A novel pentacoordinated dioxovanadium(v) salicylaldiminate: Solvent specific crystallization of dimorphs with contrasting coordination geometries, ligand conformations and supramolecular architectures

The reaction of the tridentate salicylaldimine of 2-picolylamine, C6H4(OH)CHNCH2C5H4N (HL), with bis(acetylacetonato)oxovanadium(iv) at room temperature under oxygen (or with VOSO4.5H(2)O plus H2O2) furnished crystalline greenish-yellow alpha-VO2(L) and green beta-VO2(L) from methanol (or ethanol) and acetonitrile (or dichloromethane), respectively. The dimorphs give rise to the same solution species as revealed by spectroscopic data (UV/Vis, H-1 and V-51 NMR). Both forms react with 3,5-bis(tert-butyl)catechol (H2dbcat) furnishing the known pseudooctahedral complex VO(L)(dbcat). In the a form the V(L) fragment is planar and the VO3N2 coordination sphere is distorted trigonal bipyramidal (tau = 0.55) with the two oxo oxygen atoms and the azomethine nitrogen atom defining the trigonal plane. In beta-VO2(L) the V(L) moiety, like that in VO(L)(dbcat), is not planar but consists of two planar parts rotated away from each other along the C-N single bond. In this case the coordination sphere is distorted square pyramidal (tau = 0.20) with the three donor sites of the L ligand and an oxo oxygen atom defining the tetragonal plane. Both the a and P crystal lattices consist of dimers which associate further to form supramolecules. The noncovalent intradimer and interdimer bonding interactions are predominantly of aromatic pi-pi stacking (aryl(...)pyridyl and pyridyl(...)pyridyl) and C-(HO)-O-... hydrogen bonding (azomethine(...)oxo and aryl(...)phenolato) types in the alpha and beta dimorphs, respectively. It is proposed that the low polarizability of methanol promotes pi-pi stacking of planar V(L) fragments, resulting in crystallization of the a form. When the polarizability is relatively high, as in acetonitrile, the beta form crystallizes. The geometrically isomeric coordination spheres of the a and P forms are sustained by the respective supramolecular architectures. The Schiff base HL' of 2-hydroxynapthaldehyde and 2-picolylamine has also afforded two forms Of VO2(L') in the same manner as for VO2(L). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)