MOLECULAR SPIN FRUSTRATION IN THE [FE4O2]8+ CORE - SYNTHESIS, STRUCTURE, AND MAGNETOCHEMISTRY OF [FE4O2(O2CR)7(BPY)2](CLO4) (R = ME, PH)

The structural, spectroscopic, and magnetochemical characteristics of a new tetranuclear iron-oxo complex are reported. [Fe4O2(O2CCH3)7(bpy)2](ClO4).1/4CH2Cl2.H2O (1) crystallizes in the monoclinic space group C2/c with a = 27.261 (10) angstrom, b = 11.789 (4) angstrom, c = 16.439 (5) angstrom, beta = 118.27 (2)-degrees, V = 4653.19 angstrom3, and Z = 4. The structure was refined with 2646 reflections having F > 2.33-sigma(F), giving final R factors of 0.0644 and 0.0688 for R and R(w), respectively. The [Fe4O2]8+ core of the cation is structurally similar to other [M4O.2]8+ (M = Mn, Fe) complexes which have been previously reported. The core structure consists of a tetranuclear bis(mu-3-O) cluster disposed in a ''butterfly'' arrangement. Two different Fe-O(oxide) bridge distances of 1.819 (5) angstrom (wing-body) and 1.926 (5) angstrom (body-body) are observed. These differences are reflected in the Mossbauer spectrum of the complex, which analyzes as two quadrupole-split doublets in the range of 100-300 K. Each of the doublets has parameters characteristic of high-spin Fe(III) ions. H-1 NMR spectra are reported for two [Fe4O2]8+ complexes. Assignments for all resonances were made on the basis of chemical shift data for two related complexes and one deuterated complex as well as measurements of spin-lattice (T1) relaxation times. The magnetic susceptibility of complex 1 was measured in the range of 5.01-277.4 K. The effective moment per molecule decreases gradually from 4.20-mu-B at 277.4 K to 0.82-mu-B at 5.01 K, indicating a diamagnetic S = 0 ground state. A detailed theoretical analysis of the susceptibility data using a spin Hamiltonian approach gives a value for the ''wing-body'' Fe-Fe magnetic exchange interaction parameter of J(wb) = -45 cm-1. It was interesting to find that the ''body-body'' interaction J(bb) is indeterminate and can only be described as being more positive than -15 cm-1. The lack of definition of J(bb) is due to spin frustration, where the relative magnitudes of the antiferromagnetic J(wb) and J(bb) interactions result in a net alignment of the spin vectors on the two body dioxo bridge core Fe(III) ions. The significance of these results as they pertain to exchange coupling in iron-oxo proteins is discussed.