The structure and aromaticity of lithium azaphospholide complexes

The structure and aromaticity of lithium-1,3-azaphospholide molecules have been investigated by ab initio methods using HF and MP2/6-311 ++ G(2d,p) le vels of theory. Twelve stable isomers have been found on the potential ener gy surface. The most important factor of stability is the coordination of p hosphorus: molecules 1-5 containing sigma (2),lambda (3)-phosphorus (type I ) are considerably more stable than the sigma (2),lambda (3)-derivatives 6- 12 (type II). The most stable isomer is the lithium eta (5)-complex 1. Isom ers containing divalent carbon atoms are the least stable derivatives. Thre e different aspects of the aromaticity have been studied, the molecular geo metry, the Bird and the NICS indices. Although 3H-azaphosphole is not aroma tic, complex formation enhances the aromaticity and most of the correspondi ng type II Li-complexes are highly aromatic (8-10,12). No correlation can b e found between the position of lithium, the aromaticity and the stability of the molecules. (C) 2001 Elsevier Science B.V. All rights reserved.