Synthesis, characterization and electrochemical studies of heterobometallic complexes containing redox-active molybdenum nitrosyl tris (3, 5-Dimethylpyrazolyl0 hydroborate group
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Redox active polynuclear complexes in which two or more metal centers are linked by a multiatom bridging ligand have found use in molecular electronics where elongated molecules containing electron donor and electron acceptor sites are designed to mimic the behavior of electronic components. The construction of such complexes represents an important evolution in the fields of inorganic and material chemistry because electrons are generated in situ so that there is no need to connect the molecule to a macroscopic system: electron transfer can thus be studied under controlled intramolecular conditions such as across a fixed distance, between fragments of known properties and through a conduit whose conformation and electronic characteristics are well defined. These materials are thus of potential technological interest and can be designed by controlling the bridging pathways at the construction stage. In this work, the electronic interactions were evaluated between the metal centers across a range of Schiff base ligands which were designed to allow the study of the effects of the changes in their nature, length, conformation and position of substitution. The effect of different types of' metal centers involved was also studied. Schiff base ligands formed by condensation of salicylaldehyde, dihydroxybenzaldehyde and a diamine in an alcoholic medium were refluxed with manganese(II)/iron(III) metal ions forming neutral tetradentate N2O2 type of monometallic complexes which were then refluxed with molybdenum nitrosyl metal fragment, thus affording neutral bimetallic complexes. Characterization of these compounds on the basis of their microanalytical data, molar conductance, IR, UV/Vis and MS gave results that were consistent with their formulations. Electrochemical studies showed that the redox potential of the iron(III) complexes are sensitive to changes in the length of the Schiff basc ligand. suggesting that the redox orbitals involved have considerable ligand-based character. however, in manganese(II) complexes, as the polymethylene carbon chain of the Schiff base backbone increases, the reduction potentials are constant, probably suggesting that the reduction orbitals involved are strongly metal-centered. All the monometallic complexes had one reversible reduction wave associated with the reduction of the manganese(lI)/iron(III) centers. Their corresponding bimetallic complexes, however, had two reduction waves associated with the reduction of manganesc(II)/iron(III) and Mo(NO) 2+ centers; both reduction waves being reversible in McCN solvent, while in DCM solvent, the reduction of iron/manganese metal centers was irreversible. This shows that the reduction processes are solvent dependent. p-bimetallic complexes of both iron(III) and manganese(II) were found to have more cathodic electrode potentials than their corresponding in-complexes showing that p-substituted aromatic rings are more effective in transmitting electronic interactions.