Giovanni Guglielmo

Amphiphilic Cyclodextrins as Nanocarriers of Genistein: A Spectroscopic Investigation Pointing Out the Structural Properties of the Host/Drug Complex System

Nanoggregates of nonionic amphiphilic cyclodextrin (ACyD) modified with hydrophobic chains of intermediate length [(2-oligo-ethyleneoxide-6-hexylthio)-beta-CyD, SC6OH] were prepared by emulsification-diffusion method. They are able to entrap an isoflavone, genistein (Gen), and the complexed species are studied at different host/guest molar ratio. The increased isoflavone solubility in the presence of the aggregates of SC6OH is investigated by UV-Vis spectroscopy, whereas size, charge, and structure of aggregates and their complexes with Gen are measured by means of static and quasi-elastic light scattering, and electrophoretic mobility measurements. On the other hand, preparing samples by the conventional method used for liposomes (hydration of an organic film of SC6OH and sonication) gives rise to aggregates with different sizes and lower colloidal stability. It is shown that the improved stability in water of ACyD aggregates both in the absence and in the presence of Gen, obtained by emulsification-diffusion is due to the existence of nanodomains of organic solvent (R(H) congruent with 120 nm) which cannot be completely removed by evaporation and freeze-drying and in which host/guest complexes are contained. This result shows that residues of organic solvent from preparation step favor the colloidal stability of the aggregate, but their presence must be taken into account in designing systems for drug delivery.

Photoinduced intercomponent energy transfer in covalently-linked dinuclear complexes containing Ru(II)-bipyridine and Ru(II)-biquinoline chromophores and aromatic and aliphatic spacers

Abstract We have synthesized a number of dinuclear species containing both identical or different metal-based components by employing new bridging ligands having either aliphatic or aromatic spacers and taking advantage of the “complexes as metals and complexes as ligands” synthetic strategy. The bridging ligands are dpt-S-dpt (S is 1,4-cyclohexyl, 1,4-phenyl, 4,4′-biphenyl; dpt is 4-amino-3,5-bis(2-pyridyl)- 1,2,4-triazole; the connections between S and dpt are provided by amide links). The complexes synthesized are: [(bpy)2Ru(dpt-S-dpt)Ru(bpy)2](PF6)4 (bpy=2,2′-bipyridine; biq=2,2′-biquinoline; S=1,4-cyclohexyl (1), 1,4-phenyl (4), 4,4′-biphenyl (7)); [(biq)2Ru(dpt-S-dpt)Ru(biq)2](PF6)4 (S=1,4-cyclohexyl (2), 1,4-phenyl (5), 4,4′-biphenyl (8); [(bpy)2Ru(dpt-S-dpt)-Ru(biq)2](PF6)4 (S=1,4-cyclohexyl (3), 1,4-phenyl (6), 4,4′-biphenyl 39)). The absorption spectra, luminescence properties and redox behavior of all the compounds have been studied. In the complexes containing different metal-based components, photoinduced energy transfer occurs from the higher-lying Ru→bpy CT level, centered on a metal subunit, to the lower-lying Ru→biq CT excited state, centered on the other metal component. In fluid solution at room temperature, the energy transfer is suggested to be mediated by a two-step electron transfer mechanism, whereas direct energy transfer between the chromophores most likely occurs at 77 K in rigid matrix. At the moment we arc not able to say if the energy transfer at 77 K takes place via electron exchange or coulombic mechanisms. The results obtained indicate that the efficiency of the processes depends on the donor-acceptor distance, as expected, and that occasional π bonds which are present within the bridging ligands cannot be used for speeding up electron transfer in multicomponent systems if the main skeleton of the bridge is made by σ bonds.

Absorption spectra, luminescence properties and electrochemical behaviour of ruthenium(II) complexes containing bis(pyridyl)triazole ligands

Abstract Four novel complexes of general formula Ru(L) 2 (T) 2+ ( 1 , Lbpy, Ttrz; 2 , Lbpy, Ttrz-Q; 3 , Lbiq, Ttrz; 4 , Lbiq, Ttrz-Q; bpy=2,2′-bipyridine, biq=2,2′-biquinoline; trz=4-amino-3,5-di-2-pyridyl-4 H -1,2,4-triazole; trz-Q=4(4′- N,N -dimethylamino-phenyl)imino-3,5-di-2-pyridyl-4 H -1,2,4-triazole) have been synthesized, and their absorption spectra, luminescence properties (both in fluid solution at room temperature and in rigid matrix at 77 K), and electrochemical behaviour have been investigated. The absorption spectra of the complexes show intense absorption bands in the UV region (ϵ in the range 10 4 –10 5 M −1 cm −1 ) that are assigned to ligand- centred transitions and moderately intense absorption bands in the visible (ϵ in the range 10 3 –10 4 M −1 cm −1 ) that are attributed to metal-to-ligand charge transfer (MLCT) transitions. The absorption bands in the visible of the biq-containing complexes are at lower energies than those of the bpy-containing ones. The four complexes emit from a MLCT excited state both at 77 K and at room temperature, with lifetimes in the range 10 −5 – 10 −6 s and 10 −7 –10 −8 s, respectively. The luminescence lifetimes and quantum yields are practically the same for 1 and 3 and for 2 and 4 , respectively, indicating that the presence of the N,N -dimethylamino unit on the triazole ligand does not affect the radiative and radiationless rate constants of the chromophores and does not cause an electron-transfer quenching process. On electrochemical oxidation, 1 and 3 exhibit a reversible one-electron wave at +1.22 and +1.37 V versus SCE, respectively, that are assigned to metal-centred oxidations, while 2 and 4 undergo two successive one-electron oxidations at +1.30 and +1.56 V ( 2 ) and +1.30 and +1.71 V ( 4 ). By comparison with the redox behaviour of the free ligands, in both 2 and 4 the first process is attributed to oxidation of the N,N -dimethylamino moiety, and the second one to metal-centred oxidation. Two reversible reduction processes occur in all the complexes at about −1.15 and −1.40 V ( 1 and 2 ) and at about −0.60 and −0.85 V ( 3 and 4 ). Such processes are assigned as bpy- and biq-centred reductions, respectively. The positive shift of the metal-centred oxidation on passing from 1 and 3 to 2 and 4 is attributed to electronic ‘communication’ between the chromophoric metal unit and the electron-donor N,N -dimethylamino group across the triazole ligand and the conjugate NCHC 6 H 4 bridge, and to an electrostatic term.

Photochemistry of planar four-coordinate palladium(II) complexes. I. The sterically hindered nitro(4-methyl-1,1,7,7-tetraethyldiethylenetriamine)palladium(II) complex ion

Abstract The photochemical behaviour of [Pd(MeEt4dien)NO2]+ (MeEt4dien = 4-methyl-1,1,7,7 tetraethyldiethylenetriamine) in methanol solution has been investigated. Under irradiation of different wavelengths corresponding mainly to CT and LF bands of the complex, the initial N-bonded PdNO2 compound undergoes a linkage photoisomerization to PdONO which in turn reacts photochemically and thermally with the solvent to give the final solvo-derivative [Pd(MeEt4dien)solv.]++. The quantum yields of the two photoreactions, the isomerization and the solvolysis, decrease with decreasing energy of the exciting radiations. A mechanism for both photoprocesses is proposed.

CATALYZED SUBSTITUTIONS ON CATIONIC PALLADIUM(II) COMPLEXES. I. KINETICS OF ACID-ASSISTED DISPLACEMENT OF COORDINATED AZIDO AND NITRO GROUPS

A kinetic study of the reactions of complexes of the type [Pd(AAA)X]+ (AAA = diethylenetriamine (dien); 1,1,7,7-tetraethyldiethylenetriamine (Et4dien); 4-methyl-1,1,7,7,-tetraethyldiethylenetriamine (MeEt4 dien); X = NO2 and N3) with Cl−, H+ and both H+ and Cl− in methanol at 25° C is reported. The rates of uncatalyzed displacement of nitro and azido groups by Cl− from axially blocked [Pd(Et4 dien)X]+ depend on the chloride concentration, in contrast with the kinetic behaviour found for such substrates when X = Cl, Br and l, in which only the solvolytic path controls the substitution. The rate of proton assisted displacement follows the equation: “rate” = (k1+k2[Cl−]+k′2[H+]+k′2[H+][Cl−]) [Complex] which is consistent with a mechanism involving a nucleophilic attack by solvent and entering chloride on both the substrate and its protonated form in equilibrium. All the rate constants which appear in the rate equation have been determined and the values indicate that the acid controlled paths carry the major part of the reaction. The lability of the NO2 group depends on the different orientation that it assumes with respect to the coordinate plane in unhindered or hindered complexes.

A new tetraoxa-tetraaza macrobicyclic anthracenyl luminescent receptor: control of the receptor-substrate stoichiometry

The luminescent receptor 1 incorporating anthracenyl groups as luminophores and a tetraoxa-tetraaza macrobicyclic receptor as the binding subunit has been synthesized and its interactions with CaII, investigated by fluorescence measurements, are reported. The receptor-substrate 1-Ca system is an example of multistability at the molecular level, triggered by a reversible modification of the receptor ability and evidenced by luminescence.

Photochemistry of planar four-co-ordinate palladium(II) complexes—III. Photosynthesis and crystal structure of [Pd2(PPr3n2(NCS)(SCN)2(μ-CN)] ☆

Abstract U.V. irradiation of MeOH and CH 3 CN solutions of trans -[Pd(PPr 3 n ) 2 (NCS) 2 ] causes a stepwise photochemical reaction. When the solvent is CH 3 CN the photoproduct can be conveniently isolated in a crystalline form. This compound has been shown, by X-ray analysis, to be a cyano bridged palladium(II) complex.

Photochemistry of planar four-co-ordinate palladium(II) complexes. Part 2. trans–cis Isomerization of dihalogenobis(tri-n-propylphosphine)palladium(II) in solution

The photoisomerization of trans-dihalogenobis (tri-n-propylphosphine)palladium(II) complexes has been studied in various solvents. The quantum yields depend strongly both on the nature of the co-ordinating halide and on the excitation wavelength. The photochemical results are discussed in terms of the possible mechanisms for the trans–cis photoisomerization of square-planar complexes. Photolysis of cis-[Pd(PPrn3)2Cl2] yields the chloro-bridged complex, [Pd2(PPrn3)2Cl4].

MECHANISM OF NUCLEOPHILIC SUBSTITUTION IN PALLADIUM(II) COMPLEXES OF TERPYRIDINE

Abstract Rates of nucleophilic substitution reactions (i) [terpy = terpyridine; X = Cl, Br, I, N 3 , NO2; Y = Cl, Br, N 3 , I (NH 2 ) 2 CS] have been [Pd(terpy)X] + Y − → [Pd(terpy)Y] + + X − (i) measured in methanol at 25°C and μ = 0.1. The results are compared with those relating to the analogous reactions (ii) [3-NHpd = 3-azapentane-l,5-diamine] on the quantitative basis of nucleophilic discrimination factors and intrinsic reactivities: [Pd(3-NHpd)X] + + Y − → [Pd(3-NHpd)Y] + + X − (ii) The larger reactivity of [Pd(terpy)X] + is interpreted on the assumption that the chelating terpyridine group enhances more effectively the electrophilicity of palladium(II) through π-interaction than 3-azapentane-1,5-diamine does.

Catalyzed substitution on cationic palladium(II) complexes. II. Mercury(II) and cadmium(II) catalyzed displacement of coordinated halides

Abstract A kinetic study of Hg II and Cd II catalyzed solvolysis of complexes of the type [Pd(AAA)X] + (AAA = 1,1,7,7-tetraethyldiethylenetriamine (Et 4 dien) and 4-methyl-1, 1,7,7-tetraethyldiethylenetriamine (MeEt 4 dien); X = Cl, Br, I) has been carried out in methanol at 25° C. The first-order dependence of the reaction rate on [M ++ ] is explained in terms of a solvent attack on a labile binuclear adduct of the type in equilibrium with the substrate. A correlationship is found between the catalytic efficiency and the affinity of metal ions for halide ions.