The Influence of a Single Transition Met

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 that2can lose Cr(CO)5or CO to give [Ge9R3]–1or3,

respec-tively. Further dissociation from3 leads first of all to the metalloid cluster [CrGe9R3]–4, from which different elimination routes start.

Introduction

Metalloid clusters of germanium of the general formula GenRm, wheren⬎mandRis a bulky ligand such as –Si(SiMe3)3

or –N(SiMe3)2represent 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 SiMe3groups (left), space filling model of1with view along the threefold

axis (middle) and FTICR mass spectrum of1(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.201400280or from the au-thor.

Thereby significant differences with respect to the chromium free clus-ter [Ge9R3]–1are observed, where beside C–H also Si–C and Si–Si

bond dissociation at low threshold energy take place, showing that4, with its additional chromium atom, is an ideal model system for a single atom catalyst.

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

be-havior 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 atmo-sphere have been possible giving further information about the redox chemistry of1 in the gas phase.[6] _{However in the}

fol-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

con-trast, the remaining Si(SiMe3)3 ligand can dismantle on the

cluster surface leading to the [E10(SiMe3)3]–isomer (bottom in

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 that1is 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 of1by various transition metals to yield [MGe18R6]x[M= Cu, Ag, Au forX= –1 andM= Zn, Cd, Hg

forX = 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 3the chromium atom is now part of the cluster core as shown in Figure 3 (right).[10]_{The isostructural molybdenum}

and tungsten isomers of3are known as well.[11]_{Lately further}

reactions of1 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 experi-ments as performed with1 are possible. Thereby the change in reactivity of the metalloid cluster1by 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 with2and3(see Experimental Section for details). As different transition metal fragments are bound to the cluster core in2(Cr(CO)5) and3(Cr(CO)3) also

a different reactivity of2and3might be expected and will be discussed in the following.

Figure 3.Molecular structure of [(CO)5CrGe9R3]–2(upper left) and

[(CO)3CrGe9R3]–3(upper right), without SiMe3groups. The

respec-tive experimental and calculated isotopic pattern for2and3is shown beneath.

Collision induced Decomposition Experiments (CID) on [(CO)5CrGe9R3]–2

Firstly the metalloid cluster ion [(CO)5CrGe9R3]–2was

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 of1(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]

accord-ing to quantum chemical calculations.[15] _{The other}

dissoci-ation 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 experi-ments (CID) on [(CO)5CrGe9R3]–2(m/z= 1588.6) shown as a function

of the collision energy from E_cm= 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 from2to give3seems plausible. A scheme of the reaction path starting from2 to give1and (CO)5Cr or3 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

Figure 5.Assumed reaction path with calculated minimum structures of the collision induced reaction starting from2in the gas phase either losing CO molecules step by step to give3or losing (CO)5Cr to give1. Reaction barriers and transitions states haven’t been calculated.

[Ge9R3]– 1 and [(CO)3CrGe9R3]– 3 with a comparable

inten-sity. Thus competitive pathways are present with an experi-mental dissociation threshold of ca. 1.6 eV (vide infra). The formation of 1and 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

frag-mentation of2, similar chromium containing fragment ions of lower mass are expected for2and3. This assumption proved to be true as the mass spectrum of the CID experiments of3

(Figure 6) and2(Figure 4) are quite similar. Thus in the case of3only the chromium free products are missing as the chro-mium atom in3is now part of the cluster core and seemingly not to be removed that easy anymore. Consequently with re-spect to chromium containing fragments the dissociation path-way is similar for 2 and3 and will be discussed together in the following.

Figure 6.Selected mass spectrum of the CID experiments of3.

Collision induced Decomposition Experiments (CID) on [(CO)5CrGe9R3]–2 and [(CO)3CrGe9R3]–3

Starting from [(CO)3CrGe9R3]–3one would first expect the

[(CO)2CrGe9R3]– 3-CO, [(CO)CrGe9R3]– 3-2CO and

[CrGe9R3]–4(m/z= 1448.6), which are actually observed. The

fragment ions 3-COand 3-2COare 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 for2are summarized in Figure 7 together with the experimentally determined threshold energies, showing that beside the non radical compound6also 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

disso-ciation threshold (right; [eV]).

Figure 8.Calculated isomers of [CrGe9R3]–4with relative energies. The chromium atom together with the discussed bonding interaction (see

text) is highlighted in yellow.

ing is in stark contrast to the chromium free system were radi-cal 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. Never-theless 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 isoto-pologue of [(CO)5CrGe9R3]– 2. The isotopologue is thereby

isolated by the SWIFT method. Afterwards the isolated isoto-pologue is used for similar CID experiments as described above.[16] _{The experimental fingerprint of the gained isotope}

distribution was compared with isotopic distributions calcu-lated 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]– 4plays a central

role in the formation of chromium containing fragment ions of lower mass. As4is only observed in spurious amount we con-clude that the lifetime of vibrationally excited4is 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 of4the elimination of one ligand is preferred, leading to the radical product ion [CrGe9R2]– 5, a reaction pathway not

signifi-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 ob-served 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).

Conse-quently starting from 4many different reaction pathways are possible. To get an idea about the basis of the different dissoci-ation routes, different isomers of [CrGe9R3]–4were calculated,

whose calculated minimum structures are shown in Figure 8. Thereby4a is structurally closely related to3 just without the three missing CO ligands and therefore might be the kinet-ically favored primary product. The other isomers4b,4c, and

4d are energetically more stable and underwent an obvious change in structure. Those three isomers4b,4c, and4dmight thereby be the key to understand the new reaction channels observed. Hence, in case of the isomer4bthe 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 thus4b might be an intermediate for the formation of the fragment ion [CrGe9RSi(SiMe3)2]– 8(m/z= 1128.4), where Si(SiMe3)4has

been eliminated.[18]_{In the case of the isomer4ca 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)3ligands. Thus4cmight be the

intermedi-ate for the formation of the detected fragment ion [CrGe9RSi(SiMe3)2(SiMe2CH2)]– 6 (m/z = 1200.5), where

HSi(SiMe3)3has been abstracted.[19]

The most favored isomer of4calculated so far is4d, 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 cluster4dthe incorporation of the chromium atom inside the germanium cluster core leads to the situation that the chromium atom in4dcan no longer directly interact with the Si(SiMe3)3 ligands. Nevertheless an isomer like4dcould

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 of4and thus the reactivity of the metalloid cluster.

Threshold Energies for the Dissociation Channels of [(CO)₅CrGe₉R₃]–₂

Taking a closer look onto the experimentally obtained ap-pearance energies (Figure 7) shows that for the dissociation of

2leading to1and3the threshold of 1.6 eV (154 kJ·mol–1_{) is}

much higher than the respective calculated reaction energies (90 kJ·mol–1_{for1and 110 kJ·mol}–1_{for3; see Figure 5). This}

known behavior is called kinetic shift,[21]_{as lager systems like}

2have an enormous set of vibrational degrees of freedom {414 calculated modes for the molecule [(CO)5CrGe9R3]– 2}

in-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 4and 3.5 eV for losing Si(SiMe3)4. The

loss of the ligand dimer (Si(SiMe3)3)2has a threshold energy

of 4.2 eV. At higher threshold energies the known fragmenta-tion behavior of the chromium free cluster [Ge9R3]– 1is

ob-served.

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 3is an ideal model compound to di-rectly observe the activation of C–H and Si–C bonds by a sin-gle transition metal atom. The strong impact of the transition metal atom on the reactivity of the metalloid germanium clus-ter 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 col-lision induced dissociation experiments on 2 show that two fragmentation routes of similar possibility exist, where on the one hand Cr(CO)5is lost, leading to the chromium free cluster

[Ge9R3]–1being thus the inverse reaction of its formation. On

the other hand 2 loses two CO molecules to give3. Further dissociation from3via CO elimination leads to the metalloid cluster [CrGe9R3]– 4, from which different elimination routes

start. The different routes might thereby be correlated to dif-ferent isomers of4, 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 influ-ence of the chromium atom is obvious leading to radical com-pounds at lower collision energies showing that the additional chromium atom in [CrGe9R3]–4, extremely modifies the

reac-tivity of the metalloid cluster [Ge9R3]–1being thereby an ideal

model system for a single atom catalyst.

Experimental Section

Mass spectrometric experiments were performed with an IonSpec Ul-tima FT-ICR-MS (Fourier-Transform Ion Cyclotron Resonance Mass Spectrometer), equipped with a 7 T, actively shielded, super conduct-ing magnet. The instrument was coupled to a modified electrospray ionization (ESI) source from Analytica of Branford that can be evacu-ated and flushed with an inert gas (e.g. nitrogen) to prevent highly sensitive cluster compounds from oxidation.[23]_{All givenm/zvalues}

within the text refer to the most intense isotopologue of the isotopic distribution. The cluster anion [(CO)5CrGe9R3]–2was brought intact

into the gas phase by electrospraying a solution of2(≈10 mmol·L–1)

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 hexa-pole for 3 s and were transferred into the ICR cell via a quadruhexa-pole 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

trap-ping voltages of the ICR cell were set to –1.5 V and ion detection took place by standard ICR techniques.

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 fort_ex= 500µs to increase the ions kinetic energy.

The excitation energy was varied by adjusting the excitation amplitude (Vpp) of the applied RF voltage while leavingtexconstant. The kinetic

energy of the ions after resonant, dipolar excitation is given by:

Ekin=β2

is the diameter of the ICR cell (0.0625 m) andβ 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 inHawkridgeet 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 sta-tionary gas molecules, the maximum collision energy in the center of mass frame is given by[27]

Ecm=Ekin m_g

mg+mion

wheremgis 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 timetexof 500µs.

To determine reasonable dissociation thresholds it is generally impor-tant 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 dissoci-ation 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 colli-sion time of 200 ms resulted in lower appearance energies, i.e. ca. 1.1 eV for species1and3, compared to ca. 1.6 eV att_diss= 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 prod-uctsIiwere normalized to the sum of all detected ionsI0(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

an appropriate function was fitted to the breakdown curves (see Figure S1, Supporting Information) that takes into account Doppler broaden-ing of the collision gas.[30]

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]–2is 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 in2. The Shared Electron Numbers (SENs) for bonds from an Ahlrichs-Heinzmann population analy-sis are thereby a reliable measure of the covalent bonding strength. For example, the SEN for the Ge–Ge single bond in the model compoundR₃Ge–GeR₃(R= NH2) is 1.04.

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[22] 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

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 deter-mined energy from1directly 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 from1to be 2.6 and 2.7 eV, respec-tively, and being therefore 0.7 and 0.8 eV higher than expected. It might be that by starting from2the 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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