Autoxidation of mercaptans promoted by a bifunctional catalyst

Synthesis and electrocatalytic properties of octaalkoxycobalt phthalocyanine for the oxidation of 2-mercaptoethanol

We report on the synthesis of cobalt 2,3,9,10,16,17,23,24-octakis(2-ethylhexyloxy)phthalocyanine. This complex, when pre-adsorbed onto graphite electrodes, is active for the electro-oxidation of 2-mercaptoethanol. Its activity is higher than that of cobalt phthalocyanine and cobalt tetrasulfonated phthalocyanine adsorbed on graphite. The voltammetric response of adsorbed cobalt 3,4-octaethylhexyl- oxyphthalocyanine exhibits both Co ( II )/ Co ( I ) and Co ( III )/ Co ( II ) reversible processes. However, the Co ( III )/ Co ( II ) couple shows a pH dependence that is different from that reported for other cobalt phthalocyanine derivatives. The much higher activity of cobalt 3,4-octaethylhexyloxyphthalocyanine compared with cobalt tetrasulfonated phthalocyanine and cobalt phthalocyanine is attributed to the electron-donating ability of the ethylhexyloxy substituents on the periphery of the phthalocyanine ligand.

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

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.