Effect of thiocyanate‐based ionic liquids on the dehydrogenation of amine boranes: Experimental and molecular modeling studies

Abstract

AbstractThe present investigation reports the effect of thiocyanate‐based ionic liquid (IL) on the thermal dehydrogenation of amine boranes. Fifteen ILs based on thiocyanate anion comprising cations from imidazolium, pyrrolidinium, and ammonium family were screened by a priori prediction of infinite dilution activity coefficient using the quantum chemical‐based conductor‐like screening model segment activity coefficient method. The IL with the highest predicted solubility of amine boranes converged to 1‐ethyl‐3‐methylimidazolium thiocyanate ([EMIM][SCN]) that was then used for the dehydrogenation reaction. In the IL medium, ammonia borane (AB) and ethylene diamine bisborane (EDAB) were found to produce ~1.64 equivalent of hydrogen with an induction period of 60 min and ~2.37 equivalent of hydrogen having an induction period of 30 min, respectively. It was observed that the mixture of AB and EDAB in [EMIM][SCN] produced prolonged induction period of 210 min. After 240 min, the mixed amine boranes only produce 0.43 equivalent of H2. To elucidate this slow dynamics, Density Functional Theory (DFT) optimized geometries followed by the natural bond orbital analysis revealed the formation of hydrogen bond between IL and amine borane moiety. Further, the presence of the stronger dihydrogen bond among the protic and hydridic hydrogens was also observed in AB + EDAB/[EMIM][SCN] system, resulting in longer induction period for dehydrogenation.