M. Komabayashi, T. Stiller and S. Jopp
Journal of Molecular Liquids 325 (2021) 115167
pyridinium group, we furthermore used N-methylimidazole, N-
methylpyrrolidine and N-methylpiperidine to achieve the products
Table 1
Thermal analysis overview of all ionic products.
1
2–14 (Scheme 5). These quarternisations were performed under the
Product Short name
Tg (°C) Tm (°C) Td (°C) Notes
same conditions as with pyridine and also lead to the products
smoothly. While 12 was achieved in a slightly higher yield than the
pyridinium product 8b, the yields dropped for the non-aromatic prod-
ucts 13 and 14, maybe due to the higher sterical hindrance of these
quarternary ammonium groups.
5
5
5c
a
b
β-Me-OMe-Pyr-OMs
β-Me-OEt-Pyr-OMs
β-Me-OAll-Pyr-OMs
18
5
20
135
33
–
244
244
249
Solid
Solid, hygroscopic
Liquid,
hygroscopic
Solid, hygroscopic
Solid
5
8
8
d
a
b
β-Me-OBn-Pyr-OMs
β-Me-IsoPr-Pyr-OTs
β-Me-IsoPr-Pyr-OMs
16
–
–
58
140
–
241
223
242
Liquid,
hygroscopic
Solid, hygroscopic
Solid, hygroscopic
Solid, hygroscopic
Liquid
8
8
8
8
c
d
e
f
β-Me-IsoPr-Pyr-Cl
β-Me-IsoPr-Pyr-Br
β-Me-IsoPr-Pyr-I
β-Me-IsoPr-Pyr-NTf
β-Me-IsoPr-Pyr-OTf
–
–
16
−25
−15
167
148
109
–
231
226
233
306
261
3
. Thermal analysis
The next major part of our investigations was the thermal analysis
2
via thermogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) of all our new ionic products. This will not only allow
us to categorize them as ionic liquids if applicable, but should further-
more give us some insights on the structure-property relationships as
planned. An extensive overview of the data can be found in Table 1.
Looking at the melting points and the physical state of the products
first, 11 of the overall 18 ribose based salts are ionic liquids per defini-
tion, as their melting point is below 100 °C. Out of these 11 ionic liquids,
8g
–
Liquid,
hygroscopic
Solid, hygroscopic
Liquid,
hygroscopic
Liquid,
hygroscopic
Solid, hygroscopic
Liquid,
hygroscopic
Solid
8
9
h
c
β-Me-IsoPr-Pyr-BF
β-Et-IsoPr-Pyr-OMs
4
25
–
114
–
233
228
10c
β-All-IsoPr-Pyr-OMs
β-Bn-IsoPr-Pyr-OMs
–
–
245
1
1
1c
2
21
54
25
233
263
β-Me-IsoPr-MeIm-OMs −15
8
are liquids at room temperature, meaning they are liquid at ≤25 °C). It
13
β-Me-IsoPr-MePyr-OMs
β-Me-IsoPr-MePip-OMs
–
–
102
–
276
269
is however noteworthy that many of the products are hygroscopic and
will quickly appear liquid at room temperature as well, if not kept
under an inert atmosphere after freeze drying. A few examples of DSC
measurements, showing the typical endothermic peaks for melting,
exothermic peaks for recrystallization as well as phase transitions if ap-
plicable can be seen in Fig. 1.
14
Liquid
the effects of bulkiness and size of the ions on the melting points follow
the established trend.
Lastly, 8b and 12–14 are all directly comparable as methyl
The strategic choice of our synthesis now allows us to directly com-
pare the structural elements of the products with their thermal proper-
ties. At first, the products 5a-d and 8b are all methyl ribofuranoside
mesylates with altered groups in positions 2 and 3 (Fig. 2A). Here we
observe a melting point trend of Methyl > Benzyl > Ethyl > Allyl/
Isopropylidene, with the allyl and isopropylidene products both
appearing as RTILs. In general, ionic liquids are more likely to be formed
with bulky ions, as the bulkiness of the ions may hinder interactions be-
tween cation and anion. This idea is followed in this trend, as allyl and
isopropylidene are bigger groups than ethyl, which is again bigger
than methyl. Benzyl however, even though it acts as a bulky group, al-
lows further interactions through pi stacking [10], thus showing a
higher melting point at 58 °C. The same trend, although less clearly vis-
ible, is supported by the products 9-11c (Fig. 2B). These products are all
2
,3-isopropylidene ribofuranoside mesylates with altered cationic N-
heterocycles (Fig. 2D). Here the trend is Methyl-Pyrrolidine > Methyl-
Imidazole > Methyl-Piperidine/ Pyridine, with the latter two being
RTILs. Here we can observe higher melting points of five-membered
rings, both saturated and aromatic, than the six-membered rings.
We furthermore measured and compared the onset decomposition
temperature of all of the 18 products. This temperature is often used
as an indicator of the thermal stability of ionic liquids. Lately, the cor-
rectness of this assessment has been questioned, since the measured
temperatures may vary strongly in different conditions [11]. However,
all of our samples have been measured under argon, with a heating
rate of 10 K/min, thus allowing us a direct comparison. Interestingly,
most of the ribose based ionic liquids produced during this work show
similar decomposition temperatures in a range from 223 to 249 °C.
This leads us to believe that the different structural elements such as
protecting groups used in the ribose based cation as well as the different
anions play close to no role towards the overall thermal stability of the
product. Thus, the major influence on the thermal stability could be
something not altered between the products, like the C-O bond of the
anomeric carbon or the C-N bond between the carbohydrate and the
N-heterocycle.
2
,3-isopropylidene ribofuranoside mesylates with differing groups at
the anomeric carbon. Here, the ethyl and allyl products are both RTILS,
while the benzyl product has a melting point of 54 °C.
Next we can directly compare the influence of different anions, since
the products 8a-h all bear the same methyl 2,3-isopropylidene
ribofuranoside cation (Fig. 2C). The melting point trend is as follows:
Chloride > Bromide > Tosylate > Tetrafluoroborate > Iodide > Mesy-
late/ Triflate/ Bistriflylimide, with the latter three all appearing as
RTILs. Comparing the halogens, the melting point is lower with bigger
size of the anion. Even though tosylate is a bulkier anion than the
other sulfonates mesylate and triflate, it also allows more interactions
through pi stacking, thus leading to a higher melting point. In general,
However, there are a few examples showing higher decomposition
temperatures. The products 12–14 bearing different N-heterocycles
have decomposition temperatures ranging from 263 to 276 °C. Thus
we suspect the C-N bond to be stronger in these products than in the
Scheme 5. Overview of the ribose based mesylates 12–14 with varying cationic groups.
4