Using ligand bite angles to control the hydricity of palladium diphosphine complexes.

A series of [Pd(diphosphine)(2)](BF(4))(2) and Pd(diphosphine)(2) complexes have been prepared for which the natural bite angle of the diphosphine ligand varies from 78 degrees to 111 degrees. 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 degrees as the natural bite angle increases for the [Pd(diphosphine)(2)](BF(4))(2) complexes. The dihedral angle for the Pd(diphosphine)(2) complexes varies less than 10 degrees 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)](BF(4))(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, Delta G(H*)* and Delta G(H-)*, respectively. Linear free-energy relationships are observed between pK(a) and the Pd(I/0) couple and between Delta G(H-)* and the Pd(II/I) couple. The measured values for Delta G(H*)* were all 57 kcal/mol, whereas the values of Delta G(H-)* 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 Delta G(H-)*.