Mono- and Bimetallic Alkynyl Metallocenes and Half-Sandwich Complexes: A Simple and Versatile Synthetic Approach

Article abstract of DOI:10.1002/chem.201603462

A general process for the synthesis of alkynyl mono and dimetallic metallocenes and half-sandwich complexes has been developed. This approach uses the addition of lithium derivatives of sandwich or half-sandwich complexes to arylsulfonylacetylenes. The reaction occurs in two steps (lithiation and anti-Michael addition to alkynylsulfone followed by elimination of the ArSO₂moiety) to form the corresponding mono- or bimetallic alkynes in clearly higher yields, simpler experimental procedures, and more environmentally benign conditions than those of the so far reported for the synthesis of this type of products. The electrochemical properties of the newly obtained complexes have also been studied.

Full text of DOI:10.1002/chem.201603462

A Journal of
Accepted Article
Title: Mono- and Bimetallic Alkynyl Metallocenes and Half-Sandwich Complexes: A
Simple and Versatile Synthetic Approach
Authors: Miguel A. Sierra; Luis Casarrubios; Mar ´ı a C. De La Torre; Jose Luis
Garc ´ı a-Ruano; Jos e´ Alem a´ n; Carolina Valderas; Leyre Marzo
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To be cited as: Chem. Eur. J. 10.1002/chem.201603462
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Chemistry - A European Journal
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COMMUNICATION
Mono- and Bimetallic Alkynyl Metallocenes and Half-Sandwich
Complexes: A Simple and Versatile Synthetic Approach
[
a,b,c,†]
[d,†]
[b,c]
[d]
Carolina Valderas,
Leyre Marzo,
María C. de la Torre,
José Luis García Ruano, José
[
d]
[a,b]
[a,b]
Alemán,* Luis Casarrubios,
Miguel A. Sierra*
Dedicated to Prof. José María Marinas Rubio with the occasion of his retirement
Abstract: A general process for the synthesis of alkynyl mono and
dimetallic metallocenes and half-sandwich complexes has been
developed. This approach uses the addition of lithium derivatives of
sandwich or half-sandwich complexes to arylsulfonylacetylenes. The
reaction occurs in two steps (lithiation and anti-Michael addition to
2
alkynylsulfone followed by elimination of the ArSO moiety) to form
the corresponding mono- or bimetallic alkynes in clearly higher
yields, simpler experimental procedures, and more environmentally
benign conditions than those of the so far reported for the synthesis
of these type of products. The electrochemical properties of the
newly obtained complexes have also been studied.
1
-Metallocenyl- and half-sandwich substituted alkynes have
Figure 1. Relevant structure containing metallocene and half-sandwich
substituted alkyne derivatives (Fc= ferrocenyl).
[
1]
profited as essential features or precursors of molecular wires,
linked
compounds and related fields. For example, the fullerene
derivative A forms Fc-C60/Pd and bis-Fc-C60/Pd electroactive
polymers, while B is a molecular switch controlled by light
sources (top, Figure 1). It is also possible to find interesting
metallocene compounds in biomedicine and analytical areas, as
the derivative of BAPTA C, which selectively chelates Ca ions
that allows monitoring within cells, and compounds such as D,
[
2]
donor-acceptor
arrangements,
mixed
valence
[
3]
[4]
approaches for the synthesis of such as interesting compounds.
Instead of them, their preparation requires, in general, multi-step
synthesis mainly based in two different strategies, the cross-
coupling of bromo- and iodo derivatives of the organometallic
[
5]
[
6]
[10]
[11]
moiety with alkynyl stannanes,
or alkynes,
and the
[
7]
2+
functionalization of the unsubstituted ethynyl organometallic
[
8]e
[12]
derivatives
(equations a and b, Scheme 1). Moreover, these
which have been used to determine the concentration of
bioactive compounds (such as L-ascorbic acid, tryptophan, D-
penicillamine, or dopamine) in pharmacological or biological
samples (bottom-left, Figure 1). Additionally, structures having
two metal centers, as E and F, are of great interest for obtaining
approaches are not general (only valid when M= Fe, Ru), usually
[13,14]
provide poor overall yields,
and require the use of the
scarcely ecofriendly transition metals as catalysts of the key
steps. Thus, it is clear that a general method involving a shorter
route (ideally one step) to access to ethynyl organometallic
derivatives, flexible enough to allow the synthesis of different
metallocene- and half-sandwich monometallic and bimetallic
derivatives, is desirable
[9]
"
push-pull" systems (bottom-right).
Despite the fact that these classes of organometallic
derivatives are quite important, there are not efficient general
Recently, we have reported the electrophilic behavior of
arylsulfonylacetylenes with organolithiums through an unusual α-
attack (anti-Michael addition) followed by elimination of the
2
3
2
ArSO moiety to allow the alkynylation of lithiated sp and sp
[
15]
carbon atoms, yielding the corresponding alkynes. Based on
these results, we envisioned the possibility of using lithium
[
a]
Dr. M. A. Sierra, Dr. C. Valderas, Dr. L. Casarrubios
Departamento de Química Orgánica, Facultad de Ciencias Químicas,
Universidad Complutense, 28040 Madrid, Spain. E-mail: (MAS)
sierraor@ucm.es, www.biorganomet.es
Dr. M. A. Sierra, Dr. M. C. de la Torre, Dr. C. Valderas, Dr. L.
Casarrubios. Centro de Innovación en Química Avanzada (ORFEO-
CINQA)
Dr. M. C. de la Torre, Dr. C. Valderas Instituto de Química Orgánica
General, Consejo Superior de Investigaciones Científicas (CSIC), Juan
de la Cierva 3, 28006-Madrid, Spain
Dr. J. Alemán, Prof. Dr. J. L. García Ruano, Dr. L. Marzo
Organic Chemistry Department,
metallocenes
in
reactions
with
arylsulfonylacetylenes
(
commercially available or easily prepared in one step from
[
[
[
b]
c]
d]
[
16]
alkynes ) as a general method for obtaining the coupling of
alkynyl and metallocenyl moieties (equation c, Scheme 1). In
this work, we present the results obtained in this research which
has provided different alkynyl metallocenes, half-sandwich
derivatives, and bis-metallocenyl alkynes. The clear advantages
of our procedure with respect to those so far reported,
concerning experimental simplicity, conditions and yields
determine our reaction must be considered as the method of
choice for preparing these important class of compounds. Finally,
some interesting electrochemical properties of these compounds
will be presented.
Universidad Autónoma de Madrid
28049-Madrid
E-mail: jose.aleman@uam.es, www.uam.es/jose.aleman
These authors contributed equally to the work reported in this paper.
†
Supporting information for this article is given via a link at the end of the
document.
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employed before isolation (see supporting information). (n.r.= no reaction, see
text).
Analogous Li-ruthenocene and Li-osmacene were obtained
from the corresponding metallocenes 2B and 2C, following the
procedure optimized for ferrocene (tBuLi at 0ºC in a 1:1
THF/pentane mixture). They were subsequently reacted with
phenylethynyl sulfone 2a at 0 ºC. Although the conversion to
1
both phenylethynyl metallocenes was quantitative by H NMR,
ruthenocene derivative 3Ba was isolated in 71% yield, and the
osmacene derivative 3Ca in 30% yield, due to extensive
decomposition during the chromatography purification (Table 2).
The lithiation of manganocene 2D was not possible in our hands,
and only decomposition products were detected by reaction with
[
17]
the acetylene 1a.
Scheme 1. Different general approaches for the synthesis of alkynyl half-
sandwich and metallocenes.
[
a]
Table 2. Reaction of different metallocene lithium derivatives with 1a.
We initiate the study with the preparation of different
ferrocenyl alkynes. Ferrocenyl lithium is easily available by
t
treatment of ferrocene with BuLi at 0 ºC in a 1:1 THF/pentane
mixture. Lithium derivative of ferrocene was reacted with a two-
fold excess of phenylethynyl sulfone 2a, cleanly affording the
corresponding phenylethynyl ferrocene 3Aa in a 69% isolated
yield in few minutes. In comparison to the previous work, the
reaction of 2a with phenyl-lithium was instantaneous at -78 ºC,
which indicates a lower reactivity of the ferrocenyl lithium
[a] Reactions were carried out at -78 or 0 ºC with 0.2 mmol of sulfone 1a and
0.4 mmol of the lithium derivative [b] Lithiation process was not possible, only
decomposition products were found in the crude mixture.
[
15]
derivative.
This reaction was general for aryl-substituted
sulfones (Table 1) having both electron donor (2Ab-Ad) or
electron withdrawing substituents (2Ae and 2Af). From
moderate to good yields in the corresponding arylalkynyl
ferrocenes were obtained in all cases. On the contrary, tBu and
Cy acetylene derivatives 1g and 1h led complicated crude
mixtures in which Michael type products could be identified.
Finally, ethynyl ferrocene 3Ai was obtained in a two-step
sequence (69% overall yield) starting from the TIPS-alkynyl
sulphone 2i followed by direct desilylation of TIPS-
ethynylferrocene (TBAF/THF).
The preparation of half-sandwich Re and Mn ethynyl
derivatives was next attempted. Complexes 2E-F were lithiated
under the above mentioned conditions and the corresponding Li-
1
derivatives reacted with sulfone 2a. In both cases H NMR
showed complete conversion into the desired phenylethynyl half-
sandwich derivatives, but the isolated yields for compounds 3Ea
and 3Fa were 62% and 57% (Table 3). Under similar conditions
(
addition of n-BuLi at -78 ºC and treatment with 1a at -78 ºC),
[
a]
Table 1. Reaction of ferrocenyl lithium with different arylethynyl-suflones 2.
6
tricarbonyl (η -phenyl)chromium (0) was transformed into the
ethynyl complex 3Ga with a 56% yield. Therefore, this approach
is also suitable for the preparation of alkynyl derivatives of half-
[
18]
sandwich complexes.
Table 3. Reaction of mono-sandwich lithium derivatives with the
[
a]
phenylethynyl-sulfone 1a.
[
a] Reactions were carried out at -78 or 0 ºC with 0.2 mmol of sulfone 1a and
.4 mmol of the lithium derivative [b] Both steps were carried out at – 78 ºC
and THF as solvent.
0
[
a] Reactions were carried out at -78 ºC with 0.2 mmol of sulfone 1 and 0.4
mmol of the lithium ferrocene derivative.[b] Direct desilylation by TBAF was
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Hetero-bimetalocenyl alkynes, having the two metallocenes
bridged through one ethynyl moiety were next studied. Thus,
ferrocenylethynyl sulfone 1j (prepared from 3Ai) was reacted
with ruthenocenyl lithium 2B. The resulting Fe-Ru
heterobimetallic compound 3Bj was very unstable and it could
be isolated only in a 20% yield. Analogously, the reaction of
obtained. Complexes 3Aa-Af showed a single reversible
oxidation event assigned to the Fe(II) – Fe(III) process. As
expected, due to the low conjugation of the phenyl ring with the
ferrocene moiety, the substitution in the aromatic ring resulted in
a low influence in the oxidation potential of the complex (0.61-
0.68 V, compared to 0.66 V for ethynyl ferrocene). Nevertheless,
complexes 3Aa-3Ad having donor substituents in the p-position
of the aromatic ring showed lower oxidation values than
complexes 3Ae and Ae having electron withdrawing substituents.
Albeit small, this effect should probably be due to the
stabilization of the ferrocenium radical-cation formed during the
oxidation event.
3
sulfone 1j with the lithium reagent derived from Cp(CO) Re (2F-
Li) yielded the heterobimetallic Fe-Re derivative 3Fj in a
respectable 50% isolated yield (Scheme 2). The applicability of
this approach to prepare heterobimetallic complexes was thus
demonstrated.
[
a]
Table 4. The electrochemical data of arylethynylferrocenes 3Aa-Af.
E
pa (V)
E
pc (V)
E
p1/2 (V)
Entry
Compound (R)
Aa (H)
Ab (4-Me)
Ac (4-OMe)
Ad (2,4,5-triMe)
Ae (4-CF
Af (2-Cl)
3
0.66
0.65
0.61
0.68
0.66
0.68
0.56
0.57
0.53
0.56
0.48
0.47
0.10
0.08
0.08
0.12
0.18
0.21
1
2
3
4
5
6
3
3
3
3
3
)
3
Scheme 2. Synthesis of heterobimetallic complexes 3Bj and 3Fj from alkynyl
sulfone 1j.
[a] Recorded at rt in MeCN using 0.1
4
M [NBu ]ClO4 as supporting
electrolyte and Ag/AgCl 3M as reference and a Pt wire counter electrode
(scan rate 0.1 V/s).
Being aware of the wide use of ethynyl ferrocene as an
[19]
electrochemical marker,
and the easiness of our methodology
that allow the easy incorporation of a wide range of aromatic
groups to the ethynyl sandwich derivatives (3Aa-Af) and flexible
entry developed above to diverse types of half-sandwich
derivatives (3Ea-3Ga) and heterobimetallic complexes (3Bj-3Fj),
Electrochemical data for half-sandwich complexes 3Ea and
Fa are presented in Figure 3. An irreversible oxidation wave at
.30 V (Re complex 3Ea) and 1.34 V (Mn complex 3Fa), is in
3
1
good agreement for the described oxidation values of not
we decided to undertook
electrochemical properties.
a
systematic study of their
+
substituted complexes (1.16
V
relative to Fc/Fc
for
).
[
20]
+
[21,22]
CpRe(CO)
3
and 0.92 V related to Fc/Fc for CpMn(CO)
3
In both cases, the oxidation wave is assigned to the formation of
3
Ea and 3Fa respectively. It is known that these species have a
[20-
strong tendency to dimerize, making this process irreversible.
2
2]
Figure 2. The electrochemical behavior of arylethynylferrocenes 3Aa-Af.
The electrochemical behavior of arylethynylferrocenes 3Aa-
-3
Af was first considered. In all cases room temperature (rt) 10
solutions in MeCN as solvent and 0.1M NBu ClO as supporting
electrolyte were used to register the cyclic voltammograms of
4
4
Figure 3. The electrochemical behavior of arylethynylferrocenes 3Ea-3Fa.
complexes 3Aa-Af. Figure 2 and Table 4 collect the results
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COMMUNICATION
A comparative study of the redox behavior of alkynyl
sandwich complexes was performed for complexes 3Aa, 3Ba
and 3Ca. The results are shown in Figure 4 and Table 5 (cyclic
and square wave voltamperometry). Thus, while complexes
ferrocene 3Aa and ruthenoce 3Ba showed a quasi-reversible
behavior (p = 0.10 V and 0.15 V, respectively), the osmocene
derivative 3Ca was found with a clearly irreversible wave (p =
compared with the usual methods for the synthesis of these type
of attractive products, our sequence produces better yields in
easier experimental procedures without the use of expensive
transition metals or toxic reagents. The electrochemical
properties of the newly obtained complexes have been studied.
For arylethynyl ferrocenes the influence of the substituents in the
+
oxidation wave attributed to the Fc/Fc one electron oxidation is
0
.41 V). These results are congruent with those reported in the
negligible showing a lack of communication between the aryl
group and the metal center. A similar behavior is encountered in
bimetallic Fe-Ru 3Bj and Fe-Re 3Fj complexes, which have an
electrochemical imprint almost identical to the overlapping of the
corresponding monometallic complexes.
[
22]
literature.
Figure 4. Comparison in the electrochemical behavior of 3Aa, 3Ba and 3Ca.
[
a]
Table 5. The electrochemical data of 3Aa, 3Ba and 3Ca.
E
pa (V)
E
pc (V)
E
p1/2 (V)
Entry
Compounds
3Aa
3Ba
3Ca
0.66
0.85
0.79
0.56
0.70
0.38
0.61
0.77
0.85
1
2
3
[
a] Recorded at rt in MeCN using 0.1
4 4
M [NBu ]ClO as supporting
electrolyte and Ag/AgCl 3M as reference and a Pt wire counter electrode
(scan rate 0.1 V/s).
Bimetallic Fe-Ru 3Bj and Fe-Re 3Fj complexes have an
electrochemical imprint almost identical to the overlapping of the
corresponding monometallic complexes (Figure 5), which clearly
indicates a lack of communication between the two metals of the
molecule. Both complexes showed a cathodic displacement of
Figure 5. The electrochemical behavior of heterobimetallic complex 3Bj and
comparison with 3Aa and 3Ba (top) and 3Fj with 3Fa and 3Aa (bottom).
0.11 V in the reversible Fe oxidation of the Fe-Ru complex 3n
and an analogous shift of 0.05 V in the irreversible wave of the
Ru. These small variations may be explained by the stabilizing Acknowledgements
effect of the second metal over the cation (first oxidation wave)
and the double cation (second oxidation wave) with respect to
the monometallic complexes. In the Fe-Re complex these
differences are even lower (0.03 and 0.02 V, respectively) than
in its Fe-Ru congeners.
Support for this work under grants (CTQ2015-64561-R to JA
CTQ2013-46459-C2-01-P to MAS; CTQ2013-46459-C2-02-P to
MCT, CTQ2014-51912-REDC Programa Redes Consolider)
from the MINECO (Spain) is gratefully acknowledged.
To conclude, a general protocol to access to sandwich and
half-sandwich alkynyl derivatives, including heterobimetallic
derivatives has been developed. This approach makes use of
the addition of lithium derivatives of sandwich or half-sandwich
complexes and arylsulfonylacetylenes. The process is general
and tolerates different substituents at the alkyne moiety. When
Keywords: acetylene sulfones • metallocenes • anti-Michael •
Half-Sandwich Complexes
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Chemistry - A European Journal
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COMMUNICATION
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[
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Chemistry - A European Journal
10.1002/chem.201603462
COMMUNICATION
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COMMUNICATION
Carolina Valderas, Leyre Marzo,María
C. de la Torre, José Luis García Ruano,
José Alemán*, L. Casarrubios, Miguel A.
Sierra*
Page No. – Page No.
Mono- and Bimetallic Alkynyl
Metallocenes and Half-Sandwich
Complexes: A Simple and Versatile
Synthetic Approach
A general process for the synthesis of alkynyl mono and dimetallic metallocenes
and half-sandwich complexes has been developed. This approach uses the
addition of lithium derivatives of sandwich or half-sandwich complexes and
arylsulfonylacetylenes.
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