A Nickel Complex with a Proton-Responsive PNP Pincer-Type Ligand as a Proton-Coupled Electron Transfer Reagent
Jyotima Mukherjee, Nils Ostermann, Xaiza Aniban, Inna Safianova, Nils Rotthowe, Ricardo A. Mata, Inke Siewert
Abstract
High Resolution Image Download MS PowerPoint Slide Herein, we introduce a proton-coupled electron transfer reagent consisting of a proton-responsive pyridine-4-ol PNP pincer ligand and a redox-active nickel ion ( L H = 2,6-bis((di- tert -butylphosphaneyl)methyl)pyridin-4-ol). The Ni(II) complexes with the deprotonated ([( L )NiBr], 4 ) and protonated ligand ([( L H)NiBr] +, 4H + ) were synthesized and also structurally examined. Deprotonation leads to a dearomatization in the ligand backbone and a blue-shift of the UV/vis absorption bands. 4H + has a p K a of 16.6, and reduction of 4 appears at −1.35 V in MeCN. The bond dissociation free energy (BDFE) of the OH bond in the ligand backbone of [( L H)Ni I Br], 5H, has been determined experimentally by the p K a -potential method as well as computationally by DFT, and it equals about 47 kcal mol –1 . The solution BDFE is below the BDFE of 1 / 2 H 2, and thus, H 2 loss is thermodynamically favored. However, due to the spatial separation of the redox and acid/base site, H 2 formation is kinetically disfavored and observed only in the absence of suitable substrates. Otherwise, 1H + /1e – transfer of 5H to TEMPO or MeCN as the H atom acceptor is observed.