Spectroscopic study of lanthanide(III) complexes with aliphatic dicarboxylic acids

The complexation of trivalent lanthanides with aliphatic dicarboxylic acids (malonic, succinic, glutaric and adipic) were studied at 25°C and 0.1 M (NaClO₄) ionic strength by luminescence and absorption spectroscopy and luminescence lifetime measurements. The luminescence spectra and decay constants indicate that ML and ML₂ complexes were formed. The stability constants of Eu(III) complexes with the dicarboxylic acids were calculated from the changes of the ⁵D₀←⁷F₀ excitation spectra of Eu(III). For the four dicarboxylic acids studied, both the stability constant and the number of water molecules released from the inner sphere of Eu(III) upon complexation decrease from malonate to adipate for both the ML and ML₂ complexes. The results are interpreted as reflecting an increasing tendency from chelation to monodentation as the carbon chain length increases between carboxylate groups. The trend in the oscillator strength in the hypersensitive transition of the Nd(III)and Ho(III) complexes is the same as that in the ligand basicity.     

Crystal structures, antioxidation and DNA binding properties of Eu(III) complexes with Schiff-base ligands derived from 8-hydroxyquinoline-2-carboxyaldehyde and three aroylhydrazines

Eu(III) and every newly synthesized ligand can form a binuclear Eu(III) complex with a 1:1 metal to ligand stoichiometry and nine-coordinate at Eu(III) center. Every ligand acts as a dibasic tetradentate ligand, binding to Eu(III) through the phenolate oxygen atom, nitrogen atom of quinolinato unit, the CN group (methylene) and ⁻O-CN- group (enolized and deprotonated from OC-NH- group) of the aroylhydrazine side chain. One DMF (N,N-dimethylformamide) molecule is binding orthogonally to the ligand-plane from one side to the metal ion, while another DMF and a nitrate anion (bidentate) are binding from the other. Dimerization of the monomeric unit occurs through the phenolate oxygen atoms leading to a central planar four-membered (EuO)₂ ring. On the other hand, all the ligands and Eu(III) complexes may be used as potential anticancer drugs, binding to Calf thymus DNA through intercalations at the order of magnitude 10⁵-10⁷ M⁻¹. All the ligands and Eu(III) complexes are strong scavengers of hydroxyl radicals and superoxide radicals, but Eu(III) complex containing active phenolic hydroxyl group shows stronger scavenging effects for hydroxyl radicals than others, and Eu(III) complex containing N-heteroaromatic substituent shows stronger scavenging effects for superoxide radicals than others.     

Lanthanide complexation with 1,3,5-triamino 2,4,6-trihydroxycyclohexane

The protonation constants of 1,3,5-triamino-2,4,6-trihydroxycyclohexane (taci), at 25 °C in I = 1.00 M (NaClO₄) were determined to be: pKa₁, 5.57 (0.08); pKa₂, 7.45 (0.02); pKa₃, 9.05 (0.04). The log of the stability constants, log β₃₀₂, at 25°C in I = 1.00 M (NaClO₄) for formation of were measured by potentiometry to be: Nd(III), 25.33 (0.09); Eu(III), 26.42 (0.06); Tm(III), 30.07 (0.10); Lu(III), 33.68 (0.07) ; Y(III), 28.59 (0.07). ¹H NMR spectra were consistent with formation of a single complex from pcH 6 to 10. Laser fluorescence measurements of the ⁷F_o-⁵D_o transition of Eu(III) complexed by taci indicated a single complexed species. The shift in this peak relative to that of Eu³⁺(aq) was significantly greater than the values reported for the complexes of other organic ligands with Eu(III). Luminescence lifetime measurements indicated two water molecules bound to each of the Eu(III) cations in the taci complex.     

Preparation, crystal structure and luminescent properties of lanthanide picrate complexes with N-benzyl-2-{2′-[(benzyl-methyl-carbamoyl)-methoxy]-biphenyl-2-yloxy}-N-methyl-acetamide (L)

A new amide-based ligand derived from biphenyl, N-benzyl-2-{2′-[(benzyl-methyl-carbamoyl)-methoxy]-biphenyl-2-yloxy}-N-methyl-aceamide (L) was synthesized. Solid complexes of lanthanide picrates with this new ligand were prepared and characterized by elemental analysis, conductivity measurements, IR and electronic spectroscopies. The molecular structure of [Eu(pic)₃L] shows that the Eu(III) ion is nine-coordinated by four oxygen atoms from the L and five from two bidentate and one unidentate picrates. All the coordinate picrates and their adjacent equivalent picrates form intermolecular π-π stacking. Furthermore, the [Eu(pic)₃L] complex units are linked by the π-π stacking to form a two-dimensional (2-D) netlike supramolecule. Under excitation, the europium complex exhibited characteristic emissions. The lifetime of the ⁵D₀ level of the Eu(III) ion in the complex is 0.22 ms. The quantum yield Φ of the europium complex was found to be 1.01 × 10⁻³ with quinine sulfate as reference. The lowest triplet state energy level of the ligand indicates that the triplet state energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion.     

Iron(III) complexes of tridentate N3 ligands as models for catechol dioxygenases: Stereoelectronic effects of pyrazole coordination

The iron(III) complexes of the tridentate N₃ ligands pyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L1), 3,5-dimethylpyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L2), 3-iso-propylpyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L3) and (1-methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethylamine (L4) have been isolated and studied as functional models for catechol dioxygenases. They have been characterized by elemental analysis and spectral and electrochemical methods. The X-ray crystal structure of the complex [Fe(L1)Cl₃] 1 has been successfully determined. The complex possesses a distorted octahedral coordination geometry in which the tridentate ligand facially engages iron(III) and the Cl⁻ ions occupy the remaining coordination sites. The Fe-N_pz bond distance (2.126(5) Å) is shorter than the Fe-N_py bond (2.199(5) Å). The systematic variation in the ligand donor substituent significantly influences the Lewis acidity of the iron(III) center and hence the interaction of the present complexes with a series of catechols. The catecholate adducts [Fe(L)(DBC)Cl], where H₂DBC = 3,5-di-tert-butylcatechol, have been generated in situ and their spectral and redox properties and dioxygenase activities have been studied in N,N-dimethylformamide solution. The adducts [Fe(L)(DBC)Cl] undergo cleavage of DBC²⁻ in the presence of dioxygen to afford major amounts of intradiol and smaller amounts extradiol cleavage products. In dichloromethane solution the [Fe(L)(DBC)Cl] adducts afford higher amounts of extradiol products (64.1-22.2%; extradiol-to-intradiol product selectivity E/I, 2.6:1-4.5:1) than in DMF (2.5-6.6%; E/I, 0.1:1-0.4:1). The results are in line with the recent understanding of the function of intra- and extradiol-cleaving catechol dioxygenases.     

Excitation and deexcitation processes of Eu(III) benzo-15-crown-5 complex studied on comparing with its first coordination sphere

The coordination sphere and the deexcitation mechanism of the Eu(III) benzo-15-crown-5 complex, Eu(B15C5), were studied with references of the Eu(III) complexes with a similar coordination sphere; the dibenzo-18-crown-6 complex, Eu₃(B₂18C6)₂, and the cryptand[2.2.1] complex, Eu([2.2.1]). NMR spectroscopy reveals that the Eu(B15C5) complex is quite stable in acetonitrile solution whereas only 40% of the Eu(III) ion forms the complex in the equimolar Eu(NO₃)₃ and B₂18C6 acetonitrile solution. The coordination sphere of the Eu(III) complexes in acetonitrile solutions were also discussed by the degenerate ⁷F₀→⁵D₀ transition energy levels. The Eu(B15C5) have a negative shift compared with the europium(III) nitrate in acetonitrile and it is explained by the coordination of both nitrate ions and the crown ether ligand. Energy transfer from the n–π* excited state located in the catechol structure to the central europium ion was first observed as the sensitized luminescence of ⁵D₀→⁷F_J. The excited state lifetime of the Eu(B15C5) complex was first determined as 201 μs in the present study.     

Preparation, crystal structure and luminescent properties of the 3-D netlike supramolecular lanthanide picrate complexes with 2,2′-[1,2-phenylenebis(oxy)]bis(N-benzylacetamide)

Solid complexes of lanthanide picrates with a new podand-type ligand, 2,2′-[(1,2-phenylene)bis(oxy)]bis(N-benzylacetamide) (L) have been prepared and characterized by elemental analysis, conductivity measurements, IR, electronic and ¹H NMR spectroscopies. The crystal and molecular structures of the complex NdL(Pic)₃ have been determined by single-crystal X-ray diffraction. The crystal structure shows that the Nd(III) ion is coordinated with four oxygen atoms of the ligand L and six oxygen atoms of three bidentate picrates. Furthermore, the NdL(Pic)₃ complex units are linked by the intermolecular hydrogen bonds to form a three-dimensional (3-D) netlike supermolecule. Under excitation, Eu complex exhibited characteristic emissions. The lowest triplet state energy level of the ligand indicates that the triplet state energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion.     

Synthesis,characterization and luminescent properties of lanthanide complexes with an unsymmetrical tripodal ligand

Solid complexes of lanthanide nitrates with an novel unsymmetrical tripodal ligand, butyl‐N,N‐bis[(2′‐benzylaminofomyl)phenoxyl)ethyl]‐amine ( L ) have been synthesized and characterized by elemental analysis, infrared spectra and molar conductivity measurements. At the same time, the luminescent properties of the Sm(III), Eu(III), Tb(III) and Dy(III) nitrate complexes in solid state were also investigated. Under the excitation of UV light, these complexes exhibited characteristic emission of central metal ions. Copyright © 2010 John Wiley & Sons, Ltd.     

Formation and phosphodiesterolytic activity of lanthanide(III) N,N-bis(2-hydroxyethyl)glycine hydroxo complexes

Potentiometric titrations of N,N-bis(2-hydroxyethyl)glycine (bicine) in the presence of Ln(III) cations (Ln=La, Pr, Nd and Eu) in the pH range extended to ca. 9.5 reveal formation of two types of binuclear hydroxo complexes Ln₂(bic)₂(OH)₄ and Ln₂(bic)(OH)₄ ⁺ (bicH=bicine) in addition to previously reported mononuclear mono- and bis-complexes Ln(bic)²⁺ and Ln(bic)₂ ⁺, which predominate at pH below 8. ¹H NMR titrations of La(III)-bicine mixtures in D₂O show that the complex formation with bicine is slow in the NMR time scale and confirm formation of hydroxide rather than alkoxide complexes in basic solutions. Formation of a different type of hydroxide species under conditions of an excess of metal over ligand is confirmed by studying the absorption spectra of the Nd(III)-bicine system in the hypersensitive region. The binuclear hydroxide complexes are predominant species at pH above 9 and their stabilities increase in the order La < Pr ≈ Nd < Eu. They show fairly high catalytic activity in the hydrolysis of bis(4-nitrophenyl) phosphate (BNPP) at room temperature. Comparison of concentration and pH-dependences of the reaction rates with the species distribution diagrams shows that the catalytic hydrolysis of BNPP proceeds via a Michaelis-Menten type mechanism, which involves the Ln₂(bic)(OH)₄ ⁺ complex as the reactive species. The values of the catalytic rate constants and the Michaelis constants are in the range 0.002-0.004 s⁻¹ and 0.35-1.5 mM, respectively, for all lanthanides studied. The half-life for the hydrolysis of BNPP is reduced from 2000 years to ca. 10 min at 25 °C and pH 9.2 in the presence of 5 mM La(III) and 2.5 mM bicine.     

Effect of substituents on complex stability aimed at designing new iron(III) and aluminum(III) chelators

The solution equilibria of iron(III) and aluminum(III) with two classes of hard ligands (catechol, salicylic acid and their nitro-derivatives) have been reliably studied by potentiometric, spectrophotometric and NMR spectroscopy. The effect of the nitro substituent on the binding properties of catechol and salicylic acid has been examined thoroughly. The inductive and resonance properties of the substituent that, as expected, lower the basicity of the phenolic and carboxylic groups, lead to a general decrease in both protonation and complex formation constants. This decrease causes an increase in pM of between 0.2 and 1.1 pM units for the nitro-substituted salicylates and of about 4 units for 4-nitrocatechol, with a significantly higher chelating efficacy. The influence of the substituent on catechol and salicylic acid is discussed in detail on the basis of conditional constants at pH 7.4.     

Functional model for intradiol-cleaving catechol 1,2-dioxygenase: Synthesis, structure, spectra, and catalytic activity of iron(III) complexes with substituted salicylaldimine ligands

A series of mononuclear iron(III) complexes with containing phenolate donor of substituted-salicylaldimine based ligands [Fe(L¹)(TCC)] · CH₃OH (1), [Fe(L²)(TCC)] · CH₃OH (2), [Fe(L³)(TCC)] (3), and [Fe(L⁴)(TCC)] (4) have been prepared and studied as functional models for catechol dioxygenases (H₂TCC = tetrachlorocatechol, or HL¹ = N′-(salicylaldimine)-N,N-diethyldiethylenetriamine, HL² = N′-(5-Br-salicylaldimine)-N,N-diethyldiethylenetriamine, HL³ = N′-(4,6-dimethoxy-salycyl-aldimine)-N,N-diethyl-diethylenetriamine, HL⁴ = N′-(4-methoxy-salicylaldimine)-N,N-diethyl-diethylenetriamine). They are structural models for inhibitors of enzyme-substrate adducts from the reactions of catechol 1,2-dioxygenases. Complexes 1-4 were characterized by spectroscopic methods and X-ray crystal structural analysis. The coordination sphere of Fe(III) atom of 1-4 is distorted octahedral with N₃O₃ donor set from the ligand and the substrate TCC occupying cis position, and Fe(III) is in high-spin (S = 5/2) electronic ground state. The in situ prepared iron(III) complexes without TCC, [Fe(L¹)Cl₂], [Fe(L²)Cl₂], [Fe(L³)Cl₂], and [Fe(L⁴)Cl₂] are reactive towards intradiol cleavage of the 3,5-di-tert-butylcatechol (H₂DBC) in the presence of O₂ or air. The reaction rate of catechol 1,2-dioxygenase depends on the redox potential and acidity of iron(III) ions in complexes as well as the substituent effect of the ligands. We have identified the reaction products and proposed the mechanism of the reactions of these iron(III) complexes with H₂DBC with O₂.     

Estimation of inter-binding-site distances in sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase using Eu(III) luminescence energy transfer

We have used several trivalent lanthanides as probes for the high-affinity Ca(II)-binding site of the Ca(II) + Mg(II)-ATPase of skeletal muscle sarcoplasmic reticulum. The luminescent probes Eu(III) and Tb(III) were excited directly with pulsed laser light and the energy transfer efficiencies to several lanthanide acceptors were measured, under conditions in which most donor-acceptor pair occupied high-affinity Ca(II) sites. We obtain an inter-ionic site distance of about 0.8-0.9 nm. Energy transfer measurements were also done with Eu(III) in at least one Ca(II) site and bidentate Cr-ATP complex at the ATP hydrolytic site. Quenching of Eu(III) luminescence by Cr-ATP was total under these conditions. We calculate an upper limit of 1.0 nm for the distance from the Ca(II) site(s) to the complexed Cr(III) ion at the hydrolytic site.     

Calcium binding proteins: optical stopped-flow and proton nuclear magnetic resonance studies of the binding of the lanthanide series of metal ions to parvalbumin

Optical stopped-flow techniques have been used to determine the dissociation rate constants (koff) for the lanthanide(III) ions from carp (pI 4.25) parvalbumin. For most of the 13 different lanthanides studied, the release kinetics were diphasic, composed of both a fast phase (whose rate varied across the series, La3+ leads to Lu3+, between the limits -1.2 less than or equal to log kFAST less than or equal to -0.7) and a slower phase (whose rate varied across the series, La3+ leads to Lu3+, between the limits -1.2 greater than or equal to log kSLOW greater than or equal to -2.9). In addition, the La3+- and Lu3+-induced changes in the 270-MHz proton nuclear magnetic resonance spectrum of parvalbumin were used to calculate the dissociation constants for these specific lanthanides from the two high-affinity Ca2+ binding sites. The KD for one site appears to remain constant across the lanthanide series, determined to be 4.8 X 10(-11) M for both La3+ and Lu3+. The other site, however, is evidently quite sensitive to the nature of the bound Ln3+ ion and shows a strong preference for La3+ (KD,La = 2.0 X 10(-11) M; KD,Lu = 3.6 X 10(-10) M). We conclude from these observations that reports of nearly indistinguishable CD/EF binding site affinities for parvalbumin complexes of the middle-weight lanthanides (i.e., Eu3+, Gd3+, and Tb3+) are quite reasonable in view of the crossover in relative CD/EF site affinities across the lanthanide series.     

Interaction of europium(III) with phospholipid vesicles as monitored by laser-excited europium(III) luminescence

The technique of laser-excited Eu(III) luminescence was applied to monitor Eu(III) binding to a variety of phospholipids. Eu(III) excitation spectra were similar with and without the presence of neutral phospholipids, while acidic phospholipids changed the spectrum in a concentration-dependent manner. Eu(III) appears to bind to the phosphate moiety with at least a 2:1 phospholipid:metal ion stoichiometry. Analysis of luminescence lifetimes reveals that only one or two waters of hydration are removed from Eu(III) by addition of neutral phospholipids, whereas acidic phospholipids and inorganic phosphate strip off all but one or two waters. Implications with regard to fusion and use of lanthanides as probes in membrane preparations are discussed.     

Synthesis, characterization, and DNA-binding properties of the Ln(III) complexes with 6-hydroxy chromone-3-carbaldehyde-(2'-hydroxy) benzoyl hydrazone

A new ligand, 6-hydroxy chromone-3-carbaldehyde-(2'-hydroxy) benzoyl hydrazone (L), was prepared by condensation of 6-hydroxy-3-carbaldehyde chromone (CDC) with 2-hydroxy benzoyl hydrazine. Its four rare earth complexes have been synthesized and characterized on the basis of elemental analyses, molar conductivities, mass spectra, 1H NMR, thermogravimetry/differential thermal analysis (TG-DTA), UV-vis spectra, fluorescence spectra, and IR spectra. The general formula of the complexes is [LnL2.(NO3)2].NO3 [Ln=La(1), Sm(2), Dy(3), Eu(4)]. Spectrometric titration, ethidium bromide displacement experiments, and viscosity measurements indicate that Eu(III) complex and ligand, especially the Eu(III) complex, strongly bind with calf-thymus DNA, presumably via an intercalation mechanism. The intrinsic binding constants of Eu(III) complex and ligand with DNA were 3.55 x 10(6) and 1.33 x 10(6)M(-1) through fluorescence titration data, respectively. In addition, the suppression ratio for O2-* and OH* of the ligand and its complexes was studied by spectrophotometric methods. The experimental results show that La (1), Sm (2), and Eu (4) complexes are better effective inhibitor for OH* than that of mannitol. It indicates that the complexes have the activity to suppress O2-* and OH* and exhibit more effective antioxidants than ligand alone.     

Antifeeding and insect-growth-regulating activity of certain metal complexes towardsSpodoptera litura; F

Metal complexes of divalent cobalt, nickel and copper and trivalent iron were synthesized usingN-salicylidene-3-aminocoumarin as chelating agent. The ligand behaves as a monobasic ONO donor towards Co(II), Ni(II) and Cu(II) and as an ON as well as an ONO donor towards Fe(III). All the complexes haven been proposed to have octahedral geometry on the basis of analytical, thermal conductivity, spectral and magnetic data. The complexes have been screened againstSpodoptera litura;F (Lepidoptera: noctuiidae) for antifeeding and insect-growth-regulating activity. The results show appreciable insect-growth-regulating activity associated with metal complexation.     

Oligonucleotide conjugates of Eu(III) tetraazamacrocycles with pendent alcohol and amide groups promote sequence-specific RNA cleavage

Eu(III) complexes of two neutral bifunctional tetraaaza macrocyclic ligands {1-[1-carboxamido-3-(4-nitrophenyl)propyl]-4,7,10-tris(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane and 2-(4-nitrobenzyl)-1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane} are prepared. Eu(III) complexes of the isothiocyanate derivatives of these macrocycles are treated with oligonucleotides containing 2′-O-propylamine linkers to form conjugates. Hydrolytic cleavage of an oligoribonucleotide is promoted by Eu(III) macrocyclic oligonucleotide conjugates containing complementary (antisense) sequences. Cleavage is not observed in the presence of Eu(III) conjugates containing scrambled sequences nor by free complex. Despite the fact that one of the free macrocyclic complexes is more reactive than the other, the extent of cleavage observed is similar for conjugates containing either Eu(III) macrocyclic complex.     

Synthesis and luminescence properties of polymer–rare earth complexes containing salicylaldehyde‐type bidentate Schiff base ligand

Using molecular design and polymer reactions, two types of bidentate Schiff base ligands, salicylaldehyde–aniline (SAN) and salicylaldehyde–cyclohexylamine (SCA), were synchronously synthesized and bonded onto the side chain of polysulfone (PSF), giving two bidentate Schiff base ligand‐functionalized PSFs, PSF–SAN and PSF–SCA, referred to as macromolecular ligands. Following coordination reactions between the macromolecular ligands and Eu(III) and Tb(III) ions (the reaction occurred between the bonded ligands SAN or SCA and the lanthanide ion), two series of luminescent polymer–rare earth complexes, PSF–SAN–Eu(III) and PSF–SCA–Tb(III), were obtained. The two macromolecular ligands were fully characterized by Fourier transform infrared (FTIR), ¹H NMR and UV absorption spectroscopy, and the prepared complexes were also characterized by FTIR, UV absorption spectroscopy and thermo‐gravity analysis. On this basis, the photoluminescence properties of these complexes and the relationships between their structure and luminescence were investigated in depth. The results show that the bonded bidentate Schiff base ligands, SAN and SCA, can effectively sensitize the fluorescence emission of Eu(III) and Tb(III) ions, respectively. PSF–SAN–Eu(III) series complexes, namely the binary complex PSF–(SAN)₃–Eu(III) and the ternary complex PSF–(SAN)₃–Eu(III)–(Phen)₁ (Phen is the small‐molecule ligand 1,10‐phenanthroline), produce strong red luminescence, suggesting that the triplet state energy level of SAN is lower and well matched with the resonant energy level of the Eu(III) ion. By contrast, PSF–SAN–Eu(III) series complexes, namely the binary complex PSF–(SCA)₃–Tb(III) and the ternary complex PSF–(SCA)₃–Tb(III)–(Phen)₁, display strong green luminescence, suggesting that the triplet state energy level of SCA is higher and is well matched with the resonant energy level of Tb(III).     

Lanthanide(III) complexes of amide derivatives of DOTA exhibit an unusual variation in stability across the lanthanide series

Luminescence excitation spectroscopy of the ⁷F₀→⁵D₀ transition of the Eu(III) complex of 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (TCMC, an amide derivative of DOTA) is used to measure the stability constant of the complex (K). A log K value of 10.6 is obtained for [Eu(TCMC)]³⁺ at 25 °C and an ionic strength of 0.1 M. Competition experiments with eleven other members of the lanthanide(III) series give stability constants for their complexes with TCMC. An unusual variation in stability is observed for complexes of [Ln(TCMC)]³⁺ across the lanthanide series with a pronounced optimum for the Sm(III) complex. This variation is quite different from that observed for other Ln(III) macrocyclic complexes, suggesting that the TCMC ligand is uniquely sensitive to Ln(III) ion radius.     

Tripodal amine catechol ligands: a fascinating class of chelators for aluminium(III)

Complexation constants based on potentiometric titrations and spectrophotometric measurements in an aqueous medium of 0.1 M KCl at 25 ± 1 °C for the complexes of Al(III) with multidentate tripodal polycatechol-amine ligands, cis,cis-1,3,5-tris[(2,3-dihydroxybenzylamino)aminomethyl]cyclohexane (TMACHCAT, L¹) and N¹,N³,N⁵-tris(2-(2,3-dihydroxybenzylamino)ethyl)cyclohexane-1,3,5-tricarboxamide (CYCOENCAT, L²) have been summarized in this paper. Both the ligands released six protons to form various monomeric complexes of the types AlLH₃, AlLH₂, AlLH and AlL (L = L¹ and L²). The first species AlLH₃ depicted at low pH for which a monocapped type geometry was suggested, where the ligands were coordinated through three catecholic oxygens at ortho. Other species are formed subsequently from the species AlLH₃ in steps upon deprotonation and coordination of the catecholic oxygens at meta to give encapsulated tris(catechol) type complexes. The probable structures of the metal complexes formed in solution were proposed through molecular modeling calculations. The pAl values calculated for AlL¹ and AlL² are appreciably higher than transferrin. The ligand L² showed higher affinity towards Al(III) than L¹ and desferrioxamine (DFO), the only approved drug for the treatment of aluminium intoxication.