A Pyridine Alkoxide Chelate Ligand That Promotes Both Unusually

Mar 8, 2017 - In this Account, we cover some results using the pyalk ligand and indicate the main features that make it particularly suitable as a lig...
6 downloads 7 Views 2MB Size
Download PDF
Article pubs.acs.org/accounts

A Pyridine Alkoxide Chelate Ligand That Promotes Both Unusually High Oxidation States and Water-Oxidation Catalysis Thoe K. Michaelos,† Dimitar Y. Shopov,† Shashi Bhushan Sinha,† Liam S. Sharninghausen,† Katherine J. Fisher, Hannah M. C. Lant, Robert H. Crabtree,* and Gary W. Brudvig* Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States CONSPECTUS: 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 Irbased 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 [IrIII/IV(pyalk)3] and [IrIII/IV(pyalk)2Cl2], 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 σand π-donor power strongly favors attainment of exceptionally high oxidation states. The aromatic pyridine ring with its methylprotected 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)2IrIV−O−IrIVCl(pyalk)2] 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 useful in other oxidative transformations.

■

INTRODUCTION

that minimize overpotential are necessary for efficient water oxidation if such systems are to be implemented globally.

Many important chemical transformations involve oxidation of both organic and inorganic substrates. A key example is water oxidation (eq 1), a topic that has attracted intense interest in connection with its critical role in the water-splitting step of solar fuel production by artificial photosynthesis.1 Inspired by the natural oxygen-evolving enzyme Photosystem II, artificial photosynthetic systems aim to extract protons and electrons from water to be used for fuel production, liberating free O2.2 Water oxidation, however, is both a thermodynamically and kinetically challenging reaction, so efficient and robust catalysts © 2017 American Chemical Society

2H 2O → 4H+ + 4e− + O2

E° = 1.23 V

(1)

The metal center of a molecular water oxidation catalyst (WOC) plays an important role in determining its activity.3 Some of the most active WOCs employ iridium4 or ruthenium,5 although there is also considerable interest in utilizing first-row transition metals for this task, particularly Received: December 31, 2016 Published: March 8, 2017 952

DOI: 10.1021/acs.accounts.6b00652 Acc. Chem. Res. 2017, 50, 952−959

Article

Accounts of Chemical Research

by starting from Ir(CO)2(pyalk) (TOF = 5.8s−1), showing the Cp* was not needed.13 These catalysts can be heterogenized by spontaneous attachment of the “blue solution” to metal oxide electrodes to give highly active and stable electrocatalytic WOCs, which were shown to retain bound pyalk in a molecular form. This heterogenized catalyst (“hetWOC”) is highly active and robust, with a TOF of 7.9 s−1 at 520 mV overpotential and a TON > 106 when driven electrochemically.14 Being an inseparable mixture of different oxo-bridged Ir(IV) dinuclear species, the “blue solution” defied full characterization,15,16 despite exhaustive efforts. We ascribed this problem to the great multiplicity of possible diastereomers within the oxo-dimer framework, along with the variety of possible ligation at the coordination positions unoccupied by pyalk or the oxo group. 17 This problem was resolved by moving to [IrIII/IVCl2(pyalk)2] complexes,15b where the isomers could be cleanly separated, followed by conversion to [Cl(pyalk)2IrIV− O−IrIVCl(pyalk)2] derivatives. These complexes proved to model many of the “blue solution” properties, laying the groundwork for preparation of well-defined, highly active WOCs.17 As discussed below, pyalk promotes formation of high-valent Ir(IV) or Rh(IV) species.15,18 Ligands apparently closely related to pyalk generally stabilize only moderately elevated oxidation states.19,20 For example, cyclic voltammetric studies of coordinatively saturated picolinate complexes of Ir(III) and Rh(III) indicate oxidative degradation and failure to promote stable Ir(IV) or Rh(IV) states.19 Carboxylate ligands are widely used for various applications but have much less donor power than alkoxides, as well as being much more easily oxidized via decarboxylation.21 As the key step in oxidative transformations is often the formation of high-valent metal species, understanding the electronic properties of such systems is paramount. In order to accomplish this goal, we needed a ligand scaffold able to stabilize these high oxidation states. Our research groups have found pyalk to be particularly effective for this task. This Account focuses on the design principles for our robust ligands including their donor power, oxidation resistance, and dissymmetric ligand field. Additionally, we discuss how these principles may be applied to future ligand designs for oxidation catalysis. While this work focuses mainly on applications to water-oxidation catalysis, the same design principles may be relevant to other oxidation reactions.

manganese owing to its role in Photosystem II. Just as important as the metal, however, is the ligand scaffold that supports and stabilizes the metal under the harsh oxidative conditions. Both water oxidation and other types of oxidation catalysis4,6 require appropriate ligand design, as the ligand set must favor the high oxidation states that are often needed to mediate oxidative transformations. However, a powerful oxidation catalyst is likely to oxidize and destroy the very ligand set that permits attainment of these high oxidation states.7 Thus, an oxidatively robust ligand scaffold is necessary to attain high oxidation states and efficient catalytic turnover; a classic example is Collins’ tetra-amido macrocyclic ligand (TAML) that was the result of many years of successive improvements in the oxidation resistance of the scaffold.8 Over the course of our studies on water-oxidation catalysis, we have identified a previously known pyridine alkoxide ligand, pyalk,9 (Figure 1) as a similarly robust platform for oxidation chemistry.10

Figure 1. Structure of the pyalkH ligand. Deprotonation typically occurs on binding.

In early work,11,10a we surveyed a wide range of complexes of the type Cp*Ir(chelate)Cl as precursors for water-oxidation catalysis, driven chemically or electrochemically. Detailed analysis by ourselves and others7a,12 showed that the Cp* was oxidatively cleaved and degraded in the catalyst activation step, either chemically or electrochemically, giving rise to acetate and other oxidized organic fragments. This resulted in either a blue homogeneous solution (the “blue solution,” Scheme 1) or a nanoparticulate suspension of iridium oxide materials, with both being active water-oxidation catalysts (WOCs).7a,12 Considerable effort went into distinguishing these two cases, but the ultimate outcome was the identification of pyalk as the best ligand for retaining homogeneous water oxidation activity and completely avoiding nanoparticle formation. Investigation showed that pyalk remained unoxidized by the harsh catalytic conditions and still bound to Ir, resulting in one of the most robust and active homogeneous WOCs so far reported, with a turnover frequency (TOF) of >1.8 s−1 when driven chemically with NaIO4, no deactivation being observed during the course of the assay.13 We later generated a similar “blue solution” WOC in the absence of Cp*

Scheme 1. Preparation of “Blue Solution” by Oxidative Activation of Cp*Ir(pyalk)OH Precatalysta

Adapted from ref 17. “L” is undefined and refers to ligands such as acetate derived from Cp* degradation products and other species present under catalytic conditions.

a

953

DOI: 10.1021/acs.accounts.6b00652 Acc. Chem. Res. 2017, 50, 952−959

Article

Accounts of Chemical Research

■

DESIGN PRINCIPLES

Pyridine, therefore, functions as an anchor: it readily binds to metals and is of an intermediate hardness, forming stable bonds over a broad range of oxidation states. Even if the alkoxide is protonated, pyalk is expected to remain bound through the pyridine nitrogen. The limited degree of rotational freedom in the N-bound ligand means that a second coordination position on the metal is obstructed even if the alcohol momentarily dissociates. We see this stability with iridium; Ir(pyalk)3 species protonate reversibly, but dissociation of one pyalk ligand requires strong acid and temperatures in excess of 100 °C, and further dissociation was not observable even under harsher conditions.15 The combination of pyridyl, alcohol, and methyl groups gives the ligand a strong amphiphilic character, making free pyalkH soluble in every solvent ranging from pentane to strong brine. This property frequently transfers to its complexes where neutral species are often water-soluble and charged species organic-soluble, even without specially chosen counterions. A broad range of solubility is useful in homogeneous catalysis as it allows for broad solvent variation without the need for potentially interfering cosolvents or other complications. This is indeed the case for the many pyalk complexes that are usually soluble both in water and in a wide variety of organic solvents.15b,17

Balance of Donor Power and Oxidation Resistance

The outstanding feature of pyalk is the combination of high donor strength and oxidative resistance needed to stabilize high oxidation states. Ligands that excel in one of these areas often underperform in the other:22−24 an electron-rich ligand may be a good donor but is often easily oxidized, while electron-poor ligands can be very resistant to oxidation but are often poor donors. Examples of the first type include thiolates, amides, imides, nitrides, and alkyls: these can support high oxidation states but are easily destroyed by ligand oxidation.22,23a,c Examples of the second type include fluoride, phosphate, nitrate, etc. These can also form high oxidation state compounds but only under extreme oxidizing conditions. The products themselves are so oxidizing that they are unstable in water. For example, the Pt-group metals form hexavalent fluorides on reaction with fluorine, but these complexes readily decompose on exposure to water or organic solvents and cannot be regenerated under catalytically relevant conditions.24 A common ligand in high oxidation state complexes and water oxidation catalysis is the oxo group, O2−.3,16 Its strong donor lone pairs can participate in π-donation to the metal, while at the same time being quite difficult to oxidize due to the electronegativity of oxygen. However, oxo ligands can be difficult to control synthetically, so we turned to the related alkoxide functionality. Like O2−, alkoxides are some of the strongest donor ligands,25 due to the combined σ- and πdonation, while also being difficult to oxidize at the O atom. Because benzylic positions are oxidation-sensitive, we selected gem-dimethyl substitution to suppress the ligand oxidation seen for the unprotected pyridine-methanol ligand.20 In addition, the gem-dimethyl substitution promotes metallacycle formation by the Thorpe−Ingold effect.26 Furthermore, the methyl groups provide additional electron density to the already highly donor alkoxide group. Variants of pyalk without the alkoxide moiety were less effective; for example, replacing the alkoxide by borate resulted in a rearrangement-prone ligand and a WOC that rapidly deactivated.27 In contrast, the remarkable stability of pyalk has facilitated the isolation of the stable Ir(IV) complexes, [IrIV(pyalk)3]+ and IrIV(pyalk)2Cl2.15 More impressively, pyalk even stabilizes the rare Rh(IV) state.18

Anisotropic Ligand Field

In studying the monomeric Ir−pyalk complexes of Figure 2, we observed highly geometry-dependent redox characteristics, as

Stability during Catalytic Turnover

Proton coupled electron transfer is a common reaction step that can lower reaction barriers in catalysis28 and is particularly crucial for water oxidation where four oxidation events must take place. Having a reversibly protonatable site on a ligand scaffold is thus an attractive design element; in pyalk, the ROH/RO− group can protonate in the electron-rich low oxidation states and deprotonate to provide a strong donor upon oxidation. Indeed, complexes of pyalk were found to have pH-dependent redox properties that are consistent with coupled 1H+/1e− processes.15,18 The choice of pyridine as part of the ligand may seem counterintuitive since pyridine is a poor donor, but strong donor power is not the sole requirement for an oxidatively robust ligand. A proper ligand must bind tenaciously in a variety of oxidation states and pH conditions. While alkoxides bind well to the strongly Lewis acidic high oxidation states, they become less stable on reduced states due to electronic repulsion. Passing through low oxidation states during turnover leaves the ligand prone to protonation and dissociation.

Figure 2. Structures of Ir(pyalk) monomer complexes. We have represented the apparent colors of complexes in Ir(IV) state. Reproduced with permission from ref 15b. Copyright 2017 Royal Society of Chemistry.

illustrated by the electrochemical potentials for their IrIV/III couples. For example, fac- and mer-Ir(pyalk)3 couples differ by 340 mV.15a This is so big a difference that the fac isomer is a fairly strong oxidant in the Ir(IV) state, while the mer isomer is air-sensitive in the Ir(III) state and indefinitely stable in the Ir(IV) state. This anisotropic field oxidation enhancement (AFOE) has been noted before29 but is typically of lower magnitude. AFOE occurs due to the ligands of a complex differing strongly in 954

DOI: 10.1021/acs.accounts.6b00652 Acc. Chem. Res. 2017, 50, 952−959

Article

Accounts of Chemical Research

Figure 3. Influence of ligand arrangement (isomerism) on valence electron energies for the dπ orbitals of a [ML3X3]-type d6 low-spin octahedral complex. Ligand X has a higher donor strength than L. The unequal ligand field exerted on the orbitals of the mer isomer results in a higher HOMO energy than for the fac one, thus lowering the energy needed for one-electron oxidation. Reproduced with permission from ref 15b. Copyright 2017 Royal Society of Chemistry.

Figure 4. (a) Scheme for reversible formation of Rh(IV) by chemical redox. (b) Thermal ellipsoid diagrams of the crystal structure of Rh(IV) at 50% probability level. Hydrogen atoms have been omitted for clarity. (c) X-band EPR spectrum of compound mer-RhIV measured at 8 K (black) and simulation (red). Hyperfine interactions from the I = 1/2 103Rh nucleus (100%) were simulated using principal values of 31, 53, and 70 MHz. Adapted with permission from ref 18. Copyright 2015 American Chemical Society.

donor strength, as for pyalk, where the Lever parameters25 of the pyridine and alkoxide differ by ∼0.9 V. The effect is due to the valence t2g set’s geometry-dependent interactions with the ligand field. For example, in the fac isomer, each d orbital of the set is affected by the same number of each ligand type and thus all three t2g orbitals are near-degenerate. In the mer case, the alkoxides all appear in the same plane, resulting in differentiated ligand environments for each orbital and strongly splitting the set (Figure 3). Because the t2g set is initially degenerate and its mean energy should be largely unaffected, the HOMO level, and thus proneness to oxidize, is directly dependent on the magnitude of this splitting (hence explaining why the mer isomer oxidizes more easily). AFOE applies similarly to other ligand sets: the geometries of the Ir(pyalk)2Cl2 isomers mentioned earlier lead to AFOEs that correlate well with the ranking of their measured potentials.15b With AFOE, the effects of ligand geometry extend beyond simple trans effects. Whenever highly disparate ligand types are present on the same metal, geometry has to be considered in

the context of the valence orbitals. As a result of this effect, a dissymmetric ligand like pyalk can promote high oxidation states to an extent significantly beyond what is expected from averaging its donor groups. Because AFOE affects the metal’s redox properties, it is especially significant for redox catalysis; the isomer variation in potentials that we observed with pyalk is as high as would be the case for completely changing the ligand. These observations have led us to suspect that the numerous species in our “blue solution” WOC may have dramatically different catalytic competences. While this is a detriment to the overall TOF, it may well be responsible for the low overpotential. Water oxidation is a difficult reaction, and rationally designed homogeneous catalysts generally require careful tuning of electronic properties to achieve high activity.5 However, by covering a wide swath of electronic configurations via the multitude of diastereomers, the mixture is more likely to include the optimal catalyst configuration. In fact, this kind of geometric dependence and active-site diversity may be at the 955

DOI: 10.1021/acs.accounts.6b00652 Acc. Chem. Res. 2017, 50, 952−959

Article

Accounts of Chemical Research

Figure 5. (a) Scheme for general preparation of Ir(IV,IV) mono-μ-oxo dimers. (b) Preparative TLC separation of the coupling reaction products from the green isomer (Figure 2) after elution with an acetone/water solution. (c) Thermal ellipsoid diagram of the crystal structure of the Ir(IV,IV) dimer at 50% probability level. Hydrogen atoms have been omitted for clarity. Adapted with permission from ref 17. Copyright 2016 American Chemical Society.

significantly shorter in the oxidized form, also consistent with metal-centered redox. This example not only demonstrates the ability of pyalk to stabilize high oxidation states but also exemplifies the stability of the ligand under harshly oxidative conditions, allowing for metal-centered oxidation without degradation.

root of the high activity of amorphous heterogeneous WOCs that have poorly defined structure.30

■

APPLICATIONS TO HIGH OXIDATION STATE CHEMISTRY

Stable High Oxidation State Complexes

Understanding an Active WOC

Before turning our attention to developing more active WOCs, we aimed to understand the electronic properties of high-valent transition-metal compounds, which are often invoked as key intermediates in oxidation catalysis. The combination of high donicity, oxidation resistance, and dissymmetric ligand field has allowed us to use pyalk to prepare stable complexes of elements in typically reactive high oxidation states, such as Ir(IV) and Rh(IV). Few Ir(IV) species are known, 22,23a but this combination of ideal properties of pyalk has given the series of complexes [IrIV(pyalk)3]+ and IrIV(pyalk)2Cl2, which has significantly increased the number of Ir(IV) complexes in the literature. The Ir(IV) state generally showed remarkable stability, with some isomers having lower IrIV/III potentials than in any prior cases.15 Even more unusual was the isolation and full characterization of mer-[RhIV(pyalk)3]+ (Figure 4).18 Rh(IV) is a far rarer state, previously only known in reactive, poorly characterized species such as the water-sensitive hexahalides.24a In contrast, mer[RhIV(pyalk)3]+ is stable over relatively long periods in a range of solvents, including water, and like the Ir(IV) species does not degrade under strong oxidizing conditions. Interestingly, the fac isomer does not form a stable Rh(IV) state, demonstrating the importance of AFOE in stabilizing high oxidation states. The fully reversible III/IV oxidation of the mer isomer represents the first case of stabilizing Rh(IV) with organic ligands.18 High oxidation states are sometimes artifactual with the hole residing on a noninnocent ligand; both the location of the hole and the extent of its delocalization will affect reactivity of the oxidized complex. The key point of using rhodium is that its I = 1/2 spin allows us to determine if the oxidation truly occurs at the metal. Here, EPR data support a true Rh(IV) assignment, as mer-[RhIV(pyalk)3]+ shows hyperfine coupling to 103Rh (I = 1/ 2) but not to ligand 14N (Figure 4). As with the Ir pyalk complexes, the crystallographic Rh−O bond lengths are

In our work with the “blue solution”, the multiplicity of species hindered characterization, though studies have suggested the active species are Ir(IV,IV) mono-μ-oxo dimers.16 Chromatographic separation was prevented by irreversible binding to silica, likely through the multiple open coordination sites. In addition, the “blue solution” species were not organic soluble, which further hampered isolation and crystallization. Because well-defined Ir(IV,IV) mono-μ-oxo dimers are practically unknown,23a we aimed to prepare such dimers as models for the “blue solution”. In order to alleviate the complications associated with variable ligation in the “blue solution”, we targeted dimeric complexes with two pyalk ligands per Ir instead of one as model complexes of the active catalyst. These model complexes would have several advantages. For one, we can control the geometry because all five geometric isomers of the monomeric precursor can be separated by chromatography. Furthermore, the presence of another bound pyalk ligand would result in fewer variable sites per Ir and hence reduced structural variability and surface binding, and the lower polarity would promote chromatographic separation and organic solubility. Treatment of single Ir(pyalk)2Cl2 isomers with Ag2O led to a series of geometrically defined Ir(IV,IV) mono-μ-oxo dimers [Cl(pyalk)2IrIV−O−IrIVCl(pyalk)2] (Figure 5), since the geometry of the monomeric starting complex was retained in the dimeric products. The dimers formed were separable by silica gel chromatography (Figure 5b), allowing for isolation and full characterization of distinct species. These complexes represent the first examples of well-defined Ir(IV,IV) mono-μ-oxo dimers without Ir−C bonds.17 The isolation of Ir(IV,IV) mono-μ-oxo dimers gives a clearer picture of the resting state of the “blue solution” WOC; further study of the redox properties of these complexes may facilitate 956

DOI: 10.1021/acs.accounts.6b00652 Acc. Chem. Res. 2017, 50, 952−959

Article

Accounts of Chemical Research

Figure 6. Pincer ligands 1 and 2 modified from pyalk and the proposed structure for a Mn-based WOC using pincer 1.

122 O2 h−1 (mol catalyst)−1, with our “terpy dimer” ([Mn III (terpy)(H 2 O)(μ-O) 2 Mn IV (terpy)(H 2 O)](NO 3 ) 3 ) being the fastest Mn-based oxygen-evolution catalyst to date.3 Typical turnover numbers range from 1 to 50. These data are collected under a diverse set of conditions, with a variety of sacrificial oxidants having varying potentials, and thus direct comparisons between catalysts is not straightforward. The estimation of accurate TONs and TOFs also depends on the ability to identify and quantify the active form of the catalyst. Even with this caveat in mind, the Mn-based pyalk-derived species are much less active than Ir-pyalk systems. However, our results indicate a promising level of activity for a first row metal.

understanding of other intermediates in the catalytic cycle of the active catalyst. The controlled synthesis, separation, and study of different geometric isomers lays the groundwork for rational design of more active iridium WOCs. Additionally, the general strategy should be extendable to other substitutionally inert metals. First Row Transition Metals and Modified Ligand Designs

The limited availability of second and third row transition metals makes the less expensive first row transition metals an important area for development.31 Additionally, a long-standing interest in our groups has been the stabilization and isolation of high-valent Mn species for catalytic water oxidation as functional models of the oxygen-evolving complex (OEC) of photosystem II.3,32 Manganese is a labile metal ion, particularly as high spin Mn(II), and an ideal ligand would remain bound in all oxidation states encountered during catalysis. Pyalk was thus tested as a ligand for this application, as we have found it binds tightly to both low (as pyalkH) and high (as pyalk) oxidation states.33 Initial results indicated that Mn salts and pyalk gave a small amount of oxygen with KHSO5 but quickly deactivated. However, the Mn(III) state is clearly favored with pyalk as a ligand, with [Mn(pyalk)2(pyalkH)]+ forming spontaneously in air and remaining stable against MnO2 formation.33 We, therefore, decided to extend the work to pincer ligands that favor meridional geometry and promote dimeric structures with labile coordination sites, as seen in the proposed di-μ-oxo Mn complex 3. Pincer ligands 1 and 2 (Figure 6) are prepared via similar synthetic routes as pyalkH, which itself is easily prepared from commercially available materials.10b Studies of these ligands on Mn reveal a delicate balance between the N/O ratio and reactivity; the N,N,O-type ligand promoted Mn-based O2 evolution, thought to be originating from an in situ assembled dimer (Figure 6), while the O,N,O-type ligand produced highvalent but inactive Mn species.33 Although the rate of O2 evolution was slow with an apparent TOF of 5.3 mol O2 h−1 (mol Mn)−1, the Mn-1 species continued to produce O2 for over 160 h, with an apparent TON of over 250. These numbers are scaled to the amount of Mn precatalyst present in assays. In the field of Mn-based systems for oxygen evolution, typical TOF’s range from 2 to

■

CONCLUSIONS AND FUTURE PROSPECTS The pyalk ligand has a number of useful properties: high resistance to harshly oxidizing conditions, strong donor power, compatibility with aqueous and selected organic solvents, and an internal dissymmetry that facilitates attainment of high oxidation states. It is this combination of properties that have given us the highly active Ir WOC known as the “blue solution”, and model chemistry with iridium and rhodium has emphasized the utility of such properties. Further study of the redox properties of these complexes may facilitate isolation of even more highly oxidized Ir or first-row metals. As we have started developing synthetic procedures for obtaining pure isomers of Ir-pyalk monomers and dimers, it will become possible to experimentally investigate these structure− function relationships. The ultimate prospect would be the rational design of a catalyst with low overpotential that shares the resilience of the “blue solution” but far exceeds its TOF. Heterogenization of a discrete species may result in a far more efficient hetWOC system than before.14 Other current efforts involve incorporating the pyridinealkoxide combination into application-specific architectures. The pyalk motif is well-suited to derivatization via the para position of the pyridine ring; for example, an Ir catalyst was successfully heterogenized onto semiconductor electrodes with a silatrane-based surface anchor to yield a system competent for electrocatalytic water oxidation.34 Other derivatives vary in denticity, donicity, flexibility, and coordination sites to 957

DOI: 10.1021/acs.accounts.6b00652 Acc. Chem. Res. 2017, 50, 952−959

Article

Accounts of Chemical Research accommodate other types of chemistry, an example being the pincer derivatives 1 and 2 used for Mn. We have demonstrated that pyalk is a particularly effective ligand scaffold for promoting water-oxidation catalysis, one of the most thermodynamically and kinetically challenging oxidative reactions. A promising avenue for extension involves moving to other substrates, such as unactivated CH bonds. Indeed, Ir−pyalk complexes have already proved to be active for CH oxidation.35 Thus, we believe that pyalk, its derivatives, and other ligands developed with these design principles in mind would be similarly effective for other oxidative transformations.

■

Chan and was a Miller Postdoctoral Fellow with Ken Sauer at the University of California, Berkeley, from 1980 to 1982. He has been on the faculty at Yale University since 1982, where he currently is the Benjamin Silliman Professor and Chair of Chemistry, Professor of Molecular Biophysics & Biochemistry, and Director of the Yale Energy Sciences Institute.

■

ACKNOWLEDGMENTS This work (L.S.S., S.B.S., D.Y.S., T.K.M., H.M.C.L., K.J.F.) was supported the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Award Number DESC0001059 as part of the Argonne-Northwestern Solar Energy Research (ANSER) Energy Frontier Research Center (spectroscopy and characterization) and under Award Number DEFG02-07ER15909 (synthesis). K.J.F. acknowledges support by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1122492.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

■

ORCID

Shashi Bhushan Sinha: 0000-0002-3212-7570 Liam S. Sharninghausen: 0000-0002-2249-1010 Robert H. Crabtree: 0000-0002-6639-8707 Gary W. Brudvig: 0000-0002-7040-1892

REFERENCES

(1) (a) Meyer, T. J. Chemical Approaches to Artificial Photosynthesis. Acc. Chem. Res. 1989, 22, 163−170. (b) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (2) Blankenship, R. E.; Tiede, D. M.; Barber, J.; Brudvig, G. W.; Fleming, G.; Ghirardi, M.; Gunner, M. R.; Junge, W.; Kramer, D. M.; Melis, A.; Moore, T. A.; Moser, C. C.; Nocera, D. G.; Nozik, A. J.; Ort, D. R.; Parson, W. W.; Prince, R. C.; Sayre, R. T. Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement. Science 2011, 332, 805−809. (3) (a) Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chem. Rev. 2014, 114, 11863−12001. (b) Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Molecular Catalysts for Water Oxidation. Chem. Rev. 2015, 115, 12974−13005. (4) Thomsen, J. M.; Huang, D. L.; Crabtree, R. H.; Brudvig, G. W. Iridium-based Complexes for Water Oxidation. Dalton Trans. 2015, 44, 12452−12472. (5) Duan, L.; Bozoglian, F.; Mandal, S.; Stewart, B.; Privalov, T.; Llobet, A.; Sun, L. A Molecular Ruthenium Catalyst with WaterOxidation Activity Comparable to that of Photosystem II. Nat. Chem. 2012, 4, 418−423. (6) Sun, C. L.; Li, B. J.; Shi, Z. J. Direct C-H Transformation via Iron Catalysis. Chem. Rev. 2011, 111, 1293−1314. (7) (a) Hintermair, U.; Hashmi, S. M.; Elimelech, M.; Crabtree, R. H. Particle Formation during Oxidation Catalysis with Cp* Iridium Complexes. J. Am. Chem. Soc. 2012, 134, 9785−9795. (b) Sinha, S. B.; Campos, J.; Brudvig, G. W.; Crabtree, R. H. Co(II), a Catalyst for Selective Conversion of Phenyl Rings to Carboxylic Acid Groups. RSC Adv. 2014, 4, 49395−49399. (c) Crabtree, R. H. Deactivation in Homogeneous Transition Metal Catalysis: Causes, Avoidance, and Cure. Chem. Rev. 2015, 115, 127−150. (8) Collins, T. J. TAML Oxidant Activators: A New Approach to the Activation of Hydrogen Peroxide for Environmentally Significant Problems. Acc. Chem. Res. 2002, 35, 782−790. (9) Our group regularly prepares large quantities of pure pyalk. Please contact us if you are interested in receiving a sample for your research. (10) (a) Schley, N. D.; Blakemore, J. D.; Subbaiyan, N. K.; Incarvito, C. D.; D’Souza, F.; Crabtree, R. H.; Brudvig, G. W. Distinguishing Homogeneous from Heterogeneous Catalysis in Electrode-Driven Water Oxidation with Molecular Iridium Complexes. J. Am. Chem. Soc. 2011, 133, 10473−10481. (b) Wong, Y.-L.; Yang, Q.; Zhou, Z.-Y.; Kay Lee, H. K.; Mak, T. C. W.; Ng, D. K. P. Synthesis, Structure and OxoTransfer Properties of Dioxotungsten(VI) Complexes with PyridineBased NO- and NS-Bidentate Ligands. New J. Chem. 2001, 25, 353− 357.

Author Contributions †

T.K.M., D.Y.S., S.B.S., and L.S.S. contributed equally.

Notes

The authors declare no competing financial interest. Biographies Thoe K. Michaelos received a B.A. from Stony Brook University in 2010, before spending a year as a SULI intern at Argonne National Laboratory. She is currently a Ph.D. student in the groups of Robert H. Crabtree and Gary W. Brudvig at Yale University. Dimitar Y. Shopov received B.A. in Chemistry and Physics from Colgate University in 2012 and is currently pursuing a Ph.D. under Professors Robert H. Crabtree and Gary W. Brudvig at Yale University. Shashi Bhushan Sinha received a M.S. in Chemical Sciences from Indian Institute of Science Education and Research, Kolkata, in 2012. He is currently pursuing his Ph.D. under the joint guidance of Robert H. Crabtree and Gary W. Brudvig at Yale University. Liam S. Sharninghausen received a B.A. from Oberlin College in 2011 and is currently a Ph.D. student in Robert H. Crabtree’s group at Yale University. Katherine J. Fisher received a B.S. from the California Institute of Technology in 2015. She is currently pursuing a Ph.D. as a NSF Fellow in the groups of Robert H. Crabtree and Gary W. Brudvig at Yale University. Hannah M. C. Lant received a B.A. From Grinnell College in 2014. She is currently pursuing a Ph.D. in the group of Gary W. Brudvig at Yale University. Robert H. Crabtree, educated at Oxford with Malcolm Green, did his Ph.D. with Joseph Chatt and spent four years in Paris with Hugh Felkin at the CNRS, Gif. He is now Whitehead Professor at Yale and has been an ACS and RSC organometallic chemistry awardee. He is a Fellow of the ACS, of the RSC, and of the American Academy of Arts and Sciences. Gary W. Brudvig received a B.S. in 1976 from the University of Minnesota and a Ph.D. in 1981 from Caltech working with Sunney 958

DOI: 10.1021/acs.accounts.6b00652 Acc. Chem. Res. 2017, 50, 952−959

Article

Accounts of Chemical Research

Macrocyclic Ligand Complexes − A DFT Study. Eur. J. Inorg. Chem. 2014, 2014, 728−741. (d) de Oliveira, F. T.; Chanda, A.; Banerjee, D.; Shan, X.; Mondal, S.; Que, L., Jr.; Bominaar, E. L.; Münck, E.; Collins, T. J. Chemical and Spectroscopic Evidence for an FeV-Oxo Complex. Science 2007, 315, 835−838. (24) (a) Jardine, F. H. Rhodium: Inorganic & Coordination Chemistry; John Wiley & Sons Ltd.: Hoboken, NJ, 2006. (b) Allen, G. C.; ElSharkawy, G. A. M.; Warren, K. D. Electronic Spectra of the Hexafluoroosmate(V) and Hexafluoroiridate(V) Anions. Inorg. Chem. 1972, 11, 51−56. (25) Lever, A. B. P. Electrochemical Parametrization of Metal Complex Redox Potentials, using the Ruthenium(III)/Ruthenium(II) Couple to Generate a Ligand Electrochemical Series. Inorg. Chem. 1990, 29, 1271−1285. (26) Jung, M. E.; Piizzi, G. gem-Disubstituent Effect: Theoretical Basis and Synthetic Applications. Chem. Rev. 2005, 105, 1735−1766. (27) Bloomfield, A. J.; Matula, A. J.; Mercado, B. Q.; Batista, V. S.; Crabtree, R. H. Organometallic Iridium Complex Containing a Dianionic, Tridentate, Mixed Organic−Inorganic Ligands. Inorg. Chem. 2016, 55, 8121−8129. (28) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2012, 112, 4016−4093. (29) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. Synthesis and Characterization of Facial and Meridional tris-Cyclometalated Iridium(III) Complexes. J. Am. Chem. Soc. 2003, 125, 7377−7387. (30) Bloomfield, A. J.; Sheehan, S. W.; Collom, S. L.; Crabtree, R. H.; Anastas, P. T. A Heterogeneous Water Oxidation Catalyst from Dicobalt Octacarbonyl and 1,2-bis(diphenylphosphino)ethane. New J. Chem. 2014, 38, 1540−1545. (b) Blakemore, J. D.; Schley, N. D.; Olack, G. W.; Incarvito, C. D.; Brudvig, G. W.; Crabtree, R. H. Anodic Deposition of a Robust Iridium Water-Oxidation Catalyst from Organometallic Precursors. Chem. Sci. 2011, 2, 94−98. (31) Parent, A. R.; Sakai, K. Progress in Base-Metal Water Oxidation Catalysis. ChemSusChem 2014, 7, 2070−2080. (32) Young, K. J.; Brennan, B. J.; Tagore, R.; Brudvig, G. W. Photosynthetic Water Oxidation: Insights from Manganese Model Chemistry. Acc. Chem. Res. 2015, 48, 567−574. (33) Michaelos, T. K.; Lant, H. M. C.; Sharninghausen, L. S.; Craig, S. M.; Menges, F. S.; Mercado, B. Q.; Brudvig, G. W.; Crabtree, R. H. Catalytic Oxygen Evolution from Manganese Complexes with an Oxidation-Resistant N,N,O-Donor Ligand. ChemPlusChem 2016, 81, 1129−1132. (34) Materna, K. L.; Rudshteyn, B.; Brennan, B. J.; Kane, M. H.; Bloomfield, A. J.; Huang, D. L.; Shopov, D. Y.; Batista, V. S.; Crabtree, R. H.; Brudvig, G. W. Heterogenized Iridium Water-Oxidation Catalyst from a Silatrane Precursor. ACS Catal. 2016, 6, 5371−5377. (35) (a) Zhou, M.; Hintermair, U.; Hashiguchi, B. G.; Parent, A. R.; Hashmi, S. M.; Elimelech, M.; Periana, R. A.; Brudvig, G. W.; Crabtree, R. H. Cp* Iridium Precatalysts for Selective C−H Oxidation with Sodium Periodate As the Terminal Oxidant. Organometallics 2013, 32, 957−965. (b) Huang, D. L.; Vinyard, D. J.; Blakemore, J. D.; Hashmi, S. M.; Crabtree, R. H. Cp* versus Bis-carbonyl Iridium Precursors as CH Oxidation Precatalysts. Organometallics 2017, 36, 199−206.

(11) (a) Hull, J. F.; Balcells, D.; Blakemore, J. D.; Incarvito, C. D.; Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H. Highly Active and Robust Cp* Iridium Complexes for Catalytic Water Oxidation. J. Am. Chem. Soc. 2009, 131, 8730−8731. (b) Thomsen, J. M.; Sheehan, S. W.; Hashmi, S. M.; Campos, J.; Hintermair, U.; Crabtree, R. H.; Brudvig, G. W. Electrochemical Activation of Cp* Iridium Complexes for Electrode-Driven Water-Oxidation Catalysis. J. Am. Chem. Soc. 2014, 136, 13826−13834. (12) (a) Hintermair, U.; Sheehan, S. W.; Parent, A. R.; Ess, D. H.; Richens, D. T.; Vaccaro, P. H.; Brudvig, G. W.; Crabtree, R. H. Precursor Transformation during Molecular Oxidation Catalysis with Organometallic Iridium Complexes. J. Am. Chem. Soc. 2013, 135, 10837−10851. (b) Zuccaccia, C.; Bellachioma, G.; Bolaño, S.; Rocchigiani, L.; Savini, A.; Macchioni, A. An NMR Study of the Oxidative Degradation of Cp*Ir Catalysts for Water Oxidation: Evidence for a Preliminary Attack on the Quaternary Carbon Atom of the −C−CH3 Moiety. Eur. J. Inorg. Chem. 2012, 2012, 1462−1468. (13) Huang, D. L.; Beltrán-Suito, R.; Thomsen, J. M.; Hashmi, S. M.; Materna, K. L.; Sheehan, S. W.; Mercado, B. Q.; Brudvig, G. W.; Crabtree, R. H. New Ir Bis-Carbonyl Precursor for Water Oxidation Catalysis. Inorg. Chem. 2016, 55, 2427−2435. (14) Sheehan, S. W.; Thomsen, J. M.; Hintermair, U.; Crabtree, R. H.; Brudvig, G. W.; Schmuttenmaer, C. A. A Molecular Catalyst for Water Oxidation that Binds to Metal Oxide Surfaces. Nat. Commun. 2015, 6, 6469. (15) (a) Shopov, D. Y.; Rudshteyn, B.; Campos, J.; Batista, V. S.; Crabtree, R. H.; Brudvig, G. W. Stable Iridium(IV) Complexes of an Oxidation-Resistant Pyridine-Alkoxide Ligand: Highly Divergent Redox Properties Depending on the Isomeric Form Adopted. J. Am. Chem. Soc. 2015, 137, 7243−7250. (b) Shopov, D. Y.; Rudshteyn, B.; Campos, J.; Vinyard, D. J.; Batista, V. S.; Brudvig, G. W.; Crabtree, R. H. A Full Set of Iridium(IV) Pyridine-Alkoxide Stereoisomers: Highly Geometry-Dependent Redox Properties. Chem. Sci. 2017, 8, 1642− 1652. (16) Yang, K. R.; Matula, A. J.; Kwon, G.; Hong, J.; Sheehan, S. W.; Thomsen, J. M.; Brudvig, G. W.; Crabtree, R. H.; Tiede, D. M.; Chen, L. X.; Batista, V. S. Solution Structures of Highly Active Molecular Ir Water-Oxidation Catalysts from Density Functional Theory Combined with High-Energy X-ray Scattering and EXAFS Spectroscopy. J. Am. Chem. Soc. 2016, 138, 5511−5514. (17) Sharninghausen, L. S.; Sinha, S. B.; Shopov, D. Y.; Choi, B.; Mercado, B. Q.; Roy, X.; Balcells, D.; Brudvig, G. W.; Crabtree, R. H. High Oxidation State Iridium Mono-μ-oxo Dimers Related to Water Oxidation Catalysis. J. Am. Chem. Soc. 2016, 138, 15917−15926. (18) Sinha, S. B.; Shopov, D. Y.; Sharninghausen, L. S.; Vinyard, D. J.; Brudvig, G. W.; Crabtree, R. H.; Mercado, B. Q. A Stable Coordination Complex of Rh(IV) in an N,O-donor Environment. J. Am. Chem. Soc. 2015, 137, 15692−15695. (19) Basu, S.; Peng, S. M.; Lee, G. H.; Bhattacharya, S. Synthesis, Structure and Electrochemical Properties of Tris-picolinate Complexes of Rhodium and Iridium. Polyhedron 2005, 24, 157−163. (20) Papatriantafyllopoulou, C.; Abboud, K. A.; Christou, G. Mn/Ce Clusters from the use of Pivalate and Chelate Ligands: MnIII8CeIV, MnIII2CeIV2, and MnIII4CeIII2 Products. Polyhedron 2013, 52, 196−206. (21) Cantwell, N. H.; Brown, E. V. Effect of Solvents on the Decarboxylation of Picolinic Acid. J. Am. Chem. Soc. 1953, 75, 4466− 4468. (22) Rohde, J. U.; Lee, W. T. Stabilization of Iridium(IV) by Monoanionic Dialkyldiarylguanidinato Ligands. J. Am. Chem. Soc. 2009, 131, 9162−9163. (23) (a) Lehman, M. C.; Pahls, D. R.; Meredith, J. M.; Sommer, R. D.; Heinekey, D. M.; Cundari, T. R.; Ison, E. A. Oxyfunctionalization with Cp*IrIII(NHC) (Me) (Cl) with O2: Identification of a Rare Bimetallic IrIV μ-Oxo Intermediate. J. Am. Chem. Soc. 2015, 137, 3574−3584. (b) Urano, Y.; Higuchi, T.; Hirobe, M.; Nagano, T. Pronounced Axial Thiolate Ligand Effect on the Reactivity of HighValent Oxo-Iron Porphyrin Intermediate. J. Am. Chem. Soc. 1997, 119, 12008−12009. (c) Liao, R. Z.; Li, X. C.; Siegbahn, P. E. M. Reaction Mechanism of Water Oxidation Catalyzed by Iron Tetraamido 959

DOI: 10.1021/acs.accounts.6b00652 Acc. Chem. Res. 2017, 50, 952−959