Flash Communication: Rearrangement of a tris-Carbene Ligand Precursor to an all-N-Bound tris-Imidazole Iron Complex: Flipping the Umbrella

Treatment of the ligand precursor H₃TIMMN^MesCl₃ (TIMMN^Mes = tris-[(3-mesityl-imidazol-2-ylidene)methyl]amine) with an excess of base yields the literature-known ferrous tris-N-heterocyclic carbene (NHC) complex [(TIMMN^Mes)Fe^IICl]Cl (1-Cl). In contrast, utilizing a substoichiometric amount of base initiates a unique rearrangement of all three NHC pendant arms to yield the tripodal, all-N-bound tris-imidazole [(N-TIMMN^Mes)Fe^IICl]Cl (2-Cl_sol). Divalent 2-Cl and 2-PF₆ are fully characterized, structurally by single-crystal X-ray diffraction analysis and spectroscopically by ¹H NMR and ⁵⁷Fe Mössbauer spectroscopy as well as SQUID magnetization measurements, to demonstrate the influence of the change from a soft strong-field to a hard weak-field ligand. Optimized reaction conditions for the reproducible, high-yield carbene-to-imidazole rearrangement were developed in a series of experiments.