On the feasibility of N2 fixation via a single-site FeI/FeIV cycle:: Spectroscopic studies of FeI(N2)FeI, FeIVN, and related species

The electronic properties of an unusually redox-rich iron system, [PhBp(3)(R)]Fe-N-x (where [PhBp(3)(R)] is [PhB(CH2PR2)(3)](-)), are explored by Mossbauer, EPR, magnetization, and density-functional methods to gain a detailed picture regarding their oxidation states and electronic structures. The complexes of primary interest in this article are the two terminal iron(IV) nitride species, [PhBp(3)(iPr)]Fe equivalent to N (3a) and [PhBp(3)(CH2Cy)]Fe equivalent to N (3b), and the formally diiron(I) bridged- Fe(mu-N-2) Fe species, {[PhBp(3)(iPr)]Fe}(2)(mu-N-2) (4). Complex 4 is chemicall, related to 3a via a spontaneous nitride coupling reaction. The diamagnetic iron(IV) nitrides 3a and 3b exhibit unique electronic environments that are reflected in their unusual Mossbauer parameters, including quadrupole-splitting values of 6.01(1) mm/s and isomer shift values of -0.34(I) mm/s. The data for 4 suggest that this complex can be described by a weak ferromagnetic interaction (J/D < 1) between two iron(l) centers. For comparison, four other relevant complexes also are characterized: a diamagnetic iron(IV) trihydride [PhBp(3)(iPr)]Fe(H)(3)(PMe3) (5), an S = 3/2 iron(I) phosphine adduct [PhBp(3)(iPr)]FePMe3 (6), and the S = 2 iron(II) precursors to 3a, [PhBp(3)(iPr)]Fe-Cl and [PhBP3iPr]Fe-2,3:5,6-dibenzo-7-aza bicyclo[2.2.1]hepta-2,5-diene (dbabh). The electronic properties of these respective complexes also have been explored by density-functional methods to help corroborate our spectral assignments and to probe their electronic structures further.