Synthesis, characterization and reactivity of a Mn(III)-hydroxido complex as a biomimetic model for lipoxygenase

Manganese hydroxido (Mn-OH) complexes supported by a tripodal N,N',N″-[nitrilotris(ethane-2,1-diyl)]tris(P,P-diphenylphosphinic amido) ([poat]^3-) ligand have been synthesized and characterized by spectroscopic techniques including UV-vis and electron paramagnetic resonance (EPR) spectroscopies. X-ray diffraction (XRD) methods were used to confirm the solid-state molecular structures of {Na₂[Mn^IIpoat(OH)]}₂ and {Na[Mn^IIIpoat(OH)]}₂ as clusters that are linked by the electrostatic interactions between the sodium counterions and the oxygen atom of the ligated hydroxido unit and the phosphinic (P=O) amide groups of [poat]^3-. Both clusters feature two independent monoanionic fragments in which each contains a trigonal bipyramidal Mn center that is comprised of three equatorial deprotonated amide nitrogen atoms, an apical tertiary amine, and an axial hydroxido ligand. XRD analyses of {Na[Mn^IIIpoat(OH)]}₂ also showed an intramolecular hydrogen bonding interaction between the Mn^III-OH unit and P=O group of [poat]^3-. Crystalline {Na[Mn^IIIpoat(OH)]}₂ remains as clusters with Na⁺---O interactions in solution and is unreactive toward external substrates. However, conductivity studies indicated that [Mn^IIIpoat(OH)]^- generated in situ is monomeric and reactivity studies found that it is capable of cleaving C-H bonds, illustrating the importance of solution-phase speciation and its direct effect on chemical reactivity. Synopsis: Manganese-hydroxido complexes were synthesized to study the influence of H-bonds in the secondary coordination sphere and their effects on the oxidative cleavage of substrates containing C-H bonds.