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, Fe₃TPM₂, supported by a 3-fold-symmetric 2-pyridylpyrrolide ligand [i.e., tris­(5-(pyridin-2-yl)-1H-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)-1H-pyrrol-2-yl]­methane (H₂BPM) in tetrahydrofuran (THF) results in the binuclear complex Fe₂(BPM)₂(THF)₂ in which both Fe­(II) ions are octahedrally coordinated. The coordinated THF solvent ligands are labile: THF dissociation leads to Fe₂(BPM)₂, 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)-1H-pyrrol-2-yl]­methane (H₂BPM^Me) in THF leads to the formation of four-coordinate, CDO Fe­(II) centers in Fe­(BPM^Me)₂. Further ligand truncation affords bidentate ligands 2-(1H-pyrrol-2-yl)­pyridine (PyrPyrrH) and 2-methyl-6-(1H-pyrrol-2-yl)­pyridine (Pyr^MePyrrH). Metalation of these ligands in THF affords six-coordinate complexes Fe­(PyrPyrr)₂(THF)₂ and Fe­(Pyr^MePyrr)₂(THF)₂. Dissociation of labile solvent ligands provides access to four-coordinate Fe­(II) complexes. Ligand disproportionation at Fe­(PyrPyrr)₂ results in the formation of Fe­(PyrPyrr)₃ and Fe(0). Ligand methylation suppresses this disproportionation and enables isolation of Fe­(Pyr^MePyrr)₂, which is rigorously CDO. Complete ligand truncation, by separating the 2-pyridylpyrrolide ligands into the constituent monodentate pyridyl and pyrrolide donors, affords Fe­(Pyr)₂(Pyrr)₂ 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.