Neutral water splitting catalysis with a high FF triple junction polymer cell

This document is the Accepted Manuscript version of a Published Work that appeared in final form in CS catalysis, copyright © American Chemical Society, after peer review and technical editing by the publisher and may be found at http://dx.doi.org/10.1021/acscatal.6b01036We report a photovoltaics-electrochemical (PV-EC) assembly based on a compact and easily processable triple homojunction polymer cell with high fill factor (76%), optimized conversion efficiencies up to 8.7%, and enough potential for the energetically demanding water splitting reaction (V-oc = 2.1 V). A platinum-free cathode made of abundant materials is coupled to a ruthenium oxide on glassy carbon anode (GC-RuO2) to perform the reaction at optimum potential (Delta E = 1.70-1.78 V, overpotential = 470-550 mV). The GC-RuO2 anode contains a single monolayer of catalyst corresponding to a superficial concentration (Gamma) of 0.15 nmol cm(-2) and is highly active at pH 7. The PV-EC cell achieves solar to hydrogen conversion efficiencies (STH) ranging from 5.6 to 6.0%. As a result of the solar cell's high fill factor, the optimal photovoltaic response is found at 1.70 V, the minimum potential at which the electrodes used perform the water splitting reaction. This allows generating hydrogen at efficiencies that would be very similar (96%) to those obtained as if the system were to be operating at 1.23 V, the thermodynamic potential threshold for the water splitting reaction.Peer ReviewedPostprint (author's final draft

Single Site Isomeric Ru WOCs with an Electron-Withdrawing Group: Synthesis, Electrochemical Characterization, and Reactivity

The synthetic intermediate <i>cis­(out),cis</i>-[Ru­(Cl)₂(HL)­(DMSO)₂], <b>1</b> (DMSO = dimethyl sulfoxide), and four new mononuclear ruthenium complexes with general formula <i>out/in</i>-[Ru­(HL)­(trpy)­(X)]^<i>m</i>+ (trpy = 4-<i>tert</i>-butylpyridine; X = Cl^–, <i>m</i> = 1, <b>2a</b>⁺ and <b>2b</b>⁺; X = H₂O, <i>m</i> = 2, <b>3a</b>²⁺ and <b>3b</b>²⁺) based on the ligand 1<i>H</i>-pyrazole-3-carboxylic acid, 5-(2-pyridinil)-, ethyl ester (HL), are synthesized and characterized by analytical, spectroscopic, and electrochemical methods. A linkage isomerism is observed for a DMSO moiety of <b>1</b>, and relevant thermodynamics and kinetics values are obtained through electrochemical experiments and compared to literature. Different synthetic routes are developed to obtain isomeric <b>2a</b>⁺ and <b>2b</b>⁺, with different relative yields. Water oxidation activity of <b>3a</b>²⁺ and <b>3b</b>²⁺ is analyzed by means of electrochemical methods, through foot of the wave analysis, yielding <i>k</i>_obs values of 1.00 and 2.23 s^–1, respectively. Chemically driven water oxidation activity is tested using [(NH₄)₂Ce­(NO₃)₆] as sacrificial electron acceptor, and turnover number (TON) and turnover frequency (TOF) values of TON = 10.8 and TOF_i = 58.2 × 10^–3 s^–1 for <b>3a</b>²⁺ and TON = 4.2 and TOF_i = 15.4 × 10^–3 s^–1 for <b>3b</b>²⁺ are obtained

Mechanistic Study of Amine to Imine Oxidation in a Dinuclear Cu(II) Complex Containing an Octaaza Dinucleating Ligand

Density functional theory (DFT) calculations have been carried out to elucidate the mechanism of self-oxidation of a Cu(II) complex octaaza dinucleating macrocyclic ligand. The reaction is bimolecular and spontaneous, in which amine groups of one macrocycle are oxidized and the CuII centers of a second macrocylic complex are reduced. No additional oxidation or external base agents are required. DFT calculations predict the reaction to proceed via a two-step mechanism, in which the first step is proton transfer between two reactant complexes. This is followed by a second transfer step in which an electron and proton are transferred together between the two complexes. Concurrent with this external transfer there is also an internal electron transfer in which the ligand reduces the metal center to give the imine product bound to CuI. The complexity of this final step differs from the generally accepted mechanisms for transition metal catalyzed amine to imine oxidation in which protons and electrons are transferred individually

Single Site Isomeric Ru WOCs with an Electron-Withdrawing Group: Synthesis, Electrochemical Characterization, and Reactivity

The synthetic intermediate <i>cis­(out),cis</i>-[Ru­(Cl)₂(HL)­(DMSO)₂], <b>1</b> (DMSO = dimethyl sulfoxide), and four new mononuclear ruthenium complexes with general formula <i>out/in</i>-[Ru­(HL)­(trpy)­(X)]^<i>m</i>+ (trpy = 4-<i>tert</i>-butylpyridine; X = Cl^–, <i>m</i> = 1, <b>2a</b>⁺ and <b>2b</b>⁺; X = H₂O, <i>m</i> = 2, <b>3a</b>²⁺ and <b>3b</b>²⁺) based on the ligand 1<i>H</i>-pyrazole-3-carboxylic acid, 5-(2-pyridinil)-, ethyl ester (HL), are synthesized and characterized by analytical, spectroscopic, and electrochemical methods. A linkage isomerism is observed for a DMSO moiety of <b>1</b>, and relevant thermodynamics and kinetics values are obtained through electrochemical experiments and compared to literature. Different synthetic routes are developed to obtain isomeric <b>2a</b>⁺ and <b>2b</b>⁺, with different relative yields. Water oxidation activity of <b>3a</b>²⁺ and <b>3b</b>²⁺ is analyzed by means of electrochemical methods, through foot of the wave analysis, yielding <i>k</i>_obs values of 1.00 and 2.23 s^–1, respectively. Chemically driven water oxidation activity is tested using [(NH₄)₂Ce­(NO₃)₆] as sacrificial electron acceptor, and turnover number (TON) and turnover frequency (TOF) values of TON = 10.8 and TOF_i = 58.2 × 10^–3 s^–1 for <b>3a</b>²⁺ and TON = 4.2 and TOF_i = 15.4 × 10^–3 s^–1 for <b>3b</b>²⁺ are obtained

Oxygen−Oxygen Bond Formation by the Ru-Hbpp Water Oxidation Catalyst Occurs Solely via an Intramolecular Reaction Pathway

Oxygen−Oxygen Bond Formation by the Ru-Hbpp Water Oxidation Catalyst Occurs Solely via an Intramolecular Reaction Pathwa

Single Electron Transfer Steps in Water Oxidation Catalysis. Redefining the Mechanistic Scenario

The systematic computational study of the mechanism for water oxidation in four different complexes confirms the existence of an alternative mechanism for the O–O bond formation step to those previously reported: the single electron transfer–water nucleophilic attack (SET-WNA). The calculated mechanism relies on two SET steps and features the existence of an intermediate with a (HO···OH)⁻ moiety in the vicinity of the metal center. It is operative in at least three representative copper based complexes and is the only option that explains the experimentally observed efficiency in two of them. The proposal of this reaction pathway redefines the mechanistic scenario and, importantly, generates a promising avenue for designing more efficient water oxidation catalysts based on first row transition metals

Oxo-Bridge Scenario behind Single-Site Water-Oxidation Catalysts

High-oxidation-state decay of mononuclear complexes [RuTB­(H₂O)]²⁺ (<b>X</b>²⁺, where B = 2,2′-bpy or bpy for X = 1; B = 5,5′-F₂-bpy for X = 2; B = 6,6′-F₂-bpy for X = 3; T = 2,2′:6′,2″-terpyridine) oxidized with a large excess of Ce^IV generates a manifold of polynuclear oxo-bridged complexes. These include the following complexes: (a) dinuclear [TB-Ru^IV-O-Ru^IV-(T)­(O)­OH₂]²⁺ (<b>1-dn</b>⁴⁺), [TB-Ru^III-O-Ru^III-T­(MeCN)₂]⁴⁺ (<b>1-dn-N</b>⁴⁺), and {[Ru^III(trpy)­(bpy)]₂(μ-O)}⁴⁺ (<b>1-dm</b>⁴⁺); (b) trinuclear {[Ru^III(trpy)­(bpy)­(μ-O)]₂Ru^IV(trpy)­(H₂O)}­(ClO₄)₅⁶⁺ (<b>1-tr</b>⁶⁺) and {[Ru^III(trpy)­(bpy)­(μ-O)]₂Ru^IV(pic)₂}­(ClO₄)₄ (<b>1-tr-P</b>⁴⁺, where P is the 2-pyr­i­dine­car­box­yl­ate anion); and (c) tetranuclear [TB-Ru^III-O-TRu^IV(H₂O)-O-TRu^IV(H₂O)-O-Ru^III-TB]⁸⁺ (<b>1-tn</b>⁸⁺), [TB-Ru^III-O-TRu^IV(AcO)-O-TRu^IV(AcO)-O-Ru^III-TB]⁶⁺ (<b>1-tn-Ac</b>⁶⁺), and [TB-Ru^II-O-TRu^IV(MeCN)-O-TRu^IV(MeCN)-O-Ru^II-TB]⁶⁺ (<b>1-tn-N</b>⁶⁺). These complexes have been characterized structurally by single-crystal X-ray diffraction analysis, and their structural properties were correlated with their electronic structures. Dinuclear complex <b>1-dm</b>⁴⁺ has been further characterized by spectroscopic and electrochemical techniques. Addition of excess Ce^IV to <b>1-dm</b>⁴⁺ generates dioxygen in a catalytic manner. However, resonance Raman spectroscopy points to the in situ formation of <b>1-dn</b>⁴⁺ as the active species

Heterobimetallic Dioxygen Activation: Synthesis and Reactivity of Mixed Cu−Pd and Cu−Pt Bis(μ-oxo) Complexes

Heterobimetallic CuPd and CuPt bis(μ-oxo) complexes have been prepared by the reaction of (PPh3)2MO2 (M = Pd, Pt) with LCu(I) precursors (L = β-diketiminate and di- and triamine ligands) and characterized by low-temperature UV−vis, resonance Raman, and 1H and 31P{1H} NMR spectroscopy in conjunction with DFT calculations. The complexes decompose upon warming to yield OPPh3, and in one case this was shown to occur by an intramolecular process through crossover experiments using double-labeling (oxo and phosphine). The reactivity of one of the complexes, LMe2Cu(μ-O)2Pt(PPh3)2 (LMe2 = β-diketiminate), with a variety of reagents including CO2, 2,4-di-tert-butylphenol, 2,4-di-tert-butylphenolate, [NH4][PF6], and dihydroanthracene, was compared to that of homometallic Pt2 and Cu2 counterparts. Unlike typical [Cu2(μ-O)2]2+ cores which have electrophilic oxo groups, the oxo groups in the [Cu(μ-O)2Pt]+ core behave as bases and nucleophiles, similar to previously described Pt2 compounds. In addition, however, the [Cu(μ-O)2Pt]+ core is capable of oxidatively coupling 2,4-di-tert-butylphenol and 2,4-di-tert-butylphenolate. Theoretical evaluation of the electron affinities, basicities, and H-atom transfer kinetics and thermodynamics of the Cu2 and CuM (M = Pd, Pt) cores showed that the latter are more basic and form stronger O−H bonds

Complete Mechanism of σ* Intramolecular Aromatic Hydroxylation through O₂ Activation by a Macrocyclic Dicopper(I) Complex

The present study reports the first example of a complete and detailed mechanism of intramolecular aromatic hydroxylation through O2 activation by a hexaazamacrocyclic dicoppper(I) complex, [CuI2(H3m)]2+. The reactivity of this complex has been previously studied experimentally, although only the characterization of the final μ-phenoxo-μ-hydroxo [CuII2(H3m-O)(μ-OH)]2+ product was possible. In the present theoretical study, we unravel the reaction pathway for the overall intramolecular aromatic hydroxylation, that is, from the initial reaction of O2 with the dicopper(I) species to the final [CuII2(H3m-O)(μ-OH)]2+ product using the B3LYP method. Our results indicate that a CuICuII-superoxo species is formed first, then the interaction of the O2 moiety with the second CuI center leads to a μ-η2:η2-peroxo-CuII2 intermediate. This latter species is found to be close in energy with the isomeric bis(μ-oxo) species. The relative stability of these two isomers depends on the method of calculation, and therefore, it has not been possible to reach a definite conclusion about the nature of the active species in this reaction mechanism. Notwithstanding, our B3LYP calculations indicate that it is the μ-η2:η2-peroxo species that evolves via an electrophilic σ* attack involving a concerted peroxide O−O bond cleavage and C−O bond formation to a Wheland-type intermediate. The reaction follows with a proton release assisted by the presence of a second aromatic ring yielding a μ-hydroxo-μ-oxo intermediate species, which, in the final stage of the reaction, rearranges to the product. The proton transfer path points out the possibility to design new systems with improved reactivity by properly placing a second aromatic ring to assist the deprotonation step. The lack of high energy barriers and deep energy wells explains the difficulty to trap intermediates experimentally

Systematic Evaluation of Molecular Recognition Phenomena. 3. Selective Diphosphate Binding to Isomeric Hexaazamacrocyclic Ligands Containing Xylylic Spacers

The crystal structure of 3,7,11,18,22,26-hexaazatricyclo[26.2.2.213,16]tetratriaconta-1(31),13(34),14,16(33),28(32),29-hexaene hexahydrobromide salt [(H6P3)Br6] has been determined by means of X-ray diffraction analysis. It crystallizes with an additional molecule of ethanol and half a molecule of water per molecule of the hydrobromide P3 ligand. The protonation constants of P3 and its host−guest interactions with monophospate (Ph) and pyrophosphate (Pp) have been investigated by potentiometric equilibrium methods. Ternary complexes are formed in aqueous solution as a result of hydrogen bond formation and Coulombic interactions between the host and the guest; formation constants for all the species obtained are reported and compared with the isomeric 3,7,11,19,23,27-hexaazatricyclo[27.3.1.113,17]tetratriaconta-1(33),13, 15,17(34),29,31-hexaene (Bn) ligand. For the H6P3Pp2+ those bonding interactions reach a maximum yielding a log KR6 of 5.87. The selectivity of the P3 ligand with regard to the monophosphate and pyrophosphate substrates (S) is discussed and illustrated with global species distribution diagrams showing a strong preference for the latter over the former as a consequence of the much stronger formation constants with pyrophosphate. An analysis of the isomeric effect is also carried out by comparing the P3−S versus Bn−S systems. In the best case, a selectivity of over 88% is achieved for the diphosphate complexation when using the meta isomer over the para, due solely to the size and shape of the receptors cavity