Utilization of Hydrogen Bonds to Stabilize M-O(H) Units: Synthesis and Properties of Monomeric Iron and Manganese Complexes with Terminal Oxo and Hydroxo Ligands

Non-heme iron and manganese species with terminal oxo ligands are proposed to be key intermediates in a variety of biological and synthetic systems; however, the stabilization of these types of complexes has proven difficult because of the tendency to form oxo-bridged complexes. Described herein are the design, isolation, and properties for a series of mononuclear Fe^III and Mn^III complexes with terminal oxo or hydroxo ligands. Isolation of the complexes was facilitated by the tripodal ligand tris[(N′ -tertbutylureaylato)-N-ethyl]aminato([H₃1]^3-), which creates a protective hydrogen bond cavity around the M^III-O(H) units (M^III = Fe and Mn). The M^III-O(H) complexes are prepared by the activation of dioxygen and deprotonation of water. In addition, the M^III-O(H) complexes can be synthesized using oxygen atom transfer reagents such as N-oxides and hydroxylamines. The [Fe^IIIH ₃1(O)]^2- complex also can be made using sulfoxides. These findings support the proposal of a high valent M^IV-oxo species as an intermediate during dioxygen cleavage. Isotopic labeling studies show that oxo ligands in the [M^IIIH₃1(O)]^2- complexes come directly from the cleavage of dioxygen: for [Fe^IIIH ₃1(O)]^2- the v(Fe-¹⁶O) = 671 cm^-1, which shifts 26 cm-1 in [Fe^IIIH₃1(¹⁸O)] ^2- (v(Fe-¹⁸O) = 645 cm^-1); a v(Mn- ¹⁶O) = 700 cm^-1 was observed for [Mn^IIIH ₃1(¹⁶O)]^2-, which shifts to 672 cm ^-1 in the Mn-¹⁸O isotopomer. X-ray diffraction studies show that the Fe-O distance is 1.813(3) Å in [Fe^IIIH ₃1(O)]^2-, while a longer bond is found in [Fe ^IIIH₃1(OH)]^- (Fe-O at 1.926(2) Å); a similar trend was found for the Mn^III-O(H) complexes, where a Mn-O distance of 1.771(5) Å is observed for [Mn^IIIH ₃1(O)]^2- and 1.873(2) Å for [Mn^IIIH ₃1(OH)]^-. Strong intramolecular hydrogen bonds between the urea NH groups of [H₃1]^3- and the oxo and oxygen of the hydroxo ligand are observed in all the complexes. These findings, along with density functional theory calculations, indicate that a single σ-bond exists between the M^III centers and the oxo ligands, and additional interactions to the oxo ligands arise from intramolecular H-bonds, which illustrates that noncovalent interactions may replace π-bonds in stabilizing oxometal complexes.