Mössbauer, electron paramagnetic resonance, and theoretical study of a high-spin, four-coordinate Fe(II) diketiminate...

The iron(II) complex LFeCl 2Li(THF) 2 (L = beta-diketiminate), 1, has been studied with variable-temperature, variable-field Mossbauer spectroscopy and parallel mode electron paramagnetic resonance (EPR) spectroscopy in both solution and the solid state. In zero applied field the 4.2 K Mossbauer spectrum exhibits an isomer shift delta = 0.90 mm/s and quadrupole splitting Delta E Q = 2.4 mm/s, values that are typical for the high-spin ( S = 2) state anticipated for the iron in 1. Spectra recorded in applied magnetic fields yield an anisotropic magnetic hyperfine tensor with A x = +2.3 (+ 1.0) T, A y = A z = -21.5 T ( solution) and a nearly axial zero-field splitting of the spin quintet with D = D x approximately -14 cm (-1) and rhombicity E/ D approximately 0.1. The small, positive value for A x results from the presence of residual orbital angular momentum along x. The EPR analysis gives g x approximately 2.4 (and g y approximately g z approximately 2.0) and reveals a split " M S = +/- 2" ground doublet with a gap distributed around Delta = 0.42 cm (-1). The Mossbauer spectra of 1 show unusual features that arise from the presence of orientation-dependent relaxation and a distribution in the magnetic hyperfine field along x. The origin of the distribution has been analyzed using crystal field theory. The analysis indicates that the distribution in the magnetic hyperfine field originates from a narrow distribution, sigma phi approximately 0.5 degrees , in torsion angle phi between the FeN 2 and FeCl 2 planes, arising from minute inhomogeneities in the molecular environments.