Theoretical Study On The Design And Ligand Stabilization Of Planar Hypercoordinate Molecules

Planar hypercoordinate molecules,also known as anti-van’t Hoff and Le Bel molecules,are a class of molecular systems with anti-conventional bonding modes.Their peculiar geometrical and electronic structures have attracted extensive research interest from many researchers.Up to now,a large number of planar hypercoordinate molecules with high thermodynamic stability have been predicted,synthesized and characterized.At present,the planar hypercoordinate molecules are mainly concentrated in the tetra-coordinate system,and the stabilization of the higher coordinated molecular system is still very challenging.At the same time,due to the unique bonding mode of planar hypercoordinate molecules and the high activity of ligands,it is very worthy of attention to discuss the coordinate stabilization of planar hypercoordinate molecules theoretically,which provides important support for the acquisition of condensed matter systems of planar hypercoordinate molecules.In this paper,through structure search and high-level energy confirmation,several rare planar hexacoordinate molecules Be C₆M₃^-（M=Al,Ga）and Mg C₆M₃^-（M=Ga,In,Tl）have been obtained,and effective coordination stabilization mechanisms have been discussed.It provides important theoretical clues for the enrichment of planar hypercoordinate for molecular system and its ligand stabilization.The specific research results are as follows:1)Compared with the rich planar tetracoordinate system,the theoretical design of planar molecules with higher coordinate is very challenging.We used the efficient CALYPSO structure search program to systematically search the potential energy surface of Be C₆M₃^-（M=Al,Ga）and Mg C₆M₃^-（M=Ga,In,Tl）clusters.The ground state structures of Be C₆M₃^-（M=Al,Ga）and Mg C₆M₃^-（M=Ga,In,Tl）are found to be planar hexacoordinate beryllium（ph Be）and magnesium（ph Mg）in spin singlet states.Molecular dynamics simulations show that Be C₆M₃^-and Mg C₆M₃^-have excellent kinetic stability,which is attributed to the（6σ+6π）double aromaticity of three delocalizedσbonds and three delocalizedπbonds.At the same time,accurate energy decomposition analysis reveals Be C₆Ga₃^-bonding way for spin doublet Be⁺atoms and spin doublet C₆Ga₃^2-fragments formed an electron-sharedπbond,two Be⁺（d）←[C₆Ga₃^2-]πbond,two Be⁺（p）←[C₆Ga₃^2-]σbonds and one Be⁺（s）←[C₆Ga₃^2-]σbond.Furthermore,the stabilization mechanism of the planar molecule by a B（C₆F₅）₃ligand with strongσelectron acceptance capability was investigated.2)Planar hypercoordinate ligands generally have high activity,which inhibits the large-scale expansion of planar hypercoordinate molecules and their synthesis in the laboratory.We use the theoretically obtained ground state planar pentacoordinate carbon molecule CBe₅Au₅⁺to investigate the mechanism ofσelectron donor for its ligand protection and stabilization.Large ligand complexes are constructed by bridging CBe₅Au₅⁺external Au atoms with strongσelectron donors（CO,N₂,Xe,N-heterocyclic carbene,Cycloparaphenylenes）,etc.High level calculations show thatσelectron donors can maintain the geometry and electronic structure of planar pentacoordinate carbon molecules and have good thermodynamic stability.Electronic structure analysis shows that pp C CBe₅Au₅⁺andσelectron donors form a strong covalent bond,which greatly protects the active center of planar pentacoordinate ligand and is an important driving force for the stability of the complex.At the same time,the larger size of the NHC ligand complex can also form the minimum structure of the potential energy surface,and perfectly maintain the planar pentacoordinate carbon structure,which is conducive to the synthesis and acquisition in the laboratory.