The behaviour of tricyclic fused host systems comprising seven-membered B-rings in mixed pyridines

Abstract

In this work, the selectivity behaviour of two tricyclic fused host systems with seven-membered B-rings, namely N,N′-bis(5-phenyl-5-dibenzo[a,d]cycloheptenyl)ethylenediamine (H1) and N,N′-bis(5-phenyl-10,11-dihydro-5-dibenzo[a,d]cycloheptenyl)ethylenediamine (H2), was investigated in various mixtures of pyridine (PYR) and the three C-methylated pyridine isomers (2-, 3-, and 4-MP). It was first demonstrated that H1 possessed the ability to enclathrate all four pyridines in the single guest solvent experiments while H2 was only able to form complexes with PYR and 4MP. H2 showed significantly enhanced selectivities compared with H1, consistently preferring PYR, while 2MP was the favoured guest of H1. Selectivity profiles suggested that H1 has the ability to separate mixtures of 2MP/PYR when these contain 40% 2MP (K = 11.8). H2, on the other hand, was shown to have exceptional separatory potential for PYR/2MP and PYR/3MP mixtures even when the amount of PYR in these was as low as 20%. These host compounds, therefore, are able to separate some of these pyridyl mixtures with high efficiency. SCXRD analyses on five of the six complexes prepared here demonstrated that the reason for the preferential behaviours of H1 and H2 for 2MP and PYR, respectively, was the significantly shorter hydrogen bonding interactions present between the host and guest molecules in these complexes. Thermal analyses further showed that these two complexes were more thermally stable than those with the less preferred guest compounds.