4f-4f hypersensitivity in the absorption spectra and NMR studies on paramagnetic lanthanide chloride complexes with 1,10-phenanthroline in non-aqueous solutions

The optical absorption and NMR studies of trivalent lanthanide chloride complexes with 1,10-phenanthroline (phen) are presented and discussed. The 1H NMR spectra of the complexes of La, Pr, Nd, Eu and Yb have been studied in methanol-d(4). The resonances of phen in the NMR spectra of the paramagnetic complexes have been shifted to lower as well as higher fields, which is a manifestation of dipolar interaction. The H (2) protons of the heterocyclic amine display broad resonances. The degree of broadening in Pr, Nd, and Yb complexes follows the order Pr < Nd < Yb. The inter- and intra-molecular shift ratios show that the paramagnetic shift is predominantly due to dipolar interaction. The electronic spectra of Pr, Nd, Ho and Er complexes have been investigated in methanol, pyridine, DMSO and DMF, which reveal that the hypersensitive transitions exhibit larger variation in oscillator strength values and band shapes. The change in the coordination geometry of the complexes and relative basicity of ligand are found responsible for oscillator strength and band shape variation. The interelectronic repulsion and covalency parameters show covalent nature of bonding between the metal and the ligand.     

Optical absorption and NMR spectroscopic studies on paramagnetic neodymium(III) complexes with beta-diketone and heterocyclic amines. The environment effect on 4f-4f hypersensitive transitions

The optical absorption spectra of [Nd(acac)3(H2O)2].H2O, [Nd(acac)3bpy] and [Nd(acac)3phen(H2O)2] (where acac=acetylacetone, bpy=2,2'-bipyridyl and phen=1,10-phenanthroline) complexes in the visible region, in a series of non-aqueous solvents (methanol, ethanol, isopropanol, chloroform, acetonitrile, pyridine, nitrobenzene and dimethylsulphoxide) have been analyzed. The transition 4G(5/2)<--4I(9/2) (Nd-VI) located near the middle of the visible region (17,500 cm(-1)) is hypersensitive. Its behavior is in sharp contrast to many other typically weak and consistently unvaried, normal 4f-4f transitions. The oscillator strength of this transition for the chelate as well as its adducts with phen and bpy in any of the solvent employed is larger than the oscillator strength of Nd3+ aqua-ion. It is most intense in pyridine for all the complexes studied and, therefore, pyridine is the most effective in promoting f-f spectral intensity. The band shape and oscillator strength of the hypersensitive transitions display pronounced changes as compared to Nd3+ aqua-ion. The band shapes of the hypersensitive transitions show remarkable changes on passing from aqueous solution to various non-aqueous solutions, which is the result of change in the environment about the Nd(III) ion in the various solutions and suggests change in the environment about the Nd(III) ion in the various solutions and suggests coordination of solvent molecule(s), in some cases. A comparative account of hypersensitivity in the present complexes with those of other adducts of Nd(beta-diketoenolate)3 with heterocyclic amines is discussed. The NMR signals of heterocyclic amines have been shifted to high fields while the resonances due to acetylacetone moiety have moved to low fields. The paramagnetic shift in the complexes is dipolar in nature.     

4f-4f absorption spectra of nine-coordinate Pr (III) and Nd (III) complexes in different environments

Absorption spectroscopic studies on the mixed-ligand complexes of Pr (III) and Nd (III) with 2,2'-bipyridyl and thiocyanate in pyridine, DMSO, DMF and methanol are presented. The oscillator strengths for the hypersensitive and non-hypersensitive transitions have been calculated and variation in the oscillator strength and band shapes with respect to solvent type is rationalized in terms of solvent structures and coordinating properties. A comparison is made with 1,10-phenanthroline complexes and has been shown that bpy is a weaker ligand than phen for these ions. Pyridine has been found most effective in promoting 4f-4f intensity and the increase in the oscillator strength in this solvent is due to dynamic ligand polarization mechanism.     

4f-4f absorption spectra and hypersensitivity in nine-coordinate Ho(III) and Er(III) complexes in different environments

The effect of change in the environment upon 4f-4f absorption spectra of nine-coordinate Ho(III) and Er(III) complexes with thiocyanate and 2,2'-bipyridyl in methanol, DMSO, DMF and pyridine have been investigated. The oscillator strength for hypersensitive and non-hypersensitive transitions have been calculated and variation in the intensity and band shape with respect to solvent type is rationalized in terms of solvent structure and coordinating properties. A comparison with 1,10-phenanthroline complexes is investigated and has been found that phen has a larger impact on the transition intensities for these ions. Pyridine has been found most effective in promoting 4f-4f intensity. The results indicate that it is important to consider both the static and dynamic coupling mechanism while analysing the oscillator strength of hypersensitive transition.     

顺磁性镧系金属有机配合物的~1H核磁共振研究

顺磁类的核磁共振研究大多是简单化合物,偏重理论方面的研究,对镧系配合物曾有报道。由于这类样品对空气和湿气极为敏感,在国内外研究顺磁性~1H谱甚少。本文研究了含氯桥的醚基取代环戊二烯镧系配合物二聚体的~1H化学位移,线宽,弛豫时间T_1和磁化率,从中找出了顺磁类有机镧系配合物~1H NMR的规律。     

Combining NMR of dynamic and paramagnetic molecules: fluxional high-spin nickel(II) complexes bearing bisguanidine ligands

A detailed nuclear magnetic resonance (NMR) study was carried out on a series of paramagnetic, tetrahedrally coordinated nickel(II) dihalide complexes featuring chelating guanidine ligands. A complete assignment of the NMR signals for all complexes was achieved by sophisticated NMR experiments, including correlation spectra. The effects of halide exchange, as well as the variation in the guanidine-metal bite angles on the paramagnetic shifts, were assessed. The paramagnetic shift was derived with the aid of the diamagnetic NMR spectra of the analogous Zn complexes, which were synthesized for this purpose. The experimentally derived paramagnetic shift was then compared with the values obtained from quantum chemical (DFT) calculations. Furthermore, variable-temperature NMR studies were recorded for all complexes. It is demonstrated that NMR spectroscopy can be applied to evaluate the rate constants of fast fluxional processes within paramagnetic and catalytically active metal complexes.     

Design, synthesis, and evaluation of a lanthanide chelating protein probe: CLaNP-5 yields predictable paramagnetic effects independent of environment

Immobilized lanthanide ions offer the opportunity to refine structures of proteins and the complexes they form by using restraints obtained from paramagnetic NMR experiments. We report the design, synthesis, and spectroscopic evaluation of the lanthanide chelator, Caged Lanthanide NMR Probe 5 (CLaNP-5) readily attachable to a protein surface via two cysteine residues. The probe causes tunable pseudocontact shifts, alignment, paramagnetic relaxation enhancement, and luminescence, by chelating it to the appropriate lanthanide ion. The observation of single shifts and the finding that the magnetic susceptibility tensors obtained from shifts and alignment analyses are highly similar strongly indicate that the probe is rigid with respect to the protein backbone. By placing the probe at various positions on a model protein it is demonstrated that the size and orientation of the magnetic susceptibility tensor of the probe are independent of the local protein environment. Consequently, the effects of the probe are readily predictable using a protein structure only. These findings designate CLaNP-5 as a protein probe to deliver unambiguous high quality structural restraints in studies on protein-protein and protein-ligand interactions.     

Optical absorption spectroscopic studies on holmium(III) complexes with beta-diketone and heterocyclic amines: the environment effect on 4f-4f hypersensitive transitions

The optical absorption spectra of [Ho(acac)(3)(H(2)O)(2)].H(2)O, [Ho(acac)(3)phen] and [Ho(acac)(3)bpy] (where acac is the anion of acetylacetone; phen is 1,10-phenanthroline and bpy is 2,2'-bipyridyl) complexes in the visible region, in a series of non-aqueous solvents (methanol, ethanol, isopropanol, chloroform, acetonitrile and pyridine.) have been analyzed. The largest intensity variation was observed in the (5)G(6)<--(5)I(8) (centered at 450 nm), and (5)G(5), (3)H(5), (3)H(6)<--(5)I(8) (centered at 360 nm) transition regions. The band shape and oscillator strength of the hypersensitive transitions display pronounced changes as compared to Ho(3+) aqua-ion. The band shapes of the hypersensitive transitions show remarkable changes on passing from aqueous solution to various non-aqueous solutions which is the result of change in the environment about the Ho(III) ion in the various solutions and suggests coordination of solvent molecule(s), in some cases. The results clearly show that among the solvents studied pyridine is the most effective in promoting the 4f-4f spectral intensity. It has been inferred from this study that 2,2'-bipyridyl is a stronger ligand for heavier lanthanides. A comparative account of hypersensitivity in the present complexes with those of other adducts of Ho(beta-diketoenolate)(3) with heterocyclic amines is discussed. The TGA analyses showed that the phen complex is thermally more stable over its bpy analogue.     

Paramagnetic NMR shift,spectroscopic and molecular modeling studies of lanthanide(III)-morin complexes

Studies on nine-coordinate lanthanide complexes of morin are described. The complexes were characterized by elemental analysis, molar conductance, UV–Vis spectra, IR spectra, thermal analysis and NMR spectra. Molecular modeling studies were also carried out. The complexes are non-electrolytes in DMSO. TGA showed anhydrous nature of the complexes. The electronic spectra of the complexes were recorded in methanol. ¹H NMR spectra of lanthanum, praseodymium, neodymium, samarium and dysprosium complexes have been studied in DMSO-d₆. The complexes do not dissociate in DMSO and retain their coordination. ¹H NMR spectra of paramagnetic and diamagnetic complexes exhibit downfield as well as upfield shifts of morin resonances that shows change in geometry during coordination.     

Syntheses and spectroscopic studies of volatile low symmetry lanthanide(III) complexes with monodentate 1H-indazole and fluorinated β-diketone

Synthesis, photoluminescence, 4f–4f absorption, and NMR studies of highly volatile nine-coordinate complexes [Ln(hfaa)₃(L)₃] (Ln?=?La, Pr, Nd, and Sm; hfaa is the anion of 1,1,1,5,5,5-hexafluoro-2,4-pentanedione and L?=?1H-indazole) are described. NMR spectra reveal that three L are attached to the metal. The chemical shifts of β-diketonate and Hind protons are in opposite directions and the lanthanide induced paramagnetic shifts are dipolar. The low molecular symmetry of the complexes leads to intense luminescence with prominent Stark splitting of the bands and high oscillator strength of the hypersensitive transition. The band shape of the ⁴G_5/2, ²G_7/2?←?⁴I_9/2 transition of neodymium is similar to that of nine-coordinate complexes. Coordinating solvents have pronounced effect on the oscillator strength and band shape. The experimental intensity parameter (η _Sm) for the samarium complex is calculated and analyzed. The complexes could be used as precursors for high-performance lanthanide-based materials through chemical vapor deposition.     

Nuclear magnetic resonance and optical absorption spectroscopic studies on paramagnetic praseodymium(III) complexes with beta-diketone and heterocyclic amines

The optical absorption spectra of [Pr(acac)(3)(H(2)O)(2)].H(2)O, [Pr(acac)(3)phen.H(2)O] and [Pr(acac)(3)bpy] (where acac is the anion of acetylacetone, phen is 1,10-phenanthroline and bpy is 2,2'-bipyridyl) have been analyzed in the visible region in a series of non-aqueous solvents (methanol, ethanol, isopropanol, chloroform, acetonitrile and pyridine). The complexes display four non-hypersensitive 4f-4f transitions ((3)P(2), (3)P(1)+(1)I(6), (3)P(0) and (1)D(2)) from the (3)H(4) ground state. The band shape of the transitions shows remarkable changes upon dissolving in different solvents. Distinctively different band shapes have been observed for phen and bpy complexes. The phen is more effective in producing changes and the splitting of the bands is more pronounced in phen complex since it is a stronger ligand and leads to stronger Pr-N(phen) bond. The splitting of the bands is indicative of partaking of f-orbitals in bonding. The NMR signals of heterocyclic amines have been shifted to high fields while the resonances due to acetylacetone moiety have moved to low fields which is the consequence of change in geometry of the complexes upon coordination of the heterocyclic amines and reflects the importance of geometric factor (3cos(2)theta-1) in changing sign of the shift and to a good approximation the shifts arise predominantly from the dipolar mechanism. The phen complexes have narrower line width than bpy complexes. The line broadening in the case of bpy complexes is suggestive of exchange between inter-converting forms. The bpy possesses some degree of rotational freedom about C(6)-C(6') bond and the two pyridine rings undergo scissoring motion with respect to each other.     

顺磁性镧系金属有机配合物的1H核磁共振研究

顺磁类的核磁共振研究大多是简单化合物,偏重理论方面的研究,对镧系配合物曾有报道。由于这类样品对空气和湿气极为敏感,在国内外研究顺磁性¹H谱甚少。本文研究了含氯桥的醚基取代环戊二烯镧系配合物二聚体的¹H化学位移,线宽,弛豫时间T₁和磁化率,从中找出了顺磁类有机镧系配合物¹H NMR的规律。     

Cation sensors containing a (bpy)Re(CO)3 group linked to an azacrown ether via an alkenyl or alkynyl spacer: synthesis, characterisation, and complexation with metal cations in solution

Two [(bpy)Re(CO)3L]+ complexes (bpy = 2,2'-bipyridine), where L contains an aza-15-crown-5 ether which is linked to Re via an alkenyl- or alkynyl-pyridine spacer, have been synthesised along with model complexes. Solutions of the complexes in acetonitrile have been studied by UV-Vis absorption spectroscopy, and by 1D and 2D 1H NMR spectroscopy. Strong UV-Vis bands, assigned to intraligand charge-transfer transitions localised at the L ligands, blue shift on protonation of the azacrown nitrogen atom or on complexation of alkali-metal (Li+, Na+ and K+) or alkaline-earth metal (Mg2+, Ca2+ and Ba2+) cations to the azacrown; the magnitude of the blue shift is dependent on the cation, with protonation giving the largest shift of ca. 100 nm. Cation binding constants in the range of log K= 1-4 depend strongly on the identity of the metal cation. Protonation or cation complexation causes downfield shifts in the 1H NMR resonances from most of the azacrown and L ligand protons, and their magnitudes correlate with those of the blue shifts in the UV-Vis bands; shifts in the azacrown 1H NMR resonances report on how the different metal cations interact with the macrocycle. UV-Vis and 1H NMR spectra of the free L ligands enable the effect of the Re centre to be assessed. Together, the data indicate that the alkene spacer gives a more responsive sensor than the alkyne spacer by providing stronger electronic communication across the L ligand.     

C60-多吡啶Ru(II)配合物电子光谱的密度泛函理论研究

应用密度泛函理论(DFT)方法对两种C60-多吡啶Ru(II)衍生物进行理论研究. 在TZP全电子基组优化构型基础上, 通过分析前线轨道组成, 探讨金属及配体对C60母体影响; 以LB及SAOP校正局域密度近似, 用含时密度泛函(TDDFT)方法, 考虑溶剂化效应, 计算化合物1和2的电子吸收光谱. 结果表明, 化合物1和2在气相与丙酮溶液中所对应的光谱值差异较为明显, 溶剂化效应使吸收光谱蓝移. 计算得到化合物1和2在丙酮溶液中电子光谱与实验值吻合较好, 低能跃迁多为金属参与的混合跃迁, 高能跃迁主要由C60与配体部分贡献.     

Lanthanide(III) complexes with a tetrapyridine pendant-armed macrocyclic ligand: 1H NMR structural determination in solution, X-ray diffraction, and density-functional theory calculations

Complexes between the tetrapyridyl pendant-armed macrocyclic ligand (L) and the trivalent lanthanide ions have been synthesized, and structural studies have been made both in the solid state and in aqueous solution. The crystal structures of the La, Ce, Pr, Gd, Tb, Er, and Tm complexes have been determined by single-crystal X-ray crystallography. In the solid state, all the cation complexes show a 10-coordinated geometry close to a distorted bicapped antiprism, with the pyridine pendants situated alternatively above and below the main plane of the macrocycle. The conformations of the two five-membered chelate rings present in the complexes change along the lanthanide series. The La(III) and Ce(III) complexes show a lambdadelta (or deltalambda) conformation, while the complexes of the heavier lanthanide ions present lambdalambda (or deltadelta) conformation. The cationic [Ln(L)]3+ complexes (Ln = La, Pr, Eu, Tb, and Tm) were also characterized by theoretical calculations at the density-functional theory (DFT) B3LYP level. The theoretical calculations predict a stabilization of the lambdalambda (or deltadelta) conformation on decreasing the ionic radius of the Ln(III) ion, in agreement with the experimental evidence. The solution structures show a good agreement with the calculated ones, as demonstrated by paramagnetic NMR measurements (lanthanide induced shifts and relaxation rate enhancements). The 1H NMR spectra indicate an effective D2 symmetry of the complexes in D2O solution. The 1H lanthanide induced shifts (LIS) observed for the Ce(III), Tm(III), and Yb(III) complexes can be fit to a theoretical model assuming that dipolar contributions are dominant for all protons. The resulting calculated values are consistent with highly rhombic magnetic susceptibility tensors with the magnetic axes being coincident with the symmetry axes of the molecule. In contrast with the solid-state structure, the analysis of the LIS data indicates that the Ce(III) complexes present a lambdalambda (or deltadelta) conformation in solution.     

Phototriggered S --> O isomerization of a ruthenium-bound chelating sulfoxide

We have prepared and characterized [Ru(bpy)2(OS)]+ (bpy = 2,2'-bipyridine; OS = 2-methylthiobenzoate) and the chelating sulfoxide S-[Ru(bpy)2(OSO)]+ (OSO = methylsulfonylbenzoate) by 1H NMR (1-D and 2-D COSY), IR, UV-visible spectroscopy, electrochemistry, and X-ray crystallography. Many of the metrical and crystal parameters are nearly identical between the two structures. The sulfoxide is produced from m-cpba oxidation of the thioether. Photolysis of S-[Ru(bpy)2(OSO)]+ results in a dramatic shift in the metal-to-ligand charge-transfer (3MLCT) transition from 396 to 496 nm, with a new higher-energy 3MLCT transition appearing at 355 nm. Concomitant with this change, the Ru3+/2+ reduction potential shifts from 1.25 V vs Ag/AgCl to 0.9 V vs Ag/AgCl. These changes are ascribed to phototriggered excited-state isomerization of the sulfoxide from S- to O-bonded. Examination of the 1H NMR spectra in a CD3OD solvent before, during, and after irradiation shows the presence of two O-bonded complexes that revert to the structurally characterized S-bonded ground state. This represents the first report of isomerization of a chelating sulfoxide in a photochromic Ru complex.     

Application of digital computers in NMR lanthanide shift analysis

For quantitative application of paramagnetic shifts observed in the NMR spectra of many organic molecules upon addition of rare earth chelates, the geometry of interaction between these molecules and the “shift reagent” must be known. The authors describe and evaluate a set of computer programs useful for routine analysis of data obtained by an NMR lanthanideinduced shift technique, namely for positioning of the lanthanide ion in reference to the substrate, and for automatic interpretation of the shifted spectra.     

Generation of Pseudocontact Shifts in Proteins with Lanthanides Using Small “Clickable” Nitrilotriacetic Acid and Iminodiacetic Acid Tags

Pseudocontact shifts (PCS) induced by paramagnetic lanthanide ions provide unique long‐range structural information in nuclear magnetic resonance (NMR) spectra, but the site‐specific attachment of lanthanide tags to proteins remains a challenge. Here we incorporated p‐azido‐phenylalanine (AzF) site‐specifically into the proteins ubiquitin and GB1, and ligated the AzF residue with alkyne derivatives of small nitrilotriacetic acid and iminodiacetic acid tags using the Cu^I‐catalysed “click” reaction. These tags form lanthanide complexes with no or only a small net charge and produced sizeable PCSs with paramagnetic lanthanide ions in all mutants tested. The PCSs were readily fitted by single magnetic susceptibility anisotropy tensors. Protein precipitation during the click reaction was greatly alleviated by the presence of 150 mM NaCl.     

Oxygen as a paramagnetic probe of clustering and solvent exposure in folded and unfolded states of an SH3 domain

The N-terminal SH3 domain of the Drosophila modular protein Drk undergoes slow exchange between a folded (Fexch) and highly populated unfolded (Uexch) state under nondenaturing buffer conditions, enabling both Fexch and Uexch states to be simultaneously monitored. The addition of dissolved oxygen, equilibrated to a partial pressure of either 30 atm or 60 atm, provides the means to study solvent exposure with atomic resolution via 13C NMR paramagnetic shifts in 1H,13C HSQC (heteronuclear single quantum coherence) spectra. Absolute differences in these paramagnetic shifts between the Fexch and Uexch states allow the discrimination of regions of the protein which undergo change in solvent exposure upon unfolding. Contact with dissolved oxygen for both the Fexch and Uexch states could also be assessed through 13C paramagnetic shifts which were normalized based on the corresponding paramagnetic shifts seen in the free amino acids. In the Fexch state, the 13C nuclei belonging to the hydrophobic core of the protein exhibited very weak normalized paramagnetic shifts while those with greater solvent accessible surface area exhibited significantly larger normalized shifts. The Uexch state displayed less varied 13C paramagnetic shifts although distinct regions of protection from solvent exposure could be identified by a lack of such shifts. These regions, which included Phe9, Thr12, Ala13, Lys21, Thr22, Ile24, Ile27, and Arg38, overlapped with those found to have residual nativelike and non-native structures in previous studies and in some cases provided novel information. Thus, the paramagnetic shifts from dissolved oxygen are highly useful in the study of a transient structure or clustering in disordered systems, where conventional NMR measurements (couplings, chemical shift deviations from random coil values, and NOEs) may give little information.     

13C and 15N NMR studies of iron-bound cyanides of heme proteins and related model complexes: sensitive probe for detecting hydrogen-bonding interactions at the proximal and distal sides

Studies of the 13C and 15N NMR paramagnetic shifts of the iron-bound cyanides in the ferric cyanide forms of various heme proteins containing the proximal histidine and related model complexes are reported. The paramagnetic shifts of the 13C and 15N NMR signals of the iron-bound cyanide are not significantly affected by the substitution of the porphyrin side chains. On the other hand, the paramagnetic shifts of both the 13C and 15N NMR signals decrease with an increase in the donor effect of the proximal ligand, and the 13C NMR signal is more sensitive to a modification of the donor effect of the proximal ligand than the 15N NMR signal. With the tilt of the iron-imidazole bond, the paramagnetic shift of the 13C NMR signal increases, whereas that of the 15N NMR signal decreases. The hydrogen-bonding interaction of the iron-bound cyanide with a solvent decreases the paramagnetic shift of both 13C and 15N NMR signals, and the effect is more pronounced for the 15N NMR signal. Data on the 13C and 15N NMR signals of iron-bound cyanide for various heme proteins are also reported and analyzed in detail. Substantial differences in the 13C and 15N NMR shifts for the heme proteins can be explained on the basis of the results for the model complexes and structures around the heme in the heme proteins. The findings herein show that the paramagnetic shift of the 13C NMR signal of the iron-bound cyanide is a good probe to estimate the donor effect of the proximal imidazole and that the ratio of 15N/13C NMR shifts allows the hydrogen-bonding interaction on the distal side to be estimated.