Taking too many precautions in making a catalyst is never a loss of time: A lesson we learned at our own expense

Article abstract of DOI:10.1021/om030132b

The reaction in MeOH between the bis-chelate complex [Pd(dppe)₂](OAc)₂ and Pd(OAc)₂ to give the monochelate product Pd(OAc)₂(dppe) is assisted by free acetate ion, and its rate is proportional to the concentrations of both reagents (dppe = 1,2-bis(diphenylphosphino)-ethane). The aggregation of Pd(OAc)₂ in CH₂Cl₂ and the low dielectric constant of this solvent are proposed to be important factors in accelerating the formation of Pd(OAc)₂(dppe) in CH₂Cl₂.

Full text of DOI:10.1021/om030132b

Organometallics 2003, 22, 4281-4285
4281
Ta k in g Too Ma n y P r eca u tion s in Ma k in g a Ca ta lyst Is
Never a Loss of Tim e: A Lesson We Lea r n ed a t Ou r Ow n
Exp en se
Claudio Bianchini,* Andrea Meli, and Werner Oberhauser
Istituto di Chimica dei Composti OrganoMetallici (ICCOM), Area di Ricerca CNR di Firenze,
Via Madonna del Piano, 50019 Sesto Fiorentino, Firenze, Italy
Received February 27, 2003
The reaction in MeOH between the bis-chelate complex [Pd(dppe)2](OAc)2 and Pd(OAc)2
to give the monochelate product Pd(OAc)2(dppe) is assisted by free acetate ion, and its rate
is proportional to the concentrations of both reagents (dppe ) 1,2-bis(diphenylphosphino)-
ethane). The aggregation of Pd(OAc)2 in CH2Cl2 and the low dielectric constant of this solvent
are proposed to be important factors in accelerating the formation of Pd(OAc)2(dppe) in CH2-
Cl2.
Sch em e 1
In tr od u ction
The alternating copolymerization1 of carbon monoxide
and ethene in MeOH has been recently investigated by
us using as catalyst precursor Pd(OAc)2 modified with
the chelating diphosphines 1,2-bis(diphenylphosphino)-
ethane (dppe), meso-2,3-bis(diphenylphosphino)butane
(meso-2,3-dppb), and rac-2,3-bis(diphenylphosphino)-
butane (rac-2,3-dppb).2 Under comparable experimental
conditions, the productivity in polyketone obtained with
the dppe catalyst was found to be lower than those with
either meso-2,3-dppb or rac-2,3-dppb by a factor of 10
(Scheme 1). Such a remarkable decrease in activity for
a catalyst precursor, differing from the other two only
for the methyl substituents in the ligand backbone, was
surprising and chemically intriguing. Therefore, a de-
tailed study of the chemical-physical properties of all
PdII precursors was undertaken in our laboratories.
The dppe catalyst used in the CO/C2H4 copolymeri-
zation reactions (denoted as 1*) was prepared by react-
ing a MeOH solution of Pd(OAc)2 with an equivalent
amount of dppe dissolved in CH2Cl2. The addition of
petroleum ether gave a yellow-orange crystalline prod-
uct that multinuclear NMR spectroscopy in CD2Cl2 and
a single-crystal X-ray analysis showed to be the known
complex Pd(OAc)2(dppe) (1).2 Every sample of 1* was
regularly checked by NMR spectroscopy in CD2Cl2 prior
to use in the copolymerization reactions. The complexes
Pd(OAc)2(meso-2,3-dppb) (2) and Pd(OAc)2(rac-2,3-dppb)
(3) were synthesized by a different procedure, involving
metathetical reactions of the corresponding bis-chloride
derivatives Pd(Cl)2(P-P) with silver acetate in CH2Cl2.
Multinuclear NMR spectroscopy and X-ray analyses
showed 2 and 3 to have the same primary structure of
1.
The following step was to study the kinetics of the
catalytic reactions, measuring the gas uptake with
time: None of the precursors showed an induction
period; in accord with the batch catalytic reactions, the
dppe-modified catalyst consumed less CO/C2H4 than the
other two, steadily since the early stages of the reaction.
Next, a high-pressure NMR study of the copolymeriza-
tion reactions was carried out under experimental
conditions comparable to those in the batch reactions.
A remarkable difference was immediately noticed in the
behavior of the three PdII precursors 1*, 2, and 3. The
latter two complexes dissolved in MeOH-d4 maintained
the monochelate structure, yet undergoing the exchange
of one or two acetate groups for solvent molecules.3 In
contrast, the 31P{1H} NMR spectrum of 1* in MeOH-d4
showed the presence of both 1 and the bis-chelate
complex [Pd(dppe)2](OAc)2 (4). Remarkably, the 1:4 ratio
remained constant for 1 h (longer times were not
investigated), while the addition of TsOH, alone or in
conjunction with BQ, had the only effect of converting
the acetate precursors into the tosylate derivatives Pd-
(OTs)2(P-P) (P-P ) dppe, 5; meso-2,3-dppb, 6) and [Pd-
(dppe)2](OTs)2 (4OTs). The 5:4OTs ratio remained
constant for 24 h at room temperature, and the addition
* Corresponding author.
(1) (a) Sen, A. Acc. Chem. Res. 1993, 26, 303. (b) Drent, E.;
Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663. (c) Drent, E.; van
Broekhoven, J . A. M.; Budzelaar, P. H. M. In Applied Homogeneous
Catalysis with Organometallic Compounds; Cornils, B., Herrmann, W.
A., Eds.; VCH: Weinheim, 1996; Vol. 1, p 333. (d) Sommazzi, A.;
Garbassi, G. Prog. Polym. Sci. 1997, 22, 1547. (e) Nozaki, K.; Hijama,
T. J . Organomet. Chem. 1999, 576, 248. (f) Bianchini, C.; Meli, A.
Coord. Chem. Rev. 2002, 225, 35. (g) Bianchini, C.; Meli, A.; Ober-
hauser, W. J . Chem. Soc., Dalton Trans. 2003, 2627. (h) Freisa, Z.;
van Leeuwen, P. W. N. M. J . Chem. Soc., Dalton Trans. 2003, 1890.
(2) Bianchini, C.; Lee, H. M.; Meli, A.; Oberhauser, W.; Peruzzini,
M.; Vizza, F. Organometallics 2002, 21, 16.
(3) (a) Pignat, K.; Vallotto, J .; Pinna, F.; Strukul, G. Organometallics
2000, 19, 5160. (b) Brown, J . M.; Hii (Mimi) K. K. Angew. Chem., Int.
Ed. Engl. 1996, 35, 657. (c) Stang, P. J .; Cao, D. H.; Poulter, G. T.;
Arif, A. M. Organometallics 1995, 14, 1110.
10.1021/om030132b CCC: $25.00 © 2003 American Chemical Society
Publication on Web 09/19/2003

4282 Organometallics, Vol. 22, No. 21, 2003
Bianchini et al.
Sch em e 2
Sch em e 3
of 40 bar of CO/C2H4 did not appreciably affect the 31P
NMR picture of either system.
Our interpretation of this experimental evidence was
that 1 undergoes in MeOH (ꢀ ) 32.6) the autoionization
process illustrated in Scheme 2, whereas it remains
intact in a solvent with a lower dielectric constant such
as CH2Cl2 (ꢀ ) 9.1).
Any other residual doubt of this interpretation was
seemingly dispelled by a paper published almost con-
temporaneously to the writing of our article. In this
paper,4 Mul, Bouwman, and co-workers reported that
Ni(OAc)2(dppe) undergoes autoionization in MeOH to
give equilibrium concentrations of [Ni(dppe)2]2+ and
[Ni(OAc)4]2-. A previous work by Sadler et al. was in
line with the occurrence of NiCl2(dppe) autoionization
in MeOH.5
In late 2002, Mul and co-workers published a paper
in which it was unambiguously shown that the bis-
chelate complex 4 is the kinetic product of the stoichio-
metric reaction between Pd(OAc)2 and dppe in MeOH
(Scheme 3).6 On standing in MeOH at room tempera-
ture, 4 slowly converted into the thermodynamic prod-
uct 1. This means that the autoionization of 1 in MeOH
does not occur. Therefore, the question is, what misled
us and what really was 1*?
F igu r e 1. Variable-temperature 31P{1H} NMR study (CD2-
Cl2, 81.01 MHz) of the reaction of Pd(OAc)2 with dppe: (a)
at -70 °C after dissolving Pd(OAc)2 and dppe in CD2Cl2 at
-70 °C; (b) at -20 °C after 50 min; (c) at room temperature
after 10 min.
of 1. Increasing the temperature led to the dissolution
of precipitated 4 with concomitant formation of 1 (trace
b). The first spectrum at room temperature showed the
exclusive presence of 1 (trace c). In comparable concen-
trations of Pd(OAc)2 and dppe, but in MeOH-d4, the
conversion of the kinetic product 4 into the thermody-
namic product 1 was much slower, occurring with a t1/2
value of ca. 4 h at room temperature (see below).6
The NMR study described above proves that the
reaction of Pd(OAc)2 in MeOH with a stoichiometric
amount of dppe in CH2Cl2 at room temperature, fol-
lowed by precipitation with petroleum ether, yields a
solid mixture, namely, 1*, comprising 4, Pd(OAc)2, and
1. The relative ratio between 4 and 1 in this mixture
decreases with the reaction time, yet the dissolution of
any sample of 1* in CH2Cl2 at room temperature leads
to the immediate formation of 1, which explains why
we were confident to use pure 1 in the catalytic
experiments.2 As shown by Mul and co-workers,6 the
transformation of 4 into 1 in MeOH is too slow to be
appreciated in 1 h (as we tried to do)2 and is also
immediately stopped by the addition of TsOH, which is
an essential component of the Pd(OAc)2/(diphosphine)
catalytic systems under investigation.2
Resu lts a n d Discu ssion
As reported by Mul and co-workers,6 the stoichiomet-
ric reaction of Pd(OAc)2 with dppe in CD2Cl2 at room
temperature gave 1 immediately and quantitatively
1
(31P{1H} and H NMR analysis). When a solid sample
In an attempt to understand the role of TsOH in inhi-
biting the formation of the thermodynamic monoche-
late complex 1, the bis-chelate 4OTs was prepared by
of 1 prepared in CH2Cl2 was dissolved in MeOH-d4
under nitrogen, no trace of 4 was formed over 24 h at
room temperature. Next, the reaction between Pd(OAc)2
and dppe was investigated in CD2Cl2 at low temperature
by 31P{1H} NMR spectroscopy. The two reagents were
dissolved in a 5 mm NMR tube containing degassed
CD2Cl2 at -70 °C. Immediately, a solid product pre-
cipitated,7 which was filtered off and identified as pure
4 by NMR spectroscopy in CD2Cl2. In a second experi-
ment, the solid precipitate of 4 was not removed from
the NMR tube which was placed into a probe-head
precooled at -70 °C. As shown in Figure 1 (trace a),
the solution contained 4 together with a minor amount
8
metathetical reaction of [Pd(dppe)2]Cl2 with AgOTs in
CH2Cl2. Remarkably, 4OTs was found to be fully stable
in MeOH and CH2Cl2 at room temperature for 24 h even
in the presence of Pd(OAc)2 (Scheme 4a). In contrast, 4
reacted with Pd(OAc)2 in either MeOH or CH2Cl2,
yielding 1 (Scheme 4b). Interestingly, 4 was unstable
in CH2Cl2, where it underwent slow and irreversible
reduction to the Pd0 derivative Pd(dppe)2 (7) with
concomitant formation of 1, dppedO, and acetic anhy-
dride (Scheme 4b).7,8a,b,9 This redox transformation is
(8) (a) Mason, M. R.; Verkade, J . G. Organometallics 1990, 9, 864.
(b) Lindsay, C. H.; Benner, L. S.; Balch, A. L. Inorg. Chem. 1980, 19,
3503. (c) Engelhardt, L. M.; Patrick, J . M.; Raston, C. L.; Twiss, P.;
White, A. H. Aust. J . Chem. 1984, 37, 2193.
(9) (a) Amatore, C.; Carre´, E.; J utand, A.; M’Barki, M. A. Organo-
metalllics 1995, 14, 1818. (b) Amatore, C.; J utand, A.; Thuilliez, A.
Organometalllics 2001, 20, 3241.
(4) Angulo, I. M.; Bouwman, E.; Lutz, M.; Mul, W. P.; Spek, A. L.
Inorg. Chem. 2001, 40, 2073.
(5) J arrett, P. S.; Sadler, P. J . Inorg. Chem. 1991, 30, 2098.
(6) Marson, A.; van Oort, A. B.; Mul, W. P. Eur. J . Inorg. Chem.
2002, 3028.
(7) Grushin, V. V. Organometallics 2001, 20, 3950.

Precautions in Making a Catalyst
Organometallics, Vol. 22, No. 21, 2003 4283
Sch em e 4
F igu r e 2. Plot of ln [4] versus reaction time for the
reaction of 4 with Pd(OAc)2 in MeOH at 23 °C; traces a-d
refer to the experiments 1-4 of Table 2, respectively.
Ta ble 1. Alter n a tin g Cop olym er iza tion of Ca r bon
Mon oxid e w ith Eth en e Ca ta lyzed by P d /d p p e
P r ecu r sor s in MeOHa
entry
precursor
alt-E-CO g
alt-E-CO prod.b
1
2
3
4
1c
3.20
1.20
0.14
0.06
3.01
n.c.
n.c.
[4 + Pd(OAc)2 + 1]d
[4 + Pd(OAc)2 + 1]e
4c
0.06
F igu r e 3. Plot of -ln kobs versus -ln [Pd(OAc)2] for
different initial concentrations of Pd(OAc)2 (Table 2, ex-
periments 1-4).
a
Initial p(C2H4) (20 bar), initial p(CO) (20 bar), temperature
b
(85 °C), time (3 h), stirring rate (1400 rpm). Productivity
expressed as kg of polymer (g of Pd)-1
.
c Precursor (0.01 mmol),
d
MeOH (100 mL), BQ (0.8 mmol), TsOH (0.2 mmol). See ref 2;
the precursor (6.2 mg) employed in the catalytic run was part of
a solid sample precipitated with petroleum ether from a reaction
mixture obtained by mixing Pd(OAc)2 in MeOH and dppe in
CH2Cl2. e The precursor was prepared in situ by reacting Pd(OAc)2
(0.01 mmol) and dppe (0.01 mmol) in MeOH (50 mL) for 10 min
before the addition of BQ (0.8 mmol) and TsOH (0.2 mmol) in
MeOH (50 mL).
Ta ble 2. Kin etic Da ta for th e Con ver sion 4 +
P d (OAc)2 in to 1 in MeOH
entry 4:Pd(OAc)2 ratio 102[4] M 102[Pd(OAc)2] M 10kobs h-1
1
2
3
4
5
6
7
1:1
1:1.5
1:2
1:3
1:1
1:1
1:1
2.35
2.35
2.35
2.35
1.17
3.52
4.70
2.35
3.52
4.70
7.05
1.17
3.52
4.70
1.72
2.33
3.11
5.04
1.22
1.79
2.00
known by the name of Amatore reaction and is typical
for Pd(OAc)2(phosphine)2 complexes (see below).9
The results reported in Scheme 4 prove that the
presence of free acetate ions is required to promote the
conversion of 4 into 1. This explains why the addition
of TsOH to MeOH solutions of 1*, converting OAc- into
HOAc, stops immediately the formation of the catalytic
precursor 1. Table 1 illustrates clearly the influence of
the preparation procedure of some dppe-based catalysts
on the productivity in alternating polyketone under
standard copolymerization conditions.
The highest activity was shown by isolated 1, pre-
pared by mixing equivalent amounts of Pd(OAc)2 and
dppe in CH2Cl2 (entry 1), while the precursors obtained
in either MeOH/CH2Cl2 or MeOH, being kinetic mix-
tures of 4, Pd(OAc)2, and 1, enriched in catalytically
inactive 4, were much less active (entries 2 and 3). In
particular, the activity of the kinetic mixtures decreased
by decreasing the reaction time before quenching by
either precipitation2 (entry 2) or TsOH addition (entry
3).6
As anticipated in Scheme 4, both free OAc- and Pd-
(OAc)2 are required to accomplish the transformation
of the kinetic bis-chelate product into 1. In an attempt
to understand the mechanism by which 4 converts into
1, a kinetic study was undertaken for the reaction
between 4 and Pd(OAc)2 in MeOH-d4 at 23 °C, using
31P{1H} NMR spectroscopy to monitor the variation of
the concentrations of 4 and 1 with time (Table 2).
noticed by Mul.6 We have now found that the rate of
the reaction depends also on the concentration of Pd-
(OAc)2. Indeed, increasing the initial concentration of
Pd(OAc)2, at constant concentration of 4, was found to
accelerate the formation of 1 (Table 2, entries 2-4;
Figure 2, traces b-d).
For a rate law of the type -d[4]/dt ) k[4][Pd(OAc)2]p
) kobs[4] with kobs ) k[Pd(OAc)2]p, a plot of -ln kobs
versus -ln [Pd(OAc)2] gave a straight line, indicating a
first-order dependence of the reaction rate also on Pd-
(OAc)2 (Figure 3). However, the complex nature of Pd-
(OAc)2 in MeOH (see below) biases this plot, at least to
draw out the true kinetic constant, because the initial
concentrations of Pd(OAc)2 given in the x-axis cannot
correspond to the actual concentrations of monomeric
Pd(OAc)2.
Indeed, palladium acetate is a trimer in the solid state
and may exist in solution in various forms, from
monomers to aggregates, depending on solvent, tem-
perature, and concentration.10,11 In particular, various
species, [Pd(OAc)2]n with n ) 1, 2, 3, 4, etc., have been
reported to form in MeOH at room temperature.6 The
1H NMR spectrum in MeOH-d4 of the palladium acetate
employed in this study showed several signals around
δ 2 for the acetate methyl group. Therefore, the con-
centration of Pd(OAc)2 available for the interaction with
(10) (a) Skapsky, C.; Smart, M. L. Chem. Commun. 1970, 658. (b)
Stephenson, T. A.; Morehouse, S. M.; Powell, A. R.; Heffer, J . P.;
Wilkinson, G. J . Chem. Soc. 1965, 3632.
A plot of ln [4] versus time showed the formation of 1
to proceed with first-order kinetics in 4 (Table 2, entry
1; Figure 2, trace a). This rate dependence was also
(11) Stoyanov, E. S. J . Struct. Chem. (Engl. Transl.) 2000, 41, 440.

4284 Organometallics, Vol. 22, No. 21, 2003
Bianchini et al.
Sch em e 5
4, depending on the nature of the clusters, cannot be
the theoretical one at any stage of the reaction. As a
matter of fact, when some reactions were carried out
varying the initial equimolecular concentration of both
4 and Pd(OAc)2 (from 1.17 × 10-2 to 4.70 × 10-2 M,
Table 2, entries 1 and 5-7) and the kinetic data were
treated with the fractional-life method, the seeming
second-order of the reaction was not confirmed (a value
of 1.4 was obtained, in fact). In conclusion, the rate of
formation of 1 is first-order in 4 and depends also on
the concentration of Pd(OAc)2, yet the order in the latter
reagent cannot be determined reliably.
Con clu sion s
This study has confirmed6 that the reaction of Pd-
(OAc)2 with dppe in MeOH yields the bis-chelate com-
plex [Pd(dppe)2](OAc)2 as kinetic product, which slowly
converts to the thermodynamic monochelate Pd(OAc)2-
(dppe). The formation of the kinetic bis-chelate product
is due to the instantaneous shortage of active Pd(OAc)2
released by the [Pd(OAc)2]n clusters. It has been found
that the rate of the reaction of [Pd(dppe)2](OAc)2 with
Pd(OAc)2 in MeOH (t1/2 ) 4 h at 23 °C) is first-order in
the bis-chelate and also depends on the Pd(OAc)2
concentration. The formation of the monochelate com-
plex Pd(OAc)2(dppe) from either {dppe + Pd(OAc)2} or
{[Pd(dppe)2](OAc)2 + Pd(OAc)2} is much faster in CH2-
Cl2 than in MeOH, which has been ascribed to the lower
nuclearity of Pd(OAc)2 in the former solvent, whose
small dielectric constant would favor the formation of
the rate-determining species [Pd(OAc)(η2-dppe)(η1-dp-
pe)]OAc.
The results reported in this paper may be considered
as a warning to all the chemists who intend to study a
catalytic reaction whereby the catalyst is prepared in
situ: such a procedure is correct and may provide
reliable results only when the quantitative formation
of the catalytically active precursor is unambiguously
proved to occur within the residence time preceding the
addition of the other reagents involved in the process.
Incorporation of all of the experimental data obtained
for the reaction of 4 with Pd(OAc)2 in MeOH leads to a
mechanism involving a fast and reversible opening of
one of the chelating dppe ligands in 4 by acetate,
followed by slow and rate-determining interaction be-
tween the free phosphine arm and Pd(OAc)2 (Scheme
5, upper). The formation of a four-coordinate transient
species containing coordinated OAc- and monodentate
dppe is indirectly confirmed by the occurrence of the
Amatore reaction9 in CH2Cl2 leading to phosphine
oxidation and metal reduction (Scheme 5, dashed box).
The mechanism of formation of 1 as proposed in
Scheme 5 is presumably valid also in CH2Cl2, where the
rate is too fast to allow for kinetic studies. The faster
kinetics in CH2Cl2 may be due to the fact that Pd(OAc)2
exists mainly as a linear dimer in dilute solutions of
halogenated solvents such as those employed in this
study.11 A higher concentration of active Pd(OAc)2 would
therefore accelerate the formation of 1. Another factor
that might contribute to speed up the transformation
of 4 in CH2Cl2 is the low dielectric constant of this
solvent, which should favor the coordination of the
acetate counteranion to favor the four-coordinate inter-
mediate [Pd(OAc)(η2-dppe)(η1-dppe)]OAc (Scheme 5).
This may explain why the Amatore reaction of 4 is
rather fast in CH2Cl2 and does not appreciably occur in
MeOH over 24 h.
Exp er im en ta l Section
All reactions and manipulations were carried out under an
atmosphere of dry nitrogen by using Schlenk-type techniques.
All the reagents and solvents were used as purchased from
Aldrich. The palladium complexes Pd(OAc)2(dppe)6 and [Pd-
2,6
(dppe)2](OAc)2 were prepared following literature methods.
All of the isolated solid samples were collected on sintered-
glass frits and washed with appropriate solvents before being
dried under a stream of nitrogen. Copolymerization reactions
were performed with a 250 mL stainless steel autoclave,
constructed at the ISSECC-CNR (Firenze, Italy), equipped
with a magnetic drive stirrer and a Parr 4842 temperature
and pressure controller. The autoclave was connected to a gas
reservoir to maintain a constant pressure over the catalytic
reactions. Deuterated solvents for NMR measurements were
dried over molecular sieves. 1H and 31P{1H} NMR spectra were
obtained on a Bruker ACP 200 (200.13 and 81.01 MHz,
respectively). All chemical shifts are reported in ppm (δ)
relative to tetramethylsilane, referenced to the chemical shifts
of residual solvent resonances (1H) or 85% H3PO4 (31P).
Elemental analyses were performed using a Carlo Erba Model
1106 elemental analyzer.
A perusal of the relevant literature shows that the
(kinetic) bis-chelate to (thermodynamic) monochelate
transformation is not limited to either Pd(OAc)2 or dppe.
Precedents have been reported for the reactions of
various chelating diphosphines (dppm, dppe, dppp,
12
dppb, and DIOP) with Pd(dba)2 that, like Pd(OAc)2,
may form polynuclear species in solution.13
(12) Amatore, C.; Broeker, G.; J utand, A.; Khalil, F. J . Am. Chem.
Soc. 1997, 119, 5176.
(13) (a) Takahashi, Y.; Ito, T.; Sakai, S.; Ishii, Y. J . Chem. Soc.,
Chem. Commun. 1970, 1065. (b) Mosely, K.; Maitlis, P. M. J . Chem.
Soc., Chem. Commun. 1971, 982.
NMR Stu d y of th e Rea ction of P d (OAc)2 w ith d p p e in
CD2Cl2. A. Room Tem p er a tu r e. Solid dppe (10.9 mg, 2.8 ×

Precautions in Making a Catalyst
Organometallics, Vol. 22, No. 21, 2003 4285
10-2 mmol) was added to a 5 mm NMR tube containing a
solution of Pd(OAc)2 (6.2 mg, 2.8 × 10-2 mmol) in CD2Cl2 (0.8
mL) at room temperature. 31P{1H} and 1H NMR spectra,
acquired at room temperature after ca. 10 min of mixing,
showed the quantitative formation of 1.
(31P{1H} NMR singlet of 4OTs at δ 58.8 and 56.9 in MeOH-d4
and CD2Cl2, respectively), a solid sample of Pd(OAc)2 (3.0 mg,
1.3 × 10-2 mmol) was added into the tube. The reactions were
followed by 31P{1H} and 1H NMR spectroscopy at room
temperature. No reaction involving the bis-chelate 4OTs
occurred in 24 h, as shown by the NMR spectra acquired at
intervals of 1 h.
B. Low Tem p er a tu r e. Solid dppe (10.9 mg, 2.8 × 10-2
mmol) was added to a 5 mm NMR tube containing a solution
of Pd(OAc)2 (6.2 mg, 2.8 × 10-2 mmol) in CD2Cl2 (0.8 mL) at
-70 °C. There was the immediate precipitation of an off-white
solid that was separated by filtration. The solid, dissolved in
CD2Cl2 at room temperature, was identified as 4 by 31P{1H}
and 1H NMR spectroscopy.
In a parallel experiment, the NMR tube was placed into a
probe-head precooled at -70 °C without removing the formed
solid. The reaction was followed by variable-temperature 31P-
{1H} and 1H NMR spectroscopy. A sequence of selected 31P-
{1H} NMR spectra is reported in Figure 1. The first spectrum
at -70 °C showed the solution to contain 4 together with a
minor amount of 1 in a molar ratio of 6 to 1 (trace a).
Increasing the temperature led to the progressive dissolution
of precipitated 4 with concomitant formation of 1. The exclu-
sive presence of 1 was observed when the solution reached
room temperature. The spectra acquired at -20 °C after 50
min (trace b) and at room temperature after 10 min (trace c)
are reported as examples.
Kin etic Stu d y of th e Rea ction of 4 w ith P d (OAc)2 in
MeOH-d 4. A 5 mm NMR tube was charged with a solution
prepared by mixing 4 and Pd(OAc)2 in appropriate ratios (see
Table 2) in MeOH-d4 (0.8 mL) at room temperature and placed
into the probe of a NMR spectrometer at 23 °C. The reaction
was followed with the use of 31P{1H} NMR spectroscopy by
determining the concentrations of both 4 and 1 as a function
of time. The first spectrum was acquired after ca. 5 min of
mixing, while the following spectra were recorded every 30
min. Table 2 summarizes the kinetic data.
NMR Stu d y of th e Sta bility of 4 in CD2Cl2. A 5 mm NMR
tube was charged with a solution of 4 (13.3 mg, 1.3 × 10-2
mmol) in CD2Cl2 (0.8 mL) under nitrogen. The 31P{1H} NMR
spectrum acquired 10 min after the dissolution showed ca. 5%
decomposition of 4 into an almost equimolecular mixture 1,
the known bis-chelate Pd0 complex 7 (31P{1H} NMR singlet at
δ 30.7), and dppedO (31P{1H} NMR doublets at δ 31.8 and
-12.1, J (PP) ) 48.1 Hz).7,8a,b,9 The corresponding 1H NMR
spectrum showed a signal at δ 2.21, which was indicative of
acetic anhydride production. The disappearance of 4 occurred
slowly at room temperature (ca. 30% in 1 h). In contrast, when
4 was dissolved in CD2Cl2 containing Pd(OAc)2 (2.9 mg, 1.3 ×
10-2 mmol), the immediate and quantitative formation of 1
occurred.
NMR Stu d y of th e Sta bility of 1 in MeOH-d 4. A solid
sample of 1 (11.8 mg, 1.9 × 10-2 mmol), prepared from Pd-
(OAc)2 and dppe in CH2Cl2, was dissolved in MeOH-d4 (0.8 mL)
under nitrogen at room temperature and then transferred into
a 5 mm NMR tube. The reaction was monitored by recording
1
31P{1H} and H NMR spectra every hour. No trace of 4 formed
in 24 h.
Cop olym er iza t ion of E t h en e a n d Ca r bon Mon oxid e
Ca ta lyzed by P d /d p p e P r ecu r sor s in MeOH. MeOH (100
mL) was introduced by suction into a 250 mL autoclave
containing 86.5 mg of BQ (0.8 mmol), 38.0 mg of TsOH (0.2
mmol), and 0.01 mmol of catalyst precursor previously evacu-
ated by a vacuum pump. The autoclave was charged with an
1:1 CO/C2H4 mixture to 40 bar at room temperature. After the
contents of the autoclave had been brought to 85 °C, the
pressure was then maintained at ca. 45 bar by continuous
feeding of an equimolar mixture of CO and C2H4 from a gas
reservoir. The reaction mixture was then stirred (1400 rpm)
for the required reaction time. The reaction was stopped by
cooling the autoclave to room temperature by means of an ice-
water bath. After the unreacted gases were released, the
formed insoluble ethene/carbon monoxide copolymer was
filtered off, washed with methanol, and dried in a vacuum oven
at 70 °C overnight. The results of the catalytic experiments
are reported in Table 1.
NMR Stu d y of th e Rea ction of P d (OAc)2 w ith d p p e in
MeOH-d 4 a t Room Tem p er a tu r e. Solid dppe (15.1 mg, 3.8
× 10-2 mmol) was added to a 5 mm NMR tube containing a
solution of Pd(OAc)2 (8.5 mg, 3.8 × 10-2 mmol) in MeOH-d4
1
(0.8 mL) at room temperature. 31P{1H} and H NMR spectra,
acquired at room temperature after ca. 5 min of mixing,
showed the almost quantitative formation of 4.6 The reaction
was monitored at room temperature by recording 31P{1H} and
1H NMR spectra every 30 min. Compound 4 slowly converted
into 1 with a t1/2 of ca. 4 h.
Identical kinetic data were obtained by using 1.9 × 10-2
mmol of both 4 (19.4 mg) and Pd(OAc)2 (4.2 mg) in place of
3.8 × 10-2 mmol of both dppe and Pd(OAc)2.
Syn th esis of 4OTs. To a stirred solution of PdCl2(dppe)
(287.9 mg, 0.5 mmol) and dppe (199.2 mg, 0.5 mmol) in 20
mL of CH2Cl2 was added 2 equiv of AgOTs (1 mmol). After 30
min, the mixture was passed through a column of Celite to
remove the formed AgCl. The solution was concentrated to ca.
5 mL under vacuum. Addition of diethyl ether led to the
precipitation of 4OTs in 80% yield. Anal. Calcd for C66H62P4-
PdS2O6: C, 63.64; H, 5.02. Found: C, 63.53; H, 4.99.
Attem p ted Rea ction s of 4OTs w ith P d (OAc)2. A 5 mm
NMR tube was charged with a solution of 4OTs (16.2 mg, 1.3
× 10-2 mmol) in either MeOH-d4 or CD2Cl2 (0.8 mL) under
nitrogen. After 31P{1H} and 1H NMR spectra were acquired
Ack n ow led gm en t. Thanks are due to the European
Commission for the contracts HPRN-CT-2000-00010
and HPRN-CT-2002-00196 and to COST Action D17 for
sponsoring the Working Group 0007/2000.
OM030132B