1368
Organometallics 2010, 29, 1368–1373 DOI: 10.1021/om901007f
Parametric Analysis of the Crystal Field Splitting Pattern of Pr(η5-C5Me5)3† Hanns-Dieter Amberger,*,‡ Hauke Reddmann,‡ Thomas J. Mueller,§ and William J. Evans§ ‡
Institut f€ ur Anorganische und Angewandte Chemie der Universit€ at Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany, and §Department of Chemistry, University of California, Irvine, California 92697-2025 Received November 19, 2009
By comparing the FIR, MIR, NIR, and vis spectra (pellets) of PrCp*3 (Cp* = η5-C5Me5) (1) with those of LaCp*3 (2) the energies of a number of crystal field (CF) levels of 1 could be determined. The levels of the ground multiplet 3H4 could be assigned on the basis of the polarized luminescence transition 13Γ6(1D2)f3H4 of an oriented single crystal of 1, and the CF levels of the excited multiplets by assuming equal sequences of CF levels for 1 and those of Pr(C5Me4H)3 (3) assigned previously on the basis of linear dichroism, luminescence anisotropy, and electronic Raman measurements. The free parameters of a phenomenological Hamiltonian were fitted to the thus derived truncated CF splitting pattern of 1, leading to a reduced rms deviation of 10.2 cm-1 for 18 assignments. On the basis of these phenomenological CF parameters, the global CF strength experienced by the Pr3þ central ion was estimated and seems to be the third largest one ever encountered in PrIII chemistry. The obtained nephelauxetic parameter beta and the relativistic nephelauxetic parameter beta0 allow the insertion of compound 1 into empirical nephelauxetic and relativistic nephelauxetic series, respectively, of PrIII compounds. The experimentally based nonrelativistic molecular orbital scheme (in the f range) of complex 1 was determined and compared with the results of a previous XR-SW calculation on the pseudo-trigonal-planar model compound Pr(η5-C5H5)3.
1. Introduction Three decades ago, the leading experts in parametric analyses of optical data explained the missing progress in understanding the electronic structures of f element organometallics by the lack of single crystals suitable for recording easily interpretable linear dichroism (LD),1 luminescence anisotropy (LA),2 magnetic circular dichroism (MCD),3 polarized electronic Raman (PER)4 spectra, and paramagnetic susceptibility measurements parallel as well as perpendicular to the principal molecular axis5 of highly symmetrical homoleptic complexes.6 † Part 72 of the series “Electronic Structures of Organometallic Compounds of f Elements”. For part 71, see: Amberger, H.-D.; Reddmann, H.; Evans, W. J. Inorg. Chem. 2009, 49, 10811. *To whom correspondence should be addressed. E-mail: fc3a501@ uni-hamburg.de. (1) Rodger, A.; Norden, B. Circular Dichroism and Linear Dichroism; Oxford University Press: Oxford, 1997, and references therein. (2) Feofilov, P. P. The Physical Basis of Polarized Emission: Polarized Luminescence of Atoms, Molecules, and Crystals; Fizmatgiz: Moscow, 1959; Consultants Bureau, New York, 1961, and references therein. (3) Piepho, S. B.; Schatz, B. N. Group Theory in Spectroscopy (With Applications To Magnetic Circular Dichroism); John Wiley & Sons, Inc.: New York, 1983, and references therein. (4) Koningstein, J. A.; Mortensen, O. S. Electronic Raman Transitions. In The Raman Effect; Anderson, A., Ed.; Dekker: New York, 1973; Vol. 2, p 519, and references therein. (5) Lueken, H. Magnetochemie; B. G. Teubner: Stuttgart, Leipzig 1999. (6) Carnall, W. T.; Edelstein, N. M. Personal communication in the frame of the Advanced Study Institute Organometallics of the f-Elements, Sogesta, Urbino, Italy, September 11-22, 1978.
pubs.acs.org/Organometallics
Published on Web 02/15/2010
In order to apply successfully the above-mentioned methods, the principal axes of the individual molecules in the unit cell have to be aligned not only to each other but also to the macroscopic axis of a rod-like single crystal, thus allowing an easy orientation of the principal molecular axis to outer vector or homogeneous fields. In the middle of the 1980s, when we started our work on electronic structures of organometallics of f elements, the required single crystals were not available, but we could identify a number of crystal field (CF) levels by applying the technique of MCD to solutions of monoadducts of the moiety tris(η5-cyclopentadienyl)praseodymium(III).7,8 Some years later, these assignments could be confirmed by LD measurements of oriented single crystals.9,10 Now, the originally required material is available. Single crystals of commercially available and easy crystallizing Ln(η5-C5Me4H)311-13 (Ln = La-Sm, Eu-Tb; space group R3, two molecules of molecular D3h symmetry per unit cell, rhombohedric setup) and more difficult to synthesize (7) Amberger, H.-D.; Jahn, W. Spectrochim. Acta 1984, 40A, 1025. (8) Amberger, H.-D.; Jahn, W.; Edelstein, N. M. Spectrochim. Acta 1985, 41A, 465. (9) Amberger, H.-D.; Schulz, H. Spectrochim. Acta 1991, 47A, 233. (10) Schulz, H.; Amberger, H.-D. J. Organomet. Chem. 1993, 443, 71. (11) Schumann, H.; Glanz, M.; Hemling, H. J. Organomet. Chem. 1993, 445, C1. (12) Schumann, H.; Glanz, M.; Hemling, H.; Hahn, F. E. Z. Anorg. Allg. Chem. 1995, 621, 341. (13) Evans, W. J.; Rego, D. B.; Ziller, J. W. Inorg. Chem. 2006, 45, 10790. r 2010 American Chemical Society
Article
Organometallics, Vol. 29, No. 6, 2010
LnCp*314-17 (Cp* = η5-C5Me5; Ln = La-Sm, Gd; space group P63/m, two molecules of somewhat distorted molecular D3h symmetry per unit cell) have the requested structural properties and crystallize as rods. In recent years, the truncated CF splitting patterns of pseudo-trigonal-planar Ln(η5-C5Me4H)3 complexes (Ln = Pr,18 Nd,19,20 Sm21) could be derived on the basis of LD, LA, MCD (preliminary only solutions), and PER spectra of partly oriented large single crystals and simulated by fitting the free parameters of a phenomenological Hamiltonian. In the case of LnCp*3 complexes, only smaller single crystals are presently available, which allow the recording of LA and PER but not of LD spectra with commercial instruments. The optical spectra of powdered Sm(C5Me4H)321 and SmCp*3 are not too different;22 hence we assume that this also holds for the other pairs of compounds Ln(C5Me4H)3/ LnCp*3. Accepting this assumption, the CF levels of PrCp*3 (1) (additional signals in the FIR, MIR, NIR, and vis spectra of pellets as compared to LaCp*3 (2)) can be identified by correlating these signals with the already assigned ones of Pr(C5Me4H)3 (3).18 Some of these correlations we hope to prove experimentally by LA measurements of small oriented single crystals of 1. Subsequently, it is planned to fit the free parameters of a phenomenological Hamiltonian to the thus derived CF splitting pattern of 1. The parameters obtained are discussed here in terms of CF strength and nephelauxetic and relativistic nephelauxetic effects and used to set up an experimentally based nonrelativistic molecular orbital (MO) scheme (in the f range), which is compared with the results of a previous XR-SW calculation on the pseudo-trigonalplanar model compound Pr(η5-C5H5)3.23
the recently developed Senterra instrument (Bruker) equipped with a microscope and lasers with exciting lines at 785, 632.8, and 532 nm was available. Polarizers and analyzers can be set to positions pa/pa, pa/pe = pe/pa (in the case of axially symmetric systems) and pe/pe, where pa and pe mean parallel and perpendicular to the macroscopic axis of the rod-like single crystals of 1 and 2. At the present stage, the relative orientations of the (antiparallel directed) trigonal principal axes of the two pseudo-trigonal-planar PrCp*3 molecules (see Figure SI-1) per unit cell to the rod axis are not known from X-ray analyses. A comparison of the polarizations of the vibrational Raman spectra of small oriented single crystals of LnCp*3 (Ln = La, Pr, Nd, Sm) with those of Ln(C5Me4H)3 (Ln = La,25 Pr,18 Nd,20 Sm25) shows close relations.22,26 The same also holds for the polarized luminescence spectra of NdCp*326 and Nd(C5Me4H)3.20 In case of the latter class of compounds, the 3-fold axes are parallel to the rod axes. Thus, we assume that this also holds for LnCp*3.
3. Results and Discussion 3.1. Phenomenological Hamiltonian and Selection Rules. The energy levels within the f2 configuration in D3h symmetry can be written in terms of the atomic free ion (HFI) and crystal field (HCF) Hamiltonians as follows:
H ¼ H FI þ H CF where
H FI ¼
X k ¼0, 2, 4, 6
f k F k ðnf, nfÞ þ aso ζ4f þ RLðL þ 1Þ
(14) Evans, W. J.; Gonzales, S. L.; Ziller, J. W. J. Am. Chem. Soc. 1991, 113, 7423. (15) Evans, W. J.; Seibel, G. A.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 6745. (16) Evans, W. J.; Perotti, J. M.; Kozimor, S. A.; Champagne, T. M.; Davis, B. L.; Nyce, G. W.; Fujimoto, C. H.; Clark, R. D.; Johnston, M. A.; Ziller, J. W. Organometallics 2005, 24, 3916, and references therein. (17) Evans, W. J.; Davis, B. L; Champagne, T. M.; Ziller, J. W. Proc. Natl. Acad. Sci. U.S.A. 2006, 34, 12678. (18) Amberger, H.-D.; Reddmann, H. Z. Anorg. Allg. Chem. 2008, 634, 1542. (19) Amberger, H.-D.; Reddmann, H. Z. Anorg. Allg. Chem. 2007, 633, 443. (20) Amberger, H.-D.; Reddmann, H. Z. Anorg. Allg. Chem. 2009, 635, 291. (21) Amberger, H.-D.; Reddmann, H. J. Organomet. Chem. 2007, 692, 5103. (22) Amberger, H.-D.; Reddmann, H.; Evans, W. J. Inorg. Chem. 2009, 49, 10811. (23) Strittmatter, R. J.; Bursten, B. E. J. Am. Chem. Soc. 1991, 113, 552. (24) Evans, W. J.; Davis, B. L.; Ziller, J. W. Inorg. Chem. 2001, 40, 6341.
X
þ βGðG2 Þ þ γGðR7 Þ þ
mk M k þ
k ¼0, 2, 4
2. Experimental Section Small single crystals of orange 116 and yellow 224 were synthesized at Irvine according to the literature. The absorption spectra in the FIR (polyethylene pellets), MIR (KBr pellets), and NIR/vis (KBr pellets) ranges were recorded by means of Vertex 70 (Bruker), FT-IR 1720 (Perkin-Elmer), and Cary 5e (Varian) instruments, respectively. The latter two can be combined with the Helitran LT-3-110 (Air Products) transfer cryostat and the Cary 5e apparatus additionally with a bath cryostat, using liquid N2 or liquid He as coolant. For running the Raman spectra (powdered material and single crystals sealed in glass tubes),
1369
X k ¼2, 4, 6
pk P k
(see ref 27 (p 167)) and ð2Þ
ð4Þ
ð6Þ
ð6Þ
ð6Þ
H CF ðD3h Þ ¼ B20 C 0 þ B40 C 0 þ B60 C 0 þ B66 ðC -6 þ C 6 Þ (see ref 27 (p 242)). The Fk(nf,nf)s and ζ4f represent, respectively, the radial parts of the electrostatic and spin-orbit interactions between f electrons, while fk and aso are the angular parts of these interactions. R, β, and γ are the parameters associated with the two-body effective operators of configuration interaction, G(G2) and G(R7) being the Casimir operators of the groups G2 and R7, and L is the orbital angular momentum. The Mk parameters represent the spin-spin and spin-otherorbit interactions, while the Pk parameters arise from electrostatic spin-orbit interactions with higher configurations, with mk and pk being the corresponding operators. The CF interaction for the above symmetry is represented by the Bkq parameters and the tensor operators C(k) q (see ref 27 (p 170)). The selection rules for induced electric dipole transitions of an oriented single crystal of 1 are given in Table 1. Assuming for compound 1;like 3;a CF ground state of Γ1 symmetry, only absorption transitions to excited CF (25) Amberger, H.-D.; Reddmann H. Unpublished results. (26) Amberger, H.-D.; Reddmann, H.; Mueller, T. J.; Evans, W. J. In preparation. (27) G€ orller-Walrand, C.; Binnemans, K. Rationalization of Crystal Field Parametrization. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., Eyring L., Eds.; Elsevier Science B.V.: Amsterdam, 1996; Vol. 23, Chapter 155.
1370
Organometallics, Vol. 29, No. 6, 2010
Amberger et al.
Table 1. Selection Rules for Forced Electric Dipole Transitions of Oriented Single Crystals of fn Systems (n even) in a CF of D3h Symmetry D3h
Γ1
Γ2
Γ3
Γ4
Γ5
Γ6
Γ1 Γ2 Γ3 Γ4 Γ5 Γ6
π R, σ
π R, σ
π R, σ -
π R, σ -
R, σ R, σ R, σ π
R, σ R, σ π R, σ
a
a R, σ, π mean the respective transition is allowed in the R, σ, π spectrum. For powdered material and solutions, only the transitions labeled - are forbidden.
states of Γ4 and Γ6 symmetry are allowed in the case of powdered or dissolved material (see Table 1). 3.2. Experimentally Derived CF Splitting Pattern of PrCp*3. Ground Multiplet 3H4. The CF splitting pattern of the ground multiplet 3H4 (Γ1þΓ3þΓ4þΓ5þ2Γ6 in the case of pseudo-trigonal-planar 1, see ref 27 (p 262)) of PrIII compounds is usually extracted from the energetically isolated “hot” transition 3H4f3P0.7-10,28 This CF splitting pattern is frequently confirmed by the inverse luminescence transition 3P0f3H4.7-10,28 Because of the positive inductive effect of the five methyl groups per ligand, compound 1 has an extremely low charge transfer band, which hampers not only the observation of the absorption transitions to the higher multiplets 3P0, 3P1, 1I6, 3P2, and 1S0 but also the inverse luminescence transitions. A more difficult alternative is to derive a usually truncated CF splitting pattern of 3H4 from the energetically isolated but spin-forbidden and thus weak “hot” transition 3 H4f1D2.18 Because of more than one purely electronic state of 1D2, here groups of “hot” transitions are superimposed and their separation is not always an easy task. At low temperatures, compound 1 exhibits in the range 15 000-18 000 cm-1 essentially only one absorption peak at 16 580 cm-1 (see Figure SI-2), which has to be assigned to the only symmetry-allowed “cold” absorption transition 1Γ1(3H4)f13Γ6(1D2), assuming 1 and 3 have the same totally symmetric CF ground state. At room temperature, this band is shifted to 16 575 cm-1 and additional signals at 16 410 and 16 102 cm-1 appear (see Figure SI-2), indicating excited CF levels at ca. 165 and ca. 473 cm-1 above the CF ground state, which have to be correlated in analogy to 3 with the initial levels 1Γ5 and 1Γ6, respectively. The correlation of the level at 165 cm-1 with 1Γ5 is confirmed by the observation of a PER signal at 165 cm-1 (the intensity of which increases strongly with the energy of the exciting line, as in the case of 3, ref 20 (Figure 5)) with dominant polarizations pe/pa and pe/pe, which is in accordance with the selection rules of the PER Γ1fΓ5.29 Likewise, the existence of a level at 473 cm-1 (of Γ4 or Γ6 symmetry) is proven by the appearance of a signal at 473 cm-1 in the FIR spectrum of 1, which is missing in the case of 2 (see Figure SI-3). At low temperatures, signals of f-f transitions gain intensity compared to those of vibrations,30 thus frequently allowing a separation of both types (28) Amberger, H.-D.; Y€ unl€ u, K.; Edelstein, N. M. Spectrochim. Acta 1986, 42A, 27. (29) G€ achter, B. F. J. Mol. Spectrosc. 1976, 63, 1, and references therein. (30) Amberger, H.-D.; Jank, S.; Reddmann, H.; Edelstein, N. M. Mol. Phys. 1997, 90, 1013.
Figure 1. Luminescence spectrum of PrCp*3 in the range 15 250-16 750 cm-1 (oriented single crystal, room temperature, exciting line at 532 nm): (a) σ spectrum; (b) π spectrum.
of signals. Going from the room-temperature FIR to the 90 K MIR spectrum, the presumable f-f band originally at 473 cm-1 increases in intensity and is weakly shifted to 470 cm-1 (see Figure SI-4). In contrast to compound 2, 1 exhibits a strong band at 1252 cm-1 in the low-temperature MIR spectrum. According to the selection rules (see Table 1), it has to be correlated with a transition to a terminal state of Γ4 or Γ6 symmetry. A closer experimental assignment is perhaps possible on the basis of polarized luminescence measurements of an oriented single crystal of 1. Using the laser line at 514.5 nm of an Arþ laser or the frequency-doubled line at 532 nm of a YAG:Nd3þ laser, the resulting Raman spectrum of 1 is dominated by a group of signals between ca. 15 200 and 16 600 cm-1, which has to be correlated with the luminescence transition 13Γ6(1D2)f3H4. In Figure 1, the σ (polarizer and analyzer perpendicular to the 3-fold molecular axis) and π (polarizer and analyzer parallel to the 3-fold molecular axis) spectra (of an oriented single crystal) of this transition are shown, but in addition to the only expected purely electronic transition 13Γ6 (1D2)f1Γ5(3H4) in the π and three (13Γ6(1D2)f1Γ1(3H4), 13Γ6(1D2)f1Γ6(3H4), 13Γ6(1D2)f2Γ6(3H4)) in the σ spectrum (see Table 1) some additional signals appear. Obviously this is due to polarization leaks, vibronic sidebands, and νCH vibrations (see Figure 1). In order to minimize the leaks, the single crystal was turned by some degrees in various directions, but the leaks only increased, indicating that they are not produced by misorientation but by an imperfect single crystal or a descent of molecular symmetry, which was concluded from X-ray measurements.16 The same procedure was repeated with different single crystals, leading to the same results; thus the latter possibility seems to hold. However, the polarization leaks of 3 are less pronounced18 than those of 1, although the maximal difference of Pr-C bonding distances is 0.1397 A˚13 compared to 0.107 A˚ for 1.16 The strong luminescence signal at 16 575 cm-1 in the σ spectrum corresponds to the inverse of the absorption transition 1Γ1(3H4)f13Γ6(1D2) (vide supra), and the two extremely strong bands at 16 102 and 15 320 cm-1 correspond to the symmetry-allowed transitions initiating at 13Γ6(1D2) and terminating at 1Γ6(3H4) and 2Γ6(3H4), respectively, lying 473 and 1255 cm-1 above the CF ground state. This confirms the identification of the CF level at 473 cm-1 and is consistent with the assignment of the level at
Article
1252 cm-1 concluded from an f-f transition in the 90 K MIR spectrum. Apart from the above-mentioned polarization leaks, a strong signal at 16 410 cm-1 appears in the π luminescence spectrum, which has to be correlated with the symmetry-allowed transition 13Γ6(1D2)f1Γ5(3H4) (see Table 1), 165 cm-1 above the ground state. Hence the level at 165 cm-1 concluded from the “hot” absorption transition 3H4f13Γ6(1D2) and the PER Γ1fΓ5 is confirmed additionally. Obviously, compounds 1 and 318 have the same sequence of CF levels of the ground mutliplet 3H4, but the total splitting of 1357 cm-1 of compound 313 is noticeably larger than that of 1 (1252 cm-1). Excited Multiplets. The CF splitting patterns of the lowlying multiplets 3H5 and 3H6 of organometallic PrIII compounds are usually extracted from luminescence transitions originating at 3P0 and terminating at the CF levels of these multiplets,28,31 whereas those of the higher multiplets are derived from absorption measurements making use of a vis/ NIR instrument.7,8,32,33 As 3P0 of compound 1 is not fluorescing, the CF splitting patterns of the above-mentioned lowlying multiplets have to be derived from f-f transitions in the MIR range, which, however, as yet have not been observed in the case of PrCp3B (B = THF, MeTHF, CH3CN, CNC6H11) adducts, even at ca. 30 K.34 Indeed, comparing the 90 K MIR spectra (KBr pellets) of 1 and 2, the former compound exhibits additional signals at 2441, 2616, 3049, and 4530 cm-1 (see Figure SI-5), which have to be correlated;in analogy with compound 3;with the symmetry-allowed transitions to the excited CF levels 3Γ6(3H5), 2Γ4(3H5), 4Γ6(3H5), and 5Γ6(3H6), respectively (see Table 2). In the range 5000-5400 cm-1 of the 77 K NIR spectrum of 1 one strong (5385 cm-1), one weak (5070 cm-1), and one very weak band (5138 cm-1) appear, which are correlated with transitions to the CF states 7Γ6(3F2), 6Γ6(3H6), and 3Γ4(3H6). The five expected symmetry-allowed transitions to the multiplets 3F3 and 3F4 are observed (6905, 7010, 7335, 7385, and 7434 cm-1) and are assigned;in analogy with compound 3;to transitions terminating at 8Γ6(3F3), 4Γ4(3F3), 9Γ6(3F4), 10Γ6(3F4), and 5Γ4(3F4) (see Figure SI-6). At 77 K, the spin-forbidden and therefore weak transition 3 H4f1G4 shows, instead of the three expected, only one band at 9996 cm-1, which is correlated with a transition to 11Γ6(1G4) (see Table 2). The low- and ambient-temperature spectra of the transition 3H4f1D2 already have been discussed (vide supra). Compared to compound 3, the CF levels of 1 have lower energies in the FIR and MIR ranges (see Figure SI-6) but higher ones in the vis range (see Figure SI-2). This is an experimental hint that the lower CF strength of 1 (vide supra) is overcompensated in the vis range by an increase of Slater parameters (compared to 3). 3.3. Simulation of the Experimental CF Splitting Pattern. The free parameters of a phenomenological Hamiltonian (vide supra) were fitted to the above assigned CF splitting (31) Jank, S.; Amberger, H.-D.; Edelstein, N. M.; Qian, C.; Wang, B. Spectrochim. Acta 1996, 52A, 429. (32) Jank, S.; Amberger, H.-D.; Reddmann, H. J. Alloys Compd. 1997, 250, 387. (33) Amberger, H.-D.; Edelmann, F. T.; Gottfriedsen, J.; Herbst-Irmer, R.; Jank, S.; Kilimann, U.; Noltemeyer, M.; Reddmann, H.; Sch€afer, M. Inorg. Chem. 2009, 48, 760. (34) Amberger, H.-D.; Schultze, H.; Schulz, H. Unpublished results.
Organometallics, Vol. 29, No. 6, 2010
1371
Table 2. Comparison of the Fitted and Experimental CF Splitting Pattern (in cm-1) of PrCp*3 multiplet 3
H4 H4 H4 3 H4 3 H4 3 H4 3 H5 3 H5 3 H5 3 H5 3 H5 3 H5 3 H5 3 H6 3 H6 3 H6 3 H6 3 H6 3 H6 3 H6 3 F2 3 F2 3 H6 3 F2 3 H6 3 F3 3 F3 3 F3 3 F3 3 F3 3 F4 3 F4 3 F4 3 F4 3 F4 3 F4 1 G4 1 G4 1 G4 1 G4 1 G4 1 G4 1 D2 1 D2 1 D2 3 P0 3 3
a
CF level b
1Γ1 1Γ5 1Γ6 1Γ3 1Γ4 2Γ6 1Γ2 2Γ5 3Γ6 2Γ4 2Γ3 4Γ6 3Γ5 2Γ1 4Γ5 5Γ6 3Γ3 6Γ6 5Γ5 3Γ4 3Γ1 7Γ6 2Γ2 6Γ5 4Γ1 4Γ3 8Γ6 7Γ5 4Γ4 3Γ2 9Γ6 10Γ6 8Γ5 5Γ1 5Γ4 5Γ3 11Γ6 6Γ4 9Γ5 6Γ1 12Γ6 6Γ3 13Γ6 10Γ5 7Γ1 8Γ1
Ecalc c
0 (1 (2 þ3 -3 -4 0 (1 (2 þ3 -3 -4 -5 0 (1 (2 þ3 -4 -5 -3 0 (2 -6 (1 þ6 -3 (2 (1 þ3 0 (2 -4 (1 0 -3 þ3 -4 -3 (1 0 (2 þ3 (2 (1 0 0
0 174 471 527 1015 1252 2174 2252 2449 2620 2979 3042 3301 4209 4317 4534 4653 5069 5128 5132 5170 5364 5472 5730 6051 6395 6916 6979 7012 7264 7341 7395 7396 7422 7428 7471 9992 10 202 10 248 10 341 10 583 11 056 16 580 17 114 17 344 20 279
Eexp
ΔE
0 165d 470
0 9 1
1252
0
2441 2616
8 4
3049
-7
4530
4
5070
-1
5138
-6
5385
-21
6905
11
7010
2
7335 7385
6 10
7434
-6
9996
-4
16 580
0
a Dominating Russell-Saunders multiplet 2Sþ1LJ. b The Bethe Γ notation for the double group D’3h is used. The irreps Γi are ordered in ascending energy. c Largest eigenvector component (MJ. “þM” √or “-M” are shorthand √ for the linear combinations (|þMæ þ |-Mæ)/ 2 and (|þMæ - |-Mæ)/ 2, respectively. d From the room-temperature spectrum.
pattern of complex 1 (see Table 2). As starting parameter set, that of 3 was used. In order to reduce the number of free parameters, the values of R, β, γ, the Pk, and the Mk parameters were fixed at the values used for 3.18 After some fitting cycles, a reduced rms deviation (see ref 27 (p 167)) of 10.2 cm-1 for 18 assignments was achieved. In Table 3, the final set of parameters is compared with those of complex .21 3,18 SmCp*3,22 and Sm(C5Me4H)3P The parameter Nv/(4π)1/2 = [ k,q(Bkq)2/(2k þ 1)]1/2 is considered as a relative measure of the CF strength experienced by the central Ln3þ ion.36 Inserting the CF parameters of 1 into this relation, one ends up with an Nv/(4π)1/2 value of (35) Carnall, W. T.; Crosswhite, H.; Crosswhite, H. M. Energy Level Structure and Transition Probabilities in the Spectra of the Trivalent Lanthanides in LaF3; ANL Report; 1977, unpublished; Appendix I, Table 1. (36) Auzel, F.; Malta, O. L. J. Phys. (Paris) 1983, 44, 201.
1372
Organometallics, Vol. 29, No. 6, 2010
Amberger et al.
Table 3. Parameter Values (in cm-1) for Pseudo-Trigonal-Planar PrCp*3, Pr(C5Me4H)3, SmCp*3, and Sm(C5Me4H)3 parameter F2 F4 F6 ζ4f R β γ T2 T3 T4 T6 T7 T8 M0 M2 M4 P2 P4 P6 B20 B40 B60 B66 Nv/(4π)1/2 sigma
PrCp*3 66 569 49 886 33 439 746 [23.1]d [-757] [1534]
Pr(C5Me4H)3a
SmCp*3b
Sm(C5Me4H)3c
65 789 48 057 32 772 745 [23.1] [-757] [1534]
71 817 56 751 36 292 1129 [21.6] [-724] [1700] [291] [13] [34] [-193] [288] [330] [2.4] [1.34] [0.91] [341] [256] [171] -2741 1341 1556 -2626 1717 15.0 (21)
72 824 57 547 36 801 1130 [21.6] [-724] [1700] [291] [13] [34] [-193] [288] [330] [2.4] [1.34] [0.91] [341] [256] [171] -2971 1304 1433 -2765 1813 19.1 (19)
e
[1.76] [0.99] [0.67] [275] [206] [138] -2293 811 1051 -2146 1385 10.2 (18)f
[1.76] [0.99] [0.67] [275] [206] [138] -2626 2022 463 -2279 1628 24.8 (24)
Table 4. Comparison of beta, beta0 , and Nv/(4π)1/2 Values (in cm-1) of PrCp*3 with Those of Selected PrIII Compounds (from ref 33) compound
beta
beta0
Nv/(4π)1/2
LiYF4:Pr3þ LaF3:Pr3þ LaCl3:Pr3þ Pr(HBpz3)3a Cs2NaYCl6:Pr3þ Pr(BA)3b [Pr{N(SiMe3)2}3(THF)] Pr{OC(tBu)3}3c [LaCp3(NCCH3)2:Pr3þ] PrCp*3 (this work) Pr(C5H4tBu)3 Pr{N(SiMe3)2}3 [(C8H8)Pr{HB(3,5-Me2pz)3}] [PrCp3(MeTHF)] [PrCp3(NCS)][Pr(MeCp)3(Cl)][PrCp3(NCCH3)] Pr(C5Me4H)3d
0.961 0.959 0.952 0.948 0.937 0.935 0.931 0.929 0.927 0.927 0.923 0.923 0.923 0.922 0.922 0.921 0.919 0.916
0.979 0.981 0.971 0.972 0.987 0.973 0.971 0.953 0.977 0.974 0.974 0.970 0.972 0.969 0.968 0.970 0.970 0.973
869 694 279 581 1016 1361 1002 1090 1229 1385 1481 1268 1236 1122 1030 948 1186 1628
a pz = pyrazol-1-yl. b BA = N,N0 -bis(trimethylsilyl)-4-methoxybenzamidinato. c Dissolved in MeTHF. d From ref 18.
a From ref 18. b From ref 22. c From ref 21. d Values in brackets held fixed on the values of LaCl3:Pr3þ and LaCl3:Sm3þ, respectively.35 e Three-body effective operators Tk do not appear in case of f2 systems. f Number of fitted energies in parentheses.
1385 cm-1 (see Table 3). After the CF strengths of pseudotrigonal-planar compounds 3 and Pr(C5H4tBu)3, this represents the third-highest value in PrIII chemistry (see Table 4). Nonetheless, the Nv/(4π)1/2 value of 1 is noticeably lower than that of 3. This is caused by the increased Pr-C distances of 1 (2.86 A˚ on average13) compared to those of 3 (2.80 A˚ on average15). For the same reason, the pair of compounds 1/3 has considerably lower Nv/(4π)1/2 values than the pair SmCp*3/Sm(C5Me4H)3 (2.82 A˚/2.76 A˚22) (see Table 3). A comparison of the absorption spectra of 1 and 3 in the FIR, MIR, and vis ranges suggested that the Slater parameters F2, F4, and F6 of 1 have higher values than those of 3. This is completely confirmed by the entries of Table 3. The nephelauxetic parameter beta (beta = F2(complex)/ 2 F (free ion))37 and the relativistic nephelauxetic parameter beta0 (beta0 = ζ4f(complex)/ζ4f(free ion))37 are considered as a relative measure of covalency of lanthanide compounds.38 With F2(Pr3þ) = 71 822 cm-1 and ζ4f(Pr3þ) = 766 cm-139 combined with the corresponding values of 1 one ends up with beta = 0.927 and beta0 = 0.974. In Table 4, these values are compared to those of selected PrIII compounds. Obviously, the f orbitals of compound 1 are less influenced by covalency than those of 3. This finding is opposite the Sm case.22 The eigenvalues of an energy matrix of the spin-free f1 system, into which the CF parameters of a previous parametric analysis of the compound of interest had been inserted, were defined as the experimentally based nonrelativistic MO scheme of this compound in the f range.40 (37) Joergensen, C. K. The Nephelauxetic Series. Prog. Inorg. Chem. 1962, 4, 73. (38) Tandon, S. P.; Mehta, P. C. J. Chem. Phys. 1970, 52, 5417. (39) Crosswhite, H. M.; Crosswhite, H. J. Opt. Soc. Am. 1984, B1, 246. (40) Jank, S.; Amberger, H.-D. Acta Phys. Pol., A 1996, 90, 21.
Figure 2. Experimentally based and calculated nonrelativistic MO schemes of (a) ψ trigonal-planar Pr(η5-C5H5)3 (calcd, from ref 23); (b) like (a), but B66 reduced to a third; (c) Pr(C5Me4H)3, experimentally based.; (d) PrCp*3, experimentally based. (For definitions of |þ3æ and |-3æ see footnote c of Table 2.)
In Figure 2, the experimentally based nonrelativistic MO schemes (in the f range) of complexes 1 and 3 are compared with the nonrelativistic MO scheme of the ψ trigonal-planar
Article
model complex Pr(η5-C5H5)3 calculated in the framework of the XR-SW approximation.23 Obviously, the calculated total splitting of f orbitals is considerably greater than the experimentally based ones of complexes 1 and 3, and, as expected, that of 1 is somewhat smaller than that of 3. On fitting the free parameters of the phenomenological Hamiltonian of the spin-free f 1 system to the calculated energies of f orbitals of Pr(η5-C5H5)3 one arrives at B20 = -3485 cm-1, B40 = 3044 cm-1, B60 = 842 cm-1, and B66 = -5919 cm-1. On comparing these values with those of compound 1 (see Table 3), it becomes evident that first of all the CF parameter B66 (which considers the interactions between orbitals fx(x2-3y2) and fy(3x2-y2) within the framework of CF theory) but also B40 are heavily overestimated by the model calculation. To roughly reproduce the correct total splitting of f orbitals, B66 has to be reduced to a third, giving also a correct sequence of levels for compound 3, but in the case of 1 the close-lying orbitals fz3 and fz(x2-y2) are interchanged (see Figure 2). 3.4. Vibrational Spectra. The vibrational spectra of 1, 2, and SmCp*3 in the FIR and MIR ranges are nearly identical apart from some additional f-f transitions in the case of 1 and SmCp*3.22 The former also holds for the polarized Raman spectra of oriented single crystals using the exciting line at 785 nm, but the polarization leaks are much more pronounced in the case of 1, and an additional electronic Raman transition appears at 165 cm-1 (vide supra). Using the exciting line at 632.8 nm (15 803 cm-1), the spectra are dominated by a broad band (half-width ca. 1300 cm-1) onto which a number of sharp signals of innerligand vibrations are superimposed. An additional signal of comparable intensity to the skeletal vibrations appears with a Raman shift of 483 cm-1 (corresponding to an absolute energy of 15 320 cm-1), which has to be correlated according to its energy and polarization properties with the abovementioned luminescence transition 13Γ6(1D2)f2Γ6(3H4) at 15 320 cm-1.
Organometallics, Vol. 29, No. 6, 2010
1373
4. Conclusions Cp3PrB (B = THF, MeTHF, NCCH3, CNC6H11) monoadducts do not exhibit f-f transitions in the FIR and MIR ranges, even at ca. 30 K, but compound 1 (and 3) does surprisingly already at ambient temperature. The same holds for the electronic Raman transitions. Both types of transitions strictly obey the selection rules of D3h symmetry. Additional signals or a splitting of 2-fold degenerate levels because of the deviation from strict D3h symmetry cannot be detected. However, the σ and π luminescence spectra of the transition 13Γ6(1D2)f3H4 of 1 show stronger polarization leaks. Obviously, the polarization properties are much more sensitive to a descent of symmetry than a break of selection rules or splitting effects. The more distant Cp* ligands of 1 produce;as expected;a lower CF strength and give rise to larger Slater parameters than the closer [C5Me4H]- ligands of 3. In the case of the pair of compounds SmCp*3 and Sm(C5Me4H)3, the CF strength of the former complex is weaker; the Slater parameters, however, are smaller than those of the latter. These different trends suggest a parametric analysis of the CF splitting of NdCp*3, which can be compared with the already performed one of Nd(C5Me4H)3. The calculated total splitting of f orbitals of the trigonal-planar model complex Pr(η5-C5H5)3 is much larger than the experimentally based one of compound 1. In addition, the sequence of f orbitals is strongly different from that of 1 because of a heavy overestimation of the CF parameter B66. The total splitting of f orbitals in the experimentally based MO scheme of 1 is;as expected;somewhat weaker than that of 3, and the sequence of the close-lying orbitals fz3 and fz(x2-y2) is interchanged.
Acknowledgment. The financial support of the Deutsche Forschungsgemeinschaft and the U.S. National Science Foundation is gratefully acknowledged. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
