Electronic and steric effects in manganese Schiff-base complexes as models for the water oxidation complex in photosystem II. The isolation of manganese-(II) and -(III) complexes of 3- and 3,5-substituted N,N′-bis(salicylidene)ethane-1,2-diamine (H2salen) ligands

Abstract

Manganese-(II) and -(III) complexes of substituted N,N′-bis(salicylidene)ethane-1,2-diamine (H₂salen) ligands H₂L (substituents are in the 3, 5 or 3,5 positions of the phenyl rings of the salen moiety) have been prepared and thoroughly characterised. The reaction of Mn(ClO₄)₂·6H₂O with H₂L in ethanol in air normally leads to manganese(III) complexes ligated by both the N₂O₂ ligand and water molecule(s). However, by employing electron-withdrawing substituents on the ligand, e.g. 3-Br,5-NO₂, a manganese(II) complex can be obtained. A ‘borderline’ ligand is represented by the 5-NO₂ derivative (nsalen), which produces a manganese(II) complex contaminated with a small amount of a manganese(III) species. Using a more rigorous oxidising agent in the synthesis, [Fe(η-C₅H₅)₂][FeCl₄], drives the reaction totally to a manganese(III) complex [Mn(nsalen)Cl(H₂O)]. In addition to magnetic susceptibility studies, cyclic voltammetry has been employed. All the complexes exhibit an oxidation and reduction peak, the reversible character being confirmed by pulse voltammetry. Pulse voltammetry also confirmed the nature of the manganese(II) species [Mn(bnsalen)(H₂O)₂·2H₂O [H₂bnsalen =N,N′-bis(3-bromo-5-nitrosalicylidene)ethane-1,2-diamine] and that a slight amount of a manganese(III) species is present in [Mn(nsalen)(H₂O)₂]·2H₂O. Six complexes have been crystallographically characterised. Despite the retention of an octahedral manganese environment in all of them, the supramolecular structures exhibit a wide diversity. The 3,5-dichloro and 5-bromo salen complexes containing co-ordinated water display combined π and hydrogen bonding, as well as dimerisation. The complex [{Mn(µ-dbsalen)(µ-O)}₂](dbsalen = 3,5-dibromo derivative) offers an alternative bridging arrangement, and [Mn(bsalen)(MeOH)(OClO₃)]·H₂O (bsalen = 5-bromo derivative) highlights the versatility of the manganese centre in these systems where, unexpectedly, perchlorate is co-ordinated in place of a lattice water. A more subtle rearrangement of supramolecular structure is obtained in [Mn(nsalen)Cl(H₂O)] where the usual combination of π- or hydrogen-bonding interaction is modified by the corresponding ability of the 5-NO₂ substituent.