Dinuclear calcium complex with weakly NH•••O hydrogen-bonded sulfonate ligands

The novel intramolecularly NH . . .O hydrogen-bonded Ca(II)-aryl sulfonate complex, [Ca-2(SO3-2-t-BuCONHC6H4)(2)-(H2O)(4)](n)(2-t-BUCONHC6H4SO3)(2n) (1) sulfonate anion, (HNEt3)(SO3-2-t-BuCONHC6H4) (2a), (PPh4)(SO3-2-tBuCONHC(6)H(4)) (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 H-1 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 H-1 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.