Investigations On Boron And Transition-Metal-Doped Boron Nanoclusters

Since the discovery of fullerene C₆₀ in 1985,fullerene,graphene and carbon nanotubes have been the focus of material science research widely applied in various applications.As a typical electron-deficient element,boron is the light neighbor of carbon in the periodic table,it exhibits both clear similarities and obvious defferences with carbon.The electron-deficiency determines the unique geometric structures and chemical bonding properties of boron clusters.In the past two decades,chemists have gradually revealed the structural characteristics of boron nanoclusters within a certain size range B_n^-?n = 3-28,35,36,39,40? through a large number of experiments and theoretical studies,but a few B_n^-/0 clusters?n=29,31,32,33,34,37,and 38? still remain experimentally unresolved.In addition,the reported work shows that the transition metal doping can effectively change the geometry of boron clusters.In this thesis,we have systematically studied the B₂₉^-/0 via the PES and theoretical calculation.It found that entropy effect affected the relative stability of clusters,and planar and cage structures compete with each other in different temperature.Based on the borospherene,it is found that D_2d B₄₀⁺ is the global minima of B₄₀⁺,and the structural rheology conforms to the "W-X-M" mechanism.Transition metal Ni is doped on the hexagons or heptagons of borospherene D_2d B₄₀ and found that Ni could effectively form coordination bonds with heptagons.We established the structural relationship between heteroborospherene and heteroborophene.Theoretical prediction of endohedral metal molecular rotor and endohedral metal borospherene Ta B_n^q?q =-1⁺3,n = 21-28? complexes,found that the rotary speed of molecular rotor C_s Ta@B₂₁ and C_3v Ta@B₂₂⁺ are different with temperature changes.We proposed the current chemical highest coordination number of Ta-B complexes.At the same time,we predict the metal boron nanotubues grown with Ta@B₉ as the structural unit.The main contents and results are as follows:I.The experimental and theoretical investigation on boron clusters1.Competition between quasi-planar and cage-like structures in the B₂₉^- cluster Here we report a study on the structures and bonding of the B₂₉^- and B₂₉ clusters using photoelectron spectroscopy and first-principles calculations and demonstrate the continued competition between the 2D and borospherene structures.The PES spectrum of B₂₉^- displays a complex pattern with evidence of low-lying isomers.Global-minimum searches and extensive theoretical calculations revealed a complicated potential energy surface for B₂₉^- with five low-lying isomers,among which the lowest three were shown to contribute to the experimental spectrum.A 3D seashell-like C_s B₂₉^- isomer,featuring two heptagons on the waist and one octagon at the bottom,is the global minimum for B₂₉^-,followed by a 2D C₁ B₂₉^- isomer with a hexagonal hole and a stingray-shaped 2D C_s B₂₉^- isomer with a pentagonal hole.However,by taking into account of entropic effects,the stingray-shaped C_s B₂₉^- was shown to be the lowest in energy at room temperature and was found to dominate the PES spectrum.Chemical bonding analyses showed that stingray-shaped C_s B₂₉^- is an all-boron analogue of benzo [ghi] fluoranthene(C₁₈H₁₀),whereas the seashell-like C_s B₂₉^-possesses 18-? electrons,conforming to the 2?n + 1?2?n = 2?electron counting rule for spherical aromaticity.2.Cage-Like B₄₀⁺: A Perfect Borospherene MonocationRecent discovery of the perfect cage-like D_2d B₄₀-and D_2d B₄₀?borospherene? has led to the emergence of a borospherene family.However,the geometrical and electronic structures of their cationic counterpart B₄₀⁺ previously detected in gas phase still remain unknown to date.Based on extensive first-principles theory calculations,we present herein the possibility of a perfect cage-like D_2d B₄₀⁺ for the monocation which turns out to be the global minimum of the system similar to B₄₀^- and B₄₀,adding a new member to the borospherene family.Molecular dynamics simulations indicate that D_2d B₄₀⁺ is dynamically stable at 300 K,whereas it starts to fluctuate at 500 K between the two lowest-lying isomers D₂ d B₄₀⁺?W?and C_s B₄₀⁺?M?in concerted W-X-M mechanisms via the transition state of C₁ B₄₀⁺?X?,with the forward?W?X?M?and backward?M?X?W?activation energies?Ea?of 14.6 and 6.9 kcal/mol,respectively.The IR,Raman,and UV-vis spectra of D₂ d B₄₀⁺ are computationally simulated to facilitate the future charaterizations of this important borospherene monocation.II.Theoretical investigation of transiton-metal-doped boron clusters1.Heteroborospherene clusters and heteroborophene monolayers with planar heptacoordinate transition-metal centers in ?⁷-B₇ heptagonsWith inspirations from recent discoveries of the cage-like borospherene B₄₀ and perfectly planar Co ? B₁₈^- and based on extensive global minimum searches and first-principles theory calculations,we present herein the possibility of the novel planar Ni?B₁₈,cage-like heteroborospherenes Ni_n?B₄₀?n = 1-4?,and planar heteroborophenes Ni₂?B₁₄ which all contain planar or quasi-planar heptacoordinate transition-metal?phTM?centers in ?⁷-B₇ heptagons.The nearly degenerate Ni₂?B₁₄ monolayers which may coexist in experiments are predicted to be metallic in nature,with Ni₂?B₁₄ composed of interwoven boron double chains with two ph Ni centers per unit cell being the precursor of cage-like Ni_n?B₄₀?n = 1-4?clusters.Detailed bonding analyses indicate that Ni_n?B₄₀?n = 1-4?and Ni₂?B₁₄ possess the universal bonding pattern of ? + ? double delocalization on the boron frameworks,with each ph Ni forming three lone pairs in radial direction(3d_z2,3d_zx,and 3d_yz)and two nearly in-plane 8c-2e ?-coordination bonds between the remaining tangential Ni 3d orbitals(3d_x2-y2 and 3d_xy)and the ?⁷-B₇ heptagon around it.The structural relationship between heteroborospherene and heteroborophene is established for the first time.2.Tubular-to-cage-like structural transition in metal-centered boron clusters at Ta@B₂₂^-Inspired by recent discovery of the metal-centered tubular molecular rotor C_s B₂-Ta@B₁₈^- with the record coordination number of CN = 20 and based upon extensive first-principles theory calculations,we present herein the possibility of the largest tubular molecular rotors C_s B₃-Ta@B₁₈ and C_3v B₄-Ta@B₁₈⁺ and smallest axially chiral endohedral metalloborospherenes D₂ Ta@B₂₂^-,unveiling a tubular-to-cage-like structural transition in metal-centered boron clusters via effective spherical coordination interactions at Ta@B₂₂^- with CN = 22.The highly stable Ta@B₂₂^- as an elegant superatom possesses the 18-electron configuration with a bonding pattern of ? + ? double delocalization and follows the 2?n + 1?² electron counting rule for spherical aromaticity?n = 2?.Its calculated adiabatic detachment energy of ADE = 3.88 eV represents the electron affinity of the cage-like neutral D₂ Ta@B₂₂ which can be viewed as a superhalogen.3.Cage-like Ta@B_n^q complexes in 18-electron configurations with the highest coordination number of twenty-eightExperiments show that the highest coordination numbers of CN = 10 in planar wheel-type complexes in D_10h Ta@B₁₀^- and CN = 20 in double-ring tubular species in D_10d Ta@B₂₀^- and theoretical prediction of the smallest endohedral metalloborospherene D₂ Ta@B₂₂^- with CN = 22,we present herein the possibility of larger endohedral metalloborospherenes C₂ Ta@B₂₃,C₂ Ta@B₂₄⁺,C_2v Ta@B₂₄^-,C₁ Ta@B₂₅,D_2d Ta@B₂₆⁺,C₂ Ta@B₂₇²⁺,and C₂ Ta@B₂₈³⁺ based on extensive first-principles theory investigations.These cage-like Ta@B_n^q complexes with _B6 pentagonal or B₇ hexagonal pyramids on the surface turn out to be global minima of the systems with,unveiling the highest coordination number of CN_max = 28 in spherical environments known in chemistry in Ta-B complexes.Detailed bonding analyses show that Ta@B_n^q complexes as superatoms follow the 18-electron configuration with a universal ? + ? double delocalization bonding pattern.They are effectively stabilized via spd-? coordination interactions between the Ta center and ?ⁿ-B_n ligand which match both geometrically and electronically.Such complexes may serve as embryos of novel metal-boride nanomaterials.4.High-symmetry tubular Ta@B₁₈^3-,Ta₂@B₁₈,and Ta₂@B₂₇⁺ as embryos of ?-boronanotubes with a transition-metal wire coordinated insideTransition-metal doped leads to dramatic structural changes and results in novel bonding patterns in small boron clusters.Based on the experimentally derived mono-ring planar C_9v Ta?B₉^2- and extensive first-principles theory calculations,we present herein the possibility of high-symmetry double-ring tubular D_9d Ta@B₁₈^3- and C_9v Ta₂@B₁₈ and triple-ring tubular D_9h Ta₂@B₂₇⁺.These clusters may serve as embryos of single-walled metalloboronanotube ?-Ta₃@B_48?3,0?,which are wrapped up from the recently observed most stable free-standing boron ?-sheet on Ag?111?substrate with a transition-metal wire?-Ta-Ta-?coordinated inside.Detailed bonding analyses indicate that,with an effective d_z2-d_z2 overlap on the Ta-Ta dimer along the C₉ molecular axis,both Ta₂@B₁₈ and Ta₂@B₂₇⁺ follow the universal bonding pattern of ? + ? double delocalization with each Ta center conforming to the 18-electron rule,rendering tubular aromaticity to these Ta-doped boron complexes with magnetically induced ring currents.5.Fluxional bonds in B₂-Ta@B₁₈^-,B₃-Ta@B₁₈,and B₄-Ta@B₁₈⁺Boron and metal-doped boron nanoclusters possess unique fluxional behaviors in dynamics.Detailed bonding analyses performed in this work indicate that,similar to the experimentally observed C_s B₂-[Ta@B₁₈]^-,the theoretically predicted tubular molecular rotors C_s B₃-[Ta@B₁₈] and C_3v B₄-[Ta@B₁₈]⁺ possess typical fluxional 4c-2e and 3c-2e ?-bonds atop the Ta-centered [Ta@B₁₈] double-ring tube between the B_n unit?n = 3,4?and upper B₉ ring,unveiling the fluxional bonding nature of the B_n-[Ta@B₁₈]^q complex series?n = 2-4,q = n-3?which follow the 18-electron rule in differnet charge states.Chiral conversions are involved in the fluxional processes via pseudo-rotations between the B_n unit?n = 2,3,4?and upper B₉ ring.