Measurement of Extreme Hyperfine Fields in Two-Coordinate High-Spin Fe²⁺ Complexes by Mössbauer Spectroscopy: Essentially Free-Ion Magnetism in the Solid State

Abstract

Mössbauer studies of three two-coordinate linear high-spin Fe(2+) compounds, namely, Fe{N(SiMe3)(Dipp)}2 (1) (Dipp = C6H3-2,6-(i)Pr2), Fe(OAr')2 (2) [Ar' = C6H3-2,6-(C6H3-2,6-(i)Pr2)2], and Fe{C(SiMe3)3}2 (3), are presented. The complexes were characterized by zero- and applied-field Mössbauer spectroscopy (1-3), as well as zero- and applied-field heat-capacity measurements (3). As 1-3 are rigorously linear, the distortion(s) that might normally be expected in view of the Jahn-Teller theorem need not necessarily apply. We find that the resulting very large unquenched orbital angular momentum leads to what we believe to be the largest observed internal magnetic field to date in a high-spin iron(II) compound, specifically +162 T in 1. The latter field is strongly polarized along the directions of the external field for both longitudinal and transverse field applications. For the longitudinal case, the applied field increases the overall hyperfine splitting consistent with a dominant orbital contribution to the effective internal field. By contrast, 2 has an internal field that is not as strongly polarized along a longitudinally applied field and is smaller in magnitude at ca. 116 T. Complex 3 behaves similarly to complex 1. They are sufficiently self-dilute (e.g., Fe···Fe distances of ca. 9-10 Å) to exhibit varying degrees of slow paramagnetic relaxation in zero field for the neat solid form. In the absence of EPR signals for 1-3, we show that heat-capacity measurements for one of the complexes (3) establish a geff value near 12, in agreement with the principal component of the ligand electric field gradient being coincident with the z axis.

Maximizing Electron Exchange in a [Fe3] Cluster

“…The initial communication on the all-ferrous ( tbs L)­Fe 3 (thf) cluster reported a broad spectrum at 90 K that was fit to three distinct electric field gradients (signifying three distinct iron environments) . Nonetheless, when the temperature of acquisition is lowered to 4.2 K, a multiline spectrum for ( tbs L)­Fe 3 (thf) is obtained (Figure S8), as observed in other molecular species. , The spectral features are broad and unresolved, consistent with overlapping sextets, indicative of a slowly relaxing internal magnetic field ( H int ). However, the magnetic relaxation rate exceeds that which is needed to display an out-of-phase (χ M ″ ) signature in the ac magnetic susceptibility under zero-applied dc field since no maximum is observed, even at the lowest temperatures achievable (1.8 K).…”

A Two-Coordinate Iron(II) Imido Complex with NHC Ligation: Synthesis, Characterization, and Its Diversified Reactivity of Nitrene Transfer and C–H Bond Activation

“…According to the observed A ∥ value, complex 3 possesses an internal magnetic field of ∼132 T. The measured Mössbauer parameters are comparable to those found for analogous two-coordinate high-pin ferrous complexes. 61−64…”

“…Both states are hence nearly degenerate, which explains the unusual magnetic properties observed for 3 . 61−64…”

Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

“…For instance, in the iron(II)-lithium complex [Fe(N(SiMe 3 ) 2 ) 2 (Li(bta)] 2 (26), where weak interactions occur between planar trigonal Fe II complexes via Li 2 (bta) 2 units. Based on its magnetic behavior, the ZFS and exchange interaction between the dinuclear units were estimated as J = -0.03 cm -1 , D = -10.5 cm -1 , and g = 2.2 [46].…”