Formation of [Cr(CO)x(Ph2PN(i Pr)PPh 2)]+ structural isomers by reaction of triethylaluminum with a chromium N,N -Bis(diarylphosphino)amine complex [Cr(CO) 4(Ph2PN(i Pr)PPh2)]+: An EPR and DFT investigation

Article abstract of DOI:10.1021/om400029y

The interaction of triethylaluminum (TEA) with a solution of the paramagnetic Cr(I) bis(phosphine) complex [Cr(CO)₄1][Al(OC(CF ₃)₃)₄] (1 = Ph₂PN(iPr)PPh ₂) has been studied using EPR and DFT. It was found that the TEA is responsible for the complete removal of all CO groups from the [Cr(CO) ₄1]⁺ complex, producing the [Cr(1-bis-η⁶- arene)]⁺, and this reaction occurs via a dominant pathway involving a series of [Cr(CO)_x1]⁺ (x < 4) intermediates, consistently including the cis-[Cr(CO)₃1]⁺ complex (species A) and the piano-stool -type [Cr(CO)₂1] ⁺ complex (species C). A further [Cr(CO)₂1]⁺ intermediate complex (labeled species D, which is a structural isomer of species C) was also identified experimentally, suggesting a second pathway for TEA activation may also be operative. All of these paramagnetic complexes have been characterized by CW EPR, and the spin Hamiltonian parameters were verified using DFT. The distribution and type of [Cr(CO)_x1]⁺ intermediates formed were found to be very sensitive to the experimental conditions, including the quantity and manner of TEA addition, the temperature of activation, and the aging time of the solution.

Full text of DOI:10.1021/om400029y

Article
pubs.acs.org/Organometallics
Formation of [Cr(CO)x(Ph2PN(iPr)PPh2)]+ Structural Isomers by
Reaction of Triethylaluminum with a Chromium N,N-
Bis(diarylphosphino)amine Complex [Cr(CO)4(Ph2PN(iPr)PPh2)]+: An
EPR and DFT Investigation
Emma Carter,*,† Kingsley J. Cavell,*,† William F. Gabrielli,‡ Martin J. Hanton,‡ Andrew J. Hallett,
Lucia McDyre,† James A. Platts,† David M. Smith,‡ and Damien M. Murphy*,†
†School of Chemistry, Cardiff University, Cardiff, CF10 3AT, U.K.
‡Sasol Technology (U.K.) Ltd, Purdie Building, North Haugh, St Andrews, U.K.
ABSTRACT: The interaction of triethylaluminum (TEA) with a solution of the paramagnetic Cr(I) bis(phosphine) complex
[Cr(CO)41][Al(OC(CF3)3)4] (1 = Ph2PN(iPr)PPh2) has been studied using EPR and DFT. It was found that the TEA is
responsible for the complete removal of all CO groups from the [Cr(CO)41]+ complex, producing the [Cr(1-bis-η6-arene)]+, and
this reaction occurs via a dominant pathway involving a series of [Cr(CO)x1]+ (x < 4) intermediates, consistently including the
cis-[Cr(CO)31]+ complex (species A) and the “piano-stool”-type [Cr(CO)21]+ complex (species C). A further [Cr(CO)21]+
intermediate complex (labeled species D, which is a structural isomer of species C) was also identified experimentally, suggesting
a second pathway for TEA activation may also be operative. All of these paramagnetic complexes have been characterized by CW
EPR, and the spin Hamiltonian parameters were verified using DFT. The distribution and type of [Cr(CO)x1]+ intermediates
formed were found to be very sensitive to the experimental conditions, including the quantity and manner of TEA addition, the
temperature of activation, and the aging time of the solution.
partly understood and empirically derived; a complete picture
based upon the fundamental understanding is certainly more
desirable, but as yet remains elusive. Nevertheless differences in
both activity and selectivity of the catalytic reaction can still be
tuned depending on which ligand is used.2a,4c
INTRODUCTION
■
The selective production of linear α-olefins, particularly 1-
hexene and 1-octene, is a highly desirable reaction that has
attracted significant academic and industrial interest over the
past 20 years.1 Among the many catalysts available for olefin
oligomerization, chromium complexes have emerged as
promising systems for both selective trimerization and
tetramerization.1 Ligands based on bis(phosphino)amine and
bis(sulfanyl)amine2−4 have been investigated in detail, but
despite the enormous potential of these catalysts, the precise
nature of the active species remains elusive.1
These catalysts are usually generated in situ by addition of a
cocatalyst, such as an aluminum alkyl or an alkylaluminoxane.1
The choice and indeed quantity (molar ratio of cocatalyst to
chromium) of the cocatalyst significantly affects the outcome of
the reaction,2d,e,5 and it is reported to play various roles
including an alkylating agent,6 a reducing agent for
chromium,2e,3,7 aiding the formation of a cationic com-
plex,2e,6b,8 and scavenging impurities.8 Furthermore, although
several papers have appeared on the correlation between ligand
structure and reaction selectivity,1,9 the relationship is only
In parallel with the work reported on catalyst development
and ligand design, the mechanistic aspects of the trimerization
and tetramerization reaction have also been investigated in
some detail both experimentally and theoretically.1 Evidence
has been presented that supports a Cr(II/IV) redox couple,1,10
but Cr(I/III) centers have also been implicated in the
catalysis,1b,11 which operates via a metallacycle intermediate.
While Cr(III) catalysts have certainly received considerable
attention from the mechanistic perspective, fewer studies have
explored the corresponding Cr(I) complexes.12 We therefore
recently examined the structure of a family of low-valent
paramagnetic Cr(I) complexes of general formula [Cr-
(CO)4L]+ (where L = Ph2PN(R)PPh2, Ph2P(R)PPh2) using
Received: January 14, 2013
© XXXX American Chemical Society
A
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electron paramagnetic resonance (EPR) and electron nuclear
double resonance (ENDOR) spectroscopy.13 Despite the
differences in the structure of the diphosphine ligand in these
complexes, no correlation was noted between the observed spin
Hamiltonian parameters and the catalytic ethylene tetrameriza-
tion data, indicating that the structure of the active catalyst was
significantly different compared to the [Cr(CO)4L]+ precata-
lysts.13 We then monitored the structural changes to two
specific [Cr(CO)4L]+ complexes (L = Ph2PN(iPr)PPh2 and
Ph2P(C3H6)PPh2) following the addition of 10 molar equiv of
triethylaluminum (Et3Al, hereafter labeled TEA) at room
temperature and identified the formation of a Cr(I) bis-arene
complex, labeled [Cr(L-bis-η6-arene)]+.14 This complex was
shown to arise from an intramolecular rearrangement and
coordination of Cr(I) to the ligand phenyl groups and indicated
that the TEA was responsible for complete loss of the CO
ligands (Scheme 1).14
RESULTS AND DISCUSSION
■
EPR Spectroscopy. The low-temperature continuous wave
(CW) EPR spectrum of the [Cr(CO)41]+ complex in frozen
solution is shown in Figure 1a. The main features of this
Scheme 1. Structure of the [Cr(CO)4Ph2PN(iPr)PPh2]+
Complex Investigated Here (Labeled [Cr(CO)41]+) and
a
Associated Counterion
Figure 1. X-band CW EPR spectra of (a) [Cr(CO)41]+ (1 =
Ph2PN(iPr)PPh2) dissolved in dichloromethane (recorded at 140 K)
and (b) [Cr(1-bis-η6-arene)]+ formed after addition of 10 molar equiv
of TEA to [Cr(CO)41]+ (recorded at 185 K). An expanded view of the
[Cr(1-bis-η6-arene)]+ signal is shown in the inset. The corresponding
simulations are shown in a′ and b′.
spectrum were recently discussed in detail by us, as part of a
study on a series of related [Cr(CO)4L]+ complexes (L =
Ph2PN(R)PPh2, Ph2P(R)PPh2).13 In brief, this EPR spectrum
is characterized by an axial g tensor (g⊥ > ge > g||) with a large
hyperfine coupling originating from two equivalent 31P nuclei
(Table 1). The spin Hamiltonian parameters were shown to be
consistent with a low-spin d5 Cr(I) center possessing a SOMO
of largely dxy character.13,15
Following the 298 K addition of 10 molar equiv of TEA:Cr
into a dichloromethane solution of [Cr(CO)41]+, the EPR
spectrum changes completely and a narrow symmetrical signal
with well-resolved 1H hyperfine couplings can now be observed
at 185 K (Figure 1b). For clarity only the fluid solution
a
The transformation of [Cr(CO)41]+ into a bis-arene species [Cr(1-
bis-η6-arene)]+ by TEA addition is also shown.
1
spectrum is shown in Figure 1b, to highlight the distinctive H
couplings, since the corresponding low-temperature spectrum
(recorded at 140 K) does not display any resolved hyperfine
structure.14 An analogous spectrum to that shown in Figure 1b
was recently reported by us14 and shown to arise from a Cr(I)
bis-arene complex, labeled [Cr(2-bis-η6-arene)]+ (where 2 =
Ph2P(C3H6)PPh2), but identical EPR spectra are observed with
ligand 1.14 Figure 1b was obtained in dichloromethane, since
solvent-based [Cr(bis-arene)]+ complexes are preferentially
formed in the presence of aromatic solvents, such as toluene,
upon addition of TEA.14
Following on from these initial experiments we sought to
explore further the possibility that other partially decarbony-
lated [Cr(CO)xL]+ (x < 4) intermediates may also be involved
in the above transformations and can therefore be identified by
EPR and DFT following activation of the [Cr(CO)4L]+
complexes using varying levels of TEA at specific temperatures.
Herein, we describe the structure of the paramagnetic Cr(I)
intermediates formed after activating a solution of the
[Cr(CO)4(Ph2PN(iPr)PPh2)]+ complex ([Cr(CO)41]+; see
Scheme 1) using varying ratios of TEA (from 0.5 to 10
equiv) at different temperatures. EPR spectroscopy, in
conjunction with DFT, reveals how different intermediate
[Cr(CO)31]+ and [Cr(CO)21]+ complexes can be formed in
the solution. This work highlights the structural variety of Cr(I)
complexes that may exist in these oligomerization catalysts
[Cr(CO)4L]+, even when very low levels of cocatalyst (TEA)
are present.
The well-resolved multiplet pattern in Figure 1b originates
1
from the hyperfine interaction with 10 equivalent H’s of the
η6-coordinated phenyl rings in 1, as expected in this particular
Cr(I) bis-arene species (Scheme 1).14 Under the adopted
experimental conditions (i.e., addition of 10 equiv of TEA at
298 K), ca. 80% of the original EPR signal is lost following
activation of [Cr(CO)41]+, as the signal intensity decreases
substantially from Figure 1a to 1b.16 The entire remaining
spectral intensity in Figure 1b arises from the [Cr(1-bis-η6-
arene)]+ species. No other paramagnetic Cr(I) or Cr(III)
centers were detected under these specific activation con-
B
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Table 1. Spin Hamiltonian Parameters for the Series of Cr(I) Diphenylphosphino-Based Complexes [Cr(CO)41]+,
a
[Cr(CO)31]+, [Cr(CO)21]+, and [Cr(1-bis-η6-arene)]+
b
g values
PA values
species
[Cr(CO)41]+
g1
g2
g3
giso
A1
A2
A3
aiso
refs
expt
DFT
expt
DFT
expt
DFT
2.072
2.072
1.988
2.044
78.30
−99.753
72.0
78.30
−82.695
72.0
71.00
−82.149
68.1
75.90
−88.199
70.7
13, this work
2.0470
2.063
2.0369
2.063
1.9944
1.987
2.0261
2.037
f
[Cr(CO)42]+
13
2.0436
2.032
2.0339
2.017
1.9910
1.988
2.0228
2.012
−80.400
55.44
−71.528
40.60
−69.329
41.86
−73.752
45.96
c
c
c
(species A) cis-[Cr(CO)31]+
this work
2.0541
2.0299
1.9967
2.0269
−82.04
−87.39
−119.09
−81.0
−70.49
−76.21
−95.83
−71.55
−69.73
−75.02
−94.88
−69.4
−74.09
−79.54
−103.3
−74.28
59.18
(species B) trans-[Cr(CO)31]+
(species C) [Cr(CO)21]+
DFT
2.0328
2.0176
1.962
2.0043
this work
this work
d
d
d
expt
2.043
2.010
1.994
2.015
54.88
66.40
56.28
DFT
2.0537
2.0225
1.9948
2.0237
−112.896
−7.809
86.80
−95.380
−6.855
86.80
−91.538
−4.342
98.0
−99.938
−6.335
90.50
(species D) [Cr(CO)21]+
[Cr(1-bis-η6-arene)]+
expt
2.014
2.0247
1.979
1.9771
2.007
2.010
2.104
2.134
2.014
2.0120
1.979
1.9771
1.995
1.997
2.013
2.035
1.970
1.9209
2.003
2.0027
1.979
1.975
1.994
1.997
1.999
1.9859
1.987
1.9856
1.993
1.994
2.037
2.055
this work
14
DFT
−106.39
−107.925
−129.908
−114.66
e
expt
e
DFT
[Cr(3)]+
[Cr(4)]+
[Cr(5)]+
[Cr(6)]+
20a
20b
95.20
100.80
98.00
98.00
20c, d
20e, f
a
For comparison the g values for a range of piano-stool-type complexes (labeled [Cr(3-6)]+; Scheme 2) are also given. Errors: g = 0.005; Ai =
b
c
d
0.50 MHz. All hyperfine values are given in MHz. Coupling arises from two equivalent 31P nuclei. The smaller 31P coupling was not detected in
e
f
the experimental spectrum. Only the g values are given (see ref 14 for the H and 53Cr couplings). For comparison purposes, the experimental
1
(taken from ref 13) and calculated (DFT) g/PA values for a second Cr(I) complex, [Cr(CO)42]+ (2 = Ph2P(C3H6)PPh2), are also reported.
ditions, to account for the ca. 80% loss in signal intensity.14
However, when the [Cr(CO)41]+ complex is exposed to lower
levels of TEA (less than 10 equiv) and crucially when the
activation temperature is lowered to 273 K (rather than 298 K),
a different series of EPR signals can be detected (as shown in
Figures 2 and 3).
interaction of TEA with the Cr(I) complex is difficult to
control). For example, direct addition of 10 equiv of TEA at
298 K leads to the observation of the [Cr(1-bis-η6-arene)]+
complex only (vide supra, Figure 1), but if small aliquots of
TEA (≈1 equiv) are slowly and progressively added to the
solution at 273 K, then these intermediate complexes can still
be detected even when the total cumulative TEA quantity
reaches 10 equiv; eventually after overnight aging at 298 K, the
[Cr(1-bis-η6-arene)]+ species can be detected. These results
may initially be simply the result of the slower rate of formation
of [Cr(1-bis-η6-arene)]+ at 273 K versus 298 K. In other words,
direct addition of 10 equiv of TEA at 298 K gives the bis-arene
signal immediately (with no other signals), and the stepwise
addition of 10 equiv of TEA in small aliquots at 298 K produces
a bis-arene signal within ca. 1 h, whereas small quantities of
TEA added at 273 K produce only the bis-arene after
prolonged aging (overnight). Generally, these intermediate
complexes shown in Figure 2 could not be detected under any
conditions if the TEA levels were higher than 5 equiv.
Furthermore, in an experiment performed with the direct
addition of 5 equiv of TEA at 293 K, a new additional
intermediate species was detected (Figure 3). The well-resolved
axial g/PA profile for this spectrum is quite distinct from the
spectra of the intermediate species shown in Figure 2a−c,
suggesting that this new spectrum in Figure 3 (with very
intense Cr(I) signals, comparable to the precatalyst [Cr-
(CO)41]+ signal intensities) must arise from a further
intermediate complex. The absence of any bis-arene signal in
this spectrum (Figure 3) may also imply that a different
activation pathway has been followed in this case (vide infra).
Therefore, in order to identify the nature of these newly
formed paramagnetic centers responsible for the EPR spectra
shown in Figures 2 and 3, we performed a series of DFT
First when 2 equiv of TEA is added directly to a
dichloromethane solution of [Cr(CO)41]+ at 273 K and the
sample is immediately cooled to 140 K, a new EPR spectrum
can be observed (Figure 2a). This spectrum contains
overlapping features bearing a significant contribution from
residual, unreacted [Cr(CO)41]+ (labeled Precatalyst in Figure
2a). However, additional signals are also clearly visible in this
spectrum (compare Figure 1a to 2a). As the sample is
progressively and slowly warmed from 140 K to 298 K, the EPR
spectra evolve as the distribution of these new signals changes
(from Figure 2a to 2b,c). Eventually after aging the sample
overnight at 298 K, only the anisotropic frozen solution EPR
signal of the bis-arene [Cr(1-bis-η6-arene)]+ complex remains
(Figure 2d, recorded at 140 K). It should be recalled that an
analogous EPR spectrum to Figure 2d can also be formed
simply by addition of 10 molar equiv of TEA directly at 298 K
(as in Figure 1b).
The new signals detected in Figure 2a−c most likely arise
from “intermediate” partially decarbonylated complexes, of the
form [Cr(CO)x1]+ (x < 4), since their profile is quite
distinctive and different from the EPR spectra observed for
the two limiting cases of [Cr(CO)41]+ and [Cr(1-bis-η6-
arene)]+ (Figure 1). The relative abundance of these
“intermediate” complexes, as monitored by EPR, was also
found to vary slightly from one experiment to the next and
crucially was very dependent on how the TEA was added
(suggesting they are not particularly stable species or the
C
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Figure 3. X-band CW EPR spectra of [Cr(CO)41]+ dissolved in
dichloromethane following the addition of 5 molar equiv of TEA.
These spectra have been assigned to species D ([Cr(CO)21]+). The
spectra were recorded at (a) 140 K and (b) 298 K. The corresponding
simulations are shown in a′ and b′. Note: [Cr(CO)41]+ produces a
broad, structureless signal at 298 K due to fast relaxation character-
istics, whereas a readily resolved EPR signal can still be detected at the
same temperature for species D. See Scheme 3 for structure of species
D.
Figure 2. X-band CW EPR spectra of (a) [Cr(CO)41]+ dissolved in
dichloromethane following the addition of 2 molar equiv of TEA at
273 K and immediately frozen to 140 K. The sample was then warmed
to (b) 200 K and (c) 273 K for 30 min and (d) left at 298 K overnight.
All spectra were recorded at 140 K. The simulations (shown in a′, b′,
c′, and d′) were obtained using different contributions of [Cr-
(CO)41]+, species A ([Cr(CO)31]+), species C ([Cr(CO)21]+), and
[Cr(1-bis-η6-arene)]+. The relative weightings of these four species in
the simulations were respectively (a′) 0.55:0.33:0.0:0.12, (b′)
0.0:0.66:0.19:0.15, (c′) 0.0:0.30:0.35:0.35, and (d′) 0.0:0.0:0.0:1.0.
See Scheme 3 for structures of species A and C.
With good agreement achieved between experiment and
theory on this parent complex, we next calculated the spin
Hamiltonian parameters for the two structural isomers of
[Cr(CO)31]+ (i.e., starting from the octahedral geometry of
[Cr(CO)41]+, one CO was removed from a position either cis
or trans to the P atom, and the resulting structure allowed to
relax to the geometry-optimized complex shown in Figure 4);
hereafter these two isomers are labeled species A and B (see
Figure 4). In addition we also calculated the spin Hamiltonian
calculations to ascertain the magnitude of the spin Hamiltonian
parameters for a series of plausible [Cr(CO)x1]+ intermediate
complexes.
DFT Calculations. The experimentally determined spin
P
Hamiltonian parameters (g and A) for [Cr(CO)4L]+ (L =
Ph2PN(R)PPh2, Ph2P(R)PPh2) and [Cr(1-bis-η6-arene)]+ have
been reported recently by us.13,14 We therefore decided to first
calculate these parameters as a reference point using DFT,
before examining the possible [Cr(CO)x1]+ intermediates
P
formed by addition of TEA. The calculated g and A values
for [Cr(CO)41]+ are listed in Table 1 along with the
experimental values. The spin Hamiltonian parameters for the
[Cr(CO)4L]+ complexes were experimentally found to vary
depending on the nature of L,13 specifically with variations in
the giso and aiso values (Table 1).13 Although current state-of-
P
the-art DFT cannot quantitatively predict the EPR parameters
of transition metal ions,17 the general agreement between the
experimental and calculated values for the precatalyst [Cr-
(CO)41]+ is very good. It is important to note that DFT
P
overestimates the A values in [Cr(CO)41]+ compared to the
experiments. Furthermore, the experimental and calculated
parameters for the bis-arene [Cr(1-bis-η6-arene)]+ complex14
were also found to be satisfactorily reproduced by DFT (Table
1).
Figure 4. DFT-optimized structures of the precatalyst [Cr(CO)41]+
and the proposed intermediate [Cr(CO)31]+ (species A and B) and
[Cr(CO)21]+ (species C and D) complexes.
D
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parameters for the two structural isomers of [Cr(CO)21]+,
hereafter labeled species C and D (Figure 4). The energy
differences within each set of isomers were found to be
substantial. The cis- and trans-[Cr(CO)31]+ complexes differed
in energy by only 20 kJ mol−1, with the cis-complex (species A)
being more stable; both isomers are 15-electron complexes. For
the [Cr(CO)21]+ isomers, the differences were even more
substantial, with the 17-electron piano-stool isomer (species C)
being 120 kJ mol−1 more stable compared to the 13-electron
species D. The DFT-calculated g and PA values, along with the
known g values for related Cr(I) complexes, were then used as
a starting point to assist in the interpretation and deconvolution
of the EPR spectra shown in Figures 2 and 3.
Scheme 2. Representative Series of EPR-Characterized Cr(I)
Piano-Stool-Type Complexes (3−6)
a
Nature of the Intermediate Complexes and Analysis
of the Spin Hamiltonian Parameters. A detailed review of
the EPR parameters for a wide range of low-spin d5 transition
metals, including Cr(I)-carbonyl complexes, has previously
been presented by Rieger.15 In most cases, axial or slightly
rhombic g tensors are reported for the pseudo-octahedral
complexes with a predominantly dxy ground state.18 This is
indeed the situation observed for the [Cr(CO)41]+ complex
possessing well-defined axial g and PA values (Table 1). On the
other hand, the d5 sandwich complexes of Cr(I) produce easily
a
2
recognizable EPR signals because the SOMO is largely dz in
Two further examples of structurally characterized Cr(I) dicarbonyl
character.15,19 Since the e1g orbital containing most of the dxz,
dyz character lies well above the SOMO, negative g shifts are
expected in the EPR spectrum (i.e., g⊥ < ge), as indeed observed
for [Cr(1-bis-η6-arene)]+ (Table 1).
piano-stool-type complexes bearing the diphenylphosphino ligand are
the [bis(diphenylphosphino-η-benzene)chromium]carbonyl complex
(7) and the dicarbonyl bis(di-o-tolyl)phosphinomethane Cr(I)
complex (8). See Table 1 for corresponding spin Hamiltonian
parameters and references.
By comparison, some of the low-symmetry [Cr(CO)x1]+
complexes, particularly the piano-stool species C, are expected
to produce very distinctive EPR profiles with a rhombic g
tensor15,20 and should be easily identified. For example,
representative g values for a series of Cr(I) piano-stool-type
complexes (shown in Scheme 2) are listed in Table 1. Indeed,
Cr(I) piano-stool-type complexes bearing a diphenylphosphino
ligand (Scheme 2) have been structurally characterized in the
past,21 so their formation in the current system would not be
unexpected. For [Cr(CO)21]+ (species C), hyperfine couplings
to two inequivalent 31P nuclei and a rhombic g tensor are
therefore expected,15 and this prediction was nicely reproduced
by DFT (Table 1), giving giso > geP2and inequivalent 31P
couplings (P1aiso = −99.93 MHz and aiso = −6.335 MHz).
time with the concurrent increase of species C in Figure 2c
(possessing rhombic g values, but with each g component split
P
into a doublet due to the large A coupling to the unique 31P
nucleus in the piano-stool complex). The series of spectra in
Figure 2 could therefore be satisfactorily simulated using
contributions from unreacted [Cr(CO)41]+, [Cr(CO)31]+
(species A), [Cr(CO)21]+ (species C), and [Cr(1-bis-η6-
arene)]+; excellent agreement with the experimental spectra
was obtained simply by varying the contribution (weighting) of
these four paramagnetic complexes in each spectrum.
We were unable to reproduce the EPR spectra in Figure 2 by
including any contributions from species B (i.e., characterized
by a rhombic g tensor, but with inequivalent 31P couplings).
Therefore we may conclude that under these specific
experimental conditions of TEA addition, only the intermediate
species A and C were formed in the reaction pathway as the
stable precatalyst [Cr(CO)41]+ complex is progressively
transformed into the relatively stable bis-arene [Cr(1-bis-η6-
arene)]+ complex via A and C (Scheme 3). It should also be
recalled that according to DFT, the cis-[Cr(CO)31]+ structure
(species A) was found to be more stable compared to the trans-
[Cr(CO)31]+ structure (species B), so the absence of the latter
intermediate from Figure 2 is not surprising.
Although the piano-stool complex (species C) was identified
in Figure 2, the second structural isomer of [Cr(CO)21]+
(species D) was not observed under these experimental
conditions. According to the DFT calculations, this species is
expected to possess the largest 31P couplings (aiso = −115.66
MHz), smallest Δg shifts, and giso (=1.9859) < ge, compared to
the other intermediates. These parameters generally match the
principal characteristics of the EPR spectrum shown in Figure
3a, which was recorded following addition of 5 molar equiv of
TEA at 293 K. In particular small Δg shifts, giso (=1.999) < ge,
1
The H couplings could not be experimentally detected, so we
did not consider these. Using the DFT parameters calculated
for species C and the literature values for Cr(I) piano-stool
complexes,15 the overlapping EPR spectra shown in Figure 2a−
c could then be simulated; the resulting spin Hamiltonian
parameters used in the simulation are listed in Table 1 and were
found to be in reasonable agreement with the theory (although
P
the experimental A values are significantly smaller).
Furthermore, a second intermediate complex was also found
to contribute to the mixed EPR spectra in Figure 2a−c, as
manifested by the evolving profile of the peaks as a function of
increasing temperature. We considered that species A or B
must also contribute to this mixed spectrum (since the
predicted Δg values for species D are far too small; Table 1).
Since species A was calculated to be more stable compared to
species B (by 20 kJ mol−1), the DFT spin Hamiltonian
parameters for A were used as a starting point in the simulation
of Figure 2, specifically using a rhombic g tensor and large Δg
shifts. The nearly equivalent 31P nuclei in species A are largely
responsible for the quasi 1:2:1 triplet pattern, which is just
evident in Figure 2a,b. This signal decreases in intensity over
E
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Scheme 3. Summary of the Transformations That Occur Following Activation of the Precatalyst [Cr(CO)41]+ with Cocatalyst
a
(TEA) in Dichloromethane Solvent
a
Other than species B, all of the above complexes were detected and characterized by EPR.
and large 31P couplings were found experimentally (aiso
=
It should be recalled that the Philips trimerization system is
composed of a chromium source, 2,5-dimethylpyrrole, and
Et3Al1 and that it is believed to operate via a CrII/IV cycle,
90.53 MHz). These parameters are quite distinct from the
aforementioned lower symmetry species A, B, and C. The
resulting spin Hamiltonian parameters extracted by simulation
of the EPR spectra (Figure 3) are given in Table 1. Therefore
based on the agreement between the DFT and EPR data, the
spectrum shown in Figure 3 can be confidently assigned to
species D. Since species D was calculated to be far less stable
compared to species C (by 120 kJ mol−1), the observation of D
in Figure 3 suggests that a different (or secondary) pathway
may also occur during TEA activation of [Cr(CO)41]+.
although CrI/III have also been proposed.7 11 The pyrrole ligand
c,
is thought to flip between η1 and η5 coordination throughout
the catalytic cycle, effectively compensating for changes in the
coordination environment at the chromium center.1b Intrigu-
ingly the presence of the two structural isomers of [Cr-
(CO)21]+ (species C and D), one of which is η6 coordinated to
a Ph ring of PPh2, could also imply that an intramolecular
rearrangement of Cr(I) may also be possible in these
diphenylphosphino ligand based catalysts. Certainly the facile
formation of the bis-arene [Cr(1-bis-η6-arene)]+ complex
indicates the importance of η6 coordination modes in these
systems. Clearly the structures of the paramagnetic Cr(I)
complexes presented in Scheme 3 may not exist under the high-
pressure conditions operative during catalysis nor in the
presence of ethylene or at higher cocatalyst levels. Nevertheless,
the current findings reveal how the molecular rearrangement of
the parent [Cr(CO)41]+ complex can be transformed in
solution simply by addition of low levels of cocatalyst (TEA), as
[Cr(CO)41]+ evolves into [Cr(1-bis-η6-arene)]+ through the
possible intermediates [Cr(CO)31]+ (species A) and [Cr-
(CO)21]+ (species C).
Finally it should be mentioned that the relative distribution
of the intermediate species was observed to vary depending on
how the TEA is added to a solution of the complex (in small
aliquots or as a larger aliquot), how much is added
(concentration), the temperature of addition (273 K versus
298 K), and the aging time. The relative distribution of the
paramagnetic species, as extracted by simulations of the EPR
spectra (see Figure 2 caption), therefore changes according to
these experimental variables for TEA addition. In other words,
a combination of [Cr(CO)41]+, [Cr(CO)31]+ (species A),
[Cr(CO)21]+ (species C), and [Cr(1-bis-η6-arene)]+ was
Transformations of [Cr(CO)41]+ Following TEA Addi-
tion. As mentioned earlier, the role played by TEA as a
cocatalyst for reactions in olefin oligomerization varies
enormously from an alkylating agent,6 to a reducing agent for
chromium,2e,3,7 to aiding the formation of cationic complex-
es2e,6b,8 to scavenging impurities.8 According to the current
EPR results, the addition of TEA is also responsible for the loss
of paramagnetic Cr(I) signal intensity (Figures 1 and 2). Some
of this loss is likely attributed to the formation of diamagnetic
complexes, whose identity is currently unclear. However the
residual EPR signals detected after TEA addition can certainly
be assigned to various intermediate [Cr(CO)x1]+ complexes
and eventually the stable [Cr(1-bis-η6-arene)]+ complex.
Addition of TEA therefore clearly leads to the elimination of
some or all of the CO groups from the starting [Cr(CO)41]+
complex. Preliminary 2P ESEEM experiments did not reveal
any strong 27Al modulation, so although TEA is responsible for
elimination of CO, it does not coordinate to the Cr(I) centers
(at least under the adopted experimental conditions). Although
the experimental EPR spectra can be quite complex due to the
simultaneous presence of multiple Cr(I) centers (Figure 2), the
spin Hamiltonian parameters of the different “intermediate”
species are sufficiently distinct that they can be readily
identified in the experimental spectra.
F
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to literature procedures.12,23 The Ag[Al(OC(CF3)3)4] was prepared
according to a literature procedure.24
typically observed in the EPR spectra (as in Figure 2), but the
relative weightings of each species varied slightly from one
experiment to the next. Furthermore, the observation of the
intermediate species D, obtained by using slightly different
experimental conditions for TEA addition, only emphasizes the
fact that controlling how the TEA is added, at what
temperature, and for how long will affect the nature of the
resulting species. These results therefore highlight the difficulty
of structurally characterizing the nature of the complexes
formed by addition of the cocatalyst, TEA, to a series of Cr(I)
diphenylphosphino ligand based catalysts, as the nature and
distribution of the paramagnetic Cr(I) “intermediate” com-
plexes (species A, C, D, and finally [Cr(1-bis-η6-arene)]+) can
vary greatly depending on the experimental conditions.
Instruments. NMR spectra were recorded at 298 K on Bruker
Avance AMX 400 or Jeol Eclipse 300 spectrometers. Chemical shift
values are given relative to residual solvent peak. ESI-MS were
performed on a Waters LCT Premier XE instrument. Infrared spectra
were recorded using a JASCO FT/IR-660 Plus spectrometer and
analyzed in solution (dichloromethane). Microanalyses were per-
formed by SASOL Ltd. All continuous-wave EPR spectra were
recorded on an X-band Bruker EMX spectrometer operating at 100
kHz field modulation, 10 mW microwave power, and equipped with a
high-sensitivity cavity (ER 4119HS). EPR computer simulations were
performed using the SimEPR32 program.25 g values were determined
using a DPPH standard.
Synthesis of Chromium(0) Tetracarbonyl Complex [Cr-
(CO)4(Ph2PN(iPr)PPh2)]. Toluene (20 mL) was added to chromium
hexacarbonyl [Cr(CO)6] (350 mg, 1.59 mmol) and 1 (500 mg, 1.17
mmol), and the stirred mixture was heated under reflux for 48 h. The
solution was cooled to 0 °C and filtered to remove excess [Cr(CO)6].
Solvent was removed under reduced pressure, and the product
extracted into dichloromethane (5 mL). Methanol (10 mL) was added
to precipitate the product, which was isolated by filtration and dried in
vacuo to yield the yellow solid [Cr(CO)4(Ph2PN(iPr)PPh2)]: yellow
solid (260 mg, 38%); 1H NMR (CDCl3, 400.8 MHz, 298 K) δ (ppm)
0.62 (d, 6H, CH3, JHH = 6.8 Hz), 3.52 (sept, 1H, CH, JHH = 7.0 Hz),
7.41 (m, 12H, meta-, para-C6H5), 7.69 (m, 8H, ortho-C6H5); 31P{1H}
NMR (CDCl3, 121.7 MHz, 298 K) δ (ppm) 112.70 (s); 13C NMR
(CDCl3, 125.8 MHz, 298 K) δ (ppm) 22.54 (CH3), 54.79 (CH),
127.39 (meta-C6H5), 129.52 (para-C6H5), 130.86 (ortho-C6H5),
136.09 (ipso-C6H5), 221.89 (cis-CO), 227.40 (trans-CO); high-
resolution ESIpos-MS (MeCN) found 591.0796 (calc 591.0820 dev:
−4.1 ppm); IR (CH2Cl2) ν 1887 (s) (CO), 1923 (s) (CO), 2006 (s)
(CO) cm−1. Anal. Calcd for C31H27CrNO4P2 (found): C, 62.95
(62.93); H, 4.60 (4.56); N, 2.37 (2.31).
Synthesis of Ag[Al(OC(CF3)3)4]. LiAlH4 (1.0g, 0.026 mol) was
suspended in hexane (60 mL) and cooled to 253 K, and HOC(CF3)3
(15 mL, 0.11 mol) was added slowly. The mixture was stirred for 45
min, then heated under reflux overnight using a condenser set at 253
K. The solution was filtered, the product washed with hexane, and
solvent removed in vacuo to yield the white solid Li[Al(OC(CF3)3)4]
(20.0g, 80%): 19F NMR ((CD3)2SO, 250 MHz, 298 K) δ (ppm)
−75.06. Li[Al(OC(CF3)3)4] (10.0 g, 0.01 mol) and AgF (1.7g, 0.013
mol) were suspended in CH2Cl2 (50 mL) in the dark and mixed in an
ultrasonic bath overnight. The solution was filtered, and the solvent
removed in vacuo to yield the white solid Ag[Al(OC(CF3)3)4] (8.3g,
77%).
Synthesis of Cr(I) Tetracarbonyl Species [Cr(CO)4(Ph2PN-
(iPr)PPh2][Al(OC(CF3)3)4]. Cr(0)1 (100 mg, 0.17 mmol) and
Ag[Al(OC(CF3)3)4] (220 mg, 0.23 mmol) were dissolved in
dichloromethane (5 mL) to give a dark blue solution. The Schlenk
tube was covered with foil to reduce exposure of the reaction mixture
to light. The solution was stirred at room temperature overnight, then
filtered, and the solvent removed in vacuo to yield the blue solid
[Cr(CO)4(Ph2PN(iPr)PPh2)][Al(OC(CF3)3)4]: dark blue powder
(105 mg, 40%); high-resolution ESIpos-MS (MeCN) found 591.0824
(calc 591.0820 dev: 0.6 ppm); IR (CH2Cl2) ν 1962 (s)(CO), 2032
(s)(CO), 2086 (s)(CO) cm−1; 19F NMR (CD2Cl2, 282 MHz) δ
CONCLUSION
■
A combined CW EPR spectroscopy and DFT computation
study of a Cr(I)bis(phosphine) complex, [Cr(CO)41]+, has
been carried out to investigate the progressive changes to the
tetracarbonyl complex following the addition of small quantities
of triethylaluminum. At ambient temperatures and pressures,
addition of 10 molar equiv of TEA to a dichloromethane
solution of [Cr(CO)41]+ results in the formation of a bis-arene
complex, [Cr(1-bis-η6-arene)]+, and complete loss of CO.
Approximately 80% of the starting Cr(I) signal intensity (from
[Cr(CO)41]+) is lost at this stage. However, addition of 1−2
molar equiv of TEA at slightly lower temperatures (273 K)
affords the detection of intermediate [Cr(CO)x1]+ complexes
due to the partial loss of CO. The two structural isomers of
[Cr(CO)21]+, namely, species C and D, were both detected by
EPR (albeit under different TEA activation conditions). The
more stable piano-stool-type complex, species C, was found to
be more prevalent in the experiments. Among the two possible
structural isomers of [Cr(CO)31]+ (species A and B), only the
former and slightly more stable species A was detected
experimentally. The DFT-calculated spin Hamiltonian param-
eters were used as a starting point to simulate the overlapping
EPR spectra, containing varying contributions of [Cr(CO)41]+,
[Cr(CO)31]+ (species A), [Cr(CO)21]+ (species C), and
[Cr(1-bis-η6-arene)]+. As the temperature of the [Cr(CO)41]+/
TEA (2 equiv) solution is raised, the loss of Cr(I) centers
follows the following trend: [Cr(CO)41]+ is depleted first,
followed by [Cr(CO)31]+ and [Cr(CO)21]+, until eventually
only the [Cr(1-bis-η6-arene)]+ signal is detected. The
distribution and detection of the intermediate species are very
sensitive to the quantity and manner of TEA addition. These
results highlight the complexity of the paramagnetic species
that can exist with Cr(I)bis(phosphine) complexes following
interaction with a cocatalyst. While there is some variation in
the ratios of intermediates generated during catalyst activation
with TEA, depending on reaction conditions, it is clear that
several key intermediates are consistently formed. This allows
us to gain a better understanding of the activation process and
the oxidation state(s) of the metal center that may be involved
in the catalysis.
(ppm) −76.2 (s, Δν1/2
= 8.41 Hz). Anal. Calcd for
C47H27AlCrF36NO8P2 (found): C, 36.22 (36.19); H, 1.75 (1.77); N,
0.90 (0.99).
Sample Preparation for EPR Measurements. [Cr(CO)41]+ (6
mg of the solid containing the Ag[Al(OC(CF3)3)4] counterion) was
dissolved in 200 μL of dichloromethane in the EPR tube under
anaerobic conditions (1.92 × 10−2 M). Triethylaluminum (predis-
solved in hexane or dichloromethane; 0.1 M) was added directly to the
solution using a microsyringe. A sufficient quantity was delivered to
the tube, affording different TEA:Cr molar ratios of between 2 and 10
equiv. The TEA was added to the precooled EPR tube containing the
[Cr(CO)41]+ solution (at either 298 or 273 K). The samples were
frozen immediately to 77 K by immersing in liquid nitrogen. The
EXPERIMENTAL SECTION
■
All manipulations were performed using standard Schlenk techniques
under an Ar or N2 atmosphere in an MBraun UNILAB glovebox (less
than 0.1 ppm H2O and O2). Solvents were dried using a Braun solvent
purification system and degassed prior to use. Ligand 1 was prepared
according to a literature procedure.22 The chromium(0) and
chromium(I) compounds of [Cr(CO)41]+ were prepared according
G
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frozen solution CW EPR spectra were recorded at 140 K. The samples
were then slowly and progressively annealed at increasing temper-
atures (i.e., 200, 273, and 298 K, overnight), allowing the intermediate
[Cr(CO)x1]+ species to be detected.
Details of DFT Calculations. Geometry was optimized using
Turbomole26 at the uBP86/def2-TZVP level27,28 and confirmed as a
true minimum via harmonic frequency calculation. Hyperfine coupling
and g tensor data were calculated in ORCA 2.829 using the hybrid
PBE0 functional30 and a basis set consisting of EPR-II on C and H,31
def2-TZVP on P,28 and the “Core Properties” basis (defined in ORCA
for first-row transition metals) on Cr.32
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AUTHOR INFORMATION
Corresponding Author
*E-mail: cartere4@cf.ac.uk; cavellkj@cf.ac.uk; murphydm@cf.
ac.uk.
■
(16) For spin quantification analysis, both EPR spectra in Figure 1
(of [Cr(CO)41]+ and [Cr(1-bis-η6-arene)]+) were recorded at the
same temperature (140 K) and power (5 mW).
(17) Neese, F. Coord. Chem. Rev. 2009, 253, 526.
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(b) Cummings, D. A.; McMaster, J.; Rieger, A. L.; Rieger, P. H.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
L.M. acknowledges funding from SASOL. EPSRC funding
(EP/H023879) is also gratefully acknowledged.
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