Organometallics 2010, 29, 6141–6144 DOI: 10.1021/om100516s
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New Type of Reaction of Lanthanide Hydrido Complexes with C60: Oxidative Dehydrogenation vs Addition. The Reversible σ-C60-C60 Bond Formation in Complex [{(Me3Si)2NC(NCy)2}2Lu(Et2O)þ](C60-•) Elena A. Schupak, Dmitrii M. Lyubov, Evguenii V. Baranov, Georgii K. Fukin, Olga N. Suvorova, and Alexander A. Trifonov* G. A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina Street 49, 603950, Nizhny Novgorod, Russian Federation Received May 26, 2010 Summary: A novel type of reaction of lanthanide hydrido complexes with C60 is described. The hydride anion of complex [{(Me3Si)2NC(NCy)2}2Lu(μ-H)]2 reduces C60 (1:2 molar ratio, toluene, 20 °C) to the radical anion, and the reaction results in the formation of ionic complex [{(Me3Si)2NC(NCy)2}2Lu(Et2O)þ](C60-•). At low temperature (below ∼120 K) reversible dimerization of radical anions C60-• occurs and affords the complex [{(Me3Si)2NC(NCy)2}2Lu(Et2O)þ]2[(C60-)2] 3 2(C7H8) 3 (Et2O) with the diamagnetic σ-bonded (C60-)2 dianion. Organolanthanide hydrides demonstrated enhanced and unique reactivity, and the permanent attention to this class of compounds had a considerable impact on the development of lanthanide chemistry. Addition of organolanthanide hydrides to multiple C-C bonds, a key elementary step in a wide variety of stoichiometric and catalytic transformations of unsaturated hydrocarbons, has been extensively studied.1 Several years ago we reported on the synthesis and structure of a new family of low-coordinated bis(guanidinate) hydrido complexes of lanthanides; some of them turned out to be efficient catalysts of olefin polymerization and hydrosilylation, while their stoichiometric reactions still remain unexplored.2 Fullerene C60 exhibits the chemical reactivity typical of an electron-poor olefin3 and seems to be an interesting object for investigation of the addition reaction of hydrides [Ln{(Me3Si)2NC(NR)2}2(μ-H)]2 to the conjugated system of double CdC bonds. Since one of the most characteristic features of fullerene is the ability to be reduced to an anion,4 the previous investigation of its reactivity toward lanthanide compounds has been mainly focused on the reactions with Ln(II) derivatives.5 Herein we report that the reaction of the hydrido complex [{(Me3Si)2NC(NCy)2}2Lu(μ-H)]2 with C60 resulted in the oxidation of the hydrido anion by fullerene *To whom correspondence should be addressed. Fax: (þ7)831 4633532. E-mail:
[email protected]. (1) (a) Ephritikhine, M. Chem. Rev. 1997, 97, 2193–2242. (b) Okuda, J. Dalton Trans. 2003, 2367–2378. (c) Konkol, M.; Okuda, J. Coord. Chem. Rev. 2008, 252, 1577–1591. (2) (a) Trifonov, A. A.; Skvortsov, G. G.; Lyubov, D. M.; Skorodumova, N. A.; Fukin, G. K.; Baranov, E. V.; Glushakova, V. N. Chem.—Eur. J. 2006, 12, 5320–5327. (b) Trifonov, A. A.; Fedorova, E. A.; Fukin, G. K.; Bochkarev, M. N. Eur. J. Inorg. Chem. 2004, 4396–4401. (c) Lyubov, D. M.; Bubnov, A. M.; Fukin, G. K.; Dolgushin, F. M.; Antipin, M. Yu.; Pelce, O.; Schappacher, M.; Guillaume, S. M.; Trifonov, A. A. Eur. J. Inorg. Chem. 2008, 2090–2098. (3) Hirsch, A. The Chemistry of Fullerenes; Thieme: Stuttgart, 1994. (4) Reed, C. A.; Bolskar, R. D. Chem. Rev. 2000, 100, 1075–1120. (5) (a) Bochkarev, M. N.; Fedushkin, I. L.; Nevodchikov, V. I.; Protchenko, A. V.; Schumann, H.; Girgsdies, F. Inorg. Chim. Acta 1998, 20, 138–142. (b) Zhang, J.; Cai, R.; Chen, M.; Weng, L.; Zhou, X. Eur. J. Inorg. Chem. 2005, 3298–3302. r 2010 American Chemical Society
and the formation of ionic complex [{(Me3Si)2NC(NCy)2}2Lu(Et2O)þ](C60-•) instead of the expected addition of Lu-H to the CdC bond of the C60 core. The reaction of [{(Me3Si)2NC(NCy)2}2Lu(μ-H)]2 (1) with two equivalents of C60 was carried out in toluene at room temperature under anaerobic conditions and resulted in a color change from violet to brown. Slow diffusion of diethyl ether into the reaction mixture afforded deep brown crystals of complex [{(Me3Si)2NC(NCy)2}2Lu(Et2O)þ](C60-•) 3 (C7H8) 3 0.5(Et2O) (2) (Scheme 1), which crystallizes as a solvate containing one molecule of toluene and a half molecule of diethyl ether per unit and was isolated in 74% yield. The GC analysis of the reaction gas phase showed the presence of H2. The X-ray crystal structure investigation of 2 was carried out at 100 K and revealed that it is a dinuclear complex consisting of a single-bonded (C60-)2 dimer and two [{(Me3Si)2NC(NCy)2}2Lu (Et 2O)]þ fragments (Figure 1). The 2:1 ratio between [{(Me 3Si)2 NC(NCy)2}2 Lu(Et 2O)]þ units and the (C60-)2 dimers implies that this dimer should have formal 2- charge. Complexes containing such single-bonded dimers resulting from dimerization of radical anions C60-• were previously obtained in the reactions of reduction of C60 by complexes of d-transition metals in low oxidation states.6 The length of the intercage C-C bond in 2 (1.608(4) A˚) is longer than the normal C-C bond between sp3 carbons (1.530 A˚)7 and is even longer than the length of a single interfullerene C-C bond in related complexes (1.585(5)-1.597(7) A˚).6 The interfullerene center-to-center distance in 2 is 9.368 A˚. The C-C bond lengths around the sp3 carbon excluding the intercage one are 1.506(4), 1.520(4), and 1.522(4) A˚. The bond angles for sp3 carbons (114.7(4)116.8(3)°) are indicative of the tetrahedral geometry but are somewhat larger compared to the values reported for complexes containing single-bonded (C60-)2 dimers.6 The geometry of the cationic [{(Me3Si)2NC(NCy)2}2Lu(Et2O)]þ fragments containing a five-coordinated lutetium (6) (a) Konarev, D. V.; Khasanov, S. S.; Otsuka, A.; Saito, G. J. Am. Chem. Soc. 2002, 124, 8520–8521. (b) Konarev, D. V.; Khasanov, S. S.; Saito, G.; Otsuka, A.; Yoshida, Y.; Lyubovskaya, R. N. J. Am. Chem. Soc. 2003, 125, 10074–10083. (c) Konarev, D. V.; Khasanov, S. S.; Kovalevsky, A. Y.; Saito, G.; Otsuka, A.; Lyubovskaya, R. N. Dalton Trans. 2006, 3716–3720. (d) Konarev, D. V.; Khasanov, S. S.; Saito, G.; Otsuka, A.; Lyubovskaya, R. N. Inorg. Chem. 2007, 46, 7601–7609. (e) Domrachev, G. A.; Shevelev, Yu. A.; Cherkasov, V. K.; Fukin, G. K.; Khorshev, S. Ya.; Markin, G. V.; Kaverin, B. S.; Karnatsevich, V. L.; Kirkin, G. A. Dokl. Chem. 2004, 395, 74–77. (7) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1–S19. Published on Web 10/13/2010
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Figure 1. Fragment of the crystal structure of 2 at 100 K. Cy groups at N atoms, Me groups of SiMe3 fragments, Et groups of the Et2O ligand, all H atoms, and solvate Et2O and toluene molecules are omitted. Selected bond distances [A˚] and angles [deg]: Lu(1)-O(1S) 2.228(3), Lu(1)-N(1) 2.224(4), Lu(1)-N(2) 2.271(4), Lu(1)-N(4) 2.245(4), Lu(1)-N(5) 2.260(4), C(39)-C(39A) 1.608(4); C(47)C(39)-C(39A) 114.7(4), C(43)-C(39)-C(39A) 115.7(4), C(40)-C(39)-C(39A) 116.8(3). Scheme 1
atom is similar to those formerly observed in the related bis(guanidinate) lutetium complexes, but the Lu-N distances (average Lu-N bond length is 2.250(4) A˚) in 2 are predictably shorter compared to the appropriate distances in complexes of six-coordinated lutetium, [Lu{(Me3Si)2NC(Ni-Pr)2}2(μ-Cl)2Li(THF)2 ] (2.312 A˚ ),2b [Lu{(Me 3Si)2 NC(Ni-Pr)2}2 (μ-H)]2 (2.309 A˚),2b and [Lu{(Me3Si)2NC(NCy)2}2(μ-H)]2 (2.306 A˚).2c
In the crystal of 2 anions (C60-)2 are packed in zigzag chains along the b axis of the unit cell (Figure 2). These fullerene chains are stacked on each other along the c axis. The shortest distance between parallel hexagons of neighboring fullerene dimers in the chain is 4.07 A˚. The cationic parts of the complex are disposed in the void space of the fullerene chains and have a zigzag motif.
Note
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Figure 2. Fragment of crystal packing of 2 along the bc plane of the unit cell.
)
(8) Eaton, S. S.; Eaton, G. R. Appl. Magn. Reson. 1996, 11, 155–170.
Figure 3. Temperature dependence of the integral intensity of the signal in the solid-state EPR spectrum of 2 in the temperature region 120-295 K.
radical centers calculated from D using the approximation of dot dipoles9 (r = (55600/D )1/3) is 10.5 A˚, which is very close to the distance between two centroids of each C60 core in a dimer particle (∼10 A˚). One can suppose that this minor signal is due to a small part of the (C60-C60)2molecules possessing a triplet state. The regenerative increase of the intensity of the C60-• signal with the temperature increase is consistent with the reversible C60-C60 bond formation. Thus the whole set of spectroscopic data registered for 2 at room temperature and the results of the low-temperature EPR and X-ray investigations allow suggesting that complex 2 exists at room temperature in the form [{(Me3Si)2NC(NCy)2}2Lu(Et2O)þ](C60-•) containing parmagnetic radical anion C60-•, while at temperatures below ∼120 K dimerization of C60-• occurs and affords [{(Me3Si)2NC(NCy)2}2Lu(Et2O)þ]2[(C60-)2] 3 2(C7H8) 3 (Et2O), with a diamagnetic σ-bonded (C60-)2 dianion. It is noteworthy that the C60-C60 bond in 2 is obviously weaker compared to those in the previously reported compounds containing a (C60-)2 dianion6a,b,d since it is longer (1.608(4) vs 1.585(5)1.597(7) A˚),6 and the dimer dissociation in this case starts at a lower temperature. Formerly, several examples of reactivity of late d-transition metal hydrido complexes (Pt, Ir, Rh) toward C60 have been reported.10 Unlike the reaction of 1 with C60, interaction of the derivative of the less electropositive platinum trans-[Pt(H)2(PCy3)2] occurs with reductive elimination of dihydrogen and coordination of the reduced Pt center to neutral fullerene, which acts as a η2-ligand.10a In the reactions of [MH(CO)(PPh3)3] (M = Rh, Ir)10b,c and [IrH(COD)(PPh3)2]10c )
The NIR spectrum of the reaction mixture 1-C60 measured at room temperature in toluene solution contains a strong absorption band with a maximum at 1075 nm and a band of lower intensity at 935 nm characteristic of C60-• radical anions,4 which allow an unambiguous identification of the radical-anionic state of the fullerene fragment in ionic complexes. The weak low-energy band at 1356 nm that is normally attributed to the σ-(C60-)2 dimers is not present in the spectrum of 2. The solid-state NIR spectrum of 2 recorded in KBr pellet also demonstrates the same set of absorptions (1075, 933 nm). The IR spectrum recorded in KBr pellet at 22 °C also gives evidence for the ionic ground state of complex 2. The F1u(4) C60 mode, which is sensitive to the charge transfer to the fullerene molecule, shifts from 1429 cm-1 (free C60) to 1409 cm-1 in 2, while the F1u(1-3) modes (527, 577, and 1181 cm-1) remain unshifted. 1H and 13C NMR spectroscopy was uninformative about the solution structure of complex 2 due to its low solubility and paramagnetism of the C60-• radical anions. The solid-state EPR spectra of 2 were recorded in the temperature region 105-295 K. Unlike the related derivatives of C60-• containing paramagnetic d-transition metal cations,6 which are EPR silent due to magnetic coupling between paramagnetic centers, the spectrum of complex 2, which comprises diamagnetic lutetium, at 20 °C displays a strong broad single line centered at g = 1.9990 (ΔH = 61.9 G), characteristic of a radical-anion C60-•.8 A decrease of the temperature to 180 K results in the typical line narrowing for the EPR spectra of radical-anion C60-• . In the temperature region 180-120 K a strong decrease of the signal intensity is observed, thus indicating the dimerization of C60-• radical anions leading to the formation of the diamagnetic σ-bonded (C60-)2 dianion. At 120 K the signal of C60-• disappears almost completely. At the same time, at low temperatures the appearance of a weak signal typical for compounds in the triplet spin state (S = 1) was observed. This spectrum has a zero splitting parameter D = 48 G. The estimated average distance r between
(9) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance. Elementary Theory and Practical Applications; McGraw-Hill Book Co.: New York, 1972. (10) (a) Pandolfo, L.; Maggini, M. J. Organomet. Chem. 1997, 540, 61–65. (b) Usatov, A. V.; Kudin, K. N.; Vorontsov, E. V.; Vinogradova, L. E.; Novikov, Yu. N. J. Organomet. Chem. 1996, 522, 147–153. (c) Balch, A. L.; Lee, J. W.; Noll, B. C.; Olmstead, M. M. Inorg. Chem. 1993, 32, 3577–3578.
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substitution of one phosphine ligand and formation of η2-C60 complexes was observed. Thereby we described herein the new type of reactivity of metal hydrido complexes toward C60. The hydrido complex of electropositive lutetium reacts with fullerene similarly to low-valent complexes of d-transition metals6 but not to hydrido complexes of late d-transition metals.
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Acknowledgment. This work was supported by the Russian Foundation for Basic Research (Grants No. 08-03-00391-a, 09-03-97034-r_povolzh’e_a). We thank Dr. A. Poddel’skii for recording EPR spectra. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
