Theoretical Study On The Aromaticity Of Metallacycles And All-Metal Clusters In The Lowest Triplet State

Aromaticity is one of the most important concepts in the field of chemistry,which has been widely applied to organic molecules.To date,the scope of aromatic species includes not only conventional annulenes,heteroannulenes,and polycyclic hydrocarbons,but also Mobius molecules,fullerenes,metallacycles,and all-metal clusters.Generally,ordinary planar aromatics follow the Hückel’s 4h+2 rule in the singlet ground state,while in the lowest-lying ππ*triplet state the aromatic characters are reversed according to the Baird’s rule.Transition-metal(TM)d orbitals can participate in the formation of a cyclic conjugated system,leading to unique types of aromatic behaviors.The adiabatic excitation from the lowest singlet to the lowest triplet state of TM-containing metallacycles does not always result in the ππ*fashion,thus accounting for various examples disobeying the Baird’s rule.Aromaticity in TMcontaining metallacycles and even all-metal species in the lowest-lying triplet state has not been extensively explored,whereas its significance in the field of photochemistry and photophysics is continuously inspiring us to devote ourselves to the development of the title concept.This thesis focuses on the study of aromaticity of TM-containing metallacycles and all-metal clusters in the lowest triplet state.Density functional theory(DFT)calculations and multiple aromaticity indices are used to discuss the structure,bonding,and aromatic properties of target systems.Firstly,we examined aromatic characters of osmapentalynes and osmapentalenes and found that the 16-valence-electron osmapentalene is significantly aromatic in both the lowest-lying singlet and triplet states.Such an unconventional aromatic behavior was defined as "adaptive aromaticity"by us.Next,we analyzed the potential of ruthenium as a substitute for osmium in the realization of adaptive aromaticity,by replacing the osmium center with ruthenium in osmapentalyne,osmapentalene,and osmapyridinium skeletons.Thirdly,we further extended the scope of target systems to include 16e metallapentalenes with various metal centers and ligands,revealed the origin of adaptive aromaticity in such type of metallacycles via both electronic excitation and spin delocalization mechanisms,and provided specific and effective guidelines for designing complexes with adaptive aromaticity.Finally,we evaluated the aromaticity in a series of all-metal cluster in the lowest-lying singlet/triplet states using multiple aromaticity indices,and for the first time demonstrated that the Baird’s rule can also be applied to all-metal clusters including Li3Al4-Li3Ga4-,Be2B6,Be2B7+,etc.