The N,N'-bis(trimethylsilyl)pentafluorobenzamidinate ligand: Enhanced ethene oligomerisation with a neutral V(III) bis(benzamidinate) alkyl catalyst

Article abstract of DOI:10.1039/b000397m

The pentafluorobenzamidinate ligand [C₆F₅C(NSiMe₃)₂]^- is reported, together with its bis(benzamidinate) vanadium(III) methyl derivative; the latter is considerably more active in catalytic ethene olig

Full text of DOI:10.1039/b000397m

The N,NA-bis(trimethylsilyl)pentafluorobenzamidinate ligand: enhanced ethene
oligomerisation with a neutral V(III) bis(benzamidinate) alkyl catalyst†‡
Edward A. C. Brussee, Auke Meetsma, Bart Hessen* and Jan H. Teuben
Centre for Catalytic Olefin Polymerisation, Stratingh Institute of Chemistry and Chemical Engineering, University of
Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands. E-mail: hessen@chem.rug.nl
Received (in Basel, Switzerland) 20th December 1999, Accepted 18th February 2000
The pentafluorobenzamidinate ligand [C6F5C(NSiMe3)2]2
is reported, together with its bis(benzamidinate) vanadiu-
m(III) methyl derivative; the latter is considerably more
active in catalytic ethene oligomerisation than its non-
fluorinated analogue.
[C6F5C(NSiMe3)2]Li·0.5Et2O and IR spectroscopy showed no
residual nitrile bands.
Reaction of 2 equivalents of [C6F5C(NSiMe3)2]Li with
VCl3(THF)3 in THF resulted in formation of the green
bis(amidinate) complex [C6F5C(NSiMe3)2]2VCl(THF)
1
(Scheme 1), which was isolated in 58% yield. Removal of THF
by heating solid 1 in vacuo at 120 °C for 5 h yields the orange
base-free chloride [C6F5C(NSiMe3)2]2VCl 2. The 19F NMR
spectrum of paramagnetic 2 shows a strong downfield shift of
the o-F resonance (d 2106.8, Dn1/2 380 Hz), and less
pronounced shifts for the p-F (d 2144.1, Dn1/2 85 Hz) and m-F
(d 2155.3, Dn1/2 45 Hz) resonances.
Fluorination of ancillary ligands attched to catalytically active
transition-metal centres leads in many cases to a significant
improvement in catalytic properties relative to the non-
fluorinated analogues. Examples of this can be found in the
metathesis catalysts [(CF3)nMe3 2 nCO]2W(CHCMe3)(NAr) (n
= 0, 1 2; Ar = 2,6-diisopropylphenyl)1 and in the olefin
polymerisation
catalysts
based
on
[2-pyridyl–
It proved to be difficult to obtain single crystals of 2 that were
suitable for X-ray diffraction. For structural characterisation the
benzonitrile adduct [C6F5C(NSiMe3)2]2VCl(NCPh) 3a was
prepared by addition of benzonitrile to a pentane solution of 2
followed by recrystallisation from diethyl ether. Its molecular
structure§ is shown in Fig. 1. For comparison a structure
determination of the non-fluorinated analogue 3b was also
performed.§ This compound is essentially isostructural, and its
C(CF3)2O]2Zr(CH2Ph)2.2 The electron withdrawing properties
of the fluorinated ligand renders the metal centre more
electrophilic and, for many reaction types, more reactive. In this
contribution we present the synthesis of a new fluorinated
derivative of a well known ancillary ligand, N,NA-bis(trime-
thylsilyl)benzamidinate, and show that this leads to a significant
improvement in the catalytic performance of an ethene
oligomerisation catalyst, bis(benzamidinate)vanadium alkyl.
The N,NA-bis(trimethylsilyl)benzamidinate monoanionic an-
cillary ligand, [PhC(NSiMe3)2]2, is widely used in transition-
metal chemistry and catalysis,3 for example in the olefin
polymerisation catalysts [PhC(NSiMe3)2]2MCl2/MAO (M =
Ti, Zr, Hf; MAO = methylaluminoxane)4 and Cp*M[PhC(NSi-
Me3)2]Cl2/MAO (M = Ti, Zr, Hf).5 We recently reported the
neutral paramagnetic (S = 1) 12-electron bis(benzamidinate)-
vanadium(III) alkyls [PhC(NSiMe3)2]2VR (R = Me, Et).6 This
neutral alkyl system catalyses the oligomerisation of ethene to
linear alkenes without the need for added cocatalysts. The
productivity of the catalyst system is modest, which is likely to
be due to the relatively low electrophilicity of the metal centre.
We therefore sought to increase this electrophilicity by
preparing a benzamidinate with a more electron-withdrawing
aryl group, pentafluorophenyl, on the ligand backbone.
Scheme 1 Reagents and conditions: i, Et2O, 240 to 20 °C, 16 h; ii,
VCl3(THF)3 (0.5 equiv.), THF, 278 to 20 °C, 16 h; iii, 120 °C, vacuum, 5
h; iv, MeMgCl, THF, 278 to 20 °C, 1 h; v, MeLi, Et2O, 278 to 20 °C,
1 h.
Lithium N,NA-bis(trimethylsilyl)benzamidinate is readily pre-
pared by reacting benzonitrile with the bis(trimethylsilyl)amide
LiN(SiMe3)2.7 The reaction of pentafluorobenzonitrile with
LiN(SiMe3)2 in THF was recently reported to lead to C–F
activation of the nitrile rather than to formation of the
pentafluorobenzamidinate [C6F5C(NSiMe3)2]Li.8 We observed
that changing the solvent from THF to diethyl ether suppresses
this tendency towards C–F activation. Thus, addition of 1
equivalent of C6F5CN to a suspension of Li[N(SiMe3)2] in
diethyl ether at 240 °C and gradual warming to ambient
temperature yields a clear, pale yellow solution of the desired
pentafluorobenzamidinate that is ready for further use (Scheme
1). For spectroscopic analysis, small solid samples were
obtained by simply evaporating the solvent from an aliquot of
the solution. NMR spectroscopy (1H, 13C and 19F)9 showed
clean formation of a single product with apparent composition
† Electronic supplementary information (ESI) available. Synthetic and
spectroscopic details and ethene oligomerisation experiments. See http://
www.rsc.org/suppdata/cc/b0/b000397m
‡ Netherlands Institute for Catalysis Research (NIOK) publication no. RUG
99-4-05.
Fig. 1 Molecular structure of 3a (SiMe3 methyl groups omitted for clarity).
Selected interatomic distances (Å) and angles (°): V–Cl 2.3060(9), V–N(1)
2.082(2), V–N(2) 2.067(2), V–N(3) 2.104(2), V–N(4) 2.121(2), V–N(5)
2.153(2), N(1)–C(1) 1.326(3), N(2)–C(1) 1.317(2), N(5)–C(27) 1.139(3);
Cl–V–N(5) 91.33(5), N(1)–V–N(2) 65.59(6), V–N(5)–C(27) 177.5(2).
DOI: 10.1039/b000397m
Chem. Commun., 2000, 497–498
This journal is © The Royal Society of Chemistry 2000
497

molecular structure is not shown here. The metal centre in 3a
has a distorted octahedral geometry, with two bidentate
amidinate ligands and with the chloride and benzonitrile ligands
being cis relative to each other. The pentafluorobenzamidinate
exhibits a normal bidentate coordination mode, and the aryl
substituent is perpendicular to the ligand NCN plane. In this
respect it does not differ significantly from the non-fluorinated
ligand in 3b. The largest differences between the structures of
the two complexes are found in the V–Cl distance and the Cl–
V–N(5) and N(1)–V–N(4) angles [2.306(1) Å, 91.33(3) and
164.63(7)°, respectively, in 3a; 2.350(1) Å, 86.01(8) and
168.6(1)° in non-fluorinated 3b].
the chemistry of this ligand to other catalytically active metal
centres. We are also using the synthesis route to prepare
pentafluoro-derivatives of other trimethylsilylbenzamidinates
(such as the amidinate–amine ligands reported recently by
us11).
This investigation was carried out in connection with NIOK,
the Netherlands Institute for Catalysis Research, and supported
by the Department of Economic Affairs of the Netherlands.
Notes and references
Unlike with the non-fluorinated analogue,6 reaction of 1 with
MeMgCl in THF followed by crystallisation from pentane
yields a stable THF adduct of the methyl complex, [C6F5C(NSi-
Me3)2]2VMe(THF) 4 (Scheme 1). The stability of this adduct is
an indication of the enhanced electrophilicity of the metal centre
in the fluorinated benzamidinate complexes. Reaction of the
base-free chloride 2 with MeLi in diethyl ether, followed by
extraction with and crystallisation from pentane, affords the
orange 12-electron methyl complex [C6F5C(NSiMe3)2]2VMe
5a in 52% isolated yield. NMR spectra indicate that this alkyl
compound (without b hydrogens) is thermally robust in benzene
solution at 80 °C (no noticeable decomposition after 18 h).
To illustrate the effect of the fluorination of the amidinate
ligand on the reactivity of the metal complexes, we compared
the catalytic conversion of ethene by the neutral alkyl 5a and its
non-fluorinated analogue 5b. Reactions on NMR tube scale had
already shown that at 80 °C 5b converts ethene to linear
alkenes.6 Initially, linear alk-1-enes are formed which are
isomerised to internal alkenes when the system is starved of
ethene. Autoclave experiments (80 °C, toluene, 4 or 8 bar
ethene pressure) were performed using 5a and 5b as catalysts.
Under these conditions, 5b produces a Flory–Schultz distribu-
tion of linear alk-1-enes ( > 99% by GC) with [Cn +2]/[Cn] =
0.87(2) over the range C8–C32, that is practically invariant with
ethene pressure. The catalyst productivity for 5b (as determined
from isolated material precipitated with methanol) at 8 bar
ethene (4 h run time) is 1.5 kg (mol V)21 h21 of a material with
Mn = 850 (by NMR). Under the same conditions, the
fluorinated catalyst 5a shows a productivity that is more than
five times higher, 8.1 kg (mol V)21 h21 giving a material with
higher molecular weight (Mn = 1780, Mw/Mn = 2.3 by
GPC).
§ Crystallographic data: for 3a: C33H41ClF10N5Si4V, M
= 896.44,
¯
triclinic, space group P1, a = 10.206(2), b = 13.176(2), c = 17.992(3) Å,
a = 86.41(1), b = 74.73(1), g = 67.63(1)°, U = 2148.1(7) Å, T = 130 K,
Z = 2, Dc = 1.386 g cm23, m = 4.8 cm21, Enraf Nonius CAD4-F
diffractometer, l(Mo-Ka) = 0.71073 Å, 8144 unique reflections, final
residuals wR(F2) = 0.0986, R(F) = 0.0356 for 7274 reflections with Fo
>
4s(Fo) and 650 parameters. For 3b: C33H51ClN5Si4V, M
= 716.54,
orthorhombic, space group Pbca, a = 20.778(1), b = 17.918(1), c =
21.623(1) Å, U = 8050.2(7) Å, T = 130 K, Z = 8, Dc = 1.182 g cm23, m
= 4.6 cm21, Enraf Nonius CAD4-F diffractometer, l(Mo-Ka) = 0.71073
Å, 6975 unique reflections, final residuals wR(F2) = 0.109, R(F) = 0.051
for 5016 reflections with Fo
> 4s(Fo) and 601 parameters. CCDC
182/1548. See http://www.rsc.org/suppdata/cc/b0/b000397m/ for crystallo-
graphic files in .cif format.
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A series of experiments with 5a at 4 bar ethene pressure and
run times of 2, 4 and 16 h shows that catalyst deactivation
occurs, as the observed overall productivity drops from 7.4 to
4.5 and 1.3 kg (mol V)21 h21, respectively. As vanadium-based
polymerisation catalysts are generally considered to deactivate
through reduction of the metal centre to V(II),10 it is likely that
the electron-withdrawing nature of the fluorinated ligand
accellerates this process.
In conclusion, we have prepared the new N,NA-bis(trime-
thylsilyl)pentafluorobenzamidinate ligand in a convenient man-
ner, and have shown that the fluorination of the aryl substituent
has a significant effect in the reactive properties of the metal
centre in complexes with this ligand. Presently we are extending
9 [C6F5C(NSiMe3)2]Li·0.5Et2O: 1H NMR (200 MHz, C6D6) d 3.20 (q,
2H, J 7.1, OCH2), 0.98 (t, 3H, J 7.1, ether Me), 20.02 (s, 18H, SiMe3).
13C{1H} NMR (125.7 MHz, C6D6) d 162.2 (NCN), 142.2 (d, JCF 243.4,
m-CF), 140.7 (d, JCF 253.0, p-CF), 138.0 (d, JCF 253.0, o-CF), 119.6 (br,
C ipso), 65.7 (OCH2), 14.4 (ether Me), 1.1 (SiMe3). 19F NMR (188
MHz, C6D6) d 2144.6 (dd, 2F, J 24.8, 7.9, o-F), 2160.9 (t, 1F, J 24.8,
p-F), 2164.6 (td, 2F, J 24.8, 7.9, m-F).
10 See for example: Y. Ma, D. Reardon, S. Gambarotta, G. Yap, H.
Zahalka and C. Lemay, Organometallics, 1999, 18, 2773 and references
therein.
11 M. J. R. Brandsma, E. A. C. Brussee, A. Meetsma, B. Hessen and J. H.
Teuben, Eur. J. Inorg. Chem., 1998, 1867.
Communication b000397m
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Chem. Commun., 2000, 497–498