David J. Szalda

Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures

Green plants convert CO2 to sugar for energy storage via photosynthesis. We report a novel catalyst that uses CO2 and hydrogen to store energy in formic acid. Using a homogeneous iridium catalyst with a proton-responsive ligand, we show the first reversible and recyclable hydrogen storage system that operates under mild conditions using CO2, formate and formic acid. This system is energy-efficient and green because it operates near ambient conditions, uses water as a solvent, produces high-pressure CO-free hydrogen, and uses pH to control hydrogen production or consumption. The extraordinary and switchable catalytic activity is attributed to the multifunctional ligand, which acts as a proton-relay and strong π-donor, and is rationalized by theoretical and experimental studies. When operating at near-ambient conditions, using water as a solvent, a high-turnover iridium catalyst enables a reversible hydrogen storage system that uses carbon dioxide, formate and formic acid. Proton-responsive ligands in the catalyst allow it to be turned on or off by controlling the pH of the solution.

Effects of a proximal base on water oxidation and proton reduction catalyzed by geometric isomers of [Ru(tpy)(pynap)(OH2)]2+.

Basic difference: The importance of a pendent base in promoting proton-coupled electron-transfer reactions with low activation barriers has been discussed for H(+) reduction or H(2) oxidation in acetonitrile. Investigation of the interaction between a base positioned in the second coordination sphere of a complex and a water ligand in water oxidation reactions using geometric isomers of [Ru(tpy)(pynap)(OH(2))](2+) (see picture) gave intriguing results.

An N-heterocyclic carbene as a bidentate hemilabile ligand: a synchrotron X-ray diffraction and density functional theory studyElectronic supplementary information (ESI) available: experimental details and characterization data; table of results for hydrogenation of 3-pentanone; Gaussian 98 summary for the W and Mo models; ORTEP plot of 1W and crystal data. See http://www.rsc.org/suppdata/cc/b3/b303762b/

The N-heterocyclic carbene ligand IMes was shown by synchrotron crystallography and DFT computations to adopt a hemilabile bidentate coordination mode in CpM(CO)2(IMes)+B(C6F5)4− (M = Mo, W), with a CC bond of one mesityl weakly coordinated to the metal.

Mechanism of water oxidation by [Ru(bda)(L)2]: the return of the “blue dimer”

We describe here a combined solution-surface-DFT calculations study for complexes of the type [Ru(bda)(L)2] including X-ray structure of intermediates and their reactivity, as well as pH-dependent electrochemistry and spectroelectrochemistry. These studies shed light on the mechanism of water oxidation by [Ru(bda)(L)2], revealing key features unavailable from solution studies with sacrificial oxidants.

Iron(II) and Ruthenium(II) Complexes Containing P, N, and H Ligands: Structure, Spectroscopy, Electrochemistry, and Reactivity

The purpose of this work was to explore the possibility of using iron(II) hydrides in CO(2) reduction and to compare their reactivity to that of their ruthenium analogues. Fe(bpy)(P(OEt)(3))(3)H(+) and Ru(bpy)(P(OEt)(3))(3)H(+) do not react with CO(2) in acetonitrile, but the one-electron-reduction products of Ru(bpy)(P(OEt)(3))(3)H(+) and Ru(bpy)(2)(P(OEt)(3))H(+) and the two-electron-reduction product of Fe(bpy)(P(OEt)(3))(3)H(+) do. Ru(bpy)(2)(P(OEt)(3))H(+) also reacts slowly with CO(2) to give a formate complex [as reported previously by Albertin et al. (Inorg. Chem. 2004, 43, 1336)] with a second-order rate constant of ∼4 × 10(-3) M(-1) s(-1) in methanol. The structures for the hydride complexes [Fe(bpy)(P(OEt)(3))(3)H](+) and [Ru(bpy)(2)(P(OEt)(3))H](+) and for the (η(5)-Cp)bis- and -tris-PTA complexes (PTA = 1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane) of iron(II) are reported. These and the CpFe(CO)(bpy)(+) and Fe(II)PNNP compounds have been subjected to electrochemical and UV-vis spectroscopic characterization. Fe(bpy)(P(OEt)(3))(3)H(+) exhibits a quasi-reversible oxidation at +0.42 V vs AgCl/Ag in acetonitrile; Ru(bpy)(P(OEt)(3))(3)H(+) and Ru(bpy)(2)(P(OEt)(3))H(+) are oxidized irreversibly at +0.90 and +0.55 V, respectively, vs AgCl/Ag. The reduction site for Fe(bpy)(P(OEt)(3))(3)H(+) and Fe(bpy)(P(OEt)(3))(3)(CH(3)CN)(2+) appears to be the metal and gives rise to a two-electron process. The bpy-centered reductions are negatively shifted in the ruthenium(II) hydride complexes, compared to the acetonitrile complexes. The results of attempts to prepare other iron(II) hydrides are summarized.

Striking Differences in Properties of Geometric Isomers of [Ir(tpy)(ppy)H]+: Experimental and Computational Studies of their Hydricities, Interaction with CO2, and Photochemistry

We prepared two geometric isomers of [Ir(tpy)(ppy)H](+), previously proposed as a key intermediate in the photochemical reduction of CO2 to CO, and characterized their notably different ground- and excited-state interactions with CO2 and their hydricities using experimental and computational methods. Only one isomer, C-trans-[Ir(tpy)(ppy)H](+), reacts with CO2 to generate the formato complex in the ground state, consistent with its calculated hydricity. Under photocatalytic conditions in CH3CN/TEOA, a common reactive C-trans-[Ir(tpy)(ppy)](0) species, irrespective of the starting isomer or monodentate ligand (such as hydride or Cl), reacts with CO2 and produces CO with the same catalytic efficiency.

Water Oxidation by Ruthenium Complexes Incorporating Multifunctional Bipyridyl Diphosphonate Ligands.

We describe herein the synthesis and characterization of ruthenium complexes with multifunctional bipyridyl diphosphonate ligands as well as initial water oxidation studies. In these complexes, the phosphonate groups provide redox-potential leveling through charge compensation and σ donation to allow facile access to high oxidation states. These complexes display unique pH-dependent electrochemistry associated with deprotonation of the phosphonic acid groups. The position of these groups allows them to shuttle protons in and out of the catalytic site and reduce activation barriers. A mechanism for water oxidation by these catalysts is proposed on the basis of experimental results and DFT calculations. The unprecedented attack of water at a neutral six-coordinate [Ru(IV) ] center to yield an anionic seven-coordinate [Ru(IV) -OH](-) intermediate is one of the key steps of a single-site mechanism in which all species are anionic or neutral. These complexes are among the fastest single-site catalysts reported to date.

NMR Spectroscopic and Computational Study of Conformational Isomerism in Substituted 2-Aryl-3H-1-benzazepines: Toward Isolable Atropisomeric Benzazepine Enantiomers

Certain 2-aryl-3H-1-benzazepines are conformationally mobile on the NMR time scale. Variable-temperature NMR experiments bolstered by calculations indicate that alkylation of the azepine ring will slow the interconversion of conformational enantiomers markedly. DFT studies show that, while the substitution patterns of the aryl groups at C2 and C4 do not exert large effects on the rate of enantiomerization, alkylation at C5 slows it appreciably. Alkylation at C3 slows enantiomerization even more, possibly to the extent that isolation of atropisomers might be attempted.

Crystallization and structure of a binuclear species containing the CoC(OH)OCo moiety

Abstract A binuclear species containing the CoC(OH)OCo moiety, [(CoL)2(COOH)](ClO4)3(NCCH3)2 (L=5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) was isolated from a CO2 saturated acetonitrile/diethyl ether solution of [CoL(CO2)](ClO4) and its structure was determined. The complex consists of two [CoL]+ units connected by a bridging COOH+ in which the carbon is coordinated to one CoL+ and the non-protonated oxygen to the other CoL+ unit.

Hydride transfer reactions of transition metal hydrides in the preparation of [Cp(CO)3W(η1-aldehyde)]+OTf− and [Cp(CO)3W(η1-ketone)]+OTf− complexes

Abstract The reaction of Ph(CO)Cl with Cp(CO)3WH and HOTf gives the η1-aldehyde complex [Cp(CO)3W(η1-PhCHO)]+OTf−. The structure of[Cp(CO)3W(η1-PhCHO)]+OTf− (C16H1lF3O7SW) was determined by single crystal X-ray diffraction (triclinic, space group P 1 , a = 10.639(5), b = 10.752(4), c = 10.096(3) A, α = 91.38(3), β = 117.08(3), γ = 66.01(4)°, ζ = 2). Decomposition of this compound in CH2Cl2 solution follows first-order kinetics (k = 3.6(2) × 10−4 s−1 at 25°C) and produces free PhCHO and Cp(CO)3WOTf The η1-CH3CHO complex [Cp(CO)3W (η1-CH3CHO)]+OTf− was similarly prepared from the reaction of acetyl chloride with Cp(CO)3WH and HOTf. Hydrogenation of α,β-unsaturated aldenydes by Cp(CO)3WH and HOTf produces [Cp(CO)3W(η1-RCHO)]+OTf− that were isolated and fully characterized; analogous reactions with α,β-unsaturated ketones gives [Cp(CO)3W(η1-O=CRR′)]+OTf− complexes. All of these aldehyde and ketone complexes with OTf- release free aldehyde or ketone in solution and produce Cp(CO)3WOTf, but [Cp(CO)3W(η1-2-butanone)]+BAr′4− (Ar′ = 3,5-bis(trifluoroniethyl)phenyl) is much more stable.