Metal/Ligand Proton Tautomerism Facilitates Dinuclear H₂Reductive Elimination

Abstract

Using the doubly protic bis-pyrazole-pyridine ligand (N­(NNH)2), we have synthesized an octahedral IrIII–H [HIr­(κ3-N­(NNH)­(NN–))­(CO)­( t BuPy)]+ ([1-MH]+) from an IrI 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 [ t BuPy] dependent equilibrium with a ligand protonated square-planar IrI [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., t BuPy). This equilibrium was shown to be key to the reactivity of the IrIII–H; 2 equiv of [1-MH]+ release H2, converting to the IrII dimer [[Ir­(N­(NN–)­(NNH))­(CO)­( t BuPy)]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 H2 and the IrII 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.

The underappreciated influence of ancillary halide on metal–ligand proton tautomerism

“…53 Furthermore, in related studies of MLPT, Rauchfuss, Bullock, and our group have previously demonstrated that the MLPT equilibrium can be perturbed towards the ligand-protonated side by addition of an exogenous hydrogen bonding acceptor such as BF 4 − or CF 3 SO 3 − that can engage in favorable hydrogen bonding interactions with the acidic proton on the ligand backbone. 9,17,18 A similar hydrogen bonding stabilization of the imidazolyl protons is proposed here that disrupts the MLPT equilibrium of [ LH Ir–Cl] with [H– L Ir–Cl] ( Scheme 1, C ) upon addition of excess Cl anions, precluding the formation of latter.…”

“…To consider how [H-L Ir-Cl] might originate from [ LH Ir][Cl] in solution, one can either propose that proton-transfer from the imidazole to the Ir occurs rst and is followed by Cl À coordination, or that Cl À coordination is followed by ligand to metal proton-transfer. Based on literature precedent wherein addition of electron density to the metal center (thereby increasing its basicity) has been shown to trigger proton-transfer from the ligand to the metal, 9 it is expected that proton-transfer from the…”

“…To consider how [H– L Ir–Cl] might originate from [ LH Ir][Cl] in solution, one can either propose that proton-transfer from the imidazole to the Ir occurs first and is followed by Cl − coordination, or that Cl − coordination is followed by ligand to metal proton-transfer. Based on literature precedent wherein addition of electron density to the metal center (thereby increasing its basicity) has been shown to trigger proton-transfer from the ligand to the metal, 9 it is expected that proton-transfer from the imidazole to the more electron rich neutral Ir center in [ LH Ir–Cl] ( Scheme 1, C ) should be more favorable than proton-transfer to the cationic Ir center in [ LH Ir][Cl]. In light of this proposal, it was of interest to explore the effect of added Cl anion.…”

“…À or CF 3 SO 3 À ) that engaged in favorable interactions with the acidic proton on the ligand backbone, thus favoring the ligand protonated side. 9,17,18 Some common themes emerge from the strategies described above to control the position of an MLPT equilibrium. One can either change the basicity of the metal by changing the identity of the metal, or its ancillary ligands, or add a hydrogen bond acceptor to stabilize ligand based protons.…”

“…Here, the concentration of an exogenous electron donating ligand (4- tert -butyl pyridine) was used to control the position of the MLPT equilibrium; a higher concentration of 4- tert -butyl pyridine favored the metal protonated (metal-hydride) side of an Ir-complex possessing a protic ligand. 9 In a related rhenium system, use of electron withdrawing ligands on a Re III –H species was shown to influence the MLPT equilibrium. 16 Computations showed that stronger π-acid ligands resulted in a smaller energy difference between the Re III –H and the ligand protonated tautomer.…”