Dual Functions of Water in Stabilizing Metal-Pentazolate Hydrates [M(N₅)₂(H₂O)₄]·4H₂O (M = Mn, Fe, Co, and Zn) High-Energy-Density Materials

Abstract

High-energy-density materials (HEDMs) containing the cyclopentazole anion (cyclo-N 5 ) are highly desirable due to the release of more energy and being environmentally more friendly than conventional HEDMs. However, the synthesis of stable cyclo-N 5 -containing HEDMs has been a challenge. In this study, quantum mechanical calculations were employed to elucidate the stability of [M(N 5 ) 2 (H 2 O) 4 ]•4H 2 O (M = Mn, Fe, Co, and Zn), one of the few recently reported cyclo-N 5 -contained HEDMs, under ambient conditions. The results from our study indicate that the stability is due to the presence of two types of water (coordinated H 2 O (c-H 2 O) and hydrogenbonded H 2 O (h-H 2 O)). Each type uses a unique mode to stabilize the highly reactive M(N 5 ) 2 cores. c-H 2 O binds with M to reduce the M ↔ cyclo-N 5 interaction, leading to a less activated cyclo-N 5 and higher kinetic barriers (E a s) for its decomposition. In contrast, h-H 2 O takes advantage of its permanent electrostatic interactions with cyclo-N 5 to inhibit the decomposition. The stabilizing effects of the two types of water on M(N 5 ) 2 are similar. On the basis of the lower energy cost to remove h-H 2 O from the materials and the subsequent large decrease in the E a due to this removal, we propose that h-H 2 O acts as a "safety device" that prevents the materials from becoming kinetically unstable. For future design of cyclo-N 5 -contained HEDMs, we proposed the use of various molecular building blocks, such as NH 3 , H 2 S, and PH 3 , which can tightly bind to M to reduce the M ↔ cyclo-N 5 interaction and impose permanent electrostatic interactions with cyclo-N 5 to provide the similar dual functions of H 2 O to suppress cyclo-N 5 decomposition.

Solvent effects on the geometry, electronic structure, and bonding style of Zn(N5)2: A theoretical study

“…Luo's study showed that water can improve the stability of Zn(N 5 ) 2, because the dissociation energy of [Zn(N 5 ) 2 (H 2 O) 4 ]·4H 2 O is higher than that of Zn(N 5 ) 2 . This study further reveals the stabilization mechanism of water, that is, the charge transfer and strong interactions between Zn(N 5 ) 2 and water make Zn(N 5 ) 2 more stable.…”

“…In these compounds, [M(H 2 O) 4 (N 5 ) 2 ]·4H 2 O (M = Mn, Fe, Co, and Zn) are particularly interesting. They are all metal‐pentazolate hydrates, and Luo and coworkers have recently studied the role of water in these compounds …”

Iron Pentazolate Complex: Isoelectronic Species of Ferrocene but with a Planer Structure

“…Almost all the pentazolate complexes are stabilized through hydrogen bonds by connecting with water or ammonia based on the experiments, such as Fe(H 2 O) 4 (N 5 ) 2 [4,21] . It should be noted that cyclo ‐N 5 − is the isoelectronic species of the high stable pentadienide cyclo ‐C 5 H 5 − , both containing a 5‐center‐6‐electron delocalized π bond.…”

“…(N 5 ) 6 (H 3 O) 3 (NH 4 ) 4 Cl is the first successfully synthesized complex containing cyclo ‐N 5 − with the decomposition temperature of 116.8 °C, which is a landmark achievement in this field [3,15,16] . Then a series of metal pentazolate complexes have been synthesized continuously, [17] such as [Na(H 2 O)(N 5 )] ⋅ 2H 2 O, M(H 2 O) 4 (N 5 ) 2 (M=Mn, Fe, Co and Zn) and [Mg(H 2 O) 6 (N 5 ) 2 ] ⋅ 4H 2 O [4,18–21] . Gong et al reported that the kinetic barriers of decomposition of M(N 5 ) 2 (M=Mg, Ca, Sr and Ba) are ranged from 23.4 to 25.5 kcal/mol, less than that of individual cyclo ‐N 5 − (27 kcal/mol) [8,22–23] .…”

“…However, Fe(η 5 ‐C 5 H 5 ) 2 is well‐known for its stability caused by the strong d‐π interactions between ligands and metal. However, based on the experimental structure of Fe(H 2 O) 4 (N 5 ) 2 , [4,21] where the planer [Fe(N 5 ) 2 ] is the isoelectronic species of Fe(η 5 ‐C 5 H 5 ) 2 . But cyclo ‐N 5 − as the mono‐dentate ligands coordinates with Fe 2+ to form two σ(N−Fe) bonds (simplified as Fe(η 1 ‐σ‐N 5 ) 2 ), rather than the formation of the other planer isomer with η 2 ‐σ‐bond or ‘sandwich‐like’ isomer with η 5 ‐π‐bond.…”