Synthesis and characterization of dinuclear complexes containing the Fe-III-F center dot center dot center dot(H2O)M-II motif

The dinucleating phenolate-hinged ligand 4-tert-butyl-2,6-bis[bis(2-pyridyl methyl)aminomethyl]phenolate (bpbp(-)) has been used to prepare a series of (FeMII)-M-III complexes containing independent species at the exogenous bi nding sites. These sites are occupied by fluoride and water ligands and sho w the general formulation [(bpbp)Fe(F)(2)M(H2O)(n)][BF4](2), M = Zn or Cu, n = 1; M = Co or Fe, n = 2. Two terminal fluoride ions are bound to the iro n(III) ion and one or two water ligands to the adjacent divalent metal ion. The fluoride ligands are derived from the hydrolysis of tetrafluoroborate. In the crystal structure of [(bpbp)Fe(F)(2)Cu(H2O)][BF4](2). 4H(2)O. The c opper(II) and iron(III) atoms are linked asymmetrically by the phenolic oxy gen atom hinge of bpbp(-) with Cu-O-phenolato 2.270(2) and Fe-O-phenolato 2 .041(2) Angstrom with a Cu ... Fe distance of 3.828(1) Angstrom. The two te rminal fluoride ions are bound to the Fe atom (Fe-F 1.818(2), 1.902(2) Angs trom) and one of them is strongly hydrogen bonded to the water molecule on the adjacent Cu atom (F-H ... O 2.653(4) Angstrom). The metal ions in the a quafluoride complexes [(bpbp)Fe(F)(2)M(H2O)(2)][BF4](2), M = Fe or Co, are weakly antiferromagnetically coupled (J = -8 and -10 cm(-1) respectively) a nd in [(bpbp)Fe(F)(2)Cu(H2O)][BF4](2) are weakly ferromagnetically coupled (J = 2 cm(-1)). The spectroscopic, electrochemical and magnetic properties of these complexes are compared to those of an analogous series of complexe s containing two acetate bridging groups in the exogenous site. Electrochem ical results indicate that the iron(III) ions in the bis-fluoride complexes are stabilized by about 300 mV towards reduction compared to the bis-mu-ac etate complexes. The crystal structure of one bis-mu-acetate complex, [Fe-2 (bpbp)(CH3CO2)(2)][BF4](2), shows the expected arrangement; the iron-(II) a nd -(III) atoms are triply bridged by the phenolic oxygen atom of bpbp(-) a nd two mu-acetate groups with Fe-II-O-phenolato 2.088(4) and Fe-III-O-pheno lato 1.951(5) Angstrom and an Fe ... Fe distance of 3.380(2) Angstrom. The crystal structure at 120 K indicates that the iron atoms are valence trappe d and in accordance with this Mossbauer measurements between 80 and 200 K s how clearly distinguishable iron-(II) and -(III) components. The Mossbauer spectra of [(bpbp)Fe(F)(2)Cu(H2O)][BF4](2). 4H(2)O are influenced by parama gnetic relaxation effects with relaxation times of the order of 1 ns. The r elaxation time increases when a magnetic field is applied. This effect can be explained by a model for cross-relaxation in conjunction with the crysta l symmetry of the compound.