On The Novel Structures,Chemical Bonding,Electronic Transmutation,and Dynamic Fluxionality Of Metal-Doped Boron-Based Nanoclusters

The unique 2s²2p¹ electron configuration of boron gives rise to its high coordination number and intrinsic electron deficiency in bulk crystals and compounds.Therefore,elemental boron clusters feature structural diversity,unusual electronic properties,and nonclassical chemical bonding.The bonding in boron clusters is governed by concepts such asπ/σaromaticity,antiaromaticity,conflicting aromaticity,as well as multiple aromaticity.Doping with electron-rich metals in boron-based clusters can effectively compensate for boron’s electron deficiency.Such efforts may alter the planar or quasi-planar structures of elemental boron clusters,thus leading to the linear chain or monocyclic ring structural motifs that are unknown in free-standing boron clusters.Among the key issues in metal-doped boron clusters is the nature of interactions in between foreign metal atoms,which has received relatively little attention in prior works in the literature.Exploration of the unusual structures,exotic chemical bonding,and dynamic structural fluxionality of metal-doped boron-based alloy clusters has lately emerged as one of the research frontiers in the field.Based on the consideration of these scientific issues,in this thesis work,a series of metal-doped boron-based binary clusters are theoretically explored and designed through computer global structure searches and electronic structure calculations.These alloy clusters all contains a B₇ component,and yet they assume distinctly different global minimum（GM）structures,owing to the doping of metallic elements ranging from alkali metals,alkaline-earth metals,and transition metals.In these alloy clusters,a variety of B-based structural motifs are observed,including hybrid in-plane heptacoordinate B wheel,monocyclic B₇ ring,inverse sandwich with a B₇ ring,and sandwich cluster with a hexacoordinate B wheel.The hybrid B wheel and inverse sandwich clusters can be rationalized using the concept of electronic transmutation.The sandwich alloy cluster is dictated by three-foldπ/σaromaticity,which also facilitates the dynamic fluxionality.The study of metal-metal（M-M）interaction in inverse sandwich clusters reveals direct M-M bonding,which is scarce in the literature,to our knowledge.The main research contents of this thesis are as follows:1.A plier-shaped binary molecular wheel B₇Mg₄⁺cluster:hybrid in-plane heptacoordination,doubleπ/σaromaticity,and electronic transmutationQuantum chemical calculations in combination with unbiased GM searches reveal a unique binary B₇Mg₄⁺cluster,in which a vertically-oriented Mg₂ dimer is incorporated into a ringent（or Pac-Man shaped）B₇ disk,forming a hybrid Mg₂B₇ molecular wheel with pseudo seven-fold in-plane coordination.The hybrid wheel assumes a plier shape.The remaining Mg₂ dimer floats on the hybrid wheel via electrostatic interaction,with relatively minor covalent Mg-B bonding.The B₇Mg₄⁺cluster can be described as a charge transfer[Mg₂]²⁺[Mg₂B₇]^-complex.Chemical bonding analysis indicates double 6π/6σaromaticity in B₇Mg₄⁺cluster.The plier shape of the cluster is elucidated using the concept of electronic transmutation,in which the vertical Mg₂ dimer closely mimics the chemistry of an atomic B^-or C site.2.Inverse sandwich Sc₂B₇^-cluster featuring a monocyclic B₇ ring motifBased on computational GM structure searches and density functional calculations,we report on a double transition-metal-doped Sc₂B₇^-cluster.Herein the B₇ component assumes a monocyclic ring shape,which is sandwiched in between two Sc atoms,thus forming a bipyramid inverse sandwich structure for the alloy cluster.Canonical molecular orbital（CMO）analysis and natural bond orbital（NBO）analysis show that the electron contribution of Sc₂ dimer to chemical bonding of the system amounts to five valence electrons,which is equivalent to a C^-/B^2-site.The double 6π/6σaromaticity in Sc₂B₇^-cluster is similar to that in dianionic B₈^2-cluster,as are the outer skeleton Lewis-typeσsingle bonds.Therefore,using the electronic transmutation concept,Sc₂B₇^-cluster can be regarded as the substitution of the center B atom of B₈^2-by a Sc₂ dimer.It is stressed that,in Sc₂B₇^-cluster,the inner Sc₂ dimer and outer B₇ ring interact primarily via intramolecular charge-transfer,as well as electrostatics.With this understanding,the monocyclic B₇ ring motif in this alloy cluster maintains chemical intergrity to some extent.Such a motif is not likely to survive in free-standing bare boron clusters.3.Boron-based inverse sandwich V₂B₇^-cluster:doubleπ/σaromaticity,metal-metal bonding,and chemical analogy to planar hypercoordinate molecular wheelsInverse sandwich clusters composed of a monocyclic boron ring and two capping transition metal atoms are interesting alloy cluster systems in chemistry,and yet their nature of chemical bonding has not been sufficiently elucidated to date.We shall report herein on the theoretical prediction of a new example of boron-based inverse sandwich alloy clusters,V₂B₇^-,through computational GM structure searches and quantum chemical calculations.This alloy cluster has a heptatomic boron ring,as well as a perpendicular V₂ dimer unit that penetrates through the ring.Chemical bonding analysis suggests that the inverse sandwich cluster is governed by globally delocalized 6πand 6σframeworks;that is,double 6π/6σaromaticity following the（4n+2）Hückel rule.The skeleton B-Bσbonding in the cluster is shown not to be strictly Lewis-type two-center two-electron（2c-2e）σbonds.Rather,these are quasi-Lewis-type,roof-like 4c-2e V-B₂-Vσbonds,which amount to seven in total and cover the whole surface of inverse sandwich in a truly three-dimensional manner.Intriguingly,theoretical evidence is revealed for a 2c-2e Lewisσsingle bond within the V₂dimer.Direct metal-metal bonding is rare in inverse sandwich alloy clusters,to our knowledge.Based on chemical bonding analysis,the V₂B₇^-alloy cluster turns out to be chemically analogous to planar hypercoordinate CB₇^-and B₈^2-clusters despite that the systems differ by as many as six valence electrons,of which two are responsible for a V-Vσbond as mentioned above and four are present as nonbonding V 3d electrons（albeit not lone-pairs）.Thus,the V₂B₇^-,CB₇^-,and B₈^2-clusters are indeed isovalent with each other in terms of skeleton B-Bσbonds and global B-B/B-C/B-Vπandσbonding,which collectively consume 26 electrons.In this context,the V₂ dimer in V₂B₇^-cluster,which is formally in its[V₂]^-charge-state owing to the extra charge in the anion,shall be considered as a single site with valence five;that is,it is analogous to a central C^-or B^2-site in planar molecular wheels.The V-B bonding is polar in nature with mixed covalent/ionic characters,resulting in a natural atomic charge distribution of[V₂]⁺[B₇]^2-for the cluster.The formal charge-state and the natural bond orbital charges should not be confused with each other.The present inverse sandwich alloy cluster offers a new type of electronic transmutation in physical chemistry.4.Chemical bonding and dynamic structural fluxionality of a boron-based Na₅B₇sandwich clusterDoping alkali metals into boron clusters can effectively compensate for the intrinsic electron deficiency of boron and lead to interesting boron-based binary clusters,owing to small electronegativity of the former elements.We report on the computational design of a three-layered sandwich cluster,Na₅B₇,on the bases of GM structure searches and electronic structure calculations.It is shown that the Na₅B₇ cluster can be described as a charge-transfer complex:[Na₄]²⁺[B₇]^3-[Na]⁺.In this sandwich cluster,the[B₇]^3-core assumes a molecular wheel in shape and features in-plane hexagonal coordination.The magic 6π/6σdouble aromaticity underlies the stability of the[B₇]^3-molecular wheel,following the（4n+2）Hückel rule.The tetrahedral Na₄ ligand in the sandwich has a[Na₄]²⁺charge-state,which is the simplest example of three-dimensional aromaticity,spherical aromaticity,or superatoms.Its 2σelectron counting rendersσaromaticity for the ligand,conforming to the2（n+1）² electron-counting rule.Overall,the sandwich cluster has three-fold 6π/6σ/2σaromaticity.Molecular dynamics simulation shows that the sandwich cluster is dynamically fluxional even at room temperature,with a negligible energy barrier for intramolecular twisting between the B₇ wheel and the Na₄ ligand.The Na₅B₇ cluster offers a new example for dynamic structural fluxionality in molecular systems.