Synthesis, structures, and ethylene oligomerization activity of bis(phosphanylamine)pyridine chromium/aluminate complexes

Article abstract of DOI:10.1021/om4008289

A trivalent chromium complex of a PN(pyridine) ligand system, {[(2,6-Ph₂P-NH)₂C₅H₃N]CrCl ₃}(THF)₂ (1), was prepared and tested as a catalyst for ethylene oligomerization and polymerization, with the purpose of probing the ability of a pyridine ring substituent as a stabilizing factor on catalytically active intermediates. Its nonselective catalytic behavior indicated that ready reduction of the metal center to the divalent state occurred during the activation process. To substantiate this point, we have reacted 1 with a few common aluminate activators and isolated both the divalent complexes {[2,6-Ph₂PNHC₅H₃NAlClEt₂NPPh ₂]Cr(μ-Cl)₂AlEt₂}(toluene) (3) and {[2,6-Ph₂PNHC₅H₃NAlCl-i-Bu₂NPPh ₂]Cr(μ-Cl)₂Al-i-Bu₂}₂ (toluene) (4) and the trivalent complexes {[2,6-Ph₂PNHC₅H ₃NAlClMe₂NPPh₂]CrMe(μ-Cl) ₂AlMe₂}(toluene)_1.5 (2) and {[2,6-Ph ₂PNHC₅H₃ NHNPPh₂]CrEt(μ-Cl) ₂AlEt₂}AlEtCl₃(hexane)_0.5 (5). The reaction of the ligand with the divalent chromium precursor CrCl ₂(THF)₂ in the presence of alkylaluminum afforded {[2,6-Ph₂PNHC₅H₃NCl₂EtAlNPPh ₂]Cr(μ-Cl)₂AlEt₂}₂(toluene) (6) containing aluminate residues, where the metal has preserved the initial divalent state. All of these species showed moderate to high activities toward ethylene oligomerization.

Full text of DOI:10.1021/om4008289

Article
pubs.acs.org/Organometallics
Synthesis, Structures, and Ethylene Oligomerization Activity of
Bis(phosphanylamine)pyridine Chromium/Aluminate Complexes
Ahmed Alzamly, Sandro Gambarotta,* and Ilia Korobkov
Department of Chemistry, University of Ottawa, 10 M. Curie, Ottawa, Ontario K1N 6N5, Canada
S
* Supporting Information
ABSTRACT: A trivalent chromium complex of a PN(pyridine) ligand system, {[(2,6-
Ph2P-NH)2C5H3N]CrCl3}(THF)2 (1), was prepared and tested as a catalyst for
ethylene oligomerization and polymerization, with the purpose of probing the ability of
a pyridine ring substituent as a stabilizing factor on catalytically active intermediates. Its
nonselective catalytic behavior indicated that ready reduction of the metal center to the
divalent state occurred during the activation process. To substantiate this point, we have
reacted 1 with a few common aluminate activators and isolated both the divalent
complexes {[2,6-Ph2PNHC5H3NAlClEt2NPPh2]Cr(μ-Cl)2AlEt2}(toluene) (3) and
{[2,6-Ph2PNHC5H3NAlCl-i-Bu2NPPh2]Cr(μ-Cl)2Al-i-Bu2}2 (toluene) (4) and the
trivalent complexes {[2,6-Ph2PNHC5H3NAlClMe2NPPh2]CrMe(μ-Cl)2AlMe2}-
(toluene)1.5 (2) and {[2,6-Ph2PNHC5H3 NHNPPh2]CrEt(μ-Cl)2AlEt2}-
AlEtCl3(hexane)0.5 (5). The reaction of the ligand with the divalent chromium
precursor CrCl2(THF)2 in the presence of alkylaluminum afforded {[2,6-
Ph2PNHC5H3NCl2EtAlNPPh2]Cr(μ-Cl)2AlEt2}2(toluene) (6) containing aluminate
residues, where the metal has preserved the initial divalent state. All of these species showed moderate to high activities
toward ethylene oligomerization.
explains the frequent occurrence of catalysts producing a
distribution of oligomers enriched in either 1-hexene or 1-
octene and further contaminated by variable amounts of
polymer.5a,b,11
INTRODUCTION
■
A task of strategic importance in the petrochemical industry is
the development of large-scale processes for the production of
linear α-olefins (LAOs) via ethylene oligomerization.1 Linear α-
olefins are important commodity chemicals marketed for a wide
range of applications spanning from plasticizers to surfactants,
detergents, and synthetic lubricants.2,3 Since their extraction
from crude oil cannot satisfy the market demand, there is a
compelling case for developing catalysts for their making from
abundant ethylene gas.
Trivalent chromium complexes, in the presence of
alkylaluminum coactivators, provide simple and active catalytic
systems.4 The reaction mechanism may follow two concep-
tually very different paths: redox versus nonredox. The metal
oxidation state of the catalytically active species was long
debated,5 and only in the recent past have new data contributed
to clarify the complexity of the mechanistic puzzle.5b,c,e,6 Upon
alkylation by the aluminum activator, trivalent chromium
species show a rather general inclination to reduction1,7 toward
the divalent state. In this event, nonselective oligomerization is
observed with formation of Schulz−Flory distributions of
oligomers. When further reduction to the monovalent state
occurs,8 selective oligomerization (>80%) ensues. Which
oxidation state leads to polymerization remains unclear, even
though there is general agreement that it is either the tri- or
divalent state.9 The situation is further complicated by the
chromium redox dynamism:10 i.e., the rapid interconversion
among the three oxidation states during the catalytic cycle
through series of dis- and comproportionations. In turn, this
The ligand clearly plays a pivotal role in the stabilization of a
specific oxidation state and consequently in determining the
type of catalytic behavior (selective versus nonselective). For
this reason, we and others have embarked on a systematic
screening of a series of ligands, aiming at identifying the
features of a selective catalyst.3,12 To this end, it is safe to state
that the N−P function of the phosphamine ligands is
particularly versatile at enhancing the catalytic behavior of
chromium. In fact, by using ligands based on this function, a
rather large family of performing catalysts, selective,13 non-
selective,10b,14 and switchable,10a,15 has been discovered.
In this study, we examined a pincer-type bis-
(phosphanylamine)pyridine for the synthesis and oligomeriza-
tion of chromium catalysts. We were interested in clarifying the
possible role of the pyridine substituent in stabilizing lower
oxidation states and ultimately affecting the selectivity of the
catalytic cycle. Herein we describe our results.
RESULTS AND DISCUSSION
■
The phosphanylpyridine ligand Ph2PNHPyNHPPh2 was
prepared according to a literature procedure.16 Treatment of
CrCl3(THF)3 with the ligand in THF afforded the correspond-
Received: August 16, 2013
© XXXX American Chemical Society
A
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ing complex [(2,6-Ph2P-NH)2C5H3N]CrCl3 (1) (Scheme 1).
The complex has the magnetic moment expected for the d3
electronic configuration of trivalent chromium in an octahedral
field.
Cl(3) = 2.2969(15) Å, Cl(2)−Cr(1)−Cl(3) = 93.45(6)°,
Cl(3)−Cr(1)−Cl(1) = 172.74(6)°).
Upon activation with MAO, complex 1 displays good activity
as a nonselective oligomerization catalyst. However, a
substantial amount of polymeric material was invariably present
in the oligomeric mixtures (Table 1). This tentatively suggested
the presence of dynamism between the di- and trivalent
oxidation states during the catalytic cycle. The fact that no
enrichment in 1-hexene or 1-octene was observed further
possibly suggests that no reduction to the monovalent state
occurred. The lower than expected amount of 1-hexene initially
suggested incorporation into the polymeric or heavy-oligomer
solid material. However, 13C NMR clearly showed only highly
linear chains.
Scheme 1
To get insights into the interaction of complex 1 with the
activator and aiming at isolating catalytically active species, this
complex was reacted with stoichiometric amounts of different
alkylaluminum activators (Scheme 2).
It was possible to grow crystals of sufficient quality for X-ray
analysis. The structure of 1 (Figure 1) shows a distorted-
The reactions with MAO invariably gave oily materials, while
t h e a c t i v e o l i g o m e r i z a t i o n c a t a l y s t { [ 2 , 6 -
Ph2PNHC5H3NAlClMe2NPPh2]CrMe(μ-Cl)2AlMe2}-
(toluene)1.5 (2) was obtained in crystalline form upon treating
1 with 10 equiv of Me3Al. Interestingly, the trivalent oxidation
state was preserved during the formation of this complex.
The salient features of the structure of 2 are the methylation
of the chromium center and the deprotonation of one of the
two amino functions, whose hydrogen atom was in fact
replaced by one ClAlMe2 moiety (Figure 2). Unfortunately, the
crystal quality was strained by the very high air sensitivity; this
was reflected in the borderline values of the convergence
parameters. Nonetheless, the connectivity remains demon-
strated. The trivalent chromium is in a distorted-octahedral
coordination environment (Cr(1)−N(2) = 2.052(6) Å, Cr(1)−
Cl(1) = 2.490(3) Å, Cr(1)−P(1) = 2.465(2) Å, N(2)−Cr(1)−
Cl(2) = 174.55(19)°, N(2)−Cr(1)−P(1) = 82.76(17)°, N(2)−
Cr(1)−Cl(1) = 91.41(18)°). The coordination polyhedron is
defined by the three donor atoms of the meridionally arranged
ligand system (Cr(1)−P(2) = 2.403(2) Å, N(2)−Cr(1)−P(2)
= 79.71(17)°, P(2)−Cr(1)−P(1) = 162.19(9)°). The remain-
ing three meridional coordination sites are occupied by one
carbon atom of a terminally bonded Me group (Cr(1)−C(30)
2.095(10) Å, N(2)−Cr(1)−C(30) = 87.3(3)°) and two
chlorine atoms. In turn, the two chlorines bridge a Me2Al
residue (Cl(1)−Al(1)−Cl(2) = 90.60(12)°, Al(1)−Cl(1)−
Cr(1) = 92.24(11)°, Al(1)−Cl(2)−Cr(1) = 93.41(11)°).
When the more reducing Et3Al or i-Bu3Al was used, reactions
carried out under otherwise identical conditions took an
interesting turn. The two activators generated the two nearly
i s o s t r u c t u r a l d i v a l e n t c o m p l e x e s { [ 2 , 6 -
Ph2PNHC5H3NAlClEt2NPPh2]Cr(μ-Cl)2AlEt2}(toluene) (3)
and {[2,6-Ph2PNHC5H3NAlCl-i-Bu2NPPh2]Cr(μ-Cl)2Al-i-
Bu2}2(toluene) (4) (Figure 3). Apart from the two different
alkyls, the two species are basically identical. The complexes are
also similar to 2 in the sense that they retained the aluminate
Figure 1. Thermal ellipsoid plot of 1 with ellipsoids drawn at the 50%
probability level. Bond lengths (Å) and angles (deg) for 1: Cr(1)−
N(2) = 2.108(4), Cr(1)−Cl(2) = 2.2934(15), Cr(1)−Cl(3) =
2.2969(15), Cr(1)−Cl(1) = 2.3010(15), Cr(1)−P(1) = 2.4190(15),
Cr(1)−P(2) = 2.4503(15), N(1)−P(1) = 1.680(4), N(3)−P(2) =
1.686(4); N(2)−Cr(1)−Cl(2) = 179.15(12), N(2)−Cr(1)−Cl(3) =
87.41(11), Cl(2)−Cr(1)−Cl(3) = 93.45(6), N(2)−Cr(1)−Cl(1) =
86.37(11), Cl(2)−Cr(1)−Cl(1) = 92.78(5), Cl(3)−Cr(1)−Cl(1) =
172.74(6), N(2)−Cr(1)−P(1) = 80.31(12), Cl(2)−Cr(1)−P(1) =
99.75(5), Cl(3)−Cr(1)−P(1) = 85.97(5), Cl(1)−Cr(1)−P(1) =
89.28(5), N(2)−Cr(1)−P(2) = 80.15(12), Cl(2)−Cr(1)−P(2) =
99.82(5), Cl(3)−Cr(1)−P(2) = 90.35(5), Cl(1)−Cr(1)−P(2) =
92.28(5), P(1)−Cr(1)−P(2) = 160.26(5), N(1)−P(1)−Cr(1) =
97.75(14), N(3)−P(2)−Cr(1) = 97.13(15).
octahedral trivalent chromium center surrounded by three
meridionally placed donor atoms of one ligand (Cr(1)−N(2) =
2.108(4) Å, Cr(1)−P(1) = 2.4190(15) Å, N(2)−Cr(1)−P(1)
= 80.31(12)°, P(1)−Cr(1)−P(2) = 160.26(5)°). The three
chlorine atoms occupy the three remaining positions (Cr(1)−
Cl(1) = 2.3010(15) Å, Cr(1)−Cl(2) = 2.2934(15) Å, Cr(1)−
a
Table 1. Ethylene-Catalyzed Oligomerization Reactions of 1
linear α-olefin (mol %)
catalyst
alkene (g)
22
PE (g)
3.2
Mn/DP (g mol−1
)
activity (g/((g of Cr) h))
46029
C4
C6
C8
C10
C12
C14
C16
6.2
C18
2.5
1
355/1.6
4.1
13.2
24.5
21.5
15.7
12.3
a
Conditions: 100 mL of toluene, loading 20 μmol of catalyst, reaction temperature 80 °C, 40 bar of ethylene, reaction time 30 min, 500 (Al/Cr)
MAO.
B
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Scheme 2
reducing power of TEAL and TIBA with respect to that of
TMA and the consequently lower stability of the initially
formed organochromium function.
The reaction of 1 with the more Lewis acidic DEAC (10
equiv) afforded instead alkylation and cationization. In this
case, no reduction and no deprotonation of the amino
functions were observed. As a result, the cationic, trivalent
chromium complex {[2,6-Ph2PNHC5H3NHNPPh2]CrEt(μ-
Cl)2AlEt2}AlEtCl3(hexane)0.5 (5) (Scheme 2) containing an
ethyl function directly connected to chromium was obtained in
crystalline form.
The structure of 5 shows the chromium atom in an
octahedral environment with the intact ligand arranged as in
complex 1 (Cr(1)−N(1) = 2.062(3) Å, Cr(1)−P(1) =
2.4648(13) Å, P(2)−Cr(1)−P(1) = 159.87(5)°, N(1)−
Cr(1)−P(1) = 80.15(10)°). One ethyl group and two chlorine
atoms complete the coordination sphere of the metal center
(Cr(1)−Cl(1) = 2.5069(12) Å, Cr(1)−C(30) = 2.079(4) Å,
Cl(2)−Cr(1)−Cl(1) = 84.51(4)°, C(30)−Cr(1)−Cl(2) =
89.51(13)°, C(30)−Cr(1)−Cl(1) = 173.87(13)°). An alumi-
nate counteranion is present in the lattice, unconnected to the
chromium unit (Figure 4).
To probe the stability of the divalent analogue, a similar
preparation was carried out by reacting an in situ generated
divalent adduct with 5 equiv of DEAC. The reaction afforded
the corresponding divalent {[2,6-Ph2PNHC5H3NCl2-
EtAlNPPh2]Cr(μ-Cl)2AlEt2}2(toluene) (6) (Scheme 3). Differ-
ent from the case of 5, the ligand in 6 was deprotonated, with
the nitrogen atom retaining one additional aluminate residue.
In turn, this indicates that the acidity of the ligand amino
protons is affected by more than simple inductive effects.
Even in this case, the good crystallinity allowed a
crystallographic structural investigation. The molecule features
the metal center in a square-pyramidal coordination environ-
ment with the ligand donor atoms (Cr(1)−N(1) = 2.074(4) Å,
Cr(1)−P(1) = 2.4432(14) Å, N(1)−Cr(1)−P(1) =
80.87(11)°, P(2)−Cr(1)−P(1) = 149.73(6)°) and one
chlorine (Cr(1)−Cl(2) = 2.3740(16) Å, N(1)−Cr(1)−Cl(2)
Figure 2. Thermal ellipsoid plot of 2 with ellipsoids drawn at the 50%
probability level. Bond lengths (Å) and angles (deg) for 2: Cr(1)−
N(2) = 2.052(6), Cr(1)−Cl(2) = 2.390(2), Cr(1)−P(2) = 2.403(2),
Cr(1)−P(1) = 2.465(2), Cr(1)−Cl(1) = 2.490(3), Al(1)−Cl(1) =
2.262(3), Al(1)−Cl(2) = 2.321(4), Al(2)−N(3) = 1.964(6), Al(2)−
Cl(3) = 2.238(3), N(1)−P(1) = 1.669(7), N(3)−P(2) = 1.671(6),
Cr(1)−C(30) = 2.095(10); N(2)−Cr(1)−Cl(2) = 174.55(19),
N(2)−Cr(1)−P(2) = 79.71(17), Cl(2)−Cr(1)−P(2) = 98.24(8),
N(2)−Cr(1)−P(1) = 82.76(17), Cl(2)−Cr(1)−P(1) = 99.50(8),
P(2)−Cr(1)−P(1) = 162.19(9), N(2)−Cr(1)−Cl(1) = 91.41(18),
Cl(2)−Cr(1)−Cl(1) = 83.73(9), P(2)−Cr(1)−Cl(1) = 95.43(8),
P(1)−Cr(1)−Cl(1) = 88.10(8), Cl(1)−Al(1)−Cl(2) = 90.60(12),
N(3)−Al(2)−Cl(3) = 102.2(2), Al(1)−Cl(1)−Cr(1) = 92.24(11),
Al(1)−Cl(2)−Cr(1) = 93.41(11), P(2)−N(3)−Al(2) = 122.2(4),
N(1)−P(1)−Cr(1) = 93.5(2), N(3)−P(2)−Cr(1) = 101.4(2), N(2)−
Cr(1)−C(30) = 87.3(3), N(2)−Cr(1)−Cl(2) = 174.55(19).
residues at the deprotonated N and the two chlorine atoms.
Different from the case for 2, the oxidation state of chromium
was lowered from tri- to divalent. The two alkyaluminum
reagents used for the reactions were tested by NMR for the
presence of hydrides and found to be hydride-free. Therefore,
the reduction may be only ascribed to the more enhanced
C
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Figure 3. Thermal ellipsoid plots of 3 and 4 with ellipsoids drawn at the 50% probability level. Bond lengths (Å) and angles (deg) for 3: Cr(1)−
N(1) = 2.062(2), Cr(1)−Cl(2) = 2.3865(9), Cr(1)−P(1) = 2.3952(8), Cr(1)−P(2) = 2.4290(8), Cr(1)−Cl(1) = 2.5642(9), Al(1)−Cl(1) =
2.2504(12), Al(1)−Cl(2) = 2.3003(13), Al(2)−N(2) = 1.950(2), Al(2)−Cl(3) = 2.2403(11), P(1)−N(2) = 1.686(2), P(2)−N(3) = 1.683(2);
N(1)−Cr(1)−Cl(2) = 174.36(6), N(1)−Cr(1)−P(1) = 77.86(5), Cl(2)−Cr(1)−P(1) = 99.92(3), N(1)−Cr(1)−P(2) = 80.46(5), Cl(2)−Cr(1)−
P(2) = 98.95(3), P(1)−Cr(1)−P(2) = 144.54(3), N(1)−Cr(1)−Cl(1) = 100.43(6), Cl(2)−Cr(1)−Cl(1) = 85.09(3), P(1)−Cr(1)−Cl(1) =
102.66(3), P(2)−Cr(1)−Cl(1) = 108.64(3), Cl(1)−Al(1)−Cl(2) = 94.81(4), N(2)−Al(2)−Cl(3) = 102.84(7), Al(1)−Cl(1)−Cr(1) = 88.29(4),
Al(1)−Cl(2)−Cr(1) = 91.61(3), N(2)−P(1)−Cr(1) = 102.18(7), N(3)−P(2)−Cr(1) = 96.13(7), P(1)−N(2)−Al(2) = 122.49(11). Bond lengths
(Å) and angles (deg) for 4: Cr(1)−N(2) = 2.063(3), Cr(1)−Cl(2) = 2.3874(13), Cr(1)−P(1) = 2.3931(13), Cr(1)−P(2) = 2.4417(13), Cr(1)−
Cl(1) = 2.5560(14), Al(1)−N(1) = 1.957(4), Al(1)−Cl(3) = 2.2413(19), Al(2)−Cl(1) = 2.256(2), Al(2)−Cl(2) = 2.298(2), N(1)−P(1) =
1.686(4), N(3)−P(2) = 1.684(4); N(2)−Cr(1)−Cl(2) = 174.24(10), N(2)−Cr(1)−P(1) = 77.38(10), Cl(2)−Cr(1)−P(1) = 99.44(5), N(2)−
Cr(1)−P(2) = 80.90(10), Cl(2)−Cr(1)−P(2) = 99.63(5), P(1)−Cr(1)−P(2) = 145.31(5), N(2)−Cr(1) Cl(1) = 100.58(10), Cl(2)−Cr(1)−Cl(1)
= 84.92(5), P(1)−Cr(1)−Cl(1) = 107.12(5), P(2)−Cr(1)−Cl(1) = 103.20(5), N(1)−Al(1)−Cl(3) = 103.16(12), Al(2)−Cl(1)−Cr(1) = 88.63(6),
Al(2)−Cl(2)−Cr(1) = 91.92(6), P(1)−N(1)−Al(1) = 122.8(2), N(3)−P(2)−Cr(1) = 95.58(13).
= 171.09(12)°) defining the basal plane and a second chlorine
atom in the axial position (Cr(1)−Cl(1) = 2.5757(17) Å,
N(1)−Cr(1)−Cl(1) = 103.93(11)°) (Figure 5). One of the
two amino functions was deprotonated, and the nitrogen atom
coordinated a Cl2AlEt residue (Al(2)−N(3) = 1.932(4) Å).
The two chlorine atoms also bridge a second Et2Al moiety
(Cl(1)−Al(1)−Cl(2) = 94.32(9)°).
When activation of the complexes was attempted in
methylcyclohexane, no catalytic activity could be detected.
This is most likely the result of the insolubility of all these
complexes in nonaromatic solvents. Therefore, catalytic runs
were necessarily carried out in toluene, and the results are
summarized in Table 2.
As anticipated, upon activation with MAO the divalent 3, 4,
and 6 gave slightly higher activity as nonselective ethylene
oligomerization catalysts. Low-molecular-weight polymeric
material was invariably present in the reaction mixtures. Their
high-temperature NMR spectra and GPC analysis indicated
that the broadly dispersed polymer chains were heavier linear
α-olefins with a distribution centered over the average values of
the molecular weights. In turn, this ruled out that the solid
material could simply be the tail of the lighter oligomer
distribution. Variable amounts of 1-butene were also formed by
the catalytic runs. While the quantification of 1-butene is
affected by larger error, its presence is in principle indicative of
an S−F distribution. However, the low amount of 1-hexene and
the somewhat larger amount of 1-octene (and 1-decene in one
case) are reflective of an abnormal unselective oligomerization.
To explain this complex behavior, we suggest that more than
one mechanism is at play, in turn involving more than one
chromium oxidation state. The lack of selectivity, which
dominates the distribution, indicates that the divalent state
was preserved in these complexes throughout the activation
process without further reduction to the monovalent state.
Conversely, the formation of oligomeric/polymeric material
might be attributed to the re-forming of trivalent species via
comproportionation.7c It is tempting to ascribe this behavior to
an increased resiliency of the trivalent state toward reduction to
the divalent state which makes the overall catalytic cycle less
productive and more inclined toward polymer formation. The
small enrichment in 1-octene could be attributed to
monovalent species formed in low concentration, but we
remain baffled by the lower than expected amount of 1-hexene.
The linearity of the polymeric material discourages the idea that
1-hexene might have been perhaps incorporated.
CONCLUSIONS
■
In conclusion, we have herein reported the synthesis and
characterization of organochromium complexes based on a P−
N(pyridine) ligand system. The introduction of the pyridine
ring in the PN ligand scaffold afforded nonselective catalytic
systems. The almost complete obliteration of selectivity,
otherwise commonly observed within the large family of N−
P ligands, at first glance may look surprising. On the other
hand, the stabilizing ability of the pyridine ring affects not only
the chromium divalent state but also the Cr−R function. This is
clearly visible in 2 and especially in 5, containing a relatively
rare example7e of a CrIII−Et group. Even in the case when
D
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PerkinElmer 2400 CHN analyzer. Magnetic susceptibility measure-
ments were performed with a Johnson Matthey balance at room
temperature. The samples were powdered and weighed inside a drybox
and transferred to sealed and calibrated tubes for measurements. NMR
spectra were recorded on a Varian Mercury 400 MHz spectrometer at
300 K. Infrared spectra were recorded on an ABB Bomem FTIR
instrument from Nujol mulls prepared in a VAC drybox. All chemical
reagents were purchased from commercial sources and used as
received. Diethylaluminum chloride, trimethylaluminum, triethylalu-
minum, and triisobutylaluminum were purchased from Strem and used
as received. Methylaluminoxane (MAO, 20% in toluene) was
purchased from Albemarle Corp. TMA-depleted methylaluminoxane
(DMAO) was prepared by removing all the volatiles from MAO in
vacuo (2 mmHg) with moderate heating (40 °C) for 6 h.
Preparation of {[(2,6-Ph2P-NH)2C5H3N]CrCl3}(THF)2 (1). A
solution of CrCl3(THF)3 (0.375g, 1.0 mmol) in THF (15 mL) was
added to a solution of Ph2PNHPyNHPPh2 (0.477g, 1 mmol) in THF
(5 mL). The solution immediately turned green. The solution was
refluxed for 10 min and stirred at room temperature for 2 h. The
volume of the solution was then reduced to 7 mL and the residue
allowed to stand at −40 °C for 3 days. Green crystals of 1 were thus
obtained (0.61 g, 0.96 mmol, 96%). μeff = 3.83 μB. Anal. Calcd (found)
for C37H41Cl3CrN3O2P2: C, 54.78 (54.63); H, 3.96 (3.91); N, 6.61
(6.55). IR (Nujol): νN−H 3205 cm−1.
Preparation of {[2,6-Ph2PNHC5H3NAlClMe2NPPh2]CrMe(μ-
Cl)2AlMe2}(toluene)1.5 (2). A solution of Ph2PNHPyNHPPh2 (0.48
g, 1 mmol) in toluene (10 mL) was treated with CrCl3(THF)3 (0.37 g,
1.0 mmol). After 12 h of stirring at room temperature, the mixture was
cooled to −40 °C and trimethylaluminum (0.72 g, 10.0 mmol) was
added dropwise. Stirring was continued for 30 min while the mixture
was warmed to room temperature. The suspension was then
centrifuged and the supernatant concentrated, layered with hexanes
(3 mL), and stored in the freezer at −40 °C for 3 days. The resulting
brown crystals of 3 were filtered, washed with cold hexanes (10 mL),
and dried in vacuo (0.23 g, 0.31 mmol, 31%). μeff = 3.88 μB. Anal.
Calcd (found) for C78.5H90Al4Cl6Cr2N6P4: C, 53.45 (53.12); H, 5.15
(4.95); N, 5.50 (5.49). IR (Nujol): νN−H 3212 cm−1.
Preparation of {[2,6-Ph2PNHC5H3NAlClEt2NPPh2]Cr(μ-
Cl)2AlEt2}(toluene) (3). A solution of Ph2PNHPyNHPPh2 (0.48 g,
1.0 mmol) in toluene (10 mL) was treated with CrCl3(THF)3 (0.37 g,
1.0 mmol). After 12 h of stirring at room temperature, the mixture was
cooled to −40 °C and triethylaluminum (0.57 g, 5.0 mmol) was added
dropwise. Stirring was continued for 30 min while the mixture was
warmed to room temperature. The suspension was then centrifuged
and the supernatant concentrated, layered with hexanes (3 mL), and
stored at −40 °C for 3 days. The resulting bluish green crystals of 4
were filtered, washed with cold hexanes (10 mL), and dried in vacuo
(0.52 g, 0.65 mmol, 65%). μeff = 4.81 μB. Anal. Calcd (found) for for
C44H52Al2Cl3CrN3P2: C, 55.20 (54.8); H, 5.51 (5.48); N, 5.22 (5.16).
IR (Nujol): νN−H 3244 cm−1.
Figure 4. Thermal ellipsoid plot of 5 with ellipsoids drawn at the 50%
probability level. Bond lengths (Å) and angles (deg) for 5: Cr(1)−
N(1) = 2.062(3), Cr(1)−Cl(2) = 2.3992(13), Cr(1)−P(2) =
2.4569(13), Cr(1)−P(1) = 2.4648(13), Cr(1)−Cl(1) = 2.5069(12),
Al(1)−Cl(1) = 2.2650(18), Al(1)−Cl(2) = 2.3088(18), Al(2)−Cl(5)
= 2.1485(18), Al(2)−Cl(4) = 2.1695(18), Al(2)−Cl(3) = 2.178(2),
N(2)−P(1) = 1.690(3), Cr(1)−C(30) = 2.079(4); N(1)−Cr(1)−
Cl(2) = 173.28(10), Cl(2)−Cr(1)−P(2) = 96.57(5), N(1)−Cr(1)−
P(1) = 80.15(10), Cl(2)−Cr(1)−P(1) = 103.55(5), P(2)−Cr(1)−
P(1) = 159.87(5), N(1)−Cr(1)−Cl(1) = 89.80(9), Cl(2)−Cr(1)−
Cl(1) = 84.51(4), P(2)−Cr(1)−Cl(1) = 89.54(4), P(1)−Cr(1)−
Cl(1) = 91.73(4), Cl(5)−Al(2)−Cl(3) = 105.54(7), Cl(4)−Al(2)−Cl
= (3)105.29(8), Al(1)−Cl(1)−Cr(1) = 90.28(5), Al(1)−Cl(2)−
Cr(1) = 91.98(5), N(2)−P(1)−Cr(1) = 94.94(13), N(3)−P(2)−
Cr(1) = 95.24(13), N(1)−Cr(1)−P(2) = 79.76(10).
Scheme 3
Preparation of {[2,6-Ph2PNHC5H3NAlCl-i-Bu2NPPh2]Cr(μ-
Cl)2Al-i-Bu2}2(toluene) (4). A solution of Ph2PNHPyNHPPh2
(0.48 g, 1.0 mmol) in toluene (10 mL) was treated with CrCl3(THF)3
(0.37 g, 1.0 mmol). After 12 h of stirring at room temperature, the
mixture was cooled to −40 °C, and triisobutylaluminum (0.99 g, 5.0
mmol) was added dropwise. Stirring was continued for 30 min while
the mixture was warmed to room temperature. The suspension was
then centrifuged and the supernatant concentrated, layered with
hexanes (3 mL), and stored in a freezer at −40 °C for 3 days. The
resulting blue crystals of 5 were filtered, washed with cold hexanes (10
mL), and dried in vacuo (0.25g, 0.27 mmol, 27%). μeff = 4.92 μB. Anal.
Calcd (found) for for C97H128Al4Cl6Cr2N6P4: C, 58.92 (57.88); H,
6.59 (6.52); N, 4.58 (4.54). IR (Nujol): νN−H 3239 cm−1.
Preparation of {[2,6-Ph2PNHC5H3NHNPPh2]CrEt(μ-Cl)2AlEt2}-
AlEtCl3(hexane)0.5 (5). A solution of Ph2PNHPyNHPPh2 (0.48 g, 1.0
mmol) in toluene (10 mL) was treated with CrCl3(THF)3 (0.37 g, 1.0
mmol). After 12 h of stirring at room temperature, the mixture was
cooled to −40 °C and diethylaluminum chloride (1.21 g, 10.0 mmol)
was added dropwise. Stirring was continued for 30 min while the
mixture was warmed to room temperature. The suspension was then
reduction to the divalent state does occur, the pyridine ring
most likely prevents further reduction. Only in the cases where
some enrichment in 1-octene is observed, most likely a small
concentration of the chromium intermediates have made their
way to the monovalent state. The simultaneous presence of all
these species may well account for the complexity of the
observed distributions.
EXPERIMENTAL SECTION
■
All reactions were carried out under an inert atmosphere using Schlenk
techniques or in a purified nitrogen-filled drybox. Solvents were dried
using a purification system composed of aluminum oxide. GC-MS
analysis of the oligomers was carried out with a Hewlett-Packard HP
5973 gas chromatograph using an Agilent DB1 column and dual FID
and MS detector. Elemental analysis were carried out with a
E
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Figure 5. Thermal ellipsoid plot of 6 with ellipsoids drawn at the 50% probability level. Bond lengths (Å) and angles (deg) for 6: Cr(1)−N(1) =
2.074(4), Cr(1)−Cl(2) = 2.3740(16), Cr(1)−P(2) = 2.4014(15), Cr(1)−P(1) = 2.4432(14), Cr(1)−Cl(1) = 2.5757(17), Cr(2)−N(4) = 2.068(4),
Al(1)−Cl(1) = 2.233(3), Al(1)−Cl(2) = 2.323(3), Al(2)−Cl(4) = 2.150(2), Al(2)−Cl(3) = 2.208(2), P(1)−N(2) = 1.694(4), P(1)−C(12) =
1.805(5), P(2)−N(3) = 1.684(4); N(1)−Cr(1)−Cl(2) = 171.09(12), N(1)−Cr(1)−P(2) = 78.43(11), Cl(2)−Cr(1)−P(2) = 97.35(6), N(1)−
Cr(1)−P(1) = 80.87(11), Cl(2)−Cr(1)−P(1) = 99.85(6), P(2)−Cr(1)−P(1) = 149.73(6), N(1)−Cr(1)−Cl(1) = 103.93(11), Cl(2)−Cr(1)−
Cl(1) = 84.82(6), P(2)−Cr(1)−Cl(1) = 108.69(6), P(1)−Cr(1)−Cl(1) = 97.56(6), Cl(1)−Al(1)−Cl(2) = 94.32(9), N(3)−Al(2)−C(34) =
116.9(2), N(3)−Al(2)−Cl(4) = 106.72(15), N(3)−Al(2)−Cl(3) = 105.55(14), Cl(4)−Al(2)−Cl(3) = 108.34(10), Al(1)−Cl(1)−Cr(1) =
88.54(8), Al(1)−Cl(2)−Cr(1) = 91.52(8), N(2)−P(1)−Cr(1) = 94.76(14), N(3)−P(2)−Cr(1) = 102.43(14), P(2)−N(3)−Al(2) = 119.1(2).
a
Table 2. Ethylene-Catalyzed Oligomerization Reactions of 2−6
linear α-olefin (mol %)
catalyst alkene (g) PE (g) Mn, PD (g mol−1
)
activity (g/((g of Cr) h))
C4
C6
C8
C10
C12
C14
C16
C18
2
3
4
5
6
16
48
35
18
40
2.6
2.5
1.1
2
394, 1.9
316, 2.0
407, 2.3
406, 2.2
381, 1.9
33875
92808
68115
36846
76144
15.1
9.3
5.2
8
23.4
19.9
18.1
12.9
16.8
21.1
19.7
20
14.3
15.9
17.1
15.8
17.7
10.3
12.7
13.9
14.3
14.4
7.2
9.7
5.1
7.3
3.5
5.5
6.5
9.3
6.6
10.9
12.5
11.2
8.3
16.6
20.6
10.6
8.5
1.7
4.2
a
Conditions: 100 mL of toluene, loading 20 μmol of catalyst, reaction temperature 80 °C, 40 bar of ethylene, reaction time 30 min, 500 (Al/Cr)
MAO.
centrifuged and the supernatant concentrated, layered with hexanes (3
mL), and stored in a freezer at −40 °C for 3 days. The resulting brown
crystals of 6 were filtered, washed with cold hexanes (10 mL), and
dried in vacuo (0.53 g, 0.60 mmol, 60%). μeff = 3.85 μB. Anal. Calcd
(found) for C38.5H48.5Al2Cl5CrN3P2: C, 50.68 (49.16); H, 5.17 (4.96);
N, 4.79 (4.65). IR (Nujol): νN−H 3242 cm−1
Preparation of {[2,6-Ph2PNHC5H3NCl2EtAlNPPh2]Cr(μ-
Cl)2AlEt2}2(toluene) (6). A solution of Ph2PNHPyNHPPh2 (0.48 g,
1.0 mmol) in toluene (10 mL) was treated with CrCl2(THF)2 (0.27 g,
1.0 mmol). After 12 h of stirring at room temperature, the mixture was
cooled to −40 °C and diethylaluminum chloride (0.57 g, 5.0 mmol)
was added dropwise. Stirring was continued for 30 min while the
mixture was warmed to room temperature. The suspension was then
centrifuged and the supernatant concentrated, layered with hexanes (3
mL), and stored in a freezer at −40 °C for 3 days. The resulting brown
crystals of 7 were filtered, washed with cold hexanes (10 mL), and
dried in vacuo (0.53 g, 0.65 mmol, 65%). μeff = 4.88 μB. Anal. Calcd
(found) for C77H86Al4Cl8Cr2N6P4: C, 51.68 (51.74); H, 5.08 (4.92);
N, 5.17 (5.09). IR (Nujol): νN−H 3275 cm−1.
maintaining the pressure throughout the run. Oligomerizations were
quenched by cooling to 0 °C, releasing the pressure, and adding
MeOH/HCl (80% MeOH solution). The total yield of oligomers was
1
determined by H NMR by integrating the intensity of the olefinic
multiplets against the solvent resonance. The composition of the
mixture of oligomers was determined by GC/MS with single oligomer
quantification by GC using an FID detector. The insoluble polymeric
solid was isolated by filtration and dried prior to measuring its mass.
X-ray Crystallography. Data collection results for compounds 1−
6 represent the best data sets obtained in several trials for each sample.
The crystals were mounted on thin glass fibers using paraffin oil. Prior
to data collection, crystals were cooled to 200.15 K. Data were
collected on a Bruker AXS KAPPA single-crystal diffractometer
equipped with a sealed Mo tube source (wavelength 0.71073 Å) APEX
II CCD detector. Raw data collection and processing were performed
with the APEX II software package from BRUKER AXS.17 Diffraction
data for samples of 3 and 4 were collected with a sequence of 0.5° ω
scans at 0, 120, and 240° in φ. Because of the lower unit cell symmetry
in order to ensure adequate data redundancy, diffraction data for 1, 2,
5, and 6 were collected with a sequence of 0.5° ω scans at 0, 90, 180,
and 270° in φ. Initial unit cell parameters were determined from 60
data frames with 0.3° ω scan each, collected at the different sections of
the Ewald sphere. Semiempirical absorption corrections based on
equivalent reflections were applied.18 Systematic absences in the
Oligomerization Results. Oligomerizations were carried out in a
200 mL high-pressure Buchi reactor containing a heating/cooling
̈
jacket. A preweighted amount of catalyst was dissolved in toluene (100
mL) under nitrogen prior to loading into the reaction vessel. Solutions
were heated using a thermostatic bath and charged with ethylene,
F
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diffraction data set and unit-cell parameters were consistent with
motion parameters and avoid further splitting of atomic positions, sets
of thermal parameter constraints (SIMU, RIGU for for the fragment
C(42)−C(45), EADP for all other fragments) were introduced into
the refinement routine. For the same reasons all carbon atoms of the
toluene solvent molecule were refined with isotropic thermal motion
approximations.
triclinic P1 (No. 2) for compounds 1, 2, 5, and 6, monoclinic P2 /n
̅
1
(No. 14, alternative settings) for compound 3, and monoclinic P21/c
(No. 14) for 4. Solutions in the centrosymmetric space groups for all
compounds yielded chemically reasonable and computationally stable
results of refinement. The structures were solved by direct methods,
completed with difference Fourier synthesis, and refined with full-
matrix least-squares procedures based on F2.
The structure of compound 5 contains a cationic metal moiety and
an AlCl3(C2H5) fragment located in general positions. One molecule
of hexane crystallization solvent is located at the inversion center. The
occupancy of the carbon atoms of this hexane molecule was initially
refined to obtain acceptable thermal parameter values. In the final
stages of refinement, the occupancy value was preset to 50%, resulting
in reasonable thermal motion parameter values and stable computa-
tional results. A set of geometry restraints (DFIX, DANG) was used to
maintain an acceptable molecular geometry during the refinement.
The asymmetric unit of 6 reveals two molecules of the target
complex and two partially occupied molecules of toluene, all located in
general positions. In the final refinement stages unusually large thermal
parameters for several carbon atoms in the model of 6 indicated the
presence of disorder. Positional disorder not related to symmetry
elements was introduced for the Et moieties based at C(65), C(67),
and C(69). Occupational factors for all these fragment were refined to
50:50, demonstrating reasonable isotropic thermal motion parameter
values and stable refinement results. Several sets of geometry restraints
(DFIX) were introduced to ensure an acceptable molecular geometry
for these fragments. To obtain acceptable values of anisotropic thermal
motion parameters, conserve an adequate data to parameter ratio, and
avoid further splitting of atomic positions, sets of thermal parameter
constraints (EADP) were introduced into the refinement routine for
all these disordered moieties. For the same reasons all carbon atoms of
the toluene solvent molecule were refined as rigid body models (AFIX
66) with isotropic thermal motion approximations. A close
examination of the strongest residual electron density peaks revealed
the presence of several peak with values of 1.17−0.68 electrons in
close proximity to the toluene molecules. Several other electron
density peaks were discovered in the VOID space of the unit cell. The
values of the latter peaks did not exceed 1.12 electrons (one-fifth of a
carbon atom). All of the attempts to model these electron density
peaks into molecules of acceptable geometry were unsuccessful
without an excessive amount of geometrical and rigid body restraints.
For all of the compounds all hydrogen atom positions were
calculated on the basis of the geometry of related non-hydrogen atoms.
All hydrogen atoms were treated as idealized contributions during the
refinement. All scattering factors are contained in several versions of
the SHELXTL program library, with the latest version used being
v.6.12.19
Diffraction data for 1 were collected to 0.75 Å resolution; however,
due to the small crystal size and weak diffraction it was discovered that
both R(int) and R(σ) exceeded 35% for the data below 1.10 Å
resolution. On the basis of the R(σ) value, data were truncated to 0.95
Å resolution for refinement. Refinement of the structural model for 1
revealed that all of the molecules of the compound are located in
general positions. In addition to the target complex molecule the
asymmetric unit also contains two molecules of THF crystallization
solvent with full occupancy.
Similarly to the data for 1, diffraction data for the crystal of complex
2 were collected to 0.75 Å resolution. The small crystal size and weak
diffraction contributed to the growth of both R(int) and R(σ) to over
35% for the data below 1.05 Å resolution. On the basis of the R(σ)
value, data were truncated to 0.95 Å resolution for refinement. The
structural model of 2 displays two independent molecules per one
asymmetric unit, where both molecules are located in general
positions. In addition to two molecules of the target compound,
three partially occupied molecules of toluene solvent were discovered
in the lattice. Two of the molecules appear to be positionally
disordered, where disorder is not related to the unit cell symmetry
elements. Originally atomic position occupancies were refined to
obtain reasonable thermal parameter values. However, the occupancy
was fixed at 50% for all three molecular positions of the solvent units.
All carbon atoms for toluene solvent molecules were refined in
isotropic approximations to avoid introducing an excessive amount of
constraints/restraints as well as to preserve an acceptable data to
parameter ratio. In addition to that, sets of thermal parameter
constraints (EADP), geometry (FLAT, DFIX) and rigid body
restraints (AFIX 66), and geometry constraints (SADI) were used
to maintain reasonable molecular geometry and thermal motion values
for solvent molecules. A close examination of the locations of the
strongest residual electron density peaks revealed the presence of
several peaks with values of 1.23−0.72 electrons in close proximity to
the nondisordered toluene molecule. Several other electron density
peaks were discovered in the VOID space of the unit cell. The values
of the latter peaks did not exceed 1.14 electrons (one-fifth of a carbon
atom). All of the attempts to model these electron density peaks into
molecules of acceptable geometry were unsuccessful without an
excessive amount of geometrical and rigid body restraints.
ASSOCIATED CONTENT
* Supporting Information
All structural fragments are situated in general positions in the
asymmetric unit of structural model for compound 3. Together with
the target compound molecules, two partially occupied molecules of
toluene crystallization solvent were discovered in the crystalline lattice.
Occupancy factors of the carbon atoms were initially refined to obtain
acceptable thermal motion parameter values; however, occupancy was
fixed at 50% for each molecular position at the final stages of the
refinement. Sets of constraints (SIMU, DELU) and rigid body
restraints (AFIX 66) were used to maintain reasonable molecular
geometry and thermal motion values for solvent molecules.
■
S
Tables and CIF files giving crystallographic data for 1−6. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.G.: sgambaro@uottawa.ca.
■
The structural model for the compound 4 includes two molecules of
the target complex per asymmetric unit as well as one fully occupied
molecule of toluene, all located in general positions. In the final
refinement stages for the model of 4 it was noticed that the thermal
parameters for several structural fragments suggested the presence of
disorder. Positional disorder not related to symmetry elements was
introduced for the i-Pr moieties based at C(30), C(42), C(75), and
C(83). Occupation factors for these fragment were successfully refined
to the following values: 50:50 for the fragments C(30)−C(33) and
C(75)−C(78); 60:40 for the fragment C(83)−C(86), and finally
65:35 for the fragment C(42)−C(45). Several sets of geometry
constraints (SADI) were introduced to ensure acceptable molecular
geometry for these fragments. To obtain acceptable values of thermal
Notes
The authors declare no competing financial interest.
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