Ligand effects in chromium diphosphine catalysed olefin co-trimerisation and diene trimerisation

Article abstract of DOI:10.1039/b913302j

A series of symmetric and unsymmetric N,N-bis(diarylphosphino)amine ('PNP') ligands (Ar₂PN(R)PNAr′₂: R = Me, Ar₂ = o-anisyl, Ar′₂ = Ph, 1, R = Me, Ar₂ = o-tolyl, Ar′₂ = Ph, 2, R = Me, Ar₂ = Ph(o-ethyl), Ar′₂ = Ph, 3, R = Me, Ar₂ = Ar′₂ = o-anisyl, 4, R = ⁱPr, Ar₂ = Ar′₂ = Ph, 5) and symmetric N,N′-bis(diarylphosphino)dimethylhydrazine ('PNNP') ligands (Ar₂PN(Me)N(Me)PAr₂: Ar₂ = o-tolyl, 6, Ar ₂ = o-anisyl, 7) have been synthesised. Catalytic screening for ethene/styrene co-trimerisation and isoprene trimerisation was performed via the in situ complexation to [CrCl₃(THF)₃] followed by activation with methylaluminoxane (MAO). PNNP catalytic systems showed a significant increase in activity and selectivity over previously reported PNP systems in isoprene trimerisation. Comparing the symmetric and unsymmetric variants in ethene and styrene co-trimerisation resulted in a switch in selectivity, an unsymmetric catalytic (o-anisyl)₂PN(Me)PPh ₂ (1) ligand system affording unique incorporation of two styrenic monomers into the co-trimer product distribution differing from the familiar two ethene and one styrene ω-substituted alkenes. Complexes of the type [(diphosphine)Cr(CO)₄] 8-11 were also synthesised, the single-crystal X-ray diffraction of which are reported. We propose the mechanisms of these catalytic transformations and an insight into the effect of the ligand series on the chromacyclic catalytic intermediates. The Royal Society of Chemistry.

Full text of DOI:10.1039/b913302j

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www.rsc.org/dalton | Dalton Transactions
Ligand effects in chromium diphosphine catalysed olefin co-trimerisation
and diene trimerisation†
Lucy E. Bowen, Manutsavin Charernsuk, Thomas W. Hey, Claire L. McMullin, A. Guy Orpen and
Duncan F. Wass*
Received 3rd July 2009, Accepted 21st September 2009
First published as an Advance Article on the web 16th October 2009
DOI: 10.1039/b913302j
A series of symmetric and unsymmetric N,N-bis(diarylphosphino)amine (‘PNP’) ligands
(Ar2PN(R)PNAr¢2: R = Me, Ar2 = o-anisyl, Ar¢2 = Ph, 1, R = Me, Ar2 = o-tolyl, Ar¢2 = Ph, 2, R =
i
Me, Ar2 = Ph(o-ethyl), Ar¢2 = Ph, 3, R = Me, Ar2 = Ar¢2 = o-anisyl, 4, R = Pr, Ar2 = Ar¢2 = Ph, 5) and
symmetric N,N¢-bis(diarylphosphino)dimethylhydrazine (‘PNNP’) ligands (Ar2PN(Me)N(Me)PAr2:
Ar2 = o-tolyl, 6, Ar2 = o-anisyl, 7) have been synthesised. Catalytic screening for ethene/styrene
co-trimerisation and isoprene trimerisation was performed via the in situ complexation to
[CrCl3(THF)3] followed by activation with methylaluminoxane (MAO). PNNP catalytic systems
showed a significant increase in activity and selectivity over previously reported PNP systems in
isoprene trimerisation. Comparing the symmetric and unsymmetric variants in ethene and styrene
co-trimerisation resulted in a switch in selectivity, an unsymmetric catalytic (o-anisyl)2PN(Me)PPh2 (1)
ligand system affording unique incorporation of two styrenic monomers into the co-trimer product
distribution differing from the familiar two ethene and one styrene w-substituted alkenes. Complexes of
the type [(diphosphine)Cr(CO)4] 8–11 were also synthesised, the single-crystal X-ray diffraction of
which are reported. We propose the mechanisms of these catalytic transformations and an insight into
the effect of the ligand series on the chromacyclic catalytic intermediates.
found in apple coatings, and 7,11-dimethyl-3-methylene-1,6,10-
Introduction
dodecatriene (trans-b-Farnesene), an essential oil. Hydrogenated
In recent years, catalysts have emerged capable of the selective
sesquiisoprenoid motifs are also extremely important in nature,
trimerisation of ethene to commercially valuable 1-hexene via
the side chain of vitamin E being an excellent example.
We report here a more detailed study of the systems from
catalysts based on chromium complexes with ligands of the type
our preliminary communications, in which a wider range of
ligands and reaction conditions have been explored. We also
with productivity figures over an order of magnitude better
report structural studies of model chromium complexes for these
a distinctive metallocyclic mechanism.1 In 2002, we reported
Ar2PN(Me)PAr2 (Ar = ortho-methoxy-substituted aryl group)
than previous systems.2 This unprecedented performance led to
interest both from a mechanistic viewpoint and in ligand struc-
tural modification, the most significant subsequent development
catalysts.
Results and discussion
being the report from Bollmann and co-workers that relatively
minor changes to the ligand structure and reaction conditions
can lead to ethene tetramerisation rather than trimerisation.3
Only limited studies have been conducted into the selective
oligomerisation of other olefinic substrates until we recently
reported the trimerisation of isoprene and the co-trimerisation
of ethene and styrene to primarily linear trimers.4,5 These two
novel chromium PNP systems showed far superior results to
previous catalysts in both activity and selectivity, the latter being
accounted for by the Briggs’ type metallacyclic mechanism by
which our catalysts operate.6 These systems are of potential
interest allowing the selective syntheses of isoprenoid products and
w-substituted alkenes from simple substrates. The former species,
in particular, are a vast and diverse class of natural products,
such as 3,7,11-trimethyl-1,3,6,10-dodecatetraene (a-Farnesene),
Ligand synthesis
A library of symmetric and unsymmetric PNP and symmetric
PNNP ligands were synthesised (Fig. 1). Ligands 1–3 are novel
and ligands 4–7 have been previously reported.2,3,7,8
Fig. 1 Diphosphine ligands.
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK
BS8 1TS. E-mail: duncan.wass@bristol.ac.uk
† CCDC reference numbers 739221–739224. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b913302j
Ligand 4 is a benchmark for ethene trimerisation to 1-hexene
and ligand 5 for tetramerisation to 1-octene.9
560 | Dalton Trans., 2010, 39, 560–567
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The Royal Society of Chemistry 2010
©

The ligands were synthesised by one of two common meth-
ods (Scheme 1). PNNP species were obtained by the reac-
tion of a Grignard reagent with the commercially available
Cl2PN(Me)N(Me)PCl2. Unsymmetric PNP ligands were synthe-
sised via the controlled addition of one equivalent of substituted
chlorophosphine to methylamine followed in situ by the addition
of a further equivalent of diphenylchlorophosphine. Intermediate
species of the type Ar2PN(Me)H can be isolated and purified
although this is not critical. These unsymmetric ligands gave
substituted-dodecatetraenes; this differs from the natural 2,6,11-
trimethyl-substituted-dodecatetraene products in the position of
one methyl group. We postulated that the relative stability of
tentative allyl catalytic intermediates would favour the head-to-
tail, tail-to-tail trimerisation which give rise to the unnatural
products, as shown in Fig. 2. With this in mind, we hypothesised
that an unsymmetrical ligand may destabilise the more bulky allyl
species on one side of the metallacycle, leading to the desired
product. We were also keen to explore a wider range of reaction
parameters to better understand the optimum conditions for
trimerisation.
1
a characteristic set of doublets in their 31P{ H} NMR spectra,
typically occurring in the range from 40–70 ppm. PNP ligands (1–
3) were obtained in low to moderate yields (22–36%), in general
high yields being sacrificed for high purity. The yields of the PNNP
ligands varied in a similar range from 19–52%.
Scheme 1 PNP and PNNP ligand synthesis.
Isoprene trimerisation
Fig. 2 Postulated trimerisation mechanism.
We recently reported the selective trimerisation of isoprene to form
terpenes (eqn (1)). This was far more productive [826 g (gCr h)-1]
than previous nickel based catalysts [6 g (gCr h)-1].10,11
Optimisation reactions using ligand 4
(1)
Ligand 4 was used as a benchmark, the variables of temperature
and isoprene concentration being investigated (Table 1).
Unfortunately, although excellent selectivity of isoprene trimers
could be achieved, the specific products obtained were a mixture
of cyclic species (minor product) and linear 2,6,11-trimethyl-
Generally, as the concentration of isoprene is increased (runs
1–4), the productivity increases and the total percentage of
trimer is approximately constant, albeit with a slight increase in
Table 1 Trimerisation of isoprene using ligand 4
Product distribution/wt%
Trimers
Wt% within trimer fraction
Run
Isoprene/mol dm-3
Temper-ature/◦C
Prod.a
Total trimer
Linearb
Cyclicb
Othere
1
2
3
4
5
6
0.5
2.3
4.5
6.8
6.8
6.8
6.8
6.8
6.8
70
70
70
70
57
45
25
70
70
73
82
87
69
79
75
68
100
0
32
61
77
70
75
88
73
—
—
68
39
23
30
25
12
27
—
—
18
13
31
21
25
32
0
510
530
826
400
349
19
7
8c
9d
0
0
—
—
0
Note: we have presented selectivities as wt% within the trimer fraction to facilitate comparison; this is a different format to our preliminary communications
(ref. 4 and 5). Conditions unless stated otherwise: 20 mmol ligand, 20 mmol [CrCl3(THF)3], 300 equivalents of MAO, toluene diluent, 44 mL total volume;
temperature controlled by external bath; 1 h run time.a Productivity/g (gCr h)-1. b Overall selectivity. c No [CrCl3(THF)3] added. d No MAO added. e Largely
(>80%) tetramers.
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 560–567 | 561
©

higher oligomers. The ratio of linear to cyclic trimers increases
with isoprene concentration. This ratio depends on the rate of
reductive elimination of the intermediate illustrated in Fig. 2 to
give the cyclic vs. linear isomers, suggesting that the incoming
substrate may facilitate this elimination in the latter case. It
is noteworthy (runs 4–7) that higher temperatures in general
are required for good rates in isoprene trimerisation compared
to ethene trimerisation; at 25 ◦C only low activity is observed
here whereas ligand 4 is extremely active with ethene even at
this temperature. The total trimer fraction decreases at higher
temperature at the expense of higher oligomers as does the ratio
of linear to cyclic trimers.
Ethene and styrene co-trimerisation
In 2006, we reported the co-trimerisation of styrene and ethene
using chromium PNP catalysts, co-trimers formed from one
styrene and two ethene units being formed exclusively (eqn (2)).5
The isomers obtained were a function of ligand structure, with
the linear isomer being favoured by 4 and branched isomers in
all other cases. Based on product analysis, a mechanistic scheme
was proposed in which 2,1-regiochemistry of styrene insertion is
favoured.
(2)
Using these optimised conditions (run 4), ligands 1–7,
1,2-bis(diphenylphosphino)ethane and bis(diphenylphosphino)-
methane were tested (Table 2). PNP ligand derivatives demon-
strate similar productivity with more bulky ligands in general
giving slightly better results (runs 10–14); ligand 4 remains
the most active. A more marked increase in productivity is
seen when comparing PNP to PNNP ligand systems (runs
15 and 16). The increased bite angle of these ligands (vide
infra) is an obvious point of difference and is a possible cause
for this enhanced performance. Surprisingly, dppe and dppm
were inactive for isoprene trimerisation under these conditions
(runs 17 and 18). As in ethene tri/tetramerisation, it seems
that ligands with nitrogen backbone motifs have a distinctive
performance, something we have attributed to the delocalisa-
tion of the potential nitrogen lone pair over the entire ligand
chelate. Unsymmetric PNP ligands (runs 10–12) showed very high
selectivities towards linear trimers compared to the symmetric
PNP analogues (runs 13 and 14). However, the best performance
we have observed to date is for ligand 7, a PNNP structure
with pendant MeO groups; this combines excellent productivity
with outstanding selectivity to trimeric products under these
conditions.
Using the optimised conditions from our initial communication,
we explored a broader range of ligands described here (Table 3).
The most active ligand was the previously reported symmetric
o-methoxy PNP ligand 4 which was also the only ligand to show
a major selectivity towards linear trimers (run 22). In other cases,
even when two MeO groups are present (ligand 1, run 19) the
branched trimer 3-phenylhexene resulted as observed before; this
isomer is consistent with 2,1-insertion of styrene. Somewhat sur-
prisingly, the PNNP systems showed no co-trimerisation activity
under these conditions. Of particular interest is the result for the
unsymmetrical ligand 1 (run 19); this is the only system we have
yet found which allows the incorporation of two styrene and one
ethene units, albeit in low yield. The reason for this change in
selectivity is not yet understood, but it is noteworthy that a simple
argument for reduced ligand sterics allowing more incorporation
of the bulkier styrene substrate cannot hold; the less bulky ligand 5,
and indeed the sterically equivalent 3, do not show this selectivity.
Synthesis and structural study of model [Cr(CO)4(diphosphine)]
complexes
In order to probe the structure of these ligands in greater
depth, we synthesised and structurally characterised a selection of
complexes of the type [Cr(CO)4(diphosphine)]. We have previously
reported that such model systems are useful, in that a combination
of structural study and IR spectroscopy can benchmark these
ligands against the growing portfolio of related complexes for
Analysis of the trimer products by a combination of NMR
spectroscopy and GC compared to standards reveals that in every
case the 2,6,11-trimethyl dodecene/ane products are obtained;
clearly the propensity for head-to-tail, tail-to-tail trimerisation is
not easily overcome by the ligands explored here.
Table 2 Isoprene trimerisation with various ligands
Product distribution (wt%)
Trimers
Wt% within trimer fraction
Run
Ligand
Prod./g (gCr h)-1
Total trimer
Linear
Cyclic
Other
10
11
12
13
14
15
16
17
18
1
585
669
346
826
298
1483
1103
0
98
86
94
79
95
65
100
0
99
87
86
70
74
87
87
0
1
2
14
6
21
5
35
0
2
13
14
30
26
13
13
0
3
4
5
6
7
dppe
dppm
0
0
0
0
0
0
Conditions unless stated otherwise: 20 mmol ligand, 20 mmol [CrCl3(THF)3], 300 equivalents of MAO, 6.8 mol dm-3 substrate concentration, toluene
diluent, 44 mL total volume; 70 ◦C controlled by external bath; 1 h run time.
562 | Dalton Trans., 2010, 39, 560–567
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The Royal Society of Chemistry 2010
©

Table 3 Ethene and styrene co-trimerisation
Product distribution (wt%)
2 Ethene + styrene
Run
Ligand
Prod./g (gCr h)-1
Linear
Branched
Ethene +2 styrene
19
20
21
22
23
24
25
26
27
1
2617
127
184
7926
3791
0
0
0
0
4
1
1
85
5
0
0
0
0
90
99
99
15
95
0
0
0
0
6
0
0
0
0
0
0
0
0
2
3
4
5
6
7
dppe
dppm
Conditions unless stated otherwise: 20 mmol ligand, 20 mmol [CrCl3(THF)3], 300 equivalents of MAO, 1 bar ethene, 6.8 M substrate concentration,
toluene diluent, 44 mL total volume; 25 ◦C controlled by external bath; 1 h run time.
trimerisation-active ligands. A bonus is that such complexes may
also be used as catalyst precursors.12
The complexes were synthesised in reasonable yields (27–52%)
by treatment of the ligand with [Cr(CO)6] in toluene (Scheme 2).
IR spectroscopy is unremarkable with CO stretching frequencies
in the range of previous derivatives (see Experimental).
Scheme 2 Synthetic route to complexes 4–6.
Fig. 3 Thermal ellipsoid plot of 8. Displacement ellipsoids are shown at
Crystals containing 8–11 were grown from saturated
the 50% probability level and all hydrogen atoms have been removed for
clarity.
dichloromethane/methanol solutions, and each crystallised in the
space group P21/c. The ortho-tolyl substituted diphosphines (9
and 10) crystallised as solvates: 9·0.5MeOH has one methanol
molecule per two crystallographically independent molecules of
complex, with the solvent having 50% static disorder, whilst
10·0.75CH2Cl2 has 75% occupancy of the dichloromethane.
The structures of the [Cr(CO)4{PNP}] complexes 8 and 9 are
shown in Fig. 3 and 4 and selected bond lengths and angles given
in Table 4. In these complexes the Cr(0) centre is coordinated
by four CO ligands and chelated by the bidentate ligands 1 and
2, which have bite angles of 68.37(2)◦ and 67.88(4)◦/68.03(4)◦
respectively. The conformations of the ortho substituted aryls
o-C6H4R (R = OMe, Me) can be helpfully described by their
Cr–P–Cipso–Cortho torsion angles, which are either gauche (ca. 50◦)
or anti (ca. 180◦). In the asymmetric methoxy PNP ligand in 8
the o-anisyl substituents adopt g-g- conformations (Cr–P–C–C
torsion angles -57.8(2)◦, -58.4(2)◦, see Fig. 1). The asymmetric
o-tolyl PNP ligand in 9 has an ag- conformation (torsion
angles 176.8(3)◦, -79.1(3)◦ and 175.4(3)◦, -80.0(3)◦ in the two
independent molecules of 9 present).
Fig. 4 Thermal ellipsoid plot of one of the 2 independent molecules of
9. Displacement ellipsoids are shown at the 50% probability level and all
The structures of the symmetrically ortho-substituted
[Cr(CO)4{PNNP}] complexes 10 and 11 are shown in Fig. 5 and
6 and selected bond lengths and angles given in Table 5. In these
complexes the Cr(0) centre is coordinated by four CO ligands
solvent and hydrogen atoms have been removed for clarity.
and chelated by the bidentate ligands 6 and 7 which have bites
angles of 79.87(2)◦ and 80.63(1)◦ respectively. Both the o-tolyl
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 560–567 | 563
©

◦
◦
˚
˚
Table 4 Selected bond distances (A) and angles ( ) for 8 and 9·0.5MeOH
Table 5 Selected bond distances (A) and angles ( ) for 10·0.75CH2Cl2
and 11
8
9·0.5MeOH
10·0.75CH2Cl2
11
Cr1–P1
Cr1–P2
P1–N1
P2–N1
N1–C1
P1–C2
2.3535(6)
2.3328(7)
1.6972(19)
1.6992(19)
1.464(3)
1.821(2)
1.828(2)
1.827(2)
1.825(2)
68.37(2)
101.66(10)
2.3756(11)
2.3405(11)
1.707(3)
1.704(3)
1.466(5)
1.842(4)
1.829(4)
1.822(4)
1.815(4)
67.88(4)
101.08(16)
2.3753(11)
2.3429(11)
1.709(3)
1.697(3)
1.470(5)
1.839(4)
1.830(4)
1.831(4)
1.825(4)
68.03(4)
101.58(15)
Cr1–P1
Cr1–P2
P1–N1
P2–N2
N1–N2
N1–C1
N2–C2
P1–C3
P1–C10
P2–C17
P2–C24
P1–Cr–P2
P1–N1–N2–P2
2.3858(5)
2.3924(6)
1.7171(16)
1.7342(16)
1.438(2)
1.459(2)
1.475(2)
1.8472(19)
1.835(2)
1.8385(19)
1.837(2)
79.873(19)
-51.34(14)
2.3699(4)
2.3536(4)
1.7194(12)
1.7349(12)
1.4427(16)
1.4647(18)
1.4723(18)
1.8292(15)
1.8414(15)
1.8316(15)
1.8276(15)
80.627(14)
53.43(11)
P1–C9
P2–C16
P2–C22
P1–Cr–P2
P1–N1–P2
What is clear from this study is that there is a very subtle
interplay of factors which influence activity and selectivity in these
systems. The expected larger bite angle of the PNNP derivatives
is the obvious factor determining the excellent performance of
these systems for isoprene trimerisation. However, why these
systems are then less successful for ethene/styrene co-trimerisation
is more difficult to rationalise. Similarly, the factors resulting
in the unique selectivity of ligand 1 in co-trimerisation are not
illuminated. Studies are ongoing to more fully map structure–
activity relationships in this regard.
Conclusions
Fig. 5 Thermal ellipsoid plot of 10. Displacement ellipsoids are shown
at the 50% probability level and all solvent and hydrogen atoms have been
removed for clarity.
A series of symmetric and unsymmetric PNP and PNNP ligands
have been synthesised. Catalytic screening for ethene/styrene
co-trimerisation and isoprene trimerisation reveals a number of
interesting trends. PNNP catalytic systems show a significant
increase in activity and selectivity over previously reported PNP
systems in isoprene trimerisation. Comparing the symmetric
and unsymmetric variants in ethene and styrene co-trimerisation
resulted in a switch in selectivity, an unsymmetric catalytic
(o-anisyl)2PN(Me)PPh2 ligand system giving, for the first time,
incorporation of two styrenic monomers into the cotrimer product
distribution. Structural studies of model complexes of the type
[(diphosphine)Cr(CO)4] give some insights and are helping us
to unravel the very subtle interplay of effects which determine
performance in these systems.
Experimental
General comments
Fig. 6 Thermal ellipsoid plot of 11. Displacement ellipsoids are shown
at the 50% probability level and all hydrogen atoms have been removed for
clarity.
All procedures were carried out under an inert (N2) atmosphere
using standard Schlenk techniques or an inert atmosphere (Ar)
glovebox. Chemicals were obtained from Sigma Aldrich, Fischer
Scientific, Acros or Alfa Aesar and used without further purifi-
cation unless otherwise stated. All solvents were purified using
an anhydrous engineering Grubbs-type solvent system. Infra-
red spectra were recorded on a Perkin-Elmer 1600 series FTIR
Spectrometer in methylene chloride. Bands are characterised as
(10) and o-anisyl (11) substituted diphosphines show g+aag- con-
formations (torsion angles 68.9(2)◦, 171.3(1)◦, 167.8(1)◦, -82.4(2)◦
and 82.2(1)◦, -174.4(1)◦, -170.1(1)◦, -62.6(1)◦). The backbone of
the PNNP diphosphine has one planar nitrogen (N1), while the
other (N2) has pyramidal geometry with the C2 methyl group
axial. Combined with the conformation of the ortho substituents
this reduces steric interaction between the aryl groups and the
ligand backbone.
strong (s), moderate (m), weak (w) and/or broad (br). H, 13C
1
1
{ H} and 31P NMR spectra were recorded on a Jeol ECP 300,
Lamda 300 or Varian 400 spectrometers at 400 or 300, 100.5
564 | Dalton Trans., 2010, 39, 560–567
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The Royal Society of Chemistry 2010
©

1
1
and 121 MHz respectively at room temperature. H NMR and
mass spectrum: m/z = 460.1592 [M + H]+ (calcd 460.1595). H
13C { H} chemical shifts are referenced relative to the residual
NMR (CDCl3): d = 2.45 (dd, 3JHP = 4.13 Hz, 3JHP = 2.66 Hz, 3H,
1
solvent resonances in the deuterated solvent, and 31P { H} NMR
NCH3), 3.69 (s, 6H, OCH3), 7.33–7.47 (m, 18H, ArH). 13C NMR
1
2
spectra are referenced to high frequency of 85% H3PO4. Mass
spectra were recorded on a VG Analytical Autospec or Bru¨ker
Daltonics Apex IV spectrometers. Oligomerisation products were
analysed by GC-FID and GC-MS, using a Varian L3800, using
a Varian WC07 fused silica capillary column, 25 m ¥ 0.25 mm,
ID coating CP-Sil 5CB, DF = 0.25. Co-oligermerisation method:
40 ◦C to 80 ◦C at 2 ◦C min-1, then 80 ◦C to 250 ◦C at 10 ◦◦C min-1.
(CDCl3): d = 32.62 (d, JCP = 7.79 Hz, NCH3), 54.1 (s, OCH3),
108.9 (s, CH), 119.3 (s, CH), 126.9 (d, 1JCP = 6.23 Hz, CP), 127.5
(s, CH), 129.1 (s, CH), 131.4 (s, CH), 131.7 (s, CH), 131.9 (s, CH),
138.0 (dd, 1JCP = 18.68 Hz, 3JCP = 11.68 Hz, CP), 159.6 (d, 2JCP
=
17.3 Hz, COCH3). 31P{ H} NMR (CDCl3): d = 55.7 (d, 2JPNP
=
1
284 Hz, PAr2), 69.5 (d, 2JPNP = 284 Hz, PAr2).
◦
◦
Synthesis of {2-C6H4(Me)}2PN(Me)P(C6H5)2 (2). This was
synthesised in the same way as 1 only using ortho-tolyl magnesium
bromide in place of ortho-anisyl magnesium bromide. The product
was obtained as a white powder (2.48 g, 5.8 mmol, 22.3%). ESI
Isoprene oligomerisation method: 40 C to 80 C at 2 C min-1,
then 80 ◦C to 300 ◦C at 10 C min-1. Microanalyses were
◦
carried out by the Microanalytical Laboratory of the School
of Chemistry at the University of Bristol. The syntheses of 413
and 53,7,14 were performed according to literature procedures.
Cl2PN(Me)N(Me)PCl2 is commercially available but may be very
conveniently prepared from low-cost starting materials as reported
by Reddy and Katti.15
mass spectrum: m/z = 428 [M + H]+, 450 [M + Na]+. H NMR
1
3
(CDCl3): d = 2.32 (s, 6H, ArCH3), 2.49 (dd, JHP = 3.39 Hz,
3JHP = 2.48 Hz, 3H, NCH3), 6.98–7.44 (m, 18H, ArH). 13C NMR
(CDCl3): d = 21.3 (d, 3JCP = 20.7 Hz, CH3), 33.0 (s, NCH3), 125.5
1
(s, CH), 128.2 (d, JCP = 5.2 Hz, CP), 128.9 (s, CH), 130.6 (s,
Synthesis of dichloro(diethylamine)phosphine. Diethylamine
(100 mL, 0.96 mol) was added dropwise over 3 h to a stirred
solution of PCl3 (42 mL, 0.48 mol) in ether (800 mL) at -78 ◦C. The
solution was then filtered via a cannula to give a colourless filtrate
and white crystalline Et2NH·HCl. The ether was removed by
distillation and the product was obtained by vacuum (60 ◦C at
CH), 131.8 (s, CH), 132.6 (s, CH),132.9 (s, CH), 136.6 (s, CH),
136.8 (s, CH), 138.6–138.9 (m, CH), 141.3 (d, 1JCP = 27.1 Hz, CP).
31P{ H} NMR (CDCl3): d = 58.6 (d, 2JPNP = 297 Hz, PAr2), 72.5
1
2
(d, JPNP = 296 Hz, PAr2). Elemental analysis: C27H27NP2 calcd
(%) C 75.86, H 6.37, N 3.28, found (%) C 75.93, H 6.50, N 3.30.
1
3
Synthesis of {2-C6H4(ethyl)}2PN(Me)P(C6H5)2 (3). Again, the
same method as for 1 was followed, only using ortho-ethylphenyl
magnesium bromide in place of ortho-anisyl magnesium bromide.
In this case an oily product was initially obtained, which only after
repeated trituration with methanol yielded 3 as a white powder
(1.70 g, 3.72 mmol, 31%).
0.6 Torr) distillation. H NMR (CDCl3): d = 1.18 (t, JHH
=
7.14 Hz, 6H, CH3), 3.34 (dq,2JPH = 14.0 Hz, 3JHH = 6.2 Hz, 4H,
CH2). 31P{ H} NMR (CDCl3): d = 162.9 (s, PCl2).
1
Synthesis of {2-C6H4(OMe)}2PN(Me)P(C6H5)2 (1). A solu-
tion of o-bromoanisole (17 mL, 0.136 mol) in ether (60 mL)
was added dropwise to magnesium turnings (5.27 g, 0.219 mol)
in ether (20 mL). Once addition was complete, the mixture
was heated under reflux for 2 h to give a brown solution.
The Grignard reagent, ortho-methoxyphenylmagnesium bromide
(0.136 mol) was transferred to a pressure equalising dropping
funnel and added dropwise to a stirred solution of dichlorodi-
ethylaminephosphine (9.89 mL, 68 mmol) in ether (30 mL).
After 2 h the solution was filtered via a cannula and the filtrate
was distilled to separate the ether. Hydrochloric acid (36 mL,
2.0 M in diethyl ether) was added and the solution filtered via
a cannula. Diethyl ether was removed under reduced pressure
resulting in pure chlorodi(ortho-methoxy)phenylphosphine as a
pale yellow solid (5.25 g, 18.72 mmol, 27.5%). Chlorodi(ortho-
methoxy)phenylphosphine (2 g, 6.6 mmol) was dissolved in
DCM (20 mL) and the solution added dropwise to a stirred
solution of triethylamine (20 mL) and methylamine (3.3 mL,
2.0 M solution in tetrahydrofuran) at 0 ◦C. This resulted in a
white precipitation of HCl·Et3N salt and a colourless solution.
Chlorodiphenylphosphine (1.19 mL, 6.6 mmol) was dissolved in
dichloromethane (40 mL) and added dropwise to the (di(ortho-
methoxy)phenylphosphino)methylamine solution resulting in fur-
ther precipitation of HCl·NEt3. The solution was allowed to stir
for 30 min. The solution was then filtered via a cannula and the
solvent was removed under reduced pressure resulting in an oily
product. The product was dissolved in diethyl ether (60 mL) and
passed through a neutral alumina plug under nitrogen to remove
contaminates. Removal of diethyl ether under reduced pressure
and drying in vacuo yielded 1 as a white powder (1.02 g, 2.22 mmol,
36.3%). CI mass spectrum: m/z = 460 [M + H]+, 459 [M]+. CI HR
1
ESI mass spectrum: m/z = 456 [M + H]+, 478 [M + Na]+. H
3
NMR (CDCl3): d = 1.22 (t, JHH = 6.97 Hz, 6H, CH3), 2.56–
3
3
3.04 (m, 4H, CH2), 2.46 (dd, JHP = 3.30 Hz, JHP = 2.57 Hz,
3H, NCH3), 7.23–7.42 (m, 18H, ArH). 13C NMR (CDCl3): d =
3
3
15.2 (d, JCP = 20.7 Hz, CH3), 27.3 (d, JCP = 20.7 Hz, CH2),
33.0 (s, NCH3), 125.4 (s, CH), 127.9–129.9 (m, CH), 132.1–132.9
(m, CH), 136.5–136.9 (m, CH), 138.4–138.8 (m, CH), 147.2 (d,
1JCP = 25.7 Hz, CP). 31P{ H} NMR (CDCl3): d = 57.6 (d, 2JPNP
=
1
300 Hz, PAr2), 72.5 (d, 2JPNP = 301 Hz, PAr2). Elemental analysis:
C29H31NP2·CH3OH calcd (%) C 74.84, H 7.80, N 2.64, found (%)
C 74.24, H 7.52, N 2.91.
Synthesis of {2-C6H4(Me)}2PN(Me)N(Me)P{2-C6H4(Me)}2
(6). N,N¢-bis(dichlorophosphino)dimethylhydrazine (0.68 mL,
3.8 mmol) was dissolved in ether (25 mL) and added dropwise
to ortho-tolyl magnesium bromide (60 mL in ether, 0.159 mol,
prepared as before). After 24 h of stirring, water (25 mL) was
slowly added and stirred for a further hour. The reaction mixture
was transferred to a separatory funnel and the layers separated.
The organic layer was dried over anhydrous magnesium sulfate
and the drying agent was removed via Bu¨chner filtration. The
filtrate was evaporated to dryness in vacuo to give a colourless oil.
The product was then washed with light petroleum ether and then
dissolved in ether (60 mL) and filtered through a short neutral
alumina plug. Removal of diethyl ether and drying in vacuo gives
6 as a white powder (0.34 g, 0.705 mmol, 19%). EI mass spectrum:
m/z = 484.3 [M]+. CI HR mass spectrum: m/z = 485.2272 [M +
H]+ (calcd 485.2276). 31P{ H} NMR (CDCl3): d = 47.3. 1H NMR
1
(CDCl3): d = 2.34 (s, 12H, ArCH3), 2.83 (s, 6H, NCH3), 7.11–7.27
Dalton Trans., 2010, 39, 560–567 | 565
This journal is
The Royal Society of Chemistry 2010
©

(m, 16H, ArH). 13C NMR (CDCl3): d = 21.4 (d, 3JCP = 23.08 Hz,
ArCH3), 37.7 (s, NCH3), 125.7 (s, CH), 128.6 (s, CH), 130.2 (s,
7.13 (m, 20H, ArH). 13C NMR (CDCl3): d = 21.0 (s, CH3), 56.2
1
(s, NCH3), 125.5–132.7 (m, CH), 211.1 (s, CO). 31P{ H} NMR
2
CH), 131.8 (s, CH), 138.3 (d, JCP = 20.8 Hz, CCH3), 141.3 (d,
(C6D6): d = 147.4 (s, PPho-tolyl). Satisfactory elemental analysis
1JCP = 28.27 Hz, CP).
could not be obtained; accurate ESI MS for C34H34CrN2O4P2 calcd
≡
m/z 649.140, found 649.138. IR (n-hexane): n = 1897 (s) (C O),
Synthesis
of
{2-C6H4(OMe)}2PN(Me)N(Me)P{2-C6H4-
≡
≡
1932 (s), 1955 (C O), 2015 (s) (C O).
(OMe)}2 (7). The same method as for 6 was followed only
using ortho-anisyl magnesium bromide in place of ortho-tolyl
magnesium bromide. In this case an oily product was initially
obtained, methanol trituration yielding 7 as a white powder
(1.10 g, 2.01 mmol, 52%). ESI mass spectrum: m/z = 549 [M +
H]+, 571 [M + Na]+. 1H NMR (C6D6): d = 2.70 (s, 12H, OCH3),
2.83 (s, 6H, NCH3), 6.18–6.86 (m, 16H, ArH). 13C NMR (C6D6):
d = 36.0 (d, 2JCP = 6.23 Hz, NCH3), 53.2 (s, ArOCH3), 108.7 (s,
CH), 109.0 (d, 1JCP = 1.56 Hz, CP), 119.7 (s, CH), 128.5 (s, CH),
[Cr(CO)4 {2-C6H4 (OMe)}2 PN(Me)N(Me)P{2-C6H4 (OMe)}2]
(11). 28%, ESI mass spectrum: m/z = 713 [M + H]+, 735 [M +
1
Na]+. H NMR (CDCl3): d = 2.11 (s, 6H, NCH3), 2.63 (s, 12H,
CH3), 6.64–7.25 (m, 20H, ArH). 13C NMR (CDCl3): d = 56.4 (s,
NCH3), 54.0 (s, OCH3), 109.4–131.8 (m, CH), 160.7 (s, COCH3),
1
208.1 (s, CO). 31P{ H} NMR (CDCl3): d = 135.7 (s, PPhOMe).
Elemental analysis: C34H34N2O4P2·2CH3OH calcd (%) C 55.67, H
5.45, N 3.61, found (%) C 55.20, H 5.09, N 3.61. IR (CHCl2): =
1
131.6 (s, CH), 159.6–160.8 (m, COCH3). 31P{ H} NMR (CDCl3):
≡
≡
≡
1883 (br) (C O), 1895 (s), 1916 (C O), 2005 (s) (C O).
d = 27.6. Elemental analysis: C30H34N2O4P2·2CH3OH calcd (%)
Catalytic testing
C 62.74, H 6.91, N 4.57, found (%) C 62.51, H 6.62, N 4.75.
Isoprene trimerisation. Our previously reported method was
followed.4 The ligand (20 mmol) and chromium(III) chloride
tetrahydrofuran complex, [CrCl3(THF)3], (8 mg, 20 mmol) were
dissolved in tetrahydrofuran (3 mL) and stirred at room temper-
ature in a round bottomed Schlenk fitted with a condenser under
nitrogen for 10 minutes. The solvent was removed and toluene
(10 mL) was added to give a pink mixture. Methylaluminoxane
(MAO, 10% wt solution in toluene, 4 mL, 300 equivalents, 6 mmol)
was then added, to give a green solution. Freshly distilled isoprene
(30 mL) was added and the solution was vigorously stirred at 70 ◦C
for 1 h. The Schlenk tube was then opened to air and a dilute
solution of hydrochloric acid (10% solution) in water was slowly
added to quench the reaction. The organic layer was separated
and dried over anhydrous magnesium sulfate, giving a colourless
solution of products in toluene. A sample of the solution was
analysed by GC-FID.
Chromium carbonyl complexation
The same general method was followed for all complexes, de-
scribed here for the synthesis of 8. Toluene (40 mL) was added
to chromium hexacarbonyl, [Cr(CO)6], (350 mg, 1.6 mmol) and 1
(500 mg, 1.2 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 (10 mL). Methanol
(20 mL) was added to precipitate the product, which was isolated
by filtration and dried in vacuo to yield a yellow solid (287 mg,
0.46 mmol, 44%). X-Ray-quality crystals were formed from a
concentrated dichloromethane solution layered with methanol at
0 ◦C.
[Cr(CO)4{2-C6H4(OMe)}2PN(Me)P(C6H5)2] (8). 34%, CI
+
+
≡
mass spectrum: m/z = 623 [M] , 511 [M - 4 C O] . CI HR mass
Ethene and styrene co-trimerisation. Our previously reported
method was followed.5 The ligand (20 mmol) and chromium(III)
chloride tetrahydrofuran complex, [CrCl3(THF)3], (8 mg, 20 mmol)
were dissolved in tetrahydrofuran (3 mL) and stirred at room
temperature in a Schlenk tube under nitrogen for 10 minutes. The
solvent was removed and toluene was added (10 mL) to give a pink
mixture. Methylaluminoxane (MAO, 10% wt solution in toluene,
4 mL, 300 equivalents, 6 mmol) was then added, to give a green
solution and the Schlenk tube was weighed. Next, the Schlenk
tube was put under an ethene (1 bar) atmosphere and styrene
(30 mL) was added. The solution was vigorously stirred at room
temperature for 1 h. After 1 h the ethene feed was stopped and
the Schlenk tube was re-weighed. The Schlenk tube was opened
to air and a dilute solution of hydrochloric acid (10% solution)
in water was slowly added to quench the reaction. The organic
layer was separated and dried over anhydrous magnesium sulfate,
giving a colourless solution of products in toluene. A sample of
the solution was analysed by GC-FID.
1
spectrum: m/z = 623.0724 (calcd 623.0719). H NMR (CDCl3):
d = 2.70 (t, 3JHP = 8.19 Hz, 3H, NCH3), 3.40 (s, 6H, OCH3), 6.81–
7.47 (m, 18H, ArH). 13C NMR (CDCl3): d = 23.94 (s, NCH3), 54.0
(s, OCH3), 109.8 (s, CH), 119.4 (s, CH), 119.5 (s, CP), 127.3 (s,
CH), 127.4 (s, CH), 129.18 (s, CH), (m, 130.8–131.5, CH), 158.7
1
(s, COCH3), 204.1 (s, CO). 31P{ H} NMR (CDCl3): d = 100.36
2
2
(d, JPNP = 27.3 Hz, PAr2), 111.2 (d, JPNP = 27.3 Hz, PAr2).
.
Elemental analysis: C31H27CrNO6P2 2CH3OH calcd (%) C 57.64,
H 5.13, N 2.04, found (%) C 57.45, H 4.61, N 2.37. IR (CHCl2): =
≡
≡
≡
1876 (s) (C O), 1893 (s), 1918 (C O), 2005 (s) (C O).
[Cr(CO)4{2-C6H4(Me)}2PN(Me)P(C6H5)2] (9). 31%, ESI
+
+
≡
≡
mass spectrum: m/z = 591 [M + Na - C O] , 577 [M - 4 C O] .
1H NMR (CDCl3) (MeOH of crystallisation observed but omitted
here for clarity): d = 1.93 (s, 6H, ArCH3), 2.72 (t, 3JHP = 3.39 Hz,
3H, NCH3), 7.10–7.47 (m, 18H, ArH). 13C NMR (CDCl3): d =
15.4 (s, CH3), 34.7 (s, NCH3), 126.5–132.3 (m, CH). 31P{ H}
1
2
NMR (CDCl3): d = 109.9 (d, JPNP = 29.8 Hz, PAr2), 114.1 (d,
2JPNP = 29.8 Hz, PAr2). Elemental analysis: C31H27CrNO4P2 calcd
Crystal structure determinations
(%) C 62.95, H 4.60, N 2.37, found (%) C 62.19, H 4.85, N 2.32.
≡
≡
IR (CHCl2): = 1877 (s) (C O), 1895 (s), 1916 (C O), 2008 (s)
X-Ray diffraction experiments on 8, 10·0.75CH2Cl2 and 11,
≡
(C O).
were carried out at 100 K on a Bruker Kappa Apex II CCD
˚
[Cr(CO)4{2-C6H4(Me)}2PN(Me)N(Me)P{2-C6H4(Me)}2] (10).
diffractometer, using Mo Ka radiation (l = 0.71073 A). A single
1H NMR (C6D6): d = 1.93 (s, 12H, CH3), 2.53 (s, 6H, NCH3), 6.97–
crystal was coated in inert oil and mounted on a glass fibre.
566 | Dalton Trans., 2010, 39, 560–567
This journal is
The Royal Society of Chemistry 2010
©

Table 6 Crystallographic data
Compound
8
9·0.5MeOH
10·0.75CH2Cl2
11
Colour, habit
Size/mm
Empirical formula
M
Yellow block
0.196 ¥ 0.156 ¥ 0.052
C31H27CrNO6P2
623.48
Yellow block
0.042 ¥ 0.028 ¥ 0.026
C31.5H28.5CrNO4.5P2
607.02
Yellow block
0.94 ¥ 0.85 ¥ 0.45
C34.75H35.5Cl1.5CrN2O4P2
712.27
Yellow block
0.28 ¥ 0.12 ¥ 0.10
C34H34CrN2O8P2
712.57
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
9.944(1)
15.875(1)
38.828(2)
101.416(1)
6008.5(4)
8
Monoclinic
P21/c
Monoclinic
P21/c
˚
a/A
14.712(1)
13.357(1)
20.732(1)
134.635(1)
2898.95(7)
4
11.825(1)
16.615(1)
20.683(1)
121.267(1)
3473.6(3)
4
20.708(1)
10.778(1)
15.910(1)
110.343(1)
3329.28(11)
4
˚
b/A
˚
c/A
b/◦
V/A3
Z
m/mm-1
0.550
100
0.525
120
0.577
100
0.494
100
T/K
Reflections: total/independent
R(int)
Final R1, wR2
34 319/6627
0.0636
0.0419, 0.0989
51 936/13 757
0.0897
0.0694, 0.1654
29 654/7919
0.0269
0.0374, 0.1004
34 988/7573
0.0264
0.096, 0.0744
Intensities were integrated16 from several series of exposures in
j and w calculated by the Apex II17 program after unit cell
determination.
2 A. Carter, S. A. Cohen, N. A. Cooley, A. Murphy, J. Scutt and D. F.
Wass, Chem. Commun., 2002, 858.
3 A. Bollmann, K. Blann, J. T. Dixon, F. M. Hess, E. Killian, H.
Maumela, D. McGuinness, D. H. Morgan, A. Neveling, S. Otto, M.
Overett, A. M. Z. Slawin, P. Wasserscheid and S. Kuhlmann, J. Am.
Chem. Soc., 2004, 126, 14712.
4 L. E. Bowen, M. Charernsuk and D. F. Wass, Chem. Commun., 2007,
2835.
5 L. E. Bowen and D. F. Wass, Organometallics, 2006, 25, 555.
6 J. R. Briggs, J. Chem. Soc., Chem. Commun., 1989, 11, 674.
7 M. J. Overett, K. Blann, A. Bollmann, J. T. Dixon, F. Hess, E. Killian,
H. Maumela, D. H. Morgan, A. Neveling and S. Otto, Chem. Commun.,
2005, 622.
X-Ray diffraction experiments on 9·0.5MeOH, were carried
out at 120 K on a Bruker-Nonius KappaCCD diffractometer,
˚
also using Mo Ka radiation (l = 0.71073 A) generated by a
Bruker-Nonius FR591 rotating anode. All data collections were
performed using a single crystal coated in perfluoroalkylether and
mounted on a glass fibre. Intensities were integrated18 from several
series of exposures in j and w calculated by the COLLECT19 and
DENZO20 programs after unit cell determination by the DirAx21
program.
8 A. M. Slawin, M. Wainwright and J. D. Woollins, J. Chem. Soc., Dalton
Trans., 2002, 513.
9 D. F. Wass, PCT patent appl. WO 0204119, 2002, (to BP).
10 S. Akutagawa, T. Taketomi, H. Kumobayashi, K. Takayama, T. Someya
and S. Otsuka, Bull. Chem. Soc. Jpn., 1978, 51, 1158.
11 S. Akutagawa, T. Taketomi and S. Otsuna, Chem. Lett., 1976,
485.
Absorption corrections were based on equivalent reflections
using SADABS,22 and structures were refined against all Fo
2
data with hydrogen atoms riding in calculated positions using
SHELXTL.23 Crystal structure and refinement data are given
in Table 6. Ten low angle disagreeable reflections were omitted
from 9, 10 and 11. To improve the disordered solvent model for
structure 9 the anisotropic displacement parameters for the carbon
of the methanol molecule were restrained to match the attached
oxygen atom. Attempts to model the disorder of the co-crystallised
CH2Cl2 molecule in structure 10·0.75CH2Cl did not improve the
final model, and instead an occupancy of 75% was assigned to the
solvate after refinement.
12 L. E. Bowen, M. Haddow, A. G. Orpen and D. F. Wass, Dalton Trans.,
2007, 1160.
13 N. Cooley, S. M. Green and D. F. Wass, Organometallics, 2001, 20,
4769.
14 K. Blann, A. Bollmann, J. T. Dixon, F. M. Hess, E. Killian, H.
Maumela, D. H. Morgan, A. Neveling, S. Otto and M. J. Overett,
Chem. Commun., 2005, 620.
15 V. S. Reddy and K. V. Katti, Inorg. Chem., 1994, 33, 2695.
16 SAINT v7.34A, Bruker-AXS, 2007.
17 Apex2, Bruker-AXS, 2007.
18 HKL package (DENZO, XDisplayF and Scalepack), HKL Re-
search Inc, Z. Otwinowski & W. Minor, Methods in Enzymol-
ogy, Vol. 27, Macromolecular Crystallography, part A, 307-326,
1997.
Acknowledgements
19 R. W. W. Hooft & B. V. Nonius, COLLECT data collection software,
1998.
20 Z. Otwinowski and W. Minor, Methods in Enzymology, Vol. 27,
Macromolecular Crystallography, part A, 307–326, 1997.
21 A. J. M. Duisenberg, J. Appl. Crystallogr., 1992, 25, 92–96.
22 G. M. Sheldrick (2007), SADABS V2007 (2, Bruker AXS Inc.,
Madison, Wisconsin, USA.
We would like to thank the EPSRC, CCDC and University of Bris-
tol for funding. Thanks to the National Crystallographic Service
at Southampton for collecting diffraction data for 9·0.5MeOH.
References
23 SHELXTL program system version V 6.14, Bruker AXS Inc., Madison,
1 J. T. Dixon, M. J. Green, F. M. Hess and D. H. Morgan, J. Organomet.
Chem., 2004, 689, 3641; D. F. Wass, Dalton Trans., 2007, 816.
Wisconsin, USA, 2000-3.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 560–567 | 567
©