Three-Coordinate Iron(II) Expanded Ring N‑Heterocyclic Carbene Complexes

A sterically
demanding seven-membered expanded ring N-heterocyclic
carbene (NHC) ligand allows access to rare examples of three-coordinate
iron­(II)–NHC complexes incorporating only halide coligands
of the general formula [Fe­(NHC)­X2] (NHC = 7-DiPP; X = Br
(1) Cl (2)). Reducing the steric influence
of the ancillary NHC ligand through modulation of the N-aryl substituents leads to either four- or three-coordinate complexes
of the general formula [Fe­(NHC)­Br2(THF)] (3) or [Fe­(NHC)­Br2] (4) (NHC = 7-Mes), dependent
upon the solvent of recrystallization. The further reduction of NHC
steric influence results in four-coordinate geometries at iron in
the form of the dimeric species [Fe­(NHC)­Br­(μ-Br)]2 (5) or [Fe­(NHC)­Br2(THF)] (6) (NHC = SDiPP), again dependent upon the solvent of recrystallization.
Compounds 1–6 have been analyzed
by 1H NMR spectroscopy, X-ray crystallography, elemental
microanalysis, Mössbauer spectroscopy (for 1 and 3–5), and Evans method magnetic susceptibility.
In addition to these measurements the three-coordinate species 1 and 4 have been further analyzed by SQUID magnetometry
and CASSCF calculations, which show significant magnetic anisotropy
that is extremely sensitive to the coordination geometry.