4.8 Article

Metal/Ligand Proton Tautomerism Facilitates Dinuclear H2 Reductive Elimination

Journal

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 142, Issue 51, Pages 21439-21449

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/jacs.0c10458

Keywords

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Funding

  1. National Institute of General Medical Sciences of the National Institutes of Health [F32GM126639]
  2. Department of Energy, Basic Energy Sciences [DE-SC0018057]
  3. U.S. Department of Energy (DOE) [DE-SC0018057] Funding Source: U.S. Department of Energy (DOE)

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Using the doubly protic bis-pyrazole-pyridine ligand (N(NNH)(2)), we have synthesized an octahedral Ir-III-H [HIr(kappa(3)-N(NNH)(NN-))(CO)((BuPy)-Bu-t)](+) ([1-MH](+)) from an Ir-I starting material. This hydride was generated by adding sufficient electron density to the metal center such that it became the thermodynamically preferred site of protonation. It was observed via UV-vis spectroscopy that [1-MH](+) establishes a [(BuPy)-Bu-t] dependent equilibrium with a ligand protonated square-planar Ir-I [Ir(N(NNH)(2))(CO)](+) ([2-LH](+)). This example of metal/ligand proton tautomerism is unusual in that the position of the equilibrium can be controlled by the concentration of exogeneous ligand (i.e., (BuPy)-Bu-t). This equilibrium was shown to be key to the reactivity of the Ir-III-H; 2 equiv of [1-MH](+) release H-2, converting to the Ir-II dimer [[Ir(N(NN-)(NNH))(CO)((BuPy)-Bu-t)](2)](2+) ([7](2+)) under mild conditions (observable at room temperature). Mechanistic evidence is presented to support that this dinuclear reductive elimination occurs by tautomerization of the metal hydride [1-MH](+) to a ligand protonated species [1-LH](+), from which ligand dissociation is facile, generating [2-LH](+). Subsequent reaction of [2-LH](+) with [1-MH](+) allows for production of H-2 and the Ir-II dimer [7](2+). The tautomerization between the metal-hydride and the ligand protonated species provides a low energy pathway for ligand dissociation, opening the needed coordination site. The ability to control the interconversion between a metal-hydride and a ligand-protonated congener using an exogeneous ligand introduces a new strategy for catalyst design with proton responsive ligands.

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