Molecular Design of Neutral Intramolecular Complexes Bearing Two Silicon Atoms Anchored by a Carbonyl Oxygen Atom: N,N′-Bis(silylmethyl)propylene Ureas

• Published in: Chemistry : a European journal Vol. 12; no. 7; pp. 2021 - 2031
• Main Authors: Sidorkin, Valery F., Belogolova, Elena F., Pestunovich, Vadim A.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 20.02.2006; WILEY‐VCH Verlag
• Subjects: ab initio calculations; bond nature; chelates; density functional calculations; ELF (electron localization function); silicon; substituent effects
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200500498

B3LYP and MP2 computations have been performed on a variety of Si,Si′‐substituted N,N′‐bis(silylmethyl)propylene ureas. According to electron‐density atoms‐in‐molecules (AIM) and electron localization function (ELF) quantum‐topological analyses, a transition from the unstable non‐chelate forms of these compounds to mono‐ and bis‐chelate forms results in the successive interaction of one and two tetracoordinate silicon atoms with the carbonyl oxygen and the formation, respectively, of one and two covalent, polar Si⋅⋅⋅O bonds. This previously unknown XSi←O→SiX type of bonding in isomers possessing an anchor structure may be classified as a five‐center, six‐electron (5c‐6e) bond. The factors that favor the existence of Si,Si′‐substituted N,N′‐bis(silylmethyl)propylene ureas exclusively in the form of stable, bridged complexes (the size of equatorial ligands and the electronegativity of axial substituents at the silicon atom, change in the donor capability of the carbonyl group, and effect of the polar solvent) are discussed. An insight into the nature of the O⋅⋅⋅Si coordinate bonds in isomers 1 b and 1 c of Si,Si′‐substituted N,N′‐bis(silylmethyl)propylene ureas (1; X, Y=Br, Cl, F, H, OH, Me) has been achieved by using electron localization function (ELF) analysis, which revealed differences in the sizes and characters of the domains of the electron pairs of the oxygen atom in 1 a–c.