How far is the π-electron delocalization of the phenanthrene moiety modified in the aza-analogues and their N-oxides?

Abstract

The consequences of the replacement of a CH fragment of phenantherene by N-aza, N-N-diaza and/or N-oxide groups for the cyclic π-electron delocalization are discussed. Despite substantial structural changes, the π-electron structure is modified only moderately. The aza-analogues usually exhibit more effective π-electron delocalization, while the N-oxidation leads to its decrease. Usually the presence of the nitrogen atom or N-O group lead to more efficient delocalization in neighboring fragment(s). The analysis is based on experimental geometries of four phenanthroline N-oxides and N,N-dioxides (4,7-phenanthroline 4-oxide, 1,7-phenanthroline 1,7-dioxide, 1,7-phenanthroline 7-oxide and 1,8-phenanthroline 1,8-dioxide) and is completed by the geometry optimizations with use of the DFT method at B3LYP/6-311G* * level of theory.