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 (H(2)bpb, 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 nu(NO2) at 1384 cm(-1). The reaction of 1 with 2 equiv of Et4NX (X = Cl-, Br-) affords the high-spin complexes (Et4N)[(bpb) Fe(Cl-2)] (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 x 10(-3) min(-1). The O-atom transfer rates and turnover numbers (5 > 6 > 7) are reflective of the strength of the axial sigma 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).