δ Aromaticity in [Ta_3O_3]

摘要

The concept of aromaticity was introduced into organic chemistry to describe delocalizedπbonding in planar, cyclic, and conjugate molecules possessing (4n+2)πelectrons.[1] In recent years, this concept has been advanced into main-group molecules including organometallic compounds with cyclic cores of metal atoms[2] and, in particular, all-metal clusters.[3] It has been shown that main-group clusters may exhibit multiple aromaticity ( and ), multiple antiaromaticity ( and ), and conflicting aromaticity ( aromaticity andπantiaromaticity orσantiaromaticity andπaromaticity).[4]-[6] Here, we report experimental and theoretical evidence ofδaromaticity, which is only possible in transition-metal systems. It is discovered in the [Ta_3O_3]- cluster through combined photoelectron spectroscopy and ab initio studies. Well-resolved low-lying electronic transitions are observed in the photoelectron spectra of [Ta_3O_3]- and are compared with ab initio calculations, which show that the [Ta_3O_3]- cluster has a planar D3h triangular structure. Chemical-bonding analyses reveal that among the five valence molecular orbitals involved in the multicenter metal-metal bonding, there is a completely bondingδandπorbital formed from the 5d atomic orbitals of Ta. The totally delocalized multicenterδbond rendersδaromaticity for [Ta_3O_3]- and represents a new mode of chemical bonding. [Ta_3O_3]- is the first δ-aromatic molecule confirmed experimentally and theoretically, which suggests thatδaromaticity may exist in many multinuclear, low-oxidation-state transition-metal compounds.