Metal-Ion Selectivity Produced by C-Alkyl Substituents on the Bridges of Chelating Ligands: The Importance of Short H-H Nonbonded van der Waals Contacts in Controlling Metal-Ion Selectivity. A Thermod
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- 作者：Robert D. Hancock ; Alvaro S. de Sousa ; George B. Walton ; Joseph H. Reibenspies
- 刊名：Inorganic Chemistry
- 出版年：2007
- 出版时间：May 28, 2007
- 年：2007
- 卷：46
- 期：11
- 页码：4749 - 4757
- 全文大小：211K
- 年卷期：v.46,no.11(May 28, 2007)
- ISSN：1520-510X
文摘
The effect on metal-ion selectivity of the use of cyclohexenyl bridges in ligands in place of ethylene bridges isexamined (selectivity is defined as the difference in log K1 for one metal ion relative to that of another with thesame ligand). The syntheses of N,N'-bis(2-hydroxycyclohexyl)ethane-1,2-diamine (Cy2-en), N,N'-bis(2-hydroxycyclohexyl)propane-1,3-diamine (Cy2-tn), and 1,7-bis(2-hydroxycyclohexyl)-1,4,7-triazaheptane (Cy2-dien) are reported.The crystal structures of [Cu(Cy2-tn)(H2O)](ClO4)2 (1) and [Cu(Cy2-dien)](ClO4)2 (2) are reported. Characteristics of1: monoclinic, Pn space group, a = 11.627(2) Å, b = 7.8950(10) Å, c = 12.737(8) Å, 
hars/beta2.gif" BORDER=0 ALIGN="middle"> = 98.15(3)
, Z = 2, R =0.0524. Characteristics of 2: orthorhombic, Pbca space group, a = 21.815(16) Å, b = 8.525(7) Å, c = 25.404(14)Å, Z = 8, R = 0.0821. Structure 1 has the Cu(II) atom coordinated in the plane of the ligand to the two N donorsand two O donors, with a long bond to an axially coordinated water molecule. 2 has three N donors, and onehydroxyl O donor from the ligand is coordinated in the plane around the Cu(II) atom, with the second hydroxyl Odonor of the ligand occupying an axial site with a long Cu-O bond. The salient feature of both structures is theshort H-H nonbonded distance between H atoms on the cyclohexenyl bridges and H atoms on the ethylenebridges of the ligand. These short contacts are important in explaining the metal-ion selectivities of these ligands.Formation constants, determined by glass-electrode potentiometry, for the Cy2-en (Cu(II), Ni(II), Zn(II), Cd(II), Pb(II)),Cy2-dien (Cu(II), Zn(II), Cd(II), Pb(II)), and Cy2-tn (Cu(II), Zn(II), Cd(II)) complexes are reported. These all show astrong shift in selectivity toward smaller metal ions compared with the analogous ligands, where ethylene bridgesare present in place of the cyclohexenyl bridges of the ligands studied here. Molecular mechanics (MM) calculationsare used to analyze these changes in selectivity. These calculations show that the short H-H contacts becomeshorter with increasing metal-ion size, which is suggested as the cause of the shift in the selectivity of ligands infavor of smaller metal ions when ethylene bridges are replaced with cyclohexenyl bridges. MM calculations arealso used to rationalize, in terms of short H-H contacts, the fact that when the chelate ring contains two neutralO donors, more stable complexes result with cis placement of the donor atoms on the cyclohexenyl bridge, butwith two N donors, trans placement of the donor atoms results in more stable complexes.
hars/beta2.gif" BORDER=0 ALIGN="middle"> = 98.15(3), Z = 2, R =0.0524. Characteristics of 2: orthorhombic, Pbca space group, a = 21.815(16) Å, b = 8.525(7) Å, c = 25.404(14)Å, Z = 8, R = 0.0821. Structure 1 has the Cu(II) atom coordinated in the plane of the ligand to the two N donorsand two O donors, with a long bond to an axially coordinated water molecule. 2 has three N donors, and onehydroxyl O donor from the ligand is coordinated in the plane around the Cu(II) atom, with the second hydroxyl Odonor of the ligand occupying an axial site with a long Cu-O bond. The salient feature of both structures is theshort H-H nonbonded distance between H atoms on the cyclohexenyl bridges and H atoms on the ethylenebridges of the ligand. These short contacts are important in explaining the metal-ion selectivities of these ligands.Formation constants, determined by glass-electrode potentiometry, for the Cy2-en (Cu(II), Ni(II), Zn(II), Cd(II), Pb(II)),Cy2-dien (Cu(II), Zn(II), Cd(II), Pb(II)), and Cy2-tn (Cu(II), Zn(II), Cd(II)) complexes are reported. These all show astrong shift in selectivity toward smaller metal ions compared with the analogous ligands, where ethylene bridgesare present in place of the cyclohexenyl bridges of the ligands studied here. Molecular mechanics (MM) calculationsare used to analyze these changes in selectivity. These calculations show that the short H-H contacts becomeshorter with increasing metal-ion size, which is suggested as the cause of the shift in the selectivity of ligands infavor of smaller metal ions when ethylene bridges are replaced with cyclohexenyl bridges. MM calculations arealso used to rationalize, in terms of short H-H contacts, the fact that when the chelate ring contains two neutralO donors, more stable complexes result with cis placement of the donor atoms on the cyclohexenyl bridge, butwith two N donors, trans placement of the donor atoms results in more stable complexes.