Magnetic exchange interactions propagated by polyatomic bridges

Abstract

The investigation of the mechanisms of electron transfer at long distances is one of the crucial problems in bioenergetics. The fundamental nature of electron transfer between transition metal ion sites in both electron-transfer proteins and precursor complexes for outer- and inner-sphere redox reactions needs to be determined. In order for electron transfer to occur between two transition metal ions that are well separated, there has to be an electronic interaction between the two metal ions. This interaction would be propagated by the organic material that is bridging between the two metal ions. In a binuclear complex of paramagnetic metal ions bridged by an organic species the electronic interaction that develops between the two metal ions is called a magnetic exchange interaction There are at least four instances where polyatomic bridges propagated magnetic exchange interactions: (1) Polynuclear transition metal complexes with unsaturated bridges; (2) Polynuclear transition metal complexes with saturated bridges; (3) Transition metal complexes with a paramagnetic ligand; and (4) Organic biradicals. Emphasis will be placed on the first two cases, where unsaturated polyatomic bridges include imidazolate, pyrazine, oxalate, cyanurate and aromatic diamines and saturated bridges are represented by dabco and various aliphatic dicarboxylate dianions. The distance dependence of magnetic exchange interactions will be examined. It will be shown that there is no simple distance dependence. Exchange interactions can be propagated over large distances. For example, hydrogen-bond interactions between metal complexes can propagate exchange interactions; temperature dependence in the exchange parameter J has been seen for some of these complexes by means of the observation of singlet-to-triplet EPR transitions. The relationship between magnetic exchange interactions propagated by extended polyatomic bridges and electron transfer via these same bridges will be discussed. The study of well characterized, generally binuclear, mixed-valence transition metal complexes is proving to be useful in characterizing the factors controlling electron transfer between metal ions. Magnetic exchange interactions are present in certain mixed-valence complexes, such as the trinuclear oxocentered Fe 2 IIIFe II carboxylate complexes.