Co-reporter:Tobias J. Sherbow, James C. Fettinger, and Louise A. Berben
Inorganic Chemistry August 7, 2017 Volume 56(Issue 15) pp:8651-8651
Publication Date(Web):April 12, 2017
DOI:10.1021/acs.inorgchem.7b00230
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 ligand-centered. 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 electron-transfer 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.
Co-reporter:Bruce M. Johnson;Robert Francke;R. Daniel Little
Chemical Science (2010-Present) 2017 vol. 8(Issue 9) pp:6493-6498
Publication Date(Web):2017/08/21
DOI:10.1039/C7SC02482G
Glassy carbon electrodes covalently modified with a phenanthroimidazole mediator promote electrochemical alcohol and ether oxidation: three orders of magnitude increase in TON, to ∼15 000 in each case, was observed compared with homogeneous mediated reactions. We propose the deactivation pathways in homogeneous solution are prevented by the immobilization: modified electrode reversibility is increased for a one-electron oxidation reaction. The modified electrodes were used to catalytically oxidize p-anisyl alcohol and 1-((benzyloxy)methyl)-4-methoxybenzene, selectively, to the corresponding benzaldehyde and benzyl ester, respectively.
Co-reporter:Natalia D. Loewen, Emily J. Thompson, Michael Kagan, Carolina L. Banales, Thomas W. Myers, James C. Fettinger and Louise A. Berben
Chemical Science 2016 vol. 7(Issue 4) pp:2728-2735
Publication Date(Web):05 Jan 2016
DOI:10.1039/C5SC03169A
Proton relays are known to increase reaction rates for H2 evolution and lower overpotentials in electrocatalytic reactions. In this report we describe two electrocatalysts, [Fe4N(CO)11(PPh3)]− (1−) which has no proton relay, and hydroxyl-containing [Fe4N(CO)11(Ph2P(CH2)2OH)]− (2−). Solid state structures indicate that these phosphine-substituted clusters are direct analogs of [Fe4N(CO)12]− where one CO ligand has been replaced by a phosphine. We show that the proton relay changes the selectivity of reactions: CO2 is reduced selectively to formate by 1− in the absence of a relay, and protons are reduced to H2 under a CO2 atmosphere by 2−. These results implicate a hydride intermediate in the mechanism of the reactions and demonstrate the importance of controlling proton delivery to control product selectivity. Thermochemical measurements performed using infrared spectroelectrochemistry provided pKa and hydricity values for [HFe4N(CO)11(PPh3)]−, which are 23.7, and 45.5 kcal mol−1, respectively. The pKa of the hydroxyl group in 2− was determined to fall between 29 and 41, and this suggests that the proximity of the proton relay to the active catalytic site plays a significant role in the product selectivity observed, since the acidity alone does not account for the observed results. More generally, this work emphasizes the importance of substrate delivery kinetics in determining the selectivity of CO2 reduction reactions that proceed through metal–hydride intermediates.
Co-reporter:Atefeh Taheri and Louise A. Berben
Chemical Communications 2016 vol. 52(Issue 9) pp:1768-1777
Publication Date(Web):11 Dec 2015
DOI:10.1039/C5CC09041E
Molecular approaches to the electrocatalytic reduction of CO2 to formate are varied and versatile in their methods. We discuss recent efforts to catalyse this reaction including significant progress made in the last 5 years. This Feature Article begins with a survey of molecular electrocatalysts that produce CO or H2, but have been observed under certain conditions to afford some formate. These examples are included because they provide valuable mechanistic insight for design of catalysts that produce hydrogenated products selectively from CO2. The subsequent discussion describes catalyst properties that favour C–H bond formation with CO2 and this is followed by recent advances that have been made in developing these catalysts. The focus on specific catalyst systems includes recently reported Ir PCP-type pincer complexes and Fe carbonyl clusters, such as [Fe4N(CO)12]−, that selectively produce formate from CO2 in aqueous solution. A discussion of the relevant thermochemical properties of the catalysts in the context of formate production is included.
Co-reporter:Atefeh Taheri
Inorganic Chemistry 2016 Volume 55(Issue 2) pp:378-385
Publication Date(Web):December 21, 2015
DOI:10.1021/acs.inorgchem.5b02293
The design of electrocatalysts that will selectively transfer hydride equivalents to either H+ or CO2 to afford H2 or formate is a long-standing goal in molecular electrocatalysis. In this Forum Article, we use experimentally determined thermochemical parameters, hydricity and pKa values, to rationalize our observations that the carbide-containing iron carbonyl cluster [Fe4C(CO)12]2– reduces H+ to H2 in the presence of CO2 in either acetonitrile (MeCN), MeCN with 5% water, or buffered water (pH 5–13), with no traces of formate or other carbon-containing products observed. Our previous work has shown that the closely related nitride-containing clusters [Fe4N(CO)12]− and [Fe4N(CO)11(PPh3)]− will also reduce H+ to H2 in either MeCN with 5% water or buffered water (pH 5–13), but upon the addition of CO2, they selectively generate formate. The thermochemical measurements on [Fe4C(CO)12]2– predict that the free energy for transfer of hydride, in MeCN, from the intermediate [HFe4C(CO)12]2– to CO2 is thermoneutral and to H+ is −32 kcal mol–1. In water, these values are less than −19.2 and −8.6 kcal mol–1, respectively. These results suggest that generation of both H2 and formate should be favorable in aqueous solution and that kinetic effects, such as a fast rate of H2 evolution, must influence the observed selectivity for generation of H2. The hydride-donating ability of [HFe4N(CO)12]− is lower than that of [HFe4C(CO)12]2– by 5 kcal mol–1 in MeCN and by at least 3 kcal mol–1 in water, and we speculate that this more modest reactivity provides the needed selectivity to obtain formate. We also discuss predictions that may guide future catalyst design.
Co-reporter:Thomas W. Myers, Tobias J. Sherbow, James C. Fettinger and Louise A. Berben
Dalton Transactions 2016 vol. 45(Issue 14) pp:5989-5998
Publication Date(Web):01 Jul 2015
DOI:10.1039/C5DT01541C
Phenyl-bis(imino)pyridine (PhI2P) complexes, (PhI2P)ZnCl2 (1), (PhI2P−)ZnCl (2) and (PhI2P−)Zn(py)Cl (3) were obtained with the I2P ligand in both the neutral and the one-electron reduced state. In all examples, the metal ion is Zn(II). Metrical parameters obtained from solid state structures of 2 and 3 indicate that the PhI2P− ligand exists as a radical which is supported at the carbon atom of the imino donor, and this electronic state is also apparent in the analogous one-electron reduced ligand Al(III) complex, (PhI2P−)AlCl2 (4), that we prepared for comparison. We were unable to obtain PhI2P Mg complexes, and so the more electron rich methyl-substituted bis(imino)pyridine ligand, MeI2P, was investigated. Reaction of two-electron reduced MeI2P with MgCl2 and Mg(OTf)2 did afford the two-electron reduced ligand complexes [(MeI2P2−)Mg(THF)]2(μ-MgCl2) (5) and (MeI2P2−)Mg(THF)2 (6), respectively (MeI2P = 2,6-bis(1-methylethyl)-N-(2-pyridinylmethylene)phenylamine). Complex 5 crystallizes as a trinuclear Mg complex consisting of two (MeI2P2−)Mg moieties bridged by MgCl2 and the (MeI2P2−) ligand is symmetric across the pyridine ring, but is not planar. In contrast, the (MeI2P2−) ligand in 6 is asymmetric across the pyridine ring and all atoms in the ligand are coplanar. Cyclic voltammetry measurements reveal that in complexes, 1, 4, 5, 6, the I2P0, I2P−, and I2P2− ligand charge states are accessible electrochemically.
Co-reporter:Tobias J. Sherbow, Cody R. Carr, Tomoya Saisu, James C. Fettinger, and Louise A. Berben
Organometallics 2016 Volume 35(Issue 1) pp:9-14
Publication Date(Web):December 23, 2015
DOI:10.1021/acs.organomet.5b00743
The activation of O–H bonds in alcohol substrates is the initial step in acceptor-less catalytic dehydrogenation of alcohols. At room temperature, the bis(imino)pyridine-ligated aluminum hydride compound, denoted as (PhI2P2–)AlH (1), activates O–H bonds in alcohols through a metal–ligand cooperative pathway to afford the phenoxide and benzyloxide complexes with protonated ligand: (PhHI2P1–)Al(OR)H, where R = Ph, Bn. Thermochemical measurements indicate that the amido nitrogen of the protonated ligand in (PhHI2P1–)Al(OPh)H is far more basic (pKa = 36–45) than the analogous proton for the previously reported (PhHI2P1–)Al(NHPh)H (pKa = 10–14), and this is consistent with reactivity we observe, where (PhHI2P1–)Al(OPh)H complexes do not intramolecularly liberate H2. The inability of (PhHI2P1–)Al(OR)H to release H2 upon heating precludes access to a four-coordinate Al center and results in an inability of 1 to dehydrogenate benzyl alcohol to benzaldehyde. These observations also lend further support to the mechanism for benzylamine dehydrogenation that we have previously proposed and provide insights for future catalyst design using metal–ligand cooperative pathways with Al.
Co-reporter:Atefeh Taheri, Emily J. Thompson, James C. Fettinger, and Louise A. Berben
ACS Catalysis 2015 Volume 5(Issue 12) pp:7140
Publication Date(Web):October 27, 2015
DOI:10.1021/acscatal.5b01708
C–H bond formation with CO2 to selectively form products such as formate (HCOO–) is an important step in harnessing CO2 emissions as a carbon-neutral or carbon-negative renewable energy source. In this report, we show that the iron carbonyl cluster, [Fe4N(CO)12]−, is an electrocatalyst for the selective reduction of CO2 to formate in water (pH 5–13). With low applied overpotential (230–440 mV), formate is produced with a high current density of 4 mA cm–2 and 96% Faradaic efficiency. These metrics, combined with the long lifetime of the catalyst (>24 h), and the use of the Earth-abundant material iron, are advances in catalyst performance relative to previously reported homogeneous and heterogeneous formate-producing electrocatalysts. We further characterized the mechanism of catalysis by [Fe4N(CO)12]− using cyclic voltammetry, and structurally characterized a key reaction intermediate, the reduced hydride [HFe4N(CO)12]−. In addition, thermochemical measurements performed using infrared spectroelectrochemistry provided measures of the hydride donor ability (hydricity) for [HFe4N(CO)12]− in both MeCN and aqueous buffered solution. These are 49 and 15 kcal mol–1, respectively, and show that the driving force for C–H bond formation with CO2 by [HFe4N(CO)12]− is very different in the two solvents: +5 kcal mol–1 in MeCN (unfavorable) and −8.5 kcal mol–1 in water (favorable).Keywords: carbonyl cluster; CO2 reduction; electrocatalysis; formate; hydricity; iron
Co-reporter:Emily J. Thompson ; Louise A. Berben
Angewandte Chemie 2015 Volume 127( Issue 40) pp:
Publication Date(Web):
DOI:10.1002/ange.201584061
Co-reporter:Emily J. Thompson ; Louise A. Berben
Angewandte Chemie 2015 Volume 127( Issue 40) pp:11808-11812
Publication Date(Web):
DOI:10.1002/ange.201503935
Abstract
Environmentally sustainable hydrogen-evolving electrocatalysts are key in a renewable fuel economy, and ligand-based proton and electron transfer could circumvent the need for precious metal ions in electrocatalytic H2 production. Herein, we show that electrocatalytic generation of H2 by a redox-active ligand complex of Al3+ occurs at −1.16 V vs. SCE (500 mV overpotential).
Co-reporter:Emily J. Thompson ; Louise A. Berben
Angewandte Chemie International Edition 2015 Volume 54( Issue 40) pp:
Publication Date(Web):
DOI:10.1002/anie.201584061
Co-reporter:Emily J. Thompson ; Louise A. Berben
Angewandte Chemie International Edition 2015 Volume 54( Issue 40) pp:11642-11646
Publication Date(Web):
DOI:10.1002/anie.201503935
Abstract
Environmentally sustainable hydrogen-evolving electrocatalysts are key in a renewable fuel economy, and ligand-based proton and electron transfer could circumvent the need for precious metal ions in electrocatalytic H2 production. Herein, we show that electrocatalytic generation of H2 by a redox-active ligand complex of Al3+ occurs at −1.16 V vs. SCE (500 mV overpotential).
Co-reporter:T. W. Myers and L. A. Berben
Chemical Science 2014 vol. 5(Issue 7) pp:2771-2777
Publication Date(Web):29 Apr 2014
DOI:10.1039/C4SC01035C
Herein, we report that molecular aluminium complexes of the bis(imino)pyridine ligand, (PhI2P2−)Al(THF)X, X = H (1), CH3 (2), promote selective dehydrogenation of formic acid to H2 and CO2 with an initial turnover frequency of 5200 turnovers per hour. Low-temperature reactions show that reaction of 1 with HCOOH affords a complex that is protonated three times: twice on the PhI2P2− ligand and once to liberate H2 or CH4 from the Al-hydride or Al-methyl, respectively. We demonstrate that in the absence of protons, insertion of CO2 into the Al-hydride bond of 1 is facile and produces an Al-formate. Upon addition of protons, liberation of CO2 from the Al-formate complex affords an Al-hydride. Deuterium labelling studies and the solvent dependence of the reaction indicate that outer sphere β-hydride abstraction supported by metal–ligand cooperative hydrogen bonding is a likely mechanism for the C–H bond cleavage.
Co-reporter:Emily J. Thompson;Thomas W. Myers
Angewandte Chemie 2014 Volume 126( Issue 51) pp:14356-14358
Publication Date(Web):
DOI:10.1002/ange.201407098
Abstract
The synthesis of two four-coordinate and square planar (SP) complexes of aluminum(III) is presented. Reaction of a phenyl-substituted bis(imino)pyridine ligand that is reduced by two electrons, Na2(PhI2P2−), with AlCl3 afforded five-coordinate [(PhI2P2−)Al(THF)Cl] (1). Square-planar [(PhI2P2−)AlCl] (2) was obtained by performing the same reaction in diethyl ether followed by lyphilization of 2 from benzene. The four-coordinate geometry index for 2, τ4, is 0.22, where 0 would be a perfectly square-planar molecule. The analogous aluminum hydride complex, [(PhI2P2−)AlH] (3), is also square-planar, and was characterized crystallographically and has τ4=0.13. Both 2 and 3 are Lewis acidic and bind 2,6-lutidine.
Co-reporter:Emily J. Thompson;Thomas W. Myers
Angewandte Chemie International Edition 2014 Volume 53( Issue 51) pp:14132-14134
Publication Date(Web):
DOI:10.1002/anie.201407098
Abstract
The synthesis of two four-coordinate and square planar (SP) complexes of aluminum(III) is presented. Reaction of a phenyl-substituted bis(imino)pyridine ligand that is reduced by two electrons, Na2(PhI2P2−), with AlCl3 afforded five-coordinate [(PhI2P2−)Al(THF)Cl] (1). Square-planar [(PhI2P2−)AlCl] (2) was obtained by performing the same reaction in diethyl ether followed by lyphilization of 2 from benzene. The four-coordinate geometry index for 2, τ4, is 0.22, where 0 would be a perfectly square-planar molecule. The analogous aluminum hydride complex, [(PhI2P2−)AlH] (3), is also square-planar, and was characterized crystallographically and has τ4=0.13. Both 2 and 3 are Lewis acidic and bind 2,6-lutidine.
Co-reporter:Thomas W. Myers and Louise A. Berben
Chemical Communications 2013 vol. 49(Issue 39) pp:4175-4177
Publication Date(Web):23 Nov 2012
DOI:10.1039/C2CC37208H
Redox-active Group 13 molecules possess the unusual combination of concomitant redox and acid–base reactivity. These combined properties enable regeneration of a metal hydroxide complex in a cycle for conversion of CO2 into carbonate salts. Reaction of (IP−)2Al(OH) (M = Al, Ga) with 1 atm of CO2 affords [(IP−)2Al]2(μ2κ1:κ2-OCO2). Subsequent reduction affords MgCO3 or CaCO3 and two equivalents of [(IP2−)2Al]−, which can be reoxidized to (IP−)2Al(OH) to close a cycle.
Co-reporter:An D. Nguyen ; M. Diego Rail ; Maheswaran Shanmugam ; James C. Fettinger
Inorganic Chemistry 2013 Volume 52(Issue 21) pp:12847-12854
Publication Date(Web):October 11, 2013
DOI:10.1021/ic4023882
The development of efficient hydrogen evolving electrocatalysts that operate near neutral pH in aqueous solution remains of significant interest. A series of low-valent iron clusters have been investigated to provide insight into the structure–function relationships affecting their ability to promote formation of cluster-hydride intermediates and to promote electrocatalytic hydrogen evolution from water. Each of the metal carbonyl anions, [Fe4N(CO)12]− (1–), [Fe4C(CO)12]2– (22–), [Fe5C(CO)15]2– (32–), and [Fe6C(CO)18]2– (42–) were isolated as their sodium salt to provide the necessary solubility in water. At pH 5 and −1.25 V vs SCE the clusters afford hydrogen with Faradaic efficiencies ranging from 53–98%. pH dependent cyclic voltammetry measurements provide insight into catalytic intermediates. Both of the butterfly shaped clusters, 1– and 22–, stabilize protonated adducts and are effective catalysts. Initial reduction of butterfly shaped 1– is pH-independent and subsequently, successive protonation events afford H1–, and then hydrogen. In contrast, butterfly shaped 22– undergoes two successive proton coupled electron transfer events to form H222– which then liberates hydrogen. The higher nuclearity clusters, 32– and 42–, do not display the same ability to associate with protons, and accordingly, they produce hydrogen less efficiently.
Co-reporter:Thomas W. Myers;Gereon M. Yee
European Journal of Inorganic Chemistry 2013 Volume 2013( Issue 22-23) pp:3831-3835
Publication Date(Web):
DOI:10.1002/ejic.201300192
Abstract
The control of radical reactions to afford selective carbon–carbon bond formation is a significant synthetic challenge with applications ranging from small-molecule activation to natural product synthesis. Oxidation of (IP–)2Al(CH3) (1, IP = iminopyridine) with TrBPh4 (Tr = trityl) afforded the C–C coupled product [(IP)(Tr-IP)Al(CH3)][BPh4] (2) in which the trityl radical and the IP– radical have undergone C–C bond formation. In contrast, oxidation of 1 with TrBArF24 {BArF24 = tetrakis[(3,5-trifluoromethyl)phenyl]borate} or TrB(C6F5)4 affords cationic [(IP)(IP–)Al(CH3)][BArF24] (3a) or [(IP)(IP–)Al(CH3)][B(C6F5)4] (3b), respectively . The different reaction outcomes provided by the different counteranions of Tr+ imply that a difference in stability of the products or of the intermediate mixed-valent [(IP)(IP-)Al(CH3)]+ state exists. We speculate that the most likely factor is the difference in solubility afforded by the different anions of the products that are formed. We also show that the formation of stable, cationic biradical complexes is possible and that these complexes do not undergo C–C radical coupling at the IP ligand. Cationic [(IP–)2Al(OEt2)][BArF24] (4) was obtained by protonolysis of 1 with H(OEt2)2BArF24, and two-electron oxidation of [(IP2–)2Al]– (5) afforded [(IP–)2Al(thf)][BArF24] (6).
Co-reporter:Thomas W. Myers and Louise A. Berben
Organometallics 2013 Volume 32(Issue 22) pp:6647-6649
Publication Date(Web):August 5, 2013
DOI:10.1021/om400556s
Addition of the O–H bonds in water across the aluminum–nitrogen bond of a molecular aluminum–amido complex affords an alumoxane. The reaction of (PhI2P2–)AlH (1) with water forms dimeric [(PhHI2P–)AlH](μ-O) (2) under mild conditions. Upon reaction of 2 with excess water [(PhHI2P–)Al(OH)](μ-O) (3) is formed with liberation of H2.
Co-reporter:Thomas W. Myers, Alexandra L. Holmes, and Louise A. Berben
Inorganic Chemistry 2012 Volume 51(Issue 16) pp:8997-9004
Publication Date(Web):July 27, 2012
DOI:10.1021/ic301128m
Redox active ligands are shown to facilitate a variety of group transfer reactions at redox inert aluminum(III). Disulfides can be used as a two-electron group transfer reagent, and we show that (IP–)2AlSR can be formed by reaction of [(THF)6Na][(IP2–)2Al] (1c) with disulfides RSSR (where X = C(S)NMe2, 4; SMe, 5). In a more general redox route to substitution of aluminum bis(iminopyridine) complexes, we report zinc(II) salts as a group transfer reagent. Reaction of [(RIP2–)2Al]− (R = H, 1c; Me, 1d) with ZnX2 affords (RIP–)2AlX (where IP = iminopyridine, R = H, and X = Cl, 2; CCPh, 6; N3, 7; SPh, 8; or R = Me and X = NHPh, 9). Single crystal X-ray diffraction analysis of the complexes reveal that each of the five coordinate complexes reported here has a trigonal bipyramidal geometry with τ = 0.668 – 0.858. We observed a correlation between the greatest deviations from ideal trigonal bipyramidal symmetry (lowest τ values), the bond lengths consistent with smallest degree of ligand reduction, and the least polarizable X ligand in (IP–)2AlX. Complex 4 is six-coordinate and is best described as distorted octahedral. Variable temperature magnetic susceptibility measurements indicate that each of the complexes 3–9 has a biradical electronic structure similar to previously reported 2. Magnetic exchange coupling constants in the range J = −94 to −212 cm–1 were fit to the data for 2–9 to describe the energy of antiferromagnetic interaction between ligand radicals assuming a spin Hamiltonian of the form Ĥ = −2JŜL(1)·ŜL(2). The strongest coupling occurs when the angle between the ligand planes is smallest, presumably to afford good overlap with the Al–X σ* orbital. Electrochemical properties of the complexes were probed using cyclic voltammetry and each of 3–9 displayed a reversible two-electron reduction and two quasi-reversible one-electron oxidation processes. The energy of the ligand based redox processes for 2–9 differ by about 150 mV over all complexes and show a correlation with the degree of IP– reduction observed crystallographically; more reduced IP– ligands require higher potentials for further reduction. Comproportionation constants that describe the equilibrium for the reaction (IP–)2AlX + (IP)2AlX ↔ (IP–)(IP)AlX fall in the range of Kc = 105.7 to 107.9 for 3–9.
Co-reporter:Chelsea D. Cates, Thomas W. Myers, and Louise A. Berben
Inorganic Chemistry 2012 Volume 51(Issue 21) pp:11891-11897
Publication Date(Web):October 23, 2012
DOI:10.1021/ic301792w
Reaction of M+[(IP2-)2Ga]− (IP = iminopyridine, M = Bu4N, 1a; (DME)3Na, 1b) with pyridine N-oxide affords two-electron-oxidized (IP–)2Ga(OH) (2) in reactions where the product outcome is independent of the cation identity, M+. In a second example of net two-electron chemistry, outer sphere oxidation of M+[(IP2-)2Ga]− using either 1 or 2 equiv of the one-electron oxidant ferrocenium afforded [(IP–)2Ga]+ (3) in either 44 or 87% yield, respectively. Reaction with 1 equiv of TEMPO, a one-electron oxidant, afforded the two-electron-oxidized product (IP–)2Ga(TEMPO) (4). Reduction of 2IP by 3Na and subsequent reaction with GaCl3 yielded a 1:1 mixture of (IP–)2GaCl and 1. Most remarkably, all of these reactions are overall two-electron processes and only the (IP–)2GaX and [(IP2-)2Ga]− oxidation states are thermodynamically accessible to us. Analogous aluminum chemistry previously afforded either one-electron or two-electron reactions and mixed-valent states. The thermodynamic accessibility of the mixed-valent states of (IP2-)(IP–)E, where E = Al or Ga, can be compared using cyclic voltammetry measurements. These measurements indicated that the mixed-valent state [(IP2-)(IP–)Ga]+ is not significantly stabilized with respect to disproportionation on the time scale of the electrochemistry experiment. The electrochemically observed differences in thermodynamic stability of the mixed-valent state [(IP2-)(IP–)E]+ can be rationalized by the observation that the dihedral angle between the ligand planes containing the π-system of IP is roughly 5° larger in all gallium complexes compared with aluminum analogs. Presumably, a larger dihedral angle provides weaker electronic coupling between the π-systems of IP via the E–X σ* orbital. Alternatively, the observed difference may be a result of the “inert pair effect”: a contracted Ga component in the E–X σ* orbital would also afford weaker electronic coupling.
Co-reporter:Thomas W. Myers and Louise A. Berben
Inorganic Chemistry 2012 Volume 51(Issue 3) pp:1480-1488
Publication Date(Web):January 5, 2012
DOI:10.1021/ic201729b
The combination of an electrophilic metal center with a redox active ligand set has the potential to provide reactivity unique from transition metal redox chemistry. In this report, substituted iminopyridine complexes containing monoanionic and dianionic MeIPMes ligands have been characterized structurally and electronically. Green (MeIPMes–)AlCl2 (1), (MeIPMes–)AlMe2 (2), and (MeIPMes–)GaCl2 (5) have a doublet spin state which results from the anion radical form of MeIPMes. Purple (MeIPMes2–)AlCl(OEt2) (3), (MeIPMes2–)AlMe(OEt2) (4), and (MeIPMes2–)GaCl(OEt2) (6) are each diamagnetic. We have also investigated the solvent dependence of the decomposition of the MeIPMes anion radical. Complexes 1 and 2 can be obtained from benzene and hexanes whereas the use of ether solvents results in the formation of undesirable (CH2IPMes–)AlCl2 (1a) and (CH2IPMes–)AlCl2 (2a) formed by loss of a hydrogen atom from the MeIPMes– ligand. Electrochemical measurements indicate that 1, 2, and 5 are redox active.
Co-reporter:Kristin Kowolik, Maheswaran Shanmugam, Thomas W. Myers, Chelsea D. Cates and Louise A. Berben
Dalton Transactions 2012 vol. 41(Issue 26) pp:7969-7976
Publication Date(Web):19 Mar 2012
DOI:10.1039/C2DT30112A
We have prepared a series of gallium(III) complexes of the redox active iminopyridine ligand (IP). Reaction of GaCl3 with iminopyridine ligand (IP) in the presence of either two or four equivalents of sodium metal resulted in the formation of deep green (IP−)2GaCl (1), or deep purple [(DME)3Na][(IP2−)2Ga] (2a), respectively. Complex 1 is paramagnetic with a room temperature magnetic moment of 2.3 μB which falls to 0.5 μB at 5 K. These observations indicate that two ligand radicals comprise a triplet at room temperature which becomes a singlet due to antiferromagnetic coupling at low temperature. Complex 2 is diamagnetic. Cyclic voltammograms recorded on 0.3 M Bu4NPF6 THF solutions of [Na(THF)6][(IP2−)2Ga]− (2b) indicate that oxidation of 2b occurs in two two-electron steps at −1.31 V and −0.54 V vs. SCE. The observation of two-electron redox events indicates that electronic coupling through the gallium(III) center is minimal and that the two IP ligand on 2b are oxidized concurrently. Oxidation of 2 with one equivalent of MeS–SMe afforded the two-electron oxidized product (IP−)2Ga(SMe) (3). This complex has an electronic structure analogous to 1. Accordingly, both 1 and 3 are deep green in color and magnetic susceptibility measurements performed on 3 confirm the triplet character of the complex at room temperature. Electron paramagnetic resonance experiments on 1 and 3 display a quartet signal at g = 2.0 which confirmed the triplet nature of the compounds, and a half field signal consistent with the integer spin state.
Co-reporter:Owen T. Summerscales, Thomas W. Myers, and Louise A. Berben
Organometallics 2012 Volume 31(Issue 9) pp:3463-3465
Publication Date(Web):May 1, 2012
DOI:10.1021/om300242q
Use of SiCl4 as an organometallic reagent can be complicated by access to Si3+, Si2+, and unwanted sigmatropic rearrangements. Herein we report a mild reduction route, using (IP–)2Mg(THF) (1) and Mg metal, to cleanly access (IP2–)2Si (2). Electrochemical measurements show that IP2– is stabilized by Si4+ > Al3+ > Mg2+.
Co-reporter:M. Diego Rail
Journal of the American Chemical Society 2011 Volume 133(Issue 46) pp:18577-18579
Publication Date(Web):October 27, 2011
DOI:10.1021/ja208312t
Selective reactivity of an electrocatalytically generated catalyst–hydride intermediate toward the hydrogen evolution reaction (HER) or reduction of CO2 is key for a CO2 reduction electrocatalyst. Under appropriate conditions, Et4N[Fe4N(CO)12] (Et4N-1) is a catalyst for the HER or for CO2 conversion at −1.25 V vs SCE using a glassy carbon electrode.
Co-reporter:Thomas W. Myers ; Nasrin Kazem ; Stefan Stoll ; R. David Britt ; Maheswaran Shanmugam
Journal of the American Chemical Society 2011 Volume 133(Issue 22) pp:8662-8672
Publication Date(Web):May 13, 2011
DOI:10.1021/ja2015718
Electrophilic activation and subsequent reduction of substrates is in general not possible because highly Lewis acidic metals lack access to multiple redox states. Herein, we demonstrate that transition metal-like redox processes and electronic structure and magnetic properties can be imparted to aluminum(III). Bis(iminopyridine) complexes containing neutral, monoanionic, and dianionic iminopyridine ligands (IP) have been characterized structurally and electronically; yellow (IP)AlCl3 (1), deep green (IP–)2AlCl (2) and (IP–)2Al(CF3SO3) (3), and deep purple [(IP2–)Al]− (5) are presented. The mixed-valent, monoradical complex (IP–)(IP2–)Al is unstable toward C–C coupling, and [(IP2–)Al]2−(μ-IP–IP)2– (4) has been isolated. Variable-temperature magnetic susceptibility and EPR spectroscopy measurements indicate that the biradical character of the ligand-based triplet in 2 is stabilized by strong antiferromagnetic exchange coupling mediated by aluminum(III): J = −230 cm–1 for Ĥ = −2J(ŜL(1)·ŜL(2)). Coordination geometry-dependent (IP–)–(IP–) communication through aluminum(III) is observed electrochemically. The cyclic voltammogram of trigonal bipyramidal 2 displays successive ligand-based oxidation events for the two IP1–/0 processes, at −0.86 and −1.20 V vs SCE. The 0.34 V spacing between redox couples corresponds to a conproportionation constant of Kc = 105.8 for the process (IP–)2AlCl + (IP)2AlCl → 2(IP–)(IP)AlCl consistent with Robin and Day Class II mixed-valent behavior. Tetrahedral 5 displays localized, Class I behavior as indicated by closely spaced redox couples. Furthermore, CV’s of 2 and 5 indicate that changes in the coordination environment of the aluminum center shift the potentials for the IP1–/0 and IP2–/1– redox couples by up to 0.9 V.
Co-reporter:Thomas W. Myers
Journal of the American Chemical Society 2011 Volume 133(Issue 31) pp:11865-11867
Publication Date(Web):July 20, 2011
DOI:10.1021/ja203842s
Hydrogen abstraction by aluminum(III)–oxo intermediates via reaction pathways reminiscent of late transition metal chemistry has been observed. Oxidation of M+[(IP2–)2Al]− (IP = iminopyridine, M = Na, Bu4N) yielded [Na(THF)(DME)][(IP–)(IP2–)Al(OH)] (3) or [(IP–)2Al(OH)] (4), via O-atom transfer and subsequent C–H activation or proton abstraction, respectively.
Co-reporter:Gereon M. Yee, Kristin Kowolik, Shuhei Manabe, James C. Fettinger and Louise A. Berben
Chemical Communications 2011 vol. 47(Issue 42) pp:11680-11682
Publication Date(Web):26 Sep 2011
DOI:10.1039/C1CC14758G
Synthesis of substituted phenylacetylide ligands 2,6-bis(trimethylsilyl)phenylacetylene (H1) and 2-(triphenylsilyl)phenylacteylene (H2) is reported. Ligand 1 supports tetrahedral complexes of V(III), Fe(II), and Mn(II) (3–5). Complexes 3–5 are high-spin and redox active.
Co-reporter:Thomas W. Myers
Journal of the American Chemical Society () pp:
Publication Date(Web):June 25, 2013
DOI:10.1021/ja4032874
Activation of N–H bonds by a molecular aluminum complex via metal–ligand cooperation is described. (PhI2P2–)AlH (1b), in which PhI2P2– is a tridentate bis(imino)pyridine ligand, reacts with anilines to give the N–H-activated products (PhHI2P–)AlH(NHAr) (2). When heated, 2 releases H2 and affords (PhI2P–)Al(NHAr) (3). Complex 1b catalyzes the dehydrogenative coupling of benzylamine to afford H2, NH3, and N-(phenylmethylene)benzenemethanamine.
Co-reporter:Atefeh Taheri and Louise A. Berben
Chemical Communications 2016 - vol. 52(Issue 9) pp:NaN1777-1777
Publication Date(Web):2015/12/11
DOI:10.1039/C5CC09041E
Molecular approaches to the electrocatalytic reduction of CO2 to formate are varied and versatile in their methods. We discuss recent efforts to catalyse this reaction including significant progress made in the last 5 years. This Feature Article begins with a survey of molecular electrocatalysts that produce CO or H2, but have been observed under certain conditions to afford some formate. These examples are included because they provide valuable mechanistic insight for design of catalysts that produce hydrogenated products selectively from CO2. The subsequent discussion describes catalyst properties that favour C–H bond formation with CO2 and this is followed by recent advances that have been made in developing these catalysts. The focus on specific catalyst systems includes recently reported Ir PCP-type pincer complexes and Fe carbonyl clusters, such as [Fe4N(CO)12]−, that selectively produce formate from CO2 in aqueous solution. A discussion of the relevant thermochemical properties of the catalysts in the context of formate production is included.
Co-reporter:Thomas W. Myers and Louise A. Berben
Chemical Communications 2013 - vol. 49(Issue 39) pp:NaN4177-4177
Publication Date(Web):2012/11/23
DOI:10.1039/C2CC37208H
Redox-active Group 13 molecules possess the unusual combination of concomitant redox and acid–base reactivity. These combined properties enable regeneration of a metal hydroxide complex in a cycle for conversion of CO2 into carbonate salts. Reaction of (IP−)2Al(OH) (M = Al, Ga) with 1 atm of CO2 affords [(IP−)2Al]2(μ2κ1:κ2-OCO2). Subsequent reduction affords MgCO3 or CaCO3 and two equivalents of [(IP2−)2Al]−, which can be reoxidized to (IP−)2Al(OH) to close a cycle.
Co-reporter:Gereon M. Yee, Kristin Kowolik, Shuhei Manabe, James C. Fettinger and Louise A. Berben
Chemical Communications 2011 - vol. 47(Issue 42) pp:NaN11682-11682
Publication Date(Web):2011/09/26
DOI:10.1039/C1CC14758G
Synthesis of substituted phenylacetylide ligands 2,6-bis(trimethylsilyl)phenylacetylene (H1) and 2-(triphenylsilyl)phenylacteylene (H2) is reported. Ligand 1 supports tetrahedral complexes of V(III), Fe(II), and Mn(II) (3–5). Complexes 3–5 are high-spin and redox active.
Co-reporter:Natalia D. Loewen, Emily J. Thompson, Michael Kagan, Carolina L. Banales, Thomas W. Myers, James C. Fettinger and Louise A. Berben
Chemical Science (2010-Present) 2016 - vol. 7(Issue 4) pp:NaN2735-2735
Publication Date(Web):2016/01/05
DOI:10.1039/C5SC03169A
Proton relays are known to increase reaction rates for H2 evolution and lower overpotentials in electrocatalytic reactions. In this report we describe two electrocatalysts, [Fe4N(CO)11(PPh3)]− (1−) which has no proton relay, and hydroxyl-containing [Fe4N(CO)11(Ph2P(CH2)2OH)]− (2−). Solid state structures indicate that these phosphine-substituted clusters are direct analogs of [Fe4N(CO)12]− where one CO ligand has been replaced by a phosphine. We show that the proton relay changes the selectivity of reactions: CO2 is reduced selectively to formate by 1− in the absence of a relay, and protons are reduced to H2 under a CO2 atmosphere by 2−. These results implicate a hydride intermediate in the mechanism of the reactions and demonstrate the importance of controlling proton delivery to control product selectivity. Thermochemical measurements performed using infrared spectroelectrochemistry provided pKa and hydricity values for [HFe4N(CO)11(PPh3)]−, which are 23.7, and 45.5 kcal mol−1, respectively. The pKa of the hydroxyl group in 2− was determined to fall between 29 and 41, and this suggests that the proximity of the proton relay to the active catalytic site plays a significant role in the product selectivity observed, since the acidity alone does not account for the observed results. More generally, this work emphasizes the importance of substrate delivery kinetics in determining the selectivity of CO2 reduction reactions that proceed through metal–hydride intermediates.
Co-reporter:T. W. Myers and L. A. Berben
Chemical Science (2010-Present) 2014 - vol. 5(Issue 7) pp:NaN2777-2777
Publication Date(Web):2014/04/29
DOI:10.1039/C4SC01035C
Herein, we report that molecular aluminium complexes of the bis(imino)pyridine ligand, (PhI2P2−)Al(THF)X, X = H (1), CH3 (2), promote selective dehydrogenation of formic acid to H2 and CO2 with an initial turnover frequency of 5200 turnovers per hour. Low-temperature reactions show that reaction of 1 with HCOOH affords a complex that is protonated three times: twice on the PhI2P2− ligand and once to liberate H2 or CH4 from the Al-hydride or Al-methyl, respectively. We demonstrate that in the absence of protons, insertion of CO2 into the Al-hydride bond of 1 is facile and produces an Al-formate. Upon addition of protons, liberation of CO2 from the Al-formate complex affords an Al-hydride. Deuterium labelling studies and the solvent dependence of the reaction indicate that outer sphere β-hydride abstraction supported by metal–ligand cooperative hydrogen bonding is a likely mechanism for the C–H bond cleavage.
Co-reporter:Thomas W. Myers, Tobias J. Sherbow, James C. Fettinger and Louise A. Berben
Dalton Transactions 2016 - vol. 45(Issue 14) pp:NaN5998-5998
Publication Date(Web):2015/07/01
DOI:10.1039/C5DT01541C
Phenyl-bis(imino)pyridine (PhI2P) complexes, (PhI2P)ZnCl2 (1), (PhI2P−)ZnCl (2) and (PhI2P−)Zn(py)Cl (3) were obtained with the I2P ligand in both the neutral and the one-electron reduced state. In all examples, the metal ion is Zn(II). Metrical parameters obtained from solid state structures of 2 and 3 indicate that the PhI2P− ligand exists as a radical which is supported at the carbon atom of the imino donor, and this electronic state is also apparent in the analogous one-electron reduced ligand Al(III) complex, (PhI2P−)AlCl2 (4), that we prepared for comparison. We were unable to obtain PhI2P Mg complexes, and so the more electron rich methyl-substituted bis(imino)pyridine ligand, MeI2P, was investigated. Reaction of two-electron reduced MeI2P with MgCl2 and Mg(OTf)2 did afford the two-electron reduced ligand complexes [(MeI2P2−)Mg(THF)]2(μ-MgCl2) (5) and (MeI2P2−)Mg(THF)2 (6), respectively (MeI2P = 2,6-bis(1-methylethyl)-N-(2-pyridinylmethylene)phenylamine). Complex 5 crystallizes as a trinuclear Mg complex consisting of two (MeI2P2−)Mg moieties bridged by MgCl2 and the (MeI2P2−) ligand is symmetric across the pyridine ring, but is not planar. In contrast, the (MeI2P2−) ligand in 6 is asymmetric across the pyridine ring and all atoms in the ligand are coplanar. Cyclic voltammetry measurements reveal that in complexes, 1, 4, 5, 6, the I2P0, I2P−, and I2P2− ligand charge states are accessible electrochemically.
Co-reporter:Kristin Kowolik, Maheswaran Shanmugam, Thomas W. Myers, Chelsea D. Cates and Louise A. Berben
Dalton Transactions 2012 - vol. 41(Issue 26) pp:NaN7976-7976
Publication Date(Web):2012/03/19
DOI:10.1039/C2DT30112A
We have prepared a series of gallium(III) complexes of the redox active iminopyridine ligand (IP). Reaction of GaCl3 with iminopyridine ligand (IP) in the presence of either two or four equivalents of sodium metal resulted in the formation of deep green (IP−)2GaCl (1), or deep purple [(DME)3Na][(IP2−)2Ga] (2a), respectively. Complex 1 is paramagnetic with a room temperature magnetic moment of 2.3 μB which falls to 0.5 μB at 5 K. These observations indicate that two ligand radicals comprise a triplet at room temperature which becomes a singlet due to antiferromagnetic coupling at low temperature. Complex 2 is diamagnetic. Cyclic voltammograms recorded on 0.3 M Bu4NPF6 THF solutions of [Na(THF)6][(IP2−)2Ga]− (2b) indicate that oxidation of 2b occurs in two two-electron steps at −1.31 V and −0.54 V vs. SCE. The observation of two-electron redox events indicates that electronic coupling through the gallium(III) center is minimal and that the two IP ligand on 2b are oxidized concurrently. Oxidation of 2 with one equivalent of MeS–SMe afforded the two-electron oxidized product (IP−)2Ga(SMe) (3). This complex has an electronic structure analogous to 1. Accordingly, both 1 and 3 are deep green in color and magnetic susceptibility measurements performed on 3 confirm the triplet character of the complex at room temperature. Electron paramagnetic resonance experiments on 1 and 3 display a quartet signal at g = 2.0 which confirmed the triplet nature of the compounds, and a half field signal consistent with the integer spin state.
![2,6-Bis[1-(2,6-di-i-propylphenylimino)ethyl]pyridine](/data/chemimg/6300/204203-14-5.png)
![2,6-Bis[1-(2,6-di-i-propylphenylimino)ethyl]pyridine](/data/chemimg/6300/204203-14-5_b.png)
![Poly[2,7-(9,9-di-octyl-fluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole]](/data/chemimg/133300/782469-77-6.png)
![Poly[2,7-(9,9-di-octyl-fluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole]](/data/chemimg/133300/782469-77-6_b.png)
![Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)]](/data/chemimg/107700/210347-52-7.png)
![Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)]](/data/chemimg/107700/210347-52-7_b.png)
