Lanthanide complexes formed with the tri- and tetraacetate derivatives of bis(aminomethyl)phosphinic acid: Equilibrium, kinetic and NMR spectroscopic studies

Gyula Tircsó, Ferenc K. Kálmán, Róbert Pál, István Bányai, Tamás R. Varga, Róbert Király, István Lázár, Laurent Québatte, André E. Merbach, Éva Tóth, Erno Brücher

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Abstract

The lanthanide(III) complexes formed with the tri- and tetraacetate derivatives of bis(aminomethyl)phosphinic acid, L 1 and L 2, respectively, have been studied by pH potentiometry, spectrophotometry and 1H and 17O NMR spectroscopy. L 1 forms [Ln(L 1)] -, [Ln(L 1) 2] 4-, protonated [Ln(HL 1)] and Ln(H 2L 1)] +, and [Ln(L 1)(OH)] 2- hydroxido complexes. Heptadentate L 2 forms [Ln(L 2)] 2- and protonated [Ln(HL 2)] - and [Ln(H 2L 2)] complexes in solution and it shows a strong propensity to form [Ln 2(L 2)] + dinuclear complexes, which has not been observed previously. The stability constants (log K LnL) of the complexes increase in the order [Ln(L 1)] - < [Ln(L 2)] 2- following the order of increasing number of acetate pendants attached to the bis(aminomethyl)phosphinic acid (BAP) backbone. Within the Ln III series, the log K LnL values increase from La 3+ to Gd 3+ and remain practically constant for the heavier lanthanides. Despite the lower basicity, the ligands that contain a phosphinate group generally form similar (L 1) or more stable (L 2) Ln 3+ complexes than the structurally similar N-benzylethylenediamine-N,N',N'-triacetic acid (L 3) and propylenediamine-N,N,N',N'-tetraacetic acid (L 4), respectively. This indicates that the hard phosphinate group may be coordinated to the Ln 3+ 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(L 1)] and [Ln(L 2)] 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(L 2)] 2- are at least two orders of magnitude lower than those obtained for [Ln(L 4)] -. 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(L 2)]. The chemical shifts evidenced monohydration of the complex. The water exchange rate, k ex 298 = (2.7 ± 0.4) × 10 7 s -1 is about ten times higher than that of [Ln(DTPA)] 2- (DTPA = diethylenetriamine-N,N,N',N''-pentaacetic acid). The rotational correlation time, τ RO 298 = 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. The stability constants, relaxivities and rates of the metal exchange reactions that characterize the kinetic inertness of Ln III complexes formed with tri- and tetraacetate derivatives of bis(aminomethyl)phosphinic acid (TCAP and BIMP) are reportedherein. The water exchange rate of[Gd(BIMP)] 2- was also determined.

Original languageEnglish
Pages (from-to)2062-2073
Number of pages12
JournalEuropean Journal of Inorganic Chemistry
Issue number12
DOIs
Publication statusPublished - Apr 1 2012

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Keywords

  • Imaging agents
  • Kinetics
  • Lanthanides
  • Protonation constants
  • Stability constants
  • Water exchange rate

ASJC Scopus subject areas

  • Inorganic Chemistry

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