Photophysical and Photochemical Trends in Tricarbonyl Rhenium(I) N-Heterocyclic Carbene Complexes

Replacement of the phosphodiester backbone between canonical nucleosides with a dirhenium carbonyl "click" linker-a new class of luminescent organometallic dinucleoside phosphate mimics

The first-in-class luminescent dinucleoside phosphate analogs with a [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] "click" linker as a replacement for the natural phosphate group are reported together with the synthesis of luminescent adenosine and thymidine derivatives having the [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] entity attached to positions 5' and 3', respectively. These compounds were synthesized by applying inverse-electron-demand Diels-Alder and copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition reactions in three or four steps. The obtained compounds exhibited orange emission (λ_PL ≈ 600 nm, Φ_PL ≈ 0.10, and τ = 0.33-0.61 μs) and no toxicity (except for one nucleoside) to human HeLa cervical epithelioid and Ishikawa endometrial adenocarcinoma cancer cells in vitro. Furthermore, the compounds' ability to inhibit the growth of Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacterial strains was moderate and only observed at a high concentration of 100 μM. Confocal microscopy imaging revealed that the "dirhenium carbonyl" dinucleosides and nucleosides localized mainly in the membranous structures of HeLa cells and uniformly inside S. aureus and E. coli bacterial cells. An interesting finding was that some of the tested compounds were also found in the nuclei of HeLa cells.

Rhenium(I) Complexes with Neutral Monodentate Coligands and Monoanionic 2-(1,2,4-Triazol-5-yl)pyridine-Based Chelators as Bidentate Luminophores with Tunable Color and Photosensitized Generation of 1O2: An Integrated Case Study Involving Photophysics and Theory

In this work, we describe the synthesis as well as structural, photophysical, and theoretical investigation of a new coordination chemical concept involving rhenium(I) complexes bearing monoanionic 1,2,4-triazolylpyridine-based bidentate chromophores. The X-ray diffractometric analysis of single crystals revealed particular packing features: the trifluoromethylated exemplar displayed two kinds of arrangements of the coordination centers, where the bidentate ligands at the edges of the unit cell are staggered parallel to each other, whereas those inside show antiparallel stacking with respect to the external ligands. On the other hand, the complexes bearing an adamantyl substituent yield a linear arrangement, where the bulky moiety of one luminophore points to the pyridine center of the adjacent ligand of the neighboring complex while including methanol molecules hydrogen-bonded to the triazolato unit. We observed that the photophysical properties of the complexes (photoexcited-state lifetimes, photoluminescence maxima and quantum yields) can be adjusted by tuning of the substitution pattern at the bidentate luminophore as well as by variation of the monodentate coligand. The photoluminescence spectra and photoexcited-state lifetimes of the crystalline phases were measured by phosphorescence lifetime micro(spectro)scopy. Interestingly, the vibrationally resolved emission spectra of the crystals closely resemble those of diluted frozen glassy matrixes at 77 K, in contrast with the broad bands observed in amorphous solids and in fluid solutions, where the charge-transfer character is enhanced. While the photoluminescence quantum yields (Φ_L) reach up to 15%, the complexes are able to attain up to 55% efficiency regarding the photosensitization of ¹O₂ (Φ_Δ), depending on the combination of luminophore and coligand. Theoretical calculations showed that the photoexcited triplet (T₁) state has a metal-ligand-to-ligand charge-transfer character, where promotion to the excited electronic configuration shortens the Re(I)-N bond involving the bidentate triazolylpyridine while stretching the three fac-CO-Re(I) bonds as well as the linkage to the axial monodentate coligand. The calculated vertical (E^vl) and 0-0 (E^(0-0)) radiative transition energies are in very good agreement with the experimental values (E_exp^lum).

Photoactive Metal Carbonyl Complexes Bearing N-Heterocyclic Carbene Ligands: Synthesis, Characterization, and Viability as Photoredox Catalysts

This report details the synthesis and characterization of a small family of previously unreported, structurally related chromium, molybdenum, tungsten, manganese, and iron complexes bearing N-heterocyclic carbene and carbonyl supporting ligands. These complexes have the general form [ML(CO)₃X] or [ML(CO)₃], where X = CO or Br and L = 1-phenyl-3-(2-pyridyl)imidazolin-2-ylidene. Where possible, the solid-state, spectroscopic, electrochemical, and photophysical properties of these molecules were studied using a combination of experiment and theory. Photophysical studies reveal that decarbonylation occurs when these complexes are exposed to ultraviolet light, with the CO ligand being replaced with a labile acetonitrile solvent molecule. To obtain insights into the potential utility, scope, and applications of these complexes in visible-light-mediated photoredox catalysis, their capacity to facilitate a range of photoinduced reactions via the reductive or oxidative functionalization of organic molecules was investigated. These chromium, molybdenum, and manganese catalysts efficiently facilitated atom-transfer radical addition processes. In light of their photolability, these types of catalysts may potentially allow for the development of photoinduced reactions involving less conventional inner-sphere electron-transfer pathways.

Efficient Direct Nitrosylation of α-Diimine Rhenium Tricarbonyl Complexes to Structurally Nearly Identical Higher Charge Congeners Activable towards Photo-CO Release

The reaction of rhenium α-diimine (N-N) tricarbonyl complexes with nitrosonium tetrafluoroborate yields the corresponding dicarbonyl-nitrosyl [Re(CO)2(NO)(N-N)X]+ species (where X = halide). The complexes, accessible in a single step in good yield, are structurally nearly identical higher charge congeners of the tricarbonyl molecules. Substitution chemistry aimed at the realization of equivalent dicationic species (intended for applications as potential antimicrobial agents), revealed that the reactivity of metal ion in [Re(CO)2(NO)(N-N)X]+ is that of a hard Re acid, probably due to the stronger π-acceptor properties of NO+ as compared to those of CO. The metal ion thus shows great affinity for π-basic ligands, which are consequently difficult to replace by, e.g., σ-donor or weak π-acids like pyridine. Attempts of direct nitrosylation of α-diimine fac-[Re(CO)3]+ complexes bearing π-basic OR-type ligands gave the [Re(CO)2(NO)(N-N)(BF4)][BF4] salt as the only product in good yield, featuring a stable Re-FBF3 bond. The solid state crystal structure of nearly all molecules presented could be elucidated. A fundamental consequence of the chemistry of [Re(CO)2(NO)(N-N)X]+ complexes, it that the same can be photo-activated towards CO release and represent an entirely new class of photoCORMs.

Tricarbonyl rhenium(i) tetrazolato and N-heterocyclic carbene complexes: versatile visible-light-mediated photoredox catalysts

This study demonstrates that structurally-diverse, photoactive rhenium(i) tricarbonyl complexes can mediate representative atom-transfer radical addition, hydrodehalogenation, and α-amino C–H functionalisation reactions.

Dramatic Alteration of 3ILCT Lifetimes Using Ancillary Ligands in [Re(L)(CO)3(phen-TPA)] n+ Complexes: An Integrated Spectroscopic and Theoretical Study

The ground and excited state photophysical properties of a series of fac-[Re(L)(CO)₃(α-diimine)] ⁿ⁺ complexes, where L = Br^-, Cl^-, 4-dimethylaminopyridine (dmap) and pyridine (py) have been extensively studied utilizing numerous electronic and vibrational spectroscopic techniques in conjunction with a suite of quantum chemical methods. The α-diimine ligand consists of 1,10-phenanthroline with the highly electron donating triphenylamine (TPA) appended in the 5 position. This gives rise to intraligand charge transfer (ILCT) states lying lower in energy than the conventional metal-to-ligand charge transfer (MLCT) state, the energies of which are red and blue-shifted, respectively, as the ancillary ligand, L becomes more electron withdrawing. The emitting state is ³ILCT in nature for all complexes studied, characterized through transient absorption and emission, transient resonance Raman (TR²), time-resolved infrared (TRIR) spectroscopy and TDDFT calculations. Systematic modulation of the ancillary ligand causes unanticipated variation in the ³ILCT lifetime by 2 orders of magnitude, ranging from 6.0 μs for L = Br^- to 27 ns for L = py, without altering the nature of the excited state formed or the relative order of the other CT states present. Temperature dependent lifetime measurements and quantum chemical calculations provide no clear indication of close lying deactivating states, MO switching, contributions from a halide-to-ligand charge transfer (XLCT) state or dramatic changes in spin-orbit coupling. It appears that the influence of the ancillary ligand on the excited state lifetime could be explained in terms of energy gap law, in which there is a correlation between ln( k_nr) and E_em with a slope of -21.4 eV^-1 for the ³ILCT emission.

A Robust Pyridyl-NHC-Ligated Rhenium Photocatalyst for CO2 Reduction in the Presence of Water and Oxygen

Re(pyNHC-PhCF3)(CO)3Br is a highly active photocatalyst for CO2 reduction. The PhCF3 derivative was previously empirically shown to be a robust catalyst. Here, the role of the PhCF3 group is probed computationally and the robust nature of this catalyst is analyzed with regard to the presence of water and oxygen introduced in controlled amounts during the photocatalytic reduction of CO2 to CO with visible light. This complex was found to work well from 0–1% water concentration reproducibly; however, trace amounts of water were required for benchmark Re(bpy)(CO)3Cl to give reproducible reactivity. When ambient air is added to the reaction mixture, the NHC complex was found to retain substantial performance (~50% of optimized reactivity) at up to 40% ambient atmosphere and 60% CO2 while the Re(bpy)(CO)3Cl complex was found to give a dramatically reduced CO2 reduction reactivity upon introduction of ambient atmosphere. Through the use of time-correlated single photon counting studies and prior electrochemical results, we reasoned that this enhanced catalyst resilience is due to a mechanistic difference between the NHC- and bpy-based catalysts. These results highlight an important feature of this NHC-ligated catalyst: substantially enhanced stability toward common reaction contaminates.

Properties and prospects for rhenium(i) tricarbonyl N-heterocyclic carbene complexes

Rhenium tricarbonyl complexes bound to N-heterocyclic carbene ligands are emerging as a new class of complexes with promising applications in a wide variety of areas.

Photochemical Processes in a Rhenium(I) Tricarbonyl N-Heterocyclic Carbene Complex Studied by Time-Resolved Measurements

We carried out time-resolved infrared (TR-IR) and emission lifetime measurements on a Re(I) carbonyl complex having an N-heterocyclic carbene ligand, namely, fac-[Re(CO)₃(PyImPh)Br], under photochemically reactive (in solution in acetonitrile) and nonreactive (in solution in dichloromethane) conditions to investigate the mechanism of photochemical ligand substitution reactions. The TR-IR measurements revealed that no reaction occurs on a picosecond time scale and the cationic product, namely, fac-[Re(CO)₃(PyImPh)(MeCN)]⁺, is produced on a nanosecond time scale only in solution in acetonitrile, which indicates that the reaction proceeds thermally from the excited state. Because no other products were observed by TR-IR, we concluded that this cationic product is an intermediate species for further reactions. The measurements of the temperature-dependent emission lifetime and analysis using transition-state theory revealed that the photochemical substitution reaction proceeds from a metal-to-ligand charge transfer excited state, the structure of which allows the potential coordination of a solvent molecule. Thus, the coordinating capacity of the solvent determines whether the reaction proceeds or not. This mechanism is different from those of photochemical reactions of other types of Re(I) carbonyl complexes owing to the unique characteristics of the carbene ligand.

Benzannulated Re(i)–NHC complexes: synthesis, photophysical properties and antimicrobial activity

A series of novel Re(i)(CO)₃–NHC complexes bearing unsubstituted benzimidazol-2-ylidene ligands is presented which provide strong luminescence as well as high antibacterial activity.

Synthesis, characterization, and gas-phase fragmentation of rhenium-carbonyl complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands

Five complexes with the generic formula [ReBr(CO)₃(κ²-S,S'-S₂C·NHC)] were obtained by reacting [ReBr(CO)₅] with a set of representative imidazol(in)ium-2-dithiocarboxylate zwitterions. These ligands are the adducts of N-heterocyclic carbenes (NHCs) and carbon disulfide. The monometallic Re(i) compounds were further coupled with Na[Re(CO)₅] to afford bimetallic Re(0) species. Depending on the experimental conditions, either octacarbonyl dimers [Re₂(CO)₈(μ₂-κ¹-S,κ¹-S'-S₂C·NHC)] or hexacarbonyl clusters [Re₂(CO)₆(κ²-S,S'-κ³-S,C,S'-S₂C·NHC)] were isolated. All the products were fully characterized using various analytical techniques. Single crystal XRD analysis helped establish with certainty the various binding modes exhibited by the NHC·CS₂ ligands. With bite angles ranging from ca. 104 to 130°, these zwitterions displayed a remarkable flexibility, which also permitted significant twists of the thiometallated rings to preserve a staggered arrangement of the carbonyl groups in the bimetallic systems. Monitoring the chemical shift of the CS₂^- moiety by ¹³C NMR spectroscopy was most useful to detect its change of hapticity upon decarbonylation of the octacarbonyl compounds into hexacarbonyl derivatives. IR spectroscopy was another very convenient tool to identify the type of complex formed in a reaction, based on the pattern of its carbonyl vibration bands. Advanced mass spectrometry techniques showed that all the compounds underwent partial or total decarbonylation in the gas phase with no concomitant fragmentation of the bimetallic assemblies into monometallic ions.

Electrocatalytic Reduction of CO2 to CO With Re-Pyridyl-NHCs: Proton Source Influence on Rates and Product Selectivities

A series of four electron-deficient-substituted Re(I) pyridyl N-heterocyclic carbene (pyNHC) complexes have been synthesized, and their electrocatalytic reduction of CO2 has been evaluated by cyclic voltammetry and controlled potential electrolysis experiments. All of the catalysts were evaluated by cyclic voltammetry under inert atmosphere and under CO2 and compared to the known benchmark catalyst Re(bpy)(CO)3Br. Among the four Re-NHC catalysts, Re(pyNHC-PhCF3)(CO)3Br (2) demonstrated the highest catalytic rate (icat/ip)(2) at the first and second reduction events with a value of 4 at the second reduction potential (TOF = 0.8 s(-1)). The rate of catalysis was enhanced through the addition of proton sources (PhOH, TFE, and H2O; TOF up to 100 s(-1); (icat/ip)(2) = 700). Controlled potential electrolysis shows Faradaic efficiencies (FE) for CO production and accumulated charge for the Re(pyNHC-PhCF3)(CO)3Br catalyst exceed those of the benchmark catalyst in the presence of 2 M H2O (92%, 13 C at 1 h versus 61%, 3 C for the benchmark catalyst) under analogous experimental conditions. A peak FE of 100% was observed during electrolysis with Re(pyNHC-PhCF3)(CO)3Br.

Re(I) NHC Complexes for Electrocatalytic Conversion of CO2

The modular construction of ligands around an N-heterocyclic carbene building block represents a flexible synthetic strategy for tuning the electronic properties of metal complexes. Herein, methylbenzimidazolium-pyridine and methylbenzimidazolium-pyrimidine proligands are constructed in high yield using recently established transition-metal-free techniques. Subsequent chelation to ReCl(CO)5 furnishes ReCl(N-methyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3 and ReCl(N-methyl-N'-2-pyrimidylbenzimidazol-2-ylidine)(CO)3. These Re(I) NHC complexes are shown to be capable of mediating the two-electron conversion of CO2 following one-electron reduction; the Faradaic efficiency for CO formation is observed to be >60% with minor H2 and HCO2H production. Data from cyclic voltammetry is presented and compared to well-studied ReCl(2,2'-bipyridine)(CO)3 and MnBr(2,2'-bipyridine)(CO)3 systems. Results from density functional theory computations, infrared spectroelectrochemistry, and chemical reductions are also discussed.

CO2 reduction with Re(i)–NHC compounds: driving selective catalysis with a silicon nanowire photoelectrode

Excellent selectivity was observed in CO₂ reduction using Re(i)–NHC catalysts on a silicon nanowire photoelectrode.

Enhancement of N-heterocyclic carbenes on rhodium catalyzed olefination of triazoles

A few rhodium complexes of N-heterocyclic carbenes were prepared through carbene transfer reactions and their structures were characterized by X-ray diffraction analysis.

Cationic rhenium complexes ligated with N-heterocyclic carbenes – an overview

This review provides an overview of the currently known cationic rhenium NHC complexes.

Photophysical and photochemical studies of tricarbonyl rhenium(i) N-heterocyclic carbene complexes containing azide and triazolate ligands

Rhenium NHC complexes bound to azide anions readily react with alkynes to form the corresponding triazolate complexes, a new class of photochemically active species.

Photocatalytic Reduction of CO2 with Re-Pyridyl-NHCs

A series of Re(I) pyridyl N-heterocyclic carbene (NHC) complexes have been synthesized and examined in the photocatalytic reduction of CO2 using a simulated solar spectrum. The catalysts were characterized through NMR, UV-vis, cyclic voltammetry under nitrogen, and cyclic voltammetry under carbon dioxide. The complexes were compared directly with a known benchmark catalyst, Re(bpy) (CO)3Br. An electron-deficient NHC substituent (PhCF3) was found to promote catalytic activity when compared with electron-neutral and -rich substituents. Re(PyNHC-PhCF3) (CO)3Br was found to exceed the CO production of the benchmark Re(bpy) (CO)3Br catalyst (51 vs 33 TON) in the presence of electron donor BIH and photosensitizer fac-Ir(ppy)3. Importantly, Re(PyNHC-PhCF3) (CO)3Br was found to function without a photosensitizer (32 TON) at substantially higher turnovers than the benchmark catalyst Re(bpy) (CO)3Br (14 TON) under a solar simulated spectrum.

Luminescent rhenium(I) tricarbonyl complexes with pyrazolylamidino ligands: photophysical, electrochemical, and computational studies

New pyrazolylamidino complexes fac-[ReCl(CO)3(NH[double bond, length as m-dash]C(Me)pz*-κ(2)N,N)] (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH) and fac-[ReBr(CO)3(NH[double bond, length as m-dash]C(Ph)pz*-κ(2)N,N)] are synthesized via base-catalyzed coupling of the appropriate nitrile with pyrazole, or via metathesis by halide abstraction with AgBF4 from a bromido pyrazolylamidino complex and the subsequent addition of LiCl. In order to study both the influence of the substituents present at the pyrazolylamidino ligand, and that of the "sixth" ligand in the complex, photophysical, electrochemical, and computational studies have been carried out on this series and other complexes previously described by us, of the general formula fac-[ReL(CO)3(NH[double bond, length as m-dash]C(R')pz*-κ(2)N,N)](n+) (L = Cl, Br; R' = Me, Ph, n = 0; or L = NCMe, dmpzH, indzH, R' = Me, n = 1). All complexes exhibit phosphorescent decays from a prevalently (3)MLCT excited state with quantum yields (Φ) in the range between 0.007 and 0.039, and long lifetimes (τ∼ 8-1900 ns). The electrochemical study reveals irreversible reduction for all complexes. The oxidation of the neutral complexes was found to be irreversible due to halido-dissociation, whereas the cationic species display a reversible process implying the ReI/ReII couple. Density functional and time-dependent density functional theory (TD-DFT) calculations provide a reasonable trend for the values of emission energies in line with the experimental photophysical data, supporting the (3)MLCT based character of the emissions.

Exploring the effect of axial ligand substitution (X = Br, NCS, CN) on the photodecomposition and electrochemical activity of [MnX(N-C)(CO)3] complexes

The synthesis, electrochemical activity, and relative photodecomposition rate is reported for four new Mn(i) N-heterocyclic carbene complexes: [MnX(N-ethyl-N'-2-pyridylimidazol-2-ylidine)(CO)3] (X = Br, NCS, CN) and [MnCN(N-ethyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3]. All compounds display an electrocatalytic current enhancement under CO2 at the potential of the first reduction, which ranges from -1.53 V to -1.96 V versus the saturated calomel electrode. Catalytic CO production is observed for all species during four-hour preparative-scale electrolysis, but substantial H2 is detected in compounds where X is not Br. All species eventually decompose under both 350 nm and 420 nm light, but cyanide substituted complexes (X = CN) last significantly longer (up to 5×) under 420 nm light as a result of a blue-shifted MLCT band.