Photo‐Hydrogen‐Evolving Molecular Catalysts Consisting of Polypyridyl Ruthenium(II) Photosensitizers and Platinum(II) Catalysts: Insights into the Reaction Mechanism

Abstract

The mechanism of photoinduced hydrogen evolution from water driven by the first photo-hydrogen-evolving molecular catalyst (1), given by a coupling of [Ru(bpy)(2)(5-amino-phen)](2+) and [PtCl(2)(4,4'-dicarboxy-bpy)] (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline), was investigated in detail. The H(2) evolution rate was found to obey Michaelis-Menten enzymatic kinetics with regard to the concentration of EDTA (ethylenediamine tetra-acetic acid disodium salt, sacrificial electron donor), which indicates that an ion-pair formation between the dicationic 1 and the dianionic form of EDTA (pH 5) is a key step leading to H(2) formation. A 2:1 coupling product of 1 and ethylenediamine (i.e., a {Ru(II) (2)Pt(II) (2)} complex 2) was found to show significantly higher photo-hydrogen-evolving (PHE) activity than 1, which revealed the validity of the bimolecular activation proposed in our previous study. The PHE activity of 2 was also observed to be linear to the concentration of 2, which indicates that H(2) formation through the intermolecular path competes with the intramolecular path. Molecular orbital diagrams, conformational features, and PtH(water or acetic acid) hydrogen bonds were characterized by DFT calculations.

A metal-organic cage incorporating multiple light harvesting and catalytic centres for photochemical hydrogen production

“…For the known PMDs1219202122, the N̂N-chelated N n MX 2 (M= Pd 2+ , Pt 2+ or Rh 2+ , X= Cl − , Br − or I − , n =2 or 4) coordination sphere was commonly adopted as a catalyst centre, for which dissociation of the terminal X − anions during the photolysis was also pointed out2425. Moreover, some of the works revealed that colloidal Pd has a major contribution to the photoinduced H 2 evolution in such a Ru–Pd supramolecular system42.…”

“…Moreover, integrating six catalytic Pd centres in a whole cage may endow them with synergistic durability on structural deformation during catalysis on a Pd-centre, where dissociation of Pd–N bond and formation of hydride intermediate may occur1221232425. The stability of MOC-16 is also confirmed by an Hg-test, where the 1 H NMR spectra of MOC-16 recorded before and after the treatment with excess Hg in DMSO for 9 h shows no destruction of MOC-16 (Supplementary Fig.…”

“…Remarkable progress has been made since the development of intramolecular photochemical molecular devices (PMDs) by integrating the chromophoric photosensitizer, catalytic centre and electron relay components into a single-component photocatalyst131415. Compared with the three-component intermolecular photocatalytic systems161718, the covalently or coordinatively linked single-component intramolecular PMDs are expected to improve the overall catalytic efficiency by avoiding the molecular collision before relaxation of the 3 MLCT excited state, therefore affording efficient electron and energy transfer1219202122.…”

“…To date, many active bimetallic PMDs, such as Ru–Pt121421, Ru–Pd13232425, Ru–Rh26, Ru–Co15 and Ir–Co27 complexes, have been developed for homogenous catalysis. Considering the multiple electrons transfer and accumulation required for H 2 evolution, many efforts are made to improve the effective and directional electron transfer from light absorbing centres to catalytic metal centres282930.…”

Supramolecular activation of a molecular photocatalyst

“…Route (ii) can be taken by screening properties of the catalytic system like solvent composition, pH value and ionic additives. 5,7,[23][24][25][26][27] Within this research, it has, for instance, been shown that the addition of N(C 4 H 9 ) 4 Cl to the photocatalytic system for the light induced formation of hydrogen from water based on [Ru(tbbpy) 2 (tpphz)-PdCl 2 ](PF 6 ) 2 (RutpphzPd) (ruthenium(II)-bis(4,4′-di-tert-butyl-2,2′-bipyridyl)-(μ-tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2′″,3′″-j]phenazine)-(dichloropalladium)-dihexafluorophosphate) ( Fig. 1) leads to a significant drop in catalytic activity.…”