Table 1 Catalytic ethene polymerisation with 3 and 4 in conjunction with
ethene, thus leading to bimodal product distributions. The
polymerisation behaviour of this type of catalyst and its
dependence on various parameters (ligand substitution pattern,
activator species, etc.) is subject of further study.
In conclusion, we have prepared new yttrium dialkyl species
with monoanionic tetradentate triazacyclononane-amide ancil-
[PhNMe2H][B(C6F5)4] activatora
Productivity/
103 kg(PE)
Dialkyl T/°C PE yield/g
mol(Y)21 h21 bar21 1023 Mw Mw/Mn
lary
ligands.
Reactions
of
these
dialkys
with
4
4
4
3b
30
50
80
30
5.62
9.40
14.30
11.95
0.70
1.18
1.79
0.96
471
325
98
4.0
4.9
6.0
[PhNMe2H][B(C6F5)4] generate the ionic species 5·thf-d8 and
6, rare examples of spectroscopically characterised cationic
group 3 metal alkyls.12 These cationic alkyl species are active
catalysts for the polymerisation of ethene.
This investigation was supported by ExxonMobil Chemical
Company.
c
c
—
—
a Conditions: toluene solvent (150 ml), 10 mmol Y dialkyl complex, 10
mmol activator, 5 bar ethene pressure, 10 min run time. b 15 min run time.
c Not determined.
Notes and references
arranged in such a way as to prevent the eclipsing of the ligand
Pri and Y–alkyl groups. The Y–N distances for the tacn
nitrogens span a considerable range, the shortest being the
distance to the bridgehead nitrogen Y–N(2), 2.541(5) Å, and the
longest the Y–N(4) distance, 2.740(5) Å, N(4) being practically
trans to C(23). This asymmetry appears to be retained in
solution, as the ambient temperature 1H and 13C NMR spectra
show resonances consistent with an asymmetric species (e.g.
four resonances for the diastereotopic CH2SiMe3 methylene
protons).7 The alkyl methylene carbon resonances are found at
d 33.7 and 31.0, with rather large 1JYC coupling constants (ca.
38 Hz) and relatively small 1JCH coupling constants of 95 Hz.
Reaction of the amine 2 with Y(CH2SiMe3)3(thf)2 in pentane
yielded the corresponding dialkyl complex [N,NA-Me2-tacn-NB-
(CH2CH2)NBut]Y(CH2SiMe3)2 4, analytically pure in 86%
isolated yield. The smaller size of the alkyl substituents makes
the complex more fluxional, as the room temperature 1H NMR
spectrum of 4 shows broad resonances consistent with a species
with an average Cs symmetry. Cooling a toluene-d8 solution of
4 to 260 °C slows down this dynamic process, revealing spectra
with four Y–alkyl methylene and two Y–C resonances, again
consistent with an asymmetric ground state structure.8
The dialkyl complex 4 reacts cleanly with the Brønsted acid
[PhNMe2H][B(C6F5)4]9 in C6D5Br solvent to give SiMe4, free
PhNMe2, and an ionic species formulated as {[N,NA-Me2-tacn-
NB-(CH2CH2)NBut]Y(CH2SiMe3)}[B(C6F5)4] 6.10 The 1H
NMR spectrum shows a single resonance at d 21.06 for the
YCH2 group (JYH not resolved), and the 13C NMR YCH2
resonance at d 37.0 (JYC 40.7 Hz), shifted downfield and with a
larger JYC relative to the dialkyl 4. The ionic species 6 is
thermally relatively stable, and remains essentially unchanged
over 1 h at ambient temperature in bromobenzene solution. In
§ Crystallographic data for 3: C26H61N4Si2Y, M = 574.87, triclinic, space
¯
group P1, a = 9.815(1), b = 9.859(1), c = 17.291(1) Å, a = 95.60(1), b
= 90.68(1), g = 98.63(1)°, U = 1645.7(4) Å3, T = 130 K, Z = 2, Dc
=
1.160 g cm23, m = 18.6 cm21, Enraf-Nonius CAD4-F diffractometer,
l(Mo-Ka) = 0.71073 Å, 6433 unique reflections, final residuals wR(F2) =
0.1800, R(F) = 0.0731 for 3940 reflections with Fo ! 4s(Fo) and 311
b101012n/ for crystallographic data in .cif or other electronic format.
1 For a recent review of this chemistry, see: A. L. McKnight and R. M.
Waymouth, Chem. Rev., 1998, 98, 2587.
2 P. J. Shapiro, E. E. Bunel, W. P. Schaefer and J. E. Bercaw,
Organometallics, 1990, 9, 867; P. J. Shapiro, W. D. Cotter, W. P.
Schaefer, J. A. Labinger and J. E. Bercaw, J. Am. Chem. Soc., 1994, 116,
4623; K. C. Hultzsch, P. Voth, K. Beckerle, T. P. Spaniol and J. Okuda,
Organometallics, 2000, 19, 228.
3 J. A. M. Canich, T. D. Schaffer, J. N. Christopher and K. R. Squire,
World Pat., WO0018808, 2000, (Exxon).
4 For recent examples of functionalised 1,4,7-triazacyclononane ligands
and metal complexes thereof, see: S. E. Watkins, X. Yang, D. C. Craig
and S. B. Colbran, Chem. Commun., 1999, 1539; L. M. Berreau, J. A.
Halfern, V. G. Young, Jr., and W. B. Tolman, Inorg. Chem., 1998, 37,
1091; C. Stockheim, L. Hoster, T. Weyhermüller, K. Wieghardt and B.
Nuber, J. Chem. Soc., Dalton Trans., 1996, 4409; D. A. Robson, L. H.
Lees, P. Mountford and M. Schröder, Chem. Commun., 2000, 1269;
M. A. H. Male, M. E. G. Skinner, P. J. Wilson, P. Mountford and M.
Schröder, New J. Chem., 2000, 24, 575; B. Quian, L. M. Henling and
J. C. Peters, Organometallics, 2000, 19, 2805, and references therein.
5 C. Fassbeck and K. Wieghardt, Z. Anorg. Allg. Chem., 1992, 608, 60.
6 M. F. Lappert and R. Pearce, J. Chem. Soc., Chem. Commun., 1973,
126.
7 Selected NMR data for 3: 1H NMR (500 MHz, C6D6) d 20.26 (dd, JHH
10.5, JYH 3.3 Hz, 1H, YCHH), 20.53 (dd, JHH 10.8, JYH 2.1 Hz, 1H,
YCHH), 20.83 (dd, JHH 10.8, JYH 3.0 Hz, 1H, YCHH), 21.00 (dd, JHH
10.8, JYH 2.1 Hz, 1H, YCHH). 13C NMR (125.7 MHz, C6D6) d 33.7 (dt,
JYC 36.9, JCH 95.1 Hz, YCH2), 31.0 (t, JYC 38.7, JCH 95.0 Hz,
YCH2).
contrast, reaction of the dialkyl complex
3
with
[PhNMe2H][B(C6F5)4] leads to rapid formation of propene and
2 equiv. of SiMe4 (as seen by 1H NMR spectroscopy), and an ill-
defined yttrium species. Apparently, an Pri substituent on the
ancillary ligand is metallated on one of its methyl groups,
followed by elimination of propene. When the reaction is
performed in the presence of an excess of d8-thf, the cationic
alkyl species is trapped before ligand metallation occurs, giving
8 Selected NMR data for 4: 1H NMR (500 MHz, 260 °C, C7D8) d 20.62
(d, JHH 11.0 Hz, 1H, YCHH), 20.86 (d, JHH 11.0 Hz, 1H, YCHH),
20.94 (d, JHH 11.0 Hz, 1H, YCHH), 21.06 (d, JHH 10.5 Hz, 1H,
YCHH). The JYH coupling on the YCH2 protons is unresolved. 13C
NMR (125.7 MHz, 260 °C, C7D8) d 29.8 (dt, JYC 35.4, JCH 93.3 Hz,
YCH2), 28.5 (dt, JYC 38.9, JCH 97.3 Hz, YCH2).
1
9 G. G. Hlatky, H. W. Turner and R. R. Eckman, J. Am. Chem. Soc., 1989,
111, 2728.
a species formulated (based on its H NMR characteristics) as
{[N,NA-Pri2-tacn-NB-(CH2CH2)NBut]Y(CH2SiMe3)(d8-thf)}-
[B(C6F5)4] (5·d8-thf), with YCH2 resonances at d 21.29 and
21.35 (dd, 2JHH 11.0 Hz, JYH 3.0 Hz).11
10 NMR data for the cation of 6: 1H NMR (500 MHz, 230 °C, C6D5Br) d
2.58–2.22 (m, 16H, NCH2), 2.18 (s, 6H, NMe), 1.09 (s, 9H, But), 0.11
(s, 9H, CH2SiMe3), 21.06 (br, 2H, YCH2). 13C{1H} NMR (125.7 MHz,
C6D5Br, 230 °C) d 60.09 (NCH2), 56.05 (NCH2), 53.33 (CMe3), 53.75
Ethylene homopolymerisation experiments (toluene solvent)
showed that the dialkyls 3 and 4, in combination with the
Brønsted acid activator [PhNMe2H][B(C6F5)4], yield active
ethene polymerisation catalysts, with observed productivities
up to 1.79 3 103 kg(PE) mol(Y)21 h21 bar21. Relatively short
run times (10–15 min) were chosen to minimise inhomogeneity
and mass transfer effects. Over the run period the catalysts show
a modest (25–30%) decrease in ethene uptake rate. The results
listed in Table 1 show that the productivity of the Me2-tacn
system is enhanced by increasing the reaction temperature, but
that the polydispersity of the polyethene produced also
increases substantially. One possible explanation for this is that
the initial cationic alkyl catalyst is thermally transformed into
another species that is also active in the polymerisation of
1
(NCH2), 51.78 (NCH2), 46.51 (NMe), 46.16 (NCH2), 37.02 (d, JYC
40.7 Hz, YCH2), 30.17 (CMe3), 4.31 (SiMe).
11 NMR data for the cation of 5·thf-d8: 1H NMR (500 MHz, 230 °C,
C6D5Br–thf-d8) d 3.48 (sept, JHH 6.0 Hz, 1H, CHMe2), 3.40 (t, JHH 13.0
Hz, 1H, NCH2), 2.79–2.75 (m, 2H, NCH2), 2.68–2.59 (m, 3H, NCH2),
2.55–2.48 (m, 2H, NCH2), 2.42–2.29 (m, 3H, NCH2), 2.25–2.17 (m, 3H,
NCH2), 1.27 (br, 1H, CHMe2), 1.18 (d, JHH 6.0 Hz, 6H, CHMe2), 1.15
(s, 9H, But), 0.84 (d, JHH 5.5 Hz, 3H, CHMe2), 0.80 (d, JHH 5.5 Hz, 3H,
CHMe2), 0.09 (s, 9H, Me3SiCH2), 21.29 (dd, JHH 11.0, JYH 3.0 Hz, 1H,
YCHH), 21.35 (dd, JHH 11.0, JYH 3.0 Hz, 1H, YCHH).
12 L. W. M. Lee, W. E. Piers, M. R. J. Elsegood, W. Clegg and M. Parvez,
Organometallics, 1999, 18, 2947; L. Lee, D. J. Berg, F. W. Einstein and
R. J. Batchelor, Organometallics, 1997, 16, 1819; S. Haleja, W. P.
Schaefer and J. E. Bercaw, J. Organomet. Chem., 1997, 532, 45.
638
Chem. Commun., 2001, 637–638