Phthalocyanine

Also written "phthalozyanin" in German or "phtalocyanine" in French.

A phthalocyanine is a macrocyclic compound having an alternating nitrogen atom-carbon atom ring structure. The molecule is able to coordinate hydrogen and metal cations in its center by coordinate bonds with the four isoindole nitrogen atoms. The central atoms can carry additional ligands. Most of the elements have been found to be able to coordinate to the phthalocyanine macrocycle. Therefore, a variety of phthalocyanine complexes exist.

History

The first known appearance of an unknown blue by-product (which we know by now was metal-free phthalocyanine) has been reported in 1907.
In 1927, two researchers from Switzerland accidentally synthesized copper phthalocyanine, copper naphthalocyanine and copper octamethylphthalocyanine when they tried to convert o-dibromobenzene into phthalonitrile. They remarked the enormous stability of these complexes but failed to appreciate their discovery and to fully characterize these blue complexes.
H. de Diesbach,E. von der Weid, Helevtica Chimica Acta, 1927, 10, 886.
The real discovery also started as an accident, when a blue product was found in a reaction flask where only white product was expected. However, this accident occurred in a dye company, Scottish Dyes, Ltd., Grangemouth, Scotland (later ICI) and the discovery was followed on.

Overview

The structure of a phthalocyanine molecule is closely related to that of the naturally occurring porphyrin systems.

The phthalocyanine macrocycle is also related to some other macrocyclic complexes, as e.g., the subphthalocyanine, superphthalocyanine or hemiporphyrazine.

Synthesis

You can see in the figure below (left side) that a phthalocyanine macrocycle consists of four identical corners. A synthesis strategy, therefore, starts from molecules which correspond to these corners. Such molecules are derivatives of phthalic acid: e.g., phthalonitrile, o-cyanobenzamide, phthalanhydride, phthalimide or diiminoisoindole. Several of these starting materials are shown in the figure below (right side).