Thermal Reactions of Titanium Thiolates:  Terminal Titanium Sulfides in CS 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(OC 6 H 3 -2,6- i -Pr 2 )(SBn) 2 ( 2 ) to the sulfide-bridged dimer [CpTi(OC 6 H 3 -2,6- i -Pr 2 )(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(OC 6 H 3 -2,6- i -Pr 2 )(SBn)Cl ( 3 ), CpTi(OC 6 H 3 -2,6- i -Pr 2 )(SMe) 2 ( 4 ), and CpTi(OC 6 H 3 -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 (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 CpTiCl 2 Me with LiSBn in the presence of PMe 3 gives [CpTiCl(μ-S)] 2 [PMe 3 Bn] ( 10 ) in low yield. The analogue of 2, Cp*Ti(OC 6 H 3 -2,6- i -Pr 2 )(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(OC 6 H 3 -2,6- i -Pr 2 )(μ-S)(μ-Cl)Li(THF) 2 ( 14 ) was isolated from the reaction of Cp*Ti(OC 6 H 3 -2,6- i -Pr 2 )Cl 2 ( 11 ) with Li 2 S. Treatment of ( 14 ) with PMe 3 generates the monomeric terminal sulfide complex species Cp*Ti(OC 6 H 3 -2,6- i -Pr 2 )(S)(PMe 3 ) ( 15 ), which is unstable, slowly evolving PMe 3 affording [Cp*Ti(OC 6 H 3 -2,6- i -Pr 2 )(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.