(NHCMe)SiCl4: A versatile carbene transfer reagent-synthesis from silicochloroform

Article abstract of DOI:10.1039/c2sc21214e

A new synthetic pathway for the N-heterocyclic carbene adduct (NHC ^Me)SiCl₄ (2) (NHC^Me = 1,3-dimethylimidazolidin- 2-ylidene) using silicochloroform is presented. Supported by DFT calculations, the energy for dissociation of 2 into the carbene and the SiCl₄ fragment was found to be comparable to carbene transfer reagents based on silver(i) chloride. Compound 2 was used to transfer the NHC ligand to three different phosphorus(iii) chloro compounds, resulting in the neutral complexes (NHC^Me)PCl₃ (3a), (NHC^Me)PCl₂Ph (3b) and (NHC^Me)PCl₂Me (3c). The sterically non-demanding NHC ligand allowed the phosphorus(iii) in complex 3a to be oxidized to phosphorus(v) without loss of the NHC ligand, and afford (NHC^Me)PF₄H (4). Furthermore, bis-carbene complexes of Ni(ii) (5) and Pd(ii) (6) were obtained by reacting 2 with the respective metal chlorides.

Full text of DOI:10.1039/c2sc21214e

Chemical Science
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Volume 4 | Number 1 | January 2013 | Pages 1–540
ISSN 2041-6520
EDGE ARTICLE
Gerd-Volker Röschenthaler et al.
(NHCMe)SiCl4: a versatile carbene transfer reagent – synthesis from
silicochloroform

Chemical Science
EDGE ARTICLE
(NHCMe)SiCl4: a versatile carbene transfer reagent –
synthesis from silicochloroform†
Cite this: Chem. Sci., 2013, 4, 77
¨
Tobias Bottcher, Bassem S. Bassil, Lyuben Zhechkov, Thomas Heine
*
¨
and Gerd-Volker Roschenthaler
A new synthetic pathway for the N-heterocyclic carbene adduct (NHCMe)SiCl4 (2) (NHCMe ¼ 1,3-
dimethylimidazolidin-2-ylidene) using silicochloroform is presented. Supported by DFT calculations, the
energy for dissociation of 2 into the carbene and the SiCl4 fragment was found to be comparable to
carbene transfer reagents based on silver(I) chloride. Compound 2 was used to transfer the NHC ligand
to three different phosphorus(III) chloro compounds, resulting in the neutral complexes (NHCMe)PCl3
(3a), (NHCMe)PCl2Ph (3b) and (NHCMe)PCl2Me (3c). The sterically non-demanding NHC ligand allowed
the phosphorus(III) in complex 3a to be oxidized to phosphorus(V) without loss of the NHC ligand, and
afford (NHCMe)PF4H (4). Furthermore, bis-carbene complexes of Ni(II) (5) and Pd(II) (6) were obtained by
reacting 2 with the respective metal chlorides.
Received 8th August 2012
Accepted 13th September 2012
DOI: 10.1039/c2sc21214e
www.rsc.org/chemicalscience
coordination by NHCs resulted in unusual bonding properties
and oxidation states. Bertrand and co-workers isolated a series
Introduction
N-heterocyclic carbene (NHC) complexes have been of great
interest to chemists since the 1960s, although the rst stable
NHC was only isolated in 1991.1 Since then, free and stable
NHCs have been available for preparative chemistry, and many
coordination compounds of metals and non-metals coordi-
nated by NHCs have been reported.2 Later on, Herrmann et al.
reported the catalytic property of palladium NHC complexes,
making the research of NHC-coordinated transition metals one
of the fastest growing topics in coordination chemistry ever
since.3 During the last two decades, numerous reports in liter-
ature on the application of NHC complexes in metal-mediated
catalysis, organocatalysis, pharmaceutical applications and
technical processes have appeared.4 Although carbenes have
generally become an established class of ligands, their synthesis
and isolation, either in the free form or as part of a complex, is
an ongoing challenge. The high stability of the NHC–transition-
metal bond provides effective pathways in the synthesis of the
resulting complex, including ligand exchange, in situ deproto-
nation of imidazolium, oxidative addition to halo-imidazolium,
and transmetallation.5
of NHC-stabilized phosphorus(0) compounds by addition of
different free NHCs to white phosphorus.6 Robinson and co-
workers reduced NHCdipp (NHCdipp ¼ :C{N(2,6-iPr2C6H3)CH}2)
complexes of silicon(IV) in order to form Si–Si single and double
bonds. Reduction of (NHCdipp)SiCl4 resulted in the rst complex
with silicon being in the formal oxidation state of zero,
(NHCdipp)Si]Si(NHCdipp).7 Following the same procedure, they
reduced (NHCdipp)PCl3 to give (NHCdipp)P]P(NHCdipp), as well
as (NHCdipp)AsCl3 to give (NHCdipp)As]As(NHCdipp).8 Jones
et al. followed a modied procedure to synthesize (NHCdipp
)
Ge]Ge(NHCdipp) by reduction from (NHCdipp)GeCl2.9 In addi-
tion to these diatomic allotropes, other compounds of main-
group element species stabilized by N-heterocyclic carbenes
were also synthesized.10
The starting compounds for the above mentioned reduction
reactions as well as most other adducts of NHCs to main-group
elements are prepared starting from free, uncoordinated NHCs.
This approach imposes the use of NHCs with bulky substituents
at the N,N0-positions, as they provide steric protection to the
carbene center and hence stability to the NHCs. Moreover, these
substituents allowed the isolation of the diatomic allotropes
mentioned above. On the other hand, these bulky groups do not
allow any additional reaction at the coordinated center of the
aforementioned NHC complexes because of steric hindrance.
The challenge lies therefore in a synthetic pathway, which does
not require the presence of sterically demanding NHC ligands,
in order to allow further reactions at the carbene-coordinated
center and to have a NHC-containing starting material stable
enough to be isolated but active enough to allow the NHC
ligands to undergo further coordination.
However, the coordination of NHC to main-group elements
is more constrained. Very recently, there have been remarkable
advances in the chemistry of p-block compounds, where the
School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, D-28759
Bremen, Germany. E-mail: g.roeschenthaler@jacobs-university.de; Tel: +49 421
200-3138
† Electronic supplementary information (ESI) available: Experimental and
computational details, crystallographic data, and thermogravimetric analysis.
CCDC reference numbers 895219¼895225. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2sc21214e
This journal is ª The Royal Society of Chemistry 2013
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Herein we report the synthesis of the NHC transfer reagent
(NHCMe)SiCl4 (2, NHCMe ¼ 1,3-dimethylimidazolidin-2-ylidene)
and its reactivity towards main-group and transition-metal
elements. We chose NHCMe as it is the smallest imidazolidine-
based NHC and has not yet been isolated in its free form, unlike
its unsaturated analog.11 We performed ligand transfer reaction
on 2 in order to synthesize adducts of NHCMe and phosphor-
us(III) as well as nickel(II) and palladium(II). We also proved the
accessibility of the carbene-coordinated phosphorus(III) by
oxidizing it to phosphorus(V) without loss of NHCMe. As a source
of silicon we rst used hexachlorodisilane and then silico-
chloroform, which is a raw material in chemical industry,
providing precursor 2 very inexpensively.12
Results and discussion
Synthesis of (NHCMe)SiCl4
Scheme 1 Synthesis of 2.
The reaction of 2-chloro-1,3-dimethylimidazolinium chloride
based on SiCl4 (Scheme 2) and SiBr4.7,17 The only structural
exception known so far, in which the NHC ligand occupies the
axial position, is the uorine analog recently reported by Roesky
and co-workers.17a
In the solid-state structure of 2, the C1–Si1 bond (192.8(3)
pm) is slightly longer than that in B1 (191.1(7) pm) and almost
identical to that in B2 (192.8(2) pm), no crystal structure was
reported for B3. Although 2 has a saturated NHC backbone, the
angular sum for each nitrogen is 360ꢀ and the ring is almost
planar (deviation from the N1–C1–N2 plane is ꢁ6.5 pm for C3
and 7.6 pm for C2).
(1) with hexachlorodisilane in dichloromethane gave (NHCMe
)
SiCl4 (2) in good yield. The mechanism depicted in Scheme 1 as
‘route A’ proposes the initial addition of chloride to hexa-
chlorodisilane, which leads to Si–Si bond cleavage and forma-
tion of silicon tetrachloride and trichlorosilanide. The latter
species is a very reactive nucleophile and attacks the carbon at
the 2-position in 2-chloroimidazolinium. The chlorine is then
abstracted by the Lewis-acidic silicon and 2 eventually is formed
by redox-rearrangement. The cleavage of hexachlorodisilane by
bases was described by Wilkins in 1953 and has been since then
used for trichlorosilylation reactions.13 Our group recently
introduced 2,2-diuoro-1,3-dimethylimidazolidine, which can
be considered as the uorine analog of 1, as a carbene precursor
for main-group uoro-complexes.14 We also proposed a mech-
anism involving the formation of a phosphoranide by abstrac-
tion of one of the uorides, which then attacks the carbon at the
2-position, followed by redox-rearrangement which leads to
(NHCMe)PF5.14a This is in agreement with the proposed mech-
anism depicted in Scheme 1. In the case of silicon, there is no
neutral analog to PF3, although hexachlorodisilane has just
been recently discussed as a source for :SiCl2.15 Other sources of
trichlorosilanide include silicochloroform, which has been
extensively investigated by Benkeser and others.16 Conse-
quently, the reaction of 1 with silicochloroform in presence of
triethylamine also yields to product 2 (route B in Scheme 1).
Compound 2 is a colorless, non-hygroscopic solid which is
stable under an inert atmosphere. It is sensitive towards air and
moisture, but can be handled using standard Schlenk-tech-
niques without the need of a glovebox. It is soluble in acetoni-
trile, dichloromethane and tetrahydrofuran, sparingly soluble
in toluene, and insoluble in methyl tert-butyl ether, diethyl
ether and alkanes. Complex 2 was fully characterized by
multinuclear NMR spectroscopy, elemental analysis and XRD.
Single crystals suitable for X-ray diffraction were grown by slow
diffusion of diethyl ether into a solution of 2 in acetonitrile.
Compound 2 crystallizes in the orthorhombic space group Pbca;
the structure corresponds to silicon(IV) in a trigonal-bipyramidal
geometry, with the NHC ligand at the equatorial position
The 29Si NMR singlet of 2 (ꢁ103.9 ppm, CD3CN) shows a
chemical shi similar to that reported for B1 (ꢁ105.1 ppm,
C6D6), B2 (ꢁ108.9 ppm, CD2Cl2) and B3 (ꢁ110.1 ppm, C4D8O)
and is more upeld than SiCl4 (ꢁ18.5 ppm, CCl4).18 Further-
more, its carbene 13C NMR signal (173.1 ppm, CD3CN) is more
downeld compared to B1 (153.1 ppm, C6D6) and B3
(156.9 ppm, C4D8O) (no 13C NMR data for B2 is available), which
is expected for a saturated NHC complex.19
Fig. 1 Crystal structure of compound 2 with the thermal ellipsoids set at 50%
probability level. Selected bond lengths (pm) and angles (ꢀ): Si1–C1 192.8(3), Si1–
Cl1 220.22(12), Si1–Cl2 207.27(11), Si1–Cl3 207.32(12), Si1–Cl4 221.39(12); N1–
(Fig. 1). This arrangement is consistent with known derivatives C1–N2 110.4(3), Cl1–Si1–Cl4 172.95(6), Cl2–Si1–Cl3 113.54(5).
78 | Chem. Sci., 2013, 4, 77–83
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Scheme 3 Synthesis of NHC–phosphorus(III) adducts 3a–c.
Scheme 2 Synthesis of known analogs of 2.
Fig. 2 Model compounds C and D.
Fig. 4 Crystal structures of 3a (top left), 3b (bottom) and 3c (top right) with
thermal ellipsoids set at 50% probability level. For selected bond lengths and
angles see Table 2.
Fig. 3 Fragment molecular orbitals of 2: NHC fragment (left, HOMOꢁ1 for this
fragment), SiCl4 fragment (middle, HOMO for this fragment) and the resulting
HOMO orbital of 2 (right). The isosurfaces are drawn at 0.03 a.u.
Table 1 Calculated bond dissociation energies (BDE) of the carbene–metal bond
and NBO charges in 2, 2*, C and D
NBO charges
Structure
BDE/kcal molꢁ1
C
N1
N2
Si (Ag)
2
68
69
53
55
0.156
—
0.194
0.094
ꢁ0.291
—
ꢁ0.291
—
1.108
—
1.079
0.516
C
2*
D
ꢁ0.303
ꢁ0.300
ꢁ0.318
ꢁ0.319
Table 2 Selected bond lengths (pm) and angles (ꢀ) (R ¼ Cl (3a), Ph (3b), Me (3c))
3a
3b
3c
Scheme 4 Cationic species resulting from the addition of NHC precursors to
different phosphorus(III) compounds.
P1–C1
P1–Cl1
P1–Cl2
P1–R
187.87(16)
223.32(6)
248.20(7)
205.37(6)
187.07(15)
241.61(6)
232.27(5)
184.82(15)
187.64(16)
246.91(7)
229.47(7)
184.35(18)
Quantum mechanical models of (NHCMe)SiCl4
Stability and structures of carbene-derived penta- and hexa-
coordinated silicon complexes have been studied earlier.20
However, here we focus on a peculiarity of the binding between
the SiCl4 and carbene fragments. Recently, Liu studied the
N1–C1–N2
C1–P1–R
Cl1–P1–Cl2
111.72(14)
103.22(5)
170.66(2)
111.17(13)
105.75(7)
171.88(2)
111.11(14)
104.47(7)
172.24(2)
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Scheme 5 Proposed mechanism for the formation of 3a–c.
Scheme 7 Synthesis of transition-metal complexes 5 and 6.
Scheme 6 Chloride/fluoride metathesis and subsequent addition of HF to 3a.
Fig. 7 Crystal structures of 5 (left) and 6 (right) with thermal ellipsoids set at
50% probability level. H atoms are omitted for clarity. Selected bond lengths (pm)
and angles (ꢀ) for 5: C1–Ni1 186.3(2), C6–Ni1 187.9(2), Ni1–Cl1 222.56(6), Ni1–Cl2
221.27(7); N1–C1–N2 108.8(2), N3–C6–N4 108.62(18), C1–Ni1–Cl1 174.24(7),
C6–Ni1–Cl2 170.96(7). For 6: C1–Pd1 198.8(2), C6–Pd1 197.52(2), Pd1–Cl1
238.03(2), Pd1–Cl2 237.81(2); N1–C1–N2 109.2(2), N3–C6–N4 109.2(2), C1–Pd1–
Cl1 176.28(7), C6–Pd1–Cl2 176.65(7).
Fig. 5 1H NMR spectrum of 4 in CD3CN. d 3.20 (s, 6H, NCH3), 3.71 (s, 4H, –CH2–),
5.72 (d, quint, 1H, P–H,1JPH ¼ 938 Hz, 2JHF ¼ 119 Hz). (signals at d 3.71 and 3.20
are cut off for clarity).
follow the formation of a classical Lewis-acid/Lewis-base
adduct, (ii) the Si–C bond is based on purely electrostatic
interaction, (iii) the Si–C bond is highly polarized. As the
binding between the SiCl4 and the carbene fragment is of most
importance for the chemistry presented further in this article,
we have carried out additional calculations to complement
those performed by Liu.
As a method of choice, we have employed DFT23 calculations
using PBE24 functional and full electron TZ2P ZORA25 basis set
with scalar relativistic correction, as implemented in adf2012.01.26
We have performed three type of analyses: AIM27, NBO28 and
fragment analysis.29 AIM and NBO did not contribute new infor-
mation. The fragment and EDA analysis, however, revealed that
binding between the two fragments (NHC and SiCl4) is manifested
not only by the electrostatic interaction. Although it is the
predominant term, signicant stabilization is due to the orbital
interactions, which suggest that the binding between the frag-
ments indeed has substantial donor–acceptor character (Table S5,
ESI†). The fragment analysis also indicates that part of the energy
Fig. 6 Crystal structure of 4 with thermal ellipsoids set at 50% probability level.
Selected bond lengths (pm) and angles (ꢀ): C1–P1 190.6(2), av. P1–F 163.37(14), P1–H
134(2); N1–C1–N2 110.17(17), C1–P1–H 179.3(9), F1–P1–F3 178.00(7), F2–P1–F4 stabilization of the binding between the two fragments is due to
177.71(7).
interactions of molecular orbitals which have very little or no
contribution from the atoms forming the C–Si bond (see Fig. 3).
According to the fragment analysis, the HOMO of 2 can be
bonding situation for compound C, as a simplied model of represented as emerging from mainly two fragment molecular
Robinson’s compound B2, making it the unsaturated analog of orbitals (FMOs), each contributing with different weight. The
our compound 2 (Fig. 2).21 He applied an energy decomposition two FMOs and the resulting HOMO of 2 are depicted in Fig. 3
analysis (EDA)22 on C, using DFT (BP86 functional and frozen (Fig, S27, ESI†).
core TZ2P basis set).
The SiCl4 FMO is delocalized only on the four Cl atoms of the
Liu came to three conclusions: (i) the formation of the fragment and constitutes about 90% of the HOMO in
compound from SiCl4 and the carbene fragment should not compound 2. This leads to the conclusion that a chemical
80 | Chem. Sci., 2013, 4, 77–83
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attack or deformation of the SiCl4 fragment would result in and phosphine, as already described for the reaction of the free
destabilization of the interaction between the two moieties.
carbene with chloroform.32
A clear and straightforward way to assess the effect is to
Adduct 3a crystallizes in the monoclinic space group P21/c,
compute the binding energy between the fragments for a model 3b in the monoclinic space group C2/c and 3c in the ortho-
where the SiCl4 would have an articial geometrical congura- rhombic space group Pbca.
tion. An example is where the four Cl atoms lie in one plane with
The three compounds are structurally analogous and reveal a
the Si atom (2*). The transformation from 2 to 2* has a penalty pseudo-trigonal-bipyramidal geometry around phosphorus.
of 15 kcal molꢁ1 on the binding energy between the two frag- The NHC ligand, the lone pair and the substituent R occupy the
ments. Thus, the bond dissociation energy for the activated equatorial positions and the remaining two chlorine atoms
state 2* becomes similar to that found for NHC–AgCl reside at the axial positions (Fig. 4). The carbene–phosphorus
(compound D, Fig. 2), which we included as a model for widely- bond lengths vary only slightly between the derivatives
used NHC transfer reagents.
(187.87(16) pm for 3a, 187.07(15) pm for 3b and 187.64(16) pm
The bond dissociation energies (BDE) of 2, 2*, C and D (Table for 3c) and are in good agreement with Robinson’s compound E
1) revealed almost the same value for 2 (68 kcal molꢁ1) and C (69 (187.1(11) pm).
kcal molꢁ1), even though the NHC of C is an unsaturated ring
Compound E was the only previous neutral adduct of car-
system. Unsaturated ve-membered NHCs do not exhibit full p- bene and phosphorus(III) chloride. Other reactions of sterically
delocalization, but rather two separated p-systems.30 In this non-demanding NHCs with phosphorus(III) compounds gave
case, the double bond in C has a negligible inuence on the only cationic products as shown in Scheme 4.33 The direct
silicon–carbene bond. The decrease in BDE going from 2 to the addition of free NHC to PCl3 led to the reduction of phosphorus
activated 2* indicates that NHC–SiCl4 can be used as an NHC and formation of F.34 Most likely, the free carbene attacks a
transfer reagent. With these computational results we can chloride, instead of directly coordinating to phosphorus. A
conclude that the title compound 2 has a similar carbene different approach was made by utilizing imidazolium-2-
transfer potential as the established carbene transfer reagent carboxylate (G), which has been used for synthesizing new
based on silver(I) (D).
transition-metal complexes.35 Reaction of G with substituted
chlorophosphines afforded the cationic complexes H and I.36
It should also be noted that an analog of F was reported
almost 30 years ago by Schmidpeter et al. by reaction of PCl3
NHC transfer to phosphorus(III)
The results presented above, as well as its straightforward and with electron-rich bis-imidazolidinylidenes (“Wanzlick-
inexpensive accessibility through the synthesis using silico- Olen”).37
chloroform, led to the consideration of testing 2 as a reagent for
For our NHC transfer reaction however, no cationic products
carbene transfer. Addition of PCl3, PCl2Ph and PCl2Me to a were observed, and a proposed mechanism is shown in Scheme
solution of 2 in THF at 0 ꢀC followed by stirring at room 5. Abstraction of a chloride from 2 by PCl2R leads to the imi-
temperature for 12 h gave the NHCMe adducts (NHCMe)PCl3 (3a), dazolinꢁium-2-trichlorosilyl cation (J) and the phosphoranide
(NHCMe)PCl2Ph (3b) and (NHCMe)PCl2Me (3c) in almost quan- [PCl3R] .38 Cation J has not been isolated so far, however its
titative yields (Scheme 3).
bromine and methyl analogs were reported recently.39 The
Compounds 3a–c are colorless solids, which are stable under phosphoranide itself then undergoes a nucleophilic attack on
an inert atmosphere for at least several weeks. Furthermore, TGA cation J to form the transition-state assembly K according to
measurements on these solids show decomposition between 70 Scheme 5. Loss of SiCl4 from K leads to the products 3a–c.
and 110 ꢀC (see ESI†). They are soluble in acetonitrile, dichloro-
In contrast to the phosphorus in E, which is protected
methane, chloroform and THF, but insoluble in diethyl ether and towards oxidation by the sterically demanding NHCdipp ligand,
alkanes. All compounds were characterized by multinuclear the phosphorus in 3a is exposed and can easily be oxidized
NMR, elemental analysis and XRD. The 31P NMR (CD3CN) shis without loss of the carbene ligand. Addition of triethylamine
(24.6 ppm for 3a, ꢁ21.6 ppm for 3b and ꢁ22.1 ppm for 3c) are trihydrouoride to a cooled solution of 3a in dichloromethane,
signicantly upeld compared to the starting materials by followed by an activation step with triethylamine, gave (NHCMe
)
around 200 ppm, due to electron donation of the NHC ligand. PF4H (4) (Scheme 6).40 Chloride/uoride metathesis occurs
Robinson and co-workers have previously reported the analogous during the reaction, followed by addition of HF. Complex 4 is a
complex (NHCdipp)PCl3 (E) by addition of the respective free colorless solid, which is soluble in acetonitrile and dichloro-
carbene to PCl3.8a,31 The 31P NMR upeld shi of E to 16.9 ppm methane, but insoluble in diethyl ether and alkanes. It is stable
(C6D6) is in agreement with our data. All 13C signals are split into towards air and moisture and can be washed with ethanol for
doublets by the adjacent phosphorus and reveal no signicant purication.
deviation in chemical shis and coupling constants amongst
The 1H NMR (CD3CN) spectrum of 4 (Fig. 5) shows
each other. The respective spectra showed a shi of 172.9 ppm singlet signals at 3.20 ppm (6H) and 3.71 ppm (4H), which are
(1JCP ¼ 109 Hz) for 3a, 174.9 ppm (1JCP ¼ 78 Hz) for 3b and 176.1 assigned to the coordinated NHCMe ligand. At 5.72 ppm the P–H
ppm (1JCP ¼ 82 Hz) for 3c. If kept in solution, decomposition of proton is detected as a doublet with a large coupling constant of
the three compounds to the parent phosphines can be detected 1JHP ¼ 938 Hz. The doublet is split into quintets by the four
by 31P NMR spectroscopy aer one day. The respective 1H NMR adjacent uorine atoms with a geminal coupling constant of
shows decomposition products indicating a dissociation of NHC 2JHF ¼ 118 Hz.
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¨
Complex 4 was characterized by multinuclear NMR spec- Brackmann (Universitat Bremen, Germany) for X-ray crystallo-
troscopy, HRMS and XRD. X-Ray quality single crystals were graphic measurements.
obtained aer keeping a solution of 4 in acetonitrile for 7 days
at ꢁ40 ꢀC. Compound 4 crystallizes in the triclinic space group
Notes and references
ꢀ
P1 and the crystal structure reveals an octahedral geometry
¨
1 (a) K. Ofele, J. Organomet. Chem., 1968, 12, P42–P43; (b)
around phosphorus (Fig. 6). Our group recently reported
analogs of 4 with the same octahedral geometry, in which
hydrogen is replaced by uorine, phenyl and methyl, via
oxidative addition of 2,2-diuoro-1,3-dimethylimidazolidine to
PF3, Cl2PPh and Cl2PMe, respectively.14a
¨
H. W. Wanzlick and H. J. Schonherr, Angew. Chem., Int. Ed.
Engl., 1968, 7, 141–142; (c) A. J. Arduengo III, R. L. Harlow
and M. Kline, J. Am. Chem. Soc., 1991, 113, 361–363.
¨
2 (a) W. A. Herrmann and C. Kocher, Angew. Chem., Int. Ed.
Engl., 1997, 36, 2162–2187; (b) F. E. Hahn and
M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47, 3122–3172;
(c) C. M. Crudden and D. P. Allen, Coord. Chem. Rev., 2004,
NHC transfer to nickel(II) and palladium(II)
Addition of solid Ni(PPh3)2Cl2 and PdCl2 to a solution of 2 in
THF, followed by reuxing for 12 h, gave the bis-carbene
complexes (NHCMe)2NiCl2 (5) and (NHCMe)2PdCl2 (6), respec-
tively (Scheme 7). Both complexes are isostructural and crys-
tallize in the monoclinic space group P21/n. The crystal
structures reveal square-planar geometry around the metal
centers with the NHC ligands in cis-conformation (Fig. 7).
The two carbene ligands are twisted relative to the
MCl2-plane with planar angles of 86.7 and 80.4ꢀ for 5 and 63.6
and 64.0ꢀ for 6. Complex 5 was reported by Lappert and Pye-
some some time ago by addition of a nickel(0) precursor to
bis(1,3-dimethylimidazoline) and subsequent oxidation with
chlorine.41 Though, it has not been characterized in the solid
state. Herrmann et al. reported the iodine analog of 6 with
unsaturated NHCMe ligands by deprotonation of 1,3-dimethy-
limidazolium iodide with [Pd(OAc)2].3a
´
248, 2247–2273; (d) P. de Fremont, N. Marion and
S. P. Nolan, Coord. Chem. Rev., 2009, 253, 862–892; (e)
P. L. Arnold and I. J. Casely, Chem. Rev., 2009, 109, 3599–
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Conclusions
We have synthesized the adduct (NHCMe)SiCl4 (2) using silico-
chloroform and demonstrated its application as a versatile
carbene transfer reagent. Direct addition of 2 to PCl3, PCl2Ph
and PCl2Me in THF afforded the respective neutral (NHCMe)–
phosphorus(III) complexes 3a–c in almost quantitative yields.
Previous attempts to synthesize such compounds proved
unsuccessful and only led to cationic species. Moreover, the
phosphorus(III)-center in 3a is prone to oxidation, due to the
sterically non-demanding NHCMe ligand. Without loss of
the NHCMe ligand, complex 3a was oxidized to give (NHCMe
)
PF4H (4). The ligand NHCMe was also transferred from 2 to the
transition metals nickel(II) and palladium(II) to give the bis-
carbene complexes 5 and 6, respectively. All these compounds
were fully characterized by single-crystal X-ray diffraction. The
experimental results are supported by DFT calculations, which
concluded a similar carbene transfer potential of 2 compared to
established transfer reagents based on silver(I) chloride. The
synthesis of 2 using silicochloroform makes it a low-cost alter-
native to the established yet expensive carbene transfer reagents
based on silver.
Acknowledgements
Financial support from the Deutsche Forschungsgemeinscha
(Grant RO 362/47-1) is gratefully acknowledged. Marie Curie
(call: FP7-PEOPLE-2009-IAPP) QUASINANO. We thank Peter
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