Octahedral molybdenum cluster complexes with aromatic sulfonate ligands

Article abstract of DOI:10.1039/c6dt02863b

This article describes the synthesis, structures and systematic study of the spectroscopic and redox properties of a series of octahedral molybdenum metal cluster complexes with aromatic sulfonate ligands (ⁿBu₄N)₂[{Mo₆X₈}(OTs)₆] and (ⁿBu₄N)₂[{Mo₆X₈}(PhSO₃)₆] (where X^- is Cl^-, Br^- or I^-; OTs^- is p-toluenesulfonate and PhSO₃^- is benzenesulfonate). All the complexes demonstrated photoluminescence in the red region and an ability to generate singlet oxygen. Notably, the highest quantum yields (>0.6) and narrowest emission bands were found for complexes with a {Mo₆I₈}⁴⁺ cluster core. Moreover, cyclic voltammetric studies revealed that (ⁿBu₄N)₂[{Mo₆X₈}(OTs)₆] and (ⁿBu₄N)₂[{Mo₆X₈}(PhSO₃)₆] confer enhanced stability towards electrochemical oxidation relative to corresponding starting complexes (ⁿBu₄N)₂[{Mo₆X₈}X₆].

Full text of DOI:10.1039/c6dt02863b

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A. Vorotnikov, K. Brylev, N. A. Vorotnikova, I. Novozhilov, N. Kuratieva, M. V. Edeleva, D. Benoit, N.
Kitamura, Y. V. Mironov, M. A. Shestopalov and A. J. Sutherland, Dalton Trans., 2016, DOI:
10.1039/C6DT02863B.
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DOI: 10.1039/C6DT02863B
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ARTICLE
Octahedral Molybdenum Cluster Complexes with Aromatic
Sulfonate Ligands
Olga A. Efremova,*,a,b Yuri A. Vorotnikov,c Konstantin A. Brylev,c,d Natalya A. Vorotnikova,c
Igor N. Novozhilov,c Natalia V. Kuratieva,c,d Mariya V. Edeleva,e David M. Benoit,a
Noboru Kitamura,f Yuri V. Mironov,c,d Michael A. Shestopalov,**,c,d and Andrew J. Sutherlandb
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
This article describes the synthesis, structures and systematic study of the spectroscopic and redox properties of a series
of octahedral molybdenum metal cluster complexes with aromatic sulfonate ligands (nBu4N)2[{Mo6X8}(OTs)6] and
–
(nBu4N)2[{Mo6X8}(PhSO3)6] (where X– is Cl–, Br– or I–; OTs– is p-toluenesulfonate and PhSO3 is benzenesulfonate). All the
complexes demonstrated photoluminescence in the red region and an ability to generate singlet oxygen. Notably, the
highest quantum yields (> 0.6) and narrowest emission bands were found for complexes with a {Mo6I8}4+ cluster core.
Moreover, cyclic voltammetric studies revealed that (nBu4N)2[{Mo6X8}(OTs)6] and (nBu4N)2[{Mo6X8}(PhSO3)6] confer
enhanced stability towards electrochemical oxidation relative to corresponding starting complexes (nBu4N)2[{Mo6X8}X6].
It was shown recently that complexes with a {Mo6I8}4+ cluster
core ligated by residues of some strong oxoacids – either
organic (fluorinated aliphatic acids9, 13) or inorganic acids (nitric
Introduction
Octahedral molybdenum cluster complexes of the general
formula [{Mo6X8}L6] (where X– are inner halide ligands and L
are apical organic or inorganic ligands) have been known for
several decades since the structure, luminescence and redox
properties of the [{Mo6Cl8}Cl6]2– anionic unit were established
and studied comprehensively.1-8 However, in the last decade
there has been a resurgence of interest in related octahedral
molybdenum complexes. This is due to recent reports detailing
the outstanding photoluminescence properties of some
molybdenum octahedral cluster complexes. In general, these
studies showed that upon UV-visible excitation cluster
complexes based on a {Mo6X8}4+ core demonstrated emission
in both the red and near-infrared regions with high quantum
yields and photoluminescence lifetimes of up to several
hundreds of microseconds.9-15 Moreover, these cluster
complexes also demonstrated the capability to generate
singlet oxygen efficiently.13-16 These photoemissive properties
combined with the relative chemical robustness of the
{Mo6X8}4+ cluster core make such complexes excellent
candidates for photonic (e.g. materials for lasers17 and optical
waveguides18), photovoltaic,19 biological and medical (e.g.
27
acid11,
)
–
demonstrated outstanding photophysical
properties in comparison with those based on {Mo6Cl8}4+ or
{Mo6Br8}4+ core. These include not only significantly higher
photoluminescence quantum yields at ambient conditions, but
also comparatively narrow emission spectra profiles.
Therefore, we were keen to understand whether the same
tendency in photophysical properties of {Mo6X8}4+ cluster
complexes is retained in cases, when they are coordinated by
sulfonate aromatic outer ligands that are also anions of strong
acids. Accordingly, we report the synthesis and systematic
study of the photophysical and electrochemical properties of
molybdenum cluster complexes (nBu4N)2[{Mo6X8}L6], where L–
= p-toluenesulfonate (OTs–) and X– = Cl– (
1
) Br– (
), Br– (
2
) or I– (
) or I– (
3) or L
–
= benzenesulfonate (PhSO3 ) and X– = Cl– (
4
5
6
).
Experimental section
Materials
29
Cs2[{Mo6X8}X6],28,
(nBu4N)2[{Mo6X8}X6],30-32
silver
benzenesulfonate33 and 2,3-diphenyl-p-dioxene34 were
synthesised according to previously reported procedures. All
other reactants and solvents were purchased from Fisher, Alfa
Aesar and Sigma-Aldrich and used as received.
Instrumentation
imaging agents,20, bactericides,22 photo-dynamic therapy23,
21
24) and environmental protection (e.g. hazardous organic
waste decomposition25, 26) applications.
NMR spectra of
1
-
6
were recorded on a Bruker Avance 300
solution-state dual
a.Department of Chemistry, University of Hull, Cottingham Road, Hull, HU6 7RX;
o.efremova@hull.ac.uk
NMR spectrometer equipped with
a
1
b.Aston Materials Centre, Aston University, Aston Triangle, Birmingham, B4 7ET, UK
c. Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Acad. Lavrentiev Ave., 630090
Novosibirsk, Russian Federation
channel probe working at 300.13 MHz for H and 75 MHz for
13C. NMR spectra were measured in CDCl3 or in acetone-d6 in
the standard way using the zg30 pulse program for 1H.
Elemental analyses were obtained using a EuroVector EA3000
Elemental Analyser. FTIR spectra were recorded on a Bruker
Vertex 80 as KBr disks. Absorption spectra were recorded on a
PerkinElmer Lambda35 UV-vis spectrometer.
d.Novosibirsk State University, 2 Pirogova Str., 630090 Novosibirsk, Russian
Federation
e. Novosibirsk Institute of Organic Chemistry SB RAS, 9 Acad. Lavrentiev ave.,
630090 Novosibirsk, Russian Federation
f. Department of Chemistry, Faculty of Science, Hokkaido University, 060-0810
Sapporo, Japan
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: DFT calculations data, Single
crystal data; singlet oxygen-generation data; examples of NMR spectra; Cyclic
voltammograms; the Tauc plot of the absorption spectra of (nBu4N)2[{Mo6X8}X6];
average values of the important bond lengths and angles. See
DOI: 10.1039/x0xx00000x
Cyclic voltammetry
Cyclic voltammetry (CV) was performed using a Metrohm 797
VA Computrace instrument with a glassy carbon electrode as
the working electrode and a saturated silver / silver chloride
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(Ag/AgCl) in 3.5 M KCl as a reference electrode, against which 6×2Ηmeta], 3.43 [16Η, t, 2×4CΗ2], 2.39 [18Η, s, 6×CH3], 1.81
the half-wave potential of the ferrocene / ferrocenium (Fc/Fc+) [16Η, quin, 2×4CΗ2], 1.43 [16Η, sex, 2×4CΗ2], 0.97 [24Η, t,
redox couple (E1/2, Fc/Fc+) was found to be 0.598 V. The 2×4CΗ3].
potentials were related to the standard platinum electrode. A
2
:
Yellow. Yield: 112 mg (90%) Anal. calcd. for
0.15 solution of tetra-n-butylammonium perchlorate C74H114Br8Mo6N2O18S6: C 32.6, H 4.2, N 1.0, S 7.1; found: C
M
(nBu4NClO4) in acetonitrile was used as the electrolyte. 32.6, H 4.2, N 1.0, S 6.8. UV-vis (CH2Cl2): λmax, nm (ε, M–
Solutions of the samples in the electrolyte (1-2 mM) were 1·cm–1) = 320 (5.0×103), 363 (sh, 3.4×103). FTIR (KBr, cm–1):
degassed by purging with argon prior to recording the CV νas(SO2) – 1293s, 1277s; νs(SO2) – 1158s; ν(SO) – 964. 1H
measurements. Compounds
(nBu4N)2[{Mo6X8}X6] (X–
1-
6
as well as starting compounds NMR (acetone-d6) δ: 7.64 [12H, d, 6×2Ηortho], 7.28 [12Η, d,
Cl–, Br–, I–) were investigated 6×2Ηmeta], 3.44 [16H, t, 2×4CΗ2], 2.39 [18H, s, 6×CH3], 1.81
=
voltammetrically within the potential window from –2V to [16H, quin, 2×4CΗ2], 1.43 [16H, sex, 2×4CΗ2], 0.97 [24H, t,
2.3V at 25 °C. The formal half-wave potentials (E1/2) were 2×4CΗ3].
calculated as the midpoint between the anodic and cathodic
peak potentials of the first oxidation of the cluster complex.
Photoluminescence Measurements
3
:
Orange. Yield: 95 mg (87%) Anal. calcd. for
C74H114I8Mo6N2O18S6: C 28.6, H 3.7, N 0.9, S 6.2; found: C
28.7, H 3.6, N 0.9, S 6.0. UV-vis (CH2Cl2): λmax, nm (ε, M–1
cm–1) = 347 (5.2×103), 394 (4.3×103). FTIR (KBr, cm–1):
νas(SO2) – 1269s; νs(SO2) – 1157s; ν(SO) – 982. 1H NMR
(acetone-d6) δ: 7.59 [12H, d, 6×2Hortho], 7.27 [12H, d,
6×2Hmeta], 3.43 [16H, t, 2×4CH2], 2.39 [18H, s, 6CH3], 1.81
[16H, quin, 2×4CH2], 1.43 [16H, sex, 2×4CH2], 0.97 [24H, t,
2×4·CH3].
For photoluminescence measurements, powdered samples of
the complexes were placed between two non-fluorescent glass
plates. The absorbance of acetonitrile solutions was set at <
0.1 at 355 nm. The solutions were poured into quartz cuvettes.
To deaerate, the solutions were purged with an Ar-gas stream
for 30 min and then the cuvettes were sealed. Measurements
were carried out at 298 K. The samples were excited by 355-
nm laser pulses (6 ns duration, LOTIS TII, LS-2137/3). Corrected
4
:
Yellow. Yield: 101 mg (67%). Anal. calcd. for
C68H102Cl8Mo6N2O18S6: C 35.7, H 4.5, N 1.2, S 8.4; found: C
35.7, H 4.5, N 1.3, S 8.1. UV-vis (CH2Cl2): λmax, nm (ε, M–
1·cm–1) = 348 (sh, 3.5×103). FTIR (KBr, cm–1): νas(SO2) –
1298s, 1285s; νs(SO2) – 1161s; ν(SO) – 957. 1H NMR
(acetone-d6) δ: 7.85-7.72 [12H, m, 6×2Hortho], 7.54-7.41 [18H,
m, 6×(2Hmeta + Hpara)], 3.43 [16H, t, 2×4CH2], 1.80 [16H, quin,
2×4CH2], 1.42 [16H, sex, 2×4CH2], 0.96 [24H, t, 2×4CH3].
emission spectra were recorded on
a red-light-sensitive
multichannel photodetector (Hamamatsu Photonics, PMA-11).
For emission decay measurements, the emission was analysed
using a streakscope system (Hamamatsu Photonics, C4334 and
C5094). The emission quantum yields were determined using
an
Measurement System (Hamamatsu Photonics, C9920-03),
which comprised xenon excitation light source (the
excitation wavelength was set at 400 nm), an integrating
sphere, and red-sensitive multichannel photodetector
Absolute
Photo-Luminescence
Quantum
Yield
5
:
Yellow. Yield: 90 mg (72%) Anal. calcd. for
C68H102Br8Mo6N2O18S6: C 30.9, H 3.9, N 1.1, S 7.3; found: C
30.6, H 3.7, N 1.0, S 7.6 UV-vis (CH2Cl2): λmax, nm (ε, M–1·cm–
1) = 317 (5.2×103), 367 (3.6×103). FTIR (KBr, cm–1): νas(SO2) –
a
a
1
1282s; νs(SO2) – 1162s; ν(SO) – 965. H NMR (acetone-d6) δ:
7.80–7.71 [12H, m, 6×2Hortho], 7.55–7.41 [18H, m, 6×(2Hmeta
Hpara)], 3.43 [16H, t, 2×4CH2], 1.80 [16H, quin, 2×4CH2], 1.43
[16H, sex, 2×4CH2], 0.98 [24H, t, 2×4CH3].
(Hamamatsu Photonics, PMA-12).
+
Synthesis
General procedure for synthesis of (nBu4N)2[{Mo6X8}L6] (L =
–
OTs–, X– = Cl– (1), Br– (2) or I– (3); L– = PhSO3 , X– = Cl– (4), Br–
6
:
Orange. Yield: 94 mg (87%). Anal. calcd. for
(5) or I– (6)). 100 mg of (nBu4N)2[{Mo6X8}X6] (0.064 mmol for
X– = Cl–, 0.046 mmol for X– = Br–, 0.035 mmol for X– = I–) and
6.1 equiv of AgOTs or C6H5SO3Ag were dissolved in 20 mL of
acetone. The reaction mixture was stirred for 5 days at
room temperature in a flask covered by aluminum foil. The
precipitates of AgX were sedimented by centrifugation
(7000 rpm, 5 min). The supernatant was decanted and the
solvent evaporated on a rotary evaporator. The resultant
solid residue was dissolved in 2 mL of acetone. The
products were precipitated by adding 20 mL of diethyl ether
and collected by filtration. To obtain single crystals for XRD
the acetone solutions were subjected to slow diffusion of
diethyl ether vapour.
C68H102I8Mo6N2O18S6: C 27.1, H 3.4, N 0.9, S 6.4; found: C
27.4, H 3.3, N 0.9, S 6.2. UV-vis (CH2Cl2): λmax, nm (ε, M–
1·cm–1) = 347 (5.8×103), 391 (4.7×103). FTIR (KBr, cm–1):
νas(SO2) – 1272s; νs(SO2) – 1156s; ν(SO) – 985. 1H NMR
(acetone-d6) δ: 7.75-7.67 [12H, m, 6×2Hortho], 7.52-7.43 [18H,
m, 6×(2Hmeta + Hpara)], 3.43 [16H, t, 2×4CH2], 1.81 [16H, quin,
2×4CH2], 1.43 [16H, sex, 2×4CH2], 0.97 [24H, t, 2×4CH3].
Crystal structure determination
Single-crystal X-ray diffraction data were collected at 150 K on
a Bruker Nonius X8 Apex 4K CCD diffractometer fitted with
graphite monochromatised MoKα radiation (λ = 0.71073 Å). All
crystallographic information is summarised in Table 1S.
Absorption corrections were made empirically using the
SADABS program.35 The structures were solved by the direct
method and further refined by the full-matrix least-squares
method using the SHELXTL program package.35 All non-
hydrogen atoms were refined anisotropically. The positions of
the hydrogen atoms of the tetra-n-butylammonium cation,
1
:
Yellow. Yield: 130 mg (86%). Anal. calcd. for
C74H114Cl8Mo6N2O18S6: C 37.5, H 4.8, N 1.2, S 8.1; found: C
37.5, H 4.8, N 1.2, S 8.4. UV-vis (CH2Cl2): λmax, nm (ε, M–
1·cm–1) = 350 (sh, 3.2×103). FTIR (KBr, cm–1): νas(SO2) –
1295s, 1283s; ν s(SO2) – 1159s; ν(SO) – 958. 1Η ΝΜR
(acetone-d6) δ (ppm): 7.67 [12Η, d, 6×2Ηortho], 7.29 [12Η, d,
2 | J. Name., 2012, 00, 1-3
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organic ligands and solvent molecules, apart from water has been used previously to synthesise 1 39, 41
ARTICLE
.
While Johnston
molecules, were calculated corresponding to their geometrical et al. did not succeed in the structural characterization of the
conditions and refined using the riding model. The positions of complex, Sokolov et al.41 obtained crystals of solvates
hydrogen atoms for water molecules weren't localised. In the
crystal structures of and the disorder of sulfonate groups P21/c) by slow diffusion of diethyl ether vapour into either a
was caused by the rotation of terminal groups around the C–S, CH2Cl2 or an acetonitrile solution of respectively.
S–O or Mo–O bonds. We crystallised by slow diffusion of diethyl ether vapour
1·2CH2Cl2 (space group Pbca) and 1·2CH3CN (space group
2-4
6
1
1-6
CCDC 1425980-1425985 contain the supplementary into acetone solutions to generate crystals suitable for
crystallographic data for this paper. These data can be characterization by single crystal XRD. The crystallographic
obtained free of charge from the Cambridge Crystallographic data obtained are summarised in Table 1S. In our case, all six
Data Centre (www.ccdc.cam.ac.uk/data_request/cif).
complexes crystallised in the triclinic (P
clear that the choice of solvent resulted in crystals of
attaining different space group from those published
previously.41 Moreover, in our work, crystals of compound
did not contain any solvate molecules. Indeed, only
compounds and crystallised as solvates: with acetone,
diethyl ether and water ( ) or acetone and water ( ).
Although compounds and are cluster complexes of
very similar compositions, only crystals of pairs of compounds
and , and and have similar crystal structures. In the
independent part of the crystal structures of there are two
halves of the cluster anions [{Mo6X8}L6]2– and two nBu4N+
cations, while compounds and have only a half of the
1) crystal system. It is
Singlet Oxygen Generation
1
a
The ability of 1-6 to generate singlet oxygen was investigated
1
semi-quantitatively as follows: 0.6 mL of an acetone-d6
solution containing 0.0012 mmol of a cluster complex analyte
2
3
and 0.12 mmol of the singlet oxygen trap molecule 2,3-
diphenyl-p-dioxene36,
37
2
3
was placed in a conventional NMR
1
-
3
4-6
tube and purged with oxygen gas for 5 min. The tube was
sealed and then irradiated using filtered light (λ ≥ 400 nm)
from a DRSh-500 mercury lamp. 1H NMR (200 MHz) spectra
were recorded on a Bruker Avance 200 NMR spectrometer
after 0, 1, 3 and 5 h of irradiation. The conversion of the trap
molecule was calculated as a ratio of the aliphatic hydrogen
peak integral of the oxidation product of 2,3-diphenyl-p-
dioxene, namely ethylene glycol dibenzoate to the sum of
integrals of the aliphatic hydrogen peaks of 2,3-diphenyl-p-
dioxene and the oxidation product, ethylene glycol dibenzoate
(see Figure 1S-2S).
2
3
4
5
2-5
1
6
corresponding cluster anion and a nBu4N+ cation in the
independent part. In all cases the centre of the [{Mo6X8}L6]2–
cluster anion coincides with the inversion centre.
Similar to other octahedral molybdenum cluster complexes,
the Mo6 octahedra in compounds 1-6 are coordinated by eight
μ3-type halide ligands and, additionally, each molybdenum is
coordinated by a terminal ligand – p-toluenesulfonate or
benzenesulfonate ion. The organic ligands are coordinated in a
monodentate mode via an oxygen atom of the SO3 group, as
shown in Figure 1. The important average bond lengths and
angles in the crystal structures of compounds 1-6 are
summarised in Table 2S. The average bond lengths Mo–O are
between 2.100-2.165 Å and they tend to increase with
increasing size of the cluster core. Analysis of the Cambridge
Structural Database (CSD)43 data on the typical Mo–OSO2 bond
values show that the values found for 1-6 are towards the
shorter range of these bond lengths. This observation indicates
significant covalent contribution in the bonding. The Mo–O–S
bond angles also tend to increase (i.e. deviate more
significantly from the ideal tetrahedral bond angle) with
increasing size of the cluster core, with the highest values,
Results and discussion
Synthesis and characterization of (nBu4N)2[{Mo6X8}L6] (L = OTs–, X–
–
= Cl– (1), Br– (2) or I– (3); L– = PhSO3 , X– = Cl– (4), Br– (5) or I– (6))
To obtain (nBu4N)2[{Mo6X8}L6] we used a well-established
approach, in which tetra-n-butylammonium salts of
molybdenum halide anions [{Mo6X8}X6]2– (X– = Cl–, Br– or I–)
were allowed to react with a silver salt of the corresponding
sulfonic acid
– either p-toluenesulfonic acid (HOTs) or
benzenesulfonic acid (PhSO3H). In these reactions – driven to
completion by the formation of poorly-soluble and
thermodynamically favourable silver halides
– only the
terminal halides were substituted by anionic ligands.9-15, 38-42 In
all cases the reactions proceeded smoothly and resulted in the
formation of (nBu4N)2[{Mo6X8}(OTs)6] (X– = Cl– (
and (nBu4N)2[{Mo6X8}(PhSO3)6] (X– = Cl– ), Br–
respectively. The purity of compounds was confirmed by
1
), Br– (
2
), I– (
3
))
140.6° and 142.1°, found in structures
3 and 6, respectively.
(
4
(
5
), I–
(6
)),
These values are slightly higher than the most typical bond
angles between sulfonate groups and molybdenum atoms
(128-138° according to CSD).
1-6
both elemental analysis and NMR spectroscopy.
As expected, due to the symmetry of the cluster anions, only
one set of signals, associated with the organic ligands, was
The FTIR spectra of compounds 1-6 are very similar, as bands
originate mostly from the organic ligands and Bu4N+ cations.
The strong vibrations in the sulfonate group (Figure 2) provide
a simple way of monitoring the coordination of sulfonate
n
observed in the H NMR, with the ratio of signals from Bu4N+
and the organic ligands being 1:3 (Figure 3S). Moreover, in
each case the good agreement of the elemental analysis data
with the theoretically calculated formula also supports the
proposed compositions of the compounds synthesised.
1
n
ligands to the metal centres. Namely, the FTIR spectra of 1-6
further confirm the mono-dentate coordination mode of
sulfonate groups. In comparison with the ν(SO2) bands
observed in the FTIR spectra of p-toluenesulfonic acid and
It should be mentioned that the above reaction, although
conducted in a different solvent (CH2Cl2 instead of acetone),
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voltammograms of compounds
1
-6, while those of the starting
materials are shown in Figure 5S. Table 3 summarises the most
relevant data pertaining to the redox properties of the cluster
complexes.
Oxidation of the starting complexes (nBu4N)2[{Mo6X8}X6] (X– =
Cl–, Br– and I–) has been reported previously by Kirakci et al.13
Our CV measurements confirmed the earlier data that
[{Mo6Cl8}Cl6]2– and [{Mo6Br8}Br6]2– undergo one-electron
reversible oxidation in the positive potential region, while
[{Mo6I8}I6]2– undergoes irreversible oxidation, with the
oxidation potential (Table 1).13
CV measurements of compounds 1-3, 5, 6 showed quasi-
reversible oxidation in the positive region at somewhat higher
potentials than those observed for the corresponding
(nBu4N)2[{Mo6X8}X6] complexes (Table 1). For compound
4 no
Figure 1. The structure of the cluster anions [{Mo6X8}(OTs)6]2– (a) and
[{Mo6X8}(PhSO3)6]2– (b), where X– = Cl–, Br– or I–: Mo – black; X – green; S – yellow; O –
red; C – charcoal; hydrogen atoms are omitted for clarity.
oxidation peak was recorded up to potential of 2 V. In
summary it appears that substitution of the apical halide
ligands by sulfonated aromatic ligands – benzenesulfonate and
p-toluenesulfonate
–
increases the stability of the
corresponding cluster complexes towards oxidation. Likewise,
in analogous fashion to the starting complexes, oxidation of
the complexes with heavier (iodide) cluster cores takes place
at significantly lower potential, which signifies the noteworthy
input of iodide ligand orbitals to the HOMO.
Similarly to earlier reported data on oxidation of octahedral
46
molybdenum13,
and rhenium47-49 cluster complexes, the
difference between anodic and cathodic potential peaks (ΔEp)
was higher than the “ideal” value of 59 mV in the Nernst
equation for a one electron process. The increase of ΔEp
typically occurs because of uncompensated solution resistance
and/or non-linear diffusion. Indeed, the anodic and cathodic
peak currents, Ip(a) and Ip(c), respectively, were proportional
to the square root of the scan rate, which agrees with Randles-
Sevcik theory for processes controlled by diffusion. Moreover,
Figure 2. The fingerprint regions in the FTIR spectra of 1-6.
the values of Ip(a)/Ip(c) for 1-3, 5 and 6 indicate that some
adsorption of the reactant could take place.50
benzenesulfonic acid ν(SO2) bands observed in spectra of the
obtained molybdenum cluster complexes are downshifted.44, 45
Moreover, minor changes are noticeable when comparing FTIR
spectra of complexes with the same ligands, but different
halide ligands in the cluster core (Figure 2). Specifically, in the
series Cl−Br−I there is a noticeable upshift of ν(S−O) bands
In the negative potential region, both the starting compounds
(nBu4N)2[{Mo6X8}X6] (X– = Cl–, Br– and I–) and compounds
1-6
underwent irreversible reduction. Interestingly, the reduction
of all compounds took place in a very narrow range of voltages
~
–1.1 V (Table 1, Figures 4S and 5S). Such similarity in
reduction potentials suggests that the LUMO orbital in the
studied molybdenum complexes does not have significant
input from the halide (inner) ligand orbitals. The irreversible
nature of reduction is likely to be associated with the
destruction of the cluster complexes, as it has been suggested
previously for [{Mo6Cl8}Cl6]2–.51
especially in case of compounds
3 and 6. This shift might
indicate the change of the force constants (i.e. bonding
order) for the S−O bonds, which can be additional evidence
of some increase of the ionicity of the bonding between
sulfonate groups and the molybdenum atoms. The DFT
calculations undertaken for the crystal structures of 1-6 did
It is known that cyclic voltammetry can be used to estimate
the energy gap between the HOMO and the LUMO (also
referred to as the electrochemical energy gap) as the
difference between onsets of oxidation and reduction peaks.
The values of electrochemical band gap for the starting
materials and the studied compounds decrease in the order
Cl>Br>I for given apical ligands (Table 1).
not however confirm significant changes in the ionic nature
–
of the bonding between the SO3 groups and the Mo cluster
core (see ESI for details for DFT calculations, Table 3S).
Redox properties
Oxidation and reduction of compounds 1-6 were investigated
by cyclic voltammetry in acetonitrile and the results obtained
compared with those obtained for the (nBu4N)2[{Mo6X8}X6] (X–
= Cl–, Br– and I–) starting materials. Figure 4S shows the cyclic
4 | J. Name., 2012, 00, 1-3
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Table 1. Formal half-wave potentials (E1/2), the peak current difference (ΔEp) and the ratio of anodic and cathodic peak currents (Ipa/Ipc) for [{Mo6X8}L6]−/2− couples, the potential of
reduction (Epred), onset of oxidation (Eoxonset) and reduction (Eredonset) potentials and calculated electrochemical energy gaps of starting compounds (Bu4N)2[{Mo6X8}X6] (X– = Cl–, Br–
or I–) and complexes 1-6 in acetonitrile vs. saturated Ag/AgCl couple: ΔEp is the peak separation. The scan rate was 0.5 V·s−1. The optical energy gap is given for comparison.
[Mo6X8L6]2− = [Mo6X8L6]− + e
ΔEp,
[Mo6X8L6]2− + e
Optical
energy
gap,s eV
Electrochemical
energy gap, eVa
Eoxonset, V
Eredonset, V
Compound
E1/2, V
Ipa/Ipc
Epred, V
–1.40
mV
(nBu4N)2[{Mo6Cl8}Cl6]
1.78
(1.7813)b
1.55
105
1.00
1.70
1.51
0.70
–1.21
–1.26
–1.04
2.9
2.8
1.7
2.6
2.4(2.452)
2.1
(nBu4N)2[{Mo6Br8}Br6]
(nBu4N)2[{Mo6I8}I6]
100
–
1.04
–
–1.35
–1.25
(1.5713)b
1.02c
(1.1213)b
1.81
1
2
3
4
5
110
111
94
–
0.85
0.82
0.86
–
–1.09
–1.14
–1.13
–1.15
–1.13
1.72
1.77
1.44
–0.89
–0.87
–1.00
2.6
2.6
2.4
2.7
2.6
2.3
2.7
2.4
1.88
1.54
–
1.93
97
0.85
1.81
1.42
–0.81
–0.81
2.6
2.2
6
1.54
170
0.89
–1.10
onset
2.2
a Electrochemical energy gap was estimated as a difference between Eoxonset and E red
b Recalculated vs. saturated Ag/AgCl in acetonitrile
c The first oxidation peak current potential
Absorption properties
observation from electrochemical data about the significant
input from the apical ligands in the HOMO. Taking into account
the suggested nature of the HOMO and the LUMO, the lowest
energy electronic transitions arise from ligand to metal charge
transfer (LMCT). This is directly analogous to previously
reported suggestions, based on theoretical calculations, for
[{Mo6X8}(NCS)6]2− (X– = Cl–, Br–, I–)53 and isoelectronic rhenium
clusters54-56 with 24 valence electron counts (VEC).
The UV-vis absorption spectra of all the cluster complexes
were recorded in dichloromethane and are shown in Figure 3.
Comparison of the absorption spectra of (nBu4N)2[{Mo6X8}X6]13
with those of 1-6 shows that the substitution of apical halide
ligands by sulfonate ones causes a blue shift of the absorption
bands, especially in the case of the cluster complexes based on
{Mo6I8}4+ core. These observations further support the
UV-vis spectra of 1-6 show that the maxima of the absorptions
as well as the values of the extinction coefficients do not
depend on whether the apical ligand is benzenesulfonate or p-
toluenesulfonate. A pronounced red shift of absorption band
maxima is observed in the series Cl−Br−I. Similar bathochromic
shifts of absorption maxima have been reported previously for
[{Mo6X8}X6]2–, [{Mo6X8}(NCS)6]2− (X
=
Cl–, Br–, I–) and
57, 58
[{Mo6X8}(CF3COO)6]2–.13,
These shifts signify that the
energy gap between the HOMO and the LUMO in cluster
complexes [{Mo6X8}L6]2– decreases in the order Cl>Br>I for
given apical ligands. This order was further confirmed, when
the energy gaps (also known as the optical energy gap) were
estimated via Tauc plot (see inserts in Figure 3 as well as Figure
6S). These optical energy gaps, obviously, refer to spin-allowed
(i.e. singlet state to singlet state) transitions. The above values
for the electrochemical band gap for the starting materials and
the studied compounds are comparable with the optical
energy band gaps found from absorption spectra (Table 1).
Moreover the same trend in terms of the widths of the energy
gaps is also observed, i.e. the optical energy gap decreases in
the order Cl>Br>I for given apical ligands.
Photoluminescence properties
The normalised photoluminescence spectra of samples 1-6 in
the solid state and in deaerated acetonitrile solutions are
shown in Figure 4. The photophysical characteristics, namely
Figure 3. The UV-vis spectra of 1-3 (top) and 4-6 (bottom) in dichloromethane. The
emission maximum wavelengths (λem), quantum yields (Φem)
inserts are the Tauc plots used to estimate excited state energy.
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Figure 4. Normalised emission spectra of 1-6 in the solid state (top) and in deaerated acetonitrile solutions (bottom).
Table 2. Spectroscopic and photophysical parameters of cluster complexes determined for powdered samples and both aerated (non-degassed) and deaerated
1-6
CH3CN solutions. λem is an emission maximum wavelength; fwhm is the value of full width at half maximum of the emission spectrum; τem are the photoluminescence lifetimes
with the corresponding amplitudes (A) for the photoluminescence decay equation, where intensity (I) of photoluminescence vs time is expressed as ꢀ ꢁ ꢀꢂ ꢄꢆꢇ ꢃꢄ expꢈꢉ ꢋꢊ ꢍ
;
Φe is
m
ꢅ
∑
ꢌ
photoluminescence quantum yield.
(nBu4N)2[{Mo6X8}(OTs)6]
(nBu4N)2[{Mo6X8}(C6H5SO3)6]
λem, nm
λ
em, nm
τem, ꢀs (A)
Φem
0.34
<0.01
<0.01
0.29
<0.01
0.26
0.44
0.01
0.65
τem, ꢀs
Φem
0.13
<0.01
<0.01
0.13
<0.01
0.17
0.62
0.01
0.60
(fwhm, cm−1
723
)
(fwhm, cm−1
719
)
τ1 = 228 (0.7)
τ2 = 174 (0.3)
τ1 = 5.1 (0.3)
τ2 = 1.2 (0.7)
τ1 = 9.8 (0.2)
τ2 = 3.0 (0.8)
τ1 = 185 (0.7)
τ2 = 86 (0.3)
τ1 = 127 (0.3)
τ2 = 75 (0.7)
τ1 = 3.9 (0.4)
τ2 = 0.5 (0.6)
τ1 = 9.2 (0.1)
τ2 = 2.6 (0.9)
τ1 = 124 (0.3)
τ2 = 52 (0.7)
X = Cl
powdered sample
aerated solution
(~4000)
735
(~4200)
723
(~3850)
735
(~4050)
723
deaerated CH3CN
solution
(~3850)
708
(~4050)
718
X = Br
powdered sample
(~4100)
717
(~4350)
750
aerated CH3CN
solution
15
13
(~4150)
717
(~4150)
750
deaerated CH3CN
solution
243
179
(~4150)
662
(~4150)
657
τ1 = 135 (0.6)
τ2 = 56 (0.4)
τ1 = 183 (0.6)
τ2 = 57 (0.4)
X = I
powdered sample
(~2300)
667
(~2200)
665
aerated CH3CN
solution
5.0
5.2
(~2400)
667
(~2400)
665
deaerated CH3CN
solution
305
263
(~2400)
(~2400)
and lifetimes (τem) with the corresponding amplitudes (A), of equilibrated and deaerated acetonitrile solutions are
all samples measured in the solid state as well as in air- summarised in Table 2.
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As Table 2 shows, the photophysical characteristics of the by double exponential decays that are usually explained by an
complexes are strongly dependent on the halide ligand in the efficient excitation energy migration and subsequent energy
cluster core and only slightly on the composition of the apical trap/emission in the crystalline phase.
ligands.
The emission spectra for each compound, in both aerated and
deaerated solutions, have almost identical profiles. For
solutions of the bromide and iodide complexes (2, 3, 5 and 6)
the deaerated solutions are characterised by significantly
higher quantum yields and longer lifetimes than those
observed for the corresponding aerated systems (Table 2). This
difference may be explained readily by the fact that the long-
lived luminescence of hexanuclear cluster complexes is well
known to be quenched efficiently by oxygen.9-11, 38
Chloride complexes
1
and
4
behaved differently from their
deaerated
bromide and iodide analogues. Even in
a
acetonitrile solution these complexes demonstrated very weak
and relatively short-lived luminescence despite the relatively
high τem and Φem values found for them in the solid state
(Table 2). Similar low levels of luminescence in solution with a
double exponential emission decay and relatively small
lifetimes have also been reported previously for {Mo6Cl8}4+-
based clusters coordinated by fluorinated carboxylate apical
ligands.9, 13
In contrast, as in our work, the emissions of dissolved related
bromide, and especially, iodide clusters were characterised by
very high emission quantum yields with decay profiles fitted
well by single exponential functions and τem values of
Since oxygen quenches the
13
hundreds of microseconds.9,
luminescent emissions of cluster complexes
1-6 effectively
(Table 4S), we were keen to explore the potential of these
materials in applications associated with the use of singlet
oxygen generated in situ. Therefore, we compared the ability
of each of the cluster complexes
1-6 to generate singlet
oxygen using a semi-quantitative test based on oxidation of
the singlet oxygen trap molecule, 2,3-diphenyl-p-dioxene.
Accordingly, 1H NMR was used to trace oxidation of 2,3-
diphenyl-p-dioxene in the presence of the cluster complexes.
Specifically, 1:100 molar mixtures of each cluster complex:
trap were irradiated for periods of 1, 3 and 5 h. The results
obtained unambiguously confirmed the successful generation
Figure 5. The conversion of 2,3-diphenyl-p-dioxene into ethylene glycol dibenzoate in
the presence of 1-6 after photoirradiation with λ ≥ 400 nm.
Generally, all of the complexes, in both the solid state and an
acetonitrile solution, exhibited red photoluminescence with
broad spectral emission profiles ranging from ~550 to longer
than 900 nm. These are typical photoluminescence emission
of singlet oxygen by all six complexes 1-6 (Figures 5, 1S and 2S,
profiles for compounds based on {Mo6X8}4+ (X– = Cl–, Br– or I–)
Table 4S). The rate of oxidation of the 2,3-diphenyl-p-dioxene
was dependent on the halide ligand present in the cluster core
and decreased in the order I>Br>Cl for given apical ligands.
This order of singlet oxygen generation efficiencies correlates
well with the photoluminescence quantum yields recorded in
38
cluster cores.9-15,
Notably, the emission profiles of iodide
3 and 6 are significantly narrower and blue shifted
complexes
compared with the emission profiles of the chloride and
bromide-containing clusters. In combination absorption and
deaerated acetonitrile solutions of 1-6.
photoluminescence spectra of 1-6 show that the emission in
the case of {Mo6I8}4+ occurs mostly from the triplet state that is
closer to the excited singlet state than that in the case of
{Mo6Cl8}4+ and {Mo6Br8}4+. This intriguing feature of
Conclusions
[{Mo6I8}L6]2– cluster complexes, where
L is a residue
In this work we synthesised and characterised, in detail,
octahedral molybdenum cluster complexes, where the cores
{Mo6X8}4+ (X– = Cl–, Br– or I–) are coordinated by aromatic
sulfonates – either p-toluenesulfonate or benzenesulfonate.
Our study clearly confirms the strong effect that the inner
halide ligands exert on the photophysical and redox properties
of the obtained cluster complexes. Specifically UV-vis and CV
studies showed that the energy gap between ground state and
oxygenated acid has been noticed earlier by us and other
groups.9-11, 13, 38
The solid state luminescence properties of complexes 1-6 are
highly encouraging being characterised by quantum yields,
from 0.13 to 0.62. The highest quantum yields were found for
complexes 3 and 6
, i.e. those comprising {Mo6I8}4+ cluster core.
The luminescence lifetimes of all the solid samples were fitted
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Journal Name
11.
O. A. Efremova, M. A. Shestopalov, N. A. Chirtsova, A. I.
Smolentsev, Y. V. Mironov, N. Kitamura, K. A. Brylev and
A. J. Sutherland, Dalton Trans., 2014, 43, 6021-6025.
K. Kirakci, K. Fejfarova, M. Kucerakova and K. Lang, Eur. J.
Inorg. Chem., 2014, 2331-2336.
K. Kirakci, P. Kubat, J. Langmaier, T. Polivka, M. Fuciman,
K. Fejfarova and K. Lang, Dalton Trans., 2013, 42, 7224-
7232.
the first singlet (exited) state decreases in the order Cl>Br>I,
while the gap between emissive (triplet) state and the ground
state increases in the same order. Furthermore, our data also
suggest that the HOMO orbital has a significant input from
halide ligands, especially in the case of iodide cluster
complexes, while the LUMO is predominantly Mo6 derived in
nature.
12.
13.
All of the cluster complexes displayed strong red luminescence
in the solid state, while only those with heavier cluster cores
(i.e. Br– and I– inner ligands) showed outstanding
photoluminescence in deaerated acetonitrile solutions.
Notably, cluster complexes with {Mo6I8}4+ cores showed the
highest quantum yields, the narrowest and the strongly blue-
shifted emission spectra. Moreover, [{Mo6I8}(OTs)6]2– and
[{Mo6I8}(PhSO3)6]2– also showed the highest singlet oxygen
generation efficiency in comparison with the chloride and
bromide analogues. This study thus further suggests that
synthesis of highly photoactive compounds and materials may
be generally achieved by coordinating of octahedral cluster
{Mo6I8}4+ by residues of strong oxoacids. In order to facilitate
the rational design of materials with bespoke photophysical
properties, detailed study should be undertaken to uncover
whether there is any dependence between the acidy of an
oxoacid and the photoluminescence parameters of the
corresponding octahedral molybdenum cluster complexes.
14.
K. Kirakci, P. Kubat, M. Dusek, K. Fejfarova, V. Sicha, J.
Mosinger and K. Lang, Eur. J. Inorg. Chem., 2012, 3107-
3111.
15.
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K. Kirakci, V. Sicha, P. Holub and K. Lang, Inorg. Chem.
,
2014, 53, 13012-13018.
J. A. Jackson, M. D. Newsham, C. Worsham and D. G.
Nocera, Chem. Mater., 1996, 8, 558-564.
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S. Chenais and S. Forget, Polym. Int., 2011, 61, 390–406.
H. Liu, J. B. Edel, L. M. Bellan and H. G. Craighead, Small
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,
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B. C. Rowan, L. R. Wilson and B. S. Richards, IEEE J. Sel.
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C. Vonesch, F. Aguet, J.-L. Vonesch and M. Unser, IEEE
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,
F. Vatansever, W. C. M. A. de Melo, P. Avci, D. Vecchio, M.
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Acknowledgements
P. Agostinis, K. Berg, K. A. Cengel, H. Foster, A. W. Girotti,
S. O. Gollnick, S. M. Hahn, M. R. Hamblin, A. Juzeniene, D.
Kessel, M. Korbelik, J. Moan, P. Mroz, D. Nowis, J. Piette,
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This work was supported by a Marie-Curie Intra-European
Fellowship (No 327440), by the Russian Foundation for Basic
Research (Grant no. 14-03-92612), and by the Grant of the
President of the Russian Federation (MK 4054.2015.3). K.A.
Brylev thanks the Japan Society for the Promotion of Science
(JSPS) for a post-doctoral fellowship for foreign researchers.
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DOI: 10.1039/C6DT02863B