The Study On Synthesis,structure And Properties Of N-substituted B,N-butane Analogues

N-Substituted B,N-butane analogues have attracted much attention as a key intermediate product of aminoborane hydrogen storage materials.The cyclodiborazane and the aminoborane compounds are their dehydrogenation products and can be used as precursors for the synthesis of boron nitride at low pressure.Therefore,the N-Substituted B,N-butane analogues are also potential precursors for the synthesis of boron nitride.The preparation of a few N-substituted B,N-butane analogues which has been reported needs to be carried out under low temperature conditions.The reaction time is long and the purification process is complicated,which limits the research of their properties and applications.Furthermore,only the mechanism of thermal decomposition and metal catalyst rearrangement were roughly studied.Therefore,it is important to systematically study the synthesis and properties of N-substituted B,N-butane analogues,which is of great significance for the rich B,N-butane analogues and the potential application.In this paper,a series of N-substituted B,N-butane analogues were synthesized by nucleophilic reaction of aminodiborane with 19 different kinds of amines.The products were characterized by NMR,IR,single crystal X-ray diffraction and XRD.The stability and thermal decomposition of N-substituted B,N-butane analogues were studied.The results show that: 1)The reaction of aminodiborane with amine can be carried out at room temperature,and the reaction is rapid.Whether the aminodiborane reacts with the amine may depend on the size and number of substituents on the nitrogen atom of amine.2)The B in BH₂ of N-substituted B,N-butane analogues is downfield compared to NH₃BH₂NH₂BH₃,while the chemical shift of B in BH₃ has no significant change.It is found by DFT calculations that the change of the natural charge on the terminal and the intermediate boron atom is consistent with the change trend of the actual chemical shift.The H in B1 H of N-substituted B,N-butane analogues is highfield compared to NH₃BH₂NH₂BH₃,the H in N1 H of N-substituted B,N-butane analogues is downfield compared to NH₃BH₂NH₂BH₃.Probably due to the substitution of hydrogen on the terminal nitrogen atom.3)The structures of both anti and gauche conformations of N-substituted B,N-butane analogues were observed and both inter-and intramolecular dihydrogen bonds were formed.Most of the N-substituted B,N-butane analogues have similar boron-nitrogen bond lengths at terminals,but are larger than the intermediate boron-nitrogen bond.4)The stability of the N-substituted boron-butane analogues decreases with the number of amine substituent,primary amine compounds are the most stable.In addition,the melting point of the N-substituted B,N-butane analogues is higher than that of the corresponding alkane due to the effect of the dihydrogen bonds.Based on the analysis of the thermal decomposition products of N-substituted B,N-butane analogues in different solvents,it is presumed that the decomposition mechanism of N-substituted B,N-butane analogues in THF and CH₂Cl2 is different,but both form an aminodiborane as key intermediate,it is further converted to form H₃N·BH₃,R3N·BH₃,[NHBH]3,[NH₂BH₂]x in THF,while in CH₂Cl2,R3N·BH₃,[NHBH]3,[NH₂BH₂]_x were produced.