Borospherenes: Structures, Chemical Bonding And Photoelectron Spectroscopy

Buckminsterfullerene C60 was discovered in 1985. However, there has been no experimental evidence for all-boron fullerenes before 2014. In recent years, a series of rich-boron cluster with B₃ triangles basic unit and boron double chains and the quasi-planar boron cluster with hexagonal holes were observed in gas by our group in collaboration with Lai-Sheng Wang at Brown University. It is interesting that the double chains and hexagonal holes are the basic building blocks of the stable two-dimensional boron sheets at theoretical levels. Based on these experimental and theoretical results of low-dimensional boron materials, we perform systematical theoretical and experimental researches on all-boron fullerenes. Firstly, we confirmed the existence of cage-like D2d B40-/O and C3/C2 B₃9- in gas phase by joint photoelectron spectroscopy ?PES? and first-principles calculations. The borospherenes D2d B40 and C3/C2 B₃9- are composed of twelve interwoven boron double chains with six hexagonal and heptagonal holes. These experimental evidences initiated the borospherene chemistry, and motivated the theoretically predictions of a large number of borospherene family members. The main contents and results are as follow:I. Boron-rich binary clusters with B₃ triangular basic unit and boron double-chains1. B₃Aun and B₃?BO?n-?n=1,2? clusters with the B₃ Triangular Basic UnitPhotoelectron spectroscopy and density-functional theory are combined to study the structures and chemical bonding in boron-gold alloy clusters and boron boronyl clusters:B₃Aun- and B₃?BO?n-?n=1,2?. The anion and neutral clusters turn out to be isostructural and isovalent to the B₃Hn-/B₃Hn ?n=1,2? species, which are similar in bonding owing to the fact that Au, BO, and H are monovalent a ligands. All B₃Aun-and B₃?BO?n-?n=1,2? clusters are aromatic with 2? electrons. The current results provide new examples for the Au/H and BO/H isolobal analogy and enrich the chemistry of boronyl and gold.2. B₆Aun- and B₆Aun ?n=1-3? Clusters with B₆ Boron Double-ChainsPhotoelectron spectroscopy and density-functional theory are combined to investigate the electronic and structural properties of a series of B-Au alloy clusters: B₆Aun- and B₆Aun ?n=1-3?. The calculated electronic binding energies are compared with experimental measurements to establish the anion global-minimum structures and their corresponding neutral states. The ground-state structures of these clusters are shown to be planar or quasi-planar with an elongated B₆ core. All three anion clusters are ? antiaromatic. Natural bond orbital analyses show that the B-Au bonding is highly covalent, providing new examples for the Au/H analogy in Au alloy clusters.3. Bn?BO?2-/0 ?n=5-12? Clusters with Longer Boron Double-ChainsA series of boron dioxide clusters, BxO2- ?x=7-14?, have been produced and investigated using photoelectron spectroscopy and quantum chemical calculations. The dioxide clusters are shown to possess elongated ladder-like structures with two terminal boronyl ?BO? groups, forming an extensive series of boron nanoribbons, Bn?BO?2-?n= 5-12?. Both ? and a conjugations are found to be important in the unique bonding patterns of the boron nanoribbons. The ? conjugation in these clusters is analogous to the polyenes, while the ? conjugation plays an equally important role in rendering the stability of the nanoribbons. The concept of ? conjugation established here has no analogues in hydrocarbons. The nanoribbons form a new class of nanowires and may serve as precursors for a variety of boron nanostructures.4. co-bonds and o-bonds in Oxygen-Rich B₃On-/0/+?n=2-4? ClustersWe explore the structural and bonding properties of the electron-deficient boron oxide clusters, using a series of B₃On-/0/+?n= 2-4? clusters as examples. Global-minimum structures of these boron oxide clusters show a remarkable size and charge-state dependence. An array of new bonding elements are revealed:core boronyl groups, dual 3c-4e hypervalent bonds ??-bonds?, and rhombic 4c-4e bonds ?o-bonds?. In favorable cases, oxygen can exhaust all its 2s/2p electrons to facilitate the formation of B-0 bonds. The current findings should help understand the bonding nature of low-dimensional boron oxide nanomaterials and bulk boron oxides.II. Quasi-planar Boron Cluster with Hexagonal Vacancies1. Quasi-Planar Boron Clusters B₃6-/0 with Dual ? AromaticityHere we present computational evidence for the quasi-planar all-boron aromatic B₃6 ?C6v,1A1? and B₃6 ?C2v,2A1? clusters, established as the global-minimum structures on the basis of Stochastic Surface Walking ?SSW? searches. Our global-minimum structures are in line with a recent report ?Z. A. Piazza et al., Nat. Commun.,2014,5,3113?. These structures consist of two-dimensional close-packing boron with a perfect hexagonal hole at the center, which may serve as molecular models for the monolayer boron a sheet. Chemical bonding analysis indicates that B₃6 and B₃6- are all-boron analogues of coronene ?C24H12?, featuring concentric dual ? aromaticity with an inner ? sextet and an outer ? sextet.2. Quasi-Planar Boron Clusters B₃5-/0 with Triple ? AromaticityHere we report a boron cluster with a double-hexagonal vacancy as a new and more flexible structural motif for borophene. Global minimum searches find that both B₃5- and B₃5 possess planar hexagonal structures, similar to that of B₃6, except a missing interior B atom that creates a double hexagonal vacancy. The closed-shell B₃5-is found to exhibit triple ? aromaticity with 11 delocalized ? bonds, analogous to benzo?g,h,i?perylene ?C22H12?. The B₃5 cluster can be used to build atom-thin boron sheets with various hexagonal hole densities, providing further experimental evidence for the viability of borophene.?. The Discovery of Borospherenes1. The Discovery of Borospherenes-Experimental and Theoretical Evidence of Cage-Like All-Boron Fullerenes D2d B40-/0Here we present the observation of the first all-boron fullerenes D2d B40-/0, dubbed as borospherenes, via a joint first-principles calculation and photoelectron spectroscopy investigation. These all-boron fullerenes are composed of twelve interwoven boron double chains with six hexagonal and heptagonal faces. In fact, the double chains and hexagonal holes are the building block of numerous low-dimensional boron nanomaterials. The chemical bonding analyses show that there exist a three-center two-electron ?3c-2e? ? bond on each B₃ triangle on the cage surface and twelve multicenter two-electron n bonds ?mc-2e, m= 5,6,7? over the ? skeleton. D2d B40 appears to be the first case of a plus ? double delocalization bonding pattern. The observation of D2d B40-/0 borospherenes suggests the beginning of the experimental and theoretical research of all-boron fullerenes, implies there exist a family of borospherenes which parallel to the well-known carbon fullerenes.2. Experimental and Theoretical Evidence of Axially Chiral Borospherenes C3/C2B₃9-Here we report the observation of axially chiral borospherene in the B₃9-nanocluster on the bases of photoelectron spectroscopy, global minimum searches, and electronic structure calculations. Extensive structural searches in combination with density functional and CCSD?T? calculations show that B₃9- has a C3 cage global minimum with a close-lying C2 cage isomer. Both the C3 and C2B₃9- cages are chiral with degenerate enantiomers. The C3 global minimum consists of three hexagons and three heptagons around the vertical C3 axis. The C2 isomer is built on two hexagons on the top and at the bottom of the cage with four heptagons around the waist. Both the C3 and C2 axially chiral isomers of B₃9- are present in the experiment and contribute to the observed photoelectron spectrum. The chiral borospherenes also exhibit three-dimensional aromaticity, featuring a and ? double delocalization for all valence electrons. The current findings add chiral members to the borospherene family and indicate the structural diversity of boron-based nanomaterials.3. Endohedral C3/C2 Ca@B₃9+:Axially Chiral Metalloborospherenes Based on B₃9-Using the newly discovered borospherenes C3B₃9- and C2B₃9- as molecular devices and based on extensive global-minimum searches and first-principles calculations, we present herein the possibility of the first axially chiral metalloborospherenes C3 Ca@B₃9+ and C2 Ca@B₃9+ which are the global minimum and the second lowest-lying isomer of CaB₃9+, respectively. These metalloborospherene species turn out to be charge-transfer complexes Ca2+@B₃9-in nature, with the Ca centre on the C3 or C2 molecular axis donating one electron to the B₃9 cage which behaves like a superhalogen. Molecular orbital analyses indicate that C3/C2 Ca2+@B₃9-possess the universal bonding pattern of ? plus ? double delocalization, similar to their C3/C2B₃9- parents. Molecular dynamics simulations show that both C3 Ca@B₃9+?1? and C2 Ca@B₃9+?2? are dynamically stable at 200 K, with the former starting to fluctuate strucutrally at 300 K and the latter at 400 K, again similar to C3/C2B₃9-.4. Cage-Like B41+ and B422+:Chiral Members of the Borospherene FamilyWith inspirations from the newly discovered cage-like D2d B40-/0 and C3/C2B₃9- we introduce herein two chiral members to the borospherene family based upon extensive global minimum searches and first-principles calculations:the cage-like C1 B41+ and C2 B422+, both of which are the global minima of the systems with degenerate enantiomers. These chiral borospherene cations are composed of twelve interwoven boron double chains with six hexagonal and heptagonal faces and may be viewed as the cuborenes analogous to cubane ?C8H8?, similar to the observed D2d B40-/0 and C3/C2B₃9- in geometry. Chemical bonding analyses show that there exist a three-center two-electron ?3c-2e? a bond on each B₃ triangle and twelve multi-center two-electron ? bonds ?mc-2e, m=5,6,7? over the twelve interwoven boron double chains on the cage surface. The ? plus ? double delocalization appears to be the universal bonding pattern for all borospherenes that have been studied so far.B₃9-, B40, B41+, and B422+ thus form a ?-isovalent Bn9 series in different charge states ?q=n-40?, which all have 12 delocalized mc-2e ? bonds over a ? skeleton made of n+8 delocalized 3c-2e ? bonds.5. Borospherenes B₃82-, B₃64-, B₃73- and Metalloborospherenes Ca@B₃8, Li4&B₃6, and Ca@B₃7-Based on extensive global-minimum searches and first-principles electronic structure calculations, we present the viability of an endohedral metalloborospherene Cs Ca@B₃8 which contains a Cs B₃82-dianion composed of interwoven boron double chains with a ?+? double delocalization bonding pattern. The discovery of borospherene B₃82- and metalloborospherene Ca@B₃8 indicate that electron-deficient borospherene can be effectively stabilized by doping metals. Based on the same measure, we confirmed the charge-transfer complexes Li4&B₃6 and Ca@B₃7- contain borospherenes B₃64- and B₃73-respectively. The current findings extended the Bnq ?g= n-40? borospherene family from n= 39-42 to n= 36-42.