Forum Article pubs.acs.org/IC
Control of Ligand pKa Values Tunes the Electrocatalytic Dihydrogen Evolution Mechanism in a Redox-Active Aluminum(III) Complex Tobias J. Sherbow, James C. Fettinger, and Louise A. Berben* Department of Chemistry, University of California, Davis, California 95616, United States S Supporting Information *
ABSTRACT: Redox-active ligands bring electron- and proton-transfer reactions to main-group coordination chemistry. In this Forum Article, we demonstrate how ligand pKa values can be used in the design of a reaction mechanism for a ligand-based electron- and proton-transfer pathway, where the ligand retains a negative charge and enables dihydrogen evolution. A bis(pyrazolyl)pyridine ligand, iPrPz2P, reacts with 2 equiv of AlCl3 to afford [(iPrPz2P)AlCl2(THF)][AlCl4] (1). A reaction involving two-electron reduction and single-ligand protonation of 1 affords [(iPrHPz2P−)AlCl2] (2), where each of the electron- and proton-transfer events is ligandcentered. Protonation of 2 would formally close a catalytic cycle for dihydrogen production. At −1.26 V versus SCE, in a 0.3 M Bu4NPF6/ tetrahydrofuran solution with salicylic acid or (HNEt3)+ as the source of H+, 1 produced dihydrogen electrocatalytically, according to cyclic voltammetry and controlled potential electrolysis experiments. The mechanism for the reaction is most likely two electrontransfer steps followed by two chemical steps based on the available reactivity information. A comparison of this work with our previously reported aluminum complexes of the phenyl-substituted bis(imino)pyridine system (PhI2P) reveals that the pKa values of the N-donor atoms in iPrPz2P are lower, which facilitates reduction before ligand protonation. In contrast, the PhI2P ligand complexes of aluminum are protonated twice before reduction liberates dihydrogen.
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INTRODUCTION In biological systems, it is now well-known that protein radicals play an important role in electron transfer at the active sites of enzymes,1 as in, for example, galactose oxidase, an enzyme that catalyzes the oxidation of a primary alcohol to the aldehyde. A ligand-based tyrosyl radical is essential for efficient catalysis and requires one electron from a Cu center and one from a ligand to reduce O2 and close a catalytic cycle (Chart 1A).2 Model compounds synthesized by several research groups have provided support for this understanding.3 Examples such as this one have inspired extensive study of redox-active ligands in d-block chemistry,4 and this Forum Article explores the emerging field of ligand-based electron and proton transfer in main-group coordination chemistry. Redox-Active Ligands in d-Element Chemistry. Extensive precedent for the use of redox-active ligands is found in transition-element chemistry, and some illustrative examples are included here.5 In a comprehensive study by Wieghardt and coworkers, mixed-valent bis(imino)pyridine6 and bis(α-iminopyridine)7 complexes (Chart 1B) of the first-row transition metals were synthesized and characterized, and that work discussed the way in which ligand redox properties are influenced dramatically by the electronic structure of the metal center. In an early example of reactivity from the same laboratory, electron-transfer reactions, in which both the metal and ligand become oxidized, mediated reduction of an azide to form chromium imido complexes via the loss of dinitrogen.8 More © 2017 American Chemical Society
recently, Chirik and co-workers reported that a dimolybdenum−dinitrogen complex with a redox-active terpyridine ligand has five accessible redox states centered on both the metal and ligand (Chart 1C).9 Ligand-based proton-transfer reactions with transition-metal complexes are also well-known, and the ligand employed is sometimes referred to as “cooperating”.10 As an illustrative example, Milstein and co-workers reported acceptorless alcohol oxidation to afford esters using a ruthenium complex (Chart 1D),11 where loss of dihydrogen and coupling with an additional alcohol substrate afforded an ester. Proton transfer in titanium complexes was demonstrated by Veige and coworkers using bis(phenoxy)pyrrolide ligands (Chart 1E).12 The large field of ligand-based proton relays that augment transition-metal reactivity in electrocatalysis reactions could also be considered with these examples.13 The combination of both proton and electron transfer mediated by a ligand is less studied but has precedent. Taking recent examples, Nocera and co-workers described a “hangman” nickel complex where electrons and protons are stored in chemical C−H and O−H bonds of the ligand, which then react with excess protons as an efficient hydrogen evolution catalyst (Chart 2A).14 Agapie and co-workers showed that proton and Special Issue: Advances in Main-Group Inorganic Chemistry Received: January 29, 2017 Published: April 12, 2017 8651
DOI: 10.1021/acs.inorgchem.7b00230 Inorg. Chem. 2017, 56, 8651−8660
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Inorganic Chemistry Chart 1. (A) Galactose Oxidase (Whittaker et al.),2 (B) Bis(α-iminopyridine) Complexes (Wieghardt et al.),7 (C) Molybdenum Terpyridine (Chirik et al.),9 (D) Ru-NNP (Milstein et al.),11 and (E) Titanium Pyrolide (Veige et al.)12
chemistry is driven by ligand-based processes or ligand- and metal-based processes depending on the charge and oxidation states of the ligand and U center.16d Redox-Active Ligands in Main-Group Chemistry. In main-group chemistry, ideas and methods to utilize the capacity of ligand-based proton and electron transfer, separately or in combination, are beginning to appear, and this opens new applications in main-group reactivity.21 Most commonly, maingroup metal complexes of redox-active ligands exist in just one charge state of the ligand. Ligand-Based Electron Transfer in Main-Group Chemistry. Just a handful of electron-transfer reactions have been demonstrated. Aluminum(III) complexes in which the BIAN ligand is in a doubly reduced state have been employed in electron-transfer reactions. For example, one-electron-reductive disproportionation of ketones (Scheme 1A)22 and one-electron Scheme 1. (A) Al-BIAN Redox Chemistry (Fedushkin et al.),22 (B) Al-ONO Complex (Heyduk et al.),24 (C) an Aluminum α-Diimine Complex (Graves et al.),25 and (D) Aluminum and Gallium Iminopyridine Redox Chemistry (Berben et al.)27a a
Chart 2. (A) Hangman Nickel (Nocera et al.),14 (B) Quinonoid Molybdenum (Agapie et al.)15 and (C) Zinc Thiosemicarbazole (Grapperhaus et al).4f
a
electron processes occur simultaneously at both the ligand and molybdenum in complexes supported by a catechol-substituted terphenyldiphosphine: reduction of dioxygen was observed (Chart 2B).15 Using a redox-active thiosemicarbazone ligand, Grapperhaus and co-workers synthesized a zinc complex with the ability to perform ligand-based proton reduction and dihydrogen oxidation (Chart 2C).4f Redox-Active Ligands in f-Element Chemistry. Organof-block chemistry incorporating redox-active ligands has had an increasing presence in the literature over the last 15 years. Most of these studies have focused on uranium chemistry,16 but some examples of cerium,17 ytterbium,18 europium,19 samarium,18a and thorium20 also exist. Kiplinger and co-workers synthesized 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (BIAN) complexes of uranium(III) and uranium(IV) and showed that the charge state of the ligand and the oxidation state of the metal depend on the coordination environment of the complex.16b Bart and co-workers have utilized bis(imino)pyridine ligands to perform redox chemistry with complexes of uranium. One elegant example includes the synthesis of a series of mono-, bis-, and tris(imido) complexes in which the redox
Ar = diisopropylphenyl.
reduction of oxidizing agents such as (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO)23 were observed. Heyduk and coworkers showed that bis(3,5-ditert-butyl-2-phenol)amine ([ONO]H3) complexes of aluminum(III) perform electrontransfer reactions with a variety of quinones (Scheme 1B),24 while Graves and co-workers studied doubly reduced α-diimine ligand complexes of aluminum(III) that promote one-electron transfer in the presence of oxidants like AgCl (Scheme 1C).25 In boron chemistry, phenoxide ligands have been observed to undergo reduction to a phenoxyl radical.26 In our own work on ligand-based electron transfer, αiminopyridine ligands support both aluminum(III) and gallium(III) complexes, and both one- and two-electron oxidation reactions have been demonstrated using O-atomtransfer reagents, such as pyridine-N-oxide (pyO), to afford aluminum and gallium hydroxyl complexes (Scheme 1D).27 Ligand-Based Proton Transfer in Main-Group Chemistry. Using ligands for proton transfer in main-group chemistry, we developed a bis(imino)pyridine (PhI2P) ligand aluminum 8652
DOI: 10.1021/acs.inorgchem.7b00230 Inorg. Chem. 2017, 56, 8651−8660
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Inorganic Chemistry hydride complex, (PhI2P2−)AlH(THF) (THF = tetrahydrofuran), that catalyzes the dehydrogenation of formic acid or the dehydrogenative coupling of amines via proton-transfer reactions facilitated by the ligand (Scheme 2A).28 (PhI2P2−)AlH(THF) also heterolytically activates the O−H bonds in alcohols and water by protonation of an amido N atom in Ph I2P: alkoxides or alumoxanes are formed.29
demonstrated ligand-based proton and electron transfer, hydride generation, and dihydrogen evolution.35 In this example, (PhI2P2−)AlCl(THF) can be protonated twice when the PhI2P ligand has a 2− charge to afford doubly protonated [(PhH2I2P)AlCl]2+. The twice-protonated complex is subsequently reduced, electrochemically or chemically, and this produces dihydrogen and regenerates (PhI2P2−)AlCl(THF) (Scheme 3).
Scheme 2. (A) Al-PhI2P Complexes (Berben et al.),28 (B) Activation of NH3 by a Germylene (Roesky et al.),30 (C) N− H and O−H Bond Activation with (BIAN)Al(Et2O)(Et) (Fedushkin et al.),32 (D) Alkyne Activation with (BIAN)Mg(THF)3 (Fedushkin et al.),33 and (E) Protonation of a Aluminum Amide (Bertrand et al.).34a
Scheme 3. Protonation of (PhI2P2−)Al(THF)Cl and Dihydrogen Evolution, Which Has Been Demonstrated Electrocatalyticallya
a
Ar = diisopropylphenyl.
The proton-, electron-, and hydride-transfer chemistry associated with the PhI2P ligand system has enabled us to design next-generation aluminum complexes. The approach described in this work draws on a ligand designed with lower pKa values for the N-donor atoms, so that ligand reduction precedes ligand protonation and does not promote ligand dissociation. In choosing a tridentate, pincer-type ligand, we observed that bis(pyrazolyl)pyridine (Pz2P) has lower pKa values than the bis(imino)pyridine ligand. Of the structurally similar organic molecules with published pKa values, the 1,5dimethyl-3-phenyl-substituted pyrazolium ion most closely resembles the pyrazolyl ring of Pz2P and has a pKa value of 2.59.36 Conjugated secondary amines, structurally similar to the Ph I2P2− ligand, have pKa values greater than 16.37 a
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RESULTS AND DISCUSSION Bis(pyrazolyl)pyridine ligands and their derivatives have been utilized in coordination chemistry for transition metals,38 in addition to lanthanides39 and actinides.40 In the present study, we look at the coordination chemistry of a bis(pyrazolyl)pyridine derivative, iPrPz2P, and its coordination chemistry with aluminum (Chart 3). The ligand that we discuss is functionalized at the 1 position with an isopropyl group and at the 5 position with a isobutyl group to increase the solubility of the compounds in nonpolar solvents. Other iPrPz2P ligands have been published with isopropyl groups at the 1 position,38a,41
Ar = diisopropylphenyl.
Redox-active β-diketiminate is another common ligand platform used to support main-group metal ions, and with this, Roesky et al. have shown that a molecular germylene with an unsaturated β-diketiminato ligand can activate N−H bonds of ammonia via a metal−ligand cooperative mechanism in which the β-diketiminato becomes protonated (Scheme 2B).30 Analogous reactivity is observed with Si II-Ni(CO)3 βdiketiminato complexes that promote the activation of S−H and N−H bonds.31 Aluminum(III) complexes of the BIAN ligand in its doubly reduced state also mediate ligand-based proton transfer. Heterolytic activation of N−H and O−H bonds occurs via protonation of the ligand amido donor atom (Scheme 2C).32 (BIAN)Mg complexes heterolytically activate C−H bonds in alkynes via proton transfer to the amido N atom of the ligand (Scheme 2D).33 Bertrand and co-workers earlier synthesized complexes of a tridentate amide complex that is protonated at the ligand by HCl (Scheme 2E).34 Ligand-Based Proton and Electron Transfer in MainGroup Chemistry. The chemistry of PhI2P complexes of aluminum has also provided examples where both proton and electron transfer are mediated on the ligand platform. We
Chart 3. Bis(pyrazolyl)pyridine (iPrPz2P)a
a
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The numbering of the pyrazolyl ring is included. DOI: 10.1021/acs.inorgchem.7b00230 Inorg. Chem. 2017, 56, 8651−8660
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Inorganic Chemistry
Efforts to provide H• or H+ using sacrificial donors, 1,10dihydroanthracene or weak acids, also failed. Complex 2 is formally the product of a two-electron reduction and protonation of 1, and we investigated possible routes for the interconversion of 1 and 2 along with the possibility of incorporating the two molecules into a catalytic cycle for dihydrogen evolution. To a stirring solution of complex 1 in THF was added 2 equiv of sodium, and the solution became a deep-purple color. 1H NMR confirms the same resonances as previously described for complex 2. To decipher whether the transfer of an H atom to the pyridine in this case was caused by a reaction between two ligand molecules or whether the source of the H atom was a THF molecule, the reaction was performed in C6D6. Again, we determined no deuterium incorporation, and this confirmed that the THF solvent is not necessary as a source of protons. In the case of the C6D6 reaction and most likely in the THF reaction, ligand degradation supplies needed protons. Solid-State Structures. iPrPz2P was crystallized so that the bond lengths and angles could be used as a reference in assigning charge states of the iPrPz2P in complexes 1−3. Large colorless needle-shaped crystals of iPrPz2P were grown from a concentrated hexane solution stored at −40 °C for 1 day (Figure S1). Colorless block-shaped single crystals of 1 were grown by layering a concentrated THF solution of 1 with hexanes at −25 °C overnight (Figure 1). The six-coordinate
and in those instances, the 5 positions bear methyl or phenyl groups. Synthesis of Compounds. The synthesis of [(iPrPz2P)AlCl2(THF)][AlCl4] (1) was adapted from a common synthetic route.42 A total of 1 equiv of iPrPz2P was added to a stirring THF solution of 2 equiv of AlCl3 at room temperature (Scheme 4). Colorless and cationic [(iPrPz2P)AlCl2(THF)]+ Scheme 4. Synthetic Scheme for the Syntheses of Complexes 1−3
formed within 1 h and was isolated with the [AlCl4]− counterion. Complex 1 was characterized by 1H and 13C NMR spectroscopy and single-crystal X-ray diffraction. In the 1 H NMR spectrum of the free ligand and 1, there are heptets observed at 4.04 and 5.49 ppm, respectively, for the isopropyl group on the 1 position of the pyrazolyl ring. The resonances for pyridyl protons also shift significantly when bound to aluminum. The pyridine doublet and triplet resonances shift from 8.27 and 7.34 ppm to 7.77 and 7.97 ppm, respectively, from iPrPz2P to 1. Caulton and co-workers have recently described the reactivity of a similar Pz2P ligand in which studies were performed with the free ligand and reducing equivalents.43 Following the addition of 1 or more equiv of potassium to the ligand, the paramagnetic monoanion was consistently observed. We have previously observed that the higher charges on group 13 3+ cations are effective at stabilizing further reduced ligands. The reaction of iPrPz2P with 2 equiv of sodium produced a blood red solution of “(iPrPz2P−)Na”. A total of 1 equiv of AlCl3 or AlCl2H(THF)2 was added, which caused an immediate color change from red to intense purple, and following workup, we obtained (iPrHPz2P−)AlCl2 (2) or (iPrHPz2P−)AlClH (3), respectively (Scheme 4). In the 1H NMR spectrum of complexes 2 and 3, heptet resonances of the isopropyl group at the 1 position of the pyrazolyl ring are observed at 5.96 and 5.98 ppm, respectively. There are also two triplets observed at 4.99 and 3.51 ppm (J = 3.5 Hz) for complex 2 and at 5.05 ppm (J = 3.4 Hz) and 3.62 ppm (J = 3.6 Hz) for complex 3. These resonances are assigned to the m- and p-pyridine protons, and the integration and splitting of the 1H NMR data suggests that the para position of the pyridine ring had been protonated. The low yield of the reactions (
