Tricarbonyl Diimine Complexes Research Articles

Manganese Tricarbonyl Diimine Bromide Complexes as Electrocatalysts for Proton Reduction.

Manganese tricarbonyl diimine complexes bearing pyridine and imidazole ligands have been prepared as electrocatalysts for proton reduction using acetic acid as the proton source. The electron-donor ability of the diimine ligand is found to play an important role in determining the efficiency of the electrocatalysts with [MnBr(pybz)(CO)3] (pybz = 2-(2-pyridyl)benzimidazole) exhibiting the lowest overpotential (0.28 V) toward proton reduction. The [Mn(pybz)(CO)3(MeCN)]+ cationic complex prepared via debromination of [MnBr(pybz)(CO)3] by a silver salt has also been shown to catalyze proton reduction upon its electrochemical reduction. A neutral complex [Mn(pyridine-benzimidazolate)(CO)3(MeCN)], which can be synthesized by reacting [MnBr(pybz)(CO)3] with a strong base, has been detected using IR-SEC (infrared spectroelectrochemistry) as an intermediate species in the catalytic process. Using [MnBr(pybz)(CO)3] as the model electrocatalyst, we have carried out density functional calculations to propose a proton reduction mechanism consistent with our experimental observations.

Antimicrobial Activity of Rhenium Di- and Tricarbonyl Diimine Complexes: Insights on Membrane-Bound S. aureus Protein Binding.

Antimicrobial resistance is one of the major human health threats, with significant impacts on the global economy. Antibiotics are becoming increasingly ineffective as drug-resistance spreads, imposing an urgent need for new and innovative antimicrobial agents. Metal complexes are an untapped source of antimicrobial potential. Rhenium complexes, amongst others, are particularly attractive due to their low in vivo toxicity and high antimicrobial activity, but little is known about their targets and mechanism of action. In this study, a series of rhenium di- and tricarbonyl diimine complexes were prepared and evaluated for their antimicrobial potential against eight different microorganisms comprising Gram-negative and -positive bacteria. Our data showed that none of the Re dicarbonyl or neutral tricarbonyl species have either bactericidal or bacteriostatic potential. In order to identify possible targets of the molecules, and thus possibly understand the observed differences in the antimicrobial efficacy of the molecules, we computationally evaluated the binding affinity of active and inactive complexes against structurally characterized membrane-bound S. aureus proteins. The computational analysis indicates two possible major targets for this class of compounds, namely lipoteichoic acids flippase (LtaA) and lipoprotein signal peptidase II (LspA). Our results, consistent with the published in vitro studies, will be useful for the future design of rhenium tricarbonyl diimine-based antibiotics.

Photocatalytic Reduction of CO2 to CO in Aqueous Solution under Red-Light Irradiation by a Zn-Porphyrin-Sensitized Mn(I) Catalyst.

This work demonstrates photocatalytic CO2 reduction by a noble-metal-free photosensitizer-catalyst system in aqueous solution under red-light irradiation. A water-soluble Mn(I) tricarbonyl diimine complex, [MnBr(4,4′-{Et2O3PCH2}2-2,2′-bipyridyl)(CO)3] (1), has been fully characterized, including single-crystal X-ray crystallography, and shown to reduce CO2 to CO following photosensitization by tetra(N-methyl-4-pyridyl)porphyrin Zn(II) tetrachloride [Zn(TMPyP)]Cl4 (2) under 625 nm irradiation. This is the first example of 2 employed as a photosensitizer for CO2 reduction. The incorporation of −P(O)(OEt)2 groups, decoupled from the core of the catalyst by a −CH2– spacer, afforded water solubility without compromising the electronic properties of the catalyst. The photostability of the active Mn(I) catalyst over prolonged periods of irradiation with red light was confirmed by 1H and 13C{1H} NMR spectroscopy. This first report on Mn(I) species as a homogeneous photocatalyst, working in water and under red light, illustrates further future prospects of intrinsically photounstable Mn(I) complexes as solar-driven catalysts in an aqueous environment.

Combatting AMR: A molecular approach to the discovery of potent and non-toxic rhenium complexes active against C. albicans-MRSA co-infection

Antimicrobial resistance (AMR) is a major emerging threat to public health, causing serious issues in the successful prevention and treatment of persistent diseases. While the problem escalates, lack of financial incentive has lead major pharmaceutical companies to interrupt their antibiotic drug discovery programs. The World Health Organisation (WHO) has called for novel solutions outside the traditional development pathway, with emphasis on new classes of active compounds with non-classical mechanisms of action. Metal complexes are an untapped source of antibiotic potential owing to unique modes of action and a wider range of three-dimensional geometries as compared to purely organic compounds. In this study, we present the antimicrobial and antifungal efficacy of a family of rhenium tricarbonyl diimine complexes with varying ligands, charge and lipophilicity. Our study allowed the identification of potent and non-toxic complexes active in vivo against S. aureus infections at MIC doses as low as 300 ng/mL, as well as against C. albicans-MRSA mixed co-infection. The compounds are capable of suppressing the C. albicans morphogenetic yeast-to-hyphal transition, eradicating fungal-S. aureus co-infection, while showing no sign of cardio-, hepato-, hematotoxiciy or teratogenicity.

The effect of group-substitution on the sensitization properties of alkynylrhenium(I) tricarbonyl diimine complexes adsorbed to TiO2(101) film surface: a theoretical study.

A series of dyes are designed by adding the different electron-donating (-CH3, -NH2, -OH) and electron-withdrawing groups (-Br, -Cl, -NO2) to the different ancillary ligands in the alkynylrhenium(I) tricarbonyl diimine complexes [Re(CO)3(N^N){C≡C-C6H4-CH=C(CN)(COOH)}], where N^N = 1,10-phenanthroline (phen)(1) and then investigated the sensitization properties of dyes linked to the TiO2(101) surface. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to study the electronic structure, frontier molecular orbitals, and absorption spectral properties. The effect of group-substitution on sensitization properties is obvious. When the dye molecules are combined with TiO2(101) surface, not only the absorptions of some sensitizers containing -CH3 or -OH groups have red shift but also the electrons can be directly injected into the TiO2 conduction band from the dye molecules compared with the parent molecular 1. The results indicate that the designed dyes containing electron-donating groups have smaller energy gaps, better light-harvesting efficiency, sufficient driving force, and higher charge transfer efficiency as appropriate dye sensitizers. We hope it can provide valuable hints so that we can design more efficient dye sensitizers in DSSCs.

Synthesis and Photochemical Properties of Manganese(I) Tricarbonyl Diimine Complexes Bound to Tetrazolato Ligands

Ten manganese(I) tricarbonyl diimine complexes bound to variably functionalised 5‐aryl‐tetrazolato ligands were prepared, and their photochemical properties were investigated. Upon exposure to light at 365 nm, each complex decomposed to its free diimine and tetrazolato ligands, simultaneously dissociating three CO ligands, as evidenced by changes in the IR spectra of the irradiated complexes over time. The anti‐bacterial properties of one of these complexes were tested against Escherichia coli. While the complex displayed no effect on the bacterial growth in the dark, pre‐irradiated solutions inhibited bacterial growth. Comparative studies revealed that the antibacterial properties originate from the presence of free 1,10‐phenanthroline.

Synthesis, characterization, photophysical and electrochemical properties of rhenium(I) tricarbonyl diimine complexes with triphenylphosphine ligand

In this work, the compounds fac-[ReL(CO)3(NN)]0/+1, NN=1,10-phenanthroline, phen, 4,7-dimethyl-1,10-phenanthroline, Me2phen, or 4,7-dimethoxy-1,10-phenanthroline, (MeO)2phen, and L=Cl− or triphenylphosphine, PPh3, were synthetized and characterized by 1H NMR, UV–vis and FTIR spectroscopies and their photophysical properties were investigated by steady-state and time-resolved emission spectroscopies. All complexes displayed strong absorption in the high energy UV-region assigned to intraligand transitions, ILNN, and the lower energy band was ascribed to a metal-to-ligand charge transfer, MLCTRe→NN. The two electron donor groups attached to phen ligand shifted the emission maxima of the complexes to higher energy as the substituent becomes more electron donor, consequently the contribution of the 3ILNN on the 3MLCT emission becomes more evident. This effect is enhanced by coordinating the electron-withdrawing PPh3 ligand on Re(I) complexes (λmax=515nm, ϕ=13%, τ=2.52μs for phen; λmax=512nm, ϕ=8.2%, τ=2.00μs for Me2phen; λmax=478 and 506nm, ϕ=1.1%, τ=1.12μs for (MeO)2phen). This interference is supported by the small shift of the emission maxima and two lifetimes in the μs domain observed for substituted-phen complexes in PMMA films. The photophysics of Re(I) compounds reported herein provides new insights into the understanding of electron-donor groups on the phen ligand that are relevant to practical and fundamental development of photoinduced molecular devices.

Synthesis, characterization and computational studies of luminescent rhenium(I) tricarbonyl diimine complexes with 8-hydroxyquinoline-containing alkynyl ligands

A series of 8-hydroxyquinoline-containing alkynylrhenium(I) tricarbonyl diimine complexes has been designed and synthesized. Their UV-vis absorption, emission as well as electrochemical properties have been studied. Computational studies have also been performed to provide insights into the electronic transitions, excited state origins and electrochemical properties of the complexes.

Synthesis and characterization of alkynylrhenium(I) tricarbonyl diimine complexes with fused thiophene and cyanoacrylic acid moiety

A series of alkynylrhenium(I) tricarbonyl diimine complexes with fused thiophene and cyanoacrylic acid moiety has been synthesized and characterized. A long-lived charge separated state of these complexes have been found. Preliminary studies to explore the utilization of these complexes as photosensitizers have also been made, which show photoresponse upon light illumination.

Synthesis, characterization and crystal structure of rhenium(I) tricarbonyl diimine complexes coupled with their efficiency in producing hydrogen in a photocatalytic system

Herein, we present the synthesis and characterization of two complexes, namely [ReBr(CO)3amphen] (1) (amphen=1,10-phenanthroline-5-amine) and [ReBr(CO)3pq] (2) (pq=2-(2′pyridyl)quinoxaline); complex 2 is reported for the first time. The X-ray crystal structure of 2 has been determined. Electrochemical and photophysical studies have been performed to study the effect of the variation of the diimine ligand on the redox and optical properties of this class of compounds. Both complexes were used as photosensitizers for hydrogen production in a homogeneous photocatalytic system also carrying [Co(dmgH)2] as a hydrogen reaction catalyst, triethanolamine as an irreversible reductive quencher and AcOH as proton source. Complex 1 produces H2 after only 2h irradiation while complex 2 requires 24h indicating that the differences in the electronic properties influence their photosensitizing ability.

Clean Singlet Oxygen Production by a ReI Complex Embedded in a Flexible Self-Standing Polymeric Silsesquioxane Film

Rhenium complexes are versatile molecular building blocks whose tunable photophysical properties are useful in diverse opto-related applications. Herein we report the synthesis and characterization of a novel ReI tricarbonyldiimine complex, [(phen)Re(CO)3Br] (phen: 1,10-phenanthroline), which was found to be an efficient singlet oxygen [O2(1Δg)] photosensitizer in homogeneous solution [ΦO2(1Δg) = 0.55 (dichloromethane) and 0.16 (dimethylformamide)]. The photophysical properties of [(phen)Re(CO)3Br] were thoroughly characterized in solution and modeled by means of density functional theory (DFT) and time-dependent (TD)-DFT quantum mechanical calculations. The Re complex was incorporated into a flexible polymeric silsesquioxane (SSO) film, which has excellent dopant compatibility, chemical resistance, and mechanical properties. When [(phen)Re(CO)3Br] is embedded in the SSO film, it is found to retain most of the photophysical characteristics observed for the complex in solution. In particular, the [(phen)Re...

Bichromophoric rhodamine-rhenium(I) and -iridium(III) sensory system: Synthesis, characterizations, photophysical and selective metal ions binding studies

A new class of rhenium(I) tricarbonyl diimine complexes containing rhodamine derivative as a sensory moiety, have been synthesized and characterized. Their photophysical and selective ion-binding properties have also been investigated, with the comparison of the previously reported cyclometalated iridium(III) system. One of the complex was found to exhibit selective binding toward Hg(II) ion with electronic absorption and emission spectral changes. On the other hand, the iridium(III) complexes were found to give similar electronic absorption response but different emission spectral changes upon addition of Hg(II) ion. Interestingly, efficient intramolecular energy transfer from rhodamine 6G to the cyclometalated iridium(III) moiety was suggested in one of the complexes, which could be modulated by selective ion-binding process upon ring-opening of rhodamine 6G. This report shows that the modification of rhodamine derivatives and the choice of transition metal ion could be important to influence the selective binding properties for development of colorimetric and/or luminescent probe.

DFT study of potential energy surfaces and conical intersection structures of Rhenium(I) tricarbonyl diimine complexes

Potential energy surfaces of the ground and lowest excited states of [Re(CO)3(phen)L]q (L=imH, q=+1; L=Cl, q=0) complexes were investigated by TDDFT methods in gas phase and solvent. Two (in gas phase) or three (in solvent) tautomeric structures, differing by the angle of rotation of the imidazole ligand around the Re–N bond were found for [Re(CO)3(imH)(phen)]+. Excited state minima of the lowest singlet and triplet states were found, and luminescence energies were calculated. 11 conical intersection (CI) structures in total were obtained. Three common types of CI’s were found both for [Re(CO)3(imH)(phen)]+ and [Re(CO)3(Cl)(phen)], and additional CI type was found only for [Re(CO)3(imH)(phen)]+. Every CI type of [Re(CO)3(imH)(phen)]+ corresponds to two CI structures, differing by the angle of rotation of the imH ligand.

Synthesis, characterization, electrochemistry and photophysical studies of rhenium(I) tricarbonyl diimine complexes with carboxaldehyde alkynyl ligands

A class of rhenium(I) tricarbonyl diimine complexes with carboxaldehyde alkynyl ligands, [Re(CO)3(N^N)(CC–C6H4–CHO)] (N^N=α,α′-diimine ligand), has been successfully synthesized and characterized, and the X-ray crystal structure of one of the complexes has been determined. Electrochemical and photophysical studies have been performed to study the effect of the variation of the diimine ligand on the redox and optical properties of the rhenium(I) complexes.

CO2 Capture by a Rhenium(I) Complex with the Aid of Triethanolamine

A rhenium(I) tricarbonyl diimine complex with a N,N-dimethylformamide ligand captures one CO2 molecule in the presence of triethanolamine (TEOA), giving fac-[Re(I)(bpy)(CO)3{R2N-CH2CH2O-COO}] (bpy = 2,2'-bipyridine, R = CH2CH2OH). This could be a predominant complex in various photocatalytic CO2 reduction reactions using [Re(I)(N^N)(CO)3X](n+) (N^N = diimine ligand; X = monodentate ligand; n = 0, 1) type complexes in the presence of TEOA.

Proton-Coupled Electron Transfer between 4-Cyanophenol and Photoexcited Rhenium(I) Complexes with Different Protonatable Sites

Two rhenium(I) tricarbonyl diimine complexes, one of them with a 2,2'-bipyrazine (bpz) and a pyridine (py) ligand in addition to the carbonyls ([Re(bpz)(CO)(3)(py)](+)), and one tricarbonyl complex with a 2,2'-bipyridine (bpy) and a 1,4-pyrazine (pz) ligand ([Re(bpy)(CO)(3)(pz)](+)) were synthesized, and their photochemistry with 4-cyanophenol in acetonitrile solution was explored. Metal-to-ligand charge transfer (MLCT) excitation occurs toward the protonatable bpz ligand in the [Re(bpz)(CO)(3)(py)](+) complex while in the [Re(bpy)(CO)(3)(pz)](+) complex the same type of excitation promotes an electron away from the protonatable pz ligand. This study aimed to explore how this difference in electronic excited-state structure affects the rates and the reaction mechanism for photoinduced proton-coupled electron transfer (PCET) between 4-cyanophenol and the two rhenium(I) complexes. Transient absorption spectroscopy provides clear evidence for PCET reaction products, and significant H/D kinetic isotope effects are observed in some of the luminescence quenching experiments. Concerted proton-electron transfer is likely to play an important role in both cases, but a reaction sequence of proton transfer and electron transfer steps cannot be fully excluded for the 4-cyanophenol/[Re(bpz)(CO)(3)(py)](+) reaction couple. Interestingly, the rate constants for bimolecular excited-state quenching are on the same order of magnitude for both rhenium(I) complexes.

Electronic structures and spectral properties of rhenium(I) tricarbonyl diimine complexes with phosphine ligands: DFT/TDDFT theoretical investigations

The ground and excited state geometries and frontier molecular orbital, absorption and emission properties of the four transition metal complexes Re(bpy)(CO)3(PEt3)+ (1), Re(bpy)(CO)3(PPh3)+ (2), Re(bpy)(CO)3[P(OEt3)]+ (3) and Re(bpy)(CO)3[P(OPh3)]+ (4) were studied theoretically. The introduction of different phosphine ligands significantly changed the geometric parameters of the complexes. The axial Re–C bond distance was longer than the equatorial Re–C bond distance. The lowest lying absorption bands of 1 and 2 have HOMO→LUMO transition configurations resulting in MLCT/LLCT transition characteristics. The HOMO-1→LUMO transition predominantly contributed to the lowest absorption bands of 3 with MLCT/LLCT characteristics. The absorption band of 4 at 326nm was dominated by HOMO-4→LUMO and HOMO-6→LUMO transitions with mixed MLCT/LLCT characteristics. The phosphorescence emission energies of 1–4 were also calculated and compared with the experimental values.

Synthesis, structure, photophysics and electrochemiluminescence of Re(i) tricarbonyl complexes with cationic 2,2-bipyridyl ligands

Two water soluble Re(I) tricarbonyl diimine complexes containing cationic 2,2'-bipyridyl ligands [Re(L1)(CO)(3)(AN)](2+) (1) and [Re(L2)(CO)(3)(AN)](3+) (2) (L1 and L2: a cationic 2,2'-bipyridyl ligand, AN: acetonitrile) were synthesized and characterized. Their photophysical, electrochemical and electrochemiluminescent properties were investigated. The crystal structures of the two complexes have also been determined. Electrochemiluminescence (ECL) of the two complexes 1 and 2 have been studied in aqueous buffer solution in the presence of co-reactant tri-n-propylamine (TPrA) or 2-(dibutylamino)ethanol (DBAE) at a Au working electrode. The ECL behavior of the complexes was also studied in the presence of several surfactants such as Triton X-100 and Zonyl FSN. The ECL signals of the rhenium(I) complex were enhanced about 190-fold and 70-fold at a Au electrode in the presence of Triton X-100 for the [Re(L1)(CO)(3)(AN)](2+)/TPrA and [Re(L1)(CO)(3)(AN)](2+)/DBAE systems, respectively.

A DFT exploration of luminescent rhenium(I) tricarbonyl diimine complex with a triarylboron moiety and its F derivative

The geometry, electronic structure, and spectroscopic properties together with the fluoride binding nature of the luminescent rhenium(I) tricarbonyl diimine complex with a triarylboron moiety, [(arB)Re(CO)3(phen)] were investigated by density functional theory (DFT) approach, where, arB = (2,3,5,6-tetramethyl-4-ethynyl-benzene) 3B, phen = 1,10-phenanthroline), The simulated UV–vis absorptions by M06 functional are in excellent agreement with the experimental results. All absorptions are attributed to intra-ligand charge-transfer (ILCT) within the triarylboron ligand but being disturbed by some contribution of metal-to-arylboron ligand charge-transfer (MLCT) characters. Moreover, the 676 nm phosphorescence computed by TPSSh functional originates from the 3LLCT/ 3LMCT transition. Binding the F − results in the re-configuration of the molecular geometry and the variation of the optical properties. The highest energy absorption band is intensified while the lower energy absorption bands are weakened and blue-shifted slightly. The emission maximum is red-shifted to 875 nm after binding the F −.

Photoswitching tetranuclear rhenium(i) tricarbonyl diimine complexes with a stilbene-like bridging ligand

Highly efficient photoswitching tetranuclear rhenium(I) tricarbonyl diimine complexes with a stilbene-like bridging ligand are reported. The ability to directly populate excited states localized on the bridging ligand is the key for the observed efficient photoisomerization.