Application of molybdenum bis(imido) complexes in ethylene dimerisation catalysis

Article abstract of DOI:10.1039/c2dt12061e

In combination with EtAlCl₂ (Mo:Al = 1:15) the imido complexes [MoCl₂(NR)(NR′)(dme)] (R = R′ = 2,6-Prⁱ ₂-C₆H₃ (1); R = 2,6-Prⁱ ₂-C₆H₃, R′ = Bu

Full text of DOI:10.1039/c2dt12061e

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PAPER
Application of molybdenum bis(imido) complexes in ethylene dimerisation
catalysis†
William R. H. Wright,a,b Andrei S. Batsanov,b Antonis M. Messinis,a,b Judith A. K. Howard,b
Robert P. Tooze,c Martin J. Hanton*c and Philip W. Dyer*a,b
Received 29th October 2011, Accepted 12th December 2011
DOI: 10.1039/c2dt12061e
In combination with EtAlCl2 (Mo : Al = 1 : 15) the imido complexes [MoCl2(NR)(NR′)(dme)] (R = R′ =
2,6-Pri2–C6H3 (1); R = 2,6-Pri2–C6H3, R′ = But (3); R = R′ = But (4); dme = 1,2-dimethoxyethane) and
[Mo(NHBut)2(NR)2] (R = 2,6-Pri2–C6H3 (5); R = But (6)) each show moderate TON, activity, and
selectivity for the catalytic dimerisation of ethylene, which is influenced by the nature of the imido
substituents. In contrast, the productivity of [MoCl2(NPh)2(dme)] (2) is low and polymerisation is
favoured over dimerisation. Catalysis initiated by complexes 1–4 in combination with MeAlCl2 (Mo : Al
= 1 : 15) exhibits a significantly lower productivity. Reaction of complex 5 with EtAlCl2 (2 equiv.) gives
rise to a mixture of products, while addition of MeAlCl2 affords [MoMe2(N–2,6-Pri2–C6H3)2]. Treatment
of 6 with RAlCl2 (2 equiv.) (R = Me, Et) yields [Mo({μ-N–But}AlCl2)2] (7) in both cases. Imido
derivatives 1 and 3 react with Me3Al and MeAlCl2 to form the bimetallic complexes [MoMe2(N{R}
AlMe2{μ-Cl})(NR′)] (R = R′ = 2,6-Pri2–C6H3 (8); R = 2,6-Pri2–C6H3, R′ = But (10)) and [MoMe2(N{R}
AlCl2{μ-Cl})(NR′)] (R = R′ = 2,6-Pri2–C6H3 (9); R = 2,6-Pri2–C6H3, R′ = But (11)), respectively.
Exposure of complex 8 to five equivalents of thf or PMe3 affords the adducts [MoMe2(N–2,6-Pri2–
C6H3)2(L)] (L = thf (12); L = PMe3 (13)), while reaction with NEt3 (5 equiv.) yields [MoMe2(N–2,6-
Pri2–C6H3)2]. The molecular structures of complexes 5, 9 and 11 have been determined.
upon the pioneering studies of Manyik et al. and subsequently
those of Briggs, chromium-based systems bearing nitrogen-con-
Introduction
Selective transition metal-initiated alkene oligomerisation (in
particular ethylene di- and trimerisation) has been the focus of a
substantial industrial and academic research effort, providing
simple and economical routes for the manufacture of higher
olefinic feed stocks.1–6 Of particular importance in this area is
the manufacture of linear, terminal, short-chain materials (C4–
C20) that are finding increasing uses as plasticiser co-monomers
in polyethylene manufacture and as components of synthetic
lubricant base oils.7–9 A variety of different initiator systems
have been developed for such applications, with those based on
the Group 4, 6 and 10 metals being amongst the most successful
and commercially relevant.10–14
taining ligands for selective alkene oligomerisation have been
extensively probed.15–19 A wide variety of mono-, bi-, and tri-
dentate metal scaffolds have been employed, with pyrrolyide-
supported initiators proving especially important from an indus-
trial perspective.6,20 The success of these latter systems is, in
part, due to the variable η1-/η5-coordination behaviour of the
aza-heterocycle.21 Notably, a related phenomenon has been
reported for Cr(I) bis(diphenylphosphino)propane pro-initiators,
which upon activation by alkyl aluminium reagents, form dipho-
sphine-bridged ansa-bis(arene) Cr(I) species.22 Together, these
results highlight the potential importance of ligands that may
adopt more than one binding mode (i.e. linkage isomerism) in
the field of oligomerisation catalysis.
Attempts have also been made to utilise electronically-flexible
N-donors to prepare chromium-based pro-initiators. Here efforts
have focused on well-established imido ligands (NR2−) that can
behave as 2- or 4-electron metal scaffolds.23–25 However, in
combination with a variety of aluminium co-activators these
chromium imido systems (e.g. CrX2(N–2,6-Pri2–C6H3)2, X = Cl,
But) yield high molecular weight polymer rather than the desired
olefin oligomerisation products.26
Recent years have seen a particular resurgence of interest in
oligomerisation systems incorporating Group 6 metals especially
those utilising chromium and tungsten. For example, building
aCentre for Sustainable Chemical Processes, Department of Chemistry,
Durham University, South Road, Durham, UK, DH1 3LE. E-mail: p.w.
dyer@durham.ac.uk; Fax: +44 (0) 191 334 2150; Tel: +44(0)191 384
4737
bDepartment of Chemistry, Durham University, South Road, Durham,
UK, DH1 3LE
In contrast, related tungsten imido complexes such as
cSasol Technology (UK) Ltd, Purdie Building, North Haugh, St
Andrews, Fife, UK, KY16 9ST. E-mail: martin.hanton@eu.sasol.com
†CCDC reference numbers 852768–852770. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2dt12061e
[WCl4(N–2,6-Pri2–C6H3)(thf)]
and
[WCl2(N–2,6-Pri2–
C6H3)2(dme)] are both active for selective ethylene and propy-
lene dimerisation when treated with EtAlCl2.27,28 Importantly,
5502 | Dalton Trans., 2012, 41, 5502–5511
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Table 1 Ethylene dimerisation using pre-formed molybdenum bis(imido) complexes 1–6a
Pro-initiator
Activator
Time (min)
TONb
Activityc
Mol % C4d (of liq. prod.)
% 1-C4 in C4
% PE
1
1
2
2
3
3
4
4
5
6
EtAlCl2
MeAlCl2
EtAlCl2
MeAlCl2
EtAlCl2
MeAlCl2
EtAlCl2
MeAlCl2
EtAlCl2
EtAlCl2
55
55
1.5
3
80
9
74
2
54
4
58 500
24 790
770
2020
59 510
4420
14 030
1790
56 940
120
63 820
27 250
34 350
47 250
44 870
28 500
11 360
48 660
62 880
1900
88.9
97.5
94.1
83.2
89.3
90.9
88.9
81.5
87.7
100.0
63.7
78.6
97.9
97.5
61.5
87.0
63.7
94.1
62.9
100.0
0.1
0.5
69.6
20.5
0.2
0.2
3.0
18.5
0.0
0.0
a General conditions: 0.2 mmol pro-initiator and 3.0 mmol RAlCl2 (R = Me or Et); PhCl (solvent) 74 mL; 60 °C; ethylene pressure (40 bar); stirrer
speed 1000 rpm; nonane standard (1.000 mL); catalytic runs were performed until consumption of C2H4 dropped below 0.2 g min−1, at which time
reaction was quenched by addition of dilute HCl. b TON is reported in (mol C2H4) (mol Mo)−1 c Activity is reported in (mol C2H4) (mol Mo)−1 h−1
. .
d In all runs C6 alkenes were produced (8.4–9.8% of liquid products) as well as trace ≥C8 fractions (0.1–2.4% of liquid products). PE = polyethylene.
these observations support the proposed role of tungsten imido
species in selective olefin dimerisation catalysis mediated by
commercially attractive initiator systems derived in situ from
sequential reaction of inexpensive WCl6 with aniline, triethyl-
amine, and EtAlCl2 (W : aniline : NEt3 : Al 1 : 2 : 4 : 15).29–33
These multi-component systems are not only extremely selective
to the dimer products for a range of α-olefins (>99% vs. higher
oligomers), but also demonstrate high specificity for methyl-
branched products.
Despite the importance of both chromium- and tungsten-based
initiators in catalytic olefin dimerisation, no studies have been
reported that probe the potential of analogous molybdenum
systems. This is surprising since the utility of Mo-containing
complexes in olefin metathesis chemistry has lead to the prep-
aration of a wide variety of variously-substituted molybdenum
imido, derivatives, something that could open the way for
detailed structure/property correlations to be established.34–36
Furthermore, a number of well-defined molybdenum imido com-
plexes are commercially available, making use of such systems
attractive from an industrial perspective. Consequently, herein,
we report a fundamental study of the reactivity of a number of
readily accessible molybdenum imido complexes with alkyl alu-
minium reagents and explore their potential as pro-initiators in
selective, catalytic ethylene dimerisation.
40 bar ethylene pressure, and chlorobenzene as the solvent at
60 °C.28,32
In combination with EtAlCl2 the pro-initiators 1, 3, and 4
each achieve a moderate selectivity to but-1-ene of ∼90%, which
compares favourably with the value established for the in situ
WCl6-derived system of ∼80% (Table 1, Fig. 1).32,42 A notice-
able attenuation in the product selectivity is observed for the
more active well-defined pro-initiators consistent with the incor-
poration of the higher olefinic products generated during cataly-
sis. The activity of the pro-initiators 1–4 varied according to the
nature of the imido substituents, decreasing in the order 1 > 3 >
2 ≫ 4. However, a comparison of the TONs (turnover numbers)
for each pro-initiator reveals that both complexes 1 and 3
perform well (∼60 000 (mol C2H4) (mol Mo)−1), while the life-
times of the bis(Ph)- and bis(But)-imido systems 2 and 4 are
extremely low. Notably, the phenyl imido-substituted complex 2
also exhibits a very low selectivity to dimer products, instead
affording significant quantities of polyethylene. The rapid
deterioration in performance observed for pro-initiators 2 and 4
has tentatively been attributed to steric factors that open alterna-
tive, but potentially different reaction pathways for these Ph- and
But-substituted imido systems. Although the steric demands of
Results and discussion
Catalytic ethylene dimerisation testing
The utility of a series of readily accessible molybdenum imido
complexes [MoCl2(NR)(NR′)(dme)] (R = R′ = 2,6-Pri2–C6H3
(1);37,38 R = R′ = Ph (2);39 R = 2,6-Pri2–C6H3, R′ = But (3);40
R
= R′ = But (4)41) as pro-initiators for ethylene dimerisation in
combination with an aluminium activator RAlCl2 (R = Me, Et)
has been explored. Catalytic performance for each system
was screened under conditions analogous to those used for the
WCl6-derived in situ systems, namely an Mo : Al ratio of 1 : 15,
Fig. 1 Activity ((mol C2H4) (mol Mo)−1 h−1) and TON ((mol C2H4)
(mol Mo)−1) data for the dimerisation of ethylene initiated by complexes
1–6 in the presence of an aluminium activator (for experimental details
see Table 1).
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the N–Ph and N–But imido ligands are quite different, both
motifs have a propensity to adopt bridging coordination modes,
something that is prevented by the more sterically demanding
N–2,6-Pri2–C6H3 moiety (vide infra).23
alkylation and did not react with ethylene (10 equiv.) even in the
presence of excess EtAlCl2. In contrast, reaction of complex 5
with EtAlCl2 (2 equiv.) under identical conditions afforded a
mixture of unassignable products (according to 1H and 13C
NMR spectroscopy), although it was clear that no ethane evol-
ution occurred. However, subsequent addition of ethylene (10
equiv.) to this complex mixture resulted in the formation of
small quantities of butenes indicative of the presence of a dimeri-
sation-active species.
Previously, deuterium labelling studies have indicated that
olefin dimerisation mediated by tungsten bis(imido) pro-initiators
(in combination with EtAlCl2 as activator) most likely occurs via
a chain-growth pathway. Such reactions are believed to proceed
through a tungsten hydride species, which is generated in situ
following β-hydride elimination from a tungsten ethyl group,
itself resulting from Al-to-W transmetallation.28,32 Adoption of
this type of reaction manifold for tungsten systems is supported
by the observation that replacing EtAlCl2 by MeAlCl2 as activa-
tor results in a very significant drop in catalytic performance.32
Consequently, in order to probe the nature of the dimerisation
path for molybdenum-based systems, the catalytic performance
of complexes 1–4 was examined using MeAlCl2 as activator.
Although catalysis was initiated by each system, the productiv-
ities were significantly lower than those achieved using EtAlCl2
as activator, although the levels of selectivity were retained
(Table 1, Fig. 1). Together, these observations suggest that even
when MeAlCl2 is used tungsten hydride species are formed,
albeit significantly less efficiently than with EtAlCl2, as might
be expected. Although the precise mechanistic trajectory by
which MeAlCl2 can lead to a metal hydride in such systems
remains obscure, it could involve α-hydride elimination from a
molybdenum or tungsten methyl species. Indeed, it is well
known that for the WCl6-based dimerisation systems use of
lower loadings of aluminium activator results in a switch to an
olefin metathesis pathway, although it should be noted that no
metathesis products were detected in the catalysis performed
here.27
ð1Þ
Analogous reactions of complexes 5 and 6 with MeAlCl2 (2
equiv.) were carried out (C6D6 solution) in order to probe the
potential role of ethyl group β-hydride elimination, Scheme 1.
Again, complex 6 was subject to deprotonation and afforded 7
and methane, while derivative 5 cleanly gave the known
dimethyl complex [MoMe2(N–2,6-Pri2–C6H3)2] via transmetalla-
tion; the formation of methane was not observed for the latter
reaction.45,53
Reactions of [Mo(NHBut)2(NR)2] with R′xAlCl3−x
Scheme 1 (i) MeAlCl2 (2 equiv.), C6D6, room temperature.
Previous studies have clearly exemplified the participation of
imido species in catalytic olefin dimerisation using in situ WCl6/
aniline-derived systems (vide supra). However, the potential role
of amido-ligated (R2N−) species, which may equally as readily
be generated during the formation of the active initiator, cannot
be disregarded. Consequently, the catalytic performance of the
mixed bis(amido) bis(imido) derivatives [Mo(NHBut)2(NR)2] (R
= 2,6-Pri2–C6H3 (5)43 and R = But (6)44) was explored in combi-
nation with EtAlCl2 (15 equiv.) under identical reaction con-
ditions. Here, use of the aryl imido complex 5 gave rise to a
system that initiated dimerisation with activity, productivity, and
selectivity that is identical to that for the analogous dichloride
complex [MoCl2(N–2,6-Pri2–C6H3)2(dme)] (1). In contrast, the
tert-butyl imido derivative 6 was barely active, despite its parent
complex [MoCl2(NBut)2(dme)] (4) exhibiting moderate activity.
To further probe the significant differences observed in the
catalytic performance of the 2,6-Pri2–C6H3- and But-substituted
imido tert-butyl amido complexes 5 and 6, respectively, their
reactions with EtAlCl2 were probed in detail. Treatment of
diamide 6 with two equivalents of EtAlCl2 in C6D6 solution
cleanly afforded the previously reported bimetallic species
[Mo({μ-N–But}AlCl2)2] (7) and ethane, the latter resulting from
deprotonation of the parent diamide, eqn (1).44 Notably, this
imido-bridged complex 7 was found to be inert to further
In order to explore the differences in reactivity determined for
complexes 5 and 6, attempts were made to determine their mol-
ecular structures. However, despite repeated attempts, no crystals
of 6 suitable for X-ray diffraction studies could be obtained. In
contrast, the molecular structure of complex 5 was verified by
single-crystal X-ray diffraction (Fig. 2). This proved very similar
to that of the previously reported complex [Mo(HN–2,6-Pri2–
C6H3)2(N–2,6-Pri2–C6H3)2], with the Mo atom possessing a dis-
torted tetrahedral coordination.46
As expected, the imido Mo–N bond distances (1.7609(14) and
1.7543(15) Å) in 5 are intermediate between those of an LX2
(4e−) and X2 (2e−) donor and, although the two MovN–CAr
bond angles differ significantly (155.3(1) and 170.3(1)°), they
lie within the accepted range for quasi-linear imido motifs
(150–180°).40 The Mo–N amido bond distances are comparable
with other molybdenum amido complexes (1.92–2.15 Å).46,47
Both amide nitrogen atoms N(3) and N(4) are planar, within
experimental error, and are orientated such that the N(3)–Mo–
N(4) plane and the H–N–C planes for N(3) and N(4) are at 62.7
and 70.6°, respectively. This arrangement maximises the extent
of N(pπ)→Mo(dπ) donation for each amido ligand about the
pseudo-tetrahedral Mo atom, something facilitated by the
5504 | Dalton Trans., 2012, 41, 5502–5511
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Reactions of [MoCl2(NR)(NR′)(dme)] with MexAlCl3−x
Understanding the subtle interplay between aluminium activator
and transition metal pro-initiator remains key to the elucidation
of the mechanism of catalytic olefin dimerisation. For example,
it has been proposed from computational studies that coordi-
nation of R′AlCl2 to tungsten bis(imido) fragments enhances
ethylene dimerization via an oxidative coupling pathway due to
a lowering of the energy barrier for reductive elimination.52
However, in a separate study, it has been revealed that although
coordination of various MexAlCl3−x species to tungsten bis
(imido) moieties does indeed take place, the resulting bimetallic
systems are inert towards both ethylene and propylene.28 Hence,
given the differences observed in catalytic performance between
the different molybdenum imido complexes 1–4, the reactivity
of each system with variously-substituted alkyl aluminium
reagents has been studied.
Treatment of [MoCl2(N–2,6-Pri2–C6H3)2(dme)] (1) with
excess Me3Al (6 equiv.) in hexane solution resulted in full con-
version (by NMR spectroscopy) to [MoMe2(N{2,6-Pri2–C6H3}
AlMe2{μ-Cl})(N–2,6-Pri2–C6H3)] (8), Scheme 2. Here, the
Me2AlCl generated in situ by Al/Mo transmetallation has been
sequestered by one of the Lewis basic imido nitrogens, presum-
ably via an intermolecular pathway. In a similar fashion treat-
ment of 1 with MeAlCl2 (3 equiv.) in toluene cleanly afforded
[MoMe2(N{2,6-Pri2–C6H3}AlCl2{μ-Cl})(N–2,6-Pri2–C6H3)]
(9), although further addition of MeAlCl2 yielded other (uniden-
tified) reaction products.‡ Complex 8 can also be formed directly
via reaction of [MoMe2(N–2,6-Pri2–C6H3)2] with Me2AlCl,
although the reaction also affords a significant number of other,
unidentified side-products, eqn (2).
Fig. 2 Molecular structure of [Mo(NHBut)2(N–2,6-Pri2–C6H3)2] (5). H
atoms are omitted for clarity and the thermal ellipsoids are shown at the
50% level. Selected bond distances (Å) and angles (°): Mo–N(1) 1.7609
(14), Mo–N(2) 1.7543(15), Mo–N(3) 1.9537(16), Mo–N(4) 1.9495(16),
N(1)–Mo–N(2) 112.32(7), N(3)–Mo–N(4) 111.88(7), N(1)–Mo–N(3)
105.53(7), N(1)–Mo–N(4) 111.25(7), Mo–N(1)–C(1) 155.32(13), Mo–
N(2)–C(11) 170.28(13), Mo–N(3)–C(7) 130.80(14), Mo–N(4)–C(17)
133.68(13). Neither amido ligand forms any hydrogen bonds.
pseudo-trans disposition of the two But substituents, which
reduces steric constraints.48–50
These solid-state data emphasize the electron-withdrawing
nature of the aryl imido ligands, evidenced by appreciable
1
Namido→Mo lone pair donation. This is consistent with the H
NMR spectrum of complex 5 in which the amido NH proton
appears to considerably higher frequency to that for complex 6
bearing electron-releasing tert-butyl imido ligands, δ 6.29 (5)44
versus 5.71 ppm (6). Further corroboration of this electronic dis-
parity is given by a comparison of the chemical shift difference
between the α and β carbon resonances (Δδ) of the tert-butyl
amido substituents of 5 (Δδ = 22.1 ppm) and 6 (Δδ = 19.6 ppm).
By analogy with the use of this parameter to probe the electron
density at the imido nitrogen atom of d0 tert-butyl imido com-
plexes, the greater magnitude of Δδ for complex 5 is consistent
with lower electron density at its amido nitrogen atoms, again
resulting from greater Namido→Mo donation. Together, these data
are all indicative of a superior acidity of the amido NH group of
5 versus that of 6.51 However, this clear difference in electronic
nature of the imido substituents and consequently, in the relative
acidities of the NH moieties of complexes 5 and 6, does not cor-
relate with their observed reactivity with RAlCl2 (R = Et, Me)
since only the less acidic amido group of 6 is subject to deproto-
nation. Consequently, it is likely that the greater steric demands
of the 2,6-diisopropylphenyl substituents of 5 prevent the for-
mation of imido-bridged dimeric species such as 7, which result
from amido deprotonation. Thus, the poor catalytic performance
of pro-initiator 6 in combination with the necessary alkyl alu-
minium activator is attributed to the formation of complex 7 in
situ, which prevents access to the catalytically productive mani-
fold and highlights the probable role of similarly-bridged imido
species in catalyst deactivation pathways.
Scheme 2 (i) Me3Al (6 equiv.), hexane, room temperature (r.t.); (ii)
MeAlCl2 (3.25 equiv.), toluene, r.t.
The 1H NMR spectrum of 8 is consistent with the presence of
a symmetrical MoMe2 motif, which gives rise to a single reson-
ance at δ 1.64 ppm, with the AlMe2 moiety presenting a signal
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Table 2 Selected bond distances (Å) and angles (°) for complexes 9 and 11
9, A
9, B
9, C
9, D
9, mean
11
Mo–Cl(1)
Mo–N(1)
Mo–N(2)
Mo–C(1)
Mo–C(2)
Mo⋯Al
Al–Cl(1)
Al–N(2)
Al–Cl(2)
Al–Cl(3)
Mo–N(1)–C
Mo–Cl(1)–Al
Mo–N(2)–Al
Mo–N(2)–C(11)
Al–N(2)–C(11)
Interplanar angle
i/iii
2.695(2)
1.711(5)
1.891(6)
2.136(7)
2.141(6)
3.054(2)
2.205(3)
1.878(5)
2.087(3)
2.109(3)
162.6(5)
76.39(8)
108.2(3)
129.4(4)
122.3(5)
2.708(2)
1.718(5)
1.851(6)
2.123(7)
2.118(7)
3.056(2)
2.195(3)
1.885(6)
2.091(3)
2.110(3)
163.1(5)
76.33(8)
109.8(3)
130.6(4)
119.6(4)
2.712(2)
1.712(5)
1.867(6)
2.126(7)
2.118(7)
3.058(2)
2.194(3)
1.893(6)
2.096(4)
2.089(3)
164.2(5)
76.31(8)
108.9(3)
131.7(4)
119.4(5)
2.695(2)
1.723(5)
1.865(6)
2.127(7)
2.106(7)
3.045(2)
2.198(3)
1.880(6)
2.107(3)
2.099(3)
164.3(5)
76.30(8)
108.8(3)
131.8(4)
119.3(4)
2.702(9)
1.716(6)
1.869(16)
}2.124(11)
2.7474(5)
1.7024(14)
1.8874(13)
2.1191(17)
2.1217(17)
3.1024(6)
2.1887(7)
1.8718(14)
2.1094(7)
2.1118(8)
174.6(1)
3.053(6)
2.198(5)
1.884(7)
} 2.099(9)
163.6(8)
76.33(4)
108.9(7)
131(1)
76.96(2)
111.23(6)
127.4(1)
120(1)
121.4(1)
10.4(3)
86.6(1)
5.3(3)
88.7(1)
4.1(3)
88.2(2)
6.8(3)
88.3(2)
6.7
87.9
—
89.0(1)
ii/iii
at δ − 0.09 ppm. Notably, the protons of the Mo–Me groups of 8
resonate to higher frequency of those reported previously for
[MoMe2(N–2,6-Pri2–C6H3)2] (δ 1.39 ppm), consistent with the
electron-withdrawing effect of the coordinated Me2AlCl.53 Ana-
logously, the 13C NMR spectrum for 8 shows two metal-bonded
carbons at δ 41.5 and −7.0 ppm (Mo–Me and Al–Me, respect-
ively), as expected. The 1H NMR spectrum for complex 9
revealed a resonance at δ 1.67 ppm for the Mo–Me motif at a
shift comparable to that determined for 8.
With the aim of better understanding the crucial alkyl alu-
minium chloride–metal imido interaction, single crystals of
complex 9 were grown from CH2Cl2 solution and characterised
by X-ray diffraction (Fig. 3, Table 2). The asymmetric unit of
9·½CH2Cl2 comprises four host molecules (A to D) of similar
geometry, as well as two dichloromethane molecules (one of
them chaotically disordered). The molecular structure of 9
reveals one terminal imido ligand, N(1)–2,6-Pri2–C6H3 (mean
Mo–N distance 1.715(6) Å) and an asymmetrically-bridged
imido unit, together an arrangement that closely resembles that
of the related tungsten complex [WMe2(N{2,6-Pri2–C6H3}
AlCl2{μ-Cl})(N–2,6-Pri2–C6H3)].28 The Mo atom of 9 adopts a
trigonal-bipyramidal coordination distorted in an ‘umbrella’
fashion,54 whereby the equatorial ligands (bridging N(2)–2,6-
Pri2–C6H3 and two methyl groups) are pushed away from the
terminal Mo–N(1) bond and towards the bridging chloride
ligand. The latter is rather weakly bonded, the Mo–Cl distance
being longer even than previously reported Ar–Cl→Mo dative
bonds (2.581(5) or 2.624(1) Å).55 The Al atom of 9 adopts a
near-tetrahedral geometry, with N(2) being trigonal planar (mean
Σang N(2) ∼ 360°). Arene ring (i) is roughly coplanar with the
MoN(2)AlCl metallacycle (iii), while ring (ii) adopts a perpen-
dicular orientation.
The test data reported in Table 1 and Fig. 1 reveal a marked
similarity in the catalytic performance of the bis(aryl) imido pro-
initiator 1 and the mixed aryl/tert-butyl bis(imido) complex 3,
while those for the bis(But) imido complex 4 were significantly
poorer. Thus, it was of interest to examine in detail the interplay
between the mixed imido complex [MoCl2(N–2,6-Pri2–C6H3)
(NBut)(dme)] (3) and MexAlCl(3−x) reagents. As was the case for
1, reaction of 3 with Me3Al (6 equiv.) or MeAlCl2 (3 equiv.) in
toluene smoothly afforded the Lewis acid-containing complexes
[MoMe2(N{2,6-Pri2–C6H3}AlX2{μ-Cl})(NBut)] where X = Me
(10) and X = Cl (11), respectively, Scheme 2.‡ Consistent with
the proposed structure, complex 10 exhibits both Mo–Me and
1
Al–Me resonances in its H (δ 1.39 and −0.21 ppm, respect-
ively) and 13C (δ 39.3 and −6.7 ppm, respectively) NMR
spectra. The regiochemistry of complex 10 was confirmed via a
1H/1H NOESY experiment, which revealed a unique correlation
between the Me groups of the 2,6-Pri2-C6H3 moiety and those
bound to aluminium, consistent with a static structure in which
there is coordination of Me2AlCl to the aryl imido nitrogen only.
Comparable NMR spectroscopic data were recorded for the Mo-
methyl groups of complex 11 (1H δ 1.38; 13C δ 45.1 ppm). The
weaker Lewis acidity of the bound AlMe2 residue compared to
that of the AlCl2 unit is reflected in the values of Δδ associated
Fig. 3 Molecular
structures
of
[MoMe2(N{2,6-Pri2–C6H3}
AlCl2{μ-Cl})(N–2,6-Pri2–C6H3)]·½CH2Cl2 (9)—independent molecule
A only, and [MoMe2(N{2,6-Pri2–C6H3}AlCl2{μ-Cl})(NBut)] (11). H
atoms are omitted for clarity and the thermal ellipsoids are shown at the
50% level.
5506 | Dalton Trans., 2012, 41, 5502–5511
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with the tert-butyl imido ligands of complexes 10 (45.6 ppm)
[MoMe2(N–2,6-Pri2–C6H3)(L)] (L = thf (12); L = PMe3 (13)
together with the corresponding base adducts [Me2AlCl(L)],
while reaction of 8 with NEt3 (5 equiv.) afforded the previously
reported base-free complex [MoMe2(N–2,6-Pri2–C6H3)2].28,53 In
contrast, no reaction occurred either between 8 or even its base-
free analogue, [MoMe2(N–2,6-Pri2–C6H3)2], and ethylene (5
equiv.) in C6D5Cl or C6D6 even at elevated temperature (60 °C),
something that has been ascribed to the poor nucleophilicity of
the olefin.
Analogous reactions with Lewis bases were attempted for the
mixed bis(imido) complex 10. As was the case with 8 (d6-
benzene solution) no reaction was found to occur with excess
ethylene, while reactions with thf, PMe3 and NEt3 (5 equiv.)
afforded unattributable mixtures of products. This latter obser-
vation has been ascribed to the occurrence of imido ligand
exchange, something that was reported during attempts to
prepare the base-free complex [MoMe2(N–2,6-Pri2–C6H3)
(NBut)].53
and 11 (48.2 ppm), respectively, which indicate greater Nimido
→
Mo lone pair donation for the latter.51
Recrystallisation of complex 11 from toluene afforded light
yellow crystals suitable for an X-ray diffraction study. The struc-
ture contains one independent molecule (Fig. 3, Table 2) geome-
trically similar to 9, except that the Mo–N–C bond angle of the
terminal imido ligand in 11 is greater, presumably as a result of
steric constraints.56 The trigonal bipyramidal coordination of the
Mo centre exhibits a comparable ‘umbrella-type’ distortion to
that observed for 9, with the Mo atom being displaced from the
N(2)C(1)C(2) plane towards N(1) by 0.31 Å (cf. 0.35 Å in 9).
Specific coordination of the AlCl3 moiety only to the aryl imido
nitrogen N(2) rather than the tert-butyl imido nitrogen is appar-
ent, which is in agreement with the inferences made for the ana-
logous Me2AlCl derivative 10 by NMR spectroscopy. As
determined for complex 9, the aluminium-bound nitrogen N(2)
in 11 is trigonal planar (Σang N(2) 359.95°).
Complexes 9 and 11 result from regioselective coordination of
the MexAlCl3−x (x = 0 or 2), generated in situ following transme-
tallation, to the aryl imido ligand of the [MoMe2(N–2,6-Pri2–
C6H3)(NBut)] fragment. The regioselectivity obtained here is
consistent with observations that can be made from the molecu-
lar structure determination of the parent complex [MoCl2(N–2,6-
Pri2–C6H3)(NBut)(dme)] (3). Here, the Mo–N bond distance
associated with the NBut ligand is significantly shorter than that
for the aryl imido ligand (1.728(2) vs. 1.753(2) Å), which
reflects the greater π-donation of the NBut lone pair to the mol-
ybdenum centre.57 Consequently, this suggests that in complexes
9 and 11 the NBut imido lone pair is likely to be less available
for donation to the Lewis acidic MexAlCl3−x fragments, giving
rise to the observed regiochemistry in the bimetallic complex.
Conclusions
We have shown in this study that preformed molybdenum bis
(imido) complexes bearing at least one bulky 2,6-Pri2–C6H3 sub-
stituent, in combination with alkyl aluminium dichlorides
RAlCl2 (R = Me, Et), exhibit moderate activity and productivity
for ethylene dimerisation. Here, it is believed that the sterically-
demanding imido ligand is required to inhibit imido bridge for-
mation, something that can not only prevent access to a catalyti-
cally productive manifold during pro-initiator activation, but is
also a major contributing factor to initiator degradation.
Additionally, it is noted that the catalytic performance of such
bis(imido) molybdenum systems is intimately linked to the
nature of the aluminium activator, with the highest productivities
and selectivities being achieved with EtAlCl2. It is proposed that
the significantly better catalytic performance observed when
using EtAlCl2 (compared with MeAlCl2) can be attributed to
ready access to a molybdenum hydride-based catalytic manifold
as a result of β-hydride elimination in the molybdenum coordi-
nation sphere following Et–Al to Et–Mo transmetallation.
In contrast, the low levels of catalytic dimerisation activity
determined for the MoCl2(NR)2/MeAlCl2 systems is proposed to
result from the formation of low concentrations of Mo–H species
via a much less accessible α-hydride abstraction pathway. These
proposed mechanistic differences are supported by the obser-
vation that although Me-group transmetallation from MeAlCl2 to
d0 molybdenum bis(imido) complexes is facile, the resulting
new aluminium(III) species are sequestered by one of the Lewis
basic imido nitrogen atoms to afford bimetallic complexes
[MoMe2(N{R}AlMe2{μ-Cl})(NR′)] (8–11), which do not
initiate ethylene dimerisation at low ethylene concentrations.
Moreover, no reaction is observed between ethylene and the
base-free derivative [MoMe2(N–2,6-Pri2–C6H3)2] consistent with
both the low nucleophilicity of the olefin and its low intrinsic
kinetic facility for insertion. Together these observations are in
agreement with studies of related aluminium-tungsten systems,
which have also been demonstrated to be inert towards olefins,
despite computational studies having highlighted such systems
Reactions of [MoMe2(N{2,6-Pri2–C6H3}AlMe2{μ-Cl})
(N–2,6-Pri2–C6H3)] with Lewis bases
It has been established previously that tungsten imido-bound
Al(III) species may be readily displaced through reaction with an
appropriate Lewis base to afford the corresponding acid-free
imido complexes, something that is of particular importance for
the application of these molybdenum imido systems in olefin
dimerisation.28 Consequently, the reactivity of complex 8 with a
variety of strong Lewis bases was probed in d6-benzene and d5-
chlorobenzene solution (Scheme 3). Treatment of 8 with excess
thf or PMe3 (5 equiv.) cleanly afforded the known complexes
Scheme 3 (i) thf (5 equiv.) or PMe3 (5 equiv.), C6D6, room tempera-
ture (r.t.); (ii) NEt3 (5 equiv.), C6D6, r.t.
This journal is © The Royal Society of Chemistry 2012
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as potentially playing a crucial role in WCl6/aniline-mediated
olefin dimerisation.28,52
Further studies exploring the behaviour of Group 6 imido
complexes in alkene dimerisation are on-going.
pressurized with ethylene (5 bar), and stirred (1000 rpm). Next,
the autoclave was vented to enable addition of the desired activa-
tor, EtAlCl2 or MeAlCl2 (300 μmol). After activator addition,
the vessel was pressurized with ethylene (40 bar). A constant
pressure of 40 bar was maintained throughout the reaction
period, whilst ethylene consumption was monitored via a flow-
meter. After gas uptake was observed to cease, the autoclave was
isolated from the ethylene supply and cooled to 0 °C. The
reactor was carefully vented, with a portion of the vent gas being
directly fed to a GC-FID instrument equipped with gas-sampling
loop for analysis. Next, the reactor contents were treated sequen-
tially with 1000 μL of nonane (GC internal standard) and a 10%
aqueous HCl solution. A sample of the organic phase was taken
for GC-FID analysis. Any solid formed was collected, washed
repeatedly with 10% HCl and EtOH, dried overnight and
weighed.
Experimental
All operations were conducted under an atmosphere of dry nitro-
gen using standard Schlenk and cannula techniques, or in a nitro-
gen-filled Saffron Scientific glove box. NMR-scale reactions
were conducted using NMR tubes fitted with Young’s tap valves.
Bulk solvents were purified using an Innovative Technologies
SPS facility and degassed prior to use and NMR solvents (C6D6)
were dried over P2O5, distilled and degassed before use. When
appropriate, liquid reagents were dried, distilled and deoxyge-
nated. Nitrogen gas was passed through a drying column (silica–
CaCO3–P2O5). The known complexes [MoCl2(N–2,6-Pri2–
C6H3)2(dme)] (1),37,38 [MoCl2(NPh)2(dme)] (2),39 [MoCl2(N–
2,6-Pri2–C6H3)(NBut)(dme)] (3),40 [MoCl2(NBut)2(dme)] (4),41
[Mo(NHBut)2(NBut)2] (6)44 were prepared using standard litera-
ture procedures, while neat Me3Al was purchased from Alber-
marle R & D centre and EtAlCl2 from Aldrich. Solid MeAlCl2
was obtained by slow evaporation of hexane from a hexane sol-
ution obtained from Aldrich. LiNHBut was prepared through
addition of equimolar quantities of BunLi to a cold (−78 °C)
Et2O solution of HNBut. Routine NMR spectra were collected
on a Varian Unity 300 or 200, a Varian Mercury 400, Avance
400 MHz Bruker, Varian Inova 500, or a Varian 700 MHz spec-
trometers. Chemical shifts were referenced to residual protio
impurities in the deuterated solvent (1H) or the 13C shift of the
solvent. Solvent proton shifts (ppm): C6D6, 7.15. Solvent carbon
shifts (ppm): C6D6, 128.3. Elemental analyses were performed
by the Analytical Services Department of the Chemistry Depart-
ment, Durham University.
Catalysis was performed in 250 mL volume Buchi Miniclaves
equipped with stainless steel vessels with integral thermal-fluid
jackets, internal cooling coils, and mechanical mixing via gas-
entraining stirrers. Ethylene (Grade 4.5) was supplied by Linde
and passed through oxygen and moisture scrubbing columns
prior to use; ethylene flow was measured using a Siemens
Sitrans F C Massflo system (Mass 6000–Mass 2100) and the
data logged. All catalytic tests were allowed to run until ethylene
uptake had dropped below the 0.2 g min−1 lower detection
threshold. Gas phase sample GC-FID analysis was performed
using an Agilent Technologies 6890 N GC System equipped
with a 250 μL gas sample loop and GS GasPro column (30 m ×
0.32 mm) using hydrogen as carrier gas. Liquid phase sample,
GC-FID analysis was performed using an Agilent Technologies
6850 N GC System equipped with a PONA column (50 m ×
0.20 mm × 0.50 μm) using hydrogen as carrier gas.
[Mo(NHBut)2(N–2,6-Pri2–C6H3)2] (5). To a cold (−78 °C)
Et2O (30 mL) solution of [MoCl2(N–2,6-Pri2–C6H3)2(dme)] (1)
(2.0 g, 3.28 mmol) was added a cold (−78 °C) Et2O (100 mL)
solution of LiNHBut (0.52 g, 6.57 mmol). The mixture was
allowed to warm to r.t. and allowed to stir for a further 4 h. All
volatile components were removed in vacuo and the residue
extracted with hexane (3 × 30 mL) via filtration. Concentration
of the resulting hexane solution under reduced pressure and sub-
sequent recrystallisation at −35 °C afforded 5 as orange single
crystals (0.36 g, 20%). Further concentration of the hexane sol-
ution resulted in precipitation of further quantities of 5 as an
orange powder, which was collected by filtration; yield 0.45 g
1
(45%). H NMR (CDCl3, 200 MHz): δ 6.96 (6H, br, Hmeta and
3
Hpara), 6.29 (2H, br, NH), 3.57 (4H, septet, JHH = 6.8 Hz,
3
CH3CH), 1.29 (18H, s, NCCH3), 1.11 (24H, d, JHH = 6.8 Hz,
CH3CH). 13C{1H} NMR (CDCl3, 125.6 MHz): δ 153.5 (Cipso),
140.5 (Cortho), 123.3 (Cmeta), 122.1 (Cpara), 55.2 (NCCH3), 33.1
(NCCH3), 28.3 (CH3CH), 23.1 (CH3CH). Anal. calcd for
C32H54MoN4: C, 65.06; H, 9.21; N, 9.48. Found C, 65.00; H,
9.34; N, 9.58.
[Mo({μ-NBut}2AlCl2)2]
(7).44 [Mo(NHBut)2(NBut)2]
(6)
(50 mg, 0.13 mmol) was dissolved in C6D6 (0.8 mL), EtAlCl2
1
(34 mg, 0.26 mmol) added and the reaction monitored by H
NMR spectroscopy. Within 10 min quantitative formation of pre-
1
viously reported complex 7 and ethane had occurred. H NMR
(C6D6, 500 MHz): δ 1.38 (s, NBut), 0.71 (s, C2H6).
Reaction of [Mo(NHBut)2(N–2,6-Pri2–C6H3)2] with EtAlCl2
and C2H4. [Mo(NHBut)2(N–2,6-Pri2–C6H3)2] (5) (50 mg,
0.08 mmol) was dissolved in C6D6 (0.8 mL), EtAlCl2 (22 mg,
0.17 mmol) added and the reaction monitored by 1H NMR spec-
troscopy, which indicated the formation of a number of unattri-
butable products. Subsequently, the NMR tube and its contents
were cooled (−196 °C), evacuated, and ethylene (10 equiv.,
0.80 mmol) added, which lead to the formation of butenes
according to 1H NMR spectroscopy.
General procedure for evaluating ethylene dimerisation activity
of pro-initiators 1–6
Reaction of [Mo(NHBut)2(NBut)2] with MeAlCl2. [Mo
(NHBut)2(NBut)2] (6) (50 mg, 0.13 mmol) was dissolved in
C6D6 (0.8 mL), MeAlCl2 (29 mg, 0.26 mmol) added and the
reaction monitored by H NMR spectroscopy, which indicated
the quantitative formation of 7.
The autoclave was heated under vacuum at 90 °C for 1 h, then
cooled and back-filled with Ar, before charging with PhCl
(70 mL) and the required pro-initiator (20 μmol, as a 5 mM
stock solution in PhCl). The vessel was then heated (60 °C),
1
5508 | Dalton Trans., 2012, 41, 5502–5511
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Reaction of [Mo(NHBut)2(N–2,6-Pri2–C6H3)2] with MeAlCl2.
[Mo(NHBut)2(N–2,6-Pri2–C6H3)2] (5) (50 mg, 0.08 mmol) was
dissolved in C6D6 (0.8 mL), MeAlCl2 (19 mg, 0.17 mmol)
added and the reaction monitored by 1H NMR spectroscopy,
which indicated the quantitative formation of [MoMe2(N–2,6-
Pri2–C6H3)2].53
solid state and solution, which precluded acquisition of meaning-
ful 13C NMR spectroscopic and analytical data.
[MoMe2(N{2,6-Pri2–C6H3}AlMe2{μ-Cl})(NBut)] (10). [MoCl2
(N–2,6-Pri2–C6H3)(NBut)(dme)] (3) (75 mg, 0.14 mmol) was
dissolved in C6D6 (0.8 mL) and to this solution was added
Me3Al (64 mg, 0.88 mmol). This resulted in the quantitative
(by NMR spectroscopy) formation of complex 10. 1H (C6D6,
[MoMe2(N{2,6-Pri2–C6H3}AlMe2{μ-Cl})(N–2,6-Pri2–C6H3)]
(8). [MoCl2(N–2,6-Pri2–C6H3)2(dme)] (1) (1.0 g, 1.64 mmol)
was dissolved in hexane (30 mL) and a solution of Me3Al
(0.80 g, 11.11 mmol) in hexane (30 mL) was added. This
resulted in the immediate formation of a dark brown solution.
The reaction was allowed to proceed for 16 h, after which time
the solution was filtered to remove insoluble Me3Al·dme
adducts. The resulting solution was then cooled (−35 °C), which
led to the formation of a small quantity of brown precipitate
(Me3Al–dme by-product), which was again removed via fil-
tration. Further concentration of the hexane solution resulted in
precipitation of 8 as a yellow solid, which was collected by fil-
3
500 MHz): δ 6.92 (3H, br s, aromatic), 3.45 (2H, septet, JHH
=
=
3
6.9 Hz, CH3CH), 1.39 (6H, s, MoCH3), 1.19 (6H, d, JHH
3
6.9 Hz, CH3CH), 1.18 (6H, d, JHH = 6.9 Hz, CH3CH), 0.84
(6H, s, CCH3), −0.21 (6H, s, AlCH3). 13C{1H} NMR (C6D6,
125.6 MHz): δ 156.0 (Cipso), 140.4 (Cortho), 127.2 (Cmeta), 124.5
(Cpara), 74.2 (NCCH3), 39.3 (MoCH3), 28.6 (NCCH3), 28.2
(CH3CH), 25.8 (CH3CH), 25.0 (CH3CH), and −6.7 (AlCH3). In
1
a H–1H NOESY spectrum of the reaction solution revealed a
correlation between the CH3CH (iPr) resonances and Al(CH3)2
Cl resonances.
[MoMe2(N{2,6-Pri2–C6H3}AlCl2{μ-Cl})(NBut)] (11). [MoCl2
(N–2,6-Pri2–C6H3)(NBut)(dme)] (3) (800 mg, 1.58 mmol) was
dissolved in toluene (30 mL) and a hexane solution of MeAlCl2
(4.8 mL, 1 M in hexanes, 4.76 mmol) was added. The reaction
was allowed to stir for 16 h, after which time the solution was
filtered. Concentration of the toluene solution followed by
cooling (−5 °C) gave an initial crop of yellow micro crystals of
complex 11 that were of sufficient quality for X-ray diffraction.
After collection of the micro crystals the toluene washings were
concentrated in vacuo resulting in further precipitation of
complex 11, which was collected via filtration and washed with
hexane (3 × 5 mL). This material was then recrystallized from
CH2Cl2 to give a yellow powder, yield 330 mg (40%): 1H NMR
(C6D6, 500 MHz): δ 6.90 (3H, s, Hmeta and Hpara), 3.63 (2H,
septet, 3JHH = 7.0 Hz, CH3CH), 1.38 (6H, s, MoCH3), 1.32 (6H,
1
tration and recrystallised from hexane; yield 230 mg (24%). H
NMR (C6D6, 200 MHz): δ 6.93 (6H, s, Hmeta and Hpara), 3.53
(2H, septet 3JHH = 6.8 Hz, CH3CH), 2.59 (2H, septet 3JHH = 6.6
Hz, CH3CH), 1.64 (6H, s, MoCH3), 1.17 (6H, d, 3JHH = 6.8 Hz,
3
CH3CH), 1.02 (6H, d, JHH = 6.8 Hz, CH3CH), 0.87 (12 H, d,
3JHH = 6.6 Hz, CH3CH), −0.09 (6H, s, AlCH3). 13C{1H} NMR
(C6D6, 125.6 MHz): δ 154.0 (Cipso), 150.0 (Cortho), 140.6
(Cmeta), 125.0 (Cpara), 41.5 (Mo(CH3)2), 30.1 (CH3CH), 28.7
(CH3CH), 26.2 (CH3CH), 24.2 (CH3CH), −7.0 (AlCH3). Anal.
calcd for C28H46AlClMoN2: C, 59.10; H, 8.15; N, 4.92. Found
C, 59.28; H, 8.14; N, 5.10.
[MoMe2(N{2,6-Pri2–C6H3}AlCl2{μ-Cl})(N–2,6-Pri2–C6H3)]
(9). i) [MoCl2(N–2,6-Pri2–C6H3)2(dme)] (1) (400 mg,
0.65 mmol) was dissolved in a minimal amount of toluene
(20 mL), before addition of a solution of MeAlCl2 (1.9 mL, 1.0
M in hexanes, 1.90 mmol). The reaction mixture was allowed to
stir (16 h) before being filtered and the volatile components
removed in vacuo. Recrystallisation from CH2Cl2 gave brown
micro crystals of complex 9 of sufficient quality for single
crystal X-ray diffraction analysis. However, 1H NMR spec-
troscopy of the bulk reaction material consistently indicated the
formation of multiple reaction products. Furthermore, all
attempts to purify the bulk of the material by recrystallization
repeatedly failed. Repetition of this procedure using a larger
amount of 1 (3.0 g, 4.93 mmol) also failed to produce material
of sufficient purity. ii) Addition of solid MeAlCl2 (30 mg,
0.26 mmol) to a toluene solution (0.8 mL) of 1 (50 mg,
0.08 mmol) resulted in an immediate colour change to give a
dark brown solution; which was then charged to a recrystalliza-
tion tube. Over a 24 h period large red crystals of 9 evolved and
were collected via filtration and then washed with hexane (3 ×
2 mL). Dissolution of a sample of these crystals in C6D6 allowed
9 to be characterized using 1H NMR spectroscopy; 1H NMR
(C6D6, 200 MHz): δ 6.90 and 6.79 (6H, aromatic), 3.70 (2H,
septet, 3JHH = 6.8 Hz, CH3CH), 2.37 (2H, septet, 3JHH = 6.8 Hz,
3
3
d, JHH = 7.0 Hz, CH3CH), 1.11 (6H, d, JHH = 7.0 Hz,
CH3CH), 0.68 (9H, s, NCCH3).13C{1H} NMR (C6D6,
125.6 MHz): δ 154.5 (Cipso), 140.9 (Cortho), 128.1 (Cmeta), 124.9
(Cpara), 76.3 (NCCH3), 45.1 (MoCH3), 28.6 (CH3CH), 28.1
(NCCH3), 25.7 (CH3CH), 25.3 (CH3CH). Anal. calcd for C18-
H33AlCl3MoN2: C, 42.66; H, 6.56; N, 5.53. Found: C, 42.87; H,
6.47; N, 5.56.
[MoMe2(thf)(N–2,6-Pri2–C6H3)2] (12). [MoMe2(N{2,6-Pri2–
C6H3}AlMe2{μ-Cl})(N–2,6-Pri2–C6H3)]
(8)
(50
mg,
0.07 mmol) was dissolved in C6D6 (0.8 mL) and thf (28 μL,
0.35 mmol) added. Subsequently, the reaction was monitored by
1H NMR spectroscopy, which indicated the quantitative for-
mation of [MoMe2(thf)(N–2,6-Pri2–C6H3)2] (12) together with a
broad resonance from the aluminium-containing fragment.53
[MoMe2(PMe3)(N–2,6-Pri2–C6H3)2]
(13). [MoMe2(N{2,6-
Pri2–C6H3}AlMe2{μ-Cl})(N–2,6-Pri2–C6H3)] (8) (50 mg,
0.07 mmol) was dissolved in C6D6 (0.8 mL) and PMe3 (0.1 mL,
1.10 mmol) added via vacuum transfer at −196 °C. Following
warming to r.t., the reaction was monitored by H and 31P NMR
1
spectroscopy, which indicated the quantitative formation of
[MoMe2(PMe3)(N–2,6-Pri2–C6H3)2] (13) together with broad
resonances from the aluminium-containing residues.53
3
CH3CH), 1.67 (6H, s, MoCH3), 1.33 (6H, d, JHH = 6.8 Hz,
3
CH3CH), 0.98 (6H, d, JHH = 7 Hz, CH3CH), 0.82 (12H, d,
Reaction of [MoMe2(N{2,6-Pri2–C6H3}AlMe2{μ-Cl})(N–2,6-
3JHH = 6.6 Hz, CH3CH). Complex 9 degrades slowly both in the
Pri2–C6H3)]
(8)
with
NEt3. [MoMe2(N{2,6-Pri2–C6H3}
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Table 3 Crystal data and experimental details
5
9
11
Chemical formula
Fw
C32H54MoN4
590.73
C26H40AlCl3MoN2·½CH2Cl2
652.33
C18H32AlCl3MoN2
505.73
T/K
Crystal system
Space group (no.)
a/Å
b/Å
c/Å
120
120
125
Orthorhombic
Pbca (# 61)
10.463(1)
19.225(2)
33.429(3)
90.00
6724(1)
8, 1.167
0.41
Monoclinic
P21/n (# 14, non-standard)
16.424(2)
19.713(2)
39.485(4)
97.22(1)
12683(2)
16, 1.367
0.80
Monoclinic
P21/c (# 14)
9.046(1)
16.394(2)
16.605(2)
101.76(1)
2587.3(4)
4, 1.393
0.92
β (°)
V/Å3
Z, ρc/g cm−3
μ/mm−1
No. of reflns, total
No. of reflns, unique
Rinat,b
71726
68331
28594
8928, 6757a
0.047
22278, 11385a
0.119
6418, 5474a
0.035
R1
0.033
0.079
0.066
0.135
0.025
0.066
c
wR2
2 2
2 2
a reflections with I≥2σ(I); b R1 = ΣkFo| − |Fck/Σ|Fo|; c wR2 = {Σ[w(Fo2 − Fc ) ]/Σ[w(Fo ) ]}1/2
.
AlMe2{μ-Cl})(N–2,6-Pri2–C6H3)] (8) (50 mg, 0.07 mmol) was
dissolved in C6D6 (0.8 mL) and NEt3 (50 μL, 0.35 mmol)
added. Following warming to r.t., the reaction was monitored by
1H NMR spectroscopy, which indicated the quantitative for-
mation of [MoMe2(N–2,6-Pri2–C6H3)2] together with a broad
resonance from the aluminium-containing fragment.53
Cambridge Crystallographic Data Centre, CCDC-852768 (5),
CCDC-852769 (9), and 852770 (11).
Acknowledgements
Durham University and Sasol Technology UK Ltd. (studentships
WRHW and AMM) are warmly acknowledged for financial
support of this work. We thank Dr A. M. Kenwright,
Mrs. C. Heffernan, and Mr I. McKeag for assistance with NMR
studies.
Reaction of [MoMe2(N{2,6-Pri2–C6H3}AlMe2{μ-Cl})(N–2,6-
Pri2–C6H3)]
(8)
with
C2H4. [MoMe2(N{2,6-Pri2–C6H3}
AlMe2{μ-Cl})(N–2,6-Pri2–C6H3)] (8) (50 mg, 0.08 mmol) was
dissolved in C6D6 (0.8 mL) and the solution cooled (−196 °C),
evacuated, and ethylene (10 equiv., 0.80 mmol) added, which
lead to the formation of butenes according to 1H NMR
spectroscopy.
Notes and references
‡For the preparation of complexes 7–10 excess alkyl aluminium reagent
is required since the corresponding dme adduct of MexAlCl3−x (x = 1 or
3) is obtained in each case; such species have been reported
previously.63
X-Ray crystallography
X-Ray diffraction experiments (Table 3) were carried out on
Bruker 3-circle diffractometers with CCD area detectors Pro-
teumM APEX (for 5) or SMART 1000 (for 9 and 11), using
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) from
a 60 W Mo-target microfocus Bede Microsource® X-ray genera-
tor with glass polycapillary X-ray optics (5) or a sealed tube (9,
11). The crystals were cooled using Cryostream 600 (Oxford
Cryosystems) open-flow N2 cryostats. Full sphere of the recipro-
cal space was covered with narrow-frame (0.3°) ω scans. Diffrac-
tion from 9 was very weak, mean I/σ(I) = 4.2. Reflection
intensities were corrected for absorption by semi-empirical
method based on Laue equivalents.58 The structures were solved
by direct methods and refined by full-matrix least squares
against F2 of all reflections, using SHELXTL 6.14,59 PLATON60
and OLEX261 software. In the crystal of 9, two independent
CH2Cl2 molecules of crystallisation occupied voids of 133 and
185 Å3, respectively. The latter molecule showed chaotic dis-
order, which could not be modelled satisfactorily, therefore the
refinement was completed with the observed structure factors
corrected using BYPASS procedure.62 Full crystallographic data,
excluding structure factors, have been deposited at the
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