Structure of the copper(II) complex of the reinforced ligand N,N′-bis(trans-2-hydroxycyclohexyl)- trans-cyclohexane-1,2-diamine and the metal-ion-size-based selectivity produced by cyclohexanediyl bridges *

Abstract

The compound N,N′-bis(2-hydroxycyclohexyl)-trans- cyclohexane-1,2-diamine (L¹) and its complex with Cu^II have been prepared. The structure of [CuL¹][ClO₄]₂ 1 was determined. The Cu has a distorted octahedral co-ordination, with the four donor atoms provided by L¹ giving Cu–N 1.985(7) and Cu–O 1.947(6) Å. The oxygens from two perchlorates lie on the axial positions with Cu–O 2.749 Å. The perchlorates are involved in a hydrogen bonding network holding layers of [CuL¹]²⁺ cations together with bonds to the hydrogens of the co-ordinated hydroxyls of the cyclohexanediyl groups and the NH hydrogens. Alternating layers of [CuL¹]²⁺ cations have the L¹ ligand as the SRS and RSR enantiomers. Difficulty in obtaining a sufficiently large data set for L¹ meant that R for this structure was not less than 0.13. The structure is thus not reported in any detail, but is sufficient to indicate the conformation of the ligand, which is used as the basis of a molecular mechanics generation of the structure. Both the copper(II) complex of L¹ and L¹ itself have only the RSR and SRS diastereomeric pair present in the structures, supporting the idea that syntheses involving reaction of cyclohexene oxide with polyamines result in considerable diastereoselectivity. The structure of 1 shows steric crowding, with van der Waals repulsion between hydrogens on adjacent cyclohexyl groups of the L¹ ligand. This crowding becomes more severe as the metal ion becomes larger, which is related to the strong preference the ligand shows for smaller metal ions. This latter effect is indicated by the formation constants (log K₁) of L¹: Cu^II, 11.50; Ni^II, 6.84; Zn^II, 4.77; Cd^II, 4.08; Pb^II, 4.80; in 0.1 mol dm^-3 NaNO₃ at 25 °C. The effect of cyclohexanediyl bridges on the stability of complexes formed by L¹ is discussed in relation to the stability of complexes of similar ligands that have simple ethylene bridges between their donor atoms.

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