Syntheses, characterization, and photo-hydrogen-evolving properties of tris(2,2′-bipyridine)ruthenium(ii) derivatives tethered to a cis-Pt(ii)Cl₂unit: insights into the structure–activity relationship

Abstract

The photo-hydrogen-evolving activity (activity to enhance the photochemical EDTA-reduction of water into molecular hydrogen) was evaluated for three different Ru(II)Pt(II) dimers with a general formula of [(bpy)2Ru(micro-bridge)PtCl2]2+(bpy = 2,2'-bipyridine; bridge = 4,4'-bis(N-(3-aminopropyl)carbamoyl)-2,2'-bipyridine (L1), 2,3-bis(2-pyridyl)pyrazine (L2), and 4,4'-bis(N-(4-pyridyl)methylcarbamoyl)-2,2'-bipyridine (L3); EDTA = ethylenediaminetetraacetic acid disodium salt). A new Ru(II)Pt(II) complex, [(bpy)2Ru(micro-L3)PtCl2]2+, was synthesized and characterized. It was confirmed that all three compounds are ineffective towards photochemical H2 production. In each case, an acetate-buffer solution (pH = 5) containing the Ru(II)Pt(II) dimer and EDTA was photolysed using a 350-W Xe lamp under an Ar atmosphere, during which the amount of H2 evolved was analysed by gas chromatography. Additional photolysis experiments were carried out by adding [Ru(bpy)3]2+ and methylviologen (N,N'-dimethyl-4,4'-bipyridinium) to the photolysis solutions described above to test the H2-evolving activity of the Pt(II) unit involved in these Ru(II)Pt(II) dimers. As a result, the Pt(II) units involved in the L1 and L2 compounds were found to be active as an H2-evolving catalyst, while that of the L3 compound was found to show no activity at all. The extent of intramolecular electron-transfer quenching from the 3MLCT excited state of the [Ru(bpy)3]2+ derivative to the tethering Pt(II) catalyst centre was investigated by comparison of the luminescence spectra of these compounds, together with the related compounds. The results showed that the quenching of the 3MLCT luminescence is not at all enhanced in either the L1 or the L3 compounds. On the other hand, the L2 compound is strongly quenched as previously reported. In addition to the above studies, the H2-evolving activity of some Pt(II) monomers, cis-PtCl2(NH3)2, PtCl2(en)(en = ethylenediamine), cis-PtCl2(4-methylpyridine)2, PtCl2(2,2'-bipyrimidine), PtCl2(4,4'-dicarboxy-2,2'-bipyridine), and [PtCl(terpy)]+(terpy = 2,2':6',2''-terpyridine), were similarly investigated in the presence of EDTA, [Ru(bpy)3]2+ and methylviologen, since they were regarded as structural analogues of the Pt(II) units involved in the L1-L3 compounds. The compounds having a cis-Pt(II)Cl2 unit were generally found to show high H2-evolving activity. This was interpreted in terms of the ligation of negatively charged chloride anions leading to the destabilization of the Pt(II) dz2 orbital responsible for the hydrogenic activation. Importantly, cis-PtCl2(4-methylpyridine)2 exhibited relatively high activity as an H2-evolving catalyst, suggesting the importance of the flexible rotation of the pyridyl ligands for efficient hydrogenic activation at the axial site of the Pt(II) ion. The DFT calculations also showed the validity of the structure-activity relationship discussed above for the L3 compound.

Optimization of Hydrogen‐Evolving Photochemical Molecular Devices

“…However,t he exchange of chloride by iodide leads to significant increase of catalytic activity.T hese observations support studies carried out by Sakai and co-workers,w ho highlighted the importance of the d z 2 orbital of the Pt center for the generation of PtÀHi ntermediates. [34] As the introduction of iodide ligands increases the electron density at the Pt center and therefore stabilizes such an intermediate,this is likely to lead to increased TONv alues.…”

“…Both, the catalytic behavior of 3 and the TBAX halideaddition experiments support the hypothesis that the dominant catalytic reaction mechanism is similar to that of 2.Sakai and co-workers recently reported for structurally related hydrogen evolving photocatalysts,that the d z 2 orbital of the Pt center plays am ajor role in the bond formation with the proton source (water). [34] Based on the different ligand properties,t hat is,i odo ligands are stronger p donors than chloride we assumed an increased electron density at the Pt core.T his increased electron density could potentially accelerate the initial catalytic step,w hich is the PtÀHÀOH bond formation. To further investigate the effect of the iodido ligands on the electron density at the catalytic Pt center the charge distribution at the catalytic center of 3 was calculated and compared to 2.D FT calculations indicate that there is adecrease of the positive charge on Pt (by about 0.25) going from 2 to 3 (see Figure S7 and Table S6).…”

Promoting Effect of Electrostatic Interaction between a Cobalt Catalyst and a Xanthene Dye on Visible-Light-Driven Electron Transfer and Hydrogen Production

“…1À4 In addition to the extensively studied heterogeneous photocatalysts, the homogeneous systems for photoinduced hydrogen production have been actively studied in recent years. 5À8 In most reported molecular catalyst systems, the photosensitizer (PS) and the proton reduction catalyst in the H 2 -evolving system are complexes containing precious metals such as Ru, 9,10 Ir, 11,12 Pt, 13,14 Pd, 15,16 Rh, 17,18 etc. For large-scale hydrogen production, development of catalytic systems with PSs and catalysts based on earth-abundant elements is an alluring topic.…”

Hydrogen Production from Water: Photocatalysis

“…9,12,44,47,70, 71 The use of a supramolecular design allows component modification to enhance device function and lightabsorbing properties. The systems use rigid and flexible spacer between the chromophore and the CAT.…”

“…The light-absorbing properties are very similar to the parent monometallic synthon, [(bpy) 2 2+ (py, pyridyl; Figure 19), having similar structural motifs have been reported by this group. 71 These systems incorporate aliphatic linkages to connect the chromophore to the CAT. Interestingly, these systems do not display photocatalytic activity.…”