PM-IRRAS and DFT investigation of the surface orientation of new Ir piano-stool complexes attached to Au(111).

Surface immobilization of organometallic catalysts is a promising approach to developing new catalytic systems that combine molecular catalysts with heterogenous surfaces to probe surface mechanisms. The orientation of the catalyst relative to the surface is one important parameter that must be considered in such hybrid systems. In this work, we synthesize three new sulfide-modified Ir piano-stool complexes with sulfide-modified bipyridine and phenylpyridine ligands for the attachment to Au(111) surfaces. Self-assembled monolayers made from (Cp*Ir(2,2'-bipyridine-4-sulfide)Cl)2[Cl]2 (C1m) and [Cp*Ir(2-phenylpyridine-4-sulfide)Cl]2 (C2m) were characterized by combining polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) with DFT calculations of the minimum energy orientations of the complexes on the surface. We find that the bipyridine and phenylpyridine ligands are oriented at between 73-77° relative to the surface normal, irrespective of the orientation of the other ligands. Additionally, DFT and PM-IRRAS support that there is no orientation preference for C1m and C2m, with both orientations present on the surface.

Improving Photocatalysis for the Reduction of CO2 through Non-covalent Supramolecular Assembly.

We report the enhancement of photocatalytic performance by introduction of hydrogen-bonding interactions to a Re bipyridine catalyst and Ru photosensitizer system (ReDAC/RuDAC) by the addition of amide substituents, with carbon monoxide (CO) and carbonate/bicarbonate as products. This system demonstrates a more-than-3-fold increase in turnover number (TONCO = 100 ± 4) and quantum yield (ΦCO = 23.3 ± 0.8%) for CO formation compared to the control system using unsubstituted Ru photosensitizer (RuBPY) and ReDAC (TONCO = 28 ± 4 and ΦCO = 7 ± 1%) in acetonitrile (MeCN) with 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as sacrificial reductant. In dimethylformamide (DMF), a solvent that disrupts hydrogen bonds, the ReDAC/RuDAC system showed a decrease in catalytic performance while the control system exhibited an increase, indicating the role of hydrogen bonding in enhancing the photocatalysis for CO2 reduction through supramolecular assembly. The similar properties of RuDAC and RuBPY demonstrated in lifetime measurements, spectroscopic analysis, and electrochemical and spectroelectrochemical studies revealed that the enhancement in photocatalysis is due not to differences in intrinsic properties of the catalyst or photosensitizer, but to hydrogen-bonding interactions between them.

Charged Macromolecular Rhenium Bipyridine Catalysts with Tunable CO2 Reduction Potentials.

A series of polymeric frameworks with functional assemblies were designed to alter the catalytic activity of covalently bound ReI electrocatalysts. Norbornenyl polymers containing positively charged quaternary ammonium salts, neutral phenyl, or negatively charged trifluoroborate groups were end-labelled with a ReI fac-tricarbonyl bipyridine electrocatalyst via cross metathesis. Electrochemical studies in acetonitrile under an inert atmosphere and with saturated CO2 indicate that the quaternary ammonium polymers exhibit a significantly lower potential for CO2 reduction to CO (ca. 300 mV), while neutral polymers behave consistently with what has been reported for the free, molecular catalyst. In contrast, the trifluoroborate polymers displayed a negative shift in potential and catalytic activity was not observed. It is demonstrated that a single catalytically active complex can be installed onto a charged polymeric framework, thereby achieving environmentally tuned reduction potentials for CO2 reduction. These materials may be useful as polymer-based precursors for preparing catalytic and highly ordered structures such as thin films, porous catalytic membranes, or catalytic nanoparticles.

Photocatalytic Reduction of Carbon Dioxide to CO and HCO2H Using fac-Mn(CN)(bpy)(CO)3.

Studies are reported regarding the use of Mn(CN)(bpy)(CO)3 (1) as a catalyst for CO2 reduction employing [Ru(dmb)3](2+) as a photosensitizer in mixtures of dry N,N-dimethylformamide-triethanolamine (N,N-DMF-TEOA) or acetonitrile-TEOA (MeCN-TEOA) with 1-benzyl-1,4-dihydronicotinamide as a sacrificial reductant. Irradiation with 470 nm light for up to 15 h yields both CO and HCO2H with maximum turnover numbers (TONs) as high as 21 and 127, respectively, with product preference dependent on the solvent. Further data suggests that upon single electron reduction this catalyst avoids the formation of a Mn-Mn dimer and instead undergoes a disproportionation reaction, which requires 2 equiv of [Mn(CN)(bpy)(CO)3](•-) to generate 1 equiv each of the active catalyst [Mn(bpy)(CO)3](-) and the starting compound 1. Additional characterization by cyclic voltammetry (CV) and infrared spectroelectrochemistry (IR-SEC) indicates that the stability of the singly reduced [Mn(CN)(bpy)(CO)3](•-) differs slightly in the N,N-DMF-TEOA solvent system compared to the MeCN-TEOA system. This contributes to the observed selectivities for HCO2H vs CO production.

A visible-light-driven composite photocatalyst of TiO2 nanotube arrays and graphene quantum dots.

TiO2 nanotube arrays are well-known efficient UV-driven photocatalysts. The incorporation of graphene quantum dots could extend the photo-response of the nanotubes to the visible-light range. Graphene quantum dot-sensitized TiO2 nanotube arrays were synthesized by covalently coupling these two materials. The product was characterized by Fourier-transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and UV-vis absorption spectroscopy. The product exhibited high photocatalytic performance in the photodegradation of methylene blue and enhanced photocurrent under visible light irradiation.