Mononuclear Rhenium Research Articles

Solar Light CO2 Photoreduction Enhancement by Mononuclear Rhenium(I) Complexes: Characterization and Mechanistic Insights.

The catalytic efficacy of a novel mononuclear rhenium(I) complex in CO2 reduction is remarkable, with a turnover number (TONCO) of 1517 in 3 h, significantly outperforming previous Re(I) catalysts. This complex, synthesized via a substitution reaction on an aromatic ring to form a bromo-bipyridine derivative, L1 = 2-bromo-6-(1H-pyrazol-1-yl)pyridine, and further reacting with [Re(CO)5Cl], results in the facial-tricarbonyl complex [ReL1(CO)3Cl] (1). The light green solid was obtained with an 80% yield and thoroughly characterized using cyclic voltammetry, nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectroscopy, and ultraviolet-visible (UV-vis) spectroscopy. Cyclic voltammetry under CO2 atmosphere revealed three distinct redox processes, suggesting the formation of new electroactive compounds. The studies on photoreduction highlighted the ability of the catalyst to reduce CO2, while NMR, FTIR, and electrospray ionization (ESI) mass spectrometry provided insights into the mechanism, revealing the formation of solvent-coordinated complexes and new species under varying conditions. Additionally, computational studies (DFT) were undertaken to better understand the electronic structure and reactivity patterns of 1, focusing on the role of the ligand, the spectroscopic features, and the redox behavior. This comprehensive approach provides insights into the intricate dynamics of CO2 photoreduction, showcasing the potential of Re(I) complexes in catalysis.

Synthesis and structural elucidation of cytotoxic mono and dinuclear rhenium carbonyl complexes bearing bis-{1,3-(imino pyrrolyl)-m-chloro phenyl)} ligand

A mononuclear and dinuclear rhenium tricarbonyl compounds were synthesized by the one-pot condensation reaction of [Re(CO)5Br], pyrrole-2-carboxaldehyde and 4‑chloro-m-phenylene diamine. The mononuclear rhenium tricarbonyl complex containing {3-(imino pyrrolyl)-6‑chloro phenylamine)} ligand (IPP) with formula {[Re(CO)3Br(IPP)], (1), was obtained by refluxing stoichiometric quantities of amine, aldehyde and rhenium metal precursor in methanol solution. Similarly, dinuclear rhenium tricarbonyl compound {[Re(CO)3Br]2(BIPP)], BIPP = bis-{1,3-(imino pyrrolyl)-m-chlorophenyl)}, (2) has been synthesized by refluxing appropriate amounts of aldehyde, and rhenium metal precursor in the presence of one equivalent of amine ligand. Compounds 1 and 2 were characterized by FT-IR, 1H NMR, and UV–Vis spectroscopy, elemental analysis, and mass spectrometry. The luminescence properties of the formed complexes were studied in solution at room temperature, revealing an extended lifetime for the dinuclear complex 2 in comparison to 1. DFT calculations for the optimized structures helped to understand the geometry of the complexes, whereas TD-DFT revealed the vertical transitions responsible for the photophysical properties of the complexes. The cytotoxicity of such complexes against MCF-7 breast cancer has been also reported, revealing a dose-dependent growth inhibition attributed to a DNA-intercalating mode of binding for both complexes. The pronounced effect of the two metals on the cytotoxic studies and lifetime of the excited species were investigated. The greater intercalation ability of the complex 2 in comparison to complex1 obtained from circular dichroism studies also indicates the greater efficacy of Re complex 2.

A neutral mononuclear rhenium(I) complex with a rare in situ-generated triazolyl ligand for the luminescence "turn-on" detection of histidine.

A rare in situ-generated mononuclear rhenium complex [Re(bpt)(CO)3(NH3)] (1, bpt = 3,5-bis(2-pyridyl)-1,2,4-triazolate) can be used as a "turn-on" luminescent probe for selectively sensing L-histidine against other amino acids. Compound 1 was prepared by reacting Re2(CO)10, 2-cyanopyridine and hydrazine with an in situ formed bpt ligand through cyclization via C-N and N-N couplings with its single-side chelating mode arrayed with respect to the Re center. Compound 1 was highly stable and showed a green light MLCT emission in DMF solution at 507 nm upon excitation at 360 nm. Interestingly, the emission from 1 could be quenched by the addition of metal ions such as Ni2+ and Cu2+ but the emission efficiently recovered with the introduction of histidine. However, histidine could only be selectively detected when a combination of compound 1 and Ni2+ was used. Therefore, the luminescence response of the Ni2+-modified compound 1 could be utilized as a "turn-on" probe for the selective detection of histidine. This work provides a simple method for developing new sensing platforms of a discrete metal complex based on rare in situ generation.

Prolonging the Triplet State Lifetimes of Rhenium Complexes with Imidazole-Pyridine Framework for Efficient CO2 Photoreduction.

The photocatalytic reduction of CO2 into fuels offers the prospect for creating a new CO2 economy. Harnessing visible light-driven CO2 -to-CO reduction mediated by the long-lived triplet excited state of rhenium(I) tricarbonyl complexes is a challenging approach. We here develop a series of new mononuclear rhenium(I) tricarbonyl complexes (Re-1-Re-4) based on the imidazole-pyridine skeleton for photo-driven CO2 reduction. These catalysts are featured by combining pyridyl-imidazole with the aromatic ring and different pendant organic groups onto the N1 position of 1,3-imidazole unit, which display phosphorescence under Ar-saturated solution even at ambient conditions. By contrast, {Re[9-(pyren-1-yl)-10-(pyridin-2-yl)-9H-pyreno[4,5-d]imidazole)](CO)3 Cl} (Re-4) by introducing pyrene ring at the N1 position of pyrene-fused imidazole unit exhibits superior catalytic performance with a higher turnover number for CO (TONCO =124) and >99.9 % selectivity, primarily ascribed to the strong visible light-harvesting ability, long-lived triplet lifetimes (164.2 μs) and large reductive quenching constant. Moreover, the rhenium(I) tricarbonyl complexes derived from π-extended pyrene chromophore exhibit a long lifetime corresponding to its ligand-localized triplet state (3 IL) evidenced from spectroscopic investigations and DFT calculations.

Exploring the Full Potential of Photocatalytic Carbon Dioxide Reduction Using a Dinuclear Re2Cl2 Complex Assisted by Various Photosensitizers

AbstractPhotosensitizing units have already been applied to enable light‐driven catalytic reduction of CO2 with mononuclear rhenium complexes. However, dinuclear catalytic systems that are able to activate CO2 in a cooperative bimetallic fashion have only rarely been combined with photosensitizers. We here present detailed studies on the influence of additional photosensitizers on the catalytic performance of a dirhenium complex (Re2Cl2) and present correlations with spectroscopic measurements, which shed light on the reaction mechanism. The use of [Ir(dFppy)3] (Ir, dFppy=2‐(4,6‐difluorophenyl)pyridine)) resulted in considerably faster CO2 to CO transformation than [Cu(xant)(bcp)]PF6 (Cu, xant=xantphos, bcp=bathocuproine). Emission quenching studies, transient absorption as well as IR spectroscopy provide information about the electron transfer paths of the intermolecular systems. It turned out that formation of double reduced species [Re2Cl2]2− along with an intermediate with a Re−Re bond ([ReRe]) can be taken as an indication of multi‐electron storage capacity. Furthermore, under catalytic conditions a CO2‐bridged intermediate was identified.

DNA interaction, anticancer, antibacterial, ROS and lipid peroxidation studies of quinoxaline based organometallic Re(I) carbonyls

Hetero mononuclear rhenium(I) complexes (I-V) using ligands (L1-L5) [L1-L5 = 11-((2-chlorobenzylidene)hydrazono)-11H-indeno[1,2-b]quinoxaline (L1), 8-methyl-11-((4-methyl-benzylidene)hydrazono)-11H-indeno[1,2-b]quinoxaline (L2), 11-((4-bromobenzylidene) hydrazono)-8-nitro-11H-indeno[1,2-b]quinoxaline (L3), 11-((4-bromobenzylidene) hydrazono)-8-chloro-11H-indeno[1,2-b]quinoxaline (L4), 8-bromo-11-((4-fluorobenzylidene) hydrazono)-11H-indeno[1,2-b]quinoxaline (L5)] were synthesized and characterized by spectroscopic method. All the synthesized compounds have biological importance. DNA interaction studies gave information about the modes of binding and the nucleolytic efficiency of compounds. The binding of the rhenium complexes to Herring sperm DNA (HS DNA) was monitored by UV–visible spectroscopy, viscosity measurements, and molecular docking studies; groove binding was suggested as the most possible mode. The DNA-complexes binding strength was measured in terms of intrinsic binding constants. In vivo and In vitro cytotoxicity against the eukaryotic and prokaryotic cells gave the toxic nature of the synthesized compounds. An antimicrobial study was carried out by estimating MIC (Minimum Inhibitory Concentration) against two Gram-positive (S. aureus, B. subtilis) and three Gram-negative (S. marcescens, P. aeruginosa, E. coli) bacteria. All synthesized complexes are biologically more active than the corresponding ligands. Complexes were having higher MDA and H2O2 production than ligands.

Biological activities of pyrazoline-indole based Re(I) carbonyls: DNA interaction, antibacterial, anticancer, ROS production, lipid peroxidation, in vivo and in vitro cytotoxicity studies

Hetero mononuclear rhenium(I) metal complexes (I-V) using different substituted indole-pyrazoline based ligands were synthesized and characterized by spectroscopic and analytical methods. The binding of the rhenium complexes to Herring sperm DNA was monitored by UV spectroscopy, viscosity measurements, and molecular docking studies; groove binding was suggested as the most possible mode and the DNA-binding constants of the complexes were evaluated. In vivo and in vitro cytotoxicity of compounds were evaluated against the brine shrimp and S. cerevisiae cells. An antimicrobial study was carried out by estimating MIC (Minimum Inhibitory Concentration) against two Gram-positive and three Gram-negative bacteria. All synthesized complexes are biologically more active than the corresponding ligands. The anti-proliferation activity of complexes was evaluated on MCF-7, HCT116, and A549 cancer cells by MTT assay. The toxicity profile of synthesized compounds was confirmed by H2O2 production by reactive oxygen species. The increased concentration of lipid peroxidation end products increased free radicals, which enhancing the oxidative stress level in living organisms and results in cell death.

Synthesis, characterization and DFT studies of complexes bearing [Re(CO)3]+ core and reactivity towards cyanide ion

Mononuclear rhenium(I) complexes having fac-[Re(CO)3]+ core of general formula fac-[Re(CO)3(L)Cl] have been synthesized in excellent yield by reacting [Re(CO)5Cl] with L1 and L2in a ratio of 1:1 in toluene under inert atmosphere. Here L1 and L2 are N-((quinolin-2-yl)methylene)-9H-fluoren-2-amine and N-((anthracen-10-yl)methylene)quinolin-8-amine respectively. Spectroscopic measurements such as NMR, ESI-MS and IR spectroscopy were used to ensure the formations of desired complexes. Molecular structures of fac-[Re(CO)3(L1)Cl] and fac-[Re(CO)3(L2)Cl] were confirmed by single-crystal X-ray diffraction. The ligands are emissive whereas the metal complexes are weak emitter. A remarkable change in absorption as well as emission behavior was observed in complex 2 upon addition of cyanide ion. Interruption in intraligand charge transfer in complex 2 is the probable reason for the obvious change in spectral response of the same. 1H NMR titration was performed as the evidence of the previously stated fact. The ground and excited-state geometries, absorption properties of rhenium(I) complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO) and analysis reveal the nature of excitations.

Sensitized Photochemical CO2 Reduction by Hetero-Pacman Compounds Linking a ReI Tricarbonyl with a Porphyrin Unit.

The hetero-Pacman architecture places two different metal coordination sites in close proximity, which can support efficient energy and/or electron transfer and allow for cooperative activation of small molecules. Here, the synthesis of dyads consisting of a porphyrin unit as photosensitizer and a rhenium unit as catalytically active site, which are held together by the rigid xanthene backbone, is presented. Mononuclear [(NN)Re(CO)3 (Cl)] complexes for CO2 reduction in which NN represents a bidentate diimine ligand (e.g., bipyridine or phenanthroline) lack light absorption in the visible region, resulting in poor photocatalysis upon illumination with visible light. To improve their visible-light absorption, we have focused on the incorporation of a strongly absorbing free base or zinc porphyrin unit. Resulting photocatalytic experiments showed a strong dependence of the catalytic performance on both the type of photosensitizer and the excitation wavelengths. Most notably, the intramolecular hetero-Pacman system containing a zinc porphyrin unit showed much better catalytic activity in the visible region (excitation wavelengths >450 nm) than the free base porphyrin version or the corresponding mononuclear rhenium compound or an intermolecular system comprised of a 1:1 mixture of the mononuclear analogues.

Synthesis and catalytic activity of rhenium carbonyl complexes containing alkyl-substituted tetramethylcyclopentadienyl ligands

A series of six alkyl-substituted tetramethylcyclopentadienyl mononuclear metal carbonyl complexes [(η 5-C5Me4R)Re(CO)3] [R = allyl (1), i-Pr (2), n-butyl (3), t-butyl (4), benzyl (5), CH(CH2)4 (6)] have been synthesized by treating the corresponding ligands (C5Me4R) [R = allyl, i-Pr, n-butyl, t-butyl, benzyl, CH(CH2)4] with Re2(CO)10 in refluxing xylene. The six new complexes were characterized by elemental analysis, IR, 1H NMR and 13C NMR spectroscopy. The crystal structures of all six complexes were determined by X-ray crystal diffraction analysis, showing that they have similar molecular structures, being mononuclear carbonyl complexes. In each of these complexes, the Re atom is η 5 -coordinated to the cyclopentadienyl ring. Complexes 1–5 have significant catalytic activity in Friedel–Crafts reactions of aromatic compounds with alkylation reagents. Compared with traditional catalysts, these mononuclear rhenium carbonyl complexes have obvious advantages such as lower amounts of catalyst, mild reaction conditions and environmentally friendly chemistry.

Pressure induced enhancement of the magnetic ordering temperature in rhenium(IV) monomers.

Materials that demonstrate long-range magnetic order are synonymous with information storage and the electronics industry, with the phenomenon commonly associated with metals, metal alloys or metal oxides and sulfides. A lesser known family of magnetically ordered complexes are the monometallic compounds of highly anisotropic d-block transition metals; the ‘transformation' from isolated zero-dimensional molecule to ordered, spin-canted, three-dimensional lattice being the result of through-space interactions arising from the combination of large magnetic anisotropy and spin-delocalization from metal to ligand which induces important intermolecular contacts. Here we report the effect of pressure on two such mononuclear rhenium(IV) compounds that exhibit long-range magnetic order under ambient conditions via a spin canting mechanism, with Tc controlled by the strength of the intermolecular interactions. As these are determined by intermolecular distance, ‘squeezing' the molecules closer together generates remarkable enhancements in ordering temperatures, with a linear dependence of Tc with pressure.

Structural features of monomeric octahedral d 2-rhenium(V) monooxo complexes with the oxygen atoms of monodentate acido ligands OЕR n– (n = 1, 2; Е = Si, B, S; R = Me3, F3, О2CF3)

Some structural features of mononuclear octahedral rhenium(V) oxo complexes (I–VI) with the oxygen atoms of monodentate acido ligands OЕRn– (n = 1, 2; Е = Si, B, S; R = Me3, F3, О2CF3) have been considered. The Re–O(OЕR)trans bonds in complexes I–V (Е = Si, B; R = Me3, F3) are elongated due to the structural manifestation of the trans-effect of multiply bonded oxo ligands. The Re–O{S(O2CF3)}cis bond in cis-position to the O(oxo) ligand in complex VI is shortened in comparison with Rе–O(ST).

Specific features of the structures of monomeric octahedral d 2-rhenium(V) monooxo complexes with oxygen atoms trans to the oxo ligands: Complexes with monodentate neutral ligands OR trans (OR = DMF, R'OH, L1–L3)

Specific features of the structures of ten mononuclear octahedral rhenium(V) oxo complexes (I–X) with the oxygen atoms of the ligands DMF, R'OH (R' = Me, Et, Pr), and Ln (Ln = ON4C6 ∙ C6H10 (n = 1), O–(C6H3MeCH2NH+Et2) (n = 2), O–(NH+C5H4) (n = 3)) trans to the multiply bonded oxo ligands have been considered. The Re–O(OR)trans bonds in complexes I–VII (OR = DMF, R'OH, L1) are elongated owing to the structural manifestation of the trans influence of multiply bonded oxo ligands. The Re–O–(OR) bonds with a negatively charged oxygen atom in three complexes with zwitterionic ligands L2 (in VIII) and L3 (in IX and X) are either comparable with the standard Re–O(ST) bond length (in IX and X) or strongly shortened as compared with Rе–O(ST) (in complex VIII). In the anionic complex [ReO(MeOH)Cl2(L4)]– (XI), the oxygen atom of the monomeric neutral ligand МеOH is unexpectedly cis to the oxo ligand, while the trans position to =O(oxo) is occupied by the acido oxygen atom of the bidentate chelating monodeprotonated 2-(diphenylphosphonyl) benzoate ligand (L4)–.

Specific features of the structures of monomeric octahedral d 2 rhenium(V) oxo complexes with oxygen atoms trans to the oxo ligands: Complexes with neutral ligands OER3 (E = P, As; R3 = Ph3, PhEt2) trans to O(oxo)

Specific features of the structures of mononuclear octahedral rhenium(V) oxo complexes with oxygen atoms of neutral ligands OER3 (E = P, As; R3 = Ph3, PhEt2) trans to the multiply bonded oxo ligands have been considered. The complexes contain halide ligands, as well as organic monoand bidentate ligands, in the equatorial plane. The Re–O(OER3)trans bonds are elongated owing to the structural manifestation of the trans influence of multiply bonded oxo ligands.

Structural details of monomeric octahedral d 2-rhenium(V) oxo complexes with oxygen atoms in the trans-positions to oxo ligands. Complexes with the anionic ligands OR n− (n = 1, 2) in trans-positions to O(oxo)

The paper considers details of the structures of mononuclear octahedral rhenium(V) oxo complexes with oxygen atoms of aromatic (phenolic) and other similar cyclic ligands OR− (R = Ph, Cy, C6H4R′ (R′ = OH, OMe), C10H6OH) and with other structurally studied anionic ligands ORn− (n = 1, 2) (R = P(O)(OMe)2, C(O)(CF3), OC(Me(CF3)2, BF3) located in the trans-position to the multiply bonded oxo ligands. The complexes contain halide ligands and organic mono-, bi-, and tetradentate ligands in the equatorial plane. The Re-O(OR)trans bonds as well as the bonds with aliphatic alkoxo ligands (ORalk)trans (R = Me, Et, Pr) are not elongated due to the trans-effect of doubly bonded oxo groups but are substantially shortened (with one exception). This is attributable to the fact that these Re-O(OR) bonds, like Re-O(oxo) bonds, have a higher order, i.e., d2-rhenium(V) complexes of this type should be treated as pseudo-dioxo compounds [Re(=O)(=OR)]3 − n (n = 1, 2).

Synthesis, crystal structure and magnetic properties of a novel heterobimetallic rhenium(IV)-dysprosium(III) chain.

The use of the mononuclear rhenium(IV) precursor [ReBr5 (H2 pydc)](-) (H2 pydc=3,5-pyridinedicarboxylic acid) as a metalloligand towards dysprosium(III) afforded the first heterobimetallic Re(IV) -Dy(III) complex. Crystal structures and static and dynamic magnetic properties of both rhenium-containing species are reported herein. The 5d-4f compound shows an extended 1D structure and the AC magnetic measurements reveal frequency dependence at low temperature suggesting slow relaxation of the magnetization.

Structural features of monomeric octahedral d 2 rhenium(V) oxo complexes with oxygen atoms in trans-positions to oxo ligands: Complexes with propoxo ligands in trans-positions to O(oxo)

Some structural features of mononuclear octahedral rhenium(V) oxo complexes with oxygen atoms of propoxo ligands O(Pr-iso)− in trans-positions to multiply bonded oxo ligands are considered. The complexes contain monodentate inorganic or organic ligands or bi- and tetradentate organic ligands in the equatorial plane. Re-O(OPr) trans bonds, as well as the other bonds with (ORalkoxo) trans alkoxo ligands (R = Me, Et), are considerably shortened, instead of being elongated, due to the structural manifestation of the trans-effect of multiply bonded oxo ligands. This is explained by that these Re-O(ORalkoxo) bonds also have an increased multiplicity; that is, d 2 rhenium(V) monooxomonoalkoxo complexes should be considered as pseudo-dioxo Re(=O)(=ORalkoxo) compounds.

Synthesis and characterization of rhenium(I) complexes based on O, N, N coordinating ligands: DFT/TDDFT studies on the electronic structures and spectral properties

Mononuclear rhenium(I) complexes having fac-[Re(CO)3]+ moiety of general formula fac-[Re(CO)3(L)] have been synthesized in excellent yield by reacting [Re(CO)5Cl] with HL1 and HL2 in a ratio of 1:1 in methanol. Here L− is deprotonated form of 1-[(quinolin-8-ylimino)methyl]naphthalene-2-ol (HL1) and 1-[(quinolin-8-yl-diazenyl)]naphthalene-2-ol (HL2). The elemental analysis and ESI mass spectroscopic measurements ensure the formations of desired complexes. Molecular structures of fac-[Re(CO)3(L1)] and fac-[Re(CO)3(L2)] were confirmed by single-crystal X-ray diffraction. The complexes were also characterized by different spectroscopic techniques. A remarkable change in optical properties is observed in case of fac-[Re(CO)3(L2)] over fac-[Re(CO)3(L1)] where the ligands bearing a NN function (HL2) instead of coordinated CN function (HL1). The ground and excited-state geometries, NMR, absorption, and phosphorescence properties of Re(I) complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO) and spin density difference map analysis reveals the nature of excitations. The lowest lying triplet excited is associated with the 3MLCT excited state. The emission like transition is consistent with the strong 3MLCT character.

Structures of monomeric octahedral d 2 rhenium(V) oxo complexes with oxygen atoms trans to the oxo ligands: Complexes with methoxo ligands trans to O(oxo)

Specific features of the structure of mononuclear octahedral rhenium(V) complexes with oxygen atoms of ethoxo ligands trans to the multiply bonded oxo ligands have been considered. The complexes contain inorganic or organic monodentate ligands, as well as organic bi-, tri-, and tetradentate ligands in the equatorial plane. It has been noted that the Re-O(OEt)trans bonds, like the other bonds with alkoxy ligands ORalk (in particular, Re-O(OMetrans)) are not elongated owing to the structural manifestation of the trans influence of multiply bonded oxo ligands; rather, they are significantly shortened. This fact is explained by that the Re-O(ORalk) bonds are also multiple; i.e., the oxo alkoxo complexes should been considered as pseudo-dioxo compounds Re(=O)(=ORalk).

Glycoconjugated Rhenium(I) and 99m‐Technetium(I) Carbonyl Complexes from Pyridyltriazole Ligands Obtained by “Click Chemistry”

AbstractA series of pyridyltriazole ligands containing sugar moieties have been prepared by copper(I)‐mediated 1,3‐dipolar cycloaddition (“click” reaction) of azides functionalized with D‐glucose, D‐galactose, D‐mannose, D‐xylose as well as D‐maltose residues, and 2‐ethynylpyridine as alkyne. The peracetylated saccharide residues as well as their water‐soluble deprotected derivatives were treated with Re(CO)5Cl to obtain the corresponding mononuclear rhenium(I) carbonyl complexes [LRe(CO)3Cl]. For comparison, one ReI complex bearing a tert‐butylbenzyl residue instead of a sugar moiety as well as two dinuclear rhenium complexes derived from a branched ligand containing two pyridyltriazole units were prepared. The structure and integrity of the ligands and complexes was established by NMR, IR, UV/Vis and fluorescence spectroscopy, mass spectrometry, and by elemental analysis. Coordination of the metal ion occurred by both the pyridyl nitrogen atoms and one of the triazole nitrogen atoms. Upon treatment with an excess of histidine, the ReI complexes were stable for only 2.5 h. After a longer period (24 h) ligand exchange was detected by HPLC measurements. In contrast, a complex labeled with 99mTc was found to be stable for up to 24 h against an excess of histidine. Cytotoxicity was screened for all ReI complexes against HepG2 cells using a concentration of 100 μM. All sugar‐functionalized complexes were found to be nontoxic, except for the complex derived from the pyridyl (tert‐butylbenzyl)‐triazole, which exhibited remarkable toxicity.