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

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) 2 (MeCN) 2 /external carboxylate reactants. X‐ray crystallographic studies of two diiron complexes derived in this manner from L3 revealed their formulas to be [Fe 2 L3 (μ‐OH)(μ‐O 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 2 L1 (μ‐OH)(μ‐O 2 CAr Tol )(OTf) 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 2 ‐activating carboxylate‐bridged diiron centers in biological systems.