Pushing Single-Oxygen-Atom-Bridged Bimetallic Systems to the Right: A Cryptand-Encapsulated Co–O–Co Unit

Unveiling the role of proton concentration in dinuclear metal complexes for boosting photocatalytic CO2 reduction

The reaction kinetics of photocatalytic CO2 reduction is highly dependent on the transfer rate of electrons and protons to the CO2 molecules adsorbed on catalytic centers. Studies on uncovering the proton effect in catalysts on photocatalytic activity of CO2 reduction are significant but rarely reported. In this paper, we, from the molecular level, revealed that the photocatalytic activity of CO2 reduction is closely related to the proton availability in catalysts. Specifically, four dinuclear Co(II) complexes based on Robson-type ligands with different number of carboxylic groups (-nCOOH; n = 0, 2, 4, 6) were designed and synthesized. All these complexes show photocatalytic activity for CO2 reduction to CO in a water-containing system upon visible-light illumination. Interestingly, the CO yields increase positively with the increase of the carboxylic-group number in dinuclear Co(II) complexes. The one containing -6COOH shows the best photocatalytic activity for CO2 reduction to CO, with the TON value reaching as high as 10,294. The value is 1.8, 3.4, and 7.8 times higher than those containing -4COOH, -2COOH, and -0COOH, respectively. The high TON value also makes the dinuclear Co(II) complex with -6COOH outstanding among reported homogeneous molecular catalysts for photocatalytic CO2 reduction. Control experiments and density functional theory calculation indicated that more carboxylic groups in the catalyst endow the catalyst with more proton relays, thus accelerating the proton transfer and boosting the photocatalytic CO2 reduction. This study, at a molecular level, elucidates that more carboxylic groups in catalysts are beneficial for boosting the reaction kinetics of photocatalytic CO2 reduction.

Reduction of CO2 by a masked two-coordinate cobalt(i) complex and characterization of a proposed oxodicobalt(ii) intermediate

Fixation and chemical reduction of CO2 are important for utilization of this abundant resource, and understanding the detailed mechanism of C-O cleavage is needed for rational development of CO2 reduction methods. Here, we describe a detailed analysis of the mechanism of the reaction of a masked two-coordinate cobalt(i) complex, L tBuCo (where L tBu = 2,2,6,6-tetramethyl-3,5-bis[(2,6-diisopropylphenyl)imino]hept-4-yl), with CO2, which yields two products of C-O cleavage, the cobalt(i) monocarbonyl complex L tBuCo(CO) and the dicobalt(ii) carbonate complex (L tBuCo)2(μ-CO3). Kinetic studies and computations show that the κN,η6-arene isomer of L tBuCo rearranges to the κ2 N,N' binding mode prior to binding of CO2, which contrasts with the mechanism of binding of other substrates to L tBuCo. Density functional theory (DFT) studies show that the only low-energy pathways for cleavage of CO2 proceed through bimetallic mechanisms, and DFT and highly correlated domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) calculations reveal the cooperative effects of the two metal centers during facile C-O bond rupture. A plausible intermediate in the reaction of CO2 with L tBuCo is the oxodicobalt(ii) complex L tBuCoOCoL tBu, which has been independently synthesized through the reaction of L tBuCo with N2O. The rapid reaction of L tBuCoOCoL tBu with CO2 to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO2 cleavage by L tBuCo. L tBuCoOCoL tBu is a novel example of a thoroughly characterized molecular cobalt-oxo complex where the cobalt ions are clearly in the +2 oxidation state. Its nucleophilic reactivity is a consequence of high charge localization on the μ-oxo ligand between two antiferromagnetically coupled high-spin cobalt(ii) centers, as characterized by DFT and multireference complete active space self-consistent field (CASSCF) calculations.

Benzothiazole integrated into a cryptand for ESIPT-based selective chemosensor for Zn2+ ions

A novel 2(2′-hydroxyphenyl) benzothiazole-based cryptand (L) exhibits high fluorescence intensity in the presence of Zn²⁺ ions by stopping the excited state intramolecular proton transfer (ESIPT) process with a detection limit of 0.20 μM.

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O₂ to H₂O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O₂ to water via Co(III)-O-O-Co(III) peroxo intermediates.

Chiral Cryptates Derived from a Hexaazamacrocycle

The reactions of hexaazamacrocycle 1 with 2,6-bis(bromomethyl)pyridine or 2,6-bis[(tosyloxy)methyl)]pyridine in the presence of appropriate carbonates result in the formation of derivatives of cryptand 6: enantiopure azacryptates of sodium and potassium. Crystal structures of these compounds indicate interaction of a metal ion with four pyridine nitrogen atoms and four tertiary amine atoms. The competition reactions monitored by NMR spectroscopy indicate preferential binding of Na⁺ over K⁺ as well as higher affinity of 6 for Na⁺ in comparison with the [2.2.1] cryptand.