9Be and 31P NMR analyses on Be2+ complexation with cyclo-tri-μ-imidotriphosphate anions in aqueous solution

Abstract

Be 2+ ion complexation with the cyclo -tri-μ-imidotriphosphate ligand, cP 3 O 6 (NH) 3 , in aqueous solution has been investigated by 9 Be and 31 P NMR techniques in order to clarify the microscopic coordination structures of the complexes. The NMR spectra indicate the formation of ML, ML 2 , together with M 2 L complexes (L = cP 3 O 6 (NH) 3 ), whose formation constants have been determined. The formation of these complexes by coordination with the nitrogen atoms of the cyclic framework in the cP 3 O 6 (NH) 3 molecule has been excluded. ► Stability constants of ML, ML 2 and M 2 L (L = cP 3 O 6 (NH) 3 ) complexes have been evaluated from the peak area ratio of the deconvoluted 9 Be NMR spectra. ► Formation of complexes coordinating with the nitrogen atoms of the cyclic framework in the cP 3 O 6 (NH) 3 molecule has been excluded. ► The coexistence of the complexes is expected to produce a Be 2+ ion bridged polymerized structure. Microscopic information on the complexation of Be 2+ with cyclo -tri-μ-imidotriphosphate anions in aqueous solution has been gained by both 9 Be and 31 P NMR techniques at −2.3 °C. Separate NMR signals corresponding to free and complexed species have been observed in both spectra. Based on an empirical additivity rule, i.e. , proportionality observed between the 9 Be NMR chemical shift values and the number of coordinating atoms of ligand molecules, the 9 Be NMR spectra have been deconvoluted. By precise equilibrium analyses, the formation of [BeX(H 2 O) 3 ] + and [BeX 2 (H 2 O) 2 ] 0 (X = non-bridging oxygen donor as a coordination atom in the phosphate groups) has been verified, and the formation of complexes coordinating with the nitrogen atoms of the cyclic framework in the ligand molecule has been excluded. Instead, the formation of one-to-one (ML) complexes, one-to-two (ML 2 ), together with two-to-one (M 2 L) complexes (L = cP 3 O 6 (NH) 3 ) has been disclosed, the stability constants of which have been evaluated as log K ML = 3.87 ± 0.03 (mol dm −3 ) −1 , log K ML 2 = 2.43 ± 0.03 (mol dm −3 ) −2 and log K M 2 L = 1.30 ± 0.02 (mol dm −3 ) −2 , respectively. 31 P NMR spectra measured concurrently have verified the formation of the complexes estimated by the 9 Be NMR measurement. Intrinsic 31 P NMR chemical shift values of the phosphorus atoms belonging to ligand molecules complexed with Be 2+ , together with the 31 P– 31 P spin–spin coupling constants have been determined.