Palladium versus Platinum: The Metal in the Catalytic Center of a Molecular Photocatalyst Determines the Mechanism of the Hydrogen Production with Visible Light

Abstract

To develop highly efficient molecular photocatalysts for visible light-driven hydrogen production, a thorough understanding of the photophysical and chemical processes in the photocatalyst is of vital importance. In this context, in situ X-ray absorption spectroscopic (XAS) investigations show that the nature of the catalytically active metal center in a (N^N)MCl2 (M=Pd or Pt) coordination sphere has a significant impact on the mechanism of the hydrogen formation. Pd as the catalytic center showed a substantially altered chemical environment and a formation of metal colloids during catalysis, whereas no changes of the coordination sphere were observed for Pt as catalytic center. The high stability of the Pt center was confirmed by chloride addition and mercury poisoning experiments. Thus, for Pt a fundamentally different catalytic mechanism without the involvement of colloids is confirmed.

Tuning of photocatalytic activity by creating a tridentate coordination sphere for palladium

“…1 H-NMR studies revealed that the unsubstituted Ru(tpphz)PdCl 2 reacts to the mononuclear Ru(tpphz) under the presence of Hg, meaning an extraction of the Pd(II) centre by Hg. 33 Interestingly, no reaction occurred with Ru(bmptpphz)PdCl under identical conditions underlining the increased stability of the Pd centre (see ESI Fig. S7 †).…”

“…S5 †). 32,33 It is apparent that Ru(bmptpphz)PdCl is less active giving a maximum TON of 50 after 64 hours of irradiation, compared to Ru(tpphz)PdCl 2 , which shows a maximum TON of 187 after 24 hours.…”

“…We have recently shown that also in Ru(tpphz)PdCl 2 colloid formation takes place during catalytic turnover. 33 By X-ray absorption spectroscopy (XAS), a fast and significant accumulation of Pd(0) colloids as a function of irradiation time was detected. However, from these experiments it is not finally clarified whether Pd(0) colloids function as an essential intermediate for the evolution of molecular hydrogen or are just a catalytically inactive decomposition product.…”

Functional Dimming of Pincer‐Shaped Bibenzimidazole‐Ruthenium(II) Complexes with Improved Anion‐Sensitive Luminescence

“…[7,8] On the one hand, polypyridyl-ruthenium(II) complexes are widely applicable molecular tools for the introduction of light in technological and biological applications [9][10][11][12] because of their well-known photophysical properties and generally long-lived excited states, in which light energy is absorbed and provided for chemical conversions. [13,14] On the other hand, the vast synthetic variability of the general tris(diimine) framework allows the adjustment of a chromophore for a distinct application. [15][16][17] For example, 2,2′-bibenzimidazole ligands provide an elegant direct connection between a cisdiimine function for coordination to a ruthenium center with and XRD studies.…”

π-Stacking attraction vs. electrostatic repulsion: competing supramolecular interactions in a tpphz-bridged Ru(ii)/Au(iii) complex

“…However, theoretical calculations on the electron transfer processes during catalysis were based on a planar model of the dinuclear PMD. 3,6,7 Therefore, it was highly desirable to obtain further model structures. Generally, replacing the catalyst metal by gold(III) adds an additional charge to the system while retaining the coordination environment and, hence, provides an isostructural and isoelectronic model complex with respect to the literature known photocatalysts (Scheme 1).…”