Using Ligand Bite Angles To Control the Hydricity of Palladium Diphosphine Complexes
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- 作者：James W. Raebiger ; Alex Miedaner ; Calvin J. Curtis ; Susie M. Miller ; Oren P. Anderson ; and Daniel L. DuBois
- 刊名：Journal of the American Chemical Society
- 出版年：2004
- 出版时间：May 5, 2004
- 年：2004
- 卷：126
- 期：17
- 页码：5502 - 5514
- 全文大小：269K
- 年卷期：v.126,no.17(May 5, 2004)
- ISSN：1520-5126
文摘
A series of [Pd(diphosphine)2](BF4)2 and Pd(diphosphine)2 complexes have been prepared forwhich the natural bite angle of the diphosphine ligand varies from 78
to 111. Structural studies havebeen completed for 7 of the 10 new complexes described. These structural studies indicate that the dihedralangle between the two planes formed by the two phosphorus atoms of the diphosphine ligands and palladiumincreases by over 50 as the natural bite angle increases for the [Pd(diphosphine)2](BF4)2 complexes. Thedihedral angle for the Pd(diphosphine)2 complexes varies less than 10 for the same range of natural biteangles. Equilibrium reactions of the Pd(diphosphine)2 complexes with protonated bases to form thecorresponding [HPd(diphosphine)2]+ complexes were used to determine the pKa values of the correspondinghydrides. Cyclic voltammetry studies of the [Pd(diphosphine)2](BF4)2 complexes were used to determinethe 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-dissociationfree energies, 
and 
, respectively. Linear free-energy relationships are observed between pKaand the Pd(I/0) couple and between 
and the Pd(II/I) couple. The measured values for 
were all57 kcal/mol, whereas the values of 
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 contributesapproximately 20 kcal/mol to the observed difference of 27 kcal/mol for 
.
to 111. Structural studies havebeen completed for 7 of the 10 new complexes described. These structural studies indicate that the dihedralangle between the two planes formed by the two phosphorus atoms of the diphosphine ligands and palladiumincreases by over 50 as the natural bite angle increases for the [Pd(diphosphine)2](BF4)2 complexes. Thedihedral angle for the Pd(diphosphine)2 complexes varies less than 10 for the same range of natural biteangles. Equilibrium reactions of the Pd(diphosphine)2 complexes with protonated bases to form thecorresponding [HPd(diphosphine)2]+ complexes were used to determine the pKa values of the correspondinghydrides. Cyclic voltammetry studies of the [Pd(diphosphine)2](BF4)2 complexes were used to determinethe 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-dissociationfree energies, and , respectively. Linear free-energy relationships are observed between pKaand the Pd(I/0) couple and between and the Pd(II/I) couple. The measured values for were all57 kcal/mol, whereas the values of 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 contributesapproximately 20 kcal/mol to the observed difference of 27 kcal/mol for .