Rhenium Tricarbonyl Research Articles

Catalytic Deoxydehydration of Diols to Olefins by using a Bulky Cyclopentadiene‐based Trioxorhenium Catalyst

A bulky cyclopentadienyl (Cp)-based trioxorhenium compound was developed for the catalytic deoxydehydration of vicinal diols to olefins. The 1,2,4-tri(tert-butyl)cyclopentadienyl trioxorhenium (2) catalyst was synthesised in a two-step synthesis procedure. Dirhenium decacarbonyl was converted into 1,2,4-tri(tert-butyl)cyclopentadienyl tricarbonyl rhenium, followed by a biphasic oxidation with H2 O2 . These two new three-legged compounds with a 'piano-stool' configuration were fully characterised, including their single crystal X-ray structures. Deoxydehydration reaction conditions were optimised by using 2 mol % loading of 2 for the conversion of 1,2-octanediol into 1-octene. Different phosphine-based and other, more conventional, reductants were tested in combination with 2. Under optimised conditions, a variety of vicinal diols (aromatic and aliphatic, internal and terminal) were converted into olefins in good to excellent yields, and with minimal olefin isomerisation. A high turnover number of 1400 per Re was achieved for the deoxydehydration of 1,2-octanediol. Furthermore, the biomass-derived polyols (glycerol and erythritol) were converted into their corresponding olefinic products by 2 as the catalyst.

Monometallic rhenium(I) complexes as sensor for anions

Abstract Three new tricarbonyl rhenium(I) polypyridine complexes containing amide, urea and thiourea derivatives of 2,2′-bipyridine ligands L1 – L3 have been synthesized, characterized and found to show significant UV–visible absorption spectral changes on the addition of anions CN − , F − , CH 3 COO − and H 2 PO 4 − . These spectral changes and 1 H NMR titrations of complexes 1 – 3 with anions indicate strong binding of anions with the Re(I) complexes and the binding constants are in the range of 10 3 –10 5 M − 1 . The strong binding is attributed to the H-bond formation/deprotonation of NH proton in the ligand and the selectivity of anion is in the order CN − > F − > CH 3 COO − > H 2 PO 4 − .

Ligand-Induced Structural, Photophysical, and Electrochemical Variations in Tricarbonyl Rhenium(I) Tetrazolato Complexes

Treatment of [Re(CO)5X] (X = Cl, Br) with 2-(2-tert-butyltetrazol-5-yl)pyridine yielded neutral mononuclear complexes by exchange of two CO ligands for the chelating tetrazolato ligand. Treatment of [Re(CO)5Br] with phenyltetrazolate resulted in the assembly of an anionic dinuclear rhenium tricarbonyl species bridged by three tetrazole rings. The reaction of [Re(CO)5Br] with (2-tert-butyltetrazol-5-yl)benzene formed an analogous neutral dinuclear complex bridged by a tetrazole ring as well as two bromide ligands; however this complex was found to be rather unstable in solution and was only structurally characterized via X-ray diffraction. The first three complexes were investigated for their photophysical properties, highlighting phosphorescent emission from their triplet metal-to-ligand charge transfer excited states, although in the case of the dinuclear species the quantum yield was found to be extremely low. The complexes are also characterized for their electrochemical behavior, and while the neutral...

Synthesis, Characterization, and Binding to the Translocator Protein (18 kDa, TSPO) of a New Rhenium Complex as a Model of Radiopharmaceutical Agents

AbstractA new tridentate 2‐phenyl‐imidazopyridin‐dipropylacetamide ligand (CB239‐H) with high (nanomolar) affinity for the TSPO protein was synthesized and its coordination compound with rhenium tricarbonyl, fac‐[Re(CO)3(CB239‐N,N,O)] was investigated. The procedure established for the synthesis of the 187/185Re complex can be also used for the synthesis of 99mTc and 188/186Re analogues, which find application in SPECT diagnosis and in therapy. Because of the tridentate coordination of CB239‐H and the kinetic inertness of the carbonyl ligands, the new complex was expected to exhibit low reactivity towards plasma proteins and hence greater resistance to deactivation. Being TSPO overexpressed in numerous types of cancers and in activated microglial cells occurring in inflammatory neurodegenerative diseases, TSPO ligands can be exploited as carriers for receptor‐mediated drug targeting and hence can be used in diagnosis as well as in therapy. Very suprisingly, fac‐[Re(CO)3(CB239‐N,N,O)] resulted to be not very stable in diluted human serum but maintained a good affinity towards TSPO.

Crystal structures and properties of the tetrathiafulvalene substituted bispyrazol ligand and its rhenium(I) tricarbonyl complex

Abstract The new tetrathiafulvalene-functionalized bispyrazole ligand ( L ) and the corresponding tricarbonyl rhenium(I) complex ( 4 ) have been synthesized and structurally characterized. Their absorption spectra and redox behaviors have been recorded, and the absorption spectral changes of L upon addition of NOPF 6 have also been studied in detail.

Coordination of N,N-Chelated Re(CO)3Cl Units Across a Mo2 Quadruple Bond: Synthesis, Characterization, and Photophysical Properties of a Re–Mo2–Re Triad and Its Component Pieces

2-(2-Pyridyl)-4-methylthiazole carboxylic acid (PMT-H) and rhenium tricarbonyl chloride react to form the red crystalline compound fac-Re(PMT-H)(CO)3Cl, I, which is an analog of the well-known Re(bpy)(CO)3Cl molecule, where bpy is 2,2'-bipyridine. The acids PMT-H (2 equiv) and Re(PMT-H)(CO)3Cl (2 equiv) also react with Mo2(T(i)PB)4 (T(i)PB = 2,4,6-triisopropylbenzoate) in toluene to give the red compound trans-Mo2(T(i)PB)2(PMT)2, II, and the royal blue compound trans-Mo2(T(i)PB)2[(PMT)Re(CO)3Cl]2, III, respectively. The X-ray and spectroscopic characterization of I confirms its close relationship with Re(bpy)(CO)3Cl, as does the spectroscopic characterization of compounds II and III as analogs of other compounds of the form trans-M2(TiPB)2L2, where L is a π-acceptor ligand. Electronic structure calculations on model compounds II' and III', where formate ligands substitute for T(i)PB, show that the highest occupied molecular orbital (HOMO) in II is Mo2δ. When the Re(CO)3Cl unit is attached to the PMT ligand to form III, this orbital is stabilized significantly and now becomes associated with a close in energy band of Re d(6), t2g type orbitals. Oxidation of III is shown to be Mo2-based, as evident by EPR spectroscopy, and the lowest-energy electronic absorption corresponds to a Mo2δ-to-PMT π* transition. The S1 states in both II and III are metal-to-ligand charge-transfer (MLCT), and the lowest-energy triplet sate, T1 is (3)MoMoδδ*, as evidenced by its steady state emission spectral features. The excited states of compounds I (T1) and III (S1 and T1) have been investigated by time-resolved infrared spectroscopy (TRIR). The spectral features of I parallel those for Re(bpy)(CO)3Cl, with the lowest-energy T1 state corresponding to Re dπ to PMT-H π* charge transfer, producing higher-energy CO stretching vibrations relative to the ground state. For III, the CO vibrations are shifted to lower energy, consistent with charge being located on the PMT ligand, which enhances Re-to-CO backbonding. In the MoMoδδ* T1 state, however, the backbonding is reduced to the PMT ligand, and the CO stretches are at slightly higher energy relative to the ground state.

Tricarbonyl rhenium complex of 2,6-bis(8′-quinolinyl)pyridine: Synthesis, spectroscopic characterization, X-ray structure and DFT calculations

2,6-Bis(8′-quinolinyl)pyridine (bqp) reacts with pentacarbonyl rhenium chloride in toluene to give fac,fac-[Re(bqp-κ3N)(CO)3](Cl). X-ray crystallographic data on fac,fac-[Re(bqp-κ3N)(CO)3](Cl) show that the bqp ligand and carbonyl ligands achieve a mutually facial arrangement. A mix of MLCT and π–π* ligand centered transitions is observed for the low energy UV–vis absorption bands. The complex is emissive in solution and appears to be dominated by a MLCT-based process. Electronic and structural characteristics are supported by DFT calculations.

The photochemistry of rhenium(i) tricarbonyl N-heterocyclic carbene complexes

The photophysical and photochemical properties of the new tricarbonyl rhenium(I) complexes bound to N-heterocyclic carbene ligands (NHC), fac-[Re(CO)3(N^C)X] (N^C = 1-phenyl-3-(2-pyridyl)imidazole or 1-quinolinyl-3-(2-pyridyl)imidazole; X = Cl or Br), are reported. The photophysics of these complexes highlight phosphorescent emission from triplet metal-to-ligand ((3)MLCT) excited states, typical of tricarbonyl rhenium(I) complexes, with the pyridyl-bound species displaying a ten-fold shorter excited state lifetime. On the other hand, these pyridyl-bound species display solvent-dependent photochemical CO dissociation following what appear to be two different mechanisms, with a key step being the formation of cationic tricarbonyl solvato-complexes, being themselves photochemically active. The photochemical mechanisms are illustrated with a combination of NMR, IR, UV-Vis, emission and X-ray structural characterization techniques, clearly demonstrating that the presence of the NHC ligand is responsible for the previously unobserved photochemical behavior in other photoactive tricarbonyl rhenium(I) species. The complexes bound to the quinolinyl-NHC ligand (which possess a lower-energy (3)MLCT) are photostable, suggesting that the photoreactive excited state is not any longer thermally accessible. The photochemistry of the pyridyl complexes was investigated in acetonitrile solutions and also in the presence of triethylphosphite, showing a competing and bifurcated photoreactivity promoted by the trans effect of both the NHC and phosphite ligands.

Visible light-absorbing rhenium(i) tricarbonyl complexes as triplet photosensitizers in photooxidation and triplet–triplet annihilation upconversion

We prepared N^N Re(I) tricarbonyl chloride complexes (Re-1 and Re-2) that give very strong absorption of visible light. To this end, it is for the first time that boron dipyrimethane (Bodipy) was used to prepare Re(I) tricarbonyl chloride complexes. The π-conjugation linker between the π-conjugation framework of the antenna Bodipy and the Re(I) coordination centre ensures efficient intersystem crossing (ISC). Re-0 without visible light-harvesting ligand was prepared as a model complex in the photophysical studies. Re-1 (with Bodipy) and Re-2 (with carbazole-ethynyl Bodipy) show unprecedented strong absorption of visible light at 536 nm (ε = 91700 M(−1) cm(−1)) and 574 nm (ε = 64,600 M(−1) cm(−1)), respectively. Interestingly, different from Re-0, Re-1 and Re-2 show fluorescence of the ligand, not the phosphorescence of the Re(I) coordination centre. However, long-lived triplet excited states were observed upon visible light excitation (τ(T) = 104.0 μs for Re-1; τ(T) = 127.2 μs for Re-2) vs. the short lifetime of Re-0 (τ(T) = 26 ns). With nanosecond time-resolved transient absorption spectroscopy and DFT calculations, we proved that the triplet excited states of Re-1 and Re-2 are localized on the Bodipy ligands. The complexes were used as triplet photosensitizers for two triplet–triplet-energy-transfer (TTET) processes, i.e.(1)O(2) mediated photooxidation and triplet–triplet annihilation (TTA) upconversion. With the strong visible light-harvesting ability, Re-1 proved to be a better (1)O(2) photosensitizer than the conventional triplet photosensitizer tetraphenylporphyrin (TPP). Significant upconversion was observed with Re-1 as the triplet photosensitizer. Our result is useful for preparation of Re(I) tricarbonyl chloride complexes that show strong absorption of visible light and long-lived triplet excited states and for the application of these complexes as triplet photosensitizers in photocatalysis, photodynamic therapy and TTA upconversion.

Rhenium(i) tricarbonyl complex of 5,20-bis(p-tolyl)-10,15-bis(p-methoxyphenyl)-21-selenaporphyrin: first X-ray structural characterization of metal complex of 21-selenaporphyrin

Synthesis and first structural characterization of hexa coordinated rhenium(I) tricarbonyl complex of 5,20-bis(p-tolyl)-10,15-bis(p-methoxyphenyl)-21-selenaporphyrin 3 are described. The Re(I) complex of 21-selenaporphyrin 3 was synthesized by treating free base 21-selenaporphyrin in 1,2-dichlorobenzene with Re(CO)5Cl at reflux for 7 h and analyzed using mass, NMR, FT-IR, UV-vis and electrochemical techniques. The first structure of metal complex of 21-selenaporphyrin was determined by X-ray single crystal analysis. The crystal structure revealed that the Re(CO)3 coordinates to two of the three inner nitrogens and one selenium to produce compound 3. The selenophene ring bent towards the Re(I) ion and the selenium is displaced by 0.41 Å from the mean plane of 24-atoms to coordinate with Re(I) ion in η(1)-fashion. The 21-selenaporphyrin is distorted in compound 3 compared to free base 21-selenaporphyrin. (1)H and (13)C NMR studies indicated that compound 3 exhibits fluxional behaviour in coordination mode of binding in solution. The compound 3 is highly stable and does not undergo decomplexation under acidic conditions. The absorption spectra showed three broad Q-bands and splitted Soret band and electrochemical studies indicated that compound 3 is stable under redox conditions.

Rhenium(i) phenanthrolines bearing electron withdrawing CF3 substituents: synthesis, characterization and biological evaluation

Rhenium(I) tricarbonyl complexes bearing a 2,4,7,9-tetraphenylphenanthroline ligand, where the 2,9-phenyl groups have either meta- or para-CF3 groups, have been screened against a selection of cell lines. The meta derivative shows anticancer activity against HeLa and A549 cell lines, whereas the para derivative causes proliferation of HL-60 cells, whilst showing toxicity towards A549 cells.

Electronic structures and spectral properties of rhenium (I) tricarbonyl cyclopenta[b]dipyridine complexes containing different aromatic ring groups

The ground geometries, frontier molecular orbitals, and absorption properties of [Re(CO)3(cpen-dpy)Cl] (cpen-dpy = cyclopenta[b]dipyridine) functionalized with different aromatic ring groups were theoretically studied. The introduction of different moieties insignificantly changed the geometric parameters of the complexes. However, the molecular orbital energy levels of the complexes with aromatic ring groups were strongly affected by the solvent molecules. The lowest lying absorption bands of all complexes had HOMO → LUMO transition configurations that resulted in the ligand-to-ligand charge transfer character. The results showed that all complexes displayed a comparative higher sensitivity within the long wavelength range with stronger oscillator strength, which can be beneficial to more efficient absorption of the solar spectrum. The calculated ionization potential and electron affinity revealed that the balanced transport of electrons and holes of 1 was more accessible, whereas 4 possessed good hole-transfer abilities.

Re-Mediated C–C Coupling of Pyridines and Imidazoles

Rhenium tricarbonyl complexes with three N-heterocyclic ligands (N-alkylimidazoles or pyridines) undergo deprotonation with KN(SiMe(3))(2) and then oxidation with AgOTf to afford complexes with pyridylimidazole or bipyridine bidentate ligands resulting from deprotonation, C-C coupling and rearomatization.

Electronic Structures and Spectral Properties of the Emitting Molecule Rhenium (I) Tricarbonyl Containing Naphthalimide Ring in Chemical Manufacturing System

Rhenium (I) tricarbonyl complexes comprise an important class of luminescent materials in chemical manufacturing system. A Re(I) complexes, Re(CO)3(phen-PIN)(Cl) (1), where PNI = 4-piperidinyl-1,8-naphthalimide, was investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The optimized ground structures show that the phen ring is not coplanar with the naphthalimide ring. The HOMO is π character, while the LUMO is π* orbitals of the phen ligands. The lowest lying absorption band of the complexes has the HOMO → LUMO+1 transition configurations resulting in the LLCT/ILCT characters.

Spectral Properties of the Optoelectronic Material Rhenium (I) Tricarbonyl Complexes with Bipyridine Ligand Containing Triphenylamine Moiety

The photophysical behavior of rhenium (I) complex having a bipyridine ligand with triphenylamidine moiety is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The complex shows the maximum absorption band at 417.1 nm, with ligand-to-ligand charge transfer (LLCT) character. The lowest lying triplet excited state was localized on3LLCT/ILCT state

Tricarbonyl rhenium(I) complex of benzothiazole – Synthesis, spectroscopic characterization, X-ray crystal structure and DFT calculations

The paper presents a combined experimental and computational study of a novel tricarbonyl rhenium(I) complex with benzothiazole ligand – [Re(CO)3(bt)2Cl]. The compound has been characterized by structural (single-crystal X-ray diffraction) and spectroscopic (IR, NMR, MS, UV–vis) methods. DFT and time-dependent DFT (TDDFT) calculations have been also carried out and the UV–vis spectrum of the complex has been discussed on this basis.

Rhenium(I) tricarbonyl complexes of salicylaldehyde semicarbazones: Synthesis, crystal structures and cytotoxicity

A series of N,N-disubstituted salicylaldehyde semicarbazones (SSCs), HOC6H4CHN-NHCONR2, and their rhenium(I) tricarbonyl complexes, [ReBr(CO)3(SSC)], have been synthesised and characterised by IR and 1H NMR spectroscopy. Crystallographic analysis of the complex [ReBr(CO)3(H2Bu2)] (H2Bu2=SSC where R=Bun) showed that the SSC acts as a bidentate ligand via its imino nitrogen and carbonyl oxygen atoms. The [ReBr(CO)3(SSC)] complexes exhibit moderate to high cytotoxicities towards MOLT-4 cells (IC50=1–24μM, cf. 18μM for cisplatin), and the majority of them are virtually non-toxic against non-cancerous human fibroblasts. Apoptotic assays of [ReBr(CO)3(H2Bnz2)] (Bnz=benzyl) revealed that it mediates cytotoxicity in MOLT-4 cells via apoptosis. The complex [ReBr(CO)3(H2Bnz2)] reacts with guanosine by proton transfer from the phenolic OH group to N(7) of guanosine. In (CD3)2SO, [ReBr(CO)3(H2Bnz2)] undergoes facile conversion to the dimeric complex, [Re(CO)3(HBnz2)]2, via bromide dissociation.

Dynamics of Functionalized Surface Molecular Monolayers Studied with Ultrafast Infrared Vibrational Spectroscopy.

The structural dynamics of thin films consisting of tricarbonyl (1,10-phenanthroline)rhenium chloride (RePhen(CO)(3)Cl) linked to an alkyl silane monolayer through a triazole linker synthesized on silica-on-calcium-fluoride substrates are investigated using ultrafast infrared (IR) techniques. Ultrafast 2D IR vibrational echo experiments and polarization selective heterodyne detected transient grating (HDTG) measurements, as well as polarization dependent FT-IR and AFM experiments are employed to study the samples. The vibrational echo experiments measure spectral diffusion, while the HDTG experiments measure the vibrational excited state population relaxation and investigate the vibrational transition dipole orientational anisotropy decay. To investigate the anticipated impact of vibrational excitation transfer, which can be caused by the high concentration of RePhen(CO)(3)Cl in the monolayer, a concentration dependence of the spectral diffusion is measured. To generate a range of concentrations, mixed monolayers consisting of both hydrogen terminated and triazole/RePhen(CO)(3)Cl terminated alkyl silanes are synthesized. It is found that the measured rate of spectral diffusion is independent of concentration, with all samples showing spectral diffusion of 37 ± 6 ps. To definitively test for vibrational excitation transfer, polarization selective HDTG experiments are conducted. Excitation transfer will cause anisotropy decay. Polarization resolved heterodyne detected transient grating spectroscopy is sensitive to anisotropy decay (depolarization) caused by excitation transfer and molecular reorientation. The HDTG experiments show no evidence of anisotropy decay on the appropriate time scale, demonstrating the absence of excitation transfer the RePhen(CO)(3)Cl. Therefore the influence of excitation transfer on spectral diffusion is inconsequential in these samples, and the vibrational echo measurements of spectral diffusion report solely on structural dynamics. A small amount of very fast (~2 ps time scale) anisotropy decay is observed. The decay is concentration independent, and is assigned to wobbling-in-a-cone orientational motions of the RePhen(CO)(3)Cl. Theoretical calculations reported previously for experiments on a single concentration of the same type of sample suggested the presence of some vibrational excitation transfer and excitation transfer induced spectral diffusion. Possible reasons for the experimentally observed lack of excitation transfer in these high concentration samples are discussed.

Rhenium(I) tricarbonyl complexes with multidentate nitrogen-donor derivatives

The reaction of [Re(CO)5X] (X=Cl, Br) with a series of six potentially multidentate nitrogen donor ligands (L1–L6) was studied. The ligands L1–L5 are coordinated bidentately in the neutral complexes of the type fac-[Re(CO)3X(L)] (1–5). The complex salt [Re(CO)3(L6)]Br (6) was formed by the reaction of [Re(CO)5Br] with 2-aminobenzaldehyde (aba), in which the tridentate ligand L6 was formed by the Schiff base condensation of three aba molecules. The crystal structures for complexes 1–6 were determined by X-ray single crystal diffraction. UV–Vis absorption spectra of the complexes show the intraligand and MLCT transitions. Electrochemical behaviour of the complexes was studied with cyclic voltammetry.

Novel imino rhenium(I) tricarbonyl complexes of salicylidene-derived ligands: Synthesis, X-ray crystallographic studies, spectroscopic characterization and DFT calculations

Novel rhenium(I) tricarbonyl complexes of the form fac-[Re(L,L′-Bid)(CO)3(S)] (L,L′10.1016/j.poly.2012.08.059-Bid=N,O-Sal-type bidentate ligand, S=coordinating methanol or pyridine) were obtained by the reaction of the fac-[Re(CO)3]+ moiety with a range of 2-(“T”-iminomethyl)-5-methylphenol Schiff-base ligands. Various biological functional groups (“T”) with different electronic and steric properties were introduced at the imino nitrogen atom. The complexes were characterized spectroscopically by IR, 1H and 13C NMR. Three solid state structures were elucidated by X-ray diffraction and compared to DFT calculations employing B3LYP functions in combination with LANL2DZ.