Bis[1,3-di(4-piperidinium)propane] di-hydrogensulfate mono-sulfate trihydrate: An organic molten salt containing simultaneous acceptor-donor hydrogen bonding and Brönsted acid for homogeneous catalysis

Abstract

The simultaneous presence of sulfate and hydrogen sulfate in the crystal structure of an organic molten salt was demonstrated by single-crystal X-ray diffraction. The plenty of intra-hydrogen bonding between the dication of the protonated 1,3-di(4-piperidinyl)propane (TMDP), anions of sulfate and hydrogen sulfate, and crystalline water, as well as the existence of Brönsted acid of [HSO4]− gives a strong ionic network. Spectroscopic and single-crystal studies reveal that the origin of superior binding for organic molten salt is correlated with the formation of strong hydrogen bonds between organic dications, sulfate and hydrogen sulfate anions, and crystalline waters. Though FTIR spectroscopy is a useful tool to prove the presence of sulfate or hydrogen sulfate, the existence of simultaneous [HSO4]− and [SO4]2− cannot be differentiated from the presence of [HSO4]− by the FTIR spectrum. Two protonated piperidinyl rings were in the chair conformation. The tetrahedral geometry was observed around sulfur atoms in sulfate and hydrogen sulfate ions. This salt is another sample of unique organic molten salts in the sense that both sulfate and hydrogen sulfate anions are simultaneously linked through hydrogen bonds with the protonated piperidinyl rings of TMDP in one unit cell; nevertheless, there is no direct hydrogen bond between hydrogen sulfate and sulfate anions. Its unit cell contains two protonated TMDP, one sulfate, two hydrogen sulfate, and three crystalline waters. The crystal structure is stabilized by classical N[sbnd]H⋯O and O[sbnd]H⋯O hydrogen bonds, generating a strong three-dimensional network. In addition, the catalytic activity of the organic salt was demonstrated by preparing various 3,4,5-trisubstituted furan-2-one using a catalytic amount of the organic salt under mild and greener conditions owing to the existence of donor-acceptor hydrogen bonding sites and the presence of Brönsted acid anion.