Angelo Perotti

Fluorescent sensor of imidazole and histidine

The dizinc(II) complex of an octamine containing the anthracene subunit binds both the imidazolate anion and the imidazolate moiety of L-histidine, and signals the binding through the fluorescence quenching of the fluorophore.

A study of the strongly fluorescent species formed by the interaction of the dye 1,4-dihydroxyanthraquinone (quinizarin) with Al(III)

Abstract Strongly fluorescent species form when Al(III) ions interact with 1,4-dihydroxyanthraquinone (quinizarin, QNZ), in aqueous and non-aqueous media. Speciation studies suggest the supramolecular and/or polymeric nature of these complexes as both their absorbance and fluorescence intensity increase proportionally to the Al(III) equivalents up to 60/1 Al(III)/QNZ and in certain cases even further. iso -Propanol is the solvent that yields the highest quantum yield of fluorescence ( Φ =0.51 for QNZ/Al(III) 1/75). Counterion effects were also studied and Al(acac) 3 was found to be the compound with which the highest quantum yield values were obtained, suggesting an active role for acac − in the stabilisation of the QNZ/Al(III) fluorescent species. Potentiometric studies, performed between pH 3 and 6 in dioxane/water 80/20, suggest the formation of 2/1 QNZ/Al(III) species, having global stability constant of 10 40 . Studies at higher ratios of Al(III) were precluded by precipitations.

Synthetic and equilibrium studies on copper(II), nickel(II), and palladium(II) complexes of N,N′,N″,N‴-tetrakis(2-hydroxyethyl)-1,4,8,11-tetra-azacyclotetradecane (L); kinetics of the acid dissociation of the copper(II) complex and X-ray structural characterisation of the nickel(II) complex [NiLH–1][ClO4]

The preparation of the macrocyclic ligand N,N′,N″,N‴-tetrakis(2-hydroxyethyl)-1,4,8,11-tetra-azacyclotetradecane (L) by reaction of ethylene oxide with cyclam (1,4,8,11-tetra-azacyclotetradecane) is described. The pKa values of L at 25 °C and I= 0.1 mol dm–3(NaClO4) are pK1= 8.83, pK2= 8.30, pK3= 2.65, pK4 < 2. The ligand forms very stable complexes with copper(II) and palladium(II) of the type [ML]2+ with log β110= 16.20 for copper(II) and 18.32 for palladium(II). The equilibria with nickel(II) can be represented by (i)–(iii), as follows. Ni2++ L ⇌[NiL]2+; log β110= 7.45(1)(i) Ni2++ L ⇌[NiLH–1]++ H+; log β11–1=–1.42(1)(ii) Ni2++ L ⇌[NiLH–2]+ 2H+; log β11–2=–12.39(2)(iii) The pK values of [NiL]2+ are 8.87 and 10.97 at 25 °C. The complex [NiLH–1][ ClO4](C18H39 ClN4NiO8) has been characterised in the solid state and its crystal structure determined. In 0.1 mol dm–3 sodium hydroxide solution E½ for the nickel(III)–nickel(II) couple is +0.28 V, one of the lowest values recorded for a macrocyclic complex. Unlike complexes of cyclam, complexes of L are rapidly formed and dissociate readily in acidic solution. Stopped-flow studies of the dissociation of the copper(II) complex in the acidity range 0.015–0.50 mol dm–3 HClO4 establish that rate =kH[CuL2+][H+] with kH= 3.44 dm3 mol–1 s–1 at 25 °C and I= 1.0 mol dm–3. The copper(II) and nickel(II) complexes [ML][ClO4]2 have been characterised in the solid state and their spectral properties are reported.

A study of the luminescent complexes formed by the dye 1,4-dihydroxyanthraquinone (quinizarin) and Ga(III) and In(III)

Abstract Fluorescent species form when Ga(III) and In(III) ions interact with 1,4-dihydroxyanthraquinone (quinizarin), in aqueous and non-aqueous media. Time dependence, temperature, solvent, counterion and pH effects studies have been performed in order to interpret the behaviour of this ligand in the presence of Ga(III) and In(III) ions. Quinizarin by itself is not fluorescent, but both its absorbance and fluorescence intensity increase proportionally to the complexation with Ga(III) and In(III), thus suggesting the supramolecular nature of these complexes, which could be used as fluorescence sensors. The fluorescent quantum-yield values of the QNZ complexes with Al(III), Ga(III) and In(III) ions are negatively affected by the increase of the size of M(III) ions, suggesting that the increase in metal size weakens the supramolecular interaction with QNZ. Potentiometric studies, performed between pH 3 and 6, suggest the formation of 2:1 QNZ/In(III) species, having a high β formation constant.

Crystal and molecular structure and solution behaviour of low-spin (3-methyl-1,3,5,8,12-pentaazacyclotetradecane)κ4N1,N5,N8,N12)nickel(II) diperchlorate

Template reaction of 3,7-diazanonane-1,9-diamine, formaldehyde and methylamine, in the presence of Ni(CIO4)2, gives the pentaazamacrocyclic low-spin complex [Ni(azacyclam)][CIO4]2(azacyclam = 3-methyl-1,3,5,8,12-pentaazacyclotetradecane), whose crystal and molecular structure was determined from single-crystal X-ray diffraction data collected with the use of Cu-Kα radiation: space group P212121 with a= 15.967(3), b= 13.497(3), c= 8.737(2)A, α=β=γ= 90°, Z= 4 (R= 0.0705). Only the four secondary amine nitrogen atoms of azacyclam are bound to the metal, according to a regular square-planar stereochemistry, the lone pair of the tertiary nitrogen atom, N(1), being exposed. In particular, the N(1)C3 group is flattened, probably due to a long-range electrostatic interaction between N(1) and the metal. As a consequence, N(1) displays a very small affinity towards aqueous H+(pKa < 2). The [Ni(azacyclam)]2+ complex in aqueous solution displays the high spin–low spin (blue-to-yellow) interconversion, typical of complexes of cyclam. However, in contrast, addition of HCI favours formation of the high-spin form. This unusual behaviour is ascribed to protonation of the N(1) atom, at acid concentrations of 0.5–1.0 mol dm–3, and to axial co-ordination of a hydrogen-bonded chloride ion. Addition of HCIO4 first favours formation of the high-spin species, then that of the low-spin complex: in this case, the hydrogen-bond-facilitated axial co-ordination of a water molecule is hypothesized.

A structurally characterized azide-bridged dinuclear nickel (II) cryptate

The macrobicyclic ligand L2 forms dimetallic NiII cryptate species in a delimited pH range, which are further able to include one N3-anion. The molecular structure of the azide-bridged complex shows an almost linear NiIINNNNiII arrangement and a different coordination at the two metal sites (one NiII being octahedral, the other trigonal bipyramidal).

Scandium trifluoromethanesulphonate as an active catalyst in the decarbonylation of aromatic aldehydes

Abstract Scandium trifluoromethanesulphonate efficiently catalyzes the decarbonylation of 2,4,6-trimethoxybenzaldehyde ( 1 ) which, on heating in MeOH solution, is deformylated neatly and completely in a few minutes, yielding 1,3,5 trimethoxybenzene and methyl formate. The reaction was studied by UV and NMR spectroscopy, which gave evidence for the reaction mechanism. Aromatic aldehydes less electron rich than 1 were decarbonylated more sluggishly. The unique catalytic properties of Sc(OTf) 3 were compared with other non transition metal triflates.

Acetazolamide binding to zinc(II), cobalt(II) and copper(II) model complexes of carbonic anhydrase

New zinc(II), cobalt(II) and copper(II) complexes with tris[2-(1-methylbenzimidazol-2-yl)ethyl]nitromethane (L) have been prepared. The tris(benzimidazole) donors provided by the ligand simulate the environment of the zinc(II) site of carbonic anhydrase. The structural properties of the complexes deduced on the basis of their spectroscopic characteristics correlate with those of the metal derivatives of the enzyme, at least in the solid state, while the alkyl chains connecting the donor groups of L allow a more flexible co-ordination environment of the metal sites in solution, with the adoption of five-co-ordinate structures. The complexes bind the inhibitor acetazolamide {N-[5-(aminosulfonyl)-1,3,4-thiadiazol-2-yl]acetamide} in its monoanionic form through the deprotonated sulfonamide group and these ternary complexes model the carbonic anhydrase–acetazolamide adduct recently characterized by X-ray crystallography. In the case of copper(II) it was impossible to isolate the pure ternary complex, probably because of the ease of formation of a dinuclear CuII–L–OH– complex under the reaction conditions, but the related ternary complexes with acetazolamide and diethylene- or dipropylene-triamine were readily obtained.

Scandium Trifluoromethanesulfonate as Catalyst in the Hydrogen/Deuterium Exchange of 1,3,5-Trimethoxybenzene: Evidence for a Direct Interaction of Scandium with the Aromatic Ring

Abstract Scandium trifluoromethanesulfonate in CD3OD is a very active catalyst for hydrogen/deuterium exchange at the aromatic nucleus of 1,3,5-trimethoxybenzene. The very low proticity of the system, spectrophotometric evidence, and easy iodination of the substrate point to the formation of a transient unprecedented arylscandium species as a plausible intermediate of the reaction.

The copper(II) promoted hydrolysis of benzylpenicillin. Evidence for the participation of a Cu–OH species in the hydrolysis of the β-lactam ring

The pKa of benzylpenicillin (HL ⇄ L– or H+) relating to the carboxyl group ionisation has been determined to be 2.67 ± 0.005 at 10°C and 2.60 ± 0.008 at 5 °C and I= 0.1 mol dm–3. At a 1 : 1 ligand-to-metal ratio the interaction of copper(II) with benzylpenicillin can be described by the equilibria (i) and (ii) at 10 °C. The pK for the ionization [CuLH–1]⇄[CuLH–2]–+ H+ is 4.92 at, L–+ Cu2+⇄[CuLH–1]+ H+; log β11 – 1=–1.41 ± 0.1 (i), L–+ Cu2+⇄[CuLH–2]–+ 2H+; logβ11 –2=–6.33 ± 0.08 (ii) 10°C and I= 0.1 mol dm–3. No potentiometric evidence was obtained for a species [CuL]+. Possible structures for [CuLH–1] and [CuLH –2]– are considered involving deprotonation of the side chain amide group and a water molecule co-ordinated to copper(II). The copper(II)-promoted hydrolysis of benzylpenicillin to penicilloic acid has been studied over the pH range 4.3–5.3 at 30 °C and I= 0.5 mol dm–3 using non-complexing 2,6-dimethylpyridine-3-sulphonic acid buffers. The dependence of the rate on the copper concentration and the pH strongly suggests that the complex [CuLH–2]– is the active species in the reaction. Hydrolysis of the lactam ring of benzylpenicillin occurs by intramolecular attack of a Cu–OH species on the carbonyl group and not by intermolecular attack of ‘free’ hydroxide ion on the complex. The hydrolysis of [CuLH–2]– at pH 6 is some 6 × 106 fold faster than the hydrolysis of the anion L– at 30 °C.