Oxidation potential of benzylferrocenes and related compounds: Effects of the ortho-methoxy substituent in the phenyl group

Article abstract of DOI:10.1016/S0022-328X(00)00075-9

Oxidation potentials of some ferrocenyl derivatives of type Fc-CHΦOH, Fc-CΦΦ′OH, Fc-CH₂Φ, Fc-CHΦΦ′ [Fc=Fe(η⁵-C₅H₅)(η⁵-C ₅H₄); Φ, Φ′=2,4,6-(MeO)₃C₆H₂, 2,6-(MeO)₂·C₆H₃, 2,5-(MeO)₂·C₆H₃, 2,4-(MeO)₂·C₆H₃, 2-MeOC₆H₄, 4-MeOC₆H₄, C₆H₅] were measured in acetonitrile. All of them exhibited a reversible one-electron oxidation-reduction wave based on the ferrocene-ferrocenium redox couple with a wide range of shift. Compounds having ortho-methoxy groups showed lower redox potentials than those of compounds having para-methoxy groups. The X-ray crystal structure of Fc-CHΦ₂^a showed that the ortho-methoxy oxygens were situated very close to the central carbon, and there is a possibility that one of the oxygen lone-pairs interacts with the antibonding orbital of C-Fc σ-bond. Some other possibilities are also discussed.

Full text of DOI:10.1016/S0022-328X(00)00075-9

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 601 (2000) 246–252
Oxidation potential of benzylferrocenes and related compounds:
effects of the ortho-methoxy substituent in the phenyl group
Masahiro Asahara, Satoko Natsume, Hideaki Kurihara, Takuya Yamaguchi,
Tatsuo Erabi, Masanori Wada *
Department of Materials Science, Faculty of Engineering, Tottori Uni6ersity, Koyama, Tottori 680-0945, Japan
Received 27 October 1999; received in revised form 14 January 2000; accepted 25 January 2000
Abstract
Oxidation potentials of some ferrocenyl derivatives of type Fc-CHFOH, FcꢀCFF%OH, FcꢀCH2F, FcꢀCHFF% [Fc=Fe(h5-
C5H5)(h5-C5H4); F, F%=2,4,6-(MeO)3C6H2, 2,6-(MeO)2·C6H3, 2,5-(MeO)2·C6H3, 2,4-(MeO)2·C6H3, 2-MeOC6H4, 4-MeOC6H4,
C6H5] were measured in acetonitrile. All of them exhibited a reversible one-electron oxidation–reduction wave based on the
ferrocene–ferrocenium redox couple with a wide range of shift. Compounds having ortho-methoxy groups showed lower redox
potentials than those of compounds having para-methoxy groups. The X-ray crystal structure of FcꢀCHFa2 showed that the
ortho-methoxy oxygens was situated very close to the central carbon, and there is a possibility that one of the oxygen lone-pairs
interacts with the antibonding orbital of CꢀFc s-bond. Some other possibilities are also discussed. © 2000 Elsevier Science S.A.
All rights reserved.
Keywords: Benzylferrocenes; Ferrocenyl derivatives; o-Methoxyphenyl; Oxidation potential; X-ray crystal structure
as oxidizing agents [7], redox catalysts [8], sensors [9–
12], organic magnets [13], modifiers of electrode [14–
1. Introduction
16], and second-order non-linear optical materials
[17,18]. In the present study, we report unique effects
of the ortho-methoxy substituent in ferrocenyl-
(phenyl)methanols and the related ferrocenyl deriva-
Due to the electronic and steric effects of ortho-
methoxy groups, 2,6-dimethoxyphenyl derivatives often
exhibit unusual properties. As part of our systematic
investigation on the chemistry of 2,6-dimethoxyphenyl
tives, FcꢀCHFOH (1), FcꢀCFF%OH (2), FcꢀCH2F (3),
derivatives of a variety of elements, we have recently
FcꢀCHFF% (4), on the oxidation potential [F, F%=(a)
reported the preparation and reactions of ferro-
Fa, (b) Fb, (c) 2,5-(MeO)2C6H3, (d) 2,4-(MeO)2C6H3, (e)
cenyl(phenyl)carbenium salts, [FcꢀCHF]X[Fc=Fe(h5-
2-MeOC6H4, (f) 4-MeOC6H4, (g) C6H5]. We abbreviate
C5H5)(h5-C5H4);
F=2,4,6-(MeO)3C6H2(Fa),
2,6-
here some methoxyphenyl groups, F, as shown in
Scheme 1.
(MeO)2C6H3(Fb)], which have very high stabilities,
pKR+ =3.0(Fa), 1.6(Fb) [1]. Although we were also
interested in the preparation and properties of tertiary
carbenium salts, [FcꢀCF2]X, we have not yet succeeded
in their isolation because these salts are unstable and
decompose to give 6-diphenylfulvene, C5H4ꢁCF2. On
the other hand, ferrocene derivatives are well known to
exhibit reversible one-electron oxidation [2–6], and re-
cent years have seen developments in their applications
2. Experimental
2.1. Physical measurements
1H- and 13C-NMR spectra were recorded for solu-
tions in CDCl3 using a JEOL model JNM-GX 270
spectrometer. IR spectra were recorded for Nujol®
mulls using a Shimadzu FTIR-4200 spectrophotometer.
GC–MS spectra were recorded for acetone or toluene
* Corresponding author. Tel.: +81-857-315248; fax: +81-857-
310881.
E-mail address: wada@chem.tottori-u.ac.jp (M. Wada)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00075-9

M. Asahara et al. / Journal of Organometallic Chemistry 601 (2000) 246–252
247
solutions using a Shimadzu QP-5000 mass spectrometer
2.2. Preparation of ferrocene deri6ati6es
(gasified at 250°C). Visible spectra were recorded
for 1,2-dichloroethane solutions using a Shimadzu
UV-160 spectrophotometer. The 1H- and 13C-NMR
spectral data are summarized in Tables 1 and 2, respec-
tively.
Compounds 1a, 1b, 1g, 3a, 3b, 3g, 4aa, 4bd, and 4dg,
were prepared as described elsewhere [1]. The following
compounds were prepared essentially in an analogous
manner.
Scheme 1.
Table 1
1H-NMR spectral data for ferrocenyl (phenyl) methane derivatives a
C5H5
C5H4
CcentralꢀH
Others
1c
1d
1e
4.21s
4.19s
4.19s
4.25ꢀ4.12m
4.27ꢀ4.14m b
4.27ꢀ4.13m
5.74d[5]
5.74d[5]
5.79d[5]
6.94d[2] and 6.75d[2] (3,4-H), 6.77s (6-H), 3.79s and 3.75s (2,5-MeO), 2.85d[5] (OH)
7.19d[9] (1H), 6.45ꢀ6.43br (2H), 3.81s (3H), 3.78s (3H), 2.76d[5] (1H,OH)
7.35d[8], 7.22t[8], 6.93t[7], and 6.86d[8] (4H, 3,4,5,6-H), 3.84s (6H,MeO), 2.78d[5] (1H,
OH)
1f
4.21s
4.21ꢀ4.16m
4.24br (2H),
4.08t[2] (2H)
5.44d[3]
7.30d[8] (2H), 6.85d[9] (2H), 3.79s (3H), 2.39d[3] (1H, OH)
7.08t[8] (2H), 6.54d[8] (4H), 6.27s (1H), 3.47br (12H)
2bb 4.03s
2bb c 4.08s b 4.52s (1H), 4.17s
(2H) b,3.92s (1H)
7.24t[8] and 7.06t[8] (2H, 4-H), 6.78d[8], 6.61d[8], 6.53d[8], and 6.47d[8] (4H, 3,5-H),
6.00s (1H, OH), 4.08sb, 3.42s, 3.37s, and 3.08s (3H+3H+3H+3H, MeO)
7.25ꢀ7.13m (6H, Ph and 4-H), 6.60d[8] (2H, 3,5-H), 6.11s (1H, OH), 3.47s (6H, MeO)
7.31ꢀ7.20m (12H, Ph), 3.44s (1H, OH)
2bg 4.12s b 4.19ꢀ4.09m b
2gg 4.17s
4.27t[2], 4.04t[2]
3c
3e
3f
4.15s
4.15ꢀ4.08m (4H)
3.63s (2H)
6.76ꢀ6.62m (3H, 3,4,6-H), 3.80s and 3.70s (6H, 2,5-MeO)
4.12s b 4.12 b, 4.05br (2H) 3.68s (2H)
7,15t[7], 7.03d[7], 6.85d[7], and 6.82d[7] (4H, 3,4,5,6-H), 3.85s (3H, MeO)
7.10d[9] (2H, 2,6-H), 6.81d[9] (2H, 3,5-H), 3.78s (3H, MeO)
7.06ꢀ7.01m (2H), 6.84d[8] and 6.74t[8] (2H), 6.14s (2H), 3.81s and 3.66s (12H)
7.46br (2%,6%-H), 7.23t[8] (4-H), 7.26ꢀ7.19m (3%,5%-H), 6.60d[8] (3,5-H), 3.73br (2,6-MeO),
3.10br (NMe2)
4.12s
4.08s (4H)
4.07br (4H)
4.12br
3.63s (2H)
5.98s
5.90s
4ae 4.03s
4bg 3.98s
4de 4.02s
4.11t[2] (2H),
3.98t[2] (2H)
5.86s
7.12td[8, 2] (1H); 6.95dd [8, 2] (1H), 6.88ꢀ6.76m (2H), 6.43d[2] (1H), 6.34dd[8, 2] (2H),
3.81s (3H), 3.80s (3H), and 3.76s (3H)
a In CDCI3 (l), 270 MHz; coupling constants [JHH/Hz] are given in parentheses; s, singlet; d, doublet; t, triplet; dd, double doublet; dt, double
triplet; m, multiplet; br, broad.
b Overlapped.
c At −60°C.

248
M. Asahara et al. / Journal of Organometallic Chemistry 601 (2000) 246–252
Table 2
13C-NMR spectral data for ferrocenyl (phenyl)methane derivatives a
C2H5 C5H4
Ccentral Others
1c
1d
1e
1f
69.2 93.7, 68.5, 68.4, 69.3
68.3, 68.2
154.3 and 151.4 (2,5-C), 133.6 (1-C), 114.1, 113.4, and 112.2 (3,4,6-C), 56.6 and 56.4 (2,5-MeO)
160.8 and 158.2 (2,4-C), 128.7, 125.2, 104.8, and 99.2 (1,3,5,6-C), 56.0 and (2,4-MeO) b
157.1(2-C), 132.4 (1-C), 129.1 (6-C), 128.0 (4-C), 121.3 (5-C), 111.2 (3-C), 56.2 (MeO)
158.9 (4-C), 135.6 (1-C), 127.4 (2,6-C), 113.5 (3,5-C), 55.2 (MeO)
69.2 93.9, 68.4, 68.3, 69.3
68.1, 67.1
b
69.3 93.7, 68.4, 68.4,
68.2, 67.2
68.4 94.3, 68.1, 68.0, 71.7
67.3, 65.9
2bb 69.0 100.5, 69.2, 66.9 78.2
2bg 69.3 98.8, 69.8, 69.8, 79.7
68.0, 67.2
158.8 (2,6-C), ---(1-C) b, 127.7 (4-C), 107.8 (3,5-C), 57.1 (MeO)
158.8, 150.3, 129.1, 127.5, 127.2, 126.7, 125.7, 107.8, 57.0
3c
3e
3f
68.6 87.2, 68.9, 67.4
68.6 87.5, 69.0, 67.3
68.6 88.5, 68.5, 67.4
29.9
29.8
35.1
153.4 and 151.2 (2,5-C), 131.7 (1-C), 116.3, 110.9, 110.8, (3,4,6-C), 55.8 and 55.6 (2,5-MeO)
156.9 (2-C), 130.4 (1-C), 129.5 (6-C), 127.0 (4-C), 120.3 (5-C), 110.0 (3-C), 55.2 (MeO)
157.8 (4-C), 133.8 (1-C), 129.3 (2,6-C), 113.6 (3,5-C), 55.2 (MeO)
4ae 67.5 91.1, 69.5, 68.6, 34.2
66.9, 66.4
159.3 (2%,6%-C), 159.2 (4%-C), 156.7 (2-C), 134.7 (1-C), 131.3 (3-C), 125.9 (6-C), 119.2 (4-C), 113.9
(5-C), 110.2 (1%-C), 91.4 (3%,5%-C), 55.7, 55.5 and 55.1 (MeO)
4de 69.3 92.5, 69.6, 69.5, 36.6
68.0, 67.9
159.5 and 158.2 (2%,4%-C), 157.3 (2-C), 134.9 (1-C), 130.7 and 127.3 (5%,6%-C), 130.2 (6-C), 127.2
(4-C), 120.6 (5-C), 111.2 (3-C), 104.2 (1%-C), 99.0 (3%-C), 56.2, 56.1, and 55.9 (MeO)
a In CDCl3 (l), 68 MHz.
b Not detected or overlapped.
of an acid in 32% yield; m.p. 57–59°C, MS m/z (rela-
tive intensity) 306 [100, M+].
3f: by reduction of 1f in 2-propanol in the presence of
an acid in 36% yield; m.p. 74–75°C, MS m/z (relative
intensity) 306 [100, M+].
1c: by reaction of FcꢀCHO with 2,5-
dimethoxyphenyllithium in 78% yield; mp 87–88°C; IR
3580 cm−1 (OH); MS m/z (%) 214 (100, [C5H4=
CHFc]+). Calc. for C19H20FeO3: C, 64.79; H, 5.72
Found: C, 65.02; H, 5.81%.
4ae: by a reaction of 1e with 1,3,5-trimethoxybenzene
in the presence of an acid in 86% yield; m.p. 157–
158°C, MS m/z (relative intensity) 472 [100,M+].
4de: by a reaction of 1e with 1,3-dimethoxybenzene
in the presence of an acid in 42% yield; m.p. 141–
142°C, MS m/z (relative intensity) 442 (100, M+). Calc.
for C26H26FeO3: C, 70.60; H, 5.92. Found: C, 70.88; H,
6.12%.
1d: by
a
reaction of FcꢀCHO with 2,4-
dimethoxyphenyllithium in 32% yield; m.p. 76–78°C;IR
3466 cm−1 (OH); MS m/z (%) 214 (100, [C5H4=
CHFc]+).
1e: by a reaction of FcꢀCHO with 2-methoxyphenyl-
lithium in 78% yield; m.p. 100–101°C; IR 3520 cm−1
(OH); MS (toluene) m/z (%) (1) 306 (100, 3e+),(2) 320
(100, [FcꢀCFeO]+).
1f: by a reaction of FcꢀCHO with 4-methoxyphenyl-
lithium in 45% yield; m.p. 73–74°C (reported 72–74°C
[19]); IR 3580 cm−1 (OH).
2.3. Cyclic 6oltammetric measurements
2bb: (1) by a reaction of FcꢀCOOEt with 2,6-
dimethoxphenyllithium in 70 (crude) ꢀ22% yield or (2)
by a reaction of Fb2CO [20] with ferrocenyllithium in 43
(crude) ꢀ15% yield; m.p. 178–180°C; IR 3550 cm−1
(OH); MS m/z (%) 350 (100, [C5H4=CFb2]+).
Ferrocenyl derivatives were dissolved in acetonitrile
containing 0.1 M (M=mol/dm3) tetra-n-butylammo-
nium tetrafluoroborate as a supporting electrolyte.
Cyclic voltammetric measurements were carried out at
25°C in a conventional three-electrode cell with plat-
inum wire working electrode, platinum plate counter
electrode and aqueous Ag/AgCl reference electrode.
Voltammograms were recorded with a Hokuto Denko
model HA-501 potentiostat and a Function Generator
HB-104 in connection with a Rika Denki RW-21TX-T
recorder. The formal redox potentials of these com-
pounds were estimated as the midpoint between the
anodic and cathodic peak potentials, and were referred
against the ferrocene–ferrocenium redox couple (+460
mV vs. aq. Ag/AgCl). All the relative current intensities
(iox/ired) were in the region of 1.090.15. The redox
2bg: by
a
reaction of FcꢀCPhO with 2,6-
dimethoxyphenyllithium in 78% yield; m.p. 144–145°C;
IR 3500 cm−1 (OH); MS m/z (%) (82, [C5H4=
CPhFb]+).
2gg: by a reaction of FcꢀCPhO with phenyllithium in
77% yield; m.p. 132–133°C (reported 130–131°C [21]);
IR 3540 cm−1 (OH).
3c: by reduction of 1c in 2-propanol in the presence
of an acid in 27% yield; m.p. 64–65°C, MS m/z (rela-
tive intensity) 336 [100, M+].
3e: by reduction of 1e in 2-propanol in the presence

M. Asahara et al. / Journal of Organometallic Chemistry 601 (2000) 246–252
249
potentials are summarized in Table 3, together with the
electronic spectral data.
(coefficient=3.55176e−06). The structure was solved
by direct methods (SIR92) [22] and expanded using
Fourier techniques (DIRDIF94) [23]. Hydrogen atoms
were included at calculated positions but not refined.
2.4. X-ray crystallography
All calculations were performed using the TEXSAN crys-
Single crystals of 4aa suitable for X-ray crystal struc-
ture analysis were obtained by recrystallization from
hexane. The intensity data were collected at 173 K on a
Rigaku RAXIS-IV imaging plate area detector with
tallographic software package of Molecular Structure
Corporation. Crystal structure data for C29H32O6Fe:
approximate crystal dimensions=0.8×0.3×0.1 mm3,
,
triclinic, a=11.4013(9), b=13.405(1), c=8.3098(4) A,
,
graphite-monochromated Mo–Ka (u=0.71070 A) radi-
h=97.363(8), i=102.240(5), k=81.491(4)°, V=
3
,
(
ation from a rotating-anode generator operating at 50
kV and 100 mA to a maximum 2q value of 55.2°. A
total of 17 oscillation images, each being oscillated 6°
and exposed for 40 min, were collected for 4aa. The
data were corrected for Lorentz and polarization ef-
fects. A correction for secondary extinction was applied
1221.2800 A , space group P1 (No. 2), Z=2, Dcalc
=
1.45 g cm−3, Mr=532.42, v(Mo Ka)=6.6 cm−1, no.
of observations (I=3.00|(I))=3116, no. variables=
326, residuals: R; Rw=0.059; 0.087, goodness-of-fit=
1.22, max/min peak in final difference map=
−3
,
0.77/−0.94 e A
.
Table 3
The redox potentials and visible spectral data of ferrocene derivatives
Compounds
F
F%
Redox potentials a
Visible spectra b
E0 (mV)
umax (nm)
log m
Fc-CHFOH (1)
1a
1b
1c
1d
1e
1f
Fa
Fb
Fc
Fd
Fe
Ff
−43
−30
−15
−16
−2.5
−2
440
445
445
429
443
443
444
2.30
2.15
2.15
2.23
2.11
2.08
2.18
1g
Ph
+4
Fc-CFF%OH (2)
2bb
2bg
2gg
Fb
Fb
Ph
Fb
Ph
Ph
−184
−98
+24
443
450
445
2.18
2.30
2.18
Fc-CH2F (3)
3a
3b
3c
3e
3f
Fa
Fb
Fc
Fe
Ff
−80
−70
−43
−49
−28
−20
444
450
447
440
440
432 c
2.04
2.04
2.00
2.11
2.15
2.08
3g
Ph
Fc-CHFF% (4)
4aa
Fa
Fa
Fa
Fa
Fb
Fd
Fd
Ph
Fa
Fb
Fe
Ph
Fd
Fe
Ph
Ph
−183
−179
−87
−62
−93
−43
−18
+10
0
427
445
443
445
440
427
442
2.15
2.23
2.00
2.11
2.11
2.10
2.08
4ab
4ae
4ag
4bd
4de
4dg
4gg d
FcꢀH
440
1.97
a Half-wave potentials versus ferrocene–ferrocenium measured in 1 mM acetonitrile solutions; potential sweep rate was 5 mV s−1
b Measured in 1,2-dichloroethane; d–d band.
.
c 22 400 cm−1 (446 nm) have been reported in Ref. [27].
d Data from Ref. [5].

250
M. Asahara et al. / Journal of Organometallic Chemistry 601 (2000) 246–252
protons. The resonances sharpened at +50°C with
little chemical shift change (90.01 ppm), as shown in
Fig. 1. The C5H5 proton resonance was observed as a
very sharp singlet at l 4.03, and the C5H4 proton
resonances were observed at l 4.24 as a broad peak
with 2H intensity and at l 4.08 (JH=2 Hz) as a triplet
with 2H intensity. The spectrum was also measured at
−60°C, and it showed the presence of two kinds of Fb
groups with all the four methoxy groups, all the four
3%,5%–protons, and both the two 4%-protons non-equiva-
lent, as shown in Fig. 1. The C5H5 proton resonance
was also observed as a singlet at l 4.08 with stronger
intensity due to overlapping with one of the methoxy
proton resonances. The C5H4 proton resonances were
observed at l 4.52 (1H), 4.17 (2H) and 3.92 (1H)
without magnetic coupling with each other. These re-
sults indicate that the free rotation around the C–Fb
bonds is frozen in the compound at −60°C, and the
compound takes an asymmetric structure. 1-Ferroceny-
lalcohols often aggregate by OꢀH···O hydrogen bond-
ing, and they often involve intramolecular
OꢀH···p(C5H5) interaction [26,27]. The OH proton res-
onance of 2bb was observed at l 6.27 (room tempera-
ture) or 6.00 (−60°C) as a sharp single peak,
suggesting the absence of such hydrogen bondings. The
13C-NMR spectrum measured at ambient temperature
showed only one methoxy carbon resonance and is
consistent with the compound having free rotation
around the CꢀFb bond.
Fig. 1. Temperature-dependent 1H-NMR spectra of 2bb in CDCl3; (a)
at +50°C, (b) at −60°C.
3. Results and discussion
3.1. Preparation and general properties of ferrocenyl
deri6ati6es
Compounds of types 1–4 in Scheme 1 were prepared
as described elsewhere [1] or were prepared essentially
in an analogous manner. The new compounds were
1
characterized by H- and 13C-NMR (Tables 1 and 2),
IR, and GC mass spectra. The 1H- and 13C-NMR
spectra are as expected for each compound except for
2bb (see below). With a hope of isolating the carbenium
salt, 2bb was treated with acids under a variety of
conditions, but it easily decomposed to give 6-bis(2,6-
dimethoxyphenyl)fulvene, C5H4=CFb2. We have also
failed in the reductions of 1d, 2bb, 2bg to 3 or 4 by the
method reported previously [1], possibly because these
compounds also decomposed to the fulvene in the
presence of an acid.
The rotation barrier (DG‡) around the central carbon
atom in 2bb was estimated by the temperature-depen-
1
dent H-NMR measurement in CDCl3 to give 53.2 kJ
mol−1 from the 2,5-proton resonances on the C5H4, or
52.8 or 54.9 kJ mol−1 from the methoxy protons
resonances (Tc=0°C).
The GC mass spectra of 1 showed thermal decompo-
sitions during the GC measurement to give the fulvene,
C5H4ꢁCHF, or the disproportionation products, 3 and
FcꢀCFO. The spectra of 2 also showed thermal decom-
position to give C5H4ꢁCFF%. Usage of acetone as the
solvent for GC–MS measurement should be avoided,
since it often reacted with 1 to give the adduct, Fc–
CHFCH2CMeO. Related reactions with acetone have
been reported [24,25]. Alcohols can also react with 1
and 2 to give the reduction product 3 or 4.
3.3. Oxidation potentials
All of the substituted ferrocenes studied here exhib-
ited a reversible one-electron diffusion-controlled oxi-
dation–reduction wave (E0, Table 3) based on the
ferrocene–ferricenium redox couple (iox/ired= ꢀ1.0). It
has been known that such an oxidation–reduction po-
tential of substituted ferrocene depends on the elec-
tronic effects of the substituent group [3–5]. Little et al.
have reported that successive replacement of the hydro-
gen of the methyl group in methylferrocene, FcꢀCH3,
by phenyl groups changes the potential in the order
FcꢀCH3B3gBFcꢀHB4ggBFꢀCPh3 [5]. We also ob-
served essentially the same order as seen in Table 1;
3gBFcꢀHꢀ1gB4ggB2gg. In contrast, we observed
a very different order for 2,4,6-trimethoxypheny substi-
tution: 4aaB3aB1aBFcꢀH. These results may be
because the phenyl group is electron withdrawing to the
a-carbon, while 2,4,6-trimethoxyphenyl group is elec-
tron donating.
All the compounds 3 and 4 studied here were ther-
mally stable, showing only one GC peak, and the mass
spectrum showed
respectively.
a
very strong parent peak,
3.2. NMR spectra
1
The H-NMR spectrum of 2bb measured at ambient
temperature showed the resonance due to only one kind
of Fb group with a broad peak for the 2%,6%–dimethoxy

M. Asahara et al. / Journal of Organometallic Chemistry 601 (2000) 246–252
251
Table 4
We were very interested in the difference of effects
,
Selected interatomic distances (A) for 4aa
the between 2-methoxyphenyl group and 4-
methoxyphenyl group on the oxidation potential: 1eB
1fB1g. An analogous difference was observed be-
tween the 2,6-dimethoxyphenyl group and 2,4-
O(1)···C(1)
O(3)···C(1)
Fe(1)···O(1)
2.749(4)
2.831(5)
3.934(3)
O(4)···C(1)
O(6)···C(1)
2.679(5)
2.981(4)
dimethoxyphenyl group: 1bB1d. These results indicate
that the ortho-methoxy group is more electron donating
than the para-methoxy group. A series of 3a-g, 3a and
3b bearing two ortho-methoxy substituents showed
lower redox potentials (−80 and −70 mV) than 3c
and 3e bearing only one ortho-methoxy substituent,
and the latter showed much lower redox potentials than
3f and 3g (−28 and −20 mV, respectively). Fig. 2
visualizes the effect of the number of ortho-methoxy
substituents on the redox potential of ferrocene deriva-
tives.
In contrast to the wide range of shift in the redox
potentials, the visible spectra of the derivatives 1–4
(Table 3) showed a band with umax at the analogous
region (427–450 nm) to that of ferrocene and 3g. This
may be because the presence of ortho-methoxy sub-
stituent leads to destabilization of both HOMO (a1g)
and LUMO (e1g) levels [28] of 1–4 by approximately
equal amount.
In the hope of obtaining further information con-
cerning the effect of the ortho-methoxy substituent, the
X-ray crystallographic analysis of 4aa was performed.
Fig. 2. Effect of the number and position of methoxy substituents on
oxidation potentials of benzylferrocenes and the related compounds.
3.4. Crystal structure of
bis-(2,4,6-trimethoxyphenyl)methylferrocene
The molecular structure derived from an X-ray struc-
ture analysis of 4aa is shown in Fig. 3 and the selected
interatomic distances are summarized in Table 4. The
two Fa groups were nonequivalent in the crystal, and
the fact that ortho-methoxy oxygens situated very close
to the central carbon (C1) is characteristic; 2.749 (O1),
,
2.831(O3), 2.679 (O4), 2.981 (O6) A, respectively. These
,
distances are much shorter than the sum [3.22 A] of the
van der Waals radii of these two atoms, and there is a
possibility, for example, that one of the oxygen (O4)
lone pairs interacts with the antibonding orbital of
C(1)ꢀFc s-bond. In solution the molecule should be
mobile, and the other methoxy oxygens may also take
part in such an interaction. Such an interaction can
increase the energy levels of ferrocenyl group to de-
crease the oxidation potential, as observed.
One of the methoxy oxygens (O6) of the four
methoxy oxygens is also situated very close above the
,
C5H4 ring with the interatomic distances of 2.768 A
,
(O(6)···C(20)) and 2.912 A (O(6)···C(21)). Thus, it may
also be possible to assume that there is an interaction
between the oxygen lone pairs and the p-electron or-
bitals of the cyclopentadienyl group.
Fig. 3. X-ray crystal structure of Fc-CHFa2[Fc=Fe(h5-C5H5)(h5-
C5H4); Fa=2,4,6-(MeO)3C6H2] drawn at 50% probability level. All
hydrogen atoms are omitted for clarity except for H1.

252
M. Asahara et al. / Journal of Organometallic Chemistry 601 (2000) 246–252
Zanello, J. Organomet. Chem. 506 (1996) 129.
It may also be expected that ortho-methoxy groups
can coordinate to the ion atom. However, the shortest
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4. Supplementary material
Crystallographic data for the structural analysis have
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Data Centre, CCDC no. 132015 for 4aa. Copies of this
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Director CCDC, 12 Union Road Cambridge, CB2 1EZ,
UK. Fax: +44-1223-336033 or e-mail:deposit
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This work was supported by a Grant-in-Aid for
Scientific Research (no. 08455492) from the Ministry of
Education, Science and Culture. We thank Professor K.
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ments at the Institute for Chemical Research, Kyoto
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