1646
J . Am. Chem. Soc. 1984, 106, 1646-1650
Bonding, Hyperfine Interactions, and Lattice Dynamics of Bis( pentadieny1)iron Compounds Richard D. Ernst,*+ David R. Wilson,+ and R. H. Herber*t Contribution from the Departments of Chemistry, University of Utah, Salt Lake City, Utah 841 12, and Rutgers. The State University of New Jersey, New Brunswick, New Jersey 08903. Received May 23, 1983 Abstract: The 57FeMossbauer effect parameters have been determined for five bis(pentadieny1)iron compounds and compared to the corresponding data for ferrocene and azaferrocene. The decrease in the isomer shift parameter in going from the cyclic to the open ligand structures can be accounted for by a change in the oribtal population around the metal atom resulting in an increase in the electron density at the iron nucleus. The quadrupole splitting parameter for the pentadienyl compounds at liquid nitrogen temperature is about 50% of the value observed for the corresponding cyclopentadienyl homologues. An effective Mossbauer lattice temperature for these covalent solids has been determined from the temperature dependence of the recoil-free fraction of the 57Feresonances. The s7FeMossbauer parameters for bis(2,4-dimethylpentadienyl)iron dissolved in an inert glass forming solvent are identical with those of the neat solid, suggesting that there are no significant structural changes between the molecule present in solution and that which makes up the solid, thus permitting a direct comparison between the X-ray diffraction data and the solution spectroscopic data for these “open ferrocene” compounds.
A major factor in the growth of interest in organometallic chemistry during the last three decades derives from the unusual properties of ferrocene and related “sandwich” compounds. A great deal of effort has been expended in theoretical studies to arrive a t an appropriate description of the bonding in such molecules and to account for the wealth of experimental data available for these compounds. It is interesting to note, in reference to the spectroscopic studies that have been published in the literature, that ferrocene was the first organometallic compound for which information was obtained by using S7FeMossbauer effect methods,]+ and the results of such studies are now considered to be reasonably well understood in terms of a complete MO description of (qS-CsHS),Fe. More recently, a series of “open ferrocenes” has been reported by Emst et a1.J and the electronic structures, photoelectron spectra, and semiempirical calculations for these new compounds have been discussed.7a In the present paper we report detailed 57Fe Mossbauer effect studies of five bis(pentadieny1)iron compounds, together with corresponding data for ferrocene and azaferrocene, and relate the results of this investigation to the MO descriptions of the bonding in these organometallics. Experimental Section Synthesis. Samples of bis(pentadieny1)iron (l), bis(2-methylpentadieny1)iron (2), bis(3-methylpentadieny1)iron (3), bis(2,4-dimethylpentadieny1)iron (4), and bis(2,3-dimethylpentadienyl)iron (5) were obtained as reported p r e v i o ~ s l yAzaferrocene .~~~~ was prepared by literature methods8 and generously made available to us by Prof. A. Efraty. Ferrocene was obtained commerciallyg and recrystallized repeatedly from anhydrous ethanol.I0 All compounds were stored under vacuum or in an inert atmosphere until just prior to spectroscopicexamination. 57FeMossbauer Spectroscopy. Mossbauer measurements were carried out by using the constant acceleration spectrometer described earlier.” Spectrometer calibration was effected by using 0.82-mil National Bureau of Standards SRM iron foil at 295 K, and all isomer shifts reported in this paper are with reference to the centroid of such a spectrum.I2 Data analysis was done by using the SPECTRA” program described earlier, in which line position, line width, and effect magnitude are allowed to vary as independent parameters in a matrix inversion least-squares fitting routine. The experimental samples (solids) were mounted as thin layers of powder between two 6.25 mg/cm’ foils of 99.999% aluminum, rigidly clamped to a copper sample holder. The latter, in turn, was attached to a copper sample mount that is thermally clamped to the cold head of a Heli-tran cryostatI4 and fitted with two Chromel/Au-O.O7% Fe thermocouples. Temperature control to better than h0.5 OC was achieved by the use of a proportional temperature controller. Thermocouple calibration is based on NBS data. ‘University of Utah. ‘Rutgers, The State University of New Jersey. 0002-7863/84/ 1506-1646$01.50/0
Results and Discussion The results of the 57FeMossbauer experiments are summarized in Table I and a representative spectrum is shown in Figure 1. All of the spectra consist of two well-resolved resonance lines, each of which has a full width at half-maximum (0.25 to 0.30 mm SKI) corresponding to a single iron site (Le., there is no evidence for an unresolved hyperfine interaction in any of the resonance spectra). In addition to the isomer shift (IS) and quadrupole splitting (QS) data that can be obtained from a spectrum at a single temperature, two other parameters of interest can be extracted from spectra obtained over a temperature range. These parameters elucidate the motional properties of the Mossbauer atom and provide information about the lattice temperature (0,) and the motional anisotropy ( R ) of the metal atom in the compounds under study. The temperature dependence of the recoil-free fraction for a thin absorber, for which saturation effects can be neglected, is given by the temperature dependence of the area under the resonance curve and obeys a relationship of the form d[ln.fl dIln (area)] 3E.,= A typical data set for ferrocene and 4 is shown in Figure 2. In ( l ) , Meffis the effective vibrating mass that participates in the (1) (a) Wertheim, G. K.; Herber, R. H. In ”Proceedings2nd International Conference on Mossbauer Effect”,Compton, D. M. J., Schoen, A. H., Eds.; Wiley: New York, 1962; pp 105-1 11. See also: Zahn, U.; Kienle, P.; (b) Eicher, H. In ref la, pp 271-273. (2) Wertheim, G. K.; Herber, R. H. J . Chem. Phys. 1962, 38, 2106. (3) Herber, R. H.; Kingston, W. R.; Wertheim, G. K. Inorg. Chem., 1963, 2, 153. (4) Epstein, L. M. J . Chem. Phys. 1962, 36, 2731. (5) Lesikar, A. V. J . Chem. Phys. 1964, 40, 2246. (6) Stukan, R. A.; Gubin, S. P.; Nesmeyanov, A. N.; Gol’danskii, V. I.; Makarov, E. F. Teor. Eksp. Khim. 1966, 2, 805. (7) (a) Wilson, D. R.; Dilullo, A. A.; Ernst, R. D. J . Am. Chem. SOC.1980, 102, 5928. (b) Bohm, M. C.; Eckert-Maksii., M.; Ernst, R. D.; Gleiter, R. Ibid. 1982, 104, 2699. (c) Wilson, D. R.; Ernst, R. D.; Cymbaluk, T. H. Organometallics 1983, 2, 1220. (8) Joshi, K. K.; Pauson, P. L.; Qazi, A. R.; Stubbs,W. H. J . Organomef. Chem. 1964,1,471. King, R. B.; Bisnette, M. B. Inorg. Chem. 1964, 3,796. (9) Aldrich Chemical Co., Milwaukee, W I 53233. (10) Lippincott, E. R.; Nelson, R. D. Specrrochim. Acta 1958, 10, 307. (11) Rein, A. J.; Herber, R. H. J . Chem. Phys. 1975, 63, 1021 and ref-
erences therein. (12) Spijkerman,J. J.; DeVoe, J. R.; Travis, J. C. Nafl.Bur. Srand. (US.) Spec. Publ. 1970, 260-20. Stevens, J. G.; Gettys, W. L. In “Mossbauer Isomer Shifts”; Shenoy, G. K., Wagner, F. E., Eds.; North Holland Publishing Co.: Amsterdam, 1978; pp 901 ff. (13) Viegers, M. P. A. “I9’Au Mossbauer Spectroscopy”, Ph.D. Thesis, Katholieke Universiteit Nijmegen, Nijmegen, The Netherlands, 1976. Viegers, M. P. A,; Trooster Phys. Reu. 1977, B15, 72: Nucl. Insfrum. Methods 1974, 118, 257. (14) Air Products Co., Allentown, PA.
0 1984 American Chemical Society
Bis(pentadieny1)iron Compounds
J. Am. Chem. SOC.,Vol. 106, No. 6, 1984 1641
Table I. Summary of "Fe Mossbauer Data compound
- d i ~ i / d 103 ~, ~ OM,
-
azaferrocene 0.592 t 0.005 2.494 i 0.010 3.11 134 t 10 7.4420.65 135 t 6
ferrocene 0.542 t 0.009 2.453 i 0.017 3.74 111 i: 8 8.09t0.22 130 f 6
IS ( 7 8 K), mm s-' ' QS (78 K), m m s-' -dIS/dT, lo4 mm SKI K - ' M,ff, a m b 1
KC
1
2
3
0.479 f 0.010 1.402 i 0.015 3.52 118 t 9 5.13i0.22 160 t 7
0.462 i 0.005 1.206 i 0.008 3.84 108 i 9 7.29i0.35 137 i 7
0.482 i 0.005 1.255 i 0.041 4.06 103 i 8 5.49t0.41 157 i 11
With respect to the center of a metallic iron spectrum at 295 f 2 K.
Calculated from eq 2.
5
4 0.498 i 0.007 1.516 t 0.010 3.60 116 i 9 5.48i0.25 157 t 7
0.461 i 0.005 1.261 i 0.006 2.30 1 8 2 i 15 7.94i0.46 131 t 8
Calculated from eq 3.
r
0.0
96
-
h
t
co k -0.5 .
e
h
t -1.0a C
d
g71 96
I !
-6.0
100
*. '. . ..
I
I
-4.0
I
I
-2.0
I
200
250
TEMPERATURE
300
(OK)
.
I
0.0
I
I
2.0
I
I
4.0
I
I
I
6.0
VELOCITY ( mm /s) Figure 1. 57Fe Mossbauer suectra of ferrocene (al and bis(2.4-di-
methylpentadieny1)iron (4) (6) at liquid nitrogen temperature. The isomer shift (horizontal) scale is with reference to metallic iron a t rmm temperature. '
lattice dynamical processes in the solid, and OM is a lattice temperature as probed by the Mossbauer-active atom. For many covalent solids, Meffcan be replaced by the molecular weight, provided that specific intermolecular interactions are negligible, and the solid can be approximated by point masses having the expected molecular weight. In cases where this assumption is not valid, the effective vibrating mass can be related to the temperature dependence of the isomer shift through the second-order Doppler shift, and this relationship is of the form d(IS) -=--dT
150
Figure 2. Temperature dependence of the recoil-free fraction (normalized to the 78 K value) for ferrocene (circles) and bis(2,4-dimethylpentadien yl) iron (4) (diamonds).
*.
0
- 1.5 -
3 k 2 Meffc
