The effect of nitro group on the coordination of cyclic ligands

Abstract

The coordination chemistry of a number of nitro-substituted thiazoles and nitro-substituted pyridine N-oxides with a variety of acceptors has been investigated. The stoichiometry and stereochemistry of the transition metal complexes of these ligands depend on the metal, the anion and the substituents on the ligands. The influence of the nitro-group on the bonding abilities of these ligands is the most significant.

In part I, the preparation and characterisation of complexes of 2-amino-5-nitrothiazole (ANT) with Co(II), Ni(II), Cu(II), Cd(II), Cu(I), and Ag(I) are reported. From spectral data it is inferred that the complexes fall into two groups; those in which ANT is bound through the ring nitrogen, and those in which the exocyclic amine group is bound to the metal ion. In the former group are complexes ot the type MCl₂.L₂ (M = Co, Ni, Cu, Cd); compounds containing methanol, and [Cu(AcO)₂.L]₂, CuCl.L ₂, AgNO₃.L₂ complexes. In the latter group are complexes prepared from and containing dimethylformamide. A protonated copper(II) chloride complex of ANT is also characterised. 2-Bromo-5-nitrothiazole and 2-chloro 5-nitrothiazole did not form complexes with the same metal salts.

In part II, the coordinating properties of nitro-substituted pyridine N-oxides toward copper(II) and cadmium(II) have been investigated. The ligands studied were 4-nitro 2-picoline N-oxide, 4-nitro 3-picoline N-oxide, 4-nitro 2,6-lutidine N-oxide, and 3-nitro 2,6-lutidine N-oxide. Several types of copper(II) halide complexes were isolated and characterised. These include (a) low magnetic moment 1:1 complexes, (b) a normal magnetic moment 1:1 complex, (c) distorted trans- and cis- 1:2 monomeric complexes with normal magnetic moment, (d) a polymeric halide-bridged 1:2 complex with normal magnetic moment, and (e) a grossly distorted trans-planar 1:2 complex with low magnetic moment. Among all the nitro-substituted pyridine N-oxides studied only 4-nitro 3-picoline N-oxide formed a complex with copper(II) acetate. Cadmium(II) chloride formed two series of complexes with these ligands; one with the general formula CdCl 2.L and the second with the formula CdCl₂.L₂. It was concluded that in the presence of a nitro substituent, steric factors near the donor site play a very important role in determining the structure of the complexes.