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

The electronic properties of an unusually redox-rich iron system, [PhBP3]FeONx (where [PhBP3] is [PhB(CH2PR2)3] ), are explored by Mössbauer, 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, [PhBP3]Fe'N (3a) and [PhBP3]Fe'N (3b), and the formally diiron(I) bridged-Fe( -N2)Fe species, {[PhBP3]Fe}2( -N2) (4). Complex 4 is chemically 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 Mössbauer parameters, including quadrupole-splitting values of 6.01(1) mm s and isomer shift values of 0.34(1) mm s. The data for 4 suggest that this complex can be described by a weak ferromagnetic interaction (J D < 1) between two iron(I) centers. For comparison, four other relevant complexes also are characterized: a diamagnetic iron(IV) trihydride [PhBP3]Fe(H)3(PMe3) (5), an S 3 2 iron(I) phosphine adduct [PhBP3]FePMe3 (6), and the S 2 iron(II) precursors to 3a, [PhBP3]FeOCl and [PhBP3]Fe-2,3:5,6dibenzo-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.