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 CO(2) reduction.

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 RuRe(2) is 1.7 x 10(8) 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 Ru(2)Re and RuRe(2) show excellent photocatalytic activities to the CO(2) reduction. RuRe(2) 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). Ru(2)Re 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 CO(2). 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.

A novel tripodal ligand, tris[(4'-methyl-2,2'-bipyridyl-4-yl)methyl]carbinol and its trinuclear Ru(II)/Re(I) mixed-metal complexes: synthesis, emission properties, and photocatalytic CO2 reduction.

A novel tripodal ligand, tris[(4'-methyl-2,2'-bipyridyl-4-yl)methyl]carbinol (L), has been synthesized. The spectroscopic, electrochemical, and photocatalytic properties of the new trinuclear complexes (Ru(2)Re and RuRe(2)) linked by the tripodal bridging ligand L are then investigated. In addition, 2-fold-improved photocatalytic activities were obtained in the case of these trinuclear complexes compared to the mixtures of the appropriate monometallic model complexes in the reduction of CO(2) under visible irradiation.

Architecture of supramolecular metal complexes for photocatalytic CO2 reduction: ruthenium-rhenium bi- and tetranuclear complexes.

We study the electrochemical, spectroscopic, and photocatalytic properties of a series of Ru(II)-Re(I) binuclear complexes linked by bridging ligands 1,3-bis(4'-methyl-[2,2']bipyridinyl-4-yl)propan-2-ol (bpyC3bpy) and 4-methyl-4'-[1,10]phenanthroline-[5,6-d]imidazol-2-yl)bipyridine (mfibpy) and a tetranuclear complex in which three [Re(CO)3Cl] moieties are coordinated to the central Ru using the bpyC3bpy ligands. In the bpyC3bpy binuclear complexes, 4,4'-dimethyl-2,2'-bipyridine (dmb) and 4,4'-bis(trifluoromethyl)-2,2'-bipyridine ({CF3}2bpy), as well as 2,2'-bipyridine (bpy), were used as peripheral ligands on the Ru moiety. Greatly improved photocatalytic activities were obtained only in the cases of [Ru{bpyC3bpyRe(CO)3Cl}3]2+ (RuRe3) and the binuclear complex [(dmb)2Ru(bpyC3bpy)Re(CO)3Cl]2+ (d2Ru-Re), while photocatalytic responses were extended further into the visible region. The excited state of ruthenium in all Ru-Re complexes was efficiently quenched by 1-benzyl-1,4-dihydronicotinamide (BNAH). Following reductive quenching in the case of d2Ru-Re, generation of the one-electron-reduced (OER) species, for which the added electron resides on the Ru-bound bpy end of the bridging ligand bpyC3bpy, was confirmed by transient absorption spectroscopy. The reduced Re moiety was produced via a relatively slow intramolecular electron transfer, from the reduced Ru-bound bpy to the Re site, occurring at an exchange rate (DeltaG approximately 0). Electron transfer need not be rapid, since the rate-determining process is reduction of CO2 with the OER species of the Re site. Comparison of these results with those for other bimetallic systems gives us more general architectural pointers for constructing supramolecular photocatalysts for CO2 reduction.