Inorg. Chem. 1987, 26, 2950-2954
2950
Contribution from the Departments of Chemistry, University of Sassari, Sassari, Italy, and Northwestern University, Evanston, Illinois 60208
Vibrational Frequencies Associated with the Carbide Ligand in Iron Butterfly Clusters P. L. Stanghellini,*la M. J. Sailor,Ib P. Kuznesof,Ib K. H. Whitmire,Ib J. A. Hriljac,Ib J. W. Kolis,lb Y . Zheng,lb and D. F. Shriver*Ib Received February 18, 1987
The vibrational frequencies associated with the exposed carbon atom in several tetranuclear iron carbide clusters with a butterfly arrangement of atoms were investigated by infrared and Raman spectroscopy. Vibrational assignments were confirmed in most cases by " C labeling of the carbide carbon atom. The characteristic feature of the iron butterfly carbides is a readily observed band in the infrared spectrum around 900 cm-I. An approximate normal-coordinate analysis on these molecules yields values of the metal-carbon force constant of about 250 N m-l.
Introduction
Experimental Section
The chemistry of organometallic carbide clusters took a new direction with t h e discovery of butterfly clusters containing an exposed carbide ligand.2 Unlike the previous cluster carbides, t h e new clusters possess reactivity a t t h e carbon a t ~ m . ~Low-~ coordinate carbides are often proposed as reactive intermediates in surface chemistry: and molecular clusters with exposed carbides provide models for t h e carbon a t o m in a low-coordinate m e t a l environment on m e t a l surfaces.? Previous vibrational spectroscopic characterization of m e t a l cluster carbides has been confined to species lacking reactivity a t t h e carbide These are generally high-nuclearity clusters in which t h e carbide ligand occupies an interstitial site; therefore, the lack of observed reactivity of such an enclosed carbon atom is not surprising. In contrast, t h e tetranuclear carbide cluster [Fe4C(CO),J2- displays reactivity with alkylating agents or proton sources at the carbon atom: consistent with t h e idea that lowering t h e steric constraints about the carbon a t o m enhances its reactivity." In the present research, we have investigated t h e vibrational spectroscopy of several tetranuclear metal carbide clusters. The frequencies and assignments should prove useful for the characterization of molecular metal cluster carbides and reactive surface carbides. Force constants derived from these data are useful in t h e calculation of steric interactions in butterfly carbides by molecular mechanics methods.l*
(a) University of Sassari. (b) Northwestern University. Bradley, J. S. Adu. Organomet. Chem. 1983, 22, 1. Bradley, J. S.; Ansell, G. B.; Hill, E. W. J. Am. Chem. SOC.1979, 101, 7417. Holt, E. M.: Whitmire, K. H.; Shriver, D. F. J . Am. Chem. SOC.1982, 104, 5621. Tachikawa, M.; Muetterties, E. L. J . Am. Chem. SOC.1980, 102, 4541. Biloen, P.; Helle, J. N.; Sachtler, W. M. H. J . Catal. 1979, 58, 95. Tachikawa, M.; Muetterties, E. L. Prog. Inorg. Chem. 1981, 28, 301. Braye, E. H.; Dahl, L. F.; Hiibel, W.; Wampler, D. L. J . Am. Chem. SOC.1962, 84, 4633. Bor, G.; Stanghellini, P. L. J . Chem. SOC.,Chem. Commun. 1979, 886. Dietler, U. K.; Stanghellini, P. L.; Gervasio, G.; Rossetti, R.; Bor, G.; Sbrignadello,G.; Battiston, G. A. J . Organomet. Chem. 1981, 213, 277. Stanghellini, P. L.; Rossetti, R.; D'Alfonso, G.; Longoni, G., private communication. Oxton, I. A,; Kettle, S . F. A,; Jackson, P. F.; Johnson, B. F. G.; Lewis, J. J . Mol. Struct. 1981, 71, 117. Oxton, I. A.; Powell, D. B.; Goudsmit, R. J.; Johnson, B. F. G.; Lewis, J.; Nelson, W. J. H.; Nicholls, J. N.; Rosales, M. J.; Vargas, M. D.; Whitmire, K. H. Inorg. Chim. Acta 1982, 64, L259. Johnson, B. F. G.; Lewis, J.; Nicholls, J. N.; Oxton, I. A,; Raithby, P. R.; Rosales, M. J. J . Chem. SOC.,Chem. Commun. 1982, 289. Creighton, J. A,; Della Pergola, R.; Heaton, B. T.; Martinengo, S.; Strona, L.; Willis, D. A. J . Chem. SOC.,Chem. Commun. 1982, 864. Kolis, J. W.; Basolo, F.; Shriver, D. F. J . Am. Chem. SOC.1982, 104, 5626. Stanghellini, P. L.; Cognolato, L.: Bor, G.; Kettle, S. F. A. J . Crystallogr. Spectrosc. Res. 1983, 13, 127. Bogdan, P. L.; Horwitz, C. P.; Shriver, D. F. J . Chem. SOC.,Chem. Commun. 1986, 5 5 3 .
0020-1669/87/1326-2950$01.50/0
Samples of [Fe4C(CO)12] 2-,19 [ HFe,C(CO) 12]-,20 Fe4C(CO)1 3 , 2 1 HFe4(CH)(C0)12,20[Fe3RhC(C0),2]-,22and [Fe3WC(C0),3]2-27 were prepared according to published procedures. Selective I3C enrichment at the carbide carbon of the tetrairon clusters was accomplished by enriching the carbonyls in the starting material [Fe4(C0),,l2- to ca. 60% in 13C0.23Since the carbide atom is derived from a carbonyl ligand on the starting material,I9 this leads to a product that is I3C enriched at all carbon atoms in the cluster. One of the "CO ligands was cleaved to produce a I3C carbide ligand, and then the carbonyl ligands were exchanged by stirring a CH2C12solution of the compound under an atmosphere of I2CO for 8 days, intermittently replacing the atmosphere over the solution with fresh I2CO. This final material was I3C-enriched exclusively at the carbide ligand, thus simplifying the assignment of modes associated with the C ligand. Infrared spectra were obtained on a Perkin-Elmer 580 infrared spectrometer or a Nicolet 7199 FT-IR spectrometer on N U J Omulls ~ or CsI pellets of the samples. Low-temperature spectra were obtained on samples in a standard optical cryostat. Raman data were determined with a Spex 1401 double monochromator using 676- or 647-nm Kr ion laser excitation. A 180' backscattering technique was employed on spinning samples.24 Laser power measured at the sample was typically 30 mW, and the band-pass of the spectrometer was approximately 5 cm-I. Semiquantitative depolarization ratios were measured as described elsewhere.2s
Results and Discussion Vibrational Assignments for Clusters with C2"Symmetry. For t h e idealized C, symmetry of the butterfly carbides there are four modes associated with vibrations of t h e carbon a t o m , shown schematically in Figure 1 . All of these vibrations are both Ramanand IR-active. The A,( 1) symmetry coordinate involves motion of the heavy atoms, and the frequency of t h e associated normal mode is expected t o be low. The most intense band in the Raman spectrum of [Fe4C(C0),,l2- is a t 272 cm-' (Figure 2) and is assigned to this totally symmetric Fe-C-Fe stretch, which involves very little C motion because t h e Fe-C-Fe a r r a y is nearly linear (176'). The assignment of this band t o an A, mode is firmly established by the observation t h a t it is polarized in t h e solution Raman spectrum of [PPN]2[Fe,C(CO)12]. T h e M-M stretching frequencies a r e observed in the Raman spectrum below 220 cm-I. T h e remaining t h r e e modes, which primarily involve motion of the carbon atom, are expected t o have IR and Raman transitions between 9 5 0 a n d 550 cm-l. In most cases these a r e readily observed in t h e infrared spectrum. As shown in Figure 3, there (19) Ceriotti, A,; Chini, P.; Longoni, G.; Piro, G. Gazr. Chim. Ital. 1982, 112, 353. (20) Holt, E. M.; Whitmire, K. H.; Shriver, D. F. J . Organomet. Chem. 1981, 213, 125. (21) Bradley, J. S.; Ansell, G. B.; Leonowicz, M. E.; Hill, E. W. J . Am. Chem. SOC.1981, 103,4968. (22) Hriljac, J. A,; Swepston, P. N.; Shriver, D. F. Organometallics 1985, 4, 158. (23) Drezdzon, M. A,: Shriver, D. F. J . Mol. Catal. 1983, 21, 81. (24) Strommen, D. P.; Nakamoto, K. Laboratory Raman Spectroscopy; Wiley: New York, 1984; p 57. (25) Reference 24, pp 88-89. (26) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1955.
0 1987 American Chemical Society
Iron Butterfly Clusters
Inorganic Chemistry, Vol. 26, No. 18. 1987 2951
Table I. Carbide Vibrational Data for Molecules of C, Symmetry (cm-')O
compd [PPNlz[Fe4C(C0)121
BZ
temp, K obsd calcd obsd calcd obsd calcd obsd calcd obsd calcd obsd calcd
300 100
[PPNI [H Fe4C(CO)
300 100
Fe&(CO) 13
300 100
Al(2)
BIb
1 2 c
12C
13C
1%
l3C
929 931 934 935 922 923 [928, 9221 926 928 930 936 937
896 898 90 1 902 889 891 893 893 895 897 904 904
666 673 670 673 66 1 665 [664, 6581 667 658 667 663 666
652 650 [660, 6551 65 1 648 643 649 645 645 645 650 644
608 590 611 591
Ai(1Y
izc
l3c
592 -609 593
601 572 606 573 600 573 602 574
568 601 587
550 586 550
272 290 29 1 269 287 288 252 290 292
'Vaseline mulls. Those bands observed to split at low temperature are enclosed in brackets. Calculated values were obtained by a least-squares fit of the B2 and Al(2) symmetry modes as described in the text. Both I2C and "C data were used in the fit. bMeasured by difference spectrum. 'Obtained from the Raman spectrum as polycrystalline samples. All other data are from infrared measurements.
B7
A
