DINUCLEATING, TRINUCLEATING, AND TETRANUCLEATING PYRIDYL LIGANDS WHICH FACILITATE MULTICENTER MAGNETIC EXCHANGE BETWEEN PARAMAGNETIC MOLYBDENUM CENTERS

Two series of di-, tri- and tetranucleating bridging ligands have been prepared comprising 4-pyridyl binding sites radiating out from a central aromatic core, linked by ethenyl (series A) or ethynyl (series B) conjugated spacers. All of the ligands were prepared by a simple one-pot Heck coupling reaction between 4-vinylpyridine (series A) or 4-ethynylpyridine (series B) and the appropriate polybromobenzene. Complexes with these ligands were prepared by attachment of paramagnetic {Mo(NO)Tp*Cl} fragments to some or all of the pyridyl binding sites of the ligands. Electrochemical studies showed that (i) the redox splitting Delta E(1/2) between the 17-electron/18-electron couples of the Mo centers varies with the substitution pattern of the bridging ligand, such that ortho and para relationships between two {Mo(NO)Tp*Cl} fragments result in stronger interactions than a meta relationship and that (ii) ligand series A, with ethenyl spacers between the pyridyl termini and the central aromatic ring, gives slightly stronger electrochemical interactions than does ligand series B, which contains ethynyl spacers. EPR spectra showed that in all complexes of nuclearity two or more, magnetic exchange between the unpaired electrons is occurring; i.e., all unpaired electrons apparently couple to all molybdenum nuclei. The triangular trinuclear complexes and rectangular tetranuclear complexes represent the first examples of well-defined complexes where multicenter magnetic exchange is unequivocally apparent from the EPR spectra.