Unexpected reactivity resulting from modifications of the ligand periphery: Synthesis, structure, and spectroscopic properties of iron complexes of new tripodal N-heterocyclic carbene (NHC) ligands

The chelating ligand tris-[2-(3-aryl-imidazol-2-ylidene)ethyl]amine (TIMENR, R = aryl = 2,6-xylyl (xyl), mesityl (mes)) has provided access to reactive transition metal complexes. Here, two new tripodal N-heterocyclic carbene ligands of the TIMENR system (R = aryl = tolyl (tol), 3,5-xylyl (3,5xyl)), featuring sterically less demanding aryl substituents were synthesized. With these ligands, Fe(II) precursor complexes could be obtained, namely [(TIMENtol)Fe](BF4)2 (3) and [(TIMEN3,5xyl)Fe(CH3CN)](PF6)2 (7), which showed unexpected reactivity upon reduction. Treatment of the compounds with sodium amalgam yield the tris- and bis-metallated products, [(TIMENtol∗∗∗)Fe] (4) and [(TIMEN3,5xyl∗∗)Fe] (8), respectively. While the Fe(III) complex 4 is relatively inert towards oxygen, the Fe(II) complex 8 is prone to oxidation. This oxidation of 8 can readily be observed in chlorinated solvents, producing the Fe(III) complex [(TIMEN3,5xyl∗∗)Fe](PF6) (9). All new ligand imidazolium precursors and metal complexes were characterized by single crystal X-ray structure determination.