Metal atom lability in polynuclear complexes

Abstract

The asymmetric oxidation product [((Ph)L)Fe3(μ-Cl)]2 [(Ph)LH6 = MeC(CH2NHPh-o-NHPh)3], where each trinuclear core is comprised of an oxidized diiron unit [Fe2](5+) and an isolated trigonal pyramidal ferrous site, reacts with MCl2 salts to afford heptanuclear bridged structures of the type ((Ph)L)2Fe6M(μ-Cl)4(thf)2, where M = Fe or Co. Zero-field, (57)Fe Mössbauer analysis revealed the Co resides within the trinuclear core subunits, not at the octahedral, halide-bridged MCl4(thf)2 position indicating Co migration into the trinuclear subunits has occurred. Reaction of [((Ph)L)Fe3(μ-Cl)]2 with CoCl2 (2 or 5 equivalents) followed by precipitation via addition of acetonitrile afforded trinuclear products where one or two irons, respectively, can be substituted within the trinuclear core. Metal atom substitution was verified by (1)H NMR, (57)Fe Mossbauer, single crystal X-ray diffraction, X-ray fluorescence, and magnetometry analysis. Spectroscopic analysis revealed that the Co atom(s) substitute(s) into the oxidized dimetal unit ([M2](5+)), while the M(2+) site remains iron-substituted. Magnetic data acquired for the series are consistent with this analysis revealing the oxidized dimetal unit comprises a strongly coupled S = 1 unit ([FeCo](5+)) or S = 1/2 ([Co2](5+)) that is weakly antiferromagnetically coupled to the high spin (S = 2) ferrous site. The kinetic pathway for metal substitution was probed via reaction of [((Ph)L)Fe3(μ-Cl)]2 with isotopically enriched (57)FeCl2(thf)2, the results of which suggest rapid equilibration of (57)Fe into both the M(2+) site and oxidized diiron site, achieving a 1:1 mixture.

Figure 1

Zero-field ⁵⁷Fe Mössbauer spectrum obtained at 90 K and spectral fits (δ, |ΔE_Q| (^mm/_s)) for (a) (^PhL)₂Fe₇(µ²-Cl)₄(thf)₂ (2) ((blue, 28.7%) 0.28, 2.38; (green, 24.9%) 0.17, 2.67; (gold, 29%) 0.72, 1.32; (pink, 17.4%) 1.18, 2.37); (b) (^PhL)₂Fe₆Co(µ²-Cl)₄(thf)₂ (3) (blue, 38.9%) 0.21, 2.69; (green, 38.9%) 0.73, 1.39; (gold, 22.3%) 1.16, 2.35. Solid-state structures for (^PhL)₂Fe₇(µ²-Cl)₄(thf)₂ (2) with the thermal ellipsoids set at the 50% probability level (hydrogen atoms, and solvent molecules omitted for clarity; Fe orange, C gray, N blue, O red, Cl green).

Figure 2

Solid-state structures for (a) (^PhL)Fe₂CoCl(NCCH₃) (4), (b) (^PhL)FeC_O2Cl(NCCH₃) (5), and (c) [(^PhL)FeCo₂(µ-Cl)]₂ (6) with the thermal ellipsoids set at the 50% probability level (hydrogen atoms, and solvent molecules omitted for clarity; Fe orange, Co aquamarine, C gray, N blue, O red, Cl green). Bond lengths (Å) for 4: Fe1-M2, 2.5391(7); Fe1-M3, 2.5493(8); M2-M3, 2.2934(8); Fe1-Cl, 2.3393(9); Fe1-N_ACN, 2.134(3); for 5: Fe1-Co2, 2.5253(6); Fe1-Co3, 2.5348(6); Co2-Co3, 2.2971(5); Fe1-Cl, 2.2348(9); Fe1-N_ACN, 2.129(3); for 6: Fe1A-Co2A, 2.5120(14); Fe1ACo3A, 2.5319(14); Co2A-Co3A, 2.2860(13); Fe1A-Cl1, 2.349(2), Fe1A-Cl2, 2.441(2); Fe1-Fe1, 3.4474(14); Fe1B-Co2B, 2.5009(14); Fe1B-Co3B, 2.5334(14); Co2B-Co3B, 2.2862(14); Fe1B-Cl1, 2.349(2), Fe1B-Cl2, 2.428(2); Fe1-Fe1, 3.4474(14).

Figure 3

Zero-field ⁵⁷Fe Mössbauer spectrum obtained at 90 K and spectral fits (δ, |ΔE_Q| (^mm/_s)) for (a) (^PhL)Fe₂CoCl(NCCH₃) (4) (component 1 (blue): 0.83, 1.41, 56.7%; component 2 (green): −0.01, 2.36, 22.1%; component 3 (magenta): 0.21, 2.92, 21.1%); (b) (^PhL)Fe₂CoCl(NCCH₃) (6) (0.69, 1.38). (c) X-ray fluorescence spectra (data black circles, fits represented as lines): of 1 (red); 4 (green); 6 (blue). (d) Variable-temperature magnetic susceptibility data for 4 (circles) and 5 (triangles) collected in an applied dc field of 0.1 T. Solid lines represent fits to the data as described in the text.

Scheme 1

Scheme 2

Scheme 3

Frontier molecular orbital interactions of [M_AM_B]⁵⁺ unit in 1, 4, and 6.