Click chemistry mediated synthesis of bio-inspired phosphonyl-functionalized ionic liquids

Article abstract of DOI:10.1039/c4gc01933d

This study focuses on the synthesis of a class of novel biologically-inspired ionic liquids coupled with a phosphonate group containing short to long side chains (C₃-C₁₁) via the Pudovic reaction. The ionic liquids exhibited very low glass transition temperatures and were hydrophobic in character. This method has the attributes of click chemistry with outstanding efficiency, simplicity, yields and regioselectivity. The results of their calcium(ii) ligating capability are also presented.

Full text of DOI:10.1039/c4gc01933d

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ARTICLE
Click chemistry mediated synthesis of bioꢀinspired
phosphonylꢀfunctionalized ionic liquids
a
a
b
a,§
Richard A. Braun, Joseph L. Bradfield, Codey B. Henderson, Niloufar Mobarrez,
a
b
b
b
Yinghong Sheng, Richard A. O’Brien, Alexandra C. Stenson, James H. Davis, Jr.,
and Arsalan Mirjafari*
a
Cite this: DOI: 10.1039/x0xx00000xx
This study focuses on the synthesis of a class of novel biologicallyꢀinspired ionic liquids coupled with a
phosphonate group containing short to long side chains (C3−C ) via Pudovic reaction. The ionic liquids
11
exhibited very low glass transition temperatures and were hydrophobic in character. This method has the
attributes of "click" chemistry with outstanding efficiency, simplicity, yields and regioselectivity. The
results of their calcium (II) ligating capability are also presented.
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
3a,b
Introduction
liquids. Therefore, synthesis of ionic liquids that incorporate more
hydrophobic structural elements, while keeping their melting points
Taskꢀspecific ionic liquids (TSILs) or functionalized ionic liquids below room temperature, continues to be a challenge.
are a unique subclass of conventional ionic liquids that incorporate
Phosphonic acids and their analogue esters, phosphonates, are
an ionꢀtethered functionality tailored for specific applications a valuable class of organophosphorus compounds employed in a
ranging from organic synthesis and metal extraction to material and variety of chemical and biological functions due to their structural
1
10
pharmaceutical sciences. The introduction of a functional group on similarities with phosphates (including DNA and RNA) and
11
a
cation and/or anion allows for marked changes of the phosphonopeptides. Their positions as crucial constituents of many
12
13
14
physicochemical properties of ionic liquids and conveys a particular major biomolecules, proꢀdrugs and materials have resulted in
reactivity pattern to them. The continued success of TSILs in a wide the development of various synthetic methodologies designed to
variety of applications presents challenges toward developing generate phosphonates and phosphonic acids as a mimic of the
practical synthetic routes for these functional organic materials. parent phosphate. From a synthetic viewpoint, the additions of
Since functional groups on TSILs are generally heteroatom-based, phosphorusꢀcentered radicals across the ene and yne substrates are
the challenge is to develop a highly efficient, atomꢀeconomical and one of the most versatile pathways for the formation of carbonꢀ
15
2
orthogonal methodologies, such as "click" chemistry. Underlying phosphorus bonds. The reactivity has been extensively studied using
these synthetic goals is the additional desire to develop scalable various activation methods or catalysts such as AIBN/heat,
1
6
16a
17
18
19
reactions that fall within the philosophical realm of green chemistry AIBN/UV,
Et B/O ,
microwave,
phosphine−borane,
transition metals and NiLn. Recently, Li et al. reported a radicalꢀ
3
2
2
e
20
21
for the synthesis of ILs.
Functional groups can either be coupled to the cation, anion or based phosphonofluorination of unactivated alkenes using AgNO3/
both through a linker, which is generally used for minimizing Selectfluor reagent as a catalytic system, which led to formation of
2
2
interaction between a reactive functionality and an ionic moiety. In
fact, modulation of a functionality’s reactivity can be expected as a
β
ꢀfluorinated alkylphosphonates with high stereoselectivity.
Phosphonyl and phosphinoylꢀfunctionalized TSILs are
function of the length and structure of the linker. Ionic liquids typically synthesized by the nucleophilic substitution of 1ꢀ
containing long linkers (≥C ) exhibit a high degree of selfꢀ alkylimidazole
with diethyl haloalkylphosphonates and
8
organization and mobility that increases with the extention of the haloalkyldiphenyl phosphinate (alkyl= ethyl and propyl),
side chains. Ionic liquids containing long chain appendages recently respectively.2
3,25
They were reported as an adaptive ligand for the
23bꢀg
have been exploited in a broad spectrum of applications such as complexation of 4f metals,
biomaterials, phase transfer catalysts, NiꢀNHC catalyzed olefin fuel, lubricants and corrosion inhibitors for reactive metals. Liu
and Srinivasan et al. separately reported the synthesis of
nanoparticles syntheses. Regrettably, the melting points of ionic phosphorylꢀbased ILs containing short side chains (C ꢀC ) via
deep desulfurization process of diesel
3
4
24
25
26
5
6
7
23a,25
23f
dimerization, biomass dissolution,
zeolites
and metallic et al.
8
,9
2
3
liquids begin to increase significantly once an appended Nꢀ nucleophilic substantiation of 1ꢀalkylimidaoles with diethyl
substituted linker exceeds seven carbon atoms in length for several bromoalkylphosphonates at high temperature in 24ꢀ40h.
homologous series of 1ꢀnꢀalkylꢀ3ꢀmethylimidazolium ionic Phosphoramideꢀfunctionalized imidazolium salts were synthesized
This journal is © The Royal Society of Chemistry 2013
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Jou4Grnal Name
by the Davis group via nucleophilic substantiation of phosphorus (V) economical addition of phosphonyl radicals across ene and yne
oxyhalides and 1ꢀ(3ꢀaminopropyl) imidazole in DCM and then substrates − hydrophosphonylation or the Pudovic reaction − using a
quarternized at the imidazole N(3) by the reaction of EtI, followed nonꢀprecious catalyst, Mn(OAc) , to synthesize a novel class of bioꢀ
2
23b
by metathesis.
Likewise, Quadi et al. reported preparation of inspired phosphonylꢀfunctionalized ILs (Scheme 1, III and IV) with
ammoniumꢀtype phosphoramide/phosphonate TSILs using similar C −C linkers and low glass transition temperatures (Chart 1).
3
11
approach by the reaction of corresponding amine/alcohol with
dichlorophosphate in Et O followed by the quarternization by alkyl
2
2
3d
iodide and metathesis.
Synthesis of P=O functionalized
imidazolium salts as intermediates toward nucleophilic heterocyclic
carbeneꢀphosphine complexes of Pd(II) was reported by Danopoulos
2
7
et al.
They observed the reductive elimination of
methylimidazolium salts from complex palladium and phosphine or
phosphate followed by oxidative addition. The ligand synthetic
strategy is based upon the quarternization of 1ꢀarylimidazoles with
o
(
ωꢀbromoalkyl) diphenylphosphine oxide at 150ꢀ160 C followed by
reduction with SiHCl in refluxing chlorobenzene. Nockemann et al.
3
reported a multiꢀstep approach to make the phosphine oxideꢀ
functionalized ionic liquids with moderateꢀsized linkers (C −C ) in Scheme 1. Aerobic hydrophosphonylation of various ene and yneꢀ
2
8
moderate yields (~70%) by nucleophilic addition of alkyl bromideꢀ bearing ILs (I and II) catalyzed by Mn(OAc)
bearing imidazolium ionic liquids with
potassium phosphonylꢀfunctionalized ionic liquids (III and IV).
diphenylphosphine followed by the oxidation with hydrogen
to form the
2
28
Experimental
peroxide. Due to use of KPPh as a strong base and presence of
2
acidic proton at C2 postion of imidazolium ring, only C2ꢀsubstituted
imidazolium cations were used, which limited the number of
possible ILs that could be made via this methodology. Generally, in
the case of aboveꢀmentioned nucleophilic substitution methods, only
phosphonyl/phosphinoylꢀbased ILs containing short to moderate
linkers (C −C ) are synthesizable.
Materials and methods
For the present study, all of the chemicals employed including 1ꢀ
methylimidazole (98% purity), diethyl phosphite (98% purity), and
1
,2ꢀdimethylimidazole (98% purity) were commercially available in
2
8
high purity and were purchased from Acros Organics and were used
without further purification. Solvents were used as received, without
further purification. Potassium bistriflimide, employed for
metathesis reactions, was prepared via neutralization (pH = 7.0) of
Dupont and Moores groups recently reported a radicalꢀ
mediated preparation of phosphineꢀfunctionalized ionic liquids as
versatile ligands toward the second generation of Grubbs catalysts
29
for RCM reaction and palladium nanoparticles for biphasic
HNTf (Iolitec GmbH, 80%) with KOH followed by the removal of
8
2
hydrogenation of olefins, respectively. In fact, according to the
water.
1
6a
EtemadꢀMoghadam’s report,
the major disadvantages of the
1
13
31
Characterization of the products using H, C and P NMR was
performed on JEOL 500 MHz NMR with multiꢀnuclear
capabilities using DMSOꢀd . Chemical shifts are reported relative to
photoactivation methods are long reaction time and poor
yield/regioselectivity and reaction needs be carried out under argon
atmosphere. Likewise, Grubbs and Daeffler described a method to
produce Wittig triphenylphosphonium salts via the radicalꢀmediated
a
6
1
13
TMS as the internal reference at 0.00 ppm for both the H, C and
31
P NMR data. ESIꢀMS analyses were performed by flowꢀinjection
30
photoaddition of [HPPh ][BF ] across unactivated olefins. Forsyth
3
4
on a Thermo Scientific ion trap mass spectrometer using HPLC
grade acetonitrile. As our interest was in the cations of these salts,
data was collected in positive ion mode. Glass transition
temperatures were determined by using a Q2000 TA differential
scanning calorimeter, with a heating rate of 10 min . The reported
data represents the average of at least three trials, and were
et al. reported the preparation of a number of tetraalkylphosphonium
cations coupling with phosphate and phosphinate anions and their
o
sulfur analogs that displayed a high thermal stability (above 200 C)
26
and were used for corrosion protection of reactive metals.
ꢀ1
Despite the wide range of the potential applications of P=O
bearing ionic liquids, very few investigations into an efficient
synthesis methodology have been reported to date. Notably, the
reported synthetic methods suffer from one or more drawbacks such
o
reproducible to within 1 C.
2
7,28
General synthetic procedure
A mixture of diethyl phosphite (6−9 mmol), Mn(OAc) (0.3 equiv.
2
as harsh reaction conditions
laborious and complex workꢀ
use of volatile organic solvents in
2
3a, 27
up/purification procedures,
the preparation/workup process,
2
3a,27,28
relative to each ene or yne group) was stirred at room temperature
until a homogenous mixture was observed. Then ene or yneꢀbearing
ionic liquids (1 mmol) were added to the mixture and were stirred at
ability to form limited
23,27ꢀ29
8,23a,23f
numbers of ILs,
long reaction time,
expensive inert gas atmosphere like argon,
with the occurrence of side reactions.
necessity of an
moderate yields
2
6,29,30
o
1
2
3,28
105 C for 8 hours. After completion of the reactions ( H NMR
monitoring on a sample of crude mixture), the residues were washed
To further develop this
important class of TSILs, the exploitation of additional robust,
efficient and orthogonal "click" reactions is desirable. To this intent,
we sought to take advantage of the wellꢀdocumented and atomꢀ
with hexanes (5 × 10 mL) and then H O (1 × 5 mL). Removal of
residual solvents in vacuo yielded ILs 1−19 in 81−96%.
2
2
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Ene and yneꢀbearing ILs were prepared in good yields using
multiꢀstep reactions with highꢀpurity bromoalkenes or propargyl
bromide with corresponding 1ꢀmethylimidazole, 1,2ꢀdimethyl
imidazole, trialkylamine or triphenylphosphine as starting materials
a
entry
conditions
conversion (%)
ref.
followed by the metathesis reaction with KNTf , which was reported
2
AIBN (0.2 equiv.), cyclohexane, 80
b
in reference 3b. These ILs were subjected to standard silverꢀion tests
1
o
0
16b
C, 60h, N
2
ꢀ
to ensure the completeness of the exchange of halide for the NTf2
ABCN (0.2 equiv.), cyclohexane, 80
c
anion. All gave negative results, indicating that residual halide anion
content in the salts were below the accepted threshold levels.
2
o
0
0
0
27
C, 60h, N
2
Et
air
Et
3
B (0.3 equiv.), MeOH, RT, 60h,
3
4
5
17b
17c
3
B (0.3 equiv.), cyclohexane, RT,
Results and discussion
6
0h, air
Et
3
B (0.3 equiv.), cyclohexane, 80
Synthesis and characterization. The ideal catalyst should not
contain precious elements and should bring reactions to completion
in a few hours through operationally simple procedures. In this
context, the primary focus of this work was to design an
operationally simple, highꢀyielding and nonꢀprecious metal
catalyzed addition of P−H across ene and yneꢀbearing ILs (Scheme
0
ꢀ
o
C, 60h, air
d
o
6
Microwave, 80 C, 5h, air/N
NiCl (0.3 equiv.)/dppe (0.5 equiv.),
CH CN, 82 C, 2h
NiCl (0.3 equiv.), CH
2
0
18b
21e
21e
ꢀ
2
e
7
89
73
23
58
51
o
3
o
8
9
2
3
CN, 82 C, 2h
Ni(OAc)
2
3
(0.3 equiv.), CH CN, 82
1
, I and II). Our initial studies concentrated on the coupling reaction
o
C, 2h
of 1ꢀallylꢀ3ꢀmethyl imidazolium IL (Scheme 1, I, n= 1, R= H) with
diethyl phosphite as a model reaction to optimize the reaction
conditions by examining the effects of initiator/catalyst, solvent and
temperature (Table 1). The preliminary screening of various nonꢀ
o
1
1
0
1
Mn(OAc)
Mn(OAc)
2
(0.3 equiv.), 90 C, 1h, air
31
31
2
(0.3 equiv.)/Co(OAc)
2
o
(0.1 equiv.), 90 C, 1h, air
o
precious metal catalysts (NPMCs) revealed that the catalytic amount
Mn(OAc)
2
(0.3 equiv.), 105 C, 8h,
f
1
2
100
ꢀ
ꢀ
o
of Mn(OAc) (0.3 equiv./alkene) activated at 105 C in the presence
air
2
o
of air and in the absence of solvent was the most effective reaction
conditions, which led to complete conversion to product 1 in antiꢀ
Markovnikov fashion (Table 1, entry 12). Remarkably, when the
reaction was performed in an openꢀflask manner, the complete
conversion was observed (Table 1, entry 12), whereas the yield of
the obtained product was very low in the absence of air (Table 1,
entry 13). This methodology is the modification of previously
Mn(OAc) (0.3 equiv.), 105 C, 8h,
2
1
3
<10
N2
a
1
b
Conversion measured by H NMR; AIBN: azobis(isobutyronitrile);
c
d
ABCN: azobis(cyclohexanecarbonitrile); decomposed product was
e
f
obtained; dppe: bis(diphenylphosphino)ethane; reaction conditions:
[allylmim][NTf ] (1 mmol), diethyl phosphite (6 mmol), Mn(OAc) (0.3
2
2
mmol) under openꢀflask conditions.
31
reported reaction by Ishii et al. for the hydrophosphonylation of
alkenes (Table 1, entries 10 and 11) with longer reaction time and
higher temperature. Using Ishii’s reaction conditions (Table 1, entry
Encouraged by these initial results, the scope of
hydrophosphonylation of ene and yneꢀbearing ILs was investigated
to create a library of novel phosphonylꢀfunctionalized ionic liquids
1
0), led to the low yield (58%).
In order to generate the required Pꢀcentered radical under
(Chart 1). The hydrophobic and weakly coordinating anion
oxidative reaction condition, Mn(OAc)2 catalyst was used. The
requisite Mn(III) was generated in situ from the reaction of Mn(II)
with the oxygen from the air and oxidized diethyl phosphite to the
corresponding phosphonyl radical, which then was efficiently added
across double bond. In terms of sustainability, O2 is the ideal
bis(trifluoromethanesulfonyl)imide [NTf ], which is highly stable
2
from both thermal and hydrolytic standpoints, is paired with the
cations. First, the radical additions of diethyl phosphite to a number
of aromatic (imidazolium) and nonꢀaromatic (ammonium and
phosphonium) ene-bearing ionic liquids containing C −C side
3
11
terminal oxidant because it is readily available and H O is the byꢀ
2
chains were investigated. Near quantitative yields (84−96%) along
with exclusive antiꢀMarkovnikov orientation were achieved under
the optimized reaction conditions outlined above (Chart 1, ILs 1−12
and 14−16). The key objective of this work is to develop bioꢀ
inspired materials that are highly lipophilic while being low melting
salts. This is the first time a highly efficient construction of
phosphonylꢀfunctionalized ionic liquids with long aliphatic
product of this catalytic oxidation reaction. Our preliminary
experiments demonstrated the combination of Mn(II) and air to be
potentially able to generate the chain process of Pꢀcentered radical
addition to [allylmim][NTf ] as an ene-bearing ionic liquid. The
2
Mn(II)ꢀcatalyzed click reaction is an idea synthesis platform to
systematically probe properties of ionic liquids due to its facile
synthesis process, benign reaction conditions, excellent molecular
structural units (≥C ), while maintaining them liquid at ambient
8
control and fidelity. This methodology that using Mn(OAc) as an
2
temperature, was achieved (Chart 1, ILs 6−12). Subsequently, the
scope of the study was expanded to ionic liquids with cations
bearing more than one ene group. These ene ILs were prepared
efficient catalyst in the presence of air was reported by Ishii et al. for
3
1
hydrophosphonylation of alkenes (Table 1, entries 10 and 11).
3
2
according to a general method reported by Welton et al. Reacting
Table 1. Optimization Studies.
these ILs with diethyl phosphite under the same conditions provided
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the desired polyadducts (Chart 1, ILs 13 and 17) quantitatively and
Interestingly, the solubility characteristic of the ionic products
1
isolated in very good yields (13: 85%; 17: 81%). Both the H along with the efficient nature of the Pudovic reaction creating no
13
and C NMR spectra of each 1–19 clearly illustrate complete byproducts allowed all of the ionic liquids to be purified simply by
hydrophosphonylation, which was readily ascertained on the basis of washing with hexanes and cold water. Notably, this means no
1
peak integration ( H NMR) and the disappearance of the olefinic chromatographic purification was required. Indeed, the
1
13
peaks ( H, C) of the starting cation. This was further corroborated residual/excess diethyl phosphite was readily removed by washing
by it’s the ESIꢀMS spectrum; however, in the ESIꢀMS of 17 the the reaction residue with hexanes, in which none of the starting or
expected parent ion of m/z = 730.34 is not observed. Instead, it product ionic liquids are soluble. Likewise, any residual catalyst was
produces a highꢀmass ion with m/z = 458.32, consistent with a easily removed by washing with H O, with which those ionic liquids
2
structure in which two of the four phosphonyl moieties have been are immiscible. Final products were isolated in >80% recovered
cleaved, leaving two intact while regenerating two of the original Nꢀ yields as viscous clear liquids. It is noteworthy to mention that the
tethered allyl moieties. To fully probe the efficiency of this process, purification process does not involve the use of halogenated solvents
the addition of the phosphonyl radical to imidazoliumꢀtype ionic nor chromatographic separation. Indeed, the total obtained yields
liquids containing Nꢀsubstituted triple bond was investigated were related to the length of the linkers. As observed, the yields of
(Scheme 1, II). Both the mono and bis yneꢀfunctionalized ionic the ILs with long linkers are higher in comparison to ionic liquids
liquids smoothly underwent monoꢀhydrophosphonylation, reaching containing shorter linkers and multiple phosphonate groups. This
high yields with complete antiꢀMarkovnikov selectivity (Chart 1, 18 observation can be rationalized by the fact that ionic liquids bearing
and 19).
shorter linkers and multiple phosphonate functionality are more
readily water soluble due to the Hꢀbond formation capability of
o
Chart 1. A library of phosphonylꢀfunctionalized ionic liquids including their glass transition temperatures ( C) and recovered yields (%). Only the
ꢀ
structures of cations are present and the anion, NTf
2
is not shown.
4
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phosphonate group, thus were partially lost during the subsequent et al. have proposed the former domain constitutes a symmetry-
aqueous washing step. breaking region, the impact of which is eventually outweighed by
The P=O stretching vibration in IR spectra of all ionic liquids that of the cumulative interꢀchain dispersion forces of
33
demonstrated characteristic absorption peaks ranging between 1194– progressively longer alkyl groups. Therefore, the low symmetry
ꢀ
1
1
197 cm and was not greatly affected by the length of the linkers and relatively large size of the cations combined with effective
31
and presence of a α,β−double bonds. On the other hand, the
P
charge distribution of the anion led to the destabilization of the
NMR spectra showed the P=O singlet chemical shift increased with packing efficiency of the crystal lattice of the ionic liquids.
increasing linker length ranging from 30.93 to 32.68 ppm in ILs Generally, elongation of the cation’s alkyl side chain leads to an
1
−12. For all of the synthesized TSILs, the P=O characteristic peaks increase in melting point. However, in this case the trend of
were partially split due to the formation of hydrogen bonds. increasing in Tg values with the number carbons on the side chain
According to Nockemann’s report, a Hꢀbond was formed between was not observed. For the imidazoliumꢀbased ionic liquids the
the P=O functionality as an acceptor with protons on the α carbon to hydrogen bonding between the cation and the anion is mainly
phosphorus of an adjacent cations. Despite considerable effort, an Xꢀ determined by the most acidic hydrogen at the cation in the
3
4
ray crystal structure was not obtained since the ILs could not be position C2. As such, the methylation at C2 position removes
isolated most likely because phosphonyl group diminishes the the hydrogen bond between the cation and anion, remarkably
crystallinity of IL.
affecting the melting point of ionic liquids, which is the most
35
Thermophysical properties. The thermal properties of these important feature of ionic liquids, as well as the viscosity. As
new ionic liquids were determined by differential scanning observed, higher melting points were obtained for the C2ꢀ
calorimetry (DSC), and only a glass transition temperature ( g
T
;
methylated ionic liquids relative to their C2ꢀprotonated
midpoints of phase transitions), no melting or crystallization counterparts. As proposed by Ludwig et al., formation of strong
points, was observed upon heating. The resulting glass transition HBs between the H2 position of the imidazolium cation and
temperature (Tg) data was in Chart 1. All of the synthesized anion introduces “defects” into the their Columbic network and
TSILs are viscous liquids at ambient temperature and exhibited destabilize the whole system, which leads to a lower melting
36
wellꢀdefined Tg at a very low temperature ranging from ꢀ33.6 to point and viscosity. However, another explanation, which was
o
2
2.6 C without melting (Chart 1). Indeed, the O=P(OEt) group assessed by theroretical and experimental studies, emphasizes on
2
37ꢀ39
is the main structural feature of the new ionic liquids that appears the determinant role of entropy.
In this light, the replacement
to have an important impact upon their physical state. The overall of the acidic C2 hydrogen at the imidazolium ring by a methyl
depressions in the glass transition points brought about by the group, decreases the entropy of the ionic liquids in the liquids
inclusion of phosphonate moiety are substantial regardless of phase. As a result, the loss of entropy is the main reason for their
whether the latter is Hꢀ or Meꢀbearing at the imidazolium C2 relative higher melting points and enhanced viscosities. In our
position. A comparison of the Tg values of phosphonylꢀbased study, this phenomenon is evident when comparing the Tg values
ionic liquids with Tm of corresponding allꢀcarbon reference ionic of ILs
liquids containing the same number of sideꢀchain carbons, shows substituting at C2 position can be observed. The C2ꢀmethylated
the dramatic impact of the presence of phosphonate functionality ILs and 10 are the structural isomers of their C2ꢀprotonated
upon the phase transition temperatures of the ionic liquids, which counterparts (ILs and 11) and the sole structural deference
demonstrably decreases the Tg values of new TSILs. For instance, between them is the presence of a methyl group at C2 position of
7 and 10 with ILs 8 and 11, where the effect of methyl
7
8
[
C C im][NTf ] comprising a saturated C side chain, has a Tm imidazolium ring.
ꢀ
Tg values for the C2ꢀmethylated ILs/C2ꢀ
12
1
2
12
o
9
o
o
value of 34.9 C reported by Mudring et al.
replacement with a phosphonyl unit (IL 11) yields Tg of ꢀ24.0 C respectively. IL 16
;
however, protonated ILs such as
7
/8
and 10
/11 are 3.5 C and 5.2 C,
o
,
as
a
functionalized alkyltriphenylꢀ
and converts a crystalline solid to an amorphous liquid. It is phosphonium salt, shows the greatest Tg in this series due to the
reasonable to assume that the introduction of phosphonyl large cation size. Interestingly, inclusion of the phosphonate
functional group lowered the tendency for crystallization. Indeed, group to allytriphenylphosphoniumꢀbased IL turn a crystalline
introduction of phosphonyl group caused the suppression of a solid salt to an amorphous ionic liquid, which is liquid at room
o
clear melting temperature, and only glass transitions were temperature ( g
T
= 22.6 C). According a recent Davis’s report,
observable (chart 1). Likewise, while reference [C C im][NTf ] arylphosphoniumꢀbased ionic liquids are generally more
1
0
1
2
o
3a
o
has a Tm of 18.9 C, its counterpart IL
8 has Tg of ꢀ23.0 C. Not thermally stable then their imidazolium and ammonium
surprisingly, the viscosities of ILs and 11 tangibly greater than counterparts under various conditions.
40
8
reference ILs, likely because of the more Hꢀbonding potential Molecular modeling. The molecular modeling was performed using
inherent in their cations. The melting point is controlled by three IL 1 as the model. The structure of 1 was optimized using the
41
main factors: intermolecular forces, molecular symmetry and the B3LYP exchange–correlation functional and the 6ꢀ311++G (d,p)
33
42
conformational degrees of the freedom of molecule. The likely basis set as implemented in the Gaussian09 suite of programs.
explanation for the observed complete disappearance of Stable structures were confirmed by computing analytic vibrational
crystallinity and very low Tg values is that the heightened frequencies. Figure 2 displays the most stable conformation for IL 1.
rotational freedom of the chain caused the reduced lattice energy Only frontꢀconformations were obtained, while no top conformation
of the ionic liquids, which high chain flexibility predominates was found due to the presence of the flexibility of the bulky NTf2
over the high polarity of phosphonyl groups. Besides, Rothenberg anion. The free energy associated with the formation of IL 1
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5

Green Chemistry
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ARTICLE
DOI: 10.1039/C C01933D
Jou4Grnal Name
amounts to ꢀ63.1 kcal/mol. In addition, the electron density carbonate. We hypothesized the calcium in the stationary phase
isosurface of IL 1 is shown in the electrostatic potential energy map would immobilize the ionic liquids by means of chelation between
in Figure 1. The ESP mapping illustrates the polarization of the the phosphonate functionality and calcium salts. A chromatographic
cation with the imidazolium ring as the blue region and phosphonyl column was prepared with 1.00 g of bone char powder. ILs were
group as the least positive charge region (red).
separately massed (4.1 mmol for each IL) and dissolved in
dichloromethane before running the ionic liquids through the
column. Once the ILs eluted the column, dichloromethane was used
to wash out nonꢀbonded ionic liquids. The solvent was evaporated
and the remained ionic liquids were massed to determine the amount
retained in the column (Figure 2). As shown in Figure 2,
interestingly, the increasing length of the linker between the cation
and phosphonate group appeared to increase the binding ability of
the ligand toward calcium salts. Nockemann’s report gives support
to these findings since increasing the length of the linker between the
cation core and phosphonate functional group appeared to
Figure 1. Right: the most stable conformation of IL 1. Left: the electrostatic profoundly enhance the binding ability of the ligand with transition
28
potential energy map of complex of IL 1. The ethyl group was modeled by metals. Additionally, the phosphonylꢀbearing ionic liquids with
methyl group.
longer linkers showed stronger adsorption on the metallic surface
and have better antiꢀwear ability rather than shorter chains ionic
Calcium affinity. One the objectives of the research are also to liquids, according to the Liu’s report. These TSILs displayed a high
o
investigate the calcium complexation process by using selected thermal stability up to ca. 300 C and their thermal stabilities
25
phosphonylꢀfunctionalized ILs. Normally, waterꢀsoluble calcium improved with longer linkers. Moreover, polyfunctionalized ionic
ions are separated from hydrocarbonaceous feedstock by using liquids 13, 17 and 19 as biꢀ or tetradentate ligands, demonstrated the
strong acids, which highly corrosive. The separation of oilꢀsoluble maximal bone affinities, which are related to their ability for
calcium ions (calcium naphthenate, calcium phenol, fatty acid simultaneously binding/chelating with more than one calcium site
calcium) require the usage of organic solvents and organic per compound.
decalcifying agents, which are usually toxic, flammable and volatile
organic compounds (VOCs). Their usage raises serious
environmental and safety concerns. Therefore, ionic liquids have
been proposed as suitable alternatives due to their unique physical
1
h,k
properties and environmental compatiblity.
However, the main
drawback associated with using classical ionic liquids for metal
extraction procedures is finding extractant molecules that remain
only in the IL phase as well as understanding the increased
complexity of the system upon the addition of solutes, prompting the
studies of the functionalized ionic liquids concept for metal
extraction using IL. For the first time, the Rogers and Davis groups
introduced the concept of TSILs designed for the extraction of
specific metal ions. Attaching a metal ion coordinating group
directly to the imidazolium cation makes the extractant an integral
part of the hydrophobic phase and greatly diminishes the chance for
loss to the aqueous phase. In other words, other than using ILs as
ecoꢀfriendly solvents for metal separation, the functional ILs
consisting extracting entities (functionalities), which are grafted onto
the structures of cations and combine unique physical and chemical
properties of ionic liquids. Functionalized ionic liquids were also
derived to include the task specific functionalities, such as metal
ligating groups that when used as part of the solvent dramatically
enhance the extraction of targeted metal ions. These TSILs behave
both as nonꢀvoletile solvents and extractants, overcoming the
potential issues encountered through solvent/extractant misibility
and facilitating the solvent/extractant recovery process.
MassꢀofꢀretainedꢀILꢀinꢀtheꢀcolumnꢀꢁꢄ ꢅ ꢆꢂꢀ
TotalꢀmassꢀofꢀILꢀꢁꢄꢂ
CalciumꢀaffinityꢀofꢀILꢀꢁ%ꢂ ꢃ
ꢇ 100
80
7
5.3
77.9 76.1
66.4
6
8.5 69.7
70
60
50
40
30
20
5
9.5 60.2 60.9
4
3
10
0
1
3
6
8
9
11
13
Ionic liquid
17
19
Figure 2. Calcium (II) affinity of selected phosphonylꢀbearing ionic liquids.
Organic chelating agents such as EDTA are used to bind metal
2+
2+
2+
3+
3+
ions ( Ca , Cu , Pb , Co and Fe ) in controling water hardness
and cheation therapy e.g. for treating mercury and lead poisoning.
Due to the cost the enviromnetal hazard, great efforts have been
made to replace EDTA with citric acid, inorganic polyphosphates
e.g. pentasodium tripolyphosphate (STPP), nitrilotriacetic
The affinity of the synthesized phosphonylꢀfunctionalized ionic
liquids for calcium ions was examined by utilizing them as
complexing agents to bond to the calcium ions present in bone char,
which consists mainly of tricalcium phosphate and calcium
4
4
acid (NTA) and organic polymers e.g. polyacrylic acid.
6
| J. Name., 2012, 00, 1-3
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Journal Name
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ART33ICLE
DOI: 10.1039/C4GC019 D
Polyhosphates and NTA were found to potentionally cause Acknowledgments
4
5
enviromental issues and be carcinogenic. We also conducted the
aboveꢀmentioned test to compare the calcium binding power of ILs
Authors are grateful to the Department of Chemistry and Physics of
FGCU for providing financial support. AM and ACS thank the
National Science Foundation for funding the mass spectrometer used
in this work (CHEꢀ1039944). In addition, AM would like to thank
Prof. JeanꢀLuc Montchamp at TCU and Dr. Greg Boyce at FGCU
for their helpful suggestions and Dr. Richard Sykora and his group at
University of South Alabama for the Xꢀray crystallographic studies.
1
1, 13 and 17 with some standard calcium agents (Table 2). As
shown in Figure 3, although citric acid has considerable higher
calcium (II) affinity (83.4%) than ILs, biꢀ and tetradentate ILs (13
and 17) showed the better binding affinity to calcium (II) ions in
comparison to EDTA (77.0%) and STPP (72.1%), making them
“greener” alternatives for calcium (II) traditional solvent/chelator
systems.
Notes and references
Department of Chemistry and Physics, Florida Gulf Coast University,
a
8
8
7
7
6
6
5
0
5
0
5
0
Fort Myers, Florida 33965, United States. Tel.: +1ꢀ239ꢀ590ꢀ1270; Email:
amirjafari@fgcu.edu
b
Department of Chemistry, University of South Alabama, Mobile,
Alabama 36688, United States.
§
Present address: Arthrex Inc., Naples, Florida 34108, United States.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
1
For
applications of taskꢀspecific ionic liquids, see: (
Chem. Lett., 2004, 33, 1072; ( ) P. Wasserscheid and T. Welton
Eds), Ionic Liquids in Synthesis, WileyꢀVCH Verlag GmbH,
a
comprehensive reviews of the history, properties and
IL 11
IL 13
IL 17
EDTA
STPP citric acid
a
) J. H. Davis Jr.,
b
Figure 3. Comparison of calcium (II) binding power of ILs (blue) and
(
common chelating agents (red).
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Conclusions
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011, 111, 3508; (
e
In summary, we have reported the synthesis of a series of novel bioꢀ
inspired ionic liquids containing phosphonyl functional groups as a
stable linkage for the connection of chemical and biological
components was reported. Simple reaction conditions were used to
provide good to quantitative yields. The construction of
functionalized ionic liquids using the Pudovic "click" reaction offers
considerable promise as a facile method to create structurally
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reaction coupled with synthetically powerful characteristics e.g.
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f
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,
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regioselectivity,
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l
) E. Sebastiao, C. Cook, A. Hu and M. Murugesu,
chromatographic purification/workup procedure and solventꢀfree
conditions. Mn(OAc)2 was employed as a nonꢀprecious metal
catalyst in these transformations. Accordingly, our research has
assured a progressive direction in the development of the new ionic
liquids containing phosphonate group and long aliphatic tail
structures, which are generally similar to the corresponding
phospholipids. Additionally, the preliminary results were described
in terms regarding their tunability toward calcium ligation abilities.
Increasing length of linkers between the cations and phosphonate
groups appeared to enhance the binding ability of ligands to calcium
ions. These novel TSILs have a great potential to use in the calcium
separation/extraction process (e.g. decalcification of crude oil) and
also development synthetic boneꢀseeking compounds with calcium
antiresorptive and mineralization potencies. Further studies
involving their thermal stability, density/viscosity and temperatureꢀ
dependent miscibility of IL/water with emphasize on the
demetallization of crude oil are currently in progress.
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J. Name., 2012, 00, 1-3 | 7

Green Chemistry
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ARTICLE
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DOI: 10.1039/C4GC019 D
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This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 9

Green Chemistry
Page 10 of 10
View Article Online
DOI: 10.1039/C4GC01933D
The key objective of this work is to develop bio-inspired ionic liquids that are highly lipophilic via “click” chemistry.
Graphical abstract:
"
Click" Chemistry
NTf -
NTf2-
O
2
P(OEt)2
N
N
N
N
N
N
n
n
R
O
R
Mn(OAc)2
+
(EtO) P H
2
1
05 oC, 8h
air (1 atm)
NTf2-
NTf -
O
2
N
N
P(OEt)2
n= 1-9
19 exapmels; Tg= -33.6 to 22.6
R= H, Me