Preparation of a Nickel(II) tetraazamacrocycle.


The first and most extensively studied series of synthetic tetraaza macrocycles were initially prepared by Curtis, who reported that the reaction of tris-(1,2-diaminoethane)nickel(II) perchlorate and acetone at room temperature produced a yellow crystalline product. The compound was diamagnetic and very resistant to hydrolysis. The original structure proposed was not a macrocycle but this was later corrected to fit in with the chemical inertness, the infrared spectrum and its reaction with hydrogen to give 1,2-diaminoethane and mesityl oxide. The product can occur in two noninterconvertible forms, which Curtis originally designated as A and B. The A isomer occurring in two interconvertible forms called Aa and Ab. The three isomers are similar in appearance and physical properties although some differences in their IR spectra have been noted. The first crystals of cyclic complex which crystallise from the reaction medium are isometrically pure B(ClO4)2. The Aa(ClO4)2 isomer usually begins to crystallise after about 25% of the total product has separated. The separation occurs since the B form is less soluble. The A-B isomerism comes from cis- trans arrangements of the imine linkages in the macrocyclic ring, with A corresponding to the trans- form and B the cis-isomer.
Later studies have shown that Aa is the more stable form and that it corresponds to the racemic mixture of isomers (the asymmetry exists at the tetrahedral nitrogen atoms in the diene complex) and that Ab is the meso isomer.

Isomer B is also a racemate.


Preparation of [Ni(en)3]Cl2

70% aqueous 1,2-diaminoethane (ethylenediamine, 2.8 g, 3.1 cm3) is added to a solution of nickel(II) chloride hexahydrate (2.4 g) in 10 -15 cm3 of water. The resulting purple solution is filtered to remove any insoluble material and the solution is then evaporated down to approximately 6 cm3 on a steam bath. Two drops of 1,2 diaminoethane are then added and the solution is cooled in an ice-bath.
The orchid coloured crystals are collected on a sintered glass filter funnel and washed twice with 3 cm3 portions of ethanol and finally air-dried. The mother liquor should not be discarded since further product can be obtained by addition of ethanol and further cooling.
Record the yield and show the product to a demonstrator.

Conversion to the perchlorate salt.

The perchlorate salt is more soluble in acetone than the chloride and this is used for the subsequent reaction. Tris-(1,2-diaminoethane)nickel(II) chloride (2 g) is dissolved in hot ethanol and sodium perchlorate (2 g) is then added. The solution is cooled and the complex precipitating out is collected by suction.
Record the yield and show the product to a demonstrator.
NB. Perchlorate salts have been known to detonate spontaneously and should be treated as potentially hazardous.

Preparation of the Macrocyclic complex.

Tris-(1,2-diaminoethane)nickel(II) perchlorate dihydrate (0.5 g) is dissolved in 7 cm3 of acetone and the purple solution is thoroughly shaken. The reaction mixture is left to stand for a few hours (may be left overnight) after which yellow crystals are formed. These are collected by suction and air-dried.
Record the yield and show the product to a demonstrator.
An IR spectrum is available, record the major peaks and try to account for their position and intensity.


1. Outline a possible reaction scheme for the template synthesis of the Ni(II) complex.
2. What is The Macrocyclic Effect?


L.F. Lindoy, "The Chemistry of Macrocyclic Ligand Complexes", Cambridge University Press, 1989, Chapter 2.
G.A. Melson, "Coordination Chemistry of Macrocyclic Compounds", Plenum Press, New York, 1979. Chapter 2.

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Nov-96, rjl