Pyridine and s-triazine as building blocks of nonionic organic superbases—a density functional theory B3LYP study

Abstract

It is shown by reliable DFT B3LYP/6-311+G(2df,p)//B3LYP/6-31G* method that pyridine and s-triazine can serve as useful building blocks in tailoring neutral superbases. The former compound is used in constructing the four-fragment macrocycles substituted by a number of NMe₂ substituents placed at strategic positions on the suprastructure’s backbone. The calculated proton affinities are within the range of 270–290 kcal mol⁻¹, thus belonging to the upper part of the superbasicity ladder. It is shown that proton affinities of polysubstituted derivatives follow a simple additivity rule, which facilitates prediction and interpretation of the results in multiply substituted systems. The s-triazine motif is employed in forming manxane-like systems by using guanidine fragments or 1,1-dimethylaminoethylenes, which exhibit proton affinities spread over the lower part of the superbasicity scale (256–272 kcal mol⁻¹). The amplified basicity in both families of compounds is a consequence of the strong cationic resonance in conjugate acids supported by stabilization provided by intramolecular hydrogen bonds (IMHB). The nature of the IMHBs is examined. It is found that IMHBs in large macrocycles constructed by pyridine moieties is of a bicentric type. They are not cationic resonance supported IMHBs in the first approximation. In contrast, protonation in one of the manxane-like systems yields the IMH bonds that are stabilized by substantial cationic resonance through the hydrogen H^δ+ bridge. Consequently, the cationic resonance supported hydrogen bonds (CRSHBs) do exist in some protonated species. The basicity of pyridine macrocycles is examined in acetonitrile. It is found that their pK_a values span a range between 26–30 units meaning that they are also strong superbases in MeCN solutions compared to the threshold given by pK_a(MeCN) of DMAN (18.2 units).