Hydrogen Production Using a Nickel Catalyst Combining Redox Activity and Pendent Base Effects.

With the mounting need for clean and renewable energy, catalysts for hydrogen production based on earth abundant elements are of great interest. Herein, we describe the synthesis, characterization, and catalytic activity of two nickel complexes based on the pyridinediimine ligand that possess basic nitrogen moieties of pyridine and imidazole that could potentially serve as pendent bases to enhance catalysis. Although these ligands have previously been reported to be complexed to some metal ions, they have not been applied to nickel. The nickel complex with the pendent pyridines was found to be the most active of the two, catalyzing proton reduction electrochemically with an overpotential of 490 mV. The appearance of a wave that preceded the Ni(I/0) redox couple in the presence of protons suggests that protonation of a dissociated pyridine was likely. Further evidence of this was provided with density functional theory calculations, and a mechanism of hydrogen production is proposed. Furthermore, in a light-driven system containing Ru(bpy)32+ and ascorbic acid, TON of 1400 were obtained.

Systematic evaluation of the electronic effect of aluminum-containing ligands in iridium-aluminum and rhodium-aluminum bimetallic complexes.

Pyridinemethanolate and oxyquinoline derivatives of previously reported late transition metal-aluminum heterobimetallic complexes containing iridium and rhodium have been synthesized and characterized. A combination of experimental and computational data permits a direct comparison of the electronic effects of each novel aluminum-containing ligand in our library on the late transition metal centers. Alongside electronic data of previously reported oxypyridine bridged systems, we conclude that the addition of a dialkylaluminum(X) (X = anion) fragment does not significantly perturb the electron donor ability of the bridging ligand. Anions bound to the aluminum are also shown to behave similarly. The overall library, thus, suggests that the best predictor of the electron donor ability of an alkylaluminum-containing ligand to a transition metal is the donor power of the bridging ligand.

Synthesis and Characterization of Strongly Solvatochromic Molybdenum(III) Complexes.

A series of seven molybdenum(III) complexes with the general formula of [Mo(diimine)Cl4]- were synthesized and characterized by X-ray diffraction, IR, cyclic voltammetry (CV), and UV-vis. The complexes were discovered to be highly solvatochromic, showing shifts in λmax between ∼120 and 170 nm in solvents ranging from water to acetone. Varying the substituents on the diimine ligand influenced the absorption energy such that electron-withdrawing groups induced a red shift while electron-donating groups exhibited the opposite effect. The complexes were surprisingly stable in both acidic and basic solutions, and in the case where carboxylic acid substituents were present, additional shifts in the absorption maxima were observed, corresponding to the state of protonation of these groups. Both the MoIV/III and MoIII/II redox couples were observed in CV experiments and were complemented with density functional theory (DFT) calculations.

Synthesis and Characterization of Heterobimetallic Iridium-Aluminum and Rhodium-Aluminum Complexes.

We demonstrate the synthesis and characterization of a new class of late-transition-metal-aluminum heterobimetallic complexes via a novel synthetic pathway. Complexes of this type are exceedingly rare. Joint experimental and theoretical data sheds light on the electronic effect of ligands containing aluminum moieties on late-transition-metal complexes.

Light-driven hydrogen production from aqueous protons using molybdenum catalysts.

Homogeneous light-driven systems employing molecular molybdenum catalysts for hydrogen production are described. The specific Mo complexes studied are six-coordinate bis(benzenedithiolate) derivatives having two additional isocyanide or phosphine ligands to complete the coordination sphere. Each of the complexes possesses a trigonal prismatic coordination geometry. The complexes were investigated as proton reduction catalysts in the presence of [Ru(bpy)3](2+), ascorbic acid, and visible light. Over 500 TON are obtained over 24 h. Electrocatalysis occurs between the Mo(IV)/Mo(III) and Mo(III)/Mo(II) redox couples, around 1.0 V vs SCE. Mechanistic studies by (1)H NMR spectroscopy show that upon two-electron reduction the Mo(CNR)2(bdt)2 complex dissociates the isocyanide ligands, followed by addition of acid to result in the formation of molecular hydrogen and the Mo(bdt)2 complex.

From seconds to femtoseconds: solar hydrogen production and transient absorption of chalcogenorhodamine dyes.

A series of chalcogenorhodamine dyes with oxygen, sulfur, and selenium atoms in the xanthylium core was synthesized and used as chromophores for solar hydrogen production with a platinized TiO2 catalyst. Solutions containing the selenorhodamine dye generate more hydrogen [181 turnover numbers (TONs) with respect to chromophore] than its sulfur (30 TONs) and oxygen (20 TONs) counterparts. This differs from previous work incorporating these dyes into dye-sensitized solar cells (DSSCs), where the oxygen- and selenium-containing species perform similarly. Ultrafast transient absorption spectroscopy revealed an ultrafast electron transfer under conditions for dye-sensitized solar cells and a slower electron transfer under conditions for hydrogen production, making the chromophore's triplet yield an important parameter. The selenium-containing species is the only dye for which triplet state population is significant, which explains its superior activity in hydrogen evolution. The discrepancy in rates of electron transfer appears to be caused by the presence or absence of aggregation in the system, altering the coupling between the dye and TiO2. This finding demonstrates the importance of understanding the differences between, as well as the effects of the conditions for DSSCs and solar hydrogen production.

Cobalt complexes as artificial hydrogenases for the reductive side of water splitting.

The generation of H2 from protons and electrons by complexes of cobalt has an extensive history. During the past decade, interest in this subject has increased as a result of developments in hydrogen generation that are driven electrochemically or photochemically. This article reviews the subject of hydrogen generation using Co complexes as catalysts and discusses the mechanistic implications of the systems studied for making H2. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Kinetic and mechanistic aspects of atom transfer radical addition (ATRA) catalyzed by copper complexes with tris(2-pyridylmethyl)amine.

Kinetic and mechanistic studies of atom transfer radical addition (ATRA) catalyzed by copper complexes with tris(2-pyridylmethyl)amine (TPMA) ligand were reported. In solution, the halide anions were found to strongly coordinate to [Cu(I)(TPMA)](+) cations, as confirmed by kinetic, cyclic voltammetry, and conductivity measurements. The equilibrium constant for atom transfer (K(ATRA) = k(a)/k(d)) utilizing benzyl thiocyanate was determined to be approximately 6 times larger for Cu(I)(TPMA)BPh(4) ((1.6 ± 0.2) × 10(-7)) than Cu(I)(TPMA)Cl ((2.8 ± 0.2) × 10(-8)) complex. This difference in reactivity between Cu(I)(TPMA)Cl and Cu(I)(TPMA)BPh(4) was reflected in the activation rate constants ((3.4 ± 0.4) × 10(-4) M(-1) s(-1) and (2.2 ± 0.2) × 10(-3) M(-1) s(-1), respectively). The fluxionality of Cu(I)(TPMA)X (X = Cl or Br) in solution was mainly the result of TPMA ligand exchange, which for the bromide complex was found to be very fast at ambient temperature (ΔH(‡) = 29.7 kJ mol(-1), ΔS(‡) = -60.0 J K(-1) mol(-1), ΔG(‡)(298) = 47.6 kJ mol(-1), and k(obs,298) = 2.9 × 10(4) s(-1)). Relatively strong coordination of halide anions in Cu(I)(TPMA)X prompted the possibility of activation in ATRA through partial TPMA dissociation. Indeed, no visible differences in the ATRA activity of Cu(I)(TPMA)BPh(4) were observed in the presence of as many as 5 equiv of strongly coordinating triphenylphosphine. The possibility for arm dissociation in Cu(I)(TPMA)X was further confirmed by synthesizing tris(2-(dimethylamino)phenyl)amine (TDAPA), a ligand that was structurally similar to currently most active TPMA and Me(6)TREN (tris(2-dimethylaminoethyl)amine), but had limited arm mobility due to the rigid backbone. Indeed, Cu(I)(TDAPA)Cl complex was found to be inactive in ATRA, and the activity increased only by opening the coordination site around the copper(I) center by replacing chloride anion with less coordinating counterions such as BF(4)(-) and BPh(4)(-). The results presented in this Article are significant from the mechanistic point of view because they indicate that coordinatively saturated Cu(I)(TPMA)X complexes catalyze the homolytic cleavage of carbon-halogen bond during the activation step in ATRA by prior dissociation of either halide anion or TPMA arm.

Molecular systems for light driven hydrogen production.

Recent work towards the production of hydrogen via reduction of protons is described. Most of the systems examined in this perspective use a molecular chromophore for harvesting visible light, a catalyst, which is reduced by the excited (or reduced) chromophore, and finally a sacrificial electron source to oxidatively or reductively quench the chromophore. The reduced catalyst is then responsible for the reduction of protons resulting in hydrogen evolution. Relevant mechanistic work on this topic is also discussed.

Crystal structures of Au2 complex and Au25 nanocluster and mechanistic insight into the conversion of polydisperse nanoparticles into monodisperse Au25 nanoclusters.

We previously reported a size-focusing conversion of polydisperse gold nanoparticles capped by phosphine into monodisperse [Au(25)(PPh(3))(10)(SC(2)H(4)Ph)(5)Cl(2)](2+) nanoclusters in the presence of phenylethylthiol. Herein, we have determined the crystal structure of [Au(25)(PPh(3))(10)(SC(2)H(4)Ph)(5)Cl(2)](2+) nanoclusters and also identified an important side-product-a Au(I) complex formed in the size focusing process. The [Au(25)(PPh(3))(10)(SC(2)H(4)Ph)(5)Cl(2)](2+) cluster features a vertex-sharing bi-icosahedral core, resembling a rod. The formula of the Au(I) complex is determined to be [Au(2)(PPh(3))(2)(SC(2)H(4)Ph)](+) by electrospray ionization (ESI) mass spectrometry, and its crystal structure (with SbF(6)(-) counterion) reveals Au-Au bridged by -SC(2)H(4)Ph and with terminal bonds to two PPh(3) ligands. Unlike previously reported [Au(2)(PR(3))(2)(SC(2)H(4)Ph)](+) complexes in the solid state, which exist as tetranuclear complexes (i.e., dimers of [Au(2)(PR(3))(2)(SC(2)H(4)Ph)](+) units) through a Au···Au aurophilic interaction, in our case we found that the [Au(2)(PPh(3))(2)(SC(2)H(4)Ph)](+) complex exists as a single entity, rather than being dimerized to form a tetranuclear complex. The observation of this Au(I) complex allows us to gain insight into the intriguing conversion process from polydisperse Au nanoparticles to monodisperse Au(25) nanoclusters.

Atom transfer radical addition (ATRA) catalyzed by copper complexes with tris[2-(dimethylamino)ethyl]amine (Me6TREN) ligand in the presence of free-radical diazo initiator AIBN.

In this article, we focus on the evaluation of tris[2-(dimethylamino)ethyl]amine (Me(6)TREN) ligand in copper catalyzed ATRA in the presence of free-radical diazo initiator AIBN (2,2'-azobis(2-methylpropionitrile)). The addition of carbon tetrachloride to 1-hexene, 1-octene and cis-cyclooctene proceeded efficiently to yield 89, 85 and 85% of monoadduct, respectively, using the catalyst to alkene ratio of 1 : 2500. For alkenes that readily undergo free radical polymerization, such as methyl acrylate, catalyst loadings as high as 0.4 mol-% were required. Furthermore, modest yields of the monoadduct were obtained with less active alkyl halides (chloroform and bromoform) using 250 : 1 and 500 : 1 ratios of alkene to copper(II). Interestingly, the addition of carbon tetrachloride to cis-cyclooctene produced only 1-chloro-4-(trichloromethyl)-cyclooctene, while carbon tetrabromide yielded 1,2 and 1,4-regioisomers in 75 : 25 ratio. The activity of [Cu(II)(Me(6)TREN)X][X] (X = Br(-) and Cl(-)) complexes in ATRA in the presence of AIBN was additionally probed by adding excess free ligand, source of halide anions and triphenylphosphine. The results indicated that disproportionation is a likely cause for lower activity of Me(6)TREN as compared to TPMA (tris(2-pyridylmethyl)amine).

Copper-catalyzed atom transfer radical addition (ATRA) and cyclization (ATRC) reactions in the presence of environmentally benign ascorbic acid as a reducing agent.

In recent years, copper-catalyzed atom transfer radical addition (ATRA) has emerged as a viable organic procedure for the formation of carbon-carbon bonds starting from alkyl halides and alkenes. Studies have primarily focused on the use of free-radical initiators to regenerate the copper(I) complex or activator in situ. Although these initiators led to a significant decrease in the amount of metal catalyst, they were much less effective for highly active alkenes that readily undergo free-radical polymerization. In this study, the non-radical reducing agent ascorbic acid (commonly known as Vitamin C) was effectively employed resulting in TONs as high as 15,200 in the homogenous ATRA of polyhalogenated compounds to α-olefins, and enabling selective monoadduct formation for highly active alkenes such as acrylonitrile (TONs as high as 11,800). As low as 7-20 mol% of ascorbic acid relative to substrate was sufficient for all ATRA and ATRC reactions examined. Further, product isolations for all selected syntheses were quite facile and nearly quantitative, requiring only simple liquid-liquid extraction techniques. Reactions in the presence of ascorbic acid were also examined kinetically via (1)H NMR and UV/Vis spectroscopy. A half order rate dependence on reducing agent concentration was observed, but the first order kinetic plots became nonlinear as the concentration of ascorbic acid was increased. Finally, the use of ascorbic acid circumvented otherwise necessary purging techniques, successfully furthering the utility of these reactions in organic synthesis.

Structural comparison of copper(I) and copper(II) complexes with tris(2-pyridylmethyl)amine ligand.

Copper(I) complexes with the tris(2-pyridylmethyl)amine (TPMA) ligand were synthesized and characterized to examine the effect of counteranions (Br(-), ClO(4)(-), and BPh(4)(-)), as well as auxiliary ligands (CH(3)CN, 4,4'-dipyridyl, and PPh(3)) on the molecular structures in both solid state and solution. Partial dissociation of one of the pyridyl arms in TPMA was not observed when small auxiliary ligands such as CH(3)CN or Br(-) were coordinated to copper(I), but was found to occur with larger ones such as PPh(3) or 4,4'-dipyridyl. All complexes were found to adopt a distorted tetrahedral geometry, with the exception of [Cu(I)(TPMA)][BPh(4)], which was found to be trigonal pyramidal because of stabilization via a long cuprophilic interaction with a bond length of 2.8323(12) Å. Copper(II) complexes with the general formula [Cu(II)(TPMA)X][Y] (X = Cl(-), Br(-) and Y = ClO(4)(-), BPh(4)(-)) were also synthesized to examine the effect of different counterions on the geometry of [Cu(II)(TPMA)X](+) cation, and were found to be isostructural with previously reported [Cu(II)(TPMA)X][X] (X = Cl(-) or Br(-)) complexes.

Total structure determination of thiolate-protected Au38 nanoparticles.

We report the total structure of Au(38)(SC(2)H(4)Ph)(24) nanoparticles determined by single crystal X-ray crystallography. This nanoparticle is based upon a face-fused Au(23) biicosahedral core, which is further capped by three monomeric Au(SR)(2) staples at the waist of the Au(23) rod and six dimeric staples with three on the top icosahedron and other three on the bottom icosahedron. The six Au(2)(SR)(3) staples are arranged in a staggered configuration, and the Au(38)S(24) framework has a C(3) rotation axis.

Atom transfer radical addition in the presence of catalytic amounts of copper(I/II) complexes with tris(2-pyridylmethyl)amine.

Highly efficient atom transfer radical addition of polyhalogenated compounds to alkenes catalyzed by copper(I/II) complexes with tris(2-pyridylmethyl)amine in the presence of a radical initiator [2,2'-azobis(2-methylpropionitrile)] was reported.