Theoretical Design Of Species With The Planar Hexa-coordinate Boron And 3D Extensible Sandwich Compounds

According to the principle that structures determine properties,the species with new type of chemical bonding will generally exhibit the novel properties that are different from that of species with classical bonding.Nevertheless,the experimental exploration of the species with such novel bonding are often restricted by experimental technology.In contrast,the rapid development of modern quantum chemistry theory and improvement in computer speed make it possible to predict and characterize the novel structure using quantum chemical method.With the aim to predict the molecules with novel bonding,We carried out two kind of works in this thesis:1.Based on the density functional theory?density functional theory,DFT?method and the high level ab initio calculations,we designed a series molecules containing a planar hexa-coordinated boron?phB?and the boron is absent for the first time in the ligands of this phB;2,Taking "star-benzene"polymer which previously reported by our reserch group as the two-dimensional ligand,we designed the 3D extensible transition metal sandwich structures,which are connected thoroughly by chemical bonds using DFT methods.The details are as follows:1.Planar Hexa-coordinate Boron Species:Be6HnBq?n = 3,4,5,6,q=n-5?As a typical electron deficient element,boron prefers the multicenter bonding and thus the planar hexa-coordination is common for boron in two-dimensional boron monolayer,pure boron or boron rich clusters.These phBs usually take other boron as the key ligand atoms.Correspondingly,phB without boron as the key ligand has not been reported,and the radius of boron is relatively small,which will make it rather crowded when there are six in-plane ligand atoms.Actually,except for boron itself,it is rather difficult to find the appropriate ligand atoms.Our research group recently reported a pentagonal Be₅H₅ star with a planar penta-coordinate carbon?ppC?in the center.In this molecule,with the aid of H atom,Be-Be distance is shortened to 1.93A,which is shorter than the sum of B-Be covalent radius?1.95 A?,This inspired us that similar Be-H star like structure can be used to design planar hexa-coordinated boron.Our computation results show that the phB can be stabilized in the center of the B-H hexagonal star,forming an energy minimum,D₆h Be₆H₆B⁺?1a?.By removing H atoms and adding the same number of negative charges,a series of phB species can be further designed,including C₂v Be₆H₅B?2a?,D₂h Be₆H₄B?3a?and D₃h Be₆H-3B^2-?4a?.The extensive exploration of potential energy surface revealed that la,2a and 3a are the global minima,and 4a is a high-energy local minimum;The kinetic stability of la-4a were studied by the ab-initio Born-Oppenheimer molecular dynamic simulation?BOMD?at 298k and 500k.The results revealed that the basic structures of the four phB molecules were well-maintained during the BOMD simulations and their kinetic stability was proved to be very excellent.The thermodynamic and kinetic analysis showed it is highly possible to realize la-3a in the experiment,and 4a may also be experimentally viable as a kinetically stable molecule.The AdNDP analysis results show that each of the Be atom provides a electron to form bond to center interacting with B atom in la-4a molecules.Considering the molecular charges and the needs of electrons for the Be-Be bonds without H-bridge,the stable eight electrons shell structures are formed for all boron atoms in la-4a.Obeying the octet rule is crucial for the stabilization of whole molecule.At the same time,the AdNDP analysis and the NICS calculations suggested that these phB species possessed?and?double aromaticity,which also contributes to the stability of molecular.2.3D extensible sandwich compoundsSo far,the transition metal sandwich structure reported in the literature include one dimensional extensible sandwich chain and two-dimensional extensible sandwich sheet.There also has an example of 3D extensible sandwich structure,where the sandwich chains were connected by H-bonds network.However,if the chemical bonds are required for the linkages among the sandwich chains,no such structure is known to date.Our calculations on the multilayer sandwich compounds with the grapheme-like C₄₂H₁₈ as the ligands suggested that the?electrons will be thoroughly localized when the sandwich structures were formed on both sides of molecules,which makes the whole ligand lose basis of electronic structure to maintain the planar geometry.Accordingly,we proposed that the locally delocalized?electron is desired for 2D planar ligand of 3D extensible sandwich structure and we found that the "star benzene"?D₆h C6?BeH?6?polymers reported previously by our group possess such locally delocalized?electrons and the maintenance of it's plannarity is independence of the delocalization of?electron in higher degree.Then,we tried to design the mixed ligand one-dimensional extensible sandwich chains,two-dimensional extensible sandwich sheets and three-dimensional extensible sandwich structure based on "star benzene" polymers.The AdNDP analysis showed that after the formation of the sandwich structure,C6 in graphene like ligands,that participate in the sandwich bonding,have no direct relations in?bonding,this makes the molecule lost the basis for maintaining a planar structure.As a contrast,the C-Be interactions in "star benzene" polymers shows obvious ionic characters,which deter the?electrons on one C6 ring from interacting with that on the other C6 rings.Our calculation results also indicated that the assembly of 3D-ESCs based on "star benzene" polymers in any directioon is largely exthermic,which will facilitate the future experimental synthesis.The HOMO-LUMO gaps showed different variation trends when 3D sandwich structure is extented in different directions,which makes them potentially useful in design of new electronic devices.