Benzylic oligothioethers as ditopic ligands for Group 6 transition metal carbonyls

Abstract

The co-ordination preferences of a family of novel thioethers based on (alkylsulfanylmethyl)benzene were examined. The alkyl chain length and the number and position of the thioether substituents were varied. The ligands were synthesized by coupling the alkanethiol to the appropriate benzyl bromide or via the reaction of benzyl mercaptans with bromoethane; Cs₂CO₃–DMF was employed as the base–solvent mixture. The molecular structure of hexakis(propylsulfanylmethyl)benzene (L3) and 1,2,4,5-tetrakis(propylsulfanylmethyl)benzene (L4) were obtained at room temperature. Both have a crystallographic centre of symmetry. In L3 the S-propyl substituents (“legs”) alternate “a(bove)” and “b(elow)” the plane of the benzene ring, whereas in L4 the “legs” adopt an abba pattern. Several co-ordination modes of the ligands were observed. Bismetallated tetracarbonyl complexes where the ligand bridges two monometal Group 6 carbonyl fragments (M(CO)₄) were generated on reaction of [W(CO)₄(MeCN)₂] or [Mo(CO)₃(MeCN)₃] with the ligands L1 and L2. The molecular structures of the resulting complexes [{Mo(CO)₄}₂L] (L = L1 or L2 which are the pentyl analogues of L3 and L4 respectively) show that the conformations of the ligands change radically to accommodate the octahedral geometry about the metal centres. In all cases the metal atoms are chelated by “legs” positioned ortho to each other. Changing the stoichiometry of the reactions does not significantly influence the products.