Formation in solution, synthesis, and electrochemical study of oxalato complexes of N,N′-ethylenebis(salicylideneiminato)-chromium(III) and -iron(III): crystal structures of piperidinium [N,N′-ethylenebis(salicylideneiminato)](oxalato-O1O2)-chromate(III) and ferrate(III)

Abstract

Two new mononuclear complexes of formula [Hpip][M(salen)(ox)][M = Cr^III(1) or Fe^III(2)] and the binuclear [Fe₂(salen)₂(ox)]·H₂O, (3) where Hpip = piperidinium, salen =N,N′-ethylenebis(saIicylideneiminate), 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)Å, and β= 100.95(2)° for (1) and a= 24.363(4), b= 6.991(2), c= 14.105(3)Å, and β= 98.76(2)°for (2). The structure 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 [M (salen)(ox)]^– 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 [Cr(salen)(ox)]^– as well as [Cr(salen)(H₂O)₂]⁺+ ox^2– [graphic omitted] [Cr(salen)(ox)]^–(i), [Cr(salen)(H₂O)₂]⁺ [graphic omitted] [Cr(salen)(OH)(H₂O)]+ H⁺(ii), [Cr(salen)(OH)(H₂O)] [graphic omitted] [Cr(salen)(OH)₂]^–+ H⁺(iii) the acidity constants of the complex [Cr(salen)(H₂O)₂]⁺ have been determined by potentiometry in aqueous solution: log β₁= 4.80 ± 0.03, pK_a1= 7.54 ± 0.01, and pK_a2= 10.47 ± 0.01 (25 °C, 0.1 mol dm³ 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.