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Measurement of Extreme Hyperfine Fields in Two-Coordinate High-Spin Fe<sup>2+</sup> Complexes by Mössbauer Spectroscopy: Essentially Free-Ion Magnetism in the Solid State
Mössbauer studies of three
two-coordinate linear high-spin
Fe<sup>2+</sup> compounds, namely, FeÂ{NÂ(SiMe<sub>3</sub>)Â(Dipp)}<sub>2</sub> (<b>1</b>) (Dipp = C<sub>6</sub>H<sub>3</sub>-2,6-<sup><i>i</i></sup>Pr<sub>2</sub>), FeÂ(OAr′)<sub>2</sub> (<b>2</b>) [Ar′ = C<sub>6</sub>H<sub>3</sub>-2,6-(C<sub>6</sub>H<sub>3</sub>-2,6-<sup><i>i</i></sup>Pr<sub>2</sub>)<sub>2</sub>], and FeÂ{CÂ(SiMe<sub>3</sub>)<sub>3</sub>}<sub>2</sub> (<b>3</b>), are presented. The complexes were characterized
by zero- and applied-field Mössbauer spectroscopy (<b>1</b>–<b>3</b>), as well as zero- and applied-field heat-capacity
measurements (<b>3</b>). As <b>1</b>–<b>3</b> 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 ironÂ(II) compound, specifically
+162 T in <b>1</b>. 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, <b>2</b> 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 <b>3</b> behaves similarly to complex <b>1</b>. 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 <b>1</b>–<b>3</b>, we show that heat-capacity measurements for one of the complexes
(<b>3</b>) establish a <i>g</i><sub>eff</sub> value
near 12, in agreement with the principal component of the ligand electric
field gradient being coincident with the <i>z</i> axis