The
bulky bis(carbene)borate ligand H<sub>2</sub>B(<sup>t</sup>BuIm)<sub>2</sub><sup>–</sup> allows for the synthesis of
three- and four-coordinate iron(II) complexes, including heteroleptic
H<sub>2</sub>B(<sup>t</sup>BuIm)<sub>2</sub>FeN(TMS)<sub>2</sub> and
homoleptic [H<sub>2</sub>B(<sup>t</sup>BuIm)<sub>2</sub>]<sub>2</sub>Fe. The magnetic properties of these coordinatively unsaturated complexes
have been characterized by SQUID magnetometry, but no evidence of
single-molecule magnet behavior is observed, despite large negative
uniaxial zero field splitting. The three-coordinate complex H<sub>2</sub>B(<sup>t</sup>BuIm)<sub>2</sub>FeN(TMS)<sub>2</sub> serves
as a precursor for the synthesis of the four-coordinate mixed carbene
complex H<sub>2</sub>B(<sup>t</sup>BuIm)<sub>2</sub>(<sup>i</sup>Pr<sub>2</sub>Im)FeCl, which has a coordination environment similar to that
found in tris(carbene)borate iron(II) chloride complexes. Despite
this similarity, attempts to prepare the corresponding iron(IV) nitride
were unsuccessful, suggesting that subtle structural factors are critical
to stabilizing this species
