Pentacoordinate (&mgr;-Oxo)diiron(III) Thiolate Complexes and Dimeric Iron(II) Precursors

The (&mgr;-oxo)diiron(III) complexes of N,N'-bis(2-methyl-2-mercaptopropane)-1,5-diazacyclooctane (H(2)bme-daco) and N,N'-bis(mercaptoethyl)-1,5-diazacyclooctane (H(2)bme-daco) ligands present uncommon examples of well-characterized (&mgr;-oxo)diiron(III) species in the presence of sulfur donors. Whereas the reaction of molecular oxygen with a solution of diiron(II) complex, [(bme-daco)Fe](2), affords slow formation of the &mgr;-oxo species, the presence of O(2) in the reaction of free ligand with the iron source, Fe(acac)(3), gave good yields in shorter time. With the less sterically encumbered derivative, [(bme-daco)Fe](2), the only successful route to [(bme-daco)Fe](2)O required single O-atom sources such as 4-chloropyridine N-oxide to be present during the preparation of the complex; attempted use of molecular O(2) either during the synthesis or following isolation of the iron(II) dimer [(bme-daco)Fe](2 )resulted in decomposition. That is, both the reactant [(bme-daco)Fe](2) and the product [(bme-daco)Fe](2)O are unstable toward O(2). The molecular structures of [(bme-daco)Fe](2), [(bme-daco)Fe](2)O, and [(bme-daco)Fe](2)O were determined by X-ray crystallography. The topological description of the Fe(II) dimers is that of edge-bridged square pyramids, with the Fe(II) displaced from the N(2)S(2) planes of each ligand on average by 0.59 Å and engaged in a planar Fe(2)S(2) core. For the (&mgr;-oxo)diiron(III) compounds, the square pyramids are O-vertex bridged, and the Fe(III) dome effect is 0.65 and 0.62 Å for [(bme-daco)Fe](2)O and [(bme-daco)Fe](2)O, respectively. Further characterization by ESI mass spectroscopy, cyclic voltammetry, and EPR spectroscopy (of the one-electron reduced [(bme-daco)Fe](2)O is presented. There was no indication of the ligand-based oxygen capture producing S-oxygenates which are prevalent in nickel derivatives of bme-daco and bme-daco.