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(L(N4))2](BF4)2, where L(N4) is the tetradentate N(1)(E),N(2)(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, δ(Fe), of 1.260(1) mm/s and a quadrupole splitting, ΔE(Q), 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, Θ(M), of 319(27) K and the temperature dependence of the logarithm of the Mössbauer spectral absorption area yields a Debye temperature, Θ(D), 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(L(N4))2](2+), [Mn(L(N4))2](ClO4)2, 1, [Fe(L(N4))2](2+), and [Fe(L(N4))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-N(imid) bonds to the imidazole nitrogens with an average bond distance of 2.253(2) Å and four different Fe-N(imine) bonds to the benzenediimine nitrogens, with an average bond distance of 2.432(2) Å; this large difference yields the large observed ΔE(Q). An optimization of the [Fe(L(N4))2](2+) structure leads to a highly symmetric eight-coordination environment with S4 symmetry and four equivalent Fe-N(imid) bond distances of 2.301(2) Å and four equivalent Fe-N(imine) 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 D(2d) symmetry and four equivalent Mn-N(imid) bond distances of 2.350(3) Å and four equivalent Mn-N(imine) bond distances of 2.565(3) Å.