A PNNH Pincer Ligand Allows Access to Monovalent Iron

Reaction of the unsymmetrically-armed pincer PNNH (phosphine, pyridyl, pyrazole) with FeCl2 yields a five coordinate monomer (PNNH)FeCl2, whose NH proton hydrogen bonds to capture THF. Deprotonation of this NH functionality with Li[N(SiMe3)2] does not give four coordinate (PNN)FeCl, but instead retains LiCl to yield (PNNLi)FeCl2, where Li bridges between the pyrazolate beta nitrogen and one chloride on Fe. One electron reduction of this compound under CO occurs with loss of LiCl to form square pyramidal monovalent iron in (PNN)Fe(CO)2, which is characterized by IR, Mossbauer, X-ray diffraction, EPR measurements and DFT calculations. Cyclic voltammetry of (PNN)Fe(CO)2 showed a reversible reduction wave and the reduction product was synthesized using KC8. The product (KPNN)Fe(CO)2 contains saturated, five coordinate Fe(0); the (PNN)Fe subunit is anionic, and the K+ cations cluster near the pyrazolate side of the two CO ligands, leaving the back side of potassium electrophile open to aggregate with the oxygen of CO of a neighboring unit, creating a polymeric chain. Reaction of (PNN)Fe(CO)2 with HBpin results in reduction of the metal center (by release of H2) and borylation of the pyrazole β-nitrogen. This redox active addition of the H-B bond across metal/ligand assembly is an unusual example of metal-ligand cooperativity and establishes a ligand which supports iron in three different oxidation states.