Gold(II) Phthalocyanine Revisited: Synthesis and Spectroscopic Properties of Gold(III) Phthalocyanine and an Unprecedented Ring-Contracted Phthalocyanine Analogue

In 1965, gold(II) phthalocyanine (AuPc, 1) was described to be synthesized from unsubstituted 1,3‐diiminoisoindoline and gold powder or AuBr. Compound 1 has been regarded as a rare example of a paramagnetic gold(II) complex. However, its chemistry, especially the oxidation state of the central gold ion, has not been previously explored due to the inherent insolubility of 1 caused by its unsubstituted structure. In our attempt to synthesize soluble AuPcs by using 5,6‐di‐substituted 1,3‐diiminoisoindolines, gold(III) phthalocyanine chloride (3) and a gold(III) complex of an unprecedented ring‐contracted phthalocyanine analogue ([18]tribenzo‐pentaaza‐triphyrin(4,1,1), 4) were isolated. With this discrepant result from the original literature in hand, a reinvestigation of the original AuPc synthesis by using unsubstituted 1,3‐diiminoisoindoline and various gold salts (including gold powder and AuBr) was performed, finding that only unsubstituted analogues of 3 and 4 or free‐base phthalocyanine were obtained. No gold(II)‐containing species could be isolated. Reinvestigation of a putative gold(II) phthalocyanine, which was synthesized in 1965 and has not been fully characterized, provided only gold(III) complexes, gold(III) phthalocyanine chloride, and a gold(III) complex of an unprecedented ring‐contracted phthalocyanine (see figure). Interconversion reaction of the ring‐contracted species to gold(III) phthalocyanine was also revealed.