A PNNH Pincer Ligand Allows Access to Monovalent Iron

Reaction of the unsymmetrically armed pincer PNNH (phosphine-pyridyl-pyrazole) ligand with FeCl₂ yielded the five-coordinate monomer [(PNNH)FeCl₂ ], the NH proton of which captures THF through the formation of a hydrogen bond. Deprotonation of this NH functionality with Li[N(SiMe₃ )₂ ] did not give the four-coordinate [(PNN)FeCl], but instead retained LiCl to yield [(PNNLi)FeCl₂ ], in which the lithium bridges between the pyrazolate β-nitrogen and one of the chlorides on iron. One-electron reduction of this compound under CO occurred with the loss of LiCl to form the square-pyramidal monovalent iron in [(PNN)Fe(CO)₂ ], which was characterized by IR, Mössbauer, and EPR spectroscopy, X-ray diffraction, and DFT calculations. Cyclic voltammetry studies of [(PNN)Fe(CO)₂ ] showed a reversible reduction wave and the reduction product was synthesized by using KC₈ . The product K[(PNN)Fe(CO)₂ ] contains saturated, five-coordinate Fe⁰ ; the (PNN)Fe subunit is anionic and the K⁺ cations cluster close to the pyrazolate side of the two CO ligands. Potassium electrophile complete its coordination sphere through interactions with the oxygen atom of a CO of a neighboring unit, thereby creating a polymeric chain. The reaction of [(PNN)Fe(CO)₂ ] with HBpin (4,4,5,5-tetramethyl-1,3,2-dioxaborolane) resulted in the reduction of the metal center (by release of H₂ ) and borylation of the pyrazole β-nitrogen atom. This redox-active addition of the H-B bond across the metal-ligand assembly is an unusual example of metal-ligand cooperativity and establishes a ligand that supports iron in three different oxidation states.