Activation and deactivation of neutral palladium(II) phosphinesulfonato polymerization catalysts

Article

pubs.acs.org/Organometallics

Activation and Deactivation of Neutral Palladium(II)

Phosphinesulfonato Polymerization Catalysts

Thomas Runzi,^†Ulrich Tritschler,^†Philipp Roesle,^†Inigo Gottker-Schnetmann,^†Heiko M. Moller,^†

̈

Lucia Caporaso,*^,†Albert Poater,^§Luigi Cavallo,^⊥and Stefan Mecking*^,†

^†Department of Chemistry, University of Konstanz, 78464 Konstanz, Germany

^‡Department of Chemistry, University of Salerno, Via Ponte Don Melillo, 84084-Fisciano (SA), Italy

^§Institut de Química Computacional, Department de Química, Universitat de Girona, Campus de Montilivi, E-17071 Girona, Spain

^⊥Chemical and Life Sciences and Engineering, Kaust Catalysis Center, King Abdullah University of Science and Technology,

Thuwal 23955-6900, Saudi Arabia

S

* Supporting Information

ABSTRACT: ¹³C-Labeled ethylene polymerization (pre)-

catalysts [κ²-(anisyl)₂P,O]Pd(¹³CH₃)(L) (1-¹³CH₃-L) (L =

pyridine, dmso) based on di(2-anisyl)phosphine benzenesul-

fonate were used to assess the degree of incorporation of

¹³CH₃groups into the formed polyethylenes. Polymerizations

of variable reaction time reveal that ca. 60−85% of the ¹³C-

label is found in the polymer after already 1 min polymer-

ization time, which provides evidence that the pre-equilibra-

tion between the catalyst precursor 1-¹³CH₃-L and the active

species 1-¹³CH₃-(ethylene) is fast with respect to chain growth. The fraction of 1-¹³CH₃-L that initiates chain growth is likely

higher than the 60−85% determined from the ¹³C-labeled polymer chain ends since (a) chain walking results in in-chain

incorporation of the ¹³C-label, (b) irreversible catalyst deactivation by formation of saturated (and partially volatile) alkanes

diminishes the amount of ¹³CH₃groups incorporated into the polymer, and (c) palladium-bound ¹³CH₃groups, and more

general palladium-bound alkyl(polymeryl) chains, partially transfer to phosphorus by reductive elimination. NMR and ESI-MS

analyses of thermolysis reactions of 1-¹³CH₃-L provide evidence that a mixture of phosphonium salts (¹³CH₃)_xP⁺(aryl)_4−x(2−7)

is formed in the absence of ethylene. In addition, isolation and characterization of the mixed bis(chelate) palladium complex

[κ²-(anisyl)₂P,O]Pd[κ²-(anisyl)(¹³CH₃)P,O] (11) by NMR and X-ray diﬀraction analyses from these mixtures indicate that

oxidative addition of phosphonium salts to palladium(0) species is also operative. The scrambling of palladium-bound

carbyls and phosphorus-bound aryls is also relevant under NMR, as well as preparative reactor polymerization conditions

exempliﬁed by the X-ray diﬀraction analysis of [κ²-(anisyl)₂P,O]Pd[κ²-(anisyl)(CH₂CH₃)P,O] (12) and [κ²-(anisyl)₂P,O]Pd-

[κ²-(anisyl)((CH₂)₃CH₃)P,O] (13) isolated from pressure reactor polymerization experiments. In addition, ESI-MS analyses of

reactor polymerization ﬁltrates indicate the presence of (odd- and even-numbered alkyl)(anisyl)phosphine sulfonates (14) and

their respective phosphine oxides (15). Furthermore, 2-(vinyl)anisole was detected in NMR tube and reactor polymerizations,

which results from ethylene insertion into a palladium−anisyl bond and concomitant β-hydride elimination. In addition to these

scrambling reactions, formation of alkanes or fully saturated polymer chains, bis(chelate)palladium complexes [κ²-P,O]₂Pd, and

palladium black was identiﬁed as an irreversible catalyst deactivation pathway. This deactivation proceeds by reaction of palladium

alkyl complexes with palladium hydride complexes [κ²-P,O]Pd(H)(L) or by reaction with the free ligand H[P,O] generated by

reductive elimination from [κ²-P,O]Pd(H)(L). The model hydride complex 1-H-P^tBu₃has been synthesized in order to establish

whether 1-H-P^tBu₃or H[P,O] is responsible for the irreversible catalyst deactivation. However, upon reaction with 1-⁽¹³⁾CH₃-L or

1-CH₂CH₃-PPh₃, both 1-H-P^tBu₃and H[P,O] result in formation of methane or ethane, even though H[P,O] reacts faster than

1-H-P^tBu₃. DFT calculations show that reductive elimination to form H[P,O] and (alkyl)[P,O] from 1-H/(alkyl)-P^tBu₃is

kinetically accessible, as is the oxidative readdition of the P−H bond of H[P,O] and the P−anisyl bond of (alkyl)[P,O] to

[Pd(P^tBu₃)₂]. These calculations also indicate that for a reaction sequence comprising reductive elimination of H[P,O] from

1-H-P^tBu₃and reaction of H[P,O] with 1-CH₃-P^tBu₃, 1-CH₃-dmso, or 1-CH₂CH₃-PPh₃to form methane or ethane, the rate-

limiting step is reductive elimination of H[P,O] with a barrier of 124 kJ mol⁻¹. However, a second reaction coordinate was found

for the reaction of 1-H-P^tBu₃with 1-CH₃-P^tBu₃or 1-CH₃-dmso, which evolves into bimetallic transition-state geometries with a

nearly linear H-(CH₃)-Pd alignment and which exhibits a barrier of 131 or 95 kJ mol⁻¹for the formation of methane.

Received: October 17, 2012

Published: November 13, 2012

8388

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

INTRODUCTION

■

Catalytic polymerizations of oleﬁns are practiced on a large scale.

However, an insertion polymerization of polar vinyl monomer is

a long-standing challenge.^1,2Major advances were achieved

recently by the ﬁnding that neutral κ²-P,O-phosphinesulfonato

palladium(II) complexes (cf. 1-L) promote the formation of

linear copolymers of ethylene with a broad scope of polar

monomers including not only acrylates,³but also acrylonitrile,⁴

vinylacetate,⁵vinylﬂuoride,⁶allylic monomers,⁷acrylic acid,⁸and

acrylamides⁹and also catalyze insertion homooligomerization of

acrylates.^10,11

Many features of the catalytic chain growth in these

(co)polymerizations have been revealed by detailed experimental

and theoretical investigations,¹²and a fairly comprehensive

mechanistic picture exists. Polar monomers (CH₂CHX)

reduce polymerization rates by chelating coordination of the

growing polymeryl chain via functional groups (X) of a last or

penultimate incorporated repeat unit. For many polar monomers

κ-X coordination of free monomer can also block coordination

sites required for π-coordination of monomer and chain growth.

These and other features observed are all reversible, and they slow

down but do not shut down chain growth.

No irreversible steps have been reported to occur in

polymerizations with these catalysts, although the importance

of any such steps for polymer synthesis, catalyst development, and

also mechanistic analyses is obvious. It appears that this unique

class of catalysts is considered to be essentially stable during

polymerization. Intimately related to issues of irreversible

deactivation is the eﬃciency of activation of the catalyst precursor

employed. A knowledge of the portion of metal sites that actually

enter the catalytic cycle as an active species is essential in order to

analyze decomposition pathways of the latter.

We now report on the identiﬁcation and in-depth analyses of

various reversible and irreversible deactivation reactions that

occur extensively in polymerizations and have long-ranging

implications, and provide a detailed analysis of catalyst precursor

activation as a prerequisite for a comprehensive picture.

concentration, the overall rate of polymer formation should

asymptotically approach a maximum value that corresponds to

the equilibria being entirely in favor of the oleﬁn complexes.

Indeed, such saturation kinetic behavior was observed for

1-CH₃-L at p(ethylene) > 5−10 bar (L = dmso) or >15 bar

(L = pyridine) in 30 and 60 min polymerization runs.¹⁰However,

the assessment of a rapid equilibration of 1-CH₃-L and 1-CH₃-

(ethylene) with respect to chain growth was never made.

Moreover, the picture outlined in eq 1 suggests that precatalysts

1-CH₃-L quantitatively participate in the polymerization

reaction and that no other competitive reaction pathways exist

by which fractions of 1-CH₃-L or 1-(CH₂)_2nCH₃-L may be

excluded from forming polymer.

Activation of Precatalysts under Preparative Polymer-

ization Conditions and Overall Incorporation of ¹³C-

Label in the Formed Polymer. In a ﬁrst approach to elucidate

these issues ¹³C-labeled precatalysts 1-¹³CH₃-L (L = dmso,

pyridine) were synthesized (for experimental details see

Supporting Information). These complexes were employed for

ethylene polymerizations with variable reaction times (1−10 min,

150−450 μM precatalyst, 5 bar ethylene, 333−363 K) to

determine the fraction of ¹³C-labeled methyl groups incorporated

from the precatalyst 1-¹³CH₃-L into the S1 end groups of the

isolated polymer (Figure 1 shows an exempliﬁed ¹³C NMR

spectrum including end group deﬁnitions).

RESULTS AND DISCUSSION

■

Methyl complexes 1-CH₃-L have been employed frequently

as well-deﬁned catalyst precursors (Chart 1).^4−10,12Activation

Chart 1. Natural Abundance and ¹³C-Enriched Complexes

1-¹³CH₃-L

Figure 1. S1, S2, and S3 region of the ¹³C NMR spectrum (inverse gated

decoupled, 403 K, C₂D₂Cl₄) of polyethylene (Table 1, entry 7) obtained

with 1-¹³CH₃-pyr (54 TON ethylene, ca. 0.9 chain transfers per active

1-¹³CH₃-pyr) in a pressure reactor polymerization experiment.

A higher sensitivity in these experiments was achieved by

increasing the concentration of the catalyst precursor, [1-¹³CH₃-L],

and by decreasing the ethylene concentration as well as the

polymerization time and temperature in comparison to standard

polymerization conditions (which are typically 20−50 μM, 40 bar

ethylene, 363 K, 30 min polymerization time),¹⁰thus limiting the

absolute turnover number per overall palladium.¹³Aliquots of the

polyethylenes thus obtained were subjected to quantitative ¹³C

NMR analysis,¹⁴and the fraction of ¹³C-labeled CH₃end groups

incorporated into the polymer was determined by comparing the

overall backbone integral with the S1 and S2 end group integrals,

occurs by displacement of the monodentate ligand L by ethylene

monomer and insertion in the resulting oleﬁn complex 1-CH₃-

(ethylene). Closely related to this activation pre-equilibrium

(K_eq), the growing active species 1-(CH₂)_2nCH₃-(ethylene) can

be in equilibrium with a dormant state due to competitive

binding of the oleﬁn and the ligand L introduced with the catalyst

precursor (K′_eqin eq 1).

Assuming these competitive binding equilibria to be fast, this

overall scenario should result in a saturation kinetic behavior

of the polymerization reaction. That is, with increasing ethylene

8389

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

Table 1. Ethylene Polymerization Conducted with ¹³C-Labeled Complexes 1-¹³CH₃-L

g

h

i

catalyst

precursor

T

t

yield

Me

no. of chain

M_n[10³g mol⁻¹

]

M_n/(M_w/M_n)

inc. ¹³CH₃

[ 5%]

oleﬁn to

d

e

f

j

entry

[K] [min] [mg] TON branches

transfers

(NMR)

[10³g mol⁻¹] (GPC)

S2

a

1

1-¹³CH₃-dmso 333

1-¹³CH₃-dmso 343

1-¹³CH₃-dmso 353

1

3

138

165

273

199

231

253

68

328

393

650

158

550

601

54

1.0

0.9

1

4.2

5.0

2.8

3.3

5.3

2.0

3.4

2.7

1.7

2.9

4.3

8.1

9/(1.6)

n.d.

69

59

71

41

86

76

72

73

72

1:10

1:6

1:8

1:10

1:8

1:3

1:8

1:6

1:4

a

2

a

3

10

1

5.1

11/(2)

n.d.

b

4

1.4

2

2.8

a

5

1

10.5

15.1

0.9

10/(2)

7/(2.1)

1.6/(1.2)

5.3/(1.3)

7/(1.7)

n.d.

a

6

1

4.5

0.7

1.2

2.8

2.7

b

7

1-¹³CH₃-pyr

333

343

353

1

c

8

1

151

144

232

484

180

343

552

1153

1.8

a

9

1

4.1

a

10

3

4.3

a

11

10

4.7

10/(2)

a

b

Polymerization conducted with 15 μmol of 1-¹³CH₃-L in 100 mL of toluene at 5 bar ethylene pressure. Polymerization conducted with 45 μmol of

c

1-¹³CH₃-L in 100 mL of toluene at 5 bar ethylene pressure. Polymerization conducted with 30 μmol of 1-¹³CH₃-L in 100 mL of toluene at 5 bar

d

e

ethylene pressure. Mol ethylene converted per mol precatalyst. Methyl branches per 1000 carbon atoms, determined in C₂D₂Cl₄solution at 403 K

f

in inverse gated decoupled experiments with a relaxation delay of >7 s. Determined by comparing the (¹³C)S1(¹³C)S2 and the (¹²C)S1(¹³C)S2

g

signal in inverse gated decoupled experiments with a relaxation delay of >7 s. Determined by the ratio of S1 end groups:remaining PE signals.

h

i

Determined by high-temperature GPC in trichlorobenzene at 403 K vs linear PE standards. Fraction of polymer-incorporated ¹³CH₃groups from

j

1-¹³CH₃-L determined by ¹³C NMR spectroscopy; for details see SI. Determined by ¹³C NMR spectroscopy by comparing the integrals of allylic

and S2 signals in inverse gated decoupled experiments with a relaxation delay of 7 s.

assuming that the ¹³C-label was exclusively incorporated in the S1

end groups. These data were corrected for additional occurring S1

and S2′ end groups (containing natural abundance ¹³C) resulting

from chain transfer reactions during these polymerization

experiments. A distinction of S2 end groups in the vicinity of

¹²C- or ¹³C-labeled S1 groups, respectively, can be made, since

S2′ end groups formed after chain transfer do not exhibit the

characteristic ¹J_CC= 34.8 Hz that is observed for S2 end groups

adjacent to the ¹³C-enriched S1 group (signals between 22 and

23 ppm in Figure 1; for a detailed description see Supporting

Information).

This distinction in turn allows for an estimation of the number

of chain transfers per 1-¹³CH₃-L. Chain transfer numbers per

1-¹³CH₃-L determined in this fashion increase with increasing

polymerization temperature and are in the range of ca. 1 to 5

using 1-¹³CH₃-pyr and in the range of 3 to 15 using 1-¹³CH₃-

dmso (Table 1, column 8).

the chain transfer being directly linked to the deactivation of the

catalytically active species.

We also note that the isolated polymers exhibit ratios of

unsaturated groups to aliphatic end groups that, if corrected

for methyl branches and internal oleﬁns, signiﬁcantly deviate

from the expected 1:1 ratio (Table 1, last column). While an

incomplete ¹³C relaxation of oleﬁnic ¹³C resonances and possibly

also polymer chains still attached to palladium may contribute to

this observed deviation of oleﬁnic:aliphatic end group ratios, we

believe that part of the polymer consists of fully saturated chains,

which will also contribute to deviating ratios of oleﬁnic:aliphatic

end group signals. Evidence that supports this assumption will be

presented in the following sections.

Activation of Precatalysts under NMR-Tube Condi-

tions and Position of Incorporated ¹³C-Label in the

Polymer Chains. Triggered by the observed catalyst deactiva-

tion and indication for the formation of fully saturated polymers,

NMR monitoring of the reaction of ¹³C-labeled complexes

1-¹³CH₃-pyr and 1-³CH₃-dmso with ethylene in pressure-stable

J. Young NMR tubes was conducted: In thf-d₈solution (dried

over sodium−potassium alloy) 1-¹³CH₃-pyr (6.6 mmol L⁻¹)

reacts only very slowly with ethylene (initial concentration

[ethylene]₀= ca. 46 mmol L⁻¹, 7 equiv) even at elevated tem-

perature, and consumption of ca. 6.5 equiv of ethylene requires

heating to 378 K for ca. 7 h. At this point the initially introduced

1-³CH₃-pyr is totally consumed, as jugded by ¹H NMR

spectroscopy. Traces of palladium black were visible, and no

further polymerization takes place upon repeated addition of

ethylene and heating to 378 K. However, only an estimated

ca. 65% of the initial 1-³CH₃-pyr had initiated a polymer chain, as

judged by additional resonances between δ = 13.5 and 11.5 ppm

in the ¹³C NMR spectrum, accounting for ca. 35% of the

introduced ¹³C-label.¹⁶These signals are split into a doublet of

doublets with J = ca. 35 Hz (and J = 3 Hz) at 150 and 100 MHz.

While the observed J = 35 Hz are suggestive of ¹J_CCcouplings,¹⁷

upon ³¹P-decoupling all these signals collapsed into singlets

(Figure 2). In addition they were identiﬁed as isotopomeric

All fractional incorporations of ¹³CH₃-labeled groups into the

isolated polymers even after only 1 min polymerization time

are consistently >55% for 1-¹³CH₃-dmso (Table 1, entries 1−4)

at 333 K polymerization temperature and reach ca. 85% for

1-¹³CH₃-pyr (Table 1, entry 7). They decrease with increasing

polymerization temperature within the error limits (Table 1,

entries 4−6 and 7−9) and do not increase with polymerization

time (Table 1, entries 1−3 and 9−11). These data indicate that

(a) the pre-equilibration of 1-¹³CH₃-L and 1-¹³CH₃-(ethylene)

according to eq 1 is indeed rapid as compared to chain growth,

and (b) side reactions triggered by increasing the polymerization

temperature decrease the observed S1 incorporation of ¹³C-label

into the isolated polymer.

Turnover numbers do not increase linearly with time (entries

1−3 and 9−11, columns 4 and 6) and decrease with increasing

[precatalyst], e.g., from 150 μM to 450 μM (entries 1 and 4).

This indicates that deactivation of catalytically active species

occurs to a relevant extent and may account for the observation

that the ¹³C-label is not incorporated completely into the isolated

polymer, despite the rapid pre-equilibration of 1-¹³CH₃-L

and 1-¹³CH₃-(ethylene).¹⁵The ﬁnding that increasing chain

transfer numbers also correlate with decreasing incorporation of

¹³C-label into the polymer (Table 1, columns 8 and 11) points to

1

enriched ¹³CH₃groups by phase-sensitive H,¹³C gHSQC in

1

combination with H,³¹P gHMBC and H,³¹P{¹³C} gHMBC

experiments with no further correlation in H,¹³C gHMBC

1

8390

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

(i.e., β-hydride elimination and reinsertion with opposite

regiochemistry) along the growing chain, which exchanges

¹³C-label between these two groups and is competitive to chain

growth. This chain-walking processalong the growing chain results

in dissipation of the initially introduced ¹³C-label at the chain

terminus into every other position of a linear chain or, if methyl

branching takes place, even into a methyl branch (Scheme 1).

A deeper analysis of the labeled oligomers obtained from

1-¹³CH₃-pyr in the NMR-tube experiment supports that chain

walking is operative in a polymerization-active palladium species.

A ratio of S3:S2 end groups of ca. 4:1 was found, whereas a S3:S2

ratio of 1:1 was obtained starting from unlabeled 1-CH₃-pyr.

That is, some of the ¹³C-label is indeed incorporated at the S3

position, resulting in increased S3 integrals. With respect to

methyl branches we ﬁnd an intensive ¹³C signal at δ 23.04 ppm,

which is correlated via ¹J_CHto an obscured signal at 0.88 ppm in

Figure 2. ¹³C NMR methyl region of an oligomer sample obtained with

1-¹³CH₃-pyr after 7 h at 378 K, with 7 equiv of C₂H₄in a NMR-tube

experiment. Signals marked (*) were assigned to (¹³CH₃)-phosphine

palladium complexes. Signals marked (+) were assigned to (¹³CH₃)-

phosphine palladium complexes containing one additional phosphine

ligand (vide infra, Scheme 2 and Figure 4).

1

the H NMR spectrum and which is tentatively assigned to a

¹³C-enriched isopropyl end group. With respect to the isopropyl

end groups however, we note that that they may also form after

1,2-insertion of 1-oleﬁns into 1-¹³CH₃and extensive chain walking

through a tertiary carbon center (Scheme 1 and Figure 3).¹⁸

Overall, the increased signal-to-noise ratio for many

resonances attributable to ethylene enchainment products

experiments (see SI). Thus, a substantial part of the ¹³C-labeled

CH₃groups in these experiments is transferred to phosphorus-

containing species.

obtained with 1-¹³CH₃-pyr vis a

̀

vis unlabeled 1-CH₃-pyr clearly

The identity of these species will be discussed later, as the ¹³C

NMR spectrum of this ¹³C-enriched sample also proves most

supports that these catalysts can undergo chain-walking on the

same time scale as chain growth, particularly at low ethylene

concentration and high temperature.

̀

informative with respect to the formed oligomers vis a vis the

control experiment with natural abundance 1-CH₃-pyr: First of

all we have noticed a much better signal-to-noise ratio (S/N) for

the ¹³C-enriched sample not just for the ¹³C-enriched S1 signal at

δ = 14.47 ppm, but for many other resonances attributable to

products obtained by chain growth. However, an increased S/N

was not observed for the identiﬁed signals of cis- and trans-but-2-

ene, which are formed after β-hydride elimination and insertion

of ethylene into palladium hydride species [κ²-P,O]Pd(H) under

NMR polymerization conditions (and therefore contain no

¹³C-label introduced by 1-¹³CH₃, Figure 3).

This chain-walking process, which was also observed on a

preparative scale as jugded by low but clearly quantiﬁable degrees

of branching (Table 1, column 7: Me branches) and which is

also supported by other studies,¹⁹will apparently lower the

calculated fraction of ¹³C-label (Table 1, column 11) introduced

into the polymer, as exclusively ¹³C-labeled S1 positions have

been considered. However, while chain-walking contributes to

an apparently lowered ¹³C incorporation into the polymer at

low ethylene concentration and high temperature, the formation

of ¹³C-labeled phosphorus-containing side products results in

a fraction of the ¹³C-label not being incorporated at all into the

polymer.²⁰

Furthermore, the formation of volatile saturated alkanes

constitutes another reaction path by which ¹³C-label is prevented

from incorporation into the formed polymer in the above NMR

experiments: We have unambiguously observed ¹³C-enriched

1

¹³CH₄identiﬁed by a doublet with J_CH= 125.6 Hz at δ

1

0.185 ppm in the H NMR spectrum, which is correlated via

¹J_CH(phase-sensitive gHSQC) to a carbon signal with even-

numbered hydrogen substitution at δ −4.675 ppm in the ¹³C

NMR spectrum.

In agreement with this assignment, the respective natural

abundance methane formed by using 1-CH₃-pyr exhibits a

singlet in the ¹H NMR spectrum at δ 0.184 ppm.²¹In addition to

(¹³C-enriched) methane a triplet at δ 1.118 ppm, i.e., downﬁeld

from usually observed methyl end groups, or methyl branching at

ca. 0.85 ppm was observed, which is coupled to a multiplet at

1.329 ppm partially obscured by the methylene backbone signal

of the formed oligomers (for details, see SI). We have assigned

these signals to propane after incremental addition of a genuine

sample of propane to these samples. Incremental addition of ethane

to these samples also revealed that natural carbon abundance

ethane identiﬁed by a singlet at δ = 0.851 forms during these in situ

NMR experiments.

Figure 3. S3, S2, and methyl branch ¹³C NMR region of ethylene

oligomers obtained with 1-¹³CH₃-pyr (top line) and 1-CH₃-pyr

(bottom line).

Even more striking, substantial amounts of propylene were

identiﬁed in both samples with virtually identical integral ratios of

terminal methylidene (116.0 ppm)-to-methyl signals (19.52 ppm)

of 0.8:1 in each case (see SI), while the internal methine carbon is

hardly detectable. Since propylene can be formed only by ethylene

insertion into 1-¹³CH₃(1-CH₃, respectively) and concomitant

β-hydride elimination, the observed 0.8:1 ratio of methylidene:-

methyl resonances in the case of the ¹³C-labeled precatalyst

1-¹³CH₃-pyr is indicative of a rapid chain-walking reaction

Scrambling of the Growing Alkyl Chain into the

Phosphine Ligand. With respect to the phosphorus species

8391

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

Scheme 1. Insertion and Chain Walking Reactions Explaining the Dissipation of ¹³C-Label into Every Other Position of Linear

a

Chains and into Branches Observed with 1-¹³CH₃-pyr

a

In parentheses: alternative formation of ¹³C-enriched isopropyl end groups after 1,2-insertion of 1-oleﬁns into 1-¹³CH₃.

Scheme 2. Phosphonium Salts and Methylphosphine Complexes by Thermolysis of 1-¹³CH₃-L in thf-d₈at 378 K

containing the ¹³CH₃-label (vide supra, Figure 2), several NMR

spectroscopic and mass spectrometric observations led to the

conclusion that these ¹³C-labeled species correspond to

palladium complexes of ¹³C-labeled methylphosphines and not

to palladium-¹³CH₃complexes, which implies that an exchange

of aryl substituents on phosphorus and the palladium-bound

¹³CH₃group takes place (Scheme 2 and Scheme 3):²²(a) the

phosphonium cations (¹³CH₃)_xP⁺(anisyl)_4−x(x = 1, 2) 2 and 4 by

ESI-MS and MALDI (Scheme 2, for details, see SI). (e) Despite

the reported stability of 1-CH₃-L,²⁵thermolysis of 1-¹³CH₃-pyr

and 1-¹³CH₃-dmso in thf-d₈for 18−24 h at 378 K also in the

absence of ethylene results in formation of quaternary

phosphonium salts. However, in addition to 2 and 4, tetra-

(anisyl)phosphonium 7 is also detected by mass spectrometry.

(f) The ¹³C NMR spectrum of these samples obtained in the

absence of ethylene contains several ³¹P-coupled signals between

Scheme 3. Exchange of Palladium-Bound Carbyls with

Phosphorus-Bound Carbyls by Reductive Elimination and

Oxidative Addition, and Formation of 2-Vinylanisole

1

δ = 15 and 8.5 ppm with J_PC≈ 54−62 Hz typical for methyl

phosphonium cations which are correlated to doublets of

doublets with ¹J_CH≈ 135 Hz and ²J_PH≈ 15 Hz in the ¹H NMR

spectra. Comparison with independently prepared, but unlabeled

2 and 4 clearly indicates the formation of ¹³C-labeled 2 and 4

in the aforementioned experiments starting from 1-¹³CH₃-L at

378 K. (g) Thermolysis of 1-¹²CH₃-pyr and 1-¹²CH₃-dmso in

dioxane for 18−24 h at 395 K yields the full range of

phosphonium salts 2−7 as observed by ESI-MS.

In addition to phosphonium salts, the ¹³C-labeled samples

obtained by thermolysis of 1-¹³CH₃-dmso in thf-d₈for 18−24 h

at 378 K also contain substantial amounts of the main ¹³C-labeled

side product observed under NMR-tube polymerization

conditions. It is characterized by a doublet of doublets at δ =

11.51 ppm with J_PC= 35 and 3 Hz (see Figure 2), which is

assigned to a palladium diphosphine complex of the generic

structure 8 or 9 (Scheme 2). This (¹³CH₃)phosphine species was

unambigously identiﬁed by X-ray diﬀraction analysis and NMR

characterization of the collected X-ray quality crystals, which

reveals that under polymerization conditions in the NMR

tube [κ²-(anisyl)₂P,O]Pd[κ²-(anisyl)(¹³CH₃)P,O] (10) forms

(Figure 4). The formation of 10 in NMR-tube polymerization

and its isolation after thermolysis of 1-¹³CH₃-dmso ultimately

prove that (a) palladium-bound ¹³CH₃groups can transfer to

observed J_PC= 35 Hz is neither in the range of a typical ²J_{PC, trans}

as observed in several diphosphine palladium methyl complexes

(ca. 70−120 Hz)²³nor in the range of typically observed ²J_{PC, cis}

in related phosphinesulfonato palladium methyl complexes

(ca. 0−7 Hz).²⁴(b) H chemical shifts of the observed ¹³CH₃

1

groups (ca. 0.85−1.9 ppm) are not in the range of typical

palladium methyl complexes (typically −2 to +0.85 ppm). (c)

Traces of 2-vinylanisole were detected by ¹H- and ¹H,¹H-gCOSY

NMR experiments (see SI), which are likely formed by ethylene

insertion into a palladium-(2-anisyl) complex followed by

β-hydride elimination. (d) Mass spectrometric analysis of the

obtained sample after heating 1-¹³CH₃-pyr with 7 equiv of

ethylene for 7 h at 378 K revealed the presence of quaternary

8392

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

1

Figure 4. H NMR spectrum (298 K, 400 MHz, methylene chloride-d₂) of isolated X-ray quality crystals identiﬁed as [κ²-(anisyl)₂P,O]Pd[[κ²-

(anisyl)(¹³CH₃)P,O] (10) with three characteristic methoxy singlets between 3.8 and 3.2 ppm and the P−¹³CH₃group at 1.37 ppm. The aliphatic inset

shows ¹H (bottom line), {³¹P}¹H (middle line), and {¹³C}¹H NMR traces (top line), proving two ³¹P and one ¹³C coupling to the methyl protons

marked (#).

phosphorus under these conditions and that (b) part of the

palladium-bound ¹³C-label therefore can be prevented from

incorporation into the formed polymer. This is in accordance

with the fractional incorporation of ¹³C-label determined by the

NMR polymer analyses (vide supra, Table 1) and reveals that eq 1

(vide supra) is simplistic in describing the polymerization

reaction catalyzed by 1-CH₃-L, as the palladium-bound κ²-(P,O)

ligand may be altered during the polymerization reaction. In

addition to the isolation of 10, poorly methanol-soluble material

from these samples was identiﬁed as [κ²-(anisyl)₂P,O]₂Pd (11) by

its characteristic ¹H and ³¹P resonances in dmso-d₆in comparison

to independently prepared material.

These data fully corroborate the exchange of aryl substituents

on phosphorus with methyl substituents on palladium (Scheme 3).

Such exchange of substituents in transition metal complexes is not

unprecedented and can be explained by consecutive reductive

eliminations and oxidative additions of quaternary phosphonium

salts followed, inter alia, by [P,O]-ligand exchange reactions with

palladium-CH₃fragments.²⁶It also provides a mechanistic scheme

for the aforementioned formation of 2-vinylanisole after ethylene

insertion into a palladium−(2-anisyl) bond (Scheme 3).

While proven in NMR-tube experiments, the relevance of these

reactions under preparative polymerization conditions remained

questionable at this point. We have therefore conducted additional

experiments under polymerization conditions at which the catalyst

is highly active [5 bar ethylene pressure, 20−30 min polymer-

ization time, resulting in formation of ca. 6 g of PE (from 20 μmol

of 1-¹²CH₃-dmso) and ca. 16 g of PE (from 40 μmol of 1-¹²CH₃-

dmso), respectively]. The reaction mixture was precipitated

by addition of methanol, the polymer was ﬁltered oﬀ, and the

methanol ﬁltrate was concentrated. Part of this concentrated

ﬁltrate redissolves in methanol-d₄, while the methanol-insoluble

part was dissolved in dmso-d₆and identiﬁed as [κ²-(anisyl)₂P,O]₂Pd

(11) by comparison to independently prepared material.

The methanol-d₄-soluble fraction does not contain detectable

amounts of 11 by ¹H or ³¹P NMR spectroscopy. However, several

CH₃groups coupled to phosphorus were identiﬁed by analysis of

the ¹H, ³¹P, ¹H,³¹P-gHMBC, and ¹H,¹³C-gHSQC NMR spectra,

one of which were assigned to phosphonium salt 5 (Scheme 2)

on the basis of a doublet at δ = 3.05 ppm with ²J_PH= 15.5 Hz in

the ¹H NMR spectrum, which is correlated to one phosphorus at

27.7 ppm and agrees with the data of independently prepared 5

(for details, see SI). Furthermore, traces of 2-vinylanisole and one

not fully identiﬁed styrene derivative were detected by ¹H NMR

experiments. Most intriguing, one ethyl phosphine species was

clearly identiﬁed by its characteristic splitting of the ethyl-CH₃

3

group into a doublet of triplets at δ = 1.05 ppm with J_PH

=

20.5 Hz, while the ethyl-CH₂group exhibits diastereotopic

protons resonating at δ = 1.89 and 1.78 ppm with couplings to

two phosphorus nuclei (Figure 5 and SI). This ethyl phosphine

species was identiﬁed as [κ²-(anisyl)₂P,O]Pd[κ²-(anisyl)(ethyl)

P,O] (12) by X-ray diﬀraction analysis. 12 cocrystallizes with

[κ²-(anisyl)₂P,O]Pd[κ²-(anisyl)(butyl)P,O] (13) as a methanol-d₄

solvate with identical positions in the unit cell (Figure 5).

In addition, ESI-MS analyses (negativemode) of themethanolic

polymerization ﬁltrate exhibit peaks that ﬁt the resonances

expected for (2-anisyl)(alkyl)phosphine benzenesulfonates (14)

and their respective phosphine oxides (15) (Figure 6). These data

ultimately prove that the exchange of palladium-bound alkyl

groups with phosphorus-bound carbyl substituents is also relevant

under preparative reactor polymerization experiments, even if

turnover numbers of ethylene per palladium present exceed

10.000. The ESI-MS assignments, particurlarly those of (odd-

numbered alkyl)phosphines, imply that under preparative (high

productivity) reactor polymerization conditions chain growth is

signiﬁcantly faster than exchange of phosphorus- and palladium-

bound carbon substituents, but that chain transfer and exchange of

phosphorus- and palladium-bound carbon substituents occur on

a similar time scale. While not supported by thermolysis experi-

ments of 1-⁽¹³⁾CH₃-dmso in the absence of ethylene (which takes

up to 24 h at 378−395 K), this notion is clearly supported by the

X-ray diﬀraction analysis of 12 and 13.

While [κ²-(anisyl)₂P,O]₂Pd (11), [κ²-(anisyl)₂P,O]Pd[κ²-

(anisyl)(ethyl)P,O] (12), and [κ²-(anisyl)₂P,O]Pd[κ²-(anisyl)-

(butyl)P,O] (13) separated (by crystallization) from the

polymerization ﬁltrates are not polymerization active in NMR-

tube experiments, some of the species remaining in solution

still enchain ethylene in thf-d₈NMR experiments, whereby

additional traces of 2-vinylanisole are formed apart from ethylene-

oligomers/polymers. We therefore conclude that part of these

complexes consist of palladium-anisyl complexes and that the

transfer of palladium-bound alkyl groups to phosphorus does not

8393

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

Figure 5. Aromatic and aliphatic region of the ¹H (bottom line) and {³¹P}¹H NMR spectra (298 K, 400 MHz, methanol-d₄) of isolated X-ray quality

crystals identiﬁed as a mixture of [κ²-(anisyl)₂P,O]Pd[[κ²-(anisyl)(ethyl)P,O] (12) and [κ²-(anisyl)₂P,O]Pd[[κ²-(anisyl)(butyl)P,O] (13).

#: PCH₂CH₃, +: P(CH₂)₃CH₃, *: pentane. 12 and 13 have been isolated from the ﬁltrate of preparative reactor polymerizations [20(9) μmol of

1-¹²⁽¹³⁾CH₃-dmso, 5 bar ethylene, 30 min polymerization time at 368 K, ca. 10⁴(10⁴) turnovers ethylene per present palladium]. Also given: X-ray

diﬀraction analysis of cocrystallized 12 and 13. 12 and 13 occupy symmetry-equivalent positions in the unit cell. However, they likely form pairs, since

the crystallographic void containing the P(2)-C34-C35 (ethyl-)/P(2ⁱ)-C34ⁱ-C35ⁱ-C36ⁱ-C37ⁱ(butyl-) group (and a center of symmetry) allows for an

alignment of six carbon atoms, e.g., C34ⁱ, C35ⁱ, C36ⁱ, C37ⁱ, C35, and C34, while C37 and C37ⁱare separated by only 1.247 Å. This is in accordance with

an occupancy of C36, C37, which reﬁned to 50% (for details, see SI).

Figure 6. ESI-MS analysis (negative mode) of preparative reactor polymerization residue obtained from 1-¹²CH₃-dmso after separation of the formed

polymer, indicating the presence of several alkylphosphines (14) and their respective alkylphosphine oxides (15).

necessarily result in deactivation of polymerization-active

palladium complexes in preparative reactor polymerizations.

DFT calculations of the reductive elimination and oxidative

addition of methylphosphonium salts indicate that reductive

elimination is kinetically accessible and thermodynamically

favorable, while oxidative addition of the P-CH₃moiety is

thermodynamically prohibitive (vide infra). Thus, the multiple

methylation of phosphorus to form, for example, 2 starting from

1-¹³CH₃-L and the formation of palladium anisyl complexes

(and 2-vinylanisole in the presence of ethylene) likely reﬂect the

thermodynamic stability of the palladium−aryl vs the palladium−

methyl bond, which is supported by DFT calculations (vide infra).

Details of the Formation of Saturated Polymer Chains.

In contrast to NMR-tube oligomerizations with 1-¹³CH₃-pyr and

1-⁽¹³⁾CH₃-dmso and to preparative reactor polymerizations

with 1-CH₃-dmso, these side reactions resulting in the formation

of phosphonium salts, 2-vinylanisole, and methylphosphine

(or more general: alkylphosphine) palladium complexes can be

completely avoided by using the weaker coordinated and thus

more reactive 1-¹³CH₃-dmso (5 mmol L⁻¹, ca. 39 mmol L⁻¹

ethylene, 8 equiv) in anhydrous thf-d₈at 298 K in NMR-tube

experiments: Albeit slow, 1-¹³CH₃-dmso already inserts ethylene

at room temperature and ca. 95% of 1-¹³CH₃-dmso is consumed

within 165 min at 298 K, while ca. 5.5 equiv of ethylene has been

enchained.²⁷In these room-temperature studies, chain transfer

by β-hydride elimination and chain-walking/isomerization is

negligible, as concluded from the appearance of only traces of

α-oleﬁns (<2% with respect to all palladium complexes present)

8394

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

and the absence of internal oleﬁns. In addition, palladium propyl,

pentyl, heptyl, and higher alkyl complexes were identiﬁed in

Deactivation of Catalytically Active Species by Endo-

and Exogenic Pathways. An exogenic (and thus avoidable)

catalyst deactivation pathway at low ethylene concentration has

been reported in the presence of chlorinated solvents, e.g.,

C₂D₂Cl₄or CDCl₃, and resulted in isolation of [κ²-P,O]Pd(Cl)-

(L) complexes.^12aWhile the presence of water has been reported

not to aﬀect the stability of 1-CH₃-L at temperatures of 343 to

373 K,²⁵we have now found that thermolysis of 1-⁽¹³⁾CH₃-L at

378−395 K apart from several phosphonium salts (vide supra)

also yields ⁽¹³⁾CH₄or ⁽¹³⁾CH₃D if the solvent is not rigorously

dry. Complete consumption of 1-CH₃-dmso in the presence of

30 equiv of D₂O in tetrachloroethane-d₂solution takes ca. 3 h at

368 K, while a ca. 8:1 mixture of CH₃D and CH₄forms (see SI).

We therefore consider hydrolysis of palladium−methyl/alkyl

species a further exogenic decomposition pathway.³⁰

In contrast, reductive elimination of methyl/alkyl phospho-

nium salts, e.g., 2−7 (vide supra), as observed in the NMR-tube

experiments and in preparative polymerizations, constitutes an

endogenic (and unavoidable) deactivation pathway if the formed

palladium(0) does not completely reenter the catalytic cycle by

oxidative addition of phosphonium salts, with, for example,

formation of palladium-anisyl complexes (vide supra).

Additionally, we will present evidence that there is a second

endogenic and totally irreversible catalyst deactivation pathway

that is directly linked to the reactivity of palladium hydrides

[κ²-P,O]Pd(H)(L) formed by β-hydride elimination from a

growing palladium polymeryl species and that these palladium

hydrides are (directly or indirectly) responsible for the formation

of fully saturated oligomers.

Considering the increased catalyst decomposition with

increasing concentration of 1-¹³CH₃-L in preparative ethylene

polymerizations (vide supra, Table 1, entries 1−3 and 9−11), the

most straightforward hypothesis to rationalize the formation of

saturated polymer chains and the decreasing activity over time is a

bimolecular reaction of catalytically active palladium complexes,

which is greater than ﬁrst-order in active palladium and results in

inactive palladium species according to eq 2 or eq 3.

1

the H NMR spectra to contain ¹³C-labeled CH₃end groups

exclusively on the basis of characteristic ¹J_CH= 124−125 Hz and

the absence of the respective ¹²CH₃signals. These palladium

alkyl complexes 1-(CH₂)_2n¹³CH₃-L constitute the only products

formed from 1-¹³CH₃-dmso and indicate that true activation

eﬃciencies of ca. 95% are already reached after ca. 5.5 turnovers

of ethylene. They also indicate that at 298 K the polymerization

of ethylene by 1-¹³CH₃-dmso has a living character in that

the formed 1-(CH₂)_2n¹³CH₃-L complexes do not undergo

decomposition or chain transfer even at very low ethylene con-

centration over prolonged periods of time: After an additional

12 h at 298 K ca. 7.3 of the initially 8 equiv of ethylene have been

consumed. Simultaneously, trace amounts (ca. 5% vs palladium

complexes) of 1-oleﬁns have formed by β-hydride elimination,

while the formed palladium hydride complex reinserts ethylene,

as concluded from the observation of palladium ethyl and butyl

complexes 1-Et-L and 1-But-L. The stability of these complexes

1-(CH₂)_2n¹³CH₃-L has allowed for a full characterization by 2D

NMR techniques, which reveals that the most characteristic

palladium-bound methylene group of a growing polymer chain

resonates at δ(¹H) = 0.73 ppm and δ(¹³C) = 32.75 ppm with

²J_PC= 5.8 Hz (for details, see SI).

Heating this sample for 16 h to 313 K results in depletion of all

signals corresponding to identiﬁed odd-numbered palladium

alkyl complexes 1-(CH₂)_2n¹³CH₃-L, while ca. 2% of 1-¹³CH₃-

dmso, 5% of 1-Et, and traces of 1-But are still detectable. With

respect to organic products we have unambiguously detected

1-alkenes, 2-alkenes, internal (x-)alkenes with x > 2 (4:4:1 ratio),

13

and traces of ¹³CH₃−CHCH₂and CH₃−CH CH₂(in a ca.

3:1 ratio), 2-methyl-1-alkenes, ¹³CH₃CH₂CH₃, and ¹³CH₄in the

¹H NMR spectra (for further details, see SI).

The presence of ¹³CH₄and ¹³CH₃CH₂CH₃in all these NMR

experiments with both 1-¹³CH₃-dmso and 1-¹³CH₃-pyr suggests

that also higher alkanes may form during ethylene polymer-

ization. While the ratio of 1-oleﬁns:2-oleﬁns:internal oleﬁns

(n > 2) allows for an estimate of the expected ¹³CH₃:¹²CH₃end

group ratio in these samples (internal oleﬁns contain one ¹²CH₃−

S1 end group even if initiated by 1-¹³CH₃-L), we ﬁnd ¹³CH₃:¹²CH₃

ratios that indicate the presence of additional saturated chains.

However, since chain transfer as a source of ¹²CH₃end groups is

not quantiﬁed here, an accurate proof for the formation of fully

saturated ethylene oligomers is hardly possible with these

experiments. Therefore further experiments have been devised

that circumvent the complicating issue of chain transfer and allow

for the quantiﬁcation of oleﬁnic and aliphatic chains formed by

1-¹³CH₃-dmso and ethylene (vide infra).

With respect to a coupling reaction of two palladium carbyl

complexes according to eq 3 the experimental data do not support

such a possibility:²⁹(a) 1-¹³CH₃-dmso (1-¹³CH₃-pyr, respec-

tively) does not decompose to ¹³C-enriched/natural abundance

ethane in anhydrous thf-d₈solution up to 378 K, 8 h, and

(b) INADEQUATE, 2D ADEQUATE, or ¹³C,¹³C-J-resolved

experiments conducted with the oligomer sample obtained with

[1-¹³CH₃-pyr] (6.6 mmol L⁻¹) and [ethylene]₀(50−55 mmol

L⁻¹, ca. 8 equiv) in thf-d₈under chain-walking conditions (vide

supra) revealed no measurable ¹³C−¹³C connectivity between,

for example, ¹³CH₃- and ¹³CH₂-S3-(or backbone ¹³CH₂groups),

i.e., to form, for example, 1,2-(¹³C)-butane from 1-¹³CH₃and

1-¹³CH₂CH₂CH₃[or higher even-numbered x,(x+1)-(¹³C)-

alkanes].

On the basis of these data we conclude here that the formation

of the catalytically active species 1-¹³CH₃-(ethylene) (1-CH₃-

(ethylene), 1-(CH₂)_2n¹³CH₃-(ethylene), and 1-(CH₂)_2nCH₃-

(ethylene), respectively) from 1-¹³CH₃-L (1-CH₃-L, respec-

tively) is indeed a rapid equilibrium reaction with respect to the

following ethylene insertion, which justiﬁes (a) the simpliﬁed

reaction scheme given in eq 1 and (b) the assumption of a high

activation eﬃciency. However, the reaction scheme of the overall

polymerization according to eq 1 is simplistic in that it does

not account for the observed exchange of palladium- and

phosphorus-bound alkyl and aryl substituents and the polymer-

ization activity of the species formed thereby. We also note

that there is a strong indication that part of the formed ethylene

oligomers in preparative (reactor) and analytical (NMR) experi-

ments are fully saturated.

In contrast, the formation of ¹³C-labeled methane and propane

as well as natural abundance ethane can likely be traced to a

formal reaction of palladium hydride and palladium methyl,

propyl, or ethyl complexes according to eq 2. In order to provide

8395

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

Scheme 4. Formation of ¹³C-Labeled Oleﬁns from

hard evidence that also higher saturated alkanes can form,

we have repeated the formation of 1-(CH₂)_2n¹³CH₃-dmso

under conditions where β-hydride elimination/chain-walking is

negligible (298 K), then removed excess ethylene, and ﬁnally

heated the sample in order to induce β-hydride elimination and

formation of palladium hydride complexes [κ²-P,O]Pd(H)(L).

Under these reaction conditions the overall number of oligomer

chains will not increase after β-hydride elimination since initia-

tion of a new chain requires ethylene to be present. Consequently,

easily identiﬁed ¹²CH₃-CH₂end groups are expected only for

internal (n-)oleﬁns (n > 2) and for fully saturated polymer chains

formed according to eq 2. Furthermore, a quantiﬁcation of fully

saturated chains is possible since the amount of internal oleﬁns is

accessible (for details see SI). After heating the ethylene-free

sample of 1-(CH₂)_2n¹³CH₃-dmso for 3 min at 363 K, more than

>90% of 1-(CH₂)_2n¹³CH₃-dmso is already decomposed, while a

mixture of 1-oleﬁns, 2-oleﬁns, internal oleﬁns, propane ¹³CH₄,

and alkanes forms. The sample was heated for a further 22 min

at 363 K, after which time 1-(CH₂)_2n¹³CH₃-dmso is no longer

detected, while palladium black deposited in the NMR tube and

four new signals in the ³¹P NMR spectrum at δ = 30.03, 22.81,

3.83, and −24.06 ppm in a 3:25:1:1 ratio were detected. The

signals at 30.03 and 22.81 ppm (accounting for more than 90% of

all ³¹P detected) were identical to those obtained when dissolving

the bis(chelate) palladium complex [κ²-(anisyl)₂P,O)]₂Pd (11) in

thf-d₈after prolonged heating. The other two phosphorus-

containing species at 3.83 and −24.06 ppm have not been

identiﬁed.

1-(CH₂)_2n¹³CH₃-dmso and ¹³C-Labeled Alkanes by β-Hydride

Elimination and Reaction of the Formed Palladium Hydride

with 1-(CH₂)_2n¹³CH₃-dmso

mixture corresponds to an 80% yield with respect to 1-(CH₂)_2n-

¹³CH₃-dmso accessible for the formation of saturated chains

(Scheme 4).

Palladium black, [κ²-(anisyl)₂P,O)]₂Pd (11), and the two

remaining phosphorus-containing species in this sample do not

initiate new oligomer chains upon repeated addition of ethylene;

that is, all polymerization-active species have been decomposed

at this point.³⁰However, trace amounts of ethane are clearly

detected already after 5 min at 298 K upon addition of ethylene.

While we have not detected hydrogen directly after the

1

thermolysis in the H NMR spectrum (Figure 8 and SI), we

1

Figure 7 shows the oleﬁnic and aliphatic regions of the H

NMR spectrum obtained after heating the ethylene-free mixture

Figure 8. Synthesis of 1-H-P^tBu₃by oxidative addition of P-protonated

phosphonium salt H[P,O] to Pd(P^tBu₃)₂and ORTEP plot of 1-H-

P^tBu₃at the 50% probability level. Hydrogen atoms (except for Pd-

H36) and one cocrystallized benzene molecule are omitted for clarity.

The hydride bound to palladium was located in the electron density map

and reﬁned isotropically.

1

Figure 7. Oleﬁnic and aliphatic region of the H NMR spectrum of

1-(CH₂)_2n¹³CH₃-dmso after 25 min at 363 K in the absence of

additional ethylene. I1: integral of CH₂of 1-oleﬁns, I2: integral of the

allyic CH₃of cis- and trans-2-oleﬁns, I3: integral of all allylic CH₂in 1-, 2-,

and internal oleﬁns, I4: integral of all CH₂¹³CH₃introduced by 1-¹³CH₃-

dmso, I5: integral of CH₂¹²CH₃of n-oleﬁns and fully saturated chains,

I6: integral of the ¹³C-satellite of thf-d_8−x[I2 is obtained by (I2 + I6) −

I6]; *: propane, #: CH₂¹²CH₃group of 3-oleﬁns, +: ¹³CH₄, °: grease.

believe that the observed presence of ethane after ethylene

addition results from palladium black catalyzed hydrogenation of

ethylene; that is, hydrogen is also formed during the thermolysis of

1-(CH₂)_2n¹³CH₃-dmso but not observable under these con-

ditions. This assumption is supported by incremental addition of

hydrogen (1.2 bar) to the ethylene-containing reaction mixture,

which results in rapid growth of the ethane singlet at δ =

0.851 ppm in the ¹H NMR spectrum, while no hydrogen at δ =

4.545 ppm is detectable until all ethylene has been hydrogenated.

Notably, hydrogenation of the propene present is much slower

(12 h, 363 K), as evidenced by its disappearance and the observed

growth of the propane triplet at δ = 1.118 ppm in the ¹H NMR

spectrum. In contrast to propene, 1-oleﬁns with C > 3 are mainly

isomerized to internal oleﬁns, which even after 20 h at 363 K are

not hydrogenated.

of 1-(CH₂)_2n¹³CH₃-dmso for 25 min at 363 K. Analysis of the

integrals I1 (1-oleﬁns), I2 (2-oleﬁns), and I3 (allylic CH₂groups

of all oleﬁns) reveals a 25:38:37 ratio of 1-oleﬁns:2-oleﬁns:internal

oleﬁns. In combination with the integrals I4 (¹³CH₃CH₂end

groups) and I5 (¹²CH₃CH₂end groups) a ratio of oleﬁns:alkanes

of 60:40 was determined (Scheme 4; for details, see SI).

Since formation of alkanes requires equimolar amounts of

palladium hydride [κ²-P,O]Pd(H)(L) (formed by β-hydride

elimination and release of equimolar amounts of oleﬁns) and

palladium carbyl complexes 1-(CH₂)_2n¹³CH₃-dmso according

to eq 2, a quantitative reaction according to eq 2 will result in

a 50:50 mixture of oleﬁns and alkanes. Thus, the observed 60:40

The assumed presence of hydrogen after thermolysis of

1-(CH₂)_2n¹³CH₃-dmso (which is responsible for ethane

8396

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

formation upon ethylene addition) indicates that formation of

hydrogen is another reaction path by which palladium hydride

complexes may be deactivated according to eq 4.

density map with a distance to palladium of 1.48(4) Å.³³The

coordination geometry of palladium in the solid state and in

solution is corroborated by the ¹H and ³¹P NMR solution data,

which conﬁrm the trans-disposition of both phosphorus atoms,

as concluded from a J_PP,trans= 357.4 Hz and two J_PH,cis= 13.6

and 1.9 Hz, which split the hydridic resonance into a doublet of

doublets at δ(¹H) = −18.02 ppm.

2

Reactivity of the Model Palladium Hydride Complex.

In order to understand the cross-reactivity of 1-H-P^tBu₃with

1-CH₃-L (vide infra), the reactivity of 1-H-P^tBu₃alone was also

evaluated: 1-H-P^tBu₃slowly decomposes in tetrachloroethane-

d₂solution within ca. 11 h at 373 K with concomitant evolution

of hydrogen (δ = 4.70 ppm). In addition, the bis(chelate)

palladium complex [κ²-(anisyl)₂P,O)]₂Pd (11) crystallizes from

these samples, as proven by single-crystal X-ray diﬀraction

analysis (Figure 9).³²

Thus, 1-(CH₂)_2n¹³CH₃-dmso at elevated temperature in

part β-hydride eliminates oleﬁns, while the formed palladium

hydride [κ²-P,O]Pd(H) decomposes by bimolecular reaction

with 1-(CH₂)_2n¹³CH₃-dmso to form alkanes and with further

palladium hydride [κ²-P,O]Pd(H) to form hydrogen, which is

ﬁnally responsible for the detection of ethane upon ethylene

addition. The hydrogen formed likely also participates in further

irreversible decomposition reactions with 1-(CH₂)_2n⁽¹³⁾CH₃-

dmso, as 1-¹³CH₃-dmso in the presence of hydrogen was

found to decompose to ¹³CH₄, palladium black, and mainly

[κ²-(anisyl)₂P,O)]₂Pd (11) in thf-d₈solution. However, the

reaction takes >8 h at 363 K, which indicates that under

polymerization conditions, i.e., in the presence of ethylene, (a)

hydrogen present will hydrogenate ethylene to ethane once

palladium black forms instead of cleaving growing polymer chains

in 1-(CH₂)_2n¹³CH₃-dmso, and (b) therefore palladium hydride

complexes [κ²-P,O]Pd(H) are mostly responsible for deactivating

polymerization-active species by cleaving polymer chains from

1-(CH₂)_2n¹³CH₃-dmso.

Synthesis of the Model Hydride Complex [κ²-P,O-(o-

MeOC₆H₄)₂P(C₆H₄SO₃))Pd(H)(P^tBu₃)] (1-H-P^tBu₃). As we did

not directly observe palladium hydride complexes [κ²-P,O]Pd-

(H) in any of the above-mentioned experiments, we sought to

synthesize a suitable palladium hydride complex 1-H-L that

could model the decomposition reactions, e.g., of 1-CH₃-dmso

and 1-(CH₂)_2n⁽¹³⁾CH₃-dmso to form methane, alkanes, and

hydrogen. Since the seminal work of Drent, Pugh, and co-

workers, it is known that a combination of [Pd(0)(dba)₂] and

zwitterionic, P-protonated phosphine benzenesulfonic acid

Figure 9. ORTEP plot of [κ²-(anisyl)₂P,O)]₂Pd (11). Ellipsoids are

shown at the 50% probability level. Hydrogen atoms and two

cocrystallized C₂D₂Cl₄molecules are omitted for clarity.

With these observations in hand we monitored the reactivity of

1-CH₃-dmso with 1-H-P^tBu₃by ¹H NMR spectroscopy: 1-CH₃-

dmso (30 mM) disappears in the presence of 1-H-P^tBu₃(1.2

equiv) within 1.5 h at 373 K in tetrachloroethane-d₂solution,

while methane (characteristic singlet at δ = 0.23 ppm) and traces

of hydrogen (δ = 4.71 ppm) were detected as decomposition

products.³³The formation of methane may originate from

adventious traces of water in the solvent (vide supra). However,

we note that upon reaction of partially deuterated 1-D-P^tBu₃

(ca. 71% deuteride content) with 1-CH₃-dmso under otherwise

identical reaction conditions (i.e., same solvent batch, temper-

ature, concentration), methane-d₁and methane are formed in

a 66:34 ratio, as identiﬁed by the additional 1:1:1 triplet at δ =

0.21 ppm with a ²J_HD= 1.8 Hz in the ¹H NMR spectrum. That is,

the solvent does not contain a signiﬁcant amount of H₂O, as the

amount of deuterium-label found in the methane is not signiﬁ-

cantly altered with respect to 1-D-P^tBu₃. While these data clearly

indicate that methane (mainly) originates from a reaction of

1-H-P^tBu₃with 1-CH₃-dmso, we note that in tetrachloroethane-d₂

solution only ca. 0.7 equiv of 1-H-P^tBu₃is necessary to decompose

1 equiv of 1-CH₃-dmso; that is, there is likely a second (and

unidentiﬁed) decomposition reaction operative. In contrast, such

an alternative decomposition pathway is not operative in thf-d₈

solution (ca. 24 h at 363 K), as 1-CH₃-dmso and 1-H-P^tBu₃

(2 equiv) were consumed in a 1:1 ratio, while the remaining

equivalent of 1-H-P^tBu₃only very slowly decomposes to hydrogen

(additional 120 h, 383 K).

(o-MeOC₆H₄)₂PH⁺(C₆H₄SO₃) (H[P,O]) generates active

−

species for the copolymerization of ethylene and acrylates

(dba = 1,5-diphenylpenta-1,4-dien-3-one).³While the nature of

the active species has not been investigated in detail, it is likely

a palladium hydride [κ²-P,O]Pd(H) that forms upon formal

oxidative addition of the protonated phosphine to palladium(0).

In need for a suitable [κ²-P,O]Pd(H) model complex we

therefore explored the reaction of zwitterionic phosphine

benzenesulfonic acid with palladium(0) sources. While [Pd(0)-

(dba)₂] and (o-MeOC₆H₄)₂PH⁺(C₆H₄SO₃) (H[P,O]) indeed

−

forms a palladium hydride (even though in less than 5% yield as

evidenced by in situ ¹H NMR), this hydride is not stable and at

room temperature eludes isolation. However, oxidative addition

of H[P,O] to Pd(P^tBu₃)₂generates [(κ²-P,O)-(o-MeOC₆H₄)₂P-

(C₆H₄SO₃)]Pd(H)(P^tBu₃) (1-H-P^tBu₃) in virtually quantitative

yield, which was isolated and fully characterized by NMR

techniques and X-ray diﬀraction analysis (Figure 8 and Table 2).

Notably, the palladium-bound hydride was located in the electron

Table 2. Selected Bond Distances (Å) and Angles (deg) for

Complex 1-H-P^tBu₃

Pd1−H36

Pd1−O3

Pd1−P1

Pd1−P2

1.48(4)

P1−Pd1−P2

P2−Pd1−H36

O3−Pd1−H36

O3−Pd1−P1

166.28(2)

85.8(14)

174.5(13)

92.66(5)

While much slower thaninthethermolysisof 1-(CH₂)_2n¹³CH₃-

dmso, the formation of methane and particularly of methane-d₁

(from 1-D-P^tBu₃) and hydrogen clearly supports our assumption

that palladium hydride complexes [κ²-P,O]Pd(H) are responsible

2.191(2)

2.3072(7)

2.3473(8)

8397

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

for the deactivation of polymerization-active palladium complexes

and the formation of saturated hydrocarbons. Further evidence

for this assumption also comes from the reaction of the isolated

palladium ethyl complex 1-Et-PPh₃, which models the mixture

of palladium alkyl complexes 1-(CH₂)_2n¹³CH₃-dmso obtained by

in situ NMR experiments. Upon heating 1-Et-PPh₃in tetrachloro-

ethane-d₂solution, a mixture of ethylene, ethane, and hydrogen

forms along with detectable amounts of 1-H-PPh₃characterized

by two ³¹P nuclei resonating at δ = 24.42 and 7.92 ppm with

decomposition of 1-H-P^tBu₃alone in the absence of H[P,O]

(11 h, 373 K, vide supra). Finally, 1-Et-PPh₃decomposes to

ethane, ethylene, butenes, and higher oligomers in the presence

of H[P,O] at a similar rate as in the absence of H[P,O]. While

the relatively slow formation of ethane from 1-CH₂CH₃-PPh₃

̀

and H[P,O] vis a vis the much faster formation of methane from

1-CH₃-dmso or 1-CH₃-P^tBu₃and H[P,O] is intriguiging, we

believe that it is caused by a much slower dissociation of

PPh₃from 1-Et-PPh₃than the dissociation of bulky P^tBu₃or

dmso from 1-CH₃-dmso or 1-CH₃-P^tBu₃and that the reaction

of 1-alkyl-PR₃complexes with H[P,O] only qualitatively models

a catalyst decomposition during polymerization experiments

with 1-CH₃-dmso or 1-CH₃-pyr. To verify this hypothesis,

further ethylene oligomerizations were conducted in NMR-tube

experiments. Thus, 1-¹³CH-dmso was reacted with ca. 7.5 equiv

of ethylene for 90 min at 298 K in thf-d₈, while 5.5 equiv of

ethylene was enchained to form a mixture of 1-(CH₂)_2n-¹³CH₃-

dmso complexes in 74% NMR yield and 26% 1-¹³CH-dmso

remained unreacted. At this point no signals indicating the

2

a J_PPtrans= 395 Hz and one hydridic resonance at δ(¹H) =

−17.36 ppm with observable ³J_PHcis= 16.5 Hz (for details, see SI).

Enhanced Decomposition in the Presence of Zwitter-

ionic (o-MeOC₆H₄)₂PH⁺(C₆H₄SO₃), H[P,O]. All experimental

−

data presented so far point to an inherent reactivity of palladium

hydride complexes [κ²-P,O]Pd(H), which results in a deactiva-

tion of polymerization-active species by formation of saturated

hydrocarbons and hydrogen as well as palladium black and

[κ²-(anisyl)₂P,O)]₂Pd (11) (vide supra). However, as reductive

elimination and oxidative addition of methylphosphonium salts

were proven under NMR-tube conditions as well as in high-

activity, high-pressure polymerization experiments (vide supra),

1

presence of terminal or internal oleﬁns were detectable by H

NMR spectroscopy. After addition of 1.5 equiv of H[P,O]

(which is virtually insoluble in pure thf-d₈) the complete

disappearance of 1-¹³CH-dmso and formation of ¹³CH₄were

evident within less than 5 min at 298 K, while complete depletion

of the aliphatic signals of 1-(CH₂)_2n-¹³CH₃complexes was

observed within 25 min at 298 K. Simultaneously, characteristic

signals of linear alkanes (internal methylene groups at δ = 1.293

ppm and of CH₃groups at δ = 0.891 ppm) and traces of propane

were detected. The ratio of ¹³CH₃and ¹²CH₃as well as (¹³C)S1-

S2:(¹²C)S1-S2 in these linear alkanes is virtually 1:1, while only

trace amounts of oleﬁns (<5%, mainly 1-oleﬁns) were detected by

and oxidative addition of (o-MeOC₆H₄)₂PH⁺(C₆H₄SO₃) to

−

palladium(0) is obviously a viable strategy to generate polymer-

ization-active palladium hydride species (vide supra and ref 3),

we hypothesized that the microscopic reverse, i.e., reductive

elimination of phosphonium salt (o-MeOC₆H₄)₂PH⁺-

−

(C₆H₄SO₃) (H[P,O]) from palladium hydride complexes,

may in principle be possible and that therefore H[P,O] may be

responsible for the deactivation of polymerization-active

palladium complexes and the formation of saturated polymer

chains (Scheme 5).³⁴While thermodynamically the formation

1

means of H and ¹³C NMR spectroscopy. This experiment

Scheme 5. Reductive Elimination As a Source of H[P,O]

conﬁrms that in the real catalytic system H[P,O] (if formed by

reductive elimination from a palladium hydride according to

Scheme 5) rapidly cleaves the palladium−polymeryl bond to form

saturated hydrocarbons and that palladium-coordinated PPh₃may

signiﬁcantly reduce the rate of this decomposition reaction in the 1-

CH₂CH₃-PPh₃model complex. ³¹P NMR analysis of this sample

indicates a virtually quantitative formation of [κ²-(anisyl)₂P,O)]₂Pd

(11), as concluded from two signals at δ = 30.03 and 22.81 ppm in

a 1:8 ratio, also observed with independently prepared material in

thf-d₈:dmso-d₆4:1 (vide supra).

At this point it remains unresolved whether palladium hydride

complexes [κ²-P,O]Pd(H) or H[P,O] are directly responsible for

the deactivation of polymerization-active palladium complexes

and the formation of saturated hydrocarbons. However, DFT

calculations have been conducted in order to clarify this issue

(vide infra).

Implications for Copolymerizations of Ethylene with

Polar Vinyl Monomers. The tendency of 1-alkyl-L for

β-hydride elimination under preparative reactor polymerization

conditions ultimately opens a catalyst decomposition pathway by

the inherent reactivity of formed [κ²-P,O]Pd(H) or the reduc-

tive elimination product H[P,O] formed thereof. In general,

in copolymerizations of ethylene with polar vinyl monomers

catalyzed by 1-CH₃-L polymer molecular weights tend to be

lower vs ethylene homopolymerizations, due to an enhanced

propensity for β-hydride elimination after enchainment of polar

monomer. That is, the issue of irreversible catalyst deactivation

initiated by palladium hydride complexes becomes increasingly

important. As the decomposition reaction of 1-(CH₂)_2n-⁽¹³⁾CH₃

with [κ²-P,O]Pd(H) or H[P,O] will be greater than ﬁrst-order in

palladium {due to d[H[P,O]]/dt = −k × [[κ²-P,O]Pd(H)]}, the

of 1-H-P^tBu₃from stochiometric amounts of H[P,O] and

Pd(P^tBu₃)₂is preferred (Figure 8), heating of 1-H-P^tBu₃in

dmso-d₆solution in the presence of excess P(^tBu)₃results in the

formation of [HP^tBu₃]⁺[P,O]⁻, which thermodynamically likely

reﬂects the higher basicity of P^tBu₃vs [P,O] (cf. SI) and reduces

the amount of H[P,O] accessible for oxidative addition to Pd(0).

These experiments establish that oxidative addition and reductive

elimination of P-protonated phosphonium sulfonate H[P,O] to

pallladium(0) are kinetically accessible and equilibrated.

Therefore, the reactivity of H[P,O] toward 1-CH₃-dmso,

1-CH₃-P^tBu₃, 1-CH₂CH₃-PPh₃, 1-H-P^tBu₃, and 1-(CH₂)_2n-

¹³CH₃-dmso was studied. Intriguingly, the reaction of 1-CH₃-

dmso or 1-CH₃-P^tBu₃with 1.1 equiv of H[P,O] (instead of

1-H-P^tBu₃) also yields methane, but the reaction is already

complete within 5 min at 298 K in tetrachloroethane-d₂solution,

which contrasts with ca. 1.5 h at 373 K for the complete

decomposition of 1-CH₃-dmso by 1-H(D)-P^tBu₃to form

methane(-d₁).

Decomposition of 1-H-P^tBu₃by H[P,O] to form hydrogen is

much slower than decomposition of 1-CH₃-dmso or 1-CH₃-

P^tBu₃by H[P,O] (4 h, 373 K vs <5 min, 298 K) but faster than

8398

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

most promising strategy to suppress this decomposition without

changing the nature of the catalyst will be (a) to reduce the

catalyst concentration in (co)polymerization experiments and

(b) to lower the reaction temperature, thereby reducing the

tendency for β-hydride elimination. While lowering the reaction

temperature, e.g., to 298 K, already was proven to reduce the

tendency for β-hydride elimination due to the living character of

ethylene enchainment (vide supra), the viability of this strategy

is limited for preparative polymerizations due to decreased poly-

merization rates. We have therefore focused on the eﬀect of

catalyst concentration in ethylene−methyl acrylate (MA)

copolymerization experiments.

Copolymerizations of ethylene and MA were performed with

varying concentrations of 1-CH₃-dmso (from 0.2 to 1.0 mM

under otherwise identical conditions). In these experiments,

clearly a decreasing productivity was found with increasing catalyst

loading (i.e., with increasing [Pd]). Furthermore, although

methyl acrylate incorporation did not change signiﬁcantly,

molecular weights of the obtained copolymers decreased with

increasing [Pd], which indicates that growing copolymer chains

may be terminated not only by β-hydride elimination but on a

similar time scale also by reaction with [κ²-P,O]Pd(H) or [P,O]H,

with formation of saturated copolymer chains (Table 3).

Furthermore, upon addition of 1.6 equiv of H[P,O] to complex

16 the saturated product 18 is formed in nearly quantitative NMR

yield within 1.5 h under otherwise identical reaction conditions.

Theoretical Investigations on Reversible and Irrever-

sible Catalyst Deactivation Reactions. DFT calculations were

performed with Gaussian09³⁵using the BP86³⁶⁻³⁸functional

and the LANL2DZ ECP C5³⁹with the associated valence basis

set on the Pd atom and the 6-31G (d) basis set on all the other

atoms. All geometries were localized in the gas phase at the BP86

level. Minima were localized by full optimization of the starting

structures, while transition states were approached through a

linear transit procedure and then located by a full transition-state

search. All structures were conﬁrmed as minimum or transition

state through frequency calculations. The natural bond orbital

analyses were performed by using NBO version 3.⁴⁰⁻⁴⁷The

reported energies were obtained through single-point energy

calculations on the BP86 geometries using the M06 functional,

the SDD ECP with the associated valence basis set on the Pd

atom, and the TZVP basis set on all other atoms. Dichloroethane

solvent eﬀects were included with the continuum solvation

model PCM.⁴⁸

Since products obtained after reductive elimination of

phosphonium salts (aryl)_3−xP⁺R_x+1(x = 0, 1) from [(κ²-P,O)-

(o-MeOC₆H₄)₂P(C₆H₄SO₃)]Pd(R)(L) (1-R-L) were formed

during preparative reactor polymerizations, NMR in situ

polymerizations, and thermolysis of complexes 1-R-L (vide

supra), mechanistic pathways for these reductive elimination

reactions were investigated. Due to the potential participation of

P-protonated phosphonium salt (o-MeOC₆H₄)₂(C₆H₄SO₃)-

P⁺(H) (H[P,O]) in an irreversible catalyst deactivation (vide

supra), we ﬁrst investigated the formation of H[P,O] starting

from the synthesized model complex 1-H-P^tBu₃and P^tBu₃(note

that 1-H-P^tBu₃was synthesized by oxidative addition of H[P,O]

to [Pd(P^tBu₃)₂]).

Table 3. Ethylene−Methyl Acrylate Copolymerizations at

a

Variable Concentrations of the Catalyst Precursor

conc

Pd(II)

[mM]

d

yield TON TON

M_n

b

c

d

entry

[g] C₂H₄

MA

χ_MA[10³g mol⁻¹

]

M_w/M_n

1

2

3

0.2

0.5

1.0

0.47

0.97

1.61

603

486

414

77

67

53

11.3

12.2

11.3

8000

7100

5800

1.4

1.5

a

Reaction conditions: 100 mL 0.6 M MA in toluene, 5 bar [C₂H₄];

95 °C; 30 min polymerization time, catalyst (1-CH₃-dmso) added as

b

c

methylene chloride solution. Turnover number per palladium. From

These calculations indicate that the reductive elimination of

H[P,O] occurs through a transition state (TS) lying 124 kJ mol⁻¹

higher in energy than 1-H-P^tBu₃, with the P^tBu₃ligand still

coordinated to the Pd (see Scheme 7). The transition-state

geometry 1-H-TS1 (Scheme 7 and Figure 10) exhibits a still

short palladium−hydride and palladium−oxygen bonds (Pd−H:

1.58 vs 1.48 Å and 2.28 vs 2.19 Å in 1-H-P^tBu₃, see Figure 8 and

Table 2), while the cleaving palladium−phosphine bond is

d

¹H NMR in C₂D₂Cl₄at 100 °C. From GPC at 40 °C vs polystyrene

standards.

The reactivity of the methyl acrylate diinsertion product 16,

which is isolable upon reaction of 1-CH₃-L with methyl

acrylate,^12bwas studied. Complex 16 is a suitable model for

catalyst deactivation in ethylene−methyl acrylate copolymeriza-

tions, as β-hydride elimination after acrylate insertion can be

modeled. Thus, heating 16 in tetrachloroethane-d₂solution to

373 K for 8 h results in formation of a ca. 60:40 mixture of

unsaturated 4-ethylpent-2-enedioic acid dimethyl ester (17) and

saturated 2-ethylpentanedioic acid dimethyl ester (18) (Scheme 6;

for details, see SI). This experiment proves that in ethylene−

acrylate copolymerizations palladium hydride [κ²-P,O]Pd(H) or

P-protonated phosphonium salt H[P,O] is capable of cleaving

palladium polymeryl species formed after an acrylate insertion.

Scheme 7. Energetics (kJ mol⁻¹) of the Reductive Elimination

of H[P,O] from 1-H-P^tBu_3.

Scheme 6. Formation of Unsaturated 17 and Saturated 18 by

Thermolysis of Complex 16 in the Absence or Presence of

H[P,O]

Figure 10. Transition-state geometry for the reductive elimination

reaction of H[P,O] from 1-H-P^tBu₃.

8399

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

already 2.96 Å long, as compared to 2.31 Å in 1-H-P^tBu₃. Finally

the evolving phosphine−hydrogen bond is still 1.86 Å long,

as compared to ca. 1.33−1.40 Å in reported P-protonated

phosphonium salts.⁵¹After reductive elimination of H[P,O] the

departing Pd−P^tBu₃fragment is stabilized by incoming P^tBu₃to

form [Pd(P^tBu₃)₂]. The overall reaction is essentially thermo-

neutral, with 1-H-P^tBu₃+ P^tBu₃favored by 5 kJ mol⁻¹over

H[P,O] + [Pd(P^tBu₃)₂], which is in qualitative agreement with

the synthesis of 1-H-P^tBu₃from H[P,O] + [Pd(P^tBu₃)₂].

Considering the principle of microscopic reversibility and the

calculated energies of educts, products, and the transition state

1-H-TS1, these results indicate a fully reversible formation of

1-H-P^tBu₃+ P^tBu₃from H[P,O] + [Pd(P^tBu₃)₂].

A similar reaction proﬁle was also found for the reductive

elimination of P-methylated and P-ethylated phosphonium salts

Me[P,O], and Et[P,O] from 1-CH₃-P^tBu₃and 1-CH₂CH₃-

P^tBu₃+ P^tBu₃. However, the formed products Me[P,O]

(Et[P,O], respectively) + [Pd(P^tBu₃)₂] are much more stable

than the educts by 65 (66, respectively) kJ mol⁻¹, while the

transition states 1-CH₃-TS1 (1-CH₂CH₃-TS1, respectively)

lie 164 (171) kJ mol⁻¹higher than the educts (Scheme 8).

P-ethylated complex 12 or with a calculated ground-state distance

of 1.84 (1.86) Å in Me[P,O] (Et[P,O]).

To rationalize the remarkable thermodynamic diﬀerence

between reductive eliminations from the Pd−H complex on

one hand and the Pd−Me and Pd−Et complexes on the other

hand, we compare the homolytic bond dissociation energy

(BDE) of the Pd−R bond in the Pd−H, Pd−Me, and Pd−Et

complexes. The calculated BDEs indicate that the Pd−H bond

is roughly 100 kJ mol⁻¹stronger than the Pd−C bonds. On the

other hand, the BDE of the P−H, P−Me, and P−Et bonds in the

corresponding R[P,O] zwitterions indicates that the P−H bond

is only ca. 30 kJ mol⁻¹stronger than the P−C bonds. Thus, the

main diﬀerence in the thermodynamics of the reductive elimina-

tion is caused by the diﬀerence in the strength of the Pd−H

and Pd−C bonds and can be reasonably approximated by the

calculated diﬀerences of the BDE of the Pd−R and P−R bonds.

Furthermore, the transition-state energy diﬀerences seem to

have a steric term. As indicated in Figure 11, steric interactions

between the migrating Me or Et group and one of the anisyl rings

of the ligand with close contacts of ca. 2.8 Å to the ipso-anisyl

carbon atom may be responsible for the higher energy of 1-CH₃-

TS1-P^tBu₃and 1-CH₂CH₃-TS1-P^tBu₃compared with the TS

calculated for the less encumbered 1-H-TS1 (Figure 10).

Given the strong thermodynamic preference for the reductive

elimination to form P-alkylated phosphonium salts, the micro-

scopic reverse, i.e., oxidative addition of Me[P,O] to [Pd(P^tBu₃)₂]

with an activation energy of 229 kJ mol⁻¹(237 kJ mol⁻¹for

Et[P,O], respectively), seems highly unlikely based on these

calculations. However, since mass spectrometric and NMR data

prove the formation of double-methylated phosphonium salts,

e.g., 2, and in addition 2-(vinyl)anisole was detected by NMR

experiments (vide supra and SI), the oxidative addition of a

P-anisyl bond of Me[P,O] to [Pd(P^tBu₃)₂] was investigated.

Remarkably, a transition-state geometry 1-anisyl-TS1 was found,

which lies 157 kJ mol⁻¹higher than Me[P,O] + [Pd(P^tBu₃)₂].

This barrier is similar to that of reductive elimination of Me[P,O]

from 1-CH₃-P^tBu₃(164 kJ mol⁻¹) and is therefore kinetically

accessible considering that formation of Me[P,O] takes place

under experimental conditions. The transition state 1-anisyl-TS1

evolves into the palladium anisyl complex 19-(anisyl)-P^tBu₃,

which, though thermodynamically 15 kJ mol⁻¹less stable than

Me[P,O] + [Pd(P^tBu₃)₂], is 50 kJ mol⁻¹more stable than 1-CH₃-

P^tBu₃(Scheme 9a). These results are in accordance with the

Scheme 8. Energetics (kJ mol⁻¹) of the Reductive Elimination

of Me[P,O] from 1-CH₃-P^tBu₃(a) and of the Elimination of

Et[P,O] from 1-CH₂CH₃-P^tBu₃(b)

Thus, although thermodynamically much more favorable than

reductive elimination of H[P,O], the reductive elimination of

Me[P,O] (Et[P,O], respectively) requires a 40 (47, respectively)

kJ mol⁻¹higher activation energy than reductive elimination

of H[P,O]. These signiﬁcantly increased activation energies

also manifest in the transition-state geometries 1-CH₃-TS1

(1-CH₂CH₃-TS1, respectively, Figure 11) with already short

P−CH₃(P−CH₂CH₃, respectively) distances of 1.92 (1.95,

respectively) Å for the evolving P−C bond as compared, for

example, with 1.80 Å in P-methylated complex 10 or 1.84 Å in

Scheme 9. Energetics (kJ mol⁻¹) of the Reductive Elimination of

Me[P,O] from 1-CH₃-P^tBu₃and of the P-Anisyl Oxidative

Addition of Me[P,O] to [Pd(P^tBu₃)₂] (a) and to [Pd(P^tBu₃)] (b)

more general trend observed in metal−carbon bond energies

experimentally determined by Wolczanski et al. for titanium carbyl

complexes^50aand by Jones et al. for rhodium carbyl complexes.^50b

They indicate that the respective metal−methyl bonds tend to

be ca. 30 kJ mol⁻¹(Ti) or ca. 65 kJ mol⁻¹(Rh) less stable than

the respective metal−phenyl bonds. They are also in accordance

Figure 11. Transition states 1-CH₃-TS1 and 1-CH₂CH₃-TS1 for the

reductive elimination of H[P,O] from 1-CH₃-P^tBu₃(a) and from

1-CH₂CH₃-P^tBu₃(b).

8400

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

with DFT calculations of the Wolczanski and Jones series at the

B3PW91 and BP86 levels of theory, which clearly support the

higher stability of metal−phenyl vs metal−methyl bonds.^50c

In the absence of free P^tBu₃the Pd(P^tBu₃) fragment evolving

from 1-CH₃-TS1-P^tBu₃will not be stabilized by formation

of [Pd(P^tBu₃)₂], and reaction of Me[P,O] + Pd(P^tBu₃) →

19-anisyl-P^tBu₃becomes energetically barrierless (Scheme 9b).

Simultaneously, the absence of free P^tBu₃makes 19-anisyl-P^tBu₃

a global and not just a local thermodynamical minimum when

compared to the reaction shown in Scheme 9a.⁵¹

These results indicate that reductive elimination of methyl/

alkylphosphoniumsalts Me[P,O]/(alkyl)[P,O] in the realcatalytic

system constitutes a reversible deactivation pathway of polymer-

ization-active palladium complexes to form palladium(0) species.

Reactivation of palladium(0) species, however, proceeds by

oxidative addition of a P-anisyl bond as supported by experimental

results and DFT calculations.

In contrast, the formation of fully saturated alkanes under

polymerization conditions and the additional formation of

dihydrogen in model reactions result in an irreversible catalyst

deactivation. The experimental data, however, do not allow for a

precise mechanistic scheme, since palladium hydride complexes

[(κ²-P,O]Pd(H)(L) or reductively eliminated P-protonated

phosphonium salts H[P,O] may be responsible for the formation

of inactive palladium and saturated alkanes. Therefore several

mechanistic schemes were investigated by DFT methods:

(a) the reactions of P-protonated phosphonium salt H[P,O]

with synthetically accessible 1-H-P^tBu₃, 1-CH₃-P^tBu₃, and

1-CH₂CH₃-PPh₃, (b) the reaction of 1-CH₃-P^tBu₃with 1-H-

P^tBu₃, and ﬁnally (c) the reaction 1-CH₃-dmso with 1-H-P^tBu₃.

With respect to the reactions of P-protonated phosphonium

salt H[P,O], we note that low-lying transition states were formed

only after tautomerization into the O-protonated sulfonic

acid form [P,O]H, which is 12 kJ mol⁻¹less stable than the

P-protonated H[P,O] form (Scheme 7).⁵²The reaction of 1-H-

P^tBu₃with [P,O]H is then initiated by substitution of the P^tBu₃

ligand for the phosphine [P,O]H to form 1-H-[P,O]H, which is

20 kJ mol⁻¹higher in energy than 1-H-P^tBu₃+ O-protonated

[P,O]H. 1-H-[P,O]H directly evolves to the transition state 1-

H-TS2 lying 124 kJ mol⁻¹higher than 1-H-P^tBu₃+ O-protonated

sulfonic acid (Scheme 10). Considering the tautomerization of

Figure 12. Transition-state geometry of 1-H-TS2 for the reaction

leading to [P,O]₂Pd (11) + H₂.

H[P,O], tautomerization of H[P,O] into [P,O]H, and reaction

of [P,O]H with 1-H-P^tBu₃) shows that the reactions 1-H-P^tBu₃

+ P^tBu₃→ [Pd(P^tBu₃)₂] + H[P,O] and 1-H-P^tBu₃+ H[P,O] →

1-H-P^tBu₃+ [P,O]H → [κ²-(anisyl)₂P,O)]₂Pd (11) + H₂have

very similar energy barriers, i.e., 124 and 12 + 124 = 136 kJ mol⁻¹

(Scheme 10). The energy balance of the overall decomposition

reaction, shown in Scheme 11, is endoergic by ca. 60 kJ mol⁻¹,

Scheme 11. Energetics (kJ mol⁻¹) of the Overall

Decomposition Reaction of 1-H-P^tBu₃

which is mostly caused by the formation of 11 and H₂from 1-H-

TS2. However, despite the endoergicity, release and irreversible

removal of H₂from the reaction mixture is a plausible driving

force for this reaction to proceed.

The reaction of O-protonated [P,O]H with 1-CH₃-P^tBu₃to

form complex 11 and methane proceeds along a similar reaction

coordinate to the reaction of [P,O]H with 1-H-P^tBu₃. However,

substitution of P^tBu₃by [P,O]H leads to intermediate 1-CH₃-

[P,O]H + P^tBu₃, which lie 2 kJ mol⁻¹higher than the educts

and evolve to transition state 1-CH₃-TS2 for the formation of

methane and 11, which lies 84 kJ mol⁻¹higher than the educts

(as compared to 124 kJ mol⁻¹for the formation of H₂and 11)

(Scheme 12, line (a)). The overall formation of methane and 11

Scheme 12. Energetics (kJ mol⁻¹) of the Reaction of [P,O]H

with 1-CH₃-P^tBu₃(line (a)) and with 1-CH₃-dmso (line (b))

Scheme 10. Energetics (kJ mol⁻¹) of the Reaction of [P,O]H

with 1-H-P^tBu₃

P-protonated H[P,O] into O-protonated [P,O]H, which costs

12 kJ mol⁻¹, the overall barrier amounts to 136 kJ mol⁻¹to reach

1-H-TS2. 1-H-TS2 then decomposes to form dihydrogen and

[κ²-(anisyl)₂P,O)]₂Pd (11), which lie 43 kJ mol⁻¹higher in energy

than 1-H-P^tBu₃+ O-protonated [P,O]H. The endoergicity of the

overall reaction is in accordance with a late transition state in which

the hydrogen−hydrogen distance is only 0.80 Å (Figure 12), i.e.,

very close to the distance in free H₂(0.74 Å) and certainly at the

lower limit even of dihydrogen complexes (0.8−1.0 Å).⁵⁵

is exoergic (and therefore thermodynamically favorable) by 73 kJ

mol⁻¹starting from O-protonated [P,O]H and 1-CH₃-P^tBu₃.

When [P,O]H is reacted with 1-CH₃-dmso, substitution of dmso

for [P,O]H to form the new ground state 1-CH₃-[P,O]H +

dmso is exoergic by 33 kJ mol⁻¹and evolves into the transition

state 1-CH₃-TS2, which now lies 49 kJ mol⁻¹above [P,O]H +

1-CH₃-dmso.⁵⁶The rate-limiting step to reach 1-CH₃-TS2,

Comparison of the energy proﬁle of the three reactions

involved in the deactivation process (i.e., reductive elimination of

8401

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

however, is 82 kJ mol⁻¹in energy for the transformation of

1-CH₃-[P,O]H + dmso into 1-CH₃-TS2 + dmso (Scheme 12,

line (b)) and therefore very similar to the overall barrier of

84 kJ mol⁻¹calculated for the reaction of [P,O]H with 1-CH₃-

P^tBu₃(compare lines (a) and (b) in Scheme 12).

Scheme 14. Energetics (kJ mol⁻¹) of the Decomposition

Reaction of 1-CH₂CH₃-P^tBu₃with 1-H-P^tBu₃

The overall formation of methane and 11 is also exoergic by

56 kJ mol⁻¹starting from 1-H-P^tBu₃and 1-CH₃-P^tBu₃and

23 kJ mol⁻¹starting from 1-H-P^tBu₃and 1-CH₃-dmso by

following a reaction sequence composed of reductive elimination

of H[P,O] from 1-H-P^tBu₃, tautomerization of H[P,O] into

[P,O]H, and reaction of [P,O]H with 1-CH₃-P^tBu₃or 1-CH₃-

dmso (Scheme 13).

place of P^tBu₃is considered. The main diﬀerence is in the

energy of PPh₃substitution by [P,O]H, which is disfavored by

18 kJ mol⁻¹; see Scheme 15. This is related to the lower steric

Scheme 13. Energetics (kJ mol⁻¹) for the Overall Formation

of Methane and 11 from the Reaction of 1-CH₃-P^tBu₃with 1-

H-P^tBu₃(line (a)) and from the Reaction of 1-CH₃-dmso with

1-H-P^tBu₃(line (b))

Scheme 15. Energetics (kJ mol⁻¹) of the Reaction of [P,O]H

with 1-CH₂CH₃-PPh₃

bulkiness of PPh₃compared to P^tBu₃. The stronger binding of

PPh₃compared to P^tBu₃translates into a decomposition

pathway shifted about 20 kJ mol⁻¹higher in energy; compare

Schemes 14 and 15.⁵⁷The reasonable energy barrier we found for

the decomposition of 1-CH₂CH₃-PPh₃promoted by the

protonated ligand [P,O]H is in agreement with the experimental

evidence: (a) 1-CH₂CH₃-PPh₃decomposes with formation of

ethane and trace amounts of hydrogen at 343 K, (b) the reaction

of H[P,O] with 1-CH₂CH₃-PPh₃is slower than the reaction

with 1-CH₃-P^tBu₃, and (c) the reaction of H[P,O] with

1-(CH₂)_2n¹³CH₃-dmso at 298 K is slower than with 1-¹³CH₃-

dmso (vide supra).

In conclusion, the experimentally observed reactivity diﬀer-

ences are largely not due to the diﬀerent binding strengths of the

monodentatephosphineor dmso. While thesubstitutionof mono-

dentate phosphines for [P,O]H in 1-alkyl-PR₃is essentially

thermoneutral or slightly endoergic, replacement of dmso in

1-CH₃-dmso by [P,O]H is exoergic (Schemes 12, 14, and 15).

However, all these substitution reactions lead to 1-CH₃/

CH₂CH₃-[P,O]H, which eliminates methane with a barrier of

82 kJ mol⁻¹or ethane with a barrier of 107 kJ mol⁻¹. Thus, the

experimentally observed ﬁnding, that 1-¹³CH₃-dmso reacts faster

with [P,O]H to form methane than with 1-(CH₂)_2n¹³CH₃-dmso

to form alkanes (vide supra) is mainly due to the diﬀerence in

the respective barriers for methane vs ethane elimination. However,

as shown for the model compound 1-H-P^tBu₃, reductive elimination

of H[P,O] with a barrier of 124 kJ mol⁻¹seems rate limiting for the

formation of alkanes whenever H[P,O] is not deliberately added to

catalytically active species.

However, the rate-limiting step for the formation of methane

and 11 starting from 1-H-P^tBu₃and 1-CH₃-P^tBu₃or 1-CH₃-

dmso is now reductive elimination of H[P,O] with a barrier of

124 kJ mol⁻¹, as compared to the 84 kJ mol⁻¹barrier calculated

for the reaction of O-protonated [P,O]H with 1-CH₃-P^tBu₃and

compared to the 82 kJ mol⁻¹calculated barrier for the reaction

of [P,O]H with 1-CH₃-dmso (compare Schemes 7 and 12).

A rate-limiting reductive elimination of H[P,O] is in accordance

with the experimental ﬁnding that upon addition of H[P,O] to

1-CH₃-P^tBu₃or 1-CH₃-dmso, formation of methane and 11 takes

place within <5 min at 298 K, while complete formation of methane

and 11 takes 1.5 h at 373 K starting from 1-CH₃-dmso + 1-H(D)-

P^tBu₃in tetrachloroethane-d₂(vide supra).

While formation of H₂and 11 from 1-H-P^tBu₃is thermo-

dynamically disfavored by 60 kJ mol⁻¹, but due to removal of H₂

still observable in the experimental system, formation of methane

and 11 from 1-H-P^tBu₃and 1-CH₃-P^tBu₃is favored by 56 kJ

mol⁻¹. This huge calculated energy diﬀerence of 116 kJ mol⁻¹for

the formation of H₂and methane seems odd at ﬁrst glance.

However, it is in accordance with calculated homolytic bond

dissociation energies of the Pd−CH₃bond and the Pd−H bond

in 1-CH₃-P^tBu₃and 1-H-P^tBu₃, which indicates that the Pd−

CH₃bond is ca. 100 kJ mol⁻¹weaker than the Pd−H bond (see

SI),⁵⁷while the respective H₃C−H and H−H bonds are reported

to be ca. 440 (H₃C−H) and 436 kJ mol⁻¹(H₂):⁵⁶Thus the

calculated diﬀerence of 116 kJ mol⁻¹for the formation of a H−H

bond as compared to a H₃C−H bond starting from 1-CH₃-P^tBu₃

and 1-H-P^tBu₃originates mainly from the diﬀerences in bond dis-

sociation energies of the respective Pd−H and Pd−CH₃bonds.

We move now to the decomposition of 1-CH₂CH₃-P^tBu₃

promoted by reaction with the protonated ligand [P,O]H; see

Scheme 14. Overall, the energy proﬁle is rather similar to that of

the corresponding methyl complex 1-CH₃-P^tBu₃(Scheme 12);

the main diﬀerence is in the energy barrier for ethane elimination,

which is roughly 25 kJ mol⁻¹higher than the barrier for methane

elimination.

Finally we have investigated two reaction schemes in which

1-H-P^tBu₃directly reacts with 1-CH₃-P^tBu₃or with 1-CH₃-

dmso (i.e., without prior reductive elimination of H[P,O]). The

ﬁrst reaction coordinate we investigated comprises the reaction

of 1-H and 1-CH₃formed after P^tBu₃dissociation from 1-H-

P^tBu₃and 1-CH₃-P^tBu₃. The formed intermediate 1-H-1-CH₃

exhibits a bridging hydride and methyl group with short

Pd(1)−H (1.877 Å), Pd(2)−H (1.646 Å), Pd(1)−CH₃

(2.044 Å), and Pd(2)−CH₃(2.538 Å) distances, which evolves

into transition state 1-H-1-CH₃-TS1, in which the two palladium

atoms, the hydrogen atom, and the carbon atom adopt a

A rather similar energy proﬁle is also calculated when the

experimentally employed ethyl complex with a PPh₃ligand in

8402

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

distorted four-membered ring. However, 1-H-1-CH₃+ 2 P^tBu₃

and 1-H-1-CH₃-TS1 + 2 P^tBu₃are 225 and 278 kJ mol⁻¹higher

in energy than 1-H-P^tBu₃+ 1-CH₃-P^tBu₃and thus prohibitively

high for the reaction to proceed (Scheme 16). The reason for

Scheme 16. Energetics (kJ mol⁻¹) of the Decomposition

Reaction of 1-CH₃-P^tBu₃with 1-H-P^tBu₃Initiated by P^tBu₃

Dissociation from 1-CH₃-P^tBu₃and 1-H-P^tBu₃

Figure 13. Geometry of 1-H-(P^tBu₃)-1-CH₃-P^tBu₃-TS (left) and 1-H-

(P^tBu₃)-1-CH₃-dmso-TS (right) for the methane elimination starting

from 1-H-P^tBu₃and 1-CH₃-P^tBu₃or 1-CH₃-dmso.

thesehigh energies of1-H-1-CH₃+ 2 P^tBu₃and 1-H-1-CH₃-TS +

2 P^tBu₃is likely a huge energetic penalty for the dissociation of

P^tBu₃in both educt complexes, which cannot be compensated

for by the bridging interaction of hydride and the methyl group in

1-H-1-CH₃.

In order to avoid such high energetic penalties due to ligand

dissociations, we have investigated a second reaction coordinate,

which comprises the reactions of two four-coordinate palladium

complexes, namely, 1-H-P^tBu₃+ 1-CH₃-P^tBu₃and 1-H-P^tBu₃+

1-CH₃-dmso, i.e., without dissociation of P^tBu₃or dmso.

Intriguingly, transition state 1-H-P^tBu₃-1-CH₃-P^tBu₃-TS was

located lying only 131 kJ mol⁻¹higher than 1-H-P^tBu₃+ 1-CH₃-

P^tBu₃(Scheme 17, line (a)). An even lower transition state,

associated with 1-H-P^tBu₃-1-CH₃-P^tBu₃-TS and 1-H-P^tBu₃-1-

CH₃-dmso-TS (Scheme 17 and Figure13) areprone to systematic

errors caused by the self-interaction error of DFT,⁶⁰which results

in a potentially overestimated stability of these transition states

due to artiﬁcially stabilized delocalized states. Consequently, the

real barriers might be higher than determined here.

The calculated reaction coordinate for the overall reaction

of 1-H-P^tBu₃and 1-CH₃-P^tBu₃with a barrier of 131 kJ mol⁻¹

according to Scheme 17a indicates that it may contribute to the

formation of methane since it is very similar to the rate-limiting

barrier (124 kJ mol⁻¹) of reductive elimination of H[P,O] in the

reaction sequence composed of reductive elimination of H[P,O]

from 1-H-P^tBu₃, tautomerization of H[P,O] into [P,O]H, and

reaction of [P,O]H with 1-CH₃-P^tBu₃according to Schemes 7

and 12a. For the formation of methane from 1-H-P^tBu₃and

1-CH₃-dmso a S_N2-like reaction with an activation barrier of

95 kJ mol⁻¹according to Scheme 17b is favored over reductive

elimination of H[P,O] from 1-H-P^tBu₃, tautomerization of H[P,O]

into [P,O]H, and reaction of [P,O]H with 1-CH₃-dmso with a

rate-limiting barrier of 124 kJ mol⁻¹(Scheme 7 and 12b).

Scheme 17. Energetics (kJ mol⁻¹) of the reaction of 1-H-P^tBu₃

with 1-CH₃-P^tBu₃Leading to 1-H-P^tBu₃-1-CH₃-P^tBu₃-TS

(line (a)) and of 1-H-P^tBu₃with 1-CH₃-dmso Leading to

1-H-P^tBu₃-1-CH₃-dmso-TS (line (b))

In summary, two viable reaction coordinates for the irreversible

deactivation of polymerization-active 1-(alkyl)-L/1-H-L and

formation of saturated hydrocarbons have been disclosed: (a)

A reaction sequence comprising reductive elimination of the

P-protonated H[P,O] from 1-H-P^tBu₃, followed by tautomeriza-

tion of H[P,O] into O-protonated [P,O]H (Scheme 7),

substitution of P^tBu₃in 1-(alkyl)-P^tBu₃by [P,O]H, and ﬁnally

formation of alkane and [P,O]₂Pd (11) (Schemes 12, 14, and 15).

The rate-limiting step in this sequence is reductive elimination of

H[P,O] with a barrier of 124 kJ mol⁻¹. (b) A bimolecular reaction

of fully intact 1-H-P^tBu₃with 1-CH₃-L according to Scheme 17

in which the transition-state geometries resemble a bimolecular

substitution at a sp³-carbon atom. The energy barrier found for

the reaction of 1-H-P^tBu₃with 1-CH₃-P^tBu₃along this reaction

coordinate is 131 kJ mol⁻¹, whereas it is 95 kJ mol⁻¹for the

reaction of 1-H-P^tBu₃with 1-CH₃-dmso.

On the basis of these results, both reaction channels starting

with reductive elimination of H[P,O] and starting with direct

evolution into a bimetallic S_N2-like substitution can contribute to

the overall deactivation of 1-H-P^tBu₃and 1-(alkyl)-P^tBu₃.

However, when modeling the reaction of 1-H-P^tBu₃with 1-

CH₃-dmso, which is closer to the real catalytic system 1-H-L and

1-(alkyl)-L (L = oleﬁn, dmso), the bimetallic substitution pathway

appears favorable.

1-H-P^tBu₃-1-CH₃-dmso-TS, was located for the reaction of

1-H-P^tBu₃+ 1-CH₃-dmso with a barrier of 95 kJ mol⁻¹

(Scheme 17, line (b)). Characteristic geometric features of

these two transition state evolving into the elimination of

methane comprise a nearly linear alignment of the reacting

H−CH₃−Pd2 moieties of ca. 165° and a substantial planarization

of the central CH₃group (Figure 13), which are highly reminis-

cent of bimolecular substitution reactions. While it has been

suggested in theoretical studies that evolution of hydrogen from

homoleptic manganese hydride complexes may proceed through

a similar, fully linear Mn−H−H−Mn transition-state geome-

try,⁶⁰to our knowledge a linear hydride-alkyl-metal transition

state for the reductive elimination of alkane has not been

proposed or investigated for transition metal complexes.

However, the reduction of haloalkanes by complex hydrides,

e.g., LiBEt₃H or LiAlH₄, was found to exhibit typical character-

istics of an S_N2 reaction.⁶¹We note, however, that the barriers

CONCLUSIONS

■

These studies involving (a) direct monitoring of the polymer-

ization reaction by NMR spectroscopy with isotope-labeled

catalyst precursors, (b) analysis of end groups and distribution of

8403

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

catalyst precursor-derived labeled carbon atoms in polymers

obtained under preparative polymerization conditions, (c) direct

isolation and full identiﬁcation of catalyst decomposition products

from typical polymerization mixtures, (d) clean preparation,

isolation, and full characterization of relevant intermediates of the

polymerization reaction, namely, diﬀerent palladium alkys as well

as a palladium hydride, and (e) theoretical and experimental

elucidation of the reactivity of these intermediates reveal the

following reaction and decomposition pathways:

(3) Concerning the eﬃciency of activation of the catalyst

precursor for polymerization, a weakly coordinated (L = dmso)

complex [(P,O)PdMe(L)] is activated rapidly and completely

even at room temperature and low ethylene concentrations.

Under these mild conditions, polymerization is living, which

allows for clean observation of the activation reaction. For less

labile coordinated catalyst precursors, activation typically will

also be complete, but at the elevated temperatures required not

only will the aforementioned irreversible deactivation reactions

complicate studies of activation and chain growth but also

scrambling of the isotope label from the Pd¹³CH₃initiator into

the polymer backbone by propene elimination and reinsertion

can simulate a seemingly incomplete inactivation.

These ﬁndings suggest strategies for improving catalyst

stability. As key reactions are bimolecular, low catalyst con-

centrations reduce deactivation rates, as illustrated in this study.

Site isolation, by appropriate bulky and repulsive ligand

environments, multidentate ligands, or ultimately binding to

soluble or solid supports, is the further elucidation of this concept.

While the aforementioned reductive eliminations to phospho-

nium sulfonates and Pd(0) are reversible, an agglomeration to

palladium black will render this step irreversible. Colloidal

stabilization toward further aggregation of [Pd(0)]_nat the stage of

nanoparticles or addition of further tetraalkyl/aryl phosphonium

sulfonates R[P,O] to the reaction mixture may be advantageous.

(1) Under typical polymerization conditions, extensive

formation of mixed tetraaryl/alkyl phosphonium salts occurs

by their reductive elimination from the polymerization-active

Pd(II) alkyl species. That is, the growing polymeryl chain is

transferred to the P atom of the phosphinesulfonato ligand. The

reaction is in principle reversible as long as palladium black does

not precipitate. It can aﬀord Pd(II) aryl complexes with a P-alkyl

phosphinesulfonato ligand by net scrambling of the growing

polymeryl chain and the aryl moieties introduced with the catalyst

precursor between the metal center and the phosphorus atom of

the ligand. Such a species inserts ethylene, suggesting that it is

polymerization-active. In fact, di(alkyl)phosphinesulfonate and

(aryl)(alkyl)phosphinesulfonato Pd(II) complexes are precursors

to active catalysts.⁶¹This implies that in polymerizations with this

class of catalysts for a given catalyst precursor actually a range

of active species with diﬀerent chelating P,O ligands including

P-polymeryl- and P-dipolymeryl-substituted ligands may be

involved. The overall productivity and selectivity will be the sum

of all their contributions. Thus, the catalytic properties observed for

a given phosphinesulfonato catalyst precursor may actually reﬂect

its propensity for the aforementioned net scrambling reactions.

(2) Also the Pd(II) hydride, which is an ubiquitious inter-

mediate in polymerization resulting from β-hydride elimination

as the key chain transfer step, undergoes reductive elimination to

form the free neutral (zwitterionic, P-protonated) phospho-

niumsulfonate H[P,O] and palladium(0). This reaction again is

reversible. However, once formed, the prevailing reaction of

H[P,O] is a rapid irreversible reaction with the active growing

polymeryl species to yield an alkane and the bis-chelated

[κ²-(P,O)]₂Pd. This stable compound is the ﬁnal product of

decomposition. Also, hydrogen can be formed from two equi-

valents of hydride, 2 [κ²-(P,O)]Pd(H) → H₂+ [κ²-(P,O)]₂Pd +

Pd(0). This reaction again occurs most likely via reaction of

the Pd hydride with free ligand, H[P,O], formed by reductive

elimination from another equivalent of Pd hydride. A second

irreversible catalyst deactivation pathway yielding alkanes,

[κ²-(P,O)]₂Pd, and palladium(0) was identiﬁed comprising the

direct reaction of Pd(II) hydride with growing polymeryl species

without reductive elimination of H[P,O].

ASSOCIATED CONTENT

■

S

* Supporting Information

Preparative procedures for the synthesis of complexes 1-¹³CH₃-L,

1-H(D)-P^tBu₃, and 1-Et-PPh₃, experimental procedures for

preparative and NMR-tube polymerizations of ethylene,

algorithms for the quantiﬁcation of ¹³C-label incorporation and

for the quantiﬁcation of unsaturated and saturated oligomers,

X-ray diﬀraction data of complexes 10, 11, 12 cocrystallized with

13, and 1-H-P^tBu₃(CCDD nos. 897155−897158), NMR spectra

of key experiments, ESI-MS data, and DFT data. This material is

available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

■

Corresponding Author

*E-mail: stefan.mecking@uni-konstanz.de; lcaporaso@unisa.it.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

Financial support by the DFG (Me1388/10-1) is gratefully

acknowledged. P.R. thanks the Carl Zeiss Foundation for a

research stipend. The authors thank the HPC team of ENEA

(www.enea.it) for using the ENEA-GRID and the HPC facilities

CRESCO (www.cresco.enea.it) in Portici, Italy.

These irreversible decomposition pathways originating

from Pd hydrides will be particularly pronounced in (co)-

polymerization reactions of polar vinyl monomers. The metal

carbyl insertion products of vinyl monomers, such as methyl

acrylate, are more prone to chain transfer by β-hydride elimina-

tion than unsubstituted metal alkyls from ethylene incorporation.

This is reﬂected in an overproportional number of unsaturated

end groups based on the polar monomer in ethylene copolymers.

These reactions not only limit catalyst lifetime but also result

in the formation of relevant amounts of fully saturated polymer

chains. This can falsify molecular weight analysis substantially

in NMR determination of M_nunder the common assumption

that each chain will carry an unsaturated end group (from β-H

elimination) whenever chain transfer numbers are small.

REFERENCES

■

(1) (a) Domski, G. J.; Rose, J. M.; Coates, G. M.; Bolig, A. D.;

Brookhart, M. Prog. Polym. Sci. 2007, 32, 30−92. (b) Berkefeld, A.;

Mecking, S. Angew. Chem., Int. Ed. 2008, 47, 2538−2542. (c) Nakamura,

A.; Ito, S.; Nozaki, K. Chem. Rev. 2009, 109, 5215−5244. (d) Chen, E.

Y.-X. Chem. Rev. 2009, 109, 5157−5214.

(2) (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc.

1996, 118, 267−268. (b) Mecking, S.; Johnson, L. K.; Wang, L.;

Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888−899.

(3) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I.

Chem. Commun. 2002, 744−745.

8404

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

(4) (a) Kochi, T.; Noda, S.; Yoshimura, K.; Nozaki, K. J. Am. Chem. Soc.

2007, 129, 8948−8949. (b) Nozaki, K.; Kusumoto, S.; Noda, S.; Kochi,

T.; Chung, L. W.; Morokuma, K. J. Am. Chem. Soc. 2010, 132, 16030−

16042.

note that 1-¹³CH₃-L does not form ¹³CH₄or ¹³CH₃D in rigorously dry

(sodium−potassium alloy) thf-d₈up to 378 K for >5 h and that the same

thf-d₈batch was used for these ethylene oligomerizations in J. Young

tubes.

(5) Ito, S.; Munakata, K.; Nakamura, A.; Nozaki, K. J. Am. Chem. Soc.

2009, 131, 14606−14607.

(22) Carbon−phosphorus bond cleavage is not unprecedented:

(a) MacGregor, S. Chem. Soc. Rev. 2007, 36, 67−76. (b) Garrou, P. E.

Chem. Rev. 1985, 85, 171−185. (c) Morita, D. K.; Stille, J. K.; Norton, R.

K. J. Am. Chem. Soc. 1995, 117, 8576−8581. (d) Herrmann, W. A.;

(6) (a) Weng, W.; Shen, Z.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129,

15450−15451. (b) Shen, Z.; Jordan, R. F. Macromolecules 2010, 43,

8706−8708.

̈

Broßmer, C.; Ofele, K.; Beller, M.; Fischer, H. J. Organomet. Chem. 1995,

(7) Ito, S.; Kanazawa, M.; Munakata, K.; Kuoda, J.; Okumura, Y.;

Nozaki, K. J. Am. Chem. Soc. 2011, 133, 1232−1235.

491, C1−C4. (e) Wakioka, M.; Ozawa, F. Organometallics 2010, 29,

5570−5578.

(8) (a) Runzi, T.; Frohlich, D.; Mecking, S. J. Am. Chem. Soc. 2010, 132,

̈

(23) See, for example: (a) de Graaf, W.; Boersma, J.; Smeets, W. J. J.;

Spek, A. L.; van Koten, G. Organometallics 1989, 8, 2907−2917.

(b) Ledford, J.; Shultz, C. S.; Gates, D. P.; White, P. S.; DeSimone, J. M.;

Brookhart, M. Organometallics 2001, 20, 5266−5276.

17690−17691. (b) Daigle, J.-C.; Piche, L.; Claverie, J. P. J. P. Macromolecules

2011, 44, 1760−1762.

(9) (a) Skupov, K. M.; Piche, L.; Claverie, J. P. Macromolecules 2008,

41, 2390−2310. (b) Friedberger, T.; Wucher, P.; Mecking, S. J. Am.

Chem. Soc. 2012, 134, 1010−1018.

(24) See, for example: (a) Ref 19b. (b) Zhang, D.; Guironnet, D.;

Gottker-Schnetmann, I.; Mecking, S. Organometallics 2009, 28, 4072−

̈

(10) Guironnet, D.; Roesle, P.; Runzi, T.; Gottker-Schnetmann, I.;

̈

4078.

Mecking, S. J. Am. Chem. Soc. 2009, 131, 422−423.

(25) (a) Skupov, K. M.; Hobbs, J.; Marella, P.; Conner, D.; Golisz, S.;

Goodall, B. L.; Claverie, J. P. Macromolecules 2009, 42, 6953−6963. (b)

Ref 24b.

(26) The mechanistic diversity for the exchange of phosphorus- and

metal-bound carbyls has been reviewed: see ref 22a.

(27) A similar formation of 1-(CH₂)_2nCH₃-(lutidine) characterized by

¹H, ³¹P, and ¹H,¹H-gCOSY NMR experiments has already been

reported: ref 12a.

(11) A broad scope of polar monomers can be incorporated in ethylene

copolymers with cationic phosphine-phosphineoxide Pd(II) complexes:

Carrow, B. P.; Nozaki, K. J. Am. Chem. Soc. 2012, 134, 8802−8805.

(12) Ethylene homopolymerization: (a) Noda, S.; Nakamura, A.;

Kochi, T.; Wa Chung, L.; Morokuma, K.; Nozaki, K. J. Am. Chem. Soc.

2009, 131, 14088−14100. Acrylate copolymerizations and homo-

oligomerization: (b) Guironnet, D.; Caporaso, L.; Neuwald, B.;

Gottker-Schnetmann, I.; Cavallo, L.; Mecking, S. J. Am. Chem. Soc.

̈

(28) (a) Hristov, H. I.; DeKock, R. L.; Anderson, G. D. W.; Gottker-

̈

2010, 132, 4418−4426.

Schnetmann, I.; Mecking, S.; Ziegler, T. Inorg. Chem. 2005, 44, 7806−

(13) The ratio, e.g., of [1-¹³CH₃-dmso]:[ethylene], in these

experiments is ca. 4−12 times higher than in comparable ethylene

polymerizations catalyzed by 1-¹²CH₃-dmso at an identical ethylene

pressure of 5 bar; cf. ref 10.

7818. (b) DeKock, R. L.; Hristov, I. H.; Anderson, G. D. W.; Gottker-

̈

Schnetmann, I.; Mecking, S.; Ziegler, T. Organometallics 2005, 24,

2679−2687.

(29) An unidentiﬁed chain coupling reaction of palladium carbyl

complexes has been proposed to account for the increased

polydispersity due to a bimodal distribution in living ethylene

polymerization by palladium diimine complexes. Cleavage of palladium

polymeryl species by triethylsilane after polymerization yields fully

saturated polymer chains and avoids bimodal molecular weight

distribution, thus narrowing polydispersities: Gottfried, A. C.;

Brookhart, M. Macromolecules 2001, 34, 1140−1142.

(14) On exempliﬁed polyethylene samples we have determined T₁-

relaxation times in the range of 1−1.5 s for all aliphatic carbon atoms.

Based on these T₁-values, quantitative ¹³C NMR analyses were

conducted with a relaxation delay of 7 s using inverse gated decoupling

techniques.

(15) We note that at lower [precatalyst], e.g., [1-¹³CH₃-L] = 20−50

μmol L⁻¹, under saturation kinetic behavior (>10 bar ethylene for 1-

CH₃-dmso and > 15 bar for 1-CH₃-pyr), negligible catalyst deactivation

is observed over 30 and 60 min polymerization experiments by mass

ﬂow monitoring, resulting in nearly polymerization time-independent

turnover numbers; cf. ref 10.

(30) Complete inactivity of [κ²-(anisyl)₂P,O)]₂Pd (11) in ethylene

polymerizations was also proven on a preparative scale with 10 μmol of

11 under 5 bar ethylene pressure, 60 min polymerization time, in

toluene at 363 K.

(16) Note that the respective ¹³C NMR experiments do not allow for

an accurate quantiﬁcation, since the relaxation delay was set to 1 s, and

magnetization transfer from ¹H was not suppressed during the relaxation

delay.

(31) X-ray diﬀraction analyses of ﬁve diﬀerent palladium hydride

complexes with a trans-disposition of oxygen and hydride at the

palladium center have been deposited in the Cambrigde Crystallo-

graphic Database. In all these analyses the hydride was located in the

electron density map and the palladium hydride bond distances range

from 1.46 Å for electron-deﬁcient oxygen donors (pentaﬂuoropheno-

late) to 1.57 Å (phenolate, H₂O, arene carboxylate): (a) Braga, D.;

Sabatino, P.; Di Bugno, C.; Leoni, P.; Pasquali, M. J. Organomet. Chem.

1987, 334, C46−C48. (b) Di Bugno, C.; Pasquali, M.; Sabatino, P.;

Braga, D. Inorg. Chem. 1989, 28, 1390−1394. (c) Leoni, P.; Sommovigo,

M.; Pasquali, M.; Midollini, S.; Braga, D.; Sabatino, P. Organometallics

1991, 10, 1038−1044. (d) Konnick, M. M.; Gandhi, B. A.; Guzei, I. A.;

Stahl, S. S. Angew. Chem., Int. Ed. 2006, 45, 2904−2907. (e) Konnick, M.

M.; Decharin, N.; Popp, B. V.; Stahl, S. S. Chem. Sci. 2011, 2, 326−330.

(32) The parent complex bis[(κ²-P,O)-diphenylphosphine benzene-

sulfonato]palladium has been analyzed by X-ray crystallography: (a)

CCDC no. 246480: Schultz, T.; Pfaltz, A. Synthesis 2005, 1005−1011.

(b) CCDC no. 274818: Hearley, A. K.; Nowack, R. J.; Rieger, B.

Organometallics 2005, 24, 2755−2763.

(17) ¹³C,¹³C-J-resolved NMR experiments and INADEQUATE

experiments did not support the existence of measurable quantities of

¹³C−¹³C connections for these signals under discussion, while a

¹³C−¹³C connection was established for the S1−S2 end group couple of

formed oligomers.

(18) β-Hydride elimination after such 1,2-insertion of 1-oleﬁns would

yield vinylidene end groups, which were indeed observed to form by ¹H

NMR experiments in the late stage of the oligomerization, i.e., when

most of the added ethylene was consumed.

(19) (a) Kanazawa, M.; Ito, S.; Nozaki, K. Organometallics 2011, 30,

6049−6052. (b) Vela, J.; Lief, G. R.; Shen, Z.; Jordan, R. F.

Organometallics 2007, 26, 6624−6635.

(20) The ¹³C-labeled S1 end group of P-capped polymer chains is likely

indistinguishable from S1 end groups in pure hydrocarbon polymer

chains by ¹³C NMR spectroscopy and qualiﬁes as “polymer” despite the

P-capping. However, short-chain alkylphosphine species, e.g., methyl-

and propylphosphine species, containing ¹³CH₃-label are expected to

exhibit diﬀerent chemical ¹³C NMR shifts as compared to S1 end groups

and do not qualify as “polymer”. Consequently, some ¹³C-label will not

at all be incorporated into the “polymer”.

(33) The NMR yield is likely underestimated, as some of the formed

methane and H₂will be present in the headspace of the NMR tube.

(34) Note that for related anilidotropone nickel polymerization

catalysts reductive elimination of anilinotropone from nickel hydride

intermediates has been convincingly proposed to account for the

formation of analogous, polymerization-inactive [N,O]₂Ni complexes:

Jenkins, J. C.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 5827−5842.

(21) While formation of ⁽¹³⁾CH₄may in principle result from

hydrolysis with adventious traces of water in the reaction medium, we

8405

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Organometallics

Article

(35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,

M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;

Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;

Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;

Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,

T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;

Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,

K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;

Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.;

Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.;

Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;

Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.;

Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich,

and 1-CH₃-(κ-S-dmso), we prefer to present the calculation starting

from 1-CH₃-(κ-S-dmso).

(55) For comparison see also: Ziegler, T.; Cheng, W.; Baerends, E. J.;

Ravenek, W. Inorg. Chem. 1988, 27, 3458−3464.

(56) Blanksby, S. J.; Ellison, G. B. Acc. Chem. Res. 2003, 36, 255−263.

(57) The diﬀerent behavior of 1-CH₂CH₃-PPh₃and 1-CH₂CH₃-

P^tBu₃in the reaction with the protonated ligand [P,O]H is due to a

higher dissociation energy of the PPh₃vs the P^tBu₃ligand in 1-CH₂CH₃-

PPh₃and 1-CH₂CH₃-P^tBu₃. Steric interactions between the more bulky

P^tBu₃and the ethyl group are responsible for the weaker coordination of

P^tBu₃in 1-CH₂CH₃-P^tBu₃as compared to PPh₃in 1-CH₂CH₃-PPh₃.

(58) Trinquier, G.; Hoffmann, R. Organometallics 1984, 3, 370−380.

(59) (a) Krishnamurthy, S.; Brown, H. C. J. Org. Chem. 1983, 48,

3085−3091. (b) Krishnamurthy, S.; Brown, H. C. J. Org. Chem. 1982,

47, 276−280. (c) Newcomb, M.; Curran, D. P. Acc. Chem. Res. 1988, 21,

206−214.

̈

S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.;

Fox, D. J. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT,

2009.

́

(60) (a) Cohen, A. J.; Mori-Sanchez, P.; Yang, W. Science 2008, 321,

792−794. (b) Lundberg, M.; Siegbahn, P. E. M. J. Chem. Phys. 2005, 122,

224103−224109.

(61) (a) Ito, S.; Manukata, K.; Nakamura, A.; Nozaki, K. J. Am. Chem.

Soc. 2009, 131, 14606−14607. (b) Piche, L.; Daigle, J.-C.; Rehse, G.;

Claverie, J. P. Chem.Eur. J. 2012, 18, 3277−3285.

(36) Becke, A. D. Phys. Rev. A 1988, 38, 3098−3100.

(37) Perdew, J. P. Phys. Rev. B 1986, 34, 7406−7406.

(38) Perdew, J. P. Phys. Rev. B 1986, 33, 8822−8824.

(39) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry;

Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, pp 1−28.

(b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.

(40) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066−4073.

(41) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211−

7218.

(42) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985,

83, 735−746.

(43) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736−1740.

(44) Carpenter, J. E. Extension of Lewis structure concepts to open-

shell and excited-state molecular species, Ph.D. thesis, University of

Wisconsin, Madison, WI, 1987.

(45) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (THEOCHEM) 1988,

46, 41−62.

(46) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,

899−926.

(47) Weinhold, F.; Carpenter, J. E In The Structure of Small Molecules

and Ions; Naaman, R. Vager, Z., Eds.; Plenum: New York, 1988; pp 227−

36.

(48) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105,

2999−3094.

(49) For some crystallographically determined P−H distances

(isotropically reﬁned) in R₃P⁺H, see, for example: Dureen, M. A.;

Brown, C. C.; Stephan, D. W. Organometallics 2010, 29, 6594−6607.

(50) For experimentally determined M−C bond strengths:

(a) Bennett, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1997, 119,

10696−10719. (b) Wick, D. D.; Jones, W. D. Organometallics 1999, 18,

495−505.

For calculated M−C bond strengths experimentally

ret, C.; Eisenstein,

determined in refs (a) and (b): (c) Clot, E.; Meg

́

O.; Perutz, R. N. J. Am. Chem. Soc. 2006, 128, 8350−8357.

(51) This description holds true considering that a ca. −40 kJ mol⁻¹

entropic contribution to the formation of Me[P,O] + Pd(P^tBu₃) from

one molecule 1-CH₃-P^tBu₃makes Me[P,O] + Pd(P^tBu₃) ca. 60 kJ

mol⁻¹less stable than 1-CH₃-P^tBu₃, and ca. 110 kJ mol⁻¹less stable than

19-anisyl-P^tBu₃.

(52) The reaction between 1-H-P^tBu₃and the zwitterionic H[P,O]

ligand leads to an intermediate substitution product with the oxygen

atom of H[P,O] coordinated to Pd that is about 40 kJ mol⁻¹higher in

energy than 1-H-[P,O]H. The related TS is about 60 kJ mol⁻¹less

favored than TS 1-H-TS2.

(53) Heinekey, D. M.; Oldham, W. J., Jr. Chem. Rev. 1993, 93, 913−

926.

(54) We note that we also calculated the energetic proﬁle for the

reaction of 1-CH₃-(κ-O-dmso) with 1-H-P^tBu₃. The energy diﬀerence

between the two coordination modes (κ-O-dmso and κ-S-dmso) is only

4 kJ mol⁻¹in favor of κ-O-dmso coordination. However, due to the

experimentally determined κ-S-dmso coordination mode (X-ray

diﬀraction analysis) in the related [di(2′-methoxybiphen-2-yl)phos-

phine]benzensulfonato palladium methyl dmso complex (ref 10) and

the only small diﬀerence in ground-state energies of 1-CH₃-(κ-O-dmso)

8406

dx.doi.org/10.1021/om300969d | Organometallics 2012, 31, 8388−8406

Article Doi

Activation and deactivation of neutral palladium(II) phosphinesulfonato polymerization catalysts

DOI: 10.1021/om300969d

Source and publish data:

Authors:

Article abstract of DOI:10.1021/om300969d

Full text of DOI:10.1021/om300969d

Products guided by the article

R&D Labs maybe for 859982-19-7

Relevant to this article

Hot Product