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.