Metal-Assisted In Situ Formation of a Tridentate Acetylacetone Ligand for Complexation of fac-Re(CO)3+ for Radiopharmaceutical Applications

Synthesis, structural characterization and study of antioxidant and anti-PrPSc properties of flavonoids and their rhenium(I)-tricarbonyl complexes

This study aims at the synthesis and initial biological evaluation of novel rhenium-tricarbonyl complexes of 3,3',4',5,7-pentahydroxyflavone (quercetin), 3,7,4΄-trihydroxyflavone (resokaempferol), 5,7-dihydroxyflavone (chrysin) and 4΄,5,7-trihydroxyflavonone (naringenin) as neuroprotective and anti-PrP agents. Resokaempferol was synthesized from 2,2΄,4-trihydroxychalcone by H₂O₂/NaOH. The rhenium-tricarbonyl complexes of the type fac-[Re(CO)₃(Fl)(sol)] were synthesized by reacting the precursor fac-[Re(CO)₃(sol)₃]⁺ with an equimolar amount of the flavonoids (Fl) quercetin, resokaempferol, chrysin and naringenin and the solvent (sol) was methanol or water. The respective Re-flavonoid complexes were purified by semi-preparative HPLC and characterized by spectroscopic methods. Furthermore, the structure of Re-chrysin was elucidated by X-ray crystallography. Initial screening of the neuroprotective properties of these compounds included the in vitro assessment of the antioxidant properties by the DPPH assay as well as the anti-lipid peroxidation of linoleic acid in the presence of AAPH and their ability to inhibit soybean lipoxygenase. From the above studies, it was concluded that the complexes' properties are mainly correlated with the structural characteristics and the presence of the flavonoids. The flavonoids and their respective Re-complexes were also tested in vitro for their ability to inhibit the formation and aggregation of the amyloid-like abnormal prion protein, PrP^Sc, by employing the real-time quaking-induced conversion assay with recombinant PrP seeded with cerebrospinal fluid from patients with Creutzfeldt-Jakob disease. All the compounds blocked de novo abnormal PrP formation and aggregation.

Crystal structure of fac-(acetylacetonato-κ2 O,O′)tricarbonyl(tri-m-tolyl phosphane-κP)rhenium(I), C29H28O5PRe

C29H28O5PRe, monoclinic, P21/n (no. 14), a = 10.548(5) Å, b = 22.996(5) Å, c = 11.022(5) Å, β = 98.011(5)°, V = 2647.4(18) Å3, Z = 4, R gt(F) = 0.0306, wRref (F2) = 0.0911, T = 100 K.

Crystal structure of fac-(acetylacetonato-κ2O,O′)tricarbonyl(tri(p-tolyl)phosphine-κP)rhenium(I), C29H28O5PRe

C29H28O5PRe, triclinic, P1̄ (no. 2), a = 12.733(7) Å, b = 18.668(11) Å, c = 24.779(15) Å, α = 101.442(20)°, β = 100.844(18)°, γ = 104.240(19)°, V = 5419(9) Å3, Z = 8, Rgt(F) = 0.0307, wRref(F2) = 0.0772, T = 100 K.

Crystal structure of fac-(acetylacetonato-κ2O,O′)tricarbonyl(benzyldiphenylphosphine-κP)rhenium(I), C27H24O5PRe

C27H24O5PRe, triclinic, P1̄ (no. 2), a = 9.0305(17) Å, b = 12.084(2) Å, c = 12.403(2) Å, α = 89.688(6)°, β = 70.083(6)°, γ = 76.315(6)°, V = 1232.2(4) Å3, Z = 2, Rgt(F) = 0.0180, wRref(F2) = 0.0432, T = 100(2) K.

Synthesis and characterization of fac-[M(CO)3(P)(OO)] and cis-trans-[M(CO)2(P)2(OO)] complexes (M = Re, (99m)Tc) with acetylacetone and curcumin as OO donor bidentate ligands

The synthesis and characterization of neutral mixed ligand complexes fac-[M(CO)3(P)(OO)] and cis-trans-[M(CO)2(P)2(OO)] (M = Re, (99m)Tc), with deprotonated acetylacetone or curcumin as the OO donor bidentate ligands and a phosphine (triphenylphosphine or methyldiphenylphosphine) as the monodentate P ligand, is described. The complexes were synthesized through the corresponding fac-[M(CO)3(H2O)(OO)] (M = Re, (99m)Tc) intermediate aqua complex. In the presence of phosphine, replacement of the H2O molecule of the intermediate complex at room temperature generates the neutral tricarbonyl monophosphine fac-[Re(CO)3(P)(OO)] complex, while under reflux conditions further replacement of the trans to the phosphine carbonyl generates the new stable dicarbonyl bisphosphine complex cis-trans-[Re(CO)2(P)2(OO)]. The Re complexes were fully characterized by elemental analysis, spectroscopic methods, and X-ray crystallography showing a distorted octahedral geometry around Re. Both the monophosphine and the bisphosphine complexes of curcumin show selective binding to β-amyloid plaques of Alzheimer's disease. At the (99m)Tc tracer level, the same type of complexes, fac-[(99m)Tc(CO)3(P)(OO)] and cis-trans-[(99m)Tc(CO)2(P)2(OO)], are formed introducing new donor combinations for (99m)Tc(I). Overall, β-diketonate and phosphine constitute a versatile ligand combination for Re(I) and (99m)Tc(I), and the successful employment of the multipotent curcumin as β-diketone provides a solid example of the pharmacological potential of this system.

Fluorescent, Theoretical and Structural Investigations on [Me4bim][CdCl4] (Me4bim = 1,1′,3,3′-tetramethyl-2,2′-bi-1H-imidazolium)

The compound [Me4bim][CdCl4] (Me4bim = 1,1′,3,3′-tetramethyl-2,2′-bi-1 H-imidazolium), has been prepared via a solvothermal reaction and has an isolated (0-D) structure. It displays an emission in the blue region that is ascribed to ligand-to-ligand charge transfer on the basis of a theoretical investigation.

In Situ Synthesis, X-Ray Structure, Fluorescence and Theoretical Investigations of [Cu2Cl4][(C2H5)2-4,4'-Bipyridine]

A novel N,N'-diethyl-4,4'-bipyridine copper complex, [Cu2Cl4][(C2H5)2-4,4'-bipyridine] (1), has been obtained in situ through a solvothermal reaction and structurally characterised by X-ray diffraction. Complex 1 comprises discrete Cu2Cl4 moieties and (C2H5)2-4,4'-bipyridine molecules interlinked via hydrogen bonding interactions to give a three-dimensional supramolecular open framework. Photoluminescent study shows that the complex exhibits an emission in the green region. Theoretical study reveals that the emission is attributed to the π → π* charge-transfer interaction of the (C2H5)2-4,4'-bipyridine moieties.

In Situ Synthesis, Structure, Photoluminescence and Theoretical Study of [Cu(2-PyCOOH)2(H2O)2]Cl2

A new pyridine-2-carboxylic acid complex, [Cu(2-PyCOOH)2(H2O)2]Cl2, has been prepared in situ through a hydrothermal reaction and its structure comprises discrete [Cu(2-PyCOOH)2(H2O)2]2+ cations and chloride anions which are interlinked via hydrogen bonding interactions to give a 3-D supramolecular motif. Photoluminescent study shows that the complex exhibits an emission in the blue region. Theoretical studies reveal that the emission can be ascribed to metal-to-ligand charge transfer.

Metal complex mediated conjugation of peptides to nucleus targeting acridine orange: a modular concept for dual-modality imaging agents

To target the nucleus of specific cells, trifunctional radiopharmaceuticals are required. We have synthesized acridine orange derivatives which comprise an imidazole-2-carbaldehyde function for coordination to the [Re(CO)₃](+) or [(99m)Tc(CO)₃](+) core. Upon coordination, this aldehyde is activated and rapidly forms imines with amines from biological molecules. This metal-mediated imine formation allows for the conjugation of a nuclear targeting portion with a specific cell receptor binding function directly on the metal. With this concept, we have conjugated the acridine orange part to a bombesin peptide directly on the (99m)Tc core and in one step. In addition, a linker containing an integrated disulfide has been coupled to bombesin. LC/MS study showed that the disulfide was reductively cleaved with a 60 min half-life time. This concept enables the combination of a nucleus targeting agent with a specific cell receptor molecule directly on the metal without the need of separate conjugation prior to labeling, thus, a modular approach. High uptake of the BBN conjugate into PC-3 cells was detected by fluorescence microscopy, whereas uptake into B16BL6 cells was negligible.

Curcumin as the OO bidentate ligand in "2 + 1" complexes with the [M(CO)3]+ (M = Re, 99mTc) tricarbonyl core for radiodiagnostic applications

The synthesis and characterization of "2 + 1" complexes of the [M(CO)(3)](+) (M = Re, (99m)Tc) core with the β-diketones acetylacetone (complexes 2, 8) and curcumin (complexes 5, 10 and 6, 11) as bidentate OO ligands, and imidazole or isocyanocyclohexane as monodentate ligands is reported. The complexes were synthesized by reacting the [NEt(4)](2)[Re(CO)(3)Br(3)] precursor with the β-diketone to generate the intermediate aqua complex fac-Re(CO)(3)(OO)(H(2)O) that was isolated and characterized, followed by replacement of the labile water by the monodentate ligand. All complexes were characterized by mass spectrometry, NMR and IR spectroscopies, and elemental analysis. In the case of complex 2, bearing imidazole as the monodentate ligand, X-ray analysis was possible. The chemistry was successfully transferred at (99m)Tc tracer level. The curcumin complexes 5 and 6, as well as their intermediate aqua complex 4, that bear potential for radiopharmaceutical applications due to the wide spectrum of pharmacological activity of curcumin, were successfully tested for selective staining of β-amyloid plaques of Alzheimer's disease. The fact that the complexes maintain the affinity of the mother compound curcumin for β-amyloid plaques prompts for further exploration of their chemistry and biological properties as radioimaging probes.

Tetra-ethyl-ammonium (acetyl-acetonato)bromidotricarbonyl-rhenate(I)

In the title compound, (C₈H₂₀N)[ReBr(C₅H₇O₂)(CO)₃], the Re^I atom in the rhenate anion is surrounded by three carbonyl ligands orientated in a facial arrangement, a bromide ligand and an acetyl­acetonate ligand, leading to a distorted octa­hedral ReC₃BrO₂ coordination with a O—Re—O bite angle of 85.66 (7)°. An array of C—H⋯O and C—H⋯Br hydrogen-bonding inter­actions between the cations and the surrounding rhenate anions stabilize the crystal structure.

Synthesis, Structure and Photoluminescence of [N-ethyl-4,4′-bipyridinium][CdBr4] with N-ethyl-4,4′-bipyridinium Generated in situ

A new viologen(4,4′-bipyridinium)-based complex [ N-ethyl-4,4′-bipyridinium][CdBr4], in which N-ethyl-4,4′-bipyri-dinium ( EQ2+) has been generated in situ, has been synthesised via hydrothermal reaction and structurally characterised by single-crystal X-ray diffraction. [ N-ethyl-4,4′-bipyridinium][CdBr4] features an isolated structure, based on an N-ethyl-4,4′-bipyridinium moiety and a cadmium atom terminally bound by four bromine atoms. Photoluminescence investigation reveals a broad and intense emission in blue region, which may originated from π → π* charge-transfer interaction of the N-ethyl-4,4′-bipyridinium moiety.