Importance of aliphatic C-H hydrogen bonding on anion recognition

We have designed and synthesised new anion receptors 1 and 2, both of their C-H groups were at the positions to carbonyl groups and further polarised by the attached polarising substituents. This enabled us to study hydrogen bonding donor ability of C-H bonds. The polarising substituents are electron withdrawing cyano group for host 1, while charged pyridinium group for host 2. As expected from charge effects, host 2 shows roughly an order of magnitude higher binding constants against various anion guests than those of receptor 1. Since the magnitude of polarisation change should be greatest for C-H group among various hydrogen bonding groups, this indicates the importance of C-H hydrogen bonding. In contrast, the relative order of binding constants was the same for both host 1 and 2. The order of association constants was found to be (CH3)(2)POO- > CH3COO- > C6H5COO- > Cl- > Br-. DFT calculation results were in good agreement with experimental binding constants and confirmed the importance of charged group substitution. In addition, receptor 1 showed the highest association constant for dimethyl phosphinate, which is implicated in many metabolic diseases.