Synthesis and structure of Ag(i), Pd(ii), Rh(i), Ru(ii) and Au(i) NHC-complexes with a pendant Lewis acidic boronic ester moiety

Article abstract of DOI:10.1039/c5dt00814j

Bifunctional Ag(i), Pd(ii), Rh(i), Ru(ii) and Au(i) complexes containing a NHC ligand and a pendant trivalent boron moiety have been synthesized in high yields. Fine-tuned reaction conditions were used to prevent potential ligand self-quenching or polymer

Full text of DOI:10.1039/c5dt00814j

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Synthesis and structure of Ag(I), Pd(II), Rh(I), Ru(II)
and Au(I) NHC-complexes with pendant Lewis acidic
boronic ester moiety†
Cite this: DOI: 10.1039/x0xx00000x
a
a,
,a
Received 00th January 2012,
Accepted 00th January 2012
Momar Toure, Olivier Chuzel * and Jean-Luc Parrain*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Bifunctional Ag(I), Pd(II), Rh(I), Ru(II) and Au(I) complexes NHC (Lewis base) and not hindered boron derivative (Lewis acid) in
situ (Scheme 1).
containing a NHC ligand and pendant trivalent boron moiety
have been synthesized in high yields. Fine-tuned reaction
conditions were used to prevent potential ligand self-
quenching or polymerization due to the eventual co-existence
in situ of free NHC (Lewis base) and boronic ester (Lewis
acid) in the same molecule.
N
N
or
R
oligomers
(
R'O)
2
B
1- base
2-
X
Lewis acid/base quenching
N
[
M]
N
R
?
N
R'O
B
N
N
R'O
R
or
N
R
Ln[M]
B
Boron-centered ligands have relatively large structural and
electronical diversity allowing for different coordination mode to a
B(OR')
2
[
M]Ln
R'O OR'
s
Pendant Lewis acid
-
acceptor ligand
functionality
metal center, as found, in borane, boryl, borylene or boride
1
complexes. In some particular cases of Lewis-basic metals, the Scheme 1 Possible NHC/boronic ester/metal combined product pathways.
Lewis-acidic trivalent boron atom can be coordinated to the metal
2
center by a dative bond, becoming a σ-acceptor Z-type ligand,
Here, we report the synthesis and the characterization of original
3
mainly with borane derivatives, first authenticated by Hill et al. stable bifunctional NHC-boronic ester silver, palladium, rhodium,
through the isolation of a metallaboratrane in 1999. Complexes ruthenium and gold complexes. Despite several modifications, the
4
featuring this supported M–B Z-type interactions are derived from boronic ester moiety remained pendant and no Z-type interaction to
the B–H activation of hydrido tris(methimazolyl)borates and related the metal center was observed in this series of complexes (Scheme
systems, or are obtained directly from ambiphilic phosphine borane 1).
5
ligands where the boron atom moiety is structurally maintained In the initial attempts, NHC-boron salts were obtained by reacting
close to the metal center.
There are rare examples of bidentate or bifunctional ligands dioxaborolane 2 that was prepared in one step through hydroboration
phenyl-substituted imidazole 1a with the bromopropyl
6
-9
11
containing a boronic ester moiety that are reported in the literature,
of allylbromide. Attempts to prepare the complex 5a directly from
1
2
and the only observed transition metal–Z interactions with boronic imidazolium salt 1a and classic bases such as n-BuLi, KHMDS,
ester moiety disclosed so far are the bridging borane-boryl NaH or t-BuOK did not succeed and only degradation of the starting
compounds. Because the structural feature of those bifunctional material was observed. No self-quenching resulting in cyclic NHC-
ligands does not allow the boronic ester moiety to coordinate to the boranes, polymerization processes or frustrated Lewis pair was
metal center, we designed a more flexible bifunctional ligand detected. To generate the carbenoid species and avoid
allowing the possibility of such an interaction. To increase the decomposition, we decided to use silver oxide as a soft base.
1
0
interaction, strong σ-donor N-heterocyclic carbene (NHC) ligand Gratifyingly the reaction between Ag O and the imidazolium salt 1a
2
was chosen to enhance the Lewis base character of the transition gave the desired cationic bis-NHC silver complex 4a in quantitative
metal. Synthesis of such NHC-boron transition metal complexes was yield, demonstrating the importance of the silver oxide mediated
highly challenging due to the necessary generation of coexisting free approach (Scheme 2). The structure was confirmed by NMR and
+
mass spectrometry analysis (ESI, MeOH, m/z for [M-AgBr2]
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3
3
7
31.4). Subsequent treatment of 4a with 2.5 equiv of ppm, 2H, t, J = 7.3 Hz) and 4a (4.13 ppm, 2H, t, J = 7.3 Hz), but a
PdCl (CH CN) at room temperature in dichloromethane afforded downfield shift of these protons in an unresolved broad signal
2
3
2
binuclear dichloride complexes 5a in 84% yield. Transmetallation of appeared for 5a at room temperature (4.40-4.95 ppm, 2H, m). Upon
the
silver
complex
with
other
transition cooling to 243 K in CD Cl solution, this signal split into two
2 2
distinct sets attributed respectively to the hydrogen atom
participating in an interaction with the palladium atom (4.57-4.91
ppm, 1H, m) and to the one that is not (4.27-4.57 ppm, 1H, m) (See
ESI). In the case of rhodium complex 6a the H NMR signals of one
of these two protons is strongly shifted downfield and these protons
appeared as two distinct sets of broad signals at room temperature
Bpin
N
Y
N
3
N
100 °C
Acetonitrile,
Br
Y
N
1
Y
Br
Y
3
Bpin
1
a, Y = CH
b, Y = N
2
3a, 85 % yield
3b, 66 % yield
(4.32 and 4.89 ppm for non-coordinated and coordinated hydrogen
1
atoms respectively) (see ESI). The downfield chemical shift of one
of these protons indicated a clearer rhodium–H interaction than those
observed for complex 5a, typical of metal–hydrogen bonding or
anagostic interaction. The existence of such interactions in 5a and 6a
was confirmed in the solid state by X-ray diffraction study (Fig. 2)
and their structural parameters (Pd1–H10 2.8203(4) Å, Pd1–C10
3.338(6) Å, Pd1–H10–C10 114.31(3)°; Pd2–H28 2.8580(5) Å, Pd2–
C28 3.346(8) Å, Pd2–H28–C28 112.08(4)°; (Rh1–H10) 2.9297(2)
Å, Rh1–C10 = 3.449(3) Å and angle (Rh1–H10–C10) = 114.65(2)°)
are in good agreement with those of anagostic interaction in
Bpin
N
AgBr
2
Y
N
3
Ag
2
O (0.5 equiv)
Y
Y
Ag
CH Cl rt
2
2,
N
Y
N
3
Bpin
a, 4b, quantitative yields
4
8
14
transition metal d complexes (litt. d(M–H) ≈ 2.3-2.9 Å, M–H–C ≈
10-170°). The origin of these interactions is still under debate and
Scheme 2 Synthesis of Ag-NHC boronic ester complexes.
1
Ph
may involve donation of filled dz2 or dxz/yz orbitals of the metal center
into the C–H σ* orbital. In order to obtain a more detailed
Bpin
15
N
Bpin
N
3
N
N
3
understanding of the involved orbitals, a Natural Bond Orbital
Cl
N
Cl
Cl
16
N
Pd
Ph
Pd
N
(
NBO) analysis was computed (see ESI for details). Two distinct
Cl
Pd
Cl
Cl
N
3
interactions were found, one involving the sp C–H orbitals and the
antibonding rhodium s–orbital and a second interaction between the
antibonding sp C–H with a rhodium lone pair with second order
3
Bpi
n
3
5
a
5b
−1
perturbation energies, E2P = 2.94 and 2.44 kJ mol , respectively.
Moreover, the hydrogen atom close to the rhodium atom has a
slightly higher natural charge of 0.222 than the other hydrogen atom
bonded by the same carbon atom (0.204). To conclude, the M–H
bond distance and the M–H–C angle values and the very low E (<5
Ph
N
Bpin
N
N
N
N
3
Cl
2
P
Rh
Bpin
-1
3
Cl
kJ mol ) strongly suggest the establishment of a very weak
Ru
N
Cl
17
hydrogen bond, probably led to the alkyl chain conformation. Such
-
p
cymene
interaction was not observed in the corresponding silver or gold
complex 4a and 7a respectively.
6
a
8b
The presence of privileged conformers having equivalent (4a, 7a) or
not equivalent (5a, 6a) protons bearing by the methylene group α to
the NHC is of interest. In the last case, this effect is strongly
pronounced in complex 6a and to estimate the participation of the
rhodium complex geometry and/or the boronic ester moiety in such
a)
Bpin
Bpin
N
N
N
3
N
N
3
Au
Cl
Au
Cl
N
7
a
7b
Fig.1 Structures of palladium, rhodium, gold and ruthenium NHC-boronic ester
complexes.
4
metals was also investigated. The reactivity of 4a towards [Rh(η -
cod)Cl]2 or AuCl metallic sources afforded the stable neutral
complexes 6a and 7a in 74% and 87% yield respectively (Fig. 1).
1
1
Negligible difference in chemical shift of the boron atom in
B
b)
NMR (128 MHz, CDCl ) for imidazolium salts 3a (33.3 ppm) and
3
all complexes prepared (33.3-34.0 ppm) showed that the boron
moiety was not ligated to the metal center and remained pendant.
This initial evidence was further confirmed for 5a, 6a and 7a in the
solid state by X-ray diffraction studies (Fig. 2).
Interestingly, for complexes 5a and 6a the existence of hydrogen
bonding or anagostic interaction (three-centers / four-electrons)
between the metal and the hydrogen atom of the methylene group in
1
α to the NHC was suspected by H NMR (400 MHz, CDCl3).
Hydrogen atoms of this CH group were equivalent both in 3a (4.57
2
2
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2 2
Fig. 2 ORTEP diagram of a) [PdCl NHC]
complex 5a; b) [RhClNHC] complex 6a b)
were depicted with thermal ellipsoids at 50% probability. Most of hydrogen
atoms are omitted for clarity.
preferential conformation the analogous rhodium complex 11
(scheme 3) was synthesized. It was observed in 11 that these two
protons were downfield shifted and not equivalent, but the effect was
not as important as observed in 6a (see ESI). Clearly, the boronic
ester moiety has an effect on the conformation of the alkyl chain,
leading in a rhodium complex 6a more stabilized by an additional
C–H hydrogen interaction. This, could generated a difference in the
reactivity of 6a and 11 complexes (see further).
Furthermore, to increase the Lewis basicity at the metallic center and
the possibility of Z-interaction to the boron atom, a donor pyrimidine
hemilabile ligand was incorporated at the N-position of the NHC
Fig. 3 ORTEP diagram of a) [PdCl
were depicted with thermal ellipsoids at 50% probability. Hydrogen atoms are
omitted for clarity.
2
NHC] complex 5b; b) [AuClNHC] complex 7b
1
13
instead of the phenyl group (Scheme 2). H NMR and C NMR
spectra of palladium dichloride complex 5b showed clearly three
non-equivalent pyrimidine hydrogen and carbon atoms with a
downfield shift (δH > 1) of one aromatic hydrogen atom in the α
position to the nitrogen atom. As expected, these observations
clearly indicate bidentate coordination of the metal to the NHC and
one of the nitrogen atom of the pyrimidine moiety. In the solid state,
complex 5b features a pseudo square planar geometry with a Pd1–
N1 distance of 2.049(2) Å and a N1–Pd1–C7 and N1–Pd1–Cl1
angles of 80.18(1)° and 93.47(7)°, respectively, that confirms the
N3–C7–N2 dihedral angle, -34.91(1)°). As previously observed in
this series of complexes, NMR data and X-ray characterization
revealed again the pendant character of the boron moiety in the
1
1
complex 7b ( B NMR, 128 MHz, in CDCl , 33.8 ppm) (Fig. 3).
3
Despite some evidence in the literature of transition metal–Z
1
0
interactions with boronic ester moiety,
or related reaction
1
3
intermediate found in borylation reaction mechanism, such
interactions were not observed in this series of flexible complexes,
owing to a less buttress structure that do not allow potential
recruitment of the boron atom moiety close to the metal center.
Interestingly, these stable complexes exhibit a Lewis acidic pendant
boronic ester moiety that could serve as an additional binding site in
bifunctional catalysis. An initial 1:1 equivalent mixture of complex
spectroscopic NMR data (Fig. 3). Unfortunately, attempts to
4
synthesize the rhodium complex analogue from [Rh(η -cod)Cl]
2
gave an undetermined complex mixture.
Alternatively, the treatment of 4b with [Ru(p-cymene)Cl2]2 gave
quantitatively the corresponding 18-electrons ruthenium(II) complex
1
1
5
a and Et N was analyzed by B NMR (128 MHz, CDCl ) showing
3
3
8
b. The cationic nature of the complex was correlated by high-
+
a complete upfield chemical shift of the boron atom (from 33.5 to
.85 ppm) corresponding to a borate species that revealed an
resolution mass spectrometry (ESI, MeOH, m/z 585.1742 [M-Cl] ).
Unfortunately, all attempts to crystallize complex 8b failed, but the
NMR spectroscopic data, as seen previously in the complex 5b,
provided evidence for the bidentate nature of the NHC-pyrimidine
2
unambiguous covalent B–N bond. This information let us confidant
to use the boron atom in these complexes as an additional
electrophilic activating site. In this context, stability and reactivity of
complex 6a were evaluated in a preliminary hydrosilylation reaction
1
1
ligand as well as the pendant nature of the boron moiety ( B NMR,
28 MHz, in CDCl , 34.0 ppm). As expected, reaction of 4b with
1
3
1
and 6a was found to be a very efficient catalyst (conv > 99 % by H
AuCl gold source gave the linear geometric complex 7b, where the
NMR) (Scheme 3). However, the analogous complex 11 gave the
same results in terms of kinetic (see ESI), meaning that the boronic
part of the ligand seems to behave only as a spectator in this
reaction. Nevertheless, the use of the stable palladium, rhodium,
ruthenium and gold complexes described here as potential catalysts
for the synergistic activation of reaction partners will be explored
soon, as well as extension to structurally analogous more
electrophilic NHC-borane ligands.
pyrimidine aromatic ring was twisted to hold the gold metal center
far from the nitrogen atom of the pyrimidine (Au1–N3, 3.118(1) Å,
Au1–
a)
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O
1- 6a or 11
OH
(
1 mol%)
2
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rt, 2h
+
Ph SiH
2
2
Kameo and H. Nakazawa, Chem. Asian J., 2013, 8, 1720; G. R.
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MeO
MeO
2
3
Owen, Chem. Soc. Rev., 2012, 41, 3535; M. Sircoglou, S. Bontemps,
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9
10
12
conv > 99%
Ph
Ph
N
N
N
N
Rh
Rh
Bpin
3
Cl
Cl
6
a
11
9 with analogous rhodium-NHC
3
Selected references: F. Inagaki, C. Matsumoto, Y. Okada, N.
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Scheme 3. Catalytic hydrosilylation of ketone
complexes 6a and 11
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5
3
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In summary, we have reported the synthesis and
characterization of novel bifunctional NHC-boronic ester
ligands and their corresponding silver(I), palladium(II),
rhodium(I), ruthenium(II) and gold(I) complexes. It was
observed that only the use of Ag O as a base allowed us to
obtain these complexes in very good yields, and without either
a self-quenching or a polymerization process taking place
during the generation of the in situ Lewis base (NHC) and
Lewis acid (boronic ester). It was confirmed by B NMR and
X-ray diffraction that the boron moiety is not coordinated at the
metal center and remained pendant. Complexes bearing pendant
trivalent borane derivatives are of interest and could be an
alternative to enhance potential electrophilic activation of a
substrate. Reported examples are essentially built on
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1
8
2
1
1
1
9
2
0
metallocene shapes system, but many of them must be
stabilized in situ as borate derivatives, to avoid potential
intramolecular rearrangement.
4
5
2
1
2
2
The stable complexes
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developed here would let us confidently use them in
bifunctional catalysis in the cooperative activation of small
9
molecules. These hypothesis will be examined in further detail
in due course.
6
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Acknowledgments
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, 2229.
Tetrahedron: Asymmetry, 1993,
4
This work was supported by grant from the French Research
Ministry to M.T., Aix-Marseille Université, CNRS UMR 7313.
We thanks M. Giorgi (www.spectropole.fr) for the X-ray
diffraction studies.
7
8
9
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Notes and references
1
a
Aix-Marseille Université, Centrale Marseille, CNRS, iSm2 UMR 7313,
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jl.parrain@univ-amu.fr
Electronic Supplementary Information (ESI) available: Experimental
procedures, spectroscopic data, detailed calculations and CIF for
complexes 5a 5b 6a 7a and 7b. DOI: 10.1039/c000000x/
†
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2 Wang et al. have synthesized a NHC-borane platinium complex but
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7 J. Saßmannshausen, Dalton Trans., 2012, 41, 1919.
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1
9 For bifunctional complex with pendant borane features, see: W.-J.
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0 For bifunctional complexes based on a metallocene-borane pendant
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