title: Mössbauer and computational study of an N2-bridged diiron diketiminate complex: parallel alignment of the iron spins by direct antiferromagnetic exchange with activated dinitrogen.
abstract: This work reports Mössbauer and DFT studies of the diiron-N2 complex LMeFeNNFeLMe (L = beta-diketiminate), 1a. Complex 1a, formally diiron(I), has a system spin S = 3 with an isolated MS = +/-3 quasi-doublet as a ground state; the MS = +/-2 doublet is >100 cm-1 higher in energy. Complex 1a exhibits at 4.2 K a large, positive magnetic hyperfine field, Bint = +68.1 T, and an effective g value of 16 +/- 2 along the easy magnetization axis of the ground doublet; this value is significantly larger than the spin-only value (g = 12). These results have been rationalized by DFT calculations, which show that each Fe site donates significant electron density into the pi* orbitals of dinitrogen, resulting in a configuration best described as two high-spin FeII (Sa = Sb = 2) bridged by triplet N22- (Sc = 1). In this description the minority spin electron of each iron is accommodated by two nonbonding, closely spaced 3d orbitals, z2 and yz (z is perpendicular to the diketiminate planes, x is along the Fe...Fe vector). Spin-orbit coupling between these orbital states generates a large unquenched orbital momentum along the iron-iron vector. The S = 3 ground state of 1a results from strong antiferromagnetic direct exchange couplings of the Fe spins (Sa = Sb = 2) to the N22- spin (Sc = 1) and can be formulated as ((Sa,Sb)Sab = 4, Sc = 1), S = 3>; H = J(Sa + Sb).Sc with J approximately 3500 cm-1.
authors.affiliation: Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, USA.
authors.forename: Sebastian A
authors.initials: SA
authors.lastname: Stoian
authors.forename: Javier
authors.initials: J
authors.lastname: Vela
authors.forename: Jeremy M
authors.initials: JM
authors.lastname: Smith
authors.forename: Azwana R
authors.initials: AR
authors.lastname: Sadique
authors.forename: Patrick L
authors.initials: PL
authors.lastname: Holland
authors.forename: Eckard
authors.initials: E
authors.lastname: Münck
authors.forename: Emile L
authors.initials: EL
authors.lastname: Bominaar
mesh_terms: D007501:Iron / Q000737:chemistry*; D008280:Magnetics; D009584:Nitrogen / Q000737:chemistry*; D015204:Spectroscopy, Mossbauer / Q000379:methods*
chemical_list: D007501:Iron; D009584:Nitrogen