Aromaticity Suppression by Intermolecular Coordination. Optical Spectra and Electronic Structure of Heavy Carbene Analogues with an Amidophenolate Backbone

Experimental optical (UV–vis,
Raman) and computational magnetic
(NICS, GIMIC) criteria were used to study the aromaticity of group
14 N,O-heterocyclic carbenes tBuAPE (AP = amidophenolate,
E = Ge, Sn, Pb). The UV–vis and Raman spectra of the monomeric
compound with E = Ge were shown to differ significantly from those
of dimeric compounds with E = Sn, Pb. The reason for these phenomena
is suppression in the dimeric molecules of 10 π-electron aromaticity
due to partial filling of the vacant pz orbital of the EII atom caused by intermolecular coordination
N→EII. The monomeric forms of all molecules studied
are aromatic according to all the criteria used.