Synthesis and structures of some sterically hindered zinc complexes containing 6-membered and rings

Article abstract of DOI:10.1016/j.jorganchem.2008.02.012

Zinc β-diketiminates containing the N,N′-chelating ligand [{N(SiMe₃)C(Ph)}₂CH]^- (≡LL^-) [Zn(LL)(μ-Cl)]₂ (1) and [ZnEt(LL)thf] (2) were prepared from 2ZnCl₂ + [Li(LL)]₂ and ZnEt₂ + H(LL), respectively. The new phenols 2-(N-R-piperazinyl-N′-methyl)-4,6-di-tert-butylphenol [R = Ph (3a), Me (3b)] and 2,2-[μ-N,N′-piperazindiyldimethyl]-bis(4,6-di-tert-butylphenol) (4) were obtained from 2,4-tBu₂C₆H₃OH, (CH₂O)_n and the appropriate piperazine. Zinc phenoxides 5, 7 and 8 were derived from 2ZnEt₂ with 2(3a), 2(3b) and 4, respectively. Controlled methanolysis of 5 furnished the bis(phenoxo)zinc compound Zn[OC₆H₂tBu₂-2,4-{CH₂N(CH₂CH₂)₂NPh}-6]₂ (6). The X-ray structures of the crystalline zinc compounds 1, 2, 5, 6, 7 and 8, are presented; each of 5-8 contains two {A figure is presented} six-membered rings. The centrosymmetric molecule 1 has a rhomboidal (ZnCl)₂ core with exceptionally different Zn-Cl and Zn-Cl′ bond lengths of 2.248(1) and 2.509(1) A?, respectively. None of 1, 2 or 5-8 was an effective catalyst for the copolymerisation of an oxirane and CO₂.

Full text of DOI:10.1016/j.jorganchem.2008.02.012

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 1861–1869
www.elsevier.com/locate/jorganchem
Synthesis and structures of some sterically hindered zinc
ZnNCCCN
complexes containing 6-membered
and ZnOCCCN rings
a
a
a,
*
c
James D. Farwell , Peter B. Hitchcock , Michael F. Lappert , Gerrit A. Luinstra ,
Andrey V. Protchenko a, Xue-Hong Wei b
a Department of Chemistry, University of Sussex, Brighton BN1 9QJ, UK
b School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
c BASF Aktiengesellschaft, D-67056 Ludwigshafen, Germany
Received 16 January 2008; received in revised form 13 February 2008; accepted 14 February 2008
Available online 20 February 2008
Abstract
Zinc b-diketiminates containing the N,N0-chelating ligand [{N(SiMe3)C(Ph)}2CH]ꢀ (ꢁLLꢀ) [Zn(LL)(l-Cl)]2 (1) and [ZnEt(LL)thf] (2)
were prepared from 2ZnCl2 + [Li(LL)]2 and ZnEt2 + H(LL), respectively. The new phenols 2-(N-R-piperazinyl-N0-methyl)-4,6-di-tert-
butylphenol [R = Ph (3a), Me (3b)] and 2,2-[l-N,N0-piperazindiyldimethyl]-bis(4,6-di-tert-butylphenol) (4) were obtained from 2,4-tBu2-
C6H3OH, (CH2O)n and the appropriate piperazine. Zinc phenoxides 5, 7 and 8 were derived from 2ZnEt2 with 2(3a), 2(3b) and 4, respec-
tively. Controlled methanolysis of 5 furnished the bis(phenoxo)zinc compound Zn[OC6H2tBu2-2,4-{CH2N(CH2CH2)2NPh}-6]2 (6). The
X-ray structures of the crystalline zinc compounds 1, 2, 5, 6, 7 and 8, are presented; each of 5–8 contains two
six-membered
ZnOCCCN
rings. The centrosymmetric molecule 1 has a rhomboidal (ZnCl)2 core with exceptionally different Zn–Cl and Zn–Cl0 bond lengths of
˚
2.248(1) and 2.509(1) A, respectively. None of 1, 2 or 5–8 was an effective catalyst for the copolymerisation of an oxirane and CO2.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Zinc; b-Diketiminate; Phenoxide; X-ray structure; Copolymerisation catalysis
1. Introduction
ZnOCCCN cyclic core, the b-diketiminates or 2-dialkyla-
minophenolates of formulae A or B, respectively. A recent
paper by Gibson and coworkers identified metal complexes
such as these as potential catalysts for such processes [2].
Recently, we reported on the synthesis and structures of
some bimetallic diamides derived from the 1,2-benzene-
bis(neopentylamido) ligand (L2ꢀ), including the zinc-con-
taining complexes [Li(thf)4][{Zn(l-L)}3(l3-Cl)], [{Li(OEt2)-
(l-L)Zn}2(l-L)], [Li(OEt2)(l-L)Zn(l-L)Zn(LH)}] and the
paramagnetic [Li(thf)4][Zn(L)(LÅꢀ)] [1]. That study was
part of an exploratory programme to identify compounds
which might be active catalysts for the copolymerisation
of an oxirane and carbon dioxide or the related catalytic
ring-opening polymerisation of lactide. Two classes of zinc
N,N0- or O,N0-chelate compounds and their potential for
the oxirane/CO2 copolymerisation are the focus of the
tBu
H2
C
H
C
Ph
Ph
C
N
C
N
N
tBu
O
Me3Si
SiMe3
Zn
Zn
X
X
A
B
present investigation. These have
a ZnNCCCN or
The use of zinc-based catalysts for the copolymerisation
of an oxirane (including propylene oxide) and carbon diox-
ide has a long history, as expounded in the 2004 review of
*
Corresponding author. Fax: +44 1273 678335.
E-mail address: m.f.lappert@sussex.ac.uk (M.F. Lappert).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.02.012

1862
J.D. Farwell et al. / Journal of Organometallic Chemistry 693 (2008) 1861–1869
Ph
Ph
Coates and Moore [3]. Among the landmarks were hetero-
geneous systems based on ZnEt2/H2O [4a], ZnEt2/
C6H4(OH)2-1,3 [4b], ZnEt2/PhCH2CH2NH2 [4c] and
Zn(OH)2/(CH2)3(CO2H)2-1,3 [4d].
1. C6H14
+ ZnEt2
N
N
2. Crystd. (Et2O/thf)
Me3Si
SiMe3
H
G
The use of single-site catalysts for such copolymerisa-
tions are of more recent date, starting (1995) from zinc
aryloxides such as [Zn(OC6H2tBu3-2,4,6)2(thf)2] and
[{Zn(OC6H3F2-2,6)(l-OC6H3F2-2,6)(thf)}2] [5]. Coates
and coworkers showed that certain b-diketiminatozinc
complexes, including the binuclear complex [{Zn(L0)-
ð2Þ
Ph
Ph
+ C2H6
N
N
Me3Si
SiMe3
thf
Zn
Et
2, 93%
(l-OMe)}2]
[L0 = N(C6H3Me2-2,6)C(Me)C(CN)C(Me)-
Compounds 1 and 2 were characterised by microanaly-
sis, 1H and 13C NMR spectra and EI mass spectra, and sin-
gle crystal X-ray diffraction. Surprisingly, the binuclear
zinc chloride complex 1 was resistant to Li[N(SiMe3)2] or
KOtBu in refluxing thf for 10 min. Likewise, the zinc ethyl
compound 2 proved to be unresponsive to attack by
N(C6H3iPr2-2,6)] [6a], were highly active living copolymer-
isation (CO2, cyclohexene oxide) catalysts [6]. A very recent
paper dealing with such copolymerisations featured dinu-
clear zinc complexes containing b-diiminato binding sites
such as C (Ar0 = C6H3Me2-2,3) [7]. Highly active catalysts
for the controlled polymerisation of lactide were the
crystalline complexes D and E [8]. Synthesis and structures
of a series of binuclear zinc alkyl, aryl and aryloxide
complexes, which were suggested as potential precursors
for catalysts of such processes, has been reported recently
[9].
1
HN(SiMe3)2, but iPrOH yielded a gel containing (by H
NMR spectroscopy) a significant proportion of the b-dik-
etimine G.
The dimeric structure of complex 1 has been confirmed
by the X-ray diffraction study. It is noteworthy that while
b-diketiminatozinc chloride and bromide with a ‘‘nacnac”
tBu
Me
Me
tBu
tBu
Me2N
tBu
Me
N
tBu
tBu
Me
Zn
Et
O
O
N
N
O
O
O
O
Zn
Zn
Et
Ar'
N
N
Ar' Ar'
N
Ar'
N
N
tBu
Me
tBu
Zn
Zn
Et
Me2
NMe2
Et
C [7]
Et
thf
thf
D [8]
E [8]
2. Results and discussion
2.1. b-Diketiminatozinc complexes
type ligand were regarded as dimeric [6b], their structures
had not been proved (the unsolvated Et derivative was a
monomer having a 3-coordinate planar Zn atom [6b]).
The centrosymmetric molecule 1 (Fig. 1) has a rhomboidal
(ZnCl)2 core with clearly different Zn–Cl and Zn–Cl0 bond
The crystalline b-diketiminatozinc compounds 1 and 2
were obtained in high yield by a salt-(1) or alkane-(2) elim-
ination procedure, as summarised in Eqs. (1) and (2),
respectively. The starting materials, the lithium b-diketimi-
nate F [10] (for 1) or the b-diketimine G [10] (for 2), have
been used by our group as precursors for a wide range of
metal b-diketiminates [11]
˚
lengths of 2.248(1) and 2.509(1) A, respectively; these
distances may be compared with the 2.328(9) and
˚
2.332(8) A reported for the more symmetrically bridged
[Zn(CCl2CF3)(l-Cl)(OEt2)]2 [12a]. Even in
sterically crowded complex with an NPPN chelating ligand
[{Zn(l-Cl)(NPPN)}2] [NPPN = N(C6H3iPr2-2,6)P(Ph)-
a highly
P(nBu)(Ph)N(C6H3iPr2-2,6)] the difference in bridging
˚
Zn–Cl bond lengths [2.3011(8) and 2.4124(9) A] was not
Ph
Ph
1. Et2O
so pronounced [12b]. The coordination environment of
each Zn atom in 1 can be described as trigonal pyramidal
with the N1, N2 and Cl atoms in equatorial positions
(the sum of N1–Zn–N2 [100.36(13)°], N1–Zn–Cl
[127.12(10)°] and N2–Zn–Cl [125.28(9)°] angles is 352.8°)
and the Cl0 atom in the axial position.
The molecular structure of the crystalline compound 2 is
shown in Fig. 2. The zinc atom adopts a trigonal pyramidal
coordination environment with the N1, N2 and the a-ethyl
carbon (C22) atoms in equatorial positions (the sum of the
+ 2ZnCl2
N
N
2. Crystd. (PhMe/C6H14
)
Me3Si
SiMe3
2
Li
F
ð1Þ
Ph
Ph
SiMe3 Me3Si
Cl
Ph
Ph
N
N
N
N
+
2LiCl
Zn
Zn
Cl
SiMe3 Me3Si
1, 94%

J.D. Farwell et al. / Journal of Organometallic Chemistry 693 (2008) 1861–1869
1863
Table 1
˚
Selected bond lengths (A) and angles (°) for the ZnN1C1C2C3N2 moiety
of 1 and 2
1
2
Bond lengths
Zn1–N1
Zn1–N2
N1–C1
N2–C3
C1–C2
1.965(3)
1.957(3)
1.336(5)
1.339(5)
1.407(6)
1.417(5)
2.019(2)
2.021(2)
1.328(4)
1.332(4)
1.398(4)
1.413(4)
C2–C3
Bond angles
N1–Zn1–N2
Zn1–N1–C1
Zn1–N2–C3
N1–C1–C2
N2–C3–C2
C1–C2–C3
100.36(13)
110.1(3)
110.2(2)
125.9(3)
124.2(4)
128.7(4)
95.85(9)
115.54(18)
114.83(18)
126.0(3)
125.3(2)
128.1(3)
Fig. 1. ORTEP representation of the molecular structure of 1 (50%
ellipsoids; H atoms omitted). Symmetry transformation to generate
equivalent atoms: ꢀx, ꢀy, ꢀz.
C1N1C3N2 plane, respectively. The atoms Si1 and Si2
˚
˚
are 0.65 and 0.86 A (1) or 0.89 and 0.71 A (2) on the same
side of this plane.
2.2. 2-(Piperazinyl-N-methyl)-4,6-di-tert-butyl-phenoxozinc
compounds and their phenolic precursors
The 2-(piperazinyl-N-methyl)-4,6-di-tert-butyl-phenols
3a and 3b were prepared in good yield as shown in outline
in Scheme 1. The detailed procedure was related to that
used previously for 2-[Me2NCH2CH2N(Me)CH2]-4,
6-But2C6H2OH [8], except that in place of N,N,N0-trimeth-
ylethylenediamine used for the latter, for 3a and 3b the
appropriate 1-hydrocarbylpiperazine was employed. For
3a and 3b this involved (i) heating under reflux equimolar
portions of the three reagents (slight excess of paraformal-
dehyde) in ethanol, (ii) adding aqueous HBr (and washing
the resulting salts with ethanol), (iii) neutralising with
Na[HCO3] and (iv) extracting with chloroform. The
Fig. 2. ORTEP representation of the molecular structure of 2 (20%
ellipsoids; H atoms omitted).
tBu
N1–Zn–N2 [95.85(9)°], N1–Zn–C22 [133.06(12)°] and
N2–Zn–C22 [127.79(11)°] angles is 356.7°) and the O atom
of the thf moiety in the axial position. Such a geometry
around a Zn atom is similar to that of a recently reported
ZnEt complex of a binucleating b-diketiminato ligand with
a pyridine donor ligand in place of thf in 2 [13]. The Zn–C
R
N
N
tBu
NH
(CH2O)n
tBu
OH
N
R
EtOH
3a, 84% (R = Ph)
3b, 86% (R = Me)
tBu
˚
and Zn–O distances of 1.984(3) and 2.332(2) A, respec-
OH
˚
tively, are similar to the 1.988(7) and 1.985(9) A (Zn–C)
˚
EtOH
and 2.323(5) and 2.240(5) A (Zn–O) found in [ZnEt{l-
1/2
HN
N(H)C6H2Me3-2,4,6}(thf)]2 [14]. This rather long Zn–O
NH
(CH2O)n
tBu
distance in 2 [it is significantly longer than those in complex
HO
˚
8 at 2.165(2) A] suggests that the bonding between the Zn
tBu
N
N
atom and the thf ligand is weak.
tBu
Selected geometric parameters for the boat-shaped
ZnN1C1C2C3N2 ring for compounds 1 and 2 are listed
in Table 1; the atoms Zn and C2 are ca. 0.82 (1) or 0.72
OH
tBu
4, 75%
˚
Scheme 1.
(2) and ca. 0.115 (1) or 0.105 A (2) out of the

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J.D. Farwell et al. / Journal of Organometallic Chemistry 693 (2008) 1861–1869
corresponding diphenol 4 having a piperazine-N,N0-dim-
ethylene bridge, was obtained in an analogous fashion
using a 2:2:1 molar ratio of the phenol, (CH2O)n and piper-
azine, Scheme 1.
Reaction of the phenol 3a with an equimolar portion of
diethylzinc in diethyl ether afforded the dinuclear ethylzinc
phenoxide 5, which upon methanolysis, using a stoichiom-
etric quantity of MeOH (5 + 2MeOH), in thf/hexane fur-
nished the mononuclear zinc phenoxide-thf adduct 6,
Scheme 2; the presumed co-product, Zn(OMe)2, was not
isolated and the 94% cited yield is based on the ligand.
Using a procedure similar to that for 5, the analogue 7
was obtained from 3b and diethylzinc, Eq. (3). Likewise,
the bis(phenol) 4 and 2ZnEt2 were the source of the piper-
azine-N,N0-dimethylene bridged bis(phenoxoethylzinc)
complex 8, Eq. (4). Each of the crystalline compounds
5–8 (8 being isolated diastereoselectively as meso-),
obtained in good yield, contains two O,N-chelating
[OCCC(H)2N—]ꢀ ligands bonded to the zinc atom (6) or
atoms (5, 7, 8) (cf., B) in a terminal (6, 8) or bridging (5,
7) fashion; for 8, the two zinc atoms are N,N0-joined by
the N,N0-disubstituted piperazine.
Fig. 3. ORTEP representation of the molecular structure of 5 (50%
ellipsoids; H atoms omitted). Symmetry transformation to generate
equivalent atoms: ꢀx + 1, ꢀy + 1, ꢀz + 1.
The molecular structures of the crystalline centrosym-
metric compounds 5 (Fig. 3) and 7 (Fig. 4) are similar.
Each has a fused pentacyclic core built around a central
ZnOZn‘O’ rhomboid with the angle at the zinc atoms
narrower than that at the oxygen atoms. The rhomboid
is flanked by the twist boat-shaped six-membered
ZnOC1C6C7N1 rings (C7 and O1 out of the plane), the
C1 and C6 atoms of which are part of the aromatic
C1–C6 ring; the N1 atom is a member of a piperazine ring,
the N2 atom of which has a pendant phenyl (C20–C25; 5)
or methyl (C20; 7) substituent. Selected geometrical para-
meters for 5 and 7 are given in Table 2
tBu
2 ZnEt2
Et2O
tBu
Et
2 (3a)
N
N
Ph
O
Zn
Zn
Fig. 4. ORTEP representation of the molecular structure of 7 (50%
ellipsoids; H atoms omitted). Symmetry transformation to generate
equivalent atoms: ꢀx + 2, ꢀy, ꢀz + 1.
O
Et
Ph
N
N
tBu
2 MeOH
(− 2 C2H6)
(− Zn(OMe)2)
tBu
5, 84%
tBu
C6H14/thf
tBu
tBu
tBu
N
Ph
Me
N
O
O
N
Et
Zn
Et2O
N
Zn
O
N
Zn
2 (3b) + 2 ZnEt2
ð3Þ
Et
N
O
N
Me
tBu
Ph
N
tBu
6, 94%
tBu
tBu
7, 87%
Scheme 2.

J.D. Farwell et al. / Journal of Organometallic Chemistry 693 (2008) 1861–1869
1865
Table 2
˚
Selected bond lengths (A) and angles (°) for 5 and 7
5a
7b
5a
7b
Bond lengths
Zn–O1
Zn–O10
Zn–C26
Zn–C21
Zn–N1
2.063(2)
2.056(2)
1.982(3)
2.055(2)
2.084(2)
N1–C7
C7–C4
C6–C1
C1–O1
1.505(4)
1.504(4)
1.499(4)
1.405(4)
1.373(3)
1.503(4)
1.402(4)
1.369(3)
1.982(3)
2.164(2)
2.199(2)
Bond angles
Zn–O1–Zn0
O1–Zn–O10
C26–Zn–O1
C21–Zn–O1
C26–Zn–O10
C21–Zn–O10
C26–Zn–N1
C21–Zn–N1
95.60(7)
84.40(7)
125.1(1)
95.09(8)
84.91(8)
N1–C7–C6
C7–C6–C1
C6–C1–O1
C1–O1–Zn
C1–O1–Zn0
Zn–N1–C7
O1–Zn–N1
O10–Zn–N1
110.4(2)
111.2(2)
119.5(2)
118.0(2)
115.8(2)
125.9(2)
104.6(2)
91.23(8)
104.6(1)
120.5(3)
118.1(3)
119.7(2)
123.2(2)
104.5(2)
91.67(8)
98.77(8)
128.3(1)
114.9(1)
123.8(1)
128.8(1)
118.3(1)
Symmetry transformations to generate equivalent atoms.
a
ꢀx + 1,ꢀy + 1, ꢀz + 1.
b
ꢀx + 2, ꢀy, ꢀz + 1.
tBu
HO
1. Et2O
tBu
+ 2 ZnEt2
N
N
2. C6H14/thf
tBu
OH
tBu
4
ð4Þ
tBu
(thf)
Zn
Et
O
tBu
N
N
tBu
O
Zn
(thf)
Et
8, 82%
tBu
The molecular structure of the crystalline compound 6 is
illustrated in Fig. 5 (selected geometric parameters are
listed in Table 3). The molecule 6 has the zinc atom at
the spiro centre of two N,O-centred 2,4-di-tert-butyl-6-
{N0-phenyl-N-(piperazinyl)methyl}phenoxo ligands; each
of the two boat-shaped ZnOCCCN rings has a carbon
atom (C7 or C32) and an oxygen atom (O1 or O2) out of
the plane of the remaining four atoms. All the four Zn–O
and Zn–N distances in the diphenoxo complex 6 are
shorter than the corresponding distances in the phenoxo-
ethylzinc compounds 5, 7 and 8. This fact (together with
the ease of ligand redistribution reaction upon methanoly-
sis of 5) suggests that the steric bulk of this type of N,O-
centred ligand is not sufficient to stabilise mixed-ligand
phenoxo/alkoxo zinc complexes.
Fig. 5. ORTEP representation of the molecular structure of 6 (50%
ellipsoids; H atoms omitted).
Table 3
˚
Selected bond lengths (A) and angles (°) for 6 and 8
Complex 6
Complex 8
Bond lengths
Zn–O1
Zn–O2
Zn–N1
Zn–N3
1.9182(12)
1.9111(12)
2.0914(14)
2.0982(14)
Zn–O1
Zn–O2
Zn–C18
Zn–N
1.926(2)
2.165(2)
1.978(3)
2.147(2)
The molecular structure of the crystalline compound 8 is
shown in Fig. 6 (selected geometrical parameters are listed
in Table 3). The molecule is centrosymmetric about the
mid-point of the bridging piperazine ring. Its nitrogen
atoms provide one of the four donor sites to the Zn and
Zn0 atoms, each (e.g., Zn) also bound to a thf ligand (via
O2), an ethyl group (via C18) and the O1 atom of the
Bond angles
N1–Zn–O1
N3–Zn–O2
N1–Zn–N3
O1–Zn–O2
99.47(5)
98.66(5)
133.40(5)
120.36(5)
O1–Zn–N
95.79(8)
128.54(12)
125.76(11)
93.53(8)
109.43(13)
94.95(8)
C18–Zn–O1
C18–Zn–N
O1–Zn–O2
C18–Zn–O2
O2–Zn–N

1866
J.D. Farwell et al. / Journal of Organometallic Chemistry 693 (2008) 1861–1869
zophenone (C6H6, Et2O, PhMe and thf) or sodium-potas-
sium alloy (C5H12, C6H14). [Li{N(SiMe3)C(Ph)2CH}]2
and H[{N(SiMe3)C(Ph)}2CH] were prepared as described
in the literature [10]; other chemicals were commercial sam-
ples. The NMR spectra were recorded in C6D6 or C7D8 at
298 K using a Bruker DPX 300 instrument and referenced
internally to residual solvent resonances. Electron impact
mass spectra were taken from solid samples using a Kratos
MS 80 RF instrument. Melting points were determined on
an electrothermal apparatus (Philip Harris). Elemental
analyses were by Medac. Ltd.
Copolymerisation experiments were carried out in 5 mL
pressure vessels and were conducted with 100 mg of cata-
lyst. The catalyst was loaded into the vessels in a glovebox
and sealed. Subsequently, 2 mL of epoxide (PO, CHO or a
1:1 PO/CHO mixture) was added. The vessels were loaded
with 25 bars of carbon dioxide gas and heated to 80 °C.
The pressure and the temperature of the vessels were mon-
itored in time for 4 h. The copolymerisations were in batch
mode; the pressure at reaction temperature was in the
range of 40–50 bar. A pressure decrease was interpreted
as evidence for the coupling of carbon dioxide and epox-
ides. After the reaction was completed, the reactors were
cooled down, and the volatile compounds of the reaction
mixture were removed; the solid residue was analysed for
its polymer content.
Fig. 6. ORTEP representation of the molecular structure of 8 (50%
ellipsoids; H atoms omitted). Symmetry transformation to generate
equivalent atoms: ꢀx + 1, ꢀy + 1, ꢀz + 1.
six-membered boat-shaped (C7 and C1 out of the plane)
ZnNC7C6C1O1 ring.
Each of the new zinc compounds 1, 2 and 5–8 was exam-
ined for catalytic activity for the copolymerisation of pro-
pene oxide (PO), cyclohexene oxide (CHO) or a 1:1 PO/
CHO mixture and carbon dioxide at relatively low pres-
sures (40–50 bar, 80 °C). With the b-diketiminates 1 and
2 the catalytic activity was low, while with the phenoxides
only in the case of complex 5 a small CO2 uptake was
observed; however no polymer was isolated.
3.2. Preparation of [Zn{(N(SiMe3)C(Ph))2CH}-
(l-Cl)]2 ꢂ PhMe (1)
A
solution of [Li{(N(SiMe3)C(Ph))2CH}]2 (3.01 g,
8.09 mmol) in diethyl ether (50 cm3) was added to a stirred
suspension of zinc chloride (1.10 g, 8.09 mmol) in Et2O
(20 cm3) at ambient temperature. The mixture was set aside
for 16 h, toluene (10 cm3) was added and the mixture was
filtered. Volatiles were removed from the filtrate in vacuo
yielding compound 1 (3.54 g, 94%). X-ray quality crystals
were obtained from a mixture of toluene and pentane after
cooling at ꢀ30 °C for 2 d. Compound 1, m.p. 160 °C
(C49H66Cl2N4Si4Zn2 requires: C, 57.4; H, 6.49; N, 5.47.
In conclusion, six new zinc complexes containing bulky
b-diketiminato and 2-(piperazinyl-N-methyl)-4,6-di-tert-
butylphenoxo ligands were prepared as potential catalyst
precursors for copolymerisation of cyclohexene oxide and
carbon dioxide; in the event none of compounds 5–8
showed significant activity. An attempted synthesis of a
mixed-ligand zinc alkoxide (which is considered as an
active catalyst) from N,N0-bis(trimethylsilyl)-b-diketimina-
tozinc chloride (1) and ethyl (2) was unsuccessful due to
the inertness of the Zn–Cl bond in the former or the lability
of the b-diketiminato-zinc bond in the latter. The amino-
functionalised phenoxo ligands in complexes 5, 7 and 8
were not sufficiently bulky to stabilise mixed-ligand phen-
oxo/alkoxo compounds as evident from the formation of
the homoleptic zinc phenoxide 6 (Scheme 2).
1
Found: C, 56.6; H, 6.41; N, 5.60.); H NMR (C7D8): d
0.22 [s, 18H, Si(CH3)3], 2.10 (s, ꢃ1.5H, PhMe), 5.39 (s,
1H, CH), 7.03 (m, 6H, Ph; overlapped with ꢃ2H, PhMe),
7.36 (m, 4H, Ph); 13C{1H} NMR (C6D6): d 3.41 [Si(CH3)3],
106.3 (CH); 128.0, 128.2, 129.1, 145.7 (C6H5); 178.5 (s,
CPh) [signals for PhMe solvate molecule: 21.4 (PhMe);
125.6, 128.5, 129.3, 137.8 (PhMe)]; EI-MS (M denotes
the monomer) (m/z, assignment): 466, [M+H2]+, 365
[MꢀZnCl]+.
3. Experimental
3.1. General details
3.3. Preparation of [Zn{(N(SiMe3)C(Ph))2CH}Et(thf)]
(2)
Experiments involving zinc compounds were performed
under an atmosphere of argon, or in a vacuum, using
Schlenk apparatus and vacuum line techniques. The sol-
vents used were reagent grade or better and were freshly
distilled under dry nitrogen gas and freeze/thaw degassed
prior to use. The drying agents employed were sodium ben-
The b-diketimine H[{N(SiMe3)C(Ph)}2CH] (3.22 g,
8.79 mmol) in hexane (50 cm3) was added to a stirred solu-
tion of diethylzinc (8.79 cm3, 8.79 mmol, in 1 M hexane) at
ambient temperature. Volatiles were removed after 3 h and

J.D. Farwell et al. / Journal of Organometallic Chemistry 693 (2008) 1861–1869
1867
the residue was dissolved in a mixture of Et2O and thf.
After 1 d at –30 °C, X-ray quality crystals of 2 (4.35 g,
93%) were collected (C27H42N2OSi2Zn requires: C, 60.9;
EI-MS (M denotes parent) (m/z, assignment): 319 (75%,
[M]+), 100 (85%, [MeN(CH2CH2)2NH]+), 58 (100%,
[ButH]+).
1
H, 7.95; N, 5.26. Found: C, 60.8; H, 7.34; N, 5.67%.); H
NMR (C7D8): d 0.08 [s, 18H, Si(CH3)], 0.85 (q, 2H,
CH2CH3), 1.72 (t, 3H, CH2CH3), 1.40 (m, thf), 3.57 (m,
thf), 5.32 (s, 1H, CH), 6.98–7.40 (m, 6H, Ph), 7.28–7.33
(m, 4H, Ph); 13C{1H} NMR (C7D8): d 3.11 [Si(CH3)3],
5.7 (CH2CH3), 13.4 (CH2CH3), 25.8 (thf), 67.8 (thf),
105.8 (CH); 127.8, 128.2, 128.7, 146.3 (Ph); 176.3 (s,
CPh); EI-MS (M denotes parent) (m/z, assignment): 429
[MꢀEtꢀthf]+, 350 [MꢀZnꢀEtꢀMe]+.
3.6. Preparation of 2,2-[l-(N,N0-pirerazindiyl)dimethyl]-
bis(4,6-di-tert-butyl-phenol) (4)
From piperazine (2.13 g, 25 mmol), paraformaldehyde
(1.80 g, 60 mmol based on HCHO), and 2; 4-But C6H3OH
2
(10.32 g, 50 mmol), using a procedure similar to that of 3a,
there were obtained colourless crystals of 4 (9.80 g, 75%)
(C34H54N2O2 requires C, 78.1; H, 10.41; N, 5.36. Found:
1
C, 78.0; H, 10.32; N, 5.24.), m.p. 236–238 °C, H NMR
3.4. Preparation of 2-(N-phenylpiperazinyl-N0-methyl)
ꢀ4,6-di-tert-butyl-phenol (3a)
(C6D6): d 1.38 [s, 18H, C(CH3)3], 1.72 [s, 18H, C(CH3)3],
2.07 [br, 8H, N(CH2CH2)2N], 3.14 (s, 4H, CH2C6), 6.82 (s,
2H, C6H2), 7.46 (s, 2H, C6H2), 10.70 (s, 2H, OH); 13C{1H}
NMR (C6D6): d 29.95 (CH3), 31.9 (CH3), 34.3 (CH3), 35.3
(CH3), 51.8 [N(CH2CH2)2N], 61.9 (CH2C6); 120.8, 123.2,
123.7, 135.95, 140.9, 154.8 (C6H2); EI-MS (M denotes par-
ent) (m/z, assignment): 523 (30%, [M]+), 508 (45%,
[MꢀMe]+), 219 (100%, [MꢀMeN(CH2CH2)2NH]+), 85
(75%, [MeN(CH2CH2)2NH]+).
1-Phenylpiperazine (8.11 g, 50 mmol), paraformalde-
hyde (1.81 g, 60 mmol based on HCHO) and 2,4-
But2C6H3OH (10.32 g, 50 mmol) were dissolved in ethanol
(50 cm3). The solution was heated under reflux for 20 h,
then cooled to ca. 20 °C. Hydrobromic acid (8 cm3 of a
48% aqueous solution) was added. The resultant gel was
freed from volatiles in vacuo. The residue was washed with
ethanol, thereby removing its original yellow colouration.
The colourless solid was dissolved in water, and then neu-
tralised by addition of aqueous Na[HCO3], and finally
extracted into chloroform. The extract was dried (MgSO4)
and solvent removed in vacuo, thus affording colourless
crystals of 3a (15.98 g, 84%) (C25H36N2O requires: C,
78.9; H, 9.53; N, 7.36. Found: C, 78.65; H, 9.26; N,
3.7. Preparation of bis[ethyl{l-4,6-di-tert-butyl-2-(N-
phenylpiperazinyl-N0-methyl)phenoxo}zinc] (5)
Diethylzinc (3.26 cm3 of a 1 M solution in Et2O,
3.26 mmol) was added dropwise to a solution of the phenol
3a (1.24 g, 3.26 mmol) in Et2O (40 cm3) at ꢀ35 °C. The
mixture was allowed to warm with stirring to ambient tem-
perature for 16 h, then filtered. The colourless precipitate
was washed with hexane and then extracted into thf/hex-
ane. The extract was concentrated in vacuo and stored at
ꢀ25 °C to afford colourless crystals of 5 (1.30 g, 84%)
(C27H40N2OZn requires C, 68.4; H, 8.51; N, 5.91. Found:
1
7.15%.), m.p. 160–161 °C. H NMR(C6D6): d 1.33 [s, 9H,
C(CH3)3], 1.71 [s, 9H, C(CH3)3], 2.11 (br, 4H, NCH2),
2.70 (br, 4H, NCH2), 3.28 (s, 2H, CH2C6H5), 6.60 (d, 2H,
C6H5), 6.87 (t, 1H, p-C6H5), 6.88 (s, 1H, C6H2), 7.12 (t,
2H, C6H5), 7.50 (s, 1H, C6H2), 10.92 (s, 1H, OH);
13C{1H} NMR (C6D6): d 30.0 (CH3), 32.0 (CH3), 35.4
(CH3); 49.1, 54.4 (CH2N); 63.5 (CH2C6); 116.7, 120.15,
120.4, 120.9, 123.3, 123.8, 129.3, 136.15, 140.9, 155.0
(C6H5, C6H2); EI-MS (M denotes parent) (m/z, assign-
ment): 380 (40%, [M]+), 162 (100%, [PhN(CH2CH2)2NH]+).
1
C, 67.9; H, 8.43; N, 5.68%.), m.p. 129 °C (decomp). H
NMR (C6D6): d 0.47 (q, 2H, CH3CH2), 1.30 (t, 3H,
CH3CH2), 1.40 [s, 9H, (CH3)3], 1.71 [s, 9H, (CH3)3],
2.58–3.00 [br, 8H, (CH2CH2)2], 3.88 (s, 2H, CH2C6), 6.88
(d, 2H, C6H5), 6.81 (br, 1H, C6H5), 6.94 (s, 1H, C6H2),
7.14 (t, 2H, C6H5), 7.60 (s, 1H, C6H2); 13C{1H} NMR
3.5. Preparation of 2-(N-methylpiperazinyl-N0-methyl)-4,6-
di-tert-butyl-phenol (3b)
(C6D6):
d
3.43 (CH3CH2), 13.02 (CH3CH2), 31.4
[(CH3)3], 31.9 [(CH3)3], 34.2 [C(CH3)3], 35.8 [C(CH3)3];
47.0, 47.9, 52.1, 54.4 [N(CH2CH2)2N]; 63.5 (CH2C6);
116.4, 120.3, 125.45, 127.1, 129.4, 139.0, 139.5, 150.9,
159.8 (Ph, C6H2).
From 1-methylpiperazine (4.96 g, 50 mmol), parafor-
maldehyde (1.80 g, 60 mmol based on HCHO), and
2; 4-But C6H3OH (10.32 g, 50 mmol), using the procedure
2
similar to that for 3a, there were obtained colourless crys-
tals of 3b (13.69 g, 86%) (C20H34N2O requires C, 75.4; H,
10.76; N, 8.80. Found: C, 75.6; H, 10.43; N, 8.68%.),
m.p. 112–113 °C. 1H NMR (C6D6): d 1.37 [s, 9H,
C(CH3)3], 1.73 [s, 9H, C(CH3)3], 1.95 (s, 3H, CH3N),
2.11–2.66 [br, 8H, N(CH2CH2)2N], 3.73 (s, 2H, CH2C6),
6.96 (s, 1H, C6H2), 7.34 (s, 1H, C6H2), 10.92 (s, 1H,
OH); 13C{1H} NMR (C6D6): d 30.0 (CH3)3, 34.3 (CH3)3,
35.3 (CH3)3, 45.7 (CH3N); 52.4, 54.9 [N(CH2CH2)2N];
62.3 (CH2C6); 121.1, 123.6, 135.9, 140.6, 155.1 (C6H2);
3.8. Preparation of bis[4,6-di-tert-butyl-2-(N-
phenylpiperazinyl-N0-methyl)phenoxo]zinc (6)
Methanol (5.20 cm3, of a 0.5 M solution in C6H14,
2.60 mmol) was added dropwise to a stirred solution of 5
(1.23 g, 1.30 mmol) in thf/hexane (35 cm3) at ꢀ78 °C. The
mixture was allowed to warm to ambient temperature dur-
ing 4 h. Volatiles were removed in vacuo and the residue
was extracted with thf/hexane. The extract was concen-
trated to ca. 15 cm3 and stored at 0 °C, yielding colourless

1868
J.D. Farwell et al. / Journal of Organometallic Chemistry 693 (2008) 1861–1869
crystals of the zinc phenoxide
6
(1.01 g, 94%)
3.10. Preparation of [2,2-{(l-N,N0-piperazindiyl)dimethyl}-
bis{(4,6-di-tert-butyl-phenoxo)ethyl(thf)zinc}] (8)
(C50H70N4O2Zn requires C, 72.8; H, 8.56; N, 6.80. Found:
1
C, 72.5; H, 8.32; N, 6.63%.). H NMR (C6D6): d 1.42 (s,
18H, CH3), 1.62 (s, 18H, CH3), 2.83–3.10 [br, 16H,
(CH2CH2)2], 3.75 (s, 4H, CH2C6), 6.70 (d, 4H, C6H5),
6.84 (t, 2H, C6H5), 6.89 (s, 2H, C6H2), 7.12 (t, 4H,
C6H5), 7.59 (s, 2H, C6H2); 13C{1H} NMR (C6D6): d 30.3
(CH3), 32.3 (CH3), 34.2 [C(CH3)3], 35.5 [C(CH3)3]; 46.4,
47.65, 53.5, 55.0, [N(CH2CH2)2N]; 64.7 (CH2C6); 116.4,
119.1, 120.5, 124.9, 125.9, 129.9, 135.8, 138.1, 150.7,
163.5 (Ph, C6H2).
Compound 8 (1.50 g, 82%) (C46H78N2O4Zn2 requires C,
64.7; H, 9.21; N, 3.28. Found: C, 64.7; H, 9.20; N, 3.51%.)
was obtained from the bis(phenol) 4 (1.12 g, 2.14 mmol)
and diethylzinc (4.28 cm3 of a 1 M solution in Et2O,
4.28 mmol) using a procedure similar to those for the zinc
1
complexes 5 and 7. H NMR (C6D6, C5D5N): d 0.59 (q,
4H, CH3C H2), 1.30 (t, 4H, CH3CH2), 1.43 [s, 18H,
(CH3)3], 1.49 (t, 8H, thf), 1.82 [s, 18H, (CH3)3], 2.14–2.38
[br, 8H, N(CH2CH2)2N], 3.40 (s, 4H, CH2C6), 3.55 (t,
8H, thf), 6.75 (s, 2H, CH2C6), 7.62 (s, 2H, CH2C6);
13C{1H} NMR (C6D6, C5D5N): d 3.65 (CH3CH2), 13.8
(CH3CH2), 23.65 (thf), 30.7 [(CH3)3], 31.9 [(CH3)3], 34.3
[(CH3)3], 35.3 [C(CH3)3], 51.8 [N(CH2CH2)2N], 61.9
(CH2C6), 6.5 (thf); 122.3, 123.3, 125.2, 133.55, 142.5,
158.9 (C6H2). X-ray quality crystals of 8 were isolated as
a bis-thf solvate.
3.9. Preparation of bis[ethyl{l-4,6-di-tert-butyl-2-(N-
methylpiperazinyl-N0-methyl)phenoxo}zinc] (7)
Compound 7 (1.19 g, 87%) (C44H76N4O2Zn2 requires C,
64.4; H, 9.30; N, 6.80. Found: C, 64.1; H, 9.23; N, 6.89%.)
was obtained from the phenol 3b (1.06 g, 3.33 mmol) and
an equimolar portion of diethylzinc (3.33 cm3 of a 1 M
solution in Et2O, 3.33 mmol), using a procedure similar
1
to that for the zinc complex 5. H NMR (C6D6): d 0.51
3.11. X-ray crystallographic studies
(q, 2H, CH3CH2), 1.30 (t, 3H, CH3CH2), 1.39 [s, 9H,
(CH3)3], 1.45 [s, 9H, (CH3)3], 1.97 (s, 3H, CH3N), 2.08–
2.16 [br, 8H, (CH2CH2)2], 3.56 (s, 2H, CH2C6), 6.99 (s,
1H, C6H2), 7.54 (s, 1H, C6H2); 13C{1H} NMR: d 3.43
(CH3CH2), 13.02 (CH3CH2), 29.8 [(CH3)3], 32.0 [(CH3)3],
34.05 [C(CH3)3], 35.6 [C(CH3)3], 46.1 (CH3N); 53.9, 55.2
[N(CH2CH2)2N]; 65.3 (CH2C6); 119.4, 124.7, 126.2,
135.7, 137.7, 162.1 (C6H2). X-ray-quality crystals of
7 ꢂ 2(thf) were obtained by recrystallisation from thf/
hexane.
Diffraction data were collected on a Nonius Kappa
CCD diffractometer using monochromated Mo Ka radia-
˚
tion, k 0.71073 A at 173(2) K. Crystals were coated in oil
and then directly mounted on the diffractometer under a
steam of cold nitrogen gas. The structures were refined
on all F2 using SHELXL-97 [15]; absorption corrections were
applied using MULTISCAN. In 1, the poorly defined toluene
solvate molecule was disordered across a twofold rotation
axis and was refined with constraints. The molecule 5 lies
Table 4
Crystal data and structure refinement for 1, 2, 5, 6, 7 and 8
Compound
Formula
1
2
5
6
7
8
C
42H58Cl2N4Si4Zn2
C27H42N2OSi2Zn
C54H80N4O2Zn2
C50H70N4O2Zn
ꢂ C4H8O
C44H76N4O2Zn2
ꢂ (C4H8 O)2
968.04
Monoclinic
P21/n (no. 14)
8.7705(1)
19.2908(20)
15.5111(2)
90
91.066(1)
90
2623.87(5)
2
C46H78N2O4Zn2
ꢂ (C4H8 O)2
998.05
Monoclinic
P21/c (no.14)
11.0444(2)
19.2560(3)
13.4883(3)
90
90.489(1)
90
2868.47(9)
2
ꢂ C7H8
1025.06
Monoclinic
C2/c
24.0729(10)
12.3641(5)
21.3953(6)
90
122.020(2)
90
5399.3(3)
4
M
Crystal system
Space group
532.18
947.96
Triclinic
Pi (no. 2)
896.57
Orthorhombic
Pbca (no. 61)
11.7351(2)
16.2542(2)
30.6042(5)
90
90
90
5837.6(2)
8
Triclinic
Pi (no. 2)
ꢀ
ꢀ
˚
a (A)
9.6763(2)
11.1045(3)
12.5991(4)
81.006(1)
86.528(1)
76.839(2)
1301.57(6)
1
9.9905(1)
13.2993(2)
19.9840(3)
80.325(1)
77.611(1)
78.217(1)
2517.20(6)
2
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
˚
U (A)
Z
Absorption
coefficient (mmꢀ1
Unique
reflections, Rint
Reflections
with I > 2r(I)
Final R indices
[I > 2r(I)] R1, wR2
R indices (all data)
R1, wR2
1.11
0.94
0.96
0.53
0.96
0.88
)
4748, 0.050
3730
4598, 0.061
3797
4579, 0.064
4062
8842, 0.036
8153
4901, 0.049
4442
5048, 0.044
4348
0.052, 0.125
0.070, 0.136
0.040, 0.088
0.054, 0.095
0.044, 0.113
0.052, 0.118
0.035, 0.090
0.039, 0.094
0.050, 0.135
0.056, 0.140
0.042, 0.114
0.051, 0.122

J.D. Farwell et al. / Journal of Organometallic Chemistry 693 (2008) 1861–1869
1869
[4] (a) S. Inoue, H. Koinuma, T. Tsuruta, J. Polym.Sci., Part B 7 (1969)
287–292;
on a crystallographic inversion centre; there was disorder
in one of the tert-butyl groups which was included with
SAD1 constraints. Further details are found in Table 4.
(b) S. Inoue, H. Koinuma, T. Tsuruta, Makromol. Chem. 130 (1969)
210–220;
(c) S. Inoue, M. Kobayashi, H. Koinuma, T. Tsuruta, Makromol.
Chem. 155 (1972) 61–73;
(d) K. Soga, E. Imai, I. Hattori, Polym. J. 13 (1981) 407–410.
[5] D.J. Darensbourg, M.W. Holtcamp, Macromolecules 28 (1995)
7577–7579.
[6] (a) D.R. Moore, M. Cheng, E.B. Lobkovsky, G.W. Coates, Angew.
Chem., Int. Ed. 41 (2002) 2599–2602;
(b) M. Cheng, D.R. Moore, J.J. Reczek, B.M. Chamberlain, E.B.
Lobkovsky, G.W. Coates, J. Am. Chem. Soc. 123 (2001) 8738–8749.
[7] M.F. Pilz, C. Limberg, B.B. Lazarov, K.C. Hultzsch, B. Ziemer,
Organometallics 26 (2007) 3668–3676.
Acknowledgements
We thank the BASF for financial support (J.D.F. and
X.H.W.). We wish to acknowledge the use of the EPSRC’s
Chemical Database Service at Daresbury.
Appendix A. Supplementary material
[8] C.K. Williams, L.E. Breyfogle, S.K. Choi, W. Nam, V.G. Young Jr.,
M.A. Hillmyer, W.B. Tolman, J. Am. Chem. Soc. 125 (2003) 11350–
11359.
[9] Y. Sarazin, J.A. Wright, D.A.J. Harding, E. Martin, T.J. Woodman,
D.L. Hughes, M. Bochmann, J. Organomet. Chem. doi:10.1016/
j.jorganchem.2007.10.043.
[10] P.B. Hitchcock, M.F. Lappert, M. Layh, D.S. Liu, R. Sablong,
T. Shun, J. Chem. Soc., Dallton. Trans. (2000) 2301–2312.
[11] P.B. Hitchcock, M.F. Lappert, A.V. Protchenko, Chem. Commun.
(2005) 951–953, and references therein.
CCDC 289172, 654543, 654544, 654545, 654546 and
654547 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.jorganchem.2008.02.012.
[12] (a) J. Behm, S.D. Lotz, W.A. Herrmann, Z. Anorg. Allg. Chem. 619
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