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 pKa 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 pKa 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 pKa
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

.