Electron transfer in mixed-valence polyferrocenium cations: Preparation, electrochemistry, and 57Fe M?ssbauer characteristics

Article abstract of DOI:10.1021/om9801709

Convenient new methods are developed for the preparation of 1′,1″?-disubstituted triferrocenes and tetraferrocenes that can be oxidized with iodine to a new series of mixed-valence compounds. The X-ray structures of 1′,1″?-diethyltriferrocene, 1′,1″?-dimethoxymethyltriferrocene, and 1′,1″?-dimethoxymethyltetraferrocene have been determined at 298 K. The rates of intramolecular electron transfer in these mixed-valence cations were estimated by variable-temperature ⁵⁷Fe M?ssbauer experiments. The features in all 80 and 300 K spectra include two doublets, one with a quadrupole splitting (ΔE_Q) of ～2 mm s^-1 (Fe(II) site) and the other with ΔE_Q = ～0.3 mm s^-1 (Fe(III) site). This pattern of two doublets is expected for a mixed-valence biferrocenium cation that is valence-trapped on the time scale of the M?ssbauer technique (electron-transfer rate <～10⁷ s^-1 in the solid state). The electrochemical measurements are also presented.

Full text of DOI:10.1021/om9801709

Organometallics 1998, 17, 3323-3330
3323
Electr on Tr a n sfer in Mixed -Va len ce P olyfer r ocen iu m
Ca tion s: P r ep a r a tion , Electr och em istr y, a n d 57F e
Mo1ssba u er Ch a r a cter istics
Teng-Yuan Dong,*,1 Wen-Yu Lee,1 Pau-Tang Su,1 Ling-Shao Chang,1 and
Kuan-J iuh Lin2
Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan, and
Institute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan
Received March 9, 1998
Convenient new methods are developed for the preparation of 1′,1′′′′′-disubstituted
triferrocenes and tetraferrocenes that can be oxidized with iodine to a new series of mixed-
valence compounds. The X-ray structures of 1′,1′′′′′-diethyltriferrocene, 1′,1′′′′′-dimethoxy-
methyltriferrocene, and 1′,1′′′′′-dimethoxymethyltetraferrocene have been determined at 298
K. The rates of intramolecular electron transfer in these mixed-valence cations were
estimated by variable-temperature 57Fe Mo¨ssbauer experiments. The features in all 80 and
300 K spectra include two doublets, one with a quadrupole splitting (∆EQ) of ∼2 mm s-1
(Fe(II) site) and the other with ∆EQ ) ∼0.3 mm s-1 (Fe(III) site). This pattern of two doublets
is expected for a mixed-valence biferrocenium cation that is valence-trapped on the time
scale of the Mo¨ssbauer technique (electron-transfer rate <∼107 s-1 in the solid state). The
electrochemical measurements are also presented.
Ch a r t 1
In tr od u ction
The insight provided by the rich chemistry of bridged
mixed-valence dimers, especially those of ruthenium
dimers3 and biferrocenium cations,4 has promoted a
great deal of both experimental and theoretical studies.
Considerable work has been focused on the use of mixed-
valence complexes to probe electron transfer between
well-separated metal sites. In the case of mixed-valence
biferrocenium cations (fulvalenide-bridged compounds),
it has been studied extensively in the solid state
regarding the nature of the structural factors and lattice
dynamics.5 Despite considerable work in biferrocenium
cations, however, the mixed-valence chemistry of fer-
rocenium oligomers larger than biferrocenium is virtu-
ally unknown.6,7 This paper is concerned with the
oxidation products of triferrocenes and tetraferrocenes
(Chart 1, 1-7).
Mo¨ssbauer spectra of 8 show that the ferrocene-to-
ferrocenium ratio increases, as indicated by fitting the
areas of the FeII and FeIII doublets. A migration of
electron density from the porphyrin ring at low tem-
peratures was proposed.
The first single-crystal X-ray structure of the mixed-
valence ferrocenium(III)-trisferrocenyl(II) borate, 9,
was communicated.9 The centroid-to-centroid distance
between the two cyclopentadienyl (Cp) rings in one
ferrocenyl moiety was found to be longer than the
corresponding distance in the other three metallocene
units. Unfortunately, no 57Fe Mo¨ssbauer, EPR, or
spectroscopic data other than electronic absorption data
have been reported as yet for this compound.
Few mixed-valence ferrocenyl compounds that have
been prepared up to this time contain more than two
ferrocenyl units. Tetraferrocenylporphyrin3+(I3-)3, 8,
has been well characterized to be localized on the 57Fe
Mo¨ssbauer time scale (electron-transfer rate < 107 s-1).8
Furthermore, upon lowing the temperature to 90 K, the
(1) National Sun Yat-Sen University.
(2) Academia Sinica.
(3) For reviews, see: (a) Day, P. Int. Rev. Phys. Chem. 1981, 1, 149.
(b) Mixed-Valence Compounds, Theory and Applications in Chemistry,
Physics, Geology and Biology; Brown, D. B., Ed.; Reidel Publishing
Co.: Boston, MA, 1980. (c) Creutz, C. Prog. Inorg. Chem. 1983, 30,
1-73. (d) Richardson, D. E.; Taube, H. Coord. Chem. Rev. 1984, 60,
107.
(4) Dong, T.-Y.; Lee, S. H.; Chang, C. K.; Lin, H. M.; Lin, K. J .
Organometallics 1997, 16, 2773 and references therein.
(5) Dong, T.-Y.; Chang, C. K.; Lee, S. H.; Lai, L. L.; Chiang, M. Y.
N.; Lin, K. J . Organometallics 1997, 16, 5816 and references therein.
(6) Cowan, D. O.; Kaufman, F. J . Am. Chem. Soc. 1970, 92, 219.
(7) Brown, G. M.; Meyer, T. J .; Cowan, D. O.; LeVanda, C.; Kaufman,
F.; Roling, R. V.; Raush, M. D. Inorg. Chem. 1975, 14, 506.
In 1974, Meyer and co-workers reported the electro-
chemical generation of the mixed-valence ions of poly-
(8) Wollmann, R. G.; Hendrickson, D. N. Inorg. Chem. 1977, 16,
3079.
(9) Cowan, D. O.; Shu, P.; Hedberg, F. L.; Rossi, M.; Kitsenmacher,
T. J . J . Am. Chem. Soc. 1979, 101, 1304.
S0276-7333(98)00170-8 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 06/26/1998

3324 Organometallics, Vol. 17, No. 15, 1998
Dong et al.
Ch a r t 2
Ch a r t 3
nomenon could also be observed for the valence-trapped
triferrocenium dication 1 reported before. In addition
to the studies of electron transfer, the series of trifer-
rocenes and tetraferrocenes can also provide rather
versatile electrochemical and magnetic chemistry.
Dialkyltriferrocenes and dialkyltetraferrocenes were
prepared by coupling the corresponding bromofer-
rocenes, using activated Cu as the catalyst (Schemes 1
and 2). Mixed-valence compounds were prepared by
oxidizing the neutral triferrocenes and tetraferrocenes
with I2. Before the new physical data are described, a
summary of the single-crystal X-ray structural results
obtained for 23, 25, and 29 will be presented.
Molecu la r Str u ctu r es of Tr ifer r ocen es 23 a n d 25.
The results of our crystallographic studies at room
temperature show that 23 and 25 crystallize in the
monoclinic space group P21/c and monoclinic space
group P21/n, respectively. Details of the X-ray crystal
data collections and unit cell parameters are given in
Table 1. ORTEP drawings of the molecules are shown
in Figure 1, and bond distances and angles are given
as Supporting Information. An interesting finding is
that the two ethyl substituents on the Cp rings in 23
are situated differently, and this is in contrast to com-
pound 25 in which the -CH2OCH3 groups positioned
ferrocenes (10 and 11).7 They found that a series of one-
electron waves is observed by cyclic voltammetry, cor-
responding to one wave per ferrocenyl unit. The elec-
trochemically generated mixed-valence monocation 10
(Chart 2) has an intervalence transfer electronic absorp-
tion band at 1990 nm (ꢀ ) 1560 cm-1 M-1). In the case
of dication 11, an intervalence transfer band is observed
at 1670 nm (ꢀ ) 1080 cm-1 M-1).
The present study began with efforts to prepare the
neutral disubstituted triferrocenes and tetraferrocenes,
selectively oxidize the iron centers, and then study the
electronic properties which may be compared to those
of other mixed-valence ferrocenyl cations.
Resu lts a n d Discu ssion
In the solid state, it has been demonstrated that
relatively minor perturbations caused by the substitu-
ents on the Cp ring in mixed-valence biferrocenium
cations have dramatic effects on the electronic structure
and rate of intramolecular electron transfer.4 Estimated
by 57Fe Mo¨ssbauer techniques, the mixed-valence cation
12 (Chart 3) has a valence-trapped electronic structure
at 300 K (rate < 107 s-1). However, the mixed-valence
dialkylbiferrocenium cations 13-16 give unusual tem-
perature-dependent Mo¨ssbauer spectra.10 The Mo¨ss-
bauer results indicate that cations 13-16 are valence-
detrapped (rate > 107 s-1) at temperatures above 275,
245, 275, and 260 K, respectively.11-15 Thus, we under-
took the present work in the hope that a similar phe-
(10) Dong, T.-Y.; Hendrickson, D. N.; Iwai, K.; Cohn, M. J .; Rhei-
ngold, A. L.; Sano, H.; Motoyama, S. J . Am. Chem. Soc. 1985, 107,
7996.
(11) Hendrickson, D. N.; Oh, S. M.; Dong, T.-Y.; Moore, M. F. Comm.
Inorg. Chem. 1985, 4, 329.
(12) Dong, T.-Y.; Cohn, M. J .; Hendrickson, D. N.; Pierpont, C. G.
J . Am. Chem. Soc. 1985, 107, 4777.
(13) Cohn, M. J .; Dong, T.-Y.; Hendrickson, D. N.; Geib, S. J .;
Rheingold, A. L. J . Chem. Soc., Chem. Commun. 1985, 1095.
(14) Dong, T.-Y.; Hendrickson, D. N.; Iwai, K.; Cohn, M. J .; Rhei-
ngold, A. L.; Sano, H.; Motoyama, S. J . Am. Chem. Soc. 1985, 107,
7996.
(15) Iijima, S.; Saida, R.; Motoyama, I.; Sano, H. Bull. Chem. Soc.
J pn. 1981, 54, 1375.

Mixed-Valence Polyferrocenium Cations
Organometallics, Vol. 17, No. 15, 1998 3325
Sch em e 1a
a
(i) 1.0 eq n-BuLi. (ii) RCHO. (iii) LiAlH4/AlCl3. (iv) ICH2OCH3. (v) Cu. (vi) 3 equiv of I2.
above and below the central ferrocenyl moiety are
crystallographically equivalent. Consequently, the mo-
lecular structure of 23 is different from 25. In the case
of 25, the molecular structure can be described as
steplike ferrocenyl moieties. This usual type of steplike
conformation has been seen in other triferrocenyl com-
pounds. As shown in Figure 1, the molecular structure
of 23 can be described as column of ferrocenyl moieties.
Furthermore, there is a Cp-Cp π-π interaction (∼3.3
Å) between the two terminal ferrocenyl units.
As given in Table 2, a direct comparison of the aver-
age Fe-C and Fe-Cp ring distances is made. Inspec-
tion of the average bond distance between the Fe atom
and the five carbons (Fe-C) of a given Cp ring and the
average Fe-Cp distance for a ferrocenyl moiety indi-
cates that these values are closer to the corresponding
value observed for ferrocene (2.045 Å for Fe-C, 1.65 Å
for Fe-Cp)16 than the corresponding value observed for
ferrocenium cation (2.075 Å for Fe-C, 1.70 Å for Fe-
Cp).17 The C-C bond length in the Cp rings in these
compounds also agrees well with that in ferrocene (1.42
Å).16 As shown in Table 2 (ta (dihedral angle between
the two Cp rings associated with the corresponding Fe
atom) values), the two least-squares planes of the Cp
rings for a given ferrocenyl moiety in 23 and 25 form a
nearly parallel geometry. Furthermore, the two Cp
rings for a given ferrocenyl moiety are nearly eclipsed
(see the sa (stagger angle between the two Cp rings
associated with the corresponding Fe atom) values in
Table 2), except the central ferrocenyl moiety in 25. In
25, the two Cp rings associated with Fe1 form a
perfectly staggered conformation.
Molecu lar Str u ctu r es of Tetr afer r ocen e 29. Com-
pound 29 crystallizes in the triclinic space group P1h.
Figure 2 shows the molecular structure and atomic-
labeling scheme. The site symmetry imposed on the
molecule obviously requires that the two -CH2OCH3
substituents are in equivalent positions; this is similar
to compound 25. However, the structural conformation
of 29 is different from 25. As mentioned before, the
molecular structure of 25 can be described as steplike
ferrocenyl moieties. In the case of 29, it can be
described as column of ferrocenyl moieties, similar to
compound 23. Similarly, there is a Cp-Cp π-π inter-
action (∼3.3 Å) between the Fe1 and Fe2 ferrocenyl
moieties. As shown in Table 2, the two Cp rings for a
given ferrocenyl moiety form a nearly parallel geometry
and eclipsed conformation.
Electr och em ica l Mea su r em en ts. Electrochemical
data for the neutral compounds 23-25 and 27-29, as
well as those for some other relevant compounds, are
shown in Table 3. For all of the compounds studied,
the number of waves observed by cyclic voltammetry
(16) Seiler, P.; Dunitz, J . D. Acta Crystallogr., Sect. B 1979, 35, 1068.
(17) Mammano, N. J .; Zalkin, A.; Landers, A.; Rheingold, A. L. Inorg.
Chem. 1977, 16, 297.

3326 Organometallics, Vol. 17, No. 15, 1998
Dong et al.
Sch em e 2
couple has ∆ ) 70 mV, which is used as the criterion
for reversibility. From CV, triferrocenes (23-25) un-
dergo three electrochemically reversible one-electron
oxidations. In the case of tetraferrocenes (27-29), they
undergo four electrochemically reversible one-electron
oxidations.
The effect of the substituents on the stability of the
Fe(III) state is illustrated by the shift of the half-wave
potential. In general, electron-donating groups stabilize
the ferrocenium cation, lowering the half-wave poten-
tial, while electron-withdrawing groups have the op-
posite effect. Substituent effects on E1/2 have been
shown to be additive by potential measurements.7
For compounds 23-29, there are chemically different
sites (X-Fc and -Fc-) in the polymeric units. Upon
oxidation to the mixed-valence ions, more than one
oxidation state can exist. Consequently, the isomers
may differ in free energy. For example, the dication of
triferrocenes can exist as one of two energetically
equivalent isomers, X-Fc+-Fc+-Fc-X and X-Fc-
Fc+-Fc+-X, or as the energetically nonequivalent
isomer X-Fc+-Fc-Fc+-X (Figure 3). Of course, the
distribution between the various isomers will depend
on the zero-point energy difference(s) (Eo) between them.
It has been shown that estimation of the zero-point
energy difference between isomers is possible. For
example, it is possible to estimate the Eo value between
the isomers C2H5-Fc+-Fc+-Fc-C2H5 and C2H5-Fc+-
Fc-Fc+-C2H5. From the E1/2 values in Table 3, ethyl-
ferrocene and 1′,1′′′-diethylbiferrocene are more easily
oxidized than ferrocene. Therefore, the substituent
effects of the ethyl group and ethylferrocenyl group can
be calculated.
δ-C H ) E1/2 (C2H5Fc+,0) - E1/2 (Fc+,0) ) -0.05 V
2
5
δ-C H Fc ) E1/2 ((C2H5-Fc)2+,0) -
2
5
E1/2 (Fc+,0) - δ
-C2H5
Ta ble 1. Exp er im en ta l a n d Cr ysta l Da ta for th e
X-r a y Str u ctu r es
) 0.26 - 0.4 - (-0.05) ) -0.09 V
23
25
29
formula
mol wt
cryst syst
space group
a, Å
C
34H34Fe3
C34H34Fe3O2
642.18
monoclinic
P21/n
13.738(1)
7.503(1)
14.764(2)
C44H42Fe4O2
826.20
triclinic
P1h
5.937(2)
9.4772(7)
17.874(2)
92.424(9)
97.76(2)
105.91(2)
1.609
955.0(4)
1
1.67
0.70930
50.0
0.030
The potential for the reaction C2H5Fc+-Fc+-Fc-C2H5
+ e f C2H5-Fc+-Fc-Fc-C2H5 can be calculated.
610.18
monoclinic
P21/c
14.498(5)
7.680(5)
23.62(2)
E1/2 (calcd) ) E1/2 (C2H5-Fc22+,+) + δ (C2H5-Fc) )
0.64 - 0.09 ) 0.55 V
b, Å
c, Å
R, deg
Since E1/2 (0.35 V) for the couple (C2H5-Fc-Fc-Fc-
C2H5)2+,+ is significantly lower, the experimental value
must refer, mainly, to the reaction
â, deg
94.29(5)
116.888(9)
γ, deg
F
calcd, gcm-3
1.546
2622(3)
4
1.66
0.70930
49.8
1.571
1357.2(3)
2
1.61
0.70930
54.8
V, Å3
Z
C2H5-Fc+-Fc-Fc+-C2H5 + e f
µ, mm-1
λ, Å
C2H5-Fc-Fc+-Fc-C2H5 (E ) 0.35 V)
2θ limits, deg
a
Rf
0.090
0.105
0.033
0.038
b
Rwf
0.034
By combining the half-reactions, the zero-point energy
difference between the isomers C2H5-Fc+-Fc-Fc+-
C2H5 and C2H5-Fc+-Fc+-Fc-C2H5 is estimated to be
0.2 V which is significantly larger than the Eo value
(0.12 V) between the isomers Fc+-Fc-Fc+ and Fc+-
Fc+-Fc.7 We believe that this difference of the Eo
values is a result of the electronic effect of the ethyl
substituent. The nature of the substituent has a direct
influence on the magnitude of Eo. As shown in Figure
Rf ) ∑(||Fo| - |Fc||)/∑|Fo|. Rwf ) ∑[(w(Fo - Fc)2)/∑(wFo2)]1/2
.
a
b
(CV) was equal to the number of ferrocenyl groups.
Electrochemical reversibility is demonstrated by the
peak-to-peak separation (∆ in Table 3) between the
resolved reduction and oxidation wave maxima. In fact,
∆ is larger than the theoretical value of 59 mV. Under
our experimental conditions, the ferrocene-ferrocenium

Mixed-Valence Polyferrocenium Cations
Organometallics, Vol. 17, No. 15, 1998 3327
F igu r e 1. (a) ORTEP drawings for 23 and (b) 25.
Ta ble 2. Com p a r ison of Atom ic Dista n ces (Å) a n d
An gles (d eg)
23
25
29
Fe1-Ca
2.04(1)
2.04(1)
2.04(1)
1.651(6)
1.656(6)
1.656(6)
1.9(5)
1.6(5)
1.9(6)
2.0(7)
2.9(8)
2.048(3)
2.040(3)
2.053(3)
2.051(4)
Fe2-Ca
Fe3-Ca
Fe1-Cpb
Fe2-Cpb
Fe3-Cpb
ta (Fe1)c
ta (Fe2)c
ta (Fe3)c
sa (Fe1)d
sa (Fe2)d
sa (Fe3)d
1.652(2)
1.648(2)
1.660(2)
1.652(2)
0.0
3.7(2)
1.7(2)
1.9(2)
36.0(1)
4.6(2)
1.5(2)
1.0(2)
3.6(8)
a
b
Average Fe-C distance. The distance between the Fe atom
and the Cp ring. c The dihedral angle between the two Cp rings
associated with the corresponding Fe atom. Stagger angle be-
d
tween the two Cp rings associated with the corresponding Fe atom.
3, an electron-donating substituent can exert somewhat
more influence on the X-Fc+-Fc-Fc+-X energy sur-
face than on the X-Fc+-Fc+-Fc-X energy surface. It
is important to realize that energetically it is the
X-Fc+-Fc-Fc+-X energy surface in Figure 3 which is
stabilized more by an electron-releasing substituent.
With respect to the X-Fc+-Fc-Fc+-X energy surface,
the stabilization in X-Fc+-Fc+-Fc-X energy surface
by an electron-releasing substituent will be smaller. The
net result shows an increase of the Eo value in a
triferrocenium dication with an electron-donating sub-
stitutent. As shown in Table 3, the disproportionation
F igu r e 2. ORTEP drawing for 29.
trapped cation in which the rate of intramolecular
electron transfer is less than ∼107 s-1. For 2-4, the
ferrocene-to-ferrocenium area ratio is fitted into 1:2. The
1:2 ferrocene-to-ferrocenium relative area is expected
for 2-4 if two of the ferrocenyl groups are oxidized. In
the case of 5 and 6, the ferrocene-to-ferrocenium area
ratio is fitted into 2:2. The 2:2 ferrocene-to-ferrocenium
area ratio for 5 and 6 indicates that two of the ferrocenyl
groups are oxidized. An interesting observation in the
cations of 5 and 7 is that ∆EQ values for ferrocenium
centers are much larger than those observed for trifer-
rocenium cations. The larger ∆EQ in the cations of 5
and 7 can be interpreted by assuming that the increased
constants Kc can be calculated from the values of ∆E1/2
.
57F e Mo1ssba u er Ch a r a cter istics. The rates of
intramolecular electron transfer in the mixed-valence
cations 2-7 were estimated by 57Fe Mo¨ssbauer spec-
troscopy (time scale ∼107 s-1). The various absorption
peaks were fitted to Lorentzian lines. The resulting
parameters are collected in Table 4. All of the 57Fe
Mo¨ssbauer spectra for mixed-valence compounds 2-7
show two doublets (Fe(II), ∆EQ ) 2.0 mm s-1; Fe(III),
∆EQ ) 0.3 mm s-1), which is characteristic of a valence-
2
1
amount of overlap between the E2g and A1g states of

3328 Organometallics, Vol. 17, No. 15, 1998
Dong et al.
Ta ble 3. Cyclic Volta m m etr y for Va r iou s
Bifer r ocen es
a
b
E1/2
V
,
∆E1/2
V
,
∆,c
mV
Kc
compd
(×10-5
)
ferrocene
ethylferrocene
methoxymethylferrocene
biferrocene
0.40
0.35
0.41
0.31
0.63
0.28
0.64
0.26
0.59
0.28
0.65
0.26
0.58
0.40
0.72
0.22
0.44
0.82
0.16
0.36
0.61
0.89
0.19
0.35
0.80
0.19
0.35
0.80
0.25
0.45
0.80
0.19
0.31
0.58
0.86
0.20
0.32
0.60
0.87
0.20
0.38
0.60
0.83
70
65
79
70
75
70
85
65
65
75
72
65
65
70
72
0.32
0.36
0.33
0.37
0.32
0.32
2.65
12.6
3.92
18.7
2.65
2.65
1′-ethylbiferrocene
1′,1′′′-diethylbiferrocene
1′-butylbiferrocene
1′,1′′′-dibutylbiferrocene
1′,1′′′-dimethoxymethyl-
biferrocene
triferrocene
0.22
0.38
0.0536
27.6
tetraferrocene
0.2
0.25
0.28
0.0245
0.173
0.057
F igu r e 3. Potential energy-configurational diagrams for
dication triferrocenium ions.
Ta ble 4. 57F e Mo1ssba u er Lea st-Squ a r es-F ittin g
23
24
25
27
0.16
0.45
68
73
68
71
66
65
72
69
61
65
65
68
70
57
55
62
90
64
67
57
87
0.00515
424
P a r a m eter s
a
compd
T, K
∆EQ
δb
Γc
0.16
0.45
0.00515
424
1
300
1.970
0.371
2.045
0.209
2.073
0.329
2.006
0.299
2.074
0.374
2.047
0.235
2.050
0.235
1.882
0.501
1.954
0.349
1.742
0.651
0.472
0.453
0.328
0.297
0.425
0.418
0.337
0.257
0.423
0.414
0.344
0.335
0.426
0.424
0.337
0.307
0.328
0.265
0.441
0.417
0.264; 0.256
0.416; 0.390
0.169; 0.159
0.360; 0.360
0.154; 0.147
0.245; 0.278
0.215; 0.199
0.332; 0.332
0.148; 0.142
0.210; 0.239
0.140; 0.147
0.359; 0.359
0.139; 0.150
0.255; 0.271
0.187; 0.238
0.371; 0.403
0.251; 0.251
0.402; 0.402
0.409; 0.378
0.744; 0.545
0.20
0.35
0.0245
8.55
2
3
4
300
80
0.12
0.27
0.28
0.00108
0.377
0.557
300
80
28
29
0.12
0.28
0.27
0.00108
0.557
0.377
300
80
0.18
0.22
0.23
0.00112
0.0536
0.0790
5
6
7
300
300
300
a
All half-wave potentials are referred to the SCE electrode.
Peak separation between waves. c Peak-to-peak separation be-
tween the resolved reduction and oxidation wave maxima.
b
Quadrupole-splitting in mm s-1
.
Isomer shift referenced to
a
b
iron-foil in mm s-1
.
c Full-width at half-height taken from the
the ferrocenium and ferrocene portions of the molecular
results in increased electron density at the ferrocenium
centers.
least-squares-fitting program. The width for the line at more
positive velocity is listed first for each doublet.
In flu en ce of Zer o-P oin t En er gy Differ en ce on
Electr on Tr a n sfer . The goal of this section is to
present an explanation for the differences of electron-
transfer rates between biferrocenium cations 12-16 and
polyferrocenium cations 1-7.
Recently, there has been considerable progress made
in understanding what factors control the rate of intra-
molecular electron transfer in the solid state for mixed-
valence biferrocenium salts.10-12,14,15 Hendrickson sug-
gested that what is affecting the Mo¨ssbauer spectrum
and impacting the temperature dependence is the onset
of lattice dynamics (a second-order phase transition).
In Hendrickson’s theoretical model,18 the factors that
are potentially important in controlling the rate of
intramolecular electron transfer in a mixed-valence
biferrocenium cation include the effective barrier for
charge oscillation in the anion and the intermolecular
cation-cation and cation-anion interactions. In our
primary paper,19 we found that there is a significant
influence in the electron-transfer rate in the mixed-
valence biferrocenium salt when the cyclopentadienyl
(Cp) rings in each ferrocenyl moiety are tilted from the
parallel geometry. Deviations of the Cp rings from the
parallel position were found to correlate quite well with
the critical temperature (Tc) of electronic delocalization-
localization in biferrocenium salts. In a qualitative
view, we suggest that there is an increased (dx -y , dxy)-
ring overlap as the tilt angle increases.
In fact, we believe that the most important factor in
controlling the intramolecular electron transfer is the
symmetry of the cation. For an asymmetric biferroce-
nium cation, the two irons are not in equivalent en-
2
2
(18) Kambara, T.; Hendrickson, D. N.; Dong, T.-Y.; Cohn, M. J . J .
Chem. Phys. 1987, 86, 2362.
(19) Dong, T.-Y.; Huang, C. H.; Chang, C. K.; Wen, Y. S.; Lee, S. L.;
Chen, J . A.; Yeh, W. Y.; Yeh, A. J . Am. Chem. Soc. 1993, 115, 6357.

Mixed-Valence Polyferrocenium Cations
Organometallics, Vol. 17, No. 15, 1998 3329
Ta ble 5. P h ysica l P r op er ties of Va r iou s F er r ocen es
MS (M+
at m/z)
compd
1H NMR (CDCl3, ppm)
mp
18
19
23
24
25
27
28
29
1.45 (d, 3H, -CH3), 2.01 (d, 1H, -OH), 4.12 (t, 2H, Cp), 4.22 (m, 4H, Cp), 4.42 (m, 2H, Cp),
4.66 (m, 1H, -CH)
308, 310
336, 338
610
0.93 (t, 3H, -CH3), 1.46 (m, 2H, -CH2-), 1.64 (m, 2H, -CH2-), 2.06 (s, 1H, -OH), 4.13 (s, 2H, Cp),
4.22 (m, 4H, Cp), 4.42 (m, 3H, Cp, -CH)
1.02 (t, 6H, -CH3), 2.11 (q, 4H, -CH2-), 3.81 (d, 4H, Cp), 3.83 (d, 4H, Cp), 4.02 (t, 4H, Cp),
4.07 (t, 4H, Cp), 4.10 (t, 4H, Cp), 4.16 (t, 4H, Cp)
0.84 (t, 6H, -CH3), 1.26 (m, 8H, -CH2-), 2.06 (t, 6H, -CH2-), 3.80 (d, 4H, Cp), 3.83 (t, 4H, Cp),
4.02 (t, 4H, Cp), 4.07 (t, 4H, Cp), 4.09 (t, 4H, Cp), 4.15 (t, 4H, Cp)
3.18 (s, 6H, -OCH3), 3.91 (s, 4H, -CH2-), 3.95 (d, 8H, Cp), 4.04 (d, 8H, Cp), 4.11 (s, 4H, Cp),
4.17 (s, 4H, Cp)
1.00 (t, 6H, -CH3), 2.09 (q, 4H, -CH2-), 3.79 (s, 4H, Cp), 3.80 (s, 4H, Cp), 3.98 (s, 12H, Cp),
4.03 (s, 4H, Cp), 4.07 (s, 4H, Cp), 4.11 (s, 4H, Cp)
0.82 (t, 6H, -CH3), 1.25 (m, 8H, -CH2CH2-), 2.03 (t, 4H, -CH2-), 3.77 (s, 4H, Cp),
3.79 (s, 4H, Cp), 3.97 (s, 12H, Cp), 4.01 (s, 4H, Cp), 4.04 (s, 4H, Cp), 4.09 (s, 4H, Cp)
3.17 9s, 6H, -OCH3), 3.89 (s, 4H, -CH2-), 3.92 (s, 4H, Cp), 3.94 (s, 8H, Cp), 3.97 (s, 4H, Cp),
3.98 (s, 4H, Cp), 4.01 (s, 4H, Cp), 4.08 (s, 4H, Cp), 4.13 (s, 4H, Cp)
102-103
100-101
136-137
110-112
162-163
157-158
666
642
794
850
826
desired compound (89% yield for 18 and 86% yield for 19). The
physical properties are shown in Table 5.
vironments and this asymmetry results in a zero-point
energy difference for intramolecular electron transfer.
In other words, one vibronic state of the mixed-valence
cation is energeically more stable than the other state.
This explains why a delocalized electronic structure for
an asymmetic biferrocenium cation has not been ob-
served.
As mentioned in the Introduction, the symmetric
mixed-valence compounds 13-16 have a delocalized
electronic structure at 300 K. However, the 300 K
Mo¨ssbauer spectra of 2-7 indicate the presence of a
localized electronic structure. Here, we suggest that the
most important factor in controlling the electron trans-
fer in 2-7 is the presence of the zero-point energy dif-
ference (Eo). As shown in Figure 3, the electron-transfer
products in mixed-valence cations 2-7 are energetically
unfavorable oxidation-state isomers. For example, there
are actually two equivalent unfavorable oxidation-state
isomers for mixed-valence triferrocenium dications.
Thus, one vibronic state of the mixed-valence dication
is energetically more stable than the other state. This
explains why a localized electronic structure for 2-7
was observed.
Red u ction of 18 a n d 19. The reduction reaction was
carried out by carefully adding, with stirring, a small portion
of AlCl3 to a mixture of ferrocene compound and LiAlH4 in
dry ether. After 30 min, the solution became yellow, an excess
of H2O was added to it, and the ether layer was separated.
The ether layer was washed with H2O and dried over MgSO4.
After evaporation of the solvent, the crude product was
chromatographed on Al2O3, eluting with hexane. The first
band was the desired compound (∼95% yield). These com-
pounds were identical by 1H NMR with those prepared by a
different procedure.14,21
Gen er a l Ullm a n Cou p lin g Rea ction of 20-22 w ith 17.
A mixture of the corresponding bromoferrocene (6.48 mmol),
17 (3.24 mmol), and activated Cu (10 g) was heated under N2
at 125 °C (oil bath) for 24 h. The reaction mixture was cooled
to room temperature and then subjected to the Soxhlet process
with dichloromethane. The extract was evaporated at reduced
pressure, and the residue was chromatographed on Al2O3.
Compound 23 was obtained by a cross-coupling reaction of
17 and 20. The crude product was chromatographed, eluting
with hexane. The first band was a mixture of ferrocene and
ethylferrocene. The second band was 1′,1′′′-diethylbiferrocene,
which was identical by 1H NMR with that prepared by a
different procedure.21 Continued elution with hexane:CH2Cl2
(20:1) gave the desired compound 23 (15% yield).
Exp er im en ta l Section
Compound 24 was obtained by a cross-coupling reaction of
17 and 21. The purification was carried out by following the
same procedure as described for 23.
Gen er a l In for m a tion . All manipulations involving air-
sensitive materials were carried out using standard Schlenk
techniques under an atmosphere of N2. Chromatography was
performed on neutral alumina (Merck, activity II). Solvents
were dried as follows: benzene, hexane, ether, and THF
distilled from Na/benzophenone; CH2Cl2 and (CH3)2S2 distilled
from CaH2. Samples of 1,1′-dibromoferrocene, 22, and 26 were
prepared according to the literature procedure.5,20
Syn th eses of 18 a n d 19. Dibromoferrocene (17, 8.88 g,
20 mmol) was placed in a freshly oven-dried three-necked flask
(500 mL) and dried under vacuum at 2 mmHg at 25 °C for 4
h. Dried THF (80 mL), followed by n-butyllithium (20 mmol),
was added under nitrogen. The resulting solution was stirred
at -30 °C for 30 min, during which 1-bromo-1′-lithioferrocene
gradually precipitated. Acetaldehyde (20 mmol) or butyral-
dehyde (20 mmol) was then added, and the solution was stirred
further at -30 °C for 30 min. Water (200 mL) was added,
and the mixture was then extracted with CH2Cl2. The
combined extracts were dried over MgSO4 and evaporated at
reduced pressure. The residue was chromatographed on Al2O3.
The first band eluted with hexane was ferrocenes. Continued
elution with hexane:CH2Cl2 (1:1) gave elutes which yielded the
Compound 25 was obtained from 17 and 22. The crude
product was chromatographed, eluting with hexane. The first
band was a mixture of ferrocenes. The second band eluted
with hexane/ethyl acetate (50/1) was methoxymethylferrocene.
Continued elution with hexane/ethyl acetate (100/7) gave
1′,1′′′-dimethoxymethylbiferrocene (third band) and compound
25 (fourth band, 12.5% yield).
Gen er a l Ullm a n Cou p lin g Rea ction of 20-22 w ith 26.
The coupling reaction was carried out according to the general
procedure described above, except using compound 26 instead
of 17.
Compound 27 was obtained by a cross-coupling reaction of
20 with 26. The crude product was chromatographed, eluting
with hexane. The first band was a mixture of ferrocenes. The
second band was 1′,1′′′-diethylbiferrocene, which was identical
by 1H NMR with that prepared by a different procedure.21
Continued elution with hexane:benzene (1:1) gave the desired
compound 27 (14% yield).
(21) Iijima, S.; Saida, R.; Motoyama, I.; Sano, H. Bull. Chem. Soc.
J pn. 1981, 54, 1375.
(20) Dong, T.-Y.; Lai, L. L. J . Organomet. Chem. 1996, 509, 131.

3330 Organometallics, Vol. 17, No. 15, 1998
Dong et al.
Compound 28 was obtained by a cross-coupling reaction of
21 with 26. The crude product was chromatographed, eluting
with hexane. The first band was a mixture of ferrocenes.
Continued elution with hexane/CH2Cl2 (20/1) gave 1′,1′′′-
to determine the heavy-atom positions, which phased the data
sufficiently well to permit location of the remaining non-
hydrogen atoms from Fourier synthesis. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
calculated at ideal distances. The X-ray crystal data are
summarized in Table 1. Selected bond distances and angles
are given in Tables 2, 3, and 5. Listings of the final positional
parameters for all atoms, bond distances and angles, and
thermal parameters of these compounds are given in the
Supporting Information.
Str u ctu r e Deter m in a tion of 23. An orange crystal (0.19
× 0.44 × 0.50 mm) was obtained when a layer of hexane was
allowed to slowly diffuse into a CH2Cl2 solution of 23. Cell
dimensions were obtained from 25 reflections with 14.38° <
2θ < 30.94°. The θ-2θ scan technique was used to record the
intensities for all reflections for which 1° < 2θ < 49.8°. Of
the 4593 unique reflections, there were 3201 reflections with
Fo > 2.0σ(Fo), where σ(Fo) values were estimated from counting
statistics.
Close examination of difference Fourier maps in the final
stage of the structure solution revealed a disordered ethyl
group in the Cp ring. The site occupancy factor of the ethyl
substituent was set equal to 0.5.
Str u ctu r e Deter m in a tion of 25. An orange crystal (0.16
× 0.38 × 0.50 mm) was obtained following the same procedure
as that described for 23. Data were collected to a 2θ value of
55.0°. The unit cell dimensions were obtained from 25
reflections with 14.74° < 2θ < 34.20°. Of the 3109 unique
reflections, there were 2472 with Fo > 2.0σ(Fo).
1
dibutylbiferrocene, which was identified by H NMR.14 Con-
tinued elution with hexane/CH2Cl2 (10/1) gave the desired
compound 28 (9% yield).
Compound 29 was obtained by a cross-coupling reaction of
22 with 26. The crude product was chromatographed, eluting
with hexane. The first band was a mixture of biferrocenes.
Continued elution with hexane/CH2Cl2 (10/1) gave 1′,1′′′-
dimethoxymethylbiferrocene.5 Continued elution with hexane/
CH2Cl2 (5/1) gave the desired compound 29 (9% yield).
Mixed-Valen ce Com pou n ds 2-7. Samples of these mixed-
valence compounds were prepared by adding a benzene/hexane
(1:1) solution containing a stoichiometric amount of iodine to
a solution of the corresponding polyferrocene at 0 °C. The
corresponding polyferrocene solution was prepared by dissolv-
ing the polyferrocene in a proper solvent (benzene for 23, 24,
and 27-29; CH2Cl2 for 25). The resulting dark crystals were
filtered and washed repeatedly with cold hexane. Anal. Calcd
for 2 (C34H34Fe3I6): C, 29.76; H, 2.50. Found: C, 28.94; H, 2.43.
Anal. Calcd for 3 (C38H42Fe3I6): C, 31.97; H, 2.96. Found: C,
32.02; H, 2.79. Anal. Calcd for 4 (C34H34Fe3I6O2): C, 29.09;
H, 2.44. Found: C, 29.02; H, 2.39. Anal. Calcd for 5 (C44H42
-
Fe4I6): C, 34.02; H, 2.86. Found: C, 33.97; H, 2.72. Anal.
Calcd for 6 (C48H50Fe4I6): C, 35.77; H, 3.13. Found: C, 35.80;
H, 3.13. Anal. Calcd for 7 (C44H42Fe4I11/2): C, 34.84; H, 2.75.
Found: C, 34.67; H, 2.78.
P h ysica l Meth od s. 57Fe Mo¨ssbauer measurements were
made on a constant-velocity instrument, which was previously
described.19 Velocity calibration was made using a 99.99%
pure 10-µm iron foil. Typical line widths for all three pairs of
iron foil lines fell in the range 0.24-0.27 mm s-1. Isomer shifts
are reported relative to iron foil at 300 K. Tabulated data is
provided.
Str u ctu r e Deter m in a tion of 29. An orange crystal (0.06
× 0.13 × 0.38 mm) was obtained following the same procedure
as that described for 23. The unit cell dimensions were
obtained from 25 reflections with 11.78° < 2θ < 34.18°. Of
the 3353 unique reflections, there were 2423 with Fo > 2.0σ-
(Fo).
Close examination of difference Fourier maps in the final
stage of the structure solution revealed a disordered meth-
oxymethyl group in the Cp ring. The site occupancy factor of
the methoxymethyl substituent was set equal to 0.5.
1H NMR spectra were run on a Varian VXR-300 spectrom-
eter. Mass spectra were obtained with a VG-BLOTECH-
QUATTRO 5022 system.
Electrochemical measurements were carried out with a BAS
100W system. Cyclic voltammetry was performed with a
stationary glassy carbon working electrode, which was cleaned
after each run. These experiments were carried out with a 1
× 10-3 M solution of polyferrocene in dry CH2Cl2/CH3CN (1:
1) containing 0.1 M (n-C4H9)4NPF6 as the supporting electro-
lyte. The potentials quoted in this work are relative to a SCE
electrode at 25 °C. Under these conditions, ferrocene shows
a reversible one-electron oxidation wave (E1/2 ) 0.40 V).
The single-crystal X-ray determinations of all compounds
were carried out on an Enraf-Nonius CAD4 diffractometer at
298 K. Absorption corrections were made with empirical ψ
rotation. A three-dimensional Patterson synthesis was used
Ack n ow led gm en ts are made to the National Science
Council (Grant No. NSC87-2113-M-110-010), Taiwan,
ROC, and Department of Chemistry at Sun Yat-Sen
University.
Su p p or tin g In for m a tion Ava ila ble: Complete tables of
positional parameters, bond distances and angles, and thermal
parameters for 23, 25, and 29 (19 pages). Ordering informa-
tion is given on any current masthead page.
OM9801709