Synthetic Cluster Models of Biological and Heterogeneous Manganese Catalysts for O₂Evolution

Abstract

Artificial photosynthesis has emerged as an important strategy toward clean and renewable fuels. Catalytic oxidation of water to O2 remains a significant challenge in this context. Mechanistic understanding of currently known heterogeneous and biological catalysts at a molecular level is highly desirable for fundamental reasons as well as for the rational design of practical catalysts. This article discusses recent efforts in synthesizing structural models of the oxygen-evolving complex (OEC) of photosystem II (PSII). These structural motifs are also related to heterogeneous mixed metal oxide catalysts. A stepwise synthetic methodology was developed toward achieving the structural complexity of the targeted active sites. A geometrically restricted multinucleating ligand, but with labile coordination modes, was employed for the synthesis of low oxidation state trimetallic species. These precursors were elaborated to site-differentiated tetrametallic complexes in high oxidation states. This methodology has allowed for structure-reactivity studies that have offered insight into the effects of different components of the clusters. Mechanistic aspects of oxygen-atom transfer and incorporation from water have been interrogated. Significantly, a large and systematic effect of redox-inactive metals on the redox properties of these clusters was discovered. With the pKa of the redox-inactive metal-aqua complex as a measure of Lewis acidity, structurally analogous clusters display a linear dependence between reduction potential and acidity; each pKa unit shifts the potential by ca. 90 mV. Implications for the function of the biological and heterogeneous catalysts are discussed.

What Influences the Water Oxidation Activity of a Bioinspired Molecular CoII4O4 Cubane? An In-Depth Exploration of Catalytic Pathways

“…[21][22][23][24][25] Starting with Nocera's oxygen evolving catalyst, 26 cobalt oxide has become a subject of interest to many research groups focusing their attention on Co(III)-based cubane type WOCs. [27][28][29][30][31] Wang and Van Voorhis explored a Co(III)-based cubane model complex using a quantum mechanics/molecular mechanics approach investigating an oxo-oxo coupling mechanism.…”

The effect of different metal ions between nanolayers of manganese oxide on water oxidation

“…With the pKa value of the redox-inactive metal-aqua complex as a measure of the Lewis acidity, structurally analogous clusters display a linear dependence between the reduction poten- tial and acidity; each pKa [58] unit shifts the potential by ca. 90 mV [55]. However, when we used different ions between layered Mn oxides, we could see no clear relation between Mn(III)/(IV) potential and pKa of the ions between layers (Fig.…”

“…However, the water oxidation by the compounds depends on some factors (Scheme 1) [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57].…”

“…Recently, Theodor Agapie's group has reported large and systematic effect of redox-inactive metals on the redox properties of clusters [55]. With the pKa value of the redox-inactive metal-aqua complex as a measure of the Lewis acidity, structurally analogous clusters display a linear dependence between the reduction poten- tial and acidity; each pKa [58] unit shifts the potential by ca.…”

“…Among Mn oxides, nanolayered Mn oxides with different organic or inorganic ions between layers are efficient and stable in water oxidation [41]. Inspired by the WOC, layered MnACa oxides were reported as efficient catalyst for water oxidation [42][43][44][45][46][47][48][49][50][51][52][53][54][55]. X-ray absorption spectroscopy (XAS) reveal similarities between layered oxide and MnACa cluster in PSII [42].…”