Photoelectrochemical CO2 Reduction to CO Enabled by a Molecular Catalyst Attached to High-Surface-Area Porous Silicon.

A high-surface-area p-type porous Si photocathode containing a covalently immobilized molecular Re catalyst is highly selective for the photoelectrochemical conversion of CO2 to CO. It gives Faradaic efficiencies of up to 90% for CO at potentials of -1.7 V (versus ferrocenium/ferrocene) under 1 sun illumination in an acetonitrile solution containing phenol. The photovoltage is approximately 300 mV based on comparisons with similar n-type porous Si cathodes in the dark. Using an estimate of the equilibrium potential for CO2 reduction to CO under optimized reaction conditions, photoelectrolysis was performed at a small overpotential, and the onset of electrocatalysis in cyclic voltammograms occurred at a modest underpotential. The porous Si photoelectrode is more stable and selective for CO production than the photoelectrode generated by attaching the same Re catalyst to a planar Si wafer. Further, facile characterization of the porous Si-based photoelectrodes using transmission mode FTIR spectroscopy leads to highly reproducible catalytic performance.

Homogeneous Organic Reductant Based on 4,4'-tBu2-2,2'-Bipyridine for Cross-Electrophile Coupling.

The synthesis of a new homogeneous reductant based on 4,4'-tBu2-2,2'-bipyridine, tBu-OED4, is reported. tBu-OED4 was prepared on a multigram scale in two steps from inexpensive and commercially available starting materials, with no chromatography required for purification. tBu-OED4 has a reduction potential of -1.33 V (vs Ferrocenium/Ferrocene) and is soluble in a range of common organic solvents. We demonstrate that tBu-OED4 can facilitate Ni/Co dual-catalyzed C(sp2)-C(sp3) cross-electrophile coupling reactions and is highly functional group tolerant. tBu-OED4 is expected to be a valuable addition to the set of homogeneous reductants available for organic transformations.

Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling.

A family of 4,4'-tBu2-2,2'-bipyridine (tBubpy) ligands with substituents in either the 6-position, 4,4'-tBu2-6-Me-bpy (tBubpyMe), or 6 and 6'-positions, 4,4'-tBu2-6,6'-R2-bpy (tBubpyR2; R = Me, iPr, sBu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the tBubpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of tBubpy better stabilized (tBubpyR2)NiICl species and resulted in cleaner reduction from (tBubpyR2)NiIICl2. However, bulkier substituents hindered or prevented coordination of tBubpyR2 ligands to Ni0(cod)2. In addition, by using complexes of the type (tBubpyMe)NiCl2 and (tBubpyR2)NiCl2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while (tBubpyMe)NiIICl2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing tBubpyR2 ligands. A detailed study on the catalytic intermediates (tBubpy)Ni(Ar)I and (tBubpyMe2)Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type (tBubpy)Ni(Ar)I are low spin and relatively stable, complexes of the type (tBubpyMe2)Ni(Ar)I are high-spin and less stable. Further, (tBubpyMe2)Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than (tBubpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.

Long-range electrostatic effects from intramolecular Lewis acid binding influence the redox properties of cobalt-porphyrin complexes.

A CoII-porphyrin complex (1) with an appended aza-crown ether for Lewis acid (LA) binding was synthesized and characterized. NMR spectroscopy and electrochemistry show that cationic group I and II LAs (i.e., Li+, Na+, K+, Ca2+, Sr2+, and Ba2+) bind to the aza-crown ether group of 1. The binding constant for Li+ is comparable to that observed for a free aza-crown ether. LA binding causes an anodic shift in the CoII/CoI couple of between 10 and 40 mV and also impacts the CoIII/CoII couple. The magnitude of the anodic shift of the CoII/CoI couple varies linearly with the strength of the LA as determined by the pKa of the corresponding metal-aqua complex, with dications giving larger shifts than monocations. The extent of the anodic shift of the CoII/CoI couple also increases as the ionic strength of the solution decreases. This is consistent with electric field effects being responsible for the changes in the redox properties of 1 upon LA binding and provides a novel method to tune the reduction potential. Density functional theory calculations indicate that the bound LA is 5.6 to 6.8 Å away from the CoII ion, demonstrating that long-range electrostatic effects, which do not involve changes to the primary coordination sphere, are responsible for the variations in redox chemistry. Compound 1 was investigated as a CO2 reduction electrocatalyst and shows high activity but rapid decomposition.