Cyanide bridged di- and tri-nuclear complexes with central Cr(III)-, Mn(III)- and Co(III)-salen units

Reactions between M(salen)X (M=Cr(III), Mn(III), Co(III)) and LnM′CN (LnM′=Cp(CO)2Fe, Cp(dppe)Fe, Cp(PPh3)2Ru, (CO)5Cr−) have yielded 4 dinuclear and 11 symmetrical trinuclear complexes with LnM′CNM(salen) arrangements. The hetero-trinuclear complexes with FeCNCrNCRu and FeCNCrNCCr arrays and one isomeric FeNCCoCNFe complex were also obtained. Their IR, electrochemical and UV–Vis data were recorded and compared, including those of the corresponding Fe(III)salen complexes. Characteristic differences between the systems with empty (Cr, Co) and occupied (Mn, Fe) eg levels at the central M(III)salen units were observed. The tendency to support M′C(CN) back bonding was found to decrease in the sequence Mn≈Fe≫Cr>Co for the central M(III)salen units. The cyclic voltammograms indicate long range electronic interactions between the terminal metals in the trinuclear complexes. The UV–Vis data point to a metal–metal charge transfer from the terminal to the central metals, the energy required for which increases in the sequence Mn≈Fe≪Cr≈Co.