A Novel Pentacoordinated Dioxovanadium(V) Salicylaldiminate: Solvent Specific Crystallization of Dimorphs with Contrasting Coordination Geometries, Ligand Conformations and Supramolecular Architectures

摘要

The reaction of the tridentate salicylaldimine of 2-picolylamine, C_6H_4(OH)CHNCH_2C_5H_4N (HL), with bis(acetylacetonato)oxovanadium(IV) at room temperature under oxygen (or with VOSO_4·5H_2O plus H_2O_2) furnished crystalline greenish-yellow α-VO_2(L) and green β-VO_2(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 ~(51)V NMR). Both forms react with 3,5-bis(tert-butyl)catechol (H_2dbcat) furnishing the known pseudooctahedral complex VO(L)(dbcat). In the form the V(L) fragment is planar and the VO_3N_2 coordination sphere is distorted trigonal bipyramidal (τ = 0.55) with the two oxo oxygen atoms and the azomethine nitrogen atom defining the trigonal plane. In β-VO_2(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 (τ = 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 α form. When the polarizability is relatively high, as in acetonitrile, the β form crystallizes. The geometrically isomeric coordination spheres of the α 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 VO_2(L) in the same manner as for VO_2(L).