Effect of Pyridyl Substitution on Chemical and Photochemical Water Oxidation by [Ru(terpyridine)(bipyridine)(OH₂)]²⁺ Scaffolds

Abstract

The complexes [Ru(2‐py‐tpy)(bpy)(OH2)][PF6]2 (1; bpy = 2,2′‐bipyridine), [Ru(3‐py‐tpy)(bpy)(OH2)][PF6]2 (2), and [Ru(4‐py‐tpy)(bpy)(OH2)][PF6]2 (3; n‐py‐tpy = 4′‐(n‐pyridyl)‐2,2′:6′,2′′‐terpyridine, n = 2–4), as electronic modifications of the [Ru(tpy)(bpy)(OH2)][PF6]2 scaffold, have been synthesized and characterized thoroughly by several spectroscopic methods. All three complexes were employed as catalysts for chemical and photochemical water oxidation. The free nitrogen atom of the pyridine ring in catalysts 1–3 is protonated at pH 1 and acts as an electron‐withdrawing group. For 2, the meta‐nitrogen atom has a larger withdrawing effect compared with those of 1 and 3, as the positive charge (δ+) is delocalized only at the carbon atoms. However, the positive charge (δ+) is distributed among the nitrogen and carbon atoms of the pyridine ring for the ortho‐ and para‐nitrogen atoms in 1 and 3, respectively. Electrochemical and manometry studies revealed that 1 and 3 are better catalysts owing to the weaker electron‐withdrawing effects than that for 2. All of these complexes also act as efficient catalysts towards the photo‐oxidation of water in pH 6.5 phosphate buffer at 20 °C in the presence of [Ru(bpy)3]2+ as a photosensitizer and Na2S2O8 as a sacrificial electron acceptor.

Effect of Quinoline Substitution on Water Oxidation by [Ru(Ql-tpy)(bpy)(OH2 )](PF6 )2

“…Recently our group have reported the single site ruthenium complexes [Ru(2‐py‐tpy)(bpy)(OH 2 )](PF 6 ) 2 , [Ru(3‐py‐tpy)(bpy)(OH 2 )](PF 6 ) 2 , [Ru(4‐py‐tpy)(bpy)(OH 2 )](PF 6 ) 2 , and [Ru(QCl‐tpy)(bpy)(OH 2 )](PF 6 ) 2 , which are employed as successful chemical water oxidation catalysts in CF 3 SO 3 H acid solution at pH 1, in the presence of Ce IV as an oxidant . First three complexes [Ru(n‐py‐tpy)(bpy)(OH 2 )](PF 6 ) 2 (where n = 2 to 4), exhibit electron withdrawing nature due to the protonation at the free nitrogen atom in terminal pyridine moiety, in comparison to parent [Ru II (tpy)(bpy)(OH 2 )] 2+ complex.…”

Systematic Influence of Electronic Modification of Ligands on the Catalytic Rate of Water Oxidation by a Single‐Site Ru‐Based Catalyst

“…This functional aspect of the OEC may provide a possible path for the design of efficient catalysts for artificial photosynthesis. In the last decade, various water oxidation catalysts (WOCs) have been developed based on ruthenium, [10–14] manganese, [15–19] iron, [20–23] and iridium [24–27] . Integrated systems with chromophore‐catalyst assemblies [28–31] and catalysts incorporated into metal‐organic frameworks (MOF)s were also reported [32–36] …”

Water Oxidation by a Neoteric Dinuclear Mn(II) Electrocatalyst in Aqueous Medium

“…(2) (3) Nature has already provided an excellent system of an oxygen-evolving complex (OEC, Mn 4 CaO 5 cluster) responsible for water oxidation to release O 2 , protons, and electrons. [12][13][14][15] In the past few decades, a breakthrough in molecular water oxidation catalysts (WOC) based on Ru [16][17][18][19][20][21] and Ir [22][23] metals has been explored. Still, their high cost and less availability became a serious concern among the scientific community.…”