Thermal Fragmentation of Acyl Thiolato Complexes to Reactive Metal Sulfido Intermediates. Structure of Ru(η⁶-SC₃Me₃COMe)(PPh₃)₂

In this study we have examined the thermal fragmentation of acyl thiolate complexes of the type (ring)M(SC₃Me₃COMe). While the (cymene)Ru derivative only gave insoluble materials, the thermolysis of the corresponding osmium compound, (cymene)Os(SC₃Me₃COMe) (2), proved very informative. Here it was established that the tetramethylfuran is indeed released prior to formation of the free cymene. Furthermore, two intermediates were observed, the second of which was identified as (cymene)₃Os₃S₂. This species is more stable with respect to loss of cymene than the corresponding (cymene)₃Ru₃S₂.Attempts were made to intercept intermediates in these fragmentation processes by employing PPh₃ as a trapping agent. For the Ru case, PPh₃ completely changed the course of the reaction and no tetramethylfuran was generated. Instead we obtained the structurally unusual η⁶-acyl thiolate complex (5).We propose that this unimolecular step involves the formation of (η⁴-cymene)Ru(η⁶-SC₃Me₃COMe). The forward rate for this equilibration is far slower than the rate of isomerization of the acyl thiolate and far faster than that for the unassisted thermal decomposition of (cymene)Ru(SC₃MesCOMe).This result leads to a more global assessment of the reactivity of the acyl thiolates: the orientation of the carbonyl center determines the decomposition pathway. Thus, the carbonyl in the kinetic isomer is oriented correctly for cyclization, giving tetramethylfuran, while the carbonyl in the thermodynamic isomer cannot cyclize but instead binds to ruthenium to give the η⁶-acyl thiolate complex which is trapped by PPh₃.