Table 1 Alkylation of [EMIM][Me(Me)PO3] with different alkylating
agents R*–X to form the corresponding neutral esters
Following approach (b), we subjected the binary IL mixtures
obtained in the first alkylating step to another SN2 substitution
reaction (Scheme 3).
Entry R*–Xa
Resulting ester
Yield/%
The so obtained neutral phosphonate esters (in relation
90 : 10) were distilled (T = 160 1C, p = 1 bar) to yield the
pure alkylating agent R2ꢁðMeÞPO3. Obviously, by using
Rꢁ2ðMeÞPO3 in the quaternisation of different amines, pure ionic
liquids were obtained. However, the latter carry the alkyl group
functionality R* in both the cation and the anion (detailed
synthetic protocols are given in the ESIw). For comparison of
the physico-chemical properties we synthesised in addition the
ionic liquids [EMIM][Me(Me)PO3] and [OMIM][Oc(Ph)PO3] by
reacting the commercially available dimethyl methylphosphonate
and dioctyl phenylphosphonate esters with N-ethylimidazole and
N-methylimidazole, respectively. It was found that the alkylating
power of the symmetrical phosphonate ester decreases with
increasing length of the alkyl residue as found for the asymmetric
phosphonate esters. The reaction conditions were adjusted
accordingly to realise full conversion. Whereas the methylation
reaction by the AcNMe(Me)PO3 ester (Table 1, entry 1) required
acetonitrile as a solvent and 90 1C for 20 h, the alkylation using
the dodecyl methyl methylphosphonate ester required a reaction
temperature of 160 1C for three days under solventless conditions.
Table 2 displays important physico-chemical properties of the
obtained phosphonate ionic liquids.
1
2
AcNMe(Me)PO3
MeAcMe(Me)PO3
42
74
3
4
C4H9–Br
C8H17–Br
ButylMe(Me)PO3
OctylMe(Me)PO3
85
89
5
(MeEG3)Me(Me)PO3 87
DodecylMe(Me)PO3 92
6
C12H25–I
a
R*–X was prepared according to Kuhlmann et al.6 Reaction conditions:
entry 1: 25 1C, 5 h; entries 2 and 5: 25 1C, 12 h; entry 3: 50 1C, 20 h;
entry 4: 60 1C, 20 h; entry 6: 75 1C, 36 h.
at 34–36 ppm. Among the applied alkylation agents R*–X, the
alkyl halides with longer alkyl chains were less reactive than
those with shorter ones. As expected, the use of alkyl iodides
and alkyl bromides resulted in faster transformation compared
to the reaction with alkyl chlorides. Isolation of the formed
esters R*Me(Me)PO3 was straightforward by extracting the
reaction mixture of the alkylation reaction with diethyl ether.
All esters R*Me(Me)PO3 were obtained after solvent removal
in a completely halide-free quality (confirmed by Ag[NO3] test).
The stoichiometric by-product [EMIM]X (X = Clꢀ, Brꢀ, Iꢀ,
and [Ph-SO3]ꢀ) could be isolated from the reaction mixture by
crystallisation (after cooling in the case of X = Brꢀ and Iꢀ) and
represents itself a valuable ionic liquid material.
In accordance to the trends found for functionalised alkylsulfate
ionic liquids, the density decreases with longer alkyl substituents
and increases with oxygen or nitrogen containing function-
alisations. The viscosity data demonstrate a strong influence of
the functionalisation with the ethylene glycol functionalised
ionic liquid showing the lowest viscosity at 20 1C, even lower
than [EMIM][Me(Me)PO3]. The ILs prepared in this study are
characterised by reasonable thermal stabilities with values
between 216 and 281 1C. The same temperature range was
recently reported also for the short-chain, non-functionalised
phosphonate ILs.7 Most of the synthesised phosphonate systems
show glass transition temperatures of ꢀ50 to ꢀ70 1C, only the
C12-substituted salt shows a melting point around 60 1C.
Very remarkably, the hydrolytic stability of the investigated
phosphonate ionic liquids was found to be very high. By
heating the IL for 5 h in excess water at 100 1C (reflux), no
changes in the NMR spectra were observed for all ILs except
for [EMIM][MeAc(Me)PO3]. In this latter case, there was also
no hydrolytic reaction at the phosphorous observed but
cleavage of the carboxylic ester to form the corresponding
acid functionality (for details see ESIw).
In conclusion, our paper presents a new and very general
synthetic route to long-chain and functionalised phosphonate
esters and the corresponding ionic liquids. The key-step in this
method is the SN2 reaction of a phosphonate ionic liquid with
a suitable alkylating agent R*–X to form the neutral esters
R*Me(Me)PO3 or Rꢁ2ðMeÞPO3. These esters can be isolated by
extraction or distillation and can serve as alkylating agents to
quaternise amines or phosphines. The obtained, functionalised
phosphonate salts are free of halogen impurities and display
interesting physico-chemical properties including a wide liquid
range, relatively low viscosities (taking into account their size
and molecular weight) as well as reasonable thermal and very
good hydrolytic stability. We anticipate for these new ionic
It is another remarkable finding of our research that the so
obtained esters R*Me(Me)PO3 exhibit an alkylation strength
suitable to quaternise amines to form new ionic liquids with the
functionalised group R* attached to the phosphonate ion. This
is not a trivial fact, as—for example—carboxylate esters are
also obtainable by alkylation of carboxylate ionic liquids.16
However, the latter cannot be used as alkylating agents. Thus,
by reacting the different esters R*Me(Me)PO3 of Table 1 with
ethylimidazole, the corresponding [EMIM] ionic liquids were
prepared (for reaction conditions see Table 2; for details on the
synthesis see ESIw).
While [EMIM][AcN(Me)PO3] and [EMIM][MeAc(Me)PO3]
were obtained as pure compounds, the reaction with the
phosphonate esters of entries 3–6 (Table 1) afforded defined
product mixtures under the applied quaternisation conditions
(Scheme 2). For these phosphonate esters also a small part of
the functionalised substituent of the alkylating agent was
transferred to ethylimidazole, next to the more reactive methyl
group. Scheme 2 shows the observed behaviour for R* being
an alkyl or ethylene glycol ether group.
The obtained binary mixtures of two different ILs are well
defined and may find applications as mixtures e.g. in tribology
applications. From a synthetic perspective, however, we
were interested to develop routes to a large range of pure
[R*-methylphosphonate]ꢀ ionic liquids. This requires a selective
transfer of the alkyl rest which can be realised if (a) the two
alkyl ester groups are very different in their reactivity (see
examples 1 and 2 in Table 1) or (b) if the two rests are the same.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11234–11236 11235