Experimental and Theoretical Mossbauer Study of an Extended Family of [Fe8(μ4-O)4(μ-4-R-px)12X4] Clusters

Six [Fe-8(mu(4)-O)(4)(mu-4-R-pyrazolato)(12)X-4] complexes containing an identical Fe-8(mu(4)-O)(4) core have been structurally characterized and studied by Mossbauer spectroscopy. In each case, an inner mu(4)-O bridged Fe-III cubane core is surrounded by four trigonal bipyramidal iron centers, the two distinct sites occurring in a 1:1 ratio. The Mossbauer spectrum of each of the clusters consists of two quadrupole doublets, which, with one exception (X = NCS, R = H), overlap to give three absorption lines. The systematic variation of X and R causes significant changes in the Mossbauer spectra. A comparison with values for the same clusters computed using density functional theory allows us to establish an unequivocal assignment of these peaks in terms of a nested model for the overlapping doublets. The changes in Mossbauer parameters (both experimental and computed) for the 1-electron reduced species [Fe-8(mu(4)-O)(4)(mu-4-Cl-pyrazolato)(12)Cl-4](-) are consistent with a redox event that is localized within the cubane core.