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International Journal of Mass Spectrometry 436 (2019) 59–64

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Chemistry at the rim: Differentiation of isomeric open-cage fullerenes by MALDI-LIFT and ESI-MS/MS Leanne C. Nye c , Jakob F. Hitzenberger a , Manolis M. Roubelakis b,1 , Michael Orfanopoulos b , Thomas Drewello a,∗ a

Physical Chemistry I, Department of Chemistry and Pharmacy, University of Erlangen-Nuremberg, Egerlandstrasse 3, 91058 Erlangen, Germany Organic Chemistry, Department of Chemistry, University of Crete, 71003 Voutes, Heraklion, Greece c Computation and System Medicine, Department of Surgery and Cancer, Faculty of Medicine, Imperial College, London SW7 2AZ, UK b

a r t i c l e

i n f o

Article history: Received 10 August 2018 Received in revised form 25 October 2018 Accepted 1 November 2018 Available online 12 November 2018 Dedicated to Prof. Helmut Schwarz on the occasion of his 75th birthday. Keywords: MALDI-LIFT ESI-MS/MS Open-cage fullerenes Isomers Kinetic method DCTB Pencil lead

a b s t r a c t Isomeric open-cage fullerenes (OCFs) are studied by MALDI-LIFT. The two types of isomers feature different connectivities of the heteroatoms at the rim of the orifice to the fullerene cage. The molecular ions (radical cations, M+ .) are generated by electron transfer MALDI, using DCTB as the matrix. The resulting LIFT spectra are indistinguishable. The alternative use of pencil lead is found inferior as a matrix but proves an exceptionally facile method of creating sodium and potassium adducts. The fragmentation of these metallated OCFs reveals different fragmentation pathways allowing the differentiation of the OCF isomers. This is evidently caused by the different interactions of the metal cation with the heteroatoms at the rim of the orifice. Additionally, the influence of the functionalization and isomer structure on the sodium ion affinity (SIA) is analysed by the kinetic method investigating the dissociations of sodium-bound hetero-dimers in electrospray ionization/ collision-induced dissociation (ESI-CID) experiments. © 2018 Published by Elsevier B.V.

1. Introduction The pioneering work that provided evidence of the gas-phase formation of endohedral fullerenes is certainly of particular significance among the numerous seminal scientific contributions by Helmut Schwarz and his collaborators [1–3]. In a series of keV-collision experiments, to which also other groups contributed [4–8], fast moving fullerene ions would “pick up” stationary helium atoms or vice versa [7]. In this way, stable endohedral fullerene ions of the type He@C60/70 were generated and studied. At around the same time, efforts towards the preparation of macroscopic amounts of such compounds came into full swing, mainly including noble gas- [9,10] and metal atom-containing [11] endohedral fullerenes. Besides the more recent and somewhat more efficiently produced metal nitride cluster fullerenes [12–14], the yield of such endohedral complexes has been disappointingly low.

∗ Corresponding author. E-mail address: [email protected] (T. Drewello). 1 Present address: Evonik Corporation, 7201 Hamilton Boulevard, Allentown, PA 18195-1501, USA. https://doi.org/10.1016/j.ijms.2018.11.006 1387-3806/© 2018 Published by Elsevier B.V.

To enhance the potential yield of the envisaged endohedral fullerenes, great hopes have been pinned on the so-called “molecular surgery” strategy [15–17] using open-cage fullerenes (OCFs) [18]. OCFs are characterised by cage openings that exceed the 6membered ring as the largest ring size of the fullerene cage. The array of possible OCF structures may commence with the implementation of one or two methano- or imino-bridges [19,20] which may lead to [5,6]-open and/or [6,6]-open cages and may extend up to fullerene cages with very large orifices (13-membered rings and more), held open by strategically placed heteroatoms and functional groups [18,21,22]. The “molecular surgery” approach includes the following three steps: opening of the fullerene cage, filling of the cage with the endohedral component and closure to prevent its escape. In fact, the strategy has been successful for the inclusion of small molecules, such as H2 [15,16] and H2 O [17]. While fullerenes and fullerene derivatives with exohedral ligands have been studied to a considerable extent by mass spectrometry, gas-phase investigations into the behaviour of “empty” open cage fullerenes are rather scarce. At first sight, the investigation of an OCF with a filling seems rather more attractive [23]! However, the study of “empty” OCFs by mass spectrometry is of importance for a variety of reasons. For instance, the extent and

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Ph

Ph

Py N

O

N Ph

O

Ph

Py

O O

N H H

N

NH

R

HH

R = -H x.1 -Br x.2 -OMe x.3

HN

R 1.x

For pencil lead as a MALDI matrix, 6B and 8B Staedtler branded Mars Lumograph pencils were applied individually on the target spots with a pressure equivalent to writing. Prior to sample application, the entire slide was held in an air stream for the removal of loose pencil lead. The sample solutions were applied one drop at a time and allowed to dry between applications. About 5 ␮l of sample solution was applied to each target spot. MALDI mass spectra were recorded with two MALDI instruments; the Bruker Ultraflex III and the Bruker Reflex III. For the measurements of the SIAs and MS/MS, ESI experiments were carried out with a quadrupole ion trap (QIT) mass spectrometer (esquire 6000, Bruker). 3. Results and discussion

2.x

Fig. 1. OCFs, of which there are two isomers 1.X and 2.X with three different substituents: hydrogen (1.1 and 2.1), bromine (1.2 and 2.2) and methoxy (1.3 and 2.3).

way of cage closure may depend on the method of activation. With several different heteroatoms being placed at the rim of the OCF, the closure may not only lead to pure fullerenes, but also to heterofullerenes. Cage closure may differ for neutrals and ions and may also depend on the charge. Finally, the knowledge of the behaviour of “empty” OCFs will prevent confusion of isobaric ions in low resolution experiments, such as endohedral and “empty” fullerenes with different cages. An example is the isobaric pair H2 @C60 and C59 N. The OCFs studied here are depicted in Fig. 1. Their synthesis has been published elsewhere [24]. They represent three sets of two isomeric forms. The two isomers feature a different succession of functional groups at the rim of the OCFs, each creating a 16-membered-ring orifice. Each set of the isomeric pairs features a different substituent at the para-position of the phenylhydrazone ligand. The OCFs are investigated by (matrix-assisted) laser desorption / ionization (MA)LDI in combination with LIFT [25], as a means to perform tandem mass spectrometry (MS/MS, daughter ion analysis). The dissociation behaviour of the molecular cation radicals, M+ ., is compared to the dissociations of the sodiated MNA+ , and potassiated MK+ , species. In a second set of experiments, the sodium bridged dimers of the OCF’s are formed by electrospray ionization (ESI), and in MS/MS experiments the competitive dissociation into neutral and sodiated monomer ions is studied (kinetic method) [26], providing a relative sodium ion affinity (SIA) order between the OCFs. The two sets of MS/MS data are then compared in regards to their usefulness in characterisation and differentiation of the OCF isomers. 2. Experimental For the MALDI-MS and ESI-MS experiments, stock solutions of the samples were prepared. The open-cage fullerenes 1.1 to 2.319 (Fig. 1) were dissolved separately in toluene (Tol, Fisher Chemicals, 99% laboratory grade or VWR, HPLC grade) with 1 mg/ml. For ESI, the stock solution was then diluted in DCM:MeOH (Volume ratio 1:2, Sigma-Aldrich, HPLC grade). After addition of equimolar amounts of sodium acetate (NaOAc, Alfa Aesar) and thorough mixing, the 5 × 10−6 M solution was introduced to the ESI-MS ionization chamber by direct injection. For LDI/MALDI-MS, sample and matrix were individually dissolved in toluene. DCTB (2-[(2E)-3-(4-tertbutylphenyl)2-methylprop2-enylidene]malononitrile) was prepared in a 10 mg/ml solution. The sample and matrix were mixed in a 1:50 M ratio. Between 3 and 5 ␮l per target spot were applied to the MALDI slide.

3.1. MALDI DCTB is a typical electron transfer matrix with a particular track record for the successful MALDI analysis of derivatised fullerenes [27–31]. The thermochemical frame, in which this matrix and the matrix-derived ions operate, is often ideally suited for the abundant formation of molecular ions of the fullerene derivative in both positive- and negative-ion mode [27]. Moreover, unwanted fragmentation processes are frequently absent in DCTB-MALDI, or at least, the degree of fragmentation is markedly reduced compared to the use of other matrix materials [28,28,29,30,31]. The six OCF analytes are no exception to this “rule”. DCTB-MALDI produced the molecular ions in both ion modes with little or no fragmentation. The negative-ion mode spectra were mostly of better quality. Protonation/deprotonation and metal ion attachment were neither observed. Activation of the compounds under study by increasing the laser fluence using MALDI and laser desorption / ionization (LDI) resulted in re-healing of the cage of the OCFs to either C60 and/or C59 N, as already observed in previous (MA)LDI experiments [32]. However, the DCTB-MALDI spectra obtained in both ion modes did not provide any differentiation between the isomeric OCF pairs, which is also true for the LIFT spectra of the molecular ions (see below). Therefore, LIFT experiments with the sodiated and potassiated OCF were envisaged, following the speculative idea that the metal cations would attach to the heteroatoms at the rim of each isomer in a distinct manner, so that dissociations may lead to the release of different entities. Pencil lead as a target substrate is known for the abundant formation of sodiated and potassiated analyte molecules [33]. The use of DCTB for this purpose would require an additional salt layer on the target slide or a salt solution deposited with the sample solution [29]. Pencil lead was used in this context solely as a most versatile means to produce sodiated and potassiated OCFs with the intension to produce the precursor ions for the LIFT experiments. Pencil lead consists of graphite mixed with clay and varying levels of oils and waxes. It has been reported to provide a matrix-style protection under laser ablation for peptides, polymers, steroids and sugars [33]. However, it was recently shown, that the performance of pencil lead as a matrix material for MALDI of organic derivatised fullerenes does not match the high-quality spectra obtainable by DCTB-MALDI [34]. And this is also the case here, employing the OCFs as analytes. 3.2. MALDI-LIFT (MS/MS) Isomeric OCFs provide almost identical DCTB-MALDI spectra without any indication as to which isomer was responsible for the data. As depicted in Fig. 2, the same is true for the LIFT spectra of the molecular ions. In recent investigations into the gasphase behaviour of covalent C60 /crown ether-adducts [35–37], we

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Fig. 2. LIFT (MS/MS) spectra in the positive-ion mode of the molecular ion of 1.2 (a) and 2.2 (b) using DCTB as matrix and of the sodiated 1.2 (c) and 2.2 (d) and potassiated 1.2 (e) and 2.2 (f) molecules using pencil lead as the matrix.

found substantial changes when varying the alkali cation, originating from the differences in the interaction of metal cation with the crown ether ligand. Inspired by these findings the present study aims at the elucidation of metal complexation with the heteroatoms at the rim of each isomer in a distinct manner, in order to result in a distinguishable dissociation pattern. The LIFT spectra of the OCF molecular ion pairs (DCTB-MALDI) of Sample 1.2 and 2.2 (Fig. 2) are representative for all three pairs investigated. The main fragments are observed at m/z 1049 and m/z 1036 without an isomer specific intensity order. Each ion is found abundantly without one or the other dominating. In other words, all six OCF analyte ions show exactly the same daughter ion pattern. Obviously, this must be caused by the loss of the ligand which would differentiate the three pairs in the first step of the fragmentation. The mass difference from the respective molecular ions to m/z 1049 indicates that one hydrogen atom was transferred to the released ligand, so that each pair lost a H2 N-C6 H4 -X moiety with X H / Br / OMe. We assume the neighbouring methylene group (Fig. 1) as the source of this hydrogen atom. All other hydrogen atoms are further away and more strongly bound at aromatic rings. The signal at m/z 1036 is caused by the loss of NN(H)-C6 H4 -X, i.e. the complete hydrazone ligand. Since the low mass fragment ion cannot be constructed by a logical neutral loss from the high mass fragment ion, it is evident that both fragmentations are independent reactions and do not represent a reaction sequence. In summary, all molecular ions feature the partial and complete loss of the hydrazone ligand in two independent one-step reactions without providing a distinction between individual OCF molecules.

The LIFT spectra of the sodiated and potassiated sample molecules 1.2 and 2.2 (pencil lead-MALDI) are also shown in Fig. 2. The selectivity of these MS/MS data is somewhat deceiving, as the ion gate was unable to cleanly separate the sodiated and potassiated species from one another. Therefore, contributions from both adducts were observed in each LIFT spectra. Interference-free LIFT spectra, as obtained for Fig. 2, were only possible by avoiding the unwanted metal adduct, respectively. This was achieved by the use of 6B pencil enhancing the Na+ adduct and 8B pencil to promote the K+ adduct. The LIFT spectrum of the sodium adduct of sample 1.2 (Fig. 2c)) shows two fragment ions, whereas sample 2.2 (Fig. 2d)) only shows one. In both cases, the fragment ion at m/z 1073 is formed by release of a neutral ·NH-C6 H4 Br radical from the hydrazone ligand. Thereby, contrary to the dissociation of the molecular ions, an odd electron species is formed by losing a radical. The second fragment of sample 1.2 is the second step of a sequential reaction as it only occurs after the loss of the ·NH-C6 H4 Br radical and represents the loss of 78 mass units. In contrast to the molecular ion, this second dissociation cannot possibly involve the hydrazone ligand, as no reasonable losses of that mass can be assigned. The only reasonable candidates for the loss of mass 78 would represent C6 H6 (benzene) and C5 H4 N· (pyridyl radical). Despite the lack of sufficiently high resolution/mass accuracy to undoubtedly do so, one may for two reasons fairly confidently assign the loss of 78 mass units to C5 H4 N· (pyridyl radical). Firstly, losing a second radical would turn the remainder of the molecule into the often-preferred even electron system. Secondly, the loss of benzene would require

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Fig. 3. Sketch of the different dissociation behavior of molecular ion versus sodiated isomers.

the transfer of a hydrogen atom to one of the C6 H5 (phenyl groups). The most likely candidates for such a transfer are those methylene hydrogen atoms, which have been assumed to be involved in the dissociations of the molecular ions. However, this time, these hydrogen atoms are too far away to easily interact with the phenyl groups. The dissociation behaviour for potassium adducts resembles the one of the sodiated species. Replacing sodium by potassium does not induce a distinct behaviour. The different dissociation behaviour of the two OCF isomers is sketched in Fig. 3 and can be explained through the coordination of the cation to the heteroatoms of the rim. In those isomers that show the additional loss of the pyridyl radical (1.x), the heteroatoms are in a closer proximity to this ligand. For the opposite isomers (2.x), the pyridyl-group is situated further away from the ring orifice. It can be assumed that the metal cation interacts with several of the heteroatoms simultaneously, and depending on which isomer it is bound too, will either repel or have very little interaction with the pryidine ligand, inducing fragmentation in one isomer which is not observed in the other. 3.3. ESI-MS/MS A second approach to differentiate the OCF isomers was conducted by studying mixed dimers of the OCFs bound together by one sodium cation. These sodium-bound heterodimers are often readily formed during electrospray ionization. In MS/MS experiments, the competitive dissociation of the heterodimers would produce a certain ratio of the sodiated monomers of each OCF and the OCF with the higher sodium ion affinity (SIA) would be observed more abundantly. Pairing the different OCFs in such experiments

O O

O

O N

N H

H

NH

O

Fig. 4. Reference compound (Ref) for the determination of the SIA.

leads to a relative SIA order. This approach is commonly known as the kinetic method [26]. In order to reduce the number of measurements in the present set of experiments and to measure the SIA against one fixed molecule, the OCFs were not directly paired with one another, but with a similarly structured reference compound (Ref), which is depicted in Fig. 4. Ref is also an OCF. It has a differently constructed orifice but uses the same heteroatoms for its construction. The competitive dissociation of the heterodimers (OCF-Na+ -Ref) leads to a certain ratio of the fragment ion intensities. The different formation rates of the daughter ions are caused by differences in the enthalpy of the two reaction pathways. Thereby, the ratio of the reaction rates is equal to the ratio of the fragment ion intensities,

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Fig. 5. Relative SIA of R*ln(x M/Ref M), with x = Sample 1.1–2.3, plotted against the fragmentation amplitude of the QIT.

the natural logarithm of which is linearly related to the difference in enthalpy: ln

k1 = ln k2

 [OCF + Na+ ]  (H)  + ≈ [Ref + Na ]

RT eff

(1)

Where k1 and k2 are the reaction rates, H is the enthalpy and R is the gas constant. The effective temperature Teff is only obtainable for substances with known thermodynamic properties but is then applicable for similar substances by approximation. In this work, relative SIA are measured, which does not require a value for Teff . The drawbacks encountered by the present approach included firstly the large differences in the SIA of Ref compared to the OCFs. The ratio of the sodiated monomer of Ref to one of the OCF isomers was occasionally as high as 1:25, which makes it difficult to detect minor deviations of the intensities. Secondly, there were facile secondary reactions of the primary sodiated monomeric fragment ions, in analogy to the dissociations observed in the LIFT experiments. To minimize the resulting error, all fragment ions had to be included in the calculation of the SIA. Additionally, sodium bridged heterodimers were fragmented at different fragmentation amplitudes to ensure the reproducibility of the results and absence of the influence of entropic effects and the grade of fragmentation. In Fig. 5 a graph according to eqn. (1) is shown, in which R*ln(x M/Ref M) -with X M representing the intensity of the sodiated OCF monomer and Ref M the sodiated reference compoundis plotted against the fragmentation amplitude of the QIT. Besides some expected inconsistencies at low amplitudes, the data appear otherwise in a consistent and reliable manner. Overall, isomer 1.x appears to have the lower SIA compared with isomer 2.x. Moreover, the substituent in para-position at the phenylhydrazone has an influence. For bromine as substituent, there is no clear distinction between the isomeric pair, for hydrogen, the gap in SIA increases and for methoxy, the biggest difference is observed. Unfortunately, these data merely describe the phenomenon. Understanding of the role of coordination and/or electronic effects cannot be provided. Clearly, the metal cation will coordinate to several sites leading to different, perhaps even fluctuating dimer geometries. Ironically,

the methoxy group brings about the lowest SIA for isomer 1.x and the highest for isomer 2.x. Whether this is caused by the -in generalelectron pushing character of the methoxy group or by differences in coordination, is far from being obvious given the complexity of the molecules under investigation. In summary, only two of the three isomer pairs show a clear difference in relative SIA. 4. Conclusion The isomeric OCFs could be distinguished through a clearly different fragment ion ratio in dissociations of their sodiated and potassiated adduct ions, which could not be achieved through dissociations of the isomeric molecular ions. This indicates that the cation interaction with the heteroatoms at the rim brings about a differentiation of the isomeric forms. The relative sodium ion affinities are less selective in distinguishing isomeric OCF. One in three pairs showed indistinguishable SIAs. Cage re-healing to C60 and C59 N could only be achieved during (MA)LDI at elevated laser fluences and not during MS/MS by LIFT and low-energy CID. DCTB-MALDI is excellently suited to analyse organic OCFs, pencil lead-(MA)LDI is a cheap, rapid and efficient way to produce sodiated and potassiated OCF adducts. Acknowledgements Funding form the Deutsche Forschungsgemeinschaft (DFG) – SFB 953 “Synthetic Carbon Allotropes” is gratefully acknowledged. We thank M.B. Minameyer for help with the preparation of the manuscript. References [1] T. Weiske, D.K. Böhme, J. Hrusak, W. Krätschmer, H. Schwarz, Endohedral cluster compounds: inclusion of helium within C60 .+ and C70 .+ , Angew. Chem. Int. Ed. Engl. 30 (1991) 884–886. [2] J. Hrusak, D.K. Böhme, T. Weiske, H. Schwarz, Ab initio MO calculation on the energy barrier for the penetration of a benzene ring by a helium atom. Model

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