Site-isolated redox reactivity in a trinuclear iron complex

Abstract

The symmetric, high-spin triiron complex ((Ph)L)Fe(3)(THF)(3) reacts with mild chemical oxidants (e.g., Ph(3)C-X, I(2)) to afford an asymmetric core, where one iron bears the halide ligand ((Ph)L)Fe(3)X(L) and the hexadentate ((Ph)L = MeC(CH(2)NPh-o-NPh)(3)) ligand has undergone significant rearrangement. In the absence of a suitable trapping ligand, the chlorine and bromine complexes form (μ-X)(2)-bridged structures of the type [((Ph)L)Fe(3)(μ-X)](2). In the trinuclear complexes, the halide-bearing iron site sits in approximate trigonal-bipyramidal (tbp) geometry, formed by two ((Ph)L) anilides and an exogenous solvent molecule. The two distal iron atoms reside in distorted square-planar sites featuring a short Fe-Fe separation at 2.301 Å, whereas the distance to the tbp site is substantially elongated (2.6-2.7 Å). Zero-field, (57)Fe Mössbauer analysis reveals the diiron unit as the locus of oxidation, while the tbp site bearing the halide ligand remains divalent. Magnetic data acquired for the series reveal that the oxidized diiron unit comprises a strongly coupled S = (3)/(2) unit that is weakly ferromagnetically coupled to the high-spin (S = 2) ferrous site, giving an overall S = (7)/(2) ground state for the trinuclear units.

Figure 1

Solid-state structures for (a) (^PhL)Fe₃Cl(py) (3), (b) [(^PhL)Fe₃(µ-Br)]₂ (4), and (c) (^PhL)Fe₃I(thf) (6) with the thermal ellipsoids set at the 50% probability level (hydrogen atoms, and solvent molecules omitted for clarity; Fe orange, C black, H white, N blue, O red, Cl green, Br brown, I magenta). Bond lengths (Å) for 3: Fe1-Fe2, 2.7303(8); Fe1-Fe3, 2.6534(8); Fe2-Fe3, 2.2955(8); Fe1-Cl, 2.3333(11); Fe1-N_py, 2.066(3); for 4: Fe1-Fe2, 2.5871(7); Fe1-Fe3, 2.5873(7); Fe2-Fe3, 2.3504(7); Fe1-Br1, 2.5715(6), Fe1-Br2, 2.4670(6); Fe1-Fe1, 3.5810(11); for 6: Fe1-Fe2, 2.6026(10); Fe1-Fe3, 2.6971(11); Fe2-Fe3, 2.3079(10); Fe1-I, 2.6695(9); Fe1-O, 2.024(4).

Figure 2

(a) Zero-field ⁵⁷Fe Mössbauer spectrum obtained at 90 K and spectral fits (δ, |ΔE_Q| (mm/s)) for 3 (component 1 (magenta): 0.83, 1.67, 33%; component 2 (green): 0.29, 2.44, 33%; component 3 (blue): 0.20, 2.79, 33%). (b) Variable-temperature magnetic susceptibility data for 2 (diamonds) and 3 (circles) collected in an applied dc field of 0.1 T. Solid lines represent simulations to the data as described in the text. (c) Plot of reduced magnetization for 5 between 1.8 and 10 K at applied fields of 1–7 T.

Scheme 1

Scheme 2