Comparison of the structure and dynamic properties of aniono(1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane)zinc(II) perchlorate complexes in nitromethane solutions, and the crystal and molecular structure of the chloro-complex

Abstract

Variable-temperature ¹³C n. m. r. spectroscopy in nitromethane solutions of the complex cations [ZnX(L)]⁺(X = Cl, Br, I, NCS, or ClO₄; L = 1,4,8,11 -tetramethyl-1,4,8,11 -tetra-azacyclotetradecane) demonstrates the presence of a dynamic process which is not clearly seen in their corresponding 90-MHz ¹H n.m.r. spectra. Line-shape analysis shows that for X = Cl or NCS the activation parameters are ΔH^‡= 59.3 ± 3.2 kJ mol^–1 and ΔS^‡= 15 ± 12 J K^–1 mol^–1 and ΔH^‡= 49.5 ± 1.8 kJ mol^–1 and ΔS^‡=–10 ± 10 J K^–1 mol^–1 respectively. The dynamic process is interpreted as an intramolecular rearrangement between two identical trigonal-bipyramidal structures. The crystal structure of the complex [ZnCl(L)][ClO₄] is, however, different to that proposed in solution. The complex crystallises in the monoclinic space group P2₁, Z= 2, with a= 8.230(2), b= 15.156(3), c= 8.569(2)Å, and β= 108.64(2)°. 1 923 Reflections were recorded by four-circle diffractometer and least-squares refinement produced a final R of 0.041. In the crystal the complex has square-pyramidal geometry around the zinc atom with the four nitrogen atoms of the macrocycle forming the basal plane and the zinc centrally positioned 0.57 Å above the plane [Zn–N 2.20(1)Å]. The co-ordination sphere is completed by an apical chlorine atom [Zn–Cl 2.265(4)Å]. Possible reasons for the differing solution and crystal structures are discussed.