Cis-Divacant Octahedral Fe(II) in a Dimensionally Reduced Family of 2-(Pyridin-2-yl)pyrrolide Complexes.

Four-coordinate transition-metal complexes can adopt a diverse array of coordination geometries, with square planar and tetrahedral coordination being the most prevalent. Previously, we reported the synthesis of a trinuclear Fe(II) complex, , supported by a 3-fold-symmetric 2-pyridylpyrrolide ligand [i.e., tris(5-(pyridin-2-yl)-1-pyrrol-2-yl)methane] that featured a rare cis-divacant octahedral (CDO) geometry at each Fe(II) center. Here, a series of truncated 2-pyridylpyrrolide ligands are described that support mono- and binuclear Fe(II) complexes that also exhibit CDO geometries. Metalation of the tetradentate ligand bis[5-(pyridin-2-yl)-1-pyrrol-2-yl]methane () in tetrahydrofuran (THF) results in the binuclear complex in which both Fe(II) ions are octahedrally coordinated. The coordinated THF solvent ligands are labile: THF dissociation leads to , which features five-coordinate Fe(II) ions. The Fe-Fe distance in these binuclear complexes can be elongated by ligand methylation. Metalation of bis[5-(6-methylpyridin-2-yl)-1-pyrrol-2-yl]methane () in THF leads to the formation of four-coordinate, CDO Fe(II) centers in . Further ligand truncation affords bidentate ligands 2-(1-pyrrol-2-yl)pyridine () and 2-methyl-6-(1-pyrrol-2-yl)pyridine (). Metalation of these ligands in THF affords six-coordinate complexes and . Dissociation of labile solvent ligands provides access to four-coordinate Fe(II) complexes. Ligand disproportionation at results in the formation of and Fe(0). Ligand methylation suppresses this disproportionation and enables isolation of , which is rigorously CDO. Complete ligand truncation, by separating the 2-pyridylpyrrolide ligands into the constituent monodentate pyridyl and pyrrolide donors, affords in which Fe(II) is tetrahedrally coordinated. Computational analysis indicates that the potential energy surface that dictates the coordination geometry in this family of four-coordinate complexes is fairly flat in the vicinity of CDO coordination. These synthetic studies provide the structural basis to explore the implications of CDO geometry on Fe-catalyzed reactions.