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.