Inclusion of potassium 4,4′-biphenyldicarboxylate into β-cyclodextrin: the design and synthesis of an organic secondary building unit

Abstract

The paper describes a design strategy for the preparation of a novel inclusion compound (potassium 4,4′-biphenyldicarboxylate⊂β-CD) suitable for use in supramolecular chemistry. In the first step the chemical feasibility of such an inclusion compound was evaluated by DFT calculations. Preliminary results revealed that two types of adducts are energetically favourable: 1 ∶ 1 and 2 ∶ 1 (host ∶ guest). The stability in aqueous solution of the prepared inclusion compounds, a strict requirement for their use as Secondary Building Units in supramolecular chemistry, was investigated by means of liquid NMR (Job plot) and computational resolution of the equilibrium equations. As predicted by the theoretical calculations, it was found that two types of inclusion compounds were formed. The 1 ∶ 1 adduct has a large formation constant. Synthesis led to the preparation of two unusual crystalline products depending on the pH. Compound 1 (formed at pH = 7.5) is truly unprecedented and comprises a 1 ∶ 1 inclusion compound with a large water layer. Because it is highly stable in the liquid state this novel inclusion compound is a good candidate to be employed as a rigid organic Secondary Building Units in the construction of modular coordination polymers. Compound 2 (formed at pH = 10) comprises a mixture of the same two adducts predicted by the theoretical calculations and detected by liquid NMR, and its crystal structure exhibits a rare proportion between the host and the guest (4 ∶ 3).