Synthesis, characterization, and reactivity of [((iPr)2P(CH2)3P( iPr)2)(PCy3)PdH][OR]

Article abstract of DOI:10.1021/om0008833

The preparation of (DIPPP)Pd(PR₃) and [(DIPPP)(PR₃)PdH] [OR′] (DIPPP = bis(1,3-diisopropylphosphino)propane; R = Cy, Et; R′ = Ar, CF₃SO₂) is reported. (DIPPP)Pd(PCy₃) and [(DIPPP)(PCy₃)PdH]

Full text of DOI:10.1021/om0008833

Organometallics 2001, 20, 337-345
337
Syn th esis, Ch a r a cter iza tion , a n d Rea ctivity of
[((iP r )2P (CH2)3P (iP r )2)(P Cy3)P d H][OR]
Pedro J . Perez
Departamento de Quı´mica y Ciencia de los Materiales, Universidad de Huelva,
Carretera de Palos de la Frontera, s/ n, 21819-Huelva, Spain
J oseph C. Calabrese and Emilio E. Bunel*
Central Research & Development Department, E. I. du Pont de Nemours and Company,
Experimental Station, Wilmington, Delaware 19880
Received October 13, 2000
The preparation of (DIPPP)Pd(PR3) and [(DIPPP)(PR3)PdH][OR′] (DIPPP ) bis(1,3-
diisopropylphosphino)propane; R ) Cy, Et; R′ ) Ar, CF3SO2) is reported. (DIPPP)Pd(PCy3)
and [(DIPPP)(PCy3)PdH][CF3SO3] have been structurally characterized. (DIPPP)Pd(PR3)
complexes catalyze the reaction of ethylene with carbon monoxide and phenols to give aryl
propionates. In situ 31P NMR experiments have shown that the resting state of the catalyst
in these transformations is the binuclear species [{(DIPPP)Pd}2(µ-H)(µ-CO)][OPh].
In tr od u ction
Pd center more nucleophilic, we in effect protonated the
metal with a moderately weak acid such as phenol or
even an aliphatic alcohol. These cationic Pd hydride
complexes would then be able to catalyze the synthesis
of, for example, aryl propionates from ethylene, carbon
monoxide, and phenol. There is some precedent in the
literature for the protonation of Pd(0) complexes with
alcohols and phenols. Milstein observed the formation
of [(DIPPP)PdH(η1-DIPPP)][OMe]5 when heating
(DIPPP)2Pd with MeOH, and Leoni reported trans-[Pd-
(PCy3)2(H)(OPh)]‚PhOH,4a which was obtained by reac-
tion of Pd(PCy3)2 with phenol. However, their reactiv-
ities under catalytic conditions have not been reported.
Protonation of a transition-metal center to produce a
transition-metal hydride is widely recognized as being
a crucial step in the functionalization of olefins.1 A large
number of cationic palladium hydrides have been re-
ported in the literature,2 but their role in catalytic
transformations involving olefins still remains unclear.
Among all the metals described in the literature, pal-
ladium catalyzes a wide variety of reactions, including
the hydroformylation and carbonylation of olefins and
the carbonylation of acetylenes to methyl methacrylate.3
Despite the fact that we can find a selection of well-
characterized palladium hydrides such as trans-Pd-
(PCy3)2(H)(OPh),4a [(PEt3)3Pd(H)][BPh4],4b and [(PPh3)2-
Pd(H)(CO)Pd(PPh3)2][CF3CO2],4c their behavior under
conditions relevant to catalysis is still poorly under-
stood.4d
In this contribution, we will describe a set of iso-
structural cationic complexes capable of catalyzing the
transformation of ethylene into aryl propionate by
reaction of the alkene, CO, and the appropriate phenol.
Our strategy was to prepare a nucleophilic palladium
center by attaching electron-rich phosphines such as
DIPPP (bis(l,3-diisopropylphosphino)propane) and aux-
iliary ligands such as PCy3 and PEt3. By making the
Exp er im en ta l Section
Gen er a l Com m en ts. All manipulations and preparations
were carried out inside a nitrogen-filled drybox. Dry solvents
were purchased from Aldrich and degassed before use. Pd-
(PCy3)2,6 Pd(PEt3)3,4b and DIPPP7 were prepared as described
in the literature. The alcohols and phenols were purchased
from Aldrich and used without further purification. 1H and
31P{1H} NMR spectra were recorded on GE 300 and Omega
500 MHz spectrometers. Chemical shift values are reported
1
in ppm relative to TMS for H and 85% H3PO4 for 31P. In situ,
high-pressure NMR experiments were performed in a 10 mm
sapphire tube.
Syn th esis of (DIP P P )P d (P R3) (1a , R ) Cy; 1b, R ) Et).
One molar equivalent of DIPPP (1.1 g, 4 mmol) was added to
a colorless solution of Pd(PCy3)2 (2.64 g, 4 mmol) in 60 mL of
THF. After 3 h of stirring, the resulting yellow solution was
evaporated and the orange, oily residue was extracted with
toluene (20 mL). The toluene solution was mixed with aceto-
nitrile, and the mixture was cooled at -35 °C overnight.
Orange-yellow crystals of complex 1a , along with a small
amount of PCy3, were collected upon filtration. The impurity
(1) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987; pp 80-93.
(2) (a) Grushin, V. V. Chem. Rev. 1996, 96, 2011. (b) Stromnova, T.
A.; Moiseev, I. I. Russ. Chem. Rev. (Engl. Transl.) 1998, 67, 485.
(3) Ojima, I.; Eguchi, M.; Tzamarioudaki, M. In Comprehensive
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson,
G., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 12, p 9.
(4) (a) Di Bugno, C.; Pasquali, M.; Leoni, P.; Sabatino, P.; Braga,
D. Inorg. Chem. 1989, 28, 1390. (b) Shunn, R. A. Inorg. Chem. 1976,
15, 208. (c) Zudin, V. N.; Chinakov, V. D.; Nekipelov, V. M.; Likholobov.;
V. A.; Yermakov, Y. I. J . Organomet. Chem. 1985, 289, 425. (d) During
the submission of this paper, a contribution regarding the existence
of palladium hydride species as the active catalyst in the methoxy-
carbonylation of ethylene appeared. See: Eastham, G. R.; Heaton, B.
T.; Iggo, J . A.; Tooze, R. P.; Whyman, R.; Zacchini, S. Chem. Commun.
2000, 609.
(5) Portnoy, M.; Milstein, D. Organometallics 1994, 13, 600.
(6) Grushin, V. V.; Bensimon, C.; Alper, H. Inorg. Chem. 1994, 33,
4804.
(7) Tani, K.; Tariigawa, E.; Tatsuno, Y.; Otsuka, S. J . Organomet.
Chem. 1985, 279, 87.
10.1021/om0008833 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 12/23/2000

338 Organometallics, Vol. 20, No. 2, 2001
Ta ble 1. 31P {1H} a n d 1H NMR Da ta for Com p lexes 5a -e a n d 6a -ea
Pe´rez et al.
R
R′
δA
δM
δX
2J AX
2J AM
2J MX
δ(Pd-H)
2J H-P
2J H-P
trans
cis
5a
5b
5c
5d
5e
6a
6b
6c
6d
6e
Cy
Cy
Cy
Cy
Cy
Et
Et
Et
Et
Et
p-CNC6H4
C6H5
CF3CH2
FCH2CH2
CH3
p-CNC6H4
C6H5
CF3CH2
FCH2CH2
CH3
41.9
40.9
41.6
41.8
42.7
12.5
12.3
12.3
12.6
12.6
35.6
36.5
35.7
35.8
36.8
12.0
11.9
12.0
12.2
12.3
13.8
13.6
13.9
14.0
15.1
32.2
32.5
32.3
32.6
32.8
309
308
310
309
308
321
320
320
321
321
23
24
24
23
24
27
28
28
27
27
41
40
40
40
42
42
41
41
42
41
-7.70
172
171
172
nr
17; 7
17; 7
18; 7
nr
-7.79
-7.81
-7.60 (br)
-7.65 (br)
-7.13
nr
nr
173
177
178
nr
16; 3
15; 2.5
15; nr
nr
-7.10
-7.03
-6.9 (br)
-6.8 (br)
nr
nr
a
δ values are given in ppm and J values in Hz.
was removed by sublimation, under dynamic vacuum at 90
°C. The overall process afforded the complex (DIPPP)Pd(PCy3)
in 80% yield. Anal. Calcd for C33H67P3Pd: C, 59.7; H, 10.2.
Found C, 59.7; H, 10.2.
20 and 100 equiv of each alcohol, respectively. In the case of
fluoroethanol and methanol, complex 1a was dissolved in the
neat alcohols. Similar reactions were carried out with the same
substrates and complex 1b. Selected 31P{1H} NMR data are
shown in Table 1.
The triethylphosphine derivative 1b was prepared by direct
reaction of Pd(PEt3)3 with 1 molar equiv of DIPPP in toluene.
Yellow crystals of (DIPPP)Pd(PEt3) were obtained upon crys-
tallization from an acetonitrile solution in 85% yield. Anal.
Calcd for C21H49P3Pd: C, 50.4; H, 9.8. Found C, 50.7; H, 10.1.
1a : 31P{1H} NMR (C6D6, 121 MHz) AX2 spin system, δA 47.5
(PCy3), δX 19.8 (iPr2P(CH2)3PiPr2), 2J AX ) 81 Hz; 1H{31P} NMR
(toluene-d8, 499.9 MHz) δ 1.08 (d, J ) 7 Hz, 12H), 1.24 (d, J
) 7 Hz, 12H), 1.30 (m), 1.38 (m), 1.58 (m), 1.70 (m), 1.82 (m),
2.13 (m); 13C{1H,31P} NMR (toluene-d8, 125.7 MHz) δ 25 (CH2),
31.6 (CH2), 20.3 (CH3), 18.3 (CH3), 28 (CH), 36.9 (CH), 31.7
(CH2), 28.4 (CH2), 27.3 (CH2). 1b: 31P{1H} NMR (C6D6, 121
MHz) AX2 spin system, δA 19.0 (PEt3), δX 24.0 (iPr2P(CH2)3PiPr2),
2J AX ) 92 Hz; 1H{31P} NMR (toluene-d8, 499.9 MHz) δ 1.04
(d, J ) 7 Hz, 12H), 1.14 (d, J ) 7 Hz, 12H), 1.60 (b, 9H), 1.36
(m, 4H), 1.50 (b, 6H), 1.69 (d, J ) 7 Hz, 4H), 1.82 (m, 2H);
13C{1H,31P} NMR (toluene-d8, 125.7 MHz): δ 28.1 (CH), 25.5
(CH2), 25.1 (CH2), 24.5 (CH2), 20.4 (CH3), 19.2 (CH3), 10.6
(CH3).
Syn th esis of [(DIP P P )P d H(P Cy3)][OTf] (7). A solution
of 1a (0.33 g, 0.5 mmol) in 30 mL of diethyl ether was treated
with 1 equiv of triflic acid (50 µL, 0.57 mmol). A white solid
precipitated almost instantaneously. The mixture was stirred
for 3 h before the precipitate was separated by filtration. The
solid was dissolved in a 50:50 mixture of toluene and pentane.
After 30 min at room temperature, the product separated as
an oil. Diethyl ether was added dropwise to redissolve it, and
the resulting solution was left overnight at room temperature.
Off-white crystals suitable for X-ray diffraction were obtained
in 60% yield. Anal. Calcd for C34H68F3O3P3PdS: C, 50.2; H,
8.4; S, 3.9, Found C, 50.2; H, 8.2; S, 4.2. Selected spectroscopic
data: IR (Nujol mull) 1951 cm-1 (νPd-H); 31P{1H} NMR
(toluene-d8, 121 MHz) AMX spin system, δA 41.4, δX 14.0, δM
2
2
2
36.7, J AX ) 308 Hz, J MX ) 40 Hz, J AM ) 24 Hz; 1H NMR
2
(toluene-d8, 300 MHz) δ -7.6 ppm (ddd, Pd-H, J H-P(trans)
)
171 Hz, J H-P(cis) ) 16 and 7 Hz); 13C{1H,31P} NMR (toluene-
d8, 125.7 MHz) δ 37.6 (CH), 31.0 (CH2), 28.4 (CH), 27.7 (CH2),
27.3 (CH), 26.6 (CH2), 22.6 (CH2), 21.2 (CH3), 19.1 (CH3), 18.9
(CH2), 18.2 (CH2), 17.7 (CH3), 16.9 (CH2).
2
In Situ Gen er a tion of (DIP P P )P d (CO)n (n ) 1, 2) a n d
(DIP P P )P d (C2H4). (DIP P P )P d (CO)2 (2). A toluene solution
of complex 1a (40 mg, 0.06 mmol) was transferred into a NMR
tube and pressurized with 40 psi of CO. The initial clear yellow
solution became colorless within a few seconds. IR (toluene):
2002, 1961 cm-1 (νCO). 31P{1H} NMR (toluene-d8, 121 MHz):
29.9 ppm. 13C{1H,31P} NMR (toluene-d8, 125.7 MHz): δ 26.4
Syn th esis of [{(DIP P P )P d]2(µ-H)(µ-CO)}][OP h ] (8). Com-
plex [(DIPPP)PdH(PCy3)][OPh] (5b) was generated as de-
scribed above, and the solution was pressurized with 40 psi of
CO. After 48 h, a red solution was obtained. NMR studies
revealed quantitative conversion into complex 8. IR (tolu-
ene): 1793 cm-1 (νCO). 31P{1H} NMR (toluene-d8, 121 MHz):
21.6 ppm. When 8 was generated with 13CO, this resonance
1
(CH), 23.7 (CH2), 22.6 (CH2), 18.9 (CH3), 17.8 (CH3). H{31P}
NMR (toluene-d8, 499.9 MHz): 1.52 (sept, 7 Hz, 4H), 1.45 (m,
2H), 1.02 (d, 7 Hz, 12H), 0.99 (m, 4H), 0.92 (d, 7.0 Hz, 12H).
(DIP P P )P d (CO) (3). A solution of complex 2, generated
as described above, was exposed to a nitrogen atmosphere for
3 min. The clear solution turned yellow-orange, and the IR
and NMR spectra were recorded immediately. IR (toluene):
1953 cm-1 (νCO). 31P{1H} NMR (toluene-d8, 499.9 MHz): 23.7
2
1
splits into a doublet with J PC ) 32 Hz. H NMR (toluene-d8,
2
300 MHz): -5.50 ppm (quintet, 1 H, Pd-H-Pd, J HP ) 40
Hz). 13C{1H} NMR (toluene-d8, 75 MHz): 250.3 (quintet, J PC
2
) 32 Hz).
Ca ta lytic Isom er iza tion of 1-P en ten e. The isomerization
1
of 1-pentene to 2- pentene was followed by H NMR spectros-
13
ppm. When CO was used, ν CO 1924 cm-1 was measured and
a doublet was observed in the 31P{1H} NMR (2J PC ) 32 Hz).
(DIP P P )P d (C2H4) (4). A toluene-d8 solution of complex 1a
(40 mg, 0.06 mmol) was transferred into a NMR tube and
pressurized with 40 psi of ethylene. 31P{1H} NMR (toluene-
d8, 121 MHz): 33.3 ppm.
13
copy. A solution containing 20 mg of 1a , 42 mg of 1-pentene,
and 60 mg of CF3CH2OH in 600 µL of benzene-d6 was loaded
into a 5 mm NMR tube. After the sample was heated to 80 °C
in the NMR probe, the isomerization rate was determined by
integrating the intensities of the olefinic protons of 1-pentene
and 2-pentene, respectively. To determine the effect of phos-
phine concentration on the rate of olefin isomerization, the
experiment was repeated in the presence of 10 mg of PCy3.
Ca ta lytic Exp er im en ts. o-Dichlorobenzene (0.2 g, internal
standard), phenol (1.4 g, 14.9 mmol), and complex 1a (20 mg,
0.03 mmol), 1b (15 mg, 0.03 mmol), or 7 (24 mg, 0.03 mmol)
were dissolved in 60 mL of toluene. The solution was loaded
Rea ction s of Com p lexes 1a a n d 1b w ith Alcoh ols. In
Situ Gen er a tion of th e Com p lexes [(DIP P P )P d H(P Cy3)]-
[OR] (5a -e) a n d [(DIP P P )P d H(P Et3)][OR] (6a -e). Com-
plex 1a (20 mg, 0.03 mmol) was dissolved in toluene-d8 (0.5
mL), and 5 molar equiv of p-cyanophenol was added to the
solution. The initial yellow color became colorless instanta-
neously. The NMR data displayed in Table 1 are in agreement
with the formula [(DIPPP)PdH(PCy3)][OR] (5a , R ) p-NC-
C6H40). Attempts to isolate the new complex failed because the
reaction reverses in the absence of excess alcohol.
into a 100 mL Hastelloy C autoclave under
a nitrogen
atmosphere. The autoclave was pressurized with ethylene (300
psi) and CO (600 psi) and then heated at 100 °C for about 2 h.
The content of the reactor was analyzed by GC on a HP-5890
chromatograph using a Quadrex 30 m × 320 µm cyanopropyl
methyl silicone capillary column. Conversion of phenol into
The reactions of 1a with phenol and 2,2,2-trifluoroethanol
were carried out in a similar manner, but with the addition of

[((iPr)2P(CH2)3P(iPr)2)(PCy3)PdH][OR]
Organometallics, Vol. 20, No. 2, 2001 339
Ta ble 2. Cr ysta llogr a p h ic Da ta for 1a a n d 7
X-r a y An a lysis of 7. At a crystal-to-plate distance of 85
mm, a total of 46 frames were collected with an oscillation
range of 4.0°/frame. A total of 19 408 data were collected from
2.6° < 2θ < 48.4°. After the 5648 duplicates were merged (3.0%
R(merge)), 4803 unique reflections with I > 3.0σ(I) remained
for the analysis. The structure was solved by direct methods
(MULTAN). The asymmetric unit consists of one ion pair in a
general position. The hydrogen atoms were idealized with a
C-H bond length of 0.95 Å; the Pd-H distance was set to 1.6
Å and refined. Refinement was done by full-matrix least
squares on F2. All non-hydrogen atoms were successfully
refined anisotropically, and all hydrogen atoms were refined
isotropically. The final refinement cycle included 410 param-
eters, with a data/parameter ratio of 11.62, and resulted in R
) 0.036 and Rw ) 0.036 with an error of fit of 1.17. The
maximum shift/error was 0.08, and the largest residual density
on the on the final difference map was 0.57 e/Å3, suggesting a
second orientation of the triflate.
1a
7
formula
fw
C
33H67P3Pd
C33H68F3O3P3SPd
813.30
663.22
cryst syst
space group
a (Å)
b (Å)
c (Å)
noncentrosymmetric monoclinic
Cc (No. 9)
13.174(1)
16.175(1)
16.872(1)
96.233(3)
3574
P21/n
16.137(1)
22.431(1)
11.362(1)
103.738(1)
3995
â (deg)
V (Å3)
Z
4
4
Dcalcd (g cm-3
)
1.232
1.357
cryst size (mm)
µ(Mo KR) (cm-1
2θ range (deg)
temp (°C)
cryst to plate
dist (mm)
0.06 × 0.11 × 0.85
0.2 × 0.33 × 0.25
)
6.63
6.72
2.4-48.9
-41
83
2.6-48.4
-54
85
no. of frames
45
46
oscillation range,
deg/frame
4.0
4.0
Resu lts a n d Discu ssion
exposure, min/frame 8.0
4.0
P a lla d iu m (0) Com p lexes. The reaction of Pd(PEt3)3
and Pd(PCy3)2 with DIPPP provides a convenient entry
into the (DIPPP)PdPR3 (1a , R ) Cy; 1b, R ) Et) system
(eq 1). Both complexes are easily characterized on the
no. of rflns collected
no. of indep rflns
final R index
9803
19 804
4803
0.036
1.17
0.08
11.62
0.57
2812
0.039
1.52
0.31
8.16
0.40
goodness of fit on F
largest ∆/σ
data to param ratio
largest diff, e Å-3
Ta ble 3. Selected Bon d Dista n ces (Å) for 1a a n d 7
1a
7
Pd(1)-P(1)
Pd(1)-P(2)
Pd(1)-P(3)
P(1)-C(1)
P(1)-C(11)
P(1)-C(21)
P(2)-C(3)
P(2)-C(31)
P(2)-C(41)
P(3)-C(51)
P(3)-C(61)
P(3)-C(71)
2.300(1)
2.307(2)
2.313(2)
1.856(8)
1.869(8)
1.863(8)
1.865(7)
1.894(9)
1.849(8)
1.884(6)
1.877(7)
1.883(6)
Pd(1)-P(1)
Pd(1)-P(2)
Pd(1)-P(3)
Pd(1)-H(1)
P(1)-C(1)
P(1)-C(11)
P(1)-C(21)
P(2)-C(3)
P(2)-C(31)
P(2)-C(41)
P(3)-C(51)
P(3)-C(61)
P(3)-C(71)
2.2925(10)
2.3721(9)
2.340(1)
1.532(39)
1.823(4)
1.846(4)
1.833(3)
1.825(4)
1.848(4)
1.858(4)
1.855(4)
1.851(3)
1.850(3)
phenyl propionate was 70% for complex 1a , 72% for complex
1b, and 100% for complex 7.
Cr ysta l Str u ctu r e Deter m in a tion . The X-ray crystal-
lographic analyses were based on the data collected on a
Rigaku RU300 R-AXIS IIc image plate area detector using Mo
KR radiation with a graphite monochromator. Tables 2 and 3
display the main crystallographic data, bond distances, and
bond angles for the molecules of complexes 1a and 7.
X-r a y An a lysis of 1a . At a crystal to plate distance of 83
mm, a total of 45 frames were collected with an oscillation
range of 4.0°/frame. A total of 9803 data were collected from
2.4° < 2θ < 48.9°. After the 3086 duplicates were merged (2.4%
R(merge)), 2812 unique reflections with I > 3.0σ(I) remained
for the analysis. The structure was solved by automated
Patterson analysis (PHASE) and consists of one molecule in a
general position. The hydrogen atoms were idealized with a
C-H bond length of 0.95 Å but were omitted for the C3
bridging atoms. The acentric space group indicates an optically
pure enantiomorph, which was established from an R value
test (0.0589/0.0597 vs 0.0711/0.0722). Refinement was by full-
matrix least squares on F; the final refinement cycle included
341 parameters, with a data/parameter ratio of 8.16, and
resulted in R ) 0.039 and Rw ) 0.039 with an error of fit of
1.52. The maximum shift/error was 0.31, and the largest
residual density on the final difference map was 0.40 e/Å3, near
Pd.
basis of their 31P NMR spectra, where they both exhibit
an AX2 pattern. For example, the value of J AX in
complex 1a (81 Hz) compares well to the value reported
for (DIPPP)Pd(PiPr2nBu) (84 Hz).5 Complex 1a was
further characterized by X-ray diffraction methods. The
molecular structure of 1a in the solid state (Figure 1)
shows the expected trigonal-planar coordination around
the metal center formed by the three phosphorus atoms.
Whereas the atomic distances appear to be within the
normal range for this type of compound (Table 3), the
P(1)-Pd-P(2) (101.95(6)°) bite angle is rather short
when compared with other molecules containing the

340 Organometallics, Vol. 20, No. 2, 2001
Pe´rez et al.
of the same complex, (DIPPP)Pd(C2H4) (4). This complex
F igu r e 1. View of the molecular structure of complex 1a ,
showing propane bridge disorder.
same fragment. For example, the dimeric complex
[(DIPPP)Pd]2(µ-H)(µ-CO)][Cl]8 presents a 126.1(2)° value
for that angle. This could be explained in terms of the
steric hindrance between the cyclohexyl groups in PCy3
and the isopropyl fragments on the DIPPP ligand.
Surprisingly, the Pd is nearly planar with hydrogen
atoms of the isopropyl groups yielding an 180° cone
angle.
The complexes 1a and 1b instantaneously react with
CO to produce (DIPPP)Pd(CO)2 (2) (31P{1H} NMR, δ
29.9 ppm; IR (toluene), νCO 2002, 1961 cm-1). The
monocarbonyl complex (DIPPP)Pd(CO) (3) (31P{1H}
NMR, δ 23.7 ppm; IR (toluene), νCO 1953 cm-1) was
observed when a solution of complex 3 was briefly
purged with nitrogen to remove the carbon monoxide.
When the reaction is performed in the presence of 13CO,
no coupling of 13C to 31P is observed in the 31P NMR
spectra of 3, even at temperatures as low as -80 °C.
Additionally, no resonance could be assigned to the
palladium carbonyl moiety in the 13C NMR spectra of 3
at temperatures as low as -80 °C. The lack of coupling
and the lack of a discrete resonance in the 13C NMR
spectra are probably due to the fast exchange of carbon
monoxide bonded to palladium in complex 3 and free
CO in solution. Milstein5,8 has previously suggested the
existence of 2 and 3. The related complex (BDPP)Pd-
(CO)2 (BDPP ) (2S,4S)-2,4-bis(diphenylphosphino)pen-
tane) has been proposed by Elsevier.9 (DtBPE)Pd(CO)2
(DtBPE ) bis(di-tert-butylphosphino)ethane) has been
structurally characterized by Porschke.10 To have an
independent confirmation of the assignments for com-
plexes 2 and 3, we prepared samples of both complexes
by reacting {(DIPPP)Pd}2(µ-H)2}11 with CO. The spec-
troscopic features previously described for 2 and 3 are
identical with those obtained when prepared by this
independent reaction. Attempts to isolate 2 or 3 failed
due to the loss of carbon monoxide.
has been described previously by Fryzuk.11 The ex-
change between free ethylene in solution and coordi-
nated ethylene is responsible for the absence of coupling
between the hydrogen nuclei of ethylene and the
phosphorus of the DIPPP ligand. No coupling is ob-
served, even at -80 °C. In the presence of R-olefins, no
olefin complex was detected. Ethylene is cleanly dis-
placed by carbon monoxide, giving 2 and 3 (Scheme 1).
P a lla d iu m (II) Com p lexes. Reaction of 1a and 1b
with alcohols and phenols cleanly generates complexes
of the general composition [(DIPPP)PdH(PR3)][OR] (5,
6). Attempts to isolate these complexes proved to be
unsuccessful, due to the decomposition to (DIPPP)Pd-
(PR3) and alcohol. Despite this, all the complexes were
fully characterized by 1H and 31P NMR (Table 1). Thus,
the 1H NMR spectrum of [(DIPPP)PdH(PCy3)][OPh]
displays a resonance centered at -7.8 ppm (ddd),
assigned to the hydride group (Figure 2). The splitting
of the hydride resonance occurs due to coupling with a
phosphorus atom of the DIPPP ligand occupying a trans
position relative to the hydride (J H-P ) 171 Hz) and
both the cis phosphorus nuclei of the PCy3 and DIPPP
We have also investigated the interaction of 1a and
1b with ethylene (eq 2), both leading to the formation
(8) Portnoy, M.; Frolow, F.; Milstein, D. Organometdllics 1991, 10,
3960.
(9) To´th, I.; Elsevier, C. J . Organometallics 1994, 13, 2118.
(10) Trebbe, R.; Goddard, R.; Rufinska, A.; Seevogel, K.; Porschke,
K. Organometallics 1999, 18, 2466.
(11) Fryzuk, M.; Lloyd, B.; Clentsmith, G.; Rettig, S. J . Am. Chem.
Soc. 1994, 116, 3804.
F igu r e 2. 1H NMR upfield hydride resonance for the
complex [(DIPPP)PdH(PCy3)]OPh.

[((iPr)2P(CH2)3P(iPr)2)(PCy3)PdH][OR]
Organometallics, Vol. 20, No. 2, 2001 341
Sch em e 1
F igu r e 3. Molecular geometry of complex 7 (cation only).
) 8) is required to convert >99% of (DIPPP)Pd(PCy3)
(1a ) into [(DIPPP)PdH(PCy3)][OR]. In contrast, MeOH
(pKa ) 16.0) must be the solvent to observe the forma-
tion of the cationic Pd hydride.
Even though we were not able to isolate any [(DIPPP)-
PdH(PCy3)][OR] complexes, we have prepared an analo-
gous compound (7) by reaction of (DIPPP)Pd(PCy3) and
triflic acid. The spectroscopic properties of this complex
are completely similar to those of complexes 5 and 6.
The X-ray structure of 7 (Figure 3) reveals the expected
square-planar geometry of the complex. The angles
around the palladium center show significant deviation
from orthogonality (H-Pd-P(1) ) 79°, P(1)-Pd-P(2)
) 97.69°, P(2)-Pd-P(3) ) 110°). The refined hydrogen
atom is located in the PdP3 plane, and the Pd-H bond
distance is 1.532(39) Å. This value compares well with
those reported for other structurally characterized pal-
ladium hydrides: 1.51(1) and 1.57(2) Å for trans-[Pd-
(PCy3)2(H)(H2O)]BF412 and [Pd(PCy3)2(H)(OPh)]‚PhOH,4a
respectively. The P-Pd-P angle encompassing the
hydride is highly acute (152°), suggesting a large van
der Waals interaction between the isopropyl groups of
the ligand and the cyclohexyl groups of the PCy3.
The reaction of the cationic complexes 5 and 6 with
olefins seems to be negligible, since no new resonances
are shown in the NMR spectra of the reaction mixture.
However, upon heating at 80 °C, it was possible to
observe the slow isomerization of 1-pentene into 2-pen-
tene, which suggests a very facile olefin insertion/â-
hydrogen elimination mechanism (Scheme 2). The pres-
ence of added phosphine partially inhibits this isomeri-
zation process (Figure 4), indicating that phosphine
dissociation must occur before the rate-determining
step.
ligand (J H-P ) 17 and 7 Hz). Similar patterns have been
reported for the related complexes [(DIPPP)2PdH][Cl]5
and [PdH(PPh3)3][BPh4].4b In all the cases studied,
metal protonation is a reversible process, indicated by
detection of both the Pd(0) and Pd(II) compounds in the
reaction mixture (eq 3).
The reaction of [(DIPPP)PdH(PCy3)][OPh] with CO
gives a mixture of (DIPPP)Pd(CO)2 and [(DIPPP)Pd(µ-
1
H)(µ-CO)Pd(DIPPP)][OPh] (8). The main feature in H
NMR of complex 8 at room temperature is the hydride
resonance, split as a quintet at -5.5 ppm with a J H-P
value of 40 Hz. The coupling pattern is attributed to
the presence of four equivalent phosphorus atoms. Fast
exchange is also observed for the carbonyl resonance of
[(DIPPP)Pd(µ-H)(µ-13CO)Pd(DIPPP)][OPh] at 250.3 ppm.
The resonance of the carbonyl is split into a quintet due
to coupling with four equivalent phosphorus atoms with
The equilibrium constant for the reaction depicted in
eq 3 depends on the value of the pKa of the alcohol or
phenol. For example, only 5 equiv of p-cyanophenol (pKa
(12) Leoni, P.; Sommovigo, M.; Pasquali, M.; Midollini, S.; Braga,
D.; Sabatino, P. Organometallics 1991, 10, 1038.

342 Organometallics, Vol. 20, No. 2, 2001
Pe´rez et al.
Sch em e 2
a J C-P value of 32 Hz. Variable-temperature studies by
1H and 13C NMR spectroscopy revealed that the ex-
change, which accounts for the equivalence of all four
phosphorus atoms, cannot be frozen at temperatures as
low as -80 °C. The mechanism of formation of 8 could
first involve dissociation of PhOH from [(DIPPP)PdH-
(PCy3)][OPh], followed by displacement of PCy3 by CO
to give (DIPPP)Pd(CO)2 and finally protonation by
PhOH to give the binuclear compound 8 (eq 4). Even
detected spectroscopically, a mechanism suggesting its
protonation and direct reaction with either 2 or 3 to give
complex 8 cannot be ruled out. [(DIPPP)Pd(µ-H)(µ-CO)-
Pd(DIPPP)][OPh] is totally analogous to the PPh3
derivative [(PPh3)2Pd(µ-H)(µ-CO)Pd(PPh3)2][O2CCF3] pre-
viously reported by Zudin.4c Milstein5 and Elsevier9 later
reported analogous complexes with the ligands DIPPP
and BDPP, respectively.
The presence of a binuclear palladium hydride of
structure 8 has been previously reported in the carbo-
F igu r e 4. Kinetic profile for the isomerization of 1-pentene
into 2-pentene catalyzed by (DIPPP)Pd(PCy3) (1a ) and CF3-
CH2OH at 80 °C in C6D6: (a) in the absence of PCy3; (b) in
the presence of PCy3. [1a ]/[PCy3] ) 0.85.
though the unsaturated fragment (DIPPP)Pd cannot be

[((iPr)2P(CH2)3P(iPr)2)(PCy3)PdH][OR]
Organometallics, Vol. 20, No. 2, 2001 343
F igu r e 5. 31P{1H} NMR monitoring of the catalytic conversion of a mixture of carbon monoxide, ethylene, and phenol
into phenyl propionate. The resonance marked with an asterisk has not been identified (elapsed time between spectra 30
min).
nylation of (P-P)PdMeCl to MeCOOMe13 in the pres-
ence of methanol and also in the reaction of (DIPPP)-
PdPhCl with methanol.5 The presence of this bridging
hydride in a catalytic transformation has never been
proven spectroscopically.
Ca ta lytic Exp er im en ts. To establish the participa-
tion of 8 in a catalytic transformation, we have studied
the carbonylation of ethylene with phenols in the
presence of [(DIPPP)PdH(PR3)][OPh] (R ) Cy, Et) as
the catalyst (eq 5). We have found that (DIPPP)Pd(PR3)
onates with activities on the order of 102 (mol of
product)/((mol of catalyst) h) at temperatures around
150 °C. The rate of this transformation was not affected
by the nature of PR3, which is in agreement with the
mechanism proposed in Scheme 2: i.e., phosphine
dissociation takes place immediately in the presence of
CO and ethylene. The rate of the reaction had a direct
relation to the nature of the phenol: higher pKa led to
higher turnover frequencies. Methanol and ethanol,
neat or in toluene solution, fail to give methyl and ethyl
propionates, respectively, when reacted with ethylene
in the presence of 1a and 1b. The lack of activity can
CH2dCH2 + CO + ArOH 1a , 1b8 CH3CH2COOAr (5)
(R ) Cy, Et) each catalyze the synthesis of aryl propi-
(13) To´th, I.; Elsevier, C. J . J . Am. Chem. Soc. 1993, 115, 10388.

344 Organometallics, Vol. 20, No. 2, 2001
Pe´rez et al.
Sch em e 3
be attributed to the large pKa value of methanol and
ethanol compared to the pKa of phenols. The difference
in pKa is probably responsible for the absence of
(DIPPP)Pd(L)H+OR- complexes when L ) ethylene,
which we believe necessary to enter the catalytic cycle.
In comparison, when L ) PEt3, PCy3, protonation occurs
even with methanol or ethanol.
Using in situ high-pressure NMR techniques, we
followed the transformation under conditions identical
with those used in our autoclave experiments. We were
able to detect, as shown in Figure 5, the complexes
(DIPPP)Pd(C2H4), (DIPPP)Pd(CO)2, and [(DIPPP)Pd(µ-
H)(µ-CO)Pd(DIPPP)][OPh] and free PCy3. The mono-
meric cationic hydride [(DIPPP)PdH(PR3)][OPh] was not
detected in the reaction mixture. GC analysis of the
reaction solution from the high-pressure NMR tube after
completion of the experiment confirmed the presence
of phenyl propionate.
acidity of phenols compared with strong acids. However,
a similar in situ experiment using [(DIPPP)PdH(PR3)]-
[OTf] as the catalyst was performed and concluded with
results identical with those obtained when [(DIPPP)-
PdH(PR3)][OPh] was used as catalyst. In addition, the
major species in solution was the binuclear complex 8.
This complex acts as the thermodynamic sink for the
palladium complexes involved in the catalytic cycle.
Thus, a possible mechanistic proposal based on the
pieces of information described above is depicted in
Scheme 3. The cationic hydride 5 would exchange
phosphine and olefin to give a transient alkene hydride
intermediate. A series of elementary steps would fol-
low: (a) formation of a Pd alkyl complex; (b) CO
coordination; (c) formation of an alkyl carboxylate Pd-
(II) complex or the alternative acyl alkoxy isomer; (d)
reductive elimination of the aryl propionate; (e) CO
coordination to give complexes 2 and 3, both equilibrat-
ing with 1a ; (f) reaction of 3 with CO and PhOH to
provide 8, the resting state in the catalytic cycle, which
further reacts with olefin to start the cycle. We have
It is reasonable to assume that the lack of monomeric
palladium hydrides, presumed to be the most active
component in the catalysis, are due to the much lower

[((iPr)2P(CH2)3P(iPr)2)(PCy3)PdH][OR]
Organometallics, Vol. 20, No. 2, 2001 345
not found any evidence to propose a unique intermediate
from the alkyl carbonyl intermediate. Both the alkyl
carboxylate and the acyl aryloxide find precedent in the
literature. Elsevier13 has proposed the complex (BDPP)-
Pd(Me)(COOMe) in the formation of MeCOOMe, whereas
Yamamoto has established the existence of acyl aryl-
oxide nickel and palladium intermediates and the
corresponding reductive elimination to yield the aryl
carboxylates.14
catalyst to functionalize olefins with carbon monoxide,
the resting state of the catalyst is far different from the
one desired. In the example described in this work, we
targeted [(DIPPP)(PR3)PdH][OR] as the complex most
likely to be responsible for the catalytic activity. How-
ever, in the presence of CO and ethylene, the only
observable monomeric structures were (DIPPP)PdL (L
) CO, ethylene). Because of back-bonding between Pd
and the olefin, the electronic density at the metal center
in complexes such as (DIPPP)Pd(olefin) does not favor
the formation of [(DIPPP)(olefin)PdH] [OR] required to
enter the catalytic cycle shown in Scheme 3.
Con clu sion s
The use of basic phosphines such as DIPPP in
combination with PCy3 or PEt3 makes the palladium
metal center very electron rich. Proof of this behavior
is the in situ characterization of a variety of isostruc-
tural palladium hydrides prepared by simple protona-
tion of (DIPPP)PdPR3 with alcohols and phenols. The
key in this case was the synthesis of monomeric tri-
coordinated complexes.
Ack n ow led gm en t. We are indebted to Mrs. Mar-
garet Beattie for obtaining the NMR spectra and to Dr.
Henry E. Bryndza for many helpful discussions. P.J .P.
thanks DuPont and the Universidad de Huelva (Plan
Propio de Investigacion) for financial support.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data and structure refinement parameters, atomic coordinates,
bond lengths, bond angles, anisotropic displacement param-
eters and hydrogen coordinates of 1a and 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
It is interesting to note that, in contrast to what one
might expect when trying to design a possible active
(14) Komiya, S.; Akai, Y.; Tanaka, K.; Yamamoto, T.; Yamamoto,
A. Organometallics 1985, 4, 1130.
OM0008833