Computational Studies On Small Metal-Doped Boron Clusters:Structural And Electronic Properties,Chemical Bonding,and Aromaticity

Bulk boron is known for three-dimensional（3D）polymorphic structures with cage-like building blocks,whereas boron nanoclusters have proved to prefer unique planar or quasiplanar（2D）structures over the past 20 years,via systematic spectroscopic experiments and quantum chemical calculations.Planar boron clusters are observed up to 40 atoms for the anions,which are unprecedented in cluster science,being commented as “an important landmark in chemistry”.Planar boron clusters are governed by the intrinsic electrondeficiency of boron,which leads to an array of new bonding concepts: aromaticity,multifold aromaticity,antiaromaticity,and conflicting aromaticity.Doping boron clusters with metal elements offers additional degrees-of-freedom to the boron flatland,which can alter or tune the structures,electronic properties,and chemical bonding of elemental boron clusters,as well as uncover the bonding essence of planar boron such as electron counting rules.Intuitively,metal doping is anticipated to be able to compensate for boron’s electron deficiency,and yet this simple idea has to be tested or confirmed through detailed theoretical and experimental studies.Lastly,new types of cluster structures and new forms of chemical bonding may be discovered in metal doped boron clusters.This thesis deals with a number of boron-rich boron-aluminum and boron-gold cluster systems via quantum chemical calculations,focusing on their structural and electronic properties,chemical bonding,and aromaticity/antiaromaticity.The main contents and conclusions are summarized as follows:1.A quantum chemical study on the structures,chemical bonding,and p/s aromaticity in boron-aluminum binary clusters: B₇Al_n^0/-（n = 2,3）.Binary B-Al clusters are anticipated to inherit the electron-deficiency of boron and all-metal aromaticity of aluminum,thus leading to unusual structural,electronic,and bonding properties.Here we report on an extensive computational study of a series of binary B-Al clusters: B₇ Aln and B₇Aln-（n = 2,3）.Their global-minimum structures are identified via computer searches and electronic structure calculations at the B3 LYP and single-point CCSD（T）levels.The B₇Al2,B₇Al₂^-,and B₇Al3 clusters feature a heptagonal B₇ ring with a central Al2 unit orienting perpendicular to the ring.In contrast,B₇Al3-cluster contains a hexacoordinate B₇ disk with one capping and two in-plane bridging Al atoms.Chemical bonding analyses conclude that all B₇Al_n^0/-（n = 2,3）clusters are doubly π/σ aromatic with 6π and 6σ electrons,which are the decisive factor that governs the stability of these species.The isovalent Al/B substitution in binary B-Al clusters turns out not to be completely isovalent,because the Al centers tend to preserve a number of localized electrons and/or lone-pairs.This explains why B₇Al_n^0/-（n = 2,3）clusters assume different structures than B₈Al-/B9Al-and B9-/B10-.One electron alters the potential landscapes of B₇Al3 and B₇Al3-clusters,due to distinct electron affinities of two-types of neutral structures.2.On the structures and bonding in binary B₈Al₂^0/-clusters: further examples of double π/σ aromaticity with magic 6π and 6σ electron counting.Computer structural searches and density functional calculations at the PBE0 level show that the global minimum structures of B₈Al₂^0/-clusters feature B₈ molecular wheel with a symmetric Al2 unit and B₈ molecular wheel with one capping and one bridged Al atom,respectively.The coordination of Al atom to a planar boron is thus sensitively affected by the charge state of a binary cluster.Chemical bonding analysis shows that clusters B₈Al₂^0/-are all doubly aromatic with 6π and 6σ electrons,offering further example that the magic 6π/6σ electron counting are essential for the boron-aluminum binary clusters.In the B₈Al₂^0/-clusters,isovalent Al/B substitution is not exactly true,either.Aluminum atoms retain certain number of localized electrons,making the B₈Al₂^0/-clusters different from their B100/-counterparts in terms of the number of bonding electrons.We conceive that cluster B₈Al2 can be dynamically fluctional,owing to its unique geometric structure.3.Terminal and bridging Au ligands in the boron-gold alloy system: The case of B₇Au3 cluster.We use the Coalescence Kick program to search the global minimum structure of B₇Au3 cluster.The low-lying isomers are calculated at the B3 LYP and singlepoint CCSD（T）levels to determine their energetics.The global minimum of B₇Au3 cluster contains a trapezoidal B₇ unit,that is,double-boron chains,with three Au atoms being coordinated terminally or in a bridging fashion.Since Au is similar to H as a monovalent s radical,the terminal and bridging Au ligands in B₇Au3 cluster mimic those of H.Chemical bonding analysis indicates that B₇Au3 cluster has σ aromaticity and π antiaromaticity,thus leading to a system with conflicting π/σ aromaticity,which underlies its elongated B₇ double-chain shape.Cluster B₇Au3 offers another example for Au/H isolobal analogy in binary cluster systems.