Search

Metal ion complexing properties of the ligand PDAM (1,10-phenanthroline-2,9-dicarboxamide) are reported in relation to its possible use as a functional group for solvent extractants in the separation of Am(III) from Ln(III) (lanthanide) ions. PDAM is only slightly water soluble, but variation of the intense π–π* transitions in the UV spectrum of 2×10−5 M PDAM solutions as a function of pH or metal ion concentration allowed for the determination of the protonation constant (pK) and logK 1 values with metal ions. The pK of PDAM is 0.6±0.1 in 1.0 M NaClO4, the lowest for any 1,10-phenanthroline (phen) derivative (in contrast, pK phen=5.1), which is attributed to the electron-withdrawing properties of the amide substituents of PDAM. The weak proton basicity of PDAM may be an important factor in its use as the functional group of a solvent extractant from acidic solutions. The formation constants are determined by UV-Visible spectroscopy for the Ln(III) ions from La(III) to Lu(III) (excluding Pm(III)), as well as for Y(III), Sc(III), Th(IV), and the UO2 2+ cation in 0.1 M NaClO4 at 25 °C. The logK 1 values for the Ln(III) ions show only small changes from La(III) to Lu(III) (both have logK 1 = 3.80). The amide O-donors (oxygen donors) of the amide groups of PDAM appear to cause considerable stabilization of the complexes of PDAM as compared to those of phen, consistent with the idea that the neutral O-donor is a strong Lewis base towards large metal ions such as the Ln(III) ions. A reviewer has pointed out that the amide groups would also stabilize the complexes of PDAM by virtue of the chelate effect, in that PDAM is tetradentate, while phen is only bidentate. The small change in complex stability for PDAM complexes in passing from La(III) to Lu(III) is rationalized in terms of the idea that neutral O-donors stabilize the complexes of the large La(III) ion more than the smaller Lu(III) ion, offsetting the greater affinity of Lu(III) than La(III) for N-donor ligands. The complexes of Th(IV), Sc(III), and the UO2 2+ cation with PDAM have logK 1 values slightly higher than those of the Ln(III) cations, while Y(III) forms a slightly less stable complex. The best-fit size of metal ion for coordination with PDAM is analyzed using molecular mechanics calculations. A fluorescence study shows that the free ligand PDAM fluoresces very strongly, but that the Ln(III) cations quench the fluorescence of PDAM rather than produce a chelation enhanced fluorescence (CHEF) effect as Ln(III) ions do with other phen-based tetradentate ligands. Possible reasons for the lack of a CHEF effect with PDAM are discussed.

The affinity of In(III) for N-donor ligands was investigated by differential pulse voltammetry, DFT calculations, and crystallography. The structure of [In(tpen)(CH3COO)](ClO4)2 ・ 0.5H2O (1) is reported (tpen = N,N,N´ ,N´-tetrakis(2-pyridylmethyl)ethylenediamine): Monoclinic, P21/n, a = 8.687(4), b = 7.767(8), c = 20.432(10) Å , β = 93.372(8)°, Z = 4, R = 0.0518. The In(III) center is 7-coordinate, with six In-N bonds to the tpen ligand in the range 2.306 - 2.410 Å, and a unidentate acetate group with In-O = 2.247 Å. The formation constants of In3+ in 0.1 M NaNO3+ at 25 °C are (M = In(III), L = ligand, H = proton): L = triethylenetetramine, logβ (MLH2) = 25.3±}0.3, logK1 = 14.43±}0.09, and logβ (ML(OH)2) = 27.7±}0.1; tetraethylenepentamine, logβ (MLH) = 20.8±}0.2, and ML (logβ (ML) = 20.1±}0.3); diglycolic acid, (logβ (MLH) = 8.06±}0.06), logK1 = 6.02±} 0.06, logβ2 = 9.40±}0.08; tpen, logK1 = 17.71±}0.07; N,N´-bis(2-pyridylmethyl)ethylenediamine, logK1 = 14.69±}0.05; 1,10-phenanthroline, logK1 = 6.81±}0.07, logK2 = 6.44±}0.07, logK3 = 6.20±}0.08. Correlations are shown between the determined formation constants for the polyamines and logK1(NH3) values for a wide variety of metal ions. For M(II) ions, the log K1(NH3) values are experimental data, but for M(III) ions the data are predicted by an empirical dual-basicity equation, including logK1(NH3) = 4.0 for In(III). DFT calculations are used to obtain ΔE for the reaction [M(H2O)6]n+ + NH3 ⇆[M(H2O)5NH3]n+ + H2O for M(II) through M(IV) ions in water, represented as a structureless medium with the dielectric constant of water. Correlations are found that support the predicted value of logK1(NH3) for In(III) of 4.0. The nature of the intercepts on such correlations are discussed.

The coordinating properties of structurally reinforced macrocyclic ligands are discussed. The ligand 6-1 2-aneN4 (1,4,7,1 O-tetraazabicyclo(8.2.2)tetradecane) has a very small cavity, and is able to compress the low-spin Ni(ll) Ion. In spite of the resulting high steric strain in (Ni(B-12-aneN,))2t, the complex is of high stability, because the free ligand itself is also of very high steric strain. In complexes of Cu(ll) with unreinforced macrocycles with cavities that are too large for the Cu(ll), the CU-N bonds are not stretched, but rather the coordination geometry of the Cu(ll) is distorted toward tetrahedral, and normal CU-N bonds are found. In contrast, a large cavity reinforced macrocycle such as NE-3,3-HP (1 1-methyl-1 l-nitro-1,5,9,13-tetraazabicyclo-(l1.3.2)tetradecane) is able to stretch the Cu-N bonds out to 2.09 A, since the homopiperazine reinforcing bridge prevents buckling of the ligand. However, with further increase in macrocyclic ring size, buckling of the ligand and tetrahedral distortion of the Cu(ll) occurs, showing the need for even more rigid reinforcing groups. The synthesis and properties of ligands containing the bispidine group, which has an adamantane-like structure, as a reinforcing group, is discussed.

The metal ion size selectivity of the oxygen— and nitrogen— donor macrocycles is examined. It is shown that the presence of the neutral oxygen donor in ligands leads to stronger complexation of large metal ions, with less favourable complexation of small, irrespective of whether the oxygen donor is part of a macrocyclic ring or not. Molecular Mechanics calculations indicate that the size—dependence of the complexing of metal ions by the neutral oxygen donors is controlled by a balance between the steric strain produced by the group bearing the oxygen donor, and its inductive effects. For tetraazamacrocycles the stability is controlled by the size of the chelate ring formed on complex—formation. Larger chelate rings lead to greater complex stabilisation for small metal ions, while larger metal ions show progressively greater complex destabilisation with larger chelate rings. This apparent paradox is also examined using molecular mechanics calculations. The use of neutral oxygen donors and chelate ring size to control metal ion size selectivity in ligand design is discussed.