Tricarbonyl Complexes Research Articles

Crystal structure, spectroscopic characterization and computational studies of a Re(I) tricarbonyl-diimine complex with the N,N′-bis(2-methylbenzaldehyde)-1,2-diiminoethane Schiff base

The synthesis, structure and spectroscopic properties of a tricarbonylrhenium(I) complex with the N,N′-bis(2-methylbenzaldehyde)-1,2-diiminoethane Schiff base ligand have been investigated. This complex was characterized by FT-IR, NMR, UV–Vis spectroscopy and X-ray crystallography. Optimized geometric parameters and electronic properties of the synthesized compound and some similar Re(I) tricarbonyl complexes were obtained. The low lying electronic transition energies of the complexes have been determined with the time dependent density functional theory method. The calculations showed that in the complexes the HOMOs cover the Re atom, carbonyl groups and chlorine p orbitals, while the LUMOs are mainly localized on the diimine ligand as a π∗ orbital.

Thermal and Photochemical Reactivity of Manganese Tricarbonyl and Tetracarbonyl Complexes with a Bulky Diazabutadiene Ligand

The manganese tricarbonyl complex fac-Mn(Br)(CO)3((i)Pr2Ph-DAB) (1) [(i)Pr2Ph-DAB = (N,N'-bis(2,6-di-isopropylphenyl)-1,4-diaza-1,3-butadiene)] was synthesized from the reaction of Mn(CO)5Br with the sterically encumbered DAB ligand. Compound 1 exhibits rapid CO release under low power visible light irradiation (560 nm) suggesting its possible use as a photoCORM. The reaction of compound 1 with TlPF6 in the dark afforded the manganese(I) tetracarbonyl complex, [Mn(CO)4((i)Pr2Ph-DAB)][PF6] (2). While 2 is comparatively more stable than 1 in light, it demonstrates high thermal reactivity such that dissolution in CH3CN or THF at room temperature results in rapid CO loss and formation of the respective solvate complexes. This unusual reactivity is due to the large steric profile of the DAB ligand which results in a weak Mn-CO binding interaction.

Synthesis and evaluation of a new 99mTc(I)‐tricarbonyl complex bearing the 5‐nitroimidazol‐1‐yl moiety as potential hypoxia imaging agent

The objective of this work was to develop a novel (99m) Tc complex bearing the 5-nitroimidazol-1-yl moiety with recognised selectivity towards hypoxic tissue, as a potential radiopharmaceutical for imaging tumour hypoxia. The new metronidazole derivative (2-amine-3-[2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethylthio]propanoic acid) (L) containing adequate groups to coordinate technetium through the formation of a Tc(I)-tricarbonyl complex was synthesised with adequate yield (33%) and characterised by spectroscopy. Labelling was performed by substitution of three labile water molecules of the technetium tricarbonyl precursor, fac-[(99m)Tc(CO)3 (H2O)3](+) with the ligand. A radiochemical purity higher than 90% was achieved and remained unchanged for more than 4 h. The complex has a high stability in plasma, a moderate plasma protein binding and a moderate hydrophilicity. In vitro cell uptake studies showed a ratio between the activity taken up by cells in hypoxia/normoxia of 1.6 ± 0.4 (p < 0.5). Biodistribution in normal mice showed rapid depuration and low uptake in all organs and tissues except liver. Biodistribution in mice bearing induced tumours showed a low tumour uptake, but tumour/muscle ratio was favourable thanks to depuration. Comparison with biological results of other metronidazole derivatives clearly shows that modifications of the chelator are very important and contribute to improve the biological behaviour.

CC Coupling of N‐Heterocycles at the fac‐Re(CO)3 Fragment: Synthesis of Pyridylimidazole and Bipyridine Ligands

A new family of cationic rhenium tricarbonyl complexes with either two N-alkylimidazole (N-RIm) and one pyridine (Py) ligand, or two pyridine and one N-RIm ligand, [Re(CO)3 (N-RIm)(3-x) (Py)x ](+) , has been prepared. The reaction of these complexes with a strong base, followed by an oxidant, selectively afforded 2,2'-pyridylimidazole complexes as the result of intramolecular dehydrogenative CC coupling reactions. For tris(pyridine) complexes [Re(CO)3 (Py)3 ](+) the reaction pattern upon a deprotonation/oxidation sequence is maintained, which allows the generation of complexes with 2,2'-bipyridine ligands. In the particular combination of two different types of pyridine ligand in the cationic fac-Re(CO)3 complexes only the cross-coupling products with asymmetric 2,2'-bipyridine ligands were obtained; the homocoupling products were not observed.

Re(I) tricarbonyl complex of 1,10-phenanthroline-5,6-dione: DNA binding, cytotoxicity, anti-inflammatory and anti-coagulant effects towards platelet activating factor

The complex fac-[Re(CO)3(phendione)Cl] (1) (where phendione=1,10-phenanthroline-5,6-dione) has been synthesized and fully characterized by UV–visible, FTIR, and NMR techniques. The DNA binding properties of 1 are investigated by UV-spectrophotometric (melting curves), covalent binding assay, CV (cyclic voltammetry), circular dichroism (CD) and viscosity measurements. Experimental data indicate that 1 fits into the major groove without disrupting the helical structure of the B-DNA in contrast to the free phendione which intercalates within the base pairs of DNA. Upon irradiation, complex 1 promotes the cleavage of plasmid pBR322 DNA from supercoiled form I to nicked form II via a proton coupled electron transfer mechanism. This comes as a result of experimental data in anaerobic/aerobic conditions and in the presence of DMSO. The biological activities of 1 and its precursors [Re(CO)5Cl] and phendione are tested towards a series of cancerous cell lines as glioblastoma (T98G), prostate cancer (PC3) and breast cancer (MCF-7) as well as platelet activating factor (PAF)-aggregation. Moreover, all the aforementioned compounds are tested for their ability to modulate PAF-basic metabolic enzyme activities in preparations of rabbit leukolytes. The in vitro experiments indicate that phendione has a better antitumor effect than cisplatin whereas [Re(CO)5Cl] is a better PAF inhibitor than both the phendione ligand and 1. Moreover, for the first time it is indicated that [Re(CO)5Cl], with a IC50 of 17nM is comparable to the widely used PAF receptor antagonists, BN52021 and WEB2170 with IC50 of 30 and 20nM, respectively, whereas 1 affects PAF-catabolism.

Photochemistry of Chromium Arene Tricarbonyl Complexes with Tethered Pyridinyl and Propenyl Groups: Investigations of the Effect of Ring Size on Chelate Formation, Structure, and Linkage Isomerization

Chromium arene tricarbonyl complexes with tethered pyridinyl groups [Cr{η6-C6H5(CH2)n(2-Py)}(CO)3] (4–6) (2-Py = 2-pyridinyl, n = 1–3, respectively) were synthesized and irradiated to form the chelates [Cr{η6-C6H5(CH2)n(2-Py)-κN}(CO)2] (7–9). Studies examined the effect of ring size and structure on chromophore λmax, stability, and photosensitivity, which are factors important for photochromes based on linkage isomerization of tethered functional groups. The studies also include [Cr{η6-C6H5CH(2-Py)CH2CH═CH2}(CO)3] (3), which has a bifunctional tether of propenyl and pyridinyl groups, and irradiation produces the linkage isomers [Cr{η6-C6H5(CH(2-Py)CH2CH═CH2)-κN}(CO)2] (1) and [Cr{η6-C6H5(CH(2-Py)CH2CH═CH2)(η2-CH═CH2)}(CO)2] (2). X-ray crystal structures for 7–9 show that the dihedral angle between the coordinated pyridinyl groups and the phenyl-chromium centroid increases from 1 to 73° (n = 1–3, respectively). The experimental and TDDFT computed optical changes accompanying an increase in the dihedral ang...

Rhenium and technetium bi- and tricarbonyl complexes in a new strategy for biomolecule incorporation using click chemistry.

A versatile strategy to prepare fac-[M(I)(CO)3](+) and cis-[M(I)(CO)2](+) (M = Re, (99m)Tc) complexes was developed using Huisgen click chemistry and monodentate phosphine ligands to readily incorporate biomolecules and tailor the chemical properties.

New modular manganese(i) tricarbonyl complexes as PhotoCORMs: in vitro detection of photoinduced carbon monoxide release using COP-1 as a fluorogenic switch-on probe

Five manganese(i) tricarbonyl complexes of the general formulae [Mn(bpea(N=CHC6H4R))(CO)3]PF6 and [Mn(bpea(NHCH2C6H4R))(CO)3]PF6 based on the tridentate bis(pyrazolyl)ethylamine (bpea) ligand, each containing a pendant 4-substituted phenyl group with R = H, I, and C≡C-H, were synthesized and fully characterized, including X-ray structure analysis for three compounds. All complexes are stable in the dark in aqueous buffer for an extended period of time. However, CO-release could be triggered by illumination at 365 nm, establishing these compounds as novel photoactivatable CO-releasing molecules (PhotoCORMs). The influence of the imine vs. amine group in the ligands on the electronic structure and the photophysical behavior was investigated with the aid of DFT and TDDFT calculations. Solution IR studies on selected compounds allowed identification of intermediates resulting from the photoreaction. Finally, light-induced CO release from a model compound was demonstrated both in PBS buffer and in vitro in human umbilical vein endothelial cells (HUVECs) using COP-1 as a fluorescent switch-on probe.

Towards cancer cell-specific phototoxic organometallic rhenium(i) complexes

Over the recent years, several Re(I) organometallic compounds have been shown to be toxic to various cancer cell lines. However, these compounds lacked sufficient selectivity towards cancer tissues to be used as novel chemotherapeutic agents. In this study, we probe the potential of two known N,N-bis(quinolinoyl) Re(I) tricarbonyl complex derivatives, namely Re(I) tricarbonyl [N,N-bis(quinolin-2-ylmethyl)amino]-4-butane-1-amine (Re-NH₂) and Re(I) tricarbonyl [N,N-bis(quinolin-2-ylmethyl)amino]-5-valeric acid (Re-COOH), as photodynamic therapy (PDT) photosensitizers. Re-NH₂ and Re-COOH proved to be excellent singlet oxygen generators in a lipophilic environment with quantum yields of about 75%. Furthermore, we envisaged to improve the selectivity of Re-COOH via conjugation to two types of peptides, namely a nuclear localization signal (NLS) and a derivative of the neuropeptide bombesin, to form Re-NLS and Re-Bombesin, respectively. Fluorescent microscopy on cervical cancer cells (HeLa) showed that the conjugation of Re-COOH to NLS significantly enhanced the compound's accumulation into the cell nucleus and more specifically into its nucleoli. Importantly, in view of PDT applications, the cytotoxicity of the Re complexes and their bioconjugates increased significantly upon light irradiation. In particular, Re-Bombesin was found to be at least 20-fold more toxic after light irradiation. DNA photo-cleavage studies demonstrated that all compounds damaged DNA via singlet oxygen and, to a minor extent, superoxide production.

New insights into dihydrogenphosphate recognition with dirhenium(i) tricarbonyl complexes bridged by a thiourea moiety

Three thiourea ligands and their dirhenium(i) complexes are reported that show an unusual cooperative binding to two dihydrogenphosphate anions; this could explain the high selectivity for phosphate in many reported anion receptors.

ChemInform Abstract: New Cycloaddition/Fragmentation Strategies for Preparing 5‐7‐5 and 5‐7‐6 Fused Tricyclic Ring Systems.

Abstract Tethering additional functionality to cyclobutadienyl iron tricarbonyl complexes provides new opportunities for the rapid construction of medium-ring-containing polycyclic compounds. Specifically, an intramolecular cycloaddition between cyclobutadiene and a tethered olefin, followed by an intramolecular cyclopropanation of the resulting cyclobutene-containing adduct generates highly strained pentacyclic intermediates. These compounds can then be relaxed thermally to generate 5-7-5 and 5-7-6 fused tricyclic ring systems that are shared with numerous natural products.

Re(I) and 99mTc(I) tricarbonyl complexes with ether-containing pyrazolyl-based chelators: Chemistry, biodistribution and metabolism

Tris(pyrazolyl)methane chelators, L1–L3, containing one or two ether groups at different positions of the azole rings, were synthesized and fully characterized. These chelators enabled the synthesis of fac-[99mTc(CO)3{HC[4-(ROCH2)pz]3}]+ (R = Me (Tc1), Et (Tc2)) and fac-[99mTc(CO)3{HC[3,5-(EtOCH2)2pz]3}]+ (Tc3) which were identified by HPLC in comparison with the rhenium counterparts. The evaluation of Tc1–Tc3 in CD-1 mice has shown that the number and/or nature of the ether groups greatly influence the biodistribution profile, pharmacokinetics and metabolic stability of these complexes. Tc1 and Tc2, bearing a unique ether substituent at the 4-position of the pyrazolyl ring, undergo metabolic transformation in vivo while Tc3 is not metabolized. The metabolization of Tc1 and Tc2 enhanced their rate of excretion but, most probably, also justify their negligible heart uptake in contrast with the high heart uptake of congener non-metabolizable complexes (99mTc-DMEOP and 99mTc-TMEOP), which have recently emerged as potential myocardial imaging agents.The attempts made to identify the metabolites of Tc1 and Tc2 have shown that the metabolization of these compounds must involve the ether functions with probable formation of carboxylic acid derivatives. A comparative study with the congener fac-[99mTc(CO)3{[4-(MeOCH2)pz](CH2)2NH(CH2)2NH2}]+ (Tc6) led us to confirm the formation of such type of metabolites. In fact, Tc6 is also metabolized in mice with formation of fac-[99mTc(CO)3{[4-(HOCH2)pz](CH2)2NH(CH2)2NH2}]+ (Tc7) and fac-[99mTc(CO)3{[4-(HOOC)pz](CH2)2NH(CH2)2NH2}]+ (Tc8), which were chemically identified by HPLC in comparison with the Re congeners (Re7 and Re8).

Bifunctional (Cyclopentadienone)Iron–Tricarbonyl Complexes: Synthesis, Computational Studies and Application in Reductive Amination

Reductive amination under hydrogen pressure is a valuable process in organic chemistry to access amine derivatives from aldehydes or ketones. Knölker's complex has been shown to be an efficient iron catalyst in this reaction. To determine the influence of the substituents on the cyclopentadienone ancillary ligand, a series of modified Knölker's complexes was synthesised and fully characterised. These complexes were also transformed into their analogous acetonitrile iron-dicarbonyl complexes. Catalytic activities of these complexes were evaluated and compared in a model reaction. The scope of this reaction is also reported. For mechanistic insights, deuterium-labelling experiments and DFT calculations were undertaken and are also presented.

The hapticity of octafluorocyclooctatetraene in its first-row mononuclear transition metal carbonyl complexes: effect of perfluorination

The iron tricarbonyl complex of octafluorocyclooctatetraene was synthesized by Hughes and co-workers and shown by X-ray crystallography to have a trihapto–monohapto structure (η3,1-C8F8)Fe(CO)3 in contrast to the tetrahapto structure (η4-C8H8)Fe(CO)3 formed by the non-fluorinated cyclooctatetraene. This difference has stimulated a comprehensive density functional theoretical study of the octafluorocyclooctatetraene metal carbonyl complexes (C8F8)M(CO)n (n = 4, 3, 2, 1 for M = Ti, V, Cr, Mn, and Fe; n = 3, 2, 1 for M = Co, Ni) for comparison with their hydrogen analogues (C8H8)M(CO)n. In most such systems, the substitution of fluorine for hydrogen leads to relatively small changes in the preferred structures. However, for the iron carbonyl derivatives (C8X8)Fe(CO)3 (X = H, F), the difference observed experimentally has been confirmed by theory with (η3,1-C8F8)Fe(CO)3 and (η4-C8H8)Fe(CO)3 being the lowest energy structures by 4 and 14 kcal/mol, respectively. The ligand exchange reactions C8H8 + (C8F8)M(CO)n → C8F8 + (C8H8)M(CO)n are predicted to be exothermic for almost all of the systems considered, with the (η3,1-C8X8)Fe(CO)3 system being the main exception. This suggests that the C8F8 ligand generally bonds more weakly to transition metals than the C8H8 ligand in accord with the electron-withdrawing effect of the ligand fluorine atoms.

Synthesis and Structural Characterization of Metal Compounds Incorporating the Ortho-Quinone Manganese Tricarbonyl Complex as an Organometalloligand

Three π-quinonoid compounds incorporating the organometalloligand ortho-quinone manganese tricarbonyl ([(C6H4O2)Mn(CO)3]−, o-QMTC) have been synthesized and characterized by IR and X-ray crystallography: [Pd(o-QMTC)(PPh3)2]BF4·CH2Cl2 (1), (o-QMTC)Rh(1,5-cyclooctadiene) (2), and Cu(o-QMTC)2 (3). The formally anionic o-QMTC ligand features Mn-arene bonding that is best described as η4-bound. Nevertheless, o-QMTC interplanar angular data and νco data suggest significant contribution from η5 and η6 forms. In each of the compounds, the o-QMTC organometalloligand(s) contribute to a square planar geometry about the Pd(II), Rh(I), or Cu(II) metal center.

Catalytic olefin epoxidation with a carboxylic acid-functionalized cyclopentadienyl molybdenum tricarbonyl complex

The complex CpMo(CO)3CH2COOH (1) (Cp = η5-C5H5) has been examined as a precatalyst for the epoxidation of cis-cyclooctene and α-pinene using tert-butylhydroperoxide (TBHP) as oxidant. A high turnover frequency of ca. 600 mol molMo−1 h−1 was achieved in the epoxidation of cyclooctene, giving the epoxide as the only reaction product. With α-pinene as substrate, the added-value products α-pinene oxide and campholenic aldehyde were obtained. Two different approaches to facilitate catalyst recovery and reuse were explored: (1) use of an ionic liquid (IL) as solvent, and (2) intercalation of 1 in a Zn,Al layered double hydroxide (LDH) by a direct synthesis (coprecipitation) route. Characterization of the LDH by powder X-ray diffraction, thermogravimetric analysis, FT-IR and 13C CP MAS NMR spectroscopies showed that the CpMo(CO)3CH2COO− anions intercalate in a bilayer arrangement, resulting in an interlayer spacing of 20.7 Å. In the epoxidation of cyclooctene, catalytic activity in the first batch run was very high for the catalyst/IL mixture and moderate for the LDH. Characterization of the LDH after catalysis indicated that nearly complete oxidative decarbonylation of supported complexes had occurred (by reaction with TBHP), resulting in the presence of immobilized oxomolybdenum species. However, catalytic activities for both the recovered LDH and catalyst/IL decreased in consecutive runs, due in part to progressive removal of active species during either the catalytic reaction (for the LDH) or the solvent extraction/work-up (for the catalyst/IL mixture).

Experimental and theoretical characterisation of phosphorescence from rhenium polypyridyl tricarbonyl complexes

Synthetic, spectroscopic and theoretical methods are used to probe the photophysics of five rhenium polypyridyl tricarbonyl complexes, which exhibit strong phosphorescence behaviour. Particular focus is placed on the effect of polypyridyl ligand structure on absorption and emission properties, for instance by extending conjugation or by addition of electron donating groups. Experimental methods are performed in a consistent and controlled manner, thereby giving data that is ideally suited for comparison with theory. Modifications in ligand structure give rise to large changes in absorption energies, but rather smaller differences in emission. Density functional theory (DFT) and its time-dependent formulation (TD-DFT) perform rather better in predicting emission than absorption. The effect of environment on photo-physical properties was probed theoretically by protonating nitrogen(s) of dipyrido[3,2-a:2′,3′-c]phenazine (dppz). This has a large effect on calculated spectra and leads to more complex patterns of absorption and emission that require spin–orbit coupling to be included in DFT calculations for full explanation.

Photoinduced linkage isomerization in new rhenium(I) tricarbonyl complexes coordinated to N-nitrite and O-nitrite

The ambidentate nitrite ion, NO2−, is a biologically relevant species that can be coordinated to transition metals via the nitrogen or oxygen atom. Most of its physiological effects involve metalloproteins so it is important to understand the factors that control the predominance of one or another isomer. In this work, we present the syntheses and characterization by spectroscopy, electrochemistry and DFT calculations of new rhenium(I) tricarbonyl complexes with different polypyridyl ligands in the coordination sphere and with the nitrite ion coordinated through either its N- or O-atom. For two of them we have determined their crystal structures by X-ray diffraction and we have found that in the solid when the ancilliary ligand is bpy the N-binding mode is the most stable, while dmb is the ligand, the –CH3 substituents imposed the C–H (methyl group)⋯O–NO (nitrito) interactions favoring the O-binding mode. However, at room temperature in CH2Cl2 the nitro isomer is the most stable for both complexes in agreement with DFT calculations. Both complexes exhibit photoinduced linkage isomerism which could be of interesting in the design of new materials. Although nitro complexes are precursors of nitrosyl compounds, in this work the synthesis of the nitrosyl complexes as solid could not be possible because there is higher π-backbonding from Re(I) to the carbonyl groups, or to the polypyridil ligands instead of to the NO group, which is π-acceptor, in consequence the nitrosyl complexes are unstable and decompose immediately to the solvate complexes.

Synthesis, characterization, structure and dual room-temperature fluorescent and phosphorescent emission in μ-oxalato-bridged rhenium(I) metallacycle

Abstract A new self-assembled dinuclear rhenium(I) tricarbonyl complex, [Re2(CO)6(μ-η4-C2O4)(μ-4,4′-tmdp)] (1) featuring a bis-chelating oxalato dianion (μ-η4-C2O4, H2C2O4 = oxalic acid) connects two fac-Re(CO)3 cores, which are interconnected by a neutral ditopic N-donor clip ligand, that is, 4,4′-trimethylenedipyridine (μ-4,4′-tmdp) has been elegantly synthesized in high yield in a one-step reaction. Compound 1 was characterized by elemental analysis, IR, ESI-MS, TGA and single-crystal X-ray diffraction analyses. The titled complex represents the first dirhenium(I) metallacycle which shows the coexistence of two emissive pathways at room temperature, one populates an emissive 3π–π* state and the other populates a 3MLCT state.

Molecular polarizability of the organic substances and their complexes: LX. Molecular volume and spatial structure of the series of heterocycles, their structurally nonrigid derivatives, and chromium tricarbonyl π-complexes in solutions

We have analyzed the molar volumes and structures in solutions of a series of azines, azoles, conformationally nonrigid bis(2-substituted benzimidazol-1-yl)methanes, and chromium tricarbonyl complexes of arenes. For most of the compounds, the rules of molar volume additivity with respect to the bonds and groups increments hold. Molecules of bis(2-substituted benzimidazol-1-yl)methanes exist in the form of conrotatory comformers with aryl fragments being out of the plane of bridging NCN bond angle. Strong linear correlations between the molar volume and molar refraction have been revealed within the studied compounds classes. In the case of chromium tricarbonyl complexes of arenes, the coordination bond polarity has increased with growing π-donor ability of the ligand, the molar volume increment of Cr(CO)3 has increased as well, due to transfer of π-electron density from the ligand. The simplification of dipole moment and Kerr constant determination has been demonstrated.