Density Functional Theory Study of the Structural, Electronic, and Magnetic Properties of a μ-oxo Bridged Dinuclear FeIV Complex Based on a Tetra-Amido Macrocyclic Ligand

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• 作者：Arani Chanda ; Filipe Tiago de Oliveira ; Terrence J. Collins ; Eckard Mü ; nck ; Emile L. Bominaar
• 刊名：Inorganic Chemistry
• 出版年：2008
• 出版时间：October 20, 2008
• 年：2008
• 卷：47
• 期：20
• 页码：9372-9379
• 全文大小：235K
• 年卷期：v.47,no.20(October 20, 2008)
• ISSN：1520-510X

文摘

Recently, the synthesis, crystallographic structure, and Mössbauer characterization of the first example of an [(Fe^IVTAML)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 ⁵⁷Fe 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 Å, in good agreement with the crystallographic distance 1.728 Å. To accommodate the spin-dependent reorganization energies, the conventional bilinear spin Hamiltonian has been extended with a biquadratic coupling term: Ĥ_ex = c′ + j₀Ŝ₁·Ŝ₂ + j₁(Ŝ₁·Ŝ₂)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^IVB*)₂O]²⁻. Two mechanisms for biquadratic exchange are discussed.