Covalent Photosensitizer–Polyoxometalate‐Catalyst Dyads for Visible‐Light‐Driven Hydrogen Evolution

Abstract

A general concept for the covalent linkage of coordination compounds to bipyridine-functionalized polyoxometalates is presented. The new route is used to link an iridium photosensitizer to an Anderson-type hydrogen-evolution catalyst. This covalent dyad catalyzes the visible-light-driven hydrogen evolution reaction (HER) and shows superior HER activity compared with the non-covalent reference. Hydrogen evolution is observed over periods >1 week. Spectroscopic, photophysical, and electrochemical analyses give initial insight into the stability, electronic structure, and reactivity of the dyad. The results demonstrate that the proposed linkage concept allows synergistic covalent interactions between functional coordination compounds and reactive molecular metal oxides.

Experimental and Theoretical Investigation of the Light‐Driven Hydrogen Evolution by Polyoxometalate–Photosensitizer Dyads

“…In our previous study, we have shown that Ir‐POM Mn can be employed as homogenous light‐driven HER catalyst (see above) . Here, we investigated how variation of the central metal M affects the light‐driven HER activity of the dyads.…”

“…Next, we explored the covalent linkage of bpy units to { M Mo 6 } using our established synthetic protocol . Briefly, the corresponding TRIS‐Anderson was treated with 4‐methyl‐2,2′‐bipyridine‐4′‐carboxaldehyde (bpy‐CHO) in dry acetonitrile, leading to the formation of a Schiff‐base linkage between the bpy and the TRIS moiety.…”

“…As described by us recently, the { M Mo 6 ‐bpy 2 } units are ideal platforms for the linkage of metal complexes such as Ir photosensitizers . To this end, we treated the respective { M Mo 6 ‐bpy 2 } with equimolar amounts of the iridium dimer [Ir(ppy) 2 (μ‐Cl)] 2 (ppy=2‐phenylpyridine) in a mixture of dichloromethane and methanol (5:2 v : v ), see Figure .…”

“…To this end, two 2,2′‐bipyridine ligands (bpy) were covalently attached to manganese‐centered TRIS‐Andersons using Schiff‐base linkages. Next, an iridium chromophore [Ir(ppy) 2 (μ‐Cl)] 2 (ppy=2‐phenylpyridine) was coordinated to the bpy binding sites, resulting in the formation of covalent POM–photosensitizer dyads for light‐driven HER . Notably, we could show that the covalent linkage of photosensitizer and catalyst results in improved electronic coupling between both species and higher HER activity was observed for the covalent system compared with a noncovalent reference.…”

“…We have taken first steps to explore this reactivity by designing a light‐driven covalent HER dyad in which a metal complex photosensitizer is covalently linked to the TRIS‐Anderson . To this end, two 2,2′‐bipyridine ligands (bpy) were covalently attached to manganese‐centered TRIS‐Andersons using Schiff‐base linkages.…”