Formation and structures of palladium-cyanomethyl complexes generated under unexpectedly mild conditions: quantifying the acidification of acetonitrile coordinated to a palladium center

Abstract

The reaction of the palladium-triphosphine complex [Pd(triphos)(PPh3)]2+ with guanidine and phosphazene bases in acetonitrile forms the cyanomethyl complex [Pd(triphos)(CH2CN)]+, where triphos is the triphosphine ligand PhP(CH2CH2PPh2)2. A cyanomethyl complex with a tridentate PNHP ligand, HN(CH2CH2PPh2)2, was also formed in a similar reaction with solvent, demonstrating that this acidification is relatively general. The crystal structures of these cyanomethyl complexes possess distorted square planar geometries with the cyanomethyl group trans to the central P and N atoms, demonstrating a trans influence that is significantly greater than observed in analogous halide complexes but weaker than complexes with monodentate phosphines. The bases used to form the triphosphine complex are too weak to deprotonate free acetonitrile, and the observed reaction demonstrates a dramatic acidification of the solvent in the presence of the palladium ion. The pKa of the coordinated acetonitrile is estimated to be approximately 25 from these results, which represents a decrease of at least 8 pKa units relative to the free solvent; through a thermodynamic cycle, the heterolytic M–C bond energies can also be estimated as 11 kcal/mol using the H–C bond energies as a reference.