Thermal decomposition of metal complexes IV. Lanthanide(III) complexes with dibenzo-18-crown-6

The solid-state thermal dissociation reactions of the complexes of all the lanthanide(III) nitrates and thiocyanates (except Pm) with the cyclic polyether dibenzo-18-crown-6 were investigated. Thermal analysis was carried out in a dynamic atmosphere of dry nitrogen and under reduced pressure (5·10^?2 mm Hg). In both the conditions examined, the two series of complexes exhibit different thermal behaviours. The values of enthalpy change and “activation energy” for the dissociation reactions of the complexes with lanthanide thiocyanates show a periodic trend along the lanthanide series.     

DNA-binding properties and antioxidant activity of lanthanide complexes with the Schiff base derived from 3-carbaldehyde chromone and isonicotinyl hydrazine

Structural analyses indicate that the ligand and lanthanide ions form mononuclear 10-coordinate ([Ln L₂ · (NO₃)₂] · NO₃ [Ln(III) = La, Sm, Nd, and Yb; L is chromone-3-carbaldehyde-(isonicotinoyl) hydrazone) complexes with 1 : 2 metal-to-ligand stoichiometry. DNA-binding studies show that the ligand and its lanthanide complexes can bind to calf thymus DNA via an intercalation mode with binding constants of 10⁵ (mol L^?1)^?1, and the lanthanide complexes bind stronger than the free ligand alone. Antioxidant activities of the ligand and lanthanide complexes were determined by superoxide and hydroxyl radical scavenging methods in vitro. The ligand and complexes possess strong scavenging effects, and the lanthanide complexes show stronger antioxidant activities than the ligand and some standard antioxidants, such as vitamin C.     

Chiral tripode approach toward multiple anion sensing with lanthanide complexes

A chiral tripodal ligand was demonstrated to form a series of lanthanide complexes exhibiting multiple anion-sensing profiles, which incorporated a fluorescent quinoline and a stereo-controlled substituent in a tetradentate skeleton. This mainly gave 1:2 (lanthanide cation/ligand) complexes with lanthanide triflates but 1:1 complexes with lanthanide nitrates. Since addition of external guest anion dynamically changed the preferred stoichiometry, the chiral lanthanide complexes exhibited anion-responsive fluorescence, luminescence, and circular dichroism spectral characteristics as multiple anion-sensing probes.     

Are cyclopentadienyl complexes more stable than their pyrrolyl analogues?

Metal-η⁵-cyclopentadienyl (M-Cp) and metal-η⁵-pyrrolyl (M-pyr) bond dissociation enthalpies in group 4 complexes were determined from DFT/B3LYP calculations with a VTZP basis set. Thermochemical cycles involving calculated enthalpies of ligand exchange reactions and experimental values of ligand electron affinities and M-Cl bond dissociation enthalpies were applied to [M(η⁵-X)Cl₃] piano stool complexes (M = Ti, Zr, Hf; X = pyr, Cp), allowing a comparative study of those metal-ligand bond strengths. The results indicate that both ligands establish weaker bonds with Ti than with the heavier elements, Zr and Hf. Very similar bond dissociation enthalpies were obtained for pyrrolyl and cyclopentadienyl (within 1 kcal mol⁻¹), suggesting that the well known difference in reactivity between those families of complexes should derive from kinetic rather than thermodynamic causes.     

Spectroscopic,magnetic and conductivity measurements of manganese(II) complexes with 2,5-diphenyloxazole

Summary The Mn(PPO)_nX₂ complexes (where PPO is 2,5-diphenyloxazole, n = 1, 2 or 3 and X = Cl^–, Br^–, I^– or SCN^–) have been prepared from the corresponding metal salt with the ligand in methanolic solution or with the molten ligand in the appropriate reactant ratio and studied by chemical analysis, electronic and i.r. spectroscopy, magnetic and molar conductivity values at 25°. The complexes are generally nonconducting in DMF; they are high spin and hexacoordinated. The Mn(PPO)X₂ complexes are high polymers with both ligand and anions bridging. The Mn(PPO)₂X₂ species are polymeric with PPO monodentate N-bonded and halogens and thiocyanates bridging bonded; the Mn(PPO)₃X₂ derivatives have monodentate N-bonded ligands whereas the halogens and thiocyanates are terminal and bridging, respectively.     

Mixed ligand complexes of lanthanide nitrates with acetone ferrocenecarbonylhydrazone and 2,2′-dipyridine

Transition Metal Chemistry - Acetone ferrocenecarbonylhydrazone, HL, reacts with hydrated lanthanide nitrates in absolute EtOH to give mixed ligand complexes, Ln(HL)(dipy)(NO3)3nH2O (Ln =...     

"Asymmetric" catalysis by lanthanide complexes

Catalysis with lanthanide (Ln) complexes has been underestimated for long time, although Ln(III) complexes have great advantages as Lewis acid catalysts for "asymmetric" carbon-carbon bond-forming reactions. Lanthanide complexes are highly active in ligand-substitution reactions, especially with hard ligands. The association with substrates and dissociation of products are achieved fast enough for high catalyst efficiency. The asymmetric catalysis of organic reactions can be greatly advanced by the use of Ln complexes with chiral ligands such as binaphthol (binol). Ln(II) complexes are good reducing agents, which can be used in a wide variety of synthetically important reactions; when chiral ligands are used, many of these reactions are highly stereoselective. In the context of "green chemistry", the development of asymmetric Ln catalysts, and their recyclable use, is of increasing importance. This review gives an overview of the most recent developments in catalysis with lanthanide(II) and lanthanide(III) complexes.     

Synthesis,characterization and fluorescence of lanthanide Schiff-base complexes

Six new lanthanide Schiff-base complexes were synthesized by reactions of hydrated lanthanide nitrates with H₂L (H₂L?=?N,N′-bis(salicylidene)-1,2-cyclohexanediamine) and characterized by elemental analysis, DTA–TG, IR, UV and luminescence spectra. The microanalyses and spectroscopic analyses indicate a 1D polymeric structure with the formula of [Ln(H₂L)(NO₃)₃(MeOH)₂] _n [Ln?=?La (1), Ce (2), Pr (3), Sm (4), Gd (5) & Dy (6)]. The fluorescence spectrum of complex 4 exhibited Sm³⁺ centered, Schiff-base sensitized orange fluorescence, indicating that energy levels of the triplet state of H₂L match closely to the lowest excited state (⁴G_5/2) of Sm³⁺ ion.     

Synthesis and characterization of a new unsymmetrical porphyrin liquid crystal and its lanthanide complexes

A meso-substituted unsymmetrical porphyrin liquid crystal, 5-(4-myristyloxy)phenyl-10,15,20-triphenyl porphyrin, and a series of its lanthanide complexes, (lanthanide ions: Gd, Tb, Dy, Ho and Er) with acetylacetone were synthesized and characterized by elemental analyses, molar conductances, UV-Vis, IR and ¹H?NMR spectra. A structure is proposed in which the porphyrin is as a tetradentate ligand and acetylacetonate is bidentate to the lanthanide. Luminescence spectra show that quantum yields of the Q band fluorescence are in the region 0.027–0.191. DSC data and an optical textural photo using a polarizing microscope indicates that the compounds have liquid crystalline character.     

Lanthanide radii controlled one-dimensional polymer and dinuclear complexes and their fluorescent properties

A bi-phosphonate ligand tetraethyl-(2,3,5,6-tetramethyl-1,4-phenylene) bis(methylene)diphosphonate has been designed and synthesized. The bi-phosphonate as a bridging ligand reacts with lanthanide nitrates forming four different types of 1D coordination complexes: ribbon polymer (type I), semi-ribbon polymer (type II), zigzag polymer (type III), and dinuclear-triligand short chain (type IV), which changed according to the decrease of the radius of the lanthanide. They have been characterized by IR spectroscopy, elemental analysis, and X-ray diffraction spectroscopy. The photophysical properties of Sm(3+), Eu(3+), Tb(3+) and Dy(3+) complexes at room temperature were also investigated. They exhibit strong fluorescence by excitation of the Ln(3+) ion absorption bands and the quantum yield values of Eu(3+) and Tb(3+) complexes are no less than 20%.     

4-Aminoantipyrine schiff-base complexes of lanthanide and uranyl ions

Using a new tridentate ligand, acetylacetone-4-aminoantipyrine (AAP) complexes of lanthanide nitrates having the general formula [Ln(AAP)₂](NO₃)₃ (where Ln = Pr, Nd, Sm, Gd, Dy or Y) have been prepared. Uranyl complexes of salicylaldehyde-4-aminoantipyrine (SAAP) of general formulae [UO₂(SAAP)₂](X)₂ (where X = NO₃ or I) and [UO₂(SAAP)₂(OAc)₂] have also been prepared. The complexes have been characterized by IR, electronic, NMR, conductivity and analytical techniques. While SAAP acts as bidentate ligand without using its hydroxyl group, AAP acts as tridentate ligand involving hydroxyl coordination. The covalency parameter δ has been calculated for the Pr and Nd complexes.     

Lanthanide(III) Nitrate Complexes of Two 17 Membered N3O2-Donor Macrocycles

The interaction of lanthanide(III) ions with two N₃O₃-macrocycles, L¹ and L², derived from 2,6-bis(2-formylphenoxymethyl)pyridine and 1,2-diaminoethane has been investigated. Schiff-base macrocyclic lanthanide(III) complexes LnL¹(NO₃)₃ · xH₂O (Ln = Nd, Sm, Eu or Lu) have been prepared by direct reaction of L¹ and the appropriate hydrated lanthanide nitrate. The direct reaction between the diamine macrocycle L² and the hydrated lanthanide(III) nitrates yields complexes LnL²(NO₃)₃· H₂O only for Ln = Dy or Lu. The reduction of the Schiff-base macrocycle decreases the complexation capacity of the ligand towards the Ln(III) ions. The complexes have been characterised by elemental analysis, molar conductivity data, FAB mass spectrometry, IR and, in the case of the lutetium complexes, ¹H NMR spectroscopy.     

Dual-emissive complexes: Visible and near-infrared luminescence from bis-pyrenyl lanthanide(III) complexes

The syntheses and photophysical attributes of a range of dual-emissive lanthanide complexes are described. The simple ligand architecture is based upon a diethylenetriaminepentaacetic acid (DTPA) core and appended with two aminopyrenyl chromophores to yield the fluorescent free ligand L^pyr. Reaction of the ligand with Ln(tris-trifluoromethanosulfate) gave the mononuclear complexes Ln · L^pyr (Ln = Nd, Er, Yb). Luminescence studies revealed that the complexes were emissive in both the near-IR and UV–Vis, the latter resulting from pyrene localised emission (λ_em = 390 nm), the former from pyrene-sensitised emission of the lanthanide ion (λ_ex = 337 nm). Time-resolved measurements in the near-IR indicated that the number of coordinated solvent molecules for Nd and Yb was <1, confirming the proposed coordination mode of the octadentate L^pyr. The suitability of pyrene as a sensitiser for near-IR emitting lanthanides was further demonstrated in the rare observation of Er^III emission in a non-deuteriated protic medium.     

Structural and selectivity of 18-crown-6 ligand in lanthanide-picrate complexes

The selectivity factor in the separation of lanthanide could be associated with the coordination behaviour. Thus, we observed the study in the solid phase to understand the coordination pattern of Ln(III) with the 18-crown-6 (18C6) ligand. Good selectivity of the rigid 18C6 ligand toward Ln(III) depends on gradually smaller their ionic radii of Ln(III) in the complexes formation in the presence of picrate anion (Pic⁻), i.e. lanthanide contraction and steric effects as clearly shown in the series of [Ln(Pic)₂(18C6)]⁺(Pic)⁻ {Ln = La, Ce, Pr, Nd, Sm, Gd} and [Ln(Pic)₃(OH₂)₃] · 2(18C6) · 4H₂O {Ln = Tb, Ho} complexes. The La-Gd complexes crystallized in an orthorhombic with space group Pbca, while the Ho complex crystallized in triclinic with space group . The lighter lanthanides complexes [La-Sm] had a 10-coordination number from the 18C6 ligand and the two picrates, forming a bicapped square-antiprismatic geometry. Meanwhile, the middle lanthanide complex [Gd] had a nine-coordination number from the 18C6 ligand and the two picrates, forming a tricapped trigonal prismatic geometry. The heavier lanthanide [Ho] is rather unique, since Ho(III) coordinated with nine oxygen atoms from three picrates and three water molecules in the opposite direction whereas three 18C6 molecules surrounded in the inner coordination sphere, forming a trigonal tricapped prismatic geometry. The 18C6 ligand is effective in controlling the molecular geometry and coordination bonding of Ln-O and can use a crystal engineering approach. No dissociation of Ln-O bonds in solution was observed in NMR studies conducted at different temperatures. The photoluminescence spectrum of the Pr complex has typical 4f-4f emission transitions, i.e. ³P₀ → ³F₂ (650 nm), ¹D₂ → ³F₂ (830 nm) and ¹D₂ → ³F₄ (950 nm).     

Polymer complexes XXXIV. Potentiometric and thermodynamic studies of monomeric and polymeric complexes containing 2-acrylamidosulphadiazine

2-Acrylamidosulphadiazine (ASD) was synthesized and characterized by elemental analyses, IR and 1H NMR spectra. Proton-monomeric ligand dissociation and metal-monomeric ligand stability constants of ASD with some metal ions were determined potentiometrically in 0.1 M KCl and 40% (v/v) ethanol-water mixture. In the presence of 2,2^′-azobisisobutyronitrile as initiator, the dissociation and stability constants of ASD were determined in polymeric form (PASD). The influence of temperature on the dissociation of ASD and the stability constants of their complexes in monomeric and polymeric forms were critically studied. The pK₂^H value of PASD was found to be higher than ASD, this means that the vinyl group in the monomeric form decreases the electron density and hence reduces the N-H bond strength. The stability constants of the metal complexes in polymeric form are higher than those of the monomeric form. This is quite reasonable because the ligand in a polymeric form is considered as a better complexing agent.     

Synthesis and characterization of complexes of lanthanide nitrates with N,N′-dinaphthyl-N,N′-diphenyl-3,6-dioxaoctanediamide

Solid complexes of lighter lanthanide nitrates with N,N′-dinaphthyl-N,N′-diphenyl-3,6-dioxaoctanediamide (DDD), Ln(NO₃)₃(DDD) (Ln = La---Nd, Sm) have been prepared in non-aqueous media. These complexes have been characterized by elemental analysis, conductivity measurements, IR spectra, electronic spectra and TG-DTA techniques. In all the complexes, DDD and NO₃⁻ are coordinated to the lanthanide ions as tetradentate and bidentate ligands, respectively. The differences in the IR and electronic spectra between these complexes and lanthanide nitrate complexes with N,N,N′,N′-tetraphenyl-3,6-dioxaoctanediamide (TDD) are discussed.     

Mass spectra of organolanthanide complexes

The EI mass spectra of seven 1,1′-(3-oxa-pentamethylene)-dicyclopentadienyl lanthanide and yttrium chlorides ( 1–7 ), and eleven dicyclopentadienyl lanthanide and yttrium chlorides ( 8–18 ) were investigated. Fragmentation patterns of these complexes were studied by using metastable ion measurements. The EI spectra of complexes 1–7 exhibited strong molecular ion peaks, the fragmentation of molecular ions were more complicated. The EI mass spectra of complexes 8–18 supported the dimeric structure under gaseous state. In the spectra of dimers (C₅H₅)₃LN⁺ observed were resulted from the skeletal rearrangement involving the migration of cyclopentadienyl moiety.     

Divalent lanthanide complexes: highly active precatalysts for the addition of N-H and C-H bonds to carbodiimides

Various divalent lanthanide complexes with the formula LnL2(sol)x (L = N(TMS)2, sol = THF, x = 3, Ln = Sm (I), Eu (II), Yb (III); L = MeC5H4, sol = THF, x = 2, Ln = Sm (IV); L = ArO(Ar = [2,6-((t)Bu)2-4-MeC6H2]), sol = THF, x = 2, Ln = Sm (V)), especially complexes I- III, serve as excellent catalyst precursors for catalytic addition of various primary and secondary amines to carbodiimides, efficiently providing the corresponding guanidine derivatives with a wide range of substrates under solvent-free condition. The reaction shows good functional groups tolerance. Complexes I- III are also excellent precatalysts for addition of terminal alkynes to carbodiimides yielding a series of propiolamidines. The active sequence of Yb < Eu < Sm for metal and MeC5H4 < ArO < N(TMS)2 for ligand around the metal was observed for both reactions. The first step in both reactions was supposed to include the formation of a bimetallic bisamidinate samarium species originating from the reduction-coupling reaction of carbodiimide promoted by lanthanide(II) complex. The active species is proposed to be a lanthanide guanidinate and a lanthanide amidinate.     

Lanthanide(III) Complexes of 4,10‐Bis(phosphonomethyl)‐1,4,7,10‐tetraazacyclododecane‐1,7‐diacetic acid (trans‐H6do2a2p) in Solution and in the Solid State: Structural Studies Along the Series

Complexes of 4,10‐bis(phosphonomethyl)‐1,4,7,10‐tetraazacyclododecane‐1,7‐diacetic acid (trans‐H₆do2a2p, H₆ L ) with transition metal and lanthanide(III) ions were investigated. The stability constant values of the divalent and trivalent metal‐ion complexes are between the corresponding values of H₄dota and H₈dotp complexes, as a consequence of the ligand basicity. The solid‐state structures of the ligand and of nine lanthanide(III) complexes were determined by X‐ray diffraction. All the complexes are present as twisted‐square‐antiprismatic isomers and their structures can be divided into two series. The first one involves nona‐coordinated complexes of the large lanthanide(III) ions (Ce, Nd, Sm) with a coordinated water molecule. In the series of Sm, Eu, Tb, Dy, Er, Yb, the complexes are octa‐coordinated only by the ligand donor atoms and their coordination cages are more irregular. The formation kinetics and the acid‐assisted dissociation of several Ln^III–H₆ L complexes were investigated at different temperatures and compared with analogous data for complexes of other dota‐like ligands. The [Ce( L )(H₂O)]^3? complex is the most kinetically inert among complexes of the investigated lanthanide(III) ions (Ce, Eu, Gd, Yb). Among mixed phosphonate–acetate dota analogues, kinetic inertness of the cerium(III) complexes is increased with a higher number of phosphonate arms in the ligand, whereas the opposite is true for europium(III) complexes. According to the ¹H NMR spectroscopic pseudo‐contact shifts for the Ce–Eu and Tb–Yb series, the solution structures of the complexes reflect the structures of the [Ce(H L )(H₂O)]^2? and [Yb(H L )]^2? anions, respectively, found in the solid state. However, these solution NMR spectroscopic studies showed that there is no unambiguous relation between ³¹P/¹H lanthanide‐induced shift (LIS) values and coordination of water in the complexes; the values rather express a relative position of the central ions between the N₄ and O₄ planes.     

Synthesis and structural characterization of 3d-metal complexes of pyridine-4-carboxaldehyde thionicotinoyl hydrazone

Summary Pyridine-4-carboxaldehyde thionicotinoyl hydrazone (4-PTNH) forms 1:1 adducts with metal(II) halides and 1:2 complexes (metal to ligand) with metal(II) thiocyanates. Magnetic and spectral studies indicate polymeric octahedral geometry for M(4-PTNH)X₂ (M=Co^II or Cu^II, X=Cl; M=Ni^II, X=Cl, Br or I), five coordinate geometry for Co(4-PTNH)X₂ (X=Br or I) and octahederal geometry for [M(4-PTNH)₂(NCS)₂] (M=Co^II or Ni^II). I.r. spectral studies show that 4-PTNH acts as a neutral bidentate ligand in all the complexes, the bonding sites being the thione sulphur and azomethine nitrogen.