Synthesis, Characterization, and Application of Vanadium−Salan Complexes in Oxygen Transfer Reactions

We report the synthesis and characterization of several chiral salen- and salan-type ligands and their vanadium complexes, which are derived from salicylaldehyde or salicylaldehyde derivatives and chiral diamines (1R,2R-diaminocyclohexane, 1S,2S-diaminocyclohexane, and 1S,2S-diphenylethylenediamine). The structures of H2sal(R,R-chan)2+·2Cl−·(CH3)2CHOH·H2O (1c; H2sal(R,R-chan) = N,N′-salicyl-R,R-cyclohexanediaminium), Etvan(S,S-chen) (3c; Etvan(S,S-chen) = N,N′-3-ethoxy-salicylidene-S,S-cyclohexanediiminato), and naph(R,R-chen) (6c; naph(R,R-chen) = N,N′-naphthylidene-R,R-cyclohexanediiminato) were determined by single-crystal X-ray diffraction. The corresponding vanadium(IV) complexes and several other new complexes involving different salicylaldehyde-type precursors were prepared and characterized in the solid state and in solution by spectroscopic techniques: UV−vis, circular dichroism, electron paramagnetic resonance, and 51V NMR, which provide information on the coordination geometry. The salan complexes oxidize in organic solvents to VV species, and this process was also studied using spectroscopic techniques. Single crystals suitable for X-ray diffraction were obtained for [{VVO[sal(S,S-dpan)]}2(μ-O)]·H2O·2(CH3)2CHOH (14c; sal(S,S-dpan) = N,N′-salicyl-S,S-diphenylethylenediaminato) and [{VVO[t-Busal(R,R-chan)]}2(μ-O)]·2(CH3)2CHOH (15c), both containing an OVV(μ-O)VVO moiety (V2O3 4+ core) with tetradentate ligands and one μ-oxo bridge. Both structures are the first examples of dinuclear vanadium complexes involving the VV 2O3 4+ core with tetradentate ligands, the configuration of the V2O3 unit being twist-angular. The V−salen and V−salan complexes are tested as catalysts in the oxidation of styrene, cyclohexene, cumene, and methyl phenyl sulfide with H2O2 and t-BuOOH as oxidants. Overall, the V−salan complexes show higher activity and normally better selectivity in alkene oxidation and higher activity and enantioselectivity for sulfoxidation than their parent V−salen complexes, therefore being an advantageous alternative ligand system for oxidation catalysis. The better performance of V−salan complexes probably results from their significantly higher hydrolytic stability. Mechanisms for the alkene oxidation with these newly obtained V−salan compounds are discussed, including the use of DFT for the comparison of several alternative mechanisms for epoxidation.