Kinetic and mechanistic studies of geometrical isomerism in neutral square-planar methylpalladium complexes bearing unsymmetrical bidentate ligands of α-aminoaldimines

Article abstract of DOI:10.1021/ic900296y

A series of hemilabile ligands of α-aminoaldimines and their methylpalladium complexes have been prepared and characterized. Neutral square-planar methylpalladium complexes in the form of [R¹R ²NCMe₂CH=NR]Pd(Me)CI (R=Me, R¹=R²=Me (3a); R=Me, R¹=R²=Et (3b); R=B, R¹=R ²=Me (4a); R=ⁿPr, R¹=R²=Me (5a); R='Pr, R¹=R²=Me (6a); R=ⁱPr, R ¹=R²=Et (6b); R=ⁱPr, (R¹, R ²)=c-C₄H₈ (6c); R=ⁱPr, R ¹='Pr, R²=H (6d); R=ⁱPr, R¹= ^tBu, R²=H (6e); R=^tBu, R¹=R ²= Me (7a); R=^tBu, R¹=R²=Et (7b); R=^tBu, (R¹, R²)=c-C₄H₈ (7c); R=^tBu, R¹=ⁱPr, R²=H (7d); R=^tBu, R¹=^tBu, R²=H (7e); R= Ph, R¹=R²=Me (8a); R=Ph, R¹=R²=Et (8b)) show geometrical isomerism. The relative ratios of trans/cis isomers appear to be predominated by the steric hindrance between the Pd-bound methyl group and imino or amino substituents (R and R¹ and R²). The NMR studies for the substitution reaction of (COD)Pd(Me)CI with Et ₂NCMe₂CH=NⁱPr at -20 °C indicate that c/s-6b is the major kinetic product, which isomerizes to the thermodynamic product in trans form quantitatively above -5 °C. Kinetic results show that the ligand substitution reaction likely undergoes an associative pathway, and the isomerization reaction proceeds via an intramolecular process that comprises imine dissociation and recoordination.

Full text of DOI:10.1021/ic900296y

Inorg. Chem. 2009, 48, 7639–7644 7639
DOI: 10.1021/ic900296y
Kinetic and Mechanistic Studies of Geometrical Isomerism in Neutral Square-
Planar Methylpalladium Complexes Bearing Unsymmetrical Bidentate Ligands of
r-Aminoaldimines
Feng-Zhao Yang, Yu-Heng Wang, Mu-Chieh Chang, Kuo-Hsuan Yu, Shou-Ling Huang, Yi-Hung Liu, Yu Wang,
Shiuh-Tzung Liu, and Jwu-Ting Chen*
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
Received February 13, 2009
A series of hemilabile ligands of R-aminoaldimines and their methylpalladium complexes have been prepared and
characterized. Neutral square-planar methylpalladium complexes in the form of [R1R2NCMe2CHdNR]Pd(Me)Cl
(R=Me, R1=R2=Me (3a); R=Me, R1=R2=Et (3b); R=Et, R1=R2=Me (4a); R=nPr, R1=R2=Me (5a); R=iPr, R1=R2=Me (6a);
R=iPr, R1=R2=Et (6b); R=iPr, (R1, R2)=c-C4H8 (6c); R=iPr, R1=iPr, R2=H (6d); R=iPr, R1=tBu, R2=H (6e); R=tBu, R1=R2=
Me (7a); R=tBu, R1=R2=Et (7b); R=tBu, (R1, R2)=c-C4H8 (7c); R=tBu, R1=iPr, R2=H (7d); R=tBu, R1=tBu, R2=H (7e); R=
Ph, R1=R2=Me (8a); R=Ph, R1=R2=Et (8b)) show geometrical isomerism. The relative ratios of trans/cis isomers
appear to be predominated by the steric hindrance between the Pd-bound methyl group and imino or amino
substituents (R and R1 and R2). The NMR studies for the substitution reaction of (COD)Pd(Me)Cl with
Et2NCMe2CHdNiPr at -20 °C indicate that cis-6b is the major kinetic product, which isomerizes to the
thermodynamic product in trans form quantitatively above -5 °C. Kinetic results show that the ligand substitution
reaction likely undergoes an associative pathway, and the isomerization reaction proceeds via an intramolecular
process that comprises imine dissociation and recoordination.
Introduction
hybrid functionalities of amine and imine that possess nitro-
gen donors with sp2 and sp3 configuration, respectively. We
previously found that these two functionalities appear to
provide comparable trans influence. On the other hand, the
amine and imine of R-amino-aldimines can have substituents
with different numbers and varieties and thus can afford
distinct steric influence on their vicinal ligands.4
In the cases of square-planar [Et2NCMe2CHdNR]Pd-
(Me)Cl (R=iPr (6b), Ph (8b)), the trans configuration that
is defined according to the orientation of heavier donor
Square-planar coordination compounds with unsymme-
trical bidentate ligands may carry the character of geome-
trical isomerism.1 This property is worthy of investigation,
because the structural differentiation in such isomerism
potentially may convey distinct chemical reactivity.2 The
derivatives of R-amino-aldimines in the form of R1R2N-
CMe2CHdNR have been found to serve as hemilabile
bidentate ligands of non-C2 symmetry.3 These ligands bear
*To whom correspondence should be addressed. Tel.: 8862-3366-1659.
Fax: 8862-2363-6359. E-mail: jtchen@ntu.edu.tw.
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Am. Chem. Soc. 1994, 116, 12117–12118. (b) Hamilton, D. H.; Shapley, J. R.
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Clark, C. R. J. Chem. Soc., Dalton Trans. 2001, 758–765. (e) Vicente, J.; Abad,
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119, 12779–12795. (c) Bastero, A.; Claver, C.; Ruiz, A; Castillon, S.; Daura, E.;
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R.; White, A. J. P.; Williams, D. J. J. Organomet. Chem. 2005, 690, 5113–5124.
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ꢀ
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r
2009 American Chemical Society
Published on Web 07/15/2009
pubs.acs.org/IC

7640 Inorganic Chemistry, Vol. 48, No. 16, 2009
Scheme 1. Synthesis of R-Aminoaldimines
Yang et al.
atoms of Cl and N(sp2) toward the metal center has been
found to be the sole isomer.3 DFT calculations for the
geometrical isomers gave energy preferences of -17.1 kJ/
mol for trans-6b and -20.8 kJ/mol for trans-8b to their cis
analogues. The favored stability of the trans form is attrib-
uted to the methyl ligand’s inclination to being seated at the
site cis to imine, for which the Pd-N(sp2)-R angles
(generally >120°) can undertake more steric tolerance than
the Pd-N(sp3)-R angles in amines (generally <110°). The
theme of this study details our further investigation along
the line by varying amino and imino substituents of
R-aminoaldimine ligands in the square-planar chloromethyl-
palladium complexes, which can fine-tune the geometrical
isomerism. The kinetics and mechanism of isomerization and
formation reactions of [Et2NCMe2CHdNR]Pd(Me)Cl have
also been examined.
Scheme 2. Formation of [R1R2NCMe2CHdNR]Pd(Me)Cl
yields. All products were purified by distillation and
characterized mainly by NMR techniques.
The neutral organometallic complexes in the form of
[R1R2NCMe2CHdNR]Pd(Me)Cl (R = Me, R1 = R2 =
Me (3a); R=Me, R1=R2=Et (3b); R=Et, R1=R2=Me
(4a); R=nPr, R1=R2=Me (5a); R=iPr, R1=R2=Me
(6a); R=iPr, R1=R2=Et (6b); R=iPr, (R1, R2)=c-C4H8
(6c); R=iPr, R1=iPr, R2=H (6d); R=iPr, R1=tBu, R2=
H (6e); R=tBu, R1=R2=Me (7a); R=tBu, R1=R2=Et
(7b); R=tBu, (R1, R2)=c-C4H8 (7c); R=tBu, R1=iPr,
R2=H (7d); R=tBu, R1=tBu, R2=H (7e); R=Ph, R1=
R2=Me (8a), R=Ph, R1=R2=Et (8b)) could be readily
prepared via substitution of R-aminoaldimine for COD
(1,5-cyclooctadiene) in (COD)Pd(Me)Cl (Scheme 2).6
The single crystals of trans-6d, trans-6e, cis-7d, and
trans-8a were grown from CH2Cl2/n-hexane. The crystal
data and selected bond parameters are collected in the
Tables S4 and S5 (Supporting Information). The repre-
sentative ORTEP drawings of trans-6e and cis-7d with
thermal ellipsoids at 30% probability are shown in
Figure 1. The four-coordinate molecular structures in
square-planar geometry are confirmed. In the complexes
of the trans form, the bond distances of Pd-N(sp2), Pd-
N(sp3), Pd-C, and Pd-Cl as well as the bite angles of
N(sp2)-Pd-N(sp3) and C-Pd-Cl are quite comparable
to those of known chloromethylpalladium analogues
trans-6b and trans-8b.3 The noticeable difference is that
the angles of C5-N2-Pd in 6d and 6e are substantially
larger (5-7°) than in 6b and 8b. This is presumably due to
Results and Discussion
1. Synthesis and Structures. The synthesis of R-ami-
noaldimine in the form of R1R2NCMe2CHdNR
has succeeded, as illustrated in Scheme 1. Substitution
of a secondary amine for bromine in 2,2-bromomethyl-
propanal (1) gives 2,2-aminomethylpropanal (2).5 Suc-
cessive condensation reactions between 2 and a primary
amine afford the desired products (R=Me, R1=R2=Me
(L3a); R=Me, R1=R2=Me (L3b); R=Et, R1=R2=Me
(L4a); R=nPr, R1=R2=Me (L5a); R=iPr, R1=R2=Me
(L6a); R=iPr, R1 =R2 =Et (L6b); R=iPr, (R1, R2)=
c-C4H8 (L6c); R=tBu, R1=R2=Me (L7a); R=tBu, R1=
R2=Et (L7b); R=tBu, (R1, R2)=c-C4H8 (L7c); R=Ph,
R1=R2=Me (8a), R=Ph, R1=R2=Et (8b)). When iso-
propylamine or tert-butylamine reacts with 1, substitu-
tion and condensation may be achieved in a one-pot
reaction. As a consequence, R-aminoaldimines in the
t
form of RHNCMe2CHdNR (R=iPr (L6d), Bu (L7e))
could be prepared. Using an excess of iso-propylamine
with L7e or tert-butylamine with L6d, the imino substi-
tuent could be replaced, affording R1HNCMe2CHdNR
(R=iPr, R1=tBu (L6e); R=tBu, R1=iPr (L7d)) in fair
(5) (a) Fisher, L. E.; Muchowski, J. M. Org. Prep. Proced. Int. 1990, 22,
399. (b) Sibi, M. P.; Zhang, R.; Manyem, S. J. Am. Chem. Soc. 2003, 125, 9306–
9307. (c) Sibi, M. P.; Stanley, L. M. Tetrahedron: Asymmetry 2004, 15, 3353–
3356. (d) Larionov, S. V.; Tkachev, A. V.; Savel'eva, Z. A.; Myachina, L. I.;
Glinskaya, L. A.; Klevtsova, R. F.; Bizyaev, S. N. Russ. J. Coord. Chem. 2006,
32, 250–260.
(6) (a) Stoccoro, S.; Minghetti, G.; Cinellu, M. A.; Zucca, A.; Manassero,
M. Organometallics 2001, 20, 4111–4113. (b) Liu, W.; Brookhart, M. Organo-
metallics 2004, 23, 6099–6107. (c) Wang, R.; Twamley, B.; Shreeve, J. M. J. Org.
Chem. 2006, 71, 426–429.

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7641
Figure 1. ORTEP drawing of (a) trans-6e and (b) cis-7d. All hydrogen atoms are omitted for clarity.
Table 1. Percentage Ratio of trans Isomers
the steric hindrance among the amino substituents in the
secondary amine possibly being substantially smaller
than that in tertiary amine.
complex
R
R1
R2
rel. ratioa (trans %)
K
Contrary to all trans analogues, the distance of Pd-
3a
3b
4a
5a
6a
6b
6c
6d
6e
7a
7b
7c
7d
7e
8a
8b3
Me
Me
Et
Me
Et
Me
Me
Me
Et
Me
Et
Me
Me
Me
Et
97
.99
.99
.99
93
.99
95
54
78
15
60
34
5.0
12
32
2
3
˚
N(sp ) in cis-7d is 0.11 A longer than that of Pd-N(sp ).
This surprising result suggests that the geometrical con-
figuration is a major factor affecting the bonding rather
than atomic hybridization. This gives further evidence of
that imino- and amino-nitrogen atoms are not very
different in electronic contribution to the coordination
with the metal.3 As a consequence, R-aminoaldimines
may serve the C2-unsymmetrical bidentate auxiliary
ligands with predominant coordinating differentiation
in steric control.
nPr
iPr
iPr
iPr
iPr
iPr
tBu
tBu
tBu
tBu
tBu
Ph
13
19
1.2
3.5
0.18
1.5
0.52
0.053
0.14
c-C4H8
iPr
tBu
Me
Et
H
H
Me
Et
c-C4H8
iPr
tBu
Me
Et
H
H
Me
Et
2. Geometrical Isomerism. The 1H NMR spectra
clearly indicate that there are two geometrical isomers
for the chloromethylpalladium complexes bearing
R-aminoaldimines in most cases of this study. The nuclear
Overhauser effect spectrometry technique provides
the unequivocal assignment for the isomers. The Pd-
bound methyl signals corresponding to the cis isomers
are located in the range of δ 0.5-0.7 and are of higher
field than those corresponding to their trans form (δ 0.6-
0.8). Thus, they serve as effective and convenient probes
for the identification of these isomers.7
The relative ratios for the isomers could be readily
measured by NMR integrations. The percentage ratios
of trans complexes 3-7 are listed in Table 1. At 22 °C, the
relative ratios for the trans forms of 3a, 3b, 4a, 5a, 6a, and
6b are quite comparable and show little dependence on
.99
.99
Ph
a The percentage ratios were measured by H NMR integrations in
CDCl3.
1
the amino substituents. One may conclude that, in such a
square-planar complex, the methyl or ethyl substituent on
the imino group gives indistinguishable hindrance to the
adjacent methyl ligand. For 6d and 6e, secondary amines
can substantially stabilize the cis isomers, when the imino
substituent is an isopropyl group.
The trans abundance shows remarkable depletion for
derivatives 7 in which the imino substituent is a bulky
tertiary butyl group. It appears that the cis configuration
of studied complexes may be favored when the imino
substituent becomes sufficiently bulky. In addition, the
geometrical isomerism becomes susceptible to variation
of the amino substituents. When the relative ratios of the
isomers for the complexes 7 are viewed, the cis derivatives
are predominant, when the amino moiety is of the sec-
ondary class, as in 7d and 7e. When 7b is compared with
7a, in which the two amino substituents are ethyl and
methyl, respectively, the trans percentage drops from 60
to 15. Between 7b and 7c, the amino substituents have the
same composition of C4H8, but in distinct acyclic and
cyclic skeletons. The trans percentages are 66 and 33,
respectively.
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Eur. J. Inorg. Chem. 2002, 732–742. (d) Froseth, M.; Dhindsa, A.; Roise, H.;
Tilset, M. Dalton Trans. 2003, 4516–4524. (e) Groux, L. F.; Weiss, T.; Reddy, D.
N.; Chase, P. A.; Piers, W. E.; Ziegler, T.; Parvez, M.; Benet-Buchholz, J. J. Am.
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Batsanov, A. S.; Dyer, P. W.; Howard, J. A. K. Dalton Trans. 2006, 4134–
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Acta 2006, 89, 1660–1671. (h) Canovese, L.; Visentin, F.; Santo, C.; Levi, C.;
Dolmella, A. Organometallics 2007, 26, 5590–5601. (i) Ceder, R. M.; Muller, G.;
Ordinas, M.; Ordinas, J. I. Dalton Trans. 2007, 83–90. (j) Bella, A. F.; Ruiz, A.;
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2008, 693, 1269–1275.
Such results are attributed to the competition in releas-
ing steric hindrances between Pd-bound methyl and the

7642 Inorganic Chemistry, Vol. 48, No. 16, 2009
Yang et al.
Figure 2. Time-resolved 1H NMR spectra for the reaction of (COD)Pd(Me)Cl and L6b at 0 °C in CDCl3.
Scheme 3. Kinetic and Thermodynamic Products of the Reaction of
(COD)Pd(Me)Cl with Et2NCMe2CHdNiPr (L6b)
were investigated by NMR at varied temperatures.1d,6b,9
In the case of 6b, the mixtures of (COD)Pd(Me)Cl
(14.6 mg, 0.056 mmol) and L6b (47.5 mg, 0.258 mmol)
were dissolved in 0.5 mL of CDCl3 at -30 °C and
monitored. At -20 °C, cis-6b was first observed as
the major product. When the temperature was raised to
-5 °C, the isomerization from cis to trans could be
monitored. At 25 °C, trans-6b accounts for the sole
thermodynamic product of substitution in quantitative
yield (Scheme 3). In the analogous experiment of L8b, no
other isomer could ever been observed besides trans-8b.
In other cases such as L7a, L7b, L7c, and L7e, the
substitution reactions result in geometrical isomerism to
a varied extent.
substituents of imine or amine. When the substituent on
imine is small, the methyl favors being cis to imine,
because of the more spacial vicinity around the sp2
nitrogen compared to the sp3 nitrogen. When the imine
substituent is sufficiently large, the methyl ligand might
compromise by seating at the site cis to amine. The amino
substituents will get a chance to affect the methyl ligand
and thus can fine-tune the geometrical isomerism.1b,1c,8
This is consistent with the aforementioned structural
results.
3. Kinetics and Mechanisms. In order to understand the
mechanism of the isomerization reactions of these
methylpalladium complexes with R-aminoaldimines, the
reactions of (COD)Pd(Me)Cl with R-aminoaldimines
1
Figure 2 displays a time-resolved H NMR study for
the reaction of (COD)Pd(Me)Cl and L6b at 0.0 °C. The
spectral change clearly demonstrates the transformation
of cis-6b to trans-6b. The geometrical isomerization of 6b
was monitored by measuring the signals with respect to
Pd-CH3 results for the first-order kinetics, as illustrated in
::
(9) Rulke, R. E.; Kaasjager, V. E.; Kliphuis, D.; Elsevier, C. J.; van
Leeuwen, P. W. N. M.; Vrieze, K. Organometallics 1996, 15, 668–677. (b)
Setsune, J.-i.; Yamauchi, T.; Tanikawa, S.; Hirose, Y.; Watanabe, J.-y. Organo-
metallics 2004, 23, 6058–6065. (c) Groen, J. H.; Vlaar, M. J. M.; van Leeuwen, P.
W. N. M.; Vrieze, K.; Kooijman, H.; Spek, A. L. J. Organomet. Chem. 1998, 551,
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Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7643
Figure 3. First-order kinetic plots for the isomerization reaction of 6b in
CDCl3 (2, -10 °C; b, 0 °C; (, 10 °C).
Figure 5. Pseudo-first-order kinetic plots for the substitution reactions
of (COD)Pd(Me)Cl with various R-aminoaldimines at 0 °C in CDCl3((,
L6a; 9, L6b; b, L7a; 2, L7c).
k could be evaluated. The evaluation for thermodynamic
parameters of activation was done according to Eyring
relationships. The data for kobsd and ksubs as well as
ΔHsubs‡, ΔSsubs‡, and ΔGsubs‡ corresponding to ksubs are
collected in Table 2.
The negative values of ΔSsubs‡, although small, impli-
cate that the substitution reactions likely undergo an
associative (or intermediate associative) pathway. A me-
chanism for the formation of [R1R2NCMe2CHdNR]-
Pd(Me)Cl via ligand substitution from (COD)Pd(Me)Cl
is also summarized in Scheme 4, in which the R-amino-
aldimine is considered to first attach the metal of
(COD)Pd(Me)Cl.
Figure 4. Eyring relationship for the isomerization reaction of 6b in
CDCl3.
Figure 3. The rate constants kisom could be evaluated as
7.0 ꢀ 10-5 s-1 at -10 °C, 5.3 ꢀ 10-4 s-1 at 0.0 °C, and
3.5 ꢀ 10-3 s-1 at 10 °C. The linear Eyring plot, as shown
in Figure 4, provides the estimation for the enthalpy of
activation (ΔHisom‡) as 118 kJ/mol and the entropy of
activation (ΔSisom‡) as 126 J/mol K. At 0.0 °C, the kisom’s
for 6a and 3b were evaluated similarly as 5.5 ꢀ 10-4 s-1
and 1.4 ꢀ 10-3 s-1, respectively (Figure S1, Supporting
Information), indicating that isomerization is dependent
Looking into the thermodynamic data in Table 2, the
‡
significant differences of ΔGsubs between L6a and L7a
and between L6b and L8b show the dependence on imino
substituents. In addition, the bulkier t-butyl on imine
‡
results in a larger ΔGsubs than does isopropyl. On the
other hand, the reactions for L7a and L7c show compar-
‡
able values of ΔGsubs‡. However, the difference of ΔGsubs
values between the reactions for L6a and L6b is relatively
large. Recalling the kinetic products of the cis configura-
tion, the imine is trans to the methyl ligand, which is of
strong trans effect. A pathway with imine as the entering
functionality may be favored (Scheme 4). A similar path-
way with the amine side as the entering functionality
should not be excluded. The NMR measuring difficulty
has limited experimental flooding conditions; the back-
ward reactions and the possibility of a solvent dependence
thus are overlooked in the studied cases.
Such R-aminoaldimine ligands allow the equipping of a
coordination of bidentate chelation with functionally
different but electronically comparable donors and steri-
cally distinguishable environments around the metal. The
fine-tuned geometrical isomerism in [R1R2NCMe2CHd
NR]Pd(Me)Cl of square-planar configuration simply by
changing amino and imino substituents is a good illustra-
tion of an unsymmetrical bidentate ligand being able to
result in fundamental selectivity of the coordination.
‡
on the imine substituent. The positive value of ΔSisom
suggests that the isomerization reactions likely undergo
dissociative activation. Accordingly, the geometrical iso-
merization is proposed to proceed via a mechanism
of imine dissociation and recoordination, as illustrated
in Scheme 4. Such isomerization reactions in other ana-
logous derivatives appear not as clear as these studied
cases, even at -50 °C. Another similar pathway through
dissociative activation of the amine might not be
excluded.
Kinetic studies for the substitution reactions of
(COD)Pd(Me)Cl with L6a, L6b, L7a, L7c, and L8b were
also done in CDCl3 by recording the 1H NMR integration
for the COD signals corresponding to (COD)Pd(Me)Cl
versus the reaction time. In a typical run, the mixture of
(COD)Pd(Me)Cl (6.0 mg, 0.0134 mmol) and L8b (74 mg,
0.34 mmol) was dissolved in 0.50 mL of thermostatted
CDCl3. The 1H NMR spectra were recorded with suitable
intervals for more than three half-lives. The disappear-
ance of (COD)Pd(Me)Cl follows the first-order kinetics.
The linear kinetic plots corresponding to the reactions
of (COD)Pd(Me)Cl with L6a, L6b, L7a, L7c, and L8b at
0 °C, as shown in Figure 5, give the values of kobsd. Using
the rate law, kobsd=k[L], the second-order rate constants
Experimental Section
General Procedures. Commercially available reagents were
purchased and used without further purification unless other-
wise indicated. Toluene, hexane, and diethyl ether were distilled
from purple solutions of sodium benzophenone ketyl under
nitrogen, and dichloromethane was dried over P2O5 and

7644 Inorganic Chemistry, Vol. 48, No. 16, 2009
Yang et al.
Table 2. Kinetic Data for the Substitution Reactions of (COD)Pd(Me)Cl with R-Aminoaldimines in CDCl3
ksubs (-5 °C) (M-1
s-1
k
subs (0 °C) (M-1 s-ksubs (5 °C) (M-1 s-
k
subs (10 °C) (M-1
s-1
k
subs (15 °C) (M-1 ΔHsubs‡ (kJ/ ΔSsubs‡ (J/mol ΔGsubs‡(kJ/
)
)
)
s-1
)
mol)
K)
mol)
1
1
)
L6a
L6b
L7a
L7c
L8b
1.57 ꢀ 10-2
2.18 ꢀ 10-3
3.22 ꢀ 10-3
1.95 ꢀ 10-3
7.4 ꢀ 10-3
1.9 ꢀ 10-2
3.38 ꢀ 10-3
5.32 ꢀ 10-3
4.03 ꢀ 10-3
1.12 ꢀ 10-2
3.04 ꢀ 10-2
6.05 ꢀ 10-3
8.85 ꢀ 10-3
7.68 ꢀ 10-3
1.76 ꢀ 10-2
5.21 ꢀ 10-2
50.4
75.5
78.0
77.3
60.0
-14.5
-5.45
-3.86
-4.56
-10.9
52.7
77.7
80.3
79.4
62.3
9.2 ꢀ 10-4
2.12 ꢀ 10-2
1.19 ꢀ 10-2
3.90 ꢀ 10-3
Scheme 4. Mechanism of Formation and Isomerization of [R1R2NCMe2CHdNR]Pd(Me)Cl
distilled immediately prior to use. Air-sensitive material was
manipulated under a nitrogen atmosphere in a glovebox or by
standard Schlenk techniques. The IR spectra were recorded on a
Bio-Rad FTS-40 spectrophotometer. The NMR spectra were
measured on a Bruker AC-300, AC-400, or AC-500 spectro-
meter. The corresponding frequencies for 13C NMR spectra
were 75.469, 100.625, or 125.753 MHz, respectively. Values
upfield of 1H and 13C data are given in δ (ppm) relative to
tetramethylsilane (δ 0.00) in CDCl3. All spectra were obtained at
ambient temperature unless stated otherwise. Mass spectro-
metric analyses were collected on a JEOL SX-102A or
WATERS LCT Premier XE spectrometer. Elemental analysis
was done on a Perkin-Elmer 2400 CHN analyzer. Details for the
synthesis of the ligands and complexes are given in the Support-
ing Information.
ments of linear regression for ln[(COD)Pd(Me)Cl] versus time
afford the evaluation of pseudo-first-order rate constants, kobsd
.
The second-order rate constants kat various temperatures could be
determined according to the rate law of kobsd=k[L]. The plots of
Eyring relationship, ln(k/T) = -ΔH‡/RT þ ln(kB/h) þ ΔS‡/R,
provide an evaluation of enthalpies and entropies of activation.10
For the isomerization reactions, the disappearance of the cis isomer
and the formation of the trans isomer, monitored by the 1H NMR,
give consistent first-order rate constants.
Acknowledgment. We thank the National Science Council,
Taiwan, ROC, and the NSC-NWO joint project for the financial
support.
Supporting Information Available: Kinetics data of geometric
isomerism; crystallographic data of 6d, 6e, 7d, and 8a in CIF
format; ORTEP drawings of 6d and 8a; and the characterization
and synthetic procedure of the new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
Kinetic Study. Kinetic measurements for the reactions of
(COD)Pd(Me)Cl and L6a, L6b, L7a, and L8b were done under
pseudo-first-order conditions. In a typical run, the mixtures
of (COD)Pd(Me)Cl (6.0 mg, 0.013 mmol) and L6b (74 mg, 0.40
1
mmol) in 0.5 mL of CDCl3 were set at -5 °C. The H NMR
(10) (a) Eyring, H. J. Chem. Phys. 1935, 3, 107. (b) Laidler, K. J.; King, M.
C. J. Phys. Chem. 1983, 87, 2657–2664. (c) Lente, G.; Fabian, I.; Poe, A. J. New
J. Chem. 2005, 29, 759–760.
integrations were measured at intervals of 30 s. The disappearance
of (COD)Pd(Me)Cl with time follows first-order kinetics. Treat-