Stoichiometric and Catalytic Secondary O-Atom Transfer by Fe(III)−NO₂ Complexes Derived from a Planar Tetradentate Non-heme Ligand: Reminiscence of Heme Chemistry

An Fe(III) nitro complex [(bpb)Fe(NO2)(py)] (2) of the tetradentate ligand 1,2-bis(pyridine-2-carboxamido)benzene (H2bpb, H is the dissociable amide proton) has been synthesized via addition of NaNO2 to [(bpb)Fe(py)2](ClO4) (1) in MeCN or DMF. This structurally characterized Fe(III) nitro complex exhibits its νNO2 at 1384 cm-1. The reaction of 1 with 2 equiv of Et4NX (X = Cl-, Br-) affords the high-spin complexes (Et4N)[(bpb)Fe(Cl2)] (3) and (Et4N)[(bpb)Fe(Br)2] (4), respectively. The structure of 4 has been determined. The addition of an equimolar amount of Et4NCl, Et4NBr, or Et4NCN to a solution of 2 affords the mixed-ligand complexes (Et4N)[(bpb)Fe(NO2)(Cl)] (5), (Et4N)[(bpb)Fe(NO2)(Br)] (6), and (Et4N)[(bpb)Fe(NO2)(CN)] (7), respectively. These complexes are all low spin with isotropic g values of 2.15. Under anaerobic conditions, the reactions of 5−7 with Ph3P in MeCN afford the five-coordinate {Fe−NO}7 nitrosyl [(bpb)Fe(NO)] (and Ph3PO) via secondary oxygen-atom (O-atom) transfer. The O-atom transfer to Ph3P by 5−7 becomes catalytic in the presence of dioxygen with transfer rates in the range of 1.70−13.59 × 10-3 min-1. The O-atom transfer rates and turnover numbers (5 > 6 > 7) are reflective of the strength of the axial σ donors (Cl- > Br- > CN-). The catalytic efficiencies of complexes 5−7 are limited due to formation of the thermodynamic end products [(bpb)Fe(X)2]- (where X = Cl- for 5, Br- for 6, and CN- for 7).