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 ten-membered cis-chelated ring

Abstract

The unsymmetrical Schiff-base ligand L¹(where H₂L¹ is the condensation product of 8-amino-4-methyl-5-azaoct-3-en-2-one and 3-ethoxysalicylaldehyde) has been found to rearrange into the symmetrical ligand L²[H₂L²=N,N′-bis(3-ethoxysalicylidene)propane-1,3-diamine] in the presence of manganese(III). The manganese(III) complexes of both the unsymmetrical and symmetrical ligands have been characterised by elemental analyses, infrared spectroscopy, magnetic susceptibility measurements and 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 interesting phenomenon, with two distinct metal co-ordination environments being 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 at m/z 357 corresponds to the monomeric [ML¹]⁺ ion with the amine nitrogen atom of the ligand deprotonated. The manganese(III) complex of the unsymmetrical ligand [Mn(HL¹)₂][ClO₄] has been crystallographically characterised, no water of crystallisation being observed in the selected crystal. This monomeric complex consists of a pseudo-octahedral metal 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 Å, produces a ten-membered chelate ring about the manganese ion, believed to be the first example of such a system. The crystal structure has enabled a mechanism for the rearrangement of Schiff bases about manganese(III) to be proposed. It involves a series of reversible hydrolysis reactions which are stabilised by manganese(III) acting as a template.