Synthesis and properties of a novel tripodal bipyridyl ligand tb-carbinol and its Ru(II)–Re(I) trimetallic complexes: investigation of multimetallic artificial systems for photocatalytic CO2reduction

Abstract

A novel tripodal ligand, tris[(4′-methyl-2,2′-bipyridin-4-yl)methyl]carbinol (tb-carbinol) and its homonuclear and heteronuclear Ru(II)–Re(I) complexes have been synthesized and characterized by NMR spectroscopy, elemental analysis, and mass spectroscopy. The spectroscopic, electrochemical and photocatalytic properties of the Ru(II)–Re(I) complexes have been investigated. In these supramolecular complexes with tb-carbinol as a bridging ligand, the intramolecular interaction among the terminal metal centers is very weak. In the cases of Ru(II) and Re(I) heteronuclear systems, when the Re(I) moieties are excited, the emission from the Re(I) moiety is efficiently quenched and the intensity of the emission from the Ru(II) moiety increases. The rate constant of energy transfer from Re(I) moieties to Ru(II) unit in RuRe₂ is 1.7 × 10⁸ s⁻¹. From the point of view of the free energy change, the intramolecular electron transfer from the Ru(II) moiety to the Re(I) moiety could proceed smoothly in the ground state. Both of Ru₂Re and RuRe₂ show excellent photocatalytic activities to the CO₂ reduction. RuRe₂ exhibits a turnover number of 190 for CO formation compared with 89 from the model complexes system after 16 h of irradiation (TN_CO calculated based on Ru(II) moiety concentration). Ru₂Re shows a higher turnover number than the model complexes system, 110 compared with 55 from the model system (TN_CO calculated based on Re(I) moiety concentration). The bridging ligand of Ru(II)–Re(I) heteronuclear tripodal systems, tb-carbinol, plays an important role in converting radiant energy to chemical energy in the form of CO from CO₂. Enhancement of the photocatalytic response to light in the visible region has been achieved by fabricating supramolecular systems featuring covalently linked Ru(II) and Re(I) moieties.