Accessing Alkyl- and Alkenylcyclopentanes from Cr-Catalyzed Ethylene Oligomerization Using 2-Phosphinophosphinine Ligands

Article abstract of DOI:10.1021/acs.organomet.8b00063

Desilylation of the 2-phosphinophosphinine 2-PPh₂-3-Me-6-SiMe₃-PC₅H₂ with HCl gave 2-PPh₂-3-Me-PC₅H₃, demonstrating the late-stage modification of this bidentate heterocyclic lig

Full text of DOI:10.1021/acs.organomet.8b00063

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Accessing Alkyl- and Alkenylcyclopentanes from Cr-Catalyzed
Ethylene Oligomerization Using 2‑Phosphinophosphinine Ligands
Robert J. Newland,†,∥ Alana Smith,†,∥ David M. Smith,‡ Natalie Fey,*,§ Martin J. Hanton,*,‡
and Stephen M. Mansell*,†
†Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
‡Sasol Technology U.K., Ltd., Purdie Building, North Haugh, St Andrews, Fife KY16 9ST, U.K.
§School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
S
* Supporting Information
ABSTRACT: Desilylation of the 2-phosphinophosphinine 2-PPh2-3-
Me-6-SiMe3-PC5H2 with HCl gave 2-PPh2-3-Me-PC5H3, demonstrat-
ing the late-stage modification of this bidentate heterocyclic ligand.
Group 6 metal carbonyl complexes of these ligands showed κ2 binding
and very small bite angles of 65.1−68.3°, and also demonstrated that
the donor properties of 2-phosphinophosphinines can be tuned readily
by the presence of the SiMe3 group, which gives a more π accepting
phosphinine ligand. The properties of 2-phosphinophosphinines were
compared to those of bidentate diphosphorus ligands computationally,
contextualizing them in the ligand knowledge base for bidentate P,P
donor ligands (LKB-PP), and were found to occupy an area of ligand space adjacent to Ar2PN(R)NAr2 ligands that have been
successfully used in ethylene oligomerization reactions, but with well-separated properties in the second principal component.
Testing 2-phosphinophosphinines in Cr-catalyzed ethylene oligomerization reactions showed key differences from standard PNP
ligands in that a high proportion of alkyl- and alkenylcyclopentanes were formed. This demonstrates that the different donor
properties of 2-phosphinophosphinines influence the reactivity of the key seven-membered metallacycle postulated in the
metallacyclic reaction mechanism, generating products from isomerization and subsequent ethylene insertion. Alkyl- and
alkenylcyclopentanes represent new products for the key industrial feedstock ethylene, with the alkenes having potential as new
monomers, comonomers, or additives for plastics. Computational evaluation of ligand properties and the resulting property maps
can play a role in suggesting future ligand developments to change the selectivity of this industrially relevant system, in the
pursuit of new products generated from ethylene.
Alkenes bearing cyclopentane substituents also represent
potential monomers giving access to polyolefins with new
properties, and polyethylene waxes/short-chain polymers have
materials applications as well.12
INTRODUCTION
■
Ethylene is a key starting material in the chemical industry,1,2
including in the production of valuable linear α-olefins (LAO).2
Nonselective processes for the production of LAO, such as the
SHOP process,3 have been complemented by selective
processes that can be tuned to exactly match market demand,
and industrial processes for ethylene dimerization,4,5 trimeriza-
tion,6 and tetramerization2,7 have all been commercialized.7 A
metallacyclic mechanism (Scheme 1) has been generally
accepted as the reason for high selectivities to 1-hexene and
1-octene,5,8 and it is the understanding of this distinctive
mechanism that has enabled a step change in performance in
comparison to previous unselective oligomerization processes.
Encouraged by these successes, researchers have shown
continued interest in understanding these processes in more
detail as well as developing other selective processes to higher
LAO (e.g., C10−C18)3 and to new products.9,10 Alkyl- and
alkenylcyclopentanes have been found to be produced by
tetramerization catalysts11−15 and represent a potential target
for optimization, by identifying the factors influencing their
production from the key seven-membered metallacycle.
Small-bite-angle PNP (diphosphinoamine) ligands16 have
proven to be very successful ligands in combination with simple
Cr salts and MAO coactivators for selective oligomerization
catalysis because they produce highly active catalysts both for
trimerization, where they can be extremely selective to 1-
hexene (e.g., A, Chart 1),17,18 and for tetramerization (B, Chart
1), producing 1-octene in greater than 70% selectivity (along
with 1-hexene and other products).11,19,20 Although computa-
tional and experimental insight into the mechanism has been
gained,9,11,13,14,21−25 definitive spectroscopic identification of
the active species in the catalytic cycle has remained out of
reach,26 somewhat hampering catalyst design. In a move from
nitrogen- to carbon-based ligand backbones, it was established
that dppm (C, Chart 1, R = H) does not form a selective
Received: January 31, 2018
© XXXX American Chemical Society
A
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Scheme 1. Cr-Catalyzed Ethylene Oligomerization via Metallacyclic Intermediates and Formation of Alkyl- and
a
Alkenylcyclopentanes
a
The formation of co-oligomers is not shown.
the form of an aromatic phosphinine,35 would protect the
ligand backbone from deprotonation and give unsymmetrical 2-
phosphinophosphinine ligands with potentially very different
properties.32,35,36 Ligand 1 has recently been used in the Ru-
catalyzed transfer hydrogenation of acetophenones at room
temperature as well as the catalytic “hydrogen borrowing”
Chart 1
37
i
upgrading of EtOH/MeOH to BuOH. Unsymmetrical
ligands are an interesting development in selective oligomeriza-
tion catalysis,12,38 as 1-phosphanyl methanimine ligands (I)
have recently shown excellent properties in Cr-catalyzed
ethylene oligomerization, achieving conversions of greater
than 95% to give high-value liquid 1-hexene and 1-octene.38
In this work, we present a new synthetic route to desilylated 2-
phosphinophosphinines, experimentally and computationally
characterize the donor properties of 2-phosphinophosphinines,
and assess their performance in Cr-catalyzed ethylene
oligomerization, revealing the formation of alkyl- and
alkenylcyclopentanes from the key seven-membered metal-
lacyclic intermediate.
RESULTS AND DISCUSSION
■
catalyst, instead generating a Schulz−Flory distribution of
LAO,27 most likely due to deactivation processes involving
deprotonation of the acidic methylene group.27,28 Protection of
this position with one alkyl substituent recovered modest
catalytic activity and some selectivity for tri- and tetramerization
(C, R = Me, n-hexyl, benzyl),28 along with the use of ligand D
which is more resistant to deprotonation,27 but the unsaturated
1,1-bis(diphenylphosphino)ethene ligand (E) gave an un-
selective catalyst of very low activity.27 Dppe (F, R = H) and
related ligands with hydrocarbyl backbones containing
unsaturation (G, H), either with or without backbone
substitution, also work well in selective ethylene tetramerization
catalysis.27,29
Desilylation of 2-PPh2-3-Me-6-SiMe3-phosphinine.
The double protodesilylation of 2,6-bis(trimethylsilyl)-
phosphinines using ethereal HCl has been shown to be facile
when the 3,5-substituents are simple hydrocarbyl groups,39 and
also very recently it has been shown for 2-(trimethylsilyl)-
phosphinine.40 A computational study found that o-silyl groups
should improve both the σ-donating and π-accepting properties
of phosphinines,41 and, as the impact of substituent effects on
the donor properties of phosphinophosphinine ligands has yet
to be investigated, we sought experimental evidence for this
hypothesis by comparing the donor properties of ligands 1 and
2.
Proligand 1 was synthesized as detailed previously from the
borane-protected compound 1·BH3.37 Desilylation was
achieved both with and without BH3 protection using 1 equiv
of HCl in a mixture of Et2O and CH2Cl2, cleanly generating 2·
The use of donor-functionalized phosphinines in catalysis is a
growing field,30−34 and incorporation of the ligand backbone
and one phosphorus donor in an unsaturated heterocycle, in
B
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BH3 and 2, respectively (Scheme 2). 31P{1H} NMR spectros-
copy showed the phosphinine P resonance at 229.8 ppm in 2·
perpendicularly to the phosphinine plane (the B1−P2−C1−P1
torsion angle is 108.79(13)°). The P−C bond lengths in the
ring (1.746(2) and 1.721(2) Å) are shorter than the P−C bond
to the PPh2 moiety (1.822(2) Å), as expected from the
aromatic nature of the phosphinine ring, but are noticeably
different, with P1−C1 longer by approximately 0.02 Å. The C−
C bond lengths in the phosphinine ring fall between 1.380(3)
and 1.410(3) Å, typical for those observed in benzene (1.40 Å).
Although deprotection of 2·BH3 was achieved with 0.5 equiv of
1,4-diazabicyclo[2.2.2]octane (DABCO), direct formation of 2
from 1 was preferred due to simpler purification and higher
yields.
Scheme 2. Synthesis of Proligands and Carbonyl
a
Complexes
Coordination Chemistry. Quantifying ligand properties
can help explain experimental observations and guide further
catalyst design. Coordination complexes of metal carbonyls
have long been proven to be a useful means of determining the
donor properties of ligands experimentally,43 such as Tolman’s
use of [Ni(L)(CO)3],44 trans-[Rh(Cl)(L)2(CO)] as a safer
alternative,45 or the A1 vibration in cis-[Mo(CO)4(L2)]
complexes for bidentate ligands.46 The only previously reported
chelating metal carbonyl complex of a 2-phosphinophosphinine
is 2-diphenylphosphino-3,4,6-triphenylphosphinine coordi-
nated to W(CO)4; however, only the yield (41%), melting
point, and observation of the molecular ion by MS were
reported.47 Only bridging complexes of 2 have been reported to
date.48
a
L2 is NBD (norbornadiene) for Cr and Mo and COD (1,5-
1 and 2 react in toluene solution with diene complexes of
group 6 tetracarbonyls to generate the corresponding chelating
complexes. All compounds were isolated pure after recrystal-
lization from toluene at low temperature, as determined by
elemental and spectroscopic analysis. 31P NMR spectroscopic
cyclooctadiene) for W.
BH3, approximately 25 ppm lower than in 1·BH3 (255.4
ppm),37 1H NMR spectroscopy showed a multiplet resonance
at 8.59 ppm for the 6-H in 2·BH3, and purity was also
established by elemental analysis. 2 has been previously
synthesized through a route involving Pd-catalyzed cross-
coupling of a 2-bromophosphinine.42 The molecular structure
of 2·BH3 was determined by X-ray diffraction (Figure 1) and
showed a planar phosphinine ring bearing a 2-diphenylphos-
phine substituent bound to BH3 which is oriented almost
2
analysis showed two doublets for each complex with JPP
coupling constants that increase in the order proligand (2,
31.2 Hz; 1, 31.6 Hz37) < Cr complexes (3, 38.2 Hz; 6, 33.8 Hz)
< Mo complexes (4, 72.8 Hz; 7, 71.1 Hz) < W complex (5, 78.0
Hz); 5 also showed additional 183W (I = 1/2, 14% abundant)
satellites. The phosphinine P chemical shifts are highest for the
Cr complexes, with lower frequency resonances for the Mo and
W complexes, and the PPh2 moieties displayed the same
pattern. All values for the phosphinine resonances are still
typical of P in an aromatic phosphinine environment.32
M(CO)4 complexes of the chelating 4,5-dimethyl-2-(2-pyridyl)-
phosphinine ligand are known that show 31P NMR resonances
ca. 24 ppm lower for M = Cr, Mo, and ca. 4 ppm lower for M =
W, likely due to the different substitution on the phosphinine.49
X-ray diffraction experiments on single crystals of 3−5
revealed that they all crystallize in the P1 space group and are
̅
isostructural (Figure 2a and Tables S1 and S2). The chelating
phosphinophosphinine ligands are bound to distorted-octahe-
dral M(CO)4 fragments with P1−C2−P2 angles that are
significantly decreased in comparison to the free ligand
(97.50(8)° in 3 to 99.98(7)° in 4 in comparison to
119.9(1)° in 1) indicating distortion of the ligand upon
coordination. The bite angles are all very small: between
67.99(2)° in 3 and 65.07(2)° in 5. In comparison, for
[M(dppm)(CO)4] the bite angles are 70.2° (Cr),50 67.3°
(Mo),50 and 67.1° (W).51 The M−P bond lengths are shorter
to the phosphinine P than to the phosphine P (Table 1), as
seen previously in a chelating Ru complex.37 The metal center
is offset in comparison to the orientation of the phosphinine
ring (the C3−P1−M1 angle is ca. 151°), emphasizing the
flexibility of the phosphinine donor in accommodating this
geometry, which is likely to be enhanced by the high s character
Figure 1. Thermal ellipsoid plot of the molecular structure of 2·BH3
(50% probability). Selected bond lengths (Å) and angles (deg): P2−
B1 1.934(3), P1−C1 1.746(2), P1−C5 1.721(2), P2−C1 1.822(2),
C1−C2 1.410(3), C2−C3 1.402(3), C3−C4 1.382(3), C4−C5
1.380(3); P1−C1−P2 114.94(12), C5−P1−C1 101.40(11), C2−
C1−P1 124.65(16), C3−C2−C1 121.0(2), C4−C5−P1 124.90(17);
torsion angle B1−P2−C1−P1 108.79(13).
C
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Figure 2. Thermal ellipsoid plots of the molecular structure of 3 (left, 4 and 5 are isostructural) and 6 (right). Thermal ellipoids are at 50%
probability, and hydrogen atoms are omitted for clarity.
Table 1. Selected Analytical Data for 1−7
1
2
3
4
5
6
7
phosphinine 31P NMR δ/ppm
PPh2 31P NMR δ/ppm
2JP−P/Hz
249.8
−7.5
31.6
224.9
−7.6
31.2
273.6
244.8
209.6
250.3
222.7
20.0
71.1
40.5
18.9
−0.1
78.0
41.8
38.2
72.8
33.8
M−Pphosphinine/Å
M−PPPh2/Å
bite angle/deg
2.3752(5)
2.4340(5)
67.99(2)
97.50(8)
2013
2.5028(4)
2.5599(4)
65.09(1)
99.98(7)
2026
2.4866(6)
2.5462(6)
65.07(2)
99.40(11)
2021
2.3464(11)
2.4030(13)
68.29(4)
97.7(2)
2003
a
P−C−P angle/deg
A1 CO vibration/cm−1
119.9(1)
114.9(1)
2017
a
As determined from the molecular structure of 2·BH3.
of the phosphinine lone pair.32 Single-crystal X-ray diffraction
experiments on 6 revealed the analogous Cr complex without a
SiMe3 group (Figure 2b). The Cr−P bond lengths (Cr−P1
2.3464(11) Å and Cr−P2 2.4030(13) Å) are ca. 0.03 Å shorter
than in 3, and P1−C5 is also shorter than the analogous
distance in 3 by 0.03 Å at 1.705(4) Å, but the bite angle is
similar (68.29(4)°). Comparison with (4,5-dimethyl-2-(2-
pyridyl)phosphinine)tetracarbonylchromium (Cr−P =
2.280(1) Å)49 show a longer Cr−phosphinine bond length
for 6. The two crystallographically-characterized Mo−phosphi-
nine complexes show shorter bond lengths of ca. 2.38 Å for
[Mo(PC5H5)6]52 and 2.3565(4) Å for a bridging dinuclear
complex,53 and seven crystallographically characterized W−
phosphinine compounds have bond lengths ranging from 2.36
to 2.57 Å.52,54−57
Upon desilylation, the A1 CO stretch for the Cr and Mo
carbonyl complexes (Table 1) is lowered by ca. 10 cm−1,
indicating substantially poorer π acceptance for the desilylated
ligand 2. This is in line with a previous computational
investigation which predicted that the HOMO of phosphinine
is higher in energy and the LUMO is lower in energy with o-
silyl substituents.41 However, a recent DFT comparison of 2-
SiMe3PC5H4 with PC5H5 suggested different behavior in
comparison to phosphinophosphinines, as it was calculated
that the silyl group did not change the energy of the LUMO
but instead raised the energy of the phosphinine lone pair, such
that it went from HOMO-2 to HOMO-1.40
related chelating pyridyl-phosphinine complex49 is very similar
to that for 1. The relatively small differences in comparison to
PPh3 or dppm are consistent with half of the ligand resembling
PPh3. The IR stretching data therefore suggest that these
ligands would be suitable candidates in catalysis where small-
bite-angle arylphosphine donors are predominant,16 such as
ethylene oligomerization reactions.
Stability of 2-Phosphinophosphinines. Complex 6 was
found to be thermally unstable, as shown by heating a benzene
solution to 80 °C, which formed an insoluble precipitate and
several new soluble phosphorus-containing species that could
not be characterized further. A dinuclear decomposition
product (8, Scheme 3) was identified from a reaction at
Scheme 3. Dimerization of 6 and Addition of H2O To Form
Dinuclear 8
room temperature in the presence of water vapor from the air,
alongside a number of other unidentified decomposition
products. X-ray diffraction experiments of single crystals
grown from the slow diffusion of hexane into a CH2Cl2
solution revealed the molecular structure of 8 (Figure 3),
which demonstrated that two complexes had reacted together
in a manner that resembles 4 + 2 cycloaddition, followed by
addition of H2O that would be facile with the resultant isolated
PC bonds. Although no further analytical data could be
The values for cis-[Mo(CO)4L2] complexes allow compar-
isons between ligands to be made.46 Ligand 1 (ν = 2026 cm−1)
was observed to be more π accepting than dppm (ν 2020
cm−1)58 and (PPh3)2 (ν 2022 cm−1),46 whereas ligand 2 (ν
2017 cm−1) is less π accepting. A value of 2028 cm−1 for a
D
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different coordination environments from relatively straightfor-
ward optimizations of ground state structures using standard
density functional theory (DFT) calculations.79 A range of
structural parameters, energies, and steric properties are
extracted from these calculations (Table S5) and have been
used in regression models but were also processed with a
statistical projection technique, principal component analysis
(PCA), to optimally represent ligand similarities as distances on
a so-called map of chemical space.72 As shown recently for
monodentate ligands,82 such maps, where proximity indicates
ligands with similar properties, can set unusual ligand designs,
including phosphinines,30−32,35 into context and so suggest
possible applications in catalysis.
Having established the bidentate nature of ligands 1 and 2,
we were interested in how they compared to other small-bite-
angle ligands, including some of those shown in Chart 1if
their properties are very similar to those of known bidentate
ligands, they will appear in close proximity to these on a PCA
map. In addition, we used this approach to explore syntheti-
cally-accessible structural variations computationally, with a
view to quantifying their effects on ligand properties and so
selecting the most promising targets. Chart 2 shows the ligands
Figure 3. Thermal ellipsoid plot of the molecular structure of 8 (50%
probability). The phenyl rings are displayed as capped sticks and
without hydrogen atoms for clarity.
obtained, the reaction casts light on another effect of removing
SiMe3 substitution from the phosphinine 6-position because
SiMe3 substitution would hinder the ability of the phosphinine
to act as a diene in this manner. Additional, unwanted reactivity
of a ligand contributes toward catalyst instability and represents
a very important, but often overlooked, area of catalyst design,
as well as highlighting a computationally challenging field
computational studies can be used to test mechanistic
hypotheses, but the discovery of novel reactivity cannot
normally be achieved.
a
Chart 2. Ligands Discussed for LKB-PP
Reactions of phosphinines with lithium alkyls have been
shown to give anionic phosphacyclohexadienyl anions.59−62
These species can act as ligands either through the P lone pair,
usually when another donor is present as a substituent on the
phosphinine ring and the ligand can chelate,35,61,63−67 or
through the π system.61,68−70 It has been reported that
Et2AlOEt reacts with 2,4,6-triphenylphosphinine and [Ni-
(acac)2] to generate a dinuclear complex with one Ni bound
to the π systems of two P-ethyl phosphacyclohexadienyl anions
and the second Ni binding η1 to the P atoms and an ethene
ligand.71 This reactivity with an aluminum alkyl prompted us to
explore the reaction of MAO with 1 and complex 3, as a
surrogate for the paramagnetic species formed in the catalytic
reactions, in a manner analogous to dppm reactivity studies.28
In reactions with 30 equiv of MMAO-12, 1 and 3 showed no
significant change in color, which would have been expected for
the intensely colored phosphacyclohexadienyl anions,37,61,62
and retained the high-frequency 31P{1H} NMR spectroscopic
resonances observed in the starting materials which are
characteristic of aromatic phosphinines. Extended reaction
times still did not lead to reaction of the phosphinine centers,
indicating that phosphinophosphinine ligands are resistant to
reaction with MAO.
Computational Insight. The properties of ligands can be
captured by a range of steric and electronic parameters,72,73 and
organometallic catalysis has a long tradition of relying on such
data to guide catalyst optimization.44,74 While these parameters
can be determined experimentally, such an approach becomes
particularly powerful when computational approaches can be
used to evaluate novel ligand structures,75 contextualizing their
likely properties before synthesis.76,77 The ligand knowledge
base (LKB) approach developed in Bristol and described in
detail elsewhere78−81 captures the properties of ligands in
a
Labels in boldface correspond to those used elsewhere in this work.
added to the relevant LKB (LKB-PP),79 alongside the labeling
of relevant ligands published previously; note that the labeling
of ligands is in line with previously published LKB-PP data and
so is specific to this section only. Tables of all ligands (Table
S4) and parameters (Table S5), as well as the data for new
ligands (Table S6) and further details of the principal
component analysis (Tables S7 and S8) can be found in the
Supporting Information. Attaching chemical meaning to
principal components is difficult, both because this approach
is not statistically robust, i.e. the composition changes when the
ligand set is changed, and because ligand property parameters
extracted from calculations on representative molecular systems
are rarely due to a single effect. In most settings, both steric and
electronic effects combine to give rise to an observable. Here, in
E
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the interest of brevity, discussion will focus on the maps
resulting from PCA analysis. Figure 4 shows the updated LKB-
PP map of ligand space with ligands N1−N14 added.
with N2 and N4 lying slightly farther away from the other
ligands. Where the phosphorus donor is more electron rich
(N6−N9 vs N10−N13), these differences appear less
pronounced, suggesting that such ligand modifications are a
fine balance of steric and electronic effects and that it may be
possible to fine-tune catalytic activity through changing these
substituents. This map also suggests that, although similar in
the first principal component, these new ligands are likely to
have somewhat different properties in comparison to other
small-bite-angle PNP and PCP ligands, as identified by their
differences in the second principal component.
Figures 4 and 5 focus on PC1 and PC2, which capture 58%
of the variation in this database, but it is worth noting that 28
parameters will give rise to 28 principal components and, in the
case of bidentate ligands, consideration of PC3 will capture
66% of the variation, with PC4 taking this to 73%. Without
relating this to extensive experimental data sets, it is difficult to
determine which PCs are most important in terms of the
catalysis of interest, and in the present case it will be sensible to
consider PC3 as well (Figure S59 and Figure 6). Interestingly, a
Figure 4. Ligand map generated by principal component analysis of 28
ligand parameters capturing the structures and energies of 324 P,P-
donor ligands through DFT-calculated parameters, collected in LKB-
PP. The principal components are linear combinations of the original
parameters derived to capture most of the variation in fewer
uncorrelated parameters (58% in this case). Each symbol corresponds
to a ligand, and the shape and color are determined by substituents as
shown in the legend. See Figure S58 for more detailed labels.
The ligand map shown in Figure 4 places the new ligands
(orange dots) toward the edge of ligand space, but generally
these are closer, and so more similar, to other ligand
architectures than halide-substituted ligands. Figure 5 shows
the area of the LKB-PP map closest to the phosphinine ligands,
including labels for all ligands shown in Chart 2.
Figure 6. Plot of PC1 and PC3, focusing on ligands N1−N14. See
Chart 2 for ligand structures and Figure S61 for more comprehensive
labels in this area. Red boxes highlight the exceptional PNP ligands A
(24) and B (31) for ethylene oligomerization and the phosphino-
phosphinines 1 and 2.
Considering the 2-phosphinophosphinine ligands, the
removal of the SiMe3 substituent alters the ligand properties,
plot of PC1 vs PC3 places the small-bite-angle ligands quite
close together (Figure 6), regardless of backbone, further
emphasizing that additional principal components may need to
be considered in the analysis of such bidentate ligands. While 2-
phosphinophosphinine ligands remain slightly separate from
other ligand architectures, changing the Si substituents (N5) or
making the phosphine donor more electron rich without
increasing steric hindrance near to the coordinating site (nBu,
N11) appears to move ligands toward the privileged PNP
ligands (including 31).
This analysis of ligand properties highlights that 2-
phosphinophosphinine ligand properties can be tuned by
altering substituents, which will support the selection of a more
extensive and varied set for future synthesis and testing in
catalysis. Synthetically accessible changes can be evaluated
computationally, guiding synthesis to the most promising
modifications.
Figure 5. Ligand map focusing on ligands N1−N14. See Chart 2 for
ligand labels and Figure S60 for a more comprehensively labeled
version. Red boxes highlight the exceptional PNP ligands A (24) and B
(31) for ethylene oligomerization and the phosphinophosphinines 1
and 2.
Ethylene Oligomerization Catalysis. Ligands 1 and 2
were screened for ethylene oligomerization catalysis under
F
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typical conditions (Scheme 4 and Table 2). The results quoted
for ligand 1 are an average of four reactions and for ligand 2 an
Scheme 4. Catalytic Ethylene Oligomerization
average of two reactions. Data for the individual repeats are
summarized in the Supporting Information, along with
representative GC traces and details of the catalysis procedure
and equipment used. In order to provide benchmarks for
comparison, a reaction was also performed under the same
conditions with no ancillary ligand present, along with another
reaction using the standard PNP ligand Ph2P-N(iPr)-PPh2
(ligand B in Chart 1).
As can be seen from Table 2, entry 1, when no ancillary
ligand is used, Cr(acac)3 and MMAO-3A activator under these
conditions give almost exclusively polymer (98.0 wt %) as the
reaction product, while the trace liquid fraction is a Schulz−
Flory distribution overlaid with some selective trimerization of
ethylene to 1-hexene (see Figure 7a). This highlights the
importance of blank reactions, as Cr(acac)3 is capable of a small
degree of selective trimerization with no ancillary ligand
present. Catalysis using ligands 1 and 2 (entries 3 and 6)
serves to slightly lower the activity, but the selectivity is vastly
improved, with the polymer fraction reduced to 15.1 and 36.5
wt % on average, respectively. Furthermore, the selectivity is
strongly perturbed toward selective trimerization and tetrame-
rization (see Figure 7c, d), although there is a trace of Schulz−
Flory oligomerization still present. However, further examina-
tion reveals that the formation of cyclic products is extremely
high, with cyclic species being observed throughout the product
spectrum (see GC traces in the Supporting Information) and
accounting for much of the Schulz−Flory behavior observed.
This is an interesting observation, as the formation of cyclic
products is believed to stem from the key chromacycloheptane
intermediate (see Scheme 1 in the Introduction), from which 1-
hexene may be liberated or further growth to larger
metallacycles (producing 1-octene in due course) may
occur.11 Isomerization of the chromacycloheptane intermediate
forms a methylcyclopentane ligand that can then insert
ethylene, which ultimately forms longer chain alkyl- and
alkenylcyclopentanes via two different pathways (Scheme
1).12−15 Although small amounts are produced with ligand
B,11 a high percentage of cyclopentane products has been
recently observed for several heteroditopic P,S and homo-
ditopic S,S ligands in Cr-catalyzed ethylene oligomerization.12
1-PPh2-2-SMe-C6H4 was the most active ligand and also
generated a large amount of cyclic products because it was
observed that increasing steric bulk tended to suppress
formation of C10+ oligomers.12 A comparison of the ratios of
alkenylcyclopentanes to alkylcyclopentanes showed ratios close
to 1 for PNP ligand B over many different chain lengths14
(similar to 1), whereas 1-PPh2-2-SMe-C6H4 gave ratios that
varied from 4.2 for C6 to ca. 2 for C8+, which only converged
toward 1 with much longer chain lengths of alkenyl- and
alkylcyclopentanes.12 Interestingly, the pendant ether donor
PNP ligands Ph2PN(R)PPh2, where R = o-OSiMe3C6H4,
G
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Article
Figure 7. Graphs of the liquid fraction product slate obtained for catalysis from Table 2: (a) entry 1, no ligand; (b) entry 2, PNP ligand; (c) entry 3,
ligand 1; (d) entry 6, ligand 2. The green bars represent 1-olefin, blue bars alkyl- and alkenylcyclopentanes, and white bars other structural isomers
and alkanes.
C2H4OSiMe3, C2H4OBn, have also shown high levels of alkyl-
and alkenylcyclopentanes.15 These results indicate that the
nature of the ligand can affect both the barrier heights,
producing either saturated or unsaturated cyclic products, and
the fraction of these products.
CONCLUSIONS
■
This work has shown that 2-phosphinophosphinines are a
useful class of ligands for homogeneous catalysis with extremely
small bite angles and tunable properties. Ligands with
trimethylsilyl substitution at the 6-position, which is readily
achieved synthetically, also allow access to a hydrogen atom in
the 6-position by a simple reaction with HCl, dramatically
changing the steric profile and π-accepting properties of the
phosphinine. Computational mapping of ligand properties,
using Bristol’s ligand knowledge base approach (LKB-PP), has
identified that 2-phosphinophosphinines inhabit an area of
ligand space close to aryl-substituted PNP ligands, suggesting
their application in known catalytic reactions, but with distinct
ligand properties as identified by principal component analysis.
When these ligands were used in the catalytic oligomerization
of ethylene, 1-hexene and 1-octene were observed, as
anticipated by the computational mapping of ligand space,
but in addition we have identified that these ligands promote a
third pathway from the chromacycloheptane intermediate,
producing alkyl- and alkenylcyclopentanes in significant
quantities. The computational analysis also highlights that the
properties of these ligands are responsive to changes in their
substitution pattern and that such fine-tuning can be captured
by, and illustrated on, ligand property maps, supporting the
rational selection of ligands for synthesis and testing in catalysis.
There is therefore the potential in the future to use ligand
Comparison with the benchmark Ph2P-N(iPr)-PPh2 ligand
(entry 2) reveals that ligands 1 and 2 are not competitive with
the industry standard system but rather provide a different
selection of products. In comparing the effect of silyl
substitution, ligand 2 gave a better selectivity to 1-hexene and
1-octene within the liquid fraction, but the activity was lower in
comparison to ligand 1, and the polymer formation level was
doubled. Thus, simple correlation of ligand donor properties
and steric hindrance to catalytic performance is insufficient in
this case, highlighting that ligand design is still a complex
challenge, likely requiring more extensive mechanistic study.
In order to explore the scope for improvement of
performance with ligand 1, a reaction was performed using a
different activation method. Rather than activation under dilute
conditions in situ in the reactor, a concentrated preactivation in
a Schlenk tube was employed (Table 2, entry 4). While the
activity and selectivity within the liquid fraction were essentially
unchanged, the polymer formation level doubled. Another
alternative procedure examined was to halve the chromium
concentration (Table 2, entry 5); this led to a doubling of the
activity and a slight increase in polymer formation, while the
selectivity within the liquid fraction remained unchanged.
H
DOI: 10.1021/acs.organomet.8b00063
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(EI/MS): m/z calcd for C18H16P2 294.0722 [M]+; found 294.0715.
Anal. Calcd for C18H16P2: C, 73.45; H, 5.48. Found: C, 73.39; H, 5.45.
Data match literature values.42
mapping and ligand design to target new products from the
industrially important oligomerization of ethylene.
Synthesis of [Cr(1)(CO)4] (3). [Cr(NBD)(CO)4] (72 mg, 0.273
mmol) and 1 equiv of 1 (101 mg, 0.273 mmol) were dissolved in
toluene (20 cm3) and stirred at 60 °C for 24 h, during which time the
solution turned from yellow-orange to red-orange. Volatile solvent and
NBD were evaporated under reduced pressure, forming an orange oil,
which was washed with petroleum ether to form a solid. This solid was
dissolved in toluene, and orange crystals formed when the solution was
cooled to −25 °C for 24 h. These were isolated by filtration and dried
EXPERIMENTAL SECTION
■
All experiments were performed under dry, oxygen-free N2 using
standard Schlenk-line and glovebox techniques. Dry and degassed
solvents were collected from an MBraun SPS-800 solvent purification
system (toluene, THF, CH2Cl2) or distilled from an appropriate
drying agent under N2: 40−60 petroleum spirit (Na wire), Et2O (Na/
benzophenone). Benzene-d6 was dried over molten potassium, and
CDCl3 was dried over CaH2 and vacuum-distilled prior to use. Air-
sensitive samples for NMR spectroscopy were prepared in NMR tubes
equipped with a J. Young tap. NMR spectra were recorded on Bruker
AVIII400 (400 MHz), AVI400 (400 MHz), and AVIII300 (300 MHz)
1
under vacuum (109 mg, 0.204 mmol, 75%). H NMR (400 MHz,
CDCl3): δ 7.88 (ddd, J = 23.2, 8.5, 1.5 Hz, 1H, 5-H), 7.60 (m, 4H,
PPh2), 7.47 (m, 6H, PPh2), 7.00 (app dt, J = 8.8, 3 Hz, 1H, 4-H), 1.94
(s, 3H, 3-CH3), 0.42 (s, 9H, SiMe3). 13C NMR (100.6 MHz, CDCl3):
δ 229.3 (dd, J = 13.6, 1.6 Hz, CO), 227.5 (dd, J ≈ 11, 3 Hz, CO),
221.9 (dd, J = 17.8, 11.9 Hz, CO), 166.5 (dd, J = 13.4, 4.5 Hz,
phosphinine C), 164.0 (dd, J = 52.0, 20.8 Hz, phosphinine C), 146.1
(app d, J ≈ 11, 5 Hz, phosphinine C), 144.6 (dd, J = 14.9, 3.0 Hz,
phosphinine CH), 133.1 (dd, J = 28.2, 10.4 Hz, ipso-Ph C), 131.6 (d, J
= 11.9 Hz, o-Ph CH), 130.3 (s, p-Ph CH), 128.9 (d, J = 8.9 Hz, 5.94
Hz, m-Ph CH), 127.0 (dd, J = 34.2, 6.0 Hz, phosphinine CH), 21.7
(app t, J = 5.9 Hz, CH3), −0.3 (d, J = 3.0 Hz, SiMe3). 31P NMR (162
spectrometers at 25 °C unless otherwise noted. H and 13C NMR
1
spectra were referenced to internal residual protio-solvent resonances,
11B, 29Si, and 31P NMR spectra were referenced to external samples of
BF3·OEt2, SiMe4, and 85% H3PO4 in H2O, respectively, as 0 ppm.
Mass spectrometry analysis was performed at the EPSRC UK National
Mass Spectrometry Facility at Swansea University using an
Atmospheric Solids Analysis Probe interfaced to a Waters Xevo G2-
S instrument (3 and 5−7) or EI on a MAT95 instrument (2) or at the
University of Edinburgh using EI on a ThermoElectron MAT 900
instrument (4). FTIR was performed on a Thermo Scientific Nicolet
iS5/iD5 ATR spectrometer. Elemental analyses were conducted by Dr.
Brian Hutton using an Exeter CE-440 elemental analyzer at Heriot-
Watt University or by Mr. Stephen Boyer at London Metropolitan
University. The Supporting Information gives details for the catalytic
reactions. All calculations were performed as previously described;78
see the Supporting Information for further details. The following
starting materials were synthesized according to literature procedures:
1·BH3 and 1,37 [M(CO)4(NBD)] (M = Cr, Mo),83 and [W-
(CO)4(COD)].84
2
2
MHz, CDCl3): δ 273.6 (d, JPP = 38.2 Hz, 1-P), 40.5 (d, JPP = 38.2
Hz, 2-P). 29Si NMR (79.5 MHz, CDCl3): δ −1.1 (dd, 2JSiP = 21.6, 4JSiP
= 2.6 Hz, SiMe3). HRMS (ASAP/QTof): m/z calcd for
C25H25CrO4P2Si 531.0403 [M + H]+; found 531.0404. FTIR
(ATR): ν (cm−1) 1895 (CO), 1906 (CO), 2013 (CO). Anal. Calcd
for C25H24O4P2SiCr: C, 56.60, H, 4.56. Found: C, 56.50, H, 4.63.
Synthesis of [Mo(1)(CO)4] (4). [Mo(NBD)(CO)4] (85 mg, 0.280
mmol) and 1 equiv of 1 (103 mg, 0.280 mmol) were reacted as above
at 20 °C for 2 h, yielding the product as pale yellow crystals (48 mg,
0.09 mmol, 30%). 1H NMR (400 MHz, CDCl3): δ 7.93 (ddd, J = 22.6,
8.5, 2.1 Hz, 1H, 5-H), 7.58 (m, 4H. PPh2), 7.47 (m, 6H, PPh2), 7.06
(m, 1H, 4-H), 1.93 (s, 3H, 3−CH3), 0.44 (s, 9H, SiMe3). 13C NMR
(100 MHz, CDCl3): δ 219.1 (dd, J = 31.2, 8.9 Hz, CO), 217.8 (dd, J =
26.8, 8.9 Hz, CO), 210.3 (dd, J ≈ 11, 8 Hz, CO), 168.5 (d, J = 14.9
Hz, phosphinine C), 161.1 (dd, J = 52.0, 19.3 Hz, phosphinine C),
148.0 (dd, J = 11.9, 6.0 Hz, phosphinine C), 143.8 (d, J = 17.8 Hz,
phosphinine CH), 133.0 (dd, J = 28.2, 10.4 Hz, ipso-Ph C), 131.7 (d, J
= 11.9 Hz, o-Ph CH), 130.2 (s, p-Ph CH), 128.9 (d, J = 8.9 Hz, m-Ph
CH), 127.7 (dd, J = 34.2, 4.5 Hz, phosphinine CH), 21.9 (app. t, J =
6.0 Hz, CH3), −0.3 (d, J = 3.0 Hz, SiMe3). 31P NMR (162 MHz,
CDCl3): δ 244.8 (d, 2JPP = 72.8 Hz, 1-P), 18.9 (d, 2JPP = 72.8 Hz, 2-P).
Synthesis of 2-(Diphenylphosphine−borane)-3-methyl-
37
phosphinine (2·BH3). 1·BH3 (1.01 g, 2.65 mmol, 1 equiv) was
dissolved in dry CH2Cl2 (15 cm3), and a 1 M solution of HCl in Et2O
(2.65 cm3, 2.65 mmol) was added. The reaction mixture was stirred at
room temperature for 24 h, and then the volatiles were removed under
reduced pressure, yielding a pale cream-colored solid. The product was
recrystallized from toluene at −25 °C, yielding 2·BH3 as a colorless
crystalline solid (432 mg, 1.40 mmol, 53%).
1H NMR (400 MHz, CDCl3): δ 8.59 (m, 1H, 6-H), 7.85 (m, 1H, 5-
H), 7.73 (m, 4H, o-PPh2), 7.59−7.44 (m, 7H, m- and p-PPh2 and 4-H),
2.55 (s, 3H, 3-CH3), 2.12−0.87 (bm, 3H, BH3). 13C NMR (100 MHz,
CDCl3): δ 156.6 (dd, phosphinine C), 151.4 (dd, phosphinine CH),
150.4 (m, 2 overlapping phosphinine CH), 135.3−135.1 (m, PPh2),
133.8−133.5 (m, PPh2), 131.4 (d, PPh2), 129.8−129.1 (m,
2
4
29Si NMR (80 MHz, CDCl3): δ −0.8 (dd, JSiP = 22.1, JSiP = 2.7 Hz,
SiMe3). MS (EI/MS): m/z 576.0 ([M]+, 6.3%). FTIR (ATR): ν
(cm−1) 1888 (CO), 1926 (CO), 2026 (CO). Anal. Calcd for
C25H24O4P2SiMo: C, 52.27; H, 4.21. Found: C, 52.14; H, 4.36.
Synthesis of [W(1)(CO)4] (5). [W(COD)(CO)4] (110 mg, 0.273
mmol) and 1 equiv of 1 (101 mg, 0.273 mmol) were reacted as above
at 75 °C for 4 days, yielding the product as red crystals (125 mg, 0.188
mmol, 69%). 1H NMR (300 MHz, CDCl3): δ 7.95 (ddd, J = 24.6, 8.4,
2.6 Hz, 1H, 5-H), 7.58 (m, 4H, PPh2), 7.47 (m, 6H, PPh2), 7.06 (m,
1H, 4-H), 1.93 (s, 3H, 3−CH3), 0.44 (s, 9H, SiMe3). 13C NMR (75
MHz, CDCl3): δ 210.0 (dd, J = 31.0, 6.6 Hz, CO), 209.0 (dd, J = 24.3,
7.7 Hz, CO), 203.7 (dd, J = 10.0, 6.6 Hz, CO), 172.0 (d, J = 21.0 Hz,
phosphinine C), 155.8 (dd, J = 44.2, 21.0 Hz, phosphinine C), 149.7
(dd, J = 12.2, 5.5 Hz, phosphinine C), 143.8 (dd, J = 17.7 Hz, 3 Hz
phosphinine CH), 131.9 (dd, J = 34.3, 11.1 Hz, ipso-Ph C), 131.8 (d, J
= 13.3 Hz, o-Ph CH), 130.5 (s, p-Ph CH), 128.9 (d, J = 10.0 Hz, m-Ph
CH), 126.5 (dd, J = 35.4, 5.5 Hz, phosphinine CH), 21.7 (app t, J =
6.6 Hz, CH3), −0.4 (d, J = 3.0 Hz, SiMe3). 31P NMR (121 MHz,
3
phosphinine CH), 128.8 (d, PPh2), 26.1 (d, C(CH3), JC−P = 7.4
Hz). 31P NMR (162 MHz, CDCl3): δ 229.7 (d, 2JP−P = 79.8 Hz, 1-P),
23.0 (bs, 2-P). 11B NMR (128 MHz, CDCl3): δ −36.0 (bs, BH3). Anal.
Calcd for C18H19BP2: C, 70.17; H, 6.22. Found: C, 70.11; H, 6.14.
Synthesis of 2-(Diphenylphosphino)-3-methylphosphinine
(2). Method A. A 1 M solution of HCl in Et2O (1.4 cm3, 1.4 mmol, 1
equiv) was added to a solution of 1 (512 mg, 1.40 mmol, 1 equiv) in
CH2Cl2 (10 cm3), and the reaction mixture was stirred for 24 h. All
volatiles were removed under reduced pressure, and the resulting solid
was dried under high vacuum. The product was then recrystallized
from 40−60 petroleum ether at −25 °C, producing 2 as a colorless
microcrystalline material (319 mg, 1.08 mmol, 77%).
Method B. 2·BH3 (174 mg, 0.565 mmol) was deprotected by
stirring with 0.5 equiv of DABCO (32 mg, 0.282 mmol) in toluene (10
cm3) for 2 days. The mixture was filtered, and then all volatiles were
removed under reduced pressure. The product was then recrystallized
from petroleum ether, yielding 2 as a colorless solid (144 mg, 0.489
mmol, 87%). 1H NMR (300 MHz, 25 °C, C6D6): δ 8.37 (dd, 1H, 6-H,
2
1
CDCl3,): δ 209.6 (d, JPP = 78.0 Hz; 183W satellites dd, JWP ≈ 212,
2JPP = 78 Hz, 1-P), −0.1 (d, J = 78.0 Hz; 183W satellites dd, JWP
=
1
200.3, JPP = 75.4 Hz, 2-P). 29Si NMR (79.5 MHz, CDCl3,): δ −0.7
2
3
2JH−P1 = 40.7 Hz, JH−H = 9.9 Hz), 7.46−7.40 (m, 4H, o-PPh2), 7.30
2
4
(dd, JSiP = 20.9, JSiP = 2.3 Hz, SiMe3). HRMS (ASAP/QTof): m/z
calcd for C25H25O4P2SiW 663.0509 [M + H]+; found 663.0508. FTIR
(ATR): ν (cm−1) 1883 (CO), 1914 (CO), 2021 (CO). Anal. Calcd for
C25H24O4P2SiW: C, 45.34; H, 3.65. Found: C, 45.49; H, 3.55.
(m, 1H, 5-H), 7.06−7.04 (m, 6H, m- and p-PPh2), 6.94−6.89 (m, 1H,
4-H) 2.42 (s, 3H, 3−CH3). 31P NMR (121 MHz, 25 °C, C6D6): δ
224.9 (d, 1-P, 2JP−P = 31.2 Hz), −7.6 (d, 2-P, 2JP−P = 31.2 Hz). HRMS
I
DOI: 10.1021/acs.organomet.8b00063
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Organometallics
Article
Synthesis of [Cr(2)(CO)4] (6). [Cr(NBD)(CO)4] (109 mg, 0.427
mmol) and 1 equiv of 2 (126 mg, 0.427 mmol) were reacted as above
at 65 °C for 24 h, yielding the product as red crystals (62 mg, 0.135
mmol, 32%). 1H NMR (400 MHz, C6D6): δ 7.71−7.61 (m, 1H, 6-H),
7.55−7.51 (m, 4H, o-PPh2), 7.13−6.99 (m, 7H, 5-H and m,p-PPh2),
6.37 (bs, 1H, 4-H), 1.52 (s, 3H, 3-CH3). 13C NMR (100 MHz, C6D6,):
δ 230.3 (d, J = 13.6 Hz, CO), 228.4 (dd, J = 11.2, 3.2 Hz, CO), 223.0
(dd, J = 17.6, 11.2 Hz, CO), 166.9 (dd, J = 15.2, 1.6 Hz, phosphinine
C), 149.6 (dd, J = 28.0, 20.0 Hz, phosphinine CH), 146.9 (dd, J =
12.8, 7.2 Hz, phosphinine C), 140.6 (dd, J = 16.0, 3.2 Hz, phosphinine
CH), 133.5 (dd, J = 28.8, 10.4 Hz, ipso-Ph C), 132.1 (d, J = 12.0 Hz,
PPh2), 130.8 (d, J = 2.4 Hz, PPh2), 129.5 (d, J = 9.6 Hz, PPh2), 128.6−
128.2 (m, phosphinine CH and C6D6 overlapping), 21.9 (t, J = 7.2 Hz
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for N.F.: natalie.fey@bristol.ac.uk.
*E-mail for M.J.H.: martin.hanton@eu.sasol.com.
*E-mail for S.M.M.: s.mansell@hw.ac.uk.
ORCID
2
CH3). 31P NMR (162 MHz, C6D6): δ 250.3 (d, JPP = 33.8 Hz, 1-P),
Martin J. Hanton: 0000-0002-1026-9199
Stephen M. Mansell: 0000-0002-9332-3698
2
41.8 (d, JPP = 33.8 Hz, 2-P). HRMS (ASAP/QTof): m/z calcd for
C22H17CrO4P2 459.0007 [M + H]+; found 459.0001. FTIR (ATR): ν
(cm−1) 1864 (CO), 1898 (CO), 2003 (CO). Anal. Calcd for
C22H16CrO4P2: C, 57.66; H, 3.52. Found: C, 58.08; H, 3.44.
Author Contributions
∥R.J.N. and A.S. contributed equally to this work.
Synthesis of [Mo(2)(CO)4] (7). [Mo(NBD)(CO)4] (20 mg, 0.068
mmol) and 1 equiv of 2 (20 mg, 0.068 mmol) were dissolved in
toluene (2 cm3) and stirred at 20 °C for 2 h, during which time a
yellow precipitate was observed. All volatiles were removed under
reduced pressure, and the resulting solid was dried under high vacuum,
yielding the product as an analytically pure pale yellow microcrystalline
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Sasol Technology for permission to publish
this work and the EPSRC UK National Mass Spectrometry
Facility at Swansea University and the EPSRC UK National
Crystallography Service at the University of Southampton for
assistance with sample analysis. Financial support is gratefully
acknowledged from the EPSRC (DTP studentship to R.J.N.),
the Royal Society (Research grant: RG130436), and Heriot-
Watt University as well as the UK Catalysis Hub Consortium
(funded by EPSRC grants EP/K014706/2, EP/K014668/1,
EP/K014854/1, EP/K014714/1, and EP/M013219/1) for
providing travel funding to S.M.M. and R.J.N. Additional
research data supporting this publication are available from
Heriot-Watt University’s research data repository at DOI:
10.17861/0373af39-4b4c-4916-b6c3-c01dd6f4f362.
1
powder (20 mg, 0.041 mmol, 60%). H NMR (400 MHz, C6D6): δ
7.70−7.60 (m, 1H, 6-H), 7.49 (bs, 4H, o-PPh2), 7.09−6.97 (m, 7H, 5-
H and m,p-PPh2), 6.37 (bs, 1H, 4-H), 1.52 (s, 3H, 3−CH3). 13C NMR
(100 MHz, C6D6): δ 218.8 (dd, J = 32.0, 9.6 Hz, CO), 217.5 (dd, J =
26.4, 8.8 Hz, CO), 210.3 (dd, J ≈ 11, 8 Hz, CO), 168.4 (dd, J = 16.8,
1.6 Hz, phosphinine C), 148.0 (dd, J = 12.0, 5.6 Hz, phosphinine CH),
146.4 (dd, J = 25.6, 19.2 Hz, phosphinine C), 139.1 (dd, J = 16.0, 2.4
Hz, phosphinine CH), 132.8 (dd, J = 28.8, 10.4 Hz, ipso-Ph C), 131.5
(d, J = 12.8 Hz, PPh2), 130.0 (d, J = 1.6 Hz, PPh2), 128.8 (d, J = 9.6
Hz, PPh2), 128.3−127.5 (m, phosphinine CH and C6D6 overlapping),
21.4 (s, 3-H). 31P NMR (162 MHz, C6D6): δ 222.7 (d, 2JPP = 71.1 Hz,
2
1-P), 20.0 (d, JPP = 71.1 Hz, 2-P). HRMS (ASAP/QTof): m/z calcd
for C22H17MoO4P2 504.9661 [M + H]+; found 504.9666. FTIR
(ATR): ν (cm−1) 1867 (CO), 1910 (CO), 2017 (CO). Anal. Calcd for
C22H16MoO4P2: C, 52.59; H, 3.21. Found: C, 52.46; H, 3.17.
X-ray Crystallography. Single crystals of the samples were
covered in inert oil and placed under the cold stream of a Bruker X8
APEXII four-circle diffractometer, except for 4, data for which were
collected by the EPSRC National Crystallography Service on a
XtaLAB AFC12 (RCD3) Kappa single diffractometer. Exposures were
collected using Mo-kα radiation (λ = 0.71073). Indexing, data
collection, and absorption correction were performed using the
APEXII suite of programs.85 Structures were solved using direct or
Patterson methods (SHELXS or SHELXT)86 and refined by full-
matrix least squares (SHELXL)86 interfaced with the program
OLEX287 (Table S1). There was a disordered CH2Cl2 solvate
molecule in 8 that was successfully modeled over two positions.
CCDC deposition numbers: 1584358−1584363.
REFERENCES
■
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ASSOCIATED CONTENT
* Supporting Information
■
(5) McGuinness, D. S. Olefin Oligomerization via Metallacycles:
Dimerization, Trimerization, Tetramerization, and Beyond. Chem. Rev.
2011, 111, 2321−2341.
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(7) Breuil, P.-A. R.; Magna, L.; Olivier-Bourbigou, H. Role of
Homogeneous Catalysis in Oligomerization of Olefins: Focus on
Selected Examples Based on Group 4 to Group 10 Transition Metal
Complexes. Catal. Lett. 2015, 145, 173−192.
(8) Wass, D. F. Chromium-catalysed ethene trimerisation and
tetramerisation-breaking the rules in olefin oligomerisation. Dalton
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(9) Agapie, T. Selective ethylene oligomerization: Recent advances in
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S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00063.
Spectroscopic data for 2, 2·BH3, and 3 − 7, additional
crystallographic information, further details on the
catalysis, reactivity studies of proligand 1 and complex
3 with MAO, and full computational details. (PDF)
Percent contribution to variation (XLSX)
Accession Codes
CCDC 1584358−1584363 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
J
DOI: 10.1021/acs.organomet.8b00063
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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