Carbon-Rich Trinuclear Octamethylferrocenophanes

Article abstract of DOI:10.1021/acs.inorgchem.8b03389

Several trinuclear ferrocenes are obtained by Friedel-Crafts reaction of octamethylferrocene with ferrocenoyl chloride and subsequent modifications. 1,1′-Diferrocenoyloctamethylferrocene (3) is transformed to the divinyl derivative (4a) by reaction with MeLi and AlCl₃. The reactive 4a cyclizes spontaneously to a [4]ferrocenophane with buta-1,3-diene handle (5) or in the presence of AlCl₃ to a [3]ferrocenophane with propene handle (6). Structure assignments are supported by X-ray crystallography and NMR spectroscopy, and mechanisms are proposed. Electrochemical behavior of the compounds was investigated with cyclic voltammetry, and assignments of the redox processes were carried out with the aid of density functional theory calculations. The synthesized compounds and demonstrated transformations represent useful tools for preparation of materials for charge-transport studies in metal-molecule-metal junctions.

Full text of DOI:10.1021/acs.inorgchem.8b03389

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Carbon-Rich Trinuclear Octamethylferrocenophanes
Max Roemer,*,†,§ Duncan A. Wild,† Alexandre N. Sobolev,‡ Brian W. Skelton,‡ Gareth L. Nealon,‡
Matthew J. Piggott,† and George A. Koutsantonis*,†
‡
†Chemistry, M310, School of Molecular Sciences and Center for Microscopy, Characterization, and Analysis, The University of
Western Australia, 35 Stirling Hwy, Crawley, Western Australia 6009, Australia
S
* Supporting Information
ABSTRACT: Several trinuclear ferrocenes are obtained by Friedel−
Crafts reaction of octamethylferrocene with ferrocenoyl chloride and
subsequent modifications. 1,1′-Diferrocenoyloctamethylferrocene
(3) is transformed to the divinyl derivative (4a) by reaction with
MeLi and AlCl3. The reactive 4a cyclizes spontaneously to a
[4]ferrocenophane with buta-1,3-diene handle (5) or in the presence
of AlCl3 to a [3]ferrocenophane with propene handle (6). Structure
assignments are supported by X-ray crystallography and NMR
spectroscopy, and mechanisms are proposed. Electrochemical
behavior of the compounds was investigated with cyclic voltammetry,
and assignments of the redox processes were carried out with the aid
of density functional theory calculations. The synthesized compounds and demonstrated transformations represent useful tools
for preparation of materials for charge-transport studies in metal−molecule−metal junctions.
realized by oxidation of decamethylferrocene.39 Thus, 1
represents an attractive building block for compounds used
INTRODUCTION
■
The interest in ferrocenes shows no signs of abating. Since the
discovery of the archetypal organometallic complex,1,2
in molecular electronics applications. We recently reported the
diacetylation of octamethylferrocene40 and formation of
thousands of derivatives have been prepared with the number
of publications involving ferrocenes steadily rising to over 900/
ferrocenophanes from the 1,1′-bis(1-chlorovinyl)-deriva-
tive.40,41 1,1′-Divinyl- and -ethynylferrocenes are known to
undergo cyclizations to [4]ferrocenophanes, as demonstrated
for mononuclear derivatives bearing trifluorovinyl,42−44 1-
chlorovinyl,41 vinyl,45 and ethynyl substituents.41,46,47 Herein,
we report an efficient synthetic protocol for the preparation of
trinuclear octamethylferrocenes and cyclization reactions of
1,1′-bis(1-ferrocenylvinyl)-octamethylferrocene affording [3]-
and [4]ferrocenophanes.
year in the last five years.3 Increasingly, ferrocenes4 are finding
application in a number of fields such as sensing,5−9 cancer
treatment,10−12 materials applications,13−20 and in synthetic
methodology.21−25 Their excellent redox and chemical stability
and synthetic accessibility4 make ferrocenes compounds of
choice in metal−molecule−metal junctions.26−31 Charge-
transport studies using large-area self-assembled monolayer
devices revealed diode-type behavior for a number of
ferrocenes, with binuclear ferrocenes giving high molecular
rectification.32,33 Particularly, binuclear alkynyl-bridged con-
jugated ferrocenes immobilized on platinum surfaces led to
giant rectification of electric current.34 Macrocyclic polynuclear
ferrocenes have also been proposed for molecular electronics
applications.35,36 In this regard, the role of the synthetic
chemist is to provide access to molecular architectures that will
allow functional properties to be investigated. While syntheses
of substituted ferrocenes starting from the parent compound
are relatively straightforward,37,38 the syntheses of octame-
thylferrocene (1) (Scheme 1) derivatives are much less
established. The methyl groups of the tetramethylcyclopenta-
dienyl (Me4Cp) ligand impart higher solubility compared to
Cp, which is an important practical consideration for the
preparation of larger molecules. Derivatives of 1 are
significantly more electron rich than ferrocene, which results
in lowered HOMO/LUMO energy levels and redox potentials.
This in turn alters charge-transport behavior and chemical
reactivity. For instance, Fe(IV) oxidation states have been
RESULTS AND DISCUSSION
■
Trinuclear ferrocene (3) was prepared by the Friedel−Crafts
reaction of octamethylferrocene (1) and ferrocenoyl chloride
(2) (Scheme 1). Diketone 3 was converted into the 1,1′-
divinyl derivative (4a) by addition of MeLi in THF, followed
by elimination induced by AlCl3.
The optimized reaction in solution affords 4a in near
quantitative yield with a purity of ∼95%, which we used as
method for preparation of 4a throughout this study. If the
reaction is quenched prior to the addition of AlCl3, then the
diol 4c is generated in excellent yield, and incomplete reaction
resulted in the isolation of the monovinyl tertiary alcohol 4b.
Molecular structures of 3 and 4a−c derived from single crystal
X-ray diffraction experiments are shown in Figure 1. The
Received: December 5, 2018
© XXXX American Chemical Society
A
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Scheme 1. Friedel−Crafts Reaction of Octamethylferrocene (1) and Ferrocenylcarbonyl Chloride (2) and Subsequent
Transformations Provide Access to a Variety of Trinuclear Complexes with a Central Octamethylferrocene Moiety
Figure 1. Molecular structures as determined by X-ray single crystal diffraction for compounds 3 (a), 4a (b), 4b (c), and 4c·THF (d). H-atoms and
disorder are not shown for clarity. Atomic displacement ellipsoids are shown at a probability level of 50%.
crystal structure of 4c revealed it to be the racemic chiral
diasteromer. Presumably, the intermediate monolithiumalk-
oxide directs the second equivalent of MeLi stereoselectively to
the adjacent, sterically encumbered carbonyl group. Both 4b
and 4c can be converted almost quantitatively to 4a by
dehydration, and very minor amounts of 5 and 6 are also
B
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formed, achieved simply by heating in air above the melting
point.
When a solution of 4a in THF-d8 was treated with a drop of
deuterochloric acid in an NMR experiment, the spectra
indicated formation of the product but immediate broadening
of the signals was also observed suggesting the presence of
paramagnetic material. Treatment of complex 4a with a
catalytic amount of the oxidant DDQ in DCM led to an
immediate color change from yellow-orange to deep purple. 1H
NMR spectroscopy revealed an instantaneous and complete
consumption of 4a with formation of 5 as the major product,
along with minor amounts of the [3]ferrocenophane 6.
Based on the described observations, we propose a potential
mechanism for cyclization of 4a to 5, shown in Scheme 3.
Cyclization is initiated by oxidation of the core electron-rich
octamethylferrocene moiety in 4a to the corresponding
ferrocenium ion 4a+. It is worth noting that protonation
facilitates such oxidation,48,49 potentially explaining the
observed rate acceleration of the cyclization reaction in the
presence of acids.50−52
Single electron transfer (SET) from the vinylic π-orbital into
the Fe-centered SOMO initiates or facilitates (in a concerted
fashion as shown) a radical rearrangement in which the carbon
bridge is formed. Deprotonation of the resultant radical cation
(I1) gives rise to a radical (I2), which can undergo a second
single electron oxidation, either directly or via iron oxidation
and internal SET, to give carbocation I3. A final deprotonation
gives the product 5.
This can be observed visually when heating a crystal of 4b
using a melting point microscope, which initially results in gas
evolution (presumably H2O) followed by melting and
subsequent formation of crystalline 4a above the initial melting
point of 183−185 °C. Small-scale dehydration experiments for
4b and 4c revealed the formation of 4a after heating neat
samples to 200 °C for 5 min, indicating that the divinyl
derivative 4a is temperature and air stable as we did not
observe significant decomposition.
In solution, however, 4a is very reactive and undergoes an
unexpected, spontaneous cyclization to the [4]-octamethylfer-
rocenophane (5), bearing a buta-1,3-diene handle with
ferrocenyl groups in the two α-positions (Scheme 2).
Scheme 2. Cyclization of 4a to a [4]Ferrocenophane with
Butadiene Handle (5) and to a [3]Ferrocenophane with
a
Propene Handle (6) Bearing Two Ferrocenes
Intriguingly, increasing the amount of AlCl3 from 3 to 6
equiv after treating 3 with MeLi resulted in the formation of
significant quantities of [3]ferrocenophane 6, an isomer of 4a.
Indeed, 4a was converted to 6 when treated with AlCl3 in
THF, which shows that MeLi is not required for the reaction.
A similar reaction has been reported when 1,1′-bis-
(ferrocenoyl)ferrocene was treated with a large excess of
both MeLi and AlCl3 (6b, Scheme 2).53 In that case, the
isolation of a 1,1′-divinyl species was not reported, and a
mechanism involving nucleophilic addition of a methyl group
and subsequent deprotonation followed by rearrangement was
proposed. In light of the results presented here, it seems likely
that 1,1′-bis(1-ferrocenylvinyl)ferrocene is intermediate in the
formation of 6b. We propose the mechanism depicted in
Scheme 4 for the formation of 6, supported by the above-
mentioned observations. Intramolecular attack of a vinyl group
on the α-carbon is enabled by activation of the electrophilic π-
bond by aluminum coordination. The resulting zwitterionic
intermediate is stabilized by the electron-donating octame-
thylferrocene core. Deprotonation and proto-demetalation
results in generation of 6 bearing a methyl group in the α-
position. This transformation proceeds even in the presence of
small amounts of AlCl3, and as consequence 6 is always formed
in the process of generating 4a from 3 in solution. This side-
reaction could not be avoided, but controlling the quantity of
AlCl3 used and adding it slowly suppressed its formation,
affording 4a in a purity of ∼95%.
a
Compound 4a reacts on silica to the diol 7. Compound 6b
represents the non-methylated analogue of 6.
Ferrocenophane formation from 1,1′-alkenylferrocenes is
well-known, as described in the Introduction; however, most
reported reactions proceed by addition of a nucleophile to the
α-position of a vinyl or alkynyl group, which initiates ring-
closure and rearrangement steps.42−47 In the present case, it is
noteworthy that the transformation proceeds under formal loss
of one equivalent of H2 per cyclized molecule and thus is
oxidative.
The cyclization proceeds near quantitatively in polar
solvents like CHCl3, DCM or THF, even with the exclusion
of air and at low temperatures. Attempted recrystallization of
4a from either DCM or THF, under Schlenk conditions,
resulted in crystallization of 5 and 5·THF, respectively. We
monitored the cyclization by 1H NMR spectroscopy in
chloroform-d and benzene-d6. While near quantitative (95%)
conversion occurs in CDCl3 over 16 h (Figure 2) and is
complete in 1.5 days, the reaction in C6D6 is significantly
slower with only 6% conversion after 16 h and 21% conversion
after 3 days. This suggested that the greater polarity of this
solvent, relative to C6D6, and/or adventitious acid in the
CDCl3, accelerates the reaction rate. Indeed, while 4a is stable
upon heating for at least 1 h in THF/water, addition of a few
drops of dilute HCl resulted in an immediate color change
from orange to red and quantitative formation of 5.
In support of the mechanism presented in Scheme 4, ortho-
di(1-phenylvinyl)benzene and related compounds undergo
FeCl3-mediated cyclization to give indenes,54,55 structurally
analogous to 6. That reaction is also acid catalyzed.56 It seems
that in the case of the ferrocene 4a, acid-mediated oxidation to
the ferrocenium ion is faster than acid-catalyzed cyclization to
6.
Upon attempted chromatography of compound 4a on silica,
another ferrocenophane, the diol 7 (Scheme 2), was formed
and isolated in small amounts after selective crystallization
C
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1
Figure 2. Selected ranges of the H NMR spectra (500 MHz) in CDCl3 upon sample preparation (bottom) and standing for 16 h at room
temperature. Initially, the sample contains mostly 4a and minor amounts of 5 (bottom). After standing, nearly all 4a is consumed with 5 being the
only product.
gel did not trigger a reaction, hence ruling out hydration of 5 as
the source of 7. Similarly, 4a was stable in t-BuOH/water.
Ferrocenophanes bearing OH-groups in the α-positions were
proposed as transient intermediates arising from nucleophilic
addition of HO− to a trifluorovinyl group and subsequent
cyclizations of fluorovinylferrocenes.42,44 In the current case,
the α-position is sterically crowded by the ferrocenyl and
methyl groups, which likely prevents such a mechanism. The
most likely source of 7 is interception of the carbocation I1
(Scheme 3) by water, followed by oxidation and a second
similar reaction at the α′-position. While we obtained crystals
suitable for an X-ray single crystal diffraction experiment, by
selection under a microscope, we did not have a quantity
sufficient to provide a 13C NMR spectrum, and the sample
Scheme 3. Proposed Mechanism for Spontaneous
Cyclization of 4a to a [4]Ferrocenophane in Solution
1
contained impurities visible in the H NMR spectrum (SI,
Figure S66).
Molecular structures of all trinuclear ferrocenes herein
reported were elucidated by X-ray single crystal diffraction.
Molecular structures are shown in Figures 1 and 3. Details of
the X-ray crystal structure determinations are reported in the
SI (Tables S1 and S7). The Cp rings of the octamethylferro-
cene core of 5 and 4c are essentially nontilted (α < 1°, Table 1,
see Figure 4a for definition of α), whereas 6 shows a small tilt
of ∼6° for both molecules in the asymmetric unit due to the
shorter C3 handle. As expected, none of the terminal ferrocenes
exhibit a noteworthy tilt. The central ferrocene of 6b is
reported to have a tilt angle of 8.5°.53 The slightly smaller tilt
regarding of 6 and the essentially non-tilted structures of 5 and
7 are in line with our previous observation that octamethyl-
ferrocenophanes are less tilted compared to non-methylated
ferrocenophanes of similar structure,41 likely due to steric
repulsion between the methyl groups on opposing rings.
The diol 4c crystallizes with two different enantiomers in the
asymmetric unit as a THF tetrasolvate. H-bonds, intra-
Scheme 4. Proposed Mechanism for AlCl3 Catalyzed
Cyclization of 4a to the [3]Ferrocenophane 6
from a mixture containing 4a and 5. Treatment of 5 with
water/THF and t-BuOH/water mixtures and exposure to silica
D
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Figure 3. Representations of the molecular structures of ferrocenophanes 5 (a), 6 (b), and 7 (c) as determined by X-ray single crystal diffraction.
For 6, only one of two molecules in the asymmetric unit is shown. H-atoms are omitted for clarity. Atomic displacement ellipsoids are shown at a
probability level of 50%.
1
the H NMR spectra display an individual multiplet for each
H-atom of the two substituted Cp rings, with evidence for
appreciable coupling between all hydrogens of a given ring
displayed in the COSY spectrum of 6. The methyl groups of
the octamethylferrocene moieties and each ipso-C give rise to
an individual resonance in the 13C NMR spectra, again
consistent with the low overall symmetry of 4b and 6. The
vinyl protons of 4a and 4b give rise to two doublets with
geminal 1H−1H coupling constants of 2.5 Hz. The two vinylic
protons of butadiene 5 are chemically equivalent and
consequently appear as a singlet. Chemical shifts for the
vinylic protons of 4a/b appear in the expected region, around
6 ppm, and are shifted further downfield to 6.77 and 7.06 ppm
for 5 and 6, respectively.
Table 1. Tilt and Twist Angles of the Central Ferrocenes of
Compounds 4c−7
b
compound
tilt angle α (°) twist angle β (°) O−H distance (Å)
5.97(6)
4.79(6)
0.93(4)
6.22(12)
6.12(13)
8.5
25.0
16.2
15.6
7.2
6.2
6.4
1.934(2)
1.974(2)
a
4c
5
6
−
−
a
−
a
6b (ref 53)
8.4
5.9
10.3
7
0.57(14)
1.876(4)
a
b
The asymmetric unit contains two molecules. Intramolecular H-
bond.
We investigated the redox behavior of the novel compounds
by cyclic voltammetry (CV). CV data were acquired in two
different supporting electrolytes: tetrabutylammonium hexa-
fluorophosphate (THFP) and the weakly coordinating
tetrabutylammonium tetrakis(3,5-bis(trifluoromethyl)phenyl)-
borate (TBArF) in DCM. Weakly coordinating anions have
been used successfully in measurements of polynuclear
ferrocenes to suppress ion-pairing processes making the
observation of multiple redox processes efficacious.35,42,57
CVs are shown in Figure 5a (THFP) and 5b (TBArF),
respectively. Determined oxidation potentials are listed in
Table 2.
Figure 4. Tilt angles α (a) and twist angles β (b) of the
octamethylferrocene core of the trinuclear ferrocenophanes.
molecular between both OH groups (1.974(2) and 1.934(2)
Å), and intermolecular between OH and a THF molecule
(2.020(3) and 2.000(3)Å), are present. Due to the intra-
molecular H-bonds in the solid state, the molecule resembles a
[5]-ferrocenophane. The octamethylferrocene core is slightly
tilted (α = 5.97°(6), 4.79°(6)); however, the tilt is not toward
the handle as observed for 6, but away from it. This is
presumably due to repulsion between the two terminal
ferrocenyl groups. None of the central Cps are in ideally
eclipsed positions with twist angles β varying from 5.9 to 25.0°
(see Figure 4b for definition of β).
CVs show a clean redox wave with good reversibility for all
compounds for the first redox process (Fe(II) → Fe(III);
E1/2(1)) for both supporting electrolytes. This oxidation process
emanates from the octamethylferrocene cores. The electron-
donating methyl groups lower the oxidation potential of the Fe
center significantly compared to the non-methylated ferro-
cenes, thus the first oxidation occurs here. Density functional
theory (DFT) calculations support this assignment as all
HOMOs of the neutral species are centered here (Figure 6).
The HOMO of 5 is further distributed along the butadiene
handle toward the two terminal ferrocenes. The second and
third oxidation processes, E1/2(2) and E1/2(3), are assigned to the
metals at the terminal ferrocenes and appear at significantly
higher potentials. This in turn is further supported by the DFT
calculations, as the HOMOs of the cations of 3+, 4a+, and 5+
are centered at both outer ferrocenes (Figure 6) with
All of the new compounds displayed NMR spectra
consistent with their molecular structures. Compounds 3
(Figures S13−S16, SI), 4a (Figures S23 and S24, SI), and 5
1
(Figures S45 and S46, SI) provide relatively simple H- and
13C NMR spectra due to the symmetry of the molecules. Cp
protons of the substituted terminal Cp rings give rise to two
pseudotriplets; unsubstituted Cp rings give rise to one singlet,
and the methyl groups of the central octamethylferrocene are
pairwise equivalent giving two singlets. In contrast, the NMR
spectra of compounds 4b (Figures S25−S29, SI) and 6
(Figures S52−S55, SI) are complicated. Due to the asymmetry,
distribution along the handle for 5+. E1/2 occurs for 3 at
(1)
−151/−178 mV (THFP/TBArF), which is significantly higher
compared to all other compounds (−336 to −524 mV) in the
E
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Figure 5. (a) CVs of trinuclear ferrocenes 3, 4a, 4c, 5, and 6 recorded in 0.1 M THFP in DCM. The analyte concentration was 3 mM (3) and 1
mM (4a, 4c, 5, and 6). (b) CVs of 3, 4a, 4c, 5, and 6 recorded in 0.06 M TBArF in DCM. The analyte concentration was 1 mM. All shown CVs
were recorded at a scan rate of 100 mV/s.
Table 2. Summary of Oxidation Potentials As Determined by CV for Compounds 3−6 using THFP and TBArF Supporting
a
Electrolytes
(1)
(2)
(3)
(1−2)
(2−3)
ΔE
1/2
compound
SE
E1/2
E1/2
E1/2
ΔE1/2
575
b
3
THFP
TBArF
THFP
TBArF
THFP
TBArF
THFP
TBArF
THFP
TBArF
THFP
THFP
−178
−151
−482
−524
−480
−500
−336
−343
− 415
− 506
−110
397
648
56
−
c
c
970
266
799
538
624
546
661
498
636
534
733
260
400
322
210
887
−
367
−
324
−
194
120
−
d
4a
4c
5
c
c
100
987
b
66
c
161
528
b
162
c
293
617
b
6
119
227
150
421
270
−70
1,1′-Fc(FcCCH2)2 (ref 58)
1,1′-Fc(CH2OMFc)2 (ref 58)
b
−470
a
b
All potentials are reported in mV and vs the Fc/Fc+ couple. SE: supporting electrolyte, OMFc: octamethylferrocene. The two processes are not
c
d
resolved. Redox waves not well resolved. Reported values are estimates. Irreversible oxidation; reported value is peak potential of oxidation wave.
series and attributed to the presence of the two electron-
moiety and two outer octamethylferrocenes connected via
methylene bridges was reported to show two processes, at
−470 mV and −70 mV, respectively. The first oxidation was
assigned to the simultaneous oxidation of both outer
octamethylferrocenes and the second to the central ferro-
withdrawing carbonyl groups. Notably, the first oxidation of
(1)
ferrocenophanes 5 and 6 E1/2 occurs at elevated potentials,
+146/181 mV and +67/18 mV (THFP/TBArF), respectively,
compared to the noncyclized 4a, while 5 is of nearly identical
formula with two H-atoms less and 6 is an isomer of 4a. A non-
methylated counterpart of 4a was reported to exhibit three
redox processes at −110 mV, 150 mV, and 270 mV, and the
central ferrocene moeity was assigned to the first oxidation.58
cene.58 The processes E1/2
and E1/2
show good
(2)
(3)
reversibility for 3, 4c, 5, and 6 using the HFP-based electrolyte,
but the two processes ( E1/2(2,3)) are not well resolved. The
(2)
(3)
CVs of 3 and 6 show only one wave for E1/2 and E1/2
,
(1)
(2)
The redox splitting between E1/2
and E1/2
for all
while in the case of 4c and 5 we observed shoulders, which
compounds is high (528−799 mV), compared to a bis-
octamethylferrocenophane (202 mV) we recently reported,41
and a binuclear 3,3′-di-tert-butyl-ferrocenophane bearing a
partially fluorinated n-butylene handle with a ferrocene in the
α-position (296 mV)44 and other polynuclear ferrocenes.59,60
This originates in the different chemical environment of the
iron centers (Cp vs tetramethyl Cp ligands). In line with our
observation, a trinuclear complex bearing a central ferrocene
indicates two processes in close proximity. Using the TBArF
(2)
supporting electrolyte resulted in decoupling of E1/2 and
E1/2(3) for 3, 4c, and 5, giving a clean redox wave for E1/2(2) for
(3)
4c and 5, while the wave attributed to E1/(22) is broadened.
(3)
Regarding 3, both waves attributed to E1/2 and E1/2 are
broadened. The reactive divinyl derivative 4a exhibits
irreversible oxidations for E1/2(2)/E1/2(3). Repeated scans led
to visible darkening of the solution and change of shape of the
F
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Figure 6. HOMOs of compounds 3, 3+, 4a, 4a+, 4c, 5, 5+, and 6 obtained from DFT calculations. The 6-31G* basis sets were used for lighter atoms
(H, C, O), while the LANL2DZ basis set incorporating an effective core potential was used for iron. All surfaces depicted were constructed using an
isosurface value of 0.02. All HOMOs in the neutral state are centered at the octamethylferrocene core, while the HOMO of 5 is distributed along
the handle toward the outer ferrocenes. The HOMOs of cations 3+, 4a+, and 5+ are located at both outer ferrocenes with distribution along the
handle for 5+.
solvents were distilled under reduced pressure prior to use.
Octamethylferrocene was prepared as reported previously.40 Other
compounds were purchased from Sigma-Aldrich and used as received.
NMR spectra were recorded on Bruker AVIII HD 500 and AVIII HD
600 spectrometers. Chemical shifts of residual solvent signals were
used as internal references.62 Mass spectra were recorded on a Waters
LCT Premier mass spectrometer in ESI+ or APCI+ modes. Elemental
analyses were obtained from the Elemental Analysis Service of the
London Metropolitan University. Infrared spectra were recorded on a
PerkinElmer Spectrum One FT-IR spectrometer equipped with an
ATR sampling accessory. Melting points were determined using a
Reichert melting point microscope.
Syntheses. FcCOOH. A Schlenk flask was charged with 2.0 g (10.8
mmol) ferrocene, 121 mg (1.1 mmol) t-BuOK, and 100 mL THF and
cooled to −78 °C. t-BuLi (15.8 mL, 26.9 mmol; 1.7 M in pentane)
was added dropwise over 30 min and stirring at −78 °C was
continued for 1.5 h, affording an orange precipitate. An excess of dry
ice (15 g) was added, and the mixture was allowed to warm up to rt.
The mixture was treated with 150 mL aqueous HCl (5%), and the
organic phase was collected concentrated to dryness. The solids were
curve, indicating electrochemically induced decomposition and
demonstrating the sensitivity of the compound toward
oxidation. Scan rate-dependent measurements of 3 in TBArF
(2)
showed significantly less well resolved redox-waves for E1/2
/
(3)
E1/2 that were at faster scan rates (Figure S1, SI), while
repeated scans using the THFP electrolyte likely led to
poisoning of the working electrode. On the contrary, repeated
scan rate-dependent measurements indicate good reversibil-
ity61 (ipa/ipc ≈ 1) for compounds 4c, 5, and 6 using both
electrolytes (Figure S2−S7, SI). To summarize, the use of the
BArF supporting electrolyte allows decoupling from E1/2(2) and
E1/2(3), but at the expense of broadening of E1/2(3), and for the
(2)
(3)
case of 3 both E1/2 and E1/2
.
CONCLUSIONS
■
In summary, we developed viable synthetic methods for
preparation of several trinuclear ferrocenes with a central
octamethylferrocene moiety. The divinyl derivative 4a can be
obtained in quantitative yield by different pathways. It is a
highly reactive precursor, readily cyclizing to two different
types of ferrocenophanes according to the reaction conditions.
Oxidative formation of a ferrocenophane with a buta-1,3-diene
handle proceeds by a new cyclization mechanism. The
ferrocenophanes show excellent redox and chemical stability,
which makes them attractive building blocks for metal-
molecule-metal junctions in molecular electronics applications.
suspended in 30 mL hexanes, collected using a Buchner funnel, and
̈
washed with 30 mL hexanes. Drying under high vacuum afforded
2.244 g (9.8 mmol, 91%) of ferrocenecarboxylic acid (FcCOOH) as
pale orange solid. Analytical data were in agreement with the
literature.63
3. FcCOOH (848 mg, 3.7 mmol) was suspended in 60 mL
dichloromethane in a Schlenk flask. Oxalyldichloride (0.94 mL, 3
equiv) and several drops of DMF were added under stirring. The
solids dissolved within a few minutes, resulting in a deep red solution.
The solution was stirred for 30 min, and the solvents were removed
under vacuum to afford ferrocenecarboxylic acid chloride (2) as deep
red solid, which was redissolved in 60 mL dichloromethane.
Octamethylferrocene (500 mg, 1.7 mmol) was added, and the
mixture was cooled to −50 °C. Aluminum(III) chloride (541 mg, 4.1
mmol) was added slowly over several portions. The temperature was
allowed to raise to −30 °C, and stirring was continued for 2 h before
EXPERIMENTAL SECTION
■
General. Reactions were performed under Ar atmosphere using
standard Schlenk techniques unless otherwise stated. THF was
distilled from sodium/benzophenone. Dichloromethane was purified
using an Innovative Technologies solvent purification system. Other
G
DOI: 10.1021/acs.inorgchem.8b03389
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the mixture was allowed to warm up to room temperature. Water was
added, and the phases were separated. The organic layer was dried
over sodium sulfate and concentrated to dryness. The crude red solids
were subject to column chromatography on silica using initially
dichloromethane/ethyl acetate (99:1) and slowly increasing the
amount of ethyl acetate to a ratio of 10:1. The deep red main fraction
was concentrated to dryness and dried under high vacuum, affording
731 mg (0.10 mmol, 60%) of the product as deep red solid. 1H NMR
(600 MHz, CD2Cl2): δ = 4.58 (m, 4H, Cp, H6), 4.37 (m, 4H, Cp,
H7), 4.09 (s, 10H, Cp, H8), 1.83 (s, 12H, CH3, H1a), 1.80 (s, 12H,
CH3, H2a) ppm. 13C NMR (150 MHz, CD2Cl2): δ = 202.8 (C4),
84.8 (C3), 83.5 (C5), 83.5 (C2), 81.9 (C1), 71.3 (C7), 71.2 (C6),
70.0 (C8), 11.4 (C1a), 9.5 (C2a) ppm. IR (ATR): 3102 (w), 3088
(w), 2972 (w), 2952 (w), 2897 (m), 1779 (w), 1739 (w), 1714 (w),
1651 (m), 1631 (s), 1439 (s), 1379 (s), 1387 (s), 1371 (m), 1335
(m), 1254 (s), 1207 (w), 1150 (w), 1108 (m), 1083 (m), 1050 (w),
1028 (s), 1007 (m), 956 (w), 935 (w), 887 (w), 877 (w), 850 (m),
839 (m), 819 (s), 755 (m), 726 (m), 659 (m), 615 (w), 596 (w), 561
(m), 542 (w), 533 (w). Elemental analysis: calcd for C40H42Fe3O2: C
66.51; H 5.86; found C 66.37; H 5.83. MS (ESI+): 723 (M+ + 1,
100%), 470 (20%), 242 (40%). HRMS: calcd for C40H43O2Fe3:
723.1311; found: 723.1318. Mp: 235−238 °C.
chromatography on silica using hexane/EtOAc (9:1), afforded 42
mg (0.06 mg, 41%) of the title compound as yellow-orange crystalline
1
solid after solvent evaporation and drying under high vacuum. H
NMR (600 MHz, C6D6): δ = 6.18 (d, J = 2.5 Hz, 2H, CCH2), 6.14
(d, J = 2.5 Hz, 2H, CCH2), 4.45 (m, 1H, Cp), 4.22 (m, 1H, Cp),
4.08 (s, 5H Cp; one Cp-H concealed), 3.97 (s, 5H Cp), 3.96 (m, 1H,
Cp), 3.95 (m, 1H, Cp), 3.90 (m, 1H, Cp), 3.84 (m, 1H, Cp), 3.82 (m,
1H, Cp), 2.68 (s, 1H, OH), 2.12 (s, 3H, CH3, H7), 2.11 (s, 3H, CH3),
1.87 (s, 3H, CH3), 1.82 (s, 3H, CH3), 1.81 (brd, 6H, CH3), 1.75 (s,
3H, CH3), 1.55 (s, 3H, CH3), 1.54 (s, 3H, CH3) ppm. 13C NMR (150
MHz, CDCl3): 143.1 (C20), 115.9 (C19), 102.3 (C8), 92.9 (C5),
88.8 (C18), 88.0 (C21), 80.7, 80.5, 79.7, 79.6, 79.5, 79.0, 78.1, 76.5,
72.8 (C6), 70.1, 68.84, 68.82, 68.77, 68.1, 68.0, 67.4, 67.23, 67.20,
66.4, 32.6 (C7), 13.2, 12.7, 12.3, 11.7, 10.3, 10.1, 9.9, 9.8 ppm. IR
(ATR): 3577 (w), 3092(w), 2965(m), 2939(w), 2902(m), 1759(w),
1738(w), 1687(w) 1614(w), 1472(w), 1455(m), 1426(m), 1411(m),
1374(m), 1332(w), 1290(w), 1262(m), 1209(w), 1182(w), 1160(w),
1104(s), 1080(m), 1060(m), 1023(s), 1000(s), 925(w), 913(w),
891(m), 880(m), 862(w), 811(s), 731(m), 697(m), 674(w), 640(w),
613(w), 584(w), 572(w), 555(w), 541(w, 532(w), 522(w). MS
(ESI): 736 (M+, 100%). Elemental analysis: calcd for C42H48Fe3O: C
68.51, H 6.57; found: 68.43, H 6.71. HRMS: calcd for C42H49Fe3O:
737.1831; found: 737.1832. Mp: 183−185 °C, melting with
conversion to 4a and subsequent crystallization.
4c. Compound 3 (25 mg, 0.035 mmol) was dissolved in 2 mL THF
in a Schlenk tube. Methyllithium (0.05 mL, 0.087 mmol; 2.5 equiv)
was added dropwise at 0 °C. After 20 min, the mixture was allowed to
warm up to room temperature, and 5 mL of water was added. The
mixture was extracted with 10 mL ethyl acetate, and the organic layer
was collected and dried over sodium sulfate. The yellow solution was
filtered through a PTFE syringe filter (pore size: 0.45 μm), and the
solvents were removed using a rotary evaporator, affording a yellow-
orange oil which immediately crystallized. Drying under high vacuum
afforded 24 mg (0.032 mmol, 92%) of the title compound as yellow-
orange crystalline solid. 1H NMR (600 MHz, C6D6): δ = 4.73 (m, 2H,
Cp), 4.35 (m, 2H, OH), 4.10 (s, 10H, H13), 3.90 (m, 2H, Cp), 3.84
(m, 2H, Cp), 3.80 (m, 2H, Cp), 2.25 (s, 6H, H7), 2.20 (s, 6H, CH3),
1.87 (s, 6H, CH3), 1.47 (s, 12H, CH3) ppm. 13C NMR (150 MHz,
CDCl3): δ = 101.7 (C8), 92.2 (C5), 80.9 (ipso-C), 80.6 (ipso-C), 78.4
(ipso-C), 75.9 (ipso-C), 72.9 (C6), 68.9 (C13), 67.9, 67.2, 66.8, 65.6,
32.8 (C7), 13.8, 12.7, 9.7, 9.5 ppm. IR (ATR): 3617(w), 3566(w),
3420(w), 3085(w), 2998(w), 2956(m), 2906(m), 2187(w), 2168(w),
2050(w), 1981(w), 1769(w), 1716(w), 1634(w), 1479(m), 1464(m),
1409(m), 1377(s), 1360(m), 1341(m), 1310(m), 1262(w), 1236(w),
1225(w), 1161(w), 1105(s), 1085(m), 1052(m), 1023(s), 998(s),
950(w), 926(m), 890(m), 859(w), 817(s), 732(w), 708(w), 697(w),
666(w), 623(w), 615(w), 597(w), 589(w), 579(w), 568(w), 551(w),
539(w), 532(w), 524(w), 518(w). Elemental analysis: calcd for
C42H50Fe3O2·1 EtOAc: C 65.58, H 6.94; found: 65.34, H 6.66. MS
(ESI): 754 (M++H, 100%), 737 (83%), 719 (26%). HRMS: calcd for
C42H50Fe3O2: 754.1859; found: 754.1860. Mp: 160−162 °C.
4a. A Schlenk tube was charged with 50 mg (0.07 mmol) of
compound 3 and 4 mL of THF. The solution was cooled to 0 °C and
0.11 mL (0.17 mmol, 2.5 equiv) of methyllithium (1.6 M in diethyl
ether) were added dropwise over two min. Stirring at 0 °C was
continued for 0.5 h before 28 mg (0.21 mmol) of aluminum(III)
chloride, dissolved in 2 mL THF, was added slowly over 1 h using a
syringe pump. The mixture was allowed to warm up to room
temperature, and stirring was continued for another hour. Degassed
water (5 mL) was added, and the product was extracted with 10 mL
dichloromethane, affording a deep orange extract. The solution was
dried over sodium sulfate, filtered through a PTFE-syringe filter, pore
size 0.45 μm, and concentrated to dryness under high vacuum,
affording 50 mg (0.07 mmol, ∼98%) of an orange crystalline solid
with a purity of >95%. The only impurities present were found to be
trace amounts of ferrocenophanes 5 and 6. Attempted recrystalliza-
tion from dichloromethane and THF at −10 °C resulted both in
conversion to ferrocenophane 5 and crystallization of it. Sublimation
under vacuum (160 °C, 0.05 * 10−3 mbar) resulted in sublimation of
both compound 4a and the minor amounts of 5/6. Attempted
purification column chromatography on silica did not result in
separation of the compounds, but significant oxidative decomposition,
manifested in a color change to deep green, occurred. Alternatively,
we obtained 4a by heating 4b or 4c for 5 min to 200 °C. The reported
analytical data were obtained from a sample prepared in solution as
described above. Samples prepared by heating 4b and 4c were
characterized by 1H NMR spectroscopy, which matched the data
1
reported below. H NMR (C6D6): δ = 6.05 (d, J = 2.5 Hz, 2H, C
CH2), 6.00 (d, J = 2.5 Hz, 2H, CCH2), 4.12 (pst, 4H, Cp), 3.98 (s,
10H, Cp), 3.95 (pst, 4H, Cp), 1.81 (s, 12H, CH3), 1.75 (s, 12H,
CH3) ppm. 13C NMR (C6D6): δ = 141.6 (CCH2), 116.3 (C
CH2), 89.9, 87.8, 79.1, 70.0, 68.5, 68.1, 11.3 (CH3), 9.8 (CH3) ppm.
IR (ATR): 3094 (w), 3080(w), 2921(s), 2853(m), 2717(w),
1757(w), 1660(w), 1621(m), 1588(w), 1572(w), 1462(m),
1446(m), 1429(m), 1411(m), 1377(m), 1361(m), 1336(w),
1317(w), 1262(m), 1212(w), 1192(m), 1183(w), 1153(w),
1105(s), 1023(s), 1000(s), 914(m), 886(m), 849(m), 808(s),
770(m), 739(m), 719(w), 696(m), 674(w), 645(w), 620(w),
613(w), 586(w), 567(w), 552(m), 543(m), 531(w), 518(w).
Elemental analysis: calcd for C42H46Fe3: C 70.22, H 6.45; found: C
70.11, H 6.45. MS (ESI+): 718 (M+ + H, 100%). HRMS: calcd for
C42H46Fe3: 718.1648; found: 718.1649. Mp: 225−233 °C.
5. A round-bottom flask was charged with 50 mg (0.07 mmol) of 4
and 10 mL of degassed chloroform. The solution was stirred for 16 h
at room temperature. Solvent evaporation and drying under high
vacuum gave a red solid which was flash filtered through a plug of
silica using dichloromethane as eluent. Solvent evaporation and drying
under high vacuum afforded 37 mg (0.05 mmol, 74%) of the title
compound as red crystalline material. 1H NMR (600 MHz, C6D6): δ
= 6.77 (s, 2H, H5), 4.22 (pst, 6H, H8), 4.04 (s, 10H, H9), 4.01 (pst,
6H, H7), 1.87 (s, 12H, CH3), 1.69 (s, 12H, CH3) ppm. 13C NMR
(150 MHz, C6D6): δ = 138.7 (C4), 122.8 (C5), 91.1 (C6), 81.5 (C3),
81.3 (C1), 77.7 (C2), 70.0 (C9), 68.7 (C7), 67.7 (C8), 11.1 (CH3),
9.7 (CH3) ppm. IR (6 × THF, ATR): 3088 (w), 2967(m), 2938(m),
2904(m), 2885(m), 1721(w), 1645(w), 1609(w), 1565(w), 1472(w),
1455(m), 1436(m), 1413(m), 1385(w), 1375(m), 1315(w),
1289(w), 1252(2, 1243(w), 1207(w), 1174(w), 1158(w), 1107(m),
1067(s), 1029(s), 1002(s), 966(w), 907(m), 870(m), 852(m),
812(s), 772(w), 730(w), 702(w), 684(w), 666(w), 658(w),
639(w), 614(w), 598(w), 589(w), 574(w), 561(w), 547(w),
529(w), 518(w). Elemental analysis: calcd for C42H44Fe3·2/3 DCM:
4b. A Schlenk tube was charged with 100 mg (0.14 mmol) of
compound 3 and 8 mL of THF. The solution was cooled to 0 °C, and
0.22 mL (0.34 mmol, 2.5 equiv) of methyllithium (1.6 M in diethyl
ether) was added dropwise over 2 min. Stirring at 0 °C was continued
for 0.5 h before 28 mg (0.21 mmol) of aluminum(III) chloride,
dissolved in 2 mL THF, was added over 1 h using a syringe pump.
Workup analogously to preparation of 4a, followed by flash
H
DOI: 10.1021/acs.inorgchem.8b03389
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
C 66.30, H 5.91; 66.24, H 5.58. MS (APCI+): 717 (M+ + H, 17%),
676 (5%), 426 (43%), 379 (9%), 338 (100%). HRMS: calcd for
C42H45Fe3: 717.1569 found: 717.1570. Mp: 272−290 °C.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03389.
■
S
6. A Schlenk tube was charged with 20 mg (0.03 mmol) of 4a and
2 mL THF. Aluminum(III) chloride 7 mg (0.06 mmol), dissolved in 1
mL THF, was added under stirring at 0 °C. After 0.5 h the mixture
was allowed to warm up to room temperature, and stirring was
continued for another hour. Water was added, mixture was extracted
with dichloromethane, and the organic phase was dried over sodium
sulfate. Solvent evaporation afforded a yellow-orange solid. The
solvent was evaporated to dryness, and the solid was washed with
several small portions of pentane. Drying under high vacuum afforded
11 mg (0.02 mmol, 55%) of the title compound as pale orange solid.
1H NMR (600 MHz, C6D6): δ = 7.06 (s, 1H, H7), 4.55 (m, 1H, Cp),
4.50 (m, 1H, Cp), 4.34 (m, 1H, Cp), 4.222 (s, 5H, Cp), 4.220 (s, 5H,
Cp), 4.11 (m, 1H, Cp), 4.06 (m, 1H, Cp), 4.03 (m, 1H, Cp), 4.01(m,
1H, Cp), 3.96 (m, 1H, Cp), 2.15 (s, 3H, H9), 1.94 (s, 3H, CH3), 1.81
(s, 3H, CH3), 1.67 (s, 3H, CH3), 1.66 (s, 3H, CH3), 1.61 (s, 3H,
CH3), 1.59 (s, 3H, CH3), 1.47 (s, 3H, CH3), 1.21 (s, 3H, CH3) ppm.
13C NMR (150 MHz, C6D6): δ = 144.9 (C7), 129.7 (C6), 103.6
(C15), 89.3 (C21), 86.6 (C14), 84.0 (C5), 82.1, 82.1, 81.8, 81.4,
80.4, 79.8, 79.8, 70.2, 69.7, 69.2, 68.9, 68.2, 68.2, 68.01, 68.00, 67.7,
65.1, 40.1 (C8), 33.2 (C9), 14.5, 13.1, 11.17, 11.16, 10.3, 10.1, 10.0,
9.9 ppm. MS (ESI+): 718 (M+ + H, 100%). HRMS: calcd for
C42H46Fe3: 718.1648; found: 718.1649. Elemental analysis: calcd for
C42H46Fe3: C 70.22, H 6.45; found: 70.15, H 6.34. IR (ATR): 3359
(w), 3087(w) 2953(m), 2904(m), 2855(m), 2725(w), 1714(w),
1653(w), 1633(w), 1455(m), 1444(m), 1427(m), 1411(m), 1378(s),
1308(w), 1261(w), 1250(w), 1223(w), 1185(w), 1158(w), 1106(s),
1085(m), 1049(m), 1027(s), 1018(s), 1000(s), 958(w), 927(w),
907(w), 876(m), 863(w), 813(s), 758(w), 737(w), 706(w), 696(w),
682(w), 670(w), 621(w), 601(w), 589(w), 582(w), 564(w), 537(w),
521(w). Mp: decomposition ∼290 °C.
Crystallographic information files in cif-format for
compounds 3−7 (CCDC 1837183−1837188,
1848108, and 1848109), details from the X-ray structure
determinations, NMR numbering schemes and spectral
data. Molecular graphics: OLEX2 (ref 66) (PDF)
Accession Codes
CCDC 1837183−1837188 and 1848108−1848109 contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: max.roemer@mq.edu.au.
*E-mail: george.koutsantonis@uwa.edu.au.
ORCID
Matthew J. Piggott: 0000-0002-5857-7051
George A. Koutsantonis: 0000-0001-8755-3596
■
Present Address
§Department of Molecular Sciences, Macquarie University,
Sydney, New South Wales 2109, Australia
Notes
7. Preparation of compound 5 as described above and attempted
column chromatography resulted in collection of a mixture of 5 and 7
from the deep red main fraction. Crystals of both compounds were
obtained by slow evaporation of a mixture of pentane/dichloro-
methane at room temperature. Diol 7 crystallized in form of yellow
platelets, while crystals of 5 are red. Yellow platelets were collected
under the microscope and used for X-ray structure analysis and
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported under Australian Research
Council’s Discovery Projects funding scheme (project number
DP 150104117). We acknowledge the facilities and the
scientific and technical assistance of the Australian Microscopy
and Microanalysis Research Facility at the Center for
Microscopy, Characterization and Analysis, The University of
Western Australia, a facility funded by the University, State,
and Commonwealth Governments.
1
characterization. H NMR (C6D6): δ = 4.37 (m, 2H, Cp), 4.22 (m,
2H, Cp), 4.05 (s, 10H, Cp), 3.95 (m, 2H, Cp), 3.86 (m, 2H, Cp),
3.44 (s, 2H, OH), 3.12 (m, 2H, CH2), 2.70 (m, 2H, CH2), 2.49 (s,
6H, CH3), 1.57 (s, 6H, CH3), 1.40 (s, 6H, CH3), 1.31 (s, 6H, CH3)
ppm. MS (ESI): 752 (M+ + H, 100%). HRMS: calcd for
C42H49Fe3O2: 753.1781; found: 753.1805. Mp: decomposition
∼275 °C.
Electrochemistry. Cyclic voltammetry measurements were
performed on a Princeton Applied Research VersaSTAT 3
potentiostat using a three-electrode setup with Pt working electrode
and platinum-coated titanium rods as counter and pseudoreference
electrodes. We used tetrabutylammonium hexafluorophosphate
(THFP) and tetrabutylammonium tetrakis(3,5-bis(trifluoromethyl)-
phenyl)borate (TBArF) as supporting electrolytes in concentration of
0.1 and 0.06 M in dichloromethane, respectively. We used either
ferrocene or decamethylferrocene (DMFc) as internal standards and
converted all potentials to the ferrocene/ferrocenium couple
(E1/2(DMFc) = E1/2(Fc) − 480 mV).57
DFT Calculations. DFT calculations were undertaken using the
familiar B3LYP functional, with the incorporation of the GD3
empirical dispersion correction of Grimme and co-workers.64 The 6-
31G* basis sets were used for lighter atoms (hydrogen, carbon,
oxygen), while the LANL2DZ basis set incorporating an effective core
potential was used for iron to reduce the computational burden. All
calculations were undertaken using the Gaussian 09 program suite.65
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