Formation of unexpected silicon- and disiloxane-bridged multiferrocenyl derivatives bearing Si-O-CHCH2 and Si-(CH2)2C(CH3)3 substituents via cleavage of tetrahydrofuran and trapping of its ring fragments

Article abstract of DOI:10.1039/c7dt02286g

The formation of a family of silicon- and siloxane-bridged multiferrocenyl derivatives carrying different functional groups attached to silicon, including Fc₂(CH₃)₃C(CH₂)₂SiCHCH₂ (5), Fc₂(CH₂CH-O)SiCHCH₂ (6), Fc₂(OH)SiCHCH₂ (7), Fc₂(CH₂CH-O)Si-O-Si(O-CHCH₂)Fc₂ (8) and Fc₂(CH₂CH-O)Si-O-SiFc₃ (9) is described. Silyl vinyl ether molecules 6, 8 and 9 and the heteroleptic vinylsilane 5 resulted from the competing metathesis reaction of lithioferrocene (FcLi), CH₂CH-OLi or (CH₃)₃C(CH₂)₂Li with the corresponding multifunctional chlorosilane, Cl₃SiCHCH₂ or Cl₃Si-O-SiCl₃. The last two organolithium species have been likely formed in situ by fragmentation of the tetrahydrofuran solvent. Diferrocenylvinyloxyvinylsilane 6 is noteworthy since it represents a rare example of a redox-active silyl mononomer in which two different CC polymerisable groups are directly connected to silicon. The molecular structures of the silicon-containing multiferrocenyl species 5, 6, 8 and 9 have been investigated by single-crystal X-ray diffraction studies, demonstrating the capture and storage processes of two ring fragments resulting from the cleavage of cyclic THF in redox-active and stable crystalline organometallic compounds. From electrochemical studies we found that by changing the anion of the supporting electrolyte from [PF₆]^- to [B(C₆F₅)₄]^-, the redox behaviour of tetrametallic disiloxane 8 can be switched from a poorly resolved multistep redox process to four consecutive well-separated one-electron oxidations, corresponding to the sequential oxidation of the four ferrocenyl moieties.

Full text of DOI:10.1039/c7dt02286g

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

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Formation of Unexpected Silicon- and Disiloxane-Bridged
Multiferrocenyl Derivatives Bearing Si-O-CH=CH2 and
Si-(CH2)2C(CH3)3 Substituents via Cleavage of Tetrahydrofuran and
Trapping of its Ring Fragments
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Sonia Bruña,a,b Ana Mª González-Vadillo,a Marta Ferrández,a Josefina Perles,c M. Merced Montero-
Campillo,d Otilia Mó,b,d and Isabel Cuadrado*a,b
www.rsc.org/
The formation of a family of silicon- and siloxane-bridged multiferrocenyl derivatives carrying different functional groups
attached to silicon, including Fc2(CH3)3C(CH2)2SiCH=CH2 (5), Fc2(CH2=CH-O)SiCH=CH2 (6), Fc2(OH)SiCH=CH2 (7), Fc2(CH2=CH-
O)Si−O−Si(O-CH=CH2)Fc2 (8) and Fc2(CH2=CH-O)Si−O−SiFc3 (9) is described. Silyl vinyl ether molecules 6, 8 and 9 and the
heteroleptic vinylsilane 5 resulted from the competing metathesis reaction of lithioferrocene (FcLi), CH2=CH−OLi or
(CH3)3C(CH2)2Li with the corresponding multifunctional chlorosilane, Cl3SiCH=CH2 or Cl3Si−O−SiCl3. The last two
organolithium species have been likely formed in situ by fragmentation of the tetrahydrofuran solvent.
Diferrocenylvinyloxyvinylsilane 6 is noteworthy since represents a rare example of a redox-active silyl mononomer in
which two different C=C polymerisable groups are directly connected to silicon. The molecular structures of the silicon-
containing multiferrocenyl species 5, 6, 8 and 9 have been investigated by single-crystal X-ray diffraction studies,
demonstrating the capture and storage processes of two ring fragments resulting from the cleavage of cyclic THF in redox-
active and stable crystalline organometallic compounds. From electrochemical studies we found that by changing the
anion of the supporting electrolyte from [PF6]− to [B(C6F5)4]−, the redox behaviour of tretrametallic disiloxane 8 can be
switched from a poorly resolved multistep redox process to four consecutive well-separated one-electron oxidations,
corresponding to the sequential oxidation of the four ferrocenyl moieties.
undergo deprotonation by alkyllithium bases such as t-BuLi, i-
PrLi and s-BuLi.1-4
Introduction
Metal-induced cleavage reactions of ethers are rather
complicated and, depending on the organometallic reagent
Reactions with organolithium compounds are of fundamental
and practical interest, particularly because of their valuable
role in organic and organometallic chemistry.1 The
deprotonation reactions of organolithiums are often carried
out in the presence of ethereal solvents, such as diethyl ether
and tetrahydrofuran (THF), to enhance the reactivity of the
organometallic reagents. However, it is well-known that these
polar organometallic reagents commonly decompose ethereal
solvents.1-3 In particular THF, the most widely employed cyclic
ether, is a strongly activating solvent, highly susceptible to
and conditions, can take place in many different forms.
Scheme 1 summarises an overview of the main described
degradation processes of THF. Route A represents the principal
pathway for decomposition of THF by strong organolithium
bases RLi (R = Me, t-Bu, n-Bu) and involves the deprotonation
of the α carbon, adjacent to oxygen, to generate a C-Li bond.
Subsequently, the resultant 2-furyl anion undergoes a facile
reverse [3+2] rearrangement to afford ethylene gas and the
lithium enolate of acetaldehyde (CH2=CH-OLi).3 In rare cases,
for example using
a t-BuLi/HMPA mixture (HMPA =
hexamethylphosphoric triamide ((Me2N)3PO) (Route B) ring
opening occurs but THF ring retains the five atoms forming
lithium but-3-en-1-oxide.2 Using novel and more sophisticated
synergic mixed-metal strategies, Mulvey and co-workers have
observed remarkable unprecedented degradation pathways of
THF. Thus, in the presence of the moderately reactive
bimetallic
Na/Zn
base
=
(TMEDA)Na(TMP)(CH2SiMe3)Zn(CH2SiMe3)
(TMEDA
N,N,N’,N’-tetramethylethylenediamine; TMP
=
2,2,6,6-
tetramethylpiperidide) (Route C),
α
-metallation of THF occurs
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but the α deprotonated anion remains intact without any from 1-lithioferrocene, in turn generated by transmetallation
opening of the heterocyclic OC4 ring.5 In marked contrast to of 1-(tri-n-butylstannyl)ferrocene (FcSnBu3) with n-BuLi,
this,
followed
trichlorovinylsilane. In contrast, the synthesis of homoleptic
hydrosilane was effected in a one-pot procedure, in which 1-
by
quenching
with
the
electrophile
Route A
-
O
H2C CH2
2
+
lithioferrocene was directly generated in situ, by reaction of
ferrocene and t-BuLi, in a 1:1 mixture of THF and n-hexane.17
Over the course of this reaction we found that, in addition to
O
reverse [3+2] cycloaddition
O
Li
the desired Si−H-functionalised triferrocenylsilane
diferrocenyl(3,3-dimethylbutyl)silane Fc2(CH3)3C(CH2)2SiH
2,
-H+
(3)
RLi, R= Me, t-Bu, n-Bu
O
was unexpectedly generated. Formation of heteroleptic silicon
hydride is proposed to occur via the competitive metathesis
reactive bimetallic
t-BuLi/HMPA
-H+
Route D
Route B
bases
H
H
3
O2
-
O-
O
H
O
H
Li
reaction of Cl3SiH with organolithium bases FcLi and
(CH3)3C(CH2)2Li which, in turn, has been formed in situ via the
carbolithiation of ethene, generated by degradation of THF.
moderately
reactive
bimetallic
bases
-H+
+
H
-4 H+
H
H
H
H
H
-
-
O
H
H
Encouraged by these results, we subsequently became
Route C
Scheme 1 Summary of the main known fragmentation reactions that THF ring can
experiment in the presence of different organometallic bases.
the
more
reactive
heterometallic
or
Na/Mg
Na/Mn
[(TMEDA)Na(TMP)(CH2SiMe3)Mg(TMP)]
[(TMEDA)Na(TMP)(CH2SiMe3)Mn(TMP)] bases promote
a
interested in exploring the reaction of Cl3SiCH=CH2 with 1-
lithioferrocene, directly generated from ferrocene and t-BuLi
in THF, and furthermore to test the reactivity of a more
noteworthy cleavage of six bonds in the five-membered THF
ring (Route D).6 The two anionic fragments of the THF
deconstruction shown in Route D have been trapped in complex chlorosilane, Cl3Si
−
O
−
1, the ultimate focus of this work
SiCl3. In addition to improve the
yield of triferrocenylvinylsilyl
separated complexes, which have been crystallographically
characterised.6
was to obtain new functionalised ferrocenes by the capturing
of molecular fragments generated from the fragmentation of
cyclic THF.
In a related context, deprotonative metallation has proved
to be of particular importance in the chemistry of ferrocene
since this synthetic transformation has been frequently
applied for the derivatisation of the ferrocene backbone. The
reagents classically used for this purpose are organolithiums
such as n-BuLi and t-BuLi.7 Accordingly, both 1-lithioferrocene
(FcLi)8 and dimetallated 1,1´-dilithioferrocene Fe(η5-C5H4Li)2,
(fcLi2)9 have been prepared and often used as valuable
intermediates in the synthesis of new ferrocene derivatives as
they readily undergo transmetallation or nucleophilic
substitution reactions. In addition, recent work by Mulvey and
co-workers has shown that direct multimetallation of
ferrocene with mixed bimetallic synergic bases is also
possible.10 One of the most noteworthy examples of this
chemistry was the 1,1′,3,3′-tetramagnesiation of ferrocene
Herein we report on the formation of structurally new
multiferrocenyl
Fc2(CH3)3C(CH2)2SiCH
Fc2(OH)SiCH=CH2 ( ), Fc2(CH2=CH-O)Si
and Fc2(CH2=CH-O)Si SiFc3 ). These vinylsilyl compounds
silanes
and
), Fc2(CH2=CH-O)SiCH=CH2
Si(O-CH=CH2)Fc2
disiloxanes,
namely
=
CH2
(
5
(6),
7
−
O−
(8)
−
O−
(9
have been serendipitously generated from the reaction of
ferrocene, t-BuLi and the corresponding multifunctional
chlorosilane in the presence of the cyclic THF solvent.
Formation of 6, 8 and 9, bearing the reactive silyl vinyl ether
group (-Si-O-CH=CH2), is noteworthy since not only provides
new experimental proofs about the fragmentation mechanism
of cyclic THF, but also points to a new potentially useful
synthetic way to functionalised ferrocenes. It is interesting to
note that, among vinylsilanes, those bearing oxygen
substituents at silicon are particularly useful, as these reagents
consistently provide superior reactivity compared to their
alkylsilyl counterparts.18
using
the
sodium
tris(amido)magnesiate
base
NaMg(NiPr2)3.10b,c In addition to promote unique
polymetallation of ferrocene, some of these bimetallic
reagents are particularly useful for the functionalisation of
substituted ferrocenes bearing sensitive organic functional
groups such as nitriles, esters or carboxylic acids.11-13
Results and discussion
From years ago, our research interests concern the
chemistry
of
silicon-containing
multiferrocenyl Reaction of monolithioferrocene with Cl3SiCH=CH2: formation of
compounds.14,15 As contributions to this field, we have recently Fc3SiCH=CH2 (1), Fc2(CH3)3C(CH2)2SiCH=CH2 (5), Fc2(CH2=CH-
synthesised and studied the Si-vinyl and Si-H functionalised O)SiCH=CH2 (6) and Fc2(OH)SiCH=CH2 (7)
triferrocenyl molecules Fc3SiCH
η5-C5H4)Fe(η5-C5H5). Triferrocenylvinylsilane
=CH2 (1) 2)
16 and Fc3SiH ( 17 (Fc =
Two different synthetic strategies were explored for the
formation of triferrocenylvinylsilane ( ). Both routes, A and B,
(
1
was prepared
1
2 | J. Name., 2012, 00, 1-3
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Li
iv) Cl3SiCH=CH2
THF, -40 ºC
Fe
O
Si
Si
Fe
Fe
Fe
Si
+
i) t-BuLi
THF/Hexane, 0 ºC
ii) n-Bu3SnCl
THF/Hexane, 0 ºC
iii) n-BuLi
Fe
Fe
Fe
Fe
THF, -78 ºC
1
4
SnBu3
Route A)
Fe
Fe
Fe
Fe
Fe
Fe
H2
C
CH3
Li
Si
Si
Si
Si
+
+
+
ii) Cl3SiCH=CH2
C
Route B)
Fe
C
CH3
CH3
OH
O
H2
THF, -20 ºC
Fe
Fe
Fe
Fe
Fe
i) t-BuLi
THF/Hexane, -10 ºC
7
6
5
1
Scheme 2. Alternative routes for the reaction of 1-lithioferrocene and trichlorovinylsilane.
are shown in Scheme 2 and involve the lithiation of ferrocene octasilsesquioxane cages (POSS) and linear and cyclic siloxane
followed by the low temperature salt metathesis reaction of 1- scaffolds.14c
lithioferrocene (FcLi) and the chlorosilane Cl3SiCH
these approaches, the use of pure FcLi is a key factor, in order
to exclude the formation of the dimetallated ferrocene, Fe(η5
=CH2. Within
With the aim to improve the yield of vinylsilane
1, to
-
reduce the number of reaction steps, and mainly, to check
C5H4Li)2, (fcLi2), which could produce undesirable whether new functionalised ferrocenes were formed, in an
disubstituted- and oligo-ferrocene products. While the alternative approach 1-lithioferrocene was directly generated
dilithiation of ferrocene with n-butyllithium in the presence of in situ in a single synthetic step, from the reaction between
the chelating diamine TMEDA to afford fcLi2 is well- ferrocene and t-BuLi in a 1:1 mixture of THF and n-hexane at
established,9 in marked contrast, the selective formation of
monolithioferrocene is a very challenging synthetic task in the pyrophoric solid FcLi, a THF solution of trichlorovinylsilane
organometallic chemistry. In our long experience, the yields of was added dropwise to the reaction mixture cooled at 20 C.
−10 °C (Scheme 2, Route B). Subsequently, without isolation of
−
°
this reaction were quite variable. A convenient procedure for
After separation of the solid LiCl and appropriate workup
FcLi was reported by Gillaneaux and Kagan,8a and involved the of the crude product, the 1H NMR spectrum of the reaction
use of 1-(tri-n-butylstannyl)ferrocene as an excellent precursor mixture (see the ESI, Fig. S1†) exhibited the signals of targeted
of 1-lithioferrocene, leading, through reaction with suitable compound
electrophiles, to monosubstituted ferrocenes in high yields. resonances due to unknown monosubstituted ferrocenyl
Accordingly, in our first attempt to prepare , with three species. Thus, two complex multiplets centred at 1.00 1.50
ferrocenyl moieties attached to the vinylsilane functionality, ppm characteristic of linked CH2 groups, and a very intense
1 and the apparition of new cyclopentadienyl
1
δ
−
we used a two-step synthetic procedure starting from FcSnBu3 singlet at
as the monolithioferrocene precursor (Scheme 2, Route A).16 decoupled methyl units were observed. In addition,
Subsequently, transmetallation of FcSnBu3 was achieved by informative resonances in the vinyl region also appeared.
δ 0.95 ppm, clearly attributable to protons of
treatment with n-BuLi at −78° C, followed by the quench with These key NMR spectroscopic data motivated us to isolate the
Cl3SiCH=CH2. After removing the insoluble LiCl and subsequent unknown compounds and to properly investigate their origins
purification via column chromatography, the target Si-Vinyl- and molecular structures.
terminated
(42%). During purification of the reaction mixture, column chromatography on silica gel allowed us to separate a
tetraferrocenyl disiloxane was also obtained. first band containing unreacted ferrocene, followed by other
Most likely, this initially unexpected tetrametallic compound orange bands. The first one, eluted with n-hexane/CH2Cl2
1
was isolated in high purity and reasonable yield
Purification of the crude reaction product by careful
4
4
was formed under the used reaction conditions, as a result of (10:1), contained a yellow-orange crystalline compound which
the condensation reaction of a silanol-containing diferrocenyl was isolated in 6% yield, in high purity, and unequivocally
molecule Fc2(OH)SiCH=CH2, in turn generated accidentally, identified as diferrocenyl(3,3-dimethylbutyl)vinylsilane (
either by partial hydrolysis of the starting trichlorovinylsilane heteroleptic silane supports the resonances observed at
or by hydrolysis of chlorodiferrocenylvinylsilane 1.00
1.50 and 0.95 ppm in the 1H NMR spectrum of the
Fc2(Cl)SiCH=CH2, as it is shown in Scheme S1† (see the ESI).16 reaction mixture (see the ESI, Fig. S1†). The second band,
Triferrocenyl compound has proven to be a particularly eluted with n-hexane/CH2Cl2 (10:2) was found to contain a
reactive vinylsilane and has been successfully incorporated, via mixture of the expected compound and a new mysterious
hydrosilylation chemistry, around the surface of polyhedral vinylsilane . Unfortunately, both compounds have very similar
5). This
δ
−
1
1
6
physical properties and have been therefore particularly
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difficult to separate. Luckily after repeated column Cl3SiCH=CH2 following Route B (Scheme 2) was repeatedly
chromatographies and
performed and in all the cases compounds
obtained. In addition, in some
diferrocenylvinylsilanol ( ) was also isolated. The formation of
compound provides significant evidences to affirm that
disiloxane , obtained by Route A, was generated as a result of
1
,
5
and
6 were
reactions,
7
7
4
the condensation reaction of silanol-containing intermediates,
as we previously proposed (see the ESI, Scheme S1†).16
Compound
7 was purified by column chromatography and was
isolated as an air stable, yellow-orangish crystalline solid in 5%
yield.
It must be emphasised that compounds
6 and 7 carry two
different reactive and polymerisable groups, allowing them to
participate in different chemical transformations. In addition,
the polar vinyl ether reactive group present in 6 is an attractive
functionalisation due to its versatile and rich reactivity.18
Once separated, the new multiferrocenyl vinylsilanes
Scheme 3 Pathway for the degradation of THF and reactions that presumably
occur in the system THF/t-BuLi/FcLi/Cl3SiCH=CH2.
5−7
were thoroughly characterised by elemental analysis,
multinuclear NMR spectroscopy, IR and MALDI-TOF mass
several recrystallization processes using different mixtures of
1
spectrometry. In addition to the ferrocenyl resonances, the H
n-hexane/CH2Cl2, we achieved the separation and isolation of
NMR spectra of 5−7 show three double doublets in the
expected integrated ratios, which are consistent with the vinyl
pure samples of the targeted vinylsilane
1 (in 65% yield) and
the new ferrocenyl species 6 (in 9% yield). On the basis of
1
AMX system. Furthermore, the H NMR spectrum of
5 shows
multinuclear NMR spectroscopy, mass spectrometry and X-ray
studies (see below) the unknown compound was
the peculiar pattern for the (CH2)2C(CH3)3 group, a singlet at
−
δ
0.94 ppm corresponding to the nine protons of the three
unequivocally identified as Fc2(CH2=CH-O)SiCH=CH2 (6), an
decoupled methyl groups, and two complex multiplets centred
interesting molecule bearing two ferrocenyl moieties and two
types of C=C double bonds (one directly connected to silicon,
1
and the other one to oxygen), thus explaining the striking H
at
The
6.59 and two doublets, one is centred at
one cannot be observed because it is overlapped with the C5H5
resonance at 4.16 ppm. This last signal could be identified
performing a {1H 13C} HMQC experiment (see the ESI, Fig.
S9†), as the carbon resonance of the CH=CH2 unit
correlates, not only with the multiplet at 4.58 ppm, but also
with the other that is under the C5H5 resonance at 4.16 ppm.
exhibits a singlet at
δ
1.02 and 1.44 ppm characteristic of the
CH=CH2 group of appears as a double doublet at
4.58 while the other
−CH2−CH2− unit.
−
O−
6
δ
δ
NMR resonances observed in the vinyl region in the spectrum
of the reaction mixture.
δ
The origin of the initially unexpected vinylsilanes
−
Fc2(CH3)3C(CH2)2SiCH=CH2 (5) and Fc2(CH2=CH-O)SiCH=CH2 (6)
−O−
is certainly noteworthy and deserves an explanation. Their
formation can be rationalised by the sequence of processes
outlined in Scheme 3, and presumably involves the THF
cleavage. As already mentioned in the Introduction (Scheme
1), the main pathway for the THF degradation by RLi reagents
δ
δ
Meanwhile, the spectrum of compound
2.15 ppm, distinctive of the OH group.
7
δ
Besides the characteristic carbon signals of the ferrocenyl
units and of the vinyl groups (at 133 and 137 ppm), the 13C
first comprises the deprotonation at the carbon
α to oxygen.
Then, the resultant 2-furyl anion undergoes a reverse [3+2]
cycloaddition to generate ethylene and the lithium enolate of
NMR spectrum of
dimethylbutyl group. Thus, the CH2 carbons appear at
and 38.3 ppm, the CH3 at 28.9 ppm, and the ipso-carbon at
31.3 ppm. In the spectrum of compound two resonances of
the CH=CH2 group appear at 94.2 and 146.3 ppm. The
29Si NMR spectrum displays a single resonance at
17.0 ppm
(for ), 12.1 ppm (for ), 7.8 ppm (for ), and at 8.9
ppm (for ). Therefore, the replacement of a ferrocenyl moiety
5
shows the resonances of the 3,3-
δ
9.0
acetaldehyde. Therefore, very probably, compound
6 carrying
δ
δ
both a vinyloxy and a vinyl group has been formed under the
used reaction conditions, as a result of the competing salt-
metathesis reaction of the organolithium reagents FcLi and
CH2=CH-OLi with Cl3SiCH=CH2. Meanwhile, diferrocenyl
6
−
O−
δ
δ
−
1
δ
−
5
δ
−
6
δ −
compound
5 has been likely formed as a result of the
7
competing metathesis reaction of FcLi and (CH3)3C(CH2)2Li with
Cl3SiCH=CH2. As occurred during the formation of heteroleptic
by a 3,3-dimethylbutyl shifts downfield the silicon signals.
These resonances are even more deshielded in the spectra of
hydrosilane 3 17
the only plausible source of the lithiated
,
compounds
6 and 7, due to the electronic influence of the
reagent (CH3)3C(CH2)2Li is the intermolecular carbolithiation of
ethylene arising from the decomposition of THF (Scheme 3).1-3
When comparing the two synthetic procedures used to
−
OCH=CH2 and –OH groups.
Reaction of monolithioferrocene with hexachlorodisiloxane:
formation of Fc2(CH2=CH-O)Si−O−Si(O-CH=CH2)Fc2 (8), Fc2(CH2=CH-
O)Si−O−SiFc3 (9) and Fc3Si−O−SiFc3 (10)
prepare
1 it can be concluded that Route B resulted more
efficient and less complicated than the approach using FcSnBu3
as starting material and that, in addition, newly interesting
Encouraged by the results described above and with the idea
to increase the members of the multiferrocenyl family
products, 5 and 6, have been formed. The reaction of FcLi with
4 | J. Name., 2012, 00, 1-3
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supported by Si−O−Si bridges, our efforts were next directed
toward the synthesis of a disiloxane maximally functionalised
with as many ferrocenyl units as chemically possible. Thus, we
reverse
[3+2]
cycloaddition
O
O
Li
t-BuLi
O
Li
H2C CH2
+
-H+
O
Cl3Si-O-SiCl3
FcLi
Fe
O
O
O
O
O
Si
Si
Si
Si
Fe
Fe
Fe
Fe
+
Fe
Fe
Fe
Fe
8
9
Scheme 5 Reactions that presumably occur in the system THF/t-BuLi/FcLi/Cl3Si−O−SiCl3
Scheme 4 Synthesis of polyferrocenes 8−10 with disiloxane bridges.
On the basis of the formation of silylated acetaldehyde
species
6, it seems reasonable to assume that disiloxanes 8
and 9 were formed as a result of the competing metathesis
sought to prepare the hexaferrocenyldisiloxane molecule 10
shown in Scheme 4. To this end, monolithioferrocene was
generated by treating ferrocene with t-BuLi, in a 1:1 mixture of
reactions of FcLi and CH2=CH-OLi to react with the
hexachlorodisiloxane, according to Scheme 5.
As can be observed, the reaction between FcLi and
THF and n-hexane at
−
with hexachlorodisiloxane (Cl3Si
10
°
C. Subsequently, FcLi was reacted
Cl3Si−O−SiCl3 fails to give the desired hexaferrocenyl 10 in
−
O−
SiCl3) at −20 C and the
reaction mixture was purified by column chromatography. To
°
preparative amounts, which might be due to the steric
repulsion of the six ferrocenyl moieties into close contact
across the siloxane bond. Therefore, it is very likely that the
difficulty entailed in effectively bonding six ferrocenyl units to
our surprise, the tetrametallic compound Fc2(CH2=CH-
O)Si−
product (29% yield). Compound
O
−
Si(O-CH=CH2)Fc2 (
8
) was isolated as the major reaction
is a disiloxane functionalised
8
the Si−O−Si bridge facilitates, as an alternative reaction route,
the incorporation of the less bulky vinyloxy group.
with four ferrocenyl units and two vinyloxy reactive groups,
which was isolated as an orange, crystalline and air stable
We have to note that neither disiloxane 8, nor vinylsilanes 5 and
6, are accessible by conventional organosilicon chemistry methods,
illustrating again the synthetic potential of THF-cleavage reactions.
Multinuclear NMR (1H, 13C and 29Si) analyses of the new
solid. It is interesting to note that, in addition to the peak of
8
at m/z 898.1, the MALDI−TOF mass spectrum of the reacꢀon
mixture (see Fig. 1) exhibits two peaks at m/z 1040.0 and
1182.0, thus confirming the formation of the targeted
hexaferrocenyl disiloxane 10 and of an asymmetric disiloxane
disiloxane
Thus, the protons of the
NMR spectrum at 6.82 ppm (double doublet) and at
4.26 ppm (doublets). The 29Si NMR of
presents a single
8 were consistent with the proposed structure.
1
−
O−
CH=CH2 groups appear in the H
molecule, Fc2(CH2=CH-O)Si−O−SiFc3 (9), also shown in Scheme
δ
4.68 and
4.
8
Unfortunately, significant amounts of pure samples of
multiferrocenyl disiloxanes and 10 could not be isolated, so
resonance at δ −27.7 ppm, in good agreement with values
found for other ferrocenyl disiloxanes.14
9
far, due to their similar solubility properties. However, their
existence is fully supported by the MALDI-TOF mass
Crystal structures of compounds 5−9
spectrometric study and, in the case of disiloxane
diffraction analysis (see below).
9 by X-ray
The ultimate confirmation that the molecular fragments
generated from the cleavage of the THF ring have been
certainly captured in crystalline redox-active ferrocenyl
compounds was achieved by single-crystal X-ray diffraction
Comp. 8
analysis of silanes
Diffraction-grade single crystals were obtained by
crystallisation at 4 C in n-hexane/CH2Cl2 (10:2) for , at room
temperature in CH2Cl2 for and and at 4 C in n-
hexane/CH2Cl2 (10:3) for and . The resulting structures are
shown in Figs. 2 and 3, and complete structural information is
collected in the ESI. The three bimetallic silanes and the
pentametallic disiloxane crystallise in the triclinic space
group P-1 with one ( and ) or two ( ) molecules per
asymmetric unit, while tetrametallic disiloxane belongs to
5 and 6 and disiloxanes 8 and 9.
°
5
6
7
°
8
9
1036
1038
1040
m/z
1042
1044
1
5−7
1178 1180 1182 1184 1186
m/z
1
Comp. 9
9
Comp. 10
6,
7
9
5
8
the monoclinic P21/n space group. The two crystallographic
m/z
600
800
1000
1200
1400
1600
1800
2000
Fig. 1 MALDI-TOF mass spectrum of the reaction mixture of disiloxanes 8−10. The
insets show the experimental (top) and calculated (bottom) isotopic patterns for
Th
c
isomjo
p
u
o
r
u
n
n
a
d
l
s
is
9
© T
and
1
h
0
e
.
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Journal Name
corresponds to Si−OH groups linked by hydrogen bonds. The
fact that the two diferrocenylsilanols
6 and 7 show an almost
identical molecular conformation and very similar packing
patterns (see the ESI, Fig. S32†) regardless of the difference in
supramolecular interactions, proves the important role that
the shape and size of these molecules play in their
arrangement in the crystal state. In this case, the smaller
substituent in
7 together with the existence of hydrogen
bonds, lead to a closer packing of the molecules involved in
the supramolecular interactions, which in turn yields a denser
crystal (calculated density values are 1.473 for
Mg/cm3 for
).
In disiloxanes and 9 (Fig. 3) the ferrocenyl units are
6 and 1.526
7
8
placed as distant as possible from each other in order to avoid
steric congestion around the silicon atoms, and the
Si
disiloxanes show a bent arrangement of the disiloxane linkage,
with Si Si angles of 160.0(1) for and 152.9(2) for . The
Fe atoms of the ferrocenyl substituents attached to the same
silicon centre are separated by 5.9037(5) and 6.1052(5) Å (
and by 5.444(1), 6.501(1) and 6.077(1) Å ( ). Meanwhile, the
−O−CH=CH2 groups are arranged in zig-zag chains. These two
−
O−
°
8
°
9
8
)
9
iron centres of the ferrocenyl units attached to different silicon
atoms (see the ESI, Table S11†) are separated by larger
distances, ranging from 6.3853(7) to 8.4029(6) Å (for
from 6.501(1) to 9.327(1) Å (for ).
In all of the crystal structures
nearly tetrahedral with C Si C (for
Si O (for and ) bond angles close to 109
Fig. Table S13†) and similar to the ones obtained for 1 9
8) and
9
5
−
−
9
, the silicon atoms are
), C Si O (for ) and
(see the ESI,
−
−
5
9
−
−
6−9
independent molecules in the asymmetric unit cell of
5 (5A
O−
−
8
9
°
and
5B) are shown in Fig. 2 (top);
.
they differ only slightly in their bond lengths and angles.
Fig.2 Molecular structures of silanes 5-7 with selected atoms labelled. Hydrogen atoms
(except the hydroxylic one) have been removed for clarity.
In both molecules, the two ferrocenyl groups attached to the
silicon atoms are disposed in
arrangement. The cyclopentadienyl rings in
(maximum deviation 2.22 for the Cps attached to Fe2) and
a
nearly perpendicular
5
are parallel
°
essentially eclipsed in three of the ferrocenyl moieties, while in
the fourth one (the one comprising Fe3) they are completely
staggered. The intramolecular distances between Fe atoms are
6.0556(8)
Å (Fe1-Fe2) and 6.1853(8) Å (Fe3-Fe4). The
supramolecular arrangement is achieved by van der Waals
forces, giving rise to four-molecule rings.
In the bifunctional compound
cyclopentadienyl rings are also essentially parallel (maximum
deviation 2.81 in the ferrocene containing Fe1) and eclipsed.
6 (Fig. 2, middle), the
°
Iron atoms are separated by 6.0857(6) Å and no relevant
supramolecular interactions have been found.
In diferrocenylsilanol
the Fc units is very similar to the one observed in
cyclopentadienyl rings parallel (maximum deviation 2.36
7
(figure 2, bottom) the disposition of
, with the
in
6
°
the Fc with Fe2) and eclipsed, presenting an intramolecular
distance between Fe atoms of 6.1174(6) Å. However, in this
structure hydrogen bonds between pairs of molecules are
responsible for the formation of dimeric units (see the ESI, Fig.
S31† and Table S12†). This is in agreement with the IR data,
where a ν(OH) band at 3413 cm-1 was observed, which
6 | J. Name., 2012, 00, 1-3
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Fig.3 Molecular structures of disiloxanes
8 and 9 with selected atoms labelled.
homoleptic vinylsilane
1
and heteroleptic diferrocenyl-
vinylsilane Fc2(CH3)SiCH=CH2 previously investigated by us.14d
,16
They are constituted by a vinyl reactive group and two
Hydrogen atoms have been removed for clarity.
Electrochemistry and Electronic Structure Calculations
identical ferrocenyl units, linked together by a silicon bridge,
The redox properties of 5−8, featuring ferrocenyl moieties linked by
a silicon atom and/or a disiloxane bridge, are also of considerable
interest as these molecules display multielectron redox chemistry.
and they vary in the nature of the non-electroactive −O-
CH=CH2, (CH2)2C(CH3)3 or OH substituent (R in Fig. 4 top). As
−
−
expected, this leads to qualitatively similar voltammetric
responses which differ only slightly in the half-wave potentials
E1/2, reflecting the different electronic influence of the fourth
substituent (R) at silicon. As representative examples, Fig. 4 (A-
19
The electrochemical behaviour was investigated by cyclic
voltammetry (CV) and square wave voltammetry (SWV), using
CH2Cl2 as a non-nucleophilic solvent, and tetra-n-butylammonium
hexafluorophosfate ([n-Bu4N][PF6]) or tetra-n-butylammonium
D) presents the CV and SWV responses obtained for
CH2Cl2 using either the traditional [PF6]− or the
noncoordinating electrolyte anion, the
[B(C6F5)4]−
electrochemical oxidation of takes place in two well-resolved
5. In
tetrakis(pentafluorophenyl)borate
([n-Bu4N][B(C6F5)4])
as
supporting electrolytes containing anions of different coordinating
ability. Geiger and co-workers have performed remarkable studies
on the perfluoroarylborate anion [B(C6F5)4]− and have ascribed the
improvements observed in the oxidative electrochemical processes
of organometallic compounds to its intrinsically low ion-pairing
strengths and nucleophilicities, and to the increased solubility of its
salts in lower polarity solvents as compared to classical electrolytes
such as [PF6]−, [ClO4] − and [BF4] −.20
5
voltammetric waves, which correspond to the sequential
oxidation of the two ferrocenyl moieties, giving the
ferrocenium species 5+ and 52+. Experimentally measured
1
2
potential values are E1/2 = 0.420 and E1/2 = 0.620 V (ΔE1/2
=
−
2E1/2 1E1/2 = 200 mV) (in [n-Bu4N][PF6]) and E1/2 = 0.422 and
2E1/2 = 0.737 V (ΔE1/2 = 315 mV) (in [n-Bu4N][B(C6F5)4]).
Similarly, the CVs and SWVs of and also showed two
1
Diferrocenyl silanes 5−7 constitute the simplest molecules
described here and they are structurally related to the
6
7
successive clearly defined ferrocene-based oxidation waves
separated by ΔE1/2 = 180 and 170 mV (in [n-Bu4N][PF6]) and by
ΔE1/2 = 280 and 264 mV (in [n-Bu4N][B(C6F5)4]). These findings
suggest the existence of appreciable iron−iron interactions
between the two neighboring ferrocenyl units linked to the
silicon atom. The structurally related linear and cyclic oligo-
and polyferrocenylsilanes, having similar silicon-bridged
ferrocenyl moieties, have been extensively studied by
Manners21 and Pannell22 and exhibit electronic communication
between adjacent iron centres.23
For redox-active multimetallic compounds, the magnitude
of the separation between the half-wave potentials (ΔE1/2) of
two redox sites has usually been taken as a measure of
electronic interaction between metal centres. However, one
should be extremely careful when using electrochemical data
since voltammetric separations are influenced by the effects of
the solvent/supporting electrolyte media, which can modify
the electrostatic interactions in polycationic species.
The graphical comparison of ∆E1/2 values shown in the
bottom of Fig. 4 is also representative of the changes in the
redox splitting between the first and second redox events
observed for diferrocenylsilanes 5−7 when changing the anion
of the supporting electrolyte from nucleophilic [PF6]− to weakly
coordinating fluoroarylborate anion. The fact that in the three
compounds the low ion-pairing supporting electrolyte [n-
Bu4N][B(C6F5)4] increases the separation of the two one-
electron oxidations suggests that the through-space
interaction is also significant. In addition, this figure shows that
incorporation of −
silicon of diferrocenyl
(CH2)2C(CH3)3 substituent in the bridging
results in an enlarged peak separation,
5
which can be attributed to more effective intermetallic
communication between the two ferrocenyl units.
The determination of the redox potentials for the
Fig. 4. Top: Redox processes experimented by diferrocenylvinylsilanes 5-7.
Voltammetric responses, on Pt electrode, of 5: (A) and (B) recorded in CH2Cl2
containing 0.1 M [n-Bu4N][PF6]; (C) and (D) recorded in CH2Cl2 containing 0.1 M [n-
Bu4N][B(C6F5)4]. CVs are at a scan rate of 0.1 V s-1. Bottom: ΔE1/2 values for the two
successive oxidations of 5-7 measured in CH2Cl2 in the presence of the weakly
coordinating [B(C6F5)4]− (red) or the traditional [PF6]− (blue) supporting electrolyte
successive oxidations of 5−7 allowed us to estimate the
comproportionation constant, Kc, relative to the equilibrium
anions.
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ARTICLE
[FeII FeII] + [FeIII
Journal Name
−
−
FeIII]
↔
between the first and second oxidations measured in the
[FeII
−
FeIII].24 The wave splitting (∆E1/2
)
In addition, electronic structure calculations show that
compound presents a different variation of the partial charge
5
of both Fe atoms (0.119 and 0.190 a.u.) after the first
oxidation, in agreement with the electrochemical results. This
compound exhibits slightly larger iron charge variations on
their response to oxidation with respect to values reported for
compounds
6 and 7
at the same level of theory.27 Accordingly,
spin densities on iron atoms in the radical species present
values of 0.699 and 0.398 a.u.
We have also explored the electrochemical oxidation of
since the four ferrocenyl redox moieties linked by the Si
8
−
O−
Si
bridge provide an excellent opportunity to study multi-step
electron-transfer processes. As shown in Fig. 6, the CV of in
CH2Cl2/0.1 M [n-Bu4N][PF6] is considerably more complex than
that of the diferrocenyl compounds , as it exhibits a
8
5−7
sequence of several overlapped, and poorly resolved, waves
(from about +0.45 to +0.85 V vs SCE), thereby preventing
detailed analysis of the successive individual oxidations. The
more anodic wave is not a diffusion-controlled process and a
sharp cathodic stripping wave is observed on the return
Fig. 5 (A): HOMO and SOMO orbitals of Compound 5 at the BPW91/6-31+G(d) level of
theory. (B): NCI analysis carried out by means of the NCIPLOT program. Blue and red
colors denote strong attractive and repulsive interactions, respectively, whereas green
color indicates interactions in the van der Waals range.
sweep. This indicates that for disiloxane
8, a change in
solubility accompanies the change in oxidation state, very
likely due to the rapid precipitation of the
hexafluorophosphate salt [84+][PF6]4 on the electrode surface.
On the reverse scan, this tetraoxidised cation is redissolved as
it is reduced. Such electrochemical phenomena are well known
for multi-ferrocenyl compounds,28 where solubility problems
and follow-up reactions result in deviations from ideality when
using the traditional nucleophilic electrolyte anion [PF6]– in
low-polarity solvents.
weakly coordinating supporting electrolyte [n-Bu4N][B(C6F5)4]
and the Kc are both indicative of the thermodynamic stability
of the mixed valence state of these molecules relative to other
redox systems.25 The resulting values of Kc are 211.15
(for ), 54.07 ) and 29.01 ), calculated
103 (for 103 (for
from E1/2 electrochemical values, indicating that intermetallic
interaction exists in the partially oxidised multiferrocenyl
compounds
7 19b,26
×
103
5
×
6
×
7
ꢀ
By using [n-Bu4N][B(C6F5)4] as electrolyte, we hoped to
accurately investigate the multistep electron-transfer
5- .
processes of tetrametallic 8, because the weakly coordinating
[B(C6F5)4]− anion is extremely effective in solubilising positively
Many times, a strong electronic communication between
active redox centres is allowed by the presence of unsaturated
or aromatic units in the bridge. This is nicely reflected in the
shape of the HOMO, which involves the bridge, as a sign of
charged species produced in anodic processes.
Thus, Fig. 6 compares the CV of 8 in CH2Cl2/[NBu4][PF6]
delocalisation of the charge. However, neither 6− , nor 5
7 27
with those of the same molecule in CH2Cl2 with 0.1 M [n-
Bu4N][B(C6F5)4], and shows the striking improvement observed
for the anodic reaction of this tetrametallic disiloxane. With
[B(C6F5)4]− the adsorption effects on the working electrode
were minimised, which indicates an increase in solubility of the
tetracationic species 84+ in this solvent/electrolyte medium. In
addition, in agreement with the results reported by Geiger and
coworkers, as compared to the small inorganic [PF6]− anion,
the [B(C6F5)4]− electrolyte anion has a lower coordinating
power and restrains ion pairing, allowing development of
(see Fig. 5A) present a HOMO orbital involving the silicon
bridges, something not surprising in Class II mixed-valence
compounds. When dealing with these systems in vacuum, i.e.,
looking strictly at the chemical structure, weak non-covalent
interactions in the van der Waals range are already observed
between the substituents in the silicon bridge and the
ferrocenyl units, as evidenced by the NCI (Non Covalent
Interaction) analysis represented by green isosurfaces in Fig.
5B. Undoubtedly, the solvent/electrolyte medium plays a very
important role on modulating and reinforcing these
interactions between centres, as previously observed in the
interactions between the four metallocene units in 8, which
are electrostatic in nature, that is, mostly through-space
interactions. This results in the observation of four well-
resolved one-electron oxidation waves, at different potentials,
voltammetric
measurements.
Upon
oxidation,
the
corresponding SOMO orbital occupies the next ferrocenyl unit
to be oxidised (Fig. 5A). We obtain a vertical ionisation
for each of the four ferrocenyl moieties. Likewise, SWV of
8
potential value of 6.08 eV for
values for compounds
(6.16 and 6.08 eV).27 The calculated
UV-Vis spectrum of in CH2Cl2 along with the different orbital
5, in agreement with previous
measured in CH2Cl2/[n-Bu4N][B(C6F5)4] (Fig. 6C) also shows four
well-separated waves. The height peak of the two more anodic
waves is somewhat smaller than that of the first two waves.
This effect has been previously observed in ferrocenyl29 and
fulvalenediyl multimetallic compounds30 and can be taken as a
6−7
5
contributions to each transition is provided in Fig. S35† and
Table S14† (see the ESI).
8 | J. Name., 2012, 00, 1-3
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ARTICLE
qualitative indication that the last one-electron transfers are slightly slower than that of the first two oxidations.31
ic
A
ia
-
PF6
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
B
-
B(C6F5)4
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
C
-
B(C6F5)4
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
E (V vs SCE)
Fig. 6 Cyclic and SWV responses, on platinum electrode of tetrametallic 8, recorded in CH2Cl2 containing 0.1 M [n-Bu4N][PF6] (A) or 0.1 M [n-Bu4N][B(C6F5)4] (B and C) as
supporting electrolytes. CVs at a scan rate of 0.1 V s-1
.
The mechanism for the electrochemical oxidation of this
General procedures and equipment. All reactions and
tetrametallic disiloxane is shown in Fig. 6 and involves a first compound manipulations were performed in an oxygen- and
1
oxidation (at E1/2 = +0.484 V vs SCE) corresponding to the moisture-free Ar atmosphere using standard Schlenk
generation of monocationic species 8
+. At a higher potential techniques. THF was distilled over sodium/benzophenone
(2E1/2 = +0.696 V) a second electron is removed from a under argon before use. n-Hexane and dichloromethane were
ferrocenyl moiety attached to the neighbouring silicon atom, dried by standard procedures over the appropriate drying
at the other end of the Si−O−Si bridge, yielding the dicationic agents and distilled under argon, immediately prior to use.
species 82+. The potential separation between the second and Ferrocene (Sigma-Aldrich) was purified by sublimation prior to
3
third redox processes, ∆E1/2 = E1/2
−
2E1/2 = 256 mV, is larger use. Trichlorovinylsilane, hexachlorodisiloxane and t-
2
than the E1/2
4
−
implying that the third oxidation (at E1/2 = +0.952 V) occurs at were used as received. Silica gel (70-230 mesh) (Sigma-Aldrich)
1E1/2 = 212 mV and E1/2
−
3E1/2 = 228 mV, Butyllithium (1.7 M solution in n-pentane) (Sigma-Aldrich)
3
one of the two remaining ferrocenyl moieties, the adjacent to was used for column chromatography purifications. Infrared
the last oxidised ferrocenyl subunit.14e,16 The final oxidation of spectra were recorded on
a Perkin-Elmer 100 FT-IR
the last neutral ferrocenyl centre is the most difficult one and spectrometer. Elemental analyses were performed in a LECO
takes place at more anodic potential (4E1/2 = +1.180 V), giving CHNS-932 elemental analyzer, equipped with a MX5 Mettler
the tetracationic species 84+. The considerable spread of the Toledo microbalance. All NMR spectra were recorded on a
four oxidations in disiloxane
intermetallic interactions between the Si- and Si
ferrocenyl moieties.14e,16 Observed shifts of the oxidation residual solvent resonances for 1H (
potentials values for the
8+ 82+ 83+ 84+ couples (see Fig. 6) ppm). 29Si NMR resonances were recorded with inverse-gated
8
suggests appreciable Bruker Avance 300 MHz spectrometer. Chemical shifts were
Si-bridged reported in parts per million ( ) with reference to CDCl3
7.26 ppm) and 13C (
77.2
−
O−
δ
δ
δ
8/
/
/
/
when going from [PF6]− to [B(C6F5)4]− media are consistent with proton decoupling in order to minimize nuclear Overhauser
the considerably decrease in the ion pairing of the FeIII centres effects and were referenced externally to tetramethylsilane.
with the weakly coordinating fluoroarylborate anion as the MALDI-TOF mass spectra were recorded using a Bruker-
ferrocenyl moieties are progressively oxidised.
Ultraflex III TOF/TOF mass spectrometer equipped with a
nitrogen laser emitting at 337 nm. Dichloromethane solutions
of the matrix (trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-
Experimental Section
propenylidene]malonitrile
(DCTB),
10
mg/mL)
and
dichloromethane solutions of the corresponding compound (1
General Considerations
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mg/mL) were mixed in the ratio 20:5. Then, 0.5-1
Journal Name
ꢁ
L of the compounds 5−9 have been deposited at the Cambridge
mixture was deposited on the target plate using the dried Crystallographic Data Centre with deposition numbers CCDC
droplet method. The positive ion and the reflectron mode 1556608-1556612.
were used for these analyses.
Computational Details. Electronic structure calculations were
Electrochemical measurements. Cyclic voltammetry (CV) and carried out with the Gaussian09 program.38 Structures were fully
square wave voltammetry (SWV) experiments were recorded on a optimized at the BPW91/6-31+G(d) level of theory.39 Vibrational
Bioanalytical Systems BAS CV-50W potentiostat. CH2Cl2 and CH3CN normal modes from harmonic frequency calculations allowed
(SDS, spectrograde) were freshly distilled from calcium hydride ensuring that all structures are minima of the potential energy
under Ar. The supporting electrolytes used were tetra-
n
-
surface. NCIPLOT program was used to analyse NCI (non covalent
butylammonium hexafluorophosphate (Alfa-Aesar), which was interactions) in Compound 5.40 This analysis shows which regions of
purified by recrystallisation from ethanol and dried in vacuum at the space present low reduced density gradient (RDG) values, a
60°C,
tetrakis(pentafluorophenyl)borate, which was synthesised as regions. Non-covalent interactions are characterized by both low
described in the literature,32 by metathesis of [
-Bu4N]Br with RDG and low electron density values. Also, the sign of eigenvalue λ2
Li[B(C6F5)4]·( OEt2) (Boulder Scientific Company) in methanol and of the laplacian of the density helps to distinguish between
and
tetra-n-butylammonium dimensionless quantity that assume very small values for NCI
n
n
recrystallised twice from CH2Cl2/hexane. The supporting electrolyte attractive (λ2 < 0) and repulsive (λ2 > 0) weak interactions.
concentration was 0.1 M. A conventional three-electrode cell According to this, attractive regions appear in blue colour in three-
connected to an atmosphere of prepurified nitrogen was used. The dimensional NCI representations, whereas green surfaces denote
counter electrode was a coiled Pt wire, and the reference electrode interactions within the van der Waals range. Red colour stands for
was a BAS saturated calomel electrode (SCE). All cyclic voltammetric repulsive interactions. QTAIM (Quantum Theory of Atoms in
experiments were performed using
a platinum-disk working Molecules) allows to analyse the molecular space from the topology
electrode (
A
= 0.020 cm2) (BAS). The working electrode was of the density point of view. In particular, interacting atoms are
polished on a Buehler polishing cloth with Metadi II diamond paste connected through paths containing bond critical points (BCPs).41
for about 3 min followed by sonication in absolute ethanol, rinsed Density values at BCPs are a way of describing the strength of the
thoroughly with purified water and acetone, and allowed to dry. interaction when comparing a same pair of atoms in different
Under our conditions, the ferrocene redox couple [FeCp2]0/+ is molecular environments. The sign of the laplacian at the BCPs also
+0.462, and the decamethylferrocene redox couple [FeCp*2]0/+ is allows classifying bonds as ionic or covalent. Closed-shell
−0.056 V vs. SCE in CH2Cl2/0.1 M [n-Bu4N][PF6]. Solutions were, interactions as those in Figure S43 are characterized by positive
typically, 10-3 M in the redox-active species and were purged with values.
nitrogen and kept under an inert atmosphere throughout the
Synthesis
measurements. No IR compensation was used. SWV was performed
using frequencies of 10 Hz.
Reaction of FcLi with Cl3SiCH=CH2: Synthesis of Fc3Si−CH=CH2
(1), Fc2(CH3)3C(CH2)2SiCH=CH2 (5), Fc2(CH2=CH-O)SiCH=CH2 (6) and
Fc2(OH)SiCH=CH2 (7). A 15 g amount of sublimed ferrocene (80.6
mmol) was dissolved in a mixture of 100 mL of dry THF and 100 mL
X-ray crystal structure determination. Suitable orange crystals
of compounds 5−9 were coated with mineral oil and mounted on
Mitegen MicroMounts. The samples were transferred to a Bruker
D8 KAPPA series II with APEX II area-detector system equipped with
of dry
cooled to −10 °C. To this stirred system, a 1.7 M solution of
-pentane (72.0 mL, 121.0 mmol) was added dropwise. The mixture
n-hexane, under argon at room temperature, and then
graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Full
t
-BuLi in
details of the data collection and refinement can be found in the
supplementary material. The substantial redundancy in data allows
empirical absorption corrections (SADABS)33 to be applied using
multiple measurements of symmetry-equivalent reflections. Raw
intensity data frames were integrated with the SAINT program,34
which also applied corrections for Lorentz and polarization effects.
The Bruker SHELXTL Software Package was used for space group
determination, structure solution, and refinement.35 The space
group determination was based on a check of the Laue symmetry
and systematic absences were confirmed using the structure
solution. The structures were solved by direct methods (SHELXS-
n
was stirred for another 30 min and cooled to −20 °C before a
solution of Cl3SiCH=CH2 (3.2 mL, 24.2 mmol) in 15 mL of dry THF
was added dropwise. The solution was allowed to warm to room
temperature and was stirred overnight. The mixture was filtered,
treated with n-hexane, and then filtered once again to remove the
LiCl byproduct. Solvent removal yielded a red-orange oily product,
which was purified by column chromatography on silica gel (6 cm ×
10 cm) using
ferrocene was eluted, and subsequently, a second orange band was
obtained with -hexane/CH2Cl2 (10:1). Solvent removal afforded the
n-hexane as eluent. A first band containing unreacted
n
97), completed with different Fourier syntheses, and refined with
reaction side product 5, which was obtained as an analytically pure,
2
full-matrix least-squares using SHELXS-97 minimizing
Fc2)2.36,37 Weighted
factors ( w) and all goodness of fit
on F2; conventional
factors ( ) are based on . All non-hydrogen
ω(
Fo
–
air-stable, yellow-orangish crystalline solid. Subsequently, on
R
R
S are based
eluting with
collected. Solvent removal afforded a mixture of two compounds (1
and 6). This mixture was subjected to second column
chromatography on silica gel (6 cm × 16 cm). A first band was
eluted with -hexane/CH2Cl2 (10:2) and solvent removal afforded
n-hexane/CH2Cl2 (10:2) a third major band was
R
R
F
atoms were refined with anisotropic displacement parameters. The
hydrogen atoms were calculated geometrically and allowed to ride
a
on their parent carbon or oxygen atoms with fixed isotropic U. All
n
scattering factors and anomalous dispersion factors are contained
in the SHELXTL 6.10 program library. The crystal structures of
compound 1 as an analytically pure, air-stable, orange crystalline
solid. The other fractions of this column were again mixtures of
10 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7DT02286G
ARTICLE
compounds 1 and 6. After several columns and recrystallisations in silica gel (3 cm × 11 cm). Unreacted ferrocene was firstly eluted
-hexane/CH2Cl2, compound 6 was finally isolated as an analytically with -hexane and, subsequently, a second orange band was
pure, air-stable, orange crystalline solid. In some of the repeated obtained with -hexane/CH2Cl2 (10:3). Solvent removal afforded
reactions, a final fraction was obtained from the first column disiloxane 8 as an analytically pure, air-stable, yellow-orangish
chromatography, eluted with CH2Cl2. After the appropriate solvent crystalline solid. Subsequently, on eluting with different
removal process, compound 7 was obtained as a pure, air-stable, hexane/CH2Cl2 mixtures, diverse bands were collected. After
light orange crystalline solid. MALDI-TOF analysis, these fractions were found to correspond to a
Data for 1: Yield: 9.59 g (65%). H NMR (CDCl3, 300 MHz, ppm): mixture of compounds and 10, impossible to isolate in
4.05 (s, 15H, C5 5), 4.25, 4.39 (m, 12H, C5
4), 6.08 (dd, 3J=20.2 Hz, quantitative yields.
2J=4.0 Hz, 1H, CH=C transHcis), 6.25 (dd, 3J=14.7 Hz, 2J=4.0 Hz, 1H, Data for 8: Yield: 0.94 g (29%). 1H NMR (CDCl3, 300 MHz, ppm):
CH=CHtrans =CH2). 4.16 (s, 20H, C5 cis), 4.29,
cis), 6.69 (dd, 3J=20.2 Hz, 3J=14.7 Hz, 1H, C 5), 4.26 (d, 3J = 5.8 Hz, 1H, O-CH=CHtrans
13C{1H} NMR (CDCl3, 75 MHz, ppm): 4), 4.68 (d, 3J = 13.5 Hz, 1H, O-CH=CHtrans
68.7 ( 5H5), 70.8, 74.0 ( 5H4), 4.36, 4.40 (m, 16H, C5
132.2 (CH= H2), 136.4 (
H=CH2). 29Si {1H} NMR (CDCl3, 59 MHz, Hcis), 6.82 (dd, 3J = 13.5 Hz, 3J = 5.8 Hz, 1H, O-C =CH2). 13C{1H} NMR
ppm): (C=C) 1163, (Si−C) 820. MS (CDCl3, 75 MHz, ppm): 65.8 (ipso-Fc), 68.8 ( 5H5), 71.1, 71.2, 73.6,
−17.0 (SiFc). IR (KBr, cm−1):
(MALDI-TOF): m/z 610.1 [M+]. Anal. Calcd for C32H30Fe3Si: C, 62.95; 73.8 ( H=CH2). 29Si{1H} NMR
5H4), 94.6 (O−CH= H2), 145.6 (O−
H, 4.96. Found: C, 62.68; H, 4.98. (CDCl3, 59 MHz, ppm): (C=C) 1636,
−27.7 (SiFc). IR (KBr, cm-1):
1171, (Si-O-Si) 1075,
(Si-C) 815. MS (MALDI-TOF): m/z 898.1 [M+].
5), 4.15, Anal. Calcd for C44H42Fe4Si2O3: C, 58.80; H, 4.71. Found: C, 58.95; H,
4), 5.83 (dd, 3J=20.3 Hz, 2J=4.0 Hz, 1H, 4.63.
transHcis), 6.13 (dd, 3J=14.7 Hz, 2J=4.0 Hz, 1H, CH=CHtrans
cis),
6.46 (dd, 3J=20.3 Hz, 3J=14.7 Hz, 1H, C =CH2). 13C{1H} NMR (CDCl3,
75 MHz, ppm): 9.0 (Si− H2), 28.9 ( H3), 31.3 ( −CH3), 38.3
H2−C), 68.4 ( 5H5), 68.5 (ipso-Fc), 70.76, 70.78, 73.7, 73.8 ( 5H4),
133.1 (CH= H2), 136.9 (
H=CH2). 29Si {1H} NMR (CDCl3, 59 MHz,
ppm): (C−C) 2950, (C=C) 1162,
−12.1 (SiFc). IR (KBr, cm−1):
(Si−C) 819. MS (MALDI-TOF): m/z 510.2 [M+]. Anal. Calcd for
C28H34Fe2Si: C, 65.87; H, 6.71. Found: C, 65.59; H, 6.85.
n
n
n
n
-
1
9
δ
H
H
H
H
H
δ
H
H
δ
C
C
H
C
C
H
δ
ν
ν
δ
C
C
C
C
δ
ν
1
Data for 5: Yield: 0.74 g (6%). H NMR (CDCl3, 300 MHz, ppm):
0.94 (s, 9H, C 3), 1.02, 1.44 (m, 4H –C 2–), 4.10 (s, 10H, C5
4.37 (m, 8H, C5
CH=C
δ
ν
ν
H
H
H
H
H
H
H
Conclusions
δ
C
C
C
(
C
C
C
In this work we have successfully isolated and characterised in the
solid and solution states, several silicon- and siloxane-bridged
multiferrocenyl compounds bearing different reactive groups. We
propose that species Fc2(CH3)3C(CH2)2SiCH=CH2 (5), Fc2(CH2=CH-
O)SiCH=CH2 (6), Fc2(CH2=CH-O)Si−O−Si(O-CH=CH2)Fc2 (8) and
Fc2(CH2=CH-O)Si-O-SiFc3 (9) have resulted from the competing salt-
metathesis reactions of the corresponding chlorosilanes
(Cl3SiCH=CH2 or Cl3Si-O-SiCl3) with lithioferrocene and the
organolithium reagents CH2=CH−OLi or (CH3)3C(CH2)2Li which, in
turn, have been generated in situ by degradation of THF.
Multiferrocenyl species 6, 8 and 9 are noteworthy since, for the first
time, a CH2=CH−O− fragment resulting from the cleavage of the THF
ring has been entrapped and stored in very stable and redox-active
compounds, which have been crystallographically characterised. To
the best of our knowledge, these compounds are the first redox-
active molecules containing a reactive silyl vinyl ether group. In
addition, biferrocenyl 6, carrying both a vinyl and a vinyloxy group,
is also of interest since it represents the first, and so far the only
example, of an electroactive silyl mononomer with two different
reactive polymerisable groups.
C
C
δ
ν
ν
ν
1
Data for 6: Yield: 1.02 g (9%). H NMR (CDCl3, 300 MHz, ppm):
4.16 (s, 10H, C5 cis), 4.25,
5), 4.16 (d, 3J=5.7 Hz, 1H, O−CH=CHtrans
4.29, 4.42 (m, 8H, C5 trans Hcis),
4), 4.58 (d, 3J=13.5 Hz, 1H, O−CH=C
6.11 (dd, 3J=20.0 Hz, 2J=4.2 Hz, 1H, CH=C transHcis), 6.28 (dd, 3J=14.9
Hz, 2J=4.2 Hz, 1H, CH=CHtrans cis), 6.46 (dd, 3J=20.0 Hz, 3J=14.9 Hz,
1H, C =CH2).
=CH2), 6.59 (dd, 3J=13.5 Hz, 3J=5.7 Hz, 1H, O−C
13C{1H} NMR (CDCl3, 75 MHz, ppm):
68.5 (ipso-Fc), 68.7 ( 5H5),
71.3, 71.4, 73.7, 73.8 ( 5H4), 94.2 (O−CH= H2), 134.3 (CH=
135.6 ( H=CH2), 146.3 (O−
ppm):
δ
H
H
H
H
H
H
H
H
δ
C
C
C
C
H2),
C
C
H=CH2). 29Si {1H} NMR (CDCl3, 59 MHz,
δ
−7.8 (SiFc). IR (KBr, cm−1):
ν
(C=C) 1633, 1167,
ν
(Si−C) 824.
MS (MALDI-TOF): m/z 468.1 [M+]. Anal. Calcd for C24H24Fe2SiO: C,
61.53; H, 5.17. Found: C, 61.36; H, 5.05.
Data for 7: Yield: 0.53 g (5%). 1H NMR (CDCl3, 300 MHz, ppm):
δ
H4),
2.15 (s, 1H, OH), 4.17 (s, 10H, C5
6.11 (dd, 3J=20.2 Hz, 2J=4.1 Hz, 1H, CH=C
Hz, 2J=4.1 Hz, 1H, CH=CHtrans cis), 6.51 (dd, 3J=20.2 Hz, 3J=15.0 Hz,
1H, C 67.8 (ipso-Fc),
=CH2). 13C{1H} NMR (CDCl3, 75 MHz, ppm):
68.5 ( 5H5), 71.2, 71.3, 73.4, 73.5 ( 5H4), 134.0 (CH= H2), 136.3
H=CH2). 29Si {1H} NMR (CDCl3, 59 MHz, ppm):
−8.9 (SiFc). IR (KBr,
cm−1):
H
5), 4.26, 4.29, 4.42 (m, 8H, C5
H
transHcis), 6.23 (dd, 3J=15.0
H
Electrochemical studies reveal that, in both electrolyte systems
containing either the traditional [PF6]– or the weakly coordinating
[B(C6F5)4]– anion, the silicon-bridged diferrocenylvinyl compounds
5−7 undergo two well-separated reversible one-electron oxidations,
indicating intermetallic interaction between the ferrocenyl moeties.
The introduction of the 3,3-dimethylbutyl group in the silicon bridge
of 5 seems to increase the metal–metal interaction with respect to
that of the structurally related diferrocenyl systems 6 and 7. In
addition, tretraferrocenyl disiloxane 8 displays an interesting redox
behaviour that can be switched from a poorly unresolved multistep
redox processes, to four consecutive, clearly defined, one-electron
oxidations by changing the anion of the supporting electrolyte from
[PF6]− to the weak nucleophilic and ion-pairing [B(C6F5)4]− anion.
Future studies will focus on studying the reactivity of 6, 7 and 8 as
redox-active bifunctional silyl monomers.
H
δ
C
C
C
(C
δ
ν
(O−H) 3604, 3413, ν(C=C) 1166, ν(Si−C) 822. MS (MALDI-
TOF): m/z 442.1 [M+]. Anal. Calcd for C22H22Fe2SiO: C, 59.73; H, 5.02.
Found: C, 59.60; H, 5.15.
Reaction of FcLi with Cl3Si−O−SiCl3. Formation of Fc2(CH2=CH-
O)Si−O−Si(O-CH=CH2)Fc2 (8), Fc2(CH2=CH-O)Si-O-SiFc3 (9) and
Fc3Si−O−SiFc3 (10). Following the same procedure as the one
described for the previous reaction, sublimed ferrocene (4.5 g, 24.2
mmol) was dissolved in 30 mL of dry THF and 30 mL of dry
and treated with a solution of Cl3Si−O−SiCl3 (0.69 mL, 3.6 mmol) in
mL of THF. The red-orange oily product obtained after
n-hexane
7
appropriate treatment was purified by column chromatography on
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ARTICLE
Journal Name
Organometallics, 2012, 31, 3248–3258; (d) I. Cuadrado, C. M.
Casado, B. Alonso, M. Morán, J. Losada and V. Belsky, J. Am.
Acknowledgements
Chem. Soc., 1997, 119, 7613−7614; (e) S. Bruña, A. F.
The authors are grateful to the Spanish Ministerio de
Economía y Competitividad (MINECO) (projects CTQ2012-
30728 and CTQ2015-63997-C2-1-P) for the generous support
of this work. M. M. M.-C. and O. M. thank the STSM COST
Action CM1204 and the Project FOTOCARBON-CM S2013/MIT-
2841 of the Comunidad Autónoma de Madrid. Computational
time at Centro de Computación Científica (CCC) of Universidad
Autónoma de Madrid is acknowledged.
Garrido-Castro, J. Perles, M. M. Montero-Campillo, O. Mó, A.
E. Kaifer and I. Cuadrado, Organometallics, 2016, 35, 3507–
3519; (f) I. Cuadrado in Silicon-Containing Dendritic
Polymers; ed. P. R. Dvornic and M. J. Owen, Springer,
Germany, 2009, vol 2, ch.8, pp. 141-196.
15 For interesting examples of multiferrocenyl compounds see
for instance: (a) L. Xu, Y.-X. Wang, L.-J. Chen and H.-B. Yang,
Chem. Soc. Rev., 2015, 44, 2148
Chen, L. Xu, C.-H. Wang and H.-B. Yang, Chim. Chem. Lett.,
2013, 14, 471 474; (c) Q.-J. Li, G.-Z. Zhao, L.-J. Chen, H. Tan,
C.-H. Wang, D.-X. Wang, D. A. Lehman, D. C. Muddiman and
H.-B. Yang, Organometallics, 2012, 31, 7241 7247; (d) G.-Z.
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H.-B. Yang, Organometallics, 2011, 30, 5141 5146; (e) M. S.
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Albrecht and N. J. Long, Nat. Chem. 2016, , 825–830; (f) A.
−2167; (b) J.-K. Ou-Yang, L.-J.
−
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23 When [n-Bu4N][PF6] is used as the supporting electrolyte in
the low polarity dichloromethane solvent, the strong ion
pairing between the positively charged electrogenerated
ferrocenium species and the [PF6]- anion of the electrolyte,
12 | J. Name., 2012, 00, 1-3
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leave little or any electrostatic effect. See: A. K. Diallo, C.
View Article Online
DOI: 10.1039/C7DT02286G
ARTICLE
Absalon, J. Ruiz and D. Astruc, J. Am. Chem. Soc., 2011, 133
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,
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25 Both the E1/2 and the splitting ꢀE1/2 values are strongly
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Organometallics, 2014, 33
D'Alessandro and F. R. Keene, Dalton Trans., 2004,
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,
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−
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DOI: 10.1039/C7DT02286G
Two fragments resulting from the degradation of the THF ring has been entrapped and stored in redox-active and
crystalline multiferrocenes with vinyl, O-CH=CH2 and (CH2)2C(CH3)3 groups.