Transmetalation of a nucleophilic carbene fragment: From early to late transition metals

Article abstract of DOI:10.1039/c2cc17934b

Early transition metal nucleophilic carbene complexes have been used as stoichiometric carbene transfer agents in a transmetalation process. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17934b

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Transmetalation of a nucleophilic carbene fragment: from early to late
transition metalsw
´ ´ ´
Marie Fustier-Boutignon, Hadrien Heuclin, Xavier Frederic Le Goff and Nicolas Mezailles*
Received 19th December 2011, Accepted 2nd February 2012
DOI: 10.1039/c2cc17934b
Early transition metal nucleophilic carbene complexes have been
used as stoichiometric carbene transfer agents in a transmetalation
process.
The ‘‘Transmetalation’’ elementary step is at the heart of the
extensively studied transition metal complex catalyzed cross
coupling reactions.1 Regardless of some subtleties, the transition
state is a heterobimetallic species (eqn (1), Scheme 1), whose
energetic accessibility is linked to the ability of the C atom to
increase its valency. As a consequence, transmetalation processes
involving C sp2 or C sp fragments are much more favorable
than the corresponding C sp3 which has to go through a
‘‘pentavalent-like’’ transition state.
From another standpoint, electrophilic transition metal
carbene complexes, Fischer type carbenes, have been known
for nearly 50 years and used extensively as stoichiometric
reactants in organic synthesis.2 Starting in 1970, successful
stoichiometric metal to metal electrophilic carbene transfer
has opened the way for the design of a catalytic process in the
Scheme 1 Known transmetalation processes and proposed strategy.
late 1990s involving transmetalation from group 6 Fischer carbene
to a palladium fragment as a key step (eqn (2), Scheme 1).3 DFT
calculations carried out in this process have highlighted the
energetic accessibility of related heterobimetallic complexes both
as a transition state and even as an intermediate in the process.4 In
the past years, we and others have been developing the use of
geminal dianions5 (such as A in Scheme 1) as precursors
for carbene complexes.6 Such highly sensitive and reactive
dianionic species are rare because the two lone pairs at the
same carbon center need to be efficiently stabilized by strongly
electron accepting moieties. It was shown that hypervalent
P centers were particularly suitable for this goal.5 The underlying
strategy was to utilize the dianionic ligand as the sole carrier of
the four electrons formally engaged in the M–C interaction in
carbene complexes.z So far, this strategy has been very successful
and in particular provided simple access to carbene complexes of
Ti(IV) and Zr(IV),7 of rare earth8 or U(IV),9 U(V)10 and U(VI)11
with pronounced nucleophilic behavior (reactivity toward
carbonyl derivatives, in an alkylidene like fashion). Electron
rich metal fragments could also be used (Ru(II) or Pd(II)).12
However, in some instances, despite efficient stabilization of
the dianion A, undesired and uncontrolled redox processes
became competitive with coordination, preventing isolation of the
desired complexes. We then reasoned that carbene complexes of
early transition metals could be used to transfer the carbenic
fragment to other metals (middle to late). To our surprise,
literature search revealed that only one related process has been
reported to date, which involves the alkylidene transfer from
Ta(V) to W(VI) (eqn (3)).13 In this contribution, we report the first
examples of transmetallation reaction involving the stoichiometric
transfer of the carbenic fragment from Zr(IV) and Sc(III)
complexes to group 8, 9 and 10 metal centers. In particular,
this novel strategy has provided the only access to date to a
Fe complex using these types of ligands.
In a first series of experiments, the readily available nucleophilic
Zr carbene complex 1 was used.7b In order to test the validity
of our concept, the transmetalation reaction to form known
complexes was attempted. The reactions of dimeric complex 1
with [Ru(PPh3)3Cl2] and [Pd(PPh3)2Cl2] were carried out in
THF, and most satisfyingly, led to the formation of the desired
Laboratoire ‘‘He´te´roe´le´ments et Coordination’’,
Ecole Polytechnique CNRS, Palaiseau, France.
E-mail: nicolas.mezailles@polytechnique.edu; Fax: 0033 169334440;
Tel: 0033 169334402
w Electronic supplementary information (ESI) available: Experimental
details. CCDC 848404 and 848405. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2cc17934b
c
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complex C with bridging Cl as a second intermediate. The overall
transfer is rather complex as unlike the above mentioned transfer of
an electrophilic carbene fragment, three bonds have to be broken in
complex 1 or 5, two Zr–S and the ZrQC bonds to be formed with
M in the final complex. Examples of homo or heterobimetallic
complexes are known, supporting the intermediacy of complex D
in the proposed mechanism.15 With these encouraging results
proving the concept, the synthesis of the more challenging
Fe complex was attempted. Indeed, when the dianion A was
reacted with several Fe(II) or Fe(III) precursors, only redox to
Fe(0) was observed concomitant to the formation of the
neutral ligand dppmS2 most likely by hydrogen abstraction
from the solvent. In fact, no Fe complex has been reported to
date with these dianionic ligands, although several Ru complexes
are known.16 Interestingly, the reaction of complex 1 with
stoichiometric amounts of FeCl2 did not lead to any decom-
position, showing the efficient shutting down of an undesired
redox pathway when using a carbene complex in place of the
dianion A. However, the reaction appeared very sluggish, even at
high temperatures (36 h, toluene reflux) and no Fe(II) complex
(expected to be silent by NMR spectroscopy) could be isolated
from the reaction mixture. As suggested by the proposed simplified
mechanism shown above, a more reactive carbene complex, for
which solvent coordination and/or S and/or carbene coordination
would be weaker, was therefore needed.
Scheme 2 Transmetalation involving a Zr(IV) carbene complex.
complexes 2 and 3, respectively, as shown by 31P NMR
spectroscopy (Scheme 2). The kinetics of the reaction appeared
slower than with dianion A, and the use of THF as solvent
needed to favor the formation of the ZrCl4 (THF)2 byproduct
(note that the use of THF is precluded when using the dianion A
as it readily forms the monoanion).5a The complete formation of
complexes 2 and 3 was observed within 15 minutes and several
hours, respectively, at room temperature (two sets of triplets
at 57.6 ppm and 48.8 ppm, JPP = 12 Hz for 2, and a triplet
at 21.5 ppm and a doublet at 39.8 ppm, JPP = 15 Hz for 3).12
The transmetalation to Co(II) was then attempted, and appeared
to require more forcing conditions (24 h, reflux). The reaction was
nevertheless nearly quantitative, and in the 31P NMR spectrum,
the slow increase of a singlet at 187 ppm corresponding to the
previously reported complex 4 was observed,14 concomitant to
the decrease of the signal of complex 1. The desired complex was
then isolated in 80% yield. As far as the mechanism is concerned
(Scheme 3), the formation of an early intermediate, characterized
by a singlet at 22 ppm, was observed in the three cases, which we
thus postulated to be the mononuclear Zr complex 5. Complex 5
was independently synthesized and its structure was proven
by X-ray diffraction analysis (see ESIw). Interestingly, when isolated
and dissolved in a non-coordinating solvent (such as CH2Cl2)
this complex 5 evolved back to complex 1, showing that
chloride coordination from a metal–Cl bond is competitive
to THF coordination. From this mononuclear Zr complex we
then logically propose the formation of a heterobimetallic Zr/M
We logically turned our attention to the only known
monomeric Sc carbene complex, 6, as it was shown by DFT
calculations that the C–Sc p interaction is weaker than in the
corresponding Zr complex, B.8i The same first three reactions
were carried out (Scheme 4). They all proceeded very smoothly,
being complete within 15 minutes at room temperature. The very
significant difference in the kinetics of the reaction with CoCl2 of
the two carbene complexes 1 and 6 (24 h, 60 1C vs 15 minutes,
RT respectively) proved our hypothesis right. The reaction
between complex 6 and a suspension of [FeCl2(THF)1.5] was
followed by 31P NMR. The singlet at 10 ppm had totally
disappeared within 15 minutes, with a simultaneous deposition
of red crystals. After isolation they were dissolved in C6D6 and
characterized by multinuclear NMR spectroscopy. As expected,
the complex is silent by 31P NMR, and in the 1H spectrum, the
protons of the phenyl rings are found between 4 and 10 ppm,
both indicative of coordination on paramagnetic Fe(II). The
crystals were analyzed by X-ray diffraction and proved the
formation of a dimer, with the C atom behaving as a bridging
dianionic ligand between two Fe centers. A view of the complex
Scheme 3 Proposed simplified mechanism for the transmetalation.
Scheme 4 Reactivity of the Sc(III) carbene complex.
Chem. Commun., 2012, 48, 3306–3308 3307
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Fig. 1 View of complex 7 (hydrogen atoms have been omitted for
clarity). Selected bond lengths (A) and angles (1). C(1)–P(1)
1.736(4), C(1)–P(2) 1.728(3), P(1)–S(1) 2.028(1), P(2)–S(2) 2.037(1),
C(1)–Fe(1) 2.099(3), C(1)–Fe(2) 2.082(3), Fe(1)–Fe(2) 2.5683(8),
Fe(1)–S(1) 2.380(1), Fe(2)–S(2) 2.355(1), P(1)–C(1)–P(2) 128.8(2),
C(1)–Fe(1)–C(10)–Fe(2) 0.00.
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is given in Fig. 1. The structure of complex 7 is similar to the one
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Bond lengths in the ligand in complex 7 are similar to the
ones of complex 4 (1.736(4) and 1.728(3) A vs 1.742 and 1.728 A
for the P–C bonds). The main structural difference is the metal–
metal bond. Indeed, the Fe–Fe bond is longer by ca. 0.1 A in
complex 7 than in complex 4 (2.568 A vs 2.471 A). The Fe–Fe
bond distance is however shorter than the sum of the van der Waals
radii (2.64 A), indicative of a strong interaction. In terms of
reactivity, the Fe complex 7 does not react with carbonyl
derivatives, such as benzophenone, unlike the Sc and Zr
complexes (6 and 1).7b,8e The complex is also stable toward
weak acid sources such as CH3CN or the strongly alkylating
agent MeI, in agreement with a strong electron transfer from
the formally dianionic ligand to the metal. In line with the
apparent paramagnetic nature observed by NMR, the effective
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(expected 4.0 mB). This value is in agreement with two Fe(II)
centers with S = 1 each.
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In conclusion, we show here for the first time that tridentate
SCS nucleophilic carbene complexes may be used as stoichiometric
carbene transfer agents in a transmetalation process. This
novel transformation does allow circumventing redox competitive
processes. As such it opens the way for the synthesis of ‘‘SCS’’
carbene complexes of oxidized and oxidizing metal centers.
The precise mechanism of this transformation is currently
being investigated using molecular modeling.
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´ ´
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The authors thank the CNRS and the Ecole Polytechnique
for financial support. M. F. B. and H. H. are thankful to the
Ecole Polytechnique for fellowships.
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Notes and references
z The two substituents at the C are PR2QX (X = S, O, NR0) moieties,
and therefore the complexes cannot be called alkylidene complexes
which imply alkyl (or H) substituents. They are called here carbene
complexes, and can behave as either nucleophilic (alkylidene like
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This journal is The Royal Society of Chemistry 2012