Oxidative Addition of Sn-C Bonds on Palladium(0): Identification of Palladium-Stannyl Species and a Facile Synthetic Route to Diphosphinostannylene-Palladium Complexes

Methyl-, phenyl-, and n-butyltin trichloride RSnCl3 (R = Me, Ph, Bu-n) 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 = Bu-n ([11]Cl); X = RSnCl4, R = Me ([5][MeSnCl4]), R = Ph ([6][PhSnCl4]), R = Bu-n ([11][(BuSnCl4)-Bu-n])). The SnCl2(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 beta-H elimination in its Pd-alkyl derivatives: [11][(BuSnCl4)-Bu-n], containing a Pd-Bu-n moiety, was found to be stable up to 70 degrees C. Oxidative addition of SnCl4 on [Pd(2-PyPPh2)(3)] resulted in trans-[PdCl(SnCl2(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 trichloro-stannyl 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 beta-H elimination (R = Bu-n). 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)(PPh3)(2)] (1), trans-[PdMe(Cl)(PPh3)(2)] (3), trans-{PdH(SnCl3)(PPh3)(2)] (trans-10), and trans-[PdH(Cl)(PPh3)(2)] (9).