Tetra- and dinuclear manganese complexes of xanthene-bridged O,N,O-Schiff bases with 3-hydroxypropyl or 2-hydroxybenzyl groups: ligand substitution at a triply bridging site.

Complexation properties of U-shaped ligands, L and L, which are Schiff bases of 5,5'-(9,9-dimethylxanthene-4,5-diyl)bis(salicylaldehyde) (Hxansal) with 3-amino-1-propanol or 2-hydroxybenzylamine, respectively, were investigated to construct polynuclear manganese complexes. In these ligands, two O,N,O-Schiff bases are bridged by a xanthene backbone. The reactions of HL or HL with manganese salts afforded tetra- and dinuclear manganese complexes, including the tetramanganese(ii,ii,iii,iii) complex [Mn(L)(μ-OAc)] with a MnO core exhibiting an incomplete double-cubane structure. In the MnO core, phenolate and alkoxide O atoms bridge the manganese ions. Deprotonated 3-hydroxypropyl groups were crucial to the assembly of four manganese ions because the phenolate-bridged dimanganese(iii,iii) complex [Mn(HL)] was obtained in the absence of a base, and HL, which has 2-hydroxybenzyl groups instead of 3-hydroxypropyl groups in HL, afforded the cyclic dimanganese(iv,iv) complex [Mn(L)]. We disclosed that [Mn(L)(μ-OAc)] was converted to the oxo-bridged tetramanganese(iii,iii,iii,iii) complex [Mn(L)(HL)(μ-O)(μ-OAc)] by treating with NHPF or NHBF: a triply bridging alkoxide was protonated and replaced by an oxide ligand. The cyclic voltammograms of [Mn(L)(HL)(μ-O)(μ-OAc)] suggested that the reverse reaction forming [Mn(L)(μ-OAc)] occurred in the electrochemical processes and was assisted by protonation.