A Combined Magnetic and Mössbauer Spectroscopic Study of High-Spin Half-Sandwich Fe(I) Complexes in Monomeric and Polymeric Configurations

The reduction of the iron(II) half-sandwich precursors [Cp'FeN(R)(SiMe₃)] (R = SiMe₃, 2,6-di-iso-propylphenyl (dipp)) with KC₈ yields the one-dimensional polymeric iron(I) complexes {[Cp'Fe(N(SiMe₃)₃)]K}n (4) and {[Cp'Fe(N(dipp)(SiMe₃))]K}n (5). Addition of 18-crown-6 breaks up these chains and affords the monomeric salts [Cp'Fe(N(SiMe₃)₂)][K(18-crown-6)(thf)₂] (6) and [Cp'Fe(N(dipp)(SiMe₃))][K(18-crown-6)(thf)₂] (7). All four compounds were structurally characterized by single-crystal X-ray diffraction analysis and adopt an S = 3/2 spin ground state. Zero-field ⁵⁷Fe Mössbauer spectra uncover slow magnetic relaxation at low temperatures in each complex. Complementary CASSCF/NEVPT2 calculations reveal an S = 3/2 ground-state Kramers doublet that is ca. 100 cm⁻¹ below the first excited state. These findings are confirmed by results from DC susceptibility and magnetization experiments. The slow relaxation of magnetization is further characterized via AC susceptibility measurements, unveiling detectable slow relaxation of magnetization at zero external field for 4-6. Complex 4 exhibits an unusual field dependence of the quantum tunneling of magnetization, akin to exchange-bias effects, which are commonly observed in lanthanide dimers.