New self-activating organochromium catalyst precursor for selective ethylene trimerization

Article abstract of DOI:10.1021/om200560e

Two catalytic systems based on anionic ligands with the NPN structural motif, bearing tri- and pentavalent phosphorus, respectively, have been compared vis-a-vis their ability to selectively trimerize ethylene. In the case of the trivalent phosphorus ligands, reaction of the Cr(II) catalyst precursor [(t-Bu)NPN(t-Bu)]₂Cr (1) with 2 equiv of MeLi afforded the Cr(III) species [(t-Bu)NP(Me)N(t-Bu)]₂CrLi(OEt₂) (2). The same reaction with 3 equiv of MeLi yielded instead the Cr(II) species {[(t-Bu)NP(Me)N(t-Bu)]Cr(μ-Me)}₂{Li(THF)}₂ (3). The P atoms of both 2 and 3 have been methylated. Activation of 2 and 3 with MAO produced an S-F distribution of oligomers. Conversely, activation of 3 with EtAlCl₂ exclusively afforded 1-hexene together with a small amount of polymer as a byproduct. Treatment of 2 with EtAlCl₂ did not yield an active catalyst. The reaction of CrCl₂(THF)₂ with [(t-Bu)NP(Ph)₂N(t-Bu)]^-Li⁺, containing pentavalent phosphorus, afforded the Cr(II) derivative [(t-Bu)NP(Ph) ₂N(t-Bu)]Cr(μ-Cl)₂Li(THF)₂ (4). Its alkylation with EtLi gave the ethyl-bridged dimer {[(t-Bu)NP(Ph) ₂N(t-Bu)]Cr(μ-Et)}₂ (5), which, upon thermolysis, afforded [(t-Bu)NP(Ph)₂N(t-Bu)]₂Cr (6). The structures of 5 and 3 are closely related, having in common two bridging alkyls and the same metal oxidation state but differing with respect to the P atom oxidation state and the presence/absence of alkali-metal cations. The catalytic behavior instead is remarkably different. Complex 5 is active as a self-activating selective ethylene trimerization catalyst, while complex 3 requires activation.

Full text of DOI:10.1021/om200560e

ARTICLE
pubs.acs.org/Organometallics
New Self-Activating Organochromium Catalyst Precursor
for Selective Ethylene Trimerization.
Khalid Albahily,† Valeria Fomitcheva,† Yacoob Shaikh,† Elena Sebastiao,† Serge I. Gorelsky,†,‡
Sandro Gambarotta,*,† Ilia Korobkov,§ and Robbert Duchateau*,||
†Department of Chemistry, ‡Centre for Catalysis Research and Innovation, and §X-ray Core Facility, Faculty of Science,
University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, Eindhoven 5600 MB,
The Netherlands
S Supporting Information
b
ABSTRACT: Two catalytic systems based on anionic ligands with the
NPN structural motif, bearing tri- and pentavalent phosphorus, respec-
tively, have been compared vis-ꢀa-vis their ability to selectively trimerize
ethylene. In the case of the trivalent phosphorus ligands, reaction of the
Cr(II) catalyst precursor [(t-Bu)NPN(t-Bu)]2Cr (1) with 2 equiv of
MeLi afforded the Cr(III) species [(t-Bu)NP(Me)N(t-Bu)]2CrLi(OEt2)
(2). The same reaction with 3 equiv of MeLi yielded instead the Cr(II)
species {[(t-Bu)NP(Me)N(t-Bu)]Cr(μ-Me)}2{Li(THF)}2 (3). The P
atoms of both 2 and 3 have been methylated. Activation of 2 and 3 with
MAO produced an S-F distribution of oligomers. Conversely, activation of
3 with EtAlCl2 exclusively afforded 1-hexene together with a small amount
of polymer as a byproduct. Treatment of 2 with EtAlCl2 did not yield an
active catalyst. The reaction of CrCl2(THF)2 with [(t-Bu)NP(Ph)2N(t-Bu)]ÀLi+, containing pentavalent phosphorus, afforded the
Cr(II) derivative [(t-Bu)NP(Ph)2N(t-Bu)]Cr(μ-Cl)2Li(THF)2 (4). Its alkylation with EtLi gave the ethyl-bridged dimer
{[(t-Bu)NP(Ph)2N(t-Bu)]Cr(μ-Et)}2 (5), which, upon thermolysis, afforded [(t-Bu)NP(Ph)2N(t-Bu)]2Cr (6). The structures
of 5 and 3 are closely related, having in common two bridging alkyls and the same metal oxidation state but differing with respect to
the P atom oxidation state and the presence/absence of alkali-metal cations. The catalytic behavior instead is remarkably different.
Complex 5 is active as a self-activating selective ethylene trimerization catalyst, while complex 3 requires activation.
’ INTRODUCTION
that a tetramerization with selectivity higher than that currently
provided by the few existing trimerization systems cannot be
achieved via the same ring expansion mechanism and would
require an alternative pathway.6
Linear R-olefins (LAOs) are important commodity chemicals
central to the industrial preparation of many diversified products
essential to our modern lifestyle.1 These compounds are typically
produced as a statistical distribution, which has to be fractionated
through an energy-intensive separation process to afford the
individual LAOs for specific applications.2 The problem is
particularly acute for the light LAOs, given the steadily increasing
demand of 1-hexene and 1-octene as comonomers for LLDPE
production. For these reasons, introducing selectivity into the
catalytic oligomerization cycle is particularly being pursued.3
In addition to the keen industrial relevance for discovering
selective tri- and tetramerization catalysts, developing such
systems poses genuine intellectual challenges. In fact, specific
mechanistic pathways must be engineered in order for the
desired selectivity to be introduced in the catalytic cycle. For
example, it is commonly accepted that selective trimerization is
achieved with the so-called ring expansion redox mechanism.4,5
The same mechanism has also been proposed for rationalizing
the behavior of the very few existing quasi-selective tetrameriza-
tion catalysts (70%).5 However, it becomes increasingly apparent
In any event, controlling the metal oxidation state is central to
obtain a selective catalytic system. Recent synthetic,7 spectro-
scopic,8 and theoretical9 work has established that Cr(I) is
responsible for the selective ethylene oligomerization, while Cr(II)
yields nonselective oligomerization catalysts. It remains unclear if
polyethylene is produced only by Cr(III) or also by Cr(II)
species.10 Thus, clearly defined reductions and availability of
ligands that can adequately stabilize Cr(I) may open the way to
new and better performing selective ethylene oligomerization
catalysts. There are formidable challenges, however. The +I state
of chromium is characterized by an exceptionally high reactivity,
as clearly indicated by the occurrence of dinitrogen fixation,11
arene coordination,12 metalÀmetal quintuple bond formation,13
etc. In addition, all the isolated Cr(I) species are, with only two
Received: June 28, 2011
Published: July 14, 2011
r
2011 American Chemical Society
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Scheme 1
Scheme 2
exceptions,7a,c catalytically inert. An ingenious alternative to the
problems posed by selective reduction to the +I state (dispro-
portionations, ligand fragmentations, etc.) consists of the re-
cently developed abstraction of CO from some representatives of
the ligand, given the appropriate alkylating agent, has sufficient
capability to stabilize catalytically selective Cr(I) intermediates.17
In this paper, we have analyzed the effect of the alkylation and
oxidation of the P atom on the relative stability of the +II versus
+III state and their effect on organochromium species that in
turn may act as catalyst precursors in the absence of aluminate
activators.
the large family of highly stable Cr(I) carbonyl derivatives.5m,7aÀ7c
A
pragmatic solution to the instability of the highly reactive Cr(I)
species remains designing catalyst precursors with the metal in
the stable +III state to be directly reduced to Cr(I) under the
conditions required by the oligomerization reaction. However,
even the in situ reduction of Cr(III) precursors poses a
problem, since the +II state must be necessarily bypassed.
Due to its intrinsic stability, the +II state often prevents further
reduction and furnishes only nonselective ethylene oligomer-
ization catalysts, albeit occasionally highly active.4w,14 A possi-
bility of bypassing Cr(II) consists of an initial double alkylation
of Cr(III) precursors followed by a thermal two-electron
reductive elimination.15
We have recently found that the NPN ligand may afford
chromium complexes of switchable catalytic behavior (from
polymerization to statistical oligomerization or trimerization)
depending on the activation conditions.14h,16 In turn, this
indicates that this family of ligands has the ability of stabilizing
each of the three critical chromium oxidation states and impedes
or even prevents their interconversion after activation. To this
end, we have observed that, first, the original P2N4 structure can
be both cleaved and re-formed, depending on the countercation
coordination requirements (Scheme 1).15,16 Second, the activa-
tors consistently alkylate the ligand P atom with its aluminum
residues remaining connected to the molecular frame. Third,
given the occasional appearance of selective behavior, it is clear that
’ RESULTS AND DISCUSSION
To deliberately alkylate the P(III)-containing NPN frame-
work, MeLi was the most obvious choice. The preparation and
characterization of the Cr(II) catalyst precursor [(t-Bu)NPN-
(t-Bu)]2Cr (1) was previously described.14h Its reaction with
MeLi proceeded in a remarkably different manner, depending on
the stoichiometric ratio. While large amounts of MeLi (4À5
equiv) afforded the Cr(II) derivative Me8Cr2Li4(Et2O)4,18 the
reaction with 2 equiv afforded the Cr(III) species [(t-Bu)NP-
(Me)N(t-Bu)]2CrLi(OEt2) (2). In this complex both P atoms
have been methylated and one partially solvated lithium atom
was retained to provide electroneutrality to the Cr(III) metal
center (Scheme 2).
The oxidation of chromium from the +II to the +III state upon
treatment with MeLi (certainly a reducing rather than an
oxidizing agent) is explained by assuming a disproportionation
between two Cr(II) intermediates (Scheme 2). In turn, this
requires that a low-valent complex, probably Cr(I), must have
been generated as a partner of the formation of 2 along with the
alkylated form of the ligand dilithium salt. Unfortunately, all
attempts to isolate or even trap the low-valent chromium
byproduct via reoxidation reactions did not afford any identifi-
able compound.
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The crystal structure of 2 (Figure 1) shows the Cr(III) atom
in a distorted-tetrahedral coordination environment (N(2)À
Cr(1)ÀN(1) = 77.67(8)°, N(1)ÀCr(1)ÀN(1A) = 102.89°,
N(2)ÀCr(1)ÀN(1A) = 130.72(9)°, andN(2A)ÀCr(1)ÀN(1) =
130.71(9)°) defined by the four nitrogen donor atoms of two
separate NPN anions. This distortion is likely to be attributed to
the presence of the two lithium atoms, each bridging two
nitrogens of two ligands (N(1)ÀLi(2) = 2.097(5) Å).
line broadening, but all the expected resonances with correct
intensity were clearly identified. This reaction is, from a formal
point of view, rather straightforward, implying the usual ligand
alkylation, dissociation of alkylated ligand dilithium salt, and
coordination of the third 1 equiv of MeLi prior to dimeric
aggregation. However, it is also conceivable that the reaction
may instead proceed via preliminary disproportionation and
formation of 2 (Scheme 4), since treatment of 2 with MeLi also
affords 3. In this event the additional MeLi is required for the
reduction of the metal center.
The crystal structure of 3 (Figure 2) consists of two chromium
atoms, each chelated by one methylated ligand and bridged by
two methyl groups. The Cr2(μ-Me)2 core is severely folded with
a very short CrÀCr contact (Cr(1)ÀCr(2) = 2.0148(15) Å).
The coordination environment around each chromium atom
is slightly distorted square planar (N(1)ÀCr(1)ÀC(19) =
176.71(17)°, N(1)ÀCr(1)ÀN(2) = 76.92(15)°, N(1)ÀCr-
(1)ÀC(20) = 102.85(18)°) and is defined by the two N donor
atoms of the chelating ligand (C(1)ÀN(1) = 2.061(4) Å,
Cr(1)ÀN(2) = 2.073(4) Å) as well as by the two bridging
methyl groups (Cr(1)ÀC(19) = 2.199(5) Å and Cr(1)ÀC(20) =
2.296(5) Å). Two lithium atoms, each solvated by one THF,
bridge one nitrogen atom of each ligand (N(1)ÀLi(2) =
2.122(9) Å).
Instead, the reaction of 1 with 3 equiv of MeLi afforded
an interesting dinuclear and almost diamagnetic Cr(II) complex,
formulated as {[(t-Bu)NP(Me)N(t-Bu)]Cr(μ-Me)}2{Li(THF)}2
1
(3) (Scheme 3). The H NMR spectrum of 3 shows a minor
Both 2 and 3 display a rather disappointing catalytic behavior.
Activation with MAO in toluene yielded strictly nonselective
behavior, albeit with high activity (Table 1). Complex 3 was
also tested as a single-component catalyst and was thermo-
lyzed (100 °C) under an ethylene atmosphere (35 bar). No
activity or degradation of the complex was observed. In turn,
this emphasizes the high stability of organochromium(II)
complexes and reiterates the need for bypassing this oxidation
state in order to obtain selective ethylene oligomerization
catalysts. Interestingly, though, activation of 3 with RAlCl2
(R = Me, Et) in MeCy selectively produced 1-hexene, along
with a small amount of polymer as a byproduct. This behavior
was also observed in toluene, the only difference being a
substantially lower activity. This is in agreement with the often
Figure 1. Plot of 2 with ellipsoids set at the 50% probability level and
hydrogen atoms omitted for clarity. Selected bond lengths (Å) and
angles (deg): Cr(1)ÀN(2) = 1.902(2), Cr(1)ÀN(2A) = 1.902(2),
Cr(1)ÀN(1) = 2.022(2), Cr(1)ÀN(1A) = 2.022(2), N(1)ÀLi(2) =
2.097(5), Li(2)ÀO(1) = 1.926(7); N(2)ÀCr(1)ÀN(1) = 77.67(8),
N(1)ÀCr(1)ÀN(1A)
=
102.89(12), N(2)ÀCr(1)ÀN(1A)
=
130.72(9), N(2A)ÀCr(1)ÀN(1) = 130.71(9), N(2)ÀP(2)ÀN(1) =
91.16(10), N(1A)ÀLi(2)ÀN(1) = 97.9(3).
Scheme 3
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Scheme 4
The selective behavior observed upon activation with EtAlCl2
suggests the necessity for the presence in the catalyst precursor of
both chlorine and a more reducing alkyl. The “chlorine effect” is a
primary characteristic of the commercially used pyrrolato-based
catalytic system,17 and the characterization of its catalytically
active intermediates has fully elucidated its role.7c For example, in
the case of the Cr(II) dimeric tetramethylpyrrolato catalyst
precursor [{η5-2,3,4,5-Me4C4N(AlClMe2)Cr}(μ-Me)2], also a
Me-bridged dimer,7d the very strong chlorine effect is mainly due
to its ability to trigger the disproportionation to the +III and À-I
states. Although in the present case we lack direct proof, we
reasoned that the effect might be very similar (Scheme 5).
In order to probe the viability of this mechanistic hypothesis,
DFT calculations were carried out to evaluate the free energy
variation associated with each step of the proposed mechanism
(Figure 3). Geometry optimization calculations at the spin-
unrestricted PBE level on the full structure of 3 yielded geome-
trical parameters in agreement with the experimental crystal-
lographic values. Complex 3 contains a short CrÀCr distance
and is almost diamagnetic with small residual paramagnetism. In
perfect agreement, calculations indicated that the ground-state
configuration of the complex is the open-shell singlet where two
Cr(II) ions are antiferromagnetically coupled with a Mayer bond
order for the CrÀCr interaction of only 1.22. Calculations
carried out on the closed-shell singlet with the quadruple CrÀCr
Figure 2. Plot of 3 with ellipsoids set at the 50% probability level and
hydrogen atoms omitted for clarity. Selected bond lengths (Å) and
angles (deg): Cr(1)ÀCr(2) = 2.0148(15), Cr(1)ÀC(19) = 2.199(5),
Cr(1)ÀC(20) = 2.296(5), Cr(2)ÀC(20) = 2.187(5), Cr(2)ÀC(19) =
2.295(5), Cr(1)ÀN(1) = 2.061(4), Cr(1)ÀN(2) = 2.073(4), N(1)À
Li(2) = 2.122(9); Cr(1)ÀC(19)ÀCr(2) = 53.22(11), Cr(2)ÀC(20)À
Cr(1) = 53.35(11), N(1)ÀCr(1)ÀN(2) = 76.92(15), N(1)ÀCr-
(1)ÀC(19) = 176.71(17), N(1)ÀCr(1)ÀC(20) = 102.85(18), Cr-
(2)ÀCr(1)ÀC(19) = 65.85(15), Cr(2)ÀCr(1)ÀC(20) = 60.55(13),
N(1)ÀLi(2)ÀN(3) = 125.2(4).
bond predicted a structure with a much shorter Cr Cr
3 3 3
distance (1.796 Å) and also a higher energy (ΔG = 16.5 kcal
molÀ1 relative to the open-shell singlet).
The reaction of 3 with [RAlCl2]2 removes two Li(THF) units
as [LiCl(THF)]2 and replace them with two AlRCl moieties.
Calculations on the intermediate species a were carried out and,
similar to the case for complex 3, gave the open-shell singlet as the
ground state (two Cr(II) ions are antiferromagnetically coupled).
Interestingly, the anticipated formation of chlorine bridges be-
tween chromium and aluminum is responsible for the beginning of
the disruption of the dimetallic structure, eventually leading to
heterolytic cleavage inthe followingstep. The situationis strikingly
similar to that of the aforementioned aluminum pyrrolyl Cr(II)
observed poisoning effect of aromatic solvents, possibly re-
sponsible for an excessive stabilization of intermediate Cr(I)
species.6a,b
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Table 1. Oligomerization of Ethylenea
amt (mol %)
cat.
Al:Cr
cocat.
MAO
amt of alkene (mL)
amt of PE (g)
activity (g/((mmol of Cr)/h))
C6
C8
C10À20
2b
2b
2c
2c
2c
3c,d
3b
3b
3c
3c
3c
3c
3b
3c
1000
500
5
90
63
0
0.8
2.6
traces
0
2130
1560
0
24
33
0
23
27
0
53
40
0
MAO
Et3Al
5
Et2AlCl
EtAlCl2
0
0
0
0
0
5
0
traces
0
0
0
0
0
0
0
0
0
0
1000
MAO
59
42
0
1.1
2.9
traces
0
1420
1080
0
28
29
0
25
25
0
47
46
0
500
5
MAO
Et3Al
5
Et2AlCl
EtAlCl2
EtAlCl2
EtAlCl2
MeAlCl2
0
0
0
0
0
5
2
0.2
0.9
0.4
1.15
52
99.9
99.9
99.9
99.9
0
0
2
5
143
36
0
0
2
1
0
0
2
9
287
0
0
a Conditions: loading 30 μmol of chromium complex, 35 bar of ethylene, reaction temperature 60 °C, reaction time 60 min. b 100 mL of toluene.
c 100 mL of methylcyclohexane. d Reaction temperature 100 °C.
Scheme 5
system, a highly active and selective trimerization catalyst.7d In the
present case, the process appears to be thermodynamically
highly favored. The value of ΔG becomes even more negative if
other forms of LiCl(THF) (e.g., tetrameric) are being consid-
ered. Interestingly, the calculation predicts that the replace-
ment of Li ions by aluminate cations resolves into a lengthening
of the CrÀCr distance to 2.44 Å that compares well to that of
the ethyl-bridged 5 (see below) with very similar arrangement
and no countercations.
dimethyl intermediates is thermodynamically uphill (ΔGr = +9.8
kcal molÀ1). However, the reductive elimination of ethane from
the dimethyl intermediate b is thermodynamically favorable
(ΔGr = À3.8 kcal molÀ1). If C2H4 is added to intermediate b
and methane and propylene are released as products, the ΔGr
value of this reaction step is also negative (À11.4 kcal molÀ1).7d
The metallacyclic system is, of course, the key intermediate
capable of expanding the cycle and therefore starting the selective
oligomerization.
The disproportionative splitting of the dinuclear structure by
ethylene with formation of the two Cr(III) metallacyclic and
The data above clearly indicate that the Cr(II) 3 may in fact be
used as precursor for selective trimerization but only in the
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presence of halogenated aluminum alkyl activators capable of
triggering disproportionation and consequent reduction to the
+I state responsible for the formation of the metallacycle.
Activators such as MAO do not possess such ability in this
catalytic system and gave only SchulzÀFlory (S-F) distributions
of oligomers or no activity at all. In turn, this indicates that
neither reduction of the +II state nor disproportionation to the
+III and +I states occurred.
Given the critical role of the aluminate species in determining
the catalytic behavior of NPN complexes and the noninnocent
behavior of this particular ligand system, we have now oxidized
the trivalent P atom to its pentavalent state. This modification
was aimed at verifying that an increased electron-withdrawing
effect, resulting from the higher oxidation state of phosphorus,
could possibly better stabilize Cr(I) intermediates via charge
transfer. We have recently described the synthesis of the
Cr(II) species [(t-Bu)NP(Ph)2N(t-Bu)]Cr(μ-Cl)2Li(THF)2
(4), which showed a near-record catalytic activity in producing
polymer-free S-F distributions of oligomers upon activation with
MAO.19 We were anticipating that in this case the role of the
activator would be solely that of alkylating the metal center.
Therefore, we have reacted the Cr(II) species 4 with EtLi, in the
hope that ethyl groups bonded to chromium could make the
metal more inclined toward reduction to the +I state.
The reaction proceeded smoothly at room temperature,
affording the corresponding dinuclear and paramagnetic {[(t-Bu)-
NP(Ph)2N(t-Bu)]Cr(μ-Et)}2 (5) (Scheme 6). The structure of
5 is the result of a straightforward chlorine replacement by an
ethyl group with additional dissociation of the LiCl unit. In
addition to the absence of the lithium counterions, the dimeric
arrangement with the two bridging ethyl groups in 5 is closely
reminiscent of the case for 3. Clearly, the expected reduction did
not occur.
The dimeric structure of 5 (Figure 4) consists of two
[(t-Bu)NP(Ph)2N(t-Bu)]Cr units bridged by two ethyl groups
(Cr(1)ÀC(41) = 2.143(2) Å, Cr(2)ÀC(43) = 2.145(2) Å)
forming a folded Cr2Et2 core. The coordination geometry
around each metal center is distorted square planar (N(2)À
Cr(1)ÀN(1) = 72.07(6)°, N(2)ÀCr(1)ÀC(41) = 107.29(8)°,
N(1)ÀCr(1)ÀC(41) = 144.04(7)°, N(2)ÀCr(1)ÀCr(2) =
120.45(5)°, N(1)ÀCr(1)ÀCr(2) = 150.78(5)°, C(41)À
Cr(1)ÀCr(2) = 61.72(6)°) and is defined by the two bridging
carbon atoms and each ligand’s nitrogen donors (Cr(1)ÀN(2) =
2.0542(16) Å, Cr(1)ÀN(1) = 2.0970(16) Å). The deviation
from square planarity is probably due to the long-range steric
interaction between the t-Bu groups of two units. This is in
striking contrast to the perfect planarity of the metal centers in 3
as a possible result of the rigidity introduced by the two alkali-
metal cations.
Figure 4. Plot of 5 with thermal ellipsoids drawn at the 50% probability
level. Selected bond lengths (Å) and angles (deg): Cr(1)ÀN(2) =
2.0542(16), Cr(1)ÀN(1) = 2.0970(16), Cr(1)ÀC(41) = 2.143(2),
Cr(1)ÀCr(2) = 2.3282(5), Cr(2)ÀC(43) = 2.145(2); N(2)ÀCr-
(1)ÀN(1) = 72.07(6), N(2)ÀCr(1)ÀC(41) = 107.29(8), N(1)ÀCr-
(1)ÀC(41) = 144.04(7), N(2)ÀCr(1)ÀCr(2) = 120.45(5), N(1)À
Cr(1)ÀCr(2) = 150.78(5), C(41)ÀCr(1)ÀCr(2) = 61.72(6).
Figure 3
Scheme 6
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elimination of a ethylene/ethane/butane mixture (observed
when the thermolysis was carried out in sealed and centrifuged
NMR tubes) and intermediate formation of the Cr(I) putative
intermediate “[(t-Bu)NP(Ph)2N(t-Bu)]Cr”, most likely dinuc-
lear. In the absence of ethylene, this species may rapidly regain
stability only via ligand scrambling, forming 6 and colloidal
chromium. In agreement with this proposal implying the
transient formation of Cr(I) by heterolytic splitting of the
dimer, complex 5 indeed acts, in toluene and under ethylene
pressure, as a self-activating, single-component trimerization
catalyst (Table 2). This is in striking contrast with the behavior
of 3, which required instead activation by at least a stoichio-
metric amount of RAlCl2. In what seems to be now a distinctive
feature of the (NPN)Cr catalytic systems, the solvent has a
marked effect on the catalytic behavior. An identical self-
activating experiment carried out on a solution of 5 in MeCy
doubled the activity but replaced the highly selective formation
of 1-hexene with an S-F distribution.
Figure 5. Plot of 6 with thermal ellipsoids drawn at the 50% probability
level. Selected bond lengths (Å) and angles (deg): Cr(1)ÀN(1) =
2.100(3), Cr(1)ÀN(4) = 2.104(3), Cr(1)ÀN(3) = 2.112(3), Cr-
(1)ÀN(2) = 2.111(3); N(1)ÀCr(1)ÀN(4) = 144.06(13), N(1)ÀCr-
(1)ÀN(3) = 119.53(14), N(4)ÀCr(1)ÀN(3) = 71.47(13), N(1)À
Cr(1)ÀN(2) = 71.30(12), N(4)ÀCr(1)ÀN(2) = 120.69(13), N(3)À
Cr(1)ÀN(2) = 145.59(13).
Although in the pyrrolato selective trimerization catalyst
toluene has a poisoning effect, in this case it obviously lends a mini-
mum stability for the formation of the Cr(I) species responsible
for selectivity. Lack of such a stabilizing effect, as in the case of
MeCy, clearly makes the formation of Cr(I) more problematic
with consequent retention of the +II state. As in the case of 3,
activation of 5 with MAO in toluene gave the usual nonselective
oligomerization with surprisingly high activity. Thus, it seems
that in toluene 3 and 5, when activated with MAO, give similar
catalytically active species or at least species with metal centers in
the same +II state. Complex 6 is instead a mediocre nonselective
catalyst, as expected from the fact that the activator cannot easily
extract one of the two ligands.
In conclusion, we have herein reported the synthesis and
characterization of the two closely related organochromium(II)
complexes 3 and 5. The only differences are (1) the oxidation
state of the P atom and (2) the ligand’s anionic charge (+I versus
+II). These differences affect not only the CrÀCr interaction of
dimetallic species but also the catalytic behavior. In turn, this
speaks for the different support provided by the two ligands to
the catalytically selective Cr(I). The comparison between the
two species is not as rigorous as may be desired, given the
different nature of the alkyl functions bonded to chromium in
both complexes (Me versus Et). Unfortunately, it was not
possible to prepare and characterize the ethyl 3 and methyl 5
analogues. Nonetheless, the fact that 3 may be activated into a
selective trimerization catalyst with only 2 equiv of RAlCl2
suggests that both ligands can indeed sufficiently stabilize a Cr(I)
intermediate during the catalytic cycle.
There is an interesting comparison that can be made between
the structures of 3 and 5. In complex 3, the ligand frame
is dianionic due to the alkylation of the P atoms. In turn, this
requires the Cr2R2 core to retain two lithium counterions
between the two ligands, thus providing additional bridging
interactions. In complex 5, the ligand bears instead the formal
charge of À1, for which there is no necessity for retention of the
alkali-metal cation. This has a profound effect on the CrÀCr
distance (Cr(1)ÀCr(2) = 2.3282(5) Å), being remarkably
elongated in comparison to that in 3 (Cr(1)ÀCr(2) =
2.0148(15) Å), as well as on the magnetism of the complex
(paramagnetic versus almost diamagnetic). In our opinion, this
provides once again a good illustration of the intriguing and yet
elusive nature of the CrÀCr interaction.20 In the present case, the
CrÀCr distance is affected by the ligand’s formal charge and the
presence of the alkali-metal cations. Only these two factors
appear to be responsible for the occurrence of the short CrÀCr
contact of the diamagnetic 3 and long distance of the paramag-
netic 5.
The presence of the two Et groups in the dinuclear structure of
5, the absence of alkali-metal cations, and the monoanionic
nature of the ligand altogether make this species better suited,
in comparison to 3, to form a Cr(I), catalytically active inter-
mediate. Upon having observed that solutions of 5 have the
tendency to slowly decompose at room temperature, a deliberate
attempt to flash-thermolyze the complex was carried out by
heating its solution to 110 °C for few seconds (Scheme 6). As
anticipated, the thermolysis of 5 afforded a new Cr(II) complex,
where one of the metal centers has basically extracted the other’s
ligand to form the square-planar and paramagnetic [(t-Bu)NP-
(Ph)2N(t-Bu)]2Cr (6). The reaction was accompanied by se-
paration of some colloidal metallic chromium.
’ EXPERIMENTAL SECTION
All reactions were carried out under a dry nitrogen atmosphere.
Solvents were dried using an aluminum oxide solvent purification system.
The liquid product mixtures were analyzed by using a CP 9000 gas
chromatograph (GC) equipped with a 30 mL Â 0.32 mm i.d. capillary CP
volamine column and an FID detector. The yield was determined by 1H
NMR spectroscopy (Varian Mercury 400 MHz spectrometer). Samples
for magnetic susceptibilitywerepreweighedinside a dryboxequippedwith
an analytical balance and measured on a Johnson Matthey Magnetic
Susceptibility balance. Elemental analysis was carried out with a Perkin-
Elmer 2400 CHN analyzer. Data for X-ray crystal structure determination
were obtained with a Bruker diffractometer equipped with a 1K Smart CCD
area detector. The complexes [(t-Bu)NPN(t-Bu)]2Cr (1) and [(t-Bu)NP-
(Ph)2N(t-Bu)]Cr(μ-Cl)2Li(THF)2 (4) were prepared according to a
The crystal structure of 6 (Figure 5) shows a severely flat-
tened tetrahedral chromium atom (N(1)ÀCr(1)ÀN(4) =
144.06(13)°, N(1)ÀCr(1)ÀN(3) = 119.53(14)°, N(4)ÀCr(1)À
N(3) = 71.47(13)°, N(1)ÀCr(1)ÀN(2) = 71.30(12)°, N(4)À
Cr(1)ÀN(2) = 120.69(13)°, N(3)ÀCr(1)ÀN(2) = 145.59(13)°)
surrounded by the four nitrogen donor atoms of the two ligands
(Cr(1)ÀN(1) = 2.100(3) Å, Cr(1)ÀN(4) = 2.104(3) Å, Cr-
(1)ÀN(3) = 2.112(3) Å, Cr(1)ÀN(2) = 2.111(3) Å).
The formation of 6, as a result of the thermolysis of 5, is rather
indicative of the inclination toward disproportionative decomposition
pathways. The reaction may simply be envisioned with the initial
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Organometallics
ARTICLE
Table 2. Ethylene Oligomerization Resultsa
C6
(mol
%)
C8
(mol
%)
Al:
Cr
amt of alkenes
temp
PE
activity
C10ÀC16
(mol %)
cat.
solvent
cocat.
(mL)
(°C)
(g)
(g/((mmol of Cr) h))
5
5b
toluene
toluene
MeCy
MeCy
4
106
8
100
100
60
0
93
4947
197
99
37
39
24
traces
26
traces
37
1000
1000
MAO
MAO
0
5
0.3
0
32
29
6
9
60
210
22
54
a Conditions: loading 30 μmol of complex, 35 bar of ethylene, reaction time 60 min, 100 mL of solvent. b catalyst loading 15 μmol.
published procedure.15a,16 Et3Al (93%, Strem), Et2AlCl (97%, Strem),
EtAlCl2 (1 M in hexane, Aldrich), Me2AlCl (Aldrich), and MAO (10 wt
%, Aldrich) were used as received.
vessel. Solutions were heated using a thermostatic bath and charged with
ethylene, maintaining the pressure throughout the run. The reaction
mixtures of the oligomerization runs were cooled to 0 °C prior to
releasing the overpressure and quenching with MeOH and HCl. The
activity was measured by NMR by integrating the resonances of the vinyl
protons against those of the toluene solvent methyl group. Individual
olefins were qusantified by GC. Results of catalytic runs are given in
Table 1.
Computational Details. DFT calculations were performed using
the Gaussian 09 package21 using the PBE22 functional and the TZVP23
basis set. The spin-unrestricted formalism was used for all open-shell
species. Tight SCF convergence criteria were used for all calculations.
Harmonic frequency calculations were performed on the optimized
structures to establish the nature of stationary points and to calculate
Gibbs free energies at 298 K and 1 atm. The converged wave functions
were tested to confirm that they correspond to the ground-state surface.
All calculations for the analysis of the electronic structure, including the
generation of initial wave functions, Mulliken population analysis,24 the
calculation of Mayer bond order indices,25 natural population analysis
(NPA)26-derived spin densities, and populations of fragment orbitals,27
were performed using the AOMix software package.28
X-ray Crystallography. Suitable crystals were selected, mounted
on a thin glass fiber with paraffin oil, and cooled to the data collection
temperature. Data were collected on a Bruker AXS SMART 1 k CCD
diffractometer. Data collection was performed with three batch runs at j
= 0.00° (600 frames), at j = 120.00° (600 frames), and j = 240.00°
(600 frames). Initial unit-cell parameters were determined from 60 data
frames collected at different sections of the Ewald sphere. Semiempirical
absorption corrections based on equivalent reflections were applied. The
systematic absences and unit-cell parameters were consistent for the
reported space groups. The structures were solved by direct methods,
completed with difference Fourier syntheses, and refined with full-
matrix least-squares procedures based on F2. All non-hydrogen
atoms were refined with anisotropic displacement parameters. All
hydrogen atoms were treated as idealized contributions. All scatter-
ing factors and anomalous dispersion factors are contained in the
SHELXTL 6.12 program library. Relevant crystal data and extensive
listings of bond distances and angles are given in the Supporting
Information.
Preparation of [(t-Bu)NP(Me)N(t-Bu)]2CrLi(OEt2) (2). A so-
lution of [(t-Bu)NPN(t-Bu)]2Cr (1; 0.398 g, 1.0 mmol) in THF
(10 mL) was treated with MeLi (1.31 mL, 2.1 mmol, 1.6 M in diethyl
ether) at À20 °C. The mixture was stirred at room temperature for 20 h.
Then the solvent was removed in vacuo and replaced with diethyl ether
(5.0 mL). The resulting green suspension was centrifuged, and the
supernatant was then stored at À30 °C for 3 days, forming deep green
crystals of 2. The crystals were washed with cold hexanes (10 mL) and
dried in vacuo to give analytically pure 2 (0.209 g, 0.41 mmol, 41%). μeff
= 3.93 μB. Anal. Calcd (found) for C22H52CrLiN4OP2: C, 51.86
(51.87); H, 10.29 (10.25); N, 11.00 (10.96).
Preparation of {[(t-Bu)NP(Me)N(t-Bu)]2Cr(μ-Me)}2{Li(THF)}2
(3). A solution of [(t-Bu)NPN(t-Bu)]2Cr (1; 0.398 g, 1.0 mmol) in THF
(20 mL) was treated with MeLi (2.06 mL, 3.3 mmol, 1.6 M in diethyl ether)
at À20 °C. The solution was stirred at 40 °C for 1 day and then cooled to room
temperature. Then the solvent was removed in vacuo and replaced with hexane
(5 mL). The resulting brown solution was centrifuged; storing the resulting
solution at À30 °C for 6 days afforded brown crystals of 3, which were filtered
and washed with cold hexanes and dried in vacuo (0.140 g, 0.21 mmol, 42%).
1HNMR(C6D6, 300 MHz, 300 K): δÀ0.01 (br, 6H, Cr-CH3), 1.45 (br, 36H,
C(CH3)3), 1.58 (br, 8H, THF), 2.1 (br, 6H, P-CH3), 3.59 (br, 8H, THF).
31P{1H} NMR (C6D6, 300 MHz, 300 K): δ 87.05 (s). MS (ESI) m/z (M +
H)+ 669.41. Anal. Calcd (found) for C28H64Cr2Li2N4O2P2: C 50.29 (50.25),
H 9.65 (9.62), N 8.38 (8.39).
Preparation of {[(t-Bu)NP(Ph)2N(t-Bu)]Cr(μ-Et)}2 (5).
A green-blue solution of [(t-Bu)NP(Ph)2N(t-Bu)]CrCl2Li(THF)2
(4; 0.601 g, 1.0 mmol) in THF (5 mL) was treated with slow addition
of EtLi (2.2 mL, 1.1 mmol, 0.5 M in hexanes) at À35 °C, and the
resulting brown solution was stirred for 2 h at room temperature. The
solvent was removed in vacuo and the residue redissolved in diethyl
ether (10 mL). The suspension was centrifuged and the supernatant
concentrated to 4 mL and stored at À35 °C for 3 days. Brown crystals of
5 were filtered and washed with cold hexanes (10 mL) and dried in vacuo
(0.26 g, 0.32 mmol, 64%). μeff = 2.79 μB. Anal. Calcd (found) for
C44H66Cr2N4P2: C, 64.69 (64.64); H, 8.14 (8.13); N, 6.86 (6.83).
Preparation of [(t-Bu)NP(Ph)2N(t-Bu)]2Cr (6). In a 50 mL
round-bottom Schlenk flask, 0.816 g (1.0 mmol) of complex 5 was
dissolved in toluene (20 mL). The solution was refluxed at 110 °C for 3
h, producing some insoluble black material while retaining the original
solution's brown color. The suspension was centrifuged, the supernatant
was dried in vacuo, and the brown powder was then dissolved in diethyl
ether (5 mL) and stored at À35 °C for 3 days. Brown crystals of 6
were filtered, washed with cold hexanes (10 mL), and dried in vacuo
(0.33 g, 0.47 mmol, 47%). μeff = 4.92 μB. Anal. Calcd (found) for
C40H56CrN4P2: C, 67.97 (67.89); H, 7.99 (8.03); N, 7.93 (7.96).
Polymerization and Oligomerization Results. Catalytic runs
were carried out in 200 mL high-pressure B€uchi reactors containing a
heating/cooling jacket. A preweighed amount of catalyst was dissolved
in 100 mL of toluene or MeCy under N2 prior to loading the reaction
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files and tables giving
b
crystallographic data for the complexes reported in this paper
and extensive listings of geometrical parameters. This material
is available free of charge via the Internet at http://pubs.acs.
org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: sgambaro@uottawa.ca (S.G.).
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Organometallics
ARTICLE
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This work was supported by the Natural Science and
Engineering Council of Canada (NSERC).
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