Macromolecular Rhenium–Ruthenium Complexes for Photocatalytic CO₂ Conversion: From Catalytic Lewis Pair Polymerization to Well-Defined Poly(vinyl bipyridine)–Metal Complexes

Abstract

Herein, the first catalytical polymerization of 4vinyl-4′-methyl-2,2′-bipyridine (VBpy) via Lewis pair-mediated group-transfer polymerization using different combinations of Lewis acidic trialkyl aluminum compounds and Lewis basic phosphines is reported. In this context, a broad screening of different Lewis pairs is conducted, demonstrating the necessity of an adjustment of the steric and electronic properties of the Lewis pair to the demands of the monomer. Further, end-group analysis of short-chain oligomers via electrospray ionization mass spectrometry (ESI-MS) for the experimentally determined optimum combination Al(i-Bu) 3 /PMe 3 (Đ = 1.31−1.36, I.E. = 45−51%) reveals the presence of two initiation pathways via conjugate addition and deprotonation. The well-defined polymers are subsequently loaded in a two-step synthesis protocol with different ratios of Re(CO) 5 Cl and Ru(dmb) 2 Cl 2 , forming a photocatalytically active rhenium−ruthenium polymer complex with poly(vinyl bipyridine) as the macroligand. Catalyst loadings are characterized thoroughly by means of Ultraviolet−visible (UV−vis), photoluminescence (PL), and IR spectroscopy as well as inductively coupled plasma (ICP)-MS. Finally, a comparison of the photocatalytic CO 2 reduction performance of the polymeric catalysts in irradiation experiments is presented, revealing particularly high photostabilities and activities for PVBpy 5/95 (TON = 5650, TOF = 66 h −1 ). This is due to an efficient electron transfer of the Ru(II)-one-electron-reduced species (OERS) to the rhenium centers facilitated by the spatial proximity of both metals attached to the macromolecular ligand PVBpy.

The Puzzling Question about the Origin of the Second Electron in the Molecular Photocatalytic Reduction of CO2

“…This indicates efficient quenching of the emission from the rhenium(I)‐bipyridyl‐unit, which is expected at 618 nm (Figure 2, Table S2). Energy transfer from the energetical higher lying 3 MLCT of Re(I)‐bipyridyl‐unit to the Ru(II)‐complex through the bridging linker is a feasible downhill process, which has already been described previously [5,7,9,11a,17] . The excitation energy is then released by the strong fluorescent Ru(dmb) 3 2+ via light emission in the following step.…”

“…It should be noted, that in this illustration the amount of Ru‐PS in the RuRe2 and RuRe setups is 1.5 and 3 times higher compared to the RuRe3 experiments, respectively. As mentioned earlier, higher concentrations of photosensitizer are known to stabilize the photocatalytic system, which could explain the superiority of RuRe at increased concentrations [6c,12,17] …”

Photocatalytic CO2 Conversion Using Metal-Containing Coordination Polymers and Networks: Recent Developments in Material Design and Mechanistic Details

“… Two-step synthesis of the PVBpy Re/Ru photocatalyst by loading with Re(CO) 5 Cl and Ru(dmb) 2 Cl 2 [ 44 ]. …”

“…Attachment of the rhenium and ruthenium centers to the poly(vinyl bipyridine) (PVBpy) offered proximity to these metal centers for an enhanced intramolecular electron transfer. PVBpy with a photocatalyst-to-photosensitizer ratio of 5% Re(I) compared to 95% Ru(II) showed a TON of 6088 and a turnover frequency (TOF) of 66 h −1 for the conversion of CO 2 to CO due to effective electron transfers from the one electron reduced Ru(II) to the Re(I) centers [44]. The newest approach by Rieger et al took advantage of Lewis pair mediated GTP with different Lewis acidic trialkyl aluminum compounds and Lewis basic phosphines to result in well-defined polymers with bipyridine monomer units for Re-and Ru-complexation.…”

“…Attachment of the rhenium and ruthenium centers to the poly(vinyl bipyridine) (PVBpy) offered proximity to these metal centers for an enhanced intramolecular electron transfer. PVBpy with a photocatalyst-tophotosensitizer ratio of 5% Re(I) compared to 95% Ru(II) showed a TON of 6088 and a turnover frequency (TOF) of 66 h −1 for the conversion of CO2 to CO due to effective electron transfers from the one electron reduced Ru(II) to the Re(I) centers [44]. Kim and Choi reported a molecular photosensitizer-catalyst assembly without the need of any chemical linkage, since linking the photosensitizer and the catalyst could require complex synthesis procedures.…”