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

The dinucleating phenolate-hinged ligand 4-tert-butyl-2,6-bis[bis(2-pyridylmethyl)aminomethyl]phenolate(bpb p) has been used to prepare a series of Fe(III)M(II) complexes containing independent species at the exogenous binding sites. These sites are occupied by fluoride and water ligands and show the general formulation [(bpbp)Fe(F)2M(H2O)n]-[BF4]2, M=Zn or Cu, n=1; M=Co or Fe, n=2. Two terminal fluoride ions are bound to the iron(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)2Cu(H2O)][BF4]2-4H2O. The copper(II) and iron(III) atoms are linked asymmetrically by the phenolic oxygen atom hinge of bpbp with Cu-O(phenolato) 2.270(2) and Fe-O(phenolato) 2.041(2) Å with a Cu...Fe distance of 3.828(1) Å. The two terminal fluoride ions are bound to the Fe atom (Fe-F 1.818(2), 1.902(2) Å) and one of them is strongly hydrogen bonded to the water molecule on the adjacent Cu atom (F-H...O 2.653(4) Å). The metal ions in the aquafluoride complexes [(bpbp)Fe(F)2M(H2O)2][BF4]2, M=Fe or Co, are weakly antiferromagnetically coupled (J=-8 and -10 cm-1 respectively) and in [(bpbp)Fe(F)2Cu(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 complexes containing two acetate bridging groups in the exogenous site. Electrochemical 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-acetate complexes. The crystal structure of one bis-mu-acetate complex, [Fe2(bpbp)(CH3CO2)2][BF4]2, shows the expected arrangement; the iron-(II) and -(III) atoms are triply bridged by the phenolic oxygen atom of bpbp and two mu-acetate groups with Fe(II)-O(phenolato) 2.088(4) and Fe(III)-O(phenolato) 1.951(5) Å and an Fe...Fe distance of 3.380(2) Å. The crystal structure at 120 K indicates that the iron atoms are valence trapped and in accordance with this Mossbauer measurements between 80 and 200 K show clearly distinguishable iron-(II) and -(III) components. The Mossbauer spectra of [(bpbp)Fe(F)2Cu(H2O)][BF4]2-4H2O are influenced by paramagnetic relaxation effects with relaxation times of the order of 1 ns. The relaxation time increases when a magnetic field is applied. This effect can be explained by a model for cross-relaxation in conjunction with the crystal symmetry of the compound.