Structural and spectroscopic comparisons between (mu-oxo)- and (mu-hydroxo)bis(mu-carboxylato)diiron(III) complexes that contain all-oxygen-donor ligands

Treatment of the (mu-oxo)bis(mu-carboxylato)diiron(III) complex [Fe-2(mu-O) (mu-XDK)(CH3OH)(6)](2+), where XDK2- the dianion of m-xylylenediamine bis(K emp'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 [Fe-2(mu-O )(mu-XDK)(hfac)(2)(H2O)(2)] (3) was crystallographically characterized and presents water ligands bound trans to the bridging oxo group. Omission of E t3N from, the above synthesis afforded an orange adduct, which is assigned as the (mu-hydroxo)(mu-XDK)bis(hfac)diiron(III) complex. Analytically pure samples of this [Fe-2(mu-OH)(mu-XDK)(hfac)(2)(H2O)(2)](+) (4) cation were o btained as the salt of the noncoordinating anion tetrakis(3,5-bis(trifluoro methyl)phenyl)borate. Protonation of the oxo bridge in compound 3 leads to significant structural changes that are localized to the {Fe-O-Fe} core. Th e optical spectrum of 3 contains a broad Ligand-field transition centered s imilar to 590 nm, a band that is noticeably absent in 4. The 77 K Mossbauer spectra of 3 and 4 appear as doublets with similar isomer shifts (delta ap proximate to 0.5 mm s(-1)) but very different quadrupole splittings (Delta E-Q = 1.68 and 0.58 mm s(-1) for 3 and 4, respectively). When samples of 3 and 4 were subjected to gamma-ray-induced cryoreduction, EPR-active species with g-values < 2 were generated. We attribute these signals to the format ion of the corresponding mixed-valence (FeFeIII)-Fe-II forms. The signal de rived from 3 is narrowly dispersed (g(av) = 1.90), whereas the one from 4 i s more broadly dispersed (g(av) - 1.74). Crystal data for 3 . H2O . 1.5CH(3 )CN: triclinic, P (1) over bar, a = 13.9442(3) Angstrom, b = 15.2151(4) Ang strom, c = 15.7164(3) Angstrom, alpha = 109.032(1)degrees, beta = 97.975(1) degrees, gamma = 115.697(1)degrees, V 2682.4(1) Angstrom(3), Z = 2, T = -85 degrees C.