Bifurcated N[Bond]

UC Riverside UC Riverside Previously Published Works Title The basicity of unsaturated hydrocarbons as probed by hydrogen-bond-acceptor ability: bifurcated N-H+ ...pi hydrogen bonding. Permalink https://escholarship.org/uc/item/57g48370 Journal Chemistry (Weinheim an der Bergstrasse, Germany), 14(26) ISSN 0947-6539 Authors Stoyanov, Evgenii S Stoyanova, Irina V Reed, Christopher A Publication Date 2008 DOI 10.1002/chem.200800337 Peer reviewed eScholarship.org Powered by the California Digital Library University of California DOI: 10.1002/chem.200800337 The Basicity of Unsaturated Hydrocarbons as Probed by Hydrogen-Bond- Acceptor Ability: Bifurcated NACHTUGREÀH+···p Hydrogen Bonding Evgenii S. Stoyanov,* Irina V. Stoyanova, and Christopher A. Reed*[a] Abstract: The competitive substitution rated hydrocarbons have not been ob- ing the p basicity of unsaturated hydro- of the anion (AÀ) in contact ion pairs served previously. The unsaturated hy- carbons is proposed: fused-ring aro- + ACHTUNGRE À of the type [Oct3NH ]B(C6F5)4 by un- drocarbons studied include benzene matics benzene< toluene< xylene< saturated hydrocarbons (L) in accord- and methylbenzenes, fused-ring aro- mesitylene<durene < conjugated di- + ACHTUNGRE ACHTUNGRE ACHTUNGRE ance with the equilibrium Oct3NH matics, alkenes, conjugated dienes, and enes1-alkynes< pentamethylben- À Q + À ACHTUNGRE ···A + nL [Oct3NH ···Ln]A has alkynes. From the magnitude of the zene<hexamethylbenzene <internal ACHTUNGRE been studied in CCl4. On the basis of redshifts in the NÀH stretching fre- alkynes cycloalkenes<1-methylcyclo- equilibrium constants, K, and shifts of quencies, DnNH, a new scale for rank- alkenes.ACHTUNGRE This scale is relevant to the nNH to low frequency, it has been es- discussion of p complexes for incipient tablished that complexed Oct NH +···L protonation reactions and to under- 3 n Keywords: basicity · bifurcated hy- cations with n=1 and 2 are formed standing NÀH +···p hydrogen bonding drogen bonds · IR spectroscopy · and have unidentate and bifurcated NÀ in proteins and molecular crystals. hydrogen bonds · unsaturated H +···p hydrogen bonds, respectively. hydrocarbons Bifurcated hydrogen bonds to unsatu- Introduction In the gas phase, weak XÀH···p hydrogen-bonding interactions were identified in 1981 in complexes of HCl with al- XÀH···p hydrogen-bonding interactions have been studied kynes, alkenes, and arenes.[4–6] T-shaped structures were de- for a long time. Following the suggestion by Dewar in 1946 duced for the alkene and alkyne complexes (A and B) with that p complexes might be involved in the mechanism of the XÀH bond at approximately 908 and directed to the electrophilic aromatic substitution,[1] Brown and Brady center of the multiple CÀC bond. Recent atomic deforma- showed in 1952 that HCl formed complexes with aromatic tion calculations indicate that maximum electron density lies compounds and alkenes in hydrocarbon solutions.[2] Infor- at the mid-point of the double and triple bonds.[7] With ben- mation on the molecular structures of these complexes was zene, the XÀH bond is preferentially directed near the not available, but equilibrium binding constants were deter- center of the aromatic ring in a p-face manner (Scheme 1, mined and, within families of structurally similar com- structure C). pounds, a rough correlation with other measures of basicity was found. When exceptions were noted, they were attribut- ed to differences in p versus s basicity. Stability constants became the primary method of studying p complexes,[3] but a more specific measure of p basicity in condensed phases did not develop. Scheme 1. Structures of hydrogen-bonded alkenes (A), alkynes (B), and [a] Prof. E. S. Stoyanov, I. V. Stoyanova, Prof. C. A. Reed benzene (C). Department of Chemistry University of California Riverside, Riverside, California 92521 (USA) Stronger interactions with unsaturated hydrocarbons Fax : (+1)951-827-2027 E-mail: [email protected] occur when the complex involves positively charged proton [email protected] donors, such as the N+ÀH bond of the ammonium ion.[8] 7880 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2008, 14, 7880 – 7891 FULL PAPER þ À þ À The addition of this electrostatic effect increases the binding Oct3NH ÁÁÁfF20 gþnL Ð½Oct3NH ÁÁÁnLfF20 g À1 ð1Þ enthalpy of the complex by several kcalmol such that DH Contact IP Solvated IP + À1 [9] for the gas phase NH4 ···C6H6 complex is 19.3 kcalmol . Calculations indicate that the energetics are dominated by We have recently shown that nNH bands in the IR spectra [10] + the charge/quadrupole interaction and do not significantly of the Oct3NH cation in contact IPs of the type ACHTUNGRE + À À distort the p-electron distribution. [Oct3NH ]A (A = anion) decrease in frequency as a func- The T-shaped structure of the 2-butyne·HCl complex tion of increasing hydrogen-bond-acceptor strength of the [11] + [28] (Scheme 1 structure B) and the p-face structure of NH4 anion, leading to a sensitive basicity scale for anions. The [12] ···C6H6 (Scheme 1 structure C) have been confirmed in same concept is now applied to neutral molecules. In addi- + the solid state by X-ray crystallography. In the H3O ·3C6H6 tion to changes in the frequency of the nNH band, its inten- cation,[13] the OÀH bonds are directed more closely to sity, band width, and shape give further information about + carbon atoms than to the centroid of benzene. More gener- hydrogen bonding in the Oct3NH ···L interaction. ally, the XÀH···p hydrogen-bonding interaction has become We anticipated that these two measures of the basicity of widely recognized as a weak but prevalent intermolecular L would give somewhat different rankings. They measure interaction in small molecule crystallography.[14–19] The range different properties: nNH frequencies give a measure of the of acceptors now includes nitrogen heterocycles[20] and p- NH +···L interaction strength, whereas stability constants are ligand transition-metal complexes[18] and these types of in- a composite of the interaction strength and the differences teractions are beginning to be utilized in crystal engineer- in solvation energies of all reactants and products. Neverthe- ing.[21] XÀH···p interactions have also been identified in pro- less, we did tend to choose mostly six-membered ring com- teins[19, 22,23] and NÀH···p interactions can be sufficiently pounds for this study so that solvation energy differences strong to affect secondary (folding) structure.[24] between different compounds (or classes of compounds) Estimations of the strengths of XÀH···p interactions have were minimized. We also anticipated that these rankings typically relied on the determination of the thermodynamics would not correlate with existing measures of the basicity of of complexation. This can be informative in the gas phase, unsaturated hydrocarbons, for example, gas-phase ionization but in condensed phases the difficulty of separating the in- energies, complexation enthalpies, and proton affinities[29, 30] trinsic strength of the hydrogen-bonding interaction from or condensed-phase equilibrium constants[3] and nucleophi- the contribution of solvation energies to the stability con- licities.[25] These scales are based on different thermodynam- stant has thwarted the development of useful scales of p ba- ic quantities or physical phenomena that do not readily lend sicity. Mayr and co-workers have recently developed a nu- themselves to quantitative connections between gas- and cleophilicity scale for p bases based on extensive kinetic condensed-phase data. The XÀH···p interaction under study data for reactions with electrophiles.[25, 26] In favorable cases, is a weak, localized effect, whereas the energetics of com- distance information from crystallography might be utilized plete protonation (s basicity) involve larger, more global in a comparative manner,[27] but without the precise location molecular phenomena. of the hydrogen atom, typically achieved only in neutron A specific point of interest is the possible existence of bi- diffraction studies, this method is limited. furcated p hydrogen bonds of the type XÀH+ ···2L, which In the present work, with the idea of developing a ranking are formed by a positively charged cation with two unsatu- system of interaction strength, we use IR spectroscopy to rated hydrocarbons (L). The formation of bi- and even tri- explore the NÀH···p interaction of the trioctylammonium furcated hydrogen bonds has occasionally been observed by ion with alkynes, alkenes, and arenes upon reaction with ion X-ray crystallography for trialkylammonium cations, when + ACHTUNGRE À pairs (IPs) of the type [Oct3NH ]{B(C6F5)4 } in CCl4. they interact with O or N-heteroatoms, such as the + [31,32] Carbon tetrachloride is a weakly solvating, low dielectric (C2H5)3NH cation with three hydroxyl oxygen atoms. solvent that favors contact IP formation. Of commercially Sometimes one of the acceptors of a bifurcated hydrogen available anions, the perfluorinated tetraphenylborate ion, bond is an oxygen atom, the other an alkyne[19, 33,34] or ACHTUNGRE À À [24,35] B(C6F5)4 (abbreviated to {F20 }), is the least interacting to- arene. Bi- or multifurcated hydrogen bonds are fre- + [36] wards the Oct3NH ion, as judged by nNH stretching fre- quently deduced from protein crystallography. The solu- quencies in contact IPs.[28] This allows weak acceptors (L), tion-phase existence of compounds with bifurcated p hydro- such as unsaturated hydrocarbons, to compete with the gen bonds involving

Load more