Theoretical Study On Boron-Containing Species Towards Adaptive Aromaticity And Dinitrogen Activation

Aromaticity,as one of the most fundamental and important concepts in chemistry,is a key stabilizing factor.Generally,aromatics follow Hückel’s 4n+2 rule in the ground state and Baird’s 4n rule in the excited state,respectively.Therefore,traditional cyclic species could be aromatic in one state(the ground state or the excited state).Species with aromaticity both in the singlet and triplet states(termed as adaptive aromaticity)has not been reported until our group demonstrated the first example(16-electron osmapentalene)with aromaticity in both the lowest singlet(S0)and the lowest triplet(T1)states in 2018.To date,adaptive aromaticity has been successfully used in the singlet fission material,indicating great potential in the photoelectric field.However,the reported adaptive aromatic compounds are particularly limited.On the other hand,dinitrogen activation is a great challenge and the corresponding strategy is mainly restricted to transition metal system.Due to the drawbacks of transition metal complexes including scarcity,toxicity and low environment tolerance,it is urgent to develop an alternative metal-free method for dinitrogen activation.Boron is one of important main-group elements due to the unique electronic structure.It tends to aggregate and form multicenter bonds due to the electron deficiency and then exhibits distinct delocalization.On the other hand,boron has been reported to possess the metallomimetic reactivity,due to the cooperative capability of donating and accepting electrons.For this reason,boron-containing species have been a powerful tool for the activation of small molecules.Specifically,Braunschweig group reported that borylenes can react with dinitrogen,suggesting the significance of boron in the application of metal-free dinitrogen activation.This thesis focuses on the study of aromaticity of tetraatomic boron species in the S0 and T1 state,and the dinitrogen activation by boron-containing species by density functional theory(DFT)calculations.It is found that tetraatomic boron species possessσ aromaticity both in the S0 and T1 states(termed σ adaptive aromaticity)supported by various aromatic indices,extending the concept of adaptive aromaticity into the σsystem for the first time.Furthermore,the origin of adaptive σ aromaticity in tetraatomic boron species could be attributed to the ππ*excitation,leading to negligible contribution to σ system.Thus the σ aromaticity in the T1 state could be kept.For the dinitrogen activation,the studied boron-containing species are divided into three classes,including borenium(borinium)cations,carborane-based frustrated lewis pairs and the borane.As a result,DFT calculations demonstrate that all of them are able to activate dinitrogen in a thermodynamically and kinetically favorable manner,indicating dinitrogen activation by these boron-containing species could occur under a mild condition.Our findings provide an alternative approach for a metal-free dinitrogen activation,highlighting the importance of FLP chemistry and aromaticity towards dinitrogen activation.