Design Of Chemically Stable Boron-Based Clusters With Hypercoordinate Centers Using Electron-Compensation Strategy

Number 5 in the periodic table,boron has the electronic state of 1s²2s²2p¹.There are only three electrons in its four valence orbitals,so even if all these valence electrons are paired with foreign electrons to form bonds,the valence orbitals of the boron atom are not fully filled,leading to the typical electron deficiency.Therefore,boron tends to form multicenter bonds in order to fill in more electrons in its valence shell.The multicenter bonding preference of boron makes it suitable for the design of clusters and two-dimensional materials containing nonclassical hypercoordinate centers.Corresponding clusters and materials have been widely reported in the last two decades.However,the boron atoms in these clusters and materials are generally bare and their electron deficiency has not been eliminated or effectively weakened,leading to high chemical activity and high sensitivity（to aggradation,oxidation and nucleophilic attack）,so they are difficult to synthesize in the condensed phases.In addition,the bonding preference due to the electron-deficieny of boron makes boron more suitable than carbon atoms to be located in the hypercoordinate center when designing clusters with planar hypercoordinate carbon.This can be reflected in the thermodynamic stability by the lower energy of the isomer with the boron atom at the planar hypercoordinate center.Aiming to solve the problem at the source,we proposed in this work a strategy named“electron-compensation”and designed a series of boron oxide clusters with planar hypercoordinate centers to validate its feasibility.These clusters all have the wide HOMO-LUMO energy gaps as well as a low tendency to gain/lose electrons and are all sterically surrounded by the same number of oxygen atoms as boron.Therefore,they possess the desired high chemical stability.The details of this dissertation are listed below:1.“Electron-compensation”Strategy for Stabilizing the Boron-based ClustersAiming at the origin of bad chemical stability of boron-based cluster,that is the bare electron-deficient boron atoms,we propose in this dissertation the“electron compensation”strategy,which compensates for the electron deficiency of boron atoms through the formation of X→Bπdative bonds using highly electronegative X atoms with valence lone pairs.Simultaneously,X atoms also play the role of steric protection.Therefore,the clusters designed based on our strategy are chemically stable enough for synthesis in the condensed phase.In the followed-up work,we designed a series of boron-oxide clusters with hypercoordinate center to validate the strategy.Since the boron atoms in these clusters are electronically compensated through O→Bπdative bonds and sterically protected by oxygen atoms,their chemical stability has been significantly improved.2.CB₈O₈:Boron-based Cluster with a Planar Tetracoordinate CarbonTo address the issue concerning the low thermodynamic stability of boron-based clusters with a planar hypercoordinate carbon,a neutral ternary boron-containing cluster CB₈O₈with planar tetracoordinate carbon is designed in this work by applying the electron-compensation strategy.All boron atoms in this cluster are electronically saturated and geometrically protected by oxygen atoms.This dynamically stable global energy minimum possesses a well-defined electronic structure,which can be reflected by its wide HOMO-LUMO gap（6.16eV）,high vertical detachment energy（VDE,10.35 eV）and positive vertical electron affinity energy（VEA,0.10 eV）.Therefore,this cluster has a low tendency to lose,gain or excite electrons and should be chemically stable enough for experimental synthesis in the condensed phase.3.M?B₇O₇⁺（M=Ni,Pd,Pt）:σAromatic Molecular Stars with a Planar Heptacoordinate Transition MetalDue to theσ-donor andπ-acceptor nature of boron,the previously reported boron wheels with a planar hypercoordinate transition metal inside are allσ/πdouble aromatic.In this work,applying the“electron compensation”strategy,we designed a class of cationic oxygen-surrounded boron wheels with a planar heptacoordinate transition metal inside M?B₇O₇⁺（M=Ni,Pd,Pt）,which unusually possess onlyσaromaticity rather thanσ/πdouble aromaticity.Electronic structure analysis suggests that the peripheral oxygen atoms play a key role in affecting the occupation ofπorbitals concerningπ-aromaticity,that is,the strong O→Bπdative bonds compensate for the electron deficiency of the boron atoms,thus reducing their ability to be in M→Bπdative bonds.Consequently,the 16ve cation M?B₇O₇⁺withoutπaromaticity is more stable than its correspondingσ/πdouble aromatic 18ve anionic cluster M?B₇O₇^–.As dynamically stable global energy minima,they are promising for experimental realization.4.[OB-M?B₇O₇]^-（M=Co,Rh,Ir）:Heptagonal Pyramidal Octacoordinate ComplexesBonding configurations around metals usually conform to the well-known valence shell electron pair repulsion（VSEPR）theory.Specifically,for metal complexes with coordination numbers higher than 6,spherical configurations are almost always adopted to reduce the repulsion between the ligand atoms.Accordingly,the realization of non-spherical configurations is very challenging.In particular,the octacoordinate heptagonal pyramidal configuration has not been implemented experimentally and computationally so far due to the extremely high repulsion between the seven ligand atoms densely arranged in the equatorial plane.In this section,we applied the electron-compensation strategy to design such the structure based on M?B₇O₇⁺（M=Ni,Pd,Pt）by introducing a BO group from the axial direction to MB₇O₇,leading to an anionic complex[OB-M?B₇O₇]^–（M=Co,Rh,Ir）with the synergistic coordination from an equatorial hepta-dentate centripetal ligand（B₇O₇）and an axial monodentate ligand（-BO）.The high repulsion among seven equatorial ligand B atoms has been compensated by strong B-O bonding.These complexes are dynamically stable（up to1000 K）global energy minima with HOMO-LUMO gaps between 5.12 and 5.28 eV as well as first VDEs between 3.68 and 3.79 eV,indicating their high possibility for experimental realization,both in the gas and condensed phase.The high stability originates geometrically from the peripheral oxygen atoms and electronically from the“electron-compensation”,the satisfaction of the 18ve rule and the presence of theσ+π+δtriple aromaticity.Noticeably,EDA-NOCV calculations show that the ligand-metal interactions are not dominated by conventional donation and backdonation interactions,but by electrostatic interactions and electron-sharing bonds.5.Prediction of Heptagonal Bipyramidal Nonacoordination in Highly Viable[OB-M?B₇O₇-BO]^-（M=Co,Rh,Ir）ComplexesNon-spherical distributions of ligand atoms in coordination complexes are generally unfavorable due to higher repulsion than for spherical distributions.To the best of our knowledge,non-spherical heptagonal bipyramidal nonacoordination is hitherto unreported,because of extremely high repulsion among seven equatorial ligand atoms.In this section,we report the computational prediction of such nonacoordination.Through applying an electron-compensation strategy,a BO group is introduced to the axial direction of[OB-M?B₇O₇]^–,the gear-like mono-anionic complexes[OB-M?B₇O₇-BO]^–（M=Fe,Ru,Os）are constructed among the synergetic coordination of an equatorial hepta-dentate centripetal ligand（B₇O₇）and two axial monodentate ligands（-BO）.The high repulsion among seven equatorial ligand B atoms has been compensated by the strong B–O bonding.These complexes are the dynamically stable（up to 1500 K）global energy minima with the HOMO-LUMO gaps of 7.15 to 7.42 eV and first VDEs of 6.14 to 6.66 eV（being very high for anions）,suggesting their high probability for experimental realization in both gas-phase and condensed phases.The high stability stems geometrically from the surrounded outer-shell oxygen atoms and electronically from meeting the 18ve rule as well as possessing theσ+π+δtriple aromaticity.Remarkably,the ligand-metal interactions are governed not by the familiar donation and backdonation interactions,but by the electrostatic interactions and electron-sharing bonding.6.[（OB）₂-M?B₇O₇-BO]^-（M=Mn,Tc,Re）:Chemically Stable and Triply Aromatic Ballet MotorsSingle-molecule nanomotors are generally constructed based on boron atoms to obtain structural fluxionality via possessing the delocalized multicenter bonds.However,the electron-deficient boron atoms are commonly exposed in these nanomotors,which leads to extremely high chemical reactivity that blocks the synthesis in the condensed phase.In this work,we computationally designed a series of transition-metal-doped boron oxide clusters MB₁₀O₁₀^–（in the structural configuration of[（OB）₂-M?B₇O₇-BO]^–,M=Mn,Tc,Re）,which can be vividly named as“ballet motors”to label their anthropomorphic dynamic rotational behaviors.The rotational fluxionality in ballet motors originates from the completely delocalized nature of the bonding within their MB₁₀core moieties.Remarkably,compared with single-molecule nanomotors having bare boron atoms and narrow HOMO-LUMO gaps（≤4.00 eV）as well as low VDEs,（≤4.46 eV for anions）,the ballet motors possess significantly improved chemical stability,as evidenced sterically by the absence of exposed boron atoms and electronically by much wider HOMO-LUMO gaps（5.66–5.98 eV）as well as obviously higher VDEs between 5.36 and 5.47 eV.Specifically,the ballet motors are mainly stabilized by the delicately placed peripheral oxygen atoms which can compensate for all electron-deficient boron atoms via O→Bπdative bonds and sterically protect them.Simultaneously,they are additionally stabilized by the aromatic stabilization effect from possessing the novelσ+π+δtriple aromaticity.We expect that the proposal of chemically stable ballet motors in this work can arouse the rational design of nanomotors for experimental realization in the condensed phase.