Synthesis and catalytic properties of oxalic amidinato complexes

Article abstract of DOI:10.1039/b010017j

The oxalic amidine compounds C₆H₅N=C(NH^tBu)-C(NH^tBu)=NC ₆H₅ 1 and C₆H₅N=C{N(^tBu)(SiMe₃)}- C{N(^tBu)(SiMe₃)}=NC₆H₅ 2 are described. The compound [(C₆H₅N)C(NH^tBu)-C(NH^tBu)(NHC ₆H₅)][Br] 3* has been isolated from the reaction between 1 and [NiBr₂(DME)]. Treatment of 1 with M(NMe₂)₄ (M =Zr or Ti), [Ta(NEt₂)₂Cl₃(py)] or [Zr{N(SiMe₃)₂}₃Cl] gives the bimetallic complexes [M(NMe₂)₃(C₆ H₅N)C(N^tBu)-]2 (M =Zr (4) or M =Ti (5)) and the mono-amidinato complexes [Ta(NEt₂)Cl₃{(C6 H₅N)C(N^tBu)C(HN^tBu)(NC₆H ₅)}] 6* and [Zr{N(SiMe₃)₂}₂Cl{(C₆H ₅N)C(N^tBu)C(HN^tBu)(NC₆H ₅)}] 7* respectively: * indicates the crystal structure has been determined. The activity toward ethylene polymerisation has been investigated for compounds 4, 5 and 7.

Full text of DOI:10.1039/b010017j

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Synthesis and catalytic properties of oxalic amidinato complexes
Chi-Tien Chen, Leigh H. Rees, Andrew R. Cowley and Malcolm L. H. Green*
Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR
Received 15th December 2000, Accepted 22nd March 2001
First published as an Advance Article on the web 15th May 2001
The oxalic amidine compounds C H N᎐C(NHtBu)–C(NHtBu)=NC H 1 and C H N᎐C{N(tBu)(SiMe )}–
᎐
᎐
6
5
6
5
6
5
3
C{N(tBu)(SiMe3)}=NC6H5 2 are described. The compound [(C6H5N)C(NHtBu)–C(NHtBu)(NHC6H5)][Br] 3*
has been isolated from the reaction between 1 and [NiBr2(DME)]. Treatment of 1 with M(NMe2)4 (M = Zr or Ti),
[Ta(NEt2)2Cl3(py)] or [Zr{N(SiMe3)2}3Cl] gives the bimetallic complexes [M(NMe2)3(C6H5N)C(NtBu)–]2 (M =
Zr (4) or M = Ti (5)) and the mono-amidinato complexes [Ta(NEt2)Cl3{(C6H5N)C(NtBu)C(HNtBu)(NC6H5)}]
6* and [Zr{N(SiMe3)2}2Cl{(C6H5N)C(NtBu)C(HNtBu)(NC6H5)}] 7* respectively: * indicates the crystal structure
has been determined. The activity toward ethylene polymerisation has been investigated for compounds 4, 5 and 7.
Introduction
Owing to the success of metallocenes containing one or more
cyclopentadienyl ligands as pre-catalysts for the polymerisation
of α-olefins, there is considerable interest in the development
of new ‘non-metallocene’ catalysts.1 The amidinato ligands of
general formula [R1C(NR2)(NR3)]Ϫ have been the focus of
attention because they can often be used in place of cyclo-
pentadienyl ligands to provide new metal complexes. The steric
and electronic properties of amidinato ligands are program-
mable by variation of the substituents R1, R2 and R3.2 Indeed,
over the past thirty years, a large diversity of metal–amidinato
compounds has been synthesized and these have been reviewed
recently by Barker and Kilner (on amidine)3 and Edelmann (on
benzamidine).4 However, to date there are only a few new
examples of amidinato ligands with bulky substitutents,5–7
asymmetrical substituents (R2 ≠ Ρ3),8–10 and of pendant11,12
or bridged amidinato ligands.13–15
Fig. 1 Molecular structure of [(C6H5N)C(NHtBu)C(NHtBu)(NHC6-
H5)][Br] 3. Hydrogen atoms on carbon atoms omitted for clarity.
Oxalic amidines were first described in the 1900s.16,17 They
have been employed in the organic syntheses of heterocyclic18–21
and macrocyclic22 compounds. Recently, they have attracted
renewed interest, mainly driven by the discovery that the
late transition metal diimine complexes display efficient activity
as catalysts for olefin polymerisation reaction.1,23 Since the
oxalic amidines can be deprotonated, they also have the poten-
tial to act as diamide or diamidinate ligands. The steric and
electronic effects of these versatile ligands are programmable
through variation of the substituents on the nitrogen atoms.24
However, to date only a few examples of diimine,25–27 imine–
amide26,28,29 and diimine–diamide30 complexes have been
developed.
The analytical and spectroscopic data characterising 1 and 2
and the other new compounds described in this paper are
collected in Table 1. A summary of syntheses and proposed
structures is shown in Scheme 1.
In view of the diimine property of compound 1, it was
decided to attempt to prepare a Brookhart type nickel
catalyst.23 Treatment of 1 with one equivalent of NiBr2(DME)
in CH2Cl2 at room temperature affords a pale green solid, which
was purified by diffusion of concentrated CH2Cl2 solution into
toluene to give a green crystalline solid. The analytical data
were consistent with the salt product [(C6H5N)C(NHtBu)-
C(NHtBu)(NHC6H5)][Br] 3 and no nickel adduct was formed.
A possible explanation of the formation of compound 3 is the
occurrence of hydrogen migration from the ligand or solvent
followed by reduction of [NiBr2(DME)] to form 3 in low yield
(23%).
Here we report the preparation and structural properties of
early transition metal complexes containing oxalic amidinato
ligands. Their ability to act as pre-catalysts toward olefin
polymerisation has also been investigated.
The crystal structure of [(C6H5N)C(NHtBu)C(NHtBu)(NH-
C6H5)][Br] 3 has been determined and the molecular structure
is shown in Fig. 1. The interatomic distances and angles are
listed in Table 2. The structure displays the proposed diimine–
diamine structure with one proton attaching to one of the
diimine nitrogens to form a cation. The three CCNN–N(H) bond
distances, 1.309(4), 1.356(4) and 1.324(4) Å, are between those
Results and discussion
Treatment of the bis-phenylimidoyl chloride with an excess of
tBuNH2 (6 equivalents) in toluene gives a white powder which
the data show to be a bis-amidine, tethered via the carbon–
carbon single bond, namely C H N᎐C(NHtBu)–C(NHtBu)᎐
᎐
᎐
6
5
NC6H5 1. No tautomer was observed in the H and 13C-{1H}
NMR spectra. Treatment of one equivalent of 1 with two
equivalents of n-BuLi followed by refluxing in THF with an
of the C–N single bond, 1.504(4), 1.480(4) Å, and C᎐N double
1
᎐
bond, 1.271(4) Å, in the same structure. The partial double
bond property shows the delocalisation around the CNN unit.
Bond angles around the bridged carbon, 123.5(3), 117.4(3),
119.1(3), 123.8(3), 112.0(2) and 124.2(3)Њ, display the sp2
excess of Me SiCl gives the silyl amidine compound C H N᎐
᎐
3
6
5
C{N(tBu)(SiMe )}–C{N(tBu)(SiMe )}᎐NC H 2, in high yield.
᎐
3
3
6
5
DOI: 10.1039/b010017j
J. Chem. Soc., Dalton Trans., 2001, 1761–1767
This journal is © The Royal Society of Chemistry 2001
1761

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Table 1 Analytical and spectroscopic data
Compound and analysisa
Spectroscopic datab
1
1H: 1.07 (s, 18H, NC(CH3)3), 3.96 (s, 2H, NH), 6.90 (m, 2H, p-C6H5), 7.02 (m, 4H, o-C6H5), 7.11 (m, 4H, m-C6H5)
White
13C-{1H}: 27.9 (s, NC(CH3)3), 51.8 (s, NC(CH3)3), 121.8 (s, o-C6H5), 122.5 (s, p-C6H5), 129.2 (s, m-C6H5), 149.3
(s, Cipso-Ph), 150.2 (s, CNN)
C, 75.1 (75.4)
H, 9.0 (8.6)
N, 15.9 (16.0)
2
IR: 3390m (NH), 1636s, 1590s, 1506s, 1454m, 1362m, 1324m, 1250m, 1206s, 1158m, 902m, 780m, 754s, 696s,
558m, 436m
1H: 0.27 (s, 18H, Si(CH3)3), 1.06 (s, 18H, NC(CH3)3), 6.96 (m, 2H, p-C6H5), 7.09 (m, 4H, m-C6H5), 7.48 (br, 4H,
o-C6H5)
White
C, 68.0 (68.0)
H, 8.6 (9.4)
N, 11.6 (11.3)
13C-{1H}: 2.8 (s, Si(CH3)3), 30.3 (s, NC(CH3)3), 54.9 (s, NC(CH3)3), 125.5 (s, p-C6H5), 128.6 (s, m-C6H5), 130.0 (s,
o-C6H5), 143.4 (s, Cipso-Ph), 149.9 (s, CNN)
IR: 2970s, 1646s, 1620s, 1594s, 1492s, 1358m, 1292s, 1248s, 1208s, 1146s, 848s, 750s, 702s, 680m, 622m, 586m,
556m, 420m
3
1H: 1.11 (s, 18H, NC(CH3)3), 3.98 (br, 2H, NH), 6.91 (br, 2H, p-C6H5), 7.08 (br, 4H, m-C6H5), 7.14 (br, 4H,
o-C6H5)
Yellowish green
C, 60.3 (61.3)
H, 6.7 (7.2)
N, 12.7 (13.0)
Br, 18.3 (18.5)
4
13C-{1H}:c 28.1, 30.0 (2s, NC(CH3)3), 54.3, 58.8 (2s, NC(CH3)3), 121.6, 124.0, 124.3, 127.7, 129.0, 129.3 (6s,
o,m,p-C6H5), 134.5, 140.2, 146.5, 157.5 (4s, 2Cipso-Ph ϩ 2CNN)
IR: 3244m (NH), 1638s, 1592s, 1536m, 1484m, 1372m, 1186m, 762m, 694m
1H: 1.00 (s, 18H, NC(CH3)3), 3.03 (s, 36H, N(CH3)2), 6.93 (m, 2H, p-Ph), 7.27 (m, 4H, m-Ph), 7.57 (m, 4H, o-Ph)
13C-{1H}: 30.8 (s, N(C(CH3)3), 42.5 (s, N(CH3)2), 55.2 (s, N(C(CH3)3), 123.0 (s, p-C6H5), 123.3 (s, o-C6H5), 129.4
(s, m-C6H5), 147.1 (s, Cipso-Ph), 167.6 (s, CNN)
Off-white
C, 51.4 (51.3)
H, 8.7 (8.1)
N, 17.0 (17.6)
5
IR: 2822s, 2766s, 1618m, 1592s, 1496s, 1450s, 1390m, 1360m, 1242s, 1202s, 1146m, 986m, 938s, 792m, 754s, 696s,
536s, 478m
1H: 1.08 (s, 18H, NC(CH3)3), 3.23 (s, 36H, N(CH3)2), 6.95 (m, 2H, p-Ph), 7.29 (m, 4H, m-Ph), 7.57 (m, 4H, o-Ph)
13C-{1H}:c 31.4 (s, N(C(CH3)3), 46.5 (s, N(CH3)2), 55.5 (s, N(C(CH3)3), 122.5 (s, p-C6H5), 123.5 (s, o-C6H5), 129.1
(s, m-C6H5), 148.0 (s, Cipso-Ph), 167.6 (s, CNN)
Orange
C, 57.0 (57.6)
H, 9.0 (9.1)
N, 19.2 (19.8)
6
IR: 2846s, 2762s, 1618m, 1592s, 1496s, 1440s, 1394m, 1358m, 1242s, 1198s, 1150m, 946s, 794m, 752m, 696m,
588m, 560m, 482m
1H: 0.59 (t, 6H, J = 6.9, N(CH2CH3)2), 1.16 (s, 9H, NC(CH3)3), 1.67 (s, 9H, NC(CH3)3), 4.12 (q, 4H, J = 6.9,
N(CH2CH3)2), 4.24 (s, 1H, NHC(CH3)3), 6.78–7.00 (overlapping, 6H, Ph), 7.22–7.31 (overlapping, 4H, Ph)
13C-{1H}:c 11.7 (s, N(CH2CH3)2), 28.2 (s, NC(CH3)3), 30.2 (s, NC(CH3)3), 44.8 (s, N(CH2CH3)2), 53.0 (s,
NC(CH3)3), 58.3 (s, NC(CH3)3), 122.5 (s, o-Ph), 123.5 (s, p-Ph), 124.8 (s, o-Ph), 126.2 (s, p-Ph), 128.1 (s, m-Ph),
128.8 (s, m-Ph), 143.2, 145.1, 146.7 (3s, 2 Cipso-Ph ϩ 1CNN), 166.0 (s, CNN)
Red
C, 43.8 (44.1)
H, 5.5 (5.5)
N, 9.8 (9.9)
Cl, 14.3 (15.0)
IR: 3416m (NH), 2974s, 1632s, 1592m, 1532s, 1492s, 1452m, 1370s, 1246m, 1222m, 1202m, 1164m, 1040m, 992m,
928m, 788m, 768m, 740m, 698m
7
1H: 0.18 (s, 18H, N[Si(CH3)3]2), 0.40 (s, 18H, N[Si(CH3)3]2), 1.32 (s, 9H, NC(CH3)3), 1.62 (s, 9H, NC(CH3)3), 4.72
(s, 1H, NHC(CH3)3), 6.33 (d, J = 7.5, 2H, o-Ph), 6.83–7.01 (overlapping, 8H, Ph)
13C-{1H}:c 5.4 (s, N[Si(CH3)3]2), 5.7 (s, N[Si(CH3)3]2), 28.1 (s, NC(CH3)3), 31.4 (s, NC(CH3)3), 53.0 (s, NC(CH3)3),
56.3 (s, NC(CH3)3), 121.5 (s, Ph), 122.5 (s, Ph), 125.1 (s, Ph), 125.3 (s, Ph), 129.0 (s, Ph), 129.3 (s, Ph), 143.7, 147.3,
147.7 (3s, 2 Cipso-Ph ϩ 1CNN), 168.8 (s, CNN)
Off-white
C, 50.9 (51.2)
H, 8.4 (8.2)
N, 10.5 (10.6)
Cl, 4.0 (4.5)
IR: 3450w, 3412m (NH), 2966s, 1618s, 1590s, 1488s, 1454s, 1398s, 1362m, 1252s, 1218m, 1162m, 852s, 784m,
762m, 696s, 668m, 426m
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, J in Hz. c In chloroform-d.
Scheme 1 Reagents and conditions: (i) in toluene, r.t., 48 h; (ii) 2 n-BuLi, Et2O, 3 h; followed by excess Me3SiCl, THF, 75 ЊC, 8 h; (iii) [NiBr2(DME)],
CH2Cl2, r.t., 72 h; (iv) M(NMe2)4 (two equivalents), toluene, 85 ЊC, 3 h; (v) [Ta(NEt2)2Cl3(py)] (two equivalents), toluene, 110 ЊC, 7.5 h; (vi)
[Zr{N(SiMe3)2}3Cl], toluene, 105 ЊC, 4 h.
1762
J. Chem. Soc., Dalton Trans., 2001, 1761–1767

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Table 2 Selected bond lengths (Å) and angles (Њ) for 3
Table 3 Selected bond lengths (Å) and angles (Њ) for 6
N(1)–C(1)
N(1)–C(11)
C(1)–N(21)
N(21)–C(22)
C(1)–C(2)
1.309(4)
1.504(4)
1.324(4)
1.424(4)
1.523(4)
N(2)–C(2)
1.356(4)
1.480(4)
1.271(4)
1.406(4)
Ta(1)–Cl(1)
Ta(1)–Cl(2)
Ta(1)–Cl(3)
Ta(1)–N(1)
Ta(1)–N(2)
Ta(1)–N(4)
N(2)–C(3)
N(2)–C(21)
2.367(4)
2.385(3)
2.345(3)
1.960(10)
2.249(8)
2.090(10)
1.220(20)
1.547(18)
N(4)–C(3)
N(4)–C(41)
C(3)–C(5)
N(6)–C(5)
N(6)–C(61)
N(7)–C(5)
N(7)–C(71)
1.367(17)
1.382(16)
1.519(19)
1.349(17)
1.492(16)
1.285(16)
1.414(17)
N(2)–C(31)
C(2)–N(41)
N(41)–C(42)
C(1)–N(1)–C(11)
N(1)–C(1)–N(21)
C(1)–N(21)–C(22)
N(1)–C(1)–C(2)
C(1)–C(2)–N(2)
135.1(2)
117.4(3)
128.1(3)
123.5(3)
112.0(2)
C(2)–N(2)–C(31)
N(2)–C(2)–N(41)
C(2)–N(41)–C(42)
C(2)–C(1)–N(21)
C(1)–C(2)–N(41)
125.2(3)
123.8(3)
124.7(3)
119.1(3)
124.2(3)
Cl(1)–Ta(1)–Cl(2)
Cl(2)–Ta(1)–Cl(3)
Cl(2)–Ta(1)–N(1)
Cl(3)–Ta(1)–N(2)
Ta(1)–N(2)–C(3)
C(3)–N(2)–C(21)
Ta(1)–N(4)–C(41)
N(2)–C(3)–N(4)
C(5)–N(7)–C(71)
88.42(14)
170.11(13)
89.8(3)
88.9(3)
94.2(9)
132.3(12)
137.9(8)
110.3(12)
122.1(12)
Cl(1)–Ta(1)–Cl(3)
Cl(1)–Ta(1)–N(1)
Cl(3)–Ta(1)–N(1)
N(2)–Ta(1)–N(4)
Ta(1)–N(2)–C(21)
Ta(1)–N(4)–C(3)
C(3)–N(4)–C(41)
N(6)–C(5)–N(7)
C(5)–N(6)–C(61)
89.38(14)
104.4(3)
100.1(3)
58.4(5)
133.3(10)
96.9(8)
123.2(11)
121.1(13)
126.0(11)
character. This structure also confirms the proposed structure
of the neutral ligand.
Amine elimination reactions have been employed in the syn-
thesis of metal amide complexes for many years,31–33 and the
synthesis of mixed amide complexes was attempted via this
route. Therefore, the reactions of the diimine–diamine ligand
with M(NMe2)4 (M = Zr or Ti) were investigated. Treatment
of compound 1 with one equivalent of Zr(NMe2)4 in toluene
1
at 85 ЊC affords an off-white compound in low yield. The H
NMR spectrum shows two methyl resonances assigned to the
tert-butyl and NMe2 groups with the intensity of one to two.
This indicates three NMe2 groups remain co-ordinated to
the metal centre. No N–H stretch was observed in the infrared
spectrum. The elemental analysis data are consistent with the
bimetallic amidinate compound, [Zr(NMe2)3(C6H5N)C(N-
tBu)–]2 4. The compound 4 was subsequently prepared in good
yield (82%) from the reaction of 1 with two equivalents of
Zr(NMe2)4 in toluene at 85 ЊC, followed by recrystallisation
from a concentrated toluene solution. The same route was
employed in preparing the analogous titanium compound (5) in
good yield (75%). Attempts to make metal halide complexes
by treatment of compounds 4 or 5 with Me3SiCl or Me2NHؒ
HCl34–36 have been performed. However, these reactions only
result in formation of intractable mixtures. Reactions of com-
pound 2 with [ZrCl4(THF)2] either in CH2Cl2 at room temper-
ature or in refluxing toluene also failed to yield characterisable
metal halide complexes.
The successful synthesis of the mixed amidinate–amide com-
plexes via amine elimination led us to explore the synthesis of
related metal complexes by an analogous method. Treatment of
compound 1 with [Ta(NEt2)2Cl3(py)] in the 1 : 2 molar ratio in
refluxing toluene gave a red compound (6), but it was not the
expected bimetallic amidinate complex. The 1H NMR spectrum
of 6 showed two peaks with the same intensity corresponding
to the tert-butyl groups. Irradiation of one (δ 1.16) of those two
resonances resulted in an NOE enhancement of the signal for
the N–H resonance (δ 4.24). This indicates that one amidine
function remains intact. A sharp single peak in the infrared
spectrum is observed at 3416 cmϪ1 indicative of an N–H stretch.
The elemental analysis data of compound 6 are consistent with
the mono-amidinate compound, [Ta(NEt2)Cl3{(C6H5N)C(N-
tBu)C(HNtBu)(NC6H5)}]. Variation of the stoichiometry of the
reaction of [Ta(NEt2)2Cl3(py)] and compound 1 did not yield
the bimetallic amidinate or bis-amidinate compound. Instead a
mixture of starting material and 6 was obtained.
Fig. 2 Molecular structure of [Ta(NEt2)Cl3{(C6H5N)C(NtBu)C(HN-
tBu)(NC6H5)}] 6. Details as in Fig. 1.
Ta–Cl bond distances, 2.345(3), 2.367(4) and 2.385(3) Å, are in
the range expected for Ta–Cl bond lengths. The small bite angle
N(2)–Ta(1)–N(4), 58.4(5)Њ, compared with those found in
mono-37–39 or bis-40–42 amidinato compounds, may be attributed
to the steric congestion around the metal centre. The dihedral
angle indicates two CNN planes are intersecting with an angle
of 115.59Њ. Attempts to prepare a heterobimetallic compound
by the reaction between compound 6 and Ti(NMe2)4 proved
unsuccessful.
Treatment of one equivalent of compound 1 with one equiv-
alent of [Zr{N(SiMe3)2}3Cl] in refluxing toluene yielded a
pale yellow solution. Purification from a concentrated pentane
solution afforded an off-white solid in good yield. Two peaks at
δ 0.18 and 0.40 were observed in the 1H NMR spectrum in the
regions of the SiMe3 group with the same intensity and two
peaks corresponding to the methyl groups (δ 1.32 and 1.62) on
the tert-butyl groups. This indicates two N(SiMe3)2 groups and
two tert-butyl groups should have different environments in
1
solution. The H NMR (δ 4.72) and infrared (3450 cmϪ1) data
also indicate one non-co-ordinated N–H group on the amid-
inato ligand. The elemental analysis data are consistent with the
mono-amidinate compound, [Zr{N(SiMe3)2}2Cl{(C6H5N)-
C(NtBu)C(HNtBu)(NC6H5)}] 7. Attempts to make the bi-
metallic complex by treating compound 7 with Zr(NMe2)4 in
1 : 1 ratio yielded a mixture of 4 and 7. Compound 7
rearranged into 4 completely on addition of an excess of
Zr(NMe2)4, indicating not only the amine elimination but also
the substitution reaction proceeded in solution.
The crystal structure of compound 7 has been determined
and the molecular structure is shown in Fig. 3. Similar to the
structure of compound 6, the amidinato ligand is bound to the
metal centre with one end to form square pyramidal geometry
The crystal structure of compound 6 has been determined
and the molecular structure is shown in Fig. 2. Selected bond
lengths and angles are listed in Table 3. The amidinato ligand,
occupying one edge of the octahedron, co-ordinates to the
metal centre with one end free to form a pseudo-octahedral
geometry. The NEt2 group and one chloride are trans to the
amidinato ligand with the other two chlorides occupying the
axial positions of the octahedron. The Ta–NCNN bond dis-
tances, 2.090(10) and 2.249(8) Å, are asymmetric, which leads
to the observation of inequivalent C–N bonds, 1.367(17) and
1.220(20) Å. These asymmetric bond lengths may result from
the differing trans influences of amide and chloride ligands. The
J. Chem. Soc., Dalton Trans., 2001, 1761–1767
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Table 4 Selected bond lengths (Å) and angles (Њ) for 7
and 7. Compound 6 did not exhibit any reactivity in ethylene
polymerisation.
Zr(1)–Cl(1)
Zr(1)–N(1)
Zr(1)–N(2)
Zr(1)–N(5)
Zr(1)–N(6)
N(1)–C(1)
2.4366(5)
2.2351(16)
2.2907(17)
2.0459(18)
2.0760(17)
1.353(3)
N(2)–C(1)
N(3)–C(2)
N(3)–C(9)
N(4)–C(2)
N(4)–C(19)
C(1)–C(2)
1.318(3)
1.271(3)
1.410(3)
1.357(3)
1.481(3)
1.538(3)
It has been reported that an excess of MAO can activate
some metal amide complexes for polymerisation.49,50 Others
have reported that the catalytic activity of metal amide com-
plexes can be enhanced by pre-alkylation with AlR3 (R = Me or
iBu).51–54 Therefore, the catalytic activity of 4, 5 and 7 was tested
using similar conditions to those reported by Kim and co-
workers.52–54 The ethylene polymerisation results are given in
Table 5. Compound 4 in the presence of MAO as activator
shows low activity as an ethene polymerisation catalyst. The
activity is enhanced about twelve times after pre-activation with
AlMe3. No further studies were made using compound 4 due
to the low reactivity. Compound 5 shows moderate catalytic
activity as an ethene polymerisation catalyst using the same
conditions as for 4. The activity increases after pre-alkylation
treatment with AlMe3 or Al(iBu)3. A similar enhancement in
activity is also found for compound 7. The lower activity
observed for the non-alkylated system may reflect insufficient
alkylation by MAO.52 The alkylated compound 5, without the
addition of MAO, also shows a low activity in the polymeris-
ation reaction (entry 7). It may be inferred that the pre-alkylated
system creates an electron-deficient centre. We assumed the pre-
alkylation of compound 5 gave an alkyl product and therefore
should be expected as a catalytic system of the addition to the
alkylated compound with the Lewis acid B(C6F5)3. This has
been shown primarily to act as an alkyl abstraction when com-
bined with metal alkyl, e.g. [Zr(η-C5H5)2(CH3)2].55 Indeed,
moderate activity was found when B(C6F5)3 was used as the
co-catalyst and the activity becomes even higher when
[Ph3C][B(C6F5)4] is used as co-catalyst with the pre-alkylated
system.
N(1)–Zr(1)–N(2)
Cl(1)–Zr(1)–N(5)
N(1)–Zr(1)–N(5)
N(2)–Zr(1)–N(5)
N(5)–Zr(1)–N(6)
58.60(6)
99.79(5)
112.19(7)
112.69(7)
114.39(7)
N(1)–C(1)–N(2)
N(3)–C(2)–N(4)
C(2)–N(3)–C(9)
C(2)–N(4)–C(19)
112.09(17)
122.09(19)
125.72(19)
126.55(18)
The polyethylenes produced by the mixed amidinato–amide
complexes show melting points (Tm) in the range 130–140 ЊC,
indicating that they are highly linear and highly crystalline.56
The molecular weights are too high to be measured by GPC
analysis.
In conclusion, the oxalic amidinato ligand prefers to form
early transition metal amidinato complexes. Some of the new
amidinato complexes exhibit moderate to high activity towards
ethylene polymerisation.
Fig. 3 Molecular structure of [Zr{N(SiMe3)2}2Cl{(C6H5N)C(NtBu)-
C(HNtBu)(NC6H5)}] 7. Details as in Fig. 1.
with the amidinato ligand occupying one edge, and one
N(SiMe3)2 and one chloride the other corners of the square
base. The other N(SiMe3)2 group occupies the apex of the
pyramid. The structure exhibits two N(SiMe3)2 groups in differ-
1
ent environments, consistent with two SiMe3 peaks in the H
Experimental
NMR spectrum. Selected bond lengths and angles are listed in
Table 4. The Zr–Cl bond distance, 2.4366(5) Å, is slightly longer
than that (2.394(2) Å) in the starting material, Zr{N-
(SiMe3)2}3Cl.43 The Zr–NSiMe3 bond distances, 2.0459(18)
and 2.0760(17) Å, are close to those (2.070(3) Å) in Zr{N-
(SiMe3)2}3Cl. Similar to compound 6, the Zr–NCNN bond dis-
tances, 2.2351(16) and 2.2907(17) Å, are asymmetric, which
leads to the observation of inequivalent C–N bonds, 1.353(3)
and 1.318(3) Å. The small bite angle N(2)–Zr(1)–N(1),
58.60(6)Њ, compared with those found in mixed cyclopenta-
dienyl amidinato44,45 or bis-amidinato46,47 compounds, may be
attributed to the steric repulsion around the metal centre. The
steric hindrance also reflects on the puckered angle between the
CNN and ZrNN planes, 164.03Њ. Two CNN planes are inter-
secting with a 117.02Њ dihedral angle, which is close to that
found in compound 6.
All manipulations were carried out under an atmosphere of
dinitrogen using standard Schlenk-line or dry-box techniques.
Solvents were pre-dried over molecular sieves, 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 and 13C-{1H} NMR spectra were recorded either on
Varian Unity 500 or Mercury 300 spectrometers and referenced
internally to the residual solvent peak relative to tetramethyl-
silane, infrared spectra on a Perkin-Elmer 1600 Series FT-IR
instrument in the range 4000–400 cmϪ1 as caesium iodide
pellets. Elemental analyses were performed by the Micro-
analytical Department of the Inorganic Chemistry Laboratory
in Oxford. The differential scanning calorimetry (DSC) data of
the polymers were obtained from the laboratories of the Chisso
Petrochemical Corporation (Japan). The melting point (Tm) of
the polymer was determined with a Seiko SSC-5500 system at a
heating rate of 20 ЊC minϪ1. In order to obtain reproducible
values, the polymer was first melted and cooled before the
actual measurement.
Polymerisation studies
One aim of this research was to find interesting catalysts for
olefin polymerisation. Ethylene polymerisation studies were
performed in toluene at room temperature with a monomer
pressure of 2 bar. Methylaluminoxane (MeAlO)n (MAO) was
initially tested using a known pre-catalyst, [Zr(η-C5H5)2Cl2],
[NiBr2(DME)] (Aldrich), Ti(NMe2)4 (Strem), the oxanilide
C H N᎐C(Cl)᎐C(Cl)᎐NC H (Acros), PCl5 (Lancaster) and
᎐
᎐
᎐
6
5
6
5
and gave an activity of 1.41 × 106 g PE mol MϪ1
h
Ϫ1, which is
methylaluminoxane (10 wt% in toluene, Witco) were used as
close to the reported activity of 3.57 × 106 g PE mol MϪ1 hϪ1 48
The new amidinato complexes used as pre-catalysts were 4, 5, 6
.
supplied. BuNH2 was dried over CaH2 and distilled before
t
use. Bis-phenylimidoyl chloride,24 Zr(NMe2)4,36 [Zr{N(Si-
1764
J. Chem. Soc., Dalton Trans., 2001, 1761–1767

View Article Online
Table 5 Activity toward ethylene polymerisationa
Catalyst/
AlR3
Co-catalyst
(equiv.)
Time/
min
Activityb/kg
Run
Catalyst
mol × 106
(mol × 104)
Yield/g
mol MϪ1 hϪ1 barϪ1
Tm/ЊC
1
2
3
4
5
6
7
8
9
10
11
12
13
4
4
5
5
5
5
5
5
5
5
7
7
7
6.25
6.25
6.25
6.25
12.5
12.5
14.1
6.25
12.5
12.5
6.25
6.25
25
None
AlMe3 (1.94)
None
MAO (1220)
MAO (1220)
MAO (1220)
MAO (1220)
B(C6F5)3 (2)
[Ph3C][B(C6F5)4] (2)
None
MAO (1220)
B(C6F5)3 (2)
[Ph3C][B(C6F5)4] (2)
MAO (2450)
MAO (2450)
B(C6F5)3 (2)
120
120
120
60
65
60
60
60
40
15
0.03
0.49
0.85
2.80
2.18
1.74
0.17
3.37
1.77
1.30
0.10
0.34
0.03
0.7
9.7
17.0
112.0
40.2
34.8
3.1
134.8
53.2
104.0
4.1
13.7
0.5
—
134.5
137.1
136.7
135.3
137.2
139.0
136.3
135.1
137.1
139.2
130.1
—
AlMe3 (1.94)
AlMe3 (3.88)
AlMe3 (3.88)
AlMe3 (4.2)
Al(iBu)3 (1.88)
Al(iBu)3 (3.75)
Al(iBu)3 (3.75)
None
120
120
60
AlMe3 (1.94)
AlMe3 (1.94)
a Conditions: 2 bar absolute monomer pressure, room temperature, toluene ca. 220 cm3. b mol M = Total amount of metal used.
Me3)2}3Cl],43 [Ta(NEt2)2Cl3(py)],57 B(C6F5)3,58 and [Ph3C]-
[B(C6F5)4]59 were prepared by the literature methods.
precipitate appeared. The concentrated solution was kept at
Ϫ25 ЊC overnight to yield an orange solid. Yield 1.8 g, 75%.
Preparations
[Ta(NEt2)Cl3{(C6H5N)C(NtBu)C(HNtBu)(NC6H5)}] 6. A red
solution of [Ta(NEt2)2Cl3(py)] (2.5 g, 4.9 mmol) in toluene (10
C H N᎐C(NHtBu)–C(NHtBu)᎐NC H 1. A yellow solution
᎐
᎐
6
5
6
5
ml) was treated with a pale yellow solution of C H N᎐
᎐
6
5
of C H N᎐C(Cl)–C(Cl)᎐NC H (2.7 g, 9.6 mmol) in toluene (50
᎐
᎐
C(NHtBu)–C(NHtBu)᎐NC H (0.86 g, 2.4 mmol) in toluene
6
5
6
5
᎐
ml) was treated with 6.1 ml tBuNH2 (4.2 g, 57.8 mmol) at room
temperature while stirring. After 48 h the resulting precipitate
was removed by filtration. The filtrate was dried in vacuo to give
a white solid. Yield 3.1 g, 92%.
6
5
(30 ml) at room temperature. The reaction mixture was then
heated to 110 ЊC for 7.5 h. The red solution was filtered and the
volatiles were removed from the filtrate in vacuo to afford a red
solid, which was washed with pentane (5 ml) to give a red solid.
Yield 0.49 g, 28%.
C H N᎐C{N(tBu)(SiMe )}–C{N(tBu)(SiMe )}᎐NC H 2.
A
᎐
᎐
6
5
3
3
6
5
pale yellow solution of C H N᎐C(NHtBu)–C(NHtBu)᎐NC H
᎐
᎐
[Zr{N(SiMe3)2}2Cl{(C6H5N)C(NtBu)C(HNtBu)(NC6H5)}] 7.
A white suspension of [Zr{N(SiMe3)2}3Cl] (0.96 g, 1.58 mmol)
in toluene (15 ml) was treated with a pale yellow solution of
C H N᎐C(NHtBu)–C(NHtBu)᎐NC H (0.50 g, 1.43 mmol) in
6
5
6
5
(2.0 g, 5.7 mmol) in diethyl ether (80 ml) was treated with 8 ml
n-BuLi (1.6 M, 12.8 mmol) at 0 ЊC in a dropwise manner. The
reaction mixture was stirred in an ice-bath for 1 h, then
allowed to warm to room temperature and react for 2 h. The
volatiles were removed under reduced pressure to afford a
yellow solid. To this 50 ml THF were added, followed by
2 ml Me3SiCl (1.7 g, 15.8 mmol). Then the orange solution was
heated to 75 ЊC for 8 h gradually becoming yellow. After
cooling, the volatiles were removed under reduced pressure
and the residue was extracted into pentane (30 ml). Recrystal-
lisation from concentrated pentane afforded a white crystal-
line solid. Yield 2.3 g, 80%.
᎐
᎐
6
5
6
5
toluene (10 ml) at room temperature. The reaction mixture was
then heated to 105 ЊC for 4 h. The pale yellow solution was
filtered and the volatiles were removed from the filtrate in vacuo
to afford an off-white solid that was extracted with pentane
(20 ml) to give an off-white solid. Yield 1.0 g, 88%.
Polymerisation studies
Ethylene was purified by passage through 4 Å molecular sieves.
Polymerisation reactions were carried out in a Fischer–Porter
reactor equipped with a magnetic stirrer using conditions simi-
lar to those employed by Kim and co-workers.53,54,60 Toluene
(ca. 200 ml) was added to the Fischer–Porter bottle, which was
connected to the ethylene supply via a gas supply system and
filled with ethylene. The vessel was depressurised before the
co-catalyst or the precursor complex solution was syringed.
[(C6H5N)C(NHtBu)C(NHtBu)(NHC6H5)][Br] 3. A mixture
of C H N᎐C(NHtBu)–C(NHtBu)᎐NC H (0.35 g, 1 mmol)
᎐
᎐
6
5
6
5
and [NiBr2(DME)] (0.31 g, 1 mmol) was treated with CH2Cl2
(30 ml) at room temperature. The mixture was stirred at room
temperature for 3 days. The volatiles were removed under
reduced pressure to afford a pale green solid. This was extrac-
ted with CH2Cl2 (5 ml) and diffused into toluene to afford a
yellowish green solid. Yield 0.10 g, 23%.
For the catalyst–MAO system. Methylaluminoxane (10 ml)
was then syringed, the ethylene pressure increased to 2 bar
(absolute pressure) and the mixture stirred at room temperature
until saturated with ethylene. Precursor complex (6.25 × 10Ϫ6
mol) in toluene (10 ml) was then added quickly to the reactor by
syringe.
[Zr(NMe2)3(C6H5N)C(NtBu)–]2 4. A pale yellow solution of
Zr(NMe2)4 (2.1 g, 8 mmol) in toluene (20 ml) was treated with a
pale yellow solution of C H N᎐C(NHtBu)–C(NHtBu)᎐NC H
5
(1.4 g, 4 mmol) in toluene (20 ml) at room temperature. The
reaction mixture was then heated to 85 ЊC for 3 h. The result-
ing pale yellow solution was filtered and the volatiles were
removed in vacuo to afford a pale yellow solid, which was
washed with pentane (5 ml) to afford an off-white solid. Yield
2.5 g, 82%.
᎐
᎐
6
5
6
For the pre-alkylated catalyst–MAO system. Methylalumin-
oxane (10 ml) was then syringed, the ethylene pressure
increased to 2 bar (absolute pressure) and the mixture stirred
at room temperature until saturated with ethylene. Meanwhile,
precursor complex (6.25 × 10Ϫ6 mol) in toluene (10 ml) was
added to the prescribed amount of AlR3 and stirred for 15 min
pre-alkylation. The pre-alkylated mixture was then added
quickly to the reactor by syringe.
[Ti(NMe2)3(C6H5N)C(NtBu)–]2 5.
A yellow solution of
Ti(NMe2)4 (1.5 g, 6.8 mmol) in toluene (15 ml) was treated
with a pale yellow solution of C H N᎐C(NHtBu)–C(NHtBu)᎐
᎐
᎐
6
5
NC6H5 (1.2 g, 3.4 mmol) in toluene (20 ml) at room temper-
ature. The reaction mixture was then heated to 85 ЊC for 3.5 h.
The solution was filtered and the filtrate concentrated until a
For the pre-alkylated catalyst–B(C6F5)3 or –[Ph3C][B(C6F5)4]
system. The precursor complex (6.25 × 10Ϫ6 mol) in toluene (10
J. Chem. Soc., Dalton Trans., 2001, 1761–1767
1765

View Article Online
Table 6 Summary of crystal data for compounds 3, 6, and 7
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C22H32BrN4
432.43
Monoclinic
P21/c
14.0530(6)
10.0580(6)
17.5050(10)
113.913(3)
2261.9(2)
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C26H39Cl3N5Ta
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R
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1766
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