UV-vis and binding studies of cobalt tetrasulfophthalocyanine-thiolate complexes as intermediates of the Merox process

Intermediates of the cobalt tetrasulfophthalocyanine (CoTSPc)-catalyzed thi ol autoxidation were studied by UV-vis spectroscopy. All thiolates react wi th CoTSPc resulting in the formation of 1:1 complexes. Three major factors control both the stability and aggregation of the complexes: thiolate basic ity, metal-to-ligand charge transfer (MLCT) and pi stacking. Basic thiolate s partially reduce (CoTSPc)-T-II, whereas CoTSPc complexes with low-basicit y aliphatic thiolates (pK(a) < 4) do not exhibit Co(II) reduction, based on the absence of the characteristic Co(I) charge transfer band at 450 nm. Co TSPc complexes with aliphatic and bulky aromatic thiolates appear to be agg regated in aqueous solutions and are characterized by a broad band at 650 n m. Non-bulky aromatic thiolates of low basicity (pK(a) < 6) form unique sta ble monomeric (CoTSPc)-T-II complexes. This unique spectral feature can be attributed to pi stacking between the phthalocyanine ring and thiolate. Com parison of binding constants shows that the partial reduction of Co(II) sig nificantly contributes to the thiolate binding. A combination of aromatic p i stacking and MLCT appears to be responsible for the observed 1000-fold st ronger binding of non-basic aromatic thiolates as compared with aliphatic l igands of similar basicity. Kinetic studies confirm the importance of the t hiolate binding type for catalysis. Copyright (C) 1999 John Wiley & Sons, L td.