Extremely Electron-Poor Bis(diarylmethylium)-Substituted Ferrocenes and the First Peroxoferrocenophane

Article abstract of DOI:10.1002/zaac.201900347

We have prepared and studied extremely electron-poor, deeply colored dicationic 1,1'-bis(diarylmethylium)-substituted ferrocenes [(η⁵-C₅H₄-CAr₂)₂Fe]²⁺ with various aryl substituents as their [B{C₆H₃(CF₃)₂-3,5}₄]^– salts. Due to the strong acceptor substitution, the redox potential for the ferrocene-based oxidation of the anisyl- or 2-methylanisyl-substituted congeners 1b²⁺ and 1c²⁺ is close to or even surpasses that of the second oxidation of parent ferrocene, i.e. the Cp₂Fe^+/2+ couple. The strongly Lewis-acidic character of these complexes is manifest through strong interactions with donor solvents, which lead to a significant reduction of the intensities of the charge-transfer bands in their electronic spectra and to solvatochromism. The reduced forms of the complexes tend to dimerize or oligomerize as revealed by EPR spectroscopy. Direduced 1b selectively reacts with molecular oxygen to form a peroxo-bis(diarylmethyl)[4]ferrocenophane, which was also characterized by X-ray crystallography.

Full text of DOI:10.1002/zaac.201900347

Title: Extremely Electron-Poor Bis(diarylmethylium)-Substituted
Ferrocenes and the First Peroxoferrocenophane
Authors: Rainer Friedrich Winter, Larissa A. Casper, Steffen Oßwald,
Patrick Anders, and Lisa-Catherine Rosenbaum
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10.1002/zaac.201900347
Zeitschrift für anorganische und allgemeine Chemie
Extremely Electron-Poor Bis(diarylmethylium)-Substituted Ferrocenes and the
First Peroxoferrocenophane
Larissa A. Casper, Steffen Oßwald, Patrick Anders, Lisa-Catherine Rosenbaum, and Rainer F.
Winter*
Fachbereich Chemie, Universitꢀt Konstanz, Universitꢀtsstraße 10, D-78453 Konstanz,
Germany
Dedicated to Professor Manfred Scheer on the Occasion of his 65th birthday
Abstract
We have prepared and studied extremely electron-poor, deeply coloured dicationic 1,1’-
bis(diarylmethylium)-substituted ferrocenes [(5-C5H4CAr2)2Fe]2+ with various aryl
substituents as their [B{C6H3(CF3)2-3,5}4]- salts. Due to the strong acceptor substitution, the
redox potential for the ferrocene-based oxidation of the anisyl- or 2-methylanisyl-substituted
congeners 1b2+ and 1c2+ is close to or even surpasses that of the second oxidation of parent
ferrocene, i. e. the Cp2Fe+/2+ couple. The strongly Lewis-acidic character of these complexes is
manifest through strong interactions with donor solvents, which lead to a significant reduction
of the intensities of the charge transfer bands in their electronic spectra and to solvatochromism.
The reduced forms of the complexes tend to dimerize or oligomerize as revealed by EPR
spectroscopy. Direduced 1b selectively reacts with molecular oxygen to form a peroxo-
bis(diarylmethyl)[4]ferrocenophane, which was also characterized by X-ray crystallography.
Introduction
Triarylmethylium (tritylium) substituted ferrocenes constitute the most prominent examples of
metal-organic tritylium dyes and were studied early on for their electronic structures. Particular
focus was on the ability of the iron nucleus of the ferrocene scaffold to stabilize the positively
charged, adjacent methylium center by - or -interactions as expressed by the resonance forms
IV and V in Scheme 1.[1] Such kind of interactions were initially inferred by the 20.7° bending
+
of the CPh2 plane toward the Fe2+ ion in the crystallographically determined structure of
FcCPh2+ [2] (Fc = ferrocenyl, (5-C5H5)Fe(5-C5H4)) and by quantum chemical calculations,[3]
but later refuted by the notion of a small energy barrier for rotation around the CpC+Ar2 bond[4]
and the generally large dipole moments of ferrocenyl carbenium ions.[1f]
1
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Scheme 1. Resonance formulas of ferrocenyl-substituted methylium cations.
The electron-donating ferrocenyl substituent and the strong dipolar character of such
compounds usually lead to intense charge-transfer absorptions in the visible regime of the
electronic spectrum (Vis), which are typical of donor-substituted tritylium dyes. Vinylogous or
alkynylogous expansion of the ferrocenyl arm of such dyes shifts the corresponding charge
transfer band further red and increases its oscillator strength.[5] In ferrocenyl-tritylium dyes Fc-
C+Ar2, the CT transitions from the carbocyclic substituents to the methylium center are
augmented by an additional, weaker CT band (the HOMOLUMO transition) concomitant
with the direct transfer of electron density from the Fe2+ ion to the methylium acceptor. In
agreement with this assignment, this band is bleached on oxidation, whereby the ferrocene
donor is changed into an electron-poor ferrocenium ion. The same behaviour is observed on
reduction of the tritylium entity to a trityl radical, which essentially removes its electron-
accepting capabilities. Neutral ferrocenyl(diaryl)methyl radicals FcCAr2 are prone to dimerize,
and an authentic hexaarylethane structure was proposed for such dimers.[6]
Quite surprisingly, no 1,1’-disubstituted, dicationic bis(diarylmethylium)-substituted
ferrocenes seem to be known to date. Only the 1,1’-bis(isopropylium)-derivative [(5-
C5H4CMe2)2Fe]2+ derivative was generated, either by protonation and dehydration of the
corresponding diol in FSO3H/SbF5[7] or by protonation of the bis(isopropenyl) derivative with
[8]
FSO3H in liquid SO2 and characterized by NMR spectroscopy, but found to persist only at
temperatures of -30 °C or lower. Here we present the first three representatives of such
complexes, bearing aryl substituents of differing electron-donating capabilities. We have
probed their electronic properties by means of electrochemistry and UV/Vis/NIR as well as
EPR spectroscopic studies on the dications and their reduced and oxidized forms. We also
report a unique ansa-peroxoferrocenophane, which is formed by the selective reaction of the
monoreduced radical cation of the bis(anisyl) derivative with molecular oxygen.
2
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Results and Discussion
Synthesis and spectroscopic characterization
The target 1,1’-bis(triarylmethylium)-substituted ferrocenes 1a2+ to 1c2+ were prepared in a
two-step procedure analogous to that employed for the synthesis of their monosubstituted
congeners 2a+ to 2c+ (Scheme 2). Thus, 1,1’-ferrocenedicarboxylic acid dimethyl ester[9] was
reacted with ca. 4.2 equiv. of a lithiated arene to provide the corresponding 1,1’-
bis(diarylmethylcarbinols) 1a-(OH)2 to 1c-(OH)2 after workup. On addition of Brookhart´s
acid [H(OEt2)]+ [B{C6H3(CF3)2-3,5}4]-,[10] an instantaneous colour change to blue (1a2+), dark
green (1b2+), or dark yellow (1c2+) was observed. The so-dubbed BArF24- anion, which was first
reported by Kobayshi in 1984,[11] is very weakly nucleophilic and has only little ion-pairing
capabilities,[6, 12] which is both crucial for the stabilization of strong electrophiles such as the
present ones. After solvent removal, the corresponding dicationic bis(triarylmethylium)-
substituted ferrocenes were obtained as intensely blue, red, or vividly purple solids.
Compound 1a2+ provided well-resolved NMR spectra with the characteristic resonance signals
of the monosubstituted Cp rings at  = 5.17 and 4.99 ppm, the two resonances of an AA’BB’
spin system for the para-substituted arene substituents at 7.57 and 6.69 ppm, and of the N-
-
methyl groups at 3.15 ppm in addition to the proton resonances of the BArF24 anion in their
correct integration ratios (see Figure S3 of the Supporting Information). All these resonances
are shifted to lower field when compared to the bis(carbinol) precursor 1a-(OH)2. Other
characteristic assets are the resonance signal of the carbenium centers at 181.1 ppm and the
resonances at 127.8 ppm and 84.2 ppm for the adjacent carbon atoms at the aryl and the Cp
substituents (see Figures S4 of the Supporting Information and the Experimental Section).
These signals likewise experience substantial low-field shifts with respect to the corresponding
carbinol precursor. As expected, the magnitude of this shift is largest for the newly formed
carbenium centers, where it amounts to more than 100 ppm. The methylium resonance signal
appears nevertheless at a slightly higher field when compared to  = 188.0 ppm for its
monotritylium congener 2a+ (Scheme 2).[6] Of the remaining two bis(tritylium)ferrocenes, 1b2+
1
provides rather well-resolved resonances in the H NMR spectrum at lower concentration
levels, which become paramagnetically broadened at higher concentrations (see Figure S6 of
the Supporting Information). This also precluded us from recording meaningful 13C NMR
spectra.
3
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Scheme 2. Synthesis of the 1,1‘-bis(triarylmethylium)-substituted ferrocenes 1a2+-1c2+ and
their monotritylium congeners 2a+-2c+.
In keeping with the presence of small amounts of a paramagnetic component, dicationic 1b2+
shows an isotropic, unstructured EPR signal in fluid solution at a g value of 2.0086. This signal
was initially of only weak intensity at r. t., but gradually intensified on warming the solution to
60 °C (Figure 1, left) and remained at a rather constant intensity level on cooling back to r. t.
Further cooling to temperatures of -20 °C or below, however, induces a gradual decrease of the
signal intensity until the initial stage is reached (Figure 1, right). The entire process can be
cycled several times. Such reversible intensity changes combined with a hysteretic behaviour
point to the existence of a thermally accessible paramagnetic state along with a substantial
activation barrier connecting this state to the diamagnetic ground state.
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Figure 1. Time-dependent EPR spectra of 1b2+ in CHCl3 on a) warming to 60 °C, and b) cooling
the warm solution to -20 °C.
A likely explanation for this unusual behaviour is thermal equilibration between the native 1,1’-
bis(diarylmethylium)ferrocene form Ar2C+(5-C5H4)FeII(5-C5H4)C+Ar2 and its
Ar2C+(5-C5H4)FeIII+(5-C5H4)CAr2 valence tautomer, where the positive charge has
shifted from a triarylmethylium site to the ferrocene nucleus (Scheme 3, top). This renders one
of the former triarylmethylium centers a trityl-type radical. As ferrocenium ions are EPR
inactive under these conditions, only the trityl component of this biradical is detected by EPR
spectroscopy. This valence tautomer was computationally modeled as the triplet state of 1b2+.
Our quantum chemical calculations produced indeed substantial spin densities on the ferrocene
nucleus and the methyl(ium) carbon atoms (Figure 2, left). Complex 1b2+ thus complements
Veciana´s ferrocenyl-perchlorotriphenylmethyl radicals (PCT), which were found to
equilibrate with their zwitterionic ferrocenium-perchlorotriphenylmethanide isomers.[13] These
two kinds of systems are compared in Scheme 3. They mainly differ in the identity and charge
state of the triarylmethyl(ium)-based acceptor units, i. e. cationic Ar2C+ in 1b2+ versus neutral
–C6Cl4C(C6Cl5)2 in Veciana´s systems.
5
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Scheme 3. Valence tautomeric forms of complex 1b2+ (top) and of perchlorotriphenyl-
substituted ferrocene (bottom).
Figure 2. DFT-calculated spin densities for 1b2+ in its triplet state (left), monoreduced 1b+
(middle) and neutral 1b (right).
Neither meaningful NMR nor EPR spectra were, however, obtained for complex 1c2+, such that
its formation was only verified by UV/Vis spectroscopy. As expected from their vivid colours,
all three complexes present intense electronic transitions in their Vis spectra. Their electronic
absorption spectra in CH2Cl2 are compared in Figure 3. Like in the solid state, solutions of the
complexes in CH2Cl2 are intensely blue (1a2+) or purplish red (1b,c2+). The electronic spectrum
of 1a2+ is dominated by a strong band ( = 85000 M-1 cm-1) at 653 nm, which, in analogy to its
monotritylium analog 2a+ (Scheme 2) is caused by charge-transfer (CT) from the electron-rich
4-dimethylaminophenyl substituents to the carbenium acceptor (the so-called x-band). As a
token of the increased acceptor strength owing to the presence of two tritylium-type acceptors,
this band is red-shifted by 1130 cm-1 (i. e. from 608 nm) when compared to the monotritylium-
substituted counterpart 2a+. The expected CT transition originating from the substituted Cp ring
as the donor (y-band) is presumably associated with the weaker feature at 530 nm, which is
6
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present in the spectra of all three complexes. No Fetritylium CT band (the so-called yM-band)
is, however, observed; it is likely hidden underneath the intense main band of 1a2+.
In contrast, Vis spectra of complexes 1b,c2+ with anisyl or 2-methyl-4-methoxyphenyl
substituents at the tritylium centre, show a yM band at 799 nm ( = 9500 M-1 cm-1) or at 765 nm
( = 9900 M-1 cm-1, Figure 3). According to our TD-DFT calculations, this absorption is
2
associated with charge-transfer from mainly the Fe dx -y2 orbital to the empty p-orbitals at the
methylium carbon atoms. A second feature with a similar character, but differing in the identity
of the Fe d donor orbital (dz2), is observed as a shoulder at ca. 630 nm for 1b2+ or at ca. 690 nm
for in 1c2+, i. e. on the high-energy side of the other yM band. The main absorption feature of
1b2+ is an intense ( = 65000 M-1 cm-1), structured peak with a main maximum 525 nm and
separate shoulders at 485 nm and 457 nm. According to our TD-DFT calculations and in
agreement with the behaviour of other triarylmethylium dyes, these bands are assigned as the
combined x- and y-bands with concomitant CT from the donor-substituted aryl rings or the
attached Cp ligand to the methylium centres.[14] Figure 4 shows the calculated spectrum and the
charge-density differences associated with the individual excitations of complex 1b2+. The Vis
spectrum of 1c2+ differs from that of 1b2+ mainly in that the intensity of particularly the x,y
bands at 513 nm is greatly diminished. This is a likely result of a larger torsion of the sterically
more hindered 2-methyl-4-methoxyphenyl rings with respect to the plane of the methylium
acceptors.
Figure 3. UV/Vis spectra of the bis(triarylmethylium)-substituted ferrocenes in CH2Cl2.
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Figure 4. Left: TD-DFT calculated absorption spectrum of complex 1b2+; right: electron
density difference plots (blue: decreasing electron density, red: increasing electron density) for
the most important transitions.
Complexes 1b2+ and 1c2+ show a remarkable solvatochromic behaviour. While giving purplish
red solutions in CH2Cl2 and nitromethane, their solutions in N- or O-donor solvents like acetone,
methanol, THF, Et2O, CH3CN, or pyridine assume a grass-green to orange-yellow colour. The
observable colour impressions and absorption spectra of equally concentrated solutions in
various solvents are collected in Figure 5; details to the band positions and absorptivities can
be obtained from Table S1 of the Supporting Information. Of note are the greatly diminished
absorptivities of all CT bands in any donor solvent. This indicates strong interactions between
Lewis-basic solvent molecules and the Lewis-acidic methylium centres of the solute, which
reduce the electron-accepting capabilities of the latter. The decrease of the band intensities
consequently follows, at least on a qualitative level, the Gutmann donor number DN. The latter
is defined as the negative value of H (in kcal mol-1) for adduct formation between an electron
pair donor with SbCl5 in a highly diluted solution of 1,2-dichloroethane.[15] The stronger
quenching of the bands in 1c2+ when compared to 1b2+ is consistent with an increased Lewis-
acidity of the methylium centres in the former complex as a result of a lower stabilization by
the more twisted aryl substituents. Complex 1a2+, where the positive charges at the methylium
carbons are rather well stabilized by the strongly electron-donating Me2NC6H4 substituents,
shows a strongly attenuated solvatochromism, changing its colour from deep blue in CH2Cl2 or
nitromethane to dark green in THF or MeOH.
8
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Figure 5. Solvatochromism of complexes 1b2+ and 1c2+.
Electrochemical studies
The redox properties of the bis(diarylmethylium)-substituted ferrocenes were probed by cyclic
and square wave voltammetry. The results of this study are shown in Figure 6; pertinent data
are compiled in Table 1. All complexes show two consecutive one-electron reductions for the
stepwise transformations of the cationic triarylmethylium to neutral triarylmethyl substituents,
i. e. the CpAr2C+/ couples (Figure 6, left). The splitting of half-wave potentials E1/2 amounts
to 405 mV for 1a2+ and increases to 670 mV in 1c2+ and to 740 mV in 1b2+. In every instance,
the first reduction wave appears anodic of the first reduction of the corresponding
monosubstituted diarylmethylium ferrocenes 2a+ to 2c+ (Table 1), which is a clear token of the
decreased electron density of the tritylium acceptors. The magnitude of the shift between the
first reduction of complexes 1a2+ to 1c2+ and those of their corresponding counterparts 2a+ to
2c+ increases with decreasing electron-donating capabilities of the aryl substituents. It thus
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becomes larger the more the effect of changing an electron-donating neutral ferrocenyl entity
+
in complexes 2a-c to an electron-accepting, cationic (5-C5H4)Fe(5-C4H4CAr2 ) substituent
in complexes 1a-c is felt by the methylium acceptors and is not compensated by the other aryl
substituents. One should note here that, in 1a2+ and 2a+, the 4-Me2NC6H4 substituents are
stronger donors than ferrocenyl, while ferrocenyl is the strongest donor in complexes 2b+ and
2c+. Again, a larger torsion of the 2-methyl-4-methoxyphenyl substituents of 1c2+ as compared
to the anisyl residues of 1b2+ is held responsible for the finding that 1c2+ is considerably easier
to reduce than 1b2+ despite the nominally stronger donor properties of the aryl substituents in
1c2+. This effect is even amplified with respect to the corresponding monosubstituted analogs
2b+ and 2c+ (Table 1).
Figure 6. Left: Cyclic voltammograms of complexes 1a2+ to 1c2+ in CH2Cl2/NBu4BARF24
(0.02 mM) at r. t. at a scan rate v = 100 mV/s. Right: Square wave voltammograms of complexes
1b2+ and 1c2+ in SO2(l) at -20 °C with NBu4BARF24 (0.02 mM). The peak at 0 mV is due to the
internal Cp2Fe0/+ redox standard.
In the CH2Cl2/NBu4BArF24 electrolyte, the expected oxidation of the central ferrocene
constituent can only be observed for the least electron-poor congener 1a2+ as a reversible wave
at E1/2 = 670 mV against the Cp2Fe0/+ couple. This process, however, lies outside the accessible
potential range of this electrolyte for the other two complexes. It could nevertheless be observed
as a broad square-wave peak at 1640 mV (1b2+) or 1870 mV (1c2+) in liquid SO2, using
NBu4BArF24 as the supporting electrolyte (Figure 6, right). With such positive redox potentials,
1b2+ and 1c2+ seem to be the electron-poorest ferrocenes reported to date, including other
multiacceptor-substituted ferrocenes such as 1,1’-dicyanoferrocene (E1/2 = 865 mV),
1,1’,2,2’,4,4’- and 1,1’,2,2’,3,3’-hexakis(pentafluorophenylferrocene) (E1/2 = 940 mV and
10
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951 mV, respectively), 1,1’,3,3’-tetra(methoxycarbonyl)ferrocene (E1/2 = 900 mV)[16] and even
1,1’,2,2’-tetraformylferrocene (E1/20/+ = 1145 mV).[17] In fact, their E1/2 values nearly match or
even surpass that of 1710 mV for the second oxidation of parent ferrocene (i. e. the Cp2Fe+/2+
couple) under similar conditions.[18]
Table 1. Electrochemical dataa),b) for all complexes.
reductions
oxidation
E1/2
E1/2
E1/2
Ep
Ep
Ep
(2+/3+)
89
complex
(0/+)
-1340
-880
-690
(0/+)
-1160
-780
-740
(0/+) (+/2+)
(+/2+) (2+/3+)
1a2+
1b2+
1c2+
57
-935
-140
-20
57
670
70
70
1640c)
1870c)
(+/2+)
490
-
70
100
-
(0/+)
57
(+/2+)
91
2a+
2b+
57
830
180
224
2c+
72
870
a)
Potentials in mV ( 3 mV) in CH2Cl2 at T = 293( 3) K relative to the Cp2Fe0/+ couple (E1/2 = 0.000 V).
Supporting electrolyte NBu4 B{C6H3(CF3)2}4 . b) Data for complexes 2a+ to 2c+ from ref. [6] c) Potentials were
+
-
determined by square wave voltammetry in liquid SO2 at T = 253( 3) K relative to the Cp2Fe0/+ couple (E1/2
=
+
-
0.000 V) with NBu4 B{C6H3(CF3)2}4 as the supporting electrolyte.
Spectroscopic Investigations on some oxidized and reduced forms
The three reversible redox processes of 1a2+ provided us with the opportunity to probe for the
spectral changes in the Vis/NIR spectra concomitant with these transformations. The results of
this study are depicted in Figure 7. On one-electron oxidation, the prominent Vis band of 1a2+
at 653 nm is bleached and gives way to less intense bands peaking at 790 nm, 543 nm and 450
nm. The lower-energy bands are very likely associated with CT from the C6H4NMe2 donors to
the methylium and ferrocenium acceptor entities. The red-shift of the 790 nm band with respect
to that in 1a2+ is in line with an inner array of three interconnected strong electron acceptors
after ferrocene oxidation.
The two consecutive one-electron reductions likewise bleach the prominent x-band of 1a2+ to
ultimately leave two bands at 540 nm and 439 nm. By inference from their mono-
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triarylmethylium-substituted congeners, for which a similar behaviour was observed, these
bands are assigned as a mixed transition within the ferrocene nucleus and a 1,1’-

ferrocenediylCAr2 CT transition ( = 540 nm) and as * transitions within the trityl
chromophore ( = 439 nm).[6] Of note is the absence of a specific electronic transition at low
energy for any radical cation with one neutral Ar2C and one cationic Ar2C+ substituent attached
to the same ferrocene-1,1’-diyl scaffold (see also Figures S9 and S10 of the Supporting
Information for the results of such studies on complexes 1b,c2+). In view of the substantial half-
wave potential splittings for the individual reductions one would expect such a charge transfer
absorption between differently charged pendants, may it occur through space or via the
ferrocene coupling unit. Figure S11 of the Supporting Information displays a possible structure,
which would allow for CT through space. Electronic transitions of this kind are well-known for
-stacked mixed-valent donor/acceptor dyads comprising of the reduced and the oxidized forms
of a planar, -conjugated electrophore (so-called pimers).[19] Only during the reduction of 1a2+
a suspicious feature was observed as a weak band (  1000 M-1 cm-1). This band was, however,
found to persist during the second reduction and is hence not specific to 1a+.
Figure 7. Spectroscopic changes during oxidation (top) as well as the first (bottom left) and
second (bottom right) reductions of complex 1a2+ (1,2-C2H4Cl2 / NBu4BArF24 (0.02 M) at r. t.).
The reduced forms of complexes 1a2+ and 1b2+ were also investigated by EPR spectroscopy.
Singly reduced radical cations 1a+ and 1b+ were generated by reacting the dicationic
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precursors with slightly less than 1 equiv. of cobaltocene in order to avoid overreduction. EPR
spectra of these samples were measured in CH2Cl2 solutions in the temperature range of 20 °C
to -40 °C or of 20 °C to -60 °C, respectively. As shown in Figure 8, both paramagnetic species
display an isotropic signal without any resolved hyperfine splitting at g values of 2.0115 (1a+)
or 2.0301 (1b+). Both values are slightly larger than that of ca. 2.003 expected for ordinary
trityl radicals,[20] but similar to those of e. g. the triferrocenylmethyl, phenyldiferrocenylmethyl,
the ferrocenyl-ruthenocenylmethyl, or the ferrocenyl-diruthenocenylmethyl radicals.[21] DFT-
calculated spin densities (Figure 2, middle) show indeed large contributions of the ferrocene-
1,1’-diyl entity to the singly occupied molecular orbital (SOMO) of 1b+.
During these studies we also noted an odd temperature-dependence for both radical cations.
Thus, the intensity of the EPR signal of 1a+ steadily and reversibly decreases upon cooling.
This is just the opposite of the normal Curie behaviour as expressed by equation 1, where g
denotes the isotropic g value of the compound, µB is Bohr´s magneton, B0 the magnetic field
strength and kB is Boltzmann´s constant. According to this equation, the population of the
thermally excited state N is expected to diminish with decreasing T, and this, in turn, should
lead to larger signal intensities for samples with equal spin concentrations as T is lowered. Such
odd behaviour was already observed for reduced samples of triarylmethylium-substituted
ferrocenes 2a+ to 2c+ and traced to the formation of hexaarylethane-type dimers.[6] The same
obviously applies here. We however note that the reduction in signal intensity upon cooling is
less pronounced for cationic 2a+ as compared to 1a, which is a reasonable consequence of
+
electrostatic repulsion between the positively charged CAr2 pendants of such a dimer. This
holds to an even larger degree for radical cation 1b+, whose signal intensity initially increases
on lowering T, complying with the normal Curie-behaviour, but then decreases upon cooling to
even lower temperatures. We thus conclude that dimerization only commences to a noticeable
degree at temperatures of or below -20 °C. This can be seen as a token of a lesser delocalization
of the positive charge at the remaining tritylium site onto the aryl substituents, which agrees
with our electrochemistry data.
퐵
N  N  푔µ
(eq. 1)
퐵
0
2푘
푇
퐵
13
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Figure 8. EPR spectra of monoreduced radical cations 1a+ (left) and 1b+ (right).
Fully reduced 1a, generated by treating 1a2+ with an excess over 2 equiv. of cobaltocene, proved
unfortunately too reactive to be reliably characterized by EPR spectroscopy. In the case of 1b,
however, an indicative EPR signal was obtained and its T dependence was likewise studied.
The results are shown in Figure 9. Diradical 1b produces two different isotropic signals at g
values of 2.0283 and of 2.0144. The former value falls close to that of 2.0031 observed for 1b+.
On progressive cooling to lower T, this signal gradually decreases in intensity, while that of the
signal at higher field first increases and then stays rather constant. We tentatively assign the
signal at g = 2.0283 to the dissociated diradical and the one at g = 2.0144 to a diradical dimer
or higher oligomers. We also note that, in contrast to cationic 1b+, but in agreement with neutral
2b, dimerization already sets in at or slightly below r. t. as shown by the slight intensity
decrease of the low-field EPR resonance in the T interval of 20 °C to 0 °C.
Figure 9. T-dependent EPR spectra of reduced 1b and proposed oligomer formation by spin
pairing.
14
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Reactivity studies of 1b: Formation and characterization of a unique peroxo-
[4]ferrocenophane
During attempts to reduce 1b2+ to its radical cation on a preparative scale with Cp*2Fe (E1/2 = -
550 mV) as a selective reductant and without protection from the atmosphere, we observed a
fading of the solution colour from deep red to greenish yellow. Extraction of the solid obtained
after solvent evaporation with n-hexane provided a dark yellow solution and a green, insoluble
1
residue, which was not characterized further. The H NMR spectrum of the product obtained
from the soluble fraction after solvent removal consists of four separate AB doublets at  =
7.86, 7.35, 6.90 and 6.52 ppm for the para-disubstituted anisyl rings, integrating as 4 protons
each, 4 broad singlet resonances at  = 5.11, 4.21, 4.03 and 3.91 ppm for the Cp protons with
an integral of 2 H each, and two singlets, each accounting for 6 H, for the methoxy protons at
 = 3.33 and 3.22 ppm, along with minor impurities (Figure S12 of the Supporting Information).
This pattern of resonance signals indicates the formation of a chiral 1,1’-disubstituted ferrocene
derivative with C2 symmetry. A pure product was obtained, when 2 equiv. of cobaltocene were
added to a CH2Cl2 solution of 1b2+ inside a glovebox and stirred for some minutes before the
reaction vessel was taken out of the glove box and left stirring open to the air overnight. Workup
as before gave the pure compound in quantitative yield after evaporation of the solvent. The
ESI mass spectrum of this product showed the molecular ion peak at m/z = 699.1925, which
matches with an adduct of 1b with one molecule of oxygen and one proton (Figure S14 of the
Supporting Information). Crystallization of this compound by slow evaporation of a saturated
solution in CH2Cl2 afforded yellow blocky crystals that proved suitable for X-ray structure
determination. As shown in Figure 10, the newly formed product is a neutral peroxo-
[4]ferrocenophane 1b-O2 (only one of the two enantiomers of the racemic pair in the crystal is
shown; for details of the data collection and refinement, the cell parameters and the bond lengths
and bond angles see Tables S2 to S4 of the Supporting Information).
In 1b-O2 the former trityl carbon centers and the O atoms of a molecule of O2 have combined
to form a COOC bridge, which interconnects the two Ar2C- pendants of the upper and
lower Cp decks. Within the four-atom linkage bond lengths of 1.526(2) and 1.529(2) Å for the
CCpC, 1.454(2) and 1.464(2) Å for the CO, and 1.4789(16) Å for the OO bonds, bond angles
COO of 108.1(1) and 107.0(1)°, and a dihedral angle of -134.2° at the peroxo group are
observed. The two Cp decks assume a staggered conformation with a rotation angle of 39.3°
(average values CCp-Cpcentroid-Cpcentroid-CCp) and are tilted by a 7.2°, which indicates that the
four-atom COOC-hinge induces an only moderate strain. The C-atoms at the hinges are at
15
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10.1002/zaac.201900347
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a closer distance of 3.271 Å as compared to 3.438 Å or 3.540 Å at the opposite, open side of
the [4]ferrocenophane structure. This is mirrored by differences in Fe-CCp bond lengths, which
range from 2.0229(18) to 2.0356(19) Å for the C atoms at or in the immediate vicinity of the
hinge to 2.067(2) and 2.0667(19) Å at the open side.
Figure 10. Molecule structure of one enantiomer of 1b-O2 as determined by X-ray
crystallography. H-atoms are removed for reasons of clarity. Ellipsoids are displayed at a 50%
probability level.
We note a close structural similarity to the corresponding 1,1’-bis(diphenyl)peroxo-
[4]rhodocenophanium cation, which has a tilt angle of 5.5° between the cyclopentadienyl
ligands, CCpC-, CO-, and OO bond lengths of 1.531(6)/1.521(6) Å, 1.454(4)/1.458(4) Å
and 1.487(4) Å as well as a dihedral angle COOC of 144.9°.[22] The latter compound and
three more derivatives with other, 4-substituted aryl substituents were obtained from the
reaction of homoleptic Rh(I) complexes [(4-C5H4=C(C6H4R-4)2Rh]+ (R = H, Cl, OMe, Me)
bearing differently substituted 6,6-diphenylfulvene ligands with air.[22-23] Besides the doubly
linked
(5-C5H4CPh{(C2H4)(OO)}CPh(5-C5H4)Fe,[24]
the
aforementioned
[4]rhodocenophanium cations seem to be the only peroxo-[4]metallocenophanes in the
literature. It should be noted, though, that aerial oxidation of the trityl radical to the
corresponding bistrityl peroxide was noted as early as in 1903 in one of the first treatises on
this compound.[25]
The tilt of 7.2° in 1b-O2 is larger than that of 0.7° to 2.8° found in [4]ferrocenophanes with
butane-1,4-diyl or 2-oxo-butane-1,4-diyl bridges. All-carbon-bridged [4]ferrocenophanes even
16
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show the larger interplanar CC distance at the hinge.[26] Such conformational differences
result from the much smaller angle between the Fe-O-O and the Cp planes of 11.4° and 15.1°
of 1b-O2 as compared to values of 49.0° to 70.6° for interplanar angles Fe-C-C/Cp in the all-
carbon bridged counterparts, i. e. from more “upright” conformations of the inner C2H4 or
C(O=)-CH2 connectors as opposed to a rather flat-lying peroxo group. Similar or even
smaller tilts were, however, observed in [4]ferrocenophanes with fluorinated all-carbon bridges
(up to 16.0 °),[27] or in a [4]ferrocenophane with a CH2N(Ph)-C(COOMe)2CH2 connector
(6.5°).[28]
In further explorations of direduced 1b we also investigated its reactivity with acetylene-1,4-
dicarboxylic acid dimethylester, styrene, CS2, NO, water and methanol under a protective
atmosphere of dinitrogen. From the product mixtures formed in these reactions, known 6,6-
dianisylfulvene[23, 29] and the unsymmetrically substituted (5-C5H4C(C6H4OMe)2OH)Fe(5-
C5H4C(C6H4OMe)2H (1b-HOH, for NMR and ESI mass spectra see Figures S15 to S17 of the
Supporting Information) as the product of water addition were identified besides various, but
smaller amounts of 1b-O2. The latter is obviously formed by the trapping of 1b with
adventitious oxygen. Notably, no polymer formation was observed during the reaction of 1b
with styrene. The release of a 6,6-disubstituted fulvene from a 1,1’-(5-C5H4CR2)2Fe diradical
is just the opposite reaction to that observed by Tacke and coworkers in their studies on
bis(fulvene) iron(0) derivatives. The latter can either undergo H atom abstraction from the
solvent (R = Ph, Me) or, in the case of methyl substituents, alternative C-C bond formation to
a [2]ferrocenophane (Scheme 4).[30]
Scheme 4. Formation and reactions of bisfulvene iron(0) complexes.[30]
17
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Summary and Conclusions
We here report on three 1,1-bis(diarylmethylium)-substituted ferrocenes of the type [{5-
C5H4C(C6H4R-4)}2Fe]2+ (R = NMe2: 1a2+; R = OMe: 1b2+ and [{5-C5H4C(C6H3(CH3-
2)(OMe-4))}2Fe]2+ (1c2+), which were obtained by reacting their neutral bis(carbinol)
precursors with Brookhart´s acid [H(OEt2)]+ [B{C6H3(CF3)2-3,5}4]-.[10] The dicationic
ferrocene derivatives 1b2+ and 1c2+ seem to be the most electron poor ferrocenes reported to
date. Thus, the half-wave potentials of the ferrocene-based oxidation resemble or are even
positive of that of the second oxidation of ferrocene itself, i. e. the Cp2Fe+/2+ redox couple under
similar conditions.[18] The high Lewis acidity of these compounds becomes manifest through
the bleach of the arylmethylium charge transfer band in donor solvents, which evidences the
formation of Lewis acid/base pairs with the respective solvent as the Lewis base. This leads to
pronounced solvatochromism. Reduction of the complexes occurs as two consecutive one-
electron steps with a large redox splitting E1/2 of 405 to 740 mV. Nevertheless, no intervalence
charge-transfer band for electron transfer from the reduced triarylmethyl to the remaining
oxidized triarylmethylium entity was observed, although the ferrocene-1,1’-diyl scaffold might
be expected to support through-space electronic coupling by cofacial stacking of these
substituents.
Like their monotritylium-substituted counterparts 2a-2c (Scheme 2), reduced 1a-c+/0 tend to
dimerize/oligomerize via their triarylmethyl centers with concomitant formation of CC bonds.
This tendency is attenuated for the radical cations, which is likely due to electrostatic repulsion
between the remaining triarylmethylium centers. Direduced, anisyl-substituted 1b was found to
be reactive towards molecular oxygen and to form
a
novel peroxo-
(bisdiarylmethyl)[4]ferrocenophane with a Ar2COOCAr2 linkage between the two Cp
decks. As revealed by X-ray crystallography, the four-atom linker induces an only moderate
ring strain as revealed by a 7.2° tilt between the Cp rings.
Another intriguing finding of this study is that anisyl-substituted 1b2+ exists as a mixture of a
diamagnetic and a paramagnetic form. These two isomers interconvert via a sizable energy
barrier, which gives rise to hysteresis. The paramagnetic form is obviously the result of an
electron transfer from the ferrocene nucleus to one of the triarylmethylium centres. Such
behaviour of 1b2+ is reminiscent of a magnetochemical switch, where, by action of an external
trigger, a diamagnetic state can be altered into a diradical state with two unpaired spins. Further
work in our laboratories is directed to exploring this prospect further.
18
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Experimental Section
General methods
All manipulations were carried out at room temperature under a nitrogen atmosphere using
standard Schlenk techniques, unless stated otherwise. Solvents were dried and distilled by
standard procedures and degassed by saturation with nitrogen prior to use. Ferrocene-1,1’-
dicarboxylic acid was prepared according to a literature procedure[31] and converted to its
dimethyl ester following the procedure procured for ferrocene carboxylic acid.[32] 1H NMR (400
13
31
MHz), C{1H} NMR (101 MHz) and P{1H} NMR (162 MHz) spectra of the compounds
were measured on a Bruker Avance III 400 spectrometer at room temperature in the indicated
deuterated solvent. The spectra were referenced to the signal of residual protonated solvent (1H)
or the solvent signal (13C). UV/vis/NIR spectra were recorded on a TIDAS fiber optic diode
array spectrometer (combined MCS UV/NIR and PGS NIR instrumentation) from j&m in
HELLMA quartz cuvettes with 0.1 cm optical path lengths.
All electrochemical experiments were executed in a custom-built cylindrical vacuum-tight one-
compartment cell. A spiral-shaped Pt wire and a Ag wire as the counter and pseudoreference
electrodes were sealed into glass capillaries and fixed by Quickfit screws via standard joints. A
platinum electrode was introduced as the working electrode through the top port via a Teflon
screw cap with a suitable fitting. It was polished with first 1 μm and then 0.25 μm diamond
paste before measurements. The cell was attached to a conventional Schlenk line via a side arm
equipped with a Teflon screw valve, allowing experiments to be performed under an argon
+
-
atmosphere with approximately 5 mL of analyte solution. NBu4 B{C6H3(CF3)2}4 (0.1 mM)
was used as the supporting electrolyte. Referencing was done with addition of an appropriate
amount of decamethylferrocene (Cp*2Fe, E172 = -550 mV with respect to Cp2Fe) as an internal
standard to the analyte solution after all data of interest had been acquired. Representative sets
of scans were repeated with the added standard. Electrochemical data were acquired with a
computer controlled BASi CV50 potentiostat.
The optically transparent thin-layer electrochemical (OTTLE) cell was also custom-built
according to the design of Hartl et al.[33] It consists of a Pt working and counter electrode and a
thin silver wire as a pseudoreference electrode sandwiched between two CaF2 windows of a
conventional liquid IR cell. The working electrode is positioned in the centre of the
spectrometer beam. Electron paramagnetic resonance (EPR) studies were performed on a
MiniScope MS 400 table-top X-band spectrometer from Magnettech.
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X-ray diffraction analysis was performed at 100 K on a STOE IPDS-II diffractometer equipped
with a graphite-monochromated radiation source (λ = 0.71073 Å) and an image plate detection
system. A yellow, blocky crystal of 1b-O2 obtained from slow evaporation of a solution of this
complex in CH2Cl2 was mounted on a fine glass fiber with silicon grease. The selection,
integration, and averaging procedure of the measured reflection intensities, the determination
of the unit cell dimensions and a least-squares fit of the 2θ values as well as data reduction, LP-
correction and space group determination were performed using the X-Area software package
delivered with the diffractometer. A semiempirical absorption correction was performed.[34]
The structure was solved by the heavy-atom method. Structure solution was completed with
difference Fourier syntheses and full-matrix last-squares refinements using SHELX-2017[35]
and OLEX2[36], minimizing ω(Fo2 – Fc2)2. The weighted R factor (wR2) and the goodness of
the fit GOOF are based on F2. All non-hydrogen atoms were refined with anisotropic
displacement parameters, while hydrogen atoms were introduced in a riding model.
Crystallographic data (excluding structure factors) for the structure of 1b-O2 have been
deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road,
Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the
depository number CCDC-197352 (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk,
http://www.ccdc.cam.ac.uk).
The ground state electronic structures of the full complexes were calculated by density
functional theory (DFT) methods using the GAUSSIAN 09 program package.[37] Geometry
optimizations were performed without any symmetry constraints. Open shell systems were
calculated by the unrestricted Kohn-Sham approach (UKS). Geometry optimization followed
by vibrational analysis was performed in solvent media. Solvent effects were described by the
polarizable continuum model (PCM) with standard parameters for 1,2-dichloroethane.[38] For
Fe, the ten-electron quasirelativistic effective core potential (ECP) MDF10 was used,[39] and 6-
31G(d) polarized double- basis sets[40] were employed together with the Perdew, Burke,
Ernzerhof exchange and correlation functional (PBE0).[41] The GaussSum program package
was used to analyze the results,[42] while the visualization of the results was performed with the
Avogadro program package.[43] Graphical representations of molecular orbitals were generated
with the help of GNU Parallel[44] and plotted using the vmd program package[45] in combination
with POV-Ray.[46] Electronic transitions were calculated by time-dependent DFT (TD-DFT)
method.
20
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Synthesis and characterization data
1a(OH)2
1.39 g (6.95 mmol, 4.2 eq.) of 4-bromo-N,N-
dimethylaniline were dissolved in 40 mL of dry
degassed tetrahydrofuran. 3.9 mL (1.9 M solution in
t
n-hexane, 7.3 mmol, 4.4 eq.) of BuLi were added
dropwise to the solution at -78 °C. After stirring for
15 minutes a solution of 500 mg (1.66 mmol, 1 eq.)
of dimethyl-1,1’-ferrocenedicarboxylate in 30 mL
of tetrahydrofuran was added to the reaction mixture at -78 °C. After stirring overnight with
slow warming to r. t., the reaction mixture was quenched with the addition of 40 mL of water
and the solvent was removed in vacuo. The residue was dissolved in Et2O and extracted with
water (3 x 50 mL). The aqueous layer was separated off and extracted with Et2O (3 x 100 mL).
The combined organic phases were washed with brine (3 x 20 mL), dried over MgSO4, and the
solvent was evaporated in vacuo. The dark green crude product was washed with 210 mL of
Et2O (1 x 150 mL, 3 x 20 mL) to yield 490 mg (0.678 mmol, 41 %) of 1a(OH)2 as a green
3
3
crystalline solid. 1H NMR (CDCl3, 400 MHz): δ 7.12 (d, JH-H = 8.8 Hz, 8H, H7), 6.62 (d, JH-
H = 8.8 Hz, 8H, H8), 4.15 (s, 4H, H1), 3.96 (s, 4H, H2), 3.84 (s, 2H, H5), 2.91 (s, 24H, H10)
13
ppm. C NMR (CDCl3, 101 MHz): δ 149.4 (C9), 138.8 (C6), 128.1 (C7), 111.7 (C8), 99.0
(C3), 77.7 (C4), 69.5 (C1), 68.1 (C2), 40.8 (C10) ppm.
-
1a2+  2 BArF24
1.100 g (1.01 mmol, 2.05 eq.) of Brookhart’s acid in
40 mL of dry degassed Et2O were added dropwise to
354 mg (0.49 mmol, 1 eq.) of 1a(OH)2 in 40 mL of
dry degassed Et2O at room temperature. The
reaction mixture turned from green to dark blue.
After stirring for 30 minutes the blue solution was
evaporated and the petrol-coloured solid was dried
in vacuo, yielding 1.18 g (0.489 mmol, quantitative) of 1a2+. 1H NMR (CD2Cl2, 400 MHz): δ
3
7.72 (s, 16H, BAr F24-CF3CHCF3), 7.57 (d, JH-H = 9.4 Hz, 8H, H6), 7.55 (s, 8H, BAr F24
-
CF3CHCBCHR’), 6.69 (d, 3JH-H = 9.4 Hz, 8H, H7), 5.17 (s, 4H, H1), 4.99 (s, 4H, H2), 3.15 (s,
24H, H9). 13C NMR (CD2Cl2, 101 MHz): δ 181.1 (C4), 161.79 (BArF24), 156.8 (C8), 139.3
(C6), 134.8 (BAr F24), 127.8 (C5), 117.5 (BAr F24), 112.6 (C7), 84.2 (C3), 79.2 (C2), 77.9 (C1),
21
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40.5 (C9) ppm. MS (ESI): calcd. for C44H48FeN4: 344.1607 [M]2+; found 344.1478. CHN-
analysis: calcd. for C108H72B2F48FeN4: C, 53.71 %; H, 3.00 %; N, 2.32 % N. Found: C, 52.79 %
; H, 2.29 %; N, 2.36 %.
1b(OH)2
Ferrocene (2.2 g, 12 mmol, 1.5 eq.) was dissolved in 15 mL of
dry THF and cooled to 0 °C. 5.5 mL (1.9 M solution in n-
hexane, 10.4 mmol, 1.3 eq.) of tBuLi were added dropwise to
the solution. After stirring for 15 minutes, a solution of 1.94 g
(8 mmol, 1 eq.) of 4,4′-dimethoxybenzophenone in 20 mL of
tetrahydrofuran was added to the reaction mixture at 0 °C.
After stirring for 15 minutes, the ice bath was removed and the yellow solution was stirred for
additional 25 minutes at r. t. before the reaction mixture was quenched with 30 mL of water
and the solvent was removed in vacuo. The residue was dissolved in Et2O and extracted with
water (3 x 50 mL). The aqueous layer was separated off and extracted with Et2O (3 x 100 mL).
The combined organic phases were washed with brine (3 x 20 mL), dried over MgSO4, and the
solvent was evaporated in vacuo. The residual solid was purified by column chromatography.
A petroleum ether/ethylacetate (PE/EA) 5/1 mixture eluted some monosubstituted (5-
C5H5)Fe(5-C5H4C(C6H4OMe-4)2OH (2aOH) and unreacted 4,4′-dimethoxybenzophenone).
With pure EA as eluent, the orange band of 1b(OH)2 was eluted. After solvent evaporation,
1b(OH)2 was obtained in the form of dark yellow crystals. The yield was 800 mg (1.2 mmol,
30 % with respect to 4,4′-dimethoxybenzophenone). 1H NMR (DMSOd6, 400 MHz): δ 7.07 (d,
3JH-H = 8.0 Hz, 8H, H4), 6.78 (d, 3JH-H = 8.0 Hz, 8H, H5), 6.02 (s, 2H, H3), 3.97 (d, 3JH-H = 2.7
Hz, 4H, H1), 3.91 (d, 3JH-H = 2.7 Hz, 4H, H1), 3.71 (s, 12H, H6) ppm. ESI-MS (CH3CN): calcd.
for C40H38FeO6: 670.2045 [M] +; found 670.1975.
-
1b2+  2 BArF24
320 mg (0.31 mmol, 2.05 eq.) of Brookhart’s acid in 5 mL of
dry degassed Et2O were added dropwise to 100 mg
(0.14 mmol, 1 eq.) of 1b(OH)2 in 5 mL of dry degassed Et2O
at r. t. The reaction mixture turned from yellow to dark green.
After stirring for 20 minutes the solvent was removed to obtain
1
1b2+ as a red solid in quantitative yield. H NMR (CDCl8,
3
400 MHz): δ 7.66 (s, 16H, BArF-CF3CHCF3), 7.55 (d, JH-H = 8.9 Hz, 8H, H3), 7.47 (s, 8H,
22
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BArF-CF3CHCBCHR’), 6.96 (d, 3JH-H = 8.9 Hz, 8H, H4), 5.45 (t, 3JH-H = 2.1 Hz, 4H, H1), 5.10
(t, 3JH-H = 2.1 Hz, 4H, H2), 3.92 (s, 12H, H5).
1c(OH)2
2.10 g (1.5 mL, 10.4 mmol, 4.2 eq.) of 1-bromo-4-
methoxy-1-methyl-benzene were dissolved in 200 mL
of dry degassed absolute ether (Et2O). 3.9 mL of a
t
1.9 M solution of BuLi in n-hexane (7.3 mmol,
4.4 eq.) were added dropwise to the solution at -78 °C.
After stirring for 15 minutes, a solution of 7.5 g
(2.48 mmol,
1 eq.)
of
dimethyl-1,1’-
ferrocenedicarboxylate in 200 mL of Et2O was added
to the reaction mixture at -78 °C. After stirring overnight, the reaction mixture was quenched
with 100 mL of water. The residue was dissolved in Et2O and extracted with water (3 x 50 mL).
The aqueous layer was separated off and extracted with Et2O (3 x 100 mL). The combined
organic phases were washed with brine (3 x 20 mL), dried over MgSO4, and the solvent was
evaporated in vacuo. The orange crude product was purified by recrystallization from Et2O to
yield 1.12 g (1.54 mmol, 62 %) of 1c(OH)2 as a yellow crystalline solid. 1H NMR (CDCl3, 400
MHz): δ 7.48 (broad s, 4H, H13), 6.65-6.72 (m, 4H, H12), 6.55-6.59 (m, 4H, H9), 4.18 (broad
s, 4H, H1), 4.00 (s, 4H, H2), 3.99 (s, 2H, H5), 3.78 (s, 12H, H10), 1.74 (s, 12H, H8) ppm. 1H-
NMR (toluene-d8, 400 MHz): δ 7.65 (broad s, 4H, H13), 6.58-6.63 (m, 8H, H9, H12), 4.24
(broad s, 4H, H1), 3.84 (s, 4H, H2), 3.75 (s, 2H, H5), 3.40 (s, 12H, H10), 1.89 (s, 12H, H8)
ppm. 13C NMR (CDCl3, 101 MHz): δ 158.5 (C6), 137.4 (C7), 129.9 (C13), 117.4 (C9), 109.8
(C12), 100.0 (C3), 78.1 (C4), 70.5 (C1), 68.5 (C2), 55.3 (C10), 22.1 (C8).
-
1c2+  2 BArF24
550 mg (0.51 mmol, 2.05 eq.) of Brookhart’s acid in 40 mL
of dry degassed Et2O were added dropwise to 179.4 mg
(0.25 mmol, 1 eq.) of 1c(OH)2 in 40 mL of dry degassed
Et2O at r. t. The colour of the reaction mixture changed
from yellow to dark yellow. After stirring for 30 minutes
the solvent was stripped off the ochre solution. The
resulting purple solid was dried in vacuo, yielding 590 mg
(0.246 mmol, quantitative) of 1c2+. Due to paramagnetic broadening it was not possible to
obtain meaningful NMR spectra. MS (ESI): calcd. for C44H44FeO4 346.1306 [M]2+; found
23
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10.1002/zaac.201900347
Zeitschrift für anorganische und allgemeine Chemie
346.1461 (measured in dichloromethane, non-calibrated). CH-analysis: calcd. for
C108H68B2F48FeO4: 53.62 % C, 2.83 % H; found: 52.71 % C, 3.42 % H.
1b-O2
-
Inside a glove box 1b2+  2 BArF24 (100 mg, 0.042 mmol, 1
eq.) was dissolved in 3 mL of degassed CH2Cl2 and 16 mg (0.084
mmol, 2 eq.) of cobaltocene were added. The mixture was stirred
for 10 min, then removed from the glove box, exposed to the air
and stirred at r. t. overnight. The solvent was evaporated under
vacuum and the residue was extracted with n-hexane.
Evaporation of the solvent gave 1b-O2 in quantitative yield. 1H-
3
3
NMR (400 MHz, C6D6): δ 7.87 (d, 4H, JHH = 8.85 Hz, H8), 7.35 (d, 4H, JHH = 8.85 Hz, H13),
6.90 (d, 4H, 3JHH = 8.85 Hz, H9), 6.52 (d, 4H, 3JHH = 8.85 Hz, H14), 5.11 (m, 2H, H1), 4.21 (m,
2H, H2), 4.03 (m, 2H, H4), 3.91 (m, 2H, H3), 3.33 (s, 6H, CH3-11), 3.21 (s, 6H, CH3-16). 13C NMR
(C6D6, 101 MHz): δ 159.3 (C10), 158.9 (C15), 136.9 (C12), 136.4 (C7), 130.3 (C8), 128.1
(C13), 113.5 (C14), 113.4 (C9), 94.2 (C5), 89.9 (C6), 75.1 (C4), 71.7 (C1), 70.2 (C2), 67.6
(C3), 54.8 (C11), 54.7 (C16). ESI-MS (CH3CN) calcd. for C40H36FeO6+H+ [MH] + 669.1915;
found 669.1925.
1b-HOH
Inside a glove box 1b2+  2 BArF24- (100 mg, 0.042 mmol, 1 eq.) was dissolved in 3 mL of degassed
CH2Cl2. 16 mg (0.084 mmol, 2 eq.) of
cobaltocene and 0.76 µL of water were added
and the mixture was allowed to stir at r. t. for
2 h. During this time the solution turned
green. The solvents were removed under
reduced pressure and the green solid was
extracted with n-hexane. Solvent was
stripped off the filtered extract and the solid
residue was purified by solvent chromatography (silica, PE/EA 5/1). The first fraction contained
small quantities of 6,6-diansiylfluorene. The second, orange band provided complex 1b-HOH after
solvent evaporation and washing with cold n-pentane.
1H-NMR (400 MHz, C6D6): δ 7.41 (d, 4H, 3JHH = 8.80 Hz, H15), 7.04 (d, 4H, 3JHH = 8.85 Hz,
H9), 6.76 (d, 4H, 3JHH = 8.80 Hz, H16), 6.74 (d, 4 H, 3JHH = 8.85 Hz, H10), 4.81 (s, 1H, H7),
24
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10.1002/zaac.201900347
Zeitschrift für anorganische und allgemeine Chemie
4.07 (m, 2H, H2), 4.03 (m, 2H, H1), 4.02 (m, 2H, H5), 3.92 (m, 2H, H6), 3.32 (s, 12H, H12,
13
H18), 3.01 (s, 1H, OH). C-NMR (100.61 MHz, C6D6) δ 158.6 (C17), 158.2 (C11), 140.4
(C14), 137.6 (C8), 129.6 (C9), 128.5 (C15), 113.5 (C10), 112.8 (C16), 100.2 (C4), 93.2 (C3),
76.9 (C13), 69.5 (C2), 69.4 (C5), 68.9 (C6), 68.2 (C1), 54.4 (C12, C18), 49.9 (C7). ESI-MS
(CH3CN) calcd. for C40H38FeO5: [M] + 654.2071; found: 654.1662; [M-OH]+ 637.2077; found:
637.1668.
Supporting Information (see footnote on the first page of this article): NMR spectra of the
complexes, UV/Vis NIR spectra of the reduced forms of complexes 1b2+ and 1c2+, ESI-MS of
complexes 1b-O2 and 1b-HOH, Tables with UV/Vis data in various solvents and details of
the crystallographic structure determination as well as the bond lengths and bond angles.
Keywords: ferrocene, tritylium, electrochemistry, spectroelectrochemistry, ferrocenophane,
peroxo compound, EPR spectroscopy, solvatochromism
Acknowlegements
We gratefully acknowledge financial support of this work by the Deutsche
Forschungsgemeinschaft (DFG, grant no. Wi1262/9-2). Larissa A. Casper thanks the
Studienstiftung des Deutschen Volkes for a PhD grant. We are also grateful to the DFG and
the State of Baden-Württemberg for providing us with access to the computational facilities of
the JUSTUS HPC facilities as a part of the bw_FOR network at the University of Ulm (grant
no. INST 40/467-1 FUGG). We also thank Eva Schiebel for the synthesis of compound 1b-
(OH)2 as a part of an internship.
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