A Novel pentacoordinated dioxovanadium(V) salicylaldiminate: solvent specific crystallization of dimorphs with contrasting coordination geometries, ligand conformations and supramolecular architectures

Abstract

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 VOSO₄·5H₂O plus H₂O₂) furnished crystalline greenish-yellowα -VO₂(L) and green β-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, ¹H and ⁵¹V NMR). Both forms react with 3,5-bis(tert-butyl)catechol (H₂dbcat) furnishing the known pseudooctahedral complex VO(L)(dbcat). In the α form the V(L) fragment is planar and the VO₃N₂ coordination sphere is distorted trigonal bipyramidal (t = 0.55) with the two oxo oxygen atoms and the azomethine nitrogen atom defining the trigonal plane. In β-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 (t = 0.20) with the three donor sites of the L ligand and an oxo oxygen atom defining the tetragonal plane. Both the α and β crystal lattices consist of dimers which associate further to form supramolecules. The noncovalent intradimer and interdimer bonding interactions are predominantly of aromatic π-π stacking (aryl···pyridyl and pyridyl···pyridyl) and C-H···O hydrogen bonding (azomethine···oxo and aryl···phenolato) types in the α and β dimorphs, respectively. It is proposed that the low polarizability of methanol promotes π-π stacking of planar V(L) fragments, resulting in crystallization of the form. When the polarizability is relatively high, as in acetonitrile, the β form crystallizes. The geometrically isomeric coordination spheres of the and β 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).