Symmetry of metal chelates

Symmetry of Hydrogen Bonds: Application of NMR Method of Isotopic Perturbation and Relevance of Solvatomers

Short, strong, symmetric, low-barrier hydrogen bonds (H-bonds) are thought to be of special significance. We have been searching for symmetric H-bonds by using the NMR technique of isotopic perturbation. Various dicarboxylate monoanions, aldehyde enols, diamines, enamines, acid-base complexes, and two sterically encumbered enols have been investigated. Among all of these, we have found only one example of a symmetric H-bond, in nitromalonamide enol, and all of the others are equilibrating mixtures of tautomers. The nearly universal lack of symmetry is attributed to the presence of these H-bonded species as a mixture of solvatomers, meaning isomers (or stereoisomers or tautomers) that differ in their solvation environment. The disorder of solvation renders the two donor atoms instantaneously inequivalent, whereupon the hydrogen attaches to the less well solvated donor. We therefore conclude that there is no special significance to short, strong, symmetric, low-barrier H-bonds. Moreover, they have no heightened stability or else they would have been more prevalent.

ELISA and Chemiluminescent Enzyme Immunoassay for Sensitive and Specific Determination of Lead (II) in Water, Food and Feed Samples

Lead is a heavy metal with increasing public health concerns on its accumulation in the food chain and environment. Immunoassays for the quantitative measurement of environmental heavy metals offer numerous advantages over other traditional methods. ELISA and chemiluminescent enzyme immunoassay (CLEIA), based on the mAb we generated, were developed for the detection of lead (II). In total, 50% inhibitory concentrations (IC₅₀) of lead (II) were 9.4 ng/mL (ELISA) and 1.4 ng/mL (CLEIA); the limits of detection (LOD) were 0.7 ng/mL (ic-ELISA) and 0.1 ng/mL (ic-CLEIA), respectively. Cross-reactivities of the mAb toward other metal ions were less than 0.943%, indicating that the obtained mAb has high sensitivity and specificity. The recovery rates were 82.1%–108.3% (ic-ELISA) and 80.1%–98.8% (ic-CLEIA), respectively. The developed methods are feasible for the determination of trace lead (II) in various samples with high sensitivity, specificity, fastness, simplicity and accuracy.

Synthesis and Infrared Spectroscopic Investigation of Hydrogen-Bonded Complexes between 1,8-Bis(Dimethylamino)Naphthalene and 2,6-Dihydroxynaphthalene in 2:1 and 1:2 Ratios in the Crystalline forms and in Acetonitrile

Two hydrogen bonded complexes between 1,8- bis(dimethylamino) naphthalene (DMAN) and 2,6-dihydroxynaphthalene (DHN) in 2:1 and 1:2 ratios were synthesised and studied using FTIR the spectra of the solid 2:1 complex revealed incomplete protonation of DMAN with the appearance of two broad absorptions representing ν(NHN)+ and ν(OHN) in the ranges 700–400 and 1600–700 cm−1 respectively, while the 1:2 complex indicated the complete protonation of DMAN with the appearance of two broad absorptions representing ν(NHN)+ and ν(OHO-) in the ranges 700–400 and 1600–700 cm−1 respectively. On the other hand the IR spectra in acetonitrile indicated that the 1:2 complex is more stable than the 2:1 complex.

Counterion influence on the N-I-N halogen bond

A detailed investigation of the influence of counterions on the [N-I-N]+ halogen bond in solution, in the solid state and in silico is presented. Translational diffusion coefficients indicate close attachment of counterions to the cationic, three-center halogen bond in dichloromethane solution. Isotopic perturbation of equilibrium NMR studies performed on isotopologue mixtures of regioselectively deuterated and nondeuterated analogues of the model system showed that the counterion is incapable of altering the symmetry of the [N-I-N]+ halogen bond. This symmetry remains even in the presence of an unfavorable geometric restraint. A high preference for the symmetric geometry was found also in the solid state by single crystal X-ray crystallography. Molecular systems encompassing weakly coordinating counterions behave similarly to the corresponding silver(i) centered coordination complexes. In contrast, systems possessing moderately or strongly coordinating anions show a distinctly different behavior. Such silver(i) complexes are converted into multi-coordinate geometries with strong Ag-O bonds, whereas the iodine centered systems remain linear and lack direct charge transfer interaction with the counterion, as verified by 15N NMR and DFT computation. This suggests that the [N-I-N]+ halogen bond may not be satisfactorily described in terms of a pure coordination bond typical of transition metal complexes, but as a secondary bond with a substantial charge-transfer character.

Are short, low-barrier hydrogen bonds unusually strong?

In a symmetric hydrogen bond (H-bond), the hydrogen atom is perfectly centered between the two donor atoms. The energy diagram for hydrogen motion is thus a single-well potential, rather than the double-well potential of a more typical H-bond, in which the hydrogen is covalently bonded to one atom and H-bonded to the other. Examples of symmetric H-bonds are often found in crystal structures, and they exhibit the distinctive feature of unusually short length: for example, the O-O distance in symmetric OHO H-bonds is found to be less than 2.5 Å. In comparison, the O-O distance in a typical asymmetric H-bond, such as ROH···OR(2), ranges from about 2.7 to 3.0 Å. In this Account, we briefly review and update our use of the method of isotopic perturbation to search for a symmetric, centered, or single-well-potential H-bond in solution. Such low-barrier H-bonds are thought to be unusually strong, owing perhaps to the resonance stabilization of two identical resonance forms [A-H···B ↔ A···H-B]. This presumptive bond strength has been invoked to explain some enzyme-catalyzed reactions. Yet in solution, a wide variety of OHO, OHN, and NHN H-bonds have all been found to be asymmetric, in double-well potentials. Examples include the monoanion of (±)-2,3-di-tert-butylsuccinic acid and a protonated tetramethylnaphthalenediamine, even though these two ions are often considered prototypes of species with strong H-bonds. In fact, all of the purported examples of strong, symmetric H-bonds have been found to exist in solution as pairs of asymmetric tautomers, in contrast to their symmetry in some crystals. The asymmetry can be attributed to the disorder of the local solvation environment, which leads to an equilibrium among solvatomers (that is, isomers that differ in solvation). If the disorder of the local environment is sufficient to break symmetry, then symmetry itself is not sufficient to stabilize the H-bond, and symmetric H-bonds do not have an enhanced stability or an unusual strength. Nor are short H-bonds unusually strong. We discuss previous evidence for "short, strong, low-barrier" H-bonds and show it to be based on ambiguous comparisons. The role of such H-bonds in enzyme-catalyzed reactions is then ascribed not to any unusual strength of the H-bond itself but to relief of "strain."

Isotopic perturbation of the conformational equilibrium in methylene-functionalized calixarenes

The 400 MHz 1H NMR spectrum of the tetramethoxycalixarene 2 (possessing hydroxyl groups at the bridges) in commercial acetone-d6 displays five signals (an isotopic multiplet) for a pair of methoxy groups. Inspection of the X-ray structures of 2 and its isomer 3 indicates that in the adopted 1,3-alternate conformation, the methoxy groups intramolecularly hydrogen bonded to neighboring OH groups are oriented "in" (pointing toward the cavity). Upon dissolution of 2 in acetone-d6, none, some, or all of the OH protons exchange with the deuterium atoms present in the residual water of the solvent. Several species (mutually relating as isotopomers and isotopologues) differing in the number and positions of the deuterated hydroxyl groups are possible for 2. In three of these species, the "in"-"out"/"out"-"in" conformational equilibrium of a pair of methoxy groups is nondegenerate. The four external lines of the apparent multiplet are ascribed to a single- and double-isotopic perturbation of the "in"-"out" conformational equilibrium of a pair of methoxy groups. On the basis of the assignment of the signals to the individual species and their statistical distribution, the intensities of the components of the isotopic multiplet obtained at different isotopic enrichments of the hydroxyl groups could be simulated. A sample of 2 55% deuterated at the hydroxyl groups in CDCl3 displayed an isotopic multiplet consisting of nine signals. The isotopic multiplet observed for the OH groups of 2 in acetone-d6 was simulated at different deuteration enrichments.

Symmetry of hydrogen bonds in solution

A classic question regarding hydrogen bonds (H-bonds) concerns their symmetry. Is the hydrogen centered or is it closer to one donor and jumping between them? These possibilities correspond to single- and double-well potentials, respectively. The NMR method of isotopic perturbation can answer this question. It is illustrated with 3-hydroxy-2-phenylpropenal and then applied to dicarboxylate monoanions. The 18O-induced 13C NMR splittings signify that their intramolecular H-bonds are asymmetric and that each species is a pair of tautomers, not a single symmetric structure, even though maleate and phthalate are symmetric in crystals. The asymmetry is seen across a wide range of solvents and a wide variety of monoanions, including 2,3-di-tert-butylsuccinate and zwitterionic phthalates. Asymmetry is also seen in monoprotonated 1,8-bis(dimethylamino)naphthalenediamines, N,N'-diaryl-6-aminofulvene-2-aldimines, and 6-hydroxy-2-formylfulvene. The asymmetry is attributed to the disorder of the local environment, establishing an equilibrium between solvatomers. The broader implications of these results regarding the role of solvation in breaking symmetry are discussed. It was prudent to confirm a secondary deuterium isotope effect (IE) on amine basicity by NMR titration of a mixture of PhCH2NH2 and PhCHDNH2. The IE is of stereoelectronic origin. It is proposed that symmetric H-bonds can be observed in crystals but not in solution because a disordered environment induces asymmetry, whereas a crystal can guarantee a symmetric environment. The implications for the controversial role of low-barrier H-bonds in enzyme-catalyzed reactions are discussed.

Hydrogen-Bond Symmetry in Zwitterionic Phthalate Anions: Symmetry Breaking by Solvation

The cationic nitrogen of zwitterion 1 is located symmetrically with respect to its intramolecular OHO hydrogen bond. Incorporation of one (18)O allows investigation of the H-bond symmetry by the NMR method of isotopic perturbation. In both CD(3)OD and CD(2)Cl(2) equilibrium isotope shifts are detected at the carboxyl and ipso carbons. Therefore, 1 exists as a pair of interconverting tautomers, not as a single symmetric structure with its hydrogen centered between the two oxygens. The H-bond is instantaneously asymmetric, and there is an equilibrium between solvatomers (isomers or stereoisomers that differ in solvation). The broader implications of this result regarding the role of the local environment ("solvation") in breaking symmetry are discussed.

Theoretical Study of Changes in π-Electron Delocalization in the Analogues of an ortho-Hydroxy Schiff Base When the Proton Is Replaced with Li+ or BeH+

Molecular geometries of ortho-hydroxy Schiff base in keto-enamine and enol-imine tautomeric forms, its anion, and their derivatives in which H+ was replaced with Li+ or BeH+ were optimized at the B3LYP/6-311+G level of theory. Isodesmic reactions for estimating delocalization due to H-bonding or cation chelating were calculated. Geometry-based aromaticity index HOMA and magnetism-based NICS1(zz) index were used to estimate pi-electron delocalization. Keto-enamine tautomer exhibits low aromaticity in the ring and a relatively high pi-electron delocalization in the quasi-ring. The reverse was found for enol-imine tautomer. The Li+ and BeH+ derivatives showed a relatively high pi-electron delocalization in the ring and in the quasi-ring. This may be interpreted by an extension of the electron delocalization path in the pi-electron system through low-lying unoccupied p-type orbitals of Li+ and BeH+ cations.