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

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• 作者：Joanna M. Hamilton ; Michael J. Anhorn ; Karen A. Oscarson ; Joseph H. Reibenspies ; Robert D. Hancock
• 刊名：Inorganic Chemistry
• 出版年：2011
• 出版时间：April 4, 2011
• 年：2011
• 卷：50
• 期：7
• 页码：2764-2770
• 全文大小：968K
• 年卷期：v.50,no.7(April 4, 2011)
• ISSN：1520-510X

文摘

Some metal-ion-complexing properties of the ligand 2,2鈥?6鈥?2鈥测€?terpyridyl (terpy) in aqueous solution are determined by following the 蟺鈭捪€* transitions of 2 脳 10^鈭? M terpy by UV鈭抳isible 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 (渭) = 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 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 脳 10^鈭? M free terpy at pH >5.0 with solutions of the metal ion. Log K₁(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 蟺鈭捪€* transitions of free terpy, which was attributed to complex formation affecting ligand vibrations, which in the free ligand are coupled to the 蟺鈭捪€* 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]路CH₃CH₂OH路H₂O (1) is reported as follows: triclinic, P1̅, a = 8.644(3) 脜, b = 9.840(3) 脜, c = 20.162(6) 脜, 伪 = 97.355(5)掳, 尾 = 97.100(5)掳, 纬 = 98.606(5)掳, V = 1663.8(9) 脜³, Z = 4, and final R = 0.0319. The two Ni鈭扤 bonds to the central N donors of the terpy ligands in 1 average 1.990(2) 脜, while the four peripheral Ni鈭扤 bonds average 2.107(10) 脜. This difference in the M鈭扤 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.