Precise Manipulation of Electron Transfers in Clustered Five Redox Sites.

Electron transfers in multinuclear metal complexes are the origin of their unique functionalities both in natural and artificial systems. However, electron transfers in multinuclear metal complexes are generally complicated, and predicting and controlling these electron transfers is extremely difficult. Herein, we report the precise manipulation of the electron transfers in multinuclear metal complexes. The development of a rational synthetic strategy afforded a series of pentanuclear metal complexes which composed of metal ions and 3,5-bis(2-pyridyl)pyrazole (Hbpp) as a platform to probe the phenomena. Electrochemical and spectroscopic investigations clarified overall picture of the electron transfers in the pentanuclear complexes. In addition, unique electron transfer behaviors, in which the reduction of a metal center occurs during the oxidation of the overall complex, were identified. We also elucidated the two dominant factors that determine the manner of the electron transfers. Our results provide comprehensive guidelines for interpreting the complicated electron transfers in multinuclear metal complexes.

Brønsted Acid/Base Site Isolated in a Pentanuclear Scaffold.

Invited for the cover of this issue is the group of Shigeyuki Masaoka at Osaka University. The image depicts a Brønsted acid/base pair based on a pentanuclear scaffold, and the formation thermodynamically metastable state of H+ by the steric isolation of a Brønsted acid/base site. Read the full text of the article at 10.1002/chem.202203253.

Brønsted Acid/Base Site Isolated in a Pentanuclear Scaffold.

The concept of Brønsted-Lowry acids and bases is long and widely recognized as the most reasonable theory to explain the behavior of H+ ions. Here, we report a Brønsted acid/base pair that does not follow this theory. Two heteronuclear metal complexes, in which Brønsted acid/base sites are sterically isolated, were synthesized and characterized. These sterically isolated sites exhibited anomalous behavior, wherein the H+ species encapsulated in the Brønsted acid site did not undergo a deprotonation reaction, and the corresponding protonation reaction at the Brønsted base site failed to proceed. As a result, two states that are in a relationship of a Brønsted acid/base pair stably exist over a wide pH range without any interconversion, generating a thermodynamically metastable state. Additionally, these two states exhibited distinct electron transfer abilities and reactivities. The system presented in this study is in sharp contrast with the traditional concept of Brønsted-Lowry acids and bases.

Heterogenous CO2 Reduction Photocatalysis of Transparent Coordination Polymer Glass Membranes Containing Metalloporphyrins.

Transparent and grain boundary-free substrates are essential to immobilize molecular photocatalysts for efficient photoirradiation reactions without unexpected light scattering and absorption by the substrates. Herein, membranes of coordination polymer glass immobilizing metalloporphyrins were examined as a heterogeneous photocatalyst for carbon dioxide (CO2) reduction under visible-light irradiation. [Zn(HPO4)(H2PO4)2](ImH2)2 (Im = imidazolate) liquid containing iron(III) 5,10,15,20-tetraphenyl-21H,23H-porphine chloride (Fe(TPP)Cl, 0.1-0.5 w/w%) was cast on a borosilicate glass substrate, followed by cooling to room temperature, resulting in transparent and grain boundary-free membranes with the thicknesses of 3, 5, and 9 µm. The photocatalytic activity of the membranes was in proportion to the membrane thickness, indicating that Fe(TPP)Cl in the subsurface of membranes effectively absorbed light and contributed to the reactions. The membrane photocatalysts were intact during the photocatalytic reaction and showed no recrystallization or leaching of Fe(TPP)Cl.

Three-Dimensional Metal-Organic Network Glasses from Bridging MF62- Anions and Their Dynamic Insights by Solid-State NMR.

Glassy-state coordination polymers (CPs) are a new class of network-forming glasses. In this work, we constructed glass-forming CPs composed of both anionic and neutral ligands as network formers. With the use of hexafluoro anions (MF62-) and 1,3-bis(4-pyridyl)propane (bpp), two isostructural CP crystals, [Zn(SiF6)(bpp)2] (ZnSi) and [Zn(TiF6)(bpp)2] (ZnTi), were synthesized. Solid-state 19F NMR revealed rotational motion of MF62- with dissociation and re-formation of the Zn-F coordination bonds in both CP crystals, which reflects the thermodynamic parameters related to the glass formability. The mobility of SiF62- is larger than that of TiF62-, suggesting a higher glass formability of ZnSi. When mechanical ball milling was conducted, ZnSi completely changed into a glassy state, whereas ZnTi showed incomplete glass formation. Examination of the amorphous structures elucidated retention and partial destruction of the Zn-F coordination bonds in ball-milled ZnSi and ZnTi, respectively. These results provide the relationship between the ligand dynamics and glass formability of CPs.

Effect of metal ion substitution on the catalytic activity of a pentanuclear metal complex.

A pentanuclear cobalt complex that consists of five cobalt ions and six bpp- ligands (Co5, Hbpp = 3,5-bis(2-pyridyl)pyrazole) was synthesized and crystallographically characterized. Electrochemical measurements indicate that Co5 has multielectron transfer ability. We also found that Co5 reduces CO2 to CO under photoirradiation in the presence of a photosensitizer.

Pentanuclear iron catalysts for water oxidation: substituents provide two routes to control onset potentials.

The development of robust and efficient molecular catalysts based on earth-abundant transition metals for water oxidation reactions is a challenging research target. Our group recently demonstrated the high activity and stability of a pentairon-based water oxidation electrocatalyst (M. Okamura, M. Kondo, R. Kuga, Y. Kurashige, T. Yanai, S. Hayami, V. K. K. Praneeth, M. Yoshida, K. Yoneda, S. Kawata and S. Masaoka, Nature, 2016, 530, 465-468). However, the development of strategies to decrease onset potentials for catalysis remains challenging. In this article, we report the construction of a series of pentanuclear iron complexes by introducing electron-donating (methyl) and electron-withdrawing (bromo) substituents on the ligand. Two newly synthesized complexes exhibited five reversible redox processes, similar to what is seen with the parent complex. These complexes can also serve as homogeneous catalysts for water oxidation reactions, and the faradaic efficiencies of the reactions were high. Additionally, the onset potentials of the newly developed complexes were lower than that of the parent complex. Mechanistic insights revealed that there are two methods for decreasing onset potentials: control of the redox potentials of the pentairon complex and control of the reaction mechanism.