Craig Excited-State Aromaticity in Metallabenzenes: How, When, and Why?

Area of Science:

• * Computational Chemistry
• * Quantum Chemistry
• * Materials Science

Background:

• * Excited-state aromaticity describes enhanced stability and reactivity post-photoexcitation.
• * Hückel and Möbius excited-state aromatic species are known, but Craig aromaticity is unrecognized.
• * Craig aromaticity involves [4n+2] electrons in planar metallacycles.

Purpose of the Study:

• * To identify and characterize Craig excited-state aromaticity in early transition metal-based metallabenzenes.
• * To elucidate the electronic structure and bonding responsible for this phenomenon.
• * To establish a unified framework for metallabenzene electronic structure.

Main Methods:

• * Utilized computational chemistry methods, including ab initio valence bond theory.
• * Analyzed electronic, geometric, energetic, and magnetic properties to assess aromaticity.
• * Investigated the role of specific d-orbitals in electron delocalization.

Main Results:

• * Early transition metal (Ti, Sc, Y, La, Ac) metallabenzenes exhibit Craig 6π aromaticity in their lowest singlet and triplet excited states.
• * The d_yz orbital was identified as crucial for cyclic electron delocalization and phase inversion.
• * Contrasted findings with late transition metal metallabenzenes, which show Hückel or Baird (anti)aromaticity via the d_xz orbital.

Conclusions:

• * Provided the first direct evidence and explanation for Craig excited-state aromaticity.
• * Established the dominant role of the d_yz orbital in early transition metal metallabenzene excited states.
• * Addressed a significant gap in the understanding of excited-state metalla-aromaticity.