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(oimBut)], 1-methyl-1,3-dihydro-2H-benzimidazol-2-one [H­(obenzimMe)], 1-t-butyl-1,3-dihydro-2H-benzimidazol-2-one [H­(obenzimBut)], and 1-t-butyl-1,3-dihydro-2H-benzimidazole-2-thione [H­(mbenzimBut)] 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(ximBut) 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.