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

Article abstract of DOI:10.1021/ja0395240

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, ΔG°_H. and ΔG°_H-, respectively. Linear free-energy relationships are observed between pK_a 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-.

Full text of DOI:10.1021/ja0395240

Published on Web 04/07/2004
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,‡ and Daniel L. DuBois*,†
Contribution from the National Renewable Energy Laboratory, 1617 Cole BouleVard,
Golden, Colorado 80401, and Department of Chemistry, Colorado State UniVersity,
Fort Collins, Colorado 80523
Received November 10, 2003; E-mail: dan_dubois@nrel.gov
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]+ f [Pd(diphos-
phine)2]+ + H•) and the heterolytic ([HPd(diphosphine)2]+ f [Pd(diphosphine)2]2+ + H-) bond-dissociation
•
-
free energies, ∆G° and ∆G° , respectively. Linear free-energy relationships are observed between pKa
H
H
-
•
and the Pd(I/0) couple and between ∆G° and the Pd(II/I) couple. The measured values for ∆G° were all
H
H
57 kcal/mol, whereas the values of ∆G° ranged from 43 kcal/mol for [HPd(depe)2]+ (where depe is bis-
-
H
(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 ∆G° .
H
Introduction
structures. Similarly, Dierkes and van Leeuwen performed a
careful analysis of the Cambridge Structural Database and
Transition-metal phosphine complexes are used extensively
as catalysts for many important industrial applications. Most
of the early catalysts used simple monodentate phosphine
ligands, and the reactivities of complexes are understood largely
in terms of steric and electronic effects such as the θ and ø
parameters developed by Tolman.1 In many cases, researchers
found that bidentate diphosphine ligands gave enhanced rates
and selectivities for certain reactions when compared to the
monophosphine ligands. Although Tolman cone angles can be
applied to bidentate diphosphines, the constraints of the
backbone of the ligand limit their applicability. To better explain
the reactivity of chelating diphosphine ligands, Casey and
Whiteker devised the concept of the natural bite angle.2 They
defined the natural bite angle as the chelation angle preferred
by the ligand backbone. Casey and Whiteker used molecular
mechanics calculations to determine the natural bite angle of
various diphosphine ligands, and they found good agreement
between calculated angles and observed angles in crystal
confirmed that the average bite angle in known crystal structures
agrees with the values calculated using molecular mechanics.3
The natural bite angles of a wide variety of diphosphine ligands
used in catalysis have been calculated and range from 72° to
131°.
The concept of the natural bite angle of diphosphine ligands
as applied to transition-metal phosphine catalysts has been
reviewed recently by van Leeuwen and co-workers.4,5 Ligand
bite angles significantly affect the rates and selectivities of
hydroformylation, hydrocyanation, allylic alkylation, and cross-
coupling reactions. For example, van Leeuwen and co-workers
have used a series of xanthene-based diphosphines to probe the
effect of the natural bite angle without changing steric or
electronic properties of the ligands.6 In these studies, the natural
bite angle was varied by changing the size of the middle ring
(3) Dierkes, P.; van Leeuwen, P. W. N. M. J. Chem. Soc., Dalton Trans. 1999,
1519-1529.
(4) Van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P.
Chem. ReV. 2000, 100, 2741-2769.
(5) Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Acc. Chem.
Res. 2001, 34, 895-904.
† National Renewable Energy Laboratory.
‡ Colorado State University.
(1) Tolman, C. A. Chem. ReV. 1977, 77, 313-348.
(2) Casey, C. P.; Whiteker, G. T. Isr. J. Chem. 1990, 30, 299-304.
(6) Kranenburg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J.; van Leeuwen,
P. W. N. M.; Goubitz, K.; Fraanje, J. Organometallics 1995, 14, 3081-
3089.
9
5502
J. AM. CHEM. SOC. 2004, 126, 5502-5514
10.1021/ja0395240 CCC: $27.50 © 2004 American Chemical Society

Hydricity of Palladium Diphosphine Complexes
A R T I C L E S
of the NiI or PdI complexes. In this report, we investigate the
link between the ligand bite angle of the diphosphine and the
hydride donor ability in palladium complexes.
Chart 1. Selected Diphosphine Ligands with Abbreviations and
Calculated Natural Bite Angles
We selected a series of five known ligands with a range of
natural bite angles of 33°. The ligands along with their bite
angles are shown in Chart 1 (R ) Ph). Although [Pd(dppe)2]-
(BF4)2, [Pd(dppp)2](BF4)2, and [Pd(dppx)2](BF4)2 had been
prepared previously,14 investigations with the ligands DPEphos
and Xantphos showed that the [Pd(diphosphine)2]2+ complexes
were not very stable. Therefore, we opted to use the analogous
ligands in which ethyl groups have been substituted for the
phenyl groups on the phosphines, as depicted in Chart 1. The
resulting complexes are more stable, and the natural bite angles
should be very similar because the backbones of the ligands
are identical. Using the ligands with R ) Et in Chart 1, we
have synthesized and characterized the [Pd(diphosphine)2]2+ and
Pd(diphosphine)2 complexes and determined the acidities and
hydride donor abilities (hydricities) of the corresponding [HPd-
(diphosphine)2]+ complexes in an effort to correlate the hydricity
with the natural bite angle of the ligand.
b
c
d
a Reference 7. Reference 4. Reference 3. Reference 6.
Experimental Section
of the xanthene backbone. For palladium-catalyzed allylic
alkylation reactions, they found that Xantphos (see Chart 1)
effected 100% selectivity for the desired linear products.7
Although there are several influences on the selectivities of
catalysts, the link with ligand bite angle is well established.8
Several groups have examined the kinetics of hydride transfer
from transition-metal hydride complexes. Bullock and co-
workers have examined the rates of hydride transfer for a variety
of metal carbonyl hydrides, including some with phosphine
ligands.9 Very recently, Norton et al. investigated the rate of
hydride transfer from ruthenium diphosphine complexes to
iminium cations.10 They found that smaller bite angles of the
diphosphine ligand favored rapid hydride transfer.
Recently, we have investigated the thermodynamic properties
of transition-metal hydride complexes containing diphosphine
ligands.11-13 Our work focused on nickel and platinum com-
pounds containing ligands with backbones of two or three carbon
atoms such as 1,2-bis(dimethylphosphino)ethane (dmpe) and
1,3-bis(dimethylphosphino)propane (dmpp). General methods
for the determination of solution bond-dissociation free energies
(SBDFEs) of metal hydrides were exploited to obtain the values
for the hydride donor abilities of a series of compounds.11,13
Previously, we had also examined the relationship between the
ligand bite angle and the electrochemical potentials of the II/I
and I/0 couples of nickel and palladium diphosphine com-
plexes.14 We found that larger bite angles caused distortions
from square-planar geometry that resulted in the stabilization
Syntheses. All manipulations were performed under an inert
atmosphere using standard Schlenk techniques or a glovebox. Solvents
were distilled under nitrogen using standard procedures. Diphenyl ether
(99%, Aldrich) was recrystallized from 90% EtOH, and acenaphthylene
(75%, Aldrich) was sublimed (75 °C, 50 mTorr) prior to use. Lithium
diethylphosphide was prepared by reacting diethylphosphine and
butyllithium in hexane. Triethylammonium tetrafluoroborate and
1,1,3,3-tetramethylguanidinium tetrafluoroborate were prepared by
reacting either triethylamine or 1,1,3,3-tetramethylguanidine with HBF4‚
OMe2 in ether and washing the resulting solid with ether. All other
materials were purchased and used as received.
[Pd(depe)2](BF4)2. A solution of 0.45 g (1.0 mmol) of [Pd(MeCN)4]-
(BF4)2 in 20 mL of MeCN was added to a solution of 0.42 g (2.0 mmol)
of 1,2-bis(diethylphosphino)ethane (depe) in 20 mL of CH2Cl2. The
resulting colorless solution was stirred overnight. The solution was
reduced to dryness, and the solid was redissolved in 5 mL of MeCN.
This solution was filtered, and 30 mL of EtOH was added to the filtrate.
Upon cooling the filtrate for several hours at -20 °C, a precipitate
formed. The resulting solid was collected by cannula filtration, washed
with 2 × 20 mL of EtOH, and dried under vacuum. Yield ) 0.54 g
(77%) of a white powder. 1H NMR (CD3CN): δ 2.30-2.18 (m, PCH2-
CH3), 2.10-2.00 (m, PCH2CH2P), 1.17 (m, PCH2CH3). 31P NMR (CD3-
CN): δ 65.24 (s). Anal. Calcd for C20H48B2F8P4Pd: C, 34.69; H, 6.99;
P, 17.89. Found: C, 34.52; H, 7.04; P, 17.69.
Pd(depe)2. Tris(dibenzylideneacetone)dipalladium(0) (0.44 g, 0.48
mmol, Aldrich) and depe (0.40 g, 1.9 mmol) were added to 35 mL of
THF. The red-purple reaction solution was stirred for 4 h, during which
the solution became orange-yellow. The solution was filtered to remove
a small amount of black solid, and the filtrate was reduced to dryness.
The solids were washed with 3 × 20 mL of MeCN to remove
dibenzylideneacetone and dried under vacuum. Yield ) 0.20 g (40%)
of an off-white solid. 1H NMR (toluene-d8): δ 1.55-1.38 (m,
PCH2CH2P), 1.34 (m, PCH2CH3), 1.08 (m, PCH2CH3). 31P NMR
(toluene-d8): δ 31.51 (s). Anal. Calcd for C20H48P4Pd: C, 46.29; H,
9.32; P, 23.88. Found: C, 46.37; H, 9.25; P, 23.65.
(7) Kranenburg, M.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Eur. J. Inorg.
Chem. 1998, 25-27.
(8) Freixa, Z.; van Leeuwen, P. W. N. M. J. Chem. Soc., Dalton Trans. 2003,
1890-1901.
(9) Cheng, T.-Y.; Brunschwig, B. S.; Bullock, R. M. J. Am. Chem. Soc. 1998,
120, 13121-13137.
(10) Guan, H.; Iimura, M.; Magee, M. P.; Norton, J. R.; Janak, K. E.
Organometallics 2003, 22, 4084-4089.
(11) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc. 1999, 121,
11432-11447.
[Pd(depp)2](BF4)2. A solution of 0.95 g (2.2 mmol) of [Pd(MeCN)4]-
(BF4)2 in 20 mL of MeCN was added to a solution of 0.95 g (4.4 mmol)
of 1,3-bis(diethylphosphino)propane (depp) in 20 mL of CH2Cl2. The
resulting pale yellow solution was stirred overnight. The solution was
reduced to dryness, and the solid was redissolved in 10 mL of CH2Cl2.
The resulting solution was filtered, and 30 mL of EtOH was added to
(12) Berning, D. E.; Miedaner, A.; Curtis, C. J.; Noll, B. C.; Rakowski DuBois,
M. C.; DuBois, D. L. Organometallics 2001, 20, 1832-1839.
(13) Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. J. Am. Chem. Soc.
2002, 124, 1918-1925.
(14) Miedaner, A.; Haltiwanger, R. C.; DuBois, D. L. Inorg. Chem. 1991, 30,
417-427.
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J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004 5503

A R T I C L E S
Raebiger et al.
the filtrate. The volume was reduced by 5 mL, resulting in a white
precipitate. The mixture was cooled to -20 °C overnight. The solid
was then collected by cannula filtration, washed with 2 × 20 mL of
EtOH, and dried under vacuum. Yield ) 1.34 g (87%) of a white
Bis(2-(diethylphosphino)phenyl)ether (depPE). The procedure for
the analogous diphenylphosphino ligand, DPEphos, was followed.6 A
solution of 4.00 g (23.5 mmol) of diphenyl ether in 15 mL of THF
was added dropwise to a mixture of 20.7 mL (51.7 mmol) of a 2.5 M
solution of butyllithium in hexanes and 7.8 mL (51.7 mmol) of
tetramethylethylenediamine (TMEDA). The reaction was stirred over-
night. The reaction vessel was maintained at room temperature in a
water bath, while a solution of 6.44 g (51.7 mmol) of ClPEt2 in 25 mL
of hexane was added dropwise. A white precipitate had formed by the
end of the addition, and the mixture was stirred overnight. Next, 30
mL of CH2Cl2 was added, causing the precipitate to dissolve. Water
(30 mL) was added, and the organic layer was removed. The organic
solution was washed with an additional 30 mL of water and dried over
Na2SO4. After filtration, the solvent was removed from the filtrate,
producing an orange liquid. The crude product was purified by vacuum
distillation at 110 °C and 50 mTorr. Yield ) 4.17 g (51%) of a yellow-
orange, viscous liquid. The product was approximately 90% pure based
1
powder. H NMR (CD3CN): δ 2.21 (m, PCH2CH2CH2P), 2.13-1.99
(m, PCH2CH3 and PCH2CH2CH2P), 1.18 (m, PCH2CH3). 31P NMR
(CD3CN): δ 2.84 (s). Anal. Calcd for C22H52B2F8P4Pd: C, 36.67; H,
7.27; P, 17.19. Found: C, 36.66; H, 7.64; P, 16.99. Colorless rods
suitable for X-ray diffraction were grown by layering ethanol on a
solution of the complex in dichloromethane and storing at -20 °C.
Pd(depp)2. A solution of sodium acenaphthylenide was prepared
by reacting 42 mg (1.83 mmol) of sodium with 279 mg (1.83 mmol)
of acenaphthylene in 50 mL of freshly distilled THF overnight. This
brown solution was added dropwise to a pale yellow solution of 0.60
g (0.83 mmol) of [Pd(depp)2](BF4)2 in 15 mL of MeCN that was cooled
in an ice bath. Once the addition was complete, the ice bath was
removed, and the green solution was stirred for 2 h while warming to
room temperature. The solution was reduced to dryness, and the solids
were extracted with 3 × 30 mL of ether. The extracts were passed
through a Celite plug to remove NaBF4. The ether solution was reduced
to dryness, and the resulting brown oil was washed with 3 × 30 mL of
cold (0 °C) MeCN to remove acenaphthylene. The remaining solid was
dried for 2 h under vacuum. Yield ) 250 mg (55%) of a pale yellow
solid. 1H NMR (toluene-d8): δ 1.79 (m, PCH2CH2CH2P), 1.50 (m,
PCH2CH3), 1.34 (m, PCH2CH2CH2P), 1.06 (m, PCH2CH3). 31P NMR
(toluene-d8): δ -2.60 (s). Anal. Calcd for C22H52P4Pd: C, 48.31; H,
9.58; P, 22.65. Found: C, 48.10; H, 9.88; P, 22.47.
1
1
on H and 31P NMR data. H NMR (CD3CN): δ 7.45-6.68 (m, Ar-
3
3
H), 1.77 (m, PCH2CH3), 0.99 (d of t, JHH ) 7.6 Hz, JPH ) 14.4 Hz,
PCH2CH3). 31P NMR (CD3CN): δ -23.83 (s).
Pd(depPE)2. A solution of 0.81 g (2.1 mmol) of depPE in 20 mL
of MeCN was added to a solution of 0.45 g (1.0 mmol) of [Pd(MeCN)4]-
(BF4)2 in 20 mL of MeCN, forming an orange solution. The reaction
was stirred for 2 h, and then 1.0 mL (32 mmol) of hydrazine was added
dropwise, resulting in a yellow precipitate. The mixture was stirred
for 4 h. The solid was isolated by cannula filtration, washed with 2 ×
20 mL of MeCN, and dried under vacuum overnight. Yield ) 0.58 g
1
(72%) of a yellow, microcrystalline solid. H NMR (toluene-d8): δ
r,r′-Bis(diethylphosphino)xylene (depx). A solution of 2.34 g (13.4
mmol) of R,R′-dichloro-o-xylene in 30 mL of THF was added dropwise
to a solution of 2.66 g (27.7 mmol) of LiPEt2 in 75 mL of THF at -78
°C. The resulting solution was stirred for 3 h while warming to room
temperature. An aqueous 0.1 M NaCl solution (100 mL) was added,
and the organic layer was removed. The aqueous solution was washed
with 2 × 50 mL of THF, and the combined THF solutions were dried
over Na2SO4. After filtration, the solvent was removed under vacuum
to produce a white solid. The crude product was dissolved in a minimal
amount of MeCN (∼50 mL), and the resulting solution was filtered.
The filtrate was stored at -20 °C to produce a pure product as white
needles. Yield ) 2.78 g (74%). The product showed no detectable
7.20 (d, 3JHH ) 7.2 Hz, Ar-H), 6.99 (t,3JHH ) 7.6 Hz, Ar-H), 6.94 (t,
3JHH ) 7.6 Hz, Ar-H), 6.86 (d, 3JHH ) 8.0 Hz, Ar-H), 1.50 (m, PCH2-
CH3), 0.77 (m, PCH2CH3). 31P NMR (toluene-d8): δ -1.33 (s). Anal.
Calcd for C40H56O2P4Pd: C, 60.11; H, 7.06; P, 15.50. Found: C, 60.12;
H, 7.45; P, 13.21. Despite several attempts with independently prepared
samples, the phosphorus analysis of this compound was consistently
low. Yellow plates suitable for X-ray diffraction were grown by layering
acetonitrile on a solution of the complex in dichloromethane.
[Pd(depPE)2](BF4)2‚2THF. To a yellow solution of 0.30 g (0.38
mmol) of Pd(depPE)2 in 50 mL of THF was added 0.23 g (0.83 mmol)
of ferrocenium tetrafluoroborate. The solution turned orange-brown,
and an orange precipitate slowly formed. The reaction mixture was
stirred overnight. The solid was isolated by cannula filtration, washed
with 2 × 40 mL of THF to remove ferrocene, and dried briefly under
vacuum. The solid was redissolved in a minimal amount of MeCN,
and the solution was filtered. THF (100 mL) was added to the filtrate.
Upon standing overnight, the product formed as orange crystals. It was
isolated by filtration, washed with 2 × 20 mL of THF, and dried under
vacuum overnight. Yield ) 0.29 g (79%) of an orange, crystalline solid.
1H NMR (CD3CN): δ 7.67, 7.59, 7.46, and 7.26 (Ar-H), 2.14 (PCH2-
CH3), 0.88 (PCH2CH3). 31P NMR (CD2Cl2): δ 22.1, 19.8, 6.6, 4.4,
AB pattern with N ) 350 Hz at -60 °C. All NMR signals are broad
at room temperature. Anal. Calcd for C48H72B2F8O4P4Pd: C, 51.61;
H, 6.50; P, 11.09. Found: C, 51.43; H, 6.65; P, 10.86. The solvent
content was confirmed by thermogravimetric analysis. Yellow plates
suitable for X-ray diffraction were grown by the diffusion of ether into
an acetonitrile solution of the compound.
impurities by 1H NMR and 31P NMR. 1H NMR (CD3CN): δ 7.08 (m,
3
Ar-H), 2.93 (s, ArCH2P), 1.41 (m, PCH2CH3), 1.03 (d of t, JHH
)
7.6 Hz, JPH ) 14.4 Hz, PCH2CH3). 31P NMR (CD3CN): δ -18.41
3
(s).
[Pd(depx)2](BF4)2. A solution of 0.45 g (1.0 mmol) of [Pd(MeCN)4]-
(BF4)2 in 30 mL of MeCN was added to a solution of 0.57 g (2.0 mmol)
of depx in 30 mL of CH2Cl2. The homogeneous yellow solution was
stirred, and, within 10 min, a precipitate began to form. The mixture
was stirred 5 h. The product was isolated by cannula filtration and
dried overnight under vacuum. Yield ) 0.65 g (77%) of a pale yellow
powder. 1H NMR (CD3CN): δ 7.20 (m, 4,5-H), 7.03 (br, 3,6-H), 3.37
(br, ArCH2P), 1.99 (br, PCH2CH3), 1.24 (m, PCH2CH3). 31P NMR (CD3-
CN): δ 18.98 (broad). Anal. Calcd for C32H56B2F8P4Pd: C, 45.50; H,
6.68; P, 14.67. Found: C, 45.56; H, 6.69; P, 14.52. Colorless prisms
suitable for X-ray diffraction were grown by diffusion of ether into an
acetonitrile solution of the complex.
Pd(depx)2. To a solution of 0.65 g (0.77 mmol) of [Pd(depx)2](BF4)2
in 30 mL of MeCN was added 1.0 mL (31.9 mmol) of hydrazine via
a syringe. The resulting solution turned from yellow to brown, and a
white precipitate formed. The mixture was stirred overnight. The solid
was isolated by cannula filtration, washed with 3 × 25 mL of MeCN,
and dried under vacuum. Yield ) 0.36 g (69%) of a white solid. 31P
9,9-Dimethyl-4,5-bis(diethylphosphino)xanthene (EtXantphos).
The procedure for the analogous diphenylphosphino ligand, Xantphos,
was followed.6 To a solution of 5.0 g (24 mmol) of 9,9-dimethylxan-
thene in 50 mL of ether was added 10.9 mL (72 mmol) of TMEDA.
Next, 52 mL (72 mmol) of a 1.4 M solution of sec-butyllithium in
cyclohexane was added dropwise. The reaction turned dark red with
some precipitate. The mixture was stirred overnight. A solution of 8.96
g (72 mmol) of ClPEt2 in 15 mL of hexane was added dropwise,
resulting in a white precipitate. The mixture was stirred overnight. The
solvent was removed under vacuum, and the resulting yellow oil was
redissolved in 100 mL of CH2Cl2. The solution was washed with 2 ×
1
NMR (toluene-d8): δ 1.3 (broad) and -0.4 (broad). H NMR signals
are extremely broad. Anal. Calcd for C32H56P4Pd: C, 57.27; H, 8.41;
P, 18.46. Found: C, 57.20; H, 8.29; P, 18.27. Colorless rods suitable
for X-ray diffraction were grown by layering acetonitrile on a solution
of the complex in dichloromethane.
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5504 J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004

Hydricity of Palladium Diphosphine Complexes
A R T I C L E S
Table 1. Crystallographic Dataa for [Pd(depp)2], [Pd(depx)2](BF4)2,
and Pd(depx)2
100 mL of water, dried over Na2SO4, and reduced to dryness. The crude
product was recrystallized from EtOH. Yield ) 7.15 g (77%) of an
off-white solid. The purity of the product was greater than 90% based
on 1H and 31P NMR spectra. 1H NMR (CD3CN): δ 7.46 (d of d, 3JHH
) 7.6 Hz, 4JHH ) 1.2 Hz, 1,8-H), 7.21 (m, 3,6-H), 7.12 (t, 3JHH ) 7.6
Hz, 2,7-H), 1.95-1.74 (m, PCH2CH3), 1.57 (s, CH3), 1.01 (d of t, 3JHH
) 7.6 Hz, 3JPH ) 14.0 Hz, PCH2CH3). 31P NMR (CD3CN): δ -26.18
(s).
[Pd(EtXantphos)2](BF4)2. A solution of 1.55 g (4.0 mmol) of
EtXantphos in 50 mL of toluene was added to a solution of 0.88 g (2.0
mmol) of [Pd(MeCN)4](BF4)2 in 20 mL of MeCN. The red homoge-
neous solution was stirred for 1 h and then reduced to dryness under
vacuum. The resulting orange solid was recrystallized by dissolving in
15 mL of CH2Cl2, filtering the resulting solution, and adding 50 mL
of EtOH to the filtrate. The resulting solution was stored at -20 °C
overnight. The product was isolated by cannula filtration and dried
[Pd(depp)2](BF ) ‚
4 2
compound
formula
EtOH
[Pd(depx)2](BF )
Pd(depx)2
4 2
C24H58B2F8OP4Pd C16H28BF4P2Pd0.5 C32H56P4Pd
formula wt
T (K)
766.60
173
422.33
173
671.05
173
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
monoclinic
P21/n
20.3790(8)
16.9052(7)
21.5603(8)
90
111.6720(10)
90
6902.7(5)
8
monoclinic
C2/c
19.58(2)
11.555(13)
16.865(19)
90
99.802(19)
90
3759(7)
8
triclinic
P
10.035(5)
10.316(5)
18.247(9)
80.890(9)
79.214(9)
66.363(8)
1692.6(15)
2
1.317
0.757
0.0377, 0.0913
0.0450, 0.0950
Z
d
calc (g/cm3)
1.475
0.785
1.492
0.727
0.0422, 0.1062
0.0448, 0.1083
µ (mm-1
)
overnight under vacuum. Yield ) 1.60 g (77%) of an orange powder.
R1, wR2 [I < 2σ(I)]b 0.0522, 0.0908
R1, wR2 (all data)b 0.1166, 0.1098
3
1H NMR (CD3CN): δ 7.85 (d, JHH ) 7.6 Hz, 1,8-H), 7.48 (t,3JHH
)
7.6 Hz, 2,7-H), 7.41 (br, 3,6-H), 2.11 and 1.85 (br, PCH2CH3), 1.67
(s, CH3), 0.76 (br, PCH2CH3). 31P NMR (CD3CN): δ 11.1 (broad).
Anal. Calcd for C46H64B2F8O2P4Pd: C, 52.47; H, 6.13; P, 11.77.
Found: C, 51.89; H, 6.18; P, 10.39. Orange-red prisms suitable for
X-ray diffraction were grown by diffusion of ether into an acetonitrile
solution of the complex.
a Obtained with graphite-monochromated Mo KR (λ ) 0.71073 Å)
2
2
radiation. b R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) {∑[w(Fo - Fc )2]/
2
∑[w(Fo )2]}1/2
.
listed in Tables 1 and 2. Unit cell parameters were obtained from a
least-squares fit to the angular coordinates of all reflections, and
intensities were integrated from a series of frames (0.3° ω rotation)
covering more than a hemisphere of reciprocal space. Absorption and
other corrections were applied by using SADABS.15 The structures were
solved by using direct methods and refined (on F 2, using all data) by
a full-matrix, weighted least-squares process. All non-hydrogen atoms
were refined by using anisotropic displacement parameters. Hydrogen
atoms were placed in idealized positions and refined by using a riding
model. Standard Bruker Nonius control (SMART) and integration
(SAINT) software was employed, and Bruker Nonius SHELXTL16
software was used for structure solution and refinement. Thermal
ellipsoid plots were generated with the program Ortep-3 for Windows.17
Pd(EtXantphos)2. To a solution of 0.30 g (0.28 mmol) of [Pd-
(EtXantphos)2](BF4)2 in 30 mL of MeCN was added 0.2 mL (6.4 mmol)
of hydrazine via a syringe. The resulting solution turned from orange
to red, and a yellow precipitate formed. The mixture was stirred for 4
h. The solid was isolated by cannula filtration, washed with 3 × 20
mL of MeCN, and dried under vacuum overnight. Yield ) 0.16 g (64%)
of a yellow solid. 31P NMR (toluene-d8): δ -0.92 (broad) and -9.87
(broad). 1H NMR signals are extremely broad. Anal. Calcd for
C46H64O2P4Pd: C, 62.83; H, 7.34; P, 14.09. Found: C, 62.69; H, 7.45;
P, 13.86. Yellow plates suitable for X-ray diffraction were grown by
layering acetonitrile on a solution of the complex in benzonitrile.
Physical Measurements. NMR spectra were recorded on a Varian
Inova 400 MHz spectrometer. 1H chemical shifts are reported relative
to tetramethylsilane using residual solvent protons as a secondary
reference. 31P chemical shifts are reported relative to external phosphoric
acid. All electrochemical measurements were carried out under an N2
atmosphere in 0.3 M Et4NBF4 in acetonitrile or in 0.3 M Bu4NBF4 in
benzonitrile or dichloromethane. Cyclic voltammetry experiments were
carried out on a Cypress Systems computer-aided electrolysis system.
The working electrode was a glassy carbon disk (2 mm diameter), and
the counter electrode was a glassy carbon rod. A platinum wire
immersed in a permethylferrocene/permethylferrocenium solution was
used to fix the potential. Ferrocene was used as an internal standard,
and all potentials are referenced to the ferrocene/ferrocenium couple.
Results
Syntheses. Both depe and depp are commercially available;
however, the ligands depx, depPE, and EtXantphos had not been
prepared previously. These ligands were prepared by variations
of established literature procedures. The ligand depx was
synthesized by reaction of lithium diethylphosphide with R,R′-
dichloro-o-xylene. This procedure is similar to that used for other
diethylphosphine ligands, such as in the synthesis of depp.13
The ligands depPE and EtXantphos were prepared by methods
nearly identical to those for the diphenylphosphino analogues.6
Lithiation of either diphenyl ether or 9,9-dimethylxanthene
followed by reaction with chlorodiethylphosphine produced the
desired diphosphine molecules. The diphenyl ether ligand,
depPE, was a viscous liquid, whereas the xanthene ligand,
EtXantphos, was an off-white solid.
The most convenient route to the PdII complexes was the
reaction of [Pd(MeCN)4](BF4)2 with 2 equiv of the ligands. The
complexes [Pd(depe)2](BF4)2, [Pd(depp)2](BF4)2, [Pd(depx)2]-
(BF4)2, and [Pd(EtXantphos)2](BF4)2 could be prepared by this
method in yields of 77-87%. This procedure did not work well
for depPE, which produced an oily product that despite several
attempts could not be purified. It was found that the complex
pKa of [HPd(diphosphine)2]+. In a typical experiment, Pd(diphos-
phine)2 and either triethylammonium tetrafluoroborate (Et3NH+BF4
)
)
-
or 1,1,3,3-tetramethylguandinium tetrafluoroborate ((Me2N)2CNH2+BF4-
were accurately weighed into an NMR tube and dissolved in benzoni-
trile. A known quantity of conjugate base (either triethylamine or
tetramethylguanidine) was added via a gastight syringe. The solutions
were mixed and monitored by 31P NMR (unlocked). The reactions came
to equilibrium within 20 min and were checked every several hours
for 24 h to ensure that the ratios had not changed. The signals of [HPd-
(diphosphine)2]+ and Pd(diphosphine)2 were integrated, and the ratio
was used to determine the equilibrium constant for the reaction. At
least three separate experiments were run to confirm the result. The
equilibrium constant was used to calculate the pKa of the palladium
hydride.
X-ray Crystallography. X-ray diffraction data from selected crystals
were recorded on a Bruker Nonius SMART CCD diffractometer
employing Mo KR radiation (graphite monochromator). Selected details
related to the crystallographic experiment for seven compounds are
(15) Sheldrick, G. M. SADABS - a program for area detector absorption
corrections.
(16) Sheldrick, G, M. SHELXTL, v. 6.12; Bruker AXS: Madison, WI, 1999.
(17) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
9
J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004 5505

A R T I C L E S
Raebiger et al.
Table 2. Crystallographic Dataa for [Pd(depPE)2](BF4)2, Pd(depPE)2, [Pd(EtXantphos)2](BF4)2, and Pd(EtXantphos)2
[Pd(depPE)2](BF ) ‚
[Pd(EtXantphos)2](BF ) ‚
4 2
4 2
compound
formula
formula wt
T (K)
EtOH
Pd(depPE)2
2MeCN‚1/2Et2O
Pd(EtXantphos)2
C42H62B2F8O3P4Pd
1018.82
173
C40H56O2P4Pd
799.13
173
C104H150B4F16N4O5P8Pd2
2344.08
164
C46H64O2P4Pd
879.25
173
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
monoclinic
P21/n
12.423(10)
22.881(17)
16.794(13)
90
100.713(15)
90
4690(6)
monoclinic
C2/c
24.902(15)
10.638(6)
17.118(10)
90
118.091(9)
90
4000(4)
4
triclinic
P
triclinic
P
14.0903(19)
18.715(3)
23.130(3)
106.886(2)
106.065(3)
90.951(3)
5577.1(13)
2
1.396
0.516
0.0775, 0.2142
0.1141, 0.2393
11.087(2)
14.170(3)
15.859(3)
90.503(3)
109.192(3)
106.287(3)
2244.7(7)
2
1.301
0.591
0.0376, 0.0949
0.0421, 0.0978
Z
d
4
calc (g/cm3)
1.443
0.601
0.0674, 0.1700
0.1041, 0.1914
1.327
0.656
0.0305, 0.0939
0.0328, 0.0973
µ (mm-1
)
R1, wR2 [I < 2σ(I)]b
R1, wR2 (all data)b
2
2
2
a Obtained with graphite-monochromated Mo KR (λ ) 0.71073 Å) radiation. b R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
.
Table 3. 31P NMR Data for [Pd(diphosphine)2]2+ and
Pd(diphosphine)2 at Various Temperatures
[Pd(depPE)2](BF4)2 could be prepared in 79% yield by oxidation
of the Pd0 complex with ferrocenium tetrafluoroborate in THF.
31P δ (ppm)a
The Pd0 complexes were prepared primarily by reduction of
the PdII species. For the complexes [Pd(depx)2](BF4)2, [Pd-
(depPE)2](BF4)2, and [Pd(EtXantphos)2](BF4)2, which have
reduction potentials more positive than -1.0 V versus ferrocene
(see below), hydrazine was a convenient reducing agent.
Because the neutral Pd0 complexes are insoluble in acetonitrile,
reaction of the PdII complexes with hydrazine in MeCN solution
produced the desired products as pure, microcrystalline solids.
Using this method, the compounds Pd(depx)2 (69%) and Pd-
(EtXantphos)2 (64%) were isolated in the indicated yields.
Because of the problem of isolating the PdII complex of depPE,
the compound Pd(depPE)2 was made directly by adding
hydrazine to a solution of [Pd(MeCN)4](BF4)2 and depPE in
MeCN and isolating the yellow product in 72% yield. Because
of its more negative reduction potential, the complex Pd(depp)2
was prepared using the stronger reducing agent, sodium
acenaphthylenide; workup afforded the product in 55% yield.
Pd(depe)2 was most readily prepared by reacting the Pd0 starting
material tris(dibenzylideneacetone)dipalladium(0) with 4 equiv
of depe in THF solution.
c
compound
solvent
20 °C
65.24
31.51
2.84
-2.60
18.98(br)
low tempb
high temp
[Pd(depe)2]2+
Pd(depe)2
CD3CN
toluene-d8
CD3CN
toluene-d8
CD3CN
[Pd(depp)2]2+
Pd(depp)2
[Pd(depx)2]2+
23.59 d
13.58 d
3.54 e
18.13
0.89
Pd(depx)2
toluene-d8
1.3(br)
-0.4(br)
20.9 f
-0.62 e
20.95 g
5.47 g
[Pd(depPE)2]2+
CD2Cl2
5.5 f
Pd(depPE)2
toluene-d8
CD3CN
-1.33
11.10(br)
[Pd(EtXantphos)2]2+
15.71 h
13.56h
0.60 i
11.89
Pd(EtXantphos)2
toluene-d8
-0.92(br)
-9.87(br)
-5.65
-9.76 i
a Peaks are sharp singlets except where indicated (br ) broad).
b Measured at -40 °C in CD3CN and CD2Cl2 or -60 °C in toluene-d8.
c Measured at +60 °C in CD3CN or +80 °C in toluene-d8. d Triplets with
a splitting of 25.5 Hz. e Triplets with a splitting of 14.1 Hz. f Apparent AB
quartet with broad peaks. g Apparent AB quartet with N ) 350 Hz.
h Apparent AB quartet with N ) 348 Hz. i Triplets with a splitting of 19.6
Hz.
The diethylphosphino ligands in Chart 1 were found to
produce stable compounds with both PdII and Pd0. Of all the
compounds prepared, only Pd(depe)2 and Pd(depp)2 are very
air-sensitive; they turn black within minutes of exposure to air.
However, air was excluded from all samples due to the
possibility of oxidizing the phosphine moieties on the ligands.
sharp peak due to equivalence of the phosphorus atoms.
However, at low temperature, the phosphorus atoms become
nonequivalent, and [Pd(depx)2](BF4)2, Pd(depx)2, and Pd-
(EtXantphos)2 each have two well-separated triplets. This pattern
is typical of a degenerate A2X2 spin system. At -40 °C, [Pd-
(EtXantphos)2](BF4)2 exhibits a pattern similar to an AB quartet,
again consistent with a degenerate AA′XX′ system, but one that
has significantly larger coupling constants than the other three
complexes. As discussed under structural studies, the two
phosphorus atoms of each EtXantphos ligand occupy trans
positions in a tetrahedrally distorted square-planar complex. The
fact that the two nonequivalent phosphorus atoms of the same
diphosphine ligand are trans to each other is consistent with a
larger P-P coupling. At room temperature in CD3CN, [Pd-
(depPE)2](BF4)2 exhibits no signals in the 31P spectrum, but in
CD2Cl2 has an apparent AB quartet similar to [Pd(EtXantphos)2]-
(BF4)2 at low temperature, suggesting similar structures in
solution. The lack of peaks in acetonitrile is believed to be due
to a fluxional process that exchanges the nonequivalent phos-
1
NMR Spectroscopy. All compounds were analyzed by H
and 31P{1H} NMR spectroscopy. The 31P shifts of the com-
pounds are gathered in Table 3. The compounds [Pd(depe)2]-
(BF4)2, Pd(depe)2, [Pd(depp)2](BF4)2, Pd(depp)2, and Pd(depPE)2
exhibit simple 31P NMR spectra at room temperature with single,
sharp peaks indicative of all four phosphorus atoms being
equivalent. The 31P spectra are not as straightforward for the
other compounds. [Pd(depx)2](BF4)2 and [Pd(EtXantphos)2]-
(BF4)2 have single broad peaks; the corresponding zero-valent
compounds, Pd(depx)2 and Pd(EtXantphos)2, have two broad
peaks at room temperature (20 °C). All four species were
examined at temperatures between -60 and +80 °C (neutral
complexes - toluene-d8) or -40 and +60 °C (2+ complexes
- CD3CN). At high temperatures, all spectra display a single,
9
5506 J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004

Hydricity of Palladium Diphosphine Complexes
A R T I C L E S
Table 4. Selected Structural Data for the Compounds in Tables 1
and 2
diphosphine ligand are trans to each other. However, the smallest
average P-Pd-P angle is 95.8° with a dihedral angle between
the planes defined by these angles of 55.4°, resulting in a
geometry that is intermediate between square planar and
tetrahedral.
average ligand
bite angle (deg)
ligand-to-ligand
dihedral angle (deg)
average Pd−P
distance (Å)
compound
[Pd(depp)2]2+
87.0
90.1
94.5
139.7
95.8a
22.3
32.1
38.0
89.0
55.4b
2.3655(19)
2.342(2)
2.381(2)
2.338(3)
[Pd(depx)2]2+
Crystal structures of three Pd0 compounds were obtained: Pd-
(depx)2, Pd(depPE)2, and Pd(EtXantphos)2; Figure 2 shows the
structures of these molecules. The three neutral species have
distorted tetrahedral geometries typical of d10 complexes. As
seen from the data in Table 4, the ligands have bite angles that
are larger than those in the PdII species and closer to that
expected for tetrahedral geometry (109.5°). For the same ligand,
the bite angles increase by about 13° as the oxidation state of
the metal changes from PdII to Pd0. The dihedral angles between
the ligand planes are between 80° and 90°. With an increase in
the bite angle, there is a small decrease in the dihedral angle,
presumably due to steric effects. All Pd-P distances are about
2.34 Å, which is similar to that observed for the PdII compounds.
Pd(depPE)2 exhibits Pd-P bonds that are shorter by more than
0.04 Å when compared to [Pd(depPE)2]2+. The shortening of
the M-P bond upon reduction has been observed previously.11
As mentioned above, the low-temperature NMR spectra of
compounds containing depx, depPE, and EtXantphos display
peaks characteristic of two sets of phosphorus atoms. It is
evident from the structural analysis of the compounds that one
phosphorus atom of one ligand is closer to the ring backbone
of the other ligand. This is clearly apparent in the drawing of
[Pd(depx)2]2+ (Figure 1B), where P1 is closer than P2 to the
xylene ring of the rear ligand. For all three ligands, the
nonequivalence of the phosphorus atoms is a result of ring
conformations of the ligand backbone that interconvert rapidly
at high temperature in solution, but slowly at or below room
temperature.
[Pd(depPE)2]2+
[Pd(EtXantphos)2]2+
Pd(depx)2
Pd(depPE)2
Pd(EtXantphos)2
103.2
106.3
108.3
89.3
84.6
80.6
2.3404(13)
2.3371(10)
2.3432(9)
a For the smallest P-Pd-P angle. b For the angle between the planes
defined in footnote a.
phorus atoms of the diphosphine ligands. This process is
promoted by acetonitrile and could occur by a chelation/
dechelation mechanism or by acetonitrile promoting a confor-
mational rearrangement.
In the crystal structures discussed below, conformations of
the ligand backbones render the phosphorus atoms nonequiva-
lent. In these distorted tetrahedral structures, the phosphorus
atoms are not equivalent because one phosphorus atom of one
ligand is closer to the ring backbone of the other ligand. Above
room temperature, there is enough thermal energy to allow the
ligand conformations to interconvert, thus causing the phos-
phorus atoms to become equivalent, as evidenced by the
coalescence into one, sharp 31P NMR peak in all four com-
pounds. A similar type of nonequivalence is observed for [RuH-
(solvent)(diphosphine)2]+ complexes using Xantphos-type
ligands.18 Another example is found in the compound (µ-o-
SCH2C6H4CH2S)Fe2(CO)6 where the o-xylyl group of the
bridging dithiolate ligand is fixed in position, lowering the
symmetry of the molecule.19
Structural Studies. Single-crystal X-ray structures have been
obtained for seven compounds listed in Tables 1 and 2. Selected
structural data are found in Table 4. Compounds [Pd(depp)2]-
(BF4)2 and [Pd(EtXantphos)2](BF4)2 have two cations and four
anions in the asymmetric unit, and values are reported as the
average for both cations.
Electrochemistry. Cyclic voltammograms have been ob-
tained for all compounds in at least two different solvents; the
results are listed in Table 5. Except for [Pd(depp)2](BF4)2 in
MeCN, all compounds exhibit reversible waves (ipa ≈ ipc and
∆Ep ) 60 mV) in all solvents. Because all compounds have at
least some solubility in benzonitrile, it was possible to measure
the cyclic voltammograms in this solvent, which enables facile
comparison. The cyclic voltammograms of the five PdII species
in PhCN solution are shown in Figure 3. [Pd(depe)2]2+ and [Pd-
(depp)2]2+ exhibit a single two-electron wave at -1.47 and
-1.23 V, respectively, for the reduction to the Pd0 species. The
other complexes exhibit two one-electron waves, with varying
separations between the waves. The difference in the waves
(∆E1/2 in Table 5) changes from 0 mV for [Pd(depp)2]2+ to 410
mV for [Pd(EtXantphos)2]2+ in benzonitrile. Across the series
of compounds, there are small differences in the potentials of
the 1+/0 couples and larger differences in the 2+/1+ couples.
When comparisons are made in the same solvent, the cyclic
voltammograms of the PdII species were identical to those of
the corresponding Pd0 species.
The structures of four PdII compounds have been obtained:
[Pd(depp)2](BF4)2‚EtOH, [Pd(depx)2](BF4)2, [Pd(depPE)2](BF4)2‚
EtOH, and [Pd(EtXantphos)2](BF4)2‚2MeCN‚1/2Et2O. Figure 1
shows the structures of the cations [Pd(depp)2]2+, [Pd(depx)2]2+
,
[Pd(depPE)2]2+, and [Pd(EtXantphos)2]2+. The PdII complexes
all exhibit tetrahedrally distorted square-planar geometries
typical of d8 metals. The tetrahedral distortion increases with
increasing ligand bite angle, as evidenced by the increase in
the dihedral angle between the two planes defined by the
palladium atom and the two phosphorus atoms of each diphos-
phine ligand (Table 4). The average Pd-P distance in these
compounds ranges from 2.34 to 2.38 Å, which is typical of
palladium phosphines.20,21 The structure of [Pd(EtXantphos)2]-
(BF4)2 is different in that the ligand has a very large bite angle
of almost 140°, and the two phosphorus atoms of each
As shown in Table 5, the wave separation changes slightly
in different solvents, but these differences are minor. The more
polar solvents display smaller wave separations, so the separa-
tion increases in the order MeCN < PhCN < CH2Cl2. On
average, the Pd(II/I) couples become more negative by 68 mV
on changing from dichloromethane to acetonitrile, and the Pd-
(I/0) couples become more positive by 72 mV. This leads to
(18) Len˜ero, K. A.; Kranenburg, M.; Guari, Y.; Kamer, P. C. J.; van Leeuwen,
P. W. N. M.; Sabo-Etienne, S.; Chaudret, B. Inorg. Chem. 2003, 42, 2859-
2866.
(19) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M. Y. J.
Am. Chem. Soc. 2001, 123, 3268-3278.
(20) Mason, M. R.; Verkade, J. G. Organometallics 1992, 11, 2212-2220.
(21) Edelbach, B. L.; Vicic, D. A.; Lachicotte, R. J.; Jones, W. D. Organome-
tallics 1998, 17, 4784-4794.
9
J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004 5507

A R T I C L E S
Raebiger et al.
Figure 1. Structures of [Pd(depp)2]2+ (A), [Pd(depx)2]2+ (B), [Pd(depPE)2]2+ (C), and [Pd(EtXantphos)2]2+ (D) showing 50% probability ellipsoids and the
atom numbering scheme. H atoms have been omitted for clarity. Half of the [Pd(depx)2]2+ cation is related by symmetry. In (D), labels on the carbons of
the ethyl groups on P4 have been omitted for clarity.
expected potentials of -1.19 and -1.23 V, respectively, for
the (I/0) and (II/I) couples of [Pd(depp)2]2+ as compared to a
single wave at -1.22 V in acetonitrile. We have used the value
and maintained a constant ratio of [HPd(diphosphine)2]+ to Pd0-
(diphosphine)2 for at least 24 h. This ratio was determined by
integration of the 31P NMR signals and used to calculate an
equilibrium constant for the reaction. Using eq 2, the pKa of
the palladium hydrides in acetonitrile could be calculated using
the pKa values of the acids (BH+) above. These values are listed
in Table 6. The larger errors assigned to the values for [HPd-
(depPE)2]+ and [HPd(EtXantphos)2]+ are due to the fact that
the 31P peaks for the hydride and the Pd0 complexes overlap,
resulting in the integration values being less accurate.
-
•
of -1.22 V in our calculations of ∆G° and ∆G° discussed
H
H
below as any errors introduced by this small difference in
potential would be small (less than 0.7 kcal/mol).
Acid-Base Equilibria and Solution Bond-Dissociation
Free Energies. The pKa values of the palladium-hydride
complexes were measured in benzonitrile solution by reacting
Pd(diphosphine)2 with an appropriate acid (BH+) of known pKa
according to reaction 1. It was found that triethylammonium
(pKa ) 18.5 in acetonitrile)22 worked well for Pd(depx)2, Pd-
(depPE)2, and Pd(EtXantphos)2, and 1,1,3,3-tetramethylguani-
dinium (pKa ) 23.3 in acetonitrile)22 worked well for Pd(depe)2
and Pd(depp)2. All reactions came to equilibrium within 20 min
Pd(diphosphine)2 + BH+ h [HPd(diphosphine)2]+ + B
(1)
pKa [HPd(diphosphine)2]+ ) pKa (BH+) + log Keq (2)
The free energies associated with the heterolytic (reaction 3)
and homolytic (reaction 5) cleavage of an M-H bond in solution
(22) Kolthoff, I. M.; Chantooni, M. K., Jr.; Bhowmik, S. J. Am. Chem. Soc.
1968, 90, 23-28.
9
5508 J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004

Hydricity of Palladium Diphosphine Complexes
A R T I C L E S
Table 5. Cyclic Voltammetry Data for [Pd(diphosphine)2]2+ and
a
Pd(diphosphine)2
compound
solvent
E1/2(II/I)
E1/2(II/0)
E1/2(I/0)
∆E
1/2
[Pd(depe)2]2+
MeCN
PhCN
CH2Cl2
PhCN
CH2Cl2
MeCN
PhCN
PhCN
MeCN
PhCN
CH2Cl2
PhCN
MeCN
PhCN
CH2Cl2
PhCN
CH2Cl2
MeCN
PhCN
CH2Cl2
PhCN
-1.48
-1.47
-1.47
-1.47
Pd(depe)2
[Pd(depp)2]2+
-1.16
-1.26
-1.22b
-1.23
0.10
-1.22b
-1.23
-1.23
Pd(depp)2
[Pd(depx)2]2+
-0.94
-0.92
-0.84
-0.92
-0.73
-0.71
-0.66
-0.71
-0.49
-0.55
-0.54
-0.49
-0.53
-1.02
-1.01
-1.06
-1.02
-0.92
-0.93
-0.99
-0.93
-1.02
-0.94
-0.95
-1.01
-0.95
0.08
0.09
0.22
0.10
0.19
0.22
0.33
0.22
0.53
0.39
0.41
0.52
0.42
Pd(depx)2
[Pd(depPE)2]2+
Pd(depPE)2
[Pd(EtXantphos)2]2+
Pd(EtXantphos)2
a All potentials are reported as volts versus the ferrocene/ferrocenium
couple and were measured at a scan rate of 50 mV/s. b Quasi-reversible
(∆Ep ) 110 mV, ipa * ipc).
dissociation free energies. For complexes with two one-electron
processes, E1/2(II/0) is the average of the two E1/2 values. The
-
•
calculated values of ∆G° and ∆G° are found in Table 6.
H
H
MH f M+ + H-
(3)
(4)
-
∆G° ) 1.37 pKa + 46.1 E1/2(II/0) + 79.6
H
MH f M• + H•
(5)
(6)
•
∆G° ) 1.37 pKa + 23.06 E1/2(I/0) + 53.6
H
Discussion
Structural Studies. The objective of this study is to
determine the influence of the natural bite angle on the
thermodynamic properties of [Pd(diphosphine)2]2+ and [HPd-
(diphosphine)2]+ complexes. To that end, structural determina-
tions of these complexes are desirable for determining how
natural bite angles actually contribute to important geometric
features. Ten new palladium diphosphine compounds have been
prepared, and seven have been characterized by X-ray crystal-
lography. Of the three compounds without structural data, two
are extremely air-sensitive, Pd(depe)2 and Pd(depp)2, and
purifications did not produce crystalline materials. The third,
[Pd(depe)2](BF4)2, produced only microcrystalline material,
despite numerous attempts to recrystallize it. Thus, structural
data were obtained for complexes containing four of the five
ligands studied.
[Pd(diphosphine)2]2+ complexes are normally regarded as
square-planar complexes. However, as can be seen from the
data for the four Pd(II) complexes in Table 4, many of these
complexes are neither square nor planar. The observed bite
angles actually vary from 87.0° to 139.7° (a range of 53°),
whereas the natural bite angles vary from 86° to 111° (a range
of 25°). As mentioned above, the compound [Pd(EtXant-
phos)2]2+ is unique in that it contains a ligand bite angle of
nearly 140°, which is much larger than the natural bite angle
Figure 2. Structures of Pd(depx)2 (A), Pd(depPE)2 (B), and Pd(EtXant-
phos)2 (C) showing 50% probability ellipsoids and the atom numbering
scheme. H atoms have been omitted for clarity. Half of the Pd(depPE)2
molecule is related by symmetry. In (C), labels on the carbons of the ethyl
groups on P3 have been omitted for clarity.
are given by eqs 4 and 6, respectively. Equations 4 and 6 are
based on thermodynamic cycles,11,23 and they require pKa values
and half-wave potentials for calculating the solution bond-
(23) Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1989, 111, 6711-6717.
Correction: Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1990, 112, 2843.
9
J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004 5509

A R T I C L E S
Raebiger et al.
Figure 3. Cyclic voltammograms of 2 mM solutions of [Pd(depe)2]2+ (A), [Pd(depp)2]2+ (B), [Pd(depx)2]2+ (C), [Pd(depPE)2]2+ (D), and [Pd(EtXantphos)2]2+
(E) in 0.3 M Bu4NBF4 benzonitrile solution. Scan rate is 50 mV/s.
Table 6. Half-Wave Potentials of [Pd(diphosphine)2]2+ Complexes and the Thermodynamic Properties of the Corresponding
[HPd(diphosphine)2]+ Complexes
1/2(II/0)a
E
1/2(I/0)a
pK
a
∆G°
-
G° d
•
pK (H )e
∆pK
a
b
c
f
diphosphine
E
a
2
H
H
depe
depp
depx
depPE
EtXantphos
-1.48
-1.22
-0.98
-0.83
-0.75
-1.24g
-1.22
-1.02
-0.92
-0.94
23.2 ( 0.2
22.9 ( 0.2
19.8 ( 0.2
18.3 ( 0.5
18.5 ( 0.5
43.2
54.7
61.5
66.4
70.4
56.8
56.8
57.2
57.5
57.3
23.9
15.5
10.6
7.0
-0.7
7.4
9.2
11.3
14.4
4.1
a Potentials (V vs FeCp2/FeCp2+) for [Pd(diphosphine)2]2+ in acetonitrile. E1/2(II/0) values are the average of E1/2(II/I) and E1/2(I/0) where two one-
electron waves are present. b Values for [HPd(diphosphine)2]+ determined experimentally from equilibrium reaction 1 and calculated from eq 2. c Calculated
from eq 4; value is in kcal/mol. d Calculated from eq 6; value is in kcal/mol. e Calculated from eq 10. f ∆pKa ) pKa - pKa(H2). g Calculated from eq 7.
of 111° calculated for Xantphos. The flexibility range (the range
of bite angles in which strain is less than 3 kcal/mol)2 for this
ligand is calculated to be 97-135°.6 Apparently, the ligand
cannot achieve an angle close to 90° in the distorted square-
planar structure, and instead spans pseudo-trans positions with
a P-Pd-P angle of 139.7°. The smallest P-Pd-P angle in
the structure is 95.8° with an associated dihedral angle of 55.4°.
These angles are consistent with a significantly distorted square-
planar complex, but indicate a less dramatic distortion than
suggested by the bite angles and dihedral angles based on the
EtXantphos ligand.
example of a palladium depe complex, and it has a P-Pd-P
angle of 84.5°.21 Therefore, the expected bite angle of depe in
[Pd(depe)2]2+ is approximately 85°. It is also anticipated that
the dihedral angle between the two planes defined by the two
depe ligands and Pd will be near 0°. In the analogous nickel
complex, [Ni(depe)2]2+, the dihedral angle is 0.0°.11 Thus, a
chelate bite angle of 85° and a dihedral angle of near 0° is
expected for [Pd(depe)2]2+. Using these estimated values for
[Pd(depe)2]2+, the values for [Pd(EtXantphos)2]2+ based on the
smallest P-Pd-P angle, and the remaining values shown in
Table 4, the average bite angle appears to increase monotonically
from 85° to 96° and the dihedral angle from 0° to 55° as the
natural bite angle increases from 86° to 111°. Clearly, the metal
complex restricts the range of observed bite angles so that the
natural bite angles are not completely expressed. A more
pronounced manifestation of the natural bite angle for these [Pd-
(diphosphine)2]2+ complexes is the dihedral angle, which
increases from near 0° for [Pd(depe)2]2+ to 55° for [Pd-
(EtXantphos)2]2+. This angle appears to amplify the natural bite
angle. The large tetrahedral distortions observed for these [Pd-
(diphosphine)2]2+ complexes have significant effects on the
potentials of the Pd(II/I) redox couples of these complexes and
on the hydride donor abilities of the corresponding [HPd-
(diphosphine)2]+ complexes (as discussed below).
Because crystals suitable for diffraction studies were not
obtained for the [Pd(depe)2]2+ cation, we were unable to obtain
structural data for this complex. However, there are numerous
examples of the analogous ligand dppe (see Chart 1), and a
CSD search conducted in 1999 found that the average ligand
bite angle for metal complexes containing this ligand was 82.6°.3
For Pd(dppe)Cl2 and Pd(dppe)(SCN)(NCS), the chelate angles
are 85.8° and 85.1°, respectively.24,25 To our knowledge, the
organometallic compound (depe)Pd(2,2′-biphenyl) is the only
(24) Steffen, W. L.; Palenik, G. J. Inorg. Chem. 1976, 15, 2432-2439.
(25) Palenik, G. J.; Mathew, M.; Steffen, W. L.; Beran, G. J. Am. Chem. Soc.
1975, 97, 1059-1066.
9
5510 J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004

Hydricity of Palladium Diphosphine Complexes
A R T I C L E S
that both oxidation and protonation are similar processes
involving the highest occupied molecular orbital (HOMO) of
these tetrahedral M(diphosphine)2 complexes.
pKa ) -15.5 E1/2(I/0) + 4.0
(7)
Structural studies of the isoelectronic [Ir(dppf)2] and [Ir(dppf)2]-
complexes (where dppf is bis(diphenylphosphino)ferrocene)
indicate that structural rearrangements associated with the (d9/
d10) couple are small with average chelate bite angles of 99.9°
and 102.3°, respectively, and dihedral angles of 74.7° and 85.0°,
respectively.28 Because the geometries of the [Pd(diphos-
phine)2]+ and Pd(diphosphine)2 complexes are expected to be
very similar, it is anticipated that E1/2(I/0) will not be strongly
dependent on the chelate bite angle. As seen in Table 5, the
potentials of the Pd(I/0) couples of [Pd(EtXantphos)2]2+ (-0.94
V) and [Pd(depPE)2]2+ (-0.92 V) are nearly the same. The same
is true for the potentials of the (I/0) couples for [Pd(depp)2]2+
(-1.22 V) and [Pd(depe)2]+ (-1.24 V, see following paragraph).
These data are consistent with our previous observations for
[Ni(diphosphine)2]2+ complexes, that the potential of the (I/0)
couple is independent of the chelate bite angle for complexes
with five- and six-membered chelate rings.
The same concepts appear to hold for protonation reactions,
as well. Protonation of Pd(diphosphine)2 complexes to form
[HPd(diphosphine)2]+ derivatives results in electron transfer
from palladium to the proton. On the basis of theoretical and
X-ray diffraction studies of [HM(diphosphine)2]n+ complexes
[where M ) Co (n ) 0), Pd (n )1), or Pt (n ) 1)], these hydride
complexes can be described as distorted trigonal bipyramidal
or face-capped tetrahedral complexes.29-31 The distorted tetra-
hedral arrangement of the phosphorus ligands in these structures
demonstrates that protonation results in a relatively small
structural rearrangement of the MP4 core. The variation in the
pKa values for the [HPd(diphosphine)2]+ complexes shown in
Table 6 and the E1/2(I/0) values of the [Pd(diphosphine)2]2+
complexes appear to arise from inductive effects of the
substituents on the diphosphine ligands. As a result, the pKa
values of [HPd(EtXantphos)2]+ (18.5) and [HPd(depPE)2]+
(18.3) are nearly the same, as are those of [HPd(depp)2]+ (22.9)
and [HPd(depe)2]+ (23.2). Substitution of the aryl bridge of [Pd-
(EtXantphos)2]2+ and [Pd(depPE)2]2+ with the trimethylene and
ethylene bridges of [Pd(depp)2]2+ and [Pd(depe)2]+ results in
more negative potentials for the (I/0) couples and larger pKa
values for the corresponding hydride complexes, as expected.
Because oxidation and protonation of Pd(diphosphine)2 com-
plexes involve removal of electron density from palladium and
small structural rearrangements of the phosphorus atoms, both
of these processes depend on ligand bite angles and ligand
substituents in the same way, and a linear relationship between
the potentials of the Pd(I/0) couples and pKa values is reason-
able.
Figure 4. Plot of pKa values for [HPd(diphosphine)2]+ in benzonitrile versus
the potentials of the E1/2(I/0) couple of the corresponding [Pd(diphos-
phine)2]2+ complexes in benzonitrile. The line is the best linear fit of the
data with eq 7.
For the Pd(0) complexes in Table 4, all of the geometries
are best described as distorted tetrahedra. In an ideal tetrahedron,
all of the P-Pd-P bond angles would be 109.5°, and the
dihedral angles between the two planes defined by two sets of
two phosphorus atoms and palladium should be 90°. For four-
coordinate 18-electron (d10) complexes containing diphosphine
ligands, the actual chelate bite angle adopted is a compromise
between the ideal tetrahedral angle of 109.5° preferred by the
metal and the natural bite angle of the ligand. The ligands with
the smallest natural bite angles, depe and depp, are expected to
deviate from the ideal tetrahedral angles the most. For Pd(depp)2,
estimates of the bite angle and dihedral angle are 99 ( 3° and
88 ( 3°, respectively. These estimates are based on observed
bite angles for Pt(dmpp)2 (99.8°),11 Pt(dppp)2 (97.8°),26 and Pd-
(dppp)2 (97.6°).20 The corresponding dihedral angles for Pt-
(dmpp)2 and Pt(dppp)2 are 89.7° and 87.2°, respectively. A
slightly smaller bite angle is anticipated for Pd(depe)2, but with
a very similar dihedral angle. For the Pd(diphosphine)2 com-
plexes described in this study, the observed bite angles vary by
slightly more than 10° from ideal tetrahedral geometry depend-
ing on the natural bite angle of the diphosphine ligand, and the
dihedral angles vary by approximately 10°. It therefore appears
that the tetrahedral angles preferred for Pd(0) complexes also
reduce the range of angles observed from those expected on
the basis of natural bite angles alone. For Pd(0) complexes, the
dihedral angle is much less sensitive to the natural bite angle
than is observed for Pd(II) complexes.
Thermodynamic Studies. Linear Free-Energy Correlation
of pKa with E1/2(I/0). Angelici and co-workers have shown that
heats of protonation for several series of complexes exhibit a
linear correlation with the oxidation potentials of the com-
plexes.27 Similarly, for [HNi(diphosphine)2]+ complexes, the
pKa values were shown to be linearly dependent on the potentials
of the Ni(I/0) couple of the corresponding conjugate base of
the hydride, Ni(diphosphine)2.12 A plot of the pKa values of the
[HPd(diphosphine)2]+ complexes versus E1/2(I/0) of the corre-
sponding Pd(diphosphine)2 complexes shows an excellent linear
correlation (see Figure 4 and eq 7). This correlation suggests
Correlations such as those shown in Figure 4 and given by
eq 7 are very useful for obtaining accurate estimates of
(28) Longato, B.; Riello, L.; Bandoli, G.; Pilloni, G. Inorg. Chem. 1999, 38,
2818-2823.
(29) Aresta, M.; Dibenedetto, A.; Amodio, E.; Papai, I.; Schubert, G. Inorg.
Chem. 2002, 41, 6550-6552.
(30) Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; Rakowski DuBois, M. J. Am.
Chem. Soc. 2002, 124, 2984-2992.
(26) Asker, K. A.; Hitchcock, P. B.; Moulding, R. P.; Seddon, K. R. Inorg.
Chem. 1990, 29, 4146-4148.
(27) Wang, D.; Angelici, R. J. J. Am. Chem. Soc. 1996, 118, 935-942.
(31) Miedaner, A.; DuBois, D. L.; Curtis, C. J. Organometallics 1993, 12, 299-
303.
9
J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004 5511

A R T I C L E S
Raebiger et al.
thermodynamic properties that cannot be obtained directly by
experiment or that have not been measured. For example, eq 7
can be used to calculate an E1/2(I/0) potential of -1.24 V for
[Pd(depe)2]2+ from the measured pKa value of 23.2 for [HPd-
(depe)2]+. The potential of the (I/0) couple for [Pd(depe)2]2+
cannot be determined from cyclic voltammetry, because the
observed wave is a two-electron wave. This estimated potential
can be used in eq 6 to calculate a homolytic bond-dissociation
free energy of 56.8 kcal/mol for [HPd(depe)2]+, which agrees
very well with those of the other complexes shown in Table 6.
Similarly, the pKa and homolytic bond-dissociation free energies
of [HPd(dppx)2]+ (where dppx is R,R′-bis(diphenylphosphino)-
xylene) can be predicted to be 12.8 (using eq 7) and 58 kcal/
mol (using eq 6), respectively, on the basis of the observed
potential of the (I/0) couple for this complex (-0.57 V).14 These
estimates are expected to be accurate within 1.5 pKa units and
Figure 5. Plot of ∆G° for [HPd(diphosphine)2]+ versus the potential of
the E1/2(II/I) couples foHr the corresponding [Pd(diphosphine)2]2+ complexes
in benzonitrile. The line is the best linear fit of the data with eq 8.
-
•
within 2.5 kcal/mol for ∆G° .
H
Homolytic Bond-Dissociation Free Energies. The homolytic
cleavage of the Pd-H bond of [HPd(diphosphine)2]+ complexes
to form [Pd(diphosphine)2]+ species is not expected to produce
a large structural rearrangement of the phosphorus atoms, as
discussed in the two preceding paragraphs. Because there is no
large change in geometry, the bite angle of the diphosphine
ligand should have little effect on the homolytic bond-dissocia-
tion free energies. Similarly, because there is no change in
charge, no large charge reorganization is expected, and ligand
substituent effects should also be small. As seen in Table 6,
neither the substituents nor the chelate bite sizes have a
significant effect on the homolytic bond-dissociation free
energies of [HPd(diphosphine)2]+ complexes, which are 57 (
1 kcal/mol for all of the complexes studied. For comparison,
increasing the electron-donating ability of the diphosphine
ligands from all aryl substituents for [HNi(dppv)2]+ (where dppv
is bis(diphenylphosphino)ethene) to all alkyl substituents for
[HNi(dmpe)2]+ (where dmpe is bis(dimethylphosphino)ethane)
increased the homolytic bond-dissociation free energies by 3
kcal/mol.12 For these same two complexes, the pKa values
electron-donating ability of the substituents on phosphorus. The
energy of this orbital will also be affected by the ligand bite
angle. For the [Pd(diphosphine)2]2+ complexes, as the natural
bite angle increases, there is an increase in the dihedral angle
between the planes defined by the phosphorus atoms of the
ligand and the metal. As the natural bite angle is increased, the
geometry becomes increasingly tetrahedral. This tetrahedral
distortion decreases the overlap between the dx -y orbital and
the ligand σ orbitals, lowering the energy of the LUMO. Based
on these simple arguments, the potential of the (II/I) couple is
expected to depend on both the substituents on phosphorus and
the chelate bite angle of the diphosphine ligand. Because the
binding of a hydride by [Pd(diphosphine)2]2+ complexes
involves a change in charge and geometry very similar to that
of a one-electron reduction, the excellent linear correlation
observed for these two thermodynamic properties is expected.
2
2
-
∆G° ) 23.5 E1/2(II/I) + 83.4
(8)
H
-
increased by 12 units, and the ∆G° values decreased by 16
kcal/mol. Clearly, substituent effects have a much larger effect
H
Equation 8 can be used to calculate an E1/2(II/I) potential of
-1.71 V for [Pd(depe)2]2+ from the measured ∆G° value of
-
H
-
on pKa and ∆G° values than on homolytic bond-dissociation
H
43.2 kcal/mol for [HPd(depe)2]+. As discussed above, the
free energies. The somewhat greater sensitivity to substituents
potential of the (I/0) couple (-1.24 V) for [Pd(depe)2]2+ was
calculated using eq 7 and the pKa value of [HPd(depe)2]+. These
results indicate that the potential of the (II/I) couple is 0.47 V
negative of the (I/0) couple, which is consistent with the two-
electron reduction wave observed experimentally. It is important
to note that the average of the two calculated values (-1.48 V)
agrees well with the experimentally measured potential (-1.47
V). The hydride donor ability of [HPd(dppx)2]+ (70 kcal/mol)
can also be estimated from eq 8 and the previously reported
(II/I) potential (-0.57 V).14
of the homolytic bond-dissociation free energies of [HNi-
(diphosphine)2]+ complexes as compared to [HPd(diphos-
phine)2]+ complexes may be a reflection of the slightly higher
electronegativity of PdII (øP ) 2.20) as compared to NiII (øP )
1.91).32
Linear Free-Energy Correlation of ∆G° and E1/2(II/I).
H-
The hydride donor abilities of the palladium hydride complexes
exhibit a linear dependence on the potentials of the (II/I) couples
of the corresponding [Pd(diphosphine)2]2+ complexes (Figure
5 and eq 8). Similar correlations have been observed previously
for [HNi(diphosphine)2]+ complexes.12 The potentials of the (II/
I) couples of the [Pd(diphosphine)2]2+ complexes are a measure
of the energy of the lowest unoccupied molecular orbital
Acidity, Hydricity, and pKa Stability Ranges. With an acid
of sufficient strength, hydride complexes become unstable with
respect to loss of hydrogen, according to reaction 9. A
-
thermodynamic cycle generates eq 10, which uses ∆G° to
H
2
2
(LUMO) of these complexes. This orbital is the dx -y orbital
in ligand field theory, and it is antibonding with respect to ligand
σ orbitals. The energy of this orbital will be affected by the
basicity of the phosphine ligands, which is controlled by the
calculate pKa(H2), the pKa required to protonate the hydride to
form H2 and [Pd(diphosphine)2]2+ 12,13
Thus, the hydricity value
.
of [HPd(dppx)2]+ can be used in eq 10 to determine a pKa(H2)
value of 4.4. [HPd(dppx)2]+ should be stable to acids with pKa
values higher than 4.4, and it should be unstable with respect
to hydrogen evolution in the presence of acids with pKa values
(32) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry; W. H.
Freeman: New York, 1990; p 641.
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5512 J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004

Hydricity of Palladium Diphosphine Complexes
A R T I C L E S
in acetonitrile of less than 4.4. Because [HPd(dppx)2]+ is
expected to have a pKa value of 12.8, as discussed above,
it will be deprotonated in solutions containing bases with
pKa(BH+) values higher than 12.8. Thus, [HPd(dppx)2]+ is
expected to be stable over a pKa range from 12.8 to 4.4 in
acetonitrile. This can be very useful when designing synthetic
reactions to prepare such a hydride.
[HPd(diphosphine)2]+ + BH+ h
[Pd(diphosphine)2]2+ + H2 + B (9)
-
pKa(H2) ) (76 - ∆G° )/1.37
(10)
H
Similarly, the hydricity of [HPd(depe)2]+ can be used to
calculate a pKa(H2) of 23.9 for H2 formation from this hydride.
Because the pKa of this complex is 23.2, it is unstable with
respect to itself (by -0.7 pKa units); that is, it is acidic enough
to cause elimination of H2. Indeed, an equimolar mixture of
[Pd(depe)2]2+ and Pd(depe)2 in benzonitrile under an atmosphere
Figure 6. Plot of ∆pKa versus ligand-to-ligand dihedral angle for the
compounds in this study; the line is the best linear fit to the data (∆pKa )
0.27 angle + 0.18). Inset: Plot of ∆pKa versus natural ligand bite angle.
of hydrogen results in an equilibrium mixture of [Pd(depe)2]2+
,
Pd(depe)2, and [HPd(depe)2]+ (as observed by 31P NMR)
according to eq 11, with Keq ) 0.01. Reaction 11 could occur
either by formation of Pd(I) intermediates followed by homolytic
H2 cleavage or by heterolytic cleavage in which the Pd(0)
complex acts as a base. Heterolytic activation of H2 using
nitrogen bases and [M(diphosphine)2]2+ (where M ) Ni and
Pt) has been observed previously.13
results in a total variation of the hydricities for these [HPd-
(diphosphine)2]+ complexes of 27 kcal/mol. Clearly, chelate bite
effects can rival or exceed substituent effects in controlling the
hydride donor abilities of these complexes. The ability to use
the natural bite angle to control hydricity while having a minimal
effect on acidity provides an attractive approach to designing
complexes that have tailored combinations of acidities and
hydricities suitable for a specific application.
Pd(depe)2 + [Pd(depe)2]2+ + H2 h 2[HPd(depe)2]+ (11)
Summary
The hydricities of the remaining complexes in Table 6 can
be used to calculate the pKa(H2) at which hydrogen formation
will occur from these hydrides. The values of pKa(H2) and the
pKa values of the hydrides give a pKa range, ∆pKa, for each of
these complexes over which the hydrides are stable. These ∆pKa
values are shown in the last column of Table 6. As discussed
above, the pKa values of the [HPd(diphosphine)2]+ complexes
are dependent on the electron-donating ability of the substituents
on phosphorus, and they are independent of the chelate bite
size. The hydride donor abilities depend on both the substituents
on phosphorus and the chelate bite size. The pKa range over
which these hydrides are stable is therefore dependent only on
the chelate bite effects. The ∆pKa values shown in Table 6
represent the difference in the hydricities and acidities if the
substituents remain the same and only the bite angle of the
ligand is changed. Figure 6 shows a plot of ∆pKa as a function
of the observed dihedral angle, which appears to be the
geometric parameter that is a manifestation of the natural bite
angle. The inset plot also shows ∆pKa as a function of the
calculated natural bite angle. It is apparent that the plot of ∆pKa
versus the dihedral angle is more linear than the plot of ∆pKa
versus the natural bite angle, but both plots are relatively smooth
and well behaved. From the plot in Figure 6 and the data in
Table 6, it can be seen that ∆pKa varies from -0.7 to 14.4 on
changing the natural bite angle from 78° to 111°. This means
that this range of natural bite angles can be used to decouple
the acidity and hydricity of these complexes by 20 kcal/mol.
The combination of natural bite angles and substituent effects
A series of five diphosphine ligands with natural ligand bite
angles ranging between 78° and 111° have been used to prepare
compounds [Pd(diphosphine)2]n (n ) 2+, 0). Values of pKa and
-
∆G° have been determined for the corresponding [HPd-
H
(diphosphine)2]+ complexes. It was found that the acidity of
the compounds varies with the electron-donating or -withdraw-
ing ability of the substituents on phosphorus, but that it is
independent of the natural bite angle of the ligand. The
homolytic bond-dissociation free energies of these [HPd-
(diphosphine)2]+ complexes are independent of both the phos-
phine substituents and the natural bite angle. The hydricities of
these compounds vary over a range of 27 kcal/mol, and they
are dependent on both the natural bite angle and the phosphine
substituents. The origin of the dependence of the hydricities
and potentials of the Pd(II/I) redox couples on the natural bite
angles can be traced to structural distortions. With increasing
natural bite angles, the geometry around the metal becomes more
distorted toward a tetrahedron. In palladium(II) complexes, the
geometry of complexes with small bite angles is very close to
square planar, but is more intermediate between square planar
and tetrahedral for complexes containing diphosphine ligands
with large bite angles. This tetrahedral distortion stabilizes the
LUMO and results in compounds that are easier to reduce and
that are energetically more favorable for binding a hydride
ligand. Thus, palladium diphosphine hydride complexes with
large bite angles are good hydride acceptors and poor hydride
donors. By varying the natural bite angle over a range of 33°,
the hydride donor abilities can be tuned over a range of 27 kcal/
9
J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004 5513

A R T I C L E S
Raebiger et al.
mol. The variation of the hydricity of metal hydrides with the
natural bite angle is undoubtedly important in many catalytic
processes.
Supporting Information Available: X-ray structural data for
the compounds listed in Tables 1 and 2, including tables of
crystal and refinement data, atomic positional and thermal
parameters, and interatomic distances and angles (CIF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This research was supported by the United
States Department of Energy, Office of Science, Chemical and
Biological Sciences Division under Contract No. DE-AC36-
99GO10337.
JA0395240
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5514 J. AM. CHEM. SOC. VOL. 126, NO. 17, 2004