Spectroscopic and Crystal Field Consequences of Fluoride Binding by [Yb⋅DTMA]3+ in Aqueous Solution

Yb?DTMA forms a ternary complex with fluoride in aqueous solution by displacement of a bound solvent molecule from the lanthanide ion. [Yb?DTMA?F]²⁺ and [Yb?DTMA?OH₂]³⁺ are in slow exchange on the relevant NMR timescale (<2000 s^?1), and profound differences are observed in their respective NMR and EPR spectra of these species. The observed differences can be explained by drastic modification of the ligand field states due to the fluoride binding. This changes the magnetic anisotropy of the Yb^III ground state from easy‐axis to easy‐plane type, and this change is easily detected in the observed magnetic anisotropy despite thermal population of more than just the ground state. The spectroscopic consequences of such drastic changes to the ligand field represent important new opportunities in developing fluoride‐responsive complexes and contrast agents.     

A direct nitrogen-15 NMR study of neodymium(III)-nitrate complex formation in aqueous solvent mixtures

A study of contact ion-pair formation between the neodymium (III) and nitrate ions in aqueous solvent mixtures has been carried out by a direct, low temperature, nitrogen-15 (¹⁵N) nuclear magnetic resonance (NMR) technique. At low temperatures, –90 to –120°C ligand exchange is slow enough to permit the observation of¹⁵N NMR signals for uncomplexed nitrate ion, and this anion in the primary solvation shell of Nd(III). In aqueous mixtures with inert acetone and Freon-12, resonance signals for Nd(NO₃)²⁺, Nd(NO₃) ₂ ¹⁺ , and two higher complexes are observed. Signal areas indicate these additional species are possibly a combination of the tetra-, penta-, and hexanitrato complexes, but not the trinitrato. In water-methanol, a medium of higher dielectric constant, complexation is much less and signals only for the mono-and dinitrato complexes are observed. The effect of solvent on complexation is demonstrated more clearly by a series of measurements in water-methanol-acetone mixtures.     

Synthesis,structures and luminescence properties of two gallium(III) complexes with 5,7-dimethyl-8-hydroxyquinoline

Luminescent gallium(III) complexes featuring 5,7-dimethyl-8-hydroxyquinoline (DimOx) are systematically compared and their structural features are correlated with their photophysical properties. The two complexes are chemically identical; however, contain various number of solvent molecules in the crystalline lattice which is representative of the bulk material confirmed by both nuclear magnetic resonance and elemental analysis. Detailed structural comparisons highlight the effect which the solvent molecules have on the intra- and intermolecular interactions. A distinct number of interactions are found for the gallium complex (1) containing more than one solvent molecule for unit cell. Variation in complex morphology is similarly observed via SEM micrographs. The distinct luminescent properties of the two gallium complexes appear directly related to octahedral coordination of the 8-hydroxyquinoline ligand as well as the number of identical coordinated solvent molecules.     

Mass spectrometric studies on small open-chain piperazine-containing ligands and their transition metal complexes

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry was used to characterize the complexes formed between open-chain piperazine-containing ligands and transition metal salts (Cobalt, Copper, Zinc, and Cadmium as chlorides, nitrates, and acetates). Only single-charged complexes were observed, formed of one ligand (L) and mainly one metal ion (M). Since the net charge of the complexes was one, a counterion (X) was attached to some of the complexes, with formation of [L + M + X]+ complexes, and a proton was lost from others, as in [L - H + M]+ complexes. In most cases the composition of the complexes was more dependent on the ligand than the metal salt. Collision-induced dissociation measurements showed that complexes with related composition often differed in structure, or that interactions between the ligand and the metal ion were not alike. The metal ion influenced considerably the fragmentation pathways of the ligands, so that the fragmentation products could be used to deduce the binding sites of the metal. The variations observed in fragmentation behavior of complexes possessing the same ligand but different metal ions can mostly be explained by the ionic radius and electronic configuration of the metal ion. The results indicated a preference of the piperazine ring of the coordinated ligand for the boat conformation.     

Synthesis,characterization and oxidase catalytic activity of vanadium(IV) and oxovanadium(IV) complexes with Schiff base ligands derived from β-diketones

New complexes of vanadium(IV) and oxovanadium(IV) with Schiff base ligands derived from -diketones and ethanolamine or o-aminophenol have been prepared and characterized by elemental analyses, electrical conductance, magnetic moment measurements, and by i.r., u.v.–vis. and e.p.r. spectroscopy. A distorted octahedral environment was proposed for the vanadium(IV) and oxovanadium(IV) complexes. The spectroscopic results were utilized to compute the important ligand field parameters. Three adduct complexes were isolated owing to the interaction of one oxovanadium complex with Lewis-bases in MeOH. Vanadium(IV) complexes exhibit promising catalytic activity towards the aerobic oxidation of p-phenylenediamine (PPD) to the corresponding semi-oxidized form (PPD⁺). A linear correlation exists between the oxidase catalytic activity and the Lewis acidity of the central vanadium(IV) ion created by the donating properties of the parent ligand.     

Synthesis,characterization and superoxide dismutase mimetic activity of ruthenium(III) oxime complexes

Vicinal carbonyl-oxime and oxime-imine ligands were used in the synthesis of new RuIII oxime complexes and the isolated chelates were characterized by elemental analysis, electrical conductance and magnetic moment measurements. I.r., u.v.–vis. and e.s.r. spectroscopic analysis methods were also employed. The spectral data were utilized to compute the important ligand field parameters B, β and Dq. The carbonyl-oxime ligand coordinates through the nitrogen of =N-OH to form a five-membered chelate ring. Replacement of the C=O group by C=N-NH2 induces the =N-OH group to coordinate through the oxygen, forming thereby a six-membered chelate ring. The quadridentate tetraaza ligand (L7) coordinates with RuIII through its nitrogen donors in the equatorial position with loss of one of the oxime protons and concomitant formation of an intramolecular hydrogen bond. The spectral and magnetic results suggest a slightly distorted octahedral environment around the RuIII ion. The superoxide dismutase (SOD) mimetic activity of the prepared complexes was assessed for their ability to inhibit the reduction of nitroblue tetrazolium (NBT). The results demonstrate that most of the complexes have promising SOD-mimetic activity. A probable mechanism for the catalytic scavenging of O2− by RuIII oximes is proposed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Pentacoordinate NiII Complexes: Preparation,Magnetic Measurements,and Ab Initio Calculations of the Magnetic Anisotropy Terms

Two novel mononuclear five‐coordinate nickel complexes with distorted square‐pyramidal geometries are presented. They result from association of a tridentate “half‐unit” ligand and 6,6′‐dimethyl‐2,2′‐bipyridine according to a stepwise process that highlights the advantage of coordination chemistry in isolating an unstable tridentate ligand by nickel chelation. Their zero‐field splittings (ZFS) were studied by means of magnetic data and state‐of‐the‐art ab initio calculations. Good agreement between the experimental and theoretical axial D parameters confirms that large single‐ion nickel anisotropies are accessible. The synthetic process can also yield dinuclear nickel complexes in which the nickel ions are hexacoordinate. This possibility is facilitated by the presence of phenoxo oxygen atoms in the tridentate ligand that can introduce a bridge between the two nickel ions. Two different double bridges are characterized, with the bridging oxygen atoms coming from each nickel ion or from the same nickel ion. This coordination change introduces a difference in the antiferromagnetic interaction parameter J. Although the magnetic data confirm the presence of single‐ion anisotropies in these complexes, these terms cannot be determined in a straightforward way from experiment due to the mismatch between the principal axes of the local anisotropies and the presence of intersite anisotropies.     

Syntheses, structures, and properties of trinuclear complexes [M(bpca)(2)(M'(hfac)(2))(2)], constructed with the complexed bridging ligand [M(bpca)(2)] [M, M' = Ni(II), Mn(II); Cu(II), Mn(II); Fe(II), Mn(II); Ni(II), Fe(II); and Fe(II), Fe(II); Hbpca = Bis(2-pyridylcarbonyl)amine, Hhfac = Hexafluoroacetylacetone

Five trinuclear complexes [M(bpca)(2)(M'(hfac)(2))(2)] (where MM'(2) = NiMn(2), CuMn(2), FeMn(2), NiFe(2), and FeFe(2); Hbpca = bis(2-pyridylcarbonyl)amine; and Hhfac = hexafluoroacetylacetone) were synthesized almost quantitatively by the reaction of [M(bpca)(2)] and [M'(hfac)(2)] in 1:2 molar ratio, and their structures and magnetic properties were investigated. Three complexes, with M' = Mn, crystallize in the same space group, Pna2(1), whereas two complexes, with M' = Fe, crystallize in P4(1), and complexes within each set are isostructural to one another. In all complexes, [M(bpca)(2)] acts as a bis-bidentate bridging ligand to form a linear trinuclear complex in which three metal ions are arranged in the manner M'-M-M'. The central metal ion is in a strong ligand field created by the N(6) donor set, and hence the Fe(II) in the [Fe(bpca)(2)] moiety is in a low-spin state. The terminal metal ions (M') are surrounded by O(6) donor sets with a moderate ligand field, which leads to the high-spin configuration of Fe(II). Three metal ions in all complexes are almost collinear, and metal-metal distances are ca. 5.5 A. The magnetic behavior of NiMn(2) and NiFe(2) shows a weak ferromagnetic interaction between the central Ni(II) ion and the terminal Mn(II) or Fe(II) ions. In these complexes, sigma-spin orbitals of the central Ni(II) ion and those of terminal metal ions have different symmetry about a 2-fold rotation axis through the Ni-N(amide)-M'(terminal) atoms, and this results in orthogonality between the neighboring sigma-spin orbitals and thus ferromagnetic interactions.     

Stereochemistry of new nitrogen containing heterocyclic aldehyde. IX. Spectroscopic studies on novel mixed-ligand complexes of Rh(III)

Mono, bis and tris complexes of rhodium(III) oxine (systematic name 8-hydroxy-7-quinolinecarboxaldehyde) and mixed ligand have been prepared. The amine exchange reaction of coordinated Schiff base in these complexes has also been carried out, which gives symmetrical tetradentate Schiff base complexes. The complexes are characterized by elemental and thermal analysis, IR, magnetic and electronic spectral analysis methods were also employed as well as conductivity measurements. An octahedral structure is proposed for all the new complexes in which chloride is attached to the metal ion in 1:1; 1:2 (metal:ligand) ratio. The spectral data were utilized to compute the important ligand field parameter B, beta and Dq. The B-values suggest a strong covalency in the metal-ligand sigma-bond and the Dq-values indicate a medium strong ligand field. 1H NMR spectra show that the tris (ligand) complex is cis isomer. IR spectra show that the ligand is mono-basic bidentate.     

The magnetocaloric effect and the heat capacity of aqueous suspensions of porphyrin complexes of rare earth elements according to microcalorimetric data

The magnetocaloric effect (MCE) and heat capacity during the magnetization process of (5,10,15,20-tetraphenylporphyrinato)acetatogadolinium(III), (AcO)GdTPP, and (5,10,15,20-tetraphenylporphyrinato)chlorogadolinium(III) complexes, (Cl)GdTPP, in the form of 6%-aqeous suspensions are determined by the microcalorimetric method in a range of temperatures from 278 to 318 K and magnetic fields from 0 to 1 T. It is found that MCE for all the complexes are positive, i.e., at applying a magnetic field in the adiabatic conditions temperature of a suspension of complex increases. It is established that MCE increases with an increase in magnetic induction at all temperatures and decreases with an increase in temperature at all magnetic fields. It is shown that the substitution of chloride ligand by acetate in (X)GdTPP leads to a significant increas in MCE and its temperature dependence; in the case of (Cl)GdTPP actually MCE does not depend on temperature. Relationships between magnetothermal properties and structure of the complexes are analyzed. The conclusion is argumented that the reason of changes in magnetothermal properties after the replacement of axial ligand in gadolinium complexes and complexes of lanthanides with an unsymmetrically filled f-shell is non-planar geometry of the coordination site and specific electronic properties of the central ion. It is concluded that heat capacity of the complexes slightly increases with an increase in temperature and more noticeably in the case of (AcO)GdTPP; a magnetic component of heat capacity is revealed only in (AcO)GdTPP at temperatures above 298 K, which is connected perhaps with a temperature change in the crystal lattice of the complex and influence of the magnetic properties of gadolinium ion on this change.     

Preparation,thermoanalytical and IR study of mixed ligand complexes formed in water-1,2-ethanediol- nickel(II)-sulfate systems

Summary Mixed ligand nickel(II) complexes of different compositions were prepared with water, sulfate ion and 1,2-ethanediol as ligands. The magnetic susceptibility data, the IR spectra and the thermoanalytical curves of the complexes were recorded. Oxygen atoms bound by one or two coordinate bonds to the metal ion, or by hydrogen-bonds in the crystal state were observed. All complexes are sensitive for moisture. The bis complex proved to be the more stable complex.     

Tunable intramolecular multicenter H-bonding interactions in first-row metal complexes bearing bidentate redox-active ligands

Abstract

In this research article, we report the synthesis and structural characterization of a family of first-row metal complexes bearing redox-active ligands with tunable H-bonding donors. We observed that these coordination complexes can adopt three different geometries and that they are stabilized by intramolecular multicenter H-bonding interactions, which are systematically modified by changing the metal ion (Co, Ni, Cu, Zn), the ligand scaffold (variations in the diamine and ureanyl substituents used) and the solvent of crystallization.     

Lithium-7 nuclear magnetic resonance and calorimetric study of lithium crown complexes in various solvents

Lithium-7 NMR studies have been carried out on lithium ion complexes with crown ethers 12C4, 15C5, and 18C6 in water and in several nonaqueous solvents. In all cases the exchange between the free and complexed lithium ion was fast on the NMR time scale, and a single, population average, resonance was observed. Both 1:1 and 2:1 (sandwich) complexes were observed between lithium ion and 12C4 in nitromethane solution. The stability of the complexes varied very significantly with the solvent. With the exception of pyridine, the stability varies inversely with the Gutmann donor number of the solvent. In general, the stability order of the complexes was found to be 15C5·Li⁺>12C4·Li⁺>18C6·Li⁺. Calorimetric studies on these complexes show that, in most cases, the complexes are both enthalpy and entropy stabilized.     

Synthesis and magnetic properties of poly(p‐phthaloyl‐disacetylaceton‐ethylenediimine) metal complexes

Macrocyclic Schiff‐base ligand, bisacetylaceton‐ethylenediimine (BAE) and its transition metal complexes M(BAE) (M = Cu²⁺, Ni²⁺) were synthesized. The complexes having characteristics of aromatic systems and well‐defined one‐dimensional structures, reacted with p‐phthaloyl chloride, to obtain polymer complexes. The complexes were characterized by elemental analysis, inductively coupled plasma (ICP), FT‐IR, and thermal analysis and show good thermal stability. ESR spectra analysis discovered that there are free radicals in the chain of polymers, indicating that a weak magnetic spin‐exchange interaction operates between the metal ions and free radicals. It is found that, as the bridging p‐phthaloyl group is able to propagate the magnetic exchange interaction, the polymer complexes show paramagnetic properties by measurement of temperature dependence of the magnetic property, and obey Curie–Weiss law. Copyright © 2001 John Wiley & Sons, Ltd.     

Slow magnetic relaxation in mononuclear tetrahedral cobalt(II) complexes with 2-(1H-imidazol-2-yl)phenol based ligands

Three mononuclear cobalt(II) complexes based on the 2-(4,5-diphenyl-1H-imidazol-2-yl)phenol ligand and its nitro and methoxy derivatives have been synthesized and investigated. For two of these complexes, which were derived from the parent ligand and its nitro derivative, the molecular structures have been determined by X-ray crystallography. A distorted tetrahedral coordination environment is observed for the cobalt(II) centers with two bidentate N,O ligands. The magnetic properties have been studied by susceptibility and magnetization measurements revealing the presence of high-spin cobalt(II) ions with a considerable zero-field splitting in the order of D = –35 to –41 cm⁻¹ for the three complexes. With an applied dc field the ac susceptibility data reveal a slow magnetic relaxation which is characteristic for single-molecular magnet (SMM) behavior.     

Ligand field-tuned single-molecule magnet behaviour of 2p-4f complexes

Three new 2p-4f complexes of [Ln(acac)(3)(NIT-2Py)]·0.5NIT-2Py [Ln(III) = Gd(1), Dy(2)] and [Dy(tfa)(3)(NIT-2Py)]·0.5C(7)H(16) (3) (NIT-2Py = 2-(2'-pyridyl)- 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; acac = acetylacetonate and tfa = trifluoroacetylacetonate) have been synthesized, and structurally and magnetically characterized. The X-ray structural analysis exhibits that the three complexes show similar mononuclear structures, in which NIT-2Py radical chelates the Ln(III) ion through the oxygen atom of the NO group and the nitrogen atom from the pyridine ring. The static magnetic measurements on the three complexes exhibit ferromagnetic coupling between the lanthanide ion and the radical. Compared to the silence of the out-of-phase ac susceptibility of complex 3, the magnetic relaxation behavior of complex 2 is observed, suggesting single-molecule magnet behavior. The different magnetic relaxation behaviours of 2 and 3 are due to their slightly different crystal structure around the Dy(III) ions. It was demonstrated that the spin dynamic can be modified by the careful adjustment of the ligand field around the metal center.     

Detailed ab initio first-principles study of the magnetic anisotropy in a family of trigonal pyramidal iron(II) pyrrolide complexes

A theoretical, computational, and conceptual framework for the interpretation and prediction of the magnetic anisotropy of transition metal complexes with orbitally degenerate or orbitally nearly degenerate ground states is explored. The treatment is based on complete active space self-consistent field (CASSCF) wave functions in conjunction with N-electron valence perturbation theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for treatment of magnetic field- and spin-dependent relativistic effects. The methodology is applied to a series of Fe(II) complexes in ligand fields of almost trigonal pyramidal symmetry as provided by several variants of the tris-pyrrolylmethyl amine ligand (tpa). These systems have recently attracted much attention as mononuclear single-molecule magnet (SMM) complexes. This study aims to establish how the ligand field can be fine tuned in order to maximize the magnetic anisotropy barrier. In trigonal ligand fields high-spin Fe(II) complexes adopt an orbitally degenerate (5)E ground state with strong in-state spin-orbit coupling (SOC). We study the competing effects of SOC and the (5)E?ε multimode Jahn-Teller effect as a function of the peripheral substituents on the tpa ligand. These subtle distortions were found to have a significant effect on the magnetic anisotropy. Using a rigorous treatment of all spin multiplets arising from the triplet and quintet states in the d(6) configuration the parameters of the effective spin-Hamiltonian (SH) approach were predicted from first principles. Being based on a nonperturbative approach we investigate under which conditions the SH approach is valid and what terms need to be retained. It is demonstrated that already tiny geometric distortions observed in the crystal structures of four structurally and magnetically well-documented systems, reported recently, i.e., [Fe(tpa(R))](-) (R = tert-butyl, Tbu (1), mesityl, Mes (2), phenyl, Ph (3), and 2,6-difluorophenyl, Dfp (4), are enough to lead to five lowest and thermally accessible spin sublevels described sufficiently well by S = 2 SH provided that it is extended with one fourth order anisotropy term. Using this most elementary parametrization that is consistent with the actual physics, the reported magnetization data for the target systems were reinterpreted and found to be in good agreement with the ab initio results. The multiplet energies from the ab initio calculations have been fitted with remarkable consistency using a ligand field (angular overlap) model (ab initio ligand field, AILFT). This allows for determination of bonding parameters and quantitatively demonstrates the correlation between increasingly negative D values and changes in the σ-bond strength induced by the peripheral ligands. In fact, the sigma-bonding capacity (and hence the Lewis basicity) of the ligand decreases along the series 1 > 2 > 3 > 4.     

Fourier Transform Ion Cyclotron Resonance Mass Resolution and Dynamic Range Limits Calculated by Computer Modeling of Ion Cloud Motion

Particle-in-Cell (PIC) ion trajectory calculations provide the most realistic simulation of Fourier transform ion cyclotron resonance (FT-ICR) experiments by efficient and accurate calculation of the forces acting on each ion in an ensemble (cloud), including Coulomb interactions (space charge), the electric field of the ICR trap electrodes, image charges on the trap electrodes, the magnetic field, and collisions with neutral gas molecules. It has been shown recently that ion cloud collective behavior is required to generate an FT-ICR signal and that two main phenomena influence mass resolution and dynamic range. The first is formation of an ellipsoidal ion cloud (termed “condensation”) at a critical ion number (density), which facilitates signal generation in an FT-ICR cell of arbitrary geometry because the condensed cloud behaves as a quasi-ion. The second phenomenon is peak coalescence. Ion resonances that are closely spaced in m/z coalesce into one resonance if the ion number (density) exceeds a threshold that depends on magnetic field strength, ion cyclotron radius, ion masses and mass difference, and ion initial spatial distribution. These two phenomena decrease dynamic range by rapid cloud dephasing at small ion density and by cloud coalescence at high ion density. Here, we use PIC simulations to quantitate the dependence of coalescence on each critical parameter. Transitions between independent and coalesced motion were observed in a series of the experiments that systematically varied ion number, magnetic field strength, ion radius, ion m/z, ion m/z difference, and ion initial spatial distribution (the present simulations begin from elliptically-shaped ion clouds with constant ion density distribution). Our simulations show that mass resolution is constant at a given magnetic field strength with increasing ion number until a critical value (N) is reached. N dependence on magnetic field strength, cyclotron radius, ion mass, and difference between ion masses was determined for two ion ensembles of different m/z, equal abundance, and equal cyclotron radius. We find that N and dynamic range depend quadratically on magnetic field strength in the range 1–21 Tesla. Dependences on cyclotron radius and Δm/z are linear. N depends on m/z as (m/z)^–2. Empirical expressions for mass resolution as a function of each of the experimental parameters are presented. Here, we provide the first exposition of the origin and extent of trade-off between FT-ICR MS dynamic range and mass resolution (defined not as line width, but as the separation between the most closely resolved masses).     

A Polarographic Study of Tl+, Pb2+ and Cd2+ Complexes with Aza-18-crown-6 and Dibenzopyridino-18-crown-6 in some Binary Mixed Non-aqueous Solvents

The complexation of Tl⁺, Pb²⁺and Cd²⁺ cations by macrocyclic ligands, aza-18-crown-6 (L1) and dibenzopyridino-18-crown-6 (L2) was studied in some binary mixtures of methanol (MeOH), n-propanol (n-PrOH), nitromethane (NM) and acetonitrile (AN) with dimethylformamide (DMF) at 22 °C using DC (direct current) and differential pulse polarographic techniques (DPP). The stoichiometry and stability constants of the complexes were determined by monitoring the shifts in half-waves or peak potentials of the polarographic waves of metal ions against the ligand concentration. In all of the solvent systems, the stability of the resulting 1:1 complexes was found to be L1 > L2. The selectivity order of the L2 ligand for the cations was found to be Pb²⁺ > Tl⁺ > Cd²⁺ and the selectivity of the L1 ligand for Pb²⁺ ion was greater than that of Tl⁺ ion. The results show that the stability of the complexes depends on the nature and composition of the mixed solvents. There is an inverse relationship between the stability constants of the complexes and the amount of dimethylformamide in the mixed solvent systems.     

Thermodynamic Data for the Formation of 1:1 and 2:1 Complexes of α,ω-Diamino Dihydrochlorides with 18-Crown-6 in Aqueous Solution

Calorimetric titrations are used to study the interactions between the crown ether 18-crown-6 and several α,ω-diamino dihydrochlorides in aqueous solution. These complexes are formed by ion-dipole interactions between the positively charged nitrogen atoms and the oxygen donor atoms of the crown ether. Depending on the experimental conditions, the formation of 1:1 or 2:1 complexes (ligand:diamines) can be studied. The solvation of the ligand and the amines are responsible for the observed thermodynamic values. The number of water molecules released during the reaction were calculated from the determined reaction entropies. Formation of 1:1 complexes distorts the solvation shell around the molecules. As a result, the number of solvent molecules released during the formation of the 2:1 complexes is slightly smaller than the number released from formation of the 1:1 complex. No experimental evidence is observed for the formation of complexes between one crown ether and two protonated amino groups.