Niobium- And tantalum-benzamidinato complexes with trimethylphosphine, imido, or η-cyclopentadienyl derivatives

Article abstract of DOI:10.1039/a909333h

The mononuclear benzamidinato compounds, [M{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}Cl4] (M = Nb (1) or Ta (2)), [Nb{(2)4,6-tn-iPr)C6H2C(NSiMe3)2}Ci4(PMe3)](4), [M{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(N'Bu)(H2N'Bu)Cl2] (M = Nb (5), M = Ta (6)), [Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(N'Bu)(L)Cl2] (L = THF (7), L = pyridine (8)), and [M{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(n-C5H5)Cl3] (M = Nb (9), M = Ta (10), and a nitrile tantalum complex, [Ta{(2,4,6-tri-'Pr)C6H2CN}Cl5] (3), obtained in a side-reaction have been prepared:indicates the crystal structures have been determined. Variable temperature NMR studies show there is hindered rotation of the aryl group around Cipso-C bonds for compounds 5 and 6; ΔC = 55.2 kJ mol'1 at 268 K for 5. Hindered rotation about the N-Si bonds for compounds 9 and 10 occurs with the ΔG= 51.0 kJ mol~' at 273 K and 40.6 kJ mol"' at 213 K for 10. The compounds 1,5 and 9 are very poor pre-catalysts for ethene polymerisation. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a909333h

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Niobium- and tantalum-benzamidinato complexes with
trimethylphosphine, imido, or ꢀ-cyclopentadienyl derivatives
Chi-Tien Chen,a Linda H. Doerrer,†b V. Cliff Williamsa and Malcolm L. H. Green*a
a Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR
b Chemistry Department, Columbia University, 3009 Broadway, New York, NY, USA
Received 25th November 1999, Accepted 27th January 2000
Published on the Web 24th February 2000
The mononuclear benzamidinato compounds, [M{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}Cl4] (M = Nb (1) or Ta (2*)),
[Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}Cl4(PMe3)] (4*), [M{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(NtBu)(H2NtBu)Cl2]
(M = Nb (5), M = Ta (6)), [Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(NtBu)(L)Cl2] (L = THF (7), L = pyridine (8)),
and [M{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(η-C5H5)Cl3] (M = Nb (9), M = Ta (10*), and a nitrile tantalum complex,
[Ta{(2,4,6-tri-iPr)C6H2CN}Cl5] (3*), obtained in a side-reaction have been prepared:* indicates the crystal
structures have been determined. Variable temperature NMR studies show there is hindered rotation of the aryl
group around Cipso–C bonds for compounds 5 and 6; ∆G‡ = 55.2 kJ molϪ1 at 268 K for 5. Hindered rotation about
the N–Si bonds for compounds 9 and 10 occurs with the ∆G‡ = 51.0 kJ molϪ1 at 273 K and 40.6 kJ molϪ1 at 213 K
for 10. The compounds 1, 5 and 9 are very poor pre-catalysts for ethene polymerisation.
show to be the lithium salt of the highly sterically demanding
Introduction
2,4,6-triisopropyl-substituted benzamidinato anion, namely
Metallocenes containing one or more cyclopentadienyl
ligands are exceptionally active and selective pre-catalysts for
homogeneous catalysis for the iso-specific polymerisation of
α-olefins.1 This has stimulated research into ligands with prop-
erties similar to the cyclopentadienyl group.2–7 The benzamid-
inato ligand RЈC(NR)2 has been a focus of much attention
because it can readily be used in place of cyclopentadienyl
ligands whilst the steric demands are considered to lie between
those of the cyclopentadienyl and pentamethylcyclopenta-
dienyl ligands.8,9 Benzamidinato ligands are hard four-electron
donors8 which are expected to generate an electron-deficient
metal centre, allowing the isolation of Lewis acidic compounds
or providing sufficient electronic unsaturation for further
reactivity to occur.10,11 Some benzamidinato compounds have
already been shown to provide active catalysts for olefin
polymerisation.11–17
Benzamidinato ligands of the general formula RЈC(NR)2
(where R = alkyl, silyl; RЈ = Ph, substituted Ph) have been
extensively studied.18 They can be synthesised readily and
variation of the substituents R and RЈ is facile providing
the opportunity of tailoring these ligands to achieve specific
properties, such as bite or cone angles. However, to date only
para-substituted, 2,4,6-trimethyl-substituted, and 2,4,6-tris(tri-
fluoromethyl)-substituted benzamidinato ligands19 have been
used as ancillary ligands, whilst studies of bulkier complexes
have not been made. Many examples of bis-benzamidinato
transition metal complexes are known, however, there are few
mono-benzamidinato complexes, especially of the Group 5
metals.
[Li{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}].
Treatment of the pentachlorides MCl5 (M = Nb, Ta) with one
equivalent of [Li{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}] in toluene
affords the compounds, [M{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}-
Cl4]; where M = Nb (1) and Ta (2), in good yields. Reaction
of MCl5 with two equivalents of the lithium salt does not yield
the expected bis-benzamidinato compounds. Instead a com-
plex mixture of products is formed from which compounds 1
and 2 were isolated, in low yield. Thus, the bulky isopropyl
substituents on the aryl groups of the benzamidinato ligands
prevent the formation of the bis-benzamidinato metal com-
pounds which are commonly formed with unsubstituted aryl
benzamidinato ligands. Further, the mono-benzamidinato
compounds 1 and 2 show no tendency towards ligand redis-
tribution.
The analytical and spectroscopic data characterising com-
pounds 1 and 2, and all the other new compounds described
below, are given in Table 1. A summary of the syntheses and
proposed structures is shown in Scheme 1.
The crystal structure of [Ta{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}-
Cl4] (2) has been determined and the molecular structure is
shown in Fig. 1 and selected interatomic distances and angles
are listed in Table 2. The asymmetric unit of 2 contains two
independent molecules. Compound 2 displays a pseudo-
octahedral geometry with the benzamidinato ligand occupying
one edge of the octahedron. The two Ta–N bond lengths,
2.093(5) and 2.109(4); 2.108(4) and 2.130(4) Å, are shorter than
those normally found in the literature for which typical values
are in the range 2.15(1) to 2.29(1) Å for compounds of the type
[Ta{p-CH3-C6H4C(NSiMe3)2}2X3]20 (X = Me) and 2.133(2) and
2.199(2) Å for compound 10. These shortened bond lengths
may be attributed to the lack of severe steric repulsion, which is
usually observed in bis-benzamidinato and cyclopentadienyl
benzamidinato compounds.20,21 Likewise the Ta–Cl bonds dis-
tances, 2.3505(14) and 2.3580(15), 2.3739(15) and 2.3456(15);
2.3125(14) and 2.3200(16), 2.3287(16) and 2.3287(16) Å, are all
towards the shorter end of the range expected for Ta–Cl bond
lengths20–23 (vide infra).
In this paper we report the preparation and structural
properties of a series of niobium and tantalum complexes con-
taining a bulky benzamidinato ligand, and a brief study of their
ability to act as pre-catalysts towards olefin polymerisation.
Results and discussion
Treatment of the carbodiimide Me SiN᎐C᎐NSiMe with
᎐ ᎐
3
3
Li[(2,4,6-tri-iPr)C6H2] gives a white powder which the data
During the synthesis of compound 2, pale-brown crystals of
another compound (3) were obtained, in 5% yield, for which
only resonances corresponding to the tris-isopropyl phenyl
† Present address: Chemistry Department, Columbia University, 3009
Broadway, New York, NY, USA.
DOI: 10.1039/a909333h
J. Chem. Soc., Dalton Trans., 2000, 967–974
This journal is © The Royal Society of Chemistry 2000
967

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Table 1 Analytical and spectroscopic data
Compound and
analysisa
Spectroscopic datab
1
1H: 0.27 (s, 18H, Si(CH3)3), 1.07 (d, 6H, J = 7.0, 4-CH(CH3)2), 1.15 (d, 12H, J = 7.0, 2,6-CH(CH3)2), 2.63 (m, 1H, 4-CH-
(CH3)2), 3.01 (m, 2H, 2,6-CH(CH3)2), 6.89 (s, 2H, 3,5-C6H2{CH(CH3)2}3)
Red
C, 42.3 (42.3)
H, 6.6 (6.6)
N, 4.5 (4.5)
Cl, 22.9 (22.7)
2
13C{1H}: 0.9 (s, Si(CH3)3), 24.1 (s, 4-CH(CH3)2), 24.5 (s, 2,6-CH(CH3)2), 31.4 (s, 2,6-CH(CH3)2), 34.7 (s, 4-CH(CH3)2), 121.6
(s, 3,5-C6H2{CH(CH3)2}3), 133.2 (s, Cipso-Ph), 145.2 (s, 2,6-C6H2{CH(CH3)2}3), 153.4 (s, 4-C6H2{CH(CH3)2}3), 188.4
(s, C{NSi(CH3)3}2)
IR: 2965s, 2903m, 1343s, 1256s, 847vs, 665s
1H: 0.23 (s, 18H, Si(CH3)3), 1.07 (d, 6H, J = 6.5, 4-CH(CH3)2), 1.17 (d, 12H, J = 6.5, 2,6-CH(CH3)2), 2.63 (m, 1H,
4-CH(CH3)2), 3.05 (m, 2H, 2,6-CH(CH3)2), 6.89 (s, 2H, 3,5-C6H2{CH(CH3)2}3)
Yellow
C, 36.9 (37.1)
H, 5.6 (5.8)
N, 3.9 (3.9)
Cl, 20.0 (19.9)
3
13C{1H}: 0.9 (s, Si(CH3)3), 24.1 (s, 4-CH(CH3)2), 24.5 (s, 2,6-CH(CH3)2), 31.1 (s, 2,6-CH(CH3)2), 34.7 (s, 4-CH(CH3)2), 121.6
(s, 3,5-C6H2{CH(CH3)2}3), 133.7 (s, Cipso-Ph), 145.3 (s, 2,6-C6H2{CH(CH3)2}3), 153.2 (s, 4-C6H2{CH(CH3)2}3), 188.0
(s, C{NSi(CH3)3}2)
IR: 2965s, 2934m, 2907m, 2874m, 1607m, 1487m, 1462m, 1422m, 1350s, 1256m, 995s, 951s, 835s, 766m
1H: 0.92 (d, 6H, J = 7, 4-CH(CH3)2), 1.04 (d, 12H, J = 6.8, 2,6-CH(CH3)2), 2.43 (m, 1H, 4-CH(CH3)2), 3.12 (m, 2H, 2,6-
CH(CH3)2), 6.75 (s, 2H, 3,5-C6H2{CH(CH3)2}3)
Pale-brown
C, 33.0 (32.7)
H, 4.3 (4.0)
N, 2.4 (2.4)
4
13C{1H}: 23.1 (s, 2,6-CH(CH3)2), 23.2 (s, 4-CH(CH3)2), 33.4 (s, 2,6-CH(CH3)2), 35.2 (s, 4-CH(CH3)2), 103.0 (s, Cipso-Ph), 122.3
(s, 3,5-C6H2{CH(CH3)2}3), 125.0 (s, NC-), 156.9 (s, 2,6-C6H2{CH(CH3)2}3), 159.2 (s, 4-C6H2{CH(CH3)2}3)
IR: 2967s, 2253s, 1601s, 1464m, 883s, 719m
2
1H: 0.43 (br, 18H, Si(CH3)3), 1.13 (d, 6H, J = 7, 4-CH(CH3)2), 1.33 (d, 12H, J = 7, 2,6-CH(CH3)2), 1.39 (d, 9H, JPH = 12,
Red
P(CH3)3), 2.68 (m, 1H, 4-CH(CH3)2), 3.57 (m, 2H, 2,6-CH(CH3)2), 6.96 (s, 2H, 3,5-C6H2{CH(CH3)2}3)
13C{1H}: 2.9 (s, Si(CH3)3), 16.2 (d, 1JPC = 30, P(CH3)3), 24.1 (s, 4-CH(CH3)2), 24.8 (s, 2,6-CH(CH3)2), 30.3 (s, 2,6-CH(CH3)2),
34.7 (s, 4-CH(CH3)2), 121.4 (s, 3,5-C6H2{CH(CH3)2}3), 135.8 (s, Cipso-Ph), 145.7 (s, 2,6-C6H2{CH(CH3)2}3), 152.1 (s, 4-
C6H2{CH(CH3)2}3), 188.8 (s, C{NSi(CH3)3}2)
C, 42.8 (43.0)
H, 6.9 (7.2)
N, 4.0 (4.0)
Cl, 19.8 (20.2)
31P{1H}: 47.1 (PMe3)
IR: 2963s, 2901m, 1667m, 1607m, 1462s, 1381s, 1302m, 1252s, 984s, 955s, 839s, 780s, 683s
5
1H: 0.14 (s, 18H, Si(CH3)3), 1.16 (d, 6H, J = 7, 4-CH(CH3)2), 1.26 (d, 12H, J = 7, 2,6-CH(CH3)2), 1.37 (s, 9H, H2NC(CH3)3),
1.50 (s, 9H, NC(CH3)3), 2.50 (s, 2H, H2NC(CH3)3), 2.72 (m, 1H, 4-CH(CH3)2), 3.12 (m, 2H, 2,6-CH(CH3)2), 6.94 (s, 2H,
3,5-C6H2{CH(CH3)2}3)
Yellow
C, 51.7 (51.6)
H, 8.5 (8.8)
N, 7.8 (8.0)
Cl, 10.1 (10.2)
13C{1H}:c 2.2 (s, Si(CH3)3), 24.2 (s, 4-CH(CH3)2), 24.9 (s, 2,6-CH(CH3)2), 30.4 (s, NC(CH3)3), 31.1 (s, 2,6-CH(CH3)2), 32.1
(s, H2NC(CH3)3), 34.7 (s, 4-CH(CH3)2), 51.7 (s, H2NC(CH3)3), 69.2 (s, NC(CH3)3), 121.4 (s, 3,5-C6H2{CH(CH3)2}3), 136.2
(s, Cipso-Ph), 143.9 (s, C6H2{CH(CH3)2}3), 151.1 (s, 4-C6H2{CH(CH3)2}3), 178.2 (s, 2,6-C{NSi(CH3)3})
IR: 2971s, 2905m, 1462m, 1422s, 1246s, 1215m, 1086m, 988s, 905m, 806vs, 762s, 714s
6
1H: 0.10 (s, 18H, Si(CH3)3), 1.15 (d, 6H, J = 6.8, 4-CH(CH3)2), 1.26 (d, 12H, J = 6.8, 2,6-CH(CH3)2), 1.32 (s, 9H,
H2NC(CH3)3), 1.51 (s, 9H, NC(CH3)3), 2.55 (s, 2H, H2NC(CH3)3), 2.71 (m, 1H, 4-CH(CH3)2), 3.07 (br, 2H, 2,6-CH(CH3)2),
6.93 (s, 2H, 3,5-C6H2{CH(CH3)2}3)
Pale-green
C, 45.9 (45.9)
H, 7.9 (7.8)
N, 7.0 (7.1)
Cl, 9.0 (9.0)
13C{1H}: 2.1 (s, Si(CH3)3), 24.1 (s, 4-CH(CH3)2), 24.9 (s, 2,6-CH(CH3)2), 31.0 (s, NC(CH3)3), 32.1 (s, 2,6-CH(CH3)2), 32.6
(s, H2NC(CH3)3), 34.6 (s, 4-CH(CH3)2), 52.0 (s, H2NC(CH3)3), 65.5 (s, NC(CH3)3), 121.5 (s, 3,5-C6H2{CH(CH3)2}3), 136.0
(s, Cipso-Ph), 144.0 (br, 2,6-C6H2{CH(CH3)2}3), 151.2 (s, 4-C6H2{CH(CH3)2}3), 176.4 (s, C{NSi(CH3)3}2)
IR: 2959s, 2930m, 2899m, 1462m, 1404s, 1275m, 1250m, 1094m, 995m, 847s, 766m, 718m
7
1H: 0.14 (s, 18H, Si(CH3)3), 1.16 (d, 6H, J = 6.9, 4-CH(CH3)2), 1.26 (d, 12H, J = 6.9, 2,6-CH(CH3)2), 1.45 (m, 4H, THF),
1.51 (s, 9H, NC(CH3)3), 2.73 (m, 1H, 4-CH(CH3)2), 3.18 (m, 2H, 2,6-CH(CH3)2), 4.08 (m, 4H, THF), 6.96 (s, 2H,
3,5-C6H2{CH(CH3)2}3)
Yellow
C, 51.3 (51.7)
H, 8.6 (8.4)
N, 6.0 (6.0)
Cl, 9.4 (10.2)
13C{1H}:d 2.3 (s, Si(CH3)3), 24.1 (s, 4-CH(CH3)2), 25.0 (s, 2,6-CH(CH3)2), 25.2 (s, C4H8O), 30.4 (s, NC(CH3)3), 30.8 (s,
2,6-CH(CH3)2), 34.6 (s, 4-CH(CH3)2), 70.5 (s, C4H8O), 121.5 (s, 3,5-C6H2{CH(CH3)2}3), 136.2 (s, Cipso-Ph), 144.1 (s, 2,6-
C6H2{CH(CH3)2}3), 150.9 (s, 4-C6H2{CH(CH3)2}3), 178.9 (s, C{NSi(CH3)3}2)
IR: 2969s, 2895m, 1462m, 1424s, 1250s, 990s, 880m, 837vs, 764m, 714m
8
1H: 0.16 (s, 18H, Si(CH3)3), 1.15 (m, 18H, 2,4,6-CH(CH3)2), 1.61 (s, 9H, NC(CH3)3), 2.71 (m, 1H, 4-CH(CH3)2), 2.85 (br, 2H,
2,6-CH(CH3)2), 6.69 (m, 2H, m-NC5H5), 6.91 (s, 2H, 3,5-C6H2{CH(CH3)2}3), 6.94 (m, 1H, p-NC5H5), 9.45 (d, 2H, J = 5.4,
o-NC5H5)
Yellow
C, 52.9 (52.9)
H, 8.1 (7.9)
N, 7.9 (8.0)
Cl, 9.8 (10.1)
13C{1H}: 2.5 (s, Si(CH3)3), 24.1 (s, 2,6-CH(CH3)2), 25.0 (s, 4-CH(CH3)2), 30.3 (s, NC(CH3)3), 30.6 (s, 2,6-CH(CH3)2), 34.6
(s, 4-CH(CH3)2), 69.3 (s, NC(CH3)3), 121.4 (s, 3,5-C6H2{CH(CH3)2}3), 123.2 (s, m-NC5H5), 136.3 (s, Cipso-Ph), 137.5
(s, p-NC5H5), 144.3 (s, 2,6-C6H2{CH(CH3)2}3), 150.9 (s, 4-C6H2{CH(CH3)2}3), 151.2 (s, o-NC5H5), 178.8 (s, C{NSi(CH3)3}2)
IR: 2961s, 2928m, 2872m, 1603m, 1460m, 1404s, 1360m, 1250s, 990s, 839s, 770m, 712m
9
1H: Ϫ0.01 (s, 9H, Si(CH3)3 (close to Cp)), 0.65 (br, 9H, Si(CH3)3 (close to Cl)), 1.12 (d, 6H, J = 7.0, 4-CH(CH3)2), 1.23 (d, 6H,
J = 7.0, 2,6-CH(CH3)2 (Cp side)), 1.38 (d, 6H, J = 7.0, 2,6-CH(CH3)2 (Cl side)), 2.67 (m, 1H, 4-CH(CH3)2), 3.60 (m, 2H, 2,6-
CH(CH3)2), 6.60 (s, 5H, C5H5), 6.89 (s, 2H, 3,5-C6H2{CH(CH3)2}3)
Dark-red
C, 48.9 (49.5)
H, 6.8 (7.1)
N, 4.1 (4.3)
Cl, 16.4 (16.3)
13C{1H}:c 1.9 (br, Si(CH3)3), 3.1 (s, Si(CH3)3), 24.1 (s, 4-CH(CH3)2), 24.7 (s, 2,6-CH(CH3)2 (Cp side)), 25.1 (s, 2,6-CH(CH3)2
(Cl side)), 29.6 (s, 2,6-CH(CH3)2), 34.7 (s, 4-CH(CH3)2), 121.4 (s, 3,5-C6H2{CH(CH3)2}3), 124.5 (s, C5H5), 135.8 (s, Cipso-Ph),
146.5 (s, 2,6-C6H2{CH(CH3)2}3), 152.0 (s, 4-C6H2{CH(CH3)2}3), 184.8 (s, C{NSi(CH3)3}2)
IR: 2963s, 2905m, 1447m, 982s, 839s, 766m, 700m, 662m
10
1H: 0.00 (s, 9H, Si(CH3)3 (close to Cp)), 0.55 (br, 9H, Si(CH3)3 (close to Cl)), 1.12 (d, 6H, J = 6.9, 4-CH(CH3)2), 1.24 (d, 6H,
J = 6.8, 2,6-CH(CH3)2 (Cp side)), 1.37 (d, 6H, J = 6.7, 2,6-CH(CH3)2 (Cl side)), 2.68 (m, 1H, 4-CH(CH3)2), 3.57 (m, 2H, 2,6-
CH(CH3)2), 6.41 (s, 5H, C5H5), 6.94 (s, 2H, 3,5-C6H2{CH(CH3)2}3)
Yellow
C, 43.7 (43.7)
H, 6.5 (6.3)
N, 3.7 (3.8)
Cl, 14.2 (14.3)
13C{1H}:c 2.0 (br, Si(CH3)3), 3.3 (s, Si(CH3)3), 24.1 (s, 4-CH(CH3)2), 24.7 (s, 2,6-CH(CH3)2 (Cp side)), 25.0 (s, 2,6-CH(CH3)2
(Cl side)), 29.6 (s, 2,6-CH(CH3)2), 34.7 (s, 4-CH(CH3)2), 121.35 (s, C5H5), 121.42 (s, 3,5-C6H2{CH(CH3)2}3), 136.2 (s, Cipso-Ph),
146.5 (s, 2,6-C6H2{CH(CH3)2}3), 152.0 (s, 4-C6H2{CH(CH3)2}3), 183.9 (s, C{NSi(CH3)3}2)
IR: 2965m, 1472m, 1348s, 1337s, 991s, 845s
a Analytical data given as: found (required)%. b All the data were taken in benzene-d6 at room temperature unless otherwise stated. Chemical shifts
are quoted in ppm and coupling constants in Hz. IR data/cmϪ1 c In toluene-d8. d The quaternary carbon cannot be observed.
.
1
group could be detected by H and 13C{1H} NMR spectro-
scopy. An X-ray structure determination identified the crystals
as the compound [Ta{(2,4,6-tri-iPr)C6H2CN}Cl5] (3) and the
molecular structure is shown in Fig. 2. Compound 3 adopts a
pseudo-octahedral geometry, with the nitrile binding end on
with an almost linear Ta–N(1)–C(1) angle of 172.4(2)Њ which
is typical for other nitrile compounds such as the cluster
[Ta6Cl12(PrCN)6][(Ta6Cl12)Cl6]ؒ2PrCN (175(7)Њ and 170(7)Њ).24
968
J. Chem. Soc., Dalton Trans., 2000, 967–974

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Table 2 Selected bond lengths (Å) and angles (Њ) for 2
Table 3 Selected bond lengths (Å) and angles (Њ) for 3
Ta(1)–Cl(1)
Ta(1)–Cl(2)
Ta(1)–Cl(3)
Ta(1)–Cl(4)
Ta(1)–N(1)
Ta(1)–N(2)
Ta(2)–Cl(5)
Ta(2)–Cl(6)
Ta(2)–Cl(7)
2.3505(14)
2.3739(15)
2.3125(14)
2.3287(16)
2.093(5)
Ta(2)–Cl(8)
Ta(2)–N(3)
Ta(2)–N(4)
N(1)–Si(2)
N(2)–Si(1)
N(1)–C(10)
N(2)–C(10)
C(10)–C(11)
2.3287(16)
2.108(4)
2.130(4)
1.790(5)
1.787(5)
1.363(7)
1.345(7)
1.481(7)
Ta(1)–Cl(1)
Ta(1)–Cl(2)
Ta(1)–Cl(3)
Ta(1)–Cl(4)
2.2668(6)
2.3196(6)
2.3348(6)
2.3393(6)
Ta(1)–Cl(5)
Ta(1)–N(1)
N(1)–C(1)
C(1)–C(11)
2.3315(6)
2.207(2)
1.147(3)
1.422(3)
2.109(4)
Cl(1)–Ta(1)–Cl(2)
Cl(2)–Ta(1)–Cl(3)
Cl(3)–Ta(1)–Cl(4)
Cl(1)–Ta(1)–N(1)
95.69(2)
90.32(2)
88.54(2)
177.74(6)
Cl(2)–Ta(1)–N(1)
Ta(1)–N(1)–C(1)
N(1)–C(1)–C(11)
82.21(6)
172.4(2)
177.3(3)
2.3580(15)
2.3456(15)
2.3200(16)
Cl(1)–Ta(1)–Cl(2)
Cl(3)–Ta(1)–Cl(4)
Cl(1)–Ta(1)–Cl(3)
N(1)–Ta(1)–N(2)
Ta(1)–N(1)–C(10)
Ta(1)–N(2)–C(10)
178.40(6)
108.21(6)
90.23(5)
63.75(17)
93.2(3)
N(1)–C(10)–N(2)
Ta(1)–N(1)–Si(2)
Ta(1)–N(2)–Si(1)
C(10)–N(1)–Si(2)
C(10)–N(2)–Si(1)
110.1(5)
134.1(2)
134.5(2)
132.6(4)
132.5(4)
93.0(3)
Fig. 2 Molecular structure of [Ta{(2,4,6-tri-iPr)C6H2CN}Cl5] (3).
Hydrogen atoms omitted for clarity.
nitrile ligand when compared to the other chloride ligands. The
N(1)–C(1) 1.147(3) Å bond length is consistent with a C–N
triple-bond character. The infrared spectrum for 3 shows a
strong band at νCN = 2253 cmϪ1, some 34 cmϪ1 higher than for
Fig. 1 Molecular structure of one of the two crystallographically
independent molecules of [Ta{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}Cl4] (2).
Hydrogen atoms omitted for clarity.
the free nitrile (2,4,6-tri-iPr)C6H2CN where νCN = 2219 cmϪ1 25
.
A possible explanation for the formation of compound 3
is the formation of (2,4,6-tri-iPr)C6H2CN by elimination of
LiN(SiMe3)2 from the lithium salt of the benzamidinato ligand
to leave the nitrile. This would require the migration of one
trimethylsilyl group to the other nitrogen to form lithium bis-
(trimethylsilyl)amide with the subsequent elimination of a
molecule of (2,4,6-tri-iPr)C6H2CN.26 In order to test this
hypothesis, the lithium salt of the benzamidinato ligand was
heated up to 85 ЊC in benzene-d6 in a sealed NMR tube. Upon
heating, a new set of resonances gradually appeared besides
those corresponding to [Li{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}].
After 160 hours of heating, the lithium salt had been com-
pletely converted into (2,4,6-tri-iPr)C6H2CN as identified by 1H
and 13C{1H} NMR spectra in comparison with literature
values.25
In view of the low formal electron count of 14 for com-
pounds 1 and 2, the reaction with Lewis bases was explored.
Compound 1 was treated with neat PMe3 to give red crystals for
the compound [Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}Cl4(PMe3)]
(4). The crystal structure of 4 was determined and the molecu-
lar structure is shown in Fig. 3. Selected bond lengths and
angles are listed in Table 4. The structure can be described as a
distorted pentagonal biprism with the PMe3 and benzamidi-
nato ligands both occupying equatorial sites, in accord with
their larger cone angles in comparison to chloride. The two
chlorides which are eclipsed by the two N–SiMe3 groups are
moved towards the phosphine such that the Cl(3)–Nb(1)–Cl(4)
angle is 132.43(4)Њ instead of the 144Њ expected. However, it
appears that in order to relieve the strain created by these chlor-
ides, the phosphine lies below the equatorial plane by 27Њ with
the result that it approaches Cl(1) (moving it towards the nitro-
gen ligand) resulting in the angle Cl(1)–Nb–Cl(2) = 169.72(4)Њ.
Such steric demands could also account for the Nb–Cl(1) bond
length, 2.4509(8) Å, being slightly longer than the other Nb–Cl
bonds, 2.361(1), 2.3875(9) and 2.3984(8) Å. Not surprisingly,
the Nb–P bond length, 2.6711(9) Å, is considerably longer than
Scheme 1 Reagents and conditions: (i) in toluene, Ϫ78 ЊC to rt, 18 h;
t
(ii) neat PMe3, rt 2.5 h; (iii) BuNH2 (four equivalents) in CH2Cl2,
Ϫ78 ЊC to rt, 16 h or LiNHtBu (two equivalents) in pentane, rt, 13 h;
(iv) Cp2Mg (half an equivalent) in toluene, Ϫ78 ЊC to rt, 16 h; (v) in thf,
rt, 1 h for 7; excess pyridine, in toluene, rt, 16 h for 8.
Selected bond lengths and angles are listed in Table 3. The
length of the Ta–Cl bond trans to the nitrile ligand is 2.2668(6)
Å and this is shorter than the others, 2.3196(6)–2.3393(6) Å.
This may be associated with greater π-bonding in the tantalum
chloride bond due to the greater σ-donor ability of the trans-
J. Chem. Soc., Dalton Trans., 2000, 967–974
969

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Table 4 Selected bond lengths (Å) and angles (Њ) for 4
groups (δ 30.4 and 32.1 in 5; δ 31.0 and 32.6 in 6) along with two
quaternary carbon resonances for each compound (δ 51.7 and
69.2 for 5; δ 52.0 and 65.5 for 6). Those resonances are in the
range reported for other M–amino and M–imido compounds
Nb(1)–Cl(1)
Nb(1)–Cl(2)
Nb(1)–Cl(3)
Nb(1)–Cl(4)
Nb(1)–P(1)
Nb(1)–N(1)
Nb(1)–N(2)
2.4509(8)
2.361(1)
2.3875(9)
2.3984(8)
2.6711(9)
2.183(2)
2.155(2)
P(1)–C(31)
N(1)–Si(1)
N(2)–Si(2)
N(1)–C(1)
N(2)–C(1)
C(1)–C(11)
1.809(4)
1.781(3)
1.783(3)
1.333(4)
1.329(4)
1.498(4)
t
(δ 51–52 for M–NH2 Bu and δ 64–70 for M = NtBu).31–34 The 1H
NOESY spectra of 5 and 6 show the two NH protons correlate
only to one of the tert-butyl resonances, confirming that the
compounds comprise one imido and one amine ligand. A
variable temperature NMR study showed that the resonances
corresponding to the proton on 2,6-iPr-Me, 2,6-HCMe2 and
3,5-Ph decoalesce as the temperature of the sample is reduced.
No splitting of the other resonances was observed. The NMR
data show the barrier to rotation of the aryl ring around the
Cipso–C bond is 55.2 kJ molϪ1 at 268 K in 5.
Crystals of compound 5 were grown by the slow evaporation
of a pentane solution. Severe disorder is present in the crystals
as a result of an averaging of the positions of the imido and
amino ligands. Consequently, the thermal ellipsoid of the
niobium atom is elongated in the axis defined between the
two nitrogens because of the difference in Nb–N bond
lengths. This prevented a satisfactory refinement from being
achieved. Compound 6 was also crystallised in a similar
manner, however refinement of the structure was also prob-
lematic due to the same type of disorder and an adequate
model proved unattainable.
Cl(1)–Nb(1)–Cl(2) 169.72(4)
Cl(1)–Nb(1)–Cl(3) 101.28(4)
Cl(2)–Nb(1)–Cl(3)
Cl(1)–Nb(1)–Cl(4)
Cl(2)–Nb(1)–Cl(4)
P(1)–Nb(1)–N(1)
P(1)–Nb(1)–N(2)
N(1)–Nb(1)–N(2)
Nb(1)–N(1)–C(1)
Nb(1)–N(2)–C(1)
N(1)–C(1)–N(2)
Nb(1)–N(1)–Si(1)
C(1)–N(1)–Si(1)
Nb(1)–N(2)–Si(2)
C(1)–N(2)–Si(2)
Cl(1)–Nb(1)–N(2)
Cl(2)–Nb(1)–N(2)
Cl(3)–Nb(1)–N(2)
Cl(4)–Nb(1)–N(2)
142.21(7)
138.45(7)
61.40(9)
92.30(18)
93.65(17)
112.6(2)
135.55(13)
132.0(2)
134.37(13)
131.8(2)
81.33(7)
91.26(7)
82.91(7)
143.84(7)
84.75(5)
95.99(4)
85.76(4)
Cl(3)–Nb(1)–Cl(4) 132.43(4)
Cl(1)–Nb(1)–P(1)
Cl(2)–Nb(1)–P(1)
Cl(3)–Nb(1)–P(1)
Cl(4)–Nb(1)–P(1)
Cl(1)–Nb(1)–N(1)
Cl(2)–Nb(1)–N(1)
Cl(3)–Nb(1)–N(1)
Cl(4)–Nb(1)–N(1)
72.60(3)
117.46(4)
71.50(3)
72.13(3)
83.07(7)
87.12(7)
143.17(7)
82.44(7)
The tert-butyl amino ligand of compound 5 was found to be
substitutionally labile, since the THF adduct [Nb{(2,4,6-tri-
iPr)C6H2C(NSiMe3)2}(NtBu)(THF)Cl2] (7) could be prepared
by stirring compound 5 in THF for one hour followed by
1
recrystallisation from pentane. The H NMR spectrum of 7 in
benzene-d6 showed two multiplets at 1.45 ppm (4H) and 4.08
ppm (4H) corresponding to the coordinated THF, the latter
peak being shifted downfield by 0.4 ppm relative to the free
solvent. No resonance corresponding to the coordinated
tBuNH2 group was observed. This preference for the oxygen of
THF over the nitrogen of the amine, is consistent with the
‘hard’ character of the electron deficient metal centre. The
pyridine adduct [Nb{(2,4,6-tri-
Fig. 3 Molecular structure of [Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}-
Cl4(PMe3)] (4). Hydrogen atoms omitted for clarity.
iPr)C H C(NSiMe ) }(NtBu)-
6
2
3 2
(py)Cl2] (8) could also be prepared by the reaction of
compound 5 with an excess of pyridine.
the Nb–P bond length, 2.5462(12) Å, in [Nb(η-C5H5)(2-tBu-
C6H4N)(PhCCPh)(PMe3)]27 and those, 2.601(1) and 2.604(2) Å,
in the compound [Nb(η-C5H5)(NMe)Cl2(PMe3)].28
Treatment of compounds 1 or 2 with half an equivalent of
Cp2Mg in toluene at Ϫ78 ЊC forms the compounds [Nb{(2,4,6-
tri-iPr)C6H2C(NSiMe3)2}(η-C5H5)Cl3] (9) and [Ta{(2,4,6-tri-
1
Evidence of steric crowding in the solid state structure of 4 is
also reflected in the spectra. Thus the 1H NMR spectrum of 4 at
25 ЊC shows that the SiMe3 resonance occurs as a broad peak,
10.8 Hz (fwhh), which may be attributed to a hindered rotation
of the SiMe3 group. Attempts to prepare the metal–imido
iPr)C6H2C(NSiMe3)2}(η-C5H5)Cl3] (10). The H NMR spectra
of compounds 9 and 10 both show one sharp and one broad
resonance (δ Ϫ0.01 and 0.65 in 9; δ 0.00 and 0.55 in 10) corre-
sponding to dissimilar SiMe3 groups. They also show three
doublet resonances (δ 1.12, 1.23 and 1.38 in 9; δ 1.12, 1.24
and 1.37 in 10) corresponding to the methyl groups of the
iso-propyl groups of the benzamidinato ligand, but only two
septets with an intensity ratio of two to one for the isopropyl
C–H(methine) groups. The 13C{1H} NMR spectra closely
reflect the 1H NMR spectra. Thus there are two resonances for
the SiMe3 groups and three resonances for the methyl carbons
on the isopropyl groups but only two resonances corresponding
to C–H(methine) carbon on the isopropyl groups. These obser-
vations are readily reconciled by considering the structure of
compound 10 shown in Fig. 4 (vide infra). Based on this struc-
ture, we can define a plane in which the phenyl ring, C11 and C1
all lie. The two SiMe3 groups are located on opposite sides of
this plane: hence, two SiMe3 resonances are observed. This
plane also bisects the isopropyl groups, and provided that the
aryl ring is slow to rotate on the NMR timescale, the two
methyl groups on the same isopropyl group (2,6-positions) are
then in different environments. This leads to two doublet sig-
nals. The two methyl groups of the isopropyl group in the 4
position then constitute the third doublet (assuming that this
substituent is free to rotate). However, the two C–H(methine)
carbons (C2 and C5) of these groups (in the 2 and 6 positions)
are located in this plane, and are thus equivalent. This leads to
compounds
[M{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(NtBu)Cl2]
(M = Nb, Ta) by treatment of 1 and 2 with three equivalents
t
of BuNH2 proved unsuccessful, and these reactions gave the
tBuNH2 adducts [M{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(NtBu)-
t
(NH2 Bu)Cl2] (M = Nb (5), Ta (6)) in low yields. The com-
pounds 5 and 6 have been subsequently prepared in good yield
(50–60%) from the reaction of compounds 1 or 2 with four
equivalents of tBuNH2, followed by crystallisation from
pentane. Alternatively, treatment of compound 1 with two
equivalents of LiNHtBu also afforded compound 5, however in
lower yield (32%). Formation of mixed amino–imido com-
plexes such as 5 and 6 is well-known, however, the mechanism
of formation is unsure. Possible pathways include both intra-
molecular reactions (bis-amido-compound formation, followed
by α-abstraction of a NH proton by one of the amido groups)
and intermolecular reactions, the former having been postu-
lated in the formation of metal multiple bonds in related
complexes.29,30
The 1H NMR spectra of the compounds 5 and 6 exhibit two
methyl resonances for the tert-butyl groups with equal intensity.
The 13C{1H} NMR spectra also show two methyl carbon
resonances corresponding to tert-butyl imido and amino
970
J. Chem. Soc., Dalton Trans., 2000, 967–974

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Table 5 Selected bond lengths (Å) and angles (Њ) for 10
Ta(1)–Cl(2)
Ta(1)–Cl(3)
Ta(1)–Cl(4)
Ta(1)–N(1)
Ta(1)–N(2)
Ta(1)–Cp(ctr.)
2.4408(7)
2.4530(7)
2.4151(7)
2.199(2)
2.133(2)
2.147
N(1)–C(1)
N(2)–C(1)
C(1)–C(11)
N(1)–Si(1)
N(2)–Si(2)
1.318(4)
1.359(4)
1.504(4)
1.778(2)
1.778(2)
N(1)–Ta(1)–N(2)
Ta(1)–N(1)–C(1)
Ta(1)–N(2)–C(1)
N(1)–C(1)–N(2)
Ta(1)–N(1)–Si(1)
C(1)–N(1)–Si(1)
Ta(1)–N(2)–Si(2)
61.68(8)
92.20(17)
93.96(17)
112.2(2)
134.60(13)
133.2(2)
C(1)–N(2)–Si(2)
Cp–Ta(1)–Cl(2)
Cp–Ta(1)–Cl(3)
Cp–Ta(1)–Cl(4)
Cp–Ta(1)–N(1)
Cp–Ta(1)–N(2)
128.5(2)
102.32
101.04
102.61
172.89
111.21
137.58(13)
Fig. 4 Molecular structure of [Ta{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}-
(η-C5H5)Cl3] (10). Hydrogen atoms omitted for clarity.
Fig. 6 The two structure types observed for compounds of the general
formula [Ta{RЈC(NR)2}CpЈX3] (CpЈ = Cp* or Cp, RЈ = Me, p-MeO-
C6H4 or 2,4,6-(iPr)3-C6H2; R = iPr or SiMe3; X = F, Cl or Me).
but at different coalescence temperatures (273 K and 213 K).
This is consistent with two different methyl environments for
each SiMe3 group at low temperature. The barriers to rotation
of the SiMe3 groups around the N–Si bonds in solution have
been determined: 51.0 kJ molϪ1 (for the SiMe3 group close to
Cl, Tc = 273 K) and 40.6 kJ molϪ1 (for the SiMe3 group close to
Cp, Tc = 213 K).35
The crystal structure of compound 10 [Ta{(2,4,6-tri-iPr)-
C6H2C(NSiMe3)2}(η-C5H5)Cl3] has been determined and the
molecular structure is shown in Fig. 4. Selected bond lengths
and angles are listed in Table 5. The cyclopentadienyl group is
bound in a η5-manner and the benzamidinato ligand lies on the
opposite side of the metal to form a “bent sandwich” structure.
One SiMe3 group lies in close proximity to the cyclopentadienyl
group and the other SiMe3 group is situated on the other side of
the metal centre, trans to the C5 ring and close to Cl(4). This
results in a strong steric interaction between the cyclopenta-
dienyl ring and the proximate silyl group, producing a Ta–N(2)–
Si(2) angle of 137.6Њ, which is 3Њ larger than the corresponding
Ta–N(1)–Si(1) angle; this being reflected in the rotational
energy barriers observed in the 1H NMR spectra. The Ta–
N(benzamidinato) bond lengths, 2.199(2) and 2.133(2) Å, are
typical of those found in the literature,22 but again are unsym-
metrical due to the orientation of the prolate benzamidinato
ligand relative to the Cp group.
Previous reports of the structures of η-cyclopentadienyl–
amidinato complexes fall into two structure types (a and b), as
shown in Fig. 6. The compound [Ta{ArC(NSiMe3)2}(η-C5Me5)-
F3], has a structure type b in which a mirror plane exists
through the metal and the cyclopentadienyl and benzamidinato
ligands: the SiMe3 groups are symmetrical and therefore only
1
one SiMe3 resonance is observed in the H NMR spectrum.21
However, compound 10 has structure type a and is isostructural
to [Ta{MeC(NiPr)2}(η-C5Me5)Cl3] and [Ta{MeC(NiPr)2}-
(η-C5Me5)(CH3)3].22 The factors determining the two structure
types are unclear.
The compounds presented in this paper were designed to
yield a highly electron deficient metal centre through use of the
bulky benzamidinato ligand. The activity of compounds 1, 5
and 9 as ethene polymerisation catalysts was tested, using MAO
(methylaluminoxane) as a co-catalyst. Ethene polymerisations
were performed in toluene at room temperature with a mono-
mer pressure of 2 bar, in accordance with the procedures used
Fig. 5 Variable-temperature 1H NMR spectra of compound 10 in
toluene-d8 at 300 MHz (* = impurity).
only two resonances corresponding to these C–H(methine)
protons being observed at room temperature.
The variable temperature 1H NMR spectra of compound 10
in toluene-d8 between 293 and 193 K are shown in Fig. 5. Each
SiMe3 signal decoalesces into two singlets of relative ratio 2:1,
J. Chem. Soc., Dalton Trans., 2000, 967–974
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[Ta{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}Cl4] (2). A white sus-
Table 6 Activity toward ethylene polymerisation
pension of lithium N,NЈ-bis(trimethylsilyl)-2,4,6-triisopropyl-
benzamidinate (1.19 g, 3.0 mmol) in 30 ml toluene at room
temperature was added, with stirring, to a suspension of TaCl5
(1.08 g, 3.0 mmol) in 20 ml toluene cooled to Ϫ78 ЊC. The
resulting white suspension was allowed to warm to room tem-
perature and stirred for a further 18 h to give a yellow suspen-
sion. The mixture was filtered and the volatiles were removed
under reduced pressure to afford a yellow solid. Extraction with
20 ml pentane, filtering and cooling to Ϫ25 ЊC afforded the
product as yellow crystals. Yield, 1.16 g, 54%.
Activity/g PE
(mol M)Ϫ1 hϪ1
Compound
Time/min
Yield/mg
1
5
9
120
120
120
22
42
44
1.76 × 103
3.36 × 103
3.52 × 103
At 2 bar absolute monomer pressure, rt, 220 cm3 toluene, 10 ml 10%
MAO, Al/M = 2448. mol M: total amount of metal used (6.25 × 10Ϫ6
mol for each run).
by Kaminsky and co-workers.36 The activity data are given in
Table 6. All three compounds (1, 5 and 9) show extremely low
activity as ethene polymerisation catalysts, similar to that
reported for the zirconium systems.11,37 Owing to the poor
reactivity of these catalyst precursors, further studies were
discontinued.
In conclusion, the presence of bulky 2,4,6-tri-isopropyl
substituents on the aryl rings of the benzamidinato ligand
favours the formation of mono-benzamidinato–metal com-
pounds. However, these compounds are not active pre-catalysts
for olefin polymerisation.
[Ta{(2,4,6-tri-iPr)C6H2CN}Cl5] (3). TaCl5 (3.58 g, 10.0 mmol)
in toluene was reacted with lithium N,NЈ-bis(trimethylsilyl)-
2,4,6-triisopropylbenzamidinate (3.97 g, 10.0 mmol) as
described for the synthesis of compound 2. The pentane extract
was concentrated until some solid precipitated, then cooled
to Ϫ25 ЊC for two days. Decantation of the yellow solution
allowed isolation of a mixture of pale-brown crystals and a
yellow solid. The mixture was washed with 20 ml pentane and
the residue was dried in vacuo to afford pale-brown crystals (3).
Yield, 0.29 g, 5%. Evaporation of the supernatant to dryness,
yielded further yellow solid (compound 2), which was washed
with 10 ml cold pentane and dried. Yield, 2.56 g, 36%.
Experimental
[Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}Cl4(PMe3)] (4). Tri-
methylphosphine (ca. 3 ml) was condensed into an ampoule
charged with [(2,4,6-tri-Pri)C6H2C(NSiMe3)2]NbCl4 (0.97 g, 1.6
mmol). The reaction mixture was warmed to room temperature
and stirred for a further 2.5 h. After this time, the excess PMe3
was removed in vacuo and the residue was extracted with 40 ml
pentane. The extract was concentrated until some solid precipi-
tated, then cooled to Ϫ25 ЊC to afford a red solid. Yield, 0.54 g,
50%.
All manipulations were carried out under an atmosphere of
dinitrogen using standard Schlenk-line or drybox techniques.
Solvents were pre-dried over molecular sieves and refluxed over
the appropriate drying agent and distilled prior to use. NMR
solvents were dried over calcium hydride or molten potassium,
distilled under vacuum and stored under dinitrogen in Young’s
ampoules.
1H, 13C{1H} and 31P{1H} NMR spectra were recorded either
on Varian Unity 500 or Varian Mercury 300 spectrometers and
referenced internally to the residual solvent peak relative to
tetramethylsilane. 31P{1H} NMR spectra were referenced exter-
[Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(NH2tBu)(NtBu)Cl2] (5).
(1) By amine hydrochloride elimination. To a red solution of
(2,4,6-tri-iPr)C6H2C(NSiMe3)2NbCl4 (0.89 g, 1.4 mmol) in 20
ml CH2Cl2, stirred at Ϫ78 ЊC, was added a solution of tert-
butylamine (0.6 ml, 5.7 mmol) in 20 ml CH2Cl2. The reaction
mixture was allowed to warm to the room temperature and
stirred for 16 hours. The yellow mixture was filtered and the
volatiles were removed in vacuo to afford a yellow solid. Yield,
0.58 g, 59%.
(2) By lithium chloride elimination. A colourless solution of
LiNHtBu (0.16 g, 2.0 mmol) in 15 ml pentane was added drop-
wise, at room temperature, to a red solution of (2,4,6-tri-
iPr)C6H2C(NSiMe3)2NbCl4 (0.62 g, 1.0 mmol) in 20 ml pentane.
After stirring for 13 h, the yellow mixture was filtered and the
volatiles were removed in vacuo to afford a yellow solid that
was purified by crystallisation from pentane at Ϫ25 ЊC. Yield,
0.22 g, 32%.
1
nally to H3PO4. Variable temperature H NMR experiments
were recorded on a Varian-VXR-300S spectrometer. Infrared
spectra were recorded on a 6020 Galaxy series FT-IR instru-
ment in the range 4000–400 cmϪ1 as caesium iodide pellets.
Elemental analyses were performed by the Microanalytical
Department of the Inorganic Chemistry Laboratory in Oxford.
NbCl5, TaCl5, 1,3-bis(trimethylsilyl)carbodiimide, tBu-
NH2, pyridine (Aldrich), 2-bromo-1,3,5-triisopropylbenzene
(Lancaster) and methylaluminoxane (Witco) were used as
supplied. Cp2Mg,38 PMe3,39 LiNHtBu,40 and 2,4,6-triisopropyl-
phenyllithium41 were prepared by the literature methods.
Preparations
Lithium N,NЈ-bis(trimethylsilyl)-2,4,6-triisopropylbenzamid-
inate. This compound was prepared as a white powder from
the reaction between (Me SiN᎐C᎐NSiMe ) and [Li(2,4,6-tri-
᎐ ᎐
3
3
[Ta{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(NH2tBu)(NtBu)Cl2] (6).
To a yellow solution of (2,4,6-tri-iPr)C6H2C(NSiMe3)2TaCl4
(0.82 g, 1.2 mmol) in 20 ml CH2Cl2, stirred at Ϫ78 ЊC, was
added a colourless solution of tert-butylamine (0.49 ml, 4.8
mmol) in 20 ml CH2Cl2. The yellow solution was allowed to
warm to the room temperature and react overnight. After 19
hours of stirring, the pale-green mixture was filtered and the
volatiles were removed in vacuo to afford a pale-green solid that
was purified by crystallisation from pentane. Yield, 0.48 g, 53%.
iPrC6H2)] in ether at room temperature followed by crystallis-
ation from ether according to the procedure given for the
2,4,6-trimethylphenyl analogue.42 It was characterised from the
1H NMR spectrum and elemental analysis.
[Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}Cl4] (1). A white sus-
pension of lithium N,NЈ-bis(trimethylsilyl)-2,4,6-triisopropyl-
benzamidinate (2.61 g, 6.6 mmol) in 20 ml toluene at room
temperature was added to a stirred suspension of NbCl5 (1.78 g,
6.6 mmol) in 20 ml toluene cooled to Ϫ78 ЊC. The resulting red
suspension was allowed to warm slowly to room temperature
and stirred for a further 19 h. The suspension was filtered and
the volatiles were removed under reduced pressure to afford a
red solid. Extraction with pentane, filtering and slow cooling
to Ϫ25 ЊC afforded the product as a red powder. Yield, 1.81 g,
44%.
[Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(THF)(NtBu)Cl2] (7). A
sample of compound 5 (0.40 g, 0.6 mmol) was dissolved in 10
ml THF and stirred at room temperature for 1 hour. The sol-
vent was removed in vacuo to afford a yellow solid that was
recrystallised from pentane to form a yellow crystalline solid.
Yield, 0.21 g, 52%.
972
J. Chem. Soc., Dalton Trans., 2000, 967–974

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Table 7 Summary of crystal data for compounds 2, 3, 4, and 10
2
3
4
10
Formula
FW
Crystal system
Space group
C22H41Cl4N2Si2Ta
712.52
Orthorhombic
Pca21
C16H23Cl5NTa
587.58
Monoclinic
P21/n
C25H50Cl4N2NbPSi2
700.55
Monoclinic
P21/c
C27H46Cl3N2Si2Ta
742.16
Monoclinic
P21/c
a/Å
b/Å
c/Å
19.289(4)
15.548(2)
20.389(4)
12.722(4)
10.598(2)
15.931(4)
97.829(6)
2127.9(6)
4
1.83
125
5.74
11112
4606
4066
0.0237
0.0257
18.388(4)
10.256(2)
18.799(7)
96.701(6)
3521.0(5)
4
1.32
270
0.78
16427
7484
5739
0.0523
0.0537
16.852(5)
15.111(4)
13.049(2)
75.653(5)
3219.3(3)
4
1.53
125
3.72
16990
6635
5902
0.0290
0.0316
β/Њ
V/Å3
6114.8(5)
8
1.55
150
4.00
27302
6889
5903
0.0243
0.0281
Z
ρcalc/g cmϪ3
T/K
µ(Mo-Kα)/mmϪ1
Total no. of data
No. of unique data
No. of observed data
Ra
Rwa
2
a R = [Σ(|Fo| Ϫ |Fc|)/Σ|Fo|]; Rw = [Σw(Fo2 Ϫ Fc2)2/Σw(Fo )2]1/2
.
[Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(py)(NtBu)Cl2] (8). To a
yellow solution of compound 5 (0.70 g, 1.0 mmol) in 20 ml
toluene, a colourless solution of pyridine (0.168 g, 2.1 mmol) in
10 ml toluene was added at room temperature. After 16 hours
of stirring, the volatiles were removed in vacuo to afford a
yellow solid that was recrystallised from pentane to form a
yellow crystalline solid. Yield, 0.26 g, 37%.
The contents of the reactor were then transferred into a conical
flask and 250 cm3 of conc. HCl in ethanol was added (20% by
volume) and the mixture was stirred overnight. The polymer
was then separated by filtration and washed with water and
ethanol then dried in vacuo at 60 ЊC.
Crystal structure data
[Nb{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(ꢀ-C5H5)Cl3] (9). A solu-
tion of Cp2Mg (0.20 g, 1.3 mmol) in 20 ml of toluene was added
to a red solution of (2,4,6-tri-iPr)C6H2C(NSiMe3)2NbCl4 (1.65
g, 2.6 mmol) in 20 ml toluene at Ϫ78 ЊC. The reaction mixture
was allowed to warm to the room temperature and was stirred
for a further 17 hours. The dark-red suspension was filtered and
the solvent was evaporated under reduced pressure to form a
dark red solid. This was washed with 5 ml pentane to afford
a dark-red powder. Yield, 0.82 g, 48%.
Crystals were grown from a saturated pentane solution (2, 3, 4)
or dichloromethane–pentane solution (10) at 248 K, and
isolated by filtration. Specimens were mounted on the end of
a glass fibre, and data were collected on an Enraf-Nonius
DIP2000 image plate diffractometer. The images were
processed with the DENZO and SCALEPACK programs.43
Corrections for Lorentz and polarisation effects were
performed.
Solution, refinement, and graphical calculations were per-
formed using the CRYSTALS44 and CAMERON45 software
packages. The structures were solved by direct methods
using the SIR92 program.46 Crystallographic information is
summarised in Table 7.
[Ta{(2,4,6-tri-iPr)C6H2C(NSiMe3)2}(ꢀ-C5H5)Cl3] (10). To a
yellow solution of (2,4,6-tri-iPr)C6H2C(NSiMe3)2TaCl4 (1.42 g,
2.0 mmol) in 20 ml toluene was added a solution of Cp2Mg
(0.15 g, 1.0 mmol) in 10 ml toluene at Ϫ78 ЊC. The reaction
mixture was allowed to warm to room temperature and was
stirred for a further 16 hours. The dark-brown mixture was
filtered and the volatiles were removed in vacuo to afford a dark-
brown solid that was washed with 5 ml pentane forming a
yellow powder. Yield, 0.28 g, 19%.
CCDC reference number 186/1825.
See http://www.rsc.org/suppdata/dt/a9/a909333h/ for crystal-
lographic files in .cif format.
Acknowledgements
We would like to thank the ORS for a grant and for leave
of absence from the Department of Chemistry, National
Chung-Hsing University, Taiwan, ROC (to C.-T. C.),
St. John’s College, Oxford for a Junior Research Fellowship
(to L. H. D.) and a post-doctoral fellowship sponsored by Col-
ebrand Ltd (to V. C. W.). We are also grateful to Dr Daniel
Haüssinger for helpful discussions and NMR spectra, as well as
Dr Leigh Rees for helpful discussions about X-ray structure
refinement.
Polymerisation studies
Ethylene was purified by passage through 4 Å molecular sieves
and then over finely divided potassium metal supported on
glass wool. Polymerisation reactions were carried out in a
Fischer-Porter reactor equipped with a magnetic stirrer using
conditions similar to those employed by Kaminsky et al.36
Methylaluminoxane(MAO) (9.4 ml) was syringed into the
Fischer-Porter bottle, which was connected to the ethylene
supply via a gas supply system and filled with ethylene. Toluene
(200 cm3) was then added, the ethylene pressure increased to
2 bar (absolute pressure) and the mixture was stirred at room
temperature until saturated with ethylene. Meanwhile, 0.6 ml of
MAO was added to a solution of the benzamidinato catalyst
precursor complex (6.25 × 10Ϫ6 mol) in 10 cm3 of toluene and
stirred for 15 min pre-activation. The catalyst–MAO mixture
was then added quickly to the reactor by syringe. The reaction
mixture was stirred vigorously at room temperature under 2 bar
pressure of ethylene for 2 hours. The reaction was quenched by
venting the ethylene and adding a small amount of ethanol.
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