The Influence of a Single Transition Met
ARTICLE
DOI: 10.1002/zaac.201400280
The Influence of a Single Transition Metal Atom on the Reactivity of Main
Group Metal Clusters in the Gas Phase
Marco Neumaier,[a] Christian Schenk,[b] Hansgeorg Schnöckel,[a] and
Andreas Schnepf*[b]
Dedicated to Professor Martin Jansen on the Occasion of His 70th Birthday
Keywords: Catalysis; Collision induced dissociation; Germanium; Quantum chemical calculations
Abstract. Collision induced dissociation experiments of the metalloid
clusters [(CO)5CrGe9R3]– 2 and [(CO)3CrGe9R3]– 3 are presented,
showing that 2 can lose Cr(CO)5 or CO to give [Ge9R3]– 1 or 3, respectively. Further dissociation from 3 leads first of all to the metalloid
cluster [CrGe9R3]– 4, from which different elimination routes start.
Thereby significant differences with respect to the chromium free cluster [Ge9R3]– 1 are observed, where beside C–H also Si–C and Si–Si
bond dissociation at low threshold energy take place, showing that 4,
with its additional chromium atom, is an ideal model system for a
single atom catalyst.
Introduction
area is also important for nanoscience as the realm between
molecules and the solid state is the essence of nanotechnology.
Gas phase investigations open the way to observe the behavior
of an isolated cluster without disturbance of other clusters or
solvent molecules.[3] In the case of the metalloid cluster anion
[Ge9R3]– [R = Si(SiMe3)3][4] (1) (Figure 1) the dissociation behavior in the gas phase after collision induced dissociation
(CID) via on and off resonant excitation has given first insights
into the reactivity of group 14 metalloid clusters.[5]
Additionally oxidation reactions of 1 in a chlorine atmosphere have been possible giving further information about the
redox chemistry of 1 in the gas phase.[6] However in the fol-
Metalloid clusters of germanium of the general formula
GenRm, where n ⬎ m and R is a bulky ligand such as –Si(SiMe3)3
or –N(SiMe3)2 represent a novel class of cluster compounds in
group 14 chemistry, being ideal model compounds to get an
insight into the area between molecules and the solid state.[1]
This borderland is of particular interest, especially for metals
or semimetals, as drastic changes of physical properties
take place during reduction from salt-like oxidized species
(e.g. oxides, halides: non-conducting) to the bulk elemental
phase (metal: conducting; semimetal: semiconducting).[2] This
Figure 1. Molecular structure of [Ge9(Si(SiMe3)3)3]– (1) without SiMe3 groups (left), space filling model of 1 with view along the threefold
axis (middle) and FTICR mass spectrum of 1 (right).
* Prof. Dr. A. Schepf
E-Mail: andreas.schnepf@uni-tuebingen.de
[a] KIT - IMK-ASF
Engesserstr. 15
76131 Karlsruhe, Germany
[b] Mathematisch Naturwissenschaftliche Fakultät
Universität Tübingen
Auf der Morgenstelle 18
72076 Tübingen, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201400280 or from the author.
Z. Anorg. Allg. Chem. 2014, 640, (14), 2701–2707
lowing we will focus only on the CID experiments and shortly
sum-up the so far obtained results for 1: Thereby the first
dissociation step is the elimination of [Si(SiMe3)3]2 to give
[Ge9R]–.[7] As shown in Figure 2 two isomers of [Ge9R]– can
be formed that show a different fragmentation behavior. Hence
the isomer with an intact Si(SiMe3)3 ligand (top in Figure 2)
further reacts to [Ge9Si(SiMe3)]–, [Ge9Si]–, and [Ge9]–. In contrast, the remaining Si(SiMe3)3 ligand can dismantle on the
cluster surface leading to the [E10(SiMe3)3]– isomer (bottom in
Figure 2), where now three SiMe3 ligands are bound to an
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Neumaier, C. Schenk, H. Schnöckel, A. Schnepf
Figure 2. Schematic presentation of the reaction paths of the collision induced dissociation experiments of [Ge9R3]– [R = Si(SiMe3)3] (1) after
collision with argon atoms.
E10 cluster core (E10 = Ge9Si). Further elimination from
[E10(SiMe3)3]– leads to [E10(SiMe3)]– and [E10]–.[8]
Beside these investigations on the redox chemistry of
[Ge9(Si(SiMe3)3)3]– (1) in the gas phase it could be recently
shown that 1 is also a good starting material for further buildup
reactions in solution, where the easy accessible germanium
atoms of the cluster core of 1 (Figure 1 middle) are used to
connect two clusters of 1 by various transition metals to yield
[MGe18R6]x [M = Cu, Ag, Au for X = –1 and M = Zn, Cd, Hg
for X = 0; R = Si(SiMe3)3].[9] Furthermore the reaction of a
solution of 1 with (CO)5Cr(COE) (COE = cyclooctene) or
(CO)3Cr(CH3CN)3 gave the adducts [(CO)5CrGe9R3]– 2 and
[(CO)3CrGe9R3]– 3, respectively as crystalline compounds. In
case of 3 the chromium atom is now part of the cluster core
as shown in Figure 3 (right).[10] The isostructural molybdenum
and tungsten isomers of 3 are known as well.[11] Lately further
reactions of 1 with reactive main group molecules have been
conducted too, leading to neutral cluster compounds.[12]
As the chromium adducts 2 and 3 are accessible in high
yield[10] and as they can be transferred intact into the gas phase
via electrospray ionization (Figure 3) comparable CID experiments as performed with 1 are possible. Thereby the change
in reactivity of the metalloid cluster 1 by the addition of one
transition metal atom is most interesting and can be directly
observed, being thus an ideal model compound for a single
atom catalyst.[13]
Results and Discussion
To get an idea if and how the reactivity of the metalloid
cluster [Ge9(Si(SiMe3)3)3]– (1) is changed by the addition of a
transition metal fragment similar CID experiments to those of
1 (vide supra) were carried out with 2 and 3 (see Experimental
Section for details). As different transition metal fragments are
bound to the cluster core in 2 (Cr(CO)5) and 3 (Cr(CO)3) also
a different reactivity of 2 and 3 might be expected and will be
discussed in the following.
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Figure 3. Molecular structure of [(CO)5CrGe9R3]– 2 (upper left) and
[(CO)3CrGe9R3]– 3 (upper right), without SiMe3 groups. The respective experimental and calculated isotopic pattern for 2 and 3 is shown
beneath.
Collision induced Decomposition Experiments (CID) on
[(CO)5CrGe9R3]– 2
Firstly the metalloid cluster ion [(CO)5CrGe9R3]– 2 was focus of the CID experiments, which are summarized in Figure 4. It was found that 2 shows two different main decomposition pathways, i.e. via the breaking of the Cr–Ge bond in 2,
leading to [Ge9R3]– 1 and its known subsequent fragment ions
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 2701–2707
The Influence of a Single Transition Metal Atom on the Reactivity of Main Group Metal Clusters in the Gas Phase
like [Ge9R]–, or via the loss of two CO molecules to give the
cluster ion [(CO)3CrGe9R3]– 3, whereby [(CO)4CrGe9R3]– is
only detected with low intensity {i.e. up to 5 % of the
[(CO)3CrGe9R3]– signal}. The formation of 1 (m / z = 1396.7)
is thereby not much of a surprise as the Cr–Ge bond in 2 is
the weakest bond to a ligand in the cluster molecule[14] according to quantum chemical calculations.[15] The other dissociation pathway, where two CO molecules split off to give 3
reminds of the synthesis of 3 in solution, where 1 is treated
with (CO)3Cr(CH3CN)3 and where presumably acetonitrile is
set free step by step during the reaction.[10]
Figure 4. Mass spectra of collision induced decomposition experiments (CID) on [(CO)5CrGe9R3]– 2 (m/z = 1588.6) shown as a function
of the collision energy from Ecm = 1.0 eV to 4.5 eV. The different
product species are indicated by their elemental formulae [R =
Si(SiMe3)3].
Therefore the consecutive loss of CO molecules from 2 to
give 3 seems plausible. A scheme of the reaction path starting
from 2 to give 1 and (CO)5Cr or 3 and two CO molecules is
emphasized in Figure 5 together with the calculated relative
energies and structures. As can be seen in Figure 4, the CID
experiments of [(CO)5CrGe9R3]– 2 lead to the main products
[Ge9R3]– 1 and [(CO)3CrGe9R3]– 3 with a comparable intensity. Thus competitive pathways are present with an experimental dissociation threshold of ca. 1.6 eV (vide infra). The
formation of 1 and 3 in nearly equal parts is thereby in line
with a similar calculated reaction energy of 90 kJ·mol–1 and
110 kJ·mol–1 for the formation of 1 and 3 respectively (see
Figure 5).
As now the Cr(CO)3 adduct 3 is obtained during the fragmentation of 2, similar chromium containing fragment ions of
lower mass are expected for 2 and 3. This assumption proved
to be true as the mass spectrum of the CID experiments of 3
(Figure 6) and 2 (Figure 4) are quite similar. Thus in the case
of 3 only the chromium free products are missing as the chromium atom in 3 is now part of the cluster core and seemingly
not to be removed that easy anymore. Consequently with respect to chromium containing fragments the dissociation pathway is similar for 2 and 3 and will be discussed together in
the following.
Figure 6. Selected mass spectrum of the CID experiments of 3.
Collision induced Decomposition Experiments (CID) on
[(CO)5CrGe9R3]– 2 and [(CO)3CrGe9R3]– 3
Starting from [(CO)3CrGe9R3]– 3 one would first expect the
elimination of the CO ligands, leading to the product ions
Figure 5. Assumed reaction path with calculated minimum structures of the collision induced reaction starting from 2 in the gas phase either
losing CO molecules step by step to give 3 or losing (CO)5Cr to give 1. Reaction barriers and transitions states haven’t been calculated.
Z. Anorg. Allg. Chem. 2014, 2701–2707
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M. Neumaier, C. Schenk, H. Schnöckel, A. Schnepf
[(CO)2CrGe9R3]– 3-CO, [(CO)CrGe9R3]– 3-2CO and
[CrGe9R3]– 4 (m/z = 1448.6), which are actually observed. The
fragment ions 3-CO and 3-2CO are thereby detected at low
intensity during the CID experiments, whereby the intensity of
the signal is sometimes so low that it can hardly be identified
due to a poor signal to noise ratio.
The observed dissociation channels for 2 are summarized in
Figure 7 together with the experimentally determined threshold
energies, showing that beside the non radical compound 6 also
a radical compound [CrGe9R2]– 5 (m/z = 1201.5) is present
within chromium containing ions at low collision energies
starting from a dissociation threshold of ca. 3.0 eV. This find-
Figure 7. Fragmentation channels for the CID experiments of
Ge9R3Cr(CO)5– 2, together with the experimentally determined dissociation threshold (right; [eV]).
ing is in stark contrast to the chromium free system were radical products like [Ge9]– 13 (m/z = 653.3) are only gained at
higher collision energies.[5] Consequently the chromium atom
seems to catalyze reaction pathways to radical species, leading
to a significant reduction of the dissociation threshold. Nevertheless the unambiguous determination of a radical compound,
whose ion signals might be superposed by closed shell ion
fragments of a mass with one hydrogen atom more or less,
needs extensive gas phase experiments on one single isotopologue of [(CO)5CrGe9R3]– 2. The isotopologue is thereby
isolated by the SWIFT method. Afterwards the isolated isotopologue is used for similar CID experiments as described
above.[16] The experimental fingerprint of the gained isotope
distribution was compared with isotopic distributions calculated by the software IsoPatrn.[17] This combination allowed
the precise determination of the presence of radicals and closed
shell systems, showing that indeed radical fragments are
formed.
Now taking a look onto the different dissociation products
listed in Figure 7 it appears that [CrGe9R3]– 4 plays a central
role in the formation of chromium containing fragment ions of
lower mass. As 4 is only observed in spurious amount we conclude that the lifetime of vibrationally excited 4 is too short to
be effectively detected under the experimental conditions. The
different dissociation pathways of the short lived 4 might
thereby be described as follows: On the one hand complete
Si(SiMe3)3 ligands are eliminated, being thus similar so the
dissociation pathway as found for [Ge9R3]– 1. However in the
case of 4 the elimination of one ligand is preferred, leading to
the radical product ion [CrGe9R2]– 5, a reaction pathway not
observed for 1, directly showing that the reactivity of 1 signifi-
Figure 8. Calculated isomers of [CrGe9R3]– 4 with relative energies. The chromium atom together with the discussed bonding interaction (see
text) is highlighted in yellow.
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Z. Anorg. Allg. Chem. 2014, 2701–2707
The Influence of a Single Transition Metal Atom on the Reactivity of Main Group Metal Clusters in the Gas Phase
cantly changed by the addition of the chromium atom. The
change in reactivity is further corroborated by the
fact that completely different fragmentation routes are observed for 4 with respect to 1, i.e. in the case of 4 ligand
dismantling is taking place where also C–H and Si–C
bond breaking takes place, leading to fragment ions like
[CrGe9RSi(SiMe3)2(SiMe2CH2)]– 6 (m/z = 1200.5). Consequently starting from 4 many different reaction pathways are
possible. To get an idea about the basis of the different dissociation routes, different isomers of [CrGe9R3]– 4 were calculated,
whose calculated minimum structures are shown in Figure 8.
Thereby 4a is structurally closely related to 3 just without
the three missing CO ligands and therefore might be the kinetically favored primary product. The other isomers 4b, 4c, and
4d are energetically more stable and underwent an obvious
change in structure. Those three isomers 4b, 4c, and 4d might
thereby be the key to understand the new reaction channels
observed. Hence, in case of the isomer 4b the Cr atom interacts
with a SiMe3 group, leading to an elongated Si–Si bond of
262.8 pm (in contrast to 238.2 pm in an unaffected Si–Si
bond), i.e. the Si–Si bond is weakened and thus 4b might be
an intermediate for the formation of the fragment ion
[CrGe9RSi(SiMe3)2]– 8 (m/z = 1128.4), where Si(SiMe3)4 has
been eliminated.[18] In the case of the isomer 4c a C–H bond
is already broken, leading to a Cr–Ge–Si–Si–C five
membered ring, where the Cr–H bond is in beta position to
one of the Si(SiMe3)3 ligands. Thus 4c might be the intermediate for the formation of the detected fragment ion
[CrGe9RSi(SiMe3)2(SiMe2CH2)]– 6 (m/z = 1200.5), where
HSi(SiMe3)3 has been abstracted.[19]
The most favored isomer of 4 calculated so far is 4d, where
the chromium atom is located in the center of the cluster core.
A comparable arrangement is also observed in the case of the
Zintl anion Ge94–, where NiGe93– is observed as a product of
the reaction of Ge94– with Ni(cod)2 (cod = 1,5-cyclooctydiene)
as the transition metal source.[20] In the case of the metalloid
germanium cluster 4d the incorporation of the chromium atom
inside the germanium cluster core leads to the situation that
the chromium atom in 4d can no longer directly interact with
the Si(SiMe3)3 ligands. Nevertheless an isomer like 4d could
be responsible for the increased amount of detected radicals,
as it might be able to stabilize radicals like [CrGe9R2]– 5. Thus
although the chromium atom is not directly available it might
significantly change the further dissociation pathway of 4 and
thus the reactivity of the metalloid cluster.
creasing the possibility that not enough of the collision energy
is located in the adequate vibrational mode to cleave the
bonds.[22] The dissociation energy threshold for the formation
of further chromium containing products starts with 2.8 eV for
losing two Me groups and MeSi(SiMe3)3. A threshold energy
of 3.0 eV is observed for the abstraction of radical Si(SiMe3)3
or HSi(SiMe3) from 4 and 3.5 eV for losing Si(SiMe3)4. The
loss of the ligand dimer (Si(SiMe3)3)2 has a threshold energy
of 4.2 eV. At higher threshold energies the known fragmentation behavior of the chromium free cluster [Ge9R3]– 1 is observed.
In summary the presence of the Cr atom opens new reaction
pathways for decomposition, where also C–H and C–Si bonds
break and where additionally radical fragments are commonly
obtained. Consequently 3 is an ideal model compound to directly observe the activation of C–H and Si–C bonds by a single transition metal atom. The strong impact of the transition
metal atom on the reactivity of the metalloid germanium cluster can also be seen on the reaction with reactive gases like
chlorine or oxygen which will be presented in a forthcoming
paper.
Conclusions
We described first reactions of the metalloid clusters
[(CO)5CrGe9R3]– 2 and [(CO)3CrGe9R3]– 3 in the gas phase
via collision induced dissociation experiments (CID). The collision induced dissociation experiments on 2 show that two
fragmentation routes of similar possibility exist, where on the
one hand Cr(CO)5 is lost, leading to the chromium free cluster
[Ge9R3]– 1 being thus the inverse reaction of its formation. On
the other hand 2 loses two CO molecules to give 3. Further
dissociation from 3 via CO elimination leads to the metalloid
cluster [CrGe9R3]– 4, from which different elimination routes
start. The different routes might thereby be correlated to different isomers of 4, where the chromium atom has a significant
influence on the reactivity of the metalloid cluster, i.e. beside
Si–Si also Si–C and C–H bond dissociation, catalyzed by the
chromium atom is observed. Additionally an electronic influence of the chromium atom is obvious leading to radical compounds at lower collision energies showing that the additional
chromium atom in [CrGe9R3]– 4, extremely modifies the reactivity of the metalloid cluster [Ge9R3]– 1 being thereby an ideal
model system for a single atom catalyst.
Experimental Section
Threshold Energies for the Dissociation Channels of
[(CO)5CrGe9R3]– 2
Taking a closer look onto the experimentally obtained appearance energies (Figure 7) shows that for the dissociation of
2 leading to 1 and 3 the threshold of 1.6 eV (154 kJ·mol–1) is
much higher than the respective calculated reaction energies
(90 kJ·mol–1 for 1 and 110 kJ·mol–1 for 3; see Figure 5). This
known behavior is called kinetic shift,[21] as lager systems like
2 have an enormous set of vibrational degrees of freedom {414
calculated modes for the molecule [(CO)5CrGe9R3]– 2} inZ. Anorg. Allg. Chem. 2014, 2701–2707
Mass spectrometric experiments were performed with an IonSpec Ultima FT-ICR-MS (Fourier-Transform Ion Cyclotron Resonance Mass
Spectrometer), equipped with a 7 T, actively shielded, super conducting magnet. The instrument was coupled to a modified electrospray
ionization (ESI) source from Analytica of Branford that can be evacuated and flushed with an inert gas (e.g. nitrogen) to prevent highly
sensitive cluster compounds from oxidation.[23] All given m/z values
within the text refer to the most intense isotopologue of the isotopic
distribution. The cluster anion [(CO)5CrGe9R3]– 2 was brought intact
into the gas phase by electrospraying a solution of 2 (≈10 mmol·L–1)
in THF. The potential of the endplate and of the nickel coated entrance
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Neumaier, C. Schenk, H. Schnöckel, A. Schnepf
of the fused silica desolvation capillary were typically held at ca.
3.2 kV and ca. 3.3 kV relative to the grounded electrospray needle,
respectively. The potential of the capillary exit was held at ca. –30 V.
After passing a skimmer (–5 V) the ions were pre-trapped in a hexapole for 3 s and were transferred into the ICR cell via a quadrupole
ion guide. By keeping the potential difference between the capillary
exit and the skimmer low, decomposition of [(CO)5CrGe9R3]– to
[(CO)3CrGe9R3]– (+ 2CO) was effectively suppressed.[6,24] The trapping voltages of the ICR cell were set to –1.5 V and ion detection took
place by standard ICR techniques.
an appropriate function was fitted to the breakdown curves (see Figure
S1, Supporting Information) that takes into account Doppler broadening of the collision gas.[30]
In order to perform CID experiments [(CO)5CrGe9R3]– ions were first
isolated by the SWIFT (Stored Waveform Inverse Fourier Transform)
excitation technique[25] followed by a resonant dipolar excitation pulse
that was applied for tex = 500 µs to increase the ions kinetic energy.
The excitation energy was varied by adjusting the excitation amplitude
(Vpp) of the applied RF voltage while leaving tex constant. The kinetic
energy of the ions after resonant, dipolar excitation is given by:
References
Ekin = β2
q2 Vpp2 tex2
8mion d2
(1)
where mion is the mass of the ion (1588.6 u for [(CO)5CrGe9R3]–), d
is the diameter of the ICR cell (0.0625 m) and β is a cell geometry
parameter that was estimated to be 0.89.[26] Equation (1) was verified
by determining the diameter of the ICR cell via the method described
in Hawkridge et al.[26]
After the excitation the ions were brought to collision with Argon
(99.9990 %, Air Liquide) at a static argon pressure of ca.
2 ⫻ 10–8 mbar, that was introduced into the ultra-high vacuum chamber
(base pressure ca. 5 ⫻ 10–10 mbar) by a needle valve (Huntington). The
collision gas was present during the whole experiment. Assuming stationary gas molecules, the maximum collision energy in the center of
mass frame is given by[27]
Ecm = Ekin
mg
mg + mion
where mg is the mass of the collision gas (39.95 u for argon). The time
delay between the dipolar excitation pulse and ion detection, i.e. the
collision time (tdiss), was typically 75 ms and was much longer than
the excitation time tex of 500 µs.
To determine reasonable dissociation thresholds it is generally important to minimize the fraction of multiple collisions and to work under
single collisions,[28] i.e. to work at low pressures and keep the collision
times short. For large molecule, with a large number of vibrational
degrees of freedom, as in the case of [(CO)5CrGe9R3]–, the appearance
energy can strongly deviate from the real dissociation energy as a large
amount of excess energy can be required to cause a detectable dissociation within the experimental time window.[29] A collision time of
75 ms was found to be appropriate in terms of fragmentation efficiency
and minimization of the fraction of multiple collisions. A longer collision time of 200 ms resulted in lower appearance energies, i.e. ca.
1.1 eV for species 1 and 3, compared to ca. 1.6 eV at tdiss = 75 ms,
which can be explained by the influence of multiple collisions or might
be due to kinetic shift effects.
Dissociation breakdown curves were measured by taking mass spectra
as a function of collision energy. The signals of the dissociation products Ii were normalized to the sum of all detected ions I0 (educt anion
2 + all observed fragment ions) in order to account for ion loss and
for instabilities of the ESI source. To determine dissociation thresholds
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Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft (DFG) for
financial support.
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[14] The two center (2c) shared electron number (SEN) for the Cr–Ge
bond in [(CO)5CrGe9R3]– 2 is with 1.01 lower than the 2c-SEN
for the Ge-Si (1.1) bonds, thus indicating that the Cr–Ge bond is
weaker than the Ge–Si bonds in 2. The Shared Electron Numbers
(SENs) for bonds from an Ahlrichs-Heinzmann population analysis are thereby a reliable measure of the covalent bonding
strength. For example, the SEN for the Ge–Ge single bond in the
model compound R3Ge–GeR3 (R = NH2) is 1.04.
[15] Quantum-chemical calculations were carried out with the RI-DFT
version of the Turbomole program package, by employing the
Becke–Perdew 86-functional. The basis sets were of SVP quality.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 2701–2707
The Influence of a Single Transition Metal Atom on the Reactivity of Main Group Metal Clusters in the Gas Phase
[16]
[17]
[18]
[19]
[20]
[21]
[22]
The electronic structure was analyzed with the AhlrichsHeinzmann population analysis based on occupation numbers.
Turbomole: O. Treutler, R. Ahlrichs, J. Chem. Phys. 1995, 102,
346–354; BP-86-functional: J. P. Perdew, Phys. Rev. B 1986, 33,
8822–8824; A. D. Becke, Phys. Rev. A 1988, 38, 3098–3100; RIDFT: K. Eichkorn, O. Treutler, H. Öhm, M. Häser, R. Ahlrichs,
Chem. Phys. Lett. 1995, 240, 283–290; SVP: A. Schäfer, H. Horn,
R. Ahlrichs, J. Chem. Phys. 1992, 97, 2571–2577; AhlrichsHeinzmann population analysis: E. R. Davidson, J. Chem. Phys.
1967, 46, 3320–3324; K. R. Roby, Mol. Phys. 1974, 27, 81–104;
R. Heinzmann, R. Ahlrichs, Theor. Chim. Acta 1976, 42, 33–45;
C. Erhardt, R. Ahlrichs, Theor. Chim. Acta 1985, 68, 231–245.
See Supporting Information for mass spectrum of the CID experiments of a single isotopologue of [(CO)5CrGe9R3]– 2.
L. Ramaley, L. C. Herrera, Rapid Commun. Mass Spectrom. 2008,
22, 2707–2714.
A similar isomer could be responsible for the formation of the
fragment ion [CrGe9RSi2(SiMe3)2]– (m/z = 1156) where four
SiMe3 groups were lost.
A similar elimination was recently seen in subsequent reactions of
the metalloid germanium Cluster [Ge14(Si(SiMe3)3)5][Li(thf)2]3,
where the ion [H2Ge14(Si(SiMe3)3)5]– was detected in the gas
phase and where two neutral HSi(SiMe3)3 fragments can be abstracted by CID experiments; C. Schenk, A. Kracke, K. Fink, A.
Kubas, W. Klopper, M. Neumaier, H. Schnöckel, A. Schnepf, J.
Am. Chem. Soc. 2011, 133, 2518–2524.
J. M. Goicoechea, S. C. Sevov, Angew. Chem. 2005, 117, 4094–
4096; Angew. Chem. Int. Ed. 2005, 44, 4026–4028.
C. Lifshitz, Eur. J. Mass Spectrom. 2002, 8, 85–98.
This seems to have an impact on the chromium free products as
well because even when taking removal of (CO)5Cr with 1.6 eV
Z. Anorg. Allg. Chem. 2014, 2701–2707
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
into account, which leads to the known cluster 1 the adjusted
threshold energies for the reaction towards [Ge9R]– is with 2.4 eV
(4.0 eV minus 1.6 eV) about 0.4 eV higher than the afore determined energy from 1 directly with 2.0 eV.[6] This is found to be
even stronger the case for [Ge9Si(SiMe3)]– with 3.3 eV (4.9 eV
minus 1.6 eV) and [Ge9]– with 3.5 eV (5.1 eV minus 1.6 eV),
which were measured starting from 1 to be 2.6 and 2.7 eV, respectively, and being therefore 0.7 and 0.8 eV higher than expected.
It might be that by starting from 2 the separation of the neutral
(CO)5Cr fragment from the cluster leeches more than 1.6 eV at
higher induced energies cooling down the remaining cluster and
therefore might lead to higher threshold energies for the formation
of all further chromium free fragment ions.
R. Burgert, Dissertation, Universitätsverlag, Karlsruhe, 2007.
O. Hampe, M. Neumaier, M. N. Blom, M. M. Kappes, Chem.
Phys. Lett. 2002, 354, 303–309.
T. L. C. Wang, T. L. Ricca, A. G. Marshall, Anal. Chem. 1986,
58, 2935–2938.
A. M. Hawkridge, A. I. Nepomuceno, S. L. Lovik, C. J. Mason,
D. C. Muddiman, Rapid Commun. Mass Spectrom. 2005, 19,
915–918.
S. A. McLuckey, J. Am. Soc. Mass Spectrom. 1992, 3, 599–614.
C. E. C. A. Hop, T. B. McMahon, G. D. Willett, Int. J. Mass Spectrom. Ion Processes 1990, 101, 191–208; A. B. Vakhtin, E. M.
Markin, K. Sugawara, Chem. Phys. 2000, 262, 93–104.
C. Lifshitz, Eur. J. Mass Spectrom. 2002, 8, 85–98.
H. L. Sievers, H.-F. Grützmacher, P. Caravatti, Int. J. Mass Spectrom. Ion Processes 1996, 152–159, 233–247; J. P. Chantry, J.
Chem. Phys. 1971, 55, 2746.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: June 6, 2014
Published Online: September 16, 2014
www.zaac.wiley-vch.de
2707
DOI: 10.1002/zaac.201400280
The Influence of a Single Transition Metal Atom on the Reactivity of Main
Group Metal Clusters in the Gas Phase
Marco Neumaier,[a] Christian Schenk,[b] Hansgeorg Schnöckel,[a] and
Andreas Schnepf*[b]
Dedicated to Professor Martin Jansen on the Occasion of His 70th Birthday
Keywords: Catalysis; Collision induced dissociation; Germanium; Quantum chemical calculations
Abstract. Collision induced dissociation experiments of the metalloid
clusters [(CO)5CrGe9R3]– 2 and [(CO)3CrGe9R3]– 3 are presented,
showing that 2 can lose Cr(CO)5 or CO to give [Ge9R3]– 1 or 3, respectively. Further dissociation from 3 leads first of all to the metalloid
cluster [CrGe9R3]– 4, from which different elimination routes start.
Thereby significant differences with respect to the chromium free cluster [Ge9R3]– 1 are observed, where beside C–H also Si–C and Si–Si
bond dissociation at low threshold energy take place, showing that 4,
with its additional chromium atom, is an ideal model system for a
single atom catalyst.
Introduction
area is also important for nanoscience as the realm between
molecules and the solid state is the essence of nanotechnology.
Gas phase investigations open the way to observe the behavior
of an isolated cluster without disturbance of other clusters or
solvent molecules.[3] In the case of the metalloid cluster anion
[Ge9R3]– [R = Si(SiMe3)3][4] (1) (Figure 1) the dissociation behavior in the gas phase after collision induced dissociation
(CID) via on and off resonant excitation has given first insights
into the reactivity of group 14 metalloid clusters.[5]
Additionally oxidation reactions of 1 in a chlorine atmosphere have been possible giving further information about the
redox chemistry of 1 in the gas phase.[6] However in the fol-
Metalloid clusters of germanium of the general formula
GenRm, where n ⬎ m and R is a bulky ligand such as –Si(SiMe3)3
or –N(SiMe3)2 represent a novel class of cluster compounds in
group 14 chemistry, being ideal model compounds to get an
insight into the area between molecules and the solid state.[1]
This borderland is of particular interest, especially for metals
or semimetals, as drastic changes of physical properties
take place during reduction from salt-like oxidized species
(e.g. oxides, halides: non-conducting) to the bulk elemental
phase (metal: conducting; semimetal: semiconducting).[2] This
Figure 1. Molecular structure of [Ge9(Si(SiMe3)3)3]– (1) without SiMe3 groups (left), space filling model of 1 with view along the threefold
axis (middle) and FTICR mass spectrum of 1 (right).
* Prof. Dr. A. Schepf
E-Mail: andreas.schnepf@uni-tuebingen.de
[a] KIT - IMK-ASF
Engesserstr. 15
76131 Karlsruhe, Germany
[b] Mathematisch Naturwissenschaftliche Fakultät
Universität Tübingen
Auf der Morgenstelle 18
72076 Tübingen, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201400280 or from the author.
Z. Anorg. Allg. Chem. 2014, 640, (14), 2701–2707
lowing we will focus only on the CID experiments and shortly
sum-up the so far obtained results for 1: Thereby the first
dissociation step is the elimination of [Si(SiMe3)3]2 to give
[Ge9R]–.[7] As shown in Figure 2 two isomers of [Ge9R]– can
be formed that show a different fragmentation behavior. Hence
the isomer with an intact Si(SiMe3)3 ligand (top in Figure 2)
further reacts to [Ge9Si(SiMe3)]–, [Ge9Si]–, and [Ge9]–. In contrast, the remaining Si(SiMe3)3 ligand can dismantle on the
cluster surface leading to the [E10(SiMe3)3]– isomer (bottom in
Figure 2), where now three SiMe3 ligands are bound to an
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 2. Schematic presentation of the reaction paths of the collision induced dissociation experiments of [Ge9R3]– [R = Si(SiMe3)3] (1) after
collision with argon atoms.
E10 cluster core (E10 = Ge9Si). Further elimination from
[E10(SiMe3)3]– leads to [E10(SiMe3)]– and [E10]–.[8]
Beside these investigations on the redox chemistry of
[Ge9(Si(SiMe3)3)3]– (1) in the gas phase it could be recently
shown that 1 is also a good starting material for further buildup
reactions in solution, where the easy accessible germanium
atoms of the cluster core of 1 (Figure 1 middle) are used to
connect two clusters of 1 by various transition metals to yield
[MGe18R6]x [M = Cu, Ag, Au for X = –1 and M = Zn, Cd, Hg
for X = 0; R = Si(SiMe3)3].[9] Furthermore the reaction of a
solution of 1 with (CO)5Cr(COE) (COE = cyclooctene) or
(CO)3Cr(CH3CN)3 gave the adducts [(CO)5CrGe9R3]– 2 and
[(CO)3CrGe9R3]– 3, respectively as crystalline compounds. In
case of 3 the chromium atom is now part of the cluster core
as shown in Figure 3 (right).[10] The isostructural molybdenum
and tungsten isomers of 3 are known as well.[11] Lately further
reactions of 1 with reactive main group molecules have been
conducted too, leading to neutral cluster compounds.[12]
As the chromium adducts 2 and 3 are accessible in high
yield[10] and as they can be transferred intact into the gas phase
via electrospray ionization (Figure 3) comparable CID experiments as performed with 1 are possible. Thereby the change
in reactivity of the metalloid cluster 1 by the addition of one
transition metal atom is most interesting and can be directly
observed, being thus an ideal model compound for a single
atom catalyst.[13]
Results and Discussion
To get an idea if and how the reactivity of the metalloid
cluster [Ge9(Si(SiMe3)3)3]– (1) is changed by the addition of a
transition metal fragment similar CID experiments to those of
1 (vide supra) were carried out with 2 and 3 (see Experimental
Section for details). As different transition metal fragments are
bound to the cluster core in 2 (Cr(CO)5) and 3 (Cr(CO)3) also
a different reactivity of 2 and 3 might be expected and will be
discussed in the following.
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Figure 3. Molecular structure of [(CO)5CrGe9R3]– 2 (upper left) and
[(CO)3CrGe9R3]– 3 (upper right), without SiMe3 groups. The respective experimental and calculated isotopic pattern for 2 and 3 is shown
beneath.
Collision induced Decomposition Experiments (CID) on
[(CO)5CrGe9R3]– 2
Firstly the metalloid cluster ion [(CO)5CrGe9R3]– 2 was focus of the CID experiments, which are summarized in Figure 4. It was found that 2 shows two different main decomposition pathways, i.e. via the breaking of the Cr–Ge bond in 2,
leading to [Ge9R3]– 1 and its known subsequent fragment ions
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 2701–2707
The Influence of a Single Transition Metal Atom on the Reactivity of Main Group Metal Clusters in the Gas Phase
like [Ge9R]–, or via the loss of two CO molecules to give the
cluster ion [(CO)3CrGe9R3]– 3, whereby [(CO)4CrGe9R3]– is
only detected with low intensity {i.e. up to 5 % of the
[(CO)3CrGe9R3]– signal}. The formation of 1 (m / z = 1396.7)
is thereby not much of a surprise as the Cr–Ge bond in 2 is
the weakest bond to a ligand in the cluster molecule[14] according to quantum chemical calculations.[15] The other dissociation pathway, where two CO molecules split off to give 3
reminds of the synthesis of 3 in solution, where 1 is treated
with (CO)3Cr(CH3CN)3 and where presumably acetonitrile is
set free step by step during the reaction.[10]
Figure 4. Mass spectra of collision induced decomposition experiments (CID) on [(CO)5CrGe9R3]– 2 (m/z = 1588.6) shown as a function
of the collision energy from Ecm = 1.0 eV to 4.5 eV. The different
product species are indicated by their elemental formulae [R =
Si(SiMe3)3].
Therefore the consecutive loss of CO molecules from 2 to
give 3 seems plausible. A scheme of the reaction path starting
from 2 to give 1 and (CO)5Cr or 3 and two CO molecules is
emphasized in Figure 5 together with the calculated relative
energies and structures. As can be seen in Figure 4, the CID
experiments of [(CO)5CrGe9R3]– 2 lead to the main products
[Ge9R3]– 1 and [(CO)3CrGe9R3]– 3 with a comparable intensity. Thus competitive pathways are present with an experimental dissociation threshold of ca. 1.6 eV (vide infra). The
formation of 1 and 3 in nearly equal parts is thereby in line
with a similar calculated reaction energy of 90 kJ·mol–1 and
110 kJ·mol–1 for the formation of 1 and 3 respectively (see
Figure 5).
As now the Cr(CO)3 adduct 3 is obtained during the fragmentation of 2, similar chromium containing fragment ions of
lower mass are expected for 2 and 3. This assumption proved
to be true as the mass spectrum of the CID experiments of 3
(Figure 6) and 2 (Figure 4) are quite similar. Thus in the case
of 3 only the chromium free products are missing as the chromium atom in 3 is now part of the cluster core and seemingly
not to be removed that easy anymore. Consequently with respect to chromium containing fragments the dissociation pathway is similar for 2 and 3 and will be discussed together in
the following.
Figure 6. Selected mass spectrum of the CID experiments of 3.
Collision induced Decomposition Experiments (CID) on
[(CO)5CrGe9R3]– 2 and [(CO)3CrGe9R3]– 3
Starting from [(CO)3CrGe9R3]– 3 one would first expect the
elimination of the CO ligands, leading to the product ions
Figure 5. Assumed reaction path with calculated minimum structures of the collision induced reaction starting from 2 in the gas phase either
losing CO molecules step by step to give 3 or losing (CO)5Cr to give 1. Reaction barriers and transitions states haven’t been calculated.
Z. Anorg. Allg. Chem. 2014, 2701–2707
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Neumaier, C. Schenk, H. Schnöckel, A. Schnepf
[(CO)2CrGe9R3]– 3-CO, [(CO)CrGe9R3]– 3-2CO and
[CrGe9R3]– 4 (m/z = 1448.6), which are actually observed. The
fragment ions 3-CO and 3-2CO are thereby detected at low
intensity during the CID experiments, whereby the intensity of
the signal is sometimes so low that it can hardly be identified
due to a poor signal to noise ratio.
The observed dissociation channels for 2 are summarized in
Figure 7 together with the experimentally determined threshold
energies, showing that beside the non radical compound 6 also
a radical compound [CrGe9R2]– 5 (m/z = 1201.5) is present
within chromium containing ions at low collision energies
starting from a dissociation threshold of ca. 3.0 eV. This find-
Figure 7. Fragmentation channels for the CID experiments of
Ge9R3Cr(CO)5– 2, together with the experimentally determined dissociation threshold (right; [eV]).
ing is in stark contrast to the chromium free system were radical products like [Ge9]– 13 (m/z = 653.3) are only gained at
higher collision energies.[5] Consequently the chromium atom
seems to catalyze reaction pathways to radical species, leading
to a significant reduction of the dissociation threshold. Nevertheless the unambiguous determination of a radical compound,
whose ion signals might be superposed by closed shell ion
fragments of a mass with one hydrogen atom more or less,
needs extensive gas phase experiments on one single isotopologue of [(CO)5CrGe9R3]– 2. The isotopologue is thereby
isolated by the SWIFT method. Afterwards the isolated isotopologue is used for similar CID experiments as described
above.[16] The experimental fingerprint of the gained isotope
distribution was compared with isotopic distributions calculated by the software IsoPatrn.[17] This combination allowed
the precise determination of the presence of radicals and closed
shell systems, showing that indeed radical fragments are
formed.
Now taking a look onto the different dissociation products
listed in Figure 7 it appears that [CrGe9R3]– 4 plays a central
role in the formation of chromium containing fragment ions of
lower mass. As 4 is only observed in spurious amount we conclude that the lifetime of vibrationally excited 4 is too short to
be effectively detected under the experimental conditions. The
different dissociation pathways of the short lived 4 might
thereby be described as follows: On the one hand complete
Si(SiMe3)3 ligands are eliminated, being thus similar so the
dissociation pathway as found for [Ge9R3]– 1. However in the
case of 4 the elimination of one ligand is preferred, leading to
the radical product ion [CrGe9R2]– 5, a reaction pathway not
observed for 1, directly showing that the reactivity of 1 signifi-
Figure 8. Calculated isomers of [CrGe9R3]– 4 with relative energies. The chromium atom together with the discussed bonding interaction (see
text) is highlighted in yellow.
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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 2701–2707
The Influence of a Single Transition Metal Atom on the Reactivity of Main Group Metal Clusters in the Gas Phase
cantly changed by the addition of the chromium atom. The
change in reactivity is further corroborated by the
fact that completely different fragmentation routes are observed for 4 with respect to 1, i.e. in the case of 4 ligand
dismantling is taking place where also C–H and Si–C
bond breaking takes place, leading to fragment ions like
[CrGe9RSi(SiMe3)2(SiMe2CH2)]– 6 (m/z = 1200.5). Consequently starting from 4 many different reaction pathways are
possible. To get an idea about the basis of the different dissociation routes, different isomers of [CrGe9R3]– 4 were calculated,
whose calculated minimum structures are shown in Figure 8.
Thereby 4a is structurally closely related to 3 just without
the three missing CO ligands and therefore might be the kinetically favored primary product. The other isomers 4b, 4c, and
4d are energetically more stable and underwent an obvious
change in structure. Those three isomers 4b, 4c, and 4d might
thereby be the key to understand the new reaction channels
observed. Hence, in case of the isomer 4b the Cr atom interacts
with a SiMe3 group, leading to an elongated Si–Si bond of
262.8 pm (in contrast to 238.2 pm in an unaffected Si–Si
bond), i.e. the Si–Si bond is weakened and thus 4b might be
an intermediate for the formation of the fragment ion
[CrGe9RSi(SiMe3)2]– 8 (m/z = 1128.4), where Si(SiMe3)4 has
been eliminated.[18] In the case of the isomer 4c a C–H bond
is already broken, leading to a Cr–Ge–Si–Si–C five
membered ring, where the Cr–H bond is in beta position to
one of the Si(SiMe3)3 ligands. Thus 4c might be the intermediate for the formation of the detected fragment ion
[CrGe9RSi(SiMe3)2(SiMe2CH2)]– 6 (m/z = 1200.5), where
HSi(SiMe3)3 has been abstracted.[19]
The most favored isomer of 4 calculated so far is 4d, where
the chromium atom is located in the center of the cluster core.
A comparable arrangement is also observed in the case of the
Zintl anion Ge94–, where NiGe93– is observed as a product of
the reaction of Ge94– with Ni(cod)2 (cod = 1,5-cyclooctydiene)
as the transition metal source.[20] In the case of the metalloid
germanium cluster 4d the incorporation of the chromium atom
inside the germanium cluster core leads to the situation that
the chromium atom in 4d can no longer directly interact with
the Si(SiMe3)3 ligands. Nevertheless an isomer like 4d could
be responsible for the increased amount of detected radicals,
as it might be able to stabilize radicals like [CrGe9R2]– 5. Thus
although the chromium atom is not directly available it might
significantly change the further dissociation pathway of 4 and
thus the reactivity of the metalloid cluster.
creasing the possibility that not enough of the collision energy
is located in the adequate vibrational mode to cleave the
bonds.[22] The dissociation energy threshold for the formation
of further chromium containing products starts with 2.8 eV for
losing two Me groups and MeSi(SiMe3)3. A threshold energy
of 3.0 eV is observed for the abstraction of radical Si(SiMe3)3
or HSi(SiMe3) from 4 and 3.5 eV for losing Si(SiMe3)4. The
loss of the ligand dimer (Si(SiMe3)3)2 has a threshold energy
of 4.2 eV. At higher threshold energies the known fragmentation behavior of the chromium free cluster [Ge9R3]– 1 is observed.
In summary the presence of the Cr atom opens new reaction
pathways for decomposition, where also C–H and C–Si bonds
break and where additionally radical fragments are commonly
obtained. Consequently 3 is an ideal model compound to directly observe the activation of C–H and Si–C bonds by a single transition metal atom. The strong impact of the transition
metal atom on the reactivity of the metalloid germanium cluster can also be seen on the reaction with reactive gases like
chlorine or oxygen which will be presented in a forthcoming
paper.
Conclusions
We described first reactions of the metalloid clusters
[(CO)5CrGe9R3]– 2 and [(CO)3CrGe9R3]– 3 in the gas phase
via collision induced dissociation experiments (CID). The collision induced dissociation experiments on 2 show that two
fragmentation routes of similar possibility exist, where on the
one hand Cr(CO)5 is lost, leading to the chromium free cluster
[Ge9R3]– 1 being thus the inverse reaction of its formation. On
the other hand 2 loses two CO molecules to give 3. Further
dissociation from 3 via CO elimination leads to the metalloid
cluster [CrGe9R3]– 4, from which different elimination routes
start. The different routes might thereby be correlated to different isomers of 4, where the chromium atom has a significant
influence on the reactivity of the metalloid cluster, i.e. beside
Si–Si also Si–C and C–H bond dissociation, catalyzed by the
chromium atom is observed. Additionally an electronic influence of the chromium atom is obvious leading to radical compounds at lower collision energies showing that the additional
chromium atom in [CrGe9R3]– 4, extremely modifies the reactivity of the metalloid cluster [Ge9R3]– 1 being thereby an ideal
model system for a single atom catalyst.
Experimental Section
Threshold Energies for the Dissociation Channels of
[(CO)5CrGe9R3]– 2
Taking a closer look onto the experimentally obtained appearance energies (Figure 7) shows that for the dissociation of
2 leading to 1 and 3 the threshold of 1.6 eV (154 kJ·mol–1) is
much higher than the respective calculated reaction energies
(90 kJ·mol–1 for 1 and 110 kJ·mol–1 for 3; see Figure 5). This
known behavior is called kinetic shift,[21] as lager systems like
2 have an enormous set of vibrational degrees of freedom {414
calculated modes for the molecule [(CO)5CrGe9R3]– 2} inZ. Anorg. Allg. Chem. 2014, 2701–2707
Mass spectrometric experiments were performed with an IonSpec Ultima FT-ICR-MS (Fourier-Transform Ion Cyclotron Resonance Mass
Spectrometer), equipped with a 7 T, actively shielded, super conducting magnet. The instrument was coupled to a modified electrospray
ionization (ESI) source from Analytica of Branford that can be evacuated and flushed with an inert gas (e.g. nitrogen) to prevent highly
sensitive cluster compounds from oxidation.[23] All given m/z values
within the text refer to the most intense isotopologue of the isotopic
distribution. The cluster anion [(CO)5CrGe9R3]– 2 was brought intact
into the gas phase by electrospraying a solution of 2 (≈10 mmol·L–1)
in THF. The potential of the endplate and of the nickel coated entrance
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Neumaier, C. Schenk, H. Schnöckel, A. Schnepf
of the fused silica desolvation capillary were typically held at ca.
3.2 kV and ca. 3.3 kV relative to the grounded electrospray needle,
respectively. The potential of the capillary exit was held at ca. –30 V.
After passing a skimmer (–5 V) the ions were pre-trapped in a hexapole for 3 s and were transferred into the ICR cell via a quadrupole
ion guide. By keeping the potential difference between the capillary
exit and the skimmer low, decomposition of [(CO)5CrGe9R3]– to
[(CO)3CrGe9R3]– (+ 2CO) was effectively suppressed.[6,24] The trapping voltages of the ICR cell were set to –1.5 V and ion detection took
place by standard ICR techniques.
an appropriate function was fitted to the breakdown curves (see Figure
S1, Supporting Information) that takes into account Doppler broadening of the collision gas.[30]
In order to perform CID experiments [(CO)5CrGe9R3]– ions were first
isolated by the SWIFT (Stored Waveform Inverse Fourier Transform)
excitation technique[25] followed by a resonant dipolar excitation pulse
that was applied for tex = 500 µs to increase the ions kinetic energy.
The excitation energy was varied by adjusting the excitation amplitude
(Vpp) of the applied RF voltage while leaving tex constant. The kinetic
energy of the ions after resonant, dipolar excitation is given by:
References
Ekin = β2
q2 Vpp2 tex2
8mion d2
(1)
where mion is the mass of the ion (1588.6 u for [(CO)5CrGe9R3]–), d
is the diameter of the ICR cell (0.0625 m) and β is a cell geometry
parameter that was estimated to be 0.89.[26] Equation (1) was verified
by determining the diameter of the ICR cell via the method described
in Hawkridge et al.[26]
After the excitation the ions were brought to collision with Argon
(99.9990 %, Air Liquide) at a static argon pressure of ca.
2 ⫻ 10–8 mbar, that was introduced into the ultra-high vacuum chamber
(base pressure ca. 5 ⫻ 10–10 mbar) by a needle valve (Huntington). The
collision gas was present during the whole experiment. Assuming stationary gas molecules, the maximum collision energy in the center of
mass frame is given by[27]
Ecm = Ekin
mg
mg + mion
where mg is the mass of the collision gas (39.95 u for argon). The time
delay between the dipolar excitation pulse and ion detection, i.e. the
collision time (tdiss), was typically 75 ms and was much longer than
the excitation time tex of 500 µs.
To determine reasonable dissociation thresholds it is generally important to minimize the fraction of multiple collisions and to work under
single collisions,[28] i.e. to work at low pressures and keep the collision
times short. For large molecule, with a large number of vibrational
degrees of freedom, as in the case of [(CO)5CrGe9R3]–, the appearance
energy can strongly deviate from the real dissociation energy as a large
amount of excess energy can be required to cause a detectable dissociation within the experimental time window.[29] A collision time of
75 ms was found to be appropriate in terms of fragmentation efficiency
and minimization of the fraction of multiple collisions. A longer collision time of 200 ms resulted in lower appearance energies, i.e. ca.
1.1 eV for species 1 and 3, compared to ca. 1.6 eV at tdiss = 75 ms,
which can be explained by the influence of multiple collisions or might
be due to kinetic shift effects.
Dissociation breakdown curves were measured by taking mass spectra
as a function of collision energy. The signals of the dissociation products Ii were normalized to the sum of all detected ions I0 (educt anion
2 + all observed fragment ions) in order to account for ion loss and
for instabilities of the ESI source. To determine dissociation thresholds
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Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft (DFG) for
financial support.
[1] A. Schnepf, Coord. Chem. Rev. 2006, 250, 2758–2770; A.
Schnepf, Chem. Soc. Rev. 2007, 36, 745–758.
[2] H. Schnöckel, Chem. Rev. 2010, 110, 4125–4163; A. Schnepf,
New J. Chem. 2010, 34, 2079–2092.
[3] R. Burgert, H. Schnöckel, Chem. Commun. 2008, 18, 2075–2089;
M. Neumaier, M. Olzmann, B. Kiran, K. H. Bowen, B. Eichhorn,
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[7] This is in contrast to similar gas phase dissociation experiments
with the metalloid gallium cluster {Ga19[C(SiMe3)3]6}–, where
Ga[C(SiMe3)3] units are eliminated, indicating significant differences between metalloid group 13 and group 14 clusters. However, as different ligands (Si(SiMe3)3 and C(SiMe3)3) are present
this might also significantly influence the dissociation behavior,
which has to be clarified by a future comprehensive analysis: K.
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[8] Ge9– is thereby not observed as the incorporated silicon atom
cannot be eliminated due to a higher binding energy with respect
to the germanium atoms.
[9] C. Schenk, A. Schnepf, Angew. Chem. 2007, 119, 5408–5410;
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[14] The two center (2c) shared electron number (SEN) for the Cr–Ge
bond in [(CO)5CrGe9R3]– 2 is with 1.01 lower than the 2c-SEN
for the Ge-Si (1.1) bonds, thus indicating that the Cr–Ge bond is
weaker than the Ge–Si bonds in 2. The Shared Electron Numbers
(SENs) for bonds from an Ahlrichs-Heinzmann population analysis are thereby a reliable measure of the covalent bonding
strength. For example, the SEN for the Ge–Ge single bond in the
model compound R3Ge–GeR3 (R = NH2) is 1.04.
[15] Quantum-chemical calculations were carried out with the RI-DFT
version of the Turbomole program package, by employing the
Becke–Perdew 86-functional. The basis sets were of SVP quality.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 2701–2707
The Influence of a Single Transition Metal Atom on the Reactivity of Main Group Metal Clusters in the Gas Phase
[16]
[17]
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L. Ramaley, L. C. Herrera, Rapid Commun. Mass Spectrom. 2008,
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A similar isomer could be responsible for the formation of the
fragment ion [CrGe9RSi2(SiMe3)2]– (m/z = 1156) where four
SiMe3 groups were lost.
A similar elimination was recently seen in subsequent reactions of
the metalloid germanium Cluster [Ge14(Si(SiMe3)3)5][Li(thf)2]3,
where the ion [H2Ge14(Si(SiMe3)3)5]– was detected in the gas
phase and where two neutral HSi(SiMe3)3 fragments can be abstracted by CID experiments; C. Schenk, A. Kracke, K. Fink, A.
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This seems to have an impact on the chromium free products as
well because even when taking removal of (CO)5Cr with 1.6 eV
Z. Anorg. Allg. Chem. 2014, 2701–2707
[23]
[24]
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[28]
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into account, which leads to the known cluster 1 the adjusted
threshold energies for the reaction towards [Ge9R]– is with 2.4 eV
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even stronger the case for [Ge9Si(SiMe3)]– with 3.3 eV (4.9 eV
minus 1.6 eV) and [Ge9]– with 3.5 eV (5.1 eV minus 1.6 eV),
which were measured starting from 1 to be 2.6 and 2.7 eV, respectively, and being therefore 0.7 and 0.8 eV higher than expected.
It might be that by starting from 2 the separation of the neutral
(CO)5Cr fragment from the cluster leeches more than 1.6 eV at
higher induced energies cooling down the remaining cluster and
therefore might lead to higher threshold energies for the formation
of all further chromium free fragment ions.
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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: June 6, 2014
Published Online: September 16, 2014
www.zaac.wiley-vch.de
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