Structural and Spectroscopic Comparisons between (μ-Oxo)- and (μ-Hydroxo)bis(μ-carboxylato)diiron(III) Complexes That Contain All-Oxygen-Donor Ligands

Treatment of the (μ-oxo)bis(μ-carboxylato)diiron(III) complex [Fe2(μ-O)(μ-XDK)(CH3OH)6]2+, where XDK2- = the dianion of m-xylylenediamine bis(Kemp's triacid imide), with 2 equiv of 1,1,1,5,5,5-hexafluoroacetylacetone (hfacH) and Et3N led to substitution of two methanol ligands by a bidentate hfac- anion at each metal center. The green bis(aqua) complex [Fe2(μ-O)(μ-XDK)(hfac)2(H2O)2] (3) was crystallographically characterized and presents water ligands bound trans to the bridging oxo group. Omission of Et3N from the above synthesis afforded an orange adduct, which is assigned as the (μ-hydroxo)(μ-XDK)bis(hfac)diiron(III) complex. Analytically pure samples of this [Fe2(μ-OH)(μ-XDK)(hfac)2(H2O)2]+ (4) cation were obtained as the salt of the noncoordinating anion tetrakis(3,5-bis(trifluoromethyl)phenyl)borate. Protonation of the oxo bridge in compound 3 leads to significant structural changes that are localized to the {Fe−O−Fe} core. The optical spectrum of 3 contains a broad ligand-field transition centered ∼590 nm, a band that is noticeably absent in 4. The 77 K Mössbauer spectra of 3 and 4 appear as doublets with similar isomer shifts (δ ≈ 0.5 mm s-1) but very different quadrupole splittings (ΔE Q = 1.68 and 0.58 mm s-1 for 3 and 4, respectively). When samples of 3 and 4 were subjected to γ-ray-induced cryoreduction, EPR-active species with g-values < 2 were generated. We attribute these signals to the formation of the corresponding mixed-valence FeIIFeIII forms. The signal derived from 3 is narrowly dispersed (g av = 1.90), whereas the one from 4 is more broadly dispersed (g av = 1.74). Crystal data for 3·H2O·1.5CH3CN: triclinic, P1̄, a = 13.9442(3) Å, b = 15.2151(4) Å, c = 15.7164(3) Å, α = 109.032(1)°, β = 97.975(1)°, γ = 115.697(1)°, V = 2682.4(1) Å3, Z = 2, T = −85 °C.