Communication
Chemistry—A European Journal
doi.org/10.1002/chem.202001658
contribute to thermal instability and decomposition behavior
of cubane and other cage systems. Together, iodinated and
carboxylic acid derivatives raise concerns, as many of these are
common synthetic intermediates within the cubane, BCP, and
BCO classes.
Table 1. Summary of hammer test results and exotherms observed in
scDSC (endotherms not listed).
[a,b]
In addition to cubane examples, the BCP framework is also
known to be thermally sensitive. Namely 1,3-diethynyl[1.1.1]bi-
cyclopentane (11), of which a sample exploded during at-
[
20]
tempted sublimation. Subsequently, others have successfully
sublimed 11 at 508C with no incident, however cautious han-
[
21]
dling of this BCP was strongly recommend.
In light of the emerging and increasing popularity of these
cage motifs, it is timely to review and investigate key function-
al groups used in synthesis that potentially instill sensitivity. Es-
pecially, in terms of increasing practitioner awareness and en-
hancing general safety (e.g., handling and scale up).
Reported herein are the results of impact sensitiveness test-
ing, as well as thermal sensitiveness and behavior of selectively
substituted cubane, BCP and BCO systems involving carboxyl-,
hydroxymethyl- and iodo-substituents (Table 1), as evaluated
through hammer test and sealed cell differential scanning calo-
rimetry (scDSC). These results build a safety perspective, unob-
tainable through commonly deployed in silico methods,
around key functional groups such that informed precautions
can be taken.
Impact Sensitiveness: Hammer tests were performed on all
system derivatives listed in Table 1. This test is commonly used
in high energy density materials research, and provides infor-
mation about any precautions that should be taken with stor-
[
22]
age, handling, and reaction scale up. The two possible out-
comes of a hammer test are 1) a go event, which is defined as
any one of: an audible report, flame or visible light, smoke, or
definite evidence of discoloration of the sample due to decom-
position (e.g., charring); or 2) a no-go event, which is the ab-
sence of any of the above. Sample size for this type of assess-
ment is generally in the range of 5 mg.
[a] The synthesis of the carboxyl-, hydroxymethyl- and iodo-substituted
cubane, BCP and BCO systems followed standard literature procedures,
which afforded known and novel cubanes (1a–j), BCPs (2a–j) and BCOs
(
3d–g) (See Supporting Information); [b] Each entry represents one exo-
Most cage derivatives investigated were not sensitive to
impact and none of the BCO compounds displayed go events.
thermic event in the heatflow curve, compounds with two entries dis-
played two exotherms; [c] Liquids at room temperature were not impact
tested; [d] Exotherm was ongoing at 3008C so QDSC and thermal potential
could not be determined.
1,4-Diiodocubane (1d) was by far the most impact sensitive
material observed in this study, requiring only a light tap to re-
liably and repeatedly give a go event, which consisted of a
loud crack, smoke, and charring. 4-Iodocubane-1-carboxylic
acid (1 f) also displayed a go event, consisting of charring and
an audible pop. Overall, both iodinated cubanes 1d and 1 f
that results can vary from batch to batch. Diacid 1c, for exam-
ple, was quantitatively determined to have an impact sensitive-
[13]
ness similar to a secondary explosive, but did not always dis-
play a go event in our hands, that is, depending on the batch,
impurities, and the experimenter.
[23]
were markedly easier to initiate than diacid 1c (Table 1).
Of the BCP derivatives it was again the diiodo 2d which dis-
played the most obvious go event, consisting of a popping
noise and visible charring. This is in keeping with the high
impact sensitivity of 1d and suggests that iodinated cages
may be particularly susceptible to impact initiation. Acid-meth-
anol 2j also gave a go event, albeit only consisting of a color
change (slight charring) (Table 1).
Thermal testing: Sealed cell differential scanning calorime-
try was employed to investigate the thermal sensitiveness and
behavior of all derivatives, using the method described by
[24]
Sperry et al. Surprisingly, the interpretation and reporting of
DSC data is currently not standardized, although the technique
is gaining popularity due to its utility as an early indication of
[25]
The occurrence of a go event for BCP 2j, but not for the
cubane equivalent 1j, was slightly surprising, given the higher
energy of the cubyl system and the tendency for cubanes to
be more sensitive to impact. It should be noted, however, that
many factors affect impact sensitivity (e.g., particle size), and
thermal sensitivity. Herein, the onset temperature (TDSC) re-
ferred to is the left limit onset temperature rather than the ex-
trapolated onset temperature, as this is a more useful parame-
ter in thermal hazard assessments (See Supporting Informa-
tion).
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Chem. Eur. J. 2020, 26, 1 – 6
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Dallaston, Madeleine A.
