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Materials

Switchable compound shows promise for safer explosives

Transition-metal complex’s sensitivity to impact changes in response to spin state

by Tien Nguyen, special to C&EN
February 18, 2020 | A version of this story appeared in Volume 98, Issue 8

Side by side photos of iron complex in purple low spin state and white high spin state.
Credit: J. Am. Chem. Soc.
The iron spin-crossover material can reversibly switch between its low-spin state (purple) with low impact sensitivity and its high-spin state (white) with high impact sensitivity.

Materials used inside bomb detonators must strike a balance between two extreme states: one maintaining stability and one triggering spectacular self-destruction. Today’s detonator materials shift between these states in irreversible fashion, which means the bomb they serve is permanently armed and could explode accidentally. Now, chemists report a material that has a fully reversible switching mechanism. If future iterations with a more defined on-off switch can be produced, they may one day be used to build safer detonation devices (J. Am Chem. Soc. 2020, DOI: 10.1021/jacs.9b13835).

“Our goal was to make something that was very safe and difficult to initiate in one state but then being able to push it, on-demand, to something that would be easier to initiate only when you want it to,” says study coauthor Thuy-Ai D. Nguyen of Los Alamos National Laboratory.

Ball-and-stick structures of monomers for a spin-crossover material [Fe(Htrz)<sub>3</sub>]<sup>2+</sup> in the low-spin and high-spin states.
Credit: J. Am. Chem. Soc.
These structures show the low-spin state (left) and the high-spin state (right) of the monomer, [Fe(Htrz)3]2+, for a material with switchable explosive properties.

The approach relies on spin-crossover (SCO) complexes, which have attracted interest for use in sensors and displays. These complexes typically have a transition-metal center, coordinating ligands, and a counterion. In response to changes in temperature, pressure, light, or magnetic field, these complexes transition from a low-spin to high-spin electronic state as the number of unpaired electrons on the transition-metal center increases. This transition changes the complex’s properties including volume, bond length, and enthalpy. The authors proposed that with the right kind of SCO complex, one with explosion-prone ligands and counterions, the transition might change the material’s sensitivity to impact, allowing it to act as a detonator in one of its spin states but not in the other. This property had not been explored, so the authors decided to see if they could exploit this feature for explosive applications.

The team synthesized an explosive SCO material called [Fe(Htrz)3]n[ClO4]2n in which iron is nestled between energetic nitrogen-rich rings common to explosives and an oxidizing counterion of perchlorate, a typical component of fireworks. This complex changes from the low- to the high-spin state in response to increased temperature. So to test impact sensitivity of the spin states, they dropped weights on the material inside an anvil cell at various temperatures. They found that the Fe-based complex indeed exploded under less force in the high-spin state at 60 °C—with an impact sensitivity of 5 J—compared to the low-spin state, at 25 °C, which had an impact sensitivity of 7.5 J. In contrast, a Ni-based analog, which cannot undergo spin crossover, showed no change in impact sensitivity in this temperature range.

The moderate temperature for switching of 60 °C means the complex can reversibly switch between states without breaking down.

The team says they’ll continue to explore this new class of SCO materials to find a candidate that has a larger spin state difference in its sensitivity to stimuli, thus making it safer to handle and better suited for practical use. Future studies will also evaluate light to trigger the spin crossover, which could be safer yet because it’s easier to control than temperature is, says study coauthor Jacqueline M. Veauthier, also of Los Alamos.

“This work presents an interesting and unconventional application of spin-state switching materials,” says Michael Shatruk, a materials expert at Florida State University. “Given the availability of a variety of spin-crossover materials with nitrogen-containing ligands, I expect that this approach will be rapidly exploited to optimize the properties of such explosives.”

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