Chiral Schiff base complexes of copper(II), vanadium(IV) and nickel(II) as oxidation catalysts.: X-ray crystal structures of [Cu(R-salpn)(OH2)] and [Cu(±-busalcx)]

Several new complexes of chiral and achiral tetradentate Schiff bases with copper(II), nickel(II) and oxovanadyl(IV) ions have been synthesised and characterised and the structures of [Cu(R-salpn)(OH2)] and [Cu(+/- -busalcx)] have been elucidated by X-ray analysis. [Cu(R-salpn)(OH2)] is essentially square pyramidal with an unusually long bond [2.494(4) Angstrom] from Cu to the oxygen of the apical water molecule. Molecules are linked by hydrogen bonding between the coordinated water and the salpn oxygens of neighbouring molecules. A long contact between a salpn CH, hydrogen and a neighbouring copper atom [H(3B)... Cu = 2.966(1) Angstrom] might be regarded as a CH ... Cu hydrogen bond and completes a pseudo-octahedral coordination about copper. [Cu(+/- -busalcx)] displays tetrahedrally-distorted planar coordination about the copper atom and has no significant intermolecular contacts. The copper, nickel and vanadyl complexes were screened as homogeneous catalysts for the oxidation by (BuOOH)-O-t or H2O2 of PhMeS to PhMeSO. ALL of them are active catalysts but of the chiral complexes only [Cu(R,R-busalcx)] produced a reasonable enantiomeric excess (14%). Supporting the complexes on silica improved the enantioselectivity of the less sterically hindered [Cu(R-salpn)(OH2)] (from 2 to 10%), but diminished that of the more hindered [Cu(R,R-busalcx)], probably by slowing the reaction to such an extent that the uncatalysed oxidation could compete effectively. Finally [Cu(salen)], [Cu(R-salpn)] and [Ni(R-salpn)] were synthesised in the pores of zeolites X and Y. These were shown to be active heterogeneous catalysts for the sulfide to sulfoxide oxidations, but no significant e.e. resulted from the use of the chiral catalysts. (C) 1998 Elsevier Science Ltd. All rights reserved.