Metal-ion template effects in the synthesis of polydentate Schiff-base ligands. Part 1. Complexes of a potentially octadentate ‘N6O2’ ligand and the crystal and molecular structure of a ten-co-ordinate barium(II) complex

Abstract

The Sr²⁺ and Ba²⁺ ions are effective templates for the Schiff-base condensation of 2 mol of 2,6-diacetylpyridine with 1 mol of 3,6-diazaoctane-1,8-diamine to give the complexes [ML(ClO₄)₂] where M = Sr or Ba and L is an open-chain potentially octadentate ‘N₆O₂’ ligand. No macrocyclic products were obtained, and attempts to effect ring closure in the diketone L by reaction of [BaL(ClO₄)₂] with a range of primary diamines of varying chain length were unsuccessful. Crystals of [BaL(ClO₄)₂] are orthorhombic with a= 17.651(9), b= 13.870 (7), c= 12.710(11)Å, Z= 4, and space group Pnaa. 1 468 Reflections above background have been measured by diffractometer, of which 1 062 were used in refinement to R 0.068. The Ba²⁺ ion is ten-co-ordinate being bonded to all eight donor atoms of L and to two ClO₄^– ions. Ba–N and Ba–O bond lengths are in the range 2.81(2)–2.92(2)Å. The conformation of the ligand is such that the two planar keto-pyridyl-imine segments are inclined at 80.2° to each other forming a cleft in which the two ketonic carbon atoms are separated by 5.21 Å. It is suggested that the failure of L to ring-close may be due to a steric barrier to the entry of the difunctional nucleophile into the cleft. Metal-exchange reactions carried out on solutions of [BaL(ClO₄)₂] afforded the complexes [ML][ClO₄]₂(M = Mn^II, Fe^II, Co^II, or Ni^II) and the binuclear complex [Cu₂L(OH)(OH₂)][ClO₄]₃. Physical data show the iron(II), cobalt(II), and nickel(II) complexes to have distorted octahedral structures in which the unco-ordinated acetyl groups are twisted out of the plane of the pyridine rings. The iron(II) complex is low spin (S= 0) while the cobalt(II) complex exists in a thermally controlled (S= 2)⇌(S = 4) spin equilibrium in the temperature range 90–350 K. In the manganese (II) complex the carbonyl groups of L are co-ordinated and the metal ion appears to be eigtht-co-ordinate. An intramolecular hydroxo-bridge is suggested for the binuclear copper(II) complex which shows weak antiferromagnetic exchange.