J . Phys. Chem. 1988, 92, 4316-4319
4316
High-Pressure Spectrbscopic Studies of Ferrocene, Nkkeiocene, and Ruthenocene R. T. Roginski, J. R. Sbapley, and H. G. Drickamer* School of Chemical Sciences, Department of Physics, and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61 801 (Received: November 19, 1987)
The solid-state infrared spectra (700-1300cm-I) of ferrocene, nickelocene, and ruthenocene exhibit significant changes with hydrostatic pressure (0-120 kbar). A number of bands show a continuous increase in intensity with increasing pressure; also, several new bands appear in this spectral region at elevated pressures. These changes are discussed in terms of intermolecular interactions between molecules within the unit cell, whereby the motions of the molecules are coupled to such an extent as to impart intensity to vibrational modes that were previously dipole forbidden.
Introduction The infrared and Raman spectroscopies of metallocenes have been well studied but are not without controversy.'-" The initial work by Lippincott and Nelson'-3 and Winter et al.4 made strong use of the vibrational assignment of the tropylium ion (C7H7+) and of benzene; the similarity in the force constants obtained by a normal-coordinate analysis essentially confirmed the aromatic nature of the cyclopentadienyl rings. The assignments of Fritz6v7 differed somewhat from the earlier studies, as a result of polarized Raman measurements. However, more recent stt~dies*-~ have upheld the original assignments by Lippincott and Nelson. It is the assignments of Lippincott and Nelson that will be used here.' Under the selection rules for either D5*or D5d symmetry, vibrations that are Raman allowed are infrared forbidden and vice versa. However, a striking feature of the vibrational spectra of metallocenes is that there are several coincidences (by accidental degeneracy) of frequencies in the infrared and Raman spectra. This phenomenon has been attributed to a lack of interaction between the rings, and the analysis of B ~ n k e rtreating ,~ ferrocene as a nonrigid molecule, showed these coincidences to be genuine. The symmetry theory of nonrigid molecules, originally forwarded by Longuet-Higgins,'* assumes no torsional barrier. The barrier to internal rotation has been determined to be approximately 0.9 kcal mol-', which is on the order of kT.* The effect of hydrostatic pressure up to 120 kbar on the solid-state infrared spectra of ferrocene, nickelocene, and ruthenocene is presented in this paper. The appearances of new bands in the spectra are discussed in terms of an intermolecular coupling of motion within the unit cell.
Experimental Procedure Ferrocene (Alfa Inorganics) was purified by sublimation before use. Ruthenocene (Aldrich) and nickelocene (Alfa Inorganics) were used without further purification. Infrared spectra were taken on a Nicolet 7199 FTIR with an 800-cm-I HgCdTe detector and a Perkin-Elmer 4:1 beam condenser in a gasketed diamond anvil cell (DAC) employing type IIA diamonds. The beam (1) Lippincott, E. R; Nelson, R. D. Spectrochim. Acra 1958, 10, 307. (2) Lippincott, E. R.; Nelson, R. D. J . Chem. Phys. 1953, 21, 1307. (3) Liuuincott, E. R.; Nelson, R. D. J . Am. Chem. Soc. 1954, 76. 1970. (4) W'mter, W. k.;Curnutte, B., Jr.; Whitcomb, S. Spectrochim. Acra 1959, 12, 1085. (5) Bunker, P. R. Mol. Phys. 1965, 9,247. ( 6 ) Fritz, H. P. Chem. Ber. 1959, 92, 780. (7) Fritz, H. P. Adv. Organomer. Chem. 1964, 1, 239. (8) Hartley, D.; Ware, M. J. J . Chem. Soc. A 1969, 138. (9) Long, T. V.;Huege, F. R. Chem. Commun. 1968, 1239. (10) Aleksanyan, V. T.; Vyshihskii, N. N.; Grinval'd, I I.; Arseneva, T. I. Izv. Akad. Nauk SSSR, Ser. Khim. 1981, 20, 303. ( I I ) Bom, V.;Hechter, B.; Shushani, M.;Konigstein, I. A.; Smirnova, E. M.; Kimel'fel'd, Ya. M.; Bykova, E. V.;Aleksanyan, V T. Izu. Akad. Nauk SSSR,Ser. Khim. 1915, 24, 572. (12) Longuet-Higgins, H. C. Mol. Phys. 1963, 6, 445. (13) Haaland, A.; Nilsson, J. E. Chem. Commun. 1968, 8 8 .
0022-3654/88/2092-4316$01.50/0
t
Ferrocene Solid State Infrared Spectrum Atmospheric Pressure
8
-.5 0 c
.-
*
0
1150 950 Wavenu m bers
1350
750
Figure 1. Solid-state infrared spectrum of ferrocene (750-1350 cm-I).
TABLE I band location, cm-I
a~sianmentl-~
1257
A,, 11 CH bend
*IO
El, 11 CH bend A2, asym ring breath E2"I CH bend El, /I CH bend
1190 1106 1048,
1057
1002
description
*5
*3 1 "18
843, 855
A*, I bend
condenser was mounted on a fixed bracket in the normal sample area of the FTIR. An X Y Z mount was used to position the DAC at the focus. Nujol was used as a pressurizing fluid the working area of the DAC was flushed with nitrogen when loading nickelocene to minimize oxidation. All changes observed were both reversible and reproducible.
Results and Discussion Figure 1 shows the infrared spectrum (750-1350 cm-l) of polycrystalline ferrocene at atmospheric pressure; the bands are labeled for ease of discussion with the assignments given in Table I. Figure 2 exhibits the effect of pressure on the ferrocene IR spectrum up to 120 kbar, with the pressure shifts of the bands in the energy range 800-1350 cm-I shown in Figure 3. The pressure shifts were both reproducible and reversible; the shifts up to 50 kbar compare quite favorably with the results of Adams and Williams.I4 Of considerably more interest than the pressure shifts are the changes in the spectrum with pressure (Figure 2 ) . Some of the changes are summarized as follows: (1) v5 (Al- /I C H bend, 1254 cm-l) shows a large increase in intensity with pressure. (14) Adams, D. M.; Williams, A. D. J. Phys. Chem. Solids 1980,41, 1073.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4317
Ferrocene, Nickelocene, and Ruthenocene Ferrocene
1303
I
1 Ferrocene
0
20
-I
60 80 100 120 Pressure (kbar)
40
140
Figure 3. Pressure shifts of bands appearing in the IR spectrum of ferrocene (800-1300 cm-'). TABLE II: New Bands Appearing in the Region 820-920 cm-' pressure shift, location, pressure, cm-'/ kbar cm-' kbar 916 88 1 853 833
I
I
I
I
1150 950 Wavenumbers
I
I
750
Figure 2. Solid-state infrared spectrum of ferrocene at several pressures; the arrows indicate the appearance of new bands or large intensity increases.
(2) vl0 (Azu asymmetric ring breathing mode, 1106 cm-') broadens considerably with increasing pressure. (3) The band initially located at 1002 cn-' (v18, El, II CH bend) broadens a great deal with increasing pressure and splits into a doublet. (4)Several bands appear with pressure (see Figures 2 and 3) in the region 820-920 cm-'; Table I1 lists the initial positions of these bands a t the pressure at which they first appear. Figures 4 and 5 show the effect of pressure on the IR spectra of nickelocene and ruthenocene, respectively. The changes exerted by pressure are similar to that of ferrocene both qualitatively and quantitatively (the pressure shifts of the IR bands of nickelocene are presented in Figure 6 ) . Some of the minor differences are as follows:
29.5 64.0 29.5 51.4
0.54 0.17 0.26 0.12
(1) For both nickelocene and ruthenocene, v18 splits into a triplet; this band splits into a doublet for ferrocene. (2) In the region 800-900 cm-', the spectrum of ruthenocene shows much more structure than Ni(cp)z and Fe(cp)z (cp = cyclopentadienyl). (3) Rather than increasing significantly in intensity, u5 splits into a doublet for ruthenocene. In an attempt to explain these changes, use will be made of two experimental facts; (a) the crystal structures of Fe(cp)z, Ni(cp),, and R ~ ( c p are ) ~ such that there are at least two molecules per unit ell,'^-'^ and (b) there exist several coincidences between Raman and infrared frequencies. Consider u3, the Al, symmetric ring breathing mode located at 1105 cm-' in the Raman spectrum of ferrocene.' Obviously, from the symmetry properties of this mode (totally symmetric), the carbon-carbon stretches on the two cyclopentadienyl rings occur "in phase", as opposed to ul0. Now, if there is an intermolecular interaction that couples the vibrational motions of two molecules in the unit cell, a vibration that normally appears only in the Raman spectrum may become dipole allowed and therefore appear in the infrared spectrum. Again, from the example of v3, consider the result of an intermolecular coupling such that the v3 modes are 180° out of phase for the molecules in the unit cell. Since the motions are strongly coupled, the symmetry of this coupled mode must be determined from the motion of both molecules moving as one. For a phase difference of 180°,the coupled A', modes clearly become ungerade (odd with respect to an inversion center), and hence they become dipole allowed. It is also quite reasonable to expect that as the strength of this interaction is increased the intensity of "dipole-allowed symmetric (15) (16) (17) (18)
Eiland, P. F.; Pepinsky, R. J. Am. Chem. SOC.1952, 74, 4971. Seiler, P.; Dunitz, J. D. Acra Crysrallogr., Sect. B 1980, 36, 2255. Seiler, P.; Dunitz, J. D. Acra Crysrallogr., Sect. B 1980, 36,2946. Dunitz, J. D.; Orgel, L. E. J . Chem. Phys. 1953, 23, 954.
4318 The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 N ickelocene
Roginski et al.
n
I
r
Ruthenocene
Q
C
C
-
c e
E C
P
I
1: 1
I
1150
I
I
950 Wavenum bers
1
I
I
I
I
1150 950 Wavenumbers
I
I
750
750
Figure 4. Solid-state infrared spectrum of Ni(cp), at several pressures; the arrows indicate the appearance of new bands or large intensity increases.
modes” increases. This could account for the broadening of uI0. This model will now be used to attempt to explain the changes seen in the spectra of ferrocene, nickelocene, and ruthenocene. The band at 1254 cm-I in ferrocene, u5, shows a considerable increase in intensity with pressure; u5 appears at 980 cm-I in ferrocene-dlo (Figure 7), with similar changes in intensity with pressure. This band shows similar behavior in nickelocene, while it splits into a doublet for ruthenocene. The latter observation is interesting in light of the symmetry of mode u5 being A2,,,hence it is singly degenerate. However, u-, (AZg11 CH bend) is calculated from a normal-coordinate analysis to be coincident to u5.I The mode u, is Raman inactive, but coupling of the type described earlier would render this mode dipole allowed.
Figure 5. Solid-state infrared spectrum of Ru(cp)* at several pressures; the arrows indicate the appearance of new bands or large intensity increases.
The band at 1002 cm-I in ferrocene, broadens considerably with pressure and splits into a doublet. In N i ( c ~ and )~ Ru(~p)~, this band becomes very broad and splits into a triplet. Coincident with is 1’13, an E,, 11 CH bend that appears at 1001 cm-I in the Raman spectrum.’** The intermolecular coupling described could account for up to four bands in this region, with the loss in degeneracy resulting from site group splittings. The first band listed in Table I1 extrapolates to a location near 900 cm-I at atmospheric pressure. This band also shifts at a rate very near that of a band initially appearing at 892 cm-I in nickelocene, which is assigned as v33 (E2,, 1) ring deformation). The band u j 3 is readily observed in both nickelocene and ruthenocene6 but apparently has been observed only in a single crystal of ferrocene aligned such that the electric vector was parallel to the t2 axis;4 the angle between the tz axis and the molecular
J. Phys. Chem. 1988, 92, 4319-4323
4319
dol Ferrocene Solid StateInfrored Spectrum
Nickelocene
130G+
p=3kb,o,r
L
A
r
0
coo
cco 1000,
L 43 ooo
LBSBV
I
AAA
0
A y 1 t
20
40
60
/
I
,
/
,
800 Wavenumbers
900
850
I
/
750
,
,
~
000
Figure 7. Solid-stateinfrared spectrum of ferrocene-dloat two pressures; the arrows point out the increase in intensity of the band assigned to v5.
r
0
/
950
80
Pressure (kbar)
100
120
Figure 6. Pressure shifts of the bands appearing in the IR spectrum of
Ni(CP)z. symmetry axis is approximately 4 5 O . Coincident with v33 is v2, (E2g (1 ring deformation). The band initially appearing at 833 cm-' at 51 kbar in ferrocene extrapolates to approximately 820 cm-' at atmospheric pressure. A reasonable assignment for this band is v2 (Ai, I C H bend), which is seen in the Raman spectrum at 815 cm-l. The two remaining bands in Table I1 may be assigned to v14 (El, I CH
bend), which is coincident with v19. Finally, all of the arguments presented for the changes occurring in ferrocene are completely consistent with those observed in both nickelocene and ruthenocene. Although this model is rather simple, it appears to consistently account for the changes seen in the solid-state infrared spectra of Fe(cp),, Ni(cp),, and Ru(cp), with pressure. Two particular experiments that could provide additional information that would complement the results shown here are (a) the IR spectra of ferrocene-dio below 800 cm-' and (b) the Raman spectra of ferrocene, nickelocene, and ruthenocene as a function of pressure. Both of these could further show the utility of the coupling model; in particular, changes in the Raman spectra with pressure could determine if the ungerade modes couple to become Raman allowed.
Acknowledgment. This work was supported in part by the Materials Science Division of the Department of Energy under Contract DE-AC02-76ER01198.
Pressure Tuning of the Electronic Energy Levels of Ferrocene, Cobaltocenium Hexafluorophosphate, and Nickelocene R. T. Roginski, A. Moroz, D. N. Hendrickson, and H. G. Drickamer* School of Chemical Sciences, Department of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received: November 19, 1987)
The effects of pressure on various d-d transitions for ferrocene, cobaltocenium hexafluorophosphate, and nickelocene in the solid state are presented here. The bands are found to shift in a manner such as to increase the symmetry-imposedsplittings between the d orbitals, which is consistent with a qualitative molecular orbital picture. These splittings are found to increase between 2 and 8% in 100 kbar, as compared with 7-15% for simple ionic compounds. The Racah parameter B is almost independent of pressure, in contrast to the results seen for simple ionic compounds. These results are discussed in terms of the large degree of covalency for these compounds.
Introduction
The electronic structure of d6 and d8 metallocenes has long been the object of both experimental and theoretical considerations, the bulk of which dealt with ferrocene. The electronic structure of ferrocene has been studied by qualitative molecular orbital
treatment^,'^ crystal and ligand field treatments (combined with qualitative molecular orbital consideration^),^-^ Wolfsberg(1) Jaffe, H. H. J . Chem. Phys. 1954, 21, 156. (2) Dunitz, J. D.;Orgel, L. E. Nature (London) 1953, 171, 121.
0022-3654/88/2092-43 19%01.50/0 0 1988 American Chemical Society
