title: Density functional theory study of the structural, electronic, and magnetic properties of a mu-oxo bridged dinuclear Fe(IV) complex based on a tetra-amido macrocyclic ligand.
abstract: Recently, the synthesis, crystallographic structure, and Mossbauer characterization of the first example of an [(Fe(IV)TAML)2O](2-) (TAML = tetra-amido microcyclic ligand) complex were reported. Here, we elucidate the prominent structural, electronic, and magnetic properties of this complex on the basis of density functional theory (DFT) calculations. While the torsion between the molecular halves is caused by hydrogen bonding between the TAML moieties, the bending of the Fe-O-Fe unit is an intrinsic property of the bridge. The values for the (57)Fe isomer shift and quadrupole splitting obtained with DFT are in good agreement with experimental results and indicate that the irons have intermediate spin states (S1 = S2 = 1). The iron spins are coupled by strong antiferromagnetic exchange to yield a ground state with system spin S = 0. The Fe-O distances in the excited S > 0 states are significantly longer than in the ground state. Since the wave function of the ground state, in which the iron spins are antiferromagnetically coupled to give system spin S = 0, is a linear combination of Slater determinants that cannot be treated with existing DFT codes, the Fe-O distance for the S = 0 state has been estimated by extrapolation from the optimized geometries for the ferromagnetic state (S = 2) and the broken symmetry state to be 1.748 A, in good agreement with the crystallographic distance 1.728 A. To accommodate the spin-dependent reorganization energies, the conventional bilinear spin Hamiltonian has been extended with a biquadratic coupling term: H(ex) = c' + j0S1 x S2 + j1(S1 x S2)(2). A computational scheme is presented for estimating the exchange parameters, yielding the values j0 = 199 cm(-1) and j1 = -61 cm(-1) for [(Fe(IV)B*)2O](2-). Two mechanisms for biquadratic exchange are discussed.
authors.affiliation: Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania, 15213, USA.
authors.forename: Arani
authors.initials: A
authors.lastname: Chanda
authors.forename: Filipe Tiago
authors.initials: FT
authors.lastname: de Oliveira
authors.forename: Terrence J
authors.initials: TJ
authors.lastname: Collins
authors.forename: Eckard
authors.initials: E
authors.lastname: Münck
authors.forename: Emile L
authors.initials: EL
authors.lastname: Bominaar