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Preparation and structure of ferrocene derivative C60 adduct
Synthetic Metals 109 Ž2000. 239–244 www.elsevier.comrlocatersynmet
Preparation and structure of ferrocene derivative C 60 adduct M. Popławska a , P. Byszewski a
b,c,)
, E. Kowalska c , R. Diduszko c , J. Radomska
c
Faculty of Chemistry, Warsaw UniÕersity of Technology, ul.Noakowskiego 3, 00-664 Warsaw, Poland b Institute of Physics PAS, al.Lotnikow ´ 32 r 46, 02-668 Warsaw, Poland c Institute of Vacuum Technology, ul.Długa 44 r 50, 00-241 Warsaw, Poland Received 26 June 1999; received in revised form 15 July 1999; accepted 10 September 1999
Abstract Experiments on preparation of C 60 ONCFn cycloadduct ŽFn s ferrocene. are reported. The adduct was prepared in the reaction between C 60 and ferrocene oxime. The ferrocene derivative is bound to C 60 at the 6–6 bond by a heterocyclic oxygen–nitrogen–carbon ring. The reaction may lead either to the mono- or a diadduct. To determine structure of the molecules at various heat treatment stages, IR absorption and 1 H and 13 C NMR were measured. Thermal stability of the compound was analyzed by thermogravimetric and differential scanning calorimetry methods. The analyses showed that thermal treatment may be applied to remove organic groups to obtain sample containing FeC 60 , with C 60 arranged in the fcc lattice and iron dispersed between fullerenes. The experimental results are compared with calculations based on the semi-empirical quantum chemistry models ZINDO1 and PM3. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Fullerenes; Fullerene adduct; Infrared absorption; NMR
1. Introduction Since the discovery of an efficient method of fullerenes production, many experiments were devoted to preparation of various C 60 based compounds, though relatively few to those containing transition metals Že.g., Refs. w1–9x.. Our particular interest was to attach iron directly to the C 60 cage. The high melting temperature and low vapor pressure of iron makes it impossible to diffuse the metal into the fullerites lattice. Several chemical reactions between fullerenes and ferrocene ŽFn s Fecp 2 , cp s C 5 H 5 . used as iron source as well as some properties of such samples are described in Refs. w10–14x. Here, we report results of experiments on preparation and characterization of C 60 ONCFn adduct, where ferrocene is bound to C 60 at the 6–6 bond by heterocyclic oxygen–nitrogen–carbon ring. The experiments are compared with the semi-empirical quantum chemical calculation performed within the terms of ZINDO1 and PM3 models using commercially available molecular modeling package w15x.
) Corresponding author. Tel.: q48-22-831-17-40; fax: q48-22-831-2160; e-mail: [email protected]
The ZINDO1 model is based on the Intermediate Neglect of Differential Overlap approximation while the PM3 model on the Neglect of Diatomic Differential Overlap approximation. In the models, the Slater type orbitals are used and electron–electron interaction is accounted for by the Self Consistent Field method. The models were parameterized by Anderson et al. w16x and Stewart w17,18x, respectively, to reproduce structure and heat of formation of the organic molecules containing also transition metals. The computational chemistry helped us to determine structure of the complexes that could be produced during the reaction, structure and charge distribution in FeC 60 complexes; moreover, simulated IR spectrum facilitated interpretation of the IR measurements.
2. Synthesis of C 60 adduct The adduct was prepared by a chemical method in a three-step process using C 60 and ferrocenecarboxaldehyde ŽFnCHO. as the substrates. In the first step, the commercially available ferrocenecarboxaldehyde FnCHO was transformed to ferrocenecarboxaldehyde oxime ŽFnCHs NOH.. The FnCHO and hydrochloride hydroxylamine NH 2 OH:HCl were dissolved in ethanol. To the solution
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sodium hydroxide NaOH was successively added. It was maintained at boiling temperature for 2 h. Then the solution was cooled and water was added. Water caused partial precipitation of the reaction products, the looked for ferrocenecarboaldehyde oxime remained in the solution. The 1 H and 13 C NMR were used to confirm that FnCHs NOH was indeed formed in the reaction. In the second step, nitrile oxide FnC s NqOy was generated in situ reacting with N-chloriomide succinic acid ŽC 4 H 2 O 2 NCl. in chloride methylene ŽCH 2 Cl 2 . on FnCHs NOH. Finally, FnC s NqOy was reacted with C 60 . This cycloaddition reaction was carried out in the fullerene toluene solution. The reaction took place at reflux in the presence of triethylamine ŽC 2 H 5 . 3 N. The molar ratio of the main substrates C 60 rŽFnCHs NOH. was equal to 1r4. The product C 60 ONCFn was separated by a chromatography using silica gel column with toluene as an eluent. The very similar chemical process that was applied to prepare the monoadduct C 60 ONCFn may also lead to formation of a diadduct of the composition C 60 ŽONCFn. 2 .
3. Identification of the adduct We were unable to grow single crystals of the compound; thus, we could not unambiguously determine structure of the molecule by diffraction methods and had to rely on indirect information. One of the possible structures of the C 60 ONCFn molecule optimized using the semi-empirical PM3 model is shown in Fig. 1; most of the experiments support the proposed structure. 3.1. Nuclear magnetic resonance The 1 H NMR exhibits one sharp resonance line with the chemical shift d s 4.20 ppm and two lines at d s 4.53 and 5.20 ppm, the ratio of the intensity of the lines equals to 5:2:2. The number and intensities of the 1 H NMR lines well correspond to the model on Fig. 1 because all protons in the non-substituted cp ring are equivalent and in the singly substituted ring there are two types of proton. The 13 C NMR spectrum includes resonance lines at d s 68.1, 69.7, 69.8 ppm ascribed to cp; at d s 73.2, 75.4, 102.7 ppm ascribed to cp or to C in the isoxazoline ring and 28 lines in the range of d s 135.2–152.4 ppm ascribed to C 60 . 3.2. Visible light absorption There are only minor differences in the absorption spectra in the 400–600 nm region of the C 60 ONCFn cycloadduct dissolved in toluene in comparison to pure C 60 . The absorption is stronger at 496 nm, weaker at 540 and 600 nm, disappears the band at 408 nm and a new band at 430 nm appears. This band is characteristic for the closed w6x C 60 -adducts w19x. The changes are caused by the
Fig. 1. Structure of C 60 ONCFn optimized by PM3 method.
distortion of fullerene; lowering of its symmetry lifts the degeneracy of the electronic levels and activates orbitally forbidden electronic transitions. 3.3. Infrared absorption The IR absorption spectra were measured in the wavenumber range of 400–3000 cmy1 using FT-IR Perkin Elmer 1725X spectrometer. The pellets were prepared from the mixture of the sample and KBr powder. The IR absorption of the monoadduct sample is shown in the top panel in Fig. 2. The large number of the observed absorption lines does not allow for an unambiguous assignment of each of them. However, one can distinguish the four IR active C 60 modes, though their relative intensities differ from those observed in pure C 60 spectra Žthe tangential mode at 527 cmy1 is the most intense one.. There are absorption lines coinciding with those observed in pure C 60 ascribed in Ref. w20x to the modes activated by the symmetry lowering of the fullerene, here the C 60 symmetry is lowered by the adduct. Some of the IR absorption lines can be identified as originating from ferrocene; they are shifted to higher energies in comparison to pure ferrocene. The tentative assignment of some of the absorption lines to the vibrations of the two main components of the complex is listed in Table 1. The first three columns contain position of the observed absorption lines Žin cmy1 . ascribed to: Ž1. pure C 60 , Ž2. C 60 modes activated by the adduct and Ž3. adducted complex, respectively. The interpretation of the vibrational modes is given in the fourth column. The calculated IR spectra of the isolated C 60 , ferrocene derivative FnCHs NOH and C 60 ONCFn are shown in the three lower panels in Fig. 2. The bars in the bottom fragment of each panel show the energy of all vibrational modes, the bars extending above zero in each panel represent the IR active modes, and their length symbolizes the intensity. The calculations correctly predict that only four C 60 modes are IR active. The energy of the calculated modes agrees within the accuracy of 20% with the experimental value. The calculations also predict several strong
M. Popławska et al.r Synthetic Metals 109 (2000) 239–244
241
Fig. 2. IR spectrum of the monoadduct C 60 ONCFn annealed to 2008C and computed spectra of respective complexes.
lines at 500–600, at 800 and at 1600 cmy1 in general agreement with the observation. Visualization of the vibrations helps in some cases, to interpret the measured spectra. Several of the lines in the calculated spectrum in the range of 542–625 cmy1 arise almost exclusively from vibrations of the atoms constituting C 60 , they do not show in the spectrum of the isolated C 60 . We are inclined to ascribe the lines at 543 and 563 cmy1 to C 60 , in line with the analyzes presented in Ref. w20x. The calculated modes
that could be interpreted in such a way are listed in the last two columns of Table 1; they are arranged according to the interpretation suggested in the fourth column. The main difference between the infrared absorption of C 60 ONCFn and C 60 ŽONCFn. 2 adducts was observed in the wavenumber range corresponding to the C–H out of plane bending, the C–C stretching, the C–H in plane bending and the C–O vibrations. In the diadduct, only broad absorption bands were observed in those regions.
Table 1 List of characteristic IR modes position Žin cmy1 . observed and calculated in C 60 , C 60 ONCFn and FnCHs NOH C 60 activated
ONCFn
Interpretation
Calculated C 60
480 496 527
Fe–cp stretch cp tilt C 60 cage deformation
557
C 60 ONCFn 602–629 483–542 542–556
543, 563 576 750, 959 803–860 1002 1182
1107 1156, 1168, 1215 1300–1260 1409
1428 1500–1600 2849–3092
C 60 symmetric breathing C 60 asymmetric deformation C–H out of plane bend C–H in plane bend cp symmetric deformation C 60 pentagons asymmetric deformation C 60 pentagon and cp asymmetric deformation C–O stretch C–C stretch, C 60 pentagons deformation C 60 pentagon pinch CsN C–H stretch
733 500–750 800–900 1000–1200 1254–1364 1377 1378–1439 1267–1288 1693–1742 1708 1640, 1838 3162–3195
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M. Popławska et al.r Synthetic Metals 109 (2000) 239–244
We suppose that broadening of the lines resulted from binding the ferrocene cp rings directly to C 60 . The deformation of the cp rings and C 60 in such a case would lead to various Ževen within a single complex. C–H and C–C bonds strength determining the vibration frequency. The broadening of the absorption bands may also result from an overlap of neighboring absorption lines arising from various possible isomers of the diadduct characterized by different absorption.
4. Structural and thermal analyses The crystalline structure of the samples was analyzed by Siemens D500 diffractometer. The diffraction pattern was very poor Žsee Fig. 3., though sufficient to suggest an arrangement of molecules in the lattice. Using TREOR program, all of the reflections could be indexed assuming a monoclinic lattice with the unit cell parameters: a s 1.873 nm, b s 1.414 nm, c s 1.419 nm, b s 108.38. The unit cell volume V s 3.647 nm3 is sufficient to accommodate four C 60 ONCFn molecules if the ferrocene fragments were directed towards the interstitial sites. The crystalline structure did not change after annealing up to 2008C in spite of the observed mass loss, that suggested that the volatile compounds did not constitute the unit cell of the main component of the sample, but probably were located between the crystalline grains. The samples annealed to 450–5008C adopted the cubic close packed lattice with the lattice constant larger than in pure C 60 . There were no diffraction lines that could be ascribed to iron grains. Increase of the lattice constant may result from iron dissolved in the fullerene-based lattice rather than from any residual hydrocarbon C 60 adducts. Thermal investigations were performed by Differential Scanning Calorimetry ŽDSC. and Thermogravimetry ŽTG. methods using a Du Pont 1090 Thermal Analyzer. The experiments were carried out at 108Crmin heating rate
under helium atmosphere. The DSC scan ŽFig. 4a. shows three overlapping endothermic peaks in the temperature range 100–2008C, connected with 8% weight loss observed by TG method. We ascribe the effect appearing at approximately 1208C to evaporation of toluene and the one at 1708C to evaporation of free ferrocene derivative that were not detected by other experiments. A single step process observed by both methods at 3308C originates from the decomposition of the cycloadduct. The observed 15% weight loss compares favorably with the 18% mass loss that should be expected if C 5 H 5 and C 5 H 4 CNO of the C 60 ONCFn complex completely evaporated. One might expect an endothermic peak in the DSC scans in this temperature region because of the energy needed to break the C 60 ONCFn and cp–Fe–cp ferrocene bonds, while in the experiment the exothermic effect occurred. It indicates that the freed organic fragments may dimerize or polymerize and the heat released in the process prevails over the endothermic process. We suppose that chemically inactive polymers evaporate from the sample leaving most of iron and C 60 . According to the X-ray fluorescence the fresh monoadduct samples contained less iron than expected, corresponding to 5% of pure, nonadducted C 60 . In the annealed samples, there remained iron at the concentration of 1Fer71C, in agreement with the TG analyses. There were neither 1 H NMR nor IR absorption lines in samples heated to 3508C, that could be ascribed to residual hydrocarbon groups. It is of importance to add that one should not expect much difference in the IR absorption between C 60 and FeC 60 , the IR absorption calculated using ZINDO1 and PM3 models is almost identical in the measured wavenumber range in both cases. The DSC and TG measurements of the diadduct sample Žshown in Fig. 4b. revealed a multi-step decomposition process in the investigated temperature range. The observed total weight loss of 30% is consistent with the diadduct formula C 60 ŽONCFn. 2 . The process in the temperature range of 200–3508C is similar to the one observed in the monoadduct sample. However, the 14% weight loss occurring at relatively high temperature of 4708C indicates that fragments of the adduct are strongly bound to fullerenes. The IR absorption proved that hydrocarbon groups remained in the diadduct sample after annealing to 3508C. The chemical calculations showed that the cp ring might be also directly bound to C 60 , the bonding in that case is much stronger than in the molecule shown in Fig. 1.
5. Modeling of C 60 Fe complexes
Fig. 3. The comparison of the X-ray powder diffraction pattern from the as-prepared sample and annealed to various temperatures.
Both adducts after high temperature annealing, exhibited ferromagnetic properties at liquid helium temperature and paramagnetic behavior above 100 K. In order to find
M. Popławska et al.r Synthetic Metals 109 (2000) 239–244
243
Fig. 4. TG and DSC plots of the monoadduct C 60 ONCFn Žleft. and diadduct ŽC 60 ONCFn. 2 Žright..
the origin of magnetic moments in the solid consisting of complexes containing even number of electrons, we tried to optimize FeC 60 complexes applying the open-shell unrestricted Hartree–Fock approximation. Unfortunately, the optimization procedure within the terms of the PM3 model converged only in a few cases. Then the structure of the complexes and orbital populations were close to those found using the ZINDO1 model. Therefore, in the first approximation, the latter method was used to determine properties of FeC 60 complexes of various structures and spin states. The results are summarized in Table 2. The calculations were performed for a few hypothetical exohedral complexes with iron at various sites relatively to fullerene: Fe opposite a hexagon, a pentagon or between two fullerenes, they are denoted by Žh. or Žp., respectively. The difference between number of spin up and spin down electrons Ž n ≠ –n x. populating iron s, p and d atomic orbitals, describes spin distribution over the complex. It is
interesting to note that transformation from a singlet to a quintet state involves Žas expected. transfer of electrons from Fe 4s to 3d orbitals in FeC 60 Žh. or Žp. complexes, though not in the C 60 FeC 60 ones. The calculations point to higher stability of the C 60 FeC 60 complexes in the high spin state than in the low spin state, the material may therefore exhibit magnetic properties depending on an interaction between the complexes. The comparison of the respective heats of formation indicates that at elevated temperatures iron may easily migrate between equivalent sites on C 60 or even diffuse between fullerenes, once it is introduced into the fullerite lattice, probably until iron clusters are formed. The FeC 60 complexes may easily be arranged in the fcc lattice if iron was directed towards the interstitial sites even those in the Ž111. planes. Thus, despite the presence of iron diluted in the fullerites, it may not distort the lattice and may not be detected by the X-ray diffraction.
Table 2 Properties of FeC 60 complexes calculated by ZINDO1 method; heat of formation E relative to isolated C 60 and Fe in kcalrmol and q in electron charge Spin
E
Fe q
FeC 60 Žh. FeC 60 Žp. FeC 60 Fe Žh. FeC 60 Fe Žp. C 60 FeC 60 Žh. C 60 FeC 60 Žp.
Ss0 Ss2 Ss0 Ss2 Ss0 Ss0 Ss0 Ss2 Ss0 Ss2
y219 y193 y190 y154 y435 y336 y309 y382 y266 y347
0.81 0.61 0.74 0.63 0.79 0.72 0.97 1.00 0.93 0.91
Fe atomic orbital population n ≠ –n x 4.18 4.04
3.96 3.95
s
p
d
1.717 0.906 1.745 0.917 1.719 1.747 1.784 1.954 1.927 1.910
1.069 0.982 0.965 0.873 1.045 0.946 1.122 0.981 0.934 0.926
6.027 6.722 6.031 6.841 6.029 6.03 6.060 6.069 6.072 6.078
244
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References w1x W.I.F. Dawid, R.M. Ibberson, J.C. Matthewman, K. Prasides, T.J.S. Dennis, J.P. Hare, H.W. Kroto, R. Taylor, D.R.M. Walton, Nature 353 Ž1991. 156. w2x K. Hedberg, L. Hedberg, D.S. Bethune, C.A. Brown, H.C. Dorn, R.D. Jonhson, M. de Vries, Science 254 Ž1991. 410. w3x A.L. Balch, V.J. Catalano, D.A. Costa, W.R. Fawcett, M. Federco, A.S. Ginwalla, J.W. Lee, M.M. Olmstead, B.C. Noll, K. Winkler, J. Phys. Chem. Solids 58 Ž1997. 1633. w4x F. Banim, Ch.J. Cardin, D.J. Cardin, M. Pistocchi, A. Todd, J. Phys. Chem. Solids 58 Ž1997. 1991. w5x J.M. Hawkins, A. Meyer, T.A. Lewis, S. Loren, F.J. Hollander, Science 252 Ž1991. 312. w6x L.M. Roth, Y. Huang, J.T. Schwedler, C.J. Cassady, D. Ben-Amotz, B. Khar, B.S. Freiser, J. Am. Chem. Soc. 1991 Ž113. Ž1991. 8186. w7x T. Pradeep, G.U. Kulkarni, K.R. Kannan, N.Y. Vasanthacharya, T.N. Guru Row, C.N.R. Rao, Indian J. Chem. 31 Ž1992. F17, A and B. w8x M.W. Ruckman, B. Xia, D. Shih, Phys. Rev. B 50 Ž23. Ž1994. 17682. w9x M. Biermann, B. Kessler, S. Krummacher, W. Eberhardt, Solid State Commun. 95 Ž1995. 1.
w10x P. Byszewski, R. Diduszko, E. Kowalska, Recent advances in the chemistry and physics of fullerenes and related materials, in: K. Kadish, R. Ruoff ŽEds.., The Electrochemical Society, Vol. 24, 1994, p. 1392. w11x P. Byszewski, R. Diduszko, M. Baran, Acta Phys. Pol., A 2 Ž85. Ž1994. 297. w12x P. Byszewski, Z. Kucharski, Fullerene Sci. Technol. 5 Ž1997. 1261. w13x E. Kowalska, P. Byszewski, P. Dłuzewski, R. Diduszko, Z. Kucharski, J. Alloys Compd. 286 Ž1999. 297. w14x P. Byszewski, E. Kowalska, J. Radomska, Z. Kucharski, R. Diduszko, A. Huczko, H. Lange, R. Kochkanjan, A. Zaritowskij, A. Bondarenko, V. Chabanenko, XII International Winterschool, Kirchberg, Tyrol, Austria 1998, March, Electronic Properties of Novel Materials, in: H. Kuzmany et al. ŽEds.., Proc. of XII International Winterschool, The American Institute of Physics, 1998, p. 348. w15x HyperChem 5.1, license No. 500-10009206. w16x T. Anderson, J. Edwards, M. Zerner, Inorg. Chem. 25 Ž1986. 2728. w17x J.J.P. Stewart, J. Comput. Chem. 10 Ž1989. 209. w18x J.J.P. Stewart, J. Comput. Chem. 10 Ž1989. 221. w19x A. Komori, M. Kubota, T. Ishida, H. Niwa, T. Nogami, Tetrahedron Lett. 37 Ž23. Ž1996. 4031. w20x M.C. Martin, X. Du, J. Kwon, L. Mihaly, Phys. Rev. B 50 Ž1994. 173.
Preparation and structure of ferrocene derivative C 60 adduct M. Popławska a , P. Byszewski a
b,c,)
, E. Kowalska c , R. Diduszko c , J. Radomska
c
Faculty of Chemistry, Warsaw UniÕersity of Technology, ul.Noakowskiego 3, 00-664 Warsaw, Poland b Institute of Physics PAS, al.Lotnikow ´ 32 r 46, 02-668 Warsaw, Poland c Institute of Vacuum Technology, ul.Długa 44 r 50, 00-241 Warsaw, Poland Received 26 June 1999; received in revised form 15 July 1999; accepted 10 September 1999
Abstract Experiments on preparation of C 60 ONCFn cycloadduct ŽFn s ferrocene. are reported. The adduct was prepared in the reaction between C 60 and ferrocene oxime. The ferrocene derivative is bound to C 60 at the 6–6 bond by a heterocyclic oxygen–nitrogen–carbon ring. The reaction may lead either to the mono- or a diadduct. To determine structure of the molecules at various heat treatment stages, IR absorption and 1 H and 13 C NMR were measured. Thermal stability of the compound was analyzed by thermogravimetric and differential scanning calorimetry methods. The analyses showed that thermal treatment may be applied to remove organic groups to obtain sample containing FeC 60 , with C 60 arranged in the fcc lattice and iron dispersed between fullerenes. The experimental results are compared with calculations based on the semi-empirical quantum chemistry models ZINDO1 and PM3. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Fullerenes; Fullerene adduct; Infrared absorption; NMR
1. Introduction Since the discovery of an efficient method of fullerenes production, many experiments were devoted to preparation of various C 60 based compounds, though relatively few to those containing transition metals Že.g., Refs. w1–9x.. Our particular interest was to attach iron directly to the C 60 cage. The high melting temperature and low vapor pressure of iron makes it impossible to diffuse the metal into the fullerites lattice. Several chemical reactions between fullerenes and ferrocene ŽFn s Fecp 2 , cp s C 5 H 5 . used as iron source as well as some properties of such samples are described in Refs. w10–14x. Here, we report results of experiments on preparation and characterization of C 60 ONCFn adduct, where ferrocene is bound to C 60 at the 6–6 bond by heterocyclic oxygen–nitrogen–carbon ring. The experiments are compared with the semi-empirical quantum chemical calculation performed within the terms of ZINDO1 and PM3 models using commercially available molecular modeling package w15x.
) Corresponding author. Tel.: q48-22-831-17-40; fax: q48-22-831-2160; e-mail: [email protected]
The ZINDO1 model is based on the Intermediate Neglect of Differential Overlap approximation while the PM3 model on the Neglect of Diatomic Differential Overlap approximation. In the models, the Slater type orbitals are used and electron–electron interaction is accounted for by the Self Consistent Field method. The models were parameterized by Anderson et al. w16x and Stewart w17,18x, respectively, to reproduce structure and heat of formation of the organic molecules containing also transition metals. The computational chemistry helped us to determine structure of the complexes that could be produced during the reaction, structure and charge distribution in FeC 60 complexes; moreover, simulated IR spectrum facilitated interpretation of the IR measurements.
2. Synthesis of C 60 adduct The adduct was prepared by a chemical method in a three-step process using C 60 and ferrocenecarboxaldehyde ŽFnCHO. as the substrates. In the first step, the commercially available ferrocenecarboxaldehyde FnCHO was transformed to ferrocenecarboxaldehyde oxime ŽFnCHs NOH.. The FnCHO and hydrochloride hydroxylamine NH 2 OH:HCl were dissolved in ethanol. To the solution
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sodium hydroxide NaOH was successively added. It was maintained at boiling temperature for 2 h. Then the solution was cooled and water was added. Water caused partial precipitation of the reaction products, the looked for ferrocenecarboaldehyde oxime remained in the solution. The 1 H and 13 C NMR were used to confirm that FnCHs NOH was indeed formed in the reaction. In the second step, nitrile oxide FnC s NqOy was generated in situ reacting with N-chloriomide succinic acid ŽC 4 H 2 O 2 NCl. in chloride methylene ŽCH 2 Cl 2 . on FnCHs NOH. Finally, FnC s NqOy was reacted with C 60 . This cycloaddition reaction was carried out in the fullerene toluene solution. The reaction took place at reflux in the presence of triethylamine ŽC 2 H 5 . 3 N. The molar ratio of the main substrates C 60 rŽFnCHs NOH. was equal to 1r4. The product C 60 ONCFn was separated by a chromatography using silica gel column with toluene as an eluent. The very similar chemical process that was applied to prepare the monoadduct C 60 ONCFn may also lead to formation of a diadduct of the composition C 60 ŽONCFn. 2 .
3. Identification of the adduct We were unable to grow single crystals of the compound; thus, we could not unambiguously determine structure of the molecule by diffraction methods and had to rely on indirect information. One of the possible structures of the C 60 ONCFn molecule optimized using the semi-empirical PM3 model is shown in Fig. 1; most of the experiments support the proposed structure. 3.1. Nuclear magnetic resonance The 1 H NMR exhibits one sharp resonance line with the chemical shift d s 4.20 ppm and two lines at d s 4.53 and 5.20 ppm, the ratio of the intensity of the lines equals to 5:2:2. The number and intensities of the 1 H NMR lines well correspond to the model on Fig. 1 because all protons in the non-substituted cp ring are equivalent and in the singly substituted ring there are two types of proton. The 13 C NMR spectrum includes resonance lines at d s 68.1, 69.7, 69.8 ppm ascribed to cp; at d s 73.2, 75.4, 102.7 ppm ascribed to cp or to C in the isoxazoline ring and 28 lines in the range of d s 135.2–152.4 ppm ascribed to C 60 . 3.2. Visible light absorption There are only minor differences in the absorption spectra in the 400–600 nm region of the C 60 ONCFn cycloadduct dissolved in toluene in comparison to pure C 60 . The absorption is stronger at 496 nm, weaker at 540 and 600 nm, disappears the band at 408 nm and a new band at 430 nm appears. This band is characteristic for the closed w6x C 60 -adducts w19x. The changes are caused by the
Fig. 1. Structure of C 60 ONCFn optimized by PM3 method.
distortion of fullerene; lowering of its symmetry lifts the degeneracy of the electronic levels and activates orbitally forbidden electronic transitions. 3.3. Infrared absorption The IR absorption spectra were measured in the wavenumber range of 400–3000 cmy1 using FT-IR Perkin Elmer 1725X spectrometer. The pellets were prepared from the mixture of the sample and KBr powder. The IR absorption of the monoadduct sample is shown in the top panel in Fig. 2. The large number of the observed absorption lines does not allow for an unambiguous assignment of each of them. However, one can distinguish the four IR active C 60 modes, though their relative intensities differ from those observed in pure C 60 spectra Žthe tangential mode at 527 cmy1 is the most intense one.. There are absorption lines coinciding with those observed in pure C 60 ascribed in Ref. w20x to the modes activated by the symmetry lowering of the fullerene, here the C 60 symmetry is lowered by the adduct. Some of the IR absorption lines can be identified as originating from ferrocene; they are shifted to higher energies in comparison to pure ferrocene. The tentative assignment of some of the absorption lines to the vibrations of the two main components of the complex is listed in Table 1. The first three columns contain position of the observed absorption lines Žin cmy1 . ascribed to: Ž1. pure C 60 , Ž2. C 60 modes activated by the adduct and Ž3. adducted complex, respectively. The interpretation of the vibrational modes is given in the fourth column. The calculated IR spectra of the isolated C 60 , ferrocene derivative FnCHs NOH and C 60 ONCFn are shown in the three lower panels in Fig. 2. The bars in the bottom fragment of each panel show the energy of all vibrational modes, the bars extending above zero in each panel represent the IR active modes, and their length symbolizes the intensity. The calculations correctly predict that only four C 60 modes are IR active. The energy of the calculated modes agrees within the accuracy of 20% with the experimental value. The calculations also predict several strong
M. Popławska et al.r Synthetic Metals 109 (2000) 239–244
241
Fig. 2. IR spectrum of the monoadduct C 60 ONCFn annealed to 2008C and computed spectra of respective complexes.
lines at 500–600, at 800 and at 1600 cmy1 in general agreement with the observation. Visualization of the vibrations helps in some cases, to interpret the measured spectra. Several of the lines in the calculated spectrum in the range of 542–625 cmy1 arise almost exclusively from vibrations of the atoms constituting C 60 , they do not show in the spectrum of the isolated C 60 . We are inclined to ascribe the lines at 543 and 563 cmy1 to C 60 , in line with the analyzes presented in Ref. w20x. The calculated modes
that could be interpreted in such a way are listed in the last two columns of Table 1; they are arranged according to the interpretation suggested in the fourth column. The main difference between the infrared absorption of C 60 ONCFn and C 60 ŽONCFn. 2 adducts was observed in the wavenumber range corresponding to the C–H out of plane bending, the C–C stretching, the C–H in plane bending and the C–O vibrations. In the diadduct, only broad absorption bands were observed in those regions.
Table 1 List of characteristic IR modes position Žin cmy1 . observed and calculated in C 60 , C 60 ONCFn and FnCHs NOH C 60 activated
ONCFn
Interpretation
Calculated C 60
480 496 527
Fe–cp stretch cp tilt C 60 cage deformation
557
C 60 ONCFn 602–629 483–542 542–556
543, 563 576 750, 959 803–860 1002 1182
1107 1156, 1168, 1215 1300–1260 1409
1428 1500–1600 2849–3092
C 60 symmetric breathing C 60 asymmetric deformation C–H out of plane bend C–H in plane bend cp symmetric deformation C 60 pentagons asymmetric deformation C 60 pentagon and cp asymmetric deformation C–O stretch C–C stretch, C 60 pentagons deformation C 60 pentagon pinch CsN C–H stretch
733 500–750 800–900 1000–1200 1254–1364 1377 1378–1439 1267–1288 1693–1742 1708 1640, 1838 3162–3195
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We suppose that broadening of the lines resulted from binding the ferrocene cp rings directly to C 60 . The deformation of the cp rings and C 60 in such a case would lead to various Ževen within a single complex. C–H and C–C bonds strength determining the vibration frequency. The broadening of the absorption bands may also result from an overlap of neighboring absorption lines arising from various possible isomers of the diadduct characterized by different absorption.
4. Structural and thermal analyses The crystalline structure of the samples was analyzed by Siemens D500 diffractometer. The diffraction pattern was very poor Žsee Fig. 3., though sufficient to suggest an arrangement of molecules in the lattice. Using TREOR program, all of the reflections could be indexed assuming a monoclinic lattice with the unit cell parameters: a s 1.873 nm, b s 1.414 nm, c s 1.419 nm, b s 108.38. The unit cell volume V s 3.647 nm3 is sufficient to accommodate four C 60 ONCFn molecules if the ferrocene fragments were directed towards the interstitial sites. The crystalline structure did not change after annealing up to 2008C in spite of the observed mass loss, that suggested that the volatile compounds did not constitute the unit cell of the main component of the sample, but probably were located between the crystalline grains. The samples annealed to 450–5008C adopted the cubic close packed lattice with the lattice constant larger than in pure C 60 . There were no diffraction lines that could be ascribed to iron grains. Increase of the lattice constant may result from iron dissolved in the fullerene-based lattice rather than from any residual hydrocarbon C 60 adducts. Thermal investigations were performed by Differential Scanning Calorimetry ŽDSC. and Thermogravimetry ŽTG. methods using a Du Pont 1090 Thermal Analyzer. The experiments were carried out at 108Crmin heating rate
under helium atmosphere. The DSC scan ŽFig. 4a. shows three overlapping endothermic peaks in the temperature range 100–2008C, connected with 8% weight loss observed by TG method. We ascribe the effect appearing at approximately 1208C to evaporation of toluene and the one at 1708C to evaporation of free ferrocene derivative that were not detected by other experiments. A single step process observed by both methods at 3308C originates from the decomposition of the cycloadduct. The observed 15% weight loss compares favorably with the 18% mass loss that should be expected if C 5 H 5 and C 5 H 4 CNO of the C 60 ONCFn complex completely evaporated. One might expect an endothermic peak in the DSC scans in this temperature region because of the energy needed to break the C 60 ONCFn and cp–Fe–cp ferrocene bonds, while in the experiment the exothermic effect occurred. It indicates that the freed organic fragments may dimerize or polymerize and the heat released in the process prevails over the endothermic process. We suppose that chemically inactive polymers evaporate from the sample leaving most of iron and C 60 . According to the X-ray fluorescence the fresh monoadduct samples contained less iron than expected, corresponding to 5% of pure, nonadducted C 60 . In the annealed samples, there remained iron at the concentration of 1Fer71C, in agreement with the TG analyses. There were neither 1 H NMR nor IR absorption lines in samples heated to 3508C, that could be ascribed to residual hydrocarbon groups. It is of importance to add that one should not expect much difference in the IR absorption between C 60 and FeC 60 , the IR absorption calculated using ZINDO1 and PM3 models is almost identical in the measured wavenumber range in both cases. The DSC and TG measurements of the diadduct sample Žshown in Fig. 4b. revealed a multi-step decomposition process in the investigated temperature range. The observed total weight loss of 30% is consistent with the diadduct formula C 60 ŽONCFn. 2 . The process in the temperature range of 200–3508C is similar to the one observed in the monoadduct sample. However, the 14% weight loss occurring at relatively high temperature of 4708C indicates that fragments of the adduct are strongly bound to fullerenes. The IR absorption proved that hydrocarbon groups remained in the diadduct sample after annealing to 3508C. The chemical calculations showed that the cp ring might be also directly bound to C 60 , the bonding in that case is much stronger than in the molecule shown in Fig. 1.
5. Modeling of C 60 Fe complexes
Fig. 3. The comparison of the X-ray powder diffraction pattern from the as-prepared sample and annealed to various temperatures.
Both adducts after high temperature annealing, exhibited ferromagnetic properties at liquid helium temperature and paramagnetic behavior above 100 K. In order to find
M. Popławska et al.r Synthetic Metals 109 (2000) 239–244
243
Fig. 4. TG and DSC plots of the monoadduct C 60 ONCFn Žleft. and diadduct ŽC 60 ONCFn. 2 Žright..
the origin of magnetic moments in the solid consisting of complexes containing even number of electrons, we tried to optimize FeC 60 complexes applying the open-shell unrestricted Hartree–Fock approximation. Unfortunately, the optimization procedure within the terms of the PM3 model converged only in a few cases. Then the structure of the complexes and orbital populations were close to those found using the ZINDO1 model. Therefore, in the first approximation, the latter method was used to determine properties of FeC 60 complexes of various structures and spin states. The results are summarized in Table 2. The calculations were performed for a few hypothetical exohedral complexes with iron at various sites relatively to fullerene: Fe opposite a hexagon, a pentagon or between two fullerenes, they are denoted by Žh. or Žp., respectively. The difference between number of spin up and spin down electrons Ž n ≠ –n x. populating iron s, p and d atomic orbitals, describes spin distribution over the complex. It is
interesting to note that transformation from a singlet to a quintet state involves Žas expected. transfer of electrons from Fe 4s to 3d orbitals in FeC 60 Žh. or Žp. complexes, though not in the C 60 FeC 60 ones. The calculations point to higher stability of the C 60 FeC 60 complexes in the high spin state than in the low spin state, the material may therefore exhibit magnetic properties depending on an interaction between the complexes. The comparison of the respective heats of formation indicates that at elevated temperatures iron may easily migrate between equivalent sites on C 60 or even diffuse between fullerenes, once it is introduced into the fullerite lattice, probably until iron clusters are formed. The FeC 60 complexes may easily be arranged in the fcc lattice if iron was directed towards the interstitial sites even those in the Ž111. planes. Thus, despite the presence of iron diluted in the fullerites, it may not distort the lattice and may not be detected by the X-ray diffraction.
Table 2 Properties of FeC 60 complexes calculated by ZINDO1 method; heat of formation E relative to isolated C 60 and Fe in kcalrmol and q in electron charge Spin
E
Fe q
FeC 60 Žh. FeC 60 Žp. FeC 60 Fe Žh. FeC 60 Fe Žp. C 60 FeC 60 Žh. C 60 FeC 60 Žp.
Ss0 Ss2 Ss0 Ss2 Ss0 Ss0 Ss0 Ss2 Ss0 Ss2
y219 y193 y190 y154 y435 y336 y309 y382 y266 y347
0.81 0.61 0.74 0.63 0.79 0.72 0.97 1.00 0.93 0.91
Fe atomic orbital population n ≠ –n x 4.18 4.04
3.96 3.95
s
p
d
1.717 0.906 1.745 0.917 1.719 1.747 1.784 1.954 1.927 1.910
1.069 0.982 0.965 0.873 1.045 0.946 1.122 0.981 0.934 0.926
6.027 6.722 6.031 6.841 6.029 6.03 6.060 6.069 6.072 6.078
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