J. Phys. Chem. 1994, 98, 5404-5407
5404
Vibrational Overtone Spectroscopy of Gaseous Ferrocene, Ruthenocene, and Related Compounds Todd Van Marter,t Carl Olsen,* and Deanne L. Snavely’ Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403 Received: May 20, 1993; In Final Form: January 17, 1994”
The vibrational overtone spectra of ferrocene, acetylferrocene, ruthenocene, and cyclopentadienyltitanium trichloride have been recorded in the third overtone region of the C-H stretch vibration, 4UC-H. All of the spectra display four peaks in this region. The transition wavenumbers and relative intensities of the metallocene spectra are invariant to changes of the metal (iron, ruthenium, or titanium) or the ligand (chloride or acetylcyclopentadienyl). The overtone spectrum of gaseous cyclopentadiene in the near infrared and visible regions has been recorded for comparison to the metallocene spectra. The aliphatic, V C - H ~ , and olefinic, YC-HO, local mode vibrational progressions were identified. The 4VC-HO in cyclopentadieneoccurs a t the same wavenumber as the lowest energy peakin the metallocenes. Shoulders on the 4YC-HO band contour in cyclopentadienecorrespond to the metallocene transition wavenumbers. It is argued that the transitions corresponding to these shoulders were dark in the cyclopentadiene overtone spectrum but became bright in the metal-coordinated ring.
Introduction Metallocenesl are sandwich compounds consisting of two cyclopentadienyl rings joined by a metal atom. These compounds have been very important in the development of the theory of inorganic bonding. Infrared vibrational spectra2 are used in the identification and characterization of these compounds. The changes observed in the vibrational spectrum for a free ligand when it coordinates with a metal are indicative of the type of bonding. Requisite to this comparison are the frequencies and assignments for the normal modes of vibration. In the case of ferrocene and ruthenocene, the vibrational assignments remain incomplete. The symmetric “sandwich” structure in D5d or D5h symmetry possesses 15 Raman and 10 infrared fundamentals.3 Gaseous ferrocene was observed by Lippincott and Nelson;4 however, due to low vapor pressure, only eight fundamentals wereassigned. Solution- and solid-phase data were used to assign other fundamentals. In ferrocene solution 10 infrared and 11 Raman fundamentals were assigned, while in ruthenoceneonly 9 infrared and 11 Raman modes were identifieda5 A normal coordinate analysis yielded force constants for the cyclopentadienyl ring similar to those in benzene.5 The infrared spectrum of ruthenocene was similar to that of ferrocene except for the higher ring-metal-ring vibrational frequency, indicating a stronger ring-metal bond. Other vibrational studies@ on ferrocene and ruthenocene have added assignments but remained largely in agreement with the assignments of Lippincott and Nelson. Many of the fundamentals of ferrocene were both infrared and Raman active.6 This is thought to indicate the isolation of the two rings from one another, prohibiting the formation of symmetric and antisymmetric motions of the rings which would obey the mutual exclusion rule. The spectra of ferrocene and ruthenocene can be thought of as the vibrations of the cyclopentadienide rings plus those of the complex. The C-H stretch vibrational frequencies taken from the infrared and Raman spectra of KC5H59for the cyclopentadienide ion shift about 50 cm-I higher while other vibrations shift as much as 100 cm-1.10 Using the free cyclopentadienide force constants in the total normal coordinate analysis of ferrocene,” these shifts are reproduced via internal vibrational coupling. Similarly, the infrared spectra of matrix-isolated cyclopentadit Current address: Department of Chemistry, Emory University, Atlanta, GA -~~30322. . . .
~ ~
f Current address: Department of Chemistry, Princeton University, Princeton, NJ 08542. Abstract published in Aduance ACS Absiracrs, April 15, 1994.
0022-365419412098-5404sO4.50f 0
enyltitanium trichloride and several dicyclopentadienyltitanium dichlorides were characterized12 by vibrations of the isolated cyclopentadienyl ring and a tetrahedral C3,-type molecule with the ring as a point unit. A complete understanding of the infrared spectrum of ferrocene and its derivatives, especially in the gas phase, is still unavailable. Vibrational overtone spectra indicate the structure and vibrational coupling of molecules possessing high internal energies.13 The predominant absorption features in overtone spectra of hydrocarbons are vibrational progressions for each type of C-H bond in the m01ecule.I~ The transition frequencies for each local mode fit a BirgeSponerl5 plot according to
AE = (we- W,X,)V
- (w,x,)v2
where v is the vibrational quantum number, w ~is ,the anharmonicity, and we is the mechanical frequency. If vibrational coupling is strong, the simple Birge-Sponer pattern is disrupted and intensity borrowing gives rise to new absorptions. In some cases vibrational coupling arises from a Fermi resonance between the CH stretch and bend 1e~els.I~ In all overtone spectra of hydrocarbons there are weak features which usually cannot be assigned. These are either hot bands or combinations of the local mode stretches plus some other vibration. Assignment of these weak absorptions is difficult because the high density of states allows for many possible assignments and the anharmonicities for the various combinations are not known. The hot bands can be distinguished by their temperature dependence, but very little work has been done in this area.I6 Lewis17 reported the first C-H overtone transition (at 6105 cm-I) in ferrocene solution using a photoacoustic method. He also assigned combinations of the C-H stretch and bend (3945 cm-I), the C-H stretch and ring breathing motion (4167 cm-I), the C-H and C=C stretch (4495 cm-I), and the first C-H stretch overtone. Other weak peaks in this spectrum could not be assigned. Blackburn, Snavely, and OrefI* observed the visible vibrational spectrum of ferrocene using laser vibrational overtone spectroscopy. Two peaks at 11 669 and 11 712 cm-1 were observed and assigned to the third overtone of the C-H oscillator. The appearance of two peaks was not expected. If the aromatic cyclopentadienyl ring is n-bonded, all the C-H bonds would be equivalent. If the C-H bonds are equivalent, only one transition at each quantum level would be observed, according to eq 1. Blackburn et a1.I8suggested that the two absorptions in ferrocene originated from the two conformations, staggered and eclipsed. 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, NO. 21, 1994 5405
Vibrational Overtone Spectroscopy
I
I
I
I
I
I
11500
11600
11700
11800
11900
12000
Wavenumbers / cm-1 Figure 1. Ovetone spectrum of gaseous (top) cyclopentadiene, (second) ferrocene, (third) ruthenocene, (fourth) acetylferrocene, and (bottom) cyclopentadienyltitaniumtrichloride. To test this hypothesis we have reinvestigated, with improved signal-to-noise, the overtone spectrum of ferrocene (I) and also recorded, for the first time, the overtone spectra of acetylferrocene (11),ruthenocene (111),and cylcopentadienyltitanium trichloride
(W
*
at constant temperature in an intracavity oven (25.5 by 12.5 by 6.5 cm.) which regulated a constant temperature up to 150 OC. The spectra for ferrocene and acetylferrocene were recorded at 60 OC, while the spectra for cyclopentadienyltitanium trichloride (100 "C) and ruthenocene (120 "C) required higher temperatures. Since thevapor pressure of these solids at these temperatures was about 150 mTorr, approximately 300 Torr of argon was added to the cell to increase the signal-to-noise ratio. Thedicyclopentadiene, purchased from Aldrich Chemical Co., was refluxed at 120 "C for 45 min, converting the dimer to monomer. The monomer was collected through distillation between 37 and 43 "C. The product was verified by infrared spectros~opy.~~ The sample was dried over magnesium sulfate and degassed by repetitive freeze-pumpthaw cycles. The photoacousticcelloperated intracavity in a Spectra Physics Model 375B argon-pumped tunable dye laser outfitted with a three-plate birefringent filter (2-cm-1 bandwidth). An Oriel motor mike with the 18007 control unit operated the birefringent filter rotation. A Spex Industries Inc. Model 1401 double monochromator was used to detect the initial and final wavelengths of the dye laser. The monochromator had a resolution of 0.2 cm-l, so the measurements were still limited by the 2-cm-1 resolution of the birefringent filter. A PTI Model 03-OC4000optical chopper chopped the argon laser at 250 Hz, providing the reference signal for an EG&G Brookdeal Electronics Model 5207 lock-in amplifier. An IBM personal computer interfaced with both the lock-in amplifier and the Oriel motor mike recorded the acoustic signal while it controlled the laser scanner. The styryl 9 dye, covering the region from 11 300 to 12 800 cm-l, was used to record the metallocene spectra. For the cyclopentadiene the u = 5 and u = 6 data were originally recorded on a similar apparatus20 at UC Berkeley using DCM/LD700, rhodamine B, and rhodamine 6G dyes by Dr. J. M. Jasinski. We confirmed thesespectraatBGSUusingrhodamine6G(17 850-15 400cm-'), DCM (16 400-13 300 cm-I), pyridine 2 (14 500-12 650 cm-I), and styryl 9 (12 579-11 300 cm-I). Gas-phase near-infrared spectroscopy was performed on cyclopentadiene. A 9.75-m pathlength cell was used on a Cygnus 100 Mattson FTIR Spectrometer equipped with a quartz beamsplitter, InGaAs detector, and tungsten lamp. The pressure of cyclopentadiene used in the cell was 15 Torr. Results and Discussion
I
11
111
Iv
These improvements resulted from the use of an intracavity oven which heats the photoacoustic cell as high as 150 "C, increasing the vapor pressure of these solid materials. Four transitions, not just two, were observed, all belonging to the third overtone of the C-H. The experimental evidence presented here is inconsistent with the internal rotation hypothesis. By comparison of the overtone spectra of these cyclopentadienylcomplexes to that of gaseous cyclopentadiene, we argue that the metal bonding to the five-membered ring brings intensity to dark states which lie at the same energy as the fourth C-H stretch level. Experimental Section Ferrocene, ruthenocene, and cyclopentadienyltitaniumtrichloride (98%) were purchased from the Aldrich Chemical Co. Acetylferrocene was purchased from Strem Chemicals. Further purification by vacuum sublimation was required for all samples. These samples were sublimed into a 20 by 1.5 cm photoacoustic cell furnished with a 1741 Knowles Electronic Inc. microphone and Brewster's angle quartz windows. The entire cell was held
The third vibrational overtone spectrum of ferrocene is shown in Figure 1. Four closely spaced peaks appear in the 11 60011 900-cm-1 region. The transition wavenumbers and splittings are tabulated in Table 1. Our expectation for the overtone spectrum of ferrocene derives from the overtone spectrum of gaseous benzene.2' The low-resolution spectrum of roomtemperature benzene possesses one major absorption at each quantum level of the C-H stretching vibration. Since the cyclopentadienyl ring is also aromatic, by analogy to benzene, we expected a transition for each quantum level of the C-H stretch vibration. There are six infrared-active C-H stretch normal modes in ferrocene. Four of these bands observed in the solid spectrum occur at 3099, 3085, 3075, and 3100 cm-l. LewisI7 assigned the first overtone in ferrocene solution to 6105 cm-1 but failed to address the assignment of the small peak at 9009 cm-1, which we believe is the second overtone of the pure C-H stretch. Using the averageof the four infrared fundamental wave number^,^ and the first and second overtone transition wavenumbers of Lewis," our new transitions fall approximately on the B i r g e Sponer line (see eq 1) at the u = 4 position. However, the appearance of four peaks rather than one means that the simple Birge-Sponer description is not applicable in this case. Originally we suggested18 that these bands arise from the internal rotation22 within ferrocene at it rotates from the staggered to eclipsed conformation over a barrier of about 0.9 kcal/m01~~
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Van Marter et al.
The Journal of Physical Chemistry, Vol. 98, No. 21, 1994
TABLE 1: Transition Wavenumben (cm-I), Intensities, and SDlittines (A) for Gaseous Metallocenes
ferrocene
ruthenocene
acetylferrocene
cyclopentadienyltitanium trichloride
transition wavenumber
intens
11 659
m
A
+
724
S
65
802
W
78
86 1
W
59
1 665 1 732 1 806 1 866
m
1 1 1 1
66 1 724 803 863
S
W W
assgnmt 3vc-n us (1257") + v20 (1414') = 11682 3Vc4 -tv5 + v26 (1 560b) = 11826 3 ~ ~ 4 2u20 4 = 11837 3vc-n + v20 + v 2 6 = 11985
+
+
67 14 60
m S
W W
1 658
m
11 723 11 800 11 861
S W
W
spectra to that of cyclopentadiene is useful in understanding these peaks. The survey spectrum of cyclopentadiene (Figure 2) reveals progressions of the aliphatic and olefinic C-H vibrations. The entire spectrum from 6400 to 17 000 cm-l was recorded with three dark regions around 10 000,12 500, and 14 800 cm-I. Using these peaks along with the fundamental freq~encies,'~ a BirgeSponer plot was constructed (Figure 3). Table 2 summarizes all of the observed transition wavenumbers and assignments. Some minor peaks are assigned to combinations of aliphatic and olefinic stretches (progression A). The third overtone of the aliphatic stretch was inaccessible in a dark spectral region around 12 500 cm-I. The mechanical frequencies and anharmonicities are tabulated in Table 3. The four peaks might arise from olefinic and aliphatic stretch combinations, for example, 4vC-HO, 3vC-HO VC-H.,~ V C - H Q ZVC-H', VC-HO 3VC-Ha. However, this explanation does not fit theobserved data since these combinations would be separated by over 100 wavenumbers rather than 60-70 cm-I as is the case in the metallocenes. Because of the aromaticity of the cyclopentadienyl rings, the 4-0 C-H transition should lie intermediate between the aliphatic and olefinic C-H transitions of cyclopentadiene. In fact, the lowest energy peak of the four falls exactly where the 4-0 olefinic stretch absorbs in cyclopentadiene. This is similar to gaseous cyclopentadienylZ5where the fundamental C-H stretch at 3096 cm-I is closer to the olefinic rather than the aliphatic C-H stretch. Furthermore, a careful examination of the cyclopentadiene 4VC-HO band shape indicates that two of the three weak shoulders (Table 2) are identical to the transitions in ferrocene. It is possible that the four peaks in the metallocenes arise from combinations of principally the VC-H with other vibrational modes of the ring. These combinations are dark in cyclopentadiene itself but brighten in the metallocenes under the influence of the q5 ring-to-metal atom bond. In our search for combinations we chose only those involving C-C stretches and C-H bends. This choice was motivated by the extensive theoretical aimed at describing the large bandwidths in the overtone spectrum of benzene. The primary interaction of the energetic C-H oscillator occurs between the C-C stretches and the C-H bends. Through an examination of the vibrational spectra of ferrocene and cyclopentadiene, certain combinations are possiible. For example, for cyclopentadiene we find that taking the two C-C stretches observed in the infrared spectrum, v4 at 1505 cm-1 and ~ 1 at7 1586 cm-l, and the two highest frequency C-H bends, vg at 1437 cm-1 and v7 at 1370 cm-l, we can form combinations with 3VC-HO at 8895 cm-l (see Table 2) which fall at the same wavenumbers as the 4VC-Ho. The C-C stretch and C-H bend anharmonicities are smaller than the stretch anharmonicity but would account for some of the energy mismatch. If we follow the same reasoning for ferrocene using the 3VC-H transition frequency of 9009 cm-1 l 7 and the observed frequencies of 1560 cm-l for v26 (C-C stretch),
63 79 60
65 77 61
Values from ref 8. Values from ref 5.
in the gas and 1.8 kcal/mol in the solid.24 If this is the case the spectra of the ferrodene derivatives with different barriers to rotation and moments of inertia should reveal these changes in the internal rotation potential. However, the spectrum of ruthenoene (Figure 1) with an internal rotation barrier in the solid of 2.3 kcal/m01~~ has the same transition wavenumbers and relative intensites as those of ferrocene. Neither does substitution by an acetyl group on the ring (acetylferrocene, Figure 1) change the spectrum. (We searched for but did not observe the C-H stretch overtone transitions for the acetyl group in acetylferrocene. These absorptions are expected to be weak relative to those of the ring overtones and, given the signal-to-noise ratio of our spectra, would be out of our observable intensity range.) Even cyclopentadienyltitanium trichloride with both rotor and barrier changed has a spectrum identical to that of ferrocene. Table 1 tabulates these transition wavenumbers. Clearly, variation of the rotor characteristics does not alter the observed spectrum, so the overtone spectra are not the result of internal rotation. Furthermore, the spectrum does not depend on the particular metal since the frequencies are very similar for the iron, ruthenium, and titanium compounds. The four absorptions in these metallocene spectra arise from the cyclopentadienyl ring itself. Comparison of these overtone
+
+
Wavenumbers I cm-1 Figure 2. Overtone spectrum of cyclopentadiene. The intensity of the peaks within each section of the survey scan are relative to that section only. The sections are arbitrarily scaled to one another to magnify the weak, high-energy spectrum.
The Journal of Physical Chemistry, Vol. 98, No. 21, 1994 5407
Vibrational Overtone Spectroscopy
Conclusion I The visible vibrational absorption spectra of four gaseous
3100 3000
.
\
2900
>
W 4
2800 2700 0
0
2600
0
aliphatic olefinic progressionA 1
2
3
4
5
6
7
Quantum Number (v)
metallocenes have beenobserved. Theabsorptions around 1 1 700 cm-I arise from the 4VC-H of the cyclopentadienyl ring. Since the third overtone of the olefinic C-H stretch in gaseous cyclopentadiene lies at 11 652 cm-1, the absorptions in the metallocenes are related to the C-H stretches of the aromatic ring. In a true aromatic hydrocarbon only one peak would be observed. The cyclopentadienylring must therefore be influenced by the presence of the metal so that the vibrational potential couples several other vibrations with the olefinic stretch. The C-C stretch and the C-H bending motions are the most likely candidates. This is the first observation of this type of vibrational mixing for a *-bonded ligand in a organometallic complex. We believe that this may be a general phenomenon which could elucidate the nuclear changes of an organic substrate when it ?r bonds to a metal.
Figure 3. BirgeSponer plot for cyclopentadiene. The A E / v is in wavenumbers (cm-I).
TABLE 2 Transition Wavenumbers (cm-l) and Assignments of Cyclopentadiene transition wavenumber 2895" and 2913' 3055' and 3083" 6764 6934 7034 7150 7288 7362 7439 8322 8488 8573 8658 8725 8770 8837 8895 11652
intens
assgnt 2904 3069
vw vw W
W
vw vw vw S
3vc-n'
2774
W W
progression A
2858
vw vw shld shld shld
2965 2913
S
shld shld shld shld 13252 13352 13583 14264 15197 15505 15854 16160 16780
2650
S
vw 2717 2853
W
S
m 2584
S
m
vvw
2693 2797
S
Values from ref 19.
TABLE 3: Anharmonicities (cm-l) and Mechanical Frequencies (cm-1) for Cyclopentadiene assgnmts
mechanical freq
anharmonicity
aliphatic olefinic progression A
3031 3180 3080
64 54 57
1414cm-1 for ~20(C-Cstretch),and 1257cm-for v5 (C-H bend), the resulting combinations fall right at the 4vC-H energy (Table 1).
Acknowledgment. This work was supported in part by the Office of Naval Research. C.O. acknowledges support by the REU Sites Grant a t BGSU funded by the National Science Foundation. We would like to thank Dr. Joseph Jasinski for sharing his unpublished data on cyclopentadiene and Dr. Valerie Walters for the near-infrared spectrum. References and Notes (1) Elschenbroich, C.; Salzer, A. Organometallics, A Concise Introduction, 2nd ed.; VCH Publishers: New York, 1992. (2) Fritz, H. P. In Advances in Organometallic Chemistry; Stone, F. G . A., West, R., Eds.; Academic Press: New York, 1964; Vol. 1. (3) Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward, R. B. J.Am. Chem.Soc.1952,74,2125. Woodward, R. B.; Rosenblum, M.; Whiting, M. C. J. Am. Chem. SOC.1952, 74, 3458. (4) Lippincott, E. R.;Nelson, R. D. J . Chem. Phys. 1953,21,1307-308. (5) Lippincott, E. R.; Nelson, R. D. Spectrochim. Acta 1958,10, 307329. ( 6 ) Hartley, D.; Ware, M. J. J . Chem. SOC.A 1969, 138. (7) Bodenheimer, J. S.; Low, W. Spectrochim. Acta 1973,29A, 17331743. (8) Winter, W. K.;Curnutte, B., Jr.; Whitcomb, S. E. Spectrochim. Acta 1959, 12, 1185-1 102. (9) Fritz, H. P.; Schafer, L. Chem. Ber. 1964, 97, 1829. (10) Adams, D. M. Metal-Ligand and Related Vibrations: A Critical Survey of the Infrared and Raman Spectra of Metallic and Organometallic Compounds; St. Martin's Press: New York, 1968. (1 1) Brunvoll, J.; Cyvin, S. J.; Schafer, L. J . Organomet. Chem. 1971,27, 107. (12) Balducci, G.; Bencivenni, L.; De Rosa, G.; Gigli, R.; Martini, B.; Nunziante Cesaro, S. J . Mol. Struct. 1980, 64, 163. (13) Quack, M. Annu. Reu. Phys. Chem. 1990, 41, 839. (14) Henry, B. R. In Vibrational Spectra and Structure; During, J. R.; Ed.; Elsevier: Amsterdam, 1981; Vol. 10, pp 269-319. (15) Birge, R. T.; Sponer, H. Phys. Reu. 1926, 28, 259. (16) Hassoon, S.; Snavely, D. L. J . Chem. Phys. 1993, 99 (4), 251 1. (17) Lewis, L. N. J . Organomet. Chem. 1982, 234, 355. (18) Blackburn, F. R.; Snavely, D. L.; Oref, I. Chem. Phys. Lett. 1991, 178(5,6), 538. (19) Gallinella, E.; Fortunato, B.; Mirone, P. J . Mol. Spectrosc. 1967,24, 345. (20) Jasinski, J. M.; Frisoli, J. K.; Moore, C. B. J. Phys. Chem. 1983,87, 2209. (21) Reddy, K. V.; Heller, D. F.; Berry, M. J. J. Chem. Phys. 1982,76(6), 2814. (22) Flygare, W. H. Molecular Structure and Dynamics; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1978. (23) Haaland, A.; Nilsson, J. E. Acta Chem. Scand. 1968, 22(8), 17. (24) Hohn, C. H.; Ibers, J. A. J . Chem. Phys. 1959, 30, 885. (25) Sado, A.; West, R.; Fritz, H. P.; Schafer, L. Spetrochim. Acta 1966, 22, 509. (26) Sibert, E. L., 111; Reinhrdt, W. P.; Hynes, J. T. J. Chem. Phys. 1984, 81(3), 1 1 1 5 .
(27) Guan, Y.; Thompson, D. L. J . Chem. Phys. 1988, 88(4), 2355. (28) Lu, D.; Hase, W. L. J . Chem. Phys. 1988,89(11), 6723. (29) Wyatt, R. E.; Iung, C. J. Chem. Phys. 1993, 98(7), 5191.
