Published on Web 04/22/2009
Decamethylytterbocene Complexes of Bipyridines and Diazabutadienes: Multiconfigurational Ground States and Open-Shell Singlet Formation Corwin H. Booth,*,† Marc D. Walter,†,‡ Daniel Kazhdan,†,‡ Yung-Jin Hu,‡,§ Wayne W. Lukens,† Eric D. Bauer,# Laurent Maron,⊥ Odile Eisenstein,| and Richard A. Andersen†,‡ Chemical Sciences DiVision and Nuclear Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720, Department of Chemistry, UniVersity of California, Berkeley, California 94720, Materials Physics and Applications DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, UniVersite´ de Toulouse, INSA-UPS-LPCNO and CNRS-LPCNO, 135 aVenue de Rangueil, F-31077 Toulouse, France, Institut Charles Gerhardt, UniVersite´ Montpellier 2, place E. Bataillon, 34095 Montpellier, France, and Institut Charles Gerhardt CNRS, UMR 5253 CNRS-UM2-ENSCM-UM1, UniVersite´ Montpellier 2, Montpellier, France Received December 9, 2008; E-mail:
[email protected] Abstract: Partial ytterbium f-orbital occupancy (i.e., intermediate valence) and open-shell singlet formation are established for a variety of bipyridine and diazabutadiene adducts with decamethylytterbocene, (C5Me5)2Yb, abbreviated as Cp*2Yb. Data used to support this claim include ytterbium valence measurements using Yb LIII-edge X-ray absorption near-edge structure spectroscopy, magnetic susceptibility, and complete active space self-consistent field (CASSCF) multiconfigurational calculations, as well as structural measurements compared to density functional theory calculations. The CASSCF calculations indicate that the intermediate valence is the result of a multiconfigurational ground-state wave function that has both an open-shell singlet f13(π*)1, where π* is the lowest unoccupied molecular orbital of the bipyridine or diazabutadiene ligands, and a closed-shell singlet f14 component. A number of other competing theories for the unusual magnetism in these materials are ruled out by the lack of temperature dependence of the measured intermediate valence. These results have implications for understanding chemical bonding not only in organolanthanide complexes but also for f-element chemistry in general, as well as understanding magnetic interactions in nanoparticles and devices.
1. Introduction
The canonical view of magnetism and bonding in lanthanide organometallic compounds is that of localized f-orbitals split by interactions with ligand orbitals, but not participating in bonding. This rule, of course, has some well-known exceptions, particularly in solid-state intermetallics.1-3 Two features of f-orbital bonding in these intermetallic systems are noteworthy. First, a reduced magnetic moment at low temperatures is often observed, and second, a theoretical understanding cannot be obtained without including higher-order (many-body) interactions than are typically provided by a molecular theoretical method such as density functional theory (DFT). Recently, several N-heterocyclic base adducts of ytterbocene of the form †
Chemical Sciences Division, Lawrence Berkeley National Laboratory. University of California, Berkeley. § Nuclear Sciences Division, Lawrence Berkeley National Laboratory. # Los Alamos National Laboratory. ⊥ LPCNO, CNRS-UPS-INSA, INSA Toulouse. | Universite´ Montpellier 2 and CNRS. (1) Hewson, A. C. The Kondo Problem to HeaVy Fermions; Cambridge University Press: Cambridge, UK, 1993. (2) Lawrence, J. M. Intermediate Valence Compounds. In Encyclopedia of Physics, 2nd ed.; Lerner, R. G., Trigg, G. L.,Eds.; VCH Publishers: New York, 1991; p 548. (3) Stewart, G. R. ReV. Mod. Phys. 1984, 56, 755. ‡
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Cp*2Yb(L), where Cp* is pentamethylcyclopentadienyl, C5Me5, and L is one of several 2,2′-bipyridine (bipy),4,5 1,4-diazabutadiene (dad),6 and related7,8 adducts, have been shown to display unusual magnetic properties involving an apparently reduced magnetic moment of the complex, corresponding to an extremely strong antiferromagnetic coupling constant. For instance, using the temperature of the peak in the magnetic susceptibility as a rough estimate of twice the coupling constant, J, the bipy and dad adducts have J ≈ -100 cm-1, whereas J between a lanthanide and a radical is typically smaller than -10 cm-1.9-15 The origin of the reduced moment in these molecules has been (4) Schultz, M.; Boncella, J. M.; Berg, D. J.; Tilley, T. D.; Andersen, R. A. Organometallics 2002, 21, 460. (5) Walter, M. D.; Berg, D. J.; Andersen, R. A. Organometallics 2006, 25, 3228. (6) Walter, M. D.; Berg, D. J.; Andersen, R. A. Organometallics 2007, 26, 2296. (7) Veauthier, J. M.; Schelter, E. J.; Carlson, C. N.; Scott, B. L.; Da Re, R. E.; Thompson, J. D.; Kiplinger, J. L.; Morris, D. E.; John, K. D. Inorg. Chem. 2008, 47, 5841. (8) Veauthier, J. M.; Schelter, E. J.; Kuehl, C. J.; Clark, A. E.; Scott, B. L.; Morris, D. E.; Martin, R. L.; Thompson, J. D.; Kiplinger, J. L.; John, K. D. Inorg. Chem. 2005, 44, 5911. (9) Benelli, C.; Caneschi, A.; Gatteschi, D.; Sessoli, R. Inorg. Chem. 1993, 32, 4797. (10) Benelli, C.; Caneschi, A.; Gatteschi, D.; Sessoli, R. J. Appl. Phys. 1993, 73, 5333. 10.1021/ja809624w CCC: $40.75 2009 American Chemical Society
Ytterbocene Complexes of Bipyridines and Diazabutadienes
controversial. The heart of the controversy has been whether the reduced moment is caused by some type of antiferromagnetic coupling (mostly intramolecular, as shown by correlating solidstate magnetism and solution 1H NMR chemical shifts5,6) or is due to the electronic structure on the metal center or the molecule. In this article, we explore the ubiquity of such behavior in this class of molecules and the nature of this unusual magnetic interaction with measurements of the magnetic susceptibility and the X-ray absorption near-edge structure (XANES). A systematic relationship is observed between the effective valence of the ytterbium atoms and the magnetism in these molecules, indicating that as the ytterbium valence moves from Yb(III), 4f13, with an electron in the lowest unoccupied molecular orbital (LUMO) π* of the bipyridine or diazabutadiene ligands, toward Yb(II), 4f14, an open-shell singlet ground state develops that becomes more stable as the f14 contribution increases. The magnetism of these materials is thus related to the degree of intermediate valence, where the term “intermediate valence” denotes a fractional, or noninteger, valence. For example, the valence of ytterbium in Cp*2Yb(pyridine)2 is two, Yb(II), and the Yb atom has a closed-shell 4f14 electron configuration, consistent with its physical properties. Similarly, the valence of ytterbium in [Cp*2Yb(bipy)+][I-] is unambiguously three, Yb(III), and the atom has an open-shell 4f13 electron configuration, consistent with its physical properties. However, the valence of the ytterbium atom in Cp*2Yb(bipy) is neither two nor three; rather, it is in between these extreme values, i.e., it has an intermediate valence.16 The physical studies reported in this article give the value of the valence from which a physical model, supported by calculations, is developed to rationalize why the complex has intermediate valence. The presence of intermediate valence in the ytterbium atoms, and the lack of a temperature dependence of this valence, rules out several other interpretations of the magnetism, such as those invoking valence tautomerism or crystal-field interactions. These data therefore point to the formation of a multiconfigurational, open-shell singlet ground state with anomalously strong antiferromagnetic coupling between the moments on the metal and the aromatic ligand. Similar behavior was predicted for cerocene17-19 (Ce(cot)2, cot ) cyclooctatetraene ) C8H8) and supportedbyexperimentsoncerocene,16,20 substitutedcerocenes,21,22 the isoelectronic compounds Pn*2Ce (Pn* ) permethylpentalene ) C8Me6)23 and Ce[C8H4(SiiPr3-1,4)2]2,24 as well as [Ce(η(11) Benelli, C.; Gatteschi, D. Chem. ReV. 2002, 102, 2369. (12) Caneschi, A.; Dei, A.; Gatteschi, D.; Massa, C. A.; Pardi, L. A.; Poussereau, S.; Sorace, L. Chem. Phys. Lett. 2003, 371, 694. (13) Caneschi, A.; Dei, A.; Gatteschi, D.; Poussereau, S.; Sorace, L. Dalton Trans. 2004, 1048. (14) Sanada, T.; Suzuki, T.; Yoshida, T.; Kaizaki, S. Inorg. Chem. 1998, 37, 4712. (15) Tsukuda, T.; Suzuki, T.; Kaizaki, S. J. Chem. Soc., Dalton Trans. 2002, 1721. (16) Booth, C. H.; Walter, M. D.; Daniel, M.; Lukens, W. W.; Andersen, R. A. Phys. ReV. Lett. 2005, 95, 267202. (17) Neumann, C.-S.; Fulde, P. Z. Phys. B 1989, 74, 277. (18) Dolg, M.; Fulde, P.; Ku¨chle, W.; Neumann, C.-S.; Stoll, H. J. Chem. Phys. 1991, 94, 3011. (19) Dolg, M.; Fulde, P.; Stoll, H.; Preuss, H.; Chang, A.; Pitzer, R. M. Chem. Phys. 1995, 195, 71. (20) Walter, M. D.; Booth, C. H.; Lukens, W. W.; Andersen, R. A. Organometallics 2009, 28, 698. (21) Edelstein, N. M.; Allen, P. G.; Bucher, J. J.; Shuh, D. K.; Sofield, C. D.; Kaltsoyannis, N.; Maunder, G. H.; Russo, M. R.; Sella, A. J. Am. Chem. Soc. 1996, 118, 13115. (22) Amberger, H. D.; Reddmann, H.; Edelmann, F. T. J. Organomet. Chem. 2005, 690, 2238.
ARTICLES Scheme 1. Schematic Diagram Showing Bipyridine and Diazabutadiene Adducts Discussed in the Present Work
C5H5)3]+,25 Cp*2Yb(terpy) (terpy ) 2,2′:6′,2′′-terpyridine),7 and Cp*2Yb(bipy).16 This interpretation is further supported by a complete active space self-consistent field (CASSCF) method calculation on (C5H5)2Yb(bipy) and (C5H5)2Yb(dad(H)-t-Bu) molecules. These calculations also highlight the structural implications of such intermediate valent Yb states. Therefore, these results emphasize the importance of a higher-order, multiconfigurational approach in determining the nature of the chemical bond in lanthanide organometallics in which an f electron on the metal center can couple with delocalized π electrons on the ligand. Yb LIII-edge XANES and magnetic susceptibility measurements are reported below on several bipy and dad adducts of Cp*2Yb of the type Cp*2Yb(L) (Scheme 1), where L is either bipy, a 4,4′-disubstituted bipyridine, or a 1,4-diazabutadiene derivative, including purely di- and trivalent Yb complexes. These bipy and dad ligands were chosen over related 4,4′dialkyl-substituted 2,2′-bipyridines, such as the 4,4′-dimethyl2,2′-bipyridine or 4,4′-di(tert-butyl-2,2′-bipyridine),5 since the former do not display any obvious phase transitions or other hysteretic behavior, and, as will be shown below, do not exhibit any obvious temperature dependence of the Yb valence. Examples of ytterbocene complexes that display strong temperature dependence of the Yb valence will be considered in a future paper. 2. Experimental Section General Synthesis. Samples were prepared as previously described,4-6,26 except for the compounds noted below, which were previously unknown. Preparation of Cp*2La(OTf)(pyridine). Cp*2Mg (6.11 g, 2.7 mmol) and dry La(OTf)3 (12.15 g, 2.7 mmol) in 50 mL of a 1:10 pyridine/toluene solution were stirred overnight. The green-yellow solution was then taken to dryness, and the green-yellow residue was dissolved in 300 mL of diethyl ether. The suspension was (23) Ashley, A.; Balazs, G.; Cowley, A.; Green, J.; Booth, C. H.; O’Hare, D. Chem. Commun. 2007, 1515. (24) Balazs, G.; Cloke, F. G. N.; Green, J. C.; Harker, R. M.; Harrison, A.; Hitchcock, P. B.; Jardine, C. N.; Walton, R. Organometallics 2007, 26, 3111. (25) Coreno, M.; de Simone, M.; Green, J. C.; Kaltsoyannis, N.; Narband, N.; Sella, A. Chem. Phys. Lett. 2006, 432, 17. (26) Walter, M. D.; Schultz, M.; Andersen, R. A. New J. Chem. 2006, 30, 238. J. AM. CHEM. SOC.
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filtered, the filtrate was concentrated to 100 mL and then warmed to redissolve the precipitate, and the solution was cooled to -20 °C. Light green crystals of Cp*2La(OTf)(pyridine) formed: yield 2.40 g (18%, mp 363-367 °C. 1H NMR (20 °C, C6D6): δ 1.92 (s, 30H), 6.56 (t, 1H, J ) 7 Hz), 6.84 (t, 2H, J ) 8 Hz), 8.32 (d, 2H, J ) 5 Hz). 19F NMR (20 °C, C6D6): δ -76.7 (s). Anal. Calcd for C26H35NF3LaO3S: C, 48.98; H, 5.53; N, 2.20. Found: C, 49.12; H, 5.26; N, 2.20. Preparation of Cp*2La(2,2′-bipyridine)(OTf). Cp*2La(OTf)(pyridine) (1.2 g, 1.9 mmol) and bipy (0.29 g, 1.9 mmol) were mixed in 50 mL of pentane with stirring. After stirring of the suspension overnight, the solvent was filtered, the red powder was dissolved in 5 mL of CH2Cl2, and the filtrate was layered with pentane. Red crystals formed over the course of a week: yield 0.48 g (36%). 1H NMR (20 °C, CD2Cl2): δ 1.73 (s, 30H), 7.70 (t, 2H, J ) 6 Hz), 8.14 (td, 2H, J ) 8 Hz, J ) 2 Hz), 8.26 (d, 2H, J ) 8 Hz), 8.92 (2H, υ1/2 ) 40 Hz). 19F NMR (20 °C, CD2Cl2): δ -76.99 (s). Anal. Calcd for C31H38N2F3LaO3: C, 52.10; H, 5.36; N, 3.92. Found: C, 51.74; H, 5.00; N, 3.81. Preparation of Cp*2La(2,2′-bipyridine). Cp*2La(2,2′-bipyridine)(OTf) (1.1 g, 1.6 mmol) was added to a sodium amalgam formed by dissolving Na (0.04 g, 1.9 mmol) in Hg (38 g, 188 mmol). Toluene (100 mL) was added, and the dark red solution was stirred overnight. The toluene suspension was filtered, and the filtrate was taken to dryness. The dark red residue was dissolved in 60 mL of pentane, the suspension was filtered, and the filtrate was concentrated to 20 mL, heated to redissolve the Cp*2La(2,2′-bipyridine), and then cooled to -20 °C. Dark red crystals were obtained: yield 0.70 g (79%), mp 310-312 °C. 1H NMR (20 °C, C6D6): 3.39 (υ1/2 ) 120 Hz); the 2,2′-bipyridine resonances were not found. IR (Nujol mull): 2917 (vs), 2724 (w), 1539 (w), 1463 (s), 1417 (w), 1377 (s), 1276 (m), 1260 (m), 1205 (m), 1169 (w), 1146 (m), 1076 (m), 998 (m), 942 (s), 822 (w), 743 (w), 715 (s), 676 (m), 642 (m), 604 (w). UV-vis (C6H12) (λmax, ε × 10-3 L mol-1 cm-1): 930 (1.36), 812 (2.07), 368 (10.10). Anal. Calcd for C30H38LaN2: C, 63.71; H, 6.77; N, 4.95. Found: C, 63.45; H, 6.97; N, 4.87. Preparation of Cp*2Yb(OTf)(pyridine). MgCp*2 (6.1 g, 20.8 mmol) was stirred with dry Yb(OTf)3 (12.9 g, 20.8 mmol) in 50 mL of a 1:10 mixture of pyridine/toluene. The purple solution was warmed and stirred overnight. The solution was then taken to dryness, and the purple residue was dissolved in 100 mL of diethyl ether. The suspension was filtered, and then the filtrate was concentrated to 50 mL, warmed to dissolve the Cp*2Yb(OTf)(pyridine), and cooled to -20 °C overnight. Dark purple block crystals of Cp*2Yb(OTf)(pyridine) formed: yield 12.6 g (90%), mp 211 °C. 1H NMR (20 °C, C6D6): δ 3.83 (30H, υ1/2 ) 60 Hz), 25.54 (1H, υ1/2 ) 80 Hz). The ortho and meta hydrogens were not observed at 20 °C. 19F NMR (20 °C, C6D6): δ -51.0 (s). Anal. Calcd for C26H35NF3O3SYb: C, 46.49; H, 5.25; N, 2.09. Found C, 46.35; H, 4.94; N, 2.43. Preparation of [Cp*2Yb(N,N′-bis(p-tolyl)-1,4-diazabutadienyl)][BPh4]. Cp*2Yb(OTf)(pyridine) (0.65 g, 0.97 mmol), N,N′bis(p-tolyl)-1,4-diazabutadiene (0.24 g, 0.97 mmol), and NaBPh4 (1.0 g, 2.8 mmol) were stirred in 50 mL of pentane overnight. The pentane was filtered, the brown residue was dissolved in 10 mL of CH2Cl2 and filtered, and the filtrate was layered with 50 mL of pentane. Brown plate-like crystals of [Cp*2Yb(dad(H)-p-tolyl)][BPh4] formed: yield 0.36 g (37%). 1H NMR (20 °C, CD2Cl2): δ -35.53 (2H, υ1/2 ) 80 Hz), -3.21 (30H, υ1/2 ) 200 Hz), -0.649 (4H, υ1/2 ) 60 Hz), 1.19 (8H, υ1/2 ) 40 Hz), 2.24 (8H, υ1/2 ) 40 Hz), 24.54 (6H, υ1/2 ) 4 Hz), 53.92 (2H, υ1/2 ) 800 Hz). One resonance due to 4H’s, perhaps those of the ortho-H’s of the p-tolyl ring, was not observed at 20 °C. 11B NMR (20 °C, CD2Cl2): δ -12.78 (υ1/2 ) 8 Hz). Anal. Calcd for C60H66N2BYb: C, 72.14; H, 6.66; N 2.80. Found: C, 69.82; H, 6.26; N, 2.97.27 A crystal suitable for X-ray diffraction was selected; full crystallographic details are available as Supporting Information. X-ray Absorption Near-Edge Structure (XANES) Spectroscopy. The samples for X-ray absorption measurements were 6482
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prepared by mixing 3-10 mg of sample with dried boron nitride in an inert atmosphere (Ar or N2) glovebox and loading the mixture into a slotted, multiple-sample aluminum holder with indium-sealed aluminized mylar windows. These holders provide sufficient sample protection that the sample integrity was not compromised during transportation in a container backfilled with nitrogen or argon to the Stanford Synchrotron Radiation Lightsource (SSRL). X-ray absorption data were collected primarily on beamline 11-2 using an unfocused, uncollimated beam with energy resolution (
