Structural Differences and Redox Properties of Unsymmetric Diiron PDIxCy Complexes

Andreas J Hofmann,Corinna R Hess,Christian Jandl

doi:10.1002/ejic.201901173

Andreas J Hofmann, Corinna R Hess + Show 1 more

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https://doi.org/10.1002/ejic.201901173

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Abstract

We present two bimetallic iron complexes, [Fe2(PDIeCy)(OTf)4] (1) and [Fe2(PDIpCy)(THF)(OTf)4] (2) coordinated by an unsymmetric ligand. The new ligand, PDIeCy (PDI = pyridyldiimine; e = ethyl; Cy = cyclam), is a variant of the previously reported PDIpCy (p = propyl) ligand, featuring a shorter linker between the two metal coordination sites. The structural and electronic properties of 1 and 2, both in the solid and solution state, were analyzed by means of X‐ray crystallography, and spectroscopic methods, including 19F‐NMR. The two ligand platforms yield markedly different diiron structures: the PDIeCy ligand permits formation of a bridged, µ‐OTf complex, while the two iron centers of the PDIpCy‐based 2 remain unconnected, directly, under all conditions examined. Both compounds contain electronically non‐coupled high‐spin (S = 2) ferrous centers, as established by Mössbauer spectroscopy and magnetic susceptibility studies. Cyclic voltammetry demonstrates the rich redox chemistry of the compounds, involving both ligand and metal‐centered redox processes. Moreover, we synthesized the two‐electron reduced [Fe2(PDIeCy)]2+ form of 1, which contains the dianionic PDI2– ligand, and represents a two‐electron charge localized complex.

Highlights

Binuclear iron sites are employed by enzymes for varied biological processes that entail dioxygen activation, NO reduction (FNORs), and H2 chemistry, and feature in the active sites of hydrolases (PAPs, GliJ).[1,2,3,4,5,6,7,8,9,10] The combination of two redox-active iron centers in close proximity confers advantages for catalysis, including cooperative binding and activation of substrates, and the use of both metal ions for multi-electron processes
PDIeCy was generated according to a similar procedure as reported for the synthesis of PDIpCy, involving the reaction of 2,6-diacetylpyridine with 2,6-diisopropylaniline and 1-(2-aminoethyl)-1,4,8,11-tetraazacyclotetradecane, respectively (Scheme S1)
We present two new unsymmetric binuclear iron complexes, wherein the individual coordination environments confer unique properties to each site

Summary

Introduction

Binuclear iron sites are employed by enzymes for varied biological processes that entail dioxygen activation (hemerythrin, MMO, RNRs), NO reduction (FNORs), and H2 chemistry, and feature in the active sites of hydrolases (PAPs, GliJ).[1,2,3,4,5,6,7,8,9,10] The combination of two redox-active iron centers in close proximity confers advantages for catalysis, including cooperative binding and activation of substrates, and the use of both metal ions for multi-electron processes. Similar J-values were reported for other μ-triflate diiron complexes.[47,48,49] The magnetic susceptibility data for 2 gave similar values, where the best fit was obtained with g = 2.17, J = 0.07 cm–1, |D| = 17.7 cm–1, and E/D = 0.33 (Figures S7 and S9).

Results

Conclusion