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Formation in Solution, Synthesis, and Electrochemical Study of Oxalato Complexes of N,N'-Ethylenebis(salicylideneiminato)-chromium(III) and -iron(III): Crystal Structures of Piperidinium (oxalato-O1O2)-chromate(III) and ferrate(III)

Two new mononuclear complexes of formula and the binuclear *H2O, (3), where Hpip=piperidinium, salen=N,N'-ethylenebis(salycilideneiminate), and ox=oxalate, have been synthesized.Compounds (1) and (2) are isostructural, monoclinic, space group P21/n, Z=4, with a=24.425(3), b=6.847(1), c=14.271(2) Angstroem, and β=100.95(2) deg for (1) and a=24.363(4), b=6.991(2), c=14.105(3) Angstroem, and β=98.76(2) deg for (2).The stucture of (1) was solved by direct methods whereas that of (2) was solved by isomorphous replacement from the co-ordinates of (1).Both structures consist of (1-) mononuclear anions and piperidinium cations.The presence of the bidentate oxalate ligand in both complexes forces the salen ligand to adopt the non-planar cis-β configuration.The metal ions exhibit distorted octahedral geometry with the two co-ordinated oxygen atoms of the oxalate ligand and an oxygen and a nitrogen atom from the salen defining the best equatorial plane.The remaining two co-ordinating atoms of the quadridentate Schiff base are bent away from the oxalate ligand.The stability constant of the complex (1-) as well as the acidity constants of the complex (1+) have been determined by potentiometry in aqueous solution: log β1=4.80+/-0.03, pKa1=7.54+/-0.01, and pKa2=10.47+/-0.01 (25 deg C, 0.1 mol dm-3 NaNO3).Complexes (1) and (2) undergo one-electron reduction at a platinum electrode in dimethyl sulphoxide solution.The reduction process is totally irreversible due to an inner-sphere redox reaction in the case of Cr(III) and to the dissociation of the anionic oxalate ligand in the case of Fe(III).A reactivity scheme is proposed to explain their different electrochemical behaviour.