Theoretical Studies On The Assembly And Stabilization Of Novel Nonstoichiometric Molecules

At the late of 20th century, several great science and technology areas (information technology, life science, nanotechnology) have the same tendencies, i.e., the miniaturization of components have developing fast. Such tendency urgently needs the manipulation of single atoms. The physics and chemistry of single molecules have got vast development. Currently, one promising concept that has emerged is the possibility of creating new materials using atomic or compound clusters as the building blocks. Additionally, the cluster-assembly and"bottom-up"growth from a small building unit to the nanoscale species, or even bulk compounds have become a focus in various fields. These useful ideas might open the capacious space and opportunity for the design of molecular materials with unique properties and functions. On the other hand, the nonstoichiometric chemistry has made vast progress in the past two decades, a series of nonstoichiometric clusters, featuring exotic electronic and structural properties, have been experimentally observed and theoretically characterized. These unique nonstoichiometric clusters might be used as building blocks to architecture and devise novel molecular materials.In this thesis, in order to design and assemble molecular materials featuring exotic nonstoichiometric clusters, on the basis of large number of experimental and theoretical works, we proposed the"hetero-decked sandwich"concept and considered the"fusion stability"that has never been reported previously. Based on the two concepts, we have systematically performed theoretical investigations by using the DFT methods on various cluster-assembled sandwich-type complexes with the incorporation of some representative nonstoichiometric clusters (planar tetra-/hyper-coordinate carbon, all-metal aromaticity and antiaromaticity, bare all-boron cluster, aluminum-nitrogen). Additionally, we make the novel interpretations and developments on the concept of sandwich-type compounds. We propose that the sandwich interactions are the interactions of the deck–core–deck in various forms. The decks can be organic, inorganic, all-metal, homocyclic and heterocyclic ligands, etc. The cores can not only be transition metals but also main group metals and nonmetal atoms, even clusters. The coordination number of the ligand in the sandwich-type compounds could beη1–8. The following is the main results:(1) The assembly and stabilization of planar tetracoordinate carbon (ptC) units (CAl₄^2-, CAl₄^-, CAl₃Si^-, CAl₃Si and CSi₂Ga₂) have been investigated. For M=Na and K, the CAl₄^2- unit can be assembled in both the traditional"homo-decked sandwich"[(CAl4)2M]q? (M=Li, Na, K, q=3; M=Be, Mg, Ca, q=2) and the novel"hetero-decked sandwich"schemes. For other situations, CAl₄^2- can only be assembled in our newly proposed"hetero-decked sandwich"scheme (e.g. [Cp(CAl4)M]q? (M=Li, Na, K, q=2; M=Be, Mg, Ca, q=1)) so as to avoid cluster fusion. For other ptC units, only in the"hetero-decked sandwich"scheme can these units be assembled and stabilized in the cluster-assembled compounds.(2) The assembly and stabilization of planar hypercoordinate carbon units (CB₆^2-) have been studied. The results show that the"homo-decked sandwich"structures are apt to transform to the low-lying fusion isomers, with large energies released. Many fusion isomers are energetically lower than that of the"homo-decked sandwich"complexes by at least 100 kcal/mol. Whether the counterions exist or not, the"homo-decked sandwich"compounds are not stable, thus, the"homo-decked sandwich"scheme is less likely for the assembly and stabilization of the hypercarbon unit CB₆^2-. Although the"hetero-decked sandwich"structures are not the ground states, the configurations between the"hetero-decked sandwich"compounds and fusion isomers are very large. The isomerization and transformation between the"hetero-decked sandwich"compounds and fusion isomers are very difficult. We can think that they are kinetically stable. Under certain conditions, the"hetero-decked sandwich"compounds might be observed. Thus, the"hetero-decked sandwich"scheme is more viable than"homo-decked sandwich"scheme.(3) The assembly and stabilization of all-metal aromatic units (Al₄^2-, Al₃^-, SiAl₃^-, Al62? and Ga3?) have been investigated. The calculated results suggest that the introducing doping atom (Si) leads to the complexity of assembly and stabilization process. The"homo-decked sandwich"structures are apt to transform to the low-lying fusion isomers, thus, the"homo-decked sandwich"complexes are not thermodynamically stable, while the"hetero-decked sandwich"compounds are the ground state structures. The"hetero-decked sandwich"scheme can effectively assemble and stabilize the all-metal aromatic units.(4) The assembly and stabilization of all-metal antiaromatic unit (Li3Al4?) have been studied. The results demonstrated that the"homo-decked sandwich"compounds are not thermodynamically stable and are apt to fuse together. Thus, the"homo-decked sandwich"scheme is not suitable for assembly and stabilization of all-metal antiaromatic unit (Li₃Al₄^-). The"hetero-decked sandwich"complexes are the ground state structures, featuring almost intact all-metal antiaromatic units.(5) The assembly and stabilization of bare all-boron units (B₃^– and B₆^2-) have been investigated. All the"homo-decked sandwich"compounds are not stable. For the B3– system, the"hetero-decked sandwich"complexes are the ground state structures. For the B₆^2- system, although the"hetero-decked sandwich"compounds are not the lowest-lying structures, they are kinetically stable.(6) The assembly and stabilization of aluminum-nitride units (Al4N?) have been studied. The"homo-decked sandwich"compounds are not stable, whereas the"hetero-decked sandwich"complexes are the global minima.