A Resilient Platform for the Discrete Functionalization of Gold Surfaces Based on N-Heterocyclic Carbene Self-Assembled Monolayers.

We describe the synthesis and characterization of a versatile platform for gold functionalization, based on self-assembled monolayers (SAMs) of distal-pyridine-functionalized N-heterocyclic carbenes (NHC) derived from bis(NHC) Au(I) complexes. The SAMs are characterized using polarization-modulation infrared reflectance-absorption spectroscopy, surface-enhanced Raman spectroscopy, and X-ray photoelectron spectroscopy. The binding mode is examined computationally using density functional theory, including calculations of vibrational spectra and direct comparisons to the experimental spectroscopic signatures of the monolayers. Our joint computational and experimental analyses provide structural information about the SAM binding geometries under ambient conditions. Additionally, we examine the reactivity of the pyridine-functionalized SAMs toward H2SO4 and W(CO)5(THF) and verify the preservation of the introduced functionality at the interface. Our results demonstrate the versatility of N-heterocyclic carbenes as robust platforms for on-surface acid-base and ligand exchange reactions.

Dehydrogenation and Transfer Hydrogenation of Alkenones to Phenols and Ketones on Carbon-Supported Noble Metals.

The catalytic dehydrogenation of substituted alkenones on noble metal catalysts supported on carbon (Pt/C, Pd/C, Rh/C, and Ru/C) was investigated in an organic phase under inert conditions. The dehydrogenation and semihydrogenation of the enone starting materials resulted in aromatic compounds (primary products), saturated cyclic ketones (secondary products), and cyclic alcohols (minor products). Pd/C exhibits the highest catalytic activity, followed by Pt/C and Rh/C. Aromatic compounds remain the primary products, even in the presence of hydrogen donors. Joint experimental and theoretical analyses showed that the four catalytic materials stabilize a common dienol intermediate on the metal surfaces, formed by keto-enol tautomerization. This intermediate subsequently forms aromatic products upon dehydrogenation. The binding orientation of the enone reactants on the catalytic surface is strongly metal-dependent, as the M-O bond distance changes substantially according to the metal. The longer M-O bonds (Pt: 2.84 Å > Pd: 2.23 Å > Rh: 2.17 Å > Ru: 2.07 Å) correlate with faster reaction rates and more favorable keto-enol tautomerization, as shorter distances correspond to a more stabilized starting material. Tautomerization is shown to occur via a stepwise surface-assisted pathway. Overall, each of the studied metals exhibits a distinct balance of enthalpy and entropy of activation (ΔH°‡, ΔS°‡), offering unique possibilities in the realm of enone dehydrogenation reactions that can be achieved by suitable selection of catalytic materials.

Small, Electron-Donating Substituents Give CO2 Activation by Permethylpentalene Zirconium Amido Complexes the Upper Hand: A DFT Study of Distortion and Interaction.

An insight into factors controlling CO2 activation is necessary to develop molecular systems that utilize CO2 as a chemical feedstock. Two permethylpentalene zirconium cyclopentadienyl (mono)amido complexes, Pn*ZrCp(NR2), were previously assessed for CO2 activation, and a strong dependence on the amido substituent was observed. The R = Me analogue reacted rapidly and quantitatively at room temperature to form the carbamato complex, while the R = Ph species was inert. Here, we investigate the origin of this reactivity difference using DFT and the distortion-interaction model to characterize steric and electronic contributions to the activation barrier. We find that the barrier for CO2 insertion with R = Me (19.1 kcal/mol) is lower than with R = Ph (36.6 kcal/mol), explaining the inertness of the Ph-substituted analogue. The distortion energy trend follows the steric bulk of the amido substituents, and the bulkier Ph-substituted complex has a consistently higher distortion energy along its potential energy surface than that of the Me-substituted complex. The interaction energy trend follows the electronics, and a more electron-donating Me-substituted complex shows a consistently lower interaction energy. The balance of these effects at the corresponding TS gives a reduced activation barrier. Small, electron-donating substituents therefore facilitate CO2 activation in these complexes.

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.

Distorted Copper(II) Complex with Unusually Short CF···Cu Distances.

We find a Cu(II)-(L-CF3)2 complex (L-CF3 = 2,2,2-trifluoro-N-[2-(pyridin-2-yl)propan-2-yl]acetamide) with a distorted "seesaw" geometry. It has the shortest crystallographic CF···Cu distances yet reported, to the best of our knowledge (<2.6 Å), for which computational and experimental data indicate a secondary bonding interaction. A comparison with a CCl3 version and one without ligand backbone gem-dimethyl groups suggests a steric origin for the distorted geometry, resulting from the specific ligand interactions.

Intrinsic electronic conductivity of individual atomically resolved amyloid crystals reveals micrometer-long hole hopping via tyrosines.

Proteins are commonly known to transfer electrons over distances limited to a few nanometers. However, many biological processes require electron transport over far longer distances. For example, soil and sediment bacteria transport electrons, over hundreds of micrometers to even centimeters, via putative filamentous proteins rich in aromatic residues. However, measurements of true protein conductivity have been hampered by artifacts due to large contact resistances between proteins and electrodes. Using individual amyloid protein crystals with atomic-resolution structures as a model system, we perform contact-free measurements of intrinsic electronic conductivity using a four-electrode approach. We find hole transport through micrometer-long stacked tyrosines at physiologically relevant potentials. Notably, the transport rate through tyrosines (105 s-1) is comparable to cytochromes. Our studies therefore show that amyloid proteins can efficiently transport charges, under ordinary thermal conditions, without any need for redox-active metal cofactors, large driving force, or photosensitizers to generate a high oxidation state for charge injection. By measuring conductivity as a function of molecular length, voltage, and temperature, while eliminating the dominant contribution of contact resistances, we show that a multistep hopping mechanism (composed of multiple tunneling steps), not single-step tunneling, explains the measured conductivity. Combined experimental and computational studies reveal that proton-coupled electron transfer confers conductivity; both the energetics of the proton acceptor, a neighboring glutamine, and its proximity to tyrosine influence the hole transport rate through a proton rocking mechanism. Surprisingly, conductivity increases 200-fold upon cooling due to higher availability of the proton acceptor by increased hydrogen bonding.

Surprisingly big linker-dependence of activity and selectivity in CO2 reduction by an iridium(i) pincer complex.

Here, we report the quantitative electroreduction of CO2 to CO by a PNP-pincer iridium(i) complex bearing amino linkers in DMF/water. The electrocatalytic properties greatly depend on the choice of linker within the ligand. The complex 3-N is far superior to the analogues with methylene and oxygen linkers, showing higher activity and better selectivity for CO2 over proton reduction.

Robust Binding of Disulfide-Substituted Rhenium Bipyridyl Complexes for CO2 Reduction on Gold Electrodes.

Heterogenization of homogenous catalysts on electrode surfaces provides a valuable approach for characterization of catalytic processes in operando conditions using surface selective spectroelectrochemistry methods. Ligand design plays a central role in the attachment mode and the resulting functionality of the heterogenized catalyst as determined by the orientation of the catalyst relative to the surface and the nature of specific interactions that modulate the redox properties under the heterogeneous electrode conditions. Here, we introduce new [Re(L)(CO)3Cl] catalysts for CO2 reduction with sulfur-based anchoring groups on a bipyridyl ligand, where L = 3,3'-disulfide-2,2'-bipyridine (SSbpy) and 3,3'-thio-2,2'-bipyridine (Sbpy). Spectroscopic and electrochemical analysis complemented by computational modeling at the density functional theory level identify the complex [Re(SSbpy)(CO)3Cl] as a multi-electron acceptor that combines the redox properties of both the rhenium tricarbonyl core and the disulfide functional group on the bipyridyl ligand. The first reduction at -0.85 V (vs. SCE) involves a two-electron process that breaks the disulfide bond, activating it for surface attachment. The heterogenized complex exhibits robust anchoring on gold surfaces, as probed by vibrational sum-frequency generation (SFG) spectroscopy. The binding configuration is normal to the surface, exposing the active site to the CO2 substrate in solution. The attachment mode is thus particularly suitable for electrocatalytic CO2 reduction.

Search for Catalysts by Inverse Design: Artificial Intelligence, Mountain Climbers, and Alchemists.

In silico catalyst design is a grand challenge of chemistry. Traditional computational approaches have been limited by the need to compute properties for an intractably large number of possible catalysts. Recently, inverse design methods have emerged, starting from a desired property and optimizing a corresponding chemical structure. Techniques used for exploring chemical space include gradient-based optimization, alchemical transformations, and machine learning. Though the application of these methods to catalysis is in its early stages, further development will allow for robust computational catalyst design. This review provides an overview of the evolution of inverse design approaches and their relevance to catalysis. The strengths and limitations of existing techniques are highlighted, and suggestions for future research are provided.