Oxoselenide triangular molybdenum clusters: Synthesis and characterization of [Mo3SeO3(acac)3(py)3]PF 6

Article abstract of DOI:10.1016/j.ica.2011.05.007

New cluster complex [Mo₃SeO₃(acac) ₃(py)₃]⁺ was obtained by ligand substitution in the aqua complex [Mo₃SeO₃(H₂O) ₉]⁴⁺. Crystal structure was det

Full text of DOI:10.1016/j.ica.2011.05.007

Inorganica Chimica Acta 375 (2011) 314–319
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Note
Oxoselenide triangular molybdenum clusters: Synthesis and characterization
of [Mo3SeO3(acac)3(py)3]PF6
P.A. Abramov a,b, M.N. Sokolov a,b, , E.V. Peresypkina a, I.V. Mirzaeva a, N.K. Moroz a, C. Vicent c,
⇑
R. Hernandez-Molina d, M.C. Goya e, M.C. Arévalo e, V.P. Fedin a,b
a Nikolayev Institute of Inorganic Chemistry, Prospekt Lavrentyeva 3, 630090 Novosibirsk, Russia
b Novosibirsk State University, ul. Pirogova 2, 630090 Novosibirsk, Russia
c Serveis Centrals d’Instrumentació Científica, Universitat Jaume I, Av. Sos Baynat s/n, 12071 Castelló, Spain
d Departamento de Quımica Inorganica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Islas Canarias, Spain
e Departamento de Química Física, Facultad de Química, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
a r t i c l e i n f o
a b s t r a c t
New cluster complex [Mo3SeO3(acac)3(py)3]+ was obtained by ligand substitution in the aqua complex
[Mo3SeO3(H2O)9]4+. Crystal structure was determined for [Mo3SeO3(acac)3(py)3]PF6ꢀC6H5CH3. The com-
plex was characterized by 77Se NMR, electrospray mass-spectrometry, and cyclic voltammetry. DFT cal-
culations were used to confirm the assignment of chemical shift and to study Mo–Mo bonding in the
cluster core.
Article history:
Received 25 January 2011
Received in revised form 29 April 2011
Accepted 5 May 2011
Available online 14 May 2011
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Molybdenum clusters
Selenide clusters
Oxide clusters
77Se NMR
NMR shift calculations
Electronic structure
1. Introduction
the aqua complex and preparation and characterization of the first
complex of the Mo3SeO3 cluster core with organic ligands,
[Mo3SeO3(acac)3(py)3]PF6ꢀC6H5CH3.
4+
4+
Triangular clusters Mo3Q4 (Q = S, Se) are key compounds in
the synthesis of a vast range of homo and heterometal chalcogen-
ide clusters. They can undergo reduction to give tetra-, hexa- and
heptanuclear clusters of different types, and they are capable of
incorporating some 20 transition and post-transition metals to give
heterometal cuboidal clusters with unique reactivity and promis-
ing catalytic properties [1–3]. Their oxygen analogs with the
Mo3O44+ core are also known [4]. They possess a completely differ-
ent reactivity and show neither tendency to clustering upon reduc-
tion nor to the incorporation of heterometals. Chemistry of the
mixed oxo-sulfido, and especially of the oxo-selenido clusters is
poorly explored, mainly because of the lack of the convenient
2. Experimental
2.1. Synthesis
The starting cluster aqua complex [Mo3(l3-Se)(l
-O)3(H2O)9]4+
was prepared as described in [6]. The stock solutions were kept
at 4 °C under Ar. All the manipulations were done in air. Pyridine
was distilled over BaO, and acetylacetone was distilled in vacuo.
Other reagents were of commercial quality and used as purchased.
DMF (Fluka) for electrochemical studies was dehydrated over so-
dium carbonate for 1 week, and was doubly distilled thereafter un-
der N2 atmosphere and reduced pressure.
4+
and selective synthetic routes to the individual Mo3Q4ꢁxOx
(x = 1–3) species [5]. Recently we have reported a simple and selec-
4+
tive synthesis of Mo3SeO3 clusters by reductive self-assembly
from [MoOCl5]2ꢁ and H2Se in situ. The aqua complex [Mo3SeO3
(H2O)9]4+ now can be easily and selectively prepared in this way
in moderate yields [6]. In this paper we report derivatization of
2.2. Synthesis of [Mo3(l3-Se)(l
2-O)3(acac)3(py)3]PF6ꢀC6H5CH3 (1)
To a stirred red solution of [Mo3(
l
3-Se)O3(H2O)9]4+ in 4 M HCl
(15 ml, 0.8 mM) 100 l of Hacac (0.97 mmol) was added, followed
by portionwise addition of KHCO3 (ca. 1 g) until the color of the
solution turned brown. Then 100 l of pyridine (1.2 mmol) and
l
⇑
Corresponding author at: Nikolayev Institute of Inorganic Chemistry, Prospekt
Lavrentyeva 3, 630090 Novosibirsk, Russia. Fax: +7 383 3309489.
E-mail address: caesar@niic.nsc.ru (M.N. Sokolov).
l
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.05.007

P.A. Abramov et al. / Inorganica Chimica Acta 375 (2011) 314–319
315
Table 1
Crystal data, data collection and structure solution details.
Crystal data
Chemical formula
C
37H44Mo3N3O9SeF6P
Mr
1232.57
triclinic, P1
100
ꢀ
Crystal system, space group
Temperature (K)
a (Å)
b (Å)
c (Å)
14.1567(3)
15.1171(5)
15.4344(8)
111.584(1)
110.983(1)
100.690(1)
2669.51(17)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
F(0 0 0)
1226
Radiation type
l
Mo K
1.47
a
(mmꢁ1
)
Crystal size (mm)
Data collection
0.27 ꢂ 0.23 ꢂ 0.15
Diffractometer
Bruker X8Apex CCD detector diffractometer
Absorption correction
empirical (using intensity measurements) based on intensities (SADABS,
Bruker, 2005)
Tmin, Tmax
0.663, 0.802
Number of measured, independent and observed [I > 2
r(I)]
30 126, 12 069, 9759
reflections
Rint
0.020
Range of h, k, l
Refinement
h = ꢁ18 ? 13, k = ꢁ19 ? 19, l = ꢁ19 ? 20
R[F2 > 2
r
(F2)], wR(F2), S
0.040, 0.146, 1.09
Number of reflections
Number of parameters
Number of restraints
H-atom treatment
12 069
653
5
H-atom parameters constrained
Weighting scheme
w = 1/[
1.23, ꢁ0.46
r
2(Fo2) + (0.0781P)2 + 7.6908P], where P = (Fo2 + 2Fc2)/3
D
qmax, D )
qmin (e Åꢁ3
Computer programs: Apex2 V.1.27 (Bruker, 2005), SHELXS97 (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997), SHELXTL V6.22 (Bruker, 2000–2005), local
programs.
0.07 g of NH4PF6 (0.43 mmol) were added in a quick succession and
the solution was left stirring for 1 h. The brownish slurry was ex-
tracted with 20 ml of CH2Cl2 for 30 min. The extract was separated,
dried over CaCl2 and the volume was reduced to 3 ml by rota-
evaporation. This solution was carefully layered with 2 ml of tolu-
ene in a thin tube, to give a crop of brown single crystals of 1 in a
few days. Yield 70%. The solid can be handled for some time in air,
but slowly converts into [Mo3(l3-O)(l2-O)3(acac)3(py)3]PF6 and
Fig. 1. View of the cluster cation [Mo3(l3-Se)(l
2-O)3(acac)3(py)3]+. Ellipsoids are shown with 50% probability.

316
P.A. Abramov et al. / Inorganica Chimica Acta 375 (2011) 314–319
Table 2
ADF2008 [7]. We used zero order regular approximation (ZORA) [8]
to account for scalar relativistic effects together with BP density
functionals [9–11]. Basis set consisted of relativistic Slater type
orbitals of triple-f quality augmented with a set of polarized orbi-
tals (TZP/ADF). Inner atomic shells were treated with frozen core
approximation (Mo – 3d; Se – 3p; C, N, and O – 1s). NMR shieldings
were calculated with DFT–GIAO method [12].
Selected bond lengths and valence angles (Å, °) (experimental).
Mo1–Mo2
Mo1–Mo3
Mo1–Se1
Mo1–O1
Mo1–O2
Mo1–N1
Mo1–O11
Mo1–O12
Mo2–Mo3
Mo2–Se1
Mo2–O2
2.6107 (5)
2.6050 (5)
2.4983 (5)
1.911 (3)
1.917 (3)
2.239 (4)
2.099 (3)
2.071 (3)
2.6033 (5)
2.5006 (5)
1.912 (3)
Mo2–O3
Mo2–N2
Mo2–O21
Mo2–O22
Mo3–Se1
Mo3–O1
Mo3–O3
Mo3–N3
Mo3–O31
Mo3–O32
1.908 (3)
2.227 (4)
2.102 (3)
2.085 (3)
2.4996 (6)
1.913 (3)
1.908 (3)
2.239 (4)
2.086 (3)
2.095 (4)
To characterize chemical bonding in the system we used topo-
logical analysis of Electron Localization Function (ELF) [13,14]. This
method allows partitioning of the real space into chemically mean-
ingful regions: core electrons, lone pairs, and covalent bonds. Val-
ues of ELF are unitless. The upper limit ELF = 1 corresponds to
perfect localization and the value ELF = 0 means that no electrons
are localized at this point. Since ELF is a scalar function, analysis
of its gradient field can be carried out in order to locate its attrac-
tors (the local maxima) and corresponding basins (the region of
space around the attractor that contains all ELF gradient trajecto-
ries that end at this attractor). Core basins (surrounding nuclei)
are denoted by C(A) where A stands for the atomic symbol of the
atom to which it belongs. V(A, B, . . ., X) denotes a valence basin
shared by atomic centers A, B, . . ., X. Synaptic order of a valence ba-
sin is the number of core basins that have a common boundary
with it. If there is a proton within this valence basin, it counts
one for the synaptic order. Localization domain is the volume sur-
rounded by the isosurface ELF = const. At low values of ELF, there is
a single localization domain that contains the whole molecule.
While ELF value increases, the single domain divides into several
domains, enabling definition of chemical units within the system.
O1–Mo1–Se1
O1–Mo1–O2
O1–Mo1–N1
O1–Mo1–O11
O1–Mo1–O12
O2–Mo1–Se1
O2–Mo1–N1
O2–Mo1–O11
O2–Mo1–O12
N1–Mo1–Se1
O11–Mo1–Se1
O11–Mo1–N1
O12–Mo1–Se1
O12–Mo1–N1
O12–Mo1–O11
O2–Mo2–Se1
O2–Mo2–N2
O2–Mo2–O21
O2–Mo2–O22
O3–Mo2–Se1
O3–Mo2–O2
O3–Mo2–N2
O3–Mo2–O21
O3–Mo2–O22
N2–Mo2–Se1
O21–Mo2–Se1
104.01 (9)
95.36 (13)
81.61 (13)
88.68 (13)
163.87 (13)
103.39 (9)
83.43 (13)
166.66 (13)
91.86 (13)
170.52 (9)
87.88 (9)
O21–Mo2–N2
O22–Mo2–Se1
O22–Mo2–N2
O22–Mo2–O21
O1–Mo3–Se1
O1–Mo3–N3
O1–Mo3–O31
O1–Mo3–O32
O3–Mo3–Se1
O3–Mo3–O1
84.20 (13)
87.98 (8)
86.01 (13)
81.05 (12)
103.90 (9)
83.41 (13)
165.56 (14)
89.36 (15)
103.89 (9)
94.39 (12)
81.67 (13)
91.65 (14)
165.55 (14)
170.25 (10)
87.30 (9)
84.51 (13)
81.71 (16)
88.68 (10)
84.92 (14)
62.968 (15)
62.830 (15)
62.750 (15)
85.88 (12)
85.97 (12)
86.02 (12)
O3–Mo3–N3
84.60 (13)
88.25 (9)
O3–Mo3–O31
O3–Mo3–O32
N3–Mo3–Se1
O31–Mo3–Se1
O31–Mo3–N3
O31–Mo3–O32
O32–Mo3–Se1
O32–Mo3–N3
Mo1–Se1–Mo2
Mo1–Se1–Mo3
Mo3–Se1–Mo2
Mo1–O1–Mo3
Mo2–O2–Mo1
Mo3–O3–Mo2
84.93 (13)
81.24 (13)
103.45 (9)
82.76 (13)
165.99 (13)
92.95 (12)
103.83 (9)
95.75 (12)
80.91 (13)
87.19 (12)
163.23 (13)
171.59 (10)
89.06 (9)
2.5. X-ray crystallography
Crystallographic data and refinement details are given in Ta-
ble 1. Crystals of 1 quickly lose solvent and become amorphous
in air. Mineral oil was used to suppress the decay. The diffraction
data were collected on a Bruker X8Apex CCD diffractometer with
must be kept under Ar. IR (KBr, cmꢁ1): 1608s, 1568s, 1529s, 841s,
558m. ESI-MS (+) (CH2Cl2): [Mo3( 3-Se)(
2-O)3(acac)3(py)3]+ (m/
z = 950), [Mo3( 3-Se)(
2-O)3(acac)3(py)2]+ (m/z = 870), [Mo3(l3
Se)(
2-O)3(acac)3(py)]+ (m/z = 792). 77Se NMR (d, ppm, CDCl3, stan-
l
l
Mo Ka radiation (k = 0.71073 Å) by doing u and x scans of narrow
l
l
-
(0.5°) frames at 100 K. Structure of 1 was solved by direct methods
and refined by full-matrix least-squares treatment against |F|2 in
anisotropic approximation with SHELXTL programs set [15]. Absorp-
tion corrections were applied empirically with SADABS program [16].
All non-hydrogen atoms of main structural units were refined
anisotropically. The hydrogen atoms were refined in their geomet-
rically calculaꢁted positions; a riding model was used for this pur-
pose. The PF6 anion was found to be disordered over two close
positions with 0.6/0.4 occupancy ratio. Since solvent toluene mol-
ecules are severely disordered, most of their carbon atoms were re-
fined in isotropic approximation, while the hydrogen atoms were
not localized.
l
dard D2SeO3): 1356. Anal. Calc. for 1: C, 36.0; H, 3.6; N, 3.4. Found:
C, 35.4; H, 3.1; N, 3.2%.
2.3. NMR studies
NMR studies of [Mo3(l3-Se)(l
2-O)3(acac)3(py)3]+ in CDCl3 solu-
tion at room temperature were carried out on a Bruker Avance 500
spectrometer operating at 95.38 MHz for 77Se nucleus. 77Se chem-
ical shift was referenced to external H2SeO3 (d = 1282 ppm) and
then recalculated versus SeMe2 (d = 0 ppm). The total range of
scanned 77Se shifts runs from ꢁ1000 to 2000 ppm.
2.6. Electrochemistry
2.4. DFT calculations
Electrochemical measurements were carried with a Mod House
273 A EG and G PARC potentiostate/galvanostate. Glassy carbon
electrode (GC, CHI 104, geometrical exposed area 12.6 mm2) was
Geometries of [Mo3(l3-Se)(l
2-O)3(acac)3(py)3]+ cation and of
the reference compounds SeMe2 and H2SeO3 were optimized with
Table 3
Selected bond lengths and valence angles (Å, °) (optimized).
Bond
Bond length (Å)
Angle
Angle value (°)
Angle
Angle value (°)
Mo1–Mo2
Mo1–Se1
Mo1–O1
Mo1–O11
Mo1–N1
2.66
2.55
1.94
2.12
2.28
O1–Mo1–Se1
O1–Mo1–O2
O1–Mo1–N1
O1–Mo1–O11
O1–Mo1–O12
N1–Mo1–Se1
103
95
82
90
165
172
O11–Mo1–Se1
O11–Mo1–N1
O12–Mo1–O11
Mo1–Se1–Mo2
Mo1–O1–Mo3
89
85
82
63
87

P.A. Abramov et al. / Inorganica Chimica Acta 375 (2011) 314–319
317
Scheme 1. ELF domain reduction in [Mo3(l3-Se)(l
2-O)3(acac)3(py)3]+.
Table 4
Calculated 77Se isotropic shielding riso and chemical
shifts dcalc compared to experimental values dexp (ppm).
SeMe2 H2SeO3 [Mo3(l3-Se)(l2-
O)3(acac)3(py)3]+
riso 1665
350
1315
1282
275
1390
1356
dcalc
dexp
0
0
used as working electrode, and Pt wire as counter electrode. The
reference electrode was Ag/AgCl/TMA chloride (saturated solution
in 3:1 acetonitrile–DMF), however the potentials reported are re-
ferred to SCE in water. Scans were run at 5, 10, 25, 50, 100, 250,
350, 500 and 750 mV sꢁ1. The parameters used in SWV were: fre-
quency 2 Hz, pulse height 25 mV and scan increment of 4 mV.
Background electrolyte was 0.1 M (Bu4N)ClO4. Ten milliliters of
0.53 mM solution of 1 in DMF was used in the experiments.
3. Results and discussion
The preparation of 1 is based upon general approach for prepa-
ration of [M3Q4(acac)3(py)3]PF6 salts from the aqua complexes
[M3Q4(H2O)9]4+ (M = Mo, W; Q = S, Se) as described in [19]. The
only-chalcogenide bridged M3Q44+ cores are more robust then their
mixed oxo-chalcogenide derivatives. Neutralization with KHCO3
during the preparation must be done by very careful portionwise
addition of the reagent with the amount just enough to make the
pH sufficiently high for complexation to proceed, and the interme-
diate species, presumably [Mo3SeO3(acac)3(H2O)3]+, must be
immediately converted into the pyridine-substituted derivative
[Mo3SeO3(acac)3(py)3]+, followed by rapid precipitation with
4+
NH4PF6. In this way formation of the oxo-cluster Mo3O4 through
Se loss is averted.
The IR spectrum of 1 shows bands at 1608 cmꢁ1 attributable to
coordinated py (
m (C@C + C@N)), and two characteristic acac bands
(m
(C@C + C@O)) at 1568 and 1529 cmꢁ1. The PF6ꢁ anion gives very
Fig. 2. ELF plots for selected planes: (a) plane containing three Mo atoms, (b) the
plane containing one Se and two Mo atoms, and (c) the plane containing Se, Mo and
intense bands at 841 and 558 cmꢁ1. ESI mass spectrum at low cone
voltages shows dominant presence of the intact cluster cationic
species [Mo3SeO3(acac)3(py)3]+. Upon harsher ionization condi-
tions the spectra show characteristic fragmentation patterns from
the sequential loss of the coordinated pyridine molecules, up to
[Mo3SeO3(acac)3(py)]+. Aged samples also show a signal from
[Mo3O4(acac)3(py)3]+.
l2-O atoms (the plane is perpendicular to Mo–Mo bond). Isoline step is 0.05.
anions and toluene solvent molecules occupy the voids of the cat-
ionic arrangement. Selected bond distances and angles in the clus-
4+
ter complex are listed in Table 2. The Mo3SeO3 core in 1 appears
to be slightly expanded in comparison with the same core in the
The crystal structure of 1 is built of triangular cluster cations
supramolecular adduct with cucurbit[6]uril (Q6) {[Mo3(
O)3(H2O)6Cl3]2{Q6}}Cl2ꢀ15H2O (Mo–Mo 2.565(1) Å, Mo-l2
(av) = 1.908(6) Å, Mo-
[M3Q4(acac)3(py)3]+ complexes [19], the acacꢁ ligands occupy
l
3-Se)(
l
-
-
[Mo3(
with three
one apical selenium ligand (Mo-
l3-Se)(l
2-O)3(acac)3(py)3]+ (Fig. 1, Mo–Mo(av) = 2.606 [3] Å)
l2-bridged oxo-ligands (Mo-
l
2-O(av) = 1.912[4] Å) and
O
l3-Se(av) = 2.492(2) Å) [6]. In all the known
ꢁ
l
3-Se(av) = 2.500 [1] Å). The PF6

318
P.A. Abramov et al. / Inorganica Chimica Acta 375 (2011) 314–319
Fig. 6. Square wave voltammetry of 1.
Fig. 3. Bonding between molybdenum atoms: valence basins V(Mo, Mo) and V(Mo,
Mo, Mo). Isosurface ELF = 0.395.
cis-positions to the capping selenium atom (Mo–O(av) = 2.09(2) Å),
and the pyridine ligands occupy the trans-positions (Mo–
N
(av) = 2.233(6) Å). The Mo–O and Mo–N bonds in 1 are distinc-
-20
5 mVs-1
tively shorter than in the sulfide cluster cation [Mo3S4(acac)3
Ep=-0.641 V
10 mVs-1
(py)3]+ (2.121(3) and 2.262(5) Å), respectively.
-1-54
-1-02
25 mVs-1
We found one narrow 77Se NMR signal at d = 1356 ppm in solu-
tions of 1 in CDCl3. Half-width of the signal was 4.3 Hz. This proves
that only one type of Se-containing species is present in the solu-
Ep=0.288 V
50 mVs-1
100 mVs-1
250 mVs-1
350 mVs-1
500 mVs-1
750 mVs-1
tion. Observed shift value is much higher then known
ical shifts in the trinuclear molybdenum clusters with one capping
Se and three bridging Se2 ligands (76.9 ppm in [Mo3(
l3-Se chem-
-5
0
l
3-Se)(l2
-
-
Se2)3{Se2P(OCH2CH3)2}3]Br [13], 666.4 ppm in [Mo3(
l3-Se)(l2
0
Se2)3(C2O4)3]2ꢁ [14]). Nevertheless, it falls in the known range of
NMR shifts for
l3-Se atoms in transition metal clusters (from
52
ꢁ608.1 ppm [17] to 2066 ppm [18]).
In order to confirm that the observed value of 77Se chemical
l3-Se)(l
2-O)3(acac)3(py)3]+
104
0.4
shift really corresponds to the [Mo3(
species, DFT calculations have been carried out. The geometry of
[Mo3( 3-Se)(
2-O)3(acac)3(py)3]+ was optimized starting from
Ep=-0.662 V
0.0
-0.8
-1.2
0.4
0.0 -0.4
-0.4
-0.8
l
l
Potential / V vs SCE
atomic coordinates taken from the X-ray data. Resulting cluster
has C3 symmetry. Interatomic distances (Table 3) turned out to
be slightly longer then the experimental values (Table 2), while va-
lence angles were almost the same. We calculated 77Se isotropic
magnetic shielding and NMR chemical shifts in the optimized
Fig. 4. CV for freshly prepared solution of 1 in DMF at different scan rates.
[Mo3(l3-Se)(l
2-O)3(acac)3(py)3]+ cation, and in two reference com-
-30
pounds SeMe2 and H2SeO3 (Table 4). All calculated values are in
good agreement with experimental data and thus unequivocally
confirm the identity of the Se-containing species in solution as
10 mVs-1
25 mVs-1
50 mVs-1
100 mVs-1
500 mVs-1
c2
c3
-20
-10
0
[Mo3(
Bonding hierarchy inside the [Mo3(
l3-Se)(l
2-O)3(acac)3(py)3]+.
l
3-Se)(l
2-O)3(acac)3(py)3]+
c1
cation is shown by electron localization domain reduction scheme
(Scheme 1). Core domains of light atoms and valence domains of
pyridine and acetylacetone separate form {Mo3(
cluster core at ELF = 0.15–0.16, thus indicating that the {Mo3(l3
Se)(
2-O)3}4+ core is an individual chemical unit. If we further as-
sume that, like 2-O atoms, 3-Se atom takes up two electrons to
l3-Se)(l
2-O)3}4+
-
l
a3
l
l
become Se2ꢁ, there are two electrons left for each molybdenum
atom. In consequence, we might expect three ordinary two-center
two-electron Mo–Mo bonds and no multicenter bonds (the con-
cept of multicenter bonding was developed for electron deficient
systems [20]). Absence of V(Se, Mo) valence basins confirms essen-
tially ionic nature of bonding between selenium and three molyb-
denum atoms. Nevertheless, along with three disynaptic V(Mo,
Mo) basins, there is a trisynaptic basin V(Mo, Mo, Mo) that indi-
cates presence of three-center bond (Fig. 2) [22]. The V(Mo, Mo)
a2
10
aII1
aI1
0.4
0.0
-0.4
-0.8
-1.2
Potential /V vs SCE
Fig. 5. CV for freshly prepared solution of 1 in DMF at different scan rate in terms of
normalized current.

P.A. Abramov et al. / Inorganica Chimica Acta 375 (2011) 314–319
319
Table 5
Peak potential variations (Ep and
DEp) as function of scan rate.
2
3
v/(V sꢁ1
)
Ep aI1/V
Ep aII1/V
Epa2/V
Epa3/V
Epc1/V
Epc2/V
Epc3/V
D
Ep (a2–c2)
D
Ep (a3–c3)
0.005
0.01
0.025
0.05
0.1
0.250
0.350
0.5
0.009
0.0043
0.000
ꢁ0.005
ꢁ0.011
–
–
–
–
–
–
–
ꢁ0.623
ꢁ0.620
ꢁ0.627
ꢁ0.621
ꢁ0.620
ꢁ0.604
ꢁ0.598
ꢁ0.589
ꢁ0.567
–
–
–
–
ꢁ0.333
ꢁ0.322
ꢁ0.362
ꢁ0.415
ꢁ0.445
–
–
–
–
ꢁ0.705
ꢁ0.697
ꢁ0.702
ꢁ0.697
ꢁ0.689
ꢁ0.686
ꢁ0.698
ꢁ0.708
ꢁ0.731
ꢁ0.881
ꢁ0.883
ꢁ0.880
ꢁ0.883
ꢁ0.894
ꢁ0.907
ꢁ0.905
ꢁ0.917
ꢁ0.939
0.082
0.077
0.075
0.076
0.074
0.082
0.100
0.119
0.164
–
–
–
–
ꢁ0.184
ꢁ0.166
ꢁ0.126
ꢁ0.114
ꢁ0.096
ꢁ0.071
ꢁ0.812
ꢁ0.811
ꢁ0.801
ꢁ0.798
ꢁ0.768
0.082
0.096
0.104
0.119
0.171
0.7
basins have common borders with the V(Mo, Mo, Mo) basin and
the difference between ELF values at the turning points and at
the attractors is 0.02 (Fig. 3). Thus, as it was suggested in [21], it
is more chemically meaningful to consider the union of the corre-
sponding basins rather than the individual basins themselves. Sim-
ilar bonding picture was observed in model sulfide cluster
Acknowledgments
The work was supported by RFBR Grant 09-03-00413, State
Contract No. 02.740.11.0628, and Haldor Topsøe. Also this work
was partially supported by Grant of FFP ‘‘SSESIR 2009-2013’’
(2010-1.2.2-210-004-036).
[Mo3(l3-S)(l
2-S)3(PH3)6Cl3]+ [22].
Appendix A. Supplementary material
3.1. Cyclic voltammetry
CCDC 786056 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.ica.2011.
05.007.
Fig. 4 shows CV for freshly prepared solution of 1 in DMF at dif-
ferent scan rate. Three cathodic peaks (c1, c2 and c3) and four ano-
dic peaks (aI1, aII1, a2 and a3) are observed at low scan rates.
Increase in the scan rate causes gradual disappearance of c1 and
aI1, as can be better seen in Fig. 5, where normalized current value
(i.e., divided by v1/2), is given. Fig. 6 shows square wave voltamme-
try results. Clearly distinguishable are two reversible peaks in the
negative area and one quasi reversible peak in the positive area.
The negative peaks agree well with the two most negative peaks
in the CV. All the results are summarized in Table 5, where the val-
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D
Ep (Ea ꢁ Ec ) are also given. From all these observations we
p
p
can conclude that c2/a2 and c3/a3 couples correspond to quasi
reversible processes. At low scan rates the peak potentials are vir-
tually the same in both cases, however,
DEp is higher than is re-
quired for a perfectly reversible couple. Increase in scan rates
displaces the potentials to more negative, and more positive val-
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respectively. In agreement with the published electrochemical
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4+
behavior of various M3Q4 clusters, including those with the clo-
sely related {Mo3(l3-S)(l
2-O)3}4+ core [23], we can assign the neg-
ative potential waves to two consecutive one-electron reductions
in the cluster core:
Mo3IVSeO34þ ! Mo2IVMoIIISeO34þ ! MoIVMo2IIISeO43þ
:
In [Mo3Se4(acac)3(py)3]+ corresponding processes were observed at
E1/2 ꢁ0.85 and ꢁ0.98 V, respectively [19]. As could be expected,
when more electronegative oxide ligands replace selenides in the
cluster core, the latter becomes more easily reducible. The c1 and
aI1 peaks can be tentatively assigned to cluster core oxidation
which gives unstable species.