6632
J . Phys. Chem. 1986, 90, 6632-6638
pared to the cluster for which the ligand is added to the NH4+ (VI) by about 2 kcal/mol. Third, the 6-31G* values can be used as an upper limit and the 3-21G values can be used as a lower limit for the relative energies of the isomeric clusters. Fourth, distinct solvent shells can be distinguished for (HzO),H+ and (NHJ,,,H+ but not for (H,O),(NH,)H+. Fifth, mixtures of isomeric clusters can be present at equilibrium when n = 4 and, ions, Sixth, the possibly, when = 3 for the (H,o),(NH,)H+ electrostatic contribution to the stabilization energy appears to be larger for the (H,O),(NH,)H+ clusters for which the ligand
is added to the outer shell, and the delocalization contribution appears to be larger for the (H,O),(NH,)H+ clusters for which the ligand is added to the inner shell. This compensatory effect, which is not observed for the (NH3),,,H+ or (HzO),H+8*10ions, explains the smaller 6hE$ found for the (H20),(NH3)H+ isomers.
Acknowledgment. I thank Dr. H. B. Schlegel for helpful discussions. The support of the Air Force Geophysics Laboratory Information Resources Center is gratefully acknowledged. Registry No. NH4+,14798-03-9; NH3, 7664-41-7; HzO, 7732-18-5.
Electron Paramagnetic Resonance Study of the Electronic Structure and Dynamic Jahn-Teller Effect in Decamethylmetallocenes L. Zoller,+ E. Moser,* and J. H. Ammeter Institute of Inorganic Chemistry, University of Zurich, CH-8057 Zurich, Switzerland (Received: April 30, 1986)
Decamethylcobaltocene and the isoelectronicdecamethylnickeloceniumcation have been diluted in several diamagnetic host lattices and studied by electron paramagnetic resonance (EPR) spectroscopy at low temperature as single crystals and polycrystalline samples. From the analysis of the g tensor and the cobalt hyperfine tensor and extended-Huckel molecular orbital calculations a quantitative comparison of covalency and dynamic Jahn-Teller effects with cobaltocene and nickelocenium cation was possible. In bojh decamethylmetallocenes the product of covalent and vibronic reduction factor kllV turned out to be notably smaller than in their metallocene counterparts. The reduction is due to both a quenched orbital angular momentum of the ligand part of the highest occupied molecular orbital ylIand a larger Jahn-Teller coupling constant kJT. Covalency, Le., spin distribution, however, is mainly unaffected by the methyl substitution. Further evidence is given that the low-temperature EPR line widths in orbitally near-degenerate sandwich compounds are determined by random strain effects.
Introduction Paramagnetic sandwich compounds, mostly diluted in suitable diamagnetic hosts, have been the subject of many electron paramagnetic resonance (EPR) studies in recent Of special interest in this series of compounds are the low-spin d5 and d7 systems since they have a degenerate or nearly degenerate electronic ground state and are therefore expected to be subjected to Jahn-Teller effects., Within the framework of a vibronic molecular orbital model, EPR experiments can give information on Jahn-Teller effects via so-called Ham type reduction factors multiplying orbital angular momentum contributions to the g value^.^ Other valuable information can be obtained on the covalency of the highest occupied molecular orbital via magnetic hyperfine coupling constants. In all d5 and d7 systems was observed a pronounced dependence of the EPR parameters (g tensor and metal hyperfine tensor) upon the crystalline host lattice or the glassy Most of the variation could be attributed to changes of the splitting of the two pseudodegenerate electronic states due to changes of the lowsymmetry components of the crystal field and to variations in the amplitudes of dynamic Jahn-Teller distortion^.^,^ Among the best known and most thoroughly studied sandwich compounds are the metallocenes, M(cp), (bis(v5-cyclopentadieny1)metal). Although they have been known for more than 30 years,6 they still draw a lot of attention from crystallographic,' chemical,* and theoretical9 points of view. Recently Robbins et a1.I0 and Koelle et al." prepared a large series of decamethylmetallocenes, M ( ~ p m e (bis(v5-pentamethylcyclo~)~ pentadieny1)metal) and found these compounds and their cationic derivatives [M(cpme5),]+ closely related to their metallocene and metallocene cation counterparts. From magnetic susceptibility studies and preliminary EPR results,I0 it was concluded that they *To whom correspondence should be addressed at the University of Fribourg, Institute of Inorganic Chemistry, CH-1700 Fribourg, Switzerland. 'Present address: Wild Heerbrugg, CH-9435 Heerbrugg, Switzerland.
0022-3654/86/2090-6632$01.50/0
each have the same electronic ground state. X-ray studies and gas-phase electron diffraction revealed very similar molecular structures with almost identical metal-to-ring d i ~ t a n c e s . l ~ - ' ~ However, some notable differencies exist as far as electrochemical properties and chemical reactivity are concerned.]' In this paper we discuss EPR spectra of the d7 metallocenes decamethylcobaltocene and decamethylnickelocenium cation ( 1 ) Ammeter, J. H. J. Magn. Reson. 1978, 30, 299 and references therein. (2) Rajasekhasan, M. V.; Giezynski, S.; Ammeter, J. H.; Oswald, N.; Michaud, P.; Hamon, J. R.; Astruc, D. J. Am. Chem. SOC.1982, 104,2400 and references therein. (3) See,e.g.: Warren, K. D. Strucf. Bonding (Berlin) 1976, 27,45. Clack, D. W.; Warren, K. D. Ibid. 1980, 39, 1 and references therein. (4) Ham, F. S. Phys. Reu. 1965, 138, 1727; 1968, 166, 307. (5) Ammeter, J. H.; Zoller, L.; Bachmann, J.; Baltzer, Ph.; Gamp, E.; Bucher, R.; Deiss, E. Helu. Chim. Acta 1981, 64, 1063. (6) (a) Kealy, T. J.; Paulson, P. L. Nature (London) 1951,168, 1039 (b) Miller, S. A,; Tebboth, J. A,; Tremaine, J. F. J . Chem. SOC.1952, 632. (c) Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward, R. B. J. Am. Chem. SOC.1952, 74, 2125. (d) Fischer, E. 0.;Pfab, W. 2.Naturforsch. B: Anorg. Chem., Org. Chem. Biochem., Biophys., Bioi. 1952, 7B, 377. (7) (a) Seiler, P.; Dunitz, J. D. Acta Crystallogr., Sect. 8: Strucf. Crystallogr. Crysf. Chem. 1979, 835, 1068; 1979, 835, 2020; 1982, B38, 1741. (b) Bbrar, J. F.; Calvarin, G.; Weigel, D.; Chhor, K.; Pommier, C. J . Chem. Phys. 1980, 73, 438. (c) Calvarin, G.; Clec'h, G.; BCrar, J. F.; AndrC, D. J . Phys. Chem. Solids 1982,43, 785. Calvarin, G.; Btrar, J. F.; Clec'h, G. Ibid. 1982, 43, 791. (8) See, e.g.: Perevalova, E. G.; Nikitana, T. V. Organomet. Reactions 1972, 4, 63. (9) Liithi, H. P.; Ammeter, J. H.; Almlof, J.; Korsell, K. Chem. Phys. Lett. 1980,69, 540. Liithi, H. P.; Ammeter, J. H.; Almlof, J.; Faegri, K. J . Chem. Phys. 1982, 77, 2002. (10) Robbins, J. L.; Edelstein, N.; Spencer, B.; Smart, J. C. J. Am. Chem. SOC.1982, 104, 1882. (11) Koelle, U.; Khouzami, F. Chem. Ber. 1981, 114, 2929. Koelle, U.; Khouzami, F.; Lueken, H. Ibid. 1982, 115, 1978. (12) Freyberg, D. P.; Robbins, J. L.; Raymond, K. N.; Smart, J. C. J. Am. Chem. SOC.1979, 101, 892. (13) Almenningen, A.; Haaland, A.; Samdal, S.; Brunvoll, J.; Robbins, J. L.; Smart, J. C. J. Organomet. Chem. 1979, 173, 293. Fernholt, L.; Haaland, A,; Seip, R.; Robbins, J. L.; Smart, J. C. Ibid. 1980, 194, 351. (14) Haaland, A. Acc. Chem. Res. 1979, 12, 415.
0 1986 American Chemical Society
Jahn-Teller Effect in Decamethylmetallocenes
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 6633
TABLE I: EPR Parameters for Decamethvlcobaltocene in Various Host Lattices and Frozen Solutions gx
hexane decameth ylferrocene pentane I11 pentane I1 isooctane decamethylruthenocene (mecp)Mn(CO)3 pentane I decameth y lmagnesocene
OIn
1.676 (3)
1.935 (3)
gY
1.62 1.737 (1) 1.74 1.81 1.36 1.992 (1) 2.02 2.03 2.05
gz
AP
1.721 (1)
12.0 (1.0)
1.772 (1)
28.1 (9)
cm-I. bCalculated with kll= 0.77 (see text). c260 ={:c
A,' 132 135.4 (1) 135 139 141 143.6 (2) 144 144 146
A,'
kllv6
tan
LY
lO'x
2dOc
84.0 (4)
0.26
1.51
8.6
0.54
74.2 (3)
0.42
3.47
8.3
1.2
tan a,with { = 0.533 X lo3 cm-I, c: = 0.67.15
diluted in various molecular host lattices and compare the results with earlier studies of c ~ b a l t o c e n e and ' ~ nickelocenium cationI6 and we discuss the effects of methylation upon the electronic ground state and Jahn-Teller effect in these compounds.
Experimental Section Synthesis. All preparations were carried out under an atmosphere of argon and generally the procedures described in the l i t e r a t ~ r e ' ~were ~ ' ' ~adopted. ~~ W e are greatly indebted to Dr. J. Robbins and Dr. U. Koelle for generous gifts of samples. Various salts of [Ni(cpme,),]+ and [Co(cpme5),]+ were prepared by rapid precipitation from aqueous solutions of [Ni(cpme5),]BF4 or [Co(cpme5),] BF4 with NH4PF6, KAsF,, KSbF,, or NaBPh4. Dilute polycrystalline samples of ca. 1% [Ni(cpme5),]+ in [Co( ~ p m e ~ ) salts ~ ] + were obtained by quick cooling to -80 O C of a saturated acetone solution. Separated microcrystalline material was filtered off and dried (room temperature, lo4 mbar). Single were crystals of ca. 1% Co(cpme5), in F e ( ~ p m eor ~ )Ru(cpme5), ~ grown from hexane solution by applying a temperature gradient to a sealed rectangular glass tube filled with the saturated solution and some excess host material. By convection, crystals started to grow on the cold end of the tube. Details are given elsewhere.'* EPR Measurements. EPR spectra were recorded on a Varian E-line spectrometer operating at X-band frequencies. The magnetic field was calibrated with a Varian N M R gaussmeter and the microwave frequency was measured with an EIP frequency counter. Temperature control was achieved by an Oxford Instruments ESR 10 helium flow cryostat. Temperature measurements were made by using a gold/iron-chromel thermocouple situated immediately below the sample tube. Quartz sample tubes (4-mm-0.d.) were charged in a nitrogen-filled glovebox (Mecaplex) and sealed under ca. 500 mbar of H e gas in order to improve thermal conductivity. N o decomposition of any sample was observed while keeping them for several weeks. EPR powder spectra were simulated by using a simulation program described by Mohos et al.I9 Approximate error estimates of g and A values were derived from the disagreement of experimental and calculated spectra. A modified Varian goniometer was used for the single-crystal measurements. Single crystals were mounted on a piece of Perspex rod by using a silicon grease film. Details are described elsewhere.,O Results We observed EPR spectra of C o ( ~ p m e doped ~ ) ~ in single crystals of Fe(cpme5), and Ru(cpme5), and diluted in a number (15) Weber, J.; Goursot, A.; Ptnigault, E.; Ammeter, J. H.; Bachmann, J. J. Am. Chem. SOC.1982, 104, 1491. (16) Rajasekharan, M. V.; Bucher, R.; Deiss, E.; Zoller, L.; Salzer, A. K.; Moser, E.; Weber, J.; Ammeter, J. H. J. Am. Chem. SOC.1983, 105, 7516. (17) King, R. B.; Bisnette, M. B. J . Organomet. Chem. 1967, 8, 287. Koelle, U.; Salzer, A. K. Ibid. 1983, 243, C27. (18) Hulliger, J. Ph.D. Thesis, University of Zurich, Zurich, Switzerland 1984. (19) Dad, C.; Schltipfer, C. W.; Mohos, B.; Ammeter, J. H.; Gamp, E. Compur. Phys. Commun. 1981, 21, 385. The program was modified by R. 0. Kiihne (Ph.D. Thesis, University of Zurich, Zurich, Switzerland, 1984). allowing for anisotropic line widths. R.O.K. has also written the program used to analyze and simulate single-crystal spectra. (20) Gamp, E. Ph.D. Thesis, ETH Zurich, Zurich, Switzerland, Nr. 6673, 1980.
c _
20 mT
Slle 1 site 2
SIle 1
Figure 1. Single-crystal EPR spectrum of Co(cpmes)* in Ru(cpmes)2 [4 K]: (a) static field parallel to g,, principal axis direction of site 1; (b) static field parallel to g, principal axis direction of site 1. The g, principal axes of site 1 and 2 are parallel. The angle between g,, (g,) directions of site 1 and site 2 is 40°.
of frozen solutions (Table I). The crystal structure of Fe(cpme5), was determined by Freyberg a t al.I2 whereas only preliminary crystal structure data of Ru(cpme,), are available.,I Typical single-crystal spectra are shown in Figure 1. As expected from the crystal structure, two magnetically inequkalent sites can be seen, each showing a typical eight-line spectrum of an I = '1, nucleus. For orientations of the crystal associated with small cobalt hyperfine splittings, additional "forbidden" Am # 0 EPR transitions occur (Figure lb). This phenomenon was observed in a number of low-spin Co(I1) complexes and is assigned to quadrupole interactions of the central ion.,, The single crystal data were analyzed by standard procedure^.^^ The influence of the cobalt quadrupole tensor was neglected, leading to uncertainties concerning g, and A,. Principal g- and hyperfine tensor axes were found to be parallel within experimental accuracy ( 3000 cm-I). Such splittings are not expected from crystal and molecular structure data or molecular orbital calculations. Otherwise, if the ground state is assumed to be near degenerate, an unreasonably small angular momentum reduction factor (k,,< 0.2) had to be invoked in order to account for the observed g tensor. Second, no indication for large anharmonic terms in the effective Hamiltonian leading to the coexistence of several Jahn-Teller minima with distorted configurations was found in any experiments or MO calculations of sandwich compounds. With large anharmonic terms involved it would be difficult to account for the smooth variation of Vwith tan a, which is illustrated in Figures 6 and 7 for cobaltocene and nickelocenium cation. Furthermore, it was shown that secondorder terms vanish for symmetry reasons in systems having fivefold ~ymmetry.~’Possible Jahn-Teller coupling modes for d7 metallocenes were discussed in earlier paper^.^?^ On the basis of extended-Huckel (EHMO) and a b initio SCF calculations,2s we found for Co(cp), two dominant active vibrations, viz., an in-plane C-C stretch and a C-C-C out-of-plane torsion (see Figure 6i in ref 2). The two models predict similar Jahn-Teller coupling (26) Englman, R. The Jahn-Teller Effect in Molecules and Crystals; Wiley: New York, 1972. Sturge, M. D. Solid State Phys. 1967, 20, 91. (27) Engelking, P. C.; Lineberger, W. C. J. Chem. Phys. 1977,67, 1412. (28) Zoller, L.; Luthi, H. P.; Ammeter, J. H., unpublished results. EHMO calculations were carried out by using standard procedures.29 The following parameters were employed: Basis set consisted of double-r functions for Co(0) [3d’4s2](3d,4s) and C(0)[2s22p2](2s,2p)taken from Clementi and Roetti;’O Co(II)[3d64p] (4p) taken from Richardson et al.;” H(1s) f = 1.2. VSIP: values for Co determined by SCCC procedures with parameters given by Basch et al.’* and values for C and H taken from Skinner and Pritchard.)’ Ab initio SCF calculations were carried out by using the program employing a split valence basis set Co(l1,7,5/4,3,2); C(8,4/3,2); H(2/2). The following bond lengths were empl~yed:’~metal-to-ring, 1.73 A; C-C, 1.42 A; C-H, 1.10 A.
TABLE III: Calculated Orbital Angular Momentum Reduction Factors and Coefficients CoCo[Ni[Ni0.76
cpme5. 0.64
1.0
1.0
CP’
yda
k c;lb
C,2C
( c P ) ~ (cpme512 0.62 0.54 0.82 0.77 0.48 0.51 0.69 0.65
( c P ) ~ I + (cpme5)d+ 0.71 0.62 0.78 0.70 0.76 0.78 0.35 0.32
OkIl = 1 - ~ ~ ~-’ yll); ~ ( c,’~ 1 = 0.50 for Co(cp), and Co(cpme5),.I5 c,‘* = 0.64 for [ N i ( ~ p ) ~ and ] + [ N i ( ~ p m e ~ ) ~ ] +bc,’2: . ’ ~ ligand coeffi-
cient. cc,2: metal 3d coefficient.
strengths k j = ~ (2EJT/hv)’12, and both favor the torsion mode = as most dominant (C-C stretch (hv = 1350 cm-’ 36) kJTEHMo 0.57, kJTSCF= 0.48; C-C-C torsion (hv = 600 cm-’ 36) kjTEHMo = 0.77, kJTSCF= 0.93). The same modes are expected to be dominant in [ N i ( ~ p ) ~ ] +Co(cpme,),, ,l~ and [Ni(cpme5),]+, too. Comparing the data of Tables I and I1 with g tensor data of cobaltocenei5 and nickelocenium cation,I6 we note a considerable difference as far as the dependence of g, on g,,is concerned. In Figure 8 is plotted g, vs. g,,for various nickelocenium and decamethylnickelocenium cations, illustrating that g,,is shifted toward higher values in decamethylmetallocenes. It can easily be seen from eq 4 that this shift is due to smaller reduction factors k,, or V. This is illustrated in Figures 6 and 7 , where we notice that the product k,,V for decamethylcobaltocene and decamethylnickelocenium cation are both reduced by ca. 12% compared with the values of cobaltocene and nickelocenium cation at the same tan a. In order to decide experimentally whether the reduction is due to a reduced angular momentum reduction factor k,,or to a smaller vibrational overlap V (and therefore a larger
(29) Haberditzl, W. Quantenchemie, Vol. 4, Hiithig, A., Ed.; Verlag: Heidelberg, 1979. See.also: Ammeter, J. H.; Burgi, H. B.; Thibeault, J. C.; Hoffman, R. J. Am. Chem. SOC.1978, 100, 3686. (30) Clementi, E.; Roetti, C. A t . Data Nucl. Data Tables 1974, 14. (31) Richardson, J. W.; Powel, R. R.; Nieuwpoort, W. C. J. Chem. Phys. 1963, 38, 796. (32) Basch, H.; Viste, A.; Gray, H. B. J. Chem. Phys. 1966, 44, 10. (33) Skinner, H. A,: Pritchard, H. 0. Chem. Rev. 1955, 55, 745. (34) Almlaf, J.; Faegri, K.;Korsell, K. J. Comput. Chem. 1982, 3, 385. (35) Almcnningen, A.; Gard, E.; Haaland, A,; Brunvoll, J. J. Organomet. Chem. 1976,107,273. Hedberg, A. K.; Hedberg, L.; Hedberg, K. J . Chem. Phys. 1975, 63, 1262. (36) Aleksanyan, V. T.; Lokshin, B. V. J . Organomet. Chem. 1977, 131, 113.
Jahn-Teller Effect in Decamethylmetallocenes
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 6637
--
-
~ , 2 c , , ? k , ~ *{ ~/ A( E ) being independent of the host system. From eq 4 then only a slight decrease of 6g is expected from the more
I
g
:
,* /
0,s
1.0
1.5
Figure 9. A, vs. g, for Co(cp), (0) and Co(cpme5)2( 0 ) .
Jahn-Teller coupling constant k j T ) we had to examine the hyperfine data. Unfortunately we could only get two complete cobalt hyperfine tensors. The [Ni(cpme,),]+ salts were prepared with Ni isotopes in natural abundance (99% 6 w i with I = 0), and our EPR measurements did not reveal any nickel hyperfine data. Therefore, we tried to rationalize these findings on the basis of extended-Huckel MOs3' by calculating k,,with the help of a program written by Rauk and I c h i m ~ r a . k,, ~ ~can be written as k,, = 1 - c,', (1 - y,,), where ylI = ($4L~lz~i$sL)fz-' is a memure for the deviation of the ligand part of the molecular orbitals from axial symmetry, approaching one, when in the limit of cyclindrical symmetry the orbitals become eigenfunctions of lz.39 The results of our calculations are summarized in Table 111. We note that c,'* and c 2 do not differ more than a few percent in the decamethylated and nonmethylated compounds, respectively. Indeed, the hyperfine data of decamethylcobaltocene do suggest that c 2 is very similar in Co(cp), and Co(cpme5),. This is shown in Figure 9 where the experimental A , values are plotted vs. g,. The data for both compounds fit onto the same straight line, indicating that K thus c 2 ) are not notably the Fermi contact terms P C ~(and different. The orbital angular momentum yII is considerably smaller in the permethylated compounds. In the free ligand cpmes', Co(cpmeS), and [Ni(cpmeS),]+, yIIis reduced by ca. 13% and k,, is decreased on the order of 5-7%. However, the total observed reduction of the product k,,Yisca. 12% (from Figures 6 and 7), indicating that a reduction of the vibrational overlap Y must also be considered. I/ can principally vary between zero (limit of large Jahn-Teller coupling) and one (static limit of negligible Jahn-Teller coupling). A smaller Vindicating a larger vibronic coupling coefficient kJTcan be caused by either a larger Jahn-Teller energy EJTor a lower frequency of the active mode. The latter is difficult to argue about because we are dealing with a multimode coupling situation. However, methylation may well increase the effective mass of some of the active modes without effecting the force constants to the same extent and thus lower their frequency. Unfortunately no conclusive infrared or Raman data of the studied metallocenes are known. From our calculations of the Jahn-Teller effect of Co(cp), we found that the C-C-C ring torsion mode mainly optimizes the M-C bond orders whereas the C-C stretch mode optimizes the C-C bond orders. The Jahn-Teller energy of the torsion mode is therefore not likely to be changed by the methyl groups whereas steric effects do not favor a larger stabilization in the case of the stretching mode. Yet it can be argued that the three active modes mainly involving C-CH3 distortions may become more important than the corresponding C-H modes in Co(cp), due to lower frequencies and maybe higher amplitudes, thus being responsible for an increased Jahn-Teller effect, Le., a reduction of the vibrational overlap on the order of a few percent. Table I1 does not show a significant variation of 6g = gy - g, among the different host lattices. This can be understood in terms of x = (37) Procedure and parameters are given in ref 28; respective parameters for NI were employed, 1.83 A was taken for the metal-to-ring distance and 1.50 A for the C-C' bond lengths. (38) Ichimura, H.; Rauk, A. J . Chem. Phys. 1973, 59, 5720. (39) Bishop, D. M.; Dingle, T.W. J . Chem. Phys. 1968, 48, 541.
axial systems with small tan (Y to the stronger orthorhombically destorted systems with larger tan (Y due to the decreasing Ycos (Y term. However, this smooth variation is completely masked by experimental errors, as it was in the case of [Ni(cp),]+. The average value x = 0.059 in Table I1 is some 5% smaller than the average x = 0.062 found for [ N i ( ~ p ) , ] + . Considering ~~ the experimental errors the derived figures might not be to accurate. Nevertheless, the EHMO calculations of [Ni(cpmes),]+ point into the same direction since we calculated not only a smaller k,,value but also on a few percent - smaller orbital angular momentum reduction factor kLUs6( A E is left unchanged). Hence, the oneelectron excitation energy seems to be nearly unaffected in agreement with preliminary optical studies of various methylated c o b a l t o ~ e n e s .In ~ ~this study the d-d bands were not found to shift by methylation, in contrast to the ones assigned to chargetransfer transitions. It must be mentioned however, that the methylated ds and d6 metallocenes have a larger ligand field splitting than the unmethylated ~nes.~O,~O Line Broadening. We found the same characteristic temperature dependence of line widths in the EPR spectra of Co(cpmes), and [Ni(cpmes),]+ as has been observed in earlier EPR studies of orbitally degenerate sandwich compounds.2J6 The line widths in single-crystal and powder spectra are constant below a characteristic temperature that depends on the host system and to a lesser extent on the preparation of the sample. In this low-temperature range the EPR transitions show saturation effects at usually moderate microwave intensities (
