Isolation of Monomeric MnIII/II−OH and MnIII−O Complexes from Water: Evaluation of O−H Bond Dissociation Energies

Abstract

The syntheses and properties of the monomeric [Mn^III/IIH₃1(OH)]^-/2- and [Mn^IIIH₃1(O)]^2- complexes are reported, where [H₃1]^3- is the tripodal ligand tris[(N‘-tert-butylureaylato)-N-ethyl)]aminato. Isotope-labeling studies with H₂¹⁸O confirmed that water is the source of the terminal oxo and oxygen in the hydroxo ligand. The molecular structures of the [Mn^IIH₃1(OH)]^2- and [Mn^IIIH₃1(O)]^2- complexes were determined by X-ray diffraction methods and show that each complex has trigonal bipyramidal coordination geometry. The Mn^III−O distance in [Mn^IIIH₃1(O)]^2- is 1.771(4) Å, which is lengthened to 2.059(2) Å in [Mn^IIH₃1(OH)]^2-. Structural studies also show that [H₃1]^3- provides a hydrogen-bond cavity that surrounds the Mn^III−O(H) units. Using a thermodynamic approach, which requires pK_a and redox potentials, bond dissociation energies of 77(4) and 110(4) kcal/mol were calculated for [Mn^IIH₃1(O−H)]^2- and [Mn^IIIH₃1(O−H)]^-, respectively. The calculated value of 77 kcal/mol for the [Mn^IIH₃1(O−H)]^2- complex is supported by the ability of [Mn^IIIH₃1(O)]^2- complex to cleave C−H bonds with bond energies of <80 kcal/mol.