Theoretical Investigation On Planar Pentacoordinate Boron And Carbon Compounds Stabilized By Sulfur-bridge Bonding

Since 1970 Hoffmann et al.theoretically designed a planar tetracoordinate carbon molecule（ptC）with lower energy than its tetracoordinate geometry for the first time,planar chemistry has attracted widespread attention.The corresponding study can help to better understand chemical bonds,and these units can be possibly used to build new two-dimensional materials,so it is significant to design novel stable planar structures.On the basis of planar tetracoordinate carbon,emerging molecules containing planar pentacoordinate carbon（ppC）and planar hexacoordinate carbon（phC）have also generated great interest,and gradually expanded to the planar multi-coordinated structure of other elements.Boron is a typical electron-deficient element.In the past few decades,the scientific community has also conducted many studies on planar multi-coordinated structure.In this paper,sulfur is used as the external bridging element to study the stable plane tetracoordinate and pentacoordinate central structure boron,and in-depth exploration.The density functional theory was used to study the structure and stability of the sulfur-bridged stable pentacoordinate boron molecule B₆S₅ⁿ（n=0,+1,+2）system,and the 18-electron rule of the design plane tetracoordinate carbon molecule was verified.Feasibility.Based on this,the structure and stability of the sulfur-bridged pentacoordinate carbon molecule XY₅S₅ⁿ（X=C,B;Y=B,Al;n=0,+1,+2）were further verified and explored the effect of different charges on the system structure.In this paper,my results show that sulfur atoms can stabilize plane pentacoordinate clusters as a new peripheral framework element.The main contents of the two studies are as follows:The main research contents are summarized as follows:1.At the level of B3LYP/def2-TZVP optimization and CCSD single point energy,B₆S₅ⁿ（n=0,+1,+2）has been utilized to validate the valuable effect of peripheral sulfur atom on constructing planar coordinated boron.The structures of global energy minimum（GEM）and the second lowest energy minimum in three charge states are purely planar ones,and their structures are very similar.Only their order is exchanged with the loss of the electrons. There exists a planar pentacoordinate boron in the GEM structure of D5h-B₆S₅ due to the 18-valent electronic rule,and there are all the planar tetracoordinate borons in the GEM structure of B₆S₅ⁿ（n=0,+1,+2）. Meanwhile,the sulfur-bridging bonding is another key factor to maintain their planar molecular structures.2.The sulfur bridged stable pentacoordinate XY5S5（X=C,B;Y=B,Al）system were further studied,and the system stability and aromaticity were analyzed and discussed in detail.The 18-valent electron CB₅S₅⁺,BAl₅S₅,and CAl₅S₅⁺ were designed to obtain three clusters with a quasi-planar pentacoordinate structure,while the 18-valent electron CAl₅S₅²⁺and BAl₅S₅⁺that have lost an electron have a planar pentacoordinate structure with D_5hh symmetry.By analyzing the bond length and bond level effects of XY₅S₅（X=C,B;Y=B, Al）clusters,the 17-valent electronic rule also has its unique validity on designing planar pentacoordinate clusters.When one electron is lost to form CAl₅S₅²⁺and BAl₅S₅⁺,there are still 1e electrons occupying the delocalized, and the double Hückel aromatic rule is still valid.When the electron is lost again,the delocalized 6c-2eπbond will completely lose the electron,which does not satisfy the（4n+2）double Hückel aromaticity rule,so the planar pentacoordinate structure will no longer exist.It is also the key role for stabilizing the entire planar skeleton by 18-or 17-valent electronic.This provides a more theoretical basis for finding the planar multi-coordinated structures of C and B centers.