Exploration of efficient water oxidation catalysts (WOCs) is the primary challenge in conversion of renewable energy into fuels. Here we report a molecularly well-defined heterogeneous WOC with Aza-fused, pi-conjugated, microporous polymer (Aza-CMP) coordinated single cobalt sites (Aza-CMP-Co). The single cobalt sites in Aza-CMP-Co exhibited superior activity under alkaline and near-neutral conditions. Moreover, the molecular nature of the isolated catalytic sites makes Aza-CMP-Co a reliable model for studying the heterogeneous water oxidation mechanism. By a combination of experimental and theoretical results, a pH-dependent nucleophilic attack pathway for O-O bond formation was proposed. Under alkaline conditions, the intramolecular hydroxyl nucleophilic attack (IHNA) process with which the adjacent -OH group nucleophilically attacks Co4+=O was identified as the rate-determining step. This process leads to lower activation energy and accelerated kinetics than those of the intermolecular water nucleophilic attack (WNA) pathway. This study provides significant insights into the crucial function of electrolyte pH in water oxidation catalysis and enhancement of water oxidation activity by regulation of the IHNA pathway.

Intercatalyst coupling has been widely applied in the functional mimics for binuclear synergy in natural metal enzymes. Herein, we introduce two facile and effective design strategies, which facilitate the coupling of two catalytic units via electrostatic interactions. The first system is based on a catalyst molecule functionalized with both a positively charged and a negatively charged group in the structure being able to pair with each other in an antiparallel manner arranged by electrostatic interactions. The other system consists of a mixture of two different of catalysts modified with either positively or negatively charged groups to generate intermo-lecular electrostatic interactions. Applying these designs to Ru(bda) (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) water-oxidation catalysts improved the catalytic performance by more than an order of magnitude. The intermolecular electrostatic interactions in these two systems were fully identified by H-1 NMR, TEM, SAXS, and electrical conductivity experiments. Molecular dynamics simulations further verified that electrostatic interactions contribute to the formation of prereactive dimers, which were found to play a key role in dramatically improving the catalytic performance. The successful strategies demonstrated here can be used in designing other intercatalyst coupling systems for activation and formation of small molecules and organic synthesis.

Significant advances during the past decades in the design and studies of Ru complexes with polypyridine ligands have led to the great development of molecular water oxidation catalysts and understanding on the O-O bond formation mechanisms. Here we report a Ru-based molecular water oxidation catalyst [Ru(bds)(pic)(2)] (Ru-bds; bds(2-) = 2,2-bipyridine-6,6 ' -disulfonate) containing a tetradentate, dianionic sulfonate ligand at the equatorial position and two 4-picoline ligands at the axial positions. This Ru-bds catalyst electrochemically catalyzes water oxidation with turnover frequencies (TOF) of 160 and 12,900s(-1) under acidic and neutral conditions respectively, showing much better performance than the state-of-art Ru-bda catalyst. Density functional theory calculations reveal that (i) under acidic conditions, the high valent Ru intermediate Ru-V=O featuring the 7-coordination configuration is involved in the O-O bond formation step; (ii) under neutral conditions, the seven-coordinate Ru-IV=O triggers the O-O bond formation; (iii) in both cases, the I2M (interaction of two M-O units) pathway is dominant over the WNA (water nucleophilic attack) pathway. Developing efficient molecular water oxidation catalysts for artificial photosynthesis is a challenging task. Here the authors introduce a ruthenium based complex with negatively charged sulfonate groups to effectively drive water oxidation under both acidic and neutral conditions.

Organic semiconductors provide significant potentials for the construction of photoelectrochemical (PEC) cells for solar hydrogen production because of their highly tunable properties. Herein, on carbon fiber paper (CFP) surface, pyridyl (Py), and 4,4'-bipyridin-1-ium (Py-2(+)) groups were introduced into polythiophene (pTH) semiconductor by electrochemical copolymerization, respectively. After assembly with the Co(dmgBF(2))(2) type catalyst (CoB, dmgBF(2) = difluoroboryldimethylglyoximate), the CoB@Py-2(+)-pTH/CFP photocathode displayed nearly twice the photocurrent enhancement (550 mu A cm(-2) at 0.15 V vs reversible hydrogen electrode, RHE) comparing to that generated by the CoB@Py-pTH/CFP photocathode (290 mu A cm(-2) at 0.15 V vs RHE) for light-driven H-2 generation under AM 1.5 solar illumination. Investigation of the mechanism revealed that the introduction of the positively charged pyridinium groups could improve the intrinsic Co(dmgBF(2))(2) catalyst activity for the H-2 generation reaction. Meanwhile, the positively charged pyridinium groups serve as p-type dopants to increase the semiconductor bulk charge transfer rate and act as electron transfer mediators to promote the interfacial charge transfer kinetics between the catalyst and the pTH-based organic semiconductor.

Understanding the seven coordination and O–O coupling pathway of the distinguished Ru-bda catalysts is essential for the development of next generation efficient water-oxidation catalysts based on earth-abundant metals. This work reports the synthesis, characterization and catalytic properties of a monomeric ruthenium catalyst Ru-bnda (H2bnda = 2,2′-bi(nicotinic acid)-6,6′-dicarboxylic acid) featuring steric hindrance and enhanced hydrophilicity on the backbone. Combining experimental evidence with systematic density functional theory calculations on the Ru-bnda and related catalysts Ru-bda (H2bda = 2,2ʹ-bipyridine-6,6ʹ-dicarboxylic acid), Ru-pda (H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid), and Ru-biqa (H2biqa = (1,1ʹ-biisoquinoline)-3,3ʹ-dicarboxylic acid), we emphasized that seven coordination clearly determines presence of RuV[dbnd]O with high spin density on the ORuV[dbnd]O atom, i.e. oxo with radical properties, which is one of the necessary conditions for reacting through the O–O coupling pathway. However, an additional factor to make the condition sufficient is the favorable intermolecular face-to-face interaction for the generation of the pre-reactive [RuV[dbnd]O···O[dbnd]RuV], which may be significantly influenced by the secondary coordination environments. This work provides a new understanding of the structure–activity relationship of water-oxidation catalysts and their potential to adopt I2M pathway for O–O bond formation.

Water oxidation is a vital anodic reaction for renewable fuel generation via electrochemical- and photoelectrochemical-driven water splitting or CO2 reduction. Ruthenium complexes, such as Ru-bda family, have been shown as highly efficient water-oxidation catalysts (WOCs), particularly when they undergo a bimolecular O-O bond formation pathway. In this study, a novel Ru(pda)-type (pda(2-) = 1,10-phenanthroline-2,9-dicarboxylate) molecular WOC with 4-vinylpyridine axial ligands was immobilized on the glassy carbon electrode surface by electrochemical polymerization. Electrochemical kinetic studies revealed that this homocoupling polymer catalyzes water oxidation through a bimolecular radical coupling pathway, where interaction between two Ru(pda)-oxyl moieties (I2M) forms the O-O bond. The calculated barrier of the I2M pathway by density-functional theory (DFT) is significantly lower than the barrier of a water nucleophilic attack (WNA) pathway. By using this polymerization strategy, the Ru centers are brought closer in the distance, and the O-O bond formation pathway by the Ru (pda) catalyst is switched from WNA in a homogeneous molecular catalytic system to I2M in the polymerized film, providing some deep insights into the importance of third coordination sphere engineering of the water oxidation catalyst.

A molecular [Ru(bda)]-type (bda = 2,2’-bipyridine-6,6’-dicarboxylate) water oxidation catalyst with 4-vinylpyridine as the axial ligand (Complex 1) was immobilized or co-immobilized with 1-(trifluoromethyl)-4-vinylbenzene (3F) or styrene (St) blocking units on the surface of glassy carbon (GC) electrodes by electrochemical polymerization, in order to prepare the corresponding poly-1@GC, poly-1+P3F@GC, and poly-1+PSt@GC functional electrodes. Kinetic measurements of the electrode surface reaction revealed that [Ru(bda)] triggers the O–O bond formation via (1) the radical coupling interaction between the two metallo-oxyl radicals (I2M) in the homo-coupling polymer (poly-1), and (2) the water nucleophilic attack (WNA) pathway in poly-1+P3F and poly-1+PSt copolymers. The comparison of the three electrodes revealed that the second coordination sphere of the water oxidation catalysts plays vital roles in stabilizing their reaction intermediates, tuning the O–O bond formation pathways and improving the water oxidation reaction kinetics without changing the first coordination structures. 

From the study of supramolecular dimers of [(RuO)-O-V(pda)](+) (pda = 1,10-phenanthroline-2,9-dicarboxylic acid) and [(RuO)-O-V(bda)](+) (bda = 2,2'-bipyridine-6,6'-dicarboxylate) complexes, the O-O bond-forming intermediates in water oxidation, we found orientational distinction induced by the pda and bda ligands. The bda complex prefers the front-to-front geometry, while the pda complex favors the front-to-back geometry in the formation of prereactive geometry. In the bda complex, the hydrophobic oxo will point at another oxo with the bda directed toward water, which favors the I2M mechanism. In the pda complex, the hydrophobic oxo instead is directed toward a more hydrophobic phenanthroline moiety of the pda of another species, which disfavors I2M. The binding free energy of the nonproductive front-to-back of pda is 3 kcal mol(-1) more stable than that of the prereactive dimer. This incorrect orientation leads to an additional rearrangement required before the O-O bond can be formed. Estimation of the rate constant shows 2 orders of magnitude lower reactivity for the I2M mechanism of the pda complex relative to the bda complex, which makes the water nucleophilic attack mechanism competitive.

Ammonia production consumes ~2% of the annual worldwide energy supply, therefore strategic alternatives for the energy-intensive ammonia synthesis through the Haber-Bosch process are of great importance to reduce our carbon footprint. Inspired by MoFe-nitrogenase and the energy-efficient and industrially feasible electrocatalytic synthesis of ammonia, we herein establish a catalytic electrode for artificial nitrogen fixation, featuring a carbon fiber cloth fully grafted by boron-doped molybdenum disulfide (B-MoS2/CFC) nanosheets. An excellent ammonia production rate of 44.09 μg h–1 cm–2 is obtained at −0.2 V versus the reversible hydrogen electrode (RHE), whilst maintaining one of the best reported Faradaic efficiency (FE) of 21.72% in acidic aqueous electrolyte (0.1 M HCl). Further applying a more negative potential of −0.25 V renders the best ammonia production rate of 50.51 μg h–1 cm–2. A strong-weak electron polarization (SWEP) pair from the different electron accepting and back-donating capacities of boron and molybdenum (2p shell for boron and 5d shell for molybdenum) is proposed to facilitate greatly the adsorption of non-polar dinitrogen gas via N≡N bond polarization and the first protonation with large driving force. In addition, for the first time a visible light driven photo-electrochemical (PEC) cell for overall production of ammonia, hydrogen and oxygen from water + nitrogen, is demonstrated by coupling a bismuth vanadate BiVO4 photo-anode with the B-MoS2/CFC catalytic cathode.

The considerable rate enhancements along with the increase in pH values may be due to the direct involvement of hydroxide anion in attacking electrophilic [Ru-V(tda)(py)(2)O](+) (1; tda = [2,2':6',2 ''-terpyridine]-6,6 ''-dicarboxylate, py = pyridine). The enhanced reaction rate is well in agreement with the descending activation barriers in our calculation. The addition of four extra water molecules in the geometry optimization plays a key role in stabilizing hydroxide anion as well as building a reasonable hydrogen-bonding network, and three of these molecules are required to stabilize the OH as an anion instead of a radical.