The influence of exocyclic lone pairs on the bonding and geometry of type A mesoionic rings

Abstract

Based on structures determined by X-ray crystallography, ab initio MP2 calculations on type A mesoionic rings give geometries in good agreement with observed values. A study of four mesoionic ring systems, each with exocyclic oxygen, nitrogen or carbon groups, shows that the presence and configuration of exocyclic lone pairs significantly influences the geometry and configurational preference. Using a localised bond model and NBO analysis, these effects are rationalised in terms of an anomeric interaction of lone pairs with the antibonding orbitals of adjacent σ bonds. In agreement with experiment, similar effects are calculated for pyran-2-imines.

Synthesis of mesoionic triazolones via a formal [3+2] cycloaddition between 4-phenyl-1,2,4-triazoline-3,5-dione and alkynes

“…19 Such an interaction was further supported by recent calculations by Oziminski and Ramsden. 20 Indeed, natural bond orbital (NBO) analysis of 3a unveiled a conspicuous n–σ* hyperconjugation (44.7 kcal mol −1 ) between the unshared electron pair on the carbonyl O1 atom and the antibonding orbital of the C1–N1 bond, corroborating the structural deformations observed in 3a .…”

Mesoionic compounds: the role of intermolecular interactions in their crystalline design

“…Compound 18 was characterized by unidimensional 1 H and 13 C NMR spectroscopy ( 1 H at 600.130 MHz and 13 C at 150.903 MHz) performed in a Bruker Advance III spectrometer at 298 K using TMS as an internal reference in CDCl 3 as the solvent at 298 K. The spectrum data are in the ESI † (Fig. S1 and S2).…”

“…In structural chemistry, mesoionic compounds are the subject of numerous structural studies using experimental/theoretical data to shed more light on their electronic structures. [10][11][12][13][14][15][16][17] Despite all these studies, there are few supramolecular data obtained by a systematic approach to experimental data on the crystal structures of mesoionic compounds.…”

Mesoionic Imines (MIIs): Strong Donors and Versatile Ligands for Transition Metals and Main Group Substrates

“…The electronic properties go beyond the intriguing formulation of structures “on paper” as the mesoionic character strongly influences many of the investigated physical properties. In this regard, quantum chemical calculations, [40] crystallographic data, [40] heteronuclear NMR studies [30, 41] and intense colouring [42] of the compounds point to the mesoionic property. Applications in coordination chemistry of MIIs besides the “classical” and non‐mesoionic NHI has been to the best of our knowledge not reported yet.…”