Investigations on Boron and Transition-Metal-Doped Boron Nanoclusters

Author:Li Hai Ru

Supervisor:li si dian


Degree Year:2019





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 Bn-(n = 3-28,35,36,39,40) through a large number of experiments and theoretical studies,but a few Bn-/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 D2d 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 D2d 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 Bnq(q =-1+3,n = 21-28) complexes,found that the rotary speed of molecular rotor Cs Ta@B21 and C3v 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 Cs 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 Cs B29- isomer with a pentagonal hole.However,by taking into account of entropic effects,the stingray-shaped Cs 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 Cs B29- is an all-boron analogue of benzo [ghi] fluoranthene(C18H10),whereas the seashell-like Cs B29-possesses 18-π 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 D2d B40-and D2d 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 D2d 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 D2d 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 Cs 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 η7-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 Nin∈B40(n = 1-4),and planar heteroborophenes Ni2∈B14 which all contain planar or quasi-planar heptacoordinate transition-metal(phTM)centers in η7-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 Nin∈B40(n = 1-4)clusters.Detailed bonding analyses indicate that Nin∈B40(n = 1-4)and Ni2∈B14 possess the universal bonding pattern of σ + π double delocalization on the boron frameworks,with each ph Ni forming three lone pairs in radial direction(3dz2,3dzx,and 3dyz)and two nearly in-plane 8c-2e σ-coordination bonds between the remaining tangential Ni 3d orbitals(3dx2-y2 and 3dxy)and the η7-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 Cs 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 Cs B3-Ta@B18 and C3v 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 σ + π 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@Bnq 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 D10h Ta@B10- and CN = 20 in double-ring tubular species in D10d 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+,C2v Ta@B24-,C1 Ta@B25,D2d Ta@B26+,C2 Ta@B272+,and C2 Ta@B283+ based on extensive first-principles theory investigations.These cage-like Ta@Bnq 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 CNmax = 28 in spherical environments known in chemistry in Ta-B complexes.Detailed bonding analyses show that Ta@Bnq complexes as superatoms follow the 18-electron configuration with a universal σ + π double delocalization bonding pattern.They are effectively stabilized via spd-π coordination interactions between the Ta center and ηn-Bn ligand which match both geometrically and electronically.Such complexes may serve as embryos of novel metal-boride nanomaterials.4.High-symmetry tubular Ta@B183-,Ta2@B18,and Ta2@B27+ as embryos of α-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 C9v Ta?B92- and extensive first-principles theory calculations,we present herein the possibility of high-symmetry double-ring tubular D9d Ta@B183- and C9v Ta2@B18 and triple-ring tubular D9h Ta2@B27+.These clusters may serve as embryos of single-walled metalloboronanotube α-Ta3@B48(3,0),which are wrapped up from the recently observed most stable free-standing boron α-sheet on Ag(111)substrate with a transition-metal wire(-Ta-Ta-)coordinated inside.Detailed bonding analyses indicate that,with an effective dz2-dz2 overlap on the Ta-Ta dimer along the C9 molecular axis,both Ta2@B18 and Ta2@B27+ follow the universal bonding pattern of σ + π 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 Cs B2-[Ta@B18]-,the theoretically predicted tubular molecular rotors Cs B3-[Ta@B18] and C3v B4-[Ta@B18]+ possess typical fluxional 4c-2e and 3c-2e σ-bonds atop the Ta-centered [Ta@B18] double-ring tube between the Bn unit(n = 3,4)and upper B9 ring,unveiling the fluxional bonding nature of the Bn-[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 Bn unit(n = 2,3,4)and upper B9 ring.