A new cubane-type Ru4(CO)12(μ3-Se)4 tetramer tailored for water photooxidation catalysis

Article abstract of DOI:10.1002/1099-0682(200109)2001:10<2489::AID-EJIC2489>3.0.CO;2-5

In an effort to tailor a transition metal complex for photoinduced water splitting, a new cubane-like ruthenium chalcogenide tetramer, Ru₄(CO)₁₂(μ₃-Se)₄, has been synthesized and structurally characterized by si

Full text of DOI:10.1002/1099-0682(200109)2001:10<2489::AID-EJIC2489>3.0.CO;2-5

FULL PAPER
A New Cubane-Type Ru (CO) (µ -Se) Tetramer Tailored for Water
4
12
3
4
Photooxidation Catalysis
[
a]
[a]
[a]
[b]
[c]
Edgar Rupp, Frank Nowak, Sebastian Fiechter,* Günter Reck, Volker Eyert,
[d]
[a]
Nicolas Alonso-Vante, and Helmut Tributsch
Keywords: Cubanes / Ruthenium / Structure determination / Density functional calculations / Catalysts
In an effort to tailor a transition metal complex for photoin-
dimensional frameworks are formed. First principles band
duced water splitting, a new cubane-like ruthenium chalco- structure calculations reveal the strong influence of both the
genide tetramer, Ru
structurally characterized by single-crystal X-ray diffraction.
The orange-red compound crystallizes in the cubic space The latter gives rise to several very sharp Ru d bands of t2g
group I 4¯ 3m. The geometry of the Ru
Se units remains like symmetry just below the valence band maximum E
undistorted in the cubic symmetry 3m. Such a high symmetry which are expected to be useful for oxidation catalysis. While
4
(CO)12(µ
3
-Se)
4
, has been synthesized and C−O bonding, which leads to a large splitting into bonding
and antibonding states, as well as of metal−ligand bonding.
-
4
4
v
,
is exceptional among cubane-type cluster compounds. Ruthe- these states are bonding below −1.3 eV, they are antibonding
nium atoms are octahedrally coordinated by three selenium near the edge of the valence band. For this reason, we expect
atoms and three CO groups. Ru
via van der Waals forces by oxygen atoms which belong to
4
Se
4
(CO)12 clusters are linked
a stabilization of the cubane core on depopulation of the
highest occupied bands. Consequently, it is likely that a dy-
CO ligands of two different cluster units. By stacking of the namic interrelation between electron transfer and crystal
cluster units along the cubic axes two weakly coupled three-
structure occurs.
[
5]
Introduction
pasteurianum, contains redoxable thiocubane Fe S cores.
4
4
One of these cores is covalently bridged by a cysteinate thiol
Photoinduced water splitting for fuel generation is a sub-
ject which over the years has received significant atten-
[5,6]
to an Fe S subcluster (H-cluster).
Besides sulfur, car-
2
2
bonyl and cyanide ligands coordinate the iron atoms. It is
thought that multi-electron transfer, for example bi-electron
transfer to reduce protons to yield dihydrogen, is corrobor-
ated with the presence of d-metal centers, which can vary
their oxidation state over a wide range. In the past, Chevrel-
type compounds were investigated, the characteristics of
which are octahedral metal cores consisting of Re, RhϪMo,
PtϪMo, OsϪMo and RuϪMo, respectively, linked by chal-
[
1]
tion. The process has been demonstrated using UV-ab-
sorbing catalysts such as TiO and SrTiO , but it turned
2
3
out to be very difficult to identify a catalyst which split
water using visible light. The semiconducting compound
RuS , however, photo-oxidizes water with high quantum ef-
2
ficiency under assistance of an electrical potential involving
Ru-centerd interfacial coordination chemistry. RuS has an
2
energy gap of 1.3 eV, which is too low for water splitting,
so a supporting potential is needed, for energetic reasons to
provide the theoretical minimum energy (including losses)
which ranges from 1.6 to 2.1 eV depending on the catalytic
performance of the material concerned. Synthesizing suit-
able transition metal compounds with d-energy bands, sep-
arated by 2 eV, is therefore a significant challenge for solar
[7]
cogen and halogen atoms. Light enhanced oxygen reduc-
[8]
tion was observed in Re S Cl
while Re Mo Se and Ru
6
8
2
4 2 8 2-
Mo Se are of interest as catalysts for the reduction of mo-
4
8
[9,10]
lecular oxygen in acidic media.
A facile chemical syn-
thesis route using carbonyl compounds and elemental
chalcogenides to tailor novel metal clustering compounds
[11]
in the form of powder was recently reported. To prepare
[
2,3]
fuel generation
and the aim of this contribution.
new ruthenium- and molybdenum-containing cluster com-
pounds, which are known to be highly catalytic for many
purposes, under increased pressures and temperatures, the
Ru (CO) was treated in a selenium-saturated xylene solu-
In the past decades numerous efforts have been made to
synthesize cubane-like cluster compounds in order to elu-
cidate the relationship between geometry and electronic
3
12
[
4]
configuration. Our interest was stimulated by the fact that
tion. Using this preparation the new compound
hydrogen-producing hydrogenase, e.g. from Clostridium
Ru (CO) (µ -Se) could be obtained.
4
12
3
4
[
a]
Hahn-Meitner-Institut, Abteilung Solare Energetik,
Glienicker Str. 100, 14109 Berlin, Germany
E-mail: fiechter@hmi.de
Bundesanstalt für Materialforschung und -prüfung,
Rudower Chaussee 5, 12489 Berlin, Germany
Institut für Physik, Universität Augsburg,
Cubane-Type Compounds
[
[
[
b]
c]
Cubane-like clusters (ML ) (µ -E) , which contain tetra-
3 4 3 4
8
6135 Augsburg, Germany
Universit e´ de Poitiers, Lab. Electrocatalysis, UMR CNRS
503, 40,
Avenue du Recteur Pineau, 86022 Poitiers, France
hedral structural units of four transition metal atoms, M4,
d]
and four main group non-metal atoms, E , combined to
6
4
give a cubic moiety, M (µ -E) , have been the subject of
4
3
4
Eur. J. Inorg. Chem. 2001, 2489Ϫ2495

WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ1948/01/1010Ϫ2489 $ 17.50ϩ.50/0
2489

S. Fiechter et al.
FULL PAPER
numerous papers.[4,12Ϫ28] Most of the cubic clusters show a bonding cluster orbitals are used for MϪL and MϪE
distortion from the idealized T symmetry to D (with 4 bonding. The occasionally observed distortion of the
d
2d
short and 2 long distances or 2 short and 4 long distances) metalϪmetal tetrahedron can be explained by an incom-
or D2 symmetry. In a series of papers, Dahl and co- plete occupation of the bonding or antibonding set of
[
12Ϫ20]
workers
have analyzed the molecular and electronic metalϪmetal orbitals.
structure of Fe- and Co-based cubane-like clusters such as
n
[12,13]
[4]
n
[
0
0
Cp Fe (µ -S) ] (n ϭ 0 to ϩ2),
[Cp Fe (µ -CO) ] (n ϭ
4
4
[
3
4
4 4 3 4
14,15]
n
, ϩ1),
, Ϫ1),
Fe (CO) (µ -Se) , [(NO) Fe (µ -S) ] (n ϭ The Model Catalyst
4
12
3
4
4
4
3
4
[
16,17] [4] n
(CO) Fe (µ -S) , [Cp Co (µ -S) ] (n ϭ 0,
1
2
4
3
4
[19]
4
4
3
4
[
18]
[20]
ϩ1), (CO) Co (µ -Sb)
and (NO) Co (µ -NR) .
In Ru (CO) (µ -Se) , described in this paper, the 80
4 12 3 4
12
4
3
4
4
4
3
4
6
2ϩ
Based on molecular orbital calculations using the available valence electrons (i.e., 24 from the four d -Ru
FenskeϪHall model[ Dahl and co-workers have success- atoms, 24 from the 12 CO ligands and 32 from the four
fully explained many of the structural variations. Em- bridging Se2 atoms) completely fill the four lone pairs of
ploying qualitative MO diagrams they suggested that for the Se atoms, the 24 (4 ϩ 12 ϩ 8) bonding metalϪligand
the clusters of cubane-like structures one set of four orbitals orbitals and the six bonding (a ϩ e ϩ t ) as well as the six
4]
Ϫ
1
2
may be considered as the nonbonding lone pair localized antibonding metalϪmetal MOs (Figure 1). From this one
on the four terminal ligands at low energy. There are groups would expect the tetrahedral bonds to be metalϪmetal non-
of 24 orbitals corresponding to metalϪCp and metalϪS bonding and the cubane core to be stabilized by the
n
bonds in [Cp Fe (µ -S) ] (Figure 1). At higher binding en- metalϪligand bonds. This energy level scheme nicely ex-
4
4
3
4
ergies it follows a group of 12 orbitals mainly localized on plains the observed distances and symmetry of cubane-like
the metal atoms. These metal-based orbitals have primarily clusters with 80 valence electrons such as (CO) Ru Se ,
1
2
4
4
[21]
[4]
[4]
d-orbital character and are responsible for the binding to Cp Co S ,
(CO) Fe S , (CO) Fe Se
and (CO)12-
4
4
4
19]
12
4
4
12
4
4
[
the framework of the cluster, namely the bonds between the Co Sb .
4
4
¯
Ru Se (CO) crystallizes in the cubic space group I43m.
4 4 12
metal atoms along the edge of the tetrahedral metal core.
The 24 antibonding combinations formed by MϪL and The coordination of the cubane-like Ru Se4 core is
4
MϪE interactions are located at high energies. The interac- illustrated in Figure 2 and the framework formed by the
tion between the 12 metal-based orbitals forms two bond- clusters in Figure 3. The ruthenium center is octahedrally
ing sets, one of metalϪmetal bonding orbitals of symmetry coordinated by three selenium atoms and three CO groups
a ϩ e ϩ t and a second set of metalϪmetal antibonding (Figure 2). As is visible from Figure 3 the clusters are
1
2
orbitals of symmetry t ϩ t .
coupled by two pairs of oxygen atoms forming a common
tetrahedron of 2.955 A distance. The bonding angles
1
2
˚
RuϪSeϪRu and SeϪRuϪSe are 95.05° and 84.71°. The six
˚
equivalent RuϪRu distances amount to 3.778 A corres-
ponding to a nonbonding distance. The RuϪSe distance of
˚
2
.561 A is comparable to the sum of covalent radii. The
clusters are three-dimensionally interconnected via van der
Waals bonds along the cubic axes and form an orthogonal
framework. Due to the body-centerd Bravais lattice, a se-
cond identical framework appears diagonally translated by
1
/2, 1/2, 1/2 with the origin in the center of the unit cell
(
Figure 2). Each oxygen atom of framework II (e.g. in clus-
˚
ter 3) is separated by the distance of 3.354 A from four oxy-
Figure 1. Qualitative energy orbital diagram of the cubane-type gen atoms belonging to framework I. These latter atoms
cluster unit Fe -Se) , inferred from molecular orbital cal-
4
(CO)12(µ
3
4
[4]
show the distance to the cluster units 1 and 2, as well as the
culations using the FenskeϪHall model (see ref. ); the diagram
can also be used to qualitatively explain the bonding conditions in two oxygen atoms to cluster 2 and the two oxygen atoms to
the ruthenium counterpart, investigated in this work
cluster 1 and a third cluster of framework I, not shown in
Figure 4 for clarity reasons. Crystallographic data and
atomic parameters are given in Tables 1 and 2. Bond
lengths and angles are summarized in Table 3.
Lauher estimated the bonding capabilities of various
The presented structure is a highly symmetrical example
transition metal clusters, based upon extended Hückel cal- of a structure with cubane-like Ru Se cores. Such a high
4
4
culations on bare Rh clusters.[ He proposed the molecu- symmetry is exceptional among cubane-type cluster com-
21]
lar orbitals of a tetrahedral cluster to have six high-lying pounds and to the best of our knowledge Ru Se (CO) is
4
4
12
antibonding orbitals termed as t ϩ t corresponding to the the first cubane-type compound of cubic symmetry de-
1
2
antibonding orbitals of metalϪmetal interaction of Dahl’s scribed. All other known structures crystallize in lower
model. Additionally, the 30 bonding cluster orbitals contain symmetric space groups. For example in Ru Se (C-
4
4
six orbitals of metalϪmetal interaction of symmetry a ϩ e O) (Ph PCH PPh ), whose cubane-like unit is closely re-
1
10
2
2
2
ϩ t in agreement with Dahl’s model. The remaining 24 lated to our compound, the RuϪSe distances vary from
2
2490
Eur. J. Inorg. Chem. 2001, 2489Ϫ2495

4 3 4
New Cubane-Type Ru (CO)12(µ -Se) Tetramer
FULL PAPER
Figure 2. Cubane-type cluster unit Ru
4
(CO)12(µ
3
-Se)
4
Figure 4. Section of Figure 3 showing the van der Waals bonds
between cluster units of different frameworks
4 3 4
Table 1. Crystallographic data for Ru (CO)12(µ -Se) (estimated
standard deviations are given in parentheses)
Empirical formula
Formula mass [g molϪ1]
Crystal system
4 4
Ru Se (CO)12
1055.42
cubic
¯
Space group
I43m (no. 217)
Ϫ1
µ [cm
]
9.391
R values R1 and wR2
0.015, 0.046
10.308(1)
1095.4(2)
25 °C
˚
a [A]
˚
3
V [A ]
Temperature of data collection
Z
ρ
2
calcd. [g cmϪ3
]
3.200
Figure 3. Arrays of cluster units of Ru
4
(CO)12(µ
3
-Se)
4
intercon-
F(000)
959
nected via van der Waals bonds along the cubic axes forming an
orthogonal framework; due to the body-centerd Bravais lattice
type, a second identical framework appears diagonally translated
by 1/2, 1/2, 1/2
Table 2. Atomic coordinates and equivalent isotropic thermal para-
meters of Ru (CO)12(µ -Se)
4
3
4
˚
2
.552 to 2.577A, the inner bond angles SeϪRuϪSe and
Atom
x/a
y/b
z/c
B
eq
RuϪSeϪRu range from 83.6 to 86.1° and from 93.4 to
6.4°. The average values (2.564 A, 84.8° and 95°, respect-
ively) are close to the angles and distances found in the
present structure.
˚
9
Ru
Se
C
0.12958(4)
0.11836(7)
0.1260(5)
0.1229(5)
0.12958(4)
0.11836(7)
0.1260(5)
0.1229(5)
0.87042(4)
0.11836(7)
0.6833(6)
0.5741(5)
0.0203(2)
0.0219(3)
0.0279(11)
0.0476(14)
[
27]
O
Density Functional Calculation
ergy interval shown. In particular, while at lower energies
The partial, i.e. orbital-projected densities of states, as there exist C- and O-2s states we find Ru-4s bands above
determined from self-consistent calculations, are displayed 7 eV. In order to study the octahedral-like crystal-field split-
in Figures 5 and 6, where we have included Ru-4d, Se-4p as ting at the Ru sites we have separated the 4d partial DOS
2
2
2
2
well as C- and O-2p states; all other states gave only negli- into its xy, xz, yz and 3z Ϫ r , x Ϫ y components, which,
gible contributions to the total density of states in the en- in an ideal octahedron, correspond to the t2g and e or-
g
Eur. J. Inorg. Chem. 2001, 2489Ϫ2495
2491

S. Fiechter et al.
FULL PAPER
4 3 4
Table 3. Distances and angles in Ru (CO)12(µ -Se)
˚
Distance [A]
Angle [°]
RuϪRu
RuϪSe
RuϪC
CϪO
OϪO
OϪO
3.7780(7)
2.5611(9)
1.929(6)
1.127(7)
2.955(7)
3.354(7)
SeϪRuϪSe
RuϪSeϪRu
RuϪCϪO
84.71(3)
95.05(3)
179.2(8)
bitals. Finally, we give the calculated partial crystal orbital
overlap populations (COOP) for selected bonds in Figure 7.
Figure 7. Partial crystal orbital overlap populations for selected
bonds
with the bonding CϪO orbitals as well as the low symmetry
of the crystal field. According to additional analysis of the
COOP, the two groups of bands at Ϯ5 eV result from strong
splitting of the C- and O-2p states into bonding and anti-
bonding bands due to π-type overlap across the short bond
˚
of only 1.12A. As expected, the occupied and unoccupied
states have larger contributions from O and C, respectively.
Since the bonding and antibonding states resulting from
this π-type overlap are completely filled and empty, respect-
ively, the CϪO bonding adds the largest contribution to the
Figure 5. Partial Ru-4d-t2g and -e
states; here and in the following figure energies refer to the valence
band edge E
g
as well as Se-4p densities of stability of the compound. Furthermore, since these states
are located well below and above the Fermi energy the cor-
responding bonds cannot be broken by simple electron
transfer. This striking feature of the CϪO cubane core in-
teraction is suspected to be also relevant for the CϪO ter-
minated ferredoxins in an iron-only hydrogenase.[
v
5]
An even larger splitting into bonding and antibonding
states of about 16 eV is observed for the O- and C-2s and -
2
pσ states, where it gives rise to rather sharp peaks at about
Ϫ24 and Ϫ8 eV, outside the energy range displayed in
Figures 5Ϫ7. Yet, in contrast to the 2p bands, both these
π
bonding and antibonding orbitals are fully occupied and
thus neither contribute to the overall stability nor determine
the low energy excitations of the compound. Finally, we
observe a fifth group of bands derived from C- and O-2p
states at approximately Ϫ6 eV below the valence band max-
imum. It comprises predominantly C- and O-2p as well as
σ
Ru-4d orbitals.
Bonding of Ru to its neighboring ligands is strongly in-
fluenced by the presence of two different types of ligands,
namely, three selenium atoms and three CϪO pairs, as well
as their pseudo-octahedral coordination of the ruthenium
sites. Although the particular arrangement of the ligands
reduces the local symmetry at the Ru sites to C3v the
Figure 6. Partial C- and O-2p states of a single CϪO pair separated
into the p and p states
π
σ
In Figure 6 we display the C- and O-2p partial densities pseudo-octahedral coordination still allows the interpreta-
separated into their p and p states. We observe two rather tion of the Ru-4d partial DOS in terms of its t2g and eg
π
σ
conspicuous groups of bands at Ϯ5 eV, each separated into components as they would grow out of an ideal cubic crys-
four sub-bands due to the small overlap of the Ru-4d states tal field. Whereas the t2g states appear mainly in the energy
2492
Eur. J. Inorg. Chem. 2001, 2489Ϫ2495

4 3 4
New Cubane-Type Ru (CO)12(µ -Se) Tetramer
FULL PAPER
range from Ϫ4.2 eV to E , the e orbitals contribute to the
v
g
Electron-Induced Structure Dynamics
sharp peaks at Ϫ6 eV as well as in the energy range from
.8 to 6 eV above the valence band maximum. Interestingly,
a finite t2g partial DOS is also observed between 4.5 and Ru-4d-t2g and -e states in the whole energy spectrum has
2
The different weighting of the ligand contributions to the
g
6
eV.
Due to the peculiarities of the crystal structure with two ground for the possible catalytic activity of this material.
striking consequences for the bonding properties and lays
different types of ligands, selenium atoms and To be specific, while the metalϪcarbonyl bonding occurs
carbonϪoxygen pairs, the ligand contributions to both the mainly far away from the valence band edge at Ϯ5 eV,
Ru-4d-t2g and -e states change across the energy spectrum. bonding and antibonding RuϪSe states are found predom-
g
To be specific, the occupied Ru-4d-t2g states occurring in inantly in the energy range from Ϫ3.2 eV to E . Strong
v
the upper part of the valence band are complemented with bonding both within the carbonyl groups, as well as be-
contributions from the Se-4p states resulting from π-type tween these groups and the metal d states, shifts the corres-
overlap. In contrast, contributions from the CϪO-2p states ponding states away from the gap. Furthermore, due to the
to π-type metalϪligand bonding are rather small in this en- rather weak π bonds both the Ru-4dϪSe-4p bonding and
ergy region. This is due to the bondingϪantibonding split- antibonding states fall into the uppermost region of the oc-
ting of the 2p states discussed above, which shifts these cupied bands and give rise to the high density of states ob-
π
states to Ϯ5 eV. As a consequence, Ru-4d-t2g states, which served in this energy range. In particular, the antibonding
form π bonds with the CϪO-2pπ states, are also found sub-bands show up between Ϫ1.3 eV and E , hence, just
v
near Ϯ5 eV.
below the Fermi energy. If these states are depopulated the
Α σ-type metalϪligand bonding is signalled by the ligand RuϪSe bonds are strengthened and the cubane core that is
p contributions to the peak at approximately Ϫ6 eV as well built from penetrating coordination tetrahedra of both
as to the unoccupied Ru-4d-e dominated bands. Again, types of atoms will contract. In contrast, if these states are
g
due to the special octahedral-like coordination of the metal again populated, the bonding will decrease, thus causing a
sites by two different types of ligands the dϪp overlap slight expansion of the cubane core. As a consequence, we
changes across the spectrum. While the bonding Ru-4d-eg expect substantial electron-lattice interaction, leading to a
states overlap almost exclusively with the CϪO-2pσ or- significant relationship between electron transfer and struc-
bitals, the Ru-4d-eg partial DOS above 2.8 eV is tural properties. To conclude, the presence of both strong
complemented by both Se-4p and small CϪO-2p contribu- CϪO and rather weak RuϪSe bonds, which push the CϪO
tions. This finding is supported by the calculated COOP bonding and antibonding states apart and leave room for
(Figure 7): Whereas the peak at approximately Ϫ6 eV re- antibonding RuϪSe states in the uppermost part of the val-
sults mainly from RuϪCϪO bonding, the energy range ence band, thus provide the basis for a working mechanism
from 2.8 to 4 eV is dominated by antibonding RuϪSe for catalytic processes.
states. In conclusion, the cubane framework is predomin-
antly stabilized by σ-type interaction between Ru and CO
and π-type interactions between Ru and Se as depicted in Conclusion
Figure 8.
A new Ru (CO) (µ -Se) tetramer has been synthesized
4
12
3
4
and characterized experimentally and theoretically. The
compound fits nicely into a series of other transition metal
cluster compounds possessing cubane-like cores. However,
in contrast to the companion compounds, the present mat-
erial crystallizes with an unusual high symmetry (space
¯
group I43m). The cluster units are weakly coupled by van
der Waals interactions. In order to investigate the electronic
structure we combined molecular orbital considerations
and state-of-the-art band structure calculations. The theo-
retical results confirm the wide band gap of the material of
E ϭ 2.44 eV. In the cubane core Ru is pseudo-octahedrally
g
coordinated by 3 Se and 3 CO ligands. The electronic struc-
ture of Ru (CO) (µ -Se) is dominated by three different
4
12
3
4
types of bonds each having a characteristic strength: (i)
strong π-type CϪO overlap shifts the corresponding bond-
ing and antibonding states far below and above the valence
band maximum and is responsible for the overall stability
of the compound; (ii) σ-type overlap of Ru-4d-e states with
g
O/C-2p as well as Se-4p states as resulting from the pseudo-
octahedral coordination of Ru likewise places these states
at energies well below and above the edge of the valence
4 3 4
Figure 8. Qualitative energy orbital diagram of Ru (CO)12(µ -Se)
Eur. J. Inorg. Chem. 2001, 2489Ϫ2495
2493

S. Fiechter et al.
FULL PAPER
band; (iii) finally, rather weakly p-bonded Ru-4d-t2g and Se- employing a micro KRS5 crystal at 45°. Ϫ To determine the energy
gap of the compound an absorption measurement was performed
in the range from 400 to 800 nm using a Leitz microscope. Ϫ X-
ray data collection was carried out with an EnrafϪNonius CAD-
4
p states dominate in the upper part of the valence band.
The respective antibonding states are found at binding ener-
gies below 1.3 eV. As a consequence, on depopulation of
these states the bonding within the cubane core will increase
and the core itself will contract while repopulation will lead
to a corresponding expansion. The thus expected coupling
between electron and structural dynamics might support
4
diffractometer at 293 K using Mo-K (λ ϭ 71.073pm) radiation
α
monochromatized by a graphite crystal. Data were collected em-
ploying the 2Θ-ω scan technique in the range 3° Ͻ 2Θ Ͻ 60°. The
total number of reflections was 936 from which 191 were independ-
i
ent [R ϭ 0.035, 169 observed (I Ͼ 2σ(I)]. The dimensions of the
electron transfer processes and, hence, might be relevant for prismatic crystal were 0.12 ϫ 0.10 ϫ 0.09 mm. The structure was
catalytic processes.
The prepared Ru (CO) (µ -Se) compound fulfils the I23. But an inspection of the resulting atomic coordinates showed
solved by direct methods using SHELXS-90[ in the space group
31]
4
12
3
4
¯
that the higher symmetry I43m was also fulfilled. The refinement
most important theoretical criteria to act as photocatalyst
for water oxidation: (i) a sufficiently large energy gap
2.44 eV) with valence states, which Ϫ as a consequence of
photooxidation Ϫ are able to transfer electrons via Ru-d
states; (ii) the oxidation states of ruthenium atoms in the
compound can be increased to allow formation of peroxo
complexes, which are able to release oxygen; (iii) the coup-
¯
[31]
of 23 parameters in I43m with SHELX-97 converged with R
1
(on
(on F ) ϭ 0.046. The crystallographic data
excluding structure factors) for the structure reported in this pub-
2
F) ϭ 0.015 and wR
2
(
(
lication have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication no. CCDC-155020.
Copies of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: (in-
ling between electronic and structural dynamics may favour ternat.) ϩ 44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk]. Ϫ
multi-electron transfer. In order to obtain more insight into the electronic properties of
However, preliminary photoelectrochemical experiments solid Ru (CO)12(µ -Se) , band structure calculations were per-
4
3
4
formed as based on density functional theory and the local density
approximation (LDA). In doing so, we account in an optimal bal-
ance for both the electronϪlattice and the electronϪelectron inter-
action, both of which are known to strongly influence the proper-
ties of transition metal compounds. For the LDA the parametriza-
tions by both Moruzzi, Janak and Williams as well as by Vosko,
using small crystals of the synthesized complex have indic-
ated a material property which at the moment seriously
limits an application: A very high electrical resistivity was
found, which has to be overcome by suitable doping tech-
niques, by substituting ligands or by preparation of ultra-
thin layers.
Wilk and Nusair were used without essentially any difference in the
results.[
32,33]
As a calculational scheme we employed the augmented
[34]
[35]
spherical wave (ASW) method (see ref. for a more recent treat-
ment), which is one of the most efficient band structure methods
in use today and offers the advantage of a minimal basis set similar
to atomic orbitals. The latter fact allows, in particular, for an intui-
tive interpretation of the results. The basis set included Ru-5s, -5p,
Experimental Section
4 3 4 8
(CO)12(µ -Se) ; ALFA,
: 18 mg of selenium (2.8·10Ϫ2 mmol Se
Ru
powder, Ϫ325 mesh, 5N) was dissolved in 100 mL of xylene
Merck, mixture of isomers, scintillation grade) under reflux at 140
(
°
-4d, Se-4s, -4p as well as C- and O-2s, -2p states. Care was taken to
Ϫ1
3
C. Then 72.87 mg of Ru (CO)12 (1.14·10 mmol; STREM crys-
ensure convergence of the results with respect to the completeness
of the basis set as well as the fineness of the k space grid; the
Brillouin zone sampling used up to 158 points within the irredu-
cible wedge. The interpretation of the electronic properties from
the calculated band structure and partial densities of states was
complemented by an analysis of the crystal orbital overlap popula-
tion (COOP), which allows access to the chemical bonding in more
talline, 99%) was added to the solution and dissolved. The solution
was then brought to ambient temperature. This solution (ca.
35Ϫ40 mL) was used to fill an ampoule of Duran glass and sealed
under vacuum using a water-jet pump. Further treatment was per-
formed in a high-pressure Morey-type autoclave with a Teflon inset
and a Teflon seal. The ampoule was placed in the autoclave and
heated. A counterpressure was applied filling the autoclave vessel
to 80 vol% with water. The autoclave was heated to 250 °C for 30
d. After cooling, the ampoule was broken and the product sepa-
rated using a PTFE filter. The residue was washed with small por-
tions of diethyl ether. Optical analysis using a light microscope re-
vealed that besides a black powder, which was identified as ele-
mental ruthenium by X-ray powder diffractometry, orange red
crystals had been formed with a yield of 5% (4.5 mg). Ϫ Depending
on the reaction time orange-red crystals up to 100 µm length grew
in a black powder of Ru particles of nanometer size. This growth
behaviour is very similar to the preparation of the carbidocarbonyl
[36]
detail. Ϫ In the present work we performed two different sets of
calculations. In the first set we used the experimentally determined
lattice constants and atomic positions. In contrast, in the second
set of calculations we used a hypothetical crystal structure where
the Se atoms were adjusted to positions corresponding to an ideal
cubic RuϪSe core and where, hence, some degeneracies were re-
stored. This way we were able to make closer connection to sym-
metry considerations employed in previous work, which were based
on an ideal cubic arrangement, and thus to cross-check our results
obtained for the measured crystal structure.
Ru
6
C(CO)17 described by Johnson et al.[ and by Braga et al.
29]
[30]
which could also only be obtained in a moderate yield. Ϫ The
purity of the crystals and the Ru/Se ratio of unity was confirmed
Acknowledgments
within an error bar of 3% by using Energy Dispersive X-ray fluor- The authors thank Prof. Dr. K. Langer (TU Berlin) for absorption
escence. The carbon monoxide content was measured thermogravi-
measurements and Dr. M. Hilgendorff (HMI Berlin) for FTIR
metrically and the gases released were analysed by mass spectro- measurements of the cubane phase.
metry. Ϫ FTIR measurements were performed using a Bruker IFS
[1]
1
13v spectrometer. By means of attenuated total reflection, two
J. O’M. Bockris, S. U. M. Kahn, Surface Electrochemistry,
Plenum Press, 1993, chapters 5.16Ϫ5.18.
-
1
strong peaks at 2079 and 2008cm in the CO region were detected
2494
Eur. J. Inorg. Chem. 2001, 2489Ϫ2495

4 3 4
New Cubane-Type Ru (CO)12(µ -Se) Tetramer
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2
7
Received January 11, 2001
[I01016]
9
Eur. J. Inorg. Chem. 2001, 2489Ϫ2495
2495