Triptycene-Based, Carboxylate-Bridged Biomimetic Diiron(II) Complexes

Abstract

A triptycene-based bis(benzimidazole) ester ligand, L3, was designed to enhance the electron-donating ability of the heterocyclic nitrogen atoms relative to those of the first-generation bis(benzoxazole) analogs, L1 and L2. A convergent synthesis of L3 was designed and executed. Three-component titration experiments using UV/Vis spectroscopy revealed that the desired diiron(II) complex could be obtained with a 1:2:1 ratio of L3/Fe(OTf)[subscript 2](MeCN)[subscript 2]/external carboxylate reactants. X-ray crystallographic studies of two diiron complexes derived in this manner from L3 revealed their formulas to be [Fe[subscript 2]L3(μ-OH)(μ-O[subscript 2]CR)(OTf)2], where R = 2,6-bis(p-tolyl)phenyl (7) or triphenylmethyl (8). The structures are similar to that of a diiron complex derived from L1, [Fe[subscript 2]L1(μ-OH)(μ-O[subscript 2]CAr[superscript Tol])(OTf)[subscript 2]] (9), a notable difference being that, in 7 and 8, the geometry at iron more closely resembles square-pyramidal than trigonal-bipyramidal. Mössbauer spectroscopic analyses of 7 and 8 indicate the presence of high-spin diiron(II) cores. These results demonstrate the importance of substituting benzimidazole for benzoxazole for assembling biomimetic diiron complexes with syn disposition of two N-donor ligands, as found in O[subscript 2]-activating carboxylate-bridged diiron centers in biological systems.