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 4-vinyl-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/PMe3 (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)5Cl and Ru(dmb)2Cl2, 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 CO2 reduction performance of the polymeric catalysts in irradiation experiments is presented, revealing particularly high photostabilities and activities for PVBpy5/95 (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.