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.     

Spectroscopic studies of aqueous gallium(III) and aluminum(III) citrate complexes

Aqueous gallium(III) citrate complexes have been studied in the 10(-2) M concentration range with extended X-ray absorption fine structure (EXAFS) and FTIR techniques. From EXAFS data, one mononuclear and one oligomeric species were identified at different Ga(III) to citrate ratios. The first shell of the mononuclear complex was found to be distorted, with average Ga-O bond lengths of 1.95 and 2.06 A, in agreement with the solid-state structure of Ga(Cit)2(3-) (Cit=citrate). Also the oligomeric species was found to have a distorted first shell, with average Ga-O bond lengths of 1.95 and 2.04 A. This complex was found to contain two Ga-Ga distances at 3.03 and 3.56 A, typical for edge and corner sharing GaO6 octahedra, respectively. The gallium(III) and aluminum(III) citrate systems were compared by means of FTIR, and were found to be analogous. The IR results suggest that the bond lengths derived from EXAFS for the 1:2 gallium(III) citrate complex also provide a good estimate of the corresponding distances in the mononuclear 1:1 complex. Direct coordination of citrate to the metal ions in the oligomeric gallium(III) citrate complex was indicated from both EXAFS and IR results, and this complex is stoichiometrically analogous to the Al3(H-1Cit)3(OH)(H2O)4- complex, which has been structurally determined. However, while the formation of the aluminum trimer has been shown to be slow, the gallium trimer was significantly more labile with a rate of formation indicated to be in the order of seconds or faster.     

An FTIR Spectroscopic Study of Calf-thymus DNA Complexation with Al(III) and Ga(III) Cations

Abstract

The interaction of calf-thymus DNA with trivalent Al and Ga cations, in aqueous solution at pH =6–7 with cation/DNA(P) (P=phosphate) molar ratios (r) 1/80, 1/40, 1/20, 1/10, 1/4 and 1/2 was characterized by Fourier Transform infrared (FTIR) difference spectroscopy.

Spectroscopic results show the formation of several types of cation-DNA complexes. At low metal ion concentration (r=l/80, 1/40), both cations bind mainly to the backbone PO₂ group and the guanine N-7 site of the G-C base pairs (chelation). Evidence for cation chelate formation comes from major shifting and intensity increase of the phosphate antisymmetric stretch at 1222 cm-¹ and the mainly guanine band at 1717 cm¹. The perturbations of A-T base pairs occur at high cation concentration with major helix destabilization. Evidence for cation binding to A-T bases comes from major spectral changes of the bands at 1663 and 1609 cm-¹ related mainly to the thymine and adenine in-plane vibrations. A major reduction of the B-DNA structure occurs in favor of A-DNA upon trivalent cation coordination.     

Spectroscopic and structural characterizations of ammonium peroxo-carboxylato molybdate(VI) complexes

New ammonium derivatives of peroxo-carboxylato molybdenum(VI) complexes of general formula (NH₄)₂[MoO(O₂)₂(H_xL)] · nH₂O with L=oxalate (ox), citrate (cit), tartrate (tart), glycolate (glyc) and malate (mal) and (NH₄)₂[MoO₂(O₂)(L)] with L=oxalate (ox) have been prepared and characterized on the basis of elemental and thermal analysis as well as by IR and ¹³C NMR spectroscopy. These last two spectroscopic methods have been used to suggest the coordination mode of the ligand in the complexes. The X-ray crystal structures of the compounds (NH₄)₂[Mo₂O₂(O₂)₂(OH)₂(ox)₂], (NH₄)₂[MoO(O₂)₂(ox)] and (NH₄)₂[MoO(O₂)₂(glyc)] · 0.5EtOH have been determined, all showing a sevenfold-coordinated Mo atom with bidentate peroxides and carboxylate ligands.     

Sugar complexes with neodymium nitrate. Spectroscopic and structural studies of two Nd(NO3)3-galactitol complexes

Two different Nd(NO3)3-galactitol complexes, 2Nd(NO3)3.C6H14O6.8 H2O and Nd(NO3)3.C6H14O6 have been obtained and were characterized by FT-IR and X-ray diffraction techniques. The spectral differences of the two complexes were consistent with the crystal-structure data.     

Spectroscopic and structural characterizations of water-soluble peroxo complexes of niobium(V) with N-containing heterocyclic ligands

New water-soluble heteroleptic peroxo complexes of niobium^V have been prepared with N-containing heterocyclic ligands. The compounds correspond to the general formula (gu)_x[Nb(O₂)₃(L)] · nH₂O (gu = guanidinium). Three different ligands (L) have been used: picolinic acid (Hpic), picolinic acid N-oxide (HpicO) and pyrazine 2,5-dicarboxylic acid (2,5-H₂pzdc). The Nb^V complexes have been characterized on the basis of elemental and thermal analysis as well as by IR and ESI-mass spectrometry. The crystal structure of the peroxo complex with the picolinato N-oxide ligand, (gu)₂[Nb(O₂)₃(picO)] (II) has been determined, showing an eightfold-coordinated Nb atom surrounded by three peroxide groups and a picO ligand bound in a bidentate mode via the carboxylato and the N-oxide groups. The coordination polyhedron around the metal is a triangular dodecahedron.     

Spectroscopic investigation of europium(III) nitrato complexes in anhydrous and aqueous acetonitrile

Electronic absorption and emission spectra, along with lifetime measurements and vibrational spectra, are used to investigate the interaction between nitrate and trivalent europium ions in dilute solutions in anhydrous and aqueous acetonitrile. Upon addition of increasing quantities of nitrate, the complexes [Eu(NO₃)_n]^(3?n)+, with n = 1–5, form quantitatively in anhydrous acetonitrile. In solution, the pentanitrato species is not further solvated and its spectroscopic properties are similar to those of solid samples, indicating a similar structure with five bidentate nitrates bonded to the 10-coordinate Eu(III) ion. The lifetimes of the ⁵D₀ level are 1.35(5) and 1.25(5) ms for Eu(NO₃)₃ and (Me₄N)₂Eu(NO₃)₅ 0.05 M in CH₃CN. The quantum yield of Eu(NO₃)₃ in CH₃CN is 27.4%.The addition of small quantities of water to Eu(NO₃)₃ solutions does not result in the dissociation of the nitrate ions, provided R_w = [H₂O]_t/[Eu³⁺]_t is smaller than 8; the apparent equilibrium rations for [Eu(NO₃)₃(H₂O)_n] are K₃ = 40 ± 15 M^?1 and K₄ = 9 ± 3 M^?1; K₁ and K₂ are too large to be determined. The formation of nitrato complexes is studied in mixtures containing increasing amounts of water and nitrate. Deconvolution of the different components of the ⁵D₀ → ⁷F₀ transition allows a semi-quantitative estimate of the relative concentration of the nitrato complexes. The total number of coordinated nitrate ions per europium ion can be determined on the basis of fluorescence lifetime measurements. The apparent equilibrium ratios for the formation of the mono- and dinitrato species amount to K₁ = 23 ± 3, 15 ± 5 and 5 ± 1 for R_w = 44, 94 and 304, respectively, and to K₂ = 17 ± 8 for R_w = 44 and 94.     

Synthesis and structural studies of gallium(III) and indium(III) complexes of 2-acetylpyridine thiosemicarbazones

Four 2-acetylpyridine ⁴N-alkyl thiosemicarbazones, and their Ga(III) and In(III) complexes have been prepared and characterised by fluorescence, UV-Vis, IR, ¹H and ¹³C NMR spectroscopy, mass spectrometry and X-ray crystallographic analysis. Comparison of the crystal structures gave an insight into the nature of the complexes formed, demonstrating a preference for [ML₂]⁺ type complexes with gallium and [MLX₃] species with indium. Stability studies on two candidates indicated that complex [InL³Cl₂MeOH] was stable to chemical degradation for prolonged periods in human serum, giving this complex potential for further biological evaluation.     

Silver(I) methanesulfonate complexes containing diphosphine ligands: Spectroscopic and structural characterization

1:1 and 2:1 adducts of diphosphine ligands R₂P(R′)_nPR₂ (dppm: R = Ph, R′ = CH₂, n = 1; dppe: R = Ph, R′ = CH₂, n = 2; dppp: R = Ph, R′ = CH₂, n = 3; dppb: R = Ph, R′ = CH₂, n = 4; dppf: R = Ph, R′ = ferrocenyl, n = 1) with silver(I) methanesulfonate have been synthesized and characterized both in solution (¹H, ³¹P NMR) and in the solid state (IR, single crystal X-ray structure analysis). The two different stoichiometries have been found to depend on the molar ratio of ligand to metal employed and the nature of the diphosphine ligand. In AgO₃SMe:dppp,dppb (1:1)₂, in the [Ag(P^P)₂Ag] arrays, the silver atoms are also bridged by anion oxygen atoms, in disparate fashion commensurate with the different Ag?Ag distances.     

Spectroscopic analysis of chlorophyll model complexes: methyl ester ClFe(III)pheophorbides

As models for chlorophyll a (Chl a), methyl ester ClFe(III)pheophorbides (1, pheophorbide a; 2, mesopheophorbide a; and 3, mesopyropheophorbide a) were examined by Fourier transform infrared (FTIR) absorption and resonance Raman (RR) spectroscopy. The infrared (IR) chlorin band above 1600 cm-1, assigned as a Ca-Cm mode (Andersson et al. (1987) J. Am. Chem. Soc. 109, 2908-2916) is shown to be metal-sensitive and responsive to spin state and coordination number for dihydroporphyrins, as well as being diagnostic for the chlorin vs. porphyrin or bacteriochlorin macrocycle. Frequency variations for this metallochlorin IR band thus parallel those of the v10 RR mode of porphyrins in their predictive utility. Qy excitation SERRS spectra of Chl a were compared with Qy excitation RR spectra of 1 and methyl Ni(II)pyropheophorbide a. The data demonstrate that 5-coordinate ClFe(III)pheophorbides are better models for chlorophylls than are ruffled 4-coordinate Ni(II)pheophorbides. Major spectral differences between the three chlorophyll models are associated with the C-9 keto and/or C-10 carbomethoxy vibrational modes. The approx. 1700 cm-1 IR band was formerly assigned solely to v(C = O) of the C-9 keto group. However, this IR feature shifts down to approx. 1685 cm-1 and nearly doubles in intensity when the C-10 carbomethoxy is removed, as for 3. Similar frequency downshifts coupled with intensity increases in the IR are found in the literature on chlorophylls. RR spectra of pheophorbides having the C-10 carbomethoxy group (1 and 2) have bands at both approx. 1700 and approx. 1735 cm-1. However, the C-9 keto v(C = O) mode of pyrophorbins also downshifts to approx. 1685 cm-1, as in the IR spectra. The approx. 1735 cm-1 ester RR mode disappears in the case of pyrophorbins, and is never RR active for nonconjugated esters of porphyrins or chlorins. These data demonstrate an interaction between the C-10 and C-9 carbonyls of phorbins. They also indicate that phorbins tend toward conjugation of the C-10 ester. Biological examples of such conjugation effects have recently been reported, e.g., for the Chl a pi-cation radical (Heald et al. (1988) J. Phys. Chem. 92, 4820-4824). Because the phorbin E ring is the major structural feature distinguishing chlorophylls from non-photosynthetic systems, the participation of the C-10 ester in ring conjugation is suggestive of its biological importance.     

Synthesis, characterization, antioxidative and antitumor activities of solid quercetin rare earth(III) complexes

Eight rare earth metal(II) complexes with quercetin ML3 x 6H2O [L=quercetin (3-OH group deprotonated); M = La, Nd, Eu, Gd, Tb, Dy, Tm and Y] have been synthesized and characterized by elemental analysis, complexometric titration, thermal analysis, conductivity, IR, UV, 1HNMR and fluorescence spectra techniques as well as cyclic voltammetry. The quercetin:metal stoichiometry and the equilibrium stability constant for metal binding to quercetin have been determined. The antioxidative and antitumor activities of quercetin x 2H2O and the complexes were tested by both the MTT and SRB methods. The results show that the suppression ratio of the complexes against the tested tumour cells are superior to quercetin x 2H2O. The property of LaL3 x 6H2O reacting with calf thymus DNA was studied by fluorescence methods. The La-complex binding to DNA has been determined by fluorescence titration in 0.05 M Tris-HCl, 0.5 M NaCl buffer (pH 7.0). The results indicate that the interaction of the complex with DNA is very evident.     

Quantitative 27Al NMR spectroscopic studies of Al (III) complexes with organic acid ligands and their comparison with GEOCHEM predicted values

The toxic inorganic monomeric forms of aluminium (Al) that limit plant growth have been shown to be effectively detoxified by complexation with organic acid ligands released by breakdown of added organic materials. The binding capacity of these acids is dependent on the degree of dissociation of their carboxyl groups and their ability to form bonds with Al. ²⁷Al NMR spectroscopy provides a non-invasive technique to study the bonding of Al with potential ligands without disturbing the equilibrium of the system. In single ligand systems containing oxalic acid, three ²⁷Al resonance peaks were observed at 6.4, 11.4 and 16.0 ppm downfield from the Al³⁺ reference peak at 0 ppm. These were assigned to Alox, Alox₂ and Alox₃ complexes respectively and were observable at pH values down to 3.5. In the presence of the citrate ligand, two ²⁷Al resonance peaks at 6.1 and 11.3 ppm, assigned respectively to the Alcit and Alcit₂ complexes, were observed at pH 3.4. At pH 4.3 and an Al:citrate molar ratio of 1:2, the 6.1 ppm peak was not visible, and the second peak further downfield was split into two unresolved peaks at 10.8 and 12.4 ppm indicating the presence of two forms of the Alcit₂ complex. Distribution of Al between the various species, based on integration of the resonance peaks and equilibrium calculations carried out using GEOCHEM, is discussed in light of the stability constants present in the database of GEOCHEM version (v.) 1.23 and GEOCHEM-PC v. 2.0. Large discrepancies between the computed values and the NMR measured values indicate the need to incorporate more recent literature values in the database for realistic equilibrium calculations in systems containing organic acid ligands. The potential of using quantitative ²⁷Al NMR measurements to calculate stability constants is discussed.     

(Catecholato)iron(III) complexes: structural and functional models for the catechol-bound iron(III) form of catechol dioxygenases

Catechol dioxygenases are mononuclear non-heme iron enzymes that catalyze the oxygenation of catechols to aliphatic acids via the cleavage of aromatic rings. In the last 20 years, a number of (catecholato)iron(III) complexes have been synthesized and characterized as structural and functional models for the catechol-bound iron(III) form of catechol dioxygenases. This review focuses on the structural and spectroscopic characteristics and oxygenation activity of the title complexes.     

Performance assessment of semiempirical molecular orbital methods in the structural prediction of Sb(III) and Bi(III) complexes

In this paper we carried out a systematic study in order to assess the quality of some semiempirical methods (AM1, PM3 and PM6), comparing predicted structural properties of many Sb(III) and Bi(III) complexes with the corresponding experimental data, indicating which one is more appropriate to describe the structure of such compounds. Root-mean squared deviation (RMSD) and unsigned mean error (UME) were used to evaluate the accuracy of the semiempirical methods to predict the ground state geometries of complexes with many ligand types. Our results have shown that, in general, PM3 predicts more accurately the geometry of Sb(III) complexes, being considered by us as the method of choice to study Sb(III) complexes with a great variety of ligands. PM6 is indicated as the method of choice to study Bi(III) complexes with many types of ligands and also to study Sb(III) thiocompounds, even though PM6 showed an inability to reproduce Sb-N bonds for complexes with flexible ligands, presenting an average deviation of 71.5 % compared the X-ray data.     

nmr and infrared spectroscopic investigations of the Al(III), Ga(III), and Be(II) complexes of ATP

The interaction of Al(III) with ATP has been examined by ³¹P and ¹H nmr and infrared spectroscopy. At pH 6.2, Al(III) forms a long-lived complex with ATP, in which chemical exchange between free and complexed ATP is slow on the nmr time scale. Infrared spectra of the Al(III)-ATP complex exhibit large perturbations in the band corresponding to the -PO₃^2? antisymmetric stretching mode. At higher pH values, equilibria involving Al(III) and OH^? become favored with the result that Al(III) no longer influences the spectroscopic properties of ATP. Similar spectroscopic results are obtained for the Ga(III) and Be(II) complexes of ATP.     

Diphosphinoazine rhodium(III) and iridium(III) octahedral complexes

Rhodium(III) and iridium(III) octahedral complexes of general formula [MCl₃{R₂PCH₂C(Bu^t)NNC(Bu^t)CH₂PR₂}] (M = Rh, Ir; R = Ph, c-C₆H₁₁, Prⁱ, Bu^t; not all the combinations) were prepared either from the corresponding diphosphinoazines and RhCl₃ · 3H₂O or by the oxidation of previously reported bridging complexes [{MCl(1,2-η:5,6-η-CHCHCH₂CH₂CHCHCH₂CH₂)}₂{μ-R₂PCH₂C(Bu^t)NNC(Bu^t)CH₂PR₂}] with chlorine-containing solvents. Depending on the steric properties of the ligands, complexes with facial or meridional configuration were obtained. Crystal and molecular structures of three facial and two meridional complexes were determined by X-ray diffraction. Hemilability of ligand in the complex fac-[RhCl₃{(C₆H₁₁)₂PCH₂C(Bu^t)NNC(Bu^t)CH₂P(C₆H₁₁)₂}] consisting in reversible decoordination of the phosphine donor group in the six-membered ring was observed as the first step of isomerization between fac and mer isomers.     

Synthetic, structural and solution speciation studies on binary Al(III)–(carboxy)phosphonate systems. Relevance to the neurotoxic potential of Al(III)

Efforts to delineate the interactions of neurotoxic Al(III) with low molecular mass substrates relevant to neurodegenerative processes, led to the investigation of the pH-specific synthetic chemistry of the binary Al(III)-[N-(phosphonomethyl) iminodiacetic acid] (Al-NTAP), Al(III)-[nitrilo-tris(methylene-phosphonic acid)] (Al-NTA3P), and Al(III)-[1-hydroxy ethylidene-1,1-diphosphonic acid] (Al-HEDP) systems, in correlation with solution speciation studies. Reaction of Al(NO₃)₃·9H₂O with NTAP at pH 7.0 and 4.0 afforded the new species (CH₆N₃)₄[Al₂(C₅H₆NPO₇)₂(OH)₂]·8H₂O (1) and (NH₄)₂[Al₂(C₅H₆NPO₇)₂(H₂O)₂]·4H₂O (2), while reaction of Al(NO₃)₃·9H₂O with NTA3P led to K₈[Al₂(C₃H₆NP₃O₉)₂(OH)₂]·20H₂O (3). Complexes 1–3 were characterized by elemental analysis, FT-IR, ¹³C, ³¹P, ¹H NMR (for 1–2 solid state and solution NMR where feasible), and X-ray crystallography. The structures of 1–3 reveal the presence of uniquely defined dinuclear complexes of octahedral Al(III) bound to fully deprotonated phosphonate ligands, water and hydroxo moieties. The aqueous solution speciation studies on the aforementioned binary systems project a clear picture of the binary Al(III)–(carboxy)phosphonate interactions and species under variable pH-conditions and specific Al(III):ligand stoichiometry. The concurrent solid state and solution work (a) exemplifies essential structural and chemical attributes of soluble aqueous species, reflecting well-defined interactions of Al(III) with phosphosubstrates and (b) strengthens the potential linkage of neurotoxic Al(III) chemical reactivity toward O,N-containing (carboxy)phosphate-rich cellular targets.     

Spectroscopic,structural and antibacterial properties of copper(II) complexes with bio-relevant 5-methyl-3-formylpyrazole N(4)-benzyl-N(4)-methylthiosemicarbazone

The coordination behaviour of the title ligand, 5-methyl-3-formylpyrazole N(4)-benzyl-N(4)-methylthiosemicarbazone(HMPz4BM), is reported with solid state isolation of copper(II) complexes, [Cu(HMPz4BM)X₂] (X = Cl, Br, NO₃, ClO₄ and BF₄) which have been spectroscopically and structurally characterised. I.r. data for the free ligand and its Cu(II) complexes indicate that HMPz4BM exhibits a neutral NNS tridentate function via the pyrazolyl nitrogen(tertiary), azomethine nitrogen and thione sulphur. Electronic spectral data are suggestive of a square pyramidal environment for the seemingly pentacoordinated Cu(II) species. E.s.r parameters (RT and LNT) of the reported copper(II) complexes are indicative of a dx_x2–y2 ground state for the reported species. Cyclic voltammograms of Cu(II) complexes show a quasireversible Cu^II/Cu^III couple and also an irreversible Cu^II/Cu^I couple. X-ray crystallography of a representative species, [Cu(HMPz4BM)(NO₃)₂] (C2/c, monoclinic ), has unambiguously documented the conjectural findings from i.r. data that coordinating sites of the title ligand are pyrazolyl (tertiary)nitrogen, azomethine nitrogen and the thione sulphur (NNS); and the oxygen of one of the nitrate ions has occupied the basal plane; the fifth coordination position has been occupied by the oxygen of another nitrate ion in a square pyramidal geometry. The antibacterial properties of the ligand and its copper(II) complexes studied on microorganism, Staphylococcus aureus have pointed out that most of the complexes have higher activities than that of the free ligand.     

DFT/TDDFT computational study of the structural,electronic and optical properties of rhodium (III) and iridium (III) complexes based on tris-picolinate bidentate ligands

The electronic structures and spectroscopic properties of two complexes [M(pic)₃] (M = Ir, Rh) containing picolinate as bidentate ligands have been calculated by means density functional theory (DFT) and time-dependent DFT/TD-DFT using three hybrid functionals B3LYP, PBE0 and mPW1PW91. The PBE0 and mPW1PW91 functionals, which have the same HF exchange fraction (25%), give similar results and do not differ drastically from B3LYP results. Calculated geometric parameters of the complexes are in good agreement with the available experimental data. The UV absorptions observed in acetonitrile were assigned on the basis of singlet state transitions. The most intense band observed in the UV-C region corresponds to ligand-to-ligand charge transfer states (LLCT) in both complexes. The theoretical spectrum of the rhodium complex is characterized by a large degree of mixing between metal-to-ligand-charge-transfer (MLCT), LLCT and metal centered (MC) states in the UV-A region. The presence of low-lying excited states with MC character affects the absorption spectrum under spin-orbit coupling (SOC) effects and play important roles in the photochemical properties.

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Graphical abstract Frontier molecular orbital diagram of mer-M(pic)₃ (M=Ir, Rh).