Structure and stability of complexes of macrocyclic ligands bearing 2-hydroxycyclohexyl groups. Structure of the copper(II) complex of 1-(2-hydroxycyclohexyl)-1,4,7,10-tetraazacyclododecane and the strontium(II) complex of 7,16-bis(2-hydroxycyclohexyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadec ane

Abstract

The compound 1-(2-hydroxycyclohexyl)-1,4,7,10-tetraazacyclododecane (L ¹ ) and its complex [CuL ¹ ][ClO ₄ ] ₂ 1 have been prepared, as well as the complex [SrL ² (H ₂ O)][NO ₃ ] ₂ 2 from the previously reported ligand L ² (L ² = D,L -7,16-bis(2-hydroxycyclohexyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctade cane). X-Ray studies of 1 and 2 gave for 1, triclinic, space group P, a = 8.632(1), b = 14.677(2), c = 17.797(4) Å, α = 86.80(2), β = 78.71(2), γ = 83.32(1)°, Z = 4, R = 0.0606, while 2 gave monoclinic, space group Cc, a = 16.809(6), b = 8.889(2), c = 21.232(6) Å, β = 101.47(2)°, Z = 4, R = 0.0314. The structure of 1 showed Cu–N and Cu–O bond lengths which were in the normal range. The structure shows some steric crowding of the co-ordinated ligand, with short H · · · H contacts between hydrogens on the cyclohexyl group and adjacent hydrogens on the macrocyclic ring. This acts to press the 2-hydroxycyclohexyl group towards the macrocyclic ring, and to have a compressive effect on the metal ion. The occurrence of 2 in space group Cc indicates spontaneous resolution of the complex into crystals with (R,R) or with (S,S) diastereomers of the ligand only. The Sr ^II is nine-co-ordinate, with a water molecule occupying a co-ordination site. An extensive hydrogen-bonding network involving hydrogens from the co-ordinated water on Sr ^II and nitrate oxygens, and hydrogens from the co-ordinated hydroxyls of the 2-hydroxycyclohexyl groups and nitrate oxygens, appears to be responsible for the spontaneous resolution. Ligands where ethylene bridges between donor atoms have been replaced by cyclohexanediyl bridges tend to show greater selectivity for smaller metal ions. This has been interpreted in terms of greater steric crowding on the outside of the ligand as the metal ion increases in size and decreases the curvature of the ligand. The structure of 2 shows six rather short H · · · H distances (2.05–2.2 Å) between hydrogens on the cyclohexyl group, and on the macrocyclic ring, which are much shorter than similar contacts in complex 1, supporting this suggestion. The protonation constants (log K) of L ¹ are 10.65, 9.51 and 4.03, while the formation constants (log K ₁ ) are 13.85 (Zn ^II ), 14.58 (Cd ^II ) and 11.40 (Pb ^II ), all in 0.1 mol dm ^-3 NaNO ₃ at 25 °C. The effect of the 2-hydroxycyclohexenyl bridge on the stability of complexes is discussed.