Journal of Nuclear and Radiochemical Sciences, Vol. 9, No. 2, pp. 33-36, 2008

ArticlesHyperfine Interactions and Metal Atom Dynamic Effects of Pentafluorophenyl Substituentson Ferrocene Complexes
Rolfe H. Herber* and Israel Nowik
Racah Institute of Physics, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel
Received: June 4, 2008; In Final Form: July 7, 2008
Seven ferrocenyl complexes, in which one or more of the Cp ring protons have been substituted by strongly electron-withdrawing pentafluorophenyl groups, have been examined by temperature-dependent 57 Fe Mössbauer
spectroscopy (MES) to determine both the electronic and dynamic effects of such substitution. The mean-squarevibrational-amplitudes of the metal atom have been expressed in terms of the dimensionless parameter₣z,T =k2 <xave2 >, where z=M for the Mössbauer data, z=X for the X-ray data, k is the wave vector of the gamma
radiation, and <xave2 > is the mean square amplitude of the metal atom vibration. The differences between the M
and X values can be related to the effects of local (atomic) and non-local (molecular) contributions to the metal
atom dynamics. The MES data also elucidate the vibrational anisotropy of the metal atom motion, which in all of
the complexes examined is isotropic within the experimental error.

1. Introduction
The recent development of imaginative new synthetic techniques for the preparation of ferrocenes, in which one or moreof the Cp ring protons has been replaced by electron-withdrawing per-fluoro substituents,1 has made it possible to investigate
in detail the effects on both the s-electron density at the ironnucleus as well as the dynamics of the metal atom of this substitution. In addition, temperature-dependent Mössbauer effectspectroscopy (MES) has facilitated the comparison of the
dynamical behavior of the iron atom in such complexes as
extracted from single crystal X-ray diffraction data and thatderived from MES measurements and thus to effect a greater
understanding of the differences between the local (atomic)
and non-local (molecular) motions in such systems.
In the present study, a number of compounds have been
examined, including C 2 2 H 8 F 10 Fe, 2 (1); C 2 4 H 8 F 14 Fe, 3 (2);
C24H14F8Fe, (3); C42H 22F 20Fe·CHCl3,4 (4); C46H4F 30Fe·C6H6,5(5); C38H16F14Fe2,6 (6); and C30H6F16FeN4,7 (7). For enhanced
clarity, the structures of these compounds are collected inscheme 1. In the present study, these compounds have been
investigated by temperature-dependent MES and the dynamical data concerning the iron atom(s) have been compared tothe temperature factor Ui,j values determined by single crystal
X-ray diffraction. A comparison between the MES and X-ray
based data relating to the vibrational amplitudes of the metal
atom in organometallics should consider the effect of latticeimperfections which play a more significant role in the magnitude of the Ui,j values extracted from X-ray diffraction data
than in the mean-square-amplitude-of-vibration data contributing the the recoil-free fraction (MES) values. Inter-molecularmotions will contribute more to the former than the latter,
since the X-ray measurements sample all atomic positions overa lengthy period of time while the MES measurements involve
only a single atom during the scattering event. It should also
be noted that the presence of solvate molecules in the unit cell
will influence the atomic positions sensed by the X-ray technique relative to the MES measurements. This point will be
addressed again in the discussion to follow. Hence, in general

the vibrational amplitudes calculated from the X-ray data arelarger than those derived from the MES data at the same temperature, as will be discussed in greater detail, below. Thisobservation has also been examined in detail in relation to protein dynamics by Parak et al.8
The second point relates to the temperature dependence ofthe vibrational amplitudes. The frequency of such vibrations
can be expressed as ω ∝ [k/M]1/2 where k is the appropriate
force constant and M is the vibrating mass. For intra-molecular motions, k is large and M is small (relatively), whereas forinter-molecular motions, k is small and M is large. Thus, at
low temperatures where the phonon frequencies are low, themajor contribution to the vibrational amplitude will be due tothe inter-molecular motions, while with increasing temperature the intra-molecular motions become more appreciable.
These considerations lead to the expectation that the tempera-

Fe

Fe
F

1

CH3

F

Fe
C6F5

F

CF3

2

3

CH3

C6F5
C6F5
C6F5
Fe
C6F5
C6F5

t-Bu

C6F5

t-Bu

C6F5
Fe
C6F5
C6F5

4

F

Fe
C6F5

C6F5

6

F

F

F

N

N

F

F
F

F

F

F

F

F

C6F5

5

F

F
F

Fe

F
N

N
F

*Corresponding author. E-mail: HERBER@VMS.HUJI.AC.IL
Fax: 972 2 6586 347

CF3

F

C6F5

F

F

F

7

scheme 1© 2008 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/05/2008

Fe

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HerberJ. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

ture dependence of the vibrational amplitudes (recoil-free fraction) will depart increasingly from a linear function as thetemperature is increased as has been discussed in detail by
Gol'danskii and Makarov. 9 This expectation is, in fact,observed in all of the compounds of the present study.
2. Experimental(a) The compounds examined in the present study have been
generously made available by Prof. P. Deck and his researchgroup who have also described the synthetic techniques
involved, as well as the details of the X-ray diffraction results.
These samples, which are air stable powders at room temperature, were used as received.
(b) Mössbauer effect spectroscopy. The experimental procedures involved in the MES studies have been detailed previously10 and were carried out on neat powdered samples held in
Perspex sample holders and examined in transmission geometry. Spectrometer calibration was effected using an α-Feabsorber at room temperature, and all isomer shifts are herereported with respect to the centroid of such calibration spectra
acquired both before and after the sample runs. The MES
spectra consisted in all cases (except where noted) of wellresolved doublets, indicating a single iron crystallographic site,
and a typical spectrum is shown in Figure 1. Typical line
widths (FWHM) of the MES absorption lines in the absenceof saturation effects are 0.22–0.25 mm s -1, compared to thetheoretical value of 0.194 mm s-1, indicative of single iron site

RELATIVE TRANSMISSION

1.00

0.96

0.92

0.88
-2

0

2

VELOCITY / mm s

4

-1

Figure 1. 57Fe Mössbauer Spectrum of compound 6 at 89.4 K. Theline width (FWHM) is 0.313±0.002 mm s-1 despite the presence of
two iron atoms per molecular unit, showing the high crystallographic
similarity of the two metal sites.

absorptions in these solids.
3. Results and discussion
Compound 1: Because of the very limited amount of sample available for study, the temperature range appropriate to 1
is severely limited. The resonance effect magnitude at 90 K
was only 0.86%, necessitating long data collection times at thehigher temperatures. The hyperfine interaction parameters,that is, the isomer shift (IS) and quadrupole splitting (QS) at
90 K, as well as the derived parameters are summarized inTable 1. Ring proton substitution by strongly electronegative
groups such as C6F5 has only a minor effect on the s-electron
density at the metal center, and the replacement of two Cp ring
protons by C6F5 changes the IS from that observed in the parent ferrocene by only ~ 0.01 mm s-1 indicating that the iron dorbitals do not sense the electron withdrawing power of theelectronegative groups. The temperature-dependence of the IS
is reasonably well fitted by a linear regression (correlation
coefficient 0.934 for 7 data points) and leads to a calculated
effective vibrational mass,11 Meff, of 140±4 daltons(Da). As
noted previously, the difference between this value and the 57
Da associated with a "bare" iron atom reflect the covalency ofthe metal atom-ligand bonding. The area ratio, {R = [A(+)/A()], where A(+) and A(-) are the areas under the resonance curveof the doublet component at more positive and more negative
velocities than the spectrum centroid, respectively}, is temperature independent (0.97±0.03) over the entire temperature
range, from which it inferred that the iron atom motion is isotropic over the interval 90<T<210 K.
The area under the resonance curve, A, for an optically"thin" absorber scales with the recoil-free fraction, f, and hencecorrelates with the mean-square-amplitude-of-vibration,<xave2 >, through the relationship ln A ∝ ln f = exp(-k2 <xave2 >),where k is the wave vector of the 57Fe Mössbauer line. In thepresent summary, k2<xave2> is represented by the symbol ₣, andfor 1 this parameter is a linear function of T. Making theassumption that in the high temperature regime, namely above
0.5 ΘM, ₣ is a linear function of T and extrapolates to zero as
T→0, makes it possible to calculate ₣ at all temperatures in the
stipulated range. The value calculated from the MES data at
173 K is ₣ M,173 =1.39(9). The ₣ parameter can also be calculated from the Ui,j values at 173 K reported in the crystallographic study by Hughes et al.12 This value is ₣ X,173 = 1.51(4),and the difference in the two values can be ascribed to the
influence of the static distribution of iron atom positions due
to lattice imperfections to which the X-ray data are sensitive,
but the MES data are not. In the present case, the small difference in the two values implies that the static lattice distortions
are negligibly small in the indicated temperature range. The₣ M,100 value is calculated to be 0.75±0.05.
Compound 2 : The hyperfine parameters at 90 K are

TABLE 1: 57Fe Mössbauer parameters and derived quantities discussed in the text
Compd.
-1
IS(90) /mm s-1
QS(90) /mm s

10.546(7)2.290(5)20.513(3)2.253(3)30.533(3)2.286(3)40.622(4)2.491(4)50.581(4)2.326(4)60.544(3)2.240(3)70.543(4)2.137(4)-dIS/dT-4-1-1/10 mm s K

2.97(28)

3.78(8)5.00(23)

4.66(17)7.47(17)4.84(3)3.39(9)7.55(50)

6.95(9)7.96(42)7.04(9)7.76(30)

5.85(5)6.36(8)0.99(2)86(1)₣X,298=1.97(2)₣M,298=1.74(2)0.996(20)
123(3)-d(ln A)/dT
-3
-1
/10 K
R
Meff/Da₣z,T0.97(3)1.03(2)(T depend)
0.94(2)0.996(17)140(4)
110(2)
84(4)
89(3)
90(3)
₣X,173=1.51(4) ₣X,100=0.889(51)₣X,120=0.928(9) ₣X,100=1.07(1)₣M,173=1.39(9) ₣M,100=0.695(12)₣M,120=0.845(11) ₣M,100=0.78(3)Hyperfine Interactions and Metal Atom Dynamic Effects ofRoot-mean-square-amplitude / Angstrom

0.22
0.20
0.18
0.16
0.14
0.12
100

150

200

250

300

TEMPERATURE / K

Figure 2. Root-mean-square-amplitudes of vibration as a function oftemperature for compound 4 (filled data points) and ferrocene (stars).
The half-filled data points are the values calculated from the X-ray
Ui,j data as discussed in the text.

0.0-3

ln [A(T)/A(90)]d ln A/ d T = -(5.32±0.19)×10 K
corr. coeff. = 0.997 (6)

-1

-0.5

-1.0

-3

-1

(d ln A/ d T = -(7.76±0.30)×10 K )corr. coeff. = 0.999 (6)

-1.5
100

150

200

250

300

TEMPERATURE / K

Figure 3. Temperature-dependence of the area under the resonancecurve, normalized to the 90 K value, for compound 5. The value of₣ M,298 reported in Table 1 is taken from the higher temperature
regime.

1.04

AREA RATIO [ R=A(+)/A(-)]

included in Table 1 and are not otherwise remarkable.
Although there is a fair amount of scatter in the data, the hightemperature limiting slope of d(ln A)/dT is -(6.95±0.12)×10 -3
K -1 with an r value of 0.998 for 10 data points. The X-ray data
at 100 K have been reported by Deck et al.13 and the comparison of the MES and X-ray data lead to ₣ X,100 = 0.89(5) and₣ M,100 = 0.70(1) in modest agreement with each other. Again,
making the assumption that the linear extrapolation leads inthe low temperature limit ₣ M →0, the mean square amplitudesof vibration of the iron atom at the same temperature for 1 and
2 are nearly identical (see above), so that replacement of a phenyl ring F by a CF3 group has essentially no effect on the ironatom dynamics.
Compound 3: Both the IS and ln A temperature dependencies for this compound are well fitted by linear regressions andlead to a value of Meff = 83±4 Da. The Mössbauer lattice temperature, ΘM, is 108±6 K. The area ratio, R, shows a negative
temperature dependence over the range 96 < T < 315 K. The
hyperfine parameters, included in Table 1, are not otherwise
remarkable.
Compound 4: The temperature-dependence of the IS
parameter for 4 shows significant curvature in the low temperature region, but the data in the interval 169<T<249 K are reasonably well fitted by a linear regression with r = 0.996 for 5
data points. This high temperature slope yields a calculated
Meff = 89(3) Da. The temperature-dependence of the ln A data
are closely fitted by a linear regression over the whole range(86.5<T<272 K) with r= 0.999 for 10 data points. From thisslope and the dIS/dT value, the Mössbauer lattice temperature,ΘM , is 110 (10) K justifying the "high temperature limit"
assumption. Moreover the d(ln A)/dT data yield ₣ M,120 =0.848(2), whereas ₣x,120 = 0.928(9) based on the X-ray data of
Deck et al.14 The calculated temperature dependencies of theroot-mean-square-amplitude-of-vibration (rmsav) of the ironatom in 4, as well as the comparable data for ferrocene, aresummarized graphically in Figure 2. The values extracted
directly from the X-ray Ui,j values are indicated by the halffilled data points. The area ratio parameter, R, for 4 is nearlytemperature-independent [mean value 0.94(2) over the whole
T range] suggesting that anisotropy in the iron atom motion
does not make a significant contribution to the metal atom
motion.
Compound 5: This compound was examined by both MES
and X-ray methods as the tetrakis benzene solvate. The IS andQS are included in Table 1, and are otherwise unremarkable
except to note that this compound, which has the largest number of electronegative ring substituents (6), evidences no particularly large effect on the s-electron density at the Fe nucleus.
The temperature-dependence of the recoil-free fraction data issummarized in Figure 3 from which it is noted that the 11 data
points can be separated into two linear correlations, both withexceptionally high r values. Even making use of the high temperature slope [-(7.76±0.30)×10 -3 K-1] leads to a value of ₣M,100= 0.78(4) much smaller than the X-ray value of ₣ X ,10 0 =1.071(9). This large difference may reflect the presence of thefour benzene solvate molecules in the unit cell of the solid, andtheir effect on the X-ray Ui,j values, as noted above. The area
ratio, R, is shown graphically in Figure 4, and is effectively
temperature independent over the temperature range 90<T<299
K [mean value 0.996(17) for 11 data points] indicating again
the isotropic nature of the Fe atom vibrations, in agreementwith the X-ray Ui,j values which are essentially equal for thethree principal components of the thermal factor matrix. It
should be noted that the "break" in the ln f data for 5 is notreflected in the R data summarized in Figure 3 nor is a discontinuity noted in the temperature dependencies of the IS and QS
parameters accompanying the ln f data.
Because of the presence of six C6 F5 groups it was thought
desirable to determine the sign of the QS parameter in this35

J. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

1.02

1.00

0.98

0.96
50

100

150

200

250

300

TEMPERATURE / K

Figure 4. The area ratio R for compound 5 as a function of temperature. The two dashed lines delineate the mean value (solid line) plus/
minus one standard deviation and show that over the indicated temperature interval the iron atom motion is essentially isotropic.

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HerberJ. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

compound, using the magnetic field technique, similar to thestudies of octamethyl- and decamethyl-ferrocene reported earlier.15 As in the case of these two ring substituted compounds,
as well as for the parent ferrocene, for compound 5 QS is positive at room temperature.
Compound 6: This compound has two C6F5 Cp Fe Cp moieties linked by a C6F4 fragment and this is expected to be similar in its hyperfine parameter values to those observed for 1
and 2, and inspection of Table 1 shows that this is indeed thecase. Moreover, although there are two iron atoms per molecule, the observed narrow linewidth of the absorption lines,
0.233(2) mm s -1, shows clearly that the metal atoms are in
identical environments with respect to the MES parameters.
The QS parameter shows the normal negative temperaturedependence due to thermal expansion effects. Both the IS andln A parameters follow a linear temperature dependence. The
high temperature limiting slope of ln A, taken to minimize saturation effects, is -d(ln A)/dT =(5.85±0.05)×10 -3 K -1 with
r= 0.997 for 9 data points. This leads to a value of ₣ M,298 =
1.743(15) while the X-ray data at 298 K yield ₣X,298 = 1.974(20).
The area ratio, R, is essentially temperature independent up to321 K, again indicating isotropic vibrational motion of the ironatom in the solid.
Compound 7: This compound was studied over the temperature range 90<T<294 K and the lower temperature regime
spectra are well fitted by a single doublet, as noted in Table 1.
At temperatures above ~ 130 K the MES spectra are better fitted by including a small amount of an impurity site, which
accounts for up to ~17% of the spectral area at 294 K. The
impurity resonance has a QS of ~0.61 mm s -1 compared to~2.13 mm s-1 for the majority site, but has not been otherwise
identified by the presently available data. The presence of thissecond site has been corrected for in the subsequent data analysis. The T-dependence of the IS of the majority site shows
significant curvature over the entire range, so that a reliablevalue of Meff cannot be calculated. However the d(ln A)/dT
data are well accounted for by a linear regression (r=0.98 for
12 data points) and the slope is similar to those observed forthe other compounds reported here. Similarly, the area ratio,R, is temperature independent as noted above.
4. Summary and Conclusions
Seven ferrocenoid compounds in which one or more Cp ring
protons have been replaced by electron-withdrawing C 6 F5groups, have been investigated by temperature-dependent
MES. It has been noted earlier in the literature 16 that the electrochemical potentials for ferrocenes with a variable numberof C6 F5 substituents was a linear function of the number ofsuch substituents. Consequently, it was expected that the IS
parameter extracted from the MES data would show a similarlinear correlation. However, as the data in Table 1 amply demonstrate, such a correlation is rather weak and shows only asmall increase in IS(90) as the number of C 6 F5 groups (n)increases. Moreover, this increase – if real – would imply asmaller s-electron density at the Fe nucleus as n increases. The
expectation here is that electron withdrawal by the C6F5 groups
would decrease the d-electron shielding, and thus lead to anincrease in the s-electron density and thus a decreasing IS(90).
Clearly, the electron withdrawing effects of the C6 F5 groups
are more complex than this simplified picture predicts, andthus is worthy of further detailed study.
Finally, it should be noted that the agreement in the ₣ factors between the X-ray and MES data in general is quite good,
and that, as expected, the ratio of the X-ray to the MES value

increases from about 1.04 at 100 K to about 1.13 at 298 K
reflecting the increasing influence of low-lying intra-molecular modes, such as ring rotations and librations and Cp-Fe-Cp
deformation17 modes to the vibrations of the iron atom as T
increases. In addition, these data clearly show that the ironatom motion in these complexes is predominantly isotropic in
the temperature regimes employed in this study.
Acknowledgements. The authors are especially grateful toProf. P. A. Deck for his generosity in providing the compounds
examined in this study. The authors are indebted to Dr. S.
Cohen for numerous fruitful discussions concerning the interpretation of the X-ray data reported in the literature. Thanks
are also due to Prof. S. Biali for helpful discussions andgraphic assistance.
References(1) P. A. Deck, Coord. Chem. Rev, 250, 1032 (2006) and references therein.
(2) P. A. Deck, W. F. Jackson, a nd F. R. Fronczek,
Organometallics 15, 5287 (1996); M. D. Blanchard, R. P.
Hughes, T. E. Concolino, and A. L. Rheingold, Chem.
Mater. 12, 1604 (2000).
(3) P. A. Deck, B. D. McCauley, and C. Slebodnick, J.
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Fronczek, J. Organomet. Chem. 637-9, 107 (2001).
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Fronczek, Organometallics 19, 5352 (2000).
(6) Ref 5, see Table S5 of supporting information therein.
(7) P. A. Deck, private comm. to be published.
(8) F. Parak and H. Formanek, Acta Cryst. A, 27, 573 (1971);
F. Parak, J. Phys. Colloque C-1m 71 (1980); F. Parak, H.
Hartmann, K. D. Aumann, H. Reuscher, G. Rennenkamp.
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30, 319 (2001).
(9) V. I. Gold a nsk i i a nd E . F. Ma ka rov, Ch em ical
Applications of Mössbauer Spectroscopy, Eds. V. I.
Goldanskii and R. H. Herber, Plenum Press, New York(1968), pp33ff.
(10) A. R. Kudinov, R. H. Herber. P. Zanello, D.S. Perekalin, I.
V. Glukhov, I. Nowik, M. Corsini, S. Fedi, and F. Laschi,
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D. A. Loginov, Z. A. Starikopva, and A. R. Kudinov, Eur.
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(11) R. H. Herber, Chemical Mössbauer Spectroscopy, Ch.
VII, Ed. R. H. Herber, Plenum Press, New York (1984).
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Rheingold, Chem. Mater. 12,1604 (2000).
(13) P. A. Deck, B.D. McCauley, and C. Slebodnick, J.
Organomet. Chem. 691, 1973 (2006).
(14) P. A. Deck, C. E. Kroll, W. G. Hollis, and F. R. Fronczek,
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(2006).
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Fronczek, Organometallics 19, 5352 (2000); see also P. G.
Gassman and C. H. Winter, J. Am. Chem. Soc. 108, 4228
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