Structure, Magnetism, and Mössbauer Spectrum of the Five-Coordinate Complex Chlorobis(N-methylbenzothiohydroxamato)iron(III)

A detailed study of the structural, magnetic, ESR, and Mössbauer spectroscopic properties of a five-coordinate iron(III) thiohydroxamate complex has been made. The complex, chlorobis(N-methylbenzothiohydroxamato)iron(III), contains two trans bidentate {SO} donor groups and one CI atom in each mononuclear unit and has bond distances Fe-Cl = 2.221 (2) Å, Fe-S = 2.389 (2) Å (average), and Fe-O = 1.946 (3)Å. This high-spin d5 molecule has an overall ligand field strength provided by the {S₂O₂Cl} donor set similar to that in related β-diketonate {O₄Cl} complexes but weaker than that in intermediate-spin dithiocarbamate {S₄Cl} species. Magnetic moment measurements between 4.2 and 300 K are generally consistent with the presence of large zero-field splitting effects although the decrease in μ_Fe at low temperatures is unusually rapid. In line with the low molecular symmetry and observation of g = 4.3 and 9.9 ESR spectral lines, a spin-Hamiltonian analysis, which includes rhombic and axial components, was carried out. The best-fit parameters were D = 10 ± 0.5 cm^-1 and E = 3 ± 0.5 cm^-1. Weak antiferromagnetic coupling (J = -0.1 ± 0.05 cm_-1) was also evident in the low-temperature susceptibility data and possibly occurs via a (Fe-Cl→Cl-Fe) superexchange pathway. Exchange coupling of this kind is common among other chloro-iron(III) chelate complexes containing a variety of oxygen, nitrogen, or sulfur donor groups. Zero-field Mössbauer spectra measured between 4.2 and 300 K on the present complex support the S = 5/2 spin-state assignment and show unusual temperature-dependent area and peak height asymmetry. Applied magnetic field spectra at 4.2 K confirm the large and positive zero-field splitting of the ⁶A₁ ground state.