Nickel and palladium complexes with new phosphinito-imine ligands and their application as ethylene oligomerization catalysts

Article abstract of DOI:10.1021/om201088y

Phosphinito-imines, a new class of P,N donors, are readily generated by reaction of bulky arylamide anions [R²CONAr]^- (R ² = Me or t-Bu; Ar = 2,6-i-Pr₂C₆H₄) with chlorophosphines ClP(R¹)₂. In solution, free phosphinito-imines exist in equilibrium with the corresponding amidophosphine tautomers, containing a nitrogen-bound P(R¹)₂ group. However, reacting the tautomer mixtures with metal precursor complexes, such as NiBr₂(dme) or PdCl₂(cod), selectively affords stable phosphinito-imine complexes MX₂(P-N) (M = Ni, Pd) in excellent yields. These complexes are diamagnetic and exhibit square-planar structures in the solid state, but in solution, the Ni derivatives exchange with a small amount of the corresponding high-spin tetrahedral isomers. On treatment with MMAO or DEAC, NiX₂(P-N) complexes become active ethylene oligomerization catalysts, affording mainly butenes along with smaller amounts of hexenes and octenes. The activity and the selectivity of these catalysts depend on the structure of the phosphinito-imine ligand and the cocatalyst used. When activated with DEAC, complexes containing the P(i-Pr)₂ moiety are extremely active, achieving TOFs over 10⁶ mol C₂H ₄/mol Ni·h and high selectivity for butenes.

Full text of DOI:10.1021/om201088y

Article
pubs.acs.org/Organometallics
Nickel and Palladium Complexes with New Phosphinito-Imine
Ligands and Their Application as Ethylene Oligomerization Catalysts
́
Laura Ortiz de la Tabla, Inmaculada Matas, Pilar Palma, Eleuterio Alvarez, and Juan Campora*
́
Instituto de Investigaciones Químicas (IIQ), Consejo Superior de Investigaciones Científicas - Universidad de Sevilla, Avda. Amer
́
ico
Vespucio 49, 41092 Sevilla, Spain
S
* Supporting Information
ABSTRACT: Phosphinito-imines, a new class of P,N donors,
are readily generated by reaction of bulky arylamide anions
[R2CONAr]− (R2 = Me or t-Bu; Ar = 2,6-i-Pr2C6H4) with
chlorophosphines ClP(R1)2. In solution, free phosphinito-
imines exist in equilibrium with the corresponding amidophos-
phine tautomers, containing a nitrogen-bound P(R1)2 group.
However, reacting the tautomer mixtures with metal precursor
complexes, such as NiBr2(dme) or PdCl2(cod), selectively
affords stable phosphinito-imine complexes MX2(P-N) (M =
Ni, Pd) in excellent yields. These complexes are diamagnetic
and exhibit square-planar structures in the solid state, but in solution, the Ni derivatives exchange with a small amount of the
corresponding high-spin tetrahedral isomers. On treatment with MMAO or DEAC, NiX2(P-N) complexes become active
ethylene oligomerization catalysts, affording mainly butenes along with smaller amounts of hexenes and octenes. The activity and
the selectivity of these catalysts depend on the structure of the phosphinito-imine ligand and the cocatalyst used. When activated
with DEAC, complexes containing the P(i-Pr)2 moiety are extremely active, achieving TOFs over 106 mol C2H4/mol Ni·h and
high selectivity for butenes.
selectivity of the catalyst. For instance, heterocyclic fragments,
such as pyridine or oxazoline, readily incorporated in the
catalyst design, have been shown to enhance the selectivity for
α-olefins.22,26 In addition, imine donor groups bearing bulky N-
aryl substituents offer the possibility of a rational control of the
molecular weight of the products based on the same principles
established by Brookhart for α-diimine catalysts.35−40 Such
phosphino-imine complexes constitute a versatile class of
catalysts giving access to products ranging from light oligomers
to high-molecular-weight polymers. However, a straightforward
relationship between ligand structure and catalyst activity or
selectivity is not always observed, as similar ligand designs can
lead to very different results.8 One of the possible causes of
such irregular behavior is the tendency of some of these ligands
to undergo enolization, especially when basic cocatalysts, such
as MAO, are used.35 By using nonenolizable phosphine-imine
ligands, more stable catalysts are obtained that produce higher-
molecular-weight polymers.35 The enolization problem can be
avoided by replacing the H atoms on the position β to the
nitrogen donor with alkyl substituents. Alternatively, this
problem can be circumvented by inserting a heteroatom
between the phosphorus and the imine functionality. However,
direct replacement of the phosphino (R2P−) for a phosphinito
(R2P−O−) fragment increases the chelate ring size, and this
may induce changes in the geometry of the coordination sphere
INTRODUCTION
■
Hybrid ligands containing hard and soft donor atoms have been
widely studied and applied in organometallic chemistry and
homogeneous catalysis due to the structural diversity of their
complexes and their ability to impart unusual chemical
reactivity.1−5 Ligands containing P,O, P,N, or N,O donor sets
introduce significant differentiation at the trans positions in the
coordination sphere of a metal center, which has important
consequences for the catalytic activity of such complexes.2,6−10
Many nickel(II) complexes containing hybrid ligands have been
reported to catalyze ethylene oligomerization, by favoring chain
transfer over propagation during the catalytic cycle.11−13
A
reference for these are the shell higher olefin process (SHOP)
catalysts. These are based on Ni complexes with anionic P,O
donors and produce Schulz−Flory mixtures of oligomers with
high selectivity for α-olefins.14,15 Modification of the P,O
ligands system leads to very significant changes on the catalyst
selectivity16 that can be shifted from ethylene oligomerization
to polymerization, in some cases, with activities comparable to
metallocene derivatives.17 Despite the important developments
achieved in this area, metal catalysts producing short-chain α-
olefins with narrow Schulz−Flory distributions are still main
targets.18−20
Nickel and palladium catalysts containing P,N ligands have
received much attention in the context of ethylene oligomeriza-
tion8,10,21−35 or polymerization.28,29,35 In general, the presence
of a phosphorus donor enhances the stability of the complexes,
while the imine fragment can be used for tuning the activity and
Received: November 6, 2011
Published: January 19, 2012
© 2012 American Chemical Society
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Article
around the metal center.10,21,26,41 In contrast, phosphinito-
imine ligands, depicted in Scheme 1, give rise to five-membered
instability. On standing at room temperature, the 31P{1H}
spectra of the mixtures show that the major/minor tautomer
ratio gradually decreases with simultaneous appearance of a
number of signals arising from decomposition products.
Products 1/2 are particularly unstable, and all attempts of
isolation failed. The pivalamide derivatives 3/4 are somewhat
more stable, and after extraction with diethyl ether and
evaporation of the solvent, essentially pure samples containing
the major isomer, 3, were obtained, which enabled us to gather
Scheme 1
1
complete H and 13C{1H} NMR data for this compound (see
the Experimental Section).
rings. These substances are tautomers of phosphinoamides, in
which the phosphorus is bound to nitrogen. In recent years, a
number of these phosphinoamide ligands have been prepared
by reacting amide-based nucleophiles with chlorophos-
phines,42−46 but to the best of our knowledge, neither free
nor complexed phosphinito-imine tautomers have been
described so far. Despite their closely related structures, the
binding properties of phosphinito-imines and phosphinoamides
are expected to be very different. The latter are known to
coordinate solely through the phosphorus atom,43−49 and
although they can form P,O chelates,45−47,49,50 these usually
exhibit hemilabile behavior, the oxygen atom being easily
displaced by incoming donor molecules.51 However, phosphi-
nito-imines should form strong P,N chelates. In this paper, we
show that phosphinito-imine tautomers, readily prepared from
carboxylic acid amides, are stabilized by steric factors when the
P and N atoms bear bulky substituents. Nickel and palladium
complexes of these nonenolizable P,N ligands have been
prepared, and we report on their behavior as ethylene
oligomerization catalysts.
The reaction of the lithium salt of N-(2,6-
diisopropylphenyl)acetamide with ClP(t-Bu)2 is considerably
slow, requiring 3 days at 100 °C to complete, but the products
are thermally robust and do not decompose under such
conditions. Again, the 31P{1H} NMR spectrum of the reaction
mixture shows two signals at δ 142 and 65 ppm in a 95:5
intensity ratio for the corresponding tautomers (5/6).
Crystallization from diethyl ether at −20 °C gave a small
amount of a solid containing the minor component and some
starting amide. Evaporation of the mother liquor afforded
compound 5 as a yellow oil in 95% yield. Solutions of ligand 5
in THF are indefinitely stable at room temperature.
Although the 1H and 13C{1H} NMR data obtained for 3 and
5 confirmed the presence of the PR2 and the N-(2,6-
diisopropylphenyl)amido groups, they provide no direct
indications of whether the P atom is bound to O or N.
However, the deshielding of the 31P resonance of the major
isomers (1, 3, or 5) suggests the presence of an electronegative
O-bound group. Therefore, these can be reasonably assigned to
the phosphinito-imine tautomers, whereas minor species 2, 4,
or 6 would have the phosphinoamide structure. This hypothesis
finds support in the comparison with literature 31P{1H} NMR
phosphinites (E2P-OR) and phosphinoamides (R2P-NR2). For
example, the chemical shifts of the phosphinoamide-phosphin-
ites depicted in Figure 1 are very similar to those formed in our
RESULTS AND DISCUSSION
■
Synthesis of Ligands. The new ligands were prepared by
deprotonation of N-(2,6-diisopropylphenyl)acetamide or N-
(2,6-diisopropylphenyl)pivalamide with n-BuLi or NaH at −78
°C, followed by reaction with the corresponding chlorophos-
phines, as shown in Scheme 2. The amide anions react
smoothly with chlorodiisopropylphosphine at low temper-
atures. In both cases, the 31P{1H} NMR spectra of the reaction
mixtures taken after stirring for 30 min at −78 °C showed two
new species in a ca. 9:1 ratio, corresponding to the tautomers
1/2 or 3/4. Major resonances appear at ca δ 140 ppm for both
the acetamide (1) and the pivalamide (3) derivatives, whereas
those of secondary products are shifted upfield, at δ 90 ppm (2)
and 65 ppm (4). These products show signs of thermal
Figure 1. Typical 31P chemical shifts for R2P−N and R2P−O linkages.
system. They give rise to low-field resonances at δ 110−150
ppm for the alkoxophosphine group and high-field ones at δ
Scheme 2
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Article
Figure 2. ORTEP views of complexes 7−12.
Table 1. Selected Bond Distances and Angles for Complexes 7−12 (M = Ni or Pd, X = Br or Cl)
a
7
8
9
10
11
12
M−P1
M−N1
M−X2
M−X1
P1−O1
O1−C13
2.0974(9)
1.917(2)
2.3027(5)
2.3318(5)
1.683(2)
1.341(4)
1.289(4)
1.454(4)
94.710(18)
97.02(8)
83.81(3)
168.06(8)
178.50(3)
84.47(8)
123.36(2)
116.4(2)
2.14
2.089(2)
1.955(7)
2.3140(13)
2.3371(11)
1.675(7)
1.339(11)
1.295(9)
1.440(9)
93.28(5)
99.41(17)
83.43(7)
166.34(17)
173.14(9)
84.47(18)
118.7(4)
122.0(7)
8.24
2.1315(10)
1.943(3)
2.3168(7)
2.3477(6)
1.685(3)
1.333(5)
1.301(5)
1.449(5)
92.46(2)
96.36(10)
87.32(3)
169.54(10)
173.73(4)
84.53(10)
125.3(2)
115.6(3)
8.84
2.1685(8)
2.056(2)
2.2960(8)
2.3552(8)
1.6696(11)
1.3533(17)
1.282(3)
1.456(3)
93.98(3)
95.66(6)
88.38(3)
170.17(6)
177.33(3)
81.94(6)
123.28(2)
117.9(2)
2.14
2.1576(3)
2.0769(10)
2.2972(3)
2.3748(3)
1.6663(9)
1.3583(14)
1.2916(15)
1.4454(15)
93.545(12)
97.54(3)
2.2042(5)
2.0570(12)
2.2995(4)
2.3637(5)
1.6696(11)
1.3533(17)
1.2873(18)
1.4533(18)
90.735(14)
94.56(3)
N1C13
N1−C1
X2−M−X1
X1−M−N1
X2−M−P1
X2−M−N1
X1−M−P1
P1−M−N1
M−N1−C1
C13−N1−C1
X1−X2−P1−N1
87.077(12)
168.34(3)
177.284(12)
82.02(3)
92.866(15)
174.60(3)
176.155(13)
81.87(3)
116.41(7)
124.3(1)
123.04(9)
117.9(1)
b
4.81
1.85
a
b
One of two independent molecules. 7.08° in molecule 2.
50−60 ppm for the phosphinoamide.52 It seems very likely that
the relative stability of the phosphinito-imine and phosphinoa-
mide tautomers is determined by steric factors. The
phosphinoamide tautomer, usually favored over the phosphi-
nito-imine,42−46 becomes destabilized in our system by the
steric repulsions posed by the bulky PR2 and N-Ar groups.
Nickel and Palladium Phosphinito-Imine Complexes.
Reacting freshly prepared mixtures of tautomers (1/2 or 3/4)
or the purified ligand 5 with equimolar amounts of NiBr2(dme)
(dme = 1,2-dimethoxyethane) or PdCl2(cod) (cod = 1,5-
cyclooctadiene) leads exclusively to the Ni(II) and Pd(II)
complexes 7−12 containing the P,N phosphinito-imine ligands
(Scheme 2). The 31P{1H} spectra of crude reaction mixtures
display a single resonance, confirming that only one isomer is
produced. Thus, a rapid isomerization of the minor
phosphinoamide tautomer takes place on complexation to the
metal center. After workup, the complexes were isolated as
crystalline, diamagnetic solids. In contrast with the free ligands,
their complexes are thermally stable and can be exposed to air
for short periods of time without noticeable decomposition.
The diamagnetism of the nickel derivatives indicates that the
phosphinito-imine ligand favors square-planar structures both
in the solid state and in solution. This is somewhat unusual, for
most P,N ligands form paramagnetic complexes with NiX2
moieties.21−37
The P,N coordination mode is supported by the 31P{1H} and
IR data and confirmed by the X-ray structures of the complexes.
The phosphorus spectra show low-field resonances at ca. 200
ppm for the palladium derivatives and 166−198 ppm for their
nickel analogues, which is more consistent with an R2P-O than
with an R2P-N fragment. In addition, ν(CN) absorption
bands are observed in the 1590−1650 cm−1 region of the
infrared spectra. The imine absorption of free ligand 5 at 1679
cm−1 shifts by ca. 20 cm−1 to lower frequency upon
coordination to Ni and Pd in complexes 9 and 12.
All complexes 7−12 have been characterized by X-ray
diffraction. Their crystal structures are shown in Figure 2, and
main bond distances and angles are collected in Table 1. All
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Figure 3. Room-temperature spectra of complexes 7 and 10 (CD2Cl2 high-field region only).
molecules are characterized by the flat five-membered N,P
chelate and a square-planar coordination environment, with
very small tetrahedral distortion. The distortion degree is
measured by the torsion angle formed by the halide ligands and
the P and N atoms (X1X2P1N1). This is very small and
virtually identical for both complexes of ligand 1, but the
derivatives of the relatively bulky ligands 3 and 5 show a slight
tetrahedral distortion, somewhat larger for the Ni (ca. 8°) than
for the Pd complexes. The intra-annular P−O, O−C, and C
N distances are unexceptional and essentially identical in all the
complexes. The aryl ring lies perpendicular to the coordination
plane, leaving the isopropyl groups pointing toward the axial
positions of the metal center. In the pivalamide derivatives 8
and 11, the aryl ring is forced by the steric pressure of the bulky
t-Bu substituent to come closer to the metal fragment. This is
reflected in the wider C13−N1−C1 and narrower C1−N1−M
angles in these complexes. The lengths of M−X bonds sense
the different trans influence of the P and N donors, and those
opposed to the more strongly donor phosphinito group are
appreciably longer than those trans to the imine fragment.
Although the lengths of Ni−Br and Pd−Cl are similar, the
differential effect of trans groups is less pronounced for Ni
complexes (ca. 1.2% of the average length) than in the Pd
counterparts. The spectra of the latter show sharp and well-
defined signals, whereas the 31P signals of the Ni complexes are
broad and H−P couplings in the H spectrum appear partially
1
unresolved. Notably, signals arising from the P-bound isopropyl
groups of the Ni complex appear broadened, whereas those of
the 2,6-substituents of the aryl group are sharp and well-
resolved. These differences can be clearly seen in the high field
1
region of the H spectra of compounds 7 and 10, represented
in Figure 3, and suggest the occurrence of some fluxional
1
process. To ascertain its nature, variable-temperature H and
31P NMR studies were carried out on complex 7 and its Pd
analogue, 10. The spectrum of the latter is essentially
temperature-independent within the studied temperature
range, but those of the nickel compound display significant
shape variations. Below 253 K, the slow exchange limit is
reached and the spectrum shows well-resolved H−P couplings.
As the temperature rises, couplings to phoshorus progressively
fade away in the proton spectra while the 31P signal becomes
broader. At 323 K, the latter disappears in the baseline, and H−
1
P couplings are totally lost in the H spectrum.
Selective loss of couplings is a common phenomenon in the
NMR spectra of coordination or organometallic complexes.
This phenomenon normally stems from the reversible
dissociation of the bonds responsible for the transmission of
the coupling. Obviously, this is not the case here since
reversible P−C bond dissociation cannot occur. A rapid
exchange between the diamagnetic square-planar complex
with small amounts of a high-spin (S = 1) tetrahedral isomer
might account for the observations and is in good agreement
with the absence of similar effects in the spectra of the Pd
derivatives (Figure 4). Because the 31P atom is directly bound
to the metal center, hyperfine coupling to the electronic spin
provides an extremely efficient relaxation mechanism, leading
to the observed broadening of the P resonance and to the
effective cancellation of heteronuclear couplings to phosphorus.
Assuming this explanation, a simulation of the methyl region of
the 1H spectrum line shape has been carried out (Figure 5; see
the Experimental Section for details). Figure 6 shows the
Eyring plot for the computed exchange rates, which provide the
following activation parameters for the exchange process: ΔH⧧
derivatives (2−3%). Complex 11 exhibits the largest dM‑X1
−
dM‑X2 difference, due to the extra elongation of the Pd−Cl1.
This is probably caused by the steric pressure originated by the
t-Bu substituent of the phosphinito-imine backbone, trans-
mitted by the aryl ring to the nearby chloride ligand. The
pressure of the aryl ring on the Ni−Br1 bond of 8 has no
noticeable effect on its length because it is efficiently released
by the significant tetrahedral distortion of this complex.
As discussed above, the diamagnetic, square-planar config-
uration of the Ni complexes is unusual among (P-N)NiX2
complexes. Some examples of square-planar Ni(II) derivatives
of phosphinito-pyridine ligands have been reported, which
display square-planar structures in the solid state,23,26 but are
paramagnetic in solution.23 The NMR spectra of the
phosphinito-imine complexes of Ni and Pd in CD2Cl2 are
very similar and typical of diamagnetic compounds, showing
that their square-planar configurations are retained in solution.
However, the room-temperature spectra of the Ni complexes
reveal some intriguing differences with those of their Pd
= 10.4(9) kcal/mol, ΔS⧧ = −4(1) cal/mol·K, and ΔG⧧
=
11.66(15) kcal/mol at 298 K. According to Hammond’s
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state splitting energies.53 The structures representing the
square-planar and tetrahedral forms of 7 shown in Figure 4
correspond to the BP86-optimized geometries.
Catalytic Behavior of Phosphinito-Imine Complexes.
Catalytic Oligomerization of Ethylene. The catalytic
activity of complexes 7−12 was investigated using modified
methylalumoxane (MMAO) or diethylaluminum chloride
(DEAC) as cocatalysts in toluene as a solvent. The reactions
were performed in 250 mL Fischer-Porter glass reactors fitted
with an internal thermocouple and a septum-capped port for
catalyst injection. The reactors were immersed in a thermostatic
water bath at a preset temperature. Ethylene consumption and
the internal reactor temperature were monitored continuously
during the experiments.
Figure 4. Square-planar and tetrahedral geometries for complex 7
(BP86-optimized structures).
The catalysts were screened in a series of experiments at 30
°C and an ethylene pressure of 5 bar (standard conditions),
which showed that the Ni complexes actively catalyze the
oligomerization of ethylene into mixtures of butenes, hexenes,
and octenes, whereas the Pd complexes are catalytically
inactive. No polyethylene precipitates when the reaction
mixtures are treated with acidified methanol. In general, the
results of quantitative GC analysis are in good agreement with
the ethylene consumption data, confirming that no high-
molecular-weight oligomers or polymers are produced. The Ni
catalysts display low thermal stability, since their activity is
sensibly reduced at 50 °C and becomes negligible at 70 °C.
The preliminary study was extended to determine the
optimum [Ni]/[Al] ratios (Table 2). Complex 7 achieves its
Figure 5. Experimental and simulated 1H NMR spectra of compound
7 recorded at variable temperatures (methyl zone).
Table 2. Influence of the Catalyst/Cocatalyst Ratio on the
Activity of Complexes 7−9
a
co-
[Al]/
[Ni]
TOF
co-
[Al]/ TOF ×
catalyst catalyst
× 10−3 catalyst catalyst [Ni]
10−3
b
7
7
7
7
7
8
8
9
9
9
9
9
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
1000
750
500
250
150
1000
500
700
600
500
400
200
10.4
14.9
17.5
20.1
12.8
7.4
7
7
7
7
7
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
250
200
175
150
100
50
186.9
389.7
834.4
866.2
528.3
518.1
22.9
b
b
b
b
b
7
9.8
8
9
9
9
9
175
300
200
100
50
2.2
54.6
2.6
36.9
2.2
20.2
1.8
14.0
1.9
a
Experimental conditions: [Ni] = 2 × 10−4 M; solvent, toluene; total
Figure 6. Eyring plot (253−323 K) of the computed rate constants
and activation parameters.
volume ≈ 20 mL; 30 °C; 4 bar. Reaction time, 1 h, unless otherwise
stated. Reaction time 3−5 min.
b
postulate, the energy of the tetrahedral form probably is only
slightly lower than the barrier of the observed exchange
process. To assess this hypothesis, DFT calculations were
performed to determine the relative energies of the square-
planar and tetrahedral forms. As a first option, the hybrid
functional B3LYP was used, but the calculation incorrectly
predicted that the tetrahedral isomer should be more stable
than the square-planar by 3.0 kcal/mol. However, a second
calculation with the pure functional BP86 provides the correct
energy order with the tetrahedral species lying 10.6 kcal/mol
above the diamagnetic ground state, in good agreement with
the experiment. It is known that, at least for some types of
transition-metal complexes, pure gradient-corrected functionals,
such as BP86, perform better for the purpose of estimating spin
best performance with relatively low loads of either MMAO or
DEAC (200 or 150 equiv, respectively). Although 8 and 9 can
also be activated with similar amounts of DEAC (8: 150 equiv;
9: 200 equiv), with MMAO, a higher [Al]/[Ni] ratio of ca. 600
is required for efficient performance. In general, the effect of
the catalyst/cocatalyst ratio on the activities is not dramatic, but
DEAC behaves as a much more effective cocatalyst than
MMAO.
Tables 3 and 4 collect data for a set of 60 min experiments
carried out to compare the effect of catalyst structure, pressure,
and temperature on the catalytic activity and selectivity of the
three nickel complexes. Complexes 7 and 8 exhibit very similar
catalytic properties and are much more active than 9. Upon
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a
Table 3. Activity and Selectivity Data for Ethylene Oligomerization with Complexes 7−9 Activated with MMAO
b
b
c
d
e
f
g
h
i
j
[Ni]
×
[Al]
×
P
T
yield
TOF × 10−3
max TOF
×
TOF60
×
t1/2
ΔT
C4:1-C4H8/2-
trans/2-cis
cat
10−5
10−2
(bar) (°C)
(grs)
(h−1)
10−3 (h−1)
10−3 (h−1)
(min)
(°C)
C4:C6:C8
1
2
7
7
7
7
8
8
8
8
8
9
9
9
9
21.0
2.6
4.4
4.1
5
5
9
5
5
5
9
5
5
5
5
9
5
30
30
30
0
3.41
0.33
0.52
3.76
1.39
0.16
4.62
2.77
1.10
0.10
0.23
0.09
2.18
13.5
8.9
37.8
15.9
25.0
37.1
71.1
28.9
102.3
36.7
36.8
2.2
4.0
8.0
12
60
>60
13
8
4.7
0.2
2.1
5.0
9.9
0.3
7.3
5.3
1.9
1.4
1.7
0.5
6.3
78:20:3
71:29:0
71:29:0
83:12:5
64:23:13
77:23:0
77:20:3
80:17:3
81:17:2
50:40:10
54:42:4
66:34:0
13:30:57
29:11:60
41:9:50
8:25:67
27:16:58
61:11:28
16:14:70
19:23:58
29:20:51
100:0:0
94:3:4
3
2.6
4.1
16.6
14.8
16.4
5.2
12.4
10.0
6.0
4
21.0
20.1
2.6
4.4
5
12.1
12.5
12.3
12.0
12.0
12.0
12.0
12.0
12.0
30
30
30
0
6
2.1
7
7
10.3
20.0
10.2
20.0
20.0
20.0
20.0
36.6
11.0
14.7
0.4
23.6
6.9
11
9
8
k
9
0
55.3
>60
8
10
11
12
13
30
30
30
0
0.0
0.0
0.0
0.9
7.7
7
0.4
9.0
7
94:3:4
8.6
26.8
2.8
14
65:31:4
94:2:3
a
b
c
d
e
Reaction time, 60 min; total volume, 42 mL; solvent, toluene. mol·L−1. Ethylene pressure. External bath temperature. Oligomer yield, from GC
f
g
h
i
analysis. Average activity (TOF, in mol ethylene/mol Ni·h). Maximum TOF, from ethylene consumption curves. Activity after 60 min. Time for
50% activity decay. Increase of internal reactor temperature. Reaction rate increases continuously during the experiment.
j
k
a
Table 4. Activity and Selectivity Data for Ethylene Oligomerization with Complexes 7−9 Activated with DEAC
b
b
c
d
e
f
g
h
i
j
[Ni]
×
[Al]
×
P
T
yield
TOF × 10−3
max TOF
×
TOF60
×
t1/2
ΔT
(°C) C4:C6:C8
C4:1-C4H8/2-
trans/2-cis
−1
cat
10−5
10−2
(bar) (°C)
(grs)
(h−1)
10−3 (h )
10−3 (h−1)
(min)
1
2
7
7
7
7
8
8
8
8
9
9
9
2.6
1.0
3.1
3.1
3.1
3.1
3.5
3.1
3.1
3.1
3.9
3.9
3.9
5
5
9
5
5
5
9
5
5
9
5
30
30
30
0
2.43
1.94
3.13
0.31
18.57
3.05
4.26
12.57
2.63
3.69
0.81
77.5
154.1
240.2
9.9
1662.5
738.0
3837.6
12.5
0.0
0.0
0.0
5
27.0
5.9
80:20:0
88:12:0
87:12:0
88:12:0
75:23:0
82:18:0
84:14:1
81:18:2
90:10:0
98:2:0
48:31:21
40:23:37
50:20:30
17:34:49
17:46:37
32:30:18
36:31:33
4:51:45
6
3
1.0
9
25.3
0.9
k
4
2.6
12.5
>60
22
4
5
19.9
2.6
30
30
30
0
73.5
96.8
135.3
99.7
10.4
14.7
322.9
1812.6
1685.3
490.3
13.8
5.0
0.0
40.6
30.1
45.2
54.8
1.2
6
7
2.6
5.2
4
8
10.3
19.7
19.7
19.7
0.0
9
9
30
30
0
7.0
60
41
>60
33:41:25
0:77:23
10
17.5
14.6
0.9
k
11
2.2
6.8
1.4
5.3
93:14:0
42:25:33
a
b
c
d
e
Reaction time, 60 min; total volume, 42 mL; solvent, toluene. mol·L−1. Ethylene pressure. External bath temperature. Oligomer yield, from GC
f
g
h
i
analysis. Average activity (TOF, in mol ethylene/mol Ni·h). Maximum TOF, from ethylene consumption curves. Activity after 60 min. Time for
50% activity decay. Increase of internal reactor temperature. Reaction rate increases continuously during the experiment.
j
k
Figure 7. Representative examples of activity profiles observed with phosphinito-imine catalysts. The lower curve (blue) represents the activity
variation (TOF) and the upper (red), the internal reactor temperature. (A) Typical activity curve, corresponding to entry 1, Table 3. (B) “Pulsed”
activity, showing a narrow activity maximum, entry 3, Table 4. (C) Slow catalyst activation at low temperature, entry 4, Table 4.
activation with DEAC or MMAO, 7 and 8 give rise to very
active catalysts, achieving ethylene turnover frequencies well
above 106 mol ethylene/mol Ni·h−1 under favorable conditions.
Monitoring ethylene consumption shows that, upon activation
at 30 °C, the activity reaches its peak within a few seconds or
minutes, decaying afterward during the rest of the experiment
(Figure 7A). The activation process is accompanied by a visible
color change of the mixture from the characteristic purple-red
of the complexes to yellow. However, as discussed below, the
use of diluted catalyst solutions or lower operating temper-
atures may lead to very different activity profiles. Apart from
the typical example A, Figure 7 shows other two significant
examples of such profiles, B and C.
Although catalysts produced with DEAC are more active
than those generated with MMAO, the latter are longer lived.
For example, the time required by the catalytic activity to fall to
50% of its maximum value (t1/2) is 4−6 min with 7 or 8 when
activated with DEAC and >7 min with MMAO under standard
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conditions. In contrast, the catalyst formed with 9 and DEAC
shows low, but sustained, activity under the same conditions
(Table 4, entry 9). A possible explanation for the faster
deactivation of 7−8/DEAC is that heat released by the high
activity levels favors catalyst decomposition. As can be seen in
Figure 7, the internal reactor temperature parallels the activity
profile, temperature peaks of 30−40 °C being observed when
high activity levels are reached. In most cases (profile A), after
the initial activity peak, the temperature declines and the
catalyst performance reaches an almost stable regime in which
the activity decay is relatively slow. With MMAO, the catalysts
retain significant activity after 60 min runs (TOF60), whereas
DEAC-activated systems are usually inactive before the end of
the experiments. To avoid excessive heating, the dose of the
nickel catalyst was decreased according to the activity observed
in preliminary experiments, while maintaining other parame-
ters, such as the cocatalyst concentration and reaction volume,
constant. However, even under high dilution conditions,
systems generated from 7 or 8 and DEAC are active enough
to induce significant temperature peaks (entries 2, 3, 6, and 7,
Table 4). After the sharp activity maximum, the catalyst
deactivates, resulting in a fast reaction “pulse” rather than a
sustained process (profile B). In an attempt to increase the rate
of heat transfer, we used external cooling to 0 °C, but at this
temperature, catalyst performance becomes limited by incom-
plete activation (profile C). In these experiments (Table 3,
entries 4 and 9, and Table 4, entry 9), ethylene consumption
curves show that the activity rises slowly, in some cases, without
even reaching a true maximum. In experiment 8, Table 4, the 0
°C bath was used in combination with a larger load of catalyst
8. This induced faster catalyst activation, but the internal
temperature rose to nearly the same level observed under
standard conditions, resulting in a similar deactivation rate.
The influence of the ethylene pressure on the activity of
complexes 7−9 is not clear-cut. In general, the average activity
increases by a factor of 1.5−2 when the pressure rises from 5 to
9 bar (compare, for example, entries 2 and 3 in Table 3, or
entries 6 and 7 in Table 4). The origin of this effect is not
evident, because global productivity figures do not depend
exclusively on the specific activity of the catalyst but also on its
decay rate, which can be, in turn, influenced by monomer
concentration, as well as by changes in the internal reactor
temperature. Complex relationships between these factors lead
to the different activity profiles, such as those illustrated by
Figure 7, which makes it very difficult to extract any conclusion
from the mere comparison of average experiment activity. In
part, the positive influence of pressure on global productivity
might originate in a decrease of the catalyst decay rate. Indeed,
catalysts survive somewhat longer in experiments carried out at
9 bar than at 5 bar, as evidenced by increases in t1/2 or TOF60.
Because catalyst deactivation probably has only a minor effect
on the maximum activity level that the catalysts achieve shortly
after the beginning of the experiment (TOFmax), this is a better
descriptor for the effect of pressure on the specific catalyst
activity. However, for very active systems, TOFmax could be
biased by mass transport effects and by the simultaneous
increase of the internal temperature. Thus, experiments
developing mild activities are probably best suited for the
analysis of the pressure effect. Thus, for complexes 7 and 8
activated with MMAO, the TOFmax parameter increases
significantly on going from 5 to 9 bar, suggesting that the
rate-limiting step is directly dependent on the monomer
concentration. However, the less-active catalyst 9 is insensitive
to pressure changes (with either MMAO or DEAC). This
difference suggests that the rate-determining step of the
ethylene oligomerization process could be different for the
latter.
The C4/C6/C8 selectivity ratio of the Ni phosphino-imine
catalysts is hardly influenced by temperature or pressure, but
there is a dramatic effect of the nature of the cocatalyst.
Activation with DEAC increases the selectivity for C4 products
and leads to lower amounts of hexenes and octenes than
MMAO. Diagrams shown in Figure 8 represent the average
Figure 8. Selectivity of complexes 7−9 for ethylene oligomerization
upon activation with MMAO and DEAC, calculated as averages for all
experiments. Percent values (%) are given on the top of each column.
product distributions obtained for each of these catalyst/
cocatalyst combinations in the studied ranges of pressure and
temperature. As can be seen, complexes 7 and 8 bear a
remarkable resemblance in terms of selectivity, whereas the
profile of the mixtures of oligomers generated with 9 is less
alike. Curiously, whereas the latter forms the least-selective
catalyst when activated with MMAO, with DEAC, it produces
almost exclusively butenes with >90% selectivity.
The ratio of 1-butene and 2-cis- and 2-trans-butene is widely
variable, probably due to secondary isomerization reactions that
are difficult to control. Complex 9 is again exceptional on this
regard, since, in combination with MMAO, the C4 fraction
produced consistently contains >94% 1-butene, suggesting that
the isomerization process does not operate in this case. This
selectivity is not seen with DEAC.
The preference of the nickel phosphinito-imine catalysts for
ethylene dimerization is rather surprising considering that these
compounds contain a 2,6-diisopropylphenylimino group, which
is specifically designed to increase the molecular weight of the
products by hindering chain transfer to the monomer.54−58
Indeed, structurally related phosphino-imine complexes con-
taining the same bulky aryl substituent produce either heavy
Schulz−Flory mixtures of oligomers37 (with α = 0.8−0.9) or
medium- to high-molecular-weight polyethylenes.27,29 The
similarity of the product distributions obtained with 7 and 8
and the differences observed for 9 suggest that selectivity is
controlled by the PR12 group, while it is relatively insensitive to
changes in the α substituent of the imine fragment (Me or t-
Bu). The different behavior of phosphinito-imine and
phosphine-imine complexes and the lack of significant effects
of substituents on the ligand backbone could be taken as an
indication of significant alterations of the phosphinito-imine
ligand on activation by aluminum cocatalysts, that is, cleavage
of the P−O bond leading to imine-free species. However, the
oligomer distribution is reminiscent of those obtained with Ni
catalysts containing phosphinito-heterocycle10,26 and other P,N
ligands.23,25 Furthermore, Braunstein has shown that phosphi-
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Ligand 3. Sodium hydride (14 mg, 0.6 mmol) was suspended in a
solution of the amide 2,6-iPr2C6H3NHC(O)tBu (150 mg, 0.57 mmol)
in 10 mL of THF. The mixture was stirred at 40 °C for 1 h. After
removing the NaH excess by centrifugation, the solution was cooled to
−78 °C, and 0.5 mL of a 1 M solution of iPr2PCl (0.5 mmol) in THF
was added with stirring. The mixture was allowed to react at the same
temperature for 30 min. The resulting solution, containing a ca. 90:10
mixture of the phosphinito-imine (3) and phosphinoamide (4)
tautomers, was used directly for the synthesis of the complexes.
31P{1H} NMR (25 °C, THF): δ 140.2 (s, isomer 3), 65.0 (s, isomer
4). A spectroscopically pure sample of 3 was obtained by evaporating
the solvent under reduced pressure, extracting the residue with diethyl
nito-type ligands form stable complexes with aluminum alkyls
without P−O cleavage.26 If the phosphinito-imine chelates
maintain their integrity under our catalysis conditions, an
unusually low energy barrier for the chain transfer process
would be necessary to explain the selectivity for low-molecular-
weight products.
CONCLUSIONS
■
A new family of phosphinito-imine ligands is readily available
from the reaction of the anions of the bulky amides N-(2,6-
diisopropylphenyl)acetamide or N-(2,6-diisopropylphenyl)-
pivalamide with chlorodiisopropylphosphine or chlorodi-tert-
butylphosphine. The new ligands are formed together with
minor amounts of the corresponding amidophosphine
tautomers. However, upon reaction with suitable precursors,
such as NiBr2(dme) or PdCl2(cod), the mixtures are cleanly
transformed into the phosphinito-imine complexes 7−12,
which are obtained in excellent yields and are thermally stable
in solution and in the solid state. These have square-planar
structures characteristic of a low-spin configuration, although in
solution, the nickel complexes undergo exchange with the high-
spin tetrahedral isomer with ΔG⧧ = 11.66(15) kcal/mol. Upon
activation with MMAO or DEAC, nickel complexes 7−9
become active ethylene oligomerization/polymerization cata-
lysts, producing mainly butenes, and minor amounts of hexenes
and octenes. Both the activity and the selectivity of these
catalysts are strongly influenced by the nature of the cocatalyst.
In general, DEAC leads to higher activities and better
selectivities for ethylene dimerization. Complexes 7 and 8
have very similar catalytic properties, reaching extremely high
catalytic activities (TOF), over 106 h−1 when activated with
DEAC. Catalysts generated from 9 are considerably less active
but offer somewhat improved selectivity control, allowing >90%
butenes when activated with DEAC, and high selectivity for α-
olefins with MMAO.
1
ether, filtering, and taking the solution to dryness. H NMR (25 °C,
3
3
CD2Cl2, 300 MHz): δ 1.07 (dd, 6H, JHH = 7 Hz, JHP = 14 Hz,
3
3
PCHMeMe), 1.16 (dd, 6H, JHH = 7 Hz, JHP = 11 Hz, PCHMeMe),
1.24 (s, 9H, tBuC(O)N), 1.33 (d, overlapping signal, aryl-CHMeMe),
1.35 (d, overlapping signal, aryl-CHMeMe), 1.83 (m, 2H, PCHMe2),
3.09 (hept, 2H, 3JHH = 7 Hz, aryl-CHMe2), 7.08 (t, 1H, 3JHH = 7 Hz, p-
CHar), 7.14 (d, 2H, JHH = 7 Hz, m-CHar). 13C{1H} NMR (25 °C,
3
2
CD2Cl2, 75 MHz): δ 17.8 (d, JCP = 3 Hz, PCHMeMe), 17.9 (s,
PCHMeMe), 22.6 (s, aryl-CHMeMe), 23.9 (s, aryl-CHMeMe), 27.9
1
(d, JCP = 23 Hz, PCHMe2), 28.8 (s, Me3CC(O)N), 29.1 (s, aryl-
CHMe2), 30.4 (s, Me3CC(O)N), 122.6 (s, m-CHar), 123.2 (s, p-CHar),
136.4 (s, N-Car), 143.2 (s, o-Car), 160.7 (d, 2JCP = 6 Hz, Me3CC(O)N).
Ligand 5. A glass ampule with a Young Teflon tap and magnetic
stirrer was charged with a THF solution of 2,6-iPr2C6H3NHC(O)Me
(1.2 g, 5.86 mol) in 20 mL of THF. The solution was cooled to −78
°C and treated with 3.66 mL of a 1.6 M solution of nBuLi in hexane
(5.86 mmol). After stirring for 5 min at this temperature, 1.017 g (5.86
mmol) of tBu2PCl was added. The cooling bath was removed, the tap
was closed, and the mixture was stirred at 100 °C for 3 days. The
resulting solution contains a 95:5 mixture of 5 and 6. 31P{1H} NMR
(25 °C, THF, 121 MHz): δ 142.3 (s, 5), δ 65.4 (s, 6). The solvent was
then removed under reduced pressure, and the residue was extracted
with 50 mL of CH2Cl2. The solution was evaporated again, and the
residue was dissolved in diethyl ether. A white solid containing the
phosphinoamide tautomer 6 and some starting amide precipitated
when the solution was stored at −30 °C. The solution was filtered and
taken to dryness, leaving 5 as a yellow oil. Yield: 95% (2.02 g). IR
1
(THF solution): v(CN) 1679 cm−1. H NMR (25 °C, C6D6, 300
MHz): δ 1.11 (d, 6H, 3JHH = 7 Hz, aryl-CHMeMe), 1.14 (d, 6H, 3JHH
= 7 Hz, aryl-CHMeMe), 1.21 (d, 18H, 3JHP = 12 Hz, P(t-Bu)2), 1.76 (s,
3H, MeC(O)N), 2.87 (hept, 2H, 3JHH = 7 Hz, aryl-CHMeMe), 7.02 (t,
1H, 3JHH = 7 Hz, p-CHar), 7.1 (d, 2H, 3JHH = 7 Hz, m-CHar). 13C{1H}
NMR (25 °C, C6D6, 75 MHz): δ 17.7 (s, MeC(O)N), 23.1 (s, aryl-
EXPERIMENTAL SECTION
■
General Considerations. All experiments were carried out under
dry nitrogen using standard Schlenk techniques. Solvents were
rigorously dried and degassed before use. Microanalyses were
performed by the Microanalytical Service of the Instituto de
2
CHMeMe), 23.5 (s, aryl-CHMeMe), 27.6 (d, JCP = 17 Hz, PCMe3),
1
28.3 (s, aryl-CHMeMe), 35.1 (d, JCP = 28 Hz, PCMe3), 123.1 (s, m-
́
Investigaciones Quımicas (Sevilla, Spain). Infrared spectra were
CHar), 123.3 (s, p-CHar), 138.1 (s, o-Car), 144.3 (s, N-Car), 160.7 (d,
2JCP = 6 Hz, MeC(O)N).
recorded on a Bruker Vector 22 spectrometer, and NMR spectra on
Bruker DRX 400 and 500 MHz and Bruker DPX 300 MHz
Synthesis of Ni[N-(2,6-iPr2C6H3)C((CH3)OPiPr2)]Br2 (7). A THF
solution of ligand 1 (1 mmol) prepared in situ as described above was
added to a suspension of NiBr2(dme) (308 mg, 1 mmol) in 10 mL of
THF cooled to −40 °C. The reaction mixture was allowed to warm to
room temperature. After stirring for 30 min, the solvent was
evaporated under vacuum, and the red residue was extracted with 60
mL of CH2Cl2 and filtered. The solution was then taken to dryness,
and the residue was washed with 2 × 20 mL of hexane and
recrystallized from a mixture CH2Cl2/toluene (2:1) at −10 °C to
afford compound 7 as a red crystalline solid. Yield: 71% (0.39 g). Anal.
Calcd for C20H34Br2NNiOP: C, 43.36; H, 6.19; N, 2.53. Found: C,
1
spectrometers. The H and 13C{1H} NMR resonances of the solvent
were used as the internal standard, but the chemical shifts are reported
with respect to TMS. 31P NMR resonances are referenced to external
85% H3PO4. NMR chemical shifts are given in parts per million and
coupling constants in hertz. GC analyses were performed in an Agilent
model HP 6890 chromatograph equipped with a HPGASPRO column
and a TCD detector, using CH2Cl2 as the internal standard.
Compounds NiBr2(dme)59 and PdCl2(cod),60 were prepared
according to literature procedures. The amides 2,6-iPr2C6H3NHC-
(O)Me and 2,6-iPr2C6H3NHC(O)-t-Bu were synthesized by standard
procedures.61
Ligand 1. A solution of the amide 2,6-iPr2C6H3NHC(O)Me (219
mg, 1 mmol) in THF (20 mL), stirred at −78 °C, was successively
treated with 0.63 mL (1 mmol) of a 1.6 M solution of nBuLi in hexane,
and 0.167 mL (1 mmol) of iPr2PCl. The stirring was continued for 30
min at the same temperature, and the solution was directly used for
the synthesis of the complexes. This solution contains a mixture of the
corresponding phosphinito-imine (1) and phosphinoamide (2)
tautomers. 31P{1H} NMR (25 °C, THF, 121 MHz): two signals in a
ca. 90:10 intensity ratio at δ 139.0 (s, isomer 1) and 90.0 (s, isomer 2).
1
43.09; H, 6.28; N, 2.51. IR (Nujol mull): v(CN) 1596 cm−1. H
3
NMR (25 °C, CD2Cl2, 300 MHz): δ 1.14 (d, 6H, JHH = 7 Hz, aryl-
3
CHMeMe), 1.51 (d, 6H, JHH = 7 Hz, aryl-CHMeMe), 1.62 (bs, 6H,
PCHMeMe), 1.75 (bs, 6H, PCHMeMe), 1.79 (s, 3H, MeC(O)N),
3
2.70 (m, 2H, PCHMe2), 3.27 (hept, 2H, JHH = 7 Hz, aryl-CHMe2),
7.13 (d, 2H, 3JHH = 7 Hz, m-CHar), 7.24 (t, 1H, 3JHH = 7 Hz, p-CHar).
13C{1H} NMR (25 °C, CD2Cl2, 75 MHz): δ 16.6 (s, PCHMeMe),
17.7 (s, MeC(O)N), 18.5 (s, PCHMeMe), 23.5 (s, aryl-CHMeMe),
23.8 (s, aryl-CHMeMe), 29.4 (s, aryl-CHMe2), 29.8 (s, PCHMe2),
123.9 (s, m-CHar), 128.1 (s, p-CHar), 140.6 (s, N-Car), 141.6 (s, o-Car),
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173.7 (s, MeC(O)N). 31P{1H} NMR (25 °C, CD2Cl2, 121 MHz): δ
178.0 (bs).
Synthesis of Pd[N-(2,6-iPr2C6H3)C(C(CH3)3OPiPr2)]Cl2 (11). A
THF solution of ligand 3 (0.5 mmol) prepared in situ as described
above was added at −78 °C to 143 mg of PdCl2(cod) (0.5 mmol)
suspended in 10 mL of THF. The reaction mixture was stirred and
allowed to warm to room temperature over 30 min. The solvent was
evaporated under reduced pressure, affording a yellow solid that was
extracted with 30 mL of CH2Cl2. The solution was taken to dryness,
and the residue was recrystallized from THF at −10 °C to yield
compound 11 as a yellow crystalline solid. Yield: 97% (0.27 g). Anal.
Calcd for C23H40Cl2NOPPd: C, 49.79; H, 7.27; N, 2.52. Found: C,
Synthesis of Ni[N-(2,6-iPr2C6H3)C((CH3)3OPiPr2)]Br2 (8). A THF
solution of ligand 3 (0.5 mmol) prepared in situ as described above
was added at −78 °C to 154 mg of NiBr2(dme) (0.5 mmol)
suspended in 10 mL of THF. The reaction mixture was stirred and
allowed to warm to room temperature over 30 min. The solvent was
evaporated under reduced pressure, affording a red solid that was
extracted with 40 mL of CH2Cl2. The solution was taken to dryness,
and the residue was washed with 3 × 20 mL of hot diethyl ether (to
remove any N-arylacetamide remaining in the mixture) and recrystal-
lized from a mixture of CH2Cl2/toluene (2:1) at −10 °C to yield
compound 8 as a red crystalline solid. Yield: 89% (0.26 g). Anal. Calcd
for C23H40Br2NNiOP: C, 46.35; H, 6.76; N, 2.35. Found: C, 46.85; H,
6.98; N, 2.27. IR (Nujol mull): v(CN) 1646 cm−1. 1H NMR (25 °C,
CD2Cl2, 300 MHz): δ 1.09 (s, 9H, tBuC(O)N), 1.30 (d, 6H, 3JHH = 7
Hz, aryl-CHMeMe), 1.63 (bs, 6H, PCHMeMe), 1.73 (d, 6H, 3JHH = 7
Hz, aryl-CHMeMe), 1.75 (bs, 6H, PCHMeMe), 2.68 (m, 2H,
1
49.63; H, 7.31; N, 2.58. IR (Nujol mull): v(CN) 1647 cm−1. H
NMR (25 °C, CD2Cl2, 300 MHz): δ 1.11 (s, 9H, tBuC(O)N), 1.31 (d,
6H, 3JHH = 7 Hz, aryl-CHMeMe), 1.51 (dd, 6H, 3JHH = 7 Hz, 3JHP = 20
Hz, PCHMeMe), 1.52 (d, 6H, 3JHH = 7 Hz, aryl-CHMeMe), 1.61 (dd,
6H, 3JHH = 7 Hz, 3JHP = 20 Hz, PCHMeMe), 2.71 (m, 2H, PCHMe2),
3
3
2.97 (h, 2H, JHH = 8 Hz, aryl-CHMe2), 7.10 (d, 2H, JHH = 8 Hz, m-
CHar), 7.24 (t, 1H, JHH = 7 Hz, p-CHar). 13C{1H} NMR (25 °C,
3
CD2Cl2, 75 MHz): δ 16.3 (d, 2JCP = 2 Hz, PCHMeMe), 17.3 (d, 2JCP
=
3
PCHMe2), 3.23 (h, 2H, JHH = 7 Hz, aryl-CHMe2), 7.04 (d, 2H,
3 Hz, PCHMe2), 23.3 (s, aryl-CHMeMe), 24.1 (s, aryl-CHMeMe),
3
3JHH = 8 Hz, m-CHar), 7.19 (t, 1H, JHH = 7 Hz, p-CHar). 13C{1H}
1
29.1 (s, aryl-CHMe2), 29.3 (s, Me3CC(O)N), 30.2 (d, JCP = 26 Hz,
1
NMR (25 °C, CD2Cl2, 75 MHz): δ 16.8 (s, PCHMeMe), 18.7 (s,
PCHMeMe), 23.4 (s, aryl-CHMeMe), 24.5 (s, aryl-CHMeMe), 29.2 (s,
Me3CC(O)N), 29.3 (s, aryl-CHMe2), 30.1 (s, PCHMe2), 41.6 (s,
Me3CC(O)N), 123.3 (s, m-CHar), 128.4 (s, p-CHar), 140.5 (s, N-Car),
141.5 (s, o-Car), 177.5 (s, tBuC(O)N). 31P{1H} NMR (25 °C, CD2Cl2,
121 MHz): δ 166.6 (bs).
PCHMe2), 41.9 (d, JCP = 4 Hz, Me3CC(O)N), 123.4 (s, m-CHar),
128.2 (s, p-CHar), 139.6 (s, N-Car), 141.2 (s, o-Car), 177.8 (s,
tBuC(O)N). 31P{1H} NMR (25 °C, CD2Cl2, 121 MHz): δ 194.5 (s).
Synthesis of Pd[N-(2,6-iPr2C6H3)C((CH3)OPtBu2)]Cl2 (12). A 0.9
mL (0.5 mmol) portion of a 0.5 M THF solution of ligand 5 was
added to 143 mg of PdCl2(cod) (0.5 mmol) suspended in 20 mL of
THF. After stirring for 30 min, the solvent was removed under
vacuum. The yellow solid thus obtained was washed with 2 × 10 mL
of hexane and recrystallized from THF to afford compound 12 as a
yellow crystalline solid. Yield: 91% (0.25 g). Anal. Calcd for
C22H38Cl2NOPPd: C, 48.86; H, 7.08; N, 2.59. Found: C, 48.81; H,
6.74; N, 2.67. IR (Nujol mull): v(CN) 1611 cm−1. 1H NMR (25 °C,
CD2Cl2, 300 MHz): δ 1.15 (d, 6H, 3JHH = 7 Hz, aryl-CHMeMe), 1.38
Synthesis of Ni[N-(2,6-iPr2C6H3)C((CH3)OPtBu2)]Br2 (9). A 2 mL
(1 mmol) portion of a 0.5 M THF solution of ligand 5 was added to a
suspension of NiBr2(dme) (308 mg, 1 mmol) in 10 mL of THF
cooled to −60 °C. The reaction mixture was allowed to warm to room
temperature while stirring. After 30 min, the solvent was removed
under vacuum, and the red solid thus obtained was extracted with 60
mL of CH2Cl2 and filtered. The solution was then taken to dryness.
The residue was washed with 2 × 20 mL of hexane and recrystallized
from a mixture CH2Cl2/toluene (2:1) at −10 °C to afford compound
9 as a red crystalline solid. Yield: 72% (0.42 g). Anal. Calcd for
C22H38Br2NNiOP: C, 45.50; H, 6.58; N, 2.41. Found: C, 45.62; H,
6.44; N, 2.36. IR (Nujol mull): v(CN) 1606 cm−1. 1H NMR (25 °C,
CD2Cl2, 300 MHz): δ 1.14 (d, 6H, 3JHH = 7 Hz, aryl-CHMeMe), 1.51
(d, 6H, 3JHH = 7 Hz, aryl-CHMeMe), 1.78 (bs, 18H, PCMe3), 1.80 (s,
3H, MeC(O)N), 3.23 (m, 2H, aryl-CHMe2), 7.12 (d, 2H, 3JHH = 8 Hz,
3
3
(d, 6H, JHH = 7 Hz, aryl-CHMeMe), 1.64 (d, 18H, JHP = 16 Hz,
PCMe3), 1.99 (s, 3H, MeC(O)N), 2.99 (m, 2H, aryl-CHMe2), 7.19 (d,
2H, 3JHH = 8 Hz, m-CHar), 7.31 (t, 1H, 3JHH = 8 Hz, p-CHar). 13C{1H}
NMR (25 °C, CD2Cl2, 75 MHz): δ 17.2 (d, 3JCP = 4 Hz, MeC(O)N),
23.5 (s, aryl-CHMeMe), 23.6 (s, aryl-CHMeMe), 27.7 (d, 2JCP = 4 Hz,
1
PCMe3), 29.1 (s, aryl-CHMe2), 42.5 (d, JCP = 12 Hz, PCMe3), 124.0
(s, m-CHar), 128.5 (s, p-CHar), 139.6 (s, N-Car), 141.8 (s, o-Car), 174.2
(s, MeC(O)N). 31P{1H} NMR (25 °C, CD2Cl2, 121 MHz): δ 206.8
(s).
3
m-CHar), 7.23 (t, 1H, JHH = 8 Hz, p-CHar). 13C{1H} NMR (25 °C,
CD2Cl2, 75 MHz): δ 18.1 (s, MeC(O)N), 23.6 (s, aryl-CHMeMe),
23.9 (s, aryl-CHMeMe), 28.9 (s, PCMe3), 29.4 (s, aryl-CHMe2), 42.7
(s, PCMe3), 123.9 (s, m-CHar), 128.1 (s, p-CHar), 140.9 (s, N-Car),
141.8 (s, o-Car), 173.6 (s, MeC(O)N). 31P{1H} NMR (25 °C, CD2Cl2,
121 MHz): δ 198.0 (bs).
Synthesis of Pd[N-(2,6-iPr2C6H3)C((CH3)OPiPr2)]Cl2 (10). A 20
mL portion of a THF solution of ligand 1 (1 mmol) prepared in situ as
described above was added at −78 °C to a suspension of PdCl2(cod)
(285 mg, 1 mmol) in 10 mL of THF. The reaction mixture was stirred
while being allowed to warm to room temperature over 1 h. The
solvent was evaporated under vacuum, and the residue was extracted
with 50 mL of CH2Cl2. The solution was then taken to dryness,
affording a yellow solid that was recrystallized from THF at −10 °C to
yield compound 10. Yield: 96% (0.56 g). Anal. Calcd for
C20H34Cl2NOPPd: C, 46.48; H, 6.68; N, 2.73. Found: C, 46.36; H,
6.57; N, 2.86. IR (Nujol mull): v(CN) 1604 cm−1. 1H NMR (25 °C,
CD2Cl2, 300 MHz): δ 1.15 (d, 6H, 3JHH = 7 Hz, aryl-CHMeMe), 1.40
Computational Details. Geometry Optimizations. Guess
structures for the square-planar (S = 0) and tetrahedral (S = 1)
structures of complex 7 were obtained with the semiempirical PM3
method implemented in the Spartan 08 software.62 The structures
were subjected to full optimization with DFT methods with the
Gaussian package.63 Two series of calculations were carried out using
the B3LYP or BP86 functionals. In either case, the metal atom and all
atoms directly bound were described with the 6-311G* basis set, and
the 6-31G* basis was used for the rest. Geometry minima were
checked with frequency calculations.
NMR Spectral Simulation. Spectral simulations of the variable-
1
temperature H NMR spectra of 7 were carried out with the gNMR
program.64 Full line-shape analysis was assisted by a least-squares
optimization of the simulated and experimental spectra. The fluxional
process was modeled as an exchange between the diamagnetic
complex, containing the spin system observed in the slow limit, and
the paramagnetic species, which was described as two independent
spin systems, one of them containing the P atom and the other, the H
atoms, in order to ensure complete loss of coupling information. Even
though the parameters for the paramagnetic species are unknown, for
the purpose of the simulation, their precise values are not important
and proof values were used to describe a very small equilibrium
concentration and very large chemical shifts typical of such substances.
The fact that the thermal drift of the average resonances changes very
little over the studied temperature range confirms that the
concentration of the paramagnetic species is small. In this case, the
exchange rate equals the product k[SP], where k is the rate constant
(square-planar to tetrahedral) and [SP] is the concentration of the
(d, 6H, 3JHH = 7 Hz, aryl-CHMeMe), 1.43 (dd, 6H, 3JHH = 7 Hz, 3JHP
=
3
3
20 Hz, PCHMeMe), 1.60 (dd, 6H, JHH = 7 Hz, JHP = 14 Hz,
PCHMeMe), 1.97 (s, 3H, MeC(O)N), 2.71 (d hept, 2H, 3JHH = 7 Hz,
3
2JHP = 14 Hz, PCHMe2), 2.99 (hept, 2H, JHH = 7 Hz, aryl-CHMe2),
7.20 (d, 2H, 3JHH = 8 Hz, m-CHar), 7.32 (t, 1H, 3JHH = 8 Hz, p-CHar).
2
13C{1H} NMR (25 °C, CD2Cl2, 75 MHz): δ 16.4 (d, JCP = 3 Hz,
2
PCHMeMe), 17.2 (s, MeC(O)N), 17.2 (d, JCP = 1 Hz, PCHMeMe),
23.4 (s, aryl-CHMeMe), 23.6 (s, aryl-CHMeMe), 29.2 (s, aryl-
1
CHMe2), 30.0 (d, JCP = 24 Hz, PCHMe2), 124.0 (s, m-CHar), 128.6
(s, p-CHar), 139.3 (s, N-Car), 141.7 (s, o-Car), 174.2 (s, MeC(O)N).
31P{1H} NMR (25 °C, CD2Cl2, 121 MHz): δ 201.5 (s).
1014
dx.doi.org/10.1021/om201088y | Organometallics 2012, 31, 1006−1016

Organometallics
Article
Table 5. Summary of Crystallographic Data and Structure Refinement Results for 7−12
compound
7
8
9
10
11
12·0.5THF
chemical formula
formula mass
cryst syst
a/Å
C20H34Br2NNiOP
553.98
C23H40Br2NNiOP
596.06
C22H38Br2NNiOP
582.03
C20H34Cl2NOPPd
512.75
C23H40Cl2NOPPd
554.83
C22H38Cl2NOPPd·0.5(C4H8O)
1153.71
monoclinic
10.7905(7)
14.5510(9)
15.0980(9)
90.00
orthorhombic
15.7309(13)
11.6766(7)
14.6596(10)
90.00
monoclinic
11.2189(9)
14.3721(12)
15.8988(11)
90.00
monoclinic
10.884(2)
14.625(3)
17.988(4)
90.00
orthorhombic
17.5777(5)
10.4053(3)
14.3919(4)
90.00
triclinic
9.8987(14)
14.667(2)
b/Å
c/Å
20.625(3)
α/deg
69.682(4)
β/deg
94.0900(10)
90.00
90.00
95.171(2)
90.00
122.881(3)
90.00
90.00
88.388(5)
γ/deg
unit cell volume/Å3
90.00
90.00
77.260(4)
2364.5(3)
100(2)
2692.7(3)
173(2)
2553.1(3)
173(2)
2404.5(8)
298(2)
2632.30(13)
100(2)
2735.1(7)
temp/K
100(2)
space group
P21/n
Pna21
P21/n
P21/c
Pna21
P1
̅
no. of formula units per
4
4
4
4
4
2
unit cell, Z
radiation type
Mo Kα
Mo Kα
Mo Kα
Mo Kα
Mo Kα
Mo Kα
absorption coefficient,
μ/mm
4.277
3.761
3.965
1.070
0.983
0.950
−1
no. of reflns measured
15298
30958
4231
49005
7573
20505
4160
19357
7174
42159
16583
0.0270
0.0253
no. of independent reflns 5987
Rint 0.0289
0.0600
0.0562
0.0445
0.0636
0.0275
0.0231
0.0173
0.0145
a
final R1 values [F2 >
0.0347
2σ(F2)]
b
final wR(F2) values [F2
0.0901
0.1420
0.1607
0.0615
0.0368
0.0673
> 2σ(F2)]
a
final R1 values (all data) 0.0476
0.0692
0.1503
0.0755
0.1725
0.0266
0.0628
0.0154
0.0373
0.0319
0.0692
b
final wR(F2) values (all
0.0931
data)
c
goodness of fit on F2, S
0.987
1.144
1.061
2
1.042
1.045
1.080
a
b
c
R1(F) = ∑(|Fo| − |Fc|)/∑|Fo|. wR2(F2) = {∑[w(Fo2 − Fc2)2]/∑w(Fo )2}1/2. S = {∑[w(Fo2 − Fc2)2]/(n − p)}1/2 (n = number of reflections, p =
number of parameters).
square-planar species, which can be approximated to the total
concentration of the complex. It was independently checked that
different proof values of the paramagnetic chemical shifts had no
significant effect on the spectral line shape, provided that these are
very large in comparison with the normal diamagnetic shifts of the
diamagnetic compound. For a final refinement of the simulation, the
equilibrium constant was estimated from the free activation energy,
which is not far from the energy difference between the square-planar
and the tetrahedral species.
X-ray Structure Analysis for 7, 8, 9, 10, 11, and 12·THF. A
summary of crystallographic data and structure refinement results for
these new crystalline compounds are given in Table 5. Crystals coated
with dry perfluoropolyether were mounted on glass fibers and fixed in
a cold nitrogen stream. Intensity data were collected on a Bruker-AXS
Apex CCD diffractometer (7 and 10) or on a Bruker-Nonius X8Kappa
Apex II CCD diffractometer (8, 9, 11, and 12·0.5THF), operating
both with graphite monochromated Mo Kα radiation (λ = 0.71073 Å).
The data were reduced by SAINT65 and corrected for absorption
effects by the multiscan method (SADABS).65 The structures were
solved by direct methods (SIR-2002, SHELXS)66,67 and refined
against all F2 data by full-matrix least-squares techniques (SHELXTL-
Ethylene Oligomerization Reactions: General Procedure. These
reactions were carried out in 250 mL Fischer-Porter glass reactors
provided with a septum-capped injection port, an internal
thermocouple probe, and magnetic stirring. The reactor was previously
oven-dried. It was then degassed in the vacuum line and charged with
the prescribed amounts of solvent (toluene) and catalyst, the latter as a
standard solution in CH2Cl2 (4.5 × 10−3 or 2.25 × 10−4 M). It was
then immersed in a thermostat water bath and connected to the
ethylene line. After purging three times with ethylene to remove the
original N2 atmosphere, the device was allowed to equilibrate at the
required temperature and pressure. At this point, a cocatalyst solution
(MMAO 1.9 M in heptane, Akzo-Nobel, or DEAC 0.25 M in toluene)
was added. A color change from purple to yellow can be noticed.
Ethylene was continuously fed into the reactor from a external
reservoir with a calibrated volume to maintain a constant pressure in
the reactor. The pressure drop in the reservoir was used to monitor the
monomer consumption. The experiments were terminated by
releasing the pressure and pouring the mixture in a beaker. These
were stored in closed containers at −20 °C until GC analyses were
performed. Quantitative GC analyses were made using the CH2Cl2
peak as an internal standard. The reaction mixtures were only slightly
hazy and, upon treatment with acidified methanol, did not produce
any significant precipitate of polyethylene. Key experiments were
repeated to ensure the reproducibility of the data, which was found to
be within 10−15%.
6.12)65 minimizing w[Fo − Fc2]2. All non-hydrogen atoms were
2
refined with anisotropic thermal parameters. Hydrogen atoms were
included in calculated positions and allowed to ride on the attached
carbon atoms with the isotropic temperature factors (Uiso values) fixed
at 1.2 times those Ueq values of the corresponding carbon atoms (1.5
times for the methyl groups).
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF file containing crystallographic data for complexes 7−11
and 12·0.5THF. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: campora@iiq.csic.es.
ACKNOWLEDGMENTS
■
Financial support from the DGI (Project CTQ2009-11721),
́
Junta de Andalucıa (Project P09-FQM5074), and EU (FEDER
1015
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Organometallics
Article
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