Theoretical Studies On Novel Borospherenes And Structural Transitions In Boron Nanoclusters

The observation of all-boron fullerene D2 d B₄₀-/0,dubbed borospherene,in 2014 by professors Si-Dian Li and Hua-Jin Zhai at Shanxi University,Lai-Sheng Wang at Brown University,and Jun Li at Tsinghua University has once again aroused widespread interests in boron nanoclusters.Both experimental and theoretical investigations on Bn+ cationic boron clusters in the size range between n = 3-27 have systematically revealed the structural evolution from quasi-planar to tubular.However,to date,the structural transition from tubular to cage-like structures in medium-sized cationic boron clusters Bn+?n = 28-39?remains unknown,and the structures of many bigger boron clusters?n > 42?are not clear yet.Recently,increasing attentions are paid to the structural features,properties,and potential applications of metal-doped borospherenes.However,the question whether B₄₀ can be used as an inorganic ligand to form complexes with transition metals remains to be further explored.In this thesis,we systematically investigated the structures and properties of medium-sized cationic boron clusters B₂₉⁺,B31+ and neutral boron cluster B32 and found that B₂₉⁺,B31+ and B32 all possessed sea-shell-like cage structures,which are obviously different from their counterparts in anionic and neutral states,unveiling an interesting charge-induced structural transition from an anion boron cluster to a cationic or neutral boron cluster.Furthermore,we studied the structures and properties of a series of complexes?C₆H₆Cr?n&B₄₀?n = 1-6?,in which,B₄₀ and benzene?C₆H₆?serves as a multi-dentate inorganic ligand and an organic ligand,respectively,and Cr as the metal coordination center.The research points out possible directions for the study of other transitionmetal-borospherene complexes,and the results can give instructions to the synthesis of new complexes,where borospherene can form with transition metals.We also predicted the existence of bilayer B₅₄,B₆₀,and B62 in the last part of the thesis,and the cage-like to bilayer structural transition in medium-sized boron nanoclusters was founded.The bilayer structures can be extended to a stable two-dimensional bilayer material.The main contents and results are as follows: 1.Structure and bonding of sea-shell-like B₂₉⁺C₂B₂₈ is the smallest neutral borospherene reported to date,while seashell-like Cs B29-as a minor isomer competes with its quasi-planar counterparts in B29-cluster beams.Extensive global minimum searches and first-principles theory calculations performed in this work indicate that,with two electrons detached from B29-,the B₂₉⁺ monocation favors a seashell-like Cs B₂₉⁺ obviously different from Cs B29-in geometry which is overwhelmingly the global minimum of the system with three B7 heptagonal holes in the front,on the back,and at the bottom,respectively,unveiling an interesting chargeinduced structural transition from Cs B29-to Cs B₂₉⁺.Detailed bonding analyses show that,with one less ? bond than B29-,Cs B₂₉⁺ also possesses nine delocalized ?-bonds over its ?-skeleton on the cage surface with a ? + ? double delocalization bonding pattern and follows the 2?n + 1?2 electron counting rule for 3D aromaticity?n = 2?.B₂₉⁺ is therefore the smallest borospherene monocation reported to date which is ?-isovalent with the smallest borospherene neutral C₂B₂₈.2.Geometrical and electronic structures of axially chiral sea-shell-like borospherenes: B31+ and B32Since the discovery of the cage-like borospherenes D2 d B₄₀-/0 and the first axially chiral borospherenes C3/C2 B39-,a series of fullerene-like boron clusters in different charge states have been reported in theory.Based on extensive global minimum searches and first-principles theory calculations,we present herein two new axially chiral members C2 B31+ and C2 B32 to the borospherene family.C2 B31+ features two equivalent heptagons on the top and one octagon at the bottom on the cage surface,while C2 B32 possesses two equivalent heptagons on top and two equivalent heptaogns at the bottom.Detailed bonding analyses show that both sea-shell-like B31+ and B32 follow the universal ? + ? double delocalization bonding pattern of the borospherene family,with ten delocalized ? bonds over a ? skeleton,rendering spherical aromaticity to the systems.Extensive molecular dynamics simulations show that these novel borospherenes are kinetically stable below 1000 K.3.B₄₀ as a multi-dentate inorganic ligand in?C₆H₆Cr?n&B₄₀?n = 1-6?complexesThe newly discovered cage-like borospherene D2 d B₄₀ with two ?6-B6 hexagons and four ?7-B7 heptagons on the surface may serve as an effective multi-dentate ligand to coordinate transition metals.Based upon extensive density functional theory calculations,we present herein the possibility of the exohedral complex series?C₆H₆Cr?n&B₄₀?n = 1-6?with n Cr centers sandwiched between the central ?6/7-B₄₀ unit and n planar ?6-C₆H₆ ligands.C₆H₆ Cr fragments in?C₆H₆Cr?n&B₄₀ occupy the ?6-B6 hexagonal coordination site atop the B₄₀ cage first,the four neighboring ?7-B7 heptagons on the waist next,and finally the ?6-B6 hexagon at the bottom,forming a multinuclear complex series effectively stabilized by n C₆H₆ Cr fragments.The two ?6-B6 hexagons and four ?7-B7 heptagons on the spherical surface of B₄₀ can be practically viewed as six independent coordination sites to coordinate Cr centers with almost the same coordination energies.Detailed bonding analyses indicate that the eclipsed C2 v C₆H₆Cr & B₄₀ possesses a coordination bonding pattern similar to that of dibenzenechromium,with the B₄₀ ligand inheriting the ? + ? double delocalization bonding pattern of the parent borospherene.4.Prediction of bilayer B₅₄,B₆₀,and B62 nanoclustersInspired by the newly reported D2 h B₄₈ and based on extensive global searches and first-principles calculations,we present herein the possibility of bilayer boron nanoclusters C2 B₅₄,C2 h B₆₀,and C1 B62 which all contain a B38 bilayer hexagonal prism at the center,with one,two,and three B6 hexagonal windows at the corners,respectively.These bilayer structures form effective B–B ? interactions between the inward buckled upper and lower monolayers to stabilize the systems.Detailed orbital and bonding analyses indicate that these aromatic nanoclusters follow the same bonding pattern of ? + ? double delocalization.They can be extended to construct the highly stable twodimensional B34 bilayer allotrope.