Expedient Access to Normal- and Abnormal- N-Heterocyclic Carbene (NHC) Magnesium Compounds from Imidazolium Salts

Article abstract of DOI:10.1002/zaac.201600308

Treatment of imidazolium salts (IPrH)Cl (1) and (IPrH)I (2) with MeMgI leads to the formation of normal N-heterocyclic carbene (nNHC) compounds (IPr)MgCl(I) (3) and (IPr)MgI₂(4) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], respectively. By employing a similar strategy, the first magnesium compound (aIPr^Ph)MgI₂(OEt₂) (6) featuring an abnormal-NHC [aIPr^Ph= 1,3-bis(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene] is prepared by the treatment of a C2-arylated imidazolium salt (IPrPh)I (5) with MeMgI. While the deprotonation of 1 and 2 can be accomplished at room temperature, an elevated temperature is required to deprotonate 5 with MeMgI. This is most likely due to the higher pK_avalue of the C4-H than that of the C2-H hydrogen atom. Salt metathesis reaction of 6 with KN(SiMe₃)₂cleanly leads to the formation of a magnesium amide derivative (aIPr^Ph)Mg{N(SiMe₃)₂}₂(7). Compounds 3, 4, 6, and 7 were characterized by elemental analyses and NMR spectroscopic studies. The molecular structures of 3, 4, and 6 were determined by single-crystal X-ray diffraction analyses.

Full text of DOI:10.1002/zaac.201600308

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Zeitschrift für anorganische und allgemeine Chemie
DOI: 10.1002/zaac.201600308
Expedient Access to Normal- and Abnormal- N-Heterocyclic Carbene
(NHC) Magnesium Compounds from Imidazolium Salts
Rajendra S. Ghadwal,*[a,b] Dennis Rottschäfer,[a] and Christian J. Schürmann[c]
Dedicated to Professor Robert Mulvey
Keywords: Carbene; Heterocycles; Magnesium; Structure
Abstract. Treatment of imidazolium salts (IPrH)Cl (1) and (IPrH)I (2) room temperature, an elevated temperature is required to deprotonate
with MeMgI leads to the formation of normal N-heterocyclic carbene
5 with MeMgI. This is most likely due to the higher pKa value of the
(nNHC) compounds (IPr)MgCl(I) (3) and (IPr)MgI2 (4) [IPr = 1,3- C4-H than that of the C2-H hydrogen atom. Salt metathesis reaction
bis(2,6-diisopropylphenyl)imidazol-2-ylidene], respectively. By em-
ploying similar strategy, the first magnesium compound
(aIPrPh)MgI2(OEt2) (6) featuring an abnormal-NHC [aIPrPh = 1,3-
bis(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene] is prepared
by the treatment of a C2-arylated imidazolium salt (IPrPh)I (5) with
MeMgI. While the deprotonation of 1 and 2 can be accomplished at
of 6 with KN(SiMe3)2 cleanly leads to the formation of a magnesium
amide derivative (aIPrPh)Mg{N(SiMe3)2}2 (7). Compounds 3, 4, 6, and
7 were characterized by elemental analyses and NMR spectroscopic
studies. The molecular structures of 3, 4, and 6 were determined by
single-crystal X-ray diffraction analyses.
a
groups independently reported some NHC-magnesium (metal-
locene) complexes. Moreover, Arnold and others reported sev-
eral magnesium compounds with functionalized NHCs[8] and
investigated their catalytic activity for the polymerization of
lactides[9]. Structurally characterized NHC compounds featur-
ing a dialkylmagnesium, magnesium amide or Grignard rea-
gent type species are also known.[10] Nevertheless, NHC-mag-
nesium halides have remained rather scarce.[3b,11]
Introduction
N-Heterocyclic carbenes (NHCs) are ubiquitous ligands in
current organometallic chemistry[1] and have also been recog-
nized as very efficient organocatalysts[2] in their own right.
Besides their applications as versatile ancillary ligands in
homogeneous catalysis, a remarkable progress has also been
made in molecular main group chemistry with the utilization
of NHCs[3] and related carbon-donor ligands.[4] In this context,
donor-acceptor stabilization of molecular species with a low-
valent main group element is noteworthy. In contrast to transi-
tion metals and p-block elements, NHC compounds featuring
an alkaline earth metal are scarce.[3b] The first stable NHC-
magnesium compounds were reported by Arduengo et al. as
early as in 1993.[5] Later, Arduengo’s[6] and Schumann’s[7]
A relatively new class of NHCs that binds to metals at the
imidazol-backbone, i.e. to the C4 (or C5) carbon instead of the
C2 carbon atom, has been developed during the past few
years.[12] These so-called abnormal NHCs (aNHCs) are also
known as mesoionic carbenes (MICs) as no canonical form of
these carbenes without the introduction of formal charges can
be written.[13] While Crabtree et al. already reported the first
aNHC complex in 2001,[14] albeit as a result of a serendipitous
discovery, it was only in 2009 when Bertrand and co-workers
* Priv.-Doz. Dr. R. S. Ghadwal
E-Mail: rghadwal@uni-bielefeld.de
[a] Anorganische Molekülchemie und Katalyse am Lehrstuhl für isolated the first metal-free stable aNHC.[15] In recent years,
Anorganische Chemie und Strukturchemie
several research programs have shown rational approaches to
Fakultät für Chemie
aNHC compounds, however they are so far limited to some
specific elements.[4a,16] Experimental and theoretical findings
suggest that aNHCs are even stronger electron donors than
Arduengo’s NHCs as well as Bertrand’s cyclic alkyl amino
carbenes (CAACs).[17] Nevertheless, only a few aNHC com-
pounds, in particular with a main group element,[4a,16] have
been reported and, to the best of our knowledge, no magne-
sium compound featuring an aNHC is known to date. Herein,
we report very convenient routes to nNHC- as well aNHC-
magnesium compounds by using air stable imidazolium salts
and MeMgI in a good to excellent yield.
Universität Bielefeld
Universitätsstr. 25
33615, Bielefeld, Germany
[b] Centrum für Molekulare Materialien
Fakultät für Chemie
Universität Bielefeld
Universitätsstr. 25
33615, Bielefeld, Germany
[c] Institut für Anorganische Chemie
Georg-August-Universität Göttingen
Tammannstr. 4
37077, Göttingen, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201600308 or from the au-
thor.
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atom of the imidazol ring have been recently reported,[4a] com-
pound 6 is indeed the first aNHC-Mg compound. In the past
few years, magnesium amides have been widely used as pre-
catalysts in a variety of chemical transformations[20] as well as
for the synthesis of organic solvent soluble molecular magne-
sium hydrides.[21] Such compounds are of a high impact in
catalysis and material science. Therefore, we carried out the
reaction of 6 with KN(SiMe3)2 and isolated compound
(aIPrPh)Mg{N(SiMe3)2}2 (7) in 80% yield (Scheme 2). Cata-
lytic and stoichiometric reactivity studies of 7 are currently
undergoing in this laboratory and results will be published in
due course. Compound 6 and 7 are soluble in C6H6 and THF
and no sign of ligands exchange was observed at room tem-
perature even after one week.[22]
Results and Discussion
Treatment of a toluene suspension of (IPrH)Cl (1) and
(IPrH)I (2) with MeMgI leads to the formation of compound
(IPr)MgCl(I) (3) and (IPr)MgI2 (4), respectively [IPr = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene] (Scheme 1). Al-
ternatively, compound 4 is also accessible by reacting free IPr
with anhydrous MgI2. Although this method requires a longer
reaction time and the product is contaminated with some unre-
acted IPr along with a small amount of intractable solid resi-
due. Compounds 3 and 4 are stable solids, which are soluble
in THF but sparingly soluble in toluene and benzene.
1H NMR spectra of compounds 3 and 4 exhibit almost sim-
ilar signals for the NHC moiety (all δ values in ppm). The
isopropyl groups of 3 and 4 exhibit two doublets and one mul-
tiplet, whereas the imidazol-backbone protons appear as a sin-
1
glet at 6.41 and 6.40, respectively. As expected, the H NMR
spectrum of compound 6 shows four-doublets for the isopropyl
groups. This is due to the lack of C2 symmetry in compound
6. Similarly, the methine protons of the HCMe2 groups exhibit
two septets. The remaining imidazol-backbone proton (C5-H)
of 6 appears as a singlet at δ = 7.01 ppm. The aromatic
hydrogen atoms of the Dipp groups show two sets of reso-
nances along with the signals due to the C2 bound phenyl sub-
stituent. In addition, compound 6 also exhibits resonances for
an ethyl ether molecule that is coordinated to the magnesium
atom. 13C NMR spectra of compounds 3, 4, and 6 exhibit cor-
responding resonances for the carbene ligands, which are con-
sistent with their 1H NMR signals. Like other NHC-
Mg{N(SiMe3)2}2 compounds,[21,23] compound 7 is very good
Scheme 1. Synthesis of normal-NHC magnesium compounds 3 and 4.
A similar strategy was applied to prepare aNHC-magnesium
compound (aIPrPh)MgI2(OEt2) (6) [aIPrPh = 1,3-bis(2,6-diisop-
ropylphenyl)-2-phenyl-imidazol-4-ylidene] by employing a
C2-arylated imidazolium salt (5)[18] and MeMgI (Scheme 2).
However, the reaction was found to be extremely slow and
only 10–20% conversion of 5 into 6 was noticed after 16 h at
room temperature. Therefore, the reaction temperature was
raised to 80 °C, which enabled complete deprotonation of 5
and the formation of compound 6 after 6 h. Interestingly, com-
pound 6 was found to be stable in a toluene solution and no
decomposition or formation of side-products was observed at
90 °C. Compound 6 was isolated as a white solid. While some
diatopic carbanionic-NHC (dc-NHC) compounds featuring a
1
soluble in benzene. The H NMR spectrum of 7 shows two-
doublets and one pseudo-triplet (that arises from two overlap-
ping doublets) for the isopropyl groups, whereas SiMe3 groups
appear as a singlet at δ = 0.40 ppm. Compound 7 exhibits a
29Si{1H} NMR signal at δ = 8.98 ppm. The 13C NMR spec-
trum of 7 reveals corresponding signals for the carbene and
amide ligands.
Molecular structures of 3, 4, and 6 were determined by sin-
gle-crystal X-ray diffraction studies. As revealed, compound
3 (Figure 1) and 4 (Figure 2) exist as halide-bridged dimers,
featuring a fourfold coordinated magnesium atom. Selected
bond lengths and angles are given in Table 1. In compound 3,
magnesium atom[19] that is coordinated with the C4 carbon chlorides are bridging ligands and feature Mg–Cl bond lengths
Scheme 2. Synthesis of abnormal-NHC magnesium compounds 6 and 7.
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of av. 2.388 Å, which are comparable with the reported magne- Table 1. Selected bond lengths /Å and angles /° of compounds 3–5.
sium compounds consisting Mg–Cl–Mg (ca. 2.37 Å) moieties,
but considerably longer than that observed for terminal Mg–
Cl bonds (ca. 2.28 Å).[8c,11] A positional disorder of the chlor-
ide atoms is present. The residual density at the bridging posi-
tions of 3 was modeled with a 7% disorder of iodine.[24] This
suggests that the halide ions scrabbling cannot be excluded
under these reaction conditions. The Mg–I bond lengths of 3
(av. 2.66 Å) are consistent with those observed for known
magnesium compounds featuring a terminal Mg–I bond.[25]
The C(carbene)–Mg bond lengths of 3 and 4 (from 2.19 to
2.20 Å) are comparable with other NHC-magnesium com-
pounds.[11]
Compound C–Mg
Mg–Xa)
Mg–Xb)
X–Mg–X
3
2.203(2)
2.195(2)
2.662(2)
2.668(2)
2.388(2)
2.388(2)
2.381(2)
2.397(2)
91.09(6)c)
91.48(6)c)
112.27(4)
111.92(5)
c)
4
6
2.194(2)
2.165(3)
2.6708(7)
2.7808(8)
2.8030(8)
95.55(3)
110.24(2)
2.682(2)
2.703(2)
–
114.80(4)
a) Terminal. b) Bridging. c) Mg2X2 ring.
6% occupancy. Iodide ligands occupy two coordination sites,
whereas one Et2O molecule and the aNHC ligand bind to the
remaining two positions. The Mg–I bond lengths of 6 are com-
parable with the terminal Mg–I bond lengths of 3 and 4 as
well as with other reported magnesium compounds.[25] The
Mg–O bond length [2.058(4) Å] of 6 can be correlated with
that of the reported magnesium compounds featuring an ethe-
real solvent molecule.[25,26] The most pertinent feature is the
Mg–C(carbene) bond length of 6 [2.165(4) Å], which is signifi-
cantly shorter than those of 3 and 4 (Table 1).
Figure 1. Molecular structure of compound 3. Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths and bond angles
are given in Table 1.
Figure 3. Molecular structure of compound 6. Anisotropic displace-
ment parameters are depicted at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths and bond angles
are given in Table 1.
Conclusions
Four magnesium compounds featuring an nNHC- (3 and 4)
or an aNHC (6 and 7) ligand are reported. As discussed, air
Figure 2. Molecular structure of compound 4. Anisotropic displace- stable imidazolium salts can be used to prepare these magne-
ment parameters are depicted at the 50% probability level. Hydrogen
sium compounds. This methodology offers three major advan-
atoms are omitted for clarity. Selected bond lengths and bond angles
tages: (i) the isolation of free NHC is not required, (ii) new
are given in Table 1.
compounds can be prepared in case a free carbene (e.g.,
In contrast to compounds 3 and 4, compound 6 (Figure 3) aNHC) is not stable, and (iii) unreacted starting material can
is monomeric featuring a fourfold coordinated magnesium be easily separated from the product. All the compounds were
atom with a distorted tetrahedral arrangement. The molecular fully characterized and the structures of 3, 4, and 6 were estab-
structure of 6 contains some residual electron density, which lished by X-ray diffraction analyses. The iodo-derivatives 3, 4,
was modeled as disordered I3MgOEt2 imidazolium salt with and 6 are appealing candidates for preparing compounds fea-
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uum. Yield: 3.30 g, 57%. C37H50N2I2MgO (816): calcd. C 54.40; H
6.17; N 3.43%; found: C 54.19; H 6.03; N 3.30%. 1H NMR
(300 MHz, [D8]THF, 25 °C): δ = 0.77 (d, 6 H, J = 6.8 Hz, HCMe2);
0.91 (d, 6 H, J = 6.8 Hz, HCMe2); 1.11 (t, 6 H, J = 7.0 Hz, H2CMe);
1.22 (d, 6 H, J = 6.7 Hz, HCMe2); 1.36 (d, 6 H, J = 6.7 Hz, HCMe2);
2.60 (sept, 2 H, J = 6.8 Hz, HCMe2); 2.86 (sept, 2 H, J = 6.8 Hz,
HCMe2); 3.37 (q, 4 H, J = 7.0 Hz, H2CMe); 6.88 (d, 2 H, J = 7.5 Hz,m-
C6H5); 7.01 (s, 1 H, HCCN); 7.05–7.21 (m, C6H3, C6H5); 7.26 (t, 1
H, J = 7.3 Hz, p-C6H3); 7.33 (d, 2 H, J = 7.8 Hz, m-C6H3); 7.50 (t, 1
H, J = 7.7 Hz, p-C6H3) ppm. 13C NMR (75 MHz, C6D6, 25 °C): δ =
15.75 (H2CMe3); 22.94, 23.54, 26.12, 26.42 (HCMe2); 29.21, 29.57
(HCMe2); 66.48 (H2CMe3); 124.91, 125.64, 125.79, 126.20, 129.06,
129.28, 129.83, 130.34, 130.79, 131.38, 131.83, 133.31, 137.07 (C6H5,
C6H3); 145.08, 145.75, 146.11 (ipso-C6H3, ipso-C6H5); 134.97
(HCCN); 166.15 (HCCN) ppm.
turing a low-valent magnesium atom as well as other magne-
sium compounds.
Experimental Section
All syntheses and manipulations were performed in an inert gas (nitro-
gen or argon) atmosphere using an MBraun glove box or Schlenk line
techniques. Toluene, benzene, n-hexane, ethyl ether, and THF were
dried by refluxing over Na-K/benzophenone-ketyl and purified by dis-
tillation in a nitrogen atmosphere. (IPrH)Cl (1),[27] (IPrH)I (2),[28] and
(IPrPh)I (5)[18] were prepared by adopting literature methods. MeMgI
(3 m solution in Et2O from Sigma-Aldrich), MgI2 (Sigma-Aldrich), and
KN(SiMe3)2 (Sigma-Aldrich) were used without further purification.
1H and 13C NMR spectra were recorded with a Bruker Avance III 500
or a Bruker Avance III 300 or a Bruker DPX 200 spectrometer. 1H
and 13C NMR resonances were assigned with respect to the residual
solvent peak(s) (for C6D6: 1H, 7.16; 13C, 128.06 and for [D8]THF: 1H,
3.58, 1.72; 13C, 67.21, 25.31) and reported in δ ppm.
Synthesis of (aIPrH)Mg{N(SiMe3)}2 (7): To a solid mixture of 6
(0.87 g, 1.06 mmol) and KN(SiMe3)2 (0.43 g, 2.15 mmol) was added
40 mL of toluene at room temperature and the resulting solution was
stirred at room temperature for 4 h. Filtration through a plug of Celite
afforded a colorless solution, which was dried under reduced pressure
to yield compound 7 as a white solid (0.70 g, 81%). The complete
conversion and purity of compound 7 was ascertained by NMR stud-
ies. C45H76N4MgSi4 (809): cald. C 66.75; H 9.46; N 6.92%; found: C
Synthesis of (IPr)MgCl(I) (3): To a 100 mL toluene suspension of
(IPrH)Cl (1) (3.71 g, 8.73 mmol) was added dropwise a 3 m ethyl ether
solution of MeMgI (3.0 mL, 9.0 mmol) at room temperature. The re-
sulting light brown solution was stirred at room temperature for 12 h.
Filtration through a plug of Celite gave a yellow solution. All the
volatiles from the filtrate were removed under vacuum to yield an off-
white solid, which was washed with n-hexane (2ϫ20 mL) and dried
under vacuum. Compound 3 was isolated as colorless crystals on stor-
age of a saturated toluene solution at –30 °C for two weeks. Yield:
4.12 g (82%). C27H36N2ClIMg (575): calcd. C 56.37; H 6.31; N
4.87%; found: C, 56.12; H, 6.22; N, 4.75%. 1H NMR (200 MHz,
C6D6, 25 °C): δ = 0.93 (d, J = 6.3 Hz, 12 H, HCMe2); 1.55 (d, 12 H,
J = 6.6 Hz, HCMe2); 2.83 (m, 4 H, HCMe2); 6.41 (s, 2 H, HCN);
7.00–7.09 (m, 6 H, C6H3) ppm. 13C NMR (125 MHz, C6D6, 25 °C):
δ = 23.94, 24.21 (HCMe2); 28.48(HCMe2); 124.15 (NCCN); 124.46,
130.71, 131.44, 145.07 (C6H3,), 194.67 (NCN) ppm.
1
66.62; H 9.34; N 6.75%. H NMR (300 MHz, C6D6, 25 °C): δ = 0.40
(s, 36 H, SiMe3); 0.70 (pseudo-triplet, 12 H, HCMe2); 1.21 (d, 6 H, J
= 6.8 Hz, HCMe2); 1.44 (d, 6 H, J = 6.8 Hz, HCMe2); 2.59 (sept, 2 H,
J = 6.8 Hz, HCMe2); 2.93 (sept, 2 H, J = 6.8 Hz, HCMe2); 6.48–6.61
(m, 3 H, m-C6H5, p-C6H5); 6.82 (d, 2 H, J = 7.2 Hz, o-C6H5); 6.90 (d,
2 H, J = 7.7 Hz, m-C6H3); 7.01–7.18 (m, 4 H, m-C6H3, p-C6H3); 7.93
(s, 1 H, HCCN) ppm. 13C NMR (75 MHz, C6D6, 25 °C): δ = 6.78
(SiMe3); 22.72, 23.47, 25.44 (HCMe2); 28.71, 28.95 (HCMe2); 124.06,
125.07, 125.63, 129.63, 130.57, 130.71, 131.31, 132.08, 136.25,
136.99 (C6H5, C6H3); 144.32, 145.20, 145.58 (ipso-C6H3, ipso-C6H5);
134.95 (HCCN); 165.18 (HCCN) ppm. 29Si NMR (59 MHz, C6D6,
25 °C): δ = 8.98 ppm.
Synthesis of (IPr)MgI2 (4): Compound 4 was prepared as colorless
crystals by employing a similar method as described for 3 using
(IPrH)I (2) (2.22 g, 4.30 mmol) and 3 m MeMgI solution (1.5 mL,
4.50 mmol). Yield: 2.43 g, 85%. C27H36N2I2Mg (666): calcd. C 48.64;
H 5.44; N 4.20%; found: C 48.42; H 5.26; N 4.01%. 1H NMR
(200 MHz, C6D6, 25 °C): δ = 0.90 (d, J = 6.3 Hz, 12 H, HCMe2); 1.57
(d, 12 H, J = 6.6 Hz, HCMe2); 2.82 (m, 4 H, HCMe2); 6.40 (s, 2 H,
HCN); 7.01–7.05 (m, 6 H, C6H3) ppm. 13C NMR (125 MHz, C6D6,
25 °C): δ = 24.09, 24.67 (HCMe2); 28.51(HCMe2); 124.27 (NCCN);
124.24, 130.19, 131.44, 145.99 (C6H3,); 196.32 (NCN) ppm.
Supporting Information (see footnote on the first page of this article):
Crystallographic details of compounds 3, 4, and 6; NMR plots of com-
pounds 3, 4, 6, and 7.
Acknowledgements
Support from the Deutsche Forschungsgemeinschaft (DFG) is grate-
fully acknowledged. We are thankful to Professor Norbert W. Mitzel
for his generous support.
Alternative Synthesis of 4: A 200 mL toluene suspension of IPr
(3.60 g, 9.26 mmol) and MgI2 (2.60 g, 9.26 mmol) was stirred at room
temperature. After 2 d, a slightly yellow solution along with a small
amount of sticky residue was obtained. The solution was decanted into
another flask and the volume was reduced to ca. 50 mL under vacuum.
The flask was stored at 4 °C for one week. Colorless crystals of 4
were isolated by filtration and dried under vacuum. The filtrate was
concentrated (20 mL) and stored at 4 °C for the second crop. Com-
bined yield: 4.29 g, 69% yield.
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Synthesis of (aIPrPh)MgI2(OEt2) (6): To a 100 mL toluene suspen-
sion of (IPrPh)I (5) (4.20 g, 7.08) was added MeMgI (3M Et2O solu-
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