3-COORDINATE IRON COMPLEXES - X-RAY STRUCTURAL CHARACTERIZATION OF THE AMIDE-BRIDGED DIMERS [FE(NR2)2]2 (R = SIME3, C6H5) AND THE ADDUCT FE[N(SIME3)2]2(THF) AND DETERMINATION OF THE ASSOCIATION ENERGY OF THE MONOMER FE(N(SIME3)2)2 IN SOLUTION

The X-ray structural characterization of the iron(II) amides [Fe{N(SiMe3)2}2]2 (1) and [Fe(NPh2)2]2 (2) and the Lewis base adduct Fe[N(SiMe3)2]2(THF) (3), as well as the syntheses of the last two compounds, is described. These complexes are rare examples of the coordination number 3 for iron. Compounds 1 and 2 are both dimeric in the solid state, with each trigonal-planar ion bound to one terminal and two bridging amides. They closely resemble the corresponding Mn(II) and Co(II) compounds. Compound 3 is monomeric in the solid state, with one THF and two amides arranged in a trigonal-planar fashion. The terminal Fe-N bond lengths in 1-3 are similar to those reported for two-coordinate iron(II) amides. The Fe-N bonds in 1 are somewhat longer than those in 2, indicating weaker association in 1. This is borne out in the solution behavior of the two compounds. Thus 1, which was studied by variable-temperature H-1 NMR spectroscopy, was seen to be a monomer in solution at 30-degrees-C. Increasing amounts of the dimer were observed at lower temperature, and calculations based on the monomer-dimer equilibrium indicate an association energy of approximately + 3 kcal mol-1 for [Fe{N(SiMe3)2}2]2. Crystal data with Mo K-alpha (lambda = 0.71069 angstrom) radiation at 130 K: 1, C24H72- Fe2N4Si8, a = 17.978 (4) angstrom, b = 14.691 (4) angstrom, c = 18.564 (5) angstrom, beta = 120.15 (2)-degree, Z = 4, monoclinic, space group C2/c, R = 0.031; 2, C48H40Fe2N4, a = 9.579 (5) angstrom, b = 10.264 (5) angstrom, c = 10.482 (6) angstrom, alpha = 91.78 (4)-degrees, beta = 110.68 (4)-degrees, gamma = 85.67 (4)-degrees, Z = 1, triclinic space group P1BAR, R = 0.039; 3, C16H44FeN2OSi4, a = 11.225 (5) angstrom, b = 13.391 (5) angstrom, c = 17.903 (8) angstrom, Z = 4, orthorhombic, space group Pcan, R = 0.061.