Synthesis, solid state and DFT structure and olefin polymerization capability of a unique base-free dimeric methyl titanium dication

Article abstract of DOI:10.1039/b925781k

Reaction of Cp*Ti{NC(ArF₂)NⁱPr ₂}Me₂ (1, ArF₂ = 2,6-C₆H ₃F₂) with [Ph₃C][B(C₆F ₅)₄] gave the base-free structurally authenticated dication [Cp*₂Ti₂{NC(ArF₂)N ⁱPr₂}₂(μ-Me)₂][B(C ₆F₅)₄]₂ (3-[BF₂₀] ₂) containing two doubly α-agostic bridging methyl groups. 3-[BF₂₀]₂ is a highly effective ethylene-propylene polymerization catalyst at 90 °C, and its performance is identical to the catalyst generated in situ from 1 and [Ph₃C][B(C₆F ₅)₄].

Full text of DOI:10.1039/b925781k

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Synthesis, solid state and DFT structure and olefin polymerization
capability of a unique base-free dimeric methyl titanium dicationw
Edwin G. Ijpeij,a Betty Coussens,a Martin A. Zuideveld,a Gerard H. J. van Doremaele,*a
Philip Mountford,b Martin Lutzc and Anthony L. Spekc
Received 11th December 2009, Accepted 19th March 2010
First published as an Advance Article on the web 7th April 2010
DOI: 10.1039/b925781k
Reaction of Cp*Ti{NC(ArF2)NiPr2}Me2 (1, ArF2 = 2,6-C6H3F2)
with [Ph3C][B(C6F5)4] gave the base-free structurally authenti-
cated dication [Cp*2Ti2{NC(ArF2)NiPr2}2(m-Me)2][B(C6F5)4]2
(3-[BF20]2) containing two doubly a-agostic bridging methyl
groups. 3-[BF20]2 is a highly effective ethylene–propylene poly-
merization catalyst at 90 1C, and its performance is identical to
the catalyst generated in situ from 1 and [Ph3C][B(C6F5)4].
Following the spectacular success of Group 4 metallocenes
as single-site olefin polymerization catalysts,1–4 numerous
half-sandwich and non-cyclopentadienyl systems have been
developed by replacing one or both cyclopentadienyl ligands
by heteroatom donor moieties.4–11 An essential feature of
Group 4 (and most other) metal catalysts is a highly electron-
deficient alkyl cation [(L)M–R]+ (R = initiating group or
polymeryl chain) which is the initiating or active species. The
Scheme 1 Synthesis of the new titaniuꢀm methyl cations derived from
nature of these cations and other species which may be formed
i
Cp*Ti{NC(ArF )N Pr2}Me2 (1). [BF20
] anions omitted.
2
prior to or during the polymerization process continues to be a
topic of considerable interest and importance.4,12–16
spectroscopic data and combustion analysis.w The NMR data
are fully consistent with the presence of the base-stabilized
titanium methyl monocation 2+ (depicted in Scheme 1) and a
separated [BF20]ꢀ anion. A number of half-sandwich, base-
stabilized Group 4 alkyl cations have been reported previously.4
Reaction of 1 with TBF20 in C6H5F in the absence of added
Lewis base, followed by cooling to ꢀ20 1C, gave red crystals of
[Cp*2Ti2{NC(ArF2)NiPr2}2(m-Me)2][BF20]2 (3-[BF20]2) in 53%
isolated yield.w Compound 3-[BF20]2 contains a dinuclear
methyl-bridged dication and two separated [BF20]ꢀ anions
according to X-ray crystallography (see below).z When followed
by 1H and 19F NMR in situ in C6D5Cl, 3-[BF20]2 forms
quantitatively from 1 and TBF20. As expected, addition of
POPh3 to either isolated or in situ generated 3-[BF20]2 in
Half-sandwich Group 4 complexes have proven to be highly
efficient co-polymerization catalysts.5–8 DSM recently introduced
(as Keltan ACEt)17 a new class of half-sandwich k1-amidinate
titanium complexes of the type (Z-C5R5)Ti{NC(Ar)NR0 }X2
2
(X = Me or Cl), which are extremely active pre-catalysts for
the commercial homo- and co-polymerization of olefins.18,19
i
One example is Cp*Ti{NC(ArF )N Pr2}Me2 (1, ArF = 2,6-
C6H3F2; Cp* = Z-C5Me5, Scheme 1). While investigating the
underlying chemistry of this compound we prepared the first
base-free, isolable and highly stable Group 4 m-methyl dication
dimer that is as active a pre-catalyst for olefin polymerization
as the system obtained by in situ activation of the corresponding
2
2
dimethyl pre-catalyst. We describe here our preliminary results.
Reaction of Cp*Ti{NC(ArF )N Pr2}Me2 (1) with [Ph3C]-
i
2
C6D5Cl formed 2-BF20 in quantitative yield.
i
Addition of Cp*Ti{NC(ArF )N Pr2}Me2 (1, 2 equiv.) to
2
[B(C6F5)4] (TBF20) in fluorobenzene in the presence of POPh3
resulted in good yields of [Cp*Ti{NC(ArF2)NiPr2}Me(OPPh3)]-
[BF20] (2-BF20), which was characterised on the basis of
3-[BF20]2 in C6D5Cl formed [Cp*2Ti2{NC(ArF2)NiPr2}2–
Me2(m-Me)][BF20] (4-BF20, Scheme 1) in quantitative yield.
This compound contains a dimeric monocation (4+) with one
bridging methyl group and terminal methyl groups. No evidence
for separated 1 and 32+ was found in the NMR spectra of
a DSM Research B.V., P.O. Box 18, 6160MD Geleen, the
Netherlands. E-mail: Gerard.Doremaele-van@dsm.com
b Chemistry Research Laboratory, Department of Chemistry,
University of Oxford, Mansfield Road, Oxford OX1 3TA, UK.
E-mail: philip.mountford@chem.ox.ac.uk
1
4-BF20. According to H and 19F NMR spectroscopy, cation
4+ exists as a mixture of diastereoisomers (arising from
different orientations of the amidinate NiPr2 and ArF substi-
2
c Bijvoet Centre for Biomolecular Research, Crystal and Structural
Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, the
Netherlands
w Electronic supplementary information (ESI) available: Experimen-
tal, crystallographic and computational details. CCDC 758120. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b925781k
tuents) in a ca. 2 : 3 ratio. Nonetheless, addition of 1 equiv.
POPh3 quantitatively formed 1 and 2-BF20. As has been
found for several compounds of the type [(L)2M2Me2(m-Me)]+
(M = Group 4 metal),14a,15c,20 4-BF20 can also be made on the
preparative scale by the addition of 0.5 equiv. TBF20 to 1.
ꢁc
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Fig.
(62+) emphasising the unusual m-methyl group geometry.23
2
The structure of [Cp*2Zr2{MeC(NtBu)(NEt)}2(m-Me)2]2+
the reported data finds the Hꢂ ꢂ ꢂH separation between the
agostic hydrogens to be only 1.32 A (sum of the van der Waals
radii = 2.40 A).
Fig. 1 Displacement ellipsoid plot (50% probability level) of 32+
.
Hydrogen atoms, others than those of the m-Me groups omitted.
Selected distances (A) and angles (1): Ti1–N1 1.7913(13), Ti1–Cg1
2.042(1), Ti1–C24 2.2575(15), Ti1–C24i 2.2838(15), Ti1–H24A 2.14(2),
Ti1–H24Bi 2.27(2), Ti1–N1–C1 170.97(12), Cg1–Ti1–C24 118.83(5),
Cg1–Ti1–C24i 114.49(5), C24–Ti1–C24i 94.83(5), Ti1–C24–Ti1i
85.18(5). Cg1 represents the computed Cp* centroid. Symmetry
operation i = ꢀx, ꢀy, ꢀz.
Because the arrangement of the m-methyl group hydrogens
in 62+ is very much at odds with those in 32+ and 5+ (and
neutral analogues [(L)M(m-Me)2M(L)]), we sought to gain
further information on these systems by carrying out DFT
calculations on both 32+ and 62+.w The minimum energy
structure for the titanium dication showed excellent agreement
with experiment (e.g., Ti–N: calcd 1.794, found 1.7913(13) A;
avg. Ti–Me: calcd 2.272, found 2.2575(15) and 2.2838(15) A;
avg. Ti–Hagostic: calcd 2.20, found 2.14(2) and 2.27(2) A). On
the other hand, regardless of the starting m-CH3 group geo-
metry, the DFT structure of 62+ minimized to be analogous to
the m-CH3 geometry in 32+ and 5+. Note that apart from the
differing arrangements of these H atoms, the computed and
experimental parameters for 62+ were in excellent agreement
(e.g., avg. Zr–Me: calcd 2.432, found 2.447 A). On the basis
of the structures of 32+ and 5+ and the DFT calculations
the m-Me geometries of 32+ are without doubt ‘‘normal’’ with
regard to other compounds of the type (L)M(m-Me)2M(L).
The unusual geometry reported for 62+ is inconsistent with
these other results.
Unfortunately, 4-BF20 forms as a waxy solid when isolated
and its solid state structure could not be determined.
The remarkable solid state structure of the centrosymmetric
dication [Cp*2Ti2{NC(ArF2)NiPr2}2(m-Me)2]2+ (32+) is shown
in Fig. 1. 32+ contains pseudo-tetrahedral titanium centres,
each bound to an amidinate and Cp* ligand and two approxi-
mately symmetrically bridging methyl groups. The H atoms of
the latter were located from a Fourier map and positionally
refined. Each m-methyl group in 32+ forms two a-agostic
contacts, one to each Ti. The Ti–Me distances in 32+
(2.2575(15) and 2.2838(15) A) are much longer than in 1
(avg. 2.119 A). This is expected since the Ti–Me–Ti bonding
in 32+ is based only on 3-centre, 2-electron interactions, and
there is also substantial coulombic repulsion between the two
cationic centres.
Although it was speculated previously23 that 62+ could have
relevance to catalysis, this was not demonstrated, the precursor
solutions decomposing at ambient temperature. The solution
stability of [Cp*2Ti2{NC(ArF2)NiPr2}2(m-Me)2][BF20]2 (3-[BF20]2)
offered a unique opportunity to assess the polymerization
initiating capability of a non-stabilized dialkyl-dication for
the first time.
As mentioned, the nature of the species present in Ziegler–
Natta polymerization catalysis is a topic of considerable
interest.4,12–16 Although neutral homobimetallic complexes of the
type (L)M(m-Me)2M(L) have been structurally authenticated,21
none has been reported for Group 4. The only structurally
authenticated Lewis base- or coordinated anion-free titanium
analogue of 32+ is the very recently reported AlMe3-stabilized
heterobimetallic monocation [Ti(Me3[9]aneN3)(NtBu)(m-Me)2-
AlMe2]+ (5+).16 Like 32+, this has two square based pyramidal
m-methyl groups, each forming an a-agostic contact to Ti
(H atoms positionally refined). The DFT computed structure
of 5+ confirmed the one determined crystallographically. Coordi-
nation of AlMe3 to [Ti(Me3[9]aneN3)(NtBu)Me]+ significantly
inhibits polymerization activity.22
Table 1 compares the ethylene–propylene co-polymerization
capability at 90 1C of in situ activated Cp*Ti{NC(ArF2)NiPr2}Me2
(1) (using TBF20) and that of isolated [Cp*2Ti2{NC(ArF2)-
NiPr2}2(m-Me)2][BF20]2 (3-[BF20]2) and [Cp*2Ti2{NC(ArF2)-
NiPr2}2Me2(m-Me)][BF20] (4-BF20). All three have overall
comparable productivities, and key polymer characteristics such
as molecular weight, molecular weight distribution and compo-
sition (ethylene and propylene content) are indistinguishable
within experimental error. It is evident that both the in situ 1 +
TBF20 and isolated 3-[BF20]2 catalyst systems give rise to the
same equilibrium level amounts of the same monomeric active
species [Cp*Ti{NC(ArF2)NiPr2}R]+ (R = polymeryl chain), in
the same way that [Cp*Ti{NC(ArF2)NiPr2}Me(OPPh3)][BF20]
(2-BF20) can be formed from all three systems 1 + TBF20,
3-[BF20]2 and 4-[BF20] (Scheme 1).
Despite the aforementioned intense international efforts around
the chemistry of alkyl cations over the last two decades, only one
dicationic analogue of 32+ has been reported, namely Sita’s
dizirconium dication [Cp*2Zr2{MeC(NtBu)(NEt)}2(m-Me)2]2+
(62+, Fig. 2).23 The structure of 62+ has attracted much
attention owing to postulated implications for Ziegler–Natta
catalysis, and also the very unusual reported arrangement of
the Zr2(m-Me)2 hydrogens. Each m-methyl group was described
as having a trigonal bipyramidal geometry and an unprecedented
doubly-bridging agostic C–H bond (Fig. 2; none of the H atoms
were positionally refined). Remarkably, close examination of
In conclusion, we have reported a base-free single site Ziegler–
Natta catalyst system of commercial relevance that can be
isolated and stabilized in the solid state. The solid state m-methyl
group geometry of [Cp*2Ti2{NC(ArF2)NiPr2}2(m-Me)2]2+ (3-[BF20]2)
ꢁc
This journal is The Royal Society of Chemistry 2010
3340 | Chem. Commun., 2010, 46, 3339–3341

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i
F2
i
Table 1 Ethylene–propylene co-polymerization data using compounds Cp*Ti{NC(ArF )N Pr2}Me2 (1), [Cp*2Ti2{NC(Ar )N Pr2}2(m-Me)2][BF20 2
]
2
i
a
(3-[BF20]2) and [Cp*2Ti2{NC(ArF )N Pr2}2Me2(m-Me)][BF20] (4-BF20
)
2
Polymer compositionb
(wt%)
Molar mass distribution (GPC, g molꢀ1
)
C
C3
Mn
Mw/Mn
Entry Compound Co-catalyst Catalyst dosing (mmol Ti) Polymer yield/g
2
1
2
3
1
3-[BF20 2
4-BF20
TBF20
None
None
0.20
0.20
0.40
10.7c
12.5d
8.95e
48
47
50
52
53
50
215 000
210 000
213 000
2.0
2.0
2.3
]
a
Experimental conditions: total pressure 8 bar, T = 90 1C, reaction time 10 min, solvent pentamethylheptane (1 L). AliBu3 scavenger (Al : Ti
ratio = 2250 modified by 4-methyl-2,6-di-tert-butylphenol (1 : 1 molar ratio). The ethylene : propylene molar ratio in the head space was kept at
1 : 2 by continuously feeding the gas to the reactor equipped with a purge to prevent polymer composition drift. After polymerization 500 ppm of
b
Irganox 1076 stabilizer was added to the solution. Volatiles were removed under reduced pressure at 100 1C. C2 = ethylene content, C3
=
.
c
propylene content, as determined by FT-IR spectroscopy. Equivalent to 4.01 ꢃ 104 kg (polymer) mol(Ti)ꢀ1 hꢀ1 (total pressure/bar)ꢀ1
d
e
Equivalent to 4.69 ꢃ 104 kg (polymer) mol(Ti)ꢀ1 hꢀ1 (total pressure/bar)ꢀ1
.
Equivalent to 3.36 ꢃ 104 kg (polymer) mol(Ti)ꢀ1 hꢀ1 (total
pressure/bar)ꢀ1
.
(supported by DFT calculations for 32+) suggests that the
unusual structure reported for the dizirconium system 62+
may warrant further investigation.
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z Crystal data for 4: [C48H70F4N4Ti2](C24BF20)2 + disordered solvent,
ꢀ
Fw = 2232.98 [*], triclinic, P1, a = 13.1212(4), b = 13.2408(3), c =
18.5640(3) A, a = 105.738(1)1, b = 105.857(2)1, g = 96.491(2)1, V =
2924.53(13) A3, T = 150(2) K, 53 377/12 979 measured/unique refl.,
Rint = 0.021, Z = 1, Dx = 1.268 g cmꢀ3 [*], m = 0.25 mmꢀ1 [*].
R1/wR2 [I > 2s(I)]: 0.0415/0.1335. R/wR2 [all refl.]: 0.0465/0.1388.
S = 1.074. [*] derived values do not contain the contribution of the
disordered solvent treated using the SQUEEZE routine of
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ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 3339–3341 | 3341