Covalently linked ferrocenyl quinones: Proton-dependent redox behavior and charge redistribution

Article abstract of DOI:10.1021/om060040x

The proton-dependent redox chemistry of dyads comprised of a ferrocenyl electron donor directly linked to a hydroquinonyl electron donor or to a quinone electron acceptor by a single covalent bond has been characterized. Ferrocenyl-1,4-hydroquinone (2), ferrocenyl-1,4-benzoquinone (3), 3-ferrocenyl-l,2catechol (5), and the precursors ferrocenyl-1,4-dimethoxybenzene (1) and 3-ferrocenyl-l,2-dimethoxybenzene (4) were studied; also the unstable compound 3-ferrocenyl-l,2-benzoquinone (6) was observed in the spectroelectrochemistry of 5. Detailed cyclic voltammetry, coulommetry, and UV-vis-NIR spectroelectrochemistry experiments allied with EPR, NMR, and Moessbauer spectroscopy were used to probe the pH-dependent redox chemistry and electron distribution within the compounds.

Full text of DOI:10.1021/om060040x

2216
Organometallics 2006, 25, 2216-2224
Covalently Linked Ferrocenyl Quinones: Proton-Dependent Redox
Behavior and Charge Redistribution
Stephen B. Colbran,* Sang Tae Lee, David G. Lonnon, Felicia J. D. Maharaj,
Andrew M. McDonagh,† Katherine A. Walker, and Rowan D. Young
School of Chemistry, UniVersity of New South Wales, Sydney, NSW 2052, Australia
ReceiVed January 12, 2006
The proton-dependent redox chemistry of dyads comprised of a ferrocenyl electron donor directly
linked to a hydroquinonyl electron donor or to a quinone electron acceptor by a single covalent bond has
been characterized. Ferrocenyl-1,4-hydroquinone (2), ferrocenyl-1,4-benzoquinone (3), 3-ferrocenyl-1,2-
catechol (5), and the precursors ferrocenyl-1,4-dimethoxybenzene (1) and 3-ferrocenyl-1,2-dimethoxy-
benzene (4) were studied; also the unstable compound 3-ferrocenyl-1,2-benzoquinone (6) was observed
in the spectroelectrochemistry of 5. Detailed cyclic voltammetry, coulommetry, and UV-vis-NIR
spectroelectrochemistry experiments allied with EPR, NMR, and Mo¨ssbauer spectroscopy were used to
probe the pH-dependent redox chemistry and electron distribution within the compounds.
insights into unusual structural and spin separation processes
driven by proton-coupled intramolecular electron transfer8-10
and has highlighted the role of hydrogen bonding and ion pairing
on the stabilities and lifetimes of charge-separated states formed
by thermal or photoinduced electron transfer within such dyads.5
Species with proton-switchable fluorescence or nonlinear optical
properties have been reported,6,7 with other examples proposed
as pH/metal ion-dependent photochromic switches for single-
molecule electronic or sensor applications.5,11 Ferrocenyl-
quinones with a direct covalent bond between the ferrocene
donor and (hydro)quinone (donor/)acceptors also have potential
applications in homogeneous catalysis, antitumor therapeutics,
and molecular electronic technologies.9,12,13
Despite all of the interest in ferrocenyl-(hydro)quinones, the
redox chemistry of the directly linked systems has not been
studied. In response, we have undertaken a detailed electro-
chemical and spectroelectrochemical investigation of the proton-
dependent redox chemistry of the prototypal ferrocenyl-(hydro)-
quinones, 1-6 (Chart 1).
Introduction
The archetypal reversible organometallic redox couple is
perhaps the FeIII-FeII couple of ferrocene,1,2 and the quinone-
semiquinone-hydroquinone redox system3,4 provides an ex-
emplar for proton-dependent redox chemistry. It is little surprise,
therefore, that dyads comprised of ferrocene electron donors
indirectly tethered by amide,5 ethynylene,6-9 or vinylene7,9,10
linkages to quinone acceptors have recently been targeted for
investigation. Research involving these dyads has provided new
* Corresponding author. E-mail: s.colbran@unsw.edu.au.
† Current address: School of Chemistry, Materials and Forensics,
University of Technology Sydney, Sydney, NSW 2007, Australia.
(1) (a) Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980,
19, 2854-2855. (b) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96,
877-910.
(2) Togni, A. Ferrocenes; VCH: New York, 1995.
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(4) (a) Bauscher, M.; Mantele, W. J. Phys. Chem. 1992, 96, 11101-
11108. (b) Chambers, J. Q. Electrochemistry of Quinones. In The Chemistry
of the Quinonoid Compounds; Rappoport, Z., Patai, Z., Eds.; J. Wiley &
Sons: New York, 1988; Vol. II, Chapter 12, pp 719-757. (c) Laviron, E.
J. Electroanal. Chem. 1986, 208, 357-372. (d) Eggins, B. R.; Chambers,
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Results and Discussion
(5) (a) Okamoto, K.; Fukuzumi, S. J. Am. Chem. Soc. 2004, 126, 13922-
13923. (b) Fukuzumi, S.; Okamoto, K.; Yoshida, Y.; Imahori, H.; Araki,
Y.; Ito, O. J. Am. Chem. Soc. 2003, 125, 1007-1013. (c) Moriuchi, T.;
Shen, X. L.; Saito, K.; Bandoh, S.; Hirao, T. Bull. Chem. Soc. Jpn. 2003,
76, 595-599. (d) Fukuzumi, S.; Yoshida, Y.; Okamoto, K.; Imahori, H.;
Araki, Y.; Ito, O. J. Am. Chem. Soc. 2002, 124, 6794-6795. (e) Fukuzumi,
S.; Okamoto, K.; Imahori, H. Angew. Chem., Int. Ed. 2002, 41, 620-622.
(6) Butler, I. R.; Callabero, A. G.; Kelly, G. A.; Amey, J. R.; Kraemer,
T.; Thomas, D. A.; Light, M. E.; Gelbrich, T.; Coles, S. J. Tetrahedron
Lett. 2004, 45, 467-472. Rangel-Rojo, R.; Kimura, K.; Matsuda, H.;
Mendez-Rojas, M. A.; Watson, W. H. Opt. Commun. 2003, 228, 181-
186. McGale, E. M.; Murray, R. E.; McAdam, C. J.; Morgan, J. L.;
Robinson, B. H.; Simpson, J. Inorg. Chim. Acta 2003, 352, 129-135.
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22, 931-939.
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Kubo, K.; Kobayashi, Y.; Nishihara, H. J. Am. Chem. Soc. 2001, 123,
12903-12904. (b) Murata, M.; Fujita, T.; Yamada, M.; Kurihara, M.;
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2001, 40, 4-5.
Synthesis. The syntheses of 1-3 have been previously
published by Kasahara et al., although this report is not widely
available.13 The synthesis of 1 involves coupling of the
diazonium salt of 2,5-dimethoxyaniline to ferrocene under acidic
conditions. After chromatographic purification, 1 was obtained
in 25% yield. Cleavage of the methoxy substituents using boron
tribromide yielded crude 2 as an oily yellow solid. Purification
of 2 was achieved by first oxidizing with 2,3-dichloro-5,6-
(11) (a) Beer, P. D.; Crowe, D. B.; Ogden, M. I.; Drew, M. G. B.; Main,
B. J. Chem. Soc., Dalton Trans. 1993, 2107-2116. (b) Wagner, R. W.;
Brown, P. A.; Johnson, T. E.; Lindsey, J. S. J. Chem. Soc., Chem. Commun.
1991, 1463-1466. (c) Beer, P. D.; Kurek, S. S. J Organomet. Chem. 1989,
366, C6-C8.
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C. Organometallics 2004, 23, 3694-3700. (b) Cotton, H. K.; Huerta, F.
F.; Ba¨ckvall, J.-E. Eur. J. Org. Chem. 2003, 2756-2763. (c) Zora, M.;
Yucel, B.; Acikalin, S. Tetrahedron Lett. 2003, 44, 2237-2241. Chan, K.
S.; Zhang, H. C. Synth. Commun. 1995, 25, 635-639. (d) Kasahara, A.;
Shimizu, I.; Ookura, K. Chem. Ind. (London) 1986, 8, 285.
(13) Kasahara, A.; Izumi, T.; Ogata, H.; Tanno, S.; Fujisawa, T.; Ogata,
T. Bull. Yamagata UniV. 1985, 18, 125-141.
10.1021/om060040x CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/25/2006

CoValently Linked Ferrocenyl Quinones
Organometallics, Vol. 25, No. 9, 2006 2217
Chart 1. Ferrocenyl-(hydro)quinones Investigated in This
Work
Figure 1. UV-vis-NIR spectra of 2 (solid trace) and 3 (dashed
trace) in acetonitrile at 298 K.
Table 1. UV-Vis Data for 1-5 in Acetonitrile
λ
max, nm (ꢀmax, cm-1 M-1
)
1
2
3
4
5
276 (6870),
270 (6170),
268 (6250),
242 (4190),
241 (4380),
314 (6450),
309 (6150),
393 (3040),
284 (3490),
280 (3680),
446 (325)
444 (280)
713 (1640)
445 (120)
452 (190)
dicyano-1,4-benzoquinone (DDQ) to give 3 as dark green
microcrystals in 57% yield after recrystallization from hexanes.
Reduction of 3 in dichloromethane with aqueous sodium
dithionite afforded a clean sample of 2.
Cleavage of the methyl substituents from previously re-
ported14 4 using boron tribromide afforded the ferrocenylcat-
echol 5. This new compound was difficult to isolate, as it
decomposed rapidly in air, and more slowly on storage under
an inert atmosphere at -15 °C, to intractable material that
showed no evidence for a ferrocene moiety. Attempts to
synthesize ortho-quinonylferrocene (6) by oxidation of 5 with
oxidants such as silver oxide, DDQ, and phenyliodoso(diacetate)
resulted only in decomposition. The instability of 5 (and 6) may
relate to the ability of the catecholate chelate ligand to strip
iron from ferrocenes.15
Electronic, Mo1ssbauer, and NMR Spectroscopy. Repre-
sentative UV-vis-NIR spectra, those of 2 and 3, are shown
in Figure 1. The spectra of 1, 2, 4, and 5 are all very similar,
while the spectrum of 3 is significantly different (see Table 1
and Figure 1). The spectra of 1, 2, 4, and 5 are typical of those
for aryl-ferrocene derivatives; the bands at ca. 240-320 and at
445 nm are attributed to Fe (e2g) f Cp (e1g) charge-transfer
and symmetry-forbidden Fe (a1g) f Fe (e1g) transitions,
respectively.16-18 These bands are more intense in ferrocenes
with electron-withdrawing (e.g., aryl) substituents. In contrast,
the spectrum of 3 is dominated by an intense band at 260 nm,
which is typical of quinone (n, π f π*) transitions,19 and a
Figure 2. Zero-field 57Fe Mo¨ssbauer spectrum of Fc-q at 298 K.
very strong band at 395 nm and a strong, broad band at 713
nm, neither of which are observed in the spectra of the other
ferrocenes nor in the spectra of benzoquinones. The lower
energy band at ∼713 nm is solvatochromic, whereas the 395
nm band shows little change in energy with solvent. The lower
energy band distinctly trends to higher energy in more highly
solvating solvents (a plot of band wavelength against Reichardt’s
“overall solvation scale” parameter, ET(30),20 is given as
Supporting Information). Accordingly, this band is attributed
to an intramolecular charge-transfer transition between the
ferrocenyl donor and quinonyl acceptor centers [eπ(HOMO-Fc)
f eπ*(LUMO-q)]. The assignment is analogous to that of the
well-characterized ferrocene-acceptor charge-transfer bands16,21,22
such as the ferrocene eπ(HOMO) f DDQ eπ*(LUMO) charge-
transfer band22 and concurs with the disappearance of the band
upon one-electron oxidation of the ferrocene center or one-
electron reduction of the quinone center (discussed below).
The zero-field 57Fe Mo¨ssbauer spectrum of 3, Figure 2,
reveals a typical ferrocene doublet; the isomer shift is 0.43 (
0.01 mm s-1 and the quadrupole splitting is 2.20 ( 0.01 mm
s-1. Under identical conditions, the measured values for
ferrocene were 0.44 and 2.38 mm s-1, respectively. The
pronounced reduction in the quadrupole splitting for 3 compared
to ferrocene16,23 indicates net ground-state charge transfer from
(14) Beer, P. D.; Sikanyika, H.; Blackburn, C.; McAleer, J. F.; Drew,
M. G. B. J. Chem. Soc., Dalton Trans. 1990, 3295-3300.
(15) (a) Bashkin, J. K.; Kinlen, P. J. Inorg. Chem. 1990, 29, 4507-
4509. (b) Prins, R.; Korswage, A. R.; Kortbeek, A. G. J. Organomet. Chem.
1972, 39, 335-344.
(16) (a) Barlow, S.; Bunting, H. E.; Ringham, C.; Green, J. C.; Bublitz,
G. U.; Boxer, S. G.; Perry, J. W.; Marder, S. R. J. Am. Chem. Soc. 1999,
121, 3715-3723. (b) Barlow, S.; Ohare, D. Chem. ReV. 1997, 97, 637-
669.
(17) (a) Gordon, K. R.; Warren, K. D. Inorg. Chem. 1997, 17, 9987-
9995. (b) Dowben, P. A.; Driscoll, D. C.; Tate, R. S.; Boag, N. M.
Organometallics 1988, 7, 305-308.
(18) Rosenblum, M.; Santer, J. O.; Howells, W. G. J. Am. Chem. Soc.
1962, 85, 1450-1456.
(19) Itoh, T. Chem. ReV. 1995, 95, 2351-2368.
(20) Reichardt, C. Chem. ReV. 1994, 94, 2319-2358.
(21) Miller, J. S.; Epstein, A. J.; Reiff, W. M. Chem. ReV. 1988, 88,
201-220.
(22) Gebert, E.; Reis, A. H., Jr.; Miller, J. S.; Rommelmann, H.; Epstein,
A. J. J. Am. Chem. Soc. 1982, 104, 4403-4410.

2218 Organometallics, Vol. 25, No. 9, 2006
Colbran et al.
Table 2. Potential Data for Redox Processes from Cyclic
Voltammetrya
compound
E
1/2(FeIII-FeII)/V
Ep(aryl or quinone centered)/V
+0.95d
1
2
3
4
5
-0.04b
-0.07b
+0.17b
-0.06b
-0.03b
+0.54d (+0.09d,e
-0.92,b -1.49c
+0.91d
)
+1.11d
a In acetonitrile-0.1 M [Bun N][PF6] at 298 K; scan rate 100 mV s-1
.
)
4
b Reversible process (∆Ep ) ∆Ep(Fc+-Fc) ) 65 mV; ipreverse/ip
forward
1.0). c Quasi-reversible process (∆Ep ) 90 mV at ν ) 100 mV s-1 and
a
c
increased with ν; ip /ip ≈ 1.0). d Irreversible process (no peak for the couple
in the return sweep). e Cathodic peak in the reverse sweep that arises from
the irreversible oxidation process.
the ferrocenyl group to the quinone center, consistent with the
above analysis of the UV-vis data.
The NMR data for 1-5 (see the Experimental Section) also
concur with these conclusions. The 1H NMR spectra of 1, 2, 4,
and 5 show pairs of triplets for Hâ and Hγ (see Chart 1) at δ
4.3-4.4 and 4.5-4.8. These are downfield of the singlets
assigned to the unsubstituted cyclopentadienyl ring at ca. δ 4.1,
which is characteristic for ferrocenes with electron-withdrawing
substituents.2,18 In the spectrum of 3, the cyclopentadienyl singlet
again appears at δ 4.10, whereas the cyclopentadienyl triplets
are significantly further downfield at δ 4.58 and 4.91, due to
additional deshielding with the increased delocalization of
electron density toward the quinone substituent. In the 13C NMR
spectrum of 3 the carbonyl carbon signals appear at δ 186.4
and 185.9 (cf. those for 1,4-benzoquinone at δ 187.06).
Electrochemistry and Spectroelectrochemistry. The elec-
trochemical behavior of 1-5 was surveyed by cyclic voltam-
metry with that of 2-5 studied in more detail by UV-vis-
NIR spectroelectrochemistry. Table 2 presents a summary of
the potential data for the observed redox couples (relative to
the Fc+-Fc couple with E1/2(Fc+-Fc) ) 0 V), and typical cyclic
voltammograms, those for 2 and 3, are shown in Figure 3. Cyclic
voltammograms of 1, 2, 4, and 5 each exhibit a fully reversible
one-electron process at ca. -0.05 V in acetonitrile corresponding
to the FeIII-FeII couple. The 240 mV shift to higher potential
in the FeIII-FeII couple for 3 compared to that of 2 is indicative
of electronic delocalization from the ferrocene to the quinone
in accord with the aforementioned spectroscopic properties.
Redox processes centered on the aryl groups are observed
for 1, 2, 3, and 4. Both 1 and 4 show an irreversible anodic
process at ∼ +1 V, which are assigned to oxidation of the
dimethoxyphenyl groups to give the corresponding cation
radicals, eq 1, a process that for simple poly(alkoxy)arenes is
electrochemically and chemically reversible and occurs at ∼ +1
V.24 That the second oxidations for 1 and 4 are irreversible
indicates that the open-shell di(radical) dication products, Fc+-
Figure 3. Cyclic voltammograms of (a) 2 (1 mM) before (solid
trace) and after (dashed trace) addition of 3.0 equiv of KOBut and
(b) 3 (1 mM). Conditions: acetonitrile-0.1 M [Bun N][PF6] at 298
4
K; 0.5 mm glassy carbon minidisk working electrode; scan rate
100 mV s-1
.
Table 3. Species Characterized by UV-Vis-NIR
Spectroelectrochemistrya
parent
complex
species
λ )
max, nm (ꢀmax, cm-1 M-1
Fc+-q
3
3
3
259 (22 700), 340 (sh, 5150), 644 (480)
268 (13 200), 393 (3185), 710 (1680)
277 (11 460), 323 (10 500), 427 (5080),
448 (5140), 535 (br sh)
271 (9960), 317 (8110), 450 (br sh, 1020)
273 (9460), 314 (7525), 444 (580)
253 (17 800), 284 (16 000), 365
(4240), 431 (2140), 598 (490), 858 (620)
242 (4190), 284 (3490), 445 (120)
250 (4670), 272 (sh, 4090), 364 (1040),
442 (810), 585 (75), 920 (220).
Fc-q (3)
Fc-sq-
Fc-hqH-
3
3
2
Fc-hqH2 (2)
Fc+-hqH2
Fc-catMe2 (4)
Fc+-catMe2
4
4
Fc-catH2 (5)
Fc+-catH2
5
5
242 (4600), 280 (3680), 450 (190)
245 (4570), 278 (3835), 362 (880), 442
(620), 605 (80), 730 (170)
Fc-oq (6)
5
250 (vs), 432 (s), 690 (w)
hqMe2 and Fc+-catMe2+, are unstable species that rapidly
+
a In acetonitrile-0.2 M [Bun N][PF6]; the listed extinction coefficients
decompose.
4
are estimated from the absorbance of the listed band relative to those in
the spectrum of the corresponding parent compound.
Fc+-ArMe2 a Fc+-ArMe2+ + e-
(1)
arises from the proton dependence of the chemical steps.3,4 The
small cathodic peak at +0.1 V in the reverse sweep is more
pronounced when the solution is buffered with acid (see below)
and is attributed to reduction of the quinone (reverse eq 2).
Notably the quinone-centered reduction occurs prior to reduction
of the ferrocenium center.
In the cyclic voltammograms of 2 and 5, the FeIII-FeII couple
is followed by an anodic peak at ∼ +0.6 V for 2 and ∼ +0.9
V for 5. This peak arises from two-electron oxidation of the
hydroquinone or catechol group to the corresponding quinone,
eq 2. The irreversibility of the 2e-, 2H+ oxidation in acetonitrile
Fc+-ArH2 f Fc+-(o)q + 2H+ + 2e-
(2)
(23) Bagus, P. S.; Walgren, U. I.; Almlof, J. J. Chem. Phys. 1976, 64,
2324-2334.
(24) (a) Zweig, A.; Hodgson, W. G.; Jura, W. H. J. Am. Chem. Soc.
1964, 86, 4124-4129. (b) Le Berre, V.; Angely, L.; Simonet, G.; Mousset,
G.; Bellec, M. J. Electroanal. Chem. 1987, 218, 273-279.
Complex 3 also exhibits successive one-electron reductions
of the quinone substituent, first to the semiquinone radical anion

CoValently Linked Ferrocenyl Quinones
Organometallics, Vol. 25, No. 9, 2006 2219
Figure 4. UV-vis spectroelectrochemistry for controlled-potential
oxidation of 4 in acetonitrile-0.2 M [n-Bu4N][PF6] at 298 K at
+0.5 V (vs Fc+-Fc). Arrows mark the direction of absorbance
change.
Figure 6. UV-vis-NIR spectroelectrochemistry for controlled-
potential oxidation of 2+ at +800 mV (vs Fc+-Fc) in acetonitrile-
0.2 M [Bun N][PF6] at 298 K.
4
oxidation of the ferrocene center proceeds prior to aryl-centered
oxidation as discussed above and affords the ferrocenium cation
4+. The oxidation progressed with distinct isosbestic points at
243, 287, 302, and 650 nm until complete conversion of 4 into
the oxidation product. Coulommetry revealed that the electroly-
sis consumed 0.97 F mol-1. The oxidation was reversed by
application of an electrolysis potential of -0.3 V. The electronic
spectra of 2+, 4+, and 5+ are all similar but unusual, as the
2
2
distinctive lowest energy e2g (Fe) f e1u (C5R5) band for a
ferrocenium ion at ∼600 nm16-18 does not dominate the visible
region. Rather, the electronic spectra are characterized by strong
bands at ca. 370 and 450 nm, a very weak ferrocenium band at
ca. 600 nm, and a broad band that extends into the NIR at ca.
920 nm for 4+, at 840 nm for 2+, and at 730 nm for 5+ that are
atypical of ferrocenes, ferrocenium cations, and (hydro)quinones.
These spectra are very similar to those very recently reported
by Robinson and co-workers for ethynyl-linked ferrocenium-
polyaromatic dyads:25 they attributed the lowest energy band
to the characteristic16,25 intervalent charge transfer from the
electron-rich aryl group to the oxidizing ferrocenium centers,
which in the present case would be Fc+-ArR2 f {Fc-ArR2+}*
transitions, and used resonance Raman spectroscopy to reveal
the bands at ∼370 and ∼440 nm to also have considerable
intercenter charge-transfer character.
In unbuffered acetonitrile, the oxidations of 2 and 5 were
not completely reversible; reduction of the cations 2+ and 5+
at -400 mV afforded the starting ferrocenyl-hydroquinone (2
or 5) contaminated by up to 20% of the corresponding
ferrocenyl-quinone (3 or 6), e.g., Figure 5. The spectrum of 6
may be obtained by subtraction of that of 5 from that of the
product mixture from reduction of 5+. It has intense bands at
255 and 432 nm and a broad band in the visible region at 690
nm and is very similar to the spectrum of 3.
Figure 5. UV-vis-NIR spectroelectrochemistry for controlled-
potential reduction 5+ at -480 mV (vs Fc+-Fc) showing the
development of the strong charge-transfer band attributed to 6 at
690 nm (inset). The solvent is acetonitrile-0.2 M [Bun N][PF6] at
4
298 K.
(at -0.92 V), eq 3, and then to the hydroquinone dianion, eq 4
(a quasi-reversible process at E1/2 ) -1.49 V: ∆Ep ≈ 100 mV
at scan rate 100 mV s-1, cf. ∆Ep(Fc+-Fc) ) 65 mV). In
comparison, under identical conditions 1,4-benzoquinone is also
reduced in two steps,3,4 first forming the semiquinone anion (at
E1/2 ) -1.09 V) and then the hydroquinonyl dianion (at E1/2
-1.77 V).
)
Fc-q + e- a Fc-sq-
(3)
(4)
Fc-sq- + e- a Fc-hq2-
The UV-vis-NIR spectroelectrochemical behavior during
the second oxidation of 2 is shown in Figure 6. The bands
assigned to Fc+-hqH2, 2+, including the broad charge-transfer
band at 850 nm, disappear as new bands at 260, 340, and 644
nm appear. The appearance of the 644 nm band is consistent
with a ferrocenium center, and the 260 nm band is indicative
for a quinone center,19 suggesting the product is Fc+-q, 3+, in
accord with the coulommetry, which revealed 1.8 F mol-1 were
consumed, and with the cyclic voltammetry discussed above.
UV-vis-NIR spectroelectrochemical experiments confirm
these assignments of the redox processes and provide spectro-
scopic evidence for production of the unstable ortho-quinone
complex 6, which we could not obtain by chemical oxidation.
Table 3 presents a summary of the UV-vis-NIR spectroscopic
data for the species that were characterized in these experiments.
The UV-vis-NIR spectra of the one-electron oxidation
products of 2, 4, and 5 were obtained from in situ UV-vis-
NIR spectroelectrochemical experiments and are very similar.
Figure 4 shows UV-vis-NIR spectra acquired from a repre-
sentative experiment, the oxidation of 4 at an electrolysis
potential of +0.5 V. As expected from the cyclic voltammetry,
(25) Cuffe, L.; Hudson, R. D. A.; Gallagher, J. F.; Jennings, S.; McAdam,
C. J.; Connelly, B. T. R.; Manning, A. R.; Robinson, B. H.; Simpson, J.
Organometallics 2005, 24, 2051-2060.

2220 Organometallics, Vol. 25, No. 9, 2006
Colbran et al.
Figure 7. UV-vis-NIR spectroelectrochemistry for controlled-
potential oxidation of 3 at +0.90 V (vs Fc+-Fc) in acetonitrile-
Figure 8. UV-vis-NIR spectroelectrochemistry for reduction of
3 at -1.0 V (vs Fc+-Fc) in acetonitrile-0.2 M [Bun N][PF6] at
4
0.2 M [Bun N][PF6] at 298 K.
298 K. The changes are (∼85%) reversed by electrolysis at -0.15
V. The spectrum of the reduction product (dashed trace) was
obtained by normalizing the first (3) spectrum and the final
(reduction product + 3) spectrum at 705 nm, subtracting the
normalized first spectrum from the normalized final spectrum, and
then scaling the absorbances so that they match those in all spectra
at the isosbestic points.
4
Reduction of 3+ at -400 mV cleanly produces the ferrocenyl-
hydroquinone 2. That reduction of the quinone group occurs in
the present case and not for 3+ produced by the oxidation of 3
(see below) is attributed to the stoichiometric production of
protons during the oxidation of 2+ according to eq 2. Thus, the
pH in the solution is lowered and reduction of the quinone center
occurs at higher potential3,4 so that 3+ is reduced to 2.
Figure 7 reveals that the ferrocenium-quinone 3+ is also
cleanly produced by electrochemical oxidation of 3. The
spectrum is identical to that of the second oxidation product of
2, Figure 6. Isosbestic points are observed at 239, 378, 472,
and 526 nm, and all of the spectroscopic changes are fully
reversed by electrolysis at +100 mV, which is consistent with
a fully reversible oxidation process as predicted from the cyclic
voltammetry of 3. This highlights the role of the released protons
(pH) during the reduction of 3+ produced from oxidation of 2.
Noteworthily, chemical oxidation of 3 also gave 3+. The UV-
vis spectrum of the product from 3 and Ag[SbF6] in tetrahy-
drofuran, after filtration to remove the precipitated Ag, was
identical with that for 3+ produced by electrolysis; however,
3[SbF6] tenaciously retained solvent and an analytically pure
sample was not obtained.
Figure 8 depicts the spectroelectrochemical behavior of 3 at
-1.0 V. This potential is poised just beyond the potential of
the first reduction couple of 3. The electrolysis was slow and
incomplete due to the low overpotential; isosbestic points were
observed at 285, 380, 406, and 620 nm, although the reduction
was not fully reversible due to some slight decomposition of
the reduction product. Arithmetic manipulation of the spectra
(see Figure 8) reveals the reduction product has a band at 280
nm, as do all of the ferrocene derivatives reported in this work,
plus bands at 326, 427, 448, 484 (sh), and 540 nm with the
characteristic profile for a semiquinone (sq-) radical anion.26
This confirms the reduction product is Fc-sq-, eq 3. Figure 9
shows the EPR spectrum of the Fc-sq- radical. The spectrum
shows a quartet at g ) 2.0056 due to hyperfine coupling with
the three protons of the semiquinonyl ring. The coupling at 0.227
mT is slightly, but significantly, lower than that of other
semiquinone radicals, typically 0.241 mT,27 indicative of
delocalization of spin density from the now electron-rich
semiquinone center onto the ferrocene.
Figure 9. X-band EPR spectrum of Fc-sq- obtained during in
situ electrolysis of 3 at -1.0 V (vs Fc+-Fc) in acetonitrile-0.1 M
[Bun N][PF6] at 298 K. Inset: X-band EPR spectrum of the
4
semiquinone (sq-) anion obtained from 1,4-benzoquinone under
identical conditions.
Figure 10 shows the spectroelectrochemical behavior of 3 at
the more reducing potential of -1.8 V. The first reduction
process proceeded rapidly and was followed by a second
process. The spectra in Figure 10a show the disappearance of
the bands of 3 and the growth of the new bands for the
semiquinone radical, Fc-sq-. Figure 10b shows spectra obtained
immediately following those shown in Figure 10a and corre-
sponds to completion of the second reduction process. The
resultant spectrum is quite similar to that of 2 and may be
assigned to a ferrocene-hydroquinonyl species. This second
reduction is not reversed upon electrolysis at -0.15 V. A
plausible reason is that the basic hydroquinone dianion is
unstable with respect to protonation under the neutral conditions
employed and reacts to form the hydroquinonyl monoanion, Fc-
hqH-, eq 5 (EH ) solvent or protic impurity, e.g., water).
Overall, the reduction chemistry of Fc-q is thus described by
eqs 3-5.
(26) Okamoto, K.; Hirota, N.; Tominaga, T.; Terazima, M. J. Phys. Chem.
A 2001, 105, 6586-6593.
(27) Fiedler, S. L.; Eloranta, J. Magn. Reson. Chem. 2005, 43, 231-
234.
Fc-hq2- + H-E a Fc-hqH- + E-
(5)

CoValently Linked Ferrocenyl Quinones
Organometallics, Vol. 25, No. 9, 2006 2221
Figure 11. Cyclic voltammograms of 3 before (a) and after (b)
addition of 2,4,6-collidinium triflate (2.0 equiv). Conditions:
acetonitrile-0.1 M [Bun N][PF6] at 298 K, glassy carbon 0.5 mm
4
minidisk working electrode, scan rate 100 mV s-1
.
Figure 10. UV-vis-NIR spectroelectrochemistry for controlled-
potential reduction of 3 at -1.8 V (vs Fc+-Fc): (a) first six spectra;
(b) subsequent spectra. Conditions: acetonitrile-0.2 M [Bun N]-
4
[PF6] at 298 K; repetition time between spectra is 45 s.
Figure 12. UV-vis-NIR spectroelectrochemistry for controlled-
potential electrolysis of a solution containing 3 and excess 2,4,6-
collidinium triflate (4.0 equiv) at -0.9 V (vs Fc+-Fc) in acetonitrile-
Effect of Acid and Bases. The cyclic voltammograms of 2
and 3 were unaffected by the weak base 2,4,6-collidene (pKa
7.3). Figure 3a shows a cyclic voltammogram of 2 before and
after the addition of excess potassium tert-butoxide [pKa 18,
cf. pKa(1,4-hydroquinone) 9.96]. The electrochemical behavior
of 2 in the presence of the base is similar to that of 3, Figure
3b, suggesting that the hydroquinonyl group is doubly depro-
tonated to afford the dianion, Fc-hq2-, which is stable under
these strongly basic conditions. This is consistent with the initial
anodic current in the dotted trace in Figure 3a because Fc-
hq2- is spontaneously oxidized to Fc-q at the initial potential
(-0.5 V). The behavior of the Fc-sq-/Fc-hq2- couple is
suggestive of adsorption/precipitation of the potassium salt of
the dianion on the working electrode. The distinct positive shifts
in the Fc-q/Fc-sq- and Fc-sq-/Fc-hq2- couples for the
hydroquinone dianion Fc-hq2-, Figure 3a, compared to the
neutral quinone Fc-q, Figure 3b, result from the tighter anion-
potassium counterion pairing28 and anion-tert-butanol hydrogen
bonding3 associations caused by the presence of potassium ion
and tert-butyl alcohol after reaction of 2 and potassium tert-
butoxide.
0.2 M [Bun N][PF6] at 298 K.
4
collidinium triflate. Addition of collidinium triflate does not
affect the ferrocene-ferrocenium couple, indicating that 3 is
not protonated in solution in this case, consistent with the
unperturbed electronic spectrum. However, the q-sq- and sq--
hq2- couples are replaced by an irreversible, two-electron
process at -0.54 V. This reveals the effect of weak acid on the
stability of the semiquinone radical anion, which is rapidly
protonated and disproportionates3,4 according to eqs 6-8:
Fc-q+ e- a Fc-sq-
(6)
Fc-sq- + H+ f Fc-sqH
(7)
(8)
2 Fc-sqH f Fc-q + Fc-hqH2
Since the disproportionation regenerates 3, provided all steps
are fast relative to the cyclic voltammetric (µs-ms) time scale,
an overall two-electron reduction to the hydroquinone Fc-hqH2
is observed. The proton dependence of the process renders it
electrochemically irreversible.
Figure 12 shows the spectroelectrochemical behavior of 3
during reduction at -0.9 V in the presence of excess 2,4,6-
collidinium triflate. As the electrolysis proceeds, the absorption
bands of 3 are replaced by those for 2 (cf. Figure 1). Isosbestic
Addition of weak acids did not affect the color or the UV-
vis-NIR spectra of solutions containing 3. Figure 11 shows
cyclic voltammograms before and after addition of 2,4,6-
(28) LeSuer, R. J.; Geiger, W. E. Angew. Chem., Int. Ed. 2000, 39, 248-
250.

2222 Organometallics, Vol. 25, No. 9, 2006
Colbran et al.
Figure 13. UV-vis-NIR spectra of (a) 3 in acetonitrile, (b) after
adding excess aqueous 40% perchloric acid (0.1 mL into 10 mL of
solution) to solution (a), and (c) after adding excess 2,4,6-collidine
to solution (b). The spectra are not compensated for dilution effects;
2,4,6-collidene absorbs strongly below 300 nm.
Figure 15. UV-vis-NIR spectroelectrochemistry for controlled-
potential oxidation of 3 in the presence of excess aqueous perchloric
acid (0.1 mL in 10 mL of solution) at +0.8 V (vs Fc+-Fc) in
acetonitrile-0.2 M [Bun N][PF6] at 298 K. (No change was
4
observed when an electrolysis potential of +0.3 V was applied to
the solution.) The dashed trace was obtained from the arithmetic
manipulation, 2 × (is - 0.5fs), of the initial (is) and final (fs)
spectra.
controlled rates.3,4 The observation of bands for Fc+-q and
Fc+-hqH2 confirms that Fc+-sqH disproportionates. Equations
9 and 10 thus summarize the response of 3 to strong acid.
Fc-q + H+ a Fc+-sqH
(9)
2 Fc+-sqH a Fc+-hqH2 + Fc+-q
(10)
The cyclic voltammetry and UV-vis-NIR spectroelectro-
chemistry for 3 in the presence of excess perchloric acid is
entirely consistent with internal electron transfer and dispro-
portionation to afford a 1:1 mixture of Fc+-hqH2 and Fc+-q
ions. Addition of excess perchloric acid to 3 results in a cyclic
voltammogram very similar to that of 2 but with the current
cathodically displaced, Figure 14. The cathodic displacement
of the initial current is indicative of spontaneous reduction of
the ferrocenium and quinone centers in the 1:1 mixture of Fc+-
Figure 14. Cyclic voltammograms of 3 before (a) and after (b)
addition of 40% aqueous perchloric acid (0.1 mL in 10 mL of
solution). Conditions: acetonitrile-0.1 M [Bun N][PF6] at 298 K,
4
glassy carbon 0.5 mm minidisk working electrode, scan rate 100
mV s-1
.
+
hqH2 and Fc+-q to afford 2 [or possibly Fc-hqH3 (2H+) in
points at 297, 350, and 520 nm reveal this to be a clean
transformation without accumulation of semiquinone intermedi-
ates.
strong acid], which is the thermodynamically stable species in
acid at the initially applied potential (-0.4 V). The observed
FeIII-FeII couple on adding the strong acid thus arises from 2,
not 3, and accordingly is shifted to lower potential as discussed
above. After acid is added, the anodic peak at +0.46 V
corresponds to the 2e-, 2H+ oxidation of the hydroquinone
group, eq 5, and the coupled cathodic peak at +0.18 V to the
reverse process. In electrolysis, no current flowed and no change
was observed when the electrolysis potential was set at +0.3 V
(i.e., between the FeIII-FeII couple and the potential for
oxidation of the hydroquinone), consistent with only ferrocenium
centers in the species in solution. Figure 15 shows the changes
in the UV-vis-NIR spectrum upon electrolysis at +0.8 V,
which consumed 0.97 F mol-1; that is, overall it is a one-electron
process. The bands for Fc+-q at 256, 354, and 640 nm increase
in intensity as those at 296, 378, 441, 600, and 854 for Fc+-
hqH2 disappear; isosbestic points were observed at 223, 286,
322, 383, 608, and 709 nm for this clean transformation. The
dotted trace in Figure 15 is the spectrum for Fc+-hqH2 obtained
by subtraction of the spectrum of Fc+-q (the final spectrum)
from that of the 1:1 mixture of Fc+-q and Fc+-hqH2 and
corresponds to that obtained from direct oxidation of 2 at +0.3
V (see above).
In contrast, 3 exhibits quite different behavior upon addition
of a strong acid. Addition of perchloric acid (pKa -7) to
solutions of 3 in acetonitrile caused an immediate color change
from intense green to pale straw-yellow. In the corresponding
UV-vis-NIR spectra, Figure 13, the intense charge-transfer
band at 712 nm disappears and bands at 256, 354, and 640 nm
for Fc+-q (see above) plus bands at 296, 378, 441, 600, and
854 nm for Fc+-hqH2 (see above) appear. The change induced
by the perchloric acid was completely reversed by neutralization
with a weak base, e.g., collidine. The pKa of 1,4-benzoquinone3,4
is estimated to be -7, and the pKa of the benzoquinonyl group
in 3 will be raised by electron delocalization with the donor
ferrocene. Therefore, the quinone group will be protonated at
an oxygen after addition of perchloric acid, thereby raising its
oxidation potential above that of the ferrocene center (see Figure
14 and the accompanying discussion below) and so inducing
intramolecular electron transfer to produce the protonated
semiquinone cation, Fc+-sqH. As already mentioned, neutral
(protonated) semiquinones are unstable with respect to dispro-
portionation, which typically proceeds at close to diffusion-

CoValently Linked Ferrocenyl Quinones
Organometallics, Vol. 25, No. 9, 2006 2223
modified UV-vis-NIR cuvette with a Pt gauze working electrode,
a Pt wire counter electrode, and an Ag/AgCl reference electrode.
Coulommetry was performed in a conventional H-cell using Pt
gauze working and ancillary electrodes and the same reference
electrode as used in the cyclic voltammetry experiments; the stirred
solvent-electrolyte solution in the working electrode compartment
was exhaustively electrolyzed at the potential of interest prior to
adding the compound for electrolysis, and the small residual current
was subtracted from the integration of the current for the experi-
ment. Infrared spectra were recorded using either a Perkin-Elmer
280B or Hitachi 260-10 instrument. High-resolution mass spectra
(HR-MS) were obtained on a Bruker BioApex-II 7 T FT-ICR mass
spectrometer, and low-resolution mass spectra were recorded using
a VG Quattro mass spectrometer. Elemental microanalyses were
carried out by the Microanalytical Service Unit at the Research
School of Chemistry, Australian National University. Mo¨ssbauer
spectra were recorded in transmission mode with a constant
Conclusion
The electrochemical behavior of the complexes investigated
here is beyond that of the individual centers. The ferrocenyl-
quinones 3 and 6 are deep brilliant green, the result of intense,
solvatochromic ferrocene-donor to quinone-acceptor charge-
transfer transitions in the visible region, which disappear upon
one-electron reduction to Fc-sq- (3-) or oxidation to Fc+-q
(3+). Weaker intercenter charge-transfer transitions in the reverse
direction are observed in the ferrocenium-acceptor, aryl-donor
species 2+, 4+, and 5+.
In contrast to the results for ethynyl-linked ferrocene-quinone
conjugates that suggest the centers are largely isolated in the
ground state,6,7,26 considerable delocalization of electron density
from the donor ferrocene to acceptor quinone center is indicated
by the physicochemical properties of 3. The distribution of
charge between the ferrocene and quinone centers of 3 is
delicately balanced. Solutions of 3 become pale yellow after
adding strong acid. In these low-pH solutions, the quinone group
within 3 becomes a strong oxidant due to protonation at oxygen.
This in turn leads to intramolecular electron transfer followed
by disproportionation of the resulting semiquinone to afford a
1:1 mixture of Fc+-hqH2 and Fc+-q cations. The reaction is
completely reversed upon raising the pH of the solution.
We anticipate that these new insights into this archetypal
redox system will facilitate progress in applications and redox-
based devices. The new ferrocenyl-catechol 5 is potentially a
redox-switchable chelate ligand to metal ions29 and metal oxide
surfaces.30 Potential uses for the 2-3 system in molecular
electronics applications are suggested by the range of accessible
oxidation states, the redox-switchable low-energy charge-transfer
transitions, and the pH-controlled switching from ferrocene-
centered to (hydro)quinone-centered redox chemistry.
acceleration spectrometer at room temperature equipped with a 57
-
Co/Pd source and a Wissel drive unit with associated Ortec
electronics. The velocity scale was calibrated with metallic iron
foil and sodium nitroferricyanide(III), and the isomer shift value
quoted is relative to the midpoint of the iron spectrum at room
temperature. The spectral parameters were extracted from least-
squares fits of the data to Lorentzian line shapes.
Syntheses. Fc-hqMe2 (1). The diazonium salt of 2,5-dimethoxy-
aniline was prepared by the slow addition of sodium nitrite (6.1 g,
0.10 mol) to a stirred solution of 2,5-dimethoxyaniline (12.24 g,
0.08 mol) in aqueous sulfuric acid (200 mL, 1 M). The resultant
solution was added rapidly to ferrocene (18.6 g, 0.10 mol) dissolved
in glacial acetic acid (900 mL) under an inert atmosphere. The
resulting green/brown solution was stirred overnight at room
temperature, and then poured into a saturated aqueous solution of
sodium bisulfate (2000 mL). The mixture was extracted with
dichloromethane (3 × 200 mL) and the combined organic extracts
treated with aqueous sodium carbonate until neutral and then dried
over magnesium sulfate. The solvent was removed and the product
purified by silica column chromatography. Elution with dichlo-
romethane/hexane (1:3) yielded 8.60 g (25%) of 1 as a red oil.
HR-MS: m/z 322.06524 (calc M+ (C18H18FeO2+) m/z 322.06507).
1H NMR (CDCl3): δ 7.13 (d, JHH 3 Hz, C6H3(OMe)2, 1 H), 6.82
(d, JHH 9 Hz, C6H3(OMe)2, 1 H), 6.74 (dd, JHH 3, 9 Hz, C6H3-
(OMe)2, 1H), 4.79 (t, C5H4, 2 H), 4.31 (t, C5H4, 2 H), 4.08 (s,
C5H4, 5 H), 3.84 (s, CH3, 3H), 3.82 (s, CH3, 3H). 13C NMR
(CDCl3): δ 153.3, 151.1, 128.5, 115.2, 112.1, 111.0, 82.5, 69.4,
69.9, 69.3, 55.6, 55.5. LR-MS: 323 (M+ + 1, 23%), 322 (M+,
Experimental Section
General Comments. All reactions were performed under a
nitrogen atmosphere in solvents dried and distilled using routine
procedures. UV-vis-NIR spectra were recorded using a Cary 5
spectrophotometer. Solutions for electrochemistry and spectroelec-
trochemistry experiments were prepared in a nitrogen-filled Braun
glovebox operating with water and oxygen levels both below 2
ppm; the compounds were ∼1.0 mM in anhydrous acetonitrile
(Aldrich, used as received) with 0.1 or 0.2 M tetra-n-butylammo-
nium hexafluorophosphate for cyclic voltammetry or spectroelec-
trochemistry and coulommetry experiments, respectively. Cyclic
voltammetry measurements were performed in a conventional three-
electrode cell using a computer-controlled Pine Instrument Co.
AFCBP1 bipotentiostat as described in detail elsewhere.31 The
reported data are from cyclic voltammograms recorded with a 0.5
100), 307 (M+ - CH3, 26), 292 (M+ - 2CH3, 73), 227 (M+
-
(2CH3 + C5H5), 39). IR (neat (cm-1)): 3097 (w), 2999 (w), 2953
(m), 2940 (m), 2908 (w), 2836 (s), 1614 (m), 1513 (vs), 1473 (vs),
1441 (s), 1415 (w), 1393 (w), 1311 (m), 1291 (s), 1271 (s), 1236
(vs), 1221 (vs), 1030 (s), 1005 (m), 820 (s), 737 (m), 504 (s).
mm glassy carbon working electrode at a scan rate of 100 mV s-1
,
Fc-hqH2 (2). Compound 3 (0.50 g, 1.7 mmol) was dissolved
in dichloromethane and added to a separating funnel containing
saturated aqueous sodium dithionite (100 mL). The mixture was
shaken until the green dichloromethane layer became completely
orange. The dichloromethane layer was washed with water and dried
over anhydrous magnesium sulfate, and the solvent was removed
in vacuo. Compound 2 was recovered as an orange solid (0.50 g,
99%). HR-MS: m/z 294.03386 (calc M+ (C16H14FeO2+) m/z
and the potentials in this paper are quoted relative to the ferroce-
nium-ferrocene (FeIII-FeII) couple measured under identical
experimental conditions (same concentrations, solvent, support
electrolyte, electrodes, temperature, and scan rate). The UV-vis-
NIR spectroelectrochemical experiments were performed using a
(29) (a) Allgeier, A. M.; Slone, C. S.; Mirkin, C. A.; Liable-Sands, L.
M.; Yap, G. P. A.; Rheingold, A. L. J. Am. Chem. Soc. 1997, 119, 550-
559. (b) Pierpont, C. G. Coord. Chem. ReV. 2001, 216, 99-125. (c) Dei,
A.; Gatteschi, D.; Sangregorio, C.; Sorace, L. Acc. Chem. Res. 2004, 37,
827-835.
1
294.03377). H NMR (CDCl3): δ 6.85 (d, JHH 9 Hz, C6H3(OH)2,
1 H), 6.78 (d, JHH 3 Hz, C6H3(OH)2, 1 H), 6.72 (dd, JHH 3, 9 Hz,
C6H3(OH)2, 1H), 4.52 (t, JHH 2 Hz, C5H4, 2 H), 4.39 (t, JHH 2 Hz,
C5H4, 2 H), 4.22 (s, C5H4, 5 H). LR-MS: 295 (M+ + 1, 18), 294
(M+, 80), 293 (M+ - H, 5), 292 (M+ - 2H, 10), 228 (M+ - C5H5),
100). IR (Nujol (cm-1)): 3319 (s), 1626 (w), 1510 (s), 1242 (w),
1196 (vs), 1127 (w), 1108 (m), 1073 (w), 1043 (w), 1020 (w), 1004
(m), 933 (w), 846 (m), 818 (s), 782 (s), 726 (m), 497 (s).
(30) (a) Rice, C. R.; Ward, M. D.; Nazeeruddin, N. K.; Gra¨tzel, M. New
J. Chem. 2000, 24, 651-652. (b) Lucas, N. T.; McDonagh, A. M.; Hook,
J. M.; Colbran, S. B. Eur. J. Inorg. Chem. 2005, 496-503. (c) Lucas, N.
T.; McDonagh, A. M.; Dance, I. G.; Craig, D. C.; Colbran, S. B. Dalton
Trans. 2006, 680-685.
(31) He, Z. C.; Colbran, S. B.; Craig, D. C. Chem. Eur. J. 2003, 9, 116-
129.

2224 Organometallics, Vol. 25, No. 9, 2006
Colbran et al.
Fc-q (3). Boron tribromide (5.9 mL, 62 mmol) was added to a
solution of 1 (4.0 g, 12 mmol) in dichloromethane (60 mL) cooled
in an acetone/dry ice bath. The resulting dark red solution was
allowed to warm to room temperature and stirred overnight. The
solvent was removed in vacuo and the residue cooled in an ice
bath. Methanol (30 mL) was added via cannula followed by
saturated aqueous sodium carbonate (30 mL). The mixture was
extracted quickly in air with dichloromethane (3 × 50 mL), and
the combined organic extracts were combined and dried over
anhydrous magnesium sulfate. The solvent was removed, and the
resulting solid was dissolved in dichloromethane (40 mL). 2,3-
Dichloro-5,6-dicyano-1,4-benzoquinone (2.26 g, 10 mmol) was
added, whereupon the solution immediately changed from orange
to green. The solution was filtered and the solvent removed in vacuo
to give a green solid. The product was purified by thin-layer
chromatography with dichloromethane as eluant. Subsequent re-
crystallization of the green band from hexane gave 2.10 g (57%)
of 3 as dark green crystals. Anal. Found: C 65.15, H 4.41. Calc:
C 65.78, H 4.14. Mp: 128-130 °C. 1H NMR (CDCl3): δ 6.81 (d,
orange solid. Anal. Found: C 67.22, H 5.66. Calc: C 67.10, H
5.66. H NMR (CDCl3): δ 7.14 (s, C6H3, 1H), 7.07 (d, JHH 9 Hz,
1
C6H3, 1 H), 6.81 (d, JHH 8 Hz, C6H3,1 H), 4.57 (s, C5H4, 2 H),
4.27 (s, C5H4, 2 H), 4.05 (s, C5H4, 5 H). LR-MS: 323 (M+ + 1,
23%), 322 (M+, 100), 307 (M+ - CH3, 10). IR (KBr pellet (cm-1)):
3087 (w), 3004 (w), 2956 (m), 2931 (m), 2833 (m), 1639 (vs),
1584 (vs), 1523 (s), 1506 (m), 1486 (m), 1472 (m), 1439 (s), 1268
(w), 1247 (vs), 1218 (s), 1206 (w), 1172 (vs), 1140 (vs), 1026 (vs),
815 (vs), 755 (m), 740 (m).
Fc-catH2 (5). Boron tribromide (0.35 mL, 2.8 mmol) was added
to a solution of 414 (609 mg, 1.89 mmol) in dichloromethane (30
mL) cooled to -78 °C in an acetone/dry ice bath. The resulting
solution was stirred for 30 min at -78 °C and then for another 30
min at room temperature. The reaction was quenched by the
cautious addition of methanol. The solution was extracted with
aqueous sodium thiosulfate and then with water and dried over
magnesium sulfate, and the solvent was removed in vacuo. The
residue was treated with methanol, whereupon an orange impurity
precipitated. The mixture was filtered, and the filtrate was reduced
in volume in vacuo to give 393 mg (75%) of 5 as an orange-brown
solid. The product is unstable in air and decomposes over time.
JHH 3 Hz, C6H3O2, 1 H), 6.70 (d, JHH 8 Hz, C6H3O2,1 H), 6.65 (dd,
JHH 3, 8 Hz, C6H3O2, 1H), 4.90 (t, JHH 2 Hz, C5H4, 2 H), 4.57 (t,
JHH 2 Hz, C5H4, 2 H), 4.09 (s, C5H4, 5 H). 13C NMR (CDCl3): δ
186.4, 185.9, 148.5, 137.2, 136.3, 126.7, 75.9, 72.4, 69.7, 70.6.
LR-MS: 293 (M+ + 1, 20), 292 (M+, 100), 226 (M+ - C5H5, 75).
IR (Nujol (cm-1)): 3058 (w), 1671 (s), 1647 (vs), 1615 (s), 1579
(vs), 1353 (w), 1329 (m), 1290 (vs), 1271 (m), 1109 (m), 1062
(m), 1039 (w), 985 (w), 917 (m), 904 (w), 839 (m), 826 (m), 502
(m), 487 (w), 431 (m).
Fc-catMe2 (4). The diazonium salt of 1,2-dimethoxyaniline was
prepared by the slow addition of sodium nitrite (83 mg, 1.2 mmol)
to a stirred solution of 1,2-dimethoxyaniline (180 mg, 1.17 mmol)
in aqueous sulfuric acid (0.2 M). The resultant solution was added
rapidly to ferrocene (270 mg, 1.17 mmol) dissolved in glacial acetic
acid (10 mL) under an inert atmosphere. The resulting green-brown
solution was stirred overnight at room temperature and then poured
into water. The mixture was extracted with dichloromethane (3 ×
20 mL), the combined organic extracts were treated with aqueous
sodium carbonate until neutral and dried over magnesium sulfate,
and the solvent was removed. The crude product was recrystallized
from dichloromethane/methanol to yield 65 mg (14%) of 4 as an
1
Anal. Found: C 64.26, H 4.99. Calc: C 65.34, H 4.99. H NMR
(d6-acetone): δ 7.69 (s, OH, 1 H), 7.67 (s, OH, 1H), 6.97 (s, C6H3,
1 H), 6.82 (d, JHH 8 Hz, C6H3,1 H), 6.74 (d, JHH 8 Hz, C6H3, 1H),
4.66 (s, C5H4, 2 H), 4.31 (s, C5H4, 2 H), 4.06 (s, C5H4, 5 H). 13C
NMR (d6-acetone): δ 205.2, 120.1, 117.8, 114.9, 113.3, 69.8, 68.5,
65.9. LR-MS: 295 (M+ + 1, 5%), 294 (M+, 17), 292 (M+ - 2H,
5). IR (KBr pellet (cm-1)): 3345 (vs), 3090 (w), 1604 (s), 1539
(s), 1468 (s), 1444 (s), 1387 (s), 1362 (w), 1296 (s), 1259 (s), 1207
(w), 1185 (w), 1104 (s), 1015 (w), 809 (s), 778 (m).
Acknowledgment. This research was supported by the
Australian Research Council (Grant: DPO557462) and The
University of New South Wales.
Supporting Information Available: A figure showing the
solvent dependence of the lowest energy visible band for 3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM060040X