Structural characterization of 2-imidazolones: Comparison with their heavier chalcogen counterparts

The molecular structures of 1-t-butyl-1,3-dihydro-2H-imidazol-2-one [H(oim^But)], 1-methyl-1,3-dihydro-2H-benzimidazol-2-one [H(obenzim^Me)], 1-t-butyl-1,3-dihydro-2H-benzimidazol-2-one [H(obenzim^But)], and 1-t-butyl-1,3-dihydro-2H-benzimidazole-2-thione [H(mbenzim^But)] have been determined by single crystal X-ray diffraction. Consideration of the C-O bond lengths in the 2-imidazolones, together with the respective values for 2-thiones and 2-selones, indicates that the C-E bonds in these compounds are intermediate between those of formal C-E single and double bonds, an observation that may be rationalized in terms of a significant contribution of zwitterionic structures that feature single C ⁺-E^- dative covalent bonds. In this regard, a natural bond orbital (NBO) analysis of the bonding in H(xim^But) derivatives demonstrates that a doubly bonded C=E resonance structure is most significant for the oxygen derivative, whereas singly bonded C⁺-E^- resonance structures are dominant for the tellurium derivative, despite the fact that oxygen is more electronegative. The C-E bonding in these compounds is, therefore, significantly different from that in chalcogenoformaldehyde derivatives for which the bonding is well represented by a H₂C=E double bonded resonance structure. Comparison of the C-E bond lengths of the imidazolechalcogenones with those of C-E single bonds indicates that the C-O bonds are anomalously short. This observation may be rationalized in terms of the oxygen derivative having not only the most significant π-component but also a substantial ionic component. The latter results from the C-O bond being the most polar due to a substantial polarization of the σ-bond in the direction of oxygen, which thereby supplements the π-polarization and increases the negative charge on oxygen. In contrast, the σ-polarization for the heavier chalcogens opposes the zwitterionic C⁺-E^- π-polarization and thereby reduces the negative charge on the chalcogen. As such, the C-E bond becomes less polar as the chalcogen becomes heavier, despite the fact that the zwitterionic C⁺-E^- contribution increases.