ISSN 10619348, Journal of Analytical Chemistry, 2010, Vol. 65, No. 14, pp. 1495–1503. © Pleiades Publishing, Ltd., 2010. Original Russian Text © V.Yu. Markov, N.A. Samokhvalova, P.S. Samokhvalov, V.A. Ioutsi, P.A. Khavrel’, N.S. Ovchinnikova, L.N. Sidorov, 2010, published in Zhurnal Mass spektrometria, 7(1), pp. 21–28.

ARTICLES

Mass Spectrometric Identification of Multicage Fullerene Cycloadducts V. Yu. Markov, N. A. Samokhvalova, P. S. Samokhvalov, V. A. Ioutsi, P. A. Khavrel’, N. S. Ovchinnikova, and L. N. Sidorov Department of Chemistry, Moscow State University, Moscow, 119991 Russia email: [email protected] Received September 16, 2009

Abstract—Multicage fullerene cycloadducts have been detected by MALDI mass spectrometry; they have been found as admixtures in the products of reactions of the trifluoromethylation of fullerene samples doped with metallic sodium, the reaction between fullerenes and a mixture of trifluoromethylfullerenes, and the synthesis of fulleroproline esters. As a result, over 75 new compounds of this type have been identified. The optimization of the synthesis procedures and chromatographic fractionation allowed us to extract five com pounds in the pure form: (C60)2(CF2)2(CF3)8, (C60)2(CF2)2(CF3)3C2F5, (C60)2(CF2)2(CF3)5C2F5, (C60)2(CF2)2(CF3)2O, and C60CH2N(CH2C60)CCOOtBu. Chemical structures of two of them, proposed on the basis of post source decay mass spectra, have been further confirmed by NMR spectra. Keywords: multicage fullerene cycloadduct, MALDI, trans2[3(4tertbutylphenyl)3methyl3prope nylidene]malononitrile, post source decay DOI: 10.1134/S1061934810140091

INTRODUCTION Among the great number of fullerene derivatives, a special place is occupied by the derivatives whose mol ecules consist of several fullerene cages. The increased interest in such compounds is primarily associated with the structural features of their nanosized mole cules, which, in addition to the purely scientific inter est, create some applied prospects for the construction of different nanosized formations, different parts of which have opposite physical or chemical properties. The main impulse initiating the development of the studies in this filed was provided by experiments in which such compounds were detected as admixtures. For example, the (C60)2 fullerene dimer was found in irradiated films of C60 [1] and the derivative C120O was present in some commercial samples of fullerene [2]. In the C120O molecule, the oxygen atom forms a bridge between the fullerene cages, while the other binding element is the C–C bond between the carbon atoms from different cages, which provides the forma tion of a 5membered cycle similar to furan. The C120CH2 compound has a similar structure with the bridging methylene group [2]. Only mass spectrometry provides the successful identification of such compounds. A large value of molecular mass (at least 1440) and a typical isotopic abundance, in which the second or the third monoiso tope peak is dominant, are signs that allow the assign ment of peaks in mass spectra to multicage com pounds.

The aim of the present study was the mass spectro metric identification of compounds whose molecules consist of several fullerene cages in the products of fullerene derivatization. Because of the thermal labil ity of these compounds [3], the socalled soft ioniza tion methods should be applied to them; in other words, the ionization methods which do not employ thermal evaporation. Nowadays, two such methods are widely used, electrospray ionization and matrix assisted laser desorption/ionization (MALDI). How ever, the application of the first method is limited by the solubility of the analytes in the suitable solvent; the solubility of multicage fullerenes is usually lower than that of singlecage ones [3]. Therefore, in our study MALDI was chosen for the identification of multicage derivatives. For examples, its application has shown that fluorofullerene identified as C60F16 on the basis of electronionization mass spectra appeared to be a two cage cycloadduct C60(C60F16) [4]; this structure was confirmed by Xray structural analysis [5]. It was also found that, as a result of heating under vacuum, C60/70(CF2)n undergoes oligomerization, leading to the formation of two and threecage derivatives [4]. These preliminary results have shown that, under MALDI conditions, multicage derivatives do not undergo complete fragmentation and, consequently, can be identified. For the present study, the following samples were selected: products of the trifluoromethy lation of samples of fullerene doped with metallic sodium, products of the reaction between fullerene

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and a mixture of trifluoromethylfullerenes, and also products of fulleroproline ester synthesis with the gross formula 1. H N

O OR

1

EXPERIMENTAL Sample Preparation Trifluoromethylation of fullerene, doped with sodium. Doping C60 fullerene (TermUSA, 99.98%) with sodium was carried out according to the well known procedure [6], using sodium hydride (Aldrich, USA, 95%) as the source of metallic sodium. The products were characterized by Xray powder diffrac tion and 13C and 23Na nuclear magnetic resonance spectroscopy (NMR). As a result, the composition of the doped fullerene samples was found to be NaxC60 where x = 1, 3, 10. These samples were treated with tri fluoroiodomethane (Apollo, UK, 98%) according to the known procedure earlier described for C60 and C70 fullerenes [7]. The reactions were carried out for 2– 30 days in sealed threesection ampoules made of molybdenum glass. The trifluoromethylated sample was placed in one of the end sections and liquefied CF3I, in the other end section. The section with the precipitate was in the hottest part of the oven (300– 420°C) and the section with liquefied CF3I was left out of the oven at room temperature (the pressure of CF3I saturated vapor was ~525 Pa). Therefore, the reaction ampoule had a natural temperature gradient. When the synthesis was completed, the products were col lected from the two sections of the ampoule: from the outer section with the trifluoromethylated sample (the socalled ‘hot zone product’) and from the central section where the sublimated products were con densed (‘cold zone product’). Reaction between C60 and C60(CF3)12/14/16/18. A mixture of trifluoromethylfullerenes containing deriv atives, including derivatives from the 12 to 18 CF3 groups in the molecule, was obtained according to the procedure described above [7]. Its molecular compo sition was determined from the MALDI mass spec trum (negative ions). The reaction of this mixture with C60 was conducted in a sealed preevacuated ampoule at 430–440°C for 2 days. Synthesis of fullerene esters. Samples of nonsub stituted fulleroproline ester (1, R = C2H5, tC4H9) were obtained by the reaction of the [2+3]cycloaddi

tion of azomethine ylides, generated in situ from gly cine ethyl or tertbutyl ester hydrochlorides (Fluka, Switzerland, ≥99%) and parapharmaldehyde (Acros, USA, 96%), with C60 fullerene catalyzed by lithium perchlorate (Acros, USA, ≥99%) [8]. The reactions were carried out in boiling toluene (chemically pure) with a reflux condenser for 2 days. Chromatography The products of the reaction between C60 and C60(CF3)12/14/16/18, as well as some products of the [2+3]cycloaddition of azomethine ylide, were sepa rated by highperformance liquid chromatography (HPLC) with the Cosmosil Buckyprep 10 × 250 mm column (Nacalai Tesque, Japan) using toluene (chemically pure) as the eluent (the flow rate was 2.0–4.6 mL/min). The products of [2+3] cycloaddition in some cases were also purified by flash chromatography using silica gel as the carrier (Aldrich, 7–230 mesh, 6 nm) and toluene or tolu ene/ethylacetate (chemically pure) as eluents. MALDI Mass Spectrometry The MALDI mass spectra were registered on a Bruker AutoFlex II reflectron timeofflight mass spectrometer (Germany) equipped with a 1ns pulse nitrogen laser λ = 337 nm. Its control systems also allowed registering postsource decay mass spectra for each ion [9]. As fullerenes and their derivatives are known to have strong electronaccepting properties (electron affinity for the C60 molecule is 2.667 ± 0.001 eV [10]), negative ion mass spectra were regis tered for all of the listed derivatives and we managed to obtained positive ion mass spectra only in some cases, which are considered below. The matrix was trans2[3(4tertbutylphenyl)2methyl2prope nyliden]malononitrile (DCTB, Fluka, ≥99%), which has an electron exchange ability and has been earlier successfully applied for the registration of mass spec tra of different fullerene derivatives [4]. Sample treat ment was carried out according to the ‘dried drop’ method [11]. For the treatment DCTB was used as its solution in toluene (chemically pure), while the analyte was dissolved in toluene (chemically pure), nhexane (chemically pure), tetrahydrofurane (Scharlau Chemie, HPLCgrade, without the addi tion of a stabilizer, ≥99.9%), or chloroform (chemi cally pure). The molar ratio matrix/analyte in the deposited samples was ≥1000/1. Nuclear Magnetic Resonance Spectroscopy Some chromatographic fractions obtained by the separation of the reaction products of C60 and C60(CF3)12/14/16/18 and the reaction of the [2+3] cycloaddition of azomethine ylide to fullerene were characterized by 19F and 1H NMR spectroscopy, respectively. The analysis employed a Bruker Avance

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F F2 C – [(C60)2(CF2)2](CF3)30

– [(C60)2(CF2)2](CF3)26

[(C60)2(CF2)2](CF3)25F–

n = 19

n = 11 n = 12 n = 13 n = 14

C60DCTB–

[(C60)2(CF2)2](CF3)29F–

[(C60)2(CF2)2](CF3)27F–

n = 18 n = 17

C60(CF3)–n

n = 15 n = 16

C– 60

– [(C60)2(CF2)2](CF3)28

MASS SPECTROMETRIC IDENTIFICATION

–CF3

3423

C F2 3560 3564 (CF3)27 –(F + 2CF3) –(F + 4CF3) –F 3560 3564

3300 3400 3500 3600 3700 3100 3150 3200 3250 3300 3350 3400 3450 m/z 3473

800 1200 1600 2000 2400 2800 3200 3600 4000 m/z

F2 C

Fig. 1. Negative ion MALDI mass spectrum of one of the ‘hot zone products’ formed upon the trifluoromethylation of Na10C60 at 300°C. The large inset shows its enlarged part in the mass range 3270–3780. The small insets show its enlarged part in the mass range 3558.5–3565.5 (above) and the theoretically calculated isotopic abundance for (C60)2(CF2)2(CF3)29F (below).

–CF3

C F2 (CF3)28

400 NMR spectrometer (Germany) with a frequency of 376.5 (19F) and 400.1 (1H) MHz. The solvents were CDCl3 (Aldrich, 99.8%) in the case of the products of the reaction between C60 and C60(CF3)12/14/16/18 and C6D5Br for the products of [2+3]cycloaddition. C6D5Br was obtained by the bromination of d6ben zene (Aldrich, 99.9%) with molecular bromine (Acros, ≥99%) in the presence of aluminum chloride followed by distillation under atmospheric pressure (boiling temperature 153°C).

3150 3200 3250 3300 3350 3400 3450 3500 m/z Fig. 2. Post source decay mass spectra of ions with the mass number of the main monoisotope peak 3423 (above) and 3473 (below) from negative ion MALDI mass spectra of the ‘hot zone products’ formed upon the trifluoromethyla tion of Na10C60 at 300°C. The insets schematically present possible models of the chemical structure of the corre sponding molecules.

RESULTS AND DISCUSSION Products of NaxC60 trifluoromethylation. The MALDI mass spectra (negative ions) for the products of the cold zone contained mainly peaks of – – C60(CF3 ) n , n = 11–18 and, in some cases, C 60 . The –

distribution of C60(CF3 ) n over n resembles a distribu tion typical for the products of the reaction between – C60 and CF3I [7]. Probably, the peak C 60 has a molec ular origin, because it was found earlier [4] that the – C60(CF3 ) n in this range of n do not undergo complete –

decomposition to produce C 60 under the conditions of MALDI. A complex peak with the C60DCTB– matrix, which was registered in some cases, also pro vides evidence for the molecular origin of this peak. –

In addition to the dominating peaks of C60(CF3 ) n with n = 11–18, the mass spectra of the ‘hot zone products’ obtained in the trifluoromethylation of JOURNAL OF ANALYTICAL CHEMISTRY

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–

Na10C60 (Fig. 1) also contained peaks of C 60 and C60DCTB– corresponding to unreacted fullerene and the peaks in the mass range corresponding to twocage ions. The observed isotopic abundance clearly indi cates their twocage origin (inset in Fig. 1). In this mass range, two series of peaks can be distinguished; they alternate in 69.00 Da, the value corresponding to the CF3 group. The distance between the main peaks in the two different series is 50.00 Da, the value corre sponding to the CF2 group. At the same time, the number of carbon and fluorine atoms in the ions in both series does not allow placing all the fluorine atoms in the CF3 groups. The problem of the chemical structure of the revealed twocage fullerene derivatives was clarified using post source decay mass spectra for the main peaks of both twocage series (Fig. 2). For example, the decay of the ion in one of the series occurred as a simultaneous abstraction of a CF3 group or an F atom. No. 14

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Table 1. Multicage cycloadducts of fullerene derivatives identified by MALDI mass spectrometry Reaction system and procedure

Main products

Side products

C60(CF2)n, n = 1–6, 250–320°C, 10–5–10–4 Pa

C60, C60(CF2)n, n = 1–6

(C60)2(CF2)m, m = 1–9, (C60)3(CF2)l, l = 1–9

Na10C60 + CF3I, 300°C

C60 and C60(CF3)2/12/14/16/18

(C60)2(CF2)2(CF3)25/27/29F and (C60)2(CF2)2(CF3)26/28/30

Na3C60 + CF3I, 400°C

(C60)2(CF2)4(CF3)17/19/21/23C2F5 C60(CF3)14/16/18 and (C60)2(CF2)4(CF3)2k, k = 9–13 and (C60)2(CF2)4(CF3)19/21/23/25F

NaC60 + CF3I, 400°C

(C60)2(CF2)4(CF3)16/18/20/22 and C60(CF3)10/12/14/16

(C60)2(CF2)4(CF3)2l – 1C2F5 and (C60)2(CF2)4(CF3)2l + 1F, l = 8–11, (C60)3(CF2)6(CF3)2k, (C60)3(CF2)6(CF3)2k + 1F, and (C60)3(CF2)6(CF3)2k – 1C2F5, k = 1–14

(C60)2(CF3)12/14/16/18 + C60, 410–440°C

C60 and C60(CF3)2/4/6/8

(C60)2(CF2)2, (C60)2(CF2)2(CF3)2k, (C60)2(CF2)2(CF3)2k – 1C2F5, (C60)3(CF2)4(CF3)2k, (C60)3(CF2)4(CF3)2k – 1C2F5, k = 1–5

C60 + NH2CH2COOEt + (CH2O)n, C60, C60CH2NHCHCOOEt Et3N, toluene, 110°C, cat. LiClO4 and C60CH2N(CH2C60)CCOOEt

C60CH2N(CH2C60CH2NHCHCOOEt)CCOOEt

C60 + NH2CH2COOtBu + (CH2O)n, C60, C60CH2NHCHCOOtBu C60CH2N(CH2[C60CH2N(CH2C60)CCOOtBu])CCOOtBu Et3N, toluene, 110°C, cat. LiClO4 and and C60CH2N(CH2[C60CH2NCHCOOtBu])CCOOtBu C60CH2N(CH2C60)CCOOtBu

Then the ion, which has lost the fluorine atom, abstracts two CF3 groups. Such behavior is typical for the ions bearing fluorine atoms and CF3 groups bound to the fullerene cage [12]. Since the abstraction of only one fluorine atom occurs, probably, only one fluorine atom is attached to the fullerene skeleton. The decay of ions in the other series occurs as the abstraction of one CF3 group; such a situation is typical for molecu lar ions of trifluoromethylfullerenes [4]. The abstrac tion of the CF2 group was not observed for any series. For this reason, it is reasonable to suppose that these groups are bridging and bing fullerene cages similar to the CH2 group [2]. Therefore, in the molecules of the studied compounds in both series, the fullerene cages were bound via two CF2 groups with the formation of a sixmembered cycle, while the cages bore CF3 groups and an F atom in one series or only CF3 groups in the other series, as is schematically illustrated in Fig. 2 (see inset). The change in the degree of doping with sodium allowed us to increase the relative concentration of compounds with two fullerene cages in the ‘hot zone products’ (Table). The increase in the temperature of trifluoromethylation resulted in a product whose mass spectrum (Fig. 3) exhibited dominant peaks corre sponding to derivatives with two fullerene cages; in addition, there were peaks with the m/z values and iso topic abundance referring to the derivatives with three fullerene cages. In the mass range corresponding to ions with two fullerene cages, several peak series can be distinguished, which alternate in the CF3 group mass, with one series being dominant. For its main peak, a postsource decay mass spectrum was registered (Fig. 3), which demonstrated only the successive abstraction of CF3 groups. No abstraction of CF2

groups and the F atom was registered. It means that only CF3 groups are bound to the fullerene cages. However, the numbers of carbon and fluorine atoms in the ion indicate that, in this case, the fullerene cages are bound via four CF2 groups with the very probable formation of two sixmembered cycles (Fig. 3). As for the peaks in the mass ranges corresponding to the ions with three fullerene cages, we can distin guish two basic series among them; their main peaks differ by the mass of the CF2 group. On the basis of the number of carbon and fluorine atoms and the special features of the addition of groups to fullerene cages we revealed for the twocage derivatives, it may assumed that, in this case, the fullerene cages are bound via six CF2 groups, probably with the formation of three six membered cycles. Two cages were bound into two cycles and the third cage was attached to one of them by one cycle, or each cage was bound with two other ones forming one cycle between every two cages, resembling a triangle, the corners of which are cage centers and the sides of which are cycles linking the cages. The CF3 groups (or the F atom in some series) play the role of addends attached to fullerene cages. Because of the cyclic nature of the binding of the fullerene cages, the revealed derivatives should be called multicage cycloadducts. Products of reaction between C60 and C60(CF3)12/14/16/18. Unlike the trifluoromethylation of NaxC60, for the products of the reaction of C60 with C60(CF3)12/14/16/18, MALDI mass spectra were regis tered not only for negative ions but for positive ones as well. In both cases, the m/z values and the isotopic abundances of the registered peaks allow us to assign them to ions with two fullerene cages (Fig. 4). How ever, in the spectra of the positive and negative ions,

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MASS SPECTROMETRIC IDENTIFICATION

1499

⎯

n = 22

n = 28

n = 24

n = 26

m = 18

[(C60)3(CF2)6](CF3 ) n – 1 [(C60)3(CF2)6](CF3)n–

F2C

n = 14

n = 12

4300

[(C60)2(CF2)4](CF3)m – 1F– [(C60)2(CF2)4](CF3)m–

–CF3

n = 16

m = 16

m = 22

–3CF3

n = 13

2883

2650

–2CF3

2700

2750 2800 m/z

2850

3600

4000

4400

2900

n=8

n = 10

(CF3)18

4400

n = 11

C60(CF3)n–

4200 m/z

4100

CF2 CF2

m = 20

4000

F2C

1600

2000

2400

2800 m/z

3200

Fig. 3. Negative ion MALDI mass spectra of some of the ‘hot zone products’ formed upon the trifluoromethylation of NaC60 at 400°C. The insets show its enlarged part in the mass range 3920–4450 (on the left), postsource decay mass spectra for the ion with the mass number of the main monoisotope peak 2883 (right bottom), and schematic drawing of possible models of the chem ical structure of the corresponding molecule.

the distributions of the peaks were different. This is associated with the earlier reported [4] discrimination effects caused by the differences in the ionization energy and electron affinity for molecules with differ ent numbers of addends and different numbers of fullerene cages. Probably, the addition of a fullerene cage reduces the ionization energy, since in the posi tive ion spectrum, the ratio of the total intensity of ions of singlecage derivatives is somewhat higher than that in the spectrum of negative ions. Moreover, some ions with three fullerene cages were registered in the spec trum of positive ions. In the mass range corresponding to ions with two fullerene cages, two main peak series alternating by the mass of two CF3 groups can be distinguished. The dis tance between the main peaks in this series corre sponds to the mass of the CF2 group. At the same time, the ratio of peaks in different series is different for the spectra of positive and negative ions. For example, in the first case, the peak series with a mass greater by 50.00 Da appears to be weaker and cannot strictly be considered as the main one. Postsource decay mass spectra for the main peaks in both series in the negative ion spectrum (Fig. 5) revealed the successive abstrac tion of the CF3 groups as well as the abstraction of the JOURNAL OF ANALYTICAL CHEMISTRY

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C2F5 group for the series that differed for the mass of the CF2 group. However, the abstraction of the latter was not registered in any case. Therefore, the decay mass spectra as well as the number of the atoms of car bon and fluorine allow assuming that the revealed two cage compounds have a structure (Fig .5) which is similar to that of the twocage cycloadducts obtained by trifluoromethylation of NaxC60 at 300°C. The cages are bound by CF2 groups with the formation of a six membered cycle. CF3 groups only in one series of CF3 groups and the C2F5 group in the other act as addends that are situated in the cages. As the revealed twocage adducts were formed only in the presence of non derivatizated C60 in the reaction system, it is reason able to assume that one of the fullerene cages origi nates from C60 and, therefore, the addends are present in one fullerene cage only. In its absence, the heating to the indicated temperatures did not lead to the for mation of these compounds [7]. On the basis of the number of carbon and fluorine atoms and special fea tures of addend attachment to multicage cycloadducts discussed above, a conclusion can be made on the structure of the positive ions of threecage cycload ducts which have been revealed in the spectrum. The cages successively bound through a sixmembered No. 14

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cycle and the addends on the cages are CF3 groups in one series and CF3 groups and C2F5 groups in the other series. One of the products of the reaction between C60 and C60(CF3)12/14/16/18 was separated into ten chromato graphic fractions. According to the mass spectra, four of them are rather pure twocage bicycload ducts: (C60)2(CF2)2(CF3)8, (C60)2(CF2)2(CF3)3C2F5, (C60)2(CF2)2(CF3)5C2F5, and (C60)2(CF2)2(CF3)2O. Neither positive, nor negative fragment ions with one fullerene cage were observed in the mass spectra of these four fractions. For the fraction of (C60)2(CF2)2(CF3)3C2F5, a 19F NMR spectrum was registered; the splitting of the signals in this spectrum indicates the presence of CF2 fragments with the mutual position of fluorine atoms close to the one observed in fluorinated cyclohexane derivatives 2 JFF ~ 240 Hz. This fact confirms the hypothesis about the binding of fullerene skeletons via a pair of CF2 fragments with the bound part of the molecule being a sixmembered ring. Products of the synthesis of fulleroproline esters. In addition to trifluoromethyl derivatives of fullerenes, multicage compounds were found in the products of the synthesis of fulleroproline esters by the reaction of

type (1). For example, in study [8], the presence of admixtures that should be recognized as twocage compounds was observed. We supposed that com pounds are formed there as the result of reaction (2). Azomethine ylide a generated in the reaction mixture adds to the C60 cage with the formation of a fullerene containing azomethine ylide b, which further adds to one more molecule of fullerene or a molecule of fullereneproline ester 1 with the formation of a two cage bicycloadduct 2 or a similar compound, one of the cages of which bears one more proline cycle. The optimization of the synthesis procedure and the chro matographic separation allowed us to obtain a pure sample of compound 2; it was confirmed by the MALDI mass spectrum with a dominating molecular peak. Postsource decay mass spectrum demonstrated that the decomposition of the molecular ion of this twocage bicycloadduct occurs mainly as the cleavage of one of the two fulleropyrrolidine cycles with the for – mation of C60CH2N(CH2)CCOOtBu– or C 60 ions [8]. Such behavior is rather typical for fulleropyrrolidines. Therefore, postsource decay confirmed the assump tion about the chemical structure of these twocage derivatives. O

H N

OR CH2O

+

–

n

+ Cl H3N

O OR

LiClO4 Et3N

H2C

Li O N+ –

C60

(1)

OR

Azomethine ylide R = Et, tBu

HO

2H2C=O + O

H2N

HO

N

Et3N –OH–

OtBu

O +

H2C

N

–

OtBu

a

OtBu

HO

H2C CH2

O

N+ –

N

O

(2)

OtBu

OtBu C60

HO

O

N C60

Et3N –OH–

b JOURNAL OF ANALYTICAL CHEMISTRY

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2 Vol. 65

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MASS SPECTROMETRIC IDENTIFICATION

n=8

2400

2800

3100

3200

1650 1700 1750 1800 1850 1900 1950 m/z

2000 m/z

C2F5

2005 F2 C

1954 1958 1954 1958

2400

2800

3200 C F2 – CF3

1600

m=8 m = 10

1200

m=6

n=8

800

– 2CF3

2900 m/z

– (2CF2 + F)

2000 m/z

2700

– 3CF3

m=8

m=4 m=6

2500

[(C60)2(CF2)2](CF3)m– [(C60)2(CF2)2](CF3)m – 1C2F5–

m=2 m=4

n=0 n=2

C60(CF3)– n

1600

(CF3)8

– 4CF3

1200 n=4

800

m=0 m=2

n=4 n=6

[(C60)2(CF2)2](CF3)m+

– CF3

C F2

n=6

n=2

C60(CF3)+ n

1955

F2 C

n=2

n=0

n=4

n=6

[(C60)3(CF2)4](CF3)+ n [(C60)3(CF2)4](CF3)n – 1C2F5+

1501

In the present study, we managed to register a 1H NMR spectrum for one of the pure samples of com pound 2. This spectrum consisted of one singlet and two duplets of multiplets with a ratio of integral inten sities of 9 : 2 : 2. The singlet should be assigned to methyl group protons of the tertbutyl group and the duplets of multiplets should be assigned to methylene protons, which are not equivalent because of their spa tial positions and the influence of the fullerene cages. Therefore, the 1H NMR spectrum is in complete accordance with the chemical structure earlier pro posed on the basis of postsource decay spectra. As a result of the optimization of the synthesis procedure, a sample of the ethyl ester of the twocage bicycloadduct C60CH2N(CH2C60)CCOOEt was obtained, which was similar to compound 2 (Table 1). For the molecular ion of this compound, the postsource decay mass spectrum was registered. This spectrum also revealed the cleavage of one of the two fulleropyrrolidine cycles with the formation of C60CH2N(CH2)CCOOEt– or –

C 60 ions. Therefore, the assumption made earlier in [8] about the formation of twocage cycloadducts in processes of type (2) was confirmed. JOURNAL OF ANALYTICAL CHEMISTRY

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– C2 F5

– 2CF3

– 3CF3

– (CF3+ C2 F5)

– 4CF3

– (2CF3+ C2 F5)

(CF3)8

Fig. 4. MALDI mass spectra of positive (top) and negative ions (bottom) of one of the products of the reaction between C60 with a mixture of C60(CF3)2k, k = 6–9. The inset into the upper mass spectrum shows its enlarged part in the mass range 2460–3200. The insets into the lower spectrum show its enlarged part in the mass range 1953.4– 1959.4 (left) and theoretically calculated isotopic abun dance for (C60)2(CF2)2(CF3)6 (right).

1700 1750 1800 1850 1900 1950 2000 m/z

Fig. 5. Postsource decay mass spectra for ions with the mass number of the main monoisotope peak 1955 (top) and 2005 (bottom) from MALDI mass spectra of the neg ative ions of one of the products of the reaction between C60 and the mixture of C60(CF3)2k, k = 6–9. The insets schematically present the possible models of the chemical structure of the corresponding molecules.

The further optimization of the synthesis proce dure allowed us to obtain a product also containing a compound with three fullerene cages in addition to the dominating twocage bicycloadduct (see the mass spectrum in Fig. 6). The evidence was provided by the peak with the corresponding m/z and the typical isoto pic abundance. CONCLUSIONS The application of MALDI mass spectrometry allowed us to detect and identify over 75 multicage cycloadducts listed in Table 1. As a result of the opti mization of synthetic procedures and chromato graphic fractionation, pure samples of five com pounds (C60)(CF2)2(CF3)8, (C60)2(CF2)2(CF3)3C2F5, (C60)2(CF2)2(CF3)5C2F5, (C60)2(CF2)2(CF3)2O, and 2 No. 14

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C60CH2N(CH2)CCOOtBu–

– C60

C60CH2N(CH2C60)CCOOtBu–

N

O OtBu

2470 2474

m/z

N

O OtBu

2470 2474

m/z C60CH2N[(CH2C60)CH2N(CH2C60)CCOOtBu]CCOOtBu– C60CH2N(CH2C60)CCOOtBuDCTB–

800

1200

1600 m/z

2000

2400

Fig. 6. Negative ion MALDI mass spectrum of one of the products of the reaction between C60 with glycine tertbutyl ester and paraformaldehyde. The insets show its enlarged part in the mass range 2469.5–2476.5 (left top), one of the possible models of the chemical structure of the molecule, corresponding to the peaks of this mass range, (right), and theoretically calculated isotopic abundance for it (left bottom).

were extracted. Two of them were characterized by NMR spectra, which confirmed the chemical struc ture proposed on the basis of post source decay mass spectra. Therefore, it may be concluded that, for mul ticage cycloadducts, postsource decay spectra provide rather precise identification of the addends bound to fullerene cages and the bridges between them. ACKNOWLEDGMENTS This study was partially supported by the Russian Foundation for Basic Research, project no. 0903 00353. REFERENCES 1. Wang, Y., Holden, J.M., Dong, Z.H., Bi, X.X., and Eklund, P.C., Chem. Phys. Lett., 1993, vol. 211, nos. 4– 5, p. 341.

2. Taylor, R., Barrow, M.P., and Drewello, T., Chem. Com mun., 1998, no. 22, p. 2497. 3. Segura, J.L. and Martín, N., Chem. Soc. Rev., 2000, vol. 29, no. 1, p. 13. 4. Markov, V.Yu., Pimenova, A.S., Petukhova, G.G., Kozlov, A.A., Ovchinnikova, N.S., Ioutsi, V.A., Dor ozhkin, E.I., Goryunkov, A.A., Tamm, N.B., Troya nov, S.I., and Sidorov, L.N., Massspektrometria, 2007, vol. 4, no. 2, p. 103. 5. Goryunkov, A.A., Ioffe, I.N., Khavrel, P.A., Avdosh enko, S.M., Markov, V.Yu., Mazej, Z., Sidorov, L.N., and Troyanov, S.I., Chem. Commun., 2007, no. 7, p. 704. 6. Rosseinsky, M.J., Murphy, D.W., Flemming, R.M., Tycko, R., Ramirez, A.P., Siegrist, T., Dabbagh, G., and Barret, S.E., Nature, 1992, vol. 356, no. 6368, p. 416. 7. Troyanov, S.I., Goryunkov, A.A., Dorozhkin, E.I., Ignat’eva, D.V., Tamm, N.B., Avdoshenko, S.M., Ioffe, I.N., Markov, V.Yu., Sidorov, L.N., Scheurel, K.,

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MASS SPECTROMETRIC IDENTIFICATION and Kemnitz, E., J. Fluorine Chem., 2007, vol. 128, no. 5, p. 545. 8. Markov, V.Yu., Ioutsi, V.A., Lanskikh, M.A., Apenova, M.G., Ovchinnikova, N.S., Ioffe, I.N., and Sidorov, L.N., Massspektrometria, 2009, vol. 6, no. 2, p. 121. 9. Spengler, B., J. Mass. Spectrom., 1997, vol. 32, no. 10, p. 1019.

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10. Brink, C., Andersen, L.H., Hvelplund, P., Mathur, D., and Voldstadt, J.D., Chem. Phys. Lett., 1995, vol. 233, nos. 1–2, p. 52. 11. Streletskii, A.V., Cand. Sci. (Chem.) Dissertation, Mos cow, 2005. 12. Troyanov, S.I., Goryunkov, A.A., Tamm, N.B., Markov, V.Yu., Ioffe, I.N., and Sidorov, L.N., Dalton Trans., 2008, no. 19, p. 2627.

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