Moessbauer spectroscopy of bucky ferrocenes: Lattice dynamics and motional anisotropy of the metal atom

Article abstract of DOI:10.1021/ic050251e

Temperature-dependent ⁵⁷Fe Moessbauer effect spectroscopy has been used to elucidate the metal atom dynamics in three neutral and two cationic bucky ferrocenes. For the three diamagnetic complexes Fe(C ₆₀H₅)Cp (1), Fe(C₆₀Me₅)Cp (2), and Fe(C₆₀Ph₅)Cp (3), the metal atom motion is anisotropic and the temperature dependence of the mean-square amplitude of vibration of the metal atom at a number of temperatures is reported. The Moessbauer lattice temperatures have been determined and compared to the parent ferrocene (6). The synthesis and X-ray crystal structure of 3 have been determined at 153(2) K, and the ¹H and ¹³C NMR spectra have been recorded. The cationic complexes derived from 2 and 3 show spin-lattice relaxation. The relaxation rate appears insensitive to the nearest-neighbor environment of the metal atom in this pair.

Full text of DOI:10.1021/ic050251e

Inorg. Chem. 2005, 44, 5629−5635
Mo1ssbauer Spectroscopy of Bucky Ferrocenes: Lattice Dynamics and
Motional Anisotropy of the Metal Atom
Rolfe H. Herber,*,† Israel Nowik,† Yutaka Matsuo,‡ Motoki Toganoh,§ Yoichiro Kuninobu,§ and
Eiichi Nakamura*,‡,§
Racah Institute Of Physics, The Hebrew UniVersity of Jerusalem, 91904 Jerusalem, Israel, and
Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency,
and Department of Chemistry, The UniVersity of Tokyo, Hongo, Bunkyo-Ku,
Tokyo 113-0033, Japan
Received February 18, 2005
Temperature-dependent 57Fe Mo¨ssbauer effect spectroscopy has been used to elucidate the metal atom dynamics
in three neutral and two cationic bucky ferrocenes. For the three diamagnetic complexes Fe(C60H5)Cp (1),
Fe(C60Me5)Cp (2), and Fe(C60Ph5)Cp (3), the metal atom motion is anisotropic and the temperature dependence
of the mean-square amplitude of vibration of the metal atom at a number of temperatures is reported. The Mo¨ssbauer
lattice temperatures have been determined and compared to the parent ferrocene (6). The synthesis and X-ray
crystal structure of 3 have been determined at 153(2) K, and the 1H and 13C NMR spectra have been recorded.
The cationic complexes derived from 2 and 3 show spin−lattice relaxation. The relaxation rate appears insensitive
to the nearest-neighbor environment of the metal atom in this pair.
Introduction
C5H5), has been the subject of an extremely large array of
physicochemical techniques designed to lead to a full
understanding of the structure, bonding, thermodynamic, and
kinetic properties of this unique molecule, first described5
by Kealy and Pauson in 1951. Among these techniques was
the phenomenon of resonant γ-ray scattering (Mo¨ssbauer
effect, ME); in fact, ferrocene was the first iron-organo-
metallic compound studied by this spectroscopy,6 and a large
literature has developed in this field.
The quest for the design and construction of molecular
devices which can function as active components in elec-
tronic circuits has received a great deal of attention in recent
years. In particular, the incorporation of C60 (buckyball) in
such structures has been the focus of intense research in this
field due to the possibilities of modifying the buckyball
structure with chemical ligands which themselves provide
possible electron pathways and connections to the larger
environment.1-4 Among such ligands are organometallic
moieties which can provide the intermolecular connectivity
needed for such devices.
Of particular interest in this context is the possibility of
using temperature-dependent ME spectroscopy to elucidate
the detailed motion of the iron atoms in such compounds
and relating this to the numerous detailed single-crystal X-ray
diffraction studies which have also been reported7 at a variety
of temperatures. The recent discovery of the synthesis of
In view of its centrality in the development of modern
organometallic chemistry, ferrocene, Fe(η5-Cp)2 (Cp )
* Authors to whom correspondence should be addressed. E-mail:
herber@vms.huji.ac.il (R.H.H.); nakamura@chem.s.u-tokyo.ac.jp (E.N.).
† The Hebrew University of Jerusalem.
(5) (a) Kealy, T. J.; Pauson, P. L. Nature (London) 1951, 168, 1039-
1040. See also the special volume devoted to ferrocene: J. Organomet.
Chem. 2001, 637-639, 3-26 and references therein.
(6) Herber, R. H.; Werthewim, G. K. In Proceedings of the Second
International Conference on the Mo¨ssbauer Effect, Saclay, France,
1961; Compton, D. M. J., Schoen, A. H., Eds.; J. Wiley and Sons:
New York, 1962. See also: Zahn, U., Kienle, P.; Eicher, H. In
Proceedings of the Second International Conference on the Mo¨ssbauer
Effect, Saclay, France, 1961; Compton, D. M. J., Schoen, A. H., Eds.;
J. Wiley and Sons: New York, 1962.
‡ Nakamura Functional Carbon Cluster Project, ERATO, JST.
§ The University of Tokyo.
(1) Porath, D.; Levy, Y.; Tarabiah, M.; Millo, O. Phys. ReV. B 1997, 56B,
9829-9833.
(2) Zeng, C.; Wang, H.; Wang, B.; Yang, J.; Hou, J. G. Appl. Phys. Lett.
2000, 77, 395-397.
(3) Gutierrez, R.; Fagas, G.; Cuniberti, G.; Grossmann, F.; Schmidt, R.;
Richter, K. Phys. ReV. B 2002, 65, 34101-34104.
(4) Cunniberti, G.; Gutierrez, R.; Fagas, G.; Grossmann, F.; Richter, K.;
Schmidt, R. Physica 2002, E12, 749-752.
(7) Brock, C. P.; Fu, Y. Acta Crystallogr. B 1997, 53, 928-938 and
references therein.
10.1021/ic050251e CCC: $30.25
Published on Web 07/07/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 16, 2005 5629

Herber et al.
Synthesis of Fe(η5-C60Ph5)(η5-Cp) (3). A mixture of C60Ph5H
(200 mg, 0.181 mmol) and [FeCp(CO)2]2 (320 mg, 0.900 mmol)
in benzonitrile (40.0 mL) was stirred at 180 °C for 20 h. After
evaporation of the solvent, the crude mixture was diluted with
toluene, and the solution was passed through a pad of silica gel.
Solvent was removed in vacuo, and the mixture was recrystallized
from CS2/hexane to obtain the title compound as air-stable dark
red microcrystals (171 mg, 77% yield): 1H NMR (400 MHz,
CDCl3) δ 3.21 (s, 5H, Cp), 7.33-7.37 (m, 10H, Ph), 7.39-7.40
(m. 5H, Ph), 7.93-7.95 (m, 10H, Ph); 13C NMR (100 MHz, CDCl3/
CS2 ) 1/1) δ 58.23 (5C), 73.20 (5C), 92.47 (s, 5C), 127.41 (5C),
127.47 (10C), 128.97 (10C), 142.82 (5C), 142.92 (10C), 143.66
(10C), 147.03 (5C), 147.81 (10C), 148.06 (5C), 152.14 (10C); IR
(CS2) νC-H (Cp)/cm-1 3101; UV-vis (1.0 × 10-5 mol‚L-1 in
CH2Cl2) λmax (ꢀ) 260 (84 700), 295 (57 300, shoulder), 351 (23 600),
394 (12 200), 475 (4330, shoulder) nm; HRMS (APCI+) calcd for
C95H30Fe (M+) 1226.1697, found 1226.1695.
Chart 1
bucky ferrocenes,8,9 in which the local bonding environment
around the iron atom is similar to that in ferrocene itself,
but in which the more distant environment can take on a
variety of architectures, has made it desirable to study the
details of the lattice dynamics and motional anisotropy of
the iron atom in these ferrocenoids. For instance, it is
expected that motion of the iron atom of a bulky bucky
ferrocene (i.e. pentaaryl bucky ferrocene 3) is much smaller
than that of a less bulky one (i.e. pentahydro bucky ferrocene
1) (Chart 1). This study was conceived to elucidate these
properties of the metal atom in both neutral and one-electron
oxidation products of a number of bucky ferrocenes and to
relate this to the molecular level architecture of these
compounds. Among the compounds for which ME param-
eters are discussed in detail in the present discussion are
Fe(C60H5)Cp (1), Fe(C60Me5)Cp (2), Fe(C60Ph5)Cp (3),
[Fe(C60Me5)Cp]+[SbCl6]- (4), and [Fe(C60Ph5)Cp]+[SbCl6]-
(5) as well as comparison data for ferrocene (6).
Preparation of Fe(III) Complexes. To a solution of 2 (151 mg,
0.164 mmol) in THF (300 mL) was added [(4-BrC6H4)3N][SbCl6]
(147 mg, 0.181 mmol). After the solution was stirred at 25 °C for
5 h, the solvent was removed in vacuo. The solid was washed
several times with H2O, EtOH, and CHCl3. After the solid was
dried in vacuo, [Fe(η5-C60Me5)(η5-Cp)][SbCl6] (4) (170 mg, 83%)
was obtained as a reddish brown solid. Characterization of 4 was
obtained by means of an ESR measurement, showing g-values of
3.94 (g|) and 1.57 (g ) at 4.4 K. Similarly, oxidation of 3 (99.2
mg, 0.0808 mmol) with [(4-BrC6H4)3N][SbCl6] (72.6 mg, 0.0889
mmol) in THF (70.0 mL) gave [Fe(η5-C60Ph5)(η5-Cp)][SbCl6] (5)
(62.2 mg, 49%) as a reddish brown solid. Samples of these cationic
complexes 4 and 5 contain starting materials 2 and 3, respectively,
as minor components as judged by Mo¨ssbauer spectroscopic
analyses.
Experimental Section
General Procedure for the Synthesis of Bucky Ferrocenes.
Syntheses and manipulation of materials were carried out under
nitrogen or argon atmosphere using standard Schlenk techniques.
THF was distilled from Na/K alloy and thoroughly degassed by
trap-to-trap distillation. Benzonitrile was distilled from CaH2.
C60Ph5H,10 C60Me5H,11 Fe(η5-C60H5)(η5-Cp) (1),8 and Fe(η5-
C60Me5)(η5-Cp) (2)9 were prepared according to the literature.
[FeCp(CO)2]2 and [(4-BrC6H4)3N][SbCl6] were purchased from
Mo1ssbauer Spectroscopy. The details of temperature-dependent
ME spectroscopy, making use of the 14.4 keV radiation of 57Fe,
have been discussed earlier.12 Spectrometer calibration was effected
using a ∼50 mCi 57Co source (Rh matrix) in conjunction with an
R-Fe absorber at room temperature, and all isomer shifts reported
herein are with reference to the centroid of such spectra. Temper-
ature control was effected using a high-gain feedback controller
together with a Cu-Constantan thermocouple, and the temperatures
reported are judged to be stable to (0.2 deg over the time intervals
(5-24 h) required to accumulate the ME data at a given temper-
ature. Powdered samples of the air-stable compounds were trans-
ferred to plastic sample holders and subjected to spectroscopic
examination without further treatment.
1
Acros Organics and Aldrich and used as received. H (400 MHz)
and 13C (100 MHz) NMR spectra were recorded using a JEOL EX-
400 spectrometer. Proton chemical shift are reported relative to
Me4Si (CDCl3) at δ 0.00 ppm or residual solvent peaks (CDCl3 at
δ 7.26 ppm; THF-d8 at δ 1.73 and 3.58 ppm). Carbon chemical
shift values are reported relative to CDCl3 at δ 77.00 ppm or THF-
d8 at δ 25.20 and 67.40 ppm. Other spectra were recorded on the
following instruments: IR, JASCO IR-420 and ReactIR 1000; UV/
vis spectra, HITACHI U3500 and Shimadzu SPD-6A; mass spectra,
Shimadzu LCMS-QP8000 and JEOL Accu TOF (JMS-T100LC).
X-ray Crystallographic Analysis. Single crystals of 3 were
obtained by recrystallization by slow diffusion of ethanol into a
CS2 solution of 3. The X-ray data sets were collected at 153 and
293 K on a MacScience DIP2030 imaging plate diffractometer
equipped with graphite-monochromated Mo KR radiation (λ )
0.710 69). Crystal data and data statistics for 3 are summarized in
Table 3. The structure of 3 was solved by the direct methods, SIR-
97.13 The positional parameters and thermal parameters of non-
hydrogen atoms were refined anisotropically on F2 by the full-
matrix least-squares method, using SHELXL-97.14 All non-
hydrogen atoms of 3 were anisotropically refined. Hydrogen atoms
(8) Toganoh, M.; Matsuo, Y.; Nakamura, E. J. Am. Chem. Soc. 2003,
125, 13974-13975.
(9) (a) Sawamura, M.; Kuninobu, Y.; Toganoh, M.; Matsuo, Y.; Ya-
manaka, M.; Nakamura, E. J. Am. Chem. Soc. 2002, 124, 9354-9355.
(b) Nakamura, E. Pure Appl. Chem. 2003, 75, 427-434. (c) Matsuo,
Y.; Nakamura, E. Organometallics 2003, 22, 2554-2563. (d) Matsuo,
Y.; Kuninobu, Y.; Ito, S.; Nakamura, E. Chem. Lett. 2004, 33, 68-
69.
(10) (a) Sawamura, M.; Iikura, H.; Nakamura, E. J. Am. Chem. Soc. 1996,
118, 12850-12851. (b) Iikura, H.; Mori, S.; Sawamura, M.; Nakamura,
E. J. Org. Chem. 1997, 62, 7912-7913. (c) Sawamura, M.; Iikura,
H.; Ohama, T.; Hackler, U. E.; Nakamura, E. J. Organomet. Chem.
2000, 599, 32-36.
(11) (a) Sawamura, M.; Toganoh, M.; Kuninobu, Y.; Kato, S.; Nakamura,
E. Chem. Lett. 2000, 270-271. (b) Nakamura, E.; Sawamura, M. Pure
Appl. Chem. 2001, 73, 355-359.
(12) (a) Herber, R. H.; Nowik, I. Hyperfine Interact. 2001, 136/137, 699-
703. (b) Herber, R. H.; Nowik I. Solid State Sci. 2002, 4, 691-694.
(13) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115-119.
(14) Sheldrick, G. M. Program for the Solution of Crystal Structures;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
5630 Inorganic Chemistry, Vol. 44, No. 16, 2005

Mo1ssbauer Spectroscopy of Bucky Ferrocenes
ligand and the five phenyl groups of the C60Ph5 ligand which
can be seen in the 3D structure of 3 (Figure 1b,c).
Cationic bucky ferrocenes, [Fe(η5-C60Me5)(η5-Cp)][SbCl6]
(4) and [Fe(η5-C60Ph5)(η5-Cp)][SbCl6] (5), were obtained by
oxidation of 2 and 3 with an aminium salt, [(4-BrC6H4)3N]-
[SbCl6] (eq 2). As the oxidant was added to a THF solution
of 2 or 3, the color of the solution changed from light orange
to dark orange, while the dark blue color of the aminium
salt disappeared immediately. Compounds 4 and 5 are soluble
to some extent in benzonitrile but are insoluble in common
solvents (i.e, chloroform, THF, toluene, hexane, methanol,
ethanol, acetonitrile, and carbon disulfide). Compounds 4
and 5 are ESR active. At 4.4 K, g-values for 4 are 3.94 (g|)
and 1.57 (g ), which are comparable to those of ordinary
Figure 1. Crystal structure of 3: (a) ORTEP drawing with 30% probability
level; (b) top view of a CPK model; (c) side view of a CPK model of 3
ellipsoids. In (a), the solvent molecule (CS2) in the unit cell is omitted for
clarity.
+ 15
ferrocenium cations such as FeCp2 .
were placed at calculated positions and refined “riding” on their
corresponding carbon atoms. In the subsequent refinement, the
2
function Σω(Fo - Fc2)2 was minimized, where |Fo| and |Fc| are
the observed and calculated structure factor amplitudes, respectively.
The agreement indices are defined as R1 ) Σ(||Fo| - |Fc||)/Σ|Fo|
4
and wR2 ) [Σω(Fo2 - Fc2)2/Σ(ωFo )]1/2. Data have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-271687.
Mo1ssbauer Studies. The ME spectra of neutral ferro-
cenoids, incorporating a single metal atom/molecule, consist
of a well-resolved doublet which can be analyzed in terms
of its isomer shift (IS), quadrupole splitting (QS), area under
the resonance curve (A), the doublet asymmetry ratio (R),
and parameters derived therefrom and are summarized in
Table 1.
Results and Discussion
Synthesis and X-ray Structure Analysis of Bucky Fer-
rocenes. We communicated earlier the synthesis and char-
acterizations of pentahydro bucky ferrocene, Fe(C60H5)Cp
(1),8 and pentamethyl bucky ferrocene, Fe(C60Me5)Cp (2).9
Pentaphenyl bucky ferrocene Fe(C60Ph5)Cp (3) was obtained
in a manner similar to the synthesis of 2 (eq 1). Heating
C60Ph5H and [FeCp(CO)2]2 in benzonitrile at 180 °C for 20
h directly afforded 3, which was obtained as air- and
moisture-stable dark red crystals in 77% isolated yield after
purification by silica gel column chromatography and re-
crystallization from a mixture of CS2 and ethanol. The
structure of 3 was determined by 1H and 13C NMR and high-
resolution APCI-TOF-MS as well as single-crystal X-ray
Compound 1. A typical spectrum of 1 at 90 K is shown
in Figure 2. The total number of counts/channel in this
spectrum is ∼1.3 × 106, and it is clear that this is an optically
“thin” absorber. The ME hyperfine parameters at 90 K for
1 are summarized in Table 1 which also lists the correspond-
ing values for the parent ferrocene (6) for comparison. It
will be seen that both the IS(90 K) and QS(90 K) values are
very similar for the two solids. Both the temperature
dependencies of the IS and ln(A) parameters suggest a
slightly “harder” bonding environment for the iron atom in
the bucky ferrocene than in the parent solid, but the
difference is small. It is noteworthy that the temperature
dependence of the mean-square amplitude of vibration
(msav), proportional to ln A(T), for 1 is well fitted by a linear
regression, with a slope of -(6.50 ( 0.22) × 10-3 K-1, R
) 0.986 for 14 data points, in contrast to the corresponding
data for ferrocene which shows two crystallographic phase
transitions at ∼170 and ∼250 K, as noted earlier.16
1
diffraction study. The H and 13C NMR spectra of bucky
ferrocene 3 exhibited C5V symmetric signal patterns for the
C60Ph5 and Cp ligands. This result suggests that the C60Ph5
and Cp ligands rotate rapidly in solution.
It is, of course, evident from Figure 2 that the area ratio,
R, defined as the ratio of the spectral area at velocities more
positive than the spectrum centroid, A(+), to that at more
negative velocities, A(-), is significantly different from unity.
There are two principal reasons for this phenomenon: texture
Figure 1 shows the crystal structure of 3. Whereas the
averaged bond lengths Fe-C(C60Ph5) (2.084 Å) of 3 are quite
similar to those of 2 (2.087 Å),9a the bond lengths Fe-C(Cp)
(2.062 Å) are much longer than those of 2 (2.033 Å). This
is probably because of the steric hindrance between the Cp
(15) Prins, R.; Reinders, F. J. J. Am. Chem. Soc. 1969, 91, 4929-4931.
(16) (a) Naruse, M.; Sorai, M.; Sakiyama, M. Mol. Cryst. Liq. Cryst. 1983,
101, 219-234. (b) Edwards, J. W.; Kington, G. L.; Mason, R. Trans.
Faraday Soc. 1960, 56, 660-667. (c) Ogasahara, K.; Sorai, M.; Suga,
H. Chem. Phys. Lett. 1979, 68, 457-460.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5631

Herber et al.
Table 1. Mo¨ssbauer Parameters of the Compounds Discussed in the Texta
1
2
3
4
5
6
units
IS(90)
0.547(5)
2.401(5)
4.73(14)b
6.50(22)c
87(3)
0.575(4)
2.475(4)
5.36(5)d
7.42(17)e
78(2)
0.586(4)
2.471(4)
5.55(5)f
4.71(5)g
75(1)
0.537(3)
0.185(3)
curv
0.612(3)
0.108(3)
4.97(24)
∼7.0(5)c
84 ( 4e
0.537(1)
2.418(1)
5.12(6)
8.78(14)
81(2)
mm/s
QS(90)
-d(IS)/dT
-d ln A/dT
Meff
mm/s
×10-4 mm/s K
∼7.2(2)b
×10-3/K
94 ( 10d
Da
K
ΘM
116
116
148
104
a The parenthetical numbers are the errors in the last place(s) of the reported value. b T range ) 170-320 K. c T range ) 90-320 K. d T range )
210-370 K. e T range ) 210-370 K. f T range ) 210-375 K. g T range ) 90-375 K.
Figure 3. Temperature dependence of the recoil-free fraction for 1 (open
points) and 2 (filled points) normalized to the 90 K data.
Figure 2. 57Fe Mo¨ssbauer spectrum of 1 at 90 K. The zero point of the
velocity scale is the centroid of a room-temperature R-Fe calibration absorber
spectrum.
amplitude perpendicular to this axis, (x2)perp as is true of
ferrocene itself. The assumptions underlying the relationship
and vibrational anisotropy. It is readily possible to distinguish
between the temperature dependence and the recoil-free
between these two effects, since the former is essentially
fraction have been discussed previously.19 A further simpli-
temperature independent, whereas the latter evidences a clear
fying assumption made in this analysis is that T > ΘM/2,
change with T, as is the case with the presently discussed
where ΘM is the Mo¨ssbauer lattice temperature,20 so that the
complexes. Moreover, this T dependence can be quantita-
tively related to the motional anisotropy of the iron atom
with respect to the (local) molecular symmetry axis, the so-
called Gol’danskii-Karyagin effect (GKE), and thus to the
temperature dependence of f scales directly with the area
under the resonance curve. Since f ) exp(-k2 x2 ), where k
is the wavenumber of the 14.4 keV γ ray of 57Fe, these data
permit the calculation of x2
for the iron atom. The
ave
difference in the vibrational amplitudes in the two directions,
that is, parallel and perpendicular to the local C5 axis through
the metal atom. This vibrational anisotropy has been
reported17 in a number of other ferrocene related solids and
appears to be a general phenomenon in these organometallics
as will be discussed more fully below for 2 and 3. In this
context it is obviously clear that the “texture” and GKE
effects are not mutually exclusive. To estimate the contribu-
tion of the former, a ME spectrum of 1 was acquired at 4.2
K, a temperature at which the GKE is assumed to be
negligible. In this spectrum, R was found to be 1.049 (
0.005, and this 5% contribution to R due to texture has been
corrected for in the vibrational amplitude calculations to be
discussed below.
In a separate ME experiment at room temperature, using
the magnetic field effect discussed in detail by Collins,18 it
was determined that the quadrupole interaction in these
compounds is positive, and hence, the measured R > 1 means
that the motion parallel to the (local) molecular symmetry
axis, (x2)para , is smaller than the corresponding vibrational
temperature dependence of the recoil-free fraction, f , for 1
is shown graphically in Figure 3, and the significance of these
results will be discussed in further detail below. Further, from
the temperature dependence of the area ratio, R, and making
the assumption that x2
) 1/3 x2
+ 2/3 x2 , it is
ave
para
perp
possible to calculate the difference in vibrational amplitude
parallel and perpendicular to the (local) molecular (C5)
symmetry axis. The results of these calculations for 1 are
summarized graphically in Figure 4 in which the temperature
dependence of k2 x2 (2) and k2 x2
(1) are compared
perp
par
to the values calculated for ΘM ) 116 K and Meff ) 87 Da,
as indicated by the solid line.
Compound 2. The ME hyperfine parameters and data
derived therefrom are included in the summary of Table 1.
It is clear that both the IS and QS parameters at 90 K are
little different from those of compound 1 and the substitution
of methyl groups for hydrogen in the five-membered ring
ligated to the metal atom has little effect on the electronic
environment around Fe. This is in consonance with earlier
data which showed that ring substitution does not appreciably
(17) The interested reader is referred to the following: J. Organomet. Chem.
2001, 637-639. This includes numerous X-ray crystal structure data
which reflect this iron atom motional anisotropy.
(19) Nowik, I.; Herber, R. H. J. Phys. Chem. Solids 2003, 64, 313-317.
(20) Herber, R. H. In Chemical Mo¨ssbauer Spectroscopy; Herber, R. H.
Ed.; Plenum Press: New York, 1984; Chapter VII.
(18) Collins, R. L. J. Chem. Phys. 1965, 42, 1072-1080.
5632 Inorganic Chemistry, Vol. 44, No. 16, 2005

Mo1ssbauer Spectroscopy of Bucky Ferrocenes
Figure 6. Temperature dependence of k2 x2 for 3. The solid line reflects
the calculation for Meff ) 75 Da and ΘM ) 148 K as discussed in the text.
Figure 4. Temperature dependence of k2 x2 for 1. The solid line reflects
the calculation for Meff ) 89 Da and ΘM ) 116 K as discussed in the text.
Figure 7. 57Fe Mo¨ssbauer spectrum of 4 at 4.1 K. The doublet absorbance
is ascribed to the presence of a small quantity of the neutral parent compound
(2) used to prepare 4.
Figure 5. Temperature dependence of k2 x2 for 2. The solid line reflects
the calculation for Meff ) 78 Da and ΘM ) 116 K as discussed in the text.
affect the ring bonding to the metal atom. The temperature
dependence of the IS and the recoil-free fraction are slightly
different in 2 as compared to 1, leading to a slightly smaller
major constituent, no further attempt was made to identify
its nature more exactly. As before, the ME hyperfine
parameters are summarized in Table 1 and with the exception
of the temperature dependence of the recoil-free fraction are
similar to those listed for 1 and 2. For 3, the vibrational
anisotropy is reflected in the ratio of the Ui,j values which
correspond to 1:1.03:1.23 at 293 K and 1:1.13:1.46 at 153
K. Of interest with respect to the pentaphenyl compound is
the temperature dependence of f, which is significantly
smaller than what was observed for the C60H5 and C60Me5
compounds, as will be discussed below. As before, calcula-
20
Meff but having the same ΘM. As in the case of 1, above,
a 4.2 K ME spectrum yielded an R value of 1.05 ( 0.01,
and again this “texture” contribution was corrected for in
the subsequent data analysis. Using the linear approximation
discussed above, it is again possible to extract the temperature
dependence of f for the metal atom in this compound, and
the results are again included in Figure 3 and in Table 1. As
in the case of 1, R is a temperature-dependent parameter and
2
2
can be used to extract xperp and xpara from the data; see
below. In this context it should be noted that the Ui,j values
extracted from single-crystal X-ray diffraction data for 2 are
in the ratio of 1:1.05:1.16 at 293 K and 1:1.25:1.76 at 153
tion of the parallel and perpendicular values for k2 x2 are
j
summarized graphically in Figure 6, again using the same
convention as in Figure 4. The solid line corresponds to
k2 x2
calculated for Meff ) 75 Da and ΘM ) 148 K.
ave
K. The x2 values are summarized graphically in Figure 5
From the data summarized in Figures 4-6, it is im-
j
using the same conventions as given for Figure 4. The solid
mediately seen that while there is relatively little difference
line in this figure corresponds to k2 x2 calculated for Meff
in k2 x2 for the C60H5 and C60Me5 compounds, a significant
ave
j
) 78 Da and ΘM ) 116 K.
decrease in this parameter at all temperatures is noted for
the C60Ph5 homologue.
Compound 3. In the case of the pentaphenyl bucky
ferrocene, the ME data showed evidence for the presence of
a small impurity in the spectra, with an IS(90) and QS(90)
of 0.282(9) and 0.369(7) mm s-1, respectively. The fractional
area due to this impurity was found to be ∼5% at 90 K but
larger at the higher temperatures and has been corrected for
in the calculation of the recoil-free fraction data. Except for
the observation that this impurity has a higher ΘM than the
Compound 4. A typical ME spectrum of the one-electron
oxidation product of 2 at 90 K is shown graphically in Figure
7 in which the major resonance reflects the nearly complete
collapse of the QS interaction in this cation, as has been
reported21 for numerous other ferrocenium-related complexes,
in consonance with the pioneering study of Collins.18 While
the IS(90) is nearly the same as it is for the parent 2, the
Inorganic Chemistry, Vol. 44, No. 16, 2005 5633

Herber et al.
Figure 9. Temperature dependence of the spin-lattice relaxation rates of
the iron atom in 4 (open points) and 5 (closed points). The two data sets
are normalized to the 90 K values to afford intersample comparison as
discussed in the text.
Figure 8. Temperature dependence of the recoil-free fraction for 2, 4,
and 5. While there is some curvature noted in these experimental points at
low temperatures, only the high-temperature-limiting slopes were used to
calculate Meff and ΘM from these data.
QS(90) value has decreased from 2.48 to 0.19 mm s-1 on
the removal of one electron. It is also noted from Figure 7
that there is evidence for a small but nonnegligible second
iron site, especially at the lower temperatures. The hyperfine
parameters for this minor component at 90 K are 0.582(3)
and 2.486(3) mm s-1 for the IS and QS, respectively. The
similarity between these values and those reported (Table
1) for pure 2 leave little doubt that this minor component is
unoxidized starting material 2 in the sample. The fractional
area due to this compound is ∼18% at 4.2 K and falls to
∼8.5% at 300 K, and corrections for the presence of this
material have been made in the data reduction where
applicable.
s), and this ratio (1.09) has been applied over the total T
range. Over the indicated temperature range, the two
relaxation rates are quite comparable, and clearly the
substitution of a phenyl for a methyl group on the five-
membered ring ligated to the metal atom does not play a
significant role in the spin-lattice relaxation rate of these
complexes.
Compound 5. As was true for the ME spectra of 4, those
of 5 evidence the presence of a small starting material 3 with
an IS and QS at 90 K of 0.583 ( 0.008 and 2.458 ( 0.008
mm s-1, respectively. The similarity between these values
and those observed for the neutral parent, 3, leave little doubt
that this second resonance is due to unoxidized parent
compound which has survived the purification process
described above. In the subsequent data analysis, the presence
of this second site has been taken into consideration.
The temperature dependence of the IS for 4 over the
temperature range 90 < T < 320 K shows some curvature
while the corresponding data for 5 are fit by a linear function
over the temperature interval 90 < T < 298 K, but the
differences in the two data sets are quite small. The value
of Meff calculated for 5 is 84 ( 4 Da, while that for 4 taken
over the total temperature range (ignoring the curvature in
the data) is 94 ( 10 Da. Clearly the replacement of the five
methyl groups in 4 by five phenyl groups in 5 plays a
negligible role in the effective vibrating mass of the metal
atom in these compounds in agreement with earlier data
related to ring substitutions in ferrocenoids.22
The temperature dependence of f for 5 is summarized
graphically in Figure 8, which also includes the correspond-
ing data for 6 and 2. From this figure it is seen that the iron
atom in the cationic complex is embedded in a slightly
“softer” lattice than is true for the corresponding neutral
diamagnetic 2, although the difference is quite small, an
effect which has been previously noted for other ferrocene
complexes and their one electron oxidation products. Clearly
-
the presence of the large SbCl6 anion in the lattice plays
only a very small role in the vibrational characteristics of
the iron atom in these structures.
ME spectra of ferrocenium salts can be analyzed in terms
of the spin-lattice relaxation rate as has been previously
reported from this laboratory.19 The assumptions inherent
in these data treatment have been reviewed earlier, and it is
clear that the absolute values of the relaxation rates depend
on the values assigned to the magnetic hyperfine field at the
metal center. In the absence of detailed information concern-
ing Hi, a value of 500 kOe has been arbitrarily assigned to
this parameter, so that only comparative values between
related compounds have any significance in the present
discussion. The temperature dependences of the spin-lattice
relaxation rates for 4 and 5 are summarized graphically in
Figure 9 in which the 90 K τ datum for 5 (3.83 × 10-9 sec)
has been normalized to the 90 K value for 4 (3.52 × 10-9
As noted above, the temperature dependence of the recoil-
free fraction for 5 is shown in Figure 8. Although there is
some slight curvature in both data sets, it is evident that the
data for the two compounds 4 and 5 are quite similar.
Moreover, -d ln[A(T)/A(90)]/d T for 2 and 4 are quite
similar, but this similarity does not obtain for the 3, 5 pair.
The relative spin-lattice relaxation rate for 5 as a function
of temperature is included in Figure 9.
Significance of the Rmsav Data. As noted above, the
recoil-free fraction of the metal atom in these compounds is
given by f ) exp(-k2 x2 ). The comparison of values for
(21) (a) Nowik, I.; Herber, R. H. Inorg. Chim. Acta 2000, 310, 191-195.
(b) Herber, R. H.; Hanusa, T. P. Hyperfine Interact. 1997, 108, 563-
575.
(22) Herber, R. H.; Bildstein, B.; Denifl, P.; Schottenberger, H. Inorg.
Chem. 1997, 36, 3586-3594 and references therein.
5634 Inorganic Chemistry, Vol. 44, No. 16, 2005

Mo1ssbauer Spectroscopy of Bucky Ferrocenes
Table 3. Crystal Data and Structure Analysis Results for 3‚CS2
formula
cryst system
space group
a, Å
b, Å
c, Å
C96H30FeS2
orthorhombic
P212121 (No. 19)
15.188(5)
17.395(5)
20.561(5)
90
90
90
5432(1)
4
R, deg
â, deg
γ, deg
V, Å3
Z
T, K
153(2)
cryst size, mm
2θmin, 2θmax, deg
no. reflcns measd (unique)
no. reflcns measd (I > 2σ(I))
no. params
R, Rw (I > 2σ(I))
R1, wR2 (all data)
GOF on F2
0.52, 0.18, 0.12
4.6, 51.18
9394
8207
893
0.0536, 0.0619
0.1373, 0.1445
1.03
Figure 10. Root-mean-square amplitudes of vibration (rmsav) for the iron
atom for the four neutral compounds examined in this study. The linear
segment reflects the calculated data for ferrocene with Meff ) 81 Da and
ΘM ) 104 K.
Table 2. Root-Mean-Square Amplitudes of Vibration (Rmsav) in Å of
the Iron Atom at Selected Temperatures for the Compounds Discussed
in the Texta
Summary and Conclusions
temp (K)
1
2
3
6
The synthesis and crystal structure of Fe(C60Ph5)Cp is
1
reported together with H and 13C NMR and IR data. This
90
150
200
250
320
0.099
0.131
0.151
0.176
0.200
0.097
0.121
0.139
0.154
0.181
0.091
0.115
0.130
0.148
0.171
0.122
0.157
0.181
0.203
0.230
compound, together with its C60H5 and C60Me5 homologues,
has been examined by temperature-dependent 57Fe Mo¨ss-
bauer spectroscopy in an effort to elucidate the dynamics
and anisotropy of the metal atom motion. The vibrational
amplitude of the iron atom of 3 was found to be clearly
smaller than that of 1. In addition, two cationic complexes
a The experimental errors are estimated at ∼(4%.
the compounds discussed above is summarized graphically
in terms of the rmsav in Å for 1-3 in Figure 10 together
with the corresponding data for the parent ferrocene, 6 for
which the calculated data for Meff ) 81 Da and ΘM ) 104
K is indicated by the solid line. It is immediately noted that
not only are the values of k2 x2 smaller at all temperatures
for the bucky ferrocenes, as compared to 6, but the values
decrease at a given temperature as the formula weight of
the ring substituents (H, Me, and Ph) increases. The inference
to be drawn from these regularities is that both the presence
of the bulky C60 group itself and the molecular weight and/
or steric requirements of the ring substituents influence the
mean square vibrational amplitude of the metal atom.
Moreover, these data permit the calculation of the rmsav of
the metal atom as a function of temperature for these
compounds, and representative values are given in Table 2.
It should be noted, however, that while the Fe-C(Cp)
bond lengths in 3 are significantly larger than those in 2, as
detailed above (X-ray structure), the rmsav of the metal atom
in 3 is smaller than that in 2 at all temperatures employed in
this study; see Figure 10. The a priori expectation is that the
longer Fe-C(Cp) bond lengths in 3 (compared to 2) would
result in a larger k2 x2 in the former, but this is not what is
observed. The origin of this unexpected difference is not
apparent from the presently available experimental data.
-
(SbCl6 salts) derived from the C60Me5 and C60Ph5 neutral
precursors have been similarly characterized. In the case of
the neutral complexes, the observation of a temperature-
dependent area ratio parameter (Gol’danskii-Karyagin ef-
fect) has made it possible to extract root-mean-square
vibrational amplitudes of vibration of the metal atom parallel
and perpendicular to the local C5 axis. The two cationic
complexes (4 and 5) relax primarily by a spin-lattice
mechanism which appears to be insensitive to the detailed
local molecular architecture around the Fe atom.
Acknowledgment. This research was supported by a
Grant-in-Aid for Scientific Research (Specially Promoted
Research) and the 21st Century COE programs for Frontiers
in Fundamental Chemistry from Monbukagaku-sho, Japan.
We thank Frontier Carbon Corp. for a generous supply of
[60]fullerene.
Supporting Information Available: CIF file including lists of
positional parameters, thermal displacement parameters, bond
lengths, and bond angles for 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC050251E
Inorganic Chemistry, Vol. 44, No. 16, 2005 5635