STRUCTURE/FUNCTION RELATIONSHIPS IN LIGAND-BASED SO2/O-2 CONVERSION TO SULFATE AS PROMOTED BY NICKEL AND PALLADIUM THIOLATES
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abstract
The dithiolate complex [1,5-bis(mercaptoethyl)-1,5-diazacyclooctane]Pd(II), (bme-daco)PdII or Pd-1, whose structure was determined by X-ray crystallography (monoclinic P21/m space group with a = 6.1680(10) , b = 15.715(5) , c = 6.5930(10) , = 107.090(10), Z = 2, R = 0.0291, Rw = 0.0718), has been added to a group of metal thiolates which form sulfur-site SO2 adducts. Exposure of the Pd-1 complex to SO2 in methanol results in the precipitation of yellow/orange crystalline Pd-1SO2: monoclinic space group, P21/c (No. 14), with a = 8.928(2) ,b= 14.655(4) , c = 11.067(2) , = 97.29(2), Z = 4, R = 0.0348, Rw = 0.0944. Analogous thiolate-SO2 adducts based on (bme-daco)NiII, Ni-1-SO2, (Ph2PCH2CH2S)2NiII, Ni-2SO2, and (bme*-daco)NiII, Ni-1*-SO2, also precipitate from methanol. To explore the transformation of SO2 to SO42 in these adducts, the following three factors expected to control the sulfate-forming reaction have been examined: (i) the stability of SO2 adducts; (ii) the oxidizability of the metal thiolate or its tendency to generate disulfide products on oxidation; and (iii) the ability of the metal thiolates to react with O2 and produce sulfur-oxygenated products. The studies indicate that the last factor is the most important influence on SO2 oxygenation. A possible mechanism involves the transient formation of an SO2-stabilized sulfperoxide intermediate, which behaves as a nucleophile and further reacts with SO2 to produce SO42. The use of the aforementioned metal thiolate complexes as catalysts for SO2 oxygenation in the presence of a sacrificial electron donor has also been explored; simple salts such as NiCl2 and NiSO4 are more efficient than the complexes. 1995, American Chemical Society. All rights reserved.
