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.