| 1. |
- Bedin, Michele, et al.
(författare)
-
Counterion influence on the N-I-N halogen bond
- 2015
-
Ingår i: Chemical Science. - 2041-6539 .- 2041-6520. ; 6:7, s. 3746-3756
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 2. |
- Karim, Alavi, 1986, et al.
(författare)
-
The nature of [N-Cl-N]+ and [N-F-N]+ halogen bonds in solution
- 2014
-
Ingår i: Chemical Science. - 2041-6520 .- 2041-6539. ; 5, s. 3226-3233
-
Tidskriftsartikel (refereegranskat)abstract
- Halonium ions are synthetically useful, transient species that may be stabilized by attachment to two electron donors. Whereas studies of [C–X–C]+-type ions have greatly contributed to the fundamental understanding of chemical bonding and reaction mechanisms, investigations of the corresponding [N–X–N]+ halogen bond complexes are only at an early stage. Herein we present solution NMR spectroscopic and theoretical evidence for the nature of [N–Cl–N]+ and [N–F–N]+ complexes, and we discuss their geometries and stabilities in comparison to their iodine and bromine-centered analogues as well as the corresponding three-center [N–H–N]+ hydrogen bond. We show the chlorine-centered halogen bond to be weaker but yet to resemble the symmetric geometry of the three-center bond of heavier halogens. In contrast, the [N–F–N]+ bond is demonstrated to prefer asymmetric geometry analogous to the [N–H–N]+ hydrogen bond. However, the [N–F–N]+ system has a high energy barrier for interconversion, and due to entropy loss, its formation is slightly endothermic.
|
|
| 3. |
- Carlsson, Anna-Carin, 1976, et al.
(författare)
-
Solvent Effects on Halogen Bond Symmetry
- 2013
-
Ingår i: CrysteEngComm. - 1466-8033. ; 15:16, s. 3087-3092
-
Tidskriftsartikel (refereegranskat)abstract
- The symmetric arrangement of the iodine and bromine centred 3-center–4-electron halogen bond is revealed to remain preferred in a polar, aprotic solvent environment. Acetonitrile is unable to compete with pyridine for halogen bonding; however, its polarity weakly modulates the energy of the interaction and influences IPE-NMR experiments.
|
|
| 4. |
- Carlsson, Anna-Carin, 1976, et al.
(författare)
-
Substituent Effects on the [N–I–N]+ Halogen Bond
- 2016
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 138:31, s. 9853-9863
-
Tidskriftsartikel (refereegranskat)abstract
- We have investigated the influence of electron density on the three-center [N–I–N]+ halogen bond. A series of [bis(pyridine)iodine]+ and [1,2-bis((pyridine-2-ylethynyl)benzene)iodine]+ BF4– complexes substituted with electron withdrawing and donating functionalities in the para-position of their pyridine nitrogen were synthesized and studied by spectroscopic and computational methods. The systematic change of electron density of the pyridine nitrogens upon alteration of the para-substituent (NO2, CF3, H, F, Me, OMe, NMe2) was confirmed by 15N NMR and by computation of the natural atomic population and the π electron population of the nitrogen atoms. Formation of the [N–I–N]+ halogen bond resulted in >100 ppm 15N NMR coordination shifts. Substituent effects on the 15N NMR chemical shift are governed by the π population rather than the total electron population at the nitrogens. Isotopic perturbation of equilibrium NMR studies along with computation on the DFT level indicate that all studied systems possess static, symmetric [N–I–N]+ halogen bonds, independent of their electron density. This was further confirmed by single crystal X-ray diffraction data of 4-substituted [bis(pyridine)iodine]+ complexes. An increased electron density of the halogen bond acceptor stabilizes the [N···I···N]+ bond, whereas electron deficiency reduces the stability of the complexes, as demonstrated by UV-kinetics and computation. In contrast, the N–I bond length is virtually unaffected by changes of the electron density. The understanding of electronic effects on the [N–X–N]+ halogen bond is expected to provide a useful handle for the modulation of the reactivity of [bis(pyridine)halogen]+-type synthetic reagents.
|
|
| 5. |
- Carlsson, Anna-Carin, 1976, et al.
(författare)
-
Symmetric Halogen Bonding is Preferred in Solution
- 2012
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 134, s. 5706-5715
-
Tidskriftsartikel (refereegranskat)abstract
- Halogen bonding is a recently rediscovered secondary interaction that shows potential to become a complementary molecular tool to hydrogen bonding in rational drug design and in material sciences. Whereas hydrogen bond symmetry has been the subject of systematic studies for decades, the understanding of the analogous three-center halogen bonds is yet in its infancy. The isotopic perturbation of equilibrium (IPE) technique with 13C NMR detection was applied to regioselectively deuterated pyridine complexes to investigate the symmetry of [N−I−N]+ and [N−Br−N]+ halogen bonding in solution. Preference for a symmetric arrangement was observed for both a freely adjustable and for a conformationally restricted [N−X−N]+ model system, as also confirmed by computation on the DFT level. A closely attached counterion is shown to be compatible with the preferred symmetric arrangement. The experimental observations and computational predictions reveal a high energetic gain upon formation of symmetric, three-center four-electron halogen bonding. Whereas hydrogen bonds are generally asymmetric in solution and symmetric in the crystalline state, the analogous bromine and iodine centered halogen bonds prefer symmetric arrangement in solution.
|
|
| 6. |
- Karim, Alavi, 1986, et al.
(författare)
-
Carbon's Three-Center, Four-Electron Tetrel Bond, Treated Experimentally
- 2018
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 140:50, s. 17571-17579
-
Tidskriftsartikel (refereegranskat)abstract
- © 2018 American Chemical Society. Tetrel bonding is the noncovalent interaction of group IV elements with electron donors. It is a weak, directional interaction that resembles hydrogen and halogen bonding yet remains barely explored. Herein, we present an experimental investigation of the carbon-centered, three-center, four-electron tetrel bond, [N-C-N]+, formed by capturing a carbenium ion with a bidentate Lewis base. NMR-spectroscopic, titration-calorimetric, and reaction-kinetic evidence for the existence and structure of this species is reported. The studied interaction is by far the strongest tetrel bond reported so far and is discussed in comparison with the analogous halogen bond. The necessity of the involvement of a bidentate Lewis base in its formation is demonstrated by providing spectroscopic and crystallographic evidence that a monodentate Lewis base induces a reaction rather than stabilizing the tetrel bond complex. A vastly decreased Lewis basicity of the bidentate ligand or reduced Lewis acidity of the carbenium ion weakens - or even prohibits - the formation of the tetrel bond complex, whereas synthetic modifications facilitating attractive orbital overlaps promote it. As the geometry of the complex resembles the SN2 transition state, it provides a model system for the investigation of fundamental reaction mechanisms and chemical bonding theories.
|
|
| 7. |
- Karim, Alavi, 1986
(författare)
-
Investigation of [N-X-N]+ and [N-C-N]+ complexes in solution : Exploring Geometry, Stability and Symmetry
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Halogen bonding is a weak interaction. In this thesis the three center four electron halogen bond, [N−X−N]+, has been studied. The lighter halogens form highly unstable halonium ions that are reactive towards nucleophiles and their complexes were therefore investigated at low temperatures. Whereas the chlorine-centered halogen bond was found to be symmetric, the fluorine-centered one is shown to be asymmetric in solution. These geometries have been determined by NMR spectroscopic evidences and computations at the DFT level. For determining the influence of the counterion on the iodine-centered halogen bond, the isotopic perturbation of equilibrium (IPE) technique was applied with 13C {1H,2H} NMR detection in solution, and X-ray diffraction in the solid state. The symmetric arrangement of [N−I−N]+ complexes possessing two equal N−I halogen bonds remains undisturbed, independent of the choice of counterion and also when it has been scavenged. In comparison, silver centered [N−Ag−N]+ complexes although similar in size to the iodonium center, show direct counterion coordination to the metal center. The three center four electron complex of a positively charged carbenium ion trapped between two nitrogenous donors forming a thermodynamically stable pentavalent [N−C−N]+ complex has also been studied. The structure and properties of this complex is discussed based on NMR spectroscopic and reaction kinetic evidences in comparison to the analogous three-centered [NXN]+ halogen bond. A geometrically restrained bidentate Lewis base is shown to be necessary for the formation of this pentavalent complex. NMR spectroscopic and X ray crystallographic evidences indicate that a monodentate Lewis base induces a reaction instead of stabilizing the reactive species as a thermodynamically stable complex. As the geometry of the pentavalent complex greatly resembles the SN2 transition state, it affords a smoothly modifiable model system for the investigation of fundamental reaction mechanisms and chemical bonding theories
|
|
| 8. |
- Kleinmaier, Roland, et al.
(författare)
-
Solvent effects on 15N NMR coordination shifts
- 2013
-
Ingår i: Magnetic resonance in chemistry : MRC. - : Wiley. - 1097-458X .- 0749-1581. ; 51:1, s. 46-53
-
Tidskriftsartikel (refereegranskat)abstract
- (15)N NMR chemical shift became a broadly utilized tool for characterization of complex structures and comparison of their properties. Despite the lack of systematic studies, the influence of solvent on the nitrogen coordination shift, Δ(15)N(coord), was hitherto claimed to be negligible. Herein, we report the dramatic impact of the local environment and in particular that of the interplay between solvent and substituents on Δ(15)N(coord). The comparative study of CDCl(3) and CD(3)CN solutions of silver(I)-bis(pyridine) and silver(I)-bis(pyridylethynyl)benzene complexes revealed the strong solvent dependence of their (15)N NMR chemical shift, with a solvent dependent variation of up to 40ppm for one and the same complex. The primary influence of the effect of substituent and counter ion on the (15)N NMR chemical shifts is rationalized by corroborating Density-Functional Theory (nor discrete Fourier transform) calculations on the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) level. Cooperative effects have to be taken into account for a comprehensive description of the coordination shift and thus the structure of silver complexes in solution. Our results demonstrate that interpretation of Δ(15)N(coord) in terms of coordination strength must always consider the solvent and counter ion. The comparable magnitude of Δ(15)N(coord) for reported transition metal complexes makes the principal findings most likely general for a broad scale of complexes of nitrogen donor ligands, which are in frequent use in modern organometallic chemistry.
|
|
| 9. |
- Peuker, Sebastian, et al.
(författare)
-
Efficient Isotope Editing of Proteins for Site-Directed Vibrational Spectroscopy
- 2016
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 138:7, s. 2312-2318
-
Tidskriftsartikel (refereegranskat)abstract
- Vibrational spectra contain unique information on protein structure and dynamics. However, this information is often obscured by spectral congestion, and site-selective information is not available. In principle, sites of interest can be spectrally identified by isotope shifts, but site-specific isotope labeling of proteins is today possible only for favorable amino acids or with prohibitively low yields. Here we present an efficient cell-free expression system for the site-specific incorporation of any isotope-labeled amino acid into proteins. We synthesized 1.6 mg of green fluorescent protein with an isotope-labeled tyrosine from 100 mL of cell free reaction extract. We unambiguously identified spectral features of the tyrosine in the fingerprint region of the time-resolved infrared absorption spectra. Kinetic analysis confirmed the existence of an intermediate state between photoexcitation and proton transfer that lives for 3 ps. Our method lifts vibrational spectroscopy of proteins to a higher level of structural specificity.
|
|
