A Pyridine Alkoxide Chelate Ligand That Promotes Both Unusually High Oxidation States and Water-Oxidation Catalysis

Abstract

Water-oxidation catalysis is a critical bottleneck in the direct generation of solar fuels by artificial photosynthesis. Catalytic oxidation of difficult substrates such as water requires harsh conditions, so the ligand must be designed both to stabilize high oxidation states of the metal center and to strenuously resist ligand degradation. Typical ligand choices either lack sufficient electron donor power or fail to stand up to the oxidizing conditions. Our research on Ir-based water-oxidation catalysts (WOCs) has led us to identify a ligand, 2-(2'-pyridyl)-2-propanoate or "pyalk", that fulfills these requirements. Work with a family of Cp*Ir(chelate)Cl complexes had indicated that the pyalk-containing precursor gave the most robust WOC, which was still molecular in nature but lost the Cp* fragment by oxidative degradation. In trying to characterize the resulting active "blue solution" WOC, we were able to identify a diiridium(IV)-mono-μ-oxo core but were stymied by the extensive geometrical isomerism and coordinative variability. By moving to a family of monomeric complexes [Ir(pyalk)] and [Ir(pyalk)Cl], we were able to better understand the original WOC and identify the special properties of the ligand. In this Account, we cover some results using the pyalk ligand and indicate the main features that make it particularly suitable as a ligand for oxidation catalysis. The alkoxide group of pyalk allows for proton-coupled electron transfer (PCET) and its strong σ- and π-donor power strongly favors attainment of exceptionally high oxidation states. The aromatic pyridine ring with its methyl-protected benzylic position provides strong binding and degradation resistance during catalytic turnover. Furthermore, the ligand has two additional benefits: broad solubility in aqueous and nonaqueous solvents and an anisotropic ligand field that enhances the geometry-dependent redox properties of its complexes. After discussion of the general properties, we highlight the specific complexes studied in more detail. In the iridium work, the isolated mononuclear complexes showed easily accessible Ir(III/IV) redox couples, in some cases with the Ir(IV) state being indefinitely stable in water. We were able to rationalize the unusual geometry-dependent redox properties of the various isomers on the basis of ligand-field effects. Even more striking was the isolation and full characterization of a stable Rh(IV) state, for which prior examples were very reactive and poorly characterized. Importantly, we were able to convert monomeric Ir complexes to [Cl(pyalk)Ir-O-IrCl(pyalk)] derivatives that help model the "blue solution" properties and provide groundwork for rational synthesis of active, well-defined WOCs. More recent work has moved toward the study of first-row transition metal complexes. Manganese-based studies have highlighted the importance of the chelate effect for labile metals, leading to the synthesis of pincer-type pyalk derivatives. Beyond water oxidation, we believe the pyalk ligand and its derivatives will also prove use...

Solar‐Driven Production of Hydrogen Peroxide from Water and Dioxygen

“…The standard one-electron oxidationp otentialo fH 2 Ot op roduce OHC is 2.73 Vv ersus SHE [Eq. (5)], [71] which is significantly higher than the two-electron/two-proton oxidation potential of H 2 1)]. [36,37]…”

“…(1)] is the key step for artificial photosynthesis as well as natural photosynthesis. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]…”

“…[ [24][25][26][27][28][29][30][31][32][33][34][35][36] Extensive efforts have so far been devoted to develop efficient catalysts for the four-electron/four-proton oxidation of H 2 Ot oe volve O 2 . [1][2][3][4][5][6][7][8][9][10][11] In contrastt ot he four-electron/four-proton oxidation of H 2 Ot oe volve O 2 ,t he two-electron/two-proton oxidation of water to produce hydrogen peroxide [Eq. (2)] has merited much less attention.T he standard oxidation potentialo fH 2 Ot op roduce H 2 O 2 is 1.77 Vv ersus SHE, which is significantly highert han the E 0 value of the fourelectron/four-proton oxidation of H 2 O[ Eq.…”

Beyond Solar Fuels: Renewable Energy‐Driven Chemistry

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Therefore, the term "solar" is considered in its broader meaningo fR E, andt hus is not limitedt ot he directu se of solar radiation. Most such studies have focusedo nt he production of H 2 by water splitting, but growingi nterestconcerns the conversion of CO 2 ,e ven thoughi ti so ften focusedo ns imple two-electron reductions( to CO or formic acid).…”

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“…It was decided to test them using NaIO 4 as sacrificial oxidantbecause, being less aggressive than cerium ammonium nitrate (CAN) [22][23][24] and not showingb yi tself any tendency to form nanoparticles (NPs), as CAN does, it avoids complications in the kinetic studies. [25] The following catalysts were selected for this study (Scheme 1): cis-[Ir(ppy) 2 (H 2 O) 2 ]OTf (ppy = 2-phenylpyridine) (1), the first Ir molecular WOC, which is also the first organometallic WOC in general,r eported by Bernhard and co-workersi n 2008; [26] [Cp*Ir(H 2 O) 3 ]NO 3 (Cp* = 1,2,3,4,5-pentamethyl-cyclopentadienyl anion) (2), the tris-acquoC p* complex independently reportedb yC rabtree and co-workers [27] and our group in 2010; [28] [Cp*Ir(bzpy)Cl] (bzpy = 2-benzoylpyridine) (3), [28][29][30] analogoust ot he first IrCp* WOC reported by Crabtree and coworkers [Cp*Ir(ppy)Cl] in 2009, [31] but slightly more active and more soluble in water;[Cp*IrCl 2 (Me 2 -NHC)] (Me 2 -NHC = N-dimethylimidazolin-2-ylidene) (4), [32][33][34] as ar epresentative of the Ir carbene WOCs; [35,36] [Cp*Ir(pyalk)Cl] (pyalk = 2-pyridine-isopropanoate) (5), [37,38] perhaps the most investigated Ir WOC; [39] [Cp*Ir(pic)NO 3 ]( pic = 2-pyridine-carboxylate) (6), apparently the Water oxidation (WO) is ac entral reactioni nt he photo-and electro-synthesis of fuels. [25] The following catalysts were selected for this study (Scheme 1): cis-[Ir(ppy) 2 (H 2 O) 2 ]OTf (ppy = 2-phenylpyridine) (1), the first Ir molecular WOC, which is also the first organometallic WOC in general,r eported by Bernhard and co-workersi n 2008; [26] [Cp*Ir(H 2 O) 3 ]NO 3 (Cp* = 1,2,3,4,5-pentamethyl-cyclopentadienyl anion) (2), the tris-acquoC p* complex independently reportedb yC rabtree and co-workers [27] and our group in 2010; [28] [Cp*Ir(bzpy)Cl] (bzpy = 2-benzoylpyridine) (3), [28][29][30] analogoust ot he first IrCp* WOC reported by Crabtree and coworkers [Cp*Ir(ppy)Cl] in 2009, [31] but slightly more active and more soluble in water;[Cp*IrCl 2 (Me 2 -NHC)] (Me 2 -NHC = N-dimethylimidazolin-2-ylidene) (4), [32]…”