Subtle Changes to Peripheral Ligands Enable High Turnover Numbers for Photocatalytic Hydrogen Generation with Supramolecular Photocatalysts

Abstract

The photocatalytic generation of hydrogen (H2) from protons by two cyclometalated ruthenium-platinum polypyridyl complexes, [Ru(bpy)2(2,5-bpp)PtIS](2+) (1) and [Ru(dceb)2(2,5-bpp)PtIS](2+) (2) [where bpy = 2,2'-bipyridine, 2,5-bpp = 2,2',5',2″-terpyridine, dceb = 4,4'-di(carboxyethyl)bipyridine, and S = solvent], is reported. Turnover numbers (TONs) for H2 generation were increased by nearly an order of magnitude by the introduction of carboxyethyl ester units, i.e., from 80 for 1P to 650 for 2P after 6 h of irradiation, with an early turnover frequency (TOF) increasing from 15 to 200 h(-1). The TON and TOF values for 2P are among the highest reported to date for supramolecular photocatalysts. The increase correlates with stabilization of the excited states localized on the peripheral ligands of the light-harvesting Ru(II) center.

Enhancing Photocatalytic Hydrogen Generation: the Impact of the Peripheral Ligands in Ru/Pd and Ru/Pt Complexes

“…The complexes reported in this contribution are part of a larger investigation of Ru/Pd and Ru/Pt dyads with various bridging (see Figure ) and peripheral ligands performed by our group under the same experimental conditions . As was observed for the compounds investigated in this contribution, the catalytic activity of these dyads in hydrogen evolution was dependent on changes to the bridging or the peripheral ligands.…”

“…In a series of studies, we investigated the introduction of carboxylate groups at the peripheral ligands of Ru(bpy) 2 ‐based photosensitisers (bpy=2,2’‐bipyridine) on the photocatalytic hydrogen‐generating capacity of these molecular assemblies . The results showed that the peripheral ligands were actively involved in the catalytic activity; they determined early excited‐state processes and dramatically changed the electron flow in the photocatalyst . The bridging ligands 2,3‐dpp, 2,5‐dpp and 2,5‐bpp (Figure ) were used, and introduction of carboxylate moieties onto the peripheral ligands increased the turnover number (TON) from 0 to 400 (18 h; PdCl 2 catalyst), 0 to 44 (18 h; PtCl 2 catalyst) and 80 to 650 (6 h, PtI 2 catalyst) .…”

“…The results showed that the peripheral ligands were actively involved in the catalytic activity; they determined early excited‐state processes and dramatically changed the electron flow in the photocatalyst . The bridging ligands 2,3‐dpp, 2,5‐dpp and 2,5‐bpp (Figure ) were used, and introduction of carboxylate moieties onto the peripheral ligands increased the turnover number (TON) from 0 to 400 (18 h; PdCl 2 catalyst), 0 to 44 (18 h; PtCl 2 catalyst) and 80 to 650 (6 h, PtI 2 catalyst) . The commonality between these ligands is that the photosensitiser and catalytic centre are both coordinated to the central ring.…”

Peripheral ligands as electron storage reservoirs and their role in enhancement of photocatalytic hydrogen generation

“…[9] Definitely, the present work provides the first photophysical evidence challenging the conventional design paradigm based on a bridging ligand acting as electron density storage reservoir. Establishing a connection between storage of electron density at the peripheral ligands at early-times and the high H 2 TON and TOF values is not straightforward considering the multiple photochemical reactions occurring at later times.…”

“…EtOOCRuPt shows a significantly improved performance (TON = 650 after 6 h of irradiation at 470 nm) as compared to RuPt (TON = 80) in the same reaction conditions. [9] For RuPd, a TON of 120 was reported.…”