Preparation and characterization of [Re(bpy)(CO) 3L][SbF6] (L = phosphine, phosphite)

Antimicrobial and anticancer properties of carbon monoxide releasing molecules of the fac-[Re(CO)3(N-N)L]+ family

The toxicity profile of fac-[Re(CO)3(N-N)L]+ complexes against microbial and tumoral cells has been extensively studied, primarily focusing on modifications to the bidentate diimine (N-N) ligand. However, less attention has been paid to modifications of the axial ligand L, which is perpendicular to the Re-N-N plane. This study reveals that the high toxicity of the fac-[Re(CO)3(bpy)(Ctz)]+ complex may be attributed to the structural effect of the trityl (CPh3) group present in clotrimazole, as removal of phenyl rings causes a significant decrease in the activity against Staphylococcus aureus (S. aureus). Moreover, substitution of the 1-tritylimidazole ligand by the structurally related ligands PPh3 and PCy3 maintains similarly high activity levels. These findings contribute to understanding the interactions of toxic complexes with bacterial membranes, suggesting that the ligand structures play a crucial role in inhibiting cell wall synthesis processes, potentially including Lipid II synthesis. Compounds with Ph3E (E = C-imidazole; P) groups also showed to be 10 times more toxic than cisplatin against three mammalian cell lines (IC50: 2-4 μM). In contrast, the analogue 1-benzylimidazole and 1-tert-butylimidazole derivatives were as toxic as cisplatin. We observed that the decomposition of the [Re(I)(CO)3] fragment inside mammalian cell lines liberates CO, which is expected to exert biological effects. Therefore, compounds of this family possessing the structural motif Ph3E seem to combine high antimicrobial and antitumoral activities, the latter being much higher than that of cisplatin.

Reaction Chemistry at Discrete Organometallic Fragments on Black Phosphorus

Black phosphorus (bP) is a two-dimensional van der Waals material unique in its potential to serve as a support for single-site catalysts due to its similarity to molecular phosphines, ligands quintessential in homogeneous catalysis. However, there is a scarcity of synthetic methods to install single metal centers on the bP lattice. Here, we demonstrate the functionalization of bP nanosheets with molecular Re and Mo complexes. A suite of characterization techniques, including infrared, X-ray photoelectron and X-ray absorption spectroscopy as well as scanning transmission electron microscopy corroborate that the functionalized nanosheets contain a high density of discrete metal centers directly bound to the bP surface. Moreover, the supported metal centers are chemically accessible and can undergo ligand exchange transformations without detaching from the surface. The steric and electronic properties of bP as a ligand are estimated with respect to molecular phosphines. Sterically, bP resembles tri(tolyl)phosphine when monodentate to a metal center, and bis(diphenylphosphino)propane when bidentate, whereas electronically bP is a σ-donor as strong as a trialkyl phosphine. This work is foundational in elucidating the nature of black phosphorus as a ligand and underscores the viability of using bP as a basis for single-site catalysts.

Electrochromic behavior of fac-tricarbonyl rhenium complexes

Tricarbonyl rhenium complex shows good electrochromic performance with a colored stage of green, rapid response and good switching stability.

Photocatalytic CO2 reduction using metal complexes in various ionic liquids

Aiming to diversify photocatalytic systems for CO2 reduction using metal complexes, this study investigated the use of various ionic liquids as reaction solvents. The photophysical properties of an Ir(iii) complex, functioning as a photosensitiser, and the photocatalytic ability of mixed systems consisting of the Ir(iii) photosensitiser and a Re(i) catalyst in twelve kinds of ionic liquids were systematically investigated by comparison with those in N,N-dimethylacetamide (DMA), which is a standard solvent for photocatalytic CO2 reduction. Even though the photophysical properties of the Ir(iii) complex in ionic-liquid solutions were quite similar to those in DMA, both the photosensitising ability of the Ir complex and the photocatalytic abilities of the systems strongly depended on the structures of the ionic liquids. Several ionic liquids were successfully used as new solvents for the photocatalytic systems showing durability similar to or higher than DMA solutions. The results demonstrated that even a small modification of the molecular structures of ionic liquids can control the efficiencies of both the photosensitising cycles and the catalytic cycles for CO2 reduction.

Tracking CO release in cells via the luminescence of donor molecules and/or their by-products

Carbon monoxide (CO) is a bioactive signalling molecule that is produced endogenously via the breakdown of heme. Beneficial health effects associated with the delivery of CO gas have spurred the development of CO-releasing molecules (CORMs) that can be used to provide specific amounts of the gas. In addition to their potential use as therapeutics, CORMs are needed to provide insight into the biological targets of CO. In this regard, light-activated CO-releasing molecules (photoCORMs), are valuable for examining the effects of localized CO release. Herein we examine luminescent CORMs and photoCORMs that have been reported for tracking CO delivery in cells. A variety of motifs are available that exhibit differing luminescence properties and cover a wide range of wavelengths. Trackable CO donors have been successfully applied to targeting CO delivery to mitochondria, thus demonstrating the feasibility of using such molecules in detailed investigations of the biological roles of CO.

Synthesis of Re(I) Rings Comprising Different Re(I) Units and Their Light-Harvesting Abilities

Trimethylamine N-oxide (Me₃NO) could selectively remove only one CO ligand from fac-[Re(N^N)(CO)₃(PR₂R')]⁺ (N^N = diimine ligand), whereby only the CO ligand in the trans position to the phosphorus ligand was selectively removed to give cis,trans-[Re^I(N^N)(CO)₂(PR₂R')(L)] ⁿ⁺ in good yields. This decarbonylation reaction using Me₃NO was found to be especially useful for synthesizing biscarbonyl Re(I) complexes with electron-withdrawing groups in the diimine ligand, which could not be synthesized or were obtained only in low yields by the photochemical method. Me₃NO also selectively removed the carbonyl ligands in the trans position to the phosphorus ligands from the edge Re(I) complex units, which have the fac-[Re(N^N)(CO)₃(PR₂R')]⁺ structure, in linear-shaped Re(I) multinuclear complexes. This reaction was successfully applied to synthesize a novel precursor with ring-shaped multinuclear Re complexes (Re-rings) comprising different kinds of Re(I) units. The newly synthesized Re-rings, which consist of one Re unit with a 4,4'-bis(trifluoromethyl)-2,2'-bipyridine (CF₃bpy) ligand and one or two Re unit(s) with a 2,2'-bipyridine (bpy) ligand, showed almost quantitative excitation-energy harvesting ability from the Re unit(s) with bpy to that with CF₃bpy.

Covalent-Organic Frameworks Composed of Rhenium Bipyridine and Metal Porphyrins: Designing Heterobimetallic Frameworks with Two Distinct Metal Sites

The incorporation of homogeneous catalysts for CO₂ reduction into extended frameworks has been a successful strategy for increasing catalyst lifetime and activity, but the effects of the linkers on catalysis are underexplored. In this work, a novel rhenium bipyridine complex was synthesized for the purpose of designing a covalent-organic framework (COF) with both metalloporphyrin and metal bipyridine moieties. Investigation of the rhenium complex as a homogeneous catalyst shows a faradaic efficiency of 81(8)% for the electrocatalytic conversion of CO₂ to CO upon the addition of methanol as the proton source. Treatment of the rhenium complex with tetra(4-aminophenyl)porphyrin under Schiff base conditions produces the desired COF, as indicated by powder X-ray diffraction (PXRD) studies. Metalation of the porphyrins was accomplished through postsynthetic modification with CoCl₂ and FeCl₃ metal precursors. The retention of the PXRD peaks and appearance of new Co and Fe peaks in the corresponding X-ray photoelectron spectroscopy spectra suggest the successful incorporation of a secondary metal site into the framework. Cyclic voltammetry measurements display increases in current densities when the atmosphere is changed from N₂ to CO₂. Controlled potential electrolyses show that the cobalt-postmetalated COF has the highest activity toward CO₂ reduction, reaching a faradaic efficiency of 18(2)%.

Rhenium tricarbonyl complexes with arenethiolate axial ligands

Pyta and Tapy-based [Re(N^N)(CO)₃X] complexes with para-substituted benzenethiolates as axial ligand are reported along with their electrochemical and photophysical properties.

Modulation of the Photophysical, Photochemical, and Electrochemical Properties of Re(I) Diimine Complexes by Interligand Interactions

The photophysical and photochemical properties of transition metal complexes have attracted considerable attention because of their recent applications as photocatalysts in artificial photosynthesis and organic synthesis, as light emitters in electroluminescent (EL) devices, and as dyes in solar cells. The general control methods cannot be always used to obtain transition metal complexes with photochemical properties that are suitable for the above-mentioned applications. In the fields of solar energy conversion, strong metal-to-ligand charge-transfer (MLCT) absorption of redox photosensitizers and/or photocatalysts in the visible region with long wavelength is essential. However, the usual methods, i.e., introduction of electron-withdrawing groups into the electron-accepting ligand and/or weak-field ligands into the central metal, have several drawbacks, including shorter excited-state lifetime, lower emission efficiency, and lower oxidation and reduction power. Herein we describe a new method to control the photophysical, photochemical, and electrochemical properties of Re(I) diimine carbonyl complexes that have been widely used in various fields such as photocatalysts for CO₂ reduction and emitters in EL devices and sensors. This method involves the introduction of interligand interactions (π-π and CH-π interactions) into the Re(I) complexes; the aromatic diimine ligand coordinating to the Re center approaches the aryl groups on the phosphine ligand or ligands at the cis position, which "compulsorily" induces a weak interaction between these aromatic groups. As a result of this interligand interaction, the Re complexes with the aromatic diimine ligand and the arylphosphine ligand(s) exhibit red-shifted ¹MLCT absorption but afford blue-shifted emission from the triplet metal-to-ligand charge-transfer (³MLCT) excited state. This increases the oxidation power and lifetime of the ³MLCT excited state. These unique property changes are favorable, particularly for redox photosensitizers. The interligand interaction is strongly expressed by the ring-shaped multinuclear Re(I) complexes (Re-rings). In the case of Re-rings with high steric hindrance due to a small inner cavity, the lifetime of the ³MLCT excited state is up to 8 μs and the emission quantum yield is up to 70%. These properties cannot be obtained by the corresponding mononuclear Re(I) complexes, which generally exhibit shorter lifetimes (<1 μs) and lower emission quantum yields (<10%). Some of the Re-rings could be successfully applied as efficient photosensitizers in photocatalytic systems for CO₂ reduction; the highest quantum yields for CO₂ reduction were achieved by using photocatalytic systems composed of Re-rings as the photosensitizers and Re(I) (82%), Ru(II) (58%), and Mn(I) (48%) complexes as catalysts. This interligand interaction potentially provides unique and useful methods for controlling the photophysical, photochemical, and electrochemical functions of various metal complexes, paving the way to create new functions for metal complexes.

Theoretical study on mechanism of the photochemical ligand substitution of fac-[ReI(bpy)(CO)3(PR3)]+ complex

The mechanism of the CO ligand dissociation of fac-[Re^I(bpy)(CO)₃P(OMe)₃]⁺ has theoretically been investigated, as the dominant process of the photochemical ligand substitution (PLS) reactions of fac-[Re^I(bpy)(CO)₃PR₃]⁺, by using the (TD-)DFT method.

Synthesis, photophysical properties and application in organic light emitting devices of rhenium(i) carbonyls incorporating functionalized 2,2′:6′,2′′-terpyridines

Photophysics of [ReCl(CO)₃(4′-R-terpy-κ²N)].

Photocatalytic reduction of CO₂: from molecules to semiconductors

We are facing three serious problems related to fossil resources, i.e., shortage of energy, shortage of carbon resources, and the global worming problem. Development of practical systems for converting CO₂ to useful chemicals using solar light, i.e., photocatalytic CO₂ reduction systems, should be one of the best solutions for these problems. In this article, we review photocatalytic CO₂ reduction systems, which are classified in two categories: (1) homogeneous reaction systems mainly using transition metal complexes, and (2) heterogeneous systems mainly using inorganic semiconductor as a light absorber.

Control of Photochemical, Photophysical, Electrochemical, and Photocatalytic Properties of Rhenium(I) Complexes Using Intramolecular Weak Interactions between Ligands

Intramolecular interactions between ligands have been successfully applied as a novel tool for controlling various properties of a series of cis,trans-[Re(dmb)(CO)(2)(PR(3))(PR'(3))](+)-type complexes (dmb = 4,4'-dimethyl-2,2'-bipyridine), in the ground state and in the excited state and in the one-electron reduced form. For rhenium complexes with two triarylphosphine ligands, P(p-XPh)(3), the dmb ligand was sandwiched by four aryl rings having CH(aryl)-pi(pyridine)-pi(aryl) interactions. On the other hand, complexes with one triarylphosphine ligand and one trialkylphosphite ligand, P(OR)(3), had pi-pi and CH-pi interactions between each pyridine ring in the dmb ligand and the aryl group in the P(p-XPh)(3). Various properties of these two series of rhenium complexes were compared with those of complexes having two trialkylphosphite ligands, which do not interact through space with the dmb ligand. Properties of the complexes associated mainly with the dmb ligand are strongly affected by the intramolecular interactions: (1) UV/vis absorptions to the pi-pi and (1)MLCT excited states were both red-shifted, but (2) emission from the (3)MLCT excited state was blue-shifted; (3) the lifetime of the (3)MLCT excited state was prolonged up to 3-fold; (4) the reduction potential in the ground state was positively shifted by 110 mV with pi-pi and CH-pi interactions and by 180-200 mV with the CH-pi-pi interactions. (5) In the excited states, the oxidation power of the complex was also enhanced by the intramolecular interactions. (6) In the corresponding one-electron-reduced species cis,trans-[Re(dmb(-.)(CO)(2)(PR(3))(PR'(3))], the intramolecular interactions are maintained and strongly affected their UV/vis spectra. (7) Photocatalysis for CO(2) reduction was significantly enhanced only by the CH-pi-pi interaction.

Direct Tyrosine Oxidation Using the MLCT Excited States of Rhenium Polypyridyl Complexes

Rhenium(I) polypyridyl complexes have been designed for the intramolecular photogeneration of tyrosyl radical. Tyrosine (Y) and phenylalanine (F) have each been separately appended to a conventional Re(I)(bpy)(CO)(3)CN framework via an amide linkage to the bipyridine (bpy) ligand. Comparative time-resolved emission quenching and transient absorption spectra of Re(bpy-Y)(CO)(3)CN and Re(bpy-F)(CO)(3)CN show that Y is oxidized only upon its deprotonation at pH 12. In an effort to redirect electron transport so that it is more compatible with intramolecular Y oxidation, polypyridyl Re(I) complexes have been prepared with the amide bond functionality located on a pendant phosphine ligand. A [Re(phen)(PP-Bn)(CO)(2)](PF(6)) (PP = bis(diphenylphosphino)ethylene) complex has been synthesized and crystallographically characterized. Electrochemistry and phosphorescence measurements of this complex indicate a modest excited-state potential for tyrosine oxidation, similar to that for the (bpy)Re(I)(CO)(3)CN framework. The excited-state oxidation potential can be increased by introducing a monodentate phosphine to the Re(I)(NN)(CO)(3)(+) framework (NN = polypyridyl). In this case, Y is oxidized at all pHs when appended to the triphenylphosphine (P) of [Re(phen)(P-Y)(CO(3))](PF(6)). Analysis of the pH dependence of the rate constant for tyrosyl radical generation is consistent with a proton-coupled electron transfer (PCET) quenching mechanism.

Effect of intramolecular π–π and CH–π interactions between ligands on structure, electrochemical and spectroscopic properties of fac-[Re(bpy)(CO)3(PR3)]+(bpy = 2,2′-bipyridine; PR3= trialkyl or triarylphosphines)

Intramolecular pi-pi and CH-pi interactions between the bpy and PR3 ligands of fac-[Re(bpy)(CO)3(PR3)]+ affect their structure, and electrochemical and spectroscopic properties. Intramolecular CH-pi interaction was observed between the alkyl groups on the phosphine ligand (R =nBu, Et) and the bpy ligand, and intramolecular pi-pi and CH-pi interactions were both observed between the aryl group(s) on the phosphorus ligand (R =p-MeOPh, p-MePh, Ph, p-FPh, OPh) and the bpy ligand, while no such interactions were found in the trialkylphosphite complexes (R = OiPr, OEt, OMe). The intramolecular interactions distort the pyridine rings of the bpy ligand as long as 3.7 x 10(-2)A in crystals. Molecular orbital calculations of the bpy ligand suggest that this distortion decreases the energy gap between its pi and pi* orbitals. An absorption band attributed to the pi-pi*(bpy) transition of the distorted rhenium complexes, measured in a KBr pellet, was red-shifted by 1-5 nm compared to the complexes without the distorted bpy ligand. Even in solution, similar red shifts of the pi-pi*(bpy) absorption were observed. The redox potential E1/2(bpy/bpy*-) of the complexes with the trialkylphosphine and triarylphosphine ligand are shifted positively by 110-120 mV and 60-80 mV respectively, compared with those derived from the electron-attracting property of the phosphorus ligand. In contrast with these properties, three nu(CO) IR bands, which are sensitive to the electron density on the central rhenium because of pi-back bonding, were shifted to higher energy, and a Re(I/II)-based oxidation wave was observed at a more positive potential according to the electron-attracting property of the phosphorus ligand.

Reactivity of the organometallic fac-[(CO)3ReI(H2O)3]+ aquaion. Kinetic and thermodynamic properties of H2O substitution

The water exchange process on [(CO)(3)Re(H(2)O)(3)](+) (1) was kinetically investigated by (17)O NMR. The acidity dependence of the observed rate constant k(obs) was analyzed with a two pathways model in which k(ex) (k(ex)(298) = (6.3 +/- 0.1) x 10(-3) s(-1)) and k(OH) (k(OH)(298)= 27 +/- 1 s(-1)) denote the water exchange rate constants on 1 and on the monohydroxo species [(CO)(3)Re(I)(H(2)O)(2)(OH)], respectively. The kinetic contribution of the basic form was proved to be significant only at [H(+)] < 3 x 10(-3) M. Above this limiting [H(+)] concentration, kinetic investigations can be unambiguously conducted on the triaqua cation (1). The variable temperature study has led to the determination of the activation parameters Delta H(++)(ex) = 90 +/- 3 kJ mol(-1), Delta S(++)(ex) = +14 +/- 10 J K(-1) mol(-1), the latter being indicative of a dissociative activation mode for the water exchange process. To support this assumption, water substitution reaction on 1 has been followed by (17)O/(1)H/(13)C/(19)F NMR with ligands of various nucleophilicities (TFA, Br(-), CH(3)CN, Hbipy(+), Hphen(+), DMS, TU). With unidentate ligands, except Br(-), the mono-, bi-, and tricomplexes were formed by water substitution. With bidentate ligands, bipy and phen, the chelate complexes [(CO)(3)Re(H(2)O)(bipy)]CF(3)SO(3) (2) and [(CO)(3)Re(H(2)O)(phen)](NO(3))(0.5)(CF(3)SO(3))(0.5).H(2)O (3) were isolated and X-ray characterized. For each ligand, the calculated interchange rate constants k'(i) (2.9 x 10(-3) (TFA) < k'(I) < 41.5 x 10(-3) (TU) s(-1)) were found in the same order as the water exchange rate constant k(ex), the S-donor ligands being slightly more reactive. This result is indicative of I(d) mechanism for water exchange and complex formation, since larger variations of k'(i) are expected for an associatively activated mechanism.

Mechanism of the photochemical ligand substitution reactions of fac-[Re(bpy)(CO)(3)(PR(3))](+) complexes and the properties of their triplet ligand-field excited states

We report herein the mechanism of the photochemical ligand substitution reactions of a series of fac-[Re(X(2)bpy)(CO)(3)(PR(3))](+) complexes (1) and the properties of their triplet ligand-field ((3)LF) excited states. The reason for the photostability of the rhenium complexes [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) was also investigated. Irradiation of an acetonitrile solution of 1 selectively gave the biscarbonyl complexes cis,trans-[Re(X(2)bpy)(CO)(2)(PR(3))(CH(3)CN)](+) (2). Isotope experiments clearly showed that the CO ligand trans to the PR(3) ligand was selectively substituted. The photochemical reactions proceeded via a dissociative mechanism from the (3)LF excited state. The thermodynamical data for the (3)LF excited states of complexes 1 and the corrective nonradiative decay rate constants for the triplet metal-to-ligand charge-transfer ((3)MLCT) states were obtained from temperature-dependence data for the emission lifetimes and for the quantum yields of the photochemical reactions and the emission. Comparison of 1 with [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) indicated that the (3)LF states of some 3- and 4-type complexes are probably accessible from the (3)MLCT state even at ambient temperature, but these complexes were stable to irradiation at 365 nm. The photostability of 3 and 4, in contrast to 1, can be explained by differences in the trans effects of the PR(3), py, and Cl(-) ligands.

New synthetic routes to biscarbonylbipyridinerhenium(I) complexes cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)n+ (X2bpy = 4,4'-X2-2,2'-bipyridine) via photochemical ligand substitution reactions, and their photophysical and electrochemical properties

Photochemical ligand substitution of fac-[Re(X2bpy)(CO)3(PR3)]+ (X2bpy = 4,4'-X2-2,2'-bipyridine; X = Me, H, CF3; R = OEt, Ph) with acetonitrile quantitatively gave a new class of biscarbonyl complexes, cis,trans[Re(X2bpy)(CO)2(PR3)(MeCN)]+, coordinated with four different kinds of ligands. Similarly, other biscarbonylrhenium complexes, cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)]n+ (n = 0, Y = Cl-; n = 1, Y = pyridine, PR'3), were synthesized in good yields via photochemical ligand substitution reactions. The structure of cis,trans-[Re(Me2bpy)(CO)2[P(OEt)3](PPh3)](PF6) was determined by X-ray analysis. Crystal data: C38H42N2O5F6P3Re, monoclinic, P2(1/a), a = 11.592(1) A, b = 30.953(4) A, c = 11.799(2) A, V = 4221.6(1) A3, Z = 4, 7813 reflections, R = 0.066. The biscarbonyl complexes with two phosphorus ligands were strongly emissive from their 3MLCT state with lifetimes of 20-640 ns in fluid solutions at room temperature. Only weak or no emission was observed in the cases Y = Cl-, MeCN, and pyridine. Electrochemical reduction of the biscarbonyl complexes with Y = Cl- and pyridine in MeCN resulted in efficient ligand substitution to give the solvento complexes cis,trans-[Re(X2bpy)(CO)2(PR3)(MeCN)]+.