Determinant Role of Electrogenerated Reactive Nucleophilic Species on Selectivity during Reduction of CO2 Catalyzed by Metalloporphyrins.

This work provides insights to understand the selectivity during the reduction of CO2 with metalloporphyrin (MP) catalysts. The attack of a nucleophile on the carbon of the CO2 appears as an important event that triggers the catalytic reaction, and the nature of this nucleophile determines the selectivity between CO (or further reduced species) and HCOOH/HCOO-. For MP, the possible electrogenerated nucleophiles are the reduced metal-center and the hydride donor species, metal-hydride and phlorin-hydride ligand. The reduced metal-center activates the CO2 with the formation of the metal-carbon bond, which then gives rise to the formation of CO. The hydride donor species trigger the CO2 reduction by the attack of the hydride on the carbon of the CO2 (formation of a C-H bond), which results in the formation of HCOOH/HCOO- (formation of the metal-bonded formate intermediate is not involved). The MP with the metals Ni, Cu, Zn, Pd, Ag, Cd, Ga, In, and Sn are predicted to only form the phlorin-hydride intermediate and are thus suitable to produce HCOOH/HCOO-. This agrees well with the available experimental results. The MP with the metals Fe, Co, and Rh can form both the reduced-metal center and the hydride donor species (metal-hydride and phlorin-hydride), and thus are able to form both CO and HCOOH/HCOO-. The production of CO for Fe and Co is indeed observed experimentally, but not for Rh, probably due to the presence of axial ligands that may hinder the formation of the metal-bonded intermediates and thus drive the CO2RR to HCOOH/HCOO- via the phlorin intermediate.

Proton-coupled electron transfer in the electrocatalysis of CO2 reduction: prediction of sequential vs. concerted pathways using DFT.

Herein we investigate computationally in detail the mechanism of the formation of the carboxylate adduct during the electroreduction of CO2 in water catalysed by cobalt porphyrin complexes. Specifically, we address qualitatively the competition between the concerted and sequential pathways for the proton-coupled electron transfer. We use a simple methodology for accurate computation of the pKa of the neutral and anionic carboxylate intermediates, [CoP-COOH] and [CoP-COOH]- (where CoP is a cobalt porphine complex), based on the isodesmic proton-exchange reaction scheme. The predicted values are used as in input for a theoretical model that describes the transition between the sequential and concerted pathways. The activation of the sequential pathway (ET-PT) that leads to the formation of the neutral [CoP-COOH] intermediate at pH ≈ 3.5 (pKa[CoP-COOH] = 3.5 ± 0.4), as predicted by the calculations, is in good agreement with the drastic increase in the faradaic efficiency of the CO2 reduction reaction towards CO at pH = 3 compared to pH = 1, as experimentally observed. This confirms the existence of the CO2 anionic adduct [CoP-CO2]- as a viable intermediate at pH = 3 and its crucial role for the pH dependence of the faradaic efficiency for the CO2 reduction. The analysis also shows that when the pH is significantly higher than the pKa of the neutral carboxylate adduct, the CO2 reduction has to go through an alternative pathway with the formation of the anionic carboxylate intermediate [CoP-COOH]-. It is formed through a concerted proton-electron transfer step from the anionic CO2 adduct [CoP-CO2]- when the pH is below ∼8.6 (pKa[CoP-COOH]- = 8.6 ± 0.4). At pH ≈ 8.6 and above, another decoupled ET-PT is predicted to take place, leading to the formation of a dianionic CO2 adduct [CoP-CO2]2-.

Pivotal Role of a Pentacoordinate (3)MC State on the Photocleavage Efficiency of a Thioether Ligand in Ruthenium(II) Complexes: A Theoretical Mechanistic Study.

A mechanistic study of the photocleavage of the methylthioethanol ligand (Hmte) in the series of ruthenium complexes [Ru(tpy)(N-N)(Hmte)](2+) (tpy = 2,2':6',2″-terpyridine, N-N = bpy (2,2'-bipyridine), biq (2,2'-biquinoline), dcbpy (6,6'-dichloro-2,2'-bipyridine), dmbpy (6,6'-dimethyl-2,2'-bipyridine)) was performed using density functional theory. These studies reveal the decisive role of two quasi-degenerate triplet metal-centered states, denoted (3)MChexa and (3)MCpenta, on the lowest triplet potential energy surface. It also shows how the population of the specific pentacoordinate (3)MCpenta state, characterized by a geometry more accessible for the attack of a solvent molecule, is a key step for the efficiency of the photosubstitution reaction. The difference in the photosubstitution quantum yields experimentally observed for this series of complexes (from φ = 0.022 for N-N = bpy up to φ = 0.30 for N-N = dmbpy) is rationalized by the existence of this (3)MCpenta photoreactive state and by the different topologies of the triplet excited-state potential energy surfaces, rather than by the sole steric properties of these polypyridinyl ligands.

Preparation, stability, and photoreactivity of thiolato ruthenium polypyridyl complexes: Can cysteine derivatives protect ruthenium-based anticancer complexes?

Ruthenium polypyridyl complexes may act as light-activatable anticancer prodrugs provided that they are protected by well-coordinated ligands that i) prevent coordination of other biomolecules to the metal center in the dark and ii) can be removed by visible light irradiation. In this paper, the use of monodentate thiol ligands RSH as light-cleavable protecting groups for the ruthenium complex [Ru(tpy)(bpy)(OH2)](PF6)2 ([1](PF6)2; tpy=2,2';6',2″-terpyridine, bpy=2,2'-bypyridine), is investigated. The reaction of [1](2+) with RSH=H2Cys (L-cysteine), H2Acys (N-acetyl-L-cysteine), and HAcysMe (N-acetyl-L-cysteine methyl ester), is studied by UV-visible spectroscopy, NMR spectroscopy, and mass spectrometry. Coordination of the monodentate thiol ligands to the ruthenium complex takes place upon heating to 353 K, but full conversion to the protected complex [Ru(tpy)(bpy)(SR)]PF6 is only possible when a large excess of ligand is used. Isolation and characterization of the two new thiolato complexes [Ru(tpy)(bpy)(κS-HCys)]PF6 ([2]PF6) and [Ru(tpy)(bpy)(κS-HAcys)]PF6 ([3]PF6) is reported. [3]PF6 shows a metal-to-ligand charge-transfer absorption band that is red shifted (λmax=492 nm in water) compared to its methionine analogue [Ru(tpy)(bpy)(κS-HAmet)](Cl)2 ([5](Cl)2, λmax=452 nm; HAmet=N-acetyl-methionine). In the dark the thiolate ligand coordinated to ruthenium is oxidized even by traces of oxygen, which first leads to the sulfenato, sulfinato, and disulfide ruthenium complexes, and finally to the formation of the aqua complex [1](2+). [3]PF6 showed slow photosubstitution of the thiolate ligand by water under blue light irradiation, together with faster photooxidation of the thiolate ligand compared to dark conditions. The use of thiol vs. thioether monodentate ligands is discussed for the protection of anticancer ruthenium-based prodrugs.

Unravelling the S → O linkage photoisomerization mechanisms in cis- and trans-[Ru(bpy)2(DMSO)2](2+) using density functional theory.

A mechanistic study of the intramolecular S → O linkage photoisomerization in the cis and trans isomers of [Ru(bpy)2(DMSO)2](2+) was performed using density functional theory. This study reveals that for the cis isomer the linkage photoisomerization of the two DMSO ligands occurs sequentially in the lowest triplet excited state and can either be achieved by a one-photon or by a two-photon mechanism. A mechanistic picture of the S → O photoisomerization of the trans isomer is also proposed. This work especially highlights that both adiabatic and nonadiabatic processes are involved in these mechanisms and that their coexistence is responsible for the rich photophysics and photochemical properties observed experimentally for the studied complexes. The different luminescent behavior experimentally observed at low temperature between the cis and trans isomers is rationalized based on the peculiarity of the topology of the triplet excited-state potential energy surfaces.

Adiabatic versus nonadiabatic photoisomerization in photochromic ruthenium sulfoxide complexes: a mechanistic picture from density functional theory calculations.

Polypyridine ruthenium sulfoxide complexes are intriguing compounds which can display both photochromic and electrochromic properties. These properties are based on the Ru-S → Ru-O linkage isomerization capability of the sulfoxide group. The photoisomerization mechanism is of particular importance in order to understand the photophysical properties of such molecules. Density functional theory calculations demonstrate that the main photoisomerization mechanism is nonadiabatic for the system under study in agreement with the experimental observations. Indeed, funnels for efficient radiationless decay back to the ground state are shown to be easily accessible compared to transition states on the adiabatic triplet potential energy surface. However, we highlight for the first time that triplet metal-centered states play a central role in the photoisomerization mechanism of these compounds.