A Family of Mononuclear Manganese(IV) Complexes: An Mn < sup > IV < /sup > O < inf > 4 < /inf > N < inf > 2 < /inf > Sphere Assembled via Phenolate–Imine–Carboxylate Coordination

Abstract

Manganese(IV) complexes (MnL2) of tridentate Schiff bases (H2L) carrying a carboxylate function are described. The specific ligands H2als, H2ans, and H2ann are derived from salicylaldehyde and β-alanine, salicylaldehyde and anthranilic acid, and 2-hydroxynaphthaldehyde and anthranilic acid, respectively. The reaction of H2L with manganese(III) acetate dihydrate affords KMnL2•H2O, which upon chemical (S2O82−, Ag+) or electrochemical (0.75 V vs SCE) oxidation in methanol affords MnL2 in high yields. The structure of Mn(als)2 has been determined (R = 0.0438 and Rw = 0.0581) by X-ray crystallography: space group C2/c;Z = 4; a = 17.314 (5) Å, b = 7.884 (3) Å, c = 13.104 (3) Å, β = 91.86 (2)°, V= 1787.8 (9) Å3. In the octahedral MnO4N2 coordination sphere each ligand acts as a meridional ONO donor utilizing phenolic (Mn–O = 1.860 (2) Å) and carboxylate oxygen (Mn–O = 1.907 (2) Å) atoms and the azomethine nitrogen (Mn–N = 2.004 (3) Å) atom. Each salicylaldimine fragment is planar, but the manganese(IV) atom is displaced by 0.64 Å from it. The β-alanine residue forms a chelate ring with a boatlike appearance, the dimethylene fragment being in gauche configuration. The magnetic moments (3.96–4.13 μB) of the MnL2 complexes correspond to the d3 configuration. Their EPR spectra (77 K) consist of strong and weak signals near g = 4 and g = 2 (55Mn hyperfine structure), respectively. There is strong axial distortion associated with a small rhombic component. This is rationalized in terms of elongation of the MnO4N2 coordination sphere along the MnN2 axis. Analogy is drawn between MnL2 and the manganese(IV) center of photosystem II. The MnIVL2–MnmL2− couple has a reduction potential of ~0.5 V vs SCE (cyclic voltammetry). A second couple, presumably MnIIIL2−–MnIIL22−, occurs near −0.2 V vs SCE, but the manganese(II) species is unstable. The effect of carboxylate coordination on the reduction potentials is noted. The role of X–O− (X = monovalent group, possibly with additional coordinating atoms for chelation) ligands in manganese(IV) binding is stressed. The present ligands belong to this class. © 1990, American Chemical Society. All rights reserved.