In:
European Journal of Inorganic Chemistry, Wiley, Vol. 2012, No. 12 ( 2012-04), p. 2062-2073
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 1 H and 17 O NMR spectroscopy. L 1 forms [Ln( L 1 )] – , [Ln( L 1 ) 2 ] 4– , protonated [Ln(H L 1 )] and Ln(H 2 L 1 )] + , and [Ln( L 1 )(OH)] 2– hydroxido complexes. Heptadentate L 2 forms [Ln( L 2 )] 2– and protonated [Ln(H L 2 )] – and [Ln(H 2 L 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 17 O 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″ , 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.
Type of Medium:
Online Resource
ISSN:
1434-1948
,
1099-0682
DOI:
10.1002/ejic.v2012.12
DOI:
10.1002/ejic.201101299
Language:
English
Publisher:
Wiley
Publication Date:
2012
detail.hit.zdb_id:
1475009-0
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