Mossbauer, Electron Paramagnetic Resonance, and Density Functional Theory Studies of Synthetic S=1/2 FeIII-O-FeIV=O Complexes. Superexchange-Mediated Spin Transition at the FeIV=O Site

Previously we have characterized two high-valent complexes [LFeIV(mu-O)(2)(FeL)-L-III], 1, and [LFeIV(O)(mu-O)(OH) (FeL)-L-IV], 4. Addition of hydroxide or fluoride to 1 produces two new complexes, 1-0H and 1-F. Electron paramagnetic resonance (EPR) and Mbssbauer studies show that both complexes have an S = 1/2 ground state which results from antiferromagnetic coupling of the spins of a high-spin (S-a = 5/2) Fe-III and a high-spin (Sb = 2) Fe-IV site. 1-OH can also be obtained by a 1-electron reduction of 4, which has been shown to have an Fe-IV=O site. Radiolytic reduction of 4 at 77 K yields a Mossbauer spectrum identical to that observed for 1-0H, showing that the latter contains an Fe-IV=O. Interestingly, the FeO moiety has Sb = 1 in 4 and Sb = 2 in 1-0H and 1-F,From the temperature dependence of the S = 1/2 signal we have determined the exchange coupling constant J(.7(= JSa " Sb convention) to be 90 20 cm ' for both 1-0H and 1-F. Broken-symmetry density functional theory (DFT) calculations yield J= 135 cmT1 for 1-0H and J= 104 cm-1 for 1-F, in good agreement with the experiments. DFT analysis shows that the Sb = 1 Sb = 2 transition of the Felv=0 site upon reduction of the Fe' OH site to high-spin Fe" is driven primarily by the strong antiferromagnetic exchange in the (Sa = 5/2, Sb = 2) couple.