Electronic Structure of Open-Shell, First-Row Transition Metal Complexes for C–H Functionalization

Abstract

The electronic and steric factors that enable the isolation of open-shell iron dipyrromethene compounds containing ligand-centered radicals were explored. In addition to isolating these complexes, the relationship between their electronic structure and functional reactivity was investigated. In particular, upon addition of several different diazoalkane substrates to FeI-supported dipyrromethene complexes, a series of iron-diazoalkane adducts were isolated. While the coordination geometry of the diazoalkane ligand varied, one electron reduction of the diazoalkane upon binding was universally observed, as supported by X-ray crystallography, EPR spectroscopy, DFT calculations, and reactivity trends. In contrast, reaction between a sterically accessible FeII dipyrromethene compound and diphenyldiazomethane does not yield the corresponding diazoalkane adduct. Instead, the metal center promotes dinitrogen extrusion to furnish the respective four-coordinate iron-carbene, (tBuL)FeCl(CPh2) (tBuL = 1,9-di-tert-butyl-5-(2,6-dichlorophenyl)-dipyrromethene). X-ray crystallography, DFT calculations, 57Fe Mössbauer, EPR, and X-ray absorption spectroscopies support the assignment of this species as an intermediate FeII/FeIII complex, featuring some small amount of carbene-radical character, consistent with its observed singlet- and radical-type reactivity. Reduction of (tBuL)FeCl(CPh2) with potassium graphite results in full reduction of the carbene ligand. The corresponding reduced species, (tBuL)Fe(CPh2)(solv), are best described as high-spin FeII complexes antiferromagnetically coupled to carbene-centered ligand radicals. In line with this assignment, these species undergo hydrogen atom abstraction from hydroxyl-2- azaadamantane. As shown with diazoalkanes and carbenes, open-shell iron dipyrrin complexes can promote electron transfer from the metal center to redox-active ligands upon coordination. The reactivity of these transiently generated ligand-centered radicals was further explored. Of note, addition of long-chain α-diazo-β-ketoesters to a catalytic amount of (tBuL)FeCl(Et2O) furnished a variety of substituted 2-alkylidene-tetrahydrofurans in modest yields. While mechanistic studies are still underway, a bidentate radical coordinated to the metal center through two oxygen atoms from the diazoester substrate is proposed as the reactive intermediate.