The synthesis of binuclear half-open 1,1′-biferrocene: Redox behavior and an interpretation of the 57Fe Moessbauer data

Article abstract of DOI:10.1016/S0022-328X(99)00270-3

Replacing the two Cp rings of 1,1′-biferrocene with pentadienyl ligands results in a cathodic shift of the redox potential for the ferrocene/ferrocenium couple and decreased quadrupole splitting in the Moessbauer spectrum and these findings can be accounted for by a change in the orbital population around the metal atom.

Full text of DOI:10.1016/S0022-328X(99)00270-3

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 587 (1999) 46–48
The synthesis of binuclear half-open 1,1%-biferrocene: redox
57
behavior and an interpretation of the Fe Mo¨ssbauer data
Teng-Yuan Dong *, Chung-Kay Chang, Ching-Hung Cheng
Department of Chemistry, National Sun Yat-Sen Uni6ersity, Kaohsiung, Taiwan, ROC
Received 23 March 1999; received in revised form 11 May 1999
Abstract
Replacing the two Cp rings of 1,1%-biferrocene with pentadienyl ligands results in a cathodic shift of the redox potential for the
ferrocene/ferrocenium couple and decreased quadrupole splitting in the Mo¨ssbauer spectrum and these findings can be accounted
for by a change in the orbital population around the metal atom. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Half-open ferrocene; Half-open biferrocene; Redox behavior
1. Introduction
2. Results and discussion
There is considerable interest in bridged metal-
locenes arising from the unusual properties of mixed-
valence fulvalenyl bridged binuclear metallocenes and
the potential for developing new types of catalysts
involving electron transfer [1,2]. Moreover, the area of
transition metal pentadienyl chemistry has received
growing attention and one interesting aspect involves
comparing electronic structural data for related open
and closed metallocenes [3–7]. To date, it has been
demonstrated that the (pentadienyl)iron systems are
usually h5 complexes with similar Fe–C distance ex-
hibited in the cyclic counterpart ferrocene. However,
the introduction of pentadienyl ligand in half-open
(h5-2,4-dimethylpentadienyl)-(h5-cyclopentadienyl)iron
and bis(h5-2,4-dimethylpentadienyl)iron complexes re-
sults in drastic change in their redox potentials and
ESR behavior [7]. Successive opening of one or both
Cp rings of ferrocene results in cathodic shift of the
redox potential, increased chemical lability, and vastly
decreased g anisotropy in ESR spectra. As opposed to
closed ferrocenium [8–10], ESR spectra of the half-
open and open ferrocenium ions can be observed at
room temperature [7]. In connection with our investi-
gations on the electron transfer in mixed-valence bifer-
rocenium cations [11], we became interested in the
synthesis of binuclear half-open ferrocene (1).
Compound 1 was prepared by a straightforward
‘one-pot’ procedure. Sodium cyclopentadienide was
converted to fulvalene dianion 2 according to the pro-
cedure given by Mueller-Westerhoff et al. [12]. After
the slow addition of a 1:1 mixture of K(2,4-C7H11)
and 2 in THF to a slurry of FeCl2 in THF solvent at
−78°C under nitrogen was completed, the mixture
was slowly warmed to room temperature and stirred
for 1 h. The THF solvent was then removed under
vacuum, and the product was extracted into hexane
and filtered. The solvent was removed under vacuum
and the red residue was chromatographed on neutral
alumina (activity I) under N2. The first band eluted
with 50:1 hexane–THF solvent was a mixture of fer-
rocene, half-open ferrocene, and open ferrocene (ꢀ7%
yield). The second band was the desired compound 1
(ꢀ20% yield) (see Section 3). A sample of bis(fulva-
lene)diiron was also obtained by further elution with
benzene (Scheme 1).
This approach starts with simple material and is
capable of being carried out without the isolation of
any intermediates. Attempts to apply the substitution
reaction on binuclear cyclopentadienyl-arene iron com-
pound 3 to yield 1 were unsuccessful. However, fer-
rocene 5 was obtained from the cyclopentadienyl
substitution reaction of (arene)iron cation 4 in 73%
yield (Eq. (1)).
* Corresponding author. Fax: +886-7-5253908.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00270-3

T.-Y. Dong et al. / Journal of Organometallic Chemistry 587 (1999) 46–48
47
rocene 8 and half-open biferrocene 1 reveals that the
magnitude of interaction between two Fe sites in 1 is
smaller. Thus, the interaction between the two Fe sites
is sensitive to the nature of the h5-2,4-dimethyl-pentadi-
enyl moiety. It must be the effect of ring opening on the
electronic structure. A simple correlation of the MOs of
closed and open ferrocenes reported by Gleiter et al.
suggested [5] that the Fe 3d orbitals is strongly coupled
to pentadienyl ligand in the open ferrocenes.
(1)
Table 1 lists electrochemical data obtained by cyclic
voltammetry for 1 and some other relevant compounds
6–8 (Chart 1). Like the closed biferrocene [13], half-open
biferrocene 1 also undergoes two successive one-electron
oxidation to yield the mono- and then the dication. From
the first E1/2 value, half-open biferrocene 1 is oxidized
easier than ferrocene and half-open ferrocene. As a
substituent, the half-open ferrocenyl group clearly acts as
net electron donor. A substituent effect can be calculated
for the half-open ferrocenyl group (lFc) by:
A primary driving force for the 57Fe Mo¨ssbauer
spectroscopic studies of compound 1 has been needed to
increase the understanding of the bonding. 57Fe Mo¨ss-
bauer spectrum was run at 300 K for 1. In this case, one
quadrupole-split doublet is observed. The resulting
fitting parameters are given in Table 1. It is clear from
the date summarized in Table 1 that the difference
between half-open ferrocene 6 and half-open biferrocene
1 lies in the fact that the latter compound shows a smaller
value of DEQ (quadrupole splitting). Appropriate molec-
ular orbital descriptions of the bonding in ferrocene,
half-open ferrocene, and open ferrocene have been
worked out by Gleiter and co-workers al. [5] and Ernst
et al. [17] and account for related observations arising
lFc=E1/2(Fc2+,0)−E1/2(Fc+,0)=0.22−0.16=0.06 V
It has been demonstrated that the magnitude of the
peak-to-peak separation (DE1/2 in Table 1) gives an
indication of the interaction between two Fe sites [14–
16]. Comparing the magnitude of DE1/2 between bifer-
Scheme 1.

48
T.-Y. Dong et al. / Journal of Organometallic Chemistry 587 (1999) 46–48
Table 1
Cyclovoltammetric data and Mo¨ssbauer fitting parameters
a
b
c
d
e
Compound
Ferrocene
E1/2
Characteristics (°C)
Rev, 25
DE1/2
Kc
l
DEQ
0.42
0.542
2.453
6 f
7
0.222
0.114
Rev, 20
Irrev, 20
0.474
0.498
1.946
1.516
8
0.33
0.65
Rev, 25
Rev, 25
0.32
0.20
2.65×105
2.45×103
0.520
2.360
1
0.16
0.36
Rev, 25
Semirev, 25
0.290
1.912
a Half-wave potential referenced to Ag ꢀ AgCl.
b Peak separation between half-wave potentials.
c Disproportionation equilibrium constant of (2, 2)+(3, 3)=2(2, 3) in which the abbreviations (3, 3), (2, 3) and (2, 2) denote the dioxidized salt,
the monooxidized salt and the neutral compound, respectively.
d Isomer shift referenced to iron-foil in mm s−1
.
e Quadrupole splitting in mm s−1
f From Ref. [7], solvent dimethoxyethane/(C4H9)4NClO4.
.
References
[1] D.O. Cowan, C. LeVanda, J. Park, F. Kaufman, Acc. Chem.
Res. 1 (1973) 1.
[2] U. Mueller-Westerhoff, P. Eilbracht, J. Am. Chem. Soc. 94
(1972) 9272.
[3] R.D. Ernst, Chem. Rev. 88 (1988) 1255.
[4] R.D. Ernst, Structure and Bonding, vol. 57, Spriger Verlag,
Berlin, 1984, p. 2.
[5] M.C. Bo¨hm, M. Eckert-Maksic, R.D. Ernst, D. Wilson, R.
Gleiter, J. Am. Chem. Soc. 104 (1982) 2699.
[6] R. Gleiter, I. Hyla-Kryspin, M.L. Ziegler, G. Sergeson, J.C.
Green, L. Stahl, R.D. Ernst, Organometallics 8 (1989) 298.
[7] C. Elschenbroich, E. Bilger, R.D. Ernst, D. Wilson, M.S. Kra-
lik, Organometallics 4 (1985) 2068.
[8] R. Prins, F.J. Reinders, J. Am. Chem. Soc. 91 (1969) 4929.
[9] R. Prins, Mol. Phys. 19 (1970) 603.
(Chart 1)
from the 57Fe Mo¨ssbauer and ESR studies. Ernst sug-
gested [17] that the dxy and dx
(e2 set) electron
2
2
density gives a positive contributio−nyto DEQ and the dxz
and dyz (e1 set) electron density gives a negative contri-
bution. Thus, the observed decrease in the DEQ going
from ferrocene to open ferrocene is seen.
[10] A. Horsfield, A. Wassermann, J. Chem. Soc. Dalton Trans.
(1972) 187.
[11] T.-Y. Dong, S.H. Lee, C.K. Chang, H.M. Lin, K.J. Lin,
Organometallics 16 (1997) 2773 and references therein.
[12] U.T. Mueller-Westerhoff, P. Eilbracht, J. Am. Chem. Soc. 94
(1972) 9272.
[13] G.M. Brown, T.J. Meyer, D.O. Cowan, C. LeVanda, F. Kauf-
man, P.V. Roling, M.D. Rausch, Inorg. Chem. 14 (1975) 506.
[14] W.H. Morrison, Jr., S. Krogsrud, D.N. Hendrickson, Inorg.
Chem. 12 (1973) 1998.
3. Supplementary information
1
The physical properties of 1 are as follows. H-NMR
(C6D6, ppm): −0.56 (d, 4H, endo-CH2), 1.76 (s, 12H,
-CH3), 2.50 (d, 4H, exo-CH2), 4.02 (t, 4H, Cp), 4.07 (t,
4H, Cp), 5.06 (s, 2H, ꢀCH). MS: M+ at m/z 420. M.p.:
116–117°C.
[15] E.E. Bunel, P. Campos, J. Ruz, L. Valle, I. Chadwick, M.S.
Ana, G. Gonzalez, J.M. Manriquez, Organometallics 7 (1988)
474.
[16] R. Chukwu, A.D. Hunter, B.D. Santarsiero, S.G. Bott, J.L.
Atwood, J. Chassaignac, Organometallics 11 (1992) 589.
[17] R.D. Ernst, D.R. Wilson, R.H. Herber, J. Am. Chem. Soc.
106 (1984) 1646.
Acknowledgements
We thank the National Science Council (ROC) for
support.
.
.