Substitution Kinetics of Palladium Pincer Complexes

Nasir Sallau Lawal

Centre for Analysis and Synthesis (CAS)

Department of Chemistry

Lund University 2012

ACKNOWLEDGEMENT

My sincere gratitude goes to my supervisor, Professor Ola F. Wendt, whose guidance and

wisdom helped me to shape my material and research work. His ability to identify errors in my

research work and writing continuously challenged my thinking. I am indebted to André

Fleckhaus for his guidance and interesting discussions in the lab and in compiling this thesis.

I also appreciate the contribution of the Wendt group their assistance in the lab has been very

helpful. I would not forget my late father, Alh. Lawal Sallau, the person of whom has supported

me morally, socially and financially throughout my academic pursuit. Thanks to my mother,

family members, relatives and friends for being around whenever I need them. You know I can’t

include all your names here and I hope you will forgive me.

I would like to thank Adbelrazek Mousa, Halilu Sale, Attahir Abubakar, and my inlaw, Shehu, for

their kindness, friendship and support. Thanks to all my friends in Lund University.

This thesis would not have been possible without the help, support and guidance of all the

persons mentioned above.

CONTENTS

Abstract i

Abbreviations ii

Introduction 1

Project Aim 3

Experimental 3

Results 4

Discussion 8

Conclusion 11

References 12

Appendix 14

ABSTRACT

The substitution reaction of complexes, (PCNR)PdCl, where R = (methyl) (1), (ethyl) (2) and

(propyl) (3), with iodide where studied in 1-10 mM LiCl in methanol. The substitution was

followed under pseudo first order conditions with an excess of incoming and leaving ligand. The

high reactivity of complex 1 was attributed to the low steric bulkiness of the methyl

substituent. The order of reactivity of the complexes followed 1 > 2 ≈ 3. The large negative

values reported for entropy of activation for the complexes confirmed an associative

substitution mode. The substitution reaction proceeds via the solvolytic pathway.

ABBREVIATIONS

PCNnPr 1-((dipropylamino)methyl)-3-((di-tert-butylphosphino)methyl)-benzene.

PCNEt 1-((diethylamino)methyl)-3-((di-tert-butylphosphino)methyl)-benzene.

PCNMe 1-((dimethylamino)methyl)-3-((di-tert-butylphosphino)methyl)-benzene.

dien diethylentriamine.

MeEt4dien 4-methyl-1, 1,7,7-tetraethyldiethylenetriamine.

TLtBu 2,6-bis[(1,3-di-tert-butylimidazolin-2-imino)methyl]pyridine.

tpdm terpyridinedimethane.

bpma bis(2-pyridylmethyl)amine.

terpy 2,2’,6’,2’’-terpyridine.

Std standard deviation.

L Ligand (two electron donor ligand).

M Metal (square planar metal complex).

X Halide or one electron donor ligand.

1

INTRODUCTION

Ligand substitution is often the first step in a stoichiometric and catalytic cycle of

organometallic reactions. It is the first step in the catalytic cycle of for instance, asymmetric

hydrogenation of carbon-carbon double bond and beta-ketoesters which results in the

synthesis of enantiometrically pure compounds used in pharmaceutical industries. It appears in

coupling of carbon-carbon bonds to be specific, the ligand exchange step in Heck reaction and

Stille cross-coupling, and the generation of palladium zero in Sonogashira cross-coupling.1 This

carbon-carbon bond formation has led to the synthesis of complex organic natural products

such as pyranicin.2 In anti-cancer therapy, drugs such as cisplatin are designed to inhibit tumor

growth by substituting the chloride ligand with guanosine base of DNA.3

Steric hindrance and electronic properties of ligand trans and/or cis to the leaving group play a

great role on the reactivity of square planar Pd (II) complexes. For instance, the rate of

substitution in [Pd(dien)Cl]+ is approximately 105 orders of magnitude higher than

[Pd(MeEt4dien)Cl]+.4 Also, nucleophilic substitution in [(TLtBu)PdCl]+ complex is about four times

faster than its sterically hindered analogue [Pd(tpdm)Cl]+.5 In both cases, the decrease in

reactivity was rationalized in terms of steric hindrance.

Numerous publications have been done on synthesis and catalysis of palladium pincer

complexes6 but little has been done on the kinetics of nucleophile substitution reaction and

mechanism of these complexes. With the advances in science, scientists have become more

interested in getting all the details of a reaction. It is one thing to synthesize a compound and

put into use but having a detailed mechanistic insight of the compound is the key to

improvements and explanations on how it behaves.7

Pincer palladacycles are a well-established family of organometallic complexes with many

applications in synthesis, catalysis and material science. Although most pincer complexes are

symmetrical, unsymmetrical pincer palladacycles are now reported. These complexes are easy

to handle and are air and moisture stable with high thermal stability. The pincer ligands (i.e.

donor atoms) can be tuned to change the electronic and steric properties of the metal center

which provides an opportunity for improvement.8 Pincer ligands coordinated to the metal

center are abbreviated as EYE, where E = atom on side arm and Y = central atom. For instance, a

complex with amino ligands (E = NR2) in the side arms and carbon atom in the center would be

called NCN complex.6

Palladium pincer complexes catalyze many important reactions. They are extensively used in

cross-coupling reactions, mostly in Heck and Suzuki-Miyaura coupling reactions as well as in

Sonogashira, Stille, Negishi, and Hiyama coupling reactions.6 Most of the synthetic results of

these reaction are characterized by high turnovers9 and interesting asymmetric applications10.

2

In aldol and Michael reactions palladium pincer complexes are used as Lewis acid catalyst. Their

main application has been towards the synthesis of enantiomeric pure compounds, such as the

synthesis of oxazolines and its derivatives11 and Michael addition of alpha-cyanoester to methyl

vinyl ketone12. In both aldol and Michael reactions enantiomeric pure pincer based catalyst

were used. These reactions have shown moderate to high enantioselectivity. Palladium pincer

complexes are also used in the allylation of aldehydes, carbon dioxide and imines. The PCP type

of pincer complexes have shown to be successful in these reactions.13 Moreover, palladium

pincer complexes are used in the synthesis of organometallic reagents. The palladium pincer

complex with σ-donor heteroatoms in the side arms such as in NCN and SeCSe type pincer

complexes and their analogues can be used to synthesize allenyl stannanes and silanes as well

as the selective preparation of allyl stannanes and boronates.6

Substitution reactions in square planar complexes have been extensively investigated and

generally found to obey a two term rate law of the form.7

( ) ( )

where = solvent path and = direct path.

The rate law is normally interpreted by the mechanism in scheme 1.

Scheme 1.

The reaction proceeds through two parallel reaction paths (i.e. the solvolytic and direct path)

and/or proceed to equilibrium rather than completion. This reaction depends on the nature of

the ligands attached to the metal center and the reaction conditions under which the reaction

proceeds. The ligand substitution proceeds mostly via a five coordinate transition state (i.e an

associative mode of activation) with few cases of dissociative mode.7 Determination of ligand

substitution mechanisms relied on kinetic probes such as the enthalpy, entropy and volume of

activation. The enthalpy and entropy can be determined from temperature dependence of a

rate constant from the Eyring equation (Eq. 2), where is the Boltzmann’s and is the

Planck’s constant.

(

) (

)

( )

3

The volume of activation can be determined from the pressure dependence of a rate constant

using eq. (3).14

( )

Substitution reaction of square planar Pt (II) and Pd (II) complexes have been well established

to proceed mostly via an associative mode of activation.15 Most of the complexes reported in

the literature that undergo substitution reaction are not of the pincer type and the Pd (II)

systems are mainly cyclometalated complexes of tridentate ligands such as diethylentriamine

(dien), bis(2-pyridylmethyl)amine(bpma) or 2,2’,6’,2’’-terpyridine (terpy). Palladium (II)

cyclometalated complexes having a nitrogen or carbon bond trans to the leaving ligand have

been shown to obey an associative mode of activation.5,16 Studies on cyclometalated complexes

having trans Pt (II)-carbon bond coordination also obey an associative mode of activation.17 To

the best of my knowledge reports on the substitution reaction of Pd (II) pincer type complexes

having a trans Pd-C bond are scarce. Herein we report the equilibrium behavior of three pincer

palladacycles.

PROJECT AIM

The objective of this project is to study the substitution reaction of palladium (II) pincer

complexes and provide detailed mechanistic insight on their behaviors.

EXPERIMENTAL

Materials: Compounds 1, 2, and 3 were synthesized in our lab by André Fleckhaus. Sodium

iodide and lithium chloride were obtained from Merck. Methanol was obtained from Sigma

Aldrich. Reaction solutions of sodium iodide were freshly prepared for each kinetic run.

Kinetics: The stopped-flow experiments were performed on an Applied Photo Physics Bio

sequential SX-17 MX stopped-flow spectrophotometer. The substitution of chloride by iodide

was studied in methanol by observing the increase in absorbance at 316 nm. The complex

solution (0.1 mM) contained chloride (1-10 mM) and was mixed with at least a ten-fold excess

of iodide (2.5-50 mM), assuring the reaction condition is under pseudo first order conditions.

The kinetic traces were fitted to single exponentials using the software provided by Applied

Photo Physics spectrophotometer. This gave observed rate constants at different

concentrations of leaving and incoming ligands. Rate constants are given as an average of at

least 5 runs. Variable temperature measurements were made between 20 and 52˚C. Time

resolved spectra were also recorded on the applied photo physics spectrophotometer.

UV/Vis Equilibrium Measurements: UV/Visible spectra were obtained on a Cary 100 Bio UV-

Visible spectrophotometer. The equilibrium absorbance for the reaction was measured for

4

different iodide concentrations and the values were used in fitting to the rate law. The

equilibrium absorbance for the different temperature runs was measured at a constant chloride

concentration of 5 mM.

NMR Mesurements: NMR measurements were performed in C6D6. 1H and 31P NMR spectra

were recorded on a Varian Unity INOVA 500 spectrometer working at 499.77 MHz (1H).

Chemical shifts are given in ppm downfield from TMS using residual solvent peaks (1H NMR) or

using H3PO4 as an external reference (31P).

RESULTS

Equilibria: The reaction (scheme 2) was studied in methanol with iodide as incoming ligand

giving 4, 5 and 6 products.

Scheme 2: Substitution reaction of the complexes.

Complexes 4, 5 and 6 were extracted from the reaction mixture and characterized by 31P-NMR

and 1H-NMR. The 31P-NMR of complexes 4, 5 and 6 has a singlet at 93.89, 93.78 and 96.89 ppm

respectively. Compared to the 31P-NMR of complexes 1, 2 and 3 which has a singlet at 90.99,

90.84 and 93.79 ppm respectively,18 there is approximately a difference in chemical shift of 3

ppm for all the complexes. Also, there is a small difference in chemical shifts for the 1H-NMR of

4, 5 and 6 compared to 1, 2 and 3 respectively, in almost all regions of the spectrum. Although,

there is a small difference in chemical shift of both 31P-NMR and 1H-NMR of complexes 4, 5 and

6 indicating a small change in the electronic environment, we cannot say that it is the products

(4, 5 and 6) not the reactants (1, 2 and 3).

The equilibrium constant for the reactions were determined using spectrophotometry by fitting

to eq. (5) at different concentration of incoming and leaving ligands (cf. figure 1 and Appendix

figure 1) and the data obtained were used in fitting to the rate law.

( )

where = equilibrium absorbance, = absorbance before reaction takes place.

5

0 20 40 60

0.30

0.33

0.36

0.39

A

[NaI]/mM

(a)

0 20 40 60

0.28

0.32

0.36

A

[NaI]/mM

(b)

0 20 40 60

0.30

0.33

0.36

A

[NaI]/mM

(c)

Figure 1: Equilibrium absorbance as a function of iodide concentration at 5 mM chloride concentration.

The solid lines represent best fits to equation (5). a) (PCNnPr)PdCl, ( ) T = 22˚C, ( ) T = 24 ˚C, ( ) T = 28 ˚C,

( ) T = 35 ˚C and ( ) T = 42 ˚C. b) (PCNEt)PdCl, ( ) T = 24˚C, ( ) T = 28 ˚C, ( ) T = 35 ˚C, ( ) T = 42 ˚C and ( ) T

= 52 ˚C. c) (PCNMe)PdCl ( ) T = 24˚C, ( ) T = 28 ˚C, ( ) T = 35 ˚C, ( ) T = 42 ˚C and ( ) T = 52 ˚C.

was determined at different temperatures and eq. (6) was fitted to these data giving ΔS˚

and ΔH˚ for the over-all process, (cf. figure 2).

( )

( )

The resulting equilibrium constants and thermodynamic parameters are reported in table 1 and

2 respectively.

6

0.0032 0.0033 0.0034

1.92

1.96

2.00

2.04

2.08

ln(K

eq)

1/T (K-1)

(a)

0.0031 0.0032 0.0033 0.0034

1.26

1.32

1.38

1.44

1.50

ln(K

eq)

1/T (K)

(b)

0.0031 0.0032 0.0033 0.0034

1.36

1.40

1.44

1.48

1.52

1.56

ln(K

eq)

1/T (K-1)

(c)

Figure 2: Van’t Hoff plots for the complexes. The solid lines represent best fits to equation (6). a)

(PCNnPr)PdCl. T = 22-42˚C. b) (PCNEt)PdCl. T = 20-42˚C. c) (PCNMe)PdCl. T = 20-52˚C. ( ) data excluded from

fitting to equation (6).

Kinetics: The reaction in scheme 2 was studied at different concentrations of leaving and

incoming ligands. Plots of observed rate constant vs. [I-] for different [Cl-] give a nonlinear

curve, (cf. figure 3).

7

0 10 20 30 40 50

0.2

0.3

0.4

0.5

k0

bs/s

-1

[NaI]/mM

(a)

0 10 20 30 40 50

0.2

0.3

0.4

0.5

ko

bs/s

-1

[NaI]/mM

(b)

0 10 20 30 40 50

12

18

24

ko

bs/s

-1

[NaI]/mM

(c)

Figure 3: Observed pseudo first order rate constants for the solvolytic path as a function of entering

ligand at different chloride concentrations. T = 25°C. The solid lines represent best fits to equation (10).

a, (PCNnPr)PdCl, b, (PCNEt)PdCl, c, (PCNMe)PdCl. ( ) 1.0 mM, ( ) 2.5 mM, ( ) 5.0 mM, and ( ) 10.0 mM

LiCl.

The rate law of the form of eq. (7) was derived assuming steady state for the solvento complex

in scheme 1.19

( )

The expression for the overall equilibrium constant can be employed to form eq. (8) and (9)

containing only three variables.

( ( )⁄

( )

8

( ( )⁄

( )

where,

( )

The enthalpies and entropies of activation were determined by fitting eq. (2) to the observed

rate constant at different temperatures (cf. Appendix figure 2). The resulting activation

parameters are reported in table 2. All fittings were performed using OriginPro 8 software.

DISCUSSION

Rate Laws and mode of activation: The reaction in scheme 2 is a reversible reaction and the

forward and reverse reactions contribute to the over-all rate expression. The rate law in eq.

(10) represents only the solvolytic pathway in scheme 1 assuming steady state for the solvento

species and the fact that figure 4 shows a well-defined isobestic point indicates that there is the

presence of small amount of solvento complex, validates this approximation, (cf, figure 4). A

rate law of this form was also reported previously.19-22

Scheme 3: Direct vs. solvolytic pathways for the substitution reaction.

9

In each series of experimental runs [Cl-] was kept constant, whereas [I-] varied (figure 3). This

means that both the denominator and nominator of the solvolytic path (eq. (10)) increase with

increase in iodide [I-], resulting in an approximately constant value of ⁄ as observed

(table 1).

280 300 320 340

0.0

0.5

1.0

Abs

Wavelenght (nm)

Figure 4: Time-resolved spectra for the substitution reaction with iodide as nucleophile. T = 25˚C,

[(PCNMe)PdCl] = 0.1 mM, [Cl-] = 1 mM, [I-] = 10 mM. Scans at 0.125, 0.4, 0.6, 0.8, 1.3, 1.9, 2.5, 3.1, 3.8,

and 5.0 s.

Table 1: Reaction conditions and rate constants determined after iteration for complex 1, 2, and

3. T = 25°C. (PCNR)PdCl = 0.1 mM.

(PCNR)PdCl [Cl-]

(mM) (s

-1) Ave.

(s-1)

⁄ Ave.

⁄

(s-1)

3 1.0 0.43±0.01 0.49±0.04 0.63±0.05 0.60±0.04 0.13 2.27±1.99 3 2.5 0.48±0.01 0.62±0.03

3 5.0 0.51±0.01 0.53±0.03 3 10.0 0.52±0.01 0.62±0.03

2 1.0 0.53±0.003 0.54±0.02 0.62±0.02 0.63±0.05 0.19 1.84±0.48 2 2.5 0.53±0.003 0.71±0.02 2 5.0 0.54±0.005 0.56±0.01 2 10.0 0.57±0.006 0.62±0.01

1 1.0 25.52±0.25 27.42±1.37 1.04±0.07 0.82±0.14 9.47 2.36±1.22 1 2.5 26.97±0.30 0.83±0.03 1 5.0 27.91±0.39 0.72±0.03 1 10.0 29.28±0.57 0.67±0.03

10

a. Obtained by substituting values and in the equilibrium expression,

.

Table 2: Activation and thermodynamic parameters determined from the Eyring and Van’t Hoff

plots of the substitution reaction of the complexes.

(PCNR)PdCl

Observed Rate

constant

(kJmol-1)

(JK-1mol-1)

(kJmol-1)

(JK-1mol-1)

(kJmol-1)

3 67.27±0.18 -26.62±0.58 -5.54±0.09 -1.46±0.31 -5.11

2 65.16±0.20 -32.15±0.66 -7.30±0.07 -11.92±0.21 -3.75

1 37.64±0.92 -89.40±2.97 -4.35±0.002 -2.11±0.01 -3.72

The activation parameters are typical of associative mechanism (Ia). All the entropies are large

and negative especially for complex 1 which has the largest value. Thus, the reaction proceeds

via a five coordinate transition state where bond formation is more prominent than bond

breaking. The effect of steric blocking and/or weakening of the Pd-Cl bond and the high trans

effect associated with metal-aryl bonds in this case do not induce a mechanistic changeover to

a dissociative mechanism as reported previously.23 Thus, changing the cis-ligands in these

complexes has no effect on the mode of activation and also as reported recently for

cyclometalated palladium complexes with nitrogen donor cis-ligands.5

The thermodynamic parameters for the reaction of complex 2 has the highest entropy and

enthalpy change compared to the reaction of complex 1 and 3. The high entropy change for the

reaction of complex 2 was compensated by its high enthalpy change thereby having a similar

Gibbs free energy change compared to the reaction of complex 1. The reaction of complex 3

has high products than the reaction of complex 2 and 1 as indicated from their Gibb’s free

energy, which is opposite of what was expected.

Solvolytic vs. direct path: The first part of the rate law in eq. (9) represents the direct pathway

while the second part represents the solvolytic pathway (in scheme 3). The two pathways are

reversible. Fitting data obtained from the observed rate constant and equilibrium constant

determined to the whole rate law gives , and the ratio of (appendix table 2).

The values of seems to decrease and become negative with increase in chloride

concentration, this indicates that the direct part is negligible. Fitting data to the solvolytic path

in eq. (10) gave good results (table 1). The possibility of the substitution reaction proceeding via

the direct pathway can be rule out in this case. Thus, the complexes follow only the reversible

solvolytic pathway in scheme 3.

11

Steric and electronic effects on the rate: All the three complexes are square planar. The metal

center is bond to phosphine, nitrogen, carbon and chloride atoms. The carbon atom is

coordinated trans to the chloride. The ground state Pd-Cl bond strength is similar for complexes

1, 3 and 2, as indicated from the Pd-Cl bond length of 2.40 Å, 2.40 Å and 2.41 Å, respectively.

Also the bond length of the Pd-nitrogen and Pd-phosphine bonds are similar.24 The main

difference between the three complexes are the substrates coordinated to the nitrogen donor

atom. Thus, the complexes have similar electronic properties as such electronic effect on the

reactivity of the metal complexes can be ruled out. The reactivity of the complexes can be

rationalized in terms of steric hindrance by the cis ligands. For example, complex 3 reacts

slower than 2 due to steric blocking by the two propyl groups above and below the plane of the

complex and is 10 orders of magnitude slower than 1 which has two methyl groups attached to

the nitrogen substrate.

CONCLUSION

In the kinetic study of the nucleophilic substitution reaction of the palladium pincer type

complexes, it was found that the reaction is reversible. The substitution reactions of the

complexes obey only the solvolytic pathway of the two term rate law of eq. (9) (i.e. eq. 10). The

possibility of the reaction proceeding through direct pathway can be ruled out. The complexes

are activated via associative (Ia) mode and tuning of the metal complex does not change the

substitution mechanism, which remains associative, as supported by the large and negative

entropy values. Steric hindrance from the cis ligands is responsible for the low reactivity of

complex 3 and high reactivity of complex 1.

12

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14

Appendix

Table 1: Observed pseudo first order rate constant for complex 1, 2, and 3 (PCNR)PdCl = 0.1

mM. T = 25.0±0.2˚C.

(PCNR)PdCl [Iodide] (mM)

Kobs (1.0 mM LiCl)

Kobs (2.5 mM LiCl)

Kobs (5 mM LiCl)

Kobs (10 mM LiCl)

3 2.5 0.31292 0.25248 0.19496 0.18004

3 5.0 0.35732 0.33388 0.25996 0.23708

3 10 0.38842 0.3834 0.31788 0.28496

3 25 0.41364 0.42792 0.40976 0.38376

3 50 0.42748 0.44808 0.4444 0.41696

2 2.5 0.3930 0.34028 0.2504 0.24128

2 5.0 0.4386 0.39306 0.29814 0.28236

2 10 0.4751 0.44732 0.3571 0.3354

2 25 0.5071 0.4903 0.4363 0.42482

2 50 0.5182 0.50764 0.48402 0.47482

1 2.5 21.300 17.366 13.294 10.58

1 5.0 23.798 20.646 16.89 13.634

1 10 24.018 22.92 20.148 17.234

1 25 25.000 24.984 23.594 21.608

1 50 25.192 26.054 25.496 24.124

15

Table 2: Reaction conditions and rate constants determined after iteration for complex 1, 2, and

3. T = 25°C.

(PCNR)PdCl [LiCl] (s

-1) std ⁄ std (mM-1s-1) std

3 1.0 0.39653 0.07304 0.49396 0.17625 0.04023 0.0775

3 2.5 0.86354 0.23193 1.09587 0.23238 -0.15991 0.09467

3 5.0 0.58226 0.04925 0.61643 0.09541 -0.01606 0.01148

3 10 0.72529 0.10853 0.94089 0.15015 -0.02392 0.01191

2 1.0 0.54517 0.01934 0.34333 0.03056 -0.01255 0.02177

2 2.5 0.70956 0.03844 0.43469 0.02255 -0.07034 0.01656

2 5.0 0.75705 0.03519 0.3469 0.01949 -0.03926 0.00816

2 10 1.13748 0.08141 0.43077 0.02001 -0.05466 0.00891

1 1.0 25.48318 0.16537 1.19617 0.06239 0.02a 0

1 2.5 26.6104 4.43937 0.95782 0.20727 0.09576 1.8526

1 5.0 29.0476 2.46157 0.92972 0.12535 -0.3357 0.54278

1 10 32.76339 2.35456 1.10294 0.12914 -0.54126 0.27132

a, value fixed because of large errors.

16

0 10 20 30 40 50

0.12

0.16

0.20

0.24

A

[NaI]/mM

(a)

0 10 20 30 40 50

0.25

0.30

0.35

0.40

A

[NaI]/mM

(b)

0 20 40 60

0.24

0.30

0.36

A

[NaI]/mM

(c)

Figure 1: Equilibrium absorbance as a function of iodide concentration at different chloride

concentrations. T = 25˚C. The solid lines represent best fits to equation (5). a, (PCNnPr)PdCl. b,

(PCNEt)PdCl. c, (PCNMe)PdCl. ( ) 1.0 mM, ( ) 2.5 mM, ( ) 5.0 mM, and ( ) 10.0 mM LiCl.

17

0.0032 0.0033 0.0034

-6.4

-5.6

-4.8

ln(K

ob

s/T

)

T-1 (K)

(a)

0.00315 0.00320 0.00325 0.00330 0.00335 0.00340 0.00345

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

ln(K

ob

s/T

)

T-1 (K)

(b)

0.0031 0.0032 0.0033 0.0034

-2.4

-1.8

-1.2

ln(K

0b

s/T

)

T-1 (K)

(c)

Figure 2: Eyring plots of the palladium complexes. a, (PCNnPr)PdCl. T = 22-42˚C b, (PCNEt)PdCl. T = 20-

42˚C. c, (PCNMe)PdCl. T = 24-52˚C.