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

Abstract

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 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 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(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 trichlorostannyl function has an octahedral coordination geometry. In contrast, oxidative addition of the Sn-Cbond 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)(PPh3)2] (1), trans-[PdMe(Cl)(PPh3)2] (3), trans-[PdH(SnCl3)(PPh3)2] (trans-10), and trans-[PdH(Cl)(PPh3)2] (9).