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 thermodynamic, molecular mechanics, and crystallographic study

Robert D Hancock, Alvaro S de Sousa, George B Walton, Joseph H Reibenspies
Inorganic Chemistry 2007 May 28, 46 (11): 4749-57
The effect on metal-ion selectivity of the use of cyclohexenyl bridges in ligands in place of ethylene bridges is examined (selectivity is defined as the difference in log K1 for one metal ion relative to that of another with the same 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 of 1: monoclinic, Pn space group, a=11.627(2) A, b=7.8950(10) A, c=12.737(8) A, beta=98.15(3) degrees, Z=2, R=0.0524. Characteristics of 2: orthorhombic, Pbca space group, a=21.815(16) A, b=8.525(7) A, c=25.404(14) A, Z=8, R=0.0821. Structure 1 has the Cu(II) atom coordinated in the plane of the ligand to the two N donors and two O donors, with a long bond to an axially coordinated water molecule. 2 has three N donors, and one hydroxyl O donor from the ligand is coordinated in the plane around the Cu(II) atom, with the second hydroxyl O donor of the ligand occupying an axial site with a long Cu-O bond. The salient feature of both structures is the short H-H nonbonded distance between H atoms on the cyclohexenyl bridges and H atoms on the ethylene bridges 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 a strong shift in selectivity toward smaller metal ions compared with the analogous ligands, where ethylene bridges are present in place of the cyclohexenyl bridges of the ligands studied here. Molecular mechanics (MM) calculations are used to analyze these changes in selectivity. These calculations show that the short H-H contacts become shorter with increasing metal-ion size, which is suggested as the cause of the shift in the selectivity of ligands in favor of smaller metal ions when ethylene bridges are replaced with cyclohexenyl bridges. MM calculations are also used to rationalize, in terms of short H-H contacts, the fact that when the chelate ring contains two neutral O donors, more stable complexes result with cis placement of the donor atoms on the cyclohexenyl bridge, but with two N donors, trans placement of the donor atoms results in more stable complexes.

Full Text Links

Find Full Text Links for this Article


You are not logged in. Sign Up or Log In to join the discussion.

Related Papers

Remove bar
Read by QxMD icon Read

Save your favorite articles in one place with a free QxMD account.


Search Tips

Use Boolean operators: AND/OR

diabetic AND foot
diabetes OR diabetic

Exclude a word using the 'minus' sign

Virchow -triad

Use Parentheses

water AND (cup OR glass)

Add an asterisk (*) at end of a word to include word stems

Neuro* will search for Neurology, Neuroscientist, Neurological, and so on

Use quotes to search for an exact phrase

"primary prevention of cancer"
(heart or cardiac or cardio*) AND arrest -"American Heart Association"