transfer Properties of Triarylamine Integrated Dimolybdenum Dyads

Article abstract of DOI:10.1021/acs.inorgchem.7b00913

Three quadruply bonded dimolybdenum complexes equipped with a triarylamine pendant, [(DAniF)₃Mo₂(μ-O₂CC₆H₄N(C₆H₄CH₃)₂] (DAniF = N,N′-di(p-anisyl)formamidinate; [OO-ph-N]), [(DAniF)₃Mo₂(μ-OSCC₆H₄N(C₆H₄CH₃)₂] ([OS-ph-N]), and [(DAniF)₃Mo₂(μ-S₂CC₆H₄N(C₆H₄CH₃)₂] ([SS-ph-N]), have been synthesized and characterized by single crystal X-ray diffraction. In electrochemical measurements, the redox couple for the organic amine group becomes irreversible, reflecting the substantially strong electronic interaction between the dimetal center and organic redox site. The potential difference for the two successive redox events, ca. ?E_1/2(E_1/2(2)(N/N⁺) - E_1/2(1)(Mo₂^IV/V)), falls in the range of 0.5-0.8 V as estimated from the differential pulse voltammograms. For the monocation radicals [OO-ph-N]⁺, [OS-ph-N]⁺, and [SS-ph-N]⁺, obtained by chemical oxidation of the neutral precursor, a broad ligand (amine) to metal (Mo₂) charge transfer (LMCT) absorption band is observed in the near-IR region. Interestingly, analogous to the intervalence charge transfer (IVCT) bands for mixed-valence complexes, the LMCT absorption bands, which are solvent dependent, decrease in energy and bandwidth as the electronic coupling between the two redox sites increases in an order of increasing S content in the chelating group. The electronic coupling matrix elements (H_ab) are determined by optical analyses from the generalized Mulliken-Hush (GMH) theory, falling in the range of 400-800 cm^-1 in CH₂Cl₂. These results indicate that in these radical cations the charge is localized. Time-dependent DFT calculations show that the frontier molecular orbitals for these asymmetrical donor-acceptor systems have unbalanced distribution of electron density, and the LMCT bands arise from an electronic transition from the pendant ligand-based to metal-based molecular orbitals, corresponding to donor (N)-acceptor (Mo₂) charge transfer.

Full text of DOI:10.1021/acs.inorgchem.7b00913

Article
pubs.acs.org/IC
Charge Transfer Properties of Triarylamine Integrated
Dimolybdenum Dyads
Lu Chen, Suman Mallick, Ying Ning Tan, Miao Meng, and Chun Y. Liu*
Department of Chemistry, Jinan University, 601 Huang-Pu Avenue West, Guangzhou-510632, China
*
S Supporting Information
ABSTRACT: Three quadruply bonded dimolybdenum complexes
equipped with a triarylamine pendant, [(DAniF) Mo (μ-O CC H N-
3
2
2
6
4
(
C H CH ) ] (DAniF = N,N′-di(p-anisyl)formamidinate; [OO−ph−
6
4
3 2
N]), [(DAniF) Mo (μ-OSCC H N(C H CH ) ] ([OS−ph−N]), and
3
2
6
4
6
4
3 2
[
(DAniF) Mo (μ-S CC H N(C H CH ) ] ([SS−ph−N]), have been
3
2
2
6
4
6
4
3 2
synthesized and characterized by single crystal X-ray diffraction. In
electrochemical measurements, the redox couple for the organic amine
group becomes irreversible, reflecting the substantially strong electronic
interaction between the dimetal center and organic redox site. The
potential difference for the two successive redox events, ca.
•
+
IV/V
ΔE (E (2)(N/N ) − E (1)(Mo
)), falls in the range of
1/2 1/2
1/2
2
0
.5−0.8 V as estimated from the differential pulse voltammograms. For
+ +
the monocation radicals [OO−ph−N] , [OS−ph−N] , and [SS−ph−
+
N] , obtained by chemical oxidation of the neutral precursor, a broad
ligand (amine) to metal (Mo ) charge transfer (LMCT) absorption band is observed in the near-IR region. Interestingly,
2
analogous to the intervalence charge transfer (IVCT) bands for mixed-valence complexes, the LMCT absorption bands, which
are solvent dependent, decrease in energy and bandwidth as the electronic coupling between the two redox sites increases in an
order of increasing S content in the chelating group. The electronic coupling matrix elements (H ) are determined by optical
ab
−1
analyses from the generalized Mulliken−Hush (GMH) theory, falling in the range of 400−800 cm in CH Cl . These results
2
2
indicate that in these radical cations the charge is localized. Time-dependent DFT calculations show that the frontier molecular
orbitals for these asymmetrical donor−acceptor systems have unbalanced distribution of electron density, and the LMCT bands
arise from an electronic transition from the pendant ligand-based to metal-based molecular orbitals, corresponding to donor
(
N)−acceptor (Mo ) charge transfer.
2
INTRODUCTION
electron transfer by optical analyses of the intervalence charge
■
4
,7,8
transfer (IVCT) absorption band.
Aiming at this goal,
Study of charge transfer spectra is of fundamental importance
in chemistry because it provides an indispensable approach to
probe the electronic structures of the molecule or molecular
system, on the basis of which its chemical and chemically
related properties are elucidated. Intensive research practices in
this direction have developed an active intersection of modern
chemistry for experimentalists and theorists, that is, donor−
acceptor electron transfer (ET) crossing a significant
interpretation of the spectroscopic behavior of various D−B−A
systems, in terms of electronic and vibronic transitions from the
ground state to the excited state, are the essentials of
4
,9,10
investigation.
The majority of the studied D−B−A systems are dinuclear
metal MV complexes with the Creutz−Taube ion, [(NH ) Ru-
3
5
5+
11,12
(
pyrazine)Ru(NH ) ] , as the prototypes.
MV compounds
3
5
1
−3
consisting of two organic redox termini, typically bridged
distance,
which is ubiquitous in artificial and naturally
5
,13
tertiaryl diamine,
become an important subgroup of this
existing compounds, materials, and systems. Long-distance
electron transfer has been studied using the simplest mixed-
valence model (MV) compounds, of which the reduced redox
site serves as the electronic donor (D) and the oxidized one as
the acceptor (A), and ET proceeds crossing the bridge (B)
array. With two identical chemical motifs as the donor and
acceptor, most of the D−B−A systems are structurally
symmetrical. Asymmetrical MV D−B−A systems may result
from heterodinuclear metal centers with similar electronic
14
15
4
,5
configurations, such as Ru−Os, Ru−Fe, and organic radicals
connecting D and A.
According to the Marcus−Hush
1
,6
in which the two different structural termini have the same
theory,
electron transfer may occur nonadiabatically
1
6,17
redox centers.
Asymmetrical D−B−A systems also include
18
(
optically) or adiabatically (thermally) depending on the D−
A electronic coupling, which is quantitatively evaluated by the
metal−organic hybrids and various redox active molecular
electronic coupling matrix element H . A strong research
ab
motivation in this field is to derive the kinetic data, e.g.,
Received: April 10, 2017
activation energy (ΔG*) and rate constant (k ), for D−A
et
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b00913
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
9
+
+
dyads, which strictly speaking, may not be termed as mixed-
valence compounds. Overall, relative to symmetrical systems,
studies of asymmetric D−B−A systems are much less
documented, especially those with mixed inorganic−organic
redox sites. Therefore, we are not quite knowledgeable on the
nature of charge transfer between the two distinct redox
partners and the pertinent electronic and vibronic transitions.
While development of novel D−B−A systems has long been a
research focus in the hope of expanding the libraries, creation
and investigation of structurally and electronically well-defined
asymmetrical electro-active donor−acceptor systems are of
particular interest.
N] , and [SS−ph−N] , were obtained by one-electron
chemical oxidation of the neutral compounds. Our study
shows that this electro-active donor−acceptor system can be
treated as asymmetrical mixed-valence systems, in terms of
donor−acceptor charge transfer behavior. For each of them, a
broad ligand (amine) to metal (Mo ) charge transfer (LMCT)
2
absorption band with significant vibronic IVCT band
characters, which is absent in the neutral precursors, was
observed in the near-IR spectra. Optical analyses based on the
generalized Mulliken−Hush (GMH) theory gave the
electronic coupling constants (H ) falling in the range of
00−800 cm . In the series, the electronic coupling increases
with increasing the S content, as indicated by the electro-
chemical and spectroscopic properties and the magnitude of
2
3
ab
−1
4
For the reasons mentioned above, we are interested in the
study of electronic coupling and electron transfer in asym-
metrical MV D−B−A systems. Recently, by employing a
quadruply bonded Mo unit as the electron donor and acceptor,
we have investigated the thermally induced electron transfer in
weakly coupled asymmetrical [Mo ]−bridge−[Mo ] systems
by optical analyses within the semiclassical theoretical frame-
work, which validates the existing theories. We also explored
the mixed-valence properties for three strongly coupled
asymmetrical Mo dimers. In these systems, symmetrical
IVCT absorption bands are observed, which correspond in
energy to the electronic transition between two δ-based
molecular orbitals. Significantly, it is found that in these
strongly coupled systems, donor−acceptor electron transfer
occurs because the redox asymmetry creates two charge-
unbalanced Mo centers, in contrast to symmetrical Class III
systems in Robin-Day’s scheme, in which charge is equally
distributed over the two redox centers.
H . DFT calculations show that the LMCT band arises from
ab
electronic transition between the molecular orbitals (MOs)
involving the two redox centers, which corresponds to N(π) →
2
Mo (δ) charge transfer, analogous to the IVCT bands in MV
2
2
2
systems. This study offers detailed understanding on the
donor−acceptor charge transfer characteristics in metal−
organic electro-active D−B−A systems.
20
2
1
2
RESULTS AND DISCUSSION
■
Molecular Design and Synthesis. With well-defined
redox properties, both the quadruply bonded Mo complex
unit and organic triarylamine group are favorable compo-
nents for construction of D−B−A assembling for the study of
mixed valency and electron transfer.
these symmetrical Mo −Mo and bridged diamine compounds
has increased the diversity of the D−B−A systems and enriched
the chemistry in these field. In an attempt to incorporate these
two different redox sites to a single molecule to create the
inorganic−organic D−B−A system, the triarylamine moiety
should be functionalized with a chelating group that is able to
2
8
13
8
,24,25
The involvement of
2
2
2
2
2
In this report, a series of three dimolybydenum compounds
with mixed inorganic−organic redox centers was developed by
integrating a triarylamine group via a carboxylate or thiolated
carboxylate chelating group, which are [(DAniF) Mo (μ-
3
2
bind to a Mo unit by equatorial coordination. Obviously,
2
O CC H N(C H CH ) )] (DAniF = N,N′-di(p-anisyl)-
2
6
4
6
4
3 2
carboxylate is the most suitable candidate that can be readily
introduced. Thus, 4-(di-p-tolylamino)benzoic acid
HO CC H N(C H CH ) is considered to be a free ligand
formamidinate) ([OO−ph−N]), [(DAniF) Mo (μ-
3
2
OSCC H N(C H CH ) )] ([OS−ph−N]), and [(DAn-
6
4
6
4
3 2
2
6
4
6
4
3 2
iF) Mo (μ-S CC H N(C H CH ) )] ([SS−ph−N]). These
3
2
2
6
4
6
4
3 2
for assembling with a Mo unit. Stepwise thiolation of the
carboxylic group yields the derivatives, which changes redox
potentials of the Mo unit. This strategy has been successfully
used to expand the family of Mo dimers in our recent study,
2
compounds were characterized by single crystal X-ray
diffractions; the molecular skeleton for the series is shown in
Figure 1. The monocation radicals, [OO−ph−N] , [OS−ph−
+
2
8
2
because the energy of the δ electrons of the Mo−Mo quadruple
bond is very sensitive to variation of the coordination
environment. The free acidic ligands in this study can be
generally denoted as HEE′CC H N(C H CH ) (EE′ = OO
6
4
6
4
3 2
(
4), OS (6), or SS (9)) and were synthesized as schematized in
Figure 2.
Acid 4 can be easily prepared by hydrolysis of the methyl
ester 3 that can be synthesized by the coupling reaction of di-p-
tolylamine 1 with p-iodo-methylbenzoate (2) in the presence of
26
the copper catalyst. Preparation of the thiobenzoic acid 6
started with 4, which was first converted to the chloride
derivative 5 using oxalyl chloride. Thiolation of 5 with
27
thioacetamide gave the target molecule in good yield. For
9
, the dithiocarboxylic group was introduced by the Grignard
28
precursor 8 reacting with CS2. For these reactions (Figure 2),
generally good yields can be obtained as long as the necessary
reaction conditions are retained. For example, for the reaction
with the Grignard reagent involved, the solvents and reagents
used must be dried thoroughly. With the availability of these
free ligands, a general procedure was adopted for preparation of
the Mo2 complexes with an arylamine pendant,
Figure 1. A molecular skeleton for the studied [Mo (DAniF) ][N-
2
3
(
(
(
C H CH ) ] (μ-O CC H ) ([OO−ph−N]), [Mo (DAniF) ][N-
6
4
3
2
2
6
4
2
3
C H CH ) ](μ-OSCC H ) ([OS−ph−N]), and [Mo (DAniF) ][N-
6
4
3
2
6
4
2
3
C H CH ) ](μ-S CC H ) ([SS−ph−N]) complexes.
6
4
3
2
2
6
4
B
DOI: 10.1021/acs.inorgchem.7b00913
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Inorganic Chemistry
Article
Figure 2. (i) Cu, anhydrous K CO , crystalline iodine, p-xylene, 200 °C reflux, 24 h. (ii, a) KOH, 60 °C, 12 h (b) HCl. (iii) (COCl) , DCM, r.t., 15
2
3
2
h. (iv, a) CH CSNH , THF, r.t., 12 h; (b) NaOH, HCl. (v) Mg, crystalline iodine, THF, 80 °C reflux, 2 h. (vi) (a) CS , r.t., 2 h; (b) HCl.
3
2
2
[
=
=
Mo (DAniF) (μ-EE′CC H N(C H CH ) ], [OO−ph−N] (E
The sum of the bond angles (∠C−N−C) about the amine N
atom in [OO−ph−N], [OS−ph−N], and [SS−ph−N] is
about 360°, which is compatible with the triarylamine
2
3
6
4
6
4
3 2
E′ = O), [OS−ph−N] (E = O, E′ = S), and [SS−ph−N] (E
E′ = S), described as follows:
18,19
structures in other systems,
atom adopts essentially an sp hybridization which endows a
indicating that the amine N
2
[
Mo (DAniF) (O CCH )] + HEE′CC H N(C H CH )
2
3
2
3
6
4
6
4
3 2
significant participation in the N−Mo charge-transfer. It is also
2
+
NaOCH → [Mo (DAniF) (μ‐EE′CC H
3
2
3
6
4
noted that there is a small dihedral angle (ϕ ) defined by the
1
Mo chelating ring and the phenyl group, 9.0° for [OO−ph−
N(C H CH ) ] + CH CO Na + CH OH
2
6
4
3 2
3
2
3
N], 6.1° for [OS−ph−N], and 12.0° for [SS−ph−N].
Relatively large torsion angles (ϕ ) between the central phenyl
The usage of the mixed-ligand Mo2 complex
Mo (DAniF) (O CCH )] as the starting material guarantees
2
group and the plane defined by the three N−C bonds, 39.6°,
[
2
3
2
3
3
1.5°, and 59.4°, are found for [OO−ph−N], [OS−ph−N],
a convergent assembling between the two redox components
under mild reaction conditions, a procedure similar to
and [SS−ph−N], respectively. The smallest ϕ and ϕ in [OS−
1
2
29
ph−N] is an indication of better conjugation of this compound
along the charge transfer axis at least in the solid state. The
C(4)−N(7) distances, ∼5.7 Å, which may be defined as the
preparation of the dicaboxylate bridged Mo dimers.
2
Molecular Structures. Single crystals suitable for crystallo-
graphic studies were obtained by slow diffusion of ethanol into
a dichloromethane solution of the compound. Crystals of
“
edge to edge” distance with respect of the two redox sites, are
close for the three compounds. As expected, the “center to
center” distance between the two redox sites increases in order
as the S atom is stepwise introduced, that is, 8.435 Å for [OO−
ph−N], 8.657 Å for [OS−ph−N], and 8.949 Å for [SS−ph−
N].
[
OO−ph−N] and [OS−ph−N] belong to the triclinic Pı space
̅
group with the molecules residing in a general position (Z = 2)
in the unit cell, while with the [SS−ph−N] crystal, the
molecule sits in a special position of the monoclinic C2/c space
group (Z = 4). The molecular structures are shown in Figure 3.
The crystallographic data and refinement parameters are given
in Table S1, and the selected bond parameters are summarized
in Table S2. The Mo−Mo bonds of these three complexes are
Electrochemical Studies. Figure 4 shows the cyclic
voltammograms (CVs) and differential pulse voltammograms
(DPVs) for [OO−ph−N], [OS−ph−N], and [SS−ph−N] in
24
about 2.10 Å in length, as expected for the quadruple bond
CH Cl . All compounds display two redox processes within the
2 2
and increase slightly with introduction of the S atom(s) to the
chelating ligand. In [OO−ph−N], the Mo−O bonds (2.143(4)
and 2.136(4) Å) are significantly longer than those of [OS−
ph−N] (2.106(4) Å). A plausible reason for this is the degree
of d(δ)−p(π) orbital interaction between atoms Mo and O in
the thiolated species. A similar phenomenon is also observed
for the Mo−S bonds of [OS−ph−N] (2.483(2) Å and [SS−
ph−N] (2.446(2) Å). The triarylamine segment of the three
measuring window, one reversible redox couple at 0.3−0.35 V
and one irreversible peak separated by 0.5−0.75 V. The half-
wave potentials are estimated from the DPVs and summarized
in the Table 1. As is known, both the Mo complex unit and
2
arylamine segment are electrochemically reversible redox
3
0
centers in the symmetrical systems. Oxidation of Mo2
complexes [Mo (DAniF) (EE′CC H )] (EE′ = O, S) occurs
2
3
6
5
at 0.4−0.7 V, increasing as the carboxylate group is stepwise
25a
compounds adopts the common propeller-shaped structure.
thiolated (Figure S1). The half-wave potentials (E1 ) for acidic
/2
C
DOI: 10.1021/acs.inorgchem.7b00913
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Inorganic Chemistry
Article
Figure 4. CVs and DPVs of [OO−ph−N] (red), [OS−ph−N] (blue),
and [SS−ph−N] (green).
Table 1. Data from the Electrochemical Measurements
a
compound
E1/2 (1; V) E1/2 (2; V) ΔE1/2 (mV)
KC
8
1
1
[
[
[
OO−ph−N]
OS−ph−N]
SS−ph−N]
0.29
0.28
0.36
0.81
1.03
0.97
520
750
610
6.5 × 10
5.1 × 10
2.1 × 10
2
0
a
Comproportionation constant K was determined as K = eΔE1/2/25.69.
C
c
the dithio chelating group is able to lower the electron
deficiency of the Mo25+ center. Therefore, the electrochemical
data indicate that in the monocation radicals, the organic redox
center serves as the electron donor, while the Mo center is the
2
acceptor.
The potential differences (ΔE ) between the two redox
1/2
centers, 0.52 V for [OO−ph−N], 0.75 V for [OS−ph−N] and
0
.61 V for [SS−ph−N], are determined by Taube−Richardson
31
methods from the DPV working curves. It is noted that in the
series, the monothiolated species, rather than [SS−ph−N], has
the largest ΔE
value, in disagreement with the trend
1/2
8
observed in other O/S series with Mo centers. For the
2
asymmetrical systems, there are three major terms that
contribute to the ΔE1 value, that is, electrostatic interaction,
/2
electron resonance, and internal potential difference. The
electron resonance effect accounts for the electronic coupling,
whereas the internal potential difference between the donor
and acceptor defines the redox asymmetry. When similar Mo ···
2
N separation is assumed for the series, the electrostatic effect
should be similar for the three complexes. The internal
Figure 3. X-ray crystal structures for [OO−ph−N] (top), [OS−ph−
N] (middle), and [SS−ph−N] (below).
potential difference (ΔE ) refers to the redox potential
IP
difference between the noninteracting donor and acceptor,
which corresponds to the free energy difference (ΔG°) for the
ligands HEE′CC H N(C H CH ) (EE′ = O, S) range from
6
4
6
4
3 2
0.9 to 1.1 V and decrease with the acidic group changing from
diabatic states. One of the practical ways to determine ΔE is
IP
carboxylic to dithiocarboxylic (Figure S1). By referring to these
data, it is clear that the low potential one-electron oxidation is
to measure the potential difference for the reference
compounds which are structurally and electronically similar to
due to the Mo24 → Mo2 process, which is reversible.
+
5+
32
the donor and acceptor.
In this study, complexes
•+
Notably, by assembling with the Mo unit, the N/N redox
2
[
Mo (DAniF) (EE′CC H )] and free ligands HEE′CC H N-
2
3
6
5
6
4
process of the amine component becomes irreversible.
(
C H CH ) (EE′ = OO, OS, and SS) can be used as the
•
•
6
4
3 2
Presumably, significant
N
→ Mo25+ charge transfer occurs in
reference compounds for the Mo and arylamine redox centers,
2
the electrochemically generated radical species, and thus, the
irreversibility is attributed to electronic interaction between the
two redox centers. This hypothesis is supported by the better
respectively. Then, the ΔE values are estimated to be 0.75
IP
([OO−ph−N]), 0.65 ([OS−ph−N]), and 0.25 V ([SS−ph−
N]) (Figure S1). Clearly, the smallest ΔE for [SS−ph−N]
IP
•+
N/N redox reversibility for [SS−ph−N] (Figure 4), because
lowers its ΔE1 value to less than that for [OS−ph−N].
/2
D
DOI: 10.1021/acs.inorgchem.7b00913
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Inorganic Chemistry
Article
Spectroscopic Properties of the Neutral and Mixed-
Valence Complexes. As shown in Figure 5, the electronic
Figure 5. Electronic absorption spectra of [OO−ph−N] (red), [OS−
ph−N] (blue), and [SS−ph−N] (green).
Figure 6. X-band EPR spectra of the radical cations generated by
single oxidation of the neutral compounds. Samples were measured in
CH Cl solution.
spectra of these three compounds are quite simple, showing
only one pronounced absorption band in the visible region.
2
2
This band is likely due to the δ(Mo ) to π*(bridging ligand)
2
8
,24,33
transitions (MLCT) according to prior studies.
The
+
ph−N] . These results show that for each of them, the odd
electron is localized on the metal-based orbital, rather than the
amine N center, consistent with the electrochemical analyses.
This is different from asymmetrical Ru−N mixed-valence
MLCT energies (E ) decrease and the intensities increase as
ML
the S content increases, in a trend parallel to the reported
8
results. For [OO−ph−N], the intense absorption is observed
−
1
18a,b
at 407 nm with an extinction coefficient (ε ) of 14 410 M
ML
systems reported by the Zhong group,
in which a single
−1
−1
−1
cm , and at 514 nm (ε , 17 040 M cm ) for [OS−ph−N],
ML
electron resides on the p orbital of the amine group. The
π
−1
−1
and at 605 nm (ε , 30 093 M cm ) [SS−ph−N], as listed in
ML
variation trend of g values indicates that the extent of charge
delocalization increases in order as the chelating group changes
from OO to OS to SS. Furthermore, the g values for this series
Table 2. An additional absorption band at 407 nm is observed
Table 2. Spectroscopic Data of the IVCT Bands for [OO−
are larger than those for symmetrical and asymmetrical Mo
2
+
+
+
20,24
ph−N] , [OS−ph−N] , and [SS−ph−N] in Different
MV complexes,
indicating a significant valence detrapping
Solvent
effect due to the donor−acceptor electronic coupling.
The absorption spectra of the monocation radials are shown
in Figure 7, and the spectral data are summarized in Table 3. All
a
E
Δν (IV)
ε
IT
Hab
IT
1/2
compound
solvent
DCM
(cm−1)
(cm
4570
−1
)
(M−1cm−1) (cm−1)
[
OO−ph−
11680
13850
14513
10245
11862
12593
8605
3280
2580
2950
6900
3647
4510
10350
6270
6538
442
403
499
701
444
549
739
553
581
+
N]
THF
4675
5000
3655
4025
3699
3118
3448
3470
ACTONE
DCM
[
[
OS−ph−
+
N]
THF
ACTONE
DCM
SS−ph−
+
N]
THF
9479
ACTONE
10370
a
Calculated by eq 1.
for [SS−ph−N], which can be assigned as the intraligand
charge transfer within the pendent triarylamine ligand on the
basis of DFT calculation (vide infra).
+
Figure 7. Vis−Near-IR absorption spectra for [OO−ph−N] (red),
Single electron oxidation of these compounds with one
equivalent of ferrocenium hexafluorophosphate yielded the
+
+
[
OS−ph−N] (blue), and [SS−ph−N] (green).
+
corresponding radical monocation [OO−ph−N] , [OS−ph−
+
+
N] , and [SS−ph−N] . These radial species were characterized
the radical species display an absorption band in the range of
by X-band electron paramagnetic resonance spectra (EPR).
The EPR signals of [OO−ph−N] , [OS−ph−N] , and [SS−
ph−N] were recorded in situ at 173 K, as shown in Figure 6.
−1
1
6 000−25 000 cm with the energy and intensity similar to
those for the MLCT band for the neural compound;
accordingly, this band is assigned to the MLCT transition.
For [OO−ph−N] and [SS−ph−N] , the MLCT band is
slightly red-shifted relative to that for the neutral precursor
(Figure S2). For the two thiolated complexes [OS−ph−N]
and [SS−ph−N] , there is an intense absorption appearing at
+
+
+
+
+
All the species show symmetrical isotropic EPR signals with a g
factor less than 2.0023 for an organic radical. In the series,
species with more S atom(s) have a larger g value, 1.945 for
+
+
+
+
[
OO−ph−N] , 1.955 for [OO−ph−N] , and 1.960 for [SS−
E
DOI: 10.1021/acs.inorgchem.7b00913
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Inorganic Chemistry
Article
Table 3. Calculated Energy Levels of the Selected Frontier Molecular Orbitals by DFT/TD-DFT Calculations
MLCT
LMCT
calcd (nm)
calcd (cm−1)
EML (cm−1)
EML(ex)
a
ΔEH−L
ELM (cm−1
)
ELM(ex)
a
ΔEH−1−H
compound
[
[
[
[
[
[
OO−ph−N]
OS−ph−N]
SS−ph−N]
OO−ph−N]
24570
19455
16530
23180
19420
16080
23740
20210
16800
21700
18250
15800
23390
19920
17180
10645
10080
7905
+
11680
10245
8605
10010
9890
7940
+
OS−ph−N]
+
SS−ph−N]
a
EML(ex) and ELM(ex) represent the excitation energies calculated by TD-DFT. ΔEH−L and ΔEH−1−H are the energy differences between the two
orbitals.
−1
+
2
4 810 and 21 900 cm , respectively. The assignment of this
N] . Therefore, the observed solvent dependences of spectra
band is based on the DFT calculations (vide infra).
for the series demonstrate that unlike the MLCT band, the
LMCT band shows a vibronic IVCT band character and signals
the extent of electronic coupling between the dimetal and
amine redox centers.
Calculation of Electronic Coupling Matrix Elements
Hab. Considering the vibronic features of the LMCT bands in
All these metal−organic radical cations exhibit a broad
absorption band in the near-IR region, which is absent for the
neutral complexes, as shown in Figure 7. We can safely assign
this band to the LMCT absorption due to the charge transfer
5
+
from the pendant amine ligand to the oxidized Mo2 center. In
previous work, it was observed that a LMCT band appears
upon one-electron oxidation of the bridged Mo2 dimers,
ascribable to the electronic transition from the high-lying
bridging ligand orbital to the singly occupied δ orbital, and the
transition energy (ELM) decreases as donor−acceptor coupling
the spectra, the generalized Mulliken−Hush (GMH) theory
(
eq 1), which is applicable for both symmetric and asymmetric
17,34,37,38
MV compounds,
is employed to determine the
electronic coupling matrix element (H ) from the LMCT
ab
band parameters.
increases. For the phenylene bridged Mo dimers, stepwise
2
|μ |
erab
ab
thiolation of the bridging ligand shifts the IVCT band toward
H =
ν
ab
max
−1
8
(1)
(2)
low energy by 800 cm . Here, introducing the S atom onto
the chelating group lowers the LM transition energy by about
−1
3hcε ln 10
2000π N (n + 2)
9n
ε
ν
1
500 cm , while the band intensity increases. For each of the
0
μ =
dν
̃
∫
ab
2
2
2
three complexes, the LMCT band is appreciably broader than
the MLCT band (Figure 7; Table 2). The highest transition
̃
−
1
In eq 1, |μab| is the diabatic transition dipole moment of the
energy (E , 11682 cm ) and lowest band intensity (ε , 3380
IT
IT
−1
−1
−
1
−1
IVCT band, νmax is the absorption maximum (cm ) of the
M cm ) and broadest band profile (Δν , 4570 cm ) are
1
/2
+
+
IVCT band, e (esu) is the elementary charge of an electron, and
r (Å) is the effective ET distance. The |μ | can be calculated
observed for [OO−ph−N] ; for [SS−ph−N] , these band
parameters vary toward the opposite direction (Table 2).
Interestingly, the LMCT bands show vibronic transition
features, resembling an IVCT band in the MV analogues.
The Gaussian-shaped band profile is also similar to that for
ab
ab
from eq 2, in which h is Planck’s constant, c is the speed of
light, ε is the permittivity of the vacuum, N is the Avogadro’s
0
number, n is the refractive index of the solvent, ε is the
extinction coefficient, and the integration part is the integrated
intensity of the IVCT band. In principle, making good use of
this equation requires reasonable estimation of the effective
21,34
asymmetrical MV compounds,
bands is generally observed for moderately strongly coupled
symmetrical MV Mo dimers,
while the “cut-off” of IVCT
8a,c,33,35
as predicted from the
2
electronic coupling distance (r ). The distance between the
3
6
ab
two-state model.
two redox sites is usually used as the ET distance, which would
Solvent dependence of the IVCT absorption is a practical
criterion for distinguishing Class II and III mixed-valence
systems. In this study, in addition to dichloromethane, more
polar solvents THF and acetone were also used for the
spectroscopic measurements of the obtained monoradical
cations. As shown in Table 2, all three complexes show solvent
dependence of the LMCT band, similar to the spectral property
of the IVCT band for Class II MV compounds. Notably, in
contrast, the MLCT bands do not show similar solvent
dependences. It is found that in the polar solvents, the
lead to an underestimated Hab value because the effective
39
distance is shorter than the geometric D−A separation. For
the three complex systems, the N(amine)−C(4) distance (ca.
5
.7 Å), or the “edge to edge” separation, is used as the effective
electron transfer distance to calculate the Hab considering
d(δ)−p(π) conjugation within the [Mo ] complex unit. The
2
corresponding H values in CH Cl , THF, and acetone are
ab
2
2
−1
determined in the range of 400−800 cm , as listed in Table 2.
In the series, the variation trend of H is in accordance with the
ab
coupling strength predicated from the S content on the
chelating group. For example, in dichloromethane, the smallest
transition energy (E ) is increased and the absorption band is
LM
−
1
+
broadened (Figure S3). For each of the complex systems, the
LMCT band in acetone is higher in energy and broader (Table
one (442 cm ) is found for [OO−ph−N] and the largest H
ab
−1
+
(739 cm ) for [SS−ph−N] . The H values for present series
are compatible with those for phenylene bridged Mo dimers
ab
2). Furthermore, solvent polarity affects the band parameters of
2
+
these complexes to different extents, which reflects roughly the
coupling strength of the complex. For example, for [OO−ph−
{[Mo (DAniF) ] (μ-EE′CC H CEE′)} , namely, [OO−ph−
2
3 2
6
4
+
−1
+
−1
OO] (H , 560 cm ), [OS−ph−OS] (H , 730 cm ), and
ab
ab
+
−1
+
−1 8b
N] , the difference of E in acetone and CH Cl is 2899 cm ,
[SS−ph−SS] (H , 870 cm ). The smaller H values for
LM
2
2
ab
ab
and 2326 cm− for [OS−ph−N] , and 1791 cm for [SS−ph−
1
+
−1
these inorganic−organic radical systems reflect the weaker
F
DOI: 10.1021/acs.inorgchem.7b00913
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Inorganic Chemistry
Article
Figure 8. DFT calculated frontier molecular orbitals of [SS−ph−N], [OS−ph−N], and [OO−ph−N] models.
+
+
+
Figure 9. TD-DFT calculated frontier molecular orbitals and dominant transitions for [OO−ph−N] , [OS−ph−N] , and [SS−ph−N] models.
coupling in comparison with the related Mo dimers because
nation of H constants for metal−organic D−B−A systems, in
2
ab
the redox asymmetry increases the charge localization. In the
combining with the application of eq 2, which accounts for the
charge transfer bandwidth and intensity.
+
symmetrical series, [SS−ph−SS] is considered in the Class
II−III transition regime, judged by the half cutoff of IVCT band
DFT and TD-DFT Calculations. To gain insights into the
ground state geometries, electronic structures, and nature of
frontier molecular orbitals (FMOs) for the series, DFT and
time-dependent DFT (TD-DFT) calculations on the simplified
molecular models were performed assuming an S = 0 spin state.
The calculation models Mo (NHCHNH) [μ-EE′CC H N-
4
,8b
at 2H .
In our recent study on strongly coupled
ab
+
asymmetrical series {[Mo (DAniF) ] (μ-EE′CCEE′)} (E, E′
2
3 2
2
1
−1
=
O, S),
H
values of 1098, 1600, and 1615 cm are
ab
+
+
estimated from eq 1 for complexes [OO−OS] , [SS−OO] ,
+
and [SS−OS] , respectively. These results validate the
2
3
6
4
generalized Mulliken−Hush expression (eq 1) for determi-
(C H CH ) ] (EE′ = OO, OS, or SS) are derived by replacing
6
4
3 2
G
DOI: 10.1021/acs.inorgchem.7b00913
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Inorganic Chemistry
Article
2
the p-anisyl groups in the DAniF ligands with H atoms. Figure
the central sp N atom and extended to the chelating group
(Figure 9). This indicates that this MO is contributed by the
8
shows that the HOMO is constructed mainly by the δ
bonding orbital of the model [Mo (HNCHNH) ], while
fourth ligand of the Mo center and has nothing to do with the
2
3
2
LUMO is dominated by the π* orbital of bridging moiety
EE′CC H − (E/E′ = O or S). HOMO−1 involves the p
DAniF ligands. On this basis, the assignment of the LMCT
−
band to the β-HOMO to β-LUMO transition is confirmed.
6
4
π
−1
orbital of the triarylamine nitrogen center (−NAr , Ar =
Calculations show that the intense band at 21 900 cm for
2
+
−
C H CH ) with some admixture of the phenylene π orbitals.
[SS−ph−N] results from linear combinations of two
6
4
3
The composition of the HOMO confirms that the lower redox
potential wave (∼0.3 V) in electrochemical measurements is
transitions, α-HOMO−1 → α-LUMO and β-HOMO → β-
LUMO+1 characters, as shown in Figure S4 and Table S4.
Since the MOs involved in these transitions are contributed by
the triarylamine pendant ligand, this band may be termed as
intraligand charge transfer (ILCT) absorption. Similarly, for
5
+/4+
attributed to the Mo
redox process. Importantly, the
2
frontier orbitals prove that the complex monocations are
electronically redox active D−B−A systems with an amine N
+
+
donor and a Mo acceptor.
[OS−ph−N] and [OO−ph−N] , linear combinations of α-
HOMO−1 → α-LUMO+1 and β-HOMO → β-LUMO+1
characters give rise to the ILCT band.
2
For each of the neutral complex models, the energy gap
between the HOMO and LUMO (ΔEH−L) corresponds to
energy of the pronounced absorption band (MLCT) in the
spectrum (Table 3). TD-DFT calculations confirm that this
band arises from the HOMO → LUMO transition, thus
showing the MLCT character (Table S3). The intense high
energy absorption (407 nm) for [SS−ph−N] corresponds to
the HOMO−1 → LUMO transition in energy, as shown in
Figure 8, and thus is due to the charge transfer within the
pendant ligand, or internal ligand charge transfer (ILCT). For
Interestingly, the TD-DFT calculations show that the vertical
transition energy matches well with not only the observed
charge transfer energy but also the charge transfer between the
ground and excited states. For the monoradicals, the β-HOMO
and β-LUMO may be viewed as the ground and excited states,
respectively, which have the electron density concentrated on
the amine and dimolybdenum sites, respectively. The β-
HOMO to β-LUMO transition leads to the charge transfer
[
OO−ph−N] and [OS−ph−N], this band does not exhibit in
from the donor (N) to the acceptor (Mo
LMCT absorption band shows IVCT band characters for the
Mo MV analogues. Therefore, these results conform well to
the two-state model in semiclassical theory.
). Consistently, the
2
the spectra because the transition energies are high.
Furthermore, in symmetric Mo dimeric systems, the electronic
2
2
2
−4
coupling between two Mo centers increases with the HOMO−
However, it
2
1
−HOMO energy gap (ΔE
). Large ΔE
is found in
should be noted that this correlation between computational
and experimental results does not exist for symmetrical,
moderately strongly coupled MV systems because the derived
symmetrical electronic states (MOs) do not account for the
dynamic process of charge transfer. The LMCT band appears
with vibronic features of IVCT bands because the pendant
ligand is redox noninnocent and the redox center is
geometrically far from the dimetal unit and the LM electronic
transition states are coupled with the vibrational states.
H−1−H
H−1−H
3
3
Class III, which corresponds to the IVCT band energy.
However, this criterion is not valid for the asymmetrical [EE−
ph−N] systems, although the orbital interactions in the frontier
MOs are established in a similar manner. Here, the ΔE
H−1−H
value is correlated to the measured LMCT band energy (E )
LM
but decreases as the coupling strength increases in the series, as
shown in Table 3. These theoretical results reflect the
electronic nature of the asymmetrical, metal−organic radical
systems, with a substantial difference from the bridged Mo2
dimers.
CONCLUSIONS
In the present work, assembling a quadruply bonded Mo unit
with an arylamine segment has generated a series of three
■
The electronic properties and optical behaviors of the singly
oxidized complexes were studied by TD-DFT calculations on
the radial cation models (S = 1/2). The calculated excitation
2
compounds with a general formula of [Mo (DAniF) (μ-
2
3
+
+
+
energies for [OO−ph−N] , [OS−ph−N] , and [SS−ph−S]
EE′CC H )N(C H CH ) ] (EE′ = OO, OS, or SS), which
6
4
6
4
3 2
are listed in Table 3. For the radical species, α- and β-MOs are
named for orbitals in different electron spin states, as shown in
Figure 9. For example, the HOMO of the neutral model
corresponds to α-HOMO and β-LUMO, while the HOMO−1
becomes α-HOMO−1 and β-HOMO. It is noted that the α-
LUMOs for the three models of the series are not in the same
are all characterized by X-ray diffraction. To achieve equatorial
linkage of the amine pendant to the Mo center, the organic
2
moiety is functionalized with a carboxylate or thiocarboxylate
group; meanwhile, variation of this chelating group tunes the
electronic communication between the two redox sites. Single
oxidation of these compounds allowed systematic examination
of the optical behavior and electronic properties of the
asymmetrical D−B−A systems in which the amine and the
+
+
order. For [OO−ph−N] and [OS−ph−N] , α-LUMO+1
shows the bridging π orbital character, whereas for [SS−ph−
N] , the corresponding orbital is α-LUMO. Therefore, metal
+
Mo redox sites serve as the electron donor and acceptor,
2
(
Mo ) to bridging ligand (π*) charge transfer (MLCT)
respectively. In this series, the dimetal and amine redox sites are
moderately strongly interacted, which reduces the reversibility
2
absorption corresponds to the transition of α-HOMO → α-
LUMO for the dithiolated species, but for the other two, this
band is due to the α-HOMO → α-LUMO+1 transition. The β-
HOMO → β-LUMO transition corresponds to the LMCT
absorption in energy and electron density distribution as well,
as shown in Figure 9. For all three systems, the calculated
transition energies E (ex) are in excellent agreement with the
observed LMCT band energies (E ); for the series, both of
the calculated and measured energies decrease with increases of
the electronic coupling (Table 3). The electron density on the
β-HOMO is spread over the three phenyl groups surrounding
•
+
of the redox couple for the amine site (N/N ). The
inorganic−organic conjugation through δ−π orbital interaction
increases with increasing the S content of the chelating group,
as expected. Both of the neutral and radiccal complexes exhibit
a characteristic metal (δ) to bridging ligand (π*) charge
transfer absorption band with similar transition energies. The
monoradical species display a low energy, solvent-dependent
LMCT absorption band due to electronic transition from the
LM
LM
pendant amine ligand to the Mo center. The transition energy
2
−1
of this band decreases from 11 680 to 8605 cm with
H
DOI: 10.1021/acs.inorgchem.7b00913
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Inorganic Chemistry
Article
decreasing bandwidth and increasing intensity as the S atom is
introduced stepwise. Therefore, the LMCT band presents
significantly vibronic features of an IVCT band in MV systems.
The generalized Mulliken−Hush (GMH) formalism has been
employed to estimate the electronic coupling matrix elements
Hamiltonian was used to take account of the relativistic effect in the
4
6
calculations. These calculations employed the resolution of identity
4
7
(
RI) approximation developed by Neese. The coordinates of all DFT
energy minimized models of the complexes presented in this study are
included in Tables S5−S10. Electronic transition energies and
intensities were computed for neutral model compounds as well as
oxidized model compounds using the time-dependent DFT (TD-
−1
H , falling in the range of 400−800 cm . DFT and TD-DFT
ab
48
calculations on the reasonably simplified models show that the
key molecular orbitals are asymmetrical with unequal
distribution of the electron density over the two redox centers.
Remarkably, for all three complex systems, the observed LMCT
band energy is in good agreement with the calculated HOMO−
DFT) method within the Tamm−Dancoff approximation. These
4
9
calculations employed the B3LYP functional and ZORA (Mo),
TZVP (S, O, and N), and SVP (C and H) basis sets. Forty excited
states were calculated by including all one-electron excitations within
an energy window of ±3 hartree with respect to the HOMO/LUMO
energies. Isosurface plots of molecular orbitals were generated using
the gOpenMol program using isodensity values of 0.04.
1
(amine based) to HOMO (metal based) transition energy,
corresponding to N(π) → Mo (δ) charge transfer. Therefore,
2
Preparation of 4-(Di-p-tolylamino)-benzoic acid (4). 4-(Di-p-
in this metal−organic radical system, the LMCT band is
analogous to the vibronic IVCT band in mixed-valence systems.
This study demonstrates that the same methods of optical
analysis used for MV compounds can also be used to study the
charge transfer bands in donor−acceptor compounds.
tolylamino)-benzoic acid (4) was synthesized by a similar procedure in
26
the literaure. Di-p-tolylamine (0.90 g, 4.56 mmol), p-iodo-
methylbenzoate (1.20 g, 4.58 mmol), anhydrous K CO (0.74 g),
2
3
copper bronze (0.23 g), p-xylene (0.5 mL), and a small amount of
iodine crystals were mixed in a 100 mL flask. Under an argon
atmosphere and in the dark, the mixture refluxed at 200 °C for 24 h.
The reaction mixture was cooled down to room temperature and
filtered. The solid materials were washed with diethyl ether. The
filtrate was evaporated under reduced pressure, yielding a sticky oil
product. Purification by column chromatography (DCM/n-hexane =
EXPERIMENTAL SECTION
■
Materials and Methods. All manipulations were performed in a
nitrogen-filled glovebox or by using standard Schlenk-line techniques.
All solvents were freshly distilled over appropriate drying agents and
collected for further use under a nitrogen atmosphere. 4-Bromo-4′,4″-
dimethyltriphenylamine is obtained from a commercial resource.
1:1) gave 1.21 g of the compound (3) as viscous yellow oil (yield,
1
80%). H NMR δ (ppm in CDCl ): 7.88 (d, 2H, aromatic C−H), 7.15
3
(q, 8H, aromatic C−H), 6.99 (d, 2H, aromatic C−H), 3.90 (s, 3H,
−COOCH ), 2.38 (s, 6H, −CH ).
40
29
HDAniF and Mo (DAniF) (O CCH ) were synthesized according
2
3
2
3
3
3
to published methods.
A solution of 3 (1.21 g, 3.82 mmol) in ethanol (30 mL) was mixed
with KOH solution (1 M, 30 mL). The mixture was stirred at 60 °C
for 12 h. After completion of the reaction, the solvents were removed
under reduced pressure. To the mixture, HCl solution (1.0 M) was
added to neutralize the excess KOH by controlling pH = 2. The
solution was extracted with ethyl acetate (3 × 20 mL), and the
collected extract was dried over Na SO . Removal of the solvent gave
Physical Measurements. UV−vis−near-IR spectra were meas-
ured in CH Cl solutions using IR quartz cells with a light path length
2
2
of 2 mm on a Shimadzu UV-3600 UV−vis−near-IR spectropho-
tometer. Cyclic voltammograms (CVs) and differential pulse
voltammograms (DPVs) were performed using a CH Instruments
Model-CHI660D electrochemical analyzer in a 0.10 M CH Cl
2
2
2
4
n
1
solution of Bu NPF with a Pt working and auxiliary electrodes, a
Ag/AgCl reference electrode, and a scan rate of 100 mV/s . All
potentials are referenced to the Ag/AgCl electrode. H NMR spectra
compound 4 as a white solid. Yield: 0.93 g, 80%. H NMR δ (ppm in
4
6
−1
CDCl ): 7.90 (d, 2H, aromatic C−H), 7.14 (q, 8H, aromatic C−H),
3
1
6.96 (d, 2H, aromatic C−H), 2.36 (s, 6H, −CH ).
3
were recorded on a Bruker Avance 300 spectrometer. Elemental
analyses were determined using an Elementar Vario EL elemental
analyzer. EPR spectra were measured using a Bruker A300-10-12
electron paramagnetic resonance spectrometer. Measurements for the
mixed-valence complexes were carried out in situ after single oxidation
of the corresponding neutral compounds using ferrocenium
hexafluorophosphate (Cp FePF ).
Preparation of 4-(Di-p-tolylamino)-thiobenzoic acid (6). To a
solution of compound 4 (1.08 g, 3.46 mmol) in CH Cl (30 mL) was
2
2
added oxalyl chloride (0.72 mL, 4.20 mmol) followed by few drops of
DMF at 0 °C under a nitrogen atmosphere. The color of the mixture
changed to yellow immediately. After stirring the mixture at room
temperature for 15 h, the solvent was removed under reduced
pressure, which gives a yellow solid of compound 5. This material was
directly used for the next step without further purification.
2
6
X-ray Structure Determinations. Single-crystal data for [OS−
ph−N]·CH Cl were collected on an Agilent Xcalibur Nova
The crude product of 5 was mixed with thioacetamide (0.65 g, 8.6
mmol) in 30 mL of THF at room temperature and stirred for 12 h.
After that, NaOH solution (1M, 30 mL) was added, and the reaction
was stirred for another 1 h. Then, solvents were removed under
reduced pressure, and HCl solution (1.0 M) was added to neutralize
the excess NaOH by controlling pH = 2. The solution was extracted
with ethyl acetate (3 × 20 mL), and the collected extract was dried
over Na SO . Removal of the solvent gave compound 6 as a green
2
2
diffractometer with Cu Kα radiation (λ = 1.54178 Å) at 173 K and
for [OO−ph−N]·CH Cl and [SS−ph−N], the data were collected
2
2
on an Agilent Gemini S Ultra diffractometer with Cu Kα radiation (λ =
1
.54178 Å) at 173 K. The empirical absorption corrections were
applied using spherical harmonics, implemented in the SCALE3
41
ABSPACK scaling algorithm. All the structures were solved using
direct methods, which yielded the positions of all non-hydrogen
atoms. Hydrogen atoms were placed in calculated positions in the final
structure refinement. Structure determination and refinement were
carried out using SHELXS-2014 and SHELXL-2014 programs,
2
4
solid. The solid was purified by column chromatography (EtOAc/n-
1
hexane = 1:15). Yield: 0.87 g, 75%. H NMR δ (ppm in CDCl ): 7.73
3
(d, 2H, aromatic H), 7.15 (q, 8H, aromatic H), 6.92 (d, 2H, aromatic
42
respectively. All non-hydrogen atoms were refined with anisotropic
H), 2.37 (s, 6H,−CH ).
3
displacement parameters.
Preparation of 4-(Di-p-tolylamino)-dithiobenzoic Acid (9). 4-
(Di-p-tolylamino)-dithiobenzoic acid (9) was synthesized by a similar
Computational Details. The ORCA 2.9.1 software package was
43
28
used for all DFT computations. The geometry of the model
complexes was optimized in the gas phase without imposing any
symmetry constraints. Geometry optimizations for the complexes were
converged to the S = 0 or 1/2 spin and employed the Becke−Perdew
procedure in the literature. Magnesium (0.09 g, 3.69 mmol) and a
small amount of iodine crystals were introduced in a three-neck round-
bottomed flask. A solution was prepared separately by dissolving 4-
bromo-4′,4″-dimethyltriphenylamine (1.07 g, 2.84 mmol) in 5 mL of
dry THF. A volume of 1 mL of this solution was added to the Mg and
I2 containing flask under a nitrogen atmosphere. The mixture was
heated at 80 °C to reflux to initiate the reaction; then, the remaining
solution was added. The reaction was heated for an additional 2 h. The
reaction mixture was cooled down to room temperature, and carbon
44
(
BP86) functional and the SV(P) (Ahlrichs split valence polarized)
basis with the SV/J auxiliary basis for all atoms except for S, N, O, and
Mo atoms where the larger TZVP (Ahlrichs triple-valence polarized)
4
5
basis in conjunction with the TZV/J auxiliary basis was used. For Mo
atoms, the scaled-ZORA (zeroth-order regular approximation)
I
DOI: 10.1021/acs.inorgchem.7b00913
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
disulfide (3 mL) was added. The mixture was stirred for another 2 h at
room temperature. After completion of the reaction, HCl solution (1.0
M) was added to control the pH = 2. The solution was exacted with
ethyl acetate (3 × 20 mL), and the collected extract was dried over
Na SO . Removal of the solvent gave compound 9 as a brown oil.
ACKNOWLEDGMENTS
■
We are greatly thankful to the National Natural Science
Foundation of China (No. 21371074 and 90922010) and Jinan
University for the financial support.
2
4
1
Yield: 0.82 g, 83%. H NMR δ (ppm in CDCl ): 7.06 (m, 12H,
3
aromatic C−H), 2.33 (s, 6H, −CH ).
3
REFERENCES
General Procedure for Preparation of [OO−ph−N], [OS−
ph−N], [SS−ph−N]. A solution of sodium methoxide (0.50 mmol) in
■
(1) (a) Creutz, C.; Taube, H. Direct approach to measuring the
Franck-Condon barrier to electron transfer between metal ions. J. Am.
Chem. Soc. 1969, 91, 3988−3989. (b) Hush, N. S. Qualitative Evidence
for Intervalence-Transfer Absorption in Inorganic Systems in Solution
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1
0 mL of methanol was transferred to a solution of
Mo (DAniF) (O CCH ) (0.25 mmol) and 4-(di-p-tolylamino)-
2
3
2
3
benzoic acid (0.25 mmol) for [OO−ph−N] or (4-(di-p-tolylamino)-
thiobenzoic acid for [OS−ph−N] or 4-(di-p-tolylamino)-dithioben-
zoic acid for [SS−ph−N]) in 30 mL of THF. The solution was stirred
at room temperature for 5 h. All volatiles were removed under a
vacuum, and the residue was extracted with 20 mL of dichloromethane
and filtered through a Celite-packed funnel. After removal of the
solvent from the filtrate, the residue was washed with ethanol (3 × 15
mL) and collected by filtration. The resulting product was dried under
a vacuum.
(
2) (a) Crutchley, R. J. Intervalence Charge Transfer and Electron
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1
Yield of [OO−ph−N]: 0.21 g (67%). Yellow powder. H NMR δ
(
8
1
ppm in CDCl ): 8.48 (s, 1H, −NCHN−), 8.38 (s, 2H, −NCHN−),
3
(
3) (a) Newton, M. D. Thermal and optical electron transfer
.14 (d, 2H, aromatic C−H), 7.10 (q, 10H, aromatic C−H), 6.64 (q,
involving transition metal complexes: insights from theory and
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6H, aromatic C−H), 6.48 (d, 4H, aromatic C−H), 6.27 (d, 4H,
aromatic C−H), 3.74 (d, 18H, −OCH ), 2.34 (s, 6H, −CH ). UV−vis,
3
3
−
1
−1
λ
nm (ε, M
cm ): 407 (30387). Anal. Calcd for
max
C H N O O Mo : C, 63.67; H, 5.06; N, 7.88. Found: C, 63.57;
6
6
63
7
6
2
2
(
4) (a) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. The
H, 5.13; N, 7.91.
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88.
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A Playground for Electrons and Holes. Angew. Chem., Int. Ed. 2012, 51,
1
Yield of [OS−ph−N]: 0.24 g (74%). Red powder. H NMR δ (ppm
in CDCl ): 8.46 (s, 1H, −NCHN−), 8.37 (s, 2H, −NCHN−), 8.13
3
(
d, 2H, aromatic C−H), 7.11 (q, 8H, aromatic C−H), 6.97 (d, 2H,
5
(
aromatic C−H), 6.63 (m, 20H, aromatic C−H), 6.20 (m, 4H, aromatic
C−H), 3.73 (t, 18H, −OCH ), 2.35 (s, 1H, −CH ). UV−vis, λ nm
3
3
max
−1
−1
(ε, M cm ): 514 (35521). Anal. Calcd for C H N O OSMo : C,
66 63 7 6 2
3
26−392.
6
2.07; H, 4.94; N, 7.68. Found: C, 62.00; H, 4.89; N, 7.81.
Yield of [SS−ph−N]: 0.28 g (85%). Green powder. H NMR δ
1
(6) (a) Marcus, R. A. The Theory of Oxidation−Reduction Reactions
Involving Electron Transfer. I. J. Chem. Phys. 1956, 24, 966−978.
(
8
ppm in CDCl ): 8.46 (s, 1H, −NCHN−), 8.33 (s, 2H, −NCHN−),
3
(
8
(
b) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta, Rev. Bioenerg. 1985,
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.24 (d, 2H, aromatic C−H), 7.10 (q, 8H, aromatic C−H), 6.95 (d,
H, aromatic C−H), 6.64 (q, 16H, aromatic C−H), 6.44 (d, 4H,
2
aromatic C−H), 6.12 (d, 8H, aromatic C−H), 3.74 (d, 18H, −OCH ),
2
3
−1
−1
.35 (s, 6H, −CH ). UV−vis, λ nm (ε, M cm ): 604 (60522).
3
max
1
(
Anal. Calcd for C H N O S Mo : C, 60.49; H, 4.81; N, 7.49. Found:
66
63
7
6
2
2
C, 60.41; H, 4.30; N, 7.62.
Spectroscopic Study of δ Electron Transfer between Two Covalently
Bonded Dimolybdenum Units via a Conjugated Bridge: Adequate
Complex Models to Test the Existing Theories for Electronic
Coupling. J. Phys. Chem. C 2013, 117, 19859−19865. (b) Xiao, X.;
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00913.
■
*
S
1H NMR spectra and DFT calculations data (PDF)
2
2
Accession Codes
1
2633.
9) Creutz, C.; Newton, M. D.; Sutin, N. J. Metal-ligand and metal-
metal coupling elements. J. Photochem. Photobiol., A 1994, 82, 47−59.
10) Brunschwig, B. S.; Creutz, C.; Sutin, N. Optical transitions of
CCDC 1551557−1551559 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(
(
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AUTHOR INFORMATION
■
*
Corresponding Author
Phone: +86-020-85222191. E-mail: tcyliu@jnu.edu.cn.
1
086−1094.
5
6
(12) (a) Creutz, C. Mixed Valence Complexes of d -d Metal
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Lahiri, G. K. Unconventional Mixed-Valent Complexes of Ruthenium
and Osmium. Angew. Chem., Int. Ed. 2007, 46, 1778−1796.
ORCID
Chun Y. Liu: 0000-0001-6908-9929
Notes
The authors declare no competing financial interest.
J
DOI: 10.1021/acs.inorgchem.7b00913
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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