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, L¹ and L², which are Schiff bases of 5,5′-(9,9-dimethylxanthene-4,5-diyl)bis(salicylaldehyde) (H₂xansal) 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 H₄L¹ or H₄L² with manganese salts afforded tetra- and dinuclear manganese complexes, including the tetramanganese(ii,ii,iii,iii) complex [Mn₄(L¹)₂(μ-OAc)₂] with a Mn₄O₆ core exhibiting an incomplete double-cubane structure. In the Mn₄O₆ 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 [Mn₂(H₂L¹)₂]²⁺ was obtained in the absence of a base, and H₄L², which has 2-hydroxybenzyl groups instead of 3-hydroxypropyl groups in H₄L¹, afforded the cyclic dimanganese(iv,iv) complex [Mn₂(L²)₂]. We disclosed that [Mn₄(L¹)₂(μ-OAc)₂] was converted to the oxo-bridged tetramanganese(iii,iii,iii,iii) complex [Mn₄(L¹)(HL¹)(μ₃-O)(μ-OAc)₂]⁺ by treating with NH₄PF₆ or NH₄BF₄: a triply bridging alkoxide was protonated and replaced by an oxide ligand. The cyclic voltammograms of [Mn₄(L¹)(HL¹)(μ₃-O)(μ-OAc)₂]⁺ suggested that the reverse reaction forming [Mn₄(L¹)₂(μ-OAc)₂] occurred in the electrochemical processes and was assisted by protonation.