Investigations On Core-shell Boron-based Nanoclusters And Hypercoordinate Planar Complexes

Carbon-based nanomaterials such as fullerenes,carbon nanotubes and graphene have attracted much attention in the last four decades.In the periodic table of elements,boron is a prototypical electron-deficient element.It exhibits unique geometric structures,chemical bonding and physicochemical properties.Boron-based nanomaterials complementary to carbon nanomaterials have received extensive attention in recent years.In the past two decades,the structures and properties of small anionic bare boron clusters B_n^-（n=3-42,48）has been extensively probed by joint experimental and theoretical investigations.It has been confirmed that the anionic boron clusters with atomic number n≤48 mainly possess four kinds of structural patterns:planar or quasi-planar,tubular,cage-like and bilayer.Extensive density-functional theory（DFT）investigations indicate that there exists a competition between core-shell and bilayer structures in medium-sized boron nanoclusters with n≥50 which remains to be experimentally confirmed.However,the thermodynamically most stable core-shell boron clusters and their bonding patterns still remain unknown.The experimentally known B-centered wheel-like B₉^-(（33）?（33）^-_（23）)and transition-metal-centered Ta?B₁₀^- and Nb?B₁₀^- possess the planar coordination numbers of CN=8 and 10,respectively.The highest coordination numbers identified to date in planar species are CN=13/12/11 in the metal-centered monocyclic wheel clusters La?C₁₃⁺/Y?B₆C₆⁺/Sc?B₅C₆.Using the recently experimentally observed polyynic cyclo[18]carbon and the theoretically predicted cyclo[14]carbon and cyclo[16]carbon as ligands,it is highly possible that planar nanoclusters with even higher coordination numbers can be realized in experiments.With the inspiration in mind,in this thesis we predict the thermodynamically most stable core-shell boron nanoclusters with a B₁₂ icosahedron at the center at first-principles theory level based on the structural motif of D_5h C₇₀.Detailed bonding analyses clearly show that icosahedral B12^2-core exhibits typical superatomic behaviors.The research provides a theoretical basis for predicting the larger size core-shell structures.The structure and properties of the core-shell boron clusters B₁₈₄ containing two B₁₂ icosahedrons at the center were investigated,and the icosahedral B12^2-cores were found to exhibit superatomic behaviors,respectively.Based on the similarities and differences of boron and carbon,we proposed the core-shell boron-carbon binary clusters containing one or two C_nB_12-n（n=0,1,2）cores at the center and explored their geometric structures and bonding patterns.The results of this study lay the foundation for the establishment of bottom-up structural growth model of boron-based nanoclusters.We have also explored the geometry,electronic structures,bonding patterns of alkali metal-or alkali-earth metal-doped carbon and boron-carbon binary wheel planar clusters with record coordination numbers of CN=14?18 in planar species.The main contents are summarized as follows:1.Mononuclear bare boron nanoclusters B₁₁₁,B₁₁₂,B₁₁₃,and B₁₁₄The stable boron allotropes are known to be predominately constructed by the interconnected icosahedral B₁₂ cages,while icosahedral-B₁₂ stuffing in the size range between B₉₈-B₁₀₂ proves to effectively improve the stability of fullerene-like boron nanoclusters.Based on the structural motif of D_5h C₇₀ and extensive first-principles theory calculations,we predict herein the high-symmetry C_5v B₁₁₁⁺ which satisfies the Wade’s n+1and n+2 skeletal electron counting rules exactly and the approximately electron sufficient C_s B₁₁₁,C_s B₁₁₂,C_s B₁₁₃,and C_s B₁₁₄ which are the most stable neutral core-shell borospherenes with a B₁₂ icosahedron at the center reported to date in the size range between B₆₈-B₁₃₀,with C_s B₁₁₂ being the thermodynamically most favorite species in the series.Detailed orbital and bonding analyses illustrate that these spherically aromatic species all contain a negatively charged icosahedral B₁₂^2-core at the center which exhibits typical superatomic behaviors in the electronic configuration of 1S²1P⁶1D¹⁰1F⁸,with its dangling valences saturated by twelve radial B-B 2c-2eσbonds between the B₁₂ inner core and the B₇₀ outer shell.The IR and Raman spectra of the concerned species are computationally simulated to facilitate their future characterizations.2.Mononuclear B-C binary C₅₀B₅₄ and binuclear nanoclusters C₈₈B₇₈ and B₁₈₄The Neutral carborane B₁₀C₂H₁₂ has important applications in the area of chemistry and materials.Extensive density functional theory calculations performed herein predict the mononuclear C_i C₅₀B₅₄(C₂B₁₀@C₄₈B₄₄),C₁ C₅₀B₅₄(CB₁₁@C₄₉B₄₃),and S₁₀ C₅₀B₅₄(B₁₂@C₅₀B₄₂)which contain one icosahedral-C_nB_12-n core（n=0,1,2）at the center following the Wade’s skeletal electron counting rules and the approximately electron sufficient binuclear C_s C₈₈B₇₈((C₂B₁₀)₂@C₈₄B₅₈),C_s C₈₈B₇₈((CB₁₁)₂@C₈₆B₅₆),C_s C₈₈B₇₈((B₁₂)₂@C₈₈B₅₄),C_s B₁₈₀((B₁₂)₂@B₁₅₆),C_s B₁₈₂((B₁₂)₂@B₁₅₈),and C_s B₁₈₄((B₁₂)₂@B₁₆₀)which encapsulate two interconnected C_nB_12-n icosahedrons inside.These novel core-shell boron-carbon binary clusters and bare boron nanoclusters appear to be the most stable species in thermodynamics in the corresponding cluster size ranges reported to date.Detailed bonding analyses indicate that the icosahedral B₁₂^2-,CB₁₁^-,and C₂B₁₀ cores in these core-shell structures possess the superatomic electronic configuration of1S²1P⁶1D¹⁰1F⁸,rendering spherical aromaticity and extra stability to the systems.Such superatomic icosahedral-C_nB_12-n stuffed B-C binary clusters and pure boron clusters with spherical aromaticity may serve as embryos to form bulk boron allotropes and their B-C binary counterparts in bottom-up approaches.3.Hypercoordinate planar complexes Cs?C₁₈⁺/Cs?C₁₇B and Na?C₁₄⁺/Na?C₁₃BSearching and discovering for the maximum coordination number（CN）in planar nanoclusters with novel bonding patterns has fascinated chemists for many years.Using the experimentally observed cyclocarbon D_9h C₁₈ and theoretically predicted cyclocarbon D_7hC₁₄ as effective ligands and based on extensive first-principles theory calculations,we predict herein their perfect planar alkaline-metal-doped complexes D_9h Cs?C₁₈⁺ and D_7hNa?C₁₄⁺ which,as the global minima of the systems with an alkaline metal atom located exactly at the center,possess the record coordination numbers of CN=18 and 14 in planar species,respectively.More interestingly,detailed energy decomposition and adaptive natural density partitioning bonding analyses indicate that the hypercoordinate alkaline-metal centers in theseσ+πdually aromatic complexes exhibit obvious transition metal behaviors,with effective in-plane（π-6s）σ,（π-7p）σ,and（π-5d）σcoordination bonds formed in Cs?C₁₈⁺ and（π-3s）σ,（π-3p）σ,and（π-3d）σcoordination interactions fabricated in Na?C₁₄⁺ to dominate the overall attractive interactions between the metal center and its cyclo[n]carbon ligand,respectively.Similar dually aromatic alkaline-metal-centered planar C_s Cs?C₁₇B,C_2v Cs?C₁₇^-,C_s K?C₁₂B₃,C_2v Na?C₁₃B,and C_2v Na?C₁₃^- have also been obtained with CN=18,17,15,14,and 13,respectively.D_8h Ba?C₁₆,C_8v Ra?C₁₆,D_8hK?C₁₆^-（11）and C_8v Rb?C₁₆^- with out-of-planeπor in-planeσaromaticity have been predicted based on theoretically predicted anti-aromatic D_8h C₁₆ as effective ligands,which possess high coordination numbers of CN=16.