of yet unlimited scope or even as an alternative to some of the
polymer synthesis approaches to energetic oligomers.
This research was supported by the Air Force Office of
Scientific Research (Grant F49620-03-1-0357).
Notes and references
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¨
Scheme 3 Top: synthesis of zwitterionic 10 from 8: (a) acrylonitrile
(1 eq.), Et3N (cat.), toluene, reflux, 30 h; (b) chloroacetonitrile, ethyl
acetate, RT, 48 h; (c) NaN3 (2.2 eq.), AcOH, 70 1C, 48 h. Bottom:
synthesis of 11 and 12 from zwitterionic 10. (d) Hydrogen bis(trifluoro-
methanesulfonyl)amide (HNTf2), MeOH/water 1/1, RT, 72 h; (e) 1,3-
dimethylimidazolium-2-carboxylate, MeOH/water 1/1, DMSO (cat.),
RT, 72 h.
1-(2-(5-tetrazolidyl)ethyl)-3-(5-1H-tetrazolyl)methylimidazolium
(10), in a single step.
The ready isolation of zwitterion 10 in good yield and purity
via crystallization allowed us to easily prepare two ILs from
this precursor with synthetic operations we normally use to
prepare ILs. The protonation of 10 with hydrogen bis(trifluoro-
methane sulfonyl)amide resulted in quantitative formation of
1-(2-(5-1H-tetrazolyl)ethyl)-3-(5-1H-tetrazolyl)methylimidazolium
bis(trifluoromethane sulfonyl)amide (11). Deprotonation of
10 was realized by reaction with l,3-dimethylimidazolium-2-
carboxylate (as described in procedures published by our group13)
leading to 1,3-dimethylimidazolium 1-(2-(5-tetrazolidyl)ethyl)-
3-(5-tetrazolidyl)methylimidazolium (12) in quantitative yield
in a single step. Each of the salts derived from the parent
zwitterion were liquid at room temperature (Tm = 10.5 1C and
ꢀ25.4 1C for 11 and 12, respectively), thereby demonstrating
how one may selectively obtain either cationic or anionic
components for ILs from this single zwitterionic precursor
utilizing known IL-based strategies.
The design strategy for a systematic and versatile multi-
heterocyclic synthetic platform allows for formation of highly
variable anions, cations, and zwitterions for potential appli-
cation as novel IL components or precursors. We believe
that the ability to extend the number, identity, and bridging
of heterocycles, especially to asymmetric, singly-charged
molecular ions will be a more useful strategy to make ILs than
any of the known oligomeric approaches to multi-heterocyclic
IL ions. We have demonstrated this here with low-valent,
bridged bi- and tricyclic compounds, including the synthesis of
the crystalline zwitterion 1-(2-(5-tetrazolidyl)ethyl)-3-(5-1H-
tetrazolyl)methylimidazolium and its conversion into room
temperature ionic liquids as either the cation or the anion.
Given the richness of this design space—from available multi-
heterocyclic precursors to the diversity enabled by IL-based
modifications—we foresee this platform as a powerful tool
that may be utilized towards new multi-heterocyclic IL targets
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ꢁc
This journal is The Royal Society of Chemistry 2010
3546 | Chem. Commun., 2010, 46, 3544–3546