Reaction of the non-innocent dinucleating ligand 2,5-bis[N,N-bis(carboxymethyl) aminomethyl]hydroquinone (H(6)bicah) with VO2+ and VO43- salts in water in the pH range 2 to 4.5 provides a series of novel tetranuclear V-IV and/or V-V macrocycles with the main core consisting of the anions [(V4O4)-O-V(mu-O)(2)(mu-bicah)(2)](4-) isolated at pH = 2.5 and [V(1V)2V(2)(V)O(4)(mu-O)(2)(mu-bicas)(mu-bicah)](5-) and [(V4O4)-O-IV(mu-O)(2)(mu-bicas)(2)](6-) isolated at pH = 4.5 (bicas(center dot 5-) = 2,5-bis[N,N-bis(carboxymethyl) aminomethyl]-p-semiquinonate), whereas at pH = 2 the dinuclear [((VO)-O-IV)(2)(OH2)(2)(mu-bicah)](2-)was obtained. All vanadium compounds have been characterized, and the charge of the ligand has been assigned in solid state by X-ray crystallography and infrared spectroscopy. The structures of the tetranuclear anions consist of four vanadium atoms arranged at the corners of a rectangle with the two bridging bicas(center dot 5-) and/or bicah(6-) ligands on the long and the two V-IVN-O-V-IVN bridges on the short sides of the rectangle. UV-vis, V-51 and H-1 NMR spectroscopy and electrochemistry showed that these complexes interconvert to each other by varying the pH. This pH induced redox transformation of the tetranuclear anions has been attributed to the shift of the reduction potential of the bicas(center dot 5-) to higher values by decreasing the pH. The electron is transferred intramolecularly from the metal ion to the electron accepting semiquinones resulting in reduction of bicas(center dot 5-) to bicah(6-) and concurrent oxidation of the V-IV to V-V. The resulting complexes are further oxidized by atmospheric oxygen. This system as a model for the H+ coupled redox reactions in metalloenzymes and its relevance is discussed briefly.