Monomeric MnIII/II and FeIII/II Complexes with Terminal Hydroxo and Oxo Ligands: Probing Reactivity via O−H Bond Dissociation Energies

Publisher Website
Google Scholar
Abstract
Non-heme manganese and iron complexes with terminal hydroxo or oxo ligands are proposed to mediate the transfer of hydrogen atoms in metalloproteins. To investigate this process in synthetic systems, the monomeric complexes [MIII/IIH31(OH)]-/2- and [MIIIH31(O)]2- have been prepared, where MIII/II = Mn and Fe and [H31]3- is the tripodal ligand, tris[(N‘-tert-butylureaylato)-N-ethyl)]aminato. These complexes have similar primary and secondary coordination spheres, which are enforced by [H31]3-. The homolytic bond dissociation energies (BDEsO-H) for the MIII/II−OH complexes were determined, using experimentally obtained values for the pKa(M−OH) and E1/2 measured in DMSO. This thermodynamic analysis gave BDEsO-H of 77(4) kcal/mol for [MnIIH31(O−H)]2- and 66(4) kcal/mol for [FeIIH31(O−H)]2-. For the MIII−OH complexes, [MnIIIH31(OH)]- and [FeIIIH31(OH)]-, BDEsO-H of 110(4) and 115(4) kcal/mol were obtained. These BDEsO-H were verified with reactivity studies with substrates having known X−H bond energies (X = C, N, O). For instance, [FeIIH31(OH)]2- reacts with a TEMPO radical to afford [FeIIIH31(O)]2- and TEMPO−H in isolated yields of 60 and 75%, respectively. Consistent with the BDEO-H values for [MnIIH31(OH)]2-, TEMPO does not react with this complex, yet TEMPO−H (BDEO-H = 70 kcal/mol) reacts with [MnIIIH31(O)]2-, forming TEMPO and [MnIIH31(OH)]2-. [MnIIIH31(O)]2- and [FeIIIH31(O)]2- react with other organic substrates containing C−H bonds less than 80 kcal/mol, including 9,10-dihydroanthracene and 1,4-cyclohexadiene to produce [MIIH31(OH)]2- and the appropriate dehydrogenated product in yields of greater than 80%. Treating [MnIIIH31(O)]2- and [FeIIIH31(O)]2- with phenolic compounds does not yield the product expected from hydrogen atom transfer but rather the protonated complexes, [MnIIIH31(OH)]- and [FeIIIH31(OH)]-, which is ascribed to the highly basic nature of the terminal oxo group.