Dinuclear Calcium Complex with Weakly NH···O Hydrogen-Bonded Sulfonate Ligands

The novel intramolecularly NH···O hydrogen-bonded Ca(II)−aryl sulfonate complex, [Ca2(SO3-2-t-BuCONHC6H4)2(H2O)4]n(2-t-BuCONHC6H4SO3)2n (1), sulfonate anion, (HNEt3)(SO3-2-t-BuCONHC6H4) (2a), (PPh4)(SO3-2-t-BuCONHC6H4) (2b), (n-Bu4N)(SO3-2-t-BuCONHC6H4) (2c), and sulfonic acid, 2-t-BuCONHC6H4SO3H (3), were
synthesized. The structures of 1, 2a, and 2b depict the presence of the formation of NH···O hydrogen bonds
between the amide NH and S−O oxygen for a series of compounds as determined by IR and 1H NMR analyses
both in the solid state and in the solution state. Thus, the NH···O hydrogen bonds with neutral amide groups are
available for investigation of the electronic state of the O- anion. The combined data from the IR and 1H NMR
spectra indicate that the sulfonic acid, sulfonate anion, and Ca(II) complex have a substantially weak intramolecular
NH···O hydrogen bond between the SO3 oxygen and amide NH. In the detailed comparison with the intense
NH···O hydrogen bonds for the carboxylate, weak NH···O hydrogen bonds for sulfonate is due to the strong
conjugation of the SO3- group with the lower nucleophilicity.