Abstract
UV/vis absorption titrations have been used to investigate the formation of H-bonded complexes between anionic H-bond acceptors (HBAs) and neutral H-bond donors (HBDs) in organic solvents. Complexes formed by three different HBDs with 15 different anions were studied in chloroform and in acetonitrile. The data were used to determine self-consistent HBA parameters (β) for chloride, bromide, iodide, phosphate diester, acetate, benzoate, perrhenate, nitrate, triflimide, perchlorate, hexafluorophosphate, hydrogen sulfate, methyl sulfonate, triflate, and perfluorobutyl sulfonate. The results demonstrate the transferability of H-bond parameters for anions between different solvents and different HBD partners, allowing reliable prediction of anion recognition properties in other scenarios. Carboxylates are the strongest HBAs studied, with β parameters (≈ 15) that are significantly higher than those of neutral organic HBAs, and the non-coordinating anion hexafluorophosphate is the weakest acceptor, with a β parameter comparable to that of pyridine. The effects of ion pairing with the counter-cation were found to be negligible, provided small polar cations were avoided in the less polar solvent (chloroform). There is no correlation between the H-bonding properties of the anions and the pKa values of the conjugate acids.
Highlights
Molecular recognition events involving anions are fundamental to a wide range of biological processes.1 The regulation of noncovalent interactions formed in a specific but transient manner between biomolecules is widely exploited within the body to perform essential tasks and achieve function.2 Synthetic systems have extensively employed the formation of H-bonding interactions to anions as a key molecular recognition motif in supramolecular chemistry,3,4 finding applications in a wide range of processes including catalysis,5 responsive materials,6 ion extraction,7 transportation,8 and sensing.9 there are numerous factors that influence the properties of noncovalent interactions, complicating the analysis of the operational basis of complex systems
The H-bond donor (HBD) parameters span a range of values that are all near the top of the α scale, and so stable complexes are formed even with less polar anions in competitive solvents
Phosphine oxide 4 was employed as a H-bond acceptors (HBAs) to align with previous studies undertaken for neutral solutes
Summary
Molecular recognition events involving anions are fundamental to a wide range of biological processes. The regulation of noncovalent interactions formed in a specific but transient manner between biomolecules is widely exploited within the body to perform essential tasks and achieve function. Synthetic systems have extensively employed the formation of H-bonding interactions to anions as a key molecular recognition motif in supramolecular chemistry, finding applications in a wide range of processes including catalysis, responsive materials, ion extraction, transportation, and sensing. there are numerous factors that influence the properties of noncovalent interactions, complicating the analysis of the operational basis of complex systems. Abraham developed quantitative scales for the H-bond acceptor (HBA) and H-bond donor (HBD) strengths of neutral organic functional groups.10 These scales were established through analysis of experimentally determined association constants (K) for formation of H-bonded complexes in carbon tetrachloride solution.− The Hbonding properties of neutral molecules were described using eq 1. H-bond wide range descriptors of different αco2Hmapnodunβd2sH10h−a1v2eanbdeehnavme ebaeseunreudsefodrtoa construct linear free energy relationships (LFERs) to predict the behavior of neutral solutes in a number of biological and physicochemical processes.13 We have extended this approach to explicitly include the influence of solvent on solution-phase equilibria between H-bonded solutes. The influence of ion pair formation on the HBA properties of the anions has been quantified through systematic variation of the counter-cation
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