STRUCTURE FUNCTION RELATIONSHIPS IN LIGAND-BASED SO2/O-2 CONVERSION TO SULFATE AS PROMOTED BY NICKEL AND PALLADIUM THIOLATES/

The dithiolate complex ,5-bis(mercaptoethyl)-1,5-diazacyclooctane]Pd(I I), (bme-daco)Pd-II or Pd-1, whose structure was determined by X-ray c rystallography (monoclinic P2(1)/m space group with a = 6.1680(10) Ang strom, b = 15.715(5) Angstrom, c = 6.5930(10) Angstrom, beta = 107.090 (10)degrees, Z = 2, R = 0.0291, R(W) = 0.0718), has been added to a gr oup of metal thiolates which form sulfur-site SO2 adducts. Exposure of the Pd-l complex to SO2 in methanol results in the precipitation of y ellow/orange crystalline Pd-1 SO2: monoclinic space group, P2(1)/c (No . 14), with a 8.928(2) Angstrom, b = 14.655(4) Angstrom, c = 11.067(2) Angstrom, beta = 97.29(2)degrees, Z = 4, R = 0.0348, R(W) = 0.0944. A nalogous thiolate-SO2 adducts based on (bme-daco)Ni-II, Ni-1 . SO2, (P h(2)PCH(2)CH(2)S)(2)Ni-II, Ni-2 . SO2, and (bme-daco)Ni-II, Ni-1*. SO 2, also precipitate from methanol. To explore the transformation of SO 2 to SO42- in these adducts, the following three factors expected to c ontrol the sulfate-forming reaction have been examined: (i) the stabil ity of SO2 adducts; (ii) the oxidizability of the metal thiolate or it s tendency to generate disulfide products on oxidation; and (iii) the ability of the metal thiolates to react with O-2 and produce sulfur-ox ygenated products. The studies indicate that the last factor is the mo st important influence on SO2 oxygenation. A possible mechanism involv es the transient formation of an SO2-stabilized sulfperoxide intermedi ate, which behaves as a nucleophile and further reacts with SO2 to pro duce SO42-. The use of the aforementioned metal thiolate complexes as catalysts for SO2 oxygenation in the presence of a sacrificial electro n donor has also been explored; simple salts such as NiCl2 and NiSO4 a re more efficient than the complexes.