Variable Photon Energy Photoelectron Spectroscopy and Magnetism

Jun 18, 2010 - Robert G. Denning , Jeffrey Harmer , Jennifer C. Green , and Mark Irwin ... Alison B. Altman , Joseph I. Pacold , Jian Wang , Wayne W. ...
0 downloads 0 Views 1MB Size
Download PDF
4752

Organometallics 2010, 29, 4752–4755 DOI: 10.1021/om100240m

Variable Photon Energy Photoelectron Spectroscopy and Magnetism of YbCp3 and LuCp3† Marcello Coreno,‡ Monica de Simone,§ Rosemary Coates,^ Mark S. Denning,4 Robert G. Denning,4 Jennifer C. Green,*,4 Charlene Hunston,^ Nikolas Kaltsoyannis,^ and Andrea Sella^ ‡

CNR-IMIP, GasPhase Beamline@Elettra and Monterotondo, 00016 Roma, Italy, §CNR-IOM, Laboratorio TASC, 34149 Trieste, Italy, ^Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, U.K., and 4Department of Chemistry, Oxford University, South Parks Road, Oxford OX1 3QR, U.K. Received March 28, 2010 Summary: The first examples of gas-phase photoelectron spectra of ytterbium and lutetium complexes are reported. Variable photon energy photoelectron spectroscopy of YbCp3 (Cp = η-C5H5) shows that f electron ionization gives ion states corresponding to both 4f12 and 4f13 configurations, whereas that of LuCp3 reveals only 4f13 ion states. Magnetic data on crystalline YbCp3 display Curie behavior between 20 and 305 K with a magnetic moment of 3.53 μB. Both the photoelectron spectra and the magnetic moment are consistent with the proposal that YbCp3 has a mixed configuration ground state with the unpaired spin residing both in an f orbital and on the cyclopentadienyl ligands. Photoelectron (PE) spectroscopy is a powerful probe of electronic structure in general and, more specifically, that of lanthanide-containing species. Ionization of the 4f valence electrons gives very characteristic final state structures that depend on the electronic ground state adopted by the 4fn configuration. Moreover varying the photon energy, as is possible with a synchrotron source, can identify, unambiguously, f ionization bands by their intensity variation.1 PE studies2 of CeCp3 (Cp = η-C5H5) suggested that the cation [CeCp3]þ has a multiconfigurational ground state with an admixture of 4f0 and 4f1 configurations, and the multiconfigurational character of this state was later confirmed by CASSCF/CASPT2 calculations.3 PE investigations of intermetallic compounds of Yb show complex 4f bands characteristic of both f12 and f13 configurations,4 which are interpreted as demonstrating mixed valency in these materials. However, there appear to be no similar gas-phase studies of Yb complexes. The formally Yb(II) complexes YbCp2L, where L is a non-innocent ligand such as bipyridyl and phenanthroline, have been shown to be paramagnetic, suggesting exchange coupling between a Yb(III) center and a radical † Part of the Dietmar Seyferth Festschrift. This paper is dedicated to Dietmar Seyferth in gratitude for his editorship of Organometallics. *To whom correspondence should be addressed. E-mail: jennifer. [email protected]. (1) Green, J. C.; Decleva, P. Coord. Chem. Rev. 2005, 249, 209. (2) Coreno, M.; de Simone, M.; Green, J. C.; Kaltsoyannis, N.; Narband, N.; Sella, A. Chem. Phys. Lett. 2006, 432, 17–21. (3) Coates, R.; Coreno, M.; DeSimone, M.; Green, J. C.; Kaltsoyannis, N.; Kerridge, A.; Narband, N.; Sella, A. Dalton Trans. 2009, 5943–5953. (4) Moreschini, L.; Dallera, C.; Joyce, J. J.; Sarraro, J. L.; Bauer, E. D.; Fritsch, V.; Bobev, S.; Carpene, E.; Huotari, S.; Vank o, G.; Monaco, G.; Lacovig, P.; Panaccione, G.; Fondacaro, A.; Paolicelli, G.; Torelli, P.; Grioni, M. Phys. Rev. B 2007, 75, 035113.

pubs.acs.org/Organometallics

Published on Web 06/18/2010

anion based on L.5 Such ambiguities in Yb electronic structure led us to investigate YbCp3 and for comparison LuCp3, where the absence of a hole in the 4f shell should lead to a straightforward description of the ground state. The valence region of the PE spectrum of LuCp3 was measured with a range of photon energies from 25 to 250 eV. Three spectra taken with incident photon energies of 30, 55, and 200 eV are shown in Figure 1. Vertical ionization energies (IE) are given in Table 1. The ligand-based bands A-E may be clearly distinguished from the f ionizations (2F7/2 and 2F5/2) by the changes in relative intensity.1,2 Assignment of the ligand bands follows that of CeCp3,2,3 where effective D3h symmetry may be assumed. The upper occupied Cp π orbitals transform as a20 þ a200 þ e0 þ e00 . The 5d orbitals are a symmetry match only for the degenerate representations e0 and e00 , whereas the 4f AOs can mix with both of the nondegenerate irreducible representations in addition to the e0 and e00 combinations. Band A is assigned to the a20 ionization and band B to the a200 , e0 , and e00 ionizations. The separation of the a20 band from the other Cp π bands arises because this combination of Cp π orbitals cannot mix with either the metal’s 5d or 6p orbitals (even in reduced C3 symmetry). An interesting comparison may be made with the analogous bands in the PE spectrum of CeCp3 (Table 1). For CeCp3 the a20 band has a higher IE (7.45 eV) compared with LuCp3 (7.28 eV) because the Ce 4f orbitals provide a symmetry match for, and are sufficiently radially extended to mix with, the ring a20 combination. In LuCp3 the 4f shell is full. By contrast for CeCp3 the other ring π orbitals have a lower IE (8.58 eV) compared with LuCp3 (8.81 eV). In this case the greater effective nuclear charge of the Lu makes the 5d and 6p orbitals more effective at covalent bonding with the ring orbitals. The cross section of Lu 4f ionizations is predicted to maximize at a photon energy around 150 eV6, while at this photon energy the cross sections of both C and H are very low. The PE spectrum at 200 eV (Figure 1) shows only two strong bands of relative intensity 4 to 3, which may be readily assigned to the 2F7/2 and 2F5/2 ion states of the 4f13 configuration of Lu4þ. It is noteworthy that no shake down bands associated with a 4f14 configuration are observable; these (5) Schultz, M.; Boncella, J. M.; Berg, D. J.; D., T. T.; Andersen, R. A. Organometallics 2002, 21, 460. (6) Yeh, J. J. Atomic and Nuclear Data Tables; AT & T, Gordon Breach: Langhorne, 1993. r 2010 American Chemical Society

Communication

Organometallics, Vol. 29, No. 21, 2010

4753

Figure 1. PE spectrum of LuCp3 with photon energies of 30, 55, and 200 eV. Table 1. IE of Valence and 4f Bands of YbCp3 and LuCp3a band (ion state)

YbCp3

LuCp3

CeCp32

A B C D E 2 F7/2 2 F5/2 f1 (3H6) f2 (3F4þ3H5) f3 (3H4þ3F3þ3F2) f4 (1G4) f5 (1D2) f6 (3P0þ3P1þ1I6þ3P2)

7.16 8.46 12.4 16.7 21.5 7.26 (1.2) 8.58 (1.0) 12.93 (4.6) 13.73 (5.4) 14.91 (3.2) 15.98 (0.05) 16.71 (0.83) 17.66 (3.3)

7.28 8.81 12.4 17.2 21.5 14.30 (1.3) 15.74 (1.0)

7.45 8.58

a

Ion states and relative intensities of the 4f bands are given in parentheses.

would result from filling the core hole. This contrasts with the CeCp3 PE spectrum, which we have interpreted as containing bands associated with both 4f0 and 4f1 configurations.2,3 The spectra of YbCp3, acquired with photon energies of 25, 56, and 240 eV (Figure 2a, Table 1), show a similar pattern of ligand ionizations and may be similarly assigned. Bands A and B have a lower IE than found for comparable bands of LuCp3, whereas bands C, D, and E have similar IE. The 4f bands of YbCp3, distinguished by their visibility at high photon energies, are much more complex than found for LuCp3. The structure shown by bands f1-f6 is characteristic of ionization to the states of an ion with a 4f12 ion state configuration, and the lower energy doublet, that with a 4f13 configuration. The latter may be directly assigned to 2F7/2 and 2F5/2 ion states, and the former region affords superior resolution to that achievable in solid-state studies. Calculations

Figure 2. (a) PES of YbCp3 with photon energies of 25, 56, and 240 eV. (b) Experimental PES spectrum in the IE range 10-20 eV (black line and right axis) and simulated relative intensities and energies of the Yb4þ final states (red bars and left axis) (details of the simulation are given in the SI).

based on scaling the relative energies of the states of Tm3þ for a 4þ charge give excellent agreement with the PE band positions (Figure 2b).7 The doublet splitting of the 2F7/2 and 2F5/2 ion states (1.32 eV) corresponds to that found for the 4f PE bands of Yb(η-C5Me5)28 and also to that (1.25 eV) in the optical spectra of single crystals of YbX63- (X = F, Cl, Br).9 Given the perturbative nature of 4f ionization in lanthanide complexes, the question arises as to whether the presence of both 4f12 and 4f13 signals in the PE spectrum is an initial or a final state effect, or both. In interpreting the PE spectrum of CeCp3 we proposed that access to final states (7) Thorne, J. R. G.; Zeng, Q.; Denning, R. G. J. Phys.: Condens. Matter 2001, 13, 7403–7419. (8) Andersen, R. A.; Boncella, J.; Green, J. C.; Burns, C. J.; Hohl, D.; R!sch, N. J. Chem. Soc., Chem. Commun. 1986, 405. (9) Zhou, X.; Reid, M. F.; Faucher, M. D.; Tanner, P. A. J. Phys. Chem. B 2006, 110, 14939–14942.

4754

Organometallics, Vol. 29, No. 21, 2010

from both a 4f0 configuration and a 4f1 configuration on f ionization was caused by extensive configuration interaction in the CeCp3þ ion state.2 Subsequent high-level quantum chemical calculations3 showed that the ground and key excited states of CeCp3þ are indeed multiconfigurational, though instabilities in the configurational admixtures as a function of the computational state-averaging procedure precluded firm conclusions as to the nature of the contributing configurations. By contrast, the PE spectrum of LuCp3 shows no trace of a 4f14 ion state on f ionization, suggesting that by the end of the lanthanide series the core hole has become uncoupled from the ligand valence electrons. In addition, the propensity of Yb to form divalent complexes supports the idea of an effect in the initial state, i.e., the neutral molecule, in this case, and we propose that YbCp3 has a mixed configuration ground state with contributions from |Lf13æ and |L-1f14æ, where L represents a full ligand shell and L-1 a hole in that shell. In compounds of the type Cp*2Yb(bipy), there is magnetic and spectroscopic evidence that both the |f14(bipy)æ and |f13(bipy)-æ configurations are represented in the ground state.10 If comparison is made with YbCp3, Cp* is more electron donating than Cp, so will raise relatively the energy of the 4f orbitals; however, this is offset by the more reducing character of the bipy anion relative to that of the Cp anion, so the presence of configurations representing a similar admixture of Yb(f14) and Yb(f13) appears plausible. Contributions from configurations of the type |L-1fnþ1æ are necessary to give a good account of anomalies in the energies of the states of TmCl63- 11 as well as in those of YbX63- (X = F, Cl, Br). In the former case the admixture of |L-1f13æ states is typically 0.5-4%. The extent of admixture of a |L-1fnþ1æ configuration is inversely proportional to its energy separation from |4fnæ, which for TmCl63 is ∼4.8 eV. For comparison ligandto-metal charge-transfer (LMCT) transitions indicate that the equivalent interval for YbBr63- is ∼3.0 eV. The intense green color of YbCp3 is due to a LMCT transition, which is found at 1.89 eV in benzene solution.12 Thus the admixture of the |L-1f14æ configuration should be much larger than in the halide complexes discussed above. Such a mixed configuration ground state should be manifest in the magnetic susceptibility. Birmingham and Wilkinson report a value of 4.0 μB for μeff at 77, 194, and 295 K and note no dependence of the magnetic moment on field strength.13 This value is significantly lower than 4.54 μB, which is calculated for a free Yb3þ ion with a 2F7/2 ground state. Unusually for the unsubstituted tris-cyclopentadienyl lanthanides, YbCp3 has a molecular structure in the solid state,14 and thus the solidstate magnetism should be a good guide to the gas-phase electronic structure. The magnetic susceptibility of powdered crystalline YbCp3 was measured between 2 and 305 K at a flux density of 0.01 T. The zero-field-cooled and field-cooled magnetization and molar susceptibility showed identical traces, confirming the absence of any cooperative interaction between the paramagnetic Yb centers. Figure 3a is a plot of χmol vs 1/T in (10) Booth, C. H.; Walter, M. D.; Daniel, M.; Lukens, W. W.; Andersen, R. A. Phys. Rev. Lett. 2005, 95, 267202. (11) Faucher, M. D.; Tanner, P. A.; Mak, C. S. K. J. Phys. Chem. A 2004, 108, 5278–5287. (12) Schesener, C. J.; Ellis, A. B. Organometallics 1983, 2, 529–534. (13) Birmingham, J. M.; Wilkinson, G. J. Am. Chem. Soc. 1956, 78, 42. (14) Eggers, S. H.; Kopf, J.; Fischer, R. D. Acta Crystallogr. C 1987, 43, 2288–2290.

Coreno et al.

Figure 3. Plot of χmol vs 1/T for YbCp3 (a) in the range 20300 K and (b) in the range 4-300 K.

the range 20-300 K, which shows excellent agreement with the Curie law over the entire temperature range and implies an associated magnetic moment of 3.53 μB. The susceptibilities of LnCp3 compounds at 300 K match the free ion values closely for the majority of Ln3þ.9 However the magnetic moment of Yb3þ in magnetically dilute materials usually decreases markedly at low temperature, due to depopulation of the various crystal field components of the 2 F7/2 free ion state. For example the moment in a Yb(β-ketoiminate)3 complex decreases from 4.41 μB at 295 K to 3.51 μB at 5.17 K.15 The susceptibility of the octahedral YbCl63- ion in Cs2NaYbCl6 has been analyzed in detail. Below 10 K the moment approaches that of the Γ6 ground state (2.24 μB), and although the first excited state Γ8 is 250 cm-1 above the ground state and its population is negligible at 25 K, the second-order (Van Vleck) contribution to the susceptibility is sufficient to cause a substantial increase in the apparent moment even at this low temperature.16 In the stronger field of oxide ligands in Yb3Ga5O12 the crystal field splitting is inverted and the first excited state is at 550 cm-1. Between 30 and 100 K the moment is found to be 2.94 μB, but above this temperature the Curie plot becomes nonlinear and the apparent moment rises to 4.27 μB at 300 K. Both first-order and second-order Zeeman effects contribute to this increase.17 Against this background the linearity of the Curie plot between 20 and 300 K in Figure 3a is remarkable (see also the (15) Rees, W. S.; Just, O.; Castro, S. L.; Matthews, J. S. Inorg. Chem. 2000, 39, 3736–3737. (16) Dunlap, B. D.; Shenoy, G. K. Phys. Rev. B 1975, 12, 2716–2724. (17) Brumage, W. H.; Lin, C. C.; Van Vleck, J. H. Phys. Rev. 1963, 132, 608–610.

Communication

implied moments as a function of temperature in Figure S2). It is also striking that the dimeric compound Cp3Yb(pyrazine)YbCp3, which is described as green-brown in color, has a moment of 3.48 μB in the range 3-100 K and also shows little deviation from the Curie law.18 (A slight curvature in the 1/χ vs T plot in this work is likely to be due to the neglect of the increasingly important contribution of the temperatureindependent part of the susceptibility at high temperatures.) Structurally this pyrazine-bridged dimer is closely related to YbCp3 in that the centroids of the Cp rings and the Yb atom are approximately coplanar in both cases, while their magnetic properties also suggest that critical features of their electronic structure are also comparable. The insensitivity of the moment in Figure 3a to temperature can be interpreted in one of two ways. Any crystal field components are separated either by energies , kT at 20 K or, alternatively, by energies . kT at 300 K. Given the low value of the magnetic moment and the low symmetry of the site, the former hypothesis appears untenable. We are therefore forced to conclude that any excited states must be located at such high energies that their thermal population is negligible at 300 K. If the perturbation of the free ion states of the f13 ion were to be attributed to the anisotropy in the potential provided by the Cp- ligands, it is hard to see how a single diffuse negative charge on each of the three ligands could lead to a crystal field splitting that must greatly exceed that in an oxide lattice. On the other hand, anisotropic perturbations of the appropriate magnitude, leading to a reduced moment, would arise naturally if the ground-state wave function including a strong admixture of the |L-1f14æ charge-transfer configuration, within which the orbital angular momentum would be largely quenched. At 20 K there is a sharp discontinuity in the plot of χmol vs 1/T, followed by a substantial departure from linearity in the region from 20 to 2 K (Figure 3b). No saturation of the (18) Baker, E. C.; Raymond, K. N. Inorg. Chem. 1977, 11, 2710–2714. (19) Wong, C.; Lee, T.; Lee, Y Acta Crystallogr. B 1969, 25, 250. Eggers, S. H.; Hinrichs, W.; Kopf, J.; Jahn, W.; Fischer, R. D. J. Organomet. Chem. 1986, 311, 313–323.

Organometallics, Vol. 29, No. 21, 2010

4755

susceptibility should occur at 0.01 T, even at 2 K. Nor is the 20 K anomaly compatible with a continuous change in the thermal population of the crystal field components of a 2 F7/2 state. In the absence of any evidence of cooperative magnetic order, we propose that the anomaly is caused by a structural phase change at 20 K, which influences the relative weight of the f13 and f14 configurations in the ground state and therefore its magnetic moment. The potential for structural instability at low temperature is suggested by the fact that YbCp3 has a different crystal structure from both SmCp3 and LuCp3.19 In conclusion, both the PE spectra and the magnetic measurements strongly support a mixed configuration ground state for YbCp3 with the unpaired spin residing predominantly in the Yb f shell but also having a significant probability of being found on the cyclopentadienyl ligands.

Experimental Section YbCp3 and LuCp3 were prepared by the method of Birmingham and Wilkinson and sublimed at 200 C/0.1 mmHg to remove coordinated THF.11 PE measurements were carried out at the gasphase photoemission beamline of the ELETTRA storage ring in Trieste, Italy. The samples were held in an oven close to the ionization region and evaporated through a small hole by holding the temperature of the oven at around 161 C. Magnetic susceptibility measurements were recorded on a Quantum Design MPMS XL magnetometer. The sample was sealed in a specially constructed cylindrical 5 mm o.d. suprasil quartz tube. The magnetization of the sample was measured as a function of temperature between 2 and 305 K in an applied magnetic field of 100 Oe. The susceptibilities were corrected for diamagnetic contributions using Pascal’s constants.

Acknowledgment. We thank UCL for a Teaching Assistantship to R.C. and a DTA to C.H. Supporting Information Available: Method for calculating relative ion state energies for Yb4þ, detailed experimental procedures, and plot of μeff vs T. This material is available free of charge via the Internet at http://pubs.acs.org.