Aminolysis of bis[bis(trimethylsilyl)amido]iron and -cobalt as a versatile route to N-heterocyclic carbene complexes

Article abstract of DOI:10.1021/om200951m

A range of new N-heterocyclic carbene complexes of iron(II) and cobalt(II) have been conveniently obtained by the aminolysis of bis[bis(trimethylsilyl) amido]iron and -cobalt precursors with imidazol(in)ium salts. Whereas sterically less hindered salts produced the tetrahedral complexes [M(carbene) ₂Cl₂] (M = Fe), bulkier salts gave the three-coordinate [M(carbene){N(SiMe₃)₂}Cl] (M = Fe, Co), which serve as versatile precursors to a range of derivatives; mechanistic aspects of the reaction are discussed.

Full text of DOI:10.1021/om200951m

Communication
pubs.acs.org/Organometallics
Aminolysis of Bis[bis(trimethylsilyl)amido]iron and -cobalt as a
Versatile Route to N-Heterocyclic Carbene Complexes
Andreas A. Danopoulos,* Pierre Braunstein, Neoklis Stylianides, and Marcel Wesolek
Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universite
F-67081 Strasbourg Cedex, France
́
de Strasbourg, 4 rue Blaise Pascal,
S
* Supporting Information
ABSTRACT: A range of new N-heterocyclic carbene complexes
of iron(II) and cobalt(II) have been conveniently obtained by the
aminolysis of bis[bis(trimethylsilyl)amido]iron and -cobalt
precursors with imidazol(in)ium salts. Whereas sterically less
hindered salts produced the tetrahedral complexes [M-
(carbene)2Cl2] (M = Fe), bulkier salts gave the three-coordinate
[M(carbene){N(SiMe3)2}Cl] (M = Fe, Co), which serve as
versatile precursors to a range of derivatives; mechanistic aspects
of the reaction are discussed.
he chemistry of the N-heterocyclic carbene complexes of
Tiron and cobalt is less developed than that of other
transition metals. Recently, important catalytic reactions have
Scheme 1. Summary of the NHC Complexes Obtained by
Aminolysis of [M{N(SiMe3}2] (M = Fe, Co) with Azolium
a
Salts
Figure 1. Molecular structure of 1b. Selected bond lengths (Å) and
angles (deg): C(1)−Fe(1), 2.132(3); C(1)−N(1), 1.348(3); C(1)−
N(2), 1.349(3); N(1)−C(1)−N(2), 106.0(2); C(1)#1−Fe(1)−C(1),
103.33(13); Cl(1)#1−Fe(1)−Cl(1), 106.90(4); C(1)#1−Fe(1)−
Cl(1), 104.90(7).
been reported in which the in situ formed catalyst, from an
imidazolium salt and a basic organometallic reagent in the
presence of iron or cobalt precursors, is presumably an N-
heterocyclic carbene metal complex.1,2 Furthermore, a few
examples where well-defined iron NHC complexes are involved
in catalytic transformations3−6 or structurally mimicking [FeFe]
hydrogenase enzymes have also appeared.7,8 However, the
majority of well-characterized NHC complexes of Fe and Co
bear carbonyl,9−11 cyclopentadienyl,3,11−16 or nitrosyl17 stabi-
a
Legend: (i) 2 equiv per Fe of azolium salt in THF, room temperature,
12 h; (ii) 1 equiv of 1,3-bis(tert butyl)imidazolinium chloride, toluene,
room temperature, 12 h; (iii) 1 equiv of azolium or azolinium chloride,
toluene, room temperature, 12 h.
Received: October 10, 2011
Published: December 2, 2011
© 2011 American Chemical Society
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Scheme 2. Selected Chemical Transformations Starting from
a
Complexes 2 and 3
Figure 2. Molecular structure of 2b (one of the two independent
molecules in the asymmetric unit is shown). Selected bond lengths (Å)
and angles (deg): C(1)−Fe(1), 2.138(3); C(1)−N(1), 1.340(4);
C(1)−N(2), 1.345(4); Cl(1)−Fe(1), 2.2353(10); Fe(1)−N(3),
1.932(3); N(3)−Fe(1)−C(1), 127.38(12); N(3)−Fe(1)−Cl(1),
118.05(9); C(1)−Fe(1)−Cl(1), 114.57(9).
a
a denotes an unsaturated and b a saturated NHC. Legend: (i)
TlOSiPh3/toluene; (ii) HOSiPh3/toluene; (iii) CyNHC(Me)NCy/
toluene; (iv) LiNHDiPP/ether; (v) H2NDiPP/octane.
conditions. Since the silylamido species responsible for the C2
metalation of the azolium salts and the operating metalation
mechanism are not established, we decided to further explore
the scope and the details of the reaction, especially using simple
azolium salt precursors to monodentate NHCs, with well-
understood steric and electronic properties.27
The reaction of [Fe{N(SiMe3)2}2] with 1,3-Pri2-imidazolium
chloride (2 equiv per Fe) in THF at room temperature gave
good yields of the colorless [Fe(NHC)2Cl2] (1a; NHC = 1,3-
diisopropylimidazol-2-ylidene) (see Scheme 1). It is analogous
to a complex previously obtained6,24 by direct substitution or
association methods.
However, the versatility and scope of the aminolysis
methodology is best demonstrated by the synthesis of
[Fe(NHC)2Cl2] (1b; NHC = 1,3-bis(2-isopropylphenyl)-
imidazolin-2-ylidene), since in this case the free NHC is not
stable under the substitution reaction conditions. Complex 1b
was obtained by the reaction of 1,3-bis(2-isopropylphenyl)-
imidazolinium chloride with [Fe{N(SiMe3)2}2] in THF and
was crystallized from THF/pentane. It was characterized
analytically and crystallographically (see Figure 1). Its structure
is consistent with the observed magnetic behavior for a high-
spin tetrahedral Fe center (5.2 μB); the bond lengths and angles
fall within the range observed for similar complexes in the
literature.6,19,24
In contrast, the reaction of [Fe{N(SiMe3)2}2] with 1,3-
bis(aryl)imidazol(in)ium chlorides (1 equiv per Fe) at room
temperature in THF or, preferentially, toluene gave excellent
yields of the colorless, air-sensitive three-coordinate species
[Fe(NHC){N(SiMe3)2}Cl] (NHC = IPri (2a), SIPri (2b),
IMes (3a), SIMes (3b); IPri = 1,3-(DiPP)2-imidazol-2-ylidene,
SIPri = 1,3-(DiPP)2-imidazolin-2-ylidene; IMes =1,3-(Mes)2-
imidazol-2-ylidene; SIMes =1,3-(Mes)2-imidazolin-2-ylidene,
Mes = mesityl, DiPP = 2,6-diisopropylphenyl). Interestingly,
aminolysis of only one N(SiMe3)2 group took place,
irrespective of the stoichiometry of the added reagents.
Magnetic measurements show the presence of one high-spin
iron center (5.5 μB). The structure of 2b is shown in Figure 2.
Figure 3. Molecular structure of 4. Selected bond lengths (Å) and
angles (deg): Cl(1)−Fe(1), 2.3043(8); Fe(1)−N(3), 1.938(2);
Fe(1)−N(4), 1.947(2); N(3)−Fe(1)−N(4), 129.38(10); N(3)−
Fe(1)−Cl(1), 112.58(8); N(4)−Fe(1)−Cl(1), 117.99(8).
lizing coligands or have the NHC donor as part of a chelating
framework.18−23 In both cases, the reactivity of the complex can
be modulated by these features.
The synthesis of NHC complexes of Fe from preformed
6
NHCs and FeCl2 is associated with problems of poor
stoichiometry control, long reaction times, and forcing reaction
conditions that are incompatible with the stability of certain
preformed NHC reactants. Very recently, a synthetic method
based on the substitution of tmed in [FeCl2(tmed)] (tmed =
N,N,N′,N′-tetramethylethylenediamine) by the free N-hetero-
cyclic carbene 1,3-R2-3,4-dimethylimidazol-2-ylidene (R = Me,
Et, Pri) was reported. The scope of this method is not yet fully
known.24 The aminolysis of the Fe and Co silylamides
[M{N(SiMe3)2}2] (M = Fe, Co36)25,26 by suitable imidazolium
halides cleanly afforded “pincer” NHC complexes; this
methodology was applied very recently also to the synthesis
of chelating dicarbene complexes.18−20 However, its scope
appeared to be limited to imidazolium halides; imidazolium
−
−
salts associated with noncoordinating anions (e.g., BF4 , PF6 )
were unreactive or gave intractable mixtures under forcing
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The iron center adopts a distorted-trigonal-planar geometry
with the NHC plane at an angle of ca. 45° from the trigonal
coordination plane. Complexes 2 and 3 can be considered as
Lewis base adducts of “FeCl{N(SiMe3)2}” and constitute rare
examples of neutral mononuclear three-coordinate Fe com-
plexes.28−31 It is plausible that the combination of the steric
requirements of the SIPri and the silylamide and their strong σ-
donating properties are responsible for the coordinative and
electronic unsaturation of the complexes.
The steric parameter %Vbur for the 1,3-bis(2-
isopropylphenyl)imidazolin-2-ylidene and 1,3-diisopropylimida-
zol-2-ylidene (25.4% and 29.2%, respectively)32 compared with
those for SIPri/IPri and Imes/SImes (35% and 33.3%,
respectively) is a useful tool to quantify the observed reactivity.
In order to test further the hypothesis that steric effects are
dictating the degree of substitution, we carried out the reaction
with 1,3-bis(tert-butyl)imidazolinium chloride (%Vbur = 39.5%)
with [Fe{N(SiMe3)2}2]. Surprisingly, we isolated the ionic
complex [SIButH][Fe{N(SiMe3)2}2Cl] (4), in almost quanti-
tative yield (see Figure 3),33 which proved to be stable toward
aminolysis at least up to 70 °C in toluene.
It became therefore reasonable to assume that the anionic
[Fe{N(SiMe3)2}2Cl]− may be the initial intermediate that is
activated toward aminolysis by the azolium salts; this is also
consistent with the expected increased basicity of the
coordinated silylamide in the more electron-rich anionic
species. In order to verify this assumption, we attempted the
aminolysis of [Fe{N(SiMe3)2}2] with (SIPriH)+(BF4)−. Under
conditions comparable to those of the formation of 2b, we
could not observe any reaction. However, after prolonged
heating of a mixture of [Fe{N(SiMe3)2}2] and (SIPriH)+(BF4)−
(1/1) in octane/THF (95/5) (120 °C for 48 h), we isolated
colorless crystals of the adduct SIPri·BF3, which was
characterized spectroscopically and crystallographically (see
the Supporting Information), presumably resulting from
dehydrofluorination of the imidazolinium salt facilitated by
the presence of the iron amide. The mechanism of this
transformation is not clear. Dehydrofluorination of imidazolium
tetrafluoroborates has recently been reported under more
forcing conditions (T > 230 °C, at 10−4 mbar).34
The new three-coordinate complexes 2, 3, and 5 provide
versatile entries for the synthesis of numerous new NHC
complexes by either metathetical and/or further aminolytic
reactions. Selected preliminary examples for iron are shown in
Scheme 2. All complexes in the scheme have been characterized
by analytical, spectroscopic, and diffraction methods and will be
reported in detail in forthcoming papers. The chemistry and
catalytic properties of all new complexes are currently being
investigated in our group.
In conclusion, the methodology described above provides a
new route to the synthesis of versatile Fe and Co monodentate
NHC complexes, some with low coordination numbers and
possessing functional groups, opening the way for the mild and
selective synthesis of numerous derivatives suitable for further
chemical studies. The method is particularly advantageous in
cases where the generation and/or stability of the NHCs are
problematic.37
ASSOCIATED CONTENT
* Supporting Information
■
S
Text and figures giving experimental details and character-
ization data of the complexes reported and CIF files giving
crystallographic data for 1b, 2b, 4, and the complex SIPriBF3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: danopoulos@unistra.fr. Tel: (+33)-0368851319.
ACKNOWLEDGMENTS
■
We are grateful to Dr. P. Rabu (IPCMS) for magnetic studies
and Dr. L. Brelot for assistance with the X-ray crystallography.
This work was supported by the CNRS, the Universite
Strasbourg, and the Ministere de l′Enseignement Superieur et
de la Recherche (Paris). A.A.D. is grateful to the Region Alsace,
the Departement du Bas-Rhin, and the Communaute Urbaine
́
de
̀
́
́
́
́
de Strasbourg for the award of a Gutenberg Excellence Chair
(2010−2011).
Interestingly, H bonding between azolium cations and
various anions is well established crystallographically,35a
spectroscopically, and computationally and has been implied
as a factor responsible for the selectivity in the formation of the
normal or abnormal NHC complexes by direct metalation.35b
The occurrence of this interaction would lead to weakening of
the azolium C−H bond and facilitate a C2 metalation assisted
by a coordinated base. However, in the aminolysis reactions
described here an increase of the basicity of the silylamide and
the steric accessibility of the C2−H acidic site by the
coordinated base (i.e., silylamide) may be the crucial factors
determining reactivity and chemoselectivity.
Extension of the aminolysis methodology to cobalt seems
also to support these ideas. For example, the reaction with
(IMesH)+Cl− or (SIMesH)+Cl− produces the three-coordinate
[Co(IMes){N(SiMe3)2}Cl] (5a) and [Co(SIMes){N-
(SiMe3)2}Cl] (5b), respectively, in excellent yields, while the
reaction of [Co{N(SiMe3)2}2] with (SIPriH)+Cl− at room
temperature leads to the ion pair [SIPriH][Co{N(SiMe3)2}2Cl]
(6), which crystallized together with minor amounts of
[SIPriH][Co(SIPri)Cl3] (both characterized crystallographi-
cally).
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