Isolation and Reactivity of an Antiaromatic s-Block Metal Compound

Abstract

The concepts of aromaticity and antiaromaticity have a long history, and countless demonstrations of these phenomena have been made with molecules based on elements from the p, d, and f blocks of the periodic table. In contrast, the limited oxidation-state flexibility of the s-block metals has long stood in the way of their participation in sophisticated π-bonding arrangements, and truly antiaromatic systems containing s-block metals are altogether absent or remain poorly defined. Using spectroscopic, structural, and computational techniques, we present herein the synthesis and authentication of a heterocyclic compound containing the alkaline earth metal beryllium that exhibits significant antiaromaticity, and detail its chemical reduction and Lewis-base-coordination chemistry.

Keywords: antiaromaticity; aromaticity; beryllium; heterocycles; s-block metals.

Figure 1

Synthesis and reactivity of the antiaromatic beryllole 1, and solid‐state structures of 1‐3. ^[78] Ellipsoids of the crystallographically derived structures are shown at the 50 % probability level. All hydrogens and ellipsoids of peripheral groups have been removed for clarity. Atom colors: N blue, Be green, Li red, O pink.

Figure 2

A) Frontier molecular orbitals, B) scan of the zz‐components of the nucleus independent chemical shifts (NICS_zz‐scan), C) aromatic stabilization energies, and D) anisotropy of the induced current density (ACID) plots of selected (anti‐/non‐)aromatic compounds.

Figure 3

Relevant molecular orbitals calculated for truncated cyclic systems [XC₄H₄] (X=[HC]⁺, to [HB], [(CAAC)Be]), highlighting the extent of participation of the X fragment in the ring π system.