名称:
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
摘要:
Manganese-(II) and -(III) complexes of substituted N,N'-bis(salicylidene)ethane-1,2-diamine (H(2)salen) ligands H(2)L (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) . 6H(2)O with H(2)L 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(In) species. Using a more rigorous oxidising agent in the synthesis, [Fe(eta-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)]. 2H(2)O [H(2)bnsalen = 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)]. 2H(2)O. 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 pi and hydrogen bonding, as well as dimerisation. The complex [{Mn(mu-dbsalen)(mu-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 pi- or hydrogen-bonding interaction is modified by the corresponding ability of the 5-NO2 substituent.