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

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.