Preparation and characterization of organotin-oxomolybdate coordination polymers and their use in sulfoxidation catalysis.

The organotin-oxomolybdates [(R(3)Sn)(2)MoO(4)].n H(2)O (R=methyl, n-butyl, cyclohexyl, phenyl, benzyl) have been prepared and tested as catalysts for the oxidation of benzothiophene with aqueous hydrogen peroxide, at 35 degrees C and atmospheric pressure. In all cases, the 1,1-dioxide was the only observed product. The kinetic profiles depend on the nature of the tin-bound R group and also on the addition of a co-solvent. For the tribenzyltin derivative, the apparent activation energies for sulfoxidation as a function of the co-solvent are in the order 1,2-dichloroethane (5 kcal mol(-1))<ethanol<acetonitrile<water (27 kcal mol(-1)). The solid could be recovered by centrifugation from the three-phase (solid-liquid-liquid) system containing 1,2-dichloroethane. The catalyst was reused in a second reaction cycle with no significant loss of activity. Increasing the oxidant/substrate ratio from 2:1 to 6:1 allows the corresponding sulfone to be obtained in quantitative yield within 24 h. Changing the nature of the tin-bound R group in the catalyst precursors modifies their physical properties and hence their catalytic performance. The variation in hydrophobic/hydrophilic character is important, since the Me, nBu and Cy derivatives crystallize as anhydrous compounds, whereas the other two derivatives are hydrates. The polymers also have different structures, as evidenced by Xray powder diffraction. Mo K-edge and Sn K-edge EXAFS spectroscopy confirmed that the structures arise from the self-assembly of tetrahedral [MoO(4)](2-) subunits and [R(3)Sn](+) spacers. The Mo...Sn separation in the trimethyltin derivative is a uniform 3.84 A. By contrast, the EXAFS results revealed the coexistence of short (3.67-3.79 A) and long (3.93-4.07 A) Mo.Sn separations in the other coordination polymers. The catalyst precursors were also characterized in the solid state by thermogravimetric analysis, FTIR, and Raman spectroscopy, and MAS NMR ((13)C, (119)Sn) spectroscopy.