Computational Design Of Molecules Containing Ultrashort Be-Be Distances

Recently,many structures containing ultrashort metal-metal distance(USMMD,d_M-M<1.900?)had been proposed both theoretically and experimentally.Though USMMDs were mainly realized between transition metals,the USMMDs between main group metal beryllium had also been proved to be possible by our group.However,one of the species reported?NHC?Be2H2?NHC?may be relatively unstable because it HOMO energy level is very high and the HOMO-LUMO gap is very small.We applied the strategies of covalent bonding and electrostatic attraction to this species,achievingamoreviablespeciesalsopossessingultrashort beryllium-beryllium distances.It was found that addition of-CH₂-group onto the bridging position of two beryllium atoms leads to species IH?Be₂H₂CH₂?IH.The original electrons in delocalized?bond had been directed into two C-Be?bonds,which greatly lower the orbital energy level.The resulted species not only possesses the ultrashort Be-Be distances in-Be₂H₂CH₂-moiety,but also has relatively low HOMO energy level.ReplacingtheIHligandswithNH₃andPH₃formspecies NHC?Be₂H₂CH₂?NHC?L=NH₃,PH₃?with similar characteristics.The CMO analysis,NBO analysis,AdNDP analysis and other properties showed that the USMMD in this species was the combined effect from both the covalent bond and the interaction of electrostatic attraction.Remarkably,when the IH,NH₃ and PH₃ ligands are replaced by large ligands with bulky groups,such as 1,3-bis?2,6-diisopropyl phenyl?imidazol-2-ylidene?IDip?,triphenylamine?NPh₃?,and triphenylphoshpine?PPh₃?,respectively,the resulted species IDip?Be₂H₂CH₂?IDip,NPh₃?Be₂H₂CH₂?NPh₃ and PPh₃?Be₂H₂CH₂?PPh₃ could have good steric protection around the-Be₂H₂CH₂-core,which provides the promising targets for experimental realization of species with ultrashort metal-metal distances between main group metals.Superalkalis and superhalogens may exhibit the highly reductive and oxidative properties,respectively,which both play the important role in reactivity.The previously reported superalkali cations and superhalogen anions generally have the atom?s?or functional group with electronegative and electropositive properties as the core structures,which are surrounded by alkali metals and halogens,respectively.In present work,we design super-alkali metal cations and superhelogen anions based on the Be₂H₃⁺structure form species[BO?Be₂H₃?BO]^–,[NHC?Be2H3?NHC]⁺,[NHB?Be₂H₃?BHN]^–,[BO?Be₂?BO?₃?BO]^–.The VDE values of the cations are much higher than 3.62 eV?VDE of Cl^–?,in which the value for[BO?Be₂?BO?₃?BO]^–,is as high as 8.34 eV at OVGF/aug-cc-pVTZ level,the VEA value[NHC?Be₂H₃?NHC]⁺is 2.04 eV,much smaller than 3.89eV?VEA of Cs⁺?.All these species possess the ultrashort Be-Be distances(d_Be-Be=1.756 to 1.847?),which are achieved by the formation a Be₂H₃pseudo triple bond,combined from three Be-H-Be 3c-2e bonds.The molecular dynamics simulations performed 4 K,298 K and 500 K suggested that all species can maintain their basic structure.Finally,when the NHC and NHB ligands are replaced by large ligands with bulky groups,such as1,3-bis?2,6-diisopropylphenyl?imidazol-2-ylidene?IDip?and1,3-bis?2,6-diisopropylphenyl?1H-1,3,2-diazaborole-2-ylidene?DDip?,respectively,theresultedspecies[IDip?Be₂H₃?IDip]⁺and[DDip?Be₂H₃?DDip]^–could have good steric protection around the-Be₂H₃-core,which provides the promising targets for experimental realization.