9Be and ³¹P NMR analyses on the influence of imino groups on Be²âº complex stabilities of a series of cyclo-µ-imido triphosphate anions.

The complexation behaviors of Be²âº with cyclo-µ-imido triphosphate anions, cP3O9-n(NH)n(3-)n= 1, 2),have been investigated by both 9Be and ³¹P NMR techniques at -2.3 °C in order to clarify the coordination structures of the complexes. The spectra showed that cP3O9n(NH)n (n = 1, 2) ligands form ML, ML2, and M2L complexes with Be²âº ions, and the formation of complexes coordinating with nitrogen atoms of the cyclic framework in the ligand molecule has been excluded. These complexation trends are very similar to those of Be²âº-cP3O6(NH)⁻³3system, which has been reported by us. The peak deconvolution of 9BeNMR spectra made these beryllium complexes amenable to stability constant determinations. The stability constants of the complexes increase with an increase in the protonation constants of the ligands as the number of imino groups, which constitute the ligand molecules, is ascended. This increase is primarily attributable to the lower electronegativity of nitrogen atoms than oxygen atoms, which are directly bonded to central phosphorus atoms; moreover, tautomerism equilibrium in the entire of the imidopolyphosphate molecule is also responsible to the higher basicity. ³¹P NMR spectra measured concurrently have verified the formation of the complexes estimated by the 9Be NMR measurement. Intrinsic ³¹P NMR chemical shift values of the phosphorus atoms belonging to ligand molecules complexed with Be²âº cations have been determined. Not only the protonation constants but also the stability constants of all Be²âº complexes increase approximately linearly with an increase in the number of imino groups.

Stabilities of the Divalent Metal Ion Complexes of a Short-Chain Polyphosphate Anion and Its Imino Derivative.

ABSTRACT: The stability constants of ML-type complexes of the two linear triphosphate ligand anion analogues triphosphate ([Formula: see text]) and diimidotriphosphate ([Formula: see text]) were investigated thermodynamically using potentiometric titrations according to Schwarzenbach's procedure. The stability constants of the ML-type complexes of different divalent metal ions with [Formula: see text] are larger than those of the corresponding complexes with [Formula: see text] because of the greater basicity of the imino group. The order of the stability constants for the ML-type complexes follows the Irving-Williams order, indicating that only non-bridging oxygen atoms are coordinated directly to the different metal ions in both ligands, and that the imino groups cannot participate in coordination to the metal ions. In the complexation reactions of the Ca2+, Sr2+, Ba2+-[Formula: see text] and Cu2+, Zn2+, Ni2+-[Formula: see text] systems, each metal ion forms an enthalpically stable complex, and there was no suggestion of a conspicuous entropic effect based on the chelate effect. Monodentate complexes that are strongly coordinated with the ligands were therefore formed, whereas entropically stable bidentate complexes were formed in the complexation reactions of the Cu2+, Zn2+, Ni2+-[Formula: see text] and Ca2+, Ba2+, Sr2+-[Formula: see text] systems. According to the HSAB concept, hard metal cations such as Ca2+, Ba2+ and Sr2+ should bind to the harder oxygen atoms rather than the softer nitrogen atoms of the imidopolyphosphate anions, preventing direct coordination to the imino nitrogen atom.

Intrinsic 31P NMR Chemical Shifts and the Basicities of Phosphate Groups in a Short-Chain Imino Polyphosphate.

ABSTRACT: The stepwise protonation constants of two linear triphosphate ligand anions, triphosphate, [Formula: see text] and di-imidotriphosphate, [Formula: see text], were investigated by potentiometric titration, and the intrinsic chemical shifts of the stepwise protonated species of these anions were determined from the pH-dependence of the 31P NMR chemical shifts. All stepwise protonation constants of [Formula: see text]were found to be larger than those of [Formula: see text], and the 31P NMR signals due to P3O8(NH)2 always appeared at a lower magnetic field compared to the signals due to P3O10. These results indicate higher basicity of the P3O8(NH)2 ligand, because it contains two imino groups in the ligand molecule. The 31P NMR signals for the end phosphate groups appear at a lower magnetic field than those for the middle phosphate groups, indicating that the basicity of the end phosphate group is higher than that of the middle phosphate group. It can be expected that the high basicity of the P3O8(NH)2 ligand brings about the formation of high stability complexes with various metal ions. Furthermore, the only 31P NMR signal due to the middle phosphate group of P3O8(NH)2 ligand molecule clearly showed a low-field shift in the range of pH < 2.5. The reason for this peculiar low-field shift should be the change of the localization of imino protons around the nitrogen atom in P3O8(NH)2.