Synthesis and spectroscopy of binuclear phosphine bridged palladium hydrides: Pd2HX3[dppm]2 (X = Br, I; dppm = bis[diphenylphosphino]methane)

Article abstract of DOI:10.1016/j.jorganchem.2003.09.017

Reactions of orange-red dichloromethane solutions of Pd₂ X₂dppm₂ (X=Br, I; dppm=bis{diphenylphosphino} methane) with aqueous, concentrated HBr or HI at ambient temperature yields dark green solids which analyze for Pd₂HX₃ dppm₂ (1a X=Br and 1b X=I). No reaction is observed between Pd₂X₂dppm₂ and aqueous, concentrated HCl. Line shape analysis of dynamic ³¹P-NMR spectra for 1a and 1b over a 100 °C range indicates that in each case a system involving two sets of chemically equivalent ³¹P nuclei, mutually coupled, is exchanging ³¹P environments via initial exchange with a less populated intermediate system in which all four ³¹P nuclei are equivalent. From lineshape analysis of the ³¹P spectra, activation parameters for the rates going to the symmetrical intermediates are as follows: 1a: Δ G ^?(-78°)=7.8±0.2 kcal mol^-1, Δ H ^?=7.5 kcal mol^-1, Δ S ^?=-1.3 e.u. and for 1b: Δ G ^? (-78°)=9.7±0.3 kcal mol^-1, Δ H ^?=8.9 kcal mol^-1, Δ S ^?=-3.9 e.u. Similar analysis of dynamic ¹H spectra for 1b over a 80 °C range reveals two exchanging Pd-H sites with activation parameters for the exchange: Δ G ^?(-78°)=9.5±0.3 kcal mol^-1, Δ H ^?=8.2 kcal mol^-1, Δ S ^?=-6.9 e.u. Compounds 1a and 1b decompose to a 1:2 mixture of Pd₂X₂dppm₂ and PdX ₂dppm in solution with the evolution of hydrogen. Compound 1b reacts with PPh₃ yielding [HPPh₃⁺] [I^-] and Pd₂I₂dppm₂ while reaction of 1b with KOH also yields Pd₂I₂dppm ₂. Decomposition of 1b is unchanged in the presence of styrene with no evidence for the formation of iodoethylbenzene or ethylbenzene.

Full text of DOI:10.1016/j.jorganchem.2003.09.017

Journal of Organometallic Chemistry 688 (2003) 206ꢁ215
/
www.elsevier.com/locate/jorganchem
Synthesis and spectroscopy of binuclear phosphine bridged palladium
hydrides: Pd2HX3[dppm]2 (XꢀBr, I; dppmꢀ
bis[diphenylphosphino]methane)
/
/
Rein U. Kirss *, David A. Forsyth, Marc A. Plante
Department of Chemistry, Northeastern University, Boston, MA 02115, USA
Received 14 July 2003; received in revised form 3 September 2003; accepted 4 September 2003
Abstract
Reactions of orangeꢁ
aqueous, concentrated HBr or HI at ambient temperature yields dark green solids which analyze for Pd2HX3dppm2 (1a Xꢀ
I). No reaction is observed between Pd2X2dppm2 and aqueous, concentrated HCl. Line shape analysis of dynamic 31P-NMR
/
red dichloromethane solutions of Pd2X2dppm2 (Xꢀ
/
Br, I; dppmꢀ/bis{diphenylphosphino}methane) with
/Br and
1b Xꢀ
/
spectra for 1a and 1b over a 100 8C range indicates that in each case a system involving two sets of chemically equivalent 31P nuclei,
mutually coupled, is exchanging 31P environments via initial exchange with a less populated intermediate system in which all four
31P nuclei are equivalent. From lineshape analysis of the 31P spectra, activation parameters for the rates going to the symmetrical
intermediates are as follows: 1a: DG$(ꢂ
788)ꢀ9.79
0.3 kcal molꢂ1, DH$ꢀ8.9 kcal molꢂ1, DS$ꢀ
range reveals two exchanging Pdꢁ
H sites with activation parameters for the exchange: DG$(ꢂ
8.2 kcal molꢂ1, DS$ꢀ
6.9 e.u. Compounds 1a and 1b decompose to a 1:2 mixture of Pd2X2dppm2 and PdX2dppm in solution
/
788)ꢀ
/
7.89
/
0.2 kcal molꢂ1, DH$ꢀ
3.9 e.u. Similar analysis of dynamic H spectra for 1b over a 80 8C
788)ꢀ9.59
0.3 kcal molꢂ1, DH$ꢀ
/
7.5 kcal molꢂ1, DS$ꢀ
/
ꢂ
/
1.3 e.u. and for 1b: DG$(ꢂ
/
1
/
/
/
/
ꢂ
/
/
/
/
/
/
/
ꢂ
/
with the evolution of hydrogen. Compound 1b reacts with PPh3 yielding [HPPh3ꢃ][Iꢂ] and Pd2I2dppm2 while reaction of 1b with
KOH also yields Pd2I2dppm2. Decomposition of 1b is unchanged in the presence of styrene with no evidence for the formation of
iodoethylbenzene or ethylbenzene.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Dinuclear palladium hydrides; Oxidative addition; Dynamic NMR spectroscopy
1. Introduction
In neither case are Pd-H intermediates from oxidative
addition to Pd(I) detected. The oxidative addition of
The oxidative addition of small molecules (e.g. H2,
HX (XꢀCl, Br) to Pd2Cl2dppm2 forming
/
Pd2HX3dppm2 has been inferred from spectroscopic
studies [5]. These dinuclear palladium hydrides were not
isolated but react further with additional HX to form
Pd2X4dppm2 and H2. Nevertheless, a series of hydride-
bridged dipalladium dppm complexes [Pd2R2(m-
H)dppm2]PF6 [6] can be prepared by reduction of
[Pd2R2(m-Cl)dppm2]PF6 compounds, suggesting that
additional examples of stable dinuclear palladium
hydride complexes might be isolated [7]. Encouraged
by the latter work we have re-investigated the reactions
HX, or X2 where Xꢀ
/
halides and Rꢀalkyl or aryl) to
/
transition metal compounds is characteristic of coordi-
natively unsaturated, sixteen valence electron, d8 metal
complexes and is often implicated in catalytic processes
involving these compounds [1]. The palladium(I) dimers,
Pd2X2dppm2 (Xꢀ
/
Cl, Br, I, dppmꢀbis(diphenylpho-
/
sphino) methane) react with X2 to form thermally
unstable palladium(II) dimers, [Pd2X4dppm2] [2]. Reac-
tion of Pd2Cl2dppm2 with H2S yields Pd2Cl2(m-S)dppm2
and is accompanied by the evolution of hydrogen [3]
while Pd2X2dppm2 acts as a hydrogenation catalyst [4].
between Pd2X2dppm2 and HX (XꢀBr, I). In the
/
present paper we describe the synthesis and character-
ization of two binuclear palladium hydrides,
Pd2HBr3dppm2 and Pd2HI3dppm2, from reactions be-
* Corresponding author. Fax: ꢃ1-617-373-8795.
/
E-mail address: rkirss@neu.edu (R.U. Kirss).
tween Pd2X2dppm2 (XꢀCl, Br, I) and concentrated
/
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.09.017

R.U. Kirss et al. / Journal of Organometallic Chemistry 688 (2003) 206ꢁ
/215
207
aqueous hydrobromic or hydroiodic acid suggesting
oxidative addition of HX and retention of the dppm
bridged structure.
C50H45Br3P4Pd2: C, 49.13; H; 3.71; Br, 19.61. Found:
C, 49.48; H, 3.78; Br, 18.42%.
The yield of 1a is improved by reducing the reaction
time. For example, reaction of 130 mg (0.12 mmol)
Pd2Cl2dppm2 with excess aqueous HBr for 5 min yielded
87 mg (58% yield) of 1a. The fine powdery product
sticks to the walls of the reaction vessel rendering
isolation of the product difficult and lowers the percent
yield. The aqueous layer was evaporated to dryness. No
phosphorous signals were observed in the 31P-NMR
spectrum of the residue.
2. Experimental
2.1. General procedures
All compounds described in this work were handled
using Schlenck techniques, in a M. I. Braun glove box
under a purified argon or nitrogen atmosphere or on a
vacuum line equipped with oil diffusion and mechanical
pumps (10ꢂ3 Torr) [8]. Solvents were purified by
refluxing over Na/benzophenone (benzene, toluene,
tetrahydrofuran), Na (hexane) or P2O5 (dichloro-
methane) and distilled prior to use. Deuterated dichlor-
omethane (CD2Cl2) was purchased from Cambridge
Isotope Laboratories and dried as described above.
PdCl2, PdBr2, triphenylphosphine, and bis(diphenylpho-
sphino)methane (dppm), were purchased from Strem
Reaction of 170 mg (0.16 mmol) Pd2Cl2dppm2 in 10
ml CH2Cl2 with 4 ml (72 mmol) of 48% aqueous DBr in
D2O yielded 150 mg of Pd2DBr3dppm2 (75% yield). IR
(KBr): 1540 cmꢂ1, 1487, 1433, 1262, 1097, 1035, 999,
1
781, 737, 688, 516, 482. H (CD2Cl2): d 4.36 (br m, 4H,
PCH2P), 7.1ꢁ7.9 (four m, 40H, Ph2PCH2PPh2).
/
2.3. Preparation of Pd2HI3dppm2 (1b)
As described for 1a, addition of 10 ml, (55 mmol)
degassed, 48% aqueous HI to a solution containing 131
mg (0.11 mmol) Pd2I2dppm2 dissolved in 30 ml of
CH2Cl2 caused the orange organic layer to turn dark
green within 30 s. After stirring for 15 min, the CH2Cl2
layer was transferred by cannula to a clean vessel and
the solvent was removed under vacuum. A dark green
solid (79 mg, 55% yield), 1b, was isolated. After
verifying the molar conductivity of [Pd2(CH3)2(m-
I)dppm2][I] in our apparatus as a control experiment,
Chemicals and used as received. Pd2X2dppm2 (Xꢀ
/
Cl,
Br, I) were prepared by literature methods [9].
1
NMR spectra were recorded at 300 MHz for H and
121.4 MHz for 31P{1H} on a Varian XL300 spectro-
meter in 5 mm tubes equipped with a Teflon valve
(Wilmad Glass, Inc.). Proton chemical shifts are refer-
enced to TMS at 0 ppm with the residual protons in the
solvent (CDHCl2 at d 5.24 ppm) as a calibrant.
Phosphorus chemical shifts are reported relative to
external 85% H3PO4 (0.0 ppm) with Pd2X2dppm2 as a
calibrant for low temperature spectra. Low temperature
spectra were obtained from samples in 5-mm tubes in a
10 mm probe pre-calibrated for temperature determina-
LM of 1b was measured as 1 cm2 molꢂ1 Vꢂ1. UVꢁ
(CH2Cl2): lmax
663 nm. IR (KBr): 1721 cmꢂ1, 1572,
1482, 1433, 1263, 1095, 1026, 998, 777, 736, 689, 541,
/vis
ꢀ
/
1
515, 509, 485. H (CD2Cl2, 20 8C): d ꢂ
PdꢁH), 4.94 (br m, 4H, PCH2P), 7.2ꢁ7.8 (four m, 40H,
Ph2PCH2PPh2); 31P{1H} (CD2Cl2, 20 8C): d ꢂ
2.77 s.
/
6.14 (br s, 1H,
tion. UVꢁ/vis spectra were recorded on a Perkin Elmer
/
/
Lambda 4B UVꢁ/vis spectrometer. Conductivity mea-
/
surements were performed in CH2Cl2 solution using a
Beckman Instruments Model RC 16B2 Conductivity
Bridge. Elemental analyses were performed by Desert
Analytics, Tucson, AZ.
Anal. Calc. for C50H45I3P4Pd2: C, 44.05; H, 3.33; I,
27.92. Found: C, 43.90; H, 3.34; I, 27.59%.
Compound 1b can also be prepared from reactions of
Pd2Cl2dppm2 and excess aqueous HI in comparable
yields.
2.2. Preparation of Pd2HBr3dppm2 (1a)
Addition of 10 ml, (180 mmol) degassed, 48%
aqueous HBr to a solution containing 390 mg (0.34
mmol) Pd2 Br2dppm2 dissolved in 25 ml of CH2Cl2
caused the orange organic layer to turn dark green
within 30 s. After stirring for 15 min, the CH2Cl2 layer
was transferred by cannula to a clean vessel and the
solvent was removed under vacuum. A dark green solid
(105 mg, 25% yield), 1a, was isolated. IR (KBr): 1572
2.4. Attempted metathesis of 1b with NaPF6 and
NH4PF6
A solution of 1b was prepared by addition of 10 ml
(55 mmol) of 48% aqueous HI to 137 mg (0.13 mmol)
Pd2Cl2dppm2 in 50 ml CH2Cl2 at 0 8C. After 5 min, the
green organic layer was decanted onto excess NH4PF6
and stirred for 15 min. The green solution was washed
cmꢂ1, 1483, 1434, 1188, 1097, 1026, 999, 775, 737, 688,
with 2ꢄ10 ml water, dried over Na2 SO4 and evapo-
/
1
516, 485. H (CD2Cl2, 20 8C): d ꢂ
/
8.46 (br s, 1H, Pdꢁ
/
rated to dryness yielding 40 mg of a green solid. 31P-
NMR (CD2Cl2) reveals a mixture of 1b, Pd2I2dppm2
and PdI2dppm. Substitution of NaPF6 for NH4PF6
leads to the same result. Longer reaction times lead to
H), 4.36 (br m, 4H, PCH2P), 7.1ꢁ
/
7.9 (four m, 40H,
Ph2PCH2PPh2); 31P{1H} (CD2Cl2, 20 8C): d 3.15 s UV/
vis (CH2Cl2): lmax 607 nm. Anal. Calc. for

208
R.U. Kirss et al. / Journal of Organometallic Chemistry 688 (2003) 206ꢁ215
/
complete decomposition to a 2:1 mixture of PdI2dppm
to Pd2I2dppm2.
2.8. Reaction of 1b with styrene
Compound 1b (100 mg, 0.075 mmol) and 16 ml (0.14
mmol) of styrene were dissolved in 2 ml CD2Cl2. H-
1
2.5. Thermal decomposition of 1a and 1b
NMR indicates that 1b is the sole palladium species
present. The green color is replaced by a red color upon
refluxing for 2 h. 1H-NMR reveals a 2:1 mixture of
PdI2dppm to Pd2I2dppm2. The ratio of styrene to
palladium compounds remains constant. There is no
evidence for formation of (iodoethyl)benzene.
Compound 1b (110 mg, 0.086 mmol) was heated to
100 8C under vacuum (100 mTorr) for 8 h. Analysis of
the green solid by H-NMR revealed that 50% of the
starting material had decomposed to a 1:1 ratio of
Pd2I2dppm2 to PdI2dppm.
1
Compound 1a (400 mg, 0.33 mmol) was dissolved in
10 ml of DMF in a 50-ml Schlenck tube. The solution
The same results were observed when a solution of 31
mg (0.023 mmol) 1b and 10 ml (0.088 mmol) styrene were
heated under 1 atm. of H2 in CD2Cl2.
was frozen at ꢂ196 8C and evacuated to a base pressure
/
of 10ꢂ2 Torr. After thawing the solution was stirred at
ambient temperature until the green color of 1a was
replaced by the red color of Pd2Br2dppm2. The evolu-
tion of hydrogen gas was followed by gas chromato-
graphy using the method described for H2 evolution in
reactions of Pd2Cl2dppm2 and H2S [3].
3. Results
Addition of aqueous, concentrated HBr or HI to a
dichloromethane solution of Pd2X2dppm2 (XꢀCl, Br,
/
A solution of 25 mg (0.019 mmole) 1b in CD2Cl2 was
monitored by 31P-NMR over 3 days. The green solution
turns brown and finally red. The intensity of the broad
I) at ambient temperature leads to a rapid color change
from orange red to dark green for the upper organic
layer. No reaction is observed with aqueous, concen-
trated HCl. Separation of the layers and evaporation of
the dichloromethane yields dark green solids of formula
resonance at ꢂ
appearance of signals for Pd2I2dppm2 at ꢂ
and PdI2dppm at ꢂ62.0 ppm. Integration of the latter
/
2.77 ppm decreases accompanied by the
/9.84 ppm
/
Pd2HX3dppm2 (Eq. (1), 1a Xꢀ
/
Br and 1b XꢀI).
/
resonances indicates a 1:2 ratio of dinuclear to mono-
nuclear products.
2.6. Reaction of 1b with triphenylphosphine
ð1Þ
A 5-mm NMR tube equipped with a Teflon valve
(Wilmad Glass Co.), was charged with 25 mg (0.019
mmole) of 1b and 5 mg (0.019 mmole) PPh3 in the glove
box. CD2Cl2 was transferred under vacuum to the tube
cooled in liquid nitrogen (ꢂ
/
196 8C). After sealing, the
Compounds 1a and 1b can also be prepared from
Pd2Cl2dppm2 and excess concentrated aqueous HX.
The use of concentrated acid is critical as reactions
between Pd2Cl2dppm2 and less concentrated of HBr or
gaseous HBr fails to produce the green color character-
istic of Pd2HBr3dppm2 leading instead to halide ex-
change and isolation of a 2:1 mixture of Pd2Br2dppm2
and PdBr2dppm.
tube was thawed immediately prior to recording the 1H-
and 31P-NMR spectra. A rapid color change from green
to red was observed within 10 min of thawing. 1H-NMR
indicated formation of a 4:1 ratio of Pd2I2dppm2 to
PdI2dppm by integration of the Ph2PCH2PPh2 reso-
nances (quintet at 4.18 and triplet at 4.44 ppm,
respectively). 31P-NMR spectra confirm the formation
of Pd2I2dppm2, PdI2dppm and [PPh3Hꢃ][Iꢂ] (31P
The green solids are stable for weeks at ꢂ
/
20 8C but
resonance at ꢂ
/
10.3 ppm by comparison with the
heating 1b at 100 8C under vacuum for 8 h leads to :
/
spectra of authentic samples.
50% decomposition to a 1:1 ratio of Pd2I2dppm2 to
PdI2dppm. Dichloromethane solutions of both 1a and
1b decompose to a 1:2 mixture of Pd2X2dppm2 and
PdX2dppm between 24 h (1a) and 72 h (1b) at ambient
temperature. Formation of H2 was measured by gas
chromatography. The decomposition of 1a,b in solution
has thwarted attempts to grow crystals suitable for X-
ray diffraction. The elemental analyses for 1a,b are
consistent with addition of HX to Pd2X2dppm2 with no
evidence for addition or retention of water. The
measured bromine content of 1a is lower than the
calculated value, however, it is consistent with the loss
2.7. Reaction of 1b with potassium hydroxide
A 5.8 mM solution (10 ml, 58 mmol) of 1b was added
to excess, pulverized KOH by syringe and stirred for 1 h.
The green color of 1b is replaced by a dark red color.
The solution was filtered to remove KOH. Solvent was
evaporated from the filtrate yielding a dark red solid.
An 8:1 ratio of Pd2I2dppm2 to PdI2dppm was deter-
mined by integration of the CH2 resonances of the dppm
1
ligands in the H-NMR spectrum of the product.

R.U. Kirss et al. / Journal of Organometallic Chemistry 688 (2003) 206ꢁ
/215
209
of HBr from the thermally unstable compound 1a. The
halide analysis for the more stable 1b is well within
acceptable limits. The similar spectral properties of 1a
and 1b, (vide infra) support the assigned composition
for 1a.
Reaction of 1b with triphenylphosphine yields triphe-
nylphosphonium iodide, [PPh3H][I], and a 4:1 mixture
of Pd2I2dppm2 to PdI2dppm. Addition of a CH2Cl2
solution of 1b to pulverized KOH leads to recovery of
nearly 90% Pd2I2dppm2 with the remaining 10% as
PdI2dppm. Exposure of solutions of 1b to neutral
alumina leads to an immediate discharge of the green
frequency triplet (Fig. 2, Table 1). The relative inten-
sities of the triplet resonances to the singlet are 4:1,
identical to the ratio of PdꢁH resonances in the low
/
temperature 1H spectra. At higher temperatures, the two
triplets and singlet broaden and eventually merge to a
singlet. As can be seen in Fig. 2, the singlet appears to
merge into the high frequency triplet as the temperature
is raised, creating a larger, distorted triplet. The two
triplets merge as the temperature is raised further.
Again, the line shape changes are entirely reproducible
and reversible. Similarly, two triplets are seen in Fig. 3 at
the lowest temperature of ꢂ118 8C (155 K, Table 1) in
/
color and isolation of Pd2I2dppm2ꢁ/PdI2dppm mixtures
the 31P{1H} spectrum of 1a. The higher frequency triplet
in this case is larger and distorted on the high frequency
side, indicating the melding in of a higher frequency
signal which is not resolved at this temperature due to
exchange broadening.
making it difficult to apply strategies used in the
preparation of [Pd2R2(m-H)dppm2ꢃ][PF6ꢂ] [6] in the
synthesis of cationic derivatives of 1a,b.
The nature of 1a,b was investigated by variable
temperature 1H- and 31P-NMR spectroscopy. 1H-
NMR spectra for 1b at 20 8C reveal a broad singlet at
Complete line shape analysis of the dynamic NMR
spectra for 1a and 1b indicates that in each case a system
involving two sets of two chemically equivalent 31P
nuclei, mutually coupled, is exchanging 31P environ-
ments via initial exchange with a less populated inter-
mediate system in which all four 31P nuclei are
equivalent. The complete line shape analysis was ac-
complished with the gNMR program [12]. Plots of
theoretical spectra for different exchange rates are
shown beside the experimental spectra in Figs. 2 and
3. Eyring plots of log k/T versus 1/T constructed from
the kinetic data were satisfactorily linear and gave
activation parameters for the rates going to the symme-
ꢂ
/
6.14 (width at half height :
H resonance. Additional resonances in the H spectrum
at 4.94 (br m), and 7.2ꢁ7.8 ppm (four br m) are assigned
/
42 Hz) assigned to a Pdꢁ
/
1
/
to the methylene and aryl protons, respectively, of the
green complex, Pd2HI3dppm2. The ratio of the reso-
nances at ꢂ
consistent with a single Pdꢁ
Similar spectra are observed for 1a with a broad Pdꢁ
resonance observed at ꢂ8.46 ppm. Substitution of DBr
/
6.14 and 4.94 ppm is found to be 1:4,
/
H bond per molecule of 1b.
/
H
/
for HBr in equation 1 yields a dark green product
spectroscopically identical to 1a with the exception of
the missing Pdꢁ
chemical shifts for the Pdꢁ
the range reported for palladium hydride complexes [6]
[10] [11].
Dynamic NMR studies were carried out for 1a and 1b
in CD2Cl2 solution. Cooling a sample of the latter
/
H resonance at ꢂ
/
8.46 ppm. The
trical intermediates as follows: 1a DG$(ꢂ
0.2 kcal molꢂ1, DH$ꢀ7.5 kcal molꢂ1, DS$ꢀ
e.u.; for 1b DG$(ꢂ 0.3 kcal molꢂ1, DH$ꢀ
788)ꢀ9.79
8.9 kcal molꢂ1, DS$ꢀ
3.9 e.u. The exchange process
/
788)ꢀ
/
7.89
/
/
H resonances are well within
/
/
ꢂ
/1.3
/
/
/
/
/
ꢂ
/
can not involve any substantial amount of direct
exchange of the nuclei giving rise to the two triplets.
Direct exchange would give rise to line shape changes
that would not match the observed spectra. Fig. 4 is an
example that shows the line shape simulation for the
occurrence of only a direct exchange process for 1b at a
rate of 66 sꢂ1. The singlet assigned to the intermediate is
not broadened in Fig. 4. If direct exchange were
competitive with exchange through the intermediate,
the singlet would broaden and blend with the other
signals and, depending on the relative rates, spectra
would show various line shapes between the limiting
case in Fig. 4 and the limiting case in Fig. 2.
compound from 20 to ꢂ
shift of the PdꢁH resonance accompanied by a doubling
in line width (Fig. 1) in the H spectra at 300 MHz. At
78 8C the PdꢁH resonance splits into two broad
singlets at ꢂ6.54 and ꢂ7.5 ppm in a 4:1 ratio,
respectively. The chemical shift values for the two Pdꢁ
H resonances are shifted further upfield at ꢂ98 8C but
/
58 8C leads to a steady upfield
/
1
ꢂ
/
/
/
/
/
/
there is no further change in their relative intensities.
The line shape changes are entirely reproducible and
reversible. The absence of better resolved PdꢁH reso-
nances does not allow a bridging hydride to be
distinguished from a terminal hydride based on phos-
/
1
The variable temperature H-NMR data for 1b agree
with the results from the 31P spectra. Two Pdꢁ
H
phorusꢁ
/
proton coupling. Limited solubility of 1b in
/
CDFCl2 and other low melting solvents prevents further
investigations at even lower temperatures.
environments exchange rapidly above ꢂ90 8C. Activa-
/
tion parameters calculated from Eyring plots as above
are within experimental error of the 31P data: DG$
/
Proton decoupled 31P spectra at 121.4 MHz show line
shape changes over a temperature range of more than
(ꢂ
6.9 e.u.
The IR spectra of 1a,b offer little insight into the
nature of the PdꢁH bond. A definitive assignment for
/
788)ꢀ
/
9.5 kcal molꢂ1, DH$ꢀ
/
8.2 kcal molꢂ1, DS$ꢀ
100 8C. At ꢂ
/
90 8C and below, the 31P spectra for 1b
consist of two mutually coupled triplets and a singlet
ꢂ
/
that occurs at a slightly lower frequency than the highest
/

210
R.U. Kirss et al. / Journal of Organometallic Chemistry 688 (2003) 206ꢁ215
/
Fig. 1. Dynamic 1H-NMR spectra at 300 MHz 1b in CD2Cl2. Scale in ppm is arbitrarily referenced to tetramethylsilane with residual protons in the
solvent (CDHCl2) as a calibrant.
Fig. 3. Dynamic 31P-NMR spectra at 121.4 MHz 1a in CH2Cl2. Scale
in Hz is arbitrarily referenced; see Table 1 for correct chemical shifts in
ppm at the lowest temperature measured. The small non-exchanging
Fig. 2. Dynamic 31P-NMR spectra at 121.4 MHz 1b in CH2Cl2. Scale
in Hz is arbitrarily referenced; see Table 1 for correct chemical shifts in
ppm at the lowest temperature measured. The small non-exchanging
singlet at 1250 Hz corresponds to a minor side-products Pd2Br4dppm2.
singlets at 1440 and 350 Hz correspond to minor side-products
Pd2I4dppm2 and Pd2I2dppm2, respectively.
of mononuclear palladium(II) compounds with mono-
dentate and chelating bidentate phosphine ligands gives
Pdꢁ
terminal,
[Pd(PCy3)2H(L)][BF4] (Lꢀ
characterized by two absorptions at :
PdꢁH) and :2080ꢁ
2115 cmꢂ1 (n Pdꢁ
/
H absorptions can not be made. IR spectra of
rise to dinuclear hydride complexes such as
Pd2(PPh3)4(m-H)(m-CO][CF3CO2], Pd2dppp2(m-CO)(m-
H)ꢃ, and Pd2dippp(m-H)2 but IR spectra for the
bridging hydride complexes are not reported [14].
Similarly no assignment of IR absorptions for bridging
mononuclear palladium hydrides,
/
H2O, CH3CN) [13], are
/
720 cmꢂ1 (d
/
/
/
/H) neither of
which are observed in the IR spectra of 1a,b. Reduction

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211
Table 1
Summary of 31P-NMR chemical shift data For 1a and 1b
Compound
T (K)
Chemical shift (ppm)
unsymm. int.
sym. int.
7.78 b
Pd2X4dppm2
Pd2X2dppm2
1a
1b
155
6.93 a, ꢂ
1.00
/
2.62 a
8.95 c
4.90
0.15
rel. conc.
165 K
rel. conc.
2.84 c, ꢂ
1.00
/
2.30
ꢂ/0.67
0.24
ꢂ/9.61
a
Mutually coupled with Jꢀ38 Hz.
From line shape analysis.
/
b
c
Mutually coupled with Jꢀ/36 Hz.
hydride ligands in [Pd2R2(m-H)dppm2][PF6] [6] nor
bridging hydride moieties in dinuclear platinum hydride
complexes are reported [15].
aqueous HX solutions. The products in Eq. (1) (1a,b)
are extracted into the lower density, dichloromethane
phase driving the reaction to completion and allowing
for the facile separation of 1a,b from excess acid before
further reactions can occur. The failure of gaseous HX
or lower concentrations of aqueous HX to produce
Pd2HX3dppm2 suggests that the formation constants for
1a,b are small and that high concentrations of HX are
required to drive the reaction to products. Even so, 1a
and 1b are thermally unstable, decomposing to mixtures
of Pd2X2dppm2 and PdX2dppm (Eq. (2)).
After verifying the molar conductivity of
Pd2I2(CH3)2dppm2 [16] in our apparatus, the molar
conductivity of 1b (LMꢀ
1 cm2 molꢂ1 Vꢂ1) suggests
that the latter is a non-electrolyte in CH2Cl2. The molar
/
conductivity of 1b is similar to that of Pd2I2dppm2 and
than
Pd2I4dppm2
and
Pd2I2(CH3)2dppm2 (Table 2).
much
smaller
for
(2)
4. Discussion
Addition of base (PPh3 or KOH) to 1a,b allows for
the removal of HX but this pathway competes with the
loss of hydrogen and formation of a mixture of
Pd2X2dppm2 and PdX2dppm. The facile deprotonation
of mononuclear palladium hydrides such as [(Cy3-
P)2Pd(H)(H2O)ꢃ] by excess phosphine or hydroxide
has been reported [17].
4.1. Synthesis and Reactivity of 1a,b
Reactions between concentrated HI or HBr and
bis(diphenylphosphino) bridged palladium(I) dimers
yield new dinuclear palladium hydrides, Pd2HX3dppm2
The reaction between 1b and styrene suggests that
reductive elimination of HI is preferred over insertion
(XꢀBr, I) as determined by elemental analysis and
/
NMR spectrosocopy. These results confirm the brief
reports of such compounds in the literature where
apparently transient complexes with formula
into the PdꢁH bond, a reaction typical of some
/
palladium hydrides [18]. The preferential loss of HI is
consistent with the reported reaction between dimethy-
lacetylene dicarboxylate and the mononuclear palla-
dium hydride [(Cy3P)2Pd(H)NO3] [19]. The isolation of
Pd2HX3dppm2 (XꢀBr, I) were proposed in reactions
/
between Pd2X2dppm2 and HX [5]. Our ability to isolate
and characterize 1a,b lies in the use of concentrated,

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/
Pd(PCy3)2(MeO2CCÄ
/
CCO2Me) reflects a preference for
4.2. Solution structures of 1a,b
reductive elimination of HX.
Palladium hydride resonances in di- and polynuclear
palladium phosphine hydride complexes have been
observed over a wide range of chemical shift values
ranging from 5.01 ppm (septuplet) for the bridging
A precedent for reversible oxidative addition/reduc-
tive elimination of HX from dinuclear, platinum group
metal complexes has been reported. The dinuclear
platinum(I) compound, Pt2Cl2dppm2, is protonated by
HCl in chloroform yielding [Pt2Cl2(m-H)dppm2][Cl] [20].
The protonation reaction is reversible; stirring [Pt2Cl2(m-
H)dppm2][Cl] in CH2Cl2 regenerates Pt2Cl2dppm2 with
the loss of HCl. Reductive elimination of HX from
mononuclear bis(phosphine)palladium(II) hydrido
chlorides is often implicated in catalytic cycles [21].
The mechanism of hydrogen elimination from 1a,b
remains unclear. It has been proposed that 1a,b react
with HX to form H2 and Pd2X4dppm2 which in turn,
decomposes to two equivalents of PdX2dppm [5]. The
demonstrated loss of HX from 1a,b forming
Pd2X2dppm2 provides a source of HX for reaction
with additional 1a,b as shown in Eq. (3). Eq. (3) predicts
a 2:1 ratio of PdX2dppm to Pd2X2dppm2, as observed in
the thermal decomposition experiments involving 1a,b
(Eq. (2)). The reaction of Pd2X2dppm2 with HX also
hydride in [Pd4dppm4H2]Cl2 [10] to ꢂ12.42 ppm
/
(triplet) for the terminal palladium hydride [Pd2(m-
Cl)H(CH3)dppm2][BPh4] [11]. The bridging hydride
ligands in cationic [Pd2R2(m-H)dppm2)ꢃ][PF6ꢂ] [6], are
distinguished by quintets for the Pdꢁ/H resonances
between ꢂ
/
7.8 to ꢂ
/
9 ppm. Details of variable tem-
perature H- or 31P-NMR experiments for these dppm
bridged compounds are not reported. The room tem-
perature singlet in the 31P-NMR spectrum of
1
Pd2(dippp)2(m-H)(m-CO][Cl] [14c,d] broadens at
80 8C but a limiting spectrum was not achieved.
ꢂ
/
The dynamic NMR spectra for 1a and 1b are
consistent with two sets of two mutually coupled,
chemically equivalent 31P nuclei exchanging 31P envir-
onments with a less populated intermediate system in
which all four 31P nuclei are equivalent. Thus the
solution structure of 1a and 1b involves rapid equilibra-
tion of a symmetrical and an unsymmetrical structure.
The bulk of the literature on dppm bridged palladium
dimers suggests an equilibrium between a symmetrical
hydride bridged dimer A (Fig. 5) and an unsymmetrical
halide bridged dimer B where both A and B are ionic
palladium(II) derivatives or tight ion pairs. For exam-
ple, protonation of the Pd2I2(m-CH2)dppm2 with HBF4
yields the ionic Pd(II) dimer [Pd2I(m-I)(CH3)-
resembles the reactions of Pd2X2dppm2 with H2E (Eꢀ
/
S, Se) and X2. [22] Hydrogen sulfide and hydrogen
selenide react with Pd2Cl2dppm2 by oxidative addition
followed by dehydrogenation to the stable, isolable A-
frame complexes Pd2Cl2(m-E)dppm2 (Eꢀ/S, Se) (Eq.
(4)). The formation of H2 is proposed to involve
deprotonation of the terminal EH ligand followed by
reaction between Hꢃ and the Pdꢁ
H bond.
/
(3)
(4)

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213
Table 2
Molar Conductivities of dppm bridged palladium dimers in CH2Cl2
Compound
L
(cm2 molꢂ1 Vꢂ1
)
M
1b
1
Pd2I2dppm2
Pd2I4dppm2
Pd2I2Me2dppm2
0.2
0.2
a
b
53
a
At
decomposition is expected.
ꢂ40 8C LM at ambient temperature was 0.6 but some
/
b
From Ref. [17] and verified in this work.
dppm2ꢃ][BF4ꢂ] [16]. The ambient temperature 31P{1H}-
NMR spectrum of [Pd2I(m-I)(CH3)dppm2ꢃ][BF4-] clo-
sely resembles that of 1a,b below ꢂ90 8C. The singlet in
/
the 31P-NMR spectrum of Pd2I2(m-CH2)dppm2 is re-
placed by two triplets at 13.1 and 3.1 ppm in [Pd2(m-
I)(CH3)(I)dppm2ꢃ][BF4ꢂ]. The presence of a bridging
Fig. 4. Calculated 31P-NMR spectra at 121.4 MHz 1b for direct
exchange of phosphorus nuclei at a rate of 66 sꢂ1
Fig. 5. Possible Solution Structures For 1a,b.

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R.U. Kirss et al. / Journal of Organometallic Chemistry 688 (2003) 206ꢁ215
/
iodide ligand [Pd2(m-I)(CH3)(I)dppm2ꢃ][BF4ꢂ] was ver-
ified by single crystal X-ray analysis. The room tem-
perature 31P-NMR spectra of the symmetrical
dipalladium(II) cations [Pd2R2 (m-Cl)dppm2ꢃ][PF6ꢂ]
tight ion pair. This is quite different from palladium
(alkyl)hydride complexes [Pd2R2(m-H)dppm2ꢃ][PF6ꢂ],
palladium (alkyl) halide complexes Pd2I2(CH3)2dppm2
and
platinum
hydride
complexes
Cl, PF6) all of which behave as
[Pt2Cl2(m-
(Rꢀ
/
CH3, Ph) and [Pd2R2(m-H)dppm2ꢃ][PF6ꢂ] (Rꢀ
/
H)dppm2ꢃ][Xꢂ] (Xꢀ
electrolytes. Further studies on the origin of the intense
green color and structure of 1a,b are in progress.
/
CH3, Ph) are also singlets while the 31P spectra of
unsymmetrical compounds [Pt2(CH3)Ph (m-Cl)dppm2ꢃ]-
[PF6ꢂ] and [PtPd(CH3)2(m-Cl)dppm2ꢃ][PF6ꢂ], consist of
two triplets [6]. Equilibration of A and B can be
achieved through an unbridged intermediate C similar
to that proposed for the fluxionality observed in
[Pt2H2Cl dppm2ꢃ] [20] and [PtPd(CH3)2(m-Cl)dppm2ꢃ]
[PF6ꢂ] [23].
References
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conductivity for 1b and our inability to exchange one of
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1 cm2 molꢂ1 Vꢂ1) is significantly
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[Pd2(m-
59 cm2 molꢂ1 Vꢂ1
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or Pd2I2(CH3)2dppm2 (LMꢀ
53 cm2 molꢂ1 Vꢂ1) in the
same solvent [16]. The molar conductivity of 1b is
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0.2 cm2 molꢂ1
Vꢂ1) and Pd2I4dppm2 (LMꢀ0.2 cm2 molꢂ1 Vꢂ1 at
40 8C, 0.6 cm2 molꢂ1 Vꢂ1 at ambient temperature.
/
/
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/
/
ꢂ
/
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/
distinguishing between A and E.
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Sabatino, Organometallics 10 (1991) 1038.
Two new dinuclear palladium hydride complexes 1a,b
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halides to formally palladium(I) centers in phosphine
bridged palladium dimers Pd2X2dppm2 similar to the
oxidative addition of H2S or halogens to Pd2Cl2dppm2.
The resulting hydride complexes exhibit dynamic beha-
vior
The solution structures of 1a,b are consistent with
covalent bonding between palladium, hydride and all
three halide ligands or the existence of an extremely
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[23] C. Xu, G.K. Anderson, Organometallics 15 (1996) 1760.

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215
[24] (a) M.D. Fryzuk, B.R. Lloyd, G.K.H. Clentsmith, S.J. Rettig, J.
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