UV-vis and Binding Studies of Cobalt Tetrasulfophthalocyanine–Thiolate Complexes as Intermediates of the Merox Process

Abstract

Intermediates of the cobalt tetrasulfophthalocyanine ( CoTSPc )-catalyzed thiol autoxidation were studied by UV-vis spectroscopy. All thiolates react with CoTSPc resulting in the formation of 1:1 complexes. Three major factors control both the stability and aggregation of the complexes: thiolate basicity, metal-to-ligand charge transfer (MLCT) and π stacking. Basic thiolates partially reduce C oII TSPc , whereas CoTSPc complexes with low-basicity aliphatic thiolates ( p K a < 4) do not exhibit Co (II) reduction, based on the absence of the characteristic Co (I) charge transfer band at 450 nm. CoTSPc complexes with aliphatic and bulky aromatic thiolates appear to be aggregated in aqueous solutions and are characterized by a broad band at 650 nm. Non-bulky aromatic thiolates of low basicity ( p K a < 6) form unique stable monomeric Co II TSPc complexes. This unique spectral feature can be attributed to π stacking between the phthalocyanine ring and thiolate. Comparison of binding constants shows that the partial reduction of Co (II) significantly contributes to the thiolate binding. A combination of aromatic π stacking and MLCT appears to be responsible for the observed 1000-fold stronger binding of non-basic aromatic thiolates as compared with aliphatic ligands of similar basicity. Kinetic studies confirm the importance of the thiolate binding type for catalysis.