Combined Mössbauer Spectral and Density Functional Study of an Eight-coordinate Iron(II) Complex


The iron-57 Mössbauer spectra of the eight-coordinate complex, [Fe(LN4)2](BF4)2, where LN4 is the tetradentate N1(E),N2(E)-bis[(1-methyl-1H-imidazol-2-yl)methylene]-1,2-benzenediimine ligand, have been measured between 4.2 and 295 K and fit with a quadrupole doublet. The fit at 4.2 K yields an isomer shift, δFe, of 1.260(1) mm/s and a quadrupole splitting, ΔEQ, of 3.854(2) mm/s, values that are typical of a high-spin iron(II) complex. The temperature dependence of the isomer shift yields a Mössbauer temperature, θM, of 319(27) K and the temperature dependence of the logarithm of the Mössbauer spectral absorption area yields a Debye temperature, θD, of 131(6) K, values that are indicative of high-spin iron(II). Nonrelativistic single point density functional calculations with the B3LYP functional, the full 6-311++G(d,p) basis set, and the known X-ray structures for [Mn(LN4)2]2+, [Mn(LN4)2](ClO4)2, 1, [Fe(LN4)2]2+, and [Fe(LN4)2](BF4)2, 2, yield small electric field gradients for the manganese(II) complexes and electric field gradients and s-electron densities at the iron-57 nuclide that are in good to excellent agreement with the Mössbauer spectral parameters. The structure of 2 with a distorted square-antiprism C1 iron(II) coordination symmetry exhibits four different Fe-Nimid bonds to the imidazole nitrogens with an average bond distance of 2.253(2) Å and four different Fe-Nimine bonds to the benzenediimine nitrogens, with an average bond distance of 2.432(2) Å; this large difference yields the large observed ΔEQ. An optimization of the [Fe(LN4)2]2+ structure leads to a highly symmetric eight-coordination environment with S4 symmetry and four equivalent Fe-Nimid bond distances of 2.301(2) Å and four equivalent Fe-Nimine bond distances of 2.487(2) Å. In contrast, an optimization of the [Mn(LN4)2]2+ structure leads to an eight-coordination manganese(II) environment with D2d symmetry and four equivalent Mn-Nimid bond distances of 2.350(3) Å and four equivalent Mn-Nimine bond distances of 2.565(3) Å. (Graph Presented).



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