Combined Mossbauer Spectral and Density Functional Study of an Eight-Coordinate Iron(II) Complex

The iron-57 Mossbauer spectra of the eight-coordinate complex, [Fe-(L-N4)(2)] (BF4)(2), where LN4 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, delta(Fe), of 1.260(1) mm/s and a quadrupole splitting, Delta E-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 Mossbauer temperature, Theta(M) of 319(27) K and the temperature dependence of the logarithm of the Mossbauer spectral absorption area yields a Debye temperature, Theta(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(LN4)(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 Mossbauer spectral parameters. The structure of 2 with a distorted square-antiprism C-1 iron(II) coordination symmetry exhibits four different Fe-N-imid bonds to the imidazole nitrogens with an average bond distance of 2.253(2) angstrom and four different Fe-N-imine bonds to the benzenediimine nitrogens, with an average bond distance of 2.432(2) angstrom; this large difference yields the large observed Delta E-Q, An optimization of the [Fe(L-N4)(2)](2+) structure leads to a highly symmetric eight-coordination environment with S-4 symmetry and four equivalent Fe-Nd-imid bond distances of 2.301(2) angstrom and four equivalent Fe-N-imine bond distances of 2.487(2) angstrom. In contrast, an optimization of the [Mn(L-N4)(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) angstrom and four equivalent Mn-N-imine bond distances of 2.565(3) angstrom.