Experimental and Theoretical Studies on the Mechanism of the C-S Bond Activation of PdII Thiolate/Thioether Complexes
Résumé
Two equivalents of L (L = 4-methylthio-2-thioxo-1,3-dithiole-5-thiolate or Medmit) react with cis-Pd(PR3)2Cl2 (R = Ph and Et) to afford Pd(PR3)(η1-L)(η2-L) (R = Et: 1 ; R = Ph: 2) complexes, which have been characterized by X-ray crystallography. These compounds are dynamic in solution due to an exchange of the thiomethyl groups on palladium. Variable-temperature 1H NMR spectroscopy reveals a low coalescence temperature (173 K). Treatment of Pd(diphos)Cl2 (diphos = dppe or dppm) with 2 equiv of L affords thiolato complexes Pd(dppe)(η1-L)2 (3) and Pd(dppm)(η1-L)2 (4). Whereas 3 is rigid in solution with firm η2-coordination of dppe and η1-coordination of the thiolate, two linkage isomers Pd(η2-dppm)(η1-L)2 and Pd(η1-dppm)(η1-L)(η2-L) coexist in a solution of 4. L coordinated on PdII undergoes a S-demethylation reaction leading to dithiolene complexes and MeL. This transformation requires high temperature, and its efficiency depends on the nature of the phosphines as well as the nature of the metal (Pd vs Pt). DFT calculations reveal that the most likely mechanism depends on the lability of phosphines. Starting from M(PR3)2(η1-L)2 (M= Pd and Pt; R = Ph and Et), the favored sequence implies decoordination of one triethyl phosphine (M(PEt3)(η1-L)(η2-L)2 as intermediate) or two triphenylphosphines (Pd(η2-L)2 as intermediate) followed by oxidative addition and reductive elimination (OA/RE) reactions. In the case of PEt3, this OA/RE sequence can also compete with an intramolecular nucleophilic addition (AN), which can be described as an attack of a thiolate sulfur atom on a CH3+ carbocation. An intramolecular SN2 process was found to be the most feasible, starting from M(dppe)(η1-L)2 (M= Pd and Pt), with the nucleophile approaching the thiomethyl group at an angle of 180° with respect to the CCH3–S bond. The influence of the coligand has also been studied experimentally. Structurally characterized disulfide L–L dimer has been isolated upon reaction of 2 equiv of L with MCl2 (M = Pd and Pt).