The important effect of ligand architecture on the selectivity of metal ion recognition in An(III)/Ln(III) separation with N-donor extractants

The existence of a strong correlation between ligand architecture and metal ion binding selectivity is demonstrated through large differences in the separation efficiencies found in the selective extraction of Am3+ from an acidic mixture of Am3+ and Eu3+ for three new tetrapodal hexadentate ligands containing four 2-pyrazinylmethyl groups attached to three different diamino spacers.     

A non-heme dinuclear iron(II) complex containing a single, unsupported hydroxo bridge

Complexation of the tetrapodal pentadentate NN4 ligand 2,6-C5H3N[CMe(CH2NH2)2]2 (I) with iron(II) perchlorate hydrate in methanol, in the presence of N-methylimidazole, produces a diferrous complex with a single, unsupported mu-OH ligand between two {(I)FeII} coordination modules.     

Light-harvesting macroring accommodating a tetrapodal ligand based on complementary and cooperative coordinations

A porphyrin macroring, mimicking the light-harvesting complex of photosynthetic purple bacteria, was synthesized by self-assembling trisporphyrinatoZn(II) complexes with imidazolyl substituents at both molecular terminals. Very strong complementary coordination of imidazolyl to Zn(II) afforded exclusively the cyclic trimer of trisporphyrin under dilute conditions at 27 degrees C in CHCl3/MeOH = 9/1 (v/v). 1H NMR spectra of the macroring indicate the existence of two topological isomers, one symmetric and one asymmetric. Use of three noncoordinated porphyrinatoZn(II) sites allows a tetrapodal ligand to be incorporated into the cavity of the macroring. A Job plot and the clear bending behavior on UV-vis titration indicated the formation a 1:1 complex; the heterogeneity of two topological isomers was not observed, and both isomers similarly accommodated the tetrapodal ligand. The association constant obtained by curve fitting analysis was 8 x 108 M-1 in toluene. This large association constant reflects the cooperative nature of three coordination sites.     

Lanthanide 8-hydroxyquinoline-based podates with efficient emission in the NIR range

The novel hydroxyquinoline-containing tetrapodal ligand forms water soluble and stable chelates and is a good sensitizer of the NIR luminescence of its Nd(III) and Yb(III) complexes; its easy synthesis opens the way for potential biomedical applications.     

Noncovalent interactions: defining cooperativity. Ligand binding aided by reduced dynamic behavior of receptors. Binding of bacterial cell wall analogues to ristocetin A

Changes in the relative populations of the monomer and asymmetric dimer forms of ristocetin A, upon binding of two molecules of ligand, suggest that ligand binding is negatively cooperative with respect to dimerization. However, strong hydrogen bonds formed in the binding sites of the ligands are reinforced in the dimer relative to the monomer, and the barrier to dissociation of the dimer is increased upon binding of the ligands. It is concluded that the interactions which are common in the binding of both ligands are made with positive cooperativity with respect to those involved in dimerization. The conclusions are relevant to the binding of ligands to proteins, where ligand binding energy can be derived from stabilization of the protein in its ligand-bound form.     

A Non-Covalent Dimer Formation of Quaternary Ammonium Cation with Unusual Charge Neutralization in Electrospray-Ionization Mass Spectrometry

Specific and nonspecific non-covalent molecular association of biomolecules is characteristic for electrospray-ionization mass spectrometry analysis of biomolecules. Understanding the interaction between two associated molecules is of significance not only from the biological point of view but also gas phase analysis by mass spectrometry. Here we reported a formation of non-covalent dimer of quaternary ammonium denatonium cation with +1 charge detected in the positive ion mode electrospray ionization mass spectrometry analysis of denatonium benzoate. Hydrogen deuterium exchange of amide and carbon-bonded hydrogens revealed that charge neutralization of one denatonium cation is the consequence of amide hydrogen dissociation. DFT (Density Functional Theory) calculations proved high thermodynamic stable of formed dimer stabilized by the short and strong N..H-N hydrogen bond. The signal intensity of the peak characterizing non-covalent dimer is low intensity and does not depend on the sample concentration. Additionally, dimer observation was found to be instrument-dependent. The current investigation is the first experimental and theoretical study on the quaternary ammonium ions dimer. Thus the present study has great significance for understanding the structures of the biomolecules as well as materials.     

A new type of C2-symmetric bisphospine ligands with a cyclobutane backbone: practical synthesis and application

[reaction: see text] A highly efficient and practical optical resolution of anti-head-to-head racemic coumarin dimer, (+/-)-5, by molecular complexation with TADDOL (6) through hydrogen bonding and a convenient transformation of enantiopure 5 to a new type of C(2)-symmetric bisphosphine ligand (3) have been achieved. The asymmetric induction efficiency of these chiral bisphosphine ligands was evaluated in Pd-catalyzed asymmetric allylic substitution, affording the allylic substitution products in excellent yield (up to 99%) and enantioselectivity (up to 98.9% ee).     

Comparative structural studies of iodide complexes of uranium(III) and lanthanide(III) with hexadentate tetrapodal neutral N-donor ligands

The syntheses, the solution structures, and the crystal structures of the two new tetrapodal N-donor ligands N,N,N',N'-tetrakis(2-pyrazylmethyl)-1,3-trimethylenediamine (tpztn), 1, and N,N,N',N'-tetrakis(2-pyrazylmethyl)-trans-1,2-cyclohexanediamine (tpzcn), 2, are described. Two different geometric isomers of the cation [La(tpztn)I(2)](+) were isolated in which the ligand adopts two different conformations leading to strong differences in the metal-ligand bond distances. The crystal structure of isostructural complexes of La, U, Ce, and Nd were determined by X-ray diffraction studies for the ligands tpztn and tpzcn. In both series of complexes the two methylpyrazyl arms and the diamine spacer (trimethylene or cyclohexane) around each aliphatic nitrogen adopt the same helical configuration. The complexes crystallize as a racemic mixture of Lambda,Lambda and Delta,Delta enantiomers with distorted square antiprism geometries. In these complexes the M-N(pyrazine) distances show a decrease from La to Ce and from La to Nd which corresponds well to the decrease in ionic radius as expected in a purely ionic bonding model. Conversely the mean value of the U-N(pyrazine) distances is shorter (0.043(3) A for tpztn and 0.054(11) A for tpzcn) than the mean value of the La-N(pyrazine) distances. These differences are significantly larger than the decrease expected from the variation of the ionic radii and can be interpreted in terms of a stronger M-N interaction for U(III). Previously reported extraction studies have shown that while the tripod tris[(2-pyrazyl)methyl]amine (tpza) containing three pyrazyl nitrogens extracts An(III) preferentially to Ln(III), tpztn and tpzcn display no selectivity despite the presence of four pyrazyl groups connected to a different spacer. The structural studies described here show that despite the lack of selectivity observed in the extraction conditions, the arrangement of pyrazyl nitrogens in the tetrapodal architectures of tpztn and tpzcn allows for metal-ligand interaction similar to that observed for tpza.     

Stable gas-phase radical cations of dimeric tryptophan and tyrosine derivatives

Stable radical cations of dimeric amino acid derivatives of tryptophan and tyrosine were generated by collision-induced dissociation of [Cu(II)(diethylenetriamine)(amino acid derivative)2]*2+. The yields of the dimer radical cations were dependent on both the auxiliary ligand and the tryptophan or tyrosine derivatives used. Amino acid derivatives with an unmodified carboxylic acid group did not generate dimer radical cations. For the amino acid derivatives Ac-Trp-OMe and Ac-Trp-NH2 (Ac is N-acetyl; OMe and NH2 are the methyl ester and amide modifications of the C-terminal carboxylic group), no auxiliary ligand was required for generating the dimer radical cations. Collision-induced dissociation of the [Cu(II)(amino acid derivative)4]*2+ precursor generated the dimer radical cation [(amino acid derivative)2]*+. Stabilizing interactions, most likely involving hydrogen bonding, between the two amino acid derivatives are proposed to account for observation of the dimer radical cations. Dissociation of these ions yields protonated or radical cationic amino acid derivatives; these observations are consistent with the expectation of proton competition between monomeric units, whose proton affinities were calculated using density functional theory.     

Synthesis,Photophysical, and Electrochemical Properties of RuII Polypyridyl Complexes Bridged with two Tetrapodal Symmetric and one Asymmetric Ligands

Two symmetric tetrapodal ligands L^1–2 and one asymmetric tetrapodal ligand L³ based on 4,5‐diazafluoren have been synthesized and characterized. Ligands L^1–2 formed by the condensation of pentaerythrityl tetratosylate with 4,5‐diazafluoren‐9‐oxime and 9‐(4‐hydroxy)phenylimino‐4,5‐diazafluorene, respectively. L³ was prepared by two steps, 9‐(4‐hydroxy)phenylimino‐4,5‐diazafluorene reacted with pentaerythrityl tetratosylate affording 1,1′,1"‐tris[(4,5‐diazafluoren‐9‐ylimino)phenoxymethyl]‐1"′‐(p‐tosyloxymethyl)‐methane, which reacted with 4,5‐diazafluoren‐9‐oxime affording the asymmetric ligand L³. Three tetranuclear Ru^II complexes [(bpy)₈L^1–3Ru₄](PF₆)₈ (bpy = bipyridine) were obtained by the reaction of Ru(bpy)₂Cl₂ · 2H₂O with ligands L^1–3. Spectroscopic studies of these complexes exhibit metal‐to‐ligand charge transfer absorptions at 440–445 nm and emissions at 575–579 nm. The electrochemical behaviors of these complexes are consistent with one Ru^II‐based oxidation couple and three ligand‐centered reduction couples.     

Molecular path for ligand search

A ligand is a small molecule bind to several residues of a receptor.We adapt the concept of molecular path for effective ligand search with its contacting residues.Additionally,we allow wild type definitions on atoms and bonds of molecular paths for fuzzy algorithms on structural match.We choose hydrogen bond interactions to characterize the binding mode of a ligand by several proper molecular paths and use them to query the deposited ligands in PDBe that interact with their residues in the same way. Expression of molecular path and format of database entries are described with examples.Our molecular path provides a new approach to explore the ligand-receptor interactions and to provide structural framework reference on new ligand design.     

A Structural Study of Tautomerism and Hydrogen-Bonding in Supramolecular Assemblies

The synthesis and X-ray structures of four new crystalline materials incorporating ’dimers’ assembled from two different units possessing complementary hydrogen bonding motifs are reported; namely, phthalimide and 3-iminoisoindolinone or 2-guanidinobenzimidazole (or selected methylated derivatives) and 2-guanidinobenzoxazole. The bonding within each dimer involves a triplet of hydrogen bonds. The extended supramolecular structures are compared with each other as well as with two related structures described previously. The effect of using complementary DAD/ADA motifs that are not symmetrical on the respective supramolecular structures is also examined as is the prospect of incorporating different tautomeric components into the supramolecular structures. The presence of a very short, proton-transferred hydrogen bond within the respective triplets is also discussed.     

Syntheses and crystal structures of 3-tert-butyl-4-cyano pyrazole and its complexes with cobalt(II), manganese(II), and copper(II)

A new pyrazole ligand, 3-tert-butyl-4-cyano pyrazole (Hpz^t-Bu,4CN), has been synthesized. The crystal structure of this pyrazole, along with the syntheses and crystal structures of Co, Cu, and Mn complexes of this ligand, are reported. The uncoordinated pyrazole shows the formation of a cyclic hydrogen-bound dimer. The Co complex is octahedral, with four coordinated pyrazoles and two coordinated waters. The Mn complex is octahedral, with two coordinated pyrazoles, two coordinated triflates and two coordinated waters. A hydrogen bonding network involving the triflates and waters results in a linear double chain of Mn complexes. The Cu complex has two coordinated pyrazoles and two coordinated chlorides in a slightly distorted square-planar geometry, with a long bond to the cyano N atom of a neighboring Cu complex, forming a pseudodimer.     

Reactivity of Cr(III) μ-oxo compounds: catalyst regeneration and atom transfer processes

Oxidation of CpCr[(XylNCMe)(2)CH] (Xyl = 2,6-Me(2)C(6)H(3)) with pyridine N-oxide or air generated the μ-oxo dimer, {CpCr[(XylNCMe)(2)CH]}(2)(μ-O). The μ-oxo dimer was converted to paramagnetic Cr(III) CpCr[(XylNCMe)(2)CH](X) complexes (X = OH, O(2)CPh, Cl, OTs) via protonolysis reactions. The related Cr(III) alkoxide complexes (X = OCMe(3), OCMe(2)Ph) were prepared by salt metathesis and characterized by single crystal X-ray diffraction. The interconversion of the Cr(III) complexes and their reduction back to Cr(II) with Mn powder were monitored using UV-vis spectroscopy. The related CpCr[(DepNCMe)(2)CH] (Dep = 2,6-Et(2)C(6)H(3)) Cr(II) complex was studied for catalytic oxygen atom transfer reactions with PPh(3) using O(2) or air. Both Cr(II) complexes reacted with pyridine N-oxide and γ-terpinene to give the corresponding Cr(III) hydroxide complexes. When CpCr[(DepNCMe)(2)CH] was treated with pyridine N-oxide in benzene in the absence of hydrogen atom donors, a dimeric Cr(III) hydroxide product was isolated and structurally characterized, apparently resulting from intramolecular hydrogen atom abstraction of a secondary benzylic ligand C-H bond followed by intermolecular C-C bond formation. The use of very bulky hexaisopropylterphenyl ligand substituents did not preclude the formation of the analogous μ-oxo dimer, which was characterized by X-ray diffraction. Attempts to develop a chromium-catalyzed intermolecular hydrogen atom transfer process based on these reactions were unsuccessful. The protonolysis and reduction reactions of the μ-oxo dimer were used to improve the previously reported Cr-catalyzed radical cyclization of a bromoacetal.     

Asymmetric synthesis of chiral tectons with tetrapodal symmetry: fourfold asymmetric reactions

The diastereoselective fourfold addition to Ellman-type imines furnished after deprotection the tetrapodal amines in excellent yields. The unprecedented asymmetric fourfold addition of hydride and alkylzinc reagents to tetrapodal ketones and aldehydes, respectively, is achieved by employing CBS-reduction or [2.2]paracyclophane-based ketimine ligands with good to excellent global enantiomeric ratios.     

Near IR overtone spectral investigations of cyclohexanol using local mode model--evidence for variation of anharmonicity with concentration due to hydrogen bonding

The near infrared vibrational overtone absorption spectrum of liquid phase cyclohexanol in carbon tetrachloride in different concentrations are examined in the region Deltav=2, 3 and 4. The free and bonded OH local mode mechanical frequency values and anharmonicity values obtained from fitting the overtones are analysed. The observation supports the conclusions drawn from earlier experimental studies on anharmonicity variation of OH-stretching vibrations of alcohols due to intermolecular hydrogen bonding. Our observation is also in agreement with the ab initio calculations on water dimer and trimer. Mechanical anharmonicity of bonded OH-stretching bands tends to increase as a consequence of strong hydrogen bonding at higher concentrations.     

A new bis(imidazolyl)(alkylthiolate) tripodal ligand and the spontaneous formation of a disulfide-linked,hydroxo-bridged dinuclear zinc complex

The new sterically encumbered, tripodal N2S(alkylthiolate) ligand, LIm2SH, has been synthesized and used to prepare [(LIm2S)ZnCH3], which upon protonolysis under acidic conditions leads to the synthesis of a novel dinucleating ligand and a zinc dimer with an unusual structure.     

Structural diversity in the coordination of amidines and guanidines to monovalent metal halides

A series of structurally characterised, monovalent metal-halide complexes incorporating neutral amidine and guanidine ligands is reported. N,N'-diphenylbenzamidine reacted with copper(I) chloride to afford the bis-ligand complex [CuCl(PhC[NPh][NHPh])2]2 (1), that exists as a chlorine bridged dimer in the solid state, with a non-symmetrical distribution of NH...Cl interactions within the 'Cu2Cl2' metallacycle. In contrast, only one equivalent of the guanidine, Me2NC[NiPr][NHiPr] (2), is coordinated in the copper(I) iodide complex [CuI(Me2NC[NiPr][NHiPr])]2 (3), which was also isolated as the dimer with bridging halide atoms. The molecular structure of the bicyclic guanidine, 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-alpha]pyrimidine (hppH), is reported, revealing a hydrogen bridged dimer with extensive delocalisation throughout the ligand framework. Coordination of hppH to lithium chloride afforded the dimeric bis-ligand complex [LiCl(hppH)2]2 (4) in which each hppH molecule interacts with a different chlorine atom of the central 'Li2Cl2' core of the molecule via NH...Cl hydrogen bonding. In contrast the 2:1 ligand to metal complex is formed with silver(I) chloride to afford AgCl(hppH)2 (5), a unique example of a monomeric, three-coordinate silver chloride supported by nitrogen-based ligands. The series of mixed ligand complexes [CuX(hppH)(PPh3)]n (6, X = Cl, n= 1; 7, X = Br, n= 2; 8 X = I, n= 2) have also been synthesised and structurally characterised, allowing comparisons of the relative coordinating behaviour of hppH and PPh3 as neutral donors at copper(I) centres to be made.     

Synthesis of a complementary dimer from mono(imidazolyl)-substituted cobalt(II) porphyrin as a new artificial T-form hemoglobin

Mono(imidazolyl)-substituted Co(II) porphyrin dimer with a ‘picket fence’ structure was synthesized as a new artificial hemoglobin model containing two binding sites. The dimer was confirmed by UV-vis, resonance Raman and ESR spectral measurements to bind two dioxygen molecules reversibly. The dioxygen binding affinity of the dimer was lower than that of the corresponding monomer. The decrease in this affinity is discussed in terms of steric hindrance and orientational effect of the axial ligand.     

The geometry of nonmetal hydrides and the ligand radius of hydrogen

The aim of this paper was to investigate why the geometries of nonmetal hydrides are often not in accordance with the VSEPR model. From a consideration of interligand distances in a variety of BX(4), CX(4), and NX(4) molecules where X is a ligand or a lone pair and in which there are at least two H ligands we have shown that the hydrogen ligands are essentially close-packed. For each of the central atoms we have obtained a value for the ligand radius of hydrogen. These radii decrease with decreasing negative charge and increasing positive charge of the hydrogen ligand as the electronegativity of the central atom increases, as has been found previously for other ligands such as F and Cl. We show that ligand-ligand intractions are an important factor in determining bond angles in hydrides and that the ligand close-packing (LCP) model gives a better explanation of bond angles than the VSEPR model according to which bond angles depend on the electronegativity of the ligand rather than on its size. For example, although the very small angles in PH(3) and SH(2) are not in accord with the VSEPR model, they are consistent with the LCP model in that they are a consequence of the small size of hydrogen ligands which are pushed together by the lone pairs until they are almost close-packed.