Helical Oxidovanadium(IV) Salen-Type Complexes: Synthesis, Characterisation and Catalytic Behaviour

• Published in: European journal of inorganic chemistry 2013-11, Vol.2013 (33), p.5708-5717
• Main Authors: Barman, Sanmitra, Patil, Smita, Desper, John, Aikens, Christine M., Levy, Christopher J.
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Catalytic oxidation; Circular dichroism; Dichroism; Electronics; Helical structures; Ligands; Metallofoldamers; Schiff base ligands; Solvents; Spectrum analysis; Synthesis; Synthesis (chemistry); Vanadium
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201300635

Chiral [VO(salen)] complexes (R,R)‐7 and (R,R)‐8 were synthesised by ligand replacement from [VO(acac)2] with the resolved (1R,2R)‐cyclohexyl ligands 3,3′‐[(1R,2R)‐1,2‐cyclohexane‐diylbis(nitrilomethylidyne)]bis‐4‐phenanthrenol [(R,R)‐3] and 3,3′‐[(1R,2R)‐1,2‐cyclohexanediylbis(nitrilomethylidyne)]bisbenz[a]anthracen‐1‐ol [(R,R)‐4], respectively. Similarly, complexes (R)‐9 and (R)‐10 were synthesised using the (R)‐binaphthyl ligands 3,3′‐[(1R)‐(1,1′‐binaphthalene)‐2,2′‐diylbis(nitrilomethylidyne)]bis‐4‐phenanthrenol [(R)‐5] and 3,3′‐[(1R)‐(1,1′‐binaphthalene)‐2,2′‐diylbis(nitrilomethylidyne)]bis‐benz[a]anthracen‐1‐ol [(R)‐6], respectively. These complexes were characterised by IR spectroscopy, ESI‐TOF‐MS, electronic absorption spectroscopy and circular dichroism (CD). Single crystal X‐ray analysis of (R,R)‐7 and (R,R)‐8 revealed distorted square pyramidal geometries and a 1:1 ratio of diastereomeric M and P helical conformers. Solution CD studies in THF combined with DFT calculations indicate that the M conformer dominates in solution for (R,R)‐7. Preliminary catalytic oxidation of thioanisole with H2O2 and 1 % [VO(salen)] showed good selectivity for sulfoxides over sulfones but low enantioselectivities (8 to 33 %) which are solvent dependent but insensitive to the catalyst used. Chiral helical [VO(salen)] complexes have been synthesised, characterised and examined as catalysts for the asymmetric sulfoxidation of thioanisole with 30 % hydrogen peroxide.