Oxidative addition of Sn-C bonds on palladium(0): Identification of palladium-stannyl species and a facile synthetic route to diphosphinostannylene- palladium complexes

Article abstract of DOI:10.1021/om1007067

Methyl-, phenyl-, and n-butyltin trichloride RSnCl₃ (R = Me, Ph, ⁿBu) react selectivily with palladium(0)-phosphine precursors through the unprecedented oxidative addition of the Sn-C bond. With [Pd(2-PyPPh₂)₃] (2-PyPPh₂ = 2-pyridyldiphenylphoshine), the reaction cleanly leads to stable cationic dichlorostannylene palladium complexes of the general formula trans-[PdR(SnCl₂(2-PyPPh₂)₂)][X] (X = Cl, R = Me ([5]Cl), R = Ph ([6]Cl), R = ⁿBu ([11]Cl); X = RSnCl₄, R = Me ([5][MeSnCl₄]), R = Ph ([6][PhSnCl₄]), R = ⁿBu ([11][ⁿBuSnCl₄])). The SnCl ₂(2-PyPPh₂)₂ fragment, formed by intramolecular coordination of the pyridyl groups to the dichlorostannylene moiety, can be considered as a self-assembled pincer-type ligand with a remarkable ability to suppress β-H elimination in its Pd-alkyl derivatives: [11][ ⁿBuSnCl₄], containing a Pd-ⁿBu moiety, was found to be stable up to 70 °C. Oxidative addition of SnCl₄ on [Pd(2-PyPPh₂)₃] resulted in trans-[PdCl(SnCl ₂(2-PyPPh₂)₂)]Cl ([7]Cl) and trans-[PdCl(SnCl₃(2-PyPPh₂)₂)] (8). The molecular structure of 8 was determined by single-crystal X-ray crystallography, indicating that the Sn atom of the trichlorostannyl function has an octahedral coordination geometry. In contrast, oxidative addition of the Sn-C bond of RSnCl₃ on [Pd(PPh₃)₄] resulted in palladium trichlorostannyl complexes that were not stable toward cis-trans isomerization, (partial) elimination of SnCl₂ (R = Me, Ph), or β-H elimination (R = ⁿBu). The resulting mixtures of palladium alkyl and palladium hydride species were analyzed by multinuclear NMR, resulting in the identification of novel cis-[PdMe(SnCl₃)(PPh₃) ₂] (cis-4), trans-[PdMe(SnCl₃)(PPh₃) ₂] (trans-4), and cis-[PdH(SnCl₃)(PPh₃) ₂] (cis-10) along with previously observed trans-[PdPh(Cl)(PPh ₃)₂] (1), trans-[PdMe(Cl)(PPh₃)₂] (3), trans-[PdH(SnCl₃)(PPh₃)₂] (trans-10), and trans-[PdH(Cl)(PPh₃)₂] (9).