Numerous biological reactions such as water splitting in plants and algae, involve manganese-dependent enzymes. There is evidence for involvement of hydroxo-manganese species, Mn-lipoxygenase (MnLOX) and oxygen evolving complex (OEC) in proton-coupled electron-transfer (PCET) reactions occurring in photosystem II. It has also been proposed that MnLOX involves in a concerted proton-electron transfer (CPET) process, a sub category of PCET while, OEC involves in a stepwise PCET mechanism, the latter is still being substantially debated. This research starts by studying [MnIII(OH)(dpaqH)]+, a complex comprising of N5 anionic ligand dpaqH (dpaqH = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) that can abstract a hydrogen atom from a substrate having O-H or C-H bonds with moderate strengths (BDFE &lt; 79 kcal/mol). Further investigations focus on the thermodynamics of these reactions which are not yet fully understood. This includes preparation and analysis of [MnII-(OH2)(dpaqH)]+ and [MnIV(OH)(dpaqH)]2+ to provide an understanding of the impact of reduction potential while maintaining the ligand environment unchanged. In addition, the analysis of [MnIII(OH)(dpaq)2-Me (dpaq2-Me = 2-[bis(pyridin-2-ylmethyl)]amino-N-2-methyl-quinolin-8-yl-acetamidate) gives the effect of steric hindrances on the reactions showing that the MnIII complex with the dpaq2-Me has a larger driving force for CPET compared to that of with dpaqH. Additionally, dimeric manganese species are also examined. Investigations are done mainly by UV-Vis spectrometry, mass spectrometry, electronic paramagnetic resonance spectroscopy, EXAFS, electrochemical and computational calculations to get insights on the intermediates, transition states and reaction mechanisms.

Thermodynamics of reactivity of a series of hydroxo-manganese complexes in H atom abstraction

253rd National Meeting (2017)

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