On the feasibility of N 2 fixation via a single-site Fe I /Fe IV cycle: Spectroscopic studies of Fe I (N 2 )Fe I , Fe IV N, and related species

The electronic properties of an unusually redox-rich iron system, [PhBP R 3 ]Fe N x (where [PhBP R 3 ] is [PhB(CH 2 PR 2 ) 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, [PhBP i Pr 3 ]Fe N (3a) and [PhBP CH2Cy 3 ]Fe N (3b), and the formally diiron(I) bridged-Fe(μ-N 2 )Fe species, {[PhBP i Pr 3 ]Fe} 2 (μ-N 2 ) ( 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 [PhBP i Pr 3 ]Fe(H) 3 (PMe 3 ) ( 5 ), an S = 3/2 iron(I) phosphine adduct [PhBP i Pr 3 ]FePMe 3 (6), and the S = 2 iron(II) precursors to 3a, [PhBP i Pr 3 ]Fe Cl and [PhBP i Pr 3 ]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.