Thermal Reactions of Titanium Thiolates: Terminal Titanium Sulfides in CS Bond Cleavage Reactions

Abstract

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-Pr2)(SBn)2 (2) to the sulfide-bridged dimer [CpTi(OC6H3-2,6-i-Pr2)(μ-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 × 10-6 s-1). The related species CpTi(OC6H3-2,6-i-Pr2)(SBn)Cl (3), CpTi(OC6H3-2,6-i-Pr2)(SMe)2 (4), and CpTi(OC6H3-2,6-i-Pr2)(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(μ-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(μ-S)]2[PMe3Bn] (10) in low yield. The analogue of 2, Cp*Ti(OC6H3-2,6-i-Pr2)(SBn)2, 12, is thermally stable; however, thermolysis of Cp*TiCl(SBn)2 gave [Cp*TiCl(μ-S)]2 (13). Ultimately, the LiCl adduct of a terminal sulfide species Cp*Ti(OC6H3-2,6-i-Pr2)(μ-S)(μ-Cl)Li(THF)2 (14) was isolated from the reaction of Cp*Ti(OC6H3-2,6-i-Pr2)Cl2 (11) with Li2S. Treatment of (14) with PMe3 generates the monomeric terminal sulfide complex species Cp*Ti(OC6H3-2,6-i-Pr2)(S)(PMe3) (15), which is unstable, slowly evolving PMe3 affording [Cp*Ti(OC6H3-2,6-i-Pr2)(μ-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.