Polymer-bound oxidovanadium(IV) and dioxidovanadium(V) complexes:synthesis, characterization and catalytic application for the hydroamination ofstyrene and vinyl pyridine

摘要

The Schiff base (Hfsal-aepy) derived from 3-formylsalicylic acid and 2-(2-aminoethyl)pyridine has beencovalently bonded to chloromethylated polystyrene cross-linked with 5% divinylbenzene(PS-Hfsal-aepy). Treatment of [V~(IV)O(acac)_2] with PS-Hfsal-aepy in dimethylformamide (DMF) gavethe oxidovanadium(IV) complex PS-[V~(IV)O(fsal-aepy)(acac)] 1, which on oxidation yielded thedioxidovanadium(V) PS-[V~VO_2(fsal-aepy)] 2 complex. The corresponding neat complexes,[V~(IV)O(sal-aepy)(acac)] 3 and [V~VO_2(sal-aepy)] 4 have also been prepared. The compounds arecharacterized in solid state and in solution, namely by spectroscopic techniques (IR, UV-Vis, EPR, ~1H,~(13)C and ~(51)V NMR), thermal as well as field-emission scanning electron micrograph (FE-SEM) studies.The crystal and molecular structure of [V~(IV)O(sal-aepy)(acac)] was solved by single-crystal X-raydiffraction. It is a monomeric complex with the tridentate sal-aepy ligand bound equatorially and thetwo 0-atoms of acac~- bound at equatorial and axial positions. These complexes catalyze thehydroamination of styrene and vinyl pyridine with amines (aniline and diethylamine) yielding a mixtureof two hydroaminated products in good yield. Amongst the two hydroaminated products, theanti-Markovnikov product is favored over the Markovnikov one. Plausible intermediates involved inthese catalytic processes are established by UV-Vis, EPR and ~(51)V NMR studies, and an outline of themechanism is proposed. The EPR spectrum of the polymer supported V~(IV)O-complex 1 is characteristicof a magnetically diluted V~(IV)O-complex, the resolved EPR pattern indicating that the oxidovanadium(IV) centers are well dispersed in the polymer matrix. Neat complexes exhibit lowerconversion along with lower turnover frequency as compared to their polymer-anchored analogues.The polymer-anchored heterogeneous catalysts are free from leaching during catalytic action and arerecyclable.