Christian Laurence

Halogen-bond geometry: a crystallographic database investigation of dihalogen complexes

X-ray crystal structures of 141 halogen-bonded complexes Y-X.B formed between homo- and heteronuclear dihalogens Cl(2), Br(2), I(2), IBr and ICl with O, S, Se, N, P and As Lewis bases show remarkable and constant geometrical features. The metrics of the halogen bond found in the gas phase for simple complexes [Legon (1999a). Angew Chem. Int. Ed. Eng. 38, 2686-2714] is supported (i). in the solid state, (ii). for new Lewis acids (I(2) and IBr), (iii). for new basic centers (Se, As and =N-) and (iv). for more complicated bases. The Y-X...B arrangement is more linear than the corresponding Y-H...B hydrogen bond and the axis of the Y-X molecule lies in the plane of the B lone pair(s), with a preference for the putative lone-pair direction within that plane. However, exceptions to this lone-pair rule are found for sterically hindered thiocarbonyl and selenocarbonyl bases. A bond-order model of the halogen bond correctly predicts the observed correlation between the shortening of the X...B distance and the lengthening, deltad(Y-X), of the Y-X bond. The expectation that the solid-state geometric parameters d(X...B) and deltad(Y-X) reflect the strength of the interaction is supported by their significant relationships with the solution thermodynamic parameters of Lewis acidity and basicity strength, such as the Gibbs energy of 1:1 complexation of Lewis bases with diiodine. This analysis of halogen-bonded complexes in the solid state reinforces the similarities already known to exist between hydrogen and halogen bonding.

The Diiodine Basicity Scale: Toward a General Halogen‐Bond Basicity Scale

The new diiodine basicity scale pK(BI2) is quasi-orthogonal to most known Lewis basicity scales (hydrogen-bond, dative-bond and cation basicity scales). The diiodine basicity falls in the sequence N>P≈Se>S>I≈O>Br>Cl>F for the iodine-bond acceptor atomic site and SbO≈NO≈AsO>SeO>PO>SO>C=O>-O->SO(2) or PS≫-S->C=S≫N=C=S for the functionality of oxygen or sulfur bases. Substituent effects are quantified through linear free energy relationships, which allow the calculation of individual complexation constants for each site of polybases and thus the classification of aromatic ethers as carbon π bases and of aromatic amines, thioethers and selenoethers as N, S and Se bases, respectively. The pK(BI2) values of nBu(3)N(+)-N(-)C≡N, 2-aminopyridine and 1,10-phenanthroline reveal a superbasic nitrile, a hydrogen-bond-assisted iodine bond and a two-centre iodine bond, respectively. The diiodine basicity scale is a general inorganic but family-dependent organic halogen-bond basicity scale because organic halogen-bond donors such as IC≡N and ICF(3) have a stronger electrostatic character than I(2). The family independence can be restored by the addition of an electrostatic parameter, either the experimental pK(BHX) hydrogen-bond basicity scale or the computed minimum electrostatic potential.

An Enthalpic Scale of Hydrogen-Bond Basicity. 4. Carbon π Bases, Oxygen Bases, and Miscellaneous Second-Row, Third-Row, and Fourth-Row Bases and a Survey of the 4-Fluorophenol Affinity Scale

The thermodynamics of the O-H...B hydrogen bond (HB) has been determined in CCl(4) by FTIR spectrometry for a wide variety of carbon pi bases, oxygen bases, and miscellaneous first- to fourth-row bases, using 4-fluorophenol as a reference hydrogen-bond donor (HBD). After inclusion of previously studied nitrogen, sulfur, and halogen bases, this 4-fluorophenol affinity scale contains 314 varied organic bases and ranges over 40 kJ mol(-1). The 4-fluorophenol affinity scale in CCl(4) is shown to be applicable to most HBDs in most media, provided a small family dependence is taken into account. The HB affinity orders are quantitatively established according to the atomic acceptor site or to its bearing functional group. A comprehensive survey of the influence of substituents on these affinity orders is then achieved, considering electronic and steric effects, as well as effects of vinylogy or iminology. Iminology is found to be more efficient than vinylogy for transmitting resonance effects. Steric effects are shown to be less important in HB affinity than in HB basicity since they mainly act on the HB entropy. The spatial proximity of two acceptor sites can favor complexation through three-center hydrogen bonds, leading to superhydrogen-bond bases on the affinity scale.

Influence of the structure of polyfluorinated alcohols on Brønsted acidity/hydrogen-bond donor ability and consequences on the promoter effect.

The influence of substituents on the properties of tri- and hexafluorinated alcohols derived from 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was examined. Measurements of specific solvent-solute interactions revealed that H-bond donation (HBD) of fluorinated alcohols is sensitive to the steric hindrance of the OH group, whereas their Brønsted acidity is dependent only on the number of fluorine atoms. For hexafluorinated alcohols (HFAs), their association with amines characterized by X-ray diffraction showed that the balance between HBD and acidity is influenced by their structure. Moreover, the ability of HFAs to donate H-bonds is exerted in synclinal (sc), synperiplanar (sp), and also antiperiplanar (ap) conformations along the C-O bond. Comparison of the effects of fluorinated alcohols as promoting solvents in three reactions is reported. The positive correlation between rate constants and H-bonding donation ability for sulfide oxidation and imino Diels-Alder reaction brings to light the role of this property, while acidity might have a minor influence. In the third reaction, epoxide opening by piperidine, none of these properties can clearly be put forward at this stage.

The Hydrogen‐Bond Basicity pKHB Scale of Peroxides and Ethers

Using 4-fluorophenol as a reference hydrogen-bond donor, equilibrium constants, Kf, for the formation of 1:1 hydrogen-bonded complexes have been obtained by FTIR spectrometry for 39 ethers of widely different structure (cyclic and acyclic ethers, crown ethers, glymes, acetals, orthoesters, and disiloxane) and 3 peroxides, in CCl4 at 298 K. The pKHB scale of monoethers extends from 1.44 for 2,3-diadamant-2-yloxirane to –0.53 for hexamethyldisiloxane. The main effects explaining the variation of the hydrogen-bond basicity of sp3 oxygen atoms are (i) the electron-withdrawing field-inductive effect [e.g. in (CF3)2CHOMe], (ii) the electron-withdrawing resonance effect (e.g. in EtOCH=CH2) (iii) the steric effect (e.g. in tBu2O), (iv) the lone-pair–lone-pair repulsion (e.g. in cyclic peroxides), and (v) the cyclization giving the basicity order: oxetane > tetrahydrofuran > tetrahydropyran > oxirane. A spectroscopic scale of hydrogen-bond basicity is constructed from the infrared frequency shift Δν(OH) of methanol hydrogen-bonded to peroxides and ethers. The thermodynamic pKHB scale does not correlate with the ν(OH) scale because of (i) statistical effects in polyethers and peroxides (ii) secondary hydrogen-bond acceptor sites (e.g. in benzyl ether), (iii) variations of the s character of oxygen lone pairs either by conjugation or cyclization, (iv) steric effects, (v) lone-pair–lone-pair repulsions, and (vi) anomeric effects. The ν(OH···O) band shape reveals two stereoisomeric complexes, the most stable being tetrahedral at the ether oxygen atom.

Stéréochimie de la liaison hydrogène sur le groupe carbonyle

Abstract The asymmetric shape or splitting of the ν(XH) or ν(XD) bands of OH(OD), NH, SH and CH proton donors hydrogen bonded to the carbonyl group of ketones, aldehydes, esters, amides, ureas and carbamates have been explained by the existence of two stereoisomeric complexes: a linear complex (along the axis of the carbonyl bond) and an angular complex (in the direction of a lone pair). Bulky substituents on the carbonyl group or near the XH bond destabilize the angular arrangement. Inductive electron-withdrawing substituents on the carbonyl group favour the linear arrangement.

A database of dispersion-induction DI, electrostatic ES, and hydrogen bonding α1 and β1 solvent parameters and some applications to the multiparameter correlation analysis of solvent effects.

For about 300 solvents, we propose a database of new solvent parameters describing empirically solute/solvent interactions: DI for dispersion and induction, ES for electrostatic interactions between permanent multipoles, α1 for solute Lewis base/solvent Lewis acid interactions, and β1 for solute hydrogen-bond donor/solvent hydrogen-bond acceptor interactions. The main advantage over previous parametrizations is the easiness of extension of this database to newly designed solvents, since only three probes, the betaine dye 30, 4-fluorophenol, and 4-fluoroanisole are required. These parameters can be entered into the linear solvation energy relationship A = A0 + di(DI) + eES + aα1 + bβ1 to predict a large number of varied physicochemical properties A and to rationalize the multiple intermolecular forces at the origin of solvent effects through a simple examination of the sign and magnitude of regression coefficients di, e, a, and b. Such a rationalization is illustrated for conformational and tautomeric equilibria and is supported by quantum-mechanical calculations.

Amino and cyano N atoms in competitive situations: which is the best hydrogen-bond acceptor? A crystallographic database investigation

The relative hydrogen-bond acceptor abilities of amino and cyano N atoms have been investigated using data retrieved from the Cambridge Structural Database and via ab initio molecular orbital calculations. Surveys of the CSD for hydrogen bonds between HX (X = N, O) donors, N-T-C identical with N (push-pull nitriles) and N-(Csp(3))(n)-C identical with N molecular fragments show that the hydrogen bonds are more abundant on the nitrile than on the amino nitrogen. In the push-pull family, in which T is a transmitter of resonance effects, the hydrogen-bonding ability of the cyano nitrogen is increased by conjugative interactions between the lone pair of the amino substituent and the C identical with N group: a clear example of resonance-assisted hydrogen bonding. The strength of the hydrogen-bonds on the cyano nitrogen in this family follows the experimental order of hydrogen-bond basicity, as observed in solution through the pK(HB) scale. The number of hydrogen bonds established on the amino nitrogen is greater for aliphatic aminonitriles N-(Csp(3))(n)-C identical with N, but remains low. This behaviour reflects the greater sensitivity of the amino nitrogen to steric hindrance and the electron-withdrawing inductive effect compared with the cyano nitrogen. Ab initio molecular orbital calculations (B3LYP/6-31+G** level) of electrostatic potentials on the molecular surface around each nitrogen confirm the experimental observations.

Halogen-bond interactions: a crystallographic basicity scale towards iodoorganic compounds

Halogen bond (X-bond) interactions involving organic iodine have been investigated using data retrieved from the Cambridge Structural Database (CSD) and Density Functional Theory (DFT) calculations. The analysis of the mean normalised intermolecular distances involving Csp–I, Csp2–I and Csp3–I X-bond donors first shows that, for interactions with the same acceptor site, the shortest mean distance is always measured for Csp–I X-bond donors. This experimental trend is rationalised through molecular electrostatic potential calculations on the iodine surface, along the σ-hole of the iodine atom, since Csp–I donors are characterised by the strongest VS,max values, that is to say the more electron poor iodine atoms. In agreement with the trends revealed from the analysis of the I⋯N distances, the X-bond with Csp–I donors appear more linear (mean of 169.5°) than with Csp2–I and Csp3–I donors, their respective values being close to 164°. From a survey of the geometries of the X-bond contacts observed in the most extended dataset (Csp2–I: 1213), a crystallographic order of X-bond acceptor strength has been obtained through a careful consideration of the chemical functions and subfunctions to which the acceptor atom belongs. This order is in good agreement with the one observed in solution on the diiodine basicity scale pKBI2. An exception to this trend is thioureas, which show unexpected long (weak) S⋯I distances. However, these observations are rationalised from the sulfur behaviour, which acts simultaneously as a X-bond acceptor and hydrogen-bond (H-bond) acceptor in these structures. Finally, an interesting correlation between the pKBI2 and the I⋯Y normalised intermolecular distances is found for a wide and varied collection of organic bases, since we have been able to delineate 22 families of organic compounds covering more than four pK units on the pKBI2 scale.

Determination of a Solvent Hydrogen-Bond Acidity Scale by Means of the Solvatochromism of Pyridinium‑N‑phenolate Betaine Dye 30 and PCM-TD-DFT Calculations

Empirical parameters of solvents describing their hydrogen-bond (HB) acidity (e.g., the Kamlet-Taft α parameter) are often difficult to determine for new solvents because they are not directly related to a single definition process. Here, we propose a simple method based on one probe, the betaine dye 30, and one reference process, the solvatochromism of this dye, measured by its first electronic transition energy, ET(30). These ET(30) values are calculated within the time-dependent density functional theory framework, using a polarizable continuum solvent model (PCM). The part of ET(30) values that is not included in the PCM calculation is taken as the HB component of the measured ET(30) values, allowing us to deduce a solvent HB acidity parameter α1. The validity of this simple model is assessed by good linear correlations between α1 and a variety of solute properties mainly depending on the solvents HB acidity. The quality of fit observed with α1 is at least comparable with that obtained by previous solvent HB acidity scales. The simplicity of our method is illustrated by the determination of α1 and of its companion, the electrostatic solvent parameter ES, for some new green solvents derived from glycerol.