Title

Electronic and Molecular Structure of Variable-spin Metal Complexes

Alternative Title

Spin state dependent crystal and molecular structures of [Feᴵᴵᴵ(5-OCH₃Salmeen)₂]PF₆ (S = ₅/₂), [Feᴵᴵᴵ(3-OCH₃Salmeen)₂]PF₆ (S = ½), and [Feᴵᴵᴵ(5-NO₂Salmeen)₂]PF₆ (S = ½)

Abstract

Spin state dependent crystal structure determinations of three six-coordinate 3d5 complexes of the [FeIII(X-Salmeen)2]PF6 series of spin-equilibrium compounds (X = 5-OCH3,3-OCH3, and 5-NO2) are reported. At room temperature, the 5-OCH3 derivative is a S = 5/2 high-spin species (μeff 295 = 5.75 μB), whereas the 5-NO2 (μeff 295 = 2.58 μB) and 3-OCH3 (μeff 298 = 3.20 μB) derivatives are at or near the S = 1/2 low-spin extreme. The structural determinations reveal that the three [FeIII(X-Salmeen)2]+ cations possess the same general structure with an N4O2 donor atom set from two tridentate X-Salmeen ligands forming a distorted octahedron about the metal ion. In each case the terminal oxygen atoms occupy cis positions and the remaining four nitrogen atoms (two cis amines and two trans imines) complete the coordination sphere. The fine structural details for the three 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 = 5/2 complex, the average Fe-ligand bond length (δav) is longer by about 0.12 Å relative to the two S = 1/2 cases, but the difference is not uniform: the Fe-N bonds vary far more [δ(amino) = 0.16 Å; δ(imino) = 0.16 Å] than the Fe-O bonds (δ = 0.02 Å). The spin state dependent structural changes observed for the [FeIII(X-Salmeen)2]+ spin-equilibrium cations are compared to results previously reported for the electronically and structurally related hexadentate cations [FeIII(Sal)2trien]+ and [FeIII(Acac)2trien]+, for which δav is also 0.12-0.13 Å. In addition, variable-spin structural information now available for a variety of other 3d5 (and 3d6) spin-equilibrium compounds is also discussed in the context of the present results for the [FeIII(XSalmeen)2]+ series of complexes. Crystal data for [FeIII(5-OCH3Salmeen)2]PF6: FePF6O4N4C22H30, space group Pbca, Z = 8, a = 10.263 (3) Å, b = 19.696 (6) Å, c = 26.768 (6) Å, V = 5411 Å3, R = 5.1% for 2712 reflections. Crystal data for [FeIII(3-OCH3Salmeen)2]PF6: FePF6O4N4C22H30, space group P21/c, Z = 4, a = 11.627 (2) Å, * = 10.412 (5) Å, c = 21.632 (9) Å, β = 92.62 (2)°, V = 2616 Å3, R = 6.3% for 2341 reflections. Crystal data for [FeIII(5-NO2Salmeen)2]PF6: FePF6O6N6C20H24, space group Pbcn, Z = 4, a = 12.609 (7) Å, b = 12.927 (4) Å, c = 16.122 (2) Å, V = 2628 Å3, R = 4.7% for 1648 reflections. © 1981 American Chemical Society.

Department(s)

Chemistry

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 1981 American Chemical Society (ACS), All rights reserved.


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