On the Homogeneity of a Cobalt-Based Water Oxidation Catalyst

The homogeneity of molecular Co-based water oxidation catalysts (WOCs) has been a subject of debate over the last 10 years as assumed various homogeneous Co-based WOCs were found to actually form CoO_x under operating conditions. The homogeneity of the Co(HL) (HL = N,N-bis(2,2′-bipyrid-6-yl)amine) system was investigated with cyclic voltammetry, electrochemical quartz crystal microbalance, and X-ray photoelectron spectroscopy. The obtained experimental results were compared with heterogeneous CoO_x. Although it is shown that Co(HL) interacts with the electrode during electrocatalysis, the formation of CoO_x was not observed. Instead, a molecular deposit of Co(HL) was found to be formed on the electrode surface. This study shows that deposition of catalytic material is not necessarily linked to the decomposition of homogeneous cobalt-based water oxidation catalysts.

Magnesium, calcium and zinc [N2N'] heteroscorpionate complexes

A new family of sterically demanding N2N' heteroscorpionate pro-ligands (HC(tBu2pz)2SiMe2N(H)R (R = iPr, tBu, Ph, Xyl)) has been prepared via a straightforward modular synthetic route. An extensive study into the synthesis and characterisation of lithium, magnesium, calcium and zinc complexes supported by both 3,5-tBu and 3,5-Me substituted N2N' ligand families has been conducted. Attempted deprotonation of the pro-ligands with nBuLi afforded the corresponding lithium salts Li{HC(tBu2pz)2SiMe2NR} (R = iPr (1), tBu (2), Ph (3) and Xyl (4)) but air- and thermal-sensitivity limited the yields of these potentially useful precursors; only the sterically encumbered ligand system allowed clean reactivity. Magnesium methyl complexes Mg{HC(tBu2pz)2SiMe2NR}Me (R = iPr (5) and R = Ph (6)) were prepared using an excess of the Grignard reagent MeMgCl. Magnesium butyl complexes were synthesised in good yields using the dialkyl precursor MgnBu2 to afford Mg{HC(R'2pz)2SiMe2NR}nBu (R' = Me; R = iPr (7), tBu (8), Ad (9), Ph (10). R' = tBu; R = iPr (11), Ph (12)). Protonolylsis reactions were used to synthesise magnesium and calcium amide complexes Mg{HC(R'2pz)2SiMe2NR}{N(SiHMe2)2} (R' = Me; R = iPr (13), tBu (14), Ph (15). R' = tBu; R = Ph (16)) or Mg{HC(R'2pz)2SiMe2NR}{N(SiMe3)2} (R' = Me; R = iPr (17), tBu (18), Ph (19). R' = tBu; R = Ph (20)), and Ca{HC(R'2pz)2SiMe2NR}{N(SiMe2)2} (L) (R' = Me; L = thf; R = iPr (21), tBu (22), Ph (23). R' = tBu; L = none; R = Ph (24). Zinc methyl complexes Zn{HC(R'2pz)2SiMe2NR}Me (R' = Me; R = iPr (25), tBu (26), Ph (27). R' = tBu; R = Ph (28)) were prepared by reaction of the N2N' heteroscorpionate pro-ligands with ZnMe2. In preliminary studies, magnesium amide complexes 16 and 20 were evaluated as initiators for the ring-opening polymerisation (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA). Although the overall polymerisation control was poor, 16 and 20 were found to be active initiators.

Role of Organic Components in Electrocatalysis for Renewable Energy Storage

Carbon dioxide electroreduction and water splitting are known as two promising strategies to convert renewable intermittent electrical energy into chemical energy. Thus, the three half-reactions, namely, CO₂ reduction reaction, hydrogen evolution reaction, and the oxygen evolution counter reaction, in these two electrolytic processes have attracted wide research interest. Organic polymer electrocatalysts or electrocatalysts containing organic components play important roles in these catalytic processes. It has been shown that the organic molecules can efficiently catalyze the reactions themselves, and modulate the active sites towards high selectivity and efficiency. The roles of the organic molecules in conducting polymers, the metal complexes, and the framework materials are extracted for the three half-reactions mentioned above, and this comprehensive review will serve as a guide for future research and aid in the design of electrocatalysts related to organic molecules.

Intramolecular hydrogen-bonding in a cobalt aqua complex and electrochemical water oxidation activity

Water oxidation is catalysed in Nature by a redox cofactor embedded in a hydrogen-bonded network designed to orchestrate proton transfer throughout the challenging 4 electron reaction.

Water oxidation is catalysed in Nature by a redox cofactor embedded in a hydrogen-bonded network designed to orchestrate proton transfer throughout the challenging 4 electron reaction. In order to mimic aspects of this microenvironment, [CoL^DMA(CH₃CN)₂][BF₄]₂ (2) was synthesized, where L^DMA is a dipyridyldiamine ligand with two dimethylamine bases in the secondary coordination sphere. Structural characterization of the corresponding aqua complexes establish hydrogen bonding between the bound water and pendant base(s). Cyclic voltammetry of [CoL^DMA(CH₃CN)₂][BF₄]₂ (2) reveals enhanced oxidative current upon titration with water and controlled potential electrolysis confirms evolution of O₂. The related complex [CoL^H(CH₃CN)₂][BF₄]₂ (1), which has the same primary coordination environment as 2 but lacks pendant bases, is inactive. The structural and electrochemical studies illustrate the role positioned proton relays can play in promoting redox reactivity.

Proton switching as a driving force for the metal–organic complex-mediated growth of gold colloids

A mono-protonated (bis(4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine)iron(ii)) complex [[Fe(ptpy)(Hptpy)](PF₆)₃] acts as a structure directing agent for larger gold colloids in the present investigation.

Electrocatalytic water oxidation by a molecular cobalt complex through a high valent cobalt oxo intermediate

Biuret-modified tetraamidomacrocyclic cobalt complex [Co^III-bTAML]⁻ is shown to catalyze electrochemical water oxidation at basic pH leading to the formation of O₂.

How a [Co(IV) a bond and a half O](2+) fragment oxidizes water: involvement of a biradicaloid [Co(II)-(⋅O⋅)](2+) species in forming the O-O bond

The mechanism of water oxidation performed by a recently discovered cobalt complex [Co(Py5)(OH2)](ClO4)2 (1; Py5=2,6-(bis(bis-2-pyridyl)-methoxymethane)pyridine) was examined using quantum chemical models based on density functional theory. The computer models were first benchmarked against the experimental cyclic voltammetry data to identify the catalytically competent resting state of the catalyst, which was thought to contain a Co(IV) -oxyl complex. The electronic structure calculations suggest that the low-spin doublet state is energetically most favorable, but the catalytically most active species is the intermediate-spin quartet complex that is almost isoenergetic with the doublet state. The electronic structure of the quartet state shows significant spin polarization on the terminal oxygen atom, which is consistent with an intramolecular electron transfer from the oxygen to the metal. Based on the calculated spin densities, the formally [Co(IV) a bond and a half O] can be viewed as a biradicaloid [Co(II)-(⋅O⋅)](2+), that is, a cobalt-oxene moiety. This electronic structure is reminiscent of many other systems where similar electronic patterns were proposed to be responsible for the oxidative reactivity. In this context, this first-row transition-metal system constitutes a logical extension, because the oxyl-radical character is maximized by using the more easily accessible high-spin configurations in which two half-filled Co-dπ orbitals can work in concert to maximize the oxyl-radical character to ultimately afford a new reactive intermediate that can be characterized as carrying a biradicaloid oxene moiety with a formal oxidation state of zero. This conceptual proposal for the catalytically active species provides a plausible rationale for the remarkable oxidative reactivity.

Electrocatalytic water oxidation by a monomeric amidate-ligated Fe(III)-aqua complex

The six-coordinate Fe(III)-aqua complex [Fe(III)(dpaq)(H2O)](2+) (1, dpaq is 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamido) is an electrocatalyst for water oxidation in propylene carbonate-water mixtures. An electrochemical kinetics study has revealed that water oxidation occurs by oxidation to Fe(V)(O)(2+) followed by a reaction first order in catalyst and added water, respectively, with ko = 0.035(4) M(-1) s(-1) by the single-site mechanism found previously for Ru and Ir water oxidation catalysts. Sustained water oxidation catalysis occurs at a high surface area electrode to give O2 through at least 29 turnovers over an 15 h electrolysis period with a 45% Faradaic yield and no observable decomposition of the catalyst.