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, des...