Rhenium Cluster Complexes Research Articles

Can Re cluster complexes be an efficient catalyst for hydrogen evolution reaction? Insights from experiments and computations.

To date, researchers in chase of economic cost-efficiency are faced with the problem of developing effective catalysts for water splitting without the use of platinoids. Herein, catalytic properties of hexanuclear rhenium cluster complexes are investigated in application to the hydrogen evolution reaction (HER). A paste composite electrode containing the cluster complexes was obtained, producing a current density of 10 mA cm-2 at an extraordinarily low overpotential of 90 mV (RHE). The {Re6Se8}-based complexes have shown very favorable reaction kinetics via 102 mV dec-1 value of the Tafel slope for HER reaction within the composition of the paste electrode. Model calculations of kinetic parameters using density functional theory also support the experimental findings. This work underscores the perspectivity of rhenium cluster compounds in HER and opens a promising avenue toward the practical implementation of hydrogen production through electrochemical water splitting.

Multispin superatoms: seven-nuclear rhenium clusters with unusual magnetic properties

Two seven-nuclear rhenium cluster complexes [{Re3S4(CN)9}{Re4S4}(CN)9]7− and [{Re3Se4(CN)9}{Re4Se4}(CN)9]7− with a new structural type and unique magnetic behavior have been synthesized.

Selective Oxidation of Inner Pnictogenide Ligands in Mixed-Ligand Rhenium Cubane-Type Cluster Complexes.

Reactivity of new tetrahedral rhenium cluster complexes with pnictogenide inner ligands μ3-As3-, μ3-Sb3-, and μ3-Bi3- has been investigated in reactions with aqueous H2O2. It has been found that the oxidation of clusters [{Re4As3Q}(CN)12]7- (Q = S or Se) led to the formation of stable clusters with μ3-(AsO)3- ligands. Under the same conditions, the oxidation of [{Re4As2S2}(CN)12]6- cluster led to substitution of μ3-As3- ligands to μ3-O2-. The resulting cluster [{Re4O2S2}(CN)12]4- easily undergoes further oxidation, and even at room temperature, a unique {Re4} to {Re3} rearrangement occurs with the formation of the new triangular cluster [{Re3(μ3-S)(μ-O)2(μ-SO2)}(CN)9]5-. Upon heating, this process proceeds faster and the triangular cluster can be isolated as individual compounds. Cluster anions [{Re4SbSe3}(CN)12]5- and [{Re4BiS3}(CN)12]5- reacted with H2O2, yielding clusters containing μ3-O2- ligands, namely, [{Re4OSe3}(CN)12]4- and [{Re4OS3}(CN)12]4-. This indicates that oxidized forms of μ3-Sb3- and μ3-Bi3- ligands can be easily substituted.

Octahedral Rhenium Cluster Complexes with 1,2-Bis(4-pyridyl)ethylene and 1,3-Bis(4-pyridyl)propane as Apical Ligands.

A series of eight new octahedral rhenium cluster complexes with the general formula trans-[{Re6Q8}L4X2] (Q = S or Se; L = 1,2-Bis(4-pyridyl)ethylene (bpe) or 1,3-Bis(4-pyridyl)propane (bpp); X = Cl or Br) was synthesized and investigated. While bpe is a ligand with a conjugated aromatic system, bpp represents a molecule of opposite type and has independent aromatic systems of the two pyridine rings. It was shown that this difference in the electronic structure of the ligands has a fundamental effect on the electronic structure, electrochemical and luminescent properties of the corresponding cluster complexes. Specifically, the [{Re6Q8}(bpe)4X2] complexes in solutions show multiple quasi-reversible electrochemical transitions associated with reduction of the organic ligands. At the same time, the trans-[{Re6Q8}(bpp)4X2] complexes show multielectron quasi-irreversible reduction processes, which correlate with the mixed character of the lowest unoccupied molecular orbitals of these complexes. All the obtained new compounds exhibit red photoluminescence. The photophysical parameters (emission lifetimes and quantum yields) measured for the bpp complexes exceed those revealed for bpe complexes by more than an order of magnitude.

Coordination Polymers Based on Rhenium Octahedral Chalcocyanide Cluster Anions and Ag+ Cations with Bipyridine Analogs.

A series of six coordination polymers based on octahedral cluster anions [Re6Q8(CN)6]4- (Q = S or Se) and Ag+ cations coordinated by bipyridine analogs were synthesized under solvothermal conditions. Their structures have been characterized by single crystal X-ray diffraction. Compounds 1 and 2 described by the general formula [{Ag(phen)}4Re6Q8(CN)6] (Q = Se (1), S (2); phen = 1,10-phenanthroline) exhibit layered structures assembled into a supramolecular network by CH…π contacts. At the same time, compounds [{Ag(bipym)}2Ag2Re6Se8(CN)6] (bipym = 2,2'-bipyrimidine) (3), [{Ag2(bipy)}Ag2Re6Se8(CN)6]·CH3CN (bipy = 4,4'-bipyridine) (4) and [{Ag(dpbp)}4Re6Q8(CN)6]·2H2O·2CH3CN (Q = Se (5), S (6); dpbp = 4,4'-Di(4-pyridyl)biphenyl)) evince framework structures. In 1, 2, 5 and 6 weak Ag⋯Ag interactions are observed. All the compounds show luminescence in the red region. The luminescence quantum yields and lifetimes were found to be notably higher than those for most of the coordination polymers based on the octahedral rhenium cluster complexes.

Thermally Controlled Synthesis of Octahedral Rhenium Clusters with 4,4′-Bipyridine and CN− Apical Ligands

The selective preparation, structural and spectroscopic study of two new rhenium cluster complexes trans-[Re6S8(bpy)4(CN)2] and trans-[Re6S8(bpy)2(CN)4]2− (bpy = 4,4′-bipyridine) obtained by reactions of corresponding hexarhenium cyanohalides with molten bpy are reported. The complexes were crystallized as solvates, displaying supramolecular structures based on cluster units linked by numerous weak interactions with bpy molecules. The molecular compound trans-[Re6S8(bpy)4(CN)2] (1) is insoluble in water and common organic solvents, while the ionic compound trans-Cs1.7K0.3[Re6S8(bpy)2(CN)4] (2) is somewhat soluble in DMSO, DMF and N-methylpyrrolidone. The presence of the redox-active ligand bpy leads to the occurrence of multi-electron reduction transitions in a solution of 2 at moderate potential values. The ambidentate CN− ligand is the secondary functional group, which has potential for the synthesis of coordination polymers based on the new cluster complexes. In addition, both new compounds show a weak red luminescence, which is characteristic of complexes with a {Re6S8}2+ cluster core.

Direct observation of x-ray excited optical luminescence from a Re6 metal cluster in true aqueous solution: The missing link between material characterization and in vivo applications

Phosphorescent octahedral metal cluster complexes are highly attractive in terms of biomedical applications, particularly in the area of luminescent diagnostics and photodynamic therapy (PDT). The ability of clusters to be excited by x-radiation followed by red/near infrared emission makes them a new promising class of photosensitizers for deep-PDT. This work demonstrates a direct observation of x-ray-excited optical luminescence of hexanuclear clusters, by example of rhenium cluster complex Na4[{Re6Se8}(CN)6], in a true aqueous solution. The article also describes point-by-point the details and pitfalls of studying cluster radioluminescence in aqueous solution.

Синтез, строение и физико-химические свойства молекулярных кластерных комплексов рения с терминальными лигандами — молекулами 4-фенилпиридина

Четыре новых кластерных комплекса рения с общей формулой транс-[Re6Q8(ppy)4X2]n(ppy) (Q = S, Se; X = Cl, Br; ppy = 4-фенилпиридин) получены взаимодействием солей Csn[Re6Q8X6]·2H2O (Q = S, n = 4; Q = Se, n = 3; X = Cl или Br) с расплавом 4-фенилпири­дина. Исследование монокристаллов соединений методом РСА показало, что их структуры содержат молекулярные фрагменты транс-[Re6Q8(ppy)4X2], упакованные вместе с сольватными молекулами 4-фенилпиридина и связанные с этими молекулами слабыми взаимодействиями, которые включают водородные связи и p-стекинг ароматических колец. Обнаружено, что тип упаковки различается для разных галогенидных и халькогенидных лигандов кластерного комплекса. Полученные соединения нерастворимы в воде и органических растворителях. Изучение люминесценции кристаллических образцов показало, что вещества проявляют эмиссию в красной области спектра с максимумами на 708—718 нм, квантовыми выходами эмиссии 0.01—0.03 и временами жизни эмиссии порядка 4 мкс. В электронной структуре соединений присутствуют лигандцентрированные уровни LUMO—LUMO+3, локализованные между металлцентрированными HOMO и LUMO+4.

CLUSTER COMPLEXES OF RHENIUM WITH CARBONYL AND AMINE CONTAINING LIGANDS

Methods for the synthesis of rhenium cluster complexes based on carbonyl and amine containing organic bifunctional ligands have been developed. The structure of the obtained cluster combinations were determined on the basis of IR spectroscopy data, methods of thermogravimetry and elemental analysis. Samples of ligands I and II were obtained by condensation of cyclopentane and cyclohexane carboxylic acids chloroanhydrides with ethylene, followed by the replacement of the chlorine atom by amine groups. To obtain cluster complexes of rhenium with the synthesized ligands, an ultradisperse solution of rhenium in distilled water was prepared in advance. To this end, the rhenium trichloride salt (ReCl3) was dissolved in water and the calculated amount of sodium borohydride in a nitrogen atmosphere was added in portions to the resulting solution with vigorous stirring. Rapidly arising black dispersed nanoparticles of metallic rhenium were not deposited. When organic ligands I and II are added, the corresponding cluster compounds III and IV are formed, which gradually over 30 minutes. precipitated from aqueous solution. The resulting black-brown precipitates were washed with distilled water and dried in a nitrogen atmosphere at a temperature of 35-40 °C. The melting points of the synthesized compounds are determined, which are components for cluster III-195 °С and cluster IV- 212 °С (with decomposition). In the IR spectra of cluster compounds, intense absorption bands were found, which characterize the presence of both a ketone carbonyl group (1718 sm–1, 1720 sm–1) and an amine fragment (2727 sm–1, 2720 sm–1 and 2613 sm–1, 2609 sm–1). The absorption bands of ketone groups in cluster compounds are shifted toward higher frequencies compared to the initial ligands. A similar picture is observed when comparing IR vibrations of C – N bonds in the initial ligands and the corresponding cluster compounds. The results of elemental analysis confirm the structure of cluster compounds and are in complete agreement with the notion that the reduction of rhenium salts with metal hydrides in an aqueous solution forms cluster compounds. Apparently, in this case, the most stable rhenium clusters with a tetrahedral structure are formed. Thermogravimetric analysis made it possible to establish the presence of a peak at a temperature of 318 °С with a mass number of 744.8 c.u. corresponding to the cluster combination of four rhenium atoms. At each stage of decomposition, the experimental mass loss agrees well with the calculated values.

CRYSTAL STRUCTURE AND LUMINESCENT PROPERTIES OF RHENIUM CLUSTERS [{Re6(μ3-Q)8}(imz-[1,2-a]py)6]Br2

Octahedral rhenium cluster complexes of the composition [{Re6(μ3-Q)8}(imz-[1,2-a]py)6]Br2, where Q = S, Se, are obtained by the interaction of Csn[{Re6(μ3-Q8)}Br6] (n = 4 for Q = S; n = 3 for Q = Se) with an excess of imidazo-[1,2-а]pyridine in a sealed glass ampoule at 180 °С within two days. Both compounds are isostructural and crystallize in the trigonal space group $$R\bar{3}$$ . The crystal structure of the complexes represents three differently directed chains of [{Re6Q8}(imz-[1,2-a]py)6]2+ cations and bromine anions linked with each other by С–H⋯Br⋯H–C hydrogen bonds. The compounds obtained have pronounced luminescence with emission maxima at 652 nm and 678 nm for Q = S, Se respectively.

Water-Soluble Rhenium Clusters with Triazoles: The Effect of Chemical Structure on Cellular Internalization and the DNA Binding of the Complexes.

Here we explore the effect of the nature of organic ligands in rhenium cluster complexes [Re6 Q8 L6 ]4- (where Q=S or Se, and L=benzotriazole, 1,2,3-triazole or 1,2,4-triazole) on the biological properties of the complexes, in particular on the cellular toxicity, cellular internalization and localization. Specifically, the study describes the synthesis and detailed characterization of the structure, luminescence and electrochemical properties of the four new Re6 clusters with 1,2,3- and 1,2,4-triazoles. Biological assays of these complexes are also discussed in addition to those with benzotriazole using cervical cancer (HeLa) and immortalized human fibroblasts (CRL-4025) as model cell lines. Our study demonstrates that the presence of hydrophobic and π-bonding rich units such as the benzene ring in benzotriazole significantly enhances cellular internalization of rhenium clusters. These ligands facilitate binding of the clusters to DNA, which results in increased cytotoxicity of the complexes.

Apically homoleptic octahedral rhenium cluster complexes with 3-methylpyrazole

Octahedral rhenium cluster complexes with organic apical ligands have a high potential to be used in different fields due to their intrinsic luminescence and X-ray contrast properties. A series of new compounds based on cations [{Re6Q8}(3-Me-pzH)6]2+, where Q is S or Se and 3-Me-pzH is 3-methylpyrazole, were obtained and characterized in the solid state (FTIR, elemental and single crystal diffraction analysis) and in solution (1H NMR and ESI-MS). The compounds possess luminescence properties that mainly depend on inner Q ligands indicating the {Re6Q8}2+ core-centered nature of emission.

Soluble Molecular Rhenium Cluster Complexes Exhibiting Multistage Terminal Ligands Reduction.

Substitution of terminal halide ligands of octahedral rhenium cluster complexes [Re6Q8X6]4- in a melt of 4,4'-bipyridine (bpy) led to us obtaining four new compounds with the general formula trans-[Re6Q8(bpy)4X2] (Q = S or Se; X = Cl or Br) in high yield. In contrast to most of the known molecular rhenium cluster complexes with heteroaromatic terminal ligands, compounds 1-4 are soluble in organic solvents. This made it possible to carry out a detailed characterization of the new compounds both in solids and in solutions. In particular, it was shown that compounds 1-4 in the DMSO solution exhibit four reversible reduction processes. A comparison of the obtained data with the results of DFT calculations of the electronic structure suggests that these processes correspond to two-electron reduction of all four bpy ligands. The reduction potentials are shifted to the positive region compared with the potential of free bipyridine, and the value of the shift depends on the composition of the cluster core. The presence of four transitions also suggests that electronic exchange between terminal ligands in the cis-position is possible. The study of the luminescence of the compounds showed that emission maxima of selenide clusters almost coincide with those of sulfide ones, while luminescence spectra of the complexes with chloride terminal ligands (1 and 3) are slightly blue-shifted relative to the spectra of the complexes with bromide ligands (2 and 4).

{Re6Q8}(SO3)6]10- (Q = S or Se): Facile Synthesis and Properties of the Most Highly Charged Octahedral Cluster Complexes and High Magnetic Relaxivity of Their Colloids with Gd3+ Ions.

New octahedral rhenium cluster complexes [{Re6Q8}(SO3)6]10- (Q = S or Se) were synthesized starting from [{Re6Q8}(H2O)4(OH)2]·12H2O. The complexes were crystallized as sodium salts and characterized by X-ray single-crystal diffraction and elemental analyses, IR, UV/vis and luminescence spectroscopies. Magnetic relaxation data demonstrate the complex formation of the cluster units with gadolinium ions. The analysis of the magnetic relaxation rates measured at various Gd:cluster ratios and different concentrations revealed the conversion of the aggregates (Gdx[{Re6Se8}(SO3)6]y)n- into a nanoparticulate form even at x = 1 and y ≥ 1. Thus, the self-assembly of the cluster units into the nanoparticles is greatly facilitated by counterion binding with sodium cations. The concentration conditions were optimized for the formation and hydrophilization of NaxGdy[{Re6Q8}(SO3)6]-based colloids with the magnetic relaxivity values of r1(2) = 21.0(24.1) and r1(2) = 25.9(29.8) mM-1 s-1 for the {Re6S8}2+ and {Re6Se8}2+ derivatives, respectively.

PO23- and AsO3-: Pnictogenide Ligands in the Highly Charged Re4 Cluster Anions [{Re4(PO)3(PO2)}(CN)12]8-, [{Re4As2(AsO)2}(CN)12]8-, and [{Re4(AsO)4}(CN)12]8.

New tetrahedral anionic rhenium cluster complexes [{Re4(PO)3(PO2)}(CN)12]8- (1) and [{Re4As2(AsO)2}(CN)12]8- (2) have been synthesized by the reaction of ReI3 with NaCN and Pred or KCN and Asgray, respectively. Compound 2 was further oxidized by aqueous H2O2 to produce the cluster anion [{Re4(AsO)4}(CN)12]8- (3). The anions contain new pnictogen/oxygen-based ligands, PO23- and AsO3-, which were stabilized by the coordination to the triangular faces of the tetrahedral Re4 metallocluster in μ3-mode. All three compounds were crystallized as either sodium or potassium salts, and their crystal structures were determined by single crystal X-ray diffraction analysis.

Tetrahedral Rhenium Cluster Complexes with Mixed-Ligand Cores {Re4As3Q}5+ (Q = S, Se) and {Re4As2S2}6+

A high-temperature reactions of equimolar amounts of ReI3, As and S or Se with KCN following by recrystallization of the soluble products from water led to the formation of new cyanometallate cluster complexes K7[{Re4As3S}(CN)12]·10H2O (1) and K7[{Re4As3Se}(CN)12]·9.5H2O (2), respectively. Involvement of As2S3 instead of the mixture of As and S resulted in the formation of a cluster complex K6[{Re4As2S2}(CN)12]·3.5H2O (3). Crystal structures of the obtained compounds were investigated by the X-ray diffraction analysis. It was found that compounds 1–3 contain a new cubane cluster cores composed of the rhenium tetrahedron surrounded by the µ3-As and µ3-Q (Q = S, Se) ligands.

Singlet Oxygen Production and Biological Activity of Hexanuclear Chalcocyanide Rhenium Cluster Complexes [{Re6Q8}(CN)6]4– (Q = S, Se, Te)

Octahedral rhenium cluster complexes have recently emerged as relevant building blocks for the design of singlet oxygen photosensitizing materials toward biological applications such as blue-light photodynamic therapy. However, their singlet oxygen generation ability as well as biological properties have been studied only superficially. Herein we investigate in detail the singlet oxygen photogeneration, dark and photoinduced cytotoxicity, cellular uptake kinetics, cellular localization and in vitro photoinduced oxidative stress, and photodynamic cytotoxicity of the series of octahedral rhenium cluster complexes [{Re6Q8}(CN)6]4-, where Q = S, Se, Te. Our results demonstrate that the selenium-containing complex possesses optimal properties in terms of absorption and singlet oxygen productivity. These features coupled with the cellular internalization and low dark toxicity lead to the first photoinduced cytotoxic effect observed for a molecular [{M6Q8}L6] complex, making it a promising object for further study in terms of blue-light PDT.

Facile Substitution of Bridging SO22- Ligands in Re12 Bioctahedral Cluster Complexes.

Selective substitution of μ-SO22- groups by either O2- or Se2- ions occurs upon heating the bioctahedral rhenium cluster complex K6[Re12CS14(μ-SO2)3(CN)6] in air atmosphere or in the presence of a Se source, respectively, manifesting the remarkable lability of SO22- ligands bound to a transition-metal cluster. A series of compounds based on the new mixed-ligand anions, [Re12CS14(μ-O)3(CN)6]6-, [Re12CS14(μ-Se)3(CN)6]6-, and [Re12CS14(μ-O)3(OH)6]6-, were isolated and their solid-state structures were elucidated by single-crystal X-ray diffraction analysis. Along with the previously reported μ-sulfide clusters, the new species constitute a series of rhenium anionic complexes with the common formula [Re12CS14(μ-Q)3L6]6- (Q = O, S, Se, L = CN-; Q = O, S, L = OH-), within which the total charge and number of cluster valence electrons (CVEs) are constant. The article presents insights into the mechanistic and synthetic aspects of the substitution process, and it comprehensively discusses the influence of inner ligand environment on the structure, spectroscopic characteristics, and electrochemical behavior of the novel compounds.

Synthesis, structure, and properties of compounds containing both octahedral rhenium cluster cations and anions

The compound [{Re6S8}(3,5-Me2pzH)6]2[{Re6S8}Br6]•9H2O•4EtOH containing a cluster cation and a cluster anion in a 2: 1 ratio was synthesized by mixing an ethanol solution of the rhenium cluster complex [{Re6S8}(3,5-Me2pzH)6]Br2•2(3,5-Me2pzH) (pzH is pyrazole) with an aqueous solution of Cs4[{Re6S8}Br6]•2H2O. The resulting compound was characterized by a combination of physicochemical methods. In particular, luminescence properties were studied and compared with those of the parent cluster complexes. A crystal suitable for single-crystal X-ray diffraction was obtained by layering these solutions. According to single-crystal X-ray diffraction analysis, this compound is composed of a cluster cation and a cluster anion (in a ratio of 1: 1) and also of Zundel cations (H5O2 +) and solvate water molecules and is described by the formula (H5O2)2[{Re6S8}(3,5-Me2pzH)6][{Re6S8}Br6]•4H2O.

A comparative study of hydrophilic phosphine hexanuclear rhenium cluster complexes' toxicity.

The octahedral rhenium cluster compound Na2H8[{Re6Se8}(P(C2H4CONH2)(C2H4COO)2)6] has recently emerged as a very promising X-ray contrast agent for biomedical applications. However, the synthesis of this compound is rather challenging due to the difficulty in controlling the hydrolysis of the initial P(C2H4CN)3 ligand during the reaction process. Therefore, in this report we compare the in vitro and in vivo toxicity of Na2H8[{Re6Se8}(P(C2H4CONH2)(C2H4COO)2)6] with those of related compounds featuring the fully hydrolysed form of the phosphine ligand, namely Na2H14[{Re6Q8}(P(C2H4COO)3)6] (Q = S or Se). Our results demonstrate that the cytotoxicity and acute in vivo toxicity of the complex Na2H8[{Re6Se8}(P(C2H4CONH2)(C2H4COO)2)6] solutions were considerably lower than those of compounds with the fully hydrolysed ligand P(C2H4COOH)3. Such behavior can be explained by the higher osmolality of Na2H14[{Re6Q8}(P(C2H4COO)3)6] versus Na2H8[{Re6Se8}(P(C2H4CONH2)(C2H4COO)2)6].