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

Abstract : 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 s-1 is about ten times higher than that of [Ln(DTPA)]2- (DTPA = diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid). The rotational correlation time, τRO298 = 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.
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Submitted on : Wednesday, August 29, 2012 - 3:25:08 PM
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G. Tircso, F. K. Kalman, R. Pal, I. Banyai, T. R. Varga, et al.. Lanthanide Complexes Formed with the Tri- and Tetraacetate Derivatives of Bis(aminomethyl)phosphinic Acid: Equilibrium, Kinetic and NMR Spectroscopic Studies. European Journal of Inorganic Chemistry, Wiley-VCH Verlag, 2012, 2012 (12), pp.2062-2073. ⟨hal-00726292⟩

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