Molecular Recognition in Electron and Oxygen Transfer Reactions of Metal Complexes in Solid and in Solution

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We have shown that the isomerization between the diastereomeric pair of an optically active Schiff base-oxovanadium (IV) complex is catalyzed by the oxidized oxovanadium (V) species. The electron-transfer reaction between the oxovanadium (IV) and oxovanadium (V) complexes was suggested to have an important role in the isomerization. Chiral Schiff base-oxovanadium (IV) complexes such as [VO {sal-(RR)-chxn}] (1) have been found to be enantioselective catalysts for the asymmetric oxidation of sulfides to sulfoxides. Accordingly, we studied the electron-transfer reaction between [VO {sal-(RR)-chxn}]Cl (2) and [VO{3-MeOsal-(RR) or (SS)-chxn}] (3-a or 3-b) which gives 1 and [VO {3-MeOsal-(RR) or (SS)-chxn}] Cl (4-a or 4-b) at equilibrium.The stereoselectivity of the reaction depends on temperature. For example, 3-b reacts 2.0 times faster than 3-a at 25ﾟC,while 1.6 times at 5ﾟC.The activation parameters were DELTAH<thermodynamics>_<RR-RR>=15.2(]SY.+-.])0.6kJ mol^<-1>, DELTAS(]SY.thermod.[)_< … More RR-RR>=-70.2(]SY.+-.])1.9 J mol^<-1> K^<-1>, DELTAH(]SY.thermod.[)_<SS-RR>=21.4(]SY.+-.])2.4kJ mol^<-1>, and DELTAS(]SY.thermod.[)_<SS-RR>=-43.7(]SY.+-.])8.1 J mol^<-1> K^<-1>.Although we do not have any conclusive evidence which determines that the present reaction proceeds by the inner-sphere mechanism, the two reacting vanadium atoms must be brought sufficiently close together when the electron-transfer occurs. It should be reasonable that the oxo ligand of oxovanadium (IV) complex interacts at the position trans to the oxo ligand of the oxovanadium (V) complex because the latter prefers a six-coordinate structure. The dinuclear structure of [(salen)V^<IV>OV^VO (salen)]I_5・CH_3CN (H_2salen=N,N'-disalicyl-idene-1,2-ethanediamine) was reported, which has a V^<IV>=O・・・V^V=O bond. A similar dinuclear structure may be expected for the intermediate of the electron-transfer reaction. The chiral recognition must be due to the difference in the non-bonding steric interaction between the ligands in the two intermediate structures. Less