Photochemical CO2 Reduction Catalyzed by Phenanthroline Extended Tetramesityl Porphyrin Complexes Linked with a Rhenium(I) Tricarbonyl Unit.

A series of heterodinuclear complexes (M-1-Re) based on a phenanthroline (phen) extended tetramesityl porphyrin ligand (H2-1) has been prepared. The phen moiety of this ligand selectively coordinates a Re(I) tricarbonyl chloride unit, whereas the metal in the porphyrin moiety has been varied: namely, Cu, Pd, Zn, Co, or Fe was used. These dinuclear complexes were fully characterized by standard analytical methods. Additionally, a crystal structure of Cu-1-Re·5.5(C7H8)·0.5(C6H6) could be obtained, and extended time-resolved emission lifetime measurements were conducted. Furthermore, their ability to catalyze the photochemical reduction of CO2 to CO was investigated. Light-driven CO2 reduction experiments were performed in dimethylformamide (DMF) using triethylamine (TEA) as the sacrificial electron donor. The TONs (turnover numbers) of CO were determined and revealed a surprising catalytic activity that is obviously independent from the redox activity of the porphyrin metal. We have recently shown that the parent M-1 compounds are active photocatalysts, but the catalytic activity was dependent on the redox activity of the porphyrin metal. In the case of the new heterodinuclear complexes M-1-Re reported in this study, the catalytic active center seems to be the Re(I) moiety and not the porphyrin. Surprisingly, Zn-1-Re proved to be the most active compound in this series showing a TONCO of 13 after 24 h of illumination using a >375 nm cutoff filter while all other compounds showed minimal activity under this condition.

Photochemical CO2-reduction catalyzed by mono- and dinuclear phenanthroline-extended tetramesityl porphyrin complexes.

We here present a comprehensive study on the light-induced catalytic CO2 reduction employing a number of mono- and dinuclear complexes with a phenanthroline-extended tetramesityl porphyrin ligand (). A stepwise synthesis of heterodinuclear complexes is possible because the phenanthroline moiety of the ligand can selectively coordinate a second metal center, e.g. Ru(tbbpy)2(2+) fragment, while any other metal can reside in the porphyrin cavity. We expanded our former studies to cobalt and iron compounds and synthesized the complexes , and , . Thorough catalytic investigation on the light-driven CO2 reduction of all compounds (M = 2H, Cu, Pd, Co, FeCl) was performed in a DMF solution in the presence of triethylamine (TEA) as a sacrificial electron donor. A very surprising wavelength dependence of the catalytic performance was observed. Turnover numbers (TONs) of CO were quantified and showed that redox active metals (i.e.M = Co and FeCl) in the porphyrin cavity caused the highest catalytic activity. After 24 hours of illumination with light λ > 305 nm reached a TONCO of 11.4 with our experimental setup without showing much decomposition. This value is twice as high as the TONCO determined for CoTPP (5.8) under the same conditions, which represented the most active porphyrinic system so far for photocatalytic CO2 reduction.

Electrocatalytic carbon dioxide reduction by using cationic pentamethylcyclopentadienyl-iridium complexes with unsymmetrically substituted bipyridine ligands.

Eight [Ir(bpy)Cp*Cl](+) -type complexes (bpy= bipyridine, Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) containing differently substituted bipyridine ligands were synthesized and characterized. Cyclic voltammetry (CV) of the complexes in Ar-saturated acetonitrile solutions showed that the redox behavior of the complexes could be fine tuned by the electronic properties of the substituted bipyridine ligands. Further CV in CO2 -saturated MeCN/H2 O (9:1, v/v) solutions showed catalytic currents for CO2 reduction. In controlled potential electrolysis experiments (MeCN/MeOH (1:1, v/v), Eapp =-1.80 V vs Ag/AgCl), all of the complexes showed moderate activity in the electrocatalytic reduction of CO2 with good stability over at least 15 hours. This electrocatalytic process was selective toward formic acid, with only traces of dihydrogen or carbon monoxide and occasionally formaldehyde as byproducts. However, the turnover frequencies and current efficiencies were quite low. No direct correlation between the redox potentials of the complexes and their catalytic activity was observed.

The activation of sulfur hexafluoride at highly reduced low-coordinate nickel dinitrogen complexes.

The greenhouse gas sulfur hexafluoride is the common standard example in the literature of a very inert inorganic small molecule that is even stable against O2 in an electric discharge. However, a reduced ß-diketiminate nickel species proved to be capable of converting SF6 into sulfide and fluoride compounds at ambient standard conditions. The fluoride product complex features an unprecedented [NiF](+) unit, where the Ni atom is only three-coordinate, while the sulfide product exhibits a rare almost linear [Ni(µ-S)Ni](2+) moiety. The reaction was monitored applying (1)H NMR, IR and EPR spectroscopic techniques resulting in the identification of an intermediate nickel complex that gave insight into the mechanism of the eight-electron reduction of SF6.

Synthesis and characterization of mono- and dinuclear phenanthroline-extended tetramesitylporphyrin complexes as well as UV-Vis and EPR studies on their one-electron reduced species.

The syntheses of mononuclear compounds based on the fused porphyrin phenanthroline ligand (H(2)-1) and their corresponding dinuclear porphyrin bis-bipyridine ruthenium complexes are reported. The extended π-system of the ligand is able to store electron equivalents as could be proven by the single electron reduction with KC(8) (and/or Na(Hg)) followed by subsequent UV-Vis and EPR analysis. Electron reduction could also be achieved under light illumination in a dichloromethane-triethylamine mixture. The two coordination spheres of the ligand are different so that mononuclear or (hetero)dinuclear complexes can be isolated depending on the reaction conditions. We could successfully introduce Zn, Cu and Pd into the porphyrinic unit leading to a series of mononuclear (M-1) compounds. Further, we could attach the bis-(4,4'-di-tert-butyl-2,2'-bipyridine) ruthenium fragment (Ru(tbbpy)(2)(2+)) to obtain dinuclear (M-1-Ru) metal complexes. In the case of Zn-1 an X-ray crystal structure could be obtained confirming the selective metallation in the porphyrinic unit. All metal complexes were isolated and characterized with standard analytical tools (elemental analysis, mass spectrometry and NMR (or EPR) spectroscopy).