Electronic and Molecular Structure of Variable-spin Iron(III) Chelates with Hexadentate Ligands Derived from Triethylenetetramine and Β-diketones Or Salicylaldehyde

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Spin state dependent crystal and molecular structures of [Fe(acac)2trien]PF₆(S = 5/2), [Fe(acacCl)₂trien]PF₆(S = 5/2), [Fe(sal)₂trien]Cl.2H₂O(S = 1/2), and [Fe(sal)₂trien]NO₃.H₂O(S = 1/2)


Six-coordinate iron(III) complexes derived from triethylenetetramine (trien) and acetylacetone (acac), 3-chloroacetylacetone (acacCl), or salicylaldehyde (sal), [Fe(acac)2trien](X), [Fe(acacCl)2trien](PF6), and [Fe(sal)2trien](Y) (X = PF6 -, BPh4 -; Y = PF6 -, Cl-, NO3 -) have been shown by variable-temperature magnetochemical and Mössbauer spectroscopy measurements to be members of a family of low-spin (S = 1/2; 2T2) ⇄ high-spin (S = 5/2; 6A1) spin-equilibrium compounds in the solid state. The acac and acacCl compounds with X = PF6 - are essentially high spin (∼5.9 μB) at room temperature, whereas the sal complexes with Y = Cl-·2H2O and NO3 -·H2O) are mainly low spin (1.94 and 2.47 μB). X-ray crystallographic structure determinations of these four compounds show that, except for the differences between the acac, acacCl, and sal ligand moieties, the cations possess the same general structures with an N4O2 donor atom set of a hexadentate ligand forming a distorted octahedron about the metal atom. In each case the terminal oxygen donor atoms occupy cis positions and the remaining four nitrogen atoms (two cis amine and two trans imine) complete the coordinate sphere. The fine structural details for the four compounds reveal significant differences in the metal atom environments which can be largely attributed to the differing S = 5/2 and S = 1/2 spin states. For the S = 1/2 sal complexes, the average metal-ligand bond length (δav) is shorter by about 0.12-0.13 Å relative to the S = 5/2 cases, but this difference is not uniform: the Fe-N bonds vary far more [δ(amine) 0.17 Å; δ(imine) 0.17 Å] than the Fe-O bonds (δ 0.04 Å). In the low-spin [Fe(sal)2trien]Cl·2H2O complex, the Cl- ion is hydrogen bonded to the two water molecules, one of which is in turn hydrogen bonded to an amine donor atom; this hydrogen bonding network in the solid state is consistent with earlier solution state studies where strong [solvent⋯HN] interactions were shown to promote the low-spin state. The hydrogen bonding forms a polymeric chain of anions and cations. A similar hydrogen bonding scheme also exists in the low-spin NO3 - compound. The spin state dependent structural changes observed for the present spin-equilibrium system are compared to those for the electronically similar, but structurally different, tris(dithiocarbamato)-iron(III) complexes, and the possible consequences of these spin-related structural changes discussed in terms of molecular volume and dynamic spin lifetime measurements in the solution state. Crystal data for [Fe(acac)2trien]PF6: FeP-F6O2N4C16H28, space group C2/c, Z = 8, a = 24.44 (1) Å, b = 13.315 (6) Å, c = 15.202 (7) Å, β = 113.51 (3)°, V = 4536 Å3, R = 5.5%, 2849 reflections. Crystal data for [Fe(acacCl)2trien]PF6: FeCl2PF6O2N4C16H 26, space group C2/c, Z = 8, a = 24.14 (1) Å, b = 13.404 (9) Å, c = 15.98 (1) Å, β = 111.56 (7)°, V = 4810 Å3, R = 6.7%, 1186 reflections. Crystal data for [Fe(sal)2trien]Cl·2H2O: FeClO4N4C20H28, space group P21/c, Z = 4, a = 18.58 (1) Å,* = 10.026 (3) Å, c = 12.061 (5) Å, β = 103.28 (3)°, V = 2187 Å3, R = 3.4% 2614 reflections. Crystal data for [Fe(sal)2trien]NO3·H2O: FeO6N5C20H26, space group P21/a, Z = 4, a = 11.979 (7) Å, b = 9.967 (4) Å, c = 18.472 (7) Å, β = 98.38 (5)°, V = 2182 Å3, R = 4.5%, 2408 reflections. © 1978 American Chemical Society.



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© 1978 American Chemical Society (ACS), All rights reserved.