Electronic Perturbations of Iron Dipyrrinato Complexes via Ligand β-Halogenation and meso -Fluoroarylation

Abstract

Systematic electronic variations were introduced into the monoanionic dipyrrinato ligand scaffold via halogenation of the pyrrolic β-positions and/or via the use of fluorinated aryl substituents in the ligand bridgehead position in order to synthesize proligands of the type 1,9-dimesityl-β-R4-5-Ar-dipyrrin [R = H, Cl, Br, I; Ar = mesityl, 3,5-(F3C)2C6H3, C6F5 in ligand 5-position; β = 2,3,7,8 ligand substitution; abbreviated (β,ArL)H]. The electronic perturbations were probed using standard electronic absorption and electrochemical techniques on the different ligand variations and their divalent iron complexes. The free-ligand variations cause modest shifts in the electronic absorption maxima (λmax: 464–499 nm) and more pronounced shifts in the electrochemical redox potentials for one-electron proligand reductions (E1/2: −1.25 to −1.99 V) and oxidations (E1/2: +0.52 to +1.14 V vs [Cp2Fe]+/0). Installation of iron into the dipyrrinato scaffolds was effected via deprotonation of the proligands followed by treatment with FeCl2 and excess pyridine in tetrahydrofuran to afford complexes of the type (β,ArL)FeCl(py) (py = pyridine). The electrochemical and spectroscopic behavior of these complexes varies significantly across the series: the redox potential of the fully reversible FeIII/II couple spans more than 400 mV (E1/2: −0.34 to +0.50 V vs [Cp2Fe]+/0); λmax spans more than 40 nm (506–548 nm); and the 57Fe Mössbauer quadrupole splitting (|ΔEQ|) spans nearly 2.0 mm/s while the isomer shift (δ) remains essentially constant (0.86–0.89 mm/s) across the series. These effects demonstrate how peripheral variation of the dipyrrinato ligand scaffold can allow systematic variation of the chemical and physical properties of iron dipyrrinato complexes.