The crystal structures of the previously reported 2,2-difluoro-1,3-dimethylimidazolidine and its acyclic analog were determined by in situ crystallization. Both liquid fluorinating agents have ...recently been introduced as potent precursors for the synthesis of p-block element carbene complexes. These fluorine compounds were compared with chloro-amidinium chlorides, which are already established carbene precursors. The chlorides were characterized as ion pairs and contrasted to the previous interpretation as charge-transfer complexes between carbene and dichlorine. Furthermore, the two precursors are used for the synthesis of a series of new carbene complexes of phosphorus(v) fluorides substituted with perfluoroalkyl-groups. These unusually stable hexacoordinate phosphorus(v) complexes were treated with anhydrous hydrogen fluoride to yield imidazolium phosphates. This procedure allows the synthesis of salts used as ionic liquids in high yield and high purity.
Abstract
The addition of BCl
3
to the carbene‐transfer reagent NHC→SiCl
4
(NHC=1,3‐dimethylimidazolidin‐2‐ylidene) gave the tetra‐ and pentacoordinate trichlorosilicon(IV) cations (NHC)SiCl
3
+
and ...(NHC)
2
SiCl
3
+
with tetrachloroborate as counterion. This is in contrast to previous reactions, in which NHC→SiCl
4
served as a transfer reagent for the NHC ligand. The addition of BF
3
⋅
OEt
2
, on the other hand, gave NHC→BF
3
as the product of NHC transfer. In addition, the highly Lewis acidic bis(pentafluoroethyl)silane (C
2
F
5
)
2
SiCl
2
was treated with NHC→SiCl
4
. In acetonitrile, the cationic silicon(IV) complexes (NHC)SiCl
3
+
and (NHC)
2
SiCl
3
+
were detected with (C
2
F
5
)SiCl
3
−
as counterion. A similar result was already reported for the reaction of NHC→SiCl
4
with (C
2
F
5
)
2
SiH
2
, which gave (NHC)
2
SiCl
2
H(C
2
F
5
)SiCl
3
. If the reaction medium was changed to dichloromethane, the products of carbene transfer, NHC→Si(C
2
F
5
)
2
Cl
2
and NHC→Si(C
2
F
5
)
2
ClH, respectively, were obtained instead. The formation of the latter species is a result of chloride/hydride metathesis. These compounds may serve as valuable precursors for electron‐poor silylenes. Furthermore, the reactivity of NHC→SiCl
4
towards phosphines is discussed. The carbene complex NHC→PCl
3
shows similar reactivity to NHC→SiCl
4
, and may even serve as a carbene‐transfer reagent as well.
The addition of BCl sub(3) to the carbene-transfer reagent NHC arrow right SiCl sub(4) (NHC=1,3-dimethylimidazolidin-2-ylidene) gave the tetra- and pentacoordinate trichlorosilicon(IV) cations ...(NHC)SiCl sub(3) super(+) and (NHC) sub(2)SiCl sub(3) super(+) with tetrachloroborate as counterion. This is in contrast to previous reactions, in which NHC arrow right SiCl sub(4) served as a transfer reagent for the NHC ligand. The addition of BF sub(3)OEt sub(2), on the other hand, gave NHC arrow right BF sub(3) as the product of NHC transfer. In addition, the highly Lewis acidic bis(pentafluoroethyl)silane (C sub(2)F sub(5)) sub(2)SiCl sub(2) was treated with NHC arrow right SiCl sub(4). In acetonitrile, the cationic silicon(IV) complexes (NHC)SiCl sub(3) super(+) and (NHC) sub(2)SiCl sub(3) super(+) were detected with (C sub(2)F sub(5))SiCl sub(3) super(-) as counterion. A similar result was already reported for the reaction of NHC arrow right SiCl sub(4) with (C sub(2)F sub(5)) sub(2)SiH sub(2), which gave (NHC) sub(2)SiCl sub(2)H(C sub(2)F sub(5))SiCl sub(3). If the reaction medium was changed to dichloromethane, the products of carbene transfer, NHC arrow right Si(C sub(2)F sub(5)) sub(2)Cl sub(2) and NHC arrow right Si(C sub(2)F sub(5)) sub(2)ClH, respectively, were obtained instead. The formation of the latter species is a result of chloride/hydride metathesis. These compounds may serve as valuable precursors for electron-poor silylenes. Furthermore, the reactivity of NHC arrow right SiCl sub(4) towards phosphines is discussed. The carbene complex NHC arrow right PCl sub(3) shows similar reactivity to NHC arrow right SiCl sub(4), and may even serve as a carbene-transfer reagent as well. One way or another: NHC arrow right SiCl sub(4) is not only a carbene-transfer reagent that is useful for the synthesis of p-block complexes with N-heterocyclic carbene (NHC) ligands that are not available in their free form, but it also serves as a potent precursor for NHC-stabilized silicon(IV) cations (see scheme). Selected examples are shown, in which NHC arrow right SiCl sub(4) reacts as a source of carbene or silicon cations, depending on the reaction conditions. NHC: 1,3-dimethylimidazolidin-2-ylidene.
The crystal structures of the previously reported 2,2-difluoro-1,3-dimethylimidazolidine and its acyclic analog were determined by
in situ
crystallization. Both liquid fluorinating agents have ...recently been introduced as potent precursors for the synthesis of p-block element carbene complexes. These fluorine compounds were compared with chloro-amidinium chlorides, which are already established carbene precursors. The chlorides were characterized as ion pairs and contrasted to the previous interpretation as charge-transfer complexes between carbene and dichlorine. Furthermore, the two precursors are used for the synthesis of a series of new carbene complexes of phosphorus(
v
) fluorides substituted with perfluoroalkyl-groups. These unusually stable hexacoordinate phosphorus(
v
) complexes were treated with anhydrous hydrogen fluoride to yield imidazolium phosphates. This procedure allows the synthesis of salts used as ionic liquids in high yield and high purity.
Solid state structures of volatile fluorinating agents used as carbene precursors are discussed. A new synthetic protocol for the synthesis of ionic liquids is presented.
