N,N,O Pincer Ligand with a Deprotonatable Site That Promotes Redox‐Leveling, High Mn Oxidation States, and a Mn2O2 Dimer Competent for Catalytic Oxygen Evolution

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149230/1/ejic201801343.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149230/2/ejic201801343_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149230/3/ejic201801343-sup-0001-SupMat.pd

Transition Metal Complexes for Glycerol Dehydrogenation and Study of Water Oxidation Catalysis

This dissertation describes the study of transition metal complexes in relation to two types of oxidation catalysis, namely dehydrogenation and water oxidation. Chapters 1 and 2 explore dehydrogenation catalysis as a means of glycerol valorization. Glycerol is the major byproduct of biodiesel production (~10%), and there is thus intense interest in developing methods to convert this waste glycerol to more valuable products. One such product is lactic acid, which is commonly used in the food and detergent industries, and is a platform chemical that is seeing increasing demand. All prior methods for convening glycerol to lactic acid employed heterogeneous catalysts, which often require high temperatures and give generally poor selectivity and catalytic activity. In this work, I describe our study of homogeneous catalysts for glycerol conversion to lactic acid. Our Ir bis-NHC (NHC = N-heterocyclic carbene) precatalysts are superior to the previous systems in terms of selectivity and activity, and function in neat glycerol without the need for a co-solvent. These complexes can convert samples of crude glycerol from the biodiesel industry without the need for prior purification, suggesting their possible industrial application. Additionally, hydrogen is produced as a valuable byproduct. Chapter 2, carried out in collaboration with Professor Nilay Hazari (Yale), describes the study of catalysts based on non-precious metals for this reaction. A family of Fe precatalysts with bifunctional PNP pincer ligands give excellent selectivity and activity, and represent the first examples of homogeneous base-metal catalysts for glycerol conversion to lactic acid. In studies of Ir species formed from our Ir bis-NHC precatalysts during glycerol dehydrogenation, we isolated a series of unusual NHC-rich Ir polyhydride clusters (Chapter 3). These compounds are unprecedented in terms of their high NHC content, and were fully characterized using a variety of methods. Chapters 4 and 5, carried out in collaboration with Shashi Sinha and Dimitar Shopov, joint BrudvigCrabtree students, describe the study of model complexes related to resting states and high oxidation state intermediates in water oxidation catalysis. Water oxidation has garnered intense interest because of its potential application in the production of solar fuels, but effective catalysts are needed to carry out the reaction with low overpotentials. Our group previously found that upon oxidative activation, the Cp*Ir(pyalk)OH precatalyst (pyalk = 2-pyridyl-2-propanolate) generates one of the most active and robust water oxidation catalysts reported to date. Previous spectroscopic characterization and DFT studies revealed that the Cp* ligand is oxidatively degraded, and the catalyst resting state likely consists of a mixture of related species with a (pyalk)2IrIV-O-IrIV(pyalk) core. However, these species completely resisted purification and crystallization by standard methods. Therefore, we developed a protocol to more selectively prepare related CI(pyalk)2IrIV-O-IrIV(pyalk) 2CI complexes, which can be isolated and crystallographically characterized. These complexes are unusual examples of well-defined Ir(IV,IV) mono-μ-oxo dimers, and are stable under ambient conditions, in contrast to previous examples of Ir(IV,IV) mono-μ-oxo dimers containing organometallic ligands. Our study of these complexes sheds light on the resting state of our Ir water oxidation catalyst, and opens the door to future development of well-defined Ir-oxo dimers for water oxidation catalysis. In a related study (Chapter 5), we use techniques and insights that build on our Ir oxo-dimer study to synthesize unprecedented Ir(V) coordination complexes with organic ligands. Study of such well-defined high oxidation state complexes is of interest in relation to oxidation catalysis, where Ir(V) species have been proposed as key intermediates. In order to access Ir(V), we developed the ligand dpyp, an N,O,Odonor analogue of pyalk. Importantly, dpyp forms coordination complexes with four coplanar alkoxogroups, an arrangement that favors attainment of high oxidation states based on our previous work. Indeed, oxidation of IrIV(dpyp)2gives IrV(dpyp) +2+, which was fully characterized including by X-ray crystallography and DFT methods

The neutron diffraction structure of [Ir4(IMe)8H10]2+ polyhydride cluster: Testing the computational hydride positional assignments

The hydride positions not being located in our prior X-ray single crystal studies of [Ir(IMe)(CO)H], [Ir(IMe)(CO)H] and [Ir(IMe)H] (IMe = 1,3-dimethylimidazol-2-ylidene) a computational approach was adopted. Our computational positional assignments have now been tested by a single crystal neutron diffraction study of the closely related [Ir(IMe)H] cluster. The prior theoretical and subsequent experimental positions are in close agreement, validating the computational method, at least in this case.This work was supported by the US DoE, Office of Science, Office of Basic Energy Sciences, under catalysis award (L.S.S., DE-FG02-84ER13297). Work performed at the ORNL Spallation Neutron Source's TOPAZ single-crystal diffractometer was supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC. D.B. acknowledges the support from the Norwegian Research Council through the Centre of Excellence for Theoretical and Computational Chemistry (CTCC; grant No. 179568/V30), the Norwegian Metacenter for Computational Science (NOTUR; grant nn4654k) and the EU Research Executive Agency for a Marie Curie Fellowship (grant CompuWOC/618303)

A Stable Coordination Complex of Rh(IV) in an N,O-Donor Environment

We describe facial and meridional isomers of [Rh^III(pyalk)₃], as well as meridional [Rh^IV(pyalk)₃]⁺ {pyalk =2-(2-pyridyl)-2-propanoate}, the first coordination complex in an N,O-donor environment to show a clean, reversible Rh^III/IV redox couple and to have a stable Rh­(IV) form, which we characterize by EPR and UV–visible spectroscopy as well as X-ray crystallography. The unprecedented stability of the Rh­(IV) species is ascribed to the exceptional donor strength of the ligands, their oxidation resistance, and the meridional coordination geometry

A Dinuclear Iridium(V,V) Oxo-Bridged Complex Characterized Using a Bulk Electrolysis Technique for Crystallizing Highly Oxidizing Compounds

We report a general method for the preparation and crystallization of highly oxidized metal complexes that are difficult to prepare and handle by more conventional means. This method improves typical bulk electrolysis and crystallization conditions for these reactive species by substituting oxidation-prone organic electrolytes and precipitants with oxidation-resistant compounds. Specifically, we find that CsPF₆ is an effective inert electrolyte in acetonitrile, and appears to have general applicability to electrochemical studies in this solvent. Likewise, CCl₄ is not only an oxidation-resistant precipitant for crystallization from MeCN but it also enters the lattice. In this way, we synthesized and characterized an Ir­(V,V) mono-μ-oxo dimer which only forms at a very high potential (1.9 V vs NHE). This compound, having the highest isolated oxidation state in this redox-active system, cannot be formed chemically. DFT calculations show that the oxidation is centered on the Ir–O–Ir core and facilitated by strong electron-donation from the pyalk (2-(2-pyridinyl)-2-propanolate) ligand. TD-DFT simulations of the UV–visible spectrum reveal that its royal blue color arises from electron excitations with mixed LMCT and Laporte-allowed d–d character. We have also crystallographically characterized a related monomeric Ir­(V) complex, similarly prepared by oxidizing a previously reported Ir­(IV) compound at 1.7 V, underscoring the general applicability of this method

High Oxidation State Iridium Mono-μ-oxo Dimers Related to Water Oxidation Catalysis

The highly active iridium “blue solution” chemical and electrochemical water oxidation catalyst obtained from Cp*IrL­(OH) precursors (L = 2-pyridyl-2-propanoate) has been difficult to characterize as no crystal structure can be obtained because of the multiplicity of geometrical isomers present. Other data suggest complete loss of the Cp* ligand and the formation of a LIr-O-IrL unit. We have now developed a route to a series of well-defined Ir­(IV,IV) mono-μ-oxo dimers, containing the closely related L₂Ir-O-IrL₂ unit. Unlike the catalyst, these model compounds are separable by silica gel chromatography and readily form single crystals. We report three stereoisomers with the formula ClL₂Ir-O-IrL₂Cl, which are fully characterized, including by X-ray crystallography, and are compared to the “blue solution”. To the best of our knowledge, these species represent the first examples of structurally characterized dinuclear μ-oxo Ir­(IV,IV) compounds without metal–carbon bonds

High Oxidation State Iridium Mono-μ-oxo Dimers Related to Water Oxidation Catalysis

The highly active iridium “blue solution” chemical and electrochemical water oxidation catalyst obtained from Cp*IrL­(OH) precursors (L = 2-pyridyl-2-propanoate) has been difficult to characterize as no crystal structure can be obtained because of the multiplicity of geometrical isomers present. Other data suggest complete loss of the Cp* ligand and the formation of a LIr-O-IrL unit. We have now developed a route to a series of well-defined Ir­(IV,IV) mono-μ-oxo dimers, containing the closely related L₂Ir-O-IrL₂ unit. Unlike the catalyst, these model compounds are separable by silica gel chromatography and readily form single crystals. We report three stereoisomers with the formula ClL₂Ir-O-IrL₂Cl, which are fully characterized, including by X-ray crystallography, and are compared to the “blue solution”. To the best of our knowledge, these species represent the first examples of structurally characterized dinuclear μ-oxo Ir­(IV,IV) compounds without metal–carbon bonds

Redox Activity of Oxo-Bridged Iridium Dimers in an N,O-Donor Environment: Characterization of Remarkably Stable Ir(IV,V) Complexes

Chemical and electrochemical oxidation or reduction of our recently reported Ir­(IV,IV) mono-μ-oxo dimers results in the formation of fully characterized Ir­(IV,V) and Ir­(III,III) complexes. The Ir­(IV,V) dimers are unprecedented and exhibit remarkable stability under ambient conditions. This stability and modest reduction potential of 0.99 V vs NHE is in part attributed to complete charge delocalization across both Ir centers. Trends in crystallographic bond lengths and angles shed light on the structural changes accompanying oxidation and reduction. The similarity of these mono-μ-oxo dimers to our Ir “blue solution” water-oxidation catalyst gives insight into potential reactive intermediates of this structurally elusive catalyst. Additionally, a highly reactive material, proposed to be a Ir­(V,V) μ-oxo species, is formed on electrochemical oxidation of the Ir­(IV,V) complex in organic solvents at 1.9 V vs NHE. Spectroelectrochemistry shows reversible conversion between the Ir­(IV,V) and proposed Ir­(V,V) species without any degradation, highlighting the exceptional oxidation resistance of the 2-(2-pyridinyl)-2-propanolate (pyalk) ligand and robustness of these dimers. The Ir­(III,III), Ir­(IV,IV) and Ir­(IV,V) redox states have been computationally studied both with DFT and multiconfigurational calculations. The calculations support the stability of these complexes and provide further insight into their electronic structures

Redox Activity of Oxo-Bridged Iridium Dimers in an N,O-Donor Environment: Characterization of Remarkably Stable Ir(IV,V) Complexes

Chemical and electrochemical oxidation or reduction of our recently reported Ir­(IV,IV) mono-μ-oxo dimers results in the formation of fully characterized Ir­(IV,V) and Ir­(III,III) complexes. The Ir­(IV,V) dimers are unprecedented and exhibit remarkable stability under ambient conditions. This stability and modest reduction potential of 0.99 V vs NHE is in part attributed to complete charge delocalization across both Ir centers. Trends in crystallographic bond lengths and angles shed light on the structural changes accompanying oxidation and reduction. The similarity of these mono-μ-oxo dimers to our Ir “blue solution” water-oxidation catalyst gives insight into potential reactive intermediates of this structurally elusive catalyst. Additionally, a highly reactive material, proposed to be a Ir­(V,V) μ-oxo species, is formed on electrochemical oxidation of the Ir­(IV,V) complex in organic solvents at 1.9 V vs NHE. Spectroelectrochemistry shows reversible conversion between the Ir­(IV,V) and proposed Ir­(V,V) species without any degradation, highlighting the exceptional oxidation resistance of the 2-(2-pyridinyl)-2-propanolate (pyalk) ligand and robustness of these dimers. The Ir­(III,III), Ir­(IV,IV) and Ir­(IV,V) redox states have been computationally studied both with DFT and multiconfigurational calculations. The calculations support the stability of these complexes and provide further insight into their electronic structures