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.
Highlights
The concept of antiaromaticity in molecules was presented in the mid-1960s by Breslow and co-workers[1,2] as a simple counterpoint to aromaticity, that is, aromatic compounds are those in which cyclic delocalization of electrons has a stabilizing effect, while cyclic delocalization imparts destabilization in antiaromatic compounds
Bond lengths in the five-membered ring are unequal. This picture is corroborated by density functional theory (DFT) in the unrestricted formalism as well as by high-level complete active space self-consistent field (CASSCF)/NEVPT2 calculations, the latter indicating a biradical character of 37.4 % for [(CAAC)BeC4Ph4]2À, which is of the same order as that of the recently isolated parent aluminene [(CAAC)2AlH],[64] and a singlettriplet gap (DT–S) of 3.3 kcal molÀ1
Our results indicate a dramatic decrease in the antiaromaticity in the ring systems [XC4H4] upon moving left from carbon in the periodic table, that is, from X = [HC]+, to [HB], and to [(CAAC)Be]
Summary
Interest in the chemistry of carbene-stabilized beryllium species has exploded over the past few years, leading to the discovery of a range of Be0, BeI, and BeII complexes with fascinating properties and reactivity, as well as providing a host of precursors containing stabilized [DBeL] scaffolds for the construction of compounds with unusual electronic structures.[41,42,43,47] The [DBeL] fragment we chose was [DBe(CAAC)], bearing a neutral cyclic (alkyl)(amino)carbene (CAAC) donor, inspired by our previous synthesis of the precursor [(CAAC)BeCl2] and its use to form a compound in which the formally zerovalent Be atom takes part in strong multiple bonding with carbon, [Be(CAAC)2].[41] we present the synthesis and isolation of a Lewis-base-stabilized beryllole, a heterocyclic organoberyllium compound, which was subsequently determined by spectroscopic, structural, and computational techniques to have significant antiaromaticity in its cyclo-BeC4 ring. Its facile reactivity, including chemical reduction and the addition of a second Lewis donor, provides stable, non-antiaromatic products
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