Synthesis and catalytic activity of binuclear titanium imido complexes

Article abstract of DOI:10.1039/b315711c

Treatment of the N-P ligand Ar^PN(Si Me₃)₂ with TiCl₄ affords the imido-bridged binuclear titanium complex [TiCl₂(THF)(μ- NAr^P)]₂ (Ar^P = m-C₆H₄ PR₂) which reacts with Ni(0) or Pd(II) to give heterotrinuclear compounds, while activation with methylaluminoxane generates a new type of imido-based ethene polymerisation catalyst that is tolerant of -PR₂ functional groups.

Full text of DOI:10.1039/b315711c

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Synthesis and catalytic activity of binuclear titanium imido
complexes
Musa Said, David L. Hughes and Manfred Bochmann*
Wolfson Materials and Catalysis Centre, School of Chemical Sciences,
University of East Anglia, Norwich, UK NR4 7TJ. E-mail: m.bochmann@uea.ac.uk;
Fax: (ϩ44)1603 592044; Tel: (ϩ44) 1603 592044
Received 3rd December 2003, Accepted 17th December 2003
First published as an Advance Article on the web 7th January 2004
Treatment of the N–P ligand ArPN(SiMe3)2 with TiCl4
affords the imido-bridged binuclear titanium complex
[TiCl2(THF)(ꢀ-NArP)]2 (ArP ؍ m-C6H4PR2) which reacts
with Ni(0) or Pd(II) to give heterotrinuclear compounds,
while activation with methylaluminoxane generates a new
type of imido-based ethene polymerisation catalyst that is
tolerant of –PR2 functional groups.
The synthesis of soluble alkene polymerisation catalysts based
on non-metallocene ligand frameworks continues to attract
considerable attention.1,2 Numerous structural motifs are
known, most commonly employing N,N and N,O chelate
ligands, such as diamides (N–N)2Ϫ and iminophenoxides
(O–N)Ϫ. Notable examples are octahedral titanium and
zirconium bis(phenoxyimine) complexes of the type (O–N)2-
MCl2.3 We have been interested in the possiblity of generating
catalysts based on binuclear complexes and have recently
reported an asymmetric example, (N–O)TiCl(µ-Cl)3TiCl3.4 We
report here the synthesis and reactivity of binuclear imido-
bridged compounds of the type [TiCl2(µ-NAr)2]2.
The phosphinoaryl amine ligand 1 was readily prepared from
m-ClMgC6H4N(SiMe3)2 and R1 PCl in 85% yield [R1 = 3,5-
2
(CF3)2C6H3]. The ligand was chosen to explore the possibility
of forming early–late transition metal heteronuclear complexes
by selective coordination to the N and/or P donor. Ligand 1
reacts with TiCl4 in dichloromethane at Ϫ78 ЊC under dehalo-
silylation to give the imido complex 2a as a deep red micro-
crystalline solid. Recrystallising 2a from THF or conducting
the reaction in THF instead of CH2Cl2 affords the bis-THF
complex 2b, which is more soluble in organic solvents (Scheme
1).†
Scheme 1 Reagents and conditions: (i), TiCl4, CH2Cl2, Ϫ78 ЊC to room
The reaction of 2b with Ni(cod)2 or PdCl2(cod) (cod = 1,5-
cyclooctadiene) afforded the corresponding heteronuclear
complexes 2aؒNi(cod) (3) and 2bؒPdCl2 (4), respectively, as
microcrystalline solids. The compounds were identified on
the basis of their elemental analyses but proved insoluble in
dichloromethane and are evidently coordination polymers.
Unfortunately attempts to grow crystals of 2–4 suitable
for X-ray diffraction proved unsuccessful. The spectrocopic
parameters of 2 did not allow us to distinguish unequivocally
between a mononuclear structure of the type RN᎐TiCl (L),5 or
temperature; (ii), THF; (iii), Ni(cod)2, dichloromethane, 16 h, 23 ЊC;
(iv) PdCl2(cod), dichloromethane, 16 h, 23 ЊC; (v) MeLi, Et2O, 0 ЊC to
room temperature, 5 h.
᎐
2
a dimeric chloro- or imido-bridged alternative. However, alkyl-
ation of 2b with methyllithium in diethyl ether at 0 ЊC afforded
the corresponding titanium dimethyl complex 5 as red crystals.
The X-ray structure of 5 (Fig. 1) confirms the binuclear struc-
ture, with a core Ti2N2 four-membered ring.‡ The formation of
a cyclic structure is in contrast to the formation of the mono-
nuclear imido compound Ti(NBut)Cl2(py)3 or the exclusively
chloride-bridged [TiCl2(NBut)(H2NBut)]4, both products of the
reaction of TiCl4 with ButN(H)SiMe3.6,7 It is also noted that
the the phosphinoaniline m-Ph2PC6H4N(H)SiMe3 has been
reported to react with TiCl4 to afford an amido complex,
TiCl[N(SiMe3)C6H4PPh2]3.8 Each titanium atom in 5 occupies
the centre of a trigonal bipyramid, with one imido-N trans to
the THF ligand, while the other N atom occupies an equatorial
position. The Ti–N distances within the ring are almost
Fig. 1 Structure of compound 5, showing the atomic numbering
scheme. Ellipsoids are drawn at 50% probability. H atoms are omitted
for clarity. Selected bond lengths (Å) and angles (Њ): Ti–C(3) 2.147(7),
Ti–C(4) 2.125(8), Ti–N(1) 1.971(6), Ti–N(1Ј) 1.921(6), Ti–O(2)
2.188(5); N(1)–Ti–N(1Ј) 83.4(2), Ti–N(1)–TiЈ 96.6(2), C(3)–Ti–N(1)
122.3(3), C(4)–Ti–N(1) 120.0(3), C(3)–Ti–C(4) 117.2(3), N(1Ј)–Ti–O(2)
169.1(2), N(1)–Ti–O(2) 87.8(2), C(3)–Ti–N(1Ј) 92.6(3), C(4)–Ti–N(1Ј)
101.8(3).
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 3 5 9 – 3 6 0
359

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Table 1 Ethene polymerisations with titanium imido complexesa
filtration and dried in vacuo, yield 1.50 g (71%). 1H NMR (300.13 MHz,
THF-d8): δ 6.66–8.07 (Ph). 13C{1H} NMR (75.46 MHz, CD2Cl2):
δ 120.00–162.37 (Ph ϩ CF3). 31P NMR (121.49 MHz, THF-d8): δ 1.39.
Anal.: Found (calcd.): C, 39.42 (39.67); H, 1.47 (1.51); N, 2.05 (2.10);
Cl, 10.92 (10.65). 2b: Recrystallisation of 2a from THF or conducting
the reaction in THF instead of CH2Cl2 led to the formation of 2b which
is more soluble in aromatic solvents. Anal.: Found (calcd.): C, 42.95
(42.30); H, 3.09 (2.46); N, 1.86 (1.90); Cl, 10.15 (9.61).
3: A mixture of 2b (1.0 g, 0.65 mmol) and Ni(cod)2 (0.18 g,
0.65 mmol) was stirred in dichloromethane (30 mL) overnight at room
temperature. The resulting precipitate was collected on a sintered glass
funnel, washed with dichloromethane (5 mL) and dried in vacuo, yield
0.55 g (57%). Anal.: Found (calcd.): C, 41.47 (41.66); H, 2.62 (2.16); N,
1.97 (1.87); Cl, 10.19 (9.47).
Complex
Ti/µmol
Al/Ti ratio
Activityb
Mw
2b
2b
2b
3
5
10
20
20
1000
1000
1000
2000
5.5
4.2
6.2
4.0
n.d.
n.d.
190,000
n.d.c
a Conditions: 50 mL toluene, 20 ЊC, 15 min, 1 bar ethene. Reactions
were terminated by methanol injection. b In 105 g-PE (mol Ti)Ϫ1 hϪ1
barϪ1 c Insufficiently soluble for GPC analysis.
.
4: Following the procedure for 3, a solution of 2b (0.4 g, 0.27 mmol),
was treated with PdCl2(cod) (0.08 g, 0.27 mmol) to give a deep red
precipitate, yield 0.3 g (67%). Anal.: Found (calcd.): C, 38.45 (37.77); H,
2.88 (2.19); N, 1.56 (1.69); Cl, 12.57 (12.86).
identical, 1.971(6) and 1.921(6) Å, slightly longer than the Ti–N
distances of 1.904(2) and 1.916(2) Å in the structurally related
mono-Cp complex [CpRTi(Cl)(µ-NBut)]2 [CpR
= C5H4B-
(C6F5)2(H2NBut)].9 The TiC2N(1) moiety deviates little from
planarity (angle sum 359.5Њ). The Ti–C bond lengths are
slightly longer than those found in TiMe4(THF) (average 2.096
Å).10 The Ti–O(THF) bond in 5 of 2.188(5) is 0.2 Å shorter
than in TiMe4(THF) but longer than in [Cp*2TiMe(THF)]ϩ
(2.154(6) Å).11
5: To a cold solution (0 ЊC) of 2b (1.02 g, 0.69 mmol) in Et2O was
added dropwise MeLi (2.76 mmol, 1.6 M in Et2O). When the addition
was complete, the solution was allowed to warm to room temperature
and stirred for 5 h. The filtrate was concentrated to one third and light
petroleum (5 mL, bp 40–60 ЊC) was added. Cooling to Ϫ20 ЊC yielded
red crystals, 0.55 g (57%), some of which were suitable for crystallo-
graphy. Anal.: Found (calcd.): C, 48.58 (48.23); H, 3.59 (3.47); N, 2.30
(2.01). 1H NMR (300.13 MHz, CD2Cl2): δ 0.84 (s, TiMe), 1.73 (br,
THF), 3.68 (br, THF), 6.23–8.22 (Ph). 13C{1H} NMR (75.46 MHz,
CD2Cl2): δ 27.63 (s, TiMe), 123.52–143.60 (Ph, CF3). 31P NMR (121.49
MHz, THF-d8): δ Ϫ0.50.
On activation with methylaluminoxane (MAO), complex 2b
catalyses the polymerisation of ethene under mild conditions
(Table 1). Within the test range productivities are independent
of [Ti], and hence not mass-transport limited. In some cases
high molecular weight polyethene was obtained which proved
to be insufficiently soluble for gel permeation chromatography
at 160 ЊC. Very similar productivities were obtained with the
heterobinuclear complex 3, i.e. the catalysts are unaffected by
the presence of complexed or uncomplexed diarylphosphine
substituents.12
The results show that titanium imido complexes constitute a
new type of readily accessible and comparatively robust poly-
merisation catalysts. The formation of well-defined alkyls sug-
gests that the Ti2N2 framework is likely to be unaffected by
activators such as MAO under polymerisation conditions, and
high molecular weight polymers are obtained without the need
for sterically highly hindered or specifically tailored ligand
environments. Studies exploring this versatile class of catalysts
in more detail are in progress.
‡ Crystal data for 5: C56H48F24N2O2P2Ti2, M = 1394.7. Triclinic, space
¯
group P1 (no. 2), a = 9.572(4), b = 9.754(10), c = 16.736(14) Å, α =
77.03(7), β = 86.91(2), γ = 87.63(1)Њ, V = 1520(2) Å3. Z = 1, Dc = 1.524 g
cmϪ3, F(000) = 704, T = 140(1) K, µ(Mo-Kα) = 4.3 cmϪ1, λ(Mo-Kα) =
0.71069 Å, 7860 reflections measured, 4957 unique (Rint = 0.144), F 2
refinement, R1 = 0.119 (I > 2σ(I )), wR2 = 0.301 (all data). Crystals were
not single. For the sample measured (Rigaku MSC R-Axis IIc image
plate diffractometer), data for the major crystal were selected, and dif-
fraction from the seondary crystal(s) were ignored. Also, there was site
disorder in one of the CF3 groups. CCDC reference number 225816.
See http://www.rsc.org/suppdata/dt/b3/b315711c/ for crystallographic
data in CIF or other electronic format.
1 G. J. P. Britovsek and V. C. Gibson, Angew. Chem. Int. Ed., 1999, 38,
429; V. C. Gibson and S. K. Spitzmesser, Chem. Rev., 2003, 103, 283.
2 S. D. Ittel, L. K. Johnson and M. Brookhart, Chem Rev., 2000, 100,
1169.
3 See for example R. Furuyama, J. Saito, S. Ishii, M. Mitani,
S. Matsui, Y. Tohi, H. Makio, N. Matsukawa, H. Tanaka and
T. Fujita, J. Mol. Catal. A: Chem., 2003, 200, 31 and cited references.
P. D. Hustad and G. W. Coates, J. Am. Chem. Soc., 2002, 124, 11578;
S. Reinartz, A. F. Mason, E. B. Lobkovsky and G. W. Coates,
Organometallics, 2003, 22, 2542 and cited references.
Acknowledgements
We thank the Engineering and Physical Science Research
Council for support.
4 D. A. Pennington, D. L. Hughes, M. Bochmann and S. J. Lancaster,
Dalton Trans., 2003, 3480.
5 L. H. Gade and P. Mountford, Coord. Chem. Rev., 2001, 216, 65;
P. Mountford, Chem. Commun., 1997, 2127.
Notes and references
† Synthesis and spectroscopic data: 1: To a 1.0 M THF solution of
3-N,N-bis(trimethylsilyl)amino)phenylmagnesium chloride (23.75 mL)
was added bis-[(3,5-bis(trifluoromethyl)phenyl] chlorophosphine
(11.7 g, 23.75 mmol) via a syringe at ambient temperature. After 1 h of
stirring, volatiles were removed in vacuo and the residue extracted with
diethyl ether (70 mL). The filtrate was concentrated and light petroleum
(30 mL) was added. Cooling to Ϫ20 ЊC gave solid 1, yield 14 g (85%).
1H NMR (300.13 MHz, C6D6): δ Ϫ0.031 (s, SiMe3), 7.05–7.85 (Ph).
13C{1H} NMR (75.46 MHz, C6D6): δ 1.95 (s, SiMe3), 123.36–150.30
(Ph ϩ CF3). 31P NMR (121.49 MHz, C6D6): δ Ϫ4.45. Anal.: Found
(calcd.): C, 48.57 (48.48); H, 4.15 (4.07); N, 2.17 (2.02).
6 P. E. Collier, S. C. Dunn, P. Mountford, O. V. Shishkin and
D. Swallow, J. Chem. Soc., Dalton Trans., 1995, 3743.
7 C. J. Carmalt, A. C. Newport, I. P. Parkin, A. J. P. White and
D. J. Williams, J. Chem. Soc., Dalton Trans., 2002, 4055.
8 Q. F. Mokuolu, A. G. Avent, P. B. Hitchcock and J. B. Love,
J. Chem. Soc., Dalton Trans., 2001, 2551.
9 S. J. Lancaster, S. Al-Bennah, M. Thornton-Pett and M. Bochmann,
Organometallics, 2000, 19, 1599.
10 H. Windisch, K. H. Thiele, G. Kociok-Köhn and H. Schumann,
Z. Allg. Anorg. Chem., 1995, 621, 861.
11 M. Bochmann, A. J. Jaggar, L. M. Wilson, M. B. Hursthouse and
M. Motevalli, Polyhedron, 1989, 8, 1838.
12 Similar phosphine tolerance has been found in some zirconium
catalysts: M. Bochmann and M. Said, results to be published.
2a: To a cold (Ϫ78 ЊC) solution of 1 (2.20 g, 3.17 mmol) in dichloro-
methane (20 mL) was added dropwise TiCl4 with continued stirring.
Product 2a precipitated as a deep red solid which was collected by
D a l t o n T r a n s . , 2 0 0 4 , 3 5 9 – 3 6 0
360