Complexation of Metal Ions, Including Alkali-Earth and Lanthanide(III) Ions, in Aqueous Solution by the Ligand 2,2′,6′,2′′-Terpyridyl

Some metal-ion-complexing properties of the ligand 2,2′,6′,2′′-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 NaClO4 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 K1 values were determined for terpy with alkali-earth metal ions MgII, CaII, SrII, and BaII and LnIII (Ln = lanthanide) ions LaIII, GdIII, and LuIII by titration of 2 × 10−5 M free terpy at pH >5.0 with solutions of the metal ion. Log K1(terpy) was determined for ZnII, CdII, and PbII 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 K1(terpy) for a wide variety of metal ions correlates well with log K1(NH3) values for the metal ions. The latter include both experimental log K1(NH3) values and log K1(NH3) values predicted previously by density functional theory calculation. The structure of [Ni(terpy)2][Ni(CN)4]·CH3CH2OH·H2O (1) is reported as follows: triclinic, P1̅, a = 8.644(3) Å, b = 9.840(3) Å, c = 20.162(6) Å, α = 97.355(5)°, β = 97.100(5)°, γ = 98.606(5)°, V = 1663.8(9) Å3, 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.