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

James W. Raebiger, Alex Miedaner, Calvin J. Curtis, Susie M. Miller, Oren P. Anderson, Daniel L DuBois

Research output: Contribution to journalArticle

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Abstract

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, ΔG°H. and ΔG°H-, respectively. Linear free-energy relationships are observed between pKa and the Pd(I/0) couple and between ΔG°H- and the Pd(II/I) couple. The measured values for ΔG°H. were all 57 kcal/mol, whereas the values of ΔG°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 ΔG°H-.

Original languageEnglish
Pages (from-to)5502-5514
Number of pages13
JournalJournal of the American Chemical Society
Volume126
Issue number17
DOIs
Publication statusPublished - May 5 2004

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Palladium
Bites and Stings
Ligands
Dihedral angle
Free energy
Xanthenes
Ethane
Hydrides
Phosphorus
Cyclic voltammetry
Atoms

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Using Ligand Bite Angles to Control the Hydricity of Palladium Diphosphine Complexes. / Raebiger, James W.; Miedaner, Alex; Curtis, Calvin J.; Miller, Susie M.; Anderson, Oren P.; DuBois, Daniel L.

In: Journal of the American Chemical Society, Vol. 126, No. 17, 05.05.2004, p. 5502-5514.

Research output: Contribution to journalArticle

Raebiger, James W. ; Miedaner, Alex ; Curtis, Calvin J. ; Miller, Susie M. ; Anderson, Oren P. ; DuBois, Daniel L. / Using Ligand Bite Angles to Control the Hydricity of Palladium Diphosphine Complexes. In: Journal of the American Chemical Society. 2004 ; Vol. 126, No. 17. pp. 5502-5514.
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N2 - 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, ΔG°H. and ΔG°H-, respectively. Linear free-energy relationships are observed between pKa and the Pd(I/0) couple and between ΔG°H- and the Pd(II/I) couple. The measured values for ΔG°H. were all 57 kcal/mol, whereas the values of ΔG°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 ΔG°H-.

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