Reactivity of bis(pyridyl)- N -alkylaminato methylpalladium complexes toward ethylene: insights from experiment and theory

Article abstract of DOI:10.1021/om5001293

A series of novel neutral and cationic methylpalladium complexes bearing N-alkyl-2,2-dipyridylaldiminato ligands were prepared and characterized. In the presence of ethylene, the cationic complexes were active as dimerization catalysts, producing a mixture of 1- and 2-butenes. A Pd-ethyl π-ethylene species was identified as the catalyst resting state by low-temperature spectroscopic and DFT studies, which provided insights into the effect of both steric and electronic factors on the observed reactivity.

Full text of DOI:10.1021/om5001293

Article
pubs.acs.org/Organometallics
Reactivity of Bis(pyridyl)‑N‑alkylaminato Methylpalladium
Complexes toward Ethylene: Insights from Experiment and Theory
Andrew J. Swarts,† Feng Zheng,† Vincent J. Smith,† Ebbe Nordlander,‡ and Selwyn F. Mapolie*,†
†Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa
‡Inorganic Chemistry Research Group, Chemical Physics, Center for Chemistry and Chemical Engineering, Lund University, P.O.
Box 124, SE-22100 Lund, Sweden
S
* Supporting Information
ABSTRACT: A series of novel neutral and cationic methylpalladium complexes bearing N-alkyl-2,2′-dipyridylaldiminato ligands
were prepared and characterized. In the presence of ethylene, the cationic complexes were active as dimerization catalysts,
producing a mixture of 1- and 2-butenes. A Pd−ethyl π-ethylene species was identified as the catalyst resting state by low-
temperature spectroscopic and DFT studies, which provided insights into the effect of both steric and electronic factors on the
observed reactivity.
They found that initial ethylene insertion rates increased with
an increase in electrophilicity of the metal center, as a result of a
decrease in σ-donor ability of the ligand (imidazole > pyridine
> pyrazole), as well as steric bulk at the metal center. Also,
catalytic dimerization proceeded via a mechanism analogous to
that reported by Brookhart and co-workers. In addition, an
increase in steric bulk around the metal center led to branched
polymer formation, thereby inhibiting associative chain transfer.
Herein we report the development of neutral and cationic
methylpalladium complexes bearing N-alkyl-2,2′-dipyridyl-
amine ligands and the evaluation of their reactivity toward
ethylene, employing low-temperature spectroscopic and mo-
lecular modeling techniques. Our specific aims were to evaluate
the effect of changing the bridgehead atom from carbon to
nitrogen (in comparison to the systems reported by Jordan) as
well as the effect of the introduction of alkyl groups ortho to the
metal coordination sphere on insertion and chain transfer rates.
1. INTRODUCTION
The development of olefin oligo- and polymerization catalysts
bearing neutral bidentate nitrogen donor ligands has seen
phenomenal research interest over the past 20 years.1 Of
particular significance are the contributions by Brookhart and
co-workers with respect to the development of diimine late-
transition-metal catalysts of nickel and palladium, which
depending on the catalyst structure produced ethylene
oligomers and polyethylene as well as copolymers of ethylene
with polar and nonpolar α, internal, and cyclic olefins.2 In
particular, their research efforts gave fundamental insight into
the mechanism of olefin oligo-/polymerization employing these
catalyst systems. It was demonstrated that chain transfer occurs
via associative olefin exchange and that the introduction of
steric bulk in the ortho position of the diimine chelate ring
positioned these substituents above and below the axial
coordination sites within the square plane of the metal. This
effectively inhibits associative ligand exchange, disfavoring chain
transfer and allowing the production of polyolefins. In addition,
it was demonstrated that, in the catalytic dimerization of
ethylene by a [(N−N)PdR]+ complex (N−N = phenanthro-
line), dimerization involves an insertion/β-H elimination
mechanism and that the ethyl ethylene complex is the catalyst
resting state.3 In relation to this, Jordan and co-workers
examined the catalytic behavior of a series of cationic palladium
alkyl complexes bearing bis(heterocycle)methane ligands.4
2. EXPERIMENTAL SECTION
2.1. General Considerations. All transformations were per-
formed using standard Schlenk techniques under a nitrogen
atmosphere. Solvents were dried by distillation prior to use, and all
other reagents were employed as obtained. NMR (1H, 300 and 400
MHz; 13C, 75 and 100 MHz) spectra were recorded on Varian
Received: February 4, 2014
Published: April 22, 2014
© 2014 American Chemical Society
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VNMRS 300 MHz and Varian Unity Inova 400 MHz spectrometers;
chemical shifts are reported in ppm, referenced relative to the residual
protons of the deuterated solvents and tetramethylsilane (TMS) as
internal standard. ESI-MS (positive and negative mode) analyses were
performed on Waters API Quattro Micro and Waters API Q-TOF
Ultima instruments by direct injection of the sample. FT-IR analysis
was performed on a Thermo Nicolet AVATAR 330 instrument, and
spectra were recorded neat (ATR) unless otherwise specified. Melting
point determinations were performed on a Stuart Scientific SMP3
melting point apparatus and are reported as uncorrected. [(COD)-
PdCl2] and [(COD)PdMeCl] were prepared according to literature
procedures.5 Access to computational hardware employed in the
molecular modeling study was provided by the Centre for High-
Performance Computing (CHPC) via the “Sun Hybrid System”.
Details of ligand (a−e) and complex syntheses (1a−e, 2a−e, and 3a−
e) are given in the Supporting Information.
internal standard, δ 1.99) were determined by careful integration. A
plot of ln(APdMe/A0,PdMe) versus time (at −10 °C), where APdMe
=
IPdMe/IMeCN and A0,PdMe = I0,PdMe/IMeCN, was linear with the slope of
the line equal to −kinsert,Me. For 4e kinsert,Me = [5.5( 0.5)] × 10−4 s−1 at
−10 °C.
2.4. Kinetics of Insertion of Ethylene into the Pd−Me Bond
of [PdEt(N−N)(CH2CH2)]+[BAr′4]− Species 6a,e. 2.4.1. Proce-
dure for 6a. A solution of 4a was generated as described in the
presence of ethylene. The formation of butenes was allowed to
proceed at −10 °C with 1H NMR spectra recorded every 30 min for a
period of 195 min. Values of Ibutenes and IMeCN, where Ibutenes = the
integration of the methyl resonances of cis- and trans-2-butene (δ
1.59) and 1-butene (δ 0.98) and IMeCN = the integration of the methyl
resonance of free MeCN (δ 1.99), were determined by integration of
each spectrum. These values were used to determine (mol of butenes
produced)/(mol of catalyst) (turnovers). A plot of turnovers versus
time was linear (r2 = 0.997). The slope of this plot equals kinsert,Et. For
6a, kinsert,Et = 4 × 10−4 s−1.
2.2. Generation of [PdMe(N−N)(CH2CH2)]+[BAr′4]− Species
4a,e. 2.2.1. [PdMe(2,2′-dipyridyl-N-methylamine)(CH2
CH2)]+[BAr′4]− (4a). A valved NMR tube was charged with cationic
complex 2a (30 mg, 0.0248 mmol) and CD2Cl2 (0.7 mL). After the
tube was purged with argon, the complex solution was frozen at −196
°C and subjected to three freeze−pump−thaw cycles to remove
dissolved oxygen. To the frozen solution was added excess ethylene,
and the solution was warmed to −78 °C. A 1H NMR spectrum
recorded (−60 °C) after 90 min showed the quantitative formation of
species 4a (100% relative to free MeCN). Under these conditions
exchange of free and coordinated ethylene was observed to be fast on
the NMR chemical shift time scale. 1H NMR (CD2Cl2, 400 MHz, −60
°C): δ 8.20 (poorly resolved d, 1H, H1/1′); δ 7.93 (poorly resolved d,
1H, H1/1′); δ 7.83 (br m, 2H, H3/3′); δ 7.24 (m, 3H, H4/4′/2); δ 7.16
(poorly resolved t, 1H, H2′); δ 5.39 (br s, coordinated and free
ethylene); δ 3.49 (s, 3H, H6); δ 1.97 (s, 3H, free MeCN); δ 0.60 (s,
3H, Pd−Me).
2.2.2. [PdMe(6,6′-dimethyl-2,2′-dipyridyl-N-methylamine)(CH2
CH2)]+[BAr′4]− (4e). A valved NMR tube was charged with cationic
complex 2e (30 mg, 0.0248 mmol) and CD2Cl2 (0.7 mL). After the
tube was purged with argon, the complex solution was frozen at −196
°C and subjected to three freeze−pump−thaw cycles to remove
dissolved oxygen. To the frozen solution was added excess ethylene,
and the solution was warmed to −40 °C. A 1H NMR spectrum
recorded (−60 °C) after 300 min showed the quantitative formation
of species 4e (100% relative to free MeCN). Under these conditions
exchange of free and coordinated ethylene was observed to be slow on
the NMR chemical shift time scale. 1H NMR (CD2Cl2, 400 MHz, −60
°C): δ 7.74 (masked t, 1H, H3′); δ 7.67 (poorly resolved t, 1H, H3); δ
7.09 (m, 4H, H2,2′,4,4′); δ 5.41 (br s, coordinated and free ethylene); δ
3.50 (s, 3H, H6); δ 2.74 (s, 3H, H7′); δ 2.51 (s, 3H, H7); δ 0.47 (s, 3H,
Pd−Me).
2.4.2. Procedure for 6e. A solution of 4e was generated as
described in the presence of ethylene. The formation of butenes was
1
allowed to proceed at −10 °C with H NMR spectra recorded every
30 min for a period of 195 min. Values of Ibutenes and IMeCN, where
Ibutenes = the integration of the methyl resonances of cis- and trans-2-
butene (δ 1.59) and 1-butene (δ 0.98) and IMeCN = the integration of
the methyl resonance of free MeCN (δ 1.99), were determined by
integration of each spectrum. These values were used to determine
(mol of butenes produced)/(mol of catalyst) (turnovers). A plot of
turnovers versus time was linear (r2 = 0.994). The slope of this plot
equals kinsert,Et. For 6e, kinsert,Et = 0.8 × 10−4 s−1.
2.5. Details of High-Pressure Catalytic Investigations. A 250
mL Parr stainless steel autoclave was sealed under an inert atmosphere
in a glovebox. Under a stream of ethylene, a solution of the precatalyst
(30 μmol) in dichloromethane (20 mL) was added. The reactor was
pressurized to the required amount with ethylene, and the reaction was
allowed to proceed at the required temperature for 18 h. The volume
of ethylene consumed throughout the reaction was monitored by
employing a mass flow meter. After the allotted time, the reactor was
cooled to −78 °C and the reaction mixture quenched with MeOH (5
mL), taking care to maintain the temperature below −20 °C to
minimize the loss of any volatile reaction products. A liquid sample
was filtered through a syringe filter, and the filtrate was analyzed by
GC-FID employing p-xylene as internal standard. Turnover
frequencies were calculated from the volume of ethylene consumed
as a function of time. Relative amounts of oligomers were obtained
from the GC analysis using authentic GC standards for the oligomers
and employing an internal standard. The remainder of the reaction
mixture was subjected to workup at low temperature to determine
whether long-chain oligomers had formed. No long-chain oligomers or
polymers were formed during the reactions.
2.6. Details of Molecular Modeling. All intermediates and
transition states along the reaction coordinate were calculated using
the Dmol3 density functional theory (DFT) code6 as implemented in
the Accelrys MaterialsStudio (version 5.5) software package. The
nonlocal generalized gradient approximation (GGA) exchange-
correlation functional was employed in all geometry optimizations,
specifically the PW91 functional of Perdew and Wang.7 The Dmol3
code utilizes a basis set of numeric atomic functions, which are exact
solutions to the Kohn−Sham equations for the atoms.8 In general,
these basis sets are more complete than a comparable set of linearly
independent Gaussian functions. In addition, these basis sets have
been demonstrated to have small basis set superposition errors. The
computational investigation employed an all-electron polarized split
valence basis set, termed double numeric polarized (DNP). All
geometry optimizations employed highly efficient delocalized internal
coordinates.9 This has the advantage of significantly decreasing the
number of iterations required for larger molecules during geometry
optimization in comparison to traditional Cartesian coordinates. The
tolerance for convergence of the self-consistent field (SCF) density
was set to 1 × 10−5 hartree, while the tolerance for energy convergence
was set at 1 × 10−6 hartree. Additional convergence criteria include the
2.3. Kinetics of Insertion of Ethylene into the Pd−Me Bond
of [PdMe(N−N)(CH2CH2)]+[BAr′4]− Species 4a,e. 2.3.1. Proce-
dure for 4a. A solution of species 4a was generated as described above
in the presence of ethylene. The solution was kept at −10 °C for 5
1
min, equilibrated at −78 °C for 3 min and a H NMR spectrum
recorded at −60 °C. This procedure was repeated at 5 min intervals.
Values corresponding to I0,PdMe (I0,PdMe = the integral of the Pd−Me
resonance at the start of the experiment, δ 0.60 ppm), IPdMe (IPdMe
=
the integral of the Pd−Me resonance after each 5 min interval) and
IMeCN (IMeCN = the integral of free MeCN as internal standard, δ 1.99)
were determined by careful integration. A plot of ln(APdMe/A0,PdMe
)
versus time (at −10 °C), where APdMe = IPdMe/IMeCN and A0,PdMe
=
I0,PdMe/IMeCN, was linear with the slope of the line equal to −kinsert,Me
.
For 4a kinsert,Me = [9( 1)] × 10−4 s−1 at −10 °C.
2.3.2. Procedure for 4e. A solution of species 4e was generated as
described above in the presence of ethylene. The solution was kept at
1
−10 °C for 10 min and equilibrated at −78 °C for 3 min, and a H
NMR spectrum was recorded at −60 °C. This procedure was repeated
in 10 min intervals. Values corresponding to I0,PdMe (I0,PdMe = the
integral of the Pd−Me resonance at the start of the experiment, δ 0.60
ppm), IPdMe (IPdMe = the integral of the Pd−Me resonance after each
10 min interval) and IMeCN (IMeCN = the integral of free MeCN as
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Scheme 1. Synthesis of Ligands and Neutral and Cationic Methylpalladium Complexes
tolerance for converged gradient (0.002 hartree/Å) and the tolerance
for converged atom displacement (0.005 Å).
Optimized geometries were subjected to full frequency analyses at
the same GGA/PW91/DNP level of theory to verify the nature of the
stationary points.
whereas the preparations of c and e have not been reported.
The ligands were isolated, after column chromatography, as
yellow oils (a, c−e) or a solid (b) in 60−90% yield. The ligands
displayed solubility in most organic solvents and were
1
characterized by FT-IR and H NMR spectroscopy (a, b, d)
Equilibrium geometries were characterized by the absence of
imaginary frequencies, while optimized transition state geometries
exhibited a single imaginary frequency in the reaction coordinate.
Preliminary transition state geometries were obtained by employing
either the Dmol3 PES scan functionality in Cerius10 or the integrated
linear synchronous transit/quadratic synchronous transit (LST/QST)
algorithm11 available in MaterialsStudio. All computations were
performed with the exclusion of solvent effects, and the results
obtained were mass-balanced for the isolated system in the gas phase.
All reported energies incorporate Gibbs free energy corrections to the
total electronic energies at 298.15 K and 1 atm.
2.7. X-ray Crystal Structure Determination. Single crystals of
complexes 1e, 3e, and 3e-A were mounted on glass fibers or nylon
loops and centered in a stream of cold nitrogen at 100(2) or 150(2) K.
Crystal evaluation and data collection was performed on a Bruker-
Nonius SMART Apex II CCD diffractometer with Mo Kα radiation (λ
= 0.71073 Å). Data collection, reduction, and refinement were
performed using SAINT12 and SADABS,13 which forms part of the
APEX II software package. The structures were solved by direct
methods and refined by full-matrix least squares on F2 using SHELX-
9714 within the X-Seed graphic user interface.15 All non-hydrogen
atoms were refined anisotropically, and all hydrogen atoms were
placed using calculated positions and riding models.
as well as 13C{1H} NMR spectroscopy, ESI-MS, and elemental
analysis (c, e).
The 2,2′-dipyridyl-N-alkylamine ligands a−e were reacted
with the palladium precursor [(COD)PdMeCl] to afford the
neutral methylpalladium complexes 1a−e (Scheme 1). The
complexes were isolated as pure orange or pale yellow solids in
60−80% yield, displaying solubility in chlorinated organic
solvents while being insoluble in aromatic solvents, alkanes,
alcohols, and ethers. These complexes were characterized by
FT-IR spectroscopy, 1H and 13C{1H} NMR spectroscopy, mass
spectrometry, melting point determination, and elemental
analysis.
Single crystals of complex 1e suitable for SCD analysis were
grown by vapor diffusion of diethyl ether into a concentrated
dichloromethane solution of the complex. The molecular
structure of complex 1e is shown in Figure 1, while selected
crystallographic data are given in the Supporting Information
(Table S1). The geometry around palladium is distorted square
planar, with geometric distortion as a result of diimine
chelation. The most significant bond lengths and angles are
observed in the expected range for these types of complexes.19
The cationic methylpalladium complexes 2a−e were
obtained by reaction of the neutral complexes with a chloride
abstractor, NaBAr′4 (Ar′ = 3,5-bis(trifluoromethyl)phenyl), in
the presence of acetonitrile as coordinating solvent (Scheme 1).
The cationic complexes were isolated after workup as solids in
60−80% yield; they were soluble in chlorinated solvents and
ethers and were insoluble in alcohols, aromatic solvents, and
alkanes.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization of Ligands and
Complexes. A series of N-alkyl 2,2′-dipyridylamine ligands of
general formula (2-C5H3NR)2NR′ (a, R = H, R′ = Me; b, R =
H, R′ = benzyl; c, R = H, R′ = methylcyclohexyl; d, R = H, R′ =
neopentyl; e, R = Me, R′ = Me) were prepared by a modified
method involving base-mediated N-alkylation (Scheme 1).16
Ligands a,17 b,16 and d18 have been reported previously,
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Figure 1. Molecular structure of complex 1e, drawn with 50%
probability ellipsoids. Hydrogen atoms are omitted for clarity.
The complexes are prone to decomposition to Pd black, as
evidenced during the reaction and during workup procedures.
The isolated cationic complexes were characterized by FT-IR
Figure 2. Molecular structure of complex 3e, drawn with 50%
probability ellipsoids. The BAr′4 counterion and hydrogen atoms are
omitted for clarity.
1
spectroscopy, H and 13C{1H} NMR spectroscopy, and mass
spectrometry.
Following the observation that the cationic acetonitrile-
ligated methylpalladium complexes undergo decomposition
both in the solid state and in solution, an attempt was made to
prepare more stable analogues. It was envisaged that an increase
in the basicity of the coordinating ligand would enhance the
stability of the electrophilic cationic metal center and prevent
the observed decomposition. To this end, the neutral
complexes 1a−e were reacted with NaBAr′4 in the presence
of pyridine (Scheme 1). The cationic pyridine analogues 3a−e
were isolated after workup and recrystallization from chloro-
form/pentane as off-white air- and moisture-stable crystalline
solids in high yields. The isolated complexes displayed solubility
in polar organic solvents and were insoluble in alkanes. The
molecular structure of complex 3e was unambiguously
determined by SCD analysis, with selected crystallographic
data (Table S1, Supporting Information) tabulated. The
molecular structure of the complex shows a dissociated ion
pair (Pd−B bond distance 8.095 Å), with the cationic fragment
in which the coordination sphere of the metal is occupied by
the neutral N-alkyl dipyridylaldimine ligand and the anionic Me
group, whereas the vacant coordination site is occupied by a
pyridine molecule (Figure 2). The geometry around the
palladium center is distorted square planar, with the largest
deviation observed in the N1−Pd1−N1′ angle (83.41°) as a
result of complexation. Significant bond lengths and angles are
in the range observed for analogous N,N-methyl palladium
complexes.20,21 The three fluorine atoms on C16, C18A, and
C18C of the tetravalent anion show positional disorder, with
each fluorine atom of the group occupying two positions within
the crystal lattice as a result of rotational motion about the
Scheme 2. Effect of Pyridine Ratio on Product Formation
during Attempted Synthesis of 3a−e
proton resonances showed the formation of two products in an
approximately 1/1 ratio, identified as the desired complex 3e
and the new complex 3e-A, the structure of which was
unambiguously established by SCD analysis. Single crystals
were grown by diffusion of pentane into a solution of the
isolated material in dichloromethane.
The molecular structure of 3e-A (Figure 3) shows a cation−
anion pair with the cation consisting of a slightly distorted
square planar palladium(II) center in which the coordination
sphere is occupied by three neutral pyridine ligands and an
anionic methyl group. The ion pair is effectively dissociated, as
evidenced by the large Pd−B bond distance of 13.639 Å. The
Pd−N(pyridine) bond distances are 2.028(2) Å (Pd−N1),
2.160(1) Å (Pd−N2), and 2.030(2) Å (Pd−N3), with the Pd−
N2 bond distance being elongated due to the trans influence of
the Me group. The Pd−Me bond distance is 2.015(2) Å, with
both Pd−N and Pd−Me bond distances being in the range
previously observed for analogous Pd(II) complexes.22
Piers and co-workers made a similar observation during their
attempted preparation of palladium complexes derived from
aryltrifluoroborate−phosphine ligands with pyridine derivatives
as coordinating solvents.23 They found that reaction of the
palladium precursor with the potentially bidentate trifluor-
oborate phosphine ligand in the presence of pyridine generated
exclusively complexes of the type in which the coordination
sphere of palladium is occupied by two pyridine molecules, the
phosphine moiety of the ligand, and the Me group initially
present in the precursor.
CPh−CCF bond.
3
During the synthesis of the cationic pyridine analogues 3a−e,
it was observed that the ratio of pyridine to neutral complex
had a significant effect on the product formed. Figure S1
1
(Supporting Information) shows a H NMR spectrum of the
material isolated during the attempted synthesis of complex 3e,
in which pyridine as coordinating solvent was added in excess
(4/1 dichloromethane/pyridine, Scheme 2). The observed
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highest TOF (nethylene consumed/(nPd h)) value of 7.4 h−1 obtained
for complex 2d.
The observed TOF values are significantly lower than what
has been reported previously for a number of cationic
palladium systems. Kress and co-workers reported the
application of unsymmetrical diimine-ligated cationic palladium
complexes in ethylene oligomerization. Their catalytic system
showed activities of up to 105 kgoligomer molPd h−1 with a
−1
Schulz−Flory distribution of oligomeric products.24 Brookhart
and co-workers reported ethylene oligomerization catalyzed by
a cationic palladium phosphine−oxazoline complex. Complexes
with no axial steric bulk yielded only butenes as oligomerization
products with no reported TOF values, while the complexes
with axial steric bulk yielded a Schulz−Flory distribution of α-
olefins with TOF values of up to 15760 h−1.2e
Jordan and co-workers have previously demonstrated the
effect of temperature on the activity of Pd(II) bis(pyrazolyl)-
methane complexes in ethylene oligomerization and found that
increased reaction temperatures (room temperature vs −10 °C)
led to a significant decrease in activity.4a
Figure 3. Molecular structure of complex 3e-A, drawn with 50%
probability ellipsoids. The BAr′4 counterion and hydrogen atoms are
omitted for clarity.
It should be noted that the observed catalytic activity in
ethylene oligomerization is still higher than that reported for
cationic palladium complexes bearing pyridine−phosphine
ligands.25 The low activity of complexes 2a−e over an extended
period of time (18 h) may be attributed to their inherent
instability and specifically their thermal instability in solution, as
evidenced by observable decomposition to palladium black
during catalysis, within 2 h after commencing the reaction. This
was also evidenced by the volume of ethylene consumed
slowing down dramatically after approximately 2 h of reaction.
Analysis of the reaction products by GC-FID showed that the
only products were 1-butene and cis- and trans-2-butenes
(Supporting Information, Figure S1). The selectivity for 1- and
2-butenes may be rationalized on the basis of an enhanced rate
for chain termination in comparison to chain propagation.
Chain termination rates have previously been shown to increase
with a decrease in axial steric bulk of the coordinated ligand,
even when the complexes with and without steric bulk
displayed similar electronic properties.2e,3a,26 No significant
differences in the ratio of 1-butene to 2-butenes were observed
for complexes 2a−d (Table 1, entries 1−4), while for 2e this
ratio was lower (Table 1, entry 5). In addition, reactions at
ambient and increased pressures (1 and 10 bar, respectively)
showed very little change in activity, thereby suggesting zero-
order kinetics in ethylene.27 Our preparative-scale catalytic
results showed that the complexes displaying the most
significant differences in activity were 2a,e. We postulated
that the differences in steric bulk around the palladium metal
center may have an effect on initial ethylene coordination and
insertion kinetics. To gain insight into these processes, we
reacted complexes 2a,e with excess ethylene at low temper-
atures and monitored the formation of the π-ethylene species
and its subsequent insertion kinetics. The reaction of the
cationic acetonitrile-coordinated complexes 2a,e with ethylene
at −78 °C generated the cationic π-ethylene species 4a,e,
respectively (Scheme 4). For both species 4a,e the ligand
backbone proton resonances remained inequivalent on going
from the acetonitrile to the π-ethylene adduct, indicating that
ethylene exchange is stereospecific and is a result of ethylene
exchange being fast on the NMR time scale.4b For species 4a
the pyridyl ring imine protons are observed as two poorly
resolved doublets integrating for a single proton each at δ 8.20
and 7.93 ppm, respectively. The remaining aromatic resonances
3.2. Preparative-Scale Ethylene Oligomerization and
Ethylene Insertion Kinetics. Preparative-scale ethylene
oligomerization reactions were conducted at room temperature,
employing cationic complexes 2a−e as precatalysts (Scheme 3
Scheme 3. Preparative-Scale Ethylene Oligomerization
Employing Cationic Complexes 2a−e
and Table 1). We rationalized that olefin exchange with
coordinated acetonitrile would be more facile in comparison to
the pyridine-ligated analogues, owing to a decrease in basicity of
the coordinated ligand.23 Our results showed that these
complexes oligomerize ethylene with low activities, with the
Table 1. Preparative-Scale Ethylene Oligomerization
a
Employing Complexes 2a−e as Catalyst Precursors
b
c
d
entry
complex
V
TOF
selectivity (1-C4:2-C4)
1
2
3
4
5
2a
2b
2c
2d
2e
17
15
18
20
12
5.6
5.5
6.7
7.4
4.4
1:9
1:8
1:7
1:9
1:2
a
Reaction parameters: ethylene pressure, 5 bar; solvent DCM, 20 mL;
b
reaction temperature, 25 °C; reaction time, 18 h. Volume of ethylene
c
−1
consumed in mL. TOF in units of molethylene molPd
h−1
.
d
Determined by GC-FID. The ratio of cis- to trans-2-butene is
approximately 1:2 in all cases.
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species 4a,e at −10 °C, the formation of the Pd−propyl species
5a,e could be observed. It should be noted that the proton
resonances for this species were not assigned, except for the
terminal Me group. The kinetic plots of ethylene insertion into
the Pd−Me bond of species 4a,e are shown in the Supporting
Information (Figures S3 and S4, respectively). For species 4a
Scheme 4. Generation of Pd−Methyl π-Ethylene Species
k
insert,Me was found to be [9( 1)] × 10−4 s−1. This value is in a
range similar to that reported for the insertion of ethylene into
the Pd−Me bond of cationic bis(5-methylpyridyl)methane
palladium complexes and suggests that changing the bridgehead
atom from carbon to nitrogen has a negligible effect on the
electronic nature of the metal center.4b
In addition, the obtained insertion rate was 3 times faster
than that observed for the cationic phenanthroline-ligated
palladium complex reported by Brookhart.3a In the case of
species 4e, kinsert,Me was found to be [5.5( 0.5)] × 10−4 s−1. In
comparison to species 4a the insertion rate is approximately 1.5
times slower for species 4e. This decrease may be attributed to
a decrease in the electrophilicity of the metal center due to the
electron-donating ability of the methyl substituents with a
concomitant decrease in reactivity toward ethylene.
A similar observation was made by Milani and co-workers
when evaluating the insertion aptitude of methyl-substituted
phenanthroline palladium complexes.28
The reaction of species 5a,e with ethylene at −10 °C allowed
for the detection of ethylene oligomers and identification of the
Pd−ethyl π-ethylene species 6a,e, as the catalyst resting state
(Scheme 6 and Supporting Information, Figure S5). In species
are observed as a multiplet at δ 7.24 ppm (H2,4,4′) and a poorly
resolved triplet at δ 7.16 ppm (H2′). The Pd−Me resonance
was shifted upfield in comparison to that of the acetonitrile
adduct and was observed as a singlet at δ 0.60 ppm.
In the case of species 4e the pyridyl ring aromatic resonances
corresponding to H3/3′ were observed as two broad triplets at δ
7.74 and 7.67 ppm, respectively, while the remaining aromatic
proton resonances were observed as a multiplet integrating for
four protons at δ 7.09 ppm (H2,2′,4,4′). An analogous upfield
shift of the Pd−Me resonance was observed as a singlet at δ
0.47 ppm. In the case of 2a, ethylene exchange was observed to
be fast on the NMR time scale, as evidenced by a broad
resonance at δ 5.40 ppm corresponding to free and coordinated
ethylene. In contrast, the introduction of steric bulk as in the
case of 2e results in ethylene exchange being slower, as
evidenced by the presence of both free and bound ethylene (δ
4.82 and 4.50 ppm). This was also observed in the time
required for quantitative conversion to 4a,e. In the case of 2a in
the presence of ethylene, quantitative formation of 4a was
observed after 90 min at −78 °C, while in the case of 2e the
ratio of the starting acetonitrile complex to π-ethylene species
was approximately 1:3 after 150 min at −78 °C (Supporting
Information, Figure S2). Analogous observations of decreased
associative exchange rates with increased steric bulk have been
reported previously for other palladium complexes.2a,3b
Scheme 6. Reaction showing the Formation of the Pd−Ethyl
π-Ethylene Species as the Catalyst Resting State
The rate of ethylene insertion into the Pd−Me bond was
determined for both species 4a,e at −10 °C (Scheme 5). By
monitoring the disappearance of the methyl resonance in
6a the PdCH2Me resonances are diastereotopic and thus were
observed as poorly resolved pentets at δ 2.06 and 1.35 ppm,
respectively. The PdCH2Me resonance was observed as a triplet
(3JH−H 7.42) at δ 0.58 ppm. Analogously, for species 6e the
PdCH2Me resonances were observed at δ 1.90 and 1.44 ppm,
while the PdCH2Me resonance was observed as a triplet (3JH−H
= 7.42 Hz) at δ 0.57 ppm.
Scheme 5. Ethylene Insertion for Pd−Methyl π-Ethylene
Species 4a,e and Generation of the Catalyst Resting State
The identification of the Pd−ethyl π-ethylene species as the
resting state confirms the nature of the reaction being first
order in palladium and zero order in ethylene.26b Since the
kinetics of the catalyst system is zero order in ethylene,
quantifying the moles of butenes formed as a function of time
allows for an estimate of the rate of ethylene insertion into the
Pd−Et bond (Supporting Information, Figures S6 and S7,
respectively).
In the case of 6a, kinsert,Et was found to be 4.0 × 10−4 s−1,
while for 6e, kinsert,Et was found to be 0.83 × 10−4 s−1. It should
be noted that, in the NMR experiments conducted at −10 °C,
dimerization continues until all of the ethylene is consumed,
evidence that the catalyst maintains activity at low temperatures
for a significantly longer period of time. The difference in
insertion rates is comparable to the difference observed for
kinsert,Me values and reflects the differences in electrophilicity
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Figure 4. Computed energy profile for comparative ethylene oligomerization starting from the catalyst resting state, 6a,e. Gibbs free energies
(ΔG298.15K) are reported at 298.15 K and 1 atm.
between these two complexes. Furthermore, significant differ-
ences were observed between 6a and 6e, for the propensity to
isomerize 1-butene to 2-butenes. In the case of 6a, the ratio of
2-butenes to 1-butene after 45 min of reaction at −10 °C was
found to be 12:1, which remained largely unchanged during the
reaction period of 195 min (Supporting Information, Figure
S8). In the case of 6e, the ratio of 2-butenes to 1-butene after
45 min of reaction was found to be approximately 1:1, varying
only slightly throughout the reaction period (Supporting
Information, Figure S9). This observation demonstrated that
the introduction of steric bulk in the coordination sphere of the
active species increases the barrier to rotation required for the
formation of 2-butenes, as is the case for 6e. Thus, evaluating
the reactivity of these complexes at lower temperatures allowed
for the observation of differences in product selectivities as a
result of differences in steric bulk.
Our spectroscopic investigation showed that (i) an
introduction of steric bulk decreases associative olefin exchange
rates significantly, (ii) the ensuing decrease in electrophilicity
decreases ethylene insertion rates, and (iii) the propensity for
1-butene isomerization is significantly decreased. Also, ethylene
insertion and subsequent oligomerization via the catalyst
resting state, Pd−ethyl π-ethylene species, is significantly
slower for 6a than for 6e. To gain further insight, the ethylene
oligomerization mechanism was investigated by density func-
tional theory (DFT) calculations.
alternative chain termination pathways are given in the
Supporting Information.
The insertion of ethylene from the catalyst resting state, 6a,e,
was calculated to proceed via TS[6ainsertion] and TS[6einsertion
]
endothermically by 18.8 kcal/mol (relative to 6a) and 17.4
kcal/mol (relative to 6e), respectively (Figure 4). As observed
for analogous insertion transition state structures (Supporting
Information), the coordinated ethylene is rotated in plane with
a Cβ(ethylene)−Cα(ethyl) bond distance of 2.102 Å (for 6a)
and 2.098 Å (for 6e). The insertion products 7a,e were
optimized as cationic Pd−butyl species stabilized by a β-agostic
interaction with Pd−Hβ bond distances of 1.815 and 1.813 Å,
respectively. Species 7a was calculated to be exothermic relative
to 6a by 13.7 kcal/mol, whereas 7e was found to be
endothermic by 5.2 kcal/mol relative to 6e. The overall results
of our calculations regarding chain termination via BHE or
BHT to generate 1-butene or further insertion to generate
longer chain Pd−alkyl species were similar to those obtained
for the elimination of 1-propene from species 5a,e (see
Supporting Information). Chain termination was calculated to
proceed via β-hydride elimination, with β-hydrogen transfer or
further ethylene insertion calculated to be significantly more
endothermic. For both species 7a,e the late-stage transition
states TS[7aBHE] and TS[7eBHE] were located and calculated to
be endothermic by 5.7 kcal/mol in the case of 7a, while
TS[7eBHE] was calculated to be exothermic by 9.5 kcal/mol
relative to 7e. This significant difference in the propensity for
BHE may be attributed to steric differences, whereas the driving
force for elimination is the generation of a π-olefin complex in
which the coordinated olefin is oriented perpendicular to the
square plane of the metal center. This preferred orientation
significantly reduces the o-Me/olefin interactions in species 7e,
interactions which are absent in the case of 7a. The optimized
3.3. Reactivity Differences Probed by DFT. Since our
kinetic studies identified the Pd−ethyl π-ethylene species 6a,e
as the catalyst resting state, we sought to evaluate the relative
energetics associated with 1- and 2-butene formation computa-
tionally. Details of our calculations to generate the catalyst
resting state, side reactions to form longer chain α-olefins, and
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structures of the transition states TS[7a/7eBHE] are charac-
terized by in-plane rotation of the butyl fragment, analogous to
that observed for TS[5aBHE] and TS[5eBHE], and displayed a
Cβ−Hβ interaction of 1.778 Å (in the case of 7a) and 1.951 Å
(in the case of 7e). The products of BHE, 8a,e, were calculated
to be endothermic by 5.4 kcal/mol (in the case of 7a) and
exothermic by 10.3 kcal/mol (in the case of 7e) and were
characterized by the perpendicular orientation of the eliminated
1-butene as well as a Pd−H bond distance of 1.555 Å and a
Pd−Cα and Pd−Cβ bond distances of 2.221 and 2.300 Å,
respectively.
Finally, we sought to evaluate the energetics of 1-butene
reinsertion and β-hydride elimination from species 8a,e to
generate 2-butenes, with a specific emphasis on the effect steric
differences would have on these two catalyst precursors, since
2-butenes are the major catalysis products observed. Scheme 7
shows the intermediates and their computed energies relative to
8a,e, which we evaluated along the reaction coordinate to
generate cis-2-butene.
the cis- and trans-Pd-sec-butyl(Pd−CH2 agostic) species 10acis
and 10etrans, exothermic by 8.6 and 8.2 kcal/mol, respectively
(relative to 8a). The optimized geometries displayed a Pd−Hβ
agostic interaction with a distance of 1.813 Å for the cis
analogue and 1.812 Å for the trans analogue. In contrast, for 9e,
isomerization via rotation of the Pd−C bond via TS[9einsertion
]
was calculated to be exothermic by 10.3 kcal/mol (relative to
8e). This difference in behavior can be attributed to the release
of steric pressure as the alkyl chain is rotated out of the
coordination sphere of the palladium center, as evidenced by a
deviation from planarity (N1−Pd−Cα−Cβ dihedral angle
124.141°) in comparison to 9e (N1−Pd−Cα−Cβ dihedral
angle −169.508°). As expected from this rationalization,
formation of the isomerization products 10ecis and 10etrans
was calculated to be endothermic by 5.9 and 6.0 kcal/mol,
respectively. In the case of 10e, the optimized geometries
displayed a Pd−Hβ agostic interaction with a distance of 1.812
Å for the cis analogue and 1.807 Å for the trans analogue. For
both 10a and 10e, the results of our calculations showed no
significant energy differences between the cis and trans isomers.
The elimination of 2-butene via β-hydride elimination was
evaluated from the cis isomers 10acis and 10ecis. Our calculations
showed that the elimination proceeded via TS[10eBHE] and
TS[10BHE]. In the case of 10a, the barrier for elimination was
found to be endothermic by 6.9 kcal/mol (relative to 10acis).
The product of elimination, the Pd-hydride π-2-butene species
11a, was calculated to be endothermic by 7.1 kcal/mol (10acis).
The optimized structure showed a Pd−H bond distance of
1.555 Å and Pd−Cβ and Pd−Cβ′ distances for the π-bound
olefin of 2.271 and 2.298 Å, respectively. In the case of 10e, the
effect of steric pressure was once again observed in our
calculations. In contrast to 10a, BHE via TS[10eBHE] was
calculated to be exothermic by 6.9 kcal/mol (relative to 10ecis).
Relaxation to the BHE product 11e was calculated to be
exothermic by 3.1 kcal/mol relative to 8e. The optimized
structure showed a Pd−H bond distance of 1.553 Å and Pd−Cβ
and Pd−Cβ′ distances for the π-bound olefin of 2.269 and 2.297
Å, respectively.
Scheme 7. Comparative Reinsertion and Elimination To
Generate cis-2-Butene
For species 8a,e reinsertion of 1-butene with 1,2-
regiochemistry was calculated to proceed via the transition
states TS[8ainsertion] and TS[8einsertion], endothermic by 1.6 and
2.8 kcal/mol, respectively (Figure 4). The transition state
structures were characterized by a Cα−H distance of 1.881 Å
(in the case of TS[8ainsertion]) and 1.825 Å (in the case of
TS[8einsertion]). In the case of 8a, formation of the Pd−sec-
butyl(Pd−CH3 agostic) species 9a was calculated to be
exothermic by 7.6 kcal/mol, relative to 8a. In contrast,
formation of the species 9e was significantly endothermic,
with the computed ground state energy endothermic by 8.9
kcal/mol, relative to 8e. This difference is as a result of
increased steric pressure in the case of 9e, with the reinserted
alkyl chain positioned in the plane of the palladium
coordination sphere. The result thereof is a significant increase
in the ground state energy of the insertion product. The effect
of steric pressure was also observed in the formation of the Pd−
sec-butyl(Pd−CH2 agostic) species, cis- and trans-10a,e. In the
case of 9a, isomerization via rotation about the Pd−C bond was
calculated to proceed via TS[9arotation] with an energy barrier of
6.8 kcal/mol. This value is lower than the isomerization barrier
(14−16 kcal/mol) determined for sterically bulky Ni(II) and
Pd(II) diimine catalysts.26a,29 This difference may be attributed
to a lack of steric bulk in the current system in comparison to
the Brookhart diimine catalyst systems,26a which would result in
a decrease in the isomerization energy relative to a sterically
encumbered precatalyst. Relaxation to the product generated
We modeled the associative displacement of cis-2-butene
from 11a,e (Figure 4) to regenerate the catalyst resting state.
Consistent with our previous results, no transition state was
located. Instead the five-coordinate intermediates 11aπ‑ethylene
and 11eπ‑ethylene were successfully optimized. Whereas olefin
displacement was moderately endothermic by 5.6 kcal/mol
relative to 11a, the same process for 11e was significantly more
endothermic, with an energy difference of 59.7 kcal/mol on
going from 11e to 11eπ‑ethylene, making associative exchange
from 11eπ‑ethylene highly unlikely. Once again, steric effects are
shown to have a significant impact on the relative energetics of
2a,e in the catalytic cycle.
3.4. Discussion of Experimental and Theoretical
Investigation. Our experimental and theoretical investigation
showed that the introduction of some measure of steric bulk in
the ortho position of the palladium coordination sphere resulted
in a decrease in catalytic activity. Both the kinetic and DFT
investigations identified associative ethylene exchange to
generate the Pd−methyl π-ethylene species as a key step, in
terms of energy differences between 2a and 2e, in the
oligomerization cycle. Low-temperature kinetic studies showed
that this process was facilitated at higher temperatures (−40
°C) for 2e, in comparison to 2a. Computed values for
associative olefin exchange from 2a,e was consistent with this
observation, with the energy difference for this step being 11.6
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kcal/mol. Ethylene associative exchange was also identified as
playing a crucial role in the formation of the catalyst resting
state, from which butene formation was shown to proceed
experimentally. The formation of the five-coordinate inter-
mediates, Pd−hydride π-propylene π-ethylene species 5a-
BHEπ‑ethylene and 5e-BHEπ‑ethylene, was calculated to be
significantly more endothermic for 2e than for the unsub-
stituted analogue 2a, with an energy difference of 7.8 kcal/mol,
which suggests that the formation of the catalyst resting state is
energetically more favorable for 2a in comparison to 2e. These
two associative exchange processes are partially responsible for
the experimental observation that complex 2a oligomerizes
ethylene with higher activity than 2e, despite a lower calculated
energy barrier for ethylene insertion in 2e in comparison to that
for 2a. In addition, our calculations showed that chain transfer
proceeded by β-hydride elimination and that reinsertion and
isomerization to generate Pd−isobutyl species, which would
eliminate 2-butenes, was energetically more favorable for 2a
than for 2e, consistent with what we observed during our
kinetic investigation. No significant energy differences were
observed in the computed ground state energies of the cis and
trans isomers, which suggests that the catalyst itself does not
influence the selectivity for cis- or trans-2-butene. These
theoretical observations are consistent with those made
experimentally.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF and .xyz files giving full
experimental details, oligomerization products and kinetic
plots, details and a discussion of computations generating the
catalyst resting states 6a,e, computed energies and xyz
coordinates for all computed ground states and transition
states, and crystallographic data for 1e, 3e, and 3e-A. This
material is available free of charge via the Internet at http://
pubs.acs.org. Crystallographic data for structural analysis have
also been deposited with the Cambridge Crystallographic Data
Centre, CCDC Nos. 985030 (1e), 985031 (3e), and 985032
(3e-A). Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, U.K. (fax, +44-1223-336063; e-mail, deposit@ccdc.
cam.ac.uk; web, http://www.ccdc.cam.ac.uk).
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.F.M.: smapolie@sun.ac.za.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Research Foundation
(NRF), DST-NRF Centre of Excellence in Catalysis
(c*change), and the Erasmus Mundus EUROSA program is
gratefully acknowledged for financial support. The Center for
High Performance Computing (CHPC) is gratefully acknowl-
edged for providing computational infrastructure.
4. CONCLUSIONS
A series of N-alkyl 2,2′-dipyridylamine ligands and their neutral
and cationic palladium methyl complexes were synthesized and
characterized. In an attempt to prepare inherently more stable
cationic complexes, pyridine was employed as the coordinating
solvent. It was found that the ratio of pyridine to neutral metal
complex had a significant effect on the product isolated. An
excess of pyridine in the reaction mixture resulted in the
isolation of a mixture of complexes with the cationic fragments
[PdMe(N−N)(C5H5N)]+ and [PdMe(C5H5N)3]+ in a near 1:1
ratio. The structure of the tris(pyridine) analogue was
unambiguously confirmed by SCD analysis. The acetonitrile-
coordinated analogues 2a−e were evaluated as catalyst
precursors in ethylene oligomerization. Our results showed
that these complexes catalytically dimerized ethylene with low
activity to a mixture of 1- and 2-butenes, with negligible
differences in product selectivities. Low-temperature kinetic
studies of complexes 2a,e allowed for the determination of
ethylene insertion rates, with 2a inserting ethylene approx-
imately 1.5× faster than 2e. Low-temperature spectroscopic
techniques also allowed for the identification of the Pd−ethyl π-
ethylene species as the catalyst resting state. Complementary
DFT calculations of the catalytic cycle identified two key steps,
both involving ethylene associative exchange, which could
explain the observed reactivity differences for complexes 2a,e.
Isomerization via reinsertion, rotation, and elimination to form
2-butenes was calculated to be quite facile for both 2a and 2e,
even though steric bulk significantly affected the relative
energetics associated with each intermediate of the isomer-
ization process. Consistent with our experimental results, our
calculations showed that the nature of the catalyst precursors
had a negligible effect on the ratio of cis- to trans-2-butene
isomers.
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