Structural anchoring of highly reactive CBH− for high-performance hypergolic coordination polymers with excellent thermal stability

Abstract

The energetic BH₃CN⁻ anions (named CBH⁻) have attracted significant interest in hypergolic materials due to their high energy density and strong reducibility. However, most CBH⁻-based hypergolic materials typically suffer from low stabilities. Structurally stabilizing the CBH⁻ anion to design materials with stable and excellent hypergolic performance continues to present significant challenges. To resolve these issues, we herein propose the first strategy to structurally anchor the high-activity CBH⁻ anion within the coordination polymer (CPs) platform to obtain three hypergolic and structurally similar CPs [M(CBH)₂(BIM)₂]_n (M = Cd 1, Mn 2, Zn 3; BIM = bis(1-imidazolyl) methane). Compounds 1–3 exhibit remarkable stability, outstanding high volumetric energy densities (E_v), and short ignition delay (ID) times. The E_v values of 1–3 are all greater than 36 kJ cm⁻³, which are significantly higher than that of commercial unsymmetrical dimethylhydrazine (UDMH) with E_v = 25.60 kJ cm⁻³. In particular, the Mn-based 2 demonstrates the highest thermal stability (T_dec = 317 °C) among all CBH-based hypergolic materials, attributed to the unique coordination polymerization method for CBH⁻ anions. Among the three compounds, the Cd-based 1 exhibits the shortest ID time (12 ms) when ignited with white fuming nitric acid (WFNA), which might be ascribed to the highest molecular polarity and smallest band gap of 1 by theoretical calculations. This study presents a precise structural design strategy for the rational design of highly active and stable hypergolic fuels for propellant applications.