Organometallics 2010, 29, 4787–4789 DOI: 10.1021/om1004385
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The First Linear, Homoleptic Triple-Decker Sandwich Complex of an f-Element: A Molecular Model for Organolanthanide Nanowires† Volker Lorenz, Steffen Blaurock, Cristian G. Hrib, and Frank T. Edelmann* Chemisches Institut der Otto-von-Guericke-Univerit€ at Magdeburg, Universit€ atsplatz 2, D-39106 Magdeburg, Germany Received May 7, 2010 Summary: The first structurally authenticated linear, homoleptic triple-decker sandwich complex of an f-element, (η8COT00 )Nd(μ-η8:η8-COT00 )Nd(η8-COT00 ) (2; COT00 = [C8H6(SiMe3)2-1,4]2-) has become available through a rather unexpected synthetic pathway. Treatment of [Li(THF)4][Nd(COT00 )2] (1) with anhydrous cobalt(II) chloride (molar ratio ca. 2:1) in a toluene suspension afforded 2 in 72% isolated yield. The most notable structural feature of 2 is the near-linear arrangement of the COT00 rings with (ring centroid)-Nd-(ring centroid) angles of 176.1 and 175.6°, respectively, and a Nd-(ring centroid)-Nd angle of 177.9°, making the title compound the first true molecular model for lanthanide-based nanowires. Molecular spintronics is an exciting new and emergent subarea of spintronics that benefits from achievements in molecular electronics and molecular magnetism.1 During the past few years, molecular spintronics using single molecules has attracted enormous attention both experimentally and theoretically, since it holds promise for the next generation of electronic devices with enhanced functionality and improved performance, especially in high-density information storage and quantum computing.2 In this context, various onedimensional organometallic sandwich molecular wires (SMW’s) have been extensively studied due to their unique electronic and magnetic properties. Among the promising examples are lanthanide-based multidecker sandwich complexes of the type Lnn(COT)nþ1 (COT = η8-cyclooctatetraenyl), † Part of the Dietmar Seyferth Festschrift. *To whom correspondence should be addressed. E-mail: frank.
[email protected]. (1) (a) Emberly, E. G.; Kirczenow, G. Chem. Phys. 2002, 281, 311– 324. (b) Rocha, A. R.; Garcia-Suarez, V. M.; Bailey, S. W.; Lambert, C. J.; Ferrer, J.; Sanvito, S. Nat. Mater. 2005, 4, 335–339. (c) Sanvito, S.; Rocha, A. R. J. Comput. Theor. Nanosci. 2006, 3, 624–642. (2) (a) Bogani, L.; Wernsdorfer, W. Nat. Mater. 2008, 7, 179–186. (b) Coronado, E.; Epstein, A. J. J. Mater. Chem. 2009, 19, 1670–1671. (c) Mas-Torrent, M.; Crivillers, N.; Mugnaini, V.; Ratera, I.; Rovira, C.; Veciana, J. J. Mater. Chem. 2009, 19, 1691–1695. (d) Ferrer, J.; GarciaSuarez, V. M. J. Mater. Chem. 2009, 19, 1696–1717. (3) (a) Kurikawa, T.; Negishi, Y.; Hayakawa, F.; Nagao, S.; Miyajima, K.; Nakajima, A.; Kaya, K. J. Am. Chem. Soc. 1998, 120, 11766– 11772. (b) Kurikawa, T.; Negishi, Y.; Hayakawa, F.; Naguo, S.; Miyajima, K.; Nakajima, A.; Kaya, K. Eur. Phys. J. D 1999, 9, 283–287. (c) Miyajima, K.; Kurikawa, T.; Hashimoto, M.; Nakajima, A.; Kaya, K. Chem. Phys. Lett. 1999, 306, 256–262. (d) Nakajima, A.; Kaya, K. J. Phys. Chem. A 2000, 104, 176–191. (e) Hosoya, N.; Takegami, R.; Suzumura, J.; Yada, K.; Koyasu, K.; Miyajima, K.; Mitsui, M.; Knickelbein, M.; Yabushita, A.; Nakajima, A. J. Phys. Chem. A 2005, 109, 9–12. (f) Takegami, R.; Hosoya, N.; Suzumura, J.; Yada, K.; Nakajima, A.; Yabushita, A. Chem. Phys. Lett. 2005, 403, 169–174. (g) Miyajima, K.; Knickelbein, M.; Nakajima, A. Polyhedron 2005, 24, 2341–2345. (h) Miyajima, K.; Knickelbein, M.; Nakajima, A. J. Phys. Chem. A 2008, 112, 366–375. (i) Xu, K.; Huang, J.; Lei, S.; Su, H.; Boey, F. Y. C.; Li, Q.; Yang, J. J. Chem. Phys. 2009, 131, 104704. (j) Zhang, X; Ng, M.-F.; Wang, Y.; Wang, J.; Yang, S.-W. ACS Nano 2009, 3, 2525–2522.
r 2010 American Chemical Society
which can be synthesized via a combination of laser vaporization and molecular beam methods.3 This way nanowires up to 8 nm in length (Ln = Eu, n = 18) have been obtained. A major obstacle making these oligomers difficult to manipulate and to investigate is their low solubility. It has recently been pointed out by Cloke et al. that lanthanide triple-decker sandwich complexes would provide excellent molecular models for the Ln(COT) sandwich nanowires.4 Previously reported triple-deckers based on a COT bridging ligand include a series of lanthanide(II) complexes of the type (μ-COT)[Ln(CpR)]2 (R = Me5, Ln = Sm, Yb, Eu; R = iPr5, Ln = Yb, Eu), as well as a number of solvated and COT-ring-substituted analogues.4,5 However, unlike the case for the nanowires, all these triple-decker sandwich complexes comprise significantly bent molecular structures.4,5 Homoleptic lanthanide triple-deckers of the type Ln2(COT)3 (Ln = Ce, Nd, Eu, Lu), for which a linear configuration is more likely, have been known since 1976,6 and Ce2(COT)3 has recently been reinvestigated by EPR and EXAFS methods, with the latter showing consistency with a triple-decker sandwich structure.7 A series of ring-substituted derivatives of the type Ln2(COT00 )3 (Ln = Ce, Nd, Sm; COT00 = [C8H6(SiMe3)21,4]2-) were reported by us in 1998.8 Thus far, unfortunately, positive proof of the linear triple-decker structure is lacking, as no X-ray diffraction data are known to exist for any of these species. We report here a novel preparation and the first structural authentication of a homoleptic, linear tripledecker sandwich complex of an f-element. Reactions of anhydrous lanthanide trichlorides with Li2(COT00 ) have been shown to strongly depend on the reaction conditions and starting material ratios. Products isolated from such reactions include not only the anionic sandwich complexes [Ln(COT00 )2]- 9 and chloro-bridged dimers [(COT00 )Ln(μ-Cl)THF)]210 but also unusual cluster-centered (4) Summerscales, O. T.; Jones, S. C.; Cloke, F. G. N.; Hitchcock, P. B. Organometallics 2009, 28, 5896–5908 and references cited therein. (5) (a) Evans, W. J.; Clark, R. D.; Ansari, M. D.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 9555–9563. (b) Evans, W. J.; Johnston, M. A.; Greci, M. A.; Ziller, J. W. Organometallics 1999, 18, 1460–1464. (c) Walter, M. D.; Wolmersh€auser, G.; Sitzmann, H. J. Am. Chem. Soc. 2005, 127, 17494– 17503. (6) (a) Ely, S. R.; Hopkins, T. E.; DeKock, C. W. J. Am. Chem. Soc. 1976, 98, 1624–1625. (b) Greco, A.; Cesca, S.; Bertolini, G. J. Organomet. Chem. 1976, 113, 321–330. (7) Walter, M. D.; Booth, C. H.; Lukens, W. W.; Andersen, R. A. Organometallics 2009, 28, 698–707. (8) (a) Poremba, P.; Edelmann, F. T. J. Organomet. Chem. 1998, 553, 393–395. (b) Edelmann, F. T. New J. Chem. 1995, 19, 535–550. (c) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Chem. Rev. 2002, 102, 1851– 1896. (9) Poremba, P.; Reissmann, U.; Noltemeyer, M.; Schmidt, H.-G.; Br€ user, W.; Edelmann, F. T. J. Organomet. Chem. 1997, 544, 1. Published on Web 08/09/2010
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Lorenz et al. Scheme 1
multidecker sandwich complexes11 as well as the bimetallics Ln2(COT00 )3.8 Although the latter could be isolated from LnCl3/Li2(COT00 ) reactions in a molar ratio of 2:3, all attempts to grow single crystals failed within the course of our initial investigation.8 In a continuation of this work we have now found a rather unexpected and novel access to the triple-decker molecules. Treatment of [Li(THF)4][Nd(COT00 )2] (1)8 with anhydrous cobalt(II) chloride (molar ratio ca. 2:1) in a toluene suspension produced a dark green solution accompanied by formation of a black precipitate of metallic Co (admixed with LiCl).12 Crystallization of the toluenesoluble material from cyclopentane afforded dark green, prismlike crystals, which were identified as the pure tripledecker sandwich complex (η8-COT00 )Nd(μ-η8:η8-COT00 )Nd(η8-COT00 ) (2). Its formation (72% isolated yield) in this unexpected redox process is illustrated in Scheme 1. In the course of the reaction, one of the dianionic COT00 ligands in 1 is oxidized to 1,4-bis(trimethylsilyl)cyclooctatetraene (admixed with its 1,6 isomer), which was isolated from the concentrated mother liquid and identified unambiguously by comparison with the NMR data (1H, 13C) of an authentic specimen.10b,13 Apparently two important factors contribute to the success of this novel synthetic route. One is certainly the enhanced solubility of the product imparted by the use of the (10) (a) Burton, N. C.; Cloke, F. G. N.; Hitchcock, P. B.; de Lemos, H. C.; Sameh, A. A. J. Chem. Soc., Chem. Commun. 1989, 1462–1464. (b) Burton, N. C.; Cloke, F. G. N.; Joseph, S. C. P.; Karamallakis, H.; Sameh, A. A. J. Organomet. Chem. 1993, 462, 39–43. (11) Lorenz, V.; Edelmann, A.; Blaurock, S.; Freise, F.; Edelmann, F. T. Organometallics 2007, 26, 4708. (12) Synthesis of 2: solid [Li(THF)4][Nd(COT00 )2] (1; 2.13 mmol, 2.0 g) was added to a stirred suspension of CoCl2 (1.16 mmol, 0.15 g) in toluene (150 mL) at room temperature. The reaction mixture was stirred for 24 h and refluxed for an additional 5 h, causing the formation of a black precipitate. After filtration the solvent was evaporated under vacuum. The solid residue was extracted with 150 mL of cyclopentane. Ca. 75 mL of cyclopentane was evaporated under vacuum. (η8COT00 )Nd(η8-COT00 )Nd(η8-COT00 ) (2) crystallized upon standing at room temperature as dark green prisms (0.85 g, 72%), which were suitable for X-ray crystallography. Anal. Calcd for C47H82Nd2Si6 (Mr = 1104.2): C, 51.13; H, 7.49. Found: C, 50.57; H, 7.32. IR (KBr disk): ν 2995 w, 2953 s, 2895 m, 1445 w, 1404 m, 1313 w, 1247 vs, 1213 m, 1046 s, 1037 m, 981 m, 937 m, 909 m, 840 vs, 782 m, 749 s, 733 s, 686 m, 651 w, 636 m, 548 m, 501 w cm-1. Mass spectrum (EI): m/z 1032 (100%) [{C8H6(SiMe3)2}3Nd2], 1017 (14%), 959 (18%) [{C8H6(SiMe3)2}3Nd2 Si(CH3)3], 784 (58%) [{C8H6(SiMe3)2}2Nd2], 711 (18%), 639 (13%) [{C8H6(SiMe3)2}2Nd]. 1H NMR (600 MHz, d8-toluene, 25 °C): δ 1.01 (br s, outer ring Si(CH3)3), -1.35 (br s, inner ring Si(CH3)3) (intensity ratio 2:1); -3.86 (br s, COT-H), -7.77 (br s, COT-H), -9.01 (br s, COT-H), -13.77 (br s, COT-H), -14.57 (br s, COT-H), -18.25 (br s, COT-H) ppm. 13C NMR (100.6 MHz, d8-toluene, 25 °C): δ 177.3, 170.5, 163.6, 136.3, 103.9 (br, COT00 ), -1.5, -1.7 (Si(CH3)3) ppm. 29Si NMR (79.5 MHz, d8-toluene, 25 °C): δ -44.7 (inner ring SiMe3), -63.3 (outer ring SiMe3) ppm (intensity ratio 1:2). (13) (a) Wadepohl, H.; Merkel, R.; Pritzkow, H.; Ruhm, S. Dalton Trans. 2001, 3617–3626. (b) Wadepohl, H.; Gebert, S.; Merkel, R.; Pritzkow, H. J. Organomet. Chem. 2002, 641, 142–147.
Figure 1. Molecular structure of 1. Selected bond lengths (A˚) and angles (deg): XCOT0 0 A-Nd1 = 1.894, Nd1-XCOT0 0 B = 2.164, XCOT0 0 B-Nd2 = 2.156, Nd2-XCOT0 0 C = 1.906, CCOT0 0 A-Nd1 = 2.622(4)-2.688(4), Nd1-CCOT0 0 B = 2.811(4)-2.917(4), CCOT0 0 BNd2 = 2.805(4)-2.885(4), Nd2-CCOT0 0 C = 2.629(4)-2.673(4); XCOT0 0 A-Nd1-XCOT0 0 B = 176.1, XCOT0 0 B-Nd2-XCOT0 0 C = 175.6, Nd1-XCOT0 0 B-Nd2 = 177.9. XCOT0 0 = COT00 ring centroid.
bulky, silyl-substituted COT00 ligands.10 The other is the heterogeneous reaction in a nonpolar solvent (toluene), which prevents the formation of solvated species. It was shown in early work by DeKock et al. that Ln2(COT)3 tripledeckers formed by cocondensation techniques afforded the solvated species [Ln(COT)(THF)2][Ln(COT)2] upon contact with THF.6a A mass spectrum of 2 showed the molecular ion of the triple-decker at m/z 1032 with 100% relative intensity. Due to the paramagnetic nature of the Nd3þ ions, the 1H NMR resonances are spread over a range of ca. 20 ppm. Dark green, prismlike, X-ray-quality crystals were grown by slow cooling of a saturated solution in cyclopentane. An X-ray diffraction analysis of 214 clearly established the presence of the first homoleptic, linear triple-decker sandwich complex of an f-element (Figure 1). All three COT00 rings are η8-coordinated to neodymium, with the central ring acting as a μ-η8:η8 bridging ligand. The (14) Crystal data for complex 2: C47H82Nd2Si6 3 c-C5H10; Mr = 1104.15, monoclinic, space group P21/n, a = 13.2914(3) A˚, b = 17.2442(3) A˚, c = 24.3534(4) A˚, β = 102.6801(15)°, Z = 4, T = 153(2) K, μ = 2.041 mm-1, green prism. Of 45 491 reflections measured, 11 010 were independent (Rint = 0.0819). Final R1 = 0.0407, wR2 = 0.1017 (all data). Supplementary crystallographic data can be found in the Cambridge Crystallographic Data file CCDC-740282.
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Nd-C distances to the outer rings are in the ranges of 2.622(4)-2.688(4) A˚ for Nd1 and 2.629(4)-2.673(4) A˚ for Nd2. This can be compared to average Nd-C distances of 2.660(24), 2.673(16), and 2.787(19) A˚ reported for the Nd-C(η8-COT) bond lengths in [Nd(COT)(THF)2][Nd(COT)2].6a Significantly longer Nd-C distances (2.811(4)2.917(4) A˚ for Nd1 and 2.805(4)-2.885(4) for Nd2) are found for the bridging COT00 ligand. The corresponding Nd-(ring centroid) distances are 1.894 A˚ (Nd1) and 1.906 A˚ (Nd2) to the outer rings and 2.164 A˚ (Nd1) and 2.156 A˚ (Nd2) to the bridging ring. These values can be favorably compared to corresponding Nd-C(ring centroid) distances of, for example, 1.957 A˚ in (COT)NdI(THF)315 or 1.981 and 2.027 A˚ in (COT)Nd(μ-η8:η2-COT)Li(THF)3.15 The most notable structural feature of 2, however, is the near-linear arrangement of the COT00 rings with (ring centroid)-Nd(ring centroid) angles of 176.1° (Nd1) and 175.6° (Nd2), respectively, and a Nd-(ring centroid)-Nd angle of 177.9°. The observed minor deviations from perfect linearity can be traced back to repulsive interactions between methyl groups of the ring substituents. Thus far, a comparable near-linear arrangement of rings has only been achieved in the Yb(II) quadruple-decker complex Cp*Yb(μ-η8:η8-COT000 )Yb(μ-η8:η8-COT000 )YbCp*, which contains the very bulky 1,3,6-tris-
(trimethylsilyl)cyclooctatetraenyl (=COT000 ) bridging ligand.16 In contrast, all other triple-decker compounds containing lanthanide elements comprise bent configurations.4 It should be noted at this stage that the reaction was initially carried out with the intention to make a heterobimetallic Nd/Co quadruple-decker sandwich complex. In summarizing the main results of the present study, the first linear, homoleptic triple-decker sandwich complex of an f-element, (η8-COT00 )Nd(μ-η8:η8-COT00 )Nd(η8-COT00 ) (2), has been made available through a novel high-yield synthetic route involving oxidation of the anionic sandwich complex [Li(THF)4][Nd(COT00 )2] (1) with CoCl2. Future work in this direction will show if this method can be employed to pepare other Ln2(COT00 )3 derivatives and perhaps even the parent triple-decker sandwiches Ln2(COT)3. In any case the Nd complex 1 represents the first true molecular model for organolanthanide-based nanowires.
(15) Meermann, C.; Ohno, K.; T€ ornroos, K. W.; Mashima, K.; Anwander, R. Eur. J. Inorg. Chem. 2009, 76–85. (16) Edelmann, A.; Blaurock, S.; Lorenz, V.; Hilfert, L.; Edelmann, F. T. Angew. Chem., Int. Ed. 2007, 46, 6732–6734.
Supporting Information Available: CIF file giving X-ray structural data for 2. This material is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was generously supported by the Deutsche Forschungsgemeinschaft (SPP 1166 “Lanthanoid-spezifische Funktionalit€ aten in Molek€ ul und Material”). Financial support by the Ottovon-Guericke-Universit€at Magdeburg is also gratefully acknowledged.
