Combined Mossbauer Spectral and Density Functional Theory Determination of the Magnetic Easy-Axis in Two High-Spin Iron(II) 2-Pyrazinecarboxylate Complexes

A combination of density functional theory (DFT) calculations and Mossbauer spectroscopy has been used to determine that the magnetic easy-axis is coincident with its crystallographic c-axis in [Fe(pca)(2)(py)(2)]center dot py, where pac is the 2-pyrazinecarboxylate ligand. This easy-axis bisects the approximately axial O-Fe-O coordination axes of molecules adjacent to each other along the b-axis. In {[Fe(pca)(2)(H2O)]center dot H2O}(n) the easy magnetic axis is not coincident with any of its crystallographic axes nor with the Fe-O(water) coordination axis, but is coincident with one of the Fe center dot center dot center dot Fe axes in the crystal structure. The DFT calculations, which use the B3LYP functional and the 6-311 + +G(d,p) basis set, yield s-electron probability densities and electric field gradient tensors for the iron(II) ion that are in excellent agreement with the observed iron-57 Mossbauer spectral isomer shifts and quadrupole interactions. The gas phase results are very similar for calculations based either on the X-ray structures of the two complexes or on their optimized structures; the optimized structures indicate that the iron to ligand bond distances increase in the absence of any solid-state lattice interactions. The results of a normal coordinate vibrational mode analysis of the two optimized structures are compared with the observed infrared spectra.