Ln Ligand Research Articles

Deep Red Emitting Heteroleptic Ir(III) Complexes that Incorporate Unsymmetrical 4‐quinoline Carboxylic Acid Derived Ligands

AbstractSix disubstituted ligands based upon 2‐(2′‐pyridinyl/pyrazinyl)quinoline‐4‐carboxylic acids have been synthesised, solvent‐free, in one step from a range of commercially available isatin derivatives. These species behave as ancillary chelating ligands for Ir(III) complexes of the form [Ir(C^N)2(N^N)]PF6 (where C^N=cyclometalating ligand; N^N=2‐(2′‐pyridinyl/pyrazinyl)quinoline‐4‐carboxylic acids). An X‐ray crystallographic study on one complex shows a distorted octahedral geometry wherein a cis‐C,C and trans‐N,N coordination mode is observed for the cyclometalating ligands. DFT calculations predicted that variations in N^N ligand from 2,2′‐bipyridine to L1–6 should localise the LUMO on to the Ln ligand and that the complexes are predicted to display MLCT/LLCT character. All complexes displayed luminescence in the deep red part of the visible region (674–679 nm) and emit from triplet states, but with little apparent tuning as a function of L1–6. Further time‐resolved transient absorption spectroscopy supports the participation of these triplet states to the excited state character.

Featuring a new computational protocol for the estimation of intensity and overall quantum yield in lanthanide chelates with applications to Eu(III) mercapto-triazole Schiff base ligands

Computational studies on luminescent properties of lanthanides chelates are important for the pre-synthetic design of new luminescent materials. However, the development of suited computational methods and protocols is still in its infancy. Here we present a new computational protocol for a systematic description and analysis of luminescent properties of Ln(III) chelates. The new methodologies herein presented are divided in three major categories: (1) Utilization of local vibrational mode theory for obtaining local Ln–ligand force constants; (2) Calculation of ligand effective polarizabilities through complex localized molecular orbitals; and (3) Extended formulation of excited state donor–acceptor RL distances accounting for excited molecular orbitals weights. The protocol was applied to understand the underlying photophysical processes in two Eu(III) complexes with mercapto-triazole Schiff bases as main ligands, utilizing time-dependent density functional theory (ωB97X-D/MWB52(Eu)/Def2-TZVP). The introduction of local force constants into lanthanide spectroscopy led to a unique explanation of the inverse relationship between the Ln–ligand strength and ligand effective polarizability. This new protocol will contribute to a better understanding of Ln–ligand bond properties, to more accurate results in terms of Judd–Ofelt intensity parameters and overall quantum yield, and will bridge the current gap between available theoretical results and experimental data.

Photophysical characteristics of Phenylimidazo(4,5-f)1,10-phenanthroline Rhenium(I) complexes - A theoretical approach to their potential applications in lighting devices

In the search for new luminescent semiconductors to be applied in energy conversion devices, we report via DFT/TD-DFT calculations the structure, UV–Vis absorption, and phosphorescent properties of Re(I) complexes denoted as [ReCl(CO)3(Ln)] and [ReTMP(CO)3(Ln)]+, where TMP is 2,4,6-trimethylpyridine and Ln is a bidentate ligand that corresponds to phenylimidazo(4,5-f)1,10-phenanthroline. This last ligand was analyzed for their electron-donating and -withdrawing characters with different substituents on the peripheral phenyl ring (Ln = -H, –CH3, o –F). Regarding the absorptions, at lower energy in the [ReCl(CO)3(Ln)], it was possible to identify a mixture of metal-to-ligand and ligand-to-ligand charge-transfer excitations, while the [ReTMP(CO)3(Ln)]+ series showed ligand centered transitions as responsible for absorption maximum. The phosphorescence of [ReCl(CO)3(Ln)] complexes exhibits an emission wavelength of 609–623 nm, whose deactivation path comes from π* orbital located at Ln ligands, towards the metal and ligands orbitals. By contrast, the emission process of the [ReTMP(CO)3(Ln)]+ complexes are mainly due to ligand-centered electron promotion (70%) in line with absorption features, showing emission wavelengths in the range of 521–527 nm. The change of the substituent Cl for TMP significantly influences the absorption and emission energies, unlike substitution on the Ln ligand. However, this last substitution pattern becomes relevant when considering injection/transport of holes-electrons where the reorganization energy (λ) for [ReCl(CO)3(L3)] neutral complexes improves due to the more balance charge transfer ability required for organic light-emitting diodes (OLED), while [ReTMP(CO)3(L2)]+ is the best choice for a light-emitting electrochemical cell (LEEC). Then, rhenium complexes can be considered as promising luminophores for device applications.

Synthesis, structural characterization, and density functional theory calculations of the two new Zn (II) complexes as antibacterial and anticancer agents with a neutral flexible tetradentate pyrazole‐based ligand

Two new mono‐ and dinuclear zinc (II) complexes with a formula of [Zn (LN)OMeCl] (1) and [Zn2(LN)(N3)2Cl2] (2) [where LN = 1,1,3,3‐tetrakis(3,5‐dimethyl‐1‐pyrazolyl)propane)] have been synthesized in methanol as a solvent and fully characterized by the single crystal X‐ray diffraction, elemental analysis, FT‐IR, UV–Vis, density functional theory (DFT) calculations, antibacterial, and anticancer properties. In both of them, zinc ion is tetrahedrally coordinated to two nitrogen atoms from one of the chelating bidentate bis(3,5‐dimethylpyrazolyl)methane units of the LN ligand and one terminal chlorine. The forth coordination site was occupied by methoxy group in 1 and azide co‐ligand in complex 2. In contrast to 1, in the structure of 2 two metal centers are bridged by the tetradentate chelating LN ligand to form a dinuclear complex. Interestingly, the environment around the central atom in these complexes are mimicked with the core structure of the zinc metalloproteinases, Thermolysin {[NNO]ZnX} and Carbonic anhydrase {[NNN]ZnX} for complexes 1 and 2, respectively. Data analysis of the antibacterial activities demonstrated that the gram positive species showed more sensitivity toward the target title agents. Although, both complexes exhibit antibacterial activities and kill bacteria at low concentration, but complex 1 has higher DNA cleavage activity than 2. Additionally, the cytotoxicity of the free ligand and its complexes was evaluated against the human carcinoma cell line (Caco‐2) by the MTT method and the IC50 values were calculated for all of them. Due to the presence of pyrazole rings in the structure of the ligand and its complexes, these materials have a substantial potential to inhibit cancer cell metastasis. The energetic stabilization due to donor‐acceptor interactions in terms of the natural bond orbital (NBO) analyses and nonlinear optical properties were also examined by using the DFT calculations.

Synthesis, chemical and biological investigations of new Ru(III) and Se(IV) complexes containing 1,3,5-triazine chelating derivatives

A series of vital complexes of [Se (Ln) (Cl)x]·yCl (where n = L1, L2, L3 and L4; x = 1, 2, or 3 and y = 1, 2, or 3) and [Ru (Ln) (Cl)x (H2O)y]zCl (where x = 2 or 3; y = 0,1 or 2 and z = 0 or 1) have been synthesized from the chemical reactions between ruthenium (III) or selenium (IV) salts and tri-substituted s-triazine derivatives (L1 = N2-(4-chlorophenyl)-N4,N6-di (pyrimidin-2-yl)-1,3,5-triazine-2,4,6-triamine; L2 = N2-(4-chlorophenyl)-N4- (pyrimidin-2-yl)- N6- (thiazol-2-yl)-1,3,5-triazine-2,4,6-triamine; L3 = 6-chloro-N2-(pyrimidin-2-yl)-N4-(1H-1,2,4-triazol-3-yl)-1,3,5-triazine-2,4-diamine and L4 = 6-chloro-N2-(4-chlorophenyl)-N4-(pyrimidin-2-yl)-1,3,5-triazine-2,4-diamine). The structural was investigated using of elemental analyses, molar conductance, IR, UV–Vis, magnetic susceptibility, 1H, 13C NMR spectra and thermal analyses. The surface morphology behaviors of selenium (IV) and ruthenium (III) complexes were studied based on scanning and transmittance electron microscopes. The crystalline nature of s-triazine complexes have been investigated using X-ray powder diffraction spectra. Spectral results were concluded that the ligand acts as a neutral bidentate for (L1 and L4) and tridentate for (L2 and L3), and coordinates through the nitrogen atom of triazine ring and nitrogen atoms of (pyrimidine, thiazole, and triazole) rings towards metal ion. The cytotoxic IC50 results of the free Ln ligands and its selenium (IV) complexes in vitro against the human colon and lung cancer cell lines introduced a promising efficiency.

Self‐Assembly of Discrete Copper(I)‐Halide Complexes with Diacylthioureas

Three diacylthioureas 1,4‐C6H4[C(O)NHC(S)NHAr]2 (Ar = 2,6‐iPr2C6H3) (L1, 1), 1,3‐C6H4[C(O)NHC(S)NHAr]2 (L2, 2), and 1,3‐C6H4[C(O)NHC(S)NHAr′]2 (Ar′ = 2,6‐Me2C6H3) (L3, 3) were synthesized and characterized. The CuI complexes from the reactions of bipodal ligands Ln with CuX (X = Cl, Br, I) were structurally investigated by single‐crystal X‐ray diffraction methods. Treatment of L1 with CuX gave the metallamacrocyclic complexes (L1CuX)2 [X = Cl (4), Br (5), I (6)] with the ligand to metal in a ratio of 2:2, where both sulfur and halide anions function as terminal substituents. In contrast, when L2 or L3 was reacted with CuBr, the two Ln ligands coordinate to four copper atoms each in a bridging and terminal fashion to yield [Ln(CuBr)2]2 [n = 2 (7), 3 (8)]. The obtained S4Cu4Br4 core contains all four bromide anions in bridging positions. The reaction of L3 with CuX (X = Cl, I) gave the 3:3 trinuclear complexes (L3CuX)3 [X = Cl (9) I (10)], interconnected by halide bridges. The obtained diacylthioureas (1–3) and their CuI complexes (4–10) were also characterized by elemental analysis, FT‐IR, 1H and 13C NMR spectroscopy.

Schiff pentadentate ligands based on an [ON2O2] core displaying structural isomerism and their coordination to dibutyltin moieties

Two tin compounds [Sn(nBu)2(Ln)], where L1{O(C6H4-2-NC(H)C6H4-2′-O)2}2− and L2{O(C6H4-2-C(H)NC6H4-2′-O)2}2− enclosing five electron donors in an [ON2O2] core, were synthesized and structurally characterized by infrared and NMR spectroscopy and by single crystal X-ray crystallographic studies at room temperature. The hepta-coordinate geometry around the Sn centers is achieved through the bonding of two monodentate n-butyl groups and the coordination of the pentadentate Ln ligands, where the [ON2O2] core is forming a pentagonal base. The two types of oxygen atoms displayed different bonding situations around the Sn atom. DFT calculations with Gaussian 09 using PBEPBE/6-31G**/SDD level were carried out for both compounds. The topological analysis indicated that bond critical points are present along the SnOether direction.

‘Lanthanide contraction’ in [Ln(BTP)3](CF3SO3)3 complexes

The FTIR spectra of the complexes of lanthanides (Ln = La − Lu, except for the radioactive Pm) with 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine have been measured in both the mid- and far-IR ranges. The spectra revealed some characteristic bands shifting towards higher wavenumbers with increasing lanthanide atomic number. X-ray diffraction results on several (La, Pr, Nd, Sm, Eu, Tm and Yb) complexes revealed decreasing Ln–ligand distances in the same direction. DFT calculations using the M062X exchange–correlation functional have been performed on selected model complexes in order to uncover the ‘lanthanide contraction’ phenomenon and to assist the interpretation of the FTIR spectra. The stronger bonding due to ‘lanthanide contraction’ is supported by the computed charge transfer interactions. The vibrational study confirmed that the blueshifting bands can be attributed to significant Ln–ligand vibrations.

Intracluster interactions in butterfly{Fe3LnO2}molecules with the non-Kramers ions Tb(III) and Ho(III)

The intracluster exchange interactions within the ``butterfly'' $[{\mathrm{Fe}}_{3}\mathrm{Ln}({\ensuremath{\mu}}_{3}\ensuremath{-}\mathrm{O}{)}_{2}({\mathrm{CCl}}_{3}\mathrm{COO}{)}_{8}({\mathrm{H}}_{2}\mathrm{O})(\mathrm{THF}{)}_{3}]$ molecules, where Ln(III) represents a lanthanide cation, have been determined by a combination of x-ray magnetic circular dichroism (XMCD) and vibrating sample magnetometry (VSM) along with an interaction model. We have studied the compounds with $\mathrm{Ln}=\mathrm{Tb}$ and Ho, both non-Kramers lanthanides and with high uniaxial anisotropy, and $\mathrm{Ln}=\mathrm{Lu}$(III) and Y(III) as pseudolanthanides, which supply nonmagnetic Ln reference cases. At low temperature, the three Fe atoms can be considered as a self-unit with total spin ${S}_{{\mathrm{Fe}}_{3}}=5/2$. Using the element selectivity of the XMCD magnetometry, measured at the Ln ${L}_{2,3}$ edges, together with the VSM measurements, the local magnetization of the Ln ion and the ${\mathrm{Fe}}_{3}$ subcluster, as a function of the field and low temperature ($T\ensuremath{\approx}2.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$), has been determined separately. These results are described quantitatively in the framework of a theoretical model based on an effective spin Hamiltonian, which considers the competing effects of intracluster interactions and the external applied magnetic field. The $\mathrm{Ln}\ensuremath{-}{\mathrm{Fe}}_{3}$ exchange interaction within the ${{\mathrm{Fe}}_{3}{\mathrm{LnO}}_{2}}$ cluster has been determined to be antiferromagnetic, in both Tb and Ho compounds, with ${\mathcal{J}}_{\mathrm{FeTb}}/{\mathrm{k}}_{\mathrm{B}}=\ensuremath{-}0.13(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${\mathcal{J}}_{\mathrm{FeHo}}/{\mathrm{k}}_{\mathrm{B}}=\ensuremath{-}0.18(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, respectively. In both cases, a field-induced reorientation of the ${\mathrm{Fe}}_{3}$ and Ln spins from antiparallel to parallel orientation takes place at a threshold field ${\ensuremath{\mu}}_{0}H=1.1$ and 2 T, for the ${{\mathrm{Fe}}_{3}{\mathrm{TbO}}_{2}}$ and ${{\mathrm{Fe}}_{3}{\mathrm{HoO}}_{2}}$ compounds, respectively. By comparison with other compounds of the series with uniaxial anisotropy, it is concluded that the polarizability of the ${\mathrm{Fe}}_{3}$ subcluster magnetic moment decreases in the trend ${{\mathrm{Fe}}_{3}{\mathrm{YO}}_{2}}\phantom{\rule{0.28em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.28em}{0ex}}{{\mathrm{Fe}}_{3}{\mathrm{TbO}}_{2}}\phantom{\rule{0.28em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.28em}{0ex}}{{\mathrm{Fe}}_{3}{\mathrm{HoO}}_{2}}\phantom{\rule{0.28em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.28em}{0ex}}{{\mathrm{Fe}}_{3}{\mathrm{DyO}}_{2}}$, because of the increasing opposition of the exchange antiferromagnetic field caused by the Ln ion. In the $\mathrm{Ln}=\mathrm{Tb}$, Ho, and Dy, the magnetization of the whole molecule is dominated by the anisotropy of the Ln ion. The intracluster ${\mathrm{Fe}}_{3}\ensuremath{-}\mathrm{Ln}$ exchange interactions are very weak compared to the Ln ligand field and Fe-Fe exchange interactions.

Biological activities of pyrenyl-derived thiosemicarbazone half-sandwich complexes

Pyrenyl-derived thiosemicarbazone half-sandwich complexes of the types [(η6-p-MeC6H4Pri)Ru(Ln)Cl]+ and [(η5-C5Me5)M(Ln)Cl]+ (M = Rh, Ir; L1 = pyrenecarboxaldehyde-3-thiosemicarbazone, L2 = pyrenecarboxaldehyde-4-methyl-3-thiosemicarbazone, L3 = pyrenecarboxaldehyde-4-phenyl-3-thiosemicarbazone) were synthesized and isolated as their chloride salts by reacting in THF the dinuclear complexes [(η6-p-MeC6H4Pri)2Ru2(μ-Cl)2Cl2] and [(η5-C5Me5)2M2(μ-Cl)2Cl2] with the corresponding Ln ligand. All complexes were isolated in good yields and were fully characterized by spectroscopic methods, including single-crystal X-ray structure analyses of the ruthenium complex [(η6-p-MeC6H4Pri)Ru(L1)Cl]Cl and the rhodium derivative [(η5-C5Me5)Rh(L2)Cl]Cl. The complexes were found to have IC50 values in the micromolar range against cancer cells (A-549, DU-145, HeLa, MCF-7), with a lower cytotoxicity in the noncancerous human HEK-293 embryonic kidney cells. The best two candidates, the rhodium derivatives [(η5-C5Me5)Rh(L1)Cl]Cl and [(η5-C5Me5)Rh(L3)Cl]Cl, show anticancer activities comparable to doxorubicin. These two complexes were further evaluated, showing an inhibition of the cell cycle at the G2/M and subG1 stages.

Phosphoramides bearing isoxazole derivative: spectroscopic and structural characterization, study of hydrogen-bonding interactions and two lanthanide complexes (LnIII = Ce and Eu).

In this study, the synthesis and spectroscopic characterization of new phosphoramides based on 3-amino-5-methylisoxazole with the formula R2P(O)[NH-C4H4NO], R = C6H5O (1), C6H5 (2), RP(O)[NH-C4H4NO]2, R = C6H5O (3), CH3-C6H4O (4), C6H5NH (5), (C6H5)ClP(O)[NH-C4H4NO] (6) and two lanthanide complexes [Ln(2)2(NO3)3(EtOH)]·EtOH, Ln(III) = Ce (7) and Eu (8), have been reported. The structural study of (3) shows the presence of two conformers (crystallographically independent molecules) in the crystalline lattice, caused by different orientations of the phenyl and isoxazole rings. For (3), the intermolecular interactions have been studied by Hirshfeld surface analysis and fingerprint plots. Furthermore, the electronic and energy aspects of hydrogen bonds between molecules of (3) have been explored by density functional theory (DFT) calculations. X-ray crystallography of complexes (7) and (8) reveals that two phosphoramide ligands take part in coordination to the metal, one as monodentate from O(phosphoryl), and the other one as chelate through O(phosphoryl) and N(ring). The complexes are also composed of two conformers in the solid-state structure. Quantum theory of atoms in molecules (QTAIM) analysis discloses the electrostatic nature of the Ln-ligand interaction.

Structure and Spectroelectrochemical Response of Arene–Ruthenium and Arene–Osmium Complexes with Potentially Hemilabile Noninnocent Ligands

Nine of the compounds [M(L2–)(p-cymene)] (M = Ru, Os, L2– = 4,6-di-tert-butyl-N-aryl-o-amidophenolate) were prepared and structurally characterized (Ru complexes) as coordinatively unsaturated, formally 16 valence electron species. On L2–-ligand based oxidation to EPR-active iminosemiquinone radical complexes, the compounds seek to bind a donor atom (if available) from the N-aryl substituent, as structurally certified for thioether and selenoether functions, or from the donor solvent. Simulated cyclic voltammograms and spectroelectrochemistry at ambient and low temperatures in combination with DFT results confirm a square scheme behavior (ECEC mechanism) involving the Ln ligand as the main electron transfer site and the metal with fractional (δ) oxidation as the center for redox-activated coordination. Attempts to crystallize [Ru(Cym)(QSMe)](PF6) produced single crystals of [RuIII(QSMe•–)2](PF6) after apparent dissociation of the arene ligand.

Assembly of organometallic networks with multinuclear silver(i)–ethynediide supramolecular synthon, trifluoroacetate and ligands derived from isomeric dicyanobenzenes

Six new silver ethynediide complexes containing trifluoroacetate and ligands derived from isomeric dicyanobenzenes, namely 2Ag2C2·10AgCF3CO2·3L1 (L1 = 1,2-dicyanobenzene) (1), 2Ag2C2·10AgCF3CO2·AgNO3·4L2 (L2 = 1,3-dicyanobenzene) (2), Ag2C2·2AgCF3CO2·2AgL3H·H2O (L3H2 = benzene-1,3-dicarboxylic acid) (3), Ag2C2·2AgCF3CO2·AgL3H·AgL4 (L4H = 3-cyanobenzenecarboxylic acid) (4), Ag2C2·4AgCF3CO2·2AgL5·4H2O (L5H = 4-cyanobenzenecarboxylic acid) (5) and 2Ag2C2·6AgCF3CO2·4AgL5·Ag2L6 (L6H2 = benzene-1,4-dicarboxylic acid) (6), have been synthesized under hydrothermal conditions and characterized using single crystal X-ray analysis. The Ln (n = 1–6) ligands incorporated into the silver ethynediide reaction system were generated in situ from hydrolysis of the o-, m- or p-isomers of dicyanobenzene. In the crystal structures of compounds 1–6, there are various types of silver(I) cages each enclosing a C22− dianion, which can be regarded as a multinuclear metal–ligand supramolecular synthon and symbolized as C2@Agn (n = 6–8). The silver cages are fused into infinite silver chains, which are connected by dicyanobenzene derivatives to yield two- and three-dimensional coordination networks in 1 and 2–6, respectively. The coordination capacity and structure-directing effect of the Ln ligands, as well as the temperature and pH value of the reaction mixture on the crystal structure of the resulting products, were also investigated.

Structure, bonding, electronic and energy aspects of a new family of early lanthanide (La, Ce and Nd) complexes with phosphoric triamides: Insights from experimental and DFT studies

A new family of isostructural early lanthanide(III) complexes (LnXPA) of the general formula Ln(XPA)(2)Cl(3)(solv)(2), where Ln = La, Ce and Nd, XPA = (4-X-C(6)H(4)NH)P(O)(NC(4)H(8)O)(2), X = H, F, Cl and Br, and solv = H(2)O and CH(3)OH, is introduced. X-ray crystallography shows that the replacement of the coordinated water by a methanol molecule may reduce the symmetry level of the unit cell from the orthorhombic crystal system and the space group Fdd2 to monoclinic and C2/c. DFT calculations, at B3LYP, PBE and B3PW91 levels, have been carried out to get a better insight into the structural, electronic and energy aspects of the compounds. The large cation attraction energy (-ΔE) values in the range 269-273 kcal mol(-1), at the B3PW91/ECP/6-311+G** level for the model complexes XPA-La(3+) with stoichiometry 1 : 1, represent new ligands XPA as efficient complexant agents for lanthanides. The electronic nature of para substituent X has no significant effect on the Ln-ligand bonding and cation affinity of the ligands XPA. The results of atoms in molecules (AIM) analysis reveal a partial covalent contribution of the Ln-ligand interaction for the models XPA-La(3+) in the absence of counterions and coordinated solvents. In the real complexes LnXPA, a closed-shell Ln-ligand interaction is established. Increasing the charge difference between nitrogen and phosphorus atoms (by ~0.06 e) associated with a weakening of the Lp(O(P))→σ*(P-N) electronic delocalization (Lp(O(P)) being the lone pair of the phosphoryl oxygen atom) may lead to an increase in partial multiple bond character of the P-N bonds in coordinated ligands, agreeing with the increase in ν(P-N) and (2)J(PH) coupling constant values. The changes in electron density (ρ) and electronic energy density (H(r)) values confirm these structural reorganizations upon complexation.

Structure and emission spectra of dinuclear lanthanide(III) β-diketonate complexes with a bridging 2,2′-bipyrimidine ligand

The 2,2′-bipyrimidine (bpm) adducts of the β-diketonate complexes of Eu(III), Sm(III), or Yb(III) with 2,2′,6,6′-tetramethyl-2,4-heptanedione (tmhd) resulted in the formation of dinuclear species. The synthesis and X-ray structure of these three new dinuclear lanthanide complexes are found to be similar. Each lanthanide ion is eight coordinate, bound to six O-atoms from the β-diketonates and 2N atoms from the bridging bpm ligand. They exhibit Ln–Ln distances (Sm(III): 6.935 Å, Eu(III): 6.901 Å, Yb(III): 6.679 Å) and Ln–ligand distances that are consistent with the decrease in radii across the lanthanide series. Absorption spectra of the complexes are dominated by ligand absorptions. Both the solution and solid state emission spectra of the complexes resemble ordinary monomeric lanthanide species, indicating independent ions in the dinuclear species. Cyclic voltammetry of all the complexes appear almost identical with discernable ligand centered redox reactions. The complex with Eu(III) ions, having the lowest possible lanthanide redox potential, was not found to display a signal corresponding to metal reduction.

Sol–gel materials with trapped trinuclear class-II mixed-valence macrocyclic complexes that mimic their solution redox behaviour

The redox chemistry of mixed-valence CoIII/FeII/CoIII complexes in solution and trapped in sol–gel matrices has been compared. Whereas the redox S2O82−/OH− reactivity cycling of the complexes [{LnCoIII(µ-NC)}2FeII(CN)4]2+ physically immobilized into the pore network of TEOS-derived xerogels, where Ln represents a polyaza macrocycle, mimic perfectly the reactivity shown in solution without leaching of the complex to the reaction medium, all attempts to immobilize smaller and negatively charged dinuclear [{LnCoIII(µ-NC)}FeII(CN)5]− related complexes have been found unsuccessful. A fine tuning of the pore size of the matrix, the electrostatic interactions between the matrix’s surface and the complexes, as well as possible chemical adduct formation between the OR groups of the matrix and the cyanide ligands, seem to be responsible for the effects observed. The chemical preparation of the mixed-valence complexes inside the sol–gel matrix has also been tried by using a silylated modified Ln ligand with four hydrolysable-Si(OEt)3 groups. Following the anchoring of the ligand to a TEOS-derived matrix, the initial formation of the LnCoIII building block has been achieved, but diffusion of the negatively charged [Fe(CN)6]4− species to the {LnCoIII}3+ centre is severely hindered unless high acidity is used. Nevertheless, under such acidic conditions the mixed-valence [{LnCoIII(µ-NC)}FeII(CN)5]− compounds have been successfully formed inside the matrix and have been characterized by IR and UV-Vis spectroscopy. Despite this fact, the redox S2O82−/OH− reactivity cycling carried out on the latter materials has proved to be much more slow than for xerogels containing the trinuclear [{LnCoIII(µ-NC)}2FeII(CN)4]2+ species physically trapped.

Di(μ-oxo) Binuclear Manganese(III,IV) Poly(bipyridyl) Complexes Bearing Four Ruthenium(II) Photoactive Units: Synthesis, Characterization, and Photoinduced Electron-Transfer Properties

In order to model the photoinduced electron-transfer reactions from the manganese cluster to the photoactive P680 chlorophylls in photosystem II, three heterohexanuclear complexes, [Mn2III,IVO2[RuII(bpy)2(Ln)]4]11+ [bpy = 2,2'-bipyridine, n = 2 (1a), 4 (1b), 6 (1c)], in which one MnIII,IV(micro-O)2 center is covalently linked to four RuII(bpy)3-like moieties by bridged bis(bipyridine) Ln ligands, have been synthesized and characterized. The electrochemical, photophysical, and photochemical properties of these complexes have been investigated in CH3CN. The cyclic voltammograms and rotating-disk electrode curves of the three complexes show the presence of two very close successive reversible oxidation processes corresponding to the Mn2III,IV/Mn2IV,IV and RuII/RuIII redox couples (estimated E1/2 approximately 0.82 and 0.90 V, respectively). The lower potential of the Mn2III,IV subunit compared to those of the RuII moieties indicates that the RuIII species can act as an efficient oxidant toward the Mn2III,IV core. The two oxidized forms of the complexes [Mn2IV,IVO2[RuII(bpy)2(Ln)]4]12+ (2a-c) and [Mn2IV,IVO2[RuIII(bpy)2(Ln)]4]16+ (3a-c) obtained in good yields (>90% for 2a-c and >85% for 3a-c) by sequential electrolyses are very stable. Photophysical studies show that the 3MLCT excited state of the Ru(bpy)3 centers is moderately quenched by the Mn2III,IV(micro-O)2 core (15-25% depending on the length of the bridging alkyl chain). Nevertheless, this energy transfer can be easily short-circuited in the presence of an external irreversible electron acceptor like the (4-bromophenyl)diazonium cation, by an electron transfer leading, in a stepwise fashion, to the stable one- and five-electron-oxidized species 2a-c and 3a-c, respectively, also in good yields, under continuous irradiation of the solutions. Electro- and photoinduced oxidation experiments have been followed by UV-visible and electron paramagnetic resonance spectroscopy.

Heterometallic trinuclear CuIIMIII2 (M = Fe or Cr) complexes with novel bridges and unusual magnetic properties

Three new trinuclear complexes based on [Cu(Ln)]2+ (L1 = 1,5,8,12-tetramethyl-1,3,6,8,10,13-hexaazacyclotetradecane, L2 = 1,3-propanediamine (tn)) and [M(bpb)(CN)2]− (bpb2− = 1,2-bis(pyridine-2-carboxamido)benzenate, M = Fe or Cr) have been synthesized and characterized structurally and magnetically. The Cu(II) ions adopt an axially-elongated octahedron with four N atoms from the Ln ligands occupying the equatorial plane and another two atoms (cyano nitrogen atoms or amide oxygen atoms) from two [M(bpb)(CN)2]− moieties at the axial positions, resulting in a neutral linear trinuclear complex. Magnetic studies show that the CuFe2 compound bridged by the amido group exhibits ferromagnetic behaviour. The two cyano-bridged CuFe2 or CuCr2 compounds show unusual magnetic properties: weak antiferromagnetic Cu(II)–Fe(III) coupling and negligible Cu(II)–Cr(III) magnetic exchange, proved by the results of the field dependence of magnetization.

Kinetic and spectroscopic studies on α- and β-cyclodextrin rotaxanes with (µ-N,N′- bis(4-pyridinylmethylene)-α,ω-alkanediimine)bis[pentacyanoferrate(II)] threads

[2]Pseudorotaxanes have been prepared by threading N,N′-bis(4-pyridinylmethylene)-1,2-ethanediimine (L2), -1,4-butanediimine (L4), and -1,6-hexanediimine (L6) ligands through α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD), and have subsequently been converted to the corresponding [2]rotaxane species by coordinating bulky [Fe(CN)5]3– end groups. The stability constants for the [2]pseudorotaxanes were determined by 1H NMR chemical shift titrations and increase with the polymethylene chain length n. The rate constants for both the formation of the [Fe(CN)5(Ln)]3– complexes from the [Fe(CN)5OH2]3– ion and Ln, and the rate constants for the dissociation of Ln from the metal complexes, exhibit significant diminutions in the presence of α- and β-CD, owing to inclusions of the free and coordinated ligands, respectively. The lability of the iron(II)–pyridine bonds also permits the spontaneous self-assembly of the [2]rotaxane upon the addition of cyclodextrin to the iron dimer complexes. The mechanism for this process involves the rate-determining dissociation of a [Fe(CN)5]3– unit from [(NC)5Fe(Ln)Fe(CN)5]6–, followed by CD inclusion of the Ln ligand to form a semirotaxane, and subsequent recomplexation by the [Fe(CN)5OH2]3– ion. Key words: cyclodextrins, rotaxanes, pentacyanoferrate(II), ligand substitution, kinetics.

Comparison of the spectroscopic behaviour of single crystals of lanthanide halides (X = Cl, Br)

Halides belong to large band gap matrices that have the perspective of wide applications when doped by optical active ions. This paper presents the results of spectroscopic studies of single crystals of halides (Cl, Br) of Ce, Pr and Nd. Their spectroscopic behaviour: electron–phonon coupling, ion pair interactions and the effect of covalency, is compared. Absorption, emission and emission excitation spectra of single crystals of LnCl3·yH2O (Ln = Nd, Pr, Ce; y=6, 7) were recorded at room temperatures and low temperatures down to 4.2K. The intensities of the electronic lines and the Judd–Ofelt parameters were calculated (Nd, Pr) and compared to those of LnBr3·yH2O presented earlier by us. The relationship between the hypersensitivity and covalency was discussed. With increasing soft character of the halides (Br− > Cl−), the covalent character of Ln–ligand bond increases and the hypersensitive bands become more intense. The Judd–Ofelt intensity analysis resulted in a set of τλ parameters evaluated with quite low standard deviations. The temperature dependences of the intensities have been found and the vibronic coupling in the f–f transitions were analysed. At the low temperature (4.2K), strong vibronic components occur in the electronic lines of the Nd(III) and Pr(III) ions, mainly with the Ln–X vibrations. The modes, which are in resonance with the splitting of the ground state multiplet, mediate in the cooperative transitions.Vibrational studies of the compounds under test were performed at the ambient temperature using IR and Raman spectroscopy. The assignment of the bands was done on the basis of the factor group analysis. The spectral features below 300cm−1 point at the differences between the spectra of the bromides and chlorides of Nd and Pr. Although the spectral features within the FIR region are complex, the bands of the praseodymium monocrystals originated by halogen bridges are clearly visible.