Quantum Chemical Studies On The Structures,Bonding,and Aromaticity In Boron-based Alloy Clusters

Elemental boron clusters show structural uniqueness and diversity,which include planar or quasi-planar structures,double-ring tubes,borophenes,and borospherenes(all-boron fullerenes).These types of boron clusters have been systematically established and confirmed in an array of theoretical and experimental works during the past 30 years.In boron-based binary or alloy clusters,the doping of a metal element not only helps compensate for the intrinsic electron-deficiency of boron,but also gives rise to new types of cluster structures and new physical chemistry.The main-group metal elements have low electronegativities and are therefore good electron donors.When combined with boron in boron-based alloy clusters,the former elements will make a perfect match.In this thesis work,we use quantum chemical calculations to study the structural,electronic,and bonding properties of a series of binary Be₂B₅^q(q=+1,0,-1)and Be₂B₄^q(q=+1,0,-1)clusters.The computational data offer new types of boron cluster structures and enriches boron chemistry.The main contents and conclusions are as follows:1.On the structures,chemical bonding,and aromaticity of boron-based Be₂B₅^+/0/-alloy clusters.We carried out a quantum chemical study on a series of alloy clusters,Be₂B₅^q(q=+1,0,-1),at the density functional theory(DFT)and coupled-cluster CCSD(T)levels.The cationic cluster assumes an inverse sandwich geometry,being composed of a pentagonal B₅ ring and two capping Be atoms.It can be formally described as a charge-transfer[Be]²⁺[B₅]^3-[Be]²⁺complex,with a minor covalent component between the B₅ ring and two Be atoms.The[B₅]^3-unit is shown to be chemically robust with double 6?/2? aromaticity,suggesting the potential to be developed as a new inorganic ligand.Its 6? framework mimics that of cyclopentadienyl anion(C₅H₅^-),although apsextet is not possible in bare boron clusters of similar sizes.The neutral inverse sandwich is similar to the cation,except for a distortion to lower symmetry.Anionic cluster has a molecular wheel structure,governed by double 2?/6?aromaticity.Thus,sequential reduction in the series leads to a structural transformation from inverse sandwich to molecular wheel,further demonstrating the robustness of the[B₅]^3-unit with exactly 18 electrons.Although the cationic inverse sandwich and anionic molecular wheel(with 20electrons)differ by two electrons,they are the same in terms of total number of delocalized?/?electrons:6?/2?versus 2?/6?.The bicapping Be atoms in an inverse sandwich interact with boron ring in peculiar ways,thus effectively stabilizing the delocalizedpframework with respect to the?one.Double?/?aromaticity appears to be a crucial bonding concept in boron-based alloy clusters.2.Boron-based Be₂B₄^+/0/-clusters:geometric structures,bonding,and aromaticity.We have computationally explored the potential energy surfaces of the Be₂B₄^+/0/-series of alloy clusters,using the Coalescence Kick(CK)algorithm.Low-lying candidate isomers are subsequently reoptimized at the DFT level and all local minima verified by vibrational frequency analyses.The global-minimum structures in three charge states are established,which turn out to be similar to each other,irrespective of their charge states.They are composed of a B₄ ring and two bridging Be atoms on the periphery,which assume a planar hypercoordinate configuration.It is found that the first three isomers for neutral cluster are energetically competitive.Therefore,we mainly focus on these three isomeric structures.Basing on chemical bonding analysis,it is shown that the first isomer(that is,the global-minimum structure)is a system with 2?/6?double aromaticity.The second isomer features 4?/2?conflicting aromaticity,whereas the third isomer has 2?/2?double aromaticity.Thus,a geometric configuration can greatly alter the total number of delocalizedp/selectrons,as well as their specific electron counting.In the second isomer,two Be atoms participate in the delocalized?bonds in a special way,similar to the inverse sandwich Be₂B₅^+/0/-clusters.We also attempt to shed light on the issue why the neutral cluster no longer prefers an inverse sandwich geometry.The global-minimum structure with planar hypercoordinate configuration appears to be dictated by its magic 2?/6?double aromaticity,whose total number of delocalized electrons seems to conform to the octet rule.