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

Abstract

The chelating ligand tris-[2-(3-aryl- imidazol-2-ylidene) ethyl]ami ne (TIMEN R, 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 TIMEN R 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 [(TIMEN tol)Fe](BF 4) 2 ( 3) and [(TIMEN 3,5xyl)Fe(CH 3CN)](PF 6) 2 ( 7), which showed unexpected reactivity upon reduction. Treatment of the compounds with sodium amalgam yield the tris- and bis-metallated products, [(TIMEN tol∗∗∗)Fe] ( 4) and [(TIMEN 3,5xyl∗∗)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 [(TIMEN 3,5xyl∗∗)Fe](PF 6) ( 9). All new ligand imidazolium precursors and metal complexes were characterized by single crystal X-ray structure determination.