Low-Coordinate Iron(II) Amido Half-Sandwich Complexes with Large Internal Magnetic Hyperfine Fields.

The half-sandwich complex [Cp'Fe{N(dipp)(SiMe 3 )}] ( Fe-dipp ; Cp' = 1,2,4-tri- tert -butylcyclopentadienyl and dipp = 2,6-diisopropylphenyl) and the mixed metallocene [Cp'Fe{(η 5 -C 6 H 3 i Pr 2 )═N(SiMe 3 )}] ( Fe-chd ) formed in the reaction between [{Cp'Fe(μ-I)} 2 ] and [Li{N(dipp)(SiMe 3 )}] 2 were characterized by NMR spectroscopy and X-ray diffraction analysis. Fe-dipp complements the series of low-coordinate, quasi-linear iron amido half-sandwich complexes [Cp'Fe{N( t Bu)(SiMe 3 )}] ( Fe- t Bu ) and [Cp'Fe{N(SiMe 3 ) 2 }] ( Fe-tms ) reported earlier, and all three compounds were characterized in the solid state by zero-field 57 Fe Mössbauer spectroscopy and magnetic susceptibility measurements, confirming their S = 2 electronic ground state. Moreover, the Mössbauer absorption spectra reveal slow paramagnetic relaxation at low temperatures with large internal magnetic hyperfine fields of B hf = 96.4 T ( Fe-dipp , 20 K), B hf = 101.3 T ( Fe- t Bu , 15 K), and B hf = 96.9 T ( Fe-tms , 20 K). The magnetic measurements further confirm that the presence of significant axial zero-field splitting and slow relaxation of magnetization is detected, which is revealed even in the absence of a static magnetic field in the case of Fe- t Bu . Supplementary ab initio and density functional theory calculations were performed and support the experimental data.