Solid state synthesis, structures and redox properties of the new [Mo 6Bri7TeiBra 6]3- and [Mo6Bri7Se iBra6]3- octahedral cluster units: Crystallochemistry of the Rb2+

Article abstract of DOI:10.1016/j.jssc.2005.07.026

The solid state synthesis and the structure of the Rb₂Mo ₆Brⁱ₈Br^a₆ bromide (C2/c, a=20.376(5)A, b=15.297(5)A, c=9.757(5)A, β=115.969(5) °, Z=4, R1=0.037; wR2=0.057) and Rb_2+xMo ₆Brⁱ_8-xYⁱ _xBr^a₆ chalcohalides built up from molybdenum octahedral clusters (x=0.5 for Y=Te; 0.25≤x≤0.7 for Y=Se) are presented. The crystallochemistry results are based on combined single-crystal and powder X-ray diffraction investigations from several preparations with different loaded compositions. A continuous solid state solution is evidenced between the two limit compositions Rb_2.25Mo₆Br_13.75Se _0.25 (Pn-3, a=13.999(2)A, Z=4, R1=0.039; wR2=0.058) and Rb _2.7Mo₆Br_13.3Se_0.7 (Pn-3, a=14.144(2)A, Z=4, R1=0.029; wR2=0.048). On the other hand, in the case of tellurium (data: Rb_2.5Mo₆Br_13.5Te _0.5, Pn-3, a=14.0936(5)A, Z=4, R1=0.028; wR2=0.048) no solid state solution is observed as previously reported for Rb_2.5Re ₆S_6.5Cl_7.5 and K_2.5Re ₆S_6.5Br_7.5. The structural features that act in the stabilization of the new [Mo₆Br₁₃Y]^3- units (Y=Se, Te) are evidenced and discussed by comparison with those obtained for related compounds in particular in Rb_2.5Re₆S _6.5Cl_7.5, K_2.5Re₆S _6.5Br_7.5, and A₂W₆Br₁₄ series (A=monovalent cation). The substitution of chalcogen for halogen strongly modifies the intrinsic properties of the M₆L₁₄ unit. Indeed, the electrochemistry performed on the [Mo₆Br ₁₃Y]^3- unit after dissolution of the crude solid samples isolated reveals a new and chemically reversible oxidation process at E1/2=0.46V vs. SCE for [Mo₆Br₁₃Se]^3- whilst a similar oxidation process centred at 0.38 V is observed for [Mo₆Br ₁₃Te]^3-.

Full text of DOI:10.1016/j.jssc.2005.07.026

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 3117–3129
www.elsevier.com/locate/jssc
Solid state synthesis, structures and redoxproperties of the new
[Mo6Br7i TeiBr6a]3ꢀ and [Mo6Bri7SeiBra6]3ꢀ octahedral cluster units:
Crystallochemistry of the Rb2+xMo6Bri8ꢀxYixBr6a series (x ¼ 0.5
for Y ¼ Te; 0.25pxp0.7 for Y ¼ Se) and Rb2Mo6Br14
Ã
Kaplan Kirakcia, Stephane Cordiera, , Olivier Hernandeza, Thierry Roisnela,
Frederic Paulb, Christiane Perrina
a
´ ´
Laboratoire de Chimie du Solide et Inorganique Moleculaire, UMR 6511 CNRS-Universite de Rennes 1, Institut de Chimie de Rennes,
Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France
b
´ ´ ´
Laboratoire Organometallique et Catalyse: Chimie et Electrochimie Moleculaire, UMR 6509 CNRS-Universite de Rennes 1,
´ ´
Institut de Chimie de Rennes, Avenue du General Leclerc, 35042 Rennes Cedex, France
Received 4 May 2005; received in revised form 22 July 2005; accepted 26 July 2005
Available online 25 August 2005
Abstract
i
a
(
(
(
The solid state synthesis and the structure of the Rb2Mo6Br8Br6 bromide (C2/c, a ¼ 20:376ð5Þ A, b ¼ 15:297ð5Þ A, c ¼ 9:757ð5Þ A,
b ¼ 115:969ð5Þ1, Z ¼ 4, R1 ¼ 0:037; wR2 ¼ 0:057) and Rb2+xMo6Bri8ꢀxYixBr6a chalcohalides built up from molybdenum octahedral
clusters (x ¼ 0:5 for Y ¼ Te; 0:25pxp0:7 for Y ¼ Se) are presented. The crystallochemistry results are based on combined single-
crystal and powder X-ray diffraction investigations from several preparations with different loaded compositions. A continuous
(
solid state solution is evidenced between the two limit compositions Rb2.25Mo6Br13.75Se0.25 (Pn-3, a ¼ 13:999ð2Þ A, Z ¼ 4,
(
R1 ¼ 0:039; wR2 ¼ 0:058) and Rb2.7Mo6Br13.3Se0.7 (Pn-3, a ¼ 14:144ð2Þ A, Z ¼ 4, R1 ¼ 0:029; wR2 ¼ 0:048). On the other hand, in
(
the case of tellurium (data: Rb2.5Mo6Br13.5Te0.5, Pn-3, a ¼ 14:0936ð5Þ A, Z ¼ 4, R1 ¼ 0:028; wR2 ¼ 0:048) no solid state solution is
observed as previously reported for Rb2.5Re6S6.5Cl7.5 and K2.5Re6S6.5Br7.5. The structural features that act in the stabilization of the
new [Mo6Br13Y]3ꢀ units (Y ¼ Se, Te) are evidenced and discussed by comparison with those obtained for related compounds in
particular in Rb2.5Re6S6.5Cl7.5, K2.5Re6S6.5Br7.5, and A2W6Br14 series (A ¼ monovalent cation). The substitution of chalcogen for
halogen strongly modifies the intrinsic properties of the M6L14 unit. Indeed, the electrochemistry performed on the [Mo6Br13Y]3ꢀ
unit after dissolution of the crude solid samples isolated reveals a new and chemically reversible oxidation process at E1=2 ¼ 0:46 V
vs. SCE for [Mo6Br13Se]3ꢀ whilst a similar oxidation process centred at 0.38 V is observed for [Mo6Br13Te]3ꢀ
.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Mo6 cluster compound; Halide; Chalcogenide; Solid state synthesis; Crystal structure
1. Introduction
according to the Schafer and Schnering notation [1]).
Such units are stabilized for 24 valence electrons per
cluster (VEC). Crystal structures of Mo6 compounds are
built up from units that are either discrete or inter-
connected by shared inner and/or apical ligands [1–2].
Recently, we have entered the field of the molybdenum
octahedral cluster chalcohalide chemistry, synthesized
by solid state chemistry techniques, containing discrete
[(Mo6X8ꢀxYx)X6]nꢀ units with the following formula:
The molybdenum octahedral cluster chemistry is
based on [(Mo6Li8)L6a] units in which the Mo6 cluster is
face-capped by eight inner ligands (Li) and additionally
bonded to sixapical ones (L a) (a ¼ apical, ¼ inner
i
Ã
Corresponding author. Fax: +33 2 23 23 67 99.
E-mail address: Stephane.Cordier@univ-rennes1.fr (S. Cordier).
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.07.026

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K. Kirakci et al. / Journal of Solid State Chemistry 178 (2005) 3117–3129
3118
Ay[(Mo6X8ꢀxYx)X6] with A ¼ cation, X ¼ halogen,
Y ¼ chalcogen [3]. The aim of this work is to control
the chalcogen/halogen ratio and consequently the charge
of the [(Mo6X8ꢀxYx)X6]nꢀ final anionic unit in the
Ay[(Mo6X8ꢀxYx)X6] series through the choice of counter
cation and its loaded stoichiometry. After dissolution,
this type of anions could further be involved in
coordination chemistry for the design of new molecular
materials [4] or compounds with extended open frame-
works [5]. Interestingly, the substitution of chalcogen for
halogen enables the modification of the electronic
properties of the Mo6L14 unit and in particular the
decrease of their oxidation potentials [6–8]. Recently, we
have isolated the Cs4Mo6Bri6Y2i Br6a (Y ¼ S and Se) series
based on [(Mo6Br6i Y2i )Bra6]4ꢀ units [3], in which chalco-
gen and bromine atoms are randomly distributed on
the inner positions. Subsequently, cyano [(Mo6Bri6Y2i )
(CN)a6]4ꢀ substituted units have been obtained by the
interaction of the Cs4Mo6Bri6Y2i Bra6 solid state precursor
with aqueous solution of KCN [3]. Attempts to obtain
the ‘Cs3Mo6Br13Y’series based on [(Mo6Bri7Y1i )Br6a]3ꢀ
units have been unfruitful. In the present work, we
present the synthesis and crystallochemistry of the
Rb2Mo6Bri8Bra6 bromide and Rb2+xMo6Bri8ꢀxYixBr6a
chalcohalides built up from molybdenum octahedral
clusters (x ¼ 0:5 for Y ¼ Te; 0:25pxp0:7 for Y ¼ Se).
The Rb2Mo6Bri8Br6a bromide is based on [Mo6Br8i Bra6]2ꢀ
units whilst the Rb2.5Mo6Bri7.5Tei0.5Bra6 and Rb2+xMo6-
Bri8ꢀxSeixBra6 (0:25oxo0:7) chalcobromides are based
on a mixture of [Mo6Bri8Br6a]2ꢀ and [Mo6Br7i YiBra6]3ꢀ
units. The phase breadth of the Rb2+xMo6Bri8ꢀxSeixBr6a
series is studied by combined single-crystal and powder
X-ray diffraction investigations from several prepara-
tions with different loaded compositions. The structural
features that act in the stabilization of the new
[Mo6Br13Y]3ꢀ units (Y ¼ Se, Te) are evidenced and
discussed by comparison with those obtained for related
compounds in particular in Rb2.5Re6S6.5Cl7.5 [9],
and MoBr2 according to the same procedure described
for the synthesis of Cs2Mo6Br14 [13] substituting RbBr
for CsBr. Single-crystals suitable for X-ray diffraction
studies were obtained directly during such syntheses.
2.1.2. Synthesis of Rb2+xMo6Bri8ꢀxYixBr6a series
Typically, a powdered mixture (0.5 g) of RbBr, Mo,
MoBr2 and Y (Y ¼ Te (Alfa Aesar 99.9%), Se (Alfa
Aesar 99.99%)) was ground, formed as a pellet and
placed into a silica tube. Once sealed under vacuum, the
tube was heated for 3 days at 900 1C. Single-crystals of
Rb2.5Mo6Br13.5Te0.5 and Rb2.7Mo6Br13.3Se0.7 have been
initially obtained from reactions designed to synthesize
the stoichiometric chalcobromide with the following
formula: ‘Rb3Mo6Br13Y’. After preliminary structural
determinations, by single-crystal X-ray diffraction tech-
nique, several syntheses were performed with various
‘Rb2+xMo6Bri8ꢀxYixBr6a’ loaded compositions in order
to determine whether the Rb2.5Mo6Br13.5Te0.5 and
Rb2.7Mo6Br13.3Se0.7 stoichiometries, found in these
preliminary studies, correspond to definite compositions
or whether they correspond to a particular composition
within phase breadths. In the ‘Rb2+xMo6Bri8ꢀxYixBr6a’
loaded compositions, the rubidium and chalcogen
contents were adjusted to maintain a VEC value of 24
on the Mo6 cluster, that corresponds to a full electronic
occupation of the Mo–Mo bonding levels of the Mo6L14
molecular orbital diagram [2]. Consequently, such
compositions correspond to compounds built up
from [M6Bri8ꢀxYixBr6a](2+x)ꢀ average anionic units—
corresponding to a mixture of (1ꢀx)[Mo6Br14]2ꢀ with
x[Mo6Br13Y]3ꢀ—associated with (2+x) Rb+ counter
cations. In order to homogenize the powders and to
increase the yield of reaction, after a first heating, the
compounds were ground in inert atmosphere and heated
once more at the same temperature for 3 days. The
single-crystals for X-ray diffraction investigations were
all issued from syntheses of the first heating since the
single-crystals issued from the second heating were too
small for such investigations. As discussed below,
combined single-crystal and powder X-ray diffraction
investigations of the resulting products revealed a phase
breadth for the Rb2+xMo6Bri8ꢀxSeixBr6a series (with
0:25pxp0:7) and a defined composition for Rb2.5Mo6
Bri7.5Tei0.5Bra6.
K
2.5Re6S6.5Br7.5 [10], and A2W6Br14 bromides [11].
Cyclic Voltammetry reveals a reversible 24/23 elec-
trons process at E1=2 ¼ 0:46 V vs. Standard Calomel
Electrode (SCE) for [Mo6Br13Se]3ꢀ and at 0.38 V vs.
SCE for [Mo6Br13Te]3ꢀ
.
2. Experimental part
2.2. Energy dispersive spectrometry (EDS) analysis
2.1. Synthesis
Elemental content has been determined first by
chemical analyses of single-crystals at the ‘‘Centre for
Scanning Electron Microscopy and Microanalyses of
Rennes 1 University, France’’ by Energy Dispersive
Spectrometry (EDS) using a scanning electron micro-
scope JEOL JSM 6400 equipped with a microprobe
EDS OXFORD LINK ISIS. The atomic percentages
obtained for several single-crystals for Rb2Mo6Br14
2.1.1. Synthesis of Rb2Mo6Br14
First, MoBr3 tribromide was obtained by reaction
of molybdenum powder (Plansee, 99.95) in Br2
flow (Prolabo, technique) at 680 1C. MoBr2 was then
prepared by decomposition of MoBr3 at 680 1C under
N2 flow [12]. Rb2Mo6Br14 was synthesized from a
stoichiometric mixture of RbBr (Alfa Aesar 99.9%)

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3119
were: Rb: 8%, Mo: 31%, Br: 61% (theoretical Rb: 9%,
Mo: 27.27%, Br: 63.63%). Analysis performed on
several selected single-crystals of Rb2.5Mo6Bri7.5Te0i .5Br6a
from several ‘Rb2+xMo6Bri8ꢀxTeixBra6’ preparations
showed that they all contained the expected elements
and that they exhibit the same loaded compositions with
the following atomic percentages: Rb: 12, Mo: 29, Br:
56, Te: 3 (theoretical for Rb2.5Mo6Bri7.5Tei0.5Br6a: Rb:
11.11, Mo: 26.66, Br: 60, Te: 2.22). The refined cell
parameters determined afterwards on these single-
crystals did not deviate within the s.u.’s corroborating
a definite composition. Contrary to what has been
observed for Rb2Mo6Br14 and Rb2.5Mo6Bri7.5Te0i .5Br6a,
the EDS analysis performed on selected single-crystals
of ‘Rb2+xMo6Bri8ꢀxSeixBra6’ series revealed some fluc-
tuations in the composition depending on the loaded
one. However, for several single-crystals from the
‘Rb2.5Mo6Bri7.5Se0i .5Bra6’ loaded composition, the ele-
mental analysis gave the following results: Rb: 11, Mo:
27, Br: 60, Se: 2 (theoretical for Rb2.5Mo6Bri7.5Se0i .5Br6a:
Rb: 11.11, Mo: 26.66, Br: 60, Se: 2.22). The cell
parameters of the analysed single-crystals, determined
by X-ray diffraction, do not deviate from 14.070(5) A
within the s.u.’s in agreement with the Rb2.5Mo6Br7i .5
Sei0.5Bra6 composition found by single X-ray diffrac-
tion analysis. Single-crystals from other ‘Rb2+xMo6
Bri8ꢀxSeixBra6’ (xa0:5) loaded compositions revealed
fluctuations in the final compositions. In order to
determine the range of homogeneity, it was decided to
collect single-crystal for high and low x ‘Rb2+xMo6
Bri8ꢀxSeixBra6’ loaded compositions. A single-crystal of
Rb2.7Mo6Bri7.3Sei0.7Bra6 was collected in a preparation
with a ‘Rb3Mo6Bri7SeiBra6’ loaded composition. The
structure has been determined and afterwards an EDS
analysis has been performed leading to the following
results: Rb: 13, Mo: 29, Br: 54, Se: 3 (theoretical for
Rb2.7Mo6Bri7.3Sei0.7Bra6: Rb: 11.89, Mo: 26.43, Br: 58.59,
Se: 3.08). A same procedure has been performed for
Rb2.25Mo6Bri7.75Se0i .25Br6a but for a lower x loaded
composition (x ¼ 0:25). As stressed below, it turns out
that the x composition exhibits a linear variation versus
the a parameter. This evolution is the consequence of
the correlation between the Rb, Br and Se contents in
order to maintain a 24 VEC value on the Mo6 cluster.
2.3. Diffraction studies
2.3.1. Single-crystal X-ray diffraction studies
Single-crystal X-ray diffraction data of Rb2Mo6Br14,
Rb2.5Mo6Br13.5Te0.5 and Rb2+xMo6Br14ꢀxSex (with the
following final refined x value: 0.24(1), 0.48(1) and
0.69(1)) were collected at room temperature on a Nonius
KappaCCD area-detector X-ray diffractometer with
(
Mo Ka radiation (l ¼ 0:71069 A) (Centre de Diffracto-
metrie de l’Universite de Rennes 1, France). Details of
data collections and structure refinements are reported
in Tables 1 and 2. The data processing was performed by
the EvalCCD analysis software [14] and an absorption
correction was made through SADABS program [15].
The calculation of E-statistics performed on the data
suggested a centrosymmetric space group for all
compounds [16]. Among the possible space groups
deduced from the observed systematic extinctions, the
refinement procedure allowed to retain unambiguously
Table 1
Crystallographic data for the structure determinations of Rb2Mo6Br14 and Rb2.5Mo6Br13.5Te0.5
Empirical formula
Rb2Mo6Br14
1865.32
C2/c No. 15
20.376(5)
15.297(5)
9.757(5)
115.969(5)
2734 (2)
4
Rb2.44(1)Mo6Br13.63(2)Te0.37(2)
1920.56
Pn-3 No. 201
Formula weight
Space group
a (A)
14.0936(5)
b (A)
c (A)
b (1)
V (A3)
Z
2799.4(2)
4
4.557
Dcalc (g cmꢀ3
)
4.531
Crystal dimensions (mm3)
Total reflections collected
Unique reflections
0.078 Â 0.098 Â 0.117
17195
4741
0.063 Â 0.066 Â 0.110
37655
1079
Rint (all reflections)
0.0588
26.655
20
0.0767
26.645
20
m (mmꢀ1
T (1C)
l (A)
)
0.71069
3146
106
0.71069
908
43
Observed reflections [I42s (I)]
Refined parameters
R1,a [I42s(I)]
0.037
0.0568
0.0285
0.0477
ꢀ1.404/1.325
wR2,a all data
Drmin/Drmax (e Aꢀ3
)
ꢀ0.997/1.413
P
P
P
P
aR1 ¼ hkl9FoꢀFc9/ hkl9Fo9; wR2 ¼ [ hkl[w(Fo ꢀF2c)2]/ hkl[w(Fo2)2]]1/2
.
2

ARTICLE IN PRESS
K. Kirakci et al. / Journal of Solid State Chemistry 178 (2005) 3117–3129
3120
Table 2
Crystallographic data for the structure determinations of Rb2.24Mo6Br13.76Se0.24, Rb2.5Mo6Br13.5Se0.5 and Rb2.8Mo6Br13.3Se0.7
Empirical formula
Formula Weight
Space group
a (A)
Rb2.24(1)Mo6Br13.76Se0.24
Rb2.48(1)Mo6Br13.52Se0.48
Rb2.69(1)Mo6Br13.31Se0.69
1885.60
Pn-3 No. 201
13.999(2)
2743.4(2)
4
1905.89
Pn-3 No. 201
14.0782(3)
2791.2(4)
4
1923.64
Pn-3 No. 201
14.144(2)
2829.5(5)
4
V (A3)
Z
Dcalc (g cmꢀ3
)
4.565
4.535
4.512
Crystal dimensions (mm3)
Total reflections collected
Unique reflections
0.063 Â 0.074 Â 0.092
35871
1060
0.056 Â 0.059 Â 0.64
31715
1076
0.072 Â 0.074 Â 0.140
31391
1059
Rint (all reflections)
0.1312
26.958
20
0.1210
26.882
20
0.1089
26.850
20
m (mmꢀ1
T (1C)
l (A)
)
0.71069
838
38
0.71069
832
38
0.71069
792
42
Observed reflections [I42s(I)]
Refined parameters
R1,a [I42s(I)]
0.0397
0.0585
0.0335
0.0485
0.0295
0.0479
wR2,a all data
Drmin/Drmax (e Aꢀ3
)
ꢀ1.225/1.324
ꢀ1.193/0.985
ꢀ0.771/0.992
P
P
P
P
aR1 ¼ hkl9FoꢀFc9/ hkl9Fo9; wR2 ¼ [ hkl[w(Fo ꢀF2c)2]/ hkl[w(Fo2)2]]1/2
.
2
the C2/c centrosymmetric space group for Rb2Mo6Br14
bromide and the Pn-3 centrosymmetric space group for
all the chalcobromides. The structures have been solved
by direct methods using SIR97 program [17]. Subse-
quent structural refinements by least-squares techniques,
combined with Fourier difference syntheses, were
performed using SHELXL-97 program [18].
randomly distributed on all the inner ligand positions
(L1 and L2, respectively) for the three Rb2+xMo6
Br14ꢀxSex compositions for which structural determina-
tions have been performed (refined x value: 0.24(1),
0.48(1) and 0.69(1)). This assumption is supported by
the close ionic radii for bromine and selenium [19].
Moreover, let us recall that previous results on Mo6
chalcohalides indicate that Se cannot be located in
apical position [20] and in Rb2.5Re6S6.5Cl7.5 and
In Rb2.5Mo6Br13.5Te0.5, an inner ligand position is
randomly occupied by bromine and tellurium atoms
(Br1 and Te1), whilst the other ligand positions (8e, 24h)
are fully occupied by bromine (Br2, Br3, respectively).
In a first step, it was assumed that Br1 fully occupied a
24h position but after several cycles of refinements, it
turned out that an electronic peak remained close to Br1
at a distance from molybdenum corresponding to a
Mo–Te bond length. Tellurium Te1 was then introduced
on this residue with the same atomic displacement
parameter as Br1. The sum of the occupancies of Br1
and Te1 was restricted to the value corresponding to a
fully occupied 24h position. Afterwards, the restraint on
atomic displacement parameter was relaxed during
the convergence, leading to final positions in agreement
with reliable Mo–(Br, Te) inter-atomic distances. Owing
to the small occupancy of tellurium, this atom was
refined isotropically. The final formula deduced
from structural determination is Rb2.44(2)Mo6Br13.63(2)
Te0.37(2). It has been rounded to Rb2.5Mo6Br13.5Te0.5 for
clarity since this refined formula is very close to the
point x ¼ 0:5 of the general Rb2+xMo6Br14ꢀxTex
formula considering that the number of VEC is constant
and equal to 24.
K
2.5Re6S6.5Br7.5 related compounds [9,10], halogen
and chalcogen atoms have been found randomly
distributed on all inner positions. The presence of
selenium in the ‘Rb2+xMo6Br14ꢀxSex’ series as well as
the variations in atomic composition depending on the x
loaded values was firstly established by EDS analysis
(see Section 2.2). Furthermore, the number of valence
electrons VEC is constant and equal to 24 that implies a
correlation between the Rb, Br and Se content.
Consequently, the selenium (x) content could also be
deduced from that of rubidium (2+x) in the Rb2+x
Mo6Br14ꢀxSex formula.
The atomic positional parameters and the equivalent
displacement parameters of these compounds are
reported in Tables 3–6. Relevant interatomic distances
for discussion are reported in Tables 7 and 8. Further
details of the crystal structure investigation can be
obtained from the Fachinformationszentrum Karlsruhe,
76344 Eggenstein-Leopoldshafen, Germany (fax: (49)
7247-808-666; e-mail: crysdata@fiz.karlsruhe.de), on
quoting the depository numbers CSD__415564
(Rb2Mo6Br14), CSD__415565 (Rb2.5Mo6Br13.5Te0.5),
CSD__415566 (Rb2.25Mo6Br13.75Se0.25), CSD__415567
(Rb2.5Mo6Br13.5Se0.5) and CSD__415568 (Rb2.7Mo6
Br13.3Se0.7), respectively.
Although X-ray diffraction techniques are not appro-
priate to discriminate Se and Br owing to their close
scattering factors, these two elements were assumed

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3121
Table 3
Positional and equivalent displacement parameters (A2) for Rb2.5Mo6Br13.5Te0.5
Atom
Position
x
y
z
Occupancy
Ueq
Mo
Te1
Br1
Br2
Br3
Rb1
Rb2
24h
24h
24h
8e
0.53143(3)
0.6986(10)
0.69305(12)
0.36930(4)
0.57547(6)
0.37894(3)
0.3906(12)
0.39012(12)
0.36930(4)
0.20748(4)
0
0.04246(3)
0.9521(11)
0.95509(12)
0.13070(4)
0.09799(5)
0
1
0.062(4)
0.0259(1)
0.024(4)a
0.0389(5)
0.0489(3)
0.0627(2)
0.0415(3)
0.0522(5)
0.938(4)
1
24h
4c
1
1/2
3/4
1
0.957(4)
6d
1/4
1/4
ꢀ
ꢁ
1P P
aUeq
¼
Uijaiaj~ai Á ~aj except for Te1—refined isotropically—in which case the isotopic displacement parameter is given.
i
j
3
Table 4
Positional and equivalent displacement parameters (A2) for Rb2.7Mo6Br13.3Se0.7
Atom
Position
x
y
z
Occupancy
Ueq
Mo
L1
24h
24h
8e
0.53128(4)
0.69183(4)
0.37002(5)
0.57548(7)
0.37910(3)
0.39110(5)
0.37002(5)
0.20768(5)
0
0.04212(4)
0.95410(5)
0.12998(5)
0.09733(6)
0
1
0.0301(1)
0.0417(2)
0.0570(4)
0.0667(3)
0.0502(4)
0.0546(6)
0.068(6)
1
L2
1
Br3
Rb1
Rb2
Rb3
24h
4c
1
1/2
3/4
1
6d
8e
1/4
0.1522(7)
1/4
0.1522(7)
1
0.096(5)
0.3478(7)
L ¼ 0.063Se+0.937Br.
Table 5
Positional and equivalent displacement parameters (A2) for Rb2.5Mo6Br13.5Se0.5
Atom
Position
x
y
z
Occupancy
Ueq
Mo
L1
24h
24h
8e
0.53139(4)
0.69267(4)
0.36948(5)
0.57545(7)
0.37878(3)
0.39075(5)
0.36948(5)
0.20708(5)
0
0.04238(4)
0.95436(5)
0.13052(5)
0.09794(6)
0
1
0.0234 (1)
0.0338(2)
0.0456(3)
0.0569(3)
0.0390(4)
0.0521(6)
1
L2
1
Br3
Rb1
Rb2
24h
4c
1
1/2
3/4
1
0.987(5)
6d
1/4
1/4
L ¼ 0.063Se+0.937Br.
Table 6
Positional and equivalent displacement parameters (A2) for Rb2.2Mo6Br13.8Se0.2
Atom
Position
x
y
z
Occupancy
Ueq
Mo
L1
24h
24h
8e
0.53153(4)
0.69321(5)
0.36920(5)
0.57611(8)
0.37838(4)
0.39030(6)
0.36920(5)
0.20683(6)
0
0.04234(4)
1
0.0255(1)
0.0355(2)
0.0457(4)
0.0573(3)
0.0409(4)
0.0501(7)
0.95394(5)
0.13080(5)
0.09826(7)
0
1
L2
1
Br3
Rb1
Rb2
24h
4c
1
1/2
3/4
1
0.829(6)
6d
1/4
1/4
L ¼ 0.063Se+0.937Br.
2.3.2. Powder X-ray diffraction studies
Linear Position Sensitive Detector (Braun L-PSD). A
whole-powder-pattern profile fitting procedure without
structural constraints (Le Bail refinement, Thompson–
Cox–Hastings profile function [21]) was used through
FullProf/WinPLOTR [22,23] in order to refine the
cell as well as the profile parameters. For each powder,
The powder X-ray diffraction patterns of the resulting
products from various ‘Rb2+xMo6Bri8ꢀxYixBr6a’ loaded
compositions were recorded on a BRUKER AXS D8
Advance diffractometer (y–y Bragg–Brentano geome-
try) using a monochromatic CuKa1 radiation and a

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3122
Table 7
Interatomic distances (A) in Rb2.4Mo6Br13.6Te0.4, Rb2.2Mo6Br13.8Se0.2, Rb2.5Mo6Br13.5Se0.5 and in Rb2.7Mo6Br13.3Se0.7
Rb2.4Mo6Br13.6Te0.4
Rb2.2Mo6Br13.8Se0.2
Rb2.5Mo6Br13.5Se0.5
Rb2.7Mo6Br13.3Se0.7
Mo6 cluster
2.6325(7)
2.6328(8)
Mo1–Mo1
Mo1–Mo1
2.6234(10)
2.6264(10)
2.6322(8)
2.6324(9)
2.6357(9)
2.6368(8)
Mo6Br14ꢀxYx unit
2.594(2)
2.685(2)
Mo1–Br1
Mo1–Te1
Mo1–Br1
Mo1–Te1
Mo1–Br1
Mo1–Te1
Mo1–Br2
Mo1–Br3
2.5850(10)
2.5911(9)
2.6032(9)
2.5924(8)
2.5974(8)
2.6099(8)
2.5953(9)
2.6002(8)
2.6105(8)
2.607(2)
2.651(2)
2.623(2)
2.669(2)
2.605(1)
2.6148(7)
2.5911(13)
2.6019(10)
2.5988(11)
2.6154(8)
2.6006(12)
2.6227(8)
Rubidium environment
3.4044(6) Â 6
3.591(2) Â 4
Rb1–Br3
Rb2–Br1
Rb2–Te1
Rb2–Br3
Rb3–Br2
Rb3–Br3
3.3779(8) Â 6
3.5553(7) Â 4
3.3958(7) Â 6
3.5858(6) Â 4
3.4152(7) Â 6
3.6046(6) Â 4
3.54(2) Â 4
3.3165(7) Â 4
3.2869(9) Â 4
3.3150(8) Â 4
3.3338(8) Â 4
3.113(8) Â 3
3.404(9) Â 3
Table 8
Interatomic distances (A) in Rb2Mo6Br14
all the coexisting phases were introduced, leading to
satisfactory profile Rp-factors. Subsequently, a Rietveld
refinement was carried out in order to evaluate the yield
of reaction of the obtained Rb2+xMo6Bri8ꢀxYixBr6a
compound versus the loaded stoichiometries (rough
quantitative phase analysis). Owing to the complexity of
the powder patterns in terms of number of coexisting
phases and to the induced high degree of accidental
overlapping of diffraction lines, combined with the
medium instrumental angular resolution of the PSD, no
attempt to refine the atomic parameters were made
through this rough analysis, the profile parameters being
set to the previously obtained Le Bail refinement values.
The evolutions of the unit-cell parameters as well as of
the reaction yield versus the loaded composition are
reported in Table 9.
The refined a parameters obtained by powder X-ray
diffraction for the three loaded ‘Rb2+xMo6Bri8ꢀxTeix
Bra6’ compositions (x ¼ 0:25, 0.5 and 0.75) are very close
within the s.u.’s. They are similar to the value obtained
by single-crystal X-ray diffraction for Rb2.5Mo6Br7i .5
Tei0.5Bra6. This feature indicates that Rb2.5Mo6Br7i .5Tei0.5
Bra6 corresponds to a defined composition that is
obtained whatever the loaded composition. On the
other hand, in the selenium series, the a cell parameter of
the compounds issued from the three ‘Rb2+xMo6
Bri8ꢀxSeixBra6’ loaded compositions (x ¼ 0:25, 0.5 and
1.25) varies within the range 14.057(1)–14.158(1) A from
the powder diffraction analysis. This range of values is
included in that found by single-crystal diffraction
investigations (in this instance the x values could be
refined): 13.999(2)–14.144(2) A. As one can see in Fig. 7,
the evolution of x versus a parameter for the three
Rb2Mo6Br14
Mo6 cluster
2.6279(9)
2.6297(8)
2.6365(8)
2.6389(15)
2.6319(8)
2.6353(10)
Mo1–Mo2
Mo1–Mo2
Mo1–Mo3
Mo1–Mo3
Mo2–Mo3
Mo2–Mo3
Mo6Br14 unit
2.5940(10)
2.5856(8)
2.5964(10)
2.6070(8)
2.6097(8)
2.6041(8)
2.5948(15)
2.6084(15)
2.6024(8)
2.6233(9)
2.5974(10)
2.5834(10)
2.5970(9)
2.5944(8)
2.5918(11)
Mo1–Br1
Mo1–Br2
Mo1–Br3
Mo1–Br4
Mo1–Br6
Mo2–Br1
Mo2–Br2
Mo2–Br3
Mo2–Br4
Mo2–Br5
Mo3–Br1
Mo3–Br2
Mo3–Br3
Mo3–Br4
Mo3–Br7
Rubidium environment
3.3721(18) Â 2
3.4582(11) Â 2
3.4163(10) Â 2
3.718(2) Â 2
Rb1–Br6
Rb1–Br6
Rb1–Br7
Rb2–Br3
Rb2–Br4
Rb2–Br5
Rb2–Br5
Rb2–Br5
Rb2–Br5
3.672(2) Â 2
3.267(2) Â 2
3.296(2) Â 2
3.322(2) Â 2
3.452(2) Â 2

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3123
Table 9
Variation of the refined a parameter and yield of Rb2+xMo6Br8i ꢀxYixBra6 using X-ray powder diffraction data versus the loaded composition
Loaded composition
‘Rb2.25Mo6Bri7.75Tei0.25Bra6’
‘Rb2.5Mo6Bri7.5Te0i .5Bra6’
‘Rb2.75Mo6Bri7.25Tei0.75Bra6’
a (A), yield (%)
14.1005(1), 27(2)
14.0998(1), 62(2)
14.1001(1), 44(2)
Loaded composition
‘Rb2.25Mo6Bri7.75Sei0.25Bra6’
‘Rb2.5Mo6Bri7.5Se0i .5Bra6’
‘Rb3.25Mo6Bri6.75Sei1.25Bra6’
a (A), yield (%)
14.0572(1), 73(2)
14.0743(1), 61(2)
14.1582(1), 32(2)
single-crystals follows a linear law. The a parameters
obtained after reaction of the three loaded ‘Rb2+xMo6
Bri8ꢀxSeixBra6’ powder compositions (x ¼ 0:25, 0.5 and
1.25) are reported on the straight line (points A, B and
C, respectively). The three corresponding x values
extrapolated from the ‘single-crystal’ xðaÞ evolution are
0.42, 0.47 and 0.73, respectively, for the powder
samples. The analysis of the results obtained by single-
crystal and powder X-ray diffractometry enables to
deduce the phase breadth of the seleniobromide with the
general formula Rb2+xMo6Bri8ꢀxSeixBr6a. Indeed, the
upper x value of the phase breadth is close to x ¼ 0:7
whilst the lower x value is less than or equal to 0.24, but
cannot reach 0 since another structure-type is observed
for this value (see Rb2Mo6Br14, Section 3).
Bri8ꢀxSeixBra6 selenium series are slightly more soluble in
dichloromethane and also exhibit two new waves at 0.46
and 1.13 V vs. SCE (supplementary material), in
addition to the irreversible wave around 1.30 V,
attributed to the oxidation of ‘Mo6Bri8Br6a’ [7]. Notably,
no distinct reduction peaks are observed in the region as
low as –1.2 V vs. SCE in this solvent. Again, at
potentials above 1.3 V, clusters undergo an irreversible
multielectron oxidation followed by decomposition,
along with the passivation of the working electrode.
The chemically reversible wave at 0.46 V (Fig. 8) is
attributed to the oxidation of a Mo6Br13Se1 unit by
analogy with that observed for Mo6Te1Br13. The second
oxidation wave (1.13 V) has been attributed to a soluble
byproduct and more precisely Rb12Mo9Br31Se [24].
The rough quantitative phase analysis reveals that the
yield of reaction of the ‘Rb2+xMo6Bri8ꢀxSeixBr6a’ series
decreases as the x value increases. The presence and
yield of secondary phases (Mo6Br10Se, Rb2Mo6Br14,
Rb4Mo6Br12Se2 or Rb12Mo9Br31Se [24]) depends on the
loaded composition, richer rubidium phases being
preferentially formed for high x values. In the case of
the Rb2+xMo6Bri8ꢀxTeixBra6 series, the maximum yield is
obviously obtained for the ‘Rb2.5Mo6Bri7.5Te0i .5Br6a’
loaded composition that corresponds to the stoichio-
metry of the defined compound. For x ¼ 0:25, the
main product is Rb2Mo6Br14 whilst for x ¼ 0:75, it is
Rb12Mo6Br31Te [24].
3. Results and discussion
3.1. Crystal structures of Rb2Mo6Br14
The crystal structure of Rb2Mo6Br14 is isostructural
with those of Rb2W6Br14 and Rb2Mo6Cl14 [26]. The
Mo6Br14 unit (Fig. 1a) is centred on a ꢀ1 site symmetry
and exhibits similar interatomic distances as those found
in Cs2Mo6Br14 [13]. The description of Rb2W6Br14
structure has been previously drawn following ‘a hcp
pattern parallel (100) with the Rb+ cations between the
layers’ [11]. This model is partially true since the units
are effectively staked according to a ABAB succession of
units along this direction. However, such a stacking
should generate octahedral and tetrahedral sites of units.
This feature is in contradiction with the prismatic unit
environment of Rb1. The unit layer and unit-cell are
represented in Figs. 1b and 2a, respectively. It turns out
that the b angle of the monoclinic unit-cell is too far
from 901 to consider that the stacking occurs along the
[0 0 1] direction. A closer look to the Rb2M6Br14
structure evidences that it can be described in a
pseudo-hexagonal unit-cell with the following cell para-
2.4. Electrochemistry
Given the very poor solubility of the isolated solid
samples in dichloromethane, the solubility of the crude
Rb2.5Mo6Bri7.5Tei0.5Bra6 extract was however sufficient to
record a cyclic voltammogram (supplementary material)
from saturated solutions after sonication under an inert
atmosphere (nitrogen or argon). Two irreversible waves
could be detected around 1.35 and 0.38 V vs. S.C.E.,
respectively. The first waves correspond to the oxidation
of Mo6Br14 and is close to the values reported in the
literature [7]. The second wave becomes chemically
reversible when the potentials are not scanned above
0.7 V vs. SCE (Fig. 8). It has been attributed to the
oxidation of a Mo6Te1Br13 unit since this range of
values has already been observed for other monosub-
stituted Mo6 chalcohalides [3,7,8,25]. The Rb2+xMo6
(
(
(
meters: a ¼ 9:072ð3Þ A, b ¼ 9:757ð5Þ A, c ¼ 9:169ð24Þ A,
a ¼ 87:391, b ¼ 91:401, g ¼ 122:531 (Figs. 2a and b). In
this pseudo unit-cell, the units are stacked according
to a AAA arrangement generating distorted prismatic
environments of units within which are located the
rubidium cations. Rb1 lies at the centre and Rb2 on

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3124
Fig. 1. (a) Mo6Br14 unit. (b) Perpendicular projection of an unit layer in Rb2Mo6Br14
.
Fig. 2. (a) Projection along the [010] direction, the pseudo-hexagonal unit-cell is represented in dashed lines. (b) Schematic representation of a
pseudo-hexagonal unit-cell in Rb2Mo6Br14. The centroid of cluster is represented as grey balls.
the three parallelepipedic faces of this prismatic site
(Figs. 3a and b, respectively). Contrary to Cs2Mo6Br14
[13] (P-31c, ABAB close-packed hexagonal stacking) in
which the ternary axis of the unit is perpendicular to the
unit layer, in Rb2Mo6Br14 the angle between the ternary
axis of the unit and the unit layer is equal to 103.161.
This enables closer inter-unit distances within and
between the layers in C2/c structures-types compared
to those of other related halides with Pn-3 or P-31c
structure-types (see Table 10).
3.2. Crystal structures of Rb2.5Mo6Br13.5Te0.5
The Rb2.5Mo6Br13.5Te0.5 tellurobromide crystallizes
in the cubic system (space group Pn-3, Fig. 4). Its
structure is strongly related to those of Rb2.5Re6S6.5Cl7.5
and K2.5Re6S6.5Br7.5 [10] that are also formulated
(Rb5)5+[Re6Si6Cl2i Cl6a]2ꢀ[Re6S7i CliCl6a]3ꢀ and (K5)5+[Re6
Si6Bri2Br6]2ꢀ[Re6S7i BriBr6]3ꢀ, respectively. The structure
is based on [Mo6Bri8Br6a]2ꢀ and [Mo6Br7i TeiBra6]3ꢀ
discrete anionic units in which the Mo6 cluster is

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3125
Fig. 3. Rubidium environments. (a) Prismatic environment of unit for Rb1; (b) Rb2 is located on the unit prism faces.
Table 10
Inter-cluster distances in some A2+xM6Br8i ꢀxYixBra6 (Y ¼ Se, Te) compounds according to [11, 13 and this work]
Compound
Space group
Unit diameter
Bra–Bra (A)
Inter-cluster distances (centroid to
centroid) within layers (A)
Inter-cluster distances (centroid to
centroid) between layers (A)
Rb2Mo6Br14
C2/c
8.940(16)
8.915(17)
8.953(13)
8.973(15)
8.952(16)
8.919(2)
8.908(2)
8.926(2)
8.907(1)
9.072(2)ꢀ9.757(5)
9.899(2)
9.956(3)
9.169(24)
Rb2.25Mo6Br13.75Se0.25
Rb2.5Mo6Br13.5Se0.5
Rb2.7Mo6Br13.3Se0.7
Rb2.5Mo6Br13.5Te0.5
Cs2Mo6Br14
Pn-3
Pn-3
Pn-3
Pn-3
P-31c
Pn-3
C2/c
P-31c
9.899(2)
9.956(3)
10.001(3)
9.966(1)
9.56(1)
10.001(3)
9.966(1)
10.192(1)
9.764(1)
K2W6Br14
Rb2W6Br14
Cs2W6Br14
9.764(1)
9.159(24)
9.578(1)
9.104(1)ꢀ9.720(5)
10.18(1)
face-capped by eight inner ligands. The average unit is
characterized by a random distribution of bromine and
tellurium on sixinner positions with a Br:Te ratio equal
to 0.94:0.06. The two other inner ligand positions
located on the ternary axis of the unit as well as the
apical positions are fully occupied by bromine. Such a
distribution implies that the [Mo6Bri7TeiBr6a]3ꢀ units are
statistically disordered around the three-fold axis of the
unit. On the other hand, in Rb2.5Re6S6.5Cl7.5 and
their ternary axis perpendicular to the unit layers, in
Rb2.5Mo6Br13.5Te0.5, only the units oriented along the
[111] direction exhibit their ternary axis perpendicular to
the unit layers. The other units are tilted by 451 versus
the [111] direction corresponding to an orientation along
the [1ꢀ11], [ꢀ1ꢀ11] and [ꢀ111] directions. This
disposition of units enable equal inter-cluster distances
between and within the layers (Table 10). The Rb1
cations are located in the octahedral sites generated by
this ABC close-packed cubic stacking (4c Wyckoff
position) whilst Rb2 cations occupy the sixover the
eight tetrahedral sites (6d Wyckoff position). The two
remaining tetrahedral sites of units (2a Wyckoff posi-
tion), located on the (111) axis, are unoccupied. Rb1 is
bonded to sixapical bromine atoms, belonging to six
K
2.5Re6S6.5Br7.5, a random distribution of chalcogen
and halogen on all the inner sites has been observed that
implies no preferential static disorder.
The layers are arranged according to a ABC close-
packed cubic stacking along the [111] direction (Fig. 5).
Contrary to Cs2Mo6Br14, for which all the units exhibit

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3126
respectively. Owing to the tilting of units, the distance
between the centre of the remaining tetrahedral sites (2a
Wyckoff position) and the four closest Bri ligands
(2.912(1) A) is too short to enable their occupation by
other rubidium cations. Attempts to reach the
‘Rb2.5A00.5Mo6Br13Te1’ composition in order to fill the
2a site by the use of smaller A0 cations, as for instance
lithium, have failed.
3.3. Crystal structures of Rb2+xMo6Bri8ꢀxSeixBr6a series
The structure of Rb2.5Mo6Br13.5Se0.5 is closer to the
structures of Rb2.5Re6S6.5Cl7.5 and K2.5Re6S6.5Br7.5 [10]
than Rb2.5Mo6Br13.5Te0.5. Indeed, the similar size of Se and
Br should favour a random distribution on all inner
positions. The analysis of the interatomic distances does not
indicate any Se/Br ordering. Rb2.25Mo6Br13.75Se0.25 is
characterized by a smaller occupancy of the Rb2 sites
compared to Rb2.5Mo6Br13.5Se0.5. Contrary to one could
expect first, the additional cations in Rb2.69(1)Mo6
Br13.31(2)Se0.69(2) are not located in the remaining tetrahedral
sites of units (2a Wyckoff position). Indeed, Rb3 statistically
occupies a 8e Wyckoff position that corresponds to the
triangular sites of units generated within the layers. Closely
related triangular environment of units have already been
found in Cs2Mo6X14 halides and related compounds [13].
Attempts to reach ‘Rb2.5A00.5Mo6Br13Se1’ by the occu-
pation of 25% (instead of 9.6% in Rb2.69(1)Mo6
Br13.31(2)Se0.69(2)) of the 8e Wyckoff position using small
A0 cations gave unfruitful results.
Fig. 4. Projection of the Rb3Mo6Br13.5Te0.5 structure. The unit-cell is
represented in bold.
3.4. Evolution of the interatomic distances
As previously observed for Cs4Mo6Bri6Se2i Br6a [3],
the replacement of bromine ligands by selenium does
not affect significantly the Mo–Mo and Mo–Bri
distances. In other words, the variation of the Mo–Mo
and Mo–Bri bond lengths is not submitted to matrix
effect and depends only on the number of valence
electrons per cluster. The main difference concerns the
Mo–Bra bond lengths that slightly increase as the
average charge of the unit increases. This effect can be
related to a lower cationic charge of the (Mo6Li8)n+
cluster core (2+ for Cs4Mo6Br12Se2 (Mo–Bra: 2.655 A);
3.3+ for Rb2.7Mo6Br13.3Se0.7 (Mo–Bra: 2.6227(8) A);
3.5+ for Rb2.5Mo6Br13.5Se0.5 (Mo–Bra: 2.6154(8) A);
3.75+ for Rb2.25Mo6Br13.75Se0.25 (Mo–Bra: 2.6019(10) A);
4+ for Cs2Mo6Br14 (Mo–Bra: 2.607A)). Indeed, the higher
the charge the shorter the Mo–Bra bond length owing to
greater electrostatic interactions.
Fig. 5. Projection of an unit layer along the [111] direction.
adjacent Mo6 clusters, that build a slightly deformed Br6
octahedron (Fig. 6a, Rb1-Bra: 3.4043(6) A). Rb2 is
bonded to four apical and four inner bromine atoms
belonging to four Mo6 clusters (Fig. 6b, Rb1-Bra:
3.3165(7) A). These Rb–Bra distances are in agreement
with those found in Rb2Mo6Br14. The Rb2-Bri and
Rb2–Tei bond lengths are 3.593(2) and 3.53(2) A,
3.5. Stacking evolution versus anionic charge and counter
cations
The analysis of inter-cluster distances (centroid to
centroid) reported in Table 10 evidences a higher unit

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3127
Fig. 6. Rubidium environment in Rb2.5Mo6Br13.5Te0.5. (a) Rb1 octahedral environment of units. (b) Rb2 tetrahedral environment of units.
Fig. 8. Cyclic voltammograms of Rb2+xMo6Bri8ꢀxYixBr6a and
Rb2.5Mo6Bri7.5Yi0.5Bra6 series, with Y ¼ Se (a) and Y ¼ Te (b) in
Fig. 7. x values versus refined cell parameters for the
Rb2+xMo6Br14ꢀxSex series. a, b and c correspond to the x value
versus a parameter obtained by X-ray single-crystal diffraction data.
A, B and C (black triangles) correspond to powder compounds. A, B
and C points have been reported on the line previously defined by a, b
and c starting from a ‘single-crystal’ parameters in order to determine
the corresponding x value. Standard uncertainties along x and a are
included within the symbols.
dichloromethane/tetrabutylammonium hexafluorophosphate (0.1 M)
at 0.5 V sꢀ1. The ferrocene/ferrocenium (Fc/Fc+) complexwas used
as an internal reference for potential measurements (Fc/Fc+ at 0.460 V
vs. SCE).
structure generates octahedral and tetrahedral sites of
3
units between the layers. Only of the tetrahedral sites
4
can be occupied by cations to reach the
A2.5Mo6Br13.5Se0.5 formula. For higher ratio, triangular
sites within the layers are filled but the ‘A3Mo6Br13Se1’
composition has not been reached although it could be
obtained by a 25% occupation of these triangular sites.
High ratio of RbBr and Se in the loaded composition
favours the formation of Rb4Mo6Br12Se2 or
Rb12Mo9Br31Se together with a significant decrease of
the reaction yield of the Rb2+xMo6Bri8ꢀxSeixBr6a phase.
compactness in the C2/c structure-type. This feature
must be related to the tilt of the ternary axis of units
within the unit layers. This leads to closer inter-cluster
distances within and between the unit layers. Indeed, the
crystallization of Rb2Mo6Br14 in the C2/c structure-type
favours a higher space filling compared to a crystal-
lization in the P-31c or Pn-3 structure-types. The
replacement of some bromine by selenium ligands
involves an increase of the cation content within the
structure in order to maintain charge balance in relation
with the 24 VEC value. Additional cations cannot be
hosted in the C2/c structure-type. As stressed above in
the structural results section, the stacking in the Pn-3
3.6. Solution chemistry and electrochemistry
The A2Mo6Br14 series are hardly soluble in most
organic solvents. The Rb2+xMo6Bri8ꢀxYixBr6a series and

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Table 11
Oxidation potentials E1/2 for selected Mo6 complexes
a
Processes
DEp (V)
ic/ia
E1/2 (V vs.SCE)
2ꢀ
[Mo6Bri8Bra
]
Cs2Mo6Br14
Rb2+xMo6Brꢀ13xSex
Rb2.5Mo6Br13.5Te0.5
24/23
24/23
24/23
24/23
24/23
24/23
24/23
/
NRb
+1.38
+0.46
+0.38
6
[Mo6Bri7SeiBra
]
0.09
0.09
/
1
3ꢀ
6
[Mo6Bri7TeiBra
]
1
3ꢀ
6
[Mo6Bri6Si2Bra
]
Cs4Mo6Br12S2
Cs4Mo6Br12Se2
NRb
NRb
1
Solvation in solution
Solvation in solution
–0.056
4ꢀ
6
[Mo6Bri6Sei2Bra
]
/
4ꢀ
6
[Mo6Bri6Si2(CN)a
]
Cs0.4K0.6(Et4N)11[(Mo6Br6S2)(CN)6]3 Á 16H2O
Cs0.4K0.6(Et4N)11[(Mo6Br6Se2)(CN)6]3 Á 16H2O
0.09
0.09
4ꢀ
6
[Mo6Bri6Sei2(CN)a
]
1
–0.068
4ꢀ
6
aAll E values in V vs SCE Conditions: CH2Cl2 solvent, 0.1 M (NnBu4)(PF6) supporting electrolyte, 20 1C, Pt electrode, sweep rate 0.100 V sꢀ1. The
ferrocene/ferrocenium (Fc/Fc+) at 0.460 V vs SCE complexwas used as an internal reference for potential measurements.
bProcess not chemically reversible.
Rb2.5Mo6Bri7.5Yi0.5Bra6 are only sparingly soluble in
dichloromethane upon sonication and slightly more in
acetonitrile. Beyond the separation of insoluble impu-
rities, their solubility in the former has also allowed to
study the electrochemistry of the novel (Mo6Br13Se)3ꢀ
and (Mo6Br13Te)3ꢀ anionic units issued from Rb2+x
Mo6Br8i ꢀxSeixBr6a and Rb2.5Mo6Bri7.5Tei0.5Br6a solid state
precursors. The presence of selenium among the ligands
apparently drastically decreases the oxidation potential
relative to a (Mo6Br14)2ꢀ unit, as already observed in
previous work [3 and references therein]. In Table 11,
comparison of E1/2 of ‘Mo6Bri7SeiBr6a’ unit with other
Mo6 units shows that the substitution of inner halogen
ligands by chalcogen ones drastically decreases oxida-
tion potentials (for example, +1.38 V for (Mo6Br8),
+0.55 V for (Mo6Br7S) and –0.056 V for (Mo6Br6S2))
vs. SCE, whilst the nature of chalcogen (S or Se) itself
seems to affect the potential only slightly. In the case of
tellurium the oxidation potential is also decreased,
leading to a reversible oxidation process centred at
0.38 vs. SCE. Hitherto no isolated mixed tellurium
halide Mo6 units has been reported. In previous work
[6–8], mixed sulphur and selenium halide soluble Mo6
units have been obtained from MoX2 halide and NaSeH
and NaSH. Nothing has been made until the present
work on molybdenum tellurium derivatives. Reversible
oxidation processes have also been reported for related
[Re6Te8L6]4ꢀ anionic units [21].
compositions Rb2.25Mo6Br13.75Se0.25 and Rb2.7Mo6
Br13.3Se0.7. On the other hand, in the case of tellurium
no solid state solution has been observed as previously
reported for Rb2.5Re6S6.5Cl7.5 and K2.5Re6S6.5Br7.5. The
electrochemistry performed on the [Mo6Br13Y]3ꢀ units
reveals a reversible 24/23 electrons process with E1=2
0:46 and 0.38 V vs. SCE for [Mo6Br13Se]3ꢀ and [Mo6
¼
Br13Te]3ꢀ, respectively.
Acknowledgments
We are indebted to the French Research Ministry for
‘ACI Nanosciences 2001—No. 18-01’ contract and to
‘‘Fondation Langlois’’ for financial support.
Appendix A. Supplementary materials
The online version of this article contains addi-
tional supplementary data. Please visit doi:10.1016/
j.jssc.2005.07.026
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