Unexpected conformational conse- quences of weak hydrogen bonds on 1,3,7,9,13,15,19,21-octaazapenta- cyclo[19.3.1.1.1.1]octa- cosane monohydrate

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Author	Rivera, Augusto Ríos-Motta, Jaime Dušekb, Michal Jarošováb, Markéta
Copyright Year	2010
Abstract	Polyaza macrocyclic compounds with tetrahedrally arranged nitrogen lone-pair orbitals (LPOs) have been exploited for complexation with ions (Mazloum et al., 2002). Some of these compounds have attracted intense attention due to their molecular diamond lattice conformation. In fact, both theoretical (Galasso et al., 2001) and experimental studies (Dale et al., 1991, 1992) of the molecular structures and spectroscopic properties of this class of compounds have been reported. The results of these studies revealed that 1,3,7,9,13,15,19,21-octaazapentacyclo[19.3.1.1.1.1]octacosane exists in two conformations, one in which the eight N atoms are diequatorially substituted (D2d symmetry) and one with an equatorial–axial substitution in each 1,3-diazane ring (S4 symmetry). Density functional theory (DFT) calculations (Galasso et al., 2001) showed that the S4 conformer is more stable than the D2d conformer by 2.6 kcal mol 1 (1 kcal mol 1 = 4.184 kJ mol ), due to dipole–dipole repulsion between the axial lone pairs of the N atoms. A previous X-ray structural investigation of a 1:1 complex of 1,3,7,9,13,15,19,21-octaazapentacyclo[19.3.1.1.1.1]octacosane and benzene showed that the 16-membered pentacyclooctaaza ring exists in a less symmetric but more stable conformation (Murray-Rust, 1975). However, the most favourable conformation is avoided when 1,3,7,9,13,15,19,21octaazapentacyclo[19.3.1.1.1.1]octacosane crystallizes as a 1:2 dichloromethane complex (Dale et al., 1991), where the D2d conformation exhibits a syn-axial orientation of the Natom lone pairs, showing the so-called rabbit-ear effect (Hutchins et al., 1968). Repulsion is avoided due to the presence of intermolecular interactions between the N atoms and the H atoms of the dichloromethane molecules (Dale et al., 1991).
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