Palladium(II) β-agostic alkyl cations and alkyl ethylene complexes: Investigation of polymer chain isomerization mechanisms

J. Am. Chem. Soc. 2001, 123, 11539-11555

11539

Palladium(II) â-Agostic Alkyl Cations and Alkyl Ethylene

Complexes: Investigation of Polymer Chain Isomerization

Mechanisms

Leigh H. Shultz, Daniel J. Tempel,^†and Maurice Brookhart*

Contribution from the Department of Chemistry, UniVersity of North Carolina at Chapel Hill,

CB 3290 Venable Hall, Chapel Hill, North Carolina 27599-3290

ReceiVed April 26, 2001

Abstract: A series of stable dialkyl complexes of Pd, (R-diimine)PdR₂(R-diimine ) aryl-substituted diimine,

R ) n-Pr, n-Bu, i-Bu), have been prepared via Grignard alkylation of the corresponding (R-diimine)PdCl₂

complexes. Protonation of these dialkyl species at low temperature results in loss of alkane and formation of

cationic Pd â-agostic alkyl complexes, which have been observed as intermediates in the polymerization of

ethylene and propylene by these Pd catalysts. Studies of the structure and dynamic behavior of these alkyl

complexes are presented, along with the results of trapping reactions of these species with ligands such as

NCMe, CO, and C₂H₄. Trapping with ethylene results in formation of cationic alkyl ethylene complexes which

model the catalyst resting state in these systems. These complexes have been used to obtain mechanistic details

and kinetic parameters of several processes, including isomerization of the alkyl ethylene complexes, associative

and dissociative exchange with free ethylene, and migratory insertion rates of both primary and secondary

alkyl ethylene species. These studies indicate that the overall branching observed in polyethylenes produced

by these Pd catalysts is governed both by the kinetics of migratory insertion and by the equilibria involving

the alkyl ethylene complexes.

Introduction

Unique features of these diimine systems include (1) the

ability to convert ethylene to high polymers with microstructures

varying from nearly linear (semicrystalline) to hyperbranched

(amorphous) with control over the degree of branching and

While several Ni(II)- and Co(III)-based complexes have been

previously shown to function as ethylene polymerization

catalysts,^1-8the report⁹from these laboratories in 1995 of highly

versatile Ni(II)- and Pd(II)-R-diimine-based catalysts of type

1 (Figure 1) has led to a resurgence of interest in development

of late metal catalysts for olefin polymerizations and copolym-

erizations.^6-8,10-32

(14) Killian, C. M.; Tempel, D. J.; Johnson, L. K.; Brookhart, M. J.

Am. Chem. Soc. 1996, 118, 11664-11665.

(15) Killian, C. M.; Johnson, L. K.; Brookhart, M. Organometallics 1997,

16, 2005-2007.

(16) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem.

Soc. 1998, 120, 888-899.

^†Current address: Air Products and Chemicals, Inc., Allentown, PA

18195-1501.

(1) Keim, W.; Appel, R.; Storeck, A.; Kruger, C.; Goddard, R. Angew.

Chem., Int. Ed. Engl. 1981, 20, 116-117.

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1001-1003.

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Transition Metal Organometallic Catalysis Olefin Polymerization [Proceed-

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Springer: Berlin, 1988; pp 349-360.

(4) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123-134.

(5) Brookhart, M.; DeSimone, J. M.; Grant, B. E.; Tanner, M. J.

Macromolecules 1995, 28, 5378-5380.

(6) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,

1169-1203. This reference contains a comprehensive summary of the patent

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(17) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc.

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1999, 121, 10634-10635.

(20) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65-74.

(21) Tempel, D. J.; Brookhart, M. Organometallics 1998, 17, 2290-

2296.

(22) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart,

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Macromolecules 1998, 31, 6705-6707.

Published on Web 11/01/2001

11540 J. Am. Chem. Soc., Vol. 123, No. 47, 2001

Shultz et al.

Scheme 2. Equilibria in Propyl Agostic Species and Propyl

Ethylene Complexes

Figure 1. Ni(II)- and Pd(II)-based olefin polymerization catalysts.

Scheme 1. Proposed Mechanism for Propagation and Chain

Walking

intermediate via a series of â-hydride elimination/readdition

reactions,²as shown in Scheme 1.

(3) Loss of ethylene from the “trapped” alkyl ethylene species

is faster than migratory insertion of these species; i.e., 5 a 6 is

a reversible reaction.

(4) Primary alkyl agostic species are less stable than the

secondary agostic species, while the primary alkyl ethylene

complexes are more stable than the secondary alkyl olefin

complexes.²¹This latter trend was demonstrated with the simple

palladium propyl complexes shown in Scheme 2.

The overall free energy diagram constructed for Scheme 2 is

shown in Figure 2. We have established that the transition state

for conversion of 9b to 10b involves trapping of 8b and not

conversion of the secondary agostic complex 7b to the primary

agostic species 8b via â-elimination/readdition. Only an upper

barrier limit of ∆G^q< 10.7 kcal/mol for conversion of 7b to

8b (â-elimination) could be estimated. However, using a labeled

agostic ethyl complex, 11b, an accurate barrier for â-elimination

of ∆G^q) 7.1 kcal/mol (-55 °C) has been determined (Scheme

3).¹⁸

While the propyl complexes shown in Scheme 2 have

provided significant insight into the mechanism of polymeri-

zation, the three-carbon chain is not a particularly good model

for polymerization intermediates where agostic alkylpalladium

complexes may bear alkyl substituents at both C_Rand C_â. As a

more realistic model, we report here the generation, structure,

chemistry, and dynamics of (R-diimine)Pd butyl complexes and

their corresponding ethylene complexes. These investigations

illuminate many new mechanistic features of the polymerization

reaction and provide a much more detailed description of the

polymer architecture through ligand design and reaction vari-

ables,^9,12,23,26(2) polymerization of R-olefins and internal olefins

to polymers with unusual branching characteristics due to

monomer insertion followed by migration of the metal to the

terminal carbon of the chain prior to the next insertion,^9,28,33

(3) copolymerization of ethylene and R-olefins with alkyl

acrylates to produce functionalized olefins with ester groups

residing primarily on the ends of branches,^13,16and (4) achieve-

ment of living polymerizations for both the Ni(II) and Pd(II)

systems.^14,34

While mechanistic investigations of both Ni(II)¹⁹and

Pd(II)^9,18,22diimine systems have been reported, the less reactive

Pd(II) systems have thus far provided the most detailed

information. The general mechanism for chain propagation of

ethylene by a diimine palladium catalyst is shown in Scheme

1.

The following features have been clearly established through

low-temperature NMR studies:

(1)The catalyst resting state is the alkyl ethylene complex.

The turnover-limiting step is the migratory insertion of this

species, where ∆G^qfor insertion ranges from 17.5 to 18.8 kcal/

mol and decreases with ligand bulk.

(2) Following insertion, the cationic palladium intermediate

formed is a highly dynamic â-agostic alkyl complex.^21,35,36

Formation of branches results from “chain-walking” in this

(31) Pappalardo, D.; Mazzeo, M.; Pellechia, C. Macromol. Rapid

Commun. 1997, 18 (12), 1017.

(32) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.;

Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460-462.

(33) Leatherman, M. D.; Brookhart, M. Macromolecules 2001, 34, 2748-

2750.

(34) Gottfried, A. C.; Brookhart, M. Macromolecules 2001, 34, 1140-

1142.

(35) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250,

395-408.

(36) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem.

1988, 36, 1-124.

Figure 2. Free energy profile for the isomerization of propyl ethylene

complex 9b.

Pd(II) Alkyl Cations and Alkyl Ethylene Complexes

J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11541

Scheme 3. Isomerization of the Agostic Ethyl Cation 11b

Scheme 4. Thermal Decomposition of Palladium(II) Dibutyl

Complex 14b

it is likely that they are rapidly isomerized by a Pd species to

the more stable internal alkenes. These results are consistent

with the products of decomposition seen from thermolysis of

(2,2′-bipyridine)PdEt₂, an electronically similar d⁸system with

accessible â-hydrogens.^39,40

Generation of Agostic Alkyl Species. (r-Diimine)Pd-

(propyl)⁺Complexes. Treatment of the di-n-propyl complexes

13a,b with H(OEt₂)₂BAr′₄(Ar′ ) 3,5-(CF₃)₂C₆H₃)⁴¹in CDCl₂F

at -80 °C results in formation of clear orange solutions; propane

is evident in the ¹H NMR spectra. The products of these

protonations were identified as the agostic isopropyl complexes

7a,b by comparison of their spectral data with that published

for the identical complexes made by insertion of ethylene into

the Pd methyl cation (eq 2).^21,22

way in which branching is controlled by a combination of the

kinetics for migratory insertion and the relative stabilities of

primary and secondary alkyl olefin complexes. These findings

are in good general agreement with the results of DFT

calculations by Ziegler et al.^37,38

Results

Synthesis of (r-Diimine)palladium(II) Dialkyl Species.

Synthesis of the di-n-propyl (13a,b), di-n-butyl (14a,b), and

diisobutyl (15a,b) complexes was accomplished by addition of

2 equiv of the corresponding Grignard reagent (n-PrMgCl,

n-BuMgCl, or i-BuMgCl in Et₂O) to a cold slurry of the (R-

diimine)PdCl₂complex (12a or 12b) in Et₂O (eq 1).

The dialkyl complexes were isolated as red-brown solids in

moderate yields after filtration of the reaction mixture through

a column of Florisil under argon. H NMR spectra of 13a,b-

1

The triplet observed at -8.00 ppm at -120 °C in the H

NMR spectrum exhibits a ²J_HH) 17 Hz (7b), diagnostic of an

agostic alkyl species.^35,36No doublet, corresponding to the

n-propyl species, is observed, as was the case with the propyl

complexes observed from insertion of ethylene into the Pd

methyl cation. Complex 7a shows similar behavior; the agostic

hydrogen resonates as a triplet (²J_HH) 16 Hz) at -7.85 ppm

at -120 °C.

Generation of (r-Diimine)Pd(butyl)⁺Complexes via In-

sertion. In the same way that a single insertion of ethylene into

a Pd-methyl bond was used to generate Pd-propyl com-

plexes,^21,22low-temperature preparation of the methyl propylene

complex 16b followed by warming to -13 °C to induce

insertion yields Pd-butyl complexes. As shown in Scheme 5,

insertion can occur in a 1,2 or 2,1 manner, where complexes

15a,b in CD₂Cl₂at ambient temperature indicate that they

possess two planes of symmetry, as expected: one containing

the square plane of the metal center and one bisecting the two

cis alkyl moieties. All of the species show one set of acenaphthyl

ortho protons between 6 and 7 ppm, a sensitive indicator of

symmetry. The two alkyl groups on the Pd center are equivalent,

and the Pd center shields the R-CH₂protons, often moving them

upfield of the â-CH₂(e.g. the â-CH₂in 13a resonates at 1.18

ppm, while the R-CH₂signal appears at 0.89 ppm). The dialkyl

complexes are surprisingly stable in solution but do decompose

after days at room temperature to give alkanes, alkenes, and

unidentified Pd(0) species due to â-H elimination followed by

loss of olefin and reductive elimination; the di-n-butyl complexes

14a,b, for example, decompose to give butane and internal

butenes (Scheme 4).

Although 1-butenes should be the initial product of â-H

elimination from a dibutyl species, they are not observed, and

(39) Sustmann, R.; Lau, J. Chem. Ber. 1986, 119, 2531-2541.

(40) Sustmann, R.; Lau, J.; Zipp, M. Recl. TraV. Chim. Pays-Bas 1986,

105, 356-359.

(41) Brookhart, M.; Grant, B.; Volpe Jr., A. F. Organometallics 1992,

11, 3920-3922.

(37) Michalak, A.; Ziegler, T. Organometallics 1999, 18, 3998-4004.

(38) Michalak, A.; Ziegler, T. Organometallics 2000, 19, 1850-1858.

11542 J. Am. Chem. Soc., Vol. 123, No. 47, 2001

Shultz et al.

Scheme 5. Formation of Agostic Butyl Complexes via

Insertion of Propylene into a Pd-Methyl Bond

Scheme 6. Trapping of Agostic tert-Butyl Complexes 18a,b

with NCMe

17b (1,2 insertion) or 19b (2,1 insertion) are the initial products.

Although a complex mixture of isomers is formed, the Pd-

tert-butyl complex, 18b (formed from isomerization of the initial

1,2 insertion product, 17b), clearly constitutes 80% of the

mixture. In the static ¹H NMR spectrum (-110 °C in CDCl₂F),

the agostic proton appears as a well-resolved triplet at -7.12

ppm (²J_HH) 15 Hz). Although the â-CH₂is obscured, the two

nonagostic methyl groups are equivalent and appear as a broad

6H singlet at 0.60 ppm. As the sample is warmed, the agostic

H signal broadens due to the rotation of the agostic methyl group

(C_R-C_ârotation), while the methyl signals at 0.60 ppm also

broaden due to rotation about the Pd-C_Rbond, which inter-

changes the agostic and nonagostic methyl groups. At 0 °C,

the three methyl groups are now exchanging rapidly on the NMR

time scale and appear as an averaged 9 H singlet at -0.20 ppm.

The ligand peaks remain sharp (for example, two well-resolved

¹H NMR signals for 18b generated by protonation are

identical to those observed from insertion of propylene. At -110

°C, the agostic hydrogen of 18a resonates as a triplet (²J_HH

)

15 Hz) at -7.03 ppm, with the nonagostic methyl groups

appearing at 0.56 ppm. The variable-temperature behavior of

18a is similar to that previously described for 18b. Overall,

signals for 18a,b are sharp and well-resolved at -110 °C, and

no agostic isobutyl species (<5%) can be detected. These

observations are consistent with the previous observation that

the â-agostic Pd-isopropyl complex is strongly favored over

the n-propyl complex.

1

ortho H signals for 19b appear at 6.33 and 6.63 ppm) and

indicate that there is no rapid side-to-side isomerization of the

tert-butyl alkyl group, consistent with the previously observed

behavior of the Pd-isopropyl complex.²¹The structural assign-

ments above have been verified by quantitative generation of

18b by an independent route (see below).

Trapping Reactions of 18a,b. Trapping the agostic tert-butyl

complexes (18a,b) with acetonitrile leads to formation of the

isobutyl acetonitrile complexes, 22a,b (Scheme 6).

No tert-butyl acetonitrile complex is observed, though, in

trapping the agostic Pd-isopropyl complex, the isopropyl

acetonitrile complex is not only the kinetic product but is also

thermodynamically favored over the n-propyl acetonitrile

complex.²²It is unclear whether the isobutyl acetonitrile complex

is formed by trapping of the small (unobservable) amount of

agostic isobutyl complex in equilibrium with the agostic tert-

butyl complex or if its formation results from initial trapping

of the tert-butyl species and subsequent isomerization of the

tert-butyl acetonitrile complex to the trapped isobutyl species

via loss of acetonitrile, â-H elimination, reinsertion, and

retrapping with acetonitrile.

The minor, complex series of resonances from 2,1 insertion

are largely obscured by 18b and are difficult to assign in these

spectra. However, these isomers have been independently

generated through the protonation of (R-diimine)Pd((CH₂)₃CH₃)₂

complexes, and a full description of their structure and dynamics

is outlined below.

Generation of (r-Diimine)Pd(tert-butyl)⁺Complexes via

Protonation of (r-Diimine)Pd(CH₂CH(CH₃)₂)₂Complexes.

Protonation of the (R-diimine)Pd(diisobutyl) complexes (15a,b)

at -80 °C in CDCl₂F results in formation of the agostic tert-

butyl complexes 18a,b (eq 3).

In an attempt to trap the tert-butyl species, the smaller, more

Lewis basic ligand CO was used; ¹³CO was purged through a

solution of the tert-butyl complex 18a at -80 °C for 5 min.

(¹³CO was chosen for ease of product identification.) Use of

CO does result in trapping of the agostic tert-butyl complex as

the tert-butyl carbonyl complex 23a, though a small (ca. 15%)

amount of the isobutyl carbonyl species 24a is initially present

(Scheme 7).

1

At -90 °C in the H NMR spectrum, the acenaphthyl para

protons of 23a are inequivalent, as expected, appearing as

doublets at 8.08 and 8.06 ppm; likewise, the ortho hydrogens

appear at 6.78 and 6.23 ppm. The two aryl rings on the diimine

ligand are also inequivalent; the two sets of aryl methyl groups

appear as singlets at 2.27 and 2.16 ppm. The tert-butyl group

appears as a 9 H singlet at 0.99 ppm. The bound ¹³CO resonates

at 180.9 ppm in the ¹³C NMR spectrum and is quite broad (ca.

60 Hz at half-height), possibly due to exchange with free ¹³CO.

Pd(II) Alkyl Cations and Alkyl Ethylene Complexes

J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11543

Scheme 7. Trapping the Agostic tert-Butyl Complex 18a

with CO

Scheme 8. Formation and Interconversion of Pd(butyl)⁺

Complexes via Protonation

Warming the solution to -70 °C results in facile isomeriza-

tion of the tert-butyl carbonyl complex (23a) to the isobutyl

carbonyl complex 24a (eq 4). Although the acenaphthyl para

solution. A new isobutyl group appears in the alkyl region; a

sharp doublet at 0.56 ppm is indicative of equivalent isobutyl

methyl groups, while the methylene group appears as a triplet

1

at 2.64 ppm from coupling to both the methine H and ¹³C-

labeled acyl carbon (²J_CH) 6 Hz). 25a decomposes upon further

warming, with Pd(0) formation evident.

Formation of (r-Diimine)Pd(butyl)⁺Complexes from

Protonation of (r-Diimine)Pd((CH₂)₃CH₃)₂Complexes. Pro-

tonation of the di-n-butyl species 14a,b produces only linear

isomers and thus facilitates the identification of the products of

2,1-insertion of propylene into a Pd-methyl bond (see Scheme

5). The possible products, 21, 20, and 19, are shown in Scheme

8 together with their modes of interconversion.

Loss of n-butane via protonolysis will initially form the

agostic n-butyl complex 21. â-H elimination and reinsertion will

produce the agostic sec-butyl complex 20, possessing an ethyl

substituent at C_R. The second sec-butyl isomer, 19, is produced

by simple release of the agostic interaction, rotation about the

Pd-C_Rbond, and agostic bond formation at C3. Species 21 is

expected to be a negligible component of the mixture on the

basis of the much greater stability of the agostic Pd-isopropyl

complex relative to the Pd-n-propyl complex.²²

protons are partially obscured by those of the tert-butyl carbonyl

complex 23a, the ortho protons of 24a are distinctly different,

appearing at 6.83 and 6.49 ppm and growing with time. The

methyl groups on the imine aryl rings at 2.31 and 2.19 are also

distinct from those of the tert-butyl carbonyl complex. The Pd-

isobutyl group exhibits a doublet at 2.01 ppm (methylene), a

multiplet at 1.56 ppm (methine), and another doublet at 0.75

ppm (two methyls). Bound ¹³CO appears as a sharp singlet at

175.0 ppm in the ¹³C NMR spectrum. Quantitative formation

of the isobutyl carbonyl complex 24a from the tert-butyl

carbonyl complex 23a is not observed, since insertion of ¹³CO

into the Pd-isobutyl bond occurs competitively at this temper-

ature (-70 °C), partially consuming the isobutyl carbonyl

complex and forming a Pd-acyl carbonyl complex, 25a. Only

one set of acenaphthyl ligand peaks appear (two doublets at

8.12 and 8.09 ppm for the para protons and two doublets at

6.82 and 6.57 ppm for the ortho protons), indicating one major

product. This species has a resonance in the ¹³C NMR spectrum

at 210.7 ppm, consistent with formation of an acyl species via

insertion of ¹³CO.⁴²A broad peak is observed at 172.2 ppm,

consistent with bound ¹³CO exchanging with free ¹³CO in

Treatment of 14b with H(OEt₂)₂BAr′₄(CDCl₂F, -110 °C)

results in formation of a mixture of â-agostic butyl isomers

1

whose H NMR spectrum in the high-field region is shown in

Figure 3.

Kinetic trapping of these species by ethylene yields only

(R-diimine)Pd(CH(CH₃)(CH₂CH₃))(CH₂dCH₂)⁺complexes, con-

firming that 21b is not present in appreciable amounts in this

mixture. The broad triplet at -8.01 ppm, which sharpens and

2

appears only below -100 °C and shows a typical J_HH) 16

Hz, can be assigned to the agostic sec-butyl species 20b, leaving

the two signals between -8.3 and -8.1 ppm which sharpen

below -80 °C as attributable to 19b.

(42) Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137-

1138.

11544 J. Am. Chem. Soc., Vol. 123, No. 47, 2001

Shultz et al.

Figure 3. High-field region of the ¹H NMR spectrum (CDCl₂F, -110

°C) resulting from protonation of 14b.

Figure 5. Variable-temperature ¹H NMR spectra of cyclopentyl agostic

cation 27a at -110 °C (top) and -80 °C (bottom).

changes H_band H_cand results in the appearance of H_aas a

triplet with an averaged J value of J ) (J_ac+ J_ab)/2 ) ca. (16

Hz + 0 Hz)/2 ) ca. 8 Hz.

Integration of the triplet at -8.01 ppm (20b) versus the two

resonances upfield due to 19b-trans and 19b-cis yields the

equilibrium constant for the species, K_eq) 8.9, corresponding

to a free energy difference of -0.7 kcal/mol (eq 5).

Figure 4. Cis and trans isomers of 19a,b.

Scheme 9. Dynamic Processes in 19b-cis That Equilibrate

H_band H_c

In an attempt to synthesize an agostic complex restricted to

a cis conformation to compare to 19b-cis, we were able to

prepare and protonate a crude dicyclopentyl species 26a (eq 6)

to produce an agostic cyclopentyl complex, 27a.

Despite several attempts, we were unable to obtain a

completely pure sample of either the dicyclopentyl complex 26a

or the cyclopentyl agostic species 27a; however, variable-

temperature ¹H NMR analysis of the impure cyclopentyl

complex 27a provides strong support for the proposed structure

of 19b-cis, so we present these data. At -110 °C in CDCl₂F,

the cyclopentyl agostic proton appears as a doublet at -8.04

ppm, ²J_HH) 16 Hz.³⁰Upon warming of the sample to -80 °C,

however, the resonance collapses to a triplet, with a coupling

Since there is a significant barrier to rotation about the C_R-C_â

bond^18,21and because C_Rand C_âeach possess a methyl

substituent, cis and trans rotational isomers are expected to be

observable, as shown in Figure 4 (each isomer exists as a pair

of enantiomers). The cis isomer yields a cis-2-butene hydride

complex upon â-elimination, while the trans isomer yields a

trans-2-butene hydride complex, as shown in Figure 4.

2

constant J_HH∼ 8 Hz (Figure 5).

The cyclopentyl complex is constrained in a cis configuration,

making it quite similar to the cis sec-butyl rotamer 20b-cis

(Figure 6).

We assign the doublet at -8.19 ppm, with a typical geminal

²J_HH) 16 Hz, to the static trans isomer, 19b-trans. The unusual

2

triplet at -8.27 ppm with a geminal J_HHof only ca. 7 Hz is

Thus, the similarity of the spectroscopic behavior of the sec-

butyl rotamer 19b-cis and the cyclopentyl agostic cation 27a,

which must be a cis species, support the structural assignment

of 19b-cis.

assigned to the fluxional cis isomer, 19b-cis. The agostic proton

from the cis rotamer appears as a triplet due to rapid â-H

elimination and reinsertion (Scheme 9). This process inter-

Pd(II) Alkyl Cations and Alkyl Ethylene Complexes

J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11545

Figure 6. Structures of the cis sec-butyl rotamer 19b-cis vs the

cyclopentyl agostic cation 27a.

in CD₂Cl₂at -80 °C show two inequivalent sides of the square

planar complex as expected; for example, two sets of methyl

groups (from the imine aryl rings) appear at 2.26 and 2.21 ppm.

Bound ethylene (rotating rapidly on the NMR time scale)

appears as a broad singlet at 4.60 ppm, upfield of free ethylene

(5.4 ppm), while the Pd-ethyl group exhibits a quartet at 1.34

ppm (methylene) and a triplet at 0.43 (methyl). The data for

30b[BAr′₄] are similar, indicating fast rotation of the bound

ethylene and a plane of symmetry containing the square plane

of the metal center.

Scheme 10. Formation of Ethyl Ethylene Complexes

30a,b[BAr′₄]

Mechanism of Alkyl Ethylene Isomerization. When the Pd

isopropyl ethylene complex 9b is warmed, isomerization to the

n-propyl ethylene complex 10b occurs with a rate independent

of [ethylene] at moderate ethylene concentrations. Though we

believed this to be a process involving dissociation of ethylene,

the lack of rate suppression by ethylene led us to investigate

this general mechanism of isomerization further. In the case of

the ethyl ethylene complexes, the isomerization is a degenerate

one. Two mechanisms are possible, which are illustrated for a

¹³C-labeled complex in Scheme 11.

Protonation of the di-n-butyl complex 14a gives similar

results; the cis and trans rotational isomers are again observed,

but none of the sec-butyl isomer 20a, having an ethyl group on

C_R, is present in the product mixture. The butyl complexes which

are the products of protonation of 14a,b (28a,b) can be obtained

on a preparatory scale and give satisfactory elemental analyses.

The static and dynamic ¹H NMR spectra of these cationic

complexes are identical to those observed from in situ proton-

ation. These isolated agostic butyl complexes can be used to

generate butyl ethylene species as discussed below and will

polymerize ethylene in an fashion identical with that of the Pd

methyl ethylene complexes ((R-diimine)Pd(CH₃)(CH₂dCH₂)⁺)

previously published.^9,22

Scheme 11. Possible Mechanisms for Alkyl Ethylene

Isomerization

Generation and Chemistry of (r-Diimine)palladium(II)

Alkyl Ethylene Complexes. (r-Diimine)Pd(CH₂CH₃)(CH₂d

CH₂)⁺Complexes. Addition of 1 or more equiv of ethylene to

either the ethyl agostic cation [((2,6-(i-Pr)₂C₆H₃)NdC(An)C-

(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(CH₂CH₂-µ-H)]BAr′₄(11b) or to

the ethyl ether complexes, [((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)d

N(2,6-(CH₃)₂C₆H₃))Pd(CH₂CH₃)(OEt₂)]BAr′₄(29a) and [((2,6-

(i-Pr)₂C₆H₃)NdC(An)C(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(CH₂CH₃)-

(OEt₂)]BAr′₄(29b), results in trapping of 11b or displacement

of the diethyl ether ligand, respectively, and quantitative

formation of the ethyl ethylene cations 30a,b[BAr′₄] (Scheme

10).¹⁸

Alternatively, the neutral diethyl complexes 31a,b can be

treated with B(C₆F₅)₃, a powerful Lewis acid known to abstract

methide from transition metal methyl complexes.⁴³Like Ph₃C⁺,⁴⁴

B(C₆F₅)₃abstracts hydride from C_âof the diethyl species 31a,b,

resulting in formation of the ethyl ethylene cations 30a,b[HB-

(C₆F₅)₃] (eq 7).⁴⁵

The first involves dissociation of ethylene to form the agostic

ethyl complex 11, which rapidly isomerizes before it is retrapped

by ethylene. The second involves â-H elimination from the 16-

electron ethylene complex, 30, to produce a 5-coordinate,

trigonal bipyramidal bis-olefin hydride species, 32. Such a

species could be the transition state; this process has been

proposed by Ziegler, on the basis of DFT calculations, to account

for chain transfer in the polymerization reactions of the Ni

analogues.⁴⁷Reinsertion of the olefin with opposite regiochem-

istry into the Pd-H bond would result in formation of the

isomerized ethyl ethylene cation. Clearly, insertion of the other

ethylene unit may also occur. We reasoned that if the actual

mechanism involved a 5-coordinate species (Scheme 11ii), we

should be able to isotopically label either the alkyl group or

the bound ethylene moiety, and some of the label should

eventually appear in the other moiety. Treatment of the singly

¹³C-labeled agostic ethyl species 11b-¹³C (generated from

The ¹H NMR spectra of 30a,b[BAr′₄] and 30a,b[HB(C₆F₅)₃]

are nearly identical (with the exception of the counterion

resonances), differing only slightly in chemical shift. For brevity,

only the spectra of 30a,b[BAr′₄] will be discussed here; the

reader is referred to the Experimental Section for the spectral

data of 30a,b[HB(C₆F₅)₃].⁴⁶The ligand resonances for 30a[BAr′₄]

(43) Chen, E. Y.; Marks, T. J. Chem. ReV. 2000, 100, 1391-1434 and

extensive references therein.

(44) For examples of â-H abstraction from transition metal alkyl

complexes with trityl cation, see: (a) Cousins, M.; Green, M. L. H. J. Chem.

Soc. 1963, 889-894 (b) Mandon, D.; Toupet, L.; Astruc, D. J. Am. Chem.

Soc. 1986, 108, 1320-1322 and ref 10.

(45) To our knowledge, this is the first example of â-H abstraction in a

transition metal alkyl using B(C₆F₅)₃. We have also observed this process

in similar Pd alkyl species such as (bpy)PdEt₂.

(46) The ^-HB(C₆F₅)₃anion is a good hydride donor above ca. -40 °C,

resulting in decomposition of the Pd complexes above this temperature.

(47) Deng, L.; Margl, P.; Ziegler, T. J. Am. Chem. Soc. 1997, 119, 1094-

1100.

11546 J. Am. Chem. Soc., Vol. 123, No. 47, 2001

Shultz et al.

Table 1. Rate Constants for Isomerization of 30b-¹³C_R[HB(C₆F₅)₃]

in the Presence of Varying Equivalents of Ethylene at -55 °C

[Pd complex], M

[C₂H₄], M

k (s^-1

)

0.014

0.015

0.014

0

3.64 × 10^-5

1.67 × 10^-5

1.44 × 10^-5

0.297

0.440

protonation of 31b-¹³C_R) with unlabeled ethylene in CD₂Cl₂at

-80 °C affords the ethyl ethylene cation 30b-¹³C[BAr′₄], having

a

¹³C label only on the ethyl moiety. Upon warming of this

species to -55 °C and acquisition of successive ¹³C{¹H} NMR

spectra over hours, no ¹³C label is observed in the bound

ethylene; this observation supports the proposal that isomer-

ization occurs via dissociation of ethylene (eq 8).²²

Associative Exchange Rates. With evidence in hand that

ethylene dissociates from the metal center during alkyl olefin

isomerization, we wished to reexamine the mechanism of

ethylene exchange with these ethyl ethylene cations. We have

previously used the corresponding methyl ethylene complexes

to obtain associative exchange rate constants in the presence of

excess ethylene; steric bulk on the ligand aryl rings slows

associative exchange.²²In the case of a higher alkyl, however,

exchange of bound ethylene with free ethylene in solution could

possibly occur rapidly in a dissociative fashion (Scheme 12),

Scheme 12. Associative vs Dissociative Exchange with Free

Ethylene

The same ¹³C-labeled diethyl complex (31b-¹³C_R) used to

produce the labeled agostic ethyl complex 11b-¹³C above can

be treated with B(C₆F₅)₃to afford an ethyl ethylene complex,

30b-¹³C_R[HB(C₆F₅)₃], having one ¹³C label at the R-carbon of

the ethyl moiety and another ¹³C label in the bound ethylene

(eq 9).

as formation of the â-C-H agostic interaction may aid in

displacement of ethylene from the Pd center. This agostic

participation is not possible in the methyl ethylene complexes,

which do not have â-C-H bonds available.

The line width in the absence of ethylene exchange was

measured in CD₂Cl₂for the bound ethylene in 30a,b[HB-

(C₆F₅)₃]. Generation of these complexes in situ allowed easy

access to the ethyl ethylene complexes with no excess ethylene

in solution, since only 1 equiv of ethylene is generated per metal

center by hydride abstraction. Line widths at half-height (ω)

were then measured (in Hz) for the bound ethylene resonance

in the presence of added ethylene at -85 °C, and rate constants

for exchange were determined from the equation for the slow-

exchange approximation, k ) π(∆ω)/[C₂H₄], where [C₂H₄] is

the concentration of free ethylene in solution (determined by

integration against the ligand resonances) (Table 2). The bound

ethylene resonance is only distinct with less than ca. 5 equiv of

ethylene in solution, after which it merges with the free ethylene

resonance. The rate of ethylene exchange clearly increases as

the concentration of ethylene in solution increases, consistent

with associative exchange. Rate constants for exchange in the

Use of B(C₆F₅)₃to abstract hydride allows direct formation

of the ethyl ethylene complex before significant isomerization

can occur. Equilibration of the ¹³C label between C_Rand C_â

1

can then be followed by H NMR spectroscopy (eq 10).

Analysis of the kinetics yields k₁) k_-1) 3.6(0.1) × 10^-5

s^-1(-55 °C) and a barrier to dissociation of ethylene from the

ethyl ethylene cation of ∆G^q) 17.1 ( 0.1 kcal/mol. Addition

of large excesses of ethylene does slightly slow the rate of

isomerization in this case (Table 1).

Pd(II) Alkyl Cations and Alkyl Ethylene Complexes

J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11547

Table 2. Second-Order Rate Constants for Ethylene Exchange

at -85 °C

Scheme 13. Comparison of 1° vs 3˚ Butyl Migration Rates

Determined by GC

rate const

complex

(M^-1s^-1

)

complex

(M^-1s^-1

)

31a (Et)

33a (Me)

5570

2600

31b (Et)

33b (Me)

950

560

corresponding methyl ethylene complexes 33a,b are shown for

comparison.²²

Ethyl Ethylene Migratory Insertion Rates. The ethyl

ethylene complexes 30a,b are unique in this system in that they

are degenerate species; isomerization of the complex does occur

but produces an identical ethyl ethylene complex. We had

previously been unable to measure rates for migratory insertion

of ethylene into different types of Pd-C bonds due to rapid

equilibration of numerous alkyl isomers prior to insertion. Using

turnover frequencies, only subsequent insertion barriers were

obtained, which are average barriers for insertion into all types

of Pd-C bonds in the system.²²Using the ethyl ethylene

complexes, however, barriers for migratory insertion of ethylene

into only Pd-primary carbon bonds can be measured (eq 11).

may still be in equilibrium with a small amount of this 3° alkyl

ethylene species. To investigate whether any migratory insertion

of ethylene into the tert-butyl-Pd bond occurs, the isobutyl

ethylene complexes 35a,b were generated in situ on a prepara-

tory scale and allowed to undergo migratory insertion at -20

°C for 10 min (ca. 1 half-life) (Scheme 13).

The reaction was quenched by the addition of excess Et₃-

SiH, which cleaves the alkyl chain from the Pd center, liberating

the corresponding alkane. Thus, insertion of ethylene into the

Pd-isobutyl bond of 35a,b would produce 2-methylpentane;

insertion of ethylene into the Pd-tert-butyl bond of 34a,b would

produce 2,2-dimethylbutane. The volatile organics were vacuum

transferred away from the Pd residue and analyzed by GC. Only

2-methylpentane was observed. No 2,2-dimethylbutane was

detected (<5%) from either 34a or 34b, indicating that little to

no insertion occurs into 3° alkyl-Pd bonds in this system. This

observation is consistent with branching analyses for polyeth-

ylenes produced from (R-diimine)Pd catalysts.²³

The lack of insertion into Pd-tert-butyl bonds indicates that

all insertion in 35a,b occurs via the isobutyl ethylene species,

allowing quantification of insertion rates using NMR spectros-

copy. In the presence of 20 equiv of ethylene, monitoring the

loss of the isobutyl methyl groups in 35a,b with respect to time

yields k ) 6.5(0.1) × 10^-4s^-1at -17 °C for 35a, corresponding

to ∆G^q) 18.7 ( 0.1 kcal/mol, and k ) 8.5(0.2) × 10^-4s^-1at

-22 °C for 35b, corresponding to ∆G^q) 18.1 ( 0.1 kcal/mol.

In the presence of 20 equiv of ethylene, loss of the ethyl

resonances in the ¹H NMR spectra of 30a,b[BAr′₄] was

measured with respect to time, yielding k ) 3.31(0.05) × 10^-4

s^-1at -19 °C for 30a, corresponding to ∆G^q) 18.9 ( 0.1

kcal/mol, and k ) 2.77(0.06) × 10^-4s^-1at -24 °C for 30b,

corresponding to ∆G^q) 18.5 ( 0.1 kcal/mol.

(r-Diimine)Pd(CH(CH₃)(CH₂CH₃))(CH₂dCH₂)⁺and (r-

Diimine)Pd((CH₂)₃CH₃)(CH₂dCH₂)⁺Complexes. When eth-

ylene is added to CD₂Cl₂solutions of the agostic sec-butyl

complexes 28a,b at -78 °C, isomerization to the n-butyl

(r-Diimine)Pd(CH₂CH(CH₃)₂)(CH₂dCH₂)⁺Complexes.

When CD₂Cl₂solutions of the agostic tert-butyl complexes

18a,b were treated with 1 or more equiv or more of ethylene at

-80 °C, no tert-butyl ethylene complexes (34a,b) were observed

by ¹H NMR spectroscopy. Instead, only the isomerization

products, the isobutyl ethylene complexes 35a,b, are observed.

This is consistent with the results seen when acetonitrile, a

smaller ligand, is used to trap these species. The ligand

resonances for complex 35b indicate the expected asymmetry

at the metal center; two doublets appear for both the acenaphthyl

para protons (8.06 and 8.02 ppm) and the ortho protons (6.52

and 6.44 ppm). Bound ethylene resonates at 4.58 ppm as a

singlet, indicating rapid rotation at this temperature. The isobutyl

methylene hydrogens appear as a doublet (J ) 5.6 Hz) at 1.38

ppm, the methine hydrogen appears as a multiplet at 0.95 ppm,

and the two equivalent methyl groups appear as a sharp doublet

at 0.61 ppm. The spectrum of 35a is similar and is reported in

the Experimental Section.

1

ethylene cations is observed by H NMR spectroscopy. In an

attempt to observe the kinetic product of ethylene trapping,

ethylene was added via syringe to a CDCl₂F solution of 28b at

1

ca. -130 °C in a pentane/N₂bath. At -90 °C, the H NMR

spectrum of this sample reveals trapping and formation of the

sec-butyl ethylene complex, 36b (eq 12).

The acenaphthyl para protons of 36b appear as doublets at

8.01 and 7.97 ppm; the ortho protons also appear as doublets,

at 6.50 and 6.37 ppm. Bound ethylene is observed as second-

order multiplet centered at 4.64 ppm. This is likely due to the

chiral center at C_Rof the sec-butyl group, which, even in the

regime of rapid ethylene rotation, creates two distinct sets of

ethylene protons. The methine proton of the Pd-sec-butyl group

resonates at 2.05 ppm; one methyl group appears as a doublet

at 0.70 ppm, while the other appears as a multiplet at 0.36 ppm

(the methylene group is obscured). The absence of (R-diimine)-

Pd((CH₂)₃CH₃)(CH₂dCH₂)⁺, 37b, is further indication of the

Though no tert-butyl ethylene complex is observed by NMR

spectroscopy at low temperature, the isobutyl ethylene complex

11548 J. Am. Chem. Soc., Vol. 123, No. 47, 2001

Shultz et al.

Scheme 14. Comparison of 1˚ vs 2˚ Butyl Migration Rates,

As Determined by GC Experiments

absence of 21 in the equilibrium mixture of Pd-butyl complexes

(see Scheme 8). When 36b is warmed to -50 °C, the sec-butyl

ethylene complex isomerizes rapidly to the n-butyl ethylene

complex, 37b. The acenaphthyl para protons for this species

appear as doublets at 8.07 and 8.03 ppm; the ortho protons

appear at 6.59 and 6.55 ppm. Bound ethylene appears as a sharp

singlet at 4.68 ppm, while resonances for the n-butyl group

appear at 1.56 (R-CH₂), 0.97 (γ-CH₂), and 0.58 (CH₃); the

â-CH₂is obscured. Integration of the acenaphthyl ortho protons

of 37b with respect to those of the sec-butyl ethylene complex

36b yields an equilibrium constant K_eq) 125 at -50 °C

(eq 13).

Scheme 15. Comparison of 1˚ vs 2˚ Propyl Migration Rates,

as Determined by GC

rates of insertion of a second 1 equiv of ethylene. Indeed, the

n-hexane/3-methylpentane ratio remained invariant though

somewhat different amounts of octane were observed for

different runs. Clearly Curtin-Hammett conditions apply (equi-

librium is rapid, and insertion is rate determining),^48,49and

after extrapolation of the applicable equilibrium constants to

-20 °C (assuming ∆S ca. 0), ratios for k(2°)/k(1°) of ca. 2/1

for 36a/37a and ca. 11/1 for 36b/37b are obtained.

(r-Diimine)Pd(propyl)(CH₂dCH₂)⁺Complexes. The pro-

pyl ethylene complexes 9b and 10b were generated by addition

of ethylene to the isopropyl agostic cation 7b and have been

previously characterized.²²GC experiments identical to those

discussed above were done using an in-situ-generated mixture

of the isopropyl ethylene complex 9b and the n-propyl ethylene

complex 10b (Scheme 15).

The case is similar for the sec-butyl ethylene/n-butyl ethylene

complexes 36a/37a, except that K_eqfor this system is 50 at

-50 °C, reflecting decreased steric interaction in the sec-butyl

complex.

With the equilibrium constants for the sec-butyl/n-butyl

ethylene complexes in hand, GC experiments similar to those

done in the tert-butyl/isobutyl ethylene case were carried out

to determine the rate of insertion into 2° alkyl centers relative

to insertion into 1° alkyl centers. Mixtures of the sec-butyl

ethylene and n-butyl ethylene complexes were generated in situ

(preparatory scale) by addition of ca. 1 equiv of ethylene to the

isolated agostic salts 28a,b dissolved in CH₂Cl₂at -78 °C. The

mixtures were warmed to -20 °C for 10 min and then quenched

by addition of excess Et₃SiH. Insertion into the Pd-n-butyl bond

in 37 produces n-hexane after silane quenching, while insertion

into the Pd-sec-butyl bond in 36 produces 3-methylpentane.

Volatiles products were vacuum transferred and analyzed by

GC; the ratio of n-hexane to 3-methylpentane for 36a/37a was

9.3, while the ratio for 36b/37b was 6.7 (Scheme 14).

GC analysis also revealed a significant amount of n-butane,

indicating that not all of the butyl complexes had undergone

insertion. A negligible amount of n-octane was observed,

resulting from double ethylene insertion, indicating that the

n-hexane/3-methylpentane ratios were not skewed by differential

Analysis of the volatile products after quenching and vacuum

transfer revealed a ratio of n-pentane to 2-methylbutane of 10.7.

Using an extrapolated K_eqof 0.09 at -20 °C (K_eq) 0.05 at

-50 °C), the ratio of k(2°)/k(1°) is calculated to be ca. 1 for

9b/10b.

Discussion

Dynamics and Relative Stabilities of Agostic Pd(alkyl)⁺

Cations. The relative stabilities of agostic (R-diimine)Pd(butyl)⁺

complexes reported here support the trend initially noted for

(48) Seeman, J. I. Chem. ReV. 1983, 83, 83-134.

(49) Zefirov, N. S. Tetrahedron 1977, 33, 2719-2722.

Pd(II) Alkyl Cations and Alkyl Ethylene Complexes

J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11549

Scheme 16. Dynamic Processes for Agostic Ethyl

Complex 11b

the isopropyl and n-propyl complexes, namely, that alkyl

substitution is preferred at C_Rrelative to C_â. This unexpected

trend⁵⁰is most dramatic for the tert-butyl system, in which the

â-agostic Pd(tert-butyl)⁺isomers, 18a,b, are much more stable

than the Pd(isobutyl)⁺isomers, 17a,b. In the case of the linear

butyl isomers, the agostic Pd(n-butyl)⁺system, 21a,b, bearing

an ethyl substituent at C_â, is disfavored relative to sec-butyl

isomers 20a,b, which bear ethyl groups at C_R, a situation

analogous to the isopropyl versus n-propyl system. In this butyl

system, however, the cis and trans sec-butyl isomers, 19a,b-cis

and 19a,b-trans, bearing single methyl substituents at both C_R

and C_â, are favored. Since the C_R-C_âbond exhibits partial

double-bond character, the greater stability of these isomers

could be rationalized on the basis of analogy with relative alkene

stabilities (2-butenes are more stable than 1-butene). The

preference for substituents to reside at C_Rrelative to C_âappears

to be electronic in origin and has been addressed by Ziegler

using DFT calculations (see below).

Figure 7. C-C bond rotation in agostic ethyl (11b) and isopropyl

(7b) complexes.

Scheme 17. Isomerization of Pd(alkyl)⁺Complexes via â-H

Elimination and Bond Rotation

The fact that the sec-butyl isomers 19a,b are favored over

20a,b suggests that in the polymerization system the preferred

Pd(alkyl)⁺isomers will be those in which Pd resides in positions

such that both C_Rand C_âbear substituents.

step for this process is likely Pd-C bond rotation, as this barrier

was measured to be ca. 9.6 kcal/mol in the isopropyl system.

Thus, the barrier to Pd migration along the polymer chain

involves not only â-H elimination but also rotation around the

Pd-C_Rbond, which is the higher barrier.

The observation of a stable, thermodynamically favored

agostic tert-butyl complex is also of key significance. To migrate

back and forth through a branch point on the polymer backbone

and to migrate onto branches themselves, the Pd center must

be able to pass easily through 3° alkyl centers. The high stability

of the tert-butyl complexes (18a,b) shows that formation of a

Pd-tertiary alkyl species poses no barrier to migration through

a branch point in the polymer.

Equilibria in (r-Diimine)Pd(alkyl)(CH₂dCH₂)⁺Com-

plexes. The trend observed for the alkyl ethylene complexes is

opposite that seen in the agostic alkyl complexes: 1° alkyl

ethylene complexes are favored over 2° alkyl ethylene com-

plexes and 3° alkyl ethylene complexes (specifically, the Pd-

(tert-butyl)(CH₂dCH₂)⁺complex). This thermodynamic pref-

erence is likely steric in nature: the η²-bound olefin adds

significant steric bulk to the fourth coordination site relative to

the â-C-H agostic interaction and favors isomerization of the

alkyl group to the least sterically demanding species. This is

supported by the observation that the n-alkyl ethylene cations

are more heavily favored (n-butyl/sec-butyl ) 125 for 37b/36b

at -50 °C) when the imine aryl rings bear isopropyl groups

than when they bear less bulky methyl groups (n-butyl/sec-butyl

) 50 for 37a/36a at -50 °C).

The dynamics of the cationic alkyl complexes are also

important to consider due to their intermediacy in isomerization

of the alkyl ethylene complexes. Studies of the ethyl agostic

complex 11b indicate that the barrier to â-H elimination is 7.1

kcal/mol (∆G^q, -108 °C), while the barrier to rotation of the

C

_R-C_âbond is ca. 8.4 kcal/mol (Scheme 16).¹⁸

The barrier to Pd-C_Rrotation cannot be measured using this

complex but can be quantified in the isopropyl agostic cation

7b; measurement of the barrier to interconversion of the two

isopropyl methyl groups gives a Pd-C_Rbond rotation barrier

of 9.6 kcal/mol.²¹The barrier to rotation of the C_R-C_âbond in

this species is 9.2 kcal/mol, consistent with the fact that methyl

group rotation in 7b develops an eclipsing interaction between

a C-H bond of the rotating methyl group and the methyl

substituent on C_R(Figure 7).

The butyl complexes 19-21 serve to illustrate the barriers

involved in Pd migration along the backbone of a hydrocarbon

chain during polymerization (Scheme 17).

In order for Pd to walk from C1 to C2, â-H elimination and

reinsertion must occur, with a barrier of ca. 7-8 kcal/mol. (The

identity of the agostic hydrogen remains fixed throughout this

process.) For Pd to move to C3, however, the agostic interaction

must be released, Pd-C2 bond rotation must occur, and a new

agostic interaction must be formed; then â-H elimination and

reinsertion can occur, moving Pd to C3. The rate-determining

(50) Primary alkyl metal complexes are generally thought to be more

stable than secondary and tertiary ones; steric factors are regarded as

responsible for this trend. See: Collman, J. P.; Hegedus, L. S.; Norton, J.

R.; Finke, R. G. Principles and Applications of Organotransition Metal

Chemistry. University Science: Mill Valley, CA, 1987; p 99.

Secondary alkyl ethylene complexes such as the isopropyl

ethylene species 9b and the sec-butyl ethylene species 36a,b

can be observed as the kinetic products of trapping the

11550 J. Am. Chem. Soc., Vol. 123, No. 47, 2001

Shultz et al.

Table 3. Average Insertion Barriers versus 1˚ Alkyl Insertion

Barriers for Cationic (R-diimine)Pd Ethylene Polymerization

Catalysts

av insertion

(kcal/mol)

1° alkyl insertion

Pd complex

(kcal/mol)

30a

30b

35a

35b

18.6

18.0

18.6

18.0

18.9

18.5

18.7

18.1

Figure 8. Probable 5-coordinate transition state for associative

exchange of ethylene.

corresponding alkyl agostic cations with ethylene at low

temperature. Upon warming, these species isomerize to the

thermodynamically favored n-alkyl complexes. Experiments

with the ethyl ethylene cation 30b indicate that this isomerization

occurs via initial loss of ethylene, with a barrier of 17.1 kcal/

mol for ethylene dissociation. The overall isomerization barrier

for the isopropyl ethylene complex 9b has been measured as

15.3 kcal/mol (see Figure 2). This indicates that the barrier to

dissociation of ethylene from the isopropyl species is at a

minimum 1.8 kcal/mol lower than the barrier measured in the

ethyl ethylene case. This lower barrier can be ascribed to more

crowding in the square plane of the 2° alkyl ethylene species

relative to the 1° alkyl species. The barrier to ethylene

dissociation for the ethyl ethylene cation 30b can be compared

to the barrier to insertion of ethylene in the same species. The

migratory insertion barrier is 1.4 kcal/mol more than that for

dissociation of ethylene for the case of a 1° alkyl ethylene

species, which indicates that Pd can migrate across many

carbons on the polymer chain before undergoing an insertion

event.

Figure 9. Free energy diagram for insertion and isomerization in

36b/37b.

Associative Exchange Rates. Although alkyl olefin isomer-

ization occurs via loss of olefin, associative exchange rates

measured for the ethyl ethylene complexes 30a,b indicate that

bound ethylene undergoes associatiVe exchange with free

ethylene at rates much higher than rates for dissociative ethylene

loss (even at low ethylene concentrations). For example, at

[C₂H₄] ) 10 mM, the rate ratio of associative loss versus

dissociative loss is ca. 10⁸:1 at -85 °C. The associative

exchange rates measured for these species track well with those

measured for the analogous methyl ethylene complexes (Table

2) in that addition of bulk to the ortho positions on the imine

aryl rings slows associative exchange. Experiments with the

ethyl ethylene complexes indicate that addition of bulk in the

square plane of the complex (i.e. ethyl rather than methyl)

increases the rate of ethylene exchange. Associative exchange

of ethylene in these systems most likely occurs via a 5-coor-

dinate transition state in which the two olefin moieties are in

the axial sites (Figure 8).

ethylene cations 35a,b, barriers for migratory insertion of 1°

alkyl ethylene complexes were obtained and are also shown in

Table 3 for comparison.

The barrier to 1° alkyl migratory insertion is higher than the

average, significantly so for the n-alkyl ethyl ethylene complexes

30a,b. The data from the isobutyl ethylene cations 35a,b indicate

that the barrier to insertion is slightly higher than the average

even when C_âis a tertiary carbon center. Because no insertion

occurs from 3° alkyl ethylene species (as indicated by the

isobutyl ethylene/tert-butyl ethylene GC experiment), the barrier

to insertion in 2° alkyl complexes must therefore be lower than

the averagesin some cases much lower, to counterbalance the

higher barrier to insertion in the n-alkyl ethylene complexes.

Taking advantage of the Curtin-Hammett kinetic situation

and the ability to measure the equilibrium ratio of the Pd(n-

butyl)(ethylene)⁺to Pd(sec-butyl)(ethylene)⁺complexes, the

relative rates of insertion of ethylene into the 1° vs 2° butyl

groups could be determined by GC analysis of product ratios

as described in the Results. These experiments indicate that the

rate of secondary insertion for 36b is nearly 11 times faster

than primary insertion. Reducing the steric bulk on the ligand

(36a) lowers the k(2°)/k(1°) ratio significantly, to ca. 2 to 1.

Less crowding in the case of the 2,6-dimethyl-substituted ligand

likely results in lower ground-state energies for these species,

raising insertion barriers and decreasing the difference in ground-

state energies between the n-alkyl and sec-alkyl ethylene

complexes. This is reflected in the lowering of K_eq(n-butyl

ethylene/sec-butyl ethylene) upon changing the ligand aryl

substituents from isopropyl groups to methyl groups.

As noted previously,^9,22addition of bulk near these axial sites

(via an increase the size of the alkyl groups in the ortho positions

on the imine aryl rings) should raise the energy of this transition

state with respect to the ground-state alkyl olefin complex,

raising the barrier to associative exchange as observed. The fact

that increasing the size of the Pd alkyl substituent increases the

rate of associative exchange suggests that the alkyl group

experiences more steric crowding in the square planar ground

state than the transition state. If the transition state shown in

Figure 8 is a good model for the transition state, reduced

crowding may result from widening the N-Pd-R angle.

Primary versus Secondary Alkyl Migratory Insertion

Barriers. Prior to this study, average barriers for migratory

insertion during polymerization had been obtained using turn-

over numbers from the methyl ethylene complexes 33a,b.²²

Those barriers, termed “subsequent insertion” barriers, are

shown in Table 3 and are average barriers for migration of all

types of 1° and 2° alkyl groups to bound ethylene in these

systems. Using the ethyl ethylene cations 30a,b and isobutyl

Using data obtained from the ethyl ethylene species to model

insertion and ethylene dissociation in the butyl ethylene

complexes, the free energy diagram shown in Figure 9 can be

constructed for complexes 36b/37b.

NMR spectroscopic data have established that the sec-butyl

ethylene complex, 36b in Figure 9, is thermodynamically

favored over the agostic n-butyl species 37b by 2.1 kcal/mol.

Pd(II) Alkyl Cations and Alkyl Ethylene Complexes

J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11551

Using the ethyl ethylene complex 30b as a model for the butyl

ethylene complex 37b, the barrier to dissociation of ethylene

(and thus to isomerization from n-butyl to sec-butyl) can be

estimated as 17.1 kcal/mol; this is 1.4 kcal/mol lower than the

barrier to migratory insertion (18.5 kcal/mol), which is also

estimated from the results for 30b. GC product ratios from

insertion of ethylene in 36b/37b allow calculation of ∆∆G^q)

1.0 kcal/mol for the difference in the overall barriers to product

formation (Curtin-Hammett kinetics apply). With these pieces

of information in hand, the barriers to sec-butyl ethylene

isomerization and migratory insertion can be deduced. The

barrier to isomerization of 36b to 37b is 17.1 - 2.1 ) 15.0

kcal/mol; this compares well with the barrier to 2° alkyl olefin

isomerization of 15.3 kcal/mol measured for the transformation

of the isopropyl ethylene complex 9b to the n-propyl ethylene

complex 10b.²²The barrier to 1° alkyl olefin insertion plus

the overall difference in barriers to product formation (18.5 +

0.9 ) 19.4 kcal/mol) minus the difference in free energy

between 36b and 37b then gives an estimate of the barrier to

migratory insertion for a 2° alkyl olefin species, 19.4 - 2.1 )

17.3 kcal/mol. The magnitude of this barrier indicates that 2°

alkyl migratory insertion is almost 10 times faster than 1° alkyl

migratory insertion at -20 °C, likely due to crowding in the

ground state of 36b. This much lower barrier to 2° alkyl

migratory insertion is necessary to observe a high degree of

branching in the resulting polymer, considering that the n-alkyl

ethylene complexes are the most likely catalyst resting states.

Comparison to Theoretical Results. Ziegler et al. have

reported extensive density functional calculations on the Ni^47,51

and Pd^37,38catalyst systems. Using a generic R-diimine ligand,

Ziegler calculates that the Pd agostic isopropyl cation (38,

R, R′ ) H, CH₃) should be favored over the Pd n-propyl cation

(39, R, R′ ) H, CH₃) by 1.96 kcal/mol, and the agostic tert-

butyl cation (38, R, R′ ) CH₃, CH₃) should be heavily favored

thermodynamically (by 3.42 kcal/mol over the isobutyl cation,

39, R, R′ ) CH₃, CH₃) (eq 14).³⁷

the calculated ordering matches that observed experimentally

suggests that electronic effects are the largest factor controlling

relative agostic alkyl stabilities.

Use of the generic R-diimine ligand is less satisfactory for

predicting relative stabilities of the alkyl ethylene complexes

and their insertion barriers. The 1° alkyl ethylene cations are

calculated to be only slightly more stable (by 0.64 kcal/mol)

relative to the isomeric 2° alkyl ethylene species, and the lowest

insertion barrier calculated is that into a Pd-1° alkyl bond

(lower than into the Pd-2° alkyl bond by 1.32 kcal/mol).³⁷This

insertion barrier, calculated for an n-propyl ethylene cation, is

18.83 kcal/mol, very close to our measured insertion barriers

in the actual catalyst system.²²

Changing the diimine backbone to acenaphthyl and the imine

substituents to 2,6-(ⁱPr)₂C₆H₃in the calculation shows an

enhanced preference for the 2° alkyl agostic cations; the

isopropyl cation 7b is calculated to be more stable than the

n-propyl cation 9b by 2.65 kcal/mol. The n-alkyl ethylene

complexes also become much more stable relative to 2° alkyl

ethylene complexes; for example, the n-propyl ethylene complex

10b is favored over the isopropyl ethylene complex 9b by 2.01

kcal/mol.³⁸These results using the actual ligand set suggest that

the observed preference for the n-alkyl ethylene complexes is

steric rather than electronic in nature.

Summary and Conclusions

Cationic (R-diimine)Pd(II) alkyl species are intermediates in

the polymerization of ethylene and R-olefins to high molecular

weight polyolefins. These â-agostic alkyl complexes have been

synthesized independently via protonation of (R-diimine)Pd

dialkyl complexes, which has allowed study of their structures,

dynamic behavior, and the equilibria between isomeric species.

Addition of ethylene to these cations produces alkyl ethylene

complexes and provides a means of investigation of the

equilibria and rates of migratory insertion for these resting state

species. Studies of (R-diimine)Pd-ethyl, -propyl, -n-butyl,

and -isobutyl agostic complexes and their corresponding

ethylene complexes have yielded new mechanistic information

about these diimine Pd(II) polymerization catalysts. Key findings

are as follows:

(1) The most stable isomeric agostic Pd alkyl cation is that

having the highest degree of substitution at C_R. For example,

the Pd tert-butyl cation is favored over the isobutyl complex,

and the Pd isopropyl cation is favored over the isomeric Pd

n-propyl complex. In the case of the n-butyl system, the

thermodynamically favored isomers are those with methyl

substituents at both C_Rand C_â. Electronic effects appear to

govern these equilibria.

(2) Addition of ethylene to the agostic alkyl complexes results

in trapping as the alkyl ethylene complexes, which can

equilibrate via Pd chain walking prior to insertion. Here, the

opposite trend in relative stabilities is observed: 1° alkyl

ethylene species are heavily favored over 2° alkyl ethylene

complexes, and no 3° alkyl ethylene complexes have been

observed. These relative stabilities appear to be controlled by

steric effects.

The sec-butyl cations (41 and 42) are also calculated to be

more stable than the n-butyl agostic cation 40 by almost 3 kcal/

mol, with the sec-butyl cation having a methyl branch on C_R

(42) being the more favored of the two sec-butyl cations by

1.24 kcal/mol (eq 15).

(3) The kinetic products of ethylene trapping of 2° alkyl

agostic species are the more sterically demanding 2° alkyl

ethylene complexes. However, these species isomerize rapidly

to the 1° alkyl ethylene species via a mechanism that involves

initial dissociation of ethylene, isomerization of the resulting

alkyl agostic cation, and reassociation of ethylene. The barrier

to dissociation of ethylene in 1° alkyl ethylene cations (modeled

with an ethyl ethylene species) is 1.4 kcal/mol less than the

All of these calculated ground state energies are in good

agreement with the relative energy ordering of the actual propyl

and butyl agostic cations described in this work. The fact that

the model imine ligand bears no bulky aryl substituents and

(51) Deng, L.; Woo, T. K.; Cavallo, L.; Margl, P. M.; Ziegler, T. J. Am.

Chem. Soc. 1997, 119, 6177-6186.

11552 J. Am. Chem. Soc., Vol. 123, No. 47, 2001

Shultz et al.

Materials. All solvents were deoxygenated and dried via passage

over a column of activated alumina.⁵⁴Dichlorofluoromethane-d (CDCl₂F)

was prepared according to the literature,⁵⁵dried over CaCl₂, degassed

by repeated freeze-pump-thaw cycles, vacuum-transferred, and stored

over 4 Å molecular sieves at -30 °C under argon. Methylene chloride-

d₂was dried over P₂O₅, degassed by repeated freeze-pump-thaw

cycles, vacuum-transferred, and stored over 4 Å molecular sieves under

argon. The R-diimine ligands,⁵⁶(PhCN)₂PdCl₂,⁵⁷and H(OEt₂)₂BAr′₄,⁴¹

were prepared as previously reported. NaBAr′₄(Ar′ ) 3,5-(CF₃)₂C₆H₃)

(Boulder Scientific) and B(C₆F₅)₃(Strem) were stored in an argon-

filled drybox and used as received. n-PrMgCl, n-BuMgCl, and

i-BuMgCl were purchased from Aldrich and stored at -30 °C. Ethylene

and propylene (CP grade) were purchased from National Welder’s

Supply Co., Inc. and used without further purification. The synthesis

and characterization of the dichloride complexes 12a,b, the diethyl

complexes 31a,b, the ethyl ether complexes 29a,b, and the agostic ethyl

complex 11b have been reported.¹⁸

corresponding migratory insertion barrier. The barrier to ethylene

dissociation in 2° alkyl ethylene species is estimated at 15.0

kcal/mol, almost 3 kcal/mol lower than insertion. This finding

confirms that ethylene loss and alkyl isomerization are facile

processes compared to chain propagation via insertion and result

in formation of highly branched polyethylene.

(4) The alkyl ethylene complexes undergo associative ex-

change of bound ethylene with free ethylene in solution.

Increasing the steric bulk of the imine aryl substituents slows

the rate of associative exchange, while increasing the bulk of

the Pd-alkyl chain (-CH₃vs -CH₂CH₃) slightly increases the

exchange rate.

(5) No migratory insertion is observed to occur through 3°

alkyl ethylene species, consistent with established branching

patterns in the polymers produced from the Pd catalysts.²³

(6) The rate of migratory insertion of 2° alkyl ethylene

complexes is generally faster than that for 1° alkyl ethylene

complexes. For the bulkier system bearing aryl substituents

with ortho isopropyl groups (series b), the 2°:1° ratio is 11:1

(-20 °C), whereas for the less bulky system bearing ortho

methyl groups (series a) this rate ratio falls to 2:1 (-20 °C).

The ground state energy of a secondary alkyl ethylene complex

relative to a primary alkyl ethylene complex is apparently raised

more through steric interactions in the case of the isopropyl-

substituted catalyst than in the case of the less bulky methyl-

substituted catalyst. On this basis, it might be anticipated that

the methyl-substituted catalyst system would yield polyethylene

with fewer branches (i.e., more chain propagation would occur

through migratory insertions of 1° alkyl ethylene complexes

which produce no branch). However, alkyl olefin complexes

equilibrate prior to insertion, and these 1° vs 2° rate differentials

for insertion are nearly exactly counterbalanced by a greater

thermodynamic preference for 2° alkyl ethylene complexes in

the case of the ortho methyl-substituted system relative to the

ortho isopropyl-substituted system. Thus, similar degrees of

branching of the polyethylenes produced from the two catalyst

systems are observed.⁵²At the single insertion level, this is

illustrated in Scheme 14, where the ratios of n-hexane (linear)

to 3-methylpentane (branched) generated from insertion and

cleavage of the equilibrating (R-diimine)Pd(butyl)(ethylene)⁺

isomers are nearly within experimental error for the two catalyst

systems.

Synthesis of Dialkyl Complexes. ((2,6-(CH₃)₂C₆H₃)NdC(An)C-

(An)dN(2,6-(CH₃)₂C₆H₃))Pd(n-C₃H₇)₂(13a). A clean, flame-dried

Schlenk flask was charged with ((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)d

N(2,6-(CH₃)₂C₆H₃))PdCl₂(12a) (0.435 g, 0.769 mmol) in an argon-

filled drybox. The flask was placed under argon and cooled to -78 °C

(dry ice/isopropyl alcohol), and Et₂O (20 mL) was added via syringe.

n-PrMgCl was added as a solution in Et₂O (2.0 M, 0.77 mL, 1.54 mmol)

via syringe, and the mixture was stirred at -78 °C for 2 h (the orange

suspension gradually changed to dark brown). MeOH (0.1 mL) was

added via syringe to quench any excess Grignard reagent, and the dark

mixture was cannulated onto Florisil and flash-filtered into a clean,

flame-dried Schlenk flask at 0 °C. The Florisil was extracted with Et₂O

(1 × 10 mL) and pentane (1 × 10 mL), and the filtrate was reduced in

vacuo to give a red-brown foamy solid, which was dried under reduced

pressure for 1 h at 25 °C and stored at -30 °C in the drybox freezer.

1

Yield: 0.203 g (45%). H NMR (CD₂Cl₂, 400 MHz, 25 °C): δ 8.04

(d, J ) 8.3, 2H, An p-H), 7.42 (dd, J ) 7.3, 8.3, 2H, An m-H), 7.26

(m, 6H, Ar H), 6.71 (d, J ) 7.3, 2H, An o-H), 2.26 (s, 12H, ArCH₃),

1.18 (tq, J ) 7.2, 7.4, 4H, PdCH₂CH₂CH₃), 0.89 (t, J ) 7.4, 4H, PdCH₂-

CH₂CH₃), 0.68 (t, J ) 7.2, 6H, PdCH₂CH₂CH₃). ¹³C NMR (CD₂Cl₂,

125 MHz, -50 °C): δ 166.6, 144.5, 142.4, 130.8, 129.8, 128.8, 128.0,

127.5, 125.5, 122.7, 24.2 (PdCH₂CH₂CH₃), 21.9 (PdCH₂CH₂CH₃), 19.3

(PdCH₂CH₂CH₃), 17.7 (Ar-CH₃). Anal. Calcd for C₃₄H₃₈N₂Pd: C,

70.27; H, 6.59; N, 4.82. Found: C, 70.50; H, 6.53; N, 4.96.

Compounds 13b, 14a,b, and 15a,b were prepared in an analogous

fashion to 13a. All give satisfactory analyses; experimental details and

spectral characterization appear in the Supporting Information.

Synthesis of Agostic Butyl Complexes. [((2,6-(CH₃)₂C₆H₃)NdC-

(An)C(An)dN(2,6-(CH₃)₂C₆H₃))Pd(C₄H₉)]BAr′₄(28a). A clean, flame-

dried Schlenk flask was charged with ((2,6-(CH₃)₂C₆H₃)NdC(An)C-

(An)dN(2,6-(CH₃)₂C₆H₃))Pd(C₄H₉)₂(14a) (50.3 mg, 82.6 µmol) and

H(OEt₂)₂BAr′₄(84.3 mg, 83.3 µmol) in an argon-filled drybox. The

flask was placed under argon and cooled to -78 °C (dry ice/isopropyl

alcohol), and CH₂Cl₂(5.6 mL) was added via syringe. The solids

dissolved immediately to give a yellow-orange solution. The solution

was allowed to stir at -78 °C for 20 min; it was then warmed to 0 °C

and reduced in vacuo to a yellow-orange powder, which was dried under

reduced pressure for 2 h at 25 °C. The product is a mixture of isomers;

see the text for details. Yield: 95.0 mg (81%). Ligand resonances for

the isomers are coincident and are reported as if the complex were a

single species. ¹H NMR (CDCl₂F, 400 MHz, -110 °C (static)): δ 8.11

(m, 2H, An p-H), 7.79 (br s, 8H, BAr′₄o-H), 7.51 (br s, 4H, BAr′₄

p-H), 7.31 (m, 8H, An m-H, Ar H), 6.80 (m, 2H, An o-H), 2.25 (m,

12H, ArCH₃). Trans isomer: 1.72 (br m, 1H, Pd(CH(CH₃)CH-µ-

H(CH₃)), -7.98 (br d, J ) 13, 1H, Pd(CH(CH₃)CH-µ-H(CH₃)), Pd-

(CH(CH₃)CH-µ-H(CH₃)) obscured. Cis isomer: 1.44 (br m, 2H,

Pd(CH(CH₃)CH-µ-H(CH₃)), -8.10 (br m, 1H, Pd(CH(CH₃)CH-µ-

Experimental Section

General Considerations. All manipulations of compounds were

performed using standard high-vacuum or Schlenk techniques. Argon

was purified by passage through columns of BASF R3-11 catalyst

(Chemalog) and 4 Å molecular sieves. Solid organometallic compounds

were transferred in an argon-filled MBraun drybox. NMR spectra were

acquired with Bruker DRX400 or DRX500 spectrometers. NMR probe

temperatures were measured using an Omega type T thermocouple

1

immersed in anhydrous methanol in a 5 mm NMR tube. H and ¹³C

chemical shifts are reported in ppm downfield of TMS and were

referenced to residual ¹H NMR signals and to the ¹³C NMR signals of

the deuterated solvents, respectively. All coupling constants are reported

in Hz. Elemental analyses were performed by Atlantic Microlab, Inc.,

of Norcross, GA.

Activation parameters (∆G^q) were calculated from measured rate

constants and temperatures using the Eyring equation. Error analysis

of ∆G^qobtained for dynamic processes was based on Binsch’s

derivation of σ∆G^qand incorporated an estimate of 10% error in k

and (1 °C error in temperature.⁵³Error calculations for coalescence

data incorporated a larger (5 °C error in the calculation.

(54) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;

Timmers, F. J. Organometallics 1996, 15, 1518-1520.

(55) Siegel, J. S.; Anet, F. A. L. J. Org. Chem. 1988, 53, 2629-2630.

(56) van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151-

239.

(52) Gottfried, A. C.; Brookhart, M. Unpublished results.

(53) Binsch, G. In Dynamic Nuclear Magnetic Resonance Spectroscopy;

Jackman, L., Cotton, F. A., Eds.; Academic Press: New York, 1975; p 77.

(57) Kharasch, M. S.; Seyler, R. C.; Mayo, F. R. J. Am. Chem. Soc.

1938, 60, 882-884.

Pd(II) Alkyl Cations and Alkyl Ethylene Complexes

J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11553

H(CH₃)), Pd(CH(CH₃)CH-µ-H(CH₃)) obscured. ¹H NMR (CDCl₂F, 400

MHz, 0 °C (dynamic)): δ 8.10 (d, J ) 8.4, 1H, An p-H), 8.07 (d, J )

8.0, 1H, An p-H′), 7.71 (br s, 8H, BAr′₄o-H), 7.50 (m, 2H, An m-H),

7.47 (br s, 4H, BAr′₄p-H), 7.27 (m, 6H, Ar H), 6.81 (d, J ) 7.6, 1H,

An o-H), 6.79 (d, J ) 7.6, 1H, An o-H′), 2.26 and 2.24 (2 s, 6H each,

ArCH₃), all Pd-butyl resonances broadened into baseline by exchange.

In Situ Formation of Agostic Alkyl Complexes. With the exception

of the agostic butyl complexes, whose synthesis is described in the

previous section, agostic alkyl complexes were studied by in situ

generation from the corresponding dialkyl complex and H(OEt₂)₂BAr′₄.

The byproducts of these reactions (generally an alkane and Et₂O) are

not included in the NMR data for the species, though they may in some

cases obscure resonances from the agostic complexes themselves.

[((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)dN(2,6-(CH₃)₂C₆H₃))Pd(CH-

(CH₂-µ-H)(CH₃))]BAr′₄(7a). See ref 22 for spectral data.

1

¹³C NMR (CDCl₂F, 100 MHz, 0 °C): δ 161.7 (q, J_CB) 49.7, BAr′₄

2

C_ipso), 131.56, 134.6 (BAr′₄C_ortho), 129.86, 129.62, 129.25 (q, J_CF

)

28.3, BAr′₄C_meta), 128.02, 127.74, 127.29, 125.43, 125.38, 125.22,

1

[((2,6-(i-Pr)₂C₆H₃)NdC(An)C(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(CH-

(CH₂- µ-H)(CH₃))]BAr′₄(7b). See ref 21 for spectral data.

124.41 (q, J_CF) 272, BAr′₄CF₃), 117.5 (BAr′₄C_para), 31.85, 25.05,

22.81, 18.56, 18.38, 17.94 (ArCH₃), 17.63 (ArC′H₃), 15.08, 13.98,

13.70. Not all carbons were accounted for. Anal. Calcd for C₆₄H₄₅N₂-

BF₂₄Pd: C, 54.31; H, 3.21; N, 1.98. Found: C, 54.47; H, 3.43; N,

1.96.

[((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)dN(2,6-(CH₃)₂C₆H₃))Pd(C-

(CH₂- µ-H)(CH₃)₂)]BAr′₄(18a). ¹H NMR (CDCl₂F, 400 MHz, -110

°C (static)): δ 8.08 (m, 2H, An p-H), 7.77 (br s, 8H, BAr′₄o-H), 7.49

(br s, 4H, BAr′₄p-H), 7.39-7.22 (m, 8H, An m-H, Ar H), 6.79 (d,

J ) 6.8, 1H, An o-H), 6.66 (d, J ) 7.2, 1H, An o-H′), 2.23 and 2.15

(2 s, 6H each, ArMe₂, Ar′Me₂), 0.56 (br s, 6H, Pd(C(CH₂-µ-H)(CH₃)₂)),

-7.03 (br t, J ) 15, 1H, Pd(C(CH₂-µ-H)(CH₃)₂), Pd(C(CH₂-µ-H)-

[((2,6-(i-Pr)₂C₆H₃)NdC(An)C(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(C₄H₉)]-

BAr′₄(28b). A procedure identical to that used for 28a was followed,

using ((2,6-(i-Pr)₂C₆H₃)NdC(An)C(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(C₄H₉)₂

(14b) (100 mg, 0.139 mmol) and H(OEt₂)₂BAr′₄(143 mg, 0.414 mmol)

in CH₂Cl₂(10 mL). Yield: 176 mg (83%) of a yellow-orange powder.

The product is a mixture of isomers; see the text for details. Ligand

resonances for the isomers are coincident and are reported as if the

1

(CH₃)₂) obscured. H NMR (CDCl₂F, 400 MHz, 0 °C (dynamic)): δ

8.13 (d, J ) 8.4, 1H, An p-H), 8.10 (d, J ) 8.4, 1H, An p-H′), 7.76 (br

s, 8H, BAr′₄o-H), 7.52 (br s, 4H, BAr′₄p-H), 7.44-7.29 (m, 8H, An

m-H, Ar H), 6.86 (d, J ) 7.6, 1H, An o-H), 6.76 (d, J ) 7.2, 1H, An

o-H′), 2.31 and 2.24 (2 s, 6H each, ArMe₂, Ar′Me₂), -0.14 (br s, 9H,

Pd(C(CH₃)₃).

Trapping the agostic tert-butyl complex with acetonitrile yields the

isobutyl acetonitrile complex, [((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)d

N(2,6-(CH₃)₂C₆H₃))Pd(CH₂CH(CH₃)₂)(NCCH₃))]BAr′₄(22a). ¹H NMR

(CD₂Cl₂, 400 MHz, -60 °C): δ 8.12 (d, J ) 8.4, 1H, An p-H), 8.10

(d, J ) 8.4, 1H, An p-H′), 7.72 (br s, 8H, BAr′₄o-H), 7.53 (br s, 4H,

BAr′₄p-H), 7.47 (m, 2H, An m-H), 7.36-7.18 (m, 6H, Ar H), 6.96 (d,

J ) 7.2, 1H, An o-H), 6.50 (d, J ) 7.2, 1H, An o-H′), 2.27 and 2.18

(2 s, 6H each, ArMe₂, Ar′Me₂), 1.80 (s, 3H, Pd-NCMe), 1.48 (d, J )

7.2, 2H, Pd(CH₂CH(CH₃)₂)), 0.69 (d, J ) 6.4, 6H, Pd(CH₂CH(CH₃)₂)),

Pd(CH₂CH(CH₃)₂obscured.

Trapping the agostic tert-butyl complex with ¹³CO at -80 °C initially

yields the tert-butyl carbonyl complex, [((2,6-(CH₃)₂C₆H₃)NdC(An)C-

(An)dN(2,6-(CH₃)₂C₆H₃))Pd(C(CH₃)₃)(¹³CO))]BAr′₄(23a). ¹H NMR

(CDCl₂F, 400 MHz, -90 °C): δ 8.08 (d, J ) 7.6, 1H, An p-H), 8.06

(d, J ) 7.6, 1H, An p-H′), 7.77 (br s, 8H, BAr′₄o-H), 7.52 (br s, 4H,

BAr′₄p-H), 7.50-7.21 (m, 8H, An m-H, Ar H), 6.78 (d, J ) 7.6, 1H,

An o-H), 6.23 (d, J ) 7.6, 1H, An o-H′), 2.27 and 2.16 (2 s, 6H each,

ArMe₂, Ar′Me₂), 0.99 (s, 9H, Pd(C(CH₃)₃). ¹³C NMR (CDCl₂F, 100

MHz, -90 °C): δ 180.9 (Pd(¹³CO), broadened due to exchange with

free ¹³CO).

1

complex were a single species. H NMR (CDCl₂F, 400 MHz, -110

°C (static)): δ 8.20 (m, 2H, An p-H), 7.73 (br s, 8H, BAr′₄o-H), 7.61

(m, 2H, An m-H), 7.50 (br s, 4H, BAr′₄p-H), 7.47 (m, 6H, Ar H),

6.93 (m, 1H, An o-H), 6.76 (m, 1H, An o-H′), 3.40 and 3.32 (2

overlapping septets, J ) 6.8 and 6.8, CHMeMe′), 3.13 (septet, J )

6.4, CHMeMe′), 2.87 (m, CHMeMe′),1.28 (br d, 12 H, CHMeMe′),

1.01 (br, 6H, C′HMeMe′), 0.93 (br, 6H, C′HMeMe′). Trans isomer:

1.72 (m, 1H, Pd(CH(CH₃)CH-µ-H(CH₃)), 0.15 (br d, 3H, Pd(CH(CH₃)-

CH-µ-H(CH₃)), -8.20 (br d, J ) 17, 1H, Pd(CH(CH₃)CH-µ-H(CH₃)),

Pd(CH(CH₃)CH-µ-H(CH₃)) obscured. Cis isomer: 1.35 (m, 2H, Pd-

(CH(CH₃)CH-µ-H(CH₃)), -8.27 (t, J ) 7, 1H, Pd(CH(CH₃)CH-µ-

H(CH₃)), Pd(CH(CH₃)CH-µ-H(CH₃)) obscured. sec-Butyl agostic iso-

mer with ethyl branch: -8.01 (t, J ) 16, 1H, Pd(CH(CH₂CH₃)CH₂-

1

µ-H)), Pd(CH(CH₂CH₃)CH₂-µ-H) obscured. H NMR (CDCl₂F, 400

MHz, 0 °C (dynamic)): δ 8.08 (d, J ) 8.4, 1H, An p-H), 8.05 (d, J )

8.4, 1H, An p-H′), 7.72 (br s, 8H, BAr′₄o-H), 7.52 (m, 2H, An m-H),

7.49 (br s, 4H, BAr′₄p-H), 7.41 (m, 6H, Ar H), 6.80 (d, J ) 7.6, 1H,

An o-H), 6.60 (d, J ) 7.6, 1H, An o-H′), 3.15 (m, 4H, CHMeMe′,

C′HMeMe′), 1.35 (d, J ) 6.8, 12 H, CHMeMe′), 1.06 (d, J ) 6.8, 6H,

C′HMeMe′), 0.96 (d, J ) 6.8, 6H, C′HMeMe′), all Pd-butyl resonances

broadened into baseline by exchange. ¹³C NMR (CDCl₂F, 100 MHz, 0

°C): δ 161.7 (q, ¹J_CB) 49.7, BAr′₄C_ipso), 151.1, 146.0, 144.2, 142.1,

142.0, 139.3, 138.4, 137.5, 134.6 (BAr′₄C_ortho), 133.5, 133.0, 131.7,

130.2, 130.0, 129.25 (q, ²J_CF) 28.3, BAr′₄C_meta), 128.4, 127.6, 126.3,

The tert-butyl carbonyl complex isomerizes at -60 °C to give the

isobutyl carbonyl complex, [((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)dN-

1

125.8, 125.5, 125.3, 125.0, 124.41 (q, J_CF) 272, BAr′₄CF₃), 117.5

1

(2,6-(CH₃)₂C₆H₃))Pd(CH₂CH(CH₃)₂)(¹³CO))]BAr′₄(24a). H NMR

(BAr′₄C_para), 31.8, 30.3, 29.8, 29.6, 29.5, 25.0, 24.0, 23.5, 23.4, 23.3,

23.2, 23.0, 22.8, 22.7, 22.3, 14.0, 13.7. Not all carbons were accounted

for. Anal. Calcd for C₇₂H₆₁N₂BF₂₄Pd: C, 56.61; H, 4.03; N, 1.83.

Found: C, 56.75; H, 4.15; N, 1.88.

(CDCl₂F, 400 MHz, -90 °C): δ 8.13-8.08 (m, 2H, An p-H, partially

obscured by tert-butyl carbonyl complex), 7.77 (br s, 8H, BAr′₄o-H),

7.52 (br s, 4H, BAr′₄p-H), 7.52-7.22 (m, 8H, An m-H, Ar H), 6.83

(d, J ) 7.2, 1H, An o-H), 6.49 (d, J ) 7.2, 1H, An o-H′), 2.31 and

2.19 (2 s, 6H each, ArMe₂, Ar′Me₂), 2.01 (d, J ) 7.2, 2H, Pd(CH₂-

CH(CH₃)₂)), 1.56 (m, 1H, Pd(CH₂CH(CH₃)₂)), 0.75 (d, J ) 6.0, 6H,

Pd(CH₂CH(CH₃)₂)). ¹³C NMR (CDCl₂F, 100 MHz, -90 °C): δ 175.0

(sharp, Pd(¹³CO)).

General Procedures for Variable-Temperature NMR Experi-

ments. In a drybox under an argon atmosphere, a tared NMR tube

was charged with ca. 0.01 mmol each of the desired dialkyl complex

and H(OEt₂)₂BAr′₄or ca. 0.01 mmol of the isolated agostic alkyl

complex. The tube was capped with a rubber septum in the drybox

and secured with Teflon tape and Parafilm once removed from the

drybox. The tube was then cooled to -78 °C (dry ice/acetone bath),

and CD₂Cl₂was added via gastight syringe (22-gauge needle, ∼700

µL). Alternatively, CDCl₂F was used for lower temperature work; it

was added to the NMR tube via a 22-gauge cannula. The tube was

then removed from the bath briefly and shaken to facilitate dissolution

of the solids (and subsequent reaction, in the case of the dialkyl

complexes). The tube was kept in the bath until it could be transferred

to a precooled (-80 °C) NMR probe for acquisition of spectra. The

concentrations of all species were calculated using the BAr′₄or

acenaphthyl ¹H signals (ortho or para) as internal standards. Acetonitrile

was added to the NMR tube via a 10-µL syringe to produce acetonitrile

complexes; ethylene was added via a gastight syringe with a 22-gauge

needle. CO was bubbled directly through the solution in the NMR tube

using a 22-gauge needle.

The isobutyl carbonyl complex undergoes insertion between -60

and -40 °C to give the isobutyl acyl carbonyl complex, [((2,6-

(CH₃)₂C₆H₃)NdC(An)C(An)dN(2,6-(CH₃)₂C₆H₃))Pd(¹³C(O)CH₂CH-

1

(CH₃)₂)(¹³CO))]BAr′₄(25a). H NMR (CDCl₂F, 400 MHz, -40 °C):

δ 8.12 (d, J ) 8.4, 1H, An p-H), 8.09 (d, J ) 8.4, 1H, An p-H′), 7.76

(br s, 8H, BAr′₄o-H), 7.55-7.48 (m, 2H, An m-H), 7.53 (br s, 4H,

BAr′₄p-H), 7.36-7.25 (m, 6H, Ar H), 6.83 (d, J ) 7.2, 1H, An o-H),

2

3

6.57 (d, J ) 7.2, 1H, An o-H′), 2.64 (dd, J_CH) 6.0, J_HH) 6.0, 2H,

Pd(¹³C(O)CH₂CH(CH₃)₂)), 2.32 and 2.31 (2 s, 6H each, ArMe₂, Ar′Me₂),

0.56 (d, J ) 6.4, 6H, Pd(¹³C(O)CH₂CH(CH₃)₂)), Pd(¹³C(O)CH₂CH(CH₃)₂)

obscured. ¹³C NMR (CDCl₂F, 100 MHz, -40 °C): δ 210.7 (sharp,

Pd(¹³C(O)CH₂CH(CH₃)₂)), 172.2 (broad, Pd(¹³CO)).

[((2,6-(i-Pr)₂C₆H₃)NdC(An)C(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(C(CH₂-

µ-H)(CH₃)₂)]BAr′₄(18b). ¹H NMR (CDCl₂F, 400 MHz, -130 °C

(static)): δ 7.92 (d, J ) 7.2, 1H, An p-H), 7.89 (d, J ) 7.2, 1H, An

p-H′), 7.71 (br s, 8H, BAr′₄o-H), 7.44 (br s, 4H, BAr′₄p-H), 7.46-

11554 J. Am. Chem. Soc., Vol. 123, No. 47, 2001

Shultz et al.

7.34 (m, 8H, An m-H, Ar H), 6.63 (d, J ) 7.2, 1H, An o-H), 6.33 (d,

J ) 7.2, 1H, An o-H′), 2.97 (2 overlapping septets, 2H each, CHMe₂,

C′HMe₂), 1.31, 1.23, 0.98, and 0.72 (4 br d, 6H each, CHMeMe′,

CHMeMe′, C′HMeMe′, C′HMeMe′), 0.60 (br s, 6H, Pd(C(CH₂-µ-H)-

(CH₃)₂)), -7.12 (br t, J ) 15, 1H, Pd(C(CH₂-µ-H)(CH₃)₂), Pd(C(CH₂-

µ-H)(CH₃)₂) obscured. ¹H NMR (CDCl₂F, 400 MHz, 0 °C (dy-

namic)): δ 8.04 (d, J ) 7.5, 1H, An p-H), 8.01 (d, J ) 7.8, 1H, An

p-H′), 7.71 (br s, 8H, BAr′₄o-H), 7.48 (br s, 4H, BAr′₄p-H), 7.54-

7.36 (m, 8H, An m-H, Ar H), 6.75 (d, J ) 7.2, 1H, An o-H), 6.50 (d,

J ) 7.2, 1H, An o-H′), 3.11 and 3.10 (2 septets, 2H each, J ) 6.9 and

6.6, CHMe₂, C′HMe₂), 1.36, 1.32, 1.05, and 0.82 (4d, 6H each, J )

6.9, 6.9, 6.6, 6.6, CHMeMe′, CHMeMe′, C′HMeMe′, C′HMeMe′),

-0.20 (sharp s, 9H, Pd(C(CH₃)₃).

Trapping the agostic tert-butyl complex with acetonitrile yields the

isobutyl acetonitrile complex, [((2,6-(i-Pr)₂C₆H₃)NdC(An)C(An)dN-

(2,6-(i-Pr)₂C₆H₃))Pd(CH₂CH(CH₃)₂)(NCCH₃)]BAr′₄(22b). ¹H NMR

(CD₂Cl₂, 400 MHz, -60 °C): δ 8.13 (d, J ) 7.8, 1H, An p-H), 8.11

(d, J ) 7.8, 1H, An p-H′), 7.72 (br s, 8H, BAr′₄o-H), 7.53 (br s, 4H,

BAr′₄p-H), 7.48-7.39 (m, 8H, An m-H, Ar H), 6.98 (d, J ) 6.9, 1H,

An o-H), 6.37 (d, J ) 7.5, 1H, An o-H′), 3.20 and 3.12 (2 septets, 2H

each, J ) 6.9 and 6.0, CHMe₂, C′HMe₂), 1.77 (s, 3H, Pd-NCMe),

1.63 (d, 2H, J ) 7.2, Pd(CH₂CH(CH₃)₂)), 1.38, 1.34, 1.00, and 0.84

(4d, 6H each, J ) 6.9, 6.9, 6.6, 6.6, CHMeMe′, CHMeMe′, C′HMeMe′,

C′HMeMe′), 0.76 (d, J ) 6.6, 6H, Pd(CH₂CH(CH₃)₂)), Pd(CH₂CH(CH₃)₂

obscured.

Alkyl Ethylene Complexes. [((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)d

N(2,6-(CH₃)₂C₆H₃))Pd(CH₂CH₃)(CH₂dCH₂)]BAr′₄(30a[BAr′₄]). ¹H

NMR (CD₂Cl₂, 400 MHz, -80 °C): δ 8.17 .(d, J ) 8.0, 1H, An p-H),

8.13 (d, J ) 8.0, 1H, An p-H′), 7.71 (br s, 8H, BAr′₄o-H), 7.53 (m,

2H, An m-H), 7.53 (br s, 4H, BAr′₄p-H), 7.35 (m, 6H, Ar H), 6.71 (d,

J ) 7.2, 1H, An o-H), 6.64 (d, J ) 7.2, 1H, An o-H′), 4.60 (br s, 4H,

Pd(CH₂dCH₂)), 2.26 (s, 6H, ArCH₃), 2.21 (s, 6H, Ar′CH₃), 1.34 (q,

J ) 7.2, 2H, PdCH₂CH₃), 0.43 (t, J ) 7.2, 3H, PdCH₂CH₃).

[((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)dN(2,6-(CH₃)₂C₆H₃))Pd(CH₂-

CH₃)(CH₂dCH₂)]HB(C₆F₅)₃(30a[HB(C₆F₅)₃]). ¹H NMR (CD₂Cl₂,

500 MHz, -80 °C): δ 8.18 (d, J ) 8.3, 1H, An p-H), 8.13 (d, J )

8.3, 1H, An p-H′), 7.52 (m, 2H, An m-H), 7.34 (m, 6H, Ar H), 6.66

(d, J ) 7.3, 1H, An o-H), 6.59 (d, J ) 7.3, 1H, An o-H′), 4.54 (s, 4H,

Pd(CH₂dCH₂)), 3.4 (v br, 1H, HB(C₆F₅)₃), 2.24 (s, 6H, ArCH₃), 2.19

(s, 6H, Ar′CH₃), 1.23 (q, J ) 7.3, 2H, PdCH₂CH₃), 0.38 (t, J ) 7.3,

3H, PdCH₂CH₃).

[((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)dN(2,6-(CH₃)₂C₆H₃))Pd(CH₂-

CH₂CH₂CH₃)(CH₂dCH₂)]BAr′₄(37a). ¹H NMR (CDCl₂F, 400 MHz,

-50 °C): δ 8.13 (d, J ) 8.4, 1H, An p-H), 8.09 (d, J ) 8.0, 1H, An

p-H′), 7.77 (br s, 8H, BAr′₄o-H), 7.51 (br s, 4H, BAr′₄p-H), 7.38 (m,

2H, An m-H), 7.35 (m, 6H, Ar H), 6.72 (d, J ) 7.2, 1H, An o-H), 6.71

(d, J ) 7.2, 1H, An o-H′), 4.60 (s, 4H, Pd(CH₂dCH₂)), 2.28 (s, 6H,

ArCH₃), 2.22 (s, 6H, Ar′CH₃), 1.28, 1.01, and 0.72 (m, 2H each,

PdCH₂CH₂CH₂CH₃), 0.56 (t, J ) 7.2, 3H, PdCH₂CH₂CH₂CH₃).

[((2,6-(i-Pr)₂C₆H₃)NdC(An)C(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(CH-

(CH₃)CH₂CH₃)(CH₂dCH₂)]BAr′₄(36b). ¹H NMR (CDCl₂F, 400

MHz, -90 °C): δ 8.01 (d, J ) 8.4, 1H, An p-H), 7.97 (d, J ) 8.4, 1H,

An p-H′), 7.78 (br s, 8H, BAr′₄o-H), 7.57 (br s, 4H, BAr′₄p-H), 7.48

(m, 6H, An m-H, Ar H), 6.50 (d, J ) 7.2, 1H, An o-H), 6.37 (d, J )

7.2, 1H, An o-H′), 4.68 (m, 2H, Pd(CHH′dCHH′)), 4.60 (m, 2H, Pd-

(CHH′dCHH′)), 3.15 and 3.05 (2 septets, 2H each, ArCHMeMe′,

ArC′HMeMe′), 2.05 (m, 1H, Pd(CH(CH₃)CH₂CH₃)), 1.40, 1.35, 0.90,

and 0.80 (4 d, J ) 6.5, 6.5, 6.8, and 6.8, 6H each, CHMeMe′,

CHMeMe′, C′HMeMe′, C′HMeMe′), 0.70 (d, J ) 6.4, 3H, Pd(CH-

(CH₃)CH₂CH₃)), 0.36 (m, 3H, Pd(CH(CH₃)CH₂CH₃)), Pd(CH(CH₃)CH₂-

CH₃) obscured.

[((2,6-(i-Pr)₂C₆H₃)NdC(An)C(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(CH₂-

CH₂CH₂CH₃(CH₂dCH₂)]BAr′₄(37b). ¹H NMR (CDCl₂F, 400 MHz,

-50 °C): δ 8.07 (d, J ) 8.0, 1H, An p-H), 8.03 (d, J ) 8.0, 1H, An

p-H′), 7.77 (br s, 8H, BAr′₄o-H), 7.54 (br s, 4H, BAr′₄p-H), 7.5 (m,

6H, An m-H, Ar H), 6.59 (d, J ) 8.0, 1H, An o-H), 6.55 (d, J ) 8.0,

1H, An o-H′), 4.68 (s, 4H, Pd(CH₂dCH₂)), 3.08 and 3.00 (2 septets,

J ) 6.8 and 6.8, 2H each, ArCHMeMe′, ArC′HMeMe′), 1.56 (m, 2H,

Pd(CH₂CH₂CH₂CH₃)), 1.42, 1.38 (2 d, J ) 6.8, 6.8, 6H each,

CHMeMe′, CHMeMe′), 0.97 (m, 2H, Pd(CH₂CH₂CH₂CH₃)), 0.92, 0.89

(2 d, J ) 6.8, 6.8, 6H each, C′HMeMe′, C′HMeMe′), 0.58 (t, J ) 7.2,

3H, Pd(CH₂CH₂CH₂CH₃)), Pd(CH₂CH₂CH₂CH₃) obscured.

[((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)dN(2,6-(CH₃)₂C₆H₃))Pd(CH₂CH-

1

(CH₃)₂)(CH₂dCH₂)]BAr′₄(35a). H NMR (CD₂Cl₂, 400 MHz, -80

°C): δ 8.14 (d, J ) 8.4, 1H, An p-H), 8.10 .(d, J ) 8.4, 1H, An p-H′),

7.72 (br s, 8H, BAr′₄o-H), 7.51 (br s, 4H, BAr′₄p-H), 7.49 (m, 2H,

An m-H), 7.35-7.26 (m, 6H, Ar H), 6.67 (d, J ) 7.2, 1H, An o-H),

6.62 (d, J ) 7.2, 1H, An o-H′), 4.54 (s, 4H, Pd(CH₂dCH₂)), 2.19 and

2.16 (2 s, 6H each, ArMe₂, Ar′Me₂), 1.20 (d, J ) 6.4, 2H, Pd(CH₂-

CH(CH₃)₂)), 0.62 (d, J ) 5.6, 6H, Pd(CH₂CH(CH₃)₂)), Pd(CH₂CH(CH₃)₂)

obscured.

[((2,6-(i-Pr)₂C₆H₃)NdC(An)C(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(CH₂CH-

1

(CH₃)₂)(CH₂dCH₂)]BAr′₄(35b). H NMR (CD₂Cl₂, 400 MHz, -80

[((2,6-(i-Pr)₂C₆H₃)NdC(An)C(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(CH₂-

1

°C): δ 8.06 (d, J ) 8.4, 1H, An p-H), 8.02 (d, J ) 8.4, 1H, An p-H′),

7.72 (br s, 8H, BAr′₄o-H), 7.51 (br s, 4H, BAr′₄p-H), 7.47 (m, 2H,

An m-H), 7.40 (m, 6H, Ar H), 6.52 (d, J ) 7.6, 1H, An o-H), 6.44 (d,

J ) 7.2, 1H, An o-H′), 4.58 (s, 4H, Pd(CH₂dCH₂)), 2.98 and 2.94 (2

septets, J ) 6.4 and 6.4, 2H each, ArCHMeMe′, ArC′HMeMe′), 1.38

(d, J ) 6.8, 2H, Pd(CH₂CH(CH₃)₂)), 1.33, 1.28 (2 d, J ) 6.4, 6.4, 6H

each, CHMeMe′, CHMeMe′), 0.95 (m, 1H, Pd(CH₂CH(CH₃)₂)), 0.80

(m, 12H, C′HMeMe′, C′HMeMe′), 0.61 (d, J ) 5.6, 6H, Pd(CH₂CH-

(CH₃)₂)).

CH₃)(CH₂dCH₂)]BAr′₄(30b[BAr′₄]). H NMR (CD₂Cl₂, 400 MHz,

-80 °C): δ 8.07 (d, J ) 7.6, 1H, An p-H), 8.03 .(d, J ) 8.0, 1H, An

p-H′), 7.71 (br s, 8H, BAr′₄o-H), 7.55-7.38 (m, 8H, An m-H, Ar H),

7.53 (br s, 4H, BAr′₄p-H), 6.50 (d, J ) 7.2, 1H, An o-H), 6.44 (d,

J ) 7.6, 1H, An o-H′), 4.58 (br s, 4H, Pd(CH₂dCH₂), dynamic), 2.99

and 2.92 (2 septets, J ) 6.4 and 6.4, 2H each, ArCHMeMe′,

ArC′HMeMe′), 0.56 (q, J ) 7.2, 2H, PdCH₂CH₃), 1.33, 1.28, 0.85,

and 0.79 (4 d, J ) 6.4, 6.4, 6.4, and 6.4, 6H each, CHMeMe′,

CHMeMe′, C′HMeMe′, C′HMeMe′), 0.33 (t, J ) 7.2, 3H, PdCH₂CH₃).

[((2,6-(i-Pr)₂C₆H₃)NdC(An)C(An)dN(2,6-(i-Pr)₂C₆H₃))Pd(CH₂-

CH₃)(CH₂dCH₂)]HB(C₆F₅)₃(30b[HB(C₆F₅)₃]). ¹H NMR (CD₂Cl₂,

400 MHz, -80 °C): δ 8.17 (d, J ) 8.0, 1H, An p-H), 8.13 (d, J )

8.0, 1H, An p-H′), 7.48 (m, 8H, An m-H, Ar H), 6.53 (d, J ) 7.6, 1H,

An o-H), 6.45 (d, J ) 7.6, 1H, An o-H′), 4.59 (s, 4H, Pd(CH₂dCH₂)),

3.4 (v br, 1H, HB(C₆F₅)₃), 3.00 and 2.93 (2 septets, J ) 6.8 and 6.4,

2H each, ArCHMeMe′, ArC′HMeMe′), 1.57 (q, J ) 7.3, 2H, PdCH₂-

CH₃), 1.35, 1.29, 0.86, and 0.81 (4 d, J ) 6.8, 6.8, 6.4, and 6.4, 6H

each, CHMeMe′, CHMeMe′, C′HMeMe′, C′HMeMe′), 0.35 (t, J ) 7.3,

3H, PdCH₂CH₃).

[((2,6-(CH₃)₂C₆H₃)NdC(An)C(An)dN(2,6-(CH₃)₂C₆H₃))Pd(CH-

(CH₃)CH₂CH₃)(CH₂dCH₂)]BAr′₄(36a). ¹H NMR (CD₂Cl₂, 400 MHz,

-90 °C): δ 8.10 .(d, J ) 8.0, 1H, An p-H), 8.06 .(d, J ) 8.4, 1H, An

p-H′), 7.77 (br s, 8H, BAr′₄o-H), 7.53-7.45 (m, 2H, An m-H), 7.51

(br s, 4H, BAr′₄p-H), 7.40-7.21 (m, 6H, Ar H), 6.63 (d, J ) 7.2, 1H,

An o-H), 6.60 (d, J ) 6.8, 1H, An o-H′), 4.56 (m, 2H, Pd(CHH′d

CHH′)), 4.53 (m, 2H, Pd(CHH′dCHH′)), 2.25, 2.21, and 2.18 (3 br s,

12H total, ArCH₃, ArCH₃′, Ar′CH₃, Ar′CH₃′), 1.67 (m, 1H, Pd-

(CH(CH₃)CH₂CH₃)), 0.61 (m, 3H, Pd(CH(CH₃)CH₂CH₃)), 0.50 (m, 3H,

Pd(CH(CH₃)CH₂CH₃)), Pd(CH(CH₃)CH₂CH₃) obscured.

Rates of Migratory Insertion, Dissociation, and Association:

NMR Spectroscopy. (a) Ethyl Ethylene Migratory Insertion. Rates

for migratory insertion of ethylene into the Pd-ethyl bond were

determined by adding 20 equiv of ethylene to the ethyl ethylene

complexes (BAr′₄counterion; spectra are described above) and

monitoring the loss of the Pd(CH₂CH₃)₂resonance (ca. 0.4 ppm) over

time (BAr′₄p-H was used as an internal standard). The natural logarithm

of the methyl integral was plotted versus time (first-order treatment)

to obtain kinetic plots (see Supporting Information). Three kinetic runs

were done for each species, and the averages are reported in Table 3.

(b) Isobutyl Ethylene Migratory Insertion. A similar method was

used for determining the rate of migratory insertion for the isobutyl

ethylene complex 35b. Loss of the isobutyl methyl resonance was

monitored with respect to time, and the average barriers obtained are

also reported in Table 3.

(c) Ethylene Dissociation. Rates for ethylene dissociation from the

ethyl ethylene complex 30b[HB(C₆F₅)₃] were determined by monitoring

1

the loss of the labeled CH₂resonance in the H NMR spectrum with

time. These data was treated as a first-order reaction approaching

equilibrium (see Supporting Information for plot).

Pd(II) Alkyl Cations and Alkyl Ethylene Complexes

J. Am. Chem. Soc., Vol. 123, No. 47, 2001 11555

(d) Associative Exchange of Ethylene. The rate of exchange of

bound ethylene with free ethylene in complexes 30a,b[HB(C₆F₅)₃] was

determined by NMR line broadening experiments at -85 °C in CD₂-

Cl₂. Samples were prepared by the reaction of the diethyl complexes

31a,b with a stoichiometric amount of B(C₆F₅)₃, generating the ethyl

ethylene complexes 30a,b[HB(C₆F₅)₃] in situ. After the ¹H NMR

spectrum was acquired in the absence of exchange, ethylene was added

to the NMR tube via gastight syringe and a second spectrum was

obtained at the same temperature. The amount of ethylene in solution

was calculated using the acenaphthyl ortho hydrogens as an internal

standard. Line widths (ω) were measured at half-height in units of Hz

for complexed ethylene and were corrected for line widths (ω_o) in the

absence of exchange. Second-order associative exchange rates were

determined from the standard equation for the slow exchange ap-

proximation, k ) π(ω - ω_o)/[C₂H₄], where [C₂H₄] is the molar

concentration of free ethylene. A pulse delay of 60 s was used. (The

T₁of free ethylene is 15 s.) These experiments were repeated twice,

and the average second-order rate constants are reported in Table 2.

Migratory Insertion Rate Ratios: Gas Chromatography. Ratios

of rate constants were obtained by GC for primary versus secondary

insertion using the butyl ethylene complexes (36a/37a and 36b/37b)

and propyl ethylene complexes (9b/10b) described above. A representa-

tive procedure is outlined here for complexes 36b/37b. A small, flame-

dried Schlenk flask was charged with 28b (38.3 mg, 0.025 mmol) in

an argon-filled drybox. The flask was cooled to -78 °C, and CH₂Cl₂

(2.5 mL) was added via syringe. Ethylene (0.61 mL, 0.025 mmol) was

added to the orange solution via gastight syringe. The flask was rapidly

warmed to -20 °C and was stirred at this temperature for 10 min. The

reaction was then quenched by the addition of excess HSiEt₃(0.1 mL).

The volatile products were then vacuum transferred away from the Pd

material and analyzed by GC. The following temperature program was

employed: injector temperature, 250 °C; detector temperature, 250 °C;

initial temperature, 35 °C; initial time, 15 min; ramp rate, 20 °C/min;

final temperature, 250 °C; final time, 15 min. Peaks were identified by

elution time on the basis of standards run separately as solutions in

CH₂Cl₂. Retention times were as follows: 5.63 min (ethylene); 5.95

min (n-propane); 6.43 min (2-methyl propane); 6.77 min (n-butane);

8.00 min (2-methyl butane); 8.86 (n-pentane); 9.04 min (diethyl ether);

9.55 min (methylene chloride); 10.14 min (2,2-dimethylbutane); 11.92

min (2-methylpentane); 12.89 min (3-methylpentane); 14.30 min (n-

hexane); 20.21 min (n-heptane); 22.07 min (triethylsilane); 24.09 min

(n-octane).

Acknowledgment. The authors wish to thank the NSF

(Grant CHE-0107810) and DuPont for funding. L.H.S. acknowl-

edges the NSF for a graduate research fellowship. M.B. thanks

the Humboldt Foundation for support during preparation of the

manuscript.

Supporting Information Available: Preparation and spec-

tral details for 13b, 14a,b, and 15a,b and graphs for the

determination of rates of migratory insertion for 30a,b[BAr′₄]

and 35a,b and ethylene dissociation in 30b[HB(C₆F₅)₃]. This

material is available free of charge via the Internet at

http://pubs.acs.org.

JA011055J

Article Doi

Palladium(II) β-agostic alkyl cations and alkyl ethylene complexes: Investigation of polymer chain isomerization mechanisms

DOI: 10.1021/ja011055j

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