An extended X-ray absorption fine structure study of bis(μ-oxo) bridged dinuclear manganese co-ordination compounds containing alkali- and alkaline-earth-metal cations in a crown ether moiety

An X-ray absorption spectroscopic study was made of seven different manganese co-ordination compounds, six of which are dinuclear and contain the structural bis(μ-oxo) unit. In all the compounds each manganese is ligated by one crown ether Schiff-base moiety. Alkali- or alkaline-earth-metal cations were introduced into this crown ether moiety in dinuclear complexes, bringing the cations (Na⁺, K⁺, Ca²⁺ and Ba²⁺) into the vicinity (<4 Å) of the manganese. The position of the X-ray absorption edge follows the order expected for the different oxidation states of manganese: 6551.6 eV for a manganese(III) monomer, 6552.1 eV for a Mn^IIIMn^IV(μ-O)₂ dimer and 6553.5 eV for a manganese(IV) dimer. Analysis of the extended-X-ray absorption fine structure of the compounds yielded information about the immediate ligation to manganese. The first shell consists of light atoms (O/N) and is composed of two sub-shells, with average distances of 1.9 Å and 2.3 Å from the Mn. The dinuclear compounds clearly show the characteristic 2.7 Å Mn-Mn distance. In the cation-containing compounds a manganese-metal distance of approximately 3.6 Å is found. The introduction of Ba²⁺ in the crown ether moiety induces substantial changes in the ligation pattern of the light elements. The properties of the compounds provide a model for discussion of the properties of the photosynthetic manganese complex, and the role and position of the Ca²⁺ cofactor.