Syntheses and redox properties of complexes with Mo3S4 Cores and tridentate schiff base ligands

Article abstract of DOI:10.1246/bcsj.20140275

A series of potentially tridentate Schiff base ligands, {HOPhC=N-(CH₂)₃-XMe: X = S (HLSMe), Se (HLSeMe)}, was prepared by condensation reactions of salicylaldehyde and the corresponding functionalized propylamines. These ligands were

Full text of DOI:10.1246/bcsj.20140275

Syntheses and Redox Properties of Complexes with
Mo S Cores and Tridentate Schiff Base Ligands
3
4
1
1
2,3
Keisuke Kawamoto,* Akio Ichimura, Hideki Hashimoto,
Isamu Kinoshita,1,3 and Takanori Nishioka*1
1Department of Chemistry, Graduate School of Science, Osaka City University,
-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585
3
2Department of Physics, Graduate School of Science, Osaka City University,
-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585
3
3The OCU Advanced Research Institute for Natural Science and Technology (OCARINA),
Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585
E-mail: nishioka@sci.osaka-cu.ac.jp
Received: September 12, 2014; Accepted: November 3, 2014; Web Released: November 10, 2014
A series of potentially tridentate Schiff base ligands, {HOPhC=N-(CH2)3-XMe: X = S (HLSMe), Se (HLSeMe)},
was prepared by condensation reactions of salicylaldehyde and the corresponding functionalized propylamines. These
+
+
+
ligands were used to form the monocationic complexes, [Mo3S4(LXMe)3] (X = S (1 ), Se (2 )), through treatment of
[
4
+
Mo3S4(H2O)9] with the appropriate Schiff base ligand in methanol. Single-crystal X-ray structural analysis of 1-PF6
revealed that one Schiff base ligand coordinates to each Mo via the O, N, and SMe donor atoms, and then the mono
+
cationic cluster [Mo3S4(LSMe)3] has a pseudo C3 symmetry if the chirality around the coordinated sulfur atoms is not
taken into account. Because of the arrangements of the asymmetric Schiff base ligand, this monocationic cluster possibly
adopts one of two axial chiralities. Furthermore, each sulfur atom of the three coordinated SMe groups in the complex
cation is also chiral affording the two enantiomers observed in the unit cell. Dynamic behaviors of complexes 1-PF6 and
2-PF6 in solution were examined using line shape analyses of variable temperature NMR spectra revealing that the rate
constant of the chiral inversion at the S atoms in 1-PF6 is greater than that at the Se atoms in 2-PF6 and the difference
‡
¹1
¹1
‡
in the ¦H values of 1-PF6 (47.1 kJ mol ) and 2-PF6 (56.7 kJ mol ) and the negative ¦S values (¹39.9 and ¹57.5
¹1
¹1
J mol
K
for 1-PF6 and 2-PF6, respectively) suggest that the inversion processes involve bond cleavage between the
metal centers and S or Se atoms followed by coordination of solvent molecules. The electrochemical properties of 1-PF6
+
+
and 2-PF6 were evaluated using cyclic voltammetry and this revealed that both 1 and 2 exhibit two consecutive,
reversible one-electron reduction waves that are assigned to formal Mo(IV IV IV)/Mo(III IV IV) and Mo(III IV IV)/
+
Mo(III III IV) couples, respectively. The ability of 1-PF6 and 2-PF6 to catalyze the electroreduction of H was also
examined using CH3CO2H or CF3CO2H as a proton source. Noteworthy, the redox potentials for the catalytic wave
depend on the acidity of the added acids. Thus, the catalytic current around the first reduction wave is observed in the
presence of the stronger trifluoroacetic acid as a proton source, while the current around the second reduction wave
appears when the weaker acetic acid is used.
Currently, very large amounts of hydrogen are produced and
consumed on a daily basis by industry for a range of impor-
the active center of one of the nitrogenases, and nanoparticles
of MoS have attracted much attention as excellent hydrogen
2
4
tant processes including the production of NH . However, this
evolution reaction (HER) catalysts.
3
hydrogen is almost invariably produced by using fossil fuels
as a raw material and source of energy. The development of
Molybdenum can adopt a wide range of oxidation states
(from 0 to +6) and its complexes exhibit a variety of reactivity.
One class of molybdenum complexes that show intriguing
chemistry are those based on the incomplete cuboidal clusters
(Mo X : X = O, S, or Se) in which the oxygen, sulfur, or
1
efficient and sustainable methods for the production of hydro-
gen that do not utilize fossil fuels is an area of research that is
2
under intensive investigation. It has been demonstrated that
3
4
5
,6
platinum particles or complexes can catalyze the electrochem-
ical reduction of protons affording hydrogen.3 In nature,
enzymes such as hydrogenase and nitrogenase catalyze the
reduction of protons and nitrogen, respectively, and many
coordination compounds that mimic the active centers of these
enzymes have been synthesized and investigated to develop
efficient catalysts for hydrogen production using nonprecious
metals. Molybdenum­sulfur units are essential components of
selenium ligands, bridge the molybdenum(IV) ions. Among
these complexes, those based on the Mo S cluster core become
3
4
more stable when the molybdenum ions are reduced from IV
to III, as might be expected by consideration of the HSAB
principle. Importantly, Shibahara et al. showed that the Mo S
3
4
2
¹
cluster complex [Mo S (Hnta) ] catalyzes the reduction of
protons to afford hydrogen. Also, Chorkendorff et al. reported
that a Mo S complex, [Mo S (η -Cp¤) ] (Cp¤: methylcyclo-
3
4
3
7
5
+
3
4
3
4
3
292 | Bull. Chem. Soc. Jpn. 2015, 88, 292–299 | doi:10.1246/bcsj.20140275
© 2014 The Chemical Society of Japan

pentadienyl), adsorbed on a p-type silicon semiconductor
worked as a heterogeneous catalyst leading to quantitative
Chi model 680 electrochemical analyzer. The working and
counter electrodes were a glassy carbon and a Pt wire, respec-
8
+
hydrogen evolution on irradiation by visible light and Hong
tively. The reference electrode was a Ag/Ag electrode. All
4+
+
et al. showed that a composite material of [Mo S (H O) ]
the potential values were corrected referenced to the Fc/Fc
3
4
2
9
and NaTaO catalyzes proton reduction and hydrogen evolution
during water oxidation at the valence band edge of NaTaO3.
couple. Cyclic voltammograms were recorded at a scan rate of
100 mV s at room temperature using tetra-n-butylammonium
3
9
¹1
While there have been many electrochemical studies of the
redox and electrocatalytic properties of numerous hydrogenase
model compounds in organic solvents such as acetonitrile,
hexafluorophosphate as a supporting electrolyte. Ligand prep-
arations were performed under a N atmosphere using Schlenk
2
1
0
techniques.
there have been only a few reports of the catalytic reduction
of protons by complexes containing the Mo S core in organic
Synthesis of HLSMe (Scheme 1). To a solution of salicyl-
aldehyde (520 mg, 4.26 mmol) in deaerated methanol (10 mL),
3-(methylthio)propylamine (477 mg, 4.25 mmol) was added.
The mixture was refluxed for 6 h. After the mixture was allowed
to cool to room temperature, the solvent was removed under
reduced pressure. The residue was dissolved in chloroform and
the solution was washed with water (3 © 10 mL). The separated
organic layer was dried with sodium sulfate. The solvent was
3
4
media.
In this study, we describe the synthesis of the new complexes
+
+
+
[
Mo S (LXMe) ] (X = S or Se, 1 or 2 ; LXMe = Schiff
3 4 3
base ligand) that contain the Mo3S4 core (Charts 1 and 2). The
Schiff base ligands coordinated to the Mo S core of these
3
4
complexes were used not only to increase solubility in organic
solvents but also to enhance the tuning properties of the com-
plexes by the chemical modification of the substituents of
the salicylaldimine moiety and the S or Se donating atoms.
Coordination of these Schiff base ligands to the Mo center is
stabilized by the chelate effect due to a bidentate coordination
mode of the salicylaldimine moiety, and the S or Se donating
atoms connected to the salicylaldimine moiety with flexible
chains also help the formation of a stable coordination environ-
ment and, in addition, provide a vacant site on the metal center
by dissociation from the metal center. We report details of the
structures and redox properties of 1-PF and 2-PF .
removed on a rotary evaporator to afford HLSMe as an orange
1
oily material in 81% yield (743 mg). H NMR (CD CN): ¤
3
13.51 (s, br, 1H, Ph-OH), 8.48 (s, 1H, azomethine-H), 7.4­7.3
(m, 2H, Ph-H), 6.92 (t, 1H, J = 8.1 Hz, Ph-H), 3.70 (t, 2H,
J = 7.3 Hz, CH ), 2.59 (t, 2H, J = 7.2 Hz, CH ), 2.10 (s, 3H,
2
2
SCH3), 2.0 (m, 2H, CH2).
Synthesis of HLSeMe (Scheme 1).
To a solution of
dimethyl diselenide (1.0 mL, 10.6 mmol) in ethanol (5 mL), a
suspension of sodium borohydride (0.52 g, 13.8 mmol) in
ethanol (5 mL) was added. After the mixture was stirred for 1 h
at room temperature, a suspension of potassium tert-butoxide
6
6
(
(
1.45 g, 13.0 mmol) and 3-chloropropylamine hydrochloride
1.32 g, 10.2 mmol) in ethanol (40 mL) was added. The mixture
Experimental
General Procedures and Materials.
3-(Methylthio)-
was refluxed for 3 hours, and then a solution of salicylaldehyde
(1.24 g, 10.1 mmol) in ethanol (10 mL) was added, and the
mixture was refluxed for an additional 12 h. The mixture was
cooled to room temperature and the organic phase was washed
with water (3 © 10 mL). The separated organic layer was dried
with sodium sulfate and then the solvent was removed on a
propylamine, salicylaldehyde, and 3-chloropropylamine hydro-
chloride were purchased from Tokyo Chemical Industry Co.,
Ltd. Ammonium hexafluorophosphate was purchased from
Wako Pure chemicals. Sodium borohydride and potassium tert-
butoxide were purchased from Nacalai Tesque. Dimethyl
diselenide was purchased from Aldrich. All chemicals were
used as received. NMR spectra were recorded on a LAMBDA
rotary evaporator to give HLSeMe as an orange oily material
in 92% yield (2.5 g). H NMR (CD CN): ¤ 13.47 (s, br, 0.8 H,
1
3
3
00 FT-NMR spectrometer. Chemical shifts are expressed in
Ph-OH), 8.45 (s, 1H, azomethine-H), 7.4­7.3 (m, 2H, Ph-H),
6.70 (t, 2H, J = 7.5 Hz, Ph-H), 3.67 (t, 2H, J = 6.7 Hz, CH2),
2.60 (t, 2H, J = 7.3 Hz, CH ), 2.02 (t, 2H, J = 7.10 Hz, CH ),
ppm downfield from SiMe and are referenced to the proteo-
4
impurity peaks in the deuterated solvents. Elemental analyses
were performed by the Analytical Research Service Center at
Osaka City University on a J-SCIENCE LAB JM10. FAB+
mass spectra were measured on a JMS-700T mass spectrometer
using 3-nitrobenzylalcohol (NBA) as a supporting matrix.
Electrochemical measurements were performed with an ALS/
2
2
7
7
1.97 (s, 3H, SeCH3). Se NMR (CD CN): ¤ 183.13.
3
Synthesis of [Mo S (LSMe) ]PF ¢2CH CN (1-PF6¢
3
4
3
6
3
2CH CN). A sample of solid material containing the cation
3
4+
[Mo S (H O) ] (0.12 mmol), which was obtained by removal
3
4
2
9
4
+
of the solvent from a 5.61 mM solution of [Mo S (H O) ]
3
4
2
9
Scheme 1. Synthesis of HLSeMe and HLSMe.
Bull. Chem. Soc. Jpn. 2015, 88, 292–299 | doi:10.1246/bcsj.20140275
© 2014 The Chemical Society of Japan | 293

(21.0 mL) in hydrochloric acid (concentration of the solution
was determined using absorption spectrum, ­ = 620 nm,
max
¹
1
¹1 7
¾
= 315 M cm ), was suspended in methanol (2 mL). A
solution of HLSMe (78 mg, 0.37 mmol) in methanol (10 mL)
was added to the suspension, and the mixture was stirred for
Chart 1. Fundamental framework of ligands (X = S, Se).
2
0 h at room temperature. After the solvent was removed under
reduced pressure, the residue was dissolved in 2 mL of aceto-
nitrile. After the insoluble solid was removed by filtration,
NH PF (20 mg, 0.12 mmol) was added to the filtrate. Diethyl
4
6
ether was then carefully layered on top of this solution and
the mixture was allowed to stand to give dark green crystals
of [Mo S (LSMe) ]PF ¢2CH CN (1-PF ¢2CH CN). Yield: 24
3
4
3
6
3
6
3
+
mg (17%). FAB-MS (NBA): m/z 1041 ([1] ). Anal. Found
(calcd for C H N O F Mo PS ): C, 34.90 (35.05); H, 3.87
37 48 5 3 6 3 7
(
3.82); N, 5.61 (5.52)%.
Synthesis of [Mo S (LSeMe) ]PF ¢1.5CH CN (2-PF ¢
3
4
3
6
3
6
1
.5CH CN). A sample of solid material containing the cation
3
4+
[Mo S (H O) ] (0.11 mmol), which was obtained by removal
3 4 2 9
4+
of the solvent from a 5.61 mM solution of [Mo S (H O) ]
3
4
2
9
in hydrochloric acid (20.0 mL) was suspended in methanol
(
(
2 mL). A solution of HLSeMe (90 mg, 0.35 mmol) in methanol
10 mL) was added to the suspension, and the mixture was
+
+
Chart 2. One of isomers of 1 (X = S) and 2 (X = Se).
stirred for 12 h at room temperature. After the insoluble solid
was removed by filtration, NH PF (23 mg, 0.14 mmol) was
atom to the Schiff base moiety are such that these tridentate
ligands can adopt either meridional or facial coordination
mode. Similar flexible Schiff base ligands coordinated in either
meridional or facial fashion have been reported for multi-
nuclear cubane Na(I) clusters and mono- and trinuclear Ni(II)
4
6
added to the filtrate. Diethyl ether was then carefully layered on
top of this solution and the mixture was allowed to stand to
afford dark green crystals of [Mo S (LSeMe) ]PF ¢1.5CH CN
3
4
3
6
3
(
2-PF ¢1.5CH CN). Yield: 30 mg (20%). FAB-MS (NBA):
6 3
+
17
m/z 1182 ([2] ). Anal. Found (calcd for C H N O F -
complexes. The ligands HLSMe and HLSeMe reported in
36
47 4.5
3 6
Mo PS Se ): C, 31.12 (31.15); H, 3.57 (3.38); N, 4.38 (4.54)%.
Se NMR (CD3CN): ¤ 3.97.
X-ray Crystallography. A single-crystal X-ray structure
this paper were prepared from the condensation reaction of
salicylaldehyde and either 3-(methylthio)propylamine or 3-
(methylseleno)propylamine. It was expected that the flexible
propylene chains that connect the sulfur or selenium donor
atoms in these tridentate ligands could facilitate coordination in
either meridional or facial fashion.
3
4
3
77
determination was performed for complex 1-PF . A single
6
crystal was attached to Cryoloop using Paraton N. Diffraction
data were collected at 123 K on a VariMax Saturn diffractometer
with graphite-monochromated Mo Kα radiation (­ = 0.7107 ¡)
using a rotation method. The data were integrated, scaled,
sorted, and averaged using CrystalClear software.11 Absorption
corrections were applied using the multiscan method. The struc-
+
+
The cationic complexes, [Mo S (LXMe) ] (X = S (1 ), Se
3
4
3
+
4+
(2 )), were prepared from the reactions of [Mo S (H O) ]
3
4
2
9
and HLSMe or HLSeMe in methanol. The starting cationic
4+
material [Mo3S4(H2O)9] was prepared according to a pre-
12
7
tures were solved using SHELX97, expanded using Fourier
viously reported procedure, and stored in conc. hydrochloric
1
3
+
+
techniques, and refined by using full-matrix least-squares
acid. For the syntheses of 1 and 2 , the hydrochloric acid
solvent was first removed under reduced pressure to give a
2
against F with SHELXL97 equipped in the CrystalStructure
1
4
4+
4
.0.1 software. All hydrogen atoms were located at the calcu-
solid material that contained the cation [Mo S (H O) ] . This
3
4
2
9
lated positions and refined as riding models. Crystallographic
data have been deposited with Cambridge Crystallographic
Data Center: Deposition number CCDC-1023632.
solid was then treated directly with the ligands HLSMe or
HLSeMe to give 1-PF or 2-PF , respectively, after precip-
6
6
itation by the addition of NH PF . Removal of the excess acid
4
6
before addition of HLSMe or HLSeMe was essential because
acid suppresses the proton loss from the ligand that is required
for tridentate coordination to the Mo centres and also greatly
accelerates the rate of imine hydrolysis.
Results and Discussion
Syntheses and Solid-State Structure of Complexes.
Schiff base ligands, obtained from the reactions of salicylalde-
hyde derivatives and a wide range of different amines, usually
behave as multidentate ligands that provide very stable coor-
dination environments, and most of their complexes adopt
meridional fashion as previously reported.15 On the other hand,
Amosova et al. reported a different type of tridentate Schiff
base ligands, synthesized using the condensation reactions of
salicylaldehyde derivatives and functionalised alkyl amine
derivatives (Chart 1).16 In these Schiff base ligands, the length
and flexibility of the alkyl chains that tether the third donor
+
+
The solubility of the monocationic complexes 1 and 2
(Chart 2) depends on the counter anions. The chloride salts
1-Cl and 2-Cl are soluble in methanol but not in acetonitrile,
while the PF salts 1-PF and 2-PF are soluble in acetonitrile
6
6
6
but not in methanol. All measurements of the complexes were
performed using the PF salts.
6
Crystallographic data and refinement details for complex
1-PF are summarized in Table 1, and an ORTEP drawing of
6
¹
the complex cation (with the counter [PF ] anion omitted) is
6
294 | Bull. Chem. Soc. Jpn. 2015, 88, 292–299 | doi:10.1246/bcsj.20140275
© 2014 The Chemical Society of Japan

Table 1. Crystallographic and Refinement Data for
[
Mo3S4(LSMe)3] (1)¢PF6¢2CH3CN
Formula
Formula weight
Crystal system
monoclinic
a/¡
C37H48F6Mo3N5O3PS7
1268.02
monoclinic
Pn (#7)
12.999(2)
11.240(2)
15.826(3)
91.501(3)
2311.4(7)
2
b/¡
c/¡
¢
V/¡
/degree
3
Z
¹3
Dcalcd/g cm
1.822
12.180
¹
1
®
(Mo Kα)/cm
G.O.F.
R1 (I > 2.00·(I))
R; wR2
1.050
0.0331
0.0350; 0.0882
a)
Figure 2. 1H NMR spectra of complexes 1-PF6 and 2-PF6
in CDCl2CDCl2 at room temperature.
b)
2
2 2
a) R1 = ∑ ««Fo« ¹ «Fc««/∑ «Fo«. b) wR2 = [∑(w(Fo ¹ Fc ) )/
2
2 1/2
∑
w(Fo ) ]
.
The Mo­Mo distances (2.7719(6), 2.7519(7), and
.7600(11) ¡) are in the range of reported values (2.726(3)­
2
2
1
9
.8314(3) ¡) for Mo S incomplete cubane complexes. No
3
4
trends in the Mo­Mo distances depending on the properties
of the different ancillary ligands such as donating ability are
observed. The Mo­μ -S and Mo­μ -S distances, and the
3
2
Mo­μ -S­angles in the Mo S core of 1-PF are very similar
3
3
4
6
to the corresponding distances and angles in the other pre-
viously reported Mo S complexes (see Supporting Informa-
3
4
tion Tables S1 and S2).
Structures and Dynamic Behavior of the Mo S Com-
3
4
plexes in Solution. The identities of complexes 1-PF and 2-
6
PF6 in solution were confirmed by using FAB mass spectrom-
1
77
etry, H (for 1-PF and 2-PF ) and Se (for 2-PF ) NMR
6
6
6
spectroscopy. In the FAB mass spectra of complexes 1-PF
6
+
+
and 2-PF , peaks attributed to the cationic moieties 1 and 2
6
7
7
were observed at m/z 1041 and 1182, respectively. Se NMR
spectrum of complex 2-PF , which contains the selenoether
6
donor group, in CD CN at room temperature exhibits one
3
singlet signal at considerably higher magnetic field (¤ 3.97)
relative to the free HLSeMe ligand (¤ 183.13) implying the
differences in chemical shifts for the different configurational
arrangements possible for the three selenium atoms are too
small to detect.
Figure 1. An ORTEP drawing of [Mo3S4(LSMe)3]
1)¢PF6¢2CH3CN. The counter anion of PF6 and CH3CN
molecules were omitted for clarity.
(
1
shown in Figure 1. The three Mo atoms and four bridging S
atoms form an incomplete cubane-type core. Each Mo atom
bonds to a LSMe ligand in a tridentate manner via the O, N,
and S donor atoms. The S atoms of the three Schiff base ligands
are located in the positions trans to the triply bridging sulfido
ligand in the Mo S core. The monocationic Mo S cluster has
H NMR spectra of 1-PF and 2-PF in deuterated 1,1,2,2-
6
6
tetrachloroethane (Figure 2) did not exhibit two sets of signals
for the LXMe ligands in each complex with a 1:2 ratio of their
integrated intensities as expected by the result of the structural
analysis of 1-PF6 showing the pseudo C3 symmetry of the
monocationic complex with the P-(R, S, S)- or M-(S, R, R)-
configuration. The signals for the SMe protons in 1-PF6
appeared in the spectrum as two broad signals at room
temperature. The broadening of the signals for the SMe protons
suggested a flipping motion of the methyl group at the chiral
sulfur center between the R and S configurations (Scheme 2).
On the other hand, two sets of signals for the SeMe protons of
2-PF6 were observed at room temperature.
3
4
3 4
the pseudo C symmetry if the coordinated thioether moieties
3
containing chiral S atoms are not taken into account. Because
of the arrangements of the asymmetric Schiff base ligand, each
molybdenum center possibly adopts one of two chiralities,
which are designated by clockwise (P) or anticlockwise (M)
as they relate to the rotation of the oxygen atoms around the
1
8
C3 axis with the capping sulfur pointing towards the viewer.
Furthermore, each sulfur atom of the three coordinated SMe
groups in the complex cation is also chiral, which is defined as
R or S. As a result, the two enantiomers, P-(R, S, S) and M-(S,
R, R), were observed in the unit cell.
To examine the details of the chiral inversion process,
variable temperature (VT) H NMR spectroscopy was per-
1
formed for both 1-PF and 2-PF (Figures 3 and 4). The SMe
6
6
protons of 1-PF were observed as two sets of triplet signals
6
Bull. Chem. Soc. Jpn. 2015, 88, 292–299 | doi:10.1246/bcsj.20140275
© 2014 The Chemical Society of Japan | 295

Scheme 2. Chiral inversion between R and S configurations
at X atom (X = S, Se).
Figure 5. Eyring plots for 1-PF6 (
obtained from line shape analysis.
) and 2-PF6 (
)
Table 2. Activation Parameters for S and Se Inversion in
Complexes 1-PF6 and 2-PF6 in CDCl2CDCl2
‡
¹1
¦S /J mol K¹1
‡
¹1
¦
H /kJ mol
1
2
-PF6
-PF6
47.1 « 0.9
56.7 « 4.0
¹39.9 « 3.1
¹57.5 « 11.8
Some of the isomers with a series of the combination of S
and R configuration around the sulfur center, such as P-(S, S,
S), P-(R, S, S), P-(R, R, S), P-(R, R, R), and their enantiomers,
possibly exist in solution. Although these isomers possibly give
some signals of the SMe or SeMe protons, only two signals for
the SMe or SeMe protons with a 0.7:1 ratio were observed.
This result suggests that there are only small influences by the
adjacent Schiff base ligands and the chemical shifts of the
XMe protons are scarcely affected by the neighboring LXMe
ligands. In other words, all XMe groups were located in similar
environments in the Mo S core, and the chemical shifts of the
1
Figure 3. VT- H NMR spectra of the SMe region of 1-PF6
(
right) and the simulated spectra (left).
3
4
XMe protons depend only on the R and S configurations at
the chalcogenide centers. In addition, the other signals for the
LXMe ligands except for the signals of the imine protons in the
complexes were indistinguishable even at low temperatures
(
described above). These observations revealed that these
cationic clusters maintain the pseudo C3 symmetry in solution
and the dynamic behavior of the inversion process can be
simplified as the inversion between P-R and P-S or M-S and
M-R, respectively (Scheme 2).
Line shape analyses were applied to the VT-NMR data to
obtain rate constants of these inversion processes for each
complex (Figures 3 and 4). The Eyring plots using the rate con-
stants for each temperature are presented in Figure 5 providing
‡
‡
the activation parameters, ¦S and ¦H , listed in Table 2. The
¦
‡
H value for 2-PF is larger than that for 1-PF reflecting the
6 6
stronger Mo­Se bond in 2-PF than the Mo­S bond in 1-PF .
6
6
In general, s character of the lone pairs on chalcogen atoms
increases for heavier elements, thus the lone pair on a Se atom
has more s character than S, which affects σ-donating ability
1
Figure 4. VT- H NMR spectra of the SeMe region of 2-PF6
(
2
0
right) and the simulated spectra (left).
of the ligands containing Se and S donating atoms resulting
‡
in the stronger Mo­Se bond for 2-PF . Both the ¦S values for
6
coupled with the protons of the methylene adjacent to the S or
Se atom at ¹30 °C and these signals merged into one sharp
1-PF6 and 2-PF6 are negative implying that the chiral inver-
sion at the S or Se atom involves aggregation processes of the
complexes and solvent molecules. These results suggest that the
inversion processes proceed via Mo­S or Mo­Se bond cleavage
accompanied by the coordination of the solvent molecules.
signal at 100 °C. In the case of 2-PF , two sets of signals for the
6
SeMe protons appeared at room temperature and the signals
merged affording one broad signal even at 135 °C.
296 | Bull. Chem. Soc. Jpn. 2015, 88, 292–299 | doi:10.1246/bcsj.20140275
© 2014 The Chemical Society of Japan

Figure 7. Cyclic voltammograms of 1-PF6 recorded with
variable equivalents of CH3COOH.
Figure 6. Cyclic voltammograms of 1-PF6 (0.5 mM) and
-PF6 (0.5 mM) in CH3CN containing 0.1 M n-Bu4NPF6
2
¹1
(scan rate: 100 mV s ).
There might be a possibility of the existence of the other
complex dynamic processes involving the adjacent coordina-
tion sites in solution because the Mo­S and Mo­Se bonds in
the trans position to the triply-bridging S atom in the com-
plexes are less labile than the others as reported. Stabilization
by the chelate effect of the coordination of the salicylaldimine
probably overcomes this disadvantage.
Electrochemical Properties of Complexes of 1-PF and 2-
6
PF6. The electrochemical properties of complexes 1-PF and
6
2
-PF were examined using cyclic voltammetry in acetonitrile
Figure 8. Cyclic voltammograms of 2-PF recorded with
6
6
and the results are shown in Figure 6. Complexes 1-PF and
variable equivalents of CH COOH.
6
3
2
-PF gave similar cyclic voltammograms (Figure 6) and each
6
complex exhibited two consecutive reversible one-electron
reduction waves at ¹1.2 and ¹1.9 V, clearly showing that the
thioether and selenoether moieties in 1-PF and 2-PF afford
6
6
no significant influence on the reduction potentials. The fact
that the two one-electron reductions proceeded stepwise sug-
gests the existence of electronic communication among the
Mo centers. Results of DFT calculations (vide infra) imply that
the reductions occurred at the Mo (μ -S) moiety. Therefore, it
3
2
3
is probably better to use the combined formal oxidation states
of the three metal ions in the Mo3S4 units (i.e. XII, XI, and
X in the monocationic, neutral, and monoanionic complexes,
respectively) rather than the hypothetical individual metal
oxidation states (IV IV IV), (III IV IV), and (III III IV), which
are presented in Figure 6 for clarity.
Figure 9. Cyclic voltammograms of complex 1-PF6
recorded with variable equivalents of CF3COOH.
To investigate the potential catalytic ability of the Mo3S4
complexes in a proton reduction reaction, cyclic voltammom-
proton reductions catalized by 1-PF or 2-PF were smaller
6
6
etry experiments were carried out for complexes 1-PF and
than those for the heterogeneous proton reduction of CH CO H
3 2
6
2
-PF6 in the presence of 1, 2, 3, 10, 20, 30, and 40 equiv. of
in the absence of 1-PF6 or 2-PF6. On the other hand, the cyclic
voltammogram of 1-PF in the presence of CF CO H as a
CH CO H as a proton source (Figures 7 and 8). The results
3
2
6
3
2
1
of the VT H NMR measurements revealed that the sulfur
or selenium atoms in the flexible arms of the propyl chains
dissociate from the metal centers in solution providing the
vacant site on the metal centers. These results also implied that
the free sulfur and selenium atoms dissociated from the metal
center possibly work as a proton relay for the proton reduction
reaction, however no differences in the reactivity with proton
between 1-PF and 2-PF were observed suggesting the sulfur
proton source showed catalytic current around the potentials
of the first reduction (Figure 9). These results can be explained
+
if the pK of the protonated form of 1 lies between the pK ’s
a
a
of CF CO H and CH CO H, and protonation of the com-
3
2
3
2
plex therefore occurred in the presence of the stronger acid,
CF CO H before reduction. As a consequence, a CECE pro-
3
2
cess is observed, while an ECEC process is dominant in the
presence of a weaker acid, CH CO H.
6
6
3
2
+
and selenium atoms do not perform such roles. Catalytic
currents were observed around the potentials of the second
reduction for 1-PF and 2-PF , and the overpotentials for the
DFTcalculations of complex 1 and its one and two electron-
reduced species were performed to rationalize the observed
+
21
reactivity of these species with H in the CV measurements.
6
6
Bull. Chem. Soc. Jpn. 2015, 88, 292–299 | doi:10.1246/bcsj.20140275
© 2014 The Chemical Society of Japan | 297

metal cubane-type sulfide clusters.19c These facts show the
nucleophilicity of the doubly bridging sulfido ligands. In our
case, while the nucleophilicity of the doubly bridging sulfido
ligands is not basic enough to capture protons before the reduc-
tion of the complex, the nucleophilicity is probably increased
after the reduction resulting in the protonation at the doubly
bridging sulfido ligands.
Conclusion
We have introduced the two kinds of Schiff base ligands,
HLXMe (X = S or Se) into Mo S complexes as a fundamental
3
4
framework for the tuning of electrochemical properties of the
complexes. X-ray structural analysis of thioether complex 1-
PF showed pseudo C symmetry for the complex and P-(R, S,
6
3
S)- or M-(S, R, R)-configurations around the sulfur atoms of
the thioether moieties in the complex. Variable temperature
1
H NMR spectroscopy of complexes 1-PF and 2-PF revealed
6
6
that the R and S configurations are under equilibrium in solu-
tion and the kinetic constants of 1-PF are larger than those of
6
2
-PF at each temperature due to the weaker σ donating ability
6
Figure 10. MOs of the isolated monocationic species (top)
and the one (middle) or two (bottom) electron reduced
species of complex 1 .
for the coordinated sulfur atoms of the thioether moieties than
that of the selenium atoms of the selenoether. Cyclic voltam-
metry exhibited two consecutive reversible one-electron reduc-
tion waves for each complex and showed catalytic current in
the presence of a proton source. The reduction potentials for the
catalytic waves depended on the acidity of acids used as proton
sources. DFT calculations of the complexes suggested that the
protonation reaction should occur at the doubly bridging
sulfido ligands. These experimental and theoretical results will
supply some information about the roles of the doubly bridging
sulfur ligands of the Mo3S4 core on the catalytic proton
reduction.
+
Optimized structures (Figure 10) were obtained using the
B3LYP density functional method with the LanL2DZ basis set
for Mo and the 6-31g** basis set for the others. The optimized
+
structure of 1 is well reproduced in the X-ray structure. There
are no significant differences among the optimized structures of
+
complex 1 and its one and two electron-reduced species. These
structural features of the complexes are consistent with the
reversibility of the reduction waves observed in the CV mea-
surement of complex 1-PF suggesting small structural changes
6
upon the reductions. The energy levels of HOMO and LUMO
We thank Professor Takashi Shibahara of Okayama Univer-
sity of Science for valuable suggestions regarding the prepara-
+
+
and their gaps for complexes 1 and 2 (See Table S7 in the
Supporting Information) are similar to each other, despite
having different donating atoms of S or Se in the flexible arms.
These results are consistent with the similar redox potentials
of 1-PF6 and 2-PF6. On the other hand, no contribution of the
S and Se atoms in the flexible arms to the HOMO and LUMO
was found. The bonding orbitals of the coordination bonds of
the S and Se atoms in the flexible arms toward the molybdenum
centers appeared as HOMO¹10 at ¹9.1673 eV and HOMO¹9
at ¹9.0168 eV, respectively. This difference may be attributed
to the donating abilities of the S and Se atoms and probably
contributes to the cleavages of the Mo­S and Mo­Se bonds
4
+
tion of [Mo S (H O) ] . We also thank Professor L. James
3
4
2
9
Wright of University of Auckland for helpful discussions.
Supporting Information
NMR spectra, Detail of crystallographic analysis of 1, and
DFT calculations. This material is available electronically on
J-STAGE.
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Bull. Chem. Soc. Jpn. 2015, 88, 292–299 | doi:10.1246/bcsj.20140275
© 2014 The Chemical Society of Japan | 299