Using Ligand Bite Angles To Control the Hydricity of Palladium Diphosphine Complexes

A series of [Pd(diphosphine)₂](BF₄)₂ and Pd(diphosphine)₂ complexes have been prepared for which the natural bite angle of the diphosphine ligand varies from 78° to 111°. Structural studies have been completed for 7 of the 10 new complexes described. These structural studies indicate that the dihedral angle between the two planes formed by the two phosphorus atoms of the diphosphine ligands and palladium increases by over 50° as the natural bite angle increases for the [Pd(diphosphine)₂](BF₄)₂ complexes. The dihedral angle for the Pd(diphosphine)₂ complexes varies less than 10° for the same range of natural bite angles. Equilibrium reactions of the Pd(diphosphine)₂ complexes with protonated bases to form the corresponding [HPd(diphosphine)₂]⁺ complexes were used to determine the pK_a values of the corresponding hydrides. Cyclic voltammetry studies of the [Pd(diphosphine)₂](BF₄)₂ complexes were used to determine the half-wave potentials of the Pd(II/I) and Pd(I/0) couples. Thermochemical cycles, half-wave potentials, and measured pK_a values were used to determine both the homolytic ([HPd(diphosphine)₂]⁺ → [Pd(diphosphine)₂]⁺ + H^•) and the heterolytic ([HPd(diphosphine)₂]⁺ → [Pd(diphosphine)₂]²⁺ + H^-) bond-dissociation free energies, Δ and Δ , respectively. Linear free-energy relationships are observed between pK_a and the Pd(I/0) couple and between Δ and the Pd(II/I) couple. The measured values for Δ were all 57 kcal/mol, whereas the values of Δ ranged from 43 kcal/mol for [HPd(depe)₂]⁺ (where depe is bis(diethylphosphino)ethane) to 70 kcal/mol for [HPd(EtXantphos)₂]⁺ (where EtXantphos is 9,9-dimethyl-4,5-bis(diethylphosphino)xanthene). It is estimated that the natural bite angle of the ligand contributes approximately 20 kcal/mol to the observed difference of 27 kcal/mol for Δ .