A MECHANISM FOR THE REARRANGEMENT OF UNSYMMETRICAL TETRADENTATE (N2O2) LIGANDS BOUND TO MANGANESE(III) - THE ISOLATION AND CRYSTAL-STRUCTURE OF A MANGANESE(III) COMPLEX CONTAINING A 10-MEMBERED CIS-CHELATED RING

The unsymmetrical Schiff-base ligand L1 (where H2L1 is the condensatio n product of 8-amino-4-methyl-5-azaoct-3-en-2-one and 3-ethoxysalicyla ldehyde) has been found to rearrange into the symmetrical ligand L2 [H 2L2 = N,N'-bis(3-ethoxysalicylidene)propane-1,3-diamine] in the presen ce of manganese(III). The manganese(III) complexes of both the unsymme trical and symmetrical ligands have been characterised by elemental an alyses, infrared spectroscopy, magnetic susceptibility measurements an d fast atom bombardment (FAB) mass spectrometry and have been assigned the stoichiometries [Mn(HL1)2][ClO4].H2O and (MnL2][ClO4].3H2O based on these data. The FAB mass spectrum of the former exhibits an interes ting phenomenon, with two distinct metal co-ordination environments be ing observed. The ion of [Mn(HL1)2]+ at m/z 661 is observed, as would be expected from the experimental data; however, the parent ion peak a t m/z 357 corresponds to the monomeric [ML1]+ ion with the amine nitro gen atom of the ligand deprotonated. The manganese(III) complex of the unsymmetrical ligand [Mn(HL1)2][ClO4] has been crystallographically c haracterised, no water of crystallisation being observed in the select ed crystal. This monomeric complex consists of a pseudo-octahedral met al centre with an N2O4 donor environment. The non-co-ordination of the amine nitrogen atom, confirmed by the Mn ... N(2) separation of 3.51 angstrom, produces a ten-membered chelate ring about the manganese ion , believed to be the first example of such a system. The crystal struc ture has enabled a mechanism for the rearrangement of Schiff bases abo ut manganese(III) to be proposed. It involves a series of reversible h ydrolysis reactions which are stabilised by manganese(III) acting as a template.