Thermal Decomposition Of Amine Boranes And Synthesis And Reactivity Of Pincer Borane Complexes

Amine-boranes are an important class of compounds arising from the reactions between Lewis bases and Lewis acids.Amine-boranes have high hydrogen content and can be thermally decomposed to release a large amount of hydrogen,so they are considered to be potential hydrogen storage materials.Thermal decomposition of the amine boranes can produce a variety of products,such as polyborazylene,polyaminoboranes[HRBNR'H]_n,boron nitride BN,which are important polymers in the field of materials.Among them,polyaminoborane is a polymer with B-N main-chain units.According to the isoelectric characteristics of BN/CC,polyaminoborane is regarded as a BN analog of polyolefin.Due to the great application of polyolefins in the field of materials,it is expected that polyaminoboranes can be well developed in the field of materials.Therefore,thermal decomposition of amine boranes is an important research topic.Although thermal decomposition of amine boranes has been extensively studied,the mechanism is still unclear.The thermal decomposition of amine boranes is a complicated process,the products are a mixture of various polymers,which are difficult to be separated.These facts make it difficult for the study of the application of BN polymers.Here,we have studied in detail the thermal decomposition of ammonia borane?NH₃BH₃?and N-methyl substituted amine boranes Me_nNH_3-nBH₃?n=1 and 2?.We studied the thermal decomposition processes of amine boranes in a Teflon reactor which is located in a sealed stainless-steel autoclave and examined the effect of temperature on the thermal decomposition of the amine boranes.It is found that in the autoclave:?1?the pyrolysis products of ammonia borane changed with temperatures,and the main solid product at 80-100?is polyaminoborane.When the temperature is higher than 150?,polyborazylene and borazine are produced.?2?the pyrolysis of MeNH₂BH₃ is a two-step reaction in which one equiv.of hydrogen is released at the first step and[MeNHBH₂]_3^x?cyclic polymers?and[MeNHBH₂]_2^x?liner polymers?were obtained.Then another equiv.of hydrogen is released and the final product[MeN-BH]₃ is produced.?3?[Me₂N-BH₂]₂ is the main product in the pyrolysis reaction of Me₂NHBH₃ in the high-pressure reactor,and?Me₂N?₃B is produced as the final product after a long period of reflux of[Me₂N-BH₂]₂.The structure of a pincer ligand consists of a backbone and two'arms'which typically contain a P or N atom.They are tridentate ligands that can coordinate to a metal center in a meridional configuration.The rigid nature of the pincer framework imparts kinetic stability to the resulting coordination complex while simultaneously modulating the reactivity at the open sites on the metal center.In addition to the thermal decomposition,we also studied the reactions of boranes with pincer complexes.Bis?phosphinite??2,6-?^tBu₂PO?₂C₆H₄,POCOP?and Bis[2-?diisopropylphosphino?ethyl]amine?NH?CH₂CH₂P?ⁱPr?₂?₂,PNP?were selected as the main ligands.Both ligands could well react with borane to produce stable borane adducts.The POCOP can react with two equiv.of THF·BH₃ to form[2,6-?^tBu₂PBH₃O?₂C₆H₄],and a crystal structure is obtained.The PNP can react with three equiv.of THF·BH₃ to form a pincer complex H₃BNH?CH₂CH₂PBH₃?ⁱPr?₂?₂ in which both P and N atom coordinated to BH₃.The compound is capable of reacting with amines to remove BH₃ coordinated to N to form NH?CH₂CH₂PBH₃?ⁱPr?₂?₂.We also found that the PNP pincer ligands are more prone to coordination with metals rather than borane.