Enzyme-like water preorganization in a synthetic molecular cleft for homogeneous water oxidation catalysis

Abstract

Inspired by the proficiency of natural enzymes, mimicking of nanoenvironments for precise substrate preorganisation is a promising strategy in catalyst design. However, artificial examples of enzyme-like activation of H2O molecules for the challenging oxidative water splitting reaction are hardly explored.Here, we introduce a mononuclear Ru(bda) complex (M1, bda: 2,2'-bipyridine-6,6'-dicarboxylate) equipped with a bipyridine-functionalized ligand to preorganize H2O molecules in front of the metal center as in enzymatic clefts. The confined pocket of M1 accelerates chemically driven water oxidation at pH 1 by facilitating a water nucleophilic attack pathway with a remarkable turnover frequency of 140 s −1 that is comparable to the oxygen-evolving complex of photosystem II. Single crystal X-ray analysis of M1 under catalytic conditions allowed the observation of a 7 th H2O ligand directly coordinated to a Ru III center.Via a well-defined hydrogen-bonding network, another H2O substrate is preorganized for the crucial O-O bond formation via nucleophilic attack.

Folding‐Induced Promotion of Proton‐Coupled Electron Transfers via Proximal Base for Light‐Driven Water Oxidation

“…For dimer 2 C , similar experiments could not be performed due to the poor solubility of the complex in organic media. To probe for the effect of protonation of the bipy units, [23] we compared 1 H NMR spectra for 2 C – 4 C at pD 7.0 and 1.0 (1 : 1 TFE‐ d 3 /D 2 O, 0.1 M CF 3 SO 3 D) (Figures S13–S15 and Tables S3–S5). For all three macrocyles, the respective blue‐labelled protons show a moderate downfield shift at pD 1.0, whereas the signals of the red‐labelled protons only changed negligible for the larger complexes 3 C and 4 C .…”

“…These results indicate a more dynamic rotation of the Ru(bda) units due to protonation of the bipy units under acidic conditions. [23,33] Due to repulsive interactions between the charged bipyridinium units, the folded structures open up, which diminishes any close intramolecular interactions between the axial and equatorial ligand sphere. This is also evidenced by the absence of NOE cross signals between the green-labelled protons of the bda units and the purple-labelled protons protons of the axial bipy units (Figures S16-S18).…”

“…[45] Based on the combined analytical and kinetic data, we propose the following mechanistic picture for the excellent water oxidation catalysis with 2 C-4 C. As shown previously, a hydrogen-bonded H 2 O network can either be stabilized by cooperative effects within the cavities of macrocyclic Ru-(bda) assemblies of different size by utilizing the carboxy groups of the bda ligands as directional bonding sites or via hydrogen-bonding interactions to a protonated bipy site located opposite to the active site within a synthetic molecular cleft. [22,23,42,46] In this work, we demonstrate the powerful effect of an auxiliary base in the ligand sphere on the catalytic performance of more flexible macrocycles of varying size. Single-crystal X-ray analysis of the multinuclear series 2 C-4 C showed that within the larger complexes 3 C and 4 C, intramolecular π-π interactions of the trans-oriented bipy units put the proximal base in close proximity to the active sites.…”

Theoretical study on the mechanism of water oxidation catalyzed by a mononuclear copper complex: important roles of a redox non-innocent ligand and HPO42−anion

“…[1][2][3] To date, WOCs based on noble metals, such as Ru and Ir, show relatively high activities and stabilities, while their widespread applications on the scale of industrialization are limited. [4][5][6][7][8][9][10] Due to the low cost and high earth abundance, copper-based complexes are good candidates for catalyzing water oxidation. Great progress has been made during the last few decades since the Mayer group reported the rst molecular Cu-based catalyst.…”

Self‐assembled Ru(bda) Coordination Oligomers as Efficient Catalysts for Visible Light‐Driven Water Oxidation in Pure Water

“…To improve both performance and stability, recent studies emphasized the importance of supramolecular interactions between closely arranged molecular WOCs. [ 21 , 22 , 23 , 24 , 25 , 26 , 27 ] Therefore, we envision higher TOF and TON values can be achieved by the accumulation of individual catalysts in coordination oligomers. [ 28 , 29 ] As an added benefit, such materials are typically easier to synthesize and purify than defined mono‐ or multinuclear molecular species.…”