Synthesis and catalytic activity of three-coordinate zinc cations

Article abstract of DOI:10.1039/b208165m

Synthesis and catalytic activity of three-coordinate zinc cations were investigated. It was found that the polymerization of propene oxide (PO) proceeds more slowly producing polymers with molecular weight of 30-40000 g mol^-1. The polymerizatio

Full text of DOI:10.1039/b208165m

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Synthesis and catalytic activity of three-coordinate zinc cations
Mark D. Hannant, Mark Schormann and Manfred Bochmann*
Wolfson Materials and Catalysis Centre, School of Chemical Sciences, University of East Anglia,
Norwich, UK NR4 7TJ. E-mail: m.bochmann@uea.ac.uk
Received 20th August 2002, Accepted 8th October 2002
First published as an Advance Article on the web 16th October 2002
Reaction between DADZnR2 and either B(C6F5)3 or ZnR2
and [DADH][B(C6F5)4] affords three-coordinate alkyl and
amide zinc cations which are active for the ring opening
polymerisation of epoxides and ꢀ-caprolactone [DAD ؍
tion for the formation of the methyl-bridged binuclear cation
[{(DAD)ZnMe}2(µ-Me)]ϩ, even at Ϫ80 ЊC.
The X-ray crystal structure of 2b confirmed the trigonal-
planar coordination geometry of the metal centre (angle sum
359.6Њ) (Fig. 1).‡ The Zn–N distances of, on average, 2.040(2) Å
(MeC᎐NC H Pri -2,6) ].
᎐
6
3
2
2
Zinc complexes of sterically hindered ligands have attracted
considerable attention in recent years as catalysts for the
formation of polyesters. For example, zinc aryloxides,1–3 carb-
oxylates4,5 and bulky diketiminato complexes6 catalyse the
alternating copolymerisation of cyclohexene oxide with carbon
dioxide to give polycarbonates. Coates et al. and Chisholm et al.
found a number of diketiminato,7,8 tris(pyrazolato)borate9 and
iminophenolato10 complexes that were highly active catalysts
for the ring-opening polymerisation of lactides but were found
to be unreactive towards the homopolymerisation of propene
oxide (PO) and even cyclohexene oxide (CHO). We report
here the synthesis of cationic three-coordinate zinc complexes
that are suitable for the homopolymerisation of epoxides and
lactones.
The addition of the diazadiene ligand (MeC᎐NC H Pri -2,6)
᎐
6
3
2
2
Fig. 1 Structure of the [ZnMe(DAD)]ϩ cation with thermal ellipsoids
at 50% probability. H atoms are omitted for clarity. Selected bond
lengths (Å) and angles (Њ): Zn–C(81) 1.914(4), Zn–N(1) 2.035(2), Zn–
N(2) 2.045(2), N(1)–C(11) 1.286(3), N(1)–C(1) 1.458(3); C(81)–Zn–
N(1) 136.3(2), C(81)–Zn–N(2) 143.2(2), N(1)–Zn–N(2) 80.10(8).
(DAD) to a solution of ZnMe2 in light petroleum affords the
complex ZnMe2(DAD) 1 as a deep orange precipitate. This
adduct is susceptible to dissociation under vacuum.11 Treatment
of a solution of 1 in hexanes with B(C6F3)3 in a 1 : 1 ratio at
0 ЊC generates [ZnMe(DAD)]ϩ[MeB(C6F5)3]Ϫ 2a as an orange
solid.† Alkyl/C6F5 exchange, as seen in the reaction of zinc
alkyls with B(C6F5)3 in the absence of donor ligands,12 is not
observed. Alternatively, the alkyl and amido zinc cations are
readily prepared by the reaction of [DADH]ϩ[B(C6F5)4]Ϫ with
ZnR2 [R = Me, Et, N(SiMe3)2] to give the [B(C6F5)4]Ϫ salts 2b,
3 and 4, respectively (Scheme 1).
are longer than those of the neutral diketiminato complexes
{HC(MeC᎐NAr) }ZnX (X = NPri , OBut),7,8 while the N(1)–
᎐
2
2
Zn–N(2) angle is more acute, 80.10(8)Њ. Although a number
of three-coordinate neutral and anionic zinc alkyl complexes
are known,14,15 complex 2a appears to be the first structurally
characterised example of a cationic zinc centre with such a low
coordination number.16,17
Complexes 2 catalyse the polymerisation of epoxides and
ε-caprolactone under mild conditions. The exotherm of the
rapid polymerisation of CHO was moderated by dilution with
toluene and ice bath cooling. The polymer molecular weight
increases with time, with a polydispersity of about 2 (Table 1).
Activities increase with time due to the rise in temperature;
there is no induction period.
The polymerisation of PO proceeds more slowly, producing
polymers with molecular weights of 30–40000 g molϪ1, regard-
less of reaction time (Table 2).
The polymerisation of ε-caprolactone at room temperature
was slow but accelerated on heating to 60 ЊC. The resulting
polymer gave Mw values of up to 40000, with polydispersities
close to 1 (Table 3). Methyl methacrylate was slowly poly-
merised at 25 ЊC to give polymer with 70% syndiotacticity.
This high catalytic activity, compared to neutral zinc com-
plexes, is no doubt due to the increased Lewis acidity of the
cationic metal centre. Zinc dialkyls activated with water or
alcohols are known to polymerise epoxides, most probably via a
coordinated anionic mechanism.18 Investigations on the poly-
merisation mechanism of these cationic systems are currently
the subject of further studies.
Scheme 1 Reagents and conditions: i, hexanes, ZnMe2 0 ЊC; ii,
B(C6F5)3, hexanes, 2 h, 23 ЊC; iii, ZnR2, dichloromethane, 30 min, 23 ЊC.
1
The H and 11B NMR spectra of 2a show singlets at δ 0.45
and Ϫ11.9, respectively, indicative of a non-coordinated
[MeB(C6F5)3]Ϫ anion. This is confirmed by the 19F NMR data.13
The metal centre is therefore three-coordinate, even in the case
of the [MeB(C6F5)3]Ϫ anion which is well-known for its ten-
dency to give methyl-bridged zwitterions. In spite of the steric
hindrance of the DAD ligand the cation in 2a is still very
reactive, and in solutions containing an excess of 1 rapid
exchange of all methyl ligands is observed. There was no indica-
We thank the Engineering and Physical Science Research
Council and BP Chemicals Ltd. for support.
DOI: 10.1039/b208165m
J. Chem. Soc., Dalton Trans., 2002, 4071–4073
This journal is © The Royal Society of Chemistry 2002
4071

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Table 1 Polymerisation of cyclohexene oxide with complex 2ba
Time/min
Conversion (%)
Activity/kg molϪ1 hϪ1
Mw
53000
69000
97000
113000
117000
119000
145000
—
Mw/Mn
0.5
1
1.5
2
2.5
3
3.5
4
2.6
10.8
22.3
28.7
39.0
44.9
48.7
49.0
50.9
54.3
3036
6361
8771
8493
9224
8843
8220
7247
6683
6419
4.1
2.0
1.5
2.3
1.9
1.5
1.9
—
4.5
5
163000
177000
2.3
1.8
a Conditions: CHO (CHO : Zn = 10000 : 1) was injected into a stirred suspension of 10 µmol of 2b in 50.5 mL toluene precooled to 0 ЊC. 2.5 mL
aliquots were removed every 30 s and quenched with HCl–MeOH to precipitate the polymer.
Table 2 Polymerisation of propylene oxide with complex 2ba
Time/h
Conversion (%)
Activity/kg molϪ1 hϪ1
Mw
Mw/Mn
1
7
24
72
0.8
23.4
51.6
73.3
4.76
19.41
12.49
5.92
33000
38000
35000
29000
6.4
4.0
1.9
1.9
a Conditions: 25 µmol of 2b was stirred with 10000 equiv. of propylene oxide at 23 ЊC. Aliquots of 2.5 mL were removed at the times shown and
quenched in methanol.
Table 3 Polymerisation of ε-caprolactone with complex 2ba
Time/min
Conversion (%)
Activity/kg molϪ1 hϪ1
Mw
Mw/Mn
30
45
60
4.4
9.9
12.7
10.1
15.1
14.7
23000
38000
40000
2.0
1.1
1.1
a Conditions: 93.5 µmol of 2b was stirred in 40 mL toluene at 60 ЊC. 10 mL of ε-caprolactone was injected. 5 mL aliquots were removed at times
shown and quenched in HCl–MeOH to precipitate the polymer.
4: Preparation analogous to 2b. Yield 3.8 g (32.3 mmol, 97%). 1H
Notes and references
NMR (CD2Cl2, 20 ЊC, 300.13 MHz): δ Ϫ0.20 (s, 18 H, SiMe3). 13C
† Synthesis and spectroscopic data: 2a: To a stirred solution of DAD
(1.25 g, 3.1 mmol) in 25 mL hexanes at 0 ЊC was injected ZnMe2 (1.54
mL of a 2 M solution in hexanes, 3.08 mmol) followed by B(C6F5)3
(1.58 g, 3.1 mmol, in 25 mL hexanes) to give a pale orange suspension.
After stirring for 2 h, the solid was filtered off, washed with hexanes and
dried to give an orange powder (2.42 g, 2.39 mmol, 77%). 1H NMR
(CD2Cl2, 20 ЊC, 300.13 MHz): δ 7.5–7.3 (m, 6 H, Ar-H), 2.50 (s, 6 H,
NMR (CD2Cl2, 20 ЊC, 75.48 MHz): δ 5.0 (s, 18 H, SiMe3). (DAD
and borate resonances omitted). Anal. calc. for C58H58BF20N3Si2Zn:
C, 53.20; H, 4.46; N, 3.21. Found: C, 52.47; H, 4.45; N, 3.05%.
‡ Crystal data for 2b: C53H43BF20N2Zn, M = 1164.1, monoclinic, space
group P21/n (no. 14), a = 16.234(3), b = 19.980(4), c = 17.443(4) Å, β =
112.81(3)Њ, V = 5215(2) Å3, Z = 4, Dc = 1.483 g cmϪ3, F(000) = 2360, T =
140(1) K, µ(Mo-Kα) = 5.8 cmϪ1, λ(Mo-Kα) = 0.71069 Å, 15871 reflec-
tions measured, 8851 unique (Rint = 0.028), F 2 refinement, R1 = 0.0417
[I > 2σ(I)], wR2 = 0.115 (all data). CCDC reference number 186246.
See http://www.rsc.org/suppdata/dt/b2/b208165m/ for crystallographic
data in CIF or other electronic format.
N᎐CCH3), 2.45 (sept, 4 H, J = 6.8 Hz, MeCHMe), 1.26 (d, 12 H, J = 6.8
᎐
Hz, CH3CHCH3), 1.20 (d, 12 H, J = 6.8 Hz, CH3CHCH3), 0.45 (br s,
3H, BMe), Ϫ0.38 (s, 3H, ZnMe). 13C NMR (CD2Cl2, 20 ЊC, 75.48
MHz): δ 175.0 (C᎐N), 138.3 (o-C), 137.7 (ipso-C), 130.1 (p-C), 125.6
᎐
(m-C), 30.2 (Me2CH), 24.2 (Me2CH), 22.8 (Me2CH), 19.9 (N᎐CCH3),
᎐
Ϫ14.6 (ZnMe). 11B NMR (CD2Cl2, 20 ЊC, 96.29 MHz): δ Ϫ11.9. 19F
NMR (CD2Cl2, 20 ЊC, 282.4 MHz): δ Ϫ133.52 (d, 6 F, J = 12.8 Hz, o-F),
Ϫ165.54 (t, 3 F, J = 20.6 Hz, p-F), Ϫ168.22 (t, 6 F, J = 20.6 Hz, m-F).
Anal. calc. for C48H46BF15N2Zn: C, 56.97; H, 4.58; N, 2.77. Found:
C, 56.30; H, 4.50; N, 2.65%.
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2b. From ZnMe2 (3.5 mmol) and [DADH]ϩ[B(C6F5)4]Ϫ (3.65 g,
3.37 mmol) in 75 mL dichloromethane at room temperature. Workup as
for 2a afforded 2b as a fine orange powder (3.8 g, 32.3 mmol, 97%).
Crystals suitable for X-ray analysis were grown from dichloromethane
1
at Ϫ26 ЊC. H NMR (CD2Cl2, 20 ЊC, 300.13 MHz): δ Ϫ0.37 (s, 3 H,
ZnMe). 13C NMR (CD2Cl2, 20 ЊC, 75.48 MHz): δ Ϫ14.6 (ZnMe). 11B
NMR (CD2Cl2, 20 ЊC, 96.29 MHz): δ Ϫ13.7. 19F NMR (CD2Cl2, 20 ЊC,
282.4 MHz): δ Ϫ133.51 (br d, 8 F, o-F), Ϫ164.06 (t, 4 F, J = 20.6 Hz,
p-F), Ϫ167.96 (br t, 8 F, J = 20.6 Hz, m-F). (DAD resonances are
essentially identical to 2a and are omitted here). Anal. calc. for
C53H43BF20N2Zn: C, 54.68; H, 3.72; N, 2.41. Found: C, 54.46; H, 3.67;
N, 2.35%.
1
3: Preparation analogous to 2b. Yield 0.58 g (0.49 mmol, 74%). H
NMR (CD2Cl2, 20 ЊC, 300.13 MHz): δ 0.86 (t, 3 H, J = 8.0 Hz, Zn–
CH2CH3), 0.54 (q, 2 H, J = 8.0 Hz, Zn–CH2CH3). 13C NMR (CD2Cl2,
20 ЊC, 75.48 MHz): δ 10.8 (Zn–CH2CH3) 0.88 (Zn–CH2CH3). (DAD
and borate resonances omitted). Anal. calc. for C54H45BF20N2Zn: C,
55.05; H, 3.85; N, 2.38. Found: C, 53.72; H, 3.82; N, 2.20%.
4072
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