Synthesis, Characterization, and NMR Study of Oxo-Centered Trinuclear Complexes [Fe2IIINiIIO(O2CC2H5)6(py)3]•py and [Fe2IIINiIIO(O2CC2H5)6(H2O)3]•H2O

Two mu(3)-oxo carboxylate-bridged heteronuclear complexes, [(Fe2NiO)-Ni-III-O-II(O2CC2H5)(6)(H2O)(3)]center dot H2O and [(Fe2NiO)-Ni-III-O-II(O2CC2H5)(6)(py)(3)]center dot py, were prepared and characterized by several spectroscopic techniques including NMR, X-ray diffraction, IR, ESR, and UV. X-ray diffraction measurements demonstrated that the Fe2NiO clusters of the complexes were close to threefold symmetry in crystals. The ligands coordinated to different metal atoms were almost equivalent in the IR timescale but inequivalent in the NMR timescale. The NMR results showed that the largest H-1 NMR chemical shift of the complex was 95.1 ppm, implying its paramagnetic property, which was weakened by the antiferromagnetic interaction of metal ions through the mu(3)-O bridge. NMR and IR studies indicated that the complexes were stable in various nonpolar and moderately polar solvents, such as CDCl3 and d(3)-MeCN, but they were decomposed into metal ions and the corresponding ligands in strong polar solvents, such as water, at room temperature. Assignments of the H-1 NMR spectra of the complexes were made on the basis of relative intensities, broadening, variable temperature experiments, spin-lattice relaxation times, and substitution by appropriate ligands. The H-1 spin-lattice relaxation time T-1 and variable-temperature NMR experiments were also applied to investigate the solution structures and dynamics of the complexes. It is worth noting that the H-1 chemical shift of the pyridine coordinated to the metals could be greater than 90 ppm.