Thermal reactions of titanium thiolates: Terminal titanium sulfides in C-S bond cleavage reactions

The thermolysis of monocyclopentadienyltitanium thiolate complexes leads to ligand redistributions and C-S bond cleavage reactions. Kinetic study of the C-S bond thermolysis reaction of CpTi(OC6H3-2,6-i-Pr-2)(SBn)(2) (2) to the sulfide-bridged dimer [CpTi(OC6H3-2,6-i-Pr-2)(mu-S)](2) (1) is shown to be first order in 2, consistent with an intramolecular process proceeding via a terminal sulfide intermediate and rapid dimerization (k = 2.8 x 10(-6) s(-1)). The related species CpTi(OC6H3-2,6-i-Pr-2)(SBn)Cl (3), CpTi(OC6H3-2,6-i-Pr-2)(SMe)(2) (4), and CpTi(OC6H3-2,6-i-Pr-2)(SPh)(2) (5) are thermally stable, although compounds 4 and 5 undergo thermally induced ligand redistribution reactions. These redistribution reactions are thought to occur via a dimeric intermediate in which bridging thiolate ligands are exchanged between two metal centers. A dimeric intermediate is supported by the characterization of the species [CpTi(SR)(2)(mu-SR)](2) (R = Et (7), Bn (8)) which are dimeric in solution at low temperature and in the solid state. In contrast, Cp*Ti(SBn)(3) (9) is monomeric. Efforts to intercept a terminal sulfide intermediate in the formation of 1 were unsuccessful, although reaction of CpTiCl2Me with LiSBn in the presence of PMe3 gives [CpTiCl(mu-S)](2)[PMe3Bn] (10) in low yield. The analogue of 2, Cp*Ti(OC6H3-2,6-i-Pr-2)(SBn)(2), 12, is thermally stable; however, thermolysis of Cp*TiCl(SBn)(2) gave [Cp*TiCl(mu-S)](2) (13). Ultimately, the LiCl adduct of a terminal sulfide species Cp*Ti(OC6H3-2,6-i-Pr-2)(mu-S)(mu-Cl)Li(THF)(2) (14) was isolated from the reaction of Cp*Ti(OC6H3-2,6-i-Pr-2)Cl-2 (11) with Li2S. Treatment of (14) with PMe3 generates the monomeric terminal sulfide complex species Cp*Ti(OC6H3-2,6-i-Pr-2)(S)(PMe3) (15), which is unstable, slowly evolving PMe3 affording [(CpTi)-Ti-*(OC6H3-2,6-i-Pr-2)(mu-S)](2) (16). Compound 16 is also obtained directly by heating solutions of 14. Crystallographic studies of 8, 10,12, 13, 14, and 16 are reported herein. The intermediacy of terminal metal sulfides in C-S processes are discussed.