Stable Ferrous Mononitroxyl {FeNO}8 Complex with a Hindered Hydrotris(pyrazolyl)borate Coligand: Structure, Spectroscopic Characterization, and Reactivity Toward NO and O2

The iron(II)-nitroxyl complex [Fe(NO)(L3)] (1) (with L3– = a hindered hydrotris(pyrazolyl)borate ligand), a high-spin (hs)-{FeNO}8 complex in the Enemark–Feltham notation, is surprisingly stable and is the first of its kind that could be structurally characterized. We further studied this compound using a variety of spectroscopic methods. These results indicate a hs iron(II) center with a bound 3NO– ligand where the spins are antiferromagnetic coupled (St = 1). Vibrational data show that this complex has a very strong Fe−NO bond. DFT calculations support this result and link it to very strong π-donation from the 3NO– ligand to the iron(II) center. Furthermore, a very unusual equilibrium between the hs-{FeNO}8 complex and a dinitrosyl iron complex (DNIC) of {Fe(NO)2}9 type is observed. The O2 reactivity of the complex is finally reported.