Published: March 02, 2011 r2011 American Chemical Society 2764 dx.doi.org/10.1021/ic101742x | Inorg. Chem. 2011, 50, 2764–2770 ARTICLE pubs.acs.org/IC Complexation of Metal Ions, Including Alkali-Earth and Lanthanide(III) Ions, in Aqueous Solution by the Ligand 2,2 0 ,6 0 ,2 00 -Terpyridyl Joanna M. Hamilton, † Michael J. Anhorn, † Karen A. Oscarson, † Joseph H. Reibenspies, ‡ and Robert D. Hancock* ,† † Department of Chemistry and Biochemistry, University of North Carolina at Wilmington, Wilmington, North Carolina 28403, United States ‡ Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States b S Supporting Information ’ INTRODUCTION 2,2 0 ,6 0 ,2 00 -Terpyridyl (terpy) and terpy-like ligands (Figure 1) have become of considerable interest in relation to the selective separation of Am III and Cm III from Ln III (Ln = lanthanide) ions in the treatment of nuclear waste. 1 Musikas 2 pioneered the use of such ligands with studies on the selective extraction of Am III from Ln III ions by N-donor ligands. Other N-donor ligands of interest in such separations are BTP, 3 TPEN, 4 4,7-diphenylphen, 5 as well as TPTZ. 2,6-9 The use 2 of N-donor ligands for the proposed separation of Am III and Cm III from Ln III ions arises because the main difference in the ligand affinity between the similarly sized An III (An = actinide) and Ln III ions is due to the fact that An ions are slightly softer in the hard and soft acids and bases (HSAB) sense than the analogous Ln ions. 2,10,11 Thus, the N donors of BTP or TPTZ, for example, are softer than the usual O donors used as solvent extractants for Ln III or An III ions such as tributylphosphate. The greater affinity of Am III , and some other An ions of interest, for N-donor ligands is supported by density functional theory (DFT) calculations that suggest 12,13 the fol- lowing log K 1 (formation constant) values for NH 3 complexes in aqueous solution: Metal ion : La 3þ Gd 3þ Lu 3þ Am 3þ Y 3þ Sc 3þ Th 4þ Pu 4þ UO 2 2þ log K 1 ðNH 3 Þ : 0:3 0:7 1:0 2:3 0:1 0:9 0:1 2:7 2:0 Ligands containing pyridyl groups, such as 2,2 0 -bipyridyl (bipy), have been extensively studied, as evidenced by 2003 structures for complexes of bipy with metal ions reported in the Cambridge Structural Database (CSD) 14 and 84 papers reported on the thermodynamic stabilities of complexes of bipy with metal ions in aqueous solution. 15 Although 873 structures of terpy complexes Received: August 26, 2010 ABSTRACT: Some metal-ion-complexing properties of the ligand 2,2 0 ,6 0 ,2 00 - terpyridyl (terpy) in aqueous solution are determined by following the π-π* transitions of 2  10 -5 M terpy by UV-visible spectroscopy. It is found that terpy forms precipitates when present as the neutral ligand above pH ∼5, in the presence of electrolytes such as NaClO 4 or NaCl added to control the ionic strength, as evidenced by large light-scattering peaks. The protonation constants of terpy are thus determined at the ionic strength (μ) = 0 to avoid precipitation and found to be 4.32(3) and 3.27(3). The log K 1 values were determined for terpy with alkali-earth metal ions Mg II , Ca II , Sr II , and Ba II and Ln III (Ln = lanthanide) ions La III , Gd III , and Lu III by titration of 2  10 -5 M free terpy at pH >5.0 with solutions of the metal ion. Log K 1 (terpy) was determined for Zn II , Cd II , and Pb II by following the competition between the metal ions and protons as a function of the pH. Complex formation for all of these metal ions was accompanied by marked sharpening of the broad π-π* transitions of free terpy, which was attributed to complex formation affecting ligand vibrations, which in the free ligand are coupled to the π-π* transitions and thus broaden them. It is shown that log K 1 (terpy) for a wide variety of metal ions correlates well with log K 1 (NH 3 ) values for the metal ions. The latter include both experimental log K 1 (NH 3 ) values and log K 1 (NH 3 ) values predicted previously by density functional theory calculation. The structure of [Ni(terpy) 2 ][Ni(CN) 4 ] 3 CH 3 CH 2 OH 3 H 2 O(1) is reported as follows: triclinic, P 1, a = 8.644(3) Å, b = 9.840(3) Å, c = 20.162(6) Å, R = 97.355(5)°, β = 97.100(5)°, γ = 98.606(5)°, V = 1663.8(9) Å 3 , Z = 4, and final R = 0.0319. The two Ni-N bonds to the central N donors of the terpy ligands in 1 average 1.990(2) Å, while the four peripheral Ni-N bonds average 2.107(10) Å. This difference in the M-N bond length for terpy complexes is typical of the complexes of smaller metal ions, while for larger metal ions, the difference is reversed. The significance of the metal-ion size dependence of the selectivity of polypyridyl ligands, and the greater rigidity of ligands based on aromatic groups such as pyridyl groups, is discussed.