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.