Lanthanide Complexes Formed with the Tri- and Tetraacetate Derivatives of Bis(aminomethyl)phosphinic Acid: Equilibrium, Kinetic and NMR Spectroscopic Studies

The lanthanide(III) complexes formed with the tri- and tetraacetate derivatives of bis(aminomethyl)phosphinic acid, L1 and L2, respectively, have been studied by pH potentiometry, spectrophotometry and 1H and 17O NMR spectroscopy. L1 forms [Ln(L1)], [Ln(L1)2]4, protonated [Ln(HL1)] and Ln(H2L1)]+, and [Ln(L1)(OH)]2 hydroxido complexes. Heptadentate L2 forms [Ln(L2)]2 and protonated [Ln(HL2)] and [Ln(H2L2)] complexes in solution and it shows a strong propensity to form [Ln2(L2)]+ dinuclear complexes, which has not been observed previously. The stability constants (log?KLnL) of the complexes increase in the order [Ln(L1)] < [Ln(L2)]2 following the order of increasing number of acetate pendants attached to the bis(aminomethyl)phosphinic acid (BAP) backbone. Within the LnIII series, the log?KLnL values increase from La3+ to Gd3+ and remain practically constant for the heavier lanthanides. Despite the lower basicity, the ligands that contain a phosphinate group generally form similar (L1) or more stable (L2) Ln3+ complexes than the structurally similar N-benzylethylenediamine-N,N',N'-triacetic acid (L3) and propylenediamine-N,N,N',N'-tetraacetic acid (L4), respectively. This indicates that the hard phosphinate group may be coordinated to the Ln3+ ions in the complexes, whereas the larger negative charge of the BAP derivatives may also have an extra stabilizing effect. The kinetic inertness of [Ln(L1)] and [Ln(L2)] is lower than that of similar [Ln(EDTA)] (EDTA = ethylenediamine-N,N,N',N'-tetraacetic acid), but the rate constants that characterize the dissociation of [Ln(L2)]2 are at least two orders of magnitude lower than those obtained for [Ln(L4)]. Variable-temperature 17O transverse and longitudinal relaxation rates and NMR spectroscopic chemical shifts have been measured to assess the water exchange and rotational dynamics of [Gd(L2)]. The chemical shifts evidenced monohydration of the complex. The water exchange rate, kex298 = (2.7 +/- 0.4)+/- 107 s1 is about ten times higher than that of [Ln(DTPA)]2 (DTPA = diethylenetriamine-N,N,N',N?,N?-pentaacetic acid). The rotational correlation time, tRO298 = 270 +/- 30 ps, is long considering the small size of the chelate, which points to aggregation in aqueous solution, in accordance with the high value of the proton relaxivity measured.