Trihapto–hexahapto fluxional behaviour of a macrocyclic ligand: template synthesis, proton nuclear magnetic resonance spectra, and the crystal and molecular structure of an eleven-co-ordinate barium(II) complex

Abstract

The alkaline-earth-metal ions Ca²⁺, Sr²⁺, and Ba²⁺, but not Mg²⁺, are templates for the high-yield synthesis of 1 : 1 complexes of the 20-membered macrocycle L¹ derived from the cyclic condensation of 2 mol of 2,5-diformylfuran with 2 mol of 1,3-diaminopropane. Lead(II) is also effective as a template for the macrocycle synthesis. In the case of Ba^II only, complexes having a 2 : 1 ligand : metal stoicheiometry could also be prepared by anion exchange. Crystals of [BaL¹₂(H₂O)₂][Co(NCS)₄] are monoclinic with a= 11.57(1), b= 19.99(1), c= 23.18(1)Å, β= 107.5(1)°, Z= 4, and space group P2₁/c. 4 329 Reflections above background were measured by diffractometer and refined by least-squares methods to R 0.062. The barium atom in the cation is bonded to all six heteroatoms of one macrocycle but only to three of the second macrocycle, this being severely folded so that one furan di-imine moiety is unco-ordinated. Two water molecules are also bonded to the barium atom giving an overall co-ordination number of 11. This structure is discussed in relation to those of other eleven-co-ordinate compounds. The hexahapto–trihapto co-ordination observed in the solid state is retained in solution as determined from the temperature-dependent ¹H n.m.r. spectrum of the analogous complex [BaL¹₂(MeCN)₂][BPh₄]₂ in CD₃CN. Below ca. 20 °C the resonances due to the imino-protons (HCN) and the methylene protons (CN–CH₂) adjacent to the imine group each appear as pairs of well separated lines having a 3 : 1 intensity ratio. These are attributed to slowly exchanging co-ordinated and unco-ordinated furan di-imine moieties, respectively. Increase in temperature causes coalescence of the components of each doublet into a single line. The two sets of temperature-dependent spectra are described by the same free energy of activation (ΔG^‡=ca. 62.5 kJ mol^–1) indicating that they are associated with the same dynamic process, viz. interconversion of four equivalent configurations of the complex cation. The results are discussed in relation to the sizes of the metal cations and the flexibility and cavity dimensions of this and related macrocycles.