Isolation of monomeric Mn(III/II)-H and Mn(III)-complexes from water: evaluation of O-H bond dissociation energies

The syntheses and properties of the monomeric MnIII/IIH31(OH)]-/2- and MnIIIH31(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 MnIIH31(OH)]2- and MnIIIH31(O)]2- complexes were determined by X-ray diffraction methods and show that each complex has trigonal bipyramidal coordination geometry. The MnIII-O distance in MnIIIH31(O)]2- is 1.771(4) A, which is lengthened to 2.059(2) A in MnIIH31(OH)]2-. Structural studies also show that H31]3- provides a hydrogen-bond cavity that surrounds the MnIII-O(H) units. Using a thermodynamic approach, which requires pKa and redox potentials, bond dissociation energies of 77(4) and 110(4) kcal/mol were calculated for MnIIH31(O-H)]2- and MnIIIH31(O-H)]-, respectively. The calculated value of 77 kcal/mol for the MnIIH31(O-H)]2- complex is supported by the ability of MnIIIH31(O)]2- complex to cleave C-H bonds with bond energies of <80 kcal/mol.